Spectroelectrochemistry (SEC) is currently among one of the most promising emerging analytical techniques. Although its potential was never in doubt, the various equipment needed to perform measurements, the use of up to three computers for data processing, and the complexity of the cell setups discouraged many researchers from utilizing SEC for their research despite its advantages. The introduction of the state-of-the-art SPELEC line of instruments—fully integrated, perfectly synchronized, and controlled by a single software—has filled this gap, making SEC even more accessible. However, one key requirement still needed for SEC to be suitable for all laboratories is the availability of user-friendly cells for different configurations: transmission, reflection, and flow conditions. This article describes these different kinds of SEC cells in detail.
Addressing the need to overcome limitations
Instrumental limitations are still found, e.g., in the development of SEC cells. Some spectroelectrochemical devices have drawbacks such as: strict design specifications (shape, size, and electrode material) where more conventional options cannot be used, the devices require larger volumes of samples solution, the cells are made of many pieces requiring complex and tedious assembly/disassembly protocols, etc.
In order to facilitate the adoption of this technique, new and innovative SEC cells with updated setups have been developed. These devices offer several advantages:
- easy handling
- versatility for working with different electrodes
- chemical resistance to different media
- simple and fast assembly and disassembly
- low ohmic drop resistance
- and more!
Furthermore, opaque and closed cells eliminate environmental interferences. This also functions as a safety feature when a laser is used as a light source, as the beam is prevented from leaving the confines of the cell.
Raman SEC: a fingerprint technique with the right cell setup
Raman spectroelectrochemistry is a hyphenated technique that studies the inelastic scattering (or Raman scattering) of monochromatic light related to chemical compounds involved in an electrochemical process. This technique provides information about the vibrational energy transitions of molecules by using a monochromatic light source (usually a laser) that must be focused on the electrode surface at the same time as the scattered photons are collected (Figure 1).
When the scattering is elastic, the phenomenon is denoted as Rayleigh scattering, and when it is inelastic, it is called Raman scattering. This concept is illustrated in Figure 2.
Raman spectroelectrochemistry is quickly becoming one of the most promising analysis techniques due to its inherent fingerprint properties which allow the identification and differentiation of chemical species present in the system under study. As such, optimization of the system setup conditions is an important factor to obtaining the desired results. For example, adjusting the distance between the probe and the sample (according to the optical properties of the probe) is required to obtain the highest Raman intensity.
The following Raman cells from Metrohm have an improved and simplified design that enhances usability and facilitates measurement optimization (jump directly to each cell type by clicking below):
A novel black cell with an easy open-close magnet system is employed to carry out spectroelectrochemical experiments in aqueous and organic solvents (Figure 3). This cell consists of two PEEK (polyether ether ketone) pieces. The top piece contains a central hole for introducing the tip of the Raman probe, and four recesses with different depths (1, 1.5, 2, and 2.5 mm) to optimize the focal distance between the probe and the working electrode (WE). Furthermore, it has four holes for the CE (counter electrode), RE (reference electrode), and inlet and outlet air flux, but these can also be capped closed.
The upper part of the bottom piece has a compartment for adding 3 mL of solution. This volume ensures the proper contact of WE, RE, and CE with the solution while also preventing the immersion of the Raman probe. The underside of the bottom piece contains a small recess for placing an O-ring which prevents leakages. In addition, the WE is fixed by threading in the clamping piece. Finally, a holder is used in order to maintain the stability of the cell and enhance the performance of the measurements. Figure 4 gives an overview of the various parts of this Raman spectroelectrochemical cell.
Designed in black PEEK, this cell only consists of two parts. The bottom piece is used to place the SPE, while the top piece has a hole designated to introduce the Raman probe (Figure 5). Focal distance of the probe is easily modified using spacers of varying thickness (0.5, 1, and 1.5 mm).
The easy assembly of the cell combined with the small volume required (60 µL) makes this configuration ideal for inexperienced users. Furthermore, this cell has a small crucible holder to facilitate precise optical characterization of solid and liquid samples without requiring electrochemistry (Figure 6).
Flow spectroelectrochemistry can be easily carried out thanks to the development of thin-layer flow-cell screen-printed electrodes with a circular working electrode (TLFCL-CIR SPEs). The design of these SPEs allows one channel (height 400 µm, volume 100 µL) to transport the solution through the WE, CE, and RE (Figure 7).
Assembly of the Raman cell consists of two easy steps. First, place the SPE in the defined position of the bottom piece. Then, simply put the top piece on and the cell is ready for use. The top part of the cell has a hole specifically designed to introduce the Raman probe and focus the laser on the WE surface.
This system overcomes any leakage of the sample solution since liquids are only located in the channel of the electrode.
Reflect or transmit light with UV-Vis and NIR spectroelectrochemical cells
When studying a chemical process, the simultaneous recording of the evolution of the UV-Vis (200–800 nm) and near-infrared (800–2500 nm) spectra along with the electrochemical reaction allows researchers to obtain information related to the electronic (UV-Vis) and vibrational (NIR) levels of the molecules involved. Development of new spectroelectrochemical cells for this purpose has allowed the expansion of this hyphenated technique in several industrial sectors.
When working with a reflection cell setup, the light beam travels in a perpendicular direction to the working electrode surface on which the reflection occurs (Figure 8, left). The reflected light is collected to be analyzed in the spectrometer (Figure 8, right). However, it is also possible to work with other incidence and collection angles. This configuration is useful for non-transparent electrodes.
Manufactured from black PEEK, this reflection cell allows SEC experiments to be performed with either aqueous or organic solvents (Figure 9). The top piece is designed for ideal placement of the reference and counter electrodes as well as the optical fiber. The clamping piece optimizes the distance between the fiber and the working electrode. Additionally, inlet and outlet channels are also present in the top part of the cell.
The bottom piece has a specific compartment for adding 3 mL of solution which is where the working electrode is placed. The open-close magnet system eliminates any need for screws and makes cell assembly easy.
Performing spectroelectrochemistry with SPEs requires a simple experimental setup, enabling this analytical technique to be used for routine analysis. This cell consists of two pieces – the bottom part with a small recess to place the SPE, and the top one to hold the optical fiber while maintaining optimal focal distance (Figure 10).
This cell is advantageous for various projects because a large amount of information is provided from a small sample volume (less than 100 µL). The cell has an innovative open-close magnet system (no screws required) for easy replacement of the sensors, facilitating the performance of the UV-Vis and NIR spectroelectrochemical experiments.
This cell is a suitable support for spectroelectrochemical measurements in flow conditions with TLFCL-CIR SPEs. Its simple design features a hole to place the reflection probe in the proper position for the analysis of the electrochemical reaction (Figure 11).
TLFCL SPEs are suitable for spectroelectrochemical measurements – due to the transparent cover that defines one channel (height 400 µm, volume 100 µL), a thin layer is formed over the electrochemical cell.
Transmission experiments require that the light beam passes through an optically transparent electrode (Figure 12). This gathers information about the phenomena that take place both on the surface of the electrode and in the solution adjacent to it. Electrodes in this configuration must be composed of materials that have great electrical conductivity and adequate optical transparency in the spectral region of interest.
Optically transparent electrodes (OTEs) allow users to simultaneously perform spectral and electrochemical measurements directly through the working electrode. Spectroelectrochemical techniques can be used to easily obtain spectra through transparent conductive layers at the same time as an electrochemical experiment is carried out.
The transmission cell for SPEs comes in two parts, with the bottom piece containing a lens (Figure 13). This lens collimates the light arriving from the light source thanks to a transmission fiber. The OTE is placed on the bottom piece, allowing the light to pass through. Transmitted light is collected with a reflection fiber that is positioned within the top piece of the cell, obtaining information of the processes that take place on the electrode surface. The small volume required (100 µL) and the easy to assemble cell facilitate the performance of UV-Vis and NIR spectroelectrochemical experiments in transmission configuration.
Transmission spectroelectrochemistry can be easily performed using a traditional quartz cuvette with a 1 mm optical pathlength, as shown in Figure 14. The cell also includes a platinum mesh WE, platinum wire CE, and Ag/AgCl RE. In addition, the robust and easily manageable cuvette holder allows highly accurate, repeatable absorbance and fluorescence (90°) measurements.
The development of the presented novel cells makes spectroelectrochemical measurements even easier to perform. Their closed configuration as well as fabrication from an opaque, inert material avoids interferences and overcomes safety issues. No complex protocols are required for the assembly, disassembly, or cleaning of the cells. Finally, their simplicity and easy handling facilitates their use, which in combination with the SPELEC integrated solutions, makes spectroelectrochemistry more accessible to a wider audience.