Environ. Sci. Technol. 43, 9525–9530
A Solar-Powered Microbial Electrolysis Cell with a Platinum Catalyst-Free Cathode To Produce Hydrogen KYU-JUNG CHAE, MI-JIN CHOI, KYOUNG-YEOL KIM, FOLUSHO F. AJAYI, IN-SEOP CHANG, AND IN S. KIM* Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea
Received July 23, 2009. Revised manuscript received October 30, 2009. Accepted November 3, 2009.
This paper reports successful hydrogen evolution using a dyesensitized solar cell (DSSC)-powered microbial electrolysis cell (MEC) without a Pt catalyst on the cathode, indicating a solution for the inherent drawbacks of conventional MECs, such as the need for an external bias and catalyst. DSSCs fabricated by assembling a ruthenium dye-loaded TiO2 film and platinized FTO glass with an I-/I3- redox couple were demonstrated as an alternative bias (Voc ) 0.65 V). Pt-loaded (0.3 mg Pt/cm2) electrodes with a Pt/C nanopowder showed relatively faster hydrogen production than the Pt-free electrodes, particularly at lower voltages. However, once the applied photovoltage exceeded a certain level (0.7 V), platinum did not have any additional effect on hydrogen evolution in the solar-powered MECs: hydrogen conversion efficiency was almost comparable for either the plain (71.3-77.0%) or Pt-loaded carbon felt (79.3-82.0%) at >0.7 V. In particular, the carbon nanopowdercoated electrode without Pt showed significantly enhanced performance compared to the plain electrode, which indicates efficient electrohydrogenesis, even without Pt by enhancing the surface area. As the applied photovoltage was increased, anodic methanogenesis decreased gradually, resulting in increasing hydrogen yield.
Introduction The microbial electrolysis cell (MEC), an electrically driven hydrogen evolution process that was modified from a microbial fuel cell (MFC), has attracted considerable attention because of its competitive advantage for hydrogen production from the oxidation of diverse organic compounds by exoelectrogens (1-5). However, the need for precious metal catalysts, such as platinum (Pt), and an external bias to supply a small voltage to overcome the thermodynamic barrier of nonspontaneous proton reduction are major inherent constraints for its practical application. MECs typically use Pt as a cathode catalyst to reduce the high overpotential for proton reduction in the cathode (1-4, 6-9). However, Pt has several drawbacks for use in MECs, even though it has been proven to be an efficient catalyst for electrochemical reduction of oxygen and protons in fuel cell-associated technology. Pt is an expensive catalyst * Corresponding author phone: +82 62 970 2436; fax: +82 62 970 2434; e-mail:
[email protected]. 10.1021/es9022317
2009 American Chemical Society
Published on Web 11/17/2009
($36.59/g as of May 2009) and an extremely rare metal, with projected concentrations of only 0.003 ppb in the Earth’s crust. Moreover, it is easily poisoned by carbon monoxide and sulfur compounds. These drawbacks outweigh the benefits of its use. Therefore, finding an alternative catalyst that is cheaper and more environmentally friendly is a research challenge. In MFCs, recently, successful current generation has been reported using noble metal-free catalysts, such as pyrolyzed iron phthalocyanine or cobalt tetramethoxyphenylporphyrin (10), and even noncatalyzed graphite granules (11). Several biocathodes utilizing different final electron acceptors, such as oxygen or nitrate, have also been suggested as a possible alternative to the Pt-catalyzed cathode in MFCs (12, 13). However, studies on hydrogen production using MECs primarily have applied Pt as a cathode catalyst (1, 3, 4, 7). Recently, the possibility of hydrogen production using a poised microbial biocathode was demonstrated after anodic acclimation of naturally selected electrochemically active microorganisms to hydrogen and acetate (8). This is an encouraging finding because this precious catalyst can be substituted with microbes as biocatalyst, which makes hydrogen production through MECs a great deal more costeffective. In particular, MECs produce too little current per amount of platinum load: i.e., several orders of magnitude less (∼1 to 10 A/m2) than the current density for a hydrogen fuel cell (∼103 to 104 A/m2) (8). This raises strong doubt about the use of expensive Pt in MECs. Current worldwide annual energy demand is approximately 13 terawatts (TW), and an additional 10 TW of energy to maintain current lifestyles will be needed by 2050. Among the many options, solar energy is considered to be the most viable choice for meeting the energy demand because a huge amount of solar energy (120000 TW) strikes the earth daily (14). Therefore, solar cells were employed in this study as a substitute for currently used external power sources, such as a potentiostat or a DC power supply, because they can supply sufficient power for MEC applications. Among various solar cells, dye-sensitized solar cells (DSSCs), which mimic the way that plants convert sunlight into energy, were employed because they are relatively inexpensive and easy to produce (15-18). This study examined the bioelectrochemical hydrogen production using a solar-powered MEC without a Pt catalyst. DSSCs were used to harvest light energy for use as an external power.
Materials and Methods Solar-Powered MEC Construction and Operation. H-shaped two-chambered glass bottle MECs were run with the assistance of DSSCs as an external power source (Figure 1). The anode and cathode compartments, with a working volume of 100 mL each, were separated by a 1.77 cm2 Nafion 117 membrane (DuPont, Wilmington, DE), which was held in place between two glass O-ring joints to prevent leakage. The anode was inoculated with anaerobic digester sludge from a sewage treatment plant and fed with acetate as an electron donor (2 mM). The aqueous electrolyte for the anode and cathode compartment contained an autoclaved nutrient mineral buffer (pH 7.0) and a phosphate buffer (50 mM, pH 7.0), respectively (19). The anode was carbon felt (12 cm2, 6 mm in thickness), and the cathode was either a Pt-loaded or free electrode (detailed below). For the acclimation of naturally selected electrochemically active microorganisms, the MECs were first operated in oxygen-breathing MFC mode at a 100 Ω for 3 months, and were then changed to MEC VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Solar-powered microbial electrolysis cell for hydrogen production. (A) Photograph. (B) Schematic diagram. mode with an external bias (DSSCs) over a 5 month period. The MECs were run in fed-batch mode in a temperature controlled room at 25 ( 1 °C. Dye-Sensitized Solar Cell (DSSC) Fabrication. The DSSCs were fabricated by adsorbing N719 dye, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)bistetrabutylammonium, onto a TiO2 film formed on a fluorine doped SnO2-coated conducting glass (FTO glass, 8 Ω/sq, Pilkington, Toledo, OH). For the fabrication of a nanocrystalline TiO2 film, a commercially available TiO2 paste (Solaronix SA, Switzerland) was coated on a FTO glass substrate with 15 µm transparent film composed of ∼25 nm nanoparticles using the doctor-blade technique. The hermetically sealed sandwich DSSCs, with an effective area of 26 cm2, were fabricated by assembling the dye-loaded TiO2 film as the working electrode and platinized FTO glass as the counter electrode separated by a hot-melt Surlyn film. The electrolyte was a standard acetonitrile-based organic solution (0.1 M LiI + 0.05 M I2) containing an iodide/tri-iodide (I-/I3-) redox couple (detailed in Supporting Information). The DSSC was irradiated using a Newport 300 W xenon lamp. Cathode Fabrication (Pt-loaded and Pt-free). In order to examine the effect of Pt on hydrogen evolution, carbon felt (12 cm2, 6 mm thickness, Morgan, UK) and graphite (12 cm2, 2 mm thickness, Woojin carbon, Korea) with, and without, a Pt catalyst were used as the cathode material (Table S1). For cathode preparation, the bare electrodes were cleaned by immersion in 0.5 M H2SO4 for 24 h. The prepared samples were rinsed thoroughly with deionized water, dried overnight in air at 105 °C, and then heat treated in an oven at 450 °C for 2 h to burn off any organic contaminants present and enhance their hydrophilicity. Among them, one carbon felt and one graphite electrode were coated with carbonsupported platinum nanoparticles (Pt/C) on either side of 9526
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each electrode with a Nafion solution as the binding material. For the Pt-loading on the electrodes, Pt/C nanopowder (40 wt % Pt deposited on Vulcan XC-72 as the carbon support, E-TEK, Somerset, NJ) was mixed with a Nafion ionomer solution (8 wt % in alcohols/water, Sigma-Aldrich Co., St. Louis, MO), and the prepared slurry was sprayed at 80 °C to achieve a Pt loading of a 0.30 mg/cm2 of the geometric area of the electrode (Figure S1). As an alternate to Pt/C nanopowder, a carbon (without Pt) nanopowder-coated carbon felt electrode (hereinafter CNP-coated) was also prepared to examine the effect of the enhanced surface area on MEC performance because the Pt/C coating leads to both Pt loading and an increased surface area. Photophysical and Electrical Characterization of the DSSCs. The photocurrent density-voltage curves were measured using a Keithley 4200 source meter. The cell performance was measured under standard conditions of AM 1.5G (AM: air mass, 100 mW/cm2, 25 °C) simulated solar light generated from an Oriel solar simulator (1 kW) with in a N2-filled glovebox. The incident photon-to-current efficiency (IPCE) was measured by illuminating the cells with a tungsten quartz halogen light source, as described previously (20). Cyclic Voltammetry. Cyclic voltammetry (CV) of the prepared electrodes, which were cleaned prior to use, was performed using a potentiostat (BAS Epsilon, Bioanalytical systems, Inc., West Lafayette, IN) with a Pt-wire counter electrode and an Ag/AgCl reference electrode in a single Pyrex glass cell (150 mL) containing a 50 mM NaH2PO4 electrolyte solution (pH 7.0). CV initiated at the most positive potential was conducted in a potential range from 0.8 to -0.8 V at a scan rate of 20 mV/s, under mild stirring with a magnetic bar (250 rpm). Before each experiment, the liquid and headspace of the reactor were sparged with N2 gas for 30 min to remove the dissolved oxygen. Analyses and Calculation. Specific surface areas of samples were estimated from the nitrogen adsorption isotherms at 77 K using the Brunauer-Emmer-Teller (BET) model (21) on a BELSORP-mini II sorptometer (BEL Inc., Japan). Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX) analyses were conducted, as previously described (19). Gas and acetate were analyzed by gas chromatography and high performance liquid chromatography, respectively (see Supporting Information). The Coulombic efficiency (CE; acetate to e- in the anode), cathodic hydrogen efficiency (CHE; e- to H2 in the cathode), and overall hydrogen efficiency (OHE; acetate to H2) were calculated, as previously described (1, 2).
Results and Discussion Cathode Characterization. On the basis of the EDX results shown in Figure S2, the plain electrodes of carbon felt and graphite showed only a carbon (C) peak. In contrast, Pt was detected in the Pt-loaded electrodes with relative atomic percentages of 1.25% for carbon felt and 0.86% for graphite, respectively, indicating the successful loading of Pt. Spraying Pt/C nanopowder on either side of each bare electrode for Pt loading resulted in a much rougher and increased surface area due to the formation of a nanoporous structure on the coated layer, which may provide an enhanced catalytic reaction (Figure S2). The measured surface area was 5.91 () 7.15 × 107) for the Pt-loaded carbon felt, 4.8 () 5.80 × 107) for CNP-coated carbon felt and 1.15 m2/g () 1.35 × 107 m2/m3) for plain carbon felt, respectively (Figure S3). Pore size distribution analysis showed that there was a wide distribution of mesopores between 2.0-200 nm, but mainly ranged below Sustainable and efficient biohydrogen production via electrohydrogenesis Proc. Natl. Acad. Sci. U.S.A. 2007, 104 (47), 18871–18873. (4) Liu, H.; Grot, S.; Logan, B. E. Electrochemically assisted microbial production of hydrogen from acetate. Environ. Sci. Technol. 2005, 39 (11), 4317–4320. (5) Logan, B. E.; Call, D.; Cheng, S.; Hamelers, H. V. M.; Sleutels, T. H. J. A.; Jeremiasse, A. W.; Rozendal, R. A. Microbial Electrolysis Cells for High Yield Hydrogen Gas Production from Organic Matter. Environ. Sci. Technol. 2008, 42 (23), 8630–8640. (6) Call, D.; Logan, B. E. Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane. Environ. Sci. Technol. 2008, 42 (9), 3401–3406. (7) Rozendal, R. A.; Hamelers, H. V. M.; Molenkmp, R. J.; Buisman, J. N. Performance of single chamber biocatalyzed electrolysis with different types of ion exchange membranes. Water Res. 2007, 41 (9), 1984–1994. (8) Rozendal, R. A.; Jeremiasse, A. W.; Hamelers, H. V. M.; Buisman, C. J. N. Hydrogen production with a microbial biocathode. Environ. Sci. Technol. 2008, 42 (2), 629–634. (9) Kim, I. S.; Chae, K. J.; Choi, M. J.; Verstraete, W. Microbial Fuel Cells: Recent Advances, Bacterial Communities and Application Beyond Electricity Generation. Environ. Eng. Res. 2008, 13 (2), 51–65. (10) Zhao, F.; Harnisch, F.; Schro?der, U.; Scholz, F.; Bogdanoff, P.; Herrmann, I. Application of pyrolysed iron(II) phthalocyanine and CoTMPP based oxygen reduction catalysts as cathode materials in microbial fuel cells. Electrochem. Commun. 2005, 7 (12), 1405–1410.
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