Performance Of Flow Batteries Research Articles

High energy density electrolytes for H2/Br2 redox flow batteries, their polybromide composition and influence on battery cycling limits.

Hydrogen–bromine redox flow batteries (H2/Br2-RFB) are a promising stationary energy storage solution, offering energy storage densities up to 200 W h L−1. In this study, high energy density electrolytes of concentrated hydrobromic acid of up to 7.7 M are investigated. Particular polybromide ion (Br2n+1−; n = 1–3) concentrations in the electrolyte at different states of charge, their effect on the electrolytic conductivity and cell operation limits are investigated for the first time. The concentrations of individual polybromides in the electrolytes are determined by Raman spectroscopy. Tribromide (Br3−) and pentabromide (Br5−) are predominantly present in equal concentrations over the entire concentration range. Besides Br3− and Br5−, heptabromide (Br7−) exists in the electrolyte solution at higher bromine concentrations. It is shown that polybromide equilibria and their constants of Br3− and Br5− from literature are not applicable for highly concentrated solutions. The conductivity of the electrolytes depends primarily on the high proton concentration. The presence of higher polybromides leads to lower conductivities. The solubility of bromine increases disproportionately with increasing bromide concentration, since higher polybromides such as Br7− or Br5− are preferably formed with increasing bromide concentration. Cycling experiments on electrolyte in a single cell are performed and combined with limitations due to electrolyte conductivity and bromine solubility. Based on these results concentrations of the electrolyte are defined for potential operation in a H2/Br2-RFB in the range 1.0 M < c(HBr) < 7.7 M and c(Br2) < 3.35 M, leading to a theoretical energy density of 196 W h L−1.

Interfacial co-polymerization derived nitrogen-doped carbon enables high-performance carbon felt for vanadium flow batteries

Nitrogen-doped carbon felt has exhibited great promise in enhancing the cycling performance and lifespan of vanadium flow batteries (VFBs).

Contact resistance stability and cation mixing in a Vulcan-based LiNi1/3Co1/3Mn1/3O2 slurry for semi-solid flow batteries

The Semi-Solid Flow Battery (SSFB) is an interesting energy storage system (ESS) for stationary applications but, in spite of the significant work presented on this technology so far, understanding the chemical and physical factors limiting its electrochemical performance is still blurred by measurements under static conditions rather than under real operando conditions. In this study, we have used Vulcan carbon as a conductive additive to formulate LiNi1/3Co1/3Mn1/3O2 (NCM) based slurries as the catholyte to characterize electrical and electrochemical performances using a 3-electrode flow cell by electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge (GCD), respectively. The results are correlated with post-mortem analyses of recovered slurries using Scanning Electron Microscopy (SEM), Raman spectroscopy and Rietveld refinement of the NCM crystal structure. Due to the improved electrochemical cycling stability of the Vulcan-based NCM slurry and cell configuration used for measurements, we have been able to characterize the system in terms of electrical contributions and correlate them with particle degradation as well as detect antisite defect formation on cycling. The electrical stability of the contact resistance and cation mixing are identified as factors limiting the performance of the semi-solid slurry. The latter is frequently reported in porous electrodes for Li-ion batteries but, to our knowledge, it has not been reported for SSFBs to date.

Rapid wet-chemical oxidative activation of graphite felt electrodes for vanadium redox flow batteries.

To boost the performance of vanadium redox flow batteries, modification of the classically used felt electrodes is required to enable higher cycling performance and longer life cycles. Alternative approaches to the standard thermal oxidation procedure such as wet chemical oxidation are promising to reduce the thermal budget and thus the cost of the activation procedure. In this work we report a rapid 1 hour activation procedure in an acidified KMnO4 solution. We show that the reported modification process of the felt electrodes results in an increase in surface area, density of oxygenated surface functionalities as well as electrolyte wettability, as demonstrated by N2-physisorption, XPS, Raman spectroscopy as well as contact angle measurements. The activation process enables battery cycling at remarkably high current densities up to 400 mA cm−2. Stable cycling at 400 mA cm−2 over 30 cycles confirms promising stability of the reported activation procedure.

The improvement of organic redox flow battery performance by spherical mesoporous carbon prepared by sol-gel polymerization in water-oil emulsification technique

Spherical mesoporous carbon (SMC) was prepared by water in oil emulsification combined with sol-gel polymerization and activated with carbon dioxide. The SMCs had an average particle size of 44 μm, a specific surface area of 479 m2 g−1 and with a large mesopore volume of 1.12 cm3 g−1. The slit-shaped mesopores were embedded into the wall surface of the SMCs. As electrocatalysts, the electrochemical properties of SMCs toward the anthraquinone-2-sulfonic acid (AQS) and 1,2-benzoqinone-3,5-disulfonic acid (BQDS) redox couples were examined. The SMCs facilitated the fast reversible-electrochemical reaction with the kinetic rate constant of 9.73 × 10−4 cm s−1 for BQDS, and of 4.39 × 10−3 cm s−1 for AQS. Subsequently, the electrodes of an aqueous organic flow battery were fabricated onto carbon paper and carbon cloth using SMCs as the electrocatalysts. The flow battery performance was improved by around 1.7-fold, when the SMCs had a mass loading of 2 mg cm−2 with 20 wt% Nafion, fabricated onto carbon cloth. However, there was no significant performance improvement when the SMCs were fabricated onto carbon paper. Moreover, SMCs provided the most outstanding power density over other carbon electrocatalysts (i.e., graphene, Vulcan XC-72 and Vulcan XC-72R), from 2 to 3 fold, according to the superior physical and electrochemical properties.

Tuning the Performance of Aqueous Organic Redox Flow Batteries via First-Principles Calculations.

Aqueous organic redox flow batteries have many appealing properties in the application of large-scale energy storage. The large chemical tunability of organic electrolytes shows great potential to improve the performance of flow batteries. Computational studies at the quantum-mechanics level are very useful for guiding experiments, but in previous studies, explicit water interactions and thermodynamic effects were ignored. Here, we applied the computational electrochemistry method based on ab initio molecular dynamics and thermodynamic integration to calculate redox potentials of quinones and their derivatives. The calculated results are in excellent agreement with experimental data. We mixed side chains to tune their reduction potentials and found that solvation interactions and entropy effects play a significant role in side-chain engineering. On the basis of our calculations, we proposed several high-performance negative and positive electrolytes. Our first-principles study paves the way toward the development of large-scale and sustainable electrical energy storage.

An alternative HCMS carbon catalyst in bromine reduction reaction for hydrogen-bromine flow batteries

In this study, significantly active hollow core mesoporous shell (HCMS) carbon catalysts are synthesized for the hydrogen-bromine flow battery cathode electrode. As an alternative to cathode catalysts such as Pt/C or carbon black, high surface area HCMS carbons are synthesized and its effect on hydrogen-bromine flow battery performance has been investigated for grid-scale energy storage for the first time. HCMS carbon is synthesized by template replication of the solid core mesoporous shell silica spheres. By changing the amount of TEOS (1 mL, 6 mL, 10 mL) used during silica template formation, carbons with different core/shell structures have been produced (HCMS1, HCMS6, HCMS10). Structural characterizations of the synthesized cathode catalysts have been performed with SEM, TEM, and N2 adsorption analyses. Electrochemical characterization of catalysts has been performed by cyclic voltammetry analyses and flow battery performance tests. 0.50 W/cm2 peak power density obtained with HCMS1 carbon is promising compared to Pt-based catalysts and commercial carbons.

The effects of temperature and membrane thickness on the performance of aqueous alkaline redox flow batteries using napthoquinone and ferrocyanide as redox couple

The mixture of naphthoquinone-4-sulfonic acid sodium salt and 2-hydroxy-naphthoquinone (NQSO) and ferrocyanide dissolved in potassium hydroxide (KOH) electrolyte was used as catholyte and anolyte, respectively. We evaluated the effects of temperature and membrane thickness on the performance of aqueous organic redox flow batteries (AORFB) using the NQSO and ferrocyanide dissolved in alkaline electrolyte. Regarding temperature effect, when the electrochemical properties of NQSO and ferrocyanide are evaluated with 25 and 40 °C, their redox reactivity is enhanced with increased temperature due to the proportional relation of reaction rate and temperature. In addition, their electron transfer rate is also improved with increased temperature due to the proportional relation of electron transfer rate and temperature. These are proven by Nyquist plots showing the reciprocal relationship of resistance and temperature. In AORFB full cell tests performed at 25 and 40 °C, although capacity decay rate observed at 40 °C (0.067 Ah·L−1 per cycle) is larger than that observed at 25°C (0.034 Ah·L−1 per cycle), energy efficiency (EE) was improved from 86% at 25 °C to 89% at 40 °C. Regarding membrane thickness effect, the performance of AORFB using thin Nafion 212 membrane is better than that of AORFBs using thick Nafion 117 and Nafion 1110 membranes in voltage efficiency (VE) and EE, while its capacity retention is vice versa. This is because thinner membrane induces lower resistance.

Modifying Carbon Felt Electrodes Using High-Temperature Ammonia Treatment to Improve the Performance of All-Vanadium Redox Flow Batteries

High temperature ammonia treatments were performed on carbon felt electrodes in this study. Their physical and electrochemical properties were investigated.Carbon felt electrodes are often used in vanadium redox flow batteries (VRFBs). Without intervention, carbon felt has poor wettability which frustrates electrochemical activity. This material must be modified to make it more effective, which has been historically accomplished with thermal treatments to promote oxygen functional groups. [1] Many attempts have been made to improve performance of carbon felt in VRFBs, but some of the more successful modifications have resulted in an increase in the specific surface area. [2-4] Preliminary experiments on carbon felt performed by this lab with ammonia at high temperatures have shown promising results. [5]For our experiments, commercially available carbon felts (SIGRACELL® GFD3 by SGL Carbon, Meitingen, Germany) were modified by exposure to flowing ammonia gas through a furnace at 900°C for varying lengths of time. The samples were tested in a single-cell flow battery using cyclic voltammetry (CV), polarization curves, and electrochemical impedance spectroscopy (EIS). This treatment results in a significant increase in electrochemical surface area and performance.The physical properties were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and Raman spectroscopy. A pitting effect was observed in SEM images, preceded by surface blistering at shorter lengths of exposure.Greatly increased active surface area, along with a substantial increase in cell performance, was achieved with the ammonia-modified carbon felts described in this work. Sun, B. and M. Skyllas-Kazacos, Modification of graphite electrode materials for vanadium redox flow battery application—I. Thermal treatment. Electrochimica Acta, 1992. 37(7): p. 1253-1260.Lu, W., et al., High-performance porous uncharged membranes for vanadium flow battery applications created by tuning cohesive and swelling forces. Energy & Environmental Science, 2016. 9(7): p. 2319-2325.Zhou, X.L., et al., A high-performance dual-scale porous electrode for vanadium redox flow batteries. Journal of Power Sources, 2016. 325: p. 329-336.Wei, L., et al., Highly catalytic hollow Ti3C2Tx MXene spheres decorated graphite felt electrode for vanadium redox flow batteries. Energy Storage Materials, 2020. 25: p. 885-892.Pezeshki, A.M., "Impedance-Resolved Performance and Durability in Redox Flow Batteries. " PhD diss., University of Tennessee, 2016. Figure 1

Combining Electrochemical, Fluid Dynamic, and Imaging Analyses to Understand the Effect of Electrode Microstructure and Electrolyte on Redox Flow Batteries

Understanding the interplay between electrode microstructure and cell performance of electrochemical devices is important both for modelling and experimental design. Redox Flow Batteries (RFBs) are an electrochemical energy storage technology with potential for grid-scale energy storage applications, although costs need to be further reduced to be competitive. One pathway to lowering the costs involves increasing the power density of each cell, such that fewer cells are required. This can be achieved in a variety of ways, including by improving the design of the electrode microstructures. The aim of this work is to better understand the relationship between electrode microstructure and RFB performance and, ultimately, to make inferences about electrode utility.The performances of a variety of commercially available carbon electrodes are examined via a series of commonly used microstructural and electrochemical analyses (Figure a-c). [1] We present a comprehensive study of pore-scale mass-transport processes occurring in each of the electrodes and rationalize their effect on the overall cell performance. A matrix of electrochemical tests were carried out in a flow-through RFB cell using incremental flow rates (Figure b-c) and two non-aqueous TEMPO electrolytes with distinct viscosity and diffusivity properties. Scanning electron microscopy (SEM) was used to image the electrodes and large 3 mm samples of each were scanned using X-ray computed tomography (Figure a). A customized segmentation technique was subsequently developed that resamples the image data to ensure the fiber dimensions agree with SEM images, improving the validity of the various extracted metrics. From these images, calculations and electrochemical tests, several microstructural parameters were extracted and a pore network model was used to calculate the permeabilities of the electrodes. A 1D model was developed across half the symmetric membrane electrode assembly and, using the parameters extracted from XCT, fit to galvanostatic polarization measurements to obtain the mass transfer coefficients at each unique operating condition – these were found to be in the range of and m s-1. Transport processes and distributions through the cell are analysed and correlations between dimensionless Reynolds and Sherwood numbers are subsequently discussed, as demonstrated in previous work. [2, 3]Given a ‘good performance’ is associated with a system being able to reach a high current with a low overpotential, the following findings were made. Firstly, while volume-specific surface area, porosity and tortuosity are useful descriptors for evaluating porous media, they are not good indicators of performance in these systems. Instead the permeability and, by extension, the ease with which convective transport can occur is found to be directly correlated with performance in this system. It is asserted that systems using solvents with higher viscosity have a reduced performance due, in part, due to regions in the electrode being starved of new electrolyte. Consistently, across all electrodes and in each electrolyte a better performance is observed when a higher flow rate is used. This effect is, in part, attributed to the reduction in size of the boundary layer at the electrode surface. Finally, in agreement with previous works, [1, 4] the carbon cloths tested in this work are shown to have higher mass-transfer coefficients and permeabilities than the carbon papers and we conclude that the best performing electrode is the thicker carbon cloth, which has the lowest resistance to convective flow, highest permeability, and highest mass-transfer coefficients in both solvents.[1]: A. Forner-Cuenca, E. E. Penn, A. M. Oliveira and F. R. Brushett, Exploring the Role of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries. J. Electrochem. Soc., 166, 2230-2241 (2019).[2]: Y. A. Gandomi, D. S. Aaron, T. A. Zawodzinski and M. M. Mench, In Situ Potential Distribution Measurement and Validated Model for All-Vanadium Redox Flow Battery. J. Electrochem. Soc., 163, A5188-A5201 (2016).[3]: K. M. Tenny, A. Forner-Cuenca, Y.-M. Chiang and F. R. Brushett, Comparing Physical and Electrochemical Properties of Different Weave Patterns for Carbon Cloth Electrodes in Redox Flow Batteries. J. Electrochem. En. Conv. Stor., 17, 041108 (2020).[4]: M. A. Sadeghi, M. Aganou, M. D. R. Kok, M. Aghighi, G. Merle, J. Barralet and J. Gostick, Exploring the Impact of Electrode Microstructure on Redox Flow Battery Performance Using a Multiphysics Pore Network Model. J. Electrochem. Soc., 166, A2121-A2130 (2019). Figure 1

Accessible Surface Area in Porous Carbon Electrodes and Its Impact on Redox Flow Battery Performance

Carbon-fiber electrodes exhibit many desirable material properties including conductivity, porosity, and chemical stability, and, accordingly, are employed in a range of electrochemical technologies.1 Such electrodes are particularly important in redox flow battery (RFB) systems, as they serve multiple critical functions including providing surface area for electrochemical reactions, distributing liquid electrolytes, and conducting electrons and heat. While commercially available materials possess suitable permeability and conductivity, the performance of pristine substrates is limited, thus oxidative treatment prior to use is common practice.2,3 Partial oxidation of the electrode surface has been shown to add a range of oxygen functional groups, which improve hydrophilicity and, in some cases, catalyze redox reactions, as well as to increase available surface area for the electrochemical reactions.4,5 Both effects are posited to improve electrode performance but their relative importance and effectiveness remains unclear.6 Here, we critically assess the nature of the surface area generated by thermal pretreatment in air and its role on RFB electrode performance. Using binder-free Freundenberg H23 as a model substrate, we systematically vary pretreatment time and temperature to create different surface morphologies and develop structure-function relations. First, we use Brauner Emmanuel Teller gas adsorption (e.g., N2, Kr, Ar/CO2) and mercury porosimetry to estimate physical surface area and pore size distribution. In parallel, we use voltammetry and impedance spectroscopy to determine electrochemically active surface area with different electrolyte compositions. In general, we find that only a fraction of the physical surface area is accessible for electrochemical redox reactions due, in large part, to the nanoscopic dimensions of the added surface area. Second, we develop a simple mathematical model to assess the effectiveness of the available surface area of carbon fibers for Faradaic reactions. We find that, under typical RFB operating conditions, external mass transfer limitations govern performance, limiting the utilization of recessed areas generated during pretreatment. Ultimately, these results highlight the potential limits of performance improvement strategies based on increasing surface area of fibrous substrates and motivate new approaches for electrode development. Acknowledgements The authors acknowledge the financial support of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the United States Department of Energy. K.V.G acknowledges additional funding from the National Science Foundation Graduate Research Fellowship. The authors acknowledge the Center for Nanoscale Systems and the NSF’s National Nanotechnology Infrastructure Network (NNIN) for the use of Nanoscale Analysis facility for electrode property characterization. References M. H. Chakrabarti et al., Journal of Power Sources, 253, 150–166 (2014).T. J. Rabbow, M. Trampert, P. Pokorny, P. Binder, and A. H. Whitehead, Electrochimica Acta, 173, 17–23 (2015).N. Pour et al., The Journal of Physical Chemistry C, 119, 5311–5318 (2015).B. Sun and M. Skyllas-Kazacos, Electrochim. Acta, 37, 8 (1992).T. J. Rabbow and A. H. Whitehead, Carbon, 111, 782–788 (2017).K. V. Greco, A. Forner-Cuenca, A. Mularczyk, J. Eller, and F. R. Brushett, ACS Applied Materials & Interfaces, 10, 44430–44442 (2018).

A Zero-Dimensional Electrochemical Model for Organic Flow Cells

We develop and experimentally validate a zero-dimensional model of the electrochemical performance of a flow cell. The model takes into account voltage losses due to Faradaic charge transfer, ohmic and mass transport resistance, as well as the effect of spatial variations in state-of-charge between the cell and electrolyte reservoir. We validate the model by comparing voltage-current data during cycling to equivalent electrochemical measurements from a symmetric cell with organic reactants. With the exception of the mass transfer coefficient, all relevant model parameters are determined independently prior to the experiment using electrochemical impedance spectroscopy and cyclic voltammetry measurements. We find excellent agreement between the model and experiment for constant-current and constant-voltage cycling, across a wide range of current densities (40 – 200 mA/cm2) and volumetric flow rates (~ 10 – 140 mL/min). The relationship between the fitted mass transport coefficient and flow rate conforms to expectations from the literature [1-3], and reasonably approximates the analogous relationship from single-reservoir cell measurements. This work supports the notion that zero-dimensional electrochemical models can yield simple but predictive frameworks for optimizing organic flow battery performance and understanding how reactant decomposition and crossover contribute to capacity fade.

Investigations on Effect of Fe on Vanadium Redox Reaction and Vanadium Redox Flow Battery Performance

The vanadium redox flow battery (VRFB) is one of the most promising energy storage technologies for large scale commercialization. The vanadium electrolyte is the main component, determining the VRFB’s energy density and capacity. The quality of the vanadium electrolyte is key to VRFB’s operation, as presence of soluble impurities will affect VRFB’s performance and durability. Understanding the impact of these impurities may ultimately lead to a specification for the impurity levels a VRFB can tolerate without compromising its performance and durability. Fe is an impurity element typically found in the vanadium ores, which is also present in the commercially available vanadium electrolytes. It was reported that Fe in positive electrolyte in a narrow concentration range (< 0.0286 M or 0.12 wt. %) could slightly affected the VRFB performance [1]. At high concentration (1.0 - 1.4 M Fe), Fe was reported to stabilize the positive electrolyte at high temperature (50oC) [2]. However, systematic studies on the effect of Fe on vanadium redox reactions, over a wider range of concentrations, are needed in order to achieve a good understanding of how it affects the VRFB performance.This work reports on the effect of Fe on vanadium redox reactions, with concentrations ranging from 0.05 wt. % or 0.012 M to 2 wt. % or 0.48 M, investigated by cyclic voltammetry and VRFB single cell cycling. In the present study, CV response of vanadium electrolyte on glassy carbon, Pt disk and graphite electrode was compared. On glassy carbon and Pt disk electrodes, vanadium redox reaction has shown irreversible or partially reversible CV response, while on graphite rod electrode, reversible redox reaction response was observed. Thus, a graphite rod was selected as working electrode for the subsequent investigations. The cyclic voltammogram on graphite rod electrode is dependent on electrode pre-treatment. Redox peak current, and peak separation are different on freshly polished graphite electrode compared to an electrode that has been used for a certain period of time (e.g. half hour). The difference is dependent on Fe concentration. In 0.05 wt. % (or 0.012 M) Fe electrolyte, electrode passivation was observed (Fig. 1), where the freshly polished electrode shows higher redox peak current than the one that has been used for CV for a certain time. However, in electrolytes with higher Fe content (e.g. 0.5 wt. % or 0.12 M), the freshly polished electrode presents lower redox peak current, indicating that electrode activation can occur during CV testing (Fig 2).VRFB performance was further evaluated, and it was found that the effect of Fe on VRFB capacity, capacity change profile with cycling, and efficiency is dependent on Fe concentration. Low Fe concentration affected more on the efficiency, while higher Fe concentration shows significant effect on capacity change during cycling. AC impedance and vanadium crossover were used to diagnosis the VRFB performance and degradation, and it was found that Fe concentration affects VRFB degradation and water transfer. The tolerance level of Fe in vanadium electrolyte can be deduced from this study, which may provide guidance on the design of low purity vanadium electrolyte.

Modeling the Impact of Sequential Two-Electron Transfer Compounds on Redox Flow Battery Performance

Redox flow batteries (RFBs) are promising candidates for grid-scale energy storage, but further cost reductions are needed for widespread use.1 As such, redox-active organic molecules and metal-coordination complexes are emerging as charge storage materials considering their redox potential, solubility, and stability can be tailored through molecular functionalization.2,3 Moreover, a number of these compounds can support multiple electron transfers, affording dramatic increases in energy storage capacity.4,5 However, the nature of the electron transfer events, concerted or sequential, plays a key role in determining whether the added capacity can be accessed efficiently. Specifically, depending on molecular features and cell operation, voltaic inefficiencies may be incurred which, in turn, offset any benefits of increased capacity. Therefore, we sought to quantify these losses and their relationship to molecular- and cell-level parameters to better understand the feasibility of multi-electron transfer in RFBs.In this presentation, we use a zero-dimensional electrochemical model, accounting for thermodynamic, kinetic, and mass transfer, to simulate the performance of two-electron compounds in redox flow cells. First, using a single half-cell, we show that voltaic efficiency is significantly influenced by the average redox potential and potential difference between the redox events. We also discuss the effects of different mass transfer coefficients and comproportionation reaction rates. Second, using a full cell, we consider the effects of utilizing two-electron compounds in both half-cells and subsequent effects of charge imbalance, both resulting in further inefficiencies. By understanding the operational tradeoffs of multi-electron compounds, we can develop design criterion to guide molecular engineering efforts towards more efficient high energy density electrolytes for RFBs.AcknowledgementsThis work was funded by the National Science Foundation (NSF) under Award Number 1805566. B.J.N gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.References(1) Chalamala, B. R.; Soundappan, T.; Fisher, G. R.; Anstey, M. R.; Viswanathan, V. V.; Perry, M. L. Redox Flow Batteries: An Engineering Perspective. Proceedings of the IEEE 2014, 102 (6), 976–999.(2) Hogue, R. W.; Toghill, K. E. Metal Coordination Complexes in Nonaqueous Redox Flow Batteries. Current Opinion in Electrochemistry 2019.(3) Ding, Y.; Zhang, C.; Zhang, L.; Zhou, Y.; Yu, G. Molecular Engineering of Organic Electroactive Materials for Redox Flow Batteries. Chem. Soc. Rev. 2018, 47 (1), 69–103.(4) Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A. A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries. J. Mater. Chem. A 2017, 5 (46), 24371–24379.(5) Sevov, C. S.; Fisher, S. L.; Thompson, L. T.; Sanford, M. S. Mechanism-Based Development of a Low-Potential, Soluble, and Cyclable Multielectron Anolyte for Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc. 2016, 138 (47), 15378–15384.

Ferrocene/anthraquinone based bi-redox molecule for symmetric nonaqueous redox flow battery

The crossover of anolyte and catholyte during cycling places a serious obstacle for the performance of non-aqueous redox flow batteries (NARFBs). Employing symmetric RFB is an effective strategy to alleviate this issue. A new bi-redox material, 1-(ferrocenylmethyl-amino)-anthraquinone (FcMeAAQ) composed of ferrocene and anthraquinone two moieties, is designed for symmetrical NARFB. Both experimental and density functional theory calculational results show the relatively good stability of FcMeAAQ when one-electron redox reaction is involved. The assembled symmetric NARFB with the Daramic AA-250 porous membrane delivers an open circuit voltage of 1.42 V, Coulombic efficiency of 90.8%, and energy efficiency of 81.8% over 100 cycles at the current density of 2 mA cm−2. Furthermore, a peak power density of ~16 mW cm−2 is achieved at the state of charge of 100%.

Nitrogen and sulfur co-doped graphene composite electrode with high electrocatalytic activity for vanadium redox flow battery application

Nitrogen and sulfur co-doped graphene has been decorated on graphite felt electrode via a hydrothermal treatment using thiourea as sulfur and nitrogen sources for vanadium redox flow battery application. Our results demonstrate that the modification of sulfur and nitrogen co-doped graphene on graphite felt provides an effective way to improve surface area and introduce functional groups for vanadium redox reaction, resulting in a significant improvement in energy efficiency (85.37% at a current density of 80 mA/cm2), power density (0.4925 W/cm2), discharge capacity and capacity retention. These results reveal that the modification of nitrogen and sulfur co-doped graphene on graphite felts is a promising method for the enhancement of vanadium redox flow battery performance.

The numerical study of vanadium redox flow battery performance with different electrode morphologies and electrolyte inflow patterns

Electrode morphological features and the electrolyte inflow pattern are important factors in determining the performance of vanadium redox flow batteries. In this study, a 2-D numerical model that couples electrolyte flow, ion diffusion, and electrochemical reaction is proposed to quantitatively investigate the mass transport and electrochemical reactions at various electrode structures, including the effects of porosity, fiber size, and fiber array pattern, as well as the electrode compression and the electrolyte inflow pattern. The influence of stochastic seeds on the reconstructed geometry was considered and assemble average value was conducted. The established model with the reconstructed geometry has been validated by experiment. The specific surface area, flow resistance, and VO2+ consumption rate increase with a decreasing in electrode porosity and fiber diameter. The fiber array pattern with a “outlet dense and inlet sparse” structure shows the optimum performance of rapid VO2+ consumption rate and high electrode utilization. The mass transport is dominated by convection in a wide flow path within the electrode, and the collective effects of convection and diffusion are found in a narrow flow path. The main advantage of the compressed electrode is the enhanced electrode utilization. A turning point at a stoich (the ratio of input reactant moles versus the consumed moles at a certain applied current) about 1000 was identified, below which the consumption rate of the flow-through pattern is higher than that of the flow field pattern and above which the consumption rate of the former pattern is slower than the latter. A faster fuel feed in the electrode with a narrower flow channel contributes to a faster species consumption rate in the last stage of the discharge process. The present study provides practical guidance for the design and optimization of VRFB electrode.

Ion‐Selective Electrocatalysis on Conducting Polymer Electrodes: Improving the Performance of Redox Flow Batteries

AbstractThe selective ion transport characteristics of a conducting polymer electrode, based on poly(3,4‐ethylenedioxythiophene) (PEDOT), is evaluated with respect to its electrocatalytic performance, specifically targeting redox switching of quinone couples. Employing model organic redox quinones, here, the novel phenomenon of ion‐selective electrocatalysis (ISEC) is conceptualized. The effect of ISEC is studied and evaluated using two forms of PEDOT electrodes, which differ in their ion‐exchange characteristics, by comparing the redox transformations of catechol and tiron. It is rationalized that the choice of the specific redox couple and the ion selectivity characteristics of the conducting polymer electrode impacts the activation losses in aqueous organic redox‐flow batteries. By carefully selecting and designing the conducting polymer electrodes, high conversion efficiency on acid‐resistant electrodes is obtained. As far as it is known, this is the first redox flow battery to include conducting polymer electrodes operating in both the posolyte and negolyte configurations, thus the first “all‐organic” RFBs.

Electrochemical performance of graphene oxide modified graphite felt as a positive electrode in all-iron redox flow batteries

In this study, we demonstrate that coating a layer of graphene oxide (GO) onto graphite felts (GF) by electrostatic spraying can substantially increase the performance of all-iron redox flow batteries (IRFBs). Graphite felts are extensively used as electrodes but they do not have the desired electrochemical properties. GO has good electrochemical features. Hence, GO was synthesized from graphite powder and applied onto graphite felts. Chemical and structural features of the bare graphite felt electrode (BGF), thermally treated graphite felt electrode (TTGF), and graphene oxide modified graphite felt electrode (GOMGF) were characterized using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDX), Transmission Electron Microscopy (TEM), Raman Spectroscopy (RS), X-Ray Photoelectron Spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) surface area analysis. Similarly, the electrochemical performance was evaluated using Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), Tafel analysis and charge–discharge experiments. The Charge–discharge experiments were performed at 1 to 5 mgcm−2 weight of GO on the modified graphite felt electrode and varying the current densities from 10 to 40 mAcm−2. The coulombic efficiency (ηC) and energy efficiency (ηE) of the cell determined at 20 mAcm−2 for 4 mgcm−2-GOMGF electrodes were found to be 64.61% and 50.27%, respectively. Among the three different types of electrodes, the GOMGF electrode showed better electrocatalytic performance mainly due to the excellent conducting network of the defective edges of oxygen on the surface of layered flakes of the GO. After twenty cycles, the average ηC and ηE of the cell using a 4 mgcm−2-GOMGF electrode were found to be 62.06% and 42.02%, respectively.

(Invited) Factors Influencing the Performance of Vanadium Flow Batteries: Electrodes and Electrolytes

There is presently great interest in redox flow batteries1-8 (RFBs) for large-scale energy storage9 due to advantages over other electrical energy storage (EES) technologies, and research activities in this area have grown exponentially in recent years. The vanadium flow battery (VFB), also known as the vanadium redox flow battery (VRFB), is commonly regarded as one of the most promising flow batteries.2-3,10-12 Active areas of research on VFBs include electrolytes,13-16 electrodes,5-7 membranes,8 cell design and modelling,8 and performance and state-of-charge (SoC) monitoring.17-18 In this paper, we will review our work in two areas: kinetics on carbon electrodes and thermal stability of electrolytes.The kinetics of the VII-VIII, VIII-VIV and VIV-VV redox couples have been studied for a range of different carbon materials using a variety of techniques, and it is clear that the kinetic rates depend strongly both on the type of carbon used and on the preparation of the electrode surface. We will discuss our investigations5-7 of the effects of anodic and cathodic pretreatments of carbon on electrode kinetics in the VFB. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), we showed for four different types of carbon that electrode treatments at negative potentials enhance the kinetics of VIV-VV and inhibit the kinetics of VII-VIII while electrode treatments at positive potentials inhibit the kinetics of VIV-VV and enhance the kinetics of VII-VIII. These observations may explain conflicting reports in the literature. We examined in detail the potentials required for activation and deactivation of electrodes. The results suggest that interchanging the positive and negative electrodes in a vanadium flow battery (VFB) would reduce the overpotential at the negative electrode and so improve the performance. This is supported by flow-cell experiments. Thus, periodic catholyte-anolyte interchange, or equivalent alternatives such as battery overdischarge, show promise of improving the voltage efficiency of VFBs.We have developed13-16 a standard methodology for measuring the thermal stability of vanadium catholytes based on their induction time for precipitation of V2O3. Using this, we have investigated the thermal stability of typical vanadium flow battery (VFB) catholytes at temperatures in the range 30–70°C for VV concentrations of 1.4–2.2 mol dm-3 and sulfate concentrations of 3.6–5.4 mol dm-3. In all cases, V2O5 precipitates after an induction time, which decreases with increasing temperature. Plots of the logarithm of induction time versus the inverse of temperature (equivalent to Arrhenius plots) show excellent linearity and all have similar slopes, yielding a value of 1.791±0.020 eV for the activation energy. The logarithm of induction time also increases linearly with sulfate concentration and decreases linearly with VV concentration. The slopes of these plots give values of concentration coefficients β S and β V5 which quantify the effects of concentration on induction times. Combining these with the Arrhenius slope, we have constructed a model to predict the stability of sulfate-based vanadium catholytes. The model can also be used as a basis for accelerated testing of the thermal stability of electrolytes.We have investigated a range of additives and shown that many suggested additives are ineffective. However, the Group V elements phosphorus and arsenic in the +5 oxidation state have a significant stabilizing effect. We have also quantified their effect in the temperature range 30-70°C. REFERENCES A.Z. Weber et al., J. Appl. Electrochem. 41, 1137 (2011)M. Skyllas-Kazacos et al., J. Electrochem. Soc. 158, R55 (2011)M.J. Watt-Smith et al., J. Chem. Technol. Biotechnol. 88, 126 (2013)S. Roe et al., J. Electrochem. Soc. 163, A5023 (2016)A. Bourke et al., J. Electrochem. Soc. 163, A5097 (2016)A. Bourke et al., J. Electrochem. Soc. 162, A1547 (2015)M.A. Miller et al., J. Electrochem. Soc. 163, A2095 (2016)D. Reed et al., J. Electrochem. Soc. 163, A5211 (2016)R. M. Darling et al., Energy Environ. Sci ., 7, 3459 (2014)R. M. Darling et al., J. Electrochem. Soc .,163, A5014 (2016)A. K. Manohar et al., J. Electrochem. Soc., 163, A5118 (2016)A. M. Pezeshki et al., J. Electrochem. Soc., 163, A5202 (2016)D. Oboroceanu et al., J. Electrochem. Soc ., 163, A2919 (2016)D. Oboroceanu et al., J. Electrochem. Society, 164, A2101 (2017)D. N. Buckley et al., J. Electrochem. Society, 165, A3263 (2018) Oboroceanu et al., J. Electrochem. Soc ., 166, A2270 (2019)C. Petchsingh et al., J. Electrochem. Soc. 163, A5068 (2016)D. N. Buckley et al., J. Electrochem. Soc., 161 A524 (2014)