Porous Co₉S₈/Nitrogen, Sulfur-Doped Carbon@Mo₂C Dual Catalyst for Efficient Water Splitting

The exploration of highly efficient and stable bifunctional electrocatalysts for overall water splitting is currently of extreme interest for the efficient conversion of sustainable energy sources. Herein, we provide an earth-abundant, low-cost, and highly efficient bifunctional electrocatalyst composed of cobalt sulfide (Co₉S₈) and molybdenum carbide (Mo₂C) nanoparticles anchored to metal-organic frameworks (MOFs)-derived nitrogen, sulfur-codoped graphitic carbon (Co₉S₈-NSC@Mo₂C). The new composite mode of the electrocatalyst was realized through simple pyrolysis processes. The composite electrocatalyst shows outstanding hydrogen evolution reaction (HER) performance and excellent stability over the entire pH range. For example, it has a lower overpotential of 74, 89, and 121 mV with the Tafel slopes of 69.3, 86.7, and 106.4 mV dec^-1 to achieve a current density of 10 mA cm^-2 in 0.5 M H₂SO₄, 1.0 M KOH, and 1.0 M phosphate-buffered saline solutions, respectively. Moreover, it shows a small overpotential of 293 mV with a Tafel slope of 59.7 mV dec^-1 to reach 10 mA cm^-2 for the oxygen evolution reaction (OER) in 1.0 M KOH. The significantly enhanced HER and OER activities of Co₉S₈-NSC@Mo₂C are mainly attributable to the electron transfer from Co to Mo₂C, resulting in a lower Mo valence and a higher Co valence in Co₉S₈-NSC@Mo₂C. Furthermore, using the Co₉S₈-NSC@Mo₂C bifunctional electrocatalyst as both the anode for the OER and the cathode for the HER for overall water splitting, a cell voltage of only 1.61 V is needed to derive a current density of 10 mA cm^-2. This interesting work offers a general method for designing and fabricating highly efficient and stable non-noble electrocatalysts for promising energy conversion.