In an electrolyzer powered by electric current (preferably from renewable energy sources such as photovoltaics),CO2 can be reduced at the cathode to various products (CO, formic acid, methanol, methane, ethylene, etc.). In general, hydrogen is also formed from the reduction of protons present in the reaction mixtures. These protons are essential to the reaction, as shown by a number of significant examples. The diversity of possible products therefore raises the question of how to control reaction selectivity. However, these reactions are subject to strong kinetic constraints and must be catalyzed. The question of catalysis is central toCO2 electroreduction. Catalysts can reduce voltage surges, improve reaction selectivity and efficiency, and can be used in electrolysis devices if they are stable.
This lecture presents the main catalysts currently under investigation. These include soluble molecular catalysts, such as organometallic complexes of rhodium, ruthenium or iridium, carbonyl complexes of rhenium or manganese, iron porphyrins and nickel cyclams. Some of the methods of cyclic voltammetry and controlled-potential electrolysis data processing, with carbon or mercury electrodes presented in the lecture, make it possible to determine rate constants and overvoltages and thus to compare different catalysts quantitatively.
Solid metal electrodes can also be used, providing both electron conduction and catalytic functions. The most widely studied example is that of copper electrodes, which have the advantage of conducting hydrocarbons, albeit with very high overpotentials. However, recent developments have led to electrodes based on copper and copper oxide that are more stable, more efficient and more selective.
