EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

In recent years, δ-MnO2 birnessite appeared as a promising positive electrode material for both hybrid supercapacitors and aqueous batteries due to its layered structure, which favors ionic diffusion, its low cost, high abundance and environmental friendly character. The δ-MnO2 phase possesses a high theoretical capacity (~300 mAh/g), however its low electronic conductivity strongly limits the electrochemical applications of the material [1]. In order to increase intrinsic conductivity of the birnessite, several works have been carried out to incorporate other transition metal like cobalt and nickel in the Mn network [2,3]. So as to use an abundant and eco-responsible element, we have chosen to incorporate iron by two different synthetic approaches, aiming at reducing the band gap and thus, improving the intrinsic electronic conductivity [4]. Iron was introduced either as a precursor during the high temperature synthesis of birnessite (HT-MnO2) or via ionic exchange by dispersing δ-MnO2 in rich iron medium during few days, involving different morphologies of initial δ-MnO2 (nano-platelets, nano-veils and micro-platelets (HT-MnO2)). Combining several characterization techniques (XRD, Raman, Mössbauer, XANES, EXAFS), we have shown that Fe3+ ions are incorporated in the structure. Depending on the various synthetic routes, iron is located in different sites in δ-MnO2 : i) only in the metal oxide layer when Fe is incorporated as a precursor during high temperature synthesis of birnessite, ii) only in the interlayer space when Fe is inserted by dispersing HT-MnO2 in Fe nitrate or iii) in both sites when MnO2 nano-veals and platelets are dispersed in Fe nitrate. In all cases, the incorporation of iron lead to a significant increase of the capacity especially due to new redox contributions involving Fe during the cycling. However, this capacity increase is counterbalanced by the limited stability upon cycling.