Understanding the charge storage mechanism of supercapacitors: in situ/operando spectroscopic approaches and theoretical investigations

Deciphering the charge storage mechanism of conventional supercapacitors (SCs)can be a significant stride towards the development of high energy density SCs with prolonged cyclability, which can ease the energy crisis to a great extent. Although ex situ characterization techniques have helped determine the charge storage mechanism of SCs, large unexplored grey areas with unknown ensembles still exist, which cannot be neglected. Over the past decade, in situ analytical characterization tools such as in situ X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS), in situ X-ray photoelectron spectroscopy (XPS), in situ Raman, in situ infrared/Fourier transform infrared spectroscopy (IR/FTIR), in situ nuclear magnetic resonance (NMR), in situ atomic force microscopy (AFM), in situ scanning electron microscopy (SEM), in situ tunnelling electron microscopy (TEM), and in situ electrochemical quartz crystal microbalance (EQCM)techniques have exclusively come to the forefront to shed light on the charge storage mechanism of SCs. This review emphases the insights into the charge storage mechanism interpreted from in situ characterization techniques together with the theoretical investigation validations. Various charge storage parameters obtained from electronic structure simulations such as quantum capacitance, voltage induced by electrolyte ions, and diffusion energy barrier of electrolyte ions are detailed with pertinent examples. The amalgamation of in situ techniques and theoretical simulations can efficiently elucidate the ion dynamics and charge transfer in SC electrode systems, giving a whole new perspective. A comprehensive classification of SCs based on their mechanism, choice of electrodes and device configuration, and explanation of the charge storage mechanism based on in situ/operando techniques together with theoretical explorations can be obtained herein.