(569f) Effect of Surface Chemistry in Electric Double Layer Capacitance of Nitrogen Doped Ordered Mesoporous Carbon

            In addition
to surface area, pore size and pore interconnectivity, surface chemistry can
play a significant role in electrochemical double layer capacitance [1]. Introduction
of surface oxygen groups has been shown to improve capacitance of carbon
materials by increasing wettability and adding sites for pseudocapacitance [1].
On the other hand, oxygenated groups also promote carbon corrosion, leading to an
increase in series resistance and decrease in capacitance over time [2]. Nitrogen
functional groups are a suitable alternative to oxygen functionals as N-doped
carbons have been shown to be highly stable and have pseudocapacitive properties
similar to oxygen groups in electrochemical systems [3, 4]. However, the effect
of nitrogen surface groups on capacitance is not well understood.

            Ordered
mesoporous carbon (OMC) synthesized by hard-templating methods has well
connected mesoporous system with a very narrow pore size distribution [5].
Furthermore, by using a nitrogen-containing carbon precursor, nitrogen surface
groups can be readily introduced onto the surface of the OMC while possessing a
nearly identical structure to its N-free and oxygen-containing analogs.  Hence,
nitrogen doped ordered mesoporous carbon (NOMC) is an ideal system to study the
effects of both the pore structure and surface chemistry on capacitance.

            In this
study, the hard template, SBA-15, was prepared by self-assembly of the triblock
copolymer Pluronic® P123 (E₂₀PO₇₀EO₂₀, BASF)
and tetraethyl orthosilicate (TEOS), whose hydrolysis and polycondensation was
catalyzed by 2 M HCl.  Pyrrole was then vacuum infiltrated into the template,
polymerized to polypyrrole, and carbonized at high temperature for 3 h in inert
atmosphere. Finally, the template was removed in hot 10 M KOH. The NOMC obtained
was divided into four batches and heated to 800, 1000, 1200 and 1400 °C. The
carbons were characterized using TEM, BET and XPS. Electrochemical measurements
were performed in a three electrode cell in 0.5 M H₂SO₄
at room temperature.

            A typical
rectangular CV at various scan rates for NOMC-800 are shown in Figure 1. The
ideal rectangular shape conforms to the double layer charge storage/discharge
mechanism. The capacitance was calculated using the slope of i. vs scan rate
(υ) plots at constant potential.  The double layer capacitance of the
NOMCs was found to vary as a function of nitrogen content.  In addition, the
top-performing NOMC possessed a double layer capacitance nearly an order of
magnitude greater than commercial carbon. 

Figure
1. Typical CVs of nitrogen doped ordered mesoporous carbon taken in N₂
saturated 0.5 M H₂SO₄ at 25 °C with different scan rates.

References

(1)   E. Frackowiak, Phys.
Chem. Chem. Phys., 9 (2007) 1774-1785.

(2)   P. Simon and Y.
Gogotsi, Nature Mater., 7 (2008) 845-854.

(3)   S. Shrestha and
W. E. Mustain, J. Electrochem. Soc. 157 (2010) B1665-B1672.

(4)   F. Béguin, K.
Szostak, G. Lota, and E. Frackowiak, Adv. Mater., 17 (2005)
2380-2384.

(5)  
R.
Ryoo, S. H. Joo, M. Kruk, and M. Jaroniec, Adv. Mater., 13 (2001)
677-681.