High-Performance Vanadium Redox Flow Batteries with Graphite Felt Electrodes

Journal of C Carbon Research

Article High-Performance Vanadium Redox Flow Batteries with Graphite Felt Electrodes

Trevor J. Davies * and Joseph J. Tummino

Department of Natural Sciences, University of Chester, Thornton Science Park, Pool Lane, Ince, Chester CH2 4NU, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-1244-512297

Received: 11 December 2017; Accepted: 18 January 2018; Published: 25 January 2018

Abstract: A key objective in the development of vanadium redox flow batteries (VRFBs) is the improvement of cell power density. At present, most commercially available VRFBs use graphite felt electrodes under relatively low compression. This results in a large cell ohmic resistance and limits the maximum power density. To date, the best performing VRFBs have used carbon paper electrodes, with high active area compression pressures, similar to that used in fuel cells. This article investigates the use of felt electrodes at similar compression pressures. Single cells are assembled using compression pressures of 0.2–7.5 bar and tested in a VRFB system. The highest cell compression pressure, combined with a thin Nafion membrane, achieved a peak power density of 669 mW cm−2 at a flow rate of 3.2 mL min−1 per cm2 of active area, more than double the previous best performance from a felt-VRFB. The results suggest that felt electrodes can compete with paper electrodes in terms of performance when under similar compression pressures, which should help guide electrode development and cell optimization in this important energy storage technology.

Keywords: vanadium redox ﬂow battery; graphite felt; ﬂow battery power density; cell ohmic resistance; electrode compression

1. Introduction The accelerating growth of renewable intermittent electricity supplies (wind, solar, tidal) requires an accompanying growth of energy storage installations [1]. At present, lithium-ion batteries dominate electrochemical energy storage applications, their success driven by low cost, high energy density and proven reliability [2]. However, redox flow batteries (RFBs) are seen as an attractive alternative to lithium-ion. Despite their higher price per kWh, RFBs have longer lifetimes, reduced fire risk, and are recyclable [3,4]. More importantly, power and capacity are completely decoupled in a flow battery, with power determined by the size of the stack and capacity determined by the volume and concentration of electrolyte (i.e., size of the electrolyte tanks). Consequently, RFBs offer a versatile approach to energy storage, capable of both long and short duration applications, with their market impact expected to rapidly increase over the next 10–20 years. Conservative estimates value the cumulative global RFB market at €50 billion by 2030, corresponding to ~€3 billion per annum by that time [5]. Of the several flow battery chemistries available, the vanadium redox flow battery (VRFB) is by far the most commercialized due to its long lifetime and ease of recycling [6]. Indeed, the world’s largest battery will be a 200 MW/800 MWh VRFB in China [7]. Despite the success of VRFBs, system cost is a major issue and must decrease for the technology to be competitive. A key strategy in VRFB cost reduction is the improvement of cell power density and performance [8]. Compared to polymer electrolyte fuel cells (PEFCs), which possess similar cell architecture, VRFBs have incredibly poor performance. For example, conventional PEFCs can easily achieve current densities of 1.5 A cm−2 whereas most VRFBs installed today operate at current densities less than 0.2 A cm−2, an order of

C 2018, 4, 8; doi:10.3390/c4010008 www.mdpi.com/journal/carbon C 2018, 4, 8 2 of 17 magnitude lower. Although a direct comparison is unfair (because VRFBs operate at higher efﬁciencies than PEFCs), the relatively poor performance of VRFB cells compared to PEFCs requires much larger cells for a given power. For example, a conventional 5 kW VRFB stack is of similar size to the 114 kW fuel cell stack in the Toyota Mirai. A number of researchers have analyzed the cost factors associated with VRFBs and found the stack contributes 30–32% of the overall system cost [6,9,10]. Thus, if fuel cell-like performance can be achieved, the size of VRFB stacks could decrease by an order of magnitude, with a dramatic effect on system cost. The electrode material plays a key role in determining the performance of a flow battery cell. Due to its high conductivity, favorable permeability, electrochemical stability, and large surface area, graphite (or carbon) felt is presently the most favored electrode material for VRFBs [11,12]. In one of the earliest studies on VRFBs, Kazacos and Skyllas–Kazacos achieved a peak power density of 85 mW cm−2 using graphite felt electrodes [13]. Since then, a large number of researchers have improved performance via kinetic effects, modifying graphite felt electrodes to increase their reactivity and decrease the over potential associated with driving the electrochemical reactions [11,12]. Some studies improved cell performance by introducing more favorable mass transport of electrolyte through the felt electrodes [14,15]. Considerable gains in performance have been achieved by reducing the ohmic resistance of the cell, via increasing the compression of the felt electrode and decreasing the contact resistance within the cell [16–19]. Park et al. found compression values in the range 20–30% gave favorable performance (where the compression is the percentage reduction in thickness—i.e., 20% compression corresponds to a felt compressed to 80% of its initial thickness) [18], whereas Chang et al. further improved power density by increasing the compression to 40% [16]. One of the highest peak power densities recorded with felt electrodes was obtained by Chen et al., who achieved 311 mW cm−2 with a GFA5 felt electrode at a compression of 33% [20]. Their performance was slightly bettered by Zhao and coworkers, who achieved a peak power density of 350 mW cm−2 with a VRFB utilizing GFA5 electrodes at elevated temperature (55 ◦C) [21]. However, the compression of the electrodes was not reported. More recently, Brown et al. studied felt compressions up to 75% [17]. At a current density of 200 mA cm−2, an increase in felt compression from 25% to 75% raised power density from 157 to 200 mW cm−2. The researchers also showed the performance benefit was mostly caused by a decrease in contact resistance between the felt and the back plate. This suggests peak power density values greater than 350 mW cm−2 are possible with felt electrodes. The most dramatic improvement in VRFB performance was made by researchers at the University of Tennessee, who used carbon paper (GDL 10AA) instead of graphite felt as the electrode material, achieving a peak power density of 557 mW cm−2 [22]. Their cell design, named “zero gap”, was essentially a conventional PEFC without the catalyst coating on the membrane. Consequently, the compression pressure on the active area was like that in PEFCs (8–10 bar), greatly reducing the ohmic resistance of the cell. Shortly after, the Tennessee group further increased peak power density to 767 mW cm−2 using pre-treated carbon paper electrodes in combination with thinner membranes [23]. More recently, the same group achieved a power density of 1290 mW cm−2 at current densities of over 1 A cm−2 by improving mass transport to the carbon paper electrodes [24]. Following the success of the “zero gap” VRFB, several groups performed studies into improving the kinetics and/or mass transport properties of carbon paper electrodes in VRFBs [8,25–31]. The most noticeable contribution came from United Technologies Research Centre (UTRC), who reported a peak power density of 1350 mW cm−2 at a current density of 1.6 A cm−2, although the exact cell details were not disclosed [32]. Reviewing the published results on VRFBs using graphite felt and carbon paper electrodes, the literature suggests the latter are required to achieve high power densities. However, there are suggestions that felt electrodes can perform better if the compression pressure is like that used with paper electrodes [17]. This work seeks to establish the performance limits of VRFBs with felt electrodes by conducting a thorough study of the effect of felt compression (from mild to extreme) on cell power density and a range of other important properties. The results will help guide the development of electrodes for high-performance ﬂow batteries. C 2018, 4, 8 3 of 17

2. Results and Discussion

2.1. Ex-Situ Tests Figure1a illustrates the effect of compression pressure on the thickness of the graphite felt GFD 4.6EA and the carbon paper GDL 10AA (where the compression pressure is the pressure experienced by the sample), recorded using the compression rig described in Section 3.4. Up to a compression pressure of 2 bar, there is a sharp decrease in felt thickness, which is followed by a gentler decrease in thickness from 2–12 bar. The blue zone represents 10–30% compression, which is often the range recommended by felt manufacturers for RFB applications. Figure1b,c illustrate the corresponding compression and porosity relationships, respectively, where compression is deﬁned as the percentage reduction in thickness (compared to the original felt sample) and porosity, φ, is estimated using:

m φ = 1 − × 100% (1) DcfV where m is the mass of the felt sample, Dcf is the density of carbon fibers in the sample (assumed to be 1.8 g cm−3 [33]) and V is the total volume of the felt sample. As observed, the carbon paper displays a much higher compressive strength than the felt. In a typical felt electrode VRFB (i.e., 10–30% compression), the compression pressure will be 1 bar or less corresponding to a porosity of ~93%. This compares to ~83% for carbon paper electrodes (i.e., GDL 10AA) used in high-performance VRFBs, where the applied pressure is likely to be 5–10 bar. These “paper-like” porosities are also achievable for felt electrodes at compression pressures of 5–10 bar, corresponding to felt compression values above 65%. Figure1d shows the area specific resistance of the electrode material, sandwiched between 2 gold-coated steel plates (details in Section 3.4), under different compression pressures (where the active area is 31 cm2). For both materials the profile mimics that of Figure1a, with a steep decrease in resistance up to 2 bar applied pressure, followed by a gentler decrease in resistance as the assembly is further compressed. At higher compression pressures the resistance of the paper electrode assembly is approximately 6 times less than the felt assembly. At the same pressures, the paper thickness is approximately five times less than that of the felt. This suggests the resistivities of the two materials are not too dissimilar and the main reason for the large difference in resistance is due to the thickness of the electrodes. Assuming the gold-coated steel plates make a negligible contribution to the resistance of the assembly, for a given applied pressure, p, the measured resistance, R, can be approximated as:

ρt R = + 2R (2) A c

2 where ρ is the felt resistivity, t is the felt thickness, A is the felt area (31 cm ) and Rc is the contact resistance between the felt and the gold-coated steel plate. In this case, it is possible to estimate Rc and ρ by repeating the measurement with a cut-down piece of the same felt, for example a section of GFD 4.6EA sliced to half its original thickness. For the same applied pressure, p, the resistivity and contact resistance of the cut-down felt should be the same as the original sample. Therefore, the resistance of the assembly containing the cut-down felt, R2, can be written as:

ρt R = 2 + 2R (3) 2 A c where t2 is the thickness of the cut-down felt. C 2018, 4, 8 4 of 17 C 2018, 4, x FOR PEER REVIEW 4 of 18

Figure 1.1. Plots of (a) thickness;thickness, ( b) compression;compression, ( c) porosity and (d) total areaarea specificspecific resistanceresistance against compression pressure for samplessamples of GFD 4.6EA and GDL 10AA;10AA. (e) BulkBulk feltfelt areaarea specificspecific resistance and (f) totaltotal areaarea specificspecific contactcontact resistanceresistance vs.vs. compressioncompression pressurepressure forfor samplessamples ofof GFDGFD 4.6EA. Blue zone represents thethe 10–30%10–30% feltfelt compressioncompression range.range.

When R, R2, A, t and t2 are all known for a given applied pressure, Equations (2) and (3) can be When R, R2, A, t and t2 are all known for a given applied pressure, Equations (2) and (3) can solved to obtain estimates for the felt resistivity and the contact resistance. Accordingly, samples of be solved to obtain estimates for the felt resistivity and the contact resistance. Accordingly, samples GFD 4.6EA and cut-down GFD 4.6 EA were used to estimate Rc and ρ for a range of pressures. On of GFD 4.6EA and cut-down GFD 4.6 EA were used to estimate Rc and ρ for a range of pressures. average, the resistivity of the bulk felt was ~2 mΩ cm and displayed a weak dependence on compression, with a change of around 5% over the range of applied pressures. This agrees with the X-ray tomography analysis of Brown et al. who found no significant increase in contact between C 2018, 4, 8 5 of 17

COn 2018 average,, 4, x FOR thePEER resistivity REVIEW of the bulk felt was ~2 mΩ cm and displayed a weak dependence5 of on 18 compression, with a change of around 5% over the range of applied pressures. This agrees with the carbonX-ray tomography fibers in a analysishighly compressed of Brown et al.graphite who found felt electrode no significant [17]. increaseThe resistivity in contact was between then used carbon to calculatefibers in athe highly bulk compressed resistance of graphite GFD 4.6EA. felt electrode This is [illustrated17]. The resistivity in Figure was 1e thenand usedthe total to calculate contact resistancethe bulk resistance (2Rc) is shown of GFD in 4.6EA.Figure This1f, both is illustrated in terms of in area Figure specific1e and resistance. the total contactIn each resistancecase, the results (2 Rc) foris shown three separate in Figure felt1f, bothsamples in terms (i.e., three of area GFD specific 4.6EA resistance. vs. cut-down In each GFD case, 4.6EA the combinations) results for three are given,separate which felt samples represent (i.e., the three spread GFD observed 4.6EA vs. using cut-down this method. GFD 4.6EA Both combinations) the felt bulk are resistance given, which and contactrepresent resistance the spread decrease observed as the using applied this method.pressure Bothincreases, the felt with bulk a similar resistance profile and to contact that for resistance the total assemblydecrease as resistance. the applied On pressure average, increases, the contact with resistance a similar profileaccounts to thatfor approximately for the total assembly 20% of resistance. the total assemblyOn average, resistance. the contact resistance accounts for approximately 20% of the total assembly resistance. The high-performance carbon carbon paper paper electrode electrode VRFB VRFBss reported reported in the literature most likely use cell compression pressures of 8–10 bar, similar to that usedused inin conventionalconventional PEFCs.PEFCs. The results in Figure 11 suggestsuggest conventionalconventional feltfelt electrodeelectrode VRFBsVRFBs useuse cellcell compressioncompression pressurespressures ofof approximatelyapproximately 1 bar or less, which are associated with high ohmic resistance (both bulk and contact). Increasing the felt compression pressure pressure to 8–10 bar decreases the po porosityrosity to a value like that of paper electrodes (~80%) andand reduces reduces the the associated associated ohmic ohmic resistance resistan by twoce by thirds. two However, thirds. However, even at high even compression at high compressionpressures, the pressures, felt electrode the stillfelt possesseselectrode astill significantly possesses higher a significantly ohmic resistance higher ohmic than the resistance carbon paper than thedue carbon to its much paper larger due to thickness. its much larger thickness. A major disadvantage of increasing felt compression is shown in Figure 22aa wherewhere pumpingpumping pressure is plotted against water flow flow rate for a range of felt compression pressures (obtained by varying thethe half-cellhalf-cell well well depth) depth) in thein flowthe flow rig described rig described in Section in Section3.4. For all3.4. compression For all compression pressures, pressures,the relationship the relationship between pumping between pressurepumping andpressure flow and rate flow is linear rate (linesis linear of best(lines fit of are best plotted), fit are plotted),in agreement in agreement with Darcy’s with Darcy’s Law [34 Law]. As [34]. felt As compression felt compression pressure pressure increases, increases, the porositythe porosity of the of thematerial material decreases, decreases, making making fluid flowfluid throughflow through the medium the medium more difficult. more Thisdifficult. is further This illustratedis further illustratedby Figure2 byb, whereFigure pumping2b, where pressure pumping required pressure for requ a waterired for (at a 22 water◦C) (at flow 22 rate°C) flow of 100 rate mL of min 100− mL1 is minplotted−1 is againstplotted the against felt compression the felt compression pressure forpressure a range for of a half-cells range of of half-cells varying wellof varying depth. well In this depth. case, Inincreasing this case, the increasing compression the compression pressure from pressure 1 to 9 bar from caused 1 to the9 bar pumping caused the pressure pumping to increase pressure over to increase7 times. Givenover 7 thistimes. was Given a one this inlet-one was a outlet one inlet- flowone through outlet arrangement, flow through the arrangement, use of a flow the field use could of a flowconsiderably field could decrease considerably the pumping decrease pressure the andpump possiblying pressure improve and the possibly mass transport improve properties the mass of transportthe electrode, properties as seen of with the carbonelectrode, paper as s electrodeseen with carbon [24,26 ].paper electrodes [24,26].

Figure 2. 2. (a()a Plot) Plot of pumping of pumping pressure pressure vs. flow vs. rate ﬂow fo rater water for at water ambient at ambient temperature temperature through samples through ofsamples GFD of4.6EA GFD at 4.6EA different at different compression compression pressures pressures (% compression (% compression given given in inbrackets). brackets); (b (b) )Plot Plot of of −1 pumping pressure for a 100 mL minmin−1 flowﬂow rate rate of of water water at at ambient ambient temperature temperature through through GFD GFD 4.6EA at different compression pressures.

2.2. VRFB Cell Performance Table 1 lists details of the 10 cell builds studied in this report. The compression pressure (i.e., pressure on the active area) was determined using Figure 1a and the measured electrode well depth. Only untreated GFD 4.6EA was used as the electrode material for cell performance tests. C 2018, 4, 8 6 of 17

2.2. VRFB Cell Performance Table1 lists details of the 10 cell builds studied in this report. The compression pressure (i.e., pressure on the active area) was determined using Figure1a and the measured electrode well depth. Only untreated GFD 4.6EA was used as the electrode material for cell performance tests.

Table 1. Details of the cell builds used in vanadium redox ﬂow batteries (VRFB) tests. The electrode material was GFD 4.6EA.

Electrode Compression Cell Number Membrane % Compression Thickness/mm Pressure/bar 1 Nafion 117 1.43 6.95 71 2 Nafion 117 2.21 2.20 54 3 Nafion 117 3.47 0.77 29 4 Nafion 117 4.37 0.17 10 5 Nafion 212 1.40 7.49 71 6 Nafion 212 2.07 2.52 57 7 Nafion 212 3.26 0.93 33 8 Nafion 212 4.42 0.15 9 9 Nafion 117 2.21 2.23 55 10 Nafion 117 4.31 0.21 11

Initially, Nafion N117 (~0.2 mm thickness) was used as the membrane material. Figure3 illustrates the effect of electrode compression on cell performance at a flow rate of 100 mL min−1, corresponding to 3.2 mL min−1 per cm2 of electrode area (31 cm2 cell active area). Similar performance results were observed at different flow rates (50 and 200 mL min−1) and are given in AppendixA. Figure3a illustrates the steady state current density-voltage curves, with the corresponding power density-voltage curves given in Figure3b. At current densities below 0.2 cm 2, there is little difference between the four cells, the voltage drops and power densities are almost identical. However, as the current density increases, the reduced ohmic resistance associated with cell compression pressure becomes apparent and the performance of the cells becomes well separated. Increasing the compression pressure on the electrode from 0.2 bar (Cell 4) to 6.9 bar (Cell 1) results in an increase in maximum power density from 276 to 429 mW cm−2 (a 55% increase). Figure4 illustrates area specific high-frequency resistance (HFR) values for each cell operating at 0.4 A cm−2 (at 50% state of charge). As the cell compression pressure increases from its lowest value there is a steep drop in HFR followed by a more gradual decline, as expected from the ex-situ resistance measurements in Figure1d. The difference in cell performance can be mostly explained by the changes in ohmic resistance. For example, from the discharge curves in Figure3a, the respective cell voltage of Cells 1 and 4 at 0.4 A cm −2 is 0.90 and 0.69 V, with corresponding HFR values of 0.753 and 1.115 Ω cm2. At this current density, the voltage-drop associated with the HFR of cell 4 is 0.145 V greater than that for Cell 1, accounting for almost 70% of the difference in operating voltage. The remaining 30% is most likely caused by electrolyte velocity and surface area effects. Table2 compares the results from Cell 1 (sixth row) with those from high performance VRFBs in the literature. The discharge voltage efficiency, VEd in column 9 is defined as the cell discharge voltage at a given current density, Vd, divided by the open circuit voltage, OCV:

V VE = d (4) d OCV

Table 1. Details of the cell builds used in vanadium redox flow batteries (VRFB) tests. The electrode material was GFD 4.6EA.

Cell Electrode Compression % Membrane Number Thickness/mm Pressure/bar Compression 1 Nafion 117 1.43 6.95 71 2 Nafion 117 2.21 2.20 54 3 Nafion 117 3.47 0.77 29 4 Nafion 117 4.37 0.17 10 5 Nafion 212 1.40 7.49 71 6 Nafion 212 2.07 2.52 57 7 Nafion 212 3.26 0.93 33 8 Nafion 212 4.42 0.15 9 9 Nafion 117 2.21 2.23 55 10 Nafion 117 4.31 0.21 11

Initially, Nafion N117 (~0.2 mm thickness) was used as the membrane material. Figure 3 illustrates the effect of electrode compression on cell performance at a flow rate of 100 mL min−1, corresponding to 3.2 mL min−1 per cm2 of electrode area (31 cm2 cell active area). Similar performance results were observed at different flow rates (50 and 200 mL min−1) and are given in Appendix A. C 2018, 4, 8 7 of 17 Figure 3a illustrates the steady state current density-voltage curves, with the corresponding power density-voltage curves given in Figure 3b. At current densities below 0.2 cm2, there is little difference powerbetween density the four (331 cells, mW the cm −voltage2) very drops similar and to thatpower of Chendensities et al. are (311 almost mW cmidentical.−2), which However, suggests as cellthe compressioncurrent density pressure increases, is a keythe factorreduced in theohmic difference resistance between associated Chen’s with peak cell power compression of 311 mW pressure cm−2 andbecomes the 429 apparent mW cm −and2 result the fromperformance Cell 1. of the cells becomes well separated. Increasing the compressionResults from pressure the first on the reported electrode high-performance from 0.2 bar (Cell paper-VRFB 4) to 6.9 bar are (Cell listed 1) in results the first in rowan increase of Table 2in. Althoughmaximum thispower cell density has a similar from 276 discharge to 429 mW voltage cm−2 efficiency (a 55% increase). to Cell 1,Figure the maximum 4 illustrates power area densityspecific forhigh-frequency the paper-VRFB resistance is 557 (HFR) mW cm values−2. The for keyeach difference cell operating between at 0.4 the A twocm−2 cells(at 50% appears state of to charge). be their ohmicAs the cell resistance, compression with HFRpressure values increases of 0.753 fromΩ cm its2 lowestfor Cell value 1 vs. there 0.50 isΩ a cmsteep2 for drop the in paper-VRFB HFR followed of Aaronby a more et al. gradual [22]. The decline, lower ohmicas expected resistance from of the the ex paper-VRFB-situ resistance is most measurements likely due to in a combinationFigure 1d. The of reduceddifference contact in cell resistance performance (i.e., acan higher be mostly cell compression explained pressure)by the changes and a lower in ohmic electrode resistance. resistance For (theexample, GDL from 10AA the electrode discharge has curves a lower in Figure thickness 3a, the and respective resistivity cell than voltage the compressed of Cells 1 and GFD 4 at 4.6EA). 0.4 A Givencm−2 is that0.90 peakand 0.69 power V, forwith both corresponding cells occurred HFR around values 0.7 of A0.753 cm −and2, the 1.115 difference Ω cm2. inAt HFRthis current values correspondsdensity, the tovoltage-drop 177 mV of extraassociated voltage with loss the for HFR Cell 1,of whichcell 4 equatesis 0.145 toV angreater additional than that power for density Cell 1, lossaccounting of 124 mW for almost cm−2 (97% 70% ofof the the difference difference in in peak operating power densities).voltage. The Other remaining factors contributing30% is most tolikely the differencecaused by inelectrolyte performance velocity could and be surface variations area in effects. OCVand mass transport.

Figure 3. PlotsPlots of of (a ()a cell) cell operating operating voltage voltage and and (b) (cellb) cell power power density density against against current current density density for Cells for C 2018Cells1–4, 4, x(labeled) FOR 1–4 (labeled)PEER with REVIEW a with range a range of active of active area co areampression compression pressures pressures and % and compression % compression values. values. 7 of 18

Figure 4. Plot of area specificspecific high-frequency resistance against active area compression pressure for Cells 1–4 (Nafion(Nafion 117 membranes) and Cells 5–8 (Nafion(Nafion 212 membranes), where the current density −2 was 0.4 A cm −2. .

Table 2 compares the results from Cell 1 (sixth row) with those from high performance VRFBs in the literature. The discharge voltage efficiency, VEd in column 9 is defined as the cell discharge voltage at a given current density, Vd, divided by the open circuit voltage, OCV: V VE = d (4) d OCV

One of the best performing felt-VRFBs was reported by Chen et al., who achieved 311 mW cm−2 with a GFA5 electrode at a compression of 33% (Table 2, row 5) [20]. Despite the much lower relative flow rate, the peak power density of 429 mW cm−2 obtained with Cell 1 is noticeably higher (a 38% increase) than the result of Chen et al. Furthermore, the discharge voltage efficiency at 0.5 A cm−2 is almost 20% higher for Cell 1. Note that Cell 3 with an electrode compression of 29%, has a maximum power density (331 mW cm−2) very similar to that of Chen et al. (311 mW cm−2), which suggests cell compression pressure is a key factor in the difference between Chen’s peak power of 311 mW cm−2 and the 429 mW cm−2 result from Cell 1. Results from the first reported high-performance paper-VRFB are listed in the first row of Table 2. Although this cell has a similar discharge voltage efficiency to Cell 1, the maximum power density for the paper-VRFB is 557 mW cm−2. The key difference between the two cells appears to be their ohmic resistance, with HFR values of 0.753 Ω cm2 for Cell 1 vs. 0.50 Ω cm2 for the paper-VRFB of Aaron et al. [22]. The lower ohmic resistance of the paper-VRFB is most likely due to a combination of reduced contact resistance (i.e., a higher cell compression pressure) and a lower electrode resistance (the GDL 10AA electrode has a lower thickness and resistivity than the compressed GFD 4.6EA). Given that peak power for both cells occurred around 0.7 A cm−2, the difference in HFR values corresponds to 177 mV of extra voltage loss for Cell 1, which equates to an additional power density loss of 124 mW cm−2 (97% of the difference in peak power densities). Other factors contributing to the difference in performance could be variations in OCV and mass transport. C 2018, 4, 8 8 of 17

Table 2. Details of high performance VRFBs reported in the literature. The last two rows concern Cells 1 and 5 from the present study.

Electrode Electrode Membrane Flow Rate/mL E at 0.5 A VE at 0.5 A Peak Power/mW HFR 4/Ω % Comp. 1 Electrolyte OCV 2/V d Ref. Material Pre-Treatment (Thickness 3) min−1 cm−2 cm−2/V cm−2 cm−2 cm2 Paper GDL 1 M V Nafion 117 None - 1.68 5 4 0.99 0.59 557 0.50 [22] 10AA 4 M H2SO4 (183 µm) Paper GDL 1 M V Nafion 212 Thermal - 1.66 5 18 1.26 0.76 767 0.219 [23] 10AA 5 M H2SO4 (50.8 µm) Paper GDL 1.7 M V Nafion 117 None - 1.43 5.6 1.0 0.70 1290 - [24] 10AA 3.3 M H2SO4 (183 µm) Felt 1.4 M V SFPAE 6 Thermal 33% 1.5 10 0.47 0.31 311 - [20] GFA5 2 M H2SO4 (45 µm) Felt 1 M V Nafion 115 None - 1.65 10 0.70 0.42 350 - [21] GFA5 4 M H2SO4 (127 µm) Felt 1.6 M V Nafion 117 None 71% 1.38 3.2 0.81 0.59 429 0.753 Cell 1 GFD 4.6EA 4 M H2SO4 (183 µm) Felt 1.6 M V Nafion 212 None 71% 1.38 3.2 0.98 0.71 669 0.374 Cell 5 GFD 4.6EA 4 M H2SO4 (50.8 µm) 1 Compression; 2 Open circuit voltage; 3 Approximate; 4 Area specific high-frequency resistance; 5 Discharge performance curves started at 100% state of charge. However, battery state of charge was 50–60% by time peak power achieved; 6 Fluorinated sulfonated poly(arylene ether). C 2018, 4, 8 9 of 17 C 2018, 4, x FOR PEER REVIEW 9 of 18

WhenWhen optimizing optimizing the the membrane membrane for for their their high-per high-performanceformance paper-VRFB, Li Liuu et al. al. found found Nafion Nafion 212212 gave gave the highest peak power density [23]. [23]. Consequently, Consequently, cell cell builds builds 1–4 1–4 above above were were repeated usingusing Nafion Nafion 212 212 as the membrane material. Figure Figure 55 illustratesillustrates thethe dischargedischarge performanceperformance ofof CellsCells 5–8,5–8, with with the the corresponding corresponding HFR HFR results results given given in Fi ingure Figure 4. The4. The trends trends mimic mimic those those for Cells for Cells 1–4, with 1–4, performancewith performance improving improving as the as cell the compression cell compression pressu pressurere is increased, is increased, mostly mostly caused caused by bychanges changes in contactin contact resistance resistance and and electrode electrode resistance resistance (i.e., (i.e., ohmic ohmic resistance resistance of the of thecells). cells). However, However, the use the useof the of 2 thinnerthe thinner membrane membrane results results in a in decrease a decrease in inHFR HFR of ofover over 0.3 0.3 ΩΩ cmcm2 forfor each each cell cell build, build, leading leading to to noticeablynoticeably higher higher peak peak power power densities densities than than Cells Cells 1–4. 1–4. Key Key figures figures for for the the best best performing performing felt-VRFB, felt-VRFB, CellCell 5, 5, are are listed listed in in the the final final row row of Table 22.. ThisThis cellcell representsrepresents thethe bestbest high-performancehigh-performance felt-VFRBfelt-VFRB −2 reportedreported to to date, date, with with a a peak power density of 669 mW cm−2 (obtained(obtained at at 50% 50% state state of of charge). charge). PerformancePerformance figures figures for for the the closest closest paper-VRFB paper-VRFB equi equivalentvalent are are listed listed in in the the second second row row of of Table Table 22,, takentaken from from a a study study into into the the effect effect of of electrode treatment treatment and and membrane membrane thickness thickness by by the the Tennessee Tennessee −2 groupgroup [23]. [23]. As As before, the paper-VRFB has a higherhigher peakpeak powerpower densitydensity (767(767 vs.vs. 669669 mWmW cmcm−2)) and and 2 lowerlower HFR HFR value value (0.219 (0.219 vs. vs. 0.374 0.374 ΩΩ cmcm2), although), although in inthis this case case the the disc dischargeharge voltage voltage efficiencies efficiencies at 0.5 at −2 A0.5 cm A− cm2 are almostare almost identical identical (0.76 (0.76 for forthethe paper-VRFB paper-VRFB vs. vs. 0.71 0.71 for for Cell Cell 5). 5). Peak Peak power power for for both both cells cells −2 occurredoccurred around around 0.95 0.95 A A cm cm−2. At. At this this current current density, density, the the differen differencece in inHFR HFR values values corresponds corresponds to toa −2 140a 140 mW mW cm cm−2 additionaladditional power power loss loss for for Cell Cell 5, 5,which which is ismore more than than the the difference difference in in peak peak power power −2 densitiesdensities (98 (98 mW mW cm cm−2). ).This This suggests suggests felt felt electrodes electrodes can can match match or orpossibly possibly better better the the performance performance of paperof paper electrodes electrodes if the if thecompression compression pressures pressures are high are high enough. enough. However, However, Cell 5 Cell is quite 5 is quitefar from far −2 achievingfrom achieving the peak the peakpower power density density of 1290 of 1290 mW mW cm cm−2 obtainedobtained with with the the Tennessee Tennessee group’s group’s best best performingperforming paper-VRFB, listedlisted in in row row 3 3 of of Table Table2[ 224 [2].4]. This This cell cell was was the the result result of a of thorough a thorough flow flow field fieldoptimization, optimization, which which could could also bealso possible be possible for the for felt-VRFBs the felt-VRFBs in this in study. this study.

FigureFigure 5. 5. PlotsPlots of of (a ()a cell) cell operating operating voltage voltage and and (b) cell (b) cellpower power density density against against current current density density for Cells for 5–8Cells (labeled) 5–8 (labeled) with a with range a rangeof active of activearea co areampression compression pressures pressures and % and compression % compression values. values.

Unlike carbon papers, where the carbon fibers mostly lie in the xy plane, graphite felts have a 3- Unlike carbon papers, where the carbon fibers mostly lie in the xy plane, graphite felts have dimensional structure. Consequently, membrane punctures and short circuits are more likely when a 3-dimensional structure. Consequently, membrane punctures and short circuits are more likely using graphite felt electrodes. Figure 6 illustrates the results of in-situ short circuit tests (recorded when using graphite felt electrodes. Figure6 illustrates the results of in-situ short circuit tests after 4 h of load testing) with Cells 1 and 4 (Nafion 117 membrane) and Cells 6 and 8 (Nafion 212 (recorded after 4 h of load testing) with Cells 1 and 4 (Nafion 117 membrane) and Cells 6 and 8 membrane). Throughout the test the cell is at open circuit and the voltage is recorded with time. (Nafion 212 membrane). Throughout the test the cell is at open circuit and the voltage is recorded Initially, the electrolytes are flowing, giving a steady open circuit voltage. The flow suddenly stops with time. Initially, the electrolytes are flowing, giving a steady open circuit voltage. The flow and the voltage is monitored. When negligible short circuits are present, the voltage gently increases, suddenly stops and the voltage is monitored. When negligible short circuits are present, the voltage due to a decrease in cell and electrolyte temperature. This was the case for all the Nafion 117 cells. gently increases, due to a decrease in cell and electrolyte temperature. This was the case for all the When electrolyte flow resumes, there is an initial increase in open circuit voltage, caused by the Nafion 117 cells. When electrolyte flow resumes, there is an initial increase in open circuit voltage, cooled electrolyte entering the cell, followed by a return to the initial value. For the Nafion 212 cells, caused by the cooled electrolyte entering the cell, followed by a return to the initial value. For the Cells 5, 6 and 7 all showed signs of short circuits, with Figure 6b illustrating the results for Cell 6 (and Cell 8, which had negligible short circuits). When electrolyte flow stopped, there was a decrease in cell voltage, indicating depletion of reactants. Thus, the high-performance results of Cell 5 are tainted C 2018, 4, 8 10 of 17

Naﬁon 212 cells, Cells 5, 6 and 7 all showed signs of short circuits, with Figure6b illustrating the results forC 2018 Cell, 4, 6x FOR (and PEER Cell REVIEW 8, which had negligible short circuits). When electrolyte ﬂow stopped, there10 wasof 18 a decrease in cell voltage, indicating depletion of reactants. Thus, the high-performance results of Cellby poor 5 are durability, tainted by and poor membrane durability, protection and membrane strategies protection must be strategies employed must if using be employed thin membranes if using thinin combination membranes with in combination graphite felts with at compression graphite felts pressures at compression greater pressures than 1 bar. greater than 1 bar.

Figure 6. PlotsPlots of of open open circuit circuit voltage voltage vs. vs. time time for for (a) ( aCells) Cells 1 and 1 and 4 (Nafion 4 (Nafion 117 117 membranes) membranes) and and (b) (Cellsb) Cells 6 and 6 and 8 (Nafion 8 (Nafion 212 212 membranes). membranes). Compression Compression pressures pressures and and % % compression compression values values are given inin bracketsbrackets forfor eacheach cell.cell. InitiallyInitially thethe flowflow raterate waswas 100100 mL mL min min−−1.. Arrows Arrows indicate indicate when when the flow flow was turned offoff andand thenthen reactivated.reactivated.

During the cell performance experiments described above, each cell went through a large During the cell performance experiments described above, each cell went through a large number number of relatively small charge-discharge cycles around 50% state of charge (SOC), i.e., between of relatively small charge-discharge cycles around 50% state of charge (SOC), i.e., between 45% and 45% and 55% SOC. Upon completion of testing, there was a small difference in the anolyte (V2+/3+) and 55% SOC. Upon completion of testing, there was a small difference in the anolyte (V2+/3+) and catholyte (V4+/5+) volumes (which were equal at the start of testing), although the total volume was catholyte (V4+/5+) volumes (which were equal at the start of testing), although the total volume was approximately equal to the initial value. As commonly observed with VRFBs, there was net transfer approximately equal to the initial value. As commonly observed with VRFBs, there was net transfer of water from the anolyte to the catholyte, caused by the cross-over of V3+ and V2+ cations [35]. of water from the anolyte to the catholyte, caused by the cross-over of V3+ and V2+ cations [35]. Interestingly, it appeared water transfer was highest for the cells with Nafion 212 membranes, Interestingly, it appeared water transfer was highest for the cells with Naﬁon 212 membranes, suggesting vanadium cross-over rates increased when using thinner membranes. However, using suggesting vanadium cross-over rates increased when using thinner membranes. However, using this this rudimentary method, it was not possible to determine if cell compression pressure affected the rudimentary method, it was not possible to determine if cell compression pressure affected the rate of rate of vanadium cross-over (i.e., membrane permeability). vanadium cross-over (i.e., membrane permeability).

2.3. VRFB Cycling The effecteffect of of compression compression pressure pressure on charge-dischargeon charge-discharge cycling cycling was investigated was investigated using Cells using 9 andCells 10 9, withand 10, 2.2 with and 0.22.2 barand compression0.2 bar compression pressure pressure on the active on the area, active respectively area, respectively (see Table (see1 for Table cell 1 details). for cell Duedetails). to the Due membrane to the membrane puncture issuespuncture associated issues withassociated Nafion with 212 membranes,Nafion 212 membranes, these cells used these Nafion cells 117used membranes. Nafion 117 Inmembranes. addition, the In highest addition, cell the compression highest cell pressure compression was avoided pressure due was to theavoided requirement due to forthe arequirement large pumping for a pressure large pumping over ~72 pressure hours’ operation. over ~72 hours’ Figure operation.7 illustrates Figure typical 7 charge-dischargeillustrates typical −2 curvescharge-discharge for the two curves cells at for 90 andthe two 174 mAcells cm at −902 (2.7and and 174 5.4mA A), cm where (2.7 theand flow 5.4 A), rate where was 100 the mL flow min rate−1. −1 Althoughwas 100 mL the min performance. Although is noticeably the performance worse than is noticeably the results worse in Figure than3 thedue results to the largein Figure difference 3 due into operatingthe large temperaturedifference in (see operating Section 3.3temperature), the beneficial (see effectSection of 3.3), increased the beneficial compression effect pressure of increased is clear. Thecompression voltage losses pressure are smalleris clear. forThe the voltage more losses compressed are smaller cell, whichfor the canmore also compressed charge and cell, discharge which can for longeralso charge within and the voltagedischarge limits, for utilizinglonger within more of the the voltage vanadium limits, electrolyte. utilizing more of the vanadium electrolyte. C 2018, 4, 8 11 of 17 C 2018, 4, x FOR PEER REVIEW 11 of 18

Figure 7. Charge-discharge curves obtained in a VRFB using Cells 9 and 10 at 90 and 174 mA cm−2 Figure 7. Charge-discharge curves obtained in a VRFB using Cells 9 and 10 at 90 and 174 mA cm−2 −1 (2.7(2.7 andand 5.45.4 A),A), withwith aa ﬂowflow raterate ofof 100 100 mL mL min min−1..

The charge-discharge cycles were analyzed to determine current efficiency (CE), voltage The charge-discharge cycles were analyzed to determine current efficiency (CE), voltage efficiency efficiency (VE) and energy efficiency (EE) given by Equations (5)–(7) [17], where Qd is the charge (VE) and energy efficiency (EE) given by Equations (5)–(7) [17], where Qd is the charge passed during passed during discharge, Qc is the charge passed during charge, Pd is the discharge power and Pc is discharge, Q is the charge passed during charge, P is the discharge power and P is the charging the chargingc power. Table 3 lists the results for Cells 9d and 10 at each current density,c i. The vanadium power. Table3 lists the results for Cells 9 and 10 at each current density, i. The vanadium electrolyte electrolyte utilization (U) is also given, defined as the percentage of the electrolyte’s total capacity U utilizationused in discharge. ( ) is also Electrode given, compression defined as theappears percentage to have of a negligible the electrolyte’s effect on total current capacity efficiency, used inin discharge.agreement Electrode with the results compression of Brown appears et al. to [17]. have Fo ar negligiblea given load, effect the on cell current with the efficiency, higher incompression agreement withdelivers the resultsthe best of voltage Brown and et al. energy [17]. For efficiency a given with load, more the cell of the with battery’s the higher capacity compression accessed. delivers However, the bestthe results voltage are and considerably energy efficiency less impressive with more than of the the battery’s recently capacity reported accessed. record-breaking However, performance the results areobtained considerably by Zhou less et impressiveal., who used than chemically the recently activated reported GDL record-breaking 10AA electrodes performance to achieve obtained EE values by Zhou et al., who used chemically activated GDL 10AA electrodes to achieve EE values of 82–88% at of 82–88% at current densities of 200–400 mA cm−2 [25]. current densities of 200–400 mA cm−2 [25]. Q =×d CE Qd 100% (5) CE = Q×c 100% (5) Qc R PdPdtdt EE = R  d × 100% (6) EE =×Pc dt 100% (6) Pdt EEc VE = × 100% (7) CE EE VE =×100% (7) Table 3. Analysis of charge-dischargeCE cycling for Cells 9 and 10.

Cell i/mA cm−2 CE/% VE/% EE/% U/% Table 3. Analysis of charge-discharge cycling for Cells 9 and 10. 90 96.1 72.4 69.5 83.7 Cell 9, 2.2 bar Cell 174i/mA cm− 97.62 CE/% VE 56.1/% EE/% 54.8U/% 68.2 55% compressed 43590 96.796.1 28.172.4 69.5 27.1 83.7 43.1 Cell 9, 2.2 bar 90174 97.397.6 67.356.1 54.8 65.5 68.2 70.8 Cell 10, 0.255% bar compressed 174 96.6 50.8 49.0 49.0 11% compressed 435 96.7 28.1 27.1 43.1 43590 -97.3 67.3 - 65.5 -70.8 - Cell 10, 0.2 bar 174 96.6 50.8 49.0 49.0 11% compressed 435 - - - -

C 2018, 4, 8 12 of 17

C 2018, 4, x FOR PEER REVIEW 12 of 18 3. Materials and Methods 3. Materials and Methods 3.1. Cell Components 3.1. Cell Components An expanded cell component diagram is given in Figure8a with an assembled cell shown in An expanded cell component diagram is given in Figure 8a with an assembled cell shown in Figure8b. A 5 cm wide by 6.2 cm long piece section of untreated GFD 4.6EA (SGL Group, Wiesbaden, Figure 8b. A 5 cm wide by 6.2 cm long piece section of untreated GFD 4.6EA (SGL Group, Wiesbaden, Germany)Germany) was was used used as theas the electrode electrode material material for for both both the the anode anode and and cathode. cathode. The The membrane membrane used used was eitherwas Nafion either N117Nafion or N117 NR212 or (IonPower,NR212 (IonPower, EU online EU store). online Before store). assembling Before assembling the cell, thethe membranecell, the wasmembrane soaked in was ultrapure soaked waterin ultrapure over night,water thenover night, transferred then transferred to 1 M H2 SOto 14 Mfor H 12SO h.4 Thefor 1 electrode h. The andelectrode membrane and weremembrane sandwiched were sandwiched between two between graphite two blocksgraphite (JP945, blocks Mersen, (JP945, Mersen, Portslade, Portslade, UK) and multipleUK) and polyolefin multiple gaskets polyolefin (Freudenberg, gaskets (Freudenberg Weinheim,, Germany).Weinheim, OneGermany). inlet and One one inlet outlet and channel one outlet were machinedchannel intowere the machined graphite into blocks theand graphite the electrode blocks and well the depth electrode was definedwell depth by the was number defined of by gaskets the used.number This resultedof gaskets in aused. “flow-through” This resulted arrangement in a “flow-through” (with no arrangement flow fields). The(with polytetrafluoroethylene no flow fields). The sheetspolytetrafluoroethylene (0.3 mm) provided ansheets insulating (0.3 mm) layer provided between an insulating the current layer collectors between (adjacent the current to the collectors graphite blocks)(adjacent and stainlessto the graphite steel endplates. blocks) and The stainless cell was stee compressedl endplates. togetherThe cell was using compressed eight M6 together bolts positioned using evenlyeight around M6 bolts the positioned square end evenly plates. around The cellthe squa activere areaend plates. was 31 The cm 2cell. active area was 31 cm2.

FigureFigure 8. 8. (a(a)) ComponentsComponents in in a a single single cell cell used used for for VRFB VRFB tests: tests: 1 stainless 1 stainless steel steel endplate; endplate; 2 2 polytetrafluoroethylenepolytetrafluoroethylene sheet; sheet; 3 3 gold-plated gold-plated coppe copperr current current collector; collector; 4 4JP945 JP945 graphite graphite block block containingcontaining flow flow inlet inlet and and outlet outlet (not (not shown); shown); 5 5 polyolefinpolyolefin gaskets; 6 GFD GFD 4.6EA 4.6EA felt felt electrode; electrode; 7 Nafion 7 Nafion membrane;membrane. (b) ( Photographb) Photograph of anof assembledan assembled cell; cell. (c) Components (c) Components in a cellin a used cell forused flow for tests: flow 1 tests: stainless 1 steelstainless endplate; steel 2 JP945endplate; graphite 2 JP945 block graphite containing block flow co inletntaining and outletflow inlet (not shown);and outlet 3 polyolefin (not shown); gaskets; 3 4 GFDpolyolefin 4.6EA gaskets; felt electrode; 4 GFD 54.6EA JP945 fe graphitelt electrode; block 5 JP945 with graphite no flow inletblock or with outlet. no flow inlet or outlet. C 2018, 4, 8 13 of 17

After assembling a cell, the width of the components between the end plates was measured using Vernier calipers. Once testing had ﬁnished, the cell was disassembled, then reassembled without the gaskets, membrane, and electrodes, and the width of the components between the end plates was measured again. Using these two measurements, it was possible to estimate the thickness of the electrodes in the cell, and hence, deduce the electrode compression.

3.2. Electrolyte Solution The electrolyte solution was provided by C-Tech Innovation (Chester, UK) and consisted of 1.6 M vanadium in 4 M H2SO4, where the average oxidation state of vanadium was +3.5 (i.e., 0.8 M vanadium(IV) and 0.8 M vanadium(III)). For performance tests, 400 mL of electrolyte was used for both the anolyte and catholyte. All discharge polarization curves were obtained using an initial open circuit voltage of 1.38 V, corresponding to 50% state of charge. Although spent electrolyte returned to the feeder tanks, the effect on the state of charge was minimal, with the decrease in open circuit voltage no more than 8 mV per test. After each discharge test, the system was charged back to 50% SOC. For charge/discharge cycles, the volume of anolyte and catholyte was 230 mL.

3.3. Cell Testing The flow battery test rig was purchased from C-Tech Innovation (UK) and is shown in AppendixB. Peristaltic pumps (Cole Parmer, St. Neots, UK) were used to pump the electrolytes at flow rates in the range 50–200 mL min−1, corresponding to 1.6–6.5 mL min−1 per cm2 of electrode area. Nalgene (Cole Parmer) and PFA (Swagelok, Manchester, UK) tubing was used to transport the electrolyte between the feeder tanks and the cell. The feeder tanks were continuously stirred (via a magnetic stirrer) and kept under nitrogen at all times (to prevent the oxidation of vanadium species). The temperature of the electrolyte in the feeder tanks was maintained at 45 ± 1 ◦C, which resulted in a cell temperature of 37 ± 1 ◦C. All electrochemical measurements were made using an HCP 803 potentiostat (Bio-Logic, Seyssinet-Pariset, France), with current cables connected to the current collectors and voltage sense cables connected to the graphite blocks of the cell. Polarization curves were obtained in both transient and steady-state modes, with little difference observed between the two. For transient discharge, the cell current was increased at a rate of 200 mA s−1 until the cell voltage decreased to 0.3 V. For steady-state measurements, each discharge current step lasted 1 min and was proceeded by a charge current step of the same coulombic value (to maintain a 50% SOC). Electrochemical impedance was performed at a discharge current of 12.5 A (corresponding to 400 mA cm−2) with a frequency range of 50 kHz to 0.1 Hz. For selected cells, charge-discharge cycles were performed three times at 2.7, 5.4, and 13.5 A (corresponding to current densities of 90, 174 and 435 mA cm−2). To avoid evaporative losses over the 2–3-day test, these experiments were conducted at room temperature (18 ± 1 ◦C). In-situ short circuit tests were performed by monitoring the change in the cell open circuit voltage when stopping and starting electrolyte flow.

3.4. Ex-Situ Testing Electrode compression/resistance tests were performed using a bespoke compression rig shown in the AppendixB. A 62 cm × 50 cm piece of electrode material was sandwiched between two gold-coated steel plates and inserted between a 100 mm diameter piston and a ﬁxed top plate. The air pressure inside the piston was gradually increased and a depth gauge recorded the change in thickness. At the same time, 2.7 A was passed through the electrode material and the voltage drop across the gold-coated steel plates was recorded. This produced a data set of applied pressure vs. thickness vs. electrical resistance. The tests were repeated with several samples of GDL 10AA (SGL Group, Wiesbaden, Germany), GFD 4.6EA and cut-down GFD 4.6EA. The latter samples were produced by slicing sections of GFD4.6EA using a skin graft blade, approximately halving the thickness of the felt. All samples were weighed on a 4-point mass balance prior to compression testing. C 2018, 4, 8 14 of 17

A series of flow tests were performed with half-cells to determine the effect of cell compression on pumping pressure. For these tests the cell assembly in Figure8c was used. A 62 cm × 50 cm piece of GFD 4.6EA was sandwiched between two graphite blocks (JP945, Mersen, Portslade, UK), one with flow inlet and outlet channels, the other flat (resulting in the same flow-through arrangement in the full cells above). The number of polyolefin gaskets (Freudenberg, Weinheim, Germany) determined the electrode well depth and felt compression. Stainless steel end plates and eight M6 bolts were used to clamp the assembly together and a depth gauge recorded the thickness of the cell (from which the felt thickness was determined). The half-cell was then placed in a bespoke flow rig. A gear pump (Micropump, Vancouver, WA, USA) supplied deionized water (20 ± 1 ◦C) at different flow rates through the half cell. Pressure sensors at the cell inlet and outlet allowed the measurement of the pressure drop across the electrode for a given flow rate.

4. Conclusions This work has demonstrated that VRFBs made with felt electrodes can perform at similar levels to the paper electrode VRFBs that have recently dominated research and development in high-performance cells. The key issue is the cell ohmic resistance. At high cell compression pressures (7.5 bar) using a relatively thin membrane (Nafion 212), an area specific HFR of 0.374 Ω cm2 was recorded, which is the lowest value reported to date for felt-VRFBs and is close to the HFRs obtained in high-performance paper-VRFBs. Consequently, a new record peak power density of 669 mW cm−2 was achieved, which is almost double that of the previous best felt-VRFB reported in the literature. Although the performance of felt-VRFBs can be further improved via flow field optimization and modifications to the electrode surface chemistry, a key issue is durability. Felt electrodes appear to be more prone to membrane punctures (and cell short circuits) than paper electrodes. Consequently, a membrane protection strategy is required if further work on high-performance felt-VRFBs is to continue.

Acknowledgments: This study was partly funded by the HEFCE (Higher Education Funding Council for England) Innovation Fund. The authors thank John Collins, Andy Leck, David Hall and John McCarthy (all C-Tech Innovation, UK) for supplying vanadium electrolyte and for their technical support. The authors also thank David Ward (University of Chester), Natasha Gunn (University of Chester), Matthew Herbert (University of Chester) and Joshua Denne (Advanced Propulsion Centre, UK) for their help and advice. Author Contributions: Trevor J. Davies conceived and designed the experiments; Trevor J. Davies and Joseph J. Tummino performed the experiments; Trevor J. Davies and Joseph J. Tummino analyzed the data; Trevor J. Davies wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A Figure A1 illustrates the transient performance of Cells 1–4 (Nafion 117 membranes) at flow rates of 50, 100 and 200 mL min−1 (corresponding to 1.6, 3.2 and 6.5 mL min−1 per cm2 of active area). Note, it was not possible to use Cell 1 at 200 mL min−1 as the required pumping pressure exceeded the maximum safe working value of the rig. Increasing flow rate improves performance for all cells, as expected, with the peak power density achieved with Cell 2 increasing from 375 to 460 mW cm−2 over the range of flow rates.

Appendix B A photograph of the VRFB test rig is shown Figure A2a. The anolyte and catholyte tanks are on the bottom left and right of the picture, respectively. The cell, in the center of the picture, is connected to the tanks via Neoprene and PFA tubing. Two peristaltic pumps provide the electrolyte ﬂow. Temperature and pressure sensors monitor the electrolyte before the inlet and after the outlet of each electrode. The bespoke rig used to measure electrode resistance and thickness under different compression pressures is shown in Figure A2b. For these measurements, the electrode material was C 2018, 4, 8 15 of 17

C 2018, 4, x FOR PEER REVIEW 15 of 18 sandwiched between two gold-coated steel plates, as shown in Figure A2c, which was then placed between a piston piston and and an an upper upper plate. plate. A A DC DC powe powerr supply supply passed passed 2.7 2.7 A A through through the the assembly assembly and and a multi-metera multi-meter recorded recorded the the voltage voltage drop drop across across the the metal metal plates. plates.

Figure A1.A1. TransientTransient cell cell performance. performance. Cell Cell operating operat voltageing voltage vs. current vs. current density density for Cells for 1–4 Cells (labeled) 1–4 −1 −1 −1 (labeled)at ﬂow rates at flow of (a )rates 50 mL of min (a) −501,( mLc) 100min mL, ( minc) 100−1 andmL min (e) 200 and mL ( mine) 200−1. mL Cell min power. Cell density power vs. density current −1 −1 vs.density current for Cellsdensity 1–4 for (labeled) Cells 1–4 at ﬂow(labeled) rates at of flow (b) 50 rates mL of min (b−) 150,( dmL) 100 min mL, min(d) −1001 and mL( minf) 200 and mL min(f) 200−1. mLFelt min compression−1. Felt compression pressures andpressures % compression and % compression values for eachvalues cell for are each given cell in are brackets. given in brackets. C 2018, 4, 8 16 of 17 C 2018, 4, x FOR PEER REVIEW 16 of 18

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