Bismuth and cerium doped cryptomelane-type manganese dioxide nanorods as bifunctional catalysts for rechargeable alkaline metal-air batteries

Abstract

Metal-air batteries are energy storage devices (ESD) used in many technological applications ranging from portable, wearable to stationary, owing to its simplicity and low cost. Manganese oxides are suitable materials to be used as electrodes in such devices presenting an electrocatalytic performance for oxygen reactions close to Pt and, most importantly, being a low cost, abundant and environmentally friend material. However, as a drawback, manganese dioxide has low electrical conductivity thus requiring the combination with some highly conductive carbon support to be efficiently used as catalyst for the oxygen reactions in ESD. The low stability of carbon for water splitting reaction, however, limits the MnO2 to act as bifunctional catalyst. In this work, to overcome this problem, three and tetravalent cation doped (Bi3+, Ce4+) manganese dioxide nanorods exhibiting higher electrical conductivity were prepared and used as carbon free electrocatalysts for the oxygen reduction and evolution reactions in alkaline medium. X-ray power diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and Diffuse Reflectance Spectroscopy (DRS) measurements were performed to determine the structure, morphology, surface chemical composition and band gap values of the catalysts confirming the formation of nanorod structures with lower band gap values (EBG). The electrochemical results of Bi3+ and Ce4+ doped MnO2 confirmed that these materials have higher conductivity than K+1 doped MnO2 in agreement with the lower EBG inferred from DRS measurements. The electrocatalytic performance of doped electrocatalysts for the OER was close to that reported for conventional MnO2/C catalysts. On the other hand, they presented a remarkable performance for the ORR, higher than that presented by a commercial state of art Pt/C catalyst. The excellent catalytic performance was confirmed by Zn-air mini battery tests which presented a peak power density of 45 mW cm−2, higher than that presented by a conventional MnO2/C 50 wt.% catalyst (40 mW cm−2).