Faraday Discussions Cite this: Faraday Discuss., 2014, 172, 139
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Effects of structural disorder and surface chemistry on electric conductivity and capacitance of porous carbon electrodes† Boris Dyatkin and Yury Gogotsi*
Received 21st March 2014, Accepted 6th May 2014 DOI: 10.1039/c4fd00048j
This article reports on changes in electric double layer charge storage capacity as a function of surface chemistry and graphitic structure of porous carbon electrodes. By subjecting 20 nm to 2.0 mm sized carbide-derived carbons (CDCs) synthesized at 800
C to high-temperature vacuum annealing at 700–1800
C, we produce three-
dimensional internal surface architectures with similar pore sizes and volumes but divergent surface chemistry and wall graphitization. Annealing increases carbon ordering and selectively removes functional groups, and both transformations affect conductivity and wettability. Contrary to an expected increase in gravimetric capacitance, we demonstrate no increases in charge storage despite increased conductivity and pore accessibility. At the same time, annealing improves the charge/ discharge rates in EMIm-TFSI ionic liquid electrolyte. The annealing process eliminates faradaic reactions that limit the voltage window, but potentially accelerates catalytic breakdown of the ions themselves. We therefore corroborate the theory that surface groups and defects in the graphitic structure act as dopants that allow facile movement of ions into pores, improve screening in the superionic state, and affect the quantum capacitance contribution from the carbon structure.
1 Introduction Electric double layer capacitors (EDLCs), also known as supercapacitors and electrochemical capacitors (ECs), are attracting substantial interest as electrical energy storage solutions for hybrid electric vehicles and other systems.1 The primary charge storage harvesting mechanism, which relies on the electrosorption of ions from an electrolyte onto a porous electrode,2 offers essential advantages such as extended cyclability, stable operation in a wide temperature range,3 versatile geometric conguration, and scalability.4 Porous carbon
A.J. Drexel Nanomaterials Institute & Department of Materials Science and Engineering, 3141 Chestnut Street, Philadelphia, PA 19104, USA. E-mail: [email protected]; Fax: +1 2158951934; Tel: +1 2158956446 † Electronic supplementary information (ESI) available: Additional materials characterization, electrochemical performance, and quantum capacitance discussions. See DOI: 10.1039/c4fd00048j
This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 172, 139–162 | 139
View Article Online
Published on 12 May 2014. Downloaded by Columbia University on 04/02/2015 09:10:29.
Faraday Discussions
Paper
electrodes can provide a tuneable, electrically conductive structure with over 3000 m2 g1 in accessible surface area,5 extending their gravimetric power densities above 10 kW kg1.6 However, the small operating potential of traditional supercapacitors (
Published on 12 May 2014. Downloaded by Columbia University on 04/02/2015 09:10:29.
PAPER
View Article Online View Journal | View Issue
Effects of structural disorder and surface chemistry on electric conductivity and capacitance of porous carbon electrodes† Boris Dyatkin and Yury Gogotsi*
Received 21st March 2014, Accepted 6th May 2014 DOI: 10.1039/c4fd00048j
This article reports on changes in electric double layer charge storage capacity as a function of surface chemistry and graphitic structure of porous carbon electrodes. By subjecting 20 nm to 2.0 mm sized carbide-derived carbons (CDCs) synthesized at 800
C to high-temperature vacuum annealing at 700–1800
C, we produce three-
dimensional internal surface architectures with similar pore sizes and volumes but divergent surface chemistry and wall graphitization. Annealing increases carbon ordering and selectively removes functional groups, and both transformations affect conductivity and wettability. Contrary to an expected increase in gravimetric capacitance, we demonstrate no increases in charge storage despite increased conductivity and pore accessibility. At the same time, annealing improves the charge/ discharge rates in EMIm-TFSI ionic liquid electrolyte. The annealing process eliminates faradaic reactions that limit the voltage window, but potentially accelerates catalytic breakdown of the ions themselves. We therefore corroborate the theory that surface groups and defects in the graphitic structure act as dopants that allow facile movement of ions into pores, improve screening in the superionic state, and affect the quantum capacitance contribution from the carbon structure.
1 Introduction Electric double layer capacitors (EDLCs), also known as supercapacitors and electrochemical capacitors (ECs), are attracting substantial interest as electrical energy storage solutions for hybrid electric vehicles and other systems.1 The primary charge storage harvesting mechanism, which relies on the electrosorption of ions from an electrolyte onto a porous electrode,2 offers essential advantages such as extended cyclability, stable operation in a wide temperature range,3 versatile geometric conguration, and scalability.4 Porous carbon
A.J. Drexel Nanomaterials Institute & Department of Materials Science and Engineering, 3141 Chestnut Street, Philadelphia, PA 19104, USA. E-mail: [email protected]; Fax: +1 2158951934; Tel: +1 2158956446 † Electronic supplementary information (ESI) available: Additional materials characterization, electrochemical performance, and quantum capacitance discussions. See DOI: 10.1039/c4fd00048j
This journal is © The Royal Society of Chemistry 2014 Faraday Discuss., 2014, 172, 139–162 | 139
View Article Online
Published on 12 May 2014. Downloaded by Columbia University on 04/02/2015 09:10:29.
Faraday Discussions
Paper
electrodes can provide a tuneable, electrically conductive structure with over 3000 m2 g1 in accessible surface area,5 extending their gravimetric power densities above 10 kW kg1.6 However, the small operating potential of traditional supercapacitors (
