Limiting processes in anion-exchange membrane fuel cells
Time: Fri 2022-12-16 14.00
Location: D2, Lindstedtsvägen 5, Stockholm
Subject area: Chemical Engineering
Doctoral student: Henrik Grimler , Tillämpad elektrokemi, Applied Electrochemistry
Opponent: Professor Marc Secanell, University of Alberta
Supervisor: Professor Göran Lindbergh, Tillämpad elektrokemi; Professor Rakel Wreland Lindström, Tillämpad elektrokemi; Professor Carina Lagergren, Tillämpad elektrokemi
Fuel cells allow for converting chemical energy stored in hydrogen into electrical energy, with only heat and water as by-products. In a sustainable energy society, hydrogen may play an important role due to its ability to act both as an energy carrier and as a valuable chemical in the process industry. The main remaining obstacles for widely available commercial fuel cells are durability and cost. One way to potentially decrease the cost is to change the fuel cell environment to an alternative chemistry by replacing the proton-exchange membrane (PEM) with an anion-exchange membrane (AEM). This thesis studies the anode reaction, the cathode reaction and water transport in an anion-exchange membrane fuel cell (AEMFC), to investigate where its performance limitations lies in the system. Electrochemical characterisation techniques together with physics-based modelling have been utilised.
The results from the study of the anode, shows that the hydrogen reaction proceeds through the Tafel-Volmer pathway, with the Tafel step starting to limit the reaction as the anode overpotential increases. Combining the anode model with a Butler-Volmer expression for the cathode reaction made it possible to model a H2:O2 fuel cell. Comparing the losses from the different processes in the fuel cell shows that the cathode is still the main contributor, but that the anode contribution cannot be neglected when predicting the fuel cell performance. Low ionic conductivity in the electrode was also identified as responsible for part of the overall resistances, leading to uneven current distribution in the catalyst layers and bad utilisation of the catalytic material.
Investigating the water transport properties of AEMs showed that not only electroosmotic drag and diffusion, but also an absorption/desorption step between gas phase and membrane phase, are necessary to get a model that can explain the experimental observations. The choice of gas diffusion layers (GDLs) used on the anode and cathode was found to be of similar importance on the water transport as doubling the membrane thickness, showing that not only the membrane is important for water transport. Under most realistic conditions, the risk of local dry-out in a cell was found to be low, as water readily diffuses from the high humidity side of the membrane to the low humidity side.