Raman Microspectroscopy as a Useful Tool for In Situ and Operando Studies of Water Transport in Perfluorosulfonic Membranes for PEMFCs

Abstract

An operating PEMFC for commercial use must be able to adapt to variations in power demands and operating conditions while maintaining near optimal performance. Current research shows that one of the main issues affecting power output, stability, and longevity is the quantity and distribution of water in the system. The last are strongly affected by the sorption and transport properties of the polymer electrolyte. Thus, the need for a fundamental understanding of water transport in the PEMFC membrane‐electrode assembly has motivated, over the past few years, the development of new operando diagnostic tools sensitive to the membrane hydration. Among these, the most recent technique is based on confocal Raman microspectroscopy. This tutorial review is devoted to the illustration of the basis of this method, with a particular focus on the Raman signal processing needed to obtain reliable through‐plane water concentration profiles across PFSA membranes. Then, the potential of in situ and operando μ‐Raman is highlighted by reviewing the few studies reported to date, completed by the most recent results obtained by our group. Finally, advantages and drawbacks of operando Raman microspectroscopy are shortly discussed.

Asymmetric bi-layer PFSA membranes as model systems for the study of water management in the PEMFC

“…S1 †). As detailed in earlier studies, 41,42 this calculation procedure represents an effective method to measure the membrane inner water content directly, i.e. from the Raman signal of sorbed water, avoiding the need of internal references and minimizing the number of external measurements.…”

“…Further, our calculation procedure implies the normalization of the sorbed water by the polymer signal, which allows correcting artificial gradients due to the progressive attenuation of the Raman signal as the probed volume moves deeper into the membrane. 41,42 Other phenomena which have also to be taken into account in order to recover artefacts-free depth-profiles are the low resolution at the interfaces (due to instrumental spreading) 43,44 and the artificial shortening of the depth-scale (due to spherical aberrations). 45 In this work, blurring effects at the interfaces are corrected with a mathematical approach which correlates apparent (raw) and true Raman intensities with the depth-resolution curve of the instrument, the latter measured under the same optical conditions encountered during the in situ Raman study.…”

“…Even if other, more rigorous, mathematical expression can be used (see Everall 45 ), our correction represents an effective approximation in the specific case of hydrated PFSA materials and Raman depthmeasurements carried out with a confocal set-up. 41 3 Results and discussion 3.1 PEMFC performance Fig. 5 compares performance data of the FC containing the asymmetric bi-layer membrane, differently oriented in the MEA, to those obtained with symmetric bi-layer materials of the same thickness i.e.…”

In Situ Raman Spectroscopy as a Tool for Structural Insight into Cation Non-Ionomeric Polymer Interactions during Ion Transport

“…One of the important factors that need to be considered is the interaction between active material in the membrane and ions passing through, exerting an influence on the transport performance. Most of the studies were performed on materials which were composed of ions and used electrochemical impedance spectroscopy (ESI) for ion transport measurements and Raman spectroscopy to record the behaviour of ions in the matrix, or cross-over effect of water and fuels [ 22 , 23 , 24 , 25 ].…”

Trends and advances in Operando methodology

“…Confocal Raman spectroscopy operates in this mode and has been applied to study water transport within proton-conducting polymer electrolyte under operando condition [27]. Along entire catalyst beds, operando space-and timeresolved DRIFTS-Raman [19], XAFS [28] and XRD [29] were used to study catalyst materials in deNO x chemistry and methanol-to-olefin (MTO) process.…”