Use of isotopically labelled compounds for the in situ IR study of the electroreduction of CO2 in aqueous hydrogencarbonate and buffered phosphate solutions

Abstract

Subtractively normalised FTIR spectroscopy studies on the electroreduction of CO₂ at a Cu electrode, using isotopically labelled CO₂ and HCO₃^– in H₂O and D₂O, indicate that as the applied potential is varied in the staircase mode from –0.6 to –1.5 V (vs. SCE), aqueous CO₂ is first transformed to hydrogencarbonate and then to carbonate rather than undergoing a reduction process. This result confirms that the ‘carbonate route’ introduces a major obstacle in the electrochemical reduction of CO₂ in aqueous systems. The use of isotopically labelled CO₂ and HCO₃^– confirms unequivocally, that the carbonate ions formed at the electrode originate from the dissolved CO₂ and not from the bulk electrolyte. Studies performed using other electrodes (Ni and Pt) show the same behaviour although in different potential ranges. Comparison with similar studies reported in recent literature suggests that the history of the electrode and pretreatment are very critical. An important isotope fractionation effect was observed which can be attributed to the different rates of hydration and ensuing reactions of ¹²CO₂ and ¹³CO₂.