The electrochemical reduction of VO2+ in acidic solution at high overpotentials

Physics, electrochemistry, chemistry, and electronics of the vanadium redox flow battery by analyzing all the governing equations

The vanadium redox flow battery has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation equations (both tank and cells, for negative and positive half cells), its thermal time variation equations, and fluid flow equations. Chemistry behavior of the electrolyte ions have also been intensively examined too. In this perspective, all of the phenomena have been examined, unified and presented together with their physical chemistry shown in the appropriate equations. This is done by providing the field equations for the battery, which are electronic, electrochemical, chemical, physics of fluid dynamics, and thermal physics of heat transport, in character. They are interpreted in new analytical equations providing fundamental scientific insights, as well as allowing engineering and manufacturing assessments. Graphs of pressure trend in electrodes, power and temperature tables for electrode and current collector loss mechanisms are provided. Current density, concentration, electrostatic, and overpotential functional dependences in the electrodes are given.

Functional materials for aqueous redox flow batteries: merits and applications

Redox flow batteries (RFBs) are promising electrochemical energy storage systems, offering vast potential for large-scale applications. Their unique configuration allows energy and power to be decoupled, making them highly scalable and flexible in design. Aqueous RFBs stand out as the most promising technologies, primarily due to their inexpensive supporting electrolytes and high safety. For aqueous RFBs, there has been a skyrocketing increase in studies focusing on the development of advanced functional materials that offer exceptional merits. They include redox-active materials with high solubility and stability, electrodes with excellent mechanical and chemical stability, and membranes with high ion selectivity and conductivity. This review summarizes the types of aqueous RFBs currently studied, providing an outline of the merits needed for functional materials from a practical perspective. We discuss design principles for redox-active candidates that can exhibit excellent performance, ranging from inorganic to organic active materials, and summarize the development of and need for electrode and membrane materials. Additionally, we analyze the mechanisms that cause battery performance decay from intrinsic features to external influences. We also describe current research priorities and development trends, concluding with a summary of future development directions for functional materials with valuable insights for practical applications.

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.

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.

Material Design of Aqueous Redox Flow Batteries: Fundamental Challenges and Mitigation Strategies

Redox flow batteries (RFBs) are critical enablers for next-generation grid-scale energy-storage systems, due to their scalability and flexibility in decoupling power and energy. Aqueous RFBs (ARFBs) using nonflammable electrolytes are intrinsically safe. However, their development has been limited by their low energy density and high cost. Developing ARFBs with higher energy density, lower cost, and longer lifespan than the current standard is of significant interest to academic and industrial research communities. Here, a critical review of the latest progress on advanced electrolyte material designs of ARFBs is presented, including a fundamental overview of their physicochemical properties, major challenges, and design strategies. Assessment methodologies and metrics for the evaluation of RFB stability are discussed. Finally, future directions for material design to realize practical applications and achieve the commercialization of ARFB energy-storage systems are highlighted.

Carbon Anode in Carbon History

This study examines how the several major industries, associated with a carbon artifact production, essentially belong to one, closely knit family. The common parents are the geological fossils called petroleum and coal. The study also reviews the major developments in carbon nanotechnology and electrocatalysis over the last 30 years or so. In this context, the development of various carbon materials with size, dopants, shape, and structure designed to achieve high catalytic electroactivity is reported, and among them recent carbon electrodes with many important features are presented together with their relevant applications in chemical technology, neurochemical monitoring, electrode kinetics, direct carbon fuel cells, lithium ion batteries, electrochemical capacitors, and supercapattery.

R, P. V, S. S, R. K. Functionalised carbazole as a cathode for high voltage non-aqueous organic redox flow batteries

Prospective high reduction potential cathode materials have been proposed that can be used in non-aqueous redox flow battery applications.

Unraveling V(V)-V(IV)-V(III)-V(II) Redox Electrochemistry in Highly Concentrated Mixed Acidic Media for a Vanadium Redox Flow Battery: Origin of the Parasitic Hydrogen Evolution Reaction

We present a mechanistic understanding of the full redox electrochemistry of V(V)-V(IV)-V(III)-V(II) and the origin of the parasitic hydrogen evolution reaction (HER) during electroreduction of either V³⁺ or VO²⁺ in a highly concentrated mixed acidic solution based on both electroanalytical and computational approaches. First, we found that the VO²⁺/VO₂⁺ redox reaction is well explained by the EC/EC square scheme. We also found that V³⁺ is electrochemically oxidized to V⁴⁺ and subsequently undergoes a transition to stable VO²⁺ via hydrolysis. In the V³⁺/V²⁺ redox reaction via voltammetric analysis at scan rates greater than 0.05 V/s, the voltammograms are well explained based on a simple 1e^- transfer reaction scheme. However, at the longer time scale observed in the chronoamperograms with constantly applied potentials where V³⁺ is electrochemically reduced to V²⁺, we found that a significant HER occurs because of possible formation of an electrocatalyst related to the V(II)O species, V(II)_catalyst. We suggest that V(II)O is kinetically formed from V²⁺ via hydrolysis only when a local concentration of V²⁺ is high in the vicinity of a GC electrode surface, and V(II)O is adsorbed on a GC surface to form V(II)_catalyst. To extend our mechanistic pathway, electroreduction of VO²⁺ to V(II) was also analyzed, revealing that VO²⁺ is electroreduced to VO⁺ and further reduced to VO in addition to disproportionation of VO⁺. Eventually, V(II)_catalyst forms on a GC electrode, resulting in a significant HER. The computational calculation strongly supports the possible formation of V(II)_catalyst. The calculation shows that neither V³⁺ nor V²⁺ can form stable intermediates during the HER, while V(II)O has the highest proton affinity compared with V(III)O⁺ and V(IV)O²⁺, indicating a plausible electrocatalytic property of V(II)O for the HER.

The Chemistry of Redox-Flow Batteries

The development of various redox-flow batteries for the storage of fluctuating renewable energy has intensified in recent years because of their peculiar ability to be scaled separately in terms of energy and power, and therefore potentially to reduce the costs of energy storage. This has resulted in a considerable increase in the number of publications on redox-flow batteries. This was a motivation to present a comprehensive and critical overview of the features of this type of batteries, focusing mainly on the chemistry of electrolytes and introducing a thorough systematic classification to reveal their potential for future development.

Temperature-related reaction kinetics of the vanadium(iv)/(v) redox couple in acidic solutions

Two diffusion equations for both VO²⁺ and VO₂⁺ ions in practical electrolyte (1.2 M V + 3.0 M H₂SO₄) have been established, which can be expressed by D_VO2+ = 4 × 10⁻⁵ exp(−4240/RT) and D_VO2+ = 0.18861 exp(−25 200/RT) (cm² s⁻¹).

Unusual phenomena in the reduction process of vanadium(v) on a graphite electrode at high overpotentials

Two unusual phenomena are observed at a transition potential (E_K) by steady-state potentiodynamic polarization and impedance spectroscopy measurements. The possible vanadium complexes such as V₂O₃³⁺ might change the reduction mechanism at high overpotentials.

Vanadium Flow Battery for Energy Storage: Prospects and Challenges

The vanadium flow battery (VFB) as one kind of energy storage technique that has enormous impact on the stabilization and smooth output of renewable energy. Key materials like membranes, electrode, and electrolytes will finally determine the performance of VFBs. In this Perspective, we report on the current understanding of VFBs from materials to stacks, describing the factors that affect materials' performance from microstructures to the mechanism and new materials development. Moreover, new models for VFB stacks as well as structural design will be summarized as well. Finally, the challenges, the overall cost evaluation, and future research directions will be briefly proposed.