Transforming active sites in nickel–nitrogen–carbon catalysts for efficient electrochemical CO2 reduction to CO

Abstract

Nitrogen-coordinated single-atom catalysts (SACs) catalyzed electrochemical reduction of CO 2 (CO 2 RR) to CO has emerged as a promising strategy in the management of the global carbon cycle. Herein, we carried out density functional theory (DFT) calculations to investigate the role of possible Ni-N x and Ni–C 4 coordinations in CO 2 RR catalysis. We discover that the free energy change for CO 2 RR is lowered with a decrease in Ni-N x coordination number, with Ni–C 4 displaying the lowest overpotential for CO 2 RR. Using these findings, we develop an effective strategy to transform Ni–N 4 to Ni–C 4 active sites by removing N moieties within Ni embedded in a hollow nitrogen-doped carbon shell (Ni@NCH). We demonstrate an improvement in CO selectivity with this transformation of active sites and the optimized Ni@NCH-1000 catalyst is capable of converting CO 2 to CO with high Faradaic efficiency for CO (FE CO ) of 96% and a current density ( j ) of −35 mA cm −2 at an applied potential of −1 V vs Reversible Hydrogen Electrode (RHE). When adopted in a high-throughput gas diffusion electrolyzer, the newly-developed superhydrophobic catalyst is capable of maintaining CO selectivity >95% over a wide range of applied cell voltages from 2.4 V to 3 V with high current densities (~100 mA cm −2 at 3 V). Our insights and findings with active site transformation in Ni–N–C SACs can serve as guidelines for designing highly active SACs for large-scale CO 2 RR systems. • We carried out density functional theory (DFT) calculations to investigate the role of possible Ni-N x and Ni-C 4 coordinations in CO 2 RR catalysis. • Free energy change for CO 2 RR is lowered with a decrease in Ni-N x coordination number, with Ni-C 4 displaying the lowest overpotential for CO 2 RR. • We develop a strategy to transform Ni-N 4 to Ni-C 4 active sites by removing N moieties within Ni@NCH catalyst. • We experimentally demonstrate an improvement in CO selectivity with this transformation of active sites. • Our insights and findings with active sites transformation in Ni-N-C SACs can serve as designing guidelines for large-scale CO 2 RR systems.