Gas evolution in activated carbon/propylene carbonate based double-layer capacitors

For the first time gas evolution in a double-layer capacitor cell employing activated carbon electrodes and a solution of 1 M (C2H5)4NBF4 in propylene carbonate (PC) has been monitored by means of on-line mass spectrometry (DEMS). During slow scan voltammetry, CO2, propene, and H2 were detected as the major gaseous decomposition products. Probably propene and H2 are formed by solvent reduction at the negative electrode, while CO2 is formed by solvent oxidation at the positive electrode. A small amount of propene is already detected at a cell voltage of below 1 V. For all species the onset of significant gas evolution coincides roughly with the rise of faradaic currents. The increase of coulombic efficiency during subsequent cycling is attributed to passivation processes taking place at both electrodes.