Platinum Group Metal-Free Direct Hydrazine Alkaline Membrane Fuel Cells for Automotive Applications
Authors: Atanassov Plamen , Serov Alexey , Artyushkova Kateryna , Matanovic Ivana , Sakamoto Tomokazu , Kishi Hirofumi , Asazawa Koichiro , Tanaka Hirohisa , Yamaguchi Susumu 
Published: February 2020

Over the last decade, research team from Daihatsu Motor Co. has introduced the concept of anion-exchange membrane fuel cell with liquid fuels for automotive applications. Switching from more commonly used proton exchange membranes (PEM) to alkaline, anion-exchange membranes (AEM) has several kinetic and materials benefits and opens the possibility of developing Fuel Cell Vehicle (FCV) entirely based on Platinum Group Metal-free (PGM-free) technology.1 This has been made possible by the adoption of hydrazine hydrate as a liquid fuel, which affords technology deployment while using the existing fuel distribution and dispensing technology. This concept pioneered the use of AEM Fuel Cells for automotive applications worldwide and created the state of the art AEM Fuel Cell stack technology2.

UNM team has been a long-term participant in the multi-institutional program led by Daihatsu Motor Co., focusing on the development of Oxygen Reduction Reaction (ORR) catalysts and selective hydrazine electro-oxidation catalysts used as cathode and anode material in AEM membrane electrode assembly (MEA)3. The theoretical electromotive force of such direct hydrazine fuel cell is 1.56V and hydrazine hydrate as a fuel can be oxidized by number of catalysts using exclusively Earth-abundant metals, such as NiZn unsupported catalysts developed at UNM4. This line of research has been continued in NiLa5 and supported Ni-allowed catalysts6 aiming not only high activity, but also extreme selectivity of hydrazine electro-oxidation to N2 and H2O only. Elucidating the mechanism of hydrazine oxidation included EXAFS/XANES study7 and let to formulating of the hypothesis of the reaction mechanism8 that involves surface hydroxyl groups on Ni as critical participants in the hydrazine dehydrogenation reaction step (see Fig. 1). The role of the nickel, secondary metal phase and the carbon support in the activity and durability of Ni-allow catalysts will be discussed.

ORR catalysts developed under this program included initially Co-polypyrrole materials system9. The mechanism of oxygen reduction and the structure-to-property relationships have been described in a recent EXAFS/XANES study10. This paper will present a family of Fe-Nitrogen-Carbon ORR catalysts that were used in the successful stack development and Daihatsu FCV prototype tests. This new generation Fe-containing PGM-free catalysts are synthesized by UNM Sacrificial Support Method (SSM) a type of for templated synthesis of hierarchically structured electrocatalysts materials.11-13 In this method the catalysts precursors are being absorbed on, impregnated within or mechanically mixed with the support (usually mono-dispersed or meso-structured structured silica), thermally processed (pyrolyzed) and then the silica support is removed by etching (in KOH or HF) to live the open frame structure of a “self-supported” material that consists of the catalysts only. 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. 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.

The high activity of these Fe-N-C cathode catalysts was confirmed in both RDE and MEA tests. As synthesized materials were extensively studied by XPS, SEM, TEM, BET and other methods, in order to elucidate the structure-to-properties correlations. This paper describes a new class of templated, self-supported PGM-free catalysts derived by sacrificial support method (SSM) and their activity in free alkaline electrolyte (KOH) and in anion-exchange MEA.

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Figure 1