Improved manganese oxide electrochemical capacitor performance arising from a systematic study of film storage/drying effects on electrochemical properties

Abstract This work is the first to systematically relate manganese oxide film storage/drying to a full gamut of electrochemical capacitor (EC) properties, with the aim of enhancing these properties and addressing this oxide's well-known stability issues. Cyclic voltammetry and galvanostatic charging-discharging show that increased film-drying temperatures result in lower resistance and higher power but at the expense of capacitance and energy. Film drying reduces physical stability and cycle life, while undried, electrolyte-stored films exhibit poor long-term aging. Novel electrolyte-stored films show a ∼102% capacitance increase, ∼75% energy increase and 32% improved film usage compared to 200 °C-dried films, while 200 °C drying results in a ∼69% power increase and ∼75% decrease in resistance compared to non-heat-treated films. Using the knowledge acquired from this systematic study, we then strategically design an innovative double-deposition in which an initial manganese oxide layer is oven-dried and subsequently recoated with a second manganese oxide layer before storing the entire electrode in electrolyte. Films made with this novel double-deposition demonstrate the optimal properties achieved from both electrolyte-stored films (high energy, capacitance, coulombic efficiency, and physical stability) and heat-treated films (low resistance, higher power and rate capability, good aging, and stability in a large potential window); they also demonstrate the smallest degree of self-discharge. This novel synthesis produces films suitable for a wider range of EC applications.