We can’t go on belching CO2 into the atmosphere at the rate we’ve been doing it. But CO2 is a product of combustion, and combustion’s not going to disappear in time to meet any climate goals. Far from it. We’ve painted ourselves into a corner on that one.
So what can we do in the meantime? The whole carbon capture and sequestration idea, which would pipe CO2 for miles and bury it underground, seems frankly kind of nuts. Not only is it literally sweeping the problem under the rug, but it also seems to help continue funneling big bucks to oil companies, who conveniently are the only ones who have the technology to shove stuff miles underground. Plus, in practical terms, it hasn’t gone well at all.
So … what else can we do in the meantime? After years of experimentation with different catalyst formulations, an Australian team of researchers has devised a cheap and scalable way — with the help of simple mechanical energy, of all things — to split CO2 into C and O2. I got so excited I split an infinitive right in my title.
They reported the finding in the October 6 edition of Advanced Materials, the top American materials science journal.
So what makes CO2 difficult to get rid of? It’s very stable and generally doesn’t like to react chemically with much of anything. In order to spark its interest in reacting, you have to expend energy to activate it, for example by forcing it to grab an electron and become CO2· ‒, where the dot means “electron” and the minus means “negative charge”. You can do that with heat, electrical, or light energy, but those approaches up to now have been energy-intensive, required high temperatures (as in >1000°F) , and suffered from slow reaction rates and catalyst clogging.
Our Aussie friends here used a totally different approach: mechanical energy. But how do you convert motion into electrochemistry? The key here was the triboelectric effect. The formal way of describing this is “a type of contact-induced electrification in which a material becomes electrically charged after it comes into frictional contact with another, dissimilar material”. But the relatable way of getting it across is this:
There are certain materials that match up really well this way, like polystyrene and cat fur, or a balloon and your hair. By rubbing them together, we can transfer electrons from one surface to another, or we can induce charge separation in a material.
So maybe there could be a way to use this phenomenon to transfer electrons to CO2 and activate it so that it will react with something and cease to be CO2. Well, there is! But it wasn’t too easy to find. That’s what these researchers have been tirelessly working on for years, and after an awful lot of tinkering, they hit on a system that really gets it done.
Before we go into the details, let’s step back and appreciate the big picture. They got a 92% efficiency of conversion of CO2 into solid carbonaceous product — at only around 100°F — with an energy input of only 230 kW∙h per ton of CO2. The cost of electricity in the U.S. is about 12 cents per kW∙h. So that’s about $27.60 to get rid of a ton of CO2. Errr, OK! I’ll buy!
There was no deterioration in performance over 100 continuous hours. The efficiency of conversion of mechanical to chemical energy was around two-thirds.
The key to the whole process is liquid gallium! Gallium is an elemental metal that, like mercury, is a liquid at relatively low temperatures. But it’s not toxic like mercury. (I hope not, for this guy’s sake...)
Droplets of liquid gallium were suspended in a solution that could dissolve a lot of CO2, (dimethylformamide and ethanolamine, if you must know) and in addition, rod-shaped silver-gallium crystals (Ag0.72Ga0.28) spontaneously formed in this solution when silver fluoride salt was added and sound waves were pumped through. No other silver salts worked, and the whole thing only worked when the crystals ended up rod-shaped. Not exactly predictable.
The sound waves helped agitate things and caused the gallium drops and the rod-shaped crystals to bump into each other from time to time. Packing peanuts, meet cat.
The authors gallantly try to explain what the fork happens when a crystal of Ag0.72Ga0.28 strikes a gallium droplet. They do know that when this happens, an ultrathin layer of gallium oxide (region A in the figure below) quickly forms on the surface of the liquid gallium droplet. The triboelectric effect of the collision induces a charge separation across layer A, and from then on this thin layer acts like a tiny battery, until its charges come back together.
Electrons are compelled to flow away from the gallium (region C), leaving some Ga+ (gallium missing an electron and hence positively charged) at the surface of the droplet. These electrons circulate up to region B, and ultimately return to C again, but on the way they encounter CO2, and convert it to CO2· ‒. So there’s the key to this whole thing. Mechanical energy — the friction between the Ag-Ga crystals and liquid gallium — makes this happen:
Ga + CO2 → Ga+ + CO2· ‒
Separation of charge, folks. The same principle behind photosynthesis, the process that literally runs all life on the planet.
At this point, that CO2· ‒ is free to react with other species, and that’s what it does, forming what overall looks like sheets of reduced graphene oxide. A lot of C=C and C-O bonds. I oversimplified a bit before when I said “solid carbon”. Some oxygen gets in there, too.
It turns out that this material isn’t just junk; it could actually be a valuable side product for use in batteries or carbon-fiber materials. No worries, mate — a startup company has already been formed to scale this up and apply it to real industrial flue gases.
Throughout this whole cycle, the crystal rods remain intact, as does the liquid gallium. Both are only catalysts, so they can be reused over and over. Besides that, the layer of carbonaceous material that forms on the surface of the liquid Ga balls is very thin and is easily dislodged by the mechanical action, keeping the liquid Ga completely active and stable.
They did some tests with simulated flue gas (not pure CO2) and observed the same kinds of conversion rates and efficiencies. Now they’re scaling it up to a semi-trailer sized reactor that could handle the offgas from a real industrial plant.
There’ll be technical hurdles, to be sure. Nobody just slaps up a plant and has everything go peachy right away. And no, of course this won’t singlehandedly solve all the world’s problems. But if this really is a practical and cost-effective way to stop CO2 pollution at large scale before it even starts, it could be a highly implementable way to help us meet the climate goals we’re all pining for. We could really use the help.
You know, there was another Aussie hit (#1, in fact) that we failed to adopt here in the U.S., back in the pre-Men at Work days. This song (actually by Kiwis) never made it to the U.S. at all, and that’s really too bad, because it’s so much fun. The first time I ever heard it was on a Qantas flight to Australia. Let’s not make the same mistake again! Crikey!