Photoangulator

Practical and commercial issues in the design and manufacture of vanadium flow batteries

The Redox Flow Battery (RFB) systems are unique chemically, mechanically, and electrically compared to other kinds of batteries. Among RFBs, the Vanadium Redox Flow Batteries (VRFBs) are the most commercialized type. VRFBs are a suitable option for large-scale energy storage with exceptional advantages like the decoupled power and energy design (scalability), long life-time, safe battery chemistry (non-toxic, and non-flammable), etc. The power and energy designs in VRFBs are decoupled, known as the scalability benefit of the VRFBs. This implies more output energy from the battery is possible only by adding more electrolytes to the reservoir tanks.There are only a few research studies in the literature about the optimized operation of the Vanadium redox flow batteries. Most of these papers are not applicable to develop in real practice. The introduced optimized operation algorithms of VRFBs in this lecture can help the participants learn how to improve the performance of the battery and operate the battery more efficiently. Different objective functions are considered in this lecture to be optimized, e.g. minimizing the time duration of battery charging (for fast charging), energy loss, voltage loss, and capacity loss of the battery. The participants can find the results of this useful in optimized operation and implementation of the VRFBs.High charging current density results in faster charging and reduces the capacity fading in Vanadium Redox Flow Batteries (VRFB). On the other hand, it leads to the reduced energy efficiency of the battery. Also, the lower electrolyte flow rate in VRFBs results in less energy consumption by the pumps leading to the higher energy efficiency of the VRFBs. However, higher flow rates have the benefit of reducing voltage loss of VRFBs. To address these trade-offs, closed-loop charge control and flow management in VRFBs are necessary. In this lecture, a multi-objective optimization is proposed in first section to optimize the charging duration and flow management of the VRFB simultaneously during its charging. An innovative method is proposed for modeling pump consumption based on affinity laws for centrifugal pumps, which leads to new electrolyte flow management. Further, three case studies are defined in charging mode on a nine-cell VRFB unit laboratory prototype to validate the proposed optimization's performance, involving the duration of charging, flow management, and energy efficiency of the VRFB. The method is compared with previously published research studies on the optimal operation of VRFBs, which shows the uniqueness and consistency of the proposed optimization method for simultaneous controlling VRFB's charging duration and flow management.Capacity fade in Vanadium Redox Flow Batteries (VRFB) relies on the loss of electrolyte volume in each of charge and discharge cycles. The loss of volume in each cycle, also known as the bulk electrolyte osmosis, is due to Vanadium ions' diffusion from the membrane. The lower electrolyte flow rate in VRFB can reduce capacity fade as the electrolyte's velocity across the membrane decreases. However, the lower electrolyte flow increases the battery’s voltage loss. A new electrolyte flow management is introduced in second section of the lecture to address this trade-off, which considers the decrease of both capacity fade and voltage loss in VRFBs simultaneously. The proposed multi-objective flow management shows a significant reduction of both capacity and voltage losses in VRFBs.Moreover, typically complex electrochemical models and equations are needed to model capacity fade in VRFBs, which are not straightforward to model. The capacity fade modeling can lead to the estimation of available capacity and the battery’s State of Health (SoH). Therefore, a new mathematical model is proposed for the VRFB’s available capacity based on the electrochemical-based capacity fade model results. The model further is developed to estimate the State of Charge (SoC) and the SoH of VRFBs per cycle of charge and discharge.

Today, the electricity industries are facing new challenges as the market is being liberalized and deregulated in many countries. Electricity storage is undoubtedly a disruptive technology that will play, in the near future, a major role in the fast developing distributed generations network. Indeed, electricity storage has many potential applications: management of the supply and demand of electricity, power quality, integration of renewable sources, improvement of the level of use of the transport and distribution network, etc. Over the years, many storage technologies have been investigated and developed, some have reached the demonstrator level and only a few have become commercially available. The pumped hydro facilities have been successfully storing electricity for more than a century; but today, appropriate locations are seldom found. Electrochemical storage is also an effective means to accumulate electrical energy; among the emerging technologies, the flow batteries are excellent candidates for large stationary storage applications where the vanadium redox flow battery (VRB) distinguishes itself thanks to its competitive cost and simplicity. In this ambitious work that encompasses the domains of electricity, electrochemistry and fluid mechanics, we have proposed a novel multiphysics model of the VRB. This model describes the principles and relations that govern the behaviour of the VRB under any set of operating conditions. Furthermore, this multiphysics model is a powerful means to identify and quantify the sources of losses within the VRB storage system; indeed, one of the purposes of this study is to propose strategies of control and operation for a greater effectiveness of the overall storage system. The electrochemical model is based on the electrochemical principles and the study of the VRB chemistry; this model determines the equilibrium voltage from the vanadium concentrations, and the associated activation, concentration, ohmic and ionic overpotentials. Furthermore, the vanadium concentrations within the tank and the stack are constantly determined as a function of the current and the electrolyte flowrate. A simplified model of the internal loss is also proposed. The electrochemical performance was then established through the simulation of a stand alone system composed of a solar source, a VRB and a load. The model determines the stack voltage, the power flows and the vanadium concentrations over a 24 h period. Furthermore, the model was successfully compared with experimental data through a series of charge and discharge cycles at constant currents. Thereafter, the properties of the electrolyte are briefly investigated: in particular their dependence upon the electrolyte composition. Indeed, the viscosity and the density are important parameters of the mechanical model. In order to analyse the battery performance, a mechanical model has been proposed to determine the mechanical power required to flow the electrolytes. This model based on fluid mechanics has an analytical part that predicts the pressure drop within the pipes and the tanks, and a numerical part. Indeed, the stack geometry is so complex that it can not be described analytically; therefore, a numerical model based on finite element method (FEM) is proposed. Hence, the mechanical power necessary to the battery operation is obtained at any operating conditions. The electrochemical and the mechanical models are finally assembled to form the original multiphysics model of the VRB. This model provides a good insight of the battery operation and offers a powerful means to enhance the battery performance. Indeed, there is at constant current an optimal flowrate that maximizes the efficiency. A second series of charge and discharge cycles has determined the efficiency of different control strategies. Finally, the battery operations at constant power were also discussed in details and an optimal operating point has been highlighted.

Redox flow batteries (RFBs) are a promising technology for grid scale stationary energy storage to complement renewable energy systems. Although RFBs have a relatively low energy density, they offer important benefits such as long lifetime, decoupled energy and power, high round-trip efficiency, scalability and design flexibility, fast response and low environmental impacts. These benefits make them superior to many other stationary energy storage technologies [1-3]. The vanadium RFB (VRFB) is the most widely used RFB in industry for grid scale energy storage and for integration of renewable energy generation systems [4]. However, due to their slow redox reaction rates, their performance and stability are limited and VRFBs would thus benefit from enhancement of the electrode kinetics [5]. Various catalysts have been employed to improve the redox reaction rates [5-6]. Bismuth is considered a suitable candidate due to its good reactivity with oxygen, low toxicity, relatively low cost and comparable catalytic properties to other metal competitors [5, 7-8].In this study, we present a novel modified VRFB electrode composed of carbonized bismuth-polyaniline (Bi-PANI) on a thermally-treated carbon paper substrate. The battery performance was evaluated in a 5 cm2 flow cell using a ‘zero-gap’ design with a 1.6 M VOSO4 in 3 M H2SO4 electrolyte. Charge-discharge of the VRFB was performed at a constant current density (ranging from 10 to 80 mA cm−2) with upper and lower voltage cut-offs of 1.65 and 0.8 V, respectively. Thermally treated carbon paper electrodes were tested as a comparison, and it was found that the VRFB efficiency was significantly enhanced, from 51% to 68% at a current density of 80 mA cm− 2, due to the modification of the electrodes with Bi-PANI. The charge-discharge capacity was also improved by around 17.2%. In addition, the stability of the battery using the modified electrodes was evaluated for over 200 cycles. The improved battery performance was attributed to the catalytic effect of the Bi particles on the VO2+/VO2 + and V2+/V3+ redox reactions leading to a significant increase in capacity, voltage, and energy efficiency.

The transition to renewable energy systems is critically dependent on the development and optimization of large-scale energy storage technologies, among which Vanadium Redox Flow Batteries (VRFBs) stand out for their scalability, flexibility, and long cycle life. However, the operational efficiency and lifespan of VRFBs are significantly undermined by shunt currents—unwanted internal currents that bypass the electrochemical reaction zones, leading to energy losses and accelerated degradation. Previous studies have predominantly employed lumped electrical circuit models to investigate these currents, a method that, while providing initial insights, substantially oversimplifies the complex interplay of ionic and electronic transport mechanisms within the VRFB electrolytes.This study proposes a novel multiphysics modeling framework that eschews the simplifications of previous models in favor of a detailed representation that captures the coupled dynamics of ionic conduction, electronic conduction, and fluid flow within the VRFB. By integrating the Navier-Stokes equations for fluid dynamics with the Nernst-Planck equation for ionic transport, and the Poisson equation for electric fields, this model aims to provide a holistic view of the factors contributing to shunt currents. This approach not only facilitates a deeper understanding of the underlying physical phenomena but also opens new avenues for identifying effective mitigation strategies.The superiority of this multiphysics approach lies in its ability to accurately simulate the complex interdependencies within the VRFB system, which are overlooked by simpler models. This enables the identification of specific structural and operational factors that contribute to shunt currents, facilitating the development of targeted optimization strategies. Through the application of this advanced modeling framework, the research can precisely quantify the impact of various physical parameters, such as membrane permeability, electrode porosity, and flow channel design, on the magnitude and distribution of shunt currents, guiding the engineering of VRFB components for minimal energy loss and enhanced efficiency.Moreover, the multiphysics model paves the way for innovative optimization techniques, such as the strategic placement of barriers or the adjustment of flow patterns, to disrupt or redirect shunt pathways effectively. These strategies, informed by a deep understanding of the underlying physics, promise to significantly improve VRFB performance, offering a path to more reliable, durable, and efficient energy storage systems.The implications of this research extend beyond academic interest, offering practical insights for the design and operation of more efficient and sustainable VRFBs. As the demand for reliable large-scale energy storage solutions grows in the context of global renewable energy integration, the findings of this study promise to contribute significantly to the advancement of VRFB technology, aligning with the broader goals of energy sustainability and environmental stewardship. References Tang, J. McCann, J. Bao, M. Skyllas-Kazacos, Investigation of the effect of shunt current on battery efficiency and stack temperature in vanadium redox flow battery, J. Power Sources, 242, (2013) 349 - 356, doi: 10.1016/j.jpowsour.2013.05.079.Viswanathan, A. Crawford, D. Stephenson, S. Kim, W. Wang, B. Li, G. Coffey, E. Thomsen, G. Graff, P. Balducci, M. Kintner-Meyer, V. Sprenkle, Cost and performance model for redox flow batteries, J. Power Sources, 247, (2014) 1040 – 1051. doi: 10.1016/j.jpowsour.2012.12.023Skyllas-Kazacos, J. McCann, Y. Li, J. Bao, A. Tang, The Mechanism and Modelling of Shunt Current in the Vanadium Redox Flow Battery, ChemistrySelect, 1(10), (2016) 2249–2256. doi: 10.1002/slct.201600432. Moro, A. Trovò, S. Bortolin, D. Del Col, M. Guarnieri, An alternative low-loss stack architecture for vanadium redox flow battery: comparative assessment, J. Power Sources, 340, (2017) 229-241.doi:10.1016/j.jpowsour.2016.11.042.Trovò, G. Marini, A. Sutto, P. Alotto, M. Giomo, F. Moro, M. Guarnieri, Standby thermal model of a vanadium redox flow battery stack with crossover and shunt-current effects, Appl Energy, 240, (2019) 893-906. doi: 10.1016/j.apenergy.2019.02.067. Trovò, F. Picano, M. Guarnieri, Comparison of energy losses in a 9 kW vanadium redox flow battery, J. Power Sources, 440, (2019) 227144. doi: 10.1016/j.jpowsour.2019.227144. Figure 1

Renewables sources of energy like solar and wind are expected to supply ~35% of the total electricity demand by 2050.1 The intermittent nature of solar and wind necessitates the development of energy storage technologies. Redox flow batteries (RFBs) are a promising technology for storing energy over several hours to days; however, RFBs suffer from inefficiencies in part due to the overvoltages at the electrode surface. Vanadium redox flow batteries (VRFBs) are the most commercialized RFB chemistry till date due to the use of same element both sides of battery, eliminating the issue of cross-contamination. Carbon felts (CFs) are the most commonly used electrodes in RFBs due to their high conductivity and porous nature. More than 70 electrode treatments for VRFBs alone have been previously shown to reduce the overvoltages and improve performance of VRFBs. Some of these CF electrode treatments include thermal, acid/base, elemental doping, and loading with electrocatalysts.2–5 However, the differences in the operating conditions between treatment studies in literature makes the comparison among different treatments very difficult. Additionally, the complexity associated with each treatment makes identification of industrially implementable treatments extremely challenging.In this talk, I will discuss our efforts to tackle the above problem by comparing treatments under similar operating conditions and accounting for the treatment complexity. We compare the VRFB performance, stability, and economic feasibility with various CF treatments, and treatment complexity to identify the most promising industrially implementable treatments. We extract the round-trip energy efficiencies (EE) of VRFBs using CFs with (and without) various treatments from literature at laboratory and industrial scale. The performance is evaluated using the EE at various operating current densities and stability is identified based on the change in EE per cycle. Economic feasibility of a treatment is measured based on the affordable capital cost (ACC) obtained from an in-house techno-economic model. ACC is defined as the maximum capital cost that can be invested in a treatment without an increase in the overall capital cost of VRFB without treated electrodes. The treatment complexity is measured using the number of steps and the process units required in a treatment. Based on carefully chosen metrics for satisfying the performance, stability, economic feasibility, and treatment complexity criteria, we identify the high performing, stable, and economic feasible treatments that are easy to implement industrially. We conduct a cost analysis of these promising treatments by utilizing the similarities in production of carbon fibers and CFs. We encourage the RFB community to standardize of RFB testing procedures for fast industrial implementation of CF treatments as we move towards a society powered by renewable energy.

Highly porous zeolitic-imidazole frameworks (ZIFs) are synthesized to produce N-doped mesoporous carbon electrocatalysts via calcination. The N-doped carbon (m-NC) and carbon nanotubes (m-NCNT) are obtained from ZIF-8 and ZIF-67, while the core-shell structure of ZIF-8@ZIF-67 produced with ZIF-8 seeds (m-NC@NCNT) is prepared by hydrothermal method. Chemical and optical evaluations of the catalysts are characterized using BET, FT-IR, XPS, XRD, Raman spectroscopy and SEM/STEM and they are used as the catalysts for redox reactions of vanadium ions and redox flow battery (VRFB) performance. In the utilization, m-NC@NCNT and m-NCNT are effective for improving VO2+/VO2+ redox reaction, although m-NC does not influence that. Even in VRFB tests using the catalysts, charge/discharge potential and energy efficiency (EE) of m-NC@NCNT and m-NCNT are highest, not to mention excellent EE resilience after undergoing tougher cycling condition. These results are due to the large graphitic-N portion of the two catalysts. Namely, electrons produced by the graphitic-N are delocalized, forming pi-conjugated system and vanadium–nitrogen transition state. This state then promotes electron transfer during VO2+/VO2+ redox reaction and VRFB performance.

Energy storage systems critically assist in the implementation of renewable energy sources. However, greenhouse gas emissions associated with the energy storage methods have received insufficient attention, especially for arid climate implementation. This paper considers three energy storage techniques that can be suitable for hot arid climates namely; compressed air energy storage, vanadium redox flow battery, and molten salt thermal storage and performs a comprehensive life cycle assessment analysis to comparatively evaluate the environmental impacts per kWh of energy. The results show that, when solar photovoltaic electricity is stored, the redox-flow battery has the highest global warming potential, corresponding to 0.121 kg CO2 eq./kWh, whereas the molten salt has the least with a value of 0.0306 kg CO2 eq./kWh. In contrast, the lowest ozone layer depletion is observed for the compressed air storage unit with a value of 7.24×10−13 kg R11 eq./kWh. In sensitivity analysis, it is found that using solar photovoltaic electricity for the considered energy storage methods rather than grid electricity critically reduces the associated environmental impacts, emphasizing the importance of implementing more renewables in the grid mix. The global warming potentials of compressed air and vanadium redox flow battery decrease by 0.599 and 0.420 kg CO2 eq,/kWh, respectively in case photovoltaic electricity is stored instead of grid electricity. It is also found that the production stage of the storage systems accounts for the highest share of carbon footprint.

A study of the effects of electrolyte flow inside the tanks on electrical performance of an industrial-size Vanadium Redox Flow Battery (VRFB) is presented. The concentration of electrolytes feeding the stacks of VRFBs is crucial in determining the device voltage and overall power performance. Usually, the assumption of perfect mixing in the tanks is assumed, but the hydraulic behavior of large VRFB can deviate significantly from this assumption depending on the electrolyte flow rate, stochiometric factor and geometry of the tanks. In this study, the State of Charge (SoC) measurements have been analyzed, starting from the signal produced by an Open Circuit Voltage (OCV) cell. An experimental campaign has been conducted during charge/discharge roundtrip cycles at different current levels and constant stoichiometric factor. Building on the delay in the voltage response of the battery, a fluid-dynamic model was developed to study how the hydraulic behavior of electrolytes inside tanks influences the electrolyte concentration and, consequently, the SoC of the electrolyte that feeds the stack and the electrical time response of the battery. The model indicated that a stratification of charged electrolyte is responsible for this delay resulting in an initial voltage plateau. A different hydraulic behavior was found in each tank and in each charge/discharge phase. These results highlight the tank role in the industrial-scale VRFB performance. References Sanchez-Diez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, “Redox flow batteries: status and perspective towards sustainable stationary energy storage”, J. Power Sources, 481, (2021) 228804.Kurilovich, A. Trovò, M. Pugach, K. J. Stevenson, M. Guarnieri. Prospect of Modeling Industrial Scale Flow Batteries – from Experimental Data to Accurate Overpotential Identification, Renew. Sust. Energ. Rev. 167, (2022) 112559.Trovò, V. Di Noto, J. E. Mengou, C. Gambaro, M. Guarnieri. Fast Response of kW-Class Vanadium Redox Flow Batteries, IEEE Trans. Sustain. Energy 12, (2021) 2413–2422.Trovò, M. Rugna, N. Poli, M. Guarnieri. Prospects for industrial vanadium flow batteries, Ceram. Int., 49 (14), (2023) 24487-24498.Maggiolo, A. Trovò, M. Guarnieri, “Reactant Flow in Flow Batteries”, in L. Cabeza (ed.), Encyclopedia of Energy Storage, vol. 4, pp. 231-242. Oxford: Elsevier. March 2022.A. Prieto-Díaz, S. E. Ibáñez, M. Vera. Fluid dynamics of mixing in the tanks of small vanadium redox flow batteries: Insights from order-of-magnitude estimates and transient two-dimensional simulations, Int. J. Heat Mass Transf. 216, (2023) 124567.Trovò, N. Poli, M. Guarnieri. New strategies for the evaluation of Vanadium Flow Batteries: testing prototypes, Cur. Opin. Chem. Eng. 37, (2022) 100853. Figure 1

The electrode is a key component to optimize in the vanadium redox flow battery (VRFB) to achieve high battery performance and market competitiveness. Generally, carbon based materials are the most prevalent type of electrode utilized in commercial and experimental VRFBs.1 Fibrous carbon materials and their derivatives are widely used as electrodes in VRFB system because they are porous to allow electrolyte penetration, resistant to the electrolyte chemical environment and relatively low-cost. Recently, significant performance improvement of VRFB has been achieved by utilization of carbon paper materials with enhanced surface properties.2 Mass transport is a major factor in the efficiency loss in VRFB, particularly at low states of charge. Ohmic, activation and mass transport overpotential losses cause efficiency loss in an electrochemical system.3The electrode loss is mainly comprised of activation and mass transport losses, while the ohmic loss is mainly due to separator resistance. The activation overpotential is caused by limited reaction rate of redox couple and limited surface area. The mass transport overpotential is caused by reaction active species depletion due to insufficient active species supply on electrode surface. Because of the slow motion of cation in solution, it is critical to understand the relationship between the electrode properties and mass transport in electrode. Future material development and system optimization can benefit from this work. The properties of the carbon electrode can have a strong influence on the active species mass transport inside the cell. In Figure 1, the IR corrected polarization curves of positive and negative redox reactions on SGL 10AA and X-2 in VRFB are shown at varying electrolyte flow rates. The electrode materials were housed in a 5 cm2 battery hardware (Fuel Cell Technologies Inc.) with serpentine flow field. The electrolyte was composed of 1.7 M vanadium ions with 5 M total sulfate/bisulfate. The state of charge was maintained at 55% during the entire polarization curve measurement. In both reactions, the polarization curves show a typical mass transport limiting current density (iMT ) at high overpotential. In both reactions, higher iMT on 10AA indicates the better mass transport towards the surface of 10AA in the experimental scenario. X-2 is advantageous in reaction kinetics, since the activation overpotential is always lower on X-2 than 10AA in both reactions. However, the current density on 10AA generally surpasses that of X-2 at elevated overpotential. This trend should be attributed to better mass transport property of 10AA, though the detailed mechanism is not clear yet. We will present a thorough study of the mass transport properties of carbon paper electrode in VRFB cell. A systematic study of the physical properties and their influence on the convective and diffusive transport of redox active species within the electrode will be discussed. The transport study will be correlated to electrochemical test to develop a protocol to evaluate the mass transport properties’ influence on battery’s performance. This work is supported by Dr. Imre Gyuk, Office of Electricity Delivery and Energy Reliability, US Department of Energy. Reference: 1. M. H. Chakrabarti, N. P. Brandon, S. A. Hajimolana, F. Tariq, V. Yufit, M. A. Hashim, M. A. Hussain, C. T. J. Low, and P. V. Aravind, J. Power Sources, 253, 150–166 (2014) 2. A. M. Pezeshki, J. T. Clement, G. M. Veith, T. A. Zawodzinski, and M. M. Mench, J. Power Sources, 294, 333–338 (2015) 3. D. S. Aaron, Z. Tang, A. B. Papandrew, and T. A. Zawodzinski, J. Appl. Electrochem., 41, 1175–1182 (2011). Figure 1. IR corrected discharging polarization curves of negative and positive redox couple on SGL 10AA and X-2 in 5 cm2 FCT flow battery cell of varying flow rates with 1.7 M V2/3+/5 M bisulfate/sulfate electrolyte at 55% SoC. Figure 1

A pluggable current collector for in-operando current measurements in all-vanadium redox flow batteries with flow field

Recent advances in redox flow batteries (RFB) have made them a viable option for grid-scale energy storage. Vanadium redox flow batteries (VRFB) are the most prominent and commercially viable, however, when compared to lithium-ion batteries they have low energy density due to low cell potentials (U = 1.5 V in VRFB). To boost cell potentials, researchers have begun exploring organic platforms. These can be engineered to exhibit extreme potentials while offering high tunability, earth abundance, and cost-effectiveness. Here, we investigated the viability of phosphorus containing organic molecules as anolytes in RFB. Through molecular engineering, we developed a series of novel, sustainably derived, organic anolytes with highly negative redox potentials (–2.0 to –2.5 V vs. Fc/Fc+) that operate in acetonitrile and dimethylformamide, allowing for a wide cell potential. Through electrochemical studies using cyclic voltammetry, charge/discharge cycling, and various mechanistic studies (decomposition studies, EPR analysis, X-ray crystallography), we found an excellent candidate. Guided by this discovery, we tuned our system to create a long-lived anolyte which exhibited no capacity fade over 350 cycles with an average coulombic efficiency of 99.3%. When paired with an appropriate catholyte (e.g. E1/2 ≥ 0.5 V), an organophosphorus-based RFB could double the cell potential (U ~ 3V) and energy density found in VRFB. This performance establishes our anolyte as one of the most negative and stable reported in non-aqueous RFB. Figure 1

Vanadium redox flow batteries (VRFBs) are an attractive technology for a variety of energy storage applications.1,2 The catholyte and the anolyte in these batteries are circulated through the electrodes from reservoirs. The active species in the catholyte are VO2+ and VO2 + (i.e. VIV and VV) while the active species in the anolyte are V3+ and V2+. VRFBs have a major advantage over other flow batteries in that cross-contamination due to transport through the separating membrane is effectively eliminated.3 Monitoring of the state of charge (SoC) is important for any battery system. Additionally, in VRFBs, transfer of vanadium ions across the membrane4 and side reactions such as hydrogen formation5 at the negative electrode can result in SoC becoming unbalanced (e.g. more VV on the positive side than VII on the negative). Therefore independent monitoring of SoC of both electrolytes is important for effective operation of VRFB technology.Both overall SoC of a VRFB and individual SoCs of the positive and negative sides (determined by the VIII/VII and VIV/VV ratios in the respective electrolytes) may be monitored in a number of ways using electrodes. However, these methods have drawbacks.6 Spectroscopic monitoring of SoC is independent of electrochemistry and offers the possibility of performing in-situ analysis. VII, VIII, VIV, and VV aqueous species have strong absorbance spectra in the visible region.6- 12 If the absorbance is a linear combination of that of the constituents UV-visible spectroscopy is a straightforward method of measuring the concentration and ratio of mixtures: e.g. for VIII-VII mixtures or very dilute VIV-VV mixtures.6-12 At higher concentrations, the absorbance of VIV-VV mixtures is a highly non-linear function of the mole fraction of VIV. For VIV-VV mixtures, this non-linearity has been shown to be due to the formation of a complex between the VIV and VV species. 6-8 Tang et al. 10 and Liu et al. 11 addressed the problem of non-linearity by developing an empirical method of estimating SoC. However, the non-linearity can be quantitatively explained6-8 allowing precise methods12 of optical monitoring of SoC in VRFBs. Factors, such as the concentration of sulphate and vanadium, can significantly affect this non-linearity.6,8 In this presentation, we present a detailed study of these factors in relation to spectroscopic SoC measurement. Acknowledgements The authors acknowledge funding from Enterprise Ireland through Commercialisation Fund CF/2013/3303. The material in this research is partly based upon works supported by Science Foundation Ireland through the Charles Parson Initiative (CPI). References H. Bindner, C. Ekman, O. Gehrke and F. Isleifsson, in Characterization of Vanadium Flow Battery, Risø Report (2011)M.J. Watt-Smith, P. Ridley, R.G.A. Wills, A.A. Shah and F.C. Walsh, J. Chem. Technol. Biotechnol. 88, 126 (2013)M. Rychcik and M. Skyllas-Kazacos, J. Power Sources 22, 59 (1988)J. Xi, Z. Wu, X. Qiu and L. Chen, J. Power Sources 166, 531 (2007)A.H. Whiteheadand M. Harrer, J. Power Sources 230, 271 (2013)D.N. Buckley, X. Gao, R.P. Lynch, N. Quill, and M.J. Leahy, J. Electrochem. Soc. 161 (4), A524 (2014)X. Gao, R.P. Lynch, M. Leahy and D. N. Buckley, ECS Transactions 45 (26), 25 (2013)P. Blanc, C. Madic and J.P. Launay, Inorg. Chem. 21, 2923 (1982)M. Skyllas-Kazacos and M. Kazacos, J. Power Sources 196, 8822 (2011)Z. Tang, D.S. Aaron, A.B. Papandrew and T.A. Zawodzinski, Jr., ECS Trans. 41 (23), 1 (2012)L. Liu, J. Xi, Z. Wu, W. Zhang, H. Zhou, W. Li and X. Qiu, J. Appl. Electrochem. 42(12), 1025 (2012)D.N. Buckley, X. Gao, R.P. Lynch, M.J. Leahy, A. Bourkeand G. Flynn, European Patent EP 13195315 (Application Date: 2 December 2013)

An increasing number of studies focus on organic flow batteries (OFBs) as possible substitutes for the vanadium flow battery (VFB), featuring anthraquinone derivatives, such as anthraquinone-2,7-disulfonic acid (2,7-AQDS). VFBs have been postulated as a promising energy storage technology. However, the fluctuating cost of vanadium minerals and risky supply chains have hampered their implementation, while OFBs could be prepared from renewable raw materials. A critical component of flow batteries is the electrode material, which can determine the power density and energy efficiency. Yet, and in contrast to VFBs, studies on electrodes tailored for OFBs are scarce. Hence, in this work, we propose the modification of commercial carbon felts with reduced graphene oxide (rGO) and poly(ethylene glycol) for the 2,7-AQDS redox couple and to preliminarily assess its effects on the efficiency of a 2,7-AQDS/ferrocyanide flow battery. Results are compared to those of a VFB to evaluate if the benefits of the modification are transferable to OFBs. The modification of carbon felts with surface oxygen groups introduced by the presence of rGO enhanced both its hydrophilicity and surface area, favoring the catalytic activity toward VFB and OFB reactions. The results are promising, given the improved behavior of the modified electrodes. Parallels are established between the electrodes of both FB technologies.

This paper focuses on technology state of the art for the charge/discharge of electric energy storage supported by vanadium redox flow battery linked to the electric grid. Properties of vanadium, the main configuration and the reaction of charge/discharge of a vanadium redox flow battery are addressed. The vanadium redox flow battery has the highest cell voltage among the other redox flow battery, implying higher power and energy density which favours application at power plants. This electric energy storage is viewed as a promising contribution to be integrated in power system due to a reasonably bulky size and to successful applications currently allowing storage of energy at power plants or at electrical grids. For instances, allowing storage of energy as an economic improvement providing spin reserve to avoid penalty for imbalances between the energy delivered and energy contracted at closing of electricity market or as an economic improvement to diminish the cost of electricity usage of a consumer. The vanadium redox flow battery has the advantages of scalability customized to meet requirements for power and energy capacity and of excellent combination of energy efficiency, capital cost and life cycle costs compared with other technology.