Mass changes accompanying the electrochemical reaction of sol-gel formed 50:50 NiCo oxide films

Cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM) technique have been employed to monitor the mass changes during potential cycling of sol-gel (SG) formed 50:50 NiCo oxide films in 0.5 M NaOH solutions. The experimentally determined pH dependence, together with the EQCM measurements, have confirmed that charging and discharging of SG NiCo oxide films is accompanied by the injection/expulsion of mobile ions, suggested to be OH− and Na+. Although an unambiguous identification of the species involved in the mass transport within the oxide films cannot be made by CV and EQCM alone, these studies provide a deeper insight into morphological changes of the SG oxide films which occur with time of cycling in alkaline solutions. A model involving water exchange during the redox process of these films is proposed, with water being expelled during oxidation and injected upon reduction. An increased amount of exchanged water with longer cycling times may indicate the early stages of the degradation of SG-formed 50:50 NiCo oxide films.