New liquid-metal membrane technology may help make hydrogen fuel cell vehicles viable

August 28, 2017
From left, Pei-Shan Yen '16 (PhD),Ravindra Datta, professor of chemical engineering, and Nicholas Deveau '17 (PhD) at Worcester Polytechnic Institute (WPI developed novel sandwiched liquid-metal membranes that could help lower the cost of hydrogen for fuel-cell-powered vehicles. Credit: Worcester Polytechnic Institute (WPI)

While cars powered by hydrogen fuel cells offer clear advantages over the electric vehicles that are growing in popularity (including their longer range, their lower overall environmental impact, and the fact that they can be refueled in minutes, versus hours of charging time), they have yet to take off with consumers. One reason is the high cost and complexity of producing, distributing, and storing the pure hydrogen needed to power them, which has hindered the roll-out of hydrogen refueling stations.

Engineers have long recognized the power—and limitless availability—of hydrogen, the most abundant element in the universe. Hydrogen occurs naturally in the environment, but it is almost always chemically bound to other elements—to oxygen in water (H2O), for example, or to carbon in methane (CH4). To obtain pure hydrogen, it must be separated from one of these molecules. Virtually all of the hydrogen produced in the United States is obtained from hydrocarbon fuels, primarily natural gas, through steam reforming, a multi-step process in which the hydrocarbons react with high-temperature steam in the presence of a catalyst to produce carbon monoxide, carbon dioxide, and molecular hydrogen (H2).

The hydrogen can then be separated from the other gases through a cumbersome, multi-step chemical process, but the cost and complexity of hydrogen production can be reduced by using a membrane to do the separation. Most of the hydrogen separation membranes currently being developed use the precious metal palladium, which has unusually high hydrogen solubility and permeance (which means that hydrogen easily dissolves in and travels through the metal, while other gases are excluded). But palladium is expensive (it currently sells for about $900 per ounce) and fragile.

For these reasons, chemical engineers have long searched for alternatives to palladium for use in hydrogen separation membranes, but so far, no suitable candidates have emerged. A pioneering study led by Ravindra Datta, professor of chemical engineering at Worcester Polytechnic Institute (WPI), may have identified the long-elusive palladium alternative: liquid metals.

A host of metals and alloys are liquid at the standard operating temperatures found in steam reforming systems (around 500 degrees C), and most of these are far less expensive than palladium. In addition, a membrane made with a film of liquid metal should not be prone to the defects and cracks that can render a palladium membrane unusable.

The WPI study, published in the Journal of the American Institute of Chemical Engineers, is the first to demonstrate that in addition to these advantages, liquid-metal membranes also appear to be significantly more effective than palladium at separating pure hydrogen from other gases, suggesting that they may provide a practical and effective solution to the challenge of supplying affordable hydrogen for fuel-cell vehicles. "The recent shift to electric cars is irreversible," said Datta. The next step after electric vehicles, he and others believe, is hydrogen-fuel vehicles—if the hydrogen supply puzzle is solved.

Like battery-powered electric cars, fuel-cell vehicles have electric motors. The motors are powered by electricity generated inside the fuel cell when hydrogen and oxygen combine in the presence of a catalyst (the only "waste" product is water). While they can pull oxygen from the air, the cars must carry a supply of pure hydrogen.

Many researchers have focused on bringing the cost of that hydrogen down by making better and thinner palladium membranes. Some of the most advanced membranes were produced by retired WPI chemical engineering professor Yi Hua "Ed" Ma, who, with considerable funding from industry and the U.S. Department of Energy, pioneered a process for binding palladium to a porous steel tube, resulting in palladium layers as thin as 5 to 10 microns.

Making the palladium layer thin increases the membrane's flux, or the rate at which pure hydrogen moves through it. "But if a membrane is too thin," Datta said, "it becomes fragile or it develops defects. And the membranes need to be defect-free. If they develop even a hairline crack or a micropore, you have to start over."

Researchers at Worcester Polytechnic Institute (WPI) tested a prototype sandwiched liquid-metal membrane with this laboratory set-up. The membrane, a thin layer of gallium sandwiched between porous ceramic supports, selectively separated hydrogen from a mixed-gas stream more efficiently than a comparable palladium membrane. Credit: Worcester Polytechnic Institute (WPI)/Curtis Sayers

Six years ago, Datta and his students began to wonder whether liquid metals might overcome some of palladium's limitations—particularly its cost and fragility—while also, potentially, offering superior hydrogen solubility and permeance. "Besides chemical affinity, permeance depends on how open a metallic crystal structure is," he said. "Liquid metals have more space between atoms than solid metals, so their solubility and diffusability should be higher."

After a literature review revealed no previous research on this topic, Datta successfully applied for a $1 million award from the U.S. Department of Energy to study the feasibility of using liquid metals for hydrogen separation. he and his team, graduate students Pei-Shan Yen and Nicholas Deveau (Yen earned her PhD in 2016; Deveau received his in May), decided to begin their exploration with gallium, a nontoxic metal that is liquid at room temperature.

They conducted fundamental work that revealed that gallium was an excellent candidate, as it demonstrated significantly higher hydrogen permeance than palladium at elevated temperatures. In fact, laboratory studies and theoretical modeling conducted by the team showed that a number of metals that are liquid at higher temperatures may have better hydrogen permeance than palladium.

While liquid gallium showed great promise as a material for hydrogen separation, creating a functioning membrane with the metal proved challenging, Datta said. "It turns out that liquid metals are very reactive," he said. "You cannot place gallium on a porous metal support, as Professor Ma did with palladium, since at higher temperatures it quickly forms intermetallic compounds that kill the permeability." The team discovered that the metal will also react with a number of ceramic materials commonly used as supports in palladium membranes.

Through modelling and experimentation, they compiled a list of materials, including carbon-based materials like graphite and silicon carbide, that do not chemically react with liquid gallium but that are also wettable by the liquid metal, meaning that the metal will spread out to form a thin film on the support material.

Aware that the surface tension of liquid metals was likely to change in response to variations in temperature and the composition of the gases they were exposed to, potentially producing leaks, they decided to insert the metal between two layers of support material to create a sandwiched liquid-metal membrane or SLiMM. A membrane consisting of a thin (two-tenths of a millimeter) layer of liquid gallium between a layer of silicon carbide and a layer of graphite, was constructed in the lab and tested for stability and hydrogen permeance.

The membrane was exposed to a hydrogen atmosphere for two weeks at temperatures ranging from 480 to 550 degrees C. The results showed that the liquid gallium film was up to 35 times more permeable to hydrogen than a comparable layer of palladium and that diffusion of hydrogen through the sandwiched membrane was considerably higher than for a typical palladium membrane. The test also showed that the membranes were selective, allowing just hydrogen to pass through.

"These tests confirmed our hypotheses that may be suitable candidate for hydrogen separation membranes," Datta said, "suggesting that these materials could be the long-sought substitute for palladium. There are a host of questions that still need to be answered, including whether the small membranes we constructed in the laboratory can be scaled up and whether the membranes will be resistant to substances present in reformed gases (including carbon monoxide and sulfur) that are known to poison membranes.

"But by demonstrating the feasibility of sandwiched liquid- membranes, we have opened the door to a highly promising new area of energy research," Datta added, "for there are many other metals and alloys, beyond gallium, that are liquid at 500 degrees C. It is a vast open field, in terms of what materials you might use. Also, it poses a host of interesting scientific questions."

Explore further: Researchers develop extremely sensitive hydrogen sensor

More information: Pei-Shan Yen et al, Sandwiched liquid metal membrane (SLiMM) for hydrogen purification, AIChE Journal (2017). DOI: 10.1002/aic.15658

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15 comments

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MR166
5 / 5 (2) Aug 28, 2017
Why on earth would one want to fuel a vehicle with H2 that was derived from fossil fuels. There is no real science left in modern society just fervent religious beliefs with H2 being just one of the deities.
MR166
1 / 5 (1) Aug 28, 2017
Again, I am willing to bet that the idea of fueling a car with fossil based H2 is the authors idea and was not mentioned in the research paper. Journalism majors are easily programmed and perform their prescribed tasks like the little politically correct Oompa Loompas that they are.
kafantaris
5 / 5 (2) Aug 28, 2017
Now way more hydrogen from steam reforming: "the liquid gallium film was up to 35 times more permeable to hydrogen than a comparable layer of palladium ... allowing just hydrogen to pass through."
Steelwolf
not rated yet Aug 28, 2017
Hydrolosis of water provides a hydrogen source that CAN be independent of fossil fuels if it is used in conjunction with solar or wind. It also has less of a problem with refinement and there are several ways to tune this effect for efficiency. Using fossil sources and producing more carbon dioxide and carbon monoxide is totally negating the reason for going to the hydrogen power in the first place.

There is an overwhelming need to get fossil fuels out of circulation as a main power (and economic power) source, we can do much better easily and it is only the present monopoly held by the whole Petro-Dollar setup where fossil fuel use is forced upon us along with petroleum based plastics, pharmaceuticals and all other facets of life where fossil fuel sources actually control while contaminating our lives.
antialias_physorg
5 / 5 (3) Aug 29, 2017
Not knocking the research, but using steam reforming from hydrocarbons is only marginally better than burning coal. You could (in theory) use hydrocarbons from plant sources, but the amount is not enough to make a serious dent in energy needs if all (or even just a part) of mobility applications were converted to hydrogen fuel cells.

That said, hydrogen will have it's use in long range applications where batteries are unlikely to reach in the near future (shipping, airplanes, maybe trucking, and possibly some trains), but from a different source than steam reforming.
dirk_bruere
3 / 5 (2) Aug 29, 2017
"The next step after electric vehicles, he and others believe, is hydrogen-fuel vehicles..."
He is deluded
Dingbone
Aug 29, 2017
This comment has been removed by a moderator.
KBK
2.3 / 5 (3) Aug 29, 2017
Research a company called SHT, or Solar Hydrogen Trends.

Bought out by the US military, the Navy, to be specific.....

You are dealing with two tier technology as a human containment system.

The public gets the stuff that doesn't work, the "deep state' develops the serious stuff and it remains off the public's radar.

The two tier system of technology development and control was enacted early in the second world war and has built itself into a shooting star that has left public science behind by thousands of years, to the point that what they are doing now tends to look like magic, as Arthur C Clark popularized in a saying....

I wish I was full of it, but I'm not.

I could deliver names, dates places of disruptive and now hidden technology, all day long, in detail, in hundreds of different examples.

All you have to do is turn and look with open eyes -dig for the details. You will be satisfied with the veracity of what you find. Horrified, as well.
Da Schneib
5 / 5 (1) Aug 29, 2017
There are quite a few options, @anti, if you have a good membrane. That's the hard part.
Eikka
2.3 / 5 (3) Aug 30, 2017
There's absolutely no need to separate the hydrogen from gasses, because a fuel cell can burn hydrocarbon fuels directly. SOFCs can burn even plain carbon.

The H2 economy is just a pipe dream because the infrastructure simply won't work unless you condense the H2 into a carrier such as methane, or preferably some liquid fuel like methanol.

antialias_physorg
5 / 5 (2) Aug 30, 2017
There are quite a few options, @anti, if you have a good membrane. That's the hard part.

Well, it needs to be a cheap membrane (which this potentially is), but I get the feeling that keeping a liquid membrane stable under operational conditions (shocks, different directions of acceleration, temperature changes) is going to be far from trivial in small scale applications (like cars). I could see the attendant stabilizing mechanisms being economical in larger systems (ships/planes)...though in those systems even expensive platinum isn't as much of an issue as the cost of the fuel cell would only be a miniscule part of the entire craft.

Or it could be useful in stationary systems (home energy storage or large grid storage systems). There storing stuff in tanks and only having a smallish active part always makes sense.
Da Schneib
5 / 5 (2) Aug 30, 2017
@anti, this is central processing; you don't put this in a car. You make the hydrogen, store it (and that's a whole other problem), and fuel things with it.

The issues you have to overcome are threefold: you have to produce the hydrogen, you have to store and transport it, and you have to burn it in a fuel cell. We're near if not at the point of making practical fuel cells to burn it; with this technique, we have the means to produce it. Now all we need is storage. They're working on that, too. Several techniques are under investigation.

In the meantime we have electric cars.
Da Schneib
5 / 5 (1) Aug 30, 2017
@Eikka is making stuff up again. Tries to sweep the carbon under the rug, always has an excuse why storage and transport won't work, makes stories up about the fuel cells.

Poor @Eikka. This is just like the stories you made up about grid stability which just got shown to be silliness here: https://phys.org/...rgy.html

Why is it, @Eikka, that every time you post you are claiming some problem you can't substantiate with getting away from fossil fuels? Just askin', 'cause I guess we all know.
antialias_physorg
5 / 5 (2) Aug 30, 2017
In the meantime we have electric cars.

True. (And by the time fuel cells come around for cars battery ranges will be there that satisfy everyone except the most die hard "I'll drive accross thousands of miles to get to a camp site"-tourists.)

I still think it would be awesome to have the ability to split hydrogen inbuilt in a car. Plug it in at night and have it reform its own hydrogen from the wastewater it created earlier. Would greatly cut down on the need for refueling stations (at least their size).
Da Schneib
5 / 5 (1) Aug 30, 2017
The problems with doing it in a car are, first, what do you do with the carbon, and second, as you already pointed out, vibration etc. Better to do the reformation either at a central site or as part of the housing.

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