H 2 Evolution at an Electrochemical “Underpotential” with an Iridium-Based Molecular Photoelectrocatalyst

Stratakes, Bethany M.; Miller, Alexander J. M.

doi:10.1021/acscatal.0c02265

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H 2 Evolution at an Electrochemical “Underpotential” with an Iridium-Based Molecular Photoelectrocatalyst

Stratakes, Bethany M.·Miller, Alexander J. M.

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Abstract

Molecular platforms capable of both photochemical H 2 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 pK a affect the kinetics and thermodynamics of the potential- and light-driven H 2 evolution reaction, conditions were identified wherein the solvento complex [Cp*Ir(bpy)(NCCH 3 )][PF 6 ] 2 (Cp* is pentamethylcyclopentadienyl; bpy is 2,2'-bipyridine) mediates photoelectrocatalytic H 2 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 H 2 evolution with Faradaic efficiencies up to 90% and overpotentials as low as –90 mV. As a result, 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.

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14 SOLAR ENERGY Catalysts Hydrogen evolution Iridium Kinetic parameters Overpotential Photoelectrocatalysis Reaction mechanisms Thermodynamics Transition metals Underpotential

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Stratakes, Bethany M., Miller, Alexander J. M.. 2020-07-15. H 2 Evolution at an Electrochemical “Underpotential” with an Iridium-Based Molecular Photoelectrocatalyst. https://doi.org/10.1021/acscatal.0c02265

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