Frontline Tracking | Biomass Modified Carbon Felt Electrodes - A New Approach to Carbon Felt Electrode Modification in Liquid Flow Batteries-Shenzhen ZH Energy Storage - Zhonghe LDES VRFB - Vanadium Flow Battery Stacks - Sulfur Iron Electrolyte - PBI Non-fluorinated Ion Exchange Membrane

Frontline Tracking | Biomass Modified Carbon Felt Electrodes - A New Approach to Carbon Felt Electrode Modification in Liquid Flow Batteries

Classification：Industrial News

- Author：Luo Xuan

- Release time：Nov-22-2022

【 Summary 】All vanadium redox flow batteries (VRFBs) have been receiving increasing attention due to their broad prospects for application in large-scale energy storage devices. The electrode material of a flow

Research background

All vanadium redox flow batteries (VRFBs) have been receiving increasing attention due to their broad prospects for application in large-scale energy storage devices. The electrode material of a flow battery, as the reaction site during the charging and discharging process of a flow battery, plays a crucial role in the charging and discharging reaction, structural stability, service life, and ultimately operational efficiency and output power of the battery. At present, research on liquid flow battery electrodes includes metal electrodes and carbon electrodes. Traditional carbon electrodes with high chemical stability, such as graphite felt electrodes, carbon felt electrodes, and carbon paper electrodes, are widely used due to their significant cost-effectiveness. However, due to the poor electrochemical activity of traditional carbon felt (CF) electrodes, liquid flow batteries often generate serious overpotentials during operation, hindering the progress of redox reactions. Therefore, developing high-performance liquid flow battery electrodes plays an important role in reducing reaction overpotentials and improving battery charging and discharging performance.

At present, due to the low hydrophilicity and poor electrochemical performance of traditional carbon electrodes, the performance of the assembled batteries is also not satisfactory. Therefore, various modification strategies have been proposed in this area, such as heteroatom doping and nanoparticle modification, to develop efficient carbon based electrode materials for VRFB. Compared with the original electrode materials, their electrochemical performance has been improved to a certain extent. Biomass modified carbon materials have the advantages of low cost, good sustainability, and high electrocatalytic activity for vanadium redox reactions, making them a powerful method for modifying VRFB electrode materials.

Research Highlights
Zhengyu Hu et al. reported the preparation of a biomass lotus seed shell modified carbon felt electrode (Bio CF), using lotus seed shells as raw materials and synthesizing hard carbon materials through pyrolysis. This electrode exhibits significant electrocatalytic effects on the redox reactions of V2+/V3+and VO2+/VO2+. The article studied the effects of auxiliary sucrose concentration and pyrolysis temperature on the performance of Bio-CF electrodes, and determined the optimal auxiliary sucrose concentration and pyrolysis temperature. Molecular dynamics simulation was used to verify the experimental data. By optimizing the preparation parameters reasonably, the prepared hard carbon is densely distributed on the surface of carbon fibers, increasing the specific surface area and enhancing the electrocatalytic activity for vanadium redox reactions. The optimized Bio CF electrode improved the performance of all vanadium flow batteries, providing high energy efficiency (83.14%) and excellent cycling stability at a current density of 100mA/cm2. This method of modifying carbon felt electrodes using low-cost biomass materials provides a new approach.

research contents
The electrode preparation method of Zhang is to immerse the pre treated carbon felt into a slurry (a mixture of lotus seed shells and sucrose solution), dry it in an oven, and transfer it to a tube furnace. The carbon felt electrode containing the covering slurry is subjected to pyrolysis treatment under high temperature and N2 atmosphere to obtain a modified carbon felt electrode made of biomass hard carbon material. The process diagram is shown below.

The CV curve shows that Bio-CF-2 prepared by pyrolysis at a concentration of 0.2 M sucrose solution and 1050 ℃ exhibits the best electrochemical performance, with smaller peak potential difference and better battery reversibility. Moreover, compared to the original carbon felt, Bio-CF-2 exhibits a smaller difference in redox peak potentials and a higher peak current density. These results indicate that the electrocatalytic activity and conductivity of Bio-CF-2 have been significantly improved.

Subsequently, the article studied the CV curves of original carbon felt and biomass modified carbon felt under different scanning speeds, and investigated the relationship between peak current and square root of scanning speed in CV. According to the Randles Sevcik equation, the good linear relationship between the oxidation and reduction peak currents and the square root of the scanning rate indicates that both the original carbon felt and Bio-CF-2 electrode are diffusion controlled redox processes, and the slope of Bio-CF-2 is greater than that of the original CF, indicating faster mass transfer processes on the Bio-CF-2 electrode. Further impedance testing can be conducted to determine the slope of the straight line through the high-frequency semicircle (charge transfer at electrolyte/electrode interface) and low-frequency linear part (diffusion of redox active substance vanadium ions) of the Nyquist plot, in order to estimate the diffusion resistance force (Zw) during the reaction process. The Rct value of Bio-CF-2 electrode (47.79 m Ω) is lower than that of CF electrode (183.63 m Ω), indicating a significant acceleration of the redox reaction. These results indicate that compared to the original CF, Bio-CF-2 exhibits excellent electrochemical performance in the redox reaction of vanadium ions.

In order to reveal the effect of sucrose concentration on the electrochemical performance of Bio CFs mentioned earlier, the author used molecular dynamics method to simulate the distribution of Bio CF microspheres in sucrose solution at different concentrations and the partial correlation function of Bio CFs in the solvent. Through simulation, it was found that when the sucrose concentration was 0.2M, the distance between microspheres remained in the optimal equilibrium state due to the electronegativity of polar groups on the surface of microspheres and the interaction of polar solvent molecules. When redox reactions occur, both distance equilibrium and electrostatic equilibrium are beneficial for ions in the electrolyte to better adhere to the electrode surface. In addition, due to this equilibrium state, charges are transferred at a lower resistance after exchange interactions, so the simulation results obtained well support the experimental data.

At the end of the article, the author compared the battery performance of Bio-CF-2 electrode and original CF electrode in all vanadium flow batteries. From the selected charging/discharging curves, VRFB batteries with Bio-CF-2 electrodes exhibit smaller polarization than those with original CF electrodes. This is mainly due to the significant decrease in Rct of the Bio-CF-2 electrode in the impedance test mentioned earlier, allowing for higher discharge capacity at the same current density and cutoff voltage. In addition, compared to the original CF (313 mW cm-2) assembled battery, the VRFB battery with Bio-CF-2 exhibits a higher peak power density (345mW cm-2), and at high current density, the Bio-CF-2 electrode exhibits better rate capability and good stability than the original CF electrode.
The improvement in catalytic performance of vanadium redox reaction on Bio-CF-2 is mainly attributed to the modification of carbon nanoparticles doped with a large number of heteroatoms on the surface of the carbon felt, thereby enhancing the reaction kinetics to ensure lower battery polarization and higher energy efficiency. When the current density is 100mA cm-2, compared with the original CF, the voltage efficiency and energy efficiency of the battery with Bio CF-2 are better than those of the original carbon felt. After 150 cycles, the energy efficiency of the battery with Bio-CF-2 electrodes is about 83%, while the energy efficiency of cells with original CF is less than 76%. It is worth noting that in the long-term cycling test, the voltage efficiency and energy efficiency of Bio-CF-2 showed slightly higher fluctuations than the original CF. The author believes that this may be due to the higher sensitivity of lotus seed carbon shells to electrolyte flow, while the flow rate has little effect on the smooth surface of the original carbon felt.

In summary, the author of this article has developed a simple and effective strategy for modifying carbon felt electrodes through the thermal decomposition reaction of biomass lotus seed shell precursors. By controlling the concentration of sucrose solution and pyrolysis temperature, the optimized electrode Bio-CF-2 exhibits significant electrocatalytic effects on the redox reactions of V2+/V3+and VO2+/VO2+. In addition, the mass transfer performance of Bio-CF-2 has been improved, and the concentration overpotential has also been reduced. Therefore, compared with the original CF, VRFB batteries with Bio-CF-2 electrodes exhibit superior performance compared to the original carbon felt electrodes. This new strategy of biomass modified carbon felt may provide new ideas for the manufacturing of VRFB electrodes and has good prospects in the future.