A closer look at water-splitting's solar fuel potential
Scientists at Berkeley Lab and the Joint Center for Artificial
Photosynthesis zero in on bismuth vanadate's role in renewable energy at the nanoscale

Date:
August 6, 2020
Source:
DOE/Lawrence Berkeley National Laboratory
Summary:
Scientists have gained important new insight into how the
performance of a promising semiconducting thin film can be optimized
at the nanoscale for renewable energy technologies such as solar
fuels.



FULL STORY ==========================================================================
In the fight against climate change, scientists have searched for ways to replace fossil fuels with carbon-free alternatives such as hydrogen fuel.


==========================================================================
A device known as a photoelectrical chemical cell (PEC) has the potential
to produce hydrogen fuel through artificial photosynthesis, an emerging renewable energy technology that uses energy from sunlight to drive
chemical reactions such as splitting water into hydrogen and oxygen.

The key to a PEC's success lies not only in how well its photoelectrode
reacts with light to produce hydrogen, but also oxygen. Few materials
can do this well, and according to theory, an inorganic material called
bismuth vanadate (BiVO4) is a good candidate.

Yet this technology is still young, and researchers in the field have
struggled to make a BiVO4 photoelectrode that lives up to its potential
in a PEC device.

Now, as reported in the journal Small, a research team led by scientists
at the Department of Energy's Lawrence Berkeley National Laboratory
(Berkeley Lab) and the Joint Center for Artificial Photosynthesis
(JCAP), a DOE Energy Innovation Hub, have gained important new insight
into what might be happening at the nanoscale (billionths of a meter)
to hold BiVO4 back.

"When you make a material, such as an inorganic material like bismuth
vanadate, you might assume, just by looking at it with the naked
eye, that the material is homogeneous and uniform throughout," said
senior author Francesca Toma, a staff scientist at JCAP in Berkeley
Lab's Chemical Sciences Division. "But when you can see details in a
material at the nanoscale, suddenly what you assumed was homogeneous
is actually heterogeneous -- with an ensemble of different properties
and chemical compositions. And if you want to improve a photoelectrode material's efficiency, you need to know more about what's happening at
the nanoscale." X-rays and simulations bring a clearer picture into
focus In a previous study supported by the Laboratory Directed Research
and Development program, Toma and lead author Johanna Eichhorn developed
a special technique using an atomic force microscope at Berkeley Lab's
JCAP laboratory to capture images of thin-film bismuth vanadate at
the nanoscale to understand how a material's properties can affect its performance in an artificial photosynthesis device. (Eichhorn, who is
currently at the Walter Schottky Institute of the Technical University
of Munich in Germany was a researcher in Berkeley Lab's Chemical Sciences Division at the time of the study.)


==========================================================================
The current study builds on that pioneering work by using a scanning transmission X-ray microscope (STXM) at Berkeley Lab's Advanced Light
Source (ALS)v/), a synchrotron user facility, to map out changes in a
thin-film semiconducting material made of molybdenum bismuth vanadate (Mo-BiVO4).

The researchers used bismuth vanadate as a case example of a
photoelectrode because the material can absorb light in the visible
range in the solar spectrum, and when combined with a catalyst, its
physical properties allow it to make oxygen in the water-splitting
reaction. Bismuth vanadate is one of the few materials that can do this,
and in this case, the addition of a small quantity of molybdenum to
BiVO4 somehow improves its performance, Toma explained.

When water is split into H2 and O2, hydrogen-hydrogen and oxygen-oxygen
bonds need to form. But if any step in water-splitting is out of sync,
unwanted reactions will happen, which could lead to corrosion. "And
if you want to scale up a material into a commercial water-splitting
device, no one wants something that degrades. So we wanted to develop a technique that maps out which regions at the nanoscale are the best at
making oxygen," Toma explained.

Working with ALS staff scientist David Shapiro, Toma and her team
used STXM to take high-resolution nanoscale measurements of grains in
a thin film of Mo- BiVO4 as the material degraded in response to the water-splitting reaction triggered by light and the electrolyte.

"Chemical heterogeneity at the nanoscale in a material can often lead to interesting and useful properties, and few microscopy techniques can probe
the molecular structure of a material at this scale," Shapiro said. "The
STXM instruments at the Advanced Light Source are very sensitive probes
that can nondestructively quantify this heterogeneity at high spatial resolution and can therefore provide a deeper understanding of these properties." David Prendergast, interim division director of the
Molecular Foundry, and Sebastian Reyes-Lillo, a former postdoctoral
researcher at the Foundry, helped the team understand how Mo-BiVO4
responds to light by developing computational tools to analyze each
molecule's spectral "fingerprint." Reyes-Lillo is currently a professor
at Andres Bello University in Chile and a Molecular Foundry user. The
Molecular Foundry is a Nanoscale Science Research Center national user facility.



========================================================================== "Prendergast's technique is really powerful," Toma said. "Often when
you have complex heterogeneous materials made of different atoms, the experimental data you get is not easy to understand. This approach tells
you how to interpret those data. And if we have a better understanding
of the data, we can create better strategies for making Mo-BiVO4 photoelectrodes less vulnerable to corrosion during water-splitting." Reyes-Lillo added that Toma's use of this technique and the work at JCAP enabled a deeper understanding of Mo-BiVO4 that would otherwise not be possible. "The approach reveals element-specific chemical fingerprints
of a material's local electronic structure, making it especially suited
for the study of phenomena at the nanoscale. Our study represents a step
toward improving the performance of semiconducting BiVO4-based materials
for solar fuel technologies," he said.

Next steps The researchers next plan to further develop the technique
by taking STXM images while the material is operating so that they can understand how the material changes chemically as a photoelectrode in
a model PEC system.

"I'm very proud of this work. We need to find alternative solutions
to fossil fuels, and we need renewable alternatives. Even if this
technology isn't ready for the marketplace tomorrow, our technique --
along with the powerful instruments available to users at the Advanced
Light Source and the Molecular Foundry -- will open up new routes for
renewable energy technologies to make a difference."

========================================================================== Story Source: Materials provided by
DOE/Lawrence_Berkeley_National_Laboratory. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Johanna Eichhorn, Sebastian E. Reyes‐Lillo, Subhayan
Roychoudhury,
Shawn Sallis, Johannes Weis, David M. Larson, Jason K. Cooper,
Ian D.

Sharp, David Prendergast, Francesca M. Toma. Revealing Nanoscale
Chemical Heterogeneities in Polycrystalline Mo‐BiVO 4 Thin
Films. Small, 2020; 2001600 DOI: 10.1002/smll.202001600 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200806101807.htm

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