Effect of potential and current on electrodeposited MnO₂ as a pseudocapacitor electrode: Surface morphology/chemistry and stability

Amorphous MnO₂ (a-MnO₂) that exhibits a high theoretical specific capacitance can be conveniently grown via electrodeposition as a pseudocapacitor electrode. However, the electrodeposited a-MnO₂ exhibits lower electrochemical stability than a-MnO₂ synthesized using other routes. We investigate the influence of the electrodeposition conditions on the evolution of the properties of a-MnO₂ during electrodeposition and their corresponding effect on the electrochemical stability. MnO₂ is electrodeposited at potentiostatic and galvanostatic mode. The different surface morphology/chemistry of a-MnO₂ is obtained by applying high and low potentials or current. The applied potential or current determines the surface morphology based on a predominance between nucleation and growth of a-MnO₂. The fine surface enables a higher specific capacitance because of the enlarged specific surface area but also leads to lower rate performance by limiting ion transport through the smaller microstructure. The opposite results are observed in a coarse MnO₂ deposit. When the applied potential or current is high, the manganese has a lower average valence number. The lifetime of a-MnO₂ is degraded by the irreversible reaction between Mn⁴⁺/Mn⁷⁺ and oxygen evolution. Particularly, finer MnO₂ is less stable than coarse one, which can arise from multiple high-energy connecting sites. A narrow potential window without Mn⁴⁺/Mn⁷⁺ interconversion promotes long-term stability.