Published on November 16th, 2013 | by James Ayre11
Low-Cost, Corrosion-Free Water Splitter Created From Silicon And Nickel
November 16th, 2013 by James Ayre
A low-cost means of producing hydrogen fuel — one that doesn’t result in the corrosion of the materials used, and uses nothing but sunlight and water — has been created by researchers at Stanford University.
The new silicon-based water splitter — essentially just a silicon semiconductor coated in an ultrathin layer of nickel — brings the commercialization of large-scale hydrogen fuel one step closer to reality, according to the researchers involved.
“Solar cells only work when the sun is shining,” stated study co-author Hongjie Dai, a professor of chemistry at Stanford. “When there’s no sunlight, utilities often have to rely on electricity from conventional power plants that run on coal or natural gas.”
A better solution — according to Dai — would be to pair effective hydrogen-powered fuel cells with the solar cells.
Stanford University provides some background:
To produce clean hydrogen for fuel cells, scientists have turned to an emerging technology called water splitting. Two semiconducting electrodes are connected and placed in water. The electrodes absorb light and use the energy to split the water into its basic components, oxygen and hydrogen. The oxygen is released into the atmosphere, and the hydrogen is stored as fuel.
When energy is needed, the process is reversed. The stored hydrogen and atmospheric oxygen are combined in a fuel cell to generate electricity and pure water. The entire process is sustainable and emits no greenhouse gases. But finding a cheap way to split water has been a major challenge. Today, researchers continue searching for inexpensive materials that can be used to build water splitters efficient enough to be of practical use.
“Silicon, which is widely used in solar cells, would be an ideal, low-cost material,” stated Stanford graduate student Michael J Kenney, co-lead author of the new study. “But silicon degrades in contact with an electrolyte solution. In fact, a submerged electrode made of silicon corrodes as soon as the water-splitting reaction starts.”
To address this, the researchers have now turned to the process of coating silicon electrodes with ordinary nickel. “Nickel is corrosion-resistant,” Kenney explained. “It’s also an active oxygen-producing catalyst, and it’s earth-abundant. That makes it very attractive for this type of application.”
For the new research, a 2-nanometer-thick layer of nickel was applied onto a silicon electrode, and then partnered with another electrode and placed in a solution of water and potassium borate — light and electricity were then applied. After the application of light and electricity, the electrodes began splitting the water into oxygen and hydrogen — importantly, even after twenty-four hours the process was still continuing, with no noticeable signs of corrosion.
To further improve the process, the researchers then mixed lithium into the solution. “Remarkably, adding lithium imparted superior stability to the electrodes,” Kenney noted. “They generated hydrogen and oxygen continuously for 80 hours — more than three days — with no sign of surface corrosion.”
“Our lab has produced one of the longest lasting silicon-based photoanodes,” Dai stated. “The results suggest that an ultrathin nickel coating not only suppresses corrosion but also serves as an electrocatalyst to expedite the otherwise sluggish water-splitting reaction. Interestingly, a lithium addition to electrolytes has been used to make better nickel batteries since the Thomas Edison days. Many years later we are excited to find that it also helps to make better water-splitting devices.”
The researchers are now planning to follow this work up with efforts to further improve the stability and durability of the nickel-treated electrodes of silicon, in addition to improving the other materials used.