This study introduces a synergistic mass-spectrometry and sample-trapping gas chromatography method to monitor product gas composition during dynamic CO₂ electrolyzer operation. It reveals how electrode–electrolyte interfaces influence the establishment of reaction equilibria at gas diffusion electrodes. The method enables highly quantitative and time-resolved tracking of changes in electrode, or electrocatalyst selectivity for broad product gas spectra during dynamic operational changes.

High-performance gas diffusion electrodes (GDEs) are crucial for large-scale CO₂-electrolysis. During initial electrode polarization, the startup procedure plays a critical role in determining the subsequent performance of electrodes. This phase influences the course and rate of catalyst formation, as well as electrode-wetting, and must be carefully managed to prevent initial electrode damage. Maintaining ideal conditions despite added constraints and complexity during scale-up requires in-depth understanding of these operational phases. However, progress is limited by the current inability to rapidly analyze product gases. To address this, we present a modified gas chromatography (GC) method capable of sampling every 30 seconds within a limited timeframe, maintaining full data quality. This method was compared and combined with online mass spectrometry (MS), which is faster but comes with calibration challenges and spectral overlaps. We explore the strengths, limitations, and potential synergies of both techniques during electrolyzer startup and for current steps on GDEs with different catalyst layer designs. Experimental results reveal that concentration gradients and additional electrode-wetting under load primarily occur within the first two minutes. Both fast-GC and MS methods are effective for product gas analysis within this critical timeframe. We demonstrate the calibration of MS fragmentation signal corrections using simultaneous GC data.