Fast and complete electrochemical conversion of solutes contained in micro-volume water droplets

The complexity involved in electrochemical experiments setups has triggered more concern from the chemists and scientists in this particular field. Take for example the high-cost involved in performing electrosynthesis using specific small volume cells. As a way of addressing this challenge, more research has been conducted to explore the possibilities of developing inexpensive reactions vessels for general-purpose electrochemical experimental analysis and results or concepts validation. The use of a hanging droplet is considered as one of the best options in performing analytical electrochemistry. The choice of the hanging droplet was as a result of its ability to remain suspended instead of being deposited on the electrodes or the substrates used in the experiment.

Louis Godeffroy, Professor Olivier Buriez and Professor Eric Labbe from PSL Research University in France in collaboration with Dr. Francois Chau at University Paris Diderot in France proposed a simple, inexpensive and complete quantitative electrolysis approach for achieving electrochemical conversions of solutes. The experiment involved solution of organic and inorganic species in water droplets. In consideration was also the effects of alkaline droplets in electrochemical reduction and oxidation of permanganate and hydroquinone respectively. Their work is currently published in the journal, Electrochemistry Communications.

The research team conducted their experiments using Autolab potentiostat for the electrolysis analysis and recorded the obtained UV-Vis spectra on Perkin Elmer Lambda 45 Spectrophotometer. The experiment setups were either 2-electrode or 3-electrodes for the cases of hanging droplets and meniscus respectively. Distilled water was used in the recrystallization of hydroquinone (HQ) and also as a solvent in the electrolyte.

The authors observed a positive result in the use of organic and inorganic substrates in carrying out quantitative electrolysis. Indeed, the experiment proved to be easily accessible and cost effective by providing an enabling and conducive environment for experiments. For example, in the experiment, only 60 seconds was required to achieve a complete electrolysis conversion of hydroquinone (HQ) to benzoquinone (BQ) in a 0.1 mole/liter concentration of the aqueous droplet potassium chloride solution.

The choice of hydroquinone in the evaluation of the efficiency of the electrolysis process was indeed a good one. The process required the use of desired and suitable experimental conditions including the concentration of the solution, and the oxidation potential (set at 1.7 V/SCE). Therefore, water oxidation was never observed  during the entire experiment for the formation of benzoquinone (BQ). The conditions also prevent any secondary electrode reaction that may occur during then experiment hence leading to a complete electrolysis process. This was further confirmed by the records of the UV- Vis spectra that were taken for the BQ sample (before electrolysis) and after the formation of HQ (after the electrolysis). On the other hand, the study qualified the possibility of performing electrochemical transformations in water droplets. The high oxidation states of manganese especially in alkaline environments favor such transformations.

According to the authors, controlled-potential 3-electrode and galvanic 2-electrode electrochemical conversion methods can be carried out using simple, inexpensive and small setups. Due to its effective nature, it can be used for small electrochemical experiments hence will promote learning in institutions and research for those who want to perform such experiments.