Water management in proton exchange membrane fuel cell at sub-zero temperatures: An in operando SANS-EIS coupled study

• The water management in PEMFC was studied in operando at sub-zero temperatures.
• The water content in the membrane was determined using Small-Angle Neutron Scattering.
• The increase of current density induces reversible membrane dehydration.
• The correlation between water content and cell resistance is very good.
• Electro-osmosis prevails on the back-diffusion at sub-zero temperature.

The water management in a proton exchange membrane fuel cell (PEMFC) was studied in operando at sub-zero temperatures combining Small-Angle Neutron Scattering (SANS) and Electrochemical Impedance Spectroscopy (EIS) techniques. The two main processes which control the water repartition within the fuel cell, namely the electro-osmosis and the back-diffusion of water in the membrane can be separated by the decrease of the operating temperature along with the use of a thick membrane compared to state-of-the-art material. It is shown that an increase of the current load at sub-zero temperature induces reversible membrane dehydration despite a higher production of water. This effect is revealed by the very good correlation between the increase of the membrane resistance measured by EIS and the decrease of water content in the membrane quantified from the SANS spectra. The predominance of the electro-osmosis on the back-diffusion, in addition to water accumulation, most probably in supercooled state, in the channel and in the gas diffusion layer, especially at the cathode side, could explain the difficulty to operate a fuel cell at sub-zero temperature.