Water Uptake and Swelling in Fuel Cell Membranes

Researchers from universities access the CNBC to obtain fundamental insights into materials used for clean energy technologies.

Source: Canadian Neutron Beam Centre (CNBC)
Contact: cnbc@cnl.ca

Cutaway of the Toyota Mirai showing the fuel cells and hydrogen tanks

Cutaway of the Toyota Mirai showing the fuel cells and hydrogen tanks

Cars powered by hydrogen instead of gasoline are starting to enter the market. The Toyota Mirai, launched in California in the fall of 2015, was one of the first such vehicles to be sold commercially. It can travel 500 kilometres on its two fuel tanks, which together store a total of 5 kilograms of hydrogen. The car generates electricity in a fuel cell by combining the hydrogen with oxygen from the air to produce water and energy.

Over 100 hydrogen fuelling stations are expected to be built in California by 2020. However, there are numerous research challenges that still require attention, including a focus on ensuring the reliability of the fuel cells and boosting their lifetimes. While fuel cells are well established for applications that require steady power, the amount of power required by a vehicle fluctuates greatly with speed and acceleration. This ‘load cycling’ in a vehicle reduces the lifetime of a fuel cell by ten times as compared to constant loads. Other factors that can impact the lifetime of fuel cells include pollutants in the intake air, low humidity, high temperatures, and cycles of drying and wetting of the fuel cell membrane.

The UBC research team observed a swelling hysteresis in the fuel cell membrane that had not been seen before.

Professor Walter Merida at the Clean Energy Research Centre at the University of British Columbia led a research team to study the effects of humidity and temperature on fuel cell membranes. The team applied the neutron reflectometry technique available at the Canadian Neutron Beam Centre to determine how humidity and temperature influenced water intake and swelling in a nafionTM model membrane that was only 15 nanometres thick. Through the use of neutron reflectometry, the researchers made two important observations. First, they observed that upon heating the film at high humidity, its thickness expanded by 48 percent. This expansion was greater than any previous observations, which had been conducted at room temperature. Secondly, the team observed that the heated membrane did not shrink back to its original thickness when cooled to room temperature, indicating an irreversible change to the membrane’s structure that had not been seen before.

Illustration of the thickness in nanometres (nm) of the nafionTM model membrane depending on conditions of humidity (RH) and temperature (°C), as determined from the scattering length density (SLD) observed by neutron reflectometry

The team is planning further experiments to investigate these findings in more detail, which may shed light on the factors affecting fuel cell performance in vehicles.

DOI: 10.1021/acs.langmuir.5b00764