Non-Platinum Group Metal Catalysts for Oxygen Reduction in Fuel Cell Applictaions

Over the last two decades the attention to alternative, low cost catalysts for fuel cells has been steadily rising. Non-Platinum Group Metal (Non-PGM) catalysts of several different types have been proposed, with those based on transition metal and nitrogen-carbon matrix (M-N-C) gaining the most attention. These catalysts are almost exclusively synthesized by pyrolysis in inert or reactive (ammonia-containing) atmosphere. Often a templating approach is used whether explicitly (hard template) or as a part of the processing conditions (soft template or pore-forming agents). Lately, such Non-PGM catalyst have started to be tested in Proton Exchange Membrane (PEM) and Alkaline Exchange Membrane (AEM) fuel cells, often aiming automotive applications. UNM has developed and demonstrated a family of Non-PGM catalysts from the M-N-C family of materials derived with the use of hard (oxide) templates. These catalysts were transferred as a technology to Pajarito Powder Co. and deployed as products for variety of applications. In this overview we will address the critical challenges that the UNM team has faced on the way to introduction of such catalysts for both acid and alkaline polymer electrolyte fuel cells.

UNM catalysts synthesis is based on a Sacrificial Support Method (SSM). This is a templated synthesis approach for obtaining hierarchically structured materials based on the "lost wax technique" practiced through pyrolysis. In this method the catalysts precursors are being absorbed on mono-dispersed silica (or other oxide powder), or on meso-structured silica material, obtained by micro-emulsion templating, aerosol synthesis routes or evaporation-induced self-assembly. This oxide template is impregnated with the solution of the catalyst precursors: transition metal salt and a nitrogen-containing organic substance (usually an amine, nitrogen-containing polymer or macrocycle). Alternatively, a mechanochemical approach is used based on mechanically mixing (ball-milling) the support (usually mono-dispersed or meso-structured structured silica) and the transition metal, and nitrogen-carbon precursor. The mix is then thermally processed (pyrolyzed) and then the silica support is removed by etching (in KOH or HF). This results in a open frame structure of a “self-supported” material that consists of the catalyst only. The support has been removed by chemical dissolution. Synthesis of Non-PGM catalysts based on transition metals and N-containing carbonaceous materials obtained by both methods will be discussed. ^1-5 Such hierarchical structures are advantageous in enhancement of the fuel cell performance since they correspond to the different levels of transport in the corrugated electrode matrixes.  

The fact that these materials consist of catalytic substance only (the support has been etched away) allows detail structural characterization. Using several methods: XPS, XAFS/XANES, Mossbauer Spectroscopy, BET and adsorption isotherm-derived pose size distribution, etc. we have built a hypothesis of the active site that includes graphene-like material with N-defects that include transition metal atomically dispersed and associated with the nitrogen defects in graphene (see Fig.). The mechanism of the oxygen reduction reaction on these M-N-C materials will be discussed as well.        

A wide variety of materials can be made by these methods in which not only the composition but also the microstructure can be varied.  It is the combination of these attributes - control over microstructure at a number of different length scales and composition, simultaneously - that is extremely important to the performance of the electrocatalyst materials in a fuel cells.  This paper will bring examples of successful practical applications of our materials in automotive technologies using both Polymer Electrolyte (PEMFC) and Alkaline Membrane (AMFC) Fuel Cells with gaseous and liquid fuels. ^6-10

References

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3  J. Ziegelbauer, et al., J. Phys. Chem. C, 112 (2008) 8839–8849

4  T. Olson, et al., J. Power Sources, 183 (2008) 557–563

5  T. Olson, et al., J. Electrochem. Society, 157 (2010) B54-B63

6  A. Serov, et al., Electrochem. Comm., 22 (2012) 53-56

7  A. Serov, et al., Appl. Catalysis B: Env., 127 (2012) 300-306

8  M. Robson, et al., Electrochim. Acta, 90 (2013) 656–665

9  P. Atanassov and A. Serov, Japanese Society of Automotive Engineers, 67 (2013) 68-71

A. Serov, et al., Advanced Energy Materials, 4 (2014) DOI: 10.1002/aenm.201301735