A Novel Fuel Cell Catalyst for Clean Energy Production Based on a Bionanocatalyst P.Yong,

Nano-scale palladium was bio-manufactured via enzymatically-mediated deposition of Pd(II) from solution. The bio-accumulated metal palladium crystals were processed and applied onto carbon paper and tested as anodes in a proton exchange membrane (PEM) fuel cell for power production. Up to 85% and 31% of the maximum power generation was achieved by Bio-Pd catalysts made using two strains of bacteria, compared to commercial fuel cell grade Pt catalyst. Therefore, it is feasible to use bio-synthesized catalysts in fuel cells for electricity production. Introduction A proton exchange membrane (PEM) fuel cell is an electrochemical cell that is fed hydrogen at the anode, and oxygen at the cathode. In the presence of platinum group metal (PGM) catalysts, hydrogen can be oxidized at the anode and the released protons transferred through a proton exchange membrane to the cathode. The electrons released from hydrogen move along the electrical circuit; an electric current is generated since the membrane is not electrically conductive. The pathway in PEM fuel cells has been described by many authors [e.g. 1, 2], and the reactions in a fuel cell are as below: Anode (with PGM catalysts): 2H2 → 4e + 4H + Cathode (with PGM catalysts): O2+ 4e + 4H + → 2H2O ______________________ Net reaction: O2 + 2H2 → 2H2O Low temperature PEM fuel cells depend critically on precious metal catalyst at anodes and cathodes to use hydrogen to produce electricity. The cost and availability of precious metals will ultimately limit the implementation of PEM fuel cells into the hydrogen economy as sustainable energy production replaces fossil fuels to overcome the production of ‘greenhouse gases’. Biosynthesis of PGM nano-particles may be an alternative way to manufacture fuel cell catalysts economically, using precious metal wastes. Bacteria can both synthesise and support nanoparticles. Hence biomanufacturing would favour biogenesis over ‘chemical’ methods of nanoparticle manufacture where agglomeration is problematic [3]. Previous studies have demonstrated that sulphate-reducing bacteria can recover Pd and Pt from liquid wastes [4, 5]. This was achieved by using hydrogenase to oxidise H2 and coupling this to the reduction of soluble Pd(II) and Pt(IV) to form metallic deposits of Pd(0) and Pt(0) onto biomass. The involvement of hydrogenase was confirmed by the use of deletion mutants [6]. The metallic deposits were shown to be nanocrystalline (~ 5 nm) [7]. E. coli, like D. desulfuricans, can reduce Pd(II) [8, 9] via hydrogenase activity. The present study shows that the sintered nanocrystallised Pd catalyst produced by resting cells of both organisms can be used in PEM fuel cells as electrode materials. Here we discuss electricity yields and the robustness of the novel bioinorganic materials compared with commercial PEM fuel cell electrode material and the scope for making cheap, clean electricity using biorecovered precious metals. Materials and Methods Cell culture. D. desulfuricans ATCC 29577 was used for metal uptake and bioreduction experiments as described as in a previous study [4]. Escherichia coli strain MC4100 was provided by Professor J. A. Cole, University of Birmingham, UK. Minimal medium used for E. coli culture Advanced Materials Research Vols. 20-21 (2007) pp 655-658 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/Jul/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.34-14/04/08,12:41:07) growth was as described previously [10]. Cells were harvested anaerobically by centrifugation, washed three times in degassed 20mM MOPS-NaOH buffer pH 7.0, and kept refrigerated at 4 o C until use, usually the next day. Biodeposition of Pd(0). The concentrated cells were resuspended anaerobically in 0.01 M HNO3 (pH 2) containing 2 mM Na2PdCl4 (to give ~5% and 25% metal : cell dry weight w/w), and suspensions were degassed with N2 for 15 min. The samples were allowed to stand for >1 h (30 °C) for the initial biosorption step and then H2 was passed through the solution (up to 30 min). The resulting black precipitate settled under gravity. The identity of the deposited metals as Pt(0) and Pd(0) was confirmed using X-ray powder diffraction as previously described [4]. Location of Pd(0) by transmission electron microscopy (TEM). Samples of Pd-loaded bacteria with or without Pd-precipitates were fixed and examined under TEM as described previously [4]. The metal loading for the TEM study was ~ 10-25% of the mass (w/w) as specified since crystals at 5% w/w were difficult to resolve clearly. Processing of Bio-Pd. The bio-Pd (5% metal/biomass w/w) was harvested by centrifugation, washed with deionised water three times and then with acetone. The samples were dried at room temperature, transferred into 10 ml alumina ceramic crucibles, and put in a furnace with a temperature control program. The temperature was increased gradually from room temperature to 700 o C within 4 h and held at 700 o C for a further 4 h. The samples were cooled to room temperature in the furnace. Preparation of electrodes. Commercial fuel cell grade Pt powder (C-Pt), commercial catalyst submicron Pd powder (C-Pd) (Sigma-Aldrich, Germany), and bio-manufactured Pd from D. desulfuricans (Bio-PdD.desulfuricans) and from E. coli (Bio-PdE.coli) (20 mg of each, as metal) were mixed separately with pure activated carbon powder (80 mg; BDH Chemicals Ltd, UK). Nafion ® perfluorinated ion-exchange resin (0.2 ml; 10 wt % in water, Sigma-Aldrich, Germany) and water (1.0 ml) were added to each sample containing 20% of Pt or Pd and 80% of C. The sample was mixed well and transferred onto 16 cm 2 teflon treated carbon paper (Fuel Cell Scientific, USA), and dried at room temperature. Testing electrodes. An ECO H2/O2 fuel cell test system (h-tec, Hydrogen Energy Systems, Luebeck, Germany) was used as described by Voigt et al. [11]. Tests were carried out for the laboratory made electrodes (i.e., containing C-Pt, C-Pd, Bio-PdD.desulfuricans, and Bio-PdE.coli) at the anode. A Pt-coated standard carbon electrode, which was provided with the fuel cell system, was used as the cathode for all the tests. Proton exchange membrane used was Nafion ® NRE-212 (Sigma-Aldrich, Germany). Electrolysed H2 and O2 are fed into the PEM fuel cell via the anode (H2) and cathode (O2). Current (I) and voltage (V) were measured and recorded against resistance (R) from R= ∞ to R= 0 Ω gradually as described by Voigt et al. [11]. P (output power in watts (W)) = I (current in amperes (A)) x V (voltage in volts (V)). Fig 1. Cells of D. desulfuricans (A, C) and E-coli. (B, D) viewed under TEM. A and B: biosorption only. C and D: Pd(II) was reduced to Pd(0) after reduction with H2. The metal loading was ~10% w/w. Metal loading on the cells was verified by assay of the supernatant after reduction. Metal unchallenged cells appeared as in A and B. Bars show 500 nm. A B C D 656 Biohydrometallurgy: From the Single Cell to the Environment