DFT Calculation on Na Doping Effects on Bismuth Ruthenate-Based Pyrochlore Oxides
Authors: Ohta Hiroto , Okazaki Aya , Morimitsu Masatsugu 
Published: December 2024

Oxygen evolution reaction (OER) is one of the most important electrochemical processes in a wide variety of fields such as water electrolysis, air batteries, electroplating, electrowinning, and others, while the overpotential is known to be 300 mV or more, which results in the increase in energy consumption and the decrease in energy efficiency. One of the authors of this paper has recently found a low overpotential of oxygen nano-catalyst, that is a bismuth ruthenate-based pyrochlore oxide [1]. This novel oxide catalyst contains ca. 5 at% of sodium, which seems to strongly affect the catalytic activity and stability for OER; e.g., the catalyst shows a Tafel slope of 40 mV/dec at 25 oC in 0.1 mol/L KOH solution, which is much lower than those of nickel-based oxide and PGMs investigated as OER catalysts for alkaline water electrolysis, although the mechanism of the fast kinetics has not been clarified. In this paper, we report the results of first-principles calculations of the electronic structure of bismuth ruthenate-based pyrochlore oxides (BRO) and discuss the effects of Na doping on the crystal structure of BRO.

Density functional theory (DFT) calculations in the generalized gradient approximation (GGA) were performed using the WIEN2k package [2] for the electronic structure of bismuth ruthenate pyrochlore Bi2Ru2O7 and its Na substituent. We substituted Na for one of the 16 Bi in the unit cell so that the Na content is close to the synthesized Na-doped BRO. Before the electronic structure calculations, structural optimization was performed within DFT with GGA using the VASP package [3-5].

We successfully optimized the structures for Bi2Ru2O7 and its Na substituent, and by Na doping, the unit cell exhibits isotropic compression in 0.12% coincident with slight distortion along a [111] direction. While the lattice shrinkage seems inconsistent with a difference in ionic size between trivalent Bi (1.17 Å) and monovalent Na (1.18 Å) [6], the local structure around Na was found to change according to the ionic size; i.e., the BiO8 polyhedron expanded with Na doping, and the NaO8 polyhedron was larger than the BiO8 polyhedron in Bi2Ru2O7 with Na. On the other hand, RuO6 octahedrons shrunk due to the Na substitution, and the degree of shrinkage was higher for the octahedron closer to Na. This shrinkage of the RuO6 octahedron may be due to the hole doping effect. The distributions of the size of the RuO6 octahedron and the electronic state of Ru are reflected in the electronic structure as complicated band dispersion. However, the energy dependence of density of states (DOS) was similar to that of Bi2Ru2O7, so the Na substitution may not have affected the electronic physical property. There is still a potential that some of the RuO6 octahedrons distributed due to the Na substitution induce the enhancement of OER activity by the Na substituent.

The authors acknowledge NEDO, Project No. JPNP20003, for their financial support.

Ref.

1) M. Morimitsu, US Patent No. 11777106-B2 (2023).

2) P. Blaha, L. Schwarz, F. Tran, R. Laskowski, G. K. H. Madsen, and L. D. Marks, J. Chem. Phys., 152, 074101 (2020).

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6) R. D. Shannon, Acta Crystallogr. A, 32, 751 (1976).