Kinetic Understanding of Catalytic Selectivity and Product Distribution of Electrochemical Carbon Dioxide Reduction Reaction

Abstract

CO₂ can be electrochemically reduced to different products depending on the nature of catalysts. In this work, we report comprehensive kinetic studies on catalytic selectivity and product distribution of the CO₂ reduction reaction on various metal surfaces. The influences on reaction kinetics can be clearly analyzed from the variation of reaction driving force (binding energy difference) and reaction resistance (reorganization energy). Moreover, the CO₂RR product distributions are further affected by external factors such as electrode potential and solution pH. A potential-mediated mechanism is found to determine the competing two-electron reduction products of CO₂ that shifts from thermodynamics-controlled product formic acid at less negative electrode potentials to kinetic-controlled product CO at more negative electrode potentials. Based on detailed kinetic simulations, a three-parameter descriptor is applied to identify the catalytic selectivity of CO, formate, hydrocarbons/alcohols, as well as side product H₂. The present kinetic study not only well explains the catalytic selectivity and product distribution of experimental results but also provides a fast way for catalyst screening.

Scheme 1. Schematic Illustration of Micro-Kinetic Model as a Bridge Connecting Electrocatalytic Performance of CO₂ Reduction and Micro Reaction Mechanism

Figure 1

Influence of binding energies and reorganization energies on the reaction activation barriers of competing COOH* and HCOO* formations on (a) In(101), (b) Sn(100), (c) Au(111), and (d) Pt(111) surfaces at U = 0 V vs SHE. The structures in the lower solid circles show stable configurations located at the potential energy curve minimum. The structures in the upper dashed circles show the adjusted configurations the same as lower solid circles after solvent reorganization.

Figure 2

Potential energy curves of two-electron reduction of CO₂ on (a) In(101), (b) Sn(100), (c) Au(111), and (d) Pt(111) surfaces at U = 0 V vs SHE.

Figure 3

Electrode potential-mediated reaction pathways of electrochemical CO₂RR. At lower potential, FA is the main product through a ΔG-controlled HCOO* pathway (blue lines). At higher potential, CO is the main product through a λ–controlled COOH* (red lines) pathway.

Figure 4

Potential-dependent reaction activation barriers of elementary steps of CO₂RR on (a) In(101), (b) Sn(100), and (c) Au(111) surfaces. Potential-dependent Faradaic efficiencies of CO and FA formations on (d) In(101), (e) Sn(100), and (f) Au(111) surfaces.

Figure 5

Comparison of product selectivity for two-electron reduction of CO₂ by thermodynamic calculations and kinetic simulations. (a) Limiting potential differences for CO and FA formations on different metal surfaces. (b) Contour graphs of log(k_COOH/k_HCOO) as a function of binding energies difference of intermediates and applied electrode potentials.

Figure 6

General reaction mechanism and pathways of electrochemical CO₂ reduction.

Figure 7

Potential-dependent potential energy curves of competing two-electron reduction of CO₂RR and HER on (a) Au(111), (b) In(101), and (c) Cu(111) surfaces. Reaction rates of CO₂RR and HER as a function of applied potential on (d) Au(111), (e) In(101), and (f) Cu(111) surfaces.

Figure 8

(a) Density of states of the analysis of CO adsorption on various metal surfaces. (b) Free energy diagram of HER occurred on various metal surfaces calculated by the RPBE functional. (c) Free energy diagrams of competing CO hydrogenation in the Langmuir–Hinshelwood mechanism and HER in the Tafel mechanism on various transition metal surfaces. The insets are adsorption configures of reaction species on the Pt(111) surface. (d) Product distribution of CO₂RR characterized by three-parameter descriptors including the binding energies of CO*, COOH*, and HCOO*.