Iron disulfide (FeS2) promises an earth-abundant, low-cost alternative to platinum group metals for the hydrogen evolution reaction (HER), but its performance is currently limited by reactivity of active sites and poor electrical conductivity. Here we employ Ketjenblack (KB) as a support to create an Fe-based electrocatalyst with high-electrical conductivity and maximal active sites. Moreover, a systematic study on the role of cobalt (Co) dopant was carried out. Electrochemical results show enhancements in HER activity of Co-doped FeS2 [FexCo1-xS2, atomic content of Fe (x) = 0.98-0.32] in comparison to un-doped FeS2 in acidic electrolyte (pH = 0). The overpotential necessary to drive a current density of 10 mA/cm(2) is -0.150 V and only decreases by 1 mV after 500 cycles of a durability test (cycling the potential between 0.0 and-0.15 V), indicating a long-term durability in acidic environment. This work suggests that FexCo1-xS2 offers a viable approach to improve the activity and durability of transition metal-sulfide electrocatalysts. (C) The Author(s) 2017. Published by ECS.