Effect of binder on the performance of carbon/carbon symmetric capacitors in salt aqueous electrolyte

The performance of AC/AC (AC = activated carbon) symmetric capacitors based on two commonly used binders–poly(vinylidene difluoride) (PVDF) and poly(tetrafluoroethylene) (PTFE)–introduced in the electrodes at same amount (10 wt%) has been compared in 1 mol·L−1 NaNO3 aqueous electrolyte. For capacitors charged up to 1.6 V, the PTFE and PVDF-based carbon electrodes exhibit capacitance values of 116 F·g−1 and 104 F·g−1, respectively. This difference in capacitance is related to the presence of the binder which, in the case of the PVDF-based electrodes, causes a more pronounced reduction of pore volume as compared to the pristine powder. The positive electrode of the AC-PTFE/AC-PTFE cell operates under the thermodynamic potential limit of di-oxygen evolution (in 1 mol·L−1 NaNO3, Eox = 0.834 V vs. SHE) up to 1.4 V voltage, while this limit is reached at a voltage of 1.2 V with the AC-PVDF/AC-PVDF cell. This shows that the porosity of the electrode strongly impacts the potential range of the positive electrode. Accelerated ageing performed by potentiostatic floating at 1.6 V for 120 h shows higher capacitance decrease and internal resistance increase in the case of the AC-PVDF/AC-PVDF cell due to a stronger oxidation of the positive electrode caused by the higher potential reached by this electrode. Hence, these results suggest that the performance of supercapacitors might differ depending on the selected binder, and discussions relating the capacitance performance to the textural parameters of the pristine carbon may be biased by a change of porosity. Therefore, it is highly recommended to take into account the gas adsorption data of the electrodes instead of the original powder.