Electrochemical conversion of CO2 into liquid fuels and value-added chemicals is considered to be one of the most promising ways to combat climate change due to anthropogenic CO2 emissions. Here, we investigated the mechanism of CO2 reduction (CO2RR) to formic acid (HCOOH) on a SnO2 electrode by a combination of density functional theory and microkinetics simulations. We used an implicit solvent model to model the influence of the electrolyte. A major finding is that hydroxylation of the SnO2 surface does not only lead to improved CO2RR activity but also to a higher HCOOH selectivity, in part due to inhibition of the competing hydrogen evolution reaction (HER). Another interesting aspect is that the main reaction pathway is via carboxyl (COOH*), while earlier postulates for formic acid formation in CO2 electroreduction found *OHCO to be the dominant intermediate for SnO2. We also explored the influence of metal doping of hydroxylated SnO2 for this COOH* mechanism. A Brønsted-Evans-Polanyi relation for the CO2 to COOH* step (i.e., the first protonation step) (Figure 1a) and COOH* to HCOOH (i.e., the second protonation step) (Figure 1b) on metal-doped (metal = Ni, Pd, Bi, and Cu) SnO2 surfaces has been identified. Among these dopants, Bi is preferred for facilitating COOH* formation. The activation barrier of the first protonation step decreased from 1.15 eV to 0.93 eV upon Bi doping. A reaction-diffusion model was used that takes into account the effect of electrochemical potential on the stability of intermediates and associated activation barriers in proton-coupled electron transfer steps as well as diffusion of protons and CO2 from the bulk electrolyte to the electrode surface. Microkinetics simulations confirm that the activity for CO2RR to HCOOH improved after Bi doping (Figure 1c). Our findings indicate that the catalytic activity and selectivity of SnO2 electrocatalysts can be improved by metal doping.

Promotional effect of surface hydroxyls and metal doping on electrochemical reduction of CO2 over SnO2 electrode

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.