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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.

Many salinity gradient energy technologies such as reverse electrodialysis (RED) rely on highly selective anion transport through polymeric anion exchange membranes. While there is considerable interest in using thermolytic solutions such as ammonium bicarbonate (AmB) in RED processes for closed-loop conversion of heat energy to electricity, little is known about membrane performance in this electrolyte. The resistances of two commercially available cation exchange membranes in AmB were lower than their resistances in NaCl. However, the resistances of commercially available anion exchange membranes (AEMs) were much larger in AmB than in NaCl, which would adversely affect energy recovery. The properties of a series of quaternary ammonium-functionalized poly(phenylene oxide) and Radel-based AEMs were therefore examined to understand the reasons for increased resistance in AmB to overcome this performance penalty due to the lower mobility of bicarbonate, 4.59 × 10–4 cm2/(V s), compared to chloride, 7.90 × 10...

Ammonium ions can be effectively removed from water using electrochemical processes such as battery electrode deionization (BDI), but previous tests have examined removal in the presence of competing ions (e.g. sodium). The recovery of NH4+ was examined here in the absence and presence of a relatively inert background electrolyte (MgCl2, 10 mM) added to only provide a conductive solution with cations that have minimal intercalation into the copper hexacyanoferrate (CuHCF) electrodes. The capacity of the CuHCF electrodes for NH4+ in the presence of MgCl2 was nearly constant at 8.4 ± 1.4 g-NH4+/g-electrode (treated stream, 0.3 V) over a range of 10 to 100 mM NH4Cl. In addition, the energy needed to remove NH4+ was constant at 11.5, at least 20% of the anions removed were phosphate ions compared to Cl−. These results demonstrate that the capacity of the BDI electrodes is relatively independent of the NH4+ concentration, and that phosphate is not selectively removed compared to Cl−.

The decreasing cost of electricity produced using solar and wind and the need to avoid CO2 emissions from fossil fuels has heightened interest in hydrogen gas production by water electrolysis. Offshore and coastal hydrogen gas production using seawater and renewable electricity is of particular interest, but it is currently economically infeasible due to the high costs of ion exchange membranes and the need to desalinate seawater in existing electrolyzer designs. A new approach is described here that uses relatively inexpensive commercially available membranes developed for reverse osmosis (RO) to selectively transport favorable ions. In an applied electric field, RO membranes have a substantial capacity for proton and hydroxide transport through the active layer while excluding salt anions and cations. A perchlorate salt was used to provide an inert and contained anolyte, with charge balanced by proton and hydroxide ion flow across the RO membrane. Synthetic seawater (NaCl) was used as the catholyte, where it provided continuous hydrogen gas evolution. The RO membrane resistance was 21.7 ± 3.5 Ω cm2 in 1 M NaCl and the voltages needed to split water in a model electrolysis cell at current densities of 10–40 mA cm−2 were comparable to those found when using two commonly used, more expensive ion exchange membranes.

Ammonium is typically removed from treated wastewaters before discharge by converting it to nitrogen gas, but its capture and reuse could provide a new strategy for energy recovery at treatment plants. A three-stage electrochemical approach was developed here to selectively remove and concentrate ammonium derived from wastewater. Each stage contained a battery electrode deionization (BDI) cell containing two copper hexacyanoferrate (CuHCF) electrodes separated into two channels using an anion exchange membrane. Through application of a low applied voltage (0.3 V) in each of the three stages, ammonium was concentrated greater than 6 times, from 5 to 32 mM (90 to 576 mg L−1), with minimal changes in the concentration of other cations (Na+, K+, Mg2+, and Ca2+) present in the water due to the high ammonium ion selectivity of CuHCF electrodes under these operating conditions. The cumulative energy use for the three-stage process was only 2.0 kW h per kg-N, compared to the 14 kW h per kg-N that would be needed to manufacture this amount of ammonium from nitrogen gas. Nitrogen recovery methods such as these will be needed to further transform used water plants into more effective resource recovery treatment plants.

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...

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...

The addition of electrically conductive materials may enhance anaerobic digestion (AD) efficiency by promoting direct interspecies electron transfer (DIET) between electroactive microorganisms, but an equivalent enhancement can also be achieved using non-conductive materials. Four high surface area brush materials were added to AD reactors: non-conductive horsehair (HB) and polyester (PB), and conductive carbon fiber (CB) and stainless steel (SB) brushes. Reactors with the polyester material showed lower methane production (68 ± 5 mL/g CODfed) than the other non-conductive material (horsehair) and the conductive (graphite or stainless steel) materials (83 ± 3 mL/g CODfed) (p

Prussian blue hexacyanoferrate (HCF) materials, such as copper hexacyanoferrate (CuHCF) and nickel hexacyanoferrate (NiHCF), can produce higher salt removal capacities than purely capacitive materials when used as electrode materials during electrochemical water deionization due to cation intercalation into the HCF structure. One factor limiting the application of HCF materials is their decay in deionization performance over multiple cycles. By examining the performance of CuHCF and NiHCF electrodes at three different pH values (2.5, 6.3, and 10.2) in multiple-cycle deionization tests, losses in capacity (up to 73% for CuHCF and 39% for NiHCF) were shown to be tied to different redox-active centers through analysis of dissolution of electrode metals. Both copper and iron functioned as active centers for Na+ removal in CuHCF, while iron was mainly the active center in NiHCF. This interaction of Na+ and active centers was demonstrated by correlating the decrease in performance to the concentration of these metal ions in the effluent solutions collected over multiple cycles at different pHs (up to 0.86 ± 0.14 mg/L for iron and 0.42 ± 0.17 mg/L for copper in CuHCF and 0.38 ± 0.05 mg/L for iron in NiHCF). Both materials were more stable (

Methane is an abundant and inexpensive feedstock that is available as natural gas and renewable biogas. However, methane has not been regarded as a good substrate for microbial fuel cells (MFCs) due to low power densities. To increase power, a two-step strategy was used based on conversion of methane into methanol, followed by electricity generation using methanol as the substrate in the MFC. To produce methanol, a methane-oxidizing culture was grown in a high phosphate buffer resulting in the accumulation of 350 ± 42 mg/L of methanol. The methanol-fed MFC produced a maximum power density of 426 ± 17 mW/m2. It was also shown that the methanol-rich medium produced from the first step can directly be supplied to the MFCs, removing the need for purification of methanol. Analysis of the microbial community suggests that acetogens first converts methanol into acetate, which is then consumed by exoelectrogens for power generation.

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...

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...

A migration electric-field assisted electrocoagulation (MEAEC) system was developed to increase phosphate removal from domestic wastewater, with reduced energy consumption, using a titanium charging (inert) electrode and a sacrificial iron anode. In the MEAEC, an electric field was applied between the inert electrode (titanium) and an air cathode to drive migration of phosphate anions towards the sacrificial anode. Current was then applied between the sacrificial anode (Fe or Al mesh) and the air cathode to drive electrocoagulation of phosphate. A MEAEC with the Fe electrode using primary clarifier effluent achieved 98% phosphate removal, producing water with a total phosphorus of 0.3 mg/L with6 min total treatment time (five cycles; each 10 s inert electrode charging, and 1 min electrocoagulation), at a constant current density of 1 mA/cm

The integration of microbial electrolysis cells (MECs) and anaerobic digestion (AD) has recently been pursued to improve methane production through a combined system (AD-MEC). Analysis of 24 different experimental results published in 13 different studies showed that the only significant correlation between increased methane generation rate and other parameters was the surface area provided by the electrodes per volume of reactor, or the S/V ratio (R2 = 0.63, p-value

High current densities have been obtained by bioanodes in electrochemical cells, but this performance has not previously been translated into higher power densities in microbial fuel cells (MFCs). The most critical factor for boosting power generation in MFCs was proven here to be OH– ion transfer from the cathode to the bioanode by comparing performance of an MFC with an anion exchange membrane (AEM), in a near zero-gap configuration and no bulk catholyte, to that obtained using a cation exchange membrane (CEM) or non-ion selective ultrafiltration membrane (UFM). In the AEM-MFC hydroxide ion transport primarily balanced charge transfer between the electrodes, enabling 8.8 ± 0.5 W m−2 (at 42 ± 1 A m−2) which is highest power density ever achieved using this anolyte (acetate in a 50 mM phosphate buffer medium). The power density with the AEM was 3 × higher than that obtained using a CEM (3.1 ± 0.1 W m−2) or UFM (2.4 ± 0.1 W m−2) as these other two membranes allowed cations (sodium and magnesium) to be transported to the cathode for balancing charge. The lack of cation transport into the cathode using the AEM without a bulk catholyte also avoided salt precipitation in the cathode, and thus enabling more stable power production over time. The large differences in power obtained using the AEM, compared to the CEM or UFM conclusively demonstrate the critical role of OH– ion transport in MFCs.

Thermally regenerative ammonia-based batteries (TRABs) have been developed to harvest low-grade waste heat as electricity. To improve the power production and anodic coulombic efficiency, the use of ethylenediamine as an alternative ligand to ammonia was explored here. The power density of the ethylenediamine-based battery (TRENB) was 85 ± 3 W m−2-electrode area with 2 M ethylenediamine, and 119 ± 4 W m−2 with 3 M ethylenediamine. This power density was 68% higher than that of TRAB. The energy density was 478 Wh m−3-anolyte, which was ∼50% higher than that produced by TRAB. The anodic coulombic efficiency of the TRENB was 77 ± 2%, which was more than twice that obtained using ammonia in a TRAB (35%). The higher anodic efficiency reduced the difference between the anode dissolution and cathode deposition rates, resulting in a process more suitable for closed loop operation. The thermal-electric efficiency based on ethylenediamine separation using waste heat was estimated to be 0.52%, which was lower than that of TRAB (0.86%), mainly due to the more complex separation process. However, this energy recovery could likely be improved through optimization of the ethylenediamine separation process.

Transition metal phosphide catalysts such as nickel phosphide (Ni2P) have shown excellent activities for the hydrogen evolution reaction, but they have primarily been studied in strongly acidic or alkaline electrolytes. In microbial electrolysis cells (MECs), however, the electrolyte is usually a neutral pH to support the bacteria. Carbon-supported phase-pure Ni2P nanoparticle catalysts (Ni2P/C) were synthesized using solution-phase methods and their performance was compared to Pt/C and Ni/C catalysts in MECs. The Ni2P/C produced a similar quantity of hydrogen over a 24 h cycle (0.29 ± 0.04 L-H2/L-reactor) as that obtained using Pt/C (0.32 ± 0.03 L-H2/L) or Ni/C (0.29 ± 0.02 L-H2/L). The mass normalized current density of the Ni2P/C was 14 times higher than that of the Ni/C, and the Ni2P/C exhibited stable performance over 11 days. Ni2P/C may therefore be a useful alternative to Pt/C or other Ni-based catalysts in MECs due to its chemical stability over time.

The silver-based thermally regenerative ammonia battery is a new technology for converting low-grade waste heat (

Accelerated pollutant degradation was examined using a new combined chemical and bioelectrochemical system, called a Bio-E-Peroxone process, based on generating ⋅OH from H2O2 produced on the cathode of a microbial fuel cell (MFC) and using ozone-enriched air. To optimize H2O2 formation, different carbon materials were examined and the highest H2O2 rates were obtained using XC-72 carbon black cathode. In E-Peroxone tests using the XC-72 cathode, methylene blue (a model pollutant) degradation rates followed first-order kinetics, with a rate constant of 0.237 min−1, 6 times higher than that obtained using only ozonation (0.032 min−1), 15 times of electrolysis+O2 system (0.015 min−1) and 46 times greater than electrolysis (0.005 min−1). In MFC tests when using the complete Bio-E-Peroxone system, the removal rate constant for methylene blue was 2.05 h−1, compared to 1.86 h−1 using only ozone and 0.41 h−1 using only MFC. Adding ozone to the air in cathode also increased power production by 47% to 170 mW m−3. The results demonstrated that this Bio-E-Peroxone system could be a feasible method for both refractory compounds degradation and wastewater electricity generation.

Microbial electrolysis cells (MECs) can generate methane by fixing carbon dioxide without using expensive catalysts, but the impact of acclimation procedures on subsequent performance has not been investigated. Granular activated carbon (GAC) was used to pre-enrich electrotrophic methanogenic communities, as GAC has been shown to stimulate direct transfer of electrons between different microbial species. MEC startup times using pre-acclimated GAC were improved compared to controls (without pre-acclimation or without GAC), and after three fed batch cycles methane generation rates were similar (P>0.4) for GAC acclimated to hydrogen (22±9.3nmolcm-3d-1), methanol (25±9.7nmolcm-3d-1), and a volatile fatty acid (VFA) mix (22±11nmolcm-3d-1). However, MECs started with GAC but no pre-acclimation had lower methane generation rates (13±4.1nmolcm-3d-1), and MECs without GAC had the lowest rates (0.7±0.8nmolcm-3d-1 after cycle 2). Microbes previously found in methanogenic MECs, or previously shown to be capable of exocellular electron transfer, were enriched on the GAC. Pre-acclimation using GAC is therefore a simple approach to enrich electroactive communities, improve methane generation rates, and decrease startup times in MECs.

Mixing entropy batteries (MEBs) are a new approach to generate electricity from salinity differences between two aqueous solutions. To date, MEBs have only been prepared from solutions containing chloride salts, owing to their relevance in natural salinity gradients created from seawater and freshwater. We hypothesized that MEBs could capture energy using ammonium bicarbonate (AmB), a thermolytic salt that can be used to convert waste heat into salinity gradients. We examined six battery electrode materials. Several of the electrodes were unstable in AmB solutions or failed to produce expected voltages. Of the electrode materials tested, a cell containing a manganese oxide electrode and a metallic lead electrode produced the highest power density (6.3 mW m(-2) ). However, this power density is still low relative to previously reported NaCl-based MEBs and heat recovery systems. This proof-of-concept study demonstrated that MEBs could indeed be used to generate electricity from AmB salinity gradients.

While most microbial electrolysis cell (MEC) tests and other bioelectrochemical tests use applied voltages (Eap) of 1 V or less, higher voltages are used in some tests that could lead to the generation of free chlorine species (FCS), from chloride ions, and hydroxyl radicals. To examine the impact of high Eap on bioanodes, MECs were acclimated at Eap = 1 V, tested for one cycle at an Eap of 3 or 4 V until the same total coulombs were achieved as Eap = 1 V, and then returned to cycles of Eap = 1 V. All biotic MECs with chloride ions showed severe biofilm damage based on the absence of current production, lack of acetate oxidation, and the absence of hydrogen gas production in subsequent cycles at 1 V. Abiotic tests conducted at Eap = 4 V, with same amount of total coulombs transferred as that which occurred in biotic tests at Eap = 4 V, showed 1.8-fold higher acetate removal than biotic cells at 4 V, suggesting 43% of generated coulombs could have contributed to microbial inactivation. FCS generation, rather than hydroxyl radical production, was concluded to be the major contribution to oxidation of organics due to small changes in acetate oxidation in the presence of a hydroxyl radical scavenger, and the measurement of FCS. These results demonstrated that high applied voltages should be avoided if bioanodes are needed in bioelectrochemical systems when chloride species are present in the solution.

Ammonium is typically removed from wastewater by converting it to nitrogen gas using microorganisms, precluding its recovery. Copper hexacyanoferrate (CuHCF) is known to reversibly intercalate alkali cations in aqueous electrolytes due to the Prussian Blue crystal structure. We used this property to create a carbon-based intercalation electrode within an electrochemical cell. Depending on the electrode potential, it can recover NH4+ from wastewater via insertion/regeneration while leaving organics. In the first phase, different binders were evaluated towards creating a stable electrode matrix, with sodium carboxymethyl cellulose giving the best performance. Subsequently, based on voltammetry, we determined an intercalation potential for NH4+ removal of + 0.3 V vs. Ag/AgCl, while the regeneration potential of the electrode was + 1.1 V (vs. Ag/AgCl). Using the CuHCF electrodes 95% of the NH4+ in a synthetic wastewater containing 56 mM NH4+ and 68 mM methanol was removed with an energy input of 0.34 ± 0.01 Wh g−1 NH4+. A similar removal of 93% was obtained using an actual industrial wastewater (56 mM NH4+, 68 mM methanol, 0.02 mM NO2−, 0.05 mM NO3−, 0.04 mM SO42− and 0.34 mM ethanol), with an energy input of 0.40 ± 0.01 Wh g−1 NH4+. In both cases, there was negligible removal of organics. The stability of CuHCF electrodes was evaluated either by open circuit potential monitoring (61 h) or by cyclic voltammetry (50 h, 116 cycles). The stability during cycling of the electrode was determined in both synthetic and real streams for 25 h (125 cycles). The charge density (C cm−1) of the CuHCF electrodes declined by 17 % and 19% after 125 cycles in the synthetic stream and the actual wastewater, respectively. This study highlights the possibility of low-cost CuHCF coated electrodes for achieving separation of NH4+ from streams containing methanol. The stability of electrodes has been improved but needs to be further enhanced for large-scale applications and long-term operation.

Current cleaning strategies for biofouling control on spiral wound membrane systems used for seawater desalination are not effective and can hinder long-term membrane performance. To enable effective cleaning of a membrane, we examined the in-situ application and the use of a sacrificial multilayer polyelectrolyte coating on the membrane surface. The membrane coating was based on a layer-by-layer assembly approach using two non-toxic linkers, poly (diallyl-dimethyl ammonium chloride) and poly(sodium-4-styrene sulfonate). This polyelectrolyte coating was effectively applied on the membrane surface under cross-flow conditions, and it was stable on the membrane surface under continuous operation. Coating removal requires only a concentrated sodium chloride solution (synthetic brine in our study) adjusted to pH 11. Using this procedure, both the biofilm and the sacrificial layer could be simultaneously removed, leaving a clean surface compared to the non-coated membrane. Biofouling tests showed that the coated membrane had two-fold higher permeate flux recovery than the control non-coated membrane. The used polyelectrolyte sacrificial coatings avoided the use of toxic linkers and harsh cleaning chemicals, and thus it is a suitable technique for biofouling control on reverse osmosis spiral wound membranes.

A microbial photoelectrochemical cell (MPEC) is a self-biased, solar-driven device combining bioenergy with solar energy to produce sustainable electricity and chemicals. Here, an MPEC with a p-type Co3O4 nanorod-arrayed photocathode and bio-anode was constructed that matched well the redox potentials of anode microorganisms with the band gap of the Co3O4. The yield of formic acid produced by this MPEC under visible light irradiation with an external resistance of 300 Ω was 239 ± 10 μmol in 10 h, which was 1.8 times that produced by a bare Co3O4 photocathode under visible light, and 4.9 times that produced by the MPEC operated in dark. The maximum power density was 331 ± 4 mW m–2 under visible light, compared to 175 ± 25 mW m–2 in the dark. This study of MPEC with the synergistic effect of light energy and bioenergy opens up new opportunities for applications in environmental treatment of wastewaters and chemicals production.

Ferricyanide is often used in microbial fuel cells (MFCs) to avoid oxygen intrusion that occurs with air cathodes. However, MFC internal resistances using ferricyanide can be larger than those with air cathodes even though ferricyanide results in higher power densities. Using a graphite fiber brush cathode and a ferricyanide catholyte (FC-B) the internal resistance was 62 ± 4 mΩ m2, with 84 ± 8 mΩ m2 obtained using ferricyanide and a flat carbon paper cathode (FC-F) and only 51 ± 1 mΩ m2 using a 70% porosity air cathode (A-70). The FC-B MFCs produced the highest maximum power density of all configurations examined: 2.46 ± 0.26 W/m2, compared to 1.33 ± 0.14 W/m2 for the A-70 MFCs. The electrode potential slope (EPS) analysis method showed that electrode resistances were similar for ferricyanide and air–cathode MFCs, and that higher power was due to the larger experimental working potential (500 ± 12 mV) of ferricyanide compared to the air cathode (233 ± 5 mV).

Deep cuts to global carbon emissions are only possible with a dramatic increase in energy storage capacity. However, the cost of the storage options available hinders the timely deployment of the necessary energy storage. Promising supplements to conventional storage are thermally regenerative ammonia batteries (TRABs), which have an additional advantage of being able to capture low-grade waste heat into dispatchable electricity for the grid. This new battery technology can potentially provide power on demand using low-cost chemistries and waste energy sources relative to their purely electric battery counterparts. Most of the research to date for TRABs has been proof-of-concept in nature. As such, little is known about how the electrolytes used in these devices impact their theoretical energy storage capacity. Here we present the general principles of TRABs from an electrochemical and aqueous chemistry perspective. Energy storage density values and Eh-pH diagrams are presented to highlight key factors that govern these complex but versatile energy storage devices. Ag-based and Cu-based TRABs are discussed, and some experimental data confirming the conclusions of the Eh-pH diagrams are presented.

As the need for a sharp reduction in carbon emissions continues to grow, many researchers are looking to electrochemical technologies to provide sustainable power to the grid. One promising technology is the silver thermally regenerative ammonia battery (Ag-TRAB), which takes advantage of low-grade waste heat (

The influence of salt concentrations on the performance of reverse electrodialysis (RED) stacks has rarely been investigated using thermolytic salts such as NH4HCO3, that can be regenerated using waste heat and can be set at any desired concentration below saturation limits. Here, power densities produced by a RED stack were first investigated using different NaCl concentrations, and then tested using NH4HCO3. The power produced by the RED stack increased with NaCl concentrations from 0.6 M to 3.6 M in the HC (high concentration) solution, but it did not increase at higher salt concentrations due to limited ion exchange membrane capacity. NaCl concentrations larger than 0.14 M in the LC (low concentration) solution decreased power primarily as a result of lower salinity ratios (

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 (

There is a lack of standardized acclimation procedures for evaluating treatability of different wastewaters, and such tests are often conducted using different types of microbial electrolysis cells (MECs). Two different types of MECs (mini or cube) were therefore acclimated using two different substrates (acetate or domestic wastewater) to see the impact of these procedures on the resulting treatment efficiency using the same cellulose fermentation effluent. COD removal was slightly larger using mini MECs (81–86%) than cube MECs (79–82%). Pre-acclimation of mini MECs to domestic wastewater increased COD removal slightly compared to non-acclimated tests with fermentation effluent, but acclimation differences for the cube MECs were not statistically significant. Gas production was not significantly different for cube pre-acclimated MECs compared to those acclimated only to the fermentation effluent. Current densities were higher for the cube reactors than the mini MECs, but they were unaffected by acclimation procedure (pre-acclimation or direct use of fermentation effluent). These results show that mini MECs acclimated to a readily available complex source of organic matter (domestic wastewater) can produce equivalent or slightly superior results for tests with a different complex wastewater (fermentation effluent), and that mini MEC performance is comparable to that of cube MECs. The similarity of reactor performance allows the use of simple and inexpensive mini MECs that can be acclimated to domestic wastewater and subsequently used to test different types of industrial effluents.

Microbial electrosynthesis (MES) systems with mixed cultures often generate a variety of gaseous and soluble chemicals. Methane is the primary end product in mixed-culture MES because it is the thermodynamically most favorable reduction product of CO 2 . Here, we show that the peptaibol alamethicin selectively suppressed the growth of methanogens in mixed-culture MES systems, resulting in a shift of the solution and cathode communities to an acetate-producing system dominated by Sporomusa , a known acetogenic genus in MES systems. Archaea in the methane-producing control were dominated by Methanobrevibacter species, but no Archaea were detected in the alamethicin-treated reactors. No methane was detected in the mixed-culture reactors treated with alamethicin over 10 cycles (∼3 days each). Instead, acetate was produced at an average rate of 115 nmol ml −1 day −1 , similar to the rate reported previously for pure cultures of Sporomusa ovata on biocathodes. Mixed-culture control reactors without alamethicin generated methane at nearly 100% coulombic recovery, and no acetate was detected. These results show that alamethicin is effective for the suppression of methanogen growth in MES systems and that its use enables the production of industrially relevant organic compounds by the inhibition of methanogenesis.

The abundance of cellulosic wastes make them attractive source of energy for producing electricity in microbial fuel cells (MFCs). However, electricity production from cellulose requires obligate anaerobes that can degrade cellulose and transfer electrons to the electrode (exoelectrogens), and thus most previous MFC studies have been conducted using two-chamber systems to avoid oxygen contamination of the anode. Single-chamber, air-cathode MFCs typically produce higher power densities than aqueous catholyte MFCs and avoid energy input for the cathodic reaction. To better understand the bacterial communities that evolve in single-chamber air-cathode MFCs fed cellulose, we examined the changes in the bacterial consortium in an MFC fed cellulose over time. The most predominant bacteria shown to be capable electron generation was Firmicutes, with the fermenters decomposing cellulose Bacteroidetes. The main genera developed after extended operation of the cellulose-fed MFC were cellulolytic strains, fermenters and electrogens that included: Parabacteroides, Proteiniphilum, Catonella and Clostridium. These results demonstrate that different communities evolve in air-cathode MFCs fed cellulose than the previous two-chamber reactors.

Microbial fuel cell (MFC) cathodes rapidly foul when treating domestic wastewater, substantially reducing power production over time. Here a wipe separator was chemically bonded to an activated carbon air cathode using polyvinylidene fluoride (PVDF) to mitigate cathode fouling and extend cathode performance over time. MFCs with separator-bonded cathodes produced a maximum power density of 190 ± 30 mW m-2 after 2 months of operation using domestic wastewater, which was ∼220% higher than controls (60 ± 50 mW m-2) with separators that were not chemically bonded to the cathode. Less biomass (protein) was measured on the bonded separator surface than the non-bonded separator, indicating chemical bonding reduced external bio-fouling. Salt precipitation that contributed to internal fouling was also reduced using separator-bonded cathodes. Overall, the separator-bonded cathodes showed better performance over time by mitigating both external bio-fouling and internal salt fouling.

Thermal energy was shown to be efficiently converted into electrical power in a thermally regenerative ammonia-based battery (TRAB) using copper-based redox couples [Cu(NH3)42+/Cu and Cu(II)/Cu]. Ammonia addition to the anolyte (2 M ammonia in a copper-nitrate electrolyte) of a single TRAB cell produced a maximum power density of 115 ± 1 W m−2 (based on projected area of a single copper mesh electrode), with an energy density of 453 W h m−3 (normalized to the total electrolyte volume, under maximum power production conditions). Adding a second cell doubled both the voltage and maximum power. Increasing the anolyte ammonia concentration to 3 M further improved the maximum power density to 136 ± 3 W m−2. Volatilization of ammonia from the spent anolyte by heating (simulating distillation), and re-addition of this ammonia to the spent catholyte chamber with subsequent operation of this chamber as the anode (to regenerate copper on the other electrode), produced a maximum power density of 60 ± 3 W m−2, with an average discharge energy efficiency of ∼29% (electrical energy captured versus chemical energy in the starting solutions). Power was restored to 126 ± 5 W m−2 through acid addition to the regenerated catholyte to decrease pH and dissolve Cu(OH)2 precipitates, suggesting that an inexpensive acid or a waste acid could be used to improve performance. These results demonstrated that TRABs using ammonia-based electrolytes and inexpensive copper electrodes can provide a practical method for efficient conversion of low-grade thermal energy into electricity.

Microbial reverse electrodialysis cells (MRECs) combine power generation from salinity gradient energy using reverse electrodialysis (RED), with power generation from organic matter using a microbial fuel cell. Waste heat can be used to distill ammonium bicarbonate into high (HC) and low salt concentration (LC) solutions for use in the RED stack, but nitrogen crossover into the anode chamber must be minimized to avoid ammonia loses, and foster a healthy microbial community. To reduce nitrogen crossover, an additional low concentration (LC) chamber was inserted before the anode using an additional anion exchange membrane (AEM) next to another AEM, and filled with different amounts of anion or cation ion exchange resins. Addition of the extra AEM increased the ohmic resistance of the test RED stack from 103 Ω cm2 (1 AEM) to 295 Ω cm2 (2 AEMs). However, the use of the anion exchange resin decreased the solution resistance of the LC chamber by 74% (637 Ω cm2, no resin; 166 Ω cm2 with resin). Nitrogen crossover into the anode chamber was reduced by up to 97% using 50% of the chamber filled with an anion exchange resin compared to the control (no additional chamber). The added resistance contributed by the use of the additional LC chamber could be compensated for by using additional LC and HC membrane pairs in the RED stack.

Given our vast methane reserves and the difficulty in transporting methane without substantial leaks, the conversion of methane directly into electricity would be beneficial. Microbial fuel cells harness electrical power from a wide variety of substrates through biological means; however, the greenhouse gas methane has not been used with much success previously as a substrate in microbial fuel cells to generate electrical current. Here we construct a synthetic consortium consisting of: (i) an engineered archaeal strain to produce methyl-coenzyme M reductase from unculturable anaerobic methanotrophs for capturing methane and secreting acetate; (ii) micro-organisms from methane-acclimated sludge (including Paracoccus denitrificans) to facilitate electron transfer by providing electron shuttles (confirmed by replacing the sludge with humic acids), and (iii) Geobacter sulfurreducens to produce electrons from acetate, to create a microbial fuel cell that converts methane directly into significant electrical current. Notably, this methane microbial fuel cell operates at high Coulombic efficiency., Microbial fuel cells generate electricity from a variety of sources, however from methane only negligible electrical power has been reported so far. Here the authors convert methane into electricity using a synthetic consortium consisting of an engineered archaeal strain, microorganisms from methane-acclimated sludge, and Geobacter sulfurreducens.

The application of immobilized redox mediators (RMs) in microbial fuel cells (MFCs) is an emerging technology for electricity generation with simultaneous azo dye decolorization due to facilitated electrons transfer from bacteria to anodes and azo dyes. The use of immobilized RMs avoids the requirement of their continuous dosing in MFCs, which has been the main limitation for practical applications. Two strategies of anthraquinones-2,6-disulphonic salt (AQDS) immobilization, AQDS immobilized with polyvinyl alcohol particles and AQDS immobilized on anodes by electropolymerization, were evaluated and compared to achieve simultaneous reactive red 2 (RR2) dye reduction and bioelectricity generation. The AQDS immobilized by electropolymerization showed the highest power density (816 ± 2 mW/m2) and extent of RR2 decolorization (89 ± 0.6%). This power density is one of the highest values yet achieved in the presence of a recalcitrant pollutant, suggesting that immobilization was important for enabling current generation in the presence of RR2.

A low cost Fe–N–C catalyst on an activated carbon (AC) support was synthesized from inexpensive ferric chloride and chitosan precursors to enhance power production by microbial fuel cells (MFCs). The direct pyrolysis of preformed Fe(III)-chitosan hydrogel as a supporting scaffold created a porous structure on AC with a uniform distribution of Fe active sites. A maximum power density of 2.4 ± 0.1 W m−2 was obtained in MFCs using Fe–N–C/AC catalyst, which was 33% higher than the control MFCs using a plain AC catalyst (1.8 ± 0.03 W m−2). The Fe–N–C/AC catalyst was closer to the more efficient four electron transfer pathway for the oxygen reduction reaction (ORR) than the plain AC or chitosan-modified AC. The adoption of chitosan as the N-containing precursor and ferric chloride for the Fe–N–C synthesis added only 6% more in material costs in cathode fabrication, but produced a 33% increase in the maximum power density. This increased power makes the use of this cathode material both economically viable and a sustainable approach to enhance power production in MFCs given the low cost and wide availability of chitosan.

A low cost poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) phase inversion coating was developed as a cathode diffusion layer to enhance the performance of microbial fuel cells (MFCs). A maximum power density of 1430 ± 90 mW m −2 was achieved at a PVDF-HFP loading of 4.4 mg cm −2 (4:1 polymer:carbon black), with activated carbon as the oxygen reduction cathode catalyst. This power density was 31% higher than that obtained with a more conventional platinum (Pt) catalyst on carbon cloth (Pt/C) cathode with a poly(tetrafluoroethylene) (PTFE) diffusion layer (1090 ± 30 mW m −2 ). The improved performance was due in part to a larger oxygen mass transfer coefficient of 3 × 10 −3 cm s −1 for the PVDF-HFP coated cathode, compared to 1.7 × 10 −3 cm s −1 for the carbon cloth/PTFE-based cathode. The diffusion layer was resistant to electrolyte leakage up to water column heights of 41 ± 0.5 cm (4.4 mg cm −2 loading of 4:1 polymer:carbon black) to 70 ± 5 cm (8.8 mg cm −2 loading of 4:1 polymer:carbon black). This new type of PVDF-HFP/carbon black diffusion layer could reduce the cost of manufacturing cathodes for MFCs.

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.

Separators are used to protect cathodes from biofouling and to avoid electrode short-circuiting, but they can adversely affect microbial fuel cell (MFC) performance. A spray method was used to apply a polyvinyl alcohol (PVA) separator to the cathode. Power densities were unaffected by the PVA separator (339±29mW/m(2)), compared to a control lacking a separator in a low conductivity solution (1mS/cm) similar to wastewater. Power was reduced with separators in solutions typical of laboratory tests (7-13mS/cm), compared to separatorless controls. The PVA separator produced more power in a separator assembly (SEA) configuration (444±8mW/m(2)) in the 1mS/cm solution, but power was reduced if a PVA or wipe separator was used in higher conductivity solutions with either Pt or activated carbon catalysts. Spray and cast PVA separators performed similarly, but the spray method is preferred as it was easier to apply and use.

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 ...

Membrane potential and permselectivity are critical parameters for a variety of electrochemically-driven separation and energy technologies. An electric potential is developed when a membrane separates electrolyte solutions of different concentrations, and a permselective membrane allows specific species to be transported while restricting the passage of other species. Ion exchange membranes are commonly used in applications that require advanced ionic electrolytes and span technologies such as alkaline batteries to ammonium bicarbonate reverse electrodialysis, but membranes are often only characterized in sodium chloride solutions. Our goal in this work was to better understand membrane behaviour in aqueous ammonium bicarbonate, which is of interest for closed-loop energy generation processes. Here we characterized the permselectivity of four commercial ion exchange membranes in aqueous solutions of sodium chloride, ammonium chloride, sodium bicarbonate, and ammonium bicarbonate. This stepwise approach, using four different ions in aqueous solution, was used to better understand how these specific ions affect ion transport in ion exchange membranes. Characterization of cation and anion exchange membrane permselectivity, using these ions, is discussed from the perspective of the difference in the physical chemistry of the hydrated ions, along with an accompanying re-derivation and examination of the basic equations that describe membrane potential. In general, permselectivity was highest in sodium chloride and lowest in ammonium bicarbonate solutions, and the nature of both the counter- and co-ions appeared to influence measured permselectivity. The counter-ion type influences the binding affinity between counter-ions and polymer fixed charge groups, and higher binding affinity between fixed charge sites and counter-ions within the membrane decreases the effective membrane charge density. As a result permselectivity decreases. The charge density and polarizability of the co-ions also appeared to influence permselectivity leading to ion-specific effects; co-ions that are charge dense and have low polarizability tended to result in high membrane permselectivity.

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...

Direct, shuttle-free uptake of extracellular, cathode-derived electrons has been postulated as a novel mechanism of electron metabolism in some prokaryotes that may also be involved in syntrophic electron transport between two microorganisms. Experimental proof for direct uptake of cathodic electrons has been mostly indirect and has been based on the absence of detectable concentrations of molecular hydrogen. However, hydrogen can be formed as a transient intermediate abiotically at low cathodic potentials (