2.4. Tailoring nanomaterial properties towards high supercapacitance
2.4.2. Porosity of carbon nanostructured materials for electrodes
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In such a set-up, the overall resistance comes from individual cells. A better contact in typical devices plays a critical role in nullifying high resistance problems. Compaction pressure used in electrode preparation reduces resistivity through contact improvements and reduction of path length. Thin films also reduce equivalent series resistance in this regard. However, excessive adherence may reduce the electrolyte concentration below desirable thresholds in the pores.
This means material packaging on electrodes will affect the overall charge stored no matter how good the nature of the carbon nanomaterial. The charge (Q) stored by each capacitor is given by equation (11) [2.77]:
π = πΆπ (11)
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nanostructured materials containing pores that cannot be reached by the electrolyte ions [2.29], the selection of a suitable carbon nanostructured materials depends mainly on either the required performance or electrolyte choice [2.28]. This is mainly because intercalation of electrolyte ions tends to be amongst the main factors that drive capacitive behaviour.
Electrolyte ion penetration into carbon nanomaterial pores depends on structural parameters, surface properties and pore size distribution [2.37]. Uniform pore size of electrode materials has been associated with clear EC behaviour [2.12]. Additionally, porous carbon nanostructured materials for use in EDLCs are the most studied due to their availability and low price, amongst other reasons [2.150]. Porous nano-based structural designs facilitate rapid ion/electron transfer and offer an enhanced active surface [2.86]. Electrolyte ion penetration into carbon nanomaterial pores, especially for organic electrolytes, is subject to ion sieving effects and size, which eventually has an effect on capacitance [2.37,2.58]. Pores sizes < 0.5 nm were reported to be unavailable to hydrated ions [2.56]. A nanoporous carbon with a large portion of pore volume and pore sizes ranging between 1-5 nm is more appropriate [2.29].
Capacitance can be optimised by use of pores which are twice the size of solvated ions [2.59].
Additionally, charge can also be stored in pores with sizes smaller than those of solvated ions and this is facilitated with the distortion of the ion solvation shell [2.58,2.152,2.153]. Solvation shell distortion depends on both the electrolyte ion and solvent [2.58]. Since intercalation is also significant in carbon nanostructures [2.58], this means electrolyte ions approach the surface of nanostructured carbons more closely [2.59]. Carbon nanomaterials with pore diameters less than 1 nm typically have discharge currents of less than 100 mA cm-1 especially upon use of organic electrolytes [2.29]. Large diameters, on the other hand, have current densities typically of not more than 500 mA cm-1 with minimum capacitance loss.
Furthermore, increasing porosity of the nanostructured carbon may be associated with a decrease in electro-conductivity due to non-compatible conductive pathways [2.150].
Appropriate pore sizes are important in quick ion diffusion and mass transfer [2.154].
Mesopores (2 - 50 nm) of nanocarbon materials participate in transportation of electrolyte ion to micropores [2.142], i.e. act as transport channels [2.133], and this way they mainly affect rate capability of ECs [2.156]. This means wider mesopores favour electrolyte ion diffusion [2.37] and high rate capability. On the other hand, the function of micropores in charge storage is to house electrolyte ions [2.142]. The micropores enhance surface area [2.150], surface to volume ratio of carbon nanostructures and they also play a crucial role in the ion selectivity
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during the double layer formation [2.37]. In addition, pores large enough to house electrolyte ions are preferred for high electro-sorption efficiency. From the above views, it means the contributions of mesopores and micropores towards enhanced charge storage of ECs are interlinked. In fact, there have been reports on the high effectiveness of micropores in double layer formation whilst mesopores increase accessibility of micropores to electrolytes [2.37,2.133,2.142,2.150,2.155]. It is also important to emphasize that charges stored in pores of a given nano-carbon based electrode have an upper limit [2.23]. Excessive existence of micropores limits electrolyte ions transportation towards the electrode, thus causing ohmic resistance [2.150]. Micropores tend to decrease Cs when the electrolyte cationic sizes are large [2.156]. In fact, adsorption/desorption becomes a surface process since cations cannot fit into the small micropores [2.37] whilst macropores ( Λ 50 nm) allow ion buffering [2.142], i.e. act as transport paths [2.37]. Based on the aforementioned views, different pore sizes have different roles in charge storage. In designing nanostructured carbons, the electrolyte ion solvation energy, electrolyte solvent system, electrolyte ion size and carbon nanomaterial pore sizes need to be matched towards high capacitance [2.59,2.152]. A mismatch will render some pores of carbon nanomaterials inaccessible to the electrolyte and this means poor utilisation of the electrolyte ions in forming the double layer [2.152]. Also, with the need to optimise pore sizes for the operational mode and electrolyte in terms of ion size and mobility [2.132,2.143], the implication is that the subject of pore sizes in ECs is a critical area that still requires focussed studies.
Several approaches can be carried out to tailor the porosity of electrode nanomaterials in order to enhance their capacitance. For example, carbonate and hydroxide co-precipitation approach to nanomaterial based electrodes have higher surface areas with suitable pore size distribution [2.66]. This is advantageous in that it increases the self-discharge stability and lowers current leakages. Such compensations imply high energy storage capacity. The characteristics of ECs, especially those derived from the nanostructure of the carbon material, highly depend on the concentration of defects, average pore sizes, pore shapes and distribution [2.58,2.59]. The norm is that a narrow pore size distribution is preferred to those broadly distributed because the relative increase in surface area is better and this means energy storage capabilities can easily be controlled [2.157]. Hence, nanotechnology enables fabrication of novel carbon nanomaterials, nano-structures and nano-devices with tailored properties for EDLCs [2.18]. In fact, from the electrode point of view, energy storage and power delivery rate of nano-structural carbons can be enhanced by tuning their porosity to match the electrolyte ion size
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[2.71,2.75,2.143,2.151]. For example, in a recent study, pores were tailored from microporosity (0.6 nm) to mesoporosity (40 nm) by varying the thickness of silica on yolk shell carbon nano-spheres [2.142].
In addition, nanostructured carbon nanomaterial can be tailored to be pseudocapacitive by the introduction of heteroatoms, such as nitrogen and phosphorus [2.23,2.97,2.123,2.155,2.158,2.159], oxygen-containing surface moieties including carboxyl and hydroxyl groups [2.91], and the use of redox materials such as MO and polymers. The pseudo-capacitive character is associated with charge/mass transfer between the electrode material and electrolyte ions [2.150]. Heteroatoms modify the electron donor-acceptor properties of carbon nanostructures [2.159]. Oxygen-containing moieties are naturally acidic and therefore induce electron acceptor features to the nanomaterials of carbon but they bring detrimental outcomes in organic electrolytes [2.159]. For example, Gu et al. [2.135] reported undesirable interactions between the electrolyte and CNO surface functionalities. The diminishing consequences were noticeable and highly undesirable current leakages with respect to solvent used. This is due to associated high surface activities culminating in irreversible reactions between oxygen and electrolyte ions [2.150]. Hence, oxygen-containing groups can be sites of electrolyte decomposition and reduce the overall cycle stability of the device especially at high scan rates [2.53]. Surface functionalities also enhance the wettability of carbon nanostructures [2.46]. Wettability is increased due to an increase in the quantity of hydrophilic polar regions on the working electrode surface [2.123]. High capability for charge accumulation at the interface depends on availability and wettability of ideal pore dimensions.
The amount and nature of functionalisation of the carbon nano-forms, in this view, can be tailored to suit a particular electrolyte in EDLCs [2.97].