Ramaraja P. Ramasamy

Impact of Initial Biofilm Growth on the Anode Impedance of Microbial Fuel Cells

Electrochemical impedance spectroscopy (EIS) was used to study the behavior of a microbial fuel cell (MFC) during initial biofilm growth in an acetate‐fed, two‐chamber MFC system with ferricyanide in the cathode. EIS experiments were performed both on the full cell (between cathode and anode) as well as on individual electrodes. The Nyquist plots of the EIS data were fitted with an equivalent electrical circuit to estimate the contributions of various intrinsic resistances to the overall internal MFC impedance. During initial development of the anode biofilm, the anode polarization resistance was found to decrease by over 70% at open circuit and by over 45% at 27 µA/cm2, and a simultaneous increase in power density by about 120% was observed. The exchange current density for the bio‐electrochemical reaction on the anode was estimated to be in the range of 40–60 nA/cm2 for an immature biofilm after 5 days of closed circuit operation, which increased to around 182 nA/cm2 after more than 3 weeks of operation and stable performance in an identical parallel system. The polarization resistance of the anode was 30–40 times higher than that of the ferricyanide cathode for the conditions tested, even with an established biofilm. For a two‐chamber MFC system with a Nafion® 117 membrane and an inter‐electrode spacing of 15 cm, the membrane and electrolyte solution dominate the ohmic resistance and contribute to over 95% of the MFC internal impedance. Detailed EIS analyses provide new insights into the dominant kinetic resistance of the anode bio‐electrochemical reaction and its influence on the overall power output of the MFC system, even in the high internal resistance system used in this study. These results suggest that new strategies to address this kinetic constraint of the anode bio‐electrochemical reactions are needed to complement the reduction of ohmic resistance in modern designs. Biotechnol. Biotechnol. Bioeng. 2008;101: 101–108.

High electrocatalytic activity of tethered multicopper oxidase–carbon nanotube conjugates

Multicopper oxidases linked to multiwall carbon nanotubes via the molecular tethering reagent, 1-pyrenebutanoic acid, succinimidyl ester, displayed high bioelectrocatalytic activity for oxygen reduction.

Electrochemical impedance spectroscopy for microbial fuel cell characterization.

Electrochemical impedance spectroscopy is an efficient, non-intrusive and semi-quantitative technique to characterize the performance of bio-electrochemical systems such as microbial fuel cells and enzymatic fuel cells. Indeed, quantitative interpretation of the impedance data can be obtained with the help of mechanistic models using meaningful equivalent circuits. The production of maximum power using such systems has been limited by their higher internal resistance. The contribution of several different resistances to the overall internal resistance of the system can be ascertained through the measurement of impedance using EIS, which is greatly required for understanding and engineering of its principle components leading to better enhancement of its performance. EIS has been successfully employed in most of the MFC researches helping in advancement of the field through emergence of many novel MFC designs with greater power generating capacity. In a nutshell, impedance spectroscopy provides a valuable addition to the existing biochemical and spectroscopic techniques to better optimize the electrochemical behavior of the biological system.

Impedance spectroscopy as a tool for non-intrusive detection of extracellular mediators in microbial fuel cells

Endogenously produced, diffusible redox mediators can act as electron shuttles for bacterial respiration. Accordingly, the mediators also serve a critical role in microbial fuel cells (MFCs), as they assist extracellular electron transfer from the bacteria to the anode serving as the intermediate electron sink. Electrochemical impedance spectroscopy (EIS) may be a valuable tool for evaluating the role of mediators in an operating MFC. EIS offers distinct advantages over some conventional analytical methods for the investigation of MFC systems because EIS can elucidate the electrochemical properties of various charge transfer processes in the bio‐energetic pathway. Preliminary investigations of Shewanella oneidensis DSP10‐based MFCs revealved that even low quantities of extracellular mediators significantly influence the impedance behavior of MFCs. EIS results also suggested that for the model MFC studied, electron transfer from the mediator to the anode may be up to 15 times faster than the electron transfer from bacteria to the mediator. When a simple carbonate membrane separated the anode and cathode chambers, the extracellular mediators were also detected at the cathode, indicating diffusion from the anode under open circuit conditions. The findings demonstrated that EIS can be used as a tool to indicate presence of extracellular redox mediators produced by microorganisms and their participation in extracellular electron shuttling. Biotechnol. Bioeng. 2009; 104: 882–891.

Time-course correlation of biofilm properties and electrochemical performance in single-chamber microbial fuel cells

The relationship between anode microbial characteristics and electrochemical parameters in microbial fuel cells (MFCs) was analyzed by time-course sampling of parallel single-bottle MFCs operated under identical conditions. While voltage stabilized within 4days, anode biofilms continued growing during the six-week operation. Viable cell density increased asymptotically, but membrane-compromised cells accumulated steadily from only 9% of total cells on day 3 to 52% at 6weeks. Electrochemical performance followed the viable cell trend, with a positive correlation for power density and an inverse correlation for anode charge transfer resistance. The biofilm architecture shifted from rod-shaped, dispersed cells to more filamentous structures, with the continuous detection of Geobacter sulfurreducens-like 16S rRNA fragments throughout operation and the emergence of a community member related to a known phenazine-producing Pseudomonas species. A drop in cathode open circuit potential between weeks two and three suggested that uncontrolled biofilm growth on the cathode deleteriously affects system performance.

Electrochemical characterization of a polypyrrole/Co0.2CrOx composite as a cathode material for lithium ion batteries☆

Polypyrrole/Co0.2CrOx, (PPy/Co0.2CrOx) composites were synthesized by polymerizing pyrrole onto the surface of cobalt chromium oxide (CrOx) in acidic media. The PPy/Co0.2CrOx composites increase the reversible capacity of the electrochemically active material up to 20%. At C/10 rate, a reversible capacity of 215 mAh/g was obtained for PPy/Co0.2CrOx composite, compared to 178 mAh/g for the virgin material, an increase of over 21%. Conductivity experiments corroborated the galvanostatic cycling tests, with the composite cathode material showing high electronic conductivity than bare material. Fitting the impedance results to an equivalent circuit reveals that addition of polypyrrole reduces the ohmic resistance due to the better conductivity and the inclusion of electrochemically active polypyrrole in the composite. Finally, the composite electrodes also showed very good rate capability and better cycling behavior compared to that of the bare material.

Characteristic Behavior of Polymer Electrolyte Fuel Cell Resistance during Cold Start

bHyundai Motor Corporation, Yongin, Korea In this study, experimental constant-current cold starts were performed on a polymer electrolyte fuel cell from �10°C to characterize high-frequency resistance behavior, water motion, and ice accumulation before, during, and after cold start. A diagnostic method for rapid and repeatable cold starts was developed and verified. Cold-start performance is found to be optimized when cell resistance is increasing prior to startup, which is indicative of polymer electrolyte membrane PEM dehydration. During cold start, cell resistance initially decreases due to PEM hydration by the product water. Interestingly, after a certain water-uptake capacity of the PEM is reached, resistance increases due to ice formation in and around the cathode catalyst layer CL, with some evidence of supercooled water flow at low currents. Utilizing lower startup currents apparently does not increase the PEM water-storage capability but does increase the total volume of ice formation in and around the CL. Lower startup currents were found to produce more total heat but at a reduced rate compared to high currents. Therefore, an acceptable current range exists for a given stack design which balances the total heat generation and time required to achieve a successful cold start.

Effect of Water on the Electrochemical Oxidation of Gas-Phase SO2 in a PEM Electrolyzer for H2 Production

Water plays a critical role in producing hydrogen from the electrochemical oxidation of SO 2 in a proton exchange membrane (PEM) electrolyzer. Not only is water needed to keep the membrane hydrated, but it is also a reactant. One way to supply water is to dissolve SO 2 in sulfuric acid and feed that liquid to the anode, but this process results in significant diffusion resistance for the SO 2 . Alternatively, we have developed a process where SO 2 is fed as a gas to the anode compartment and reacts with water crossing the membrane to produce sulfuric acid. There was concern that the diffusion resistance of water through the membrane is as significant as SO 2 diffusion through water, thus limiting the benefit of a gas-phase anode feed. We show here that water diffusion through the membrane is not as limiting as liquid-phase SO 2 diffusion. Therefore, we can control the cell voltage, the limiting current, and the sulfuric acid concentration by varying the diffusion resistance of the membrane via thickness or temperature. Catalyst loading, however, has a negligible effect on cell performance.

Synthesis, characterization and cycling performance of novel chromium oxide cathode materials for lithium batteries

Chromium oxide (CrOx) cathode material (with chromium oxidation state of +5.3) was synthesized by thermal decomposition of chromium trioxide at high temperature and pressure in oxygen atmosphere. The duration of thermal decomposition had a significant effect on the performance of these materials in terms of lithiation capacity. The detrimental effect of CrO 3 and lower oxidation state chromium oxides have been reduced considerably by reducing their amounts in the material. The operating conditions, namely, temperature, pressure and the reaction time were optimized based on synthesizing novel CrOx cathode material with superior properties, such as low irreversible capacity loss, stable capacity and low capacity fade under continuous cycling. These materials are stable intercalation hosts for lithium and were found to be reversible in the entire intercalation range (2.0–4.2 V versus Li/Li + ). The average voltage of these cells is 3 V versus Li/Li + . CrOx-B cathodes exhibit higher capacity than any of the prominent cathode materials used for lithium batteries with an initial lithiation capacity of capacity of 322 mAh/g. The material shows very low capacity fade during cycling and retained 93% of its reversible capacity after 100 cycles.

Enhanced photo‐bioelectrochemical energy conversion by genetically engineered cyanobacteria

Photosynthetic energy conversion using natural systems is increasingly being investigated in the recent years. Photosynthetic microorganisms, such as cyanobacteria, exhibit light-dependent electrogenic characteristics in photo-bioelectrochemical cells (PBEC) that generate substantial photocurrents, yet the current densities are lower than their photovoltaic counterparts. Recently, we demonstrated that a cyanobacterium named Nostoc sp. employed in PBEC could generate up to 35 mW m(-2) even in a non-engineered PBEC. With the insights obtained from our previous research, a novel and successful attempt has been made in the current study to genetically engineer the cyanobacteria to further enhance its extracellular electron transfer. The cyanobacterium Synechococcus elongatus PCC 7942 was genetically engineered to express a non-native redox protein called outer membrane cytochrome S (OmcS). OmcS is predominantly responsible for metal reducing abilities of exoelectrogens such as Geobacter sp. The engineered S. elongatus exhibited higher extracellular electron transfer ability resulting in approximately ninefold higher photocurrent generation on the anode of a PBEC than the corresponding wild-type cyanobacterium. This work highlights the scope for enhancing photocurrent generation in cyanobacteria, thereby benefiting faster advancement of the photosynthetic microbial fuel cell technology.