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A Smorgasbord of Carbon: Electrochemical Analysis of Cobalt− Bis(benzenedithiolate) Complex Adsorption and Electrocatalytic Activity on Diverse Graphitic Supports Shawn C. Eady, Molly M. MacInnes, and Nicolai Lehnert*
ACS Appl. Mater. Interfaces 2016.8:23624-23634. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/07/19. For personal use only.
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, United States S Supporting Information *
ABSTRACT: Heterogeneous dihydrogen production manifolds comprised of bulk graphite, pencil graphite, graphite powder in Nafion films, graphene, and glassy carbon electrodes with adsorbed proton reduction catalyst TBA[Co(S2C6Cl2H2)2] have been prepared and tested for their efficiency to generate dihydrogen in acidic aqueous media. The catalyst adsorbed on these inexpensive graphitic surfaces consistently displays similar electrocatalytic profiles compared to the same catalyst on highly ordered pyrolytic graphite (HOPG) supports, including high activity in moderately acidic aqueous solutions (pH < 4), moderate overpotentials (0.42 V vs platinum), and some of the highest reported initial turnover frequencies under electrolysis conditions (96 s−1). The exceptions are glassy carbon and single-layer graphene surfaces, which only weakly adsorb the catalyst, with no sustained catalytic current upon acid addition. In particular, the improved stability and good activity observed for the catalyst adsorbed on graphite powder embedded in a Nafion film shows that this is a promising H2 production system that can be assembled at minimal cost and effort. KEYWORDS: sustainable catalysis, hydrogen production, heterogeneous catalysis, graphitic materials, dithiolate complexes surfaces either covalently17,34−39 or via noncovalent interactions.24,39−47 Finally, several systems have also accomplished dihydrogen production photochemically.48,49 While the development of both solid-state and surface-immobilized catalyst manifolds has been prolific even in the past few years, a general difficulty has been the expense or complexity of such designs and the limited lifetime of the materials/catalysts for practical use. One practical solution to designing heterogeneous catalyst systems is to exploit the electrostatic adsorption of catalyst systems with aromatic moieties on graphitic surfaces, which was effective in several literature examples.40,43,44,50−52 Despite the low catalyst loadings typically seen in these systems, turnover frequencies of the catalyst-adsorbed materials have been among the largest reported, thus providing an effective H2 production material. Whereas many of these systems have been tested on more expensive highly ordered pyrolytic graphite (HOPG) electrodes, the use of inexpensive catalyst supports such as bulk graphite could drastically reduce the cost of the functionalized electrode materials. Furthermore, the simple and highly costeffective process of simply soaking the graphitic supports in the catalyst solution would be ideal for scaling and industrial use.
1. INTRODUCTION The inexpensive and non-hydrocarbon based production of dihydrogen gas, an essential feedstock for fertilizer production and oil refining and a prime candidate for a “solar fuel” (for energy storage), is a prominent focus of current renewable energy research.1−5 As of November 2014, the primary source of dihydrogen remains the reforming of fossil fuels (>90%).6 Although this remains economically viable according to the 2014 Department of Energy standards for affordable dihydrogen, the eventual depletion of natural gas and continually increasing consumption of dihydrogen calls for a cheap, abundant source without dependence on fossil fuels.7 To this end, a variety of renewable methods for dihydrogen production have been developed, including wind- and solardriven electrolysis.8,9 To most efficiently utilize this renewable energy for dihydrogen production, a large amount of research has focused on the development of heterogeneous catalyst systems for proton reduction. Noble metals are expensive but are known to have strong proton reduction catalytic abilities.10,11 In search for more cost-effective methods of proton reduction, nanoparticles and nanostructures of metals and inorganic structures have been developed.12−18 Metal sulfides have also been extensively investigated in the literature for this application12−23 as well as other inorganic materials10,13,18,20−22,24−30 and metal organic frameworks.31−33 Molecular dihydrogen evolution catalysts can be bound to © 2016 American Chemical Society
Received: May 3, 2016 Accepted: August 18, 2016 Published: August 18, 2016 23624
DOI: 10.1021/acsami.6b05159 ACS Appl. Mater. Interfaces 2016, 8, 23624−23634
Research Article
ACS Applied Materials & Interfaces
and 3,6-dichloro-1,2-benzenedithiol were purchased from Sigma and used without further purification. Graphite rods were purchase from Graphite Machining, Inc. (Grade NAC-500 purified,
