Electrocatalysts Based on Transition Metal Borides and Borates for the Oxygen Evolution Reaction

Abstract

Electrochemical water splitting is a clean and sustainable process for hydrogen production on a large scale as the electrical power required can be obtained from various renewable energy resources. The key challenge in electrochemical water splitting process is to develop low‐cost electrocatalysts with high catalytic activity for the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode. OER is the most important half‐reaction involved in water splitting, which has been extensively studied since the last century and a large amount of electrocatalysts including noble and non‐noble metal‐based materials have been developed. Among them, transition metal borides and borates (TMBs)‐based compounds with various structures have attracted increasing attention owing to their excellent OER performance. In recent years, many efforts have been devoted to exploring the OER mechanism of TMBs and to improving the OER activity and stability of TMBs. In this review, recent research progress made in TMBs as efficient electrocatalysts for OER is summarized. The chemical properties, synthetic methodologies, catalytic performance evaluation, and improvement strategy of TMBs as OER electrocatalysts are discussed. The electrochemistry fundamentals of OER are first introduced in brief, followed by a summary of the preparation and performance of TMBs‐based OER electrocatalysts. Finally, current challenges and future directions for TMBs‐based OER electrocatalysts are discussed.

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

“…The active site of transition metal borates were considered to be a combined cluster of metal oxides and borate anion. [110] Currently, the OER performance and reaction mechanism of a series of transition metal phosphates and borates based on Fe, Co, and Ni have been studied, especially in neutral or nearneutral media. [111][112][113]…”

Emerging Cocatalysts on g‐C3N4 for Photocatalytic Hydrogen Evolution

“…[ 159 ] Besides, the boron element in transition metal borides contributes to facilitating water splitting process, hence enabling them as promising cocatalysts for HER. [ 160 ] Moreover, in comparison to transition metal phosphides and sulfides, the synthetic process of transition metal borides via reduction of a metal oxide or salts with boron compounds is relatively easy, thus favoring them as accessible cocatalysts. Taking MoB as an example, owing to the metallic nature, a Schottky barrier between MoB and g‐C 3 N 4 could be formed, which acted as electron trap to improve the charge separation ( Figure a).…”

Transition‐Metal‐Based Cocatalysts for Photocatalytic Water Splitting

“…Transition metal borides have attracted considerable attention in recent years as promising hard materials, superconductors, catalysts, conductive ceramics due to their superhard, thermoelectric, and magnetic properties. [ 294 ] Furthermore, because the favorable water splitting process can be achieved under the induction by metal borides, [ 295 ] the typical nickel boride, molybdenum boride, and nickel–cobalt bimetal boride have been studied as encouraging cocatalysts for H 2 evolution ( Table 5 ). In view of behaving like metallic nature, the integration of metal borides with semiconductors can form Schottky junctions, allowing photogenerated electrons to quickly move from semiconductors to borides for H 2 evolution.…”