Comparison of Electrode Reduction Activities of Geobacter sulfurreducens and an Enriched Consortium in an Air-Cathode Microbial Fuel Cell

Abstract

An electricity-generating bacterium, Geobacter sulfurreducens PCA, was inoculated into a single-chamber, air-cathode microbial fuel cell (MFC) in order to determine the maximum electron transfer rate from bacteria to the anode. To create anodic reaction-limiting conditions, where electron transfer from bacteria to the anode is the rate-limiting step, anodes with electrogenic biofilms were reduced in size and tests were conducted using anodes of six different sizes. The smallest anode (7 cm 2 , or 1.5 times larger than the cathode) achieved an anodic reaction-limiting condition as a result of a limited mass of bacteria on the electrode. Under these conditions, the limiting current density reached a maximum of 1,530 mA/m 2 , and power density reached a maximum of 461 mW/m 2 . Per-biomass efficiency of the electron transfer rate was constant at 32 fmol cell ؊1 day ؊1 (178 mol g of protein ؊1 min ؊1 ), a rate comparable to that with solid iron as the electron acceptor but lower than rates achieved with fumarate or soluble iron. In comparison, an enriched electricity-generating consortium reached 374 mol g of protein ؊1 min ؊1 under the same conditions, suggesting that the consortium had a much greater capacity for electrode reduction. These results demonstrate that per-biomass electrode reduction rates (calculated by current density and biomass density on the anode) can be used to help make better comparisons of electrogenic activity in MFCs.Microbial fuel cells (MFCs) are devices that exploit microorganisms as "biocatalysts" of generating electric power from organic matter. MFC systems are being researched as a method of recovering energy from waste as electrical power (10,23,24,35) and generating power from aquatic sediments on the bottom of the ocean (25, 42) or from rice paddy soil (13,14). Recent technical improvements of MFC system architecture have increased power densities from Ͻ0.1 mW/m 2 to Ͼ2,400 mW/m 2 (normalized by the anode surface area) during the past several years in systems lacking exogenous electron shuttles (22,24). However, continued improvements are still needed for improved power densities, reduced costs for materials, and the development of large-scale devices (8).The two common ways of expressing MFC performance for power generation are power normalized to the projected surface area of an electrode (power density; mW/m 2 ) and power per unit of reactor volume (power output; W/m 3 ) (35). Many studies of MFCs have used power density based on the assumption that the biocatalytic activity of the anode limits power production (16,24,35,39). However, variations in the reactor volume (2, 38), composition of the proton-exchange membrane (17), catholyte reactions (32, 34), substrates (21), and anode materials (5, 33) often make it difficult to know which factors actually limit power production. There are various potential losses that can limit power output, such as microbial electron transfer to the anode, solution resistance, membrane resistance, and reduction reaction on the cathode (16, 35). The...

Microbial population and functional dynamics associated with surface potential and carbon metabolism

“…In order to obtain anode polarization curves, an Ag/AgCl reference electrode (RE-5B, BASi) was placed in the side port of the reactors, and linear sweep voltammetry analyses were conducted using a potentiostat (Reference 600 or MultEchem System, Gamry) (Tsujimura et al, 2001;Ishii et al, 2008). The anode potential was swept from open-circuit anode potential to þ 300 mV vs SHE at a scan rate of 0.5 mV s À 1 and the corresponding anodic current was recorded.…”

“…The MFC was a bottletype reactor (400 ml in capacity), with anode electrodes made of carbon cloth (108 cm 2 projected surface area; Tsukuba Materials Information Lab, Tsukuba, Japan) connected by Ti wires (Ishii et al, 2008). A Pt-catalyzed air cathode as described elsewhere (Cheng et al, 2006) was placed at the side port, providing a projected surface area of 20 cm 2 .…”

“…A Pt-catalyzed air cathode as described elsewhere (Cheng et al, 2006) was placed at the side port, providing a projected surface area of 20 cm 2 . After sterilization of the MFC, the chamber was anaerobically filled with carbonate-buffered basal salt medium described in Supplementary Methods, which is modified from the previous research (Ishii et al, 2008).…”

“…To determine the total anodic microbial cell density, part of the anode (7 Â 7 mm) was removed from the reactors (n ¼ 3). Total protein was extracted and determined by the bicinchoninic acid method (Pierce, Rockford, IL, USA) as described elsewhere (Bond and Lovley, 2003;Ishii et al, 2008).…”

“…The MFC anode electrode serves as an electron acceptor that can mimic solid metals, but does not react, and therefore serves as an unchanging solidform electron acceptor. Using electrochemical approaches in MFC systems, the EET activity in a microbial community can be monitored as operational current generation, anodic limiting (maximum) current density and per-biomass electrode reducing rate throughout system operations (Ishii et al, 2008). In addition, the anode potential can be controlled by potentiostatic set-potential (SP) operation (Wagner et al, 2010), which allows acceleration or suppression of EET rates in the community as a function of applied surface redox potential (Aelterman et al, 2008;Ishii et al, 2013).…”