High temperature proton exchange membrane fuel cells (HT-PEMFCs) are one type of promising energy device with the advantages of fast reaction kinetics (high energy efficiency), high tolerance to fuel/air impurities, simple plate design, and better heat and water management. They have been expected to be the next generation of PEMFCs specifically for application in hydrogen-fueled automobile vehicles and combined heat and power (CHP) systems. However, their high-cost and low durability interposed by the insufficient performance of key materials such as electrocatalysts and membranes at high temperature operation are still the challenges hindering the technology's practical applications. To develop high performance HT-PEMFCs, worldwide researchers have been focusing on exploring new materials and the related technologies by developing novel synthesis methods and innovative assembly techniques, understanding degradation mechanisms, and creating mitigation strategies with special emphasis on catalysts for oxygen reduction reaction, proton exchange membranes and bipolar plates. In this paper, the state-of-the-art development of HT-PEMFC key materials, components and device assembly along with degradation mechanisms, mitigation strategies, and HT-PEMFC based CHP systems is comprehensively reviewed. In order to facilitate further research and development of HT-PEMFCs toward practical applications, the existing challenges are also discussed and several future research directions are proposed in this paper.

High temperature proton exchange membrane fuel cells: progress in advanced materials and key technologies

New directions in the chemistry of shape memory polymers

A new ex situ method has been developed to mimic the degradation of the polymer membranes in polymer electrolyte membrane fuel cells (PEMFCs), caused by the cross-leakage of H2 and O2. In this ex situ setup, it is possible to expose membranes to flows of different gases with a controlled temperature and humidity. H+-form Nafion films with and without an electrode layer (Pt) have been treated in the presence of different gases in order to simulate the anode and cathode side of a PEMFC. The changes of the chemical structure occurring during the degradation tests were primarily examined by solid-state 19F NMR spectroscopy. For completion, liquid-state NMR studies and ion-exchange capacity measurements were performed. The molecular mobility changes of the ionomer membrane upon degradation were examined for the first time by variable-temperature 19F NMR line-shape, T1 and T1ρ relaxation experiments. It was found that degradation occurs only when both H2 and O2 are present (condition of gas cross-leakage) and w...

Chemical Degradation of Nafion Membranes under Mimic Fuel Cell Conditions as Investigated by Solid-State NMR Spectroscopy

Physical and chemical modification routes leading to improved mechanical properties of perfluorosulfonic acid membranes for PEM fuel cells

The effect of humidity on the degradation of Nafion® membrane

Glass transition temperature of perfluorosulfonic acid ionomers

Broadband dielectric spectroscopic characterization of Nafion® chemical degradation

H + -form Nafion 212 films were chemically degraded via Fenton's reagent and characteristic polymer chain motions were analyzed using broadband dielectric spectroscopy. The β-relaxation peak maximum frequency (f max ) shifts to higher temperatures upon degradation, reflecting slower chain motions. Perhaps this is due to an upward shift in average molecular weight caused by low molecular weight fragments being leached out of the membrane during the degradation experiment. Permittivity data for degraded and undegraded materials were fitted to the Havriliak-Negami equation. The quantities extracted from these fits were the relaxation time (r) and parameters that reflect the breadth and asymmetry of the distribution of T. These parameters were also used in the Vogel-Fulcher-Tammann-Hesse (VFTH) equation which was well-fitted to relaxation time versus temperature data. The increase in the Vogel temperature with degradation reflects more restricted chains. The distribution of relaxation time curve shifts to longer times, broadens, and is transformed from being bi- to tri-modal which reflects an increase in microstructural heterogeneity after degradation. It is concluded that dielectric spectroscopy can be a powerful tool in assessing the chemical degradation of Nafion membranes in the fuel cell environment.

David W. Rhoades

Papers

Glass transition temperature of perfluorosulfonic acid ionomers

Broadband dielectric spectroscopic characterization of Nafion® chemical degradation

Broadband dielectric spectroscopic characterization of Nafion® chemical degradation

Dielectric Spectroscopic Analysis of Amine Modified Nafion Precursor Films