The impact of climate change on human society and the natural environment is becoming more and more significant, and it has become a global consensus to reduce greenhouse gas emissions and mitigate the greenhouse effect. Governments and the scientific community are actively exploring solutions, and CO2 capture and storage technologies are one of the most important focuses.
Recently, a research team at the University of Cambridge has developed a new method to use modified activated carbon to capture carbon dioxide from the atmosphere efficiently and cost-effectively. This breakthrough provides new possibilities for tackling the growing problem of global warming and achieving the goal of carbon neutrality.
Comparison of the global recommended CO2 sequestration (all different depth grey series) relative to what has been achieved (all different depth blue series). The interception rate is more than 75% for natural gas processing plants, about 60% for other industrial projects, and about 10% for power plants. (Image source: Wiki)
Activated charcoal: the "secret weapon" in household water filters
Activated carbon, also known as activated carbon or activated carbon, is a porous substance made from carbon materials. It is usually made by carbonization of carbon-rich raw materials such as wood, coal, coconut shell and so on under high temperature and oxygen-free conditions.
In the carbonization process, the non-carbon elements in the raw material are removed, and a large number of micropores are formed inside, which makes the activated carbon have a large specific surface area and excellent adsorption performance.
Porous surface of activated carbon (Image source: Wiki)
One of the most common applications of activated carbon is household water filters. It can effectively remove chlorine, organic matter, heavy metal ions and other pollutants in the water, and provide clean and healthy drinking water. In addition to water purification, activated carbon is also widely used in air purification, food processing, chemical production and many other industries.
Its strong adsorption capacity stems from its unique porous structure: it has a surface area of 500 to 1,500 square meters per gram of activated carbon, which is equivalent to the size of two tennis courts. This highly developed pore structure allows it to adsorb and immobilize large amounts of gaseous, liquid, or solid molecules.
Schematic diagram of activated carbon adsorption dye (the cup on the right is the state before adsorption) (Image source: Wiki)
It was the excellent adsorption properties of activated carbon that inspired the Cambridge University research team. They envision that if activated carbon can be modified to increase its selective adsorption capacity for carbon dioxide, an efficient and cost-effective carbon capture technology could be developed.
So they tried to apply a specific electric field to the activated carbon in a similar way to charging a battery, enriching its surface with a charge, thereby increasing the attraction to charged carbon dioxide molecules. This is the core principle of the "rechargeable activated carbon sponge" technology.
"Capture" carbon dioxide:
The "last resort" in the fight against climate change
In order to curb global warming, the Paris Agreement aims to limit the increase in global average temperature to well below 2°C this century and strive to limit it to 1.5°C. This means that humanity must achieve net-zero CO2 emissions in the second half of this century, that is, to achieve a balance between CO2 emissions and removals through emission reductions and carbon capture.
However, it is not enough to control CO2 emissions by reducing fossil fuel use and improving energy efficiency. The Intergovernmental Panel on Climate Change (IPCC) estimates that in order to meet the 1.5°C target, we need to remove 5 billion ~ 11 billion tons of carbon dioxide from the atmosphere every year by 2050, in addition to significant emission reductions.
Currently, afforestation is the most commonly used method of carbon removal, but its potential is limited to meet such a large-scale demand. Therefore, the development of efficient and scalable carbon capture and storage technologies has become a priority.
Dr. Alexander Fowles, head of the "rechargeable activated carbon sponge" research, admits that capturing carbon dioxide from the atmosphere should be a "last resort" in the fight against climate change. After all, this approach to remediation is more costly and less efficient than reducing emissions at the source. "But given the severity of the climate crisis, this is the direction we have to explore." "Realistically, we have to do everything we can," Dr. Fowles stressed. ”
In fact, carbon capture and storage (CCS) technology has become one of the important options for the international community to combat climate change. Governments and businesses are increasing their investment and deployment of CCS. The International Energy Agency predicts that CCS will need to contribute around 13% of global emissions reductions by 2050.
However, most of the current CCS technologies target large point sources such as power plants and steel plants, and lack a cost-effective means of capturing carbon dioxide from scattered mobile sources and existing atmospheric carbon dioxide. This is the motivation for the research of "rechargeable activated carbon sponge".
Ways to use terrain and environment to absorb and fix carbon dioxide emitted by thermal power plants (Image source: Wiki)
"Rechargeable" activated carbon: simpler and more efficient
Traditional CO2 capture materials, such as aminofunctionalized porous silica gels, metal-organic frameworks, etc., typically need to be regenerated at temperatures up to 900°C to release adsorbed CO2 for storage. Not only does this consume a lot of energy, but it can also lead to rapid degradation of material properties.
In contrast, the "rechargeable" activated carbon sponge developed by the University of Cambridge team showed clear advantages. The study found that after adsorbing carbon dioxide, the activated carbon that has been "charged" can effectively release the captured carbon dioxide by heating it to 90~100°C. This temperature is much lower than that of traditional materials, and can be achieved by industrial waste heat or renewable energy sources (such as solar, geothermal, etc.), so it is more environmentally friendly and energy-saving. In addition, this heating process starts from the inside of the material, avoiding local overheating of the surface and further improving energy efficiency.
So, how does "charging" enhance the ability of activated carbon to adsorb carbon dioxide? The researchers explained that applying an electric field introduces an additional charge on the surface of the activated carbon, causing it to exert a stronger electrostatic attraction on polar carbon dioxide molecules. At the same time, the electric field may also change the pore structure of activated carbon, providing more stop points for the adsorption of carbon dioxide molecules. The combined effect of these mechanisms significantly improves the adsorption capacity and selectivity of rechargeable activated carbon for carbon dioxide.
It is worth mentioning that the "charging" process itself is not complicated. The research team used a device similar to a lithium-ion battery, with activated carbon as the positive electrode and lithium metal as the negative electrode, filled with electrolyte between the two. By applying an applied voltage, lithium ions are embedded on the surface of the activated carbon, forming a surface charge. The simple design and ease of operation of this device are expected to enable low-cost, large-scale production.
Schematic diagram of the process of "charging" the activated carbon network (Image source: Ref. 1)
Challenges and prospects
Although the "rechargeable activated carbon sponge" performs well in CO2 capture, there are still some challenges to overcome before it can be truly industrialized. The first is the further improvement of the adsorption capacity. At present, each gram of rechargeable activated carbon can adsorb up to 3~4 mmol of carbon dioxide, which is still far from the theoretical maximum. The research team is improving the adsorption performance of activated carbon by optimizing its pore structure and surface chemistry.
The second is the issue of the long-term stability of the material. Repeated adsorption-regeneration cycles may lead to the collapse of activated carbon pores and the loss of surface charge. How to ensure that the material can still maintain efficient adsorption after multiple uses is a difficult problem to overcome. The researchers plan to enhance the structure and chemical stability of activated carbon by means of surface coating and doping.
In addition, the scale-up application of technology also faces many challenges, such as reactor design, system integration, cost control, etc. Scaling up sample preparation in the laboratory to industrial production requires optimization and innovation in all aspects of materials, processes, and equipment, which requires the full cooperation and continuous investment of all sectors of industry, academia and research.
Despite the challenges, the "rechargeable activated carbon sponge" technology is still encouraging. It points in a whole new direction for the development of more efficient and environmentally friendly carbon capture materials. According to Dr. Fowles, this strategy is not limited to activated carbon, but can also be extended to other porous material systems for gas separation and purification in different fields.
As one of the key pathways to achieve the goal of carbon neutrality, CCS is being highly valued by governments, industry, and academia. The advent of "rechargeable activated carbon sponge" has undoubtedly injected new vitality into this field. Its simple, efficient, and economical characteristics are expected to promote the development and popularization of CCS technology, and contribute to the response of human society to the challenge of climate change.
Source: Popular Science China