A tunable hierarchical porous carbon structure for studying electrochemical energy conversion

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.