Redox Flow Battery Operation Research Articles

Realized potential as neutral pH flow batteries achieve high power densities

High power density operation of redox flow batteries (RFBs) is essential for lowering system costs, but until now, only acid-based chemistries have achieved such performance, primarily due to rapid membrane proton (H + ) transport. Here, we report a neutral pH RFB using the highly reducing Cr-(1,3-propylenediaminetetraacetate) (CrPDTA) complex that achieves acid-like power performance while utilizing potassium ion (K + ) transport. We investigate RFB resistance components and demonstrate the high and consistent K + conductivity of the Fumasep E-620(K) membrane. When combined with a robust bismuth electrocatalyst, this membrane enables constant voltage efficiency operation of a CrPDTA|Fe(CN) 6 RFB for 200 cycles. An optimized CrPDTA|Fe(CN) 6 RFB, which combines a high cell potential with a low area-specific resistance (0.46 Ω cm 2 ), demonstrates a maximum discharge power density of 1.63 W cm −2 and an average discharge power density over 500 mW cm −2 while maintaining 80% round-trip energy efficiency cycling, which are records for non-acid-based RFBs. • High-power flow battery operates at neutral pH with potassium membrane transport • Record discharge-power performance for a flow battery not using proton transport • Inexpensive membrane provides high conductivity and constant efficiency cycling • Chelated metal electrolyte enables high voltage battery and high-power operation High-power flow battery operation lowers system costs but has previously required proton transport. By combining high voltage with low resistance from a highly selective and conductive membrane, Robb et al. demonstrate an aqueous flow battery that achieves record non-acidic power performance while utilizing potassium membrane transport at neutral pH.

High-Energy-Density Chelated Chromium Flow Battery Electrolyte at Neutral pH.

High-concentration operation of redox flow batteries (RFBs) is essential for increasing their energy-storage capacity, but non-acidic electrolytes struggle to achieve the high concentrations of metal ions dissolved in acid, limiting the development of energy-dense neutral pH electrolytes. We report neutral pH RFB operation of chromium 1,3-propylenediaminetetraacetate (CrPDTA) at concentrations of 1.2 M at room temperature and 1.6 M at 40 °C, demonstrating 60% higher negolyte capacity, up to 42.9 Ah L-1 , than previously reported for non-additive-utilizing solutions of this promising material. With extended full cell cycling, we demonstrate the importance of buffer selection and pH when using the Fumasep E-620(K) membrane. Finally, we expand the pH operation range of CrPDTA to pH 7, which when cycled at 100 mA cm-2 against a ferrocyanide posolyte demonstrated excellent coulombic efficiencies >99.7% and energy efficiencies >87%, while operating at almost 700 mV more negative than the thermodynamic hydrogen evolution window.

A pore-scale study for reactive transport processes in double-layer gradient electrode as negative side of a deep eutectic solvent electrolyte-based vanadium-iron redox flow battery

Porous electrodes are critical component of redox flow batteries (RFBs), that decide the fluid, species/charge transport and electrochemical reaction properties during the operation of RFBs. It is a potential way to elevate the performance of RFBs by applying the porous electrodes with gradient pore structures. However, it is still lack of a suitable numerical model to understand the reactive transport processes in the heterogeneous pore structures, thus to design and develop the optimal RFB electrodes. Using a lattice Boltzmann model, this work studies the pore-scale reactive transfer behaviors in a double-layer gradient electrode assembling as the negative side of deep eutectic solvents electrolyte-based vanadium-iron RFB, that consists of a graphite felt near the current collector side and a carbon paper near the membrane side. The numerical results reveal the galvanostatic discharging performance of double-layer electrode under the different electrolyte feeding modes and flow rates. Comparing with full flow-through feeding mode, the interdigitated flow feeding mode would give rise to serious inhomogeneity of electrolyte flowing at the low flow rate condition, thereby resulting in the sluggish convective mass transfer performance near the membrane side. Moreover, the total power loss of double-layer electrode is higher than that of the mono-layer carbon paper electrode within the low flow rate range. Conversely, the double-layer electrode with interdigitated flow mode can achieve the lowest total power loss than mono-layer electrodes at the high inlet flow rate. This work provides a new pathway to investigate the RFB power performance effected by the gradient porous structures of electrodes.

Assessing the Design and Operation of Redox Flow Batteries through Levelized Cost Analysis

Energy storage is expected to play an important role in enabling deep decarbonization of the electric sector by addressing the intermittencies of renewable power generation1. The redox flow battery (RFB) is a potential energy storage solution whose unique decoupling of energy and power make it increasingly competitive, on a capital cost basis, at longer discharge durations2. While the capital cost benefits to RFBs are well-described in the literature3, there are additional economic benefits associated with operation and maintenance of open systems (e.g., tune-ups, sparing strategies)4 as compared to closed systems like lithium-ion batteries. For example, crossover, undesirable species transport through the semi-permeable membrane that separates the positive and negative electrolytes, leads to comparatively rapid capacity fade in RFBs5 but can be remediated via electrolyte rebalancing, replacement, or other servicing, all of which can be performed without sacrificing or altering the reactor components. The costs necessary to maintain battery performance over time impact its economic viability, but are not captured in the conventional capital cost estimations6. This motivates the development of techno-economic models that consider the variable operating principles of different battery formats and chemistries.In this presentation, we describe a simple levelized cost of storage (LCOS) model for RFBs that captures long-term performance changes and maintenance costs by including capacity fade and recovery7. We use this model to assess the impact of different design and operational decisions on RFB cost. Specifically, we contemplate different chemistries (symmetric vs. asymmetric, finite lifetime vs. infinite lifetime), operating strategies (e.g., rebalancing schedule), performance improvements (e.g., reducing fade rates), design decisions (e.g., battery sizing), and investment approaches (e.g., electrolyte leasing). We find that there are tradeoffs in capital and operating expenses, and in many cases upfront investments pay off in long-term savings. We anticipate this analysis will provide new insights into the cost-drivers for RFBs and motivate further research efforts in the evaluation and development of new chemistries, component materials, and reactor configurations. Acknowledgements We gratefully acknowledge funding from the MIT Energy Initiative. References Intergovernmental Panel on Climate Change. Global Warming of 1.5 C. https://www.ipcc.ch/sr15/ (2018).Darling, R. M., Gallagher, K. G., Kowalski, J. A., Seungbum, H. & Brushett, F. R. Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries. Energy Environ. Sci. 7, 3459–3477 (2014).Viswanathan, V. et al. Cost and performance model for redox flow batteries. J. Power Sources 247, 1040–1051 (2014).Yuan, X.-Z. et al. A review of all-vanadium redox flow battery durability: Degradation mechanisms and mitigation strategies. Int. J. Energy Res. 1–40 (2019) doi:10.1002/er.4607.Prifti, H., Parasuraman, A., Winardi, S., Lim, T. M. & Skyllas-Kazacos, M. Membranes for redox flow battery applications. Membranes vol. 2 275–306 (2012).US Department of Energy. Grid Energy Storage. https://www.energy.gov/sites/prod/files/2014/09/f18/Grid Energy Storage December 2013.pdf (2013).Rodby, K. E. et al. Assessing the levelized cost of vanadium redox flow batteries with capacity fade and rebalancing. J. Power Sources 460, 227958 (2020).

A combined in situ monitoring approach for half cell state of charge and state of health of vanadium redox flow batteries

Efficient vanadium redox flow battery operation requires reliable half cell specific state of charge and capacity information at each point in time. This study uses long term correlations of electrolyte densities with negative half cell capacities and half cell potentials in Nernst equations for a joint in situ measurement approach. In principle, no ex situ reference data are needed for the calibration since a combination of Coulomb counting and half cell electrolyte potentials enables a self consistent calibration during battery operation cycles. By this way state of charge results of a negative half cell feasibility study are confirmed for long term data and extended to the positive half cell. The explicit consideration of H+ concentration and acid dissociation for the positive half cell leads to consistent half cell measurements with lower errors. Experiments have been carried out with two different cation exchange membranes achieving comparable measurement errors in the few per cent range.

Systematic approaches to improving the performance of polyoxometalates in non-aqueous redox flow batteries

Polyoxometalates have been explored as multi-electron active species in both aqueous and non-aqueous redox flow batteries. Although non-aqueous systems in principle offer a wider voltage window for redox flow battery operation, realization of this potential requires a judicious choice of solvent as well as polyoxometalate properties. We demonstrate here the superior performance of N,N-dimethylformamide (DMF) compared to acetonitrile as a solvent for redox flow batteries based on Li3PMo12O40. This compound displays two 1-electron transfers in acetonitrile but can access an extra quasi-reversible 2-electron redox process in DMF. A cell containing 10 mM solution of Li3PMo12O40 in DMF produced a cell voltage of 0.7 V with 2-electron transfers (State of Charge = 60%) and showed a good cyclability. As a means to boost energy density, operation of the redox flow battery at a higher concentration of 0.1 M Li3PMo12O40 produced cells with cell voltage of 0.6 V in acetonitrile and a cell voltage of 1.0 V in DMF; both showed excellent coulombic efficiencies of more than 90% over the course of 30 cycles. Energy density was also increased by employing an asymmetric cell with different polyoxometalates on each side to extend cell voltage. Li6P2W18O62 exhibited 3 quasi-reversible 2-electron transfers in the potential range between −2.05 V and −0.5 V vs. Ag/Ag+. 10 mM Li6P2W18O62/Li3PMo12O40 in DMF produced a cell with cell voltage of 1.3 V involving 4-electron transfers (State of Charge = 50%) with coulombic efficiency of nearly 100% and energy efficiency of nearly 70% throughout the test with more than 20 cycles. These promising results demonstrate proof-of-concept approaches to improving the performance of polyoxometalates in non-aqueous redox flow batteries.

Lattice Boltzmann model for complex transfer behaviors in porous electrode of all copper redox flow battery with deep eutectic solvent electrolyte

Deep eutectic solvents (DESs) as temperature sensitive non-aqueous electrolytes have been proposed to elevate the energy density of redox flow batteries (RFBs). In this paper, a pore scale multiple-relaxation-time (MRT) lattice Boltzmann (LB) model is established to predict the electrolyte flow, species/charge transfer behaviors and electrochemical reaction in the porous electrode of all copper RFB with DES electrolyte at the different operating temperature. The effects of morphological characteristics of reconstructed porous electrode and transport properties of DES on the RFB performance during charging are investigated using this LB model. The numerical results suggest that the decreasing porosity results in a higher electrolyte utilization efficiency at the same required pumping power, and reduces the average overpotential under the galvanostatic charging condition. The operating temperature plays a key role in the operation of RFB with DES electrolyte. The flow rate of DES electrolyte are improved sufficiently by the increasing operating temperature. Meanwhile, the average overpotential diminishes observably as the elevated operating temperature. In addition, the higher operating temperature also improves the uniformity of local transfer current density especially when the operating temperature is lower than 313 K.

Membrane resistance of different separator materials in a vanadium redox flow battery

The membrane in vanadium redox flow batteries has various influences on the performance during operation due to their ohmic losses and crossover phenomena. In this study, the area specific membrane resistances of different materials (cation exchange membranes, anion exchange membranes and a porous separator) were measured under conditions relevant for practical vanadium redox flow battery operation by means of electrochemical impedance spectroscopy. The transient behavior of the membrane resistance upon current density changes was found to be strongly dependent on the thickness and the internal structure of the tested materials. In most cases, the membrane resistance decreased with increasing current density, while only the FAP-375-PP membrane showed an increasing resistance during discharge. For thin membranes and especially the Celgard® 3401 separator, a higher resistance was determined than expected from thickness and electrolyte conductivity. Regarding the influence of the state of charge on the resistance, only the separator and the FAP-375-PP membrane revealed the behavior expected from the electrolyte conductivity. Overall, our results clearly show that the processes in the membranes contributing to their resistance are highly complex and require further investigations.

Effect of channel dimensions of serpentine flow fields on the performance of a vanadium redox flow battery

Abstract For high current density operation of redox flow batteries, proper circulation of the electrolytes over the active area of the cell is important. In the present paper, we study experimentally the effect of channel dimensions of a serpentine flow field for vanadium redox flow battery applications. The study encompasses three aspects: effect on the extent of felt intrusion into the flow channels, effect on the pressure drop over a cell for a given electrolyte circulation rate, and effect on the electrochemical performance characterized by a number of parameters. Eight variants of channel and/or rib dimensions in serpentine flow fields have been studied in cells of 400 cm2 and 900 cm2 active area. A lumped parameter model, validated with water and electrolyte circulation data, has been developed to predict the pressure loss and flow apportionment in the cell. The results show that sensitivity to channel dimensions is more keenly felt in the larger cell with noticeable improvements in pressure drop, peak power density and discharge energy density. Increasing channel width and decreasing rib width is recommended as the ideal strategy to improve the overall performance of the cell.

Handling Complexity of Semisolid Redox Flow Battery Operation Principles through Mechanistic Simulations

In this paper, the semisolid redox flow battery (SSRFB) in static mode is investigated through a kinetic Monte Carlo-based mechanistic model. Electrochemical reactions are said to occur in particle...

Cardo-Polyetherketone Anion Exchange Membranes with Suitable Chemical/Mechanical Stability and Performance for All-Vanadium Redox Flow Battery

Cardo-polyethereketone (PEK-C) based anion exchange membrane (AEM) was prepared by quaternization using trimethylammonium. The structures of intermediates and AEMs were confirmed by NMR spectroscopy. The ion exchange capacity of the PEK-C AEMs was 1.4±0.1 mmol g-1, which resulted in membranes with an ionic conductivity (sulfate form, 30 oC) of 5.6 ± 0.5 mS cm-1 and a vanadium (IV) permeability (at room temperature) of 8.2±0.2 ×10-9 cm2 s-1. Stability tests performed by immersion of PEK-C based AEMs in 1.5M VO2 + oxidizing solution during two months resulted in no change in the chemical structure of the polymer as demonstrated by 1H NMR, 13C NMR and 1H-13C heteronuclear multiple quantum correlation (HMQC). PEK-C based AEMs were also stable during vanadium redox flow battery operation (30 mAcm-2) for 50 charge/discharge cycles and showed coulombic and energy efficiency of 98% and 80%, respectively.

Nanoporous Polytetrafluoroethylene/Silica Composite Separator as a High‐Performance All‐Vanadium Redox Flow Battery Membrane

AbstractA novel low‐cost nanoporous polytetrafluoroethylene (PTFE)/silica composite separator has been prepared and evaluated for its use in an all‐vanadium redox flow battery (VRB). The separator consists of silica particles enmeshed in a PTFE fibril matrix. It possesses unique nanoporous structures with an average pore size of 38 nm and a porosity of 48%. These pores function as the ion transport channels during redox flow battery operation. This separator provides excellent electrochemical performance in the mixed‐acid VRB system. The VRB using this separator delivers impressive energy efficiency, rate capability, and temperature tolerance. In additon, the flow cell using the novel separator also demonstrates an exceptional capacity retention capability over extended cycling, thus offering excellent stability for long‐term operation. The characteristics of low cost, excellent electrochemical performance and proven chemical stability afford the PTFE/silica nanoporous separator great potential as a substitute for the Nafion membrane used in VRB applications.

State of charge monitoring methods for vanadium redox flow battery control

During operation of redox flow batteries, differential transfer of ions and electrolyte across the membrane and gassing side reactions during charging, can lead to an imbalance between the two half-cells that results in loss of capacity. This capacity loss can be corrected by either simple remixing of the two solutions, or by chemical or electrochemical rebalancing. In order to develop automated electrolyte management systems therefore, the state-of-charge of each half-cell electrolyte needs to be known. In this study, two state-of-charge monitoring methods are investigated for use in the vanadium redox flow battery. The first method utilizes conductivity measurements to independently measure the state-of-charge of each half-cell electrolyte. The second method is based on spectrophotometric principles and uses the different colours of the charged and discharged anolyte and catholyte to monitor system balance and state-of charge of each half-cell of the VRB during operation.