The integration of proton exchange membrane fuel cells (PEMFCs) in heavy-duty vehicles and other demanding applications, such as aviation, would be simplified if the stacks could operate above 100 °C instead of the traditional low temperature (LT, up to 80 °C), thereby allowing a reduction in cooling system in size and power. This review offers a comprehensive compilation of experimental studies reported in the literature on PEMFCs operated in the intermediate temperature (IT)-range, here defined as above 80 °C and up to 120 °C, which represented the targeted upper temperature for PEMFCs. Membranes, electrodes and gas diffusion layers for IT-PEMFCs are discussed. Particular attention is paid to polymers in membranes and catalyst layer ionomers. Results from current state-of-the-art perfluorosulfonic acids and alternatives, including hydrocarbon polymers, are evaluated considering their properties and limitations. Further, system benefits and drawbacks of IT- compared to the traditional LT-operation are discussed, such as the interplay between vapour and oxygen pressure, hydrogen crossover and water management. We report on the lack of consistency between ex-situ and in-situ studies and underline the importance of in-situ tests, proposing guidelines to evaluate novel materials. For IT-operation, the development of stable polymers, which are the weakest components of the PEMFCs, is the most urgent challenge. As degradation happens faster at higher temperatures, further long-term tests are needed above 80 °C and accelerated stress tests should be specifically designed for IT-operation according to the polymer chemistries. We conclude that, compared to LT-, IT-operation requires improved materials and additional research.

Anion exchange membrane water electrolysis (AEMWE) has been the focus of significant research as it combines the advantages of previous electrolysis technologies. One pressing problem is the use of potassium hydroxide (KOH) as the feed in AEMWE systems due to its role in performance degradation. Asymmetric operation is one way forward, but the transport of KOH within these systems has not been extensively studied. Herein, a comprehensive study of KOH transport via pH measurement in-operando is exhibited. We have shown that KOH crossover heavily impacts the AEMWE performance. Transport number of potassium ion (K+) is calculated, indicating that K+ significantly contributes to the current. Moreover, we have shown that different alkali cations behave differently under applied electric fields. These measurements shed light on apparent mass transport limitations when near-neutral anolyte is used. Thus, the importance of KOH transport is highlighted herein, and directly contributes to the understanding of AEMWE.

The kinetics of the hydrogen electrode reactions, i.e., hydrogen- evolution (HER) and oxidation (HOR), were evaluated via a thin-platinum electrode. The 2D nature of this electrode minimizes the effect of current distribution, increasing the accuracy of the measured kinetic data. By utilizing a porous counter/reference electrode, this cell operated in both proton-exchange (PEM) or anion-exchange membrane (AEM) environments, with hydrogen as the only fuel, enabling the measurement of fast HOR/HER kinetics without mass transport limitations under more realistic fuel cell conditions. The HOR and HER activities on the Pt layer were evaluated via polarization curves at four hydrogen partial pressures (pH2) and three relative humidities, from which the reaction order of hydrogen was also calculated. The currents were normalized by the electrochemical active surface area (ECSA) obtained by CO-stripping voltammetry and electrochemical impedance results. 

The CO-stripping voltammograms in the PEM and AEM cell display a broad peak centered at 0.79 VRHE. Due to the large mobility of CO molecules in acidic media, the peak in the PEM cell is higher. For all RH, the specific HER activities on Pt in the PEM cell are between two and three orders of magnitude higher than in the AEM cell and the specific HOR activities on Pt are between one and two orders of magnitude in acid. Given the experimental mass-transport free conditions, a Tafel−Volmer (TV) mechanism is observed in PEM whereas in the AEM cell the electrochemical response appears to use the Heyrovsky−Volmer (HV) mechanism, although deviating for HOR at higher voltages due the presence of OH. Regardless of the media, the reaction order of hydrogen “m” for HER on Pt is close to 0, suggesting that the HER rate is independent of the pH2. In the case of HOR, the m value for the PEM cell is around 1, whereas for the AEM m converges around a value of 0.5, suggesting the Volmer reaction is the rate determining step. In Nyquist plots, the very high charge transfer resistance values for the AEM cell confirm the superior HOR/HER performance on Pt in acid. For the AEM cell, the presence of two semicircles may represent two kinetic processes ascribed to the electrode reaction.