title: Electrocatalysts for Fuel Cells and Water Electrolyzers: From Synthesis to Performance Evaluation at Realistic Reaction Conditions
Creators: name: Schröder, Johanna
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 540 Chemistry
subject: 570 Life sciences; biology

description: Fuel cells and electrolyzers are key aspects for the storage and conversion of renewable energy. Due to the kinetic hindered oxygen reactions at the cathode or anode of fuel cells and electrolyzers, respectively, the improvement of those Pt and Ir based nanoparticle (NP) based catalysts is crucial for their application. In a first step an understanding of the NP synthesis including a size control is necessary, then the investigation of the activity and stability of the catalysts. Therefore, this dissertation deals with “Electrocatalysts for Fuel Cells and Water Electrolyzers: From Synthesis to Performance Evaluation at Realistic Reaction Conditions”.

The polyol process enables the synthesis of “surfactant-free” NPs with tailored catalyst properties like the control of the particle size. A mechanistic investigation of the Pt NP synthesis in the polyol process can be achieved by slowing down the reduction process inducing the particle formation process by visible light. The particle size control by the OH-/Pt ratio as described previously in the polyol process can be confirmed in the visible-light-induced reduction. By systematically reducing different precursors in presence of the bases NaOH or Na(acac) it can be shown that the size of Pt NPs can be controlled additionally by the acac-/Pt ratio reaching a comparable size control at the respective OH-/Pt ratio. Besides the control by the anion/Pt ratio the character of the precursor seems to determine the particle size.

The gas diffusion electrode (GDE) setup is presented as a straightforward testing device enabling the performance of accelerated stress tests (ASTs) of Pt/C catalysts under realistic conditions with respect to catalyst loading and temperature close to the conditions applied in fuel cells. The coupling to ex situ small angle X-ray scattering (SAXS) enables degradation studies comparing the change of the particle size and the electrochemically active surface area (ECSA) during the ASTs. Applying ASTs on catalysts with different initial particle sizes revealed similar end-of-treatment sizes. For a deeper understanding of the degradation mechanism, operando SAXS is performed during the application of ASTs. One of the main challenges in coupling SAXS to ASTs is the recording of a suitable background. In this work, the background measurement in transmission configuration is achieved by the vertical movement of the cell of the gas diffusion layer (GDL) experiencing the same electrochemical treatment. Thus, a bimodal catalyst consisting of two size populations, so that the electrochemical Ostwald ripening should be induced, could be studied. The same bimodal catalyst was investigated at a synchrotron in an operando SAXS study in grazing incidence configuration enabling the establishment of a depth profile of the degradation within the catalyst layer. Surprisingly, both studies are not clearly evidencing the expected electrochemical Ostwald ripening.

“Surfactant-free” Ir NPs were synthesized with the colloidal so-called Co4Cat process performed in low boiling point solvents. These Ir NPs can be used as potential electrolyzer catalysts. Efforts to control the obtained particle size were performed by adjusting different synthesis parameters such as the temperature or water content.

Furthermore, the GDE setup was adjusted to obtain a straightforward performance testing setup for electrolyzer catalysts reaching comparable results as compared to the rotating disk electrode (RDE) method, which is currently the standard testing device in basic research. It is shown that the used carbon based GDL is stable enough to perform performance tests. In addition, a straightforward way to prepare catalyst films on top of titanium based porous transport layers (PTLs) typically used in electrolyzers was developed. Using a simple hot-pressing of a Nafion membrane to the catalyst layer leads to comparable activity results as generated using a GDL. Kinetic studies assuming Arrhenius behavior enabled the determination of the electrochemical activation energy and the estimation of the exchange current density.