The electrical conductivity of the electrode felt in a flow battery is one of the key factors affecting the battery’s efficiency and performance. The better the conductivity, the lower the resistance, and the lower the energy loss during charging and discharging, thereby improving the battery’s overall energy efficiency and power output.
Overview of Flow Batteries and Electrode Materials
As a large-scale energy storage technology, the electrodes in flow batteries not only serve as conductive current collectors but also act as the site where electrochemical reactions take place. Currently, graphite felt (GF) or carbon felt (CF) are commonly used as electrode materials in fields such as vanadium redox flow batteries (VRFB). These materials offer advantages such as low cost, good electrical conductivity, high specific surface area and excellent chemical stability. However, they also suffer from issues including insufficient solid-liquid interface compatibility, limited active sites and relatively high mass transfer resistance, which hinder further improvements in battery performance. Consequently, optimising the electrical conductivity of electrode mats—particularly by reducing their resistivity—is of significant importance for enhancing the overall performance of flow batteries.
Basic Parameters of Electrical Conductivity
Typical Range of Electrical Resistivity
Carbon-based materials: The electrical resistivity of graphite felt exhibits a wide distribution, typically ranging from 10⁻⁶ to 10³ Ω·m, depending on the degree of graphitisation and fibre structure; the electrical resistivity of carbon fibre felt is relatively stable, usually ranging from 1 to 20 Ω·m.
Metal Composite Materials: In corrosive electrolyte environments, precious metal-modified materials such as platinum-based composite fibres can maintain stable electrical conductivity within the range of 10⁻² to 1 Ω·m, although they are relatively expensive.
Other Relevant Electrical Parameters
In addition to resistivity, the electrical conductivity, contact resistance and charge transfer resistance of the electrode are also important indicators for evaluating its electrical conductivity; these three factors collectively influence the reaction kinetics and ohmic losses of the electrode.
Key Factors Affecting the Electrical Conductivity of Electrode Felt
Intrinsic Material Properties
Carbon fibre purity and degree of graphitisation: The lower the impurity content and the higher the degree of graphitisation in carbon fibres, the better the intrinsic electrical conductivity tends to be.
Fibre diameter and orientation: Fibres with smaller diameters and uniform orientation are more likely to form a continuous conductive network.
Manufacturing Process Parameters
Needling density and heat treatment temperature: An appropriate needling density facilitates the formation of stable conductive contacts between fibres; high-temperature heat treatment can enhance the degree of graphitisation and reduce electrical resistivity, but excessively high temperatures may cause the fibres to become brittle.
Surface modification: Surface conductivity and reactivity can be significantly enhanced through activation, doping, or the deposition of conductive coatings (such as carbon nanotubes or graphene).
Operating Environmental Conditions
Effect of humidity: When ambient humidity exceeds 60%, moisture adsorption on the carbon fibre surface may cause contact resistance to increase by 15–30%.
Electrolyte Wetting: The interfacial compatibility between the electrode and the electrolyte directly affects charge transport efficiency; good wetting reduces interfacial contact resistance.
Long-Term Operational Stability: During prolonged charge-discharge cycles, the electrode material may undergo corrosion, oxidation or structural collapse, leading to the degradation of the conductive network and a gradual increase in resistivity.
Technical Specifications for Resistivity Measurement
Common Measurement Methods
Four-point probe method: Suitable for block or sheet-like electrode materials; the distance between the current probes should be greater than five times the thickness of the specimen to minimise boundary effects.
Two-point probe method: Primarily used for fibre or film materials; care must be taken to correct for contact resistance.
AC impedance spectroscopy: Capable of distinguishing between bulk resistance, contact resistance and charge transfer resistance; suitable for tests simulating operational conditions.
Test Environment and Calibration
It is recommended that standard test conditions be maintained at a temperature of 23±2°C and a humidity of 45±5% to eliminate the influence of environmental fluctuations.
Prior to testing, system errors should be corrected using standard resistance standards to ensure data reliability.
Engineering Applications and Optimisation Recommendations
Guidance on Electrode Selection
For all-vanadium flow battery systems, it is recommended to select modified carbon felt with a resistivity within the range of 0.1–0.5 Ω·m, in order to balance conductivity, stability and cost.
For high-power applications, graphite felt or metal composite electrodes may be considered to further reduce ohmic polarisation.
Approaches to Performance Enhancement
Structural optimisation: Enhancing the connectivity of the conductive network by controlling fibre orientation and pore structure.
Surface modification: Techniques such as chemical vapour deposition (CVD), electroplating or chemical modification can be employed to form conductive coatings on the electrode surface or introduce catalytic active sites, thereby reducing interfacial resistance by approximately 20–40%.
Development of Composite Electrodes: By combining carbon materials with conductive polymers, carbon nanomaterials and the like, it is possible to synergistically enhance both conductivity and electrochemical activity.
Early Warning of Failure and Service Life Management
An increase in resistivity exceeding 15% of the initial value may serve as an early warning indicator that the electrode structure is beginning to deteriorate, signalling the need for maintenance or replacement.
Regular monitoring of changes in electrode resistance, combined with battery performance degradation data, provides a basis for predicting and managing electrode service life.