Relating the Electrochemical Behavior of Birnessite to the Morphology and Specific Surface: Interest of Studying the Surface Reactivity

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-MnO 2 , K-MnO 2 and HT-MnO 2 ) and protonated (H Na -MnO 2 , H K -MnO 2 and H HT -MnO 2 ), 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 SO 2 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 H K -MnO 2 with its eroded veils (85 m 2 /g). Redox reactivity was evaluated from XPS quantification comparing the S/M values, i.e., atom % of S/ atom % of Mn. HT-MnO 2 , its protonated derivative H HT -MnO 2 , and Na-MnO 2 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 H K -MnO 2 presents the best performance, 46.6 F/g at 100 mV/s, to be used as an electrode material in supercapacitor storage systems.

Manganese–cobalt geomimetic materials for supercapacitor electrode

“…The binding energy splitting value gives information regarding the oxidation state of manganese and confirms the mixed valence state Mn 3+/4+ , as reported in the literature. 41–43…”

“…The binding energy splitting value gives information regarding the oxidation state of manganese and confirms the mixed valence state Mn 3+/4+ , as reported in the literature. [41][42][43] As described by Quesne-Turin et al for spinel phases, decomposition of the Mn 2p peaks in an accurate manner allows the determination of the Mn 3+ /Mn 4+ ratio. 41…”

Incorporation of Fe3+ into MnO2 birnessite for enhanced energy storage: impact on the structure and the charge storage mechanisms

“…1,2 Among the most promising pseudocapacitive materials, the birnessite d-MnO 2 stands out due to its abundance, low cost, high theoretical capacity and stable cycling performance in aqueous electrolyte. [3][4][5][6][7][8][9][10] Moreover, its reaction mechanisms upon cycling in aqueous electrolytes (1 M K 2 SO 4 ), subject of debate, was recently elucidated by Augustyn et al 11 They demonstrated that birnessite 2D structure, with a large interlayer spacing resulting from the presence of nanoconned structural water molecules, enables extremely fast and efficient intercalation of partially hydrated alkaline ions, leading to capacitive like electrochemical signature.…”

Tuning the heterophase junction in Bi2O3 hybrid crystals with enhanced photocatalytic activity

“…Six birnessite-type phases with various morphologies were studied, and all exhibit diverse surface active sites due to their different surface features. 20 Based on those above, controlling the growth of Bi 2 O 3 with heterophase and different morphologies in a composite system and a one-pot process is a promising approach to design Bi 2 O 3 with high photocatalytic activity.…”