Can CO2 from the atmosphere really be turned into a resource?

May 17, 2024

UTokyo takes on issues close at hand
UTokyo researchers answer questions on 21 GX (Green Transformation) topics from their specialist viewpoints. Through questions that cannot simply be brushed off as someone else’s concerns, take a peek into GX and our world of research.

Q18. Can CO2 from the atmosphere really be turned into a resource?

Although CO2 is the villain causing global warming, it’s composed of carbon and oxygen, which are key elements. Isn’t there some way to utilize them?
Plants have been doing it for eons, so why not us…

Answered by Masakazu Sugiyama

Professor, Research Center for Advanced Science and Technology (RCAST)
Renewable Energy Studies

Masakazu Sugiyama

Utilizing CO2 captured by air conditioners in multi-story buildings

Artist’s impression of an urban DAC system

In photosynthesis, a process to which all animal and human life on Earth can be said to owe its existence, plants use energy from the sun to split oxygen from water and reduce (remove the oxygen from) carbon dioxide (CO2) from the atmosphere, thereby creating hydrocarbons. However, the process of creating these hydrocarbons is inefficient. Thus, if humans wanted to replace fossil fuels with plant-produced hydrocarbons, a colossal amount of land would be required.

We are investigating methods of producing hydrocarbons with greater efficiency than plants. The most used hydrocarbon in the world is ethylene (C2H4), which acts as the base component of various chemicals. Almost all ethylene is produced from oil in a process that emits a lot of CO2 into the atmosphere. CO2 is also emitted when products made from ethylene are burned as trash. Therefore, if ethylene could be made using carbon captured from the air, it would truly close the carbon loop.

Plants gradually take up CO2 through the surface of their leaves. We can create a more efficient capture process by using a large electric fan to blow air across a fluid in which CO2 easily dissolves. CO2 becomes acidic when dissolved, so to counteract this, an alkaline fluid would be used. The alkaline fluid easily dissolves the acidic CO2, which we can then release from the fluid by heating it.

In a project for the Japanese Cabinet Office’s Moonshot Research and Development Program, we are investigating a CO2 capture system using air conditioners installed in multi-story buildings. Where there are many people, there will obviously be a lot of CO2, so these buildings represent a kind of carbon treasure trove. We will install Direct Air Capture (DAC) devices inside the air conditioners to capture the CO2. Air conditioners normally bring in air from outside the building to reduce the CO2 concentration in the interior, but that is not necessary in this case as the CO2 is being captured. Air conditioning costs for cooling and heating the outside air can therefore be eliminated, offsetting the cost of capturing the CO2. This process is known as Carbon Capture and Utilization (CCU).

Schematic diagram of CO2 electrochemical reduction device
CO<sub>2</sub> electrochemical reduction device and working electrode (right)
CO2 electrochemical reduction device and working electrode (right)

The key is to separate the O from CO2 and introduce H

Capturing the CO2 is fairly simple; the bigger problem is how to reduce it. Compounds that contain oxygen are stable, and CO2 is a typical example of such a compound – the carbon and oxygen in CO2 are tightly bound to each other. In plants, low-voltage electrons generated in the leaves come into contact with CO2 and split it into carbon and oxygen. To try to replicate this, we are studying a system that combines a solar cell with an electrochemical reactor.

A calculation of the overall energy used in the production of ethylene reveals that the CO2 reduction step currently consumes about four times more energy than that needed to collect the CO2 from the air. This ratio must be cut if we want to succeed. To produce ethylene from CO2 and water, we use a device resembling a water electrolyzer. When a voltage is applied to an anode and cathode on either side of a polymer membrane, the oxygen is split off from the CO2 and the remaining carbon forms bonds with hydrogen extracted from water. While water electrolyzers boast an energy efficiency of around 70%, CO2 electrochemical reactors currently operate at around only 30%. The reaction does not proceed on its own, so a powerful catalyst – in our case, a specially engineered form of copper – is indispensable. By controlling the microstructure at the three-phase interface and improving the polymer membrane, a several-fold increase in reactivity can be achieved. A pilot plant will be constructed by 2030, with large-scale commercialization following by 2050 as we implement what is considered to be the ultimate in decarbonization strategies.


Professor Sugiyama is the director of RCAST.
This is Sentan, RCAST’s unicorn mascot. Its horn is a rocket!

Q. As oil originally comes from plants, why should we refrain from using it?
A. Because we are using it far more quickly than it was produced.

Fossil fuels (hydrocarbons) accumulated over a period of more than 100 million years, but humans have been using them for just 200 of those years. The root of the problem therefore lies in the completely different timescales for their formation and consumption. There would be no problem if our current daily usage of hydrocarbons equaled the amount produced on any particular day in the past, but that’s not really happening, is it? Advocating that the rates of consumption and production of fuels should be balanced is an example of a carbon neutral mindset.

* This article was originally printed in Tansei 46 (Japanese language only). All information in this article is as of March 2023.

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