• CORRECTION: This story was updated on June 22, 2015, to say that the number of pores leads to high surface area and that the pores are interconnected rather than aligned.
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Web Date: June 8, 2015

Designer Porous Carbon Could Boost Electrochemical Storage

Materials: An ordered hydrogel leads to highly porous carbon with desirable electronic properties
Department: Science & Technology
News Channels: Materials SCENE, Nano SCENE
Keywords: porous graphitic carbon, energy storage, electrochemical storage, batteries, supercapacitors, hydrogels
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MADE TO ORDER
A polyaniline hydrogel (left) has an ordered structure held in place by a phytic acid cross-linker (green). After carbonization, the phytic acid has been burned away, leaving behind a porous graphitic carbon that retains the hydrogel’s structure (right).
Credit: ACS Cent. Sci.
Reaction scheme for porous graphitic carbon
 
MADE TO ORDER
A polyaniline hydrogel (left) has an ordered structure held in place by a phytic acid cross-linker (green). After carbonization, the phytic acid has been burned away, leaving behind a porous graphitic carbon that retains the hydrogel’s structure (right).
Credit: ACS Cent. Sci.
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SUPER STORAGE
A porous graphitic carbon material can be printed like ink onto a gold-coated polyethylene terephthalate film to create a supercapacitor.
Credit: ACS Cent. Sci.
Photo of supercapacitor made out of porous graphitic carbon printed on a gold-coated polyethylene terephthalate film
 
SUPER STORAGE
A porous graphitic carbon material can be printed like ink onto a gold-coated polyethylene terephthalate film to create a supercapacitor.
Credit: ACS Cent. Sci.

Using a new method, scientists have made a porous graphitic carbon with record-high surface area and triple the electrical conductivity of similar carbon materials (ACS Cent. Sci. 2015, DOI: 10.1021/acscentsci.5b00149). The new graphitic carbon could be used in electrochemical energy storage devices, such as supercapacitors and battery electrodes.

For use in such devices, porous materials need to provide lots of adsorption sites for ions. A large number of microsized pores means a higher surface area, and thus a higher capacity to store electricity. What’s more, when the pores interconnect, ions pass more easily through the material, improving its ability to transport ions in an electrolyte to the electrode surface. One candidate electrode material is graphitic carbon, but it is typically made by carbonizing biological material, such as coconut husks and eggshells, and the resulting carbon has pores with bad interconnectivity. Zhenan Bao of Stanford University and colleagues overcame this problem by synthesizing a precursor for their carbon material that already had its molecules arranged in their desired order.

They first created a hydrogel by mixing an aniline monomer, an ammonium persulfate oxidizing agent, and a phytic acid cross-linker. They then freeze-dried the hydrogel to remove the water, leaving behind an aerogel with well-interconnected pores. Heating the aerogel to 600 °C in a nitrogen atmosphere began carbonizing the material while the phytic acid helped maintain its pore structure. Then, raising the temperature to 800 °C burned away the phytic acid and completed carbonization. After etching with potassium hydroxide, the resulting graphitic carbon had a surface area of 4,073 m2 per gram and more than three times the electrical conductivity of conventional activated carbon. Bao’s team used the material to create and test high-performance supercapacitors.

Bao says starting with different reactants or altering the processing temperature could allow them to tune the porosity of the final material to a particular application. Their graphitic carbon might also be used as a catalyst in fuel cells or as a sorbent to capture carbon dioxide, she says.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
Robin J. White (Wed Jun 10 10:10:29 EDT 2015)
Very interesting work. I would like to query a few things - is the following statement in the article technically true "Larger pores mean a higher surface area, and thus a higher capacity to store electricity."? Also, when a hydrogel is freeze dried, is it technically an "aerogel"? Surely it is a "cryogel"? Furthermore, regarding the reported work, I am curious as to how the pores are "aligned" and how this transfers into the final product particulary given the changes in nitrogen bonding motifs during carbonisation over the temperature range employed and the KOH "activation" step.

Thank you for the interesting report.
H.J.Metz, PhD (Thu Jun 11 08:11:35 EDT 2015)
Phytic acid is largely Phosphorous (Phosphate). Where does ist end up ? I assume that some of it must remain in this new compound, which by no means I would call "graphitic carbon", looking at its elemental composition.
H.J.Metz
Corinna Wu (Thu Jun 11 13:14:44 EDT 2015)
Thanks for your comment. For answers to your questions, I recommend checking out the paper from ACS Central Science, which is linked in the story: http://pubs.acs.org/doi/abs/10.1021/acscentsci.5b00149?source=cen. The paper is open-access, so it's freely available.
Zhenan Bao (Thu Jun 11 09:20:06 EDT 2015)
It is incorrect that "larger pores mean a higher surface area". The high surface area here is the result of small pores (micropores). The pores in this case are not aligned. They are interconnected.
Corinna Wu (Mon Jun 22 11:52:52 EDT 2015)
Dr. Bao, thank you for those clarifications. The story has been corrected.
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