Highly active MoS/carbon electrocatalysts for the hydrogen evolution reaction - insight into the effect of the internal resistance and roughness factor on the Tafel slope.

Molybdenum disulphide (MoS) nanomaterials are promising non-precious-metal electrocatalysts for the hydrogen evolution reaction. MoS/carbon electrocatalysts have been synthesized with the carbon component serving the purpose of enhancing electron transport. The impedance method of Tafel analysis has been employed to evaluate the efficiency of various carbon supports in aiding facile electron transport. A MoS/carbon nanofiber electrocatalyst has been found to be the most active towards hydrogen evolution with the lowest Tafel slope among the investigated electrocatalysts. Tafel analysis indicates that the hydrogen evolution reaction occurs through the Volmer-Heyrovsky mechanism with a rate determining Heyrovsky step in the MoS and MoS/carbon electrocatalysts. Orderly variation of the Tafel slope with the mass loading has been observed in MoS/Vulcan carbon and the cause for this has been investigated based on roughness factor measurements. A linear dependence of the Tafel slope on the roughness factor points to a concomitant increase in the limitations on mass transport. The results show that the benefit of increasing the roughness factor of the electrocatalyst is counterbalanced by increasing the Tafel slope, and hence the need for designing an optimal HER electrocatalyst balancing the roughness factor and Tafel slope is deduced.