Electrode For Vanadium Redox Flow Battery Research Articles

Engineering potential electrocatalysts for both AEM Electrolyzers and Redox Flow Batteries: Design of experiments at the different scales

Both water electrolysis and electrochemical energy storage are promising technologies for dealing with the intermittency of green electricity generation. In this work we investigated the possibility of using the same electro-catalyst for hydrogen production by Anion Exchange Membrane Water Electrolysis (AEMWE) and energy storage by Redox Flow Batteries (RFB). A complete experimental campaign was designed to investigate the effects of the catalyst nanostructure and morphology on the electrode performance at different scales. Focus was devoted to the choice of the main factors influencing the catalyst structure, its morphology and electrochemical activity; taking further into account the technique to deposit the catalyst on the Gas Diffusion Layer (GDL) and its integration inside the Membrane Electrode Assembly (MEA). Some results are reported for a spinel-based NiCo2O4 catalyst, chosen as a base nanocomposite to develop more complex materials for both the Oxygen Evolution Reaction (OER) in AEMWE and the positive electrode in Vanadium RFB in the future.

High Electrocatalytic Activities of the Electrochemically Exfoliated Graphene Deposited Carbon Paper Electrodes for Vanadium Redox Flow Batteries

The cost-effective and process-efficient preparation of highly electrocatalytic carbon electrodes on a large scale is crucial for enhancing the competitiveness of vanadium redox flow batteries (VRFBs). In this work, electrochemically exfoliated graphene oxides (Exf-GOs) are prepared form graphite foil at large scale with a multi-electrodes equipped continuous Exf-GOs production system. The Exf-GOs are deposited on the activated carbon paper (A-CP) substrate by a direct infiltration-deposition approach. Collective electro-kinetics analysis applying cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) revealed that the resulting Exf-GO/A-CP electrodes exhibit two orders magnitude greater intrinsic electrocatalytic activities than the A-CP electrode for the VO2+/VO2 + redox reactions. A trade-off between the Exf-GO deposition amount and the open pore volume through the Exf-GO/A-CP electrodes was found that have critical impacts on the overall electro-kinetic performance of the electrodes. Compared to the A-CP electrode, the Exf-GO/A-CP electrode provided the VRFB with more than two-fold enhancements in the energy storage capacity and at least 10 – 15 % higher voltage and energy efficiencies. The results collectively demonstrate the high effectiveness of the Exf-GO/A-CP electrodes for VRFBs that can be prepared cost and process efficient approach at large scales.

BiVO4-Decorated Graphite Felt as Highly Efficient Negative Electrode for All-Vanadium Redox Flow Batteries

Recently, discovering high-performance electrocatalytic materials for vanadium redox flow batteries (VRFB) has been one of the most crucial tasks. This paper successfully fabricates low-cost plate-like bismuth vanadate (BiVO4) material through a simple one-step hydrothermal route, employed as an electrocatalyst to adorn the graphite felt (GF) for use as the negative electrode in VRFB. The experimental results show that BiVO4-3h exhibits the optimal electrocatalytic activity and reversibility for the vanadium redox couples among all samples. The energy efficiency of the VRFB cell assembled with BiVO4-decorated GF as the negative electrode is found to be 75.42% at 100 mA cm−2, which is about 10.24% more efficient than that of the cell assembled with heat-treated graphite felt (HT-GF) electrode. The possible reasons for the activity enhancement can be ascribed to the existence of oxygen vacancies in the BiVO4 lattice structure and the relatively high surface area of BiVO4, which provide more active sites for facilitating the vanadium redox reactions. Furthermore, the BiVO4-GF electrode obstructs the competitive irreversible hydrogen evolution reaction on the negative side of the cell, and it also has better wettability. Impressively, BiVO4-GF as the negative electrode shows good stability over 100 cycles.

3-Dimensional numerical study on carbon based electrodes for vanadium redox flow battery

Electrode materials are critical in determining the performance of Vanadium redox flow batteries (VRFB). This work aims at elucidating the difference between carbon-based electrodes for VRFB. The electrodes considered for the present study are Graphite felt, Carbon felt, Carbon paper, and Carbon cloth. To study the characteristic behavior of different electrode materials, a 3D unit cell numerical model was developed by coupling electrolyte flow and electrochemical reactions and mass transfer module apart from considering the electrode structural and material information such as porosity, permeability, thickness, conductivity, and specific area. The simulation results of this study suggest that the Graphite felt is the optimum choice as an electrode material for VRFB as it offers higher electrocatalytic activity, lower pressure drop, better performance, and lower cost. The study is also extended to a multilayer configuration of Carbon Paper and Carbon Cloth. The performance of multi-layered electrodes by stacking single sheets of Carbon Paper and Carbon Cloth electrodes are matched to meet the performance obtained with the Graphite felt electrode. It has been shown with the help of polarization curve and the pressure drop study that the performance of five layers of carbon cloth and nine layers of carbon paper match the performance of Graphite felt, approximately. The present study provides practical guidance for the design and optimization of VRFB electrodes.

Recent Progress in our Understanding of the Degradation of Carbon‐Based Electrodes in Vanadium Redox Flow Batteries – Current Status and Next Steps

AbstractThis mini‐review summarises and discusses recent findings form the literature on the degradation of carbon‐based electrodes for vanadium redox flow batteries (VRFBs). It becomes evident that the focus of current investigations is on carbon paper, carbon felt and graphite felt electrodes, which is understandable from a practical point of view. However, the structural complexity of these materials often prohibits doubtless attribution of observed performance reduction (or increase) to changes in the electrode materials. Among the discussed major causes for degradation are formation or change of surface functional groups, changes in the carbon sp2/sp3 ratio, intercalation of ions as well as formation of inhibiting adsorbates. In order to gain deeper insight into the changes of carbon electrodes in VRFBs under relevant operation conditions, the authors suggest reducing complexity of the investigated materials and applying in situ‐studies under well‐defined and controllable conditions on model electrodes. These studies then should be extended towards more practical systems and may finally help to reduce degradation phenomena including enhanced overvoltages and thus could improve cycling and energy efficiency as well as long‐term stability of vanadium redox flow batteries.

Multiple‐dimensioned defect engineering for graphite felt electrode of vanadium redox flow battery

AbstractThe scarcity of wettability, insufficient active sites, and low surface area of graphite felt (GF) have long been suppressing the performance of vanadium redox flow batteries (VRFBs). Herein, an ultra‐homogeneous multiple‐dimensioned defect, including nano‐scale etching and atomic‐scale N, O co‐doping, was used to modify GF by the molten salt system. NH4Cl and KClO3 were added simultaneously to the system to obtain porous N/O co‐doped electrode (GF/ON), where KClO3 was used to ultra‐homogeneously etch, and O‐functionalize electrode, and NH4Cl was used as N dopant, respectively. GF/ON presents better electrochemical catalysis for VO2+/VO2+ and V3+/V2+ reactions than only O‐functionalized electrodes (GF/O) and GF. The enhanced electrochemical properties are attributed to an increase in active sites, surface area, and wettability, as well as the synergistic effect of N and O, which is also supported by the density functional theory calculations. Further, the cell using GF/ON shows higher discharge capacity, energy efficiency, and stability for cycling performance than the pristine cell at 140 mA cm−2 for 200 cycles. Moreover, the energy efficiency of the modified cell is increased by 9.7% from 55.2% for the pristine cell at 260 mA cm−2. Such an ultra‐homogeneous etching with N and O co‐doping through “boiling” molten salt medium provides an effective and practical application potential way to prepare superior electrodes for VRFB.

Nitrogen, phosphorus, and sulfur co-doped carbon nanotubes/melamine foam composite electrode for high-performance vanadium redox flow battery

The high cost and complex modification process of carbon felt electrodes limits its further popularization in vanadium redox flow batteries (VFBs). By introducing low-cost melamine foam, nitrogen, phosphorus, and sulfur co-doped carbon nanotubes/ melamine foam composite electrode (NPS-CNTs-CMF) is designed and fabricated via immersing melamine foam in a solution containing N, P, and S co-doped CNTs. The integration of modified CNTs significantly enhances the conductivity and hydrophilicity of the electrode. Moreover, the composite electrode also demonstrates outstanding electrocatalytic activity owing to the heteroatom doping that further inspired the electrocatalytic activity of CNTs. Density function theory calculations further uncover that introducing heteroatoms on CNTs not only promotes the adsorption of vanadium ions but also facilitates the electron transfer between vanadium ions and MF substrate. As a result, the battery loading with NPS-CNTs-CMF exhibits excellent battery performance, achieving energy efficiency of 80.12 % at 300 mA cm–2. Additionally, the long-term cycling stability is attained over 200 consecutive charge-discharge cycles at 300 mA cm−2. This study provides a novel melamine foam material with low cost and simple modification, and this new composite structure stimulates the development of high-performance electrodes in VFBs.

Molten salt carbon felt oxidation for VRFB electrode performance improvement

In this study, carbon felt (CF) electrodes were chemically treated using alkali metal salt (potassium nitrate) in the molten state as a mild oxidizing agent. Various temperatures in the range of 380–500 °C and oxidation duration (up to 12 h) were analyzed in order to find the most appropriate oxidation conditions. As-prepared carbon felts were electrochemically investigated in a three-electrode set-up as well as in a lab-scale vanadium redox flow battery (VRFB). The battery set-up included additional in-situ potential measurements allowing to measure the polarization of each electrode separately and compare the performance of heat- and chemically-treated electrodes. The physico-chemical properties of CFs have been analyzed using X-ray photoelectron spectroscopy (XPS) and other advanced techniques. Combination of extended oxidation time with temperature increase causes satisfactory surface functionalization and enhanced vanadium redox reaction reversibility. The chemically treated samples showed a better performance with lower overpotentials and higher efficiencies. Improved reversibility of V2+/V3+ reaction is obtained for the carbon felt treated with salt presence. What is more, a 4 times reduced CF electrode oxidation time as usually presented in literature (12 h for heat-treated electrodes vs. 3 h for salt- and heat-treated ones) can be achieved with nitrate salt oxidation.

Graphene Functionalized Carbon Felt/Graphite Felt Fabrication as Electrodes for Vanadium Redox Flow Batteries (VRBs): A Review

The growth in the development of renewable energy sources has led to tremendous attention to the research in energy storage systems. One of the electrochemical energy storage systems that have shown great potential to be used on a large scale is vanadium redox flow batteries (VRBs), as they possess flexible designs, long life cycles, and high energy density. Carbon felts (CF), and graphite felts (GF) have commonly been used as electrodes in VRBs. To improve market penetration using VRB technology, researchers have focused on electrode modifications to increase the power density and rate capabilities of VRBs. One of the carbon-based modifications which have shown significant improvements in the performance of VRBs is the use of graphene, which has outstanding electrochemical and physical characteristics as an electrocatalyst. In this review, electrochemical, physical, and other methods which have been reported in the graphene functionalization of graphite felt/carbon felt are discussed. The working principle and limiting methods were elaborated on and discussed for each method. Finally, recommendations for future developments are also highlighted.

BiVO4-Decorated Graphite Felt as Highly Efficient Negative Electrode for All-Vanadium Redox Flow Batteries

Recently, discovering high-performance electrocatalytic materials for vanadium redox flow batteries (VRFBs) has been one of the most crucial tasks. This paper details the successful fabrication of a low-cost platelike bismuth vanadate (BiVO4) material through a simple one-step hydrothermal route, employed as an electrocatalyst to adorn graphite felt (GF) for use as the negative electrode in VRFBs. The experimental results show that BiVO4-3h exhibits the optimal electrocatalytic activity and reversibility for the vanadium redox couples among all samples. The energy efficiency of the VRFB cell assembled with BiVO4-decorated GF as the negative electrode is found to be 75.42% at 100 mA cm–2, which is about 10.24% more efficient than that of the cell assembled with a heat-treated graphite felt (HT-GF) electrode. The possible reasons for the activity enhancement can be ascribed to the existence of oxygen vacancies in the BiVO4 lattice structure and the relatively high surface area of BiVO4, which provide more active sites for facilitating the vanadium redox reactions. Furthermore, the BiVO4-GF electrode obstructs the competitive irreversible hydrogen evolution reaction on the negative side of the cell, and it also has better wettability. Impressively, BiVO4-GF as the negative electrode shows good stability over 100 cycles.

Surface Treatment with CO2 to Improve Electrochemical Characteristics of Carbon Felt Electrode for VRFB

<p>The carbon felt is usually hired as electrodes for vanadium redox flow battery (VRFB). In the study, surface modification of carbon felt under CO<sub>2</sub> atmosphere with variables of operating various temperature ranges between 700<italic>°C</italic> and 900<italic>°C</italic>. The qualitative and quantitative analysis were carried out such as scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) to observe degree of surface modification. Result of XPS analysis confirmed increase of carbon and oxidation functional group on the surface with increase of temperature. SEM image was discovered similar phenomena. Electrochemical characteristics such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) revealed the improved electrode performance with increase of temperature. However, the electrochemical performance under treatments temperature of 900<italic>°C</italic> was less than that of under treatment temperature of 850<italic>°C</italic> due to weight loss at the treatment temperature of 900<italic>°C</italic>. From the CV and EIS results, the best electrochemical characteristics was at the temperature of 850<italic>°C</italic>. That of at the temperature of 900<italic>°C</italic> was decreased due to weight loss. The energy efficiencies (EE) obtained from full cell test were 69.37, 80.76, 82.45, and 75.47%, at the temperature of 700, 800, 850, and 900<italic>°C</italic>, respectively.</p>

High-activity and stability graphite felt supported by Fe, N, S co-doped carbon nanofibers derived from bimetal-organic framework for vanadium redox flow battery

High energy efficiency and cycle stability have always been the focus of research on vanadium redox flow battery (VRFB). It is noted that metal–organic framework (MOF) has many advantages due to porosity, controllable structure, and functional modification. Therefore, in this paper, a composite electrode with Fe, N, S co-doped carbon nanofibers in situ supported on graphite felt surface (GF@Fe-N/S-CNFs) was prepared by using bimetal-organic framework as precursor. GF@Fe-N/S-CNFs possess large specific surface area, abundant active centers, and high electrical conductivity, which exhibits excellent electrocatalytic activity for both VO2+/VO2+ and V2+/V3+ redox reactions. At 200 mA cm−2, energy efficiency and voltage efficiency of the modified battery using GF@Fe-N/S-CNFs are 71.7 % and 75.8 %, respectively, which are 9.7 % and 10.6 % higher than those of blank battery. Modified battery can still run smoothly at high current density of 300 mA cm−2. In addition, energy efficiency holds excellent steady during 350 charge–discharge cycles at 150 mA cm−2. Hence, this work brings a promising vision of MOF derivant for high-performance electrode of VRFB.

Characterizations and selections of electrodes with optimal performance for large-scale vanadium redox flow batteries through lab-scale experiments

Conventionally, electrodes for vanadium redox flow batteries (VRFBs) are characterized in lab-scale single cells. However, electrodes selected in such a manner may not provide the best performance in practical large-scale battery stacks, as the flow rates and electrode compression ratios in large stacks are usually much lower than those in single cells. In this work, we study the essential effects of flow rates and electrode compression ratios on the characterizations and selections of electrodes for VRFBs. It is found that the working conditions-induced performance reversal is the reason that leads to the different results in the optimal electrodes between large-scale battery stacks and lab-scale single cells. Hence, the working condition should be considered when evaluating electrodes for VRFBs with lab-scale single cells, especially under low flow rate and compression ratio. This work provides valuable insight and guidance for electrode evaluation with lab-scale single cells, which promotes lab studies to engineering applications.

A high-performance ammonia plasma-treated WO3 @carbon felt electrode for vanadium redox flow batteries

Due to the pressing need for harnessing renewable energy, sizable energy storage technologies have become increasingly critical, among which vanadium redox flow batteries (VRFBs) are considered as one of the promising technologies. However, the lack of high-performance electrodes hinders the development of VRFBs. Herein, we report a new ammonia plasma-treated WO3@carbon felt as a high-performance electrode for VRFBs. The ammonia plasma introduces not only N-contained functional groups but also oxygen deficiencies on WO3, which provide additional active sites and improve the conductivity, leading to high catalysis for both cathodic and anodic vanadium redoxes. As a result, the energy efficiency and the power density of the VRFB increase from 78.9% to 86% and from 365.5 mWcm[Formula: see text] to 389.6 mWcm[Formula: see text], respectively. Moreover, the energy efficiency of composite electrodes remains stable for more than 300 cycles. This study provides a new strategy for designing cost-effective, environmentally friendly, and high-performance electrodes for future VRFBs.

V2O5-Activated Graphite Felt with Enhanced Activity for Vanadium Redox Flow Battery

In this study, a simple and environment-friendly method of preparing activated graphite felt (GF) for a vanadium redox flow battery (VRFB) by depositing the vanadium precursor on the GF surface and calcining vanadium oxide was explored. The intermediate material, VO2, generated carbon oxidation during the calcination. In contrast to the normal etching method, this method was simple and without a pickling process. On the surface of the activated GF, multiple pores and increased roughness were noted after the calcination temperature and surface area of the activated GF reached 350 °C to 400 °C and 17.11 m2/g, respectively. Additionally, the polarization of the activated GF decreased with resistance to the charge transfer at 0.27 Ω. After a single-cell test at current density of 150 mA/cm2 was performed, the capacity utilization and the capacity retention after 50 cycles reached 70% and 84%, respectively. These results indicated the potential use of activated GF as an VRFB electrode.

Synthesis of Phosphorus Doped Graphenes via the Yucel’s Method as the Positive Electrode of a Vanadium Redox Flow Battery

Improving electrode performances in the high-current cycles is essential to increase the global competitiveness of vanadium redox flow battery (VRFB), a widespread application potential for large-scale energy storage systems. Modifying carbon electrode surfaces with graphene-based materials and even doping of graphene with heteroatoms is a reasonably critical strategy to enhance the reaction activity and reversibility of vanadium redox couples. In this study, phosphorus-doped graphene-based electrodes were produced via Yucel’s method in phosphoric acid solution, and their use as positive electrodes for VRFB was investigated. Various electrochemical, spectroscopic, and morphologic characterization methods were used to analyze the electrodes. The effects of P-doped graphene-based materials as the positive electrode of VRFB were investigated via electrochemical methods such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and cyclic charge-discharge. The VRFB with phosphorous doped graphene-based electrodes prepared in the potential range between −1.0 and +2.1 V (P-GPGE1) exhibited higher discharge capacity as 1.11 Ah l−1, and energy efficiency as 73.1% at the charge and discharge current densities of 8 and 1.6 mA.cm−2, respectively. This paper reveals a promising way to produce phosphorous doped graphene-based electrodes for the next-generation VRFB, a high-power, stable, and efficient operation.

Surface Modification of Carbon-Based Electrodes for Vanadium Redox Flow Batteries

The vanadium redox flow battery (VRFB) is one of the promising large-scale energy storage technologies. The electrode is one of the key components of the VRFB, and its design has an important effect on its electrochemical redox kinetics and battery performance. The ideal VRFB electrode material has high catalytic activity, good conductivity, and high stability. Carbon-based electrodes are the most commonly used electrode materials for VRFBs. However, its reaction kinetics and catalytic active sites are low. This paper summarizes the methods of carbon-based electrode modification of the electrochemical performance in VRFBs. Three representative methods including metal/metal oxide modification, nonmetal atom modification, and defect engineering for graphite felt in VRFBs are generally reviewed. First, metal and metal oxides have high catalytic activity, which can enhance the chemical reaction process of vanadium ions and greatly improve the reversibility of the redox reaction of vanadium ions. Then, nonmetal atom modification can generally improve the hydrophilicity of carbon-based electrodes, enhance the adsorption/desorption capacity of vanadium ions, and improve the reaction kinetics. Moreover, defect engineering can lead to the formation of the micropore structure on the surface and increase the specific surface area of the material, thus generating more redox reaction active sites. Finally, the development direction of electrode modification in VRFBs is prospected, and it is expected that this review will provide useful insights into the development of VRFBs.

In-situ functionalization of binder-free three-dimensional boron-doped mesoporous graphene electrocatalyst as a high-performance electrode material for all-vanadium redox flow batteries

The development of highly efficient and stable nanostructured electrocatalysts, capable of operating at a high current density is crucial to the broader market penetration of vanadium redox flow batteries (VRFBs). In this report, three-dimensional (3D) boron-doped mesoporous graphene functionalized carbon felt (BMG-CF) is fabricated and tested as the positive and negative electrodes for VRFB. Morphological results show that BMG-CF exhibits a homogenous distribution of boron atoms and the electrochemical testing indicates outstanding electrocatalytic activity towards VO2+/VO2+ and V2+/V3+ redox couples compared to activated-CF (A-CF) and mesoporous graphene-CF (MG-CF), ascribed to introduction of B-doped mesoporous structures and high electrical conductivity. Notably, BMG-CF attain energy efficiencies (EE) of 81.5% and 74.4% at 100 mA cm−2 and 150 mA cm−2, which are 9.4% (3.0%) and 17.3% (4.3%) higher than A-CF (MG-CF) electrodes. Furthermore, the battery can be operated at very high current densities of 175 mA cm−2 and 225 mA cm−2 with EE of 70.7% and 60.0% and exhibit excellent cycle stability for more than 100 cycles at 100 mA cm−2 with superior rate capability at current densities of 50–225 mA cm−2. The above excellent results demonstrate the practical applicability of the highly efficient and stable 3D BMG-CFs as promising electrodes for VRFB.

Impedance-Resolved Electrochemical Performance of Treated Carbon Felt Electrodes in Vanadium Redox Flow Batteries: Model Error Estimation

Vanadium redox flow batteries (VRFBs) have been targeted as a potential technology for integration of renewable energy sources onto grids as well as improving grid-level power generation efficiency. The primary reasons VRFBs are well-suited for this role are on account of their modularity, scalability, and relatively high performance [1]. There are still, however, many design parameters that can be more highly optimized to improve and specialize aspects of this technology. The kinetics at the anode of VRFBS utilizing graphite felt electrodes on are generally more limiting than at the cathode [2] as well as showing more rapid signs of performance degradation and are as such an area where sources of losses can be improved significantly.Electrochemical impedance spectroscopy is a commonly used method for characterizing sources of losses in VRFB systems. Impedance spectra are often analyzed in the context of the theory of Randles circuits, where electrochemical phenomena are treated analogously to circuit elements. Data reduction equations can then quantify these circuit elements and subsequently determine the values of physical parameters of interest in the system. The Randles circuit, however, has no geometric framework, nor any term which can capture the physical behavior of electrolyte resistance that is distributed throughout a porous medium. A model which was developed by Paasch and co-workers [3], referred to as the macrohomogeneous porous electrode (MHPE) model, addresses this phenomenon and has been used by others to characterize EIS data in systems with porous electrode where electrolyte resistance is non-negligible. Nguyen [4] et. al furthered this model in the context of inhomogeneous porous electrodes and membranes, Sun and co-workers [5] were the first to demonstrate the single MHPE model in the context of the all vanadium chemistry and Pezeshki and co-workers applied the model to quantify the effects of cell architecture in VRFBs [6].Much work on improving carbon felt-based electrodes in VRFBs does so by attempting to make charge transfer more facile [7]. As the charge transfer becomes smaller the effect of the distributed ohmic (pore) resistance begins to account for a larger portion of the cell losses and neglecting to account for it can misrepresent the series ohmic resistance (primarily membrane) and the charge transfer resistance.This work looks at the impact of using three different EIS models on different electrodes (seven treatments and one control) and the impact on the fitted parameters based on the utilization of a specific model. The first two models are both Randles circuits, which differ in the portions of the frequency spectrum which they are using. The first Randles circuit utilizes the full frequency spectrum referred to as RFS model (Randles Full Spectrum), the second Randles circuit truncates the spectrum, referred to as RTS model (Randles Truncated Spectrum), and excludes the distributed ohmic (pore) resistance, and third model is the aforementioned MHPE model. The electrode treatments include a nitric acid soak, heat treatment in 3 different gaseous environments (Oxygen, Nitrogen and Ammonia), and hydrothermal treatments in a nitric acid | sulfuric acid environment for 3 different durations. Electrodes were tested in-situ and cycled to characterize beginning-of-life stability.The results show that there are non-trivial tradeoffs between utilizing the different models. The serial ohmic (membrane) resistance cannot be predicted accurately by either of the Randles circuit models (RFS and RTS) for all of the electrode treatments where it is overestimated as seen in Figure 1. Further, if the RFS model is used, it can yield kinetics parameters that differ from the MHPE model by over 50%. However, the RTS model is in relatively high agreement with the MHPE model for kinetics parameters and mass transfer parameters that can be extracted from EIS data. Over the course of cycling the electrodes treated in the ammonia atmosphere demonstrated both the highest stability and the lowest charge transfer resistance of all of the tested electrodes (see Figure 2). Figure 1

Highly Energy-Efficient Vanadium Redox Flow Batteries with Thermo-Chemically Activated Graphite Felt Electrodes

The Vanadium Redox Flow Battery (VRFB) is a promising energy storage technology for large scale stationary applications that require a long cycle life of over 10 years. The specific feature of VRFB is the slower kinetics of redox reactions (V2+/V3+) on the negative electrode (NE) as compared to the kinetics of the redox reaction (VO2 + /VO2+) on the positive electrode (PE) and poor kinetic reversibility.. The commonly used electrodes for VRFBs are graphite felt electrodes (GFE). One of the main challenges facing these GFEs is their activation that enhances a rate of redox reactions due to the increase of GFE wettability and surface area. Another challenge is capability to maintain GFE activation over cycling.In this work, we report on thermal and thermo-chemical activation of GFEs which leads to the formation of functional groups (carboxyl C=O, phenolic C-O, and adsorbed O or H2O for the enhancement of the electrode wettability and active surface area. The physicochemical characterization of the activated GFEs was performed using X-ray photoelectron spectroscopy (XPS) and contact angle measurements. The electrochemical study was conducted by cyclic voltammetry (CV) measurements in 0.2M VOSO4/2M H2SO4 at 1 mV/s and cycling tests (50 cycles, three tests for every GFE) in a commercial 9cm2 VRFB ( Standard Energy Corp.) using a charge/discharge current density of 80 mA/cm2 at 20oC and a commercial vanadium electrolyte 1.6M [V]/2M H2SO4. The increase of temperature of GFE heat treatment of GFEs from 400 to 500oC resulted in the growth of the energy efficiency (EE) of VRFBs with such electrodes from 81.3 to 87.2%, respectively, due to the increase of adsorbed oxygen or water from 1.6 to 8.72% and C-O (C-OH) groups from 0.49 to 2.09%, respectively and wettability (contact angle (CA) decreases from 78.7 to 65.1o). Analysis of CVs of the redox reaction VO2 + /VO2+ shows the decrease of the peak of potential separation ΔEp (Epa-Epc) from 0.357 to 0.345 V and the peak of current ratio (Jpa/Jpc) from 1.406 to 1.26, indicating lower polarization due to the increase of the electrode surface area, acceleration and reversibility of these redox reaction, respectively (Table 1). The proposed additional treatment of the heat treated GFE at 400oC (GFE400) in strong oxidants H2O2, H2SO4 and HNO3 for the decomposition of C=O and the enhancement of the content C-O groups and GFE wettability didnt result in the performance improvement (VRFB with GFE400+10%H2O2 - energy efficiency of 78%).The thermal treatment of GFE at 500oC demonstrated improvement of VRFB cell performance, namely growth of energy efficiency to 87.2% and VE to 90.1%, % due to the increase of wettability and active surface area of GFEs. Figure 1