Towards high throughput PEMFC testing: Dynamic and Precise Humidification of Reaction Gases

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 1/8 A1704 Towards high throughput PEMFC testing: Dynamic and Precise Humidification of Reaction Gases Johanna Lützenkirchen*, Katharina Braun, Lukas Weiß, Michael Wensing Friedrich-Alexander-Universität Erlangen-Nürnberg, Professorship for Fluid Systems Technology, Cauerstraße 4, 91058 Erlangen/Germany; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract For the optimum operation of proton exchange membrane fuel cells (PEMFCs), controlled humidification of the reaction gases is essential and therefore also vital in PEMFC testing and test configurations. The ability to rapidly alter the humidity of the reaction gases is valuable not only for dynamic operation trials but also for cost-effective, high-throughput testing. In PEMFC tests, the time required to attain stable operating conditions is typically dominated by the humidification system. As recent studies show, conventional external humidification techniques like the bubble humidifier exhibit a lack of flexibility in adapting to changing conditions [1]. To address these limitations, this study experimentally investigates the controlled addition of liquid water into an evaporator, whereby the water is completely vaporized and then combined with a preheated gas flow in a mixing area. A comprehensive water mass balance can be derived from the quantity of water supplied by the mass flow controller and the amount collected by the condensate separator at the outlet of the fuel cell. The behavior of the evaporator during altered humidity is analyzed with respect to dynamic and stationary characteristics. Therefore, the water mass supplied to the evaporator is changed at a standard fuel cell operation point. The resulting water content measured with a humidity sensor is investigated regarding real-time behavior and reproducibility. The investigated humidification system represents an improvement on conventional humidification methods, as the amount of water in the system can be precisely controlled, thereby enhancing the dynamic and accuracy of the humidification process. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 2/8 Introduction In PEMFCs, the anode and cathode sides are separated from each other by a proton exchange membrane. Due to their chemical resistance and high proton conductivity, perfluorinated sulfonic acid (PFSA) membranes are used in various applications [2]. The proton conductivity of these membranes is dependent on the amount of water present within them. Higher conductivity results in a reduction in ohmic losses, therefore the performance of the PEMFC is dependent upon the membrane’s humidity. One water source is the product water, which is produced during the chemical reaction on the cathode side. It is not possible to set the amount of product water independently; rather, this is dependent on the fuel cell's load point. To influence the membrane humidity the reaction gas streams of air and hydrogen are typically humidified external within the balance of plant components. [3] Humidification causes the risk of excessive water flooding the gas transport channels in the cell. Condensation creates water droplets that block the channels, which causes an insufficient supply of reaction gases [4]. In addition, fluctuations in humidity lead to cyclical membrane swelling and shrinking, resulting in elevated mechanical stress. This leads to a loss of performance and can cause membrane degradation, thereby reducing the fuel cell's lifespan [5]. Consequently, uniform and controlled humidification is important in order to prevent membrane degradation and condensation within the cell. This is relevant not only in PEMFCs field application, but also in the context of fuel cell testing. To optimise fuel cell operation and lifetime, experimental testing on fuel cell test benches is done. Usually, material changes are made within the membrane electrode assembly (MEA) and then different MEAs are compared with each other. To get reliable results, the conditions imposed by the test bench have to be reproducible and constant. This approach is mandatory to ensure the comparability of measurement campaigns and the accurate interpretation of measurement results. In PEMFCs, the relative humidity is frequently used to describe the gas humidity. Relative humidity is defined as the ratio of water vapour partial pressure to saturation vapour pressure, and is correspondingly dependent on the temperature. To reach the same relative humidity at higher temperatures the absolute amount of water in the gas increases. It is therefore worthwhile to examine the water content x instead of relative humidity as a parameter for describing humidity in greater detail. The water content is defined as the mass of gaseous water contained within one kilogram of dry gas. As equation (1) shows, the ideal gas law can be utilised to demonstrate a pressure and temperaturedependent relationship between the water content and the relative humidity [6]. 𝒙 = 𝒎𝒘 𝒎𝒈,𝒅𝒓𝒚 =𝑴𝑾 𝑴𝒈,𝒅𝒓𝒚 ∙𝒑𝒔(𝑻)∙ 𝝋 𝒑 − 𝒑𝒔(𝑻)∙ 𝝋 (1) This approach is applied in the investigated humidification system by adjusting the humidity via the water content. 1. Scientific Approach Depending on the application, different humidification systems are used for the external humidification of the fuel cell’s reaction gases. As already elaborated in a previous paper [1], the following requirements are essential for humidification systems in a fuel cell test bench: Accurate and reproducible stationary behavior EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 3/8 To apply a desired humidification state and ensure that the actual humidity at the fuel cell inlet corresponds to the expected humidity, precise adjustability is essential. The application of equation (1) shows that at 80°C, even a small deviation of 3 g/h in the water load leads to a change in the relative humidity of 1%. In fuel cell test stands, it is sometimes possible to check the inlet humidity with appropriate sensors. However, conventional humidity sensors have a limited accuracy, especially at high humidity levels. Therefore, reliance on this measured value is limited. Condensation may occur at any point within the system if the gas or wall temperature is lower than the dew point temperature. The gas becomes saturated and liquid water occurs, reducing the water content of the gas-vapor mixture. This leads to a deviation in humidity, which in turn could results in a lower relative humidity in the PEMFC than expected [6]. Low time delay/inertia – high dynamics To minimize the time required between different operating points, a low inertia in changing the target humidity is important. In the event of the humidification system attaining the target humidity with high dynamics, it is possible to react to the operating status of the fuel cell with an increased response time. Concurrently, the duration of the test can be reduced, saving effort and costs. Representation of the complete operating range Furthermore, the humidification system must be capable of covering the entire operational range of a fuel cell. Low-temperature PEMFCs are typically operated within a temperature range of 65°C to 85°C. In order to achieve optimal cell performance within this temperature range, relative humidity ranging from 70% to 100% are typically established. Performing accelerated stress test, temperatures up to 90°C and low relative humidity until 30% must be feasible. In the application of commercial fuel cell test stands the humidification system of a bubble humidifier is often used. In these, water is heated in a column and the dry gas is led through the hot water from below. This humidification method is based on the assumption that the gas is fully saturated upon exiting the water column. If the water is heated homogeneously, it can be hypothesized that the gas dew point temperature corresponds to the water temperature. For humidity changes, it is therefore necessary to adjust the dew point temperature of the gas via the water temperature. As demonstrated in previous investigations [1], the dynamic of bubble humidification is limited because a large mass of water must be heated. As the gas exits the bubbler in a state of complete saturation, there is also an elevated risk of condensation, which can lead to deviations from accuracy. To overcome these limitations, we integrated a different humidification system into the design of our self-built fuel cell test bench. The following section will provide a detailed description of the optimized humidification system. 2. Experiments The experimental setup utilized a self-built fuel cell test bench capable of accommodating stacks with a power class of up to 1.5 kW. In order to measure not only individual cells but entire fuel cell stacks in this power class, it is necessary to use media flow rates of up to 100 norm liters per minute. Consequently, the amount of water necessary to achieve the required humidity levels also increases. For instance, if the reaction gas flow of 100 Nl/min is to be conditioned to 90% at 80°C and 2.4 bar(a), a water mass flow of approximately 1000 g/h is required. To fulfil these requirements for the humidification system we used an evaporator EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 4/8 manufactured by aDrop. Compared to the bubble humidifier, this evaporator has a more precise humidity control and enhanced response time which we want to show in the following section. Figure 1 shows a schematic representation of the gas conditioning prior to its entry into the fuel cell. Fig. 1 Schematics representation of the humidification system to conditionate the PEMFC gas inlet The key aspect of this humidification system lies in the controlled addition of water, which is introduced to the evaporator in liquid form and regulated by a mass flow controller (MFC 1). This mass flow controller utilizes the Coriolis principle and thereby facilitating precise regulation of the water supply. The water is vaporized within the evaporator’s core (evap. core) and which is heated to an elevated temperature (𝑻𝒆𝒗𝒂𝒑𝑪𝑶𝑹𝑬) of 200 °C. The capacity of the heater is regulated in accordance with the quantity of water supplied, thereby ensuring complete vaporization. In addition to the liquid water, dry gas is fed to the evaporator via a second mass flow controller (MFC 2). The gas is heated in the pre-heat chamber so that it can fully absorb the water vapor in the mixing unit. The temperature of the humid gas resulting from the heating/evaporation process can be set via the temperature measuring point at the vaporizer outlet (𝑻𝒆𝒗𝒂𝒑𝑶𝑼𝑻). Due to the spatial conditions, the distance between the outlet of the evaporator and the inlet of the fuel cell is 50 cm. Trace heating is therefore used to prevent condensation. To facilitate precise analysis of the gas feed at the fuel cell inlet, we installed a sensor plate. It includes three sensors to enable the simultaneous measurement of temperature (𝑻𝒊𝒏), pressure (𝑷𝒊𝒏) and the dew point temperature (𝑫𝒆𝒘𝑻𝒊𝒏). To prevent condensation in the sensor plate, the latter is also heated via a thermostat so that the sensor plate temperature (𝑻𝒑𝒍𝒂𝒕𝒆) is above the dew point temperature. The sensor plate is composed of aluminum, a material that is well-known for its ability to facilitate even heat distribution. We use humidity sensors manufactured by Vaisala, featuring a heated sensor head, which can be operated at high humidity levels without the risk of condensation. The relative humidity of the gas is calculated from the dew point temperature measured by the humidity sensor and the measured gas temperature. The water content is calculated from the dew point temperature using the measured pressure and equation (1). This value will be referred to as the sensor’s water content. This is in contrast to the evaporator’s water content which is derived from the ratio of the dry gas flow and the amount of water supplied to the evaporator. The controlled addition of liquid water ensures precise knowledge of the water content. Provided that the system is adequately heated and insulated, thereby ensuring that the gas temperature remains above the dew point temperature at all times, the water content remains uniform throughout the lines. After the EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 5/8 fuel cell, the gas flow is cooled down and the resulting condensate is collected. This process enables the determination of a water mass balance of the fuel cell. 3. Results The objective of the measurements was to analyses the developed humidification system with regards to the requirements presented in Chapter 2. Therefore, the evaporator was used to set varying water content levels under different conditions. These were then compared with the resulting measured humidity. Generally, the water content of the humidified gas can be adjusted either by changing the supplied mass of water or by changing the dry gas flow. When testing fuel cells, the gas flow rate is determined by the fuel cell's load. Therefore, the gas water content is altered via the supplied water mass. We investigated the behavior of the evaporator at a typical fuel cell operating point (gas temperature = 80°C, pressure = 2.4 bar, gas flow rate = 25 Nl/min). Therefore, we increased the water mass flow in 50 g/h steps from 50 to 300 g/h (Setpoint), thus covering a range of 20-95 % relative humidity. Figure 2 shows the water content measured by the humidity sensor compared with the water content set by the MFC’s. Fig. 2: Increase of the water content accordingly to the mass flow supplied to the evaporator in 50 g/h steps After changing the water flow the setpoint increases directly. The evaporator’s water content increases also directly as the MFC controls the desired water mass flow to match the setpoint. In comparison to the evaporator water content the sensor’s water content starts to increases with a delay of 15 seconds. The sensor’s water content shows the behavior of an e-function and approaches the setpoint asymptotically. This behavior is typical for the step response of the PT1 element and can be explained by the heating control of the evaporator. When water is added, the capacity of the heater adjusts accordingly to enable complete evaporation of the water. A comparison of the steps reveals that the three middle steps (150, duration min water content etpoint water content evaporator water content sensor as flow l min . ar(a T C EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 6/8 200, 250 g/h) exhibit the smallest time delay. Here, the sensor value approaches the setpoint and remains constant after approximately four minutes. Within the low and high humidity ranges, the time required to reach a constant state is double the duration observed in the middle humidity range (8 minutes). This indicates, that it is challenging to operate the system at its boundary conditions. The remaining deviation between the setpoint and the sensor’s water content lies within the measurement inaccuracy of the used sensors. However, it has been demonstrated that the humidification system can successfully achieve the target humidity within a shorter time frame when compared to the bubble humidifier. With a bubble humidifier a time of more than 80 minutes is required to increase the dew point temperature from 45°C to 65°C [1]. This discrepancy can be attributed to the fact that, in the bubble humidifier, the water content cannot be modified directly; rather, the dew point temperature can only be increased indirectly by increasing the temperature of the entire water column. To ascertain whether the time delay is solely influenced by the heating control of the evaporator, we also investigate how the water content is influenced by changing the gas flow. This test measurements showed that an increase in the gas flow caused a sudden reduction in the sensor’s water content. As seen in Figure 3 the sensor’s water content initially drops to a value below the setpoint, but then quickly rises again and displays constant behavior after a period of one minute. This demonstrates that the observed PT1 behavior is predominantly influenced by the evaporator’s heating and the water control by the MFC. Despite the fact that this approach attained a steady state more quickly, this type of humidity change is not used in the context of fuel cell operation, as the gas flow rate is determined by the fuel cell's load. Fig. 3: Decrease of the water content accordingly to the gas flow supplied to the evaporator in 5 Nl/min steps The accuracy of the humidity supplied by the evaporator was verified by the repeated measurement of the same variable on multiple occasions. Therefore, three water mass flows time hh mm pr water content etpoint water content Measured water content water flow h . ar(a T C duration min EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 7/8 (50, 200, 300 g/h) are added in the evaporator, whilst all other operational conditions maintained constant. Once the measured humidity has reached a steady state, we determined for each water mass flow the resulting average dew point temperature. We directly used the measured value from the humidity sensor in order to avoid influences from the calculation with other measured variables. This measurement was repeated independently on two further days. The results show as that the dew point temperatures measured on the various days differ from each other only slightly (see table 1). Table 1: repeated dew point temperature measurements in dependence on water mass In addition to the comparison of the stationary states of the final value, the course of the dew point temperature is also compared. Figure 4 illustrate the increase in dew point temperature for a water mass of 300g/h. The measurements were conducted independently and over a period of three days. However, the changes in dew point temperature were found to be almost identical. This behavior is indicative of the reproducibility and precision of the humidification system. Figure 3: repeated measurements of water mass flow increased to 300g/h To summarize, we investigated a humidification system that added a controlled water mass flow in an evaporator. The water is completely vaporized, resulting in the formation of a water content within the humid gas, in conjunction with the dry gas flow. We have analyzed the humidification system in terms of its accuracy and dynamics. It has been demonstrated through measurements that the target humidity can be attained within a brief timeframe at data point . . . . dew point temperature C dew point temp day dew point temp day dew point temp day as flow l min T C p . ar(a 50 g/h 200 g/h 300 g/h Day 1 45,46 °C 71,91 °C 80,08 °C Day 2 45,48 °C 70,85 °C 79,99 °C Day 3 45,94 °C 71,25 °C 80,07 °C EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244137 A1704 Page 8/8 the standard fuel cell operating point. Repetitions of the measuring point have confirmed the repeatability. Acknowledgements The work was carried out as part of the project H2Season which was funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy. References [1] Braun, Katharina; Lützenkirchen, Johanna; et al. Advancing Fuel Cell Testing Environments: Overcoming Limitations in Conventional Humidification Methods. SAE Technical Paper Series. Energy & Propulsion Conference & Exhibition. Columbus, Ohio, United States, 2024 [2] Safronova, E. Yu.; Lysova, A. A. Perfluorosulfonic Acid Polymer Membranes: Microstructure and Basic Functional Properties. In: Membr. Technol. 5 (6), S. 379–393, 2023 [3] Dicks, Andrew L.; Rand, David A. J. Fuel Cell Systems Explained: Wiley, 2018 [4] Li, Hui; Tang, Yanghua; et al. A review of water flooding issues in the proton exchange membrane fuel cell. In: Journal of Power Sources 178 (1), S. 103–117, 2008 [5] Madhav, Dharmjeet; Wang, Junru; et al. A Review of Proton Exchange Membrane Degradation Pathways, Mechanisms, and Mitigation Strategies in a Fuel Cell. In: Energies 17 (5), S.998, 2024 [6] Schmidt, Achim. Technical Thermodynamics for Engineers. Cham: Springer International Publishing, 2022 Keywords: EFCF2025, H2, LowTemp. Fuel Cells & Electrolysers, Gas Humidification Remark: This work is licensed under Creative Commons Attribution 4.0 International