UC Merced

Currently, anion exchange membrane fuel cells (AEMFC) are gaining strong interests from researchers because it provides great potential for applying non-precious metal in comparison to the proton exchange membrane fuel cells (PEMFC). Compared to PEMFC, AEMFC has faster reaction kinetics for the cathodic oxygen reduction reaction (ORR), which enables the utilization of non-noble catalyst. In addition, the less corrosive alkaline environment allows for low-cost stainless-steel bipolar plate. However, there are still many technical barriers related to material and transport in an AEMFC. In an AEMFC, water is generated during anodic hydrogen oxidation reaction (HOR) and is consumed during the cathodic ORR. Hydroxide ions are formed during ORR and carry water molecules from the cathode to the anode via electro-osmotic drag. During high current density operation, anode flooding can hinder hydrogen transport. Similarly, it may cause cathode drying leading to high ohmic loss due to reduction in membrane and ionomer conductivity. Therefore, optimizing water management in an AEMFC is essential for achieving good cell performance. Based on the HOR and ORR, the transport limitation may occur due to insufficient supply of one or a combination of hydrogen, oxygen, hydroxide ion, and water. This project, as discussed in Chapter 3, aims to examine and identify sources of transport limitations and quantify their effect under various operating conditions. Two types of flow fields are used to study the cell performance and water behavior in the cell. The membrane electrode assembly is made of alkaline membrane with high hydroxide ion conductivity and gas diffusion electrodes are fabricated with alkaline ionomer. Two gas diffusion electrodes (GDE) are prepared with solid (powder) and liquid (dispersion) ionomer binder, GDE-1 and GDE-2 respectively. The dispersion ionomer binder makes a very hydrophobic electrode that makes ion exchange difficult and results in poor performance although the membrane is sufficiently hydrated. Adding ethanol to ion exchange solution helps to improve exchange and consequently performance. It is found that higher RH is helpful to achieve better ohmic performance, but lower RH is preferred to achieve better concentration or high current density performance. So, it is obvious that the AEMFC performance is very sensitive to RH, especially on the anode side. Hydrogen and oxygen mass transport limitations are both observed from the performance measurement. However, the effect of reducing hydrogen concentration on AEMFC performance is greater than that caused by reducing oxygen concentration. Neutron experiments show that cathode gets more water at the beginning of the cell operation, out of which most water is then transported to the anode through electroosmotic drag, the balance shifts towards anode and anode becomes flooded at some point which causes the cell performance to reduce. All these results provide valuable insights on the AEMFC water management strategies for improving cell performance and making a significant impact towards the development of AEMFC technology.

Hydrogen-powered proton exchange membrane fuel cells (PEMFC) have great potential to replace the traditional internal combustion engines due to its inherent advantages of zero greenhouse gas emissions, better fuel efficiency, quick startup, silent operation, less required maintenance, etc. In a PEMFC, oxygen transport is a critical performance limiting factor because of the sluggish oxygen reduction reaction kinetics. Limiting current method is a well-established in-situ diagnostic tool to measure the oxygen transport resistance in a PEMFC. To obtain accurate oxygen transport resistances, a few key assumptions need to be made including: (1) the effect of temperature gradient in the diffusion media is negligible, (2) no convective flow in the porous media, (3) oxygen is diluted in a gas mixture of nitrogen and water vapor, (4) the total oxygen transport resistance combines gas diffusion layer, microporous layer, and catalyst layer, (5) the effect of membrane thickness has negligible effect, and (6) the anode side does not affect the measurement results due to fast hydrogen oxidation reaction. In this project, as discussed in Chapter 4, we perform a systematic study of the effect of membrane thickness and operating conditions on obtaining robust and reliable limiting current measurements. Standard Nafion membranes of two different thicknesses (25 and 85 µm) are tested with Toray 060 and Freudenberg H23C8 diffusion media. In addition, we further study the interaction between membrane thickness and cell temperature, asymmetric pressure and relative humidity and their effects on limiting current results. Our results show that membrane thickness and relative humidity are critical factors in obtaining reliable oxygen transport resistance due to their effect on the overall water balance in the cell.

Fuel cell durability is a key limitation for its commercialization in heavy-duty applications. Cathode catalyst layers need to be designed to demonstrate not only high oxygen reduction reaction (ORR) mass activity and cell performance but also significanlt durability during long-term operation. Ion-conducting polymers, or ionomers, play a crucial role in determining both performance and durability in a PEMFC. A highly oxygen permeable ionomer (HOPI), developed by Chemours, is explored in the project explained in Chapter 5. HOPI is known for higher ORR mass activity and lower oxygen transport resistance through its film covering the active catalyst sites. A fuel cell utilizing the HOPI ionomer in the cathode catalyst layer is compared with one using the conventional Nafion D2020 ionomer. Durability experiments demonstrate the superior durability of the HOPI ionomer. Measurements of cathode proton resistance, oxygen transport resistance, and cell performance before and after accelerated stress tests reveal the superiority of the HOPI cell in all aspects compared to the D2020 ionomer. According to the literature, the improved cell durability and performance attributed to the HOPI ionomer are due to its higher ORR mass activity, enhanced oxygen transport through the ionomer film, and higher oxygen solubility.