Abstract
Water oxidation and reduction reactions play vital roles in highly efficient hydrogen production conducted by an electrolyzer, in which the enhanced efficiency of the system is apparently accompanied by the development of active electrocatalysts. Solar energy, a sustainable and clean energy source, can supply the kinetic energy to increase the rates of catalytic reactions. In this regard, understanding of the underlying fundamental mechanisms of the photo/electrochemical process is critical for future development. Combining light-absorbing materials with catalysts has become essential to maximizing the efficiency of hydrogen production. To fabricate an efficient absorber-catalysts system, it is imperative to fully understand the vital role of surface/interface modulation for enhanced charge transfer/separation and catalytic activity for a specific reaction. The electronic and chemical structures at the interface are directly correlated to charge carrier movements and subsequent chemical adsorption and reaction of the reactants. Therefore, rational surface modulation can indeed enhance the catalytic efficiency by preventing charge recombination and prompting transfer, increasing the reactant concentration, and ultimately boosting the catalytic reaction. Herein, the authors review recent progress on the surface modification of nanomaterials as photo/electrochemical catalysts for water reduction and oxidation, considering two successive photogenerated charge transfer/separation and catalytic chemical reactions. It is expected that this review paper will be helpful for the future development of photo/electrocatalysts.
Highlights
The utilization of fossil energy produces various pollutants, increases atmospheric CO2 concentration, and causes further green-house effects and climate change
The energy conversion efficiency of this route is still very low because most of the chemical reactions for the production of fuels are endothermic reactions, which require a large energy to overcome the difference in Gibbs free energy between the reactants and products [1,2]
The generated charge carriers are trapped at these defect states on the surface. This phenomenon reduces the concentration of available charge carriers, which are consumed in catalytic reactions on the surface, and decreases the energy conversion efficiency
Summary
The utilization of fossil energy produces various pollutants, increases atmospheric CO2 concentration, and causes further green-house effects and climate change. Two processes, including the charge transfer/separation and successive catalytic chemical reaction at the surface of the materials, are critical for high energy conversion efficiency. The excited carriers obtain potential energy from solar energy and give the kinetic energy to the surface-adsorbed electron and/or hole acceptors to overcome the activation energy for the generation of valuable fuels. Maximization of photocatalytic efficiency requires understanding how the semiconductor (SC) absorbs and generates excitons (Equation (1)), how the generated excitons transfer along the semiconductors (Equation (2)) and how the electron/hole acceptors are reduced/oxidized on the electrode (Equations (3)–(8), Equations (5)–(8), * refers to the adsorption of the species on the active sites of the catalysts) [3]. Understanding the charge transfer/separation between absorber-catalysts (solid-solid) and catalysts-reactants (solid-liquid) and catalytic reaction between catalysts-electrolyte becomes significant for high photo-energy conversion efficiency. We first consider the photogenerated charge carrier transfer and separation process and consider the catalytic chemical reaction on the surface subsequently
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