In Situ Synthesis of Meso-Porous Manganese Oxides/Sulfur-Doped Graphene Sheets Hybrid As a Highly Active Bifunctional Catalyst for Oxygen Evolution and Reduction Reactions

Abstract

With the increasingly urgent energy demands of recent years, tremendous efforts have been put into the development of high-performance, low-cost, and environmental-friendly electrochemical devices for energy storage and conversion, including metal-air batteries and alkaline fuel cells. However, the efficiency and power density of these devices are seriously restricted by the sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Until now, the most efficient catalysts are Pt and other precious-metal-based materials, while the high cost on the other hand hinders the commercial application. Therefore, an effective catalyst for OER and ORR with low cost becomes the key point for future research and development. The application of carbon and functionalized carbon materials for OER and ORR catalysts has raised considerable research interest in the past decade due to their good electrochemical properties including high conductivity and stability. In particular, due to the large specific surface area and high flexibility, the catalytic activity of graphene and graphene-like sp 2-hybridized carbon materials can be further improved by doping or/and by forming hybrids. MnOx (including MnO2, Mn2O3, Mn3O4, and MnO)/graphene hybrids have been used as catalysts for OER and ORR, due to their low cost and high activity. However, the activity of these hybrids is still not comparable to traditional noble metal catalysts, and the preparation procedures are usually complex, involving at least two separate steps including the formation of the carbon skeleton and the metal oxides. The ex situ preparation also affects the synergistic interaction between graphene and metal oxides. Therefore, there is a great need for a facile preparation method of a highly efficient graphene-based catalyst. It is also noted that the electrochemical properties of the graphene based material are highly related to its specific surface area, and a meso-porous structure will enhance its catalytic activity. In addition, it is found that doping of heteroatoms (including B, N, P, and S) into graphene-like materials improves the ORR and OER catalysis, due to the change in the electron distribution and density. In this work, we propose a novel and environmental-friendly approach to synthesize in situ a meso-porous MnOx/S doped graphene sheets (MnOx/S-GS) hybrid with a specific surface area of 502.6 m2 g-1. The synthesis involves the thermal treating of polyvinyl chloride (PVC) together with KMnO4 and (NH4)2Fe(SO4)2. It is the first time that MnOx/S-GS is fabricated from a carbon source to graphene hybrid using a facile one-pot method, avoiding any intermediate products, such as graphene oxide, before graphene formation and additional chemical reagents. In addition, since PVC is the major component of plastics, this method has provided a new approach for the utilization of waste. Most importantly, the as-prepared hybrid showed a high catalytic activity towards both ORR and OER. The current density reached -3 mA cm-2 at 0.80 V vs. RHE and 10 mA cm-2 at 1.60 V vs. RHE respectively, exhibiting potential for future application in the energy field, such as metal-air batteries and alkaline fuel cells.