Metal/metal oxide nanoparticles supported on nanostructured carbons for electrochemical applications

Abstract

Among various electrochemical devices that have been developed for energy storage and conversion, electric double layer capacitors (EDLCs) and direct methanol fuel cells (DMFC) have received much research attention. Nanostructured carbon materials have been playing an important role in the development of these devices, due to such characteristics as good electrical conductivity, high chemical stability, high surface area and large pore volumes and etc. In an EDLC, nanostructured carbon electrodes, possessing pores of varied length scales, can deliver electric energy at high current loadings. This kind of pore structure also benefits the deposition of metal catalysts and the transport of reactants and products in the methanol oxidation reaction.

In order to systematically study the structural effects on the electrochemical capacitance and ionic transport, a series of three-dimensional hierarchical carbons with hollow core-mesoporous shell (HCMS) structure were template-synthesized. Periodically ordered macroscopic hollow cores of 330 nm in diameter were surrounded by a mesoporous shell containing uniform pores of 3.9 nm. The shell thickness was stepwise increased from 0, 25, 50 to 100 nm. The HCMS structure was modeled by a 5-level transmission line model to study the capacitance contribution from the pores at different length scale. Results revealed that the HCMS carbon with thicker mesoporous shells can provide high capacitance, while thinner shells could deliver high power output.

A series of HCMS carbon sphere supported Pt nanoparticles were synthesized via the “Carbonization over Protected and Dispersed Metal” (CPDM) method. Contrary to the conventional “polyol” synthetic method, whereas most of Pt nanoparticles were deposited on the external surface of carbon spheres; the Pt nanoparticles synthesized via the CPDM method were found encapsulated in the mesoporous carbon shells and highly dispersed throughout the carbon texture. „Accelerated stress tests‟ (ASTs) were conducted to investigate the nanopores confinement effect toward the electrochemical stability of these Pt catalysts. Results revealed that (1) the nanopores confined Pt nanoparticles on HCMS carbon spheres exhibited a stable electrochemical active surface area (ECSA) and catalytic activity; and (2) thick mesoporous carbon shells could provide better protection over the Pt nanoparticles. This “CPDM” method was further extended to synthesize highly alloyed PtRu nanoparticles supported electrocatalysts. It is expected that this CPDM method can also be applied to synthesize other metal/metal oxide supported catalysts with stable electrochemical performance.

WO3 has been demonstrated as a promsing co-catalyst for Pt in the methanol oxidation reaction (MOR). The synthesis of Pt-WO3/C catalyst with well-controlled nanoparticle size (2.5 nm) and composition was achieved via a microwave-assisted water-oil microemulsion reaction. Hydrogen adsorption, CO-stripping and Cu- stripping methods were used to estimate the ECSA of Pt in the Pt-WO3/C catalysts. Among these, Cu-stripping method was relatively more reliable due to the overlapping involvement of the WO3 component in the other methods. The methanol oxidation measurement shows that a 1:1 Pt:W ratio catalyst exhibits the highest Pt-mass current density of 271 mA mg-1-Pt, 1.4 times higher than that of commercial E-TEK catalyst.