Tuning the electrocatalytic activity of perovskites and related oxides and the elucidation of new catalyst design criteria
Increasing global energy demand requires greater efficiency in water electrolyzers for low cost hydrogen generation and rechargeable metal-air batteries to enable pragmatic development of these key technologies. Given that the efficiencies of these technologies are limited primarily by the sluggish kinetics of the oxygen evolution reaction (OER) and/or oxygen reduction reaction (ORR), I have made extensive efforts to reduce the overpotential for the OER and ORR in alkaline media by designing advanced non-precious metal electrocatalysts. Alkaline conditions were chosen owing the more facile kinetics of the ORR, as compared to acid, and because alkaline electrolytes enable the use of catalysts containing non-precious metals. Perovskites, represented by ABO₃± [subscript δ], in which A is a rare-earth or alkaline earth element and B is a transition metal, were selected because of their demonstrated ability to promote the OER and the ORR, owing to their high ionic and electronic conductivities, good structural stability, and synthetic versatility. This made perovskites an ideal crystal structure for the development of rational catalyst design criteria. To this goal, I designed a series of lanthanum-based perovskite electrocatalysts LaBO₃, B = Ni, Ni₀.₇₅Fe₀.₂₅, Co, Mn) that are highly active for both the OER and ORR in an aqueous alkaline electrolyte. LaCoO₃ sup7ported on nitrogen-doped carbon is shown to be ~3 times more active for the OER than high surface-area IrO₂, and was demonstrated to be highly bifunctional by having a lower total overpotential between the OER and ORR (ΔE = 1.00 V) than Pt (ΔE = 1.16) and Ru (ΔE = 1.01). I discovered a new catalyst design principle using a series of Ruddlesden-Popper (RP) La₀.₅Sr₁.₅Ni₁- [subscript x] Fe [subscript x] O₄+ [subscript δ] oxides that promote Ni-O-Fe charge transfer interactions which significantly enhance OER catalysis. Using selective substitution of Sr and Fe to control the extent of hybridization between e [subscript g] (Ni), p(O) and e [subscript g] (Fe) bands, I have demonstrated exceptional OER activity of 10 mA cm⁻² at a 360 mV overpotential and mass activity of 1930 mA mg⁻¹ [subscript ox] at 1.63 V, over an order of magnitude more than the leading precious-metal oxide electrocatalyst IrO₂. In the course of this work, I also helped discover reversible charge storage via anion-based intercalation in LaMnO₃+ [subscript δ] electrodes, and a new OER mechanism whereby redox-active lattice oxygen directly participates in the formation of O₂.