V²⁺/V³⁺ Redox Kinetics on Glassy Carbon in Acidic Electrolytes for Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow batteries are a promising technology for energy storage, yet the mechanism of the kinetically limiting V 2+ /V 3+ redox reaction remains poorly understood. Here, we elucidate the impact of anion complexation on V 2+ /V 3+ kinetics on a glassy carbon electrode in three common electrolytes: hydrochloric acid, sulfuric acid, and mixed HCl/H 2 SO 4 . The V 2+ /V 3+ kinetics are ∼2.5 times faster in HCl and have lower apparent activation energies than those in H 2 SO 4 or HCl/H 2 SO 4 . We also identify the presence of [V(H 2 O) 4 Cl 2 ] + species in HCl by UV−vis spectroscopy. We confirm that the V 2+ /V 3+ reaction proceeds via an adsorbed intermediate and propose a bridging mechanism through adsorbed *Cl (in HCl) and *OH (in H 2 SO 4 or HCl/H 2 SO 4 ). A bridging mechanism through *Cl is supported by even faster redox kinetics in HBr than in HCl, possibly due to the higher polarizability of *Br. By measuring the exchange current densities using steady-state current measurements and impedance spectroscopy, we show that the overall reaction is a two-electron process in HCl as opposed to a one-electron process in H 2 SO 4 and HCl/H 2 SO 4 .

Molecular Engineering of Azobenzene‐Based Anolytes Towards High‐Capacity Aqueous Redox Flow Batteries

“…[3] Currently,a sthe representative of aqueous RFBs using redox species dissolved in aqueous solutions,vanadium-based RFBs have been successfully commercialized. [4] In this context, aqueous RFBs have shown clear advantages compared with nonaqueous candidates,s uch as low cost, noninflammability,high power, and many available ion-selective membranes. [5] However,i nc onventional aqueous RFBs,t he choice of redox species is limited to metal-based active materials,s uch as vanadium-based ions.T he relatively high cost of transition metals and limited energy density have impeded their extensive application.…”

“…[5] However,i nc onventional aqueous RFBs,t he choice of redox species is limited to metal-based active materials,s uch as vanadium-based ions.T he relatively high cost of transition metals and limited energy density have impeded their extensive application. [4,6] Tr emendous research efforts have been made on exploring next-generation flow battery technologies as potential alternatives,s uch as zincbased flow batteries or aqueous organic RFBs. [7] Zinc-based flow batteries have high working voltage,b ut they are commonly suffering ashort cycle life because of zinc dendrite issues,e specially under high current density.…”

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

“…[ 12 ] Agarwal et al. [ 16 ] investigated the reaction mechanism of the V 2+ /V 3+ reaction in HCl, HCl/H 2 SO 4 , and H 2 SO 4 electrolytes, and found that a bridging ligand was formed during the reaction. They suggested that higher polarizability of the *Cl bridge in HCl than the *OH bridge in conventional H 2 SO 4 or HCl/H 2 SO 4 rendered an enhanced electron transfer rate.…”

“…Counter anions (such as Cl − ) can exchange with the H 2 O ligand in the primary solvation shell, which has been found to play a vital role in tuning the thermal and chemical stability of hydrated vanadium cations and their reaction pathways. [12] Agarwal et al [16] investigated the reaction mechanism of the V 2+ /V 3+ reaction in HCl, HCl/H 2 SO 4 , and H 2 SO 4 electrolytes, and found that a bridging ligand was formed during the reaction. They suggested that higher polarizability of the *Cl bridge in HCl than the *OH bridge in conventional H 2 SO 4 or HCl/H 2 SO 4 rendered an enhanced electron transfer rate.…”

Molecular Engineering of Azobenzene‐Based Anolytes Towards High‐Capacity Aqueous Redox Flow Batteries

“…Currently, as the representative of aqueous RFBs using redox species dissolved in aqueous solutions, vanadium‐based RFBs have been successfully commercialized [4] . In this context, aqueous RFBs have shown clear advantages compared with nonaqueous candidates, such as low cost, noninflammability, high power, and many available ion‐selective membranes [5] .…”