Purification of Residual Ni and Co Hydroxides from Fe‐Free Alkaline Electrolyte for Electrocatalysis Studies

Abstract It is critical to control Fe impurity concentrations in oxygen‐evolution‐reaction electrocatalysis experiments so that unambiguous assignments of activity and mechanistic details can be made. An established method to prepare Fe‐free KOH electrolyte is by using particulate Ni(OH) 2 or Co(OH) 2 as absorbents to remove the Fe from KOH or other neutral‐to‐alkaline electrolytes. However, this method yields residual Ni or Co species in the electrolyte which can be redeposited on the working electrode. Thus, current methods of Fe removal could convolute studies of OER. In this work, we compared two different methods, continuous electrolysis and nano‐filtration, to remove the Ni and/or Co species from Fe‐free alkaline electrolyte. We found the best approach is to pass the Fe‐free electrolyte through a hydrophilic 0.1 μm polyethersulfone filter which decreases the Ni species concentration in 1 M KOH to single ppb levels. This result suggests the remaining Ni or Co species are primarily particulate in nature, consistent with their small solubility as ions. In comparison, extended pre‐electrolysis of the electrolyte removed only a portion of the Ni/Co.