Electrode Treatments for Redox Flow Batteries: Translating Our Understanding from Vanadium to Aqueous-Organic

Redox flow batteries (RFBs) are a promising technology for long-duration energy storage; but they suffer from inefficiencies in part due to the overvoltages at the electrode surface. In this work, more than 70 electrode treatments are reviewed that are previously shown to reduce the overvoltages and improve performance for vanadium RFBs (VRFBs), the most commercialized RFB technology. However, identifying treatments that improve performance the most and whether they are industrially implementable is challenging. This study attempts to address this challenge by comparing treatments under similar operating conditions and accounting for the treatment process complexity. The different treatments are compared at laboratory and industrial scale based on criteria for VRFB performance, treatment stability, economic feasibility, and ease of industrial implementation. Thermal, plasma, electrochemical oxidation, CO2 treatments, as well as Bi, Ag, and Cu catalysts loaded on electrodes are identified as the most promising for adoption in large scale VRFBs. The similarity in electrode treatments for aqueous-organic RFBs (AORFBs) and VRFBs is also identified. The need of standardization in RFBs testing along with fundamental studies to understand charge transfer reactions in redox active species used in RFBs moving forward is emphasized.

Development of flow battery technologies using the principles of sustainable chemistry

Realizing decarbonization and sustainable energy supply by the integration of variable renewable energies has become an important direction for energy development. Flow batteries (FBs) are currently one of the most promising technologies for large-scale energy storage. This review aims to provide a comprehensive analysis of the state-of-the-art progress in FBs from the new perspectives of technological and environmental sustainability, thus guiding the future development of FB technologies. More importantly, we evaluate the current situation and future development of key materials with key aspects of green economy and decarbonization to promote sustainable development and improve the novel energy framework. Finally, we present an analysis of the current challenges and prospects on how to effectively construct low-carbon and sustainable FB materials in the future.

An interfacial coating with high corrosion resistance based on halloysite nanotubes for anode protection of zinc-ion batteries

Aqueous zinc-ion batteries are recognized as one of the most potential neutral aqueous batteries because of the high energy density, high specific capacity, low cost, and low pollution. However, the applications of zinc-ion batteries are seriously limited by the capacity fading, easy-corrosion, side reaction, and hydrogen evolution. Herein, we report a uniform halloysite nanotubes (HNTs) coating which can guide Zn²⁺ ions stripping/plating on the HNTs/Zn interfaces and protect the Zn anode. The HNTs coating significantly suppresses the corrosion of Zn anode and effectively reduces the hydrogen evolution and the formation of by-product. Furthermore, the HNTs-Zn anode exhibits lower resistance than bare Zn. Compared with the bare Zn anode batteries, HNTs-Zn/MnO₂ batteries exhibit good capacity retention and can increase the discharge capacity to 79% at 3 C after 400 cycles. The novel design of interfacial coating based on halloysite nanotubes through electrophoretic deposition method provides a new way to fabricate economic and stable aqueous zinc-ion batteries.

Graphene-Based Electrodes in a Vanadium Redox Flow Battery Produced by Rapid Low-Pressure Combined Gas Plasma Treatments

The development of high-power density vanadium redox flow batteries (VRFBs) with high energy efficiencies (EEs) is crucial for the widespread dissemination of this energy storage technology. In this work, we report the production of novel hierarchical carbonaceous nanomaterials for VRFB electrodes with high catalytic activity toward the vanadium redox reactions (VO²⁺/VO₂ ⁺ and V²⁺/V³⁺). The electrode materials are produced through a rapid (minute timescale) low-pressure combined gas plasma treatment of graphite felts (GFs) in an inductively coupled radio frequency reactor. By systematically studying the effects of either pure gases (O₂ and N₂) or their combination at different gas plasma pressures, the electrodes are optimized to reduce their kinetic polarization for the VRFB redox reactions. To further enhance the catalytic surface area of the electrodes, single-/few-layer graphene, produced by highly scalable wet-jet milling exfoliation of graphite, is incorporated into the GFs through an infiltration method in the presence of a polymeric binder. Depending on the thickness of the proton-exchange membrane (Nafion 115 or Nafion XL), our optimized VRFB configurations can efficiently operate within a wide range of charge/discharge current densities, exhibiting energy efficiencies up to 93.9%, 90.8%, 88.3%, 85.6%, 77.6%, and 69.5% at 25, 50, 75, 100, 200, and 300 mA cm^-2, respectively. Our technology is cost-competitive when compared to commercial ones (additional electrode costs < 100 € m^-2) and shows EEs rivalling the record-high values reported for efficient systems to date. Our work remarks on the importance to study modified plasma conditions or plasma methods alternative to those reported previously (e.g., atmospheric plasmas) to improve further the electrode performances of the current VRFB systems.

Thiourea-Grafted Graphite Felts as Positive Electrode for Vanadium Redox Flow Battery

In this paper, thiourea was successfully grafted onto the surface of acid preprocessed graphite felts [sulfuric acid-treated graphite felt (SA-GFs)] by thiol-carboxylic acid esterification. The thiourea-grafted graphite felts (TG-GFs) were investigated as the positive electrode for vanadium redox flow battery (VRFB). X-ray photoelectron spectroscopy results suggested that thiourea was grafted into the surface of graphite felts. The cyclic voltammetry showed that the peak potential separation decreased by 0.2 V, and peak currents were greatly enhanced on TG-GF electrode compared with SA-GF electrode, implying improved electro-catalytic activity and reversibility of TG-GF electrode toward VO²⁺/VO2+ redox reaction. The initial capacity of TG-GF-based cell reached 55.6 mA h at 100 mA cm⁻², 22.6 mA h larger than that of SA-GF-based cell. The voltage and energy efficiency for TG-GF-based cell increased by 4.9% and 4.4% compared with those of SA-GF-based cell at 100 mA cm⁻², respectively.

Dopamine-derived nitrogen-doped carboxyl multiwalled carbon nanotube-modified graphite felt with improved electrochemical activity for vanadium redox flow batteries

Improving the electrochemical activity of electrodes is essential to the development of vanadium redox flow battery (VRFB). In this work, we prepared a novel electrode with the modification of nitrogen-doped carboxyl multiwalled carbon nanotubes using dopamine as an eco-friendly nitrogen source (carboxyl MWCNT@PDAt). Characterization and electrochemical measurements reveal that the synthesized carboxyl MWCNT@PDAt-modified graphite felt electrode (carboxyl MWCNT@PDAt/GF) exhibits excellent electrochemical performance toward VO²⁺/ V O 2 + reaction. Superior battery performance was obtained with the energy efficiency of 80.54% at a current density of 80 mA cm^-2. Excellent durability of the carboxyl MWCNT@PDAt/GF electrode was confirmed by long-term charge/discharge tests. The enhanced reaction kinetics of VO²⁺/ V O 2 + is ascribed to the synergetic effect of oxygen and nitrogen containing groups on graphite felt surface and the presence of nitrogen-doped carboxyl multiwalled carbon nanotubes (MWCNT). The facile approach proposed in this paper provides a new route to the fabrication of electrode with excellent performance for VRFB.

A Comprehensive Study on Hydroxyl Multiwalled Carbon Nanotubes Used as Catalysts for VO2+/VO2+ Reaction in Vanadium Redox Flow Battery

A comprehensive study on the hydroxyl multiwalled carbon nanotubes (hydroxyl MWCNTs) as catalysts in a positive reaction was performed to improve the efficiency of the vanadium redox flow battery (VRFB). The physicochemical properties of the hydroxyl MWCNT-modified electrode were characterized by using a scanning electron microscope (SEM), conductivity measurement, Brunner–Emmet–Teller (BET) measurement, X-ray photoelectron spectroscopy (XPS) analysis, cyclic voltammetry (CV) tests, electrochemical impedance spectroscopy (EIS) analysis, and charge-discharge tests. The prepared composite electrode possesses a huge amount of oxygen-containing groups, high-specific surface area, high electrical conductivity, and high catalytic activity towards the VO2+/VO2+ reaction based on physicochemical characterization. The hydroxyl MWCNT-modified graphite felt (hydroxyl MWCNTs/GF) shows the best cell performance with the energy efficiency of 79.74% and remains in high stability after 50 cycles. The improved cell performance is probably ascribed to the increase in active sites, fast charge transfer, and mass transfer rate of the introduced hydroxyl MWCNTs.

Polymer Electrolytes for LIBs Based on Perfluorinated Sulfocationic Nepem-117 Membrane and Aprotic Solvents

Polymer electrolytes have been obtained by using Nepem-117 membranes in a Li⁺ form intercalated by polar aprotic solvents, such as dimethylformamide, dimethyl sulfoxide (DMSO), and dimethylacetamide (DMA), and solvent mixtures, such as ethylene carbonate-propylene carbonate (EC-PC), EC-DMA, EC-PC-DMA, and EC-PC-DMA-tetrahydrofuran. The obtained electrolytes have been characterized by IR impedance and ⁷Li pulsed field gradient NMR spectroscopy. Ion mobility was observed to increase with higher degrees of solvation of the membranes. A method is demonstrated to determine the solvent uptake corresponding to the percolation threshold. With comparable solvent uptake, materials containing a solvent with a higher permittivity and a lower viscosity have higher values of ionic conductivity. The membranes containing the three-component mixture of EC-PC-DMA show the highest ionic conductivity values (8.1 and 2.1 mS/cm at 25 and -20 °C, respectively). Such values exceed the conductivity of electrolytes on the basis of the Nafion membranes solvated with aprotic solvents.

Porous N- and S-doped carbon-carbon composite electrodes by soft-templating for redox flow batteries

Highly porous carbon-carbon composite electrodes for the implementation in redox flow battery systems have been synthesized by a novel soft-templating approach. A PAN-based carbon felt was embedded into a solution containing a phenolic resin, a nitrogen source (pyrrole-2-carboxaldehyde) and a sulfur source (2-thiophenecarboxaldehyde), as well as a triblock copolymer (Pluronic^® F-127) acting as the structure-directing agent. By this strategy, highly porous carbon phase co-doped with nitrogen and sulfur was obtained inside the macroporous carbon felt. For the investigation of electrode structure and porosity X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and nitrogen sorption (BET) were used. The electrochemical performance of the carbon felts was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The N- and S-doped carbon electrodes show promising activity for the positive side reaction and could be seen as a significant advance in the design of carbon felt electrodes for use in redox flow batteries.

Phosphorus and oxygen co-doped composite electrode with hierarchical electronic and ionic mixed conducting networks for vanadium redox flow batteries

The GF-TCN electrodes with excellent electrocatalytic activity and faster electron/ion conduction indicate outstanding rate capability and energy efficiency of VRFBs.

Hierarchical Carbon Micro/Nanonetwork with Superior Electrocatalysis for High-Rate and Endurable Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) are receiving increasing interest in energy storage fields because of their safety and versatility. However, the electrocatalytic activity of the electrode is a pivotal factor that still restricts the power and cycling capabilities of VRFBs. Here, a hierarchical carbon micro/nanonetwork (HCN) electrode codoped with nitrogen and phosphorus is prepared for application in VRFBs by cross-linking polymerization of aniline and physic acid, and subsequent pyrolysis on graphite felt. Due to the hierarchical electron pathways and abundant heteroatom active sites, the HCN exhibits superior electrocatalysis toward the vanadium redox couples and imparts the VRFBs with an outstanding energy efficiency and extraordinary stability after 2000 cycles at 250 mA cm-2 and a discharge capacity of 10.5 mA h mL-1 at an extra-large current density of 400 mA cm-2. Such a micro/nanostructure design will force the advancement of durable and high-power VRFBs and other electrochemical energy storage devices.

Graphene-decorated silica stabilized stearic acid as a thermal energy storage material

Graphene-decorated silica stabilized stearic acid composites with interesting thermal energy storage behaviors.