Overview of the factors affecting the performance of vanadium redox flow batteries
Graphical abstract
Introduction
As the world's electricity sector progresses towards high levels of renewables generation asset adoption and deployment, energy storage is critical for power reliability. For grid-scale energy storage utilising batteries (hundreds of kWh to MWh sizing), redox flow batteries (RFBs) are viable for large-scale storage applications. This battery has attractive capabilities where the energy stored and power delivered can be controlled independently [1], [2], [3], [4], [5], which is highly useful to grid operators. The amount of the energy stored in the battery can be simply modulated by raising or lowering the quantity of electrolyte while the amount of power delivered can be modulated through appropriate battery sizing. This flexibility is ideal for the battery to be used for storing energy from renewable sources during surplus generation periods and load shifting to low generation, but high demand periods [1]. Some of the popular chemistries for redox flow batteries are vanadium-vanadium, iron-chromium, zinc-bromine, zinc-iron, and hydrogen-bromine. Amongst these chemistries, vanadium-based systems (i.e., vanadium redox flow batteries (VRFBs)) are the most popular chemistry, which are utilised given the vanadium's flexible oxidation states [6]. The advantage of flow batteries over other competitive systems such as lithium arises from the lower cost per kWh due to the utilisation of more abundantly available and cheaper materials resources. Fig. 1 shows the energy cost comparison of lithium-ion and lithium polysulphide against several other redox flow batteries [7]. When compared to lithium-ion, the energy costs of all redox flow batteries are lower. With the exception of vanadium redox flow battery, all redox flow batteries generally have lower energy cost relative to lithium polysulphide.
A typical flow battery (Fig. 2) has anolyte, catholyte, graphite flow field, carbon felt electrode, current collector, and separator. The anolyte and catholyte are pumped into the respective half cells. The graphite flow field directs the electrolyte flow through carbon felt electrode [8]. The carbon felt electrode provides the surface area for the occurrence of redox reaction [9]. The function of separator is to ensure zero species cross-over and uninterrupted hydronium ion transport.
During the redox reaction, the anolyte loses an electron and gets oxidised. This electron flows through the external circuit and reaches catholyte where the catholyte will accept the electron and gets reduced. The charging and the discharging processes are illustrated in Fig. 3 for the vanadium flow battery. For every electron transferred from the anolyte to the catholyte, a hydronium ion, H+ moves across the separator to maintain electroneutrality.
The performance of any battery in an electricity grid (or any other application) has to be reliable, predictable, and produced continually on demand during the battery's installed lifetime. Each battery chemistry has unique materials, physical design, and electrical properties, which affect that particular battery chemistry's performance levels and limitations. For flow batteries, the major parameters, which govern performance are electrode activation, felt compression, electrode material, electrolyte additive, electrolyte temperature, flow field design, and membrane characteristics.
Electrode activation can be performed by utilising a thermal, chemical, or laser-based technique. Thermal and chemical activations are generally about attaching different functional groups to the electrode surface [10], [11], [12], [13], [14], [15], [16]. These functional groups are then utilised for specific redox chemistry changes required for the flow battery to work, through improved surface adherence of solution-based redox species for electron transfer reactions. For example, in the case of the vanadium system, oxygen-containing functional groups (hydroxyl and carboxyl groups) and nitrogen functional groups help to improve the electron transfer process by providing active sites for the oxidation of VO2+. The laser technique is utilised to vary the effect of pore size and pore density on the carbon felt, which has the concomitant effect of varying battery performance [17]. Felt compression is the effect of compressing the fibres of the electrode together to reduce the contact resistance between the current collector and the porous electrode material [18]. Electrolyte additives perform a major role by altering the properties of the electrolyte to sustain different operating conditions [19], [20], [21], [22], [23]. Electrolyte temperature plays a considerable role in battery performance by controlling the ionic mobility of the redox species [24,25]. The flow field design on the graphite plate significantly improves the battery performance by altering the flow patterns of the electrolyte that flows through the carbon felt [26], [27], [28], [29]. The membrane also known as separator, on the other hand, has extensive control over the battery performance by regulating the ionic transport across the membrane [30,31]. Electrode material and thickness also play a considerable role in battery performance [32], [33], [34].
At a cell level, voltage efficiency, coulombic efficiency, and energy efficiency are three characteristic parameters typically used to evaluate a battery's performance. The voltage efficiency of the battery can be enhanced by reducing the ohmic, activation, and concentration overpotential. Coulombic efficiency can be improved by reducing species cross-over across the membrane. Since energy efficiency is the product of voltage and coulombic efficiency, it can also be improved simultaneously by reducing the overpotential and the species cross-over. All these performance parameters should be considered for any comparative studies on battery. We do, however, note that upon utilisation of any battery in a real-life system, the system energy efficiency is limited by other factors such as direct current (DC) or alternating current (AC) convertors, invertors, system hardware, and software designs. Typically, battery efficiencies are higher than those of the associated system efficiencies. For example, typically a battery efficiency is in the 95–99% ranges, whereas most commercial invertor systems operate in the 80–95% range. Hence battery storage system efficiencies are typically lower than the battery cell efficiencies due to these limitations.
Previous review articles on redox flow batteries have discussed the effects of different vanadium electrolytes, battery materials, cell designs, electrocatalysts, and etc. on battery performance [1,2,19,29,[35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45]]. Xu and co-workers reviewed the evaluation criteria for RFB and suggested system energy efficiency as the preferable criterion [1]. Almheiri and co-workers reviewed various designs in membraneless RFB [2]. Skyllas-Kazacos and co-workers reviewed different electrolyte additives reported in the literature and its specific role in VRFB operation [19]. Ke and co-workers reviewed different flow field designs and their roles in optimising non-uniform flow for rechargeable redox flow batteries [29]. Skyllas-Kazacos and co-workers reviewed the research trends on the VRFB electrode for the past 30 years and novel approaches to electrode development for VRFB [34]. Kim and co-workers reviewed different vanadium electrolytes and their effect on cell performance [35]. Minke and Turek reviewed the materials, system design, and modelling approach for VRFB specified in the literature and tried analysing the associated capital cost [36]. Walsh and co-workers reviewed a broad area that covers RFB design, modelling, components, performance, and finally the scale-up [37]. Chen and co-workers reviewed different membranes, electrodes, and cell configurations in RFB [38]. Jung and co-workers reviewed the recent progress and trends in sol-gel based alkoxysilane membranes for VRFB [40]. Jirabovornwisut and Arpornwichanop reviewed the cause and effect, measurement methods, and regeneration methods for electrolyte imbalance in VRFB [41]. Tasnim and co-workers reviewed all the components, design queues, and limiting factors for VRFB. They also reviewed current research works that have been performed to overcome the limiting factors [42]. Cho and co-workers reviewed different nanostructured metal and carbon-based electrocatalyst reported in the literature for a vanadium redox reaction [43]. Yuan and co-workers reviewed the degradation mechanism for VRFB cell and cell components. They also reviewed the diagnostic tool and mitigation strategies for degradation [45].
None of these aforementioned review articles however focus on the combined effects of different parameters on RFB cell performance. They instead focussed on individual parameter effects. However, in an RFB system, all of these effects are present simultaneously and hence the system performance will be different to cell performance. Fig. 4 summarises the energy efficiencies reported in 12 similar research articles that range from 78 to 88%. Future research and development efforts should be directed to improve the efficiency to above 90%, in line with battery industry and consumer expectations.
Due to the dearth of information on overall system efficiencies of flow batteries, our review paper combines the investigations on physical and chemical parameters (particularly those related to the cell synthesis and operation) to compile and explain the effects of each and every parameter on RFB performance. This approach can be used to optimise the battery system to enable operation at maximum efficiency and peak performance through understanding the influence of different physical and chemical components on the performance parameters. Table 1 lists the parameters considered for this study.
The effects that each parameter listed in Table 1 has on the cell performance, as reported in the literature, has been compiled and will be discussed in more detail in the sections below. The parameters in Table 1 are categorised into three categories, namely, most significant, reasonably significant, and least significant. Finally, a parametric matrix has been developed to identify the role of each parameter on the overall battery performance. The outcome of this review article and methodology can be utilised by the researchers working in the field of flow batteries to gain insights and optimise cell construction for maximum performance. It can also assist researchers in appropriate parameter selection to optimise the battery for their specified applications.
Section snippets
Electrode activation
In a vanadium redox flow battery, the vanadium redox reactions occur at the electrode surface. Carbonaceous materials with high surface areas are preferentially used in the current vanadium flow battery designs. The vanadium redox reaction is as follows:
To improve the surface chemistry, electrode activation techniques can be used to enhance the electrode wettability with the electrolyte
Felt compression
During cell assembly all the battery components are mechanically clamped together, with appropriate compression for the cell components, to reduce the contact resistance between porous materials and current collectors. Electrode compression (felt compression) has a significant influence on battery performance by controlling the pressure drop, tortuosity, and porosity. Generally, carbon felt electrodes are porous and this porosity will improve the electrolyte penetration. However, increasing the
Electrode material
The nature of the electrode material plays a significant role in providing active sites for electrochemical reactions. Conductivity of the electrode governs its body resistance and the contact resistance between the electrode and bipolar plate. Carbon cloth, carbon paper, and polyacrylonitrile (PAN) type carbon felt electrodes have been reported for use as electrode materials for aqueous redox flow batteries.
PAN type carbon felt electrodes are popular electrodes for redox flow battery
Electrolyte temperature
The electrolyte temperature or operating temperature of the cell plays a major role in maintaining the electrolyte stability and determining cell performance [68]. In cyclic voltammetry experiments, when the temperature is higher, the ions will move faster and collide with each other. This supports the ion in overcoming concentration overpotential, resulting in higher cathodic and anodic peak currents. Fig. 24 shows the variation of peak current ratio of negative and positive electrolyte with
Flow field design
There are generally two methods to regulate the electrolyte distribution in the flow batteries. One is the flow-through method and the other is the flow by method [8,73]. In flow-through method, the electrolyte flows through the carbon felt electrode where it does not have any specific flow path [73]. Flow by method has channels machined on the graphite plate to ensure lower pressure drop and effective electrolyte distribution through carbon felt [74]. Popular flow field designs include the
Membrane
The purpose of the membrane in the redox flow battery is to prevent the cross-mixing of catholyte and anolyte and allow only selective ions to pass through the membrane to maintain electroneutrality [76,77]. Membranes used in VRFB can be classified as an ion exchange membrane or a non-ionic porous membrane. Ion exchange membranes have functional groups for selective ion exchange whereas non-ionic porous membranes act as a physical barrier to ions of a larger ionic radius than the target ions.
Electrolyte additive
Flow batteries are prone to have low energy density given the limited solubility of salt, poor reversibility of kinetics, and evolution of hydrogen and oxygen. In the case of vanadium flow batteries, there are four oxidation states, namely, V(V), V(IV), V(III), and V(II). The stability of V(V) can be improved by increasing the sulphuric acid concentration or reducing the temperature whereas in case of other oxidation states such as V(IV), V(III), and V(II), the stability can be improved by
Parametric matrix
A comprehensive review of the parameter effects has helped us to outline a parametric matrix as shown in Fig. 38(a). The matrix represents the attributes of every parameter considered in this study. The parameter and its corresponding effects are depicted in the matrix. Increasing the felt compression of carbon felt electrode causes drop in porosity, rise in tortuosity, rise in pressure drop, and finally, drop in cell resistance when compared to the pristine electrode without compression. For
Conclusion
The potential of redox flow batteries for stationary energy storage from renewables have been investigated widely. This battery is foreseen as a potential solution for bulk electrochemical storage. Optimising the control parameters for this battery can help in constructing an efficient electrochemical storage system. This review has discussed the influence of different parameters on the vanadium redox flow battery performance where 3 key conclusions can be summarised:
- 1
Electrode activation and
Declaration of Competing Interest
✓ All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
✓ This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue.
✓ The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
✓ The
Acknowledgements
The authors wish to acknowledge the funding support from Indian Institute of Technology Madras in the form of New Faculty Start-up Grant (NFSG) and the support from joint PhD program between Indian Institute of Technology Madras and Swinburne University of Technology.
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