Performance Improvements and Cost Considerations of the Vanadium Redox Flow Battery. Maria Skyllas-Kazacos School of Chemical Engineering, CENELEST, UNSW Sydney, NSW, Australia, 2052 Since the initial development of the All-Vanadium Redox Flow Battery (VRFB) at UNSW Sydney Australia in the mid-1980s, considerable improvements in performance have been achieved through electrode modification and new cell designs that have led to significant increases in power density and reduced stack costs. In this paper a brief overview of the main advances in electrodes and cell design will be presented, together with results from modelling and simulation studies that can be used to optimise stack design and battery performance. Introduction. The first All-Vanadium Redox Flow Battery (VFB) patent was filed by UNSW in 1986. Over the next 20 years, the UNSW group carried out extensive research and development of the VFB covering basic chemical and electrochemical studies, screening, characterisation and selection of membranes, electrodes and cell materials, development of vanadium electrolyte processes, mathematical modelling and simulation, control system development and stack design, development and field trials in an electric golf cart and a solar demonstration house in Thailand (1-10). Early commercial licensing of the UNSW VFB technology by Mitsubishi Electric Industries and Kashima-Kita Electric Power Corporation in 1993, led to the first largescale implementation of a 200 kW/800 kWh VFB in a load-levelling system in Japan in 1995. Further licensing by Sumitomo Electric Industries produced several additional grid connected VFB field trials in Japan between 2001 and 2007 in applications such as wind energy storage, emergency back-up power systems, PV energy storage and load-levelling, demonstrating energy efficiencies of 80% and a possible life of up to 200,000 cycles (9). In 2006, with the expiry of the UNSW basic VFB patent, several companies emerged as new VFB developers in Europe, USA, China and Japan, while research on VFBs has expanded around the world in a wide range of areas. By 2018, the number of VFB installations around the world has continued to grow, as illustrated in Figure 1. In May 2016, the Chinese VFB developer Rongke Power announced the planned installation of the world’s largest energy storage battery based on VFB technology (11). The battery will have a power and capacity rating of 200 MW / 800 MWh VFB and will be deployed on the Dalian peninsula. It will comprise ten (10X) 20 MW / 80 MWh vanadium Flow Battery (VFB) battery arrays and was approved by the China National Energy Administration Construction. Construction began in October 2016 and after full commissioning, will be able to peak-shave approximately 8% of Dalian's expected load in 2020. In addition, battery will form additional load centre, which will enhance grid stabilization including securing power supply and providing black-start capabilities in event of emergency.
Figure 1. Numbers and combined power rating of installed VFB storage systems around the world in January 2019. (Source: http://willigan.digital/vanitec/v2)
Amongst the other companies expanding their VFB activities, the large manufacturer, ThyssenKrupp, recently unveiled their activities aimed at developing 1 MW VRB battery stack modules using their extensive industrial electrolyser expertise and capabilities. Compared to conventional flow battery systems based on 5 kW to 50 kW stack modules, this project represents a major advance in flow battery design, with prospects for substantial automated manufacture and considerable cost reduction for GWscale systems in the future. With the expanding implementation of VFB systems around the world however, the supply of the vanadium oxide raw material used in electrolyte production has failed to keep up with demand. When combined with the increased demand for vanadium in high strength steel production globally, a dramatic surge in vanadium prices has recently occurred as illustrated in Figure 2.
Figure 2. Twelve month trend in V2O5 prices (US$/lb) in Europe. Source: https://www.vanadiumprice.com
Although prices returned to around $US18/lb by February 2019, the peak price of over $US28/lb in June 2018, created a great deal of nervousness amongst VFB producers and investors who began to question the commercial viability of the VFB for large-scale grid connected applications. At the 2018 meeting of Vanitec that was held in conjunction with the International Flow Battery Forum in Lausanne Switzerland however, several companies announced a new financial model for the marketing of their VFB technology in the future. This new financial model involves the leasing of vanadium electrolyte that now has the potential to dramatically reduce up-front investment costs, in exchange for a higher maintenance cost associated with electrolyte leasing. Under this model, at the end of a VFB project, operators and users would have the option to renew the lease or return the electrolyte to the leasing company who could then sell it on to a new user or project, also simplifying material recycling. In the present study, a techno-economic model was developed to evaluate the economic viability of the electrolyte leasing model for the VFB industry. Studies have also been conducted at UNSW to continue investigating other opportunities for cost reduction in order to increase the uptake of VFB systems in a wide range of applications around the world. Results of these studies will be presented here. Techno-Economic Modelling Techno-economic modelling studies were carried out on a VFB system with the specifications and assumptions as detailed in Table 1.
Table 1. Assumptions used in Techno-Economic Modelling of VFB System Nominal Power (kW) Voltage Efficiency at Nominal Power % Membrane life (years) Battery life (years) Cell resistance (ohms.cm2) V2O5 Cost (Series 1) ($/kg)
100 0.82 8 24 1 11
V2O5 Cost (Series 2) ($/kg) Series 3 represents V2O5 cost 2 with electrolyte leasing Membrane cost ($/m2) Graphite felt cost ($/m2) Bipolar Plate Cost ($/m2) Leasing Rate (fraction of electrolyte cost per annum) Discount Rate (fractional)
50 200 50 100 0.1 0.06
Figure 3. Simulated Capital and Levelized Costs for 100 kW VFB as a Function of Storage Capacity for Assumptions in Table 1. Effect of Electrolyte Leasing and Cell Resistance.
Figure 4. Simulated Capital and Levelized Costs for 100 kW VFB as a Function of Storage Capacity Effect of Electrolyte Leasing and Cell Resistance. Assumptions as in Table 1 except membrane cost = $50.m2, felt cost = $20/m2 and bipolar plate cost = $10/m2. The results in Figure 3 clearly demonstrated the significant reductions in both upfront and levelized costs of the VFB achievable with electrolyte leasing, the effect being more pronounced for longer storage capacities, as expected. In fact, both costs are lower
in the case of electrolyte leasing than in the $US 5/lb V2O5 scenario. Levelized costs around 5c/kWh are achievable for 8 hour storage capacities and a cell resistance of 1 ohm.cm2, even with high stack materials costs. New improvements in low cost stack materials can further reduce both capital and levelized costs are illustrated in Figure 4. With the lower stack material prices, a levelized cost of 5 cents/kWh can be achieved for storage capacities of 5 or more hours for both cell resistances of 1 and 2 ohms.cm2. Areas for Further Cost Reduction As illustrated in Figures 3 and 4, the electrolyte leasing model has the potential to dramatically reduce up-front capital costs for VFB systems. Additional opportunities to reduce cost can be identified however. The current focus for VFB researchers and developers is to further reduce stack cost. These involve: 1. Reduce component costs. - Membrane is most expensive stack component and currently used membranes vary in price from $200 to $600 per square metre. - New membranes/separators now developed with costs < $50 per square metre. Currently undergoing long-term testing 2. Increase power density - Reduce cell resistance – mainly ohmic - Reduce anode-cathode spacing without increasing pressure drop - Researchers adopting fuel cell stack architectures employing serpentine flow fields Increased power density (or higher current density), means smaller stacks and lower cost / kW. To increase power density, voltage losses need to be minimised. Overall iRcell voltage E(cell) is given by: E(cell) = E(rev) + iR + η (activation) + η (concentration) Where E(rev) is the reversible or open-circuit cell potential (OCP), i is current density, iR represents the ohmic losses in the cell and η represents the activation and concentration overpotentials at the anode and cathode. The cell voltage equation is usually simplified to include all voltage losses in a single cell resistivity term R(cell) as follows: E(cell) = OCP ± iR(cell) Ohmic losses have been shown to make up 50-80% of total cell resistance depending on cell architecture. This can be reduced with more conducting electrodes and membranes and with reduced inter-electrode distances. Activation polarisation losses represent less than 20% of total cell resistance in flow-through cells using porous carbon felt electrodes, but a much higher proportion in “zero-gap” designs that employ thin carbon paper electrodes in conjunction with serpentine or interdigitated flow fields. Graphite felt activation:
Extensive investigation of activation processes for graphite felt electrodes was undertaken at UNSW in the 1990s in an effort to allow use of lower cost felts and improve performance. Early studies by Sun and Skyllas-Kazacos [12,13] showed the importance of C-OH surface functional groups in imparting hydrophilic properties to the graphite felt, while also providing active sites for the electron transfer reactions. Since then, dozens of other studies have extended this work and further verified the importance of surface functional groups in determining the kinetics of the vanadium redox couple reactions that in turn affects power density. Further increasing the concentration of surface oxide groups could allow greater improvements in electrode kinetics, however, once a critical surface oxide concentration has been produced, the formation of CO2 becomes an undesirable side reaction that leads to the disintegration of the carbon electrode substrate. Other means of enhancing surface oxide functional groups have thus been considered and this mainly involves the introduction of metal oxide electrocatalysts to provide a higher concentration of active sites for the vanadium redox couple reactions. Electrocatalysis for Increasing Power Density This can involve the following approaches: • Introduction of oxygen functional groups • Metal/metal oxide • Mesoporous carbon and carbon nanomaterials • Doping heterologous elements • Electrolyte additives Electrolyte additives were first proposed by Skyllas-Kazacos in 1989 as a method of enhancing the kinetics for the vanadium redox cell reactions while also suppressing the hydrogen evolution side reaction during charging (2). The effects of a range of solution additives on the peak potential separation of the V(IV) ↔ V(V) redox couple reactions and the rate of hydrogen evolution are illustrated in Table 2.
Table 2. Effect of Solution Additives on Reversibility of V(IV)/V(V) Couple and Hydrogen Evolution in the VFB.
Thermal, chemical and electrochemical treatments of carbon and graphite felts were also evaluated at UNSW in the 1990s for the purpose of enhancing the electrochemical activity of these materials as electrodes in the VRB. Sun and Skyllas Kazacos [12] thermally treated samples of 3 mm thick graphite felt (Fibre Material Inc., U.S.A.) in air at various temperatures and times and found that cell resistance initially dropped with increasing activation temperature, reaching a minimum value at 400 ◦C. Significant improvements in both coulombic and voltage efficiencies were observed in cell charge– discharge cycling tests after thermal activation and XPS measurements showed that the amount of chemisorbed oxygen on the activated graphite felt surface increased dramatically after thermal treatment. This was attributed to the increased surface concentration of C- OH and C= 0 functional groups that were produced during activation. A reaction sequence was proposed in which the -O-H groups acted as mediators for the charge–discharge processes at both the positive and negative electrodes (12).
Further investigation of the chemical modification of graphite felt was carried out in sulfuric acid, nitric acid and mixtures of sulfuric and nitric acid by the same group in the 1990s and treatment involving hot concentrated sulfuric acid exhibited excellent results for the FMI (Fibre Material Inc., U.S.A.) graphite felt which were similar to those of thermal activation (13). In another study by the UNSW research group, residual graphite oxide fibre electrodes were prepared and impregnated with solutions containing Pt4+, Pd2+, Au4+, Mn2+, Te4+, In3+ and Ir3+ ions by an ion-exchange process [14]. The iridium modified electrode exhibited the best electrochemical activity for the vanadium redox couple reactions in sulfuric acid, while Mn, Te and In modified electrodes also exhibited an improved performance compared to the untreated one. Pt, Au and Pd were considered unsuitable as they catalyzed the hydrogen evolution reaction at the surface of the negative electrode (14) Carbonaceous electrodes modified by metal or metal oxides have also been intensively investigated by other groups in recent years. Among these, researchers at the Pacific Northwest National Laboratory (PNNL) achieved great performance at high current densities (beyond 100 mAcm-2 by adding Bi3+ into the electrolyte. With the addition of Bi3+, the voltage efficiency reached 87% and 80.4% at 100 mAcm-2 and 150 mAcm-2 respectively (15). After that, the performance of electrodes modified by WO3(16), W-doped Nb2O5 (17), TiO2 (18), CeO2 (19) and ZrO2 (20) and Sb3+ additive (21) at current densities higher than 100 mAcm-2 was studied and the highest voltage efficiency at 100 mAcm-2 was around 85% obtained on CeO2- and ZrO2-modified graphite felt (19, 20). The use of metal oxides as electrocatalysts for the VFB reactions was recently revisited at UNSW and excellent results were obtained by incorporating Mo oxides onto the surface of carbon paper for use as electrodes in a “zero-gap” reactor cell (22). Toray 120 carbon paper was buried in a bed of carbon black and heated in a muffle oven in air at 650℃ for 4h. The thermally treated carbon paper was then soaked in a solution of (NH4)6Mo7O24▪4H2O (3.8mg mL-1) and dried at 80℃. This was subsequently calcined at 650℃ for 2h in a bed of carbon black to produce the MoO3-CP electrode. Table 3 illustrates the voltage efficiencies of the cell incorporating the MoO3-CP modified carbon paper electrodes at various current densities and compares this with the same cell employing thermally treated carbon paper with MoO42- added to the vanadium electrolyte.
Table 3. Performance comparison of vanadium flow cell with MoO3-CP electrodes and thermally treated CP electrodes plus MoO42- additive in vanadium electrolyte Current Density (mAcm-2) 100 125 150
MoO3-CP Voltage Efficiency (%) 85.3 81.1 78.5
Power Density (mWcm-2) 143.6 172.75 199.65
MoO42- additive Voltage Efficiency (%) 85.4 81.2 77.8
Power Density (mWcm-2) 142.4 172 199.95
Both modifications, either applying MoO3- to the CP electrode or employing MoO42- as an electrolyte additive, were seen to produce MoO3 nanosheets on the surface of the carbon fibers, acting as electrocatalysts and providing active sites and for the vanadium oxidation and reduction reactions. Bipolar Electrode Substrates The bipolar electrode substrate is one of the most important components of a flow cell in that it can significantly affect cell resistance and stack cost. Two categories of bipolar electrode substrate materials have been employed in the VFB to date. Their advantages and disadvantages are listed below: Expanded Graphite, Graphite Boards and Plates Advantages: - No hot pressing is needed to attach graphite felt to substrate, therefore easier stack manufacture - High conductivity and low contact resistance with carbon felt - More resistant to overcharge Disadvantages: - Very thin sheets are brittle and difficult to handle - Cannot be welded to produce sealed stack components - 10-20 times more expensive than conducting plastic Conducting plastic substrate- carbon filled PE/PP/rubber blends: Carbon filled PE/PP/rubber blends were developed and used as bipolar electrode substrates at UNSW in the 1990s (6, 23). To obtain sufficiently low contact resistance with the carbon felt however, a tedious heat bonding process was required to allow the carbon fibres in the felt electrode to penetrate through the surface insulating skin and make electrical contact with the underlying electrical network. The advantages and disadvantages of these materials are listed below. Advantages: Low Cost ($10-20/m2) Flexible, easy handling - Can be welded to produce sealed stacks - Insulated edges possible for shunt current reduction Disadvantages:
- Early bipolar electrodes involved labour intensive manufacture and felt heat bonding (Figure 5) - Extended overcharge can cause delamination and irreversible damage - Requires monitoring of individual cells to prevent overcharge
Figure 5 Early Conducting Plastic Electrode Substrates Developed at UNSW in 1990s. The bipolar electrodes produced by this process exhibited resistivities of the order 1 ohm.cm2 that was substantially lower than the 6 ohm.cm2 resistivity for bipolar electrodes formed by compression alone, but significantly higher than can be achieved with graphite board sheets. The ideal bipolar electrode substrate should thus combine the advantage of low contact resistance achievable with graphite boards, with the mechanical flexibility and low cost of the conducting plastic substrates. Recent advances with carbon composites, has opened the possibility of achieving this, with significant cost reductions now available. Recent Conducting Plastic Improvements. On-going studies at UNSW have opened the possibility of using new low cost conducting plastic composite materials as bipolar electrode substrates with low contact resistances for the carbon felt. This would eliminate the difficult heat bonding step that was essential for the earlier materials, while also overcoming the problem of delamination of the positive electrode during extended periods of overcharge. Figure 6 illustrates the CP pellets used together with the resultant bipolar electrode formed by applying a piece of carbon felt on either side.
Figure 6 Conducting Plastic Pellets and Resultant Bipolar Electrode Several CP sheets of different thicknesses were produced by placing the CP pellets into a mould and applying the required pressure for a predetermined time in a hydraulic press (24). A piece of 4 mm thick carbon felt was then placed within frames of different thicknesses on either side of the CP sheets and the resistivity measured. Table 4 summarises the resistivity values obtained for the CP plates of different thickness and for different felt compressions (24). Table 4. Bipolar Plate Resistivity Values for Different Plate Thicknesses and Felt Compressions 2
Bipolar plate thickness (mm) 25 % compression 37.5 % compression 50 % compression
Resistivity (ohm cm ) 2.42.42.5 2.5 S1 S1R
1.71.8 S1
1.71.8 S1R
3.33.4 S1
3.33.4 S1R
3.04
2.79
2.19
1.89
5.14
4.37
1.23
1.16
1.12
1.01
2.11
1.82
0.47
0.45
0.44
0.44
0.78
0.69
Note: S1 and SR1 denotes “sample 1” and “sample 1 repeat” for each measurement.
The results show that although a 25% felt compression produces bipolar electrodes with resistivities as low as 1.9 ohm.cm2, with 37.5% felt compression, the resistivity reduces to 1.0 – 1.2 ohm.cm2 and can reach values as low as 0.44 ohm.cm2 with 50% felt compression. While such high felt compression will lead to excessive pressure drop in the flow battery stack, these results are very encouraging and highlight the possibility of investigating new cell architectures that can combine high felt compression with flow fields in the bipolar plate to reduce pressure drop and minimise pumping energy losses. Performance Optimisation - Flow rate and pump control It is well understood that poor flow-rate control will lead to large efficiency losses in flow batteries. Ideally, flow rate should be controlled using variable speed pumps to conserve energy by only supplying rate of flow required by battery as dictated by the SOC and operating current. The required flow-rate can be calculated from Faraday’s Law for the particular state-of-charge (SOC) and operating current. For a 2 M vanadium electrolyte, the stoichiometric flow rate is given by:
Fs = I/(3.2 x SOC) mL.min-1 This is minimum flow rate that needs to be supplied for each cell in stack. In practice however, actual flow rate is typically 2 to 8 x Fs for good mass transport in the cell and optimal energy efficiency. The actual flow rate required will be a function of cell design. Where a battery is used in an application with a greatly varying load, running the pumps at constant speed can result in more than 30% of the energy used to operate the pumps. An example of this is in a residential PV application where the current can vary from 100 A during peak time to only 5 Amps at night. The simplest approach to reducing pumping energy consumption with greatly varying loads, is to use simple ON/OF pump control. This approach was used in an early UNSW field trial of a VFB in a Solar Demonstration House in Thailand in the mid-1990s (Figure 7) (8).
Figure 7. VFB - Solar Demonstration House in Thailand (8) In this field trial, in order to save energy, the pump control system was designed to turn the pumps on only when the current exceeded 20 A. When the stack voltage dropped below a pre-set value, the pumps were turned on for 3 minutes to replenish the solution in the stack (8). While a drop in voltage efficiency is expected under low or zero flow conditions due to increased concentration polarisation losses, the significant savings in pumping energy far outweighed such losses in this particular application. These effects can be clearly demonstrated by mathematical modelling and simulation as described below. Modelling and Simulation of Effect of Flow Rate on Battery Performance A comprehensive mathematical model was developed by the UNSW group for the optimisation of stack design and operation for the purpose of minimising shunt currents and pumping energy losses in the VFB (25, 26). This model was employed in a series of simulations to demonstrate the effect of flow-rate on overall battery efficiency due to pumping energy losses. A 100-cell VFB stack with electrode dimensions of 50 cm (H) x 75 cm (W) was assumed. The channels and manifold dimensions were chosen so as to minimise shunt currents and the membrane used was Nafion. Other parameters used in the simulations are given in Figure 8. A number of simulations were performed for a range of currents under conditions of constant and variable flow rate and typical results are presented in Figure 9 and Table 5.
Figure 9 shows the typical outputs of the simulation that include graphs of V concentration and electrolyte flow rate as a function of time, along with the cell and stack voltage vs time plots and the individual cell currents during both charge and discharge. Also shown are the battery performance parameters of coulombic, voltage, energy efficiencies as well as the overall system efficiency that includes the pumping energy losses. In each case, a stoichiometric flow factor of 8 and an SOC range of 20-80% were assumed. The results are summarised in Table 5. Assuming a pumping efficiency of 60%, an energy efficiency loss of 6-10% can be attributed to the pumps in all cases involving 25% felt compression. At a higher felt compression of 50% however, the pumping energy loss increases to more than 20% for this particular stack design, highlighting the importance of incorporating flow fields in very large stacks where high felt compression is used to reduce contact resistance.
Figure 8. Stack parameters used in stack performance simulations
Figure 9. Typical Simulation Results - Constant vs Variable Flow Rate
Table 5. Summary of Simulation Results for Different Currents, Flow and Felt Compression. Curr ent A 100 100 100 100 150 150 150 150 300 300 300 450
Flow
Felt Compre ssion %
Cell Resisti vity ohms. cm2
Coulo mbic Eff %
Volt aic Eff %
Ene rgy Eff %
System Efficien cy%
const ant 8
25
2
87.6
90.5
79.2
68.3
50
2
87.6
90.5
79.3
43.4
25
2
87.5
88.9
77.8
75
50
2
87.5
89.1
77.9
67.3
25
1
91.8
91.8
84.3
67.4
25
1
91.8
90.1
82.7
78.2
25
2
91.8
86.5
79.4
63.4
25
2
91.8
84.5
77.5
73.3
25
2
95.9
74
71
63.3
25
1
96.2
83.4
80.3
71.6
50
1
95.5
84.1
80.4
51.2
25
1
97.6
77.1
75.3
63.2
const ant 8 varia ble 8 varia ble 8 const ant 8 varia ble 8 const ant 8 varia ble 8 varia ble 8 varia ble 8 Vari able 8 varia ble 8
Optimal Flow-rate Control As demonstrated above, significant efficiency savings can be achieved by operating a VFB with variable instead of constant flow rate. The minimum stoichiometric flow rate needed to operate a VFB between the desired SOC limits can be calculated using Faradays Law. For a cell employing a 1.6 M vanadium electrolyte, the stoichiometric flow rate will thus need to be adjusted as follows during operation at different currents, I (amps) and SOCs: F = I/(2.56 SOC) ml/min for each cell s
The pump control system thus needs to be able to calculate, monitor and continuously set the desired flow rate during charging and discharging. While flow rate can be estimated from pressure drop, an alternative approach that was proposed by Skyllas-Kazacos in the late 1980s (4). This is based on the differential open circuit
potential (OCP), measured from an open circuit cell installed at both the outlet and inlet ports of the cell stack as illustrated in Figure 10.
Figure 10. Differential OCP arrangement for flow rate control.
The required differential OCP is determined using the Nernst equation as shown below. Letting
x = SOC y = percentage conversion on a single pass through battery C’= total concentration of vanadium
We can assume that: [V5+in]
= [V2+in] = xC’ and
[V5+out] = [V2+out] = (x-xy)C’ and
[V4+in] = [V3+in] = (1-x)C’ [V4+out] = [V3+out] = ((1-x)+xy)C’
After substitution into Nernst Equation:
Eincell =
Eoutcell = The difference between the inlet and outlet open circuit potential is given by:
∆E
cell
=
This can be plotted as a function of the inlet SOC for different values of conversion per pass, or ∆SOC change per pass as illustrated in Figure 10. For example, a Delta SOC of 0.1 represents a stoichiometric flow factor of 10, that is, 10 x stoichiometric flow rate for the particular current or SOC.
Delta E(Cell) vs SOC â&#x2C6;&#x2020;E
Delta E(cell) /Volts
cell
=
0.120 0.100
Delta SOC=0.05
0.080
Delta SOC=0.1
0.060
Delta SOC=0.15
0.040
Delta SOC=0.20
0.020
Delta SOC=0.25
0.000 0
0.2
0.4
0.6
0.8
1
SOC (IN)
Figure 10. Plot of Differential OCP vs inlet SOC for different SOC changes per pass A project is currently underway at UNSW to develop a flow-rate controller based on differential OCP measurements. Other Research Projects at UNSW. A number of other research projects are also currently underway at UNSW in conjunction with Prof Jie Bao and Dr Chris Menictas. Some results have already been published, while others will be published in the near future. These projects are summarised below: -
-
Modelling and simulation of thermal behaviour of VFB (27) Dissipativity Based Distributed Economic Model Predictive Control for Residential Microgrid with Renewable Energy Generation and Battery Energy Storage (28) Studies on optimal charging conditions for vanadium redox flow batteries. (29) Control of electrolyte flow rate for VFB by gain scheduling, (30) Charging Control of VFB Energy Storage Systems with Variable Input Power (31). Dynamic response of vanadium flow batteries Frequency response analysis of VFB Fault detection in VFB Advanced control of flow batteries Inverter design for VFB applications Effect on ripple on VFB life Vanadium Oxygen redox fuel cell studies (32):
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8.
9.
10.
11.
12.
13.
14.
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