Fuel cells convert energy stored in hydrogen into electricity. As a zero-emission technology, they represent a sustainable alternative to conventional combustion engines, particularly for heavy-duty vehicles. Proton exchange membrane fuel cells (PEMFCs) are used in vehicles and typically operate up to 80 °C. To facilitate the cooling of PEMFCs, slightly rising the operating temperature to the range of 80-120 °C, defined as intermediate temperature (IT), would be desirable.

The aim of the thesis is to electrochemically analyze the impact of IT operation on commercially available PEMFC materials. Results show that increasing the temperature has multiple effects on the cell. Increased ionic conductivity, faster reaction kinetics and reduced mass transport resistance are counteracted by a negative shift of the equilibrium potential, enhanced corrosion of the carbon support, and reduced gas barrier properties. Additionally, if the humidity and the cell pressure are constant, the partial pressure of oxygen is reduced at higher temperature, which limits the cell performance. Finally, a higher temperature leads to faster degradation. The ultimate failure is attributed to the formation of pinholes in the membrane, but the polymer conductivity and the catalyst's electrochemical surface area are also negatively affected. 

Despite a scarcity of comparable data above 80 °C, the main obstacle for IT-PEMFCs is apparently the lack of stable materials. Current state-of-the-art polymers for PEMFCs are based on perfluorosulfonic acid (PFSA), whose sustainability has recently been questioned. Alternatively, fluorine-free hydrocarbon-based polymers investigated here show comparable results up to 100°C, but cannot tolerate operation at 120 °C. More research is needed to further develop sustainable materials and to allow continuous operation of PEMFCs in the intermediate temperature range. 

Bränsleceller omvandlar energin lagrad i vätgas till elektricitet. Som nollemissionsteknik utgör de ett hållbarare alternativ till konventionella förbränningsmotorer. Vätgasens höga energiinnehåll gör dem särskilt lovande för tunga fordon. I fordon används vanligtvis protonledande membranbränsleceller (PEMFC) med en driftstemperatur runt 80 °C. För att underlätta kylningen av PEMFC, vore det önskvärt att kunna använda den vid en något förhöjd temperatur, från 80 till 120 °C, definierad som en mellantemperatur (IT). 

Syftet med avhandlingen är att elektrokemiskt analysera inverkan av IT-drift på kommersiellt tillgängliga PEMFC-material. Resultaten visar att en ökning av temperaturen har flera effekter på cellen: ökad jonledningsförmåga, snabbare reaktionskinetik och minskad masstransportmotstånd motverkas av en negativ förskjutning av jämviktspotentialen, ökad korrosion av katalysatorns bärarkol och försämrade gasbarriäregenskaper hos membranet. Dessutom, om fuktigheten och celltrycket är konstanta, minskar syrepartialtrycket vid högre temperatur, vilket begränsar cellens prestanda. I förlängningen orsakar den förhöjda temperaturen en snabbare nedbrytning av PEMFC som slutligen havererar genom att hål bildas i membranet. Även polymerelektrolytens ledningsförmåga och katalysatorns aktiva yta minskar vid långvarig drift.

Det saknas jämförbara data över 80 °C, men uppenbart är det största hindret för IT-PEMFC bristen på stabila material. Nuvarande polymerelektrolyter för PEMFC är baserade på perfluorsulfonsyra (PFSA), vars miljövänlighet nyligen har ifrågasatts. Alternativa fluorfria kolvätebaserade polymerer som undersökts här visar jämförbara resultat upp till 100 °C, men tolererar inte drift vid 120 °C. Mer forskning behövs för att utveckla hållbara material för att möjliggöra kontinuerlig drift av PEMFC i mellantemperaturintervallet.

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.