Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations

Mikaela Görlin,Aleksandr Kalinko,Anders Nilsson,Oscar Diaz-Morales,Mikhail Shipilin,Sergey Koroidov,Vadim Murzin,Mia Börner,Olga V Safonova,Hsin-Yi Wang,Joakim Halldin Stenlid,Joydeep Dutta,Abdusalam Uheida,Maarten Nachtegaal,Matthias Bauer

doi:10.1038/s41467-020-19729-2

Abstract

Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Electrolyte alkali metal cations have been shown to modify the activity and reaction intermediates, however, the exact mechanism is at question due to unexplained deviations from the cation size trend. Our X-ray absorption spectroelectrochemical results show that bigger cations shift the Ni2+/(3+δ)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH > NaOH ≈ KOH > RbOH > LiOH. Here, we find that the OER activity follows the variations in electrolyte pH rather than a specific cation, which accounts for differences both in basicity of the alkali hydroxides and other contributing anomalies. Our density functional theory-derived reactivity descriptors confirm that cations impose negligible effect on the Lewis acidity of Ni, Fe, and O lattice sites, thus strengthening the conclusions of an indirect pH effect.

Highlights

Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER
Cyclic voltammograms (CVs) and activity trends from steady-state data during the in situ X-ray absorption spectroscopy (XAS) measurements are presented in Fig. 1d, e. (The spectra will be presented in the XAS section below.) We find that the activity increases in the order of LiOH < RbOH < KOH < NaOH < CsOH, does not exactly follow the overall size of the cations
Several hypotheses behind the impact of electrolyte cations on the OER activity have been put forward; (I) strong non-covalent interactions between small alkali metal cations and chemisorbed species forming cation-OHads clusters that block the active sites10. (II) perturbation of adsorption energies of the OER intermediates and the kinetics7,50. (III) promotion of peroxo-like “active oxygen” species by larger cations leading to a higher activity[8,40]

Summary

Introduction

Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. We investigate the OER activity and redox-activity using X-ray absorption spectroscopy (XAS) of a Ni–Fe oxyhydroxide (OOH) electrocatalyst in the presence of alkali metal cations (Li+, Na+, K+, Rb+, Cs+), which is one of the best performing catalysts in alkaline media[5,6]. Electrolyte cations are known to impact the oxygen evolution activity of various oxide-derived electrocatalysts[7,8,9,10,11,12], where the activity seemingly follows the trend in cation size, typically increasing from small (Li+) to large (Cs+). Zaffran et al.[7] demonstrated using density functional theory (DFT) that electrolyte cations modify the adsorption energies of OER intermediates (*OH, *O, *OOH) of the Ni–Fe catalyst, where especially small and strongly acidic cations are not beneficial for OER. Recent studies employing in situ soft XAS at the O K-edge confirmed anionic redox-activity involving the lattice oxygens in the Ni–Fe catalyst[36,37], most likely related to the “active oxygen” earlier identified in Raman spectroscopy[38,39,40,41]

Methods

Results

Conclusion