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

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 Nd0.1Ni0.9Co2O4 with excellent activities for both ORR (i.e. the half-wave potential E1/2 = 0.735 V) and OER (i.e. the overpotential at 10 mA cm-2 E10 mA.cm 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.