Minimizing the Nyquist-plot semi-circle of pseudocapacitive manganese oxides through modification of the oxide-substrate interface resistance

Abstract

Manganese oxides are promising pseudocapacitive materials for electrochemical capacitors, and their capacitance, resistance and frequency response can be characterised using electrochemical impedance spectroscopy. Often, though not universally, the Nyquist plots of these materials evidence a semi-circular feature, which is typically attributed to the pseudocapacitive reaction's charge-transfer resistance. However, we show, for the first time, that the origin of the semi-circle is in fact the oxide-substrate interface resistance. The interface is varied through heat treatment of the manganese oxide and by electrodepositing the manganese oxide on different metals (stainless steel and platinum). Additionally, our novel manganese oxide double-deposition – in which a thin layer of manganese oxide on the interface is heat treated but the bulk of the film is hydrous – separates the interface and pseudocapacitive responses. Films with a low interface resistance (e.g., with a heat-treated interface or deposited on Pt) evidence a 98% smaller semi-circle than those with a poor interface (films deposited on stainless steel with no heat treatment). Conversely, there is no correlation between the amount of psuedocapacitance and semi-circle size. Thus, the Nyquist semi-circle is associated with the interface resistance, and proper modification of this interface can reduce the performance limitations associated with this resistance.