Volumetric Current Density Research Articles

Simulation of hexavalent chromium removal by electrocoagulation using iron anode in flow-through reactor

An electrocoagulation (EC) model is developed for hexavalent chromium reduction and precipitation, using iron electrodes. Parallel removal mechanisms such as adsorption of chromium on ferrihydrite and direct reduction at the cathode is assumed negligible due to low concentration of Cr(VI). The reaction model presented for batch system represents species complexation, precipitation/dissolution, acid/base, and oxidation-reduction reactions. Batch reactor simulation is verified using experimental data obtained by Sarahney et al. (2012), where the effect of initial chromium concentration, pH, volumetric current density, and ionic strength is considered (Sarahney et al., 2012). The model couples multicomponent ionic transport in MATLAB with chemical reaction model in PHREEQC, as a widely used computational programming tool and a geochemical reaction simulator with comprehensive geochemistry databases. The suggested current density is 0.05−0.3mA/cm2 and the surface to volume ratio in batch reactor is considered 0.017 1/cm. Design parameters are presented for operation of a flow-through hexavalent chromium removal using electrocoagulation by iron electrode to treat Cr(VI) in range of 10–50 mg/L. The operational parameters for a flow-through EC reactor for Cr(VI) removal is suggested to follow 0.05mA/cm2≤3nFeQcCrVIinlet≤0.3mA/cm2.

Self-contact-electrification aerogel for all-in-one triboelectricity generation

Self-contact-electrification (SCE), induced by the charge transfer between the contact of positive and negative regions within one material, is an ideal strategy to simplify the working mechanism and device setup of conventional hetero-structured triboelectric nanogenerators (TENG). However, due to its limited electrification polarity, the low outputs of SCE makes it still very challenging to apply for triboelectric nanogeneration. Here, we design and synthesize a three-dimensional TENG aerogel to realize a pore-volume-enhanced SCE for high-performance mechanical energy harvesting. By loading fluorine and sulfone based polyimide (SF-PI) onto an ultra-flexible graphene aerogel (GA) framework, electrons can redistribute between the SF-PI layers under the deformation of aerogel, resulting in charge transfer from graphene to SF-PI to generate electricity. The large specific surface area of GA increases the self-contact area of SF-PI, resulting in high volumetric voltage and current density up to 27 × 103 kV m−3 and 35 mA m−3 under a compressive strain of 95%. This SCE-TENG aerogel can realize an all-in-one strategy for energy havesting and sensing, which may guide and accelerate the development and application of high-performance TENGs.

Cathode Catalyst Layer Design in PEM Water Electrolysis toward Reduced Pt Loading and Hydrogen Crossover.

Reducing the use of platinum group metals is crucial for the large-scale deployment of proton exchange membrane (PEM) water electrolysis systems. The optimization of the cathode catalyst layer and decrease of the cathode Pt loading are usually overlooked due to the predominant focus of research on the anode. However, given the close relationship between the rate of hydrogen permeation through the membrane in an operating cell and the local hydrogen concentration near the membrane-cathode interface, the structural design of the cathode catalyst layer is considered to be of pivotal importance for reducing H2 crossover, particularly in combination with the use of thin (≲50 μm) membranes. In this study, we have conducted a detailed investigation on the cathode structural parameters, covering the Pt wt % of the Pt/C electrocatalyst, the type of carbon support (Vulcan and high surface area carbon, HSAC), and the ionomer content, with a goal to reduce Pt loading to 0.025 mgPt/cm2 while minimizing the rate of cell hydrogen crossover. We found that the electrochemical performance is mainly influenced by the changes in the interfacial contact resistance due to variations in the cathode thickness. Both the Pt wt % in Pt/C and the ionomer content showed a positive correlation with the measured H2 in O2% in the anode outlet, whereas the Pt loading exhibited an opposite trend. The rate of hydrogen crossover was analyzed in relation to the calculated local volumetric current density within the cathode catalyst layer. Based on the obtained hydrogen mass transfer coefficient, a cathode catalyst layer comprising 40 wt % Pt on HSAC support with an ionomer-to-carbon (I/C) ratio of 0.35 was found to be an optimum configuration for achieving a low Pt loading of 0.025 mgPt/cm2 and a reduced rate of hydrogen crossover.

Experimental characterization and non-isothermal simulation of a zero-gap alkaline electrolyser with nickel-iron porous electrode

The availability of self-supported porous electrodes with excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic activity has been an important cornerstone for the performance of alkaline water electrolysers (AWEs). In addition, the management of heat and two-phase flow during ion/electron transfer in porous electrodes poses significant challenges to the stable operation of AWE electrolysers. In this work, a two-dimensional non-isothermal multiphysical model considering two-phase flow, mass, heat and charge transfer processes is developed based on COMSOL Multiphysics and well validated with reliable experimental data. The model exhibits excellent agreement with actual electrolyser operating voltages between 323–343 K, and the relative error is within acceptable 2.2% even at high current densities up to 1.5 A cm−2. In addition, the electrode kinetic data obtained based on standard three-electrode experiments allow the model to predict the corresponding HER and OER overpotentials of the porous electrodes with great accuracy, as well as to obtain the distribution of temperature and ionic species inside the porous electrodes under various volumetric flow rates and current densities. This work provides a practical and reliable guide for the design of porous electrodes and the performance enhancement of electrolysers from both experimental and simulation perspectives.

Effect of Parasitic Gas Evolution Reactions on the All-Vanadium Redox Flow Battery Performance

All-vanadium redox flow batteries (VRFB) as the most promising candidates for large-scale energy storage can overcome the inherent defect of natural energy, such as intermittent and fluctuating, et al. During the operation of VRFB, the parasitic gas evolution reactions (GERs) are unavoidable due to thermodynamic limitation, which yet will cause a series of issues, such as imbalance of state of charge (SOC), graphite electrode degradation, security risk, etc. To have a deeper understanding of the parasitic GERs in VRFB, the present work developed a transient, multidimensional, multiphysics VRFB model coupling the multiphase species transport and electrochemical reactions. The model was validated with in-house experimental data and published literature, which showed great consistency. Following, the temporal-spatial characteristics of GERs and their competition with main cell reactions were analyzed in detail under different charging current densities and electrolyte flowrate. The results showed that the rate of hydrogen evolution reaction (HER) at the negative electrode is several magnitudes higher than that of the oxygen evolution reaction (OER) at the positive electrode. Besides, the HER happens during the whole charge-discharge process since the activation overpotential remains negative while the OER occurs only at the end of the charging process under high charging current density. It is notable that the HER mainly occurs in the region near the bipolar plate and increases with increasing charging current density. Since HER is favored thermodynamically over the reduction reaction of trivalent vanadium ions, the activation overpotential of the HER remains much higher than that of the main cell reaction during the whole charge-discharge process, albeit the volumetric current density for hydrogen evolution accounts for only part of the overall current due to the limitation of the reaction kinetics. The HER increases with the SOC and causes a SOC imbalance of 0.54% within 10 charge-discharge cycles, thereby accounting for an 11.0% degradation in the VRFB’s capacity. A high current density decreases the amount of hydrogen generated during a charge-discharge cycle yet also causes hydrogen accumulation in the porous electrode due to faster hydrogen generation. The present work takes a comprehensive and detailed discussion about the characteristics of parasitic gas evolution reactions and their effect on the VRFB performance, which gives a deeper understanding of the energy loss mechanism of VRFB for its development and optimization. Figure 1

Development of methods for controlling the quality of hypochlorous acid solution obtained by electrochemical synthesis

The article presents materials on developing quality control methods for hypochlorite acid obtained by electrochemical synthesis. Electrochemical synthesis of hypochlorous acid was carried out using a combined electrochemical-pyrolytic method. The electrocatalytic activity of the obtained materials in the hypochlorite acid synthesis reaction was evaluated under the electrolysis of 0.05 M NaCl solutions in a duct cell with a Ti cathode at 25 ºС. The cathode area was varied, so the cathode current density was 40 mA/cm2. The pH of the solutions was monitored with a pX-150MI universal ionometer. The volumetric current density in the channel cell was 2.4 A/dm3, and the volumetric rate of solution supply was 9.5 l/h. The authors developed an original method of iodometric titration, which is based on the sequential determination of the content of hypochlorite ions, their total content with chlorite ions, and the total content simultaneously with chlorite and chlorate ions. The content of individual components of the mixture is determined as the difference between the total content of ions and the content of hypochlorite ions. The iodometric method of titration of oxidants is based on the reaction of replacing oxygen-containing compounds of chlorine with elemental iodine with its subsequent titration with standard solutions of sodium thiosulfate. It is proposed to fix the endpoint of the titration potentiometrically, which allows you to speed up the analysis, and it is also relatively easy to automate it using an automatic titrator. The content of oxygen-containing compounds in the solutions depends on the oxidation conditions and the specified operating modes of the electrochemical catalyst. The solution's acidity determines the presence of hypochlorous acid and can be calculated based on the equilibrium constant of its dissociation. The developed analysis methods will allow the control of the quality of veterinary drugs based on hypochlorous acid obtained by electrochemical synthesis.

Vanadium-Doped Ti4O7 Porous Transport Layers for Efficient Electrochemical Oxidation of Industrial Wastewater Contaminants

Inexpensive electrode materials and effective cell designs are needed to advance electrochemical technologies for the oxidative treatment of wastewater. Novel vanadium-doped Ti4O7 porous transport layers (PTLs) used in a compact wastewater electrolyzer are developed and characterized and their performance for the electrochemical oxidation of synthetic wastewater is evaluated. An original analytical model predicting performance with the apparent mass transfer coefficient and cell potential is developed. The influence of operating parameters such as volumetric flow, current density, and PTL composition on performance is investigated. Decolorization and chemical oxygen demand (COD) removal of 100 mg L−1 of methyl orange (MO), an azo dye, in 1,500 mgNaCl L−1 is rapid with mass transfer coefficients as great as 377 ± 24 m s−1 for MO at 15 mA cm−2. After 2.5 Ah L−1 at 10 mA cm−2, >99 decolorization and >98% COD removal are achieved with a current efficiency of 19.2% and with specific and volumetric energy consumption of 120 and 84.1 kWh kg−1 for MO and COD, respectively, and 1.34 ± 0.09 and 6.45 ± 0.97 kWh m−3 order−1, respectively. A more energy-efficient electrochemical cell design for industrial wastewater treatment using less expensive high oxidation power (HOP) electrode materials is demonstrated with these results.

Anodic Dissolution of Surface Layers as a Method of Increasing the Microhardness of Coatings by Alloys of the Iron Group Metals with Tungsten Obtained by Induced Co-Deposition

It is shown that the macroscopic dimensional effect of the composition and properties (microhardness and corrosion resistance) of coatings obtained by induced co-deposition of the iron group metals with tungsten (the effect of the surface area of electrodeposition on the composition and properties) due to the presence of oxide-hydroxide layers in the surface layer, as well as hydrogenation, is a special case of effects of this kind; this, in turn, requires a constant volumetric current density (mA/L) during electrodeposition. It has been established, using examples of electrodeposition of Fe-W and Co-W alloys from a citrate bath, that a change in the volumetric current density at a fixed electrodeposition current density leads to a change in the potential, the current output, and in the composition of the alloy in the coating. Anodic dissolution of the modified surface layer increases the microhardness, but does not eliminate the effect of dependence of the composition and properties of coatings on the electrodeposition surface area.

Scale-up and techno-economic analysis of microbial electrolysis cells for hydrogen production from wastewater

Microbial electrolysis cells (MECs) have demonstrated high-rate H2 production while concurrently treating wastewater, but the transition in scale from laboratory research to systems that can be practically applied has encountered challenges. It has been more than a decade since the first pilot-scale MEC was reported, and in recent years, many attempts have been made to overcome the barriers and move the technology to the market. This study provided a detailed analysis of MEC scale-up efforts and summarized the key factors that should be considered to further develop the technology. We compared the major scale-up configurations and systematically evaluated their performance from both technical and economic perspectives. We characterized how system scale-up impacts the key performance metrics such as volumetric current density and H2 production rate, and we proposed methods to evaluate and optimize system design and fabrication. In addition, preliminary techno-economic analysis indicates that MECs can be profitable in many different market scenarios with or without subsidies. We also provide perspectives on future development needed to transition MEC technology to the marketplace.

Synthesis of Modulated Dielectric Metasurfaces for Precise Antenna Beamforming

This paper presents an end-to-end design method for the synthesis of dielectric metasurfaces with modulated thickness that are able to realize desired radiation patterns in the far field with high precision. The method relies on integral equations and the method of moments to calculate the induced surface and volumetric current densities throughout the metasurface upon a given incident illumination. By avoiding homogenization of the dielectric layer and calculating directly the scattered fields through the method of moments, the near-field and far-field response of the metasurface can be accurately predicted and optimized. In particular, a design example of a modulated dielectric metasurface, fed by a single embedded line source, is presented for the realization of a Chebyshev array pattern with $\ensuremath{-}20$-$\mathrm{dB}$ sidelobe level. Moreover, an externally fed dielectric metasurface performing beam splitting of a normally incident plane wave is designed, three-dimensionally printed and measured, showing two reflected beams at $\ifmmode\pm\else\textpm\fi{}{30}^{\ensuremath{\circ}}$, high suppression of specular reflections ($>14\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$), and satisfactory agreement with the simulated and desired radiation patterns. The proposed design method for beamforming with dielectric metasurfaces can find application in higher frequencies where the use of copper may be problematic due to higher Ohmic losses and fabrication challenges.

Macroscopic Dimensional Effect of the Composition and Properties of Coatings With Alloys of Metals of the Iron-Tungsten Group During Induced Co-Deposition: Mechanism of Formation and Consequences

Using the example of obtaining Co-W alloys it is shown that the observed features of the induced co-deposition, including the macroscopic dimensional effect of composition and properties, as well as nanocrystallinity, are a consequence of the fact that the inducing agent (a metal-precipitator complex) is a polymer complex with a high molecular weight. Under conditions with a high current load on the electrolyte, water molecules participate in the electrode process. This leads to the formation of the surface oxide-hydroxide layers and hydrogenation, as well as to an increase in the concentration of tungsten in the alloy due to a “side” reaction of hydrogen release, alkalinization and polymerization of the inducing agent. At a large-scale transfer from laboratory tests to industrial technology, along with maintaining the usual electrochemical parameters, the presence of a macroscopic dimensional effect (dependence of composition and properties on the surface area of electrodeposition) requires to maintain a constant current load on the electrolyte (volumetric current density).

Flow-through laminar anodes with variable interlaminar distance to modulate the current density of urine-fed bio-electrochemical systems

Three-dimensional (3D) porous anodes used in urine-powered bio-electrochemical applications usually lead to the growth of electro-active bacteria on the outer electrode surface, due to limited microbial access to the internal structure and lack of permeation of culture medium through the entire porous architecture. In this study, we propose the use of 3D monolithic Ti4O7 porous electrodes with controlled laminar structures as microbial anodes for urine-fed bio-electrochemical systems. The interlaminar distance was tuned to modulate the anode surface areas and, thus, the volumetric current densities. To profit from the true area of the electrodes, urine feeding was performed as a continuous flow through the laminar architectures. The system was optimized according to the response surface methodology (RSM). The electrode interlaminar distance and the concentration of urine were selected as independent variables, with the volumetric current density as the output response to optimize. Maximum current densities of 5.2 kA.m−3 were produced from electrodes with 12 µm-interlaminar distance and 10 %v/v urine concentrations. The present study demonstrates the existence of a trade-off between the accesibility to the internal electrode structure and the effective usage of the surface area to maximize the volumetric current density when diluted urine is used as flowing-through feeding fuel.

Scalable membrane-less microbial electrolysis cell with multiple compact electrode assemblies for high performance hydrogen production

Bioelectrochemical hydrogen production via microbial electrolysis cells (MECs) is a promising method for sustainable energy production and decarbonization of energy systems. However, the application of MECs is limited by the electrochemical performance, scalability, and the cost associated with expensive materials. In this study, a scalable MEC (500 mL) with novel compact electrode assemblies and high electrode surface area to volume ratio (160 m2/m3) was designed and constructed. The use of membranes, precious metal catalyst, and current collectors with high costs was avoided. A high current density at the steady state of 49.5 ± 5.3 A/m2 was achieved using acetate as the substrate with phosphate buffer under the applied voltage of 1.01 V. The corresponding volumetric current density was 3948 ± 422 A/m3. The compact electrode assembly design limited methane production rate to 3.9 ± 0.2 L/L/D, while achieving a hydrogen production rate of 33.7 ± 1.7 L/L/D. With the suppression of microbial hydrogen consumption, the hydrogen production rate was 39.8 ± 1.9 L/L/D, higher by almost one order of magnitude than those of MECs with scaling up attempts. The compact electrode configuration reduced internal resistance to 88.5 ± 4.4 Ω cm2. The energy efficiency based on input electricity was 146 ± 7 % to 189 ± 9 % within the applied voltage range of 0.71 to 1.05 V. The results in this study demonstrated successful scaling up of high performance small MECs and offered a new possible approach of scaling up MECs by stacking high-performance subunits, with no trade-offs on electrochemical performance.

Resistance Breakdown of a Membraneless Hydrogen-Bromine Redox Flow Battery.

A key bottleneck to society’s transition to renewable energy is the lack of cost-effective energy storage systems. Hydrogen–bromine redox flow batteries are seen as a promising solution, due to the use of low-cost reactants and highly conductive electrolytes, but market penetration is prevented due to high capital costs, for example due to costly membranes to prevent bromine crossover. Membraneless hydrogen–bromine cells relying on colaminar flows have thus been investigated, showing high power density nearing 1 W/cm2. However, no detailed breakdown of resistance losses has been performed to-date, a knowledge gap which impedes further progress. Here, we characterize such a battery, showing the main sources of loss are the porous cathode, due to both Faradaic and Ohmic losses, followed by Ohmic losses in the electrolyte channel, with all other sources relatively minor contributors. We further develop and fit analytical expressions for the impedance of porous electrodes in high power density electrochemical cells to impedance measurements from our battery, which enabled the detailed cell resistance breakdown and determination of important electrode parameters such as volumetric exchange current density and specific capacitance. The insights developed here will enable improved engineering designs to unlock exceptionally high-power density membraneless flow batteries.

L1-norm vs. L2-norm fitting in optimizing focal multi-channel tES stimulation: linear and semidefinite programming vs. weighted least squares

Background and Objective: This study focuses on Multi-Channel Transcranial Electrical Stimulation, a non-invasive brain method for stimulating neuronal activity under the influence of low-intensity currents. We introduce a mathematical formulation for finding a current pattern that optimizes an L1-norm fit between a given focal target distribution and volumetric current density inside the brain. L1-norm is well-known to favor well-localized or sparse distributions compared to L2-norm (least-squares) fitted estimates.Methods: We present a linear programming approach that performs L1-norm fitting and penalization of the current pattern (L1L1) to control the number of non-zero currents. The optimizer filters a large set of candidate solutions using a two-stage metaheuristic search from a pre-filtered set of candidates.Results: The numerical simulation results obtained with both 8- and 20-channel electrode montages suggest that our hypothesis on the benefits of L1-norm data fitting is valid. Compared to an L1-norm regularized L2-norm fitting (L1L2) via semidefinite programming and weighted Tikhonov least-squares method (TLS), the L1L1 results were overall preferable for maximizing the focused current density at the target position, and the ratio between focused and nuisance current magnitudes.Conclusions: We propose the metaheuristic L1L1 optimization approach as a potential technique to obtain a well-localized stimulus with a controllable magnitude at a given target position. L1L1 finds a current pattern with a steep contrast between the anodal and cathodal electrodes while suppressing the nuisance currents in the brain, hence, providing a potential alternative to modulate the effects of the stimulation, e.g., the sensation experienced by the subject.

The mode of operation over time affects the capacitive properties of a packed anode biocapacitor

AbstractThe relationship between bioelectrochemical systems (BESs) and capacitors has been demonstrated through studies of BESs during intermittent operation. This type of BES has been named a microbial biocapacitor. This new energy storage device has been evaluated for short operational periods lasting for hours in most studies, which leaves open the question of long‐term operational feasibility for the biocapacitor. Moreover, the mode of BES operation before operation as a biocapacitor may determine the capacitive properties of the biofilm since activity or inactivity during electrical current production may affect biofilm response. In the present research, the effects of previous operational mode (rest in open circuit vs. potentiostatic current production) and operation time (seven tests in 94 days) on the capacitive properties of a biocapacitor were evaluated with key parameters such as capacitance, cell voltage, and volumetric current density. Correlations among parameters were analyzed to provide predictive equations for the packed anode biocapacitor. The equations showed that the capacitance increased after periods of biocapacitor inactivity and decreased after periods of activity, which led to equivalent variations in current density.

Quantitative analysis on cold start process of a PEMFC stack with intake manifold

To systematically explore the low-temperature operating characteristics of polymer electrolyte membrane fuel cell (PEMFC) stack, a three-dimensional PEMFC stack model with intake manifold is developed in this study. The characteristics of different cold start modes in the stack are compared and analyzed. The distribution and transmission characteristics of water, ice, and heat in each cell of the stack are analyzed in detail. The location of water accumulation in each cell of the stack is also explored. Finally, finite difference sensitivity is calculated for the cumulated charge transfer density to quantify the effects of operating parameters on the cold start process at low temperature. And how these parameters affect the operation of the PEMFC stack at low temperature is investigated. The results show that inconsistency exists in stack operation due to the position particularity of the intermediate cell. Irreversible heat is the main heat source for the cold start of the stack, and the cathode catalyst layer is the main heat-generating component. The heat production proportion of cathode catalyst layer can reach 90%, which decreases with the increment of current density and the running time, especially for the edge cell. The initial ionomer water content is most sensitive to the cold start process of the stack, followed by the porosity of cathode catalyst layer. These parameters are sensitive to the cold start process mainly because of the change in volumetric exchange current density and oxygen concentration.

Synthesis of MnO2 Carbon nanotubes catalyst with enhanced Oxygen Reduction Reaction for Polymer Electrolyte Membrane Fuel Cell

Polymer Electrolyte Membrane Fuel Cell (PEMFC), an electrochemical power generating technology, uses a precious metal Platinum (Pt) catalyst for Oxygen Reduction Reaction (ORR), which is a major hindrance in its commercialization. Using a non-precious group metal (NPGM) instead of Pt will reduces the cost of PEMFCs. Herein MnO2 carbon nanotubes (CNTs) were synthesized by impregnating the transition metal in large surface carbonaceous material CNTs by hydrothermal synthesis techniques. To enhance the catalytic reaction and increase the volumetric current density, the sample was pyrolyzed at 800 0C temperature under nitrogen atmosphere. During pyrolysis, the nitrogen was also doped in the framework of carbonaceous materials. The materials were then treated with acid, removing the unreacted metals and adding oxygen functional group to the CNT framework due to which the activity of the catalyst is amplified. The catalysts have been characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Catalyst activity has been calculated by Rotating Disc Electrode (RDE) experiment. The resulting materials are stronger in experimental conditions in alkaline environment and have high electro catalytic activity for oxygen reduction reaction (ORR). Linear Sweep Voltammetry (LSV) depicts a current density of - 4.0 mA/cm2 and over potential of -0.3V vs. Standard Calomel Electrode (SCE) in 0.1M KOH electrolyte. Rotating Disk Electrode (RDE) was conducted at 400, 800, 1200, and 1600 rpm. The results of MnO2CNT show a desirable future aspect in fuel cell commercialization.

Two-stage anaerobic digestion with direct electric stimulation of methanogenesis: The effect of a physical barrier to retain biomass on the surface of a carbon cloth-based biocathode

Two-stage anaerobic digestion (AD) is a promising method of converting organic waste into clean and sustainable energy. This work aimed to study the efficiency of a two-stage AD system with the production of hydrogen at the first acidogenic stage and methane in an electromethanogenic reactor. Two types of biocathodes based on carbon cloth were used in the electromethanogenic reactor: with and without retention of biomass on the electrode surface. A high applied voltage continuously supplied to the pair of electrodes was automatically maintained at 2.5 V. Hydrogen yield (0.11 NL/g volatile solids (VS)init or 2 mol H2/mol hexose) and hydrogen content in biogas (52%) were relatively high, even despite the extremely low pH (∼4.0). Improved retention of biomass on the surface of the carbon cloth-based biocathode led to greater stability of the AD process (no water electrolysis was observed) and a significant increase in the efficiency of electromethanogenesis, including the methane yield (by 40.5% up to 0.39 NL/g VSinit), volumetric methane production rate (by 38.8% up to 1.16 NL/L/d) and current density (by 233% up to 0.56 A/m2).

Impact of surface area and current generation of microbial electrolysis cell electrodes inserted into anaerobic digesters

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 < 0.01). Other possible relationships between hydrogen production in MECs, and methane enhancement in AD-MECs, were examined as a function of applied voltage and current density using data from previous studies. For the MECs, there was a high correlation between current density and hydrogen production rate. There was no significant relationship between methane production rate and current density in AD-MECs. A comparison of other factors between MECs and AD-MECs showed large differences in substrate concentration, substrate type (single substrates or wastewaters), and volumetric current densities. Adding electrodes increased methane production on average by 65%, but surface area was a more important contributing factor in AD-MECs than current generation. Based on our analysis of the results in these previous studies, increased area and conductive surfaces provided by the electrodes can have beneficial effects for methane production other than hydrogen gas production by increasing biomass retention and stimulating electrical syntrophy between microorganisms.