Selectivity in Electrochemical CO 2 Reduction.

The electrocatalytic CO 2 reduction reaction (CO 2 RR) to generate fixed forms of carbons that have commercial value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO 2 emissions as well as to generate carbon-based feed chemicals, which are generally obtained from the petrochemical industry. The area of electrochemical CO 2 RR has seen substantial activity in the past decade, and several good catalysts have been reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward the more chemically challenging issue of selectivity. CO 2 can be partially reduced to produce several C 1 products like CO, HCOOH, CH 3 OH, etc. before its complete 8e - /8H + reduction to CH 4 . In addition to that, the low-valent electron-rich metal centers deployed to activate CO 2 , a Lewis acid, are prone to reduce protons, which are a substrate for CO 2 RR, leading to competing hydrogen evolution reaction (HER). Similarly, the low-valent metal is prone to oxidation by atmospheric O 2 (i.e., it can catalyze the oxygen reduction reaction, ORR), necessitating strictly anaerobic conditions for CO 2 RR. Not only is the requirement of O 2 -free reaction conditions impractical, but it also leads to the release of partially reduced O 2 species such as O 2 - , H 2 O 2 , etc., which are reactive and result in oxidative degradation of the catalyst.In this Account, mechanistic investigations of CO 2 RR by detecting and, often, chemically trapping and characterizing reaction intermediates are used to understand the factors that determine the selectivity in CO 2 RR. The spectroscopic data obtained from different intermediates have been identified in different CO 2 RR catalysts to develop an electronic structure selectivity relationship that is deemed to be important for deciding the selectivity of 2e - /2H + CO 2 RR. The roles played by the spin state, hydrogen bonding, and heterogenization in determining the rate and selectivity of CO 2 RR (producing only CO, only HCOOH, or only CH 4 ) are discussed using examples of both iron porphyrin and non-heme bioinspired artificial mimics. In addition, strategies are demonstrated where the competition between CO 2 RR and HER as well as CO 2 RR and ORR could be skewed overwhelmingly in favor of CO 2 RR in both cases.