Electrochemical CO₂ Reduction over Metal-/Nitrogen-Doped Graphene Single-Atom Catalysts Modeled Using the Grand-Canonical Density Functional Theory

Abstract

Renewably driven, electrochemical conversion of carbon dioxide into value-added products is expected to be a critical tool in global decarbonization. However, theoretical studies based on the computational hydrogen electrode largely ignore the nonlinear effects of the applied potential on the calculated results, leading to inaccurate predictions of catalytic behavior or mechanistic pathways. Here, we use grand canonical density functional theory (GC-DFT) to model electrochemical CO2 reduction (CO2R) over metal- and nitrogen-doped graphene catalysts (MNCs) and explicitly include the effects of the applied potential. We used GC-DFT to compute the CO2 to CO reaction intermediate energies at −0.3, −0.7, and −1.2 VSHE catalyzed by MNCs each doped with 1 of the 10 3d block metals coordinated by four pyridinic nitrogen atoms. Our results predict that Sc-, Ti-, Co-, Cu-, and Zn-N4Cs effectively catalyze CO2R at moderate to large reducing potentials (−0.7 to −1.2 VSHE). ZnN4C is a particularly promising electrocatalyst for CO2R to CO both at low and moderate applied potentials based on our thermodynamic analysis. Our findings also explain the observed pH independence of CO production over FeN4C and predict that the rate-determining step of CO2R over FeN4C is not *CO2 – formation but rather *CO desorption. Additionally, the GC-DFT-computed density of states analysis illustrates how the electronic states of MNCs and adsorbates change non-uniformly with applied potential, resulting in a significantly increased *CO2 – stability relative to other intermediates and demonstrating that the formation of the adsorbed *CO2 – anion is critical to CO2R activation. This work demonstrates how GC-DFT paves the way for physically realistic and accurate theoretical simulations of reacting electrochemical systems.

Circumventing CO2 Reduction Scaling Relations Over the Heteronuclear Diatomic Catalytic Pair

“…As clarified by the DFT calculation, the Fe 1 -Mo 1 diatomic catalytic pair not only keeps the inherent moderate CO adsorption over the single Fe site but also greatly facilitates CO 2 activation via a bridge configuration, which effectively breaks the CO 2 reduction scaling relations (Figure b). Generally, the rate of electrochemical CO 2 reduction to CO over most of the SACs (such as Ni, Fe, and Zn) − and Au is limited by CO 2 adsorption. Moreover, it is suggested by DFT calculations that CO 2 adsorption on Au is stronger than that on most of the SACs; thus, the overpotential of most of the SACs for electrochemical CO 2 reduction to CO is larger than that of Au (Figure S36a).…”

Oxygen Reduction Reaction on Single-Atom Catalysts From Density Functional Theory Calculations Combined with an Implicit Solvation Model

“…See Table S4 for free energy contributions. See also refs and − for grand canonical simulations at the electrochemical interfaces, which inspired this work. The free energies of the reaction for each elementary step are expressed as where is the total energy of O 2 in the gas phase, Ω * ( U ) is the electronic grand potential of the clean TM–N 4 –C surfaces (*), and Ω *OOH ( U ), Ω *O ( U ), and Ω *OH ( U ) are the electronic grand potentials of the TM–N 4 surfaces with *OOH, *O, and *OH, respectively (Figure a–c).…”

“…See Table S4 for free energy contributions. See also refs and − for grand canonical simulations at the electrochemical interfaces, which inspired this work. The free energies of the reaction for each elementary step are expressed as Δ G 1 false( italicU false) = G OOH * false( italicU false) + G H 2 O false( normall false) − G O 2 false( normalg false) − G H 3 O + false( normala normalq false) − μ e − G * false( italicU false) normalΔ italicG 2 ( U ) = italicG normalO * ( U ) + 2 italicG normalH<...…”

Theoretical Understanding of Potential-Dependent Electrocatalytic CO2RR and Competition with HER upon Cobalt Single Atom Supported by Phthalocyanine Monolayer

“…This algorithm can automatically adjust the number of electrons in the system to maintain a stable work function, and as a whole, to control the system at a constant potential. This method with a polarizable continuum implicit solvation model can describe electrode–electrolyte interface reasonably, and has been used to understand many electrochemical reactions. − …”