Birnessite for supercapacitors: alkaline versus neutral electrolytes

Birnessite recharging processes in neutral (sodium sulfate) and alkaline (sodium hydroxide) solutions are compared. Birnessite is fabricated by cathodic deposition from alkaline permanganate bath on smooth carbon supports. The hypothesis is formulated and verified concerning the reason of much wider potential interval available for birnessite recharging in neutral solutions as compared to alkaline. Namely, an apparent width of this interval observed in neutral solutions is found to depend on birnessite loading and convection in solution. These observations can be explained by the changes of local pH in the course of recharging in neutral medium: seeming extension of the potential window results from the screened shift of the onset(s) of pH-dependent irreversible process(es). The effects of cathodic and anodic potential limits on birnessite recharging are addressed systematically. The total charges corresponding to reversible birnessite behavior in neutral (Na2SO4) and alkaline (NaOH) solutions are found to be very close, despite the potential interval is apparently wider for the former solution. The advantages and risks of recharging in neutral media are considered. [GRAPHICS] .

Automated summary

The recharging processes of Birnessite in neutral (sodium sulfate) and alkaline (sodium hydroxide) solutions are compared. The width of the potential interval for Birnessite recharging in neutral solutions is found to depend on Birnessite loading and convection in solution, while in alkaline solutions, the potential interval is narrower. This difference can be explained by changes in local pH during recharging in neutral solutions.