2. Fundamentals of Electrochemical CO2Reduction
2.1 Mechanism from CO2 to CO
The linear molecular structure with high carbon-oxygen bond energy made CO2 reduction an endothermic reaction in thermodynamics. Therefore, accomplishing the break of carbon-oxygen bond requires the external energy. As shown in Figure 1 a (experimental unit), the electrocatalytic method can directly supply electrons, in favor of the rapid CO2 reduction. Electrocatalytic CO2 reduction (ECR) reaction is multi-proton-electron coupling transfer process, such as two, four, six, eight, etc. electron pathways (Figure 1b).[35-38] Different electron transfer paths export different products, even multiple products may be generated in a reduction system.[35,39,40] Since the main product of Ni-N-C catalyst is CO via two-electron process, this section mainly describes the mechanism from CO2 to CO.