Performance Of Vanadium Redox Flow Battery Research Articles

Sulfonated polysulfone proton exchange membrane influenced by a varied sulfonation degree for vanadium redox flow battery

A series of sulfonated polysulfone (SPSF) proton exchange membranes with varied degree of sulfonation (DS) are prepared through controlling the molar ratio of sulfonated agent and polysulfone (PSF). An optimum DS for SPSF membranes is given by characterizing the physicochemical properties, ion selectivity and vanadium redox flow battery (VRFB) performance. SPSF membrane with DS = 62% (SPSF-62) demonstrates the best membrane performance, where the ion selectivity is 4.6 × 103 S min cm−3 that is higher than that of Nafion 117 (4.0 × 103 S min cm−3). At current density of 50 and 100 mA cm−2, coulombic efficiency (CE, 94.9% and 98.8%) and energy efficiency (EE, 89.2% and 86.2%) of SPSF-62 are much higher than that of Nafion 117 (CE, 91.3% and 96.5%; EE, 81.7% and 75.7%), respectively. In the meantime, the SPSF-62 membrane demonstrates a good durability over 100-time charging-discharging cycles. The present study of SPSF membranes proves a promising VRFB application as proton exchange membrane.

Isolation of Mass Transport and Current Distribution in Vanadium Flow Batteries Via Segmented Strip Cell

Vanadium redox flow batteries (VRFBs) have drawn considerable attention due to advantageous characteristics including flexible design, high coulombic efficiency, and long cycle life. But a number of challenges must be overcome for commercial implementation [1]. At the cell level, mass transport limitations can contribute to performance losses, limiting VRFB performance and depth of discharge, especially for cells operating at high power densities [2–4]. Mass transport losses are associated with inadequate delivery of active species to electrode surfaces and are controlled by multiple physicochemical phenomena. Therefore, understanding mass transport mechanisms in the porous electrode is crucial to understanding pathways to achieve optimal performance and a higher depth of discharge. In this study, bulk motion and concentration-driven mass transport mechanisms are qualitatively investigated utilizing a simplified test bed with a segmented strip cell architecture, having only one straight channel and a total of 1 cm2 active area as it is shown in Figure. The simple strip cell design has no gap between cell components [5], allowing the control and isolation of mass transport mechanisms by suppressing higher-dimensional behaviors, and measurement of localized current along the channel path. Thus, the impact of individual transport mechanisms on current distribution can be evaluated with precision. The diffusion-dominated and convection-dominated mass transport regimes are achieved by employing various channel depths (0.25 mm, 0.5 mm, 1 mm, 2.5 mm). Fully segmented, printed circuit board (PCB)-based, localized current distribution measurement is implemented on all strip cell configurations to obtain current distribution along the channel. Major parameters affecting current distribution considered in this work include constant voltage operation, channel depth, and flow rate. It is shown that charging is not a mass transport-limited process for VRFBs. However, once the mass transport-limited region is reached during discharge, non-uniform current distributions are observed. The highest current deviation down the channel is obtained for the deepest channel (2.5 mm) cell configuration. This type of current distribution behavior is attributed to concentration gradient-driven mass transport dominating in the electrode. As channel depth decreases, current distribution is flattened from inlet to outlet as a result of improved bulk motion of the active species in the electrode. It is also observed that the flow rate contribution for improving bulk motion of the active species in the electrode is more distinct in convection-dominated cells (0.25 mm, 0.5 mm) than diffusion-dominated cells (2.5 mm, 1 mm). This work reveals that it is possible to achieve enhanced electrochemical performance with reduced pressure drop in convection-dominated cells. Gandomi, Y. A., Aaron, D. S., Houser, J. R., Daugherty, M. C., Clement, J. T., Pezeshki, A. M., Ertugrul, T. Y., Moseley, D. P., and Mench, M. M. “Critical Review — Experimental Diagnostics and Material Characterization Techniques Used on Redox Flow Batteries” J. Electrochem. Soc. 165, no. 5 (2018): 970–1010. doi:10.1149/2.0601805jesHouser, J., Clement, J., Pezeshki, A., and Mench, M. M. “Influence of Architecture and Material Properties on Vanadium Redox Flow Battery Performance” Journal of Power Sources 302, (2016): 369–377. doi:10.1016/j.jpowsour.2015.09.095, Available at http://dx.doi.org/10.1016/j.jpowsour.2015.09.095Clement, J. T., Aaron, D. S., and Mench, M. M. “In Situ Localized Current Distribution Measurements in All-Vanadium Redox Flow Batteries” Journal of The Electrochemical Society 163, no. 1 (2016): A5220–A5228. doi:10.1149/2.029305jesHouser, J., Pezeshki, A., Clement, J. T., Aaron, D., and Mench, M. M. “Architecture for Improved Mass Transport and System Performance in Redox Fl Ow Batteries” Journal of Power Sources 351, (2017): 96–105. doi:10.1016/j.jpowsour.2017.03.083, Available at http://dx.doi.org/10.1016/j.jpowsour.2017.03.083Aaron, D. S., Liu, Q., Tang, Z., Grim, G. M., Papandrew, A. B., Turhan, A., Zawodzinski, T. A., and Mench, M. M. “Dramatic Performance Gains in Vanadium Redox Flow Batteries through Modified Cell Architecture” Journal of Power Sources 206, (2012): 450–453. doi:10.1016/j.jpowsour.2011.12.026, Available at http://dx.doi.org/10.1016/j.jpowsour.2011.12.026 Figure 1

Operando studies on through-plane cell voltage losses in vanadium redox flow battery

Through-plane potential and overpotential distributions in a vanadium redox flow battery (VRFB) is investigated in-situ using saturated Ag/AgCl reference electrodes attached to each side of the membrane with edge-type configuration. Combined with electrochemical impedance spectroscopy (EIS), this operando approach enables detailed quantification of the voltage losses contributed from the individual VRFB cell components with the magnitudes of ohmic and charge transfer losses at practical charge-discharge conditions. It is found that the overpotentials for vanadium redox reactions on graphite felt electrodes at the positive and negative half-cells are one order magnitude larger than the voltage loss through the typical cation-exchange membrane (Nafion®117) for charging-discharging at current densities from 50 to 120 mA cm−2. Within the overall half-cell overpotentials, the ohmic voltage loss accounts for about 40%. The analysis reveals that the voltage loss by the anode charge transfer reaction is about three folds larger than that by the cathode reaction, suggesting that sluggish V2+/V3+ redox kinetics in the negative half-cell is the major factor limiting the overall VRFB cell performance. This operando methodology is effective for the in-depth analysis of VRFB performance in practical conditions as well as the development of high-performance cell components and efficient total cell design by a collective and quantitative approach.

Correlations of Through-Plane Cell Voltage Losses, Imbalance of Electrolytes, and Energy Storage Efficiency of a Vanadium Redox Flow Battery.

Correlations of the through-plane voltage losses in the vanadium redox flow battery (VRFB), changes in the posolyte (positive electrolyte) and negolyte (negative electrolyte) conditions, energy storage capacity, and coulomb efficiency were investigated under an operando charge-discharge process. Collective and quantitative in situ studies with Ag/AgCl reference-electrodes inserted into a VRFB single-cell and the state of charge measuring external open-circuit voltage half-cells revealed that the overall performance of a VRFB is predominantly governed by (i) relatively high internal resistance of the anode half-cell and (ii) conditions of the negolyte as the limiting electrolyte. The studies showed that the round-trip coulomb efficiency, which is commonly used for VRFB performance analysis, has limitations in expressing the energy storage efficiency and capacity decay phenomena under a consecutive charge-discharge process. In situ and concurrent analysis of the internal cell overpotential and the electrolytes condition can provide quantitative information on the relative contributions of the coulombic round-trip loss, concentration loss, vanadium crossover, and charge imbalance between the posolyte and negolyte for the overall efficiency and capacity decay phenomena. This operando analysis approach is highly effective for gaining an in-depth and collective understanding of the complex phenomena taking place in the VRFB system.

Electrokinetic parameters of a vanadium redox flow battery with varying temperature and electrolyte flow rate

The understandings of electrochemical reaction kinetics are significant to analytically evaluate the electrical performance of vanadium redox flow battery (VRFB). In this regard, we have conducted an experimental analysis of electrochemical reactions in a VRFB which has an active area of 25 cm2. The produced voltage and electrical current have been measured by controlling the temperature between 298 and 328 K and the flow rate from 10 to 100 mL min−1. The electrical loss is resolved to activation and ohmic losses by assuming the Tafel theory. It has been found that the electron transfer coefficient increases from 0.35 to 0.78 with increasing temperature and electrolyte flow rate, while the exchange current density can be expressed by an Arrhenius-like equation resulting in an activation energy of 8.51 kJ mol−1. The area specific resistance is evaluated in the range of 843 and 1598 mΩ cm2 depending on the temperature and electrolyte flow rate. With the evaluated electrokinetic parameters of electron transfer, exchange current density, and area specific resistance, the electrochemical losses can be successfully resolved, therefore, our analysis will be a groundwork for further improvement of electrochemical performance in VRFB.

Preparation of a Nitrogen-Doped Reduced Graphene Oxide-Modified Graphite Felt Electrode for VO2+/VO2+ Reaction by Freeze-Drying and Pyrolysis Method

As one of the key factors that limit the development of vanadium redox flow battery (VRFB), the positive redox couple of VO2+/VO2+ plays an important role on the overall performance of VRFB. To improve the kinetics of a positive reaction, a new designed nitrogen-doped reduced graphene oxide-modified graphite felt (N-rGO/GF) electrode was prepared by coupling the methods of freeze-drying and pyrolysis. The characteristics of the prepared electrode were measured by scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) analysis, Raman spectroscopy (Raman), X-ray diffraction (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge tests. By coupling the methods of freeze-drying and pyrolysis, the N-rGO can be evenly dispersed on the surface of GF electrode, resulting in an excellent catalytic activity. The results demonstrate that the proposed N-rGO/GF electrode with pyrolysis temperature of 900°C shows excellent electrochemical performance and significantly improves the catalytic activity and electrochemical reversibility for the positive VO2+/VO2+ reaction, indicating that the proposed composite electrode has potential applications in the improvement of VRFB performance.

Effect of Bismuth Sulfate Coated on Acidified CNT on Performance of Vanadium Redox Flow Battery

Bismuth sulfate (BiOS) is doped on acidified carbon nanotube (ACNT) (BiOS-ACNT) using sulfuric acid treatment and this is used as catalyst to improve the performance of vanadium redox flow battery (VRFB). Its catalytic activity is compared with that of pure CNT and ACNT. Regarding the redox reactivity of vanadium ions, BiOS-ACNT whose weight ratio is 1:3 exhibits the best catalytic activity and reaction reversibility for both VO2+/VO2+ and V2+/V3+ reactions because the abundant oxygen atoms are well produced during the conversion of bismuth (Bi) nanoparticle into BiOS, and they act as active site for promoting the reactions of VO2+/VO2+ and V2+/V3+. These are confirmed by calculating kinetic parameters, such as electron transfer rate constant and diffusion coefficient, while BiOS-ACNT shows the best values (0.21 s−1 and 6.46 × 10−8 cm2 s−1). In VRFB including BiOS-ACNT, the performance of BiOS-ACNT VRFB is enhanced with voltage and energy efficiencies of 87.6 and 81.9% at 80 mA cm−2. Furthermore, in 50 cycle test of 120 mA cm−2, BiOS-ACNT VRFB shows lower overpotential (0.2 V) and capacity loss rate (0.04% cycle−1) than other VRFBs. This proves that BiOS-ACNT is beneficial for enhancements in the performance and long-term stability of VRFB.

Effect of Fe(III) on the positive electrolyte for vanadium redox flow battery.

It is important to study the effect of Fe(III) on the positive electrolyte, in order to provide some practical guidance for the preparation and use of vanadium electrolyte. The effect of Fe(III) on the thermal stability and electrochemical behaviour of the positive electrolyte for the vanadium redox flow battery (VRFB) was investigated. When the Fe(III) concentration was above 0.0196 mol l−1, the thermal stability of V(V) electrolyte was impaired, the diffusion coefficient of V(IV) species decreased from (2.06–3.33) × 10−6 cm2 s−1 to (1.78–2.88) × 10−6 cm2 s−1, and the positive electrolyte exhibited a higher electrolyte resistance and a charge transfer resistance. Furthermore, Fe(III) could result in the side reaction and capacity fading, which would have a detrimental effect on battery application. With the increase of Fe(III), the collision probability of vanadium ions with Fe(III) and the competition with the redox reaction was aggravated, which would interfere with the electrode reaction, the diffusion of vanadium ions and the performance of VRFB. Therefore, this study provides some practical guidance that it is best to bring the impurity of Fe(III) below 0.0196 mol l−1 during the preparation and use of vanadium electrolyte.

Elucidating Effects of Faradaic Imbalance on Vanadium Redox Flow Battery Performance: Experimental Characterization

Long-term performance and lifetime of vanadium redox flow batteries (VRFBs) are critical metrics in widespread implementation of this technology. One challenging issue that negatively affects these parameters is the faradaic imbalance, which is not comprehensively investigated in the literature. Faradaic imbalance is known as the shift in the average oxidation state (AOS) of the electrolyte due to side reactions. This type of imbalance requires chemical/electrochemical mitigation rather than simple electrolyte remixing. Herein, we investigate faradaic imbalance by preparing unbalanced electrolytes with different AOS values. The performance characteristics of the flow cell utilizing electrolytes with different AOS values are reported. Based on the results of charge-discharge cycling, polarization testing, and electrochemical impedance spectroscopy measurements, faradaic imbalance is found to significantly affect discharge capacity, maximum power density, cell resistances, and efficiency values. While the ratio of discharge capacity to theoretical capacity is 83% for the ideally balanced case (AOS 3.5+), it drops to 53.4% for the AOS 3.9+ case. Additionally, there is a substantial decrease of 44% in the maximum available power density for the most unbalanced case. This noticeable performance degradation highlights the importance of faradaic imbalance as a critical factor which requires further attention especially during extended cycling.

Impact of Corrosion Conditions on Carbon Paper Electrode Morphology and the Performance of a Vanadium Redox Flow Battery

The side reactions during long-term operation of vanadium redox flow batteries (VRFBs) increase the average oxidation state of the electrolyte and associated equilibrium potential of the positive half-cell. This consequently initiates the corrosion reactions at the positive side as the half-cell potential passes the critical limit. In this study, an ex-situ accelerated electrochemical corrosion protocol is performed on the carbon paper electrode to investigate the effects of corrosion conditions induced by extended cycling on electrode morphology and VRFB system performance. In terms of morphological changes of corroded electrodes, only minor mechanical degradation is observed, including disappearance of binder material and reduced mechanical properties. With regards to the cell performance, the flow cell with the corroded electrode demonstrates notably higher charge and discharge capacities which can be attributed to the enhanced active surface area of the electrode. Furthermore, the corroded case exhibits an improved capacity retention during extended cycling which can be related to the improved redox activity caused by the increased carboxyl groups and wettability. This study shows that even at these fairly aggressive corrosion conditions, the process behaves as a treatment method, which oxidizes surface functional groups, increases active surface area, and hydrophilicity, and subsequently enhances VRFB performance.

An Enhanced Equivalent Circuit Model of Vanadium Redox Flow Battery Energy Storage Systems Considering Thermal Effects

Thermal issue is one of the major concerns for safe, reliable, and efficient operation of the vanadium redox flow battery (VRB) energy storage systems. During the design of the operational strategy for a grid-connected VRB system, a suitable mathematical model is needed to predict the dynamic behaviors under various operating conditions. However, conventional VRB models usually neglect the impact of temperature variations on system performance. This work develops an enhanced VRB model with the consideration of the coupling effects between the electrochemical and the thermal behaviors. The proposed model consists of two equivalent circuits. First, the electrochemical behaviors of the VRB are modeled by a second-order RC network taking account of the effects of concentration variation of the vanadium ions and the electrochemical activation. Second, a third-order Cauer network is used to model the heat transfer process in the VRB system, and the dynamic thermal behaviors of stacks, pipes and heat exchangers are characterized. Well-designed experiments and particle swarm optimization algorithm are use to identify the parametric values of the developed model. The proposed modeling method was validated experimentally using a 5kW/3kWh VRB platform, and the results show that the model is capable of accurately predicting the VRB performance under variable temperature conditions. The developed coupled electro-thermal model is then used for simulating and analyzing the performance of a VRB system operated in conjunction with a wind power plant under real-world conditions.

Catalytic effects of heteroatom-rich carbon-based freestanding paper with high active-surface area for vanadium redox flow batteries

Owing to their scalability, flexible operation, and long cycle life, vanadium redox flow batteries (VRFBs) have gained immense attention over the past few years. However, the VRFBs suffer from significant polarization, which decreases their cell efficiency. The activation polarization occurring during vanadium redox reactions greatly affects the overall performance of VRFBs. Therefore, it is imperative to develop electrodes with numerous catalytic sites and a long cycle life. In this study, we synthesized heteroatom-rich carbon-based freestanding papers (H-CFPs) by a facile dispersion and filtration process. The H-CFPs exhibited high specific surface area (~820 m2 g–1) along with a number of redox-active heteroatoms (such as oxygen and nitrogen) and showed high catalytic activity for vanadium redox reactions. The H-CFP electrodes showed excellent electrochemical performance. They showed low anodic and cathodic peak potential separation (ΔEp) values of ~120 mV (positive electrolyte) and ~124 mV (negative electrolyte) in cyclic voltammetry conducted at a scan rate of 5 mV s–1. Hence, the H-CFP-based VRFBs showed significantly reduced polarization.

Ionic transportation and chemical stability of high-endurance porous polyethylene separator for vanadium redox flow batteries

To confirm the viability of a porous polyethylene (PE) separator for using in vanadium redox flow batteries (VRFBs), we conduct a comparative electroanalytical and chemical stability studies of the PE separator and Nafion 212 membrane. We characterize the physicochemical properties of the separator, such as water uptake, dimensional change, and ion conductivity, and analyze its structural and compositional features using thermogravimetric analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy. We also test the chemical stability of the separator against highly oxidative V5+ ions and the cell performance of VRFB using PE separator to assess practical applicability. In chemical stability, the change of VO2+ ion to VO2+ ion is considerably lowered by the PE separator (0.01 mmol/L) than the Nafion 212 (0.27 mmol/L), which indicates that the PE separator possesses higher chemical stability. The energy efficiency of the VRFB with the PE separator is lower than that obtained with Nafion 212. However, the chemical stability of PE separator is 27 times higher than that of Nafion 212, indicating that its use will promote the long-term operation of the VRFB system. Hence, the PE separator can be considered a cost-effective option for VRFB operation, with appropriate modifications to its thickness, surface properties and pore structure.

Effect of the redox reactivity of vanadium ions enhanced by phosphorylethanolamine based catalyst on the performance of vanadium redox flow battery

Phosphorylethanolamine (PHOS) doped carbon nanotube (CNT) is newly suggested as a catalyst for enhancing the redox reactions of vanadium ions and vanadium redox flow battery (VRFB) performance. For synthesizing the catalyst, CNT is linked to the phosphate group of the PHOS, forming phosphate functionalized carbon nanotube (POH-CNT). Its catalytic activity is then compared with those of pure CNT and carboxylic acid functionalized CNT (CA-CNT) catalysts. Regarding the redox reactivity of vanadium ions, POH-CNT exhibits superior catalytic activity and reaction reversibility for VO2+/VO2+ reaction to CNT and CA-CNT because of (i) the chelation ability of phosphate group, (ii) its low electron delocalization capability and (iii) its high acid dissociation constant. By the role as chelating agent of phosphate group, the density of active sites for reaction of VO2+ and VO2+ ions in POH-CNT increase. Also, the low electron delocalization and high acid dissociation constant induce effective adsorption and desorption of VO2+ and VO2+ ions. Such effects facilitate the VO2+/VO2+ reaction. In contrast, regarding V2+/V3+ reaction, POH-CNT shows equivalent reactivity to CA-CNT because phosphate and carboxyl groups form similar active sites for the V2+/V3+ reaction. The above results are supported by calculating kinetic parameters, such as charge transfer resistance and diffusion coefficient, while chemical bonding of the catalysts is examined by XPS. Finally, as the POH-CNT is used as catalyst for positive electrode prompting the VO2+/VO2+ reaction, performance of VRFB single cell is improved.

Structure and Properties of Sulfonated Poly(ether ether ketone) Hybrid Membrane with Polyaniline‐Chains‐Modified Graphene Oxide and Its Application for Vanadium Redox Flow Battery

AbstractA series of acid‐base hybrid membranes (SPEEK/PANi‐GO), composed of sulfonated poly(ether ether ketone) (SPEEK) and polyaniline‐functionalized graphene oxide (PANi‐GO), have been prepared and applied for vanadium redox flow battery (VRB). The chemical composition and structure for PANi‐GO nanosheets and SPEEK/PANi‐GO hybrid membranes are in detail characterized. Physicochemical properties such as ion exchange capacity, proton conductivity and vanadium ion permeability are highly based on the loadings of PANi‐GO nanofillers. The formed acid‐base pairs, originated from ‐SO3H of SPEEK and ‐NH group of PANi‐GO, decrease the swelling ratio and vanadium ion permeability. Ion selectivity shows a strong dependence upon the PANi‐GO content, and it presents a maximum value at 2% loading. VRB performance by SPEEK/PANi‐GO‐2 hybrid membrane exhibits a higher coulombic efficiency (CE: 98.5%) and energy efficiency (EE: 81.7%) at 30 mA cm−2, as compared with Nafion 117 (CE: 88.9% and EE: 72.5%) and SPEEK membrane (CE: 89.8% and EE: 69%). The self‐discharge time with SPEEK/PANi‐GO‐2 (58 h) is longer than that of Nafion 117 membrane (26 h), and a good chemical and oxidation stability is well demonstrated after 300 times charge‐discharge cycles for SPEEK/PANi‐GO‐2 hybrid membrane. These SPEEK/PANi‐GO hybrid membranes demonstrate a promising application for VRB system.

Local potential measurement through reference electrodes in vanadium redox flow batteries: Evaluation of overpotentials and electrolytes imbalance

Vanadium redox flow battery performance is actually hindered by increased overpotentials at high current, due to poor electrochemical activity of the most commonly used carbon electrodes and to electrolyte distribution, implying local mass transport limiting conditions. Moreover, vanadium cross-contamination leads to coulombic efficiency reduction and uncontrolled electrolytes imbalance. This work presents the application of through-plate reference electrodes at inlet and outlet of both positive and negative electrodes. The utility of the electrodes potential measurement is firstly demonstrated in the identification of a relation between electrolytes potential and the corresponding state of charge. Subsequently, local overpotentials and impedance spectra at both electrodes are evaluated, evidencing that the negative electrode is kinetically dominated and presents high overpotential even at low current, while the positive exhibits mass transport effects at high current, especially at cell outlet. Finally, during cycling operation with fixed capacity reference electrode measurements permit to monitor electrolytes imbalance induced by cross-contamination, that mainly affects negative electrolyte. Moreover, additional insights into electrodes potential loss during cycling are provided.

Redox-targeted catalysis for vanadium redox-flow batteries

Vanadium redox-flow battery (VRB) as a promising electrochemical power source for large-scale energy storage, suffers from various polarization losses despite that it has been extensively studied in the past decades. Among these losses, the sluggish interfacial charge transfer of the vanadium species on the respective electrode renders large overpotentials giving rise to inevitable hydrogen and oxygen evolutions during the charging process. In this study, we report an unprecedented method based on the redox targeting concept to tackle the above issues. Prussian blue (PB) and a Prussian blue analogues (PBA) with identical redox potentials to VO2+/VO2+ and V2+/V3+ are grafted on cathode and anode, respectively. Upon operation, the reversible proton-coupled redox targeting reactions between PB and VO2+/VO2+ on cathode, PBA and V2+/V3+ on anode facilitate the interfacial charge transfer of the vanadium species and concomitantly inhibit the hydrogen and oxygen evolutions, which improves the selectivity of the redox reactions and considerably enhances the round-trip energy efficiency and cycling performance of VRB in a wide range of current densities. The above redox-assisted catalytic reactions were scrutinized and the mechanisms are unequivocally manifested with various electrochemical and spectroscopic measurements. We anticipate the surface immobilized redox catalysis approach demonstrated here would generically provide a paradigm for improving the sluggish kinetic processes in a variety of electrochemical devices.

Exploring the Influence of Thermal Pretreatment on Carbon Paper Electrodes for Vanadium Redox Flow Batteries

Grid-scale energy storage is anticipated to play a critical role in the decarbonization of the electric power sector by enabling the reliable integration of intermittent renewable sources and increasing the efficiency of non-renewable energy processes.1 Redox flow batteries (RFBs) are a promising electrochemical device for such applications due to their independent energy and power scaling and long operational lifetime.2 Aqueous vanadium RFBs (VRBs) have been the most successful chemistry, due to their high power density and recoverable capacity from crossover, but further cost reductions are needed for ubiquitous adoption. Vanadium redox reactions have sluggish kinetics on the carbon electrodes typically employed in VRBs which motivates the implementation of pretreatment strategies to increase electrochemically active surface area and introduce catalytically active functional groups onto the electrode surface.3 While many pretreatments have been successfully demonstrated most studies are empirically-driven.4 Few have focused on systematically correlating changes in electrode structure and surface chemistry to shifts in electrochemical performance. To this end, we utilize a suite of ex-situ characterization techniques, coupled with flow cell testing, to explore structure-performance relations of thermally pretreated carbon paper electrodes. Specifically, we vary thermal pretreatment temperature from 400 °C, a commonly used temperature for VRB electrode treatments3,4, to 500 °C, to quantify changes in the physicochemical properties of the electrode, and link these changes to shifts in VRB performance. We leverage microscopic, spectroscopic, and analytical techniques to determine how microstructural, morphological, and chemical properties of electrodes are impacted by thermal pretreatment. We then evaluate the effect of these properties on flow cell performance using polarization, electrochemical impedance spectroscopy, and cell cycling. We find that many factors contribute to the observed electrode performance, including hydrophilicity, electrochemical surface area, and surface chemistry, and that not all of these factors improve with increasing temperature. Consequently, while the best overall performance is achieved with pretreatment at 475 °C, a 26% increase in maximum power density over the base case of pretreatment at 400 °C, this enhancement is based on a balance of critical properties rather than a maximization of all. 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. A.F.C. acknowledges financial support from the Swiss National Science Foundation (P2EZP2_172183). The authors acknowledge the Center for Nanoscale Systems and the NSF’s National Nanotechnology Infrastructure Network (NNIN) for the use of Nanoscale Analysis facility to collect X-ray photoelectron spectroscopy data. We thank Michael L. Perry and Robert M. Darling for guidance in experimental conditions and critical evaluation of results. References Z. Yang, J. Zhang, M. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon, J. Liu, Chem. Rev., 111, 3577–3613 (2011).M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011).B. Sun and M. Skyllas-Kazacos, Electrochim. Acta, 37, 1253–1260 (1992).M. H. Chakrabarti et al., J. Power Sources, 253, 150–166 (2014). Figure 1

A Novel Sulfonated Aromatic Polymer Membrane with Different Pendant Groups for Vanadium Redox Flow Batteries (VRFBs)

Vanadium redox flow batteries (VRFBs) have received extensive attention due to its attractive features for the large-scale energy storage because of their design flexibility, high energy efficiency, absence of intercalation/deintercalation and stress build-up in electrodes, and safe operation. One of the most critical components in VRFB is an ion exchange membrane (IEM) which can prevent cross mixing of the multivalence vanadium ions, while it still allows the transport of protons (or anions) to complete the circuit. However, the trade-off relationship of proton exchange membrane (PEM) limits the energy efficiency (EE) of the VRFB due to low coulombic efficiency (CE) caused by vanadium ion cross over through the membrane or high membrane resistances. Anion exchange membrane (AEM) suffers from poor chemical stability and low voltage efficiency due to low ion conductivity. In this study, we synthesized a series of novel sulfonated aromatic polymer membranes having high proton conductivity as well as proton selectivity. Vanadium/hydronium ion permeability, proton conductivity, area resistance, and ion exchange capacities of the membranes were measured. The single cell VRFB performance with the prepared membranes was evaluated and compared with the one using a commercial Nafion 212 membrane. The effect of the pendent group structure on the membrane ion selectivity and battery performance was also discussed. It was found that the incorporation of aromatic structures on the pendent groups effectively improved the ion selectivity of membranes which can overcome the limitations of the trade-off relationship between the proton conductivity and selectivity. The membrane with aromatic pendent groups (BP-Ar-SH) shows 7 times higher vanadium/proton ion selectivity than that without aromatic groups (BP-SH) (5.18 vs. 0.75) and 3 times higher than Nafion 212 (5.18 vs. 1.78). As a result, the VRB single cell equipped with the BP-AR-SH membrane shows significantly better performance in comparison to the cell with Nafion membranes, as high as 90.05% EE (vs. 73% EE of Nafion) at 20 mA/cm2. The cycle stability test shows that efficiencies of the membrane remain almost unchanged after running more than 50 cycles. Our results strongly suggest that the BP-Ar-SH membrane may provide a breakthrough toward the development of high-efficiency VRFB systems.

Analysis of flow field design on vanadium redox flow battery performance: Development of 3D computational fluid dynamic model and experimental validation

Homogeneous distribution of the electrolyte over the porous electrode is a critical issue hindering the commercialization of vanadium redox flow batteries, owing to increased overpotential at high current and limited power density of the system. Therefore, an understanding of the physical phenomena regulating mass transport of the electrolyte is crucial to improving system performance. The present work describes the development and experimental validation of a 3D computational fluid dynamic model of a vanadium redox flow battery in a half-cell configuration with an active area of 25 cm2. The model simulates the influence of a single serpentine and an interdigitated flow field. The adoption of the half-cell configuration allows the negative electrode to be considered as a pseudo-reference electrode with zero potential loss, leading to a reduction in computation time and the number of fitting parameters, which can be determined with reduced uncertainty. The developed model includes a traditional fluid dynamic analysis of the electrolyte in the flow field and in the porous electrode, coupled with the electrochemistry of the reactions involved. In both the experiments and the simulations, the single serpentine distributor exhibits better performance and higher pressure drops compared to those of the interdigitated geometry under all the investigated operating conditions. In the analysis of the local reaction rate, both distributors experienced increased reaction rates under the rib, induced by a by-pass flow between adjacent channels. The reaction rate shows a highly heterogeneous distribution in the serpentine geometry, while it is more uniform in the interdigitated configuration.