Tokai University's solar car. Image: "Tokai Falcon"/Wikipedia
The race to put a hydrogen fuel cell car on the market has been long and painful, almost obligatory. No matter the challenges, a car that ditches both battery storage and internal combustion is a promise that can’t be ignored. It's the genuinely oil-free auto future. And, finally, Toyota will deliver the very first hydrogen powered car to consumers next year. Despite lingering safety concerns and an extreme lack of hydrogen fueling stations, it’s here.
Should it be here? Probably not, or at least not quite yet. Elon Musk is a naysayer, but he also has quite a conflict of interest. (Musk is not shy about being a dick about conflicts of interests.) It’s true however that hydrogen is something quite a bit trickier to deliver and store than electricity and, right now, the United States is only boasting about a dozen hydrogen filling stations, mostly around Los Angeles and San Francisco. All of the California stations—which are just kind of sitting there as of today—exist because of state government funding.
Ultimately it’s an even worse chicken and egg problem than fully electric cars. Potential hydrogen fuel providers need hydrogen cars first in order to stay open, yet hydrogen cars need hydrogen fuel stations already existing in order to leave the showroom. It’s a bad scene. A bit of timely research out last week in Physical Review Letters might also suggest that the technology simply has a ways to go yet: imagine harvesting hydrogen just by using everyday sunlight.
Boron-nitride in the middle, flourine on the bottom/Jinlong Yang
First off, the real dream of hydrogen fuel cell cars involves using just plain old water as fuel, rather than paying money at the rare hydrogen filling station. The watered car then will split that water into hydrogen and oxygen. So: a car that takes water as fuel and gives off oxygen as exhaust. That’s pretty sweet. But splitting water takes energy in the form of electricity and right now, 90 percent of hydrogen is made using fossil fuels. There’s an alternate way, however, that involves using energy directly from the sun to split water rather than electricity from the grid.
Proposed first in 1972 by the Japanese chemists Akira Fujishima and Kenichi Honda, the way the method works is that a photon from the sun beams down onto some catalyst solution. A particle in this solution then absorbs the photon and an electron pops out, resulting in an electron-hole pair. This stimulates the decomposition of water, leaving us with hydrogen that can then be used in a fuel cell arrangement. The catch is that it takes a photon of a certain amount of energy to generate an electron-hole pair at the neccessary 1.23 eV, specifically a photon at about the infrared wavelength.
The sun fortunately delivers over half of its energy in the infrared spectrum. The catch is that most existing catalysts achieve band gaps in either the visible or ultraviolet ranges. The prior has proven to be unstable over time, while the latter only accounts for about 7 percent of the sun’s delivered energy. Infrared is really the only band gap up to the job. The Chinese team, led by Prof. Jinlong Yang, seems to have overcome the problem, effectively boosting the potential energy of the infrared spectrum by 10 eV, thanks using a new superthin sandwich of materials.
The result, the team claims, is a material capable of using common infrared light from the sun to split water efficiently. “If the solar energy based hydrogen generation becomes efficient enough, on board fuel generation for fuel cells may outperform direct solar cell electric cars,” Jinlong Yang told me last week. “At least, it can be used as a backup mechanism when the tank is empty but a hydrogen fuel station is not available.”
Basically, it works like this. The material has as its bottom layer flourine atoms. Flourine has a very high electronegativity, so electrons collect on this bottom layer. So the material then winds up with a reservoir of valence electrons—the electrons used for bonding between different elements—and this layer attracts the "hole" created when a photon knocks an electron out of place. The electron, meanwhile, migrates to the top of the material.
This migration induced by the flourine layer in effect adds the 10 eV to the current already induced by the photon bombing. That is quite easily within the infrared spectrum. The catch to all of this is pretty huge: it's very costly and very difficult to make the catalyst the team is describing. Sadly, it seems industrial production is a ways off. Nonetheless, the promise of hydrogen-on-the-fly is remains even more exciting than a real live car with little or no handy access to fuel.