Impedance spectroscopy: monitoring of electrochemical processes

If you want to measure the corrosion progress or the performance of a fuel cell, then impedance spectroscopy is a suitable technique. This allows you, for example, to monitor electrochemical processes 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 resistor 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 put 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 resistor; where you speak of ‘resistor’ 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 resistor – 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’.

Electrochemical processes in a solid oxide fuel cell

The degradation over time of an iron plate with a painted non-conductive corrosion-protection coating on it can be easily monitored by means of impedance spectroscopy. To do this, you expose the object at the side of the coating to a saline solution (or electrolyte) and measure it regularly, for example once a month. This involves applying an alternating voltage over a wide frequency range over the three-layer system electrolyte / coating / metal substrate and measuring the current through the system. By dividing the voltage and current, the impedance at each frequency is calculated.

The object that you measure is again a ‘black box’, of which you do not know what is inside or which electrochemical process occurs at which moment. To unravel the mystery of this ‘black box’, (with some experience) you com- pose a model that consists of several electrical resistors, capacitors and other electrical elements, and that mimics the system. With smart software, you can compare this equivalent electrical circuit with the measured impedance values. The smaller the difference between the model system and the experimental measurements, the better the model represents the actual situation, and the better you understand the underlying processes in the material.

Back to the example. Initially, when the painted coating is intact, you measure only the electrical resistance of the electrolyte (Relectrolyte), and the capacity (Ccoating) of the capacitor formed by the sandwich of the conductive metal and the electrolyte with in between the electrically insulating painted coating. Some time later, the coating has been damaged by the saline solution, and now the coating contains cracks or pores which extend to the metal substrate. At this moment you also measure the electrical resistance (Rcrack) of the electrolyte that has flowed into the crack or pore and fills it. Again some time later, the electrolyte has reacted to a certain extent with the metal surface of the substrate so that you measure a new phenomenon. Here the coating has released locally from the metal, and you measure a capacity and a resistance in the resulting cavity at the metal surface. The capacity (Cdouble layer) of a double layer capacitor is observed at the interface between the metal and the electrolyte. Depending on the location on the metal where electrons are needed or released for the corrosion reaction, there is a shortage or surplus of electrons, so that the surface layer is locally electrically charged. This charge is compensated by a layer of oppositely charged particles in the electrolyte, hence the term ‘double layer’. The measured resistance (Rcharge transfer) indicates the corroding interface. This resistance is known as the charge transfer resistance, and is a measure of the rate at which corrosion occurs, with the kinetically controlled charge transfer as rate-limiting step.

Equivalent electrical circuit vs. real system

By following the values of the above resistors and capacities over time using impedance spectroscopy, you can say something about the quality of the corrosion-protection coating, and you know which of the processes are responsible for its deterioration and when they start to occur – so when you should do something about it. Impedance spectroscopy: a great way to link electrical models to physico-chemical processes.