Enhancing porous materials with plasma
For a membrane-based fuel cell to work properly, it needs the right amount of water. Too little, and not enough hydrogen ions will be conducted across the membrane. Too much, and porous materials inside it will be flooded, preventing reactant gases from getting where they need to go.
But maintaining optimal hydration in fuel cells — or any electrochemical device, for that matter — is no easy task. Existing water management strategies (hydrophobic coatings, electroosmotic pumps, nanotubes and radiation grafts, among others) are too costly, too complex or otherwise unsuitable to serve as scalable, sustainable solutions.
Now, researchers at the University of British Columbia have demonstrated that plasma treatment — a commonly used industrial process where plasma, or ionized gas, is applied to a material before that material undergoes further treatment — offers a quick, simple and inexpensive way to control water transport not just in fuel cells, but in any device where porous materials are used. This includes electrodialysis and other energy storage and conversion technologies, which form the basis of several multibillion-dollar industries.
“Water management is a crucial aspect of everything from thermal power plants and atmospheric water harvesting to chlor-alkali processing and metal-air batteries,” says Beniamin Zahiri, the study lead and a materials scientist at UBC when the research was conducted. “Using plasma, companies may be able to easily improve the overall performance of their electrochemical devices by controlling how water-loving or water-repellent key components of those devices are.”
Commercially available porous materials have improved significantly in recent years, but they have yet to feature controllable wetting properties — that is, the ability to attract or repel liquid to different degrees on command. In a paper published in Applied Surface Science, the UBC researchers showed that by exposing the surface of a porous carbon layer (the standard electrode material in many electrochemical devices) to oxygen plasma at low power for one minute, a wettability gradient can be created across the entire thickness of the layer: most water-loving on the surface, least so on the bottom.
They then showed that using the plasma-treated porous carbon layer in a proton exchange membrane fuel cell — the kind being developed for use in motor vehicles and other applications — enhanced its performance. Conventional porous carbon layers typically flood at high current densities, blocking oxygen transport and negatively impacting fuel cell function as a result. But when a plasma-treated layer is used, it’s a different story: as water is drawn away from the porous carbon layer surface and towards the relatively hydrophobic reverse side, water droplets form and detach, clearing the pores for oxygen to travel through more easily.
Based on their research and other studies in the area, the UBC team believes that simply by changing the type of plasma gas, the rate of plasma gas flow and the duration of the plasma treatment, the wettability of porous materials used in electrodes and other critical device components may be tailored to meet the needs of a broad range of technologies. For instance, to achieve a hydrophobic gradient instead of a hydrophilic one, one may use a different gas, such as tetrafluoromethane (CF4), in place of oxygen.
“Integrating plasma treatment into the manufacturing line would be cheap and easy,” says Walter Mérida, a professor of mechanical engineering and the director of the Clean Energy Research Centre at UBC, who supervised the study. “The process is also environmentally friendly, as it generates no chemical waste.”
The researchers are currently exploring applications of plasma-based water management beyond fuel cells, in fields where its impact may be even more significant. For example, the chlor-alkali industry, which services a number of sectors — food, textiles, construction, soaps and detergents, and pulp and paper among them — has a market that was valued at approximately $80 billion (USD) in 2016 and is expected to grow to around $103 billion (USD) by 2021.