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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.

Prussian blue analogues are used in electrochemical deionization due to their cation sorption capabilities and ion selectivity properties. Elucidating the fundamental mechanisms underlying intercalation/deintercalation is important for the development of ion-selective electrodes. We examined the thermodynamic and kinetic properties of nickel hexacyanoferrate electrodes by studying different temperatures effects on intercalation/deintercalation with monovalent ions (Li

Graphite fiber brush electrodes are commonly used in microbial electrolysis cells (MECs) for simultaneous wastewater treatment and electrochemical hydrogen production. Previous brush anode designs for continuous flow systems were configured to have flow over an array of brush electrodes. Here we compared the performance of two systems, one with flow through a single smaller or larger brush anode to an MEC with multiple brushes. The single brush MECs had only a single large brush that had a diameter larger than the chamber height, so that the brush fibers were compressed to nearly (4.5 cm diameter) or completely (5.5 cm diameter) fill the 1.3 cm high anode chamber. To evaluate the time needed for acclimation of the anode potentials were continuously monitored for 138 d (4.5 cm brush) or 143 d (5.5 cm brush). The best performance was obtained using the 5.5 cm brush fibers with a volumetric current density of 554 ± 26 A/m3, compared to

Saline water represents an inexhaustible source of water for hydrogen production from electrolysis. However, direct saltwater splitting faces challenges due to chlorine evolution at the anode and the development of Nernst overpotential due to sodium ion transport competition with protons across the membrane. A new approach to minimize chlorine evolution and improve performance is proposed here by using a humidified gas stream (no liquid electrolyte) for the anode and a liquid saltwater catholyte. Charge repulsion of chloride ions by the proton exchange membrane (PEM) resulted in low chlorine generation, with anodic faradaic efficiencies for oxygen evolution of 100 ± 1% with a synthetic brackish water (50 mM NaCl, 3 g L−1) and 96 ± 2% with synthetic seawater (0.5 M NaCl, 30 g L−1). The enhanced proton transport by the electric field enabled more efficient pH control across the cell, minimizing sodium ion transport in the absence of a liquid anolyte. The vapor-fed anode configuration showed similar performance to a conventional PEM electrolyzer up to 1 A cm−2 when both anode and cathode were fed with deionized water. Much lower overpotentials could be achieved using the vapor-fed anode compared to a liquid-anolyte due to the reduced sodium ion transport through the membranes, as shown by adding NaClO4 to the electrolytes. This vapor-fed anode configuration allows for direct use of saltwater in conventional electrolyzers without additional water purification at high faradaic efficiencies.

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...

Different microbial fuel cell (MFC) configurations have been successfully operated at pilot-scale levels (100 L) to demonstrate electricity generation while accomplishing domestic or industrial wastewater treatment. Two cathode configurations have been primarily used based on either oxygen transfer by aeration of a liquid catholyte or direct oxygen transfer using air-cathodes. Analysis of several pilot-scale MFCs showed that air-cathode MFCs outperformed liquid catholyte reactors based on power density, producing 233% larger area-normalized power densities and 181% higher volumetric power densities. Reactors with higher electrode packing densities improved performance by enabling larger power production while minimizing the reactor footprint. Despite producing more power than the liquid catholyte MFCs, and reducing energy consumption for catholyte aeration, pilot MFCs based on air-cathode configuration failed to produce effluents with chemical oxygen demand (COD) levels low enough to meet typical threshold for discharge. Therefore, additional treatment would be required to further reduce the organic matter in the effluent to levels suitable for discharge. Scaling up MFCs must incorporate designs that can minimize electrode and solution resistances to maximize power and enable efficient wastewater treatment.

Microbial electrosynthesis (MES) cells use renewable energy to convert carbon dioxide into valuable chemical products such as methane and acetate, but chemical production rates are low and pH changes can adversely impact biocathodes. To overcome these limitations, an MES reactor was designed with a zero-gap electrode configuration with a cation exchange membrane (CEM) to achieve a low internal resistance, and a vapor-fed electrode to minimize pH changes. Liquid catholyte was pumped through a carbon felt cathode inoculated with anaerobic digester sludge, with humidified N

Many design and operational parameters that can impact power generation in microbial fuel cells (MFCs), such as flow over the electrodes, can only be effectively examined in larger-scale systems. A maximum power density of 0.101 ± 0.006 W m−2 (0.74 ± 0.05 W m−3) was obtained in an 85-L MFC with graphite fiber brushes (5.1 cm diameter, 61 cm long) and flat air cathode (0.62 m2 exposed area; anode-cathode spacing of 1.3 cm) in batch mode. Recirculating the anolyte diagonally through the chamber (entering the top right side of the reactor and exiting the bottom left side) further improved performance by 17% to 0.118 ± 0.006 W m−2, at a hydraulic retention time (HRT) of 22 min (3.9 L min−1), compared to static flow conditions. This power density was also higher than that obtained with parallel flow through the chamber (more evenly distributed using a manifold; 0.109 ± 0.009 W m−2). Reducing the diameter of the anode brushes from 5.1 cm to 2.5 cm did not improve the anode performance. These results demonstrate the importance of electrode spacing and hydraulic flow on large-scale MFC performance.

To scale up microbial fuel cells (MFCs), larger cathodes need to be developed that can use air directly, rather than dissolved oxygen, and have good electrochemical performance. A new type of cathode design was examined here that uses a “window-pane” approach with fifteen smaller cathodes welded to a single conductive metal sheet to maintain good electrical conductivity across the cathode with an increase in total area. Abiotic electrochemical tests were conducted to evaluate the impact of the cathode size (exposed areas of 7 cm2, 33 cm2, and 6200 cm2) on performance for all cathodes having the same active catalyst material. Increasing the size of the exposed area of the electrodes to the electrolyte from 7 cm2 to 33 cm2 (a single cathode panel) decreased the cathode potential by 5%, and a further increase in size to 6200 cm2 using the multi-panel cathode reduced the electrode potential by 55% (at 0.6 A m−2), in a 50 mM phosphate buffer solution (PBS). In 85 L MFC tests with the largest cathode using wastewater as a fuel, the maximum power density based on polarization data was 0.083 ± 0.006 W m−2 using 22 brush anodes to fully cover the cathode, and 0.061 ± 0.003 W m−2 with 8 brush anodes (40% of cathode projected area) compared to 0.304 ± 0.009 W m−2 obtained in the 28 mL MFC. Recovering power from large MFCs will therefore be challenging, but several approaches identified in this study can be pursued to maintain performance when increasing the size of the electrodes.

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.

Microbial fuel cells (MFCs) can generate electrical energy from the oxidation of the organic matter, but they must be demonstrated at large scales, treat real wastewaters, and show the required performance needed at a site to provide a path forward for this technology. Previous pilot-scale studies of MFC technology have relied on systems with aerated catholytes, which limited energy recovery due to the energy consumed by pumping air into the catholyte. In the present study, we developed, deployed, and tested an 850 L (1400 L total liquid volume) air-cathode MFC treating domestic-type wastewater at a centralized wastewater treatment facility. The wastewater was processed over a hydraulic retention time (HRT) of 12 h through a sequence of 17 brush anode modules (11 m

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.

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.

Practical application of metallurgical microbial electrolysis cells (MECs) requires efficient removal of metals and organics in larger reactors. A 40 L cylindrical single-chamber MEC fed acetate was used to achieve high removals of W(VI) and Mo(VI). In the presence of both metals, there were nearly complete removals of W (97 ‒ 98%), Mo (98 ‒ 99%), and acetate (95 ‒ 96%), along with a low level of hydrogen production (0.0037–0.0039 L/L/d) at a hydraulic residence time (HRT) of 2 d (influent ratios of W:Mo:acetate of 0.5:1.0:24 mM). The final concentrations of these conditions were sufficient to meet national wastewater discharge standards. In the controls with individual metals or acetate, lower contaminant removals were obtained (W, 2 ‒ 4%; Mo, 3 ‒ 5%, acetate, 36 ‒ 39%). Metals removal in all cases was primarily due to the biocathodes rather than the bioanodes. The presence of metals decreased microbial diversity on the anodes and increased diversity on the cathodes, based on analysis at the phylum, class and genus levels, as a function of HRT and influent concentration. This study demonstrated the feasibility of larger-scale single-chamber MECs for efficient treatment of W and Mo, moving metallurgical MECs closer to commercialization for wastewater treatment of these two metals.

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.

Although pure Ni catalysts can achieve a hydrogen production rate similar to Pt in microbial electrolysis cells (MECs), a reduction in the amount of Ni used is needed to reduce the cost. In this study, nickel powder (pNi) was blended with activated carbon (AC) to reduce the mass of Ni used, while improving catalytic activity for the hydrogen evolution reaction (HER) by increasing the active surface area. Ni powder blended AC cathodes (AC-pNi) were fabricated at different nickel powder loadings (4.8, 19, 46 mg/cm2 with AC and 77 mg/cm2 without AC as control). AC-pNi4.8 (Ni loading: 4.8 mg/cm2) produced higher hydrogen production rates (0.38 ± 0.04 L-H2/L-d) than pNi77 (0.28 ± 0.02 L-H2/L-d) with a 16 times less Ni loading. Cathodic hydrogen recovery of using the AC-pNi4.8 (98 ± 5%) was also higher than pNi77 (82 ± 4%), indicating catalytic activities were improved by AC blending. Nickel dissolution into the catholyte after completion of each cycle was negligible for AC-pNi4.8 (

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...

A vast array of microorganisms from all three domains of life can produce electrical current and transfer electrons to the anodes of different types of bioelectrochemical systems. These exoelectrogens are typically iron-reducing bacteria, such as Geobacter sulfurreducens, that produce high power densities at moderate temperatures. With the right media and growth conditions, many other microorganisms ranging from common yeasts to extremophiles such as hyperthermophilic archaea can also generate high current densities. Electrotrophic microorganisms that grow by using electrons derived from the cathode are less diverse and have no common or prototypical traits, and current densities are usually well below those reported for model exoelectrogens. However, electrotrophic microorganisms can use diverse terminal electron acceptors for cell respiration, including carbon dioxide, enabling a variety of novel cathode-driven reactions. The impressive diversity of electroactive microorganisms and the conditions in which they function provide new opportunities for electrochemical devices, such as microbial fuel cells that generate electricity or microbial electrolysis cells that produce hydrogen or methane.

Redox flow batteries are emerging as a promising method to provide grid-scale power and long-duration energy storage safely and economically. The thermally regenerative ammonia battery (TRAB) is a new redox flow battery category that can be recharged using low-grade waste heat rather than electric energy, adding further flexibility to the applicability of flow battery systems. Recently, a new TRAB with all-aqueous electroactive species (referred to as the Cuaq-TRAB), as opposed to deposition-dissolution reactions, was found to have superior energy and power densities relative to competing TRABs. The use of bromide and ammonia stabilizes for both Cu(I) and Cu(II) oxidation states, while also creating a cell potential of up to 1.0 V. The potential can be recovered by thermally separating ammonia from the electrolyte and adding it back to alternate electrolyte chambers in successive cycles. Potential improvements in overall cell performance are possible by reducing ohmic losses associated with membrane selection. However, the ideal membrane for the Cuaq-TRAB is not obvious and raises interesting transport questions relative to the dominant redox-active copper species being negatively charged in the catholyte but positively charged in the anolyte. Furthermore, membrane crossover of ammonia, a small, uncharged molecule, was previously shown to be a primary source of parasitic losses for TRABs. Therefore, we investigated how different membrane types (cation, anion, and non-selective) affected ion transport and TRAB performance. A batch symmetry cell was used to determine membrane conductivity in the Cuaq-TRAB environment and membrane diffusion coefficients of each species in the electrolytes. Flow cell experiments were also conducted to find peak power, energy density, average power during discharge, and capacity fade over successive electric charge/discharge cycles. Tradeoffs between membrane conductivity and permeability observed in the symmetry cell are manifested in flow cell results as either high peak and average power or high energy density and low capacity fade. These results expand on potential methods for controlling transport in redox flow batteries and demonstrate the potential for non-selective membranes for electrochemical energy technologies.

Anodes in microbial fuel cells (MFCs) can be chemically treated to improve performance but the impact of treatment on power generation has not been examined for different electrode base materials. Brush or flat anodes were chemically treated and then compared in identical two-chambered MFCs using the electrode potential slope (EPS) analysis to quantify the anode resistances. Flat carbon cloth anodes modified with carbon nanotubes (CNTs) produced 1.42 ± 0.06 W m−2, which was 3.2 times more power than the base material (0.44 ± 0.00 W m−2), but less than the 2.35 ± 0.1 W m−2 produced using plain graphite fiber brush anodes. An EPS analysis showed that there was a 90% decrease in the anode resistances of the CNT-treated carbon cloth and a 5% decrease of WO3 nanoparticle-treated brushes compared to unmodified controls. Certain chemical treatments can therefore improve performance of flat anodes, but plain brush anodes achieved the highest power densities.

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 (

A significant amount of the potential energy that is generated during energy harvesting worldwide is discarded as waste heat because of inefficient power generation cycles. Much of this wasted power source goes unused because it is trapped as low-grade thermal energy (< 100 °C), which traditional power cycles cannot viably harness. With the advent of electrochemical power systems such as redox flow batteries and fuel cells, researchers are investigating new methods of providing usable electric power from these unused low-grade thermal energy sources. The thermally regenerative ammonia battery (TRAB) is one promising technology in this space, as it operates with the same principles as a flow battery except that TRABs can be recharged using low-grade waste heat instead of electric power. Of the TRAB chemistries proposed, the all-aqueous copper TRAB chemistry is capable of both large power densities and energy densities, and high coulombic efficiencies. In this presentation, we discuss how key performance metrics relevant to power generation from low grade thermal energy sources are strongly influenced by electrolyte composition and battery operating parameters. Manipulating the ammonia to copper ratio demonstrated clear tradeoffs between achievable energy and power capacities. Increasing ligand concentration had a large impact on electroactive species solubility and cell potential differences, which increased the theoretical energy density limit of the battery. Increasing the amount of ammonia relative to dissolved copper raised peak power density, but adversely affected energy density. Moderate increases in discharge current density did not decrease the energy density due to reduced impact of ammonia crossover which appears to be a dominant source of energy loss in TRABs.

Magnetic seeding coagulation (MSC) process has been used to accelerate flocs sedimentation with an applied magnetic field, offering large handling capacity and low energy consumption. The interactions of three typical Al species, aluminum chloride (AlCl3), Al13O4(OH)247+ polymer (Al13), and (AlO4)2Al28(OH)5618+ polymer (Al30), with magnetic particles (MPs) were examined to clarify the MSC process. In traditional coagulation (TC) process, the aggregation of primary Ala-dissolved organic matter (DOM) complexes with in-situ-formed polynuclear species generated a large average floc size (226 μm), which was proved to be efficient for DOC removal (52.6%). The weak connections between dissolved Ala-DOM complexes and MPs led to the negligible changes of dissolved Al after seeding with MPs in AlCl3. A significant interaction between MPs and Al13 was observed, in which the MPs-Al13-DOM complexes were proposed to be responsible for the significant improvement of DOC removal (from 47% to 52%) and residual total Al reduction (from 1.05 to 0.27 mg Al L−1) with MPs addition. Al30 produced a lower floc fractal dimension (Df = 1.88) than AlCl3 (2.08) and Al13 (1.99) in the TC process, whereas its floc strength (70.9%) and floc recovery (38.5%) were higher than the others. Although more detached fragments were produced with MPs addition, the effective sedimentation of these fragments with the applied magnetic field led to the decrease of residual turbidity and colloidal Al in Al30. The dependence of coagulation behavior to MPs and different Al species can be applied to guide the application of an effective MSC process.

Two methods were examined to improve methane production efficiency in anaerobic digestion (AD) based on adding a large amount of surface area using a single electrically conductive carbon brush, or by adding electrodes as done in microbial electrolysis cells (MECs) to form a hybrid AD-MEC. To examine the impact of surface area relative to electrodes, AD reactors were fitted with a single large brush without electrodes (FB), half a large brush with two electrodes with an applied voltage (0.8 V) and operated in closed circuit (HB-CC) or open circuit (HB-OC) mode, or only two electrodes with a closed circuit and no large brush (NB-CC) (equivalent to an MEC). The three configurations with a half or full brush all had improved performance as shown by 57-82% higher methane generation rate parameters in the Gompertz model compared to NB-CC. The retained biomass was much higher in the reactors with large brush, which likely contributed to the rapid consumption of volatile fatty acids (VFAs) and therefore improved AD performance. A different microbial community structure was formed in the large-size brushes compared to the electrodes. Methanothrix was predominant in the biofilm of large-size carbon brush, while Geobacter (anode) and Methanobacterium (cathode) were highly abundant in the electrode biofilms. These results demonstrate that adding a high surface area carbon fiber brush will be a more effective method of improving AD performance than using MEC electrodes with an applied voltage.

Reducing the external resistance (Rext) for microbial fuel cell (MFC) acclimation can substantially alter the anode performance in terms of charge transfer (RCT), diffusion (Rd) and total anode resistance (RAn). Electrochemical impedance spectroscopy (EIS) was used to quantify anode impedance at different set potentials. Reducing Rext from 50 Ω to 20 Ω during acclimation reduced RCT by 31% (from 6.12 ± 0.09 mΩ m2 to 4.21 ± 0.03 mΩ m2) and Rd by 18% (from 3.4 ± 0.2 mΩ m2 to 2.8 ± 0.1 mΩ m2) at a set anode potential of −115 mV during EIS. Overall RAn decreased by 27%, to 5.13 ± 0.02 mΩ m2 for acclimation at 20 Ω, enabling the anode to achieve 38% higher current densities of 29 ± 1 A m−2. The results show a clear dependence of acclimation procedures and external resistance on kinetic and diffusion components of anode impedance that can impact overall bioelectrochemical performance.

Magnetic particles (MPs) assisted powdered activated carbon (PAC) is a promising composite material for adsorption removal of micropollutants. The fractional amount of Fe

The potential energy contained in the controlled mixing of waters with different salt concentrations (i.e., salinity gradient energy) can theoretically provide a substantial fraction of the global electrical demand. One method for generating electricity from salinity gradients is to use electrode-based reactions in electrochemical cells. Here, we examined the relationship between the electrical power densities generated from synthetic NaCl solutions and the crystal structures and morphologies of manganese oxides, which undergo redox reactions coupled to sodium ion uptake and release. Our aim was to make progress toward developing rational frameworks for selecting electrode materials used to harvest salinity gradient energy. We synthesized 12 manganese oxides having different crystal structures and particle sizes and measured the power densities they produced in a concentration flow cell fed with 0.02 and 0.5 M NaCl solutions. Power production varied considerably among the oxides, ranging from no power produced (β-MnO2) to 1.18 ± 0.01 W/m2 (sodium manganese oxide). Power production correlated with the materials' specific capacities, suggesting that cyclic voltammetry may be a simple method to screen possible materials. The highest power densities were achieved with manganese oxides capable of intercalating sodium ions when their potentials were prepoised prior to power production.

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...

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...