Electrochemical Ammonia Recovery from Anaerobic Centrate Using a Nickel-Functionalized Activated Carbon Membrane Electrode.

Ammonia (NH3) recovery from used water (previously wastewater) is highly desirable to depart from fossil fuel-dependent NH3 production and curb nitrogen emission to the environment. Electrochemical NH3 recovery is promising since it can simply convert aqueous NH4+ to gaseous NH3 using cathodic reactions (OH- generation). However, the use of a separated electrode and membrane imposes high resistances to the cathodic reaction and NH3 transfer. This study examined an activated carbon (AC)-based membrane electrode functionalized with nickel to electrochemically recover NH3 from synthetic anaerobic centrate. The membrane electrode was fabricated using nickel-adsorbed AC powder and a polyvinylidene fluoride (PVDF) binder, and the PVDF membrane layer was formed at the electrode surface by phase inversion. The NH3-N recovery flux of 50.3 ± 0.4 gNH3-N/m2/d was produced at 17.1 A/m2 with a recovery solution at pH 7, and NH3-N fluxes and energy consumptions were improved as the recovery solution became acidic (62.2 ± 2.1 gNH3-N/m2/d with 16.0 ± 1.6 kWh/kgNH3-N at pH 2). Increasing PVDF loadings did not impact the electrochemical performances of the Ni/AC-PVDF electrode, but slightly lower (7%) NH3-N fluxes were obtained with higher PVDF loadings. Ni dissolution (3.7-6.0% loss) was affected by the recovery solution pH, but it did not impact the performances over the cycles.

Regenerable Nickel-Functionalized Activated Carbon Cathodes Enhanced by Metal Adsorption to Improve Hydrogen Production in Microbial Electrolysis Cells.

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 tests showed that negligible amounts of Ni were lost into the electrolyte at pHs of 7 or 12, and the catalytic activity was restored by simple readsorption using a Ni salt solution when Ni was partially removed by an acid wash.

Impact of Ohmic Resistance on Measured Electrode Potentials and Maximum Power Production in Microbial Fuel Cells.

Low solution conductivity is known to adversely impact power generation in microbial fuel cells (MFCs), but its impact on measured electrode potentials has often been neglected in the reporting of electrode potentials. While errors in the working electrode (typically the anode) are usually small, larger errors can result in reported counter electrode potentials (typically the cathode) due to large distances between the reference and working electrodes or the use of whole cell voltages to calculate counter electrode potentials. As shown here, inaccurate electrode potentials impact conclusions concerning factors limiting power production in MFCs at higher current densities. To demonstrate how the electrochemical measurements should be adjusted using the solution conductivity, electrode potentials were estimated in MFCs with brush anodes placed close to the cathode (1 cm) or with flat felt anodes placed further from the cathode (3 cm) to avoid oxygen crossover to the anodes. The errors in the cathode potential for MFCs with brush anodes reached 94 mV using acetate in a 50 mM phosphate buffer solution. With a felt anode and acetate, cathode potential errors increased to 394 mV. While brush anode MFCs produced much higher power densities than flat anode MFCs under these conditions, this better performance was shown primarily to result from electrode spacing following correction of electrode potentials. Brush anode potentials corrected for solution conductivity were the same for brushes set 1 or 3 cm from the cathode, although the range of current produced was different due to ohmic losses with the larger distance. These results demonstrate the critical importance of using corrected electrode potentials to understand factors limiting power production in MFCs.

Effect of initial salt concentrations on cell performance and distribution of internal resistance in microbial desalination cells.

Microbial desalination cells (MDCs) are modified microbial fuel cells (MFCs) that concurrently produce electricity and desalinate seawater, but adding a desalination compartment and an ion-exchange membrane may increase the internal resistance (Ri), which can limit the cell performance. However, the effects of a desalination chamber and initial NaCl concentrations on the internal resistances and the cell performances (i.e. Coulombic efficiency (CE), current and power density) of MDCs have yet to be thoroughly explored; thus, the cell performance and Ri distributions of MDCs having different initial concentrations and an MFC having no desalination chamber were compared. In the MDCs, the current and power density generation increased from 2.82 mA and 158.2 mW/m2 to 3.17 mA and 204.5 mW/m2 when the initial NaCl concentrations were increased from 5 to 30 g/L, as a consequence of the internal resistances decreasing from 2432.0 to 2328.4 Ω. And even though the MFC has a lower Ri than the MDCs, lower cell performances (current: 2.59 mA; power density: 141.6 mW/m2 and CE: 62.1%) were observed; there was no effect of improved junction potential in the MFC. Thus, in the MDCs, the higher internal resistances due to the addition of a desalination compartment can be offset by reducing the electrolyte resistance and improving the junction potential at higher NaCl concentrations.

Pretreatments of lignocellulosic and algal biomasses for sustainable biohydrogen production: Recent progress, carbon neutrality, and circular economy.

Lignocellulosic and algal biomasses are known to be vital feedstocks to establish a green hydrogen supply chain toward achieving a carbon-neutral society. However, one of the most pressing issues to be addressed is the low digestibility of these biomasses in biorefinery processes, such as dark fermentation, to produce green hydrogen. To date, various pretreatment approaches, such as physical, chemical, and biological methods, have been examined to enhance feedstock digestibility. However, neither systematic reviews of pretreatment to promote biohydrogen production in dark fermentation nor economic feasibility analyses have been conducted. Thus, this study offers a comprehensive review of current biomass pretreatment methods to promote biohydrogen production in dark fermentation. In addition, this review has provided comparative analyses of the technological and economic feasibility of existing pretreatment techniques and discussed the prospects of the pretreatments from the standpoint of carbon neutrality and circular economy.

Enhanced wettability improves catalytic activity of nickel-functionalized activated carbon cathode for hydrogen production in microbial electrolysis cells.

A nickel-functionalized activated carbon (AC/Ni) was recently developed for microbial electrolysis cells (MECs) and showed a great potential for large-scale applications. In this study, the electroactivity of the AC/Ni cathode was significantly improved by increasing the oxygen (16.9%) and nitrogen (124%) containing species on the AC using nitric acid oxidation. The acid-treated AC (t-AC) showed 21% enhanced wettability that consequently reduced the ohmic resistance (6.7%) and the charge transfer resistance (33.3%). As a result, t-AC/Ni achieved peak values of hydrogen production rate (0.35 ± 0.02 L-H2/L-d), energy yield (129 ± 8%), and cathodic hydrogen recovery (93 ± 6%) in MECs. The hydrogen production rate was 84% higher using t-AC/Ni cathode than the control, likely due to the enhanced wettability and a higher fraction of N on the t-AC. Also, the increases in polyvinylidene fluoride (PVDF) binder loadings (from 4.6 mg-PVDF/cm2 to 7.3 mg-PVDF/cm2) demonstrated 47% higher hydrogen productions rates in MECs.

Energy-efficient removal of PFOA and PFOS in water using electrocoagulation with an air-cathode.

Electrocoagulation (EC) with a zinc anode demonstrated promising results to remove perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) from an aqueous solution. However, the energy requirement for EC is usually very high due to water electrolysis or aeration. This study aims to reduce energy consumption using an air-cathode in EC (ACEC) to supply oxygen electron acceptor without aeration for attenuating PFOA/PFOS in this new configuration. For the high PFOA concentration (0.25 mM), ACEC with 45 min of the reaction time exhibited an excellent PFOA removal (99.8 ± 0.3% removal) comparable to an EC with aeration (EC-aeration, 100% removal) while achieving much less energy consumption (0.14 kWh/m3). For the low PFOA concentration (0.1 µM), only 41.1 ± 11.6% was removed by the ACEC due to the low concentration gradient for adsorption. EC-aeration achieved higher PFOA removal (81.9 ± 15.1%) for the low PFOA concentration, possibly because air bubbles floated PFOA to the water surface, thereby concentrating PFOA. The PFOS removals in the ACEC and EC-aeration (76.4-88.5%) at the high concentration (0.25 mM) were lower than PFOA due tentatively to its micelle formation. However, PFOS was removed better than PFOA at the low concentration (0.1 µM) due to its higher hydrophobicity.

Optimization studies of bio-hydrogen production in a coupled microbial electrolysis-dye sensitized solar cell system.

Bio-hydrogen production in light-assisted microbial electrolysis cell (MEC) with a dye sensitized solar cell (DSSC) was optimized by connecting multiple MECs to a single dye (N719) sensitized solar cell (V(OC) approx. 0.7 V). Hydrogen production occurred simultaneously in all the connected MECs when the solar cell was irradiated with light. The amount of hydrogen produced in each MEC depends on the activity of the microbial catalyst on their anode. Substrate (acetate) to hydrogen conversion efficiencies ranging from 42% to 65% were obtained from the reactors during the experiment. A moderate light intensity of 430 W m(-2) was sufficient for hydrogen production in the coupled MEC-DSSC. A higher light intensity of 915 W m(-2), as well as an increase in substrate concentration, did not show any improvement in the current density due to limitation caused by the rate of microbial oxidation on the anode. A significant reduction in the surface area of the connected DSSC only showed a slight effect on current density in the coupled MEC-DSSC system when irradiated with light.

Application of phase-pure nickel phosphide nanoparticles as cathode catalysts for hydrogen production in microbial electrolysis cells.

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 ±â€¯0.04 L-H2/L-reactor) as that obtained using Pt/C (0.32 ±â€¯0.03 L-H2/L) or Ni/C (0.29 ±â€¯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.

Applying the electrode potential slope method as a tool to quantitatively evaluate the performance of individual microbial electrolysis cell components.

Improving the design of microbial electrolysis cells (MECs) requires better identification of the specific factors that limit performance. The contributions of the electrodes, solution, and membrane to internal resistance were quantified here using the newly-developed electrode potential slope (EPS) method. The largest portion of total internal resistance (120 ±â€¯0 mΩ m2) was associated with the carbon felt anode (71 ±â€¯5 mΩ m2, 59% of total), likely due to substrate and ion mass transfer limitations arising from stagnant fluid conditions and placement of the electrode against the anion exchange membrane. The anode resistance was followed by the solution (25 mΩ m2) and cathode (18 ±â€¯2 mΩ m2) resistances, and a negligible membrane resistance. Wide adoption and application of the EPS method will enable direct comparison between the performance of the components of MECs with different solution characteristics, electrode size and spacing, reactor architecture, and operating conditions.

Surfactant addition to enhance bioavailability of bilge water in single chamber microbial fuel cells (MFCs).

Effective remediation of bilge water, a shipboard oily liquid waste, is important for both commercial and military vessels due to the domestic and international regulations. In this study, bilge water was used as a substrate for exoelectrogenic bacteria and biodegradation of bilge water and concurrent electricity generation were investigated using Pseudomonas putida ATCC 49128 in single chamber microbial fuel cells (MFCs). To enhance bioavailability of the bilge water, two types of surfactants were added (100 ppm) into the oily wastewater containing 0.1% standard bilge mix (SBM) and their impacts on electricity production were evaluated under various conditions. Anionic surfactant (sodium dodecyl sulfate, SDS) addition increased soluble chemical oxygen demand (SCOD) by forming micelle, producing maximum power density of 225.3 ± 3.2 mW m-2. However, the MFC with nonionic surfactant (Triton X-100) produced only 2.3 ± 0.1 mW m-2 due to no enhancement on biodegradable SCOD. A high NaCl concentration (100-500 mM) adversely affected power production due to decrease in available SCOD caused by emulsion coalescence. This is a first study to use surfactants to enhance bioavailability of non-biodegradable oily wastewater in a single chamber MFC.

Mitigating external and internal cathode fouling using a polymer bonded separator in microbial fuel cells.

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 ±â€¯30 mW m-2 after 2 months of operation using domestic wastewater, which was ∼220% higher than controls (60 ±â€¯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.

Electrical current generation in microbial electrolysis cells by hyperthermophilic archaea Ferroglobus placidus and Geoglobus ahangari.

Few microorganisms have been examined for current generation under thermophilic (40-65°C) or hyperthermophilic temperatures (≥80°C) in microbial electrochemical systems. Two iron-reducing archaea from the family Archaeoglobaceae, Ferroglobus placidus and Geoglobus ahangari, showed electro-active behavior leading to current generation at hyperthermophilic temperatures in single-chamber microbial electrolysis cells (MECs). A current density (j) of 0.68±0.11A/m2 was attained in F. placidus MECs at 85°C, and 0.57±0.10A/m2 in G. ahangari MECs at 80°C, with an applied voltage of 0.7V. Cyclic voltammetry (CV) showed that both strains produced a sigmoidal catalytic wave, with a mid-point potential of -0.39V (vs. Ag/AgCl) for F. placidus and -0.37V for G. ahangari. The comparison of CVs using spent medium and turnover CVs, coupled with the detection of peaks at the same potentials in both turnover and non-turnover conditions, suggested that mediators were not used for electron transfer and that both archaea produced current through direct contact with the electrode. These two archaeal species, and other hyperthermophilic exoelectrogens, have the potential to broaden the applications of microbial electrochemical technologies for producing biofuels and other bioelectrochemical products under extreme environmental conditions.

Electricity from methane by reversing methanogenesis.

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.

Performance of anaerobic fluidized membrane bioreactors using effluents of microbial fuel cells treating domestic wastewater.

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.

Continuous treatment of high strength wastewaters using air-cathode microbial fuel cells.

Treatment of low strength wastewaters using microbial fuel cells (MFCs) has been effective at hydraulic retention times (HRTs) similar to aerobic processes, but treatment of high strength wastewaters can require longer HRTs. The use of two air-cathode MFCs hydraulically connected in series was examined to continuously treat high strength swine wastewater (7-8g/L of chemical oxygen demand) at an HRT of 16.7h. The maximum power density of 750±70mW/m2 was produced after 12daysof operation. However, power decreased by 85% after 185d of operation due to serious cathode fouling. COD removal was improved by using a lower external resistance, and COD removal rates were substantially higher than those previously reported for a low strength wastewater. However, removal rates were inconsistent with first order kinetics as the calculated rate constant was an order of magnitude lower than rate constant for the low strength wastewater.

The effect of flow modes and electrode combinations on the performance of a multiple module microbial fuel cell installed at wastewater treatment plant.

A larger (6.1 L) MFC stack made in a scalable configuration was constructed with four anode modules and three (two-sided) cathode modules, and tested at a wastewater treatment plant for performance in terms of chemical oxygen demand (COD) removal and power generation. Domestic wastewater was fed either in parallel (raw wastewater to each individual anode module) or series (sequentially through the chambers), with the flow direction either alternated every one or two days or kept fixed in a single direction over time. The largest impact on performance was the wastewater COD concentration, which greatly impacted power production, but did not affect the percentage of COD removal. With higher COD concentrations (∼500 mg L-1) and alternating flow conditions, power generation was primarily limited by the cathode specific area. In alternating flow operation, anode modules connected to two cathodes produced an average maximum power density of 6.0 ± 0.4 W m-3, which was 1.9 ± 0.2 times that obtained for anodes connected to a single cathode. In fixed flow operation, a large subsequent decrease in COD influent concentration greatly reduced power production independent of reactor operation in parallel or serial flow modes. Anode modules connected to two cathodes did not consistently produce more power than the anodes connected to a single cathode, indicating power production became limited by restricted anode performance at low CODs. Cyclic voltammetry and electrochemical impedance spectroscopy data supported restricted anode performance with low COD. These results demonstrate that maintaining power production of MFC stack requires higher influent and effluent COD concentrations. However, overall performance of the MFC in terms of COD removal was not affected by operational modes.

Development of carbon free diffusion layer for activated carbon air cathode of microbial fuel cells.

The fabrication of activated carbon air cathodes for larger-scale microbial fuel cells requires a diffusion layer (DL) that is highly resistant to water leakage, oxygen permeable, and made using inexpensive materials. A hydrophobic polyvinylidene fluoride (PVDF) membrane synthesized using a simple phase inversion process was examined as a low cost ($0.9/m(2)), carbon-free DL that prevented water leakage at high pressure heads compared to a polytetrafluoroethylene/carbon black DL ($11/m(2)). The power density produced with a PVDF (20%, w/v) DL membrane of 1400±7mW/m(2) was similar to that obtained using a wipe DL [cloth coated with poly(dimethylsiloxane)]. Water head tolerance reached 1.9m (∼19kPa) with no mesh supporter, and 2.1m (∼21kPa, maximum testing pressure) with a mesh supporter, compared to 0.2±0.05m for the wipe DL. The elimination of carbon black from the DL greatly simplified the fabrication procedure and further reduced overall cathode costs.

Impact of electrode configurations on retention time and domestic wastewater treatment efficiency using microbial fuel cells.

Efficient treatment of domestic wastewater under continuous flow conditions using microbial fuel cells (MFCs) requires hydraulic retention times (HRTs) that are similar to or less than those of conventional methods such as activated sludge. Two MFCs in series were compared at theoretical HRTs of 8.8, 4.4 and 2.2 h using two different brush-electrode MFC configurations: a full brush evenly spaced between two cathodes (S2C); and trimmed brush anodes near a single cathode (N1C). The MFCs with two cathodes produced more power than the MFCs with a single cathode, with 1.72 mW for the S2C, compared to and 1.12 mW for the N1C at a set HRT = 4.4 h. The single cathode MFCs with less cathode area removed slightly more COD (54.2 ± 2.3%, N1C) than the two-cathode MFCs (48.3 ± 1.0%, S2C). However, the higher COD removal was due to the longer HRTs measured for the MFCs with the N1C configuration (10.7, 5.3 and 3.1 h) than with the S2C configuration (7.2, 3.7 and 2.2 h), despite the same theoretical HRT. The longer HRTs of the N1C MFCs also resulted in slightly higher coulombic efficiencies (≤37%) than those of the S2C MFCs (≤29%). While the S2C MFC configuration would be more advantageous based on electrical power production, the N1C MFC might be more useful on the basis of capital costs relative to COD removal efficiency due to the use of less cathode surface area per volume of reactor.

Evaluation of hydrogen production and internal resistance in forward osmosis membrane integrated microbial electrolysis cells.

In order to enhance hydrogen production by facilitated proton transport through a forward osmosis (FO) membrane, the FO membrane was integrated into microbial electrolysis cells (MECs). An improved hydrogen production rate was obtained in the FO-MEC (12.5±1.84×10(-3)m(3)H2/m(3)/d) compared to that of the cation exchange membrane (CEM) - MEC (4.42±0.04×10(-3)m(3)H2/m(3)/d) during batch tests (72h). After an internal resistance analysis, it was confirmed that the enhanced hydrogen production in FO-MEC was attributed to the smaller charge transfer resistance than in the CEM-MEC (90.3Ω and 133.4Ω respectively). The calculation of partial internal resistance concluded that the transport resistance can be substantially reduced by replacing a CEM with a FO membrane; decrease of the resistance from 0.069Ωm(2) to 5.99×10(-4)Ωm(2).