Solid-state transformation of aqueous to organic electrolyte – Enhancing the operating voltage window of ‘in situ electrolyte’ supercapacitors

We introduce a holistic concept where by-product salts, which are formed during the synthesis of activated carbons, are not considered as waste products but rather upcycled to an organic electrolyte for EDLC applications. In detail, inorganic salts such as KHCO3, which accumulate inside carbon pores during chemical activation with K2CO3, are converted to the organic electrolyte KTfSI by simply treating the composite with HTfSI. This mechanochemical solid–state reaction runs in as little as one minute and the resulting composite is directly used as an electrode according to the so-called in situ electrolyte concept. Thereby, the waste production during the EDLC preparation is minimized greatly and the use of any additional electrolyte is made obsolete. EDLC electrodes are fabricated via the two most common procedures: slurry-coating on alumina foil and dry-processing with PTFE to form free-standing electrodes. The full cell devices show a good performance of 30 F g−1 at high scan rates of 10 A g−1 and a high capacitance retention of 74% after 16 000 cycles. By applying the concept the mass productivity can be increased by 15-fold.