Thin Film Electrodes and Electrochemical Reactions at Three-Phase Boundary in Alkaline Media: A DFT Approach

With increased energy demands, fuel cells offer a viable solution due to their efficient conversion of energy from various fuels. Switching to a basic electrolyte, many metals that would be susceptible to corrosion in an acidic environment become stable in an alkaline one. This allows for the inclusion of non-precious metals in the electrodes. However, the kinetics in an alkaline environment tend to be sluggish; this must be overcome for alkaline fuel cells to become commercially realizable. While many model electrodes have been investigated for possibility to improve the kinetics in alkaline fuel cells, this work focuses on thin film electrodes and reactions happening at the interphase between the substrate and the film. The introduction of three-phase boundaries can allow the various reaction intermediates to preferentially bind to different locations, thus possibly allowing one to break away from scaling relationships. This could ideally enable the catalyst to reach higher catalytic activity [1].

To determine the effectiveness of these electrodes, density functional theory (DFT) is employed. Before work was done to model the interphases, extensive work was done examining various moiré patterns for various oxidation states of transition metal films, with a focus on FeO_xH_y films. In this work, the stability of a variety of transition metal (hydr)oxide thin films on Pt and Au are first examined for stability for their future prospects of viable catalysts for fuel cell applications. In addition, phase diagrams are constructed to determine the state of the film at a given voltage, something crucial for designing optimal catalysts. A DFT + U scheme is employed to model the thin films, where U values are fitted by correcting the formation energies.

While the oxidation state and strain of the film are predicted from the full film phase diagrams, the nature of the interphase needed to be determined. The focus , hydrogen evolution and CO oxidation at these interphases with each reaction occurring at a different equilibrium potentials. This knowledge necessitated the creation of phase diagrams for the interfaces, to identify the most stable oxidation state the edge may take. With these phase diagrams, the thermodynamics and kinetics for the two reactions could be calculated with DFT along with nudged elastic band (NEB) calculations to calculate the activation barriers. These results are then compared with those on a clean Pt surface.

 [1] R. Subbaraman, D. Tripkovic, K. Chang, D. Strmcnik, A. P. Paulikas, P. Hirunsit, M. Chan, J. Greeley, V. Stamenkovic, N. M. Markovic. Nature Materials. 11, 550 (2012)