Hydrogen: Key to a Sustainable Future
Experts expect worldwide energy demand to double in the next 50 years. Many of those same experts also agree that cutting back on greenhouse emissionsparticularly carbon dioxideis the only way to avoid irreversible climate change. Considering the limited supply of fossil fuels, the trick for satisfying both realities lies not only in keeping up with today's energy needs, but in meeting the increase in ways that do not pollute the atmosphere with greenhouse gases.
"Even maintaining current fossil fuel consumption, CO2 in the atmosphere would double to Industrial Revolution levels," says SSRL postdoctoral researcher Jennifer Leisch. "The goal is carbon-free energy."
Current green energy sources have their limits. Wind and sunlight provide power without polluting, but they are not constant sources, nor can the their respective technologies be installed everywhere that demands electricity. In the conflict between constant energy demand and intermittent renewable sources, "the key is storage," says Leisch, "and hydrogen is one of the energy storage options."
Hydrogen holds promise as a clean energy: it burns easily with oxygen, leaving water as the only product. Hydrogen is everywhereit ranks as the tenth most common element on Earth. But it is not easily harnessed. At SLAC, Leisch and other researchers are looking for solutions to two major barriers to making hydrogen a viable alternative energy: producing hydrogen gas cleanly and cheaply, and storing it efficiently. But considering its abundance, hydrogen remains surprisingly hard to isolate.
For decades scientists and engineers have sought efficient, reliable ways to isolate and store hydrogen for use as a source of energy. Advances in technology have led to remarkable improvements in some of the most promising approaches, but much work remains before an efficient system for obtaining hydrogen can be made commercially available. Now, scientists at SSRL are bringing their expertise to bear on the related problems of generating and storing hydrogen by using x-rays to understand these issues on a molecular level.
"It's not a fuel we can mine," says Leisch. "It's bound in other molecules." Its simple structure lets it bond easily with all kinds of substances, making it virtually absent in its pure form. No hydrogen springs bubble up on mountainsides, and no hydrogen seams run below ground.
On Earth, the most common hydrogen source is water, with two hydrogen atoms in every molecule. Hydrogen atoms also abound in everything that is (or was) living, in carbon-based compounds called hydrocarbons. When stripped from such molecules, two hydrogen atoms merge into H2, the most stable form of the element. The trick is to extract and pack it as a gas like propane, but corralling itespecially in a green wayis no small feat.
The United States already produces some 11 million metric tons of hydrogen a year for industrial uses. Nearly all is made by steam reforming, or blasting methane with water vapor to yield H2 and carbon monoxide (CO). Steam reforming is by far the cheapest method of hydrogen production. But because CO is both a greenhouse gas and lethal, it holds less environmental appeal than electrolysis.
Electrolysis takes the H2 out of H2O with electric current. Of course, that electricity has to come from somewhere, and drawing from a conventional coal-powered plant undermines the clean energy goal. In fact, the electricity used to make H2 from electrolysis is currently more valuable than the hydrogen itself.
SLAC researchers are investigating an innovative electrolysis technology that uses sunlight as the only energy source. In "direct electrolysis," a photochemical cell captures sunlight and uses its energy to split the water directly. Unfortunately, common catalysts used to speed the process corrode within hours. At SSRL, Leisch uses x-rays to study the surface nanostructures inside these solar cells in the search for inexpensive, long-lasting catalysts that would make direct electrolysis a marketable technology.
Direct electrolysis of water may one day become the most efficient way to isolate hydrogenonce the technical obstacles are worked out. Another obstacle is what to do with the hydrogen once it is produced. These issues will need to be solved before researchers can fully address the need for more and cleaner energy.
Coming issues of SLAC Today will detail new research at SSRL that may soon contribute to the use of hydrogen as a clean, carbon-free source of energy.
Above image courtesy of Pacific Northwest National Lab.