Seeking new materials for structural components in fuel cells

Raúl Gago Fernández (Surface Engineering and Advanced Coatings Group, Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas)

There is no need to stress the actual urgency for the development of new power sources. Among the different approaches, fuel cells are already known from the 19th century but they are becoming more and more attractive. Basically, fuel cells are devices that generate electricity by a chemical reaction between protons (positively charged hydrogen ions) and oxygen or another oxidizing agent. The reactants need to be supplied continuously to sustain the reaction but, interestingly, in this way fuel cells can produce electricity steadily. There are many different types of fuel cells but they all comprise an electrolyte to allow the proton current and the electrodes (cathode and anode) for charge collection and added catalytic functionality. Hence, fuel cells are mainly catalogued according to their material components and, obviously, this factor together with the corresponding chemical reaction defines the operation temperature (ranging from tens up to hundreds of ºC).

Proton exchange membrane (also referred as polymer electrolyte membrane) fuel cells (PEMFCs) are very attractive due to their mild operation conditions (50-100ºC) (for example, in contrast to solid-oxide types with operating temperatures around 800ºC). Therefore, they are good candidates for both stationary and portable applications. Obviously, the rapid development of electric cars is one of the mayor workhorses for their growing market. Bipolar plates are key components of PEMFCs, which comprise a significant amount of cost, volume, and weight in the typical fuel cell stacks. Different materials have been investigated as bipolar plates (typically, graphite) but stainless steel (SS) is gaining attention due to its relatively high strength and machinability, good conductivity and low corrosion rate. However, insufficient corrosion resistance and surface conductivity are two main issues that plague large-scale application of SS bipolar plates in PEMFC.

The Surface Engineering and Advanced Coatings Group at the Insitute of Materials Science of Madrid (ICMM) belonging to the Consejo Superior de Investigaciones Científicas (CSIC) has been exploring the application of tantalum (Ta) based coatings for the improvement of the corrosion resistance of SS as bipolar plates for PEMFCs. The choice of Ta is straightforward since its corrosion resistance is higher than any other metallic material used as component of chemical plant equipment and can be compared to that of glass, graphite and fluoropolymers (such as teflon). However, there are many technological issues to be addressed such as the coating adhesion, compactness or architecture for final practical usage. In this study, both metal (Ta) and nitride (TaN) coatings produced by magnetron sputtering have been examined in the form of monolithic or alternating (multilayer coatings) layers. An example of the complex structure of such multilayer coatings is shown in figure (a). Here, the relevance of surface engineering has been clearly evidenced since plasma pre-cleaning of the substrate or the use of buffer (TaN) layers has been reported as critical issues for optimum coating adhesion and posterior growth of the desired crystallographic Ta phases. In general, all the coated SS substrates have shown a drastic increase of the corrosion resistance with respect of bare SS together with a remarkable decrease in the interfacial contact resistance (ICR) as shown in figure (b). Moreover, the best performance has been found for the production of certain TaN phases due to their compactness in thin film form. Finally, multilayer coatings are very promising architectures since they exploit the synergy given by the coexisting properties of both Ta and TaN layers.

Figure: (a) Cross-section image showing the alternating materials in a complex multilayer (ML) structure. (b) Reduction of ICR in coated SS with respect to bare substrates before and after exposure to potentiostatic (PS) polarization.

This study has been carried out within the project “Diseño Multiescala de Materiales Avanzados, DIMMAT” financed by Comunidad Autónoma de Madrid (CAM) in collaboration with the Department of Mining and Metallurgical Engineering at Amirkabir University of Technology in Tehran (Iran). The obtained results have been the core of the PhD thesis of Dr. Mostafa Alishahi. For the electrochemical tests, the participation of Instituto de Cerámica y Vidrio (ICV) from CSIC has also been crucial. The results have been recently published in Journal of Power Sources (http://dx.doi.org/10.1016/j.jpowsour.2016.04.133) and RSC Advances (http://dx.doi.org/10.1039/c6ra17869c) where the interested reader can find more detailed information.

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