Advancements and Applications of Redox Flow Batteries in Australia

Abstract

Redox flow batteries (RFBs) are known for their exceptional attributes, including remarkable energy efficiency of up to 80%, an extended lifespan, safe operation, low environmental contamination concerns, sustainable recyclability, and easy scalability. One of their standout characteristics is the separation of electrolytes into two distinct tanks, isolating them from the electrochemical stack. This unique design allows for the separate design of energy capacity and power, offering a significantly higher level of adaptability and modularity compared to traditional technologies like lithium batteries. RFBs are also an improved technology for storing renewable energy in small or remote communities, benefiting from larger storage capacity, lower maintenance requirements, longer life, and more flexibility in scaling the battery system. However, flow batteries also have disadvantages compared to other energy storage technologies, including a lower energy density and the potential use of expensive or scarce materials. Despite these limitations, the potential benefits of flow batteries in terms of scalability, long cycle life, and cost effectiveness make them a key strategic technology for progressing to net zero. Specifically, in Australia, RFBs are good candidates for storing the increasingly large amount of energy generated from green sources such as photovoltaic panels and wind turbines. Additionally, the geographical distribution of the population around Australia makes large central energy storage economically and logistically difficult, but RFBs can offer a more locally tailored approach to overcome this. This review examines the status of RFBs and the viability of this technology for use in Australia.

1. Introduction

Currently, society is facing an environmental and energy crisis [1,2,3]. There are many factors that are contributing to this, including the use of fossil fuels as a source of energy [4,5]. Burning fossil fuels, such as coal, oil, and natural gas, to produce energy causes air and water pollution. Fossil fuels generate a large amount of greenhouse gas (GHG) emissions, such as carbon dioxide (CO₂) [3,6,7], methane (CH₄), and nitrous oxide (N₂O) which, in addition to chlorofluorocarbons (CFC) and water vapour, form the five key greenhouse gases [8,9]. Their presence traps the sun’s heat in the earth’s atmosphere and slows down heat loss to space.

To meet increases in energy demand while reducing GHG emissions associated with electricity generation, the world needs to transition to more environmentally friendly energy sources [3,10]. This transition should be as smooth as possible to ensure minimal disruptions to industry and the global economy, as well as maintaining or improving standards of living.

Renewable energy sources such as wind and solar have been utilised as alternatives to fossil fuel-based energy sources and are increasingly being integrated into the global energy resource [3,11,12], which is helping to reduce the dependency on fossil fuels as the primary source of energy. In 2023, wind, solar, and other renewables, including hydro-based-energy sources, accounted for 36% of total energy supply globally [13,14]. The transition to renewable energy generation is expected to accelerate significantly in the coming years, with wind and solar increases from 2018 to 2050 predicted to be 14-fold and 30-fold, respectively [13]. Australia is rapidly adapting to renewable energy and currently has the largest residential photovoltaic (PV) system uptake globally, with 21% of residential homes having rooftop PV panels [15].

A notable drawback of this technology, however, is that renewable energy sources are irregular in energy generation, and there is therefore a need for reliable energy storage systems (ESSs) or electrical energy storage systems (EESs) with high safety and low cost. ESSs (or EESs) are a method of converting electrical energy from a power generating system into a form that can be stored then converted back to electrical energy when required. ESSs store the excess energy generated from renewable sources when it is in surplus and feed it back into the power grid when needed, such as during peak hours or when no energy is being generated [3,16,17]. ESSs should be regarded as essential infrastructure for the energy transition. A variety of different types of ESSs are available with the selection of a specific type depending on location, demand, capacity, and costs of investment (Figure 1).

The number of photovoltaic panels currently installed in Australian households is very large. However, despite the large amount of electrical energy generated by residential PV systems, these systems can be a threat to the stability of the overall electricity grid and when the energy provided to the grid by these resources surpasses 10–20% of the overall energy production [18,19]. In recent years, solar farms have been required to shut down when there is an oversupply of energy [20,21]. A substantial network of distributed energy storage systems could solve many of these issues with Australia’s current renewable energy system. In addition, energy storage systems are essential where there is night-time use of electricity in houses, health and aged care facilities, and any business with night-time operations that only gets its electricity from solar PV [3].

There are several technologies (Figure 1) that store electrical energy and can later return it to the grid. These energy storage systems are classified as electrochemical (batteries and supercapacitors), chemical, electrical (capacitors and superconducting magnets), thermal, and mechanical (pumped hydropower (PHS), fly wheel, and compressed air) [22].

Every energy storage system has challenges, but batteries, in general, which are electrochemical storage systems, are the most developed and flexible form of energy storage [23]. Even though PHS is considered, in certain circumstances, the most developed electricity storage technology, its utilisation is very limited [24]. Batteries are very fast to respond to demands for power not met by renewable energy generation, and they provide stability when integrated into the main electricity grid better than other energy storage systems. The calculated return on capital investments for batteries may be ten years or more [25], but this may reduce in the future as the cost of battery components reduces.

Figure 1. Classification of the principal energy storage systems (adapted from [26]).

Figure 1. Classification of the principal energy storage systems (adapted from [26]).

An increasing demand for ways to store the energy produced from renewables has led to a rapid advancement in battery technology. Flow batteries are considered promising in the large-scale commercial and grid-scale storage markets [3,27,28,29]. The recent installation of redox flow batteries (RFBs) alongside solar PV by Yadlamalka Energy [30,31,32], a first of its kind in Australia, is an excellent example of use of this technology. Australian company, Thorion energy, partnered with SMEC Power and Technology to trail the use of its redox flow batteries at mine sites in Western Australia in 2023 [33]. Capital cost analysis indicates that RFBs are cost-effective for long discharge duration applications [34] and have long cycle lives [35], making them an excellent choice for large-scale or remote energy storage. Recently, a 78 kW/220 kWh vanadium redox flow battery was deployed by Horizon Power in Kununurra, Western Australia. This battery, manufactured by Invinity Energy Systems, was supplied and commissioned by VSUN Energy [36].

2. Redox Flow Batteries (RFBs)

2.1. Basic Construction

The origins of the concept underlying redox flow batteries (RFBs) can arguably be traced to 1884 with Renard’s chlorine-chromium battery, developed for “La France” airship propulsion. However, this device was a primary cell and did not utilise electrolyte flow [37], distinguishing it from the modern RFB architecture. A process for storing electrical energy in liquids was patented as early as 1949 by Walther Kangro [3,38,39]. Several groups worldwide are currently working on the development of RFBs. Full-scale development of RFBs started in the 1970s. The principle of the RFB system was presented by L. H. Thaller in 1974 based on research conducted at the National Aeronautics and Space Administration (NASA) and focusing on the iron chromium (Fe/Cr) system [40]. The word “Redox” is an abbreviation of reduction and oxidation and describes the reactions that occur in these batteries. RFB liquid electrolytes contain electroactive redox species dissolved or suspended in a supporting electrolyte which is used to increase conductivity. The electrolyte is always kept in two separate tanks and is propelled through the electrochemical cell or the battery stack using pumps. A cooling system is also needed as charge and discharge of RFBs involves heat release [3]. Oxidation and reduction reactions between the two electroactive species occur on the surface of electrodes, where the electrical energy is converted to chemical energy during charge and vice versa during discharge (Figure 2) [3].

During charging the battery stores energy in the liquid electrolyte that flows through a stack of electrochemical cells. In the charge part of the cycle, an electron is liberated through an oxidation reaction from a high chemical potential state on the negative or anode side of the cell stacks. This electron reaches the positive or cathode side through an external connection (usually an electric wire) and is accepted through a reduction reaction at a lower chemical potential state, allowing the storage of energy [3]. Ions cross from one side of the cell to the other side through a membrane to complete the circuit. For every electron transferred from the anolyte to the catholyte, a positive charge moves across the membrane to maintain electroneutrality. The reverse process happens in the discharge cycle [3].

In practice, a flow battery system has a distinctive configuration (Figure 3). The structure of a typical RFB contains carbon felt or carbon plastic electrodes, bipolar plates usually made from graphite (these prevent direct contact between the electrolyte and current collectors), membranes (that may be an anionic exchange membrane, or a cationic exchange membrane) or a separator, a flow frame (to direct the electrolyte flow through the electrodes), and an end plate with electrolyte fittings and current collector. This is the working unit of an RFB and is separated from the bulk of the electrolyte. As such, it can be modified to suit the needs of the system independently from the electrolyte storage system [41]. For larger operations, the cells are stacked together.

The electrolyte flows from each electrolyte storage tank through the cell stack and back to that electrolyte storage tank. The energy conversions from electrical energy to chemical potential (for charging) and vice versa (for discharging) take place rapidly at the surface of the electrodes as soon as the liquid electrolytes flowing through each cell flow past one another in the stack [3].

The flow is provided via a series of pumps which in turn are controlled by a central control system. The system is connected to the grid through an inverter [41], completing the integrated battery storage system (Figure 4).

Due to the design of RFBs, the power (kW) and energy storage capacity (kWh) can be designed separately. While the size of the tanks (the volume of the electrolyte) controls the energy capacity of an RFB, the size of the electrodes and the number of cells (total surface area of all the cells) control the power of the battery [3].

A bipolar electrode design can be used to increase the specific energy of the battery. This design allows the current to travel directly through the battery stack since the electrochemical reactions take place on opposite sides of the bipolar electrodes. Unlike the encased configuration of a lithium-ion battery where energy is stored in rolled electrode sheets, a flow battery utilises electroactive species that are dissolved and freely migrate in liquid electrolytes and that are present in different oxidation states [3].

RFBs can be divided into classical or true RFBs and hybrid RFBs, which are further divided into Type I and Type II [42] (Figure 5). Classical RFBs use inert electrodes and redox species that remain in the solution. Examples of classical RFBs are the iron chromium (Fe/Cr) system [40], the bromine–polysulphide system [43,44], and the vanadium RFBs [45]. Hybrid RFB operation involves a phase change during the cell reaction. An example is the zinc/bromine system in which the plating and dissolution of zinc at the anode occur upon charge and discharge, respectively [46,47,48,49,50,51].

RFBs are well known for their long cycle life, modular design, excellent electrochemical reversibility, high round-trip efficiency, good scalability and flexibility, safety, independent sizing of power and energy, deep discharge ability, short response time, moderate maintenance cost, and low environmental impact [3,53]. With these characteristics, RFBs can be used in applications with a range of operational power and discharge time requirements and are also suitable for supporting electricity generation from a range of renewable sources (Figure 6).

RFBs can be easily integrated into applications that need energy storage systems for load-levelling, flex ramping, peak shaving, time shifting, maintaining power quality, and frequency regulation [41,56,57,58].

2.2. Redox Flow Battery Chemistry

Several combinations (more than 30) of redox pairs have been detailed in the literature as possible RFB electrolytes that contain two redox couples. Previous work summarised these combinations (

Table 1. Selected examples of inorganic redox couples for RFBs (adapted from [59]). Abbreviations are as follows: HC—half-cell study (HC); Prototype (P)—prototype tested; Commercial (C)—technology has been commercialised [59].

	Cathode	MnO2/Mn2O3	Fe(Cn)63−/FeCn6−4	Cu+/Cu	Fe3+/Fe2+	VO2+/VO2+	CIBr2−/Br−	Br2/Br−	NpO2+/NpO22+	IO3−/I2	O2/O2−	HCrO4−/Cr3+	Cl2/Cl−	PBO2/PB2+	Mn3+/Mn2+	Ce4+/Ce+3
Anode	E° (V)	0.15	0.36	0.52	0.77	0.99	1.04	1.09	1.14	1.2	1.23	1.35	1.36	1.46	1.54	1.72
Zn(OH)42−/Zn	−1.22	P	P
Zn2+/Zn	−0.76					P	P	C					P			P
Fe2+/Fe	−0.45				HC
S/S2−	−0.43							C
Cr3+/Cr2+	−0.41				C			HC				P
Cd2+/Cd	−0.4				P
V3+/V2+	−0.26				P	C	P				P				P	P
Pb2+/Pb	−0.13													P
H+/H2	0				P	P		P					P
TiO2+/Ti3+	0.04				HC		HC						HC
Cu2+/Cu+	0.15			P
Np4+/Np3+	0.15								P
Cu2+/Cu	0.34													P
I2/I−	0.54									HC

) using data from published articles and/or patents [59]. Only five of these electrolytes are used in RFBs that are produced commercially: all-vanadium (V³⁺/V²⁺ couple and V⁵⁺/V⁴⁺ couple), iron chromium (Fe/Cr), zinc bromine (Zn/Br₂), all-iron (Fe²⁺/Fe³⁺ couple and Fe²⁺/Fe⁰ couple), and polysulphide bromine (S₂/Br₂) systems. Most RFB electrolytes are acidic because many metal ions (except for zinc) precipitate as insoluble hydroxides at high pH values, and the alkaline electrolytes are therefore unsuitable [3].

Table 2. Electronic properties of selected redox flow batteries with different chemistries.

System	Open Circuit Potential (OCP) (V)	Current Density (mA/cm2)	Charge/Discharge Efficiency (%)	Reference
Fe-Cr	1.18	21.5	95 (Coulombic)	[53,60]
Fe-Ti	1.19	14	44–50 (Overall)	[53]
VRB	1.6	10–130	80 (Overall)	[53]
V–Br	1.4	20	74 (Overall)	[61,62,63,64,65,66]
V–Fe				[34,67,68,69]
V-Mn	1.66	20	63 (Overall)	[70,71,72]
V-Ce	1.5	22	90 (Coulombic)	[73,74,75,76,77,78,79]
V-glyoxal (O2)	1.2	20	66 (Coulombic)	[53]
V-polyhalide	1.3	20	83 (Coulombic) 80 (Voltaic)	[53]
Hybrid V-O2 fuel cell	-	2.4	45.7 (Overall)	[53]
Zn-Br	1.85	20	80 (Overall)	[46,47,48,49,50,51,53,61,62,80,81,82,83,84,85]
Flow-through lead battery	1.62	20	60–66 (Overall)	[86]

summarises electrical properties of redox flow batteries with different chemistries and

Table 3. Examples of flow battery companies with RFB system types [87].

Company Name	Location	System
Australian Flow Batteries	Western Australia, Australia	VRFB
AVESS Energy	Western Australia, Australia	VRFB
CellCube (Enerox GmbH)	Wiener Neudorf, Austria	VRFB
ESS Tech Inc.	Wilsonville, Oregon, U.S.A.	Fe Flow
Invinity Energy Systems	St. Helier, Jersey	VRFB
Largo Inc.	Toronto, Ontario, Canada	VRFB
Lockheed Martin Corp.	Bethesda, Maryland, U.S.A.	Synthetic metal-ligand
Primus Power Solutions	Hayward, California, U.S.A.	Zn/Br2
Rongke Power	Dalian, China	VRFB
Redflow Technologies Ltd.	Queensland, Australia	Zn/Br2 (entered voluntary administration)
SCHMID Group	Freudenstadt, Germany	VRFB
Sumitomo Electric Ind., Ltd.	Osaka, Japan	VRFB
Thorion Energy	Perth, Western Australia, Australia	VRFB
Vecco Group	Queensland, Australia	VRFB
VRB Energy	Vancouver, British Columbia, Canada	VRFB
VisBlue	Denmark	VRFB
VFlow Tech	Singapore	VRFB
VSUN Energy	Western Australia, Australia	VRFB

details the names, locations, and RFB system types of some flow battery companies.

2.3. Energy Efficiency

The system efficiency parameters for all batteries including RFBs are Coulombic efficiency, voltaic efficiency, and energy efficiency.

The Coulombic efficiency, CE, is the ratio of the amount of charge transferred upon the discharge (Q_discharge) to the amount of charge transferred upon the charge (Q_Charge). Coulombic losses are caused by several phenomena such as ion diffusion processes, irreversible reactions, and shunt currents through the electrolyte [88]. It is the purpose of the ion exchange membrane to prevent ion diffusion, and significant research has been conducted in membrane modification to ensure high selectivity without compromising performance [3,89,90,91,92,93,94,95].

$$CE={\displaystyle \frac{Q_{discharge}}{Q_{Charge}}}\times 100\mathrm{\%}={\displaystyle \frac{\int \left|i_{discharge}\left(t\right)\right|dt}{\int i_{charge}\left(t\right)dt}}\times 100\mathrm{\%}$$

(1)

The voltaic efficiency, VE, is the ratio of the average discharge voltage (${\overline{V}}_{discharge}$) to the average charge voltage (${\overline{V}}_{charge}$). VE losses occur when the average charging voltage is consistently higher than the voltage required to activate the response of the material inside the battery. Another factor that reduces VE is the internal resistance of the battery, which increases with battery size, battery age, and current, and can be dependent on the battery chemistry [3].

$$VE={\displaystyle \frac{{\overline{V}}_{discharge}}{{\overline{V}}_{charge}}}\times 100\%$$

(2)

The energy efficiency, EE, is calculated as the product of the Coulombic efficiency and the voltaic efficiency. The EE indicates how much of the energy that is supplied to the battery during charging can be extracted upon discharge [3].

$$EE={\displaystyle \frac{\int \left|i_{discharge}\left(t\right)\right|dt}{\int i_{charge}\left(t\right)dt}}\times {\displaystyle \frac{{\overline{V}}_{discharge}}{{\overline{V}}_{charge}}}\times 100\%=CE\times VE$$

(3)

It should be noted, however, that the total system energy efficiency will be different in application. There is a net energy consumption from operating the pumps for each half- cell in a RFB as well as energy loss from flowing though pipes. This can be calculated using Equations (4) and (5) [96].

$$P_{NE}=P_{NE}^{pipes}+P_{NE}^{pumps}$$

(4)

$$P_{PE}=P_{PE}^{pipes}+P_{PE}^{pumps}$$

(5)

where $P_{NE}$ is the net energy lost in the negative half-cell from the pipes ($P_{NE}^{pipes}$) and the energy consumed by the pumps $(P_{NE}^{pumps}$). Equation (5) calculates the same for the positive electrolyte ($P_{PE}$). The energy efficiency calculation (3) can be amended to account for this using Equation (6) [96].

$${EE}_{net}={\displaystyle \frac{{\int }_0^{t_d}(E_{discharge}{I}_{discharge}-\left(P_{Ne}+P_{Pe}\right)dt)}{{\int }_0^{t_c}(E_{charge}{I}_{charge}+\left(P_{Ne}+P_{Pe}\right)dt)}}$$

(6)

where EE_net is the net energy efficiency of the system, t_d is the discharging time, and t_c is the charging time. Note that the power provided to the load in the discharging process has been subtracted from the power wasted during the flow of the electrolytes in the pipe and expended by the pumps. In application, the total power required is the sum of the power expended by the pumps and pipes as well as that required by the chemical reaction.

3. Classical RFBs

3.1. All-Vanadium RFBs

The RFB type that has received the most attention is the all-vanadium redox flow battery (VRFB). VRFBs are considered to be one of the most promising grid-scale energy storage systems and are well suited for integration with renewable energy generation sources. This is due to their robustness under a wide range of operating conditions, the ability to set power and energy ratings independently, long maximum discharge and storage times, and safety, including non-flammability [3,97].

The vanadium (V) redox couple was first mentioned in a 1933 patent by P.A. Pissoort [98] and was suggested for use in batteries by NASA and by Pellegri and Spaziante in 1978 [99]. The first known successful demonstration and patenting of VRFBs was carried out in Australia by Skyllas-Kazacos and co-workers at the University of New South Wales (UNSW) who registered a patent in 1988 [100]. Australia is ideally placed to produce VRFBs as vanadium resources are abundant. VRFBs have already proven their value, but several research groups around the world are looking to improve their performance and efficiency to achieve higher power outputs [3,101].

A VRFB power cell consists of two half-cells, with each half-cell consisting of an end plate, an end gasket, a bipolar plate (extruded sheet of carbon polymer material), a porous electrode (usually carbon felt), and a frame gasket. The two halves are separated by a membrane which is usually an ion exchange membrane (that may be an anionic exchange membrane or a cationic exchange membrane) [102] or a separator [103] (Figure 3). Electrical connection to the cells is normally achieved via a metal current collector (usually copper sheet or mesh) isolated from the battery electrolyte. A VRFB stack is made up of an assembly of these power cells [3].

The VRFB uses vanadium’s capacity to exist in four different oxidation states [104] to create a battery with one electroactive element rather than two. By using vanadium redox couples in both half-cells, the issue of cross-contamination with another element caused by ion diffusion across the membrane is eliminated.

The electrochemical reactions occur on the inert carbon felt electrodes in the half-cells. Pipes and pumps are used to move the electrolyte from its storage tanks to the stack. As such, the correct pretreatment of these electrodes to ensure hydrophilicity and reduce faradaic resistance is important. The performance of a VRFB is not significantly degraded by repeated full discharge or charge rates as high as the maximum discharge rate, according to Skyllas-Kazacos and colleagues [105]. A schematic showing the reactions in a VRFB can be seen in Figure 7.

3.1.1. Chemistry of VRFB

The main reactions that occur in the battery during charge and discharge cycles are [106] as follows:

Cathode:

$$V{O}^{2+}+H_{2}O\leftrightharpoons V{O}_2^{+}+2H^{+}+e^{-};E_c^o=+1\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(7)

Anode:

$$V^{3+}+e^{-}\leftrightharpoons V^{2+};E_a^o=-0.26\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(8)

Overall process:

$$V{O}^{2+}+V^{3+}{+H}_{2}O\leftrightharpoons V{O}_2^{+}+V^{2+}+2H^{+};E_{cell}^o=1.26\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(9)

Note [106]: ${VO}^{2+}=V^{4+}+O^{2-}$; ${VO}_2^{+}=V^{5+}+2\left(O^{2-}\right)$.

The standard cell potential is 1.26 V at 25 °C, but the practical single cell voltage is between 1.4 and 1.6 V for a 1.6 M vanadium ion solution with 4.6 M sulfuric acid. VRFB net efficiency can be as high as 85%. Like other flow batteries the power and energy ratings of VRFBs are independent of each other. A typical charge–discharge curve for the VRFB bipolar electrode is shown in Figure 8 for a battery with 80% voltaic efficiency (VE) and 91% Coulombic efficiency (CE) [107]. The water (H₂O) and protons (H⁺) are required in the positive reaction to maintain the charge balance and the stoichiometry [3].

The discharged electrolyte contains an equal amount of V³⁺ and V⁴⁺ ions in single or mixed acids [108]. It is usually termed a V³˙⁵⁺ electrolyte. During the charge cycle, V⁴⁺ oxidises to V⁵⁺ in the positive compartment, and V³⁺ reduces to V²⁺ in the negative compartment and vice versa on discharge (

Table 4. Vanadium ions with their corresponding salt, their corresponding battery state, the electrolyte state, and whether the concentration of vanadium ions increases or decreases during the charge and discharge cycle.

Species	Salt	Battery state	Electrolyte	Charge	Discharge
V2+	VSO4	Charged	Anolyte	↑	↓
V3+	V2(SO4)3	Discharged	Anolyte	↓	↑
VO2+ (V4+)	VOSO4	Discharged	Catholyte	↓	↑
VO2+ (V5+)	(VO2)2SO4	Charged	Catholyte	↑	↓

). H⁺ ions are exchanged through the ion selective membrane [3].

3.1.2. Different VRFB Generation Chemistries

Since the development of VRFBs in the 1980s, the VRFB electrolyte has been constantly improved and several generations of VRFBs have emerged over decades of work aimed at enhancing performance and solving challenges in operating the batteries [109]. The main differences between these generations are the use of different electrolyte chemistries or the use of a single or mixed inorganic acid [3,110] (

Table 5. Comparison of each generation of VRFBs [101,111].

	Gen1	Gen2	Gen3
Electrolyte	V/sulphate in both half-cells	V/HBr/HCl solution in both half-cells	V/H2SO4/HCl in both half-cells
Negative couple	V3+/V2+	V3+/V2+	V3+/V2+
Positive couple	V5+/V4+	Br/ClBr2	V5+/V4+
Maximum vanadium concentration	1.5–2 M	2.0–3.5 M	2.0–2.7 M
Supporting electrolyte	H2SO4	HBr and HCl	H2SO4 and HCl
Specific energy	15–25 Wh kg−1	25–50 Wh kg−1	25–40 Wh kg−1
Energy density	20–33 Wh L−1	35–70 Wh L−1	35–55 Wh L−1
Operating temperature range	10–40 °C	0–50 °C	0–50 °C

).

The second-generation VRFB (G2 VRFB) was developed to replace the V⁵⁺/V⁴⁺ pair in the positive side with the V/HBr/HCl electrolyte, aiming to widen the operational temperature range and increase the energy density of the VRFB. Subsequently, Pacific North-Northwest Laboratories proposed using an acidic mixture (H₂SO₄ and HCl) to improve the solubility of vanadium ions in the electrolyte, leading to superior performance and higher energy density, known as the third-generation (G3 VRFB) [101]. Despite the enhanced energy density of both G2 and G3 compared to G1, the use of halides in the electrolyte increased the risk of bromine and chlorine gas evolution during operation, which increases the risk associated with the battery as well as the potential environmental impact. Currently there have been no reports of large-scale installations of G2 and G3.

3.1.3. Advantages and Disadvantages

VRFBs have discharge duration times up to 24 h. The technology has a quick response (faster than 0.001 s) with the number of maximum cycles varying in the literature, with an operational plant reporting up to 200,000 cycles over a 3 year period [112]. However, the most common number reported is 20,000 cycles [113,114,115,116]. The VRFB is the only RFB that has been used in large-scale applications around the world (Europe, Southeast Asia, and North America) for extended periods of time [55,58,117,118,119,120,121,122,123,124,125].

There are still technical challenges for VRFBs, including low electrolyte stability and solubility leading to low energy density [112,126]. Commercial systems are recommended to operate between 10 °C and 40 °C to avoid vanadium precipitation that can block flow channels. However, recent findings have suggested that when the SOC of the electrolyte is reduced to 70–80%, the temperature can be increased to 50 °C. Further testing is required to optimise the operating temperature, and notable research is being conducted into the use of additives to further stabilise the electrolyte at elevated temperatures [116,127,128,129,130,131,132,133,134,135,136].

As discussed previously, one of the leading causes of electrolyte instability in RFBs is active species permeation and crossover of the ion selective membrane. In VRFB, this can lead to a decrease in anolyte/catholyte which reduces the capacity of the battery as well as the self-discharge of the active species, reducing the state of charge of the system. In addition to the crossover of vanadium species, there can be a net transfer of water across the membrane, speculated to result from the transfer of hydrate vanadium ions between the two half-cells. This also contributes to the variation in electrolyte concentration and, if left unchecked, the rapid degradation of the system [89]. As such, significant work has been conducted to improve ion exchange membranes selectivity and performance. Sun et al. modified Nafion membranes with by introducing asymmetrical layers of tungsten oxide to hinder vanadium migration and diffusion through the ion exchange membrane. In comparison with the Nafion 212 membrane, the optimal hybrid membrane prepared with 20%wt WO₃, demonstrated higher Coulombic efficiency (93% vs. 88%), higher energy efficiency (75% vs. 65%), and higher capacity retention (62% vs. 42%) for a VRFB cell with the same current density of 60 mA/cm² [137]. Other work investigated the use of graphene oxide incorporated into Nafion membrane 117. Results indicated a similar increase in performance of the hybrid membrane an increase the Coulombic efficiency (96% vs. 91%) and energy efficiency (85% vs. 80%) for a VRFB system at a current density of 80 mA/cm² [137]. Similar findings were reported when including carbon nitride nanosheets into the Nafion matrix [138]. Alternative anion exchange membranes based on poly(phenylene oxide) with imadozolium and bis-imadazolium cations have been investigated as an alternative to the Nafion membrane for use in VRFBs. Results show that the novel membrane outperforms the Nafion membrane and achieves an impressive Coulombic efficiency of 98.5% at a current density of 140 mA/cm² [95]. Continuing the development of these membranes among others and evaluating the industrial application of this technology is critical to improve the longevity and widespread application of VRFBs.

In VRFBs, the carbon/graphite felt electrodes are critical in ensuring efficient electrochemical reactions. As such, significant research has been conducted to ensure that the felt electrodes have low resistance and are hydrophilic. Previous work has shown that electrochemically depositing bismuth onto the graphite felt induced better catalysis of the V³⁺/V²⁺ reaction, and the voltaic efficiency was 9.47% higher compared to pristine felt at a current density of 80 mA/cm² [139]. Other studies have evaluated the electrocatalytic effect of oxygen functionalisation of thermally pretreated graphite felt on the kinetics of the reactions in a VRFB. Positive effects from oxygen functionalisation as a result of thermal modification of the felt at a temperate range of 400–600 °C was observed for the negative electrode, whereas the positive electrode reaction demonstrated minimal benefits [140]. The effects of oxygen functionalisation are highly debated with contrasting results. Variation in results could be due to the amount and type of oxygen groups present [140,141,142,143,144,145,146]. A study by Esteves et al. demonstrated a dual oxidation approach using oxygen plasma followed by treatment with hydrogen peroxide to impart functional groups onto the graphite felt. By varying the oxidative techniques, they were able to study the effects of different oxygen groups. Results showed that O-C=O groups improved cell performance whereas the C-O and C=O groups degrade it. The increase in performance was attributed to a reduction in the cell overpotential after functionalisation. Regardless of the approach utilised, ensuring the optimal performance of graphite felt is critical in developing a functioning VRFB, and research continues to demonstrate novel applications [143,147].

Vulnerability of the storage systems to sealing and leakage issues is also a challenge [148]. The other major challenge for VRFB systems is their capital cost, roughly $550–600 per kWh, which is high for broad market penetration [149,150,151]. The high cost is, in part, due to the use of relatively expensive vanadium [152,153] and the high costs of membranes, but the high initial cost can be recouped as VRFBs can operate for over 20,000 full cycles and their lifespan can exceed 20 years [3,148,150].

The International Renewable Energy Agency (IRENA) reported the advantages and disadvantages of VRFBs (

Table 6. Advantages and disadvantages of VRFB electricity storage systems [3,148,154,155].

Advantages

Disadvantages

Long cycle life (20,000 + full cycles). Relative high energy efficiency (up to 85%). One of the most mature flow batteries with multiple deployments at MW scale. Design E/P ratio can be optimised to suit specific application. Long-duration (1–20 h) continuous discharge and high discharge rate possible. Quick response times. Same element in active materials on electrolyte tanks limits ion cross-contamination. Electrolyte can be recovered at end of project life. Heat extraction due to electrolyte prevents thermal runaway.

Low electrolyte stability and solubility limit energy density, and low specific energy limits use in non-stationary applications. Precipitation of V2O5 at electrolyte temperatures above 40 °C can reduce battery life and reliability, although this can be managed by controlling SOC. High cost of vanadium and current membrane designs. Unoptimised electrolyte flow rates can increase pumping energy requirements and reduce Energy efficiency. Poor selectivity in ion exchange membranes can lead to self-discharge and decrease in VRFB capacity. Graphite electrodes require pre-treatment and functionalisation to perform optimally.

) [148].

VRFB flow battery technology provides a very long operational life, low lifetime costs, and low greenhouse gas emissions. The battery technology is mature, and it can be considered to be at the commercial stage for fixed applications. The technology is well proven and is undergoing demonstration as a load levelling battery for electricity utility zone sub-stations and other transmission and distribution applications [156]. VRFBs providing MWs in power and MWhs in energy storage capacity have been demonstrated, and, recently, a 200 MW/800 MWh VRFB was built and partially operated at 100 MW/400 MWh in northeast China in Dalian [157]. H₂ Inc., a South Korean VRFB company, has installed several VRFB systems and has begun construction of a factory with 330 MWh annual manufacturing capacity [158]. Sumitomo, in Hokkaido, have a plant operating at 17 MW/51 MWH [157].

3.2. Iron Chromium RFBs

The iron-chromium redox system was initially investigated by NASA [40] and is the basis of one of the earliest proposed RFBs (Sun and Zhang, 2022). The hydrochloric acid-acidified aqueous solutions contain a ferric-ferrous redox couple and a chromium-chromic couple that serve as the positive and negative reactants, respectively [159]. Each reactant in this redox flow cell flows at a rate that is consistently higher than the stoichiometric flow requirement, allowing all the reactants to be used up in one cycle through the cell. The two flowing reactant solutions are divided by an anionic and cationic ion exchange membrane in each cell [159].

In theory, the membrane prevents the cross-diffusion of the iron and chromium ions, allowing only the chloride and hydrogen ions to freely move through the cell to complete the electrical circuit [159]. The performance of a Fe-Cr redox flow cell was investigated by a number of researchers and organisations, including NASA [40,160,161,162], a study team at the University of Alicante in Spain [160], and Shimada et al. [163], who reported that the Coulombic efficiency of a redox flow cell increased to 95% when the carbon structure in the electrodes was changed from amorphous to graphite. As with VRFBs, the electrode treatment is critical to ensure efficient battery operation.

In a recent study by Wu et al. [164], the significant capacity loss observed in iron-chromium redox flow batteries (ICRFBs) was analysed, highlighting its hindrance to further technological advancement. The authors attributed this loss to the presence of inactive Cr(H₂O)₆³⁺ ions, leading to an imbalance in active ion content between the catholyte and the anolyte. To address this issue, a novel electrolyte formulation was introduced, increasing the Cr concentration from 1 M to 1.3 M. This modification resulted in an energy efficiency (EE) of 84.51%.

3.2.1. Chemistry of Iron Chromium RFBs

The electrolyte used in ICRFBs contains chromium on the anode side and iron on the cathode side. The separate redox reactions are listed below. The ICRFB produces a standard voltage of 1.18 V [159,165,166].

Catholyte:

$$F{e}^{3+}+e^{-}\rightleftharpoons F{e}^{2+};E_c^o=+0.77\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(10)

Anolyte:

$$C{r}^{2+}\leftrightharpoons C{r}^{3+}+e^{-};E_a^o=-0.41\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(11)

Due the fast kinetics of the Fe(II)/Fe(III) redox reaction, only carbon felt is typically required. However, the Cr(II)/Cr(III) redox reaction is slower, and, for an efficient design, a catalyst is required to enhance the electrochemical kinetics. It is essential for the catalyst to have a high overpotential towards hydrogen evolution as, thermodynamically, hydrogen is more easily reduced than the Cr(III) species [159,165]. If this occurs, the cell will have a reduced Coulombic efficiency as well as an imbalance of the state of charge between the catholyte and anolyte solutions, resulting in a rapid capacity decay [159]. Typical catalysts include Bi and Au-Ti, which are deposited on the electrode surface to enhance the electrochemical kinetics of the chromium redox couple [60]. In addition to the use of catalysts, a rebalance cell can be utilised to rebalance the SOC of the electrolyte [60].

3.2.2. Advantages and Disadvantages

In addition to the general advantages and disadvantages associated with RFBs, the iron-chromium ICRFB presents a distinct set of benefits and challenges specific to its operational context. A primary advantage of ICRFB technology is that its active species—Fe²⁺/Fe³⁺ and Cr²⁺/Cr³⁺—are non-toxic to both humans and the environment [159]. Operating within a temperature range of 40–60 °C, ICRFB systems are particularly well suited for warmer climates, such as those found in many regions of Australia [159]. Furthermore, ICRFB exhibits relatively low resistance, comparable to that of conventional Generation 1 VRFBs, due to the high conductivity of hydrochloric acid, which serves as the supporting electrolyte. The use of mixed active species, in contrast to single-active-species systems like VRFBs, offers additional advantages. Notably, a highly selective permeable membrane is not necessary, which significantly lowers capital costs [159,167]. Additionally, the chemical production costs associated with chromite ore may be lower than those for pure chemicals, as the separation of iron and chromium is not required. However, accurate cost estimates per kWh are currently unavailable due to the absence of large-scale applications of this technology [159].

Since graphite felt has an important role in ICRFBs and VRFBs, significant research has been dedicated to improving the catalytic effect of the electrodes. One of the earlier examples of this is an investigation by Johnson and Reid [168] where the Fe-Cr redox system was assessed using electrodes made of 1/8 inch carbon felt, and the felt in the chromium half-cell was modified by the inclusion of 12.5 μg/cm² of gold. More recently, Niu et al. utilised silicic acid etching to intricately carve dense nano-porous structures onto the surface of carbon cloth electrodes. This innovative technique resulted in a remarkable achievement, with the battery attaining an average energy efficiency of 81.3% [169]. Chen et al. demonstrated the inclusion of SiO₂-decorated graphite after silicic acid etching which resulted in significant improvement in battery performance. The SiO₂ introduced by the decomposition of silicic acid can increase the effective specific surface area of the graphite felt while also facilitating the oxidation of the felts at 500 °C. A significant increase in the oxygen functional groups was observed and treatment the negative reaction activity of the ICRFB and voltaic efficiency increased by 6.61% at a current density of 120 mA/cm² [170]. Similar results have been observed with boric acid etching of graphite felt followed by thermal treatment at 500 °C, after which the energy efficiency of the ICRFB cell was increased by 9.5% compared to pristine felt [171]. Zhang et al. extensively studied the impact of the oxygen functional groups, the impact of graphitisation degree, and the surface area of both carbon felt and graphite felt. Both electrodes were investigated with and without BiCl₃ as a catalyst. It was found that graphite felt outperformed the thermally treated carbon felt due to the higher degree of graphitisation. In addition, although bismuth ions are believed to have the dual effect of inhibiting the hydrogen evolution reactions and catalysing the negative electrode reactions, the shorter catalytic path by oxygen functional groups is of greater benefit to improve the stability of ICRFBs [172]. There is an extensive range of treatment options to ensure excellent performance of the graphite felt in general. These can be broadly categorised as either modification of surface functional groups or modification of surface catalytic materials. Both options have been shown to be effective, and the application of the specific method would depend on the battery chemistry involved [173]. The necessary treatment of the graphite felt in an ICRFB can be a disadvantage, but as discussed, significant research has been performed to address this issue.

Despite these advantages, several disadvantages accompany the implementation of ICRFBs. The permeation of all active species through the membrane can create substantial concentration gradients between the anode and cathode tanks, resulting in decreased performance and accelerated degradation of the electrolyte [159]. The mixed electrolyte also exhibits reduced solubility, which limits the overall capacity of the system. While the solubility of FeCl₂ and CrCl₃ can reach 2 M, the solubility in the mixed system is considerably lower, with optimal electrolyte concentrations reported at 1 M FeCl₂, 1 M CrCl₃, and 3 M HCl [174]. Additionally, the system operates within a narrow voltage range of 0.7–1.2 V, further constraining its capacity [174]. The standard potential of the Cr²⁺/Cr³⁺ couple is close to that of hydrogen evolution, which can trigger parasitic side reactions, diminishing capacity and exacerbating electrolyte imbalance [159]. Although some of these issues may be mitigated through rebalancing systems, such solutions increase both the cost and complexity of maintenance for the system. Figure 9 provides an overview of an ICRFB cell [166].

3.3. Polysulphide RFBs

A polysulphide bromide battery (PSRFB) is a type of RFB that uses sodium bromide and sodium polysulphide as salt solution electrolytes. Extensive research efforts have been dedicated to PSRFB systems over numerous years [175]. Polysulphide redox species demonstrate excellent solubility in aqueous solvents, leading to their wide use in RFBs containing sulphur [176,177]. The solution chemistry of PSRFBs is governed by the intrinsic properties of tshe polysulphide compounds and their interactions with solvent molecules, which dictates the system design and cell architecture [176]. The reversibility of the formation of different polysulphide species is susceptible to the influence of different cations such as Na⁺, Li⁺, and K⁺, and this needs to be taken into consideration in cell design [176]. PSRFBs can be classified as all liquid or hybrid systems, where hybrid systems include liquid/solid, semi-solid, and liquid/gas systems. The chemistry of PSRFBs is complex and depends not only on the polysulphides but also on the solvents used and the type of system.

3.3.1. Chemistry of Polysulphide RFBs

The chemistry of PSRFBs is governed by the polysulphides which undergo reversible anodic and cathodic reactions. As with other RFBs, the electrolyte is pumped from separate anolyte and catholyte tanks into a cell where the half reactions take place [178]. The exact reactions that take place are dependent on the polysulphide species and solvent present, but an example of a sodium polysulphide RFB is shown in reactions 12–14 [176]. During the discharge, short chain polysulphides and the sulphide solution are pumped into the anodic half-cell to be oxidised to high chain polysulphides. In the cathodic half-cell, bromine is reduced to bromide with the charge compensation by the Na⁺ ion. An overview of a sodium bromine polysulphide batteries can be seen in Figure 10, and the reactions involved are the following [176]:

Anode:

$${2Na}^{+}+x{Na}_2{S}_{x+1}+2e^{-}\rightleftharpoons \left(x+1\right){Na}_2{S}_x;E_a^o=-0.27\mathrm{V},x=1-4\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(12)

Cathode:

$$2NaBr\rightleftharpoons {Br}_2+2{Na}^{+}+2e^{-};E_c^o=+1.06\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(13)

Overall:

$$\begin{array}{cc}{2Na}^{+}+x{Na}_2{S}_{x+1}+2NaBr\rightleftharpoons \left(x+1\right){Na}_2{S}_x+{Br}_2+2{Na}^{+};\hfill & \\ & E_{cell}^o=1.33\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}\hfill \end{array}$$

(14)

All Liquid PSRFBs

In all liquid PSRFBs, the electroactive species are dissolved in solvents to form the electrolyte. There are a range of solvents that are suitable for use. The commonly used solvents include 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) [179], dimethylsulfoxide (DMSO) [180], and tetrahydrofuran(THF). The choice of solvent is important as the interaction between the solvent and the polysulphide will dictate the potential window where the cell operates. For example, water can be used as an inexpensive solvent for PSRFBs; however, it has a limited electrochemical window [176]. During cell operation, hydrogen- and oxygen-evolving reactions occur, leading to the rapid decay of the electrolyte. In addition, water and sulphur can also generate unwanted species such as hydrosulphide and hydrogen sulphide under certain pH conditions. The use of organic solvents results in a larger potential window but has the drawback of increased cost [176].

In the aqueous system, the chemistry of the cell is dependent on the electrochemistry of the solvent and polysulphide species, the temperature, and the interaction between polysulphides and the solvent. Short chain polysulphides (S_n 1 ≤ n < 4) are preferred as they favour high conductivity; however, these cells often have low theoretical capacity [181]. In addition, these ionic systems have intermediate sulphur species (S_n²⁻) that convert to HS⁻ and OH⁻ through hydrolysis of the electrolyte [176]. As such, careful control of the redox potential of the electrolyte is essential to help prevent the formation of these species and hydrogen and oxygen evolution reactions. Various studies have investigated mechanisms to increase the electrochemical stability window of these electrolyte solutions.

Hybrid PSRFBs

Hybrid PSRF systems can be further divided into solid/liquid, semi-solid, and liquid/gas systems. These systems have been designed with the intention of increasing the theoretical capacity of the batteries and creating a more electrochemically stable system in comparison to aqueous PSRFBs.

Solid/liquid PSRFBs take advantage of the high energy density of lithium metal to create high-capacity systems that are easily scalable. In these systems, long chain polysulphides are often dissolved in organic solvents to form the catholyte in conjunction with metals such as lithium [180]. However, intermediate sulphur species can react with the lithium species to form short chain lithium sulphides that precipitate out of the solution, resulting in loss of active material and overall decay of the system [182].

Semi-solid PSRFBs have been proposed as another alternative to aqueous systems. These systems utilise redox-active material suspensions instead of solutions, which have the advantage of increasing the concentration of redox active species past the point of solubility, thereby creating a more energy dense system [176]. Metals commonly used include LiFePO₄, LiNi_0.5Mn_1.5O₄, and LiCoO₂ suspended in an electrolyte solution [183]. In addition, nanomaterials can be introduced into the system to improve the reaction rate of the polysulphide redox species [184]. These systems offer the benefits of higher efficiency, capacity, and cycle life compared to aqueous systems. However, the active materials are more expensive than those used in aqueous systems [176].

Liquid/Gas PSRFBs

A key strategic advantage of RFBs is the ease of scalability and low cost of the system. Although semi-solid PSRFBs have addressed many issues associated with the use of polysulphides, the overall cost of the system is significantly higher due to the use of expensive metals in the catholyte [176]. Liquid/gas PSRBs have been proposed as an alternative solution that removes the need for expensive metals in the catholyte. These RFBs pair a polysulphide anolyte with an oxygenated salt solution as the catholyte. At the anode side, the reaction proceeds as described in reaction 16 and 18. However, at the cathode side, an oxygen layer is introduced that promotes oxygen evolution/reduction in the redox active species in the catholyte [185]. In this configuration, two half reactions are paired, specifically, polysulphide oxidation and the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The half reactions taking place vary depending on the pH of the catholyte, as described in reactions 15 to 18 [185].

Acidic catholyte:

$$2H_{2}O\leftrightharpoons O_2+4H^{+}+4e^{-};E_c^o=+1.299\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(15)

Acidic anolyte:

$$x{S}_y^{2-}+2(y-x)e^{-}\leftrightharpoons y{S}_x^{2-};E_a^o=-0.447\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(16)

Basic catholyte:

$$4O{H}^{-}\leftrightharpoons O_2+2H_{2}O+4e^{-};E_c^o=+0.401\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(17)

Basic anolyte:

$$x{S}_y^{2-}+2\left(y-x\right)e^{-}\leftrightharpoons {yS}_x^{2-};E_a^o=-0.447\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(18)

To maintain electroneutrality, metal ions such as Na⁺ and Li⁺ are consumed or generated through oxygen electrochemistry. As such, the efficiency of the cell is dependent on the total concentration of metal ions in the catholyte and the concentration of polysulphides in the anolyte [185].

This design has a lower cost compared to that of the other hybrid PSRFBs; however, there are still many challenges for this design that require additional research. Specifically, the generation and consumption of H⁺ (for an acid catholyte) or hydroxyls (for an alkaline catholyte) lead to pH swings in the catholyte. In addition, the ORR and OER reactions are inefficient, reducing the economic viability of the design [176].

3.3.2. Advantages and Disadvantages

PSRFB typically have high solubility (up to 8.8 M) and low cost; however, the many versions of PSRFB have been developed in an attempt to address some of the key disadvantages of the technology [186]. When the technology was first proposed in 1983, aqueous solutions of sodium bromide and sodium polysulphide were used. Compared with some other RFB, the polysulphide bromide batteries were less expensive; however, the system had a decrease in power density and energy efficiency [186]. This is because the cation exchange membranes in the Na⁺ form have higher ohmic resistance compared to the H⁺ form [187]. This limited the use of high current densities. In addition, the S_n−1²⁻/S_n²⁻ redox reaction is characterised by retarded electron transfer, requiring the use of cobalt or a nickel-based catalyst on the electrode to facilitate efficient electron transfer, increasing the cost of the system. The initial system also had potentially significant environmental impact due to the presence of bromine compounds [187].

Over the course of its development, many of these issues in PSRFB were addressed by substituting bromide compounds for other halides. The cost of the system can also be reduced by using less expensive redox couples with minimal environmental impact, such as the use of I₃⁻/I⁻ with the S_n−1²⁻/S_n²⁻ couple [188]. The theoretical energy of such a system with 6 M KI and 3.3 M K₂S₂ solutions is 85.4 W h L⁻¹; however, the reported capacity was much lower at 49.4 W h L⁻ [188]. The system reported an energy efficiency of 63–73%, a voltaic efficiency of 73–78%, and a Coulombic efficiency of 86–93%. However, the performance of this system is still poor compared to other RFBs, and the issues with the high ohmic resistance of the membrane and the costs associated with the use of catalyst is still an issue.

3.4. Organic RFBs

Recently a new type of RFB, organic redox flow batteries (ORFB), have gained an increasing amount of interest [189,190]. ORFBs utilise redox-active organic molecules and polymers instead of inorganic molecules used in other RFBs. Methods such as molecular engineering allow a range of different organic compounds to be constructed, each with unique physical, chemical, and electrochemical properties [189]. As a result, ORFBs have a large amount of diversity stemming from the range of different organic molecules that can be utilised.

The first redox-active organic electrolyte was tested in 2009 when tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) was coupled with lead to form a Pb/tiron aqueous hybrid flow battery [191]. Since then, many more redox-active species have been investigated [189,190,192,193]. Competitive organic electrolytes typically feature highly negative or positive redox potential, high water solubility, high structural stability, rapid redox reaction kinetics, and low cost [190]. The redox-active species can be tailored through either change in the structure of the molecules themselves or through the addition of additives. For example, to acquire high potential differences, electron withdrawing groups such as -NO₂, -SO₃⁻, and -PO₃²⁻ can be added to catholyte compounds. Electron donating groups such as -NH₂ and -OH can be grafted with anolyte compounds [190]. By adding additives, the solubility of redox active species can also be increased. For example, flavin mononucleotide in 1 M KOH can be increased from 0.1 to 1.5 M by the addition of 3 M vitamin B3 [194].

A key advantage of ORFB is that potential redox couples can be designed with the help of computational modelling and the behaviour of these compounds predicted [189,190]. These compounds can feature high energy density (210 W h dm⁻³) [189] and high charge capacity (320 A h dm⁻³) [189] with some species reporting low crossover [190]. As such, ORFBs hold great promise for industrial application. However, there are still many challenges that need to be addressed. While many organic molecules show high solubility in water, they show very poor solubility in supporting electrolytes (made from inorganic salts) due to the salting out effect [190]. In addition, most of the reported electrolytes have had limited amounts of attention, and parameters such as pH [195,196], viscosity [194], temperature [197], and the range of SOC [198] still require optimisation. The physical construction of the cell, and, in particular, the membrane, still need to be further investigated and optimised [199]. In addition, although aqueous redox flow batteries generally can be considered non-flammable, the application of the electroactive organic species in ORFBs is still comparatively new, and the associated hazards will need to be investigated on a case-by-case basis. However, further research ORFBs can be adapted for industrial use. Despite a significant amount of research in the area recently, there is a notable lack of mature ORFBs, complicating the assessment of long-term advantages and disadvantages of the technology. Currently, there is no Australian industrial application of the ORFB.

4. Hybrid RFBs

4.1. Zinc Bromine RFBs

The zinc bromine battery (ZBB) is termed a hybrid flow battery because one of its electrodes, zinc, participates in the reaction after charging. Currently, this technology is being developed primarily for stationary energy storage applications [3]. The system has an excellent energy capacity (specific energy density is 65–84 Wh/kg).

The ZBB concept originated in the 1880s [200], but due to technical difficulties, particularly with bromine corrosiveness, its development was not fully realised until the mid-1970s. ZBBs have attracted the interest of researchers as a rechargeable power source due to their high energy density, high cell voltage, high degree of reversibility, and use of readily available, low-cost materials. The electrolyte is a zinc bromide salt dissolved in water, and, during charging, zinc is plated on the negative electrode, which limits the capacity of the battery [3].

Capturing the bromine produced at the positive electrode during charging is imperative due to its high solubility in the aqueous electrolyte, which poses constraints on turnaround efficiency and longevity. Various methodologies have been proposed to mitigate bromine concentration in the electrolyte and alleviate self-discharge, including complexation of generated bromine with chemicals or dissolution in organic solvents. Singh et al. [84,201] suggested the utilisation of an organic solvent where a bromine/propionitrile solution forms a biphasic system with high conductivity and favourable charge–discharge characteristics. Bloch et al. [202] investigated the reaction between low molecular weight tetraalkylammonium halides and bromine-containing aqueous solutions to produce sparingly soluble polyhalides. The complexation of bromine, generated during the charging process, with quaternary ammonium salts allows for the binding of up to nine bromine atoms per molecule [203], thereby significantly lowering the bromine vapour pressure [85] to levels similar to that of brominated zinc bromide solutions [203]. This results in an exceptionally low concentration of free bromine in the electrolyte, as the majority of the bromine exists either as polybromide ions dissolved in the aqueous phase or bound to complexing agents in a secondary phase. Under these complexed conditions, both the chemical reactivity and the evaporation rate of bromine are markedly reduced relative to its elemental state. In a recent study, ref. [204] introduced a new additive of soft–hard zwitterionic trapper that complexed bromine to form a polyhalide-complexing ‘soft’ cationic and a water-soluble ‘hard’ anionic. The generated polybromide reduces the bromine crossover and generates a homogeneous aqueous solution free from phase separation. Presently, zinc bromine flow batteries (ZBFBs) employ quaternary ammonium salts such as N-Ethyl N-methyl pyrrolidinium bromide and/or N-ethyl-N-methyl-morpholinium bromide to complex bromine [50,205]. Recently, a Prussian blue-modified nitrogen-doped carbon (PB@NC) has been engineered as a redox-targeting catalyst for the bromine cathode, facilitating improved electron transfer and increasing bromine species concentration at the reaction interface. This modification enables Prussian blue to rapidly undergo redox transformations, thereby enhancing the electrochemical kinetics and overall efficiency. Consequently, ZBFBs employing PB@NC demonstrate an energy efficiency of 85.9% at a current density of 80 mA cm² and 71.1% at 160 mA cm⁻². This work highlights a promising strategy for optimising the performance of ZBFBs through the innovative design of cathode materials [80].

At the negative electrode, zinc is redissolved to form zinc ions, and bromide ions are formed during discharge at the positive electrode [3]. To recirculate the bromine complex, a third pump or extra valve is required. The efficiencies of ZBFBs range from 60 to 75%. In terms of energy output, ZBFBs ranging from 500 kWh to 2 MWh have been built and tested over the years [46,47,48,49,51,81].

4.1.1. Chemistry of Zinc Bromine RFBs

The chemistry of the charge and discharge of ZBBs can be summarised using reactions 19–22 [82,83]. An illustration of a ZBRFB cell can be seen in Figure 11.

Anode:

$${Zn}_{\left(aq\right)}^{2+}+2e^{-}\leftrightharpoons {Zn}_{\left(s\right)};E_a^o=-0.76\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(19)

Cathode:

$$2B{r}_{\left(aq\right)}^{-}\leftrightharpoons B{r}_{2\left(aq\right)}+2e^{-};E_c^o=+1.087\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(20)

Overall:

$$2B{r}_{\left(aq\right)}^{-}+Z{n}_{\left(aq\right)}^{2+}\leftrightharpoons Z{n}_{\left(s\right)}+B{r}_{2\left(aq\right)};E_{cell}^o=1.847\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(21)

Bromine Complex:

$$QB{r}_{\left(aq\right)}^{-}+nB{r}_{2\left(aq\right)}\leftrightharpoons {Q\left({Br}_{2\left(aq\right)}\right)}_n{Br}_{\left(aq\right)}^{-};$$

(22)

$Q{Br}^{-}$ = Complexing Agent

${Q\left({Br}_2\right)}_n{Br}^{-}$ = Polybromide

4.1.2. Advantages and Disadvantages

In the past, ZBFB was popular because of the low cost, readily available reagents, and excellent energy capacity. It is the second most employed RFB after VRFB.

However, there are still several key issues with the technology. During the operation of the battery, the heterogenous redox reaction of zinc can result in an uneven plating of the metallic zinc [186]. This is affected by a range of parameters such as temperature, current densities, and concentration of active material on the electrode. This uneven plating results in the formation of zinc dendrites, which can impair membrane functionality and lead to battery failure [206]. Dendrite formation is more prevalent at higher current densities and this, coupled with the slower kinetics of the active species, reduces the current densities of the battery, especially during the charging phase [186].

The plating of zinc also limits the capacity of the cell since the surface area for the electrodeposition of zinc is limited. As a result, the capacity of the battery is determined by size of the battery stack instead of the volume of the electrolyte stored in external reservoirs. Increasing the size of the zinc deposits can also increase the transport resistance. The plating of the zinc also results in a change in the ionic strength between the anolyte and catholyte [186]. As a result, water can more easily migrate across the membrane, resulting in a change in the electrolyte concentration and a level imbalance between the anolyte and catholyte [207]. Stable Zn plating and stripping are the main keys to achieve a high-capacity ZBFBs. Further research in the ZBFB, especially in developing specialised membranes, electrodes with high affinity towards zinc and bromine, and additives to prevent water migration [207], can help create a more stabilised battery [186].

4.1.3. Alternatives to ZBFB

An alternative to ZBFB, an alkaline zinc iron flow battery (ZIFB), was also investigated. The electrolyte consists of Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ and Zn(OH)₄²⁻ active species. In 2014, ViZn Energy developed a ZIFB module with a capacity of 80 kW–160 kWh installed at Flathead Electric Cooperative [186]. In 2018, researchers demonstrated a steady ZIFB for more than 500 cycles when utilising a porous PBI membrane [208]. In 2020, the same group installed a 10 kW ZIFB system. However, the strong alkaline conditions result in not only a reduced solubility for the Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ but also lead to decomposition of ferricyanide and self-oxidation catalysed by carbon electrodes [209]. By using a chelating agent, the Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ species solubility could be increased to 1.7 M (from 0.4 M) in near neutral electrolytes. This resulted in a cell with an energy density of 74 Wh L⁻¹ at 60 °C [210]. This is only one of the many zinc-based flow batteries investigated in recent years. Others include Zn–I₂ [211], Zn–Mn [212,213], Zn–poly(TEMPO) [214], and Zn–Ce [215] flow batteries.

4.2. All-Iron RFBs

Hruska and Savinell [216] proposed all-iron RFBs (AIRFBs) in 1981. Similarly to all vanadium RFBs, AIRFBs benefit from using the same electroactive species in both the anode and cathode side of the cell. This eliminates concerns with cross contamination, which can degrade the electrolyte as discussed previously, as well as simplifying the electrochemistry of the cell, making it easier to control the redox active window. Since iron is one of the most inexpensive and common metals, AIRFBs are very inexpensive, with costs as low as $2/kWh. AIRFBs can be further divided into aqueous systems or hybrid systems, each with its own advantages and disadvantages.

4.2.1. Chemistry of All-Iron RFBs

AIRFBs employ iron in different valence states for both the positive and negative electrodes [217]. During cell operation, the electrolyte undergoes both reduction and oxidation, according to reactions 23 and 24 [218].

Anode:

$$F{e}^{2+}+2e^{-}\leftrightarrows F{e}^0;E_a^o=-0.44\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(23)

Cathode:

$$F{e}^{3+}+e^{-}\leftrightharpoons F{e}^{2+};E_c^o=0.77\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{H}\mathrm{E}$$

(24)

The three different valance states of iron make operation of AIRFBs possible. These systems are non-hazardous and inexpensive, with relatively simple engineering, which makes them good candidates for many applications [218]. A basic illustration of AIRFB operation can be seen in Figure 12.

Hybrid AIRFBs

Hybrid AIRFBs were the first type of AIRFBs to be developed and utilise solid Fe on the anode side suspended in an electrolyte and iron in an all-aqueous electrolyte on the cathode side. The anode side is typically in the form of a slurry which increases energy density and reduces the amount of water required [219,220]. However, this comes at the expense of higher ohmic resistance due to the higher viscosity of the slurry [219,220]. As a result, the depths of the charge and discharge cycles are reduced. The Coulombic efficiency can also decrease due to crossover of the active ions through the separating membrane. In addition, Fe⁰ crystals can deposit on the negative electrode to form dendrite crystals that may puncture and damage the separating membrane [219]. Overall, these effects can reduce the battery’s cycling ability over time, until it ceases to operate.

Aqueous AIRFBs

To address issues with AIRFBs, including hydrogen evolution and dendrite formation, aqueous AIRFBs have been investigated. Aqueous AIRFBs utilise organic ligands to form complexes with iron ions. This allows for the use of electrochemically stable redox agents across a much larger range of conditions. For example, it has been demonstrated that an AIRFB can operate in a pH range from near neutral to pH 8.6 using iron Fe³⁺ complexed with iron(III)-N,N’-ethylene-bis-(o-hydroxyphenylglycine) on the anode side and sodium ferrocyanide on the cathode side. This system would also be environmentally friendly. Other ligands have been examined for use in AIRFBs, including organic diacids, malic acid, malonic acid, amino acids, and dimethylsulfoxide, and were found to increase iron solubility over an even wider pH range [221]. Glycine was found to be the best candidate for use in an all-iron RFB, with faster kinetics and the ability to stabilise 0.5 M iron at pH 2 [221,222]. An electrolyte with a 1:1 glycine-to-iron ratio showed a reasonable open-circuit potential of 468 mV vs. Ag/AgCl. The cell performed best when the anolyte pH was 1 and the catholyte pH was in the range 3–4. At a discharge power density of 50 mW/cm², the cell had a Coulombic efficiency of 90; although, it had a low energy efficiency (50%). It was discovered that proton reduction during cycling caused an electrolyte imbalance, but at higher cell voltages, hydrogen evolution causes the pH to rise and Fe(OH)₂ to precipitate. Currently, energy storage systems (ESS) and electric fuel are key players in the manufacturing of iron hybrid redox batteries [219].

4.2.2. Advantages and Disadvantages

Compared to other battery RFB chemistries, the all-iron batteries have some of the lowest reagent costs and have the advantage of low chemical toxicity [186]. There have been several large-scale applications of the technology. United States-based ESS Inc. (ESS) specialises in the industrial application of AIRFB. They have numerous installations, including a 10 kW/60 kWh capacity system at the California Stone Edge Farm Winery from 2016 and two 400 kWh systems for a microgrid from 2017 [186]. In addition to the low cost, AIRFBs are also easily recyclable and are environmentally friendly compared to VRFB [223].

However, there are numerous challenges that still need to be addressed. Similarly to Fe−Cr FB, the all-iron type also needs auxiliary components to ensure the system runs stably as the severe side reaction and poor Fe⁰/Fe²⁺ activity result in penalties in both system cost and energy efficiencies. Research has been focused on decreasing the hydrogen generation through the addition of additives such as ammonium chloride, sodium citrate, and other organic compounds [218].

The problem with organic compounds is the long-term decomposition of organic complexes over time, leading to capacity degradation and cycling instability. An alternative is to use an iron complex. Recent research demonstrated the use of such Tris(4,4′-bis(hydroxymethyl)-2,2′-bipyridine) iron dichloride in the positive electrolyte and bis (3-trimethylammonio) propyl viologen tetrachloride in the negative electrolyte. The resulting electrolyte was near neutral and demonstrated excellent cycling with a decay of only 0.07% per day over 35 days [224].

The use of sodium citrate resulted in a stable Fe²⁺-citrate complex that increased cycling stability and achieved a near-100% Coulombic efficiency [225]. Although these additives, among others, show promise, the long-term stability has yet to be examined. More research is required to investigate the use of additives to mitigate hydrogen generation as well as stabilise the iron complex in an attempt to broaden the pH range in which the electrolyte is stable [186].

5. Suitability for Sustainable Powering of Australia

Australia, a country with vast renewable energy potential, continues to expand its solar and wind energy capacity. Correspondingly, the need for effective energy storage solutions will become increasingly critical. This increasingly turned Australia to advanced energy storage technologies to enhance grid stability and facilitate the transition to a low-carbon economy. A number of different energy storage technologies are employed across Australia, each with their own advantages and disadvantages. A summary of the different energy storage technologies is presented below.

5.1. Pumped Hydro Energy Storage

PHES, pumped hydro energy storage, is a method of storing energy by using two reservoirs of water at different elevations. During periods of low electricity demand, excess energy is used to pump water from the lower reservoir to the upper reservoir, effectively storing potential energy. When electricity demand is high, the stored water is released back into the lower reservoir through turbines, generating electricity. This process can be repeated, making it a reliable and efficient way to balance supply and demand in the power grid [226].

PHES is the world’s largest energy storage technology, contributing 96% of the total global energy storage capacity [226]. Approximately 616,000 potential sites for PHES have been identified with over 23,000 TWh of potential energy storage capacity [227]. Over 160 countries have some form of PHES. It has significant advantage over other energy storage technologies in scale, with typical operation ranging from 10 to 4000 MW, and robustness and longevity (40–60 years) [226]. However, due to the significant amount of infrastructure requirements, the cost of PHES is comparatively high at 2000–4300 $ kW. Due to the complexity of PHES, the lead time is typically 5–12 years depending on the location [226,227].

In Australia, PHES provides 5–7% of total electricity [228]. There are over 120 operating hydroelectric power stations, concentrated mostly in southeastern Australia [228]. There are three PHES projects connected to the national electricity grid: Wivenhoe Dam, Tumut 3, and the Shoalhaven power stations, together amounting to 1.6 GW. One of the newest projects under construction is the Borumba Hydro Project in Queensland, Australia. The project is expected to provide 2000 MW of storage with the project commencing in 2022. However, the lead time is 7 to 10 years and is expected to cost $12–14 billion AUD [228].

Although some studies have determined that Australia has adequate PHES sites to support a 100% renewable energy market, the future of the market remains uncertain. The economic viability in a rapidly changing market is a large hurdle and the long-term impact of climate change on water sources is a concern. Although suitable for medium- to long-term storage, the technology is not suitable for smaller scales and remote locations such as mining sites [226].

5.2. Solid Gravity Energy Storage Systems

Gravity energy storage systems, also known as gravitational potential energy storage, operate by using surplus energy to lift a mass, typically water or solid weights, to a higher elevation. This stored potential energy can be converted back into electrical energy when the mass is allowed to descend, driving a generator in the process. These systems are highly efficient, environmentally friendly, and provide a reliable way to balance energy supply and demand. However, their implementation requires significant infrastructure and suitable geographical locations, which can limit their widespread adoption [229]. There are currently no full-scale applications of solid gravity energy storage systems in Australia; however, Green Gravity has recently raised $9 million dollars in funding from investors to explore the application of this technology at an old mine site, Yancoal, in New South Wales [230]. The company states that they expect the technology to be suitable for mid duration storage applications of 4–24 h and deliver an energy efficiency of up to 80%.

5.3. Fly Wheel Energy Storage

Flywheel energy storage (FES) systems work by converting electrical energy into kinetic energy through a rapidly spinning rotor or flywheel. When energy is needed, the rotor’s kinetic energy is converted back into electrical energy by a generator. The flywheel is housed in a low-friction environment to minimise energy loss, allowing it to maintain its rotational speed efficiently. This method provides quick energy release and is ideal for short-duration energy storage and stabilisation of power grids.

It is characterised by high power density, quick response times, and high energy efficiency; however, it has comparatively high capital costs and the moving parts are susceptible to failure under the high operational speeds associated frequent use. In Australia, there has been very limited adoption of FES (32 kWh) [231].

5.4. Green Hydrogen Energy Storage

Green hydrogen energy storage involves producing hydrogen through electrolysis, which uses renewable energy sources like solar or wind to split water into hydrogen and oxygen. The hydrogen is then stored in pressurised tanks or underground caverns until needed. When energy demand rises, the stored hydrogen can be converted back into electricity using fuel cells or combustion engines, releasing only water vapour as a byproduct [226]. This makes green hydrogen a clean and efficient way to store and release energy, aiding in the transition to a sustainable energy system. However, the efficiency of electricity-hydrogen is only 55–75% depending on the transformation method [232].

In Australia, the existing natural gas network can be supplemented with green hydrogen to reduce greenhouse gas emissions [226]. The injection of 2% hydrogen into the existing network has already been trailed and larger commercial gas turbines are capable of processing higher percentages of hydrogen (75%) [233]. However, the injection of 10% hydrogen into the existing gas network will require significant additional infrastructure and retrofitting of the network. The use of this technology is also complicated in remote areas of Australia which would require high capital investment. The Australian government has established a $2 billion hydrogen head start initiative which aims to scale up large green hydrogen projects in Australia. However, the technology is still in its infancy in Australia [234].

5.5. Batteries

Batteries are the most used energy storage system globally. The earliest battery can be traced back to Bagdad, Iraq (Mesopotamia), and dates back over 2000 years ago. In 1938 archaeologists found jars composed of an iron rod in a copper cylinder with an asphalt stopper and it is believed that the Parthian civilisation used such jars to plate gold onto silver [235,236]. In 1800, Allesandro Volta demonstrated a device formed from an alternate stack of zinc and silver plates isolated from each other by a piece of cloth dipped in a solution of weak acid and salt. This was the first demonstration of what can be recognised as a modern battery [237].

Currently, battery application ranges from micro batteries measuring only 3 mm × 3 mm with a capacity of 2 mAh [238], to one of the most commonly used batteries in the world, the 18,560 cylindrical lithium-ion battery, measuring 18 mm by 65 mm with a capacity ranging from 1500 mAh to 3500 mAh for commercial batteries; in large-scale commercial applications, capacities are measured in MWh.

Batteries are used in everything from internal medical devices to personal devices such as watches and phones to electrical cars. Large-scale applications are used to store renewable energy storage such the massive Edwards and Sanborn solar and energy storage system used in America, which has an impressive 3287 MWh capacity battery [239]. Batteries have become ubiquitous in most regions of the world, and battery technology has evolved for specific applications. The metal ion batteries, such has lithium and sodium ion batteries, have high power and energy density, comparatively long life, and high cycle efficiencies. Lithium-ion batteries have become very popular with the advancement of electrical vehicle usage and the advancements in technology that require high capacity but low-profile energy storage. As a result, the use of LiB is expected to increase with the global demand of batteries expected to reach up to 130,000 GWh by 2040 if current trends are maintained [240]. As popular as LiBs are, there are several disadvantages. One of the largest of which is thermal runaway, which poses significant risks of fire and explosion. Numerous recent examples have linked LiB to fire outbreaks [241]. The Australia Fire and Rescue Services New South Wales, one of the world’s largest urban fire and rescue services, reported that approximately 1 in every 100 fires attended by the department involved lithium ion batteries [242]. In addition, the complex battery construction complicates the recycling and recovery of the battery material, specifically the black mass. Currently, only 10% of discarded LiBs are recycled in Australia, raising concerns about the environmental impact of these batteries [243].

In contrast, older battery technologies such as lead acid (LA) batteries have comparatively reduced power and energy density (300 W/kg and 75 Wh/kg, respectively) and concerns have been raised about the environmental and health impacts of using lead electrodes; however, the technology has several advantages [226]. LA batteries are sealed and spill-proof, can be operated at temperatures from −40 °C to 55 °C, and are easily recyclable with up to 99% recovery of the material [226,243]. The use of this technology is still common globally and remains one of the cheapest energy storage technologies.

Comparatively, RFBs offer several advantages, including scalability, a long cycle life, and the ability to independently size power and energy capacities. This makes them ideal for large-scale energy storage applications, such as balancing renewable energy sources and providing grid stability. They also have a relatively low environmental impact and can be recharged quickly. At larger capacities (>8 kWh), RFB costs are comparable with lithium iron phosphate batteries and are ideal for large-scale renewable energy storage [226,231]. A detailed overview of RFBs usage in Australia is provided below.

5.6. Redox Flow Batteries in Australia

RFBs have become significant due to their distinct characteristics and appropriateness for large-scale energy storage applications. RFBs present several economic benefits compared to other types of batteries, making them an appealing energy storage option in Australia, particularly in areas with high renewable energy generation.

Australia’s substantial vanadium reserves make VRFBs attractive, spurring interest in developing a domestic RFB industry. These resources support Australia’s goal of creating strong critical minerals supply chain for advanced technologies like batteries. Established in 2019, the Future Battery Industries Cooperative Research Centre (FBICRC) aims to promote innovation, research, and commercialisation of advanced battery technologies. It is a collaborative effort involving industry, government, and research institutions, enhancing Australia’s position as a leader in battery development.

The following are some of the benefits of RFBs in the Australian context:

• Long Cycle Life: RFBs typically have long cycle lives, often exceeding 20,000 cycles [113], reducing the need for frequent replacements and lowering the overall cost of energy storage. In comparison, lithium batteries have a 3000-cycle lifespan at an 80% deep discharge [119]. VRFBs are suggested to be more efficient in hot climates, where lithium batteries age faster and incur higher long-term costs due to the increased temperatures [244].
• Scalability: RFBs are highly scalable, allowing users to adjust the system size to meet their specific energy storage needs by increasing the electrolyte volume. This flexibility can lead to cost savings by avoiding the over-sizing of storage systems [244].
• Low Maintenance: RFBs need less upkeep than many other energy storage technologies. Separating the electrolyte storage from the cell makes maintenance simpler, lowering operational costs.
• Safety: RFBs are regarded as safer because they use non-flammable and non-toxic electrolytes [245]. Compared to other battery types, RFBs present a lower risk of thermal runaway or fire incidents, which can pose significant hazards in certain scenarios [245,246,247,248].
• Environmental Impact: RFBs can be more environmentally friendly. For example, VRFBs use vanadium, which is relatively abundant and efficiently recyclable. In contrast, the recycling processes for other battery technologies, such as lithium batteries, pose higher risks, including an increased risk of combustion, greater environmental impact, and more complexity [249,250].

While RFBs offer these advantages, it is essential to note that the viability of the technology can vary depending on factors such as location, electricity prices, system size, and the specific application. A thorough economic analysis should be conducted to assess the financial feasibility of deploying RFBs in a particular context in Australia. Due to the low rate of industrial application, the amount of information on costs of various RFBs are limited. However, recent work evaluated the commercial viability of existing RFB technologies to help guide research and development. It compared the levelised capital cost (LCOS) of storage for different RFBs to account for operating cost, capital cost, and energy throughput over the life of the project [251]. The results can be seen in

Table 7. Economic analysis of different battery types based on the power density and round-trip efficiency of each. Adapted from [251].

Battery Type	Round Trip Efficiency	Power Density (W/cm2)	Reactor Cost ($/kWh)	Chemical Costs ($/kWh)	Capital Costs ($/kWh)	LCOS ($/MWh)	Capital Loss (%/Cycle)
Lithium iron phosphate	98	0.0032	-	101	101	64	0.067
Iron chromium	67	0.18	57	36	92	73	0.222
Polysulphide permanganate flow battery	50	0.08	199	7	206	130	0.015
All vanadium redox flow battery	81	0.3	33	140	173	98	0.171
Polysulphide ferricyanide redox flow battery	62	0.14	70	23	93	76	0.075
Polysulphide sodium/bromine redox flow battery	57	0.19	111	10	121	91	0.681

. Although the LCOS for lithium iron phosphate is the lowest, it is also the most commercialised of the batteries, which helps decrease the associated costs. ZBRFB have not been included in the analysis; however, when integrated into a complete system, the cost is approximately $200 kWh, comparable with that of VRFBs [51].

In Australia, various RFB systems with distinct chemistries have been implemented. Reflow, a company specialising in zinc-bromine batteries located in Queensland, Australia, has installed multiple systems. Energy Storage Industries has implemented iron flow systems and is currently constructing a factory for iron flow batteries in Queensland. The most extensively researched and well-documented RFB is the VRFB. Significant efforts have been devoted to understanding its electrochemical properties, and global initiatives are actively progressing towards its commercial utilisation. In Australia, several VRFB systems have been built and are currently operational, with additional projects in the planning stages (Figure 13,

Table 8. List of current and announced VRFB locations around Australia [252].

Map No.	Manufacturer	Organisation	Location	Power (kW)	Hours	Capacity (kWh)	Status
1	Shanghai Electric	Household VRFB Energy Storage Projects	Northern Territories	2.5	4	10	Operational
2	UET	University of Queensland	Heron Island, Queensland	125	5	625	Decommissioned
3	StorEn Technologies	StorEn-Multicom Resources Limited	Brisbane, Queensland	30			Speculative
4	CellCube	University of New South Wales	Sydney, New South Wales	30	4	129	Operational
5	CellCube	Auckland	Auckland, New South Wales	30	4	120	Operational
6	redT Energy	Monash University	Melbourne, Victoria	180	5	900	Operational
7	VSUN Energy	Priest Bros Orchard	Pakenham, Victoria	20	4	80	Announced
8	VSUN Energy	Meredith Dairy	Meredith, Victoria	80	4	320	Announced
9	UET	University of Adelaide—Roseworthy Solar Farm	Roseworthy, South Australia	100	4	400	Operational
10	VSUN Energy	University of Adelaide	Adelaide, South Australia	135	3.33	450	Under Construction
11	Invinity Energy Systems	Yadlamalka Energy Trust	Yadlamalka, South Australia	2000	4	8000	Operational
12	CellCube	CellCube Pangea	Port Augusta, South Australia	50,000	4	200,000	Announced
13	VSUN Energy	Busselton Farm Property	Busselton, Western Australia	10	10	100	Operational
14	Protean Energy	Ozlinc industries	Perth, Western Australia	5	20	100	Operational
15	VSUN Energy	Standalone EV Battery Charger Research Project	Bayswater, Western Australia	5	6	30	Trial Completed
16	Ultra Power Systems	Thorion Energy	Perth, Western Australia	6	6.6	40	Operational
17	Avess Energy	Avess energy group	Windimurra, Western Australia	50	5	250	Announced
18	VSun Energy	IGO	Fraser Range, Western Australia	50	6	300	Under construction
19	Invinity Energy Systems	VSUN Energy/Horizon Power	Kununurra, Western Australia	78	2.82	220	Pilot
20	VRB Energy	King Island Renewable Energy Expansion VRB	Currie, Tasmania	200	4	800	Decommissioned

) [252].

Australia is well positioned to adopt VRFB technology. In 2022, about 32% of the nation’s total energy production originated from renewable sources [253]. The Australian federal government has set a target of achieving 82% renewable energy by 2030 [254], creating a demand for energy storage solutions such as VRFBs. This supports the development of vanadium electrolyte production across Australia. The country possesses one of the world’s largest vanadium reserves, accounting for 29% of global vanadium deposits [255] distributed nationwide (Figure 14). The largest deposits are in Western Australia, in which most of the geological region is Cenozoic. The majority of mining project in Western Australia focus on vanadium titanium magnetite. In Queensland, where the majority of the geological region is Mesozoic, mining operations are focused mostly on shale-hosted vanadium (

Table 9. List of current mining projects around Australia, the resource type mined, the resource output from the mining projects, and the sate/territory in which the mining projects are located [252].

Project	Organisation	Resource Type	Resource Output	Location
Mounte Peak	Tivan resources	VTM	VTMOC	Northern Territory
Vecco V + HPA project	Vecco group	SHV	BS-HVOO	Queensland
Saint Elmo	Mulitcom Resources Ltd.	SHV	CO	Queensland
Julia Creek project	Qem Ltd.	SHV	VBOS	Queensland
Toolebuc project	Toulebuc project	SHV	BS-HVOO	Queensland
The Richmond-Julia creek project	Richmond Vanadium Technology	SHV	BS-HVOO	Queensland
Speewah	Tivan Resources	VTM	VTMOC	Western Australia
Coates Project	Australian Vanadium limited	VTM	VTMOC	Western Australia
Buddadoo	Czr resources	VTM	VTMOC	Western Australia
Canegrass	Flinders mine Ltd.	VTM	VTMOC	Western Australia
Vindimurra project	Atlantic Ltd.	VTM	VTMOC	Western Australia
Younami-V-Oxide	Venus metals corporation Ltd.	VTM	VTMOC	Western Australia
Victory Bore and Unaly hill projects	Surefire Resources NI	VTM	VTMOC	Western Australia
Barrambie	Neometals Ltd.	VTM	VTMOC	Western Australia
Gabanitha-Murchison Technology metals project		VTM	VTMOC	Western Australia
Yarrabubba-Murchison Technology metals project	Technology metals Australia Ltd.	VTM	VTMOC	Western Australia
Australian Vanadium Project	Australian Vanadium Ltd.	VTM	VTMOC	Western Australia
Nowthanna Hill	Australian Vanadium Ltd.	SHV	CO	Western Australia
Balla Balla	Forge metals Ltd.	VTM	VTMOC	Western Australia

VTM: Vanadium titanium magnetite, VTMOC: Vanadium titanium magnetite ore concentrate, SHV: Shale-hosted vanadium, BS-H VOO: Black shale hosted-vanadium oxide ore, CO: Carnotite ore, VBOS: Vanadium-bearing oil shale.

).

This abundance of resources, combined with favourable renewable energy generation conditions, presents a case for implementing VRFB technology nationwide. Currently, two companies, Vecco Group and Australian Vanadium (AVL), have established a vanadium electrolyte manufacturing plant in Australia, indicating a potential for further expansion [256].

Until recently, Redflow International Pty Ltd. (Brisbane, Australia) was the sole Australian company engaged in the development of zinc bromine batteries [257] after the ZBB Energy Corporation (EnSync Energy Systems) closure. However, the company ceased operation in 2024. The estimated gross installed capacities of zinc-based FB are 2.23 MWh for Redflow, 5.67 MWh for ZBB Energy Corp., and 327 MWh for Premium Power [186]. With Redflow’s closure, there are no longer any companies in Australia producing zinc bromine batteries.

6. Conclusions

Redox flow batteries (RFBs) offer a promising solution for energy storage, particularly within the context of Australia’s expanding renewable energy sector. Since the technology’s inception in 1974, extensive research and development have led to many variations in the initial design, each with distinct advantages and limitations. These include classical or true RFBs, where all the components are in solution, as well as hybrid RFBs, which can be solid/liquid, semi-solid, or gas/liquid. The design of RFBs varies significantly due to the different electrolyte utilised. Collectively, RFB technologies present several advantages over other energy storage technologies, such as lithium batteries, owing to their ease of scalability, cost-effectiveness, long cycle life, and ease of integration into existing energy grids. These properties render RFBs an attractive option for renewable energy storage, especially in remote areas. While multiple companies have already commercialised this technology, further research is required to enhance the stability, longevity, and energy density of the electrolyte. The unique design of RFBs, which separates energy capacity and power, offers significant benefits in terms of scalability, modularity, and adaptability. Despite certain limitations, such as lower energy density and the potential use of expensive or scarce materials, the benefits of RFBs, including long cycle life, cost-effectiveness, and environmental sustainability, position them as a key technology for achieving net-zero emissions.

In Australia, the geographical distribution of the population and the increasing reliance on renewable energy sources like photovoltaic panels and wind turbines underscore the need for efficient and reliable energy storage systems. RFBs provide a locally tailored approach to energy storage, addressing the logistical and economic challenges of large central energy storage systems. The successful implementation of RFBs in Australia could serve as a model for other regions facing similar energy storage challenges. The continued development and deployment of RFB technology will be essential in supporting the transition to a more sustainable and resilient energy system in Australia and beyond.