• Energy Research

Proton exchange membrane fuel cell is one of the most promising zero-emission power sources for automotive or stationary applications. However, their cost and lifetime remain the two major key issues for a widespread commercialization. Consequently, much attention has been devoted to optimizing the membrane/electrode assembly that constitute the fuel cell core. The electrodes consist of carbon black supporting Pt nanoparticles and Nafion as the ionomer binder. Although the ionomer plays a crucial role as ionic conductor through the electrode, little is known about its distribution inside the electrode. Here we report the three-dimensional morphology of the Nafion thin layer surrounding the carbon particles, which is imaged using electron tomography. The analyses reveal that doubling the amount of Nafion in the electrode leads to a twofold increase in its degree of coverage of the carbon, while the thickness of the layer, around 7 nm, is unchanged.

Proton exchange membrane fuel cell is one of the most promising zero-emission power sources for automotive or stationary applications. However, their cost and lifetime remain the two major key issues for a widespread commercialization. Consequently, much attention has been devoted to optimizing the membrane/electrode assembly that constitute the fuel cell core. The electrodes consist of carbon black supporting Pt nanoparticles and Nafion as the ionomer binder. Although the ionomer plays a crucial role as ionic conductor through the electrode, little is known about its distribution inside the electrode. Here we report the three-dimensional morphology of the Nafion thin layer surrounding the carbon particles, which is imaged using electron tomography. The analyses reveal that doubling the amount of Nafion in the electrode leads to a twofold increase in its degree of coverage of the carbon, while the thickness of the layer, around 7 nm, is unchanged.

Abstract Ex situ and in situ fuel cell degradation of a sPAEK membrane were investigated. Post-mortem analyses of the aged membrane and of the degradation products eluted in water were carried out by NMR, IR, SEC and EDX. Ex situ agings were performed in a low concentration H2O2 solution (0.07%) without any metallic catalyst. We exemplify that ex situ accelerated aging tests in such hydrogen peroxide solution are relevant to the chemical degradation in fuel cell. We have shown that a 500 h fuel cell test at moderate temperature (60 °C) induces significant modifications on the macromolecules such as a 40% molecular weight reduction. Degradation appears heterogeneous and limited to the cathode side. The model compound approach developed in the previous article (Perrot et al. [42] ) has allowed the identification of the aging path in fuel cell. Phenolic and carboxylic acid chain ends have been identified as the main products resulting from polymer chain scissions. The ex situ lifetime (100 h) of the membrane appears very limited with respect to the in situ operating time suggesting that the low H2O2 concentration (0.07%) is still much higher than in fuel cell.

Abstract Ex situ and in situ fuel cell degradation of a sPAEK membrane were investigated. Post-mortem analyses of the aged membrane and of the degradation products eluted in water were carried out by NMR, IR, SEC and EDX. Ex situ agings were performed in a low concentration H2O2 solution (0.07%) without any metallic catalyst. We exemplify that ex situ accelerated aging tests in such hydrogen peroxide solution are relevant to the chemical degradation in fuel cell. We have shown that a 500 h fuel cell test at moderate temperature (60 °C) induces significant modifications on the macromolecules such as a 40% molecular weight reduction. Degradation appears heterogeneous and limited to the cathode side. The model compound approach developed in the previous article (Perrot et al. [42] ) has allowed the identification of the aging path in fuel cell. Phenolic and carboxylic acid chain ends have been identified as the main products resulting from polymer chain scissions. The ex situ lifetime (100 h) of the membrane appears very limited with respect to the in situ operating time suggesting that the low H2O2 concentration (0.07%) is still much higher than in fuel cell.

Abstract Nafion ® membranes stored for long periods at 80 °C under elevated relative humidity up to 95%RH exhibit large modifications of their properties attributed to the sulfonic acid end-group condensation into sulfonic anhydrides. The present study is devoted to the membrane property rejuvenation, namely the hydrolysis of the sulfonic anhydrides under different experimental conditions. Aged membranes were exposed to pure water and to acid solutions or vapors in order to check the reversibility of the condensation reaction. Indeed, the hydrolysis process is slow in pure water and limited while it is fast and complete in the presence of acid or base. The native polymer chemical structure and the main membrane properties (mechanical properties, hydrophilicity, etc.) are completely restored. No evidence of hygrothermal aging was observed after fuel cell operation and it is shown that a membrane previously aged under ex situ conditions can be completely rejuvenated when operated in fuel cell.

Abstract Nafion ® membranes stored for long periods at 80 °C under elevated relative humidity up to 95%RH exhibit large modifications of their properties attributed to the sulfonic acid end-group condensation into sulfonic anhydrides. The present study is devoted to the membrane property rejuvenation, namely the hydrolysis of the sulfonic anhydrides under different experimental conditions. Aged membranes were exposed to pure water and to acid solutions or vapors in order to check the reversibility of the condensation reaction. Indeed, the hydrolysis process is slow in pure water and limited while it is fast and complete in the presence of acid or base. The native polymer chemical structure and the main membrane properties (mechanical properties, hydrophilicity, etc.) are completely restored. No evidence of hygrothermal aging was observed after fuel cell operation and it is shown that a membrane previously aged under ex situ conditions can be completely rejuvenated when operated in fuel cell.