RESUMEN
Membranes play a crucial role in efficiency and longevity of flow batteries. Vanadium flow batteries suffer self-discharge and capacity fading due to crossover of electrolyte components through the membrane from one battery half-cell to the other. We consider the impact of vanadium species crossing ion exchange membranes on state of charge of the battery and we present a simple method to determine crossoverll open circuit potential measurements. State of s. State of charge for the negative and positive half-cell is simulated based on assumptions and simplifications for cation and anion exchange membranes and different crossover parameters. We introduce a crossover index "IndXovr" which enables the determination of crossover direction from state of charge data for the negative and positive half-cell and therewith identification of the half-cell in which predominant self-discharge occurs. Furthermore IndXovr allows statements on crossover amount in dependence on state of operation. Simulated case studies are compared to experimental state of charge values estimated from half-cell potential measurements. Our results reveal that half-cell potential monitoring respectively half-cell SOC estimation, is a simple and suitable tool for the identification of crossover direction and relative amount of crossover in VFB.
RESUMEN
The current production from the alkaline methanol electro-oxidation reaction does not reach a steady state on a smooth platinum catalyst under potentiostatic conditions. We investigated two possible explanations for this phenomenon: changes on the catalyst surface and changes in the solution near the electrode. In situ Fourier transform infrared spectroscopy experiments were conducted to evaluate the adsorbed species on the catalyst surface and a simulation model was set up to describe the changes of concentrations inside the solution. Linear- and bridge-bonded carbon monoxide are the only organic compounds which can be detected by in situ spectroscopy at fixed potentials, but their amount does not increase over time. The simulation shows that the consumption of hydroxide ions and production of carbonaceous species during alkaline oxidation causes a local pH shift near the catalyst surface. Assuming a one-electron transfer as the limiting step, this pH shift was found to contribute to the observed current loss at a potential of 0.77 V.
RESUMEN
The mechanism of the alkaline methanol electrooxidation reaction on platinum is complex and not fully understood. However, a better understanding will facilitate reaching the theoretical performance of an alkaline methanol fuel cell. Cyclic voltammetry is an often used method to investigate the mechanism of electrochemical reactions. The cyclic voltammogram of methanol electrooxidation shows a hysteresis in potential between the oxidation peaks in the forward and backward scans. The origin of this hysteresis has not been fully clarified. By means of parameter variations, such as the upper potential or the starting point of the CV measurements, and by physicochemical modeling, we investigate the formation of platinum oxides as a possible cause of this behaviour. The experiments show that the formation of platinum oxides is more likely to cause the hysteresis than the strongly adsorbed intermediates, which are formed during the forward scan. The simulation includes the formation of platinum oxide species and supports the experimental results that the hysteresis is due to the formation and reduction of platinum oxides. Besides, the simulation reveals that in the investigated potential area, different oxide forms are present.
