(168c) Electric Field-Induced Reactions in Fuel Cells From First Principles

Solid oxide fuel cells will no doubt find more widespread use in the new millennium in the conversion of chemical to electrical energy offering, as they do, very high efficiencies and a unique scalability in electricity-generation applications. To control the ongoing reactions in such a device one can modify the selectivity of the catalyst itself, which oxidizes the fuel at the anode and dissociates O₂ at the cathode. The selectivity of the catalyst in a solid oxide fuel cell will be altered by the presence of a huge electric field of the order of 1 Volts per Angstroms, which occurs near the electrode-electrolyte interface [1].  Given this background, it is surprising that the role of the electric field is often ignored in the optimization of such devices. Ab initio quantum chemistry methods coupled with detailed kinetic models allow the prediction of such field-induced effects.  Here we report density-functional theory calculations of the dehydrogenation of methyl species on the Ni(111) surface with and without an electric field. The structures, adsorption energies, and reaction barriers for all methyl species on the Ni(111) surfaces are identified. Our results show that the presence of an electric field does not affect the structures nor does it affect the favorable adsorption sites of the adsorbed species, but causes the adsorption energies of the CH_x species and the dissociation energy barrier of a CH molecule to be significantly altered by about 0.2 eV.  We expect that such field-induced effects to be even more important on a stepped surface, since the local electric field will be greatly enhanced at the step [2]. We compare these results to when a non-reactive metal (such Au) is added to the Ni catalyst with regard to the suppression of coking [3].

[1] E.M. Stuve, Ionization of water in interfacial electric fields: An electrochemical view, Chem Phys Lett, 519-520 (2012) 1-17.

[2] Y. Suchorski, N. Ernst, W.A. Schmidt, V.K. Medvedev, H.J. Kreuzer, R.L.C. Wang, Field desorption and field evaporation of metals, Prog. Surf. Sci., 53 (1996) 135.

[3] H.S. Bengaard, J.K. Nørskov, J. Sehested, B.S. Clausen, L.P. Nielsen, A.M. Molenbroek, J.R. Rostrup-Nielsen, Steam Reforming and Graphite Formation on Ni Catalysts, Journal of Catalysis, 209 (2002).