Structure, hydrolysis, and diffusion of aqueous vanadium ions from Car-Parrinello molecular dynamics

Binuclear Metal Phthalocyanines with Enhanced Activity in the Oxygen Evolution Reaction: A First-Principles Study

The rational design of efficient catalysts for the electrochemical oxygen evolution reaction (OER) critically relies on a comprehensive understanding of the reaction mechanisms. Herein, the alkaline OER on planar mononuclear metal phthalocyanines (MPc, where M = Mn, Co, Fe, and Ni) and binuclear metal phthalocyanines (bi-MPc) is studied using density functional theory (DFT) methods. Both FePc and bi-CoPc exhibit enhanced stability and OER activity, with the energy required for the leaching of central metal being as high as 2.28 and 2.45 eV and the overpotentials of the OER being 0.48 and 0.57 V, respectively. Through electronic structure analysis, it is found that, in the OER process of bi-MPc, the large macrocyclic ligand and metal ions not bonding with the intermediate can serve as hole reservoirs. Intermediate species are further stabilized by the dispersal of a positive charge, reducing the free energy. These findings underscore the significance of macrocyclic ligands in the rate-determining step of the OER catalyst.

Ion Flux Self-Regulation Strategy with a Volume-Responsive Separator for Lithium Metal Batteries

Lithium metal batteries (LMBs) are regarded as one of the most promising next-generation energy storage devices due to their high energy density. However, the conversion of LMBs from laboratory to factory is hindered by the formation of lithium dendrites and volume change during lithium stripping and deposition processes. In this work, a volume-responsive separator with core/shell structure thermoplastic polyurethane (TPU)/polyvinylidene fluoride (PVDF) fibers and SiO₂ coating layers is designed to restrict dendrite growth. The TPU/PVDF-SiO₂ separator can accommodate the volume change like an artificial lung and keep intimate contact with the electrodes, which leads to the formation of a uniform and high-density solid-electrolyte interphase. Meanwhile, the separator can regulate the transport channels and diffusion coefficients (D) of lithium ions with the change of porosity from both experimental and ab initio molecular dynamic analysis. The Li symmetric cells assembled with the TPU/PVDF-SiO₂ can run for 1000 h at the current of 1.0 mA cm^-2 without a short circuit. Moreover, the low melting point of PVDF can shut the ionic conduction down at 170 °C, guaranteeing the thermal safety of the batteries. With the above advantages, the TPU/PVDF-SiO₂ separator presents great potential to promote the commercial and industrial application of LMBs.

Tungsten oxide/fullerene-based nanocomposites as electrocatalysts and parasitic reactions inhibitors for VO2+/VO2+ in mixed-acids

The relatively high cost of all-vanadium redox flow batteries (VRFBs) limits their widespread deployment. Enhancing the kinetics of the electrochemical reactions is needed to increase the power density and energy efficiency of the VRFB, and hence decrease the kWh cost of VRFBs. In this work, hydrothermally synthesized hydrated tungsten oxide (HWO) nanoparticles, C₇₆, and C₇₆/HWO were deposited on carbon cloth electrodes and tested as electrocatalysts for the VO²⁺/VO₂⁺ redox reactions. Field Emission Scanning Electron Microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscope (HR-TEM,), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and contact angle measurements were used to characterize the electrodes’ material. The addition of the C₇₆ fullerene to HWO was found to boost the electrode kinetics towards the VO²⁺/VO₂⁺ redox reaction, by enhancing the conductivity and providing oxygenated functional groups at its surface. A composite of HWO/C₇₆ (50 wt% C₇₆) was found to be the optimum for the VO²⁺/VO₂⁺ reaction, showing a ΔE_p of 176 mV, compared to 365 mV in the case of untreated carbon cloth (UCC). Besides, HWO/C₇₆ composites showed a significant inhibition effect for the parasitic chlorine evolution reaction due to the W-OH functional groups.

Ab Initio Metadynamics Study of the VO2+/VO2+ Redox Reaction Mechanism at the Graphite Edge/Water Interface

Redox flow batteries (RFBs) are promising electrochemical energy storage systems, for which development is impeded by a poor understanding of redox reactions occurring at electrode/electrolyte interfaces. Even for the conventional all-vanadium RFB chemistry employing V²⁺/V³⁺ and VO₂⁺/VO²⁺ couples, there is still no consensus about the reaction mechanism, electrode active sites, and rate-determining step. Herein, we perform Car-Parrinello molecular dynamics-based metadynamics simulations to unravel the mechanism of the VO₂⁺/VO²⁺ redox reaction in water at the oxygen-functionalized graphite (112̅0) edge surface serving as a representative carbon-based electrode. Our results suggest that during the battery discharge aqueous VO₂⁺/VO²⁺ species adsorb at the surface C-O groups as inner-sphere complexes, exhibiting faster adsorption/desorption kinetics than V²⁺/V³⁺, at least at low vanadium concentrations considered in our study. We find that this is because (i) VO₂⁺/VO²⁺ conversion does not involve the slow transfer of an oxygen atom, (ii) protonation of VO₂⁺ is spontaneous and coupled to interfacial electron transfer in acidic conditions to enable VO²⁺ formation, and (iii) V³⁺ found to be strongly bound to oxygen groups of the graphite surface features unfavorable desorption kinetics. In contrast, the reverse process taking place upon charging is expected to be more sluggish for the VO₂⁺/VO²⁺ redox couple because of both unfavorable deprotonation of the VO²⁺ water ligands and adsorption/desorption kinetics.

First-principles study of adsorption–desorption kinetics of aqueous V2+/V3+ redox species on graphite in a vanadium redox flow battery

Vanadium redox flow batteries (VRFBs) represent a promising solution to grid-scale energy storage, and understanding the reactivity of electrode materials is crucial for improving the power density of VRFBs.