Using Aluminum & Water to Make Clean Hydrogen Fuel
Using hydrogen doesn’t generate carbon emissions, making it typically does. Today, almost all hydrogen is produced using fossil fuel–based processes that together generate more than 2% of all global greenhouse gas emissions. In addition, hydrogen is often produced in one location and consumed in another, which means its use also presents logistical challenges. Another option for producing hydrogen comes from a perhaps surprising source: reacting aluminum with water. Aluminum metal will readily react with water at room temperature to form aluminum hydroxide and hydrogen. That reaction doesn’t typically take place because a layer of aluminum oxide naturally coats the raw metal, preventing it from coming directly into contact with water. Using the aluminum-water reaction to generate hydrogen doesn’t produce any greenhouse gas emissions, and it promises to solve the transportation problem for any location with available water. Simply move the aluminum and then react it with water on-site.
MIT researchers have produced practical guidelines for generating hydrogen using scrap aluminum and water. First, they obtained specially fabricated samples of pure aluminum and aluminum alloys designed to replicate the types of scrap aluminum typically available from recycling sources. They then demonstrated ways of treating the samples to ensure that the surfaces of all the aluminum “grains” that make up the solid remain free of deposits throughout the reaction. Next, they showed that they could “tune” the hydrogen output by starting with pure aluminum or specific alloys and by manipulating the size of the internal aluminum grains. Such tuning can be used to meet demands for brief bursts of hydrogen, for example, or for lower, longer-lasting flows. The work confirms that, when combined with water, aluminum can provide a high-energy-density, easily transportable, flexible source of hydrogen to serve as a carbon-free replacement for fossil fuels.
Two problems have kept aluminum from being employed as a safe, economical source for hydrogen generation. The first problem is ensuring that the aluminum surface is clean and available to react with water. To that end, a practical system must include a means of first modifying the oxide layer and then keeping it from re-forming as the reaction proceeds.
The second problem is that pure aluminum is energy-intensive to mine and produce, so any practical approach needs to use scrap aluminum from various sources. But scrap aluminum is not an easy starting material. It typically occurs in an alloyed form, meaning that it contains other elements that are added to change the properties or characteristics of the aluminum for different uses. For example, adding magnesium increases strength and corrosion-resistance, adding silicon lowers the melting point, and adding a little of both makes an alloy that’s moderately strong and corrosion-resistant.