Percolation Effects in Supercapacitors with Thin, Transparent Carbon Nanotube Electrodes

Abstract

We have explored the effects of percolation on the properties of supercapacitors with thin nanotube networks as electrodes. We find the equivalent series resistance, R(ESR), and volumetric capacitance, C(V), to be thickness independent for relatively thick electrodes. However, once the electrode thickness falls below a threshold thickness (∼100 nm for R(ESR) and ∼20 nm for C(V)), the properties of the electrode become thickness dependent. We show the thickness dependence of both R(ESR) and C(V) to be consistent with percolation theory. While this is expected for R(ESR), that the capacitance follows a percolation scaling law is not. This occurs because, for sparse networks, the capacitance is proportional to the fraction of nanotubes connected to the main network. This fraction, in turn, follows a percolation scaling law. This allows us to understand and quantify the limitations on the achievable capacitance for transparent supercapacitors. We find that supercapacitors with thickness independent R(ESR) and C(V) occupy a well-defined region of the Ragone plot. However, supercapacitors whose electrodes are limited by percolation occupy a long tail to lower values of energy and power density. For example, replacing electrodes with transparency of T = 80% with thinner networks displaying T = 97% will result in a 20-fold reduction of both power and energy density.

Avoiding Resistance Limitations in High-Performance Transparent Supercapacitor Electrodes Based on Large-Area, High-Conductivity PEDOT:PSS Films

“…Moreover, high T simultaneously results in reduced CA, hence a reduction in the energy which can be stored by a device with a given footprint. 27,32 Quantifying these trade-offs using the described figures of merit will greatly assist with the development of high performance transparent SCs.…”

“…Whilst examples of the former are presently scarce, 16 significant efforts are underway to develop transparent/flexible SCs. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] The electrode materials under investigation for this purpose are numerous, including carbon nanotubes, 27,[32][33] graphene, 21,29,34 transition metal oxides 17, 19-20, 23, 25 and conducting polymers. 17,22,30 In some transparent SC reports, the capacitive charge storage materials have been supported by an underlying transparent indium-tin oxide (ITO) layer to provide efficient current collection.…”

“…27 To characterise this behaviour the electrode sheet resistance, Rs, can be related to T via a similar expression as for the areal capacitance: …”

Effect of Percolation on the Capacitance of Supercapacitor Electrodes Prepared from Composites of Manganese Dioxide Nanoplatelets and Carbon Nanotubes

“…We believe the current study is the first demonstration of percolation of capacitance in composite supercapacitor electrodes (although percolation scaling of capacitance has been observed in single-component nanotube networks). 79 Understanding the nature of such percolation will be critical for the optimisation of capacitive composites.…”

Relationship between Material Properties and Transparent Heater Performance for Both Bulk-like and Percolative Nanostructured Networks

“…18 These materials have been used as electrodes in a range of applications including light emitting diodes, solar cells and transparent capacitors. 19,20 However, more recently, attention has turned to using nanostructured transparent conductors as transparent heaters. [21][22][23][24][25][26][27][28][29][30] Transparent heaters are simply conducting films which are thin enough to be transparent but can be heated up on application of a voltage.…”