Recent developments on transition metal–based electrocatalysts for application in anion exchange membrane water electrolysis

Abstract

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.

An overview of co‐catalysts on metal oxides for photocatalytic water splitting

“…Compared with bulk Bi 2 O 3 , the metallic Bi obtained from quantum-sized Bi 2 O 3 has a smaller size (2-3 nm) and larger specific surface area, is in closer contact with TiO 2 , and is easier to reduce. 53 Only Bi species with small particles can increase the photocatalytic activity of semiconductors for H 2 generation. 54,55 However, the small size of co-catalysts decreases their catalytic efficacy.…”

Synergistic effect to enhance hydrogen generation of Fe 2 O 3 /Ce 0. 8 Sm 0 . 1 Gd 0 .

“…These methods always consist of complex processes 9 and require post‐hydrogen purification, 10 thus improving the investment and bringing about enormous CO 2 emissions 11 . Under such context, technologies for hydrogen generation with no CO 2 emission have attracted increasing interest as alternatives to the traditional hydrogen generation methods 12,13 . Several pathways using renewable energy as sources to generate CO x ‐free hydrogen, including photocatalytic water splitting 14 and electrochemical water splitting, 15,16 are proposed and received widespread interest.…”

“…11 Under such context, technologies for hydrogen generation with no CO 2 emission have attracted increasing interest as alternatives to the traditional hydrogen generation methods. 12,13 Several pathways using renewable energy as sources to generate CO x -free hydrogen, including photocatalytic water splitting 14 and electrochemical water splitting, 15,16 are proposed and received widespread interest. However, the photocatalytic and electrochemical processes require rare metals as catalysts 17,18 and suffer from low efficiency.…”

RuO 2 as promoter in Pt‐RuO 2 ‐ nanostructures/carbon composite, a pH ‐universal catalyst for hydrogen evolution/oxidation reactions

“…Like acid media, Pt is also a good catalyst for alkaline HOR/HER. However, a large amount of Pt is required for AEMFCs or AEMELs due to its sluggish HOR/HER activity in an alkaline environment 25,26 . The HOR or HER on Pt catalyst is 2 to 3 times lower in base than its acid media activity 27 .…”

“…However, a large amount of Pt is required for AEMFCs or AEMELs due to its sluggish HOR/HER activity in an alkaline environment. 25,26 The HOR or HER on Pt catalyst is 2 to 3 times lower in base than its acid media activity. 27 Thus, highly active HOR/HER catalysts for alkaline media are essential for their application in AEMFCs and AEMELs.…”