Electrocatalyst materials for low-temperature hydrogen fuel cells

Licentiatavhandling, 2021

Fuel cells have emerged as one alternative to satisfy the need of energy systems with net-zero emissions. Although fuel cells date back to the 1800s, it is only during the last decades that their research has boosted to ease their commercialization. This growing interest in fuel cells implementation goes hand in hand with the decrease in green H2 production cost, which makes fuel cells a cornerstone in the promising energy system based on hydrogen with water as the only by-product. For this, it is crucial that the transport sector shifts towards effective, inexpensive and carbon-free fuel alternatives which is possible with hydrogen owing to its high energy density. A broad implementation of fuel cells is, however, impeded by the high cost of fuel cell systems, which is in turn greatly attributed to the Pt-based catalyst layer currently used in low-temperature hydrogen fuel cells. As Pt is a scarce resource and an expensive material, development of new efficient, stable and inexpensive electrocatalysts is essential for large-scale fuel cells implementation. Although many strategies have been explored to reduce the amount of Pt without compromising the power output and lifetime, electrocatalyst development is currently hindered by the lack of mechanistic understanding. In order to gain a better understanding of the mechanisms behind the electrochemical reactions in proton exchange membrane fuel cells (PEMFC) and anion exchange membrane fuel cells (AEMFC), this thesis delves into both the fabrication and the characterization of electrocatalysts. A versatile platform was built to study model system catalysts with the aim to test electrocatalytic materials and establish reliable comparisons. In this way, the performance of model system catalysts can be rationalized in terms of geometric structure and electronic structure. Pt-rare earth metals thin-film alloys were studied with respect to their activity towards the oxygen reduction reaction (ORR) in PEMFCs. Similarly, NiAg nanoparticles in different morphologies were studied for the ORR in alkaline conditions for AEMFCs. Hydrogen oxidation reaction (HOR) and ORR activity of PdNi annealed thin films were investigated to pinpoint the mechanism behind the increased activity for the alloy in both reactions. This provides insights to the fundamental principles that lead to a good catalyst efficiency and effectiveness. The presented work takes a step in tailoring new electrocatalytic materials that could eventually outperform Pt in both activity and stability while reducing the total cost.