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Fig. 5 Schematic representations of different possible hybridization approaches between supercapacitor and battery electrodes and materials. — Figure 5 Schematic representations of different possible hybridization approaches between supercapacitor and battery electrodes and materials.

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Fig. 1 Ragone plot with specific energy and power for different energy storage devices. Data from ref. 2.

Fig. 2 Schematic diagram of three types of components used in the design of hybrid materials for energy storage: carbon materials, conduct- ing organic polymers (COPs) and a variety of inorganic electroactive species three of which are just shown as representative examples of extended (oxides, phosphates) or molecular (polyoxometalate) species.

Fig. 3 Black background outlines and ideal geometry with fractal granularity (see text). Inset shows scanning electron micrographs of fractal granularity in LiFePO, materials (O. Ayyad, P. Gomez-Romero, unpublished results).

Fig. 4 Cyclic voltammograms (top) and charge—discharge cycles (bottom) for different types of electrode materials: (a, b), carbon-based double-laye supercapacitors; (c, d) CozO4 pseudocapacitor; (e, f) LiFePO, battery electrode (vs. Li). The series shows the transition from a typical capacitive behavio (EDLC) (a, b) to a typical (two phase) battery electrode (e, f) with a pseudocapacitive oxide electrode as an intermediate case (c, d). It should be noted tha even for the same CozO, material different degrees of capacitive and faradaic components can be appreciated depending on the specific microstructure of the active phase.?° The conception of double-layer supercapacitors led to orders of magnitude improvement in energy density (Fig. 1) by storing A major discovery made along this line by Gogotsi and co-workers involved the demonstration that unsolvated ions can contribute to capacity in purely double-layer electrodes. These researchers used carbide-derived carbons with pore sizes smaller than 1 nm to show that ions diffuse through these ultramicropores upon desolvation and contribute to increased capacitance values.’” It is very interesting to note that this behaviour is indeed reminiscent of ion diffusion in battery materials, where the injection of electrons in a bulk electro- active phase is balanced by the diffusion of naked ions in

Fig. 6 TEM image of the nanocomposite electrode formed by Li,TisO12 nanoparticles and CNF and specific capacity of the electrode vs. Li anode showing excellent high-rate performance (data from ref. 28).

Fig. 7 Schematic representations of a single electrode system (a), capacitor (b) battery and (c) an asymmetric capacitor system with an aqueous electrolyte according to the charge—potential profile with corresponding equations for energy storage.

Fig. 8 Cyclic voltammetry curves of aqueous asymmetric capacitors based on different metal oxides/hydroxides as the positive electrode and reduced graphene oxide as the negative electrode.

Fig. 9 Schematic representation of various chemical synthetic routes used for the synthesis of hybrid electrode materials The fabrication of hybrid nanostructured composite materials involves the hybridization of battery components (oxides, con- ducting polymers or intercalation compounds) and capacitor components (primarily carbons) in a single electrode. Fig. 9 shows a few representative examples of different synthetic routes typically used to prepare this type of hybrid electrode materials. In this way we can simultaneously take advantage (in parallel) of faradaic and capacitive energy storage mechan- isms while improving kinetics and electrochemical performance at high rates. If in addition we make a series combination of capacitive and hybrid electrodes the voltage and energy density of the system will also increase. This approach represents an advanced way to get high power and energy density supercapa- citors by fabricating electrode materials through a rational design of material combination, phase morphology and particle size. More specifically, enhanced electrochemical performance of supercapacitors can be achieved by introducing faradic com- pounds, either extended or molecular into hybrid composite electrode systems based on nanostructured carbons (activated carbons, carbon nanofibers, CNTs, graphene etc.) and wisely building asymmetric combinations of them. Finally, it should be borne in mind that even the more basic studies on this type o hybrid electroactive materials are crucial for a fundamenta understanding of ground aspects such as the interactions between carbons and inorganic materials, the nature of their interfaces or electron transfer processes, all of which in turn wil provide the basis for the improvements of future devices.

Fig. 10 Schematic representations of hybrid combination of a hybrid electrode (hybridized battery and capacitor components) and a capacitor electrode according to the charge—potential profile with corresponding equations for energy storage.°° FLIO PLY VEIN CIOCUVUUE TMALeLIAlS SULIMITIAIIZEU ANUVE SllUWeU excellent electrochemical properties due to the proper hybridiza- tion of battery components (oxides or intercalation compounds) and capacitor components (carbon-based) in single electrodes. Herein we will consider theoretical aspects of this new hybrid electrode to understand the operating mechanism. Furthermore, the proper structures with ideal conditions for the hybrid elec- trode to enhance the electrochemical performance of energy storage systems are guided for future milestones. In comparison with asymmetric capacitors, we assume the hybrid electrode system containing a typical capacitor-type electrode and a hybrid nanocomposite electrode as shown in Fig. 10. The latter is formed by a battery-type component and a capacitor-type component in a single electrode which will store energy through both, faradaic and capacitive mechanisms. In this hybrid electrode, the capacitor component is first charged by electrostatic forces in the initial state (qi) until the electrode potential reaches the redox reaction potential of the battery component (AV,). Then the battery component is charged through the redox reaction while main- taining the potential of the hybrid electrode, until the faradaic component reaches its full-charge state (Q,) as seen in Fig. 10. After the full charge of the battery component (Q,) the capacitor component is charged again (q,) until the maximum potential of the hybrid electrode is reached (AV,.). Hence the total charge stored in the hybrid electrode is due to both components (battery (Qp) as well capacitor component (9. = qi + q2)). Also dealing with the core-shell approach, another group’? has reported a rational design of homogeneous core-shell nanoflake arrays for supercapacitors. By taking advantage of the high surface area of the interconnected flake-network, NiCo,0,@NiCo20,

Figure 5 – uploaded by Vanesa Ruiz

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