Advanced electrode enabled by lignin-derived carbon for high-performance vanadium redox flow battery

Vanadium redox flow batteries (VRFBs) are promising energy storage systems with the potential to bridge the gap between intermittent renewable electricity generation and continuous supply of reliable electricity. The electrodes found in VRFB cells affect their energy efficiency (EE) and power density. It is important to fabricate electrodes with intriguing properties to enable VRFBs to have high performance. Herein, the abundant and cost-effective lignin is employed as the precursor to produce amorphous carbon particles after undergoing thermal decomposition treatment. The carbon particles cover the surface of carbon felt (CF). The resulting CF modified by lignin-derived carbon particles (Lignin-CF) with increased active sites and improved hydrophilicity displays superior electrochemical activity towards the VO2+/VO2+ pair than both the pristine CF and the heated bare CF. Remarkably, the VRFB consisting of Lignin-CF which acts as the positive electrode shows high performance in terms of the average EE (83.3 %) and average voltage efficiency (VE) (85.0 %) over 1000 cycles (long cycling life) for more than 16 days at 100 mA cm-2, and high power density of 1053.2 mW cm-2. It is noted that the EE and VE are comparable to the highest reported value of CF modified by carbon-based materials, aside having evidently longer cycling life. This study provides a feasible strategy for fabricating an affordable electrode for high-performance VRFBs.

Ab initio insight into the electrolysis of water on basal and edge (fullerene C20) surfaces of 4 Å single-walled carbon nanotubes

The extreme surface reactivity of 4 Å single-walled carbon nanotubes (SWCNTs) makes for a very promising catalytic material, however, controlling it experimentally has been found to be challenging. Here, we employ ab initio calculations to investigate the extent of surface reactivity and functionalization of 4 Å SWCNTs. We study the kinetics of water dissociation and adsorption on the surface of 4 Å SWCNTs with three different configurations: armchair (3,3), chiral (4,2) and zigzag (5,0). We reveal that out of three different configurations of 4 Å SWCNTs, the surface of tube (5,0) is the most reactive due to its small HOMO-LUMO gap. The dissociation of 1 H₂O molecule into an OH/H pair on the surface of tube (5,0) has an adsorption energy of -0.43 eV and an activation energy barrier of 0.66 eV at 298.15 K in pure aqueous solution, which is less than 10% of the activation energy barrier of the same reaction without the catalyst present. The four steps of H⁺/e^- transfer in the oxygen evolution reaction have also been studied on the surface of tube (5,0). The low overpotential of 0.38 V indicates that tube (5,0) has the highest potential efficiency among all studied carbon-based catalysts. We also reveal that the armchair edge of tube (5,0) is reconstructed into fullerene C₂₀. The dangling bonds on the surface of fullerene C₂₀ result in a more reactive surface than the basal surface of tube (5,0), however the catalytic ability was also inhibited in the later oxygen reduction processes.

Why halides enhance heterogeneous metal ion charge transfer reactions

The reaction kinetics of many metal redox couples on electrode surfaces are enhanced in the presence of halides (i.e., Cl-, Br-, I-). Using first-principles metadynamics simulations, we show a correlation between calculated desorption barriers of V3+-anion complexes bound to graphite via an inner-sphere anion bridge and experimental V2+/V3+ kinetic measurements on edge plane pyrolytic graphite in H2SO4, HCl, and HI. We extend this analysis to V2+/V3+, Cr2+/Cr3+, and Cd0/Cd2+ reactions on a mercury electrode and demonstrate that reported kinetics in acidic electrolytes for these redox couples also correlate with the predicted desorption barriers of metal-anion complexes. Therefore, the desorption barrier of the metal-anion surface intermediate is a descriptor of kinetics for many metal redox couple/electrode combinations in the presence of halides. Knowledge of the metal-anion surface intermediates can guide the design of electrolytes and electrocatalysts with faster kinetics for redox reactions of relevance to energy and environmental applications.

The Role of Oxygenic Groups and sp3 Carbon Hybridization in Activated Graphite Electrodes for Vanadium Redox Flow Batteries

Graphite felt is a widely used electrode material for vanadium redox flow batteries. Electrode activation leads to the functionalization of the graphite surface with epoxy, OH, C=O, and COOH oxygenic groups and changes the carbon surface morphology and electronic structure, thereby improving the electrode's electroactivity relative to the untreated graphite. In this study, density functional theory (DFT) calculations are conducted to evaluate functionalization's contribution towards the positive half-cell reaction of the vanadium redox flow battery. The DFT calculations show that oxygenic groups improve the graphite felt's affinity towards the VO²⁺ /VO₂ ⁺ redox couple in the following order: C=O>COOH>OH> basal plane. Projected density-of-states (PDOS) calculations show that these groups increase the electrode's sp³ hybridization in the same order, indicating that the increase in sp³ hybridization is responsible for the improved electroactivity, whereas the oxygenic groups' presence is responsible for this sp³ increment. These insights can aid the selection of activation processes and optimization of their parameters.

Effect of Fluoroethylene Carbonate Additives on the Initial Formation of the Solid Electrolyte Interphase on an Oxygen-Functionalized Graphitic Anode in Lithium-Ion Batteries

The formation of a solid electrolyte interphase (SEI) at the electrode/electrolyte interface substantially affects the stability and lifetime of lithium-ion batteries (LIBs). One of the methods to improve the lifetime of LIBs is by the inclusion of additive molecules to stabilize the SEI. To understand the effect of additive molecules on the initial stage of SEI formation, we compare the decomposition and oligomerization reactions of a fluoroethylene carbonate (FEC) additive on a range of oxygen-functionalized graphitic anodes to those of an ethylene carbonate (EC) organic electrolyte. A series of density functional theory (DFT) calculations augmented by ab initio molecular dynamics (AIMD) simulations reveal that EC decomposition on an oxygen-functionalized graphitic (112̅0) edge facet through a nucleophilic attack on an ethylene carbon site (C_E) of an EC molecule (S2 mechanism) is spontaneous during the initial charging process of LIBs. However, decomposition of EC through a nucleophilic attack on a carbonyl carbon (C_C) site (S1 mechanism) results in alkoxide species regeneration that is responsible for continual oligomerization along the graphitic surface. In contrast, FEC prefers to decompose through an S1 pathway, which does not promote alkoxide regeneration. Including FEC as an additive is thus able to suppress alkoxide regeneration and results in a smaller and thinner SEI layer that is more flexible toward lithium intercalation during the charging/discharging process. In addition, we find that the presence of different oxygen functional groups at the surface of graphite dictates the oligomerization products and the LiF formation mechanism in the SEI.

DFT modelling of explicit solid–solid interfaces in batteries: methods and challenges

Density Functional Theory (DFT) calculations of electrode material properties in high energy density storage devices like lithium batteries have been standard practice for decades.

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.