(111h) Measuring the Electrode Electronic Structure Via Ultraviolet Photoelectron Spectroscopy to Explain How Electrode Composition Controls V2+/V3+ Reaction Kinetics

The charge transfer of metal ions at the electrolyte/electrode interface is important for applications including understanding and mitigating corrosion, electrodeposition, and energy storage using batteries. These reactions occur either through outer-sphere or inner-sphere mechanisms, where an inner-sphere reaction involves an intermediate species adsorbing onto the surface of the electrode.¹ Therefore, for inner-sphere mechanisms changes in the adsorption energy of the intermediate species on the electrode can change the reaction rate by more than an order of magnitude. Although the importance of the adsorption energy is widely studied for inner-sphere electrocatalytic reactions such as hydrogen evolution and oxygen reduction,² the role of the adsorption energy is less well-studied for inner-sphere charge transfer of metal ions (e.g., V²⁺/V³⁺ or Cr²⁺/Cr³⁺).³ In particular, being able to control the reaction rate for V²⁺/V³⁺ through control of the electrode has benefits for redox flow batteries.

Without knowing the exact structure of the reaction intermediate or the structure of the electrode, it is difficult to estimate the intermediate’s adsorption energy and thereby correlate adsorption energy to reaction rate on a range of electrodes. For transition metal electrodes, the structure of the d-band electrons is known to influence the adsorption strength of various intermediates and thereby the rate of reaction.⁴ Often this electrode electronic structure is described by the d-band center (first moment of the d-band) and the d-band width (based on the second moment of the d-band).⁵ These values have been calculated via density functional theory (DFT) for individual facets of all transition metals.⁵ The calculated d-band center has been shown to correlate to the adsorption energy of key intermediates and subsequently to redox reaction kinetics such as the hydrogen evolution reaction.² Recently, we have shown correlations between (1) the calculated d-band center for several metal electrodes and exchange current density for the V²⁺/V³⁺ reaction as well as (2) adsorption energy of a proposed vanadium intermediate complexed by water and sulfate anions and exchange current density for the V²⁺/V³⁺ reaction.³

However, the surfaces used for DFT calculations are not an exact representation of the experimental polycrystalline electrode surface and therefore do not fully capture nuances in the relationship between reaction kinetics and electrode electronic structure. This discrepancy is even more important when studying alloys. In this talk we discuss how to use ultraviolet photoelectron spectroscopy (UPS) to experimentally measure the electronic structure of pure transition metal and alloy electrodes. We use the V²⁺/V³⁺ half reaction as a test reaction to determine the correlation between the reaction’s exchange current density and the electrode composition as well as the electrode’s experimentally determined electronic structure.

Our kinetic results are based on rotating disk electrode studies to carefully determine the active surface area and deconvolute kinetics vs transport limited currents. To thoroughly test a wide range of theoretical d-band centers (from −4.04 eV to −0.77 eV), we present the exchange current density of the V²⁺/V³⁺ half reaction on a range of pure transition metal electrodes as well as synthesized alloys of Cu and W. We compare experimentally determined d-band structures from UPS to these reaction rates and thus help improve our understanding of the relationship between reaction rates and electrode electronic structure.

References

(1) Tanimoto, S.; Ichimura, A. Discrimination of Inner- and Outer-Sphere Electrode Reactions by Cyclic Voltammetry Experiments. J. Chem. Educ. 2013, 90 (6), 778–781. https://doi.org/10.1021/ed200604m.

(2) Jiao, S.; Fu, X.; Huang, H. Descriptors for the Evaluation of Electrocatalytic Reactions: D-Band Theory and Beyond. Adv. Funct. Mater. 2022, 32 (4), 2107651. https://doi.org/10.1002/adfm.202107651.

(3) Agarwal, H.; Florian, J.; Pert, D.; Goldsmith, B. R.; Singh, N. Explaining Kinetic Trends of Inner-Sphere Transition-Metal-Ion Redox Reactions on Metal Electrodes. ACS Catal. 2023, 2223–2233. https://doi.org/10.1021/acscatal.2c05694.

(4) Hammer, B.; Nørskov, J. K. Electronic Factors Determining the Reactivity of Metal Surfaces. Surf. Sci. 1995, 343 (3), 211–220. https://doi.org/10.1016/0039-6028(96)80007-0.

(5) Vojvodic, A.; Nørskov, J. K.; Abild-Pedersen, F. Electronic Structure Effects in Transition Metal Surface Chemistry. Top. Catal. 2014, 57 (1), 25–32. https://doi.org/10.1007/s11244-013-0159-2.