(582aw) Interconnection of Spillover and Electrochromism at Tungsten Oxide

Our research group is working to improve the efficiency of energy conversion technologies like fuel cells by developing multifunctional catalysts for electrochemical processes. In this context, we are interested in studying the related phenomena of hydrogen spillover and electrochromism in tungsten oxide. Spillover is defined as a process of transferring a species formed or adsorbed on one surface to another surface that cannot form or adsorb the same species under the same conditions [1]. Spillover can take place from a metal to metal, from oxide to oxide, from metal to oxide, or from oxide to metal, and it can occur over relatively long distances [2]. Electrochromism is a phenomenon of reversibly changing the optical properties of a material through electrochemical oxidation or reduction. Materials with cathodic coloration show color change at negative potentials while those with anodic coloration exhibit color change at positive potentials [3].

Platinum (Pt) catalyst supported on tungsten oxide (WO₃) in the presence of hydrogen gas undergoes hydrogen spillover, as first reported by Khoobiar in 1964 [4]. The product of this reaction is hydrogen tungsten bronze (H_xWO₃) which can transport protons and electrons over distances of at least a few nanometers in short timescales [5]. Electrochromism of uncatalyzed WO₃ was first reported by Deb in 1969 [6]. Applying a negative potential to WO₃ yields a blue coloration due to formation of H_xWO₃. Thus, hydrogen spillover and electrochromism at WO₃ are clearly analogous processes in that they lead to the same product. Moreover, both processes are known to be accelerated in the presence of water. Nevertheless, the mechanistic details of hydrogen spillover and electrochromism at WO₃ have not been studied under mutually similar conditions.

In this poster presentation, we highlight our recent work to understand the similarities and differences between the mechanisms of hydrogen spillover and electrochromism at WO₃. We have combined electrochemical and optical measurements to correlate color changes, corresponding to H_xWO₃ formation, with current-voltage data under conditions intended to induce or suppress hydrogen spillover or electrochromism. Based on prior work and our own studies to date, we conclude that both processes occur by essentially the same mechanism comprising discrete proton and electron-transfer steps. We have extended these mechanistic studies to practical systems targeting multifunctional reactivity where hydrogen intermediates (i.e., protons and electrons) migrate across the interface between H_xWO₃ and another catalyst component.

[1] Prins, R. Chem. Rev. 2012, 112 (5), 2714–2738.

[2] Conner, W. C.; Falconer, J. L. Chem. Rev. 1995, 95 (3), 759–788.

[3] Velevska, J.; Stojanov, N.; Pecovska-Gjorgjevich, M.; Najdoski, M. J. Electrochem. Sci. Eng. 2017, 7 (1), 27–37.

[4] Khoobiar, S. J. Phys. Chem. 1964, 68 (2), 411–412.

[5] Bond, G. C. Studies Surf. Sci. Cat. 1983, 17, 1–16.

[6] Deb, S. K. Appl. Opt., AO 1969, 8 (101), 192–195.