Frontier Tracking | PAN+PMMA Electrospinning Preparation of Porous Channel Electrodes with High Porosity-Shenzhen ZH Energy Storage - Zhonghe LDES VRFB - Vanadium Flow Battery Stacks - Sulfur Iron Electrolyte - PBI Non-fluorinated Ion Exchange Membrane

Frontier Tracking | PAN+PMMA Electrospinning Preparation of Porous Channel Electrodes with High Porosity

Classification：Industrial News

- Author：Luo Xuan

- Release time：Jan-03-2023

【 Summary 】Electrospinning technology is widely used in the field of energy storage due to its ability to effectively regulate the fine structure of fibers, increase the specific surface area and chemical stabil

Research background

At present, the most widely used renewable energy sources are wind and solar energy, but they are intermittent due to time and geographical limitations, so energy storage devices need to be configured. Vanadium redox flow batteries (VRFBs) are widely used due to their large energy storage capacity, long cycle life, environmental protection, safety, and reliability. In all vanadium flow batteries, commercial carbon materials such as carbon felt (CF) and graphite felt (GF) are mainly used as electrode materials (excellent chemical stability, high conductivity, and low cost). However, the hydrophilicity of carbon material electrodes is poor, and the effective specific surface area (<1 m2/g) is still low. The poor electrochemical activity leads to low energy efficiency, which limits the performance of VRFB and indirectly increases energy storage costs. At present, the main method of improving electrochemical activity and reducing polarization is to modify the electrode surface, thereby enhancing battery performance. However, the degree of performance improvement is limited and limited by the inherent characteristics of carbon electrodes such as fiber diameter and pore size.

Electrospinning technology is widely used in the field of energy storage due to its ability to effectively regulate the fine structure of fibers, increase the specific surface area and chemical stability of electrodes. Compared with commercial CF, the inherent characteristics of electrospun carbon nanofiber (ECNF) electrodes can be adjusted as needed, including fiber diameter, pore size, and pore shape. However, electrospun carbon nanofiber (ECNFs) electrodes have low porosity and permeability, which significantly hinder mass transfer, and this process needs to be improved.

Research Highlights

Yifan Zhang et al. prepared a porous channel carbon nanofiber electrode using electrospinning technology, which effectively increased the porosity and improved the mass transfer of the electrode. The electrode also has a high electrochemical specific surface area, which can provide more reaction sites, and can control the fiber diameter and pore size between fibers by adjusting the content of PMMA (polymethyl methacrylate) in the precursor solution. The battery assembled with this electrode achieved 81.03% energy efficiency (EE) at 200 mA cm-2 and maintained high energy efficiency (74.45%) at 300 mA cm-2. It also exhibited excellent long-term cycling stability, with only a 6% decrease in energy efficiency after 1200 cycles.

研究内容

The author first prepared porous channel electrodes with high porosity using the method shown in the above figure, and observed the electrodes prepared at different PMMA concentrations (0%, 4%, 8%, 12%, 16%) through scanning electron microscopy (SEM). It was found that after adding PMMA to the precursor, carbon nanofibers were interconnected, and the fiber diameter and pores between fibers gradually increased with the increase of PMMA concentration. As the PMMA content in the precursor solution increases, the diameter and number of internal channels in each fiber gradually increase. However, as the PMMA content increased to 16%, compared to PAM-12, the number of channels inside the fiber decreased, the unevenness of the channels increased, and larger diameter hollow channels were formed inside. Overall, we conclude that it is feasible to control the fiber diameter and pore size by controlling the content of PMMA in the precursor solution.

Next, the author characterized the pore structure of the prepared electrode. Because mesopores can serve as redox reaction sites, allowing electrolyte ions to diffuse into carbon containing materials and shorten mass transfer distances, while micropores are not easily wetted by electrolytes and are not conducive to rapid ion diffusion. The experimental results showed that compared with the absence of PMMA electrode, the addition of PMMA significantly increased the adsorption in the high-pressure region (relative pressure>0.8), especially PAM-12, indicating that the electrode has mesoporous properties, and the clear peak at 4 nm in the pore size distribution curve confirms this. The results of the contact angle test indicate that compared to other samples, the contact angle of PAM-12 is difficult to measure because droplets quickly immerse into the electrode, indicating its optimal hydrophilicity. At the same time, electrodes without PMMA have a dense fiber structure that makes it difficult for the electrolyte to quickly penetrate the electrode and hinder its flow, resulting in the highest contact angle. In summary, tests have shown that electrodes with a PMMA addition of 12% have the optimal pore structure, which can enhance wettability, shorten mass transfer distance, and reduce mass transfer resistance.

Subsequently, the author characterized it through various methods such as Raman spectroscopy, XRD, and XPS. In Raman testing, it was found that PAM-12 exhibited the highest ID/IG value, indicating that the electrode contains more active sites and defects. The low peak intensity of PAM-12 in XRD indicates the formation of more disordered structures after the addition of PMMA. The XPS results indicate that PAM-12 has the highest oxygen content (7.42%), indicating that PAM-12 has more reaction activation sites in its catalytic layer, indicating that PAM-12 is expected to have the best electrochemical activity. In summary, through these characterization methods, it can be concluded that the electrode with a PMMA addition of 12% has more active sites and higher electrochemical activity.

Therefore, the author subsequently conducted electrochemical tests on the electrodes. In a 10 mV s-1 CV test, it was found that PAM-12 had the smallest peak difference potential (Δ E p) for both VO2+/VO2+and V 2+/V 3+, indicating stronger electrochemical reversibility of the reaction. In addition, the oxidation peak current density (Ipa) and reduction peak current density (Ipc) of PAM-12 were significantly enhanced, with the value of Ipc/Ipa being closest to 1. In addition, CV tests were conducted on different samples at different scanning rates, and it was found that the CV curve of PAM-12 showed the best stability. For the VO2+/VO2+redox reaction, the Δ E p value of PAM-12 is the smallest, and the - I pc/I pa value of PAM-12 is closer to 1 than PAM-0 at different scanning rates. The above research results indicate that PAM-12 achieves the best reversibility of vanadium redox reaction, proving that PAM-12 has the best electrochemical performance. Meanwhile, it was confirmed through EIS and Randles Sevcik equation that the PAM-12 electrode has the best electrochemical activity, which is related to its larger electrochemical specific surface area and higher oxygen functional group content.

Finally, the author tested the battery performance. Through the charge discharge curves of different samples at 300 mA cm-2, it can be seen that PAM-12 has the highest charge discharge capacity. In addition, PAM-12 has the lowest initial charging level and the highest initial discharge level, indicating that it has the smallest charge discharge overpotential. The charge discharge curves of PAM-0 and PAM-12 at different current densities show that the fiber dense PAM-0 electrode cannot charge and discharge at 300 mA cm-2, while fluctuations are observed during the charging and discharging stages of PAM-0, which are caused by poor mass transfer caused by dense fibers. In contrast, PAM-12 with porous fibers maintains stable charging and discharging at 300 mA cm-2, and VRFB with PAM-12 electrodes exhibits excellent charging and discharging performance throughout the entire current density range. In addition, PAM-12 showed the lowest overpotential throughout the entire cycle, while PAM-0 had a significantly higher overpotential than other samples, mainly due to poor mass transfer within the dense fiber network, resulting in lower surface area utilization of the electrode.

Meanwhile, it is evident from the voltage efficiency (VE), Coulombic efficiency (CE), and energy efficiency (EE) graphs of VRFB at different current densities that the Coulombic efficiency of the battery remains relatively high at all current densities, with all values above 96%. This indicates that the membrane was not damaged during the assembly process and the battery was well sealed. Compared with the Coulomb efficiency, the voltage efficiency shows a decreasing trend with the increase of current density due to the increase of overpotential. Corresponding to the charge discharge curve, the voltage efficiency of PAM-12 batteries is significantly higher than other batteries, and batteries equipped with PAM-12 still exhibit the highest energy efficiency. The energy efficiency of PAM-12 is 81.03% at 200 mA cm-2, which is 10.21% higher than the battery equipped with PAM-0. It is worth noting that even at relatively high current densities (300 mA cm-2), the battery can be charged and discharged with an energy efficiency of 74.45%, thanks to enhanced mass transfer, which was not achieved by electrospinning methods in previous studies. Moreover, the energy efficiency of the battery is significantly higher than that of the widely used commercial graphite felt. In order to study the cycling stability of batteries equipped with PAM-12, the long-term cycling behavior of the batteries was investigated. After 1200 cycles at 200 mA cm-2, the energy efficiency of the battery equipped with PAM-12 only decayed by 6%, indicating that the electrodes were not damaged or chemically decomposed during the cycling process, thus proving that the battery equipped with PAM-12 has excellent long-term cycling stability. Compared with methods such as surface catalyst deposition, the electrode preparation method used here is more stable in changing the structure of the fiber itself and retaining the modification characteristics under long-term flushing conditions in acidic electrolytes, thereby achieving a stable cycle of 1200 cycles.

In summary, the author of this article constructed a high porosity porous channel electrode suitable for VRFB by adding PMMA to the electrospinning precursor solution. Compared with traditional electrospinning electrodes, the pores between the prepared electrode fibers are significantly enlarged, promoting the smooth flow of electrolyte in the electrode. Its larger electrochemical specific surface area provides more reaction sites for electrochemical reactions. Moreover, the VRFB equipped with PAM-12 exhibits excellent energy efficiency of 81.03% and long-term cycling stability of 1200 cycles at a current density of 200 mA cm-2. All results indicate that preparing high porosity porous channel structures is an effective and persistent strategy with broad application prospects, which also provides new ideas for the preparation of high-performance electrodes for all vanadium flow battery electrodes in industry.