Electrochemical evaluation of thin-layer catalysts in polymer electrolyte fuel cells
Time: Fri 2024-11-29 09.00
Location: F3 (Flodis), Lindstedtsvägen 26 & 28, Stockholm
Video link: https://kth-se.zoom.us/webinar/register/WN_UwN3VxAFRruM-foeenvOtA
Language: English
Subject area: Chemical Engineering
Doctoral student: Eva Marra , Tillämpad elektrokemi
Opponent: Professor Kaido Tammeveski, University of Tartu, Estland
Supervisor: Professor Carina Lagergren, Tillämpad elektrokemi; Professor Göran Lindbergh, Tillämpad elektrokemi; Professor Rakel Wreland Lindström, Tillämpad elektrokemi
QC 20241107
Abstract
Fuel cells emerge as a zero-emission transport option by converting chemical energy into usable electricity, with heat and water as the only by-products. In the automotive sector, acidic proton exchange membrane fuel cells (PEMFCs) are the most viable option, but their high price is still limiting the possibility to be competitive with other techniques. The anion exchange membrane fuel cells (AEMFCs) in alkaline media allow the use of less expensive materials, but these materials are not enough developed and there is a lack of established measuring protocols and stable reference materials. By focusing on electrocatalysis, this thesis describes how different experimental approaches can be utilized to evaluate the kinetics of thin-layer catalysts in fuel cell conditions, mainly in alkaline media.
By using a special membrane electrode assembly (double-MEA setup), the kinetics of oxygen reduction reaction (ORR) onto a platinum (Pt) layer in alkaline media was studied without interferences caused by mass transport and gas crossover. The ORR kinetics on a silver (Ag) layer was also evaluated and compared with Pt. In acidic media the ORR stability of a platinum yttrium (Pt3Y) layer was investigated via an accelerated stress test (AST). To test the fast kinetics of hydrogen oxidation (HOR) and hydrogen evolution (HER) onto a Pt layer without mass transport limitations, an alkaline hydrogen-cell was utilized.
The results showed that the double-MEA setup is an ideal system for kinetics studies. On Pt, it restricted hydrogen crossover which resulted in reproducible specific ORR activities, comparable to those of good porous electrodes found in the field. Compared with Pt, Ag displayed a later onset potential for ORR due to a mixed potential caused by Ag oxidation. The Ag layer performed better than Pt below 0.5 V while at higher voltages its stability was compromised due to the formation of Ag-oxides. With respect to Pt3Y, after AST, the ORR activities decreased for all voltages, being very close to those for pure Pt before AST. The loss of ORR activity on Pt3Y was due to an increase in the thickness of the Pt overlayer which induced a relaxation of the Pt overlayer, decreasing the compressive strain effect. The HER activities on Pt in acidic PEM conditions were between two and three orders of magnitude higher than in alkaline AEM media. For HOR this difference was reduced to around one order of magnitude. By correlating the experimental results to different mechanisms, the HOR/HER kinetics on Pt in acidic media can be associated with the Tafel−Volmer mechanism, with the Volmer reaction as rate determining step. In the case of Pt in alkaline media, the HOR/HER kinetics can be related to the HeyrovskyVolmer mechanism, with Volmer reaction as the rate determining step.