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 - MoS_x units - via SIM (solvothermal deposition in MOFs) of molybdenum(VI), followed by reaction with H₂S gas. The structure of the material, named MoS_x-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. MoS_x-SIM was evaluated as an electrocatalyst for evolution of molecular hydrogen from aqueous acid. Although the MoS_x-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 MoS_x 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.