Translation of Ligand-Centered Hydrogen Evolution Reaction Activity and Mechanism of a Rhenium-Thiolate from Solution to Modified Electrodes: A Combined Experimental and Density Functional Theory Study.
The homogeneous, nonaqueous catalytic activity of the rhenium-thiolate complex ReL3(L = diphenylphosphinobenzenethiolate) for the hydrogen evolution reaction (HER) has been transferred from nonaqueous homogeneous to aqueous heterogeneous conditions by immobilization on a glassy carbon electrode surface. A series of modified electrodes based on ReL3and its oxidized precursor [ReL3][PF6] were fabricated by drop-cast methods, yielding catalytically active species with HER overpotentials for a current density of 10 mA/cm2, ranging from 357 to 919 mV. The overpotential correlates with film resistance as measured by electrochemical impedance spectroscopy and film morphology as determined by scanning and transmission electron microscopy. The lowest overpotential was for films based on the ionic [ReL3][PF6] precursor with the inclusion of carbon black. Stability measurements indicate a 2 to 3 h conditioning period in which the overpotential increases, after which no change in activity is observed within 24 h or upon reimmersion in fresh aqueous, acidic solution. Electronic spectroscopy results are consistent with ReL3as the active species on the electrode surface; however, the presence of an undetected quantity of catalytically active degradation species cannot be excluded. The HER mechanism was evaluated by Tafel slope analysis, which is consistent with a novel Volmer-Heyrovsky-Tafel-like mechanism that parallels the proposed homogeneous HER pathway. Proposed mechanisms involving traditional metal-hydride processes vs ligand-centered reactivity were examined by density functional theory, including identification and characterization of relevant transition states. The ligand-centered path is energetically favored with protonation of cis-sulfur sites culminating in homolytic S-H bond cleavage with H2evolution via H atom coupling.
Department of Chemistry, University of Louisville , 2320 South Brook Street, Louisville, Kentucky 40292, United States.
Wuyu
W
Zhang
Department of Chemistry, University of Louisville , 2320 South Brook Street, Louisville, Kentucky 40292, United States.
Andrew Z
AZ
Haddad
Department of Chemistry, University of Louisville , 2320 South Brook Street, Louisville, Kentucky 40292, United States.
Brady D
BD
Garabato
Department of Chemistry, University of Louisville , 2320 South Brook Street, Louisville, Kentucky 40292, United States.|Visiting Professor, Department of Food Sciences, Medical University of Gdansk , Al. Gen. J. Hallera 107, 80-416 Gdansk, Poland.
Pawel M
PM
Kozlowski
Department of Chemistry, University of Louisville , 2320 South Brook Street, Louisville, Kentucky 40292, United States.
Robert M
RM
Buchanan
Department of Chemistry, University of Louisville , 2320 South Brook Street, Louisville, Kentucky 40292, United States.
Craig A
http://orcid.org/0000-0003-4889-2645
CA
Grapperhaus