3 In situ and operando determination of the water content distribution in proton conducting membranes for fuel cells: a critical review

Abstract

Proton exchange membrane fuel cells have been recognized as a promising zero-emission power source for portable, mobile and stationary applications. The information of water content distribution in the different components of the cell during operation, particularly the proton conducting membrane, is a critical issue for the validation of mass transfer models, the definition of optimized operating conditions and the development of efficient systems with innovative designs for efficient water management. In order to fully understand the way a fuel cell performs, water transport and distribution have to be investigated in situ and operando. In this review, we critically examine the state-of-the-art of operando diagnostics sensitive to the membrane water content, particularly those techniques able (in principle) to give insights into water transport occurring along both the in- and through-plane directions. Particular attention is devoted to experimental results obtained across the membrane thickness i.e. to the determination of water concentration profiles originating from the water activity and electrical gradients occurring through the working fuel cell. Different operando techniques have been developed for this purpose, from the early 1990s up to the last few years: internal resistance measurements, magnetic resonance and neutron imaging, neutron and X-ray scattering, confocal μ-Raman spectroscopy. These techniques can be roughly separated as either direct (i.e. the water amount can be directly derived from the detected signal, avoiding sometimes arbitrary assumptions during data processing) but intrusive (i.e. they require significant modification of the fuel cell, compared to the current design and materials) or indirect but with a significantly lower intrusiveness. It appears that operando measurements of the membrane water distribution allow a unique picture of how the internal part of the fuel cell works, thus certainly contributing to the development of more effective cell designs and materials in the near future. Nevertheless, improvement in the fundamental understanding of the actual fuel cell requires further efforts to increase spatial and, more particularly, temporal resolution of current operando techniques. Also, the comparison of limitations arising from the basic principles of the different operando approaches suggests that ultimate progress will arise from the combination of complementary techniques for simultaneous measurements.