Flow batteries can be divided into inorganic water-based flow batteries and organic water-based flow batteries. Organic water-based flow batteries have the characteristics of low cost, wide sources, and adjustable physical and chemical properties. However, due to the difference in solvent conductivity, the polarization of the battery is high, the current density is low, and there is a risk of combustion and explosion, making commercial application difficult. Inorganic flow batteries have the advantages of high conductivity, good cycling performance, and high solubility, making them the most widely used flow batteries in engineering. All vanadium, iron chromium, and zinc bromide flow battery systems have been commercially applied. In liquid flow batteries, electrodes provide a place for electrochemical reactions, which greatly affects battery performance. The methods of electrode modification can be mainly divided into two categories: one is to modify the electrode body, and the other is to introduce catalysts. We have already introduced many methods for modifying the electrode body (also known as electrode self modification) in our previous articles. This type of electrode self modification mainly involves introducing functional groups, changing the electrode structure, increasing the reaction area, thereby increasing active sites and promoting charge transfer catalytic redox reactions. The introduction of catalysts can alter the physical and chemical properties of the electrode surface, provide specific active sites, reduce charge transfer resistance, and enhance reaction kinetics. Therefore, this article will briefly introduce some research on using electrode loaded catalysts to improve the performance of new liquid flow batteries (except for VFB).
Iron chromium redox flow batteries (ICRFBs) have advantages such as high safety, long cycle time, and low cost, but are limited by the low reactivity of Cr3+/Cr2+. Currently, increasing the reactivity of Cr3+/Cr2+is widely regarded as one of the most promising strategies to improve the performance and extend the lifespan of ICRFBs. Yeonjoo Ahn et al. [1] reported an electrocatalyst containing Kochen black (KB) and embedded bismuth nanoparticles (Bi-C). By uniformly doping Bi nanoparticles into KB carbon through a simple reduction process, it can significantly promote the electrochemical redox reaction activity of Cr3+/Cr2+while delaying HER. The combination of experimental analysis and density functional theory (DFT) calculations indicates that these phenomena can be attributed to the synergistic effect of Bi and KB, which can respectively inhibit hydrogen precipitation and provide active sites to enhance the Cr3+/Cr2+redox reaction. The experimental results show that the ICRFB battery with Bi-C catalyst as the negative electrode exhibits a high energy efficiency of 86.54% and excellent capacity retention during room temperature charge discharge cycles (as shown in Figure 1).

Figure 1

In order to improve the slow reaction kinetics of the negative electrode, Xu et al. [2] prepared a defect modified carbon cloth electrode incorporating bismuth (Bi) catalyst through defect engineering method and electrochemical deposition. It provides defect sites and active sites to catalyze the redox reaction of ICRFB. The results show (as shown in Figure 2) that this modified carbon cloth electrode is easier to adsorb Cr (III) hydrates, has a more stable structure, and excellent electrocatalytic ability, which can improve the reaction rate of Cr3+/Cr2+, improve capacity retention rate, and stabilize cycling performance. The capacity decay rate of ICRFB single cell with modified electrodes at a current density of 140 mA/cm2 per cycle is only 0.23%. In addition, the energy efficiency (EE) remains around 83%, which is 8.45% higher than the original electrode pack battery after 60 cycles. According to DFT calculations, it is believed that the hydrates of Cr (III) are easily adsorbed on the surface of Bi and defect graphite, forming a more stable configuration and providing more reaction sites than the original electrode, thereby promoting the performance of ICRFB.

Another new type of flow battery is the hydrogen bromide flow battery, which has the advantages of high energy capacity, high round-trip conversion efficiency, and low cost. It also has the characteristics of high energy efficiency and low operating temperature, making it a strong candidate for energy storage. A significant challenge for hydrogen bromide flow batteries is to develop relatively low-cost catalysts to replace precious metal based catalysts, enabling the battery to exhibit high bromine reduction and oxidation (BRR/BOR) electrode kinetics in the corrosive Br2/HBr medium of the positive electrode. M. In a study by Candan Karaeyvaz et al. [4], a highly active hollow mesoporous shell (HCMS) carbon catalyst with high surface area was synthesized for the cathode electrode of hydrogen bromide flow batteries. It was used as a substitute for cathode catalysts such as Pt/C or carbon black, and its impact on the performance of hydrogen bromide flow batteries was studied for energy storage on a grid scale. HCMS carbon is synthesized by replicating the template of solid mesoporous silica spheres, and can produce carbon with different core/shell structures by changing the amount of TEOS (1 milliliter, 6 milliliters, 10 milliliters) during the formation process of silica templates (HCMS1) HCMS6、HCMS10）。 Among all the carbons synthesized and used in the experiment, HCMS1 carbon has the highest BET surface area of 1832 m2/g. The synthesized HCMS1 carbon has a hollow diameter of 181 nm and a shell thickness of 74 nm. The results showed that compared with Pt based catalysts and commercial carbon, the use of HCMS1 carbon achieved the highest power density of 0.50 W/cm 2 among all positive electrode catalysts at a battery potential of 0.7 V, indicating considerable potential for application. In addition, the bimodal porosity of HCMS carbon structure may enable effective mass transfer in the cathode catalyst layer, and the high electroactive surface area of HCMS carbon is considered the main reason for improving the performance of hydrogen bromide redox flow batteries.
Another challenge of hydrogen bromide flow batteries is the cross influence of bromide substances passing through the membrane, which can cause poisoning of the catalyst responsible for hydrogen evolution and oxidation reactions, thereby reducing the performance of H2-Br2 RFB. Kobby Saadi et al. [5] proposed a new approach for selective catalyst coating, which can reduce the impact of bromide crossing. It uses dopamine to polymerize on the catalyst surface to produce a nanoscale polydopamine layer as a semi permeable barrier to block bromide. H2-Br2 RFB with coated catalyst showed a 6% low capacity decay at 300 mA cm-2 after being exposed to a 4.5 M charged electrolyte for 2 hours, and even the initial polarization curve showed the advantage of the catalyst coating having a peak power of~550 mW cm-2.

In recent years, low-cost Zn-Br2 flow batteries (ZBB) have also been favored due to their additional advantages such as high battery voltage (1.85 V), high energy density (70-60 W/kg), and low raw material costs. Although ZBB has been successfully commercialized, their poor cycle life limits their competitiveness, especially compared to VRFB, so it is necessary to further improve the cycle life of ZBB. The cycle life of ZBB is mainly related to zinc dendrite growth, poor bromine kinetics, and bromine gas release. Therefore, the current density of ZBB is strictly limited to below 40 mA/cm2. Natesam Venkatesan et al. [6] utilized boron doped graphene (BDG) as an electrocatalyst, utilizing its high electrocatalytic activity, conductivity, large surface area, and cycling stability to improve the reversibility of bromine in Zn-Br2 redox flow batteries. Compared with the oxidation-reduction reaction of reduced graphene oxide (264 mV), BDG showed a significant improvement in the peak separation potential of 2Br -/Br2 (145 mV). In addition, for BDG based batteries, the flow battery showed a low voltage drop of 265 mV (CF: 434 mV; r-GO: 363 mV). The authors also analyzed the durability of the flow battery system using BDG/CF at 20 mA/cm2, and the battery showed a maximum Coulombic efficiency of 86% at the 100th cycle (as shown in Figure 4).

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参考文献

[1] Ahn, Yeonjoo; Moon, Janghyuk; Park, Seoung Eun; Shin, Jaeho; Wook Choi, Jang; Kim, Ki Jae (2020). High-Performance Bifunctional Electrocatalyst for Iron-Chromium Redox Flow Batteries. Chemical Engineering Journal, 127855–. doi:10.1016/j.cej.2020.127855

[2] Quan Xu , Siyang Wang , Chunming Xu , Xinyi Chen , Senwei Zeng , Chuanyuan Li , Yang Zhou , Tianhang Zhou , Yingchun Niu , Synergistic effect of electrode defect regulation and Bi catalyst deposition on the performance of iron-chromium redox flow battery, Chinese Chemical Letters (2023), doi: https://doi.org/10.1016/j.cclet.2023.108188

[3] Ren, H.-l., Su, Y., Zhao, S., Li, C.-w., Wang, X.-m., Li, B.-h., Li, Z., ChemElectroChem 2023, e202201146.

[4] Karaeyvaz, M. Candan; Duman, Berker (2020). An alternative HCMS carbon catalyst in bromine reduction reaction for hydrogen-bromine flow batteries. International Journal of Hydrogen Energy, (), S0360319920342579–. doi:10.1016/j.ijhydene.2020.11.055

[5] Kobby Saadi, Pilkhaz Nanikashvili, Zhanna Tatus-Portnoy, Samuel Hardisty, Victor Shokhen, Melina Zysler, David Zitoun (2023), Crossover-tolerant coated platinum catalysts in hydrogen/bromine redox flow battery, Journal of Power Sources, 422(84-91). doi.org/10.1016/j.jpowsour.2019.03.043