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 .
The
solvation structure of transition metal ions is important for
applications in geochemistry, biochemistry, energy storage, and environmental
chemistry. We study the X-ray absorption pre-edge and near-edge spectra
at the K-edge of a nearly complete series of hydrated first-row transition
metal ions with d orbital occupancy from d2 to d10. We optimize all of the structures at the density functional theory
(DFT) level with explicit solvation and then compute the pre-edge
X-ray absorption spectra with time-dependent DFT (TDDFT) and restricted
active space second-order perturbation theory (RASPT2). TDDFT provides
accurate results for spectra that are dominated by single excitations,
while RASPT2 correctly distinguishes between singly and doubly excited
states with quantitative accuracy compared with experiment. We analyze
the pre-edge features for each metal ion to reveal the impact of the
variations in d orbital occupancy on the first-shell coordination
environment. We also report the lowest-energy ligand field d–d
transitions using complete active space second-order perturbation
theory.
When scaling up photo-electrochemical processes to larger areas than conventionally studied in the laboratory, substrate performance must be taken into consideration and in this work, a methodology to assess this via an uncomplicated 2 dimensional model is outlined. It highlights that for F-doped SnO2 (FTO), which is ubiquitously used for metal oxide photoanodes, substrate performance becomes significant for moderately sized electrodes (5 cm) under no solar concentration for state of the art Fe2O3 thin films. It is demonstrated that when the process is intensified via solar concentration, current losses become quickly limiting. Methodologies to reduce the impact of substrate ohmic losses are discussed and a new strategy is proposed. Due to the nature of the photo-electrode current-potential relationship, operation at a higher potential where the photo-current saturates (before the dark current is observed) will lead to a minimum in current loss due to substrate performance. Crucially, this work outlines an additional challenge in scaling up photo-electrodes based on low conductivity substrates, and establishes that such challenges are not insurmountable.
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