(152s) Photothermal CO2 Capture from Air Using Amine-Modified Silica Aerogel

Direct air capture (DAC) process has gained attention as a promising method for achieving carbon neutrality [1]. The DAC processes capture trace amounts of CO₂ from ambient air using chemical adsorbents or absorbents that can react with CO₂. One common approach to the DAC process involves the use of temperature swing adsorption with amino group-containing adsorbents. This process involves two stages, adsorption and desorption, which are carried out at low and high temperatures, respectively [2]. However, the strong bindings between CO₂ and amino groups make the DAC process highly energy-intensive, with heat energy consumption during desorption process accounting for approximately 85 % of the process’s total energy consumption [3]. Therefore, there is a need to develop more energy-efficient DAC processes to promote wider adoption.

To reduce heat energy consumption in the process of steam generation for desalination systems [4] and drying of desiccants [5], photothermal heating has gained attention in recent years. This method uses light-absorbing materials such as carbon, which can be heated by light absorption. For steam generation, carbon black dispersed in a solvent and a porous carbon sheet have been reported to efficiently generate steam using solar power. Additionally, for drying desiccants, silica gels containing carbon materials can be regenerated by desorbing water after water adsorption. Similarly, this method offers a more energy-efficient approach for the regeneration of desiccants.

In our previous research, we applied the photothermal heating system to the adsorbent for DAC and reported a polyamine-based adsorbent composed of branched polyethyleneimine (PEI), fumed silica (FS), and carbon black (CB) particles [6]. This adsorbent can adsorb CO₂ and desorb only 35 % of the adsorbed CO₂ with light exposure of 2 kW m^-2, which is twice the intensity of solar power. Therefore, it is essential to develop more efficient photothermal adsorbents that can desorb more CO₂.

In this work, we developed an amine-modified aerogel with carbon black to enhance the efficiency of CO₂ desorption. The aerogel, obtained via the supercritical drying method, exhibits high surface area, porosity, excellent thermal insulation properties, and low light scattering [7], which can improve the efficiency of CO₂ desorption. In this study, we fabricated the photothermal aerogel and evaluated its adsorption and desorption performances.

METHODS:

The photothermal aerogel was fabricated through the following procedure. Firstly, a sol-gel reaction was carried out using a suspension containing tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES), 1-methyl-2-pyrrolidone (NMP), water, and CB particles to obtain a wet gel. A molar ratio of TEOS:APTES:water:NMP was set to 1:4:8:6.37. Additionally, the weight percentage of CB to NMP was 1.0 %. Then, an excessive amount of ethanol was added to replace the solvent in the gel. The wet gel was dried using supercritical CO₂ drying at 20 MPa and 40 ËšC for 3 h. Following the fabrication of the aerogel, CO₂ adsorption and desorption experiments were conducted. Prior to the adsorption test, pre-adsorbed gas was removed from the aerogel using N₂ gas at 110 ËšC for 1 h. The glass vessel containing the aerogel was exposed to air with an approximate CO₂ concentration of 400 ppm at 25 ËšC for 4 h during the adsorption process. For the desorption of CO₂, the LED light located below the adsorption glass vessel was turned on. The irradiated light intensity to the adsorbent by the LED light was 2 kW m^-2. The aerogel was exposed to air during desorption for 1.5 h. The CO₂ concentration was measured using a infrared CO₂ probe throughout the experiment to calculate the adsorption and desorption amounts. Furthermore, we investigated the morphologies of the photothermal aerogel using a field emission scanning electron microscope (FE-SEM) with a 1.0 kV accelerating voltage. The specific surface area and total pore volume of the aerogel were determined using a N₂ isothermal adsorption test. The specific surface area was calculated based on Brunauer–Emmett–Teller (BET) method.

RESULTS AND DESCUSSION:

Figure shows images of the wet gel and photothermal aerogel before and after supercritical CO₂ drying, respectively. The photothermal aerogel exhibits a self-standing property with minimal cracking. Figure also shows the SEM image of the photothermal aerogel, demonstrating a porous structure. The BET surface area and total pore volume were determined to be 287.5 m²/g and 0.78 cm³/g, respectively, through N₂ adsorption isotherm. These results indicate the successful preparation of the photothermal aerogel with high surface area and porosity. Supercritical CO₂ drying effectively removed solvents in the wet gel without causing aggregation due to surface tension, resulting in a porous structure of the photothermal aerogel with minimal cracks [8].

Then, the adsorption amount and desorption efficiency of the photothermal aerogel were compared with our previous PEI/FS/CB adsorbent [5]. Here, the desorption efficiency is defined as the total CO₂ desorption amount divided by the total CO₂ adsorption amount. According to the result of adsorption experiment, the photothermal aerogel successfully adsorbed 42.5 g-CO₂/g-adsorbent, which is competitive with the PEI/FS/CB adsorbent that can adsorb 47.3 g-CO₂/g-adsorbent. However, the desorption efficiency under light irradiation with an intensity of 2 kW m^-2 is significantly different between the photothermal aerogel and the PEI/FS/CB adsorbent. The desorption efficiencies of the photothermal aerogel and the PEI/FS/CB adsorbent were 56.3 % and 35.4 %, respectively. This high desorption efficiency of the photothermal aerogel is considered to result from its properties of high thermal insulation and low light scattering. The porous structure of the photothermal aerogel seems to result in lower heat conductivity [9], which can reduce heat loss from the aerogel and allows for efficient CO₂ desorption. Furthermore, the low light scattering in the aerogel enables efficient light absorption by the CB particles, as the irradiated light can enter the aerogel with minimal reflection or scattering. Therefore, the properties of high thermal insulation and low light scattering likely contribute to the higher desorption efficiency of the photothermal aerogel compared to the PEI/FS/CB adsorbent.

In summary, our study demonstrates the successful fabrication of a photothermal aerogel incorporating CB particles with high porosity and photothermal desorption efficiency using the supercritical CO₂ drying method. These results suggest that the use of aerogels is a suitable approach for obtaining adsorbents with high photothermal efficiency due to their properties of high thermal insulation and low light scattering. In the future, the development of a photothermal aerogel that can be regenerated by solar illumination could have the potential to significantly reduce the energy consumption in the DAC process.

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