The dream of hydrogen power refuses to die. The allure is a source of energy with no emissions other than water vapor and a little heat. That’s an exciting prospect at a time when carbon emissions threaten the very existence of the human race. But there are some problems. Most hydrogen today is made from natural gas and most of that comes from fracking, a process that injects millions of gallons of highly polluted water deep underground to release the trapped gas. Huge quantities of it escape into the atmosphere as methane, a greenhouse gas many times more powerful than carbon dioxide. So while the end product of hydrogen as a fuel is promising, getting the hydrogen is a deal with the devil that has total emissions greater than those from burning coal.
Still, hydrogen will probably have a role to play in the transition to a zero carbon environment. It can be used as a fuel for heavy trucks and cargo ships. It can also act as a battery of sorts to store excess renewable energy for use later. One way to make hydrogen is to split water into its component parts — hydrogen and oxygen. Until now, that process required pure water free of contaminants and lots of electricity.
The chemical bonds between hydrogen and oxygen are powerful and not easily broken. The thinking is that as more renewable energy becomes available, some of it will be wasted unless it is put to good use. Many people think excess electricity would be ideal for splitting water and making hydrogen.
Pure fresh water is a scarce commodity in the world today and getting more scarce all the time. Using what little is available to make hydrogen may not be the best use for it. But researchers at the University of Houston say they have developed a new catalyst composed of inexpensive non-noble metal nitrides that makes it possible to split seawater at low voltages. Their work is described in Nature Communications.
According to the University of Houston, Zhifeng Ren, director of the Texas Center for Superconductivity at UH and a corresponding author for the paper, says a major obstacle to using seawater has been the lack of a catalyst that can effectively split it to produce hydrogen without creating free ions of sodium, chlorine, calcium and other elements found in seawater. Once released, those elements can settle on the catalyst, making it inactive. Chlorine ions are especially problematic, in part because chlorine requires just slightly higher voltage to free than is needed to free hydrogen.
Professor Ren claims the new process will also work with wastewater, providing another source of hydrogen from water that is otherwise unusable without costly treatment today. “Most people use clean freshwater to produce hydrogen by water splitting,” he said. “But the availability of clean freshwater is limited.”
First author Luo Yu, a postdoctoral researcher at UH who is also affiliated with Central China Normal University, says cell voltages required to produce a current density of 100 milliamperes per square centimeter ranged from 1.564 V to 1.581 V. The voltage is significant, Yu adds, because while a voltage of at least 1.23 V is required to produce hydrogen, chlorine is produced at a voltage of 1.73 V, meaning the device had to be able to produce meaningful levels of current density with a voltage between the two levels.
And so the search for a hydrogen economy continues. Pure water is much too valuable to use to make hydrogen, but if a way can be found to use seawater or wastewater, the possibilities for hydrogen from non-polluting sources are much greater than previously thought possible.
There is a secondary benefit from this research. In addition to making hydrogen, it could also be used to purify seawater into fresh water for drinking or irrigation. As sources of fresh water become more scarce, that other purpose could turn out to be more important to people than hydrogen.
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