Investigating into the intricacies of charge storage kinetics in NbMn-oxide composite electrodes for asymmetric supercapacitor and HER applications.

In this paper, we present an effortless hydrothermal method for depositing niobium and manganese composite oxides onto Ni-foam. The formation of the desired NbMn-oxide phase is confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analytical tools. In the study, we systematically investigated the effect of varying Mn concentrations on the physicochemical properties and energy storage performance of the NbMn-oxide composite. The NbMn-oxide exhibited nanoplates assembled from sphere-shaped structures along with irregularly shaped rectangular blocks of nano/micro-structures, forming a porous framework. The electrode with an Mn concentration of 0.02 M displayed an areal capacitance of approximately 5987.8 mF/cm2 and an energy density of 0.13 mWh/cm2, even at a high current density of 10 mA/cm2. Additionally, the hydrogen evolution reaction (HER) performance of the NbMn-oxide composite electrode was evaluated and it exhibited an overpotential of 86 mV to achieve a current density of 10 mA/cm². The electrode also displayed stable performance for approximately 7 h, with only a slight increase in overpotential from − 0.218 to − 0.263 V against the reversible hydrogen electrode (RHE) during the stability test. The investigation of charge storage kinetics revealed the dominance of the diffusion process over the capacitive process, with the quasi-reversible redox reactions in the composite metal-oxides responsible for the high electrochemical performance. The assembled asymmetric supercapacitor device, utilizing NbMn-2 (positive) and activated carbon (negative) electrodes, demonstrated an impressive energy density of 0.20 mWh/cm2 at a power density of 3.75 mW/cm2, coupled with exceptional cyclability (97%). These findings highlight the potential of Nb-based composite electrodes for both supercapacitor and HER water-splitting applications and underscore their significant potential for electrochemical energy conversion and storage. [Display omitted] • Effect of varying Mn on NbMn-oxide properties and energy storage studied, informing optimization for desired performance. • NbMn-oxide displayed nano/micro-structures for efficient energy storage and conversion. • Asymmetric device with NbMn-oxide (+ve) & activated carbon (-ve) electrodes showed noticeable electrochemical performance.. • NbMn-oxide composite electrode showed stable HER performance for 7 hrs, indicating potential for water splitting. • Diffusive dominance drives high electrochemical performance in composite metal oxides, surpassing capacitive processes. [ABSTRACT FROM AUTHOR]