Solar energy
is fine when the sun is shining. But what about at night or when it is cloudy?
To be truly useful, sunshine must be converted to a form of energy that can be
stored for use when the sun is hiding.
The notion of
using sunshine to split water into oxygen and storable hydrogen fuel has been
championed by clean-energy advocates for decades, but stubborn challenges have
prevented adoption of an otherwise promising technology.
A team of
Stanford researchers may have solved one of the most vexing scientific details
blocking us from such a clean-energy future.
The team, led
by materials science engineer Paul McIntyre and chemist Christopher Chidsey,
has devised a robust silicon-based solar electrode that shows remarkable
endurance in the highly corrosive environment inherent in the process of
splitting water.
They revealed
their progress in a recent paper published in Nature Materials.
Conceptually,
splitting water could not be simpler. Scientists have long known that applying
a voltage across two electrodes submerged in water splits the water molecules
into their component elements, oxygen and hydrogen.
From an environmental standpoint, the process is a dream: an electrochemical
reaction whose only requirements are water and electricity and whose only
byproducts are pure oxygen and hydrogen, a clean-burning fuel applicable in a
promising new class of renewable energy applications. In fact, hydrogen is the
cleanest burning chemical fuel known.
Practical challenges
“In theory, water splitting is a clean and efficient energy storage
mechanism. Unfortunately, solving one problem creates another,” said
McIntyre, associate professor of materials science and engineering. “The
most abundant solar electrodes we have today are made of silicon, a material
that corrodes and fails almost immediately when exposed to oxygen, one of the
byproducts of the reaction.”
This
particular problem has vexed researchers since at least the 1970s. Many had
given up, but McIntyre and Chidsey have devised a clever solution. They coated
their silicon electrodes with a protective, ultra-thin layer of titanium
dioxide.
“Titanium
dioxide is perfect for this application,” explained McIntyre. “It is
both transparent to light and it can be efficient for transferring electricity,
all while protecting the silicon from corrosion.”
Sunlight travels through the protective titanium dioxide into the
photosensitive silicon, which produces a flow of electrons that travels through
the electrochemical cell into the water, splitting the hydrogen from the
oxygen. The hydrogen gas can be stored and then, when the sun is not shining,
the process can be reversed, reuniting hydrogen and oxygen back into water to
produce electricity.
Decades of dead ends
Other researchers had attempted to protect the electron-producing silicon
electrodes. Some tried other materials, which failed for reasons of performance
or durability. Some had even tried titanium dioxide, but those efforts also
fell short. Their layers were either materially flawed, allowing oxygen to seep
through and corrode the semiconductor, or too thick to be electrically
conductive.
Yi Wei Chen
and Jonathan Prange, the lead doctoral students on the McIntyre-Chidsey team,
discovered that the key to the titanium dioxide’s protectiveness is achieving a
very thin, yet high quality layer of material. They found that a layer just two
nanometers thick was sufficient so long as it was free of the pinholes and
cracks that doomed earlier titanium dioxide experiments.
With their electrodes successfully shielded from corrosion, the researchers
revealed yet one more engineering ace in the hole, adding a third layer of
ultra-thin iridium, a catalyst, atop the titanium dioxide. Iridium boosts the
rate of the splitting reaction and improves performance of the system.
Broader applications
In side-by-side durability experiments, the researchers put their creation to
the test. Control samples without the protective layer corroded and failed in less
than a half-hour, while those with the titanium dioxide lasted the full
duration of the test, eight hours without apparent corrosion or loss of
efficiency.
The authors
pointed out that their approach is general enough to work on other
semiconductor substrates and to integrate other catalysts, allowing for
fine-tuning of electrodes to maximize performance. Likewise, atomic layer
deposition, the technique that allowed such fine and flawless layering, is in
wide application in the semiconductor industry today. It should, therefore,
lend itself to application on a large scale. Lastly, the results were achieved
without exploring the use of other efficiency-enhancing techniques, such as
surface texturing, which could further improve performance.
“We are
excited about the possibilities of this technology,” said McIntyre,
“as much for the electrode itself, as for the process used to create
it.”
Their success might just push a promising technology one step closer to
practical application and the world one step closer to a clean-energy future.
