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US researchers evaluate viability of ‘direct’ green
hydrogen fuel production
By RYAN
KENNEDY -
Mar 22, 2023
Rather than using solar or wind to power electrolysis, researchers are
testing the competitiveness of photoelectrochemical cells to produce
emissions-free hydrogen fuels.
![](images/images_03_28_2023/pic1.jpg)
Image: Brenda Ahearn, Michigan Engineering, Communications and
Marketing
“Green”
hydrogen is the production of hydrogen fuels using emissions free
generations sources like solar and wind energy. While renewables-charged
battery-powered vehicles and power plants are expected to make up a
large share of the energy transition, green hydrogen has garnered
interest for heavy trucking and industrial use cases due it its
ability to dispatch high amounts of power.
Production of
green hydrogen typically takes the “indirect” approach using solar and
wind to power electrolyzers. These renewables-powered electrolyzers
have reached round-trip efficiencies of around 30%.
A research
team at the Helmholtz-Zentrum Berlin (HZB) Institute for Solar Fuels
is investigating another pathway to emissions-free hydrogen production
through a process they call the “direct” approach. The approach is outlined in
a recent Nature
Communications article.
The team is
developing photoelectrodes that convert sunlight into electrical
energy, are stable in aqueous solutions, and catalytically split water
into hydrogen. The photoelectrodes are coupled with catalyst
materials, creating the active component in a photoelectrochemical (PEC)
cell.
Today, the
best PEC cells are able to achieve efficiencies near 10% and are made
of low-cost and relatively stable metal oxide absorbers. Though they
are significantly less efficient than their indirect green hydrogen
counterparts, PEC cells have some advantages. The heat from the direct
sunlight can be used to further accelerate reactions. And, since the
densities range ten to 100 times lower with this approach, more
abundant and inexpensive materials can be used as catalysts, said the
researchers.
Despite these
promising elements of the PEC approach, techno-economic analysis and
net energy analysis have shown that it is not yet competitive with the
conventional green hydrogen production approach. Hydrogen fuel from
PEC systems costs about $10 per kg, about six times more than “blue”
hydrogen from fossil methane steam reforming at $1.50 per kg.
Also, cumulative energy demand for PEC water splitting is estimated to
be four to 20 times higher than for hydrogen production with renewable
energy and electrolyzers.
Despite this
gulf in cost efficacy between the approaches, the research team at HZB
Institute is testing other potential benefits of PEC. The group tested
the effects of how the hydrogen produced reacts further with itaconic
acid in the same reactor to form methyl succinic acid (MSA). MSA is
another chemical fuel that can be used to dispatch high amounts of
energy like hydrogen.
The PEC
technology enables MSA to be produced at only one-seventh of the
typical energy requirement of MSA fuel production processes.
To find this,
researchers calculated how much energy is needed to produce the PEC
cell from light absorbers, catalyst materials and other materials such
as glass, and how long the system must function to produce this energy
in the form of chemical energy as hydrogen or MSA.
For hydrogen
alone, the energy payback period is about 17 years when assuming a
modest 5% solar-to-hydrogen efficiency. If only 2% of the hydrogen
produced is used to convert itatonic acid into MSA, the energy payback
time is halved. If 30% of the hydrogen is converted into MSA, the
production energy can be regained after just 2 years.
“This makes
the process much more sustainable and competitive,” said Dr Fatwa Abdi, HZB
Institute for Solar Fuels. “This
approach offers a way to significantly reduce the production cost of
green hydrogen and increases the economic feasibility of PEC
technology. We have carefully thought through the process, and the
next step is to test in the laboratory how well the simultaneous
production of hydrogen and MSA works in practice.”
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