Force Calculation in 3-D Magnetic Equivalent Circuit Networks With a Maxwell Stress Tensor

Magnetic equivalent circuit (MEC) models are increasingly valuable for analysis and design of electromechanical devices, particularly electrical machines, because of their moderate computational effort and reasonable accuracy. Force and torque calculations in prior MEC implementations are almost exclusively based on the virtual work method (VWM) adapted to the specific device model. But VWM does not easily extend to a general MEC modeling approach. In this paper, the more direct Maxwell stress tensor (MST) method is applied to a general 3-D MEC modeling framework. MST theory and implementation are presented. Its application is discussed with the example of an electromagnet. Results are compared to both measurements and analytical and finite-element models.      

Domestic wastewater treatment in parallel with methane production in a microbial electrolysis cell

Microbial electrolysis cells (MECs) have great potential as a technology for wastewater treatment in parallel to energy production. In this study we explore the feasibility of using a low-cost, membraneless MEC for domestic wastewater treatment and methane production in both batch and continuous modes. Low-strength wastewater can be successfully treated by means of an MEC, obtaining significant amounts of methane. The results also suggest that hydrogenotrophic methanogenesis reduce the incidence of homoacetogenic activity, thus improving the overall MEC performance. However, gas production rates are low and important aspects such as methane solubility in water still remain a challenge. Overall, MECs can offer competitive advantages not only for low-strength wastewater treatment but also as an aid to anaerobic methane production by improving the chemical oxygen demand (COD) removal and methane production rates.     

Bibliometric and content analysis on emerging technologies of hydrogen production using microbial electrolysis cells

Microbial electrolysis cell (MEC) is a promising and significant approach for hydrogen production, owing to the low energy consumption and high yield/recovery efficiency. In this review, bibliometric analysis has been conducted on current research trends of MECs, followed by the content analysis of the direction and strategy for improving the hydrogen production to enlarge MECs’ applicability. Results show that energy concerns are the primary focus in MEC studies, and particular attention has been paid to decreasing internal resistance and hydrogen diffusion that may be crucial for the improvement of the yield of hydrogen production and recovery. Moreover, this study particularly reviews the development of the cathode catalysts and explores the applicability of different MEC configurations, including single-and two-chamber MECs with different kind of membranes. It also identifies the potential advantages of porous membranes as a separator in MECs and discusses the porous membrane, in conjunction with advancing hydrogen-harvesting approaches, is crucial for the improvement of hydrogen production. Finally, capital cost of MECs has been necessarily discussed, which would be significant, valuable, and beneficial to the academic research and the industrial community.     

Methanogenesis control by electrolytic oxygen production in microbial electrolysis cells

High purity H₂ production using microbial electrolysis cells (MECs) is often limited by methanogenesis. Here methanogenesis was effectively controlled by electrolytic oxygen production. Oxygen production was induced intermittently using two stainless steel electrodes, which were used as the MEC cathode during Normal operation. It was found that oxygen should be produced every 12 h or more frequently because of rapid hydrogenotrophic methanogen growth with available pure H₂. This method was also effective in an initially methanogen-dominated MEC. However, the growth of aerobic biofilms in MECs weakened methanogen control. Residual oxygen after fed-batch cycles was found to be the key indicator for effective methane control. Methane content was consistently smaller than 10% at the threshold residual oxygen volume (3 mL) or greater. MEC operation at such threshold conditions will allow high purity H₂ production, low energy consumption for O₂ production and minimal O₂ exposure on bioanodes, enabling sustainable wastewater treatment and energy recovery using MECs.     

Hydrogen production in single-chamber tubular microbial electrolysis cells using non-precious-metal catalysts

Platinum has excellent catalytic capabilities and is commonly used as cathode catalyst in microbial electrolysis cells (MECs). Its high cost, however, limits the practical applications of MECs. In this study, precious-metal-free cathodes were developed by electrodepositing NiMo and NiW on a carbon-fiber-weaved cloth material and evaluated in electrochemical cells and tubular MECs with cloth electrode assemblies (CEA). While similar performances were observed in electrochemical cells, NiMo cathode exhibited better performances than NiW cathode in MECs. At an applied voltage of 0.6 V, the MECs with NiMo cathode accomplished a hydrogen production rate of 2.0 m³/day/m³ at current density of 270 A/m³ (12 A/m²), which was 33% higher than that of the NiW MECs and slightly lower than that of the MECs with Pt catalyst (2.3 m³/day/m³). At an applied voltage of 0.4 V, the energy efficiencies based on the electrical energy input reached 240% for the NiMo MECs. These results demonstrated the great potential of using carbon cloth with Ni-alloy catalysts as a cathode material for MECs. The enhanced MEC performances also demonstrate the scale-up potential of the CEA structure, which can significantly reduce the electrode spacing and lower the internal resistance of MECs, thus increasing the hydrogen production rate.     

Influence of catholyte pH and temperature on hydrogen production from acetate using a two chamber concentric tubular microbial electrolysis cell

Microbial electrolysis cells (MECs) could be integrated with dark fermentative hydrogen production to increase the overall system yield of hydrogen. The influence of catholyte pH on hydrogen production from MECs and associated parameters such as electrode potentials (vs Ag/AgCl), COD reduction, current density and quantity of acid needed to control pH in the cathode of an MEC were investigated. Acetate (10 mM, HRT 9 h, 24 °C, pH 7) was used as the substrate in a two chamber MEC operated at 600 mV and 850 mV applied voltage. The effect of catholyte pH on current density was more significant at an applied voltage of 600 mV than at 850 mV. The highest hydrogen production rate was obtained at 850 mV, pH 5 amounting to 200 cm³_stp/l_anode/day (coulombic efficiency 60%, cathodic hydrogen recovery 45%, H₂ yield 1.1 mol/mol acetate converted and a COD reduction of 30.5%). Within the range (18.5–49.4 °C) of temperatures tested, 30 °C was found to be optimal for hydrogen production in the system tested, with the performance of the reactor being reduced at higher temperatures. These results show that an optimum temperature (approximately 30 °C) exists for MEC and that lower pH in the cathode chamber improves hydrogen production and may be needed if potentials applied to MECs are to be minimised.     

Recent advancements in the cathodic catalyst for the hydrogen evolution reaction in microbial electrolytic cells

The microbial electrochemical technology is a foremost viable technology for hydrogen production from organic matter or wastewater catalyzed by electroactive microorganisms. Developing a high-efficient and low-cost cathode for hydrogen production is crucial for the practical applications of MEC. In the present article, cathode materials and catalysts for hydrogen evolution reaction (HER) in MECs are reviewed. There is an essential requirement of cost-effective HER catalysts for improving MEC performance and as the practical findings fell short of the ideal catalyst's expectations, the density functional theory (DFT) can give essential molecular knowledge and anticipate viable catalysts. Additionally, this article provides an overview of the development of density functional theory (DFT), as well as computer simulations for HER processes using DFT, and also computational designs and virtual screens of novel HER catalysts. The development of catalysts combined with DFT simulations offers significant advancements in the near future on the path to an ideal electrocatalyst in MEC.     

Hydrogen production with nickel powder cathode catalysts in microbial electrolysis cells

Although platinum is commonly used as catalyst on the cathode in microbial electrolysis cells (MEC), non-precious metal alternatives are needed to reduce costs. Cathodes were constructed using a nickel powder (0.5–1 μm) and their performance was compared to conventional electrodes containing Pt (0.002 μm) in MECs and electrochemical tests. The MEC performance in terms of coulombic efficiency, cathodic, hydrogen and energy recoveries were similar using Ni or Pt cathodes, although the maximum hydrogen production rate (Q) was slightly lower for Ni (Q = 1.2–1.3 m³ H₂/m³/d; 0.6 V applied) than Pt (1.6 m³ H₂/m³/d). Nickel dissolution was minimized by replacing medium in the reactor under anoxic conditions. The stability of the Ni particles was confirmed by examining the cathodes after 12 MEC cycles using scanning electron microscopy and linear sweep voltammetry. Analysis of the anodic communities in these reactors revealed dominant populations of Geobacter sulfurreduces and Pelobacter propionicus. These results demonstrate that nickel powder can be used as a viable alternative to Pt in MECs, allowing large scale production of cathodes with similar performance to systems that use precious metal catalysts.     

Avenues to the financial viability of microbial electrolysis cells [MEC] for domestic wastewater treatment and hydrogen production

We propose targets, based on real world data, necessary to design a financially viable microbial electrolysis cell (MEC) for the treatment of domestic wastewater. By reducing the cost of the anode and current collecting materials by 90%, a viable organic loading rate would be between 800 and 1,400g-COD/m³/d (2–3A/m²). The anode and current collector materials account for 94% of the total material costs; consequently, cost savings in any other material are moot. If the bioanode can be reused after 20 years, further, significant savings could be achieved. To develop targets we used real world data, for the first time, to evaluate the financial viability of MECs against the current predominant method of wastewater treatment: activated sludge. We modelled net present values for eight potential scenarios and the performances required for MECs to break-even.     

The importance of OH− transport through anion exchange membrane in microbial electrolysis cells

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 (<8), resulted in a 50% lower hydrogen production rate (HPR) than that obtained using PBS (HPR = 0.7 m³-H₂ m^?3 d^?1) where the catholyte rapidly increased to pH = 12. The main reason for the decreased performance using PoB was a lack of hydroxide ion transfer into the anolyte to balance pH. The anolyte pH in MECs rapidly decreased to 5.8 due to a lack of hydroxide ion transport, which inhibited current generation by the anode, whereas the pH was maintained at 6.8 using PBS. In abiotic tests in ECHs, where the cathode potential was set at ?1.2 V, the HPR was 133% higher using PoB than PBS due to catholyte pH control, as the anolyte pH was not a factor in the performance. These results show that maintaining charge transfer to control anolyte pH is more important than obtaining a more neutral pH catholyte.     

Microbial electrolysis cell powered by an aluminum-air battery for hydrogen generation,in-situ coagulant production and wastewater treatment

Microbial electrolysis cells (MECs) are an efficient technology for generating hydrogen gas from organic matters, but an additional voltage is needed to overcome the thermodynamic barrier of the reaction. A combined system of MEC and the aluminum-air battery (Al-air battery) was designed for hydrogen generation, coagulant production and operated in an energy self-sufficient mode. The Al-air battery (28 mL) produced a voltage ranged from 0.58 V to 0.80 V, which powered an MEC (28 mL) to produce hydrogen. The hydrogen production rate reached 0.19 ± 0.01 m³ H₂/m³/d and 14.5 ± 0.9 mmol H₂/g COD. The total COD removal rate was 85 ± 1%, of which MEC obtained 75 ± 1% COD removal and 10 ± 1% COD removal was achieved by in-situ coagulating process. The microorganisms removal of MEC effluent was 97 ± 2% through ex-situ coagulating process. These results showed that the Al-air battery-MEC system can be operated in energy self-sufficient mode and recovered energy from wastewater with high quality effluent.     

Optimization of NiMo catalyst for hydrogen production in microbial electrolysis cells

Non-platinum based cathodes were recently developed by electrodepositing NiMo on carbon cloth, which demonstrated good electrocatalytic activity for hydrogen evolution in microbial electrolysis cells (MECs). To further optimize the electrodeposition condition, the effects of electrolyte bath composition, applied current density, and duration of electrodeposition were systematically investigated in this study. The developed NiMo catalysts were characterized with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) and evaluated using chronopotentiometry and in MECs. The optimal condition for electrodeposition of NiMo on carbon cloth was determined as: a Mo/Ni mass ratio of 0.65 in electrolyte bath, an applied current density of 50 mA/cm² and electrodeposition duration of 10 min. Under this condition, the NiMo catalyst has a formula of Ni₆MoO₃ with a nodular morphology. The NiMo loading on the carbon cloth was reduced to 1.7 mg/cm² and the performance of MEC with the developed NiMo cathode was comparable to that with Pt cathode with a similar loading. This result indicates that a much lower cathode fabrication cost can be achieved compared to that using Pt catalyst, and thereby significantly enhancing the economic feasibility of the MEC technology.     

A novel pico-hydro power (PHP)-Microbial electrolysis cell (MEC) coupled system for sustainable hydrogen production during palm oil mill effluent (POME) wastewater treatment

Due to accelerating global efforts toward decarbonization, a clean hydrogen (H₂) producing technology, Microbial Electrolysis Cell (MEC), has garnered considerable attention. However, MEC's external energy requirement has raised concerns about its sustainability, scalability and application costs. The objective of this research was to build a renewable energy generating system for MECs' performance enhancement during the treatment of Palm oil mill effluent (POME). A novel integration of a pico-hydro-power generator (PHP) with single-chambered MECs exceptionally improved its performance. The performance boost was observed as 1.16 m³-H₂/m³d H₂ and 113 A/m³current production in concomitant with 73% organics removal from Palm Oil Mill Effluent (POME) wastewater, which is higher than the previous single-chambered MECs studies. 78% H₂ recovery r_cat (H₂) along with 57% coulombic efficiency (CE) corroborated the removal of a high percentage of electrons from POME organic materials to generate >96% pure H₂. The MEC nourished POME wastewater degrading communities while stimulating growth of electroactive Geobacter in the anodic biofilm which produced H₂. The overall H₂ recovery, COD removal rate and energy efficiency of PHP-MEC are superior than other MECs powered by other external renewable energy sources reported to date. The PHP-MEC prototype paves the path of scale up studies to build a renewable energy dependent future.     

Process kinetics for the electrocatalytic hydrogen evolution reaction on carbon-based Ni/NiO nanocomposite in a single-chamber microbial electrolysis cell

To explore the process kinetics of hydrogen evolution reaction (HER) on carbon-based Ni/NiO nanocomposite in the microbial electrolysis cells (MECs), the performance was systematically studied by different time-course sampling of five parallel single-chamber MECs operated under identical operating conditions, which included the electrochemical performance of anodes and cathodes, and the mechanism and kinetics of HER. It was hypothesized that the decreased performance of the nickel cathodes was due to corrosion and Ni dissolution. These results provide valuable insights into the effects of long-term operation on MEC performance.     

Hydrogen production rates with closely-spaced felt anodes and cathodes compared to brush anodes in two-chamber microbial electrolysis cells

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.     

Improving microbial electrolysis stability using flow-through brush electrodes and monitoring anode potentials relative to thermodynamic minima

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 <400 A/m³ when using the smaller 4.5 cm brush or the multiple brush reactor. Full acclimation was shown by a consistent and low anode potential, for example by ?248 ± 8 mV (vs. a standard hydrogen electrode) for the 5.5 cm brush, which was only 31 ± 8 mV above the minimum estimated for acetate oxidation under standard biological conditions. These results show that brush compression into a smaller chamber can enhance MEC performance and produce anode potentials close the thermodynamic minima.     

Mathematical model of biohydrogen production in microbial electrolysis cell: A review

Microbial electrolysis cell (MEC) is a promising reactor. However, currently, the reactor cannot be adapted for industrial-scale biohydrogen production. Nevertheless, this drawback can be overcome by modeling studies based on mathematical equations. The limitation of analytical instrumentation to record the non-linearity of the dynamic behavior for biohydrogen processes in an MEC has led to the introduction of computational approach that has the potential to reduce time constraints and optimize experimental costs. Reviews of comparative studies on bioelectrochemical models are widely reported, but there is less emphasis on the MEC model. Therefore, in this paper, a comprehensive review of the MEC mathematical model will be further discussed. The classification of the model with respect to the assumptions, model improvement, and extensive studies based on the model application will be critically analyzed to establish a methodology algorithm flow chart as a guideline for future implementation.     

Estimating microbial electrolysis cell (MEC) investment costs in wastewater treatment plants: Case study

Microbial electrolysis represents a new approach for harnessing the energy contained in the organic matter of wastewater. However, before this approach can be implemented on a practical basis, a cost-effective manufacturing process for microbial electrolysis cells (MECs) must be developed. The objective of this study is to estimate an acceptable purchase cost of an MEC reactor for a domestic wastewater treatment plant. We estimate that for a full-scale MEC operating at a current density of 5 A m_a⁻² (amperes per square meter of anode) and an energy consumption of 0.9 kWh kg-COD⁻¹ (kilowatt-hour per kg of removed chemical oxygen demand (COD)), a cost of €1220 m_a⁻³ (euro per m³ of anodic chamber) can be established as a target purchase cost at which a break-even point is reached after 7 years.     

Assessment of biotic and abiotic graphite cathodes for hydrogen production in microbial electrolysis cells

Hydrogen represents a promising clean fuel for future applications. The biocathode of a two-chambered microbial electrolysis cell (biotic MEC) was studied and compared with an abiotic cathode (abiotic MEC) in order to assess the influence of naturally selected microorganisms for hydrogen production in a wide range of cathode potentials (from −400 to −1800 mV vs SHE). Hydrogen production in both MECs increased when cathode potential was decreased. Microorganisms present in the biotic MEC were identified as Hoeflea sp. and Aquiflexum sp. Supplied energy was utilized more efficiently in the biotic MEC than in the abiotic, obtaining higher hydrogen production respect to energy consumption. At −1000 mV biotic MEC produced 0.89 ± 0.10 m³ H₂ d⁻¹ m⁻³NCC (Net Cathodic Compartment) at a minimum operational cost of 3.2 USD kg⁻¹ H₂. This cost is lower than the estimated market value for hydrogen (6 USD kg⁻¹ H₂).     

The use and optimization of stainless steel mesh cathodes in microbial electrolysis cells

Microbial electrolysis cells (MECs) provide a high-yield method for producing hydrogen from renewable biomass. One challenge for commercialization of the technology is a low-cost and highly efficient cathode. Stainless steel (SS) is very inexpensive, and cathodes made of this material with high specific surface areas can achieve performance similar to carbon cathodes containing a platinum catalyst in MECs. SS mesh cathodes were examined here as a method to provide a higher surface area material than flat plate electrodes. Cyclic voltammetry tests showed that the electrochemically active surface area of certain sized mesh could be three times larger than a flat sheet. The relative performance of SS mesh in linear sweep voltammetry at low bubble coverages (low current densities) was also consistent with performance on this basis in MEC tests. The best SS mesh size (#60) in MEC tests had a relatively thick wire size (0.02 cm), a medium pore size (0.02 cm), and a specific surface area of 66 m²/m³. An applied voltage of 0.9 V produced a high hydrogen recovery (98 ± 4%) and overall energy efficiency (74 ± 4%), with a hydrogen production rate of 2.1 ± 0.3 m³H₂/m³d (current density of 8.08 A/m², volumetric current density of 188 ± 19 A/m³). These studies show that SS in mesh format shows great promise for the development of lower cost MEC systems for hydrogen production.