The effect of iron-spiked alkaline electrolyte on Ni-based electrocatalysts towards the oxygen evolution reaction

Abstract

An efficient means to store renewable energy is the electrolytic production of hydrogen. However, the anodic half-cell reaction in water electrolysis is sluggish and requires a good, cost-effective electrocatalyst, such as Ni. Nevertheless, the reason for the high electrocatalytic activity of Ni was proven to be trace amounts of Fe in the alkaline electrolyte. In recent years, numerous efforts have been made to elucidate how Fe enhances the OER activity of Ni. Controversy in the literature exists as to whether Ni or Fe is the active site, and recent literature indicates that it is surface Fe as opposed to Fe in the bulk structure that enhances the OER activity of Ni electrocatalysts. Therefore, this study aims to determine the effect of electrolytic Fe on both the OER activity and the structure of Ni electrocatalysts through electrochemical and physical characterisation. Eight electrocatalysts were investigated in iron-spiked 0.1 M KOH, i.e., colloidal nanoparticles as Ni and NiO, nanosheets as Ni, NiO and NiNiO and magnetron sputtered Ni, NiO and NiNiO. The nanoparticles were prepared with two different synthesis methods (a tailored method and chemical reduction) to produce two unique morphologies and annealing was employed to produce different oxidation states. Cyclic voltammetry (CV) was used to precondition the Ni electrocatalysts, evaluate the OER activity and to give insight into the redox features. Furthermore, in-situ Raman spectroscopy provided insight into the OER active sites. X-ray photoelectron spectroscopy (XPS) was used to analyse the oxidation states of Ni as well as the amount of Fe on the surface of the electrocatalysts before and after cycling. Similar overpotentials were achieved for nanoparticles cycled in pure KOH regardless of the preparation method or as-prepared oxidation state (390 – 415 mV). Lower overpotentials were achieved for sputtered thin films (350 mV). Increasing the electrolytic Fe concentration from 0.007 ppm to 1 mM resulted in increased OER activity with unique behaviour for electrocatalysts with different morphologies. For colloidal nanoparticles, enhanced activity was observed only at 1 mM Fe and overpotentials of 360 mV were achieved. For nanoparticles prepared by reduction (nanosheets), an electrolytic Fe concentration of 0.3 ppm already enhanced the activity. For sputtered electrocatalysts, the minimum overpotential is achieved after the addition of 0.9 ppm Fe to the electrolyte. Raman spectroscopy of nanoparticles confirmed that α-FeOOH appears on the surface of all electrocatalysts at the electrolytic Fe concentrations where enhanced activity is observed, confirmed by the presence of three new peaks (apart from the NiOOH peaks at 480 and 560 cm-1), corresponding to FeOOH. The absence of peaks corresponding to NiFe LDH confirms that the Fe species responsible for enhanced OER activity is indeed surface-bound and not bulk Fe. Furthermore, SEM-EDX and XPS results confirmed that Fe is present on the surface and is not homogeneously distributed. This investigation was successful in preparing and comparing Ni electrocatalysts of different morphologies and identifying FeOOH on the surfaces corresponding

to enhanced activities of all electrocatalysts regardless of morphology. The knowledge obtained in this study can be employed to develop more efficient OER electrocatalysts in the future.