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The Forefront of Space Science

Removing the Carbon Dioxide Using Fuel Cells
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For life support in closed spaces in the universe, oxygen supply and removal of the carbon dioxide (CO2) are very essential. Our research group is doing a research on using CO2 effectively by fuel cells. In the first stage, we do research on new fuel cells which can generate electric power while reducing CO2, and develop a next generation air regeneration system using those cells. And in the second stage, we aim at an ultimate carbon cycle system which can reduce and recycle CO2. In this paper, we are going to introduce our efforts.

Closed Spaces in the Universe and Removal of CO2

CO2 almost inexhaustibly exists on the earth and is cycling in the nature. A typical example of CO2 immobilization is photosynthesis, but the increase of CO2 concentration in atmospheric by the use of fossil fuels is faster than the CO2 immobilization. Because of this, changes in atmospheric composition are often taken as causes of global warming, which is known to everyone.

CO2 concentration in living spaces on the ground is currently about 0.03%. If the concentration is more than a few percent, human beings would suffer headache, dizziness and nausea. And if the concentration becomes higher, it might cause serious situation such as losing consciousness. It means that managing the concentration of CO2, which is often ignored in the usual living spaces, is an important issue for closed spaces with human activity in the universe from the point of view of life support.

Nowadays, technologies for removing CO2 in closed spaces in the universe include absorbing CO2 by the alkaline lithium hydroxide or by the adsorbent called zeolite. Especially for the former case, the absorbent can be used only once.

Laboratories of Nagaoka University of Technology and the ISAS research group are working together on recycling CO2 in closed spaces in the universe instead of simply removing and discarding it. For that, we must develop a new technology to reduce CO2, because CO2 is the most oxidized form and the most stable state of carbon.

With proprietary technology development, we are conducting research on new fuel cells which can generate electric power while reducing CO2. The first point is to develop the next generation air regeneration system using those fuel cells. Secondly, when this technology development is completed, it would be possible to utilize CO2 as a resource. It is expected to be an innovative technology not only for recycling CO2 in closed spaces but also for using CO2 as a carbon resource on the earth or other planets and satellites with CO2 on them.

Why It Is Difficult to Reduce CO2

In general, CO2 is stable against chemical reaction. Here, let us take a look at a a thermochemical equation showing a reaction in which CO2 is involved. Combustion of methane (CH4) can be written as

CH4+2O2 = CO2+2H2O+890kJE(1)

And the heat of formation is 890kJ/mol., meaning that when burning the methane with oxygen, heat of combustion of 890kJ/mol. is obtained.

Next, considering the reduction of CO2, we let CO2 to react with H2, and the following equation (2) is satisfied.

CO2+4H2 = CH4+2H2O+253kJE(2)

This reaction is an exothermic reaction with the heat of formation of 253kJ/mol., therefore can be considered to be a spontaneous process. But in fact, it is empirically known contrarily as an endothermic reaction.

This seemingly contradictory reaction in the equation (2) can be explained by the endothermic processes which are not shown in the equation. One of them is the bond dissociation energy. Specifically, in order to dissociate the C-O, we need to provide 348kJ/mol. of thermal energy from the outside. As shown in this example, the equation (2) can be decomposed into several elementary processes, and the slowest process (which absorbs the largest amount of heat) is called as the rate-determining step.

So, is there any way to eliminate this rate-determining step? The answer is using catalysts.

The explanation might be a little difficult. Using the correct catalyst, the rate-determining step can be eliminated. If all the elementary processes can proceed smoothly with the use of catalyst, the reduction of CO2 in the equation (2) will react as an exothermic reaction. Furthermore, part of the heat of formation can be used to generate electric power.

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