Effect of Sulfuric and Triflic Acids on the Hydration of Vanadium Cations: An ab Initio Study

Voltage prediction of vanadium redox flow batteries from first principles

Global energy demand has been increasing for decades, which has created a necessity for large scale energy storage solutions for renewable energy sources. We studied the voltage of vanadium redox flow batteries (VRFBs) with density functional theory (DFT) and a newly developed technique usingab initiomolecular dynamics (AIMD). DFT was used to create cluster models to calculate the voltage of VRFBs. However, DFT is not suited for capturing the dynamics and interactions in a liquid electrolyte, leading to the need for AIMD, which is capable of accurately modeling such things. The molarities and densities of all systems were carefully considered to match experimental conditions. With the use of AIMD, we calculated a voltage of 1.23 V, which compares well with the experimental value of 1.26 V. The techniques developed using AIMD for voltage calculations will be useful for the investigation of potential future battery technologies or as a screening process for additives to make improvements to currently available batteries.

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.

The degradation effect on proton dissociation and transfer in perfluorosulfonic acid membranes

In the operation of proton exchange membrane fuel cells (PEMFCs), the ionomer-perfluorosulfonic acid (PSFA) membrane side chains are easily attacked by free radicals, resulting in membrane degradation. In this work, the chemical degradation effect of side chains in the PSFA membrane on proton dissociation and transfer behaviors is investigated by means of the quantum chemistry calculation. The rotation of the H atom in the acid group after the degradation is evaluated. The impact of the electrostatic potential (ESP) and electronegativity of the side chains is analyzed. The results demonstrate that the membrane degradation decreases the positive potential of the proton in the acid group, leading to the proton being less active so that more water molecules are required for the spontaneous proton dissociation. The rotation of the H atom in the acid group affects the proton dissociation mode owing to the change of the hydrogen bond network. It is found that the ESP of the acid group in two side chain fragments influences each other and the water molecules between two side chains can be shared to reduce the number of water molecules for the proton dissociation.

Atomically precise vanadium-oxide clusters

Polyoxovanadate (POV) clusters are an important subclass of polyoxometalates with a broad range of molecular compositions and physicochemical properties. One relatively underdeveloped application of these polynuclear assemblies involves their use as atomically precise, homogenous molecular models for bulk metal oxides. Given the structural and electronic similarities of POVs and extended vanadium oxide materials, as well as the relative ease of modifying the homogenous congeners, investigation of the chemical and physical properties of pristine and modified cluster complexes presents a method toward understanding the influence of structural modifications (e.g. crystal structure/phase, chemical makeup of surface ligands, elemental dopants) on the properties of extended solids. This review summarises recent advances in the use of POV clusters as atomically precise models for bulk metal oxides, with particular focus on the assembly of vanadium oxide clusters and the consequences of altering the molecular composition of the assembly via organofunctionalization and the incorporation of elemental "dopants".

Systematic Investigation of the Physical and Electrochemical Characteristics of the Vanadium (III) Acidic Electrolyte With Different Concentrations and Related Diffusion Kinetics

Owing to the lack of systematic kinetic theory about the redox reaction of V(III)/V(II), the poor electrochemical performance of the negative process in vanadium flow batteries limits the overall battery performance to a great extent. As the key factors that influence electrode/electrolyte interfacial reactivity, the physicochemical properties of the V(III) acidic electrolyte play an important role in the redox reaction of V(III)/V(II), hence a systematic investigation of the physical and electrochemical characteristics of V(III) acidic electrolytes with different concentrations and related diffusion kinetics was conducted in this work. It was found that the surface tension and viscosity of the electrolyte increase with increasing V(III) concentration, while the corresponding conductivity shows an opposite trend. Both the surface tension and viscosity change slightly with increasing concentration of H₂SO₄, but the conductivity increases significantly, indicating that a lower V(III) concentration and a higher H₂SO₄ concentration are conducive to the ion transfer process. The electrochemical measurements further show that a higher V(III) concentration will facilitate the redox reaction of V(III)/V(II), while the increase in H₂SO₄ concentration only improves the ion transmission and has little effect on the electron transfer process. Furthermore, the diffusion kinetics of V(III) have been further studied with cyclic voltammetry and chronopotentiometry. The results show that an elevated temperature facilitates the V(III)/V(II) redox reaction and gives rise to an increased electrode reaction rate constant (k_s) and diffusion coefficient [D_V(III)]. On this basis, the diffusion activation energy (13.7 kJ·mol⁻¹) and the diffusion equation of V(III) are provided to integrate kinetic theory in the redox reaction of V(III)/V(II).

Proton Transfer in Phosphoric Acid-Based Protic Ionic Liquids: Effects of the Base

Electronic structure calculations were performed to understand highly decoupled conductivities recently reported in protic ionic liquids (PILs). To develop a molecular-level understanding of the mechanisms of proton conductivity in PILs, minimum-energy structures of trimethylamine, imidazole, lidocaine, and creatinine (CRT) with the addition of one to three phosphoric acid (PA) molecules were determined at the B3LYP/6-311G** level of theory with the inclusion of an implicit solvation model (SMD with ε = 61). The proton affinity of the bases and zero-point energy corrected binding energies were computed at a similar level of theory. Proton dissociation from PA occurs in all systems, resulting in the formation of ion pairs due to the relatively strong basicity of the bases (proton acceptors) and the effect of the high dielectric constant solvent in stabilizing the charge separation. The second and third PA molecules preferentially form "ring-like" hydrogen bonds with one another instead of forming hydrogen bonds at the donor and acceptor sites of the bases. Potential energy scans reveal that the bases with stronger proton affinity exert greater influence on the energetics of proton transfer between the individual PA molecules. However, the effects are minimal when shifted into a single-well from a double-well potential. Barrierless proton transfer was observed to occur in the CRT system with several PA molecules present, implying that the CRT may be a promising PA-based PIL.

Revealing the role of phosphoric acid in all-vanadium redox flow batteries with DFT calculations and in situ analysis

Phosphoric acid improves the stability of vanadium redox flow battery electrolyte and enhances the kinetics of the negative electrode.