(289c) Effect of Doping on the Activity of Nickelate Oxides Toward Surface Oxygen Exchange and Oxygen Reduction

The first-order Ruddlesden-Popper (RP) series (A₂BO₄) oxides, known as nickelate oxides (e.g. La₂NiO_4+δ, LNO), have recently attracted increasing interest due to their good mixed ionic and electrical conductivities suitable for the electrocatalysis. For instance, they have been reported as potential cathode materials for intermediate temperature (600-800 â??) solid oxide fuel cells (IT-SOFCs)¹. We have previously demonstrated that the morphology and surface structure of lanthanum nickelate oxide can significantly affect its activity toward the surface oxygen exchange/reduction.² We found that LNO nanorods highly terminated by Ni oxide (001) surfaces exhibit superior performance to the traditional nanosphere LNO catalysts. To further enhance the activity of nickelate oxides, in this contribution we explore the effect of doping on the activity toward the surface oxygen exchange/reduction process using density functional theory (DFT) calculations combined with experimental studies. We focus on the substitution of the Ni B-site with Mn, Fe, Co, and Cu transition metals, and the La A-site with Ca, Sr, and Ba to tune the activity.

The surface lattice O diffusion into the bulk to form surface O vacancy is investigated first, and two possible diffusion pathways are proposed. The apical O assisted pathway is found to be more favorable. We find that the surface chemistry of La₂NiO₄ can indeed be tuned via modification of the Ni B-site and La A-site. With the B-site metal changing from Mn, Fe, Co, to Ni, the calculated binding energy of O₂ on the surface O vacancy neighboring the transition metal becomes gradually weaker. Moreover, the barrier for O₂ dissociation becomes linearly higher as the binding energy of O₂ becomes weaker, whereas, the surface O vacancy formation energy becomes lower. A volcano-type relationship between the calculated rates and the binding energies of O₂ is found, suggesting that the binding energy of O₂ might be a good descriptor to screen for nickelate oxides with optimal oxygen exchange activity.

References

1. A. Tarancon, M. Burriel, J. Santiso, S. J. Skinner, J. A. Kilner, J. Mater. Chem. 2010, 20, 3799.

2. X. F. Ma, J. S. A. Carneiro, X. K. Gu, H. Qin, H. L. Xin, K. Sun, E. Nikolla, ACS Catal. 2015, 5, 4013.