Research
Understanding the effects of electrolyte cations on the electrochemical reduction CO2
Ringe, S., Clark, E., Resasco, J., Walton, A., Seger, B., Bell, A., Chan, K. Understanding cation effects in electrochemical CO2 reduction. Energy Environ. Sci., DOI: 10.1039/C9EE01341E (2019).
Scientific Achievement
The effects of electrolyte cation size on the electrochemical reduction of CO2 over Ag and Cu are explained theoretically.
Significance and Impact
The selectivity of Ag for CO formation and Cu for C2 product formation is due to stabilization of dipolar intermediates by the double-layer field in created by hydrated cations – the smaller the hydrated cation size, the stronger the field.
Research Details
The ion-size modified, Poisson-Boltzmann equation is solved to determine the charge on the metal surface and hence the double-layer field.
The double-layer field is used in DFT calculations of the stability of intermediates involved in CO formation on Ag and C2 product formation on Cu
Reprinted from Ringe, S., Clark, E., Resasco, J., Walton, A., Seger, B., Bell, A., Chan, K. Understanding cation effects in electrochemical CO2 reduction. Energy Environ. Sci., DOI: 10.1039/C9EE01341E (2019).
Schematic illustration of our multi-scale modeling approach to model cation effects on field-driven electrocatalysis. The process of surface charge generation as a function of potential (left panel) is simulated by a 1D-continuum electrostatic description of the electrolyte. The ion-size modified Poisson–Boltzmann approach (MPB) enables us to model the effect of ion size on the generated surface charge at a fixed potential. Surface charge density dependent reaction energetics are obtained from charge-dependent DFT calculations of the rate-limiting species (right panel). Combining the results via interpolation, we obtain the catalytic activity or current density as a function of cation size and potential of zero charge at fixed applied potential.
Reprinted from Ringe, S., Clark, E., Resasco, J., Walton, A., Seger, B., Bell, A., Chan, K. Understanding cation effects in electrochemical CO2 reduction. Energy Environ. Sci., DOI: 10.1039/C9EE01341E (2019).
Illustration of the origin of cation effects in field-driven electrocatalysis as suggested by this work. Repulsive interactions between hydrated cations at the outer Helmholtz plane reduce the local concentration of cations, the surface charge density σ (depicted by the red-colored region) and the electric double layer field. The diffuse layer that is explicitly modeled by the MPB model is depicted as well as the Helmholtz gap capacitance region and the interfacial ion diameter determined in this work.
Reprinted from Ringe, S., Clark, E., Resasco, J., Walton, A., Seger, B., Bell, A., Chan, K. Understanding cation effects in electrochemical CO2 reduction. Energy Environ. Sci., DOI: 10.1039/C9EE01341E (2019).
Partial current density of C2 products (ethanol and ethylene) at Cu(111) and Cu(100) at −1 V vs. RHE for different cations normalized to the C2 current density in the Li+ case. Filled circles represent the experimental data points, solid lines the theoretical prediction.