ARTICLE pubs.acs.org/est
Bifunctional Silver Nanoparticle Cathode in Microbial Fuel Cells for Microbial Growth Inhibition with Comparable Oxygen Reduction Reaction Activity Junyeong An, Hongrae Jeon, Jaeyoung Lee, and In Seop Chang* School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, Korea
bS Supporting Information ABSTRACT: Organic contamination of water bodies in which benthic microbial fuel cells (benthic MFCs) are installed, and organic crossover from the anode to the cathode of membraneless MFCs, is a factor causing oxygen depletion and substrate loss in the cathode due to the growth of heterotrophic aerobic bacteria. This study examines the possible use of silver nanoparticles (AgNPs) as a cathodic catalyst for MFCs suffering from organic contamination and oxygen depletion. Four treated cathodes (AgNPs-coated, Pt/C-coated, Pt/CþAgNPs-coated, and plain graphite cathodes) were prepared and tested under high levels of organics loading. During operation (fed with 50 mM acetate), the AgNPs-coated system showed the highest DO concentration (0.8 mg/L) in the cathode area as well as the highest current (ranging from 0.04 to 0.12 mA). Based on these results, we concluded that (1) the growth of oxygen-consuming heterotrophic microbes could be inhibited by AgNPs, (2) the function of AgNPs as a bacterial growth inhibitor resulted in a greater increase of DO concentration in the cathode than the other tested cathode systems, (3) AgNPs could be applied as a cathode catalyst for oxygen reduction, and as a result (4) the MFC with the AgNPs-coated cathode led to the highest current generation among the tested MFCs.
’ INTRODUCTION In recent years, interest in microbial fuel cell (MFC) technology has been increasing due to its potential application in areas such as treating wastewater contaminants, electricity generation,1 biosensors,2 chemical production,3 and the removal of nitrate4 or sulfide.5 In particular, a current issue pertains to powering electronics such as a meteorological buoys and wireless sensors using sediment MFCs.6,7 Sediment MFCs generate electricity by embedding an anode in the sediment and overlying the cathode in the water phase of lenthic or lotic systems such as rivers, estuaries, or salt marshes. For sediment MFCs, dissolved oxygen (DO) concentration in the cathode, a critical factor in determining power production, should be high enough to minimize the cathodic limitations of oxygen reduction.69 In a natural aquatic environment, however, DO concentrations in water bodies vary due to microbial activities when contaminated by organic waste.10,11 For instance, when contamination by excessive organic waste occurs, if the reoxygenation rate is slower than the oxygen consumption rate, oxygen in the water body can become depleted.12,13 However, oxygen depletion is not usually considered if the oxygen concentration is sufficient for oxygen reduction or if the organics donation to water bodies is not at a point that it limits the oxygen reduction in sediment MFCs.9 r 2011 American Chemical Society
Recently, we proposed a new concept for an MFC system for extracting energy from water bodies in which oxygen is almost depleted due to high levels of organics contamination.8,9 One such MFC is a floating-type microbial fuel cell (FT-MFC) in which the anode is located in the organics-contaminated water phase and a part of the cathode is exposed to the atmosphere;8 it was demonstrated that it is possible to directly convert organics in the water phase to electricity using the FT-MFC. The second system, applied to water bodies contaminated by organic waste, is a multiphase electrode MFC (multiphase MFC)—a combination of an FT-MFC and a sediment MFC.9 Using these MFC systems, when temporal or consistent organic pollution has occurred in water bodies, it is possible to harvest a current by utilizing the organic materials that coexist in water and sediment phases. However, it was observed that organic concentration in the cathode of the MFC systems (both FT-MFCs and multiphase MFCs) affects cell performance due to the change in DO concentration in the cathode, caused by interaction of organic matter with heterotrophic aerobic bacteria.8,9 These defects are Received: January 5, 2011 Accepted: May 2, 2011 Revised: April 25, 2011 Published: May 17, 2011 5441
dx.doi.org/10.1021/es2000326 | Environ. Sci. Technol. 2011, 45, 5441–5446
Environmental Science & Technology also common in one-chambered MFC systems with no membrane, due to the crossover of organic matter from the anode to cathode; as a result, the oxygen and substrate consumed by heterotrophic microorganisms causes a decrease in both the oxygen concentration and columbic efficiency of the MFCs.14,15 On the basis of the above considerations, development of a method for effectively minimizing the impact of heterotrophic aerobic bacterial growth on MFC performance is required. To this end, it is posited here that if we use a material to inhibit the overgrowth of heterotrophic aerobic bacteria in an organic-rich cathode, a relatively higher DO concentration could be maintained on the cathode electrode. In addition, we recognize that if the material used for the microbial inhibitor has the ability to catalyze the oxygen reduction reaction (ORR), it could also be used to increase cathodic kinetic activities as an ORR catalyst. It is known that silver nanoparticles (AgNPs) have the following distinct properties: (1) AgNPs destabilize the plasma membrane potential and deplete levels of intracellular adenosine triphosphate (ATP) by targeting bacterial membranes, thereby inhibiting bacterial cell growth,16 though AgNPs are relatively less toxic to eukaryotes;17 (2) silver can be used as an ORR catalyst in alkaline DMFCs under an alkaline atmosphere in a cathodic environment;18,19 and (3) silver is physicochemically stable as a catalyst for the long-term operation of MFCs 20 (i.e., silver is robust against galvanic corrosion, as are platinum, gold, graphite, and titanium; the electromotive force of silver (Ag/Agþ) is þ0.799 V). In this study, using FT-MFCs, we demonstrate that AgNPs can be used as a catalyst to generate electricity in an organics-rich cathode, by demonstrating that (1) AgNPs with catalytic activity for ORR inhibit bacterial growth on the cathode electrode, (2) resulting in an increase of DO concentration in the cathode, and as a result, (3) current production is possible in DO-depleted cathodes. To date, no previous studies have reported a specific catalyst to use as the cathode in MFCs suffering from oxygen depletion that is caused by aerobes growing in an organics-rich cathode.
’ MATERIALS AND METHODS Electrode Preparation. Four different cathodes were prepared using graphite felt (Electrosynthesis, Amherst, NY), coated with AgNPs (
