δ-MnO2 nanoflowers on sulfonated graphene sheets for stable oxygen reduction and hydrogen evolution reaction.

Abstract Nanoflowers are highly beneficial structural features for improving catalysis on transitional metal-based materials with regarded to the electrocatalytic activity, cost effectiveness and durability. In this study, the manganese dioxide nanoflowers onto sulfonated graphene sheets (denoted as δ-MnO 2 /SGS) is synthesized by a simple hydrothermal method for bifunctional electrocatalytic application in oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). The formation of δ-MnO 2 nanoflowers onto SGS plays a crucial role in intrinsic catalytic activity, resulting in an impressive enhancement in both ORR and HER in alkaline electrolyte. The morphological features and X-ray analysis are shown the successful sulfonation and the Mn presents in oxide form with δ-type crystallographic structure. The growth reaction mechanism of δ-MnO 2 nanoflowers is proposed in alkaline media. The robust electrocatalysis is inherited from the cave among crystalline lego-block-like petals at microscale level and the large interlayer spacing at nanoscale level of δ-MnO 2 nanoflowers with significant electrical conductivity. The δ-MnO 2 /SGS catalyst exhibits nearly same ORR and HER activity in terms of current density with electron transfer and final current density with Tafel slope, respectively, compare to the benchmark Pt/C. [ABSTRACT FROM AUTHOR]