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(School of Chemical Engineering, The University of New South Wales, UNSW Sydney, NSW 2052, Australia)

The widespread implementation of renewable energy is becoming an increasingly important issue around the world.However, because of the intrinsic intermittent and unpredictable properties of renewable energy systems such as wind and solar, suitable energy storage and supply systems are needed to make the grid more robust and reliable.

Electrochemical energy storage systems such as batteries and fuel cells are especially suited to the storage of electrical energy since they involve the direct conversion of chemical into electrical energy, allowing very high energy efficiencies to be achieved.Amongst all of the available battery technologies however, redox flow batteries (RFB) allow the greatest flexibility and versatility and have therefore attracted the most interest in recent years.The RFB is a kind of secondary battery system in which energy is stored in liquids contained in external electrolyte reservoirs with inert electrodes that only supply the reaction sites for the redox couple reactions (Fig.1).RFBs offer many advantages over conventional batteries, including independent sizing of power and capacity, simple battery structure, low capital and maintenance costs.

Fig.1 The scheme of RFB

Several redox couples were studied by NASA in 1970s, such as Fe2+/Fe3+, Cr2+/Cr3+, V2+/V3+, V4+/V5+, Br–/Br2, etc.[1-2](Fig.2).Attracted by the cost and cell potential, NASA focused their research mainly on the Fe/Cr system in a HCl supporting electrolyte.The inevitable problem of ion cross-over in the Fe/Cr RFB systems gives rise to irreversible contamination of the anolyte and catholyte, with the accompanied loss of capacity and performance [3].The problem of cross contamination of the Fe/Cr electrolytes was solved byusing a mixed electrolyte in both half-cells, but this led to a decrease in the solubility of each of the active ions in solution with a resultant reduction in energy density.

Fig.2 Potential distribution of various redox reactions

Considerable development of the Fe/Cr battery was carried out in the 1980s, leading to field testing of a 60 kW system under the NEDO funded Moonlight Project in Japan.The low efficiency and large foot print of the system compared with the Zn/Br and lead-acid batteries however, led to a phasing out of further R&D efforts on the Fe/Cr system until the mid-2000s when renewed interest in energy storage saw its resurrection by the Indian based company Deeya Energy.Despite early commercialisation efforts that produced Fe/Cr energy storage products for the telecommunications market in India however, the poor electrochemical performance of the electrode reactions of the Fe/Cr cell combined with the low energy density of the mixed electrolyte caused the company to convert their systems to the VRB chemistry, while changing their name to Immergy.Although another company, EnerVault, has more recently embarked on an extensive commercialisation effort with the Fe/Cr cell, most of their innovation has been involved with novel battery configurations using a series of cascaded stacks to improve energy efficiency [4].

The crossover problem existed in Fe/Cr systems was overcome by Skyllas-Kazacos in the mid-1990s by the use of vanadium in both half-cells, this work leading to the development of the all-vanadium redox flow battery (VRB) at UNSW [5-7]that has now been commercially implemented by several companies in Japan, China, Austria and USA.

In the meantime, however, other redox couple combinations have been investigated by various research groups in an effort to reduce cost and increase energy density.These have included the polysulfide/bromide flow battery firstly developed by Innogy in 1984 [5], the vanadium/bromide (G2 V/Br) flow battery proposed by Skyllas-Kazacos and co-workers [6-7]and the mixed Iron vanadium redox flow cell (V/Fe), proposed by researchers at the Pacific Northwest National Laboratories (PNNL) in recent years [8]as well as the all chromium redox flow battery proposed by Bae and co-workers [9-10].In each of these systems, the charged and discharged species in both half-cell electrolytes are fully soluble, so energy storage capacity (kW·h) is totally decoupled from power rating (kW).

In all redox flow batteries therefore, capacity is simply increased by increasing the volume of electrolyte.Other types of flow batteries that do not employ fully soluble redox couples however, are the zinc-based hybrid flow batteries that involve the deposition of zinc at the negative electrode during charging.These zinc-based flow batteries include the zinc-bromine, zinc-chlorine and zinc-cerium systems.Their performance is heavily linked to the quality of the zinc deposit, so their characteristics differ from those of the redox flow battery systems.

As illustrated in Fig.1, the main components of the redox flow battery:

（1）Electrolyte:This is the medium that carries the electroactive species in solution and includes the redox couple reactants, a supporting electrolyte and the solvent.Most common RFBs employ water as the solvent, so the redox couple reactions are limited by the decomposition potential of water.The supporting electrolyte is usually an acid that provide protons that migrate through the membrane to carry the current during the charging and discharging process.The supporting electrolyte is also important in determining the solubilities of the redox couple ions in the two half-cells that will in turn determine the energydensity of the electrolyte.

（2）Membrane:This acts as a separator that prevents the two electrodes from short-circuiting and the two solutions from mixing as they are pumped through the two half-cells.The membrane can be cation-selective or anion-selective and the main requirements are high conductivity and permeability for the charge carrying ions (usually protons), low permeability for the electroactive species, good chemical resistance to the electrolyte, low electrical conductivity and low cost.The selectivity of the membrane will determine the coulombic efficiency of the RFB that in turn affects overall energy efficiency.

（3）Electrodes:The electrodes provide the inert surface where the charge-discharge redox couple reactions take place.Although the electrodes in the RFBs do not take part in the actual charge-discharge reactions, their electrocatalytic properties will strongly affect the kinetics of the redox couple reactions that will in turn influence the voltage and energy efficiency of the RFB.The electrode material must therefore exhibit fast kinetics for the redox couple reactions, but inhibit undesirable side reactions that could produce hydrogen and oxygen at the negative and positive electrodes respectively during charging.Poor selectivity will not only reduce the coulombic efficiency of the RFB, but could lead to excessive gassing during charging that could lead to safety hazards in the case of hydrogen evolution, or deterioration of the positive electrode material that could potentially reduce cycle life.

The electrode is thus a critical component of the RFB that will not only determine the efficiency of the battery, but potentially its cycle life as well.Distinguished from other battery systems, the electrodes in RFBs do not react directly, but provide the reaction sites for redox couples and therefore contribute to all the types of cell polarization in the battery, i.e., concentration polarization, activation polarization and ohmic polarization.In this regard, great progress has been achieved in reducing the polarizations to maximise both energy efficiency and power density by material selection, structure design and catalytic activation of the electrodes.

This review will provide a detailed overview of the research and development of efficient electrode materials for RFB applications.This will include early screening studies of electrode materials for different flow battery systems, surface modification and electrocatalysis, surface properties, kinetic and mechanistic studies of the redox couple reactions on different electrode materials and electrode and cell design for high power density systems.

1 Flow battery chemistries

1.1 Fe/Cr redox flow battery

Most of the research work on the Fe/Cr flow battery system was conducted by NASA researchers in the 1970s and 1980s.During the screening of electrodes for the Fe/Cr RFBs by NASA [11], three broad types of electrode structures were taken into consideration:frontal electrode structure (e.g., a porous screen or paper structure contacting the membrane), recessed electrode structure (e.g., non-porous sheet or a microporous structure placed against the back plate of the cell with a sandwiched electrode-plastic screen-membrane structure) and cavity filling electrode structure (e.g., woven cloth or felt).Porous flow-through carbonaceous electrodes were chosen to be applied in the RFBs system based on their performance with respect to polarization and operating current density.Carbon or graphite felts were thereafter utilized as the electrode materials sandwiched between the membrane and current collector (Fig.1).

The Fe/Cr cell reactions are as follows:

The chromium redox reactions take place at a much slower speed at a normal electrode than the iron couples and the hydrogen evolution is quite competitive regarding their differences in standard potential.Therefore, a detailed screening of materials(including 26 kinds of metals and metal compounds, alloys, plated electrodes and teflon-bonded materials) was conducted by NASA to identify electrodes that would not enhance the H2evolution in acid environment, but show potential reversible redox activity towards chromium couples and remain stable in the corrosive environment.Although none of them show acceptable performances in positive or negative reactions,lead and gold exhibited good electrocatalytic activity towards anodic and cathodic reactions respectively.Thereby they added Pb2+and Au3+into the electrolyte solution.The testing results in a stack Fe/Cr flow cell showed enhanced coulombic and energy efficiency [12].

Yang compared the performance of ZrC and graphite electrode for the Cr2+/Cr3+reactions and found the electrical conductivity was higher than graphite and presented great potential in catalysing the redox couple.It was also suggested that transition metal carbides or nitrides might be promising due to their flexibility of modulating electronic structure and stability both chemically and mechanically.Besides, PbCl2was also proven to be a catalytic effective additive for Cr2+/Cr3+.During cycling, Pb can form on the surface of electrode and thus accelerate the reduction of Cr3+as well as inhibit H2evolution [13].

According to the research by Cheng and Hollax, addition of TlCl into the solution can not only facilitate the Cr2+/Cr3+but also increase the potential of H2liberation even more than Pb and Bi.However, the effect of Tl faded soon because of the aging caused by [Cr(H2O)6]Cl3[14].

Hollax and his co-workers investigated the kinetics of Cr3+/Cr2+and Fe3+/Fe2+reactions and studied the effect of thermal treatment particularly.They argued that the only increase of surface roughness and wetting property cannot attribute to catalytic activity after thermal treatment in 600 ℃.The enhanced catalysing effect was less stable for chromium reactions than iron reactions.Acidic surface oxygen groups were involved in the reactions because of the enhanced adsorption however, they would be reduced at the potential range close to H2evolution [15].

Schersonet alclaimed that the reaction between the redox couple containing Cr(H2O)5Cl2+was quasi-reversible on the solid Bi hydride coated gold rotating disk electrode.Compared to pure bismuth electrode, such kind of material was better in catalysing Cr2+reduction and can increase the overpotential for H2evolution [16].

In the report of Inoueet alregarding carbon fibres as electrodes in Fe/Cr flow cell, they found the surface area had no effect on the cell resistivity but the number of oxygen on surface can influence drastically.When compared the cycling performance of carbon fibres, no obvious variation was observed in terms of coulombic efficiency.However, the energy efficiency kept fading gradually [17].

Rodes and co-workerset alinvestigated a series of ad-atoms for chromium redox reactions on polyoriented gold electrodes.They found an increasing amount of Cl–in solution can modify the reversibility of Cr2+/Cr3+reactions.Atoms such as Bi, Pb, Sn, Tl and Sn all presented some electrocaltalytic function when added to the solution.Most interestingly, it would reach and maintain a maximum catalysing effect after the atom-monolayer was obtained on the surface of the electrode.They also pointed out the significance of surface control to maintain composition of the deposition.Otherwise the outcome of the catalysing would be variable all the time [18].

Lopez-Atalayaet alproved the enhancement in the reversibility of Cr2+/Cr3+reactions on C (graphite)-Au electrode by Pb and Bi again, but they also claimed that gold was not a necessary electrocatalysts for the cell property because the cell failed to display a parallel improvement in the energy efficiency of cells with graphite-gold/lead or bismuth electrodes and generated more H2instead [19].

In the early 1960s, ethylene diamine tetraacetic acid(EDTA) was reported by Pescoket al[20]and Walshet al[21]to be beneficial for the Cr3+/Cr2+redox reactions.Baeet al[9-10]also published several studies related to the chromium-EDTA couples.They found that EDTA can accelerate Cr3+/Cr2+reactionsmore than Cr5+/Cr3+reactions and utilized the conversion between Cr3+/Cr2+and Cr5+/Cr3+to fabricate an all chromium H-type redox flow cell, which performed similar efficiency as the Fe-Cr systems [9].

1.2 Polysulfide/bromine redox flow battery

Remick and Ang filed a patent on a polysulfide/polyhalide redox electrochemical system in 1981.In their invention, the reactions are the electrochemical conversions of sodium or potassium sulphide-polysulfide in the anolyte and halide (I2, Br2or Cl2)-corresponding polyhalide species in the catholyte.In their patent, the electrode material can be porous or sheet graphite, Pt, Pd, Ti, Ni.Preferably, transition metal sulphides NiS, Ni3S2, CoS, PbS and CuS can be added to the anode while reticulated vitreous carbon and transition metal dichalcogenides such as MoSe2, MoS2and WSe2for the cathode [5].

The typical sodium polysulfide/bromine energy storage cell is that:

In fact, the aqueous polysulfide redox couple shows its potential not only in redox batteries but also in photoelectrochemical cells.Two main problems in such system are the cell efficiency and stability in the long term.Therefore, studies about the electrodes or electrocatalysts for polysulfide couples had already been investigated before the invention of the PSBs.

Early in 1977, Hodeset alhad reported that CoS, NiS, Cu2S and PbS present good activity towards polysulfide redox reactions [22].In order to evaluate the properties of electrodes for practical applications, the same group started a primary material screening.Cobaltous sulphide, lead sulphide and sulphide brass (containing 30% Zn) exhibited almost the same activity but better than RuS2, carbon and Pt-black.Although brass (90% Cu) showed even lower overpotential with the increase of current density than the three former electrodes, it suffered from poor stability in the long term.Nevertheless, among those three electrodes, the stable property of PbS is the least and the thin layer of CoS did not show obvious mechanical or electrochemical instability for 4 months.Pure Cu will disintegrate in polysulfide solution easily, but with the increasing content of Zn, the mechanical stability of brass can be enhanced.

Interestingly, the slow and continuous renewal caused by the disintegration of the Cu2S surface led to a stable electrochemical activity over several months [23].Preliminary experiments were also carried out by Lessneret alto select suitable electrocatalysts in terms of smooth and reproducible surface, corrosion resistivity and polysulfide redox potential.According to their research, metal sulphides such as MoS2and NiS were unable to be polished and obtain reproducible surface.Metals such as Cu, Ag, Mo and Ni tended to oxidize themselves rather than the active species in solution.Only Co and Pt exhibited can meet three of the criteria [24].They further studied the catalysing effect of cobalt, molybdenum disulphide and nickel in polysulphide electrolyte.The results showed that nickel electrode would be faced with corrosion problems while Co and MoS2were able to meet the application requirements of an energy storage system, i.e., operating in the current density range of 10~20 mA/cm2while the overpotential was less than 50 mV [25].In the half-cell with high concentrated polysulfide anions electrolyte invented by Lichtet al, a cobalt sulphide thin film electrode was adopted.Such electrode can be prepared via electrodeposition of Co onto brass foil followed by polysulfide solution-anodic-cathodic treatment.Similar processes can be utilized to synthesise copper sulphide electrode as well.They also added K+or Cs+in electrolyte to reduce the overpotential [26].

Kegelman developed a process to prepare fused copper sulphide cathode to obtain a high porosity (enhanced electrochemical reaction area) by compacting the powder mixture of copper and sulphur at a temperature (lower than the melting point of S) with certain pressure and then heating at atmospheric pressure to form a fused cupric sulphide cathode structure [27].Similarly, Cooleyet allater filed apatent to prepare reticulated copper or nickel sulfide in a more feasible way to control the porosity and eliminate the non-stoichiometric species in the system.They applied similar sized copper (and/or nickel) sulphide powders and a water-soluble salt to make a mixture and then dissolved the salt by thermal treatment [28].

Reticulated vitrified carbon compositions with some transition metal particles such as Fe, Ni, Cu and Sn, etc.or alloys were invented to apply in sulphide-polysulfide systems.Such design offers the advantages of good stability and high electrical conductivity.Furthermore, excellent electrocatalytic behaviour can also be obtained [29].In the patent of Innogy Company, the employed electrode was composed of polyethylene impregnated with activated carbon electrode and the average cell efficiency of 45 cycles was 56% [30].Zhaoet alutilized the commercial nickel foams and carbon felts as the anodes and cathodes for polysulfide/bromine batteries and excellent catalytic activity for both redox reactions were obtained.Moreover, the energy efficiency can reach up to 77.2% within the 48 cycling tests at 40 mA/cm2[31].

It can be seen that much effort has been devoted to improving the reactions of the polysulfide and bromine redox couples for the PSB battery.However, due to the high possibility of crossover and mixing of the half-cell electrolytes, sulphur species are prone to precipitate and H2S and Br2will form, leading to reduced capacity and low energy efficiency.Thus recent interests in electrode modification or electrocatalysts for PSBs have been limited.

1.3 Other bromide based flow batteries

Other redox systems such as hydrogen/bromine, zinc/bromine and vanadium/bromine batteries also involve the Br2/Br–redox couple on one side of the cell.As intensively studied in 1960s and 1970s, bromine can be absorbed within the double-layer area and the speed of the redox reaction on platinum electrode is rapid [32-38].Therefore, Choet alapplied Pt as catalyst on carbon in the hydrogen/bromine redox batteries and proved it could be a suitable replacement of Pt for the Br electrode [32].In the zinc/bromine battery patent of Zito, activated carbon was implanted in the bromine electrode to catalyze the reaction of Br2/Br–and produced O2from water to compensate the shifting of pH [33].Considering the cost, carbonaceous materials attracted the attention of researchers.Vitreous carbon was proved to offer sufficient catalytic activity towards Br–/Br2reaction.However, its brittleness makes it impossible for practical application.Then plastic-bonded-carbon bromine electrode became an alternative.Cathro and co-workers studied a variety of carbon blacks to bind with polypropylene as bromine electrodes.They concluded that a double-layer electrode structure with a high content of carbon black could be used to achieve satisfactory electro-activity.The carbon black should possess a low bulk density and large surface area [34].

Munaiahet alinvestigated the effect of single-walled carbon nanotubes (SWCNTs) with different impurity contents in Zn/Br battery applications because of their large surface area and excellent electrical conductivity.Cyclic voltammetry demonstrated that the higher the purity of the SWCNTs, the higher the electrocatalytic activity of the modified electrode.The SWCNTs (90%, purity) modified graphite felt electrode enabled the zinc bromine cell to exhibit an energy efficiency 33% more than pure graphite felt electrodes [35].Laiet aldesigned a novel semi-solid electrode for the Br–/Br2couple in the zinc bromine cell aiming at higher energy efficiency and supressing bromine emission.They coated carbon felt with approximately 6.0 mg/cm2of active carbon slurry (a mixture of active carbon and ZnBr2solution).Together with modification of the membrane and complexation reagents, the new cell achieved 92% of coulombic efficiency and 82% energy efficiency [36].

In the study of the vanadium/bromine battery, Ruiet alused graphene oxide (GO)/polymer binders to modify the graphite electrode and achieved increased redox peak currents for Br–/Br3–couple in comparison with pure graphite electrode due to theimproved wettability and enlarged interfacial contact area.They believed that the GO accelerated the electron transfer and contributed to faster oxidation from bromine ions to bromine [37].Subsequently, the same group also found an excellent catalytic activity of CNTs especially the oxygen functionalized- SWCNTs on the electro-activity of Br–/Br3–reaction.

1.4 The all-vanadium redox flow battery

The all-vanadium redox flow battery (VRB), pioneered by the group of Skyllas-Kazacos at UNSW Australia, has become the most promising of all the current flow battery chemistries, because the same element is used in both the anolyte and catholyte, overcoming the problem of cross-contamination and ensuring greater safety and long cycle life compared to other systems.The cell reactions in the VRB are as follows:

The VRB system is now undergoing extensive commercialization and to date, dozens of VRBs have been installed in Europe, Asia, Africa and North America.Although the energy density of this system is low compared to Li-ion batteries (20～30 W·h/kg or 30～40W·h/L) [8-9], this is not an obstacle for stationary applications where size and weight are not issues.On the other hand power density has a significant effect on battery cost due to the relatively expensive membranes and electrode materials currently used in the VRFB.Much effort has thus been paid in recent years to improve the power density of the VRB stacks in order to make the system more cost competitive for grid-scale application.

1.4.1 Material selection

When selecting a suitable electrode material of the VRB, the following criteria must be satisfied:①Chemical stability in the electrolyte to ensure long cycle life; ②Good electrochemical activity for the vanadium redox couple reactions to minimise activation overvoltage losses and maximise voltage efficiency and power density; ③Poor electrochemical activity for hydrogen and oxygen evolution side reactions at the negative and positive electrodes respectively during charging; ④Good wettability in the vanadium electrolyte; ⑤Low electrical resistivity to minimise ohmic resistance losses and maximise voltage efficiency; ⑥Good mechanical properties and compressibility to prevent breakage of fibres and channel blockage during electrolyte flow through the stack; ⑦Relatively high permeability to reduce pumping energy losses at high flow-rates needed to allow high current density operation; ⑧Good electrochemical stability during overcharging to ensure long life operation; ⑨Low cost.

The first study on electrode material selection for the VRFB was published in 1987 by Rychcik and Skyllas-Kazacos [39], who found that most metals are unsuitable for use in acidic solutions since they will dissolve at the high anodic potentials experienced at the positive electrode during charging.Some inert metals such as lead and titanium were found to passivate in the sulphuric acid supporting electrolyte within the VO2+/potential range.Gold electrodes showed relatively poor electrochemical reversibility, while dimensionally stable anode (DSA), which consists of noble metal oxides coated on titanium, presented good performance during several charge-discharge cycles.DSAs were however excluded on the basis of cost, while long-term stability under continuous charge-discharge cycling was also expected to be poor.

Carbon and graphite were therefore selected as the most promising electrode material for the acidic vanadium electrolytes in the VRB.Although carbon and graphite materials undergo gradual corrosion with oxygen evolution during overcharge at the positive electrode, it was found that these materials exhibited excellent chemical and electrochemical stability during normal battery operation, allowing long cycle life to be achieved with good cell voltage control.1.4.2 Electrode modification and electrocatalysis

Early studies showed considerable variation in electrochemical performance when different graphitepaper or felts were used.Skyllas-Kazacos and co-workers compared two typical rayon- and polyacrylonitrile (PAN) - based graphite felts respectively by evaluation and comparison of their physical properties and electrochemical performance [40].They pointed out that various factors such as surface functional groups, electrical conductivity, surface area, microstructure, hydrophilicity and precursor materials affected their electrochemical performance.Blasiet al[41], Melkeet al[42]and Schweiss [43]subsequently tried to correlate the performance of different carbonaceous electrodes and the effect of thermal, chemical and electrochemical treatment with the graphitic degrees.They found that carbon electrodes with less graphitic order tended to show less or even negative effect after treatment but electrodes with higher graphitic character showed much better electrochemical behaviour.Even with the same carbon material, different sites can also show different response for vanadium redox reactions.By examining the kinetics on basal-exposed and edge-exposed graphite foil and HOPG electrodes, Pouret al[44]proved that more edge sites can significantly contribute to V2+/V3+and VO2+/VO2+redox reactions especially when the vanadium concentration is low.

Fig.3 The catalysing mechanism of C—OH groups toward the positive side reaction [45]

In addition to the intrinsic properties of carbonaceous materials, researchers are investigating the approaches such as introducing functional groups, catalysts and additives intensively to modify the electroactivity of the electrodes, to reduce the activation polarization for the charging and discharging reactions in VRFB systems.

1.4.3 Introduction of oxygen functional groups

In the early 1990s, Skyllas-Kazacos’s group [45]thermally treated the graphite electrode materials at a range of temperatures and for different times.Using XPS analysis, they found functional groups such as C—OH and C==O increased and suggested they were the contributors to the improved performance of the VRFB with treated graphite felt.They pointed out that the C—OH groups on the electrode surface could be the active sites and a catalysing mechanism for the positive side reaction was put forward as follows (Fig.3).

In this mechanism, ion exchange occurs firstly between VO2+ions from the electrolyte and H+of the phenolic functional groups on the electrode surface.Electrons from VO2+transfer to the electrode with an O atom joining VO2+to formon the electrode surface simultaneously.Finally, the VO2+ion on the graphite surface is replaced by H+from the bulk solution and forms free ions back in solution.

Later, the UNSW group [46]used sulphuric acid of different concentrations and mixed acid of H2SO4and HNO3to treat the graphite felt.Dramatic improvement of the electroactivity was observed as a result of the increased amount of the surface functional groups C—OH and C==O.The C—O—H groups obtained after the treatment were thought to contribute to the increased hydrophilicity of the felt as well as the increasing electroactivity.A catalytic mechanism for the negative half-cell reaction was also proposed (Fig.4).

V3+ions thus exchange with H+of the phenol groups in the first step.Electron transfer takes place subsequently and V+forms at the end of the C—O bond.This then exchanges with H+and diffuses into the solution.

Fig.4 The catalyse mechanism of C—OH groups toward the negative reaction [46]

The formation of these oxygen functional groups were thus shown to provide active sites for the vanadium redox couple reactions, while also enhancing wettability of the carbon electrodes through increased hydrogen bonding with the active sites [45-46].

Although stable under normal charging and discharging conditions in the cell however, both carbon and graphite will slowly oxidise and disintegrate if exposed to very high anodic voltages during extensive overcharge and oxygen evolution at the positive electrode.Careful voltage control is therefore required during operation of the VRB to prevent oxidation of the carbon positive electrodes and maintain long cycle life.Specially for this respect, Skyllas-Kazacos and co-workers [47]investigated the electrochemical behaviour of VRB under overcharge conditions and examined the surface functionality of the graphite felt electrodes.It was reported that after overcharge, the volume resistivity increased slightly, but the cell can increase up to 1.35 ?cm2which can be attributed to the loss of electrochemical activity.The XPS analysis results showed that four types of oxygen groups can be formed during overcharge process, i.e., hydroxide groups (C—OH), ether groups (C—O—C), carbonyl groups (C==O) , ether carboxyl groups (COOH), ester group (COOR) as well as carbonate groups (—CO3–).It showed that with increasing of overcharge time, more higher oxidation-state groups produced on the surface such as C==O, COOH and COOR.It seemed to suggest those higher oxides could inhibit the improvement of electroactivity.But the role of various oxygen groups in catalysing the vanadium redox reactions still remains controversial.

Since the original work was published by Skyllas-Kazacos and co-workers, various other methods such as electrochemical oxidation [48], wet-chemical treatment [49-50], corona discharge [51], plasma etching [52-53]and gamma irradiation [52], etc.have been explored to enhance the performance of the VRFBs by introducing oxygen functional groups onto the surface of carbonaceous electrodes and discussed the influence of content and types of oxygen groups and the variation of surface nature after treatment.Liet alhydroxylated the carbon paper with different content of hydroxyl groups by mixed acids of H2SO4and HNO3and confirmed that—OH did have positive effect on both VO2+/VO2+and V2+/V3+reactions.But when the content of—OH exceeded 11.9%, the carbon paper surface would be damaged seriously, resulting in a negative effect in the electrochemical performance [54].Kimet alinvestigated three treatment methods with different purposes to reveal the correlation of the surface area, functional groups and the electrochemical performance of carbon felts.Mild oxidation was utilized to increase the oxygen functional groups as well as the surface area, while oxygen plasma treatment was employed to change the felt surface physically and γ-ray irradiation was to introduce oxygen groups without much disturbance of the surface area.After comparison with the data of surface area, oxygen group type and the electrochemical behaviour of the treated felt electrodes, it was concluded that voltage efficiency (VE) of VRFB was closely related to surface area and that coulombic efficiency (CE) was affected by the surface functional groups of carbon felts.The phenolic (C—O) groups were beneficial to the improvement of electroactivity towards vanadium redox couples [52].

Oxygen groups can affect the performance in a number of ways.On the one hand, oxidation of the surface of carbon or graphite electrode can produce carbonyl, carboxylic and phenolic groups.Some hydrophilic groups attached to the carbonaceous surface can modify the wettability of the electrolyte to the electrode, increase the effective surface area and facilitate the charge transfer in return.On the otherhand, those functional groups can influence the electrical conductivity of electrodes depending on the carbon atom arrangement order in the original electrode material and the degree of oxidation.In addition, excess oxygen groups can also put electrodes at a risk of disintegration because of easier carbon monoxide and carbon dioxide evolution.Overall, a careful trade-off should therefore be made during the modification by surface oxidation, while good voltage control is also needed during charging to prevent over-oxidation of the carbon electrode surface that could lead to undesirable surface functional groups or oxygen evolution.

1.4.4 Metals and metal oxide deposition

As mentioned before, many noble metals have excellent electrochemical activity for the vanadium redox couple reactions in VRFBs.However, in consideration of the cost, they are more suitable for modification of the carbonaceous electrode surface rather than acting as the actual electrode itself.In the patent issued in 1987, Skyllas-Kazacos claimed that similar effects can be achieved by using carbon electrodes impregnated with the corresponding metals or metal ions [55].Later, Sun and Skyllas-Kazacos partially oxidised graphite fibres by cation-exchange with surface functional groups in vanadium acidic solutions containing Pt4+, Pd2+, Au4+, Mn2+, Te4+, In3+and Ir3+and then vacuum drying .Results showed that ions like Mn2+, Te4+and In3+did not exhibit much catalytic effect while Pt4+, Pd2+and Au4+enhanced the hydrogen evolution.On the other hand, Ir3+modification showed the greatest improvement in the vanadium redox couple reversibility [56].

In a later study, Wanget alsubsequently coated carbon felts with Ir metal (99.77%, by mass) by repeated immersing in H2IrCl6ethanol solution and thermally treating in the air at 450 ℃ for 15 min.By using this method, Ir metal demonstrated great coherence to the graphite fibres even after 50 charge-discharge cycles.The overpotential of VO2+/reaction and the cell resistance was decreased significantly because of the activation of Ir metal.The energy efficiency was also increased by 5.2% [57].

However, since noble metals are always accompanied by the disadvantage of high cost and increased hydrogen evolution at the negative electrode during charging, researchers have continued to find cheaper replacements.Santamaría and co-workers utilized Bi nanoparticles to modify the electrode in the positive side of VRB by immersing the felt into saturated solution of Bi2O3in 0.01 mol/L HCl.It is reported that excellent electrochemical performance in terms of increased peak current densities, better reversibility (peak separation of 0.05 V at 1mV/s) and long term stability (100 scan) can be obtained at the same time [58].

Transition metal oxides can be applied as electrocatalysts due to their ability to change between their different valency states [59]and their potential ability to absorb reactive species as active centre [60].As mentioned before, in the early screening of electrode materials for VRBs, a titanium electrode with iridium oxide on the surface (known as dimensional stability anodes, DSA) was tested by Skyllas-Kazacos and co-worker and showed excellent electrochemical performance and stability [39].Later Kimet alfiled a patent on graphite/DSA assembled electrode by rolling method for VRFBs in 2013 and claimed it can enhance the electrode lifetime, chemical stability, power density, energy efficiency and cycle performance.Again, high cost became the main factor hindering its broader application [61].Therefore, decorating carbonaceous electrodes with transition metal of low cost and high catalytic ability may be a promising way for modification.

Kimet alalso introduce Mn3O4onto carbon felts by a hydrothermal method.Carbon felt was immersed with manganese acetate solution in an autoclave and heated at 200 ℃ for 12 hours.After this reaction, Mn3O4was successfully coated onto the surface of the carbon felt.However, these nanoparticles could not remain stable during the single cell test.They therefore heat-treated the modified carbon electrodes at 500 ℃ in Ar atmosphere after the hydrothermal reaction.This time Mn3O4proved to be well attached even after more than 20 charge-discharge cycles.TheCV results showed that after Mn3O4modification, the carbon felts demonstrated better reversibility and electrochemical activity toward both the V2+/V3+and/VO2+reactions, but the more remarkable improvement was for the/VO2+reaction.In the cycle test, the vanadium redox flow cell employing Mn3O4modified carbon felts as both positive and negative electrodes exhibited increasingly higher coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) with the increasing cycle number compared to the cell using bare carbon felts.The researchers also compared the function of Mn3O4by using the modified electrode only as the positive or the negative electrode and found that Mn3O4was more effective in/VO2+reaction.They believed the improvement of electrochemical performance with Mn3O4was due to the better hydrophilicity of the carbon felt surface and the lower activation barrier for the vanadium redox reactions [62].Afterwards, several kinds of nano-sized metal oxides were introduced to the surface of graphite or carbon felt such as WO3[63]and Nb2O5[64]by similar hydrothermal method.However, although different kinds of metal oxide crystals of high quality can be prepared by using the hydrothermal method the demand for high vapour pressure during the process makes it unsuitable for modification on a large scale.

In the patent of Skyllas-Kazacos and co-workers, Pb ions displayed good catalytic ability toward the positive VRB half-cell reaction and high overpotential for oxygen evolution.Moreover, Pb ions can suppress hydrogen evolution as well [55].Accordingly, Wuet alemployed pulse electrodepostion to coat PbO2particles (2 μm, a mixture of α-PbO2and β-PbO2) onto the surface of graphite felt as a positive electrode for VRBs.From their research, there was no obvious change in the peak potential but an increase in peak current for theand VO2+redox reaction could be observed in the cyclic voltametric (CV) results.The charge transfer resistance obtained from the electrochemical impedance spectroscopy (EIS) measurements of PbO2-modified electrode was 1.85 ? less and an increase of 2.6% in voltage efficiency was demonstrated in the single cell cycle test at 70 mA/cm2.In the later charge-discharge test at different current densities and long-term cycle test (30 cycles), the modified cell maintained a good performance.It can be seen that pulse electrodeposition is a good way to coat metal crystals stably onto the surface of graphite felts, but the improvement of the electrochemical performance is limited.More investigation in crystal size and structure of PbO2might be needed [65].

CeO2modification has captured the attention of researchers recently in a variety of electrochemical applications because of its abundance and the transformation between Ce3+and Ce4+oxidation state which may promote redox reactions.Furthermore, the oxygen vacancies in the fluorite structure offer CeO2higher oxygen mobility, which might be advantageous for the VO2+/reactions.Thus, Xiet alprepared CeO2decorated graphite felts by precipitation and calcination.Enhanced hydrophilicity, improved reversibility, increased voltage and energy efficiencies and good stability in single cell test were demonstrated after modification.A catalytic mechanism was proposed as following equations (1—2) and Fig.5.During the transformation between Ce4+and Ce3+(equation 1), Ce active sites are occupied by adsorbed hydroxyl group (equation 2).Then ion exchange occurrs between vanadium ions and hydrogen ions.With the electron transfer, VO2+converts toat the surface and diffuses to the solution subsequently.The abundant hydroxyl groups on the surface of CeO2accelerates the electron and oxygen transfer faster, resulting in a higher electrochemical activity [66].

In general, it is promising to look for high electroactive metals or metal oxides of low cost to decorate the carbonaceous electrode in a way that is easy for industrial large-scale production.

Fig.5 The catalytic mechanism schematic diagram of CeO [66]

1.4.5 Mesoporous carbon and carbon nanomaterials

Mesoporous carbons and nanomaterials are regarded as promising candidates to catalyse the vanadium redox reactions due to their high surface area, good chemical stability under acidic conditions and flexibility of functionality by introducing oxygen or nitrogenous groups, loading metal or metal oxide nanoparticles and doping different elements.

（1）Mesoporous carbon

Porous carbons have been proposed as promising electrodes for electrochemical devices due to their great catalytic properties and light density.A rapid screening method was developed by Walshet alto evaluate activated carbon particles (ACPs) for RFB electrodes by mounting the powders into a thin-layer packed bed electrode.They measured the CV behaviour of this electrode in aqueous sulphuric acid and from the charge envelope in the non-faradic region at controlled scan rates they estimated the capacitance per unit area of various ACPs.The significant specific capacitance value they obtained indicated that almost all the micropores within the ACPs were utilized.Afterwards, they carried out repetitive potential step experiment to characterize the chemical-electrochemical reaction rate and constant current cycling to record the overpotential variation with time increasing.The facilitated faradaic reaction rate suggested that the porosity was beneficial for the reaction but the real active surface was mainly contributed by larger meso/macropores [67].

In the report of Shaoet al, mesoporous carbon was synthesized by soft-template method to investigate its electrochemical behaviour towards VO2+/reaction.However, its onset potential was even lower than graphite and no obvious redox peak could be observed in the CV measurement.The larger arc radius in the high frequency range also indicated the reaction resistance of mesoporous carbon is greater than graphite.It seemed that pure mesoporous carbon cannot catalyse the vanadium redox reactions from this report.However, several articles were published and showed great improvement in electrochemical properties by coating carbon paper with mesoporous carbon [68].

Zhanget alused Nafion as binder to coat a commercial super activated carbon (SAC) with high surface area (2900 m2/g) and pore cubage (1.588 mL/g) onto a SGL carbon paper.The electro-catalytic ability towards both positive and negative reactions was notably improved in terms of peak separations and peak currents.The charge transfer resistance was reduced remarkably as shown in the EIS measurement.The VE of the single VRFB was increased from 69.7% to 79.5% after coated with SAC.They owed this improvement to more active sites provided by SAC because of its high surface area, large pore volume and less graphitic structure.They further used SAC as supporting material to make WO3/SAC composite catalysts for the same carbon paper and more improvement was obtained [60].

In a recent article published by researchers from Singapore, mesoporous carbon exhibited superior performance in the VRFB with multicouple reactions.By using coconut shell as precursor, they prepared mesoporous activated carbon with BET specific surface area of 1652 m2/g about and coated them onto the Toray carbon paper with PVDF as binder.The raw Toray carbon paper showed great electrochemicalactivity to VO2+/reaction but negligible function to V2+/V3+reaction.After coating with the bio-mass derived mesoporous carbon, the modified electrode demonstrated significant improvement in catalysing the negative reactions.Moreover, the reaction peak of V3+/VO2+can also be observed when the scan rate was not greater than 15 mV/s, which indicated a good reaction kinetics and excellent reversibility.This modified electrode was further tested in a static vanadium redox batteries and an increase of 31% in the energy efficiency was achieved.The high surface area and mesoporosity derived from coconut shell were believed to be the main contributor to the improvement in the electrochemical performance [69].

Compared to graphite felt, carbon paper is much thinner and lacks enough active surface area.Mesoporous carbon can provide this and enough flow channels for the electrolyte solution as well.Therefore, this method can be expected to obtain great enhancement in the electrochemical properties of carbon paper, which may be a promising electrode for VRFB with zero-gap architecture.

（2）Carbon nanotubes

Zhuet alinvestigated the electrochemical behaviour of carbon nanotubes (CNTs) in vanadium electrolytes for the first time.They found pure CNTs were not so active towards the redox reactions of vanadium ions, but if CNTs were composited with graphite when their content was less than 5% (by mass), the reaction of VO2+/was highly catalysed compared to the bare graphite.The CV results of graphite electrode modified with CNTs under different conditions demonstrated that the anodic reactions became reversible on the electrode surface with 5% (by mass) CNTs treated at 200 ℃ in vacuum [70].Later, several investigations were carried out regarding CNTs with different types [71,108-109]and functionalities [72,111].

Yanet alcoated a glassy carbon electrodes (GCEs) with MWCNTs, hydroxyl MWCNTs and carboxyl MWCNTs respectively and reported the peak separation value of the positive VO2+reaction was in the order:hydroxyl MWCNTs/GCE < carboxyl MWCNTs/GCE

It is noteworthy that Mench and co-workers fabricated nanoporous layers (NPL) consisting of multiwall CNTs (MWCNTs) on carbon paper (without any binder) and examined the polarization behaviour.They filtered the supernatant of the sodium dodecyl sulfate (SDS) – MWCNT solution to prepare a thin film (about 4 μm) of MWCNTs and then put this layer (NPL) in four different places of a single-serpentine flow channel VRFB (as shown in Fig.6).By comparing the polarization data for charging in the VO2+/H2SO4solution under constant current (0.5 ?) and then constant voltage (1.8 V) until the current was smaller than 5 mA, the greatest improvement was observed when NPL was placed onto the surface of negative electrode facing the flow field (labelled as –ve NPL│FF), where the power density and cell voltage were increased by 8% and 65 mV respectively.They claimed that the enhancement was attributed to the increased active surface area of NPL since the 40 mV improvement in cell voltage cannot be contributed by ohimc loss only [73].This requires further verification however.Since it was the first time that a discrete carbon NPL was used in VRBs, more investigation is still needed.

Although modification with CNTs can increase the surface area and electrical conductivity of the electrode and thus achieve a better electrochemicalperformance toward vanadium redox reactions, however, metal residuals (such as Ni-based catalyst derived from the primary synthesis of CNTs) may lead to hydrogen evolution in the negative side of the cell and this requires further long-term investigation.

Fig.6 A schematic diagram of NPL orientation (FF:flow field, mem:membrane) [73]

（3）Graphene-based carbon

Graphene was first introduced into the modification of electrodes for VRFBs by Tsaiet alThey mixed different contents of thermally reduced graphene oxide with graphite and polyvinylidene fluoride (PVDF), coated the mixture onto Pt slice and measured their CV performance.It showed that the reversibility of VO2+/reaction was enhanced when the content of graphene was less than 5%(by mass) and the best improvement was obtained when the contentwas 3%(by mass)[74].The electrochemical behaviour of graphene oxides (GrO) dried under different temperatures was characterized by Hanet al.It was found that graphene oxides can improve not only the VO2+/but also the V2+/V3+redox couple reaction.Though GrO dried under 50 ℃ possessed the lowest conductivity and the smallest BET specific surface area, it still showed the best enhancement in CV behaviour.The author thus attributed this effect to the abundant oxygen groups of GrO dried under 50 ℃ [75].

Subsequently, Blanco and co-workers published a series of articles based on graphene in VRBs [76-78].Firstly, they prepared graphene (TRGO) which was thermally reduced from graphite oxide (GO) at different temperatures and compared the CV behaviour with GO.They believed the restoration of sp2domains after thermal reduction leads to the remarkable improvement of TRGO [76].Afterwards, they thermally reduced two kinds of GO into graphene (TRGO) and found the TRGO with higher sp2content showed better modifications as positive electrode in terms of catalytic ability, charge transfer resistance, CE and EE [75].They also examined the performance of two kinds of graphene, TRGO and TRGrO, which were thermally reduced from graphite oxide and graphene oxide respectively.The result showed that TRGO presented lower onset potential, less peak separation and higher current densities.Again, the author claimed it was the better restored graphitic structure that renders the better performance for the TRGO than TRGrO [78].Thus it can be seen that electrical conductivity (influenced by the sp2content or graphitic structures) is critical to the electrochemical behaviour in VRFBs.

Regarding the role of oxygen functional groups and electrical conductivity, Yanet alreduced GO electrochemically by different extents.By applying various negative reduction potential from –0.8 V to –1.6 V, they obtained ERGO (electrochemically reduced graphene oxide) with different O/C ratios.Through the comparison of CV results, ERGO obtained at –1.4 V demonstrated best catalytic ability toward both positive and negative reactions.The content of C—O (including C—O—C and C—OH) decreased dramatically when the applied reduction potential was –0.8 V, but the electrochemical catalytic performance revealed in CV showed much improvement.Thus C—O functional groups were confirmed to be not so essential in catalysing process.For the same reason, C==O (containing—COOH, C==O, COOR) started to be reduced at –1.2 V, whereas the ERGO obtained when the reduction potential was lower than –1.2 V showed slightly increase in the peak separation of VO2+/reaction.Because with the increasing of negative reduction potential, carbon atoms became more ordered, which implied better electrical conductivity, the reduction of C==O was believed to cause the decay of electroactivity.Therefore, the significance of C==O in the positive redox process was clearly confirmed.Interms of the negative reactions, C—O also presented negligible effect, whereas the hydrogen evolution was activated, when the reduction potential was below –1.2 V.A mechanism of the function of C==O towards vanadium redox reaction was proposed (Fig.7) [79].

Fig.7 Schematic mechanism of the C==O functional groups towards vanadium redox reactions [79]

As discussed before, edge planes in graphite layers are believed to be more active than basal planes due to the facilitated electron transfer.Recently Blancoet alused radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) to synthesize carbon nanowalls, graphene layers aligned vertically on a substrate (Fig.8), as positive electrodes for VRFBs.The overall electrochemical responses to the VO2+/were good and more optimization can be achieved through adjusting the intermediate thickness and edge number of the nanowalls [80].

Similarly, Choet altried dry-ice assisted ball milling method to selectively functionalize the edge of graphene without bare defects in basal plane and compared its cyclic voltammetry and cycle behaviour with that of conventional hydrazine-reduced graphene oxide.The former one exhibited superior electroactivity and cycle performance toward vanadium redox reactions compared with the latter.It was claimed that graphene materials with oxygen functionalized-edges and defect-free basal planes were the main contributors to the enhanced peak current density and charge-discharge transfer [81].

（4）Nitrogen doping and nitrogenous groups

Doping is a method typically applied in the semiconductor industry by introducing impurities into an ultra-pure semiconductor material to adjust the electrical properties.Recently, however, nitrogen- doping of carbon materials have been proved successfully in catalysing many electrochemical reactions [125].Shaoet al[68]were the first to extend this method to the VRFB system.They heat-treated the mesoporous carbon in NH3environment at a temperature of 850 ℃ to realize the doping of nitrogen atoms and achieved a much better electrochemical performance with the modified electrode than the untreated graphite, which might be contributed by three factors.Firstly, the five electrons of nitrogen atoms would lead to extra charge to the π bond in graphene layers and thereby both the basicity and electrical conductivity of carbon were increased.Secondly, the chemisorption mode of V/O atoms might be varied, thus lowering the activation energy for V—O bond formation and breaking.Lastly, the hydrophilicity increased after N-doping.Subsequently, several studies by similar method to obtain better electrocatalytic carbon electrode materials for VRFBwere reported [111-113].Nevertheless, one disadvantage of this method is that NH3is toxic and the prevention of gas leakage can be complex and difficult.

Fig.8 The proposed mechanism of carbon nanowalls towards VO2+/VO2+ reactions [80]

Other researchers [82,114-115]employed hydrothermal method to introduce nitrogenous groups to carbon materials and found these modified electrodes possessed excellent wettability and catalytic activity toward the VO2+/redox couple reaction.Specifically, Zhanget al[82]and Liuet al[83]found that among the four types of nitrogenous groups, pyridinic-N, pyrrolic-N, quaternary-N and oxidic-N, the third one is the main contributor to the enhanced properties.Notably, hydrothermal method is a much milder method and requires lower temperature, but its mass-production meets some challenge.

Recently, zein, the major protein that exists in corn, with several advantages such as high abundance, low cost, and self-assembly ability for hierarchical structures, etc.was used to synthesize highly active, electrocatalysts for VRFBs via a facile solution method.Zein powder and carbon black (CB) were mixed in ethanol and water solution and then dried at 60℃ to wrap CB with ZB film.After a high temperature treatment at 700~900 ℃ in Ar atmosphere, the obtained electrocatalysts were characterized as exhibiting high surface area, large quantities of oxygen groups, nitrogen-doped coating networks as active sites and high electron conductivity.The graphite felt with these N-doped CB particles exhibited much higher electroactivity than untreated graphite felt (GF), oxidized-GF as well as CB coated-GF [84].Dopamine, another environmental friendly source of nitrogen, was proposed by Kimet alrecently to coat onto graphite felt with abundant amine groups.Pristine graphite felt is immersed in dopamine solution (pH of 8.5) at 45 ℃ to complete the in-situ polymerization.Then the felt goes through a thermal treatment at 900 ℃ in Ar to be pyrolyzed.Compared to oxidized graphite felt, the N-doped felt enabled the VRB higher discharge capacity as well as energy efficiency when the current density was 50 mA/cm2to 150 mA/cm2[85].Introducing nitrogenous group via bio-derived sources catalysing in a feasible way showed great promising in modification of the electrodes for VRBs due to their cost-effectivity, safety and eco-friendliness.

（5）Nanocarbon composites

Hybrid carbon-based composites, for instance, graphite/CNTs [70], graphite/GO [86], GO/CNTs [87], CNT/CNFs [116-117], are capable of constructing different dimensional structures (pores or channels), modulating the contents of functional groups and electrical conductivity, and thus were investigated intensively for VRFB catalysis by various groups.Those composites were prepared by either physically mixing or in-situ CVD method.

In addition, carbon nanomaterials can be good substrate materials to support metal or metal oxide nanoparticles to catalyse the vanadium redox reactions [88-94,117,119].

Apart from monometallic nanocatalysts, nano-sized metal alloys such as CuPt3[88-89], and RuSe [90]can also reduce polarization and improve the reversibility of VO2+/reaction.As reported by Flox and coworkers, Cu-Pt alloy demonstrated better electroactivity than Pt on graphene and it was probably due to the Pt-Cu bicomponent catalytic activity and its enhanced active surface area [89-90].Considering chalcogenide (S, Se and Te) modified metals can be effectively catalyse some electrochemical reactions [126], the catalytic effect of RuSe in VO2+/reaction was explored by Cuiet alwith reduced graphene oxide as the dispersion substrate and results showed that RuSe can remarkably enhanced the reversibility and reduced the polarization of the VRFB single cell [90].

TiO2possesses a good water adsorbing ability and thus was investigated by Hsueueh and Shieuet alThey used a hydrothermal method to synthesize the TiO2nanoparticles and mixed them with CB particles in the composite electrodes.It was reported that the wettability was increased with the increasing content of TiO2.Besides, both the specific capacitance of the electrode and the efficiency of the cell with theTiO2/CB layer placed between the membrane and the negative carbon felt electrodes were promoted [93].Later,the same group published another electrochemical behaviour study of such electrodes at high current density (200 mA/cm2) and found good potential in the application of TiO2/CB as the negative catalysts [94].

Briefly, compositing can be regarded as a good way to enhance the electrochemical response of an electrode to vanadium reactions because of their numerous possibilities and potential synergetic effects.More work can be done to find a more optimized combination.

1.4.6 Electrolyte additives as electrocatalysts

In the patent filed by Skyllas-Kazacos in 1987, the use of electrolyte additives in VRFBs to enhance the vanadium redox couple reactions and suppress hydrogen evolution was proposed.Adding traces (1×10–6) of Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and /or Ag salts to the positive electrolyte can induce the corresponding metals or metal oxides to deposit on the surface of the carbonaceous electrodes during cycling, thus providing more active sites for the VO2+/reaction.In the anolyte, hydrogen evolution can be inhibited by adding traces (1×10–6) of Pb, Bi, Tl, Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg.It is stated that adding additives can achieve the same effect as impregnating electrode with relevant metals.Results from CV measurements confirmed that the kinetics of VO2+/reaction were facilitated remarkably by Au and Sb but to a less extent by Pb, Te and borax and a significant enhancement of H2overpotential was observed with Pb, Sb, Te and borax [55].

In an investigation of the modification of graphite felt by Bi nanoparticles for VRFBs, Wanget aladded a small amount of BiCl3directly into the electrolyte and conducted a series of CV test.It showed that VO2+/reaction was hardly affected while the reversibility of V2+/V3+reaction was enhanced by the Bi additives.Besides, after several cycles in the charge-discharge test, Bi nanoparticles were found to be attached only to the negative carbon electrode.They believed it was due to the standard potential of Bi/Bi3+which lied between that of VO2+/and V2+/V3+reactions, Bi metal would be formed before the reduction of V3+to V2+and it was suggested it was the Bi metal rather than Bi ions that contributes to the improved kinetics of V2+/V3+reaction.The investigation also showed the charge and discharge capacity, VE and EE were greatly enhanced with the Bi additives.In addition, no obvious decay in EE was observed after 50 cycles under the current density of 50 mA/cm2[95].Recently in the study of Sb3+additives in VRFB systems, similar effect was also achieved [96].

Electrolyte additives were also applied in VRFB systems as stabilizing agents by the UNSW group to inhibit precipitation of vanadium species in sulphuric acid for higher energy density [119-121].These additives and stabilizing agents included glucose, inositol and other organic materials containing groups of—OH, ==O,—NH and—SH [98].These additives were further studied by groups from Central South University who found that some organic compounds such as D-sorbitol and inositolcan also promote the electrochemical activity of vanadium redox couple.Nevertheless, the improvement is not so significant [99-100].

It seems that adding metal additives are more efficient than organic ones in improving the electrochemical performance of VRFBs.Moreover, compared to the other metal-based modification, adding metal ions in electrolyte is quite simple.Therefore, it is worthwhile to further explore the effect of metal additives, confirm the stability of the cell performance and ensure no adverse effects of the additives in the electrolyte at the same time.

1.4.7 Photocatalyst in all-vanadium photoelectrochemical cell

Weiet alcombined the all-vanadium flow battery and photoelectrochemical cell together for the first time in 2014.The cell utilized sunlight to activate a TiO2semiconductor and in the meantime, the holes generated oxidize VO2+toand the electrons reduce V3+to V2+.The chemical energy retained can be later transferred into electricity in the dischargeprocess.They explained it was because the band structure of TiO2can fit due to the relatively low thermodynamic requirement and overpotential of V3+/V2+compared to the hydrogen evolution [101].Later,the same group fabricated another photochemical cell with similar principle but employed a WO3/TiO2hybrid electrode instead.The results showed that by using this kind of hybrid electrode, a synergetic effect of electron storage and vanadium solutions might contribute to excellent reversibility, high capacitance and dramatically enhanced photocurrent [102].

1.4.8 Structural design

Fig.9 The architecture of “zero-gap” RFB [103]

With their very high surface area, the effective current density on the porous felt electrodes is very low, so activation polarisation tends to be relatively small component of the voltage losses in the VRB.Given that the carbonaceous felts also provide the transport channels for the reactants in this architecture design of the VRB system however, mass transport polarization is an important issue that can affect voltage efficiency as well as the pressure drop through the cell stack that in turn influences pumping energy losses.Several investigations have been carried out with regard to the influence of the thickness, porosity, compression, flow field presence of carbon or graphite felt in the overall performance of VRFB [122-123].

Most suitable carbon and graphite felts tend to be 3~5 mm thick, so high ohmic losses are experienced due to the relatively large anode-cathode distance in this type of cell architecture.In recent years however, the concept of the “zero gap” cell architecture (Fig.9) has been explored by several groups, adapting similar designs as used in fuel cells [103,124].

While dramatically reducing ohmic polarisation losses however, the zero-gap cell designs employ very thin carbon or graphite papers that have a greatly reduced surface area that leads to increased polarisation at the high operating current density.In addition, the increased dead volume will lead to declining energy efficiency as well.To address this problem, Liuet alpublished a patent on a new designed electrode structure which consists of a membrane, graphite papers, graphite felts with multi-strip structures and graphite polar plates (Fig.10).Because of the large surface area of the graphite felts and the filling of the cavity of the channel, the concentration overpotential and dead volume can be decreased, which contributed to an increased energy efficiency of VRFBs [104].

1.4.9 Catalyst coated membrane

Early in 1979, Pelligriet alapplied a patent describing a redox accumulator for liquid phase electrochemical devices to reduce the ohmic drops and to improve the power density.In the accumulator, anode and cathode were separated by an ion exchange membrane coated with thin, porous, electrically conductive and relatively chemical stable powders.In their example using Cr6+/Cr3+redox couples, graphite and platinum black were mixed to make the electrodeand then bonded to an anionic membrane.When the flow rate was 10 cm/min and charging current density was 0.1 A/cm2, the charge was extended to approximately 40 W·h/kg in the chromium system.The power density reached 5.8 kW/m2and the energy efficiency was around 90% [105].

Fig.10 Scheme of a simplified electrode assembly [104]

Later in 1993, a patent on membrane-electrode assemblies (MEA) for electrochemical cells was published by Swathirajanet alTwo layers of electrodes and ion conductive materials were coated onto or partially embedded into the membrane.The first layer of electrode is mainly composed of finely divided (90~110 ?) carbon powders with better hydrophilicity, water retention properties and a pH value within 6~7, while the second layer is made of catalytic particles supported on/in carbon particles with smaller size (60~80 ?), higher pH (8~10) and less wettability.This kind of design was aimed at enhanced catalyst utilization [106].

Actually, MEA or catalyst coated membranes (CCM) have been widely published especially for fuel cells [127].However, only few studies have been carried out for VRBs.Zhanget alintroduced the concept of CCM into VRBs recently [107].Since WO3/SAC has been proved to be catalytically active in both the positive and negative reactions and has demonstrated good performance in a single flow cell when coated onto carbon paper [60], Zhang fabricated a WO3/SAC-coated membrane by the spraying method with Nafion as binder.The performance of the CCM was tested in a cell in which the WO3/SAC catalyst layer was applied between the graphite felt and membrane, and the VE and EE obtained at the current density of 120 mA/cm2were 81.3% and 76.9% respectively.However, when the cell was assembled using current collectors (graphite plates with carved flow field) and the CCM, the VE and EE increased to 85.9% and 81.3% at the same current density.Besides, the almost constant values of EE during 300 charge and discharge cycles confirmed the good stability of the CCM [107].

It is believed that CCMs can enhance the catalyst utilization and extend the interface between catalyst and membrane, and in comparison with coating catalysts on graphite felt or paper, coating on the membrane is proposed to form a compacted catalyst layer, providing better mechanical stability and facilitating the electrochemical reactions.Nevertheless, more reliable evidence is still needed for CCMs and MEA systems in the VRB.This is therefore an area where significant performance improvements could be achieved, allowing high power density VRBs to be developed, leading to considerable stack cost savings as well.

2 Conclusions

In this review, we have briefly introduced the electrode material selection criteria and have provided a historical overview of electrode materials screening, modification and electrocatalysis for Fe/Cr, polysulfide/bromine and VRB systems.Other bromide-based flow batteries were also reviewed.Currently, there is no perfect modification and electrocatalytic method but effort continues to be devoted by research groups all around the world to the development of highly electroactive electrodes that can achieve high power densities in RFB systems.It is clear that an ideal electrode for RFBs should combinehigh electrochemical activity for the redox reactions to minimize activation polarization, with excellent electrical conductivity to reduce ohmic losses, good porosity or channel structure with high active surface area to decrease mass-transfer polarization and pumping energy losses, good chemical, mechanical and electrochemical stability in the flowing acidic electrolyte, good wettability and reasonable cost.

The VRB is one of the most promising RFB technologies, and thus intensive research has been continuously focused on electrode modification and electrocatalysts.Methodologies such as the introduction of oxygen functional groups, deposition of metals or metal oxides, fabrication of composites with carbon nanomaterials, doping heterologous elements, adding active species into the electrolyte and utilizing catalyst-coated membrane have been discussed in detail.

Introducing a certain amount of surface oxygen functional groups can enhance the wettability and electroactivity of carbonaceous electrodes, but under some conditions, this can introduce problems such as inferior electrical conductivity and electrochemical stability (under overcharge conditions).In addition, facile and controllable oxidizing methods are still necessary in order to obtain reproducible results.Many metal and metal oxides exhibit excellent catalytic activity towards the vanadium redox reactions.However, hydrogen evolution overpotential, cost-effectiveness, mechanical and chemical stability and scalable synthesis methodologies should never be ignored when characterizing the modified-electrodes.

Carbon nanomaterials can be flexibly tailored in terms of surface area, functional groups, electrical conductivity and compositing with other materials to achieve better electrochemical performance.Doping heterologous elements like N can promote the electrical conductivity, wetting property as well as electroactivity of electrodes, but more environmentally-friendly methods are needed.Other dopant elements such as P, B and S can also be explored.Adding metallic additives directly into the electrolyte is a simple way to catalyse the reactions, but potential side effects such as increased gassing rates should be avoided.More importantly, it is difficult to compare published work to identify the best catalysts for vanadium redox reactions, since in each study, improvements were characterized by different methods or under various conditions.Evaluation studies under standardised conditions are therefore essential.

Catalyst coating of membranes (CCM) is a direction worthy of further investigation for implementation in the zero-gap cell structure recently proposed for the VRB and shown to achieve dramatic improvements in power density and cell efficiencies.Despite its commercial implementation in many stationary applications to date, further improvements in the performance of VRB with higher power density, lower cost and reduced stack size will help to expand its potential use in a wider range of applications in the future.

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