X-ray absorption spectroscopic study on valence state and local atomic structure of transition metal oxides doped in mgh₂

We present a study on electric double-layer capacitors in organic electrolytes based on symmetrical bulky tetraalkylammonium cations, using activated carbons with adjusted pore size distribution. The results reveal that a close fit of electrode pore size distribution to the size of electrolyte ions can lead to a saturation of theactive surface, observable as a decrease in capacitive current beyond the saturation point. This effect arises because of maximum cation loading being reached within the electrolyte voltage window following the trend: the longer the alkyl substituent, the lower the saturation voltage. In contrast, if the pore size distribution presents mostly pores significantly larger than the ion size, the saturation effect is not observed at the expense of the lower surface charge storage. The good correspondence between the maximum charge calculated on the basis of a distorted configuration of cations, Qmax, and the measured one, Qexp, suggests that cations use a wider range of pore sizes than would be possible at their unconfined and undistorted geometries. The saturation of the electrode porosity limits the usable voltage of a supercapacitor system and consequently the deliverable energy and power. In view of optimizing the electrochemical performance, the possibility of such an effect should be considered in adapting the sizes of electrode pores and electrolyte ions.