(50d) A Tunable Hierarchical Porous Carbon Structure for Studying Electrochemical Energy Conversion

A
Tunable Hierarchical Porous Carbon Structure for Studying Electrochemical
Energy Conversion

Chunzhen YANG and Kwong-Yu CHAN*, Department of Chemistry, The University of
Hong Kong, Pokfulam Road, Hong Kong

Abstract

A well-defined porous carbon structure
with tunable parameters provides a platform for systematic

study of transport and
confinement effects on electrochemical conversion. Synthesized via silica

template techniques, the
hierarchical structure has a hollow core within a mesoporous
shell (HCMS)

with uniform pore size,
shell thickness, hollow core diameter, as shown in Fig. 1.

By varying the synthesis parameters,
the structural parameters can be tuned to study

1)   
combined
effects of surface area, mesopore size, macropore size, and hierarchy of porosity

on ionic transport and adsorption in electrochemical
capacitance behavior;

2)   
how
distribution of platinum or other electrocatalyst
nanoparticles in the porous structure

affect catalytic faradaic reactions;       

3)   
how
distribution of mixed metal nanoparticles such as PtRu
and varying of Pt and Ru
content

in the porous structure affects faradaic
reactions such as methanol oxidation; and

4)   
the
effectiveness of a mathematic model in analyzing electrochemical conversion in
porous

structures.

Electrochemical capacitance is studied
with a series with meso-shell thickness stepwise
increased

from 0, 25, 50 to 100 nm
while keeping an identical 330 nm hollow core and mesopore
of 3.9 nm.[1]

A thicker shell has a higher surface area
with a proportional increase of electrochemical capacitance

which however, can only be
fully realized at low scan rates/currents. At high currents, ionic transport

limits the electrochemical
capacitance of a thick mesoporous shell. Electrochemical
impedance

spectra (EIS) obtained on
the family of HCMS carbon structures were fitted to equivalence circuits

described by models of network
structures.[2] The results reveal the need to match the AC

frequency with the
characteristic time constant of a structure for optimum active and reactive power.

For fuel-cell electrode conversion, the
distribution of Pt and PtRu
nanoparticles in a mesoporous

carbon can vary in
different synthetic protocol. Uniform Pt particle distribution
can be achieved with a

CPDP method [3] as
shown in Fig. 2.
Different electrocatalysts with varying Pt distribution can be

compared for activity and utilization
in methanol oxidation. The particles dispersed into a thicker shell

are more stable and give
better performance over time, as shown in voltage cycling and extended

methanol oxidation.

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Fig. 1. A hollow core mesoporous shell (HCMS) carbon structure with three varying geometrical parameters: pore diameter (dp), shell thickness (S), and diameter of core (C).

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Fig. 2 TEM images of Pt nanoparticles loaded in mesoporous carbons of same hollow core diameter, C = 330 nm but different Shell thickness (a)-(c) Pt@CS-30,  S=30 nm ; (d)-(f) Pt@CS-50, S=50 nm ; (g)-(i) Pt@CS-100, S=100 nm; (j) pore size distribution of the porous structures loaded with Pt.

References:

[1]. F. Li, M. Morris and K.Y. Chan*,
?Electrochemical Capacitance and Ionic Transport in the

     Mesoporous Shell
of a Hierarchical Porous Core-Shell Carbon Structure?, J. Mater. Chem. 21

(2011)
8880-8886.

[2]
Chunzhen Yang, Chi-Ying Vanessa Li, Fujun Li, Kwong-Yu Chan*, ?Complex
Impedance with

Transmission
Line Model and Complex Capacitance Analysis of Ion Transport and Accumulation

in Hierarchical
Core-Shell Porous Carbons?, J. Electrochem. Soc. (2013) to appear.

[3] F. Li, K.Y. Chan*, and H. Yung,
?Carbonization over PFA-Protected Dispersed Platinum: An

Effective
Route to Synthesize High Performance Mesoporous-Carbon
Supported Platinum?, J.

Mater. Chem. 21 (2011) 12139-12144.