Understanding Surface-Mediated Electrochemical Reactions: CO2 Reduction and Beyond

Understanding reaction pathways and mechanisms for electrocatalytic transformation of small molecules (e.g., H2O, CO2, and N2) to value-added chemicals is critical to enabling the rational design of high-performing catalytic systems. Tafel analysis is widely used to gain mechanistic insights, and in some cases, has been used to determine the reaction mechanism. In this Perspective, we discuss the mechanistic insights that can be gained from Tafel analysis and its limitations using the simplest two-electron CO2 reduction reaction to CO on Au and Ag surfaces as an example. By comparing and analyzing existing as well as additional kinetic data, we show that the Tafel slopes obtained on Au and Ag surfaces in the kinetically controlled region (low overpotential) are consistently ∼59 mV dec–1, regardless of whether catalysts are polycrystalline or nanostructured in nature, suggesting that the initial electron transfer (CO2 + e– → CO2–) is unlikely to be the rate-limiting step. In addition, we demonstrate how initial mechanistic assumptions can dictate experimental design, the result of which could in turn bias mechanistic interpretations. Therefore, as informative as Tafel analysis is, independent experimental and computational techniques are necessary to support a proposed mechanism of multielectron electrocatalytic reactions, such as CO2 reduction.