Cycling Stability Issues Faced When Trying To Apply Anodic Films on Copper or Its Alloys as Supercapacitor Electrodes

Copper oxide/hydroxide nanomaterials are promising electrode materials for supercapacitors due to their high theoretical specific capacitance, low cost, and environmental friendliness. The anodization approach for making these nanomaterial electrodes has attracted widespread attention owing to its low energy consumption, easy morphology control, and binder-free process. However, during cycling stability tests in concentrated potassium hydroxide (KOH) electrolytes, anodic films prepared on brass and copper substrates exhibited a persistent increase in capacitance─an unusual phenomenon previously observed yet largely overlooked. As cycling stability is a key performance metric for supercapacitors, it is highly desirable to prevent the occurrence of the unusual phenomenon. Here, we thoroughly explored the mechanism behind this anomaly in capacitance upon cycling to address this issue. Our results suggested that the secondary growth of new oxides on the underlying metal substrate during cycling is responsible for the continuous increase in capacitance. Consequently, while copper substrates were replaced by copper films electroplated on other conducting substrates such as graphite, the resultant oxide films showed excellent cycling stability because of the elimination of a copper source for the secondary growth of the oxides.