NREL Talks Solar Hydrogen Splitting Best Practices For Efficiency

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There are a number of ways to make hydrogen gas. Most people know about electrolysis, because it’s both the cleanest way to make it (at present) and we probably saw a high school science teacher make some. There are also ways to get it from fossil fuels (like steam reformation). It’s also possible to get it from biological processes and other chemical reactions. The fossil fuel industry would rather we just think of all hydrogen as if it came from the cleanest sources, because they can make it from dirtier sources and don’t want people to think about that.

But, even with electrolysis using electricity from solar panels is problematic. It’s a long story, but you can’t just separate hydrogen and then use it. It needs to be collected, compressed, stored, transported, pumped into a vehicle’s tank, and then used either to generate electricity again or just to burn. Every step introduces conversion losses, and every step introduces expenses.

At the end of the day, you’re almost always better off using the solar energy to charge batteries instead of using it to separate hydrogen, because the hydrogen route uses several times more electricity.

But, batteries aren’t ideal for every application. Things like aviation, over the road trucking, and shipping all require a lot more energy density than today’s batteries can provide. So, while we shouldn’t use hydrogen for everything and make an inefficient and wasteful “hydrogen economy,” we’re still going to need to use hydrogen for a while yet in some niche applications.

But, a recent press release from the National Renewable Energy Laboratory (NREL) explains that all hope is not lost for efficiency here. Another method for getting hydrogen is Photoelectrochemical (PEC) water-splitting. Instead of using electricity, PEC uses energy directly from the sun to produce hydrogen, and thus leaves other renewable electricity sources available to power other things that are more efficient. This makes it one of the most promising and exciting ways to produce hydrogen.

The efficiency of the PEC process has been measured by different laboratories and has often varied greatly due to a lack of standardized methods. To provide confidence in comparing results obtained from different sites and groups, researchers have published a best-practices guide in Frontiers in Energy Research.

The publication provides guidance for the PEC community as researchers continue to improve the technology. To verify these best practices, both laboratories did round-robin testing using the same testing hardware, PEC photoelectrodes, and measurement procedures. Research into photovoltaics has allowed a certification of cell efficiencies, but there is not yet an widely accepted protocol for measuring efficiency in water-splitting with PECs.

“It’s really difficult to compare reported PEC water-splitting efficiency results between labs, because people tend to make measurements under different conditions,” said Todd Deutsch, a senior scientist at NREL and co-author of the new journal article. “The Department of Energy recognized this a while ago, so there have been quite a few efforts to establish standards that we’ve been involved in — multi-lab collaborative efforts and also NREL-specific efforts.”

This article provides a step by step guide so that all laboratories conducting photoelectrode experiments can follow the same uniform practices. The authors walk readers through each detail of the process, from what materials are needed for fabrication to how to measure solar-to-hydrogen (STH) efficiency. They emphasize that direct measurement of hydrogen output is essential for accurately understanding STH efficiency.

“The motivation for this protocol paper was both to serve as a guide for researchers just entering the field as well as describing subtle technique tips for more experienced scientists,” said Francesca Toma, a materials staff scientist at Berkeley Lab and a co-author of the journal article. “We leveraged the unique strengths of two national labs that together span the basic to applied science realms.”

Scientists first discovered PEC water-splitting in 1972, and research since then has only improved the process. Although no one has established standardized procedures for measuring STH (short-term holdup), NREL set a world record in 1998 of 12.4%. Unfortunately, they revised that figure downward to 10% in 2016 after realizing their original experiment had been conducted with too much light.

In 2017, the team’s design for light absorbers was more precise and resulted in a higher STH of 16.2%, setting a new world record.

The US Department of Energy’s Hydrogen and Fuel Cell Technologies Office has set 25% as the target for STH through PEC water-splitting, although preliminary cost analysis suggests that hydrogen could be achieved at a lower price with lower efficiencies. Photoelectrodes have demonstrated efficiencies from 10% to 20%.

In addition, PEC researchers are also focusing on ways to extend the durability of the semiconductor. The device that captures sunlight is submerged in an aqueous (water-based) electrolyte. However, with the pH levels in electrolytes ranging from acidic to alkaline, it causes corrosion and reduced lifespan of the semiconductor.

“Durability still is pretty much a showstopper for this technology,” Deutsch said. “There’s been some progress, but not nearly as much as there has been recently in improving efficiency.”

The new paper written by Deutsch, entitled “Long-Term Stability Metrics of Photoelectrochemical Water Splitting,” delves into the topic of highly efficient and stable unassisted PEC water-splitting. In the paper, achieving this feat is referred to as the “Holy Grail” in clean and renewable fuel generation. A framework for conducting long-term stability experiments is described with goals of obtaining ultra high stability (lasting 10,000+ hours) and efficiency greater than 15%.

Why This Matters

Having a consistent way to measure efficiency in PEC processes helps drive further improvement that the industry can trust in the future. If someone announces, “Hey! We solved PEC!” and everyone asks “How did you measure that?#8221; instead of “Yes! Let’s GOOOOO!” we’ve got a real problem. Why? Because it may, in fact, not really be time to “GOOOO!”

Solving this problem early and making it easy to not only measure, but replicate and compare experiments means the technology will be able to advance from the lab to real-world use faster (assuming it’s not a dead end for whatever reason). This way, it it’s going to able to work, we’ll probably be able to get there.

If this can be achieved, it will allow hydrogen and battery-electric technologies to develop in parallel instead of competing with each other for scarce renewable energy. That could give efforts to minimize climate change a real boost.

Featured Image: A photoelectrode is used to produce hydrogen by splitting water. Researchers have developed a best-practices guide on how to best compare water-splitting technologies across different laboratories. Photo by Dennis Schroeder, NREL.