Frank Coms

Visualizing chemical reactions and crossover processes in a fuel cell inserted in the ESR resonator: detection by spin trapping of oxygen radicals, nafion-derived fragments, and hydrogen and deuterium atoms.

We present experiments in an in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that offered the ability to observe separately processes at anode and cathode sides and to monitor the formation of HO and HOO radicals, H and D atoms, and radical fragments derived from the Nafion membrane. The presence of the radicals was determined by spin-trapping electron spin resonance (ESR) with 5,5-dimethylpyrroline N-oxide (DMPO) as a spin trap. The in situ FC was operated at 300 K with a membrane-electrode assembly (MEA) based on Nafion 117 and Pt as catalyst, at closed and open circuit voltage conditions, CCV and OCV, respectively. Experiments with H(2) or D(2) at the anode and O(2) at the cathode were performed. The DMPO/OH adduct was detected only at the cathode for CCV operation, suggesting generation of hydroxyl radicals from H(2)O(2) formed electrochemically via the two-electron reduction of oxygen. The DMPO/OOH adduct, detected in this study for the first time in a FC, appeared at the cathode and anode for OCV operation, and at the cathode after CCV FC operation of >or=2 h. These results were interpreted in terms of electrochemical generation of HOO at the cathode (HO + H(2)O(2) --> H(2)O + HOO) and its chemical generation at the anode from hydrogen atoms and crossover oxygen (H + O(2) --> HOO). DMPO/H and DMPO/D adducts were detected at the anode and cathode sides, for CCV and OCV operation; H and D are aggressive radicals capable of abstracting fluorine from the tertiary carbon in the polymer membrane chain and of leading to chain fragmentation. Carbon-centered radical (CCR) adducts were detected at the cathode after CCV FC operation; weak CCR signals were also detected at the anode. CCRs can originate only from the Nafion membranes, and their presence indicates membrane fragmentation. Taken together, this study has demonstrated that FC operation involves processes such as gas crossover, reactions at the catalyst surface, and possible attack of the membrane by reactive H or D that do not occur in ex situ experiments in the laboratory, thus implying different mechanistic pathways in the two types of experiments.

Chapter 2 – Membrane Durability: Physical and Chemical Degradation

Publisher Summary Proton exchange membranes (PEMs) are the most promising membranes for automotive applications because of their relatively high proton conductivity at low temperatures. This chapter addresses the three primary root causes of membrane failure in automotive fuel cell systems. PEM fuel cells for high power density operation utilize perfluorosulfonic acid (PFSA) membranes. Ultimately, fuel cells fail because of pinholes that develop and propagate within the polymer membranes and can also fail if electronic current passes through the membranes causing the system to short. Both chemical and mechanical degradation of PFSA membranes are extensively studied and effective mitigation strategies are developed that can significantly extend PFSA-PEM lifetimes. The approaches described in this chapter can be applied to such alternative PEMs, but new fundamental models for both chemical and mechanical degradation are likely required. Even for the case of PFSA membranes, further work is required to develop the predictive models required to enable accurate estimation of membrane life in a real system. This chapter includes the first in-depth fundamental study of membrane shorting of PEMs in subsequent thermal decay of the PEM. Results and analysis suggest that the PEM type is not the limiting factor in preventing membrane shorting, and that mitigation is best achieved by a combination of design and operating strategies.

Chemical Degradation: Correlations Between Electrolyzer and Fuel Cell Findings

Membrane chemical degradation of polymer electrolyte membrane fuel cells (PEMFCs) is summarized in this paper. Effects of experimental parameters, such as external load, relative humidity, temperature, and reactant gas partial pressure, are reviewed. Other factors, including membrane thickness, catalyst type, and cation contamination, are summarized. Localized degradations, including anode versus cathode, ionomer inside the catalyst layer, degradation along the Pt precipitation line, gas inlets, and edges are discussed individually. Various characterization techniques employed for membrane chemical degradation, Fourier transform IR, Raman, energy-dispersive X-ray, NMR, and X-ray photoelectron spectroscopy are described and the characterization results are also briefly discussed. The detailed discussion on mechanisms of membrane degradation is divided into three categories: hydrocarbon, grafted polystyrene sulfonic acid, and perfluorinated sulfonic acid. Specific discussion on the radical generation pathway, and the relationship between Fentons test and actual fuel cell testing is also presented. A comparison is made between PEMFCs and polymer electrolyte water electrolyzers, with the emphasis on fuel cells.

FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Membranes: Design and Characterization

A series of ex situ and in situ diagnostic tests have been developed to quantitatively screen proton-exchange membranes (PEMs) for automotive fuel cell applications with respect to performance and mechanical and chemical durability. A comparison of the measured lifetimes of perfluorosulfonic acid (PFSA) and sulfonated aromatic hydrocarbon membranes under accelerated test conditions reveals the inherent differences between the two membrane chemistries. Upon subjecting membranes to deep hydration–dehydration cycles, the mechanical durability of PFSA membranes is more robust compared to that of aromatic hydrocarbon membranes, which have higher modulus and lower elasticity. By contrast, under in situ conditions promoting chemical degradation, aromatic hydrocarbon membranes can display improved stability. The next generation of alternative PEMs receiving a lot of attention are low-cost, sulfonated hydrocarbon polymers having controlled molecular architectures. Improved aromatic hydrocarbon PEM performance under conditions of low relative humidity can be facilitated by mimicking the positive attributes of PFSA membranes. New design tools allow for the optimization of nanophase separation of structurally reinforced hydrophobic domains and concentrated hydrophilic domains, thereby improving the performance of aromatic hydrocarbon membranes. A design guideline for polymer scientists is presented outlining the methodology to develop new PEMs for automotive fuel cell applications, including new metrics such as the membrane humidity stability factor and the hydrophilic volume ion-exchange capacity.