Electrochemical performance of MnOx·nH2O@Ni composite foam electrodes for energy storage in KOH media

Nanostructured porous MnO₂, especially its hydrated amorphous and low crystalline form (MnO₂·nH₂O), has been one of the most promising material considered for charge storage applications, due to electrochemical similarities with RuO₂ and its relative low cost. However, the intrinsic poor conductivity of MnO₂ combined with the presence of structural water, which provides high ionic but low electronic conductivity, is a great hindrance for wider application. An effective approach to overcome this drawback involves the deposition of thin MnO₂ layers on porous, high surface area metallic scaffolds. The present work addresses this route and provides novel insights thanks to the combination of MnO_x·nH₂O with custom-made Ni foams, fabricated via one-step electrodeposition using the dynamic hydrogen bubble template (DHBT). The porous Ni foams provide a scaffold with a 3D architecture with optimized pore size and surface. The composite electrode was fabricated by anodic deposition of MnO_x·nH₂O on the 3D Ni foams. The electrochemical behaviour was tested in 1 M KOH, since there are very few studies addressing the electrochemical behaviour of MnO_x·nH₂O in alkaline media for electrochemical supercapacitors applications. In addition, thermal treatment (150–250 °C) was performed to evaluate the effect of hydration on the material properties. The results revealed that the as-obtained composites are highly stable, displaying much higher specific capacitances with 73–90% (depending on the mass load) capacitance retention compared to their de-hydrated counterparts. The charge-discharge processes were found to be highly reversible throughout 5000 cycles, maintaining almost 100% columbic efficiency. In conclusion, the MnO_x·nH₂O@Ni composite electrodes showed a very stable pseudocapacitive behaviour and exceptional cycling performance in 1 M KOH, being therefore a promising alternative charge storage electrode for electrochemical supercapacitors.