Methods for improving low temperature performance of flow batteries

 

The efficiency of liquid flow batteries will be significantly reduced at low temperatures, and divalent vanadium ions will precipitate in vanadium electrolytes at low temperatures, seriously affecting battery performance and life. The main factors causing poor performance of liquid flow batteries at  low temperatures are:

1. At low temperatures, the viscosity of the electrolyte increases, resulting in reduced conductivity;

2. At low temperatures, the charge transfer impedance at the electrode/diaphragm interface increases;
At low temperatures, the migration rate of active substances in the electrolyte decreases, and the electrode polarization increases.
Effective methods to improve the low-temperature performance of flow batteries are proposed mainly from the aspects of electrodes, electrolytes, and operating parameters.

1. Electrodes

As the reaction site of active substances, the electrode's activity, conductivity, compressibility, porosity, permeability and other performance parameters are directly related to the performance of the battery stack. Among them, the electrode activity is most significantly affected by temperature. In  vanadium flow batteries, the poor activity of the negative electrode is the main factor restricting the further improvement of the performance of vanadium flow batteries. At present, most flow battery electrodes use graphite felt as electrodes. Graphite felt is a porous material.

As an electrode, it can increase the specific surface area of ​​the electrode and can also be used as a diffusion layer. Ordinary graphite felt has poor electrode activity. Even after high-temperature treatment, the electrode activity cannot be well exerted. At present, the main research work is focused on electrode modification and modification, especially the activity of the negative electrode needs to be improved.

The main way to improve the electrochemical activity of negative electrode materials at low temperatures is to modify the electrodes. The  electrodes are modified with catalysts (such as TiN nanowires, TiC, MnO2, OTiB2, TixOy), and the electrode activity is improved by surface coating and deposition on the electrodes, which reduces the electrochemical polarization of the battery and the side reactions of the battery at the end of charge and discharge.

2. Electrolyte

As the storage place for active substances in flow batteries, the electrolyte is the capacity unit of flow batteries. The conductivity of the electrolyte increases with increasing temperature, and the viscosity decreases with increasing temperature. The comprehensive performance of the battery can be optimized by increasing the comprehensive valence state of the electrolyte and increasing the volume of the negative electrode electrolyte. See the influence of the state of the negative electrode electrolyte of the flow battery on the battery performance.

For vanadium battery electrolytes, the negative electrode electrolyte is easy to precipitate at low temperatures, the stability of the negative  electrode electrolyte is poor at low temperatures, and the viscosity increases and the conductivity decreases. At present, the main ways to improve low-temperature performance through electrolytes are: 

1) Optimizing the solvent composition, by optimizing the sulfuric acid-vanadium ion concentration ratio, to improve the stability of the negative electrode electrolyte at low temperatures; 
2) Developing mixed acid electrolytes, such as hydrochloric acid-sulfuric acid system vanadium electrolytes.
3) Additives, such as inorganic salts, organic acids, etc., through additives, destroy the precipitation/precipitation mechanism to increase the  precipitation/precipitation barrier.

3. Operation parameters
The operation parameters of flow batteries mainly include charge and discharge mode, electrolyte flow rate, temperature, SOC, etc. The performance of flow batteries can be improved by adjusting and optimizing the operation parameters. For example, at the end of charge and discharge, the  electrolyte flow rate is increased to increase the battery capacity and electrolyte utilization.

The main ways to improve the performance of flow batteries at low temperatures by optimizing the operation parameters are:

1) Increase the electrolyte flow rate, actively increase the diffusion of the electrolyte, reduce the concentration polarization of the electrolyte on the electrode, reduce the diffusion impedance, and improve the performance of the flow battery;

2) Control the SOC and operate the flow battery at a low SOC as much as possible. At low temperatures, the stability of the negative electrode electrolyte deteriorates. By reducing the concentration of divalent vanadium ions at the negative electrode, the risk of divalent vanadium ion precipitation is reduced;

3) Reduce the charge and discharge density (power). At low temperatures, the electrochemical performance decreases, and there is a risk that the liquid flow battery cannot operate normally under high density. By controlling the charge and discharge mode, high-power (density) charging and discharging is not performed at low temperatures. After running for a period of time, the heat generated is used to increase the temperature before high-power charging and discharging.

Liquid flow batteries (RFBs) generate a lot of heat during operation. If the heat cannot be dissipated in a timely and effective manner, the battery temperature will rise, thus affecting the battery performance and safety. The electrochemical reaction conditions, ion conductivity, the rate at which ions move across the membrane, and the viscosity of the electrolyte are all closely related to the temperature during operation. Specifically, increasing the temperature can increase the reaction rate constant and promote the reaction kinetics in the electrochemical reaction. At the same time, high temperature will also reduce the viscosity of the electrolyte, thereby increasing the transmission efficiency of vanadium ions from the main body to the electrode surface and reducing the concentration polarization potential. However, when the temperature exceeds a certain range, it will have a fatal effect.

 

Taking the vanadium redox flow battery (VRFB) as an example, its normal operating temperature range is 0~40°C. As the temperature increases, the hydrogen evolution reaction on the negative electrode will be significantly enhanced, resulting in a decrease in Coulombic efficiency. At the same time, the diffusion ability of vanadium ions through the ion membrane is enhanced, which intensifies the capacity fading. In addition, the vanadium active ions in the electrolyte are unstable and prone to precipitation when the temperature is abnormal. When the electrolyte of 2 mol/L VO+2+3 mol/L H2SO4 is placed at 40°C for 2 days, the VO+2 Converted into V2O5 precipitation; and after being placed at 15°C for 7 days, V2+ in the electrolyte will precipitate. This generated precipitate will block the flow channel, cover the carbon felt and ion membrane, lead to increased pump power loss and battery failure.

 

Sustained high temperature will also accelerate the aging of the internal electrodes, proton membrane and other materials of the battery, thus shortening the service life of the battery. Therefore, temperature thermal management is of great significance to maintaining the stable operation of flow batteries.

 

In order to ensure the stable and safe operation of flow batteries, it is necessary to establish a thermal model to predict and control the temperature of the electrolyte and further guide battery optimization control, which is also an important part of the thermal management system.

 

The factors that generate heat during the operation of all-vanadium liquid flow batteries include electrochemical reactions, overpotential, hydraulic friction, cross-reactions and shunts, among which electrochemical reactions and overpotential heat generation account for a larger proportion compared to the other three.

 

At present, the thermal management technology routes of electrochemical energy storage systems are mainly divided into four categories: air cooling, liquid cooling, heat pipe cooling and phase change cooling. The mainstream technology routes for thermal management of liquid flow battery energy storage in the market are air cooling and liquid cooling. The choice of these heat dissipation methods depends on the scale, design, operating conditions and cost-effectiveness of the battery.

 

1) Air cooling

Air cooling is wind cooling, which uses air as a medium to remove the heat inside the system by heat conduction and heat convection, thereby cooling the system. Air cooling is divided into natural air cooling and forced air cooling according to the driving mode. Natural air cooling uses natural conditions such as natural wind pressure, air temperature difference, and air density difference to achieve a cooling effect on the battery.

The convection heat transfer coefficient of natural air cooling is much lower than that of forced air cooling, so it is difficult to completely dissipate the heat generated by the battery. For low-rate charge and discharge of the battery, the system temperature can be controlled within a certain temperature range, but the increase in the system current density can easily cause the temperature to exceed the limit range. Therefore, although natural air cooling has the advantages of simplicity, lightness and low cost, its scope of application is extremely small and it is rarely studied now. Forced air cooling is to take away heat through forced airflow generated by a blower or fan. At this time, the heat transfer coefficient of the forced airflow is greatly improved. Compared with liquid cooling, air cooling has the advantages of simple structure, easy maintenance and low cost, but it requires a certain amount of electricity, and the heat dissipation efficiency, heat dissipation speed and temperature uniformity are poor. It is usually suitable for small or medium-sized battery systems.

 

2) Liquid Cooling

Liquid cooling (liquid cooling) uses coolant as the medium and utilizes higher specific heat and heat transfer coefficient to dissipate heat. Liquid cooling systems can provide higher heat dissipation efficiency and better temperature control effects, but the system complexity and cost are also relatively high, and are suitable for large battery systems. Commonly used coolants include water, ethylene glycol aqueous solution, pure ethylene glycol, air conditioning refrigerant, and silicone oil. Since the charge in the electrolyte of the flow battery easily flows along the coolant to the entire system, it is more dangerous, so the choice of cooling medium is also very important. However, the most common method for flow batteries is to use corrosion-resistant and non-conductive heat exchangers. The internal materials are generally the same as those of the electrolyte storage tanks, using PVC or PP, or using titanium metal tubular heat exchangers, and the inner surface is covered with a corrosion-resistant TiO2 layer to protect the heat exchanger from sulfuric acid corrosion.

 

As one of the most promising renewable energy storage technologies, the overheating problem of vanadium flow battery during operation greatly affects the efficiency and stability of the system. Therefore, various feasible methods are needed to provide a feasible solution for the VRFB thermal management system.