Water Oxidation in the Presence of a Nickel Coordination Compound: Decomposition Products, Fe Impurity in the Electrolyte, and a Candidate as a Catalyst

Sustainable
energy sources require a large-scale storage system.
Water electrolysis toward hydrogen formation is a remarkable method
for energy storage. However, oxygen-evolution reaction (OER) through
water-oxidation reaction is a bottleneck in water-splitting systems.
Different Ni-based molecular structures have been claimed to be OER
catalysts. However, the impact of Ni (hydr)­oxides formed by the degradation
of Ni-based molecular structures on OER is a challenging issue. The
effect of electrolyte impurity on OER in the presence of metal coordination
compounds is rarely investigated. In this study, five important questions
are considered regarding OER in the presence of a Ni coordination
compound: (i) In which potential does the coordination compound start
to be decomposed? (ii) After how many CVs does the decomposition occur?
(iii) What is the effect of Fe impurity on OER in the presence of
the Ni coordination compound? (iv) What is the product of the decomposition
of the Ni coordination compound? (v) What is the true catalyst for
OER in the presence of the Ni coordination compound? The roles of
both the Fe impurity in the electrolyte and the Ni (hydr)­oxide formed
in the presence of a Ni coordination compound are investigated. The
experiments showed that in the presence of the Ni coordination compound,
Ni (hydr)­oxide and Fe impurity in KOH formed a Ni–Fe oxide-based
catalyst during OER, which is an efficient catalyst for OER. Fe impurity
is a key contributor to observing OER in the presence of this Ni coordination
compound.