Metastable Phase-Controlled Synthesis of Mesoporous Molybdenum Carbides for Efficient Alkaline Hydrogen Evolution

Transition metal carbides (TMCs) have shown great promise as nonprecious metal electrocatalysts for electrochemical energy conversion reactions owing to their Pt-like catalytic activity. The major challenges in the synthesis of TMCs include the selective formation of a catalytically active phase, enlargement of the catalytically active surface area, and control of the carbonaceous overlayer, all of which collectively influence their catalytic reactivity. Herein, we present a systematic study on the catalytically active metastable phase-selective synthesis of mesoporous molybdenum carbides with controlled carbon overlayer thickness for efficient alkaline hydrogen evolution reaction (HER) electrocatalysis. The synthesis of molybdenum carbides was explored by controlling the types of Mo and C precursors and carburization conditions and the use of a mesoporous silica template. The choice of the Mo precursor and the nanospace-confined carburization were identified as critical factors for the formation of mesoporous molybdenum carbides of the metastable alpha-MoC1-x phase (MMC) with a large catalytically active surface area. The thickness of the carbon overlayer on the surfaces of the molybdenum carbides increased proportionally with the carburization temperature and decreased substantially with H-2 post annealing. Among the prepared catalysts, the H-2-annealed MMC (MMC-H-2) exhibited the highest alkaline HER activity and reaction kinetics, which is one of the best among the meal carbide alkaline HER catalysts reported so far and compared favorably with a commercial Pt/C catalyst. The MMC-H-2 also demonstrated superior durability and stability as compared to MMC and Pt/C catalysts.