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Geobacter spp. are well-known exoelectrogenic microorganisms that often predominate acetate-fed biofilms in microbial fuel cells (MFCs) and other bioelectrochemical systems (BESs). By using an amplicon sequence variance analysis (at one nucleotide resolution), we observed a succession between two closely related species (98% similarity in 16S RNA), Geobacter sulfurreducens and Geobacter anodireducens, in the long-term studies (20 months) of MFC biofilms. Geobacter spp. predominated in the near-electrode portion of the biofilm, while the outer layer contained an abundance of aerobes, which may have helped to consume oxygen but reduced the relative abundance of Geobacter. Removal of the outer aerobes by norspermidine washing of biofilms revealed a transition from G. sulfurreducens to G. anodireducens. This succession was also found to occur rapidly in co-cultures in BES tests even in the absence of oxygen, suggesting that oxygen was not a critical factor. G. sulfurreducens likely dominated in early biofilms by its relatively larger cell size and production of extracellular polymeric substances (individual advantages), while G. anodireducens later predominated due to greater cell numbers (quantitative advantage). Our findings revealed the interspecies competition in the long-term evolution of Geobacter genus, providing microscopic insights into Geobacter's niche and competitiveness in complex electroactive microbial consortia.

Power overshoot can hinder determination of maximum power densities in microbial fuel cells (MFCs). In this work, a microfluidic approach was used to study overshoot in an MFC containing a pure culture of electroactive biofilms (EAB) containing Geobacter sulfurreducens. After 1-month operation under constant flow of an ideal nutrient medium, the MFC health began to degrade, marked by voltage loss and the appearance of anomalies in the power density curves. One such anomaly was a chronic power overshoot, accompanying a loss of both measured power and current density on the high-current side of the power density curve. The degree of power overshoot was quantified while certain flow-based interventions were applied, notably the shear erosion of the EAB outer layer. Next, two approaches to acclimation were demonstrated to treat the remaining overshoot. The standard approach, which acclimates the MFC to high currents before a standard polarization test, eliminated the remaining overshoot and returned maximum power densities to initial levels, but maximum current density remained lower than the initial level. A microfluidic-assisted “long-hold polarization test” enabled efficient in situ acclimation of each external resistor during the measurement. Despite the health-compromised MFC, this method provided long-term stability during the polarization test, resulting in power and current density measurements that exceeded those made on the healthy MFC using the standard polarization test. We conclude that slower electron transfer kinetics in unhealthy MFCs can provoke overshoot by prolonging the time to reach steady state during the polarization test, but a properly designed measurement overcomes this problem.

Ion intercalation electrodes are being investigated for use in mixed capacitive deionization (CDI) and battery electrode deionization (BDI) systems because they can achieve selective ion removal and low energy deionization. To improve the thermodynamic energy efficiency (TEE) of these systems, flow-through electrodes were developed by coating porous carbon felt electrodes with a copper hexacyanoferrate composite mixture. The TEE for ion separation using flow-through electrodes was compared to a system using flow-by electrodes with the same materials. The flow-through BDI system increased the recoverable energy nearly 3-fold (0.009 kWh m-3, compared to a 0.003 kWh m-3), which increased the TEE from ∼6% to 8% (NaCl concentration reduction from 50 to 42 mM; 10 A m-2, 50% water recovery, and 0.5 mL min-1). The TEE was further increased to 12% by decreasing the flow rate from 0.50 to 0.25 mL min-1. These findings suggest that, under similar operational conditions and materials, flow-through battery electrodes could achieve better energy recovery and TEE for desalination than flow-by electrodes.

The photocatalytic activity of TiO2 nanotubes can be improved through the construction of a Z-scheme composite, where photogenerated electrons from TiO2 recombine with the photogenerated holes in a coupled semiconductor. This arrangement allows for improved oxidation due to the residual holes of TiO2 and better chemical reduction due to the greater availability of the photogenerated electrons of the coupled semiconductor. Efficient Z-scheme porous-g-C3N4/TiO2-nanotube (PCN/TNTs) composites were developed here using a solid sublimation and transition approach, with Al–O links added by an impregnation method to increase interfacial linkages between the PCN and TNTs. The best results for photocatalytic CO2 conversion were obtained using 0.7PCN/0.4Al/TNTs, as shown by production of 54.9 ± 0.70 mg L–1 h–1 of acetic acid, 42.7 ± 0.54 mg L–1 h–1 of formic acid, and 45.4 ± 0.55 mg L–1 h–1 of methanol, which were about 3.8, 4.3, and 4.2 times that produced with bare TNTs. Photocatalytic 2,4-dichlorophenol decompos...

The impact of ion migration induced by an electrical field on water flux in a forward osmosis (FO) process was examined using a thin-film composite (TFC) membrane, held between two cation exchange membranes. An applied fixed current of 100 mA (1.7 mA cm-2) was sustained by the proton flux through the TFC-BW membrane using a feed of 34 mM NaCl, and a 257 mM NaCl draw solution. Protons generated at the anode were transported through the cation exchange membrane and into the draw solution, lowering the pH of the draw solution. Additional proton transport through the TFC-BW membrane also lowered the pH of the feed solution. The localized accumulation of the protons on the draw side of the TFC-BW membrane resulted in high concentration polarization modulus of 1.41 × 105, which enhanced the water flux into the draw solution (5.56 LMH at 100 mA), compared to the control (1.10 LMH with no current). These results using this electro-forward osmosis (EFO) process demonstrated that enhanced water flux into the draw solution could be achieved using ion accumulation induced by an electrical field. The EFO system could be used for FO applications where a limited use of draw solute is necessary.

Feed spacer biofouling is a major challenge in membrane processes such as nanofiltration and reverse osmosis. The bubbling of gas using air can be effective in partially controlling biofouling, but...

For the production of TiO2 nanotubes (TNTs) efficient photoelectrodes they must have efficient charge separation by trapping holes and transfer of electrons. In this study, MnOx and Pd codecorated TNTs photoelectrodes were successfully constructed using a simple impregnation method, followed by an electrochemical deposition process. The photocatalytic activities for CO2 conversion by the optimized TNTs photoelectrode (10Pd/0.8Mn/TNTs) were increased by 2.8 times to produce 40.3 ± 2.5 mg L–1 acetic acid, and by 2.5 times to generate 24.6 ± 1.9 mg L–1 formic acid compared to a bare TNTs photoelectrode. The optimized photoelectrode also showed the highest transient photocurrent of 1.15 mA cm–2. The improved performance was due to the elevated charge separation through bidirectional modulation of photogenerated holes and electrons, on the basis of the steady-state surface photovoltage and analysis with the formed •OH concentrations, electrochemical reduction tests with N2 or CO2 atmospheres, and electrochemic...

Reverse osmosis (RO) membranes inevitably foul because of the accumulation of material on the membrane surface. Instead of trying to reduce membrane fouling by chemically modifying the membrane, we took a different approach based on adding a sacrificial coating of two polyelectrolytes to the membrane. After membrane fouling, this coating was removed by flushing with a highly saline brine solution, and a new coating was regenerated in situ to provide a fresh protective layer (PL) on the membrane surface. The utility of this approach was demonstrated by conducting four consecutive dead-end filtration experiments using a model foulant (alginate, 200 ppm) in a synthetic brackish water (2000 ppm of NaCl). Brine removal and regeneration of the PL coating restored the water flux to an average of 97 ± 3% of its initial flux, compared to only 83 ± 3% for the pristine membrane. The average water flux for the PL-coated membranes was 15.5 ± 0.6 L m–2 h–1 until the flux was decreased by 10% versus its initial flux, co...

Conventional technologies for ammonium removal from wastewaters are based on biological conversion to nitrogen gas, eliminating the possibility for ammonium recovery. A new electrochemical approach...

While nickel is a good alternative to platinum as a catalyst for the hydrogen evolution reaction, it is desirable to reduce the amount of nickel needed for cathodes in microbial electrolysis cells (MECs). Activated carbon (AC) was investigated as a cathode base structure for Ni as it is inexpensive and an excellent adsorbent for Ni, and it has a high specific surface area. AC nickel-functionalized electrodes (AC-Ni) were prepared by incorporating Ni salts into AC by adsorption, followed by cathode fabrication using a phase inversion process using a poly(vinylidene fluoride) (PVDF) binder. The AC-Ni cathodes had significantly higher (∼50%) hydrogen production rates than controls (plain AC) in smaller MECs (static flow conditions) over 30 days of operation, with no performance decrease over time. In larger MECs with catholyte recirculation, the AC-Ni cathode produced a slightly higher hydrogen production rate (1.1 ± 0.1 L-H2/Lreactor/day) than MECs with Ni foam (1.0 ± 0.1 L-H2/Lreactor/day). Ni dissolution ...

New electrochemical technologies that use capacitive or battery electrodes are being developed to minimize energy requirements for desalinating brackish waters. When a pair of electrodes is charged in capacitive deionization (CDI) systems, cations bind to the cathode and anions bind to the anode, but high applied voltages (>1.2 V) result in parasitic reactions and irreversible electrode oxidation. In the battery electrode deionization (BDI) system developed here, two identical copper hexacyanoferrate (CuHCF) battery electrodes were used that release and bind cations, with anion separation occurring via an anion exchange membrane. The system used an applied voltage of 0.6 V, which avoided parasitic reactions, achieved high electrode desalination capacities (up to 100 mg-NaCl/g-electrode, 50 mM NaCl influent), and consumed less energy than CDI. Simultaneous production of desalinated and concentrated solutions in two channels avoided a two-cycle approach needed for CDI. Stacking additional membranes between ...

Innovation in urban water systems is required to address the increasing demand for clean water due to population growth and aggravated water stress caused by water pollution, aging infrastructure, and climate change. Advances in materials science, modular water treatment technologies, and complex systems analyses, coupled with the drive to minimize the energy and environmental footprints of cities, provide new opportunities to ensure a resilient and safe water supply. We present a vision for enhancing efficiency and resiliency of urban water systems and discuss approaches and research needs for overcoming associated implementation challenges.

The CO2 concentration difference between ambient air and exhaust gases created by combusting fossil fuels is an untapped energy source for producing electricity. One method of capturing this energy is dissolving CO2 gas into water and then converting the produced chemical potential energy into electrical power using an electrochemical system. Previous efforts using this method found that electricity can be generated; however, electrical power densities were low, and expensive ion-exchange membranes were needed. Here, we overcame these challenges by developing a new approach to capture electrical power from CO2 dissolved in water, the pH-gradient flow cell. In this approach, two identical supercapacitive manganese oxide electrodes were separated by a nonselective membrane and exposed to an aqueous buffer solution sparged with either CO2 gas or air. This pH-gradient flow cell produced an average power density of 0.82 W/m2, which was nearly 200 times higher than values reported using previous approaches.

A new three-electrode electrocoagulation reactor was investigated to increase the rate of removal of phosphate from domestic wastewater. Initially, two electrodes (graphite plate and air cathode) were connected with 0.5 V of voltage applied for a short charging time (∼10 s). The direction of the electric field was then reversed, by switching the power supply lead from the anode to the cathode, and connecting the other lead to a sacrificial aluminum mesh anode for removal of phosphate by electrocoagulation. The performance of this process, called a reverse-electric field, air cathode electrocoagulation (REAEC) reactor, was tested using domestic wastewater as a function of charging time and electrocoagulation time. REAEC wastewater treatment removed up to 98% of phosphate in 15 min (inert electrode working time of 10 s, current density of 1 mA/cm2, and 15 min total electrocoagulation time), which was 6% higher than that of the control (no inert electrode). The energy demand varied from 0.05 kWh/m3 for 85% r...

Salinity-gradient energy (SGE) technologies produce carbon-neutral and renewable electricity from salinity differences between seawater and freshwater. Capacitive mixing (CapMix) is a promising class of SGE technologies that captures energy using capacitive or battery electrodes, but CapMix devices have produced relatively low power densities and often require expensive materials. Here, we combined existing CapMix approaches to develop a concentration flow cell that can overcome these limitations. In this system, two identical battery (i.e., faradaic) electrodes composed of copper hexacyanoferrate (CuHCF) were simultaneously exposed to either high (0.513 M) or low (0.017 M) concentration NaCl solutions in channels separated by a filtration membrane. The average power density produced was 411 ± 14 mW m(-2) (normalized to membrane area), which was twice as high as previously reported values for CapMix devices. Power production was continuous (i.e., it did not require a charging period and did not vary during each step of a cycle) and was stable for 20 cycles of switching the solutions in each channel. The concentration flow cell only used inexpensive materials and did not require ion-selective membranes or precious metals. The results demonstrate that the concentration flow cell is a promising approach for efficiently harvesting energy from salinity differences.

Long-term operation of microbial fuel cells (MFCs) can result in substantial degradation of activated carbon (AC) air-cathode performance. To examine a possible role in fouling from organic matter in water, cathodes were exposed to high concentrations of humic acids (HA). Cathodes treated with 100 mg L(-1) HA exhibited no significant change in performance. Exposure to 1000 mg L(-1) HA decreased the maximum power density by 14% (from 1310 ± 30 mW m(-2) to 1130 ± 30 mW m(-2)). Pore blocking was the main mechanism as the total surface area of the AC decreased by 12%. Minimization of external mass transfer resistances using a rotating disk electrode exhibited only a 5% reduction in current, indicating about half the impact of HA adsorption was associated with external mass transfer resistance and the remainder was due to internal resistances. Rinsing the cathodes with deionized water did not restore cathode performance. These results demonstrated that HA could contribute to cathode fouling, but the extent of power reduction was relatively small in comparison to large mass of humics adsorbed. Other factors, such as biopolymer attachment, or salt precipitation, are therefore likely more important contributors to long-term fouling of MFC cathodes.

Electrically conductive, graphene-coated, hollow-fiber porous membranes were used as cathodes in anaerobic electrochemical membrane bioreactors (AnEMBRs) operated at different applied voltages (0.7 and 0.9 V) using a new rectangular reactor configuration compared to a previous tubular design (0.7 V). The onset of biofouling was delayed and minimized in rectangular reactors operated at 0.9 V compared to those at 0.7 V due to higher rates of hydrogen production. Maximum transmembrane pressures for the rectangular reactor were only 0.10 bar (0.7 V) or 0.05 bar (0.9 V) after 56 days of operation compared to 0.46 bar (0.7 V) for the tubular reactor after 52 days. The thickness of the membrane biofouling layer was approximately 0.4 μm for rectangular reactors and 4 μm for the tubular reactor. Higher permeate quality (TSS = 0.05 mg/L) was achieved in the rectangular AnEMBR than that in the tubular AnEMBR (TSS = 17 mg/L), likely due to higher current densities that minimized the accumulation of cells in suspension. These results show that the new rectangular reactor design, which had increased rates of hydrogen production, successfully delayed the onset of cathode biofouling and improved reactor performance.

Different microbial electrochemical technologies are being developed for many diverse applications, including wastewater treatment, biofuel production, water desalination, remote power sources, and biosensors. Current and energy densities will always be limited relative to batteries and chemical fuel cells, but these technologies have other advantages based on the self-sustaining nature of the microorganisms that can donate or accept electrons from an electrode, the range of fuels that can be used, and versatility in the chemicals that can be produced. The high cost of membranes will likely limit applications of microbial electrochemical technologies that might require a membrane. For microbial fuel cells, which do not need a membrane, questions about whether larger-scale systems can produce power densities similar to those obtained in laboratory-scale systems remain. It is shown here that configuration and fuel (pure chemicals in laboratory media vs actual wastewaters) remain the key factors in power pro...

Methane is the primary end product from cathodic current in microbial electrolysis cells (MECs) in the absence of methanogenic inhibitors, but little is known about the archaeal communities that develop in these systems. MECs containing cathodes made from different materials (carbon brushes, or plain graphite blocks or blocks coated with carbon black and platinum, stainless steel, nickel, ferrihydrite, magnetite, iron sulfide, or molybdenum disulfide) were inoculated with anaerobic digester sludge and acclimated at a set potential of −600 mV (versus a standard hydrogen electrode). The archaeal communities on all cathodes, except those coated with platinum, were predominated by Methanobacterium (median 97% of archaea). Cathodes with platinum contained mainly archaea most similar to Methanobrevibacter. Neither of these methanogens were abundant (

Sustained current generation by anodic biofilms is a key element for the longevity and success of bioelectrochemical systems. Over time, however, inactive or dead cells can accumulate within the anode biofilm, which can be particularly detrimental to current generation. Mixed and pure culture (Geobacter anodireducens) biofilms were examined here relative to changes in electrochemical properties over time. An analysis of the three-dimensional metabolic structure of the biofilms over time showed that both types of biofilms developed a live outer-layer that covered a dead inner-core. This two-layer structure appeared to be mostly a result of relatively low anodic current densities compared to other studies. During biofilm development, the live layer reached a constant thickness, whereas dead cells continued to accumulate near the electrode surface. This result indicated that only the live outer-layer of biofilm was responsible for current generation and suggested that the dead inner-layer continued to function as an electrically conductive matrix. Analysis of the electrochemical properties and biofilm thickness revealed that the diffusion resistance measured using electrochemical impedance spectroscopy might not be due to acetate or proton diffusion limitations to the live layer, but rather electron-mediator diffusion.

The amount of salinity-gradient energy that can be obtained through capacitive mixing based on double layer expansion depends on the extent the electric double layer (EDL) is altered in a low salt concentration (LC) electrolyte (e.g., river water). We show that the electrode-rise potential, which is a measure of the EDL perturbation process, was significantly (P = 10(–5)) correlated to the concentration of strong acid surface functional groups using five types of activated carbon. Electrodes with the lowest concentration of strong acids (0.05 mmol g(–1)) had a positive rise potential of 59 ± 4 mV in the LC solution, whereas the carbon with the highest concentration (0.36 mmol g(–1)) had a negative rise potential (−31 ± 5 mV). Chemical oxidation of a carbon (YP50) using nitric acid decreased the electrode rise potential from 46 ± 2 mV (unaltered) to −6 ± 0.5 mV (oxidized), producing a whole cell potential (53 ± 1.7 mV) that was 4.4 times larger than that obtained with identical electrode materials (from 12 ± 1 mV). Changes in the EDL were linked to the behavior of specific ions in a LC solution using molecular dynamics and metadynamics simulations. The EDL expanded in the LC solution when a carbon surface (pristine graphene) lacked strong acid functional groups, producing a positive-rise potential at the electrode. In contrast, the EDL was compressed for an oxidized surface (graphene oxide), producing a negative-rise electrode potential. These results established the linkage between rise potentials and specific surface functional groups (strong acids) and demonstrated on a molecular scale changes in the EDL using oxidized or pristine carbons.

Efficient conversion of “mixing energy” to electricity through capacitive mixing (CapMix) has been limited by low energy recoveries, low power densities, and noncontinuous energy production resulting from intermittent charging and discharging cycles. We show here that a CapMix system based on a four-reactor process with flow electrodes can generate constant and continuous energy, providing a more flexible platform for harvesting mixing energy. The power densities were dependent on the flow-electrode carbon loading, with 5.8 ± 0.2 mW m–2 continuously produced in the charging reactor and 3.3 ± 0.4 mW m–2 produced in the discharging reactor (9.2 ± 0.6 mW m–2 for the whole system) when the flow-electrode carbon loading was 15%. Additionally, when the flow-electrode electrolyte ion concentration increased from 10 to 20 g L–1, the total power density of the whole system (charging and discharging) increased to 50.9 ± 2.5 mW m–2.

Air cathodes used in microbial fuel cells (MFCs) need to have high catalytic activity for oxygen reduction, but they must also be easy to manufacture, inexpensive, and watertight. A simple one-step, phase inversion process was used here to construct an inexpensive MFC cathode using a poly(vinylidene fluoride) (PVDF) binder and an activated carbon catalyst. The phase inversion process enabled cathode preparation at room temperatures, without the need for additional heat treatment, and it produced for the first time a cathode that did not require a separate diffusion layer to prevent water leakage. MFCs using this new type of cathode produced a maximum power density of 1470 ± 50 mW m–2 with acetate as a substrate, and 230 ± 10 mW m–2 with domestic wastewater. These power densities were similar to those obtained using cathodes made using more expensive materials or more complex procedures, such as cathodes with a polytetrafluoroethylene (PTFE) binder and a poly(dimethylsiloxane) (PDMS) diffusion layer, or a ...

Waste acid streams produced at industrial sites are often co-located with large sources of waste heat (e.g., industrial exhaust gases, cooling water, and heated equipment). Reverse electrodialysis (RED) systems can be used to generate electrical power and hydrogen gas using waste heat-derived solutions, but high electrode overpotentials limit system performance. We show here that an ammonium bicarbonate (AmB) RED system can achieve simultaneous waste acid neutralization and in situ hydrogen production, while capturing energy from excess waste heat. The rate of acid neutralization was dependent on stack flow rate and increased 50× (from 0.06 ± 0.04 to 3.0 ± 0.32 pH units min–1 m–2 membrane), as the flow rate increased 6× (from 100 to 600 mL min–1). Acid neutralization primarily took place due to ammonium electromigration (37 ± 4%) and proton diffusion (60 ± 5%). The use of a synthetic waste acid stream as a catholyte (pH ≈ 2) also increased hydrogen production rates by 65% (from 5.3 ± 0.5 to 8.7 ± 0.1 m3 H...

Natural mineral carbonation can be accelerated using acid and alkali solutions to enhance atmospheric CO2 sequestration, but the production of these solutions needs to be carbon-neutral. A microbial reverse-electrodialysis electrolysis and chemical-production cell (MRECC) was developed to produce these solutions and H2 gas using only renewable energy sources (organic matter and salinity gradient). Using acetate (0.82 g/L) as a fuel for microorganisms to generate electricity in the anode chamber (liquid volume of 28 mL), 0.45 mmol of acid and 1.09 mmol of alkali were produced at production efficiencies of 35% and 86%, respectively, along with 10 mL of H2 gas. Serpentine dissolution was enhanced 17–87-fold using the acid solution, with approximately 9 mL of CO2 absorbed and 4 mg of CO2 fixed as magnesium or calcium carbonates. The operational costs, based on mineral digging and grinding, and water pumping, were estimated to be only $25/metric ton of CO2 fixed as insoluble carbonates. Considering the additio...

Microbial fuel cells (MFCs) are a promising technology for energy-efficient domestic wastewater treatment, but the effluent quality has typically not been sufficient for discharge without further treatment. A two-stage laboratory-scale combined treatment process, consisting of microbial fuel cells and an anaerobic fluidized bed membrane bioreactor (MFC-AFMBR), was examined here to produce high quality effluent with minimal energy demands. The combined system was operated continuously for 50 days at room temperature (∼25 °C) with domestic wastewater having a total chemical oxygen demand (tCOD) of 210 ± 11 mg/L. At a combined hydraulic retention time (HRT) for both processes of 9 h, the effluent tCOD was reduced to 16 ± 3 mg/L (92.5% removal), and there was nearly complete removal of total suspended solids (TSS; from 45 ± 10 mg/L to

Activated carbon (AC) is a useful and environmentally sustainable catalyst for oxygen reduction in air-cathode microbial fuel cells (MFCs), but there is great interest in improving its performance and longevity. To enhance the performance of AC cathodes, carbon black (CB) was added into AC at CB:AC ratios of 0, 2, 5, 10, and 15 wt % to increase electrical conductivity and facilitate electron transfer. AC cathodes were then evaluated in both MFCs and electrochemical cells and compared to reactors with cathodes made with Pt. Maximum power densities of MFCs were increased by 9-16% with CB compared to the plain AC in the first week. The optimal CB:AC ratio was 10% based on both MFC polarization tests and three electrode electrochemical tests. The maximum power density of the 10% CB cathode was initially 1560 ± 40 mW/m(2) and decreased by only 7% after 5 months of operation compared to a 61% decrease for the control (Pt catalyst, 570 ± 30 mW/m(2) after 5 months). The catalytic activities of Pt and AC (plain or with 10% CB) were further examined in rotating disk electrode (RDE) tests that minimized mass transfer limitations. The RDE tests showed that the limiting current of the AC with 10% CB was improved by up to 21% primarily due to a decrease in charge transfer resistance (25%). These results show that blending CB in AC is a simple and effective strategy to enhance AC cathode performance in MFCs and that further improvement in performance could be obtained by reducing mass transfer limitations.

A new hybrid, air-biocathode microbial fuel cell-membrane bioreactor (MFC-MBR) system was developed to achieve simultaneous wastewater treatment and ultrafiltration to produce water for direct reclamation. The combined advantages of this system were achieved by using an electrically conductive ultrafiltration membrane as both the cathode and the membrane for wastewater filtration. The MFC-MBR used an air-biocathode, and it was shown to have good performance relative to an otherwise identical cathode containing a platinum catalyst. With 0.1 mm prefiltered domestic wastewater as the feed, the maximum power density was 0.38 W/m(2) (6.8 W/m(3)) with the biocathode, compared to 0.82 W/m(2) (14.5 W/m(3)) using the platinum cathode. The permeate quality from the biocathode reactor was comparable to that of a conventional MBR, with removals of 97% of the soluble chemical oxygen demand, 97% NH3-N, and 91% of total bacteria (based on flow cytometry). The permeate turbidity was0.1 nephelometric turbidity units. These results show that a biocathode MFC-MBR system can achieve high levels of wastewater treatment with a low energy input due to the lack of a need for wastewater aeration.

Activated carbon (AC) is a cost-effective catalyst for the oxygen reduction reaction (ORR) in air-cathode microbial fuel cells (MFCs). To enhance the catalytic activity of AC cathodes, AC powders were pyrolyzed with iron ethylenediaminetetraacetic acid (FeEDTA) at a weight ratio of FeEDTA:AC = 0.2:1. MFCs with FeEDTA modified AC cathodes and a stainless steel mesh current collector produced a maximum power density of 1580 ± 80 mW/m(2), which was 10% higher than that of plain AC cathodes (1440 ± 60 mW/m(2)) and comparable to Pt cathodes (1550 ± 10 mW/m(2)). Further increases in the ratio of FeEDTA:AC resulted in a decrease in performance. The durability of AC-based cathodes was much better than Pt-catalyzed cathodes. After 4.5 months of operation, the maximum power density of Pt cathode MFCs was 50% lower than MFCs with the AC cathodes. Pyridinic nitrogen, quaternary nitrogen and iron species likely contributed to the increased activity of FeEDTA modified AC. These results show that pyrolyzing AC with FeEDTA is a cost-effective and durable way to increase the catalytic activity of AC.

A microbial desalination cell (MDC) uses exoelectrogenic bacteria to oxidize organic matter while desalinating water. Protons produced from the oxidation of organics at the anode result in anolyte acidification and reduce performance. A new method was used here to mitigate anolyte acidification based on adding non-buffered saline catholyte effluent from a previous cycle to the anolyte at the beginning of the next cycle. This method was tested using a larger-scale MDC (267 mL) containing four anode brushes and a three cell pair membrane stack. With an anolyte salt concentration increased by an equivalent of 75 mM NaCl using the catholyte effluent, salinity was reduced by 26.0 ± 0.5% (35 g/L NaCl initial solution) in a 10 h cycle, compared to 18.1 ± 2.0% without catholyte addition. This improvement was primarily due to the increase in buffering capacity of the anolyte, although increased conductivity slightly improved performance as well. There was some substrate loss from the anolyte by diffusion into the ...

Sustainable methods are needed to recycle precious metals and synthesize catalytic nanoparticles. Palladium nanoparticles can be produced via microbial reduction of soluble Pd(II) to Pd(0), but in previous tests using dissimilatory metal reducing bacteria (DMRB), the nanoparticles were closely associated with the cells, occupying potential reductive sites and eliminating the potential for cell reuse. The DMRB Geobacter sulfurreducens was shown here to reduce soluble Pd(II) to Pd(0) nanoparticles primarily outside the cell, reducing the toxicity of metal ions, and allowing nanoparticle recovery without cell destruction that has previously been observed using other microorganisms. Cultures reduced 50 ± 3 mg/L Pd(II) with 1% hydrogen gas (v/v headspace) in 6 h incubation tests [100 mg/L Pd(II) initially], compared to 8 ± 3 mg/L (10 mM acetate) without H2. Acetate was ineffective as an electron donor for palladium removal in the presence or absence of fumarate as an electron acceptor. TEM imaging verified tha...

Commercially available activated carbon (AC) powders made from different precursor materials (coal, peat, coconut shell, hardwood, and phenolic resin) were electrochemically evaluated as oxygen reduction catalysts and tested as cathode catalysts in microbial fuel cells (MFCs). AC powders were characterized in terms of surface chemistry and porosity, and their kinetic activities were compared to carbon black and platinum catalysts in rotating disk electrode (RDE) tests. Cathodes using the coal-derived AC had the highest power densities in MFCs (1620 ± 10 mW m(-2)). Peat-based AC performed similarly in MFC tests (1610 ± 100 mW m(-2)) and had the best catalyst performance, with an onset potential of E(onset) = 0.17 V, and n = 3.6 electrons used for oxygen reduction. Hardwood based AC had the highest number of acidic surface functional groups and the poorest performance in MFC and catalysis tests (630 ± 10 mW m(-2), E(onset) = -0.01 V, n = 2.1). There was an inverse relationship between onset potential and quantity of strong acid (pKa8) functional groups, and a larger fraction of microporosity was negatively correlated with power production in MFCs. Surface area alone was a poor predictor of catalyst performance, and a high quantity of acidic surface functional groups was determined to be detrimental to oxygen reduction and cathode performance.

A better understanding of how anode and separator physical properties affect power production is needed to improve energy and power production by microbial fuel cells (MFCs). Oxygen crossover from the cathode can limit power production by bacteria on the anode when using closely spaced electrodes [separator electrode assembly (SEA)]. Thick graphite fiber brush anodes, as opposed to thin carbon cloth, and separators have previously been examined as methods to reduce the impact of oxygen crossover on power generation. We examined here whether the thickness of the anode could be an important factor in reducing the effect of oxygen crossover on power production, because bacteria deep in the electrode could better maintain anaerobic conditions. Carbon felt anodes with three different thicknesses were examined to see the effects of thicker anodes in two configurations: widely spaced electrodes and SEA. Power increased with anode thickness, with maximum power densities (604 mW/m2, 0.32 cm; 764 mW/m2, 0.64 cm; an...

Microbial fuel cells (MFCs) with hydrophilic poly(vinyl alcohol) (PVA) separators showed higher Coulombic efficiencies (94%) and power densities (1220 mW m(-2)) than cells with porous glass fiber separators or reactors without a separator after 32 days of operation. These remarkable increases in both the coublomic efficiency and the power production of the microbial fuel cells were made possible by the separator's unique characteristics of fouling mitigation of the air cathode without a large increase in ionic resistance in the cell. This new type of polymer gel-like separator design will be useful for improving MFC reactor performance by enabling compact cell designs.

Microbial fuel cells (MFCs) are an environmentally friendly method for water purification and self-sustained electricity generation using microorganisms. Microsized MFCs can also be a useful power source for lab-on-a-chip and similar integrated devices. We fabricated a 1.25 μL microsized MFC containing an anode of vertically aligned, forest type multiwalled carbon nanotubes (MWCNTs) with a nickel silicide (NiSi) contact area that produced 197 mA/m(2) of current density and 392 mW/m(3) of power density. The MWCNTs increased the anode surface-to-volume ratio, which improved the ability of the microorganisms to couple and transfer electrons to the anode. The use of nickel silicide also helped to boost the output current by providing a low resistance contact area to more efficiently shuttle electrons from the anode out of the device.

A new type of bioelectrochemical system for producing electrical power, called a microbial reverse-electrodialysis cell (MRC), was developed to increase voltages and power densities compared to those generated individually by microbial fuel cells (MFCs) or reverse electrodialysis (RED) systems. In RED systems, electrode overpotentials create significant energy losses due to thermodynamically unfavorable electrode reactions, and therefore a large number of stacked cells must be used to have significant energy recovery. This results in high capital costs for the large number of membranes, and increases energy losses from pumping water through a large number of cells. In an MRC, high overpotentials are avoided through oxidation of organic matter by exoelectrogenic bacteria on the anode and oxygen reduction on the cathode. An MRC containing only five pairs of RED cells, fed solutions typical of seawater (600 mM NaCl) and river water (12 mM NaCl) at 0.85 mL/min, produced up to 3.6 W/m(2) (cathode surface area) and 1.2-1.3 V with acetate as a substrate. Pumping accounted for2% of the produced power. A higher flow rate (1.55 mL/min) increased power densities up to 4.3 W/m(2). COD removal was 98% with a Coulombic efficiency of 64%. Power production by the individual components was substantially lower with 0.7 W/m(2) without salinity driven energy, and0.015 W/m(2) with reduced exoelectrogenic activity due to substrate depletion. These results show that the combination of an MFC and a RED stack synergistically increases performance relative to the individual systems, producing a new type of system that can be used to more efficiently capture salinity driven energy from seawater and river water.

An electrochemical system includes: (1) a battery including an anode and a cathode; (2) a first source of a first electrolyte having a first concentration of ions; (3) a second source of a second electrolyte having a second concentration of the ions, wherein the second concentration is greater than the first concentration; and (4) a fluid conveyance mechanism connected between the battery and each of the first source and the second source. During charging of the battery, the anode and the cathode are at least partially immersed in the first electrolyte, and, during discharging of the battery, the anode and the cathode are at least partially immersed in the second electrolyte. The fluid conveyance mechanism exchanges the first electrolyte with the second electrolyte between charging and discharging of the battery, and exchanges the second electrolyte with the first electrolyte between discharging and charging of the battery.

In bioelectrochemical systems (BESs), the anode potential can be set to a fixed voltage using a potentiostat, but there is no accepted method for defining an optimal potential. Microbes can theoretically gain more energy by reducing a terminal electron acceptor with a more positive potential, for example oxygen compared to nitrate. Therefore, more positive anode potentials should allow microbes to gain more energy per electron transferred than a lower potential, but this can only occur if the microbe has metabolic pathways capable of capturing the available energy. Our review of the literature shows that there is a general trend of improved performance using more positive potentials, but there are several notable cases where biofilm growth and current generation improved or only occurred at more negative potentials. This suggests that even with diverse microbial communities, it is primarily the potential of the terminal respiratory proteins used by certain exoelectrogenic bacteria, and to a lesser extent the anode potential, that determines the optimal growth conditions in the reactor. Our analysis suggests that additional bioelectrochemical investigations of both pure and mixed cultures, over a wide range of potentials, are needed to better understand how to set and evaluate optimal anode potentials for improving BES performance.

Two challenges for improving the performance of air cathode, single-chamber microbial fuel cells (MFCs) include increasing Coulombic efficiency (CE) and decreasing internal resistance. Nonbiodegradable glass fiber separators between the two electrodes were shown to increase power and CE, compared to cloth separators (J-cloth) that were degraded over time. MFC tests were conducted using glass fiber mats with thicknesses of 1.0 mm (GF1) or 0.4 mm (GF0.4), a cation exchange membrane (CEM), and a J-cloth (JC), using reactors with different configurations. Higher power densities were obtained with either GF1 (46 +/- 4 W/m(3)) or JC (46 +/- 1 W/m(3)) in MFCs with a 2 cm electrode spacing, when the separator was placed against the cathode (S-configuration), rather than MFCs with GF0.4 (36 +/- 1 W/m(3)) or CEM (14 +/- 1 W/m(3)). Power was increased to 70 +/- 2 W/m(3) by placing the electrodes on either side of the GF1 separator (single separator electrode assembly, SSEA) and further to 150 +/- 6 W/m(3) using two sets of electrodes spaced 2 cm apart (double separator electrode assembly, DSEA). Reducing the DSEA electrode spacing to 0.3 cm increased power to 696 +/- 26 W/m(3) as a result of a decrease in the ohmic resistance from 5.9 to 2.2 Omega. The main advantages of a GF1 separator compared to JC were an improvement in the CE from 40% to 81% (S-configuration), compared to only 20-40% for JC under similar conditions, and the fact that GF1 was not biodegradable. The high CE for the GF1 separator was attributed to a low oxygen mass transfer coefficient (k(O) = 5.0 x 10(-5) cm/s). The GF1 and JC materials differed in the amount of biomass that accumulated on the separator and its biodegradability, which affected long-term power production and oxygen transport. These results show that materials and mass transfer properties of separators are important factors for improving power densities, CE, and long-term performance of MFCs.