The electrode material of a flow battery is a very important component of its battery system. Although it does not directly participate as a reactant in the charging and discharging process, it provides a reaction site for the charging and discharging process of the flow battery, and also plays a crucial role in the charging and discharging reaction, structural stability, service life, and final operating efficiency and output power of the battery. Currently, high-power flow batteries require higher electrodes and largely depend on electrode performance. At present, the main focus on improving electrode performance is on improving electrocatalytic activity, increasing ECSA, increasing electronic conductivity, and reducing transmission resistance. In previous articles, we have also sorted out the main categories and modification methods of electrode materials for liquid flow batteries, including metal particle decoration, heat treatment, acid treatment, electrochemical treatment, surface carbon nanotube modification, and other methods.

       The above diagram is a schematic diagram of the structure of a liquid flow battery. Due to its structural characteristics, electrochemical reactions easily occur on the electrode near the proton exchange membrane, which is commonly known as the reaction zone. On the other hand, the electrode near the electrode plate tends to conduct electrons, which is commonly referred to as the current collection zone. Therefore, an ideal VFB electrode should have a differentiated gradient structure along the electrode thickness direction. From the principle of electrode reaction, the charge transfer step occurs on the outer surface of the electrode near the proton exchange membrane, while the electron conduction step occurs on the inner electrode near the electrode plate, which is more conducive to improving the overall efficiency of the liquid flow battery.

Research Highlights

This study proposes for the first time a highly advanced structure of dual gradient carbon nanofiber/graphite felt composite electrode for all vanadium flow batteries, and verifies the macroscopic and microscopic gradient structure of the studied dual gradient carbon nanofiber/graphite felt composite electrode through various characterizations. Moreover, this unique dual gradient structure of carbon nanofiber/graphite felt electrode exhibits excellent battery performance in electrochemical testing.

research contents

The following figure shows the proposed dual gradient structure carbon nanofiber/graphite felt electrode. The side with more carbon nanofibers on the electrode is placed on the membrane side, and the other side with less carbon nanofibers is placed on the electrode plate side. The former has good electrochemical activity due to the modification of carbon nanofibers, and serves as a catalytic layer to catalyze the redox reaction of all vanadium flow batteries; At the same time, the latter has a high conductivity as a conductive layer, which can promote electron conduction and reduce the contact resistance between the electrode and the bipolar plate. Similarly, the micro gradient distribution of carbon nanofibers along the radial direction of a single fiber can effectively improve the mass transfer process on the surface of graphite fibers with the abundant functional groups on it. The distribution of macroscopic gradient along the electrode thickness direction and its own microscopic gradient structure of this carbon nanofiber will simultaneously reduce the activation resistance, concentration resistance, and Ohmic resistance.

       The production process of this dual gradient structure carbon nanofiber/carbon felt electrode is first pre graphitized, and then prepared using an improved ethanol flame process, as shown in Figure (c). Through this flame process, the gradient distribution of carbon nanofibers at both macro and micro scales can be achieved, and the optimal gradient obtained corresponds to a combustion time of 12 minutes. The reason for this difference is that the conductive layer and catalytic layer are located in the outer and inner flames respectively during the ethanol flame combustion process. The ethanol concentration and temperature in the inner flame are prone to produce carbon nanofibers, while relatively low ethanol concentration and higher outer flame temperature are not conducive to the growth of carbon nanofibers, resulting in a double gradient structure of the electrode.

And it also tested the performance of this dual gradient structure electrode. Figure (a) shows that VFB with DG-CNFs/GF-12 (burning for 12 minutes) exhibits the lowest polarization voltage and maximum discharge capacity, indicating that DG-CNFs/GF-12 with a preferred dual gradient structure has better battery performance. Figure (b) shows that VFB with DG-CNFs/GF-12 exhibits the highest energy efficiency at different current densities, and its energy efficiency (EE) decreases consistently less than the other three samples, indicating excellent high rate discharge performance. Figure (c) shows that the discharge capacity of all samples gradually decreases with increasing current density, while VFB with DG-CNFs/GF-12 always shows the maximum discharge capacity. It is worth noting that the discharge capacity of VFB with DG-CNFs/GF-12 has almost returned to the level before the rate performance test, indicating its excellent rate performance. As shown in Figure (d), VFB with DG-CNFs/GF-12 consistently exhibits the highest energy efficiency, and its energy efficiency does not significantly decrease after 100 cycles, indicating better cycling stability.

       In addition, the study will also compare the dual gradient structure electrode with commercial carbon felt for testing. As shown in the figure below, it can be seen that the charging voltage of DG-CNFs/GF-12 is lower than that of commercial carbon felt, while the corresponding discharge voltage is much higher, indicating that its polarization is lower during the charging and discharging process, and the battery performance is better. In addition, the discharge capacity of VFB using DG-CNFs/GF-12 (30.77 Ah L-1) is about 16% higher than that using commercial carbon felt (26.55 Ah L-1), indicating an improvement in electrolyte utilization efficiency. In addition, tests have also shown that the energy efficiency of the dual gradient structure electrode is significantly higher than that of commercial carbon felt, and the decrease in energy efficiency is smaller than that of commercial carbon felt as the current density increases. And finally, at a current density of 100mA cm-2, the energy efficiency of VFB with DG-CNFs/GF-12 (82.08%) almost recovered to before the rate performance test (82.19%), while the EE of commercial CF decreased from 80.45% after the rate performance test to 79.55%. These aspects all indicate that the dual gradient structure electrode has excellent high rate discharge performance.

       From the above aspects, it can be seen that the rate performance and cycling stability of this dual gradient structure carbon nanofiber/graphite felt composite electrode are far superior to commercial carbon felt, which will also provide a promising direction for the development of efficient electrodes for the next generation of high-power density all vanadium flow batteries.

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