Electrochemical Flow Capacitor Research Articles

Direct observation of carbon slurry flow behavior and its effect on electrochemical performance in a microfluidic electrochemical flow capacitor.

Carbon slurries have been used as "flowable electrodes" in various electrochemical systems, and the slurry flow characteristics play an important role in the system electrochemical performance. For example, in an electrochemical flow capacitor (EFC), activated carbon particles must pass electrical charge from a stationary electrode to surrounding particles via particle-electrode and particle-particle interactions to store energy in the electric double layer. So far, particle behaviors under a continuous flow condition have not been observed due to the slurry's opacity, and studies of the device's performance thus have been mainly on a bulk level. To understand the relation between the hydrodynamic behavior and the electrochemical performance of carbon slurries, we have constructed a microfluidic EFC (μ-EFC) using transparent materials. The μ-EFC allows for direct observation of particle interactions in flowing carbon slurries using high-speed camera recording, and concurrent measurements of the electrochemical performance via chronoamperometry. The results indicate an interesting dependence of the particle cluster interaction on the flowrate, and its effect on the slurry charging/discharging behavior. It is found that an optimal flowrate could exist for better electrochemical performance.

Particle Dynamics-Based Stochastic Modeling of Carbon Particle Charging in the Flow Capacitor Systems

Aqueous electrochemical flow capacitors (EFCs) have demonstrated high-power capabilities and safety at low cost, making them promising energy storage devices for grid applications. A primary performance metric of an EFC is the steady-state electrical current density it can accept or deliver. Performance prediction, design improvements, and up-scaling are areas in which modeling can be useful. In this paper, a novel stochastic superparticle (SP) modeling approach was developed and applied to study the charging of carbon electrodes in the EFC system, using computational superparticles representing real carbon particles. The model estimated the exact values of significant operating parameters of an EFC, such as the number of particles in the flow channel and the number of electrolytic ions per carbon particle. Optimized model parameters were applied to three geometrical designs of an EFC to estimate their performance. The modeling approach allowed study of the charge per carbon particle to form the electric double-layer structure. The linear relationship between the concentration of SPs and the ionic charge was observed when optimized at a constant voltage of 0.75 V. The simulation results are in excellent agreement with experimental data, providing a deep insight into the performance of an EFC and identifying limiting parameters for both engineers and material scientists to consider.

MXene-based suspension electrode with improved energy density for electrochemical flow capacitors

The development of high capacitance materials with high packing density and low viscosity in suspension electrodes is critical for progressing towards high-efficiency, low-footprint electrochemical flow capacitors (EFCs). Here, we report on the first electrochemical and rheological characterization of MXene-based suspension electrodes, using multilayer Ti3C2Tx as the active material and carbon black (CB) as the conductive additive in symmetric and asymmetric EFC devices. In the case of symmetric Ti3C2Tx devices, the Ti3C2Tx concentration is fixed to 22 vol.% in the slurry and the CB concentration is varied from 0.5 to 2.0 vol.%. The symmetric device arrangement offers a high capacitance of 240 F ml−1 (2 mV s−1) and volumetric energy density of 2.65 Wh l−1 @ power density of 47.82 W l−1. Additionally, to extend the potential window, an asymmetric device assembly of activated carbon and Ti3C2Tx is investigated. This arrangement allows a stable operating potential window of 1 V with an energy density of 4.12 Wh l−1 and power density of 31.73 W l−1. Overall, multilayer Ti3C2Tx seems to be excellent candidate for flowable electrode applications, offering high capacitance, energy density and low viscosity due to its high electrochemical activity, excellent electrical conductivity, and versatile surface chemistry.

A Strategy of Designing the High-Voltage Electrochemical Flow Capacitor Via Tetrachlorohydroquinone by Lewis Base Electrocatalysis

A Strategy of Designing the High-Voltage Electrochemical Flow Capacitor Via Tetrachlorohydroquinone by Lewis Base Electrocatalysis

Activated carbon-Polyaniline composite active material slurry electrode for high capacitance, improved rheological performance electrochemical flow capacitor

Conducting polymer polyaniline (PANI) offers very high capacitance but suffers from poor cycling stability. On the other hand, electrochemical flow capacitor (EFC) systems allow replacement of electrode slurry materials whenever needed. Thus using polyaniline as active material in slurry for EFC system can enhance the performance of suspension electrode, and the flow system can itself resolve the issue of poor cyclic stability of polyaniline by replacing it with new fresh slurry whenever it decomposes. However, pristine polyaniline slurry in the flow system is highly prone to decomposition. Hence an activated carbon-polyaniline (AC-PANI) composite active material has been prepared. The porous structure of activated carbon (AC) helps in holding polymer chains upon successive cycling. AC-PANI composite in slurry results in improved capacitive performance 173.8 F-g−1 (139.04 C-g−1) @ 2 mV-s−1 during static slurry testing. After 200 cycles @ 20 mV-s−1, the AC-PANI composite slurry holds 91% capacitance. In the flow test, the composite slurry demonstrates 48% improvement in volumetric capacitance as compared to pristine AC slurry. In addition to this, AC-PANI composite slurry exhibits a gravimetric energy density of 15.46 Wh-kg−1 in static slurry testing mode and a volumetric energy density of 0.43 Wh-L−1 during the intermittent flow test. Moreover, AC-PANI composite slurry also has lower viscosity comparative to pristine AC slurry under the same mass loading. In this way, this AC-PANI composite enjoys the benefit of improved capacitive performance with better rheological behavior for suspension electrode.

Optimization of Electrochemical Flow Capacitor (EFC) design via finite element modeling

Abstract The Electrochemical Flow Capacitor (EFC) is the recently introduced electrical energy storage device for large-scale energy applications. Utilization of high surface area of carbon-based electrode suspended in the electrolyte is the key to the electrical energy storage, by forming a double layer of charges at the interface. In this study, the finite element model was used to examine the behavior of the ionic charge distribution and the maximum current density in EFCs. The current density distribution in the flow channels demonstrated significant dependence on the design and the depth of the flow channel, which ranged from 0.5 mm to 5 mm. The underutilization of the active surface area of carbon continuously increases with increasing channel depth. The geometry of flow channels affected the overall current density by strengthening it at the sharp edges between the current collector and the separating membrane. The geometries based on flow channel shape (circular and box) were tested by varying the depth of the flow channels, current collector shape, and their area in contact with the slurry electrodes to study current density distribution in the flow channel. The experiments were performed with circular flow channels (each 5 mm wide) separated by a 150 µm thick ion-conducting membrane; then, the results were used to optimize the model. The slurry was composed of 0.1 M Na2SO4, 3 wt% Carboxymethyl Cellulose (CMC), and activated carbon powder suspended in water. The studies showed that the maximum activity is observed near the current collector boundary, and slurry underutilization is observed as the distance from the edge increases. Simulations results accorded well with experimental data, including the estimation of the stationary-state current that is achieved during the charging of the slurry.

Energy storage and generation through desalination using flow-electrodes capacitive deionization

Flow electrodes-based capacitive deionization (FCDI) is a highly energy-efficient desalination process compared to conventional processes, including reverse osmosis and multi-stage flash distillation. Furthermore, it provides a continuous desalination flow with high salt-removal capacity, whereas CDI using fixed electrodes requires additional electrode rinsing. In FCDI, salt ions are adsorbed onto flow carbon electrodes and stored in the electrode slurry container, which is similar to an electrochemical flow capacitor. Therefore, stored ions can generate energy during discharging in FCDI operation. In this study, such energy generation was systematically investigated in terms of the various discharging rates, feed concentrations, and volume of flow-electrode slurry. We found that the results were correlated with the changes in the salt concentration of the effluent flow. The discharging current of 50 mA showed the highest capacity for energy generation out of all the experimental conditions; it recovered around 25% of the energy consumed during FCDI desalination. We believe that such energy recovery can greatly reduce the energy consumption needed for FCDI desalination. Such recycling of generated energy would make FCDI more attractive than other conventional desalination techniques.

Characterization Capacitive Desalinzation Cell By Using the FeIII(CN)6 3-/FeII(CN)6 4- Redox Couple As an Electrochemical Probe

Recently, a desalination technique based on porous carbon has been reported [1], merging together desalination process with electrochemical energy storage. Thanks to this new strategy, an important step forward, respect to other water desalination technologies, has been achieved. The principle is based on the capacitive adsorption of Na+ and Cl- ions in the pores of a porous activated carbon, constituting the active material of an electrochemical double layer capacitor electrode. Differently from conventional EDLC, the active material is made in the present case as slurries flowed through an electrochemical cell to achieve a continuous desalination of seat water; such system has been named Electrochemical Flow Capacitor (ELC) [2], [3]. The objective of the study is to improve the energy efficiency of the desalination process by optimizing the cell design and by selecting the proper activated carbon. To do so, we characterized a desalinization cell using different type of electrodes using the FeIII(CN)6 3-/FeII(CN)6 4- redox couple as an electrochemical probe. Our approach allowed to define a dimensionless number θ, which is proportional to the flow rate and inversely proportional to the potential scan rate, giving a guideline to sort out different operating regimes in the flow cell. Two operating regimes have been evidenced: a permanent regime for θ > 60, allowing for higher and efficient charge adsorption, and a non-permanent domain for θ < 60. Références : [1] M. E. Suss, S. Porada, X.Sun, P. M. Biesheuvel, J. Yoon and V. Presser, Energy Environ Sci., 20015, 8, 2296-2319. [2] K. B. Hatzell, E. Iwama, A. Ferris, B. Daffos, K. Urita, T. Tzedakis,F. Chauvet, P. Taberna, Y. Gogotsi, P. Simon, Electrochemistry Communications 43 (2014) 18–21. [3] Y. Gendel, A. K. E. Rommerskirchen, O. David, M. Wessling, Electrochemistry Communications 46 (2014) 152–156.

Review of supercapacitors: Materials and devices

Abstract Supercapacitors have gained a lot of attention due to their unique features like high power, long cycle life and environment-friendly nature. They act as a link for energy-power difference between a traditional capacitor (having high power) and fuel cells/batteries (having high energy storage). In this perspective, a worldwide research has been reported to address this and rapid progress has been achieved in the advancement of fundamental as well as the applied aspects of supercapacitors. Here, a concise description of technologies and working principles of different materials utilized for supercapacitors has been provided. The main focus has been on materials like carbon-based nanomaterials, metal oxides, conducting polymers and their nanocomposites along with some novel materials like metal-organic frameworks, MXenes, metal nitrides, covalent organic frameworks and black phosphorus. The performance of nanocomposites has been analysed by parameters like energy, capacitance, power, cyclic performance and rate capability. Some of the latest supercapacitors such as electrochromic supercapacitor, battery-supercapacitor hybrid device, electrochemical flow capacitor, alternating current line filtering capacitor, micro-supercapacitor, photo-supercapacitor, thermally chargeable supercapacitor, self-healing supercapacitor, piezoelectric and shape memory supercapacitor have also been discussed. This review covers the up-to-date progress achieved in novel materials for supercapacitor electrodes. The latest fabricated symmetric/asymmetric supercapacitors have also been reported.

Low concentrated carbonaceous suspensions assisted with carboxymethyl cellulose as electrode for electrochemical flow capacitor.

The search for efficient energy storage devices has recently led to the introduction of a fluid electrode material employing electrochemical flow capacitors (EFC). Unlike the classical solid electrode film containing capacitors, where the electrode material is fixed to the current collectors and capacitance is therefore limited with an active surface area of porous electrode, the flow electrodes offer new design opportunities which enable fully continuous charging/discharging processes as well as easily scalable systems. Here we describe the successful incorporation of the carboxymethyl cellulose sodium salt (CMC-Na) assisted carbonaceous suspension electrode in aqueous media for the electrochemical flow capacitor concept and demonstrate the electrochemical charge storage in flowable electrodes using a cation conductive membrane as separator in a double-pipe flow-electrode module. Experimental results were combined with computer simulations (FEM) to specify limiting processes EFC charging. The flow-electrode slurry is based on 0.1 M Na2SO4, 3 wt% CMC-Na and activated carbon powder suspended in water. During continuous operation of the system, the capacitance of the flow electrode reached to 0.3 F/L providing the energy and current densities of 7 mWh/kg and 56 mW/L, respectively. Additionally, we report a 70% round trip efficiency calculated during charging and discharging of the cell between 0 V and +0.75 V, while applying the current density of 1.6 mA/kg. The double-pipe flow-electrode module is easily expandable for transportation of large volumes of electrode material.

Computational Modeling of Particle Hydrodynamics and Charging Process for the Flowable Electrodes of Carbon Slurry

Slurry of activated carbon particles mixed with an aqueous electrolyte solution has been used as “flowable electrode” in a few recent electrochemical systems, e.g., electrochemical flow capacitors (EFCs) for energy storage, and flow-electrode capacitive deionization (FCDI) for water treatment. In these applications, the porous carbon particles with very large specific surface area adsorb ions from the electrolyte and meanwhile store electrical charges when a voltage source is added in the charging process. Under the flow condition, the motion of the particles and their mutual contact form a dynamically varying electrical network for the charge transport through the bulk material. We introduce a novel particle-based computational model using the Stokesian dynamics to simulate the hydrodynamic interaction of the carbon particles and the charge transport. An analogous electrical circuit model is developed by approximating the particles with many interconnected resistor-capacitor units, and the circuit’s topology is temporally changing depending on the instantaneous particle configuration. The Stokesian model and the circuit model are solved simultaneously to study how the hydrodynamic interaction and cluster formation affect the charge transport process of the slurry. The presence of the stationary current collector can be included to incorporate the near-wall effects on particle mobility. In the simulation, we vary the particle concentration as well as the ratio of the particle charging time to the hydrodynamic interaction time. The results shows that the charge transport in the carbon slurry is enhanced by increasing the concentration of the particles and faster particle charging. In addition, cluster formation of the particles plays an important role for the electronic transport process. After scaling, the transient electrical current from the present study generally agrees with that from previous experimental and modeling studies.

Reticulated Carbon Electrodes for Improved Charge Transport in Electrochemical Flow Capacitors

In this study, we report on an approach to flow cell design that can enable a significantly improved power output (∼10x) for electrochemical flow capacitors (EFCs), even at large flow channel gaps. Reticulated vitreous carbon (RVC) electrodes of various average pore sizes (0.43–2 mm) were integrated into EFC flow cell fixtures with channel gaps of 5 mm. Electrochemical testing under flow conditions showed a 10-fold improvement in the power density with the RVC integration (290 W/m2, 580 W/kg) for the same slurry composition. This improvement was mostly attributed to the presence of a 3D porous electrode insert (i.e., RVC) that shortened the travel distance of the electrons to the current collectors. Pressure drop in the RVC containing cells was also investigated and found to increase up to 30% depending on the pore size. However, this increase was found to be offset by the increase in the channel depth, yielding almost no change in pressure as compared to conventional narrow-gap (>0.75 mm) flow channels. RVCs having an average pore size of 0.55 mm showed the best performance out of all studied cases with improved coulombic efficiency and good specific capacity (85 F/g) under flowing conditions.

Editors' Choice—Electrochemical Impedance Spectroscopy of Flowing Electrosorptive Slurry Electrodes

In this paper, an electrochemical impedance spectroscopy (EIS) model for flowing electrosorptive slurry electrodes is developed. The model takes into account the current that is induced by the convection of surface charge carried by particles in the slurry. The use of this model facilitates the in situ characterization of critical slurry electrode properties including the conductivity, the electronic diffusivity, and the contact resistance. When applied to experimental results from an electrochemical flow capacitor (EFC), the model allowed for the discernment of a decrease in the slurry's electronic conductivity with increasing flow rate, an increase in contact resistance, and a rise in the electronic diffusivity. The in situ determination of these properties is essential for the selection of optimal slurry compositions and operating conditions for any electrochemical cell that uses flowing slurry electrodes.

Understanding Effective Electronic Conductivity of Slurry Electrodes Used in Electrochemical Flow Capacitors and All-Iron Redox Flow Batteries

Slurry electrodes are being considered in electrochemical systems1-5 for decoupling power and energy in electrochemical flow capacitors (EFCs) and for redox flow batteries (RFBs) involving electrodeposition reactions. In our all-iron slurry flow batteries (IFBs)3,4 utilizing very low-cost electrolyte materials, the slurry electrodes are used in the negative side involving iron plating/de-plating. During charging, iron is plated onto the slurry particles and then the particles carry the iron metal out of the electrochemical cell to be stored in the external electrolyte reservoirs. The slurry electrodes are synthesized by dispersing conductive carbon particles into electrolyte contained dissolved iron species. The effective electronic conductivity of slurry electrodes affects current density distributions, over-potential boundary layers, utilizations of slurry and electrochemical performance of EFCs and IFBs4. Therefore, fundamental studies of slurry electronic conductivity are needed. Possible factors affecting the effective electronic conductivity of a slurry include carbon loading, mean linear velocity or shear rate, electrolyte ionic strength, particle size, particle shape, surface chemistry, etc. In this work, we studied the effect of mean linear velocity through the slurry electrode channel ranging from stagnant to 22 cm/s on slurry effective electronic conductivity for various loadings of 660R carbon black dispersed in deionized water. A DC measurement using a chronoamperometry technique was made in a 3.5 cm2 electrode cell design. Figure 1 shows the variations of effective electronic conductivity for three different loadings, 5% w.t., 10% w.t. and 15% w.t. of 660R carbon black slurry under mean linear velocity increasing from 0 to 22 cm/s. The Figure 1 presents evidence that shear rates induced by different mean linear velocities/flow rates have certain effects on effective electronic conductivity of slurry electrodes. This is may be due to changes in the agglomeration patterns5of slurry particles. The motivation of this work is to explore possible mechanisms that lead to a better understanding of slurry electrodes. The ultimate goal is to improve the effective electronic conductivity of slurry electrodes and optimize the electrochemical performance of EFCs and IFBs. Acknowledgement This work is supported in-part by the all-iron flow battery project (award number: DE-AR0000352) funded by Department of Energy (DOE) of the United States.

Effects of High-Porosity Conductive Foams on the Electrochemical Performance of Electrochemical Flow Capacitors

Electrochemical flow capacitors (EFCs) are high-power, long-cycle life electrochemical systems that utilize flowable suspensions of activated carbon in certain electrolytes. The unique capability of flowable electrodes, which offers ease of scalability, has found many applications not only in the field of energy storage but also in capacitive deionization [1-2]. Currently, one major challenge that prohibits widespread adaptation of EFCs is the low electrical conductivity of the flowable electrodes when compared to the conductivity of the film electrodes. This is mainly caused by the high percentage of the electrolyte phase in the suspension, which is not electrically conductive. Hence, presence of readily formed electron percolation paths is of critical importance to ensure all particles have a viable electron transfer path towards the current collectors [3]. Here, we hypothesized that large pore size conductive foams such as reticulated vitreous carbons (RVCs) could offer a stationary electron transfer backbone for EFC cell fixtures and help decrease the inter-particle electron conduction distance in large channel thicknesses. Contrary to the common approach used in flowable electrochemical systems, currently proposed cell fixtures for flowable electrodes does not involve any high-surface area electrodes due to concerns over clogging and other flow related issues. Motivated by this, this study aims to investigate the feasibility of high-porosity conductive foams as three-dimensional current collectors in EFC cell fixtures. To accomplish this, flow cells containing RVCs selected from different porosities have been subjected to electrochemical testing under static and intermittent flow conditions. For the same electrode composition, a significant improvement in capacity (up to 2x) and in power density (up to 10x) has been observed with the utilization of conductive foams in the cell fixtures.

(Invited) 2D Transition Metal Carbides (MXenes) in Flow-Assisted Electrochemical Energy and Water Technologies

In recent years, 2D transition metal carbides, also known as MXenes, have attracted much attention and found applications in many different areas due to their unique physical structure and properties [1]. Previous studies have also shown that MXene can exhibit excellent potential for electrochemical energy storage and water treatment applications due to its high electronic conductivity, good chemical and structural stability, and high surface area [1-2]. More recently, our group has found significant potential in utilization of MXene electrodes for flow-assisted electrochemical systems such as capacitive deionization (CDI) and electrochemical flow capacitors (EFCs). For CDI, along with high conductivity and surface area, hydrophilicity of MXenes seems to work well for better pore accessibility. Similarly, easy dispersability in water and high surface charge for solution stability were also found to enable much higher active material loading for flowable electrode systems, which can open up a new plethora of possibilities. In this talk, some of our most recent work on these systems, potentials for further improvement, and some of the potential drawbacks will be discussed.

Enabling high-rate electrochemical flow capacitors based on mesoporous carbon microspheres suspension electrodes

Electrochemical flow capacitor (EFC) is a promising technology for grid energy storage, which combines the fast charging/discharging capability of supercapacitors with the scalable energy capacity of flow batteries. In this study, we report a high-power-density EFC using mesoporous carbon microspheres (MCMs) as suspension electrodes. By using a simple yet effective spray-drying technique, monodispersed MCMs with average particle size of 5 μm, high BET surface area of 1150–1267 m2 g−1, large pore volume of 2–4 cm3 g−1 and controllable mesopore size of 7–30 nm have been successfully prepared. The resultant MCMs suspension electrode shows excellent stability and considerable high capacitance of 100 F g−1 and good cycling ability (86% of initial capacitance after 10000 cycles). Specially, the suspension electrode exhibits excellent rate performance with 75% capacitance retention from 2 to 100 mV s−1, significantly higher than that of microporous carbon electrodes (20∼30%), due to the developed mesoporous channels facilitating for rapid ion diffusion. In addition, the electrochemical responses on both negative and positive suspension electrodes are studied, based on which an optimal capacitance matching between them is suggested for large-scale EFC unit.

High-energy-density electrochemical flow capacitors containing quinone derivatives impregnated in nanoporous carbon beads

Employment of nanoporous carbon beads impregnated with redox-active quinone derivatives enhances energy density in electrochemical flow capacitors effectively.

Nitrogen-doped carbon spheres: A new high-energy-density and long-life pseudo-capacitive electrode material for electrochemical flow capacitor

One of the most challenging issues in developing electrochemical flow capacitor (EFC) technology is the design and synthesis of active electrode materials with high energy density and long cycle life. However, in practical cases, the energy density and cycle ability obtained currently cannot meet the practical need. In this work, we propose a new active material, nitrogen-doped carbon spheres (NCSs), as flowable electrodes for EFC application. The NCSs were prepared via one-pot hydrothermal synthesis in the presence of resorcinol/formaldehyde as carbon precursors and melamine as nitrogen precursor, followed by carbonization in nitrogen flow at various temperatures. The results of EFC experiments demonstrate that NCSs obtained at 800°C exhibit a high energy density of 13.5Whkg−1 and an excellent cycle ability, indicating the superiority of NCSs for EFC application.

Capacity Enhancement of Electrochemical Flow Capacitor Using Quinonic Compounds Couple

The semi-solid flow cell system, consisting of an electrochemical charge-discharge cell connected to an external circulatory system of pumps and reservoirs, has been proposed as the grid-scale energy storage. Among the flow cells, electrochemical flow capacitor (EFC) stores electrical charge in an electrical double layer at the carbon-electrolyte interface in a flowable carbon-electrolyte suspension (slurry), so the EFC exhibits higher power density than that of semi-solid flow battery [V. Presser, et al., Adv. Energy Mater, 2, 895 (2012).]. However, compared to the battery system, EFC has the limited energy density only derived from electrical double layer capacitance.  Our previous study revealed that the impregnation of redox-active organic compounds into the nanoporous carbon enable to enhance the high energy density (＞20 Wh kg-1) of aqueous electrochemical capacitor with maintaining the high power density (＞5 A g-1) and long cycle lifetime (more than 10000 cycles) [T. Tomai, S. Mitani, D. Komatsu, Y. Kawaguchi, I. Honma., Sci. Rep., 4, 3591, (2014).]. In this study, to address the limitation of the energy density of EFC, we employed the nanoporous carbon beads holding quinonic/hydroquinonic compounds to carbon-electrolyte suspension (quinone/carbon complex slurry) for both flowable electrodes of the EFC. The flowable quinonic/hydroquinonic slurry electrode was prepared by dispersing the organic compound-carbon composite in 0.5 M H2SO4 and 0.05 M HCl aqueous solution. We employed tetrachlorohydroquinone (TCHQ) as cathode active material and dichloroanthraquinone (DCAQ) as anode active material in positive and negative slurry electrodes, respectively. The weight ratio of the quinonic/hydroquinonic compound to the nanoporous carbon bead was fixed to be 3:7. The weight ratio of the organic compound-carbon composite to 0.5 M H2SO4 and 0.05 M HCl aqueous solution was fixed to be 13:87. Charge-discharge measurement at static operation, the EFC using the quinone/carbon complex slurry shows the high energy density (19.4 Wh/kg) in the applied potential range between 0-1 V. This value is about two times larger than that of the EFC using the unmodified carbon slurry. Thus, by using the redox reactions of quinonic/hydroquinonic compounds in positive and negative slurry electrodes, we achieved the enhancement of the energy density of EFC. Notably, the EFC using TCHQ/carbon complex slurry and DCHQ/carbon complex slurry for both electrodes investigated in this study exhibits 1.6 times higher energy density than the EFC using only hydroquinone/carbon complex slurry [M. Boota, K. B. Hatzel, E. C. Kumbur, Y. Gogotsi., ChemSusChem 2015, 8, 835.]. Moreover, charge-discharge measurement at intermittent flow operation, we demonstrated the prepared slurry electrodes were applicable to semi-solid flow system.