Integrating a PEMFC Stack into an Aerobatic Plane and Boosting its Performance by Oxygen Gain

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 1/9 A1701 Integrating a PEMFC Stack into an Aerobatic Plane and Boosting its Performance by Oxygen Gain Andrej Lotrič* (1), Mitja Mori (1), Blaž Likozar (2) (1) Faculty of Mechanical Engineering, Ljubljana/Slovenia; (2) Chemical Institute of Slovenia, Ljubljana/Slovenia; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract This paper explores the integration of a low-temperature Proton Exchange Membrane Fuel Cell (LT-PEMFC) for aviation applications, with a focus on aerobatic aircraft. The research includes experimental evaluation of the effects of increased oxidant concentration at the cathode, as well as varying operating conditions such as temperature and pressure. These experimental results were extrapolated to design a PEMFC system capable of powering an aerobatic aircraft, which operates in two distinct modes: 225 kW during aerobatic maneuvers and 100 kW during regular flight. To meet the peak power demand while minimizing stack mass, the study investigated the use of pure oxygen at the cathode to enhance PEMFC performance. A numerical model was developed to simulate a fuel cell stack operating with 100% oxygen concentration and compared to the same stack operating on ambient air. Using this model, mass and energy fluxes were estimated based on the specific power required at the propeller and a variable flight power profile over an eight-minute duration. The study demonstrates the feasibility of a hydrogen-powered aerobatic aircraft. However, constructing the system with commercial off-the-shelf components would likely result in excessive weight. By engaging with component suppliers, key parts can be optimized and reduced in weight to meet the required power-to-weight ratio. With innovative approaches to aircraft design, particularly in optimizing balance-of-plant components, the overall system mass can be further reduced, offering an interesting demonstration of PEMFC technology in a unique aviation application. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 2/9 Introduction Greenhouse gas emissions from international aviation have increased dramatically over the past three decades. Although aviation accounts for only about 4% of total greenhouse gas emissions in the EU, it is the fastest-growing source of emissions contributing to climate change. Between 1990 and 2019, aviation emissions increased by 146% [1]. Although they dropped sharply in 2020 due to the COVID-19 pandemic, they have nearly returned to prepandemic levels by 2024 [2].This is primarily due to record growth in air traffic, which is the result of an increase in the number of passengers and the volume of e-commerce deliveries. The combustion of aviation fuel produces emissions of carbon dioxide, nitrogen oxides, contrails, and consequently cloud formation at high altitudes. This study is driven by the potential to replace internal combustion engines (ICE) with a more sustainable electric propulsion system. Energy for electric propulsion can be stored either in batteries or hydrogen-based via proton exchange membrane fuel cells (PEMFCs). Both approaches offer distinct advantages and challenges in terms of energy density, system complexity, and environmental impact, and are actively being investigated for application in aviation. Decarbonizing aviation through the use of hydrogen fuel cell aircraft is a promising but still technically challenging pathway. While the frequency of first flights involving hydrogenpowered aircraft has increased in recent years, the majority remain small-scale, one-off demonstrators. Experimental flight testing would need to increase substantially to make hydrogen aircraft viable in the near future. By converting conventional aircrafts to use fuel cell systems – based on hydrogen produced from renewable energy sources – it is expected to reduce lifecycle emissions by more than 85% compared to fossil fuel-powered planes [3]. The primary objective of this study is to investigate a specific case involving the replacement of a conventional ICE system with a hydrogen fuel cell system in an aerobatic aircraft. This application presents unique challenges due to the relatively short flight duration and the significantly variable power demand, with peak loads during aerobatic maneuvers reaching up to three times the cruise power. These conditions require a propulsion system that is both lightweight and capable of delivering high power output over short time intervals. 1. Scientific Approach Initially, the power requirements for aerobatic flight were determined. A simplified power demand diagram, shown in the figure, illustrates the power needed to sustain such a flight. The next objective was to assess whether a commercial PEM fuel cell system could meet the required power output. Datasheets from various automotive fuel cell manufacturers were examined. Although a few manufacturers offer fuel cell stacks capable of delivering the necessary power, the most significant challenge was weight. To address this issue, the idea of increasing power output by using pure oxygen was explored. Operating a PEM fuel cell with pure oxygen improves efficiency by significantly mitigating mass transport limitations. Although this effect is known in the literature and referred to as oxygen gain, the available data on the phenomenon is limited and, in some cases, outdated. Consequently, it was decided to experimentally investigate the effect of oxygen gain at the single-cell level and based on those measurements, estimate its impact at the stack level. The results of the power increase when operating on pure oxygen will be applied to a commercial fuel cell stack. First, we will calculate the maximum power output of the commercial fuel cell stack and the power increase due to the oxygen gain. Then, a simple 0D-mathematical model will be developed to calculate the mass and energy flows within the entire system. The relationship between these mass and energy flows and the flight profile will assist in selecting the balance-of-plant (BoP) components for the PEMFC stack. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 3/9 2. Experiments/Calculations/Simulations Building on the scientific approach and experimental work outlined below, a simple zerodimensional (0D) model incorporating mass and energy balances was developed. This is described in the following subsections. Assessment of oxygen gain The test station shown in Figure 1 is designed to measure the properties and performance characteristics of a PEMFC under various operating conditions. The station is equipped with various components that work together to ensure control and monitoring of the fuel cell environment, allowing for accurate data collection during experiments. The mass flow rates for hydrogen, air, and oxygen are regulated using mass flow controllers. The reactant flow rates are regulated using the lambda (stoichiometry) parameter, which adjusts the flow according to the specified stoichiometry at each operating point. To ensure proper membrane hydration, the gases are humidified in dedicated bubble humidifiers, with temperature regulation to achieve the desired relative humidity levels. The humidified gases are then transported to the fuel cell via PTFE-electrically heated pipes, preventing cooling and condensation of water vapor. Vaisala HUMICAP® Humidity and Temperature Probes HMP7 with RH accuracy up to 0.8 %RH are installed on both the anode and cathode sides to monitor the humidity of the gases before they enter the fuel cell. Cathode and anode pressures are continuously monitored at both the inlet and the outlet. Backpressure can be independently adjusted for the anode and cathode sides using backpressure regulators, ensuring optimal operating conditions. Figure 1: Schematic of the experimental setup for PEMFC testing The tested membrane electrode assembly (MEA) was purchased from Fuel Cell Store. It has a symmetrical platinum loading on both electrodes (0.5 mg/cm²), a 0.002” thick membrane, 410 microns thick woven carbon cloth gas diffusion layers, and an active area of 50x50 mm2. For the tests, the MEA was placed in Baltic qCf Quick Connect Fixture Unit. The fixture includes integrated electric heaters for precise temperature regulation. The contact pressure on the MEA is controlled using a pneumatic system to ensure consistent pressure across the membrane. All system components are integrated with a custom-built EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 4/9 LabVIEW program, enabling precise parameter regulation, real-time monitoring, and data collection. Hypothetical flight profile The actual aerobatic aircraft considered in this study is the Ultimate 20-300 [4], which is powered by a piston ICE that is specially designed for aerobatics, runs on aviation gasoline, and produces 225 kW of power. The empty weight of the plane (excluding fuel, passengers, and baggage) is 522 kg. The hypothetical flight profile of the aerobatic aircraft is shown in Figure 2. In the first phase of the flight, the aircraft requires maximum power for take-off, followed by lower power (100 kW) during the cruise to the aerobatic performance area. For several minutes, maximum power is needed to perform the aerobatic manoeuvres. In the final phase, lower power is sufficient for the return to the hangar. Approximately 15 litres of aviation gasoline are estimated to be required for the safe execution of the entire flight. In this study it is assumed that the modified hydrogen-based aircraft uses ambient air as the cathode reactant during low-power flight and switching to onboard oxygen when peak power is needed. Figure 2: Flight profile of the aerobatic plane Defining a PEMFC system To replace the ICE powertrain, the hydrogen-based aircraft requires not only a PEMFC stack, but also several BoP components, including a humidifier, hydrogen recirculation pump, gas storage tanks, and a cooling system. Additionally, an electric motor with its controller and a lithium-ion battery are needed to manage rapid power fluctuations. Figure 3 presents a block diagram of the system, along with estimated power losses associated with the BoP components. Based on discussions with the suppliers, literature review, and our own experience, the estimated power losses amount to approximately 15% of the stack's output power. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 5/9 Figure 3: The hydrogen-based PEMFC system and the incurred power losses of the BoP components Selecting a PEMFC stack Among commercially available products, certain PEMFC systems are suitable for this type of application. An excellent match was identified in the modular NM12 Twin PEMFC system developed by EKPO [5], which operates on hydrogen and ambient air and is capable of delivering up to 225 kW of power. Building on this data, the effect of oxygen gain was incorporated to obtain a new power output for operation with pure oxygen. Mass and energy balances The maximum power delivered to the electric motor was determined based on Equation (1), where 𝑃ST,max is the maximum allowable power of the PEMFC stack operated on air, 𝜀oxy the oxygen gain factor, and 𝜂loss denotes the efficiency related to power losses from operating the BoP components. 𝑃EM,max = 𝑃ST,max · 𝜀oxy · 𝜂loss (1) Based on the Equations (2, 3), the mass flow of hydrogen and oxygen was estimated, where I is the electric current produced by the stack, M molar mass of reactant, F Faraday’s constant, and λ stoichiometric ratio of reactants. 𝑚󰇗 H2 =𝑀H2 · 𝐼 2 · 𝐹 · 𝜆H2 (2) 𝑚󰇗 O2 =𝑀O2 · 𝐼 4 · 𝐹 · 𝜆O2 (3) Efficiency of the stack was calculated using Equation (4), where 𝑃ST is the relevant power produced by the stack, and 𝐿𝐻𝑉H2 is the lower heating value of hydrogen. 𝜂ST =𝑃ST 𝐿𝐻𝑉H2 · 𝑚󰇗 H2 (4) O2 tank PEMFC stack Stack cooling system Battery pack Electric motor EM controller H2 tank Electronics cooling system Humidifier Expansion valve Expansion valve H2 recirculation pump 5% 2% 3% 2% 3% Stack cooling pump Electronics cooling pump Legend: Oxygen Hydrogen Cooling liquid Electric connection EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 6/9 Efficiency of the entire system was estimated using Equation (5). 𝜂S=𝑃ST · 𝜂loss 𝐿𝐻𝑉H2 · 𝑚󰇗 H2 = 𝜂ST · 𝜂loss (5) 3. Results Figure 4 presents the power curves taken for the tested PEMFC measured at constant parameters, which were temperature of 70 °C, stoichiometry λH2 = 1.3 and λO2 = 2.0, and relative humidity RHH2 = 50% and RHO2 = 30%. Reactant pressure (1.0 or 1.5 bar absolute) and cathode gas composition (air or pure oxygen) were used as variable parameters. The performance enhancements attributed to oxygen gain in a single MEA were later extrapolated to the PEMFC stack level. Figure 4: Comparison of power curves for MEA operating on air or pure oxygen The power curve for the selected PEMFC stack is shown in Figure 5. At maximum allowable current the stack produces power around 225 kW for operation on air and hydrogen. It can be observed that the stack still does not operate at its peak power at this point. Consequently, the magnitude of the oxygen gain shown in Figure 4 is also estimated at the point slightly below the peak power. Assuming a 33% increase in performance due to oxygen gain, the resulting power output rises to 300 kW. Since this enhancement occurs at the same electrical current, no major modifications to the electrical or thermal management systems of the PEMFC stack are necessary. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 7/9 Figure 5: Power curve for the selected PEMFC stack and projected improvement due to oxygen gain Using Equation (1), and assuming a maximum stack power of 225 kW, an estimated oxygen gain factor of 1.33, and an efficiency of 0.85 due to BoP consumption, the maximum power delivered to the electric motor is estimated to be 255 kW. Mass of the aircraft Due to the number of BoP components required for the operation of the PEMFC stack, it was presumed that the overall aircraft mass would be higher compared to a conventional ICE setup. To compensate for the increased mass and maintain a comparable specific power ratio, the BoP components were sized based on the maximum power delivered to the electric motor. The masses of all BoP components, the PEMFC stack, and the empty airframe are summarized in Table 1. As shown, the empty aircraft in the hydrogen-based configuration is approximately 180 kg heavier than the ICE configuration. However, this baseline design was based on commercial off-the-shelf components and was not optimized. Therefore, a second optimized configuration was designed, which was based on discussions with component suppliers, literature review, and our own experience. In this configuration, potential realistic optimization opportunities have been identified: • PEMFC stack: Assuming a specific power similar to that of the Gen-2 Toyota Mirai (approximately 2.4 kW/kg), the stack mass could be reduced to around 93 kg. Further, application-specific optimizations could potentially improve this specific power even further. • Electric motor and controller: The current motor is oversized for the application at nominal power of 320 kW. Downsizing to a more appropriately rated motor would reduce its weight, and also reduce the mass of the motor controller. Beyond these realistic improvements, several more ambitious but still feasible optimizations have been considered: • Energy storage: The battery system could be replaced with supercapacitors, given that the flight is short and energy storage is primarily needed to buffer brief and rapid power fluctuations. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 8/9 • Electronics cooling system: With a smaller electric motor and the removal of batteries, the associated cooling requirements can be significantly reduced. • Stack cooling system: The PEMFC stack cooling system is not specifically designed for aviation applications, thus it could be optimized by leveraging the high-speed airflow during flight to enhance convective heat removal. • Oxygen storage: In the baseline setup, a Type II cylinder storing oxygen at 300 bar is used. By switching to a Type IV composite tank at 700 bar, properly adapted for safe oxygen storage, the weight of the tank could be significantly reduced. Table 1: Components and corresponding masses of a hydrogen-based aircraft for two design configurations Component Baseline mass (kg) Optimised mass (kg) electric motor 30 20 EM controller 13 9 batteries 33 17 electronics cooling system 36 18 electronics cooling pump 2 2 PEMFC stack 125 93 H2 recirculation pump 3.5 3.5 cathode humidifier 6 6 stack cooling system 80 53 stack cooling pump 7 5 cooling fluid 20 20 H2 tank 36.5 36.5 O2 tank 40 20 basic airframe weight 270 270 empty aircraft 702 573 Based on the estimated masses of the aircraft in Table 1, and the specific power requirements from the ICE-based aircraft. The ICE aircraft was compared with the hydrogenbased aircraft in both the baseline and optimized configurations and the result are presented in Table 2. As seen in the table, the baseline configuration of the hydrogen-based aircraft would not achieve the required maximum specific power due to the high system mass. In the optimized configuration, the reduced system mass enables the aircraft to reach the same maximum specific power as the ICE aircraft, despite the overall aircraft mass being higher. Consequently, at lower power demands, the electric motor must produce approximately 9% more power compared to the ICE aircraft. Table 1: Power required on the shaft of the propeller ICE baseline optimised max power (kW) 225 255 255 max specific power (kW/kg) 0.43 0.36 0.44 low power (kW) 100 133 109 low specific power (kW/kg) 0.19 0.19 0.19 To perform the mass and energy balance calculations, Equations (2), (3), (4), and (5) were used. In this analysis, the fuel cell stack operates on pure oxygen to achieve a maximum power output of 300 kW and on air at a lower power output of 128 kW, to account for system losses and ensure the required power is delivered to the electric motor. Table 3 presents the calculated values for the optimized system in both power modes during the flight. The total hydrogen consumption over the entire flight is approximately 1.5 kg, while 16.7 kg of oxygen is required only for the maximum power mode. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17522327 A1701 Page 9/9 Table 3: Mass and energy balances for the optimized configuration of the hydrogen-based aircraft Parameter Max power mode Low power mode Unit stack power 300 128 kW electric current 625 290 A H2 mass flow 0.0039046 0.001812 kg/s O2 mass flow 0.0557807 / kg/s time 5 3 min H2 mass 1.17 0.33 kg O2 mass 16.73 / kg stack efficiency 0.640 0.589 system efficiency 0.544 0.501 Conclusions The study demonstrated that building a hydrogen-powered aerobatic aircraft is feasible. However, assembling the aircraft using off-the-shelf components would likely result in excessive weight. By collaborating with specific component suppliers, these parts could be optimized and reduced in weight, enabling the aircraft to achieve the desired specific powerto-weight ratio. With innovative approaches to aircraft design and especially BoP components, the overall system weight could be further reduced, showcasing an interesting demonstration of PEMFC technology in a unique aviation application. References [1] European Parliament, „Topics,“ 2022. [Elektronski]. Available: https://www.europarl.europa.eu/topics/en/article/20191129STO67756/emissions-fromplanes-and-ships-facts-and-figures-infographic. [2] T&E, „Polluter pays? A large share of Europe’s aviation emissions remain unpriced,“ 2025. [3] E. J. Adler in J. R. Martins, „Hydrogen-powered aircraft: Fundamental concepts, key technologies, and environmental impacts,“ Progress in Aerospace Sciences, 2023. [4] „Ultimate biplane,“ 2025. [Elektronski]. Available: https://www.ultimatebiplane.com/ultimate-10-300--20-300.html. [5] S. Hemmer, N. Zsiga in J. Kraft, „The NM12 High-performance Fuel Cell Stack Module for Commercial Vehicles,“ MTZ Worldwide, 2023. Keywords: EFCF2025, H2, Low-Temp. Fuel Cells & Electrolysers, low-temperature PEMFC, oxygen gain, aviation Remark: This work is licensed under Creative Commons Attribution 4.0 International