Anion exchange membrane water electrolyzer (AEMWE) is a promising technology in water electrolysis for the production of green hydrogen. Unlike conventional alkaline water electrolyzers (AWE), AEMWEs utilize mild alkaline conditions and anion exchange membranes as separators and ionic conductors, respectively. However, the largest merit of AEMWE is the utilization of lowcost transition metals as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Achieving excellent AEMWE performance using highly active and robust transition metal electrocatalysts is desired for lowering the fabrication cost and realizing the successful commercialization of AEMWE. Water splitting efficiency in the AEMWE largely depends on the kinetics of the HER and OER catalysts. Such a condition requires proper understanding of the reaction mechanisms, careful design and optimization of catalyst materials, and maintenance of catalyst durability under AEMWE conditions during long-term operation. This review discussed recent transition metal HER, OER, and bifunctional water splitting electrocatalysts applied within the actual AEMWE membrane electrode assembly (MEA), where their influence toward the electrolyzer cell performance is highlighted. Additionally, the MEA fabrication methods are briefly addressed.From the review, potential electrocatalyst materials and remaining challenges are identified to provide for future improvements on the AEMWE system.

Graphene oxide (GO) and its acid-functionalized form are known to be effective in enhancing the proton transport properties of phosphoric-acid doped polybenzimidazole (PA-doped PBI) membranes utilized in high-temperature proton exchange membrane fuel cells (HTPEMFC) owing to the presence of proton-conducting functional groups. This work aims to provide a comparison between the different effects of GO with the sulfonated GO (SGO) and phosphonated GO (PGO) on the properties of PA-doped PBI, with emphasis given on proton conductivity to understand which functional groups are suitable for proton transfer under high temperature and anhydrous conditions. Each filler was synthesized following existing methods and introduced into PBI at loadings of 0.25, 0.5, and 1 wt.%. Characterizations were carried out on the overall thermal stability, acid doping level (ADL), dimensional swelling, and proton conductivity. SGO and PGO-containing PBI exhibit better conductivity than those with GO at 180 °C under anhydrous conditions, despite a slight reduction in ADL. PBI with 0.5 wt.% SGO exhibits the highest conductivity at 23.8 mS/cm, followed by PBI with 0.5 wt.% PGO at 19.6 mS/cm. However, the membrane with PGO required a smaller activation energy for proton conduction, thus less energy was needed to initiate fast proton transfer. Additionally, the PGO-containing membrane also displayed an advantage in its thermal stability aspect. Therefore, considering these properties, it is shown that PGO is a potential filler for improving PBI properties for HTPEMFC applications.

Green hydrogen production via electrocatalytic water splitting paves the way for renewable, clean, and sustainable hydrogen (H2) generation. H2 gas is produced from the cathodic hydrogen evolution reaction (HER), where the reaction is catalyzed primarily from Pt-based catalysts under both acidic and alkaline environments. Lowering the loading of Pt and the search for alternative active catalysts for HER is still an ongoing challenge. Two-dimensional MXenes are effective supports to stabilize and homogenously distribute HER-active electrocatalysts to boost the HER performance. Factors involved in the effectiveness of MXenes for their role in HER include transition metal types and termination groups. Recently, tailoring the conditions during the synthesis of MXenes has made it possible to tune the morphology of MXenes from multilayers to few layers (delaminated), formation of porous MXenes, and those with unique crumpled and rolled structures. Changing the morphology of MXenes alters the surface area, exposed active sites and accessibility of electrolyte materials/ions to these active sites. This review provides insight into the effects of varying morphology of MXenes towards the electrocatalytic HER activity of the MXene itself and MXene composites/hybrids with HER-active catalysts. Synthesis methods to obtain the different MXene morphologies are also summarized.

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