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This study investigates the effect of thermal activation of all-vanadium redox flow battery (RFB) carbon-felt electrodes on their electrode kinetics. Using X-ray photoelectron spectroscopy, thermal activation is shown to increase the content of the C−OH group, decrease the content of the CO group, and not affect the O−CO group, with all these surface moieties already being present in the nonactivated carbon felt. Rotating disk electrode studies were performed using custom electrodes fabricated using the carbon felt to investigate the kinetics of the V 2+ /V 3+ and VO 2+ /VO 2 + redox couples in H 2 SO 4 and to deconvolute the impact of thermal activation on electrode kinetics. We demonstrate that V 2+ /V 3+ kinetics is sluggish compared to VO 2+ /VO 2 + kinetics (equilibrium rate constant (k 0 ) = 4.98 × 10 −8 m•s −1 vs 8.81 × 10 −8 m•s −1 ) and that thermal activation enhanced V 2+ /V 3+ kinetics while inhibiting VO 2+ /VO 2 + kinetics. The enhancement in V 2+ /V 3+ kinetics was attributed to the oxygencontaining groups −C−OH added during thermal activation. Using thermally activated carbon-felt V 2+ /V 3+ electrodes yielded an overall increase in energy efficiency (EE) from 75 ± 3.7 to 90 ± 4.5% and voltage efficiency (VE) from 76 ± 4 to 92 ± 4.6%. On the other hand, using thermally activated carbon-felt VO 2+ /VO 2 + electrodes lowered EE from 75 ± 4 to 73 ± 3.6% and VE from 76 ± 4 to 74 ± 4%. The optimal combination of thermally activated carbon-felt V 2+ /V 3+ electrodes and untreated carbonfelt VO 2+ /VO 2 + electrodes resulted in the most efficient RFB configuration.

Development of Li–O2 cells, potentially providing ∼3 times the capacity of Li-ion cells, depends on a fundamental understanding of the oxygen reduction reaction (ORR) at the cathode. The present study investigates the mechanism and kinetics of the oxygen reduction reaction (ORR) on a glassy carbon (GC) electrode in an oxygen saturated solution of 0.1 M lithium bis-trifluoromethanesulfonimidate (LiTFSI) in 1,2-dimethoxyethane (DME) using cyclic voltammetery (CV) and the rotating ring-disk electrode (RRDE) technique. A reaction scheme considering disproportionation of LiO2 on both the cathode surface and the electrolyte bulk to form Li2O2 was proposed, and the RRDE measurements, in conjunction with an electrochemical kinetics model, were used to calculate the corresponding rate constants. The surface disproportionation reaction was found to dominate the kinetics of the ORR, and the model could explain experimental observations regarding the cell discharge products. Further, the widely reported anomalous Tafel behavior was observed over the course of these studies. Potentiostatic, point-by-point measurements of the kinetic current were carried out, and a scan rate independent evaluation of the corresponding transfer coefficient from a dimensionless CV was obtained. The measured transfer coefficient was explained invoking Marcus–Hush kinetic theory, and the solvent reorganization energy was proposed as a more comprehensive alternative to the Gutmann donor number to evaluate solvent effects on reaction kinetics. This study provides a comprehensive account of the ORR mechanism, evidence of the surface disproportionation reaction being dominant, and explains the widely reported (and previously unexplained) anomalous Tafel behavior in Li–O2 cells.

The objectives of this study were: 1) to confirm superoxide anion radical (O ) formation, and 2) to monitor in real time the rate of O generation in an operating anion exchange membrane (AEM) fuel cell using in situ fluorescence spectroscopy. 1,3-Diphenlisobenzofuran (DPBF) was used as the fluorescent molecular probe owing to its selectivity and sensitivity toward O in alkaline media. The activation energy for the in situ generation of O during AEM fuel cell operation was estimated to be 18.3 kJ mol . The rate of in situ generation of O correlated well with the experimentally measured loss in AEM ion-exchange capacity and ionic conductivity attributable to oxidative degradation.

Direct borohydride fuel cells (DBFCs) can operate at double the voltage of proton exchange membrane fuel cells (PEMFCs) by employing an alkaline NaBH 4 fuel feed and an acidic H 2 O 2 oxidant feed. The pH-gradient-enabled microscale bipolar interface (PMBI) facilitates the creation and maintenance of an alkaline environment at the anode and an acidic environment at the cathode for the borohydride oxidation and peroxide reduction reactions. However, given the need to dissociate water at the interface to ensure ionic conduction, PMBI can be efficient only when anion exchange ionomer (AEI) moieties enable fast water transport for autoprotolysis. Herein, a series of polynorbornene-based AEIs with a range of water uptake values are examined to unravel the optimum water uptake required to enable high performance DBFCs. The DBFC with PMBI configuration containing the optimal AEI composition delivers a current density of 302 mA cm −2 at 1.5 V and a peak power density of 580 mW cm −2 at 1 V. This AEI composition exhibits high hydroxide ionic conductivity of 90.7 mS cm −1 at 80 °C with an IEC of 2.01 mequiv g −1 and demonstrates impressive chemical stability by retaining 98.75% of its initial ionic conductivity after immersion into anolyte (3 M KOH and 1.5 M NaBH 4 ) at 70 °C for 536 h.

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The thermodynamic stability and relatively low free energy of formation (ΔG 0 = −239.4 kJ mol–1) of KO2 offer the possibility of K–O2 cells as a catalyst-free, low overpotential energy storage system. Having identified dimethyl sulfoxide (DMSO) as a solvent that promotes KO2 production due to its high donor number, the present study elucidates the oxygen reduction reaction mechanism of the K–O2 cell with a DMSO electrolyte. The use of DMSO-based electrolytes led to distinct first and second electron-transfer peaks, suggesting the possibility of facile voltage-based control of the cathode reaction to selectively produce KO2 as the product. However, the observed low overpotential i–E behavior on a rotating ring-disk electrode could only be accounted for by postulating further chemical reactions (disproportionation on the electrode surface and in the electrolyte) of KO2 to form K2O2. The rate of the surface disproportionation reaction to produce K2O2 was found to be competitive with the KO2 desorption step, whereas the solution disproportionation step was found to be an order of magnitude slower. Thus, DMSO is proposed as a solvent that will allow the selective production of KO2 as the reduction product in a K–O2 cell, thereby improving the reversibility of the cell. Further, the first electron-transfer rate constant in DMSO was found to be 4 orders of magnitude higher than literature values for the same in diglyme, allowing us to show that DMSO-based K–O2 cells can achieve rate capability superior to diglyme-based K–O2 cells, significantly improving on the current state-of-the-art.

Commercial fuel cell electrocatalyst degradation results from carbon electrocatalyst support oxidation at high operating potential transients. Guided by density functional theory (DFT) calculations, Nb‐doped TiO2 (NTO) was synthesized, which exhibits a unique combination of high surface area, high electrical conductivity, and high porosity. This catalyst retained 78 % of its initial electrochemically active surface area compared with 57.6 % retained by Pt/C following the DOE/FCCJ protocol for accelerated stability test. Strong metal–support interactions, which were predicted by DFT calculations and confirmed experimentally by X‐ray photoelectron spectroscopy and kinetics measurements, resulted in 21 % higher oxygen reduction reaction mass activity (at 0.9 V vs. reversible hydrogen electrode) on Pt/NTO compared with commercial Pt/C. The ex situ activity and durability of Pt/NTO translated to a fuel cell. The rise in electrode ohmic resistance and non‐electrode concentration overpotential indicate that improving the conductivity of NTO and optimizing the catalyst ink formulation are critical next steps in the development of Pt/NTO‐catalyzed proton exchange membrane fuel cells.