Electroreduction of CO2 is performed in proton exchange membrane reactors (PEMRs) with a buffer layer to investigate the critical factors that determine the cell performance. The buffer layer has the function of ensuring sufficient cathode potential (above the potential threshold of Cu, Sn, and In catalysts at around -1.3 to -1.4 V) compared with the limited cathode potential in the conventional PEMR, therefore a high hydrogenation rate (i.e., 89.8 nmol cm(-2)s(-1) at -1.8 V) is achieved. The buffer layer exhibits good ability to suppress H-2 evolution, however, the current efficiency of HCOOH decreases by over 50% after the buffer solution is saturated with protons (after 10 h reaction). Improving CO, mass transfer at the reaction interface by adding tetrahydrofuran (THF) in a buffer layer or zeolitic imidazolate framework with a leaf-like morphology (ZIFL) in the catalyst layer, the current efficiency of HCOOH can be increased by around 10-15%.