Composite Graphite–Epoxy Electrodes for In Situ Electrochemistry Coupling with High Resolution NMR

Abstract

The in situ coupling between electrochemistry and spectrometric techniques can help in the identification and quantification of the compounds produced and consumed during electrochemical reactions. The combination of electrochemistry with nuclear magnetic resonance is quite attractive in this respect, but it has some challenges to be addressed, namely, the reduction in the quality of the NMR signal when the metallic electrodes are placed close to or in the detection region. Since NMR is not a passive technique, the convective effect of the magnetic force (magnetoelectrolysis), which acts by mixing the solution and increasing the mass transport, has to be considered. In seeking to solve the aforementioned problems, we developed a system of miniaturized electrodes inside a 5 mm NMR tube (outer diameter); the working and counter electrodes were prepared with a mixture of graphite powder and epoxy resin. To investigate the performance of the electrodes, the benzoquinone reduction to hydroquinone and the isopropanol oxidation to acetone were monitored. To monitor the alcohol oxidation reaction, the composite graphite–epoxy electrode (CGEE) surface was modified through platinization. The electrode was efficient for in situ monitoring of the aforementioned reactions, when positioned 1 mm above the detection region of the NMR spectrometer. The magnetoelectrolysis effect acts by stirring the solution and increases the reaction rate of the reduction of benzoquinone, because this reaction is limited by mass transport, while no effect on the reaction rate is observed for the isopropanol oxidation reaction.

Miniaturized Carbon Fiber Paper Electrodes for In Situ High Resolution NMR Analyses

“…However, when the electrodes were positioned 1 cm within the region, the obtained FWHM was 1.43 Hz. Comparing these values with those obtained with metallic electrodes published in other works, , it is observed that the distortion caused by our carbon fiber paper electrodes is significantly lower. To obtain a quantitative spectrum, it is necessary that the standard peak has a FWHM of approximately 1 Hz.…”

“…The position of the electrodes relative to the detection region was evaluated, and Figure shows the comparison between the spectra in the main regions of interest of the NMR spectrum of the AA when the electrodes were positioned 1 mm above the detection region (black line) and 1 cm within the detection region (red line). For more details on the placement of the electrodes see the work of da Silva et al published in 2022 …”

“…For more details on the placement of the electrodes see the work of da Silva et al published in 2022. 12 Figure 4a shows the peak corresponding to the TSPd4 standard and it can be clearly seen that the insertion of the electrodes in the detection region does not significantly compromise the quality of the signal. Only a slight broadening of the peak is observed and it is not shifted when the working electrode was placed inside the detection region.…”

“…It is known that metallic electrodes, when introduced in the NMR detection region, cause significant distortions to the local magnetic field, impairing the spectra quality or even making it impossible to measure any signals. , Thus, the search for electrodes that minimize signal distortion, making it possible to introduce them in the detection region, is very important to enable the coupling of electrochemical techniques with NMR. In addition, it is important that the working electrode surface can be modified with different catalysts in order to monitor different analytes, such as alcohol and glycerol electrooxidation reactions, which are very important in studies about energy generation and often require in situ monitoring of their products …”

“…In this work, the electrodes were positioned 1 mm above the detection region to avoid any type of distortions. 12 However, as long as the reagents and products show peaks outside the water peak region, the working electrode (as well as the counter electrode) can even be positioned within the NMR detection region without major spectrum distortions.…”