Behavioural study of PEMFC during start-up/shutdown cycling for aeronautic applications
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The deployment of proton exchange membrane fuel cell (PEMFC) for aeronautic applications is a value-added energy supply alternative that not only generates useful byproducts (oxygen-depleted air, water and heat) but addresses sensitive issues such as improving health conditions of airport personnel (silent operation minimizes noise) and decreasing greenhouse gas emission (in situ zero emissions). However, the PEMFC is yet to be industrialized due to its fast degrading components. The contribution of the several start-ups and shutdowns (a PEMFC undergoes when operated in aircraft) to the degradation is not well-understood. Hence, this study seeks to explore the effects of start-up/shutdown (SU/SD) cycling on a PEMFC’s lifetime. The SU/SD cycling is incorporated with heating to 60 °C and cooling to room temperature to mimic real-life temperature changes encountered in an aircraft. The tested membrane electrode assemblies (MEAs) were characterised for performance and evolution of its components to examine the extent and nature of degradation. More than two-thirds loss of electrochemically active surface area (ECSA) of catalyst, Pt particle growth (4.71–6.41 nm) associated with Ostwald ripening and formation of PtO from adsorption of OH− by Pt–M surface were identified to be causes of the observed voltage decay at 0.196 mV h−1 rate. Hence, it is concluded that SU/SD cycling mostly affects the catalytic component of PEMFC in the aeronautic environment.
KeywordsProton exchange membrane fuel cells Degradation Accelerated stress tests Aeronautic environment Load profile State of health
The PEMFC is a device that operates silently and comprises of solid electrolyte and no moving parts [1, 2]. The PEMFC converts chemical energy from the electrochemical reaction of oxygen and hydrogen into electrical while emitting heat, oxygen-depleted air (ODA) and water as byproducts. These byproducts have useful functions in aircrafts, such as de-icing wing and heating water using emitted heat, fire retardation with the ODA and water generation on board . Furthermore, the PEMFC is an auxiliary power unit (APU) that powers small electrical systems on-board such as environmental control systems (ECS), in-flight entertainment and recharging batteries .
A study by Klebanoff et al. showed that PEMFC could successfully replace the ram air turbine (RAT) emergency backup power system . Hence, leading aircraft manufacturers are working on PEMFC-powered propulsions and APUs as part of establishing “more-electric airplanes” [1, 6]. The “more-electric airplanes” will replace the pneumatically powered and hydraulically powered systems (e.g., ECS and cargo doors) with lighter and more efficient electric systems . Consequently, PEMFC is tested for various aeronautic applications, ranging from nose wheel drive motor, hybrid with batteries, exclusively powering small manned aircrafts to emergency power systems [8, 9, 10, 11].
Developments in field and in-flight testing of PEMFC-powered aircrafts includes improving from a hybrid of 20 kW PEMFC with batteries to exclusively 33 kW PEMFC and reaching 1000–2558 m altitude without significant performance loss [8, 12]. The success observed in field testing propelled further testing of PEMFC in laboratory-simulated aeronautic conditions. The laboratory-simulated aeronautic conditions examined include orientation/inclination and high altitude/low pressure [3, 13, 14, 15, 16]. For instance, the effects’ inclinations on PEMFC performance were notable when the fuel cells were operated at a low cathode stoichiometry of 1.6 . The notable PEMFC performance losses at high altitude/low pressure (0.7 bar/2200 m altitude) were minimized by increasing the cathode stoichiometry to 1.75 and 2.5 at 1200 m and 2200 m altitude, respectively [16, 18, 19, 20]. However, the concerns of increasing cathode stoichiometry to more than 2.5 were membrane drying and power consumption by compressors . The study by Keim et al. and Werner et al. on the quality of the cathode-discharged ODA gas achieved the desired less than 12% ODA oxygen content by operating the fuel cell at 2000 m, 60 °C cell temperature and a high cathode stoichiometry of 3.6 [15, 16, 21]. Again, cathode stoichiometry above 2.5 may damage the stack. Hence, the increase of cathode stoichiometry to 2.5 is taken as an adequate measure to minimize performance losses due to low ambient pressure (0.7 bar at 2200 m) and low oxygen content (16.0% at 2200 m) at high altitudes.
It is safe to say (from the literature survey presented above) that operating and environmental/surrounding conditions have been relatively studied with respect to their effect on PEMFC performance, except for load profile. The typical load profile for aircrafts is as follows: ground taxi, take-off and climb, cruise, descend and landing and ground taxi . Exploring the effects of load profile-related operating conditions on PEMFC performance under aeronautics conditions is essential in enabling the deployment of fuel cells in the aeronautic sector.
Lifetime studies under normal operating conditions are rarely conducted due to time and resource constraints. Hence, fuel cells are tested under conditions that accelerate performance loss and degradation rates using accelerated stress tests (AST) protocols. AST apply different accelerated stressors to determine and predict the durability of fuel cell components by examining known failure mechanisms and major precursors of failure. Each of the AST protocols is designed to focus on a specific fuel cell component. For instance, open circuit voltage (OCV) facilitates chemical attack of the perfluorosulfonic acid (PSFA) membrane by peroxide radicals, whereas relative humidity (RH) cycling causes mechanical degradation of the membrane [5, 22, 23, 24, 25]. One of the load profile-related AST that is relevant to aeronautic applications is SU/SD cycling.
Factors that influence fuel cell lifetime under aeronautical conditions are operating temperature, RH and reactants’ stoichiometry. In real-world applications, PEMFC is subjected to more than one stressor, either concurrently or simultaneously. De Bruijn’s review eloquently demonstrates how a combination of different stressors contributes towards each fuel cell component’s degradation . Kim et al. showed that RH has no significant effect during SU/SD, except at 100% RH .
A study by Pei et al. profiled the life expression of a bus driven 43,000 km on a fixed route daily, particularly load-changing cycles, start-up/shutdown cycles, idling cycles and high power conditions . The start–stop cycling, with start-up operating conditions at 60 °C, 1.2/2.5 Sa/Sc, 357 mA cm−2, idling for 1 min at 10 mA cm−2, stop and purge with nitrogen until voltage was zero, caused major degradation. The degradation was attributed to high OCV. Even though there was a follow-up publication by Pei et al. on post-mortem analysis of the membrane electrode assemblies (MEAs), chemical and physical transformations of the fuel cell components caused by the degradation were not examined by common techniques such as SEM, TEM or XRD .
SU/SD cycling reported in literature examines the effect of load changes at a fixed temperature, which is not necessarily representative of the actual start-up or shutdown, as heating and cooling occur concurrently. Hence, the aim of this study is to evaluate the behaviour of PEMFC operated at 60 °C and exposed to SU/SD with heating and cooling as it would happen during aircraft operation. The PEMFC was characterised for the extent and nature of degradation to better understand its behaviour in the aeronautic environment.
Materials and methods
The single cell was tested using the Greenlight G20 Fuel Cell Test Station. The fuel cell was operated at atmospheric pressure with pure hydrogen and air supplied to the anode and cathode, respectively. The stoichiometry used was 1.8/2.5 for the anode and cathode. The MEA was activated by applying a constant voltage of 0.7 V to a cell operating at 60 °C, supplied with fully humidified (100% RH) reactants, at anode/cathode stoichiometry of 1.8/2 for 100 h. Fuel cell parameters such as operating temperature, dew point, stoichiometry/flow rates and load were controlled using Greenlight’s Emerald™ control and automation software.
The fresh and tested MEAs were characterised for electrochemical properties using cyclic voltammetry (CV). The CV measurements were taken using the Autolab PGSTAT302N potentiostat with FRA2 module controlled by Nova software. The CV was measured at 30 °C, a voltage range of 0.015–1.00 V and a sweep rate of 30 mV s−1. The gas supply for the CV was hydrogen and nitrogen at anode and cathode, respectively.
Small pieces were cut out from the MEAs and characterised using high-resolution scanning electron microscopy coupled with energy-dispersive spectroscopy (HR-SEM/EDS) and X-ray diffraction (XRD). ZEISS MERLIN HR-SEM/EDS was used to characterise cross-sections of the MEAs for chemical composition and particle migration/redeposition. Bruker D8 high-resolution X-ray diffractometer with Vantec detector was used to analyse the crystal structure, phase purity and particle size distribution (PSD). Electrode powder samples were also examined for changes in the structure of the catalyst layer by capturing Pt transformation due to particle migration, particle growth from agglomeration and changes in the structure of the membrane surface. This was done through the use of high-resolution transmission electron microscopy (HR-TEM) and HR-SEM/EDS. The HR-TEM used was Hitachi H800 200 kV instrument fitted with a digital image acquisition system.
Results and discussions
The sudden decrease observed after 100 cycles at high current densities may be caused by mass transport limitations, since PEMFC requires a higher active surface area to meet the demand of the chemical reactions compared to low current densities . Kim et al. also reported that full humidification may lead to excess water that blocks the electrode surface area and subsequently results in temporary reactants’ starvation . The review by Yu et al. highlighted that SU/SD degradation is minimal at low temperatures and humidity . Given that the rate of water generation is much faster at high current density compared to low current density, the IV curves results attest that higher humidity introduces additional water into the cell that causes flooding and subsequently promotes degradation. Full humidification was chosen as a worse-case scenario in this study, since PEMFCs operating in aeronautic conditions are likely to be periodically flooded due to varying load demands.
Exploration of degradation mechanisms
High voltages favour platinum dissolution and Wang et al. observed a higher concentration of Pt at 0.9 V . On the other hand, Pt particles are easily redeposited at reverse current conditions created by the SU/SD . Due to high equilibrium potential of dissolution, Pt ions tend to redeposit onto larger Pt particles in the cathode and thus undergo Ostwald ripening to minimize surface energy . Hence, the increased average particle size from 4.71 to 6.41 nm (Fig. 9) and HR-TEM micrographs (Fig. 7) suggest that the particles experienced Ostwald ripening, migration/dissolution and redeposition. Zhao et al. noted that narrower XRD peaks indicate larger Pt particles . The particle growth corresponds to the loss of ECSA (Fig. 6) and narrower XRD peaks (Fig. 8). Therefore, Pt particle agglomeration may be attributed to a loss of ECSA and be one of the causes of voltage decay.
Loose Pt particles, due to a collapse of support, move freely and redeposit either on other Pt particles or on the membrane. The latter was neither notable nor studied further due to unnoticeable membrane degradation and being limited by the scope of this research. The former has been shown by the different analytical techniques used in this study, such as HR-TEM and XRD to be demonstrating Pt particle growth.
Therefore, the loss of ECSA may be partly attributed to carbon support corrosion.
Elemental composition of a pristine and tested MEA as obtained from HR-SEM/EDS
Carbon leached out after 600 cycles (EoL), which indicates corrosion of the catalyst support. Carbon support corrosion is prevalent at high voltage [28, 39]. Flooding, as likely to occur at high current densities for a fully humidified cell exposed to variable load demands, is reportedly blocking catalyst reactive sites and promotes carbon corrosion [40, 41]. Pt agglomeration tends to be aggravated by carbon corrosion. Therefore, the loss of ECSA may be partly attributed to carbon support corrosion.
The aim of this study was to examine the effect of SU/SD cycling on the performance of PEMFC operating in an aeronautic environment. The aeronautic environment was simulated by incorporating heating to 60 °C and cooling to room temperature while operating at 1.8:2.5 anode/cathode stoichiometric ratio. This specific stoichiometric ratio was selected, because increasing cathode to 2.5 minimizes performance loss associated with high altitudes and improves quality of ODA for multifunctional fuel cells.
The ultimate objective of this research was to collect and document AST data relevant to aeronautic applications. In doing so, understanding of PEMFC behaviour in an aeronautic environment will be enhanced and thus its performance and state of health can be better managed.
The studied MEA was characterised for overall performance and ECSA using the IV curve and the CV. Physical and chemical properties such as elemental composition and particle size distribution were evaluated using HR-SEM/EDS, XRD and HR-TEM.
Voltage evolution recorded during the test showed average degradation of 0.196 mV h−1. The loss of ECSA revealed that the observed performance loss may be attributed to catalyst degradation. Furthermore, Pt particle agglomeration (shown by increased average particle size) suggests that Pt redeposition is one of the causes of loss of ECSA, which is evidence for catalyst degradation. The presence of oxygen from the HR-SEM/EDS results proves oxidation of Pt into PtO which covers the catalyst surface area and hinders the chemical reactions from taking place. Thus, it can be concluded that SU/SD-induced degradation on PEMFC operated in aeronautic conditions is caused by catalyst degradation in the form of Pt particle agglomeration due to redeposition and oxidation of Pt into PtO.
This work is supported by Hydrogen and Fuel Cell Technologies RDI Programme (HySA), funded by the Department of Science and Technology in South Africa (Project KP3-S03).
- 1.Seidel Jonathan, A., Sehra Arun, K., Colantonio Renato, O.: NASA aeropropulsion research: Looking forward. NASA, Glenn Research Center, Cleveland (2001)Google Scholar
- 5.Klebanoff, L.E., Cornelius, C.J.: Analysis of hydrogen storage for a fuel cell emergency power system (FCEPS) for commercial aircraft. In: Annual Merit Review and Peer Evaluation Meeting, 9–13 May, Arlington, Virginia (2011)Google Scholar
- 6.Daggett, D. L., Eelman, S. and Kristiansson, G.: Fuel cell APU for commercial aircraft. In: AIAA/ICAS International air and space symposium and exposition: the next 100 years, 14–17 July, Dayton, Ohio, pp. 1–9 (2003)Google Scholar
- 7.Eelman, S., del Pozo y de Poza, I., Krieg, T.: Fuel cell APU’s in commercial aircraft—an assessment of SOFC and PEMFC concepts. In: 24th International congress of the aeronautical sciences, 29 Aug–3 Sept, Yokohama, Japan, pp. 1–10 (2004)Google Scholar
- 9.Romeo, G.: The First European Commision Funded aircraft powered by a hydrogen fuel cell took its first flight. https://fuelcellsworks.com/archives/2010/06/15/first-european-commission-funded-aircraft-powered-by-a-hydrogen-fuel-cell-takes-first-flight/ (2010). Accessed 23 July 2015
- 10.DLR International Press Release: DLR Airbus A320 ATRA taxis uses fuel cell-powered nose wheel for the first time. Available from: http://www.dlr.de/dlr/presse/en/desktopdefault.aspx/tabid-10307/470_read-931/year-2011/#/gallery/2079 (2011). Accessed 11 June 2015
- 11.Boeing International Press Release.: 787 electrical system. Available from: http://787updates.newairplane.com/787-Electrical-Systems/787-Electrical-System (2013). Accessed 11 June 2015
- 12.Rathke, P., et al.: Long distance flight testing with fuel cell powered aircraft antares DLR-H2. https://www.dglr.de/publikationen/2014/301219.pdf (2013). Accessed 11 June 2015
- 16.Werner, C., et al.: Characteristics of PEMFC operation in ambient- and low-pressure environment considering the fuel cell humidification. CEAS Aeronaut. J. 6, 1–15 (2014)Google Scholar
- 20.Kallo, J., et al.: Fuel cell system development and testing for aircraft applications, in 18th World Hydrogen Energy Conference, 16–21 May, pp. 435–444. Essen, Germany (2010)Google Scholar
- 34.Linse, N.: Start/stop phenomena in polymer electrolyte fuel cells, pp. 1–221, PhD Thesis (2012)Google Scholar
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