Potential-Dependent Free Energy Relationship in Interpreting the Electrochemical Performance of CO₂ Reduction on Single Atom Catalysts

Abstract

Acquiring the fundamental understanding of electrochemical processes occurring at the complex electrode–liquid interface is a grand challenge in catalysis. Herein, to gain theoretical insights into the experimentally observed potential-dependent activity and selectivity for the CO2 reduction reaction (CO2RR) on the popular single-iron-atom catalyst, we performed ab initio molecular dynamics (AIMD) simulation, constrained MD sampling, and thermodynamic integration to acquire the free energy profiles for the proton and electron transfer processes of CO2 at different potentials. We have demonstrated that the adsorption of CO2 is significantly coupled with the electron transfer from the substrate while the further protonation does not show distinct charge variation. This strongly suggests that CO2 adsorption is potential-dependent and optimizing the electrode potential is vital to achieve the efficient activated adsorption of CO2. We further identified a linear scaling relationship between the reaction free energy (ΔG) and the potential for key elementary steps of CO2RR and HER, of which the slope is adsorbate-specific and not as simple as 1 eV per volt as suggested by the traditional computational hydrogen electrode (CHE) model. The derived scaling relationship can reproduce the experimental onset potential (U onset) of CO2RR, potential of the maximal CO2-to-CO Faraday efficiency (FECO), and potential where FECO = FEH2. This suggests that our state-of-the-art model could precisely interpret the activity and selectivity of CO2RR/HER on the Fe-N4-C catalyst under different electrode potentials. In general, our study not only provides an innovative insight into the theoretical explanation of the origin of the solvation effect from the perspective of charge transfer but also emphasizes the critical role of electrode potential in the theoretical consideration of catalytic activity, which offers a profound understanding of the electrochemical environment and bridges the gap between theoretical predictions and experimental results.

Modeling the Potential-Dependent Kinetics of CO2 Electroreduction on Single-Nickel Atom Catalysts with Explicit Solvation

“…In Figure a, the net Bader charge of the Ni active center is almost constant at ∼0.75 |e| at −0.71 V and same as that when U changed negatively to −0.92 and −1.11 V, suggesting that the active center, Ni, remains almost constant in the reduction process and is just an electron relay during the CO 2 activation, and instead, the graphene matrix is the electron donor when the N 4 @Gra charge increased over 0.6 |e| under three different potentials (Figure a). A similar phenomenon is reported on the FeN 4 @Gra substrate erewhile . This implies that it is the effective surface charge density that drives the charge transfer to CO 2 .…”

“…The insets in Figure a illustrate the geometric change of the TS where the C–Ni distances increased from 2.2 to 2.6 Å as U RHE = −0.71 V changes to −0.92 V; meanwhile, the O-C-O angle, averaged over the last 5 ps from the constrained MD calculation trajectories, stretched from 142.2 to 157.7°. Our results demonstrate that the location of the TS along the reaction coordinate shifted closer to the FS as the applied potential shifts to a more negative value . At the potential of −1.11 V vs RHE, the free energy barrier dropped as low as 0.13 ± 0.02 eV, making the path of CO 2 activation be more inclined to convert the IS into the FS with an exothermic reaction energy of −0.25 ± 0.04 eV.…”

“…For NiN 1 C 3 @Gra, NiN 2 C 2 @Gra, and NiN 3 C 1 @Gra, the slope is less than 1 eV per volt, which is consistent with the small charge transfer from the substrate to CO 2 upon adsorption, as shown in Table S3. For NiN 4 @Gra, the slop is significantly larger than 1 eV per volt owing to the higher charge transfer for CO 2 adsorption and the solvation effect on the free energetics …”

“…We employ the average value of the potentials at the initial state ( U IS ) and final state ( U FS ) as the potential ( U r ) of the reaction. (i.e., U r = ( U IS + U FS )/2) . The detailed electrode potentials, surface charges, and corresponding correction terms for each free energy profile are provided in Table S3.…”

“…(i.e., U r = (U IS + U FS )/2). 55 The detailed electrode potentials, surface charges, and corresponding correction terms for each free energy profile are provided in Table S3. In our simulation models, the EDLs at the electrode/electrolyte interfaces are varied by introducing different numbers of alkali Na cations at ∼3 Å (Stern layer) away from the graphene surface (Figure 1e).…”