Hydrogen may be used as fuel for transportation or as energy carrier in the future to replace fossil fuels. But its industrial production is not sustainable yet, since it´s based on CO2 emitting processes. Water electrolysis, i.e. splitting water to get hydrogen and oxygen gas, could be a valid and greener alternative to produce hydrogen gas. However, is commercialization is hampered by the scarce supply of renewable energy and the problematic oxygen evolution reaction (OER) at the anode. The latter is limiting the hydrogen-producing counterpart at the cathode (hydrogen evolution reaction, HER) due to sluggish kinetics involving four electrons instead of two.
Since oxygen is not as valuable as hydrogen, scientists are working on alternatives to the OER, such as the alcohol oxidation reaction (AOR) as shown in Figure 1. Among the alcohols, glycerol is exceptionally interesting because it is abundantly available as a byproduct of biodiesel production and it can be oxidized to many valuable compounds (the potential for circular economy is summarized in Figure 2.) Yet, glycerol oxidation can yield many different reaction products that cannot be detected by electrochemical investigations only. Therefore, during my PhD in the group of Prof. Dr. M. Muhler at RUB, I´ve developed a novel approach that couples infrared spectroscopy (IR) with typical electrochemistry to quickly derive information about the formed electrochemical alcohol oxidation products. [1, 2]
It takes two (techniques) to tango…
We chose IR spectroscopy, because it is valuable tool for the determination of various molecules such as the oxidation products of glycerol. Therefore, its coupling with electrochemical techniques seem to be straightforward. However, the obstacle of strong IR absorption of water has to be overcome by tailoring the interaction of the IR light to an optimum. This can be achieved by only investigating thin electrolyte films so that the light has to travel only short distances in the water. Another option is to apply the attenuated total reflection (ATR) technique. In this approach, a microscopic (evanescent) wave propagates into the water on a reflection plane, and allows to decouple optical from electrochemical parts as shown in Figure 3a.
Nevertheless, the required thin films lead to rapid consumption of the reactants i.e., alcohol and base. Thus, the composition in the thin layer constantly changes, rendering the comparison with typical bulk conditions challenging. To decrease the influence of the rapidly changing conditions and to achieve steady state conditions in the thin film, we designed a borehole electrode (BHE). It consists of a glassy carbon ring electrode embedded in an insulator material with a hole in the center of the ring. An electrocatalyst is immobilized on the conductive glassy carbon ring. Then, using a peristaltic pump, “fresh” electrolyte is pumped through this hole and continuously removes the “old one” from the thin layer as shown in Figure 3a. In order to ensure a uniform radial flow, we constructed a homemade, computer-controlled spectrometer lid as shown in Figure 3b which also eliminates a possible tilt of the electrode.
The residence time of the electrolyte in the thin layer can be adjusted by variation of the flow rate as well as the distance between the electrode and the internal reflection element (IRE). Under constant flow conditions, recorded IR spectra remain stable indicating that electrolyte pumped out of the thin-film has the same concentration composition of formed molecules over a long time. Thus, the cell operates under quasi steady-state conditions.
…but three are also fine
Therefore, the constant pumping allows for collecting the pumped electrolyte, which can be additionally analyzed by high performance liquid chromatography (HPLC). Similar to the separation of paint on a filter paper when some drops of solvent are added, the components formed during the electrochemical oxidation of glycerol are separated in a column and analyzed by a series of detectors. Possible oxidation products are shown in Figure 3c comprising formic acid and other carboxylic acids. Thereby, the IR spectra are validated and also minor reaction products can be determined. However, HPLC analysis is rather time consuming so it is only used when quantitative analysis is needed.
Our system can be used to investigate various electrochemical reactions using different electrocatalysts, which demonstrates its versatility. When increased precision and quantitative determination is necessary, subsequent HPLC analysis is easily applicable.
References
[1] S. Cychy, S. Lechler, Z. Huang, M. Braun, A. C. Brix, P. Blümler, C. Andronescu, F. Schmid, W. Schuhmann, M. Muhler, Optimizing the nickel boride layer thickness in a spectroelectrochemical ATR-FTIR thin-film flow cell applied in glycerol oxidation, Chinese Journal of Catalysis, 2021, accepted
[2] S. Cychy, D. Hiltrop, C. Andronescu, M. Muhler, W. Schuhmann, Operando Thin-Layer ATR-FTIR Spectroelectrochemical Radial Flow Cell with Tilt Correction and Borehole, Electrode Analytical Chemistry, 2019, 91, 14323
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