Vanadium Oxygen Fuel Cell Utilising High Concentration Electrolyte

batteries Article

Vanadium Oxygen Fuel Cell Utilising High Concentration Electrolyte Mandar Risbud 1,4 , Chris Menictas 1,4, *, Maria Skyllas-Kazacos 2,4 and Jens Noack 3,4 1 2 3 4

*

School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney 2052, Australia; m.risbud@unsw.edu.au School of Chemical Engineering, University of New South Wales, Sydney 2052, Australia; m.kazacos@unsw.edu.au Fraunhofer-Institute for Chemical Technology, Joseph-von-Fraunhofer-Str. 7, 76327 Pfinztal, Germany; jens.noack@ict.fraunhofer.de CENELEST, German-Australian Alliance for Electrochemical Technologies for Storage of Renewable Energy, School of Mechanical and Manufacturing Engineering, UNSW, Sydney 2052, NSW, Australia Correspondence: c.menictas@unsw.edu.au; Tel.: +61-2-9385 6269

Received: 20 December 2018; Accepted: 12 February 2019; Published: 19 February 2019

Abstract: A vanadium oxygen fuel cell is a modified form of a conventional vanadium redox flow battery (VRFB) where the positive electrolyte (VO2+ /VO2 + couple) is replaced by the oxygen reduction (ORR) process. This potentially allows for a significant improvement in energy density and has the added benefit of overcoming the solubility limits of V (V) at elevated temperatures, while also allowing the vanadium negative electrolyte concentration to increase above 3 M. In this paper, a vanadium oxygen fuel cell with vanadium electrolytes with a concentration of up to 3.6 M is reported with preliminary results presented for different electrodes over a range of current densities.

Using precipitation inhibitors, the concentration of vanadium can be increased considerably above the commonly used 2 M limit, leading to improved energy density.

Keywords: vanadium-oxygen fuel cell; high concentration vanadium electrolyte; high energy density; oxygen reduction

1. Introduction In recent years, redox flow batteries (RFBs) have emerged as an attractive option for grid connected, as well as off-grid, energy storage systems due to their low cost, safety and reliability. Redox flow batteries (RFB) represent energy storage devices consisting of an electrochemical reaction cell attached to two electrolyte reservoirs with redox couples having a significant electrochemical potential difference. One of the crucial advantages of redox flow batteries over other electrochemical energy storage devices is the independent scaling of power rating and energy storage capacity. Electrolyte volume, the concentration of active species and the cell voltage govern the energy density, while the power rating is determined by the kinetic behaviour of the redox species, the size of the electrodes and the number of cells in the stack. Redox flow batteries also exhibit good safety, long service life, fast response, high energy efficiency, flexible design and a relatively lower environmental impact. Of the various redox flow battery systems studied, the all-vanadium redox flow battery (VRFB) has reached the most significant level of commercialisation. Initially proposed by Skyllas-Kazacos and co-workers at UNSW [1,2] in the 1980s to avert cross-contamination issues experienced by Fe-Cr batteries developed

by NASA in the 1970s [3], VRFBs have come a long way in terms of development. In recent years, VRFBs have gained increased interest as potential stationary storage devices to compensate for the fluctuating energy production from renewable resources. The VRFB uses vanadium containing aqueous

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electrolytes on both sides (positive and negative half cells), thus reducing the cross-contamination issues associated with other redoxissues flowassociated batteries that redox couples of different elements. reducing the cross-contamination withemploy other redox flow batteries that employ redox The schematic diagram for the VRFB is shown in Figure 1. couples of different elements. The schematic diagram for the VRFB is shown in Figure 1.

Figure Figure 1.

1. Schematic Schematic diagram diagram of of vanadium vanadium redox redox flow flow battery battery (VRFB).

(VRFB).

VRFBs offer the following advantages.

VRFBs offer the following advantages.

•• Use Use the sameelectrolyte electrolyteininboth bothhalf-cells half-cellsavoids avoidsthe theissue issue of of cross-contamination. cross-contamination. This of of the same This means means that electrolytes have indefinite life. that thethe electrolytes have anan indefinite life.

•• Electrolyte Electrolyte flow throughthe thestack stackallows allowsefficient efficientheat heatexchange, exchange,thereby thereby removing removing the need for flow through for aa separate thermal management system. separate thermal management system. can easily bebe adjusted byby varying thethe solution volume.

•• Capacity Capacity can easily adjusted varying solution volume. recharging rates compared toto conventional batteries.

•• Faster Faster recharging rates compared conventional batteries.

• Possibility to monitor the battery characteristics (SOC the open open • Possibility to monitor the battery characteristics (SOCand andbattery batterycapacity) capacity) by by observing observing the circuit cell potential and monitoring the half-cell potentials. circuit cell potential and monitoring the half-cell potentials.

Despite these advantages, the VRFBs have a comparatively low specific energy. The maximum Despite these advantages, the VRFBs have a comparatively low specific energy. The maximum energy density obtained by VRFBs is 30 Wh L−1, which is due to the solubility limits of the vanadium energy density obtained by VRFBs is 30 Wh L−1 , which is due to the solubility limits of the vanadium species involved over the practical operating temperature ranges. Although this is not a major issue species involved over the practical operating temperature ranges. Although this is not a major issue in in most stationary applications, in installations where there is limited available space such as in office most stationary applications, in installations where there is limited available space such as in office or or residential buildings, the low energy density can be a major drawback. Enhanced solubility can be residential buildings, the low energy density can be a major drawback. Enhanced solubility can be obtained with the use of mixed sulphate-chloride electrolytes consisting of HCl-H2SO4 systems [4]; obtained with the use of mixed sulphate-chloride electrolytes consisting of HCl-H2 SO4 systems [4]; however, overcharging these solutions can potentially result in the release of chlorine gas, which is however, overcharging these solutions can potentially result in the release of chlorine gas, which is an an undesirable health and safety hazard, particularly in enclosed spaces. undesirable health and safety hazard, particularly in enclosed spaces.

A significant improvement in energy density can also be achieved by replacing the positive A significant improvement in energy density can also be achieved by replacing the positive reaction of a conventional VRFB with an oxygen reduction reaction (ORR) that operates similarly to reaction of a conventional VRFB with an oxygen reduction reaction (ORR) that operates similarly to fuel cells. Removing the positive half-cell electrolyte also has the added benefit of overcoming the fuel cells. Removing the positive half-cell electrolyte also has the added benefit of overcoming the solubility limits of V(V) at elevated temperatures and allowing the vanadium negative electrolyte solubility limits of V(V) at elevated temperatures and allowing the vanadium negative electrolyte concentration to be potentially increased up to 4 M [5]. The vanadium-oxygen system also gives a concentration to be potentially increased up to 4 M [5]. The vanadium-oxygen system also gives a higher standard cell potential (1.49 V compared to 1.26 V for VRFB). The Vanadium-Oxygen fuel cell higher standard cell potential (1.49 V compared to 1.26 V for VRFB). The Vanadium-Oxygen fuel cell (VOFC) concept was proposed initially by Kaneko et al. in 1992 [6] and first investigated by Menictas (VOFC) concept was proposed initially by Kaneko et al. in 1992 [6] and first investigated by Menictas and Skyllas-Kazacos in 1997 [7]. More work has been done on this technology at the Fraunhofer ICT and Skyllas-Kazacos in 1997 [7]. More work has been done on this technology at the Fraunhofer ICT [8] [8] and Twente University in the Netherlands [9] among others, to evaluate VOFC performance over and Twente University in the Netherlands [9] among others, to evaluate VOFC performance over a a range of temperatures, using different membranes to establish its feasibility. The schematic diagram range of temperatures, using different membranes to establish its feasibility. The schematic diagram representing the VOFC is shown in Figure 2. representing the VOFC is shown in Figure 2.

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Figure Figure 2.

2. Schematic Schematic diagram diagram of of aa vanadium vanadium oxygen oxygen fuel fuel cell cell (VOFC) (VOFC) system. system.

One of the significant advantages of the VOFC over the VRFB is avoiding thermal precipitation One of the significant advantages of the VOFC over the VRFB is avoiding thermal precipitation limitations of V (V), allowing operation at an increased concentration and elevated temperatures. limitations of V (V), allowing operation at an increased concentration and elevated temperatures.

Conventional VRFB electrolytes are usually restricted to vanadium concentrations between 1.5 and Conventional VRFB electrolytes are usually restricted to vanadium concentrations between 1.5 and 2 2 M, limiting their energy density and operating temperature range (to 10 to 40o ◦ C).

M, limiting their energy density and operating temperature range (to 10 to 40 C).

VOFCs thus offer the following advantages over conventional VRFBs: VOFCs thus offer the following advantages over conventional VRFBs:

•• Higher Higher standardcell cellvoltage voltageand andhigher highertheoretical theoretical energy energy density density (~60 (~60 Wh/kg) Wh/kg) due due to to the standard the significantly reduced weight and volume. significantly reduced weight and volume.

•• Halving Halving required vanadium and associated electrolyte cost. of of thethe required vanadium and associated electrolyte cost.

•• Nearly Nearly unlimited source of oxygen (ambient air) for the discharging reaction unlimited source of oxygen (ambient air) for the discharging reaction electrode reactions reactions involved involved in in the the VOFC VOFC can can be be represented representedas asfollows. follows.

The electrode electrode reaction: reaction: Negative electrode V3+ + e− ↔ V2+ V3+ + e− ↔ V2+

E0 = −0.26 V E0 = −0.26 V

Positive electrode reaction: Positive electrode reaction: O2 + 4H+ + 4e− ↔ 2H2O Overall cell reaction: Overall cell reaction:

+

−

O2 + 4H + 4e ↔ 2H2 O

4V2+ + O2 + 4H+ ↔ 4V3+ + 2H2O

E0 = +1.23 V 0

E = +1.23 V

E0 = 1.49 V

(1) (1) (2) (2)

(3)

It should be noted that2+the standard cell potential is the thermodynamically expected value 4V + O2 + 4H+ ↔ 4V3+ + 2H2 O E0 = 1.49 V (3) under standard cell conditions, and does not take into account ohmic losses and overpotentials that are encountered in practical systems.

While the negative componentsexpected of the VOFC the It should be noted that the standard cell potential is thehalf-cell thermodynamically valueare under same as those of the VRFB,and the does cathode (i.e. electrode foroverpotentials the ORR) are different. standard cell conditions, notcomponents take into account ohmic catalysts losses and that are The cathode requirements also depend on the expected mode of operation that can eitherare involve the encountered in practical systems. While the negative half-cell components of the VOFC the same use of a separate stack for recharging the spent negative half-cell solution, or may incorporate as those of the VRFB, the cathode components (i.e., electrode catalysts for the ORR) are different. Thea bifunctional electrodealso with a catalyst catalyzing both that oxygen reduction and cathode requirements depend on thecapable expectedofmode of operation can either involve theoxygen use of evolution reactions.

While the latter eliminates the need for a separate recharging system, the a separate stack for recharging the spent negative half-cell solution, or may incorporate a bifunctional development of an efficient bifunctional electrode presents a significant challenge. electrode with a catalyst capable of catalyzing both oxygen reduction and oxygen evolution reactions. al. [10] proposed modular vanadium oxygen redox battery design involving two WhileHosseiny the lattereteliminates the needafor a separate recharging system, the development of an efficient different MEAs to overcome the corrosion issues of the carbon support during the charging process. bifunctional electrode presents a significant challenge.

Pt/C Hosseiny (ORR) and /TiOproposed 2 (OER) bifunctional electrocatalysts on Ti mesh support were used for the etIrO al. 2[10] a modular vanadium oxygen redox battery design involving two oxygen evolution and oxygen reduction reactions; thesupport voltage during efficiency only different MEAs to overcome the corrosion issues ofhowever, the carbon theobtained chargingwas process.

46%. This was potentially due to cross-over of oxygen formed during the oxygen evolution process to the vanadium side. A modified unitised cell design where titanium cloth was used as a substrate

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Pt/C (ORR) and IrO2 /TiO2 (OER) bifunctional electrocatalysts on Ti mesh support were used for the oxygen evolution and oxygen reduction reactions; however, the voltage efficiency obtained was only 46%. This was potentially due to cross-over of oxygen formed during the oxygen evolution process to the vanadium side. A modified unitised cell design where titanium cloth was used as a substrate (instead of titanium felt) showed a significant improvement to the voltage efficiency, i.e., 81% with Pt/IrO2 modified catalysts [10]. However, the long-term operation has yet to be achieved.

The disadvantages or limitations of current VOFCs, therefore, are the absence of cost-effective and efficient bifunctional electrode materials, low durability of the membrane electrode assemblies used, and current cell designs with high ohmic resistance. The maximum energy efficiency obtained for the advanced rechargeable VOFC (EWE Research Centre, Germany 2015) was 41.7% at 20 mA cm−2 current density which is quite low for grid-scale energy storage application. Furthermore, to date, the maximum vanadium electrolyte concentration employed in VOFCs has been 2 M, limiting the possible energy density enhancement. There has been extensive research carried out for the development of electrodes for redox flow batteries, with work largely focused on the activation and modification of catalyst materials using physical or chemical methods like heat treatment, ion exchange, incorporation of nanomaterials, structural modifications etc. [11–15]. However, only a few of these methods provide increased power density or scalability. Thermal treatment in air at 400 ◦ C and chemical treatment with H2 SO4 and HNO3 developed by Skyllas-Kazacos group in the 1990s [16,17] remain the most effective, economical and easiest methods for activation of carbon-based materials. Surface modification of carbon-based electrodes with additives like WO3 [18], W-doped Nb2 O5 [19], TiO2 [20], CeO2 [21] and ZrO2 [22], Sb3+ additive [23], Bismuth nanoparticles [24], N- and WO3 - [25], graphene [26], TiN nanowires [27] have also been reported, resulting in improved battery performance.

In a recent publication by UNSW researchers a VRFB operating with a 3 M vanadium electrolyte was reported [28,29]. While the use of stabilising agents increases the induction time for the thermal precipitation of V (V) species at elevated temperatures, at 3 M vanadium concentrations, the recommended operating temperature range was quite limited. By eliminating the positive half-cell solution, however, this limitation is removed, so high-temperature operation becomes feasible. The aim of this study was thus to develop a high energy density VOFC that has the potential to be used in a wider range of applications including electric vehicles. In this paper, we report on the investigation of improved membrane electrode assemblies in new cell designs to reduce voltage losses, with the potential for significant energy density increases when using high concentration 3.6 M vanadium electrolyte.

In the mid-1990s, Skyllas-Kazacos and coworkers reported the use of precipitation inhibitors to stabilise highly supersaturated vanadium electrolytes. By using precipitation inhibitors like ammonium sulphate and phosphoric acid, a 3 M vanadium electrolyte could be prepared, and charge-discharge cycled in a VRFB [28,29]. A high concentration vanadium solution in sulphuric acid was thus prepared and tested in the VOFC. 2. Results & Discussion 2.1. VOFC Performance with Commercial MEAs Commercial membrane electrode assemblies comprising Pt/C catalysts with a loading of 4 mg cm−2 Pt and 2 M V(II) solution in 5 M H2 SO4 were used to study the performance of the vanadium oxygen fuel cell. Figure 3 presents the cell voltage vs time plot obtained at a current density of 5 mA cm−2 at room temperature after an initial OCV measurement for 60 s. The OCV was approx. 1.015 V, and thus, almost 500 mV lower than the differences of the standard potentials. One possible cause could have been the formation of a mixed potential in the three-phase boundary layer by diffused vanadium solution. After applying a current density of 5 mA cm−2 to the cell, the cell voltage decreased to approx. 0.885 V due to the influence of the kinetics of the reactions and the ohmic resistances, and was almost linear under load over the course of time.

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Figure 3. Effect of oxygen flow on vanadium oxygen fuel cell (VOFC) performance using a commercial Figure 3. Effect of oxygen flow on vanadium oxygen fuel cell (VOFC) performance using a Pt/C MEA (4 mg cm−2 Pt, 5 mA cm−2 , 2 M V(II), 5 M H2 SO4 , 25 ◦ C). commercial Pt/C MEA (4 mg cm−2 Pt, 5 mA cm−2, 2 M V(II), 5 M H2SO4, 25 °C).

To confirm the contribution of the ORR to the total cell voltage, the oxygen supply was turned off the behaviour with commercial MEAs voltage and with different duringTo thestudy measurement.

As canof be VOFCs seen from Figure 3, the discharge dropped after vanadium stopping concentrations, a series of discharge experiments were conducted.

Table 1 shows the comparison the oxygen flow due to ORR mass transport limitations but recovered again on the restoration ofof the VOFC oxygen feed.performance with 2 M V(II), 2.4 M V(II) and 3 M V(II) in 5–6 M sulphuric acid solutions at room temperature and Figure of 4 shows thewith polarisation curves.

To study the behaviour VOFCs commercial MEAs and with different vanadium concentrations, a series of discharge experiments were conducted. Table 1 shows the comparison of Table 1 Mean cell voltage (UCell) and corresponding area specific resistance (ASR) for VOFCs using 2 the VOFC performance with 2 M V(II), 2.4 M V(II) and 3 M V(II) in 5–6 M sulphuric acid solutions at M, 2.4 M and 3 M V(II) solution and commercial MEAs (4 mg cm−2 Pt, 25 °C), based on standard room temperature and Figure 4 shows the polarisation curves. potential difference.

Table 1. Mean cell voltage (UCellCell ) and corresponding (ASR) for using 0 area specific 10 mA resistance 20 mA 30 VOFCs mA 40 mA Vanadium −2 cm−2 Pt, −2 standard Characteristics (OCV) Concentration cm cmon cm−2 2 M, 2.4 M and 3 M V(II) solution and commercial MEAs (4cm mg 25−2 ◦ C), based 1.05 0.77 0.61 0.45 0.30 UCell/V potential difference.

2 M V(II) ASR/Ohm cm−2 77.0 30.5 15.0 7.5 Vanadium 2 −2 −2 0.51 UCell/V 0 (OCV) 1.09 Cell Characteristics 10 mA cm−20.8120 mA cm−0.67 30 mA cm0.60 40 mA cm Concentration 2.4 M V(II) 81.0 33.5 20.0 12.8 ASR/Ohm cm−2 UCell /V 0.45 0.30 1.07 0.77 0.78 0.61 0.68 0.61 0.55 UCell/V 1.05 2 M V(II) 77.0 30.5 15.0 7.5 3 M V(II) ASR/Ohm cm−2 −2 78.0 34.0 20.3 ASR/Ohm cm U /V 1.09 0.81 0.67 0.60 0.5113.8 2.4 M V(II) 3 M V(II)

Cell

ASR/Ohm cm−2 UCell /V ASR/Ohm cm−2

1.07 -

81.0 0.78 78.0

33.5 0.68 34.0

20.0 0.61 20.3

12.8 0.55 13.8

With 1.05 to 1.09 V, all cells achieved similar values for the OCV. In contrast to the previous experiment, the values were a few millivolts higher, but still approx. 400 mV below the difference of the standard potential, which indicates a non-optimal formation of the three-phase boundary layer.

Due to the influence of the activation energies, the cell voltage decreased disproportionately at current densities of 10 mA cm−2 and was linear, with higher current densities. The cell with a vanadium concentration of 2 M achieved the lowest performance values; the cells with 2.4 and 3.6 molar solutions showed similar and higher values.

With the 2 molar solution, a maximum power density of 13.5 mW cm−2 was achieved with a current density of 30 mA cm−2 and an efficiency of 30%. At the higher concentrations, higher power values were achieved due to the lower cell resistance at higher current densities. The cell with the Figure 4. Polarisation curves obtained for vanadium oxygen fuel cells (VOFC) using 2 M, 2.4 M and 3 M V(II) solutions and commercial MEAs (4 mg cm−2 Pt, 5 M H2SO4, 25 °C).

M, 2.4 M and 3 M V(II) solution and commercial MEAs (4 mg cm Pt, 25 °C), based on standard potential difference.

Cell 0 10 mA 20 mA 30 mA 40 mA Characteristics (OCV) cm−2 cm−2 cm−2 cm−2 1.05 0.77 0.61 0.45 0.30 UCell/V Batteries 2019, 24 6 of 19 2 M5,V(II) ASR/Ohm cm−2 77.0 30.5 15.0 7.5 1.09 0.81 0.67 0.60 0.51 UCell/V 2.4 M V(II) −2 − 2 − 2 81.0 33.5 20.0 ASR/Ohm cm 3.6 molar solution achieved the highest power density with 22 mW cm at 40 mA cm and 12.8 energy Cell/V 1.07 still have 0.78 0.68 0.61 0.55 U efficiency3 of 37%.

It is important to note that the cells scope for optimisation to produce higher M V(II) 78.0 34.0 20.3 13.8 ASR/Ohm cm−2 Vanadium Concentration

performance; however, the feasibility of using higher concentration electrolytes is clearly evident.

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With 1.05 to 1.09 V, all cells achieved similar values for the OCV. In contrast to the previous experiment, the values were a few millivolts higher, but still approx. 400 mV below the difference of the standard potential, which indicates a non-optimal formation of the three-phase boundary layer.

Due to the influence of the activation energies, the cell voltage decreased disproportionately at current densities of 10 mA cm−2 and was linear, with higher current densities. The cell with a vanadium concentration of 2 M achieved the lowest performance values; the cells with 2.4 and 3.6 molar solutions showed similar and higher values.

With the 2 molar solution, a maximum power density of 13.5 mW cm−2 was achieved with a current density of 30 mA cm−2 and an efficiency of 30%. At the higher concentrations, higher power values were achieved due to the lower cell resistance at higher current densities. The cell with the 3.6 molar solution achieved the highest power density with 22 mW cm−2 at 40 mA cm−2 and energy efficiency of 37%. It is important to note that the cells still have scope for optimisation to produce Figure 4.4.Polarisation curves for oxygen fuel 22M, and Figure Polarisation curvesobtained obtained forvanadium vanadium oxygen fuelcells cells(VOFC) (VOFC)using using M,2.4 2.4M Mis and higher performance; however, the feasibility of using higher concentration electrolytes clearly −−2 2 Pt, 5 M H SO , 25 ◦ C).

33M V(II) solutions and commercial MEAs (4(4mg cm Pt, 5 M H 22SO44, 25 °C).

M V(II) solutions and commercial MEAs mg cm evident.

Further experiments were conducted at an elevated temperature to enhance the kinetics of the Further experiments were conducted at an elevated temperature to enhance the kinetics of the electrode reactions and increase cell voltage. Figure 5 represents the performance of a VOFC electrode reactions and increase cell voltage. Figure 5 represents the performance of a VOFC operating operating with 2 M V(II) solution and commercial Pt/C MEA at various current densities at 60 °C. with 2 M V(II) solution and commercial Pt/C MEA at various current densities at 60 ◦ C.

◦ C with 2 M V(II) solution using Figure Figure 5.

5. Cell Cell voltage voltage of of aa vanadium vanadium oxygen oxygen fuel fuel cell cell (VOFC) (VOFC) at at 60 60 °C with 2 M V(II) solution using − 2 commercial Pt/C MEA at various current densities (4 mg cm−2 Pt, 5 M H2 SO4 ). commercial Pt/C MEA at various current densities (4 mg cm Pt, 5 M H2SO4).

As can be seen in Figure 5, the cell with a cell voltage of 1.04 V reached a value about 450 mV lower As can be seen in Figure 5, the cell with a cell voltage of 1.04 V reached a value about 450 mV than the differences of the standard potentials, which can again be attributed to non-optimal conditions lower than the differences of the standard potentials, which can again be attributed to non-optimal of the three-phase boundary layers at the cathode. As can be seen in Table 2, together with Table 1, conditions of the three-phase boundary layers at the cathode. As can be seen in Table 2, together with similar results were obtained for the performance values as for the VOFC at room temperature. The Table 1, similar results were obtained for the performance values as for the VOFC at room cell voltage was 0.64 V at 60 ◦ C and 20 mA cm−2 and 0.61 V at room−2 temperature. Reasons for the only temperature. The cell voltage was 0.64 V at 60 °C and 20 mA cm and 0.61 V at room temperature. slight increase could be an interaction of diffusion from the negative half-cell space and oxygen flow Reasons for the only slight increase could be an interaction of diffusion from the negative half-cell and pressure at an elevated temperature, which prevents an optimal reaction at the cathode. The power space and oxygen flow and pressure at an elevated temperature, which prevents an optimal reaction density was calculated by multiplying the average cell voltage by the corresponding current density. at the cathode. The power density was calculated by multiplying the average cell voltage by the corresponding current density.

Table 2. Mean cell voltage (UCell), power densities (p), area specific resistances (ASR) and energy efficiencies (EE) for a VOFC using 2 M V(II) solution and a commercial MEA (4 mg cm−2 Pt) at 60 °C, based on standard potential difference.

Vanadium Concentration 2 M V(II),

Cell Characteristics UCell/V p/mW cm−2

0 (OCV) 1.04 -

10 mA cm−2 0.8 8

15 mA cm−2 0.73 11

20 mA cm−2 0.64 12.8

25 mA cm−2 0.53 13.25

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Table 2. Mean cell voltage (UCell ), power densities (p), area specific resistances (ASR) and energy efficiencies (EE) for a VOFC using 2 M V(II) solution and a commercial MEA (4 mg cm−2 Pt) at 60 ◦ C, based on standard potential difference.

Vanadium Concentration

2 M V(II), 60

◦C

Cell Characteristics

0 (OCV)

10 mA cm−2

15 mA cm−2

20 mA cm−2

25 mA cm−2

UCell /V p/mW cm−2 ASR/Ohm cm−2 EE/%

1.04 70

0.8 8 80.0 54

0.73 11 48.7 49

0.64 12.8 32.0 43

0.53 13.25 21.2 36

In order to improve performance, the use of a catalyst/surface modification methods of carbon was considered for the negative half-cell reaction. Non-noble metal catalysts for the oxygen reduction reaction were also explored to investigate the potential to reduce the noble metals like Pt, Pt/Ir etc.

The in-house fabrication of MEAs that incorporated different oxygen reduction reaction catalysts with a suitable catalyst for the negative half-cell reaction was therefore undertaken. For this study, the electrocatalyst reported by Cao et al. [13] for the vanadium electrode reactions was selected. For the oxygen reduction electrode, a variety of electrocatalysts were screened for their potential application in VOFCs. 2.2. Membrane Electrode Assembly (MEA) Fabrication The preparation of MEAs involving non-noble electrocatalysts was initially explored, and several experiments were carried out to study the bonding process. The coating of catalyst directly on the membrane using painting brush or spray coating failed to produce a stable catalyst-coated membrane (CCM) with good adhesion.

It was observed that while a uniform coating of carbon paste by painting brush method can be obtained on carbon paper and cloth using NAFION® binder (Sigma Aldrich, Sydney, Australia) and hot press, the adhesion of these with the membrane was not successful using the hot press. Spray coat methods have been used in the preparation of catalyst coated membranes (CCM) in fuel cells where the catalyst powder is directly sprayed on to the membrane. Based on this method, the catalyst ink (carbon black powder + PTFE solution, 20 wt. %) was spray coated using the 150W Ozito mini air spray gun on the surface of wet proofed carbon paper. This was then hot pressed on to pre-treated VB1 or NAFION® membrane. Using this method, uniform adhesion of the catalyst layer was successfully achieved. Figure 6 summarises the tests results obtained during the investigation of the bonding process of MEA. Figure 6a shows the carbon black coating obtained using the paintbrush method on a VB1 membrane (properties like NAFION® 115) using 5% NAFION® solution as a binder. There was no uniform adhesion of carbon black to the membrane resulting into dislodgement of carbon black.

Tests were carried out by increasing the binder concentration to up to 15% to check the effect on the bonding process. However, a stable coating could not be obtained using this method. Shrinking and wrinkle formation of the membrane was also observed during the drying process in addition to the development of cracks in the coated layer, resulting in the removal of carbon black.

Due to problems in adhesion of the carbon black paste on to the membrane, it was decided to coat the carbon black powder directly onto Toray carbon paper 120 or wet proofed carbon cloth, which can be used as a gas diffusion layer. In this test, carbon black powder was mixed with 5% NAFION® solution in isopropanol as a binder to form a paste and spray coated on the Toray carbon paper 120 using an air spray gun. Figure 6b represents the uniform coating obtained by a spray coat method.

The coating was dried at 80 ◦ C. A uniform coating with good adhesion can be seen in Figure 6b.

Catalyst layers coated on the carbon paper (e.g., Pt/C, 0.4 mg cm−2 on carbon paper) were purchased from the Fuel cell store, USA and hot pressed against VB1 membranes at 3 MPa, 140 ◦ C. Membrane shrinking observed was very small with slight folding of the membrane observed. The obtained MEA can be seen in Figure 6c. A similar test was carried out to prepare an MEA using

membrane (CCM) with good adhesion.

It was observed that while a uniform coating of carbon paste by painting brush method can be obtained on carbon paper and cloth using NAFION® binder (Sigma Aldrich, Sydney, Australia) and hot press, the adhesion of these with the membrane was not successful using the hot press. Spray coat 2019, methods Batteries 5, 24 have been used in the preparation of catalyst coated membranes (CCM) in fuel 8 of cells 19 where the catalyst powder is directly sprayed on to the membrane. Based on this method, the catalyst ink (carbon black powder + PTFE solution, 20 wt. %) was spray coated using the 150W Ozito mini air ® 115 membrane. A catalyst layer (carbon black and NAFION® binder) was pre-treated spray gunNAFION on the surface of wet proofed carbon paper. This was then hot pressed on to pre-treated ® coated on the wet proofed carbonUsing paper this using a mini-compressor airbrush form a uniform VB1 or NAFION membrane. method, uniform adhesion of kit thetocatalyst layer was ® catalyst layer on the GDL.Figure This was then hot pressed onresults the NAFION membrane (catalyst layer successfully achieved.

6 summarises the tests obtained115 during the investigation of the ◦ C for 10 min. The obtained membrane electrode assembly onbonding the membrane side) at 3 MPa, 140 process of MEA. (MEA) is shown in Figure 6d.

Figure 6. Membrane electrode assembly fabrication tests (a): catalyst layer coated directly on the membrane using paintbrush method, no uniform adhesion with shrinking of the membrane (b) Figure 6. Membrane electrode assembly fabrication tests (a): catalyst layer coated directly on the uniform catalyst layer obtained by spray coating method (c) commercial catalyst layer on carbon paper membrane using paintbrush method, no uniform adhesion with shrinking of the membrane (b) bonded to the membrane using hot press method. (d) spray coated catalyst layer on the carbon paper uniform catalyst layer obtained by spray coating method (c) commercial catalyst layer on carbon prepared at UNSW and bonded on pre-treated NAFION® 115 membrane. paper bonded to the membrane using hot press method. (d) spray coated catalyst layer on the carbon paper prepared atwith UNSW and bonded on pre-treated NAFION® 115 membrane.

2.3. VOFC Performance Various Electrocatalysts

Preliminary tests with UNSW fabricated MEAs were carried out to measure the cell voltage obtained for different electrode combinations and values are reported in Table 3 below. For all these experiments 2 M V(II) solution in 5 M H2 SO4 was used as a negative electrolyte.

The OCV values of the measurements at 25 ◦ C were between 0.75 and 0.85 V. At temperatures of 80 ◦ C, the values for all tested combinations were in some cases significantly higher with values between 0.93 V and 1.4 V. The pure carbon-based electrodes achieved the lowest values, which could be expected due to the poor catalytic activity for the ORR. The cells with MoS2 and MoO3 as catalysts for the ORR hardly achieved higher OCV values than the carbon electrodes. The two cells with commercial Pt/C electrodes—one employed a commercial electrode, and the other utilised commercial platinum loaded carbon particles with the electrode manufactured at UNSW—achieved the highest OCV values at 80 ◦ C with 1.40 V and 1.35 V respectively, and were thus only 100–150 mV below the difference of the standard potentials. It was also observed that during discharge tests, except for commercial Pt/C electrodes used for ORR, the cell voltage for other combinations quickly dropped to the cut-off voltage of 0.1 V at current densities of 20 mA cm−2 , which was the minimum voltage measurable using the battery tester equipment. The commercial Pt/C electrode was therefore used in all further VOFC experiments.

The MEA incorporating MoO3 treated carbon paper for the negative and a Pt/C oxygen reduction electrode were initially tested at 50 ◦ C in a VOFC that employed a 2 M V(II) negative half-cell solution.

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treated Toray carbon TheHeat cell was discharged at 20 mA cm−2 and the cell voltage vs time curve is presented in Figure 7. The Iron phthalocyanine [31] 25 0.83 paper 120 (400 °C, 30 h) with a high energy density electrolyte as described below. cell was subsequently tested Heat treated Toray carbon MoO3 treated carbon paper 25 0.85 paper 3.

120 (400 °C, of 30 open-circuit h) Summary voltages (OCV) obtained for vanadium oxygen fuel cell tests (VOFC) Table ◦ C and 3 treated carbon paper MoO treated carbon 1.02 MoO using various electrode combinations at 325 80 ◦ Cpaper (2 M V(II), 5 M H2 SO80 4 ).

1.4 Pt/C Half-Cell MEA (Commercial) Pt/C MEA (commercial) ◦C Negative Half-Cell Material Positive Material Operating Temperature OCV (V)at 50 80 (0.60 4 mg cm−2, fuel cell store 4 mg cm−2, fuel cell store 2) Toray carbon paper 120 untreated Toray carbon paper 120 untreated 25 0.75 mA/cm Heat treated Toray carbon paper 120 Wet proofed CeTech carbon cloth 25 0.81.3 ◦ (400 C, 30 h) carbon Toray Pt/C (0.4 mg cm−2) coated on heatMoO3 treated Heat treated Toray carbon paper 120 80 (0.57 at 50 MoS2 coated carbon paperpaper [30] 25 0.75 paper treated carbon (400 ◦ C,120 30 h) mA/cm2) Heat treated Toray carbon paper 120 phthalocyanine [31] layer [32] 25 0.83 Toray carbon SprayIron coated microporous 80 1.15 (400 ◦ C, paper 30 h) 120 Heat treated Toray carboncarbon paper 120 Heat-treated Toray Sprayed microporous layer on MoO3 treated carbon paper 25 0.85 80 1.08 (400 ◦ C, 30 h) paper 120 (400 °C, 30 h) Toray carbon paper 120 [32] MoO3 treated carbon paper MoO3 treated carbon paper 80 1.02 Vulcan XC-72 carbon black 1.4 Pt/C MEA (commercial) Pt/C MEA (Commercial) 80 treated carbon paper MoO43mg 80 0.93 2 ) (0.60 at 50 mA/cm cm−2 , fuel cell store 4 mg cm−2untreated , fuel cell store −2

1.3 Pt/C (0.4 mg cm ) coated on 80 (0.57 at 50 mA/cm2 ) heat-treated carbon paper carbon paper 120 Spray coated microporous layer [32] 80 and a Pt/C oxygen 1.15 TheToray MEA incorporating MoO 3 treated carbon paper for the negative reduction Heat-treated Toray carbon paper 120 Sprayed microporous layer on Toray 80 1.08solution. ◦ electrode were initially tested at 50 °C in a VOFC that employed a 2 M V(II) negative half-cell carbon paper 120 [32] (400 C, 30 h) Vulcan XC-72 black −2 and The cell was discharged at 20 mA cm thecarbon cell voltage vs time curve MoO carbon paper 80 is presented in Figure 0.93 7. The 3 treated untreated MoO3 treated Toray carbon paper 120

cell was subsequently tested with a high energy density electrolyte as described below.

2 Figure 7. Cell voltage for a vanadium oxygen fuel cell (VOFC) with 70 mL 2 M V(II) at 20 mA cm−−2 Figure 7. Cell voltage for a vanadium oxygen fuel cell (VOFC) with 70 mL 2 M V(II) at 20 mA cm with a MoO3 treated carbon paper as negative half-cell electrode and a commercial Pt/C electrode with a MoO3 treated carbon paper as negative half-cell electrode and a commercial Pt/C electrode (0.4 (0.4 mg−2cm−2 Pt) on Vulcan carbon as positive half-cell electrode at 50 ◦ C. mg cm Pt) on Vulcan carbon as positive half-cell electrode at 50 °C.

For 5 min, the OCV was measured at the beginning, whereby approx. 1.08 V was achieved. The For 5 min, the OCV was measured at the beginning, whereby approx. 1.08 V was achieved. The value was therefore similar to the values with commercial MEAs and 10 times higher Pt loads, but value was therefore similar to the values with commercial MEAs and 10 times higher Pt loads, but significantly lower than with the Pt-based cells at 80 ◦ C. During the subsequent discharge process significantly lower than with the Pt-based cells at 80 °C. During the subsequent discharge process with a current density of 20 mA cm−2−2 , the cell voltage decreased to values between 0.62 and the with a current density of 20 mA cm , the cell voltage decreased to values between 0.62 and the cutcut-off voltage to 0.1 V over a period of approx. 80 min. The initial power density was 12.4 mW off voltage to 0.1 V over a period of approx. 80 minutes. The initial power density was 12.4 mW cm−2 cm−2 and was thus in about the same range as the measurements with commercial MEAs with and was thus in about the same range as the measurements with commercial MEAs with 4 mg cm−2 4 mg cm−2 Pt. The theoretical capacity of 70 mL 2 M V(II) is approx. 1.9 Ah. In this discharge Pt. The theoretical capacity of 70 mL 2 M V(II) is approx. 1.9 Ah. In this discharge test, 0.8 Ah were test, 0.8 Ah were converted, and thus, only a low charge carrier efficiency of 42 % was achieved. converted, and thus, only a low charge carrier efficiency of 42 % was achieved. The most probable causes were: diffusion of V(II) ions into the positive half-cell space, diffusion of O2 through the membrane and oxidation of V(II) by atmospheric oxygen. The experiments were conducted only at a

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Batteries 2018, 4, x FOR PEER REVIEW 10 ofof 19 The most probable causes were: diffusion of V(II) ions into the positive half-cell space, diffusion O2 through the membrane and oxidation of V(II) by atmospheric oxygen. The experiments were very low overpressure at O2 (water column), so no assessment can be given as to whether V(II) was conducted only at a very low overpressure at O2 (water column), so no assessment can be given as to lost by diffusion or by diffused oxygen. Both paths are probable and require further investigation. whether V(II) was lost by diffusion or by diffused oxygen. Both paths are probable and require further Due to the special features of the relatively complex experimental setup especially at higher investigation. Due to the special features of the relatively complex experimental setup especially at temperatures, oxidation by atmospheric oxygen could not be ruled out. An optimised setup should higher temperatures, oxidation by atmospheric oxygen could not be ruled out. An optimised setup have the capability to reduce this influence in future experiments. should have the capability to reduce this influence in future experiments.

2.4.VOFC VOFC with with High High Concentration ConcentrationVanadium VanadiumElectrolyte Electrolyte 2.4.

Figure88shows showsthe thecell cellvoltage voltageand andTable Table 4 shows performance values obtained in the Figure 4 shows thethe performance values obtained in the casecase of of 3.6 M Vanadium electrolyte in 6–8 M H 2SO4 during short-term discharge tests ◦at 50 °C. With a 3.6 M Vanadium electrolyte in 6–8 M H2 SO4 during short-term discharge tests at 50 C. With a value value of approx.

V, the OCV was significantly higher than in the previous had of approx.

1.2–1.251.2–1.25 V, the OCV was significantly higher than in the previous tests andtests thusand hadthus a better a better three-phase boundary layer. three-phase boundary layer.

Figure 8. Cell voltage values for 3.6 M VOFC with variation of current density at 50 ◦ C for UNSW Figure 8. Cell voltage values for 3.6 M VOFC with variation of current density at 50 °C for UNSW MEA (MoO3 treated carbon paper as negative half-cell electrode and commercial Pt/C (0.4 mg cm−2−2) MEA (MoO3 treated carbon paper as negative half-cell electrode and commercial Pt/C (0.4 mg cm ) on Vulcan carbon as positive half-cell electrode). on Vulcan carbon as positive half-cell electrode).

Table 4. Mean cell voltage (UCell ), power densities (p), area specific resistances (ASR) and energy The highest density was to be 34.5 mW cm−2 MEA with (4 a current 50 mA efficiencies (EE)power for a VOFC using 3.6determined M V(II) solution and a commercial mg cm−2density Pt) at 50of◦ C, cm−2based . The on power density was thus higher than in all previous tests.

The energy efficiency was 46 %. standard potential difference.

The cell could be discharged up to a current density of 70 mA cm−2, whereby still 30 mW cm−2 and an Vanadium Cell 0 10 mA 20 mA 30 mA 40 mA 50 mA 60 mA 70 mA −2 cell −2 2 −2 energy efficiency ofCharacteristics 29% could be (OCV) obtained.cmThe a cm 3.6−molar solution thus Concentration cmwith cmV(II) cm−2 cm−2achieved cm−2 the best performance values of all measurements and at the same time shows the potential for future UCell /V 1.25 1.03 0.97 0.89 0.81 0.69 0.57 0.43 −2 10.3 19.4 26.7 32.4 34.5 34.2 30.1 p/mW cm developments.

3.6 M V(II), 50 ◦ C −2 ASR/Ohm cm EE/%

84

103.0 69

48.5 65

29.7 59

20.3 54

13.8 46

9.5 38

6.1 29

Table 4 Mean cell voltage (UCell), power densities (p), area specific resistances (ASR) and energy efficiencies (EE) for a VOFC using 3.6 M V(II) solution and a commercial MEA (4 mg cm−2 Pt) at 50 The highest power density was determined to be 34.5 mW cm−2 with a current density of °C, based on standard potential difference. −2

50 mA cm . The power density was thus higher than in all previous tests. The energy efficiency was 30 cm−240 50 still 30 60mW cm 70−2 46 %.Vanadium The cell could be discharged up to0a current density20of 70 mA , whereby Cell 10 mA mA mA mA mA mA mA andConcentration an energy efficiency of 29% could be obtained.

The cell with a 3.6 molar V(II) solution thus Characteristics (OCV) cm−2 cm−2 cm−2 cm−2 cm−2 cm−2 cm−2 achieved the best performance values of all measurements and at the same time shows the potential UCell/V 1.25 1.03 0.97 0.89 0.81 0.69 0.57 0.43 for future developments. p/mW cm−2 10.3 19.4 26.7 32.4 34.5 34.2 30.1 3.6 M V(II), Figure 9 represents the continuous discharge curve at 20 mA cm−2 . In this test, the OCV of ASR / Ohm 50 °C 103.0 48.5 29.7 20.3 13.8 9.5 6.1 1.05 V was again significantly cm−2 lower than in the previous test, which indicates fragile conditions in the EE/%

84

69

65

59

54

46

38

29

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Figure 9 represents the continuous discharge curve at 20 mA cm-2. In this test, the OCV of 1.05 11 of 19 lower than in the previous test, which indicates fragile conditions in the formation of the three-phase boundary layer during the tests. The cell voltage was approx. 0.8 V at the beginning ofthree-phase the discharge test, and was thus the slightly thevoltage value was of the previous It formation of the boundary layer during tests.below The cell approx.

0.8 Vtest. at the decreased to an average value of 0.64 V in the course of the approx.

153 minute discharge.

During beginning of the discharge test, and was thus slightly below the value of the previous test. It decreased this Ahofwere (21.9 Ah/L).

With a value of 23 %, the charge carrier efficiency to anprocess, average1.53 value 0.64converted V in the course of the approx.

153 minute discharge.

During this process, was thus only half as high as in the test with 2 M V(II) at 50 °C. The converted energy density was 1.53 Ah were converted (21.9 Ah/L). With a value of 23 %, the charge carrier efficiency was thus only only 14 Wh/L, but shows the feasibility of a VOFC with highly concentrated vanadium solutions, half as high as in the test with 2 M V(II) at 50 ◦ C. The converted energy density was only 14 Wh/L, but especially in connection with the Since only one electrolyte (half the shows the feasibility of a VOFC withother highlymeasurements. concentrated vanadium solutions, especiallytank in connection volume) is used in the VOFC, this is equivalent to using an all-vanadium battery at 7.2 M vanadium with the other measurements. Since only one electrolyte tank (half the volume) is used in the VOFC, concentration, which is around 4.5 times that of VRFB concentration of 1.5–1.7 M this is equivalent to using an all-vanadium battery at 7.2 (considering M vanadium the concentration, which is around used in commercial VRFBs).

4.5 times that of VRFB (considering the concentration of 1.5–1.7 M used in commercial VRFBs).

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Figure 9. Continuous discharge curve for VOFC with 70 mL 3.6 M V(II) at 20 mA cm−2 for UNSW Figure 9. Continuous discharge curve for VOFC with 70 mL 3.6 M V(II) at 20 mA cm−2 for UNSW MEA (MoO treated carbon paper as negative half-cell electrode and commercial Pt/C (0.4 mg cm−−22 ) MEA (MoO33 treated carbon paper as negative half-cell electrode and commercial Pt/C (0.4 mg cm ) on Vulcan carbon as positive half-cell electrode) at 50 ◦ C. on Vulcan carbon as positive half-cell electrode) at 50 °C.

As seen from Table 5, the theoretical energy density values are significantly higher than those As seen from Table 5, the theoretical energy density values are significantly higher than those obtained for conventional VRFBs. The observed energy density values obtained in the above tests obtained for conventional VRFBs. The observed energy density values obtained in the above tests are are lower which can be attributed to the non-optimised cell design and factors like vanadium cross lower which can be attributed to the non-optimised cell design and factors like vanadium cross over, over, oxygen diffusion through the membrane, and higher oxygen reduction overpotential. As can oxygen diffusion through the membrane, and higher oxygen reduction overpotential. As can be seen be seen from the discharge curve, the mass transport limitation on the oxygen half-cell resulted in a from the discharge curve, the mass transport limitation on the oxygen half-cell resulted in a rapid rapid decrease in cell voltage to cut off voltage of 0.1 V. This could be caused by flooding of catalytic decrease in cell voltage to cut off voltage of 0.1 V. This could be caused by flooding of catalytic sites sites at the positive half-cell as a result of the formation of water from the oxygen reduction reaction. at the positive half-cell as a result of the formation of water from the oxygen reduction reaction. The The recovery of obtained cell voltage close to the OCV value on completion of discharge indicates recovery of obtained cell voltage close to the OCV value on completion of discharge indicates that that the electrolyte is not fully discharged and can explain the low values obtained for capacity and the electrolyte is not fully discharged and can explain the low values obtained for capacity and energy energy density. density.

Table 5. Energy density values for VOFC. Table 5. Energy density values for VOFC. Vanadium Concentration Theoretical Energy Density

Vanadium Concentration 2 M Vanadium solution For 2 MFor Vanadium solution For 3.6 M Vanadium solution For 3.6 M Vanadium solution

Theoretical Energy Density 79.9 Wh79.9 L−1Wh L−1 143.8 Wh L−1 143.8 Wh L−1

® membrane ® membrane The oxygen thethe NAFION to thetonegative side of side the VOFC The oxygendiffuses diffusesthrough through NAFION the negative of theresulting VOFC in the oxidation of V(II) species affecting cell performance.

Diffusion ofDiffusion vanadium throughions the resulting in the oxidation of V(II) speciesthe affecting the cell performance. ofions vanadium NAFION® membrane to the positive side was also observed during the discharge tests which would

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potentially flood the gas diffusion layer on the positive side (ORR) reducing the effective catalytic sites available for oxygen reduction resulting in performance loss. Investigation of parameters like electrolyte and oxygen flow rate, temperature, alternate electrocatalysts for oxygen reduction process is currently underway which may improve the energy density values further. A V(II) regeneration cell is also being designed and fabricated to allow the VOFC to be operated for longer durations in order to investigate the stability of the MEA. To avoid the vanadium cross-over issues, use of alternatives to NAFION® membranes are also under consideration.

2.5. Investigation of Mass Transport Issues To explain the observed low capacity, experiments were carried out to study the effect of mass transport issues on the VOFC performance. To identify the half-cell with mass transport issues, a reference electrode was added to the negative half-cell to measure the anode potential while overall cell voltage was also measured. The negative half-cell (V2+ /V3+ ) potential value remained nearly constant at ~−0.86 V vs. Hg/Hg2 SO4 during a discharge over a period of 10 min, while the cell voltage value reduced over the same time period from 1.03 V to 0.322 V, indicating the mass transport issues were due to ineffective transport of oxygen to the positive electrode [33].

Experimental analysis utilising individual half-cell measurements revealed that the observed low capacity is associated with mass transport problems at the oxygen half-cell of the VOFC. The individual half-cell potential measurements concluded that the mass transport issue is associated with the oxygen reduction reaction. As water is formed at the positive half-cell due to oxygen reduction, it potentially floods the catalyst active sites, preventing the diffusion of oxygen to the catalyst layer. This is confirmed with studies where only 20–25% capacity discharge was observed in single discharge test, and close to full capacity discharge was observed when water was drained from the positive half-cell.

To resolve this issue, modifications in the MEA and cell design are necessary for quicker removal of water from the positive half-cell. Cell voltage values after discharge tests were close to the OCV values indicating that the V(II) electrolyte was not fully discharged. To confirm this, successive discharge tests were carried out at a constant current density of 20 mA cm−2 to check the overall discharge time compared to the theoretical value.

Figure 10 represents the continuous discharge curve obtained for a vanadium oxygen fuel cell using 2 M V(II) electrolyte. Heat treated SIGRACELL GFD graphite felt was used as a negative half-cell electrode, while commercial Pt/C (0.4 mg cm−2 ) coated on wet proofed carbon cloth was used as a positive half-cell catalyst later. The membrane electrode assembly was prepared by hot pressing the commercial catalyst layer on the NAFION® 115 membrane. As can be seen from the discharge curve, the mass transport limitation on oxygen half-cell resulted in a rapid decrease in obtained cell voltage to the cut off voltage of 0.1 V. This could be caused by a loss of reactive oxygen at the cathode, which in turn was caused by too high or too low humidity of the three-phase boundary. Flooding of catalytic sites at the positive half-cell can be a result of the formation of water from the oxygen reduction reaction. This water formation results in a decrease in the diffusion of oxygen through the gas diffusion layer to the positive half-cell catalyst layer. A three-phase boundary layer that is too dry can be created by using dry oxygen, whereby the catalyst dries out and prevents reaction with protons. Both effects are in equilibrium with the diffusion of liquid from the negative half-cell space and a varying formation of water at different current densities together with the used oxygen supply.

The mass transport issue can be confirmed by the recovery of the cell voltage to the OCV value after completion of a discharge cycle without intermediate charging of the electrolyte. This recovery of the cell voltage can explain the low capacity value compared to the theoretical capacity. Another potential reason for the observed decreased performance and mass transport issues could be due to the cross-over of vanadium through the membrane to the positive side. The presence of vanadium species was confirmed visually at the positive half-cell. At the end of the first discharge process, the temperature of the water bath of the oxygen humidification was set to 80◦ C. In the following discharge processes, the water warmed up, providing higher humidification. During the first discharge, a time

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of 18 min was achieved. The following discharges achieved in 14, 14.3, 16 and 19.1 min. The increase 19.1 min.humidity The increase in oxygen humidity during heating of the water bathformation and led to of thethe in 19.1 oxygen occurred during heatingoccurred of the water bath and led to the better min. The increase in oxygen humidity occurred during heating of the water bath and led to the better formation of the three-phase boundary layer.

This experiment suggests that the catalyst could three-phase boundary layer.

This experiment thatexperiment the catalystsuggests could be toothe drycatalyst rather could than too better formation of the three-phase boundarysuggests layer. This that bebut too still dry more ratherdetailed than toostudies wet, butare still more detailed studies are needed to ascertain for certain the wet, needed to ascertain for certain the cause of the mass transport be too dry rather than too wet, but still more detailed studies are needed to ascertain for certain the 2 could cause of the mass transport limitation.

According to this finding, cells with a current density lowerbe limitation.

According to this finding, cells with a current density lower than 20 mA/cm cause of the mass transport limitation. According to this finding, cells with a current density lower than 20 mA/cm² could be continuously discharged with high capacities using this cathode. continuously discharged high capacities usingwith this high cathode. than 20 mA/cm² could bewith continuously discharged capacities using this cathode.

Figure 10.10.

Cell voltage (during continuous Figure Cell voltage (during continuousdischarge) discharge)for fora avanadium vanadiumoxygen oxygenfuel fuelcell cell(VOFC) (VOFC)with with 50 Figure 10. Cell voltage (during continuous discharge) for a vanadium oxygen fuel cell (VOFC) with −2 with −2 mL502 mL M V(II) at 20 mA cm a heat treated SIGRACELL GFD graphite felt as negative half-cell 2 M V(II) at 20 mA cm with a heat treated SIGRACELL graphite felt as negative half-cell 50 mL 2 M V(II) at 20 mA cm−2 with a heat treated SIGRACELL GFD graphite felt as negative half-cell −2 2Pt) electrode and a commercial Pt)on onVulcan Vulcancarbon carbonasaspositive positivehalf-cell half-cell electrode and a commercialPt/C Pt/Celectrode electrode (0.4 (0.4 mg mg cm cm−2− electrode and a commercial Pt/C electrode (0.4 mg cm Pt) on Vulcan carbon as positive half-cell ®®115 membrane, observed discharge capacity ≈ 80%. electrode using pre-treated 115 membrane, observed discharge capacity ≈ 80%. electrode using pre-treatedNAFION NAFION ® electrode using pre-treated NAFION 115 membrane, observed discharge capacity ≈ 80%.

Figure 1111 shows the vanadiumoxygen oxygenfuel fuelcell cellfor forthe the calculation Figure shows thepolarisation polarisationplot plotobtained obtained for vanadium calculation Figure 11 shows the polarisation plot obtained for vanadium oxygen fuel cell for the calculation of cell resistance.

MEA incorporating heat treated SIGRACELL of cell resistance.

MEA incorporating heat treated SIGRACELLGFD GFDfelt feltwas wasused usedatatnegative negativehalf-cell halfof cell resistance.

MEA incorporating heat treated SIGRACELL GFD felt was used at negative half−2 cm −2 Pt/C coated on wet proofed CeTech carbon cloth was used as positive half-cell cell 0.4 while 0.4 mg while mg cm Pt/C coated on wet proofed CeTech carbon cloth was used as positive half-cell cell while 0.4 mg cm−2 Pt/C coated on wet proofed CeTech carbon cloth was used as positive half-cell catalyst. NAFION®®® 115 115 membrane was hot pressed with the the positive half-cell catalyst layer. 2 M V(II) catalyst. membranewas was pressed positive half-cell catalyst 2M catalyst.NAFION NAFION 115 membrane hothot pressed withwith the positive half-cell catalyst layer. 2layer.

M V(II) solution was used as as anan electrolyte while pre-heated and pre-saturated oxygen was fedfed into thethe V(II) solution was used electrolyte while pre-heated and pre-saturated oxygen was into solution was used as an electrolyte while pre-heated and pre-saturated oxygen was fed into the positive half-cell. A constantcurrent current wasapplied, applied, and and the corresponding voltage was measured.

The positive half-cell. thecorresponding correspondingvoltage voltage was measured. positive half-cell.AAconstant constant current was was applied, and the was measured.

TheThe 2.2This can be reduced cell resistance was calculated from the slope value and was found to be 8 Ω cm 2. This cell resistance and was wasfound foundtotobe be8 8ΩΩcm cm . This reduced cell resistancewas wascalculated calculatedfrom fromthe the slope slope value and cancan be be reduced by optimisation of the cell designand and gasdiffusion diffusion pathways. pathways. byby optimisation optimisationofofthe thecell celldesign design and gas gas diffusion pathways.

Figure Polarisation plotobtained obtainedfor forvanadium vanadiumoxygen oxygen fuel cell, 22 M 2SO 4 electrolyte.

Figure 11.11.

Polarisation plot MV(II) V(II)inin55MMHH 2 SO 4 electrolyte.

Figure 11. Polarisation plot obtained for vanadium oxygen fuel cell, 2 M V(II) in 5 M H2SO 4 electrolyte.

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The cell achieved a maximum current density of 100 mA cm−2 and a maximum power density of 26 mW cm−2 with a current density of 50 mA cm−2.

The cell achieved a maximum current density of 100 mA cm−2 and a maximum power density of This mass transport issue can potentially be addressed by making the positive half-cell GDL 26 mW cm−2 with a current density of 50 mA cm−2 . more hydrophobic, thus preventing the flooding of catalytic sites at the positive half-cell. The issue This mass transport issue can potentially be addressed by making the positive half-cell GDL of vanadium cross-over through NAFION® 115 membrane can be potentially addressed by using an more hydrophobic, thus preventing the flooding of catalytic sites at the positive half-cell. The issue anion exchange membrane which will have a characteristically high blocking capacity of vanadium of vanadium cross-over through NAFION® 115 membrane can be potentially addressed by using an ions, but will still allow protons to pass through the membrane. To check the feasibility of using anion anion exchange membrane which will have a characteristically high blocking capacity of vanadium exchange membranes in VOFC, a preliminary VOFC discharge test was carried out using Fumasep ions, but will still allow protons to pass through the membrane. To check the feasibility of using anion FAP-450 anion exchange membrane to study the VOFC performance and relative comparison with exchange®membranes in VOFC, a preliminary VOFC discharge test was carried out using Fumasep NAFION 115 membrane.

FAP-450 anion exchange membrane to study the VOFC performance and relative comparison with Figure 12 represents the comparison of VOFC performance between NAFION® 115 and NAFION® 115 membrane.

Fumasep FAP-450 membrane. While no crossover of vanadium was observed ®in the preliminary Figure 12 represents the comparison of VOFC performance between NAFION 115 and Fumasep VOFC discharge test with the Fumasep FAP-450 membrane, a future detailed systematic study is FAP-450 membrane. While no crossover of vanadium was observed in the preliminary VOFC discharge needed to address the limiting mass transport aspects. Approximately 80% of the theoretical capacity test with the Fumasep FAP-450 membrane,® a future detailed systematic study is needed to address the could be achieved with the NAFION 115 membrane in this case. By comparison, this is limiting mass transport aspects. Approximately 80% of the theoretical capacity could be achieved with approximately 65% of the theoretical capacity achieved utilising the anion exchange Fumasep FAPthe NAFION® 115 membrane in this case. By comparison, this is approximately 65% of the theoretical 450 membrane. This stresses the need for the increased hydrophobicity of the positive half-cell GDL capacity achieved utilising the anion exchange Fumasep FAP-450 membrane. This stresses the need for which can be achieved by PTFE treatment. The obtained cell voltage at a discharge current density of the increased hydrophobicity of the positive half-cell GDL which can be achieved ®by PTFE treatment.

300 mA is lower for the Fumasep FAP-450 compared to that obtained at NAFION 115, which can be The obtained cell voltage at a discharge current density of 300 mA is lower for the Fumasep FAP-450 attributed to higher membrane resistance and potentially a lower proton transfer rate compared to a compared to that obtained at NAFION® 115, which can be attributed to higher membrane resistance cation exchange membrane. and potentially a lower proton transfer rate compared to a cation exchange membrane.

−2 Figure Figure 12.

12. Cell Cell voltage voltage for for aa vanadium vanadium oxygen oxygenfuel fuelcell cell(VOFC) (VOFC)with with50 50mL mL22MMV(II) V(II)atat1010mA mAcm cm−2 with GFD graphite feltfelt as negative half-cell electrode and aand commercial Pt/C withaaheat heattreated treatedSIGRACELL SIGRACELL GFD graphite as negative half-cell electrode a commercial −2 Pt) on Vulcan carbon as positive half-cell electrode. Comparison of NAFION® electrode (0.4 mg cm −2 Pt/C electrode (0.4 mg cm Pt) on Vulcan carbon as positive half-cell electrode. Comparison of 115 and Fumasep FAP-450 membrane. ® 115 and Fumasep FAP-450 membrane.

NAFION

3. Materials and Methods 3. Materials and Methods 3.1. Electrolyte Preparation 3.1. Electrolyte Preparation Vanadium trioxide (V2 O3 ) and vanadium pentoxide (V2 O5 ) powders were added to a sulphuric Vanadium trioxide (V2quantities O3) and vanadium pentoxide (V2O5)topowders added concentration to a sulphuric acid solution in equimolar and heated subsequently generatewere a required acid solution in equimolar quantities and heated subsequently to generate a required concentration of V(IV) solution by chemical dissolution. On addition of V2 O5 powder to a V2 O3 suspension, the of V(IV) solution by chemical dissolution. On addition of V2O5 powder to a V2O3 suspension, the

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V(III) ions chemically leach V2 O5 by shifting the equilibrium to form more soluble tetravalent V(IV) species [25]. The reaction can be represented as: V2 O3 (s) + 6H+ → 2V3+ + 3H2 O

(4)

V2 O5 (s) + 2H+ → 2VO2 + +H2 O

(5)

V3+ + VO2 + → 2VO2+

(6)

This resulting V(IV) solution was then electrolyzed to V(II) in a two-compartment electrolysis cell, and the final concentration of V(II) and sulphate was determined using inductively coupled plasma (ICP) analysis. The vanadium electrolyser cell consists of PVC or acrylic based endplates with copper current collector behind graphite frames with serpentine flow channels. Thermally treated (400 ◦ C, 30 h) SIGRACELL® GFD 4.6 EA graphite felts (SGL Carbon GmbH, Meitingen, Germany) were used as electrodes in negative as well as positive half-cells. A pre-treated NAFION® 115 was used as a membrane with SIGRACELL® GFD 4.6 EA graphite felts pressed against the membrane to form a membrane electrode assembly. The pre-treatment of NAFION® was done by heating the membrane in 3% H2 O2 solution at 80 ◦ C for 1 hour, followed by heating in DI water at 80 ◦ C for 2 h and finally heating it further in 0.5 M H2 SO4 at 80 ◦ C for 1 hour. After pre-treatment, the NAFION® 115 membrane was stored in DI water. To prevent the precipitation of vanadium species at higher concentration, precipitation inhibitors, ammonium sulphate (2 wt.%) and phosphoric acid (1 wt.%) were added to the 3.6 M Vanadium electrolyte solution based on previous work by Roe et al. [25]. To prevent the V(V) formation during the electrolysis process, the volume used in the positive half-cell was twice that of one used in negative half-cell.

3.2. VOFC Cell Assembly For VOFC testing, a single cell with ~30 cm2 of geometric active area was constructed with serpentine flow channels using a zero-gap design [10]. The VOFC test cell was fabricated using stereo lithographically produced end plates made of acrylic based materials with steel backing plates used for additional mechanical support. Toray carbon paper 120 (fuel cell store, USA) were used in both half-cells to reduce the contact resistance between copper current collectors and serpentine channel machined bipolar plates. On the negative half-cell, a MoO3 modified heat-treated carbon paper (Toray 120, heat-treated at 400 ◦ C, 30 h) was prepared by soaking heat-treated Toray carbon paper 120 in Ammonium molybdate ((NH4 )6 Mo7 O24 ) solution (3.8 mg mL−1 ) overnight in an oven at 80 ◦ C for 2 h [13]. The modified carbon paper was then calcined in a bed of carbon black at 650 ◦ C for 2 h. The weight ratio of carbon paper to ammonium molybdate tetrahydrate at 4:1 This MoO3 modified Toray carbon paper used as catalyst layer in negative half-cell, while on the positive side, commercial Pt on Vulcan carbon (0.4 mg cm−2 ) coated on carbon cloth (Fuel cell store, USA) was used as a combined catalyst layer gas diffusion layer. A pre-treated NAFION® 115 membrane (DuPont, Fuel cell store, Freeport, Texas, USA) (method described in Section 3.1) was utilised to prepare membrane electrode assembly (MEA).

The negative half-cell consisted of a stereo lithographically produced acrylic based flow frame with inlet and outlet channels for the V(II) electrolyte. In addition, various flat gaskets were used for the sealing of the cell. The V(II) electrolyte was pumped into the negative half-cell using a diaphragm pump at a flow rate of 50 mL/min and the gas feed to the positive half-cell was pre-heated and water pre-saturated pure oxygen gas at a flow rate of 100 mL/min. A water column was used to provide oxygen back pressure in the system to improve mass transport of oxygen to the gas diffusion electrode. The VOFC tests were carried out using a battery test system (CDS, Repower, Shenzhen, China). The schematic of proto-type VOFC and its test set up is presented in Figures 13 and 14.

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Figure 13. VOFC test cell components schematic, A: End plate, B: Current collector C: Toray carbon Figure 13. VOFC test cell components schematic, A: End plate, B: Current collector C: Toray carbon ® paper, D: VOFC flow frame, E: Catalyst coated Toray carbon paper or carbon cloth with gasket, F: NAFION Figure test cell components schematic, A: End plate, B: Current C: Toray carbon® paper, 13.

D: flow frame, E: Catalyst coated Toray carbon paper or carbon clothcollector with gasket, F: NAFION 115 membrane. paper, D: flow frame, E: Catalyst coated Toray carbon paper or carbon cloth with gasket, F: NAFION® 115 membrane.

115 membrane.

Figure Vanadium oxygen Figure 14.

14. Vanadium oxygen fuel fuel cell cell (VOFC) (VOFC) test test cell cell schematic schematic diagram. diagram.

Figure 14.

Vanadium oxygen fuel cell (VOFC) test cell schematic diagram.

3.3. VOFC Performance Measurement 3.3. VOFC Performance Measurement Different MEAs were used to investigate the behaviour of different cathode materials. Commercial 3.3. VOFC Performance Different MEAsMeasurement were used to investigate the behaviour of different cathode materials.

MEAs consisted of a composite of a membrane (NAFION® 115) and a ®catalyst-coated GDL (Pt/C Commercial MEAs of a to composite of a the membrane (NAFION 115) and a catalyst-coated MEAsconsisted were used investigate behaviour of(Fuel different cathode withDifferent different loadings coated on CeTech wet proofed carbon cloth) cell store, USA).materials.

The GDL GDL (Pt/C with different loadings coated on CeTech wet proofed carbon cloth) (Fuel cell store, USA). ® Commercial MEAs consisted of a composite of a membrane (NAFION 115) and a catalyst-coated − 2 − 2 was only on one side, and MEAs with 4 mg cm Pt or 0.4 mg cm were used.

Furthermore, own −2 were used. Furthermore, The (Pt/C GDL was only on one side, and MEAs with 4 mg cm−2 Pt or 0.4 mg cm GDL with different loadings coated on carbon (Fuel cell can store, MEAs were produced and characterised in CeTech the cell.wet Theproofed description of cloth) the fabrication beUSA). found own MEAs were and characterised thecm cell. description of the fabrication can be −2 PtThe −2 were The GDL was only produced on one side, and MEAs with 4inmg 0.4 mg cmat used.

Furthermore, in the next section.

The performance of different VOFCs wasormeasured different temperatures by found in the next section.

The performance of different VOFCs was measured at different own MEAsthe were produced characterised in the of cell.

The description of the fabrication be recording OCV for 2 minand following an application different current densities (polarisationcan curve) temperatures by recording the OCV for 2 min following an applicationwas of different current densities found in and the calculating next section.

The performance of different measured at under different for 5 min the mean cell voltage, power density VOFCs and area specific resistance load (polarisation curve) for 5 min and calculating the mean cell voltage, power density and area specific temperatures by [34]. recording the OCV tests for 2were min following application of M different current densities for comparison Full discharge carried outanfor 2 M and 3.6 Vanadium solutions and resistance under load for comparison [34]. Full tests were carried out for M and 3.6 M (polarisation curve) for 5 min and calculating the discharge mean cell voltage, power density and2 area specific the energy density values calculated.

A battery test system (CDS, Repower, Shenzhen, China) was Vanadium solutions the energy density values calculated. battery test out system Repower, resistance under load and for comparison [34]. Full discharge tests A were carried for 2(CDS, M and 3.6 M used for these measurements.

Shenzhen, China) was used for these measurements.

Vanadium solutions and the energy density values calculated. A battery test system (CDS, Repower, Shenzhen, China) was used for these measurements.

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3.4. UNSW MEA Preparation Positive half-cell: Carbon paper (Toray 120, Fuel cell store, USA), SIGRACELL® GFD 4.6 EA graphite felts (SGL Carbon GmbH, Meitingen, Germany), wet proofed CeTech carbon cloth (Fuel cell store, College Station, Texas, USA), Iron phthalocyanine (Sigma Aldrich, Sydney, Australia), NAFION® 115 (Fuel cell store, USA), Ammonium molybdate tetrahydrate (Sigma Aldrich, Australia), MoS2 (Sigma Aldrich, Sydney, Australia) were purchased and used in these tests without further modification.

The heat-treated graphite felt, and carbon paper samples were prepared using the heat treatment in Labec high-temperature furnace at 400 ◦ C for 30 h in air.

The catalyst powders (e.g., Vulcan XC-72 powder) was weighed and dispersed in isopropanol solvent along with 5% NAFION® perfluorinated resin solution as a binder (Sigma Aldrich, Sydney, Australia). The mixture was ultrasonicated to ensure uniform mixing, and then applied to the membrane using a painting brush. The solvent was allowed to evaporate, and the MEA was hot pressed using a Carver bench top standard hot press (Model 4386) under various temperature conditions. The VB1 membranes were photographed before and after the tests as well as their dimensions measured to check any dimension and appearance changes (swelling/shrinkage) during the MEA preparation and bonding.

In order to test the various electrode materials for negative and positive half cells, the catalyst materials were suspended in a mixture of isopropanol and 5% NAFION® solution (used as a binder).

The suspension was ultra-sonicated to facilitate uniform mixing and then spray coated repeatedly on the carbon paper or carbon cloth GDLs to obtain a uniform coating.

Negative half-cell: MoO3 modified carbon paper electrodes developed at UNSW have shown promise for high power density VRFB applications [13]. The modification with MoO3 was attained by soaking pretreated carbon paper in 3.8 mg mL−1 ammonium molybdate solution with the weight ratio of carbon paper to ammonium molybdate tetrahydrate maintained at 4:1. The carbon paper was then initially dried at 80 ◦ C for 2 h followed by calcination in the bed of carbon black at 650 ◦ C for 2 h.

For the VOFC test work, membrane electrode assemblies were prepared using pre-treated NAFION® 115 membranes with MoO3 modified carbon paper electrodes as negative half-cell catalyst, while several other potential ORR catalysts were used for positive half-cell.

3.5. VOFC with High Concentration Vanadium Electrolyte A high concentration vanadium electrolyte was prepared in a similar manner to that of V(II) solution preparation method described in Section 3.1. Precipitation inhibitors involving 2% ammonium sulphate and 1% phosphoric acid were added during the preparation of vanadium electrolyte to prevent the precipitation of pentavalent vanadium in the supersaturated electrolytes. Also, a higher concentration of sulphuric acid was used (7–8 M) to facilitate the dissolution of vanadium powders.

The resulting high concentration vanadium electrolyte was tested in a VOFC cell at 50 ◦ C. 4. Conclusions A series of single-cell bench top vanadium oxygen fuel cells have been designed and fabricated to test the feasibility of high concentration vanadium electrolytes in the VOFC. The performance of the VOFC was evaluated using 2 M vanadium electrolyte as well as preliminary trials with 3.6 M vanadium electrolyte. Optimisation of the VOFC for use with high concentration vanadium electrolytes can potentially result in significant improvement in energy density compared to VRFBs. The observed low energy density and capacity values can be attributed to mass transport limitations at the oxygen half-cell. Factors like vanadium cross-over through the NAFION® membrane to the positive half-cell, potential flooding of catalytic sites due to water formed from oxygen reduction, and areas of non-uniform hydration resulting in reactive areas that may be too dry, limit the supply of

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oxygen to the catalytic sites resulting in mass transport issues. The VOFC performance is also affected by the diffusion of oxygen through the membrane resulting in V(II) oxidation at the negative half-cell.

The VOFC system will be further developed in future work to optimise of the electrolyte flow rate, oxygen flow rate and operating temperature. Future optimisation of the VOFC to address the issues highlighted in this study will be focused on approaching the theoretical energy density of 144 Wh/L. Author Contributions: Conceptualisation, C.M. and M.S.-K.; investigation, M.R.; writing—original draft preparation, M.R.; writing—review and editing, C.M., M.S.-K., J.N.; supervision, C.M., M.S.-K., project administration, C.M., M.S.-K.; funding acquisition, C.M., M.S.-K. Funding: This research was funded by the Office of Naval Research Global through the Naval International Cooperative Opportunities in Science and Technology Program (NICOP).

Conflicts of Interest: The authors would like to declare the professional relationship with guest editor Prof. Maria Skyllas-Kazacos who is also co-author for this manuscript.

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