Hello everyone.
We have recently started a research on the electrochemical fixation of carbon dioxide on organic substrates (i.e. electro-carboxylation) and we are currently investigating the synthetic outcome of the reactions. These are carried out in a gas-tight single-compartment three-electrode cell (the solution is saturated with CO2 and the system is kept under CO2 pressure), with a glassy carbon rod as working electrode (cathode), a Ag/AgCl reference electrode (separated via double salt bridge from the solution), and an aluminium counter electrode (sacrificial anode), the system being controlled in potentiostatic mode.
What we are observing is a stable current at the set potential, that remains constant for a few minutes at best. Then, well before the exhaustion of the organic substrate, the current decays sharply and the applied potential starts drifting randomly towards positive values, thus practically ending the electrosynthesis. These observations apply to more than one entry that we tested.
My hypothesis is that the counter electrode becomes passivated during the initial phases of the experiment. Indeed, before starting the synthesis, we activate the surface of the aluminium wire by plunging it in 6 M HCl to dissolve the oxide layer. When vigorous hydrogen evolution is achieved, the wire is quickly rinsed with DI water and with the reaction solvent (CH3CN), and then introduced in the electrochemical cell. This activation process allows for the stable current flow at the beginning of the experiments, which can also be restored by stopping the electrolysis and re-activating the wire through the same process.
Since early and recent literature works report the thorough use of Al sacrificial anodes in such reactions, I suspect my difficulties arise from errors either in the activation of the anode and/or in the experimental setup.
I apologize for the long description, and would kindly ask for any suggestion or ideas coming from your expertise. Thanks in advance
Ruggero
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