Cool paper on the preparation of nickel doped carbon electrocatalysts from CO2.
One interesting way to deal with carbon dioxide recovered from air, in direct air capture scenarios (using, perhaps, seawater as an extractant) is to actually reverse combustion by reducing the carbon dioxide back to carbon in such a way that the carbon has economic value in materials setting.
Last March, I wrote here about a paper on this topic that caught my eye and about which I think quite a bit, because of its potential side value in performing certain otherwise difficult chemical separations. That post is here: Electrolysis of Lithium-Free Molten Carbonates
Today I came across yet another paper along the same lines. It's this one: Nickel and Nitrogen-Doped Bifunctional ORR and HER Electrocatalysts Derived from CO2 (Anna-Liis Remmel, Sander Ratso, Giorgio Divitini, Mati Danilson, Valdek Mikli, Mai Uibu, Jaan Aruväli, and Ivar Kruusenberg, ACS Sustainable Chemistry & Engineering 2022 10 (1), 134-145).
The "HER" is the hydrogen evolution reaction, and ORR is oxygen reduction reaction.
HER is useful in electrolysis of water, which is not necessarily the best pathway for generating hydrogen for captive use, but is OK if extra electricity is available as a side product of high temperature driven chemistry, where the electricity is produced as a side product to recovery exergy from a thermal system.
By contrast, the ORR is useful in fuel cells as in hydrogen (or metal)/air systems.
This paper is about a molten carbonate system. I can't spend a lot of time discussing this paper here, but perhaps a few excerpts from the full text are in order.
The beginning text is about hydrogen fuel cells for cars, an idea I actually oppose, since I regard "hydrogen economy" nonsense as jus that, nonsense, but I do believe that hydrogen is an excellent captive industrial reagent for many important processes.
The opening text:
Bifunctional catalyst materials for the oxygen reduction and hydrogen evolution reactions are synthesized by capturing CO2.
Introduction
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As the world economy continues to develop, energy consumption grows across all sectors of the economy. The transportation sector represents 21% of the global energy consumption and nearly all of the energy used for that is still based on burning fossil fuels. (1) Therefore, it is of the utmost importance to find alternatives that leave a smaller carbon footprint on the environment. A potential future power source that has attracted much attention is the PEMFC. Such a device can generate electricity efficiently from clean and renewable fuels (H2). Hydrogen in the form of compressed hydrogen or hydrides, for example, and oxygen from the air are transformed to electricity, water, and heat electrochemically. Hydrogen is oxidized on the anode, while the cathode reaction is always the ORR. Because of the sluggish kinetics of ORR, the overpotential of ORR contributes the most to the efficiency losses in a fuel cell. To overcome this, an effective electrocatalyst is needed. At the current technical stage, the most practical catalysts used in PEMFCs are platinum (Pt) or alloy-based materials, but the limited reserves, high cost, and instability over long-term operations hinder its large-scale application. Therefore, great attention is devoted to creating non-precious-metal or metal-free catalysts, to make fuel cells actually sustainable. (2,3) The reverse device for the PEMFC, the PEM electrolyzer, works by splitting water into hydrogen and oxygen using the same basic principle as the PEM fuel cell. PEM electolyzers face the same problem as PEMFCs, as catalysts are also needed to drive the oxygen evolution reaction (OER, cathode reaction) and the HER (anode reaction). (4) A variety of advanced materials, such as metallenes, two-dimensional (2D) nanomaterials, transition-metal nitrides and chalcogenides, N-doped carbon–cobalt borides, metal–organic frameworks (MOFs), nickel indium spinels, and doped aerogels have been developed to replace platinum on both the cathode and anode of the PEMFC. (5,6,15−21,7−14)...
Further on in the introduction a description of the novel feature, carbonate reduction to carbon is advanced:
...The purpose of this research is to demonstrate highly active Ni- and N-codoped bifunctional ORR and HER catalysts synthesized directly from CO2. The physical properties and electrocatalytic activity of carbon materials synthesized from CO2 were compared to create synthesis–structure–activity correlations and to note the modifications in the materials during nitrogen doping using a high-temperature carbonization with dicyandiamide (DCDA) and during the purification process using a mixture of H2SO4 and HNO3. The consequences of these aftertreatments toward the ORR and HER electrocatalytic activity are also presented...
It is important to note that the reduction of carbon dioxide to carbon requires adding all of the energy that was released in the combustion that oxidized it to CO2, plus some, particularly in air capture, to overcome the entropy of mixing. Basically it means producing all of the energy ever released by the use of dangerous fossil fuels plus some.
Nevertheless, with clean energy, of which there is as a practical matter only one source, nuclear energy, these types of processes have much to recommend themselves, as they introduce an economic benefit for removing carbon dioxide from the air..
One reagent in the synthesis is dicyanamide, a cyanoguanidine. This is made via a route involving reacting calcium carbide with nitrogen gas at high temperatures. Calcium carbide, which can be used as a source of acetylene, can be made by heating biochars with molten calcium metal.
The molten carbonate system here is a eutectic of lithium carbonate and potassium carbonate. The paper I discussed in March utilized alkaline earth molten carbonates.
This is a cool paper that might be useful to future generations as the work under extreme duress to clean up the mess we are leaving them.
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