A Smorgasbord of Carbon: Electrochemical Analysis of CobaltâBis

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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 MacInnes, and Nicolai Lehnert ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05159 • Publication Date (Web): 18 Aug 2016 Downloaded from http://pubs.acs.org on August 30, 2016

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ACS Applied Materials & Interfaces

A Smorgasbord of Carbon: Electrochemical Analysis of Cobalt- Bis(Benzenedithiolate) Complex Adsorption and Electrocatalytic Activity on Diverse Graphitic Supports Shawn C. Eady , Molly MacInnes, and Nicolai Lehnert,* Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, United States; email: [email protected] Keywords Sustainable Catalysis, Hydrogen Production, Heterogeneous Catalysis, Graphitic Materials, Dithiolate Complexes

Abstract Heterogeneous dihydrogen production manifolds comprised of bulk graphite, pencil graphite, graphite powder in Nafion films, graphene, and glassy carbon electrodes with an adsorbed proton reduction catalyst, TBA[Co(S2C6Cl2H2)2], have been prepared and tested for their efficiency to generate dihydrogen in acidic aqueous media. The catalysts adsorbed on these inexpensive graphitic surfaces consistently display similar electrocatalytic profiles compared to the

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same catalysts 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 impressive 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.

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 DOE 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 windand solar-driven 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. Nobel 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


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well as other inorganic materials10,13,18,20-22,24-30 and metal organic frameworks.31-33 On the other hand, molecular dihydrogen evolution catalysts can be bound to surfaces either covalently17,34-39 or via non-covalent 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 last few years, a general difficulty has been the expense or complexity of such designs, or 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 has shown 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 seen to be 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. Further, the simple and highly cost-effective process of simply soaking the graphitic supports in the catalyst solution would be ideal for scaling and industrial use. The ease of catalyst application in this case would also provide an alternative solution to limited catalyst lifetimes, allowing for facile reapplication of the catalyst to the graphitic supports, or simply replacement of the inexpensive support itself, after use. Previously, we reported reduced graphene oxide (RGO) coated fluorine-doped tin oxide (FTO) and

HOPG

supports

prepared

in

a

solution

of

the

proton

reduction

catalyst

(TBA)[Co(S2C6Cl2H2)2] (Chart 1).43 These heterogeneous catalyst manifolds are capable of


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dihydrogen production in moderately acidic (pH < 3) aqueous solutions of trifluoroacetic or hydrochloric acids at an overpotential (η) of 0.37 V. In this work, we report the preparation of heterogeneous dihydrogen production manifolds designed by soaking various alternative graphitic supports in catalyst solutions to provide further insight into the effect of the support on catalysis, and to test alternative, cheap forms of graphite for their abilities to bind the catalyst and support proton reduction. To assure a direct comparison between the different catalyst supports, the same proton reduction catalyst, TBA[Co(S2C6Cl2H2)2] (1), was used on all supports. Table 1 summarizes the electrochemical performances of the catalyst on the three most promising graphitic supports examined in this study. Bulk electrolysis was only performed on the bulk graphite electrode and the graphite powder embedded Nafion film (GPEN) on glassy carbon electrode. Table 1. Summary of the electrochemical performance of the most promising graphitic supports examined in this study. Bulk electrolysis was performed on bulk graphite and GPEN supports at -0.5V (vs. SHE) at pH 0.3 with H2SO4. Plateau current was reached after 8 hours with bulk graphite and after 7 hours with GPEN. Bulk Graphite Pencil Graphite GPEN on Glassy Carbon E1/2 (V) -0.46 -0.43 -0.52 Ecat/2 (V) -0.5 -0.6 -0.47 Loading (mol/cm2) 3.53x10-9 2.1x10-8 -1 Average TOF (s ) 30 3.9 Plateau current densitya 12% 37% a Percent of initial current density after 8 (bulk graphite) and 7 h (GPEN) of continuous electrolysis, respectively.

2. Experimental Section

Chart 1. Cobalt dithiolene complex 1 used for graphite adsorption (right), schematic representation of the catalyst in graphitic materials (left).


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2.1. Materials and methods. Chemicals were of highest purity grade commercially available and used without further purification (unless mentioned in the following). Methanol (anhydrous, ACS grade) was purchased from Fisher and distilled over calcium hydride, then degassed via extended dinitrogen purges prior to use. Acetonitrile (ACS grade), sodium methoxide, trifluoroacetic acid, and sulfuric acid were purchased from Fisher, and 1,2-benzenedithiol and 3,6-dichloro-1,2-benzenedithiol were purchased from Sigma and used without further purification. Graphite powder was purchased from MTI Corp. All procedures were performed under a dinitrogen atmosphere unless otherwise specified. All electrochemical measurements were conducted in 18.2 Millipore water with 0.1 M KPF6 as a supporting electrolyte. Cyclic voltammetry and controlled potential coulometry were carried out using an CHI600 electrochemical analyzer. A platinum disc (BASi, MF-2013) was used as the counter electrode in all voltammetry experiments, and the reference was an aqueous Ag/AgCl electrode (CH Instruments, with AgCl and saturated KCl fill solutions). Cyclic voltammetry acid titration experiments were initiated by analysis of the electrode prior to acid addition in electrolyte solution of pH 7±0.5. All potentials are reported vs. SHE by adding 0.205 V to the potential measured against the Ag/AgCl/saturated KCl standard. Bulk electrolysis experiments were conducted in a 2-compartment cell separated by a frit with the same working and reference electrodes previously mentioned and a carbon felt counter electrode (purchased from Alfa Aesar). To maintain a steady pH and provide a consistent activity profile during electrolysis, the solution pH was monitored and adjusted back to the starting pH after increasing by more than 0.05 units. All solutions were prepared with 0.1 M potassium hexafluorophosphate supporting electrolyte, which was purchased from Fisher and subsequently recrystallized from an aqueous


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0.1 M NaOH solution. Argon gas was used to deoxygenate all solutions for a minimum of 30 minutes prior to data collection. All X-ray photoelectron spectra were acquired with a Kratos Axis Ultra analyzer using an Al Kα (1486.6 eV) source with a monochromator. Spectra were recorded without charge neutralization at a base pressure of

A Smorgasbord of Carbon: Electrochemical Analysis of CobaltâBis

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