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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...

Air-cathodes in microbial fuel cells that can also filter wastewater provide the dual benefits of electricity production and reduction of the effluent chemical oxygen demand. Air-cathodes prepared using a novel activated carbon/graphene membrane (2, 5 or 10% graphene by weight) prepared by phase inversion have good conductivities (5.6 ± 0.5 to 7.3 ± 0.6 mS cm−1) compared to control (3.0 ± 0.4 mS cm−1, activated carbon, no graphene). The cathode with 5 wt% graphene produces the highest maximum power density of 1460 ± 10 mW m−2, which is 58% higher than that the control (928 ± 8 mW m−2). The increased power is due to an 88% reduction in charge transfer resistance of 6.0 ± 0.3 Ω (cathode with 5 wt% graphene) compared to the control. Following a cycle of treatment and current generation, 60 ± 1% of the chemical oxygen demand is removed from the remaining chemical oxygen demand, producing an effluent chemical oxygen demand concentration of 20 ± 1 mg L−1. Biomass (4.99 ± 0.02 mg-protein cm−2) is decreased by 33% compared to the control. These results demonstrate that cathodes made with graphene can produce electricity and a high quality effluent with low cathode biofouling.

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

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 ...

Flat anodes placed close to the cathode or membrane to reduce distances between electrodes in microbial electrolysis cells (MECs) could be used to develop compact reactors, in contrast to microbial fuel cells (MFCs) where electrodes cannot be too close due to oxygen crossover from the cathode to the anode that reduces performance. Graphite fiber brush anodes are often used in MECs due to their proven performance in MFCs. However, brush anodes have not been directly compared to flat anodes in MECs, which are completely anaerobic, and therefore oxygen crossover is not a factor for felt or brush anodes. MEC performance was compared using flat felt or brush anodes in two-chamber, cubic type MECs operated in fed-batch mode, using acetate in a 50 mM phosphate buffer. Despite placement of felt anodes next to the membrane, MECs with felt anodes had a lower hydrogen gas production rate of 0.32 ± 0.02 m3-H2/m3-d than brush anodes (0.38 ± 0.02 m3-H2/m3-d). The main reason for the reduced performance was substrate-limited mass transfer to the felt anodes. To reduce mass transfer limitations, the felt anode electrolyte was stirred, which increased the hydrogen gas production rate to 0.41 ± 0.04 m3-H2/m3-d. These results demonstrate brush electrodes can improve performance of bioelectrochemical reactors even under fully anaerobic conditions.

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

In two-chamber microbial electrolysis cells (MECs) with anion exchange membranes (AEMs), a phosphate buffer solution (PBS) is typically used to avoid increases in catholyte pH as Nernst equation calculations indicate that high pHs adversely impact electrochemical performance. However, ion transport between the chambers will also impact performance, which is a factor not included in those calculations. To separate the impacts of pH and ion transport on MEC performance, a high molecular weight polymer buffer (PoB), which was retained in the catholyte due to its low AEM transport and cationic charge, was compared to PBS in MECs and abiotic electrochemical half cells (EHCs). In MECs, catholyte pH control was less important than ion transport. MEC tests using the PoB catholyte, which had a higher buffer capacity and thus maintained a lower catholye pH (

Thermally regenerative ammonia batteries (TRABs) have shown great promise as a method to convert low-grade waste heat into electrical power, with power densities an order of magnitude higher than other approaches. However, previous TRABs based on copper electrodes suffered from unbalanced anode dissolution and cathode deposition rates during discharging cycles, limiting practical applications. To produce a TRAB with stable and reversible electrode reactions over many cycles, inert carbon electrodes were used with silver salts. In continuous flow tests, power production was stable over 100 discharging cycles, demonstrating excellent reversibility. Power densities were 23 W m−2-electrode area in batch tests, which was 64% higher than that produced in parallel tests using copper electrodes, and 30 W m−2 (net energy density of 490 Wh m−3-anolyte) in continuous flow tests. While this battery requires the use a precious metal, an initial economic analysis of the system showed that the cost of the materials relative to energy production was $220 per MWh, which is competitive with energy production from other non-fossil fuel sources. A substantial reduction in costs could be obtained by developing less expensive anion exchange membranes.

Low-grade heat from geothermal sources and industrial plants is a significant source of sustainable power that has great potential to be converted to electricity. The two main approaches that have been extensively investigated for converting low-grade heat to electrical energy, organic Rankine cycles and solid-state thermoelectrics, have not produced high power densities or been cost-effective for such applications. Newer, alternative liquid-based technologies are being developed that can be categorized by how the heat is used. Thermoelectrochemical cells (TECs), thermo-osmotic energy conversion (TOEC) systems, and thermally regenerative electrochemical cycles (TRECs) all use low-grade heat directly in a device that generates electricity. Other systems use heat sources to prepare solutions that are used in separate devices to produce electrical power. For example, low-temperature distillation methods can be used to produce solutions with large salinity differences to generate power using membrane-based systems, such as pressure-retarded osmosis (PRO) or reverse electrodialysis (RED); or highly concentrated ammonia solutions can be prepared for use in thermally regenerative batteries (TRBs). Among all these technologies, TRECs, TOEC, and TRBs show the most promise for effectively converting low-grade heat into electrical power mainly due to their high power productions and energy conversion efficiencies.

Long-term operation of wastewater-fed, microbial fuel cells (MFCs) with cathodes made of activated carbon and stainless steel (SS) current collectors can result in decreased performance due to cathode fouling. Copper has good antimicrobial properties, and it is more electrically conductive than SS. To demonstrate that a copper current collector could produce a more fouling resistant cathode, MFCs with air cathodes using either SS or copper current collectors were operated using domestic wastewater for 27 weeks. The reduction in biofouling over time was shown by less biofilm formation on the copper cathode surface compared to SS cathodes, due to the antimicrobial properties of copper. Maximum power densities from 17–27 weeks were 440 ± 38 mW m−2 using copper and 370 ± 21 mW m−2 using SS cathodes. The main difference in the microbial community was a nitrifying community on the SS cathodes, which was not present on the copper cathodes.

The pH of a solution has a large influence on the ion removal efficiency of the membrane capacitive deionization (MCDI) process, an electrochemical ion separation process. We developed a convolutional neural network linked with a long short-term memory (CNN-LSTM) model based on an artificial intelligence algorithm to predict the effluent pH of MCDI, as effluent pH is difficult to predict using conventional numerical modeling. The model accurately predicted effluent pH (R2≥0.998) based on the analysis of five input variables (current, voltage, influent conductivity and pH, and effluent conductivity) under standard operating conditions of MCDI using either constant-current or constant-voltage conditions. The developed model predicted effluent pH using only limited input variables, current and voltage, with high accuracy (R2≥0.997). Thus, the CNN-LSTM model can be used in practical applications as only the current and voltage of MCDI cells are often monitored in field applications.

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.

Power generation using microbial fuel cells (MFCs) must provide stable, continuous conversion of organic matter in wastewaters into electricity. However, when relatively small diameter (0.8cm) graphite fiber brush anodes were placed close to the cathodes in MFCs, power generation was unstable during treatment of low strength domestic wastewater. One reactor produced 149mW/m2 before power generation failed, while the other reactor produced 257mW/m2, with both reactors exhibiting severe power overshoot in polarization tests. Using separators or activated carbon cathodes did not result in stable operation as the reactors continued to exhibit power overshoot based on polarization tests. However, adding acetate (1g/L) to the wastewater produced stable performance during fed batch and continuous flow operation, and there was no power overshoot in polarization tests. These results highlight the importance of wastewater strength and brush anode size for producing stable and continuous power in compact MFCs.

While stainless steel (SS) cathodes have shown great promise due to their low cost and high specific surface areas in microbial electrolysis cells (MECs), they have mainly been examined under static (no flow) conditions. Several different SS materials with different 3-dimensional structures (mesh, fiber felt, wool, and brushes) were compared in the absence and presence of fluid flow (0.05, 0.1 and 0.2 cm/s) past the cathode by catholyte recirculation. MECs with wool produced the highest hydrogen production rate with 1.3 ± 0.3 L-H2/L-reactor/d, which was the same as the Pt control at a catholyte recirculation of 40 mL/min (applied voltage of 0.9 V). In the absence of flow, hydrogen production rates of SS materials decreased by 52% (wool) to 28% (brush). The high hydrogen production rate using wool was likely a result of its high specific surface area (480 m2/m3-reactor volume), and reduced cathode overpotential due to gas removal by catholyte recirculation.

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.

Understanding how current densities affect electrogenic biofilm activity is important for wastewater treatment as current densities can substantially decrease at COD concentrations greater than those suitable for discharge to the environment. We examined the biofilm’s response, in terms of viability and enzymatic activity, to different current densities using microbial electrolysis cells with a lower (0.7 V) or higher (0.9 V) added voltage to alter current production. Viability was assessed using florescent dyes, with dead cells identified on the basis of dye penetration due to a compromised cell outer-membrane (red), and live cells (intact membrane) fluorescing green. Biofilms operated with 0.7 V produced 2.4 ± 0.2 A m −2 , and had an inactive layer near the electrode and a viable layer at the biofilm-solution interface. The lack of cell activity near the electrode surface was confirmed by using an additional dye that fluoresces only with enzymatic activity. Adding 0.9 V increased the current by 61%, and resulted in a single, more homogeneous and active biofilm layer. Switching biofilms between these two voltages produced outcomes associated with the new current rather than the previous biofilm conditions. These findings suggest that maintaining higher current densities will be needed to ensure long-term viability electrogenic biofilms.

Microbial fuel cells (MFCs) need to have a compact architecture, but power generation using low strength domestic wastewater is unstable for closely-spaced electrode designs using thin anodes (flat mesh or small diameter graphite fiber brushes) due to oxygen crossover from the cathode. A composite anode configuration was developed to improve performance, by joining the mesh and brushes together, with the mesh used to block oxygen crossover to the brushes, and the brushes used to stabilize mesh potentials. In small, fed-batch MFCs (28 mL), the composite anode produced 20% higher power densities than MFCs using only brushes, and 150% power densities compared to carbon mesh anodes. In continuous flow tests at short hydraulic retention times (HRTs, 2 or 4 h) using larger MFCs (100 mL), composite anodes had stable performance, while brush anode MFCs exhibited power overshoot in polarization tests. Both configurations exhibited power overshoot at a longer HRT of 8 h due to lower effluent CODs. The use of composite anodes reduced biomass growth on the cathode (1.9 ± 0.2 mg) compared to only brushes (3.1 ± 0.3 mg), and increased coulombic efficiencies, demonstrating that they successfully reduced oxygen contamination of the anode and the bio-fouling of cathode.

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.

The use of electrically conductive membranes has recently drawn great interest in water treatment as an approach to reduce biofouling. Most conductive membranes are made by binding nanoparticles (carbon nanotubes or graphene) to a polymeric membrane using additional polymers, but this method risks leaching these nanomaterials into the environment. A new approach was developed here based on producing an electrically conductive layer of aluminum-doped zinc oxide (AZO) by atomic layer deposition. The aqueous instability of AZO, which is a critical challenge for water applications, was solved by capping the AZO layer with an ultrathin (∼11 nm) TiO2 layer (AZO/TiO2). The combined film exhibited prolonged stability in water and had a low sheet resistance of 67 Ω/sq with a 120 nm-thick coating, while the noncapped AZO coating quickly deteriorated as shown by a large increase in membrane resistance. The AZO/TiO2 membranes had enhanced resistance to biofouling, with a 72% reduction in bacterial counts in the absence of an applied current due to its higher hydrophilicity than the bare polymeric membrane, and it achieved an additional 50% reduction in bacterial colonization with an applied voltage. The use of TiO2-capped AZO layers provides a new approach for producing conductive membranes using abundant materials, and it avoids the risk of releasing nanoparticles into the environment.

Hydrogen production using two-chamber microbial electrolysis cells (MECs) is usually adversely impacted by a rapid rise in catholyte pH because of proton consumption for the hydrogen evolution reaction. While using a bipolar membrane (BPM) will maintain a more constant electrolyte pH, the large voltage loss across this membrane reduces performance. To overcome these limitations, we used an acidic catholyte to compensate for the potential loss incurred by using a BPM. A hydrogen production rate of 1.2 ± 0.7 L-H

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.

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.

Quantifying the resistances that different components of microbial electrolysis cells (MECs) contribute to the total internal resistance is important for understanding how different reactor configurations affect overall performance. The impact of the reactor architecture was examined here by varying the relative sizes of the electrodes and quantifying the changes in resistances of the electrodes and limiting current densities as a function of the applied potential (Eap). The MECs with equal cathode:anode size ratios (SR=1), showed a steady increase in current up to 1.8 mA for Eap’s ≤ 0.9 V. However, lower limiting currents were obtained for configurations with smaller anodes as shown by a lack of an increase in current for Eap > 0.7 V (limiting current of 0.8 mA, SR=16, and 1.0 mA, SR=4). The largest component of the internal resistance changed with the relative sizes of the electrodes. For example, the cathode resistance was 58% of the internal resistance for the configuration of SR=1 and 51% for SR=2, but the anode was 57% of the internal resistance for SR=16. These results show how differences in reactor architectures can be quantified in terms of individual electrode resistances and limiting currents using polarization data obtained by varying the applied potentials.

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.

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 is a new approach for converting low-grade waste heat ( A 2D numerical model was created using COMSOL Multiphysics to study the relationship between fluid flow, electrode structure, and electrodeposition in the porous electrodes using cylinders in cross-flow to represent the internal structure of a porous carbon fiber electrode. Lower void fractions resulted in 2.5% higher power density, but less uniform deposition and the pores clogging 3-5 times faster due to nonuniform deposition normal to the membrane boundary. It was found that the electric field resultant from the void fraction and fiber arrangement was critical to the performance of the electrode. The electrode microstructure can be modified to better distribute the deposition rate relative to the pore size to allow for higher power output for a longer discharge time before the pores begin to clog. An electrode with variable void fraction increased the peak power by 7.5% but the pores clogged 23% faster compared to homogenous void fraction.

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.

Salinity gradient energy can be directly converted into electrical power by using reverse electrodialysis (RED) and other technologies, but reported power densities have been too low for practical applications. Herein, the RED stack performance was improved by using 2,6-dihydroxyanthraquinone and ferrocyanide as redox couples. These electrolytes were then used in a flow battery to produce an integrated RED stack and flow battery (RED-FB) system capable of capturing, storing, and discharging salinity gradient energy. Energy captured from the RED stack was discharged in the flow battery at a maximum power density of 3.0 kW m-2 -anode, which was similar to the flow batteries charged by electrical power and could be used for practical applications. Salinity gradient energy captured from the RED stack was recovered from the electrolytes as electricity with 30 % efficiency, and the maximum energy density of the system was 2.4 kWh m-3 -anolyte. The combined RED-FB system overcomes many limitations of previous approaches to capture, store, and use salinity gradient energy from natural or engineered sources.

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.

Activated carbon (AC) is an inexpensive and sustainable catalyst for oxygen reduction in air-cathodes of microbial fuel cells (MFCs), but its electrical conductivity is relatively poor. To improve cathode performance, five different more conductive materials were added to AC: three carbon materials (carbon black, mesoporous carbon, and carbon nanotubes), and two metal powders (inexpensive copper and inert gold). Carbon-based particles improved maximum power densities by 6–14% compared to plain AC due to reduced charge transfer resistance. Copper powder had reduced performance, likely due to toxicity effects on the anode bacteria, while gold particles were similar to plain AC. Heat treated AC mixed with carbon black produced the highest power density of 1900 ± 76 mW m−2, 41% higher than the widely used Pt air-cathode (1350 ± 55 mW m−2). The use of inexpensive carbon black with heat treatment was therefore the most effective and economical approach for improving cathode performance in MFCs.

Recent estimates suggest that approximately 40% of global electricity demands can be met by capturing the potential energy contained in the mixing of seawater and freshwater at coasts. Several technologies are currently being explored to capture this energy and maximize the achievable electrical power density. Here we report a new method to capture this energy that yielded a peak power density of 12.6 ± 0.5 W m−2-membrane area (average = 3.8 ± 0.1 W m−2), which is the highest value reported to date for salinity gradient energy technologies fed with synthetic seawater (30 g L−1 NaCl) and freshwater (1 g L−1 NaCl). This unprecedentedly high power density was achieved in a concentration flow cell containing non-toxic copper hexacyanoferrate electrodes that developed electrode potentials based on Na+ activities and an anion-exchange membrane that developed a Donnan potential based on Cl− activities. The peak power density increased to 26.3 ± 1.3 W m−2 (average = 9.4 ± 0.9 W m−2) when using a highly saline synthetic brine (300 g L−1 NaCl) and freshwater (1 g L−1 NaCl). Stacking two flow cells in series doubled the total power while maintaining the same power per membrane area. Combining electrode and Donnan potentials in a concentration flow cell offers a new means to harvest salinity gradient energy that may lead to an economically viable technology.

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...

Large differences between the water and air pressure in microbial fuel cells (MFCs) can deform and damage cathodes. To avoid deformation, the cathode air pressure was controlled to balance pressure differences between the air and water. Raising the air pressures from 0 to 10 kPa at a set cathode potential of −0.3 V (versus Ag/AgCl) enhanced cathode performance by 17%, but pressures ≥25 kPa decreased current and resulted in air leakage into the solution. Matching the air pressure with the water pressure avoided cathode deformation and improved performance. The maximum power density increased by 15%, from 1070 ± 20 to 1230 ± 70 mW m−2, with balanced air and water pressures of 10–25 kPa. Oxygen partial pressures ≥12.5 kPa in the cathode compartment maintained the oxygen reduction rate to be within 92 ± 1% of that in ambient air. The use of pressurized air flow through the cathode compartments can enable closer spacing of the cathodes compared to passive gas transfer systems, which could make the reactor design more compact. The energy cost of pressurizing the cathodes was estimated to be smaller than the increase in power that resulted from the use of pressurized cathodes.

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.

Applications of microbial fuel cells (MFCs) are limited in part by low power densities mainly due to cathode performance. Successful immobilization of an Fe-N-C co-catalyst on activated carbon (Fe-N-C/AC) improved the oxygen reduction reaction to nearly a four-electron transfer, compared to a twoelectron transfer achieved using AC. With acetate as the fuel, the maximum power density was 4.7±0.2 W m(-2) , which is higher than any previous report for an air-cathode MFC. With domestic wastewater as a fuel, MFCs with the Fe-N-C/AC cathode produced up to 0.8±0.03 W m(-2) , which was twice that obtained with a Pt-catalyzed cathode. The use of this Fe-N-C/AC catalyst can therefore substantially increase power production, and enable broader applications of MFCs for renewable electricity generation using waste materials.

Electrocoagulation (EC) can be used to remove nutrients as well as suspended solids and organic carbon from wastewaters, but the high energy requirements (pumping air) and the use of precious metals in some processes has hindered widespread application of EC technologies. The use of a thermodynamically favorable activated carbon air cathode and a sacrificial aluminum anode was examined to reduce the energy needed for EC for nutrient removal. Performance of the air cathode electrocoagulation (ACEC) process was tested using raw wastewater, and a carbon-free synthetic solution (nitrogen:phosphorus ratio of 1:10 in deionized water) to simulate nutrient removal of a wastewater treated for organic matter removal. ACEC treatment of wastewater removed up to 99% of both ammonia and phosphorus, along with 72–81% COD and 78–89% TSS in 4 h (1.5 cm electrode spacing, current density of 8 A m −2 ). Nearly total removal of the nutrients required 1.8 kWh m −3 , which is lower than that previously reported for some EC processes for this application (0.4–22 kWh m −3 ). Nutrient removals using synthetic solutions (no organic matter) ranged from 74% to 93% for nitrogen (47–370 mg-N/L) and 44–76% for phosphorus. These results indicate that the ACEC can achieve good levels of nutrient removal with reduced energy demands compared to previous EC systems.

Anaerobic fluidized membrane bioreactors (AFMBRs) have been mainly developed as a post-treatment process to produce high quality effluent with very low energy consumption. The performance of an AFMBR was examined using the effluent from a microbial fuel cell (MFC) treating domestic wastewater, as a function of AFMBR hydraulic retention times (HRTs) and organic matter loading rates. The MFC-AFMBR achieved 89 ± 3% removal of the chemical oxygen demand (COD), with an effluent of 36 ± 6 mg-COD/L over 112 days operation. The AFMBR had very stable operation, with no significant changes in COD removal efficiencies, for HRTs ranging from 1.2 to 3.8h, although the effluent COD concentration increased with organic loading. Transmembrane pressure (TMP) was low, and could be maintained below 0.12 bar through solids removal. This study proved that the AFMBR could be operated with a short HRT but a low COD loading rate was required to achieve low effluent COD.

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.

Large amounts of low-grade waste heat (temperatures

In microbial fuel cells (MFCs), buffers are typically used to improve performance by stabilizing the electrode pH and increasing the electrolyte conductivity, but the importance of the buffer net charge at current densities typical of MFCs on cathode performance has received little attention. Current production results in an electric field that drives positive ions towards the cathode, and negative ions to the anode. A series of biological buffers were selected with positive, negative, and neutral charges that had pKas ranging from 5 to 10.8. Cathodic current production using these different buffers in solutions with different pHs and conductivities was compared using linear sweep voltammetry (LSV). At lower pHs, buffers with positive charge increased cathodic current by as much as 95% within certain ranges (potential windows) of cathode potentials. No difference in cathodic current was shown in current for buffers with neutral or negative charge. The reason for this increase with the net positive charge buffers was likely due to a more stable electrode pH produced by electric field driving the positively charged ions towards the cathode. The potential window for the positively charged buffers was positively correlated to the concentration of cationic buffer in the electrolyte. At a pH higher than 9, no improvement in cathodic current was shown for buffers with positive charge, indicating at these higher pHs diffusion dominated buffer transport.

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.