Aligned Carbon Nanotubes Used To Make Platinum Free Fuel Cell Catalysts for Oxygen Reduction Reaction

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University of Chicago (Chicago, IL) researchers have developed aligned carbon nanotubes with electro-catalytic activity for oxygen reduction reaction which is vital to more powerful proton exchange membrane fuel cells (PEMFCs).  The catalysts use little or no noble metals such as platinum for the catalytic reduction of oxygen.

 

In U.S. Patent Application 20100167918,  inventors Di-Jia Liu, Junbing Yang and Xiaoping Wang disclose a catalyst for an electro-chemical oxygen reduction reaction (ORR) made of a bundle of longitudinally aligned carbon nanotubes having a catalytically active transition metal incorporated longitudinally in the nanotubes.

 

 The three researchers developed new method of preparing the electrode catalyst as well as the catalyst prepared thereby for the oxygen reduction reaction (ORR) that contains mainly transition metal, carbon and nitrogen but little or no noble metal. The new catalysts also have unique geometric structures of hollow carbon nanotube bundles aligned with the same spatial orientation. These aligned carbon nanotubes (ACNT) are prepared through a chemical vapor deposition (CVD) method using precursors made of hydrocarbons, optionally nitrogen containing hydrocarbons and the organometallic compounds containing transition metal ions, preferably such as Fe and Co by also including Ni, Cr and Mn.

 

At present, the most effective catalyst for these reactions are made of platinum supported on an amorphous carbon. A typical Pt weight loading on the catalyst support ranges from 15% to 40%. Since platinum is a precious metal with limited supply, its usage adds a significant cost to a PEMFC system.

 

 Furthermore, the current method of preparing a MEA is very ineffective in utilizing platinum. An ink containing Pt/carbon catalyst mixed with a polymer solution (ionomer) is cast on the surface of the membrane, followed by hot pressing. Such a method often buries Pt/carbon catalyst particles underneath the ionomer matrix rendering them inaccessible to hydrogen or oxygen and unavailable to participate the aforementioned reactions. Fully utilizing the active catalyst is very important in reducing cost, especially for the cathode application since ORR is a more sluggish reaction than hydrogen oxidation, thus often requiring more catalyst. For example, the amount of platinum used at the PEMFC cathode typically is around 0.4 mg/cm² whereas that used at anode is about 0.14 mg/cm². 

 

The University of Chicago discovery yields a catalyst that is superior to the prior art in several aspects. First of all, the carbon made up ACNT has a graphitic phase which is different from the amorphous type produced in the prior art. By mixing transition metal organometallic compounds with hydrocarbons in liquid or in gas phase in a CVD process, a much more uniform reaction mixture is produced compared to the solid mixing method used in the prior art. This uniform mixture deposits and decomposes continuously on the substrate during the carbon nanotube growth, thus allowing a more homogenous distribution of the transition metal throughout the graphitic plane in ACNT. Second, the metal to carbon ratio is limited only by the solubility of the organometallics in the hydrocarbon for a solution-type mixture. This limit can be further enhanced for a vapor phase mixture. Such flexibility leads to very high metal-to-carbon ratio thus high density of the catalytic site. For example, an atomic ratio of Fe-to-C as high as 1:20 has been observed

 

Although a variety of metals may be catalytically active within the meaning of this invention, those preferred are iron, cobalt, nickel, manganese, chromium and mixtures and alloys.  The inventors have developed methods of making bundles of longitudinally aligned graphitic nanotubes having catalytically active transition metal incorporated longitudinally thereof. The nanotubes may be straight or spiral or bamboo in shape and may be open at one or both ends. Nitrogen atoms preferably are chemically bonded to some of the transition metals to provide catalytic activity in the acid environments of a PEMFC.

 

FIG. 1 is a schematic representation of a carbon vapor deposition reactor useful in growing carbon nanotubes showing the two zone configuration and placement for the aligned carbon nanotube (ACNT) growth to produce fuel cell catalysts. 

 

 

FIG. 2 is a SEM image of ACNT bundles as grown according to the University of Chicago invention

 

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