There’s some really exciting science and engineering going on, and it’s getting us closer to counting hydrogen among good options for clean and renewable energy. We need all the options we can get. If you’re already on board with hydrogen, just skip right down to that section because you’re going to like what’s going on, but first I have to give a little introduction…..
Renewable energy is on the rise, providing a larger percentage of our energy needs every year. One of the big keys to practical renewable energy is the ability to store it. Lithium-ion batteries have come a long way, and that has helped to extend the reach of solar and wind into times when the sun isn’t shining and the wind isn’t blowing. It’s also permitted things like electric vehicles, whose cost keeps creeping down.
One thing we are not very good at yet, though, is storing renewable energy in a chemical form, like a liquid fuel. Batteries are terrific for many things, like lights and motors, but liquid fuels are much better when you need ignition, like in jets or rockets, or for lighting your stove or your grill.
Don’t get me wrong, though; hydrogen fuel cells can absolutely make electricity (and heat along with it if you want to harness that, too). There are most assuredly hydrogen vehicles out there, and some people think they will do surprisingly well. After all, you don’t have to recharge a liquid fuel. You fill the tank in a couple minutes and you’re ready to go. And the range of a hydrogen vehicle can be significantly longer than that of an electric one. Hydrogen fuel cells haven’t yet benefitted from large-scale production like batteries, so their costs are still higher, but they too will come down.
Natural gas and propane are wonderful fuels, as we all know from experience, but they come out of the ground and release CO2 when they’re burned. Ethanol can be quasi-renewable, but it takes a fair amount of energy to produce and purify, and in the end it’s frankly a pretty lousy fuel.
The very simplest way to store energy chemically is by making hydrogen. Just H2. Notice there’s no “C” in that formula, because carbon need not be involved at all.
When you burn hydrogen, you get a lot of energy, and you only get one chemical product: WATER. That’s it. No CO2, no partial combustion, no carbon monoxide, no NOx, no SOx, no toxins, nothing. Just WATER.
Oh, and hydrogen as a fuel, if we’re judging by energy per mass (specific energy), kicks the crap out of all other fuels, and it’s not close:
NASA weighs in on hydrogen fuel quality:
Despite criticism and early technical failures, the taming of liquid hydrogen proved to be one of NASA's most significant technical accomplishments. . . . Hydrogen -- a light and extremely powerful rocket propellant -- has the lowest molecular weight of any known substance and burns with extreme intensity (5,500°F). In combination with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known rocket propellant.
OK, so you probably won’t be launching any rockets (unless you’re Mark Sumner), but this ought to reassure you that the sky is the limit on the effectiveness of hydrogen fuel.
Right now most hydrogen is made from methane (natural gas), but how can we make hydrogen renewably? There are three main ways, and I’m going to tell you about a really cool recent advance in each of them, just to provide a chunk of the encouraging stuff going on out there:
Gasification of waste — turning municipal waste into gas by blasting it with a plasma torch
Photocatalysis — using solar cells to split water into hydrogen and oxygen
Photosynthesis — harnessing a plant’s ability to split water with sunlight
Gasification of waste
This is the shortest-term answer, and it isn’t some pie-in-the-sky dream; it’s being built right now. I love this. The first large-scale green hydrogen plant in the world. A process called plasma enhanced gasification, invented by Drs. Salvador Camacho and Robert Do, will be implemented at an L.A.-area facility, and it won’t need external energy, but instead will actually produce its own energy. Construction begins in earnest next year, and operations start in 2023. It will use municipal waste (plastic, paper, textiles, etc.), which is blasted with super-hot plasma torches to break it down into its constituent atoms, which form simple molecules (H2 among them, of course) when they cool.
CO2 is among the products, too, but it’s contained, so it is not released to the air. It can be sequestered or sold as compressed CO2 for soda, industrial synthesis, etc. You can even use all-biogenic waste so that there is zero net CO2 creation. The ultimate product stream is 99.9999% pure hydrogen, and there are no toxic byproducts.
How about the economics, though? The production cost will be about 90 cents per pound, so let’s say it will sell for $1.50 per pound. Gasoline is normally about $3 per gallon (6 pounds), so that’s 50 cents a pound. But the energy density of hydrogen is about 3x that of gasoline. Folks, that means we have a draw. Wouldn’t you rather use the fuel that emits no net carbon?
The hydrogen made in this plant will be used to supply California’s 42 hydrogen filling stations. Maybe a few more will pop up.
Photocatalysis
Storing sunlight directly as hydrogen would be a dream come true. Imagine using sunlight to split water into hydrogen and oxygen and being able to utilize both products. We actually can do it now, but the efficiency of the process is still not very good. The big drawback of photocatalysis has been “recombination”, but there’s a research group that’s found a way to beat it.
To split water with sunlight, we use a semiconducting material, which only allows electrons (current) to flow when they’re excited by enough energy. Every semiconductor has a “band gap” energy, or the amount of energy needed to jolt the electrons up into the “conduction band”, where they are free to get off their home atom and move around.
Sunlight is a great way to provide this energy. That’s how solar cells make electricity. But we can also use those high-energy electrons to split water. Energy from sunlight jolts the electrons out of their comfortable seats within the semiconductor material, so now we have free electrons and “holes” (or places electrons used to be). The electrons need to migrate over to the hydrogen evolution reaction (HER) catalyst, where they’ll meet up with protons to make hydrogen. Meanwhile, the “holes” would really like their electrons back, please! So much so that they will rip electrons off of water over at the oxygen evolution reaction (OER) catalyst. But the major bugaboo is that electrons and holes really like to get back together again (“recombination”) before they reach their catalyst targets, and that defeats the purpose of the whole thing:
You lose half your incoming energy or more because of this recombination problem. Usually HER and OER catalysts are scattered all over the semiconductor, but a group led by Dr. Kazunari Domen (Shinshu University, Nagano, Japan) has done things differently, a really slick approach to get around this. Check out their impressive May 27 paper in Nature.
They found that electrons and holes will preferentially migrate down different axes of the semiconductor crystal because the conduction band energy is a bit lower on certain facets, making it easier for excited electrons to drift that way. They show this in a model here, where e— indicates electrons and h+ indicates holes.
So they found a way to deposit the HER catalyst onto the facets where electrons have an easier time reaching, and the OER catalyst onto those facets that holes have an easier time reaching. Because of this arrangement, they got greater than 96% efficiency, and when you adjust for the fact that some of the sunlight got scattered away, it’s very close to 100% efficiency. No recombination to see here, folks.
Now this particular semiconductor (aluminum-doped strontium titanate) has a band gap of 3.2 eV, so you need ultraviolet light (around 360 nm) to excite its electrons. Its efficiency drops off quickly up in the visible range, but there are other semiconductor materials that can cover the range up to 600 nm (into about orange light).
That means this approach can be used to make arrays that grab almost all the visble and UV range of sunlight and convert it to hydrogen at incredible efficiencies. There’s obviously more work to be done, but this is a result that provides a blueprint to double photocatalytic efficiency across the board. Just really awesome work.
Photosynthesis
As I’m sure you know, plants are pretty darn good at using energy from sunlight to make sugars and other things. But plants don’t really care that you and I want fuels; they care about making cellulose and starch and raspberries and stuff like that. They split water just like a photovoltaic cell does, with an OER catalyst. They then use the electrons they get from water to charge up their own little electron carrier molecules called ferredoxins, which roam around the cell donating electrons to whatever reactions the plant decides it wants to carry out. Photosynthesis doesn’t have its own HER catalyst, so no hydrogen is made.
But two groups, almost simultaneously, have found a way to add an HER catalyst right onto photosynthesis so that it uses electrons for making hydrogen instead of charging up ferredoxin. One of the groups was led by Kirstin Gutekunst (University Kiel, Germany), and the other was led by Iftach Yacoby (Tel Aviv University, Israel) and Kevin E. Redding (Arizona State University). They used cyanobacteria and algae, respectively, because those are simpler photosynthetic systems you can stick into a reactor.
Here’s a schematic of how these groups changed photosynthesis. I’m trying to use the same symbols I did in the photovoltaic diagram:
Photosynthesis creates ”holes” by letting sunlight excite electrons out of their places, just like photovoltaics. That happens within a giant protein complex called Photosystem II. Again, the holes really want their electrons back, and if water happens to be around, the hole will rip electrons from it. Meanwhile, the electrons that got excited by sunlight roll down a series of electron carriers, each step releasing some energy, until they make their way over to another giant protein complex, Photosystem I, and fall into another “hole”. Sunlight excites them once again, and usually from there they have enough energy to jump right onto ferredoxin, and that’s that.
But these research teams attached a natural HER catalyst called hydrogenase directly to the uppermost component of Photosystem I by fusing their genes — and therefore their resulting proteins — together. They had to do it just right, so that both components still worked and so that electrons could actually jump successfully from one to the other.
Hydrogenase does naturally occur in some algae, but separate from photosynthesis. In fact, it’s usually only used when there’s no oxygen around and when the algae have some extra electrons to throw away. You wouldn’t want to make hydrogen during photosynthesis if you’re algae or a plant, because you can’t really use the hydrogen for anything, and you’d just be throwing energy away. Evolution would eat you for lunch.
This is what the Yacoby-Redding group’s new Photosystem I/hydrogenase combo looks like:
So photosynthesis was already very good at keeping the charges separated without using any fancy biased crystals, but it just needed an HER catalyst to finish the water splitting and make some hydrogen.
Both groups got nice, sustained hydrogen production for hours by shining light on their engineered microorganisms. There’s more work to do, of course, in increasing productivity and lowering operating costs, but this is a very cool advance, to be sure.
Now, none of these things means that hydrogen fuel is going to be ubiquitous by next Thursday or that it’s the magic solution to all of our problems. I’m not trying to represent it that way. But I’m really impressed and encouraged with the progress that’s being made, and I know hydrogen will have a role to play in our sustainable future.