Modifying Electrodes to Enhance the Energy Density of Vanadium

Redox Flow Batteries

Kaixin Hu

, Zixiang Xia

and Kuangdi Zhu

3,*

No.2 High School of East China Normal University, Shanghai, 201203, China

Soochow University, Suzhou, 215006, China

University of British Columbia (Vancouver), Vancouver, V6T 1Z4, Canada

Keywords: Vanadium Redox Flow Battery, Electrocatalyst, Electrode Material, Energy Efficiency.

Abstract: Redox flow battery (RFB) as a promised way of large-scale energy storage has been getting more attention

recently, due to the gradually growth demand for electricity. Vanadium flow battery (VFB), an RFB

containing vanadium ions, has been proven to be a candidate of interest for industrial applications. They have

the advantages of relatively high safety, low costs and high durability against other RFBs. However, the high

costs and low energy density prohibit the application of VFBs. Promoting energy efficiency is crucial to

address those concerns since higher energy efficiency will lead to a decreased consumption of raw materials

and low energy dissipation. Electrodes, where the redox reactions occur, play a key role in improving energy

efficiency. There have existed a bunch of approaches to modify the performance of electrodes. In this paper,

recent research achievements on electrode modifications including surface modification, structural and

configuration modification, introducing electrocatalyst and electrode material modification will be discussed.

1 INTRODUCTION

To achieve the ambitious goals of reaching "peak

carbon" and attaining carbon neutrality, industries are

increasingly turning to alternative sources of

sustainable resources, including photovoltaics (PV)

and wind power, within the new energy sector (IEA,

2023). According to projections by the International

Energy Agency (IEA), onshore wind and PV are

anticipated to contribute up to one-third of global

electricity generation this year (IEA, 2023). Despite

their growing adoption, these sources of electricity

often encounter challenges related to intermittency

and instability, which can jeopardise the security and

reliability of the power grid.

Traditional batteries have several drawbacks in

these application scenarios. Firstly, they typically rely

on non-renewable energy sources such as fossil fuels

for charging, thus failing to achieve carbon neutrality

goals. Secondly, traditional batteries have relatively

low energy density, which is unable to support the

requirements of extensive energy repository.

Additionally, the lifespan of traditional batteries is

limited, requiring frequent replacement and

maintenance, thereby increasing costs and resource

consumption.

Redox flow batteries (RFBs), composed of two

electrodes, two current collectors, and a separator

(Emmett and Roberts, 2021), offer numerous

advantages including the ability to separate capacity

and power, deep charge and discharge capabilities,

rapid response times, long cycle life, high safety

standards, and adaptable designs. Consequently, they

hold immense potential for large-scale energy storage

solutions aimed at addressing the aforementioned

challenges. However, the widespread implementation

of existing RFB technology is impeded by various

drawbacks, including high operational and capital

costs, low energy density, and stability concerns

(Emmett and Roberts, 2021). Vanadium flow battery

(VFB) as a king of RFB has been obtaining more

interest recently. As the electrolytes in VFB only

contains vanadium compounds, the penetration

phenomena which is another common concern in

RFB, can be minimized.

Among these limitations, energy density and cost

efficiency are particularly crucial (Emmett and

Roberts, 2021). Obtaining a high energy efficiency is

an effective approach to address those problems

mentioned before (Emmett and Roberts, 2021). With

higher energy efficiency, the capital cost and

operation cost can be reduced by lower amount of

reactants and materials usage. Higher energy capacity

224

Hu, K., Xia, Z. and Zhu, K.

Modifying Electrodes to Enhance the Energy Density of Vanadium Redox Flow Batteries.

DOI: 10.5220/0013882300004914

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd International Conference on Renewable Energy and Ecosystem (ICREE 2024), pages 224-228

ISBN: 978-989-758-776-4

is achieved by the reduced energy dissipation (Wu et

al., 2023). Electrode, as a major part of the VFB

system, provides the place for redox reaction to

happen and charge to transfer as well. By optimizing

electrode materials and selecting designs with high

activity and stability, the ohmic potential will be

reduced while the conductivity and redox reaction

rate can be enhanced. This can be achieved through

the use of novel nanomaterials or surface

modifications such as coatings or functionalization to

improve electrode catalytic activity, thereby

enhancing battery performance and energy

efficiency. Currently, the mainstream approach for

electrode modification involves improving electrode

materials and structures. However, this requires high

electrode stability, as high potential differences may

cause corrosion or oxidation of electrode materials.

This study provides a synthesized review of recent

improvements in electrode modifications for VFBs,

including modifications to electrocatalysts and

electrode materials and surfaces, aiming to address

challenges like scant energy concentration and

vanadium flow batteries' intrinsic high-expense

through improving power efficiency.

2 REACTION PRINCIPLES AND

STRUCTURES

A typical flow battery consists of a catholyte, an

anolyte, carbon felt electrodes, graphite flow fields, a

membrane, and current collectors, shown as Figure 1.

The catholyte and anolyte are injected into their

separate half-cells. The electrolyte flow is directed

over the carbon-felt electrodes by the graphite flow

fields (Emmett and Roberts, 2021). These electrodes

provide surface area for redox reactions (Emmett and

Roberts, 2021). The separator ensures zero species

crossover and uninterrupted transport of hydrated

hydrogen ions. In the course of the redox reactions,

the anolyte is oxidized by losing electrons. These

electrons flow to the cathode fluid through the

external circuit, where they are accepted and restored

the cathode solution. The figure below shows the

charging and discharge cycle of the pillar oxidation

and reduction fluid battery. Whenever a electron

(hydrogenated ion) moves from anode fluid to

cathode fluid, H

cross the membrane to keep the

charge neutral.

Figure 1: Charging and discharging process for a typical

Vanadium flow battery (Sankaralingam et al., 2021)

Electrode materials mainly affect the energy

density of the battery through these aspects:

Conductivity, Surface area, Electrochemical activity

and Corrosion resistance.

Highly conductive electrode materials can reduce

the ohmic loss inside the battery, thereby improving

the efficiency of the battery. The specific area of the

electrode material surface significantly influences the

electrochemical reaction rate. Therefore, as long as

the specific surface area becomes larger, the more

active sites used in electrochemical reactions, which

can accelerate the charge transfer process and

increase the battery's power density and energy

density. The rate and reversibility of electrochemical

reactions are determined by the electrochemical

activity exhibited by electrode materials. Highly

electroactive materials more efficiently catalyse

vanadium ion redox reactions, reduce overpotential,

and improve battery energy efficiency. The

electrolyte used in vanadium flow batteries is usually

highly acidic (such as sulfuric acid), which requires

the electrode material to have good corrosion

resistance to ensure long-term stable operation.

Otherwise, the electrode will corrode and react, and

the battery's energy density will be reduced.

3 IMPROVEMENT METHODS

As stated before, the modification of VFBs focuses

on electrodes and electrolytes. Meanwhile, the

electrode regulates the electrochemical activity of

oxidation and reduced coupling and provides active

sites for the oxidation reaction, this is of great

significance for improving battery performance,

making a critical contribution. Modifications on the

electrode itself and configuration can manipulate the

electrochemical polarizations, concentration and

ohmic which are key factors for VFB performance

evaluation (Wu et al., 2023).

Modifying Electrodes to Enhance the Energy Density of Vanadium Redox Flow Batteries

225

3.1 Surface Modification

There are three important parameters for the

performance evaluation of VFBs. Wettability

indicates the distribution of electrolyte on electrode.

Voltage efficiency (VE) indicates the ratio of input

voltage (charging) and output voltage (discharging).

Energy efficiency (EE) refers to the energy

dissipation during the charging and discharging

process. It is defined as the ratio between the actual

energy output and the theoretical energy output.

These factors determine the performance of VFB. EE

and VE are key to the improvement of VFB’ s

performance. Higher EE will lead to a lower energy

reduction, in other words, the output energy can be

promoted. Similar effect can be expected with an

increased VE. The goal of surface modification is to

maximize all three parameters - wettability, voltage

efficiency, and energy efficiency, in order to achieve

an increase in energy density. Carbon felt (CF) and

graphite felt (GF) are commonly used electrodes in

VFBs due to they are highly conductive and stable

under acidic conditions. However, modifications are

essential for CF and GF to overcome their drawbacks

on low catalytic activity and specific surface area.

Increasing the surface functional groups by thermal

treatment and doping is one of the most common

ways to address those shortcomings. Thermal

treatment and doping can introduce more oxygen-

containing functional groups (OCFs) to the electrode

surface resulting in enhancement on wettability and

electrochemical activity. Coating is another

alternative for surface modification. By introducing

electrocatalyst to the electrode surface, the

performance of electrode becomes highly tuneable

(Wu et al., 2023).

3.2 Structural and Configure

Modification

The modification of electrode structure can

effectively increase the active surface area, enhancing

the power and energy density of the flow battery by

supplying additional active sites for reactions. In

addition to the optimization of electrode structure and

configuration, the optimization of flow field design

can also contribute to improving the performance of

redox flow batteries, such as energy density. A recent

study (Yaji et al., 2018) employed topology

optimization to refine the flow field configuration in

vanadium redox flow batteries. The study formulated

the optimization problem with the objective of

maximizing the generation rate of vanadium species,

considering the porous nature of the carbon fibre

electrode and the mass transfer coefficient dependent

on local velocity. The results indicated that the

interdigitated flow field pattern is an effective design

for VRFBs, which can help improve the charge-

discharge efficiency of VRFBs (Yaji et al., 2018).

Therefore, the energy density of flow batteries can be

improved to a certain extent by optimizing the flow

field design.

3.3 Introducing Electrocatalyst

Electrocatalysts are typically heterogenous catalysts

promoting electrochemical reactions and are widely

used in electrode modification (Raveendran et al.,

2023). The catalytic mechanism of electrocatalysts is

sophisticated and varies among different materials.

Overall speaking, electrocatalysts function at the

surface of electrode altering the reaction pathway and

reducing the activation energy of electrochemical

reactions (Raveendran et al., 2023). They play crucial

roles in VFBs offering enhancement on the reactivity

of the system and more active sites for ion exchange

to take place. Adding an electrocatalyst into the VFB

system can increase energy efficiency and lowering

costs.

3.3.1 Carbon-Based Electrocatalyst

Carbon-based electrocatalysts are responsible for

increasing the surface area of electrodes (Liu et al.,

2018) represented by carbon nanotube (CNT), carbon

felt, carbon nanosheet, graphite oxide, etc. Compared

to the other two electrocatalysts, carbon-based

electrocatalysts are stable in acidic solutions with

high conductivity, cheap and easy to manufacture.

However, they experience limitations on low specific

surface area and low catalytic activity (Wu et al.,

2023).

Modifications on carbon-based electrocatalysts

are essential for their further application. Increasing

the number of OCFs is an applicable way of

reinforcing the catalytic activity and gathering more

attention recently (Molina-Serrano et al., 2024).

Although higher numbers of OCFs introduced to the

electrocatalyst by thermal treatment as mentioned in

3.1 can increase the catalytic activity in carbon-based

electrocatalysts by enhancing the wettability, mass

transfer rate and number of active sites, there exists a

trade-off between the activity and conductivity

(Molina-Serrano et al., 2024). Heteroatom doping is

another strategy to raise the conductivity and catalytic

activity (Wu et al., 2023 & Liu et al., 2018). The

active sites on carbon electrode are regulated by the

defects introduced by the doped heteroatom on lattice

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resulting in a higher electron affinity. There exists

varies types of doping dominated by nitrogen, oxygen

and phosphorate, offering doped electrodes a high

tunability on properties including reversibility, EE,

power and durability to fit varies working conditions.

3.3.2 Metal-Based Electrocatalyst

Metal-based electrocatalyst are typically metal oxides

and metal halides. They are highly promised

materials with low cost and high conductivity. Metal-

based electrocatalysts are mainly in deposited form

and able to decrease the energy barrier of redox

reactions happens at the electrode which is one of the

major limitations of CF and GF electrodes restricting

the reactivity.

Bi deposition is an important method of

modification where the Bi ions reduced on the carbon

electrode by deposition can largely enhance the redox

reaction rate, VE and EE of VFB by implying a

greater charge exchange rate under high current

density (Liu et al., 2018). However, this

reinforcement is bounded by the amount of Bi loaded

onto the electrode, and this boundary value has not

been studied thoroughly for VFBs yet. In parallel with

ion deposition, transition metal oxide is also used for

meta-based electrocatalyst with lowest cost among

different improvement methods (Liu et al., 2018).

Due to the transition metals have various valency

states and are able to act as active sites for receiving

reactive species. Those evenly dispersed metal oxide

particles generate more surface-active OCFs and

active sites, making the electrode more hydrophilic

and permeable for electrolyte. They are also stable at

working conditions and easy to be prepared. Besides

of metal oxide, metal boride, carbide and nitride

which are covalent bonded electro-deficient

compounds, are materials of interest at present. They

can promote the electron transfer by accepting

unpaired electrons from the metal ions resulting in a

significantly higher electric conductivity than metal

oxides (Wu et al., 2023). Such high electric

conductivity enables a much higher electron transfer

rate in VFBs.

3.3.3 Composite-Based Electrocatalyst

Composite, by definition, includes a wide range of

different materials. Composite-based electrocatalysts

have a significantly higher electrocatalytic activity

than individual composition, due to the interactions

between the electrocatalyst and support. For instance,

Bi metal and carbon-based materials composite is one

of the composite-based electrocatalysts of interest

recently (Wu et al., 2023). By introducing Bi metals

into the complex and regulated carbon structure, the

diffusion pathways of electrochemical reactants can

be manipulated. With this, composite materials are

able to provide a better electrode performance than

each single materials included. As discussed in

previous, transition metal compounds act as active

sites for redox reaction to happens and carbon-based

materials acts as support being responsible for electric

conduction. Besides of metal-carbon composites,

polymer composite and metal-organic-frameworks

(MOFs) are other types of composites with huge

prospect on electrode modification (Liu et al., 2018).

MOFs are highly tuneable materials with high

porosity and specific surface area, in other words,

they are able to provide a significant number of active

sites for redox reactions and their tunability enables

them suitable for various type of operation

requirements incorporating with various type of

composited materials.

3.4 Electrode Material Modification

As discussed before, traditional electrode materials

such as GF and CF experience various of limitations

and electrode material modification is necessary to

VFBs. Similar to electrocatalysts, doping, coating,

surface functionalization and application of

composite materials are methodologies widely used

at present (Liu et al., 2018). Some examples can be

found in Table 1.

Qiao et al. (Qiao et al., 2022) successfully

prepared nitrogen-doped carbon felt using an

ammonium sulphate hydrothermal synthesis strategy,

significantly promoting the electrochemical property

of vanadium redox flow battery electrodes. This

paper indicates the importance of nitrogen doping in

strengthening the hydrophilicity and electrochemical

reactivity of electrode materials, achieving a 3.91%

increase in efficiency in energy use contrasted to the

original pure carbon felt under a charge flow density

of 80 mA ⋅ cm



. This innovative perspective

promotes progress in the field of flow batteries,

particularly in the realm of vanadium.

Deng et al. (Deng et al., 2022) fabricated a multi-

dimensional framework electrode material by the

composite of three different-dimensional carbon

materials structures (0D, 2D, and 3D), which offers

high electrocatalytic activity, rapid charge transfer,

and a large area for redox reactions. This MFC-GF

electrode, owing to the high electrocatalytic activity

of its edge carbon, exhibits excellent electrochemical

performance towards the redox pairs and suppresses

the hydrogen evolution reaction in the negative

electrolyte.

Modifying Electrodes to Enhance the Energy Density of Vanadium Redox Flow Batteries

227

Table 1. Modification of electrode material.

Electrode

material

Increase in

Reference

electrode

material

Reference

N doped CF 7.2% CF Qiao et al.

(Qiao et

al., 2022)

MnO@C/CF 8.5 % CF Chen et al.

(Chen et

al., 2023)

MFC-GF 18.3% GF Deng et

al. (Deng

et al.,

2022)

4 CONCLUSION

The activity of carbon-based electrocatalysts can be

enhanced by increasing the number of oxygen-

containing functional groups. Additionally,

introducing dopant atoms at active sites can

significantly increase electron affinity. The

deposition of Bi can effectively increase the charge

transfer rate of carbon electrodes, and covalently

bonded electro-deficient compounds combined with

unpaired electrons from metal ions can achieve higher

electrical conductivity. Composite-based

electrocatalysts incorporating metal-organic

frameworks (MOFs) can supply a wealth of active

centers for reactions, thereby significantly enhancing

the performance of batteries. The modified graphite

felt, MFC, significantly increases current density and

lifespan due to its high electrocatalytic activity and

hindrance of hydrogen evolution.

In the future, modifications in electrode materials,

such as thermal treatment, doping, and coating, will

become increasingly prevalent and play a more

significant role due to their significant enhancement

on vanadium redox flow battery performance. We

look forward to the development and application of

even better electrode materials to further enhance

flow battery performance.

AUTHORS CONTRIBUTION

All the authors contributed equally and their names

were listed in alphabetical order.

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