Electrochemistry - Measuring Methods Flashcards
Explain more about Stationary current/voltage measurement.
Can carry out both half cell and single cell measurement. It is used to measure the performance of individual electrons and the kinetics and mechanism of electrode reactions. With stationary current/voltage measurements, the electrode potential is increased until a visible current is produced. As the voltage increases, the current density also increases (where for example in DMFC, the methanol oxidation takes place) to a limiting current, at which point one reaction step is no longer rapid enough to determine the reaction rate of the entire system. This limits the achievable current density and these measurements provide detailed information about the quality of an electrode. It is essential here that the measurement is only carried out in the stationary state, as otherwise considerable measurement errors occur.
Explain Cyclic voltammetry (CV)
In cyclic voltammetry, only half-cells (electrodes) are measured. Statements about absorbed intermediates, kinetics and reaction mechanisms are obtained. Cyclic voltammetry is a method by which the electrode potential is cycled through. The current density varies greatly, depending on whether reactions take place or not. The voltage situation at which reactions take place is known in electrochemistry. In this way, if the chemical system is known, the course of the reaction can be made mapped. In particular, the exact potential position is evaluated, as well as the height of the peak, which allows statements about the intensity of the reaction. Moreover, hysteresis effects with repeated passage through the potential loop are evaluated.
Explain impedance spectroscopy.
Impedance spectroscopy picks up the impedance – hence the resistance in the complex plane of numbers – of a half-cell. This is scanned over a wide frequency range (few Hz to MHz) in the complex number plane. This allows the ohmic resistance to be measured and the polarization effects at the electrodes to be clarified (information about kinetics and mechanism). The principle is that “heavy” carriers (e.g., ions) at lower frequencies can follow the electric field as well as light carriers (electrons). With increasing frequency, charge carriers are increasingly falling out and can no longer follow the field by which the electrical signal changes. This signal response is used to analyze the processes at the electrodes. The measured impedance is divided into real and imaginary part, then plotted in a Nyquist diagram, which in simple cases can be used to derive equivalent circuit diagrams, that are used to infer material or component functions. This is because different switching elements provide their max. Part of impedance at different frequencies.
Explain shutdown Measurements.
Here, a constant current is impressed in half- or single cells, which is then switched off by means of an extremely fast electronic system. The voltage drop over time is thereby measured. An immediate drop can be interpreted as a reflection of ohmic resistance (information it gives). If the voltage continues to drop over time, it can be attributed to capacitive effects due to slower carrier motion. Can measure half cell and single cell.
Explain galvanostatic long-term measurement.
Galvanostatic measurements are used to determine the aging of electrodes or cells. These long-term measurements should not be less than 100 hours, and ideally not less than 1,000 hours. The lifetime requirements for fuel cells are between 1,000 and 40,000 hours, depending on the type. Can measure half cell and single cell.
