The global climate is changing unprecedentedly, and the primary reason behind this catastrophic change is the significant rise in global CO₂ emissions. It is critically important to lower the CO₂ concentration in the atmosphere, and this can be accomplished by CO₂ conversion technology where carbon-based materials are used. CO₂ conversion refers to the process of converting CO₂ into useful chemicals and fuels, and some common conversion methods are illustrated in Figure 1:

Different types of materials are potentially used for CO₂ conversions into useful chemicals and fuels, including carbon-based materials, namely activated carbon, graphene, and carbon nanotubes. These materials possess a high surface area and can act as catalysts, making them promising for use in CO₂ reduction reactions. However, more research is needed to optimize their performance and make the process more efficient and cost-effective.

Graphene has been studied for its potential in converting CO₂ into useful chemicals and fuels. One way for CO₂ conversion using graphene is through graphene-based catalyst usage, which can facilitate chemical reactions that convert CO₂ into CH₃OH or formic acid. Another application is membranes synthesized from graphene, which can selectively separate CO₂ from other gasses. Graphene has several qualities that make it a promising material for CO₂ conversion, and some of them are as follows [77]^[5]:

• High surface area: This characteristic makes graphene useful for catalytic reactions;
• High conductivity: Graphene is an excellent conductor of electricity and heat, which makes it useful in electrochemical reactions;
• Chemical stability: Graphene is chemically stable, and can be used in harsh environments and high-temperature reactions;
• Selectivity: Graphene membranes can be made to be highly selective, which makes graphene useful for CO₂ separation;
• Low cost: Graphene is made of carbon, which is abundant and inexpensive;
• Durability: Graphene is a strong and durable materials that can be used in long-term applications. These qualities make graphene a highly promising material for CO₂ conversion.

These materials are highly porous, lightweight, have a high surface area, are usually made from carbon nanostructures, and have been shown to have the potential for CO₂ conversion [49,77]^[5][6]. Their high surface areas allow these materials to adsorb large amounts of CO₂, and they, therefore, can be used in a variety of chemical reactions to convert CO₂ into other compounds [77]^[5].

One of the main ways ACs have been used for CO₂ conversion is in the form of adsorbents. CO₂ adsorption on carbon aerogels can be done through physisorption or chemisorption processes, which are then followed by the release of CO₂ by heating or by changing the pressure.

High surface area: CAs possess a very high surface area, typically from 300 to 2000 square meters per gram. This property allows them to adsorb large amounts of gases, liquids, and other substances [78]^[7];

Activated carbons, usually known as activated charcoal, are carbon-based materials with a highly porous surface area which makes them useful for CO₂ conversion. ACs can also be used as a catalyst to convert CO₂ into other chemical compounds. For example, they have been used to catalyze the conversion of CO₂ into formic acid and methanol, which are useful chemicals for other industrial processes [81]^[10].

ACs can be used as a CO₂ adsorbent, which involves both the physical and chemical adsorption processes. The CO₂ can then be released by heating or by changing the pressure, making it possible to capture and store CO₂ in this way. ACs are particularly effective at adsorbing CO₂ at high pressures and temperatures, making them a potential material for capturing CO₂ from industrial processes [82]^[11].

Though those materials are promising in CO₂ conversion at research and lab scales, the CO₂ conversion performance of those materials at industrial scales still need to be tested and verified.

Yellow tuff, a natural tuff, and low-cost adsorption material, has been reported to be conveniently employed in a vacuum swing for CO₂ adsorption processes [83]^[12]. MOFs are proving to be effective adsorbents for CO₂ capture due to their microporous structure and chemical and thermal stabilities [84]^[13]. MOFs are capable of providing both physical and chemical interactions with CO₂ [83]^[12], while some other chemical adsorbents, such as amine-functionalized adsorbents, are capable of interacting strongly with the acidic CO₂ molecules [83]^[12]. Zeolites have been shown to have significant potential for CO₂ adsorption due to their high porosity, ultra-small pores, structural diversity, high stability, excellent recyclability, and chemical reactivity [35]^[14].

Three main strategies exist for mitigating the consequences of climate change (illustrated in Figure 2). Traditional mitigation efforts aim to reduce carbon dioxide emissions through the use of de-carbonization technologies and methods such as renewable energy, fuel switching [85]^[15], efficiency increases, nuclear power, and carbon capture, storage, and usage. The risks associated with these devices are generally low and have been around for some time [86,87,88]^[16][17][18]. The second possibility involves the use of novel tools and methods. It is possible to remove CO₂ from the atmosphere and store it using negative emissions devices [86]^[16]. Numerous negative emissions techniques are already in use, including bioenergy carbon capture and storage, bio-char, enhanced weathering, direct air carbon capture and storage, ocean fertilization, ocean alkalinity enhancement, soil carbon sequestration, afforestation/reforestation, wetland construction/restoration, and mineral carbonation/biomass use in construction [89,90,91]^[19][20][21]. The third and final choice involves controlling the amount of radiation coming from the sun and the planet. The goal of using geoengineering techniques that rely on radiative forcing is to either keep our world’s average temperature stable or to make our planet slightly cooler. The main geoengineering techniques for radiative forcing are stratospheric aerosol injection, marine sky brightening, cirrus cloud thinning, space-based mirrors, surface-based brightness, and various radiation management approaches [92,93]^[22][23]. As a means of decarbonizing the energy and manufacturing industries, carbon capture and storage holds a great deal of potential. The burning of solid fuels such as coal, oil, or gas can produce carbon dioxide gas, which can now be collected and isolated using modern technology. The CO₂ is then transported through pipes to subterranean storage facilities, where it will stay for thousands of years. Eliminating or at least greatly lowering the pollution from burning fossil fuels is priority number one. The three possible capturing times are before, after, and during oxyfuel combustion. When it comes to CO₂ capture and storage, each method has its specific process. Post-combustion capture devices, on the other hand, are well-suited for retrofitted uses. CO₂ is captured, liquefied, and then transported via ship or conduit to long-term holding sites. It has been found that depleted oil and gas areas, coal beds, and underground saline reservoirs that are not used for potable water are all viable options.

Direct Air Carbon Capture Technology

One of the emerging technologies for artificially removing CO₂ from the atmosphere is direct air carbon capture and storage (DACCS). This technology uses molecular bonding to extract CO₂ from the atmosphere, and it is then either kept in geological reservoirs or put to some other use, such as in the production of compounds or mineral carbonates adsorption. The stability of CO₂ storage is a major issue for this technology as well, just as it is for carbon capture and storage and bioenergy carbon capture and storage. Capturing CO₂ from atmospheric air is much more challenging than capturing CO₂ from highly concentrated combustion gas streams since direct air carbon capture and storage requires more energy and materials. If unforeseen issues with large-scale implementation can be resolved, the global capacity for CO₂ removal will be between 0.5 and 5 GtCO₂ per year by 2050 and could increase to 40 GtCO₂ per year by the end of the century. The cost of CO₂ removal is estimated to run from USD 600 to 1000 per metric ton of moved (tCO₂) at the beginning but to drop to USD 100 to 300 per metric ton of CO₂ removed in the future [94]^[24]. Several companies have developed and implemented direct air capture technology at an industrial scale. These companies include Carbon Engineering, Climeworks, Global Thermostat, Hydro Cell, Sky Tree, and Infinitree. Carbon Engineering has constructed a DAC plant located in the Permian Basin in Texas, USA, with a capture capacity of 1 MtonCO₂/year [95]^[25]. There are no legislative instruments in place to aid this technology, as is the case with many of the other negative emissions options that have been investigated. The direct air capture of CO₂ by physisorbent materials, namely TEPA-SBA-15 (amine-modified mesoporous silica, chemisorbent, and Zeolite 13X (inorganic), HKUST-1, Mg-MOF-74/Mg-dobdc, and SIFSIX-3-Ni (a hybrid ultra-microporous material including four physisorbents), were found to be able to capture carbon from CO₂-rich gas mixtures [96]^[26]. The greenhouse gas removal efficiency was 79–91%, while in the best case, the removal efficacy can be as high as 97% [97]^[27]. However, competition and reaction with atmospheric moisture significantly reduced the direct air capture (DAC) performance [96]^[26], and the drawback of DAC lies in the significant amount of energy required for capturing CO₂ from the atmosphere [98]^[28]. Amine-based cellulose adsorbents or silica gel are normally used in the DAC system [99,100]^[29][30]. An estimate of US$610–780/tC CO₂ was reported for the facility using sodium hydroxide, which is a strong base [95]^[25]. The thermal energy required for a DAC adsorption system is between 1400 and 2000 kWhth/ton CO₂ [100]^[30]. Circular economy is a model that recycles all waste and byproducts discharged from a production process and generates no waste at the production end pipe. A cycle c economy is a process that involves the transforming of CO₂ from a linear economy into value-added chemicals and fuels. CO₂ is converted into methanol with enzymes or methane via electrodes and hydrogenophilic methanogenic cultures [101]^[31]. Catalysts can also be used in converting CO₂ into other chemicals. For example, activated carbons have been used as a catalyst in converting CO₂ into formic acid and methanol, as mentioned above, both of which are useful chemicals that can be used as feedstock for other industrial processes. About 200 million tons of CO₂ are utilized in different processes each year, and then CO₂ will react with ammonia to produce urea for fertilizers, while petroleum companies are also injecting CO₂ underground to help recover fossil oil. Methanol and H₂ are some of the key products produced from CO₂ conversion processes, and those chemicals can be green or gray methanol and H₂. When methanol is produced from renewable energy sources, a new term, “e-methanol”, is used. In terms of environmental impact, H_2, and methanol are known as the lowest carbon intensity chemicals, and this is best for the climate. Methanol is one of the most practical alternatives to conventional fuels in the maritime area globally, and its use in a ship’s operation can reduce SO_x emissions by 99%, PM emissions by 95%, NO_x by 60%, and CO₂ by 25% [102]^[32]. Recently, there has been an increasing number of projects that utilize sustainable feedstock such as captured CO₂ from industrial emitters and green hydrogen produced from municipal solid waste (MSW), forestry residues, or agricultural waste. In terms of market impact, methanol is available in over 100 ports today. Conventionally, by 55% of global consumption of methanol is used in the production of downstream chemicals. Increasingly, the fastest growing segment is where it is in the numerous applications where it is consumed as a fuel (~45%). Captured CO₂ can also be used for enhanced oil recovery [103]^[33]. Using methanol as a fuel would produce CO₂ emissions of 54.7 tons per day at service speed, compared to 64.7 tons per day for diesel, and even less when using renewable or bio-methanol blends. Methanol is known to be a fuel that has no sulfur emissions, very low particulate matter, and CO₂ emissions 15% lower than conventional marine fuel oil. Methanol can even blend with water to meet IMO NO_x Tier III requirements, removing the need for expensive exhaust gas treatment [104]^[34]. Renewable methanol can also be produced from renewable H₂ and captured CO₂. Globally, methanol was valued at USD 30.7 billion in 2021 and is forecasted to reach USD 36.3 billion by 2026. The option to recycle CO₂ could reduce industrial costs related to CO₂ certification, and in Germany alone, the production costs of e-MeOH or methanol produced from electricity will vary between EUR 608 and 1453 per ton. Hydrogen can also be produced from captured CO₂. An electrolyte is used in this technology. When CO₂ is injected into the aqueous electrolyte, the CO₂ will react with the cathode, making the solution more acidic, which eventually generates electricity and creates H₂. This is a new trend and has a great environmental and market impact since captured CO₂ can be recycled and turned into a clean energy source [105]^[35].