Recent Progress in Advanced Electrocatalyst Design for Acidic Oxygen Evolution Reaction

Abstract

more promising approach to generating H 2 with high purity. Electrochemical water splitting, driven by renewable electricity, proceeds through hydrogen evolution reaction (HER) at the cathode side and oxygen evolution reaction (OER) at the anode side of an electrochemical cell. [7,10] The HER, a two-electron transfer process, is the simplest electrochemical reaction and relatively easy to occur. On the contrary, OER involving four-electron transfer is a much more complicated multistep reaction. A considerably large overpotential is thus added to the actual water splitting process because of the complexity of OER, leading to sluggish reaction kinetics and distinctly reducing the energy efficiency. [11] Therefore, efficient electrocatalysts with high activity and durable stability are needed to overcome the high energy barrier.OER is an electrochemical process of producing molecular O 2 through a series of proton-electron coupled steps. [12][13][14] Depending on the pH of the electrolytes, the reaction pathways of producing O 2 are totally different. In alkaline electrolytes, hydroxyl groups (OH − ) are oxidized and converted to H 2 O and O 2 . In acidic media, two water molecules (H 2 O) are oxidized generating four protons (H + ) and O 2 . Compared to acidic OER, alkaline OER possesses more favorable reaction kinetics because of the abundant hydroxyl groups in the electrolyte. For acidic OER, however, the break of the strong covalent OH bond of H 2 O requires high energy thus leading to more sluggish kinetics. Another advantage for alkaline OER lies in the wide range of catalysts such as transition metal (e.g., Ni, Co, and Fe) based oxides and hydroxides as well as carbon materials. [15][16][17][18][19][20][21][22] The electrocatalysts for acidic OER are mostly related to iridium (Ir) and ruthenium (Ru) based materials currently. [19,[23][24][25][26][27][28][29][30][31] In spite of the more favorable kinetics and wide range of catalysts for alkaline OER, however, acidic OER is more preferable than alkaline OER because of the diversified advantages of proton exchange membrane water electrolyzers over alkaline water electrolyzers. [19,23] Proton exchange membrane (PEM) is an acidic solid polymer electrolyte membrane with much smaller gas crossover than that of the alkaline solid polymer, which can ensure a larger load range and much safer operation of an acidic electrolyzer by largely avoiding forming H 2 /O 2 mixture. [32] Furthermore, the fully developed PEMs can provide high proton conductivity, resulting in low Ohmic loss and high current density. [32] Proton exchange membrane (PEM) water electrolyzers hold great significance for renewable energy storage and conversion. The acidic oxygen evolution reaction (OER) is one of the main roadblocks that hinder the practical application of PEM water electrolyzers. Highly active, cost-effective, and durable electrocatalysts are indispensable for lowering the high kinetic barrier of OER to achieve boosted reaction kinetics. To date, a wide spectrum of advanced electroca...

Structural Variations of Metal Oxide‐Based Electrocatalysts for Oxygen Evolution Reaction

“…In the early stage of research, the catalytic performance of precious metal oxides, such as RuO 2 and IrO 2 , are widely investigated. [60,61] They are considered as benchmark OER electrocatalysts because of superior activity under both acidic and alkaline medium. However, the high cost of these noble elements impedes their large-scale commercialization.…”

Late Transition Metal Doped MXenes Showing Superb Bifunctional Electrocatalytic Activities for Water Splitting via Distinctive Mechanistic Pathways

“…The reason behind hydrogen that has not been a widely used fuel is the lack of efficient and economical catalysts. The half‐reactions of HER [ 5,11 ] and OER [ 12–17 ] which take place at cathodic and anodic sites, respectively, can be seen simultaneously in electrocatalytic water splitting. [ 18–25 ] Presently, the promising catalysts for OER and HER are Ru and Ir‐based metal oxides and Pt‐based electrocatalysts, respectively.…”

Multi‐Sites Electrocatalysis in High‐Entropy Alloys

“…[ 88,93–95 ] Most acidic OER catalysts are based on binary or ternary alloys of non‐noble metals and precious metals (such as Ir and Ru). [ 89,96 ] To improve activity, stability and reduce costs, multi‐component nano‐alloys are needed to further enhance the catalytic activity. Nanostructured HEAs contain a high specific surface area and more active sites, providing many possibilities to adjust the electronic properties and catalytic activity of the alloy surface.…”