# Anion-Exchange-Membrane Electrolysis with Alkali-Free Water Feed

Mohsin Muhyuddin, Carlo Santoro, Luigi Osmieri, Valerio C.A. Ficca, Ariel Friedman, Karam Yassin, Gioele Pagot, Enrico Negro, Anastasiia Konovalova, Grace Lindquist, et al.

► To cite this version:

Mohsin Muhyuddin, Carlo Santoro, Luigi Osmieri, Valerio C.A. Ficca, Ariel Friedman, et al. Anion-Exchange-Membrane Electrolysis with Alkali-Free Water Feed. Chemical Reviews, 2025, 125 (15), pp.6906-6976. 10.1021/acs.chemrev.4c00466. [https://doi.org/10.1021/acs.chemrev.4c00466](https://doi.org/10.1021/acs.chemrev.4c00466)

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# Anion-Exchange-Membrane Electrolysis with Alkali-Free Water Feed

Published as part of *Chemical Reviews* special issue "Green Hydrogen".

Mohsin Muhyuddin,⁺ Carlo Santoro,*,⁺ Luigi Osmieri, Valerio C.A. Ficca, Ariel Friedman, Karam Yassin, Gioele Pagot, Enrico Negro, Anastasiia Konovalova, Grace Lindquist, Liam Twight, Minkyoung Kwak, Enrico Berretti, Vito Di Noto, Frédéric Jaouen, Lior Elbaz, Dario R. Dekel, Piercarlo Mustarelli, Shannon W. Boettcher, Alessandro Lavacchi,* and Plamen Atanassov*

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Cite This: *Chem. Rev.* 2025, 125, 6906–6976

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**ABSTRACT:** Hydrogen is a green and sustainable energy vector that can facilitate the large-scale integration of intermittent renewable energy, renewable fuels for heavy transport, and deep decarbonization of hard-to-abate industries. Anion-exchange-membrane water electrolyzers (AEM-WEs) have several achieved or expected competitive advantages over other electrolysis technologies, including the use of precious metal-free electrocatalysts at both electrodes, fluorine-free hydrocarbon-based ionomeric membranes and bipolar plates based on inexpensive materials. Contrasting the analogous proton-exchange-membrane system (PEM-WE), where pure water is circulated (no support electrolyte), the current generation of AEM-WEs necessitates the circulation of a dilute aqueous alkaline electrolyte for reaching high energy efficiency and durability. For several reasons, including but not limited to lower cost of balance-of-plant, lower operating cost and improved device's lifetime, achieving high cell efficiency and performance using an alkali-free water feed is highly desirable. In this review, we develop and build a foundational understanding of AEM-WEs operating with pure water, as well as discuss the effects of operating with natural water feeds like seawater. After a discussion of the possible advantages of pure-water-fed AEM-WEs, we cover the thermodynamic and kinetic processes involved in AEM-WE, followed by a detailed review of materials and components and their integration in the device. We highlight the influence of electrolyte composition and alkali/electrolyte-free feed on the membrane-electrode assembly, ionomers, electrocatalysts, porous transport layer, bipolar plates and operating configuration. We provide evidence for how the pure water feed engenders several issues related to the degradation of device components and propose mitigation strategies.

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## CONTENTS

1. Introduction 6907

2. Advantages of Anion-Exchange-Membrane Water Electrolyzers 6909

2.1. Performance 6910

2.2. Hydrogen Pressure and Safety 6910

2.3. System Cost and Critical Raw Materials' Utilization 6911

2.4. Life Cycle Considerations 6911

3. Configurations and Operating Modes AEM-WEs 6911

3.1. AEM-WE Operating with Different Electrolytes 6912

3.2. Justification of Operating AEM-WE with Pure Water 6913

4. Thermodynamics and Kinetics of Water Electrolysis 6914

4.1. Bird's Eye View of the Energetics of Water Splitting in Alkaline Media 6914

4.2. Electrochemical Kinetics of Alkaline Electrolysis 6914

4.2.1. Hydrogen Evolution Reaction 6914

4.2.2. Oxygen Evolution Reaction 6917

5. Materials and Their Integration 6919

5.1. Electrocatalysts for the Hydrogen Evolution Reaction 6919

5.1.1. HER Electrocatalyst for AEM-WEs and Their Differences with Respect to A-WEs 6921

5.2. Electrocatalysts for Oxygen Evolution Reaction 6921

5.2.1. OER Electrocatalyst for AEM-WEs and Their Differences with Respect to A-WEs 6923

5.3. Impact of Membrane Electrode Assembly on AEM-WE Performance 6924

Received: June 18, 2024

Revised: June 30, 2025

Accepted: July 2, 2025

Published: August 1, 2025

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<table><tbody><tr><td>5.3.1. Role of the Membrane</td><td>6924</td></tr><tr><td>5.3.2. Role of Ionomers</td><td>6927</td></tr><tr><td>5.3.3. Role of Electrode and Active Layer<br>Fabrication and Structure</td><td>6931</td></tr><tr><td>5.4. Bipolar Plates and Porous Transport Layers</td><td>6933</td></tr><tr><td>6. Effect of Water Composition on AEM-WE<br>Performance</td><td>6935</td></tr><tr><td>6.1. Cost Analysis of Water Purification</td><td>6937</td></tr><tr><td>6.2. Effect of Electrolyte Composition and Pure<br>Water Feed on AEM-WE Performance</td><td>6938</td></tr><tr><td>6.3. Effect of Impurities</td><td>6943</td></tr><tr><td>6.3.1. Metallic Impurities</td><td>6943</td></tr><tr><td>6.3.2. Nonmetallic Impurities</td><td>6945</td></tr><tr><td>7. Cell and Stack Degradation</td><td>6948</td></tr><tr><td>7.1. Effect of Ionomer</td><td>6949</td></tr><tr><td>7.2. Ionomer Poisoning</td><td>6951</td></tr><tr><td>7.3. Effect of Electrocatalyst</td><td>6952</td></tr><tr><td>7.4. Advanced Active Layer Design</td><td>6952</td></tr><tr><td>7.5. Cell Preparation, Assembly, and Operation</td><td>6954</td></tr><tr><td>7.6. Accelerated Stability Tests</td><td>6954</td></tr><tr><td>8. Conclusions</td><td>6955</td></tr><tr><td>Author Information</td><td>6956</td></tr><tr><td>Corresponding Authors</td><td>6956</td></tr><tr><td>Authors</td><td>6956</td></tr><tr><td>Author Contributions</td><td>6957</td></tr><tr><td>Author Contributions</td><td>6957</td></tr><tr><td>Notes</td><td>6958</td></tr><tr><td>Biographies</td><td>6958</td></tr><tr><td>Acknowledgments</td><td>6960</td></tr><tr><td>References</td><td>6960</td></tr></tbody></table>

# 1. INTRODUCTION

Water availability and quality are critical factors in the sustainable production of hydrogen via electrolysis. While water is abundant, its use in electrolyzer systems presents several challenges, particularly in terms of purification, treatment, and overall operational cost. Traditional water electrolysis methods, such as alkaline water electrolysis (A-WE) and proton exchange membrane water electrolysis (PEM-WE), often require high-purity deionized water or the addition of strong alkaline electrolytes like potassium hydroxide (KOH) to enhance ionic conductivity. While effective, these approaches introduce issues related to system complexity, material degradation, and environmental impact due to the corrosive nature of alkaline solutions and the reliance on costly, often scarce materials.

Hydrogen is a critical element to achieve the decarbonization goal. The International Energy Agency (IEA) underscores the urgency of moving beyond fossil fuels, urging the achievement of a fully decarbonized society by 2050.¹ Aligning with this vision, both the EU and the US have incorporated this objective into their decarbonization agendas. This shift mandates a significant integration of renewable energy sources across all economic sectors. However, the intermittent nature of renewable energy poses various challenges. Variability in supply, intrinsic to natural weather patterns, necessitates advanced energy storage solutions for both short (days to weeks) and longer (seasonal) timeframes that will complement battery systems and other energy storage technologies.²⁻⁴

Hydrogen is a promising fuel for long-haul transportation via trucks, ships, and planes, both for fuel cells and internal combustion engines, while hydrogen vehicles promise

enhanced urban air quality. Overall, hydrogen can strengthen energy security as a fuel that can be produced domestically.⁵ In the chemical sector, ammonia and fertilizer manufacturing require new sources of hydrogen not derived from fossil fuels. For industrial processes using high temperatures, as in cement, iron, and steel production, hydrogen may play another important role as a reducing agent and for the decarbonization of such processes. Whether it is balancing renewable energy overproduction during certain periods, direct conversion to electricity via fuel cells, or being utilized as a chemical reactant, hydrogen's adaptability and broad applications are evident.

Traditionally hard-to-abate sectors are significant sources of greenhouse gas emissions and must be addressed for a successful climate strategy. In a 2019 assessment, emissions were distributed as follows: electricity and heat production (15.83 billion tons CO₂-equivalent), transportation (8.43 billion tons), industry processes (3.06 billion tons), and aviation and shipping (1.31 billion tons).⁶ While "green hydrogen" produced using renewable-sourced electricity is on the horizon, a 2021 report from the International Renewable Energy Agency (IRENA) showed that ~ 96% of hydrogen produced today originates from fossil fuels via steam reforming of natural gas (47%), coal (27%), and oil (22%). Only 4% of H₂ was produced through water electrolysis.⁷ To ensure hydrogen's role in decarbonization, its production must be free from greenhouse gas emissions. Consequently, only hydrogen production powered entirely by renewable energy can be classified as environmentally sustainable.⁸

Green hydrogen is produced by water electrolysis, using various technologies with different degrees of maturity. Some processes remain in the proof-of-concept phase – technology readiness level (TRL) 1, for instance, photoelectrochemical hydrogen production – others have progressed to lab or pilot-scale development (TRL 4–7), such as anion exchange membrane water electrolyzers, AEM-WE, discussed here. Some are navigating through the prototyping and scaling process, while others have matured, finding integration into industrial settings (TRL 8–9), notable examples being PEM-WE, A-WE, and solid oxide electrolyzer cells (SOEC).⁹

Desired properties of water electrolyzer systems include:

*   **High durability:** Ensuring components maintain consistent functionality over a prolonged duration

*   **High hydrogen production rates and electrical efficiency:** High rates (current) over a small footprint

*   **Low capital expenditure:** preferential use of abundant and cheap materials, scalable synthesis of functional materials

*   **Ability to electrochemically pressurize hydrogen:** Pressurized H₂ is needed for transportation applications and reducing the reliance on external compressors brings energy savings.

Water Electrolyzer systems are broadly classified into high-temperature (typically > 100 °C, where water is vapor) and low-temperature (slightly <100 °C, where water is liquid) systems. SOECs operate typically within the 500 to 900 °C range and use a solid ceramic separator.¹⁰,¹¹ While maintaining elevated temperatures causes challenges when integrating with intermittent renewable energy sources, SOECs can have exceptionally high efficiency due to fast electrode kinetics at high temperatures and do not need precious metals.¹² SOECs thus tend to integrate well with nuclear energy, where consistent baseload electricity and residual heat sidestep the challenges of intermittency.
<figure><img src="image_10.png" /><figcaption>**Figure 1.** Schematic of low-temperature WEs. (A) A-WE, (B) PEM-WE and (C) AEM-WE. The PTL is the porous transport layer, usually a metal- or carbon-based porous layer that supports the catalytic layer (CL), which is the electrocatalytically active region of the device. The CL is composed of electrocatalyst powder or self-supported electrocatalyst surface, ionomer solid electrolyte, porosity for water and gas transport, and also for ionic conduction when a support electrolyte is used, i.e., for A-WE and some operating configurations of AEM-WE.</figcaption></figure>

Low-temperature WEs can be subdivided into three main types, in the chronological order of invention: A-WE, PEM-WE, and AEM-WE. Their principles of operation are schemed in Figure 1.

A-WE is the oldest,¹³ most mature (TRL 8–9) and most widely manufactured electrolyzer technology.¹⁴,¹⁵ A-WE consists of anode and cathode electrodes separated by a diaphragm. Both electrodes are immersed in a water-based, highly concentrated (5–7 M) KOH electrolyte that is recirculated, guaranteeing continuous operations. The diaphragm is a porous material generally made of zirconium oxide stabilized with polyphenylene sulfide.¹⁶⁻¹⁸ The diaphragm is used as a passive separator, and the ionic transport is provided by the KOH electrolyte that fills its pores, typically several hundred nanometers in dimension. A-WEs can operate in the temperature range of 70–90 °C with an operating pressure of 1–30 bar.¹⁹⁻²²

The second type of low-temperature WE is the PEM-WE, which also has a TRL of 8–9.²³ PEM-WEs consist of an anode and cathode mounted on each side of a PEM forming the so-called membrane-electrode-assembly (MEA). The membrane is a proton-conducting polymeric material, which also electronically separates the anode from the cathode and strongly mitigates the cross-diffusion of H₂ and O₂. The state-of-the-art PEM is currently based on a polymer backbone of polytetrafluoroethylene (PTFE) with fluorinated vinyl ether lateral chains ending with sulfonic groups. The PEM structure consists of two main regions: 1) the hydrophobic PTFE backbone; 2) the hydrophilic end chains including sulfonic acid groups.²⁴,²⁵ PEM-WEs are fed with deionized water (DI) in the temperature range of 50–80 °C with an operating pressure of up to 70 bar.²⁶

The third and most recent low-temperature WE is the AEM-WE.¹⁹ AEM-WE shares the same architecture as PEM-WE with the MEA based on sandwiched anodelmembranelcathode. AEMs are composed of a polymeric backbone functionalized

with cationic groups that allow anions to move through the membrane. As PEM, here the AEM acts as a physical, nonporous separator. AEM-WE usually operates with supporting electrolytes, mainly KOH, potassium carbonate (K₂CO₃) or sodium carbonate (NaHCO₃), which, however, are more diluted compared to the A-WE ones. AEM-WEs operate in the range of temperature of 40–60 °C with an operating pressure of up to 35 bar.²⁰

AEM-WE is still in the developing phase and the wide range of the technology is due to the drastically different readiness levels of electrolyte-fed systems (TRL 6–7) and pure-water-fed systems (TRL ≤ 4).²⁷ Pure water-fed AEM-WEs, in principle, offer significant advantages to PEM-WE and A-WE by: (i) operating efficiently and durably without critical raw materials (CRM), containing the costs and improving the sustainability due to the reduction of CO₂ emissions caused by the energy-intensive mining of rare metals like iridium; (ii) operating without supporting electrolyte, enabling the use of inexpensive stainless steel components in the balance of plant (BoP) and simplifying BoP design, substantially reducing capital expenses, (iii) operating expenditure decrease avoiding the usage of KOH or other supporting electrolytes that have a cost and their production is energy intensive,¹⁶ (iv) operating with polymeric membranes that do not contain potentially harmful fluorinated polymers; (v) pressurizing hydrogen internally across the membrane using voltage (lowering the operation costs); and vi) efficiently maintain separated hydrogen and oxygen by using a polymeric membrane (avoiding safety issues of A-WE, especially under intermittent operation). If AEM-WE operating with a pure-water feed can match the performance metrics reached to date with electrolyte feeds, it will open the door to a competitive technology that has the potential to overcome the issues of both PEM-WEs and A-WEs while keeping their advantages. Thereby, it would offer a solution to produce green H₂ at a competitive cost and in a sustainable way. Despite the
increased interest in AEM-WE research in the past decade, progress with pure water-fed AEM-WEs has lagged behind, as most studies have hitherto been conducted with supporting electrolytes, as reported in Figure 2, where the number of publications related to devices using AEM, AEM-WE operating with supporting electrolytes and water-fed are shown.

<figure><img src="image_11.png" /><figcaption>**Figure 2.** Annual number of publications in the field of AEMs (from Web of Science, accessed June 25, 2025). Search terms: "Anion exchange membrane", "electrolyte-fed AEM-WE", "water" and "electrolysis".</figcaption></figure>

This review focuses on AEM-WE operating without the use of supporting electrolytes (e.g., KOH), using nominally pure water feed. We also discuss progress and challenges feeding abundant natural water, like seawater and the effect of elements, anions, cations, and various molecules is also discussed. In AEM-WE, KOH is typically added to the water as an electrolyte to enhance the ionic conductivity that significantly boosts the performance; however, the operational durability still remains under question due to the stability limitations of the anion exchange polymer under harsh anodic conditions.²⁸ Adding KOH even at concentrations as low as 1 M creates a very corrosive environment (especially at the typical operating temperature of 70–90 °C) that necessitates using expensive components in the system. Seawater, despite

being a practically “free” electrolyte with no water purification cost, is also quite corrosive, particularly as chloride can be converted during operation to highly oxidizing products at the anode, affecting the durability of the AEM-WE. Cells operating with pure water will have lower operational costs if the underlying stability and efficiency issues can be solved. This would be beneficial, even though the feedwater must be demineralized and/or purified, because these processes are not energy-intensive compared to the energy required to produce hydrogen. Currently, the industry is commercializing AEM-WEs operating with dilute KOH because the cost and performance are reasonably compelling. However, the interest remains extremely high in AEM-WE operating with pure water without supporting electrolytes.

This review is divided into seven main sections. Following the introduction (section 1), we discuss in section 2) the theoretical advantages of performing WE in neutral to high pH environments rather than in low pH environments. In section 3) the configurations of AEM-WE and its operation with pure water are described. The quality parameters of water and the cost of its purification are considered. In section 4) the fundamental aspects related to the thermodynamics and the kinetics of water electrolysis occurring in the presence of pure water with a circumneutral pH are highlighted. In section 5) the materials are described considering the main components of the AEM-WE, e.g., ECs, AEM and anion exchange ionomer (AEI). Their integration into MEAs and the effects of pure-water feed are discussed. Key properties and functions of the porous transport layer (PTL) and bipolar plate (BP) are presented. In section 6) the advantages and disadvantages of using pure water feed in AEM-WE are discussed in terms of performance. Comparisons with AEM-WE systems operating with supporting electrolytes are drawn and the intrinsic challenges to the use of pure water are highlighted. Last, in section 7) the durability and degradation phenomena of AEM-WEs operating with pure water feed are discussed as well as mitigation strategies.

## 2. ADVANTAGES OF ANION-EXCHANGE-MEMBRANE WATER ELECTROLYZERS

To define the roadmap for the large-scale deployment of WEs and their diffusion and integration in the various sectors, a fair and critical comparison is crucial, considering their maturity stages, or in other words, the TRL. For practical application, the performance, cost, durability and sustainability must all be considered. For cost reasons, but also environmental impacts and supply chain risk, the utilization of CRMs in electrolyzer devices and other energy-related technologies is receiving increased attention. The list of CRMs is regularly updated in the EU by the EU Commission.²⁹ While the criticality of a material can depend on a nation's perspective, one can observe that the USGS 2022 CRM list and the EU 2023 CRM list have a strong overlap.

The chemical compositions of the various components differ for different types of low-temperature devices, in particular due to their exposure to liquids of vastly different pH values. The main components of low-temperature WEs are: 1) polymeric membrane and/or separator, 2) electrode/EC (anode and cathode), 3) porous transport layer (anodic and cathodic), 4) bipolar plate (anodic and cathodic), 5) frames and sealings. The simplified chemical composition and/or architecture of each component for A-WE, PEM-WE and AEM-WE is summarized in Table 1. Many materials utilized are not
<table><caption>Table 1. Material Compositions of the Main Components of WEs<sup>a</sup></caption><thead><tr><th>Component</th><th>A-WE</th><th>PEM-WE</th><th>AEM-WE</th></tr></thead><tbody><tr><td>Electrode/EC (oxygen side)</td><td>Nickel-coated perforated stainless steel</td><td>Iridium Oxide</td><td>High surface area Nickel or NiFeCo alloy</td></tr><tr><td>Electrode/EC (hydrogen side)</td><td>Nickel-coated perforated stainless steel</td><td>Platinum nanoparticles on carbon black</td><td>High surface area nickel</td></tr><tr><td>Anode side PTL</td><td>Nickel mesh (not always Present)</td><td>Platinum-coated sintered porous titanium</td><td>Nickel foam</td></tr><tr><td>Cathode side PTL</td><td>Nickel mesh</td><td>Sintered porous titanium or carbon cloth</td><td>Nickel foam or carbon cloth</td></tr><tr><td>Anode-side bipolar plate</td><td>Nickel-coated stainless steel</td><td>Platinum-coated titanium</td><td>Nickel-coated stainless steel</td></tr><tr><td>Cathode-side bipolar plate</td><td>Nickel-coated stainless steel</td><td>Gold-coated titanium</td><td>Nickel-coated stainless steel</td></tr><tr><td>Frames and sealing</td><td>PSU, PTFE, EPDM</td><td>PTFE, PSU, ETFE</td><td>PTFE, Silicon</td></tr></tbody></table>

<sup>a</sup>Components can vary among different companies. Data is reproduced from ref 16. Copyright IRENA (2020), Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5 °C Climate Goal, International Renewable Energy Agency, Abu Dhabi.

uniformly adopted by companies but can differ significantly, as highlighted by the colored cells of Table 1. Generally, it can be seen that for PEM-WE some platinum group metals (PGMs) such as Pt and Ir and some other CRMs such as Ti, and precious metals (e.g., Au) are systematically used (Table 1).<sup>16,30</sup> In contrast, fewer (or no) precious metals are used in A-WE and this situation is also envisioned to apply to optimized AEM-WEs, without compromising the performance. However, especially in AEM-WE, Pt/C is still often used to catalyze the hydrogen evolution reaction (HER), due to the lower overpotentials achieved with Pt/C as compared to PGM-free electrocatalysts (ECs). In general, more expensive materials are used for BP, PTL and ECs on the anode side, which presents a harsh oxidative environment.

## 2.1. Performance

The significant figures of merit related to the performance of WEs are taken from the Clean Hydrogen Partnership (CHP, former Fuel Cell and Hydrogen Joint Undertaking, FCH JU, initiative) targets.<sup>31</sup> These figures are related to the stage of TRL where the low temperature WE stands. In other words, a comparison in performance between A-WE and PEM-WE can be considered fair, as the figures of merit are related to the system at the MW scale. Instead, direct comparison between A-WE and PEM-WE vs AEM-WE seems to be harsh to envision and be completely fair, as AEM-WE technology has not yet reached the readiness to be scaled up at the MW scale. However, those figures are important to take into consideration. In general, in the CHP targets, an increase in current density is envisioned concurrently with an improvement in stability and durability, a decrease in electricity consumption and a decrease in CRM used and consequently a reduction in costs.

State-of-the-art A-WEs performance metrics in 2020 are reported to operate at an imposed current density of 0.6 A cm⁻² (1.9 V). Similar current density is envisioned for 2024 despite a decrease in: (i) CRM from 3.4 to 2.1 mg cm⁻²; (ii) degradation rate from 0.12 to 0.11% per 1000 h; (iii) electricity consumption at the nominal capacity from 50 to 49 kWh kg⁻¹.<sup>31</sup> State-of-the-art PEM-WEs (in 2020) should operate at an imposed current density of 2.2 A cm⁻² (2.1 V).<sup>31</sup> Whereas the target for 2024 is set for 2.4 A cm⁻² (@1.95 V) and the target for 2030 is 3 A cm⁻² @1.9 V. Moreover, other parameters are also proposed to decrease by 2024. Particularly: (i) PGM loadings in electrodes should decrease from 2.7 to 1.25 mg cm⁻²; (ii) degradation rate from 0.19% to 0.125% per 1000 h; (iii) electricity consumption at the nominal capacity from 55 to 52 kWh kg⁻¹.<sup>31</sup> On the other hand, a rapid performance improvement is expected as important efforts and resources have been recently devoted to AEM-WEs technology. The operating current density of AEM-WE is expected to

increase from 0.5 A cm⁻² (2.07 V) in 2020 to 0.6 A cm⁻² (2.0 V) in 2024 and the final goal is 1.5 A cm⁻² (1.8 V) in 2030.<sup>32</sup> An important decrease in electricity consumption is expected over time from 55 kWh kg⁻¹ (2020) to 53 kWh kg⁻¹ in 2024 and the final goal is 48 kWh kg⁻¹ in 2030.<sup>32</sup> Importantly, the utilization of CRMs is expected to decrease from 1.7 mg W⁻¹ in 2020 to 0.4 mg W⁻¹ in 2024 and is envisioned to eventually disappear in 2030.<sup>32</sup> However, following the IRENA analysis that makes a screenshot of the current situation, it can be seen that today the operating current density of AEM-WE is variable (0.2–2 A cm⁻²) with a recorded voltage of 1.4–2.0 V.<sup>16</sup> IRENA analysis envisioned a long-term target of >2 A cm⁻² for 2050 (at a cell voltage <2 V).<sup>16</sup> The present performance attributes and the expected trends of PEM-WE at acceptable efficiency (cell voltage) are much higher compared to A-WE, although this is offset by the higher capital cost of PEM-WE, which has to be mitigated seriously.<sup>20</sup>

## 2.2. Hydrogen Pressure and Safety

Direct production of pressurized hydrogen (30–70 bar) within WEs is an important goal because it strongly reduces the additional energy needed for the final compression to 350–700 bar, needed for transportation. In WEs, the hydrogen can be pressurized internally with only a small penalty (increase) in thermodynamic cell voltage. The latter is calculated to be 34 mV at 25 °C and 48 mV at 150 °C when varying the gas pressure from 1 to 200 atm and considering that the gas pressure is the same at both electrodes. Recently, a voltage drop of 55 mV due to an increase in the differential pressure was calculated at 60 °C.<sup>33</sup>

While thermodynamics does not predict a strong performance penalty upon high internal pressurization of H₂ (and O₂), the practical WEs might undergo mechanical stress, component failure, or safety issues. A-WE does not historically possess a physically continuous separator, as the diaphragm is porous. Therefore, high differential pressure between the anode and cathode compartments leads to a high crossover of the gaseous products, with relevant safety issues and potential explosions. Moreover, as the separator is porous, the produced hydrogen in A-WE might have lower purity (99.0 to 99.5 vol. %) compared to PEM-WE, especially when operating intermittently, and it needs to go through drying and additional oxygen removal before reaching the required purity.<sup>14</sup> In practice, A-WE operates at low internal pressure (up to ~ 20 bar, balanced with O₂ and H₂) and therefore the hydrogen produced needs to be pressurized externally with additional capital (CAPEX) and operational (OPEX) costs of mechanical compressors. The use of nonporous polymeric ionomer membrane, such as in the case of PEM-WE and AEM-WE, allows the direct production of pressurized hydrogen with a relevant reduction of operating costs, without significantly
pressurizing the produced oxygen that is typically vented to the atmosphere. The polymeric membrane separator also allows the production of hydrogen with a high level of purity (typically up to 5.0 grade or 99.99999 vol.%).³² PEM-WE and AEM-WE also display higher performance than A-WEs due to relatively thin membranes, leading to lower cell ohmic resistance than A-WEs. However, thin membranes can face mechanical failure at lower operating pressure, and overly thin membranes can lead to significant H₂ or O₂ crossover. Oxygen crossover through PEM was evaluated in studies related to PEM fuel cells, where hydrocarbon membranes showed 1 order of magnitude less oxygen permeability compared to fluorinated polymers.³⁴,³⁵ Hydrogen and oxygen crossover through AEM is less studied. In PEM-WE, recombination layers are used on the anode side to enable the reaction between the hydrogen being permeated through the membrane and the generated oxygen, forming water. In this way, the concentration of hydrogen in the anodic oxygen flow remains below 4 vol.%, avoiding eventual safety issues.³⁶

A compromise between membrane thickness and operating pressure must be found to ensure optimal performance, without jeopardizing the stability and durability of the system.³⁷ Due to the maturity of PEM, know-how and greater investment in this technology, at the moment, PEM-WEs can operate at higher pressure compared to AEM-WE. According to the IRENA analysis, the general goal is to increase the current system pressure.¹⁶ The envisioned Key Performance Indicators (KPIs) by IRENA for 2050 are >70 bar (A-WE), > 70 bar (PEM-WE) and >70 bar (AEM-WE).¹⁶

## 2.3. System Cost and Critical Raw Materials' Utilization

While briefly analyzing the materials used for low-temperature WEs, as mentioned above, PEM-WE relies strongly on PGMs.³⁰ For instance, Pt is not only used in the PEM-WE cathode EC, but it is also a crucial component in the anode PTL and BP for corrosion protection in harsh oxidative environments. The loading of Pt is quite high and it heavily affects the overall cost of the device.¹⁶ Moreover, the use of CRMs is economically nonviable due to uncertain supply and skyrocketing costs since their mining is localized in a few socio-politically unstable regions. A-WEs involve CRM-free components that are stable in the alkaline operating environment; hence, cost-effectiveness and scalability can be ensured. On the other hand, although the performance attributes of AEM-WE are encouraging, it is still in the development phase and significant scientific efforts are needed to become commercially rationalized. AEM-WEs are currently relying on PGMs, especially on the cathode where Pt/C is used; however, in the medium or long-term perspective, CRM-free components are expected to be widely used in AEM-WEs as highlighted by the roadmap identified by the Clean Hydrogen Partnership (CHP) for 2030.³² Another factor is the operational durability, PGM nanoparticles supported over the carbon, typically employed at the AEM-WE's cathode, are highly unstable in the KOH electrolyte.³⁸⁻⁴² and may also undergo degradation at the AEM interface.⁴³ when subjected to potential cycling-a situation that is unpreventable during the startup and shutdown operations. Therefore, similar to A-WE, the use of CRM/PGM-free materials will not only lower the CAPEX of the AEM-WE but also help in enhancing the performance durability.

## 2.4. Life Cycle Considerations

In scaling technology, environmental impacts and sustainability are of utmost importance. CRMs imply problems related to the high cost and shortage in the supply chain in the case of large utilization, but they also mean difficulties in mining and material purification. Despite these environmental issues and recent interests, to date, there is no valid procedure or internationally shared protocols to recycle PGMs derived from WEs and fuel cells.⁴⁴⁻⁴⁶ Anyways, A-WEs and AEM-WEs can rely on CRM-free components, which should also translate to a lower carbon footprint during manufacturing and environmental safety.

Another important aspect is the use of fluorinated polymers in PEM-WEs. These polymers are based on per- and polyfluoroalkyl substances (PFAS) that are resilient to degradation, accumulate in the environment (mainly in water and soil) and can enter and accumulate in the food chain, negatively affecting the whole ecosystem.⁴⁷ PFAS can also accumulate in human bodies and cause a series of negative and deadly diseases.⁴⁸,⁴⁹ The European Union has issued a ban on PFAS for 2035,⁵⁰ and this could affect the manufacturing and sale of PEM-WE, despite the WE is a closed system where PFAS can be prevented by proper handling at the end of life and during water changes. Despite alternative hydrocarbon-based ionomers having been investigated for more than a decade,⁵¹ a robust and resilient substitution has not yet been found. The membrane chemistries in A-WE and AEM-WE typically do not use PFAS. Teflon can be used for sealing, but alternatives can be found.

A recent study on life cycle assessment (LCA) on water electrolyzers highlighted that AEM-WE technology has a lower environmental impact compared to PEM-WE in 24 impact categories (out of 27), including climate change.⁵² Naturally, this comparison is not exactly fair since PEM-WE is commercially available while AEM-WE is still not fully developed. On a positive note, a large room for improvement is foreseen and further eventual decrease in the environmental impact can be expected.

# 3. CONFIGURATIONS AND OPERATING MODES
AEM-WES

Given the considerations discussed above, AEM-WE will be suited for hydrogen production at a massive scale if the envisioned performance and durability levels can be reached. Given the low current TRL, tremendous R&D efforts are needed to bring deployable AEM-WE, particularly for systems with pure water feed that truly combine the advantages of both PEM-WE and A-WE. Operating with a water feed and locally alkaline environment, PGMs could be potentially substituted with PGM-free and CRM-free materials. This aspect is crucial because, to date, Pt/C is still the best EC for HER in alkaline media and important work is still ongoing and needed to fully replace PGMs at the cathode.⁵³,⁵⁴ The substitution of PGM with PGM-free materials is beneficial in terms of cost, usage, or rare and critical materials and so it contributes to the wide commercialization of water electrolyzers.

Compared to A-WE, the use of a polymer membrane in AEM-WE is important for driving performance due to the vicinity of the electrodes assembled on the membranes lowering the ohmic resistance of the MEA. The presence of a dense and nonporous polymeric membrane allows the pressurization of hydrogen without compromising the safety of
the device. Continued engineering and thinning of the
membrane may lead to dramatically higher currents in AEM-
WE. The lack of liquid electrolytes and the presence of flow
fields in the BPs may further facilitate gas collection compared
to A-WE.

The main configurations for feeding the water/electrolyte in
AEM-WEs are reported in Figure 3. The first one involves

<figure><img src="image_12.png" /><figcaption>**Figure 3.** Schematic of the operating configurations for AEM-WE with supporting electrolyte fed on both anode and cathode (A), or only fed to the anode side (B) or the cathode side (C). Schematic of the operating configurations for AEM-WE with pure water fed on both the anode and cathode (D), or only fed to the anode side (E) or the cathode side (F).</figcaption></figure>

feeding them with supporting electrolyte (e.g., KOH) in both
the anode and cathode compartments (Figure 3.A) or simply

to the anode side (Figure 3.B) or the cathode side (Figure 3.C). The second configuration involves the feeding of pure water that can be fed to both compartments (Figure 3.D) or is fed uniquely at the anode (Figure 3.E) or the cathode compartment (Figure 3.F). The configuration in which pure water is fed uniquely to the anode side is the one used commonly in PEM-WE, in agreement with the fact that water is the sole anode reagent in acidic OER.<sup>30,55</sup> All the configurations shown above are used and up to now, there is no clear and conclusive operational configuration that outperforms the others. Interestingly, the configuration with only anode feeding (Figure 3.B and 3.E) is counterintuitive in alkaline conditions. This is because water is a reagent for the hydrogen evolution and therefore it would be required at the cathode compartment for evolving the gas of interest. In practice, water is provided for HER by osmosis, diffusion, or drag from the anode compartment. Interestingly, this configuration is often more efficient because hydrogen is produced with the cathode electrode not submerged in a liquid and, therefore, the release of gaseous hydrogen encounters lower resistance than if water (or KOH) were fed to the cathode. The produced hydrogen also includes a lower quantity of water in such a configuration, which is beneficial because the hydrogen produced needs to undergo dewatering before being further pressurized to be stored or used. However, in some cases, other water feed modes may be advantageous.

Another important aspect is to evaluate what is actually
occurring within the core of the single-cell AEM-WE, named
the MEA. A cross-section of a single cell AEM-WE is
schematized in Figure 4. Key interfaces for both electrical
and ionic interconnections among the different layers are
identified and highlighted. Transport phenomena occurring at
different scales and involving different layers and different
electrically and ionically conductive components are also
labeled in Figure 4. Importantly, the spatial scale of the
transport phenomena taking place in the various components
is reported.

**3.1. AEM-WE Operating with Different Electrolytes**

AEM-WE operates with a wide array of feeding solutions,
including deionized water (DI),<sup>56-60</sup> seawater and brackish

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Cross section of a single cell AEM-WE identifying the key interfaces for electrical and ionic interconnection and particularly highlighting (A) catalyst electrical connection and utilization; (B) carbon/platinum interaction with the ionomer; (C) Hydroxide transport within the AEM; (D) pressure gradient within the catalytic layer.</figcaption></figure>
water,<sup>61–66</sup> impure tap water,<sup>67</sup> or water with supporting electrolytes such as potassium hydroxide (KOH),<sup>68–71</sup> sodium or potassium carbonate (Na<sub>2</sub>CO<sub>3</sub> or K<sub>2</sub>CO<sub>3</sub>)<sup>19,72</sup> and others. To date, however, none of these feed solutions, except for KOH, provide expected performance and durability metrics for commercialization. There are many possible issues with fouling of the electrodes with impure feeds. Further, any salt present will lead to concentration polarizations that reduce efficiency through a concentration overpotential. A simple example is the use of a carbonate feed. Carbonate concentration polarization leads to a pH gradient across the device and an increase in the Nernstian voltage for electrolysis.<sup>73</sup>

## 3.2. Justification of Operating AEM-WE with Pure Water

Scientists and industries are increasingly interested in utilizing pure or distilled water for the operation of AEM-WEs. In parallel with the PEM-WE, the AEM-WE operating with purified water brings a complication since purified water needs to be produced. The water available on the planet is 97% seawater and 3% freshwater.<sup>74</sup> Seawater is too salty to be consumed directly ("as it is") by humans or animals, to be used to grow crops and it is quite complicated (if not impossible) to be used for industrial purposes, except for very specific purposes (e.g., cooling). Fresh water supply is under stress due to the anthropogenic impact on the environment and this stress is increasing significantly with time. Each year, humanity consumes 4600 billion m<sup>3</sup> of water for its activity, 70% of which is spent in agriculture, 20% in industry, and 10% in domestic usage.<sup>75</sup> These numbers are the result of a steady increase in fresh water consumption of roughly 1% per year since 1980,<sup>76</sup> every single drop of which comes from the freshwater reservoirs of Earth, accounting for 3% of the global water.<sup>77,78</sup> The concern about water affordability for the future is thus real and mainly related to the increase in world population, directly translating into increased food and drinking water requests along with energy demand. Actually, within the water consumption of the industry sector, 75% of that is related to the conversion of energy, with the most demanding energy sources being biofuel, fossil fuel, and nuclear energy.<sup>79</sup> The production of renewable electricity directly from wind and solar does not need water for cooling or operating and the impact on water consumption (volume of water per MWh electricity) is *ca* 100 times lower than that of fossil fuels and nuclear power plants.

In the first decade of this century, some researchers expressed their skepticism about the hydrogen economy,<sup>79–81</sup> especially in terms of water consumption. However, in the past decade, a vast research effort improved the efficiencies of the WE, paving the way to the mass production of green hydrogen with limited impact on planet resources. Regarding the impact on Earth's water reservoirs, a distinction must be made between water withdrawn and consumed: the former is intended as the quantity of water that is extracted and then returned to the whole body of water, while the latter refers to the amount that is lost. A recent study by Oliveira et al.<sup>82</sup> estimated the future total demand for green hydrogen at 2.3 Gt per year, corresponding to 20.5 billion m<sup>3</sup> of withdrawn freshwater.<sup>83</sup> These numbers were calculated considering the stoichiometric ratio of the water splitting, which requires 9 kg of water to produce 1 kg of hydrogen. However, considering the requirement of distilled water for the WE operation, the multiplication coefficient could be higher. Other authors have calculated a global water demand of 34.5 billion m<sup>3</sup>.<sup>84</sup>

Nevertheless, in comparison, the power generation and energy production from fossil fuels withdrew 251 billion m<sup>3</sup> of freshwater and consumed 31 billion m<sup>3</sup> in annexed operations like mining, cooling, refining, etc.<sup>85</sup> At the same time, agriculture impacts on freshwater account for 2700 billion m<sup>3</sup>, more than 40% of which is consumed.<sup>85</sup> It is thus straightforward that future water utilization for hydrogen production will have a modest impact on the environment, estimated at 1.5 ppm of Earth's total freshwater.<sup>83</sup> Moreover, the circularity of the economy already sets alternatives to water withdrawal, exploiting waste products that must be processed before discarding, e.g., wastewater produced in large amounts each day. This could help to boost the spread of the technology in developing countries and especially in dry/desertic regions of the planet where access to fresh water is limited,<sup>84</sup> leading to a real zero impact on freshwater reservoirs along with reduced effects on the environment. In addition, the volume fraction of water for the projected demand of H<sub>2</sub> can be further reduced if seawater is considered instead of freshwater. In this case, desalination operations and their water consumption must be considered in the life cycle assessment of WE, leading to an amount of water required for green hydrogen that was estimated at 30 ppb of seawater per year.<sup>83</sup> The use of seawater to produce green hydrogen can thus be considered negligible.

The possibility of operating AEM-WE with impure, or at least less purified, water was recently shown<sup>86</sup> and compared to the distilled water used in PEM-based technology. Therefore, it would seem logical to operate the AEM-WE with readily available seawater due to its abundance and lower effect on resource availability. However, the presence of anions and cations, solids and microorganisms<sup>67</sup> might lead to a faster deterioration of materials, leading to a shortening of the useful lifetime of the AEM-WE<sup>86</sup> challenging the targeted durability in the order of thousands of hours.

Therefore, irrespective of the primary source, water fed to the AEM-WE must be purified. The three main methodologies for such a purpose are (i) thermal treatments, usually based on solar or geothermal heat like multiple effect distillation (MED) where seawater is evaporated, cooled, and condensed, (ii) membrane distillation, based on thermally driven physical filtration through semipermeable membranes, and (iii) reverse osmosis (RO) based on pressure-driven membrane filtration.<sup>87,88</sup> Actually, the trend of this field of research is to develop hybrid or cogenenerated systems to reduce costs and increase the efficiency of energy recovery.<sup>89–92</sup> Different purification processes, along with a comparison of cost per m<sup>3</sup> and per kWh, were extensively revised by Yadav et al.<sup>87</sup> Indeed, the MED system is considered more expensive, with 80.6 kWh of heat per m<sup>3</sup> of freshwater produced, while RO is less energy demanding with just 3–5 kWh of power per m<sup>3</sup> of purified water.<sup>93</sup> It is thus straightforward that RO will be the leading technology to be coupled with AEM-WE, with estimated costs in 2009 of 0.5 – 1.5 USD per m<sup>3</sup> of purified water,<sup>77</sup> which decreased to 0.33 USD per m<sup>3</sup> in 2018.<sup>89</sup> These numbers are not negligible but strengthen the direction of pursuing the utilization of purified water as a feeding electrolyte for AEM-WE since the cost and energy used to purify the water impact the total cost of 0.01 USD per kg of H<sub>2</sub> produced.<sup>83,94</sup> Naturally, these considerations are done considering only the operational costs, without considering the CAPEX.
# 4. THERMODYNAMICS AND KINETICS OF WATER ELECTROLYSIS

## 4.1. Bird's Eye View of the Energetics of Water Splitting in Alkaline Media

This section focuses on defining the most relevant aspects of water electrolysis that are influenced by pH variations, which can occur locally in pure water electrolysis systems. It is well-known that pH, along with other factors like impurities in the water and the nature of supporting electrolytes, significantly impacts the energetics of electrolysis. Both the HER and oxygen evolution reaction (OER) are pH-dependent, with the most notable effect observed on HER, which is much slower in alkaline and neutral electrolytes compared to acidic systems.⁹⁵ To understand and rationalize these effects, as well as to explore strategies for improving performance, this section consists of an overview of the chemical thermodynamics of water splitting and the kinetics of both OER and HER for alkaline systems.

Water is a notoriously stable molecule and the water-splitting reaction (eq 1) has a Gibbs free energy which defines a theoretical cell voltage of 1.23 V between the anode and the cathode of a WE, at standard conditions.

$$H_2O \rightarrow H_2 + \frac{1}{2}O_2 \quad (1)$$

Electrolyzers usually operate at a larger cell voltage than the theory predicts, as many other factors significantly contribute to the overall voltage when the cell delivers a current. The different contributions to the total voltage of an operating WE can be subdivided according to eq 2

$$E_{ap} = 1.23V + \eta_{HER} + \eta_{OER} + \Delta V_{other} \quad (2)$$

where $\eta_{HER}$ and $\eta_{OER}$ are the overvoltages for hydrogen and oxygen evolution, respectively. From eq 2, it can be seen how thermodynamics and kinetics both affect the operating potential quantitatively. These values depend strongly on the current density and the type of ECs, as the activation energy of both reactions is strongly affected by the nature of the catalytic surface. In addition, some potential ($\Delta V_{other}$) is needed to overcome the resistances of electrolytes, contacts, and membranes, as well as the parasitic resistance of side reactions such as electrode corrosion and oxygen reduction.⁹⁶

The dependence of the thermodynamic potential of water splitting on pressure and concentration is described by the Nernst equation (eq 3).

$$E = E_o(T) + \frac{RT}{2F} \ln \left( \frac{a_{H_2} \Delta a_{O_2}^{1/2}}{a_{H_2O}} \right) \quad (3)$$

Remarkably, eq 3 shows that the thermodynamics of water splitting does not depend on pH. Current AEM-WE technologies can reach 1 A cm⁻² even at less than 2 V using KOH electrolytes, depending strongly on whether PGM or PGM-free ECs are used. These values cannot be achieved with conventional alkaline electrolysis (A-WE), where the maximum operating currents are generally around 0.7 A cm⁻².³¹ Depending on the applied current density, overpotentials of the half-reactions (HER and OER) contribute to the overall water splitting overpotential to the extent of 30–40%. To understand the chemical basis of this, it must be considered the cathode and anode half reactions of water occurring in alkaline systems that are described by eq 4 and eq 5:

$$\text{CATHODE HALF REACTION: } 2H_2O + 2e^- \rightarrow H_2 + 2OH^- \quad (4)$$

$$\text{ANODE HALF REACTION: } 4OH^- \rightarrow O_2 + 2H_2O + 4e^- \quad (5)$$

It is worth mentioning that eq 4 and eq 5 are only the total electrochemical reactions taking place at the cathode and anode, respectively. To fully understand the relevance of the pH in the kinetics, and its implication in high or mild alkaline environments or with pure water (circumneutral pHs), the elementary steps occurring in HER and OER need to be broken down and the rate-determining steps (RDS) must be identified. A view on the current knowledge in HER and OER kinetics is the focus of the next two subsections, where, despite the extensive research efforts, many aspects remain unsolved. In fact, a universal descriptor for alkaline HER and OER able to predict the behavior of all the different EC families is still a “Chimaera”.

## 4.2. Electrochemical Kinetics of Alkaline Electrolysis

### 4.2.1. Hydrogen Evolution Reaction.
In alkaline systems, the kinetics of the HER is 2–3 orders of magnitude slower than in acids and becomes comparably limiting to the OER process at the anode, even with PGM-based ECs. A strong research focus on HER in alkaline and on PGM-free (CRM-free) ECs is hence justified, with a huge potential to deliver solutions to increase the energy efficiency of electrolysis.⁹⁷

The widely accepted breakdown of the alkaline HER into its elementary steps is described by the set of eqs 6 and 8 or eqs 7 and 8.⁹⁸

$$H_2O + e^- + M \leftrightarrow M - H_{ad} + OH^- \quad (6)$$

$$H_2O + M - H_{ad} + e^- \leftrightarrow M + H_2 + OH^- \quad (7)$$

$$2M - 2H_{ad} \leftrightarrow 2M + H_2 \quad (8)$$

Equation 6 is known as the Volmer step, which consists of the adsorption of hydrogen onto the metal. Remarkably, this step differs for alkalis from that for acids because the proton source is water, not hydronium ions.⁹⁹

The adsorbed hydrogen can then be converted into molecular hydrogen either by the Heyrovsky step (eq 7), or the chemical Tafel recombination (eq 8). The use of the Tafel slope can help in identifying the rate-determining step (RDS). Typically, it is ca. 120 mV dec⁻¹ when the Volmer step is rate-determining, while it is ca. 40 and 30 mV dec⁻¹ when the Heyrovsky or Tafel step is rate-determining, respectively.¹⁰⁰ Thus, the identification of the RDS due solely to the value of the Tafel slope might be considered. However, the surface coverage of the adsorbed hydrogen may also play a crucial role in the reaction kinetics. Indeed, Tafel slopes of 120 mV dec⁻¹ have been observed for the HER when the coverage of adsorbed hydrogen exceeds 0.6, even when the RDS is the Heyrovsky step. The Tafel slope is also potential dependent and, according to microkinetic models, this may result in turn from the potential dependence of the adsorbed hydrogen coverage. A comprehensive picture of the HER can be obtained from microkinetic models based on accurate atomistic modeling.¹⁰¹

In an alkaline system, as in acids, the HER activity follows a typical volcano plot. The Volcano diagram is obtained by plotting a parameter related to the reaction rate (e.g., current density normalized by the electrochemical surface area, or
<figure><img src="image_14.png" /><figcaption>**Figure 5.** Volcano plots of several transition metals for HER in (A) acidic and (B) alkaline conditions. In (A), $E_{M-Hads}$ is an experimental value, an operative electrochemical adsorption heat used by Trasatti for its plot, and reprinted with permission from the work of Krishtalik, ref 104. Copyright 2010, American Chemical Society (ACS). In (B), $E_{M-Hads}$ was calculated using DFT, reprinted with permission from ref 105. Copyright 2013, The Royal Society of Chemistry.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 6.** A) The dependence of the HER rate on the pH. Comparison of cyclic voltammetry scans on Pt (111) at various pH. Adapted with permission from ref 113. Copyright 2017, Springer Nature. B) Linear sweep voltammetry in the HER region of polycrystalline platinum in 0.1 M alkali with different cations. Modified with permission from ref 114. Copyright 2022 Springer Nature Limited.</figcaption></figure>

active site turnover frequency, or exchange current density) against the adsorption binding energy. This can be done by plotting the exchange current densities as a function of the hydrogen binding energies across a range of materials.<sup>102,103</sup> Figure 5.A and 5.B show that a volcano plot for the HER activity is obtained across different metallic surfaces in both acid and alkaline electrolytes.

The volcano diagram results from the free energy of hydrogen adsorption.<sup>106,107</sup> In the case of a weak binding, adsorption is the rate-determining step. This is the so-called Volmer step (eq 6). In the case of a very strong binding, desorption dominates the rate and the desorption steps, i.e., Heyrovsky/Tafel (eqs 7 and 8), control the reaction. To achieve the highest activity, the hydrogen bonding at the EC surface must be neither too strong nor too weak. Accordingly, the necessary condition for an active HER EC is that the free energy of adsorption is close to zero.<sup>106,107</sup> The Sabatier principle is the result of such just-right adsorption energy and does not hold only for the HER but also for many others. The Sabatier principle and the volcano plot give indications on how

to design ECs that have high activities. While the use of volcano plots can certainly deliver an excellent indication, it is not the only element to consider when designing ECs. More accurate and efficient techniques of computing electrochemical barriers for proton transfer processes involving both hydroxide and hydronium ions<sup>103,108–111</sup> are needed for a complete quantitative understanding of these effects.<sup>112</sup>

**Figure 6** shows the dependence of the cyclic voltammetry of Pt (111) on the pH. No significant changes in hydrogen underpotential deposition are observed in the alkaline environment (0.10–0.35 V vs RHE). However, HER shows a significant shift of the overpotential to a more negative value with the increase in pH. Remarkably, the dependence of the rate of HER on pH is much more pronounced than the dependence on hydrogen underpotential deposition. It can be noticed that the hydrogen underpotential deposition is related to the hydrogen binding energy and that accordingly, the latter does not depend on the pH. This is a clear indication that the significant variation of the HER rate at a given overpotential on Pt (111) is not related to the negligible shift in hydrogen
underpotential deposition, a fact that raises concerns about the use of the hydrogen binding energy as a single descriptor for the HER.<sup>113</sup>

The poor activity in alkali is the result of the low proton concentration and the slow water dissociation that produces a poor supply of dissociated hydrogen to the surface of platinum.<sup>115</sup> To overcome this limitation, the synergistic effects of two or more elements can be considered. It has been demonstrated that the addition of TMs, e.g., Ni, Co, Fe, to Pt, even in the form of oxides, skyrockets the Volmer step reaction rate and also increases the adsorbed hydrogen supply for the Heyrovsky and Tafel steps.<sup>116,117</sup> The role of the interface in this EC seems to be extremely relevant. Indeed, it was first speculated that the water dissociation happens mainly at the interface between platinum and the transition metals (TMs), where the adsorbed OH and H bind respectively with the transition metal and Pt. This highlights the importance of increasing the ratio between the interfacial Pt and bulk Pt atoms, which is generally very small. However, a study by Markovic et al. demonstrated that water dissociation happens mainly inside the TM regions. Accordingly, the formed adsorbed hydrogen should travel to the platinum areas where it might undergo recombination and desorb to release gaseous hydrogen.<sup>118-120</sup> This is the hydrogen spillover that might provide clear indications on how to boost the activity of HER ECs. The hydrogen spillover consists of the migration of adsorbed hydrogen from the regions where it is formed to regions where it is weakly adsorbed.<sup>121-124</sup> This is the case of platinum, which shows moderate values of H adsorption energies.

The spillover effect might be used to make ECs that are relatively insensitive to pH conditions. Indeed, in ref 125 it was argued that spillover can provide significant hydrogen evolution activities also in a neutral or basic environment. For example, oxygen-deficient tungsten oxide promotes the HER activity of ruthenium.<sup>126</sup> Also, Ni(OH)<sub>2</sub> has been shown to promote HER activity on Pt.<sup>127</sup> The presence of oxygen vacancies in tungsten oxides boosts the proton-storage capacity and the charge-transfer rate by increasing the hydrogen supply to the ruthenium metal surface, where the hydrogen can recombine and desorb as H<sub>2</sub>. Water dissociation occurs on the tungsten-oxide surface in the presence of the electrochemical double layer<sup>128-130</sup> generating protons that spill over to the ruthenium surface.

Based on changes in the Tafel slope, the spillover appears to change the RDS from the water dissociation reaction (Volmer step) to the hydrogen recombination (Tafel of Heyrovsky step). Multicomponent ECs thus offer the opportunity to tune the binding energies of the hydroxyl and hydrogen at different catalytic sites<sup>131</sup> which is central to minimizing HER overpotential in AEM-WE, and has been hypothesized to increase HER activity by simultaneously reducing the H<sub>2</sub>O dissociation and H<sub>2</sub> formation energies.<sup>132-135</sup> Both the hydrogen and hydroxyl binding energies are important in determining HER activity,<sup>136</sup> where binding hydroxyl is central to the water-dissociation step, releasing protons to form metal hydrides ultimately on the metallic catalyst surface.

Indeed, in these cases, the excessive coverage of the surface may result in EC poisoning that depresses the activity. Again, spillover may help to mitigate this effect by increasing the rate of H removal from the surface.

Related spillover effects are also central to achieving fast hydrogenation catalysis with thermal processes.<sup>137,138</sup>

Based on the above work, spillover effects are evidently an important tool in designing advanced catalysts for alkaline HER in AEM-WE. These advanced ECs might be designed to be resilient toward local variation in the pH that might occur for the combination of the low OH⁻ mobility and the use of pure water in ionomer/catalyst porous electrodes. However, developing a theoretical framework that supports the development of ECs is not trivial. The question of the factors that determine the HER rate is still debated and there is no single descriptor that can be used to explain the activity trends of ECs even in the simplest case of Pt in acidic electrolytes, let alone the complex situation of multicomponent surfaces and spillover in neutral to basic media.<sup>115</sup> Koper and co-workers reported a 3D volcano plot that considers both hydrogen and hydroxyl binding energies.<sup>139</sup> These relationships are available for a limited number of metals; however, they may represent a more comprehensive model for the HER activity.

Other authors suggest that the role of hydroxyl might be indirect, e.g., in stabilizing the transition state or modifying the interfacial water orientation. For example, some have argued that the operating potential for the reaction compared to the potential of zero free charge (PZFC) is critical. Near the PZFC, the interfacial water has high entropy, allowing facile water reorganization as ions transfer through the electrochemical double layer. In some cases, for example, the transfer of the adsorbed OH to form OH⁻ in the bulk electrolyte may be a descriptor.

All these hypotheses and underlying mechanisms have been supported by select experiments, but fail to be universal. Problems arise from the nature of many ECs, which often have complex surface structures, absorbed species (especially from impurities<sup>140</sup>), and are not easy to model. The presence of nanoparticles with surfaces rich in defects and a variety of terminations in a hybrid system of interfaces further complicates understanding. The contact between phases further introduces electronic effects due to interfacial charge transfer and Fermi energy equilibration.

The presence of metal cations in the supporting electrolyte can also significantly affect the reaction rate, and a variety of explanations have been proposed. Particularly, it has been argued that the role of the cation is linked to the role of adsorbed hydroxide in the HER. In ref 141 the authors propose that cations stabilize the OH at the interface and the transition state of the water dissociation, which is thought to be rate-limiting for the noble metals in alkali. It was shown that a Brönsted-Evans–Polanyi relationship exists between the energy of the transition state for the dissociation of water and the adsorption energy for the hydroxide.<sup>139</sup> According to this model, a cation stabilizes the transition state for water, enhancing the HER rate, particularly for metals like Au, Cu and Ag. More oxophilic metals that absorb hydroxide strongly (e.g., Ir, Pt, Pd) have a smaller activation barrier for water dissociation and the removal of the hydroxide from the surface becomes relevant. It was shown that in alkaline media, the HER rate decreases with the increase of cation size on reactive metals (Ir, Pt, Pd),<sup>141,142</sup> while an opposite trend is observed with noble metals. Even in the absence of a clear interpretation framework, it is likely that the mobility of the cation within the double layer is important. Large cations have weaker coordination with the water molecules, allowing larger mobility compared to smaller cations that bind water more rigidly.

A step forward in the understanding of the cation effect is reported by Shah et al.<sup>114</sup> Smaller solvated cations at the Pt
surface were found to increase the coverage of adsorbed OH in the HER potential window. The adsorbed OH was proposed to act as an electronically favored proton acceptor, explaining the trend of increasing HER activity with decreasing size of the cation (Figure 6.B).¹⁴⁴

For Au at pH 11, the increase in the cation size is beneficial to the HER rate, and the reaction order for the cations is 0.5. The authors concluded that the cation is central in determining the activity and that this is related to the stabilization of the transition state of the Volmer step by interacting favorably with the dissociating water molecule. Moving from pH 10 to pH 13 was thought to affect the local field strength, enhancing the HER rate by affecting the concentration of the local cation. It was also argued that excess near-surface cation concentration negatively affects the HER rate.

The discussion so far has focused on explaining HER performance of different ECs as a function of the structure and composition of the catalytic surface and also on the composition of pure and well-defined alkaline electrolytes. Yet in pure-water-fed AEM, the only cations present are those associated with the anion-conducting ionomer phase, typically quaternary ammonium groups bound to the polymeric backbone. It remains an open question how these typically larger and less mobile cations affect the interfacial water, surface-absorbed hydroxide, and local transport. Depending on the precise nanoscale and local structure and interactions between the HER EC and the ionomer, these effects could vary substantially. For example, with pure-water feed, even the local pH at the active surface of the EC is uncertain and likely depends on the magnitude of the current and specific electrode processing and ionomer character. The situation is not different from PEM technology; however, OH⁻ transport in liquid water is three times slower than proton transport, leading to larger concentration polarizations (pH gradients), providing a significant concentration gradient and eventually pH differences between the anode (locally getting more acidic with current due to hydroxide consumption) and the cathode (locally getting more basic with current due to hydroxide generation). Understanding these effects thus remains an important area for future analytical work in device-relevant architectures to enable improved design, but it is apparent that electrocatalysts resilient toward local pH variation might help in promoting reaction kinetics and improving device performance and durability.

**4.2.2. Oxygen Evolution Reaction.** The mechanism of the OER is more complex compared to HER, consisting of at least four elementary steps involving the transfer of electrons and protons (or, equivalently, hydroxide/water).¹⁴³ Correspondingly, the OER is slower with kinetic overpotentials in the best cases of ~300–400 mV at relevant current densities of ~0.5–2 A cm⁻², in porous water electrolyzer anodes with optimal loading.¹¹⁸ Despite considerable efforts in the last few decades, elucidating experimentally the details of the OER mechanism is still difficult even in “simple” systems with soluble aqueous electrolytes like KOH and with single-crystal or planar thin film electrodes. Although it is trivial to measure kinetic parameters such as the Tafel slope ζ,¹¹⁸ it is difficult to connect these parameters to specific microscopic steps of the reaction. Recent progress in computation has provided insight into the molecular OER mechanisms, particularly by coupling density functional theory approaches, including advanced solvation models with microkinetic modeling,¹⁴⁴ which is strongly affected by the pH of the environment.¹⁴⁵⁻¹⁴⁸ In a

recent study, it has been shown that pH strongly affects the oxygen evolution rate for IrO₂ ECs (Figure 7),¹⁴⁹ which has been previously shown for various first-row transition-metal oxide perovskite ECs.¹⁵⁰,¹⁵¹

<figure><img src="image_16.png" /><figcaption>**Figure 7.** Linear-sweep voltammograms of the OER region for an IrO₂ electrode at various pH: pH 1–5 (A), pH 6–10 (B), pH 11–13 (C) and pH 1, 9, 13 (D). Adapted with permission from ref 149. Copyright 2022, Elsevier. CC-BY 4.0 license, https://creativecommons.org/licenses/by/4.0/.</figcaption></figure>

The evidence reported above indicates that the lower performance of AEM-WEs operating with deionized water, compared to AEM operated with KOH electrolytes, may in part be due to slower oxygen evolution kinetics. In deionized-water systems, while the ionomer supplies OH⁻ from the anode through the AEM, the local pH at the anode EC surface is almost certainly lower than in KOH-fed AEM electrolysis, particularly under steady-state operation. The anode active layer, thus, should be engineered with transport and local pH effects to either maximize the local pH or use pH-insensitive ECs.

Indeed, in ref 152 it was shown that significant pH differences between the anode and cathode compartments affect material corrosion and the kinetics of water splitting. This aspect is not relevant in AEM-WEs fed with highly concentrated alkali due to the buffer power of the electrolyte, but it might result in significant pH variation in AEM-WEs fed with deionized water or very low supporting alkali concentrations, where the buffer power of the electrolyte is low. Transport issues might further modulate local pH variations through the depth of the catalytic layer, thus affecting the performance of the devices. This aspect is still largely unexplored. Tools capable of determining such local pH values would be particularly relevant, providing critical information to engineer the catalytic layer.¹⁵³

In both the neutral¹⁵⁴ and the alkaline¹⁴⁵ media, the OER is represented by the overall process reported in eq 5. Such a process is often assumed to be composed of four elementary steps (eqs 9–12),¹⁴⁶,¹⁵⁴,¹⁵⁵ although other microscopic mechanisms can be hypothesized.

$$M + OH_{(aq)} \rightarrow M - OH + e^{-} \quad (9)$$

$$M - OH + OH_{(aq)} \rightarrow M - O + H_{2}O_{(l)} + e^{-} \quad (10)$$
<figure><img src="image_17.png" /><figcaption>Figure 8. Volcano plot for the OER plotted using the ΔG<sub>O*</sub> − ΔG<sub>OH*</sub> and ΔG<sub>OH*</sub> descriptors. Adapted from ref 142. Copyright 2022, American Chemical Society. Licensed under CC-BY-NC-ND 4.0.</figcaption></figure>

$$M - O + OH_{(aq)}^{-} \rightarrow M - OOH + e^{-} \quad (11)$$

$$M - OOH + OH_{(aq)}^{-} \rightarrow O_{2(g)} \uparrow + H_{2}O_{(l)} + e^{-} \quad (12)$$

M represents the active site on the “surface” of the electrode, while M−OH, M-O and M-OOH identify the species adsorbed on the active site. The term “surface” here is used loosely as it is well established that essentially all catalysts for the water oxidation layer restructure and form interphases where there is some degree of volume activity and not an atomically sharp liquid/solid boundary.

It is possible to evaluate the change in the free energy associated with each of the steps in eqs 9–12.146 These are indicated respectively as

(9) → ΔG<sub>OH*</sub> [formation of adsorbed OH, release of a first electron];

(10) → ΔG<sub>O*</sub> [conversion of an adsorbed OH into an adsorbed O, release of a second electron];

(11) → ΔG<sub>OOH*</sub> [conversion of an adsorbed O into an adsorbed OOH, release of a third electron];

(12) → ΔG<sub>O2</sub> [the adsorbed OOH releases one additional electron and O<sub>2</sub> is evolved from the electrode surface].

The minimum “thermodynamic” overpotential of the OER in the neutral/alkaline medium is determined by the largest term among ΔG<sub>1</sub>, ΔG<sub>2</sub>, ΔG<sub>3</sub> and ΔG<sub>4</sub>, in compliance with eq 13.156

$$\eta_{\text{OER}} = \frac{1}{e_{n=1,2,3,4}} \max[\Delta G_n] - U_0 \quad (13)$$

where U<sub>0</sub> = ΔG<sub>0</sub>/4e = 1.23 V is the equilibrium potential for the OER, which is independent of pH and defined at p = 1 bar and T = 298.15 K.157 This is a minimum “thermodynamic” overpotential because it is set by the reaction energies of the intermediates along the reaction, not by the transition-state energies between the reaction intermediates. Experimentally the overpotential is set by the passing current, which can be

observed for any nonzero overpotential with sufficiently sensitive measurement. Reaction intermediate energies are much easier to calculate than transition states and the associated kinetic barriers. Often, the Bell-Evan-Polanyi principle, where the transition-state barrier is proportional to the reaction energy, is invoked to describe trends using simplified computational approaches of the complex electrochemical process, particularly in the approaches developed and popularized by Norskov and co-workers.158

Using DFT to compute reaction energies, OER ECs can be distinguished by the magnitude of the different ΔG<sub>x</sub> (x = 1, 2, 3, 4), that in turn is affected by the specific physicochemical features of the EC surface (e.g., chemical composition, structure and others).159,160 The magnitudes of the ΔG<sub>x</sub> values are typically not independent of each other. Computations (without explicit solvation and with the relatively simple computational hydrogen electrode) show that in most oxide-based electrodes, a scaling relationship exists between the binding energy of HOO* and HO* on M.161 These computations therefore enable one to (i) propose the rate-determining step of the OER by evaluating the ΔG<sub>x</sub> values associated with the reactions reported in eqs 6–9; and (ii) correlate those with features of the EC surface that play the most relevant role in minimizing such ΔG<sub>x</sub> values to justify a reduced OER overpotential.162,163 Connecting these predictions to real experiments is challenging due to the complexity of structural changes at the surface with potential and solvation effects that are computationally difficult to account for, which may have profound differential effects on the intermediate and transition state energies. Nonetheless, calculated ΔG<sub>OOH*</sub> using the simplest DFT computational hydrogen electrode approaches show a universal scaling relationship with ΔG<sub>OOH*</sub>161,164 described by the simple equation ΔG<sub>OOH*</sub> = ΔG<sub>OH*</sub> + 3.2 eV. Thus, the theoretical (minimum) overpotential for an EC can be estimated with only two descriptors:
$\Delta G_{O*}$ and $\Delta G_{OH*}$. The scaling law is generally accurate, at least based on computational data.<sup>164</sup>

Similar to the HER, volcano plots can be obtained for the OER. The first example was reported by Trasatti<sup>142,165</sup> who used the heat of formation of the oxides as a descriptor for the electrocatalytic activity. Similarly, the free energies for eqs 9–12 can be used to generate a volcano plot using the descriptors $\Delta G_{O*} - \Delta G_{OH*}$ (for steps 2 and 3) and $\Delta G_{OH*}$ (for steps 1 and 4). The resulting two-dimensional plot shows that the most promising ECs stay in the red zone in the middle of the diagram, where the minimum overpotential is of the order of 0.4 eV, with deviation from the scaling relationships that may be as large as $\pm$ 0.1 eV (Figure 8).<sup>142</sup> Significant information from the plot is that a relevant number of ECs range in the $\pm$ 0.1 eV within the peak and suggests a significant issue that must be overcome to increase electrolyzer performance. Considering an electrolyzer operation at 2 V, the OER kinetic overpotential accounts for ~ 20% of the electrical energy input. In Figure 8, the computational volcano plot can be divided into four regions that correspond to different steps that are rate-controlling. Among the considered ECs, most materials have the adsorbed hydroxyl oxidation to adsorbed oxygen and the oxidation of adsorbed oxygen to $OOH^*$ is predicted to be rate-determining based on this simple model.<sup>142</sup>

While these calculated thermodynamic descriptors provide useful insight, the connection to experimental systems can be tenuous. For example, alkaline solutions are often contaminated with Fe impurities, the oxide surfaces are reconstructed to form disordered surface-oxyhydroxide phases, which do not have the same properties as the underlying oxides and the dramatic differences in electrical conductivity across oxide and oxyhydroxide phases are not accounted for in these models. The intrinsic activity trends of these transition metal-based oxyhydroxide-type phases (which are derived often from parent oxides) have more recently been reconsidered taking these issues into account,<sup>166</sup> but a mechanistic understanding in the context of thermodynamic intermediate energies, particularly for the fastest ECs that are multicomponent phases like Ni(Fe)OOH and Co(Fe)OOH (discussed more below) is still unsettled.<sup>167</sup> Craig et al. showed that inhomogeneous systems in this descriptor fail to predict the electrocatalytic behavior of Ru ECs with a significant underestimation of the activity. In these cases, it is relevant to consider the variation of the oxidation state of the metal site, an element that poses serious issues in the calculation.<sup>168</sup> There is still a need to significantly improve the current interpretation framework to rationalize and predict the activity through accurate descriptors and the resulting plots. However, it is not yet clear whether fundamental limitations in the knowledge faced today can be surpassed effectively in the future.

Besides the complex theoretical framework, there are also issues with the experimental methods. Particularly, the preparation methods of the electrodes and the need for thick EC layers create significant difficulties in the definition of active surface areas, the electrical conductivity of the layer and the local surface structure. The uncontrolled experimental condition provides uncertainty in the normalization of the data and the comparison of the intrinsic OER activity of materials. The introduction of fabrication methods that will allow precise control of the EC surface and condition is recommended. Such methods would allow an easy determination of the turnover

frequency, which is a good metric for benchmarking the various materials.<sup>169</sup>

## 5. MATERIALS AND THEIR INTEGRATION

A single-cell AEM-WE is composed of several single components that are integrated into the membrane electrode assembly (MEA) that is the core of the system, as presented in Figure 9, in an AEM-WE operating with “dry-cathode”.<sup>170</sup> The

<figure><img src="image_18.png" /><figcaption>**Figure 9.** Schematic of a single cell AEM-WE including the single components and their integration. Adapted from ref 170. Copyright 2021, IOP Publishing Limited. Licensed under CC-BY 4.0.</figcaption></figure>

AEM is at the center of the AEM-WE. On the anode side, where the OER takes place, a catalytic layer (CL) composed of the EC and the AEI is in contact with the AEM on one side and on the other side with the PTL. The anodic BP closes the anode side. Similarly, on the cathode side where the HER occurs, a CL composed by the EC and the AEI is between the AEM and the cathode PTL. At the end of the cathode side, there is the BP. The CL can be integrated into the AEM or the PTL as EC Coated Membrane (CCM) or as EC Coated Substrate (CCS), respectively. In the next subsection, single components and their integrations in MEAs are discussed.

### 5.1. Electrocatalysts for the Hydrogen Evolution Reaction

As discussed above in section 4.2.1, the HER in the alkaline medium is hindered by the water dissociation process. This is a significant difference with respect to the acidic medium. Consequently, even ECs exhibiting excellent performance in the acidic medium could experience performance decline in the alkaline medium. It is generally believed that platinum-supported carbon (Pt/C) is the best HER EC both in acidic and alkaline media. This is a consequence of the optimal compromise between facile adsorption of the water reagent and an unhindered release of the $H_2$ product.<sup>171</sup> The other PGMs (e.g., Pd, see Figure 5.B) also exhibit a good HER performance in the alkaline environment. The low abundance of PGMs in Earth’s crust<sup>172</sup> challenges the scaling of the technology implementing high loadings of PGMs.<sup>173</sup> However, the use of cathodes with a low loading of PGMs (e.g., near or below 0.1 mg cm<sup>-2</sup>) may be tolerable at scale, as recently mentioned in the publication from the US Department of Energy Hydrogen and Fuel Cell Technologies Office.<sup>174</sup>

HER ECs for operation in alkaline media based on Pt have been extensively studied and implemented in AEM-WEs. ECs, including active sites only comprising Pt, yielded good performance in operating conditions in AEM-WEs fed with both pure water<sup>56</sup> and alkaline electrolyte.<sup>175</sup> With respect to Ni-based HER ECs (see below), the improved efficiency leads
<figure><img src="image_19.png" /><figcaption>**Figure 10.** (A) Impact of the chemical composition of a Raney-Ni electrode on the HER overpotential at 25 A/dm² at 25 °C. Reprinted with permission from ref 186. Copyright 2000, Elsevier. Published by Elsevier Science Limited. (B) Polarization curves of bare Ni foam, Co₃O₄ nanosheets, Co/Co₃O₄ nanosheets, and Pt wire. (C) Tafel plots derived from (B). (B) and (C) adapted with permission from ref 192. Copyright 2015, American Chemical Society. (D) Position of the different crystal surfaces of NiₓMᵧ (M = P, S, N) in the HER activity volcano plot as a function of the calculated hydrogen adsorption-free energies. (E) "Ex-situ" current-voltage characteristics of Pt, Ni₅P₄, Ni₃N, Ni₃S₂, and Ni working in 1 M KOH. (D) and (E) were adapted with permission from ref 193. Copyright 2016, WILEY-VCH Verlag GmbH. Morphology of HER ECs: (F) nickel foam; (G) detail of the surface of a branch; (H) surface of a branch after deposition of Ni(OH)₂, adapted with permission from ref 194. Copyright 2008, The Royal Society of Chemistry. (I) AEM electrolyzer cell performance at 60 °C using NiAlMo cathode and different anodes: Ni/graphite, NiAl, Ni, and NiAlMo; (J) durability test of the cell using NiAl anode for about 154 h under a current density of 1 A cm⁻², reprinted with permission from ref 195. Copyright 2019, American Chemical Society. (K) Polarization curves for AEM electrolyzer single cells with Ni–Co–S/CP and commercial Pt/C/CP cathodes, reproduced with permission from ref 196. Copyright 2020, John Wiley & Sons Limited.</figcaption></figure>

to a lower consumption of energy upon AEM-WE operation. This offsets the higher cost of the Pt-based HER EC. The HER activity of Pt/C is improved upon the combination with layered metal hydroxides; the latter are supposed to enhance the dissociation of water and the production of hydrogen intermediates.<sup>127,176</sup> A number of bimetallic Pt-based HER ECs were studied as well. The second metal promotes the kinetics of the HER either: (i) by facilitating the cleavage of the HO-H bond (e.g., in the case of PtNi<sup>177</sup> and PtCo<sup>178</sup>); or (ii) by decreasing the Pt-H* binding energy (e.g., in the case of PtRh<sup>179</sup> and PtRu<sup>180</sup>). HER ECs comprising PtRu active sites are particularly promising and have demonstrated their potential to devise AEM-WEs exhibiting an outstanding performance. For instance, an AEM-WE mounting a NiFe anode EC, a PtRu/C cathode EC, a HTMA-DAPP AEM and

fed with 1 M NaOH at 60 °C was able to operate at 5.32 A cm⁻² at a cell voltage of 1.8 V.<sup>181</sup>

Ni-based ECs are promising alternatives to PGMs to promote the HER, since they exhibit reasonable activity and a significant abundance in Earth's crust.<sup>182</sup> Several approaches were adopted to reduce the size of Ni particles composing the electrode EC and increase the interfacial area.<sup>183,184</sup> Raney Ni ECs demonstrate a high activity in the HER thanks to the porous morphology and the large surface area obtained during the synthetic process.<sup>185</sup> Further improvements were achieved by developing different Ni alloys such as Raney Ni–Sn EC (Figure 10.A).<sup>186,187</sup> In these studies, a reduction in the Tafel slope from 120 to 70 mV dec⁻¹ was achieved with the Sn-added Raney Ni ECs. Other binary and ternary Ni alloys were explored, including Ni–Co, Ni–Fe, Ni–Cr, Ni–Mo–Fe, Ni–Mo–Cu and Ni–Mo–Co, taking advantage of the good lattice
match of Ni with other metals.<sup>188–190</sup> Among such alloys, Ni–Mo systems yielded an overpotential of only 35 mV at a current density of 10 mA cm<sup>–2</sup> and a Tafel slope of 45 mV dec<sup>–1</sup>, which is comparable to the performance of a commercial Pt/C EC,<sup>187,191</sup> albeit with much higher mass loading for the Ni–Mo system. Ni–Mo HER ECs are highly promising as they have demonstrated remarkable performance in single AEM-WE.<sup>53</sup> With respect to a reference AEM-WE mounting a conventional PtRu/C HER EC, an AEM-WE comprising a nanocomposite Ni–Mo HER EC supported on oxidized Vulcan carbon black yielded only a 50–100 mV higher cell voltage at geometric current densities of 1 A cm<sup>–2</sup> or higher.<sup>53</sup> This latter Ni–Mo EC exhibited: (i) a mass-specific HER activity within 1 order of magnitude of a commercial Pt/C EC; and (ii) significant internal mass transfer limitations even at loadings as low as few tens of µg<sub>cat</sub> cm<sup>–2</sup>, suggesting that the above results are lower-bound estimates of the intrinsic HER/HOR activity of Ni–Mo composites. The remarkable performance of such Ni–Mo EC was rationalized on the basis of DFT calculations, suggesting that the introduction of Mo in Ni–Mo alloys weakens hydrogen adsorption and raises HER activity.<sup>53</sup> Additional studies are still needed to improve durability and elucidate failure modes under HER conditions, thus reaping the full benefits of Ni–Mo HER ECs for the alkaline environment.

Metal oxides by themselves are not very active in promoting the HER due to their poor electrical conductivity and hydrogen binding energies. However, when metal nanoparticles are at least partially covered by suitable metal oxides, such as in the case of Co/Co<sub>3</sub>O<sub>4</sub>, Ni/NiO and Ni/CeO<sub>2</sub>–CNT systems,<sup>192,197,198</sup> bifunctional effects are likely triggered, yielding a remarkable HER performance. Ni and NiO typically coexist on the surface of Ni-based electrodes (e.g., nickel foams).<sup>199</sup> The presence of NiO on the surface of a Ni electrode significantly raises the intrinsic kinetics of the HER in an alkaline environment. This evidence was interpreted assuming that the introduction of NiO raises the free energy of the adsorbed hydrogen intermediate and enhances the kinetics of the Volmer step.<sup>200</sup> Indeed, a lower overpotential in the HER was demonstrated for these heterostructured ECs with respect to the single metal or metal oxide systems (Figure 10.B and Figure 10.C).<sup>192</sup> Similarly, Ni/CeO<sub>2</sub>–CNT ECs demonstrated an overpotential of ca. 100 mV at a current density of 10 mA cm<sup>–2</sup>.<sup>198</sup> The introduction of heteroatoms such as S, N and P forming active sites based on transition metal sulfides, nitrides and phosphides gives rise to an enhancement of the HER activity of the EC. In particular, it was demonstrated that these heteroatoms are not just spectators and play a crucial role in the HER mechanism (Figure 10.D).<sup>193</sup> In the presence of such heteroatoms, the hydrogen adsorption-free energy approaches zero. This latter feature is strongly correlated with a high HER activity.<sup>107</sup> A few examples of such ECs include systems based on Ni<sub>3</sub>S<sub>2</sub>, Ni<sub>3</sub>N, Ni<sub>5</sub>P<sub>4</sub>, CoP, CoS<sub>x</sub>, MoP, MoS<sub>x</sub> and WP<sub>x</sub> (Figure 10.E)).<sup>193,201–204</sup> In this group of ECs, the best performance in "ex-situ" tests was obtained by devising nanocrystalline CoP nanosheets supported on carbon cloth, which: (i) were able to activate the HER with an overpotential of 48 mV at a current density of 10 mA cm<sup>–2</sup>; and (ii) exhibit a Tafel slope of 43 mV/decade.<sup>201</sup>

### 5.1.1. HER Electrocatalyst for AEM-WEs and Their Differences with Respect to A-WEs.
The key distinction between A-WEs and AEM-WEs lies in the electrolyte and the

operating conditions, such as the nature of feed<sup>205</sup> and lower operational temperature, along with the possibility of pressurizing H<sub>2</sub> within the WE.<sup>20</sup> Thus, the choice of HER EC depends on the specific type of WE and its compatibility with the operating conditions.<sup>205,206</sup> Compared to PEM-WEs, an advantage of AEM-WEs is the possibility to implement lower-cost TM in HER ECs that would be unstable in acid.<sup>59</sup> In addition, the three-dimensional design of the porous electrode is a crucial factor in ECs for AEM-WEs: nickel foam (NF), graphene (G), graphene oxide (GO) and carbon nanotubes (NTs) are often used supports for the preparation of MEAs for AEM-WEs in academic laboratories. The introduction of such supports enhances the porosity, raises the electrochemically active surface area (ECSA) and facilitates gas desorption (see Figure 10.F, Figure 10.G and Figure 10.H).<sup>194</sup> These supports often provide high electrical conductivity, thus minimizing ohmic losses. However, there are design trade-offs. Excessively thick EC layers, even if they provide high surface area, can lead to higher electrical, ionic, and mass transfer resistances that ultimately decrease cell performance.<sup>207</sup>

AEM-WEs and A-WEs usually adopt HER ECs with the same or similar chemical composition as they both operate in an alkaline environment.<sup>205</sup> Typically, PGM-free ECs for HER are based on Ni. For instance, Ni–Mo alloy EC demonstrated a good performance in full-cell at 50 °C, achieving a current of 1 A cm<sup>–2</sup> at 1.9 V.<sup>208</sup> Alloys of nickel with cobalt supported on GO were tested at room temperature, revealing a decent performance of 100 mA cm<sup>–2</sup> at 1.9 V.<sup>209</sup> In addition to binary alloys, ternary Ni alloys were developed, such as NiAlMo and NiFeCo.<sup>195,210</sup> Both performed well when in a full AEM-WE operating at 60 °C and fed with 1 M KOH, achieving a current of 2 and 1 A cm<sup>–2</sup> at 2.09 and 1.90 V, respectively (see Figure 10.I and Figure 10.J).<sup>195,210</sup> The AEM-WEs, including the NiAlMo and NiFeCo HER ECs, mounted a hexamethyl-p-terphenyl poly-(benzimidazolium) (HMT-PBI) AEM and a PBI-based AEM, respectively.

Sulfide-, oxide- and nitride-based HER ECs were also developed. A Co<sub>3</sub>S<sub>4</sub> EC showed a high current density of 0.43 A cm<sup>–2</sup> with a cell voltage of 2 V,<sup>211</sup> while a Ni–Co–S EC exhibited a current density of 1.7 A cm<sup>–2</sup> at 2.4 V (see Figure 10.K).<sup>196</sup> A NiCu mixed metal oxide EC demonstrated an AEM-WE performance of 1.85 A cm<sup>–2</sup> at 2 V in 1 M KOH at 50 °C.<sup>207</sup> Finally, an HER EC named as "NiMo-NH<sub>3</sub>/H<sub>2</sub>," including a NiMoN<sub>x</sub> component and obtained by annealing NiMoO<sub>4</sub> on nickel foam (NF) in H<sub>2</sub>/NH<sub>3</sub> (5% H<sub>2</sub>/ 95% NH<sub>3</sub>) for 2 h at 550 °C achieved a current density of 1 A cm<sup>–2</sup> with an applied potential as low as 1.57 V.<sup>212</sup> It is highlighted that the literature rarely reports the chemical composition of HER ECs after operation. Hence, a possible selective leaching of species (e.g., Al, Cu) from the ECs upon carrying out the HER is not probed. This hinders a detailed understanding of the real chemical composition of the active sites and, consequently, of the actual HER mechanism and its interplay with the physicochemical properties of the EC.

### 5.2. Electrocatalysts for Oxygen Evolution Reaction

In an alkaline medium, in principle, a broad spectrum of metal elements can be used to produce ECs to promote the OER (Figure 11.A). The elements span from PGMs (e.g., Pt, Ir, Ru)<sup>213</sup> to first-row TMs, particularly Fe, Co, Mn and Ni.<sup>214</sup>

With respect to PGMs, the activity trend is nominally Pt < Ir < Ru,<sup>213</sup> although the Ru and related oxides dissolve under anodic polarization in basic electrolyte. The combination of
<figure><img src="image_20.png" /><figcaption>**Figure 11.** (A) Activity of various OER ECs (ECs) after 2 h of operation at 10 mA·cm⁻², reprinted with permission from ref 214. Copyright 2015, American Chemical Society. (B) Free energies of OER intermediates on the IrO₂(1 1 0), NiFe₂O₄(3 1 1) and Ni₀.₇₅Fe₂.₂₅O₄(3 1 1) surfaces at U = 0 V. PDS = potential-determining step. (C) Current density at various cell voltages for Ni₀.₇₅Fe₂.₂₅O₄//(*Pt*/C) and IrO₂//(*Pt*/C) cells. (D) Comparison of AEM-WE for the Ni₀.₇₅Fe₂.₂₅O₄//(*Pt*/C) cell, other reported AEM-WE cells, and the reported IrO₂-based AEM-WE. (B–D) were reproduced with permission from ref 232. Copyright 2020, Elsevier B.V. (E, F) High-resolution TEM image of NiFe layered double hydroxide (NiFe-LDH) OER EC. (G) Current–voltage curves obtained for MEAs using (red) NiFe-LDH and (gray) IrOₓ at 80 °C as an anode EC. (E–G) were reproduced with permission from ref 233. Copyright 2020, American Chemical Society.</figcaption></figure>

different PGMs can trigger synergistic effects, raising OER performance and durability. For instance, doping RuO₂ with Ir to yield Ru₀.₉Ir₀.₁O₂ significantly raises the durability without compromising the OER performance.²¹⁵

OER ECs based on non-noble metals, in fact, have intrinsically higher OER activity than PGMs²¹⁶ and can be infinitely stable/durable, often due to dynamic dissolution-redeposition processes,²¹⁷ particularly for Fe-containing Co/Ni ECs with record activity.²⁰⁵,²¹⁴ This is a primary advantage of A-WEs and AEM-WEs over systems operating at lower temperatures in acidic conditions (e.g., PEM-WEs). At comparable energy conversion efficiencies, the operating current density increases significantly from A-WEs (ca. 0.5–1 A cm⁻²)²⁰⁵ to PEM-WEs (ca. 2–4 A cm⁻²), resulting in more compact devices.²¹⁸ However, this high-performance level is only achieved by PEM-WEs whose electrodes are based on PGMs such as Pt, Ir and Ru.²¹⁸

Over the past 50 years, there have been substantial efforts to develop high-performing ECs able to promote the OER based on earth-abundant elements (especially Fe, Co and Ni; other elements were also investigated, such as Ti, Mn and Pb).¹⁶¹ A “baseline” OER EC for the alkaline environment is based on Ni or highly porous Raney Ni deposited/applied on a stainless steel current collector.²¹⁹,²²⁰ At their surface, these ECs are transformed in the active phases, nominally consisting of Ni(Fe)OOH. The thickness and local structure of this dynamically formed active phase, along with the surface area of the initial porous metal electrode, determine the ultimate operating electrode potential as a function of current.²²¹ Several researchers have synthesized and tested various metal oxides of Ni, Co, and Fe with other elements added, including Zn, Cu, Al, and many others.²¹⁵,²²² In these cases, the active surface phases are always the disordered Ni or Co-based oxyhydroxides with Fe incorporated either intentionally or unintentionally from electrolyte impurities.²²³,²²⁴ Elements like Zn, Cu, and Al have been invoked to enhance activity, which is due to their solubility; these elements leach during operation, leading to a faster formation of the active phases discussed above with a higher degree of (initial) local disorder. A huge variety of initial precatalyst materials, including metals, alloys, oxides, phosphates, phosphides, sulfides, selenides and nitrides, have been reported and the resulting materials were all claimed to be able to yield an excellent OER performance.²²⁵,²²⁶ This is because the real active sites for the OER are the oxyhydroxides, almost uniformly containing Ni and/or Co along with Fe, that are formed during the electrochemical oxidation of water.²²⁷,²²⁸ This is true regardless of the chemical composition of the parent material (pure metal, alloy, or compound). NiFe alloys, including steel, can be used as parent materials to obtain OER electrodes.²²⁹,²³⁰ Upon suitable treatment (e.g., dealloying), highly active and stable electrodes can be obtained.²²⁹,²³¹ During operation, the exsolution of the bulk components is known to further activate the OER electrode and yield a more stable system in comparison with heterodeposited active layers. Furthermore, such a system may self-heal during operation.²²⁹,²³¹ The features of the parent material and the subsequent surface treatments, if any, are the avenues by which the physicochemical properties of the oxide and oxyhydroxide active sites formed *operando* on the EC are achieved. This modulates the apparent performance and durability in the OER of the EC, along with cell engineering parameters as discussed in more detail below.
**5.2.1. OER Electrocatalyst for AEM-WEs and Their Differences with Respect to A-WEs.** Similarly to A-WEs, AEM-WEs can run with PGM-free OER ECs and exhibit a performance high enough for practical applications.¹⁹⁵ At the same time, the baseline OER ECs used in academia for AEM-WEs are still based on PGMs such as Ru and, more commonly, Ir (typically as IrOₓ),⁵⁶,²³²,²³⁴ though it would probably be more appropriate to adopt other OER benchmark ECs, including Co₃O₄ or other oxides.¹⁹

A first family of promising PGM-free OER ECs comprises active sites based on Ni, usually coupled with other transition elements such as Fe (Figure 11.(A-D))²³² and/or Co²³⁵ similar to other OER ECs designed for A-WEs.²²³,²²⁴ The performance of PGM-free OER ECs in an operating AEM-WE is often superior to that afforded by baseline IrOₓ ECs.²³³ For example, in 1 M KOH and at T = 80 °C, an AEM-WE with a NiFe layered double hydroxide (LDH) operated at 1.59 V at 1 A cm⁻². In the same conditions, IrOₓ EC operated at 1.61 V.²³³ It is also feasible to devise entirely PGM-free AEM-WES. One such device was configured with either a binary NiFe hydroxide or a ternary NiFeCo hydroxide OER EC, coupled with a Ni mesh as the cathode.²³⁵ The resulting AEM-WE yielded 500 mA cm⁻² at 2.22 V in 1 M NaOH; such a result was obtained with a ternary NiFeCo hydroxide OER EC wherein the Ni:Fe: Co molar% ratio was 23:8:69. Co-based OER ECs also yielded very good performance. Some examples include bimetallic OER ECs wherein CoFe nanoparticles (Co: Fe molar ratio: 0.9/0.1) were deposited on highly graphitized carbon nanofibers (H–Co₀.₉Fe₀.₁-CNF).²³⁶ The corresponding AEM-WE achieved a current density of 0.794 A cm⁻² at 1.7 V. Bimetallic Cu₀.₅Co₂.₅O₄ OER ECs were synthesized by coprecipitation at pH = 11 of the corresponding precursors. The AEM-WE using Cu₀.₅Co₂.₅O₄ OER EC yielded a current density of 1.3 A cm⁻² at 1.8 V.²³⁷ Several other combinations of metals can be introduced in OER ECs; one example is a Ni/ CeO₂–La₂O₃/C system.²⁰⁶ The main differences between OER ECs for A-WEs and AEM-WEs are associated with the features of the interfaces between the electrode configurations and the separator in the two families of devices. In conventional A-WEs, the OER EC is fabricated onto a porous current collector by a variety of approaches, including electrodeposition²³⁸ and plasma spraying.²³⁹ As discussed in detail below, it is also important to distinguish between AEM-WE results where the water feed is alkaline (e.g., 0.1 M KOH) or pure water, as this affects the suitable EC properties.

On the other hand, in AEM-WEs, massive efforts are devoted to obtain highly refined “three-phase boundaries” (TPBs) on the active sites of the ECs at the interfaces between each electrode configuration and the AEM, able to: (i) minimize the sources of overpotential not associated with the intrinsic electrochemical kinetics of either the OER and the HER, and (ii) achieve a sufficient AEM-WE durability.²⁰⁶ To achieve this goal, in AEM-WEs, the electrode configurations and the AEM are fused together to obtain the MEA.²⁰⁵ In an MEA, the anodic and cathodic TPBs are very close to one another on the opposite faces of the AEM, allowing for to minimization of the ohmic losses due to the migration of OH⁻ species through the membrane and achieving the high operation current density characterizing AEM-WEs.²⁰⁵,²⁰⁶

To reduce the overpotentials due to mass transport, the OER ECs for AEM-WEs are to be supported on highly porous matrices allowing for a facile expulsion of the O₂ bubbles obtained as the product; a typical example is nickel nanofoam

(NiNF).²⁴⁰ Other possibilities include substoichiometric metal-oxides, carbides, and nitrides, which are expected to retain their porous structure even under the high oxidative potentials found at the AEM-WE anode.²⁴¹ A high AEM-WE performance is also promoted by other morphological features beyond porosity, such as the surface area and the shape of the OER EC, which ensures the access of ions to the active sites. Sample morphologies include nanoparticles,²⁴² nanoflowers,²⁴³ nanorods,²⁴⁴ and many others. Another crucial condition to achieve a high AEM-WE performance is to maximize the electrical conductivity of the OER EC (or anode active layer), to curtail the losses associated with charge transport. One possibility to achieve this outcome is to enhance the contact between the surface of the electrode and the OER EC; this result is often achieved by the direct growth (e.g., by electrodeposition) of the EC on the electrode surface.²⁴⁰ Another approach involves the introduction of carbon-based materials in the OER EC; good results are claimed, especially upon doping such carbon-based materials (e.g., graphene) with nitrogen moieties. Pyridinic N and graphitic N are claimed to raise charge delocalization, promoting the OER.²⁴⁵ It is to be highlighted that OER operation takes place at high electrode potentials (typically, E > 1.5 V). In these conditions, carbonaceous species are bound to undergo corrosion.²⁴⁶ On these bases, it must be stressed that even though carbon-supported ECs for the OER are widely studied in the scientific literature for fundamental purposes, they stand little chance of achieving in realistic conditions the durability level that is necessary for practical applications.

One key difference between A-WEs and AEM-WEs is the level of durability demonstrated in practical operating conditions. A-WEs, after decades of improvements and refinements, exhibit a durability often exceeding 100,000 h.²⁵⁰,²⁵⁰ On the other hand, the practical lifetime of AEM-WEs is not well-defined, especially beyond the 10,000 h limit and with intermittent operation.²⁶⁰ Intermittency and variable load operation are common if the electrolyzer is run by electricity obtained from renewable sources such as the sun and the wind and in response to electrical price signals.²⁵⁰ AEM-WES must exhibit a 40,000 h lifetime to compete successfully with PEM-WES.²⁶ The main reason underlying this discrepancy between A-WES and AEM-WES is that the latter technology is still under research and development and is not as technologically mature. Hence, in most instances, there are no practical conditions to carry out an extensive 100,000 h durability test on an AEM-WE including developmental components. Most durability studies on AEM-WE components, such as OER ECs, are carried out through accelerated aging tests, fixing either the operating potential or the current density (10 mA cm⁻² < *j* < 1000 mA cm⁻²) for limited timespans up to several hundred hours.²¹⁰ Another possibility is to implement cyclic voltammetry durability tests within high and low potential limits for several hundred cycles (>1000 cycles).²⁴⁷

A variety of features of the OER EC are used to reveal signs of degradation. The latter include: (i) morphological changes; (ii) compositional changes; or (iii) changes to the oxidation states of surface elements that may affect the OER activity. For instance, in nonoxide OER ECs the nonmetallic element (e.g., P in FeNiCoP systems) was shown to dissolve upon AEM-WE operation.²⁴⁸ The oxidation state of Mn could rise as well, increasing charge transfer resistance and negatively affecting the performance of the MEA.²⁴⁴ There are several additional factors negatively influencing the durability of AEM-WES, but
most are related to other components of the MEA (e.g., the binder, the AEM, the fabrication approach) and not to the intrinsic features of the OER EC. However, the OER EC and the electrode must remain in good contact during AEM-WE operation. Indeed, EC detachment is well-known to be a major source of performance loss in AEM-WEs.²⁰⁶ Such a detachment is easily triggered by the release of gas bubbles during AEM-WE operation, especially at high current densities (Figure 11.E-G).²³³,²⁴⁹ In addition, the formation of bubbles effectively raises the electrode resistance simply by inhibiting the access of the electrolyte to the surface of the OER EC.²⁴⁴ One way to address this shortcoming could be the enhancement of the OER EC porosity and hydrophilicity, to promote the expulsion of bubbles.²⁵⁰,²⁵¹

### 5.3. Impact of Membrane Electrode Assembly on AEM-WE Performance

Despite the progress in the development of individual components of the MEA, such as ECs, membranes and ionomers, the lack of careful integration of such may hinder the improvement of AEM-WE performance. This is particularly important in AEM-WE operated with pure water, since an extended interface between the electrocatalyst and the solid polymer electrolyte in both the anode and cathode catalyst layers is essential to maximize the reaction rates. Conversely, in the presence of a supporting electrolyte solution (e.g., KOH), the whole wetted EC surface constitutes the EC-electrolyte interface.²⁵² In this case, the ionomer is not crucial for $OH^-$ conduction, and it acts solely as a binder.

#### 5.3.1. Role of the Membrane.
AEMs typically consist of a polymer backbone with cationic anchoring groups that provide anion conductivity and selectivity. The hydrocarbon polymeric backbone commonly uses polysulfone or polystyrene to connect divinylbenzene, and specific ion exchange groups involving N-based groups,²⁰ with piperidinium²⁵⁵ and spirocyclic²⁵⁶ being currently state-of-the-art. New ion exchange groups with great potential to achieve improved alkaline stability were recently reported.²⁵⁷⁻²⁶⁵ Multiple rigorous reviews on the research advancements regarding the polymer chemistry of AEMs are already available and are therefore not the focus of this review.⁶⁸,²⁶⁶ Figure 12 presents a schematic representation of an AEM based on a quaternary ammonium pendant functional group and the chemical structures of selected commercial AEMs (Sustainion by Dioxide Materials, PiperION by Versogen and Aemion by Ionomr) adapted from refs 253 and 254. Table 2 summarizes key performance metrics for these AEMs. This comparison highlights varying characteristics and performance parameters, noting that specific performance differences may depend on application and testing conditions.²⁶⁷

The AEI is cast to form the AEM, which is responsible for transporting $OH^-$ from the cathode to the anode electrode and maintaining necessary water hydration at both electrodes while preventing gas crossover. Historically, AEMs have suffered from low ionic conductivity and chemical stability, which hinder the performance and durability of AEM-WE systems compared to PEM-WE systems when fed with pure water. The conductivity of $OH^-$ in AEMs is significantly lower than that of $H^+$ in PEMs. Consequently, for membranes with the same thickness, the AEM-WE system experiences considerable voltage losses from ionic conduction through the membrane compared to PEM-WE when no $OH^-$-containing supporting electrolyte is used. To improve hydroxide conductivity to

<figure><img src="image_21.png" /><figcaption>**Figure 12.** Schematic representation of an AEM based on a quaternary ammonium pendant functional group (A). Adapted from ref 253. Copyright 2017, Elsevier. Licensed under the CC BY-NC-ND 4.0. Chemical structures of selected commercial AEMs such as Sustainion X37-50 (B), PiperION (C) and Aemion series (D) adapted with permission from ref 254. Copyright 2022, American Chemical Society.</figcaption></figure>

compete with PEM-WE, the number of cationic groups in AEMs should be increased; however, this can lead to increased water uptake, resulting in swelling and mechanical instability issues. Very recently, AEMs with hydroxide conductivity as high as 300 mS cm⁻¹ were reported,²⁶⁸ thanks to their capability to operate at temperatures higher than 100 °C.²⁶⁹,²⁷⁰

Despite ongoing efforts to enhance alkaline electrolysis performance in pure water by utilizing conventional AEMs, optimal performance remains elusive due to the low ionic conductivity of these membranes in pure water. In general, the ionic conductivity of AEMs could be enhanced by increasing the ion exchange capacity (IEC); however, raising the IEC may lead to significant swelling or even dissolution of the membrane, ultimately compromising its mechanical stability. Cross-linking is commonly used to reduce dimensional changes, although this approach can adversely affect the
<table><caption>Table 2. Comparison of Commercial AEM Properties<sup>a</sup></caption><thead><tr><th>Parameter</th><th>[-]</th><th>Sustainion X37-50</th><th>PiperION</th><th>Aemion:</th></tr></thead><tbody><tr><td>IEC</td><td>meq g<sup>-1</sup></td><td>1-1.25</td><td>2.1-2.3</td><td>1.4-2.7</td></tr><tr><td>Conductivity</td><td>mS cm<sup>-1</sup></td><td>140 at 80 °C</td><td>149 at 80 °C</td><td>130 at 50 °C</td></tr><tr><td>Thickness</td><td>μm</td><td>50</td><td>20-80</td><td>25-75</td></tr><tr><td>Tensile strength</td><td>MPa</td><td>21</td><td>21-60</td><td>53-60</td></tr><tr><td>Elongation break</td><td>%</td><td>52</td><td>52-390</td><td>42-110</td></tr><tr><td>ASR</td><td>Ω or Ω cm<sup>-2</sup></td><td>0.3 Ω at RT<br>0.045 at 60 °C</td><td>0.35 Ω at RT</td><td>0.065-0.68 at 60 °C</td></tr><tr><td>Water uptake</td><td>wt %</td><td>83 @ RT</td><td>54 @ 80 °C</td><td>13-40 @ RT</td></tr><tr><td>Swelling ratio</td><td>%</td><td>15.1 @ RT</td><td>8 @ 80 °C</td><td>9-18 @ RT</td></tr><tr><td>Chemical stability</td><td></td><td>n.a.</td><td>60%-80% OH- conductivity reduction after 4 weeks in 1 M KOH at 60 °C.</td><td>39%-72% Cl conductivity reduction after 7 days in 3 M KOH at 80 °C.</td></tr></tbody></table>

<figure><img src="image_22.png" /><figcaption>**Figure 13.** (A) Schematic representation of MEA structure, featuring both single and double AEM. (B) Schematic depicting the distribution of water content (λ) within the membrane in various electrolytic cells when current density ≥ 0.6 A cm⁻². (A-B) were reprinted with permission from ref 272. Copyright 2022, Elsevier Ltd.</figcaption></figure>

solubility of the ionomer, which is essential for MEA preparation. Increasing the molecular weight of AEM polymers can also help to reduce swelling and improve mechanical stability. Consequently, developing an ideal AEM involves achieving a careful balance between moderate IEC and high molecular weight to ensure effective operation in pure water-fed conditions. While these issues for the AEM have been addressed to some extent, significant conductivity and stability issues remain for the AEI in the catalyst layer, which will be discussed later.

Nevertheless, AEMs still face water transport limitations unique to their operating requirements. For all membrane-containing WE systems, it is preferred to flow water to the anode only and operate the cathode nominally “dry” (with water only transported through the membrane to the cathode), as this reduces the amount of gas/liquid separation required in the BoP (Figure 9). However, in PEM-WEs, H⁺ is transported from the anode to the cathode, and therefore, electro-osmotic drag pulls additional water to the cathode, aiding electrode hydration. In an AEM with electrolyte or water feeding only to the anode, the flux of OH⁻ away from the cathode can lead to dehydration of the cathode interface and water transport limitations, especially at high current density and in the presence of ionic salts.²⁷¹ It is worth noting that while water at

the cathode in PEM-WE only matters for hydration, water at the cathode in AEM-WE serves as a reactant; thus, sufficient transport of water to the cathode is crucial for the AEM-WE system. However, the water transport behavior and swelling management during pure water operation are not fully understood.

Water transport or diffusion limitations significantly reduce the hydration levels in the cathode catalytic layer (λ < 6) and at the membrane/cathode interface. These low hydration levels greatly accelerate the chemical degradation of the ionomeric material in both the membrane and the cathode catalytic layer. This phenomenon is more pronounced at high current density, where the rate of water consumption exceeds the water flux from the anode to the cathode sides.²⁷³ Recently, Wang et al.²⁷² conducted an investigation into the impact of water diffusion on the performance of AEM-WE during cathode dry operation, employing both “single AEM” and “double AEM” (two AEMs of same thickness stacked on each other) in MEAs as illustrated in Figure 13.A. Their findings revealed an enhancement in cell performance when utilizing the “single AEM”, indicating improved performance with thinner AEMs. Notably, one of their key observations pertained to differences in the relative humidity of the hydrogen at the cathode, which is related to water transport. Given that water diffuses from the

<footnote><sup>a</sup>Data were taken from ref 267. Elsevier, Licensed CC BY-NC 4.0.</footnote>
<figure><img src="image_23.png" /><figcaption>**Figure 14.** *Ex-situ* methods reported in the literature to assess the alkaline stability of AEMs. Data adapted with permission from ref 274. Copyright 2021, Springer Nature.</figcaption></figure>

anode to the cathode based on the gradient in hydration number, $\lambda$, and is subsequently consumed by the cathode reaction, they depicted $\lambda$ profiles in the MEA for cells at the same current density (Figure 13.B). This figure illustrates that higher $\lambda$ values are estimated in the AEM on the cathode side when employing the 'single' AEM configuration, which is believed to be the cause for the enhancement in performance. Therefore, the introduction of electrolytes, e.g., dilute KOH, to the cathode side fails to accurately simulate the natural electrolyte-free environment of AEM-WEs, leading to misleading and inaccurate assessments of degradation rates, particularly under conditions of relatively low hydration expected during the dry cathode operation.

*Ex-situ* methods reported in the literature have been developed to assess the alkaline stability of AEMs immersed in aqueous alkali solutions for extended periods. Figure 14 provides an overview of various *ex-situ* methods found in the literature for determining the alkaline degradation of AEMs in alkaline solutions, focusing on evaluating changes in both performance and chemical structures. Generally, the hydroxide concentration in these alkali solutions is above 1 M, equivalent to hydration numbers higher than 6.²⁷⁵ Above this hydration level, sufficient quantities of water molecules are present to fill the first solvation sphere surrounding the $OH^-$ ions and influence their nucleophilicity. In contrast, the performance of AEMs in pure water conditions remains a critical area of study, as conventional AEMs may exhibit low ionic conductivity under these circumstances, impacting their effectiveness in applications such as water electrolysis.

**Figure 15.** A represents a schematic diagram illustrating nucleophilic attack onto trimethylbenzyl ammonium (TMBA), both with and without the presence of water.²⁷⁵⁻²⁷⁸ This scheme highlights the role of water molecules, which reduce their nucleophilicity when firmly bound to the hydroxide, effectively acting as a 'shield' against QA attack. To gain comprehensive insights into the chemical stability of AEMs in harsh alkaline environments, it is crucial to employ techniques that accurately replicate real-world conditions. *Ex-situ* methods, which involve characterizing AEM properties before and after their use in AEM-WE and AEM-FC, provide controlled environments where researchers can precisely manipulate hydration levels and other variables to mimic conditions experienced by AEMs in operation. This deeper understanding facilitated by *ex-situ* methods not only enhances the comprehension of AEM behavior but also aids in the selection of materials better suited for durable and efficient performance in practical applications.

Dekel and co-workers²⁷⁸⁻²⁸¹ developed a practical and reproducible *ex-situ* technique for simulating the true environment in MEAs. This method allows for measuring the harsh alkaline environment at low hydration levels (specifically at the cathode side), mimicking the true nature of AEM-WE and AEM-FC devices. The alkaline stability of AEMs has been evaluated by storing membranes in alkaline solutions at elevated temperatures.²⁸² However, the alkaline stability of AEMs was measured without using hydroxide-based electrolytes, but with various hydration levels. A comprehensive examination was conducted to identify the influence of water on the chemical stability of the QA functional group. As
<figure><img src="image_24.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_27.png" /></figure>

**Figure 15.** (A) Schematic diagram illustrating nucleophilic attack onto TMBA, with and without water. Figure reproduced from ref 276. Copyright 2017, American Chemical Society, Licensed under CC-BY 4.0. (B) Remaining TMBA fractions over time, varying with the number of water molecules per $OH^-$ ($\lambda$ = 0–8), in 0.6 M $OH^-$ DMSO-d6 solutions at room temperature. Figure reproduced with permission from ref 275. Copyright 2017, Elsevier B.V. (C) Remaining PPO-TMA fractions as a function of time with $\lambda$ = 0, 4 and 8, in 0.06 M $OH^-$ DMSO-d6 solutions at room temperature. Figure reproduced from ref 277. Copyright 2018, Royal Society of Chemistry. Licensed under CC-BY 3.0, (D) Normalized true $OH^-$ conductivity of the BTMA-LDPE AEM as a function of test time (80 °C, 100 $\mu$A, and a nitrogen flow of 500 sccm/min) at different RH levels. Figure reproduced from ref 278. Copyright 2020, American Chemical Society. Licensed under CC-BY 4.0.

illustrated in Figure 15.B, the degradation rate of TMBA exhibits a noticeable increase as the water content decreases.²⁷⁵ Furthermore, the protocol was extended to assess the stability of QA functional groups within the AEM backbone. Figure 15.C illustrates a reduction in the chemical stability of poly(phenylene oxide) quaternized with trimethylamine (PPO-TMA) as the water content decreases.²⁷⁷ Additionally, by employing this technique, the AEM degradation under varying and more rigorous environmental conditions was investigated, replicating the operational conditions.

Figure 15.D demonstrated a more rapid decline in the normalized true $OH^-$ conductivity of BTMA-functionalized LDPE-based radiation-grafted AEM as relative humidity decreased, indicating an increased chemical degradation of the AEM over time.²⁷⁸ Adopting such a method should provide further insights into the membrane behavior inside the AEM-WE. Accordingly, it offers important indications for developing highly conductive and stable AEM for liquid-electrolyte-free AEM-WE.

### 5.3.2. Role of ionomers.
AEIs are a crucial component in CLs used in AEM-WEs. These polymers contain positively charged functional groups, act as a binder for electrocatalyst particles and create “a passageway” for ions to travel by improving the contact between the mentioned components (EC, the PTL, and AEM) in the MEA. In practical terms, similar to the case of AEM-fuel cells, the use of ionomers with high ionic conductivity can broaden the three-phase region, leading to an increased active area within the EC layer during the electrolysis of pure water.²⁸³ Additionally, ionomers in the

CL facilitate the exchange of water and ionic products at the EC surface through their charged functional groups.²⁸⁴,²⁸⁵ Finally, maintaining good adhesion of the ionomer and EC to the current collector in the electrode is particularly crucial for AEM-WEs that operate with liquid water and gas evolution at the electrodes. These gases can accumulate and form bubbles between the EC layer and the membrane, which can compromise the adhesion of the layers and ultimately lead to the delamination of the MEA.²⁸⁶ Removing evolved gas bubbles from the EC-AEI interface may be a critical factor for high current density performance for DI water-fed AEM-WEs.²⁵²

Under fully hydrated conditions, ionomers with a high IEC and water uptake have a more significant dimensional change,²⁸⁷ which causes ionomer detachment from the EC.¹⁸¹ Due to the small EC-electrolyte interfacial area, this phenomenon is more common in AEM-WE operated with pure water. As a result, chemical or electrochemical degradation of the ionomer may significantly decrease the reaction rate. It has been recently revealed that higher IEC ionomers with light cross-linking are favored in the cathode electrode as this fine-tunes the water uptake and swelling of the ionomer.²⁸⁸ Particularly, a chemically tailored $OH^-$ conducting polymer, poly(2,6-dimethyl-1,4-phenylene oxide) (qPPO), was synthesized via amination and subsequent quaternization and it was blended with poly(vinyl alcohol) (PVA) to provide an environment analogous to basic water solutions.²⁶²

Huang et al.²⁸⁸ showed the application of cross-linked ionomers at the anode of an AEM-WE operated with 3%
<figure><img src="image_28.png" /><figcaption>**Figure 16.** (A) Comparison between the OH⁻ ions pathway in the anode of an AEM-WE operated with DI water (left) and with a supporting electrolyte solution (right). EC-ionomer interface is marked in black and the EC-liquid electrolyte interface is marked in red. Reproduced from ref 252. Copyright 2021, IOP Publishing Limited under CC BY-NC-ND 4.0 license. (B) AEM-WE performance of MEAs where AEIs with different IECs were used. (C) MEA performance comparison between HTMA-DAPP-bonded and TMA-53-bonded MEAs at 60 °C in water and 0.1 M NaOH. (B–C) were reproduced with permission from ref 181. Copyright 2020, Springer Nature Limited. (D) Cell voltage vs time for MEAs with various epoxy binder contents at 0.5 A cm⁻². Reproduced with permission from ref 293. Copyright 2022, Elsevier B.V. (E) CL delamination observed without Nafion binder when exposed to DI water vs no delamination observed when Nafion binder is used; observing binder-free (i) and binder-containing (ii) electrodes in 1 M NaOH, when binder-free (iii) and binder-containing (iv) electrodes immersed in DI water, and eventually the binder-free (v) and binder-containing (vi) electrodes after being left in the DI water for several hours. Reproduced with permission from ref 294. Copyright 2022, Elsevier B.V.</figcaption></figure>

$K_2CO_3$ solution. The cross-linking in ionomers with high IEC allowed them to limit hydrophilicity and water uptake, enabling them to have high ionic conductivity without the penalty of excess swelling. In this work, electrodes prepared using ionomers with IEC as low as zero outperformed ionomers with high IEC due to the excessive swelling of the

latter ones, causing an increase in ionic and electronic resistance, and EC detachment. Adding a hydrophobic agent (PTFE) to control the water content in the AEM-WE anode CL was found to be less effective than the cross-linking within the ionomer to limit swelling caused by the ionomer water uptake. In particular, cross-linked ionomers with high IEC had
comparable water uptake to non-cross-linked ionomers with low IEC but showed better performance (~150 mV lower cell voltage) when used at the AEM-WE anode. A trade-off between water uptake and IEC of the ionomer is crucial to achieving optimal AEM-WE performance, and the proposed ionomer cross-linking approach enabled us to take advantage of the benefits of a high IEC by mitigating the swelling.

Light cross-linking was also shown to independently reduce ionomer water uptake in the anode electrode while maintaining high ionic conductivity.²⁸⁸ Koch et al.²⁸⁹ showed that under dry cathode operation, the cell with high IEC ionomer (IEC (AP2-HNN8-00-X with IEC of 2.3–2.6 meq g⁻¹) in the cathode performs better than the mid ionomer IEC (AP2-HNN6-00-X, IEC = 1.8–2.2 meq g⁻¹) and has lower high-frequency resistance (HFR). This is explained by the better water retention in the MEA compared to the lower IEC ionomer over the whole current density range. It was found that dry cathode operation can cause a considerably higher degradation rate, but this effect is partially reduced when an ionomer with a higher IEC is used in the cathode EC layer. This suggests that utilizing ionomers with high IEC in the EC layer could provide an alternative to flowing liquid water to the cathode and as well as a way to reduce degradation rates associated with dry cathode operation, hence contributing to improved performance and durability. Therefore, developing high IEC ionomers with moderate water uptake that ensures high ionic conductivity and stability is critical for achieving high performance and durability in pure water-fed AEM-WE.

Recent work from Mayerhöfer et al.²⁹⁰ showed the impact of the ionomer content on the anode prepared using a PGM-free EC (Cu–Co oxide) deposited on a Ni-felt PTL. Two systems containing 10 and 30 wt % of Aemion ionomer were compared in pure water and 0.1 M KOH solution. With DI water, the electrode with higher ionomer content (30% vs 10%) performed better, while an opposite trend was observed in the presence of a supporting electrolyte (0.1 M KOH). The AEM and the AEI undergo a different swelling degree in different electrolytes having different pH (7 to 12.7), varying from above 100% in pure water in pristine condition to less than 50% in 0.1 M KOH electrolyte.

In addition, the sole presence of AEI within the electrode of the AEM-WE operated in DI water, neutral pH, may not be sufficient to provide a high enough pH, especially during operation, when OH⁻ ions are consumed at the anode. Therefore, the presence of supporting electrolytes, even in a low concentration, can increase the performance of the cell. However, an excessive amount of AEI in the anode CL can reduce the accessibility to the EC active sites, lowering the AEM-WE performance. AEIs in the AEM-WE anode CL require a different approach when operating with DI water or with a supporting electrolyte.

The work of Motz et al.²⁹¹ showed how the chemical structure and the physical properties of the ionomer influence the performance of an AEM-WE. Various AEMs and AEIs with different cationic functional groups were selected and tested in DI water, 1 wt % K₂CO₃ and 1 wt % KOH. These AEMs were: (i) alkyl trimethylammonium functionalized poly(styrene-ethylene-styrene) block copolymer (SES-TMA), (ii) alkyl trimethylammonium functionalized Diels–Alder poly(phenylene) (HTMA-DAPP) and (iii) polytetrafluoroethylene (PTFE) reinforced alkyl ammonium tethered poly(metaterphenylene) (m-TPN1) (Durion, Xergy). The AEIs tested had different cationic groups, such as Alkyl ammonium

(HTMA-DAPP and FLN55) and Benzimidazolium (HMT-PMBI). The results showed that the impact of the ionomer on the performance is larger when the AEM-WE is operated with DI water. In fact, AEI FLN55 possesses higher IEC than HTMA-DAPP AEI (2.5 meq·g⁻¹ vs 1.5 meq·g⁻¹), higher water uptake and higher hydroxide conductivity. AEM-WE with AEI FLN55 showed higher performance and higher operational stability in pure water. Another conclusion from this study is that the different AEIs tested (FLN55, HTMA-DAPP and HMT-PMBI) in DI water play a more significant role than the respective AEMs on the AEM-WE performance, pointing out the crucial importance of the EC-ionomer interaction within the electrode, even more than the AEM itself, as shown in Figure 16.A. The ionomer plays a more important role, especially in DI water operation, which is demonstrated by the larger differences in performance observed in DI water compared to operation with an alkaline supporting electrolyte solution.

In a recent work from Lindquist et al.,⁵⁶ three different commercial materials (PiperION from Versogen, Sustainion from Dioxide Materials, and Aemion from Ionomr) were tested. Here, AEIs were used in combination with the respective AEMs, and the AEI amount on the electrode was kept to 10 wt % for all three materials. The cell voltage difference for the three materials is significant in pure water operation, ranging over more than 200 mV at the same current density of 1 A cm⁻². The three materials also showed considerably different durability behavior when subjected to a durability test at a constant current density of 500 mA cm⁻² in DI water. The PiperION MEA was more durable overall and started from a lower voltage due to better performance at the beginning of the test (~1.85 V vs ~2.00 V of Sustainion and Aemion). However, all three ionomers showed a similar high degradation rate in the first 20 h of the test, in the range between 11 and 15 mV h⁻¹.⁵⁶ These results pointed out how challenging the durability of AEM systems operated in DI water is, mostly due to ionomer degradation. In fact, a high loading (2.3–2.8 mg cm⁻²) of commercial PGM EC (IrO₂) was used for the tests, which is unlikely to be the primary reason for degradation in OER catalytic activity.

Another recent work from Li et al.¹⁸¹ highlighted the huge impact on the AEM-WE performance due to the ionomer IEC and the ionomer content. With the same ionomer chemistry (trimethylammonium functionalized polystyrenes (TMA-x)) but different IEC (in the range 2.2 – 3.3 meq·g⁻¹) and by tuning the ionomer content in the electrodes, a performance improvement of ~400 mV at 0.4 A cm⁻² was achieved in pure water operation (Figure 16.B). The same paper also showed how ionomer IEC and loading impact the performance more in pure water than with supporting electrolytes (0.1 M NaOH). (Figure 16.C). Confirming earlier observations, an increase of an ionomer content (up to 20 wt %) improves the performance of DI water operation. However, an excessive amount of the ionomer (27.3 wt %) resulted in a performance decrease, which could be related to a poor EC dispersion and, in a limited ionomer-EC interface, essential for OH⁻ transport in the absence of a supporting alkaline electrolyte.¹⁸¹

The work of López-Fernández et al.²⁹² reported higher performance in a 1.0 M KOH-fed AEM-WE of an ionomer-free anode electrode in comparison with the same electrode prepared with ionomer added on the electrode surface and ionomer mixed with the EC. In this study, the electrodes were fabricated using magnetron sputtering, a physical vapor
<figure><img src="image_29.png" /><figcaption>**Figure 17.** (A) Schematic representation of MEA fabrication via CCM and CCS methods. Reproduced with permission from ref 206. Copyright 2021, John Wiley & Sons Limited. (B) proposed process for direct coating of an AEM with the CCM method using a bar coater. Reproduced with permission from ref 312. Copyright 2022, Wiley-VCH GmbH. Licensed under CC BY 4.0. (C) progression of spray coating on stainless-steel PTLs showing that slow spraying ensures ink drying between layers, and that too quick coating generates nonuniform coating with ink seeping. Reproduced with permission from ref 56. Copyright 2021, The Royal Society of Chemistry. (D) polarization curves showing the effect of the anode PTL type (Ni foam vs platinized Ti) on the performance of an AEM-WE fed with 1 wt % K₂CO₃. Reproduced with permission from ref 295. Copyright 2022, Elsevier B.V.</figcaption></figure>

deposition process avoid the EC dispersion in a solvent before
deposition onto the electrode. The results of this work
confirmed that no AEI is needed (other than for binding
purposes) when operating with a highly concentrated
supporting electrolyte.

Chen et al.<sup>293</sup> showed that the addition of a nonionic
conducting binder within the anode CL helps to increase the
durability of AEM-WE (0.1 M NaOH electrolyte), preventing
EC detachment without compromising the initial performance.
Butyl norbornene (BuNB), bromobutyl norbornene (BBNB),
and *tert*-butyl ester norbornene (terpolymer 1), norbornene-2-
propionic acid ethyl ester (terpolymer 2), or epoxyhexyl
norbornene (terpolymer 3) were the initial monomers used.
However, the insertion of covalent bonding sites for covalent
chemicals, in this case bonding of bis(phenyl)-A-diglycidyl
ether within the ionomer itself, creates a “self-adhesive”

ionomer, enabling it to obtain even better performance and
durability (Figure 16.D). Achieving a good and durable
adhesion of the anode EC onto the PTL is more challenging
than for the cathode EC, because typically the anode
compartment is fed with water or electrolyte solution, and
therefore within the anode CL, there is the creation of
additional tension forces due to the formation of O₂ gas
bubbles. On the other hand, the cathode is operated in dry
conditions, allowing the H₂ gas to evolve more easily.

Osmieri et al.<sup>294</sup> addressed the issue of detachment of a
PGM-free (LaSrCo oxide) CL from the PTL when exposed to
pure water. AEM and AEI underwent the initial ion-exchange
step in 1 M NaOH solution to exchange the HCO<sub>3</sub><sup>-</sup> ions
present. This procedure was done to convert the AEI within
the CL to hydroxide form before the test. No CL detachment
was observed in 1 M NaOH, but as soon as the electrode was
immersed in DI water, delamination of the CL layer occurred (Figure 16.E). This behavior was attributed to possible changes in AEI swelling properties at different pH and/or ionic strength. The authors mitigated this detachment issue by adding Nafion ionomer to the ink dispersion. Thanks to the superior binding properties of Nafion, the CL delamination when exposed to DI water did not occur. This resulted in better durability of the AEM-WE in DI water operation. The authors showed that a fine-tuning of the AEI-to-EC ratio and the AEI-to-Nafion ratio may improve performance. In fact, three conditions were examined: AEI/EC ratio of 0.2 and 0.4 and Binder/AEI ratio of 0.32 and 0.16. The conditions with low AEI/EC (0.2) and low Binder/AEI ratio (0.16) obtained the best performance in AEM-WE, a single polarization curve, but it did not retain its performance in short-term tests, showing a higher degradation rate, probably caused by the ionomer degradation. Attention should also be paid to the AEI-to-Nafion (AEI/Binder) ratio as Nafion operates as a binder to keep the EC in the catalytic layer, reduce the AEI and AEM swelling and avoid delamination. However, a reasonable amount of AEI is needed to favor hydroxide exchange. This aspect is particularly important for DI water operation, where $OH^-$ ion conductivity is assured by the AEI within the CL. When operating with a supporting electrolyte solution, the impact on the performance of this CL composition optimization was found to be less impactful due to the excess of $OH^-$ ions.²⁹⁴

### 5.3.3. Role of Electrode and Active Layer Fabrication and Structure.
Several variables in different electrode fabrication strategies may influence the MEA performance for electrochemical energy conversion devices like fuel cells and WEs.²⁹⁵,²⁹⁶ Many of these variables are directly related to the structure of the CL and interfaces of CL/membrane and CL/PTL.²⁹⁷⁻³⁰⁰

As it has already been discussed, ionomer plays an essential role as a binder and ion conductor in the EC layer of the device. The EC layer is deposited on the membrane (catalyst-coated membrane, CCM) or on the PTL (catalyst-coated substrate, CCS) using ink. The ink is comprised of the EC, solvent, and dispersed ionomer that acts as an EC binder and enables ion transport to the active EC surface.³⁰¹ The challenging part of the ink preparation lies in a fine balance of the formulation – increased content of the ionomer-binder often leads to the EC-particle agglomeration and sedimentation of the solids in the ink. The quality of the ink and EC layer deposited can impact device performance to the same degree as the properties of the individual components.⁵⁶ When the ink is deposited, the solvent evaporates and creates a porous layer of ionomer, EC, and void space for liquid/gas transport to/from the EC. An EC ink is a colloidal liquid with intrinsic interactions that control the electrode formation. Modeling efforts were made to describe the behavior of an EC dispersion; however, more studies must be performed to account for all the effects, such as rheology, dielectric constant and its influence on the aggregation within the ink.³⁰²,³⁰³ Additionally, the processing technique (Doctor blade coating, spraying, paint-brushing) has to be considered when discussing the ink's influence on the final performance of the device.³⁰⁴ The lack of a thorough understanding of EC ink affects the reproducibility of AEM electrolysis studies. The interactions between materials in this region directly impact device performance and durability⁵⁶,²⁸⁵,³⁰⁵ as the impedance of

electric, ionic, and reactant/product transport to/from the reaction zone reduces performance.³⁰⁶

CCM and CCS fabrication methods are shown in Figure 17.A.²⁰⁶ In the CCM method, the EC ink is deposited on the membrane, enabling a more intimate contact between the membrane and CL. It has been reported in the broad literature on PEM devices that the CCM method improves interfacial electric contacts at the EC-membrane interface and maximizes the EC utilization,³⁰⁷⁻³⁰⁹ which could work similarly in the AEM system. In the CCS method, the EC ink is deposited onto the porous substrate (PTL), and the advantage of this method is that it is typically easier to deposit the ink on the PTL substrate than on the membrane.³¹⁰ The reason for this is that AEMs have poor mechanical and/or physical properties (i.e., lower thermal stability, high water uptake and swelling), which make their handling in the CCM process more complicated compared to PFSA PEMs. In particular, polyarene-based AEIs typically used to fabricate AEMs have glass transition temperatures higher than 200 °C, which complicates the adhesion to a heated vacuum plate typically used for transferring spray-coated CL on the plate to the membrane.²⁹⁹,³¹¹ While employing the CCM method, the membrane wrinkles during spray coating and drying, or even transferring of the CL onto the membrane via a thermal decal process.³¹¹,³¹² To overcome these issues, Koch et al.³¹² recently proposed an alternative process for CCM fabrication using a bar coating deposition method on an AEM with a backing layer. The first deposited CL is covered with an adhesive backing foil, which prevents the membrane from wrinkling during the coating of the second CL. This method was demonstrated to create performing MEAs, and it is promising for the scale-up transition to roll-to-roll manufacturing highly employed in the industry (Figure 17.B). Particularly, the AEM-WE performed 1 A cm⁻² at 1.8 V with ≥ 1.9 mgIr cm⁻² anode loading that was comparable to spray-coated cells. A fully self-casted CCM with a thickness of 35 µm was able to achieve a performance of 1.4 A cm⁻² at 1.8 V (0.1 M KOH) and 2.0 A cm⁻² with 1 M KOH supporting electrolyte.

The CCS method has been more commonly used in AEM-WE so far. Lindquist et al.⁵⁶ highlighted that the quality of the EC ink deposition has a significant effect on the overall MEA performance in the CCS process. They pointed out the importance of the anode EC ink flow rate and drying time when spraying an IrO₂ EC ink onto a stainless-steel mesh PTL material using a hand-held spray gun. If the coating procedure occurs too quickly, without allowing enough time for the ink to dry between subsequent layers, the PTL becomes too wet, pulling the ink into the bulk and leaving areas with uncoated surfaces. Even after continued spraying, these areas do not coat evenly (Figure 17.C).

More recently, Osmieri et al.²⁹⁴ used an automated spray-coating system to achieve a uniform coating of a PGM-free EC (LaSrCo oxide) onto a sintered titanium PTL, as the automated system enables precise and simultaneous control of the flow rate and nozzle path. On the other hand, when the ink deposition rate is not controlled (e.g., when the ink is deposited via hand painting) the high porosity and hydrophilicity of the PTL material cause the ink to massively seep within the PTL, resulting in a largely nonhomogeneous coating, and reducing the contact between CL and membrane during the cell assembly. Tricker et al.¹⁷⁵ compared a hand airbrush and an ultrasonic spray coater using Co₃O₄ OER EC. The results of an AEM-WE fed with 1 M KOH showed better
performance for the airbrush method, which enabled obtaining
a more porous CL with fewer cracks, favoring EC accessibility.
The ultrasonic spray coating method caused many EC particles
to penetrate within the porous structure of the PTL, and
higher HFR.¹⁷⁵

As part of CCS or CCM, hot-pressing can be used to
improve the CL-membrane interface. Here again, poor thermal
stability and swelling properties of AEMs or AEIs make this
process challenging. The impact of MEA hot pressing and cell
torque on the AEM-WE performance was investigated by Lim
et al.³¹³ They showed that with a CCS fabrication method to
deposit commercial IrO₂ and Pt/C ECs onto titanium felt and
carbon paper PTLs, respectively, the hot pressing was
detrimental, and an optimum torque value should be used to
avoid solution leaking and excessive structural deformation of
the CL.

There are some reports comparing CCM and CCS methods. The work of Park et al.<sup>310</sup> showed better AEM-WE performance in 1.0 M KOH using the CCM method compared to the CCS method. The CCS method with hot pressing showed similar HFR to the CCM method, but higher mass transport resistance. As a comparison, the CCS method without hot pressing showed much higher HFR. An opposite trend was reported in the work of Gupta et al.<sup>314</sup> where better performance with the CCS method was obtained in an AEM-WE fed with 0.1 M KOH. These contrasting results in the KOH-fed system suggest that understanding and careful optimization of the EC layer deposition procedure are essential to improving the AEM-WE system. There is no study reported for direct comparison between CCM and CCS in pure water-fed systems, but key discoveries from the KOH system could help understand the pure water system. However, as hydroxide ion transport is limited to where ionomers are incorporated into the EC layer and EC utilization is expected to be different without supporting electrolytes, careful adoption from the literature of the KOH system to the pure water system will be required.

Other than the fabrication of electrodes from formulated ink
dispersions, recently, approaches to obtain self-supported
anode electrodes have been reported. Highly active Ni-Fe
oxyhydroxides ECs were directly grown onto Ni foam using a
galvanic-dissolved oxygen corrosion method from Xiao and co-
workers,<sup>315</sup> and a hydrothermal reaction from Wan and co-
workers<sup>316</sup> respectively. This approach potentially enables us
to avoid some of the CL fabrication steps previously described,
like ink formulation and deposition, and simplify the electrode
fabrication steps. For operation in DI water-fed AEM-WE, the
presence of ionomer is essential, and these two works propose
ionomer incorporation by dip-coating<sup>315</sup> and spray-coating.<sup>316</sup>
Kong et al. showed that Ni<sub>3</sub>Fe<sub>1</sub> EC layers were deposited on
membranes in MEA conditions where metal precursor and
reducing agent, NaBH<sub>4</sub>, were flown through the cell. The in
situ catalyst-coated membranes showed a cell voltage of 1.79 V
at 1 A cm<sup>-2</sup> with 1 M KOH feed and 1.91 V at 250 mA cm<sup>-2</sup>
with pure water feed.<sup>325</sup> The anode EC loading plays an
important role in the performance of AEM-WE cells. The
general trend of increasing performance with higher EC
loading when operating with a supporting electrolyte was
demonstrated by Mayerhöfer et al.<sup>290</sup> and Hassan et al.<sup>295</sup> The
relationship between EC loading and performance is currently
missing for AEM-WE; however, an increasing trend can be
expected up to a certain point, when a further increase in the

loading will lead to overly high mass transport and charge
transport resistance and will lower the cell performance.

The recent work of Tricker et al.<sup>175</sup> devoted attention to aspects related to the cathode of the AEM-WEs, which has not been largely explored so far. The authors investigated three aspects of the cathode: EC loading (using a Pt/C HER EC), the addition of a microporous layer (MPL), and GDL hydrophobicity. The AEM-WE was operated with 1 M KOH. They found that significant performance improvement was obtained when increasing the Pt/C EC loading from 0.15 to 0.3 mg<sub>Pt</sub> cm<sup>-2</sup>, but no significant difference was measured when the loading was further increased up to 0.45 mg<sub>Pt</sub> cm<sup>-2</sup>.

The microstructure and density of the EC layer also affect the performance in AEM-WE cells. For example, in the work of Park et al.,³¹⁷ despite the same EC loading, improved performance was obtained from creating a highly macroporous structure of the anode CL directly grown on the PTL, in comparison to a conventional ink-based CL prepared using the CCM method. The improvement was due to an increase in the accessible EC surface area, and it was obtained for two different ECs (IrO₂ and NiFe alloy) in an AEM-WE operated with 1.0 M KOH. Wang et al.³¹⁸ highlighted the importance of having a highly dense anode CL in close contact with the PTL to enhance performance. They used a CuCoOₓ OER EC deposited onto Ni foam PTLs with different thicknesses. With a 10% K₂CO₃ solution as a supporting electrolyte, decreasing the thickness of the PTL material and the EC loading at the same time enabled the achievement of similar AEM-WE performance due to the similar compactness of the CL.

It is of primary importance to emphasize the essential role
played by the EC-ionomer interface in dictating the perform-
ance of the AEM-WE operated with pure water. This interface
is created within the EC layer during the electrode fabrication
process, which was analyzed in detail in this section. It is
imperative to stress once again that in the case of pure water
operation, the only path for OH⁻ ions conduction is through
the AEI within the CL. Therefore, in this case (as evidenced in
Figure 16.A), the only electrocatalyst-electrolyte interface is
represented by the interface between the EC and the AEI. In
addition, a good interface between the CL and the AEM is
needed to enable continuity of the ionomer phase (and thus
the OH⁻ ions) between the CL and the AEM. This is
particularly important for the anode, where the OH⁻ ions
represent the reactant species for the OER.

It becomes thus clear that for increasing the performance in
the case of pure water operation, the thickness of the CL
should be minimized to reduce the length and the tortuosity of
the ionomer path for OH⁻ ions conduction, consequently
reducing the OH⁻ transport resistance within the CL.
However, for CLs fabricated using an EC in powder form,
the CL thickness is directly proportional to the EC loading,
and thus to the amount of (theoretically) available active sites.
Excessively reducing the EC loading, aiming to decrease the
thickness of the CL, will inevitably result in AEM-WE
performance decrease, as recently demonstrated in the work
of Kreider et al.<sup>319</sup> In this work, it was found that higher anode
EC loadings are beneficial for highly active OER ECs with high
electronic conductivity and uniform CLs, while for less
performing ECs with lower conductivity and/or less uniform
CLs, the EC loading has a minimal impact on performance.

However, typical well-established electrode materials used
for conventional A-WE operating in highly concentrated KOH
solutions may represent an “easy” and straightforward path
toward a transition to AEM-WE systems. These electrodes do not require the deposition of a CL using an ink where the EC is in powder form. Conversely, they can be represented as “self-supported” catalytic systems, or “catalyzed PTLs”, where the whole surface of the diffusion media (typically constituted by a foam, grid, mesh, or other porous structures) exposed to the electrolyte is catalytically active for the OER or the HER. The advantage of such a configuration is that it simplifies the electrode manufacturing process, without involving the CCM and CCS processes described earlier in this section. Several examples have been reported in the literature, also very recently.

In A-WE operating with supporting electrolyte at high concentration, the problem of the $OH^-$ ions conductivity does not exist. However, when the electrolyte concentration is lower, up to the extreme case of pure water operation, this configuration presents important difficulties to implement, since the catalytic material covers the entire surface of the porous electrode, and in these conditions, it is challenging to create a uniform AEI layer with low tortuosity from the AEM to the EC active sites.

Recently, an AEM-WE system operating with a supporting electrolyte (1 M KOH) in which both the anode and cathode were fabricated by direct hydrothermal deposition of PGM-free ECs (NiFe-LDH and Mo, respectively) on Raney nickel substrate has been reported. The electrodes were completely AEI-free since no ionomer is needed to conduct $OH^-$ ions with highly concentrated electrolyte.320 Another recent work by Oh et al.321 reported an improved performance and durability of an AEM-WE operating in 1 M KOH with both self-supported anode and cathode electrodes, different compared to the conventional CL electrodes.

Self-supported electrode approaches for both the anode and the cathode were also reported to be successful in achieving good performance in AEM-WE operated with pure water. The AEI, essential for operation in pure water, was incorporated into the electrode via spray coating. Two AEM-WEs prepared with self-supported and powdery anode ECs showed comparable performance when operated in pure water, although the electrode prepared with the powdery EC performed slightly better at high current densities, as expected in view of the previous considerations.322

A recent study showed that when operating the AEM-WE with supporting electrolyte feed, the type of anode PTL plays a crucial role. The authors investigated the use of different PTL materials with and without the addition of a CL. Depending on the PTL material used, the AEM-WE performance varied considerably. With a stainless steel PTL (containing Fe) without an EC, the performance was even better than the case where an Ir EC was used, confirming the much more straightforward anode configuration for operation in the presence of concentrated supporting electrolyte.323 Another recent work showed the superior performance of an AEM-WE with an anode consisting only of a stainless steel PTL (without any additional EC) compared to traditional CL-based electrodes.324

An interesting in situ formation of a CL without the use of AEI directly on the Ni fiber anode PTL with the MEA already assembled in the cell hardware was also reported, showing promising performance in 1 M KOH and also ultrapure water operation.325

Generally, it has been repeatedly observed that AEM-WE operated with pure water feed shows poor durability due to the

rapid oxidation of the AEI in contact with the OER EC within the anode CL. An interesting attempt to mitigate this issue has been recently reported in the work of Kwak et al. The authors proposed the insertion of a thin layer (2–3 nm) of Hf oxide, which is electronically insulating but $OH^-$ ions conductive, on the surface of the anode EC. This resulted in a considerable improvement in durability with only a negligible performance penalty.326

### 5.4. Bipolar Plates and Porous Transport Layers

Besides the MEA, which is the core of a membrane-based WE, other components such as the porous transport layers (PTLs) and the bipolar plates (BPs) also play an important role in determining the overall performance. The basic architecture of AEM-WEs was presented in the Introduction section and showed that to achieve high productivity, many MEAs must be interconnected among them at a stack level. There are two ways in which the interconnection between the individual cells can be realized. The first is the simplest and is the so-called monopolar design. In such a configuration, each cell is connected in parallel to form a stack. Despite its simplicity and ease of fabrication, the monopolar configuration has the major disadvantage of having high ohmic losses. To avoid the limitations associated with ohmic losses, the configuration based on BPs was introduced. With BPs, the cells are connected in series, with the voltage being equal to the sum of the voltage at each single cell and the current that is equal to the current that passes from each single cell.327 A schematic of the two configurations is reported in Figure 18.

<figure><img src="image_30.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>**Figure 18.** Water electrolyzer stack assembled in an (A) monopolar configuration and (B) bipolar configuration.</figcaption></figure>

The current PEM-WE and AEM-WE technologies employ a bipolar configuration of the cell. In PEM-WE and AEM-WE, besides connecting the cells by having isolated cathodes and anode sides, the function of the BP is to allow the transport of the electrolyte and the product gases to and from the electrodes, respectively, via carefully engineered flow fields at the surface of the BPs. The flow field can have different shapes, such as single serpentine, double serpentine, triple serpentine, parallel328 and pin type.329

BPs must also allow proper thermal management of the cell via thermal conduction to guarantee a heat removal rate
sufficient to avoid overheating.330,331 We remark that, so far, the BPs are the most expensive components of AEM-WEs, even more than the costly PGMs base ECs of PEM-WEs.16 The stack usually accounts for roughly 45% of the whole electrolysis system cost and in PEM-WE, the BPs account for more than 50% of the cost of the stack.16 In PEM-WE, the highly acidic environment together with the high potentials experienced at the anode require the use of bulk titanium, which is expensive and classified as CRM the EU.332 Moreover, to prevent the material corrosion, proper coatings can be envisaged. In this regard, alkaline technologies are less demanding, as the number of materials, also relatively inexpensive, that can resist corrosion even at more than 2 V is much larger.282 Typically, stainless steel and graphite were employed, cutting the cost compared to the use of titanium-based materials or complex-coated architectures.310 However, corrosion can still be an issue. Indeed, stainless steels with high Ni and Cr content may undergo severe passivation at the anode. This might not compromise the integrity of the device but may add ohmic drops due to the high resistance of the passivation layer, with an overall increase in the area-specific resistance of the device, as already shown in solid polymer fuel cells also in ref 333. On the other hand, the use of graphite is not suitable. That is mostly because of the high anode potential and the presence of $OH^-$ that can promote a nucleophilic attack on carbon.16,19 So far, the literature on BP materials and configuration for AEM-WE is still limited, and, basically, no studies dedicated to AEM-WE operating with pure water have been reported yet. However, it can be argued that the material selection should follow the same criteria used for AEM-WEs fed with alkali-containing electrolytes. This hypothesis can be considered a conservative one.

The PTLs are located between the BP surface and the anode and cathode sides of the MEAs. They are complex components responsible for a variety of functions. First of all, PTL provides a proper connection between the MEA and the BP to create interfaces with a low contact resistance. Additionally, one of the primary functions of PTL is an even feed distribution while allowing rapid removal of the hydrogen and oxygen gases via the flow against the supplied electrolyte. Finally, the PTLs must be thermally conductive to allow safe thermal management of the MEA.334,335 The PTL is a critical component in determining WE performance and its structure should be optimized.336 Previously, PTL was extensively studied in PEM-WEs.334,335,337,338

A recent comprehensive review analyzed the PTLs in PEM-WEs and the effects of porosity and pore size distribution, pore gradient, thickness, and pretreatment on ohmic, mass transport, activation overpotential, and PTL modeling.339 Higher porosity enhanced the mass transport but lowered the charge transport. Conversely, low porosity decreases mass transport but promotes charge transport. Optimal porosity for the PTL needs to be found case by case; however, porosity in the range between 30% and 50% is suggested.339 Optimal porosity gradients are the ones with low porosity close to the CL and larger close to the flow field since they improve charge transfer and mass transport.339 Thickness is also a critical aspect to be considered: lower thickness increases the mass and charge transport; however, it reduces the stability of the PTL. The interface between the PTL and CL is also critical to lowering the contact resistance and enhancing performance.339 PTLs undergo passivation, lowering their electrical conductivity and enhancing ohmic losses. Therefore, a protective coating layer is

usually applied to enhance the stability and durability of the PTLs. Finally, PTL modeling can enhance the understanding of the two-phase mass transport within the porous media.339

In the case of AEM-WEs, the architecture containing a microporous layer (MPL) was less explored despite much knowledge that can be inherited from the wider experience in PEM-WE. Further research efforts in this direction could lead to a significant improvement in the performance of AEM-WE.336 PTLs can also be used as a support for the active phases. It was shown that an AEM-WE with an EC deposited on a titanium nonwoven fabric modified by anodization is an excellent support for Pd-based ECs.340 Current densities of up to 2 A cm⁻² were demonstrated in the study. Interestingly, the use of modified PTLs as electrodes could enable the fabrication of electrodes with embedded nanostructures that are easy to handle and limit the potential release of nanoforms at any stage of the device life cycle.70,341,342 The deposition of the active phases on the PTL can be performed using binder-free and solvent-free methods. Razmjooei et al. showed stainless steel PTL coated with NiAl at the anode and NiAlMo at the cathode using atmospheric plasma spray.336 It was shown that this approach is effective in obtaining three-dimensional electrodes with highly productive and cost-effective processes. However, the absence of alkali in the feed solution limits the conductivity and leads to resistance problems in extended electrode architectures, where active sites can be located far from the membrane. Obtaining appropriate surface conductivity either by surface functionalization of the materials and/or by the addition of ionomer might mitigate this issue and further research is required to confirm this hypothesis. In terms of materials, in alkaline environments, nickel-based PTL (e.g., nickel foam or felt) can be used at the anode.337 The Pourbaix diagram shows that nickel undergoes passivation under anodic conditions at high pH.338,343 The use of Ti at the anode is not recommended because titanium can dissolve at high anodic potentials and high pH, leading to stability problems. Stainless steel is also very promising and its use at both the anode and cathode was reported.337 The importance of selecting an appropriate anode PTL for AEM-WE operation (with 0.3 M KOH or 1 wt % K₂CO₃ electrolyte) was highlighted in the work of Hassan et al.,295 where two different PTLs, Ni foam and platinized Ti, were used. A thorough design-of-experiment study with full statistical analysis was made investigating the effect of four different variables: EC loading, EC type, PTL type, and addition of carbon to the electrode. The PTL was found to be the factor having by far the largest impact on the AEM-WE performance, although the other variables all had a statistically significant effect.

The type of PTL, namely a Ni foam vs a sintered Ti, was found to be very impactful on the AEM-WE performance in DI water operation, with ~ 150 mV gap at 1 A cm⁻² in favor of the sintered Ti PTL. When operating with a supporting electrolyte solution (1% K₂CO₃ or 0.1 M KOH) instead, both PTLs performed similarly, with a 10–20 mV gap at 1 A cm⁻² in favor of the Ni foam. This reverse trend was explained by the partial contribution of the Ni foam PTL itself to the OER activity in the presence of supporting electrolytes.295

More recently, Hassan et al.298 investigated the impact of 14 different anode PTL materials from different manufacturers. The PTLs were classified in terms of material (Ni alloy and stainless steel) and fabrication method (sintered, fiber felt paper, sintered gradient). IrO₂ EC was deposited onto the PTL
via hand airbrushing, and the tests were done in an AEM-WE operated with 0.3 M KOH. The results demonstrated that Ni-based PTL showed ~ 100 mV lower voltage than stainless steel counterparts in the same cell fabrication and testing conditions. Little impact was instead provided by the PTL thickness and by the fibers vs sinter PTL structure. A PTL porosity in the range between 35 and 40% provided the best performance.

Li et al.³⁴⁴ showed that using an innovative type of PTL based on titanium thin foil with ordered and controlled porosity, fabricated via lithography and mask-patterned chemical wet etching, enabled considerably improved AEM-WE performance compared to a commercial platinumized Ti foam. The innovative PTL was electroplated with gold. In that study, 1 A cm⁻² at 1.8 V in a deionized water-fed AEM-WE was achieved, which is one of the highest performances recorded in operations in pure water. These outstanding results are attributed to the enhanced interfacial contacts and improved mass transport due to the well-tunable morphological features. These features include straight-through pore structure, even pore distribution, and precise pore size to achieve highly efficient permeation.³⁴⁴

It is evident that to improve the performance of AEM-WEs fed with pure water, more research should focus on MEA and electrode fabrication, since the CL structure, as far as the interface between PTL, ECs, AEI, and AEM, has a much higher impact on the performance than when the AEM-WE is operated with a supporting electrolyte solution. Similarly to BPs, not much research was done on PTLS specifically designed for AEM-WEs and even less literature is available for PTLS used in AEM-WE operating with pure water. However, it can be assumed that the operating conditions are reasonably similar or milder compared to systems fed with the alkaline solution as an electrolyte, justifying the use of the same materials. Conversely, A-WE operates with electrodes fully immersed in a liquid electrolyte, while AEM-WE best systems operate with a “dry-cathode”; therefore, the water management issues are different and need to be specifically investigated for this system. Much more effort has to be put in place into understanding the mass and charge transport within the PTL and the eventual use of an MPL identifying the effect of porosity, pore size distribution and related gradient, pore gradient, thickness, etc., on ohmic, mass transport, activation overpotential.

## 6. EFFECT OF WATER COMPOSITION ON AEM-WE PERFORMANCE

An important aspect of hydrogen production with pure-water electrolyzers is the water quality. Several sources of water can be used for electrolytic processes. The most common being groundwater, seawater, municipal tap water, and treated wastewater, or surface water. Currently, commercial WEs and most bench-scale pure-water electrolysis experiments use ultrapure water (UPW). Several standards for water purification can be found, but perhaps the most common is the American Society for Testing and Materials (ASTM) standard for reagent water (D1193), which specifies four levels of purification and their species contents (Table 3).³⁴⁵ From Table 3, there are four main subcategories to assess the water quality:

* Ions concentration

* Total organic content (TOC)

* Silica

<table><caption>Table 3. Classification of Water Type and Maximum Contaminant Levels According to the ASTM Regulation³⁴⁵</caption><thead><tr><th></th><th>Type I</th><th>Type II</th><th>Type III</th><th>Type IV</th></tr></thead><tbody><tr><td>Electrical resistivity, MΩ-cm, min (conductivity, μS cm<sup>-1</sup>)</td><td>18 (0.056)</td><td>1.0 (1.0)</td><td>4.0 (0.25)</td><td>5.0 (0.2)</td></tr><tr><td>TOC, max, μg L<sup>-1</sup></td><td>50</td><td>50</td><td>200</td><td>no limit</td></tr><tr><td>Sodium, max, μg L<sup>-1</sup></td><td>1</td><td>5</td><td>10</td><td>50</td></tr><tr><td>Chlorides, max, μg L<sup>-1</sup></td><td>1</td><td>5</td><td>10</td><td>50</td></tr><tr><td>Total silica, max, μg L<sup>-1</sup></td><td>3</td><td>3</td><td>500</td><td>no limit</td></tr></tbody></table>

* Bacteria

The primary metric used to classify water purity for water electrolysis is electrical conductivity (or resistivity), which reports on the total ion concentration in the water. While A-WEs can handle UPW with a conductivity of <5 μS cm⁻¹, AEM-WEs and PEM-WEs usually require higher purified water with less than 0.1 μS cm⁻¹. To date, there is no established water purity standard for WEs. Despite significant research on the effects of impurities in AEM-WEs, no specific studies have focused on how varying water purity levels influence their performance and durability. This may be due to the broad range of AEMs and ECs used in AEM-WEs, unlike the more limited options available for PEM-WEs. Therefore, although it may be feasible to use less pure water with AEM-WEs, the prevailing assumption is to maintain the same purity level as in PEM-WEs due to the possible sensitivity of the membranes.

As a rule of thumb, a 1 MW electrolyzer requires about 200 L h⁻¹ of UPW.⁸³ Depending on the water source, the efficiency of purification is between 30% to 70%³⁴⁶ therefore, 280–660 L h⁻¹ of raw water is required per 1 MW of water electrolyzer. In addition, more water is required for cooling, usually twice that of the UPW (about 400 L h⁻¹).³⁴⁶ Hence, the relation between the WE capacity, the available water sources, and the contamination level should be carefully considered when choosing the facility location and the purification process.

In recent years, there has been an ongoing debate regarding the necessity of UPW for alkaline electrolyzers, which is expressed in an emerging field of EC development for direct seawater electrolysis.⁶⁴,⁶⁶,⁸³,⁹⁴,³⁴⁷,³⁴⁸ The main arguments in favor of direct seawater electrolysis are: 1) eliminating the step of water purification which reduces the facility footprint and the overall energy demands; 2) reducing the cost related to the water desalination; and 3) advantage of on-site hydrogen production from seawater, for example, coupled with off-shore renewable energy such as wind farms.

Electrolysis of seawater is a very challenging task, suffering from intrinsic limitations of low activity and durability.³⁴⁹ The pH of seawater is near neutral (around 8–8.5), which means that the concentration of H₃O⁺ and OH⁻ is quite low, resulting in significantly lower electrocatalytic performance and higher resistivity, which causes an additional voltage drop in comparison to strong acidic or alkaline solutions.³⁵⁰ In addition, the buffering capacity of seawater is very poor. Hydroxide ions generation in the cathode and their consumption in the anode cause to increase in the pH in the former and a decrease in the latter. Over time, this difference is building up and pH shifting by 5 to 9 units has been reported.⁹⁴,³⁵¹,³⁵² This causes severe degradation of the ECs and membrane, in addition to lower electrocatalytic activity. In
<table><caption>Table 4. Common Ion Concentrations in Seawater, Groundwater, and City Water and Their Maximum Solubility under Alkaline Conditions (pH 13)<sup>a</sup></caption><thead><tr><th>Ion</th><th>Seawater (µg L<sup>-1</sup>)</th><th>Groundwater (µg L<sup>-1</sup>)</th><th>City Water (µg L<sup>-1</sup>)</th><th>K<sub>sp</sub><sup>b</sup></th><th>Standard potential (vs SHE)</th><th>Max. solubility @pH13 (µg L<sup>-1</sup>)<sup>c</sup></th></tr></thead><tbody><tr><td>Mg</td><td>1.3·10<sup>6</sup></td><td>5.1·10<sup>3</sup></td><td>6.3·10<sup>3</sup></td><td>5.61·10<sup>-12</sup></td><td></td><td>2.31·10<sup>-5</sup></td></tr><tr><td>Ca</td><td>4·10<sup>5</sup></td><td>1.2·10<sup>5</sup></td><td>2.6·10<sup>4</sup></td><td>5.02·10<sup>-12</sup></td><td></td><td>1.25·10<sup>-5</sup></td></tr><tr><td>Ba</td><td>30</td><td>240</td><td>2000</td><td>2.55·10<sup>-4</sup></td><td></td><td>1.86·10<sup>2</sup></td></tr><tr><td>Mn</td><td>3.4</td><td>3.5</td><td>2.5</td><td>1.9·10<sup>-13</sup></td><td></td><td>3.46·10<sup>-7</sup></td></tr><tr><td>Zn</td><td>10</td><td>4.1</td><td>62</td><td>3·10<sup>-17</sup></td><td>-0.76</td><td>4.58·10<sup>-11</sup></td></tr><tr><td>Cd</td><td>0.11</td><td>0.08</td><td>5</td><td></td><td>-0.4</td><td>1.62·10<sup>-16</sup></td></tr><tr><td>Cu</td><td>3</td><td>2</td><td>100</td><td></td><td>-0.34</td><td>1.0·10<sup>-41</sup></td></tr><tr><td>Fe</td><td>10</td><td>92</td><td>20</td><td></td><td>-0.44</td><td>4.99·10<sup>-15</sup></td></tr><tr><td>Ag</td><td>0.3</td><td>0.27</td><td></td><td></td><td>0.79</td><td>5.7·10<sup>-29</sup></td></tr><tr><td>Ni</td><td>5.4</td><td>4.7</td><td>1.6</td><td></td><td>-0.25</td><td>1.8·10<sup>-21</sup></td></tr><tr><td>Co</td><td>0.27</td><td>0.5</td><td></td><td></td><td>-0.28</td><td>1.11·10<sup>-20</sup></td></tr></tbody></table>

<sup>a</sup>The values of the ion concentration in different water sources were taken from refs 369–371. The values of the standard reduction potential and the K<sub>sp</sub> were taken from refs 246 and 372. The maximum solubility for each ion at pH 13 was calculated. <sup>b</sup>At 25 °C. <sup>c</sup>At 100 mV overpotential at the cathode. The maximal solubility was calculated by rearranging the Nernst equation - [X] = 10<sup>η/n/0.059</sup> where n is the electron number, η is the overpotential, and [X] is the ion concentration in M.

most of the published works the electrolyte was not just seawater, but rather an electrolyte simulating seawater (usually 0.5 M NaCl which has the same salt percentage as seawater ~ 3.5% wt. and the addition of some other salt for buffering),<sup>353–355</sup> seawater with added salt for buffering (usually phosphate or borate)<sup>356–358</sup> or seawater with the addition of KOH or NaOH.<sup>359–364</sup> In the few cases where only seawater was tested, the ECs performance for either HER or OER was much lower than the one obtained with KOH solutions.<sup>363,365,366</sup>

Adding KOH to seawater helps in the stabilization of the pH and shows good electrocatalytic results, but at the same time, creates another severe problem, which is the precipitation of a hard scale on the electrodes and on the membrane.<sup>367,368</sup> Considering the solubility product constant of many hydroxide complexes (Table 4), the maximum solubility of many cations that can be found in the raw water is extremely low. Assuming an overpotential of 100 mV at the cathode, the maximum solubility of selected metals was calculated based on their standard reduction potential and the Nernst equation. As can be seen, the maximum solubility is several orders of magnitude lower than their concentration in seawater. Those metals can be deposited on the cathode, causing blockage of the electrocatalytic sites and causing side reactions that result in lower activity and durability.

Figure 19 shows the maximal solubility of different cations at pH 13 and the average seawater concentration (logarithmic scale). Those values were calculated by cross-referencing with the Pourbaix diagram of those elements to find the most stable phase under the given conditions. For example, the max solubility of manganese is 3.46 × 10<sup>-7</sup> µg L<sup>-1</sup>, whereas a typical concentration in seawater is 3.4 µg L<sup>-1</sup>, a 7 orders of magnitude difference.

Yu et al. demonstrated a synthesized 3D core-shell NiMoN@NiFeN tested for OER.<sup>363</sup> After electrolysis in a mixture of seawater and 1 M KOH, a clear white deposition of insoluble salt on the electrode appears (Figure 20). The buildup of undesirable salt precipitate is critical in a cell with a constant flow of electrolytes.<sup>86</sup> Additionally, the presence of metal ions such as copper, iron, zinc, cadmium, and more can result in the deposition of metallic particles on the cathode.<sup>86</sup> This part will be discussed deeply later on in the review (section 6.3.1).

<figure><img src="image_32.png" /><figcaption>Figure 19. Logarithmic values of average concentration in seawater (blue symbols) and the maximal solubility at pH 13 (red symbols) for different cationic species.</figcaption></figure>

At the same time, the most significant side reaction at the anode is the chloride oxidation reaction (COR), which can compete with the OER. An overall assessment of the Cl<sup>-</sup> oxidation products in an AEM-WE should also consider the formation of OCl<sub>2</sub><sup>-</sup> and OCl<sub>3</sub><sup>-</sup>.<sup>67</sup> However, a mixture of ClO<sup>-</sup>, OCl<sub>2</sub><sup>-</sup> and OCl<sub>3</sub><sup>-</sup> is envisioned; therefore, the Pourbaix of the three species is reported in Figure 21. Below are reported the Nernstian expressions (V vs SHE, ignoring activity effects) for the anode reactions involving ClO<sup>-</sup> (eq 14), OCl<sub>2</sub><sup>-</sup> (eq 15) and OCl<sub>3</sub><sup>-</sup> (eq 16).

$$E_{OCl^- / Cl^-} = 1.72 - 0.059 \times pH + 0.059 \log[OCl^-] \quad (14)$$

$$E_{ClO_2^- / Cl^-} = 1.59 - 0.059 \times pH + 0.059 \log[ClO_2^-] \quad (15)$$

$$E_{ClO_3^- / Cl^-} = 1.45 - 0.059 \times pH + 0.059 \log[ClO_3^-] \quad (16)$$

OER shows the same pH dependency and the constant difference between the two is 485 mV at 25 °C at pHs above 7.5 (Figure 21C). At elevated temperatures (60 °C), this difference is further increased by 21.5 mV. Commercial A-WEs are usually working in the current density range of 0.2 to 0.8 A cm<sup>-2</sup> and up to 2 A cm<sup>-2</sup> with AEM-WE, which translates to an overpotential of about 500 mV and higher in the anode
<figure><img src="image_33.png" /><figcaption>**Figure 20.** Optical images of (A) the two electrolytes and (B) the NiMoN@NiFeN sample before and after seawater electrolysis. Figures reproduced from ref 363. Copyright 2019. Springer Nature. Licensed under CC-BY 4.0.</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>**Figure 21.** Pourbaix diagrams of H₂O and (A) OCl₃⁻, (B) OCl₂⁻, and (C) OCl⁻ in 0.5 M Cl⁻ at 25 °C. Reproduced with permission from ref 67. Copyright 2020, Elsevier Inc.</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>**Figure 22.** Schematic of a seawater desalination plant considering pretreatment, reverse osmosis and polishing.</figcaption></figure>

(based on the HER/OER exchange current density ratio). Being mechanistically simpler and kinetically faster than OER (2e⁻ vs 4e⁻), COR is a strong competitive reaction that should be avoided by engineering ECs that can suppress COR or by purifying the water.

Overall, direct seawater electrolysis is not studied enough to be considered for commercial application and additional in-depth research is needed to mitigate EC degradation, ion participation, ion poisoning, and side reactions such as COR. Significant attention is devoted to the EC degradation mitigation strategy, but deleterious effects can also occur on the AEM.⁶⁸ Importantly, to allow a true assessment of direct seawater electrolysis and its potential, the tests should be conducted under the realistic condition of constant electrolyte

circulation.³⁷³ The effect of impurities on the AEM-WE will be discussed deeply in the following sections.

## 6.1. Cost Analysis of Water Purification

The cost of water purification to UPW can have a significant impact on both CAPEX and OPEX. CAPEX for producing UPW may include the costs of purchasing and installing specialized equipment for water purification, such as reverse osmosis (RO) systems, deionizing (DI) systems, and other filtration methods.³⁷⁴ In terms of OPEX, the ongoing costs of operating and maintaining the water purification equipment must also be considered. OPEX is dominated by electricity costs and includes replacement parts, labor, and water usage.³⁴⁶ However, the benefits of using UPW in a water
electrolyzer can be substantial, as it can help improve the efficiency and longevity of the electrolyzer cells, leading to increased production and reduced maintenance costs in the long run. Therefore, while producing UPW can have a significant financial impact, the benefits of improved efficiency and reduced maintenance costs may outweigh the initial expenditures over time.

In the literature, many techno-economic analyses show that the energy saving using direct seawater electrolysis is negligible, while the disadvantages are enormous and can lead to a reduced lifetime of the electrolyzer and to overall more complicated and costly operation.16,94 The actual cost of UPW production ($ m⁻³) is heavily dependent on the electricity (rates, renewable vs grid), location (water source, lands availability), purification specification (water quality), production capacity and as a result, a range of price estimations can be found.346,375–377 The most common procedure for producing UPW is pretreatment-RO-polishing, with RO being by far the most energy-intensive346 as shown in Figure 22.

RO removes the vast majority of the contaminations (>98%) and the energy demand of the polishing step (using electro-deionization (EDI) or capacitive deionization (CDI)) is about 5 to 10 times lower than the previous steps.378,379 Therefore, the overall energy demand and related cost of producing UPW are comparable to that of desalination (including the pretreatment).16 Hence, it is convenient and more realistic to consider the actual levelized cost of water (LCOW) of plans around the world for the electrolysis step. The last 10 years reported LCOW using RO ranges between 0.46 to 1.2 US$ m⁻³ and this cost is variable in function of the location, the water characteristics and the cost of electricity.376,380–382 Taking the average price for the total UPW to be ~ 0.9 $ m⁻³ and the conversion ratio of 9 kg-UPW/kg-H₂, the water purification process is at least 2 orders of magnitude less than the target price of 1$/kg-H₂.

Accounting for thermodynamics and assuming 100% electrolyzer efficiency, the minimum energy requirement for electrolysis of 1 kg of pure water is 3.66 kWh. Even when considering the most demanding water source - seawater, the energy consumption for producing 1 kg of pure water is about 520 times less than the electrolysis process (~0.007 kWh).383 In case the source water is groundwater, the energy consumption is less than a third of seawater.374

Realistic WEs are working with much less efficiency - about 65% (alkaline) to 80% (PEM). Taking the average efficiency for AEM-WE to be 73%, the energy requirement difference between the electrolysis and purification process is at least 3 orders of magnitude. In addition, water purification technologies are getting better over time, which will be expressed in lower energy demands and overall lower LCOW. For small-scale WEs, where the water uptake is relatively low, the desalination process can have a more significant impact on the overall cost. Small-scale WEs are usually located close to the consumer (factory, for example) for on-site hydrogen production. In this case, the water source can come from municipal water, which can minimize the energy requirement during the RO step (in comparison to seawater) or even eliminate it, relying only on EDI or CDI.

## 6.2. Effect of Electrolyte Composition and Pure Water Feed on AEM-WE Performance

The composition of the circulating electrolyte is yet another essential factor in optimizing the performance of the WE.

Supporting liquid electrolytes, often comprising hydroxide or carbonate/bicarbonate salts of alkali metals, are employed to uplift the performance. Supporting liquid electrolytes being capable of facilitating the distribution of reactant hydroxide across the CL helps in plummeting the ohmic resistances and enables the EC/electrolyte interface, together with an improved ECSA.384 AEM-WE requires relatively less concentrated alkaline liquid electrolyte to rationalize the kinetic performance, whereas the usage of pure water to simplify the BoP remains challenging.385 There is literature showing cell performance in both pure water and KOH-supporting electrolytes,57,255,386–388 but there are not many studies systematically comparing the effects of pure water vs KOH-supporting electrolytes, and it is hard to summarize in simple metrics. Presumably, the EC layer design, EC-ionomer interface, ionomer/membrane, and other cell conditions have different impacts on the cell performance and durability with pure water and KOH as a feed; thus, optimizing these parameters would focus on different directions. Here, some examples of AEM-WE cell operation with a supporting electrolyte as opposed to pure water feed are introduced.

As already discussed, AEM-WE are usually fed through anolyte (solution fed at the anode) alone while keeping the cathode dried during the process, and thus water is supplied to the cathode via diffusion from the anodic side for the execution of HER and to avoid ionomer dehydration.289 Since the feed of pure water badly affects the performance due to high overpotentials and limited ionomer durability,389 the usage of secondary supporting electrolytes becomes inevitable. Recently, Kiessling et al. launched a detailed investigation to analyze the role of supporting anolyte feed on the overall performance as previously conceptualized in Figure 9.170 Using Tokuyama AEMs, the authors first ran the electrolyzer with 1 M KOH anolyte, which demonstrated the optimum performance; however, a successive circulation of Milli-Q water (DIW) as an anolyte irreversibly damaged the polarization curves with significantly higher HFR and polarization resistance. Moreover, while examining the type of cations present in the anolyte, Kiessling et al. experienced Li⁺ and Na⁺ causing voltage decay and degradation due to the OH⁻ stabilization effect.170 Cations tend to stabilize the OH⁻ ions by making noncovalent interactions, which restrict their mobility within the interfacial-double layer and hence OER activity gets worse.390 As the stabilization energy is governed by the charge density of the alkali metal cations in the order of Li⁺ > Na⁺ > K⁺, electrolytes containing K⁺ cations are considered more suitable.170 The cell potential and the HFR do not depend uniquely on the anolyte conductivity. In fact, when CO₃²⁻ was used instead of OH⁻ (e.g., K₂CO₃ vs KOH), the kinetics of the reaction were worse due to an increased cation-to-OH⁻ ratio.170 Based on the results of their work, the authors recommended the use of pure (deionized water, DIW) and concentrated KOH solutions as the best-supporting electrolyte for AEM-WE operation, due to the lack of side reactions or other factors related to intermittent operation. If the use of an electrolyte with a reduced pH is needed, CO₃²⁻ ions are recommended in case of operation at a steady state and high current density. This conclusion was drawn from comparing the HFR of AEM-WE operated with KOH and K₂CO₃ solutions with the same ionic conductivity (0.5 M KOH and 0.82 M K₂CO₃). It was found that at low current density (0.1 A cm⁻²) the HFR with K₂CO₃ was much higher than with KOH (175 vs 75 mΩ cm²), while at high current density (1 A cm⁻²) the HFR with K₂CO₃ was much lower than with KOH (175 vs 75 mΩ cm²).
<figure><img src="image_36.png" /><figcaption>**Figure 23.** Polarization curves when AEM-WE fed with anolyte while keeping the cathodic counterpart dry (A) and HFR (B) as a function of KOH concentration. In the study, cathode and anode ECs were PtRu/C and IrO₂, respectively, where HTMA-DAPP was employed as a membrane. (A-B) reproduced from ref 252. Copyright 2021, IOP. Licensed under CC BY-NC-ND 4.0. (C) Polarization curves (kinetic zone in inset) of AEM-WE operated with 1 M KOH anolyte and with different catholyte conditions: DI water, 1 M KOH, and dry with no catholyte flow. (C) reproduced from ref 394. Copyright 2022, IOP. Licensed under CC BY-NC-ND 4.0. AEM-WE polarizations comparing the different feeding conditions containing (D) 1 M KOH, (E) 0.1 M KOH, (F) pure water. (D-F) reproduced from ref 175. Copyright 2023, Elsevier B.V. Licensed under CC-BY 4.0. Comparison of polarization curves at various points when conditioning in 1 M KOH (G). Reproduced with permission from ref 56. Copyright 2021, American Chemical Society. AEM-WE operating at 1.0 A cm⁻² and 60 °C, where the initial feed of pure DIW was incrementally switched to KOH (H). The cell was configured with IrOx OER and PtNi HER ECs having GT72-10 AEM. Polarization curves were obtained as the cell operation started with 0.3 M KOH and then at different times after switching the system to DIW feed (I). (H-I) are reproduced with permission from ref 398. Copyright 2022, IOP. Licensed under CC BY-NC-ND 4.0.</figcaption></figure>

cm⁻²) the HFR values were the same. This was explained by the so-called “self-purging effect” occurring at high current densities: CO₃²⁻ anions are removed from the membrane by the high flux of OH⁻ ions flowing from the cathode to the anode. CO₃²⁻ anions are then removed at the anode by the formation of CO₂ according to the following reactions.¹⁷⁰

$$CO_3^{2-} + H_2O \rightarrow HCO_3^{-} + OH^{-} \quad (17)$$

$$HCO_3^{-} + H_2O \rightarrow H_2CO_3 + OH^{-} \quad (18)$$

While comparing different types of conventional AEMs, Pushkareva et al. reported favorable performance at various KOH concentrations, whereas the utilization of pure water led to a stark increase in membrane resistance and hence declared it insufficient for efficient electrolysis.⁷¹ With the circulation of

1 M KOH at 60 °C through both anodic and cathodic counterparts, all the analyzed AEMs, including Sustanion, Aemion and A201 exhibited noticeable current densities of 2 A cm⁻², nearly at 2.13 V, 2.26 V and 2.21 V, respectively. No doubt KOH could be expensive when compared to NaOH, but its higher conductivity, lower viscosity, and considerably greater solubility of potassium carbonate than Na₂CO₃ make it a suitable candidate for electrolysis.¹⁴,¹⁹

The ionic mobility to complete the redox reaction is determined by the solution conductivity, where pH expresses the alkalinity of the solution. Therefore, Ghoshal et al. observed the impact of the pH of the supporting electrolytes on the integrated membrane/ionomers and found a significant loss in the MEA performance with a subsequent decrease in pH, probably due to a slump in conductivity.³⁹¹ As the pH
drops, the concentration of hydroxide ions also decreases, which affects the kinetics of OER taking place at the anode.³⁹² The mechanism behind the improvement of AEM-WE performance was further explored by Liu et al. by combining the experimental study with mathematical modeling.²⁵² The cell voltage measured at 0.4 A cm⁻² was ~ 1.6, 1.7, and 1.9 V in 1.0 M KOH, 0.1 M KOH, and DI water, respectively. They witnessed a 5-fold expansion in ECSA with 1 M KOH compared to DI water, which utilizes just ~ 16.7% of the EC surface. Figure 23.A-B signifies the dependence of cell performance on pH/concentration of KOH electrolyte, where the overall activity in both regions of kinetics and mass transportation boosts as the concentration increases, while HFR gets doubled when the electrolyte is switched to DI water from 1 M KOH. As already illustrated in Figure 16A, when the KOH electrolyte circulates, additional conducting pathways (indicated by red arrows) get established, increasing the ECSA at liquid electrolyte/EC interfaces with minimization of kinetic losses, which is unlikely with DI.²⁵² Such results further underlined how creating an optimal CL structure via proper formulation, maximizing the ionomer-EC interface, can enhance performance in AEM-WE operating with pure water feed.

In addition to hydroxide-containing supporting liquid electrolytes, carbonate-based electrolytes with relatively milder alkalinity are sometimes also used to favor the stability of MEA and operational safety.¹⁹,⁷²,³⁸⁴,³⁹³ A comparison of operation with DI water and with different concentrations of K₂CO₃ (between 0.1 and 10 wt %) in the solution fed to the anode was reported by Ito et al.⁷² As expected, much better performance was achieved with higher electrolyte concentrations. The voltage gap measured at 0.1 A cm⁻² between DI water and the lower K₂CO₃ concentration (0.1 wt %) was as high as 0.6 V. The cell resistance measured in these two conditions was ~ 1.5 and ~ 0.2 Ω cm², for water and LE, respectively, explaining the large performance gap observed.⁷² On the other hand, Vincent et al. observed comparable results for 1% K₂CO₃ and 1 M KOH, while the least potential was noticed particularly with 1 M KOH electrolyte, but its corrosive nature could affect the integrity of the membrane over an extended period of time.³⁹² Furthermore, Pavel et al. compared the AEM stability with K₂CO₃/KHCO₃ and K₂CO₃ and found appreciable durability with a minimal difference between them.³⁹³ In fact, the K₂CO₃ electrolyte initially had a slightly higher conductivity and pH and thus performed slightly better in the beginning, but soon it underwent carbonation and started behaving identically to K₂CO₃/KHCO₃. With a similar thought, Hnát et al. systematically studied the influence of KOH substitution with Na₂CO₃ and NaHCO₃ on performance and stability.³⁸⁵ As predicted, KOH clearly outperformed the other two candidates due to the reduced conductivity of both the liquid and the polymeric electrolyte, together with the diminished kinetics of the anode reaction. With the help of mathematical modeling, Stanislaw et al. attempted to elucidate the reason behind the decline in the performance upon using carbonate electrolytes.⁷³ Their study moved the notion that conductivity loss due to the replacement of hydroxide with carbonate in the membrane has a minor impact on the overall device performance, but perhaps due to the shifting of carbonate ions with the OH⁻ generated by the cathode. Conversely, reduced hydroxide content in the anodic ionomer gives rise to a penalty in the Nernstian voltage. However, during operation, the carbonates

adversely pile up across the PTL/CL, leading to the insufficient utilization of EC and constraining the OER reaction closer to the membrane edge.

Owing to a strong electro-osmotic drag in alkaline conditions, water molecules also travel toward the anode from the cathode.⁷² In such a scenario, HER may undergo diffusion limitations and there is also a chance of cathodic ionomer drying out,¹⁷⁰ while catholyte (solution fed to cathode) feed may help in overcoming these issues. Cho et al. systematically investigated the effect of the catholyte feed method on the overall performance of the electrolyzer while keeping the anolyte always 0.5 M KOH.³⁸⁹ Their study concluded with an assertion that alkaline catholyte feed is distinctively better than DI water feed, owing to enhanced utilization of EC surfaces of HER, together with improved ionic conductivity. However, higher OH⁻ content delivered by KOH prefeeding recovers the electrode kinetics and, in due course, improves the AEM-WE performance. Eventually, with optimal content of anode binder, operation under dry cathodic conditions after preliminary solution feed brought about the water splitting current density of 1.07 A cm⁻² at 1.8 V. In another study, Kiessling et al.³⁹⁴ fully investigated the effect of the catholyte when the anolyte used was 1.0 M KOH. The results showed that feeding DI water or KOH solution at the cathode was beneficial at high current densities to avoid membrane dehydration and consequent HFR increase. At low current densities, instead, the presence of ions decreased HER kinetics, and better performance was obtained with dry cathode or DI water flow (Figure 23.C). Using a carbonate-containing catholyte negatively affects the performance due to anion poisoning of the HER EC. Similarly to the previous anolyte study, also in this case at low current density with the 0.82 M K₂CO₃, the HFR value was much higher than with KOH (even at a much lower concentration of just 18 mM KOH), but at high current density, the HFR decreased due to the self-purging effect. However, poor HER kinetics in the presence of a carbonate-containing electrolyte compared to OH⁻ containing ones explained the lower performance at all current densities observed for the K₂CO₃-fed AEM-WE.

Tricker et al.¹⁷⁵ compared the AEM-WE operation with 1 and 0.1 M KOH with a dry cathode (feed at the anode only), double-feed and dry anode (feed at the cathode only), and better performance was obtained for the dry cathode operation (Figure 23.D-E). These results suggest that water back-diffusion from anode to cathode is sufficient to ensure enough reactant for the HER at the cathode, even at current densities as high as 4 A cm⁻² and with a 60 µm thick membrane. The same authors also investigated an AEM-WE feed with DI water at the anode where the cathode was fed with an H₂ gas flow at different levels of humidification, showing that better performance was obtained with a relative humidity (RH) value of 80% than with 100% (Figure 23.F). This was explained by the existence of a higher water gradient from the anode to the cathode, favoring the water back-diffusion. At a lower RH value of 75%, the performance decreased due to membrane dry-out. The RH impact was higher at higher current density.

Oliveira et al.³⁹⁵ justified the better performance typically obtained with feeding only the anode by suggesting that AEM contamination from the formation of CO₃²⁻ and HCO₃⁻ ions due to dissolved CO₂ occurs, lowering the ionic conductivity. Since water can diffuse easily through the AEM, it can be fed as an anolyte. In this way, all the anions are concentrated on the anode side, where CO₃²⁻ and HCO₃⁻ ions can be easily self-