Ease of Electrochemical Arsenate Dissolution from FeAsO₄ Microparticles during Alkaline Oxygen Evolution Reaction

Transition metal-based ABO₄-type materials have now been paid significant attention due to their excellent electrochemical activity. However, a detailed study to understand the active species and its electro-evolution pathway is not traditionally performed. Herein, FeAsO₄, a bimetallic ABO₄-type oxide, has been prepared solvothermally. In-depth microscopic and spectroscopic studies showed that the as-synthesized cocoon-like FeAsO₄ microparticles consist of several small individual nanocrystals with a mixture of monoclinic and triclinic phases. While depositing FeAsO₄ on three-dimensional nickel foam (NF), it can show oxygen evolution reaction (OER) in a moderate operating potential. During the electrochemical activation of the FeAsO₄/NF anode through cyclic voltammetric (CV) cycles prior to the OER study, an exponential increment in the current density (j) was observed. An ex situ Raman study with the electrode along with field emission scanning electron microscopy imaging showed that the pronounced OER activity with increasing number of CV cycles is associated with a rigorous morphological and chemical change, which is followed by [AsO₄]^3- leaching from FeAsO₄. A chronoamperometric study and subsequent spectro- and microscopic analyses of the isolated sample from the electrode show an amorphous γ-FeO(OH) formation at the constant potential condition. The in situ formation of FeO(OH)_ED (ED indicates electrochemically derived) shows better activity compared to pristine FeAsO₄ and independently prepared FeO(OH). Tafel, impedance spectroscopic study, and determination of electrochemical surface area have inferred that the in situ formed FeO(OH)_ED shows better electro-kinetics and possesses higher surface active sites compared to its parent FeAsO₄. In this study, the electrochemical activity of FeAsO₄ has been correlated with its structural integrity and unravels its electro-activation pathway by characterizing the active species for OER.