The green hydrogen market has been taking off like a rocket, thanks mainly to the availability of low-cost wind and solar power for water electrolysis. Researchers are not resting on their laurels, though. The race is on to mimic nature’s own high-efficiency hydrogen production system, aka photosynthesis. It has been a long road, but the dream of an “artificial leaf” is finally beginning to take shape.
Many Roads To Green Hydrogen
To be clear, fossil energy resources still dominate the global hydrogen market. Not for long. Alternative sources are surging into the market, with plain old water is in the lead.
So far, most of the sustainable H2 activity has focused on electrolysis, in which renewable energy generates an electrical current that jolts hydrogen gas from water.
That’s a giant step up the sustainability ladder. However, electrolysis does require the conversion of renewable resources to electricity. With plenty of other users are crowding the field, hydrogen stakeholders will have to elbow their way into the competition for wind and solar power.
One way to make more elbow room is to improve the efficiency of electrolysis systems. However, the pressure to deploy wind and solar resources for other uses will continue to persist as the climate crisis grows worse.
Another way to relieve the pressure is to develop alternative pathways for sustainable hydrogen, such as gas from organic matter or industrial wastes.
A human-made photosynthesis system would add a powerful new tool to the alternative green hydrogen toolkit, partly because it could provide a workaround to the site selection issues that can obstruct solar development.
Across the pond, for example, a research team at the University of Cambridge is preparing to market a low cost, durable artificial leaf. Their device can be floated on canals and other bodies of water, as a land-conserving alternative to solar arrays.
What Is The Artificial Leaf?
The idea of an artificial leaf first hit the CleanTechnica radar in 2011, when we noted Harvard professor Daniel Nocera’s work on a low cost, solar powered, sustainable H2 system that could be down-scaled for home use in off-grid communities.
Water electrolysis has grabbed practically all of the media spotlight since then, but artificial leaf research has continued apace.
The basic idea behind the artificial leaf sounds simple enough. You simply fabricate a specialized solar cell called a photoelectrochemical cell, dip it in a water-based solution, and expose it to light, thereby recreating the chemical reactions in natural photosynthesis.
Depending on who’s talking, the class of photoelectrochemical cells includes two subsets, only one of which is used in direct solar-to-hydrogen production. However, we’ll follow the lead of the US Department of Energy, keep things simple, and stick with the general term photoelectrochemical. If you have an issue with that, take it up in the comment thread.
Why Is The Artificial Leaf?
Either way, it’s fair to ask why bother to produce green hydrogen with photoelectrochemical systems, if we already have water electrolysis.
Artificial leaf researchers point out that the efficiency of an electrolysis system is just one piece of the green hydrogen puzzle. The other is the efficiency of the energy inputs. When compared to electricity sourced from solar cells, the artificial leaf approach is far more efficient.
Purdue University researcher Yulia Pushkar summed it up in press release last year, when she said that “there are not fundamental physical limitations” with artificial photosynthesis.
“You can very easily imagine a system that is 60% efficient because we already have a precedent in natural photosynthesis. And if we get very ambitious, we could even envision a system of up to 80% efficiency,” Pushkar added.
In contrast, the average solar conversion efficiency of solar cells is still within the area of 20%. Specialized versions can go much higher, but they are also much more expensive than those in general use.
NREL Eyeballs Artificial Leaf For Green Hydrogen
Photoelectrochemical hydrogen production still isn’t ready for the market. Researchers know how to duplicate the reactions in photosynthesis, but durability has been an obstacle. They haven’t yet figured out how Mother Nature keeps all her balls up in the air for an extended period of time.
The main problem is that the semiconductors used in photoelectrochemical cells are corroded by the water-based solution. The Energy Department, for one, is anticipating that the durability issue is solvable.
“PEC [photoelectrochemical] water splitting is a promising solar-to-hydrogen pathway, offering the potential for high conversion efficiency at low operating temperatures using cost-effective thin-film and/or particle semiconductor materials,” the agency explains on its website, while noting that “continued improvements in efficiency, durability, and cost are still needed for market viability.”
To help move things along, the Energy Department has come up with a set of best practices, developed by the National Renewable Energy Laboratory and the Lawrence Berkeley National Laboratory. The guidelines were recently published in the journal Frontiers in Energy Research under the title, “Best practices in PEC: How to reliably measure solar-to-hydrogen efficiency of photocathodes.”
“The article spells out the path so that all laboratories can follow a uniformity of experimental practices, beginning with the materials needed for the fabrication of photoelectrodes,” NREL explained in a press release last week.
Green Hydrogen Is Not Waiting Around For Your Artificial Leaf
If you’re wondering why water electrolysis took off so quickly while direct solar-to-hydrogen technology is still waiting for its closeup, that’s a good question.
Part of the answer probably lies in the historical record. Water electrolysis is a centuries-old technology that can trace its roots back to 1789. Water electrolysis has had plenty opportunities for fine-tuning in other applications, including the production of oxygen on the International Space Station. It just never made much sense as a hydrogen production pathway until solar cells and wind turbines began to power the global economy.
In contrast, NREL points out that the artificial leaf is a relatively new development. According to the lab, a description of photoelectrochemical water-splitting did not show up in a scientific publication until 1972.
NREL cites itself as a good example of the need to establish standards and best practices in a new field of research. In 1998 NREL reported that it set a solar-to-hydrogen efficiency record of 12.4%, making it the first research institution to cross the 10% barrier. However, in 2016 the lab had to correct that figure downwards after determining that the experiment had been over-illuminated.
With more sophisticated tools in hand, an NREL research team set a new record of 16.2% efficiency in solar-to-hydrogen conversion in 2017. That could come close to doing the trick. The Energy Department is aiming for 25%, efficiency, but an NREL analysis suggests that solar-to-hydrogen systems could be economically competitive without hitting that target.
Meanwhile, water electrolysis really is taking off like a rocket. Aside from pushing fossil-sourced hydrogen out of the fuel production industry, green hydrogen stakeholders are already threatening to pry the long fingers of fossil energy out of the fertilizer market and other key sectors of the global economy.
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Photo credit (screenshot): Green hydrogen produced from a photoelectrochemical system courtesy of NREL.
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