Instead of using the sun or wind power for electrolysis, researchers are testing the competitiveness of photoelectrochemical cells to produce emission-free hydrogen fuels.
“Green” hydrogen is the production of hydrogen fuels using emission-free generation sources such as solar and wind energy. While battery-powered vehicles and power plants charged with renewable energy sources are expected to make up a large part of the energy transition, green hydrogen has attracted interest for heavy-duty transport and industrial use because of its ability to transmit large amounts of power.
Green hydrogen production typically uses an “indirect” approach, using the sun and wind as electrical power in electrolyzers. The efficiency of these electrolysis devices is about 30%.
A research team at the Solar Fuels Institute at the Helmholtz-Zentrum Berlin (HZB) is investigating another way to produce emission-free hydrogen through a process they call the “direct” approach. The approach is outlined fresh Nature communication article.
The group develops photoelectrodes that convert sunlight into electrical energy, are stable in aqueous solutions and catalytically split water into hydrogen. The photoelectrodes are connected to the catalyst material, thus forming the active component in the photoelectrochemical (PEC) cell.
Today, the best PEC cells are capable of nearly 10% efficiency and are made of inexpensive and relatively stable metal oxide absorbers. Although significantly less efficient than their indirect green hydrogen counterparts, PEC cells have some advantages. The heat of direct sunlight can be used to further speed up the reactions. And because densities are 10 to 100 times lower with this approach, more abundant and cheaper materials can be used as catalysts, the researchers said.
Despite these promising aspects of the PEC approach, techno-economic analysis and net energy analysis have shown that it is not yet competitive with conventional green hydrogen production. Hydrogen fuel for PEC systems costs about $10 per kilogram, about six times more than fossil methane steam reforming “blue” hydrogen at $1.50 per kilogram. Also, the cumulative energy requirement for PEC water splitting is estimated to be 4-20 times higher than for hydrogen production with renewable energy and electrolyzers.
Despite this cost-effectiveness difference between the approaches, the HZB Institute’s research team is testing other potential benefits of PEC. The team tested the effects of how the produced hydrogen further reacts with itaconic acid in the same reactor to form methyl succinic acid (MSA). MSA is another chemical fuel that can be used to send large amounts of energy, such as hydrogen.
Thanks to PEC technology, MSA can be produced with only one-seventh of the typical energy requirement of MSA fuel production processes.
To find out, the researchers calculated how much energy is needed to produce a PEC cell from light-absorbing, catalytic, and other materials such as glass, and how long the system must operate to produce this energy as chemical energy in the form of hydrogen, or MSA. .
For hydrogen alone, the energy payback period is about 17 years, assuming a modest 5% solar-hydrogen efficiency. If only 2% of the hydrogen produced is used to convert itatonic acid to MSA, the energy payback time is halved. If 30% of the hydrogen is converted to MSA, the production energy can be recovered after only 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 green hydrogen production costs and increase the economic viability of PEC technology. We have thought about the process carefully, and the next step is to test in the laboratory how well the simultaneous production of hydrogen and MSA works in practice.”