Holey aligned electrodes through in-situ ZIF-8-assisted-etching for high-performance aqueous redox flow batteries

New Thermoelectricity Utilization System Based on Aqueous Organic Redox Flow Battery Involving the Tripyridinium-Triazine Radicals

New energy technology is in urgent demand for waste heat harvesting and utilization for energy saving and renewable energy development. Herein, an aqueous organic redox flow battery (AORFB) with designed tripyridinium-triazine radicals ((TPyTz)··4+)/(TPyTz)·5+) as the negative redox couple is constructed as a thermoelectricity utilization system. The electrochemical test indicates that the (TPyTz)··4+/(TPyTz)·5+ couple shows an interesting potential change about 27 mV accompanied by system temperature varying from 298 K to 343 K. Nuclear magnetic resonance and electron paramagnetic resonance spectra approve that the population of high-energy (TPyTz)··4+ triplet state increases with temperature rise. This shifts the potential of the (TPyTz)··4+/(TPyTz)·5+ couple to a much more negative position. While the 2,2,6,6-tetramethylpiperidin-1-oxyl derivative (TPABPy)Cl3 is used as the positive electrolyte with no temperature-dependent potential change, the AORFB established by these two couples displays a thermally induced potential change from 1.39 V to 1.42 V.

3D cross-linked structure of dual-active site CoMoO4 nanosheets@graphite felt electrode for vanadium redox flow battery

Transition metal oxides (TMOs) can accelerate the sluggish kinetics of vanadium redox reaction, but face challenges like limited active sites and difficulties in nanometerization, highlighting the urgent need for new TMO electrocatalysts for vanadium redox flow battery (VRFB). CoMoO4 features high electrochemical activity, numerous redox sites, flexible control, and short electron pathways. Herein, a high catalytic and super stable graphite felt electrode modified in situ with network cross-linking CoMoO4 nanosheets (CoMoO4@GF) was prepared via hydrothermal and heat treatment method to enhance VRFB performance. CoMoO4@GF have large specific surface area, super hydrophilicity, and abundant reaction places, possessing well mass transfer, low charge transfer resistance, and sufficient catalytic sites. Therefore, the composite electrodes exhibit great electrocatalytic activity towards VO2+/VO2+ and V3+/V2+ redox reactions and excellent stability for VRFB. At 200 mA cm-2, the energy efficiency (EE) of the CoMoO4@GF modified VRFB improved by 19.14 % over the blank VRFB with pristine graphite felt, and remained cycle stable after 350 cycles at 150 mA cm-2. This work not only enriches the types of TMOs catalysts in VRFB, but also opens up a new direction for the research of bimetallic TMOs.

Highly active nitrogen-phosphorus co-doped carbon fiber@graphite felt electrode for high-performance vanadium redox flow battery

Heteroatom-doped electrodes offer promising applications for enhancing the longevity and efficiency of vanadium redox flow battery (VRFB). Herein, we controllably synthesized N, P co-doped graphite fiber electrodes with conductive network structure by introducing protonic acid and combining electrodeposition and high temperature carbonization. H2SO4 and H3PO4 act as auxiliary and dopant, respectively. The synergistic effect between N and P introduces additional defect structures and active sites on the electrodes, thereby enhancing the reaction rate, as confirmed by density functional theory calculations. Furthermore, the conductive network structure of carbon fibers improves electrode-to-electrode connectivity and reduces internal battery resistance. The optimized integration of these strategies enhances VRFB performance significantly. Consequently, the N, P co-doped carbon fiber modified graphite felt electrodes demonstrate remarkably high energy efficiency at 200 mA cm-2, surpassing that of the blank battery by 7.9 %. This integrated approach to in-situ controllable synthesis provides innovative insights for developing high-performance, stable electrodes, thereby contributing to advancements in the field of energy storage.

Next-Generation Ultrathin Lightweight Electrode for pH-Universal Aqueous Flow Batteries

Aqueous flow batteries (AFBs) are promising long-duration energy storage system owing to intrinsic safety, inherent scalability, and ultralong cycle life. However, due to the thicker (3-5 mm) and heavier (300-600 g m-2) nature, the current used graphite felt (GF) electrodes still limit the volume/weight power density of AFBs. Herein, a lightweight (≈50 g m-2) and ultrathin (≈0.3 mm) carbon microtube electrode (CME) is proposed derived from a scalable one-step carbonization of commercial cotton cloth. The unique loose woven structure composed of carbon microtube endows CME with excellent conductivity, abundant active sites, and enhanced electrolyte transport performance, thereby significantly reducing polarization in working AFBs. As a consequence, CME demonstrates excellent cycling performance in pH-universal AFBs, including acidic vanadium flow battery (maximum power density of 632.2 mW cm-2), neutral Zn-I2 flow battery (750 cycles with average Coulombic efficiency of 99.6%), and alkaline Zn-Fe flow battery (energy efficiency over 70% at 200 mA cm-2). More importantly, the estimated price of CME is only 5% of GF (≈3 vs ≈60 $ m-2). Therefore, it is reasonably anticipated that the lightweight and ultrathin CME may emerge as the next generation electrode for AFBs.

Carbon Structure Regulation Strategy for the Electrode of Vanadium Redox Flow Battery

Vanadium redox flow battery (VRFB) is a type of energy storage device known for its large-scale capacity, long-term durability, and high-level safety. It serves as an effective solution to address the instability and intermittency of renewable energy sources. Carbon-based materials are widely used as VRFB electrodes due to cost-effectiveness and well-stability. However, pristine electrodes need proper modification to overcome original poor hydrophilicity and fewer reaction active sites. Adjusting the carbon structure is recognized as a viable method to boost the electrochemical activity of electrodes. This review delves into the advancements in research related to ordered and disordered carbon structure electrodes including the adjusting methods, structural characteristics, and catalytic properties. Ordered carbon structures are categorized into nanoscale and macroscale orderliness based on size, leading to improved conductivity and overall performance of the electrode. Disordered carbon structures encompass methods such as doping atoms, grafting functional groups, and creating engineered holes to enhance active sites and hydrophilicity. Based on the current research findings on carbon electrode structures, this work puts forth some promising prospects for future feasibility.

Vanadium Redox Flow Battery: Review and Perspective of 3D Electrodes

Vanadium redox flow battery (VRFB) has garnered significant attention due to its potential for facilitating the cost-effective utilization of renewable energy and large-scale power storage. However, the limited electrochemical activity of the electrode in vanadium redox reactions poses a challenge in achieving a high-performance VRFB. Consequently, there is a pressing need to assess advancements in electrodes to inspire innovative approaches for enhancing electrode structure and composition. This work categorizes three-dimensional (3D) electrodes derived from materials such as foam, biomass, and electrospun fibers. By employing a flexible electrode design and compositional functionalization, high-speed mass transfer channels and abundant active sites for vanadium redox reactions can be created. Furthermore, the incorporation of 3D electrocatalysts into the electrodes is discussed, including metal-based, carbon-based, and composite materials. The strong interaction and ordered arrangement of these nanocomposites have an influence on the uniformity and stability of the surface charge distribution, thereby enhancing the electrochemical performance of the composite electrodes. Finally, the challenges and perspectives of VRFB are explored through advancements in 3D electrodes, 3D electrocatalysts, and mechanisms. It is hoped that this review will inspire the development of methodology and concept of 3D electrodes in VRFB, so as to promote the future development of scientific energy storage and conversion technology.

Two-in-one strategy for optimizing chemical and structural properties of carbon felt electrodes for vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) have received significant attention for use in large-scale energy storage systems (ESSs) because of their long cycle life, flexible capacity, power design, and safety. However, the poor electrochemical activity of the conventionally used carbon felt electrode results in low energy efficiency of the VRFBs and consequently impedes their commercialization. In this study, a carbon felt (CF) electrode with numerous nanopores and robust oxygen-containing functional groups at its edge sites is designed to improve the electrochemical activity of a carbon felt electrode. To achieve this, Ni metal nanoparticles were initially precipitated on the surface of the CF electrode, followed by etching of the precipitated Ni nanoparticles on the CF electrode using sulfuric acid. The resulting CF electrode had a specific surface area eight times larger than that of the pristine CF electrode. In addition, the oxygen-containing functional groups anchored at the graphite edge sites of the nanopores can act as robust electrocatalysts for VO2+/VO2+ and V2+/V3+ redox reactions. Consequently, the VRFB cell with the resulting carbon felt electrode can deliver a high energy efficiency of 86.2% at the current density of 60 mA cm-2, which is 20% higher than that of the VRFB cell with the conventionally heat-treated CF electrode. Furthermore, the VRFB cell with the resultant carbon felt electrodes showed stable cycling performance with no considerable energy efficiency loss over 200 charge-discharge cycles. In addition, even at a high current density of 160 mA cm-2 , the developed carbon felt electrode can achieve an energy efficiency of 70.1%.

Cobalt etched graphite felt electrode for enhanced removal of organic pollutant in aqueous solution with a solid polymer electrolyte

In this study, cobalt etched graphite felt electrodes were produced using a simple etching technique. It was used in combination with a solid polymer electrolyte (SPE) for the degradation of the target contaminant Orange II by Electro-Fenton (EF) technique in low conductivity water. In this method, 94% of Orange II in low conductivity water was removed in 90 min. The characterization analysis substantiates the hypothesis that the electrodes produced exhibit a three-dimensional porous structure, augmented defect concentration, and enhanced electron transfer capability. In addition, the potential reaction mechanism was inferred from the radical quenching experiments, and hydroxyl radicals (·OH) were deemed the main reactive substances. The combination of cobalt etched graphite felt electrodes with SPE demonstrates remarkable efficacy in the treatment of organic wastewater characterized by low electrical conductivity.

Surface Electroactive Sites of Tungstated Zirconia Catalysts for Vanadium Redox Flow Batteries

Surface electroactive sites for tungstate zirconia (WZ) were created by utilizing tungstate-immobilized UiO-66 as precursors via a double-solvent impregnation method under a mild calcination temperature. The WZ-22-650 catalyst, containing a moderate W content (22%), demonstrated a high density of surface electroactive sites. Proper heat treatment facilitated the binding of oligomeric tungsten clusters to stabilized tetragonal ZrO2, resulting in improved catalytic performance toward the VO2+/VO2+ redox couples compared to other tested samples. The substantial surface area, mesoporous structure, and establishment of new W-O-Zr bonds affirm the firm anchoring of WOx to ZrO2. This robust attachment enhances surface electroactive sites, elevating the electrochemical performance of vanadium redox flow batteries (VRFBs). Charge-discharge tests further demonstrate that the superior voltage efficiency (VE) and energy efficiency (EE) for VRFBs using the WZ-22-650 catalyst are 87.76 and 83.94% at 80 mA cm-2, which are 13.42% VE and 10.88% EE better than heat-treated graphite felt, respectively. Even at a higher current density of 160 mA cm-2, VRFBs utilizing the WZ-22-650 catalyst maintained considerable efficiency, recording VE and EE values of 76.76 and 74.86%, respectively. This facile synthesis method resulted in WZ catalysts displaying superior catalytic activity and excellent cyclability, offering a promising avenue for the development of metal-oxide-based catalysts.

Sponge-Like Microfiber Electrodes for High-Performance Redox Flow Batteries

Fabricating fiber-based electrodes with a large specific surface area while maintaining high flow permeability is a challenging issue in developing high-performance redox flow batteries. Here, a sponge-like microfiber carbon electrode is reported with a specific surface area of as large as 853.6 m² g^-1 while maintaining a fiber diameter in the range of 5-7 µm and a macropore size of ≈26.8 µm. The electrode is developed by electrospinning cross-linked poly(vinyl alcohol)-lignin-polytetrafluoroethylene precursors, followed by oxidation and pyrolysis. Applying the as-synthesized electrodes to a vanadium redox flow battery enables the battery to achieve an energy efficiency of 79.1% at the current density of 400 mA cm^-2 and a capacity retention rate of 99.94% over 2000 cycles, representing one of the best battery performances in the open literature. The strategy to fabricate sponge-like porous carbon microfibers holds great promise for versatile applications in redox flow batteries and other energy storage systems.