You may have seen headlines popping up recently from news out of the American Chemical Society’s spring meeting in Anaheim, CA about a new “artificial leaf.” So how soon are you going to be planting these leaves and leaving your energy bills far behind? Don’t cancel your utilities just yet, but let’s take a look at what all the hubbub is about. In this two parter (!) I’ll first lay out the motivation behind trying to split water, what that has to do with energy, and then I’ll outline what makes this new development so interesting.
Split it Up – What is Water Splitting?
Here is the reaction that is the crux of this hole matter. All that we are trying to accomplish is to break apart the water molecule into the elements that make it up, in this case of course it’s just hydrogen gas and oxygen gas as seen here:
Energy + 2H2O –> 2H2 + O2
“Doesn’t seem so difficult” you say… and you’d technically be correct. I can tell you that the minimum amount of energy needed to do this reaction is 1.23 V. On a relative scheme of things, it is not that much energy. For instance, you can split water in your own home with a simple 9-volt battery! Just hook up some wires to each terminal, stick in tap water, and watch hydrogen gas generate at the negative terminal and oxygen at the positive! (You can add some salt to the water to speed up the reaction if it’s taking too long). This process of using electricity to split molecules apart is called electrolysis, and here’s a nice video if you want to see done all fancy like:
(You’ll note there appears to be more hydrogen coming off than oxygen. That makes sense because there’s twice as much hydrogen in H2O than oxygen!)
Fantastic! We can make hydrogen bubbles from water… what’s the big deal?
The idea is that we can use hydrogen as a fuel to store energy when we don’t need it, and then use the fuel to produce energy when we do need it. The easiest way to do that is just to burn the hydrogen in a reaction that looks surprisingly familiar…
2H2 + O2 –> 2H2O + Energy
Hmm yes, the astute reader will note that this is just the reverse reaction to the one shown earlier. Instead of using up energy to split water apart, now we just combine hydrogen and oxygen to create water and release energy! Super! I’m now just going to put both of these reactions together to make a simple cycle that shows how we can store energy with one reaction, and use it with the other:
Neat, we provide energy (in the form of electricity) to convert low energy molecules (water in this case) to high energy molecules (hydrogen). When we burn hydrogen we convert it back into a low energy molecule (water) and get energy out out the system (either heat or electricity) which we can then use. I’ve generalized this energy storage mechanism, so let’s compare it to our current carbon-fuel based cycle:
You’ll say: “Heck, you just swapped hydrogen for oil, coal, and gasoline in the high energy molecules and added carbon dioxide to the low energy molecules!” and I’ll say “Darn right!”. Conceptually there’s no difference between hydrogen fuel based energy storage and carbon fuel based energy storage. We’re just using different molecules that happen to contain carbon as the high energy molecules, and this necessitates that we have carbon present in the form of carbon dioxide in the low energy molecules.
But here is the big difference. How much time does it take to go from CO2 to oil and coal? Well, I’m no geologist but I’m going to throw out a number on the order of 100 million years. How much time does it take to split water into hydrogen and oxygen? Well, a couple seconds to a few minutes as you just saw in that video, and that is a huge benefit to using hydrogen as an energy storage molecule. (One could reasonably argue that using the same carbon cycle above to use photosynthesis to produce simple sugars and ethanol only takes a few months – but I’d say that hydrogen production is still faster)
TIME OUT – Very Important Science Message!
If I had access to those 90’s era spinning siren animated gifs I would put them right here. This is critical. If there is one thing you take away from this post today it is this:
Hydrogen itself is not a source of energy
Remember that hydrogen is just a fuel, just a way to store the energy we got from somewhere else. There are no naturally occurring pockets of hydrogen gas that we can go mine. In order to make it with electricity we typically use fossil fuels. Many people will incorrectly report hydrogen as a source of carbon free renewable energy but this is incorrect. Hydrogen fuel is only as clean as the source of the energy that produced it.
So if you have a wind turbine that produces the electricity to make hydrogen, then yes the hydrogen can be considered clean. But if the hydrogen is created by burning coal, then the hydrogen is no more “clean” than the coal that produced it.
I kind of jumped the gun earlier but I wanted to make sure the previous point was crystal-clear. The real benefit of hydrogen is that if it can be married to a source of renewable energy – such as wind or solar. These renewable resources are great but are typically criticized for being variable, the typical catchphrase being “the sun doesn’t always shine and the wind doesn’t always blow”. BUT which hydrogen storage, the electricity could be used to create hydrogen when the sun is shinning and the wind is blowing – then the hydrogen could be burned (or used in a fuel cell) to produce the electricity on demand.
That’s Not All…
Of course, there’s a catch… making hydrogen isn’t as easy or as simple as I’ve made it seem. The 9V battery trick is not the solution to our energy problems. That would be just too easy. For one, sticking a 9V into water to make hydrogen is overkill and woefully inefficient. There are catalyst problems, storage problems, efficiency problems, all of which I will cover next time – along with this intriguing artificial leaf technology. Until then – think science!
Also – If you like this and find it useful – please share! It’s for SCIENCE!