Electrochemical Hydrogen Generation by Oxygen Evolution Reaction‐Alternative Anodic Oxidation Reactions

Abstract

Hydrogen (H2) as an environmentally friendly and sustainable energy carrier has been regarded as one of the most promising alternatives to carbon‐based fossil fuels. Electrochemical water splitting powered by renewable electricity provides a promising strategy for H2 production, but its energy efficiency is strongly limited by the kinetically sluggish anodic oxygen evolution reaction (OER), which consumes ≈90% electricity in the water‐splitting process. A new strategy is urgently needed to reduce its energy consumption. Small‐molecule electro‐oxidation reactions that replace OER have attracted increasing attention due to the advantages of low theoretical thermodynamic potential and the benefit of producing value‐added chemicals compared with OER. Hybrid electrolysis systems, by coupling cathodic hydrogen evolution reaction (HER) with anodic small‐molecule oxidation reactions, have been proposed, which can produce high‐purity H2 and value‐added products. This review aims to systematically summarize the recent research on OER‐alternative reactions at the anode for energy‐efficient water splitting. The state‐of‐the art electrocatalysts for OER‐alternative reactions are first presented. The electrolysis performance in hybrid electrolysis regarding the conversion rate, selectivity, yield, and corresponding Faraday efficiency of anodic value‐added products is then evaluated. Finally, the challenges and perspectives are discussed and it is suggested to develop energy‐efficient and economically viable hybrid electrolysis systems.

Replacing Anodic Oxygen Evolution Reaction with Organic ­Oxidation: The Importance of Metal (Oxy)Hydroxide Formation as the Active Oxidation Catalyst

“…31,56 The electrochemical surface area, number of available active sites, charge transport, etc., also affect the activity. 51 Although transition and noble metal-based catalysts were reported for the anodic oxidation of the organic compounds, 5,7,32,33 in this paper, we have considered only the transition metal-based catalysts. The improvement of the cell voltage, faradaic efficiency (FE), and selectivity for the valueadded products are particularly described when anodic OER is replaced by AOR.…”

“…obtained from the biorefinery industry can be effectively utilized to produce valuable products of industrial importance. 5,7,45 The oxidation of HMF leads to the formation of furan dicarboxylic acid (FDCA), which is used in the polymer industry to produce polyurethane, polyamide, and polyester. 45 Also, other oxidized products like maleic anhydride and 2,5-diformylfuran are important for the production of polymers and plastics.…”

“…5,7 In the conventional thermochemical method, high temperature, pressure, and strong oxidant decrease the selectivity of the product formation. 5,7 In contrast, electrocatalytic oxidation occurs at ambient pressure and temperature producing high conversion efficiency and product selectivity. 5,7 In this context, transition metal-derived electrocatalysts like alloy, [8][9][10][11] phosphide, [12][13][14][15][16] oxide, 17,18 chalcogenide, [19][20][21][22][23][24][25] boride, 26,27 , carbide, 28 nitride, 29 layered double hydroxide, 5 hydrooxide 30,31 etc., have been explored for the oxidation of alcohols (methanol, ethanol, propanol, ethylene glycol, phenol, benzyl alcohol), benzylamine, urea, and biomass-derived intermediates such as cellulose, glucose, furfural, and 5-hydroxymethylfurfural (HMF) into the value-added products.…”

“…5,7 In this context, transition metal-derived electrocatalysts like alloy, [8][9][10][11] phosphide, [12][13][14][15][16] oxide, 17,18 chalcogenide, [19][20][21][22][23][24][25] boride, 26,27 , carbide, 28 nitride, 29 layered double hydroxide, 5 hydrooxide 30,31 etc., have been explored for the oxidation of alcohols (methanol, ethanol, propanol, ethylene glycol, phenol, benzyl alcohol), benzylamine, urea, and biomass-derived intermediates such as cellulose, glucose, furfural, and 5-hydroxymethylfurfural (HMF) into the value-added products. 5,7,32,33 Transition metal-based materials play the role of precatalysts. In the alkaline medium, anodic activation of the precatalysts takes place to form the active phase metal (oxy)hydroxide [M(O)x(OH)y, M= transition metal.…”

“…5,7 At the same time, the oxidation of the organic substrate at the anode produces value-added products from inexpensive industrial feedstocks or renewable biomass sources. 5,7 In the conventional thermochemical method, high temperature, pressure, and strong oxidant decrease the selectivity of the product formation. 5,7 In contrast, electrocatalytic oxidation occurs at ambient pressure and temperature producing high conversion efficiency and product selectivity.…”