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Correlating hydration free energy and specific adsorption of alkali metal cations during CO2 electroreduction on Au

Abstract

Specifically adsorbed alkali metal cations on metal electrodes have been hypothesized to influence the reduction of CO2. However, experimental detection of these cations during CO2 reduction remains elusive. Herein, employing the asymmetric CH3 deformation band of tetramethylammonium as a vibrational probe of the aqueous electrolyte–polycrystalline Au interface, we monitored the displacement of specifically adsorbed tetramethylammonium by alkali metal cations. We found that the coverage of specifically adsorbed alkali metal cations during CO2-to-CO reduction follows the order Li+ < Na+ < K+ < Cs+ for the same bulk concentration. The alkali metal cations’ experimentally observed surface coverages correlate with their free energies of hydration. Furthermore, the rate of CO2-to-CO conversion increases with the coverage of specifically adsorbed alkali metal cations. Our observations suggest that the degree to which alkali metal cations undergo partial dehydration at the electrode–electrolyte interface plays a key role in their ability to promote CO2-to-CO reduction.

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Fig. 1: Response of the CH3 deformation band of methyl4N+ to changes in EDL structure.
Fig. 2: Comparison of CH3 deformation bands of methyl4N+ in different electrolytes.
Fig. 3: Dependence of CH3 deformation band on charge on the molecule.
Fig. 4: Response of CH3 deformation band to addition of K+ to the electrolyte.
Fig. 5: Change in area of 1,482 cm−1 band with bulk alkali metal cation concentration.
Fig. 6: Correlation between area of 1,482 cm−1 band and alkali cation hydration free energy.
Fig. 7: Dependence of electrocatalysis on area of 1,482 cm−1 integrated band.

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Data availability

Representative data and extended datasets that support the findings reported in this study are available in the manuscript and the Supplementary Information. The data in the figures shown in the main text and DFT-calculated coordinates for the optimized geometries of the cations on Au(111) are provided in machine-readable formats as supplementary files. Additional data are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was supported by a CAREER award (to M.M.W.) from the National Science Foundation (no. CHE-1847841). N.A. and M.J.J. acknowledge the National Science Foundation for support (award no. CHE-1665155).

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All authors discussed the results and commented on and revised the manuscript. M.M.W. and V.J.O. conceived and designed the experiments. V.J.O. conducted the experiments. Y.-S.H. participated in data collection. M.J.J. and N.A. contributed the DFT work.

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Correspondence to Michael J. Janik or Matthias M. Waegele.

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Nature Catalysis thanks Yanwei Lum, Wenbin Cai and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figs. 1–11, Tables 1–4 and Notes 1 and 2.

Supplementary Data 1

Coordinates for optimized geometries of cations on Au(111).

Supplementary Data 2

Machine-readable data shown in Figs. 1–7.

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Ovalle, V.J., Hsu, YS., Agrawal, N. et al. Correlating hydration free energy and specific adsorption of alkali metal cations during CO2 electroreduction on Au. Nat Catal 5, 624–632 (2022). https://doi.org/10.1038/s41929-022-00816-0

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