Cheap, sustainable hydrogen seemed like a pipe dream just a few years ago, when practically all hydrogen fuel was sourced from fossil natural gas. It still is, but researchers have been hammering away at various methods for deploying sunlight to pry hydrogen loose from alternative sources. In the latest development, a multinational team has capped a five-year research project with a breakthrough based on a nanoscale form of titanium dioxide, aka titania.
Hydrogen fuel cell EVs (FCEVs) currently face an uphill climb in terms of competing with EVs on the open road, but hydrogen fuel cells are rapidly slipping into logistics, aircraft, seaports and seacraft, and other niche markets, and that makes sustainable hydrogen sourcing a critical issue.
“Electron Highways” For Sustainable Hydrogen
The new sustainable hydrogen research comes from the University of Pennsylvania and Stanford University with Drexel University, Italy’s University of Trieste, the University of Cadiz (Spain) and the Leibniz Institute for Catalysis (Germany).
The team built on extensive, previous research demonstrating that sunlight kicks off a chemical reaction in titania, resulting in hydrogen production.
That’s simple enough. The sticky wicket is controlling the reaction and scaling it up to an efficient system that lends itself to commercial production.
Here’s how the research team describes the conundrum:
…the vehicles responsible for this response, called electrons and holes, tend to jump the gun, reacting with each other almost immediately due to their opposite charges.
In addition, getting the electrons and holes to behave according to plan is a classic case of cat herding. They each do something different. The electrons are negatively charged and they carry out reductions, while the holes are positively charged holes and do oxidations. The challenge is to get them both to do the same thing, “splitting” water into oxygen and hydrogen gas.
The research team explored the idea that titania’s reactive powers are too rapid because the electrons and holes are too close together. To slow the whole thing down, they tailored a nanoscale form of titania and used nanorods to separate the holes and electrons.
The team started with a range of 15 to 50 nanometers and determined that the longer end of the range performed more efficiently. Without each other to keep themselves busy, the electrons and holes were forced to react with other molecules, as explained by Matteo Cargnello of Stanford:
If you want to have more efficient photocatalysts, make elongated structures to create these highways for electrons to escape from holes and react much faster with the molecules.
Two Steps Forward…
Titania is an abundant, naturally occurring form of titanium. What is not so abundant is a nanoscale form of titania called brookite, upon which the new research is based.
Typically, brookite is fabricated in the lab by reducing larger pieces down to nanosize. For the new breakthrough, the research team built their own brookite from scratch, one atom at a time, using a process called solution-phase chemistry.
According to the team, this “bottom-up” approach is more precise and potentially more scalable for sustainable hydrogen production.
However, the research still has a long way to go before it gets to the water-splitting stage. So far the team has demonstrated their process on alcohols derived from biomass, resulting in the production of the undesirable byproduct carbon dioxide as well as hydrogen.
On the other hand, the team foresees that if the carbon dioxide is recycled for biomass production, the result would be a closed, carbon neutral cycle — or something close to it, at least.
You can read more about the sustainable hydrogen study by turning to the Proceedings of the National Academy of Sciences under the title “Engineering titania nanostructure to tune and improve its photocatalytic activity.”
So Much For Carbon Sequestration…
Speaking of what to do with waste carbon, last year the US Department of Energy pulled the plug on the massive FutureGen project, which was supposed to demonstrate an economical way of diverting carbon from coal fired power plants and sequestering it under ground.
Sequestration is not entirely dead, but the fate of FutureGen is another example of how the whole “out of sight, out of mind” strategy for airborne waste is slowly withering on the vine.
In addition to recycling carbon for biomass production, researchers are also looking at power plant and industrial waste gases for conversion to liquid fuel and solid plastics, with the potential for carbon-negative processes in sight.