Capabilities and Applications of Spectroelectrochemistry

Electrochemistry is an enormously valuable tool for studying the kinetics of a broad range of oxidation-reduction reactions. In these kinds of reactions, electrons are spontaneously exchanged between atoms, which forms the basis of the physical principle behind chemical batteries.

Just as specific oxidation-reduction reactions can result in electron flow, i.e. current, the same process can also function in reverse. Therefore, by running current through a substance, a chemical reaction may be generated that would not usually occur.

The most frequently known example of this, electrolysis, is shown by using an electrolytic cell to divide water into diatomic hydrogen and oxygen. In this example a voltage source is connected to an anode and cathode which are positioned in a water bath, and as current flows through the water, electrons are transferred from the oxygen to the hydrogen which causes the separation of the molecules.

Typical Spectroelectrochemisty Set-Up

Bringing together electrolysis with UV/Vis absorption spectroscopy enables real-time kinetic measurements of reactions as a function of current and voltage. It is comparable to how absorption spectroscopy is combined with titration or plasma process monitoring, but with no requirement to introduce additional dangerous chemicals or the creation of hazardous plasmas.

In a usual spectroelectrochemistry measurement, a specialized cuvette, similar to the one displayed in Figure 1, is fitted with optically transparent electrodes. These types of cells, along with the required potentiostat controller, are provided by a number of companies such as Pine Research and Metrohm.

These electrodes consist of a gold or platinum mesh for the majority of spectroelectrochemical cells, but a few more developed cells are also available with carbon or indium tin oxide electrodes. These cells are linked with an adjustable voltage source, which can be programmed to sweep the voltage across the anode and cathode while measuring the current which is being generated in the electrochemical cell at the same time. Utilizing this process, known in electrochemistry as voltammetry, combined with a light source and a spectrometer, forms the basis of the study of spectroelectrochemistry.

Figure 1. A Spectroelectrochemical cell with integrated electrodes.

Figure 2 demonstrates a schematic block diagram of a commonly used instrument. It can be observed in this diagram that the voltage driver is not just connected to the spectroelectrochemical cell, but it is also vital for the voltage driver to be able to directly trigger the spectrometer at every instance that the voltage is altered. It is not possible to make sure that the acquired absorption spectra are correlated to the appropriate drive voltage without this capability.

Figure 3 goes on to illustrate a typical triggering diagram used in measuring linear sweep voltammetry. It is important to keep in mind that sweep rate will be constrained by both the triggering capabilities and the spectrometer’s data transfer rates. Low-jitter, high-velocity triggering is also vital to eliminate latency between each voltage step and the spectrometer acquisition. An ideal candidate for this application, Avantes AvaSpec-ULS2048CL-EVO comes with a 890 nanosecond external trigger delay minimum and jitter of only 14 nanoseconds.  

Figure 2. Schematic block diagram of a spectroelectrochemistry experiment showing the AvaSpec-2048 fiber-coupled spectrometer and the Ava-Light-Hal-S fiber-coupled light source from Avantes.

Figure 3. Example linear step voltage sweep utilized in voltammetry (left) and the trigger duration for the spectrometer acquisition for each step (right).  

Software such as Avantes’ AvaSoft spectrometer acquisition software, once the spectrometer and voltage driver is appropriately synchronized, can be used to produce a waterfall plot monitoring the variations in the solution’s absorption spectrum, for each step in the applied voltage. Figure 4 provides an example of one such waterfall plot where each dot on the current-voltage (IV) curve is in direct correlation with each spectrum in the waterfall plot. The spectrometer was configured to average two 2 ms integrations for this acquisition, and the trigger was set to begin a scan every time it was edge-triggered and then autosave the spectrum.

Figure 4. Screenshot of a kinetic spectroelectrochemistry experiment performed using Avantes’ Avasoft, spectrometer acquisition software.

Application Example

Electrochemistry and by default spectroelectrochemistry, has grown in significance in more recent times because of its value to battery technology. In fact, the entire field of electrochemistry was born when the battery was discovered by Alessandro Volta in 1793.

Volta achieved this when he realized that when he positioned silver and zinc on opposite sides of a cloth soaked in brine, a voltage was generated. It is now known that this was due to a spontaneous oxidation-reduction reaction. This reaction resulted from the zinc ions having a reduction potential of -0.76V and the silver ions having a reduction potential of +0.80V, leading to electrons spontaneously flowing from the zinc to the silver, therefore generating current.

Amazingly, things have hardly changed in regards to battery technology since Volta’s original innovation, and this has resulted in most current battery technology depending on the development of improved and more efficient variations of this effect. In more recent times, teams from the chemistry departments of Macalester College and Saint Catherine University, both in Saint Paul Minnesota, working together, were able to demonstrate how spectroelectrochemistry is used in battery research.

The teams published a paper in ACS Omega illustrating how they were capable of making a shift of +0.57V to the reduction potential of pyromellitic diimides, by altering its chemical structure. Since aromatic diimides, such as these, are known to have attractive energy storing properties desirable in the photovoltaic industry, an enhancement in their reduction potential could increase the voltage of batteries assembled from such molecules.    

For this experiment, the teams employed a Pine Research WaveNow potentiostat (current monitoring adjustable voltage source) with a platinum coil electrode, a Pine Research Spectroelectrochemical Cell Kit, and an AvaSpec-ULS2048-USB2-50.  Example spectra for both singly and doubly reduced reactions, measured with this setup are displayed in Figure 5 and Figure 6 showing the variations in both the absorption spectra and the reduction potential.

Figure 5. Absorption spectra of electrochemically generated singly and doubly reduced states of 1⁺ and 2²⁺ recorded in dimethylformamide, measured using the AvaSpec-ULS2048-USB2-50 from Avantes. ^[1]

Figure 6. Cyclic voltammograms of P, 1⁺, and 2²⁺ states dimethylformamide recorded using the WaveNow potentiostat by Pine Research. ^[1]

Conclusions

The above example is only one of a great number of applications where spectroelectrochemistry is being used in the design of new battery technology. While the cases discussed previously are all using modular fiber-coupled set-ups, it should be noted that the nature of miniature spectrometer technology makes them an exemplary choice for integration into OEM systems. Avantes’ Avaspec instruments are perfectly suited to high velocity triggered or continuous measurements such as those necessary in this application.

Each of the spectrometers discussed previously are also available as OEM modules and can be integrated into turnkey laboratory spectroelectrochemistry devices, as well as work as an addition to current laboratory equipment. These types of units can communicate via Ethernet, USB, and the native digital & analog input/output capabilities of the Avantes AS7010 electronics board which offers a superior interface with other devices.

In addition, the Avantes AvaSpec DLL software development package, with sample programs in Delphi, Visual Basic, C#, C++, LabView, MatLab, and other programming environments, allows the user to develop code for their own applications.

References and Further Reading

1. A. J. Greenlee, Et al., “Pyridinium-Functionalized Pyromellitic Diimides with Stabilized Radical Anion States,” ACS Omega 2018 3 (1), 240-245.

This information has been sourced, reviewed and adapted from materials provided by Avantes BV.

For more information on this source, please visit Avantes BV.