Synthesis of Copper Sulfide and Hybrid with Two Dimensional Materials for Hydrogen Evolution Reaction

Abstract

Much effort has been made to develop cost-effective electrocatalysts for last decades to replace expensive and earth-rare platinum catalysts in hydrogen evolution reaction (HER). Copper sulfide (CuS) has been little used for an electrocatalyst but can have potential as a component of hybrid electrocatalysts. In this study, we demonstrate colloidal syntheses of CuS and its hybrid materials for HER catalysts. The colloidal CuS nanoplatelets showed poor electrocatalytic activity in HER (-526 mV for onset potential and 100 mV/dec for Tafel slope). The low HER performance can be attributed to proton-inaccessibility by hydrophobic ligand, and coverage of active sites by aggregation of nanocrystals. To improve the HER catalytic activity, CuS based hybrid materials with reduced graphene oxide (rGO) and molybdenum disulfide (MoS2) were synthesized. First, CuS/rGO hybrid was synthesized by simply mixing CuS and rGO solutions. Metallic rGO increases conductivity of the hybrid materials. Although conductive interconnecting rGO sheets enhance the performance of CuS/rGO in HER activity, onset potential (-462 mV) and Tafel slope (83.3 mV) are not comparable to those of other electrocatalysts still. In another approach, Cu2-xS/MoS2 hybrid was synthesized by ligand exchange and annealing procedures. Ammonium tetrathiomolybdate ((NH4)2MoS4) was used as a precursor for the ligand exchange. (NH4)2MoS4 was dissolved in N,N-dimethylformamide (DMF) and acted as an inorganic ligand (MoS42-). Simple sonication of a mixture solution consisting of CuS in hexane and (NH4)2MoS4 in DMF exchanged oleylamine ligand of CuS with MoS42- to give CuS@MoSx. During the ligand exchange, stacked CuS nanoplatelets were disassembled by sonication. The HER activity of CuS@MoSx was improved: -182 mV for onset potential and 88.2 mV/dec for Tafel slope. The improvement would be due to exposure of more active sites by disassembly of nanoplatelets and enhanced proton accessibility by removing oleylamine ligands attached to CuS. In addition, annealing of the as-prepared CuS@MoSx at 400 oC and 600 oC reduced onset potentials to 78 mV and 44 mV respectively although Tafel slope was same at 400 oC and even worse 9.1 mV/dec at 600 oC.