The Influence of Electric Field on Crossover in Redox-Flow Batteries

149

177

We report here the synthesis, characterization and properties of 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid (DHDMBS) as a new positive side electrolyte material for aqueous organic redox flow batteries (ORBAT). We have synthesized this material in pure form in high yield and confirmed its structure. We have determined that the standard reduction potential, the rate constant of the redox reaction, and the diffusion coefficient are ideally suited for use in ORBAT. Specifically, DHDMBS overcomes the major issue of Michael reaction with water faced with 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) and similar unsubstituted benzoquinones in the selection of positive electrolyte materials. DHDMBS can be synthesized relatively inexpensively. We have demonstrated the chemical stability of DHDMBS to repeated electrochemical cycling through NMR and electrochemical studies proving the absence of products of the Michael reaction. A flow cell with DHDMBS and anthraquinone-2,7-disulfonic acid has now been shown to operate close to 100% coulombic efficiency for over 25 cycles when continuously cycled at 100 mA/cm 2 , and can sustain current densities as high as 500 mA/cm 2 without noticeable chemical degradation. However, there was a slow decrease in the capacity of the flow cell attributable to the crossover of DHDMBS from the positive side of the cell. Thus, the present study has shown DHDMBS as a promising candidate for the positive side material for an all-organic aqueous redox flow battery in acidic media, and our future efforts will focus on understanding the crossover of DHDMBS and the effects of long-term cycling. As intermittent renewable energy sources based on solar photovoltaics and wind turbines are installed worldwide, electrical energy storage facilities will be of paramount importance to maintain the stability of the electricity grid and to meet the growing demand for clean energy. Rechargeable batteries have shown promise in meeting this large energy storage demand, because of their high efficiency and scalability.1-3 Redox flow batteries (RFBs) are especially attractive for stationary applications due to their scalability and their inherent ability to address the power and energy requirements independently.4,5 However, none of today's battery technologies can meet the demands of robustness, low-cost, and environmental-friendliness simultaneously. 6 Recently, flow batteries based on aqueous solutions of simple watersoluble organic redox couples, or ORBAT, have become the subject of great interest because of their prospect of offering such a gridscale energy storage solution. Thus, the investigation into simple organic molecules that can be readily synthesized or procured for use in ORBAT has grown significantly. 7-12Specifically, we and others have found that quinone-and anthraquinone-based molecules have the essential electrochemical characteristics for the design of ORBAT. [13][14][15][16][17][18][19] We first reported the properties of an "all-quinone" organic redox flow battery in 2014 in which the ...

Section: Resultsmentioning

confidence: 99%

A New Michael-Reaction-Resistant Benzoquinone for Aqueous Organic Redox Flow Batteries

Hoober-Burkhardt

Krishnamoorthy

Yang

et al. 2017

149

177

“…These results agree qualitatively with recent literature , where the authors also found that increased current density operation results in improved coulombic efficiency. 42 For further insight into the relative rates of migration and diffusion and how these affect the net crossover, and ultimately coulombic efficiency, see reference 42. Experiments at high SoC had more rapid crossover than those at low SoC: at high current, the coulombic efficiency of experiment D (97.4%) was lower than B (98.3%), and at low current, experiments C and F (90.2, 89.5%) had lower coulombic efficiencies than A and E (91.6, 91.4%).…”

Section: Resultsmentioning

confidence: 99%

The Cell-in-Series Method: A Technique for Accelerated Electrode Degradation in Redox Flow Batteries

Pezeshki

Sacci

Veith

et al. 2015

We demonstrate a novel method to accelerate electrode degradation in redox flow batteries and apply this method to the all-vanadium chemistry. Electrode performance degradation occurred seven times faster than in a typical cycling experiment, enabling rapid evaluation of materials. This method also enables the steady-state study of electrodes. In this manner, it is possible to delineate whether specific operating conditions induce performance degradation; we found that both aggressively charging and discharging result in performance loss. Post-mortem x-ray photoelectron spectroscopy of the degraded electrodes was used to resolve the effects of state of charge (SoC) and current on the electrode surface chemistry. For the electrode material tested in this work, we found evidence that a loss of oxygen content on the negative electrode cannot explain decreased cell performance. Furthermore, the effects of decreased electrode and membrane performance on capacity fade in a typical cycling battery were decoupled from crossover; electrode and membrane performance decay were responsible for a 22% fade in capacity, while crossover caused a 12% fade. This paper is part of the JES Focus Issue on Redox Flow Batteries-Reversible Fuel Cells.The vanadium redox flow battery (VRFB) is a potential gridscale energy storage technology that has attracted significant research interest.1-5 The cell architecture, electrodes, and membranes have been considerably optimized, resulting in increased efficiency and power density of VRFBs.3,5-10 Other research has focused on improving energy density by enhancing vanadium solubility through the use of additives in the electrolyte solution; these compounds may also enhance electrode activity. [11][12][13] As these improvements continue, the long-term durability of VRFBs becomes an issue with strong implications for commercial viability. Therefore, methods to study component durability are of growing interest.Unlike traditional secondary batteries, VRFBs do not suffer from appreciable unrecoverable capacity fade. They promise to have long lifetimes due to the ability to rebalance the electrolyte, 14-16 which negates capacity fade and may allow them to last for thousands of cycles and for several years. While the vanadium electrolyte is stable, 17 the durability of some components in the VRFB has not been studied in detail. Changes in the electrode can result in increased activation/ charge transfer resistance and decreased mass transport; changes in the membrane may result in increased ohmic resistance in the cell. These phenomena contribute to a decrease in the capacity that can be accessed at high currents as well as to reduced energy efficiency. Herein, we present a methodology and initial findings for studying performance degradation in VRFBs. The energy efficiency loss in a typical VRFB cycling experiment is deconvoluted into contributions from decreased electrode performance and increased ohmic resistance within the membrane. The capacity fade mechanism, often attributed to crossover alone...

“…[28][29][30][31][32] It has been observed in VRFBs that vanadium moves across the membrane due to the concentration gradient, while migration enhances or decreases the crossover flux depending on the relative direction of the concentration and potential gradients. 33 The crossover flux of VO 2+ has been seen to approach the diffusive flux at Open Circuit Potential (OCP) under large negative current densities, whereas it can reach the sum of migration and electro-osmotic fluxes at large positive current densities. 33 A model that explained the crossover of all ionic species in VRFBs has been recently reported, 30,31 which indicated that convection transport produces the major contribution to vanadium ions crossover.…”

mentioning

confidence: 99%

A Unit Cell Model of a Regenerative Hydrogen-Vanadium Fuel Cell

Pino-Muñoz¹,

Dewage²,

Yufit³

et al. 2017

In this study, a time dependent model for a regenerative hydrogen-vanadium fuel cell is introduced. This lumped isothermal model is based on mass conservation and electrochemical kinetics, and it simulates the cell working potential considering the major ohmic resistances, a complete Butler-Volmer kinetics for the cathode overpotential and a Tafel-Volmer kinetics near mass-transport free conditions for the anode overpotential. Comparison of model simulations against experimental data was performed by using a 25 cm 2 lab scale prototype operated in galvanostatic mode at different current density values (50 − 600 A m −2 ). A complete Nernst equation derived from thermodynamic principles was fitted to open circuit potential data, enabling a global activity coefficient to be estimated. The model prediction of the cell potential of one single charge-discharge cycle at a current density of 400 A m −2 was used to calibrate the model and a model validation was carried out against six additional data sets, which showed a reasonably good agreement between the model simulation of the cell potential and the experimental data with a Root Mean Square Error (RMSE) in the range of 0.3-6.1% and 1.3-8.8% for charge and discharge, respectively. The results for the evolution of species concentrations in the cathode and anode are presented for one data set. The proposed model permits study of the key factors that limit the performance of the system and is capable of converging to a meaningful solution relatively fast (s-min). Redox flow batteries are considered to be an exceptional candidate for grid-scale energy storage. One attractive feature is their capability to decouple power and energy.1-4 All-Vanadium Redox Flow Batteries (VRFBs) have been considered a promising system due to the limited impact of cross-contamination. However, they have faced challenges related to cost, scale-up and optimization. Current research is also focused on improvement of electrolyte stability for use over a wider temperature window and concentrations, development of electrode materials resistant to overcharge, and mitigation of membrane degradation.1,2 Cost dependency with regarding to vanadium can be mitigated through utilization of new systems that employ only half of the vanadium.1 Recently, a Regenerative Hydrogen-Vanadium Fuel Cell (RHVFC) based on an aqueous vanadium electrolyte V(V) and V(IV) and hydrogen has been introduced 5 and is illustrated schematically in Figure 1. This system contains a porous carbon layer for the positive electrode reaction, membrane and catalyzed porous carbon layer for the negative electrode reaction. Hydrogen evolution, which is an adverse reaction in VRFBs, is here the main anodic process. During discharge, V(V) is reduced to V(IV) and H 2 is oxidized, while the reverse process occurs during charge and H 2 is stored. The vanadium reaction takes place in the positive electrode (cathode), while the hydrogen reaction occurs in the catalyst layer (CL) of the negative electrode (anode). The redox reactions that occur ...