Microporous polymer material for supercapacitors with large capacitance, high energy and power densities and excellent cycle life
|Ragone plots of Aza-CMPs. Source: Kou et al., Supplementary material. Click to enlarge.
A team led by Dinglin Jiang at the National Institutes of Natural Sciences in Okazaki (Japan) reports in the journal Angewandte Chemie on the synthesis of co-operative porous frameworks based on aza-fused π(pi)-conjugated microporous polymers (CMPs) for use in supercapacitors. (Aza compounds replace a carbon atom with a nitrogen atom; π-conjugated compounds have alternating single and multiple bonds in their structure.) Aza-CMPs exhibit large capacitance, high power and energy densities (approaching those of current-generation Li-ion batteries), and enable repetitive energy storage and power supply with an excellent cycle life.
Supercapacitors work on a different charge-storage principle than rechargeable batteries, and consist of electrochemical double layers on electrodes, which are wetted by an electrolyte. When a voltage is applied, ions of opposite charge collect on both electrodes to form wafer-thin zones of immobilized charge carriers. In contrast to a battery, there is only a shift of charge; no chemical transformation occurs. Various materials are suitable for supercapacitors, but an ideal material has yet to be found.
Special microporous, framework-like, organic polymers are materials of interest in this area. Their double bonds are arranged in such a way that some of their electrons can move freely over extended regions of the framework as an “electron cloud”. Such materials are thus conducting. A large inner surface area is important for the formation of electrostatic charge-separation layers in the pores.
π-Conjugated microporous polymers (CMPs) are a class of porous frameworks consisting of an extended π-conjugated system and inherent nanopores. As high surface-area porous materials, CMPs emerge as a new medium for gas adsorption and have been developed as a new type of nanoreactors and heterogeneous catalysts upon the integration of catalytic sites into the skeletons. The extended π-conjugated system endows CMPs with noteworthy light-emitting properties and allows the construction of light-harvesting antennae that trigger efficient, rapid, and vectorial energy funneling from the skeleton to entrapped acceptors.
From a synthetic point of view, CMPs are unique because they allow the elaborate control of both skeletons and pores. In this context, a promising way to the exploration of CMPs is to combine the structural advantages of a π-conjugated system and inherent pores. Herein, we report the synthesis of such co-operative porous frameworks based on aza-fused CMPs and highlight their functions in supercapacitive energy storage and electric power supply.
Aza-CMPs comprise four features: 1) fused CMP frameworks that are conductive, 2) aza units in the skeletons that enable dipolar interaction with electrolyte cations and accumulate protons on the walls of pores, 3) inherent micropores with optimized size that allows quick ion motion during charge–discharge processes, and 4) high surface areas provide large interfaces for the formation of electrostatic charge separation layers in the pores. Ultimately, these structural features work co-operatively, leading to exceptional energy storage and power supply capacities.—Kou et al.
Jiang and his team synthesized a nitrogen-containing framework with a pore size optimal for allowing ions to flow in and out rapidly—a requirement for rapid charging and discharging. The nitrogen centers interact with the electrolyte ions, thus favoring the accumulation of charge and the movement of ions.
Supercapacitive performance was tested with galvanostatic charge–discharge cycling experiments. A general tendency is that when the current density increased, the charge and discharge times were significantly shortened. Aza-CMP@350—one of five variants of the material—can be operated at a high current density of 10 Ag-1 to allow power supply at high rates.
Among the findings:
Capacitance. The specific capacitance of Aza-CMPs was significantly higher, four times than that of activated carbon materials (<270 F g-1). Aza-CMPs are superior to nitrogen-enriched porous carbon materials (50–330 F g-1) and outperform the state-of-the-art nanostructured carbon including template porous carbon (120–350 F g-1), graphenes (120 F g-1), carbon nanotubes (50–120 F g-1), and carbon fibers (120–370 F g-1).
The capacitance is even greater than the best values for supercapacitors (720 F g-1) with a redox-active ruthenium oxide electrode.
Energy and power densities. The Ragone plot reveals that Aza-CMPs exhibit the maximum energy and power densities of 53 Wh kg-1 and 2.25 kW kg-1 respectively. The energy density is higher than those of nanostructured porous carbon materials and reaches the regime of batteries such as Pb-acid, NiCd, and lithium ion battery (10–150 Wh kg-1). The powder density of Aza-CMPs is more than one order of magnitude higher than those of batteries (< 0.3 kW kg-1).
Stability. Aza-CMP@350 exhibited excellent performance stability without loss in capacitance (397 F g-1) after 10,000 charge–discharge cycles at a current density of 5 Ag-1. AzaCMP@400 also exhibited a highly stable performance without any deterioration.
The fused skeleton, dense aza units, and well-defined micropores work co-operatively and facilitate electrostatic charge-separation layer formation. Consequently, Aza-CMPs exhibit large capacitance, high energy and power densities, and enable repetitive energy storage and power supply with an excellent cycle life. These remarkable results reported herein demonstrate the enormous potential of π-conjugated microporous polymers as high-energy storage devices.—Kou et al.
This work was supported by the Japan Science and Technology Agency (JST).
Yan Kou, Yanhong Xu, Zhaoqi Guo, and Donglin Jian (2011) Supercapacitive Energy Storage and Electric Power Supply Using an Aza-Fused π-Conjugated Microporous Framework. Angewandte Chemie International Edition doi: 10.1002/anie.201103493