High-performance hybrid supercapacitor of mixed-valence manganese oxide/N-doped graphene aerogel nanoflower using an ionic liquid with a redox additive as the electrolyte: In situ electrochemical X-ray absorption spectroscopy

Redox‐Mediator‐Enhanced Electrochemical Capacitors: Recent Advances and Future Perspectives

“…Many efforts have been devoted to the redox‐mediator‐enhanced supercapacitors. As a result, a variety of redox mediators and electrolyte systems have been developed …”

“…Therefore, the CV curve of the positive electrode now shows a pair of redox peaks and the negative one a rectangular shape (Figure g) and the GCD curve of the positive electrode exhibits the sloping potential plateaus whereas the potential of the negative electrode varies linearly with time (Figure h). After the first successful attempt of enhancing the capacitance by introducing Ag + as mediator, a large number of redox mediators were developed for supercapacitors . Based on the used redox mediators, the single‐redox‐mediator‐enhanced supercapacitors can be divided into three categories: metal‐mediator‐based supercapacitors, inorganic‐mediator‐based supercapacitors, and organic‐mediator‐based supercapacitors.…”

“…The cycling stability of MnO 2 measured in KOH containing [Fe(CN) 6 ] 3− /[Fe(CN) 6 ] 4− was much higher than that without the mediator, which was also attributed to the interaction between active materials and redox‐active mediators . In situ electrochemical X‐ray absorption spectroscopy confirmed charge transfer between MnO x and the [Fe(CN) 6 ] 3− /[Fe(CN) 6 ] 4− mediator; as a result, a wider variation in Mn oxidation states was observed in the active electrolyte . Therefore, hexacyanoferrate can serve as an electron shuttle accelerating the electron transfer of Co–Al layered double hydroxide, Co(OH) 2 , and MnO 2 .…”

“…The metal mediators here not only refer to the redox reactions of metal ions, such as Cu 2 + /Cu and Fe 3 + /Fe 2 + ,b ut also include metal complexes (e.g.,K 3 [Fe(CN) 6 ]), metal oxide clusters( e.g., silicotungstic acid), and organic transition-metal compounds (e.g.,f errocene). [33][34][35][36][37][38][39][40][41][42][43][45][46][47][48][49] The first proposed redox-mediatorenhanced supercapacitors consisted of AgNO 3 in H 2 SO 4 to im- prove the capacitance of activatedc arbon fiber cloth. [33] The capacitance wasi ncreased threefold by the pseudocapacitance attributable to the Ag + /Ag redox reaction.…”

“…[38] In situ electrochemical X-ray absorption spectroscopy confirmed charge transfer between MnO x and the [Fe(CN) 6 ] 3À /[Fe(CN) 6 ] 4À mediator; as ar esult, aw ider variation in Mn oxidation states was observed in the active electrolyte. [40] Therefore, hexacyanoferrate can serve as an electron shuttle accelerating the electron transfer of Co-All ayered double hydroxide, Co(OH) 2 Operationalm echanism for single-redox-mediator-enhanced supercapacitors during charge-discharge. a) Chargea nd b) discharge storage mechanism of supercapacitors with redox mediators at the negative electrodeonly;corresponding c) CV andd)GCD curveso fp ositive(red) andnegative (blue) electrodes in at hree-electrode system.…”