Application and Future Development of Iron-chromium Flow Batteries

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Proceedings of the 5th International Conference on Materials Chemistry and Environmental Engineering

DOI: 10.54254/2755-2721/123/2025.19567

# _Application and Future Development of Iron-chromium_ _Flow Batteries_

Minghao Huang 1,a,*

_1College of New Energy and Materials, China University of Petroleum(Beijing), Beijing, 102249,_
_China_
_a. webmaster@cup.edu.cn_

_*corresponding author_

_Abstract:_ With the transformation of the global energy structure and the rapid development
of renewable energy, large-scale energy storage technology has become the key to balancing
supply and demand and improving the stability of the power grid. Iron-Chromium Flow
Battery (ICFB), as a new type of electrochemical energy storage technology, has gradually
attracted the attention of researchers and industry. This paper summarizes the basic overview
of the iron-chromium flow battery, including its historical development, working principle,
working characteristics, key materials and technologies, and application scenarios. At the
same time, the future development of Fe-Cr flow battery is discussed, including technological
innovation and cost reduction. Finally, the working principle of the Fe-Cr flow battery is
summarized, which is based on the REDOX reaction of iron and chromium ions in different
electrolytes to achieve energy conversion. This kind of battery has the advantages of long
cycle life, high safety, environmental friendliness, low cost and easy scale, etc., which is
suitable for large-scale energy storage systems, especially in the grid connection of renewable
energy and power grid regulation. Iron-chromium flow batteries also hold the potential to
play a significant role in advancing the energy transition and meeting carbon neutrality targets.

_Keywords:_ energy storage technology, Iron-chromium flow battery, Renewable energy,
Technological challenges, Future development.

1. Introduction

With the progress and development of society, the global energy demand continues to grow. The
contradiction between the limited nature of fossil fuels and the environmental impact is becoming
increasingly prominent, which promotes the exploration and utilization of clean and renewable energy
in society. Energy storage technology plays a significant role in the development of renewable energy,
which can mitigate the output fluctuations of renewable energy sources, enhance the peak load
capacity and energy utilization efficiency of power grids, and facilitate the transition towards a more
sustainable energy structure [1].

Among many energy storage technologies, flow batteries, as an emerging energy storage
technology, has become a research hotspot. It has the advantages of no pollution, high efficiency,
long cycle life, etc. Compared with other energy storage technologies, flow battery technology has
broad development prospects [2]. Iron-chromium flow battery (ICFB) is considered as a large-scale
energy storage technology with great potential due to its advantages of wide application range, low

© 2025 The Authors. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0
(https://creativecommons.org/licenses/by/4.0/).

Proceedings of the 5th International Conference on Materials Chemistry and Environmental Engineering

DOI: 10.54254/2755-2721/123/2025.19567

cost, low environmental impact and high flexibility [2]. However, Fe-Cr flow batteries still face some
technical challenges in practical applications, including the stability of the electrolyte, the activity of
the electrode material, the selectivity of the ion conductive film, and the overall efficiency of the
battery system. To overcome these challenges, researchers have conducted a large amount of basic
and applied research to improve battery performance and reduce costs.

In this paper, the basic working principle, key technologies, application fields, current challenges
and future development direction of iron-chromium flow batteries are reviewed. By analyzing the
latest research findings and market dynamics, this paper aims to provide readers with a
comprehensive perspective on the role and potential of iron-chromium flow batteries in modern
energy systems.

2. Basic Overview of Iron-chromium Flow Batteries

2.1. History and Development of Iron-chromium Flow Batteries

The history of the iron-chromium flow battery (ICFB) can be traced back to the 1970s, when NASA
initiated research into this innovative battery technology to address the need for efficient and reliable
energy storage systems in space exploration. [3] In the 1980s, Sumitomo Electric Company of Japan
developed a 10 kW iron-chromium flow battery system prototype based on NASA technology, and
conducted system integration and scale-up studies [4]. In the 1990s, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, began to conduct in-depth research on Fe-Cr flow batteries
and successfully developed a 270-W small Fe-Cr flow battery stack [5]. However, with the increase
in global demand for renewable energy and energy storage technologies, the research of ironchromium flow batteries has received a new impetus, and researchers have begun to explore new
electrolyte formulations, electrode materials, and battery designs to improve battery performance and
reduce costs [6]. In the 2020s, advancements in technology and growing concerns about
environmental impact have led to a renewed focus on research into iron-chromium flow batteries.
Enterprises such as China State Power Investment Group have successfully developed megawatt ironchromium flow battery energy storage systems, demonstrating their potential for practical
applications [7].

The development of iron-chromium flow batteries reflects the process from proof of concept to
technology maturity, driven by continuous technological innovation and market demand. Despite the
technical challenges, its low cost and environmentally friendly characteristics make it an important
candidate for future energy storage technologies. With further research and development, ironchromium flow batteries are expected to play a key role in energy transition and sustainable
development.

2.2. The Working Principle and Components of Iron-chromium Flow Battery

Iron-chromium flow batteries store and release energy based on the conversion of active substances
between different oxidation states. As shown in Figure 1, the battery consists of two half cells, each
containing an electrolyte of iron and chromium. During the charging process, iron ions are oxidized
from Fe 2+ to Fe 3+, while chromium ions are reduced from Cr 3+ to Cr 2+ . The discharge process is
reversed, Fe 3+ gains electrons and is reduced to Fe 2+, while Cr 2+ loses electrons and is oxidized to
Cr 3+ . The Fe-Cr flow battery consists of an ion-conducting membrane, an electrode, a bipolar plate,
a flow frame, and other components. The electrolyte is a solution containing Fe 2+ /Fe 3+ and Cr 2+ /Cr 3+,
which is the key medium for battery energy storage. Electrodes are usually made of highly conductive
and catalytically active materials that facilitate the REDOX reaction. An ion exchange membrane
separates the two half cells, allowing ions to pass through while preventing the electrolyte from

Proceedings of the 5th International Conference on Materials Chemistry and Environmental Engineering

DOI: 10.54254/2755-2721/123/2025.19567

mixing. The bipolar plate serves as the support structure of the electrode and distributes the electrolyte
flowing through the electrode.

Positive reaction as: Fe 2+ -e - ⇌Fe 3+
Negative reaction as: Cr 3+ +e - ⇌Cr 2+
Total reaction of Fe-Cr flow cell as [8]: Fe 2+ +Cr 3+ ⇌Fe 3+ +Cr 2+

Figure 1: Working principle of iron-chromium flow battery[7]

3. The Key Technology of Iron-chromium Flow Battery

3.1. Electrode

The electrodes of iron-chromium flow batteries play a crucial role in this technology, as they provide
the necessary active sites for REDOX reactions to occur with the active substances present in the
electrolyte. The ideal electrode material should possess the characteristics of high electronic
conductivity, high activity, high stability, high wettability and high specific surface area.

Commonly used carbon electrode materials include a variety of forms, mainly carbon felt, graphite
felt, carbon cloth and carbon paper. On the one hand, carbon and graphite felt are made through a
textile process, and they have a porous three-dimensional matrix of fibers, which provides a rich
channel for the penetration and flow of electrolytes. Carbon paper, on the other hand, is made by
mixing short-cut carbon fiber with an organic binder and subsequently undergoing a carbonization
and curing process. Its surface is relatively flat, its thickness is similar to that of paper, and its internal
structure is compact and porous, which gives it uniform electrolyte diffusion characteristics in
electrode applications [9].

Ren et al. designed a novel electrode that uses cobalt oxide for surface modification of graphite
felt [10]. The cobalt oxide is uniformly dispersed and attached to the fiber structure of the graphite
felt, which significantly increases the number of oxygen-containing functional groups on the surface
of the graphite felt and introduces numerous electrochemically active sites. Thanks to these
improvements, the overall performance of the electrode has been significantly improved. When tested
at a current density of 140 mA/cm², the modified electrode demonstrated an 85% higher charging
capacity than untreated graphite felt. In addition, Ahn et al. successfully developed a composite
catalyst composed of Kochen carbon (KB) and bismuth nanoparticles, which has excellent
bifunctional catalytic performance[11]. In experiments, the catalyst achieved an energy efficiency of

Proceedings of the 5th International Conference on Materials Chemistry and Environmental Engineering

DOI: 10.54254/2755-2721/123/2025.19567

86.4% when the current density was set to 80 mA/cm². This study confirms that the combined use of
bismuth metal with carbon-based materials can effectively improve the overall performance of ironchromium flow batteries. In addition, Zhou Yang et al. successfully prepared a bismuth-based organic
skeleton (Bi-MOF/CC) with Bi-MOF as a precursor on carbon cloth substrate by using hydrothermal
synthesis technology, and then produced bismuth-based carbon cloth (C-Bi/CC) electrode [12]. The
impact of adding metal salt on electrode performance was thoroughly investigated, leading to the
optimization of the electrode's performance. The experimental results indicate that when the amount
of metal salt is 90 mg, the polarization resistance of the electrode is reduced to 1.069Ω, which is 8.5%
lower than that of the original carbon cloth, and the reduction overpotential of Cr 3+ is reduced to
0.25V.

3.2. Electrolyte

The electrolyte of the iron-chromium flow battery is a crucial component of this technology, which
contains a solution of iron and chromium ions, which directly affects the performance of the battery.

In order to comprehensively utilize the electrolyte of iron-chromium flow battery, Li Shuqi et al.
studied the use of oxalic acid precipitation to remove iron ions in a trivalent chromium system, and
analyzed the effects of oxalic acid addition amount, solution pH value, reaction temperature, stirring
speed, etc., on the iron removal rate [13]. Finally, when the amount of oxalic acid added is 1.2 times
the theoretical value, the solution pH value is 3.0, and the reaction temperature is 25 degrees Celsius
(Figure 2b), the iron removal rate can reach more than 97%, as shown in Figure 2a. The stirring rate
has little effect on the iron removal rate, indicating that the oxalic acid precipitation method is an
effective way to comprehensively utilize the electrolyte.

(a)

(b)

Figure 2: The effect of iron removal rate; (a) Iron removal rate and stirring rate; (b) Iron removal rate
and temperature

Wang et al. conducted experimental research on the utilization of the same electrolyte for both the
positive and negative electrode materials in iron-chromium flow batteries [14]. The study investigated
the impact of varying concentrations of iron ions, chromium ions, and hydrochloric acid on the
conductivity, viscosity, and electrochemical behavior of the electrolyte. After a series of experimental
analyses, they found that the overall performance of the electrolyte was optimal when the electrolyte
was composed of 1.0 moles per liter of ferrous chloride (FeCl), 1.0 moles per liter of chromium
chloride (CrCl) and 3.0 moles per liter of hydrochloric acid (HCl). At this specific ratio, the electrolyte
exhibits the highest conductivity, the best electrochemical activity and the best transmission
performance, and the best synergistic effect of performance is achieved.

Proceedings of the 5th International Conference on Materials Chemistry and Environmental Engineering

DOI: 10.54254/2755-2721/123/2025.19567

3.3. Ion Exchange Membrane

The ion-exchange membrane in iron-chromium flow batteries is a key component to realize the
isolation of the positive and negative electrolytes of the battery, which allows the passage of carriers
such as protons (H + ) to maintain the closed circuit of the battery while preventing cross-contamination
of the active substances in the electrolyte. The ideal ion exchange membrane should have several key
characteristics: it needs to be able to efficiently screen different ions to ensure isolation between
electrolytes, while ensuring rapid ion transport to maintain the high conductivity of the battery [15].
In addition, the film should also possess strong mechanical durability to withstand various physical
stresses during the use of the battery, and should also demonstrate good chemical stability to resist
the erosion of chemical substances in the electrolyte to ensure long-term reliability [16]. In short, an
excellent ion exchange membrane is a key factor in battery performance and longevity.

Ion-exchange membranes exhibit varying properties based on their thickness. Scientists have
carried out performance tests on ion-exchange membranes of various thicknesses and have identified
the characteristics and properties associated with each thickness. Sun et al. conducted a series of
performance tests for Nafion series ion exchange membranes of different thicknesses, specifically
Nafion212 (50 microns), Nafion115 (126 microns) and Nafion117 (178 microns) [17]. These tests
include key parameters such as ion exchange capacity, proton conductivity and ion permeability.
Their goal was to assess the specific effects of these membranes on the performance of iron-chromium
flow batteries. Their research revealed that thicker membranes, while more effective at preventing
the diffusion of ions and reducing the mixing of active substances, also resulted in decreased voltage
efficiency due to increased membrane resistance. In contrast, the thinner Nafion212 film exhibits
better voltage and energy efficiency at current densities between 40 and 120 mA/cm2 due to its lower
resistance. Therefore, from the dual point of view of cost and performance, the Nafion212 membrane
is considered to be the most suitable option for iron chromium flow batteries among the three
membranes.

4. Application of iron-chromium flow battery

For power systems, there are many application scenarios for energy storage, which can be specifically
divided into power generation measurement energy storage, grid side energy storage and user side
energy storage, but the most suitable energy storage mode should be selected in practical application

[18].

Specifically, taking the iron-chromium flow battery energy storage demonstration project of State
Power Investment Group as an example. As shown in Table 1, the project has been put into trial
operation in Zhanshigou photovoltaic power Station, Guyuan County, Zhangjiakou City, Hebei
Province, demonstrating the advantages of iron-chromium flow batteries in long-term energy storage
scenarios, and their ability to effectively adapt to adverse effects of high temperature and cold weather.
Furthermore, it exhibits significant safety performance [7].

Table 1: Major energy storage application scenarios for power systems

Scope of
Type of application Frequency/year
Application

Charging and
discharging time
(per application)

Generation-side

Thermal power unit peaking 300 4 - 8h
Thermal power unit frequency regulation >5,000 15min - 2h
New Energy Smoothing >5,000 15min - 1h
New energy power quality improvement 50 S class - 15min
New energy follow group plan output 300 0.5 - 0.4h
New Energy Peaking 300 0.5 - 0.4h

Grid-side

User-side

Proceedings of the 5th International Conference on Materials Chemistry and Environmental Engineering

DOI: 10.54254/2755-2721/123/2025.19567

Table 1: (continued).

Grid-side FM >5,000 15min - 2h
Grid-side peaking 100 4 - 16h
Delayed capacity expansion of transmission and
300 4 - 8h
distribution equipment

Black start 1-10 1h

Peak and valley arbitrage and capacity
300 4 - 8h
management

Power quality improvement 100 S class - 30min
Backup power 1-10 4 - 8h

In addition, the large-scale application of iron-chromium flow battery technology is of great
significance for promoting the green transformation of energy, ensuring energy security, promoting
the high-quality development of clean energy, and combating climate change. Finally, iron-chromium
flow batteries can also be used in industrial energy storage, optical storage systems, and commercial
and residential energy storage [18-19].

5. Conclusion

Iron-chromium flow batteries, with their advantages of low cost, long life and high safety, show great
potential in promoting global energy transformation and upgrading. Despite technical challenges such
as electrolyte stability, electrode performance and ion conductive film selectivity, recent advances in
material modification and battery design have laid a solid foundation for improving performance and
reducing costs. From renewable energy connected to smart microgrids, from peak-valley price
arbitrage to backup power systems, iron-chromium flow batteries have broad application prospects
and are expected to occupy an important position in the future energy storage market. Continuous
research, policy support and market drive will further promote its commercialization and
industrialization, and provide strategic support for achieving carbon reduction targets and optimizing
the energy structure. For the optimization of electrode materials, the performance of electrode
materials can be improved through surface modification, the use of suitable catalysts, and
hydrothermal synthesis technology. In terms of the improvement of the electrolyte, scientists greatly
improved the comprehensive utilization efficiency of the electrolyte by using the oxalic acid
precipitation method, and obtained the best ratio by changing the different ratios of the electrolyte,
so that the conductivity, activity and transmission efficiency of the electrolyte reached the best.
However, there are still some limitations in this paper, such as the lack of empirical research on the
performance and efficiency of iron-chromium flow batteries in practical applications, which may
affect the comprehensive evaluation of the practical feasibility of the technology. Although the key
technical parameters of the Fe-Cr flow battery are given in this paper, it may not cover all the factors
that affect the performance of the battery, such as environmental adaptability and long-term stability.
Therefore, in order to overcome these limitations, future research can include more extensive
empirical research, more comprehensive technical parameter analysis, and specific strategies to solve
technical challenges, so as to enhance their research capabilities and make greater contributions to
the development and application of iron-chromium flow battery technology.

With the continuous progress of technology, iron-chromium flow batteries are expected to play a
more critical role in the global energy transition, serving as an important bridge connecting clean
energy and future power systems.

Proceedings of the 5th International Conference on Materials Chemistry and Environmental Engineering

DOI: 10.54254/2755-2721/123/2025.19567

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