Worcester, Massachusetts. The Worcester Polytechnic Institute recently released a groundbreaking report on the use of liquid metal membrane technology to produce and distribute hydrogen fuel cost-effectively and quickly. The findings could make hydrogen-powered cars more affordable, reliable and even viable on an industrial scale, and will open doors for further research and developments in hydrogen energy research.
The Status Quo: Palladium
Palladium membranes have been proven to be somewhat successful in carrying a supply of pure hydrogen through a fuel cell to power a car, however the metal is both expensive to source, vulnerable to poisoning by toxic substances and very fragile. Up until now, scientists and developers have sought to reduce the costs and improve reliability by producing palladium membranes that are thinner and more sophisticated. However, concerned with the fragility of the element, a team of researchers at Worcester Polytechnic Institute were more interested in sourcing other metals for the job – ones that are more permeant, diffusible and soluble than palladium.
Six Years of Research Culminates in a Viable Solution
Ravindra Datta is a professor of chemical engineering and the director of the Fuel Cell Center at the Worcester Polytechnic Institute in Massachusetts. He has been working together with a team of researchers and PhD candidates for six years on the potential for replacing palladium with other liquid metals to produce a cost-effective, soluble and permeant solution for carrying a steady supply of pure hydrogen through a fuel cell. The U.S. Department of Energy supported his research with a grant of $1 million USD for his purposes, considering that no previous research on the feasibility of liquid metals had ever been conducted.
Could Gallium Be the Answer?
Gallium was the metal of most interest to the team at WPI. It occurs in bauxite and zinc ores and is extracted as a by-product during the processing of bauxite into alumina using the Bayer method. Of the world’s gallium supply, 98 per cent is applied to semiconductors in a high-purity form. A unique characteristic of gallium is its melting point of 29.76 degrees Celsius, slightly above room temperature.
There are only four known metals that share this characteristic, but of all of them, gallium is the least reactive and the least toxic. Up until now, gallium hasn’t been seriously considered for use in hydrogen fuel cells because it reacts with porous metals and ceramics, thus eliminating the necessary permeability. However, Ravindra Datta and his team were curious as to whether gallium could be used when combined with such materials that are both non-reactive and wettable, meaning they form a protective film around the metal.
The Development of the Metal Membrane Technology
Datta’s team created a “sandwiched liquid metal membrane” (SLiMM) to support the gallium and protect it from exposure to gases and temperature variations. The materials chosen for the membrane were silicone carbide and graphite, both of which fulfilled the criteria.
The results were very promising and the team published them in the Journal of the American Institute of Chemical Engineers in February 2017. Even over several weeks and under immense temperature variations, the liquid metal membrane containing gallium and a protective film of silicone carbide and graphite remained permeable (up to 35 times more so than palladium) and hydrogen diffusion was more effective and efficiently controlled.
Liquid Metal Opens Doors for Hydrogen Separation
Though the researchers have confirmed successfully that the sandwiched liquid metal membrane is permeable and suitable for hydrogen purification, questions will now be raised about whether SLiMMs can resist the poisons that occur in reformed gases (such as sulphur and carbon monoxide) and also whether they can be mass-produced for application in the hydrogen cars of the future. However, the doors are open for refinement, and the selection of metals and alloys that could be used for this purpose is immense.
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