Aerosol-Assisted Synthesis of Nanostructured Anode PT-RU Electrocatalysts for Direct Methanol Fuel Cells

STC.UNM, the technology-transfer arm of the University of New Mexico (UNM) (Albuquerque, NM)  received U.S.  Patent 7,670,988 for an aerosol-assisted method for the synthesis of nanostructured metallic electrocatalysts for direct methanol fuel cells.  The electrocatalyst may be formed from metals such as platinum, platinum group metals, and binary and tertiary compositions such as platinum-ruthenium and platinum-tin. The resulting unsupported electrocatalyst is homogenous and highly disperse, according to inventors Dr. Elise Switzer, Dr. Plamen Atanassov and  Dr. Abhaya Datye of the University of New Mexico Center for Micro-engineered Materials.  The catalyst show higher activity than can be obtained from commercially available catalysts. 

They developed a novel method of fuel cell electrocatalyst manufacture. In general, a precursor solution comprising metal precursors and metal oxide particles, for example silica particles, undergo aerosol-assisted self-assembly during which they are atomized ultrasonically and subjected to pyrolysis in the presence of an inert carrier gas. The resulting powder is then collected and reduced. Finally, the silica template is removed. 

The aerosol synthesis of Pt--Ru nanocomposites  procedure as shown in FIG. 1 was used to manufacture electrocatalysts.  FIG. 1 depicts a method of fuel cell electrocatalyst manufacture according to the techniques developed at UNM. 

The precursor solution consisted of tetraamineplatinum (II) hydroxide, hexaamineruthenium (III) chloride and Ludox ®. TM50 colloidal silica solution with monodisperse silica particles having an average diameter of 20 nm. 

Alternative formulations of the metallic precursor/silica template ratio in the precursor solution were investigated. The materials discussed here varied in the metallic precursors/silica template ratio in the aerosol precursor solution. This ratio was 50 wt % PtRu/SiO2 for the Aerosol Low Metal Loading (LML) sample, 70 wt % PtRu/SiO2 for the Aerosol Mid Metal Loading (MML) sample and 80 wt % PtRu/SiO2 for the Aerosol High Metal Loading (HML) sample. 

The molar ration of SiO2:Pt:Ru was 1:0.23:0.23 for the LML sample, 1:0.54:0.54 for the MML sample, and 1:0.78:0.78 for the HML sample. Inert nitrogen gas was used to pass the precursor solution through an aerosol reactor while the temperature was maintained at 125.degree. C. The powder collected on filter paper was then reduced under hydrogen flow at 300.degree. C. for 2 hours. This is followed by removal of the silica template with a 7M KOH solution for 72 hours. 

The aerosol synthesis technique also lends itself quite naturally to the synthesis of nanostructured ethanol oxidation alloys for Direct Ethanol Fuel Cells (DEFCs). Pt--Sn alloys which have shown distinct electrocatalytic activity for ethanol oxidation. The precursor solution can be easily modified to include Sn precursors. 


The most effective electrocatalyst morphology for methanol electro-oxidation can be extended to the synthesis and examination of Pt--Sn electrocatalysts for ethanol electro-oxidation. TEM micrographs of nanostructured Pt--Sn synthesized in the aerosol synthesis method are shown in FIGS. 8a and 8b, which shows the open frame structure resulting after removal of the 20 nm silica template. 

FIG. 3a is a TEM micrograph of an Aerosol LML sample of Pt--Ru network formed using th UNM aerosol assisted synthesis method. FIG. 3b is a TEM micrograph of an Aerosol MML sample of Pt--Ru network.

FIG. 3c is a TEM micrograph of an Aerosol HML Pt--Ru network. FIG. 3d is a TEM micrograph of an Aerosol HML Pt--Ru network.

FIG. 8a is a TEM micrograph of an aerosol-derived templated Pt--Sn electrocatalyst at 0.10 .mu.m.  FIG. 8b is a TEM micrograph of an aerosol-derived templated Pt--Sn electrocatalyst at 30.00 nm. 

There are many advantages of synthesizing templated electrocatalyst materials by spray pyrolysis. The final morphology and composition of the electrocatalyst is determined by the precursor solution atomized. Changing the morphology and composition of the precursor solution is extremely simple and straightforward. For alloys, the template and alloy constituent precursors are in close contact during synthesis which results in a more homogeneous final material as opposed to bulk templating methods.