(190f) Design of High Entropy Oxides Electrocatalysts for Efficient Oxygen Evolution Reaction

Hydrogen has been considered a promising substitute for fossil fuels and is ideally produced via water-splitting reaction.^1,2 However, its efficiency is largely limited by poor kinetics and low stabilities of anodes for the oxygen evolution reaction (OER). Therefore, the development of low-cost electrocatalysts without compromising electrochemical activity and stability is a great challenge for water splitting reactions.

Here, we computationally design high entropy metal-oxides (HEOs) based on spinel structure (M₃O₄) consisting of up to five different 3d-TM earth-abundant elements. We deploy high-throughput calculations to explicitly evaluate the operational stability and OER activity of this complex system. For this purpose, we design HEO surface model, shown in Figure 1a, which consists of an active M³⁺ site surrounded by another 5 nearest elements (A=Cr, B=Co, C=Mn, D=Ni, E=Fe) leading up to 120 distinct permutations for an equimolar mixture of HEOs. The calculated mixing enthalpy of this HEO models is independent of active site and largely negative­, indicating stable HEO is possible. The HEO model has Gaussian-like distribution of active sites for OH* and O*, with mean either stronger and weaker binding than pure spinel surface (Figure 1b). The overpotential calculation indicates that the Co site shows a minimum overpotential of 290 mV as compared to 340 mV for Cr and Fe active sites.

Our experimental collaborators synthesized the spinel-based HEOs with various stoichiometries using the a sol-flame method. Equimolar stoichiometry of Fe, Cr, Co, Mn, Ni exhibits significantly improved OER activity over other combinations studied, with the low overpotential of 309 mV to reach 10 mA cm^-2 and the smallest Tafel slope (30 mV dec^-1), as well as excellent long-term stability for 168 hours. This electrochemistry research is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program to the SUNCAT Center for Interface Science and Catalysis.

References

1. Olabi, A. G. Renewable energy and energy storage systems. Energy 136, 1–6 (2017)

2. Service, R. F. Hydrogen cars: Fad or the future? Science (80), 324, 1257–1259 (2009).

Figure 1. HEOs model consisting equimolar elemental mixture and its corresponding binding energies for O* and OH*. a) The HEO model with O* on active site surrounded by 5 neighboring elements labeled as A, B, C, D, E (Co, Cr, Fe, Mn, Ni respectively). b) The O* and OH* binding energy distributions for Cr, Co, and Fe active sites in the HEO system as compared to their pure spinel system.