Single Layers of WS2 Nanoplates Anchored to Hollow N-doped Carbon Nanofibers as Efficient Electrocatalysts for Hydrogen Evolution

Abstract

Since hydrogen is considered as one of the most promising energy carriers, alternative to finite fossil fuels, intensive research efforts have been devoted to developing noble metal-free electrocatalysts for hydrogen evolution reaction (HER). In particular, transition metal dichalcogenides (TMDs) such as MoS2 and WS2 have drawn tremendous attention due to their fascinating electrocatalytic activities for HER when they are single-layered or few-layered with numerous catalytically active sites. Unfortunately, their poor intrinsic conductivities hamper their use as electrocatalysts for HER. In this regard, it is an efficient strategy to make a composite, combining single-layered TMDs with highly conductive materials. Herein, single-layered WS2 nanoplates are randomly oriented and uniformly anchored to hollow N-doped carbon nanofibers (WS2@HNCNFs) via coaxial electrospinning and a subsequent two-step thermal treatment. For the coaxial electrospinning, styrene-acrylonitrile (SAN) was used for the core as a sacrificial template while polyacrylonitrile (PAN) and (NH4)2WS4 for the sheath as a robust carbon matrix and single layers of WS2, respectively. The two-step thermal treatment was conducted for the crystallization of WS2 nanoplates, and for the carbonization of the PAN with in situ N-doping. The growth of WS2 nanoplates along the c-axis direction was hindered by the surrounding amorphous carbon matrix, preventing stacking of the WS2 nanoplates. In situ N-doping in the hollow carbon nanofibers was achieved by generated ammonia gas (NH3) as by-product gas during the thermal decomposition of (NH4)2WS4. For comparison, bulk WS2 powder and single layers of WS2 embedded in nitrogen-doped carbon nanofibers were synthesized and electrochemically tested. The distinctive design of the WS2@HNCNFs enables remarkable electrochemical performances. The catalyst electrode exhibited superior electrocatalytic performance with a low overpotential with reduced charge transfer resistance to obtain significant hydrogen evolution, a small Tafel slope of approximately 45 mV/dec, a high exchange current density, and excellent durability. The extraordinary catalytic performance can be attributable to numerous exposed edges of single-layered WS2 and facile electron path ways provided by the hollow N-doped carbon network. In addition, the hollow structure showed increased the Brunauer-Emmett-Teller (BET) surface area, offering easy accessibility for electrolyte to catalytically active sites.