Title: MnS-nanoparticles-decorated three-dimensional graphene hybrid as highly efficient bifunctional electrocatalyst for hydrogen evolution reaction and oxygen reduction reaction

Abstract

The search for renewable energy resources has attracted considerable research interests in electrochemical reactions of hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) that are essential for fuel cells. Earth-abundant, eco-friendly and cost-effective transition metal compounds are emerging candidates as electrocatalysts in these reactions. Herein, we report the growth of manganese sulfide nanoparticles on three-dimensional graphene, through an easy, progressive successive ionic layer adsorption and reaction (SILAR) method, where manganese sulfide nanoparticles (MnS-NPs), diameter of 4-5 nm are homogeneously decorated on the 3D graphene matrix. SILAR has been proven to be a facile, low-cost and eco-friendly method to grow nanomaterials on 3D conductive support and it is clearly different from previous MnS synthetic routes such as spray pyrolysis, microwave irradiation, chemical bath deposition, solvothermal and hydrothermal that required time-consuming procedures and harsh conditions. Transition metals based sulfides are the most extensively studied among nanostructured electrocatalysts for overall alkaline water splitting, with the advantages of being widely available, inexpensive, chemically stable, and highly active yet with low conductivity and sensitivity to half-cell processes issue. Importantly, the active sites of transition metal sulfides based electrocatalysts are nearly always the metal atoms, while exposed S atoms play a vital direct role in the HER process via increasing the number of active sites and changing the coordination environments of adjacent metal atoms. Moreover, the indirect role of S atoms in the HER provides locations for hydrogen adhesion and separation when metal atoms act as active sites. Owing to such beneficial of transition metal sulfides based electrocatalysts with carbon based supportive materials, as formed ,MnS@3DG has indicated marvelous synergistic effects between metal sulfide based nano particles and high surface areal graphene foam with such noticeable intimacy that has markedly improved conductivity to enlist the MnS@3DG nanocomposite as efficient bifuntional electrocatalyst in alkaline electrolyte showing an η HER of just 274 mV at a current density of 10 mA cm-2, much smaller than that of the bulk MnS (800 mV). Even when the current density is increased to 40 mA cm-2, the overpotential is still low as 350 mV. While the oxygen reduction onset potential was 0.9 V (vs. RHE), and the peak potential was 0.75 V (vs. RHE), thus achieving a bifunctional catalytic performance superior to most of the reported manganese-based electrocatalysts in HER and ORR.