The development of highly sophisticated portable electronic devices and the electro-mobility revolution, require new power sources with very high power density. Consequently, we see in recent years an accelerated development of super-capacitors of several kinds.  These include electric-double layer (EDL) capacitors, based on purely electrostatic interactions, and the so-called pseudo-capacitors, which electrodes include surface red-ox species adsorbed to their surface within the porous structure and enhance the electrodes’ specific capacitance. The voltage (E) that can be applied to these devices is very important, because the energy density is proportional to E². The potential applied depends on the electrolyte solutions used. Indeed, super-capacitors can be classified also according to the electrolyte solutions used and the potential applied. Using acidic or basic aqueous solutions enable to obtain fast electrostatic interactions at relatively high specific capacitance, but the potential applied is below 1V. Aqueous super-capacitors based on neutral solutions (e.g., electrolytes like Li₂SO₄) can operate at potentials >1.5V but their specific capacitance is lower compared to systems with acidic or basic solutions. The use of non-aqueous solutions enables to operate high voltage super/pseudo-capacitors, that may reach energy density of the same order of magnitude as that of aqueous rechargeable batteries (e.g. lead acid battery systems). In this presentation we present results related to super and pseudo capacitors that reflect novel approaches. We demonstrate how the use of red-ox species in solution phase which adsorb to porous carbonaceous electrodes can enhance the specific capacity of both aqueous and non-aqueous super-capacitors¹. We will discuss what are the real limiting reactions of capacitive electrodes in non-aqueous solutions, including ionic liquids. In fact, possible intercalation of cations and anions at extreme potentials, into graphitic nano-crystallites that always exist in activated carbons is a major limiting factor in these systems, because such intercalation processes damage the structure of the activated carbon electrodes and lead to their degradation. In an  another example, we demonstrate modification of high surface area activated carbon electrodes with 3D carbon nano-dots (C-dots). We explore these composite electrodes in supercapacitor ystems.  For the first time, we propose herein the modification of activated carbon (AC) by a uniform deposition of highly crystalline carbon dots (C-dots) and used this novel material as electrode for supercapacitors. C-dots were synthesized by sonication of polyethylene glycol followed by sonochemical modification of  AC matrices with the as prepared C-dots. We studied the sonochemical effect of the C-dots deposition into the  AC and their incorporation into the pores. The porosity of the AC/C-dots and the reference AC materials and the impact of C-dots loading on the performance of electrodes comprising these AC/C-dots composites were explored. It was found that AC/C-dots electrodes demonstrate specific capacitance which is almost 3 times higher compared to that of unmodified AC electrodes. It was established that the new electrode material "activated carbon/carbon dots" exhibits very stable behavior. In all our durability experiments we test the electrodes along many thousands of cycles. We demonstrate herein highly stable , high capacity composite electrodes for super-capacitors with a Coulombic efficiency around 100%.

References

1. Borenstein, A.; Hershkovitz, S.; Oz, A.; Tsur, Y.; Luski, S.; Aurbach, D., The use of 1, 10-phenanthroline as an additive for high-performance super capacitors, J. Phys. Chem. C, 119, 12165-12173 (2015).