Surface state of NiOOH under oxidative conditions: Can dopants induce surface oxidation?

Abstract

Nickel oxyhydroxide (NiOOH), featuring redox-active

, is one of the best non-noble electro-oxidation catalyst in alkaline solution. However, NiOOH is only stable at potentials $\ge 1.5$ V vs RHE, with

being the stable reduced form at lower potentials. The potential of the phase transition from inactive

to active NiOOH can be tuned by substitutional doping. Lowering the potential for reaching the phase transition is thought to be beneficial for lowering the overpotential of oxidation reactions catalyzed by NiOOH. Here, we investigate which first row transition metals are most plausible for this purpose: first, the doped structure should be more stable than the phase-segregated system and second the potential for reaching the NiOOH-like phase should be lower compared to the pure Ni compound. Energy differences between pure and doped bulk structures reveal that substitutional doping of NiOOH is plausible (less than 0.5 eV difference) for many dopants, but only V can be incorporated exothermically compared to their pure oxyhydroxides. Furthermore, dopants lead to a substantial lowering in the potential necessary to reach the phase transition. Since catalysis is more a surface than a bulk process, we then investigate the surface state of NiOOH(0001) and the impact of substitutional doping on it. To address this question, we apply grand-canonical density functional theory (GC-DFT) in order to explicitly account for the electrochemical potential. We find that the stoichiometric surface (50% hydrogen coverage) is the most stable one over a large range of relevant potentials at pH 14. Surface oxidation lowers the hydrogen coverage and occurs at about 1.7 V vs RHE, i.e., $\sim$0.2 V less positive compared to the potential of the phase transition. At a substitutional doping level of 25%, only V and Cr stabilize

at significantly lower potentials compared to pure NiOOH (down to 1.1 V vs RHE) in the bulk. Furthermore, vanadium, chromium and manganese might be suitable choices as these metal centers, which remain in the +III or +IV state at lower potentials compared to Ni, could also be active sites in electro-oxidation reactions.