A review of carbon-based catalysts and catalyst supports for simultaneous organic electro-oxidation and hydrogen evolution reactions

Abstract

Producing organic electro-oxidation and hydrogen evolution reactions (HER) simultaneously in an electrolytic cell is an appealing method for generating valuable chemicals at the anode while also producing H2 at the cathode. Within this framework, the task of designing energy-saving electrocatalysts with high selectivity and stability is a considerable challenge. Carbon-based catalysts, along with their supports, have emerged as promising candidates due to their diverse sources, large specific surface area, high porosity and multidimensional characteristics. This review summarizes progress from 2012 to 2022, in the use of carbon-based catalysts and their supports for organic electrooxidation and HER. It delves into outer-sphere electrooxidation mechanisms involving molecule-mediated oxidation and oxidative radical coupling reactions, as well as inner-sphere electrooxidation mechanisms, encompassing both acidic and alkaline electrolytes. The review also explores prospective research directions within this domain, addressing various aspects such as the design of electrocatalytic materials, the study of the relationship between the structure and properties of electrocatalysts, as well as examining their potential industrial applications.

1. Introduction

Hydrogen (H2) has diverse sources, possesses a high calorific value (1.4×10^8 J kg−1), and generates zero emissions. Consequently, it is regarded as the “ultimate energy” to mitigate the current environmental problems and energy crises. The electrochemical conversion of water to H2 driven by clean energy (e.g., solar, wind) is one of the most promising green pathways for decarbonization. However, this process exhibits high overpotential between the anode and cathode (>1.50 V at 10 mA cm−2). Traditional electrochemical water splitting involves hydrogen evolution reaction (HER) and oxygen evolution reactions (OER), as shown below (Fig. 1a):

In an acidic or neutral medium: OER: 2H2O−4e−→4H+ +O2 (1); HER: 4H+ +4e →2H2 (2). In an alkaline medium: OER: 4OH−+4e →2H2O+O2 (3); HER: 4H2O+4e−→2H2 +4OH− (4). The HER involves a two-electron transfer, whereas the OER involves a four-electron transfer. The main bottleneck of H2 production via water splitting is the sluggish OER, where the generated O2 is not valuable and this might lead to the mixing of H2 and O2.

The addition of the oxygen atoms of H2O to organic molecules instead of their evolution as O2 is a more atom-economical approach to upgrading the anode for H2 production, which is thermodynamically more favorable than OER (Fig. 1b, c). Electrochemical alcohol oxidation reactions (AORs) involving methanol, ethanol, glycerol, glucose, 5-hydroxymethylfurfural and aryl alcohols are used in liquid fuel cells or for generating high-value fine chemicals (e.g., acids, ketones or aldehydes) along with H2 production. Despite remarkable advancements in this field, the ongoing challenge lies in enhancing the efficiency and stability of the corresponding electrocatalysts. For instance, Pt- and Pd-based catalysts exhibit excellent catalytic activity in both organic electrooxidation and HER, but have limited availability and are expensive in nature. In addition, their stability is unsatisfactory. In this context, carbon-based materials are promising catalysts for organic electrooxidation and HER.