 # Impedance spectroscopy: monitoring of electrochemical processes

If you want to measure the corrosion progress or the performance of a fuel cell, impedance spectroscopy is a suitable technique. This allows you, for example, to monitor processes within electrochemistry such as corrosion over time by taking measurements at regular intervals. Materials science meets electrical engineering.

Suppose you are an electrician and you want to know the magnitude of an electrical resistance in an electrical circuit, what can you do? You apply an electrical direct (DC) voltage over the component, and you measure the electrical current that passes through. Divide the driving force – the voltage – by the current, and the resulting value is the resistance. What you actually do is ‘trigger’ the component by applying an electrical voltage over it, and then measure what is the response of the component – here in the form of the electrical current that passes through. Using this way of measuring, you can say something about the ability of the component to resist the flow of electrons, in short the (electrical) resistance.

The same principle – triggering and measuring – is also used in the analysis technique called impedance spectroscopy. Here you consider the object that you want to comprehend – for example an iron plate with a protective paint layer on it that contains cracks – as a ‘black box’. The measurement principle is simple: you apply a small electrical alternating (AC) voltage over it, and measure the current through the object. From the impedance, which is the ratio of the AC voltage and the corresponding current, you can say something about the processes that are responsible for the electrical or electrochemical transport in and through the object. The impedance is a kind of resistance; where you speak of ‘resistance’ in the case of direct voltage, you speak of ‘impedance’ for alternating voltage. Frequency is the key word here. If you apply an alternating voltage of various frequencies over the material – hence the term ‘spectroscopy’ – then you can reveal these processes. The impedance is the equivalent of a resistance – but now it is frequency-dependent.

Electrochemical processes in or on materials extend over different time scales, and that is what impedance spectroscopy makes use of. By applying the small AC voltage over the component to be measured, you slightly disturb the processes that occur in the component. The system would like to return to its ‘normal’ state, because that is where it feels most comfortable. The return to this undisturbed state or relaxation takes some time, and this duration – or its reverse: the frequency – is characteristic of the occurring process. Take a solid oxide fuel cell (SOFC) as an example. The transport of gases such as oxygen or hydrogen in the porous electrodes has a relaxation frequency in the order of magnitude of 1 Hz. For the reactions that occur at the cathode and anode, the relaxation frequencies are in the order of magnitude of 10 resp. 1000 Hz. The electronic and ionic conductivity in the electrolyte or electrodes relax at frequencies exceeding 1 MHz. In short: different processes have their own, characteristic frequency. If you perform a measurement over the entire frequency range from (less than) 1 Hz to (more than) 1 MHz, you can encounter all of these characteristic relaxation frequencies – and thus the processes that occur in the component. This way, the ‘black box’ has revealed its secrets – without having to open or destroy this ‘box’.