Abstract
This article focuses on understanding the link between the morphologies and specific surfaces of well-known birnessites with electrochemical performance by studying the surface reactivity of each material. Our study is especially dedicated to show the impact of the material’s surface on the faradaic and pseudocapacitive mechanisms involved in the energy storage of the supercapacitors. For this purpose, a multiscale study was carried out on three birnessites, nonprotonated (Na-MnO2, K-MnO2 and HT-MnO2) and protonated (HNa-MnO2, HK-MnO2 and HHT-MnO2), to ensure the investigation of the surface reactivity on birnessites for each step of the nanocomposite conception based on birnessite. Scanning electron microscopy (SEM) was used to characterize the morphology of the materials. Coupling X-ray photoemission spectroscopy (XPS) and SO2 gas probe adsorption are especially devoted to determine the nature of active sites and the electronic structure of the material’s surface. Thus, we provide evidence that the surface properties, such as specific surface area and surface active sites, are linked to the electrochemical mechanism (faradaic or pseudocapacitive) of storage depending on the scan rate. We show that the site concentration determined by XPS can be directly linked to the contribution of the pseudocapacitive mechanism observed at low and moderate scan rates. As the capacitive mechanism is discriminated at a high scan rate, its contribution is related to the Brunauer–Emmett–Teller (BET) surface. The largest specific surface is obtained for HK-MnO2 with its eroded veils (85 m2/g). Redox reactivity was evaluated from XPS quantification comparing the S/M values, i.e., atom % of S/atom % of Mn. HT-MnO2, its protonated derivative HHT-MnO2, and Na-MnO2 exhibit the largest redox reactivity, with, respectively, S/M values of 0.49, 0.46, and 0.31, according to their smooth platelet morphology. Except for the high temperature birnessite, the change of morphology after protonation decreases the concentration of redox active sites while the BET surface increases. This work shows that HK-MnO2 presents the best performance, 46.6 F/g at 100 mV/s, to be used as an electrode material in supercapacitor storage systems.