title: A new concept for nanocomposite electrocatalysts: studying the oxygen reduction/evolution reaction and the formic acid oxidation reaction on Pt-based systems
Creators: name: Du, Jia
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 540 Chemistry
subject: 570 Life sciences; biology

description: Fuel cells are an integral part of the renewable energy concept which involves hydrogen or liquid fuels, e.g., formic acid, as energy carrier. In this thesis, a nanocomposite catalyst concept is developed to prepare fuel cell catalysts. Nanocomposite catalysts are prepared by separately depositing different (two types in this thesis) monometallic nanoparticles onto a support material (carbon black in the thesis). With respect to conventional alloys, the nanocomposites allow to individually control the physical properties of different metal nanoparticles, i.e., particle size, catalyst loading, etc., which benefits from the facile and straightforward preparation method.
The nanocomposite concept is initially introduced to prepare bifunctional Pt-Ir(IrO2)/C (with various Ir contents) catalysts. PtIry/C alloy counterparts serve as benchmark. The catalysts are tested in degradation tests, as well as for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). The measurement results show that bifunctional nanocomposite catalysts present both challenges and potentials in comparison with PtIry/C alloys, in terms of OER and ORR performances. The observed stability improvement for Pt is however at the cost of Ir dissolution, in all the studied bifunctional nanocomposites.
Ir recycling is an important topic associated not only with the use of Ir in fuel cell catalysts to improve the overall catalyst stability. It becomes also essential for the re-use of electrodes from electrolysis cells, which typically contain Ir or IrO2 at the anode. The observed instability of Ir in the nanocomposite catalysts triggered further investigations towards potential electrochemical Ir recycling schemes. It is shown that by adjusting the applied test protocols a nearly 100% selective Ir dissolution can be achieved. It is further demonstrated that selective Ir dissolution can be achieved in a simple current control setting, which enables simplified electrochemical two electrode setups.
The next part of this thesis extends the nanocomposite concept to prepare potential Pt-based ORR catalysts for proton exchange membrane fuel cells (PEMFCs). Compared with the standard work implemented with rotating disk electrode (RDE), I evaluate the ORR performance under both low (in RDE) and high (in gas diffusion electrode (GDE) setups) reactant mass transport conditions. The results demonstrate the potential for Pt-Au/C nanocomposites as fuel cell ORR catalyst, as both Pt and Au are stabilized in the nanocomposite and at the same time an improvement of the ORR activity is observed. This is in contrast to Pt-IrO2/C nanocomposites, displaying only limited performance.
Last but not least, by adjusting the added volume of Pt and Au colloidal stock solution, Pt-Au/C nanocomposites with various Au loadings are prepared. The analysis of the formic acid oxidation reaction (FAOR) is combined with detailed characterization by pair distribution function (PDF), scanning transmission electron microscopy-Energy dispersive X-ray spectroscopy (STEM-EDX), in situ small-angle X-ray scattering (SAXS), etc., to reveal that nanocomposites mixed with Pt and Au monometallic particles can be used to dynamically prepare in situ surface alloys with favorable FAOR performance. These studies are discussed with respect to the conventional preparation of alloy catalysts, highlighting the advantage of the nanocomposite concept to prepare in situ surface alloy nanoparticles.