In situ 13CO2 generation from sodium bicarbonate: A quantitative strategy for isotopic validation in photoelectrocatalytic CO2 reduction

The reliable identification of carbon-based products remains a major challenge in studies of CO2 electrochemical and photoelectrochemical reduction, especially in the case of liquid products formed at low concentrations. Misinterpretation can arise from adventitious carbon contamination, electrolyte impurities and membrane degradation, often leading to false-positive product assignments. The present work reports a simple, quantitative methodology for determining the minimum amount of NaH13CO3 required to generate sufficient 13CO2 for isotopic labeling experiments under controlled reaction conditions. The approach combines Henry's law, the Sechenov salting-out relationship, and gas-phase equilibrium calculations to accurately estimate the total amount of dissolved and gaseous 13CO2 within a closed reactor, while avoiding unnecessary consumption of the isotopically enriched precursor. For a 250 mL reactor containing 0.1 mol L-1 Na2SO4 electrolyte, at 25 °C and atmospheric pressure, only 0.75 g of NaH13CO3 was sufficient to fully replace the dissolved and headspace CO2 with its 13C-labeled counterpart. The methodology was validated experimentally by photoelectrocatalytic CO2 reduction, using a TiO2 nanotube electrode decorated with Cu2O and Au, followed by GC-MS and 13C/1H NMR analyses. The isotopic origins of methanol and ethanol were unequivocally confirmed by their characteristic mass fragmentation patterns and by the observation of 13C–13C and 1H–13C scalar couplings, providing direct structural evidence that the incorporated carbon derived exclusively from the supplied 13CO2. In addition to lowering the cost of expensive isotopically labeled reagents, this strategy offers a framework for validating carbon product assignments, minimizing analytical artifacts, and strengthening the reliability, reproducibility, and mechanistic interpretation of CO2 reduction studies.