Three-Dimensional Hierarchical Porous Carbon Foams For Supercapcitors

Background

Technology Description

The invention involves a method of fabricating 3D hierarchically porous carbon foam (PCF) derived from K2CO3-embedded chitosan aerogels. Chitosan is the second most abundant natural biopolymer, making it a sustainable precursor for the synthesis of carbonaceous materials. Moreover, on pyrolysis, chitosan forms a graphitic carbonaceous material with excellent electrical conductivity due to its intrinsic N-doping.

 Previous studies of chitosan-derived carbonaceous materials have primarily focused on the preparation of carbon powders and pellets. However, carbon powders and pellets may not be the best candidates for EDLC electrodes. The fabrication of a carbon powder electrode requires the addition of non-conductive binders and a conductive substrate as a mechanical support. In contrast, carbon pellets are self-supporting structures, but they are typically assembled with very thick sheets (~2 μm in thickness) that limit the electrode surface area.

In this invention, a glutaraldehyde is used to cross-link chitosan in solution at ambient temperature to form chitosan aerogels. Subsequent pyrolysis of the as-formed aerogel yielded a 3D porous carbon network with interconnected porous thin carbon sheets (~90 nm).

 Previously described methods for the production of porous carbon structure can be grouped into two approaches: i) template methods that involve the incorporation and removal of templates such as silica spheres  and polystyrene spheres to generate pores, and ii) KOH activation methods, in which carbonized structures are mixed with KOH powder and annealed at high temperatures (>600 ° C) to create pores . Both approaches require at least two steps to obtain the porous carbon structure. In contrast, this method simultaneously carbonizes chitosan and generates meso- and micro-pores, forming the aerogel in one step.

The porous carbon foam structure has a relatively low density of 9.92 mg/cm3 , and the porous structure and thin walls allow rapid ion diffusion and create an ultra-high surface area (1,013 m² /g) for charge storage. The self-supporting PCF electrode achieved outstanding electrochemical performance with a gravimetric capacitance of 246.5 F/g (at 0.5 A/g) based on the entire mass of the electrode. Furthermore, a capacitance retention of 67.5% was observed when the current density increased from 0.5 to 100 A/g.

A quasi-solid-state symmetric supercapacitor was assembled using two pieces of PCF, and this delivered an very high power density of 25 kW/kg at an energy density of 2.8 Wh/kg based on the total weight of two electrodes (equivalent to a power density of 6.25 kW/kg and an energy density of 0.68 Wh/kg based on the total mass of device considering a packing factor of 0.4).

Generally, the method of making the carbon foam electrode involves: mixing an SiO2 particle dispersion with a chitosan solution, adding aqueious glutarldehyde, and allowing the mixture to solidify in air to form a hydrogel. The hydrogel is lyophilized and carbonized and the SiO2 dissolved from the hydrogel to form the 3D carbon foam. 

Applications

Supercapacitors (electric double layer capacitors)

Batteries 

Catalysts

Fuel Cells (hydrogen fuel cells) 

Advantages

One-step synthesis

Inexpensive input materials

Extremely high surface area --> capacitance retention of 67.5% 

25 kW/kg power density  

Intellectual Property Information

Related Materials

• Hierarchically porous carbon foams for electric double layer capacitors - 06/07/2016