Development of a Hydrogen Fuel Cell Stack for Application in the Primary Propulsion System for Airplane Operations

The project "H2Sky" is developing a hydrogen fuel cell stack that will eventually enable the primary propulsion system of passenger aircraft to use hydrogen. The Fraunhofer ISE is developing particularly high-performance membrane-electrode assemblies (MEAs) and transferring them from the laboratory scale to full size. The focus is on materials that work efficiently and durable at temperatures above 90 °C and low humidity. This project lays a foundational basis for climate-friendly aviation propulsion and a later series production.

Initial Situation

Aviation must significantly increase its contribution to reducing global greenhouse gas emissions. Battery-powered concepts are limited in range and weight, while current PEM fuel cells are mainly designed for automotive conditions: moderate temperature, humidity, and with lower lifespan requirements. Under typical airplane conditions at operating temperatures ≥ 95 °C, low humidity, and high mechanical loads, established membrane-electrode assemblies experience significant performance losses and do not meet the required gravimetric energy density nor the lifespan of over 25,000 hours.

Objective

H2Sky aims to develop a hydrogen fuel cell stack for the primary propulsion system of passenger aircraft with a power weight of around 4 kW/kg, high efficiency, and more than 25,000 hours of operation at cell temperatures above 95 °C. The Fraunhofer ISE is optimizing the performance-determining component, the membrane-electrode assembly: the catalyst and ionomer systems are optimized for high temperatures and low humidity, and the potential of through-plane directionally graded catalyst layers is investigated. In this context, performance and degradation protocols for the developed cells under realistic operating conditions are developed.

Approach

In the H2Sky project, the Fraunhofer ISE developed membrane-electrode assemblies specifically designed for operation under aviation conditions and scaled them from the laboratory to full-size. Catalyst and ionomer systems were systematically varied, solvents, drying, and decal transfer optimized, and through-plane gradients in the catalyst layer were investigated. These were validated with performance curves that reference stack inlet, middle, and outlet conditions. At the same time, scalable production concepts for these optimized MEAs in full size were developed. New individual cell and stack testbenches, measurement setups for parallel characterization of up to four cells were developed and put into operation, as well as extended in-situ characterization methods implemented.

Results

The Fraunhofer ISE has developed catalyst layers and MEAs that enable higher performances under aviation-typical conditions. Comprehensive studies on the optimal ionomer content for various catalyst systems, solvents, drying parameters, and decal transfer provided a detailed data basis for optimizing the catalyst layer under conditions from the stack inlet to the outlet. An optimized cathode catalyst layer with 0.35 mg _Ptcm^-2 achieves voltages at 95 °C and reduced humidity that are on the level of a commercial reference with a higher platinum loading and thus offers further potential for performance improvement. Additionally, a new high-temperature individual cell test bench up to 110 °C, a 20-kW stack test stand, and parallel setups for four cells were developed and put into operation.

The “H2Sky” project is funded by the Federal Ministry for Digital and Transport.