Producing hydrogen fuel cost-effectively and sustainably has been a Holy Grail in renewable energy research for decades. In a study published in Proceedings of the National Academy of Sciences, researchers report a simple, economical new method to produce hydrogen: they use solar power and special nickel-based electrodes to directly split seawater into hydrogen and oxygen.
Hydrogen is a clean-burning fuel that would only produce water when it is burned. Right now, it is mostly produced by reforming natural gas using steam. Besides relying on a fossil fuel, this is energy-intensive, and produces carbon as a by-product.
An easy, ages-old way to produce hydrogen is to split water via electrolysis: passing an electric current through two electrodes placed in water. But the process requires purified water, which is costly on top of competing with potable water resources.
Seawater is the most abundant water resource on the Earth, write Stanford University chemist Hongjie Dai and his colleagues in the new PNAS paper. But electrolyzing it brings up problems. Negatively-charged chloride ions in saltwater corrode the catalyst and the positive electrode.
So Dai and his colleagues made an anode that repels chloride ions and resists corrosion. They start with a nickel foam core that is conductive. They coat it with a layer of nickel sulfide, and then a nickel-based catalyst. During electrolysis, the nickel sulfide undergoes chemical reactions and transforms into a negatively charged sulfate layer that repels chloride and keeps it from reaching and corroding the core.
Others have previously reported a floating device that can produce small quantities of hydrogen from salt-water, and an ink that creates hydrogen by absorbing sunlight and moisture from air. Dai and his colleagues have taken a more practical step forward, demonstrating long-term hydrogen production from water samples taken from the San Francisco Bay.
The electrolyzer, which used electricity from a small solar panel, produced hydrogen at the same rate as a system of the same size that uses current technology and purified water. And the system was stable for 1,000 hours. “Such a device provides an opportunity to use the vast seawater on Earth as an energy carrier,” the team writes.
Source: Yun Kuang et al. Solar-driven, highly sustained splitting of seawater into hydrogen and oxygen fuels. PNAS, 2019.