Björn Eriksson

Pietro Giovanni Santori

Frédéric Lecoeur

Marc Dupont

Frédéric Jaouen
email: frederic.jaouen@umontpellier.fr

Prof. Plamen Atanassov

Understanding the effects of operating conditions on the water management in anion exchange membrane fuel cells

Keywords: AEMFC, Humidity sensors, Fe-N-C, Water management, Anode flooding

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Anion exchange membrane fuel cell (AEMFC) is a promising alternative to proton exchange membrane fuel cell (PEMFC) as it holds promise for the application of non-noble metal catalysts both at the cathode [START_REF] Peng | High-Performing PGM-Free AEMFC Cathodes from Carbon-Supported Cobalt Ferrite Nanoparticles[END_REF][START_REF] Santori | High Performance FeNC and Mnoxide/FeNC Layers for AEMFC Cathodes[END_REF][START_REF] Wang | Synergistic Mn-Co catalyst outperforms Pt on high-rate oxygen reduction for alkaline polymer electrolyte fuel cells[END_REF][START_REF] Adabi | Understanding how single-atom site density drives the performance and durability of PGM-free Fe-N-C cathodes in anion exchange membrane fuel cells[END_REF] and anode sides [START_REF] Ni | An efficient nickel hydrogen oxidation catalyst for hydroxide exchange membrane fuel cells[END_REF][START_REF] Roy | Nickel-copper supported on a carbon black hydrogen oxidation catalyst integrated into an anion-exchange membrane fuel cell[END_REF], and recent developments in AEM and electrode optimization have allowed for high performances to be achieved [START_REF] Yang-Neyerlin | Editors' Choice-Examining Performance and Durability of Anion Exchange Membrane Fuel Cells with Novel Spirocyclic Anion Exchange Membranes[END_REF][START_REF] Hassan | Achieving High-Performance and 2000 h Stability in Anion Exchange Membrane Fuel Cells by Manipulating Ionomer Properties and Electrode Optimization[END_REF]. Several studies have shown that the water management was key to achieving high power performance [START_REF] Diesendruck | Water -A key parameter in the stability of anion exchange membrane fuel cells[END_REF][START_REF] He | Water limiting current measurements in anion exchange membrane fuel cells (AEMFCs); part 1: Water limiting current method development[END_REF][START_REF] Mustain | Understanding how high-performance anion exchange membrane fuel cells were achieved: Component, interfacial, and cell-level factors[END_REF][START_REF] Omasta | Importance of balancing membrane and electrode water in anion exchange membrane fuel cells[END_REF] and as such exploring the water balance in the AEMFC is important to improve the overall performance further. Too low relative humidity (RH) can lead to poor membrane and ionomer conductivity [START_REF] Duan | Water uptake, ionic conductivity and swelling properties of anion-exchange membrane[END_REF][START_REF] He | Probing Anion Exchange Membrane Fuel Cell Cathodes by Varying Electrocatalysts and Electrode Processing[END_REF][START_REF] Gerhardt | Along-the-Channel Impacts of Water Management and Carbon-Dioxide Contamination in Hydroxide-Exchange-Membrane Fuel Cells: A Modeling Study[END_REF] and accelerate their chemical degradation [START_REF] Diesendruck | Water -A key parameter in the stability of anion exchange membrane fuel cells[END_REF]16,[START_REF] Cheng | A mini-review on anion exchange membranes for fuel cell applications: Stability issue and addressing strategies[END_REF] . Too much water can instead lead to flooding, causing mass transport limitations in the electrodes, especially at the anode side [START_REF] Yang-Neyerlin | Editors' Choice-Examining Performance and Durability of Anion Exchange Membrane Fuel Cells with Novel Spirocyclic Anion Exchange Membranes[END_REF]. The more complex water management of the AEMFC compared to PEMFC is due to water being both a product in the hydrogen oxidation reaction (HOR) (Eq.1) and a reactant in the oxygen reduction reaction (ORR) (Eq. 2)

2𝐻 + 4𝑂𝐻 → 4𝐻 𝑂 + 4𝑒

Eq. 1

𝑂 + 2𝐻 𝑂 + 4𝑒 → 4𝑂𝐻

Eq. 2

These reactions lead to a net difference of six water molecules between anode and cathode, for four electrons. In comparison, in PEMFC, the electrochemical reactions lead to a net difference between cathode and anode of only four water molecules, for four electrons. Further, the electroosmotic drag leads to further transport of water from the cathode to the anode during AEMFC operation, actively enhancing the water imbalance during operation [START_REF] Berg | Towards a Consistent Interpretation of Electro-osmotic Drag in Polymer Electrolyte Membranes[END_REF]. The only mechanism for transporting water from the anode to the cathode is back diffusion, caused by the higher water activity at the anode [START_REF] Gerhardt | Along-the-Channel Impacts of Water Management and Carbon-Dioxide Contamination in Hydroxide-Exchange-Membrane Fuel Cells: A Modeling Study[END_REF].

Due to this, cathode dry out has been a concern for the operation of AEMFCs [START_REF] Gerhardt | Along-the-Channel Impacts of Water Management and Carbon-Dioxide Contamination in Hydroxide-Exchange-Membrane Fuel Cells: A Modeling Study[END_REF]. Several works have shown that AEMFC performance is tightly connected to the water management, an optimized water balance often requiring the combined use of high flow rates and high inlet RH [START_REF] Omasta | Importance of balancing membrane and electrode water in anion exchange membrane fuel cells[END_REF][START_REF] Gerhardt | Along-the-Channel Impacts of Water Management and Carbon-Dioxide Contamination in Hydroxide-Exchange-Membrane Fuel Cells: A Modeling Study[END_REF][START_REF] Gao | Enhanced water transport in AEMs based on poly(styrene-ethylenebutylene-styrene) triblock copolymer for high fuel cell performance[END_REF]. Since back diffusion of water from anode to cathode is an important mechanism to mitigate anode flooding and cathode drying, developing AEMs that are thin and with good water diffusion properties have also been identified as key aspects for achieving high power performance [START_REF] He | Water limiting current measurements in anion exchange membrane fuel cells (AEMFCs); part 1: Water limiting current method development[END_REF][START_REF] Mustain | Understanding how high-performance anion exchange membrane fuel cells were achieved: Component, interfacial, and cell-level factors[END_REF][START_REF] Yassin | The effect of membrane thickness on AEMFC Performance: An integrated theoretical and experimental study[END_REF] . Although water management has been identified as a key concern, quantification of water transport remains relatively unstudied with only a few previous studies having measured how water is distributed in a single-cell AEMFC [START_REF] Gao | Enhanced water transport in AEMs based on poly(styrene-ethylenebutylene-styrene) triblock copolymer for high fuel cell performance[END_REF][21][START_REF] Carlson | Fuel cell evaluation of anion exchange membranes based on poly(phenylene oxide) with different cationic group placement[END_REF][START_REF] Omasta | Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs[END_REF][START_REF] Luo | Water permeation through anion exchange membranes[END_REF][START_REF] Zhang | Water movement in a solid-state alkaline fuel cell affected by the anionexchange pore-filling membrane properties[END_REF][START_REF] Peng | Using operando techniques to understand and design high performance and stable alkaline membrane fuel cells[END_REF][START_REF] Oshiba | Improvement in the solid-state alkaline fuel cell performance through efficient water management strategies[END_REF] . Of these, even fewer have measured the water transport during operation [21-23, 25, 26]. In previous studies using humidity sensors to probe the RH of gas streams in and out of AEMFC, it was concluded that the amount of water increased on both anode and cathode with increasing current density [21,[START_REF] Carlson | Fuel cell evaluation of anion exchange membranes based on poly(phenylene oxide) with different cationic group placement[END_REF]. Further, these studies showed that the back diffusion from anode to cathode was greatly affected by the membrane thickness, but not so much by the type of functional groups in the AEM. However, both studies were limited to relatively low current densities (< 0.4 A•cm -

2

) due to the combination of materials used, leaving open the question whether the water transport and resulting water gradient across the cell radically changes at higher current densities, especially regarding a possible cathode dry-out at high current density. Omasta et al. [START_REF] Omasta | Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs[END_REF] used operando inplane neutron radiographic images to map the amount of liquid water in the cell during operation and concluded that increasing the inlet RH leads to an increase of the amount of liquid water on the anode side, with little to no effect on the cathode side. It was also noted that the back diffusion from anode to cathode was the main mechanism for membrane hydration during operation. However, these techniques only allow for measurements of the liquid water and does not allow for quantification of gas phase water. Furthermore, while neutron imaging is a powerful tool for visualizing the amount of water, the technique requires special facilities which makes it not widely available. Zhang et.al [START_REF] Zhang | Water movement in a solid-state alkaline fuel cell affected by the anionexchange pore-filling membrane properties[END_REF] used cold traps to capture and measure the amount of water in the gas streams during galvanostatic AEMFC operation, revealing a net water transport from cathode to anode for highly hydrophobic membranes (associated with low performance) and from anode to cathode for hydrophilic membranes (associated with better performance). However, the different membranes had varying thicknesses which could also have influenced the water management, and the current densities were low compared to the state-of-the-art for AEMFCs [START_REF] He | Water limiting current measurements in anion exchange membrane fuel cells (AEMFCs); part 1: Water limiting current method development[END_REF][START_REF] Peng | Using operando techniques to understand and design high performance and stable alkaline membrane fuel cells[END_REF][START_REF] Oshiba | Improvement in the solid-state alkaline fuel cell performance through efficient water management strategies[END_REF].

In this work we study how inlet RH, pressure, and flow rate of the anode and cathode gas streams affect the water balance in an operating AEMFC. The RH and water content at the outlet of the anode and cathode were measured online with humidity sensors and is to, the best of the authors' knowledge, the first time this has been done with a Fe-N-C cathode. The study, performed on a fixed combination of anode catalyst, cathode catalyst, membrane and ionomeric materials, provides detailed insights on the effect of the cell operating parameters on the water balance, and how this affects the cell performance in different regions of the polarisation curve.

Experimental Methods

Synthesis of the Fe-N-C cathode material

The Fe-N-C catalyst was synthesized by mixing 800 mg ZIF-8 (Basolite® Z1200, Sigma Aldrich), 200 mg 1,10-phenantroline (>99%, Sigma Aldrich), 16 mg of anhydrous iron (II) acetate (>99.99%, Sigma Aldrich), and 100 zirconia beads (5 mm diameter) in a zirconia-lined jar. The mixture was then ball milled at 400 rpm for four cycles of 25 min, with 5 min pause in-between. The obtained powder was then transferred to a quartz boat and flash pyrolyzed in Ar at 1050 °C for 1h. After the pyrolysis the sample was left to cool under Ar flow for 20 min. A second flash pyrolysis was then performed at 950 °C and with NH3 gas for 5 min. The same material has been studied by our laboratory in the past, both for PEMFC and AEMFC application, and was labelled as Fe0.5-950, Fe0.5-dry, or Fe0.5-NH3 [START_REF] Santori | High Performance FeNC and Mnoxide/FeNC Layers for AEMFC Cathodes[END_REF][START_REF] Adabi | Understanding how single-atom site density drives the performance and durability of PGM-free Fe-N-C cathodes in anion exchange membrane fuel cells[END_REF][START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogen-doped graphene materials[END_REF]. The subscript 0.5 refers to the weight percentage of iron in the complete catalyst precursor before the pyrolysis. The material was previously shown to contain iron as atomically dispersed Fe-Nx sites [START_REF] Zitolo | Identification of catalytic sites for oxygen reduction in iron-and nitrogen-doped graphene materials[END_REF],

the ammonia pyrolysis imparting the material with high amount of micropores and enhanced surface basicity, leading to markedly increased activity relative to the material obtained after Ar pyrolysis.

Electrode preparation and cell assembly

The gas diffusion electrodes (GDEs) used in this study were prepared by spray deposition. The full method can be found in our previous work [START_REF] Adabi | Understanding how single-atom site density drives the performance and durability of PGM-free Fe-N-C cathodes in anion exchange membrane fuel cells[END_REF] . In brief, inks were prepared by mixing deionized water, isopropyl alcohol, solid ionomer (in powder form) and electrocatalyst, with volumes or mass as reported in [START_REF] Adabi | Understanding how single-atom site density drives the performance and durability of PGM-free Fe-N-C cathodes in anion exchange membrane fuel cells[END_REF]. The anion exchange ionomer was an ETFE-BTMA (ethylene tetrafluoroethylene backbone -benzyl-trimethylammonium cationic groups) with an IEC of 1.24 meq•g -1 , the anode catalyst was PtRu/C (Alfa Aesar HiSPEC 10000, Pt nominally 40 wt. %, and Ru nominally 20 wt. %, supported on Vulcan XC-72R carbon) and the cathode catalyst was the Fe-N-C material previously described. After grinding and mixing the ink, it was spray deposited on a Toray 60 gas diffusion layer wetproofed with 5 wt.% polytetrafluoroethylene, to reach a target loading of 0.6 mgPtRu•cm -2 for the anode and 1.1 mgFe-N-C•cm -2 for the cathode. Afterwards, the prepared GDEs (25 cm 2 area) were cut into 4 samples of 5 cm 2 each, suitable for the cell hardware. Prior to mounting in the fuel cell hardware, the electrodes and membrane (radiation-grafted low density polyethylene (LDPE) functionalized with benzyl-trimethylammonium cationic groups, IEC of 2.54 mmol•g -1 , 22 µm thick in dehydrated form) [START_REF] Wang | Radiation-grafted anion-exchange membranes: the switch from low-to high-density polyethylene leads to remarkably enhanced fuel cell performance[END_REF] were ion-exchanged by immersion in 1.0 M KOH for 60 min while exchanging the 1.0 M KOH solution every 20 min. After ion-exchange, the excess KOH was removed and all cell components were mounted, without any hot pressing, in a Scribner fuel cell hardware with 5 cm 2 serpentine flow field. Teflon gaskets were utilized, and their thickness chosen to reach 25% compression.

Electrochemical measurements

The AEMFC was operated with a Scribner 850e Fuel cell test system. The effects of gas pressure, flow rate and dew point were investigated. If not otherwise noted, the cells were run with 500 mL•min -1 H2 and O2 fluxes, 1 bar (absolute) pressure and with humidifier dewpoints of 55 °C and a cell temperature of 60 °C. In the present study, only the operating conditions were varied (different combinations of gas pressure, flow rate and dew point at anode and cathode, but fixed cell temperature of 60 °C), while the cell assembly method and the cell components were unchanged. The gas pressure was either 1, 2 or 3 bars (absolute), the flow rate was 250, 500 or 1000 mL•min -1 , and humidifier dew point was 52, 55 or 58 °C. Both symmetric and asymmetric operating conditions were applied at anode and cathode, and Table 1 shows the list of all combinations applied as well as the sequence of measurements for each cell assembly. To avoid or mitigate possible biasing effects of irreversible electrode or membrane degradation (or due to cell shutdown and restart), in targeting the study of the effect of cell operation on cell performance and water management, a new cell assembly was prepared and utilized for each new sequence of measurements (sequence 1, 2 or 3, see Table 1). In this way, each sequence of measurement lasted only one day, and the cell was not shutdown during a sequence. The different anode and cathode samples utilized for each new assembly were however prepared during a single ink spray deposition, that resulted in 4 samples of 5 cm 2 each (see the earlier description in electrode preparation and cell assembly). The performance between different cell assemblies at the same baseline set of operating conditions can however vary slightly, making absolute comparisons among all tested combinations difficult, while the trends and absolute changes measured during each sequence (corresponding to a given cell assembly) are fully reliable.

At each set of operating condition (i.e. a given combination of gas fluxes, dewpoints, gas pressure), the following electrochemical tests were performed. First the cell was run galvanostatically at 0.6 A•cm -2 for 300 s and then let at open circuit voltage (OCV) for 10 s. This was followed by a potentiodynamic scan from OCV to 0.1 V with a scan rate of 10 mV•s -1 . After this, a galvanostatic staircase was performed by holding at 0.0/0.2/0.4/0.6/0.8/1.2/1.4/1.6/0.0 A•cm -2 for 5 min each. The sequence of galvanostatic measurements was aborted if the cell voltage went below 0.1 V. Examples of galvanostatic staircases where the experiment was aborted due to sudden drop in cell voltage below 0.1 V will be discussed and shown in the supporting information, but typically result in the absence of experimental points above a cut-off current density, for some operating conditions in the main figures. After the galvanostatic staircase, a second potentiodynamic scan was performed between OCV and 0.1 V with a scan rate of 10 mV•s -1 , in order to verify that the cell performance remained the same before and after the sequences of galvanostatic holds. To avoid excessive periods at OCV, and to maintain membrane humidification, a potentiostatic hold at 0.6 V was performed while changing from one set of operating conditions to another. The high frequency resistance (HFR) of the cell was continuously recorded at a frequency of 5 kHz using the integrated frequency resistance analyzer of the fuel cell test station.

Anode

Humidity measurement and water balance

To measure the outlet RH in the anode and cathode, HYT 939 humidity sensors (Innovative Sensor Technology IST AG) were placed at the fuel cell outlets. The inlet RH was assumed to be determined by the set temperature of the humidifier (100 % RH at the humidifier's set point temperature). This hypothesis was experimentally verified during the humidifier calibration (see Supporting information).

To avoid condensation on the sensors (which would lead to an underestimation of the total water content, the sensors being unable to capture the amount of liquid water) and to be able to quantify with the RH sensors any liquid water leaving the cell, the outlet pipes were heated to 66 °C, significantly above the cell temperature. As the pipes and sensors are warmer than the cell, the RH measured by the sensor is not the same as the RH of the gas at the cell temperature. Thus, the measured values need to be converted to a quantity that is independent of the local sensor temperature, such as partial pressure of water. The partial pressure of water vapor was calculated as follows:

𝑝 = 𝑅𝐻 * 𝑝 (𝑇) Eq. 3
Where pH2O is the partial pressure of water, RH is the relative humidity given by the sensor, 𝑝 (𝑇)

is the saturation partial pressure of water, and T the local temperature of the sensor. To calculate the saturation partial pressure of water at the sensor temperature, the Antoine equation [START_REF] Roizard | Encyclopedia of Membranes[END_REF] was utilized.

Once the partial pressure of water is known, Eq.3 can once again be used to determine the RH of the gas stream at the cell temperature. All reported RH values in the figures refer to the RH converted at the cell temperature of 60 °C.

While the water partial pressure is a useful measure of the concentration of water in the gas stream, the molar water flow normalized by geometric cell area (moles of water per second per area) is more useful for quantifying the amount of water entering and exiting the cell at each side. To calculate the molar flow of water in the outlet gas stream (either the anode or cathode outlet), the following equation was utilized, resulting from a combination of the ideal gas law and Faraday's law

𝑛 = 𝑛 , - * Eq. 4
Where nH2O is the molar flow of water at the outlet (coming partly from the humidifier, and partly from the water that is electrochemically produced in the cell), A is the geometric cell area, nDry,in is the flow of dry gas before it is humidified (i.e. corresponding to 250, 500 or 1000 mL• min -1 ), I is the cell current, z is the number of electrons involved in the reactions per molecule of H2 or O2 in the half reactions (z = 2 for the HOR and 4 for the ORR), F is Faraday's constant, pH2O is the water partial pressure at the fuel cell outlet, calculated using Eq.3, and ptot is the total absolute gas pressure at the fuel cell outlet.

Next, we define the change in the amount of water in the gas flow (at anode or cathode) as the difference between the actual water flow at the outlet of an AEMFC delivering a constant current I and the molar water flow corresponding to a 'cell bypass'. For the latter, measurements, the partial pressure of water when no current is applied and only humidified gas is flown through the cell.

Applying Eq. 4 (either the anode or cathode side), the change in water, Δ𝑛 , can be calculated according to:

Δ𝑛 = 𝑛 , - * -𝑛 , , ,
Eq. 5

Where 𝑝 , is the partial pressure of water in the gas when no current is applied. In practice, a positive value of Δ𝑛 would indicate that some water was added to the gas stream when passing through the single cell (as expected at the anode side due to production of water by the HOR), and a negative value of Δ𝑛 would indicate that some water was removed from the gas stream when passing through the single cell (possible at the cathode side, if more water is consumed by ORR and dragged with OH -ions than diffuses back from the anode). The water increase can thus be calculated when one knows the dry gas flow at inlet, the cell current, the total gas pressure at outlet, the water partial pressure at outlet and the humidifier setpoint. All those parameters are classically monitored, except the water partial pressure at outlet, which requires humidity sensors, utilized in the present study.

A particular case of Eq. 5 is worth discussing when considering changing operational parameters. As the maximum water vapor pressure, before flooding occurs, is the saturated partial pressure at the cell temperature (Tcell), Eq. 5 can be used to calculate the theoretical maximum flow of water that can be removed as gas phase from either side (anode or cathode). This is described by the following equation.

Δ𝑛 , = 𝑛 , ( ) ( ) -𝑛 , ( ) ( ) - * ( ) ( )
Eq. 6

Where 𝑇 is the humidifier temperature .The first term in Eq. 6 is the maximum amount of water which can exit in the outlet stream, the second term is the flow of water in the inlet stream, and the third term describes the indirect decrease in the water flow of water due to the electrochemical consumption of dry gas (hydrogen or oxygen) in the electrodes. As can be deduced from Eq. 6, for a given current density, increasing the dry flow linearly increases the maximum amount of water that can be removed via gas phase. Increasing the total gas pressure at the outlet causes a decrease in the maximum amount of water. Finally, decreasing the humidifier temperature decreases the flow of water in the inlet stream (second term), increasing the maximum flow of water that can be removed as gas phase. Examples of predictions using Eq.6 are shown in Figure S1 in the Supporting Information.

After the electrochemical measurements of each sequence, the humidifiers were calibrated by bypassing the single cell, setting the temperature of the anode and cathode humidifiers to a single value (60, 58, 53 or 50 °C, in chronological order) while flowing 200 mL•min -1 of N2. The calibration point was taken after a minimum of 1 h or until a stable sensor signal was obtained. The obtained calibration curve is then used to correct the sensor readings (see Supporting Figure S2a). The calibration of the sensors is required to avoid errors arising from variations of sensor temperatures and sensor readings drifting over time.

Results

Comparison of potential scans and galvanostatic measurements

Using potentiodynamic scans is a common technique to report the performance of AEMFCs. Figure 1a shows for the baseline operating conditions the polarisation curves obtained by forward (from OCV to 0.1 V) and backward (from 0.1 V to OCV) potentiodynamic scans, as well as a comparison with a polarisation curve constructed from a set of galvanostatic holds. While galvanostatic holds provide the steady-state response of the cell, a potential scan provides a transient state of the cell, when the scan rate is faster than the time needed for the slowest physico-chemical phenomenon in the cell (likely, water gradient across the cell) to reach steady-state. It should be noted that even though the highest current density achievable at low potential is uninteresting in practical fuel cell operation (due to the low efficiency of H2 conversion to electricity at such low potential), it is useful to discuss throughout the present work as a measure of the limitation of the cell by flooding effects (or other mass transport effects), since the electrochemical kinetics of the cathode and anode are fixed. As can be noted in Figure 1a, the highest current density (ca 1.5 A•cm -2 ) was achieved during the forward scan at a voltage of ca 0.3 V. At lower cell voltage, the current density started to decrease even during the forward scan, which can be assigned either to a drying of the cathode and/or flooding of the anode due to high current density, directly related with the high amount of water production/consumption in the anode/cathode. A similar shape of the forward scan had been reported previously by us with a similar Fe-N-C cathode [START_REF] Santori | High Performance FeNC and Mnoxide/FeNC Layers for AEMFC Cathodes[END_REF]. The backward scan shows a memory effect of the mass-transport limitation built up during the high current density operation of the cell, as the backward scan was significantly lower than the forward scan, between 0.1 V and ca 0.55 V (current density of ca 0.8 A•cm -2 ). As a galvanostatic hold is more comparable with a steady state measurement, this means that a fast potentiodynamic scan can, not unexpectedly, overestimate the maximum achievable performance of a cell at steady-state.

When considering the values of anode and cathode outlet RH, considered at the cell temperature of 60 °C, measured online with the sensors, Figure 1b, the varying performance of the forward and backward potentiodynamic scans can be better understood. Considering first the forward scan, the outlet RH increases on both anode and cathode sides with increasing current density, until the maximum current density of 1.5 A•cm -2 is reached. At that point, the anode outlet RH is close to 100 % while the cathode outlet RH is ca 88 %. This suggests that anode flooding is limiting the maximum achievable current density, with these operating conditions. Continuing the forward scan (further decreasing the cell voltage), the current density decreases while the anode and cathode outlet RHs still increase, further supporting that anode flooding is taking place and is the main reason for the fact that the current density decreased, instead of an expected increase, when further decreasing the cell voltage. During the backward scan, the outlet RH of both anode and cathode sides remain significantly above the RH values measured during the forward scan, when the comparison is made at a given current density. For example, at 0.5 A•cm -2 , the anode outlet RH are ca 80 and 98 % in the forward and backward scans, respectively, while the cathode outlet RH are ca 79 and 88 % in the forward and backward scans, respectively. The higher RH values measured throughout the backward scan can be assigned to the high amount of water produced during the peak of current density, flooding the anode and leading to a memory effect during the entire backward scan. Moreover, it can be noted that even though the polarisation curves measured during the forward and backward scans nearly overlap at current densities below 0.6 A•cm -2 , the RH measurements show that the humidification conditions at the anode and cathode are still very different between forward and backward scan, even below 0.6 A•cm -2 (Figure 1b). On the backward scan, the anode outlet RH became lower than 100 % at a current density of ca 0.65 A cm -2 (Figure 1b) and this coincides nicely with the closing of the hysteresis between forward and backward polarisation curves (Figure 1a). Thus, the measurement of an anode outlet RH at or above 100 % seems intimately connected with a drop in AEMFC performance, anode RH above 100 % in turn assigned to anode flooding. While anode flooding had been proposed earlier in the field as a main limitation of AEMFC [START_REF] Mustain | Understanding how high-performance anion exchange membrane fuel cells were achieved: Component, interfacial, and cell-level factors[END_REF][START_REF] Omasta | Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs[END_REF], it is the first time that a direct connection between anode flooding and AEMFC performance is demonstrated with online RH measurements. Regarding the outlet RH measurements performed during the series of galvanostatic holds, the values are systematically in-between those measured for the forward and backward scans (Figure 1b). For the forward scan, the system probably did not have sufficient time to accumulate water and to be representative of a steady-state response, relative to the series of galvanostatic holds. The entire forward scan lasts only ca 80 s, while a single galvanostatic hold (a single point on the galvanostatic series) lasts 300 s. For the backward scan, the high amount of water produced at the highest current density during the forward scan likely accounts for the higher RH outlet values as compared to RH outlet values measured during the galvanostatic holds. To further understand the differences between potentiodynamic scans and galvanostatic holds, the effect of different steady states of the AEMFC just before these measurements was tested. As can be seen in Figure 2a,c,e, starting the scan just after either a galvanostatic hold at 0.6 A•cm -2 or just after an OCV hold had an impact on the measured performance. When the inlet RH was relatively low (symmetric anode and cathode dewpoint of 52 °C, Figure 2a, or symmetric anode and cathode dewpoint of 55 °C, Figure 2c), the cell performance was improved following a previous step at 0.6 A•cm -2 compared to a previous step at OCV. However, for the measurement with higher inlet RH (symmetric inlet dewpoints of 58 °C), a lower maximum current density was observed following a previous step at 0.6 A•cm -2 (Figure 2e). The observed differences in performance and maximum current density can be understood by the corresponding measurements of anode and cathode outlet RH, as shown in Figure 2b,d,f. First it should be noted that for all three conditions of inlet dewpoints, both the anode and cathode outlet RH increase with increasing current density, and the anode outlet RH is almost always higher than the cathode outlet RH, and the increase of outlet RH with increasing current density is always stronger for the anode than for the cathode (compare slopes of blue and red solid curves, or slopes of blue and red dotted curves in Figure 2b,d,f). This is in line with the fact that water is produced at the anode side and water consumed at cathode side, in operating AEMFC. Regarding the value of peak current density reached in each condition (before the current decreases, upon further decreasing cell voltage), there is a strong correlation between its value and the moment at which the anode outlet RH reaches the exact value of 100 %. This is particularly visible in Figure 2c-d, and Figure 2e-f, while with lower inlet dewpoint of 52 °C, the anode outlet RH never reaches 100 % and accordingly, the polarisation curves do not show a peak in current density but a continuously increasing current with decreasing cell voltage. In other words, when the dew point of gases at the inlet is low (52 or 55 °C), the cell performance can be slightly improved by equilibrating the cell at 0.6 A•cm -2 before the scans (this increases slightly the RH in the electrodes at any given current density during the scans, improving OH-conductivity without leading to flooding), while when the dew point of gases at the inlet is high (58 °C), the cell performance is slightly decreased after equilibration at 0.6 A•cm -2 , due to an earlier onset of anode flooding compared to the case of a cell equilibrated at OCV. The outlet RH at the anode was approximately 88 % at the beginning of the scan after cell equilibration at OCV, while it was already 94 % after cell equilibration at 0.6 A•cm -2 prior to the scan (Figure 2f, blue curves). This leaves only a small margin to remove by gas phase any water that is electrochemically produced at the anode during cell operation, while the transfer via diffusion of water from anode to cathode is also limited by the high RH at the cathode in symmetric operation condition.

Nevertheless, among this set of experiments, the main driver for the change in cell performance remains the inlet dew point of gases, a low inlet dewpoint avoiding anode flooding (no peak in current density, Figure 2a) but low OH -conductivity in the AEM and electrodes leading to non-optimum polarisation curve, an intermediate dewpoint for incoming gases leading to a proper balance between high humidification needed for high OH-conductivity and the need to avoid anode flooding, and excessively high dewpoint for incoming gases (58 °C) leading to high OH conductivity and good cell performance at low current density but an early onset of anode flooding (at 0.9-0.95 A•cm -2 in the present case), strongly limiting the current density and peak power density.

The results of the scans and galvanostatic datapoints shows that the higher peak current density or maximum current density reached when scanning the cell voltage is due to the system not having time to accumulate water and thus not being at steady-state. The 10 mV•s -1 polarisation scans can thus incorrectly predict the steady-state response of the cell, especially when the cell is close to a full humidification (close to the peak/maximum current density, or when the inlet dewpoint is close to the cell temperature). In the remainder of this work, we chose to show only the cell performance obtained under 5 min galvanostatic control (point-by-point polarisation curve). The results also clearly show the strong interconnection between the reading of 100 % RH as anode outlet RH and the achievable peak in current density. This is in line with the view that anode flooding can limit the performance of AEMFCs [START_REF] Mustain | Understanding how high-performance anion exchange membrane fuel cells were achieved: Component, interfacial, and cell-level factors[END_REF][START_REF] Omasta | Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs[END_REF]. In contrast, the data show that the cathode outlet RH increases with current density, thereby not supporting the previous idea in the AEMFC literature that high current density can lead to a cathode drying effect. While the cathode seems always in dryer condition than the anode at a fixed current density, the cathode RH during AEMFC operation is always wetter than at OCV. The increase of cathode RH with increasing current density of AEMFC is in line with a few reports [21,[START_REF] Carlson | Fuel cell evaluation of anion exchange membranes based on poly(phenylene oxide) with different cationic group placement[END_REF]. Those studies were however performed with non-state of the art AEM and ionomer and were limited to low current density.

Effects of dewpoints

The effect of the symmetric gas inlet dewpoint (52, 55 or 58°C) on the galvanostatic polarisation curve, anode and cathode outlet RH, changes in water (calculated using Eq. 5 with input from experimental operating parameters and from pH2O measurement with humidity sensors) and cell HFR are shown in Figure 3a,b,c,d respectively. As can be seen in Figure 3a, the highest performance was achieved with an inlet dewpoint of 55 °C, leading to higher cell voltage at any current density, and in particular at high current density. This is explained by the measurements of gas outlet RH, Figure 3b, showing that with 55 °C inlet dewpoint, the outlet RH at anode surpasses 100 % only at 1.2 A•cm -2 . This implies that anode flooding was avoided, while the RH in the anode and cathode was fairly high at any current density (outlet RH of 80-100% for anode and 80-90% for cathode, for the entire series of galvanostatic holds), beneficial for OH-conductivity throughout the cell assembly and thus beneficial for the overall performance. In comparison, the measurement with the inlet dewpoint of 52 °C leads to an approximately 10 % decrease in the RH (of anode, and cathode) for all current densities (Figure 3b), which lowers the OH-conductivity (Figure 3d) and thus cell performance (Figure 3a). With a low inlet dewpoint, the current density of 1.2 A•cm -2 can be achieved, but at a lower cell voltage. In contrast, with an inlet dewpoint of 58 °C, the maximum current density that the cell can sustainably produce for at least 5 min is 0.6 A•cm -2 (Figure 3a). When applying 0.8 A•cm -2 , the cell voltage suddenly dropped below 0.1 V, and the cell was stopped. This is explained by the fact that the anode outlet reaches 100 % at 0.6 A•cm -2 , defining the maximum steady state current density achievable (Figure 3b, red circles). The calculated change in water in the gas stream, Figure 3c, shows that the increase in water at the anode with current density is nearly identical for all three operating conditions, and the same is observed for the cathode. This suggests that when switching from inlet dewpoints of 52°C to 58 °C, there is no significant changes in the water transport mechanisms for the membrane. In other words, the net positive flow of water molecules per second and per cm 2 at the anode (or at the cathode) is entirely set by the cell current density, and not influenced by the gas inlet dewpoint. Finally, the cell HFR was seen to be strongly correlated to the mean outlet dewpoint (mean of the measured anode and cathode outlet RH), which is expected as higher humidity increases the conductivity of the membrane.

Effects of gas pressure

Increasing the total pressure of a fuel cell is known to increase the overall performance as the gas activity is increased, which enhances both catalytic activity and active species diffusion. As can be seen in Figure 4a, increasing the anode gas pressure from 1 to 3 bar (absolute), while keeping the cathode pressure at 1 bar, increased the OCV slightly, due to the increased concentration of hydrogen (Figure 4a, compare red and blue curves). However, the highest achievable current density was only 0.8 A•cm - 2 , significantly lower compared to the measurement with the baseline pressure of 1 bar on both sides, achieving 1.2 A•cm -2 . When the gas pressures at both anode and cathode were increased to 3 bars, a significant performance improvement was noticed at low current density (Figure 4a, compare red and cyan curves), but the maximum current density achievable with the pressurized cell was only 0.4 A•cm -

2

. When the anode gas pressure was only 1 bar but the cathode gas pressure was 3 bars, an intermediate situation is observed, with a combination of high cell performance at low-medium current density and reasonably high maximum current density achieved (green curve in Figure 4a). As mentioned in the experimental method, the experiment was aborted when the cell voltage dropped rapidly below 0.1 V when attempting to apply the next current density in the series of galvanostatic staircases, and this appears as no data points above certain current densities in Figure 4. Examples of continuous cell voltage vs. time curves for such aborted measurements are shown in Figure S4.

The cause for the trends in performance can, as previously with the study on the effect of inlet dewpoints, be coupled to the measurement of the outlet RH values, in particular the anode one. As shown in Figure 4b, the anode outlet RH exceeded 100 % at the highest current density achieved for any given gas pressure combination, suggesting that anode flooding occurred on the next galvanostatic hold that was tried in the sequence. Similar to the previous measurements discussed in the present work, the outlet RH increased with increasing current density on both anode and cathode, for all the different combinations of anode and cathode gas pressures, and the anode outlet RH is always higher than the cathode outlet RH (Figure 4b).

Furthermore, the measurements performed with higher gas pressure show a sharper increase of outlet RH with increasing current density (compare red and cyan lines in Figure 4b). This agrees with Eq. 5, predicting that a higher total pressure decreases the amount of water that can be removed as water vapour with the exiting flow of gases. Surprisingly, increasing the anode gas pressure from 1 to 3 bar while keeping the cathode at 1 bar, blue and red curves in Figure 4c, resulted in a simultaneous increase of both the anode and cathode outlet RH values. Similarly, increasing only the cathode gas pressure, red and green curves in Figure 4c, resulted in a simultaneous increase of both the anode and cathode outlet RH values. This suggests that, at least with sufficiently thin AEM, the anode and cathode conditions are intimately interconnected, and that optimal water management can be achieved by tuning the operating conditions (pressure, inlet dewpoint, flow rate) at either side.

The calculated curves of the changes in water at the anode as a function of the current density were similar for the two experiments performed with equal gas pressures at the anode and cathode, i.e.

1bar/1 bar and 3 bar/3 bar (compare red and cyan solid curves on Figure 4c). When a positive pressure gradient is established at anode relative to cathode, the slope of the change in water at the anode vs.

current density becomes lower compared to the measurements with identical gas pressures on each side (compare for example solid blue and solid red curves in Figure 4c). In contrast, when a negative pressure gradient is established (i.e. a lower pressure anode relative to that at the cathode), the slope of the change in water vs. current density becomes higher compared to the measurements with identical gas pressure on each side (compare for example solid green and solid red curves in Figure 4c). This suggests that the rate at which water can be transported from anode to cathode through the AEM is affected by the pressure gradient. Together, the polarisation curve data and online RH measurements data show that increasing the cathode gas pressure increases the cell performance due to enhanced cathode performance, but decreases the maximum current density that can be achieved due to the modified water management and anode flooding taking place at an earlier current density (red and green data in Figure 4).

For all pressure combinations, the amount of water at anode and cathode increases linearly with current density (Figure 4c), indicating that significant back diffusion of water from anode to cathode takes place in all those conditions, leading to a positive value of ΔnH2O at the cathode at all current densities. When uneven gas pressure is applied at anode and cathode sides, the ΔnH2O value increases always faster with current density on the less pressurized side. This result suggests that the removal of water through evaporation into the dry flow is the main mechanism for removal of water from the electrode at these operating conditions.

For all combinations, the cell HFR shows a clear decreasing trend with higher mean outlet RH (Figure 4d). However, while increased anode pressure from 1 to 3 bars resulted in a lower HFR compared to the baseline 1bar/1bar (compare blue and red data in Figure 4d), increasing the cathode pressure resulted in an increased HFR (compare green and red data in Figure 4d). This is most likely due to the mean outlet RH not being the most exact descriptor of the actual RH within the cell as the membrane is in equilibrium with the local gas RH.

Effects of dry flow rates of gases

As shown in Eq. 6, increasing the flow rate of dry gases (gas flow rate before they enter the humidifiers)

should mitigate cell flooding. The humidifiers temperatures were kept constant, implying the inlet RH did not change in this set of measurements. The polarisation curves are nearly superimposed for all tested combinations of anode and cathode flow rates (Figure 5a). In contrast, the maximum achievable current density varied drastically from 0.2 to 1.0 A cm -2 (Figure . 5a). As expected, the polarisation curve with low flow rates on both sides (250 mL min -1 , green curve in Figure 5a) resulted in the lowest maximum current density (0.2 A cm -2 ), due to the low capacity of the gases exiting the anode and cathode to remove water vapor in such conditions. Increasing the anode and cathode flow rates to 1000 mL min -1 (cyan curve in Figure 5a) resulted in a maximum current density of 0.8 A cm -2 .

Such a flow rate is classical in the field when testing 5 cm 2 cell AEMFC [START_REF] Santori | High Performance FeNC and Mnoxide/FeNC Layers for AEMFC Cathodes[END_REF][START_REF] Yang-Neyerlin | Editors' Choice-Examining Performance and Durability of Anion Exchange Membrane Fuel Cells with Novel Spirocyclic Anion Exchange Membranes[END_REF][START_REF] He | Probing Anion Exchange Membrane Fuel Cell Cathodes by Varying Electrocatalysts and Electrode Processing[END_REF][START_REF] Omasta | Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs[END_REF][START_REF] Mandal | The Importance of Water Transport in High Conductivity and High-Power Alkaline Fuel Cells[END_REF]. Interestingly, the same maximum current density of 0.8 A cm -2 achieved with only 250 mL min -1 on the anode and keeping the cathode flow rate of 1000 mL min -1 . The highest maximum current density of 1 A cm - 2 was achieved with asymmetric flow rates of 1000 mL min -1 on the anode and 250 mL min -1 on the cathode. The online measurements of outlet RH shed light on these results (Figure 5), as discussed below.

Flooding conditions was approached at the anode, in the case of the measurement with an anode flow rate of 250 mL min -1 and cathode flow rate of 1000 mL min -1 (Figure 5b, blue circle at 0.8 A cm -2 , with RH outlet of ca 100%). With low flow rates on both sides (250 mL min -1 ), one can see that the anode outlet RH increases the most rapidly with increasing current density, reaching already 94 % at 0.4 A cm -2 (green circles in Figure 5b). At the same time, the cathode outlet RH is also quite high (ca 89 %, green crosses in Figure 5b), limiting the amount of water than can be removed via the cathode outlet.

This likely is the reason for rapid anode flooding at currents slightly above 0.4 A cm -2 in such conditions, since the cell voltage dropped to below 0.1 V when attempting to apply 0.6 A cm -2 with 250 mL min -1 on each side. The positive effect on the maximum achievable current density when switching from equal flow rates of 250 mL min -1 to 1000 mL min -1 on each side is evidenced by the online RH measurements on Figure 5b, showing strongly decreased slopes of the outlet RH vs. current density curves, both at anode and cathode (compare cyan and green circles for the anode, and cyan and red crosses for the cathode, in Figure 5b). Interestingly, while both the anode and cathode outlet RH are well below 100 % at 0.8 A cm -2 in the flow conditions of 1000/1000, the cell voltage dropped below 0.1 V when attempting the next current density, namely 1 A cm -2 . This suggests other limitations than electrode flooding in setting the maximum current density of AEMFC. Possibly, poor catalyst utilization due to poor ionomer conductivity at relatively low RH can cause significant performance losses at high current densities [START_REF] Gerhardt | Along-the-Channel Impacts of Water Management and Carbon-Dioxide Contamination in Hydroxide-Exchange-Membrane Fuel Cells: A Modeling Study[END_REF]. This is supported by the HFR, Figure 5d, where the flow of 1000/1000 had a higher HFR compared to the 250/250 flow suggesting that the membrane, and as such also the ionomer, is much drier and not as conductive at elevated flows. With asymmetric flow rates of 250 at anode and 1000 at the cathode, the curve of anode outlet RH vs current density is shifted upward compared to the 1000/1000 case, while the curve of cathode outlet RH vs current density is similar to that with 1000/1000 conditions. While the RH outlet measurements for these two conditions in particular are as intuitively expected, the data show that the cell can operate equally well (same maximum current density) even though the anode outlet RH is always higher in the case of the 250/1000 conditions. This suggests that outlet RH does not necessarily represent the local RH inside electrodes. Since flow of gases with less than 100% RH is a sink term (i.e. can help remove water from one electrode), it can be expected that the difference between outlet RH and local RH for a given electrode increases with increasing flow rate. In this sense, while at low flow rate, anode flooding may take place when the anode outlet RH is close to 100 %, anode flooding may take place already when the anode outlet RH is only 80%. As the outlet RH increased for all tests with increasing current density (Figure 5b), the amount of water also increases, as shown in Figure 5c. Surprisingly, the water increases on anode or cathode were not equal for the two tests with symmetrical flows (compare green and cyan in Figure 5c), being higher on the anode when both flows were increased from 250 mL min -1 to 1000 mL min -1 . This suggests that water transport from the anode active layer to the gas stream could be limiting in the overall water transport, as the higher dry gas flow lowers the RH.

Increasing the cathode flow while keeping the anode flow constant (compare dashed green and dashed blue in Figure 5c) leads to more water being removed at the cathode side, showing that increasing the cathode flow could be utilized to reduce the risk of flooding at the anode. The HFR seen in Figure 5d shows similar results as for the data set on pressure effect, with higher mean outlet RH leading to a lower HFR, when considering each combination of flows. However, the spread of HFR values between the different tests is higher, most likely due to the outlet RH not being representative of the RH across the cell (membrane electrode assembly), especially at high flows.

Similarly with the results with varying pressure and dewpoints, the outlet RH of both sides increased with increasing current density, suggesting that current induced cathode dry out will not occur for any of the tested conditions. For nearly all tested operating conditions in this study, anode flooding limited the maximum achievable current density, while a few were due to low cell performance due to the cell being too dry, causing low ionomer conductivity which reduces the achievable current density [START_REF] Gerhardt | Along-the-Channel Impacts of Water Management and Carbon-Dioxide Contamination in Hydroxide-Exchange-Membrane Fuel Cells: A Modeling Study[END_REF]. Several recent studies discussing water management came to the conclusion that the flux of water that is transported from anode to cathode via back diffusion defines the maximum current density achievable in AEMFCs [START_REF] He | Water limiting current measurements in anion exchange membrane fuel cells (AEMFCs); part 1: Water limiting current method development[END_REF][START_REF] Peng | Using operando techniques to understand and design high performance and stable alkaline membrane fuel cells[END_REF][START_REF] Mandal | The Importance of Water Transport in High Conductivity and High-Power Alkaline Fuel Cells[END_REF]. This is in line with our measurements, as anode flooding can be seen as a consequence of poor back diffusion. The challenge for AEMFCs is to have electrodes which are fully humidified, for high ionomer conductivity, while still avoiding mass transport limitations. This can be done by tuning the hydrophobicity of the electrodes. This strategy has already been employed in both Pt/C and PtRu/C electrodes to ensure higher water removal [START_REF] Mandal | The Importance of Water Transport in High Conductivity and High-Power Alkaline Fuel Cells[END_REF]. Restricting the removal of water in Fe-N-C catalysts should allow for improved performance the current densities do not cause cell flooding. However, this would most likely require modifications to the Fe-N-C carbon to make it more hydrophilic or by changing the pore structure.

Conclusions

In this work, the water management at varying operating conditions of an AEMFC based on radiationgrafted LDPE with benzyl-trimethylammonium cationic groups membranes in combination with PtRu/C anode and Fe-N-C cathode was investigated. The outlet RH was continuously measured using RH sensors, allowing the limitations to be determined. It was found that for all operating conditions the water amount increased on both anode and cathode when a current was applied. Furthermore, it was found that the increase in water amount was linear with current density, suggesting that no changes in the transport mechanisms occur with an increase in current density. It was also shown that hysteresis in potentiodynamic scans is linked to anode flooding, occurring during the negative-going scan. Further, results showed that a high inlet dewpoint of 58 °C led to anode flooding, while a low inlet dewpoint of 52 °C instead led to poor catalyst utilization. Increasing either anode or cathode gas pressure causes increases in the outlet RH on both anode and cathode. However, the performance was only increased in the case of increased cathode pressure. It was also shown that a low cathode flow led to anode flooding at low current density, while a high cathode flow seems to have led to poor catalyst utilization due to insufficient hydration of the ionomer. Overall, these results reinforce the idea that a high humidity, but still below flooding, is crucial to achieve high current densities and high performances in AEMFC and that changing operating conditions on either anode or cathode affects the water content on both electrodes, complexifying the water management of AEMFC but also giving opportunities.

Author credit statement

Figure 1 .

 1 Figure 1. Comparison of cell performance and humidity measurements obtained with potentiodynamic scans and galvanostatic holds. (a) I-V curves corresponding to forward (solid curve) and backward (dotted curve) potentiodynamic scans and the set of galvanostatic measurements (circles). (b) Corresponding RH readings at anode and cathode outlet from the sensors. The potentiodynamic scans were performed between OCV and 0.1 V with a scan rate of 10 mV•s -1 and the galvanostatic points were obtained by taking the mean of the last 30 s of each 5 min-long galvanostatic measurement. Measurements done with H2 and O2 with a symmetric flow rate of 500 mL•min -1 , symmetric gas pressure of 1 bar (absolute), symmetric inlet dewpoint of 55 °C and a cell temperature of 60 °C.

Figure 2 . 2 (

 22 Figure 2. Effect of the cell steady-state prior to the potentiodynamic scan on the cell performance and cell outlet RH. (Left) Forward potentiodynamic scan either after 5 min of OCV (solid curves) or after 5 min at 0.6 A•cm -2 (dotted curves). (Right) Corresponding RH readings at anode and cathode outlet. The potentiodynamic scans performed between OCV and 0.1 V with a scan rate of 10 mV•s -1 and the galvanostatic points were obtained by taking the mean of the last 30 s of the 5 min galvanostatic measurement. Measurements done with H2 and O2 with a flow rate of 500 mL•min -1 , symmetric gas pressure of 1 bar (absolute), and a cell temperature of 60 °C.

Figure 3 .

 3 Figure 3. Effects of humidifier dewpoint. (a) Voltage response of galvanostatic measurements. (b) RH readings at anode and cathode outlet from the sensors. (c) Change in water at anode and cathode outlets. (d) HFR as a function of the mean outlet RH (mean of anode and cathode). The galvanostatic points were obtained by taking the mean of the last 30 s of the 5 min galvanostatic measurement. Measurements done with H2 and O2 with 500 mL min -1 flow, symmetric gas pressure of 1 bar (absolute), and a cell temperature of 60 °C.

Figure 4 .

 4 Figure 4. Effects of anode and cathode gas pressures. (a) Voltage response of galvanostatic measurements. (b) RH readings at anode and cathode outlet from the sensors. (c) Change in water at anode and cathode outlets.(d) HFR as a function of the mean outlet RH. The galvanostatic points were obtained by taking the mean of the last 30 s of the 5 min galvanostatic measurement. Measurements done with H2 and O2 with 500 ml/min flow, a humidifier dewpoint of 55 °C and a cell temperature of 60 °C. Results obtained with additional combinations of gas pressures can be found in Figure S5.

Figure 5 .

 5 Figure 5. Effects of anode and cathode flow rates. (a) Voltage response of galvanostatic measurements. (b) RH readings at anode and cathode outlet from the sensors. (c) Change in water at anode and cathode outlets. (d) HFR as a function of the mean outlet RH. The galvanostatic points were obtained by taking the mean of the last 30 s of the 5 min galvanostatic measurement. Measurements done with H2 and O2, symmetric gas pressure of 1 bar (absolute), humidifier dewpoint of 55 °C and a cell temperature of 60 °C. Results obtained with additional combinations of dry flow rates can be found in Figure S6.

Björn Eriksson :

 Eriksson Conceptualization, Investigation, Methodology, Writing -Original Draft Preparation Pietro Giovanni Santori : Conceptualization, Investigation Frédéric Lecoeur : Methodology Marc Dupont : Methodology Frédéric Jaouen : Conceptualization, Writing -Original Draft Preparation

Table 1 :

 1 Operating conditions applied during each sequence, in chronological order.

		Cathode	H2 Flow Rate	O2 Flow Rate	Humidifier
	Pressure (bar)	Pressure (bar)	(mL min -1 )	(mL min -1 )	setpoint (°C)
			Sequence 1
	1	1	500	500	55/55
	1	2	500	500	55/55
	1	3	500	500	55/55
	2	3	500	500	55/55
	3	3	500	500	55/55
	3	2	500	500	55/55
	3	1	500	500	55/55
	2	1	500	500	55/55
			Sequence 2
	1	1	500	500	55/55
	1	1	250	500	55/55
	1	1	250	250	55/55
	1	1	500	250	55/55
	1	1	1000	250	55/55
	1	1	1000	500	55/55
	1	1	1000	1000	55/55
	1	1	500	1000	55/55
	1	1	250	1000	55/55
			Sequence 3
	1	1	500	500	55/55
	1	1	500	500	58/58
	1	1	500	500	52/52

Acknowledgements

This work was supported by the European Union's Horizon 2020 research and innovation programme under grant agreement CREATE [721065]. We are grateful to Prof. John R. Varcoe (University of Surrey, UK) for providing the ionomers and membranes which were produced by Dr Lianqin Wang, with funding from EPSRC (Grant EP/M014371/1).