H<sub>2</sub> Evolution at an Electrochemical “Underpotential” with an Iridium-Based Molecular Photoelectrocatalyst
Bethany M. Stratakes, Alexander J. M. Miller
Abstract
Molecular platforms capable of both photochemical H2 evolution and electrochemical catalyst regeneration hold promise for the efficient generation of solar fuels. Systems studied to date require a significant electrochemical overpotential (η), so no photon energy is being chemically stored. A detailed study of the parameters that govern the electrochemical overpotential of molecular photoelectrocatalysts is presented. By understanding how tuning the solvent, catalyst structure, and acid pKa affect the kinetics and thermodynamics of the potential- and light-driven H2 evolution reaction, conditions were identified wherein the solvento complex [Cp*Ir(bpy)(NCCH3)][PF6]2 (Cp* is pentamethylcyclopentadienyl; bpy is 2,2′-bipyridine) mediates photoelectrocatalytic H2 formation at an electrochemical “underpotential” (η < 0) driven by visible light. Under 460 nm illumination and in the presence of the weak organic acid H-PhTMG+ (2-phenyl-1,1,3,3-tetramethylguanidinium), the catalyst facilitates H2 evolution with Faradaic efficiencies up to 90% and overpotentials as low as −90 mV. Mechanistic studies enabled the construction of graphical “maps” that can guide the choice of catalyst and conditions to minimize overpotential, applicable to both photoelectrocatalysts and dark electrocatalysts.