Electrochemical study of tetraalkylammonium tetrafluoroborate electrolytes in combination with microporous and mesoporous carbon monoliths

Three electrolytes with the same anion BF4- but different cation, Et4N+, Pr4N+ and Bu4N+, are studied in combination with two binder-free carbon monoliths, one microporous and another mesoporous. The stability and working voltage window are measured. The former window broadens according to the trend Bu4NBF4 > Pr4NBF4 > Et4NBF4 for the two monoliths. The latter window coincides with the stability voltage window for the mesoporous monolith but the trend is reversed to Et4NBF4> Pr4NBF4> Bu4NBF4 for the microporous monolith. The specific capacitances associated with the cations and anions are separately measured. The specific capacitances follow the trend BF4- > Et4N+> Pr4N+ > Bu4N+ and are higher for the microporous monolith. From comparison of the ratios of the specific capacitances with those of the surface areas due to pores with sizes larger than certain values, the minimum size of the pores able to host the electro-adsorbed ions at the double layer is deduced; it results to be 0.56-0.58 nm for BF4-, 0.63-0.65 nm for Et4N+, 0.75-0.76 nm for Pr4N+ and 0.9-1 nm for Bu4N+. The surface areas that are accessible and inaccessible to the four ions are estimated for the two monoliths. The better electrolyte/monolith combinations are discussed from a practical point of view. (c) 2018 Elsevier Ltd. All rights reserved.