Ir/TiC/Pt Vs. Pt-Ir/TiC in Role of Magnetron Sputtered Thin-Film Catalyst for Anode of PEM Reversible Fuel Cell

Abstract

Ever increasing amount of installed power capacity from the volatile renewable sources (e.g. wind and solar) puts high demand on a reliable and scalable system, capable of storing the energy during the overproduction and releasing it in times of the deficit. Utilization of hydrogen as an energy vector proves to be an interesting option in this context. Overproduced electrical energy can be electrochemically converted to hydrogen by means of the water electrolyzers (WEs). Generated hydrogen can be either injected into the existing natural gas pipelines or stored for later conversion back to electricity via the fuel cells (FCs). The most suitable WEs and FCs for this task are arguably the proton exchange membrane WEs and FCs (PEM-WEs, PEM-FCs). Major factor currently hindering wider commercialization of these technologies is the high price and scarcity of noble metals, namely Pt and Ir which serve the role of a catalyst within the individual cells. One way of lowering the loading of noble metals is by using thin-film techniques for their deposition onto the high-surface conductive nanoparticles. Another approach, which is convenient in applications where the complete cycle of electricity->H2->electricity takes place, is merging the PEM-WEs and PEM-FCs into one bi-functional system – the PEM unitized reversible fuel cell (PEM-URFC). Such unification may potentially lead to significant cost savings not only on the catalyst but on the overall hardware as well. In this work, we present and discuss unorthodoxly prepared bi-functional thin-film low-loading Pt-Ir catalysts for the anode side of PEM-URFC (i.e. the oxygen evolution/hydrogen oxidation side). Two different geometries of the catalyst coated membranes (CCM) were studied; the CCM with magnetron co-sputtered Pt-Ir catalyst on top of the TiC-based support sublayer and the CCM with sandwich-like design where Pt is magnetron sputtered on the bottom and Ir on the top side of the TiC-based support sublayer. Although the loading of noble metals within both tested CCMs and the corresponding membrane electrode assemblies (MEAs) was similar, the obtained in-cell efficiency varied significantly. The sandwich-like CCM performed better in both electrolyzer and fuel cell regimes. Combination of data obtained from the in-cell performance measurements, X-ray photoelectron spectroscopy (XPS) and electrochemical atomic force microscopy (EC-AFM) helped us to identify the reasons why sandwich-like CCM performed better in both operational regimes. We concluded that thorough oxidation of Ir to IrO2, which did not occur when Ir was within Pt-Ir alloy after co-sputtering, was responsible for higher efficiency of the sandwich-like CCM in PEM-WE regime. On the other hand, the less prominent increase of efficiency in PEM-FC mode seemed to be due to the swapping of Pt thin film from position "top" (i.e. further from the PEM) to position "bottom" (i.e. closer to the PEM) which allowed for better ionic conductivity. Considering relatively high efficiencies obtained by the sandwich-like MEA with just a fraction of conventional noble metal loading, we believe that continuing in the research of thin-film, segmented catalysts is the step in the right direction.