Abstract
The impact of multi-layered electrode microstructures on the dynamic capacitance of electrochemical double layer supercapacitors is investigated. An electrochemical model that describes ion diffusion and double layer dynamics across the layered electrodes is first developed and then matched to experimental data. With TiO2 particulate and carbon nanotube layered electrodes, two knee frequencies were observed in the real and imaginary capacitance plots in both experiment and model simulations. These two knee frequencies resulted in an increase in real capacitance at high frequencies (ω≈100−102 rad s−1) but a reduction at lower frequencies (ω≈10−2 rad s−1), with the response being largely insensitive to the relative layer thicknesses. The increased capacity at high frequencies was due to increased ion mobility across the electrodes caused by the layering, allowing diffusion limitations of identical homogeneous electrodes to be overcome. These results imply the suitability of layered electrodes for applications with highly dynamic charge profiles and/or relatively thick electrodes.
Highlights
Supercapacitors with approximately ten times higher power den sities (5–50 kW kgÀ 1) and significantly longer cycle lives (>104 cycles) but lower energy densities (3–40 Wh kgÀ 1) than Li ion batteries are attractive for alternating current (AC) ripple filtering, hybrid electric vehicles and meeting peak power demands in electrical grid storage [21, 24,27]
The main benefit of storing charge on the electrode surface in this way is that it leads to high power densities, being principally limited only by the mobility of the ions across the separator and through the porous electrode
Surface only energy storage leads to relatively low energy densities, which has motivated the development of active materials with exceptionally high surface areas (500–2000 m2 gÀ 1) for supercapacitor electrodes, including activated carbon [5], carbon nanotubes (CNTs) [33], graphene [27] and metal oxide nanomaterials [19]
Summary
Supercapacitors with approximately ten times higher power den sities (5–50 kW kgÀ 1) and significantly longer cycle lives (>104 cycles) but lower energy densities (3–40 Wh kgÀ 1) than Li ion batteries are attractive for alternating current (AC) ripple filtering, hybrid electric vehicles and meeting peak power demands in electrical grid storage [21, 24,27]. A model is developed to better understand and probe the underlying ionic mobility in structured elec trodes and its links to energy storage performance, revealing the benefits of structured electrodes for applications where ionic diffusion limita tions dominate, such as in high frequency charging and the filtering of ripple current fluctuations at approximately 120 Hz. AC ripple filtering is a relevant application for the consideration of structured electrodes and involves the filtering of high order harmonics to protect against electrical power surges and spikes [26]. In contrast in this paper, the benefits of electrode layering lies in their ability to promote ion mobility across an electrode under high-frequency dynamic currents where diffusion limitations dominate deliverable capacity. We conclude: (1) Layering introduces two knee frequencies in the real capacitance that can be manipulated ad vantageously; (2) Layering can increase the capacity for high frequency operation; (3) Layering decreases capacity at lower frequencies but may reduce losses; and (4) Electrode response is surprisingly insensitive to some of the details of the layer configuration
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