The proton exchange membrane fuel cell (PEMFC) shows the interesting properties,¹ where oxygen reduction reaction (ORR) is rate-determining.² For the PEMFC, Pt-based materials are effective to be its ORR catalyst but their expensive cost and the limited availability are unavoidable,³ motivating to searching non-Pt materials. Zirconia (ZrO₂) becomes a considerable candidate for the ORR catalyst since it has a higher chemical stability than that of Pt in an acidic environment,⁴ where tetragonal ZrO₂ (t-ZrO₂) (101) and monoclinic ZrO₂ (m-ZrO₂) (-111) surfaces have been observed to be the most stable.⁵

In this study, oxygen reduction reaction (ORR) on m-ZrO₂ (-111), (-101), (110) and t-ZrO₂ (101) surfaces is investigated using the density-functional theory. Each surface is divided into two regimes, that are pristine and defective surfaces, containing defects as two nitrogen impurities and an oxygen vacancy (2N_O + V_O). Then, the effect of the defects on free energy diagrams and bonding characteristics of an adsorbed oxygen (O*) is investigated. The free energy diagrams correspond to elementary ORR steps involving four intermediates, that are O₂*, O₂H*, O*, and OH*.

As the results, the free energy diagrams show that the first electron transfer, corresponding to the formation of O₂H*, is rate-determining on the pristine surfaces. It is modified to the third and fourth electron transfers, corresponding to the formation and removal of OH*, on the defective t-ZrO₂ and m-ZrO₂ surfaces, respectively. Furthermore, the defects provide a significant effect on the bonding of O*, leading to a slight improvement of ORR catalytic activity on t-ZrO₂ but an apparent decrease on m-ZrO₂ surfaces. Moreover, due to the defects, adsorption free energies for the defective m-ZrO₂ surfaces, ΔG_O, ΔG_OH, and ΔG_OOH, are moving away from the universal scaling curve and the ideal catalyst point. On the other hand, for the defective t-ZrO₂ surface, ΔG_O, ΔG_OH, and ΔG_OOH are off the universal scaling curve, heading to the ideal catalyst point. Our result presents the significant effect of the defects, providing a new insight into the oxide-based catalysts.

References

1. E. Brightman et al., J. of Power Sources, 242, 244–254 (2013)
2. M. Beltrán-Gastélum et al., Energy, 181, 1225–1234 (2019)
3. X. Sun et al., Electrochim. Acta, 332, 135474 (2020)
4. A. Ishihara et al., J. Phys. Chem. C, 123(30), 18150–18159 (2019)
5. C. Morterra et al., Mater. Chem. Phys., 37(3), 243–257 (1994)