Water splitting toward hydrogen and oxygen production is a promising way to store energy. Oxygen-evolution reaction (OER) provides electron for hydrogen production. However, OER is a complicated reaction and is a bottleneck for water splitting. Herein, we investigate the oxygen-evolving activity of a pentanuclear iron complex and consider the role of iron compounds other than the complex in OER as a candidate for the true catalyst in the presence of this highly efficient catalyst. The structure of the complex consists of a [Fe-3(mu(3)-O)] core wrapped by two [Fe(mu-L)(3), LH: 2,2'-(1H-pyrazole-3,5-diyl)dipyridine] units. After OER in the presence of the complex, the operated electrode in the absence of the complex and in a fresh buffer solution displays OER. Using scanning electron microscopy, energy-dispersive X-ray spectroscopy, in situ visible spectroscopy, X-ray photoelectron spectroscopy, and some electrochemical experiments, it is found that under oxygen-evolution conditions, this complex is not stable and Fe oxide on the surface of the electrode could be at least one of the candidates as the true catalyst for OER. Based on our study also a turnover frequency of 0.2-0.3 s(-1) was calculated for the formed Fe oxide. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.