Andreas Glüsen

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.

Slot-Die Coating: A New Preparation Method for Direct Methanol Fuel Cells Catalyst Layers

Manufacturing of catalyst layers is one of the key processing steps in making membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs). The catalyst ink, which usually contains catalysts, ionomers, solvents, and additives, is generally applied to the substrate by a wet-coating technique. Established coating techniques which are used for manufacturing catalyst layers for fuel cells are knife-coating, screen-printing, and spraying. Slot-die coating is also an established coating technique, but not currently used for making fuel cell electrodes. For each coating technique the properties of the catalyst layer will depend on the properties of the coating technique and the properties of the substrate. Also each coating technique requires the catalyst ink to be adjusted to its specific rheological requirements. In this study, slot-die coating is developed as a new method for the continuous manufacture of catalyst layers for direct methanol fuel cells. The rheological demands for a homogeneous layer thickness are studied with model inks and a suitable catalyst ink is prepared. With this ink, decal electrodes are fabricated and CCM-type MEAs are made. Fuel cell tests show that the performance of the slot-die coated electrodes is comparable to electrodes made by knife-coating.