High Rate Capacity through Redox Electrolytes Confined in Macroporous Electrodes

Abstract

The capacitive characteristics of a model macroporous electrode, comprised of vertically aligned carbon nanotubes, with a relatively large volume /unit mass (of ∼40 cm 3 /g) are discussed. The faradaic characteristics of the capacity were evidenced through a voltage plateau in the galvanostatic discharge experiments. It is shown that the confinement of electrolyte within the spacing between the nanostructures with respect to an appropriate choice of the voltage scan rate yields a regime where both high capacity and rate capability may be realized. Additionally, the absence of diffusional processes allows for large capacitance retention even at discharge current densities of the order of 200 mA/cm 2 . For 1M K 3 Fe(CN) 6 , the maximum observed capacity was found to be ∼26 mAh/cm 3 (normalized to the pore volume) or 0.28 mAh/cm 2 (normalized to the projected area of the electrode) and it was noted that the capacity could be cycled at very high rates for 5000 cycles.Electrochemical capacitors (ECs) typically store charge either (a) electrostatically (in the double layer at the electrode/electrolyte interface, as an electrochemical double-layer capacitor: EDLC) or (b) pseudocapacitively 1 (at the electrode surface through redox reaction mediated mechanisms). The latter mode of charge storage in ECs relies on faradaic charge transfer, may occur over a relatively smaller voltage range compared to EDLCs, and varies as a function of the surface coverage area. Such pseudocapacitive characteristics have been observed in transition metal oxides (such as RuO 2 , MnO 2 , etc.) as well as conductive polymers (such as polyaniline) 2-4 and seem to be related to specific functional groups.At the very outset, a major benefit of capacitive methods of storing charge is the high power density that may be obtained due to the charge being predominantly on the surface. However, this very same attribute results in a lower energy density and consequently, there have been significant efforts to raise the energy density to the level of batteries through using interstitial spaces within the material. For instance, an intercalation pseudocapacitance 5,6 has been recently proposed as due to nanostructuring, in oxides such as TiO 2 and Nb 2 O 5 , and closely mimics battery-like behavior, while avoiding diffusional limitations. Such charge storage may also be realized through the use of high surface area nanostructured materials, such as activated carbon (AC) and carbon nanotubes (CNTs) 7-9 in conjunction with redox electrolytes. Optimized pore diameter or structure spacing mitigates diffusional constraints and could yield high charge/discharge rates without a detrimental effect on the cycling stability, which is an issue when using surface functional groups 10 for pseudocapacitance based storage. Concomitantly, a combination of redox electrolytes, 11,12 e.g., KI (for the positive electrode) and vanadyl sulfate (VOSO 4 , for the negative electrode), or through the use of an electrochemically active compound such as hydroquinone (HQ) added to a s...

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.…”

“…It should be noted that constructing 3D electrodes can optimize the rate performance of supercapacitors. A high‐rate supercapacitor was obtained when using vertically aligned carbon nanotubes as electrode and K 3 [Fe(CN) 6 ] as the mediator because the diffusion path of mediators was reduced by confining electrolyte in the space between nanostructures (Figure c and d) . Despite the many merits, K 3 [Fe(CN) 6 ] and K 4 [Fe(CN) 6 ] still suffer from thermal/light instability and toxicity.…”

“…, Royal Society of Chemistry; c, d—Ref. , Electrochemical Society; e, f—Ref. , Royal Society of Chemistry.…”

“…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.…”