PSF-based membranes possess several properties that are advantages for AEMWE applications, namely, they are chemically and mechanically stable under highly basic conditions [97] and they are also inexpensive and easy to synthesize. Xiao et al. conducted an AEMWE cell test using a self-cross-linking quaternary ammonia polysulfone (xQAPS) AEM and non-noble metal based electrodes [89]. The Xiao group developed the xQAPS based membrane in the previous study and observed only 3% swelling at 90°C while maintaining an effective OH⁻ mobility (15 and 43 mS cm⁻² at 20 and 90°C, respectively) in liquid water after self-cross-linking [98]. MEAs for the OER and HER were fabricated by pressing the self-cross-linking xQAPS membrane between Ni/Fe-coated Ni foam and Ni/Mo-coated stainless steel fiber felt, respectively, at 80°C for 2 min at a pressure of 2 MPa, and were subsequently tested in an AEMWE cell configuration [89]. The cell exhibited a water splitting current density of approximately 250 mA cm⁻² at 1.8 V and 70°C. Moreover, the voltage was stable at approximately 1.8 V over 8 h of constant current density operation at approximately 400 mA cm⁻², with only a <3% increase in voltage over this period. The authors expected that the MEA performance could be further improved by reducing the membrane/electrode contact resistance and by using a better cathode catalyst [89].

Parrondo et al. studied the MEA performance of PSF-based AEMs during AEMWE cell operation and analyzed the mechanism of AEM degradation. The AEMs were functionalized with different groups: quaternary benzyl trimethylammonium (PSF-TMA⁺OH⁻), quaternary benzyl quinuclidum (PSF-ABCO⁺OH⁻), and quaternary benzyl 1- methylimidazolium (PSF-1 M⁺OH⁻) [90]. The ionic conductivities of PSF-TMA⁺OH⁻, PSF-ABCO⁺OH⁻, and PSF-1M⁺OH⁻ AEM at 50°C in liquid water were estimated to be 17, 14, and 13 mS cm⁻¹, respectively, with all AEMs exhibiting the same theoretical IEC of 1.8 mmol g⁻¹. The MEAs were fabricated with lead ruthenate pyrochlore and Pt black catalysts for the OER and HER, respectively, and the water electrolysis was performed using ultra pure water at 50°C. The MEA utilizing the PSF-TMA⁺OH⁻ membrane exhibited the highest current density, which was estimated to be about approximately 350 mA cm⁻² at 1.8 V from its polarization curve. By contrast, the MEAs based on PSF-ABCO⁺OH⁻ and PSF-1M⁺ OH⁻ membranes both exhibited a current density of approximately 200 mA cm⁻² at 1.8 V, as shown in Fig. 3. Three repeat polarization curve measurements showed short-term performance degradation in the PSF-TMA⁺OH⁻-membrane based MEA, with the high-frequency resistance (HFR) gradually increasing from 0.60 to 1.67 Ω cm². The authors proved experimentally that the short-term performance degradation is caused mainly by the formation of carbonate anions as a result of CO₂ intrusion. They also performed long-term operation tests using a constant current density of 200 mA cm⁻² at 50°C, and the MEA based on the PSF-TMA⁺OH⁻ membrane exhibited the best performance. The cell voltage increased from 1.6 to 2.4 V over 6 h of cell operation, which was attributed to the degradation of the membrane backbone using nuclear magnetic resonance (NMR) spectroscopy. No functional group degradation was observed in this system [90].

PPO displays a better stability in alkaline media than PSF [99] and can be easily functionalized with various cationic groups [100-102]. These properties make it a good candidate for anion conducting polymer electrolytes. In order to increase the ionic conductivity and stability of AEMs under high pH conditions, Parrondo et al. developed PPO-based AEMs functionalized with TMA⁺ or ABCO⁺ groups and compared their initial performance and cell voltages during constant current operation at 100 mA cm⁻² [91]. The authors also analyzed in detail the degradation mechanism of these AEMs during cell operation. The TMA⁺- and ABCO⁺-functionalized PPO membranes exhibited higher ionic conductivities than PSF [90] at 50°C in water, with values of approximately 44 (theoretical IEC of 2.1 mmol g⁻¹) and 42 mS cm⁻¹ (theoretical IEC of 1.9 mmol g⁻¹), respectively [103]. For the single cell study, the MEAs were prepared by assembling a membrane with gas diffusion electrodes coated with IrO₂ and Pt black electrocatalysts for the OER and HER, respectively. The AEMWE cell was operated with ultrapure water at 50°C. The higher ionic conductivity of the PPO-TMA⁺OH⁻ membrane resulted in a higher initial performance (approximately 230 mA cm⁻² at 1.8 V) compared to PPO-ABCO⁺OH⁻ membrane (approximately 60 mA cm⁻² at 1.8 V), as shown in Fig. 4a. The cell voltage increased in comparison with the initial cell voltage during the continued constant current operation, as shown in Fig. 4b. Additionally, the HFR and chargetransfer resistance of both cells were greater after cell operation when compared to the initial values, as shown in Fig. 4c. Although no degradation in AEM functional groups was observed using NMR spectroscopy, catalyst particle loss was detected. This loss was attributed to poor adherence that was caused by the loss of binder in the electrode. Thus, the binder loss is the cause for the increased HFR, charge-transfer resistance, and cell voltage during operation [91].

Aili et al. measured the initial performance of AEMWE cells containing three different KOH-doped PBI membranes: untreated PBI (linear PBI), cross-linked PBI, and thermally treated PBI (thermally cured PBI). Moreover, the authors analyzed the degradation mechanism of the PBI membranes by measuring their thermo-oxidative stabilities [92]. The neutral PBI-based AEMs possess high chemical, thermal (T_g = 425-436°C), and mechanical stabilities [51] and can be rendered anion-conducting by doping with KOH. Unlike the membranes that utilize quaternary ammonium and imidazolium groups for anion conduction, PBI-based AEMs do not display degradation of cationic functional groups mediated by the hydroxyl ion attack via direct nucleophilic displacement and Hofmann-elimination reactions [104]. The cross-linked and cured cross-linked membranes were prepared by thermally treating a PBI membrane at 280 and 350°C, respectively, under N₂ atmosphere, with a cross-linking agent, p-xylene dibromide, added to control the degree of cross-linking to 13%. The MEAs were fabricated by sandwiching the membranes between polished nickel plates and the AEMWE cell tests were performed at 80°C using a 30 wt.% KOH solution. Lastly, membrane aging tests were carried out by storing the membranes in a 25 wt.% KOH solution at 85°C for 116 days and observing the polymer degradation. The conductivities of a commercial membrane (Zirfon^®) and the linear, cross-l inked, and thermal ly cured PBI membranes were determined to be approximately 77, 24, 20, and 26 mS cm⁻¹, respectively. In contrast to the results of conductivity tests, the MEA based on the cross-linked membrane exhibited the best single cell performance (approximately 63 mA cm⁻² at 1.8 V), while the MEAs based on the commercial membrane, linear PBI membrane, and thermally cured PBI membrane exhibited current densities of approximately 14, 14, and 27 mA cm⁻², respectively, at the same voltage, as shown in Fig. 5a. During the membrane aging tests, polymer degradation was found to occur via backbone hydrolysis on the C2 position of benzimidazole, with the consequent molecular weight decrease during water electrolysis resulting in performance decline (Fig. 5b). Modification of the PBI-based polymer backbone was suggested as a way of further improving its long term stability [92].

Diaz et al. conducted AEMWE cell tests using linear (untreated) and cross-linked ABPBI (poly[2,5-benzimidazole]) membranes doped with a KOH solution [50]. The cross-linking process was adopted to produce membranes with improved chemical, thermal, and mechanical stabilities relative to those of the untreated membrane. The cross-linked ABPBI membrane was prepared by benzoxazine polymerization followed by thermal treatment of the membrane at 180°C for 10 h. The MEAs were fabricated by attaching Ni-foam electrodes directly onto the membranes for both the anode and cathode. The performance of each MEA was evaluated at 50 and 70°C in an AEMWE cell. The cross-linked membrane exhibited a higher ionic conductivity (18 mS cm⁻¹) than the non-cross-linked linear membrane (17 mS cm⁻¹). The MEAs based on the cross-linked and linear ABPBI membranes exhibited similar current densities of approximately 53 mA cm⁻² at 1.8 V during cell operation at 50°C in a 1.9 M KOH solution. Both the cross-linked (approximately 89 mA cm⁻² at 1.8 V) and linear (approximately 80 mA cm⁻² at 1.8 V) ABPBI membrane based MEAs exhibited significantly improved performance at 70°C in a 3.0 M KOH solution when compared to the MEA based on the commercial membrane (Zirfon^®) (approximately 47 mA cm⁻² at 1.8 V).

As a result of the difficulties in achieving both high ionic conductivity and chemical stability in AEMs through the introduction of different functional groups, reinforced composite membranes (RCMs) [96] and inorganic membranes [21] have been developed as alternatives with the potential to attain both ionic conductivity and stability, and subsequently tested in AEMWE cells. Wu et al. evaluated the performance of an RCM-based MEA in an AEMWE single cell, where the RCM consisted of a porous PTFE-supported composite membrane filled with a polymethacrylate quaternary ammonium (QPDTB) ionomer [96]. The RCM-based AEM was ultra-thin (30 μm) and prepared by filling the PTFE membrane pores with the QPDTB ionomer, which is an anion conductor developed by Wu et al. in a previous study [105]. The RCM exhibited an ionic conductivity of 34 mS cm⁻¹ at 50°C. The MEA was fabricated by spraying a catalyst slurry, consisting of the QPDTB ionomer mixed with Cu_0.6Mn_0.3Co_2.1O₄ and Pt/C catalyst for the OER and HER, respectively, onto the RCM. The AEMWE cell stability was tested at 22°C using pure water. The current density of the MEA based on the RCM was approximately 200 mA cm⁻² at 1.8 V, as shown in Fig. 6a. It is difficult to compare the durability of this membrane with those of other developed membranes as a result of the low operating temperature (22°C) employed in the stability test. Nevertheless, the durability test revealed a performance loss of 0.3% at 100 mA cm⁻² over 120 h of AEMWE cell operation, as shown in Fig. 6b [96].

Inorganic material based layered double hydroxides (LDHs) have recently been developed for AEMWE cells and have displayed excellent stabilities and acceptable OH⁻ ion conductivities during single cell testing in alkaline media [106]. The use LDHs can be advantageous not only because of their good chemical stability, which is derived from the absence of functional groups, but also because the typical synthesis of LDH membranes does not require the use of toxic or carcinogenic reagents [21,106].

Zeng and Zhao monitored the long-term performance of an AEMWE cell containing a low-cost integrated inorganic MEA composed of an Mg-Al LDH membrane. This membrane was designed to overcome the challenges usually associated with AEMs, i.e., low ionic conductivity, chemical instability, and chemical toxicity of the corresponding synthetic process [21]. The Mg-Al LDH membrane exhibited superior stability in an alkaline environment and high OH⁻ ion conductivity (7.7 mS cm⁻¹) under high relative humidity (98%) at 60°C. The MEAs were prepared using the Mg-Al LDH membrane and commercial OER (CuCoO_x) and HER (NiM/CeO₂-La₂O₃/C) catalysts from Acta SpA. Polarization curves for the AEMWE cells fabricated with MEAs of varying thickness were measured at 50°C in 0.1 M NaOH solution, revealing current densities of approximately 60, 38, and 26 mA cm⁻² at 1.8 V for the 300-, 500- and 700-μm-thick MEAs, respectively. The decrease in water splitting current density with increasing membrane thickness was caused by the increased ionic resistance, which was determined to be 3.25, 5.11, and 7.39 Ω cm² for the 300-, 500-, and 700-μm-thick MEAs, respectively, as shown in Fig. 7 [21].

The OER kinetics are more sluggish than the HER kinetics and as a consequence, the performance of water electrolysis depends strongly on the OER (a four inner sphere electron transfer reaction). The OER activities of electrocatalysts are generally greater in higher pH solutions than in acidic or low pH solutions. As a result, non-noble metal oxides can be used as catalysts in AEMWE. Furthermore, water-electrolysis systems operating under alkaline conditions are more durable and stable than those operating under acidic conditions. For example, the OER catalysts iridium oxide and ruthenium oxide exhibit dissolution rates during potential scanning that are ca. 700 and 600 times lower, respectively, in a 0.05 M NaOH electrolyte than in acidic media [108]. Metal oxides are also often stable at high pH operating conditions as a result of the formation of a surface passivation layer [109].

Parrondo et al. studied pyrochlore-structured metal oxides, which are highly active and stable OER catalysts. The authors performed AEMWE cell tests with Pb₂Ru₂O_6.5 and Bi_2.4Ru_1.6O₇ pyrochlores, which exhibit the highest OER mass and specific activities of all pyrochlore electrocatalysts [52]. These pyrochlore catalysts exhibited electronic conductivities of 120 ± 30 and 63 ± 5 S cm⁻¹ for Pb₂Ru₂O_6.5 and Bi_2.4Ru_1.6O₇, respectively, and their OER activities in the AEMWE cell were comparable with that of IrO₂ (also tested in this study) [52]. In a half-cell evaluation, the OER mass activities of Pb₂Ru₂O_6.5 and Bi_2.4Ru_1.6O₇ pyrochlores were determined to be 202 and 10 Ag⁻¹, respectively, at 1.5 V_RHE. The MEAs were subsequently fabricated by spraying catalyst ink (IrO₂ or the pyrochlores and Pt black for the OER and HER, respectively, mixed with an AEM binder) on a commercial AEM membrane (A201, Tokuyama Co.). The AEMWE cell was operated at 50°C and exhibited a current density of approximately 250, 500, and 400 mA cm⁻² at 1.8 V_cell when employing IrO₂, Pb₂Ru₂O_6.5, and Bi_2.4Ru_1.6O₇ as the OER catalyst, respectively, as shown in Fig. 8a. The long-term AEMWE cell performance was tested using Pb₂Ru₂O_6.5, which showed the best initial performance. The cell exhibited a constant voltage of 1.75 V over 200 h of constant current operation at 200 mA cm⁻² [52]. The catalytic activity showed a strong relationship with the strength of bonding between the B-site cation of the pyrochlore and the reaction intermediate species formed during the OER, as evidenced by the reaction paths studied at the surfaces of the pyrochlore.

Seetharaman et al. studied a graphene oxide (GO) modified NiO electrode as an OER catalyst with enhanced electron conductivity and catalytic activity, and evaluated its initial performance and durability in an AEMWE cell [93]. A ternary Ni alloy was electro-chemically deposited onto a nickel foam, followed by heat treatment at 350-400°C to produce a NiO foam catalyst for the OER. GO was synthesized separately from purified graphite using the Hummers method and painted onto the NiO foam. A Selemion^TM AMV membrane (Asahi Glass Co. Ltd.) was used to prepare the MEAs and sandwiched between the Ni foam electrodes. The GO-coated NiO foam catalyst exhibited the highest OER current density when compared to the Ni foam and NiO foam. The AEMWE cell tests were performed using deionized water and various concentrations of alkaline reactant solution (0-5.36 M) at different operating temperatures (30-80°C). Increasing the concentration of the alkaline solution improved the initial cell performance, with current densities of approximately 65 mA cm⁻² and 140 mA cm⁻² observed for pure water and a 5.36 M electrolyte solution, respectively, at 1.8 V and 30°C. In addition, the current density increased in the 5.36 M KOH electrolyte at 1.8 V from 100 to 380 mA cm⁻² when the operating temperature was increased from 30 to 80°C. The stability tests were performed over 24 h using the 5.36 M KOH solution at a constant voltage of 1.9 V. A stable current density of approximately 513 mA cm⁻² was obtained at 80 °C when the MEA with a GO-coated electrode was employed, which represents almost a two-fold increase relative to the current density obtained using the MEA with uncoated electrode [93]. Application of the GO coating reduced the electrode resistance and enhanced the contact between the membrane and the electrodes, which in turn resulted in improved cell performance.

Wu et al. used nanoparticles of a Li_xCo_3-xO₄ ternary metal oxide as a non-precious metal OER catalyst [94]. Lithium shows the highest activity amongst the various binary cobalt oxides M_zCo_3-xO₄ (M = Li, Ni, and Cu) [110]. The catalytic activity was evaluated as a function of the lithium molar content and the catalyst that exhibited the highest electronic conductivity was tested in an AEMWE cell [94]. The Li_xCo_3-xO₄ nanoparticles were prepared by thermal decomposition of a mixture of LiCl·2H₂O and CoCl₂·6H₂O in deionized water and ethanol. Incorporating Li in Co₃O₄ increased its electronic conductivity by at least ~20-fold; however, no specific trend was observed on increasing the Li content in the Li_xCo_3-xO₄ nanoparticles. Li_0.21Co_2.79O₄, which exhibited the best electronic conductivity (2.1 s cm⁻¹) and OER current density (approximately 850 mA cm⁻¹ at 650 mV_Hg/HgO) was tested in a single cell. The MEAs were fabricated by spraying the catalyst ink, i.e., a mixture of the ionomer and the catalyst, on the AEMs (polymethacrylate quaternary ammonium OH⁻ membrane). A 2.5 mg cm⁻² loading of Li_0.21Co_2.79O₄ was used for the OER, while 2 mg cm⁻² loading of Ni powder was used for the HER. The AEMWE cell exhibited a current density of approximately 40 mA cm⁻² at 1.8 V using a deionized water feed at 45°C [94].

Wu et al. evaluated Cu_xMn_0.9-xCo_2.1O₄ as a bifunctional catalyst by employing varying molar ratios of Mn to prepare Mn- and Cu-doped cobaltite oxide and to improve its OER and oxygen reduction reaction (ORR) catalytic activity. Cu_0.6Mn_0.3Co_0.21O₄ exhibited the best OER performance and was therefore used to prepare MEAs, which were subsequently tested in a single cell configuration [95]. The catalyst was synthesized by addition of NaNO₃ into a solution of CuCl₂·2H₂O, MnCl₂·4H₂O, and CoCl₂·6H₂O, followed by drying at 70°C, and annealing at 400-500°C in air. The MEAs were prepared by spraying the catalyst slurry onto both sides of the membrane. In this case, the slurry was comprised of a mixture of the ionomer and the developed catalyst and Pt/C catalyst for the OER and HER, respectively. When using the developed OER catalyst, a current density of approximately 60 mA cm⁻² at 1.8 V was obtained with deionized water at 40°C [95].

The influence of non-PGM HER catalyst loading on AEMWE cell performance was investigated by Pavel et al. [107]. The anode-catalyst loading was set to 36 mg cm⁻² and the cell performance was examined as a function of increasing cathode-catalyst loading. The MEAs were fabricated using a commercial AEM (A201, Tokuyama Co.) with CuCoO_x (Acta 3030, Acta SpA) and Ni/(CeO₂-La₂O₃)/C (Acta 4030, Acta SpA) as the OER (anode) and HER (cathode) catalyst, respectively. The catalyst layers were prepared by mixing the corresponding catalyst with a PTFE binder. The mixtures were subsequently spread onto a porous Ni foam and sprayed onto a carbon cloth to prepare the anode and cathode, respectively. The HER-catalyst loading was varied from 0.6 to 7.4 mg cm⁻² and the current density exhibited by the AEMWE cell increased with increasing HER catalyst loading from approximately 40 to 300 mA cm⁻² at 1.8 V, as shown in Fig. 9 [107].

Ahn et al. constructed an efficient HER electrode using an ultra-low Pt loading (1.84 μg_Pt cm⁻²) and the MEA utilizing this electrode was tested in an AEMWE cell [63]. Typically, few mg cm⁻² [19] of Pt black have been used for HER catalysis in other studies. In this study, by contrast, electrodeposition was used to decrease the Pt loading by three orders of magnitude. Ni was first electrodeposited on carbon paper (CP) in a 0.5 M NiCl₂ solution by holding the CP at −0.95 V_SCE for 50 s. Pt was subsequently coated on the Ni-coated CP using galvanic displacement in a 1 mM K₂PtCl₆ solution at various concentrations of HCl for 180 s. The electrodeposition and galvanic displacement reactions produced ultra-thin catalyst layers on the CP gas diffusion layers (GDLs), which displayed a small ohmic resistance, fast mass transport, and high catalyst utilization. In half-cell measurements, the Pt-Ni/CP electrode exhibited higher HER activity (−73.7 mA cm_geo⁻² at −0.70 V_RHE) than the bare Ni/CP electrode (−59.4 mA cm_geo⁻² at −0.70 V_RHE). For the single-cell study, the MEAs were prepared by sandwiching a commercial membrane (A201, Tokuyama Co.) between Ni/CP (OER electrode) and Pt-Ni/CP (HER electrode). The AEMWE cell performance was evaluated at 50°C with a 1.0 M KOH solution supplied only to the cathode. When using the Pt-Ni/CP as the HER electrode, the current densities at 1.8 V were approximately 180 and 210 mA cm⁻² at 50 and 70°C, respectively, and these values are by 100 and 80 mA cm⁻² higher than the current density achieved using the Ni/CP electrode [63].