Influence of Cations on HCOOH and CO Formation during CO2 Reduction on a PdMLPt(111) Electrode

Understanding the role of cations in the electrochemical CO2 reduction (CO2RR) process is of fundamental importance for practical application. In this work, we investigate how cations influence HCOOH and CO formation on PdMLPt(111) in pH 3 electrolytes. While only (a small amount of adsorbed) CO forms on PdMLPt(111) in the absence of metal cations, the onset potential of HCOOH and CO decreases with increasing cation concentrations. The cation effect is stronger on HCOOH formation than that on CO formation on PdMLPt(111). Density functional theory simulations indicate that cations facilitate both hydride formation and CO2 activation by polarizing the electronic density at the surface and stabilizing *CO2–. Although the upshift of the metal work function caused by high coverage of adsorbates limits hydride formation, the cation-induced electric field counterbalances this effect in the case of *H species, sustaining HCOOH production at mild negative potentials. Instead, at the high *CO coverages observed at very negative potentials, surface hydrides do not form, preventing the HCOOH route both in the absence and presence of cations. Our results open the way for a consistent evaluation of cationic electrolyte effects on both activity and selectivity in CO2RR on Pd–Pt catalysts.

With the positive going scan, decreasing peaks (peak at 0.31 VRHE decreasing first) in the potential region between +0.05 and +0.35 VRHE suggest adsorbed CO.COads oxidation peaks are observed in the potential region between +0.65 and +0.9 VRHE, the peaks varying under different conditions.These different CO oxidation peaks may result from different *CO coverage or changes of local electrolyte after CO2RR.After oxidation of *CO, the typical CV features in pH 3 electrolyte are again observed in the negative going scan.In some voltammograms, CV features of PdMLPt(111) slightly deviate from the standard, which might be due to minor loss of Pd atoms during CO2RR.Moreover, peaks in the potential region between +0.05 and +0.35 VRHE decreases with increasing of *CO obtained from CO2RR, which leads to more CO oxidation current in potential region between +0.65 and +0.9 VRHE and corresponding higher CO coverage.The gray column reports the baseline case for the adsorbate without K + , while the near-K + system (with excess electron) is indicated in magenta.Solvated K + was here simulated with three water molecules within its coordination shell.

Figure S12. a.
Correlation between H δ-Bader charges and cation-induced electric field, proportional to q K+ /r 2 K+-H.b.Cation-induced electric field vs cation acidity for alkali, bi-valent, and tri-valent cations.Cation acidity affects cation accumulation, while electric field favors the formation of hydrides on the surface.]  Table S1.Calculated work function for the PdMLPt(111) (3×3) surface for clean surface and *CO2 for the K + -free, the near-K + (dK+-surf = 4 Å), and far-K + (dK+-surf = 9 Å) cases, with and without balancing OH -.Cation and H2O molecules (within bracket) were absent in the K + -free case.

System
W / eV K + -free Near K + Near K + + OH -Far K + Near K

Figure
Figure S3 a. CO stripping voltammograms on PdMLPt(111) after CO2 reduction at different vertex potentials in CO2-saturated pH 3 electrolyte without metal cations, measured at 10 mV s -1 .b. HPLC data before and after chronoamperometry at -1.2 VRHE for 10 min in CO2saturated pH 3 electrolyte without metal cations.

Figure
Figure S5 a. CO stripping voltammograms on PdMLPt(111) after CO2 reduction at different vertex potentials in CO2-saturated pH 3 with 5 mM KClO4 electrolyte, measured at 10 mV s -1 .b. HCOOH formation and the calculated CO coverage as a function of potential.

Figure
Figure S6 a. CO stripping voltammograms on PdMLPt(111) after CO2 reduction at different vertex potentials in CO2-saturated pH 3 with 10 mM KClO4 electrolyte, measured at 10 mV s - 1 .b. HCOOH formation and the calculated CO coverage as a function of potential.

Figure S9 .
Figure S9.Electrostatic potential (Φ) profile across the simulation cell (z-direction, reported on top) for the cation-free (gray line) and near-cation (dK+-surf = 4 Å) systems with and without excess electron (magenta and purple line respectively).

Figure S10 .
Figure S10.Electrostatic potential (Φ) profile across the simulation cell (z-direction, reported on top) for the cation-free (gray line) and far-cation (dK+-surf = 9 Å) systems with and without excess electron (magenta and purple line respectively).

Figure
Figure S13.a. DFT energy on PdMLPt(111) (3×3) relative to cation-induced outer-sphere CO2 activation to HCOO -for the K + -free (gray) and near-K + with and without balancing OH - (purple and magenta) for different values of metal work functions (right y-axis).b.DFT energy relative to the HCOO -formation step on PdMLPt(111) (3×3) supercell for cation-free (gray), far-K + (light purple), and near-K + (dark purple) cases at U = -0.4V vs RHE.Values of metal work function for clean surfaces are given at the right y-axis.For details on the model, see Figure 5a-b and Computational Details.