Morphological Degradation of Oxygen Evolution Reaction-Electrocatalyzing Nickel Selenides at Industrially Relevant Current Densities

We investigated electrodeposited
nanoparticulate nickel
selenide
(pre)catalysts that transform into nickel oxides/oxyhydroxides under
oxygen evolution reaction conditions in alkaline solutions. Previous
studies of this transformation were conducted at lower current densities
than those of industrial relevance (≥1 A cm–2). We used ultramicroelectrodes (UMEs) to achieve such current densities,
benefiting from their small size, ensuring low absolute currents and
low ohmic drop but high current densities. Morphological degradation
of the catalyst material was only observed at current densities exceeding 1 A cm–2 but not for
smaller ones. Using X-ray absorption, X-ray photoemission spectroscopy,
and X-ray diffraction, we confirmed that the degradation was accompanied
by the literature-known transformation of nanoparticulate Ni3Se2 (bulk)/NiSe (surface) into nickel oxyhydroxide. The
transformation of the precatalyst goes along with a significant improvement
in the charge transfer kinetics observed by decreasing Tafel slopes
with ongoing experimental time extracted from cyclic voltammetry (CV)
experiments and electrochemical impedance spectroscopy (EIS) in the
high-frequency range. However, these kinetic improvements are accompanied
by limitations in mass transport concluded from decreasing current
responses at high overpotentials in CVs and increasing impedance in
the low-frequency range of the EIS spectra after extended CV cycling.
These mass transport limitations originated from morphological degradations
at the UME exceeding 1 A cm–2 which we proved by
applying identical location scanning electron microscopy. This has
not been reported in studies that have been limited to lower current
densities before. Our findings showcase how UMEs can be used to study
(pre)catalysts (herein nickel selenides) under current densities of
industrial relevance in the absence of ohmic drop-related ambiguities,
combined with in-depth materials characterization studies, e.g., identical
location microscopy and advanced spectroscopic methods. This approach
enables direct evaluation and comparison of catalyst materials and
thus demonstrates how to overcome long-standing limitations of electrocatalyst
design and testing.