A Nd-doped NiCo spinel dual functional catalyst for both oxygen reduction reactions and oxygen evolution reactions: Enhanced activity through surface reconstruction

Abstract

The design of efficient, low-cost, highly active and thermally stable electrocatalysts is critical for both oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). While some spinel metal oxides exhibit good activities for either ORR or OER, a bifunctional spinel metal oxide that can provide decent activities for both ORR and OER would be most desirable. To date, rare earth metal-modified spinel oxides have not been well-studied, but they are thought to be able to boost both ORR and OER simultaneously. Hence, a Nd-doped NiCo₂O₄ catalyst was synthesized in this work to evaluate its potential for improving both ORR and OER reactions. We hypothesized that this catalyst would be a viable option, as the highly oxidized Co⁴⁺ (hydroxycobalt oxide) generated from surface reconstruction could be an active site for OER while Ni²⁺ is intrinsically an active site for ORR. Amazingly, our study revealed that the addition of Nd in spinel metal oxides was able to inhibit the formation of Co⁴⁺ at low potentials while the Ni species promoted the formation of Co⁴⁺ from Co²⁺, thus achieving a balance between Co²⁺ and Co⁴⁺ which resulted in a multi-step oxidation process of Co²⁺ → Co³⁺ → Co⁴⁺. In addition, by tuning the amount of Nd doped, an optimum electrocatalyst Nd_0.1Ni_0.9Co₂O₄ with excellent activities for both ORR (i.e. the half-wave potential E_1/2 = 0.735 V) and OER (i.e. the overpotential at 10 mA cm⁻² E_{10 mA·cm}⁻² = 302 mV) in alkaline conditions was developed. In summary, this work may have opened a new pathway for applying spinel metal oxides as bifunctional catalysts in future commercial ORR and OER processes.