Elucidating the Electrocatalytic CO₂ Reduction Reaction over a Model Single‐Atom Nickel Catalyst

Abstract

Designing effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) is an appealing approach to tackling the challenges posed by rising CO2 levels and realizing a closed carbon cycle. However, fundamental understanding of the complicated CO2RR mechanism in CO2 electrocatalysis is still lacking because model systems are limited. We have designed a model nickel single‐atom catalyst (Ni SAC) with a uniform structure and well‐defined Ni‐N4 moiety on a conductive carbon support with which to explore the electrochemical CO2RR. Operando X‐ray absorption near‐edge structure spectroscopy, Raman spectroscopy, and near‐ambient X‐ray photoelectron spectroscopy, revealed that Ni+ in the Ni SAC was highly active for CO2 activation, and functioned as an authentic catalytically active site for the CO2RR. Furthermore, through combination with a kinetics study, the rate‐determining step of the CO2RR was determined to be *CO2−+H+→*COOH. This study tackles the four challenges faced by the CO2RR; namely, activity, selectivity, stability, and dynamics.

Reaction mechanism and kinetics for CO2 reduction on nickel single atom catalysts from quantum mechanics

“…Recently, Gu et al 52 observed that the initial electron for reduction of CO 2 is decoupled from a proton transfer on Fe 3+ -N-C and found that CO 2 adsorption is fast, and the rate limiting step is protonation of the adsorbed CO 2to form an adsorbed COOH intermediate, with an experimental Tafel slope of 64 mV dec −1 . Moreover, experiments by Liu et al 29 for CO 2 RR on a Ni-single atom catalyst confirmed the presence of Ni + by operando spectroscopic characterization. They observed that the proton transfer to the adsorbed *CO 2 is the rate determining step with a Tafel slope of 72 mV dec −1 .…”

“…It has large surface area, high conductivity, high stability, and the capability to tune electronic properties by forming strong chemical bonds to guest atoms [22][23][24] . Several experimental studies have been published recently on the CO 2 reduction reaction (CO 2 RR) for the nickel single atom catalysts (Ni-SACs) on graphene 20,21,[25][26][27][28][29] , but the performance varies markedly perhaps because of differences in the number of carbon or nitrogen bonds to Ni.…”

Microenvironment modulation of single-atom catalysts and their roles in electrochemical energy conversion

“…2G). Results from the aberration-corrected HAADF-STEM and x-ray absorption spectroscopy (XAS) measurements clearly confirm the well-defined Ni-N 4 moiety on carbon support (66).…”

“…In the work conducted by Liu and colleagues, atomically dispersed Ni(I) has been proven as the active site for CO 2 RR (55); the asdeveloped single-Ni atom catalyst exhibits high intrinsic activity for CO 2 reduction, reaching a TOF of 14,800 hours −1 with 97% Faradaic efficiency at a mild overpotential of 0.61 V. The following research from the same group further identified the high activity of Ni + in a model Ni-based SAC for CO 2 activation through operando XAS and Raman measurements (Fig. 6C) (66).…”

“…( G ) Synthesis of model Ni SACs on carbon nanotubes, adapted with permission from Liu et al . ( 66 ). ( H ) Scheme for the formation of Ru SAs/N-C and Ru NCs/C, adapted with permission from Wang et al .…”