Electrochemical transformation of AP-CVD manganese oxide thin films into $$\delta$$-MnO$$_2$$ in alkali sulfate electrolytes for supercapacitor applications

Manganese oxides, especially birnessite-type $$\delta$$ -MnO $$_2$$ , are promising pseudocapacitive materials as electrodes in supercapacitors due to their layered structure and high specific capacitance. However, understanding how the nature of intercalated cations influences their structural evolution and charge storage mechanism remains an open challenge. In this work, $$\delta$$ -MnO $$_2$$ thin films were obtained through the electrochemical transformation of Mn $$_x$$ O $$_y$$ films deposited directly on stainless-steel current collectors by atmospheric pressure chemical vapor deposition (AP-CVD). The transformation was conducted in sulfate electrolytes containing K $$^+$$ , Na $$^+$$ , and Li $$^+$$ ions. After the transformation, the resulting $$\delta$$ -MnO $$_2$$ electrodes were evaluated as supercapacitor electrode in different alkali sulfate electrolytes to assess the influence of the testing medium on their charge storage behavior. The results demonstrate that the electrolyte cation significantly determines the crystallinity, morphology, and pseudocapacitive response of the resulting birnessite films. $$\delta$$ -MnO $$_2$$ phases were successfully formed in K $$_2$$ SO $$_4$$ and Na $$_2$$ SO $$_4$$ . K- $$\delta$$ -MnO $$_2$$ exhibited predominantly hexagonal structure with the coexistence of the monoclinic phase, while Na- $$\delta$$ -MnO $$_2$$ exhibited a monoclinic structure with well-defined nanowalls and the highest surface-controlled contribution to charge storage, achieving areal capacitances up to 6.5 mF cm $$^{-2}$$ when evaluated in Li $$_2$$ SO $$_4$$ , proving the monoclinic structure of $$\delta$$ -MnO $$_2$$ as the best for energy storage. These findings highlight the crucial role of cation species in tuning the intrinsic electrochemical behavior of $$\delta$$ -MnO $$_2$$ thin films and provide valuable insights for the rational design of high-performance birnessite-based electrodes for energy storage applications.