Redox-Mediator-Assisted Electrocatalytic Hydrogen Evolution from Water by a Molybdenum Sulfide-Functionalized Metal–Organic Framework

The Zr6-based metal–organic framework NU-1000 was successfully functionalized with candidate catalysts—MoSx units—via SIM (solvothermal deposition in MOFs) of molybdenum(VI), followed by reaction with H2S gas. The structure of the material, named MoSx-SIM, was characterized spectroscopically and through a single-crystal X-ray diffraction measurement. These measurements and others established that the catalyst is monometallic, with mixed oxygen and sulfur coordination, and that it forms from a MOF-node-supported molybdenum-based cluster featuring only oxy ligands. Notably, the formal potential for the MOF-grafted complex, like that for the metal–sulfur active site of hydrogenase, is nearly coincident with the formal potential for the hydrogen couple. Its effective concentration within the mesoporous MOF is several hundred millimolar, and its porous-framework-based immobilization/heterogenization obviates the need for aqueous solubility as a condition for use as a well-defined catalyst. MoSx-SIM was evaluated as an electrocatalyst for evolution of molecular hydrogen from aqueous acid. Although the MoSx-functionalized framework exhibits catalytic behavior, the highly insulating nature of the support inhibits high electrocatalytic performance. Introduction of an archetypal redox mediator (RM), methyl viologen (MV2+), resulted in more than 20-fold enhancement in its catalytic performance on a turnover frequency basis, implying efficient RM-assisted electron transfer to otherwise electrochemically non-addressable MoSx moieties. Electrochemical kinetic studies with additional viologens as mediators reveal an unexpected square-root dependence of overall reaction rate on mediator concentration, as well as sensitivity to the strength of RM•+ as a reductant. These observations, together with observations of potential-dependent H/D isotope effects and potential-dependent pH effects, provide useful insights into the catalysis mechanism and help to explain how the MOF-affixed monometallic catalyst can effectively catalyze a two-electron reduction reaction, i.e., hydrogen evolution from acidified water.