Cobalt Metal–Cobalt Carbide Composite Microspheres for Water Reduction Electrocatalysis

Microspheres of cobalt metal–cobalt carbide (Co–CoxC, CoxC: Co2C and Co3C) composite with carbon shells were prepared via an OH–- and Cl–-assisted polyol method and investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic media. From our transmission electron microscopy observations, the outermost surfaces of the as-prepared Co–CoxC composites were primarily covered with Co2C crystallites. Our best performing electrocatalyst exhibited superior HER activity with an overpotential of 78 mV to reach a current density of −10 mA·cm–2, a Tafel slope of 87.8 mV·dec–1, and 1 h of electrode durability. We show that this excellent HER performance is primarily due to the superior intrinsic activity of Co2C, as well as the high electrical conductivity resulting from the inclusion of cobalt metal and the presence of graphitic carbon shells in and on the composite, respectively. Using both computational and experimental approaches, we determine that the carbon-rich cobalt carbide (Co2C) phase is more favorable for the HER than the carbon-poor phase (Co3C).