Graphite Felt Electrode Research Articles

Electrochemical Evaluation of Different Graphite Felt Electrode Treatments in Full Vanadium Redox Flow Batteries

The use of flow batteries for energy storage has attracted considerable attention with the increased use of renewable resources. It is well known that the performance of a flow battery depends, among other factors, on the properties of the electrodes, which are generally composed of graphite felt (GF). In this work, thermal, chemical and plasma treatments have been employed to modify the surface of the graphite felt to improve the electrochemical activity of the redox flow cell. The influence of the variables of each of these processes on the generation of surface functional groups and on changes in the obtained surface area have been examined. In this work, the kinetics of redox reactions relevant to the VO2+/VO2+ reaction have been studied with these treated electrodes and the relationship between the nature of the surface and electrochemical activity of the GF is discussed. As a result, an enhanced electrochemical performance (reduction over 200 mV of the separation between anodic and cathodic peaks and 110 mV of the onset potential) in comparison to the untreated GF is obtained for those GF treatments with low oxygenated groups concentration.

Kinetic relevancy of surface defects and heteroatom functionalities of carbon electrodes for the vanadium redox reactions in flow batteries

Heteroatom doping of carbon electrodes is an extensively practiced approach to enhance electrokinetics of vanadium redox reactions in flow batteries, because the doped heteroatom functionalities are conventionally considered as the catalytic active sites. In this study, we conducted p type (boron), n type (nitrogen or oxygen), and p-n type (boron and nitrogen) heteroatom doping on graphite felt electrodes, and thoroughly studied their intrinsic electrokinetic effects by collective and quantitative structure-property correlation analysis. The studies reveal that the apparent kinetic enhancements observed by the p type and the n type heteroatom doping are primarily due to the surface lattice defects on carbon electrodes rather than the furnished heteroatom functionalities. Markedly, however, the B and N co-doping give rise to exceptional enhancements in the intrinsic vanadium redox kinetics, both for the VO2+/VO2+ and V2+/V3+ redox reactions, with 2–4 fold greater electrocatalytic activities than those predicted by the increase of electrochemical surface area by the surface lattice defects. Consequently, the p-n type B and N co-doped GF electrodes offers significant enhancements in the efficiency and energy storage capacity of the vanadium redox flow battery that cannot be achieved by the p type (B) or the n type (N and O) heteroatom doping.

N, B codoped graphite felt for high‐performance 1,8‐dihydroxyanthraquinone redox flow battery

AbstractIn this article, a graphite felt electrode was modified by NS double‐element and NB double‐element codoping. The surface morphology, crystal structure, element content, and surface chemical state of the modified electrode were characterized by scanning electron microscopy, X‐ray powder diffraction (XRD), Raman spectroscopy, and X‐ray photoelectron spectroscopy. The electrochemical performance of the modified electrode was evaluated by cyclic voltammetry, electrochemical impedance spectra, and a single cell. The results show that nitrogen (N) and boron (B) double‐element codoping can enhance the catalytic activity of graphite felt and create abundant defect sites, which increases the electrocatalytic activity by approximately a factor of 2. The reason why the doped graphite felt can obtain higher electrocatalytic activity may be related to the synergy between N and B atoms, especially the synergy between pyridine‐N and BC3, and the resistance of the first charge transfer reaction of 1,8‐dihydroxyanthraquinone on the doped graphite felt electrode is then reduced.

Designing high efficiency graphite felt electrode via HNO3 vapor activation towards stable vanadium redox flow battery

A new method of preparing nitric acid vapor treated graphite felt for vanadium redox flow battery (VRFB) is developed. This method outperforms traditional impregnation methods in convenience, efficacy and controllability of fabrication process. A sequence reaction mechanism is proposed for nitric acid vapor treated graphite felt reactions occurring at 200 °C. The modified-graphite felt (m-GF) with excellent electrocatalytic redox reversibility toward VO2+/VO2+ and V2+/V3+ redox couples is prepared in the conditions of VHNO3/mpGF=0.45 mL g−1 at 200 °C in 300 min (m-GF-0.45). The nitric acid vapor reaction with graphite felts occurs preferentially on the surface of amorphous carbon and at defect sites in the graphitic structure, yields N2 and carbon-oxygen surface complexes, resulting in a higher double layer capacity (DLC) from 146 μF mg−1 (p-GF) to 1514 μF mg−1 (m-GF-0.45). Thus, the energy efficiency and discharge capacity of m-GF-0.45 electrode increases significantly by 8.7% and 100% at 150 mA cm−2, respectively.

Green surface treatment of graphite felt using modified TEMPO mediated oxidation for use in vanadium redox flow batteries

MTMO (modified TEMPO((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) mediated oxidation) process was suggested as a green surface treatment to functionalize carboxylic acid group (COOH) on lower crystallinity carbon materials (LCCMs). With the proposed treatment, COOH contents on the surface of graphite felt (GF) was increased twice that of conventional heat-treated GF (CH–GF), resulting in high hydrophilicity without defects on the surface of LCCMs. These are attributed to the mild process feature of LCCMs using pre-activation of TEMPO to N-ammonium TEMPO, which changes hydroxyl groups to COOH without damaging carbon structure even on the LCCMs. Moreover, the MTMO process has environmental-friendly features such as a recyclable oxidizing agent, non-toxic effluents (NaCl and water), and room-temperature (25℃) aqueous process. Benefitting from these advantages, the maximum current densities toward vanadium ion redox reaction (VIRR) improved, and the charge transfer resistance for VIRR also decreased to 38.3 % of CH–GF. The energy efficiency for using MTMO–GF electrodes on both sides also improved by 119.4 % (200 mA cm−2) of using CH–GF during the cycling test of vanadium redox flow battery (VRFB). Furthermore, the VRFB using MTMO–GF electrodes exhibited stable operation for 500 cycles (200 mA cm−2), while VRFB using CH–GF failed at around 300th cycle.

Novel ferrocene-containing amphiphilic block copolymer nanoparticles as high-capacity charge carriers in aqueous redox flow systems

Size-exclusion polymer electrolytes are promising charge carriers to diminish the crossover and allowing commercially available low-cost porous membranes in redox flow batteries. Boosting the solubility in water and maximizing the number of redox sites to enhance the capacity of these polymeric systems is challenging. New highly water dispersed amphiphilic diblock copolymers are reported here, with an average concentration value of 1.7 10−3 mmol of Ferrocene (Fc)-linked moieties per mg of polymer, determined by total X-ray reflection fluorescence. These redox amphiphilic block copolymers are stabilized in water as spherical nanoparticles (20 nm) by using a simple phase solvent inversion procedure. We evidence a maximum polymer dispersibility value of 6 g/L in water, for long-term stable polymer nanoparticle suspensions, yielding a theoretical capacity value of 4.78 mAh at 10.5 mM Fc. Further adjustment of the ionic conductivity and pH of these stable redox block copolymer suspensions has rendered a conductivity value of 44.5 mS/cm at pH values close to a neutral one, by adding a variety of salt supports. Studies using a 3-electrode configuration cell reveal an efficient charge transport between each of the Fc motifs in the polymer nanoparticle. A capacity value of 3.1 mAh with no transient of the polymer nanoparticles crosswise the cheap porous membrane is evidenced when cycled as polycatholyte material in a Zn hybrid aqueous redox flow battery. The particle size and electronic changes of these novel amphiphilic redox block copolymer electrolytes during consecutive redox cycles have also been monitored by dynamic light scattering and ultraviolet-visible spectroscopy, respectively. The analysis of the results enables the understanding of the main mechanisms behind their non-fully reversible capacity. Among them, aggregation and sedimentation, along with retention inside the graphite felt electrode acting the latter as a filter. These insights will aid the design of future polymer electrolyte materials and redox flow battery components with better performance and cost.

Experimental study on efficient electrocatalytic reduction of U(VI) at Amine-Amidoxime bifunctional graphite felt electrode

• The TETA-PAO/GF electrode material was prepared by a simple and green method. • The TETA-PAO/GF electrode material exhibits excellent electrocatalytic reduction of U(VI) under electric field conditions. • The TETA-PAO/GF electrode material has good selectivity and reusability. • The TETA-PAO/GF electrode material can electrocatalytically reduce U(VI) in real seawater under electric field conditions. The amidoxime-based material is one of the effective materials for adsorbing uranium. The electrocatalysis can be used as an effective strategy for separating uranium. In this study, conductive graphite felt was used as substrate, amination-amidoxime modified polyacrylonitrile (PAN) graphite felt was prepared by heterogeneous method. With the successful synthesis of the material with bifunctional groups, the bifunctional modified PAN graphite felt (TETA-PAO/GF) had good uranium adsorption performance by the way of electrocatalysis technology. It was characterized by SEM, FT-IR, XPS, XRD, ICP-MS, etc. The adsorption capacity of the electrode material to uranium reached 1 142 mgU·g -1 in unsaturated state in 1 000 mg·L -1 spiked seawater. In real seawater adsorption experiment, 90.08% uranium was extracted from 16 L seawater. Ultimately, the assessment of the reusability and ion selectivity of TETA-PAO/GF revealed that it has a certain use value for uranium extraction from seawater.

Polydopamine/polypyrrole-modified graphite felt enhances biocompatibility for electroactive bacteria and power density of microbial fuel cell

The interactions between the microbes and the surface of an anode play an important role in capturing the respiratory electrons from bacteria in a microbial fuel cell (MFC). The chemical and electrochemical characteristics of the carbon material affect biofilm growth and direct electron transfer in MFCs. This study examined the electrodeposition of polydopamine (PDA) and polypyrrole (PPY) on graphite felt electrode (GF). The MFC with the modified PDA/PPY-GF reached 920 mW/m2, which was 1.5, 1.17, and 1.18 times higher than those of the GF, PDA-GF, and PPY-GF, respectively. PDA has superior hydrophilicity and adhesive force biofilm formation, while PPY provides electrochemically active sites for microbial electron transfer. Raman spectroscopy, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller surface area measurements, and contact angle analysis revealed the enhanced physicochemical properties of the carbon electrode. These results show that co-doped PDA/PPY provides a strategy for electroactive biofilm development and improves the bioelectrochemical performance in realistic MFC reactors.

Overcoming Voltage Losses in Vanadium Redox Flow Batteries Using WO3 as a Positive Electrode.

Vanadium redox flow batteries (VRFBs) are appealing large-scale energy storage systems due to their unique properties of independent energy/power design. The VRFBs stack design is crucial for technology deployment in power applications. Besides the design, the stack suffers from high voltage losses caused by the electrodes. The introduction of active sites into the electrode to facilitate the reaction kinetic is crucial in boosting the power rate of the VRFBs. Here, an O-rich layer has been applied onto structured graphite felt (GF) by depositing WO3 to increase the oxygen species content. The oxygen species are the active site during the positive reaction (VO2 +/VO2+) in VRFB. The increased electrocatalytic activity is demonstrated by the monoclinic (m)-WO3/GF electrode that minimizes the voltage losses, yielding excellent performance results in terms of power density output and limiting current density (556 mWcm-2@800 mAcm-2). The results confirm that the m-WO3/GF electrode is a promising electrode for high-power in VRFBs, overcoming the performance-limiting issues in a positive half-reaction.

Graphite Felt Electrode Modified by Quaternary Ammonium for Vanadium Redox Flow Battery with an Ultra-Long Cycle Life

Vanadium redox flow batteries (VRFBs) are one of the most attractive devices for grid-scale energy storage due to their advantages of high safety, flexible assembly, and electrolyte-class recycling. However, the conventional graphite felt electrodes usually possess inferior electrocatalytic activity for vanadium ion redox reactions, vastly limiting the rate and lifespans of VRFBs. Herein, we demonstrate a high-rate and ultra-stable vanadium redox flow battery based on quaternary ammonium salt-modified graphite felt electrodes. At a high current density of 200 mA cm−2, the constructed VRFB exhibited a superior cycling life of up to 1000 cycles. This work affords a straightforward approach for developing efficient, environmentally friendly, and low-cost graphite felt electrodes for ultra-stable and high-rate VRFBs.

Vanadium redox flow batteries using new mesoporous nitrogen-doped carbon coated graphite felt electrode

New and economical mesoporous nitrogen (N)-doped carbon structure is prepared using sodium citrate (SC) and urea precursors and then doped onto graphite felt (GF) electrode (SC/U-GF). The catalytic properties of SC/U-GF electrode are compared with those of pristine GF and SC-GF electrodes. When a large amount of mesoporous N-doped carbon is doped onto GF, the catalytic activity of SC/U-GF for improving the reactivity and reversibility of VO2+/VO2+ and V2+/V3+ redox reactions is excellent, and its proper mesoporous structure promotes the mass transfer of vanadium ions. This is proved by measuring its low charge transfer resistance and high active area. When VRFB single cell using SC/U-GF is tested, its energy efficiency (EE) is 80.2 % at 120 mA cm−2, which is 7.8 and 3.1 % better than VRFBs using pristine GF (72.4 %) and SC-GF (77.1 %). Even in a high current density (200 mA cm−2), its EE (71.6 %) is better than VRFBs using pristine GF (59.9 %) and SC-GF (66.4 %). Additionally, EE of VRFBs using SC-GF and SC/U-GF is well maintained (95.6 and 99.6 % preservation after 300 cycles). Based on that, it is obvious that SC/U-GF electrode strongly affects the enhancements in performance and stability of VRFB.

Improved performance of iron-chromium flow batteries using SnO2-coated graphite felt electrodes

Polyacrylonitrile-based graphite felt has the properties of high temperature resistance, corrosion resistance, low thermal conductivity, large surface area and excellent electrical conductivity. It has become the preferred material for flow battery electrodes, but its chemical activity is poor. In order to improve the electrochemical activity of graphite felt electrodes, the electrodes were prepared by SnO2-coated graphite felt. Scanning electron microscopy and X-ray photoelectron spectroscopy were used to analyze the microscopic morphology of SnO2-coated graphite felt electrodes. Electrochemical impedance spectroscopy, cyclic voltammetry and charge-discharge tests were performed using an electrochemical workstation to investigate the electrocatalytic activity of SnO2-coated graphite felt electrodes and their cell performance. The results show that the SnO2 coating on the graphite felt surface forms a convex and concave microstructure, which further increases the specific surface area of the electrode, and at the same time successfully introduces oxygen-containing functional groups to the electrode surface, increasing the electrochemically active spots on the surface. In addition, the presence of oxygen defects in the SnO2 crystal structure provides more electrochemically active sites and improves the electrochemical performance of the graphite felt electrode. At a current density of 142 mA cm−2, the charge-discharge capacity of the battery assembled with the SnO2-coated graphite felt electrode was significantly improved; when the current density was 250 mAcm−2, the Coulombic efficiency of the electrode (TGF-2) coated with a concentration of 0.1 M could reach 84%.

Electrochemical Testing of Carbon Materials as Bromine Electrodes for the Hydrogen-Bromine Redox Flow Battery

Hydrogen-bromine (H2-Br2) redox flow batteries (RFBs) have gained a lot of interest due to their advantages in mitigating the performance shortcomings of conventional zinc-bromine and vanadium flow batteries. Various carbon materials have been tested in H2-Br2 RFBs as bromine electrodes. However, a comparative study among the different carbon materials has not been reported in the literature. This work reports, for the first time, an evaluation of carbon papers, felt and cloth in a three-electrode half-cell setup as potential bromine electrodes, in pristine and thermally treated state. A systematic evaluation was performed by comparing the surface morphologies, kinetic parameters, polarisation curves and stability tests of different carbon electrodes. Thermally treated graphite felt electrode demonstrated the best electrochemical performance as bromine electrode owing to its improved surface area, hydrophilicity and intrinsic activity. Further in-depth studies will shed important insights, which will help understand the electrode characteristics for future bromine battery design. The current study will assist in evaluating the performance of upcoming novel electrode materials in a three-electrode assembly.

Connection of Bipolar Plate with Graphite Felt Electrode for Vanadium Flow Battery Using a Powder Thermal Unification Method

Carbon felt electrodes are usually compressed against the bipolar plates in order to decrease the contact resistance, but it leads to high pressure drop and consumption of pumping energy in conventional vanadium redox flow battery stacks. This paper describes the fabrication of an effective way to unify the bipolar plate and graphite felt electrode in order to improve the battery efficiency at lower felt compression pressure. The unification process involves a thermal fusion technique with a component design consisting of bipolar plate, graphite felt electrode and connecting powder. The powder comprises either polyvinylidene difluoride or a mixture of polyvinylidene difluoride and conductive carbon black. The interfacial bonding substance determines the electrical resistance of the resultant fabricated unified assembly. The electrical resistance results prove that the carbon black proportion in the bonding element substantially reduces the electrical resistance. The optimum quantity of carbon black in the powder is found to be 40 wt%. Micro-computed tomography analysis findings indicate that thermally unified assembly interfacial contact electrode porosity is slightly lower than that of an unbonded sample. A two-cell vanadium redox flow battery stack with integrated unified assembly achieves a rather constant energy efficiency of 76% during 100 cycles with 5% electrode compression at a current density of 80 mA cm−2.

Flexible Carbon Sponge Electrodes for All Vanadium Redox Flow Batteries.

In the present work, a flexible carbon sponge is experimentally characterized and proposed as an alternative electrode for advanced vanadium redox flow batteries. Such an electrode is prepared via directly carbonizing the commercially-available and inexpensive melamine formaldehyde resin sponge in argon, to inherit the well-defined and three-dimensional bi-continuous architecture of the melamine sponge with 99.6% porosity and 40 μm average pore size. By applying the carbon sponges as the electrodes, it is demonstrated that the vanadium flow battery at 200 mA cm-2 can yield an energy efficiency of 77.9%, significantly higher than that with commonly-used graphite felt electrodes (72.9%). After a thermal treatment in air, the energy efficiency of carbon sponge can further be improved to 81.2% at mA cm-2 due to introduction of favorable oxygen containing functionalities. The operating stability with the carbon sponge is proven by a 200 cycling test with minor efficiency decay.

Construction of anodic electron transfer chain based on CuCl2/TiOSO4 synergetic mediators for highly efficient conversion of biomass wastes into electricity at low temperature

Biomass flow fuel cells (BFFCs) are promising alternatives for large-scale conversion of biomass into electricity at low temperature. However, most of them are based on single redox-active materials in anolyte which have limited capacity density and electron transfer rate, resulting in many natural biomass residues can’t generate electricity rapidly and efficiently. Herein, we develop a new anodic electron transfer chain based on CuCl2/TiOSO4 synergetic mediators for BFFCs. The density functional theory (DFT) simulation and experiment results show that TiO2+/Ti3+ redox couple works as an intermediate medium to construct a CuCl2/TiOSO4 synergetic electron transfer chain, which strengthens the electron transfer rate between anolyte and graphite felt (GF) electrode. The CuCl2/TiOSO4 synergetic mediators can efficiently and rapidly degrade glucose, industrial lignin and other natural biomasses at low temperature, and extract electrons simultaneously for generating electricity. The maximum power density (Pmax) of CuCl2/TiOSO4-based BFFC is 146.7 mw·cm−2 when the anolyte is pre-reacted at 90 ℃ for 1 h with glucose as fuel and paired with VO2+/VO2+ catholyte. And the fuel cell can discharge electricity continuously and stably for more than 6 h at 400 mA·cm−2. Furthermore, it can stably power LED pattern with 92 bulbs. Besides, when the CuCl2/TiOSO4-based BFFC utilizes sodium lignosulfonate as fuel, its Pmax is 55.1 mW·cm−2, and the BFFC can discharge electricity stably for more than 1 h at high output current density of 1.3 A·cm−2. This work provides a promising approach for application in direct biomass wastes power generation.

Copper-Coated Graphite Felt as Current Collector for Li-Ion Batteries

Lithium-ion batteries (LIBs) undoubtedly are considered a viable option to meet the ever-increasing demands for portable consumer electronic devices and electric vehicles. To meet this requirement, intensive research is being conducted on increasing the volumetric and gravimetric energy density of LIBs as well as developing high-capacity electrode materials for LIB. In this study, a novel copper-coated graphite felt as a current collector is proposed for use as a constituent of LIB. Different type of Cu-coated graphite felt electrodes were synthesized. They were characterized by X-ray Diffractometer (XRD). To test its electrochemical performance Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry (CVA) techniques were used. Materials with a specific capacity of up to 473 mAh∙g−1 were obtained. It was found that the nature of the capacity gain of carbon felt electrodes differs from that of graphite due to a different crystal structure. The use of a copper coating reduces the charge transfer resistance and increases the capacity of the material. Therefore, such new type of anode materials may be successfully used in LIBs.

Balanced cathodic debromination and hydrogen evolution reactions endow highly selective epoxidation of styrene

The olefin epoxidation is a significant conversion that has sparked the scientific community's interest due to the importance of epoxide in the chemical industry. Conventional techniques produce hazardous oxidants and necessitate hard operating conditions and complicated separation processes. Electro-epoxidation based on Br−/Br2 is considered as an eco-friendly method, while unsatisfied electrode performance limits their selectivity and Faraday efficiency. Here, we develop an economical and highly selective electrochemical strategy for styrene epoxidation using acid and heat modified graphite felt (AHGF) electrodes, which exhibit abundant electrochemical active sites for superior anodic bromide oxidation and facilitated cathodic debromination. After optimizing reaction conditions, we obtain a 99% selectivity of styrene oxide with a Faraday efficiency (FE) of 89% at 30 mA. We discover that the remarkable performance results from the balanced cathodic debromination and hydrogen evolution reactions (HER). We explore this strategy on various styrene-based substrates and believe it has great potential for industrial applications.

Combining Electro-Fenton and adsorption processes for reclamation of textile industry wastewater and modeling by Artificial neural Networks

In the present study, coupling electro-Fenton (EF) and adsorption processes for textile industry wastewater remediation was investigated, in both batch and continuous flow modes. Sepiolite was used as an adsorbent in the coupled EF/adsorption processes. Various parameters such as reaction time, current intensity, Fe2+ concentration, sepiolite dose, and flow rate were found to affect the efficiency of the coupled processes. In comparison to the single EF process, a synergistic effect occurred in the coupled EF/adsorption processes, leading to better performance for COD and TOC removal from textile wastewater. Thus, in the single EF technology, using graphite felt electrodes, COD and TOC removal efficiencies from real textile wastewater, were 58 % and 36 %, respectively. However, in the coupled EF/adsorption processes, COD and TOC removal efficiencies increased to 85 % and to 63 %, respectively. The higher COD and TOC removals may be attributed to the combined effect of adsorption and oxidation reactions in coupled EF/adsorption process. Moreover, Artificial Neural Networks (ANN) model was built up in order to estimate COD and TOC removal efficiencies of the coupled EF/adsorption processes. A good correlation was found between the ANN model theoretical prediction, and the experimental data, for COD and TOC removals, in both batch and continuous modes. The novelty of the current work lies in the synergistic effect occurring between the EF and adsorption processes in wastewater treatment and provides the ANN model as a valuable tool for describing COD and TOC removal efficiencies under different experimental conditions.

Controlled synthesis of carbon nanonetwork wrapped graphite felt electrodes for high-performance vanadium redox flow battery

In this work, nitrogen-doped carbon nanonetwork wrapped dual-function electrode is fabricated in-situ by a simple electrodeposition method. By changing the electrodeposition time and calcination temperature, the morphology of carbon nanonetwork and the presence of nitrogen on graphite felt are controlled. The in-situ wrapped nanonetwork can increase the specific surface area of graphite felt electrode, improve the connectivity between graphite felt and carbon nanonetwork, and reduce the electrode resistance. The composite electrode is prepared without the involvement of adhesive, which enables the carbon nanofibers to exist stably on the graphite felt surface during charging and discharging. At 100 mA cm–2, the energy efficiency of modified cell is 79.2%, which is 8.8% higher than that of blank cell. Using the prepared electrode, VRFB has a discharge capacity of 350 mA h at 250 mA cm–2, while the blank cell cannot work normally. This work proposes a method for preparing a high-performance, high-stability composite electrode, which enables the carbon nanonetwork stably exist on the surface of graphite felt without a binder.