<table><caption>Table 5. Overview of Varying Electrolytic Feeds</caption><thead><tr><th>Anode EC</th><th>Cathode EC</th><th>Binder/Ionomer</th><th>AnodePTL</th><th>Cathode PTL</th><th>Memb.</th><th>Temp. (°C)</th><th>Electrolyte</th><th>Cell Volt. (V)</th><th>Current Density (A cm<sup>-2</sup>)</th><th>ref.</th></tr></thead><tbody><tr><td rowspan="2">etched copper-cobaltoxide spinel CuCo<sub>2</sub>O<sub>4</sub></td><td>Pt/C</td><td>PTFE</td><td>Ni foam</td><td>carbon cloth</td><td>QPC-TMA</td><td>70</td><td>1 M KOH</td><td>1.9</td><td>4.2</td><td>400</td></tr><tr><td>Pt/CHISPEC 4000</td><td>No</td><td>Ni foam</td><td>carbon cloth</td><td>XC37-50 grade T</td><td>45</td><td>1 M KOH</td><td>1.9</td><td>1.4</td><td>240</td></tr><tr><td rowspan="2">IrO<sub>2</sub> non-Pt-containing lead ruthenate pyrochlore</td><td>Pt/C</td><td>PTFE</td><td>-</td><td>Ti paper</td><td>A201Tokuyama</td><td>50</td><td>0.5 M KOH</td><td>1.8</td><td>1.07</td><td>389</td></tr><tr><td>Pt Black</td><td>PSF-TMA<sup>+</sup></td><td>-</td><td>Carbon paper</td><td>PSF-TMA<sup>+</sup></td><td>50</td><td>Ultrapure DIW</td><td>1.8</td><td>0.4</td><td>396</td></tr><tr><td rowspan="2">CuCoO<sub>x</sub></td><td>Pt/ CHISPEC9100</td><td>Aemion</td><td>Ni felt</td><td>carbon PTL</td><td>Aemion AP1-HNN8-50</td><td>70</td><td>0.1 M KOH</td><td>1.8</td><td>~1 (not specified)</td><td>290</td></tr><tr><td>Pt/C HiSPEC4000</td><td>Nafion</td><td>Ni foam</td><td>carbon cloth</td><td>X37-50 grade T</td><td>50</td><td>1 M KOH</td><td>1.8</td><td>1.54</td><td>401</td></tr><tr><td>NiFe</td><td>PtRu/C</td><td>quaternized ammonium polystyrene</td><td>-</td><td>platinized titanium or SGL 29 BC</td><td>HTMA-DAPP</td><td>85</td><td>Pure Water</td><td>1.8</td><td>2.7</td><td>181</td></tr><tr><td>NiCoOx:Fe</td><td>Pt Black</td><td>PAP-TP-85</td><td>Pt-coated sintered titanium frit</td><td>Carbon paper</td><td>FAA-3</td><td>50</td><td>Pure Water</td><td>2.47</td><td>1</td><td>249</td></tr><tr><td>FexNiyOOH-nF</td><td>Pt/C</td><td>AAEM</td><td>self-supported on Ni foam (not specified)</td><td>Carbon paper</td><td>PAP</td><td>90</td><td>Pure Water</td><td>1.8</td><td>1.02</td><td>315</td></tr><tr><td>Ce<sub>0.2</sub>MnFe<sub>1.8</sub>O<sub>4</sub></td><td>Ni powder</td><td>AAEM</td><td>(not specified)</td><td>(not specified)</td><td>Fumasep FAA-3-PK-130</td><td>25</td><td>DIW</td><td>1.8</td><td>0.3</td><td>402</td></tr><tr><td>CuCoOx(Acta 3030)</td><td>Acta 4030</td><td>I2</td><td>Ni foam</td><td>CarbonPaper</td><td>A201Tokuyama</td><td>60</td><td>1% K<sub>2</sub>CO<sub>3</sub></td><td>1.95</td><td>0.5</td><td>392</td></tr><tr><td>Ni<sub>0.75</sub>Fe<sub>2.25</sub>O<sub>4</sub></td><td>Pt/C</td><td>Nafion</td><td>-</td><td>Ni foam</td><td>X37-50 grade T</td><td>50</td><td>1 M KOH</td><td>1.9</td><td>2</td><td>232</td></tr><tr><td rowspan="2">IrO<sub>2</sub> Ni<sub>0.6</sub>Co<sub>0.2</sub>Fe<sub>0.2</sub></td><td>Pt/C</td><td>FAA-3-50</td><td>-</td><td>C-PTL &amp;Ti-PTL</td><td>FAA-3-Br</td><td>70</td><td>1 M KOH</td><td>1.9</td><td>1.5</td><td>310</td></tr><tr><td>Ni-MoO<sub>2</sub></td><td>Nafion</td><td>-</td><td>Au-coated Ti felt and carbon paper</td><td>Fumion</td><td>50</td><td>1 M KOH</td><td>2</td><td>1.15</td><td>403</td></tr><tr><td rowspan="2">IrO<sub>2</sub> NiCo<sub>2</sub>O<sub>4</sub></td><td>Pt/C</td><td>PFTP-8/PFBP-14</td><td>-</td><td>Carbon paper &amp;Ti felt</td><td>PFTP-13</td><td>80</td><td>1 M KOH</td><td>2</td><td>7.68</td><td>404</td></tr><tr><td>Pt/C</td><td>QPPO</td><td>-</td><td>Ti fiber felt and C PTL</td><td>QPPO</td><td>40</td><td>0.1 NaOH</td><td>2</td><td>0.814</td><td>405</td></tr><tr><td>Ni-IrO<sub>x</sub>-400</td><td>Pt/C</td><td>10 wt % FAA-3</td><td>-</td><td>Ni foam</td><td>FAA-3-50</td><td>50</td><td>1 M KOH</td><td>1.8</td><td>1.125</td><td>406</td></tr><tr><td>NiFeV LDH</td><td>Pt/C</td><td>-</td><td>-</td><td>Ni foam</td><td>X37-50 grade T</td><td>50</td><td>1 M KOH</td><td>1.8</td><td>2.1</td><td>407</td></tr></tbody></table>
purged as described before, without having to flow through the AEM, so explaining the superior anode-feed AEM-WE performance.

The ultimate goal of the AEM-WE technology is to be operated viably with pure water for competitive hydrogen generation. However, this concept is still in its early phase and requires significant scientific findings to overcome the technical drawbacks of the system. In addition to the aforementioned problems associated with DI water, i.e., high resistances and low ionic conductivity leading to kinetic losses, there are several other factors yet to be addressed. According to the perspective communicated by Li et al.,²⁸² when AEM-WE operates with DI water alone without supplementary electrolyte, the ambient is less corrosive and the durability aspects are not dictated by the stability of MEA but, linked to a high operational voltage and current density of the cell causing ionomer detachment and poisoning. On the other hand, Mayerhöfer et al.²⁹⁰ analyzed the operational dissolution of the state-of-the-art CuCoOₓ-based OER EC after integrating it with the Aemion membrane and binder. Their study suggested that Aemion is not enough to develop a satisfactory alkaline environment to thermodynamically stabilize the EC in a neutral electrolyte since both Co and Cu dissolve during OER and, therefore, a feed of alkaline nature becomes vital to safeguard the thermodynamic stability of such kind of PGM-free ECs. Being aware of this issue, Xiao and co-workers fabricated fluoride-incorporated, nickel-iron oxyhydroxide (FeₓNiᵧOOH-nF) based self-supported OER electrode, which was capable of robustly holding ionomer with high IEC without getting severely washed out during the operation.³¹⁵ With pure water feed, they were able to achieve a remarkable current density of 1.02 A cm⁻² at 1.8 V with subsequent prevention of EC detachment over 160 h of nonstop run at 0.2 A cm⁻². Not long ago, Xu et al. studied the application of (oxy)hydroxide/oxides of first-row TMs ECs for OER applications in AEM electrolysis.²⁴⁹ Despite having the best performance in an alkaline environment, NiFeOₓHᵧ turned out to be the least-performing EC when tested with AEM under pure water feeding. NiFeOₓHᵧ exhibited a large overpotential (above 2.5 V) to obtain the current density of 0.2 A cm⁻², whereas NiCoFeOₓ even outperformed the IrOₓ by delivering the 1 A cm⁻² current density at ~ 2.5 V. In fact, nickel (oxy) hydroxides-based ECs with electrolytic permeability are good conductors only in oxide form.²²¹ However, in pure water, the EC present only at the ionomer interface can undergo oxidation, where the bulk EC stays unoxidized and is thus unproductive to drive OER due to loss of electrical conductivity.²⁴⁹ Another problem associated with DI water circulating AEM-WE is the ingression of CO₂ into the system, which severely damages performance.³⁹⁶ Owing to limited mobility, their intrusion into the system can swiftly affect the ionic conductivity of both binders and membranes.³⁹⁷

In the pursuit of pure water-fed AEM-WE, where a situation of uncertainty prevails due to afore-discussed problems as well as different techniques used for the fabrication of cell components, the lack of a testing protocol also constitutes an additional bottleneck. Sometimes various electrolytic pretreatments are employed, even if the electrolyzer truly has to be operated with pure water.⁵⁶,⁵⁷,¹⁸¹,³¹³,³⁹⁸ The procedures used to switch the system from KOH to DI water are generally obscure and the effects of residual salts on the overall electrolysis remain unclear. Lindquist et al. first noticed that and thoughtfully argued that the residual electrolyte may

influence the performance of the final electrolysis, also leading to uncertain and complex degradation mechanisms involved in the ionomer.⁵⁶ They compared the AEM-WE performance measured initially with DI water and then with 1.0 M KOH. Following what was done in other works reporting high AEM-WE performance in DI water, the authors measured the performance again after flushing the cell with 0.5 L of DI water, showing that it was significantly better than the initial one in DI water. The performance kept decreasing after flushing the cell with higher volumes of DI water, up to more than 2.5 L as shown in Figure 23.G. The authors also measured the cell effluent conductivity during the flushing, observing a decrease over time. Therefore, the enhanced performance measured in DI water after the test with 1.0 M KOH should be attributed to the residual electrolyte not being completely removed from the MEA.⁵⁶ Very recently, Hassan et al.³⁹⁸ revealed that to truly switch from alkaline electrolyte to DI water feed, a large volume of water has to be circulated over an extended time to get rid of the residual salt and achieve the true performance associated with the use of DI water. To prevent misinterpretation of any artificial improvements in the performance due to trapped KOH, Hassan et al.³⁹⁸ adopted a reverse strategy where the device was fed with DI water at the start and then replaced with KOH feed with successive increments in the concentration as depicted in Figure 23.H. With each increment in the KOH concentration, the cell performance was improved, however, by the final feed of 0.3 M KOH, the operating voltage of ~ 1.75 V was achieved which was 350 mV lower than that of DI water. To further probe the impression of residual KOH, the cell was initially operated with 0.3 M KOH and then switched to DI water. The succeeding polarization curve remained nearly identical to that initially taken with 0.3 M KOH and even after 1 h of continuous DI water flushing, the kinetic region came out to be somewhat similar to the previous polarizations (see Figure 23.I). It is noteworthy that it took 13 hours of continuous DI water purging to achieve the polarization originally attributed to it. Osmieri et al.²⁹⁴ confirmed this issue, measuring the AEM-WE performance of two different MEAs in DI water at the beginning of the test and after 20 h of constant current hold, showing significant degradation. Subsequently, the performance was measured in the presence of supporting electrolytes (in sequence, 1% K₂CO₃ and 0.1 M KOH), showing the expected performance improvement. Then, the cell was purged with 2 L of DI water and the performance was better than at the beginning of the test, indicating residual presence of supporting electrolyte.

All these works suggest that in order to enable a fair comparison of results obtained in different laboratories, it is of paramount importance to establish a protocol for AEM-WE cell conditioning and performance measurement, in case DI water and different supporting electrolyte solutions are used in sequence when testing the same MEA. Table 5 summarizes the electrolytic feed in the different AEM-WEs.

The problem mainly lies in the sequence of the AEM-WE performance tests carried out using different supporting electrolyte solutions (mainly a KOH solution) and pure water. By deeply reviewing the available literature, it can be noticed that this aspect is most of the time overlooked. For example, in many papers featuring pure water operation, experimental details that would be essential to properly determine if the claimed "pure water operation" is indeed true are omitted. In our vision, this omission is so common because the main focus of the majority of the research groups
<figure><img src="image_37.png" /><figcaption>**Figure 24.** Polarization curves recorded in sequence in this order with different feeds: pure water at the beginning of life (BOL) and after 20 h durability test, 1% K₂CO₃, 0.1 M KOH, and pure water again at the end of the test (EOT) after flushing more than 2 L of pure water. Reproduced with permission from ref 294. Copyright 2023, Elsevier.</figcaption></figure>

publishing papers in the AEM-WE field has so far been the development of one of the core components of the MEA (i.e., OER or HER ECs, AEMs, AEIs), not the testing protocol or the assessment of AEM-WE performance (especially in pure water) in a comparative way. In our opinion, the essential experimental details to be reported to enable a fair assessment are: (i) the exact sequence order in which tests are conducted when the same AEM-WE (i.e., the same MEA) is fed with a supporting electrolyte and with pure water; (ii) some indication that the cell has been purged with a sufficient amount of pure water (i.e., any residual electrolyte has been rinsed off from the MEA) before starting the “pure water operation” test in case the AEM-WE was previously fed with a supporting electrolyte solution (Figure 24). In our opinion, point (ii) above can be addressed by providing in the experimental section of the paper the value of the electrolyte conductivity at the outlet of the electrolyzer cell, or the exact volume (flow rate, time) of pure water flushed through the cell to wash out the residual supporting electrolyte. To avoid this problem and eliminate any concerns regarding the validity of the reported “pure water operation” performance, we recommend carrying out all the “pure water operation” experiments before flowing any electrolyte into the cell, as done, for example, in references$^{399}$ and$^{56}$

## 6.3. Effect of Impurities

The effect of impurities on ECs in the context of pure-water-fed AEM-WE operation is a seriously understudied area of research likely stemming from a poor understanding of the chemical components of water feeds in operating electrolyzers.⁸⁶ As a result, it is not clear to what degree the removal of impurities enhances or negatively affects EC performance. On one hand, there are very low amounts of dissolved species in the ASTM Type II water used in WEs: a maximum of 5 µg L⁻¹ Na⁺, 5 µg L⁻¹ Cl⁻, 3 µg L⁻¹ silica and 50 µg L⁻¹ TOC and whatever ions are remaining which constitute the 1 µS cm⁻¹ maximum conductivity.⁸⁶ This significantly reduces poisoning and fouling compared to lower-quality water sources. However, it is precisely this lack of ionic content that makes ultrapure water sources corrosive to components of the WE plant and may permit the leaching of species into the water stream. Pourbaix diagrams, which delineate regions of thermodynamic

stability of an element according to potential, pH, and concentrations of soluble species, show that corrosion processes become more spontaneous as the concentration of the soluble species decreases.⁴⁰⁸ This may explain the corrosion of even stainless steel in ultrapure water.⁴⁰⁹ Indeed, for systems where the water flow is continuously purified, a scenario where the solution is never saturated with respect to the given corrosion product is likely and could lead to continuous leaching of impurities.

Impurities are not necessarily damaging to EC performance. For example, many common PGM-free ECs for OER display increased activity$^{223,410–412}$ and stability$^{217}$ only when Fe impurities are in solution. There may be other scenarios where impurities can inadvertently provide advantages to ECs. How different metallic and nonmetallic species interact with ECs is discussed below. The lack of information about such processes in pure water necessitates that the discussion focuses on research in alkaline electrolytes. However, this still should provide ideas about how to translate that research to the pure water context. Also, the discussion below will focus on inorganic impurities, while the effect of organic contamination is beyond the scope of this review.

### 6.3.1. Metallic Impurities.
Extensive research has been conducted on metal oxides with perovskite and spinel structures as well as layered structures such as M(OH)₂/MOOH (M = Ni, Fe, Co, and Cu), due to their structural stability and outstanding performance as ECs for OER. Perhaps the most studied PGM-free ECs for OER are nickel-based, mainly as LDH. Under high pH and potential, the nickel, at least at its surface, transforms into the oxide of oxy(hydroxide) form, which is known to be highly active toward OER. As a consequence, in recent years, the development of nickel-based ECs has focused mainly on the improvement of the highly active Ni(OH)₂/NiOOH form, which shows record-high catalytic performance. In rigorously Fe-free conditions. However, a dramatic increase in activity is always observed upon the incorporation of iron.⁴¹³ The first observations were made for nickel metal hydride batteries, where the presence of iron impurities caused a decrease in the battery capacitance, which was attributed to a decrease in the onset potential for OER at the anode.⁴¹⁴,⁴¹⁵ Trotochaud et al. showed unambiguously that the high catalytic activity of
Ni(OH)₂/NiOOH is due to the incorporation of iron impurities from the KOH solution and that the increase over time of the electrocatalytic activity (aging) is not due to phase transformation from γ-NiOOH to β-NiOOH.²²³,⁴¹⁶ Even though an increase of more than 30-fold in conductivity was observed with iron-containing ECs, the much better electrocatalytic activity could not be fully accounted for on the basis of conductivity improvement, but to changes in the electronic structure of nickel oxide by inducing partial-charge transfer. NiₓFe₂₋₁OₓHᵧ-type ECs show 3 orders of magnitude higher activity in comparison to NiOₓHᵧ with reported overpotential as low as 280 mV @ 10 mA cm⁻². The electrocatalytic performance of iron-incorporated nickel ECs is strongly dependent on the iron content, synthesis method, electrolyte composition, and various catalytic activity parameters can be found, ranging from ~ 200 mV to more than 300 mV at the current density of 10 mA cm⁻².⁴¹⁷⁻⁴¹⁹ Activity of Ni and Co oxyhydroxides increases up to around 25–30% Fe loading, after which a reduction in both activity and durability occurs. The optimal range in 0.1 M KOH was found to be between 24% to 40% Fe atoms.⁴²⁰,⁴²¹ Outside this range, the enhancement of activity in the nickel sites due to the presence of iron atoms is counterbalanced by the decreasing quantity of nickel sites (Figure 25). It was shown that Fe also phase-segregates above 25% as recently shown in ref 422.

<figure><img src="image_38.png" /><figcaption>**Figure 25.** (A) Linear-sweep voltammograms of NiFe LDH samples with various Fe contents recorded at a scan rate of 1 mVs⁻¹ in Fe-free 0.1 M KOH. (B–D) Comparison of (B) anodic peak potentials of Ni²⁺ to Ni³⁺, (C) current densities at an overpotential of 320 mV, and (D) Tafel slope. (E) Correlation of current densities acquired at η = 320 mV to Fe content. Adapted with permission from ref 421. Copyright 2020, Wiley-VCH Verlag GmbH.</figcaption></figure>

Iron also plays a crucial role in influencing the stability of nickel oxy(hydroxide) ECs. NiₓFe₂₋₁OₓHᵧ ECs were reported to have better stability than their Fe-free equivalents. On the other hand, FeOₓHᵧ was shown to be less stable than NiOₓHᵧ.⁴²¹ Since at high loading of iron new phases such as α-FeOOH and possibly γ-FeOOH start to emerge, the stability of NiₓFe₂₋₁OₓHᵧ ECs is expected to decrease at this iron loading range.

Like NiOₓHᵧ other transition metal-based LDH ECs show high electrocatalytic activity toward OER. Markovic et al. conducted a study to explore the active sites of various MOₓHᵧ ECs (M = Ni, Co, Fe) in Fe-free and Fe-containing electrolytes.²¹⁷ The investigation revealed that in Fe-free 0.1 M KOH, the electrocatalytic activity followed the sequence of FeOₓHᵧ > CoOₓHᵧ > NiOₓHᵧ, while the stability exhibited the opposite pattern with FeOₓHᵧ < CoOₓHᵧ < NiOₓHᵧ. However, when 0.1 ppm of Fe was introduced, causing Fe deposition on the EC surface, the activity trend reversed. Despite this reversal, the stability of the Fe-containing ECs remained consistent with the non-Fe-containing solution: Fe-FeOₓHᵧ < Fe-CoOₓHᵧ < Fe-NiOₓHᵧ.

The influence of iron stands out among other TMs, and numerous studies have aimed to understand its effect by coprecipitating NiOₓHᵧ with various metals such as Co, Mn, Ti, La, Cd, Zn, Y, Ce, Cr, Cu, Pb, Ag, and Mg.⁴²³,⁴²⁴ Within these studies, iron and cerium were observed to significantly reduce the overpotential by more than 140 mV, although Ce-NiOₓHᵧ showed faster degradation compared to Fe-NiOₓHᵧ. Regarding the role of lanthanum, the findings have been contradictory, ranging from no effect to an improvement of 62 mV. These discrepancies can be attributed to variations in testing conditions. On the other hand, the inclusion of Pb, Cd, Zn, and Cr was shown to have a detrimental effect on the EC, leading to higher overpotential compared to the undoped EC. Interestingly, Lyu et al. discovered that the addition of zinc as a third metal alongside iron (NiFeZn LDH) resulted in improved catalytic activity, surpassing that of NiFe LDH.⁴²⁴ At 100 mA cm⁻², NiFeZn LDH shows 30 mV lower overpotential in comparison to NiFe LDH. Similar to NiFeZn LDH, other ternary ECs (MₙMₘOₓHᵧ) were investigated, where NiFeCoOₓHᵧ is the most studied and promising. Incorporating Co into NiFeOₓHᵧ ECs leads to an enhancement in electrical conductivity, which, in turn, results in higher electrocatalytic activity.⁴²⁵,⁴²⁶ On the other hand, the presence of cobalt was found to have minimal impact on intrinsic activity.

Overall, up to a certain extent, cobalt and iron impurities were shown to have a positive effect on NiOₓHᵧ ECs for OER⁴²⁷ while metals such as Cd or Pb can poison the EC, causing a fast decline in performance. The utilization of ultrapure water with AEM-WEs needs to account for the influence of those materials. If KOH is added, its presence cannot be overlooked. Since almost all studies on the effect of impurities on the ECs were conducted in half-cell, the exact role and influence of those impurities under real operating conditions need to be determined. In any case, even when using ultrapure water, some impurities are present due to corrosion of the pipeline or an insufficient purification process.

PGM-free ECs for HER are usually nickel and nickel alloys. Nickel is considered a robust EC for HER, although its activity is lower than that of Pt/C in alkaline solution. For example, in 1 M KOH and 10 mA cm⁻², Pt/C exhibits around 100 mV less overpotential than Ni/C.⁴²⁸ Nickel as a single metal also shows
<figure><img src="image_39.png" /><figcaption>Figure 26. Pourbaix diagram for an artificial model of seawater (A). Reproduced from ref 357. Copyright 2016, Wiley-VCH Verlag GmbH. Schematic illustration of an asymmetric configuration of AEM-WE with seawater feed to the cathode while circulating KOH on the anode (B). Stability trends electrolyzers with different electrolytic feeding conditions at a fixed cell potential of 1.7 V over 12 h (C). (B–C) Reproduced from ref 451. Copyright 2020, The Royal Society of Chemistry. Licensed under CC-BY 3.0. (D) Volcano trend of repulsion between different anions and chloride ions as a function of ionic potential and charge number. (E) Schematic illustration of corrosion-avoiding mechanism due to surface adsorption. (D-E) reproduced with permission from ref 452. Copyright 2022, Elsevier B.V.</figcaption></figure>

a decline in performance over time, which is attributed to hydride formation.<sup>429,430</sup> In contrast, alloying of nickel has shown much higher activity and durability, sometimes exceeding that of PGM ECs.<sup>431–433</sup> Under reductive conditions at the cathode, metal impurities like iron, copper, and zinc can be deposited on the EC. The deposition of a metal layer on the EC leads to the immediate blockage of catalytic sites.<sup>434</sup> If the deposited metal is less active than the EC, a decline in performance is anticipated. Divisek et al. showed that under A-WE realistic working conditions, iron is deposited on the cathode, causing poisoning of the EC and an increase in the overpotential.<sup>435</sup> Under highly alkaline conditions, Fe ions are in the form of dihypoferrite and the reaction taking place at the cathode can be written as

$$HFeO_2^- + 3H^+ + 2e^- \rightleftharpoons Fe + 2H_2O, E^0 = 0.493V \text{ vs. SHE} \quad (19)$$

Conversely, several studies demonstrated that the deposition of iron can enhance the activity and durability of nickel ECs.<sup>436,437</sup> For instance, Mauer et al. conducted a long-term experiment lasting 45 h at 100 mA cm<sup>-2</sup> and observed that the uncoated nickel cathode experienced a significant increase of 380 mV in overpotential, whereas the iron-coated cathode displayed minimal signs of deactivation. This behavior was attributed to the prevention of hydride formation facilitated by the presence of iron. This phenomenon was also observed with other transition-metal-based ECs by the formation of binary and ternary alloys of Ni with different elements such as Co, Fe, Mo, Ce, Zn, and Cu.<sup>438,439</sup> While there is a wealth of literature available on the impact of metallic impurities on ECs for the OER, there is a limited number of systematic studies investigating the relationship between impurities and ECs for HER.<sup>440</sup> Even more, there are contradictory reports, and no

baseline was created to enable a fair comparison. Same as with nickel LDH ECs, it is expected that the effect of iron, for example, on hydride prevention is loading-dependent and above a certain amount, the opposite effect will take place.

Alkaline-earth metal ions impurities can affect both the anode and cathode by the formation of insoluble deposition products on the electrode, blocking the catalytic sites. Calcium and magnesium are particularly problematic under high pH conditions due to their low K<sub>sp</sub> and high presence in unpurified water. On the other side, it is relatively simple to remove them from the electrolyzer inlet water.<sup>86</sup>

### 6.3.2. Nonmetallic Impurities.
AEM-WE is expected to have a higher tolerance against impurities than PEM-WE, which has PGM-based electrodes that are susceptible to poisoning and to the presence of anions.<sup>86</sup> In addition, the use of neutral to basic media in AEM-WEs thermodynamically disfavors the oxidation of chloride ions to Cl<sub>2</sub>/HOCl/OCI<sup>-</sup> species at the anode, which would otherwise lead to the accelerated deterioration of cell components.<sup>441</sup> Moreover, chlorine and/or bromine evolution is thermodynamically more favorable under acidic conditions and causes the blocking of active sites or the dissolution of noble metal ECs.<sup>442,443</sup> Similarly, Zhang et al. reviewed the rational design of OER ECs for seawater electrolysis, including: (i) the design of 3D hierarchical porous structures, (ii) the employment of protective layers, and (iii) the engineering of the surface wettability.<sup>444</sup>

The potentials and products of the COR depend upon the Cl<sup>-</sup> ion concentration and the pH of the solution were previously discussed (Figure 21). In acidic conditions, chloride can be directly oxidized to chlorine gas (COR). Though OER is thermodynamically favored compared to COR, the potential
<figure><img src="image_40.png" /><figcaption>**Figure 27.** Left- (A) ECs optimization (left) and electrolyte optimization (right) to protect the metal substrate from $Cl^-$ corrosion; (B) stability tests recorded at a constant current of 100 $mAcm^{-2}$ for pure NF in electrolytes with different proportions of $Na_2SO_4$; (C) the durability of NF in different electrolytes. (D) and (F) optical images and (E) and (G) SEM images of NF working in two different electrolytes after 0, 20, and 50 min, the dashed circles indicate the skeleton collapse by electrolyte corrosion. SEM images of the skeletons of untreated NF. Adapted with permission from ref 454. Copyright 2021, Wiley-VCH GmbH.</figcaption></figure>

difference between the two reactions is small.357 In practice, the COR dominates in acid because it is a two-electron reaction with much faster kinetics. In neutral to alkaline media,

COR produces hypochlorite, but at a much higher potential (480 mV) than OER. Thus, alkaline pH suppresses COR if the overpotential at the anode remains below ~ 1.71 V vs SHE.357
In general, seawater oxidation under alkaline conditions is more appropriate to ensure maximum hydrogen/oxygen selectivity.

Like many other metals, nickel and nickel-based ECs are known to exhibit increased corrosion rates in contact with highly saline water where chloride levels are substantial.445,446 Several mechanisms are suggested for the corrosion of metal ECs in the presence of chloride ions.446,447 One suggested mechanism is metal chloride-hydroxide formation.448

$$M + Cl^{-} \rightleftharpoons MCl_{ads} + e^{-} \quad (20)$$

$$MCl_{ads} + Cl^{-} \rightleftharpoons MCl_{x}^{-} \quad (21)$$

$$MCl_{ads} + OH^{-} \rightleftharpoons M(OH)_{x} + Cl^{-} \quad (22)$$

Li et al. tested NiCoFe phosphide (NiCo-FeP) nanoarrays grown on the Ni foam in 1 M NaOH solution and 1 M NaOH +NaCl (sat.) for HER and OER.367 In the electrolyte containing 1 M NaOH, OER maintained a high Faradaic Efficiency (FE) of approximately 100% throughout the 1h test. This observation indicated that there was no chloride oxidation occurring in the presence of such a high concentration of hydroxide ions. However, when the electrolyte was changed to 1 M NaOH + NaCl (sat.), the FE dropped to 92% within the first hour. Furthermore, with extended electrolysis time, the FE declined even further, ranging from 46% to 65%. The decrease in FE suggested that a notable portion of the current density was being consumed for electrode corrosion, facilitated by the presence of chloride ions. Similar findings were reported for other ECs.446,449 Different strategies, such as electronic-structure modification, exposure of more electrocatalytic sites, better EC adhesion, etc., were suggested for mitigating OER ECs corrosion in the chloride-rich environment.357,450

Dionigi and co-workers357 identified and tested a design criterion of keeping the anode voltage <480 mV above the thermodynamic OER potential during the electrolysis of near-neutral (pH 9.2) borate buffered electrolyte and 0.1 M KOH with 0.5 M NaCl (Figure 26.A). They showed that the OER electrocatalyst stability in the lower pH electrolyte (using borate buffer alone) was generally worse, probably due to the EC degradation caused by the gradual support corrosion and the parallel electrocatalyst dissolution that in turn, reduced the interfacial pH as a consequence of the limited borate buffer capacity.357 The introduction of NaCl to the borate buffer enhanced the corrosion issues.357 Dresp et al. endeavored to overcome the use of the results from the group's three-electrode measurements described above to rationalize building an asymmetrically fed electrolyzer as exemplified in Figure 26.B.451 This configuration involved feeding seawater to the cathode chamber and 0.5 M KOH to the anode. NiFe-LDH, being an anode EC, demonstrated outstanding OER selective activity without Cl⁻ oxidation, delivering high power efficiencies. As can be seen in Figure 26.C, during the potentiostatic test over 12 h at the cell voltage of 1.7 V the typical symmetric feeding of 0.5 M KOH realized the best performance, whereas the asymmetric configuration with dissimilar feed to anode and cathode demonstrated better performance than the configuration containing mixed feed symmetrically and when the anode or cathode was kept dry.

One way of mitigating the degradation of anode EC by the products of chloride impurity oxidation is to screen the electrode using anionic additives,452 which have been shown, in some cases, to adsorb to metal oxyhydroxides.453 Recently,

Yu and co-workers established an anion-induced performance-enhancing approach through an inclusive screening of electrolytic additives by relating the chloride repulsion with anionic properties.452 The volcano curve presented in Figure 26.D implies the repulsion between chloride and other typical anions as a function of ionic potential and the multiplication product of charge number (z) and radius (r). Anions having a greater ionic potential and $Z \times r$ factor tend to electrostatically repel the $Cl^{-}$ ions, whereas $PO_{4}^{3-}$ demonstrate optimum electrochemical durability along with a robust interaction with water molecules via hydrogen-bonding, approving the adsorption of $PO_{4}^{3-}$ to build a fragile “semi-permeable film” with superficial water molecules that deter the flow of $Cl^{-}$ ions without inhibiting the diffusion of $OH^{-}$ as schematically illustrated in Figure 26. E. In the interim, the transition between $PO_{4}^{3-}$ and $HPO_{4}^{2-}$ may create a local buffering to recompensate the pH drops at higher current densities and therefore enhance the performance and stability of the electrode.

Kuang et al. synthesized a multilayer anode consisting of a nickel-iron hydroxide (NiFe) EC layer uniformly coated on a nickel sulfide ($NiS_{x}$) layer formed on porous Ni foam (NiFe/$NiS_{x}$/Ni), and tested its stability in 1 M KOH+real seawater (25 °C), 1 M KOH + 1.5 M NaCl (25 °C), and 6 M KOH electrolyte (80 °C).364 Negligible performance decline was observed at 400 mA cm⁻² after 1000 h of operation time. What is common to all ECs that are chloride-corrosion resistant is the requirement of high pH levels (>14). Under lower pH, those ECs show significant corrosion and fast performance decline. Despite these ongoing efforts, further research is needed to fully understand the mechanisms of chloride-induced corrosion and develop robust mitigation strategies. The optimization of electrolyte compositions, the development of corrosion-resistant materials, and the implementation of effective monitoring and maintenance practices will be crucial to minimize anode corrosion and enhance the durability and performance of AEM-WEs in the presence of chloride ions.

While extensive research has been dedicated to studying the corrosion-enhancing effect of chloride ions, there is a relative scarcity of literature discussing the impact of other commonly occurring anions, such as bromide ($Br^{-}$), sulfate ($SO_{4}^{2-}$), bicarbonate ($HCO_{3}^{-}$), and carbonate ($CO_{3}^{2-}$), on their role in facilitating or inhibiting corrosion. Such studies would contribute to a more comprehensive understanding of corrosion mechanisms and aid in the development of effective corrosion control strategies in various aqueous environments. Ma et al. studied nickel foam (NF) and Fe-LDH/NF as ECs for OER in 1 M NaOH with various concentrations of NaCl and sulfate to elucidate the role of the latter as an additive for corrosion inhibition (Figure 27).454 They demonstrated that the presence of sulfate ions in the electrolyte effectively mitigates chloride-induced corrosion on the anode in alkaline seawater electrolysis, leading to significantly improved stability of the anode during operation. This phenomenon was attributed to the selective adsorption of sulfate ions on the surface of the anode, which repelled chloride ions present in the bulk solution through electrostatic repulsion. Thanks to the repulsive effect of sulfate ions, the active NiFe-LDH nano-arrays/Ni foam anode exhibited stable performance for 500–1000 h in both simulated and real seawater environments. Once again, this highlights the significance of comprehending the role of all the species present in seawater to facilitate the
<figure><img src="image_41.png" /><figcaption>**Figure 28.** (A) Maximum performance of pure water-fed AEM-WEs. References and operational temperatures are labeled on the plot and shown in Table 6. (B) Degradation rate during long-term testing of pure water fed AEM-WE. References and operational temperatures are labeled on the plot and shown in Table 6.</figcaption></figure>

precise development of active and durable ECs for seawater electrolysis.

## 7. CELL AND STACK DEGRADATION

The long-term stability of electrolyzer cells depends on the durability of each individual component, which in turn is a function of the operating conditions. To achieve high energy efficiency and improved operation capability, the polymer electrolyte membranes must inhibit gas crossover and sustain high currents with minimal ohmic losses.²⁷⁴ In AEM-WEs, $H_2$ diffusion coefficients through the membrane are lower than in PEM ones, but can practically be more challenging to handle due to the degradation and the mechanical thinning of the ion-exchange membrane.²⁸² The morphology of the EC layer in an AEM-WE system has a significant influence on its performance, including electrochemical activity, mass transport, stability and durability, water management, and EC utilization.⁵⁴

Flow-field patterns, bubble evolution, and transport affect the overall performance and durability of AEM-WE, but due to the relative newness of the technology, it remains an understudied area of research that previously was touched upon in discussions of PEM electrolysis.⁴⁵⁵ It is important to look at the AEM-WE system as a whole, considering interactions between components. As stated, there are two primary feed conditions used in stability tests for AEM-WEs: electrolyte feed (typically 0.1 to 1.0 M KOH) and pure-water feed. In the case of electrolytes, additional hydroxide ions are introduced to the system, exposing its components to higher alkaline concentrations, possibly accelerating caustic degradation of system components in addition to the membrane and ionomer from the known chemical mechanisms.⁴⁵⁶ Moreover, ionomer failure caused by the alkaline oxidative conditions in the EC layer at the anode could be concealed by the hydroxide- or carbonate-based electrolytes. When it comes to the outer level of the cell, the attention of researchers is focused on the development of cheaper and more resistant alternatives to BPs commonly used in hydroxide-rich devices. Using carbon-coated stainless-steel BPs proved to be an efficient approach to slow down the anodic film growth in AEM-FC that causes an increased contact resistance and subsequently ohmic losses during the operation.³³⁰ However, this topic remains underdeveloped for AEM-WE in comparison to PEM-based devices, where variation of stainless-steel coatings is frequently explored to achieve a better long-term

performance of the device.⁴⁵⁷ With a pure-water feed supply, which is the ideal operational scenario for manufacturing and scaling if it is efficient and durable, hydroxide ions are only generated at the cathode and transported via AEM without supplementary anions. In this section, we focus on the pure-water operation of the AEM-WE, discussing limitations in the durability and stability of these systems, including the analysis of future research directions aimed at improving the above-named characteristics.

Initial research on AEM-WE devices in electrolyte-free (nominally pure) water as a feed electrolyte reported limited stability and relatively poor operational performance. The first implementation of AEM in pure-water electrolysis operation was reported by Xiao et al., recording 8 h of operation with 400 $mA cm^{-2}$ at 1.8–1.85 V at 70 °C with Ni–Fe anode and Ni–Mo cathode.⁴⁵⁸ Within the same time frame, the next report of the AEM-WE operation without an additional alkaline electrolyte by Leng et al. had a value of 399 $mA cm^{-2}$ at 1.80 V at 50 °C using $IrO_x$ as the anode EC and Pt black as the cathode EC.⁴⁵⁹ Analyzing HFR over time as an indicator for increased membrane resistance contribution and possible delamination in the system, a minimal decay over the 200 h of performance, varying the water feed configuration and the ionomer binder in the process of the cell optimization was reported. The authors suggest using a more durable ionomer, as well as the configurational optimization of the water-fed cell to improve the stability of the system. These findings provided the basic strategy incorporated by different research groups around the world to increase the overall durability of AEM electrolyzers with a pure water feed. Despite initial promising results, according to a 2020 review by Miller et al., initial reports on the pure water operations generally showed an average of 95 $mA cm^{-2}$ at 1.8 V and limited durability of the cell.¹⁹⁰ Early publications on pure water electrolysis lacked a tailored approach, often mimicking alkaline electrolyte-fed systems with unstandardized electrodes and polymers, leading to poorly comparable data.²⁸² Efforts toward standardization were made,¹⁷⁵,⁴⁶⁰ including publications with commercially available materials (Figure 28.A).⁵⁶

Recent work demonstrated a more competitive performance of pure water-fed AEM-WE devices reaching $1 A cm^{-2}$ at potentials below 2 V, which is twice the current density of a modern industrial alkaline water electrolysis standard at the same voltage.⁴⁶¹ Recently achieved maximum performances are 2-fold higher than results from just five years ago. High
<table><caption>Table 6. Summary of the Data Plotted in Figure 28</caption><thead><tr><th rowspan="2">Label</th><th rowspan="2">T (°C)</th><th colspan="2">Performance</th><th colspan="3">Durability</th><th rowspan="2">ref.</th></tr><tr><th>Cell Voltage(V)</th><th>Max current density (A cm<sup>-2</sup>)</th><th>Current density (A cm<sup>-2</sup>)</th><th>Degradation rate (μVh<sup>-1</sup>)</th><th>Time(h)</th></tr></thead><tbody><tr><td>a</td><td>69</td><td>1.95</td><td>1</td><td>0.5</td><td>2571</td><td>175</td><td>44</td></tr><tr><td>b</td><td>55</td><td>2.1</td><td>1</td><td>0.5</td><td>2000</td><td>250</td><td>427</td></tr><tr><td>c</td><td>55</td><td>2.1</td><td>1</td><td>0.5</td><td>300</td><td>100</td><td>462</td></tr><tr><td>d</td><td>50</td><td>2.47</td><td>1</td><td>0.5</td><td>7500</td><td>20</td><td>249</td></tr><tr><td>e</td><td>50</td><td>2.3</td><td>1</td><td>0.5</td><td>1700</td><td>20</td><td>463</td></tr><tr><td>f</td><td>60</td><td>1.9</td><td>1.9</td><td>-</td><td>-</td><td>-</td><td>316</td></tr><tr><td>g</td><td>80</td><td>1.95</td><td>1</td><td>0.2</td><td>2250</td><td>120</td><td>57</td></tr><tr><td>h</td><td>60</td><td>2</td><td>1</td><td>1</td><td>1300</td><td>85</td><td>398</td></tr><tr><td>i</td><td>50</td><td>-</td><td>-</td><td>0.2</td><td>350</td><td>2000</td><td>464</td></tr><tr><td>j</td><td>80</td><td>1.8</td><td>1.02</td><td>0.2</td><td>560</td><td>150</td><td>315</td></tr><tr><td>k</td><td>60</td><td>1.9</td><td>0.5</td><td>0.5</td><td>705</td><td>170</td><td>336</td></tr><tr><td>l</td><td>50</td><td>2</td><td>0.8</td><td>0.2</td><td>747</td><td>535</td><td>459</td></tr><tr><td>m</td><td>-</td><td>-</td><td>-</td><td>0.2</td><td>1470</td><td>170</td><td>181</td></tr><tr><td>n</td><td>25</td><td>-</td><td>-</td><td>0.2</td><td>1800</td><td>100</td><td>465</td></tr><tr><td>o</td><td>50</td><td>1.8</td><td>0.4</td><td>-</td><td>-</td><td>-</td><td>396</td></tr><tr><td>p</td><td>70</td><td>1.85</td><td>0.4</td><td>-</td><td>-</td><td>-</td><td>458</td></tr><tr><td>q</td><td>50</td><td>2.25</td><td>0.8</td><td>-</td><td>-</td><td>-</td><td>249</td></tr><tr><td>r</td><td>50</td><td>1.9</td><td>0.1</td><td>-</td><td>-</td><td>-</td><td>459</td></tr><tr><td>s</td><td>25</td><td>1.8</td><td>0.3</td><td>-</td><td>-</td><td>-</td><td>402</td></tr><tr><td>t</td><td>50</td><td>1.8</td><td>0.5</td><td>-</td><td>-</td><td>-</td><td>466</td></tr><tr><td>u</td><td>60</td><td>1.86</td><td>0.4</td><td>-</td><td>-</td><td>-</td><td>467</td></tr><tr><td>v</td><td>22</td><td>1.6</td><td>0.1</td><td>-</td><td>-</td><td>-</td><td>468</td></tr></tbody></table>

ionic concentration quaternized polystyrene ionomers with various IECs were used by Li et al. to support the key need for a local high pH environment in the EC binder for efficient HER and OER.²⁸² By removing phenyl groups from the polymer backbone, the acidic formation of phenol is prevented, ensuring the maintenance of a local alkaline pH. The system operated in pure water reached a value of 2.7 A cm⁻² at 1.8 V. However, as was discussed in later publications,²⁸² the authors were not successful in removing all the remaining alkali from the system, which affected the reported performance and durability of the electrolyzer. At the same time, the durability of the single-cell in this work remained an issue and at 60 °C and 200 mA cm⁻², only 170 h of low-rate decay performance could be observed. These studies often use individually tailored polymer membranes and ionomer binders, as well as specialized GDLs, for example, implementing fluoride-incorporated nickel-iron oxyhydroxide nanosheet arrays on a compressed Ni foam to achieve a competitive performance and reach over 160 h of a continuous operation at 200 mA cm⁻² with a 0.56 mV h⁻¹ decay rate at 70 °C. The durability of this cell at an increased current density of 500 mA cm⁻² had a higher potential and 1.81 mV h⁻¹ decay rate over 70 h.³¹⁵ While it is one of the driving forces for innovation, customization of the AEM electrolyzer components to achieve a specific performance of a single cell affects measured performance metrics and complicates the durability evaluation. Standardization of evaluation criteria is needed for a better understanding of achieved performance and durability (Figure 28.B).

Stability and degradation testing in pure water allows for researching the fundamental properties of AEM-WE components and target system development toward the ideal electrolyte-free operation. However, in this condition, the impact of the water additives can be detrimental to the system; hence, the study on the sensitivity to the water quality and

specific impurity contaminants of the AEM-WE would be useful. This part has already been treated above.

Recently, stack-level measurements on AEM-WE translated performance to a commercial magnitude. Park et al. demonstrated a 5-cell stack system that operated for 150 h at 440 mA cm⁻² per cell (28 A total current).⁴⁶⁹ This system, fed by 1 M KOH, degraded at a rate of 2 mV<sub>stack</sub> h⁻¹, bringing the initial efficiency of the cell 69% to the final 59% at the end of the stability test. To be considered a commercial-level AEM-WE stack, the device must display a degradation rate below 1 mV 1000h⁻¹ and maintain over 90% of the initial performance for 20 years of use.⁴⁷⁰ Performance and durability of the stack were influenced by the electrolyte behavior, in particular, the effect of turbulent and laminar flow was discussed. As the number of stack components increases, the behavior of the electrolyte becomes significantly different than that in a single cell. A fluid-mechanical analysis is required for a proper stack design evaluation to consider all the parameters influencing the final performance of the device. Although the stack structure itself, as well as electrolyte feed flow, poses certain limitations to the performance to be achieved, this research opens a door to the future with large-scale stack-level explorations.

## 7.1. Effect of Ionomer

As stated above, there are chiefly two types of AEI chemistry: (i) polymer containing side chains attached to the backbone of the polymer bearing fixed positively charged groups, and (ii) ionenes, charged polymers with cationic groups attached along the backbone of the polymer. The latter is considered an example of an ionomer with enhanced durability due to the advanced structure, which eliminates some of the alkaline degradation mechanisms.²⁷¹,²⁷⁵

At the cell level, AEIs historically limited the performance and durability of AEM electrolyzer devices, especially in pure water.²⁸⁸,²⁸⁹,⁴⁶⁰,⁴⁷¹,⁴⁷² Common cation exchange ionomers are PFSA-type materials with high chemical stability.²⁸² Most
<figure><img src="image_42.png" /><figcaption>**Figure 29.** (A) AEM-WE operation in different feed conditions demonstrates ionomer oxidation suppression in the presence of 0.1 M KOH. (B) XPS data show the presence of oxidized carbon after AEM-WE pure water operation, indicating ionomer oxidation. (C) Cross-sectional images of an IrOx/TP-85 anode PTE before and after operation show oxidative degradation of ionomer, causing its loss. Reproduced with permission from ref 462. Copyright 2023, The Royal Chemical Society</figcaption></figure>

anion exchange ionomers are hydrocarbon-based (e.g., polybenzimidazoles, polyethers, polyphenylenes, etc.), which are more susceptible to chemical degradation by $OH^-$ in strongly alkaline conditions. Efforts at improving the stability of AEM polymers have focused on alkaline chemical stability, including adding protecting groups near electrophilic sites,<sup>404,473</sup> partial fluorination<sup>474</sup> and tuning side chain length<sup>475</sup> or cation identity.<sup>476,477</sup> Ex-situ chemical stability, however, is not necessarily reflective of device conditions nor indicative of how a polymer will perform in an MEA, in particular during operation with electrolyte-free water feed. Multiple chemical and electrochemical degradation pathways were identified for a variety of alkaline ionomers in pure water. Alkaline ionomers are susceptible to degradation via nucleophilic substitution, elimination, phenyl oxidation, and methyl and/or proton-rearrangement reactions, depending on polymer structure.<sup>266,364,475,478,479</sup> Water-transport limitations during device operation may enhance degradation due to locally increased $OH^-$ concentration.<sup>480–482</sup> These pathways break down the membrane structure, leading to a reduced ionic conductivity due to the loss of charged end groups, either directly by degrading cations or indirectly through backbone or side chain degradation, resulting in soluble ionomer fragments.

At the anode, the ionomer is held at a strongly oxidizing potential and exposed to possible oxygen radical species produced during OER. The study that proves the presence of stable radicals, records and identifies all the short-lived radical species under alkaline conditions was first conducted for the AEM-FC application.<sup>483</sup> Superoxide and hydroxyl radicals could be spontaneously created within the polymer electrolyte material in the presence of oxygen and hydroxide ions. This work suggests the formation of stable radicals as a result of the short-lived oxygen radicals on the membrane backbone.<sup>483,484</sup>

Organic materials are disreputably difficult to stabilize under alkaline OER conditions, as has been shown for carbon

blacks.<sup>96,485</sup> It was proven that the presence of a highly active OER EC prevents carbon from oxidation in alkaline conditions due to the enhanced reaction kinetics, which reveals a closer look into carbon support behavior during this electrochemical reaction. This statement agrees with observed carbon corrosion in the presence of Pt EC, known for its sluggish kinetics in the hydroxide-rich environment.

Fenton oxidation processes (4 ppm $FeSO_4$ in 3% $H_2O_2$), forming radical oxygen species, can also degrade the ionomer and membrane.<sup>486</sup> Adopted from PEM-FC technology research, acidic Fenton processes could be a source of degradation in AEM systems with active nonprecious-metal OER ECs, which all contain Fe but have not been studied extensively in this context due to the high alkalinity of the system, tempering with a Fenton test chemistry. The oxidative and radical stability of anion exchange polymers was investigated to various extents,<sup>479,487,488</sup> but few studies have been conducted under device-relevant operating configurations and conditions. Future research on oxidative stability might be performed under relevant alkaline conditions, for example, using alkaline peroxide solutions like those used in the RCA semiconductor cleaning procedures for organics,<sup>489</sup> with a standard metal oxide EC nanopowder dispersion, for example, using commercial $CoO_x$.<sup>490</sup>

The ionomer binder in contact with the EC layer oxidizes under operating potentials at the anode.<sup>56,485</sup> The mechanism by which this oxidation occurs is poorly understood.<sup>56,254,282</sup> Oxidation could occur directly at the EC/ionomer surface or chemically through reactivity with OER intermediates or other reactive oxygen species formed, for example, through radical reactivity.<sup>491,492</sup> AEM ionomers in a pure water operation will oxidize at sufficiently high voltage regardless of EC/electrode surface,<sup>254</sup> thus degradation may only depend on the device operating voltage and feed. Others have suggested that degradation rates correlate with ionomer/EC interaction
strength,⁴⁹³ and the adsorption energies of ionomer functional groups to the EC surface are expected to vary for different oxide ECs.

One key research direction is to distinguish between degradation mechanisms of the bulk membrane, membrane interfaces, ionomer binders, and the ECs themselves in the presence of pure water, alkaline-electrolyte, and impure-water feeds. Li et al. probed the performance difference observed when varying the chemical structure of the binder polymer using the same membrane.¹⁸¹ They showed that the voltage performance and, in the long term, durability depended on the ionomer structure and loading. This was attributed to interfacial pH control. Phenyl groups in the polymer backbone can oxidize to form phenol, which was hypothesized to decrease surface pH and neutralize hydroxide associated with the cationic ionomer groups.²⁵⁵ Fully sp³-carbon polymers showed more stable performance³⁹⁸,⁴⁶⁵ and resistance to oxidation in *ex-situ* Fenton testing and during fuel cell operation,⁴⁹³ but still suffer from oxidative damage at the high operating voltage of AEM-WE (Figure 29).⁴⁶²,⁴⁹⁴

Mustain and co-workers showed how the water uptake by the AEI can affect device stability, and found that low water uptake at the anode improves stability despite the performance losses from low ionic conductivity.²⁸⁸,³⁹⁸ This hypothesis was supported by Chen et al. where a reduction in ionomer swelling via a controlled benzylation (cross-linking) of polybenzimidazole-type polymer resulted in four times prolonged operation of a pure-water operated AEM-WE system. Additionally, the above-mentioned work reports that the impact of the ionomer swelling within an EC layer is more detrimental to the stability of the device than the dimensional stability of a polymer electrolyte membrane incorporated in it.⁴⁹⁵ Water uptake will also affect how chemical OH⁻ and radical oxygen species access and interact with the polymer, and thus, polymers with lower water uptake may not degrade as rapidly. Expansion of the polymer chain network, followed by the water penetrating the bulk of the material, allows better distribution of the degradation-inducing species. At the same time, narrow channels of a less swollen ionomer could reduce the amount of hydroxides and radicals entering the polymer. PTFE-containing (polytetrafluoroethylene) anodes are resistant to oxidative damage. This type of fluorinated additive is a possible route for increasing the AEM-WE pure water operation, but not a viable commercialization pathway alone due to the high ionic transport resistance leading to low voltage efficiencies.⁴⁶² While PTFE on its own shows poor voltage performance, Alford and co-workers reported a performance of ~ 700 mA cm⁻² at 2 V using a PTFE-based hydroxide conductive membrane that degraded at a rate below 40 µV h⁻¹ over 120 h.⁴⁶⁸

The problem of ionomer oxidation is much more prominent for pure-water operation. Using X-ray photoelectron spectroscopy (XPS) analysis, Lindquist et al. showed stable cell operation in 0.1 M KOH at 3 A cm⁻² with no substantial evidence of oxidative damage after 20 h.⁴⁶² However, it is worth noting the limitations of this method, such as surficial analysis without consideration of surface roughness/uniformity, as well as ionomer presence on the spectrum have to be analyzed carefully since low polymer loading will result in a lack of signal-to-noise ratio. Additionally, signals from carbonate/bicarbonate could interfere by appearing in the binding-energy range where oxidized carbon species are found, so all samples must be carefully exchanged to chloride form

prior to the evaluation. An identical cell operating in pure water at 1 A cm⁻² (the equivalent starting voltage of the KOH cell) degraded rapidly. This difference was hypothesized to be due to changes in the local double-layer structure. Under alkaline conditions, metal-oxide surfaces are likely negatively charged due to deprotonation, which promotes the adsorption of cationic species or polymer backbone groups from the ionomer—a phenomenon previously suggested for other reasons.²⁹¹,⁴⁷⁶,⁴⁹⁶⁻⁵⁰⁰ Without supporting electrolytes, ionomer, therefore, must play a fundamental role in the formation of the double layer, requiring it to be in close vicinity to the polarized EC. The presence of mobile, soluble ions in the supporting electrolyte likely leads to displacement of the ionomer from direct involvement in the double layer, and this may be responsible for substantially reducing the degradation rate compared to pure water. It was shown that the structure of the double layer at the anode EC is critical in controlling the rate of deleterious ionomer oxidation during OER.⁴⁶² IrO₂ and Co₃O₄, the best anode ECs in pure-water AEM-WEs,⁴²⁷ are acidic or neutral oxides with a pH of zero charge of <7. In base, they thus have a negative surface charge. In the KOH electrolyte, mobile K⁺ will compensate for this charge such that the ionomer resides outside of the double layer where deleterious oxidation occurs. In electrolyte-free devices, the ionomer is absorbed electrostatically on the OER EC and thus susceptible to deleterious oxidation. This work demonstrates that controlling the reactive EC/ionomer interface enables one to slow the oxidation, e.g., by surface-charge modification with absorbed Mg²⁺ or Ca²⁺. Alternative strategies to exclude ionomers from the double-layer region might also be expected to reduce oxidation rates.

## 7.2. Ionomer Poisoning

One of the major factors that limit the durability of AEM-WEs operated with pure water is the electrochemical oxidation of the adsorbed phenyl group present in the ionomer, which occurs at the potentials required for oxygen evolution. One of the possible approaches for mitigating the impacts of phenyl group electrochemical oxidation is to use polymers with a lower affinity for these groups. For example, quaternized polyolefins, which have lower adsorption energy than quaternized polyaromatics, could be a viable option. Additionally, using nonrotatable phenyl groups like fluorene or carbazole, which have lower adsorption energy than rotatable phenyl groups like biphenyl, could help to reduce the impact of this degradation mechanism.⁵⁷,²⁸²,⁴⁷²,⁴⁷³

Water transport occurs from the anode to the cathode side, especially in a "dry cathode" configuration. Design AEMs with higher water diffusivity,⁴⁰⁴ using cathode ionomers with high IEC, and increasing the operating temperature are approaches that would help in providing more reactants to the cathode side and prevent the chemical degradation of the ionomeric materials.

Phenyl oxidation of the ionomer occurs during the operation of AEM-WE. Minimizing the adsorption of the phenyl group on oxygen evolution ECs is achievable by employing PGM-free perovskite oxides. In addition, developing new ionomers with fewer phenyl-adsorbing traits could be a potential direction for making durable AEM electrolyzers.

The development of AEMs with high hydroxide conductivity and IEC is critical while ensuring robust mechanical and chemical stability. Ionomer with high IEC and water uptake,
especially at the cathode electrode, to prevent the dehydration of the ionomer.

### 7.3. Effect of Electrocatalyst

In alkaline electrolytes, Ni–Fe oxyhydroxides have the lowest overpotential for OER and the highest per-cation turnover frequency in three-electrode studies.<sup>223,411,425</sup> However, most AEM-WE testing is conducted with expensive IrO<sub>2</sub> OER EC,<sup>56,57,249</sup> as the performance and durability of Ni–Fe oxyhydroxides have generally been poor in pure-water MEA configurations. Indeed, PGM-free ECs have shown promising performance and durability in MEA configurations, mainly with supporting electrolyte<sup>212,501,502</sup> and or complex electrode and/or EC design and preparation,<sup>181,315,460,503–505</sup> to overcome ionic and electronic conductivity issues. We expect that these observations are related to the complicated *in situ* behavior of PGM-free ECs.

For one, PGM-free transition metal oxide OER ECs are known to structurally evolve under operating conditions.<sup>411,425</sup> Initially, crystalline NiO amorphized during the OER.<sup>216</sup> *Operando* surface X-ray diffraction showed a reversible formation of a CoOOH “skin layer” at OER potentials on Co<sub>3</sub>O<sub>4</sub>.<sup>506</sup> Perovskite oxides have been reported to show a dazzling variety of dynamic behavior.<sup>507</sup> Direct evidence of CoOOH formation via leaching of surface-segregated Sr-rich areas has been reported for La<sub>0.2</sub>Sr<sub>0.8</sub>CoO<sub>3</sub>.<sup>508</sup> Such A-site leaching has also been shown to be concurrent with surface oxygen vacancy formation in related materials.<sup>412,509,510</sup> LaNiO<sub>3</sub> exchanges surface oxygen with oxygen in the electrolyte<sup>511</sup> and displays NiOOH-like redox behavior when doped with strontium.<sup>512</sup>

Restructuring could be especially detrimental in the pure-water-fed mode. Significant reconstruction may disrupt the interface between EC and ionomer/membrane, which is the only means of ionic transport to the anode. Importantly, these ionic conductivity losses should be recoverable upon the introduction of supporting electrolytes, consistent with the literature. It may even be the case that EC detachment from the ionomer in the EC layer induces oxidative degradation because the current demand cannot be effectively met by driving OER alone, but also by oxidizing the ionomer. Further, restructuring may form an insulating material such as Ni(OH)<sub>2</sub>, which must be made conductive by driving the Ni<sup>2+</sup> to Ni<sup>3+</sup> reaction in the presence of hydroxide.

In general, investigation of the extent and effect of these EC structural transformations in relevant MEA configurations is in its infancy<sup>209,401,513</sup> and complicated by membrane and ionomer instability that causes degradation independent of EC identity. Lei and co-workers reported a decrease in MEA performance for a system with amorphous NiCoFe oxide particles only under pure water-fed mode, thus concluding that the decrease was driven by EC reconstruction.<sup>514</sup> Krivina et al. tested various PGM-free OER ECs in traditional EC-coated-electrode MEA configurations in pure water. NiFe<sub>2</sub>O<sub>4</sub> nanoparticles failed rapidly during electrolyzer testing, but Co<sub>3</sub>O<sub>4</sub> nanopowders showed comparable performance and stability to high-surface-area commercial IrO<sub>x</sub>.<sup>427</sup> The enhanced performance was attributed to differences in EC electronic conductivity and changes to ionomer EC interactions during operation. Co<sub>3</sub>O<sub>4</sub> was found to have the highest electronic conductivity of all PGM-free ECs tested and showed resistance to structural rearrangement during operation. All Ni-based and mixed metal ECs showed substantial

surface rearrangement during operation, likely disrupting the EC/ionomer interface. Notably, others have shown Ni–Fe oxide ECs perform well in AEM devices when fed with KOH<sup>404,515</sup> or in pure water when using a thin layer of NiFe EC on a conductive supporting substrate, likely to compensate for the poor electronic conductivity of the material when not fully converted to the oxyhydroxide form.<sup>315,516</sup>

It has also been shown in liquid alkaline electrolytes that PGM-free OER ECs have only high activity when Fe is present.<sup>425,517,518</sup> Fe catalytic sites are not static, but dynamic. They can form by adsorption from electrolyte<sup>223</sup> and actively dissolve and redeposit during OER.<sup>217,519</sup> If there is no soluble Fe available for redeposition, the EC becomes much less active.<sup>520</sup> Maximizing Fe adsorption is not necessarily a universal strategy for high activity; the performance of an AEM with a NiFe<sub>2</sub>O<sub>4</sub> particle-based anode improved as Fe leached out.<sup>521</sup> These ECs may also uptake additional ions present in trace amounts in the electrolyte.<sup>522</sup> Thus, for Ni and Co-based ECs, management of ion adsorption/desorption processes, especially of Fe, is another understudied challenge that must be translated from three-electrode to the MEA levels.

Observed differences in ion dynamics between a three-electrode and MEA system will likely arise from the presence of the ion-selective membrane and directed electrolyte flow in the latter. The flux of OH<sup>-</sup> from cathode to anode, combined with the membrane permselectivity, drives anions to the anode. This may lead to regions of higher pH. If this led to deactivation, the addition of Fe into the electrolyte stream could be attempted to recover activity. Maximizing activity must be undertaken simultaneously with preventing the movement of dissolved metals and poisoning of the cathode. The AEM configuration may be an interesting platform because the flow of cationic impurities (e.g., M<sup>+</sup> ions leached from the OER EC) from the anode to the cathode should be retarded by the membrane permselectivity (i.e., the selective conduction of anions over cations). The flow of OH<sup>-</sup> from cathode to anode would also reduce the extent of anion exchange and crossover of anionic metal oxo anionic species. Post-mortem analysis, such as measuring elemental composition and concentrations of leached species in the outgoing electrolyte during the operation of MEAs, would be useful in understanding element speciation during operation and its relationship to durability.

### 7.4. Advanced Active Layer Design

While substantial gains have been made in advancing alkaline ionomer stability,<sup>56,57,315</sup> polymer oxidation at the anode is proving a substantial barrier that may not be solved by polymer design alone. Advanced EC layer designs that incorporate stable additives are an interesting new direction that may enable stable pure water operation. The most stable electrodes appear to be prepared with ground ionomer resin particles as opposed to the conventional dissolved/dispersed ionomer solution in the ink (Figure 30.A). The observation was first made by the Varcoe group in 2014 for the AEM fuel cell technology, where the addition of insoluble ETFE-based (ethylene tetrafluoroethylene) polymer particles to the EC layers of the systems instead of the conventionally dispersed ionomer binder improves the performance of the cell.<sup>523</sup> Continuing this research thread, in 2018, Mustain and team showed how the addition of fluorinated insoluble polymer resin improves the water management of the AEM-based fuel cell system, which is considered one of the technology
<figure><img src="image_43.png" /><figcaption>**Figure 30.** Scheme of EC (EC) layer design approaches: (A) incorporation of solid ionomer particles to facilitate ionic transport within the layer; reproduced from ref 299, copyright 2023, American Chemical Society and (B) incorporation of a passivation layer at the EC-ionomer interface; reproduced with permission from ref 326, copyright 2023, American Chemical Society.</figcaption></figure>

limitations.⁵²⁴ In order to prepare inks with incorporated insoluble ion-exchange polymer powders, an anion exchange ionomer with a particle size of 20–30 µm and an EC were ground with a mortar and pestle, followed by the addition of a solvent until a low-viscosity mixture was achieved. Deposited by ink spraying, the PTL EC layers formed have a rough structure with ionomer solids that are significantly larger than EC particles. A composite structure of the layer contributes to better water distribution within the fuel cell’s electrodes. The concept was tested using different ECs and directly comparing dispersed vs ground ionomer binder incorporation.⁵²⁵ Observed differences in ionomer processing display a trend in EC/ionomer interactions and are critical for high cell performance. The same type of system was tested in PEMFC by Holdcroft and team, incorporating nonconformal hydrocarbon-based ionomer powders into the electrocatalytic layer of the device.⁵²⁶ Improved power density performance at high current densities of the fuel cell suggests a different ionic conductivity pathway through the composite layer of the EC with insoluble polymer particles that provides a substantial ionic resistance reduction and mass activity increase. Combined with a substantial porosity of the electrodes caused by the size of insoluble ionomer particle addition, this finding led to a much improved PEMFC performance in the air in comparison to PFAS-based membrane electrode assemblies.⁵²⁶ Following the trend, these observations could be translated into the AEM-WE system to further improve it. It presents an interesting question as to the effect of EC layer geometry and morphology on ionomer degradation and the role of nonion-conducting additives as stabilizers in the EC layer. Lindquist et al. showed stable pure water AEM electrolyzer operation for 100 h when using PTFE binder at the anode in the replacement of a conventional ionomer.⁴⁶² While cell voltage was high for commercial applications, advanced electrode designs that use stable binders or additives, but maintain ionic conductivity in the EC layer, may be a viable solution for pure-water AEM operation. Such designs have been deployed as EC protective layers in PEM, enabling stable operation of earth-abundant anode ECs by using a TiO₂ protective layer to prevent EC corrosion.⁵²⁷

One strategy to stabilize EC layers and associated ionomers may be to engineer the EC surface with passivating inorganic films designed to confine EC dissolution products and prevent deleterious oxidative side reactions due to direct contact between EC and ionomer. To achieve the desired function, the engineered interface passivation layers must be thin and porous enough to allow for the transport of hydroxide ions, reactant and product, while thick enough to retard the direct oxidation of ionomer (Figure 30.B).³²⁶ If the porosity in such passivation

layers can be tuned, it may also be possible for them to allow the transport of small ions, like OH⁻, but prevent the transport of larger ions associated with hydrated dissolved EC cations. Most materials and electrode concepts related to these strategies have only been tested in half cells and not in the AEM-WE context. MEA-level testing with industrial-relevant current density will be required to demonstrate their applicability in the AEM-WE system. For acidic OER, few-nm-thick atomic layer deposition (ALD) TiOₓ coatings on IrO₂ and RuO₂ electrodes improved the OER activity and maintained their performance longer than uncoated IrO₂ or RuO₂ electrodes.⁵²⁸ Co₃O₄ anode EC coated with 4.4 nm ALD TiO₂ showed an increase in OER stability >75 h, compared to uncoated Co₃O₄ (25 h) at 10 mA cm⁻² in 1 M H₂SO₄.⁵²⁷ It was noted in both works, as well as the recent review⁵²⁹ that the thickness of protective coatings is key to understanding the trade-off between the activity and stability of the electrodes. Pt nanoparticles dispersed on carbon fibers were coated with ALD SnO₂ protective layers to prevent Pt dissolution and detachment and improve the electrode lifetime, which can be used as the cathode for PEMFC.⁵³⁰ SiOₓ overlayers have been investigated by Esposito and co-workers, where permselective SiOₓ overlayers on Pt electrodes prevented the undesirable chlorine evolution reaction (CER) while maintaining the desired OER,⁵³¹ as well as the density of SiOxCy overlayers, controlled H⁺ transport.⁵³² OH⁻ transport through the thin oxide film should be similar to H⁺ transport, opening up the possibility of the advanced AEM-WE electrode design. The CeOₓ overlayers on Pd and Pt ECs prevented metal dissolution during the hydrogen oxidation reaction (HOR), as assessed by ICP-MS, while the CeOₓ layer appeared to facilitate hydroxide transport, perhaps explaining higher catalytic activity for CeOₓ-coated Pd electrodes.⁵³³ NiFeOₓ ECs for alkaline OER also exhibited improved durability with a 100–200 nm CeOₓ protective layer.⁵³⁴ This was explained by the permselectivity of CeOₓ, enabling OH⁻ transport toward NiFeOₓ EC but preventing the diffusion of dissolved Fe species from NiFeOₓ toward electrolyte, maintaining active sites of NiFeOₓ. Deploying this concept in AEM-WE might be promising because dynamic Fe dissolution and redeposition throughout the electrolyzer system are thought to be detrimental to the durability of electrolyzers.⁴²⁷ Other than focusing on EC degradation, ALD HfO₂ coatings were used as protective layers at the anodes of AEM-WE to suppress ionomer degradation.³²⁶ Nanoscale HfO₂ thin films were shown as OH⁻ conducting but electron-insulating layers at the interface of PiperION ionomer and Ir or CoOₓ anode EC, so that less electron flow from ionomer and EC occurred, which is expected to cause electrochemical oxidation of ionomer.

To design durable AEM-WE electrodes using protective coatings, understanding and controlling the roles of the coatings (i.e., prevention of EC dissolution or detachment, ionomer oxidation, and undesirable side reactions) is required to design the coating chemistry and nanostructure/morphology. Coating materials properties, including ion conductivities (ideally selective for OH⁻), electrical conductivity, and porosity/permeability to electrolyte, are all important. Alkaline and oxidative stability of coatings is needed for stable, insoluble materials during operation. Optimizing the coating parameters of thickness, porosity, and crystallinity may enable improved AEM-WE durability.
## 7.5. Cell Preparation, Assembly, and Operation

Although individual components play a significant role in determining cell performance, preparation and assembly techniques are also crucial. Different coating techniques (e.g., airbrush versus hot-press and coated membranes versus coated electrode) result in different interactions between the EC, ionomer binder, and membrane at the triple-phase boundary, which is the point of simultaneous activity of all the above-mentioned components defining the performance of the electrochemical reaction.⁵³⁵ By careful construction of the triple-phase interface, the efficiency and selectivity of the reaction can be significantly improved. Therefore, large performance differences can be observed across MEAs of nominally identical materials.⁵³⁶ Using quartz-crystal microbalance (QCM) techniques supported by modeling and isothermal-titration-calorimetry experiments, Weber's group was able to identify key aspects to describe the relation between ionomer and EC in electrode formation.⁵³⁷ The results revealed that water-rich solvents promote ionomer adsorption to the platinum catalytic sites. How the cell is operated before analysis can also affect performance. Cell conditioning is a well-established practice for PEM electrolyzers and fuel cells,⁵³⁸,⁵³⁹ during which the cell is operated and performance gradually improves.⁵⁴⁰ Although the detailed processes that cause this improvement are not fully understood, it is hypothesized that cell conditioning further activates EC surfaces, more fully hydrates the membrane and ionomer and better establishes ion pathways through the polymer via chain rearrangement.⁵³⁸,⁵⁴¹,⁵⁴²

Sometimes different electrolyte feeds are used during conditioning, even if testing in pure water,⁵⁷,¹⁸¹,³¹³ which can also affect measured electrolyzer performance and durability. Conditioning with supporting electrolytes is done due to some advantages over pure-water conditioning. Conditioning in pure water can induce degradation before the initial measurement and therefore yield results with worse performance than the true initial state. The lower operating voltage and consistent supply of $OH^-$ to the system during conditioning in supporting hydroxide electrolytes, prevents any cell degradation during conditioning, so the measured performance in pure water is fully pristine. However, intentional care must be taken to ensure all electrolyte is flushed from the system before the pure water measurement.⁵⁶,³⁹⁸

Some observed performance losses in pure water AEM electrolyzer systems are reversible, which some have attributed to $CO_2$ gas poisoning of the system.⁵⁴³ This is a well-known effect in AEM fuel cells.⁴⁷⁷,⁵⁴³ However, the operating modes of an electrolyzer vs a fuel cell present distinctly different modes in which $HCO_3^-$/$CO_3^{2-}$ ions enter and interact with the system. In both systems, anions are conducted from the cathode to the anode. Any anions other than $OH^-$ present at the cathode will compete with $OH^-$ transport in the AEM, decreasing membrane conductivity,²⁶⁸ reducing available $OH^-$ reactants at the anode,⁷³ and increasing local pH at the anode/AEM interface.⁷³,¹⁷⁰ In AEM fuel cells, $CO_2$ and $O_2$ enter the cathode, where $CO_2$ dissolves in water to form $HCO_3^-$ (in equilibrium with $CO_3^{2-}$) anions. Impurity effects from dissolved $CO_2$ can result in performance losses up to hundreds of mV.⁴⁷⁷ Via modeling, with the increasing $CO_2$ concentration at the same operational parameters, a significant reduction in current density was observed by Stanislaw et al.⁷³ As the current increases, more $HCO_3^-$/$CO_3^{2-}$ are introduced and compete with hydroxide. Carbonate ion

induces a pH gradient that is responsible for a Nernstian (concentration overpotential) voltage loss.¹⁷⁰ However, AEM-WEs are ideally operated with liquid water feed to the anode and a dry cathode. In this configuration, any $HCO_3^-$/$CO_3^{2-}$ present at the anode is blocked from entering the membrane by the direction of the applied potential and strong flux of $OH^-$ toward the anode. There is an additional purging effect with increased current density operation, with more $OH^-$ produced. At the cathode, any gaseous $CO_2$ present is now competing with the kinetically favored HER, and therefore, $CO_2$ reduction is likely negligible. Additionally, $CO_2$ reduction reaction kinetics are slow at HER potentials. Both systems may still suffer from Nernstian voltage losses due to $HCO_3^-$/$CO_3^{2-}$ accumulation at the anode/membrane interface due to a pH gradient, but in electrolyzers, this effect is probably negligible relative to losses from ionomer oxidation at the anode. The effect of carbonation is difficult to study at the relevant device operating current in pure water without first addressing the performance-limiting issue of ionomer oxidation.

## 7.6. Accelerated Stability Tests

Stability tests for AEM-WEs are vital to assess the long-term performance of these devices. At the same time, AEM-WE's durability tests refer to the ability of the device to withstand various operating conditions inflicted by the accelerated stress tests (ASTs). These evaluation techniques provide insights into possible degradation mechanisms and enable the optimization of the system's components and overall cell design.

To date, there are no established and widely recognized procedures for the accelerated stability testing of AEM-WEs. Due to the relative newness of the technology, a lot of protocols could be translated from more mature technologies of PEM-WE or A-WE, where most test specifications are agreed upon and follow a common goal of improving water electrolysis technology for industrial demands.⁵⁴⁴

The rapid development of new components of electrolyzers with a prospective commercialization plan calls for a reliable performance that can ensure at least 50,000 h of operational time to be an industry-viable product. Therefore, there is a need for accelerated stress protocol development that could be well understood and translated into an extended longevity performance. ASTs for water electrolyzers typically focus on the chemical, mechanical, and thermal degradation aspects. Most of these could be addressed via in situ applied high current density, dynamic operation of the cell and shutdown models.⁵⁴⁵ For an AST, it is crucial to understand the degradation mechanism of materials and components used in the electrolyzer. Understanding the performance under those conditions allows the investigation of the accelerated aging process as well as the operation of the system after a failure during a real-life operation caused by external factors (ex., power supply shutdown). However, the limitation caused by the inability to perfectly mimic the operating conditions of the device suggests that tests performed ex-situ are not always translatable to the real-life system. It is of utmost importance for the AST to reflect relevant AEM-WE operating conditions, for example, performing the testing on the MEA configuration in the enclosed cell vs aqueous model system (three electrode setup) experiment, where some of the factors cannot be accounted for.⁶⁸

Ex-situ AST testing protocols and descriptions could be found in previous chapters of the manuscript; however, it is
<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /><figcaption>**Figure 31.** Suggested harmonized protocol for accelerated stability testing of water electrolyzers. (A) MEA schematic of PEM-WE and AEM-WE with corresponding stressors of systems. (B) Testing protocol scheme reproduced with permission from ref 545. Copyright 2020, Elsevier B.V.</figcaption></figure>

important to discuss findings following the in situ exposure to accelerated degradation factors. These factors should mostly focus on the actual operating strategy of a commercial device and adjust to the timeframes of such, for example, considering the surplus of renewable energy.⁵⁴⁶ Since AST must guarantee meeting the expectations of durability for the commercial systems, the development of such a protocol should include information on performance and efficiency. Developed in 2020 by Aßmann et al. AST protocols for commercial PEM water electrolyzers involve a four-step procedure described further in the paragraph (Figure 31).⁵⁴⁵ The testing should include a nonintentionally accelerated condition at a nominal current density that still would induce some degradation over time. The next component of such testing is a high current density operation or overload, the impact of which will influence the ionomer, membrane, and anode EC layer, as well as PTL if exposed. Further load cycling, which applies to hydrogen production from fluctuating renewable sources, will affect the cathode EC layer, causing its premature degradation. As a part of sudden power loss testing, open circuit operation or a complete shut-down process will mostly affect the cathode components. Similar to this development, the AST protocol for AEM-WE must be standardized, including in-between CV, EIS and steady-state polarization curves to monitor the state of health of the device.

The high chemical and mechanical durability of the AEM-WE's components under a dynamic start-stop operation is a direct pathway to reliable electrochemical energy conversion devices that function complementary with intermittent renewable power sources. An extended single-cell durability study performed by Narayanaru et al. (2023) in 1 M KOH feed at 80 °C evaluates 1000 start-stop voltage cycles between 0.1 and 2.0 V.⁵⁴⁷ In this work, the discussion is focused on the durability of MEA, which is affected by the ionomer-binder degradation issue described in previous chapters, as well as the mechanical strength of AEM in an alkaline environment challenged by the rapid bubble formation, causing nonlinear stress and shutdown. The results suggest no obvious deterioration; however, a nominal increase in the cell voltage shows an additional 9 mV requirement to reach the same

current density after 1000 cycles. Impedance spectra collected after the test indicate no changes to the resistance of the polymer electrolyte membrane and, as well as no solid EC leached out of the system. Despite these findings, when speaking about the AEM-WE with alkaline electrolyte-feed, one needs to consider a nuisance typically observed in alkaline water electrolyzers concerning EC decay caused by frequently occurring potential change (emergency shutdown/unstable supply of renewable energy) associated with reverse polarization.⁵⁴⁸ The generation of degrading reverse current in liquid alkaline systems is attributed to a significant difference between the discharge capacity of the electrodes on the same BP. Therefore, if a cathode EC on one side of a BP has low redox durability, the anode discharge capacity must be controlled to limit the generation and the effect of reverse current. One of the approaches is to manage the surface area of the electrode and work on the various metal cations EC with suitable activity to improve the design of the anode.

## 8. CONCLUSIONS

The push for sustainable energy solutions and low-emission hydrogen as an energy vector has intensified research on electrolysis and, lately, on AEM-WE to increase supply chain resilience by avoiding the usage of critical resources. So far, most of the AEM-WE investigations have been devoted to KOH-containing electrolytes. A significant interest exists in transitioning to KOH-containing electrolytes to pure water feeds to cut down the cost and limit the degradation of the electrolyzer component. Given that pure water is limited in nature, its use for AEM-WE raises environmental and logistical issues. Only 3% of earth's water is freshwater, supporting various human activities like agriculture and industry, and this supply is under growing human pressure.

Historically, doubts existed about the viability of a hydrogen economy, primarily due to concerns about water use. Yet, progress over recent years has enhanced water electrolysis efficiency, paving the way for eco-friendly hydrogen production with minimal environmental burden. Data indicates that although a hydrogen-based economy would need vast amounts of water, its actual consumption and ensuing environmental
footprint are relatively low, especially when compared to sectors like agriculture and energy production from fossil fuels.

A significant breakthrough would be adjusting AEM-WE to work with less refined water or even seawater. Such an approach would diversify water sources and decrease freshwater demand. The evolution of electrolysis technologies has allowed for the utilization of impure water sources, such as sea or tap water, to economically produce hydrogen, with a significant focus on AEM-WE. PEM-WE systems face distinct challenges when operating with these impure water sources. The primary issue stems from potential contaminants that can degrade the electrolyzer's critical components and increase maintenance costs.

A notable challenge in this context is the chloride ions present in impure water, which can cause extensive damage to cell components. When the environment becomes acidic, these ions participate in reactions that produce harmful byproducts. However, AEM-WE systems, due to their operation at a higher pH, present a certain degree of resistance against these negative effects of chloride ions. Despite this advantage, challenges persist. The degradation of ECs by elements in impure water can significantly reduce the efficiency of the electrolysis process.

Recent technological advancements offer potential solutions. One such innovation is the asymmetric configuration of electrolyzers, which shows promise in mitigating some of these challenges. However, the issue of chloride-induced corrosion remains a concern. To address this, specific anionic additives have been explored, which serve to protect against such corrosion, enhancing the lifespan of system components.

Further research has also focused on methods to deter harmful ions, effectively using anions to improve electrode stability. A notable method integrates seawater electrolysis with hydrazine degradation. This approach not only achieves more energy-efficient hydrogen production but also reduces the corrosive impacts typically associated with chloride ions. Such advancements highlight the significance of combining chemical knowledge with engineering solutions to progress toward sustainable energy outcomes and a reduction in carbon emissions. However, the use of highly impure water sources remains a major challenge with no current short-term solutions. Accordingly, Water purification for AEM-WE remains vital.

Current methods to purify water may include thermal treatments, membrane distillation, and RO. Among these, RO stands out due to its energy and cost efficiency. There's also a trend toward combining multiple purification techniques to boost efficiency and reduce expenses. Remarkably, the current energy cost for the RO purification of water per kilogram of hydrogen is negligible (0.3 kWh kg⁻¹) and somewhat acceptable for final technologies. Indeed, RO may allow the use of seawater that is extremely abundant, avoiding an impact on freshwater.

Transitioning to systems using pure water in AEM-WE is challenging because of the high resistance of the medium. Efficient ion transport between the anode and cathode is crucial for the electrocatalysis of hydrogen and oxygen. This is seamlessly handled in polymer electrolyte membrane electrolyzers using ultrapure water, thanks to membranes that facilitate efficient H⁺ transport. Yet, replicating this in AEM-WE is tough because OH⁻ ions inherently move three times slower than protons.

This reveals that the ion-conducting components—membranes and ionomers—are vital. Growing interest in AEMs, especially within AEM-WEs, hinges on their role as separators and facilitators of selective ion transport. Their effectiveness is contingent upon improving their ionic conductivity and chemical stability.

A key distinction arises when comparing operating media: alkaline solutions vs pure water. While alkaline solutions offer better conductivity and reaction rates, they pose corrosion and safety challenges. Conversely, pure water reduces these concerns but, without alkaline substances, results in higher hydroxide ion concentrations, putting membrane durability at risk.

Ionomers, whether part of the membrane or separate, can adjust ionic conductivities and might lessen high hydroxide concentrations adverse effects in pure water settings. Incorporating them thoughtfully in the MEAs can combine high conductivity with increased chemical stability.

For optimal utilization of pure water, it is crucial to redefine membrane and ionomer characteristics. Modifying the IEC can help balance conductivity and lifespan. Using advanced analytical methods can provide a better understanding of these conditions, leading to the development of superior membranes. As the pursuit of an electrolyte-free AEM-WE continues, understanding the intricate relationships between alkaline solutions, pure water, and ionomer components is critical, guiding and, ultimately, the lighthouse that has to drive the research.

According to the findings reported in this review, high current density operations have already been achieved and we believe that pure water-fed AEM-WE is feasible. The transition to pure water might simplify the electrolyzer design and operations, limiting the use of harmful alkalis and relying on abundant and inexpensive water purification technology for the upstream. To this respect, the major research challenges include the development of robust and conductive alkaline membranes and ionomers and tailored processes to integrate them with the ECs in MEAs.

## AUTHOR INFORMATION

### Corresponding Authors

Carlo Santoro – *Electrocatalysis and Bioelectrocatalysis Laboratory (EBLab)*, Department of Materials Science, University of Milano-Bicocca, 20125 Milan, MI, Italy; ● orcid.org/0000-0002-0944-4500; Email: carlo.santoro@unimib.it

Alessandro Lavacchi – *Istituto di Chimica Dei Composti OrganoMetallici (ICCOM)*, Consiglio Nazionale Delle Ricerche (CNR), 50019 Sesto Fiorentino, Firenze, Italy; Email: alessandro.lavacchi@itae.cnr.it

Plamen Atanassov – *Department of Chemical and Biomolecular Engineering, University of California, Irvine, Irvine, California 92697, United States*; ● orcid.org/0000-003-2996-472X; Email: plamen.atanassov@uci.edu

### Authors

Mohsin Muhyuddin – *Electrocatalysis and Bioelectrocatalysis Laboratory (EBLab)*, Department of Materials Science, University of Milano-Bicocca, 20125 Milan, MI, Italy

Luigi Osmieri – *Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States*; ● orcid.org/0000-0002-3111-2270
Valerio C.A. Ficca - ENEA Casaccia Research Center, Rome
00123, Italy; orcid.org/0000-0002-9817-3027

Ariel Friedman - Center for Clean Energy Engineering
(C2E2), University of Connecticut, Storrs, Connecticut
06269, United States; orcid.org/0000-0002-2453-9428

Karam Yassin - The Wolfson Department of Chemical
Engineering, Technion - Israel Institute of Technology, Haifa
3200003, Israel; The Nancy & Stephen Grand Technion
Energy Program (GTEP), Technion - Israel Institute of
Technology, Haifa 3200003, Israel

Gioele Pagot - Section of Chemistry for the Technology (ChemTech), Department of Industrial Engineering, University of Padova, I-35131 Padova, PD, Italy; orcid.org/0000-0002-4015-6670

Enrico Negro - Section of Chemistry for the Technology (ChemTech), Department of Industrial Engineering, University of Padova, I-35131 Padova, PD, Italy

Anastasiia Konovalova - Department of Energy Conversion and Storage, Technical University of Denmark, Lyngby 2800, Denmark; orcid.org/0000-0001-7301-9245

Grace Lindquist - Department of Chemistry and
Biochemistry, University of Oregon, Eugene, Oregon 97403,
United States

Liam Twight - Department of Chemistry and Biochemistry,
University of Oregon, Eugene, Oregon 97403, United States

Minkyoung Kwak - Department of Chemistry and
Biochemistry, University of Oregon, Eugene, Oregon 97403,
United States

Enrico Berretti - Istituto di Chimica Dei Composti
OrganoMetallici (ICCOM), Consiglio Nazionale Delle
Ricerche (CNR), S0019 Sesto Fiorentino, Firenze, Italy;
orcid.org/0000-0001-8200-4301

Vito Di Noto - Section of Chemistry for the Technology (ChemTech), Department of Industrial Engineering, University of Padova, I-35131 Padova, PD, Italy; orcid.org/0000-0002-8030-6979

Frédéric Jaouen - ICGM, Univ. Montpellier, CNRS, ENSCM, 34095 Montpellier, France; orcid.org/0000-0001-9836-3261

Lior Elbaz - Department of Chemistry and the Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 5290002, Israel; orcid.org/0000-0003-4989-5135

Dario R. Dekel - The Wolfson Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel; The Nancy & Stephen Grand Technion Energy Program (GTEP), Technion - Israel Institute of Technology, Haifa 3200003, Israel; Israel National Institute of Energy Storage (INIES), Technion - Israel Institute of Technology, Haifa 3200003, Israel; orcid.org/0000-0002-8610-0808

Piercarlo Mustarelli - Electrocatalysis and Bioelectrocatalysis Laboratory (EBLab), Department of Materials Science, University of Milano-Bicocca, 20125 Milan, MI, Italy; orcid.org/0000-0001-9954-5200

Shannon W. Boettcher - Department of Chemistry and
Biochemistry, University of Oregon, Eugene, Oregon 97403,
United States; Department of Chemical & Biomolecular
Engineering and Department of Chemistry, University of
California, Berkeley, Berkeley, California 94720, United
States; Energy Storage and Distributed Resources Division,
Lawrence Berkeley National Laboratory, Berkeley, California
94720, United States; orcid.org/0000-0001-8971-9123

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemrev.4c00466

Author Contributions

P.A., C.S. and A.L designed and coordinated the review. They edited all the parts. M. M., P. A. and C. S. wrote the sections ¹, ² and ³ took care of details, first submission and rebuttal. L.O. wrote sections ⁵,³,³ and ⁶,² related to the integration of electrocatalysts and membranes into MEAs. V.F. wrote section ³. A.F. and L.E. wrote sections ⁶, ⁶,¹. P.M., K.Y. and D.D. contributed to the 5.3.1 and 5.3.2 related to AEMs and related ionomers. G.P., E.N., V.D.N. and F.J. contributed to the sections ⁵,¹ and ⁵,² related to HER and OER electrocatalysts. E.B. and A.L. contributed to sections ⁴ and ⁵,⁴. E.B. took care of the references. A.K., G.L., L.T., M.K., and S.W.B. wrote section ⁷. P.M., F.J., A.L., S.W.B. and P.A. have homogenized the review.

Author Contributions

+M.M. and C.S. contributed equally. CRediT: Mohsin Muhyuddin conceptualization, formal analysis, methodology, writing - original draft, writing - review & editing; Carlo Santoro conceptualization, formal analysis, investigation, methodology, project administration, supervision, writing - original draft, writing - review & editing; Luigi Osmieri conceptualization, formal analysis, investigation, methodology, writing - original draft, writing - review & editing; Valerio C.A. Ficca conceptualization, data curation, writing - original draft, writing - review & editing; Ariel Friedman conceptualization, data curation, formal analysis, investigation, writing - original draft, writing - review & editing; Karam Yassin conceptualization, data curation, formal analysis, investigation, writing - original draft, writing - review & editing; Gioele Pagot formal analysis, investigation, writing - original draft, writing - review & editing; Enrico Negro conceptualization, formal analysis, investigation, writing - original draft, writing - review & editing; Anastasiia Konovalova conceptualization, data curation, formal analysis, investigation, methodology, writing - original draft, writing - review & editing; Grace A. Lindquist conceptualization, data curation, formal analysis, investigation, methodology, writing - original draft, writing - review & editing; Liam P. Twight conceptualization, data curation, formal analysis, investigation, methodology, writing - original draft, writing - review & editing; Minkyoung Kwak conceptualization, data curation, formal analysis, investigation, methodology, writing - original draft, writing - review & editing; Enrico Berretti conceptualization, data curation, formal analysis, investigation, methodology, software, supervision, validation, visualization, writing - original draft, writing - review & editing; Vito Di Noto conceptualization, formal analysis, methodology, writing - original draft, writing - review & editing; Frédéric Jaouen data curation, formal analysis, investigation, methodology, validation, writing - original draft, writing - review & editing; Lior Elbaz conceptualization, formal analysis, investigation, methodology, writing - original draft, writing - review & editing; Dario R. Dekel conceptualization, data curation, formal analysis, investigation, methodology, writing - original draft, writing - review & editing; Piercarlo Mustarelli conceptualization, data curation, formal analysis, investigation, methodology, project administration, validation, writing - original draft, writing - review & editing; Shannon W. Boettcher conceptualization, formal analysis, funding acquisition, investigation, methodology,
project administration, resources, supervision, writing - original draft, writing - review & editing; **Alessandro Lavacchi** conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, supervision, writing - original draft, writing - review & editing; **Plamen Atanassov** conceptualization, formal analysis, funding acquisition, project administration, resources, supervision, writing - original draft, writing - review & editing.

## Notes

The authors declare no competing financial interest.

## Biographies

Mohsin Muhyuddin is a Postdoctoral Researcher at the Electro-catalysis and Bioelectrocatalysis Laboratory in the Department of Materials Science at the University of Milano-Bicocca (UNIMIB), Italy. In September 2023, he was awarded a Doctoral degree in Materials Science at UNIMIB, where he conducted research under the supervision of Prof. Carlo Santoro, focusing on platinum group metal-free electrocatalysts for electrolyzers and fuel cells. He previously completed his Bachelor of Science and Master of Science degrees in Materials Science and Engineering at the Institute of Space Technology, Pakistan, graduating in 2016 and 2019, respectively. His research interests center on the development of nanostructured, cost-effective, and noble metal-free electrocatalysts aimed at advancing sustainable energy technologies.

Carlo Santoro obtained his PhD at the University of Connecticut in 2009, working on microbial fuel cells. He moved to the University of New Mexico in 2013 working on platinum-free electrocatalysts for oxygen reduction reaction and supercapacitive bioelectrochemical systems. He was an Associate Professor at UWE Bristol (UK) from 2017-2020. Following a spell as a Lecturer at the University of Manchester (2020), he joined the University of Milano-Bicocca in 2021 as an Assistant Professor, where he established the Electro-catalysis and Bioelectrocatalysis Lab (EBLab) that currently counts 1 Post Doc and 8 PhD students. In the past few years, he graduated 2 PhD students, 10 Master of Science students and over 10 Bachelor of Science students. He is currently an Associate Professor and his work focuses on the development of electrocatalysts based on platinum-group metal-free materials for electrochemical systems (mainly fuel cells and water electrolyzers), pursuing biomimetic and bioinspired approaches. He has published over 160 manuscripts (H index = 51, Scopus) and holds 2 patents.

Luigi Osmieri obtained his Ph.D. in chemical engineering in 2016 from Politecnico di Torino, Italy. From 2017 to 2020 he was a postdoctoral researcher at the National Renewable Energy Laboratory (USA). In 2020 he joined Los Alamos National Laboratory (USA), where he currently serves as a staff scientist. His main research interests are applied electrochemistry, electrocatalysis (oxygen reduction, oxygen evolution, hydrogen evolution, CO₂ reduction), electrode engineering and electrochemical diagnostics for electro-chemical energy conversion devices (fuel cells, electrolyzers), and ions separation for critical materials recovery.

Valerio Ficca earned his M.Sc. in Materials Science and Technology in 2017 at the Physics Department of Tor Vergata University of Rome and completed his Ph.D. in Materials Science and Electrochemistry in 2022 at the Chemistry Department of the same University. From 2022 to 2025, he was a Post-Doc at the Physics Department of Sapienza University of Rome, working on X-ray photoemission spectroscopy. Currently, he is a Research Scientist at the National Agency for New Technologies, Energy and Sustainable Economic

Development (ENEA), his interests focus on electrocatalysis and nanomaterials development for energy conversion and storage.

Ariel Friedman is an Assistant Research Professor at the University of Connecticut's Center for Clean Energy Engineering (C2E2). He earned his Ph.D. in Chemistry from Bar-Ilan University in 2022, where he studied electropolymerized molecular catalysts for oxygen reduction in fuel cells. He was named a Zuckerman Postdoctoral Scholar and pursued postdoctoral research at Northeastern University in Professor Mukerjee's group. His work focuses on the development of platinum-group-metal-free catalysts—particularly nickel- and copper-based—for water electrolysis and electrochemical synthesis. By combining molecular and materials design with advanced characterization techniques, he aims to improve the activity, durability, and cost-effectiveness of sustainable energy systems.

Karam Yassin received his Ph.D. in Chemical Engineering from the Technion – Israel Institute of Technology in 2023. During his studies, he conducted research at the Technion Electrochemical Energy Materials (TEEM) Laboratory, where he specialized in anion-exchange membrane (AEM) fuel cells and water electrolyzers. His work combines experimental and computational approaches to investigate the behavior and performance of various AEM-based electrochemical energy systems. In 2023, he joined the Grand Technion Energy Program at Technion – Israel Institute of Technology, where he leads the Hydrogen Technologies Research Laboratory, focusing on hydrogen production, system integration, and performance diagnostics.

Gioele Pagot is an Assistant Professor of Chemistry for Technologies at the Department of Industrial Engineering of the University of Padova, Italy. He earned his Master's degree in Chemistry and a Ph.D. in Science and Engineering of Materials and Nanostructures from the University of Padova. He was a visiting scientist at Hunter College of the City University of New York. His research focuses on the synthesis, characterization, and lab-scale testing of materials for fuel cells, electrolyzers, next-generation lithium batteries and beyond, as well as redox flow batteries. A key aspect of his work is investigating the relationships between composition, structure, thermal properties, and electrochemical performance in advanced materials for energy applications. Dr. Pagot has coauthored 75 peer-reviewed papers (H-index: 20), and 7 patents and has contributed to over 150 national and international scientific conferences.

Enrico Negro is an Associate Professor of Chemistry for Technologies at the Department of Industrial Engineering of the University of Padova, Italy. He belongs to the research group CheMaMSE. The research activity of Prof. Enrico Negro is focused on electrochemical energy conversion devices, and in particular on the various functional components adopted in the fabrication of low-temperature fuel cells. The expertise of Prof. Enrico Negro covers the value chain from the synthesis of the functional materials and spans to their physicochemical and electrochemical characterization. Finally, the functional materials are used in the fabrication of prototype devices, which are tested for performance and durability. Prof. Negro has coauthored 120 peer-reviewed papers (H-index: 39), 10 patents and has provided over 270 contributions to national and international scientific conferences.

Anastasiia Konovalova received a PhD in Environment Engineering at the University of Science and Technology KIST School in South Korea. After working as a postdoc at Simon Fraser University in Canada and the University of Oregon in the USA, she joined the Technical University of Denmark as a Tenure Track Assistant Professor at the Department of Energy Conversion and Storage. Her research focuses on the intersection of membrane and polymer
science, combining macromolecular engineering with electroanalytical chemistry to support a wide array of electrochemical energy systems.

Grace Lindquist received her Ph.D. in Chemistry from the University of Oregon in 2023, where she advanced anion exchange membrane electrolyzer technology through detailed studies of catalyst-layer phenomena, device instability, and operational strategies. She now applies her expertise in the development of advanced materials for alkaline water electrolysis. She continues to contribute to the green hydrogen field through collaborative research, innovation, and leadership. Her work has resulted in multiple impactful publications and patents aimed at accelerating the deployment of next-generation hydrogen technologies.

Liam Twight received a PhD in chemistry from the University of Oregon in 2024. His dissertation work investigated how surface reconstruction and impurity uptake by metal oxide electrocatalysts - nickel hydroxides and perovskite oxides - drive their oxygen evolution activity. He is currently a postdoctoral research associate at Los Alamos National Lab leveraging this knowledge to further understand and improve the performance and durability of alkaline electrolyzers.

Minkyoung Kwak earned her B.S from Yonsei University and completed her Ph.D. in Chemistry at the University of Oregon in 2025, where she conducted research under the supervision of Prof. Shannon W. Boettcher. Her work focused on developing durable anodes for pure-water-fed anion-exchange-membrane water electrolyzers through interfacial engineering. She will be joining the electrochemical technologies industry to further advance research and development in energy conversion systems.

Enrico Berretti is a researcher at the Institute of Chemistry of Organometallic Compounds (ICCOM) of the Italian National Research Council (CNR). He got its PhD in Chemical Sciences at the University of Florence in 2018, focusing on the electrodeposition of metals and semiconductors for industrial and energy conversion applications. During the years at ICCOM he managed to participate to various synchrotron light experiments, developing a strong experience in XRD and XAS in situ and operando characterization of electrocatalysts for fuel cells and electrolyzers. He was also involved in the CeME electron microscopy facility, at the florentine CNR area, where he worked on the application of fabrication methods to produce electrochemical devices. His actual work revolves around the synthesis of novel, resource-effective, electrocatalytical materials and architectures using a combination of chemical and physical methods, and their characterization via photon and particle beams.

Vito Di Noto is a Full Professor of Electrochemistry for Energy and Solid-State Chemistry at the University of Padova. He is a Fellow of The Electrochemical Society (ECS), Chair of the Publications Committee of the International Society of Electrochemistry and a recipient of the ECS Energy Technology Division Research Award. He coordinates the Italian Chemical Society's Interdivisional Group on Electrochemical Accumulation and Conversion of Energy and is a Past President of the Society's Electrochemistry Division. He heads the ChemTech Section and leads from 35 years the CheMaMSE research group (https://research.dii.unipd.it/chemamse/). His research focuses on materials for sustainable energy transition, including fuel cells, electrolyzers, lithium batteries and beyond, supercapacitors and solar cells. He has published over 365 peer-reviewed publications (H-index: 57), 15 book chapters, 1 book on electrocatalysis for fuel cells, and holds 31 industrial patents.

Frédéric Jaouen is a CNRS Research Director at the Institut Charles Gerhardt Montpellier (ICGM) in France. He obtained his Ph.D. in 2003 from the Royal Institute of Technology, Sweden, under the supervision of Prof. Lindbergh. Until 2011, he was then a research

associate in Professor Dodelet's group at Institut National de la Recherche Scientifique, Canada, before starting his position as a CNRS researcher in 2013. His research focuses on novel catalysts for hydrogen polymer-electrolyte fuel cells, water and CO₂ electrolysers based on proton- or anion-exchange membranes. He is most renowned for his work on single iron atom active sites for the oxygen reduction reaction. Dr. Frédéric Jaouen has coauthored ca. 140 research publications, including in renowned scientific journals such as Science, Nature Materials, Nature Catalysis. He has coauthored ca. 170 presentations, including ca. 40 invited, keynote, and plenary lectures.

Lior Elbaz is a Full Professor of Electrocatalysis and Sustainable Energy in the Department of Chemistry, at Bar-Ilan University, Israel. He is the head of the Israeli Fuel Cells Consortium (IFCC), composed of 17 leading Israeli laboratories and the director of the national hydrogen technologies center (H2Tech) in the framework of the National Institute for Sustainable Energy (NISE). He is the co-Founder of two Israeli start-up companies, developing reversible fuel cells and fuel cells for stationary power supply. Lior has been involved in the development of advanced catalysts for hydrogen technologies for the past 22 years, focusing on platinum group metal-free catalysts. He developed methodologies for the synthesis of 3D covalent aerogel frameworks of catalysts, reaching ultrahigh electrochemically active site densities, which allows for overcoming some key challenges relating to this family of catalysts. He has also designed and utilized high surface area ceramic catalysts' supports for increased system durability, and advanced electrochemical methods to allow in situ study of catalyst degradation and to decipher complex reactions in electrochemical devices.

Dario R. Dekel holds a M.Sc. in Chemical Engineering, and Ph.D. and MBA from Technion – Israel Institute of Technology. In 1998 he joined Rafael Ltd., where he led 50 researchers to develop high-temperature batteries. In 2007 Prof. Dekel cofounded CellEra (today Hydrolite), where as VP-R&D he led 15 researchers to pioneer and develop the Anion-Exchange Membrane Fuel Cell Technology. In 2015 Prof. Dekel joined the Technion, where he currently heads the TEEM Lab (Technion Electrochemical Energy based on Membranes), leading one of the largest worldwide research groups entirely focused on AEMs, AEMFCs, and AEMWES. Prof. Dekel is a full Professor at the Wolfson Department of Chemical Engineering, and the Director of the Grand Technion Energy Program (GTEP). Prof. Dekel holds >140 publications and >30 granted patents on battery, membrane, and fuel cell technologies.

Piercarlo Mustarelli is Full Professor of Physical Chemistry and Electrochemistry and Director of the Department of Materials Science of the University of Milano Bicocca, Italy. He has been the founder and past-President on the Italian Association for Electrochemical Energy Storage (GISEL). Recently he has been ranked among the top 2% scientists in the world for the period 1999-2019 (analysis performed by Stanford University, USA). His main research interests include: (i) solid-state NMR applied to disordered materials; (ii) electrolyte materials for electrochemical energy storage and conversion; (iii) green hydrogen production. He published more than 300 papers, with about 12000 citations and an h-index of 55 (Scopus).

Shannon W. Boettcher is the Vermeulen Professor in the Departments of Chemical and Biomolecular Engineering and Chemistry at the University of California, Berkeley, and the Deputy Director of the Energy Storage and Distributed Resources Division at Lawrence Berkeley National Laboratory. From 2010-2023, he was a Professor of Chemistry at the University of Oregon, where he founded the Oregon
Center for Electrochemistry and, along with Paul Kempler, the Nation's first master's program in Electrochemical Technology. His research spans basic understanding of interfacial processes and transport in electrochemical systems and applying those in the design of next-generation materials, components, and devices for electrochemical technology, including for green hydrogen production by water electrolysis. In 2023 he was named the Blavatnik National Award Laureat in Chemistry.

Alessandro Lavacchi is a Research Director at the Institute of Organometallic Chemistry of the National Research Council of Italy (ICCOM-CNR), where he heads both the Electron Microscopy Facility and the Technology Transfer Center. His research is centered on the development and advanced characterization of nanomaterials for electrochemical energy conversion and storage, with particular emphasis on alkaline membrane electrolysis and the electrochemical valorization of biomass. He has coauthored over 150 scientific publications, primarily in the field of energy materials, and is the coauthor of the monograph Nanotechnology in Electrocatalysis for Energy.

Plamen Atanassov graduated University of Sofia and obtained PhD from Bulgarian Academy of Sciences. He moved to the US in 1992 and joined University of New Mexico (UNM), where he founded UNM Center for Emerging Energy Technologies, was Associate Dean for Research of Engineering and director of UNM Center for Micro-Engineered Materials. In 2018 Atanassov joined University of California Irvine (UCI) as a Chancellor's Professor. His current research is focused on new electrocatalysts for fuel cells, electrolyzers, CO₂ electroreduction and valorization, as well as ammonia and urea electrosynthesis. He supervised 50 doctoral dissertations and 30 postdoctoral fellows. He is on the leadership team of Alliance for Renewable Clean Hydrogen Energy Systems (ARCHES) — the California DOE Hydrogen Hub. Atanassov is a fellow of National Academy of Inventors, The Electrochemical Society, and the International Society of Electrochemistry, of which he is currently president.

## ACKNOWLEDGMENTS

C.S. would like to acknowledge the European Union – NextGeneration EU from the Italian Ministry of Environment and Energy Security POR H2 AdP MMES/ENEA with involvement of CNR and RSE, PNRR - Mission 2, Component 2, Investment 3.5 "Ricerca e sviluppo sull'idrogeno" under the ENEA – UNIMIB agreement (Procedure 1.1.3 PNRR POR H2). C.S., M.M., P.M., E.N., G. P. and V.D.N. would like to acknowledge the National recovery and resilience Plan (PNRR), Mission 2 "Green Revolution and Ecological Transition", Component 2 "Renewable Energy, Hydrogen, Network and Sustainable Mobility", Investment 3.5 "Hydrogen Research and Development", European Union – Next Generation EU – Ministry of Ecological Transition, project AMBITION. This research has also received funding from (a) the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 - Call for tender No. 3138 of December 16, 2021 of the Italian Ministry of University and Research, funded by the European Union - NextGenerationEU [Award Number: CNMS named MOST, Concession Decree No. 1033 of June 17, 2022, adopted by the Italian Ministry of University and Research, Spoke 14 "Hydrogen and New Fuels"); (b) project "Graphene Flagship Core Project 3" n° H2020-SGA-FET-GRAPHENE-2019 within the "Graphene Flagship" of Horizon 2020 (Project: 881603); (c) Project "DURALYS - DURABLE, Scalable, and Recyclable Components

and Cell Designs for Next Generation Alkaline Exchange Membrane Water Electrolysis" funded under the German-Italian Joint call for proposals on "Green Hydrogen Research: A Collaboration to Empower Tomorrow's Energy" within the framework of the collaboration between the German Federal Ministry of Education and Research (BMBF), the Italian Ministry of Foreign Affairs and International Cooperation (MAECI) and the Italian Ministry of University and Research (MUR) (d) PRIN2022 project "Alkaline membrane-electrode assemblies for fuel cells (ALEF)" (Prot. 2022PYXHYL) of the Italian Ministry of University and Research (MUR)." This research has also received funding from the Nancy & Stephen Grand Technion Energy Program (GTEP); and the European Union's Horizon 2021 research and innovation program, under grant agreement 101071111 and the Clean Hydrogen Partnership, under grant agreement 101101479. Luigi Osmieri would like to acknowledge support by the Laboratory Directed Research and Development program of Los Alamos National Laboratory under project number 20210953PRD3. This work was authored in part by Los Alamos National Laboratory operated by Triad National Security, LLC under U.S. DOE contract no. 89233218CNA000001. The views expressed in the review article do not necessarily represent the views of the DOE or the U.S. Government.

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