Effect of Water on the Aging of Activated Carbon Based Electrochemical Double Layer Capacitors During Constant Voltage Load Tests

The effect of water contamination in 1 mol L -1 Et4NBF4 dissolved in acetonitrile or in propylene carbonate as electrolyte on the capacitance fading of activated carbon based electrochemical double layer capacitors has been investigated during constant voltage load tests at 2.75 V and 3.5 V cell voltage. For a cell voltage of 2.75 V, water contamination in the PC based electrolyte led to a fast initial cell capacitance decrease. This cell capacitance loss was predominantly due to a capacitance fading on the negative activated carbon electrode. For the AN based electrolyte a similar but less pronounced effect is observed at 2.75 V. For both electrolytes tested at 3.5 V, no influence of deliberately introduced water contamination on the capacitance loss was identified. The cause for capacitance fading at the higher cell voltage, therefore, appears to be independent on the water contamination. Post mortem X-ray photoelectron spectroscopy and surface area analysis of the negative electrode aged at 2.75 V did not provide evidence for a corresponding loss of surface area or electrode passivation. These results point toward water reduction on the negative electrode as the primary cause for capacitance loss of electrochemical double layer capacitors in water-contaminated electrolytes near nominal working voltage. The formation of trapped hydrogen within micropores of the activated carbon electrodes is proposed as the main aging mechanism in this case.