Synthesis of C<sub>2</sub> products via electrocatalytic CO<sub>2</sub> reduction

The technique of electrocatalytic CO2 reduction (ECR) is emerging as a competent candidate for neutralizing the excessive anthropogenic carbon emission, and for producing a multitude of value-added chemicals. In particular, ECR is well compatible with renewable electricity-generating technologies, and can help to alleviate their innate issue of intermittency and to level the energy output of power grid. Yet ECR itself typically suffers from the chemical inertness and low solubility of CO2, multiple products and low selectivity, and the undesired parasitic hydrogen evolution reaction in aqueous electrolytes. Therefore, electrocatalysts featuring low overpotentials, high current densities, high Faradaic efficiencies and high stabilities are in constant pursuit. From the viewpoint of Faradaic efficiency and current density that have been achieved thus far, the most promising products are CO and formic acid (among C1 products), and ethylene and ethanol (among C2 products). Different from C1 products, C2 products are formed via carbon-carbon coupling (C–C coupling) following a second-order reaction kinetics, generally with more complicated reaction mechanisms involving multiple electron transfer and therefore more stringent requirements on the electrocatalysts. As such, at the center of synthesizing C2 products via ECR is the in-depth understanding on reaction mechanisms and the rational design of advanced electrocatalysts. Up to date, the ECR catalysts that can yield C2 products are primarily based on Cu-related materials, and CO has been recognized as the most important and versatile intermediate for C–C coupling. This review summarizes the reports on the relevant underlying reaction mechanisms, and elaborates on the three most widely accepted catalytic mechanisms for C–C coupling on Cu-based catalysts: CO dimerization, CO+CHO coupling, and CH2 carbene dimerization. These mechanisms proposed on the basis of theoretical calculation, take effect in different regimes of applied electric potential, and have also found substantial support from experimental data obtained particularly via in situ and operando spectroscopies. The rational design of Cu-based catalysts can effectively improve the reaction selectivity for C–C coupling. In this regard, this review discusses Cu-based catalysts in different subcategories: bulk Cu catalysts, Cu nanocatalysts, oxide-derived Cu catalysts, and Cu-based bimetallic catalysts, and put major emphasis on the effects of exposed facet, particle size, shape, loading density, oxidation state of surface atoms and alloying on steering the selectivity towards C–C coupling pathways. It has been found that Cu-based catalysts that feature (100) facets, regular shapes, high loading areal densities and partially oxidized Cu atoms generally give superior performances in yielding C2 and C2+ products, because their optimized surface and electronic structures can effectively elevate the local concentration of CO intermediates or lower the activation barrier for C–C coupling. A few topics other than Cu-based catalysts are also briefly discussed at the end of this review. We stress the irreplaceable importance of in situ and operando characterization techniques in probing and deciphering the catalytic mechanism of ECR, the design and optimization of electrolytes and electrodes in help to alter the selectivity for different products, and most importantly, a range of novel catalysts that have emerged in recent years, including tandem catalysts, single-atomic catalysts, non-Cu-based metal catalysts, non-metal catalysts based on graphene and nanodiamond, as well as molecular catalysts, which have witnessed a resurgence after the proposal of the idea of heterogeneous immobilization.