N. Kimiaie

Interaction between the diffusion layer and the flow field of polymer electrolyte fuel cells—experiments and simulation studies

The flow distribution in fuel cells has an important influence on both the power density and efficiency of fuel cell systems. In order to effectively utilize the area, flow distribution should be as homogeneous as possible. In addition, pressure losses should be minimized with regard to the power demand of auxiliary components as pumps and compressors. In polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) the flow field is in direct contact with the diffusion layer. The main task of the diffusion layer is to distribute the reactants from the flow field towards the catalyst layer. To prevent diffusion overvoltages, the diffusion layer is in general highly porous and provides high fluxes of the reactants. Consequently, the flow distribution in the flow field can be superpositioned by a flow in the diffusion layer. In this paper, we discuss the interaction between the diffusion layer and the flow field. Experimentally, we characterized different diffusion layers with regard to their diffusion properties as well as different flow fields. Additional simulation studies help to understand the processes and to determine suitable combinations of flow fields and diffusion layers.

Results of a 20 000 h lifetime test of a 7 kW direct methanol fuel cell (DMFC) hybrid system – degradation of the DMFC stack and the energy storage

With a proven life of 20 000 operation hours in a lifetime test with a realistic dynamic load profile, the direct methanol fuel cell (DMFC) system V3.3-2 represents a milestone for the commercialization of DMFC systems. The hybrid DMFC system V3.3-2 comprises an in active serial connected 1.0 kW DMFC system and a 45 A h lithium-ion high-power battery pack. This hybrid system replaces the battery tray of a class 3 forklift truck and can supply a peak load of 7 kW. The advantages of this energy-supply module compared to conventional lead-acid batteries are its higher range (24 h use with a 20 L methanol canister instead of 8 h with battery recharging) and its higher availability (a few minutes are required to exchange methanol canisters instead of hours to recharge the battery). However, in order to ensure that use of the DMFC system V3.3-2 is economic, the DMFC stack must have a durability of at least 10 000 h. This publication describes the degradation behavior of the DMFC stack and of the energy storage system during a lifetime test of the DMFC system V3.3-2 with a dynamic load profile of a material handling vehicle. In the first-ever test worldwide lasting 25 600 h, the hybrid system is successfully operated for 20 000 h. Operation for 20 000 hours is equivalent to the life cycle of a vehicle in the material handling sector. The development and validation of the DMFC system V3.3-2 shows that this system is suitable for use in a forklift truck and that it not only meets the economic system requirements for commercialization but goes well beyond them.

The Long Way of Achieving a Durability of 20,000 h in a DMFC System

Direct Methanol Fuel Cells (DMFC) are an attractive power source for applications in the low kW-range like pallet trucks or uninterruptable power supplies. A significant problem during the past years, however, was the limited durability of DMFC systems. While single cells could be operated for thousands of hours, DMFC systems degraded significantly often within less than 1,000 hours.In an evolution of six generations of DMFC systems in the kW power range over the past decade, we identified the main reasons for degradation. Causes for fast degradation had to be removed first in order to identify what leads to slower degradation over several hundreds or thousands of hours. Interactions of cells and system components also had to be considered.As a result, the operating conditions of all cells must be carefully controlled by suitable operating algorithms and reproducible manufacturing technologies, in order to avoid high potentials on the anode, which would lead to ruthenium corrosion and subsequent poisoning of the cathode catalyst. All components of the stack and the peripheral system must be corrosion-proof and free from contaminants that might leach into the membranes. Finally, a DMFC system for a pallet truck was operated in a realistic load cycle for 20,000 hours.