REPOSITORY > RESULTS

Abstract

Electrochemical energy conversion devices such as fuel cells (FCs) and water electrolyzers (WE) have attracted attention from the scientific community for their unique ability to store electrical energy in the form of hydrogen and then generate electricity by the reverse process. Unfortunately, those devices rely on a significant amount of precious platinum group metals (PGMs) – rare and expensive raw materials – to catalyze the reactions involved, i.e. platinum-based catalysts for oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) in proton exchange membrane (PEM) FCs and iridium for oxygen evolution reaction (OER) on one side of WE and Pt for hydrogen evolution reaction (HER) on the other side. The high prices and scarcity of materials are a bottleneck for the widespread application of those technologies and slowing down the transition to clean energy. Therefore, reducing the cost and consumption of PGMs while keeping the same performances (activity and stability) is of primordial importance. Several strategies have been commonly used to optimize the utilization of PGMs, like the deposition of precious metals as nanoparticles on high surface area support or alloying with less noble metals. In the case of PEMFC (ORR and HOR), these methods have been proven effective and Pt-M (M: Co, Cu, Ni or Fe) supported on carbon black is becoming the state-of-the-art catalyst. Nonetheless, no suitable support has been widely accepted as the state-of-the-art support for OER catalysts in acidic media as carbon cannot be used in the highly oxidative environment of OER where it oxidizes to CO2. Moreover, alloying Ir with some less-noble metals leads to heavy leaching during OER and poisoning in the WE. Therefore, a conductive, high surface area and stable support still needs to be found. Another possibility to decrease the overall price of these technologies is by combining them into one unitized regenerative fuel cell (URFC) able to perform as FC when electric energy is needed and as WE when a surplus of energy is available. In this device, one catalyst should be able to perform both HOR and HER, i.e. an ultra-low amount of Pt supported on carbon. On the other side, for ORR and OER, the current state-of-the-art bifunctional catalyst is a physical mixture of unsupported Pt and Ir powders. The former one is currently not suitable for large-scale production as it still requires significant amounts of PGMs. Nonetheless, Pt/Ir nanoparticles deposited on appropriate support, able to withstand OER conditions, would ideally allow a decrease in the price of the technology while keeping the same performances. Hereby, we investigate possible support for OER and bifunctional catalysts. The material chosen as potential support was titanium oxynitride (TiONx). TiO2 is stable under OER conditions but not conductive while TiN is conductive but not as stable. Therefore, TiONx was investigated as it could exhibit the best properties of both. First, this support was studied in combination with Ir nanoparticles for OER. Modification by adding a carbon template was used to additionally increase the surface area of the catalyst. Finally, different metal nanoparticles able to catalyze both ORR and OER catalysts were deposited on our carbon-modified TiONx and the performance of this bifunctional nanocomposite was investigated. The choice of the compounds for ORR catalyst was made after a careful comparison of different Pt-alloys while Ir + Ru was used for OER.