Hi, I have recently learnt to measure the gaseous products of CO2 reduction on a copper foil electrode. The method I use is an in situ gas chromatography, which has been widely reported in the literature, such as Jaramillo's EES published in 2012 (Energy Environ. Sci., 2012, 5, 7050–7059.). However, the Faradaic efficiencies of H2 were very large, often over 100% in my measurements, depending on the polarization potential. Meanwhile, the carbonaeous products of CO2 reduction, like CO, CH4, and C2H4, were comparable with the literature results (Table 1). I tested the control experiment on a Pt electrode in the 0.5 M H2SO4 under Ar atmosphere. the Faradaic efficiencies were between 77% to 94% at different potentials (Table 2), suggesting the method we use should be right. So I'm very confused with these results, can someone give me some suggestions about this situation? Thanks!