Requiring only a single catalyst, a new water-splitting mechansim is able continuously generate hydrogen and oxygen for more than 200 hours. The device could potentially be renewable source of clean-burning hydrogen fuel.
Traditional water-splitting devices consist of two electrodes submerged in a water-based electrolyte. A low-voltage current applied to the electrodes drives a catalytic reaction that separates water molecules (H2O), releasing bubbles of hydrogen on one electrode and oxygen on the other.
Each electrode is embedded with a separate catalyst, typically platinum and iridium, both rare and costly metals.
Stanford University chemist Hongjie Dai developed in 2014 a water splitter made of low-cost nickel and iron that runs on an ordinary 1.5-volt battery.
This new device advances that technology further.
“Our water splitter is unique because we only use one catalyst, nickel-iron oxide, for both electrodes,” Haotian Wang, lead author of the study and Stanford grad student at. “This bi-functional catalyst can split water continuously for more than a week with a steady input of just 1.5 volts of electricity. That’s an unprecedented water-splitting efficiency of 82 percent at room temperature.”
“For practical water splitting, an expensive barrier is needed to separate the two electrolytes, adding to the cost of the device,” Wang says. “But our single-catalyst water splitter operates efficiently in one electrolyte with a uniform pH.”
Wang and colleagues found that nickel-iron oxide, which is cheap and easy to produce, is actually more stable than some commercial catalysts made of precious metals.
“We built a conventional water splitter with two benchmark catalysts, one platinum and one iridium,” Wang says. “At first the device only needed 1.56 volts of electricity to split water, but within 30 hours we had to increase the voltage nearly 40 percent. That’s a significant loss of efficiency.”
In search of catalytic material feasible for both electrodes, the Stanford team took a page from battery research, a technology called lithium-induced electrochemical tuning. The concept is to use lithium ions to chemically break the metal oxide catalyst into smaller and smaller pieces.
“Breaking down metal oxide into tiny particles increases its surface area and exposes lots of ultra-small, interconnected grain boundaries that become active sites for the water-splitting catalytic reaction,” says Yi Cui, an associate professor of materials science and engineering at Stanford. “This process creates tiny particles that are strongly connected, so the catalyst has very good electrical conductivity and stability.
Using one catalyst made of nickel and iron has important implications for cost, he adds.
“Not only are the materials cheaper, but having a single catalyst also reduces two sets of capital investment to one,” Cui explains. “We believe that electrochemical tuning can be used to find new catalysts for other chemical fuels beyond hydrogen.”