In recent years, anion exchange membrane fuel cells (AEMFCs) have drawn more attention by showing high performance and potentially allowing catalysts based on more abundant materials. One frequently overlooked parameter in these systems is membranes permeability, often referred to as gas crossover. This undesirable phenomenon creates mixed potentials and also hotspots with temperatures high enough to cause membrane defects and pinholes. Therefore, it is important to investigate it quantitatively at conditions where a fuel cell often operates. In this study, hydrogen and oxygen crossover of several Aemion anion exchange membranes (AEMs) are measured with mass spectrometer (MS) at various relative humidities (RHs) and temperatures. The proton exchange membrane (PEM) Nafion is also investigated for a comparison. AEMs are generally less permeable to oxygen and hydrogen than PEM. The first-generation AEMs show a decreasing trend in permeability with RH, opposite to PEM. The second-generationAEMs are much less affected by RH levels. The measured oxygen crossover was two to six times lower compared to hydrogen crossover, depending on the membrane type and RH. The influence of membrane interface on crossover is evaluated to be higher for reinforced membranes, but overall smaller compared to the bulk contribution. Overall, AEMs show lower crossover than PEM, and from that perspective making them more suitable and a safer choice for fuel cell and electrolysis applications.

The electrode's composition and structure, affecting ion-conduction and water uptake and transport, is crucial for polymer electrolyte fuel cells. This study investigates the role of particles versus dispersed ionomer based on poly (arylene piperidinium) (PAP) for AEMFC. Mixed ionomer electrodes, consisting of linear PAP ionomers and crosslinked particles, are synthesized and evaluated in AEMFC single cells through electrochemical characterizations. The addition of insoluble particles corresponding to 5 % of total electrode weight leads to an increase in peak power density of similar to 60 % in comparison to when employing electrodes based purely on the linear ionomers such as poly(terphenyl piperidinium) and poly(terphenyl piperidinium-co-trifluoroacetophenone), respectively. A deconvolution of cell resistance contributions based on electrochemical impedance spectroscopy (EIS) data, combined with a distribution of relaxation times analysis (DRT), shows a significant decrease in effective cathode charge transfer resistance. This is attributed to particles serving as bridges between the membrane and the reaction sites, leading to increased ionic conductivity and active site utilization via shortening the distance of water and ion transport through the ionomer phase. In an expansion of the study, PAP particles were added to an electrode sample based on commercial Aemion + (TM). A smaller peak power density increase of 27 % was observed, emphasizing the importance of matching the chemical structures of the particles, membrane, and linear ionomer.

There is currently a drive to operate proton exchange membrane fuel cells at temperatures above 80 degrees C. To achieve this, all aspects of the fuel cell need to be investigated to ensure that performance and durability are maintained. In this work we systematically investigated carbon corrosion from 70 degrees C to 120 degrees C and 40 and 70 % relative humidity (RH). The investigation was done by cyclic voltammetry up to 1.4 V vs RHE with simultaneous mass spectrometry measurements to quantify the produced carbon dioxide. The obtained results were modelled to differentiate the effects of temperature, water partial pressure and different types of carbon corrosion. The results show that the carbon corrosion is mainly affected by the water partial pressure, which increases exponentially with temperature if RH is kept constant. This explains why an increase of temperature (at constant humidity) and an increase of humidity (at constant temperature) lead to faster corrosion kinetics. With current materials, to contrast the effect of the water partial pressure, the upper cell voltage limit in operation must be lowered. The combination of temperatures above 100 degrees C with high relative humidity at high voltage requires material improvement or additional mitigation strategies to avoid excessive carbon corrosion.

Ionomers based on poly(arylene piperidinium)s with varying ion exchange capacities are evaluated in different combinations of anode and cathode electrodes in anion exchange membrane fuel cells. The operational conditions are chosen with an asymmetrical regime including a dry anode with 50% relative humidity at the inlet, and full humidification at the cathode. Polarization and impedance measurements are carried out in potentiostatic steps within 0.3–0.9 V and distribution of relaxation times analysis is utilized to deconvolute resistance contributions. The results show that the best cell performance is achieved with both electrodes utilizing an ionomer with the highest ion exchange capacity (IEC) of 2.79 meq g⁻¹. Cells built exclusively from this ionomer achieved a peak power density of 1.01 W cm⁻². Deconvolution of the resistance contributions revealed the impact of water content on the effective charge transfer resistance in both electrodes and a diffusion resistance associated with the movement of water from anode to cathode side. The higher conductivity and water uptake of the high IEC ionomer resulted in a reduction of both resistance contributions, leading to the highest performance under the conditions evaluated. These findings provide important insights into how to tailor the electrode layers for optimum fuel cell output.

Efficient operation and durability of polymer electrolyte fuel cells (PEFCs) depend strongly on thermodynamic conditions and materials selection. This thesis investigates water transport, gas permeability and corrosion of carbon catalyst support under realistic operating conditions in proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs).

Experiments were conducted using real-time humidity sensors, mass spectrometry, and electrochemical methods, supported with numerical modelling in COMSOL Multiphysics.

Interest in PEMFCs which operate at low temperatures (LT, <80 °C) has shifted towards intermediate temperatures (IT, 80-120 °C) for the possibility of reducing size of cooling systems. Although water accumulation was minimized at IT, increased pressure led to the opposite effect, however with performance limitations attributed to reduced oxygen partial pressure (Paper I). Hydrogen crossover was found to increase with temperature, pressure and RH, and a convective component was observed under asymmetric pressure conditions (Paper IV and V).

In AEMFCs, asymmetric inlet humidification and dynamic gas flows were effective in managing hydration. Material properties such as ionomer water uptake and GDL hydrophobicity had a minor impact on water transport but considerably affected peak power output (Paper II and III). AEMs exhibited lower gas crossover than PEMs under all conditions, particularly the reinforced second-generation membranes (Paper V).

Carbon corrosion in PEMFCs was shown to accelerate significantly with both temperature and humidity, becoming the dominant reaction above 1.1 V and almost constant at the most extreme conditions tested (120 °C and 70 % RH, Paper VI).

These findings provide new insights into optimization strategies and limitations for PEFC systems, highlighting the importance of thermodynamic control and robust materials in achieving long-term, high-performance operation.

Effektiv drift och lång livslängd hos polymerelektrolytbränsleceller (PEFC) beror starkt på termodynamiska förhållanden och materialval. Denna avhandling undersöker vattentransport, gaspermeabilitet och korrosion av kolbaserat katalysstöd under realistiska driftsförhållanden i protonutbytande membranbränsleceller (PEMFC) och anjonutbytande membranbränsleceller (AEMFC).

Experimenten genomfördes med realtidsmätning av luftfuktighet, masspektrometri och elektrokemiska metoder, kompletterade med numerisk modellering i COMSOL Multiphysics.

Intresset för PEMFC som arbetar vid låga temperaturer (LT, <80 °C) har skiftat mot intermediära temperaturer (IT, 80–120 °C) för att minska kylsystemens storlek. Även om vattenackumulering minskades vid IT, ledde ökat tryck till motsatt effekt, med prestandabegränsningar kopplade till reducerat syrgastryck (Paper I). Vätgas-crossover ökade med temperatur, tryck och relativ luftfuktighet, och en konvektiv komponent observerades under asymmetriska tryckförhållanden (Paper IV och V).

I AEMFC visade asymmetrisk fuktmatning och dynamiska gasflöden effektiv hydrering. Materialfaktorer som vattenupptag i ionomerer och GDL:s hydrofobicitet hade liten inverkan på vattentransport men påverkade maximal effekt betydligt (Paper II och III). AEM uppvisade lägre gas-crossover än PEM-membran under alla testade förhållanden, särskilt de förstärkta membranen av andra generationen (Paper V).

Kolkorrosion i PEMFC accelererade kraftigt med både temperatur och fuktighet och blev den dominerande reaktionen över 1,1 V, och nästan konstant under de mest extrema testade förhållandena (120 °C och 70 % RH, Paper VI).

Dessa resultat ger nya insikter i optimeringsstrategier och begränsningar för PEFC-system, och understryker vikten av termodynamisk kontroll och robusta material för att uppnå långvarig och högpresterande drift.

Water is a key factor in anion-exchange membrane fuel cells, since it is both a product and a reactant, and humidifies the membrane and the ionomer phase. To optimize the operation conditions preventing cathode drying and anode flooding, better knowledge on the water transport is needed. In this work, the water transport across an AemionTM membrane is quantified for different applied water partial pressure differences and current densities. Two membrane thicknesses, 25 and 50 μm, are studied, as well as two gas diffusion layers (GDLs) of different hydrophobicity: the hydrophobic Sigracet 25BC treated with polytetrafluoroethylene (PTFE), and Freudenberg H23C2 being hydrophilic as it is not treated with PTFE. The measurements show that having a hydrophilic GDL on both electrodes results in poor electrochemical performance, and restricted water transport. Although the highest water molar flux was observed for hydrophilic GDL on cathode and hydrophobic GDL on anode, the best electrochemical performance was observed for the opposite combination. A water transport model considering absorption/desorption resistance, electroosmotic drag and diffusion was deployed. The best fit of the model to the experimental data was obtained with a water drag coefficient of 2, and almost about 30% difference in absorption/desorption coefficient due to different GDLs.

Improved knowledge on Proton Exchange Membrane Fuel Cell (PEMFC) behaviour in the Intermediate Temperature (IT: 80-120 degrees C) is needed. Here, ionic conductivity and H2 permeability are analysed under H2/N2 using electrochemical impedance spectroscopy, linear sweep voltammetry for three catalyst-coated membranes (CCMs): Nafion HP (reinforced), Nafion 211 (non-reinforced) and a reinforced commercial membrane (RCM, membrane thickness 13 mu m). Multiple relative humidity (RH) levels and pressure configurations are analysed at IT. Results show that ionic conductivity and H2 permeability increase with temperature and RH. However, lower crossover is measured above 100 degrees C and wet conditions due to low H2 partial pressure. The highest crossover is measured with an overpressure on the H2 side which, especially for RCM, suggests possible convection. The membrane reinforcement might reduce the permeability and it decreases the conductivity. Mass spectrometry confirmed that sprayed CCMs suffer from higher crossover than pristine membranes, although the membrane thickness is unchanged.

The integration of proton exchange membrane fuel cells (PEMFCs) in heavy-duty vehicles would be facilitated if operating temperatures above 100 degrees C were possible. In this work, the effect of temperature in the intermediate range of 80-120 degrees C is investigated for a commercial membrane electrode assembly (MEA) through polarization curves and electrochemical impedance spectroscopy. The importance of oxygen partial pressure on voltage is systematically studied by decoupling it from humidity and temperature. The results show that adequate oper-ation at intermediate temperature is achievable if the oxygen partial pressure is sufficient. Although the cathode kinetics is faster with rising temperatures, the voltage gain is counteracted by the decreasing equilibrium po-tential. At intermediate temperature, the water transport is enhanced, levelling out the relative humidity dif-ference between anode and cathode. However, ionic conductivity in the polymer can become limiting at high currents, due to a smaller relative humidity increase at these temperatures. To conclude, a higher operating temperature does not inherently cause a decrease in obtained current density. Rather, the difficulty to simul-taneously have sufficient oxygen partial pressure and high relative humidity causes limitations within the cathode that to some extent can be solved by pressurizing the cell.