Chemical induced fragmentation of MOFs for highly efficient Ni-based hydrogen evolution catalysts

A highly efficient catalyst for the hydrogen evolution reaction (HER) composed of Ni nanoparticles encapsulated in thin layers of N-doped carbon (Ni@NC) can be obtained by NH₃ induced fragmentation of a Ni based MOF at mild temperature (450 °C). A mechanistic study shows that the high chemical reactivity of NH₃ causes ammonolysis of the ligands and allows ligand removal well below their carbonization temperature. In conventional thermal decomposition in Ar, metal-ligand bond dissociation and ligand carbonization occurs in the same temperature region, resulting in a catalyst with high carbon content and much poorer HER performance. We further improve the HER performance by combining the NH₃ induced fragmentation and destabilization of the Ni-MOF structure by low levels of Co substitution, which results in smaller, more uniform catalyst particles. Using the noble metal free HER catalyst, an overall solar-to-hydrogen efficiency of 7.9% is achieved in an electrolyzer driven by a commercial polycrystalline solar cell with an efficiency of 12.9%.