Carbon dioxide (CO₂) is the main greenhouse gas (causing climate change) that is emitted by human activities (use of fossil fuels and others). The fight against climate change includes, in this sense, the reduction of emissions of this and other greenhouse gases into the atmosphere. Another possible action to stop the increase in temperatures is the capture, capture or storage ('sequestration', in technical terms) of gases such as CO₂. The possibility of capturing CO₂ from the atmosphere or from industrial chimneys for its conversion into reusable compounds has also been studied, but until now an efficient, safe and economical method to make it a reality has not been achieved.
A team of researchers from the Massachusetts Institute of Technology (MIT) and Harvard University has developed an efficient process that can convert carbon dioxide into formate (formic acid) in a liquid or solid phase that can be used - after a relatively simple - as is done today with hydrogen or methanol to power a fuel cell and generate electricity.
It is not the first time that a method of reducing CO₂ in formate has been proposed (various international groups have been working on this issue for years, also in Spain) but, according to its promoters, from MIT and Harvard, the proposal that is now presents is the most advanced in such important aspects as simplicity of production, economy of the process and product safety. The results of this research have been published (October 30) in the journal Cell Reports Physical Science.
The new process of converting CO2 to sodium formate or potassium formate has been developed by MIT professor Ju Li and MIT doctoral students Zhen Zhang, Zhichu Ren and Alexander H. Quinn, and MIT doctoral student Harvard University Dawei Xi.
The process includes the capture and electrochemical conversion of the gas into a solid formate powder, which is then used in a fuel cell to produce electricity. At the moment, this new system has been successfully tested on a laboratory scale but researchers hope it will be scalable so that it can provide emission-free heat and power to individual homes and even be used in industrial or grid-scale applications, according to a informative note released by MIT.
Professor Ju Li explained that some of the approaches developed so far to convert carbon dioxide into fuel generally involve a two-stage process: first, the gas is chemically captured and converted into a solid form such as calcium carbonate, then that material is heated to expel carbon dioxide and convert it into a combustible raw material. That second step has a very low efficiency and typically converts less than 20% of the carbon dioxide gas into the desired product, says the lead researcher of the new proposal.
In contrast, the new process achieves more than 90% CO₂ conversion and eliminates the need for the inefficient heating step by first converting carbon dioxide into an intermediate form, liquid metal bicarbonate, which is then converted to formate usable for generate electricity.
The highly concentrated liquid solution of sodium or potassium formate produced can be dried, for example by solar evaporation, to produce a solid powder that is highly stable and can be stored in ordinary steel tanks for years or even decades, says Professor Li. .
Several optimization steps developed by the team made a difference in turning an inefficient chemical conversion process into a practical solution, reiterates Ju Li, an MIT researcher in the departments of Nuclear Science and Engineering and Materials Science and Engineering.
MIT has released some details of the new proposal stating that "the carbon capture and conversion process first involves capture based on an alkaline solution that concentrates carbon dioxide, either from concentrated streams such as emissions from power plants or from very low concentration sources, even outdoors, in the form of liquid metallic bicarbonate." "Then, by using a cation exchange membrane electrolyzer, this bicarbonate is electrochemically converted into solid format crystals with a carbon efficiency greater than 96 percent, as confirmed in the team's laboratory-scale experiments," indicates MIT.
The crystals generated have an indefinite shelf life and remain so stable that they could be stored for years, or even decades, with little or no loss. By comparison, even the best hydrogen storage tanks available allow gas to escape at a rate of about 1 percent per day, excluding any use that requires year-long storage, Professor Li says.
The authors of the new study claim that they have achieved great improvements in the efficiency of the process. "First, careful design of the membrane materials and their configuration overcomes a problem that has been encountered in previous attempts at such a system, where a buildup of certain chemical byproducts changes the pH, causing the system to lose efficiency consistently over time.
The team also built a fuel cell optimized specifically for using this fuel format to produce electricity. The stored formate particles are simply dissolved in water and pumped into the fuel cell as needed. Although solid fuel is much heavier than pure hydrogen, when you consider the weight and volume of the high-pressure gas tanks needed to store hydrogen, the end result is near-parity electricity production for a volume of determined storage, says Li.
According to the researchers, the format fuel can potentially be adapted for anything from home-sized units to large-scale industrial uses or grid-scale storage systems. Initial domestic applications could involve a refrigerator-sized electrolyzer unit to capture and convert carbon dioxide into format, which could be stored in an underground or rooftop tank.
