# Advancements in anion exchange membrane electrolysisers: from catalysts to life cycle assessment

Feriel Mustapha, Damien Guilbert, Bodo Gross

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# Advancements in anion exchange membrane electrolysers: from catalysts to life cycle assessment

Feriel Mustapha, Damien Guilbert & Bodo Gross

**To cite this article:** Feriel Mustapha, Damien Guilbert & Bodo Gross (2025) Advancements in anion exchange membrane electrolysers: from catalysts to life cycle assessment, *International Journal of Sustainable Energy*, 44:1, 2520812, DOI: 10.1080/14786451.2025.2520812

**To link to this article:** https://doi.org/10.1080/14786451.2025.2520812

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© 2025 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group

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# Advancements in anion exchange membrane electrolyses: from catalysts to life cycle assessment

Feriel Mustaphaª, Damien Guilbertᵇ and Bodo Grossª

ªIZES- Institut für ZukunftsdEnergie- und Stoffstromsysteme gGmbH, Saarbrücken, Germany; ᵇGREAH, Université Le Havre Normandie, Le Havre, France

## ABSTRACT

Anion exchange membrane (AEM) water electrolysis is a promising technology for green hydrogen production from renewable energy sources (RES). Despite its many advantages, it is not yet mature enough to compete with proton exchange membrane (PEM) electrolyses in terms of current density and durability. This review examines the state-of-the-art AEM electrolyses, focusing on technical specifications, leading manufacturers, durability challenges, and recent market developments. Additionally, life cycle assessments highlight opportunities to reduce environmental impacts by minimising reliance on fluorinated polymers and improving recycling strategies. Furthermore, this study emphasises the importance of optimising both hydrogen and oxygen evolution reactions to enhance performance. Finally, a comprehensive SWOT analysis underscores AEM's strengths, including low costs and compatibility with RES, while identifying innovation opportunities. Although AEM electrolyses provide a viable pathway for decarbonisation, overcoming technical and economic barriers is crucial for their large-scale adoption.

## ARTICLE HISTORY

Received 12 November 2024

Accepted 20 May 2025

## KEYWORDS

AEM electrolyser; renewable energy sources; membranes; materials; lifecycle assessment; performance

## Glossary

**AEL** alkaline electrolyser

**AEM** anion exchange membrane

**AEMEL** anion exchange membrane electrolyser

**BOL** beginning of life

**CAPEX** capital expenditures

**CCUS** carbon capture, utilisation, and storage

**EOL** end of life

**FAA** Fumatech anion exchange membrane series

**GDL** gas diffusion layer

**GHG** greenhouse gas

**GW** gigawatts

**HER** hydrogen evolution reaction

**HTE** high-temperature electrolyser

**KOH** potassium hydroxide

**LCA** life cycle assessment

**CONTACT** Damien Guilbert

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damien.guilbert@univ-lehavre.fr

© 2025 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.
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<table><tr><td>LDH</td><td>layered double hydroxides</td></tr><tr><td>LTE</td><td>low-temperature electrolyser</td></tr><tr><td>M</td><td>molar mass</td></tr><tr><td>MEA</td><td>membrane electrode assembly</td></tr><tr><td>NaOH</td><td>sodium Hydroxide</td></tr><tr><td>Nm³</td><td>normal cubic metre</td></tr><tr><td>ODP</td><td>ozone depletion potential</td></tr><tr><td>OER</td><td>oxygen evolution reaction</td></tr><tr><td>O-SOE</td><td>oxide-ion-conducting solid oxide electrolyser</td></tr><tr><td>ODP</td><td>ozone depletion potential</td></tr><tr><td>P-SOE</td><td>proton-conducting solid-oxide electrolyser</td></tr><tr><td>PEMEL</td><td>proton exchange membrane electrolyser</td></tr><tr><td>PGM</td><td>platinum-group metals</td></tr><tr><td>PFAP</td><td>poly (fluorenyl-co-aryl piperidinium)</td></tr><tr><td>RES</td><td>renewable energy sources</td></tr><tr><td>SEC</td><td>specific energy consumption</td></tr><tr><td>SPM</td><td>solid polymer membrane</td></tr><tr><td>SWOT</td><td>strengths, weaknesses, opportunities, and threats</td></tr><tr><td>TMA</td><td>tetramethylammonium</td></tr><tr><td>TRL</td><td>technology Readiness Levels</td></tr><tr><td>VBC</td><td>vinylbenzyl chloride</td></tr></table>

# 1. Introduction

Hydrogen is considered a key energy carrier for decarbonising the transportation and industry sectors, both of which significantly contribute to global temperature rise due to the intensive use of fossil fuels (Yue et al. 2021). Currently, 95% of hydrogen is used for industrial applications such as ammonia and fertiliser production, crude oil refining, electronics and semiconductor manufacturing, and metal processing. A large share of this hydrogen is produced from fossil fuels, particularly natural gas, resulting in approximately 900 million tons of CO₂ emissions annually (International Chamber of Shipping 2024). To reduce the carbon footprint of such thermochemical processes, carbon capture, utilisation, and storage (CCUS) technologies are often employed. However, CCUS currently accounts for less than 1% of global hydrogen production. Although this approach allows for low-cost hydrogen generation, the broader goal of producing low-carbon hydrogen has yet to be realised (International Chamber of Shipping 2024). To date, more than 50 countries have adopted strategies focused on the production and use of green hydrogen via water electrolysis powered by renewable energy sources (RES) such as wind, solar, hydro, and geothermal energy, in response to growing hydrogen demand. Most projections estimate an increase in hydrogen consumption of 20% to 40% by 2030 (International Chamber of Shipping 2024). Hydrogen produced from RES through advanced water electrolysis technology is considered essential for enabling a cleaner and more sustainable future. The electrolysis process has reached commercial maturity, as demonstrated by numerous green hydrogen plants established worldwide (Ikuerowo et al. 2024; Sayed-Ahmed, Toldy, and Santasalo-Aarnio 2024). This process is carried out using an electrolyser, whose core components are cells grouped into stacks. Each cell contains two electrodes (through which electricity enters and exits) separated by a membrane made of materials that conduct only ionic species (Yue et al. 2021).

Electrolysers are particularly well-suited to help balance the increasing share of RES in power grids. Their widespread deployment will enable greater integration of RES into
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electricity production, injection into gas networks, industrial processes, ammonia synthesis, renewable fuel production (via the Fischer-Tropsch process), and mobility applications, including both light- and heavy-duty fuel cell electric vehicles [International Renewable Energy Agency (IRENA) 2020]. Additionally, hydrogen, as a gas, can be stored and transported through various means (dedicated pipelines, tanker trucks, or railcars) contributing to the storage of excess renewable electricity and the decarbonisation of diverse industrial sectors. According to the International Renewable Energy Agency (IRENA), at least 270 GW of electrolyser capacity will be needed by 2030 to limit global temperature rise to below 2°C [International Renewable Energy Agency (IRENA) 2020]. Currently, green hydrogen accounts for less than 0.1% of global hydrogen supply, compared to grey hydrogen (produced from natural gas), which represents approximately 62% (International Chamber of Shipping 2024).

Water electrolysis technologies are typically classified into two categories based on their operating temperature: low-temperature electrolysers (LTEs) and high-temperature electrolysers (HTEs). LTEs encompass alkaline electrolyser (AEL) (Dubouis et al. 2024), proton exchange membrane electrolyser (PEMEL) (Sayed-Ahmed, Toldy, and Santasalo-Aarnio 2024), and more recently anion exchange membrane electrolyser (AEMEL) (Volk et al. 2024). In contrast, HTEs consist of oxide-ion-conducting solid oxide electrolysers (O-SOELs) and proton-conducting solid oxide electrolysers (P-SOELs) (Chi et al. 2024). AELs, AEMELs, and O-SOELs utilise anion-conducting electrolytes, while PEMELs and P-SOELs employ proton-conducting electrolytes. Each of these electrolyser technologies presents unique advantages and challenges in terms of performance, durability, and cost when integrated with low-carbon electricity sources. However, some are still in the early stages of development. Among the various types, only AEL and PEMEL currently dominate the market, while AEMEL and SOEL technologies, despite their recent emergence, remain in intensive research and development. On one hand, AELs are the most mature technology. They offer the lowest capital expenditure (CAPEX), and a longer operational lifetime compared to other available electrolyser types [International Renewable Energy Agency (IRENA) 2020]. However, their use of an alkaline liquid electrolyte (typically a potassium hydroxide (KOH) solution in the 25–30 M concentration range) requires frequent maintenance and limits their operational flexibility (typically 10–100%). This also reduces their responsiveness to dynamic loads, which is critical for adapting to the variable output of renewable energy sources such as solar and wind. On the other hand, while PEMELs have higher costs (mainly due to the use of platinum-group metals (PGMs) in their catalysts) and higher specific energy consumption (SEC, measured in kWh/kg H₂) compared to AELs, they offer higher current densities (up to 6 A/cm² as reported in the literature) and greater operational flexibility (0–100%). These characteristics make PEMELs particularly suitable for integration with renewable energy systems.

In recent years, anion exchange membrane electrolysers (AEMELs) have attracted growing interest from both industry and academia due to their potential for low-cost green hydrogen production (Lu et al. 2024). A bibliometric analysis of publications (including review articles, research papers, conference proceedings, and book chapters) related to AEMELs from 2019 to 2024 is presented in Figure 1. This analysis includes data from several major research databases: ScienceDirect, Taylor & Francis, Springer, and IEEE. These databases were selected for their high volume of relevant publications, with ScienceDirect hosting the majority. For instance, in 2024, approximately 85% of
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Evolution of publications focused on AEM electrolysers

<figure><img src="image_13.png" /><figcaption>**Figure 1.** Bibliometric overview of published works related to AEMEL from 2019 to 2024.</figcaption></figure>

all AEMEL-related publications appeared on the ScienceDirect platform. Notably, the number of publications in 2024 is eight times higher than in 2019, highlighting the growing recognition of this technology within the global scientific community.

Due to the increasing interest in this technology, several review papers have been published since 2018. These reviews can be categorised into four main areas: (1) investigations on AEMEL (Feng, Gupta, and Mamlouk 2023; Ferriday et al. 2024; Henkensmeier et al. 2021; Lu et al. 2024; Sugawara et al. 2023; Vinodh et al. 2023; Zakaria and Kamarudin 2021), (2) the state-of-the-art of AEMEL technology (El-Shafie 2023; Santoro et al. 2022; Shirvanian et al. 2021; Vincent and Bessarabov 2018), (3) materials development (Du et al. 2022; Keat Ng et al. 2023; Miller et al. 2020), and (4) summaries of conducted research activities, achievements, and remaining challenges (Chand and Paladino 2023; C. Li and Baek 2021; Hua et al. 2023; Lee et al. 2022; Q. Xu et al. 2022). Additionally, there have been publications related to modelling (including mathematical models and equivalent circuit models) and the effects of operating parameters such as current density, temperature, pressure, membrane thickness, and liquid saturation on performance (An et al. 2014; Gomez Vidales, Millan, and Bock 2023; Immanuel, Eun-Chong, and Hyung-Man 2021; Lawand et al. 2024; Panda et al. 2022). Finally, other cited works in this review focus on the design of electrocatalysts, anion exchange membranes (AEMs), ionomers, and their impact on the resulting AEMEL performance (e.g. current density, efficiency, ionic conductivity, stability, and aging).

Starting from this summary of reported works, the main contributions of this review paper are summarised below:

1. To provide an updated review of the performance and durability of AEMELs as outlined in the literature.

2. To present a comprehensive synthesis of leading AEMEL manufacturers, highlighting their key technical specifications and linking them with current research activities.

3. To present a comparative analysis of LTE technologies.
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(4) To examine the materials and components involved in AEMELs, as well as their combinations, and explore current research aimed at reducing the electrode cost of AEMELs.

(5) To discuss the role of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in AEMELs.

(6) To conduct a life cycle assessment of AEMELs, from the extraction of raw materials to the recycling process.

(7) To provide a SWOT analysis identifying both internal and external factors that influence the development and adoption of AEMEL technology.

Alongside the introduction, which outlines the state-of-the-art, motivations, and contributions, this review is structured into seven sections, followed by a conclusion. Section 2 discusses the principles, advantages, and durability challenges of AEMELs, along with an updated review of their performance. Section 3 begins with a summary of leading AEMEL manufacturers and the technical specifications of their products, followed by a comparative analysis of LTE technologies, focusing on SEC, operational flexibility, and hydrogen output. Section 4 examines the components and materials used in AEMELs, emphasising recent advancements. Section 5 outlines the HER and OER, while Section 6 explores their impact on system performance and includes a life cycle assessment. Section 7 presents a SWOT analysis of AEMELs, identifying key areas for future research. The conclusion summarises the findings and discusses remaining challenges.

## 2. AEM electrolyser technology

### 2.1. Principle of operation and benefits

AEMEL is a promising hydrogen production technology with a low technology readiness level (TRL). It belongs to the LTE category and uses a solid polymer membrane (SPM) to transport $OH^-$ between the electrodes. AEMELs are designed to combine the advantages of both AELs and PEMELs: the alkaline environment allows the use of non-precious metal catalysts and less expensive metal bipolar plates, while the SPM enables differential pressure operation, reduced maintenance, dynamic operation capabilities, and the ability to operate at higher current densities than traditional AELs. Due to the inherent alkalinity of the AEM, pure water can be fed into the electrolyser instead of an alkaline electrolyte. However, current AEMELs perform better and demonstrate higher durability when fed with an alkaline electrolyte. In parallel, life cycle assessment (LCA) studies have begun to play a critical role in evaluating the environmental performance of AEMELs, from raw material extraction to end-of-life disposal, providing insights into ways to reduce greenhouse gas emissions, energy demand, and material criticality. The chemical reactions involved in the AEMEL process are summarised below, along with their Nernst potential (also known as the open-circuit potential):

$$ \text{Cathode} \quad 2\text{H}_2\text{O} + 2\text{e}^- \rightarrow \text{H}_2 + 2\text{OH}^- \quad E_0 = -0.828 \text{ V} \quad (1) $$

$$ \text{Anode} \quad 2\text{OH}^- \rightarrow 1/2\text{O}_2 + \text{H}_2\text{O} + 2\text{e}^- \quad E_0 = 0.401 \text{ V} \quad (2) $$

$$ \text{Global} \quad 2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2 \quad E_0 = 1.23 \text{ V} \quad (3) $$
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Anion Exchange Membrane Electrolyser

<figure><img src="image_16.png" /><figcaption>**Figure 2.** Principle of operation of AEM electrolyzers.</figcaption></figure>

The principle of operation of AEMELs is illustrated in Figure 2. In the chemical reaction (1), representing the HER, two molecules of pure water or an aqueous solution (H₂O + KOH) are split into hydrogen and OH⁻ due to the presence of electrons at the cathode. In reaction (2), representing the OER, OH⁻ ions move through the AEM towards the anode, where they combine with electrons to form oxygen. The main advantages of this technology include the use of cheaper, non-noble metal catalysts (e.g. nickel (Ni), cobalt (Co), iron (Fe)), leading to a significant cost reduction; low SEC (4.2 kWh/Nm³, the lowest reported value); large operational flexibility in the nominal hydrogen flow rate (0–100%), making it suitable for integration with intermittent RES; and reduced maintenance requirements due to the use of a SPM and a mildly alkaline solution (0.1–1 M KOH). However, because this technology is relatively new (introduced in 2010 for water electrolysis) (Vinodh et al. 2023), it is still under active research and development (Lu et al. 2024). Indeed, it faces several challenges, particularly regarding durability, which is addressed in the following subsection.

**2.2. Main limitations and durability issues**

The main limitations include a low current density (up to 2 A/cm² at 2 V without using noble metal electrodes) (Du et al. 2022), compared to PEMELs (up to 6 A/cm² at 2 V) (Chen et al. 2021), and lower durability of current membranes, as
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highlighted in recent studies (Campagna Zignani et al. 2022; D. Li et al. 2021). The durability of AEMELs is typically assessed by monitoring cell voltage over time at a constant current density. Six different voltage behaviours during short-term operation (<100 h) have been reported in the literature, as shown in Figure 3 and detailed below:

(1) slight decrease in cell voltage,

(2) cell voltage increase,

(3) stepwise increase in voltage,

(4) repetitive voltage spikes,

(5) fast voltage decrease,

(6) failure causing a sudden drop in voltage.

The causes of these different voltage behaviours are summarised in D. Li et al. (2021). In addition to monitoring cell voltage over time at a constant current density, the use of impedance spectroscopy (also known as Nyquist plot) is a valuable tool, as it allows for observing changes in patterns (increase and decrease) caused by operating conditions, the membrane electrode assembly (MEA), and the materials used (Immanuel, Eun-Chong, and Hyung-Man 2021; Khan Niaz et al. 2021). Generally, the best performance in terms of chemical stability is reported for short-term operations (<100 h) under unrealistic operating conditions (low current density, low operating temperature, and low ambient pressure). Indeed, the durability of AEMELs is strongly dependent on the combination of several degradation modes, such as operating conditions (current density, temperature, pressure, type of liquid electrolyte), AEM, and materials (bipolar plates, gas diffusion layers (GDLs)) (D. Li et al. 2021; Khan Niaz et al. 2021). However, the effects of dynamic operations (start-stop cycles, intermittency) on AEMEL durability have not been sufficiently explored compared to PEMELs (Kannan, Kabza, and Scholta 2015; Kuhnert et al. 2024; Weiß et al. 2019). In fact, only one study has been

<figure><img src="image_18.png" /><figcaption>**Figure 3.** Overview of the six different voltage behaviours at a constant current density during short-term operations (<100 h) reported in the literature.</figcaption></figure>
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reported on the durability assessment of an AEMEL MEA under start-stop cycles (1000 start-stop cycles were performed in 1 M KOH solution at 80°C) (Narayanaru et al. 2023).

Over the past few years, several papers have summarised the performance and durability of AEMELs as reported in the literature (Du et al. 2022; Ferriday et al. 2024; Faid and Sunde 2022; Q. Xu et al. 2022; Sugawara et al. 2023; Vinodh et al. 2023). An updated summary is provided below, incorporating recent contributions. Table 1 offers an overview of various catalysts, membranes, and conditions used in AEMEL systems. Moreover, this table highlights key parameters, including catalyst loading, activity, durability, and testing conditions, which are essential for evaluating and comparing the performance of different systems. The data indicates that combinations such as IrO₂ with Pt/C or NiFeCo with Pt/C show promising performance in terms of both activity and durability, particularly under alkaline conditions. Additionally, membranes like Aemion+® and Sustainion® demonstrate significant stability, contributing to extended operational lifetimes. These benchmarks emphasise the importance of optimising material combinations to enhance both efficiency and durability, thereby supporting ongoing research towards high-performance, cost-effective electrolysers.

The best performance of AEMELs has been reported in Chen et al. (2021), achieving a current density of 7.68 A cm⁻² at 2 V with an operating temperature of 60°C and a durability of 1000 h. This achievement exceeds the current density of 6 A cm⁻² obtained for a PEMEL at the same temperature. However, PGM catalysts were employed, and poly(fluorenyl-co-aryl piperidinium) (PFAP)-derived AEM was used, which achieved a high rate of water diffusion and OH⁻ ionic conductivity. Recently, new and significant improvements in AEM have been reported, such as the use of Aemion+® membranes (AF2-HWP8-75-X), which feature high OH⁻ ionic conductivity, chemical stability, and mechanical toughness, with a thickness of 85 µm (Moreno-González et al. 2023). It has been demonstrated that Aemion+® membranes undergo negligible degradation (13 µV h⁻¹ at a current density of 0.6 A cm⁻², corresponding to the first voltage behaviour mentioned above) over 8900 h. Additionally, in S. Li et al. (2024), Lindquist et al. (2021), PiperION membranes have been employed for AEMELs. In both studies, it was shown that these membranes offer higher efficiency and better chemical stability compared to other types of membranes (Aemon, Sustainion). Such significant performance suggests a promising future for industrial applications in the manufacturing of high-efficiency and stable AEMELs. These aspects are discussed further in the fourth section, which focuses on the materials and components of AEMELs.

# 3. Comparative analysis of electrolysers technologies

## 3.1. Review of leading manufacturers of AEMELs

Currently, only a few companies manufacture AEMELs, with approximately eleven known manufacturers. In contrast, AELs and PEMELs have a more significant presence in the global market. Table 2 provides an overview of key AEMEL models from companies in Canada, China, Germany, France, Israel, Italy, the Philippines, Singapore, and the USA.

Most AEMEL manufacturers specify hydrogen flow rate (Nm³/h), system-SEC (given in kWh/Nm³), hydrogen output pressure, electrolyte type, and rated electrical power.
<table><caption>**Table 1.** Summary of performance and durability of AEMELS.</caption><thead><tr><th>Anode</th><th>Cathode</th><th>Membrane</th><th>Ionomer</th><th>Testing conditions</th><th>Activity</th><th>Durability</th><th>Upper catalyst load</th><th>Reference</th></tr></thead><tbody><tr><td>IrO<sub>2</sub></td><td>Pt/C</td><td>Aemion+® membranes (AF2-HWP8-75-X), 85 µm</td><td>Nafion® D520</td><td>1M KOH at 70°C</td><td>0.6 A.cm<sup>-2</sup> at 1.8 V</td><td>8900 h</td><td>0.4 mg/cm<sup>2</sup> (IrO<sub>2</sub>); 0.5 mg/cm<sup>-2</sup> (Pt/C)</td><td>Moreno-González et al. (2023)</td></tr><tr><td>Acta 3030® (CuCoO<sub>x</sub>)</td><td>Acta 4030 (Ni/(CeO<sub>2</sub> - La<sub>2</sub>O<sub>3</sub>)/C)</td><td>A-201 membrane (Tokuyama Corp., Japan), 28 µm</td><td>Acta I<sub>2</sub></td><td>1M K<sub>2</sub>CO<sub>3</sub> at 60°C</td><td>0.5 A.cm<sup>-2</sup> at 2 V</td><td>200 h</td><td>Not specified</td><td>Vincent, Kruger, and Bessarabov (2018)</td></tr><tr><td>Acta 3030® (CuCoO<sub>x</sub>)</td><td>Acta 4030 (Ni/(CeO<sub>2</sub> - La<sub>2</sub>O<sub>3</sub>)/C)</td><td>A-901 membrane (Tokuyama Corp.), 9 µm</td><td>Acta I<sub>2</sub></td><td>1M K<sub>2</sub>CO<sub>3</sub> at 50°C</td><td>0.5 A.cm<sup>-2</sup> at 2 V</td><td>200 h</td><td>Not specified</td><td>Vincent, Kruger, and Bessarabov (2018)</td></tr><tr><td>NiFeCo-OOH, NiFe-OOH</td><td>Pt/C</td><td>AEM, X37-50 Grade T, Dioxide Materials</td><td>Nafion®</td><td>1M KOH at 55°C</td><td>0.5 A.cm<sup>-2</sup> at 1.8 V</td><td>300 h</td><td>1 mg/cm<sup>2</sup> (NiFeCo); 0.5 mg/cm<sup>2</sup> (Pt/C)</td><td>Ha et al. (2024)</td></tr><tr><td>Ni-Fe Co</td><td>Ni-Mo alloy</td><td>Fumatech, FAA-3-50</td><td>–</td><td>1M KOH at 50°C</td><td>0.3A.cm<sup>-2</sup> at 2.17 V</td><td>72 h</td><td>2 mg/cm<sup>2</sup> (Ni-Mo)</td><td>Loh et al. (2023)</td></tr><tr><td>Ni-Fe-Co</td><td>Ni-Mo alloy</td><td>Fumatech, FAA-3-50</td><td>–</td><td>4M KOH at 50°C</td><td>0.3A.cm<sup>-2</sup> at 2.03 V</td><td>72 h</td><td>2 mg/cm<sup>2</sup> (Ni-Mo)</td><td>Loh et al. (2023)</td></tr><tr><td>CuCrO<sub>2</sub></td><td>Pt/C</td><td>PiperION-A60-HCO3</td><td>PiperION-A5-HCO3</td><td>1M KOH at 60°C</td><td>0.5 A.cm<sup>-2</sup> at 2.1 V</td><td>120 h</td><td>Not specified</td><td>S. Li et al. (2024)</td></tr><tr><td>HNA-CA</td><td>HNA-CA</td><td>Sustainion® X37-50 Grade RT</td><td>–</td><td>1M KOH at room temperature</td><td>1 A.cm<sup>-2</sup> at 1.89 V</td><td>500 h</td><td>Not specified</td><td>Jiang, Bouzek, and Liao (2023)</td></tr><tr><td>FeCoP</td><td>PtRu/C</td><td>XL20-rPNB-LY100, 30 µm</td><td>TMA-70</td><td>1M KOH at 80°C</td><td>1 A.cm<sup>-2</sup> at 1.89 V</td><td>100 h</td><td>Not specified</td><td>Hyun Choi et al. (2023)</td></tr><tr><td>CuCoO<sub>x</sub></td><td>Pt</td><td>A-201 membrane (Tokuyama Corp., Japan), 28 µm</td><td>Anion exchange</td><td>K<sub>2</sub>CO<sub>3</sub> (0.1M, 1M<sub>10M</sub>), KOH at 50°C</td><td>–0.1 A.cm<sup>-2</sup> at 2.2 V (KOH), – 1 A.cm<sup>-2</sup> at 1.8 V (10M)</td><td>–</td><td>1 mg/cm<sup>2</sup> (CuCoO<sub>x</sub>)</td><td>Ito et al. (2018)</td></tr><tr><td>IrO<sub>2</sub></td><td>Pt/C</td><td>A-201 membrane (Tokuyama Corp., Japan), 28 µm</td><td>–</td><td>0.5M KOH at 50°C</td><td>1.72 A.cm<sup>-2</sup> at 2.2 V</td><td>–</td><td>0.4 mg/cm<sup>2</sup> (IrO<sub>2</sub>); 0.5 mg/cm<sup>2</sup> (Pt/C)</td><td>Cho et al. (2018)</td></tr><tr><td>Ni/CP</td><td>Pt-Ni/CP</td><td>Teflon®</td><td>–</td><td>1M KOH at 50°C and 70°C</td><td>0.25 A.cm<sup>-2</sup> at 1.9 V (50°C), 0.25 A.cm<sup>-2</sup> at 1.8 V (70°C)</td><td>–</td><td>Not specified</td><td>Ahn et al. (2016)</td></tr><tr><td>IrO<sub>2</sub></td><td>Pt black</td><td>-PiperION (TP-85 50 and 40 µm, W7Energy),</td><td>PiperION</td><td>Pure water at 55°C</td><td>1 A.cm<sup>-2</sup> at 2.25 V (Aemion), 1 A.cm<sup>-2</sup> at 2 V (PiperION)</td><td>-Sustainion: 40h</td><td>Not specified</td><td>Lindquist et al. (2021)</td></tr></tbody></table>

(Continued)
<figure><img src="image_20.png" /><figcaption>**Table 1. Continued.**</figcaption></figure>

<table><thead><tr><th>Anode</th><th>Cathode</th><th>Membrane</th><th>Ionomer</th><th>Testing conditions</th><th>Activity</th><th>Durability</th><th>Upper catalyst load</th><th>Reference</th></tr></thead><tbody><tr><td>Ni</td><td>Ni</td><td>-Sustainion (X37-50, Dioxide Materials)<br/>-Aemion (AF1-HNN8-50-X)<br/>50 µm<br/>Sustainion®</td><td>-Sustainion<br/>-Aemion</td><td>-</td><td>0.5-2 M KOH at 30-60°C<br/>1 M KOH at 30-60°C<br/>C</td><td>1.2 A.cm<sup>-2</sup> at 2.4 V at 60°C, 2M KOH<br/>0.62 A.cm<sup>-2</sup> at 2.3 V (HNN8-25)</td><td>-<br/>-180h<br/>-180h</td><td>1 mg/cm<sup>2</sup> (Ni)<br/>1 mg/cm<sup>2</sup> (Ni)</td><td>Noor Azam et al. (2023)<br/>Khataee et al. (2022)</td></tr><tr><td>Ni</td><td>Ni</td><td>Aemion™ (AF1-HNN8-50, AF1-HNN8-25, AF1-HNN5-50, AF1-HNN5-80)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>
<figure><img src="image_21.png" /><figcaption>**Table 2.** Summary of leading manufacturers of AEMELs.</figcaption></figure>

<table><thead><tr><th>Manufacturer</th><th>Number of models</th><th>Hydrogen flow range (Nm³/h)</th><th>SEC (kWh/ Nm³)</th><th>Water consumption (L/h)</th><th>Power range (kW)</th><th>Notable features</th></tr></thead><tbody><tr><td>Enapter</td><td>4</td><td>0.5-210</td><td>4.8</td><td>0.42-190</td><td>2.4-1008</td><td>Small to industrial scale; flexible design</td></tr><tr><td>Gen-Hy</td><td>10</td><td>23-462</td><td>4.5-4.8</td><td>21-420</td><td>100-2000</td><td>Scalable, high efficiency at larger sizes</td></tr><tr><td>HYDROLITE</td><td>1</td><td>2-10.4</td><td>4.2-4.5</td><td>1.91-9.58</td><td>10-50</td><td>Compact range; efficient systems with low SEC</td></tr><tr><td>Sungreen H2</td><td>2</td><td>4.67-233.5</td><td>5.04</td><td>5-250</td><td>20-1000</td><td>High SEC; large-scale offering</td></tr><tr><td>Cipher Neutron</td><td>1</td><td>1.8-52</td><td>3.95</td><td>2-48</td><td>10-250</td><td>Lowest SEC; moderate capacity range</td></tr><tr><td>TaijiPower</td><td>1</td><td>115.5-462</td><td>4.8</td><td>125-500</td><td>500-2000</td><td>Focused on large, industrial applications</td></tr><tr><td>SPF Hydrogen Energy</td><td>2</td><td>0.5-2</td><td>4.3</td><td>0.6-2</td><td>2.5-10</td><td>Compact systems for small-scale use</td></tr><tr><td>HydGen</td><td>2</td><td>2-23</td><td>4.4</td><td>2-20</td><td>10-100</td><td>Medium capacity, efficient systems</td></tr><tr><td>Hyter</td><td>1</td><td>23-57.5</td><td>Not provided</td><td>20-50</td><td>100-250</td><td>Large KOH concentration (3-5M)</td></tr><tr><td>Alchemr</td><td>1</td><td>115</td><td>Not provided</td><td>100</td><td>500</td><td>High capacity, no SEC data</td></tr><tr><td>Ansaldo Green Tech</td><td>1</td><td>231</td><td>5.5</td><td>210</td><td>1000</td><td>High SEC, large capacity</td></tr></tbody></table>

Notably, only the Italian company Hyter reports a continuous operation lifetime of 8000 h for their AEMELs, unlike most AEL and PEMEL manufacturers.

SEC values are typically reported at the beginning of life (BOL), with performance declining over time due to increased cell voltages and reduced energy efficiency (Ha et al. 2024). AEMELs generally operate more efficiently at partial loads (50%) than at nominal loads (100%). All manufacturers use a slightly alkaline solution (1 M KOH) to minimise leakage risks and reduce maintenance. Hyter, however, employs a higher concentration of 3-5 M KOH, which has been shown to lower both ohmic and charge transfer resistances, enhancing energy efficiency and reducing SEC (Immanuel, Eun-Chong, and Hyung-Man 2021; Loh et al. 2023).

KOH is preferred over sodium hydroxide (NaOH) due to its higher specific electrolyte conductivity (Immanuel, Eun-Chong, and Hyung-Man 2021). The maximum hydrogen output pressure for AEMELs is typically 35 bar, compared to 50 bar for PEMELs and 30 bar for AELs. Hydrogen flow rates range from 0.5 to 462 Nm³/h, corresponding to daily hydrogen production between 1 and 1000 kg.

In terms of rated electrical power, AEMEL stacks currently reach up to 2 MW, as demonstrated by the French company Gen-Hy, whereas AEL and PEMEL stacks have achieved up to 5.5 MW. System SEC ranges from 4.2 to 5.5 kWh/Nm³, offering better energy efficiency than PEMELs. AELs and AEMELs exhibit similar SEC values.

Regarding operational flexibility, AEMELs can operate across a wider range (0-100%) compared to AELs (10-100%) due to the lower concentration of KOH, which allows for faster kinetics. Among AEMEL manufacturers, only four have provided operational
<figure><img src="image_22.png" /></figure>

flexibility data: Enapter (3–100%), Cipher Neutron (20–120%), Hyter (0–100%), and Ansaldo Green Tech (25–100%). Hyter offers the best performance in terms of operational flexibility, utilising a higher KOH concentration and minimal use of precious group metals (PGM) catalysts, which are regenerated for sustainability purposes.

In conclusion, like PEMELs, AEMELs are well-suited to be coupled with intermittent RES such as wind energy conversion systems (both onshore and offshore) due to their large operational flexibility and fast response times (in terms of seconds). However, the impacts of intermittent operations on AEMEL performance remain a considerable challenge, as no research has been conducted so far.

## 3.2. Technical specifications for AEM, PEMEL and AEMEL

Based on the technical features collected from each AEMEL manufacturer, a state-of-the-art comparison of the main LTEs (AEL, PEMEL, AEMEL) has been carried out and is summarised in Table 3. This state-of-the-art is updated compared to previous reports in Ferriday et al. (2024), El-Shafie (2023). As mentioned in the previous subsection, the SEC (stack and system) values are provided considering the BOL of the electrolysers. The longer the operating time, the higher the SEC. From Table 2, on one hand, it can be noted that AEMELs offer significant improvements compared to PEMEL technologies (especially in terms of coupling with intermittent RES to generate green hydrogen), particularly regarding cost (no use of high-cost noble metal catalysts) and SEC (lower energy consumption required to produce the same amount of hydrogen). On the other hand, despite their recent spread in the water electrolysis market, AEMELs are not yet mature enough to compete with PEMELs in terms of current density, durability, and hydrogen production. Therefore, further improvements are needed for AEMEL technology to address the challenges and limitations related to the membrane and catalysts in AEMs, while demonstrating the feasibility and wider adoption of this technology.

# 4. Materials and components innovations

AEMELs are crucial for sustainable hydrogen production and offer significant advantages over conventional electrolysers, particularly in terms of cost and material versatility. The materials and components used in AEMELs are specifically designed to optimise performance in alkaline environments, targeting both HER and OER kinetics. Table 4 highlights the key materials used in AEMELs and their respective roles within the system. However, a closer examination of the table underscores the challenges, where further advancements and changes in material selection could significantly impact the cost and performance of each component, and consequently, the overall system (Alaa et al. 2018; An et al. 2014; Faid and Sunde 2022; Hagesteijn, Jiang, and Ladewig 2018).

## 4.1. Membrane

The AEM is the central structure of the electrolyser; it not only transports ions but also physically separates the anode and cathode compartments. Conventional materials for
<figure><img src="image_23.png" /><figcaption>**Table 3.** State of the art of AELS, PEMELS, and AEMELS.</figcaption></figure>

<table><thead><tr><th>Technical specification</th><th>AEL</th><th>PEMEL</th><th>AEMEL</th></tr></thead><tbody><tr><td>Cell temperature (°C)</td><td>70-90</td><td>50-80</td><td>60-80</td></tr><tr><td>Max pressure (bar)</td><td>30</td><td>50</td><td>35</td></tr><tr><td>Current density (A.cm<sup>-2</sup>)</td><td>0,2-0,8</td><td>1-6</td><td>0,2-2</td></tr><tr><td>Cell voltage (V)</td><td>1,5-2,6</td><td>1,4-2,3</td><td>1,8-2,2</td></tr><tr><td>Cell area (m<sup>2</sup>)</td><td>3</td><td>0,3</td><td>0,03</td></tr><tr><td>Cell voltage efficiency (LHV<sup>a</sup>, HHV<sup>b</sup>) (%)</td><td>52-85%</td><td>57-83%</td><td>Up to 85%</td></tr><tr><td>System efficiency (%)</td><td>Up to 76.5%</td><td>Up to 75%</td><td>Up to 77%</td></tr><tr><td>Spec. energy consumption stack (kWh.Nm<sup>-3</sup>) (BOL)</td><td>3,8-5,2</td><td>4,3-5,8</td><td>3,95-5,2</td></tr><tr><td>Spec. energy consumption system (kWh.Nm<sup>-3</sup>) (BOL)</td><td>4-5,47</td><td>4,53-6,1</td><td>4,2-5,5</td></tr><tr><td>Operational flexibility (%)</td><td>10-100</td><td>0-100</td><td>0-100</td></tr><tr><td>Hydrogen flow rate (Nm<sup>3</sup>.h<sup>-1</sup>)</td><td>1,5-1100</td><td>0,22-1255</td><td>0,5-462</td></tr><tr><td>Lifetime stack (h)</td><td>&lt;90,000 (more than 10 years)</td><td>&lt;60,000</td><td>&lt;9,000</td></tr><tr><td>Average electrolyser stack cost<sup>c</sup> (USD/kW)</td><td>50</td><td>909</td><td>208</td></tr><tr><td>Lifetime system incl. maintenance</td><td>&gt;30 years</td><td>35,000-80,000 h (around 9 years)</td><td>-</td></tr><tr><td>Status</td><td>Large-scale application</td><td>Large-scale application</td><td>Commercial stage</td></tr></tbody></table>

AEMs are polymers with quaternary ammonium groups, which enhance the conductivity of hydroxide ions. Typical materials include poly(arylene ether sulfone) and polybenzimidazole, often with the addition of potassium hydroxide to improve conductivity (Hayat et al. 2023; Kishore and Ombretta 2023; Yuqing et al. 2023). AEMs based on imidazole-functionalised poly(ether ketone) offer better thermal stability, mechanical strength, and higher ionic conductivity of $OH^-$ (Yuqing et al. 2023; Zulfirdaus and Siti 2021). However, this material remains relatively expensive compared to those based on Nafion (Kishore and Ombretta 2023), which increases the final cost of the electrolyser.

## 4.2. Electrodes

Electrodes in AEMELS are designed to be highly conductive and catalytically active, while also maintaining stability under alkaline conditions. The choice of electrode materials directly impacts the overall efficiency of both the HER and the OER.

*   **Cathode (HER):** Ni-based alloys, such as Ni-Mo and Ni-Cu, are commonly used due to their strong catalytic activity for HER. Ni-Mo alloys, in particular, demonstrate excellent performance in reducing the overpotential required for hydrogen production (see Table 4).

*   **Anode (OER):** Transition metal oxides such as NiFeOₓ, CoOₓ, and MnOₓ are employed for their superior OER performance. The inclusion of cobalt, manganese, or iron in these oxides improves both their catalytic efficiency and stability in alkaline environments.

This balance between cost and performance is a key focus of ongoing research, as improving the activity and durability of these materials is essential to making AEM electrolyzers more competitive.

<footnote><sup>a</sup>Lower Heat Value (alkaline electrolyzers).</footnote>

<footnote><sup>b</sup>Higher Heat Value (AEM and PEM electrolyzers).</footnote>

<footnote><sup>c</sup>Electrolyser stack (>1MW).</footnote>
<figure><img src="image_24.png" /><figcaption>**Table 4.** AEMEL component materials (El-Shafie 2023; Hamish et al. 2020; Vincent and Bessarabov 2018).</figcaption></figure>

<table><thead><tr><th rowspan="2">GDL</th><th rowspan="2">Anode</th><th colspan="4">Cathode</th><th rowspan="2">Ionomer</th><th rowspan="2">Electrolyte</th><th rowspan="2">Ref</th></tr><tr><th>Catalyst</th><th>Loading (mg cm<sup>-2</sup>)</th><th>GDL</th><th>Catalyst</th><th>Loading (mg cm<sup>-2</sup>)</th></tr></thead><tbody><tr><td>Ti foam</td><td>IrO<sub>2</sub></td><td>2.9</td><td>Ti foam</td><td></td><td>Pt black</td><td>3.2</td><td>A-201, Tokuyama</td><td>AS-4</td><td>Deionized water</td><td>Yongjun et al. (2012), Midilli et al. (2005)</td></tr><tr><td>Ni-felt (Bekaert, 2NI30)</td><td>IrO<sub>2</sub> (Tanaka Co., 76% Ir)</td><td>1.2</td><td>Ni-felt (Bekaert, 2NI30-0.50)</td><td>Pt/C (TANAKA Co., 36.9% Pt)</td><td></td><td>0.7</td><td>Nafion® D520</td><td>Nafion® D520</td><td>1 M KOH</td><td>Moreno-González et al. (2023)</td></tr><tr><td>Ni foam</td><td>Ni/CeO<sub>2</sub>-La<sub>2</sub>O<sub>3</sub>/C</td><td>36</td><td>Carbon Cloth</td><td>CuCoO<sub>3</sub></td><td></td><td>7.4</td><td>A-201, Tokuyama</td><td>PTFE</td><td>1% K<sub>2</sub>CO<sub>3</sub>/ KHCO<sub>3</sub></td><td>Claudiu et al. (2013)</td></tr><tr><td>Carbon paper</td><td>Pb2Ru2O6.5</td><td>2.5</td><td>Carbon paper</td><td>Pt black</td><td></td><td>2.5</td><td>Chloromethylated PSF (CMPSF)</td><td>PSF-TMA+Cl<sup>-</sup>, PSF-ABCO+Cl<sup>-</sup> or PSF-1M+Cl<sup>-</sup> (xQAPS)</td><td>Ultra-pure water</td><td>Parrondo et al. (2014)</td></tr><tr><td>Ni foam</td><td>Ni-Fe</td><td>40</td><td>Stainless steel fibre felt</td><td>Ni-Mo</td><td></td><td>40</td><td>(xQAPS)</td><td>–</td><td>Ultra-pure water</td><td>Xiao et al. (2012)</td></tr><tr><td>Carbon paper</td><td>Ni</td><td>0.085</td><td>Carbon paper</td><td>Ni</td><td></td><td>0.085</td><td>A-201, Tokuyama</td><td>–</td><td>1 M KOH</td><td>Sang Hyun et al. (2014)</td></tr><tr><td>Ni foam</td><td>Ni/CeO<sub>2</sub>-La<sub>2</sub>O<sub>3</sub>/C</td><td>Ni/C</td><td>–</td><td>CuCoO<sub>3</sub></td><td></td><td>–</td><td>LDPE-g-VBC</td><td>AS-4</td><td>1% K<sub>2</sub>CO<sub>3</sub></td><td>Marco et al. (2012), Ahyoun et al. (2019)</td></tr><tr><td>Stainless steel mesh</td><td>Cu<sub>0.7</sub>CO<sub>2</sub>3O<sub>4</sub></td><td>3</td><td>Stainless steel mesh</td><td>Pt</td><td></td><td>1</td><td>Quaternary ammonium radiation grafted membrane</td><td>–</td><td>1 M KOH</td><td>W. Xu and Keith (2011), Yuan-Cheng, Xu, and Keith (2012)</td></tr><tr><td>Stainless steel mesh</td><td>Cu<sub>0.7</sub>CO<sub>2</sub>3O<sub>4</sub></td><td>3</td><td>Stainless steel mesh</td><td>Nano Ni</td><td></td><td>2</td><td>mm-qPVBz/Cl<sup>-</sup></td><td>QPVB/Cl<sup>-</sup></td><td>Deionized water</td><td>Yuan-Cheng, Xu, and Keith (2012)</td></tr><tr><td>Stainless steel mesh</td><td>Cu<sub>0.7</sub>CO<sub>2</sub>3O<sub>4</sub></td><td>3</td><td>Stainless steel mesh</td><td>Nano Ni</td><td></td><td>2</td><td>QPDTB</td><td>Poly (DMAEMA-co-TFEMA-co-BMA)</td><td>Deionized water</td><td>W. Xu and Keith (2012), Yongjun et al. (2012)</td></tr><tr><td>Ni foam</td><td>Ni/CeO<sub>2</sub>-La<sub>2</sub>O<sub>3</sub>/C</td><td>40</td><td>Carbon Cloth</td><td>CuCoO<sub>3</sub></td><td></td><td>40</td><td>Mg-Al layered double hydroxide</td><td>PTFE</td><td>0.1 M NaOH/ Na<sub>2</sub>CO<sub>3</sub></td><td>Zeng and Zhao (2015)</td></tr><tr><td>Ni foam</td><td>Ni-Fe</td><td>–</td><td>Carbon cloth</td><td>Ni-Mo</td><td></td><td>–</td><td>PSF Quaternary ammonium</td><td>–</td><td>–</td><td>An et al. (2014)</td></tr><tr><td>NiO</td><td>Graphene</td><td>–</td><td>NiO</td><td>Graphene</td><td></td><td>–</td><td>Selemion AMV</td><td>–</td><td>0-5.36M KOH</td><td>Seetharaman et al. (2013)</td></tr><tr><td>Ni oxide</td><td>Graphene</td><td>–</td><td>Ni</td><td>Graphene</td><td></td><td>–</td><td>Selemion AMV</td><td>–</td><td>Deionized water</td><td>Donald Joe, Siva Kumar, and Sivakumar (2014)</td></tr><tr><td>Pt coated Ti</td><td>Ce<sub>0.2</sub>MnFe1.8O<sub>4</sub></td><td>3.5</td><td>Pt coated Ti</td><td>Ni</td><td></td><td>3.5</td><td>FAA-3-PK-130</td><td>–</td><td>Deionized water</td><td>Grace et al. (2020)</td></tr></tbody></table>
### 4.3. Catalysts

Non-precious metal catalysts are preferred in AEMELs due to their cost-effectiveness and satisfactory performance. For the HER, Ni-based catalysts are standard, with Ni–Mo alloys showing considerable promise. In the OER, catalysts such as NiFe layered double hydroxides (LDH) and cobalt oxide (Co₃O₄) nanocrystals are among the most effective (Hayat et al. 2023).

There is a clear shift in catalyst development away from PGMs towards more abundant and less expensive alternatives. Cathode catalysts such as Ni-Mo alloys and NiCu oxides are particularly promising for HER, offering significantly lower costs while maintaining reasonable catalytic activity and durability. However, Ni-based catalysts are susceptible to degradation over time, especially under harsh alkaline conditions. This highlights the need for further research to enhance their stability and performance, ensuring long-term viability in practical applications.

### 4.4. Ionomer

Ionomers play a crucial role in AEMELs, functioning as binders within the catalyst layer to ensure uniform dispersion and enhance ionic conductivity. They facilitate efficient ion transport between the catalyst layer and the membrane, thereby improving overall electrochemical performance.

Commonly used ionomers in AEMELs are based on quaternary ammonium-functionalised polymers, which offer high hydroxide ion (OH⁻) conductivity while maintaining chemical stability under alkaline conditions. However, their long-term performance can be compromised due to hydroxide-induced degradation and limited mechanical robustness (Favero, Stephens, and Titirci 2024).

Recent research has focused on developing ionomers with improved durability and ionic conductivity. Novel approaches – such as the incorporation of crosslinked structures or alternative functional groups – have shown promise in enhancing chemical resistance while maintaining efficient ion transport. For example, ether-free polyfluoroenes functionalised with N,N-dimethylpiperidinium cations have demonstrated superior alkaline stability and hydroxide conductivity (Pham et al. 2020). Furthermore, optimising the integration of ionomers with catalysts is essential for minimising interfacial resistance and enhancing long-term performance. The development of high-performance anion exchange ionomers, such as those based on highly quaternized polystyrene, has led to notable improvements in AEMEL performance, achieving current densities of up to 2.7 A cm⁻² at 1.8 V (Yu et al. 2020). Looking ahead, the exploration of cost-effective and environmentally friendly ionomer alternatives is vital for the sustainable advancement of AEMEL technology. Materials derived from bio-based or recyclable sources could offer eco-friendly solutions without compromising performance. Additionally, enhancing the chemical and mechanical stability of ionomers in alkaline environments remains a critical area for ongoing research (Favero, Stephens, and Titirci 2024).

### 4.5. Gas diffusion layer

GDL materials, such as carbon cloth and nickel foam, play a critical role in facilitating gas transport and distribution within AEMELs. These materials are selected for their
<figure><img src="image_25.png" /></figure>

excellent electrical conductivity and structural integrity; however, there remains room for improvement – particularly in enhancing durability and reducing costs. For example, while nickel foam is effective, it is prone to corrosion over time, especially in alkaline environments. This underscores the need for more robust and cost-effective GDL materials that can endure the operational conditions of AEMELs.

Table 4 highlights ongoing research efforts aimed at reducing electrode costs in AEMELs by replacing expensive platinum (Pt) and iridium (Ir) (common PGMs) with more affordable and abundant alternatives, such as nickel-based materials. The studies reviewed demonstrate a growing interest in nickel and its alloys in various forms, including Ni/Fe, Ni/CeO₂-La₂O₃, and Ni/Mo, as substitutes for PGMs. This transition is driven by the urgent need to lower system costs and enable large-scale commercialisation. While nickel-based materials show promising catalytic behaviour, their long-term activity and stability in alkaline media still lag behind those of PGMs. Therefore, continued research and development are essential to improve both their short-term performance and long-term durability.

In terms of membrane materials, Table 4 (corroborating literature and market data) indicates a continued reliance on fluorinated polymers, such as Tokuyama A-201 and FAA-3-PK-130. These membranes offer excellent chemical stability and high ionic conductivity, but they are among the most expensive components of the system. Moreover, the use of fluorinated membranes raises environmental concerns due to the challenges associated with their disposal. This highlights the need to explore alternative membrane materials that provide comparable performance with lower environmental impact and manufacturing costs.

Furthermore, Table 4 reveals growing research interest in graphene-based materials. Graphene is highly conductive, possesses a large surface area, and exhibits exceptional mechanical strength (attributes that make it attractive for energy applications such as batteries, fuel cells, and solar cells). However, the rising demand for high-quality graphene across various industries creates competition for limited resources. Additionally, the high production costs and the lack of efficient, scalable, and sustainable synthesis methods pose significant challenges. As a result, there is a pressing need for the development of alternative advanced materials that offer similar benefits to graphene but are more practical for widespread use in AEMELs.

# 5. Hydrogen evolution reaction and oxygen evolution reaction in AEMELs

Both the HER and OER require catalysts that combine high activity, stability, and cost-effectiveness. Recent material developments include NiFe-based composites for OER and Ni-Mo alloys for HER, which offer more affordable alternatives to PGMs.

## 5.1. Hydrogen evolution reaction

The HER in an alkaline medium is generally slower than in acidic media due to the involvement of water molecules as the proton source. The reaction proceeds via a two-step electrochemical process:
(1) Volmer Step (Electrochemical Adsorption):

* Reaction:

$$H_2O + e^- \rightarrow H^+ + OH^- \quad (4)$$

* Mechanism: In AEMELs, water molecules are adsorbed onto the catalyst surface – typically a nickel alloy – where they undergo electrochemical reduction. This process involves the dissociation of water, leading to the formation of adsorbed hydrogen atoms (H*) on the catalyst surface and hydroxide ions (OH⁻) in the electrolyte (John et al. 2017; Junhua, Shenghai, and Suobo 2010). This step is critical, as the catalyst’s ability to effectively adsorb and retain hydrogen atoms significantly influences the overall reaction rate. Among nickel-based catalysts, Ni–Mo alloys have demonstrated particularly high effectiveness due to their optimal hydrogen binding energy and enhanced reaction kinetics, which promote more efficient proton reduction.

(2) Heyrovsky Step (Electrochemical Desorption) or Tafel Step (Recombination):

* Heyrovsky Reaction:

$$H^+ + H_2O + e^- \rightarrow H_2 + OH^- \quad (5)$$

* Tafel Reaction:

$$2H^+ \rightarrow H_2 \quad (6)$$

* Mechanism:

* In this step, a second water molecule interacts with the adsorbed hydrogen atom, dissociating to produce another proton and hydroxide ion, while simultaneously forming hydrogen gas (H₂). This step is driven by electron transfer from the electrode.

* In this pathway, two hydrogen atoms adsorbed on the catalyst surface recombine to form a hydrogen molecule (H₂), which then desorbs from the surface.

## 5.2. Catalysts in HER

**Ni–Mo Alloys:** These catalysts exhibit high activity for the HER in alkaline media, offering a balanced combination of activity, cost-effectiveness, and durability. The incorporation of molybdenum enhances catalytic performance by increasing the number of active sites and improving the hydrogen adsorption-desorption process. NiFe and NiCu alloys have also been investigated, with structural modifications designed to lower the energy barrier for hydrogen formation and enhance overall reaction kinetics.

## 5.3. Oxygen evolution reaction

The OER is the rate-determining step in water electrolysis due to its complex multi-electron transfer process (Raina et al. 2022). It involves the oxidation of water at the anode to produce oxygen gas and is typically catalysed by transition metal oxides such as NiFeOₓ and Co₃O₄.
<figure><img src="image_26.png" /></figure>

(1) Formation of Metal-OH Complex:

* Reaction:

$$M + OH^{-} \rightarrow M - OH + e^{-} \quad (7)$$

* Mechanism: Metal catalysts are initially involved in the adsorption of hydroxide ions (OH⁻) from the alkaline electrolyte. This adsorption is done with the help of the catalyst which is a transition metal oxide. For example, NiFe LDH favours this step due to the availability of active site that binds to hydroxide ions which in turn help the electron transfer to occur and the subsequent reactions (Zhang, de Respiñis, and Frei 2014).

(2) Oxidation to Metal-O:

* Reaction:

$$M - OH \rightarrow M - O + H^{+} + e^{-} \quad (8)$$

* Mechanism: Hydroxide ion gets adsorbed on the metal surface and gets oxidised to from a metal-oxo species (M-O) by releasing a proton and an electron. This step is crucial to the overall energy barrier within the OER necessities. This is true due to the following reasons: Stability and reactivity of the metal-oxo species are vital for increasing the rate of the reaction. Other catalysts such as Co₃O₄ nanocrystals enhance the stability of this intermediate so as increasing the efficiency of OER.

(3) Oxygen Evolution (O-O Bond Formation):

* Reaction:

$$2M - O \rightarrow 2M + O_{2} \quad (9)$$

* Mechanism: Finally, the metal-oxo species combine to form molecular oxygen (O₂), which is then released as gas. The catalyst is thus regenerated into the cycle in this process so that it can be used again in the next cycle. The effectiveness of this step relies on the catalyst capacity to facilitate the formation of the O-O bond and liberation of oxygen. NiFe-based composites have been also found to decrease the amount of overpotential for this step enhancing the overall OER performance in AEM system

5.4. Catalysts in OER

* NiFeOₓ: This material is highly effective in reducing the overpotential for OER and offers good stability under alkaline conditions. The synergy between nickel and iron in the oxide matrix enhances the formation and desorption of oxygen species.

* Co₃O₄: Nanocrystals: These offer high surface area and catalytic activity, particularly in promoting the formation of the metal-oxo intermediate, which is crucial for efficient oxygen evolution.

The HER and OER play a crucial role in evaluating the performance of AEMELs, as they significantly influence the polarisation curve and overall overpotential. Both reactions contribute to activation losses at low current densities. However, the OER is generally
slower than the HER due to its more complex multi-electron transfer mechanism. A study conducted by C. Li and Baek (2021) demonstrated that a NiFe oxide anode achieved a notable current density of 650 mA cm⁻² at 2 V and 50°C, emphasising the critical role of the OER in determining overall cell efficiency. Reducing overpotential can be achieved by optimising system design and improving catalyst performance, thereby lowering the kinetic barriers in both half-cells and enhancing hydrogen production efficiency in AEMELs. On the HER side, NiMo alloys have shown competitive performance compared to Pt-based catalysts. For example, a NiMo/X72 catalyst achieved a current density of 1 A cm⁻² at 1.9 V in a 1 M KOH solution, closely approaching the performance of Pt-PGM catalysts, which reached the same current density at 1.8 V (C. Li and Baek 2021). Future research should focus on further investigation of HER and OER mechanisms in alkaline media, particularly within AEMEL systems, with the aim of reducing overpotentials through the development of advanced materials and optimised components.

## 6. Life cycle assessment of AEMELs

The objective of this LCA is to evaluate the environmental impacts associated with AEMEL technology. Several recent LCA studies have been conducted (Elke et al. 2024; Subbaraman et al. 2012; Wagd, Patrícia, and Rui 2024). The life cycle of AEMELs involves several critical phases, each contributing to the overall environmental impact. In the raw material extraction and manufacturing phase, the production of membranes, such as Aemion+® (AF2-HWP8-75-X), significantly contributes to environmental burdens due to energy-intensive processes and the use of fluorinated polymers, which have a high ozone depletion potential (ODP). Electrode fabrication typically involves non-precious metal catalysts, such as Ni-Mo alloys or NiFe-based composites, which reduce reliance on critical raw materials but still involve resource-intensive mining and processing activities.

The operational phase of AEMELs demonstrates improved energy efficiency. Commercial units like the Enapter EL 4.1 consume 4.8 kWh/Nm³ of hydrogen produced, translating to a cumulative energy demand of approximately 160–200 MJ/kg H₂. This is slightly lower than that of PEMELs (180–250 MJ/kg H₂) and comparable to AEL systems (150–190 MJ/kg H₂) (Yue et al. 2021). However, indirect environmental impacts also arise during operations, such as isopropanol emissions during catalyst application and the need for regular electrolyte maintenance to minimise leakage risks. These factors can influence greenhouse gas (GHG) emissions. When powered by RES, AEMELs achieve GHG emissions in the range of 1.5–3.0 kg CO₂-eq/kg H₂, which is favourable compared to PEMELs at 2.0–4.5 kg CO₂-eq/kg H₂ and competitive with AELs at 1.0–2.8 kg CO₂-eq/kg H₂ (Du et al. 2022).

High-throughput systems, such as the Gen-Hy HC-2000 from GEN-HY French company, require significant water and electricity inputs, making the integration of RES essential to minimise life cycle emissions. In the end-of-life phase, the disposal and recycling of components, particularly membranes and stainless-steel bipolar plates, present challenges. Stainless steel, composed of chromium and nickel, is a resource-intensive material. However, companies like Hyter are implementing strategies
<figure><img src="image_27.png" /></figure>

to reclaim and recycle spent catalysts and alkaline solutions, reducing the demand for virgin materials.

Benchmarking AEMELs against PEMELs reveals that the power plant is the primary driver influencing the environmental load for both technologies. However, AEMELs slightly outperform PEMELs in material criticality due to their lower reliance on sensitive materials, despite challenges such as lower durability and more frequent component replacements. Furthermore, if the use of stainless steel, particularly its chromium, nickel, and iron content, is reduced, sustainability metrics would improve. Overall, the LCA highlights AEMELs as a promising and potentially more environmentally sustainable option for hydrogen production, contingent on continued innovation in membrane materials, catalyst recycling, and system integration with RES.

## 7. SWOT analysis of AEMELs

In the literature, the advantages and drawbacks of electrolyser technologies are generally discussed (Vincent and Bessarabov 2018; Vinodh et al. 2023; Zakaria and Kamarudin 2021). For evaluating AEMELs, the use of a SWOT analysis is highly beneficial, as it allows for the assessment of both internal and external factors that influence the technology's development and adoption. By categorising strengths, weaknesses, opportunities, and threats (SWOT), a comprehensive understanding of the current state of AEMEL technology is obtained, as shown in the table below (see Table 5). The SWOT analysis highlights opportunities to improve membrane durability and reduce hydrogen

<table><caption>**Table 5.** SWOT analysis with strengths, weaknesses, opportunities, and threats of AEMEL technology.</caption><thead><tr><th>STRENGTHS</th><th>WEAKNESSES</th></tr></thead><tbody><tr><td><ul><li>Low cost (due to cheaper catalyst materials such as Ni, Co, Fe)</li><li>Low system specific energy consumption (4,2 kWh.Nm<sup>-3</sup>, minimal reported value)</li><li>Quick response, wide operational flexibility (particularly fit with intermittent RES)</li><li>Differential pressure operation</li><li>Less critical raw materials compared to PEMEL technology</li><li>Minimal maintenance requirements due to the use of SPMs and slightly alkaline solutions</li><li>Technology supported by flexible system designs</li><li>Promising catalyst development (e.g. NiFeO<sub>x</sub>, Co<sub>3</sub>O<sub>4</sub>) enhances HER and OER performance</li></ul></td><td><ul><li>Limited current density (0,2-2 A.cm<sup>-2</sup>)</li><li>Weak ionic conductivity (OH<sup>-</sup>)</li><li>Poor durability under alkaline environments</li><li>Trace PGM catalysts still required</li><li>Efficiency losses employing pure water feed</li><li>High hydrogen crossover</li><li>Current membranes (e.g. Aemion+®, Sustainion) are expensive and have limited longevity under dynamic and high-demand conditions</li><li>Environmental concerns with fluorinated polymers.</li><li>AEMELs are at a commercial stage but still lack widespread adoption compared to mature AELs and PEMELs technologies</li></ul></td></tr><tr><td><ul><li>To develop novel and enhanced polymer/membrane material (the objective is to increase OH<sup>-</sup> conductivity and chemical stability, while reducing hydrogen crossover) such as Aemion+® membrane</li><li>Progress in the use of neutral-pH water electrolyte to avoid the use of undesirable strong acids</li><li>Dissemination of low carbon footprint hydrogen production plants supplied by intermittent renewable energy and nuclear resources</li><li>Recycling of spent catalysts and alkaline solutions by manufacturers like Hyter can reduce material costs and environmental impact</li></ul></td><td><ul><li>Degradation mechanism known as hydroxide (OH<sup>-</sup>) attack on the polymer blackbone (causing membrane collapse and catalyst layer dissolution)</li><li>Lack of highly efficient PGM-free catalysts to use neutral-pH water electrolyte</li><li>Growing development of PEMEL technology in large hydrogen production plants due to its maturity and benefits</li><li>Lack of hydrogen refuelling stations close to the hydrogen production units</li><li>Manufacturing of membranes (e.g. fluorinated polymers) and use of stainless steel contribute to environmental burdens</li></ul></td></tr></tbody></table>
crossover. Strategic research into non-PGM catalysts and advanced membrane materials will enhance the competitiveness of AEMELs.

The development of advanced membranes using next-generation polymers should focus on enhancing their ionic OH⁻ conductivity, chemical stability, and resistance to hydrogen crossover, while utilising materials that exhibit minimal degradation. Future research should prioritise both hybrid and composite membranes, incorporating sustainable alternatives to fluorinated polymers for manufacturing. Additionally, conducting a comprehensive LCA is essential to ensure that these advancements align with sustainability goals. In the area of advanced catalyst innovation, improving the performance of the OER and HER should be a priority. This can be achieved by exploring PGM-free catalysts optimised for neutral to slightly alkaline solution conditions. Moreover, hydrogen crossover during intermittent renewable energy storage must be addressed by optimising cell designs that integrate advanced flow fields and cell geometries. Finally, thorough operational testing is necessary to identify failure modes, enabling the development of solutions that ensure system reliability under diverse and challenging performance scenarios while minimising the overall environmental footprint.

## 8. Conclusion

Anion Exchange Membrane Electrolyser (AEMEL) technology offers a promising solution for green hydrogen production, with advantages in cost, environmental sustainability, and operational flexibility, especially when integrated with intermittent power sources. While AEMELs show great potential, they face challenges compared to other low-temperature electrolysis technologies like Alkaline Electrolysers (AELs) and Proton Exchange Membrane Electrolysers (PEMELs), particularly in terms of stability, performance, and current density. AEMELs can achieve lower system costs than PEMELs, mainly due to lower material costs, provided energy efficiency and durability targets are met.

AEMELs currently face technical limitations, including lower current densities (up to 2 A/cm²) and reduced durability, largely due to membrane degradation under alkaline conditions. The hydroxide ion (OH⁻) conductivity of current membranes is also lower than proton-conducting membranes in PEMEL systems, limiting efficiency.

Advances in membrane materials, such as Aemion+® and PiperION, have shown promise, demonstrating high conductivity, stability, and mechanical robustness. However, further improvements are needed for long-term viability. Catalyst development, particularly for non-noble metal catalysts, is also critical for enhancing AEMEL performance. Additionally, AEMELs' operational flexibility such as the ability to operate with lower KOH concentrations or even pure water aligns well with renewable energy integration.

Life Cycle Assessment (LCA) highlights the environmental benefits of AEMELs, although challenges remain with fluorinated polymers and stainless-steel components. Recycling strategies and non-fluorinated alternatives are needed to improve sustainability.

The SWOT analysis underscores AEMELs' strengths, including low energy consumption and low-cost catalysts, but identifies weaknesses like limited current density and poor durability in alkaline environments. Opportunities lie in developing more stable
<figure><img src="image_28.png" /></figure>

membranes and non-PGM catalysts, while threats include the dominance of PEMEL technology and the need to address membrane degradation due to hydroxide attack.

In conclusion, further research is required to develop durable, high-conductivity membranes, PGM-free electrodes, and optimised cell designs. Efforts should also focus on understanding electrolyte degradation and exploring integration strategies to enhance AEMEL technology's competitiveness. Continued multidisciplinary innovation across materials science, electrochemistry, and system engineering is essential to unlocking the full performance and cost advantages of AEMELs, enabling their role in the future hydrogen economy.

## Acknowledgments

The authors are very grateful to the German Academic Exchange Service (DAAD) for the financial support brought to Damien Guilbert for carrying out a research stay at Institut für ZukunftsEnergie- und Stoffstromsysteme IZES (Saarbrücken, Germany). Special thanks to IZES, Université Le Havre Normandie, and GREAH research laboratory for their constant support in strengthening the cooperation between Germany and France.

## Disclosure statement

No potential conflict of interest was reported by the author(s).

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