Proceedings of nanoGe Fall Meeting 2018 (NFM18)
Publication date: 6th July 2018
Using density functional theory calculations with an on-site Hubbard term (DFT+U) we explore the effect of surface orientation, termination and cation substitution on the performance of the CoxNi1-xFe2O4 (001) and (111) surfaces (x= 0.0, 0.5, 1.0) as anode materials in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni and an oxygen vacancy) were investigated at different surface terminations, e.g. the B-layer with octahedrally coordinated Co/Ni and with an additional 0.5 and 1 ML of Fe (0.5 A and A layer) at the (001) spinel surface. Our results indicate that for the latter termination the OER activity is significantly higher than for the (111) surface [1]. Moreover, Ni substitution with equal concentrations of Co and Ni (x=0.5) reduces further the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the 1 ML Fe termination exhibit the lowest theoretically reported overpotential of 0.30 V. The improved performance is correlated to the modification of binding energies of intermediate species to the surface. Analysis of the electronic properties and spin densities indicates that this beneficial effect is associated with stabilization of a bulk-like oxidation state of +2 for Co and Ni at the 0.5 ML Fe termination, while at the B-layer termination they are oxidized to +3. [2]
We acknowledge funding within SPP1613, PE883/9-2 and computational time at magnitUDE and Leibniz Rechenzentrum.
[1] K. Chakrapani, G. Bendt, H. Hajiyani, I. Schwarzrock, T. Lunkenbein, S. Salamon, J. Landers, H. Wende, R. Schlögl, R. Pentcheva, M. Behrens, S. Schulz, Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction, ChemCatChem. 9, 2988-2995, (2017).
[2] H. Hajiyani and R. Pentcheva, submitted.
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