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Silicon/nickel water splitter could lead to cheaper hydrogen

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November 18, 2013

This image shows two electrodes connected via an external voltage source splitting water i...

This image shows two electrodes connected via an external voltage source splitting water into oxygen (O2) and hydrogen (H2). The illuminated silicon electrode (left) uses light energy to assist in the water-splitting process and is protected from the surrounding electrolyte by a 2-nm film of nickel. (Illustration: Guosong Hong, Stanford University)

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While not a primary source of energy, hydrogen, because of its large energy density, provides a vehicle with which to store and transport energy. Photoelectrochemical (PEC) cells can use sunlight to sustainably split water into hydrogen and oxygen, but efficient PEC materials tend to corrode rapidly in use. A Stanford research group has been studying this problem, and has found that depositing a thin layer of nickel atoms on a silicon PEC electrode allows it to operate for over 80 hours with no sign of corrosion.

If you want to harvest the energy from sunlight on a large scale, looking into using silicon would be a good start. Cheap and workable owing largely to the massive infrastructure built to support the fabrication of integrated circuits, silicon can absorb from the near-IR into the UV, a range that covers the peak wavelengths of the Sun's radiation. Silicon solar cells have attained as much as 30 percent conversion of sunlight into electricity.

The chemical energetics of using sunlight to split water into hydrogen and oxygen (Image: ...

The figure above illustrates how a PEC cell operates. Briefly, incoming sunlight excites free electrons near the surface of the silicon electrode. These electrons are free to flow through the wires to the stainless steel electrode at the other side of the cell, where four of them react with four water molecules to form two molecules of hydrogen and 4 OH groups.

The OH groups flow through the liquid electrolyte to the surface of the silicon electrode. There they react with the four holes associated with the four photoelectrons, the result being two water molecules and two oxygen molecules. Notice that while electrons do not have to travel through the electrolyte, the OH groups do.

Only things aren't quite that simple in practice. In particular, illuminated silicon begins to corrode immediately when in contact with the electrolytes used in PEC cells. Not only does corrosion eat materials, but the operation of a PEC cell to generate hydrogen depends sensitively on the properties of the surfaces and interfaces within the cell. If chemical reactions are changing the materials of which those features are formed, the odds are tiny that the result will be an efficient PEC cell.

The lifetime of practical PEC cells must be long enough to recover construction costs and minimize routine maintenance. This requires a lifecycle measured in thousands (if not tens of thousands) of operating hours. Prior to the work being described here, eight hours of operation without noticeable corrosion was considered a significant advance, indicating a good deal of room for improvement.

This eight hour silicon electrode was protected from the electrolyte by a surface layer of titanium dioxide and iridium. Stanford professor of chemistry Hongjie Dai thought that a thin (2 nm) nanolayer of nickel, also a metal well known as resistant to corrosion, might perform better.

So his group tried it out. They built a PEC cell with 2 nanometers of nickel on a silicon electrode, paired it with a stainless steel electrode, and immersed both in a solution of potassium borate and lithium borate in water (introducing lithium in electrolytes also helped Thomas Edison in his quest for better batteries). Fresh electrolyte was introduced into the cell twice a day to avoid changes in concentration.

The resulting PEC cell split water for 80 continuous hours with on sign of surface corrosion. In the process, about 150 ml of hydrogen gas was generated, representing the storage of about 2 kilojoules of energy.

The result points out a new path toward longer-lasting PEC cells. "Our lab has produced one of the longest lasting silicon-based photoanodes," Dai said. "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." In an area of study in which "breakthroughs" seem to appear monthly, hopefully this new path will be a path toward a solution, rather than a dead end.

Source: Stanford University

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer.   All articles by Brian Dodson
4 Comments

I think this will be a boost for fuel cell vehicles and lessen our use / depency on other fuels, plus I think it is cool.

BigGoofyGuy
19th November, 2013 @ 05:24 am PST

The article did not give any figures as to how much "bias voltage" power had to be supplied, and how much sunlight equaled how much electricity. Looks to be an interesting, small small application, but not powering any service stations that refuel fuel cell cars. Without any performance figures in this article, I would nonetheless put a bet on conventional solar cells charging a battery, to drive a vehicle, would outperform this conversion -> hydrogen -> fuel cell -> vehicle on road, by about 5x.

Scott in California
20th November, 2013 @ 10:42 am PST

'.... hydrogen, because of its large energy density, ....'

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Large energy density? Energy density if energy per volume. The energy density of compressed hydrogen isn't that large. In fact it pales in comparison to current fuels....less than 1/6 the energy density of gasoline and even less compared to diesel.

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Perhaps you were thinking of specific energy; energy per mass.

BGriffin
21st November, 2013 @ 02:32 pm PST

When considering hydrogen as fuel derived from electrolysis, it would be best to consider HHO, not H. HHO is the fuel you want. What's the energy density of HHO? That might be a better question to ask. Or, what is the energy density of HHO as compared to the vaporized fuel/air mixture that is fed into the chambers in a gasoline engine? HHO when generated on demand and immediately used as fuel this way, need not be compressed at all, thereby avoiding the explosive hazards of gas or oil fuel tanks and compressed hydrogen fuel tanks. Also, HHO makes more sense to use than using hydrogen itself, which then has to look around for an oxygen molecule to combust

Jay Dillon
30th November, 2013 @ 04:11 pm PST
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