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.