(648f) High-Performance Postcombustion Carbon Capture Using Porous Single-Layer Graphene Membranes

Post-combustion carbon capture is a key process to abate CO₂ emissions and achieve net-zero transition. Membrane-based capture processes have emerged as an attractive alternative, thanks to their high modularity, continuous steady-state operation, and high energy efficiency. Membranes can reduce the power consumption required by state-of-art capture processes based on absorption via monoethanolamine from 2-3 MJ/kg to 1 MJ/kg (for coal power plant post-combustion capture).[2] Consequently, state-of-the-art capture costs between 50 and 110 $/ton can be more than halved if high-performance membranes and optimized operating conditions are considered.[1][2] Nonetheless, this requires development of high-performance membranes yielding large CO₂ permeance (defined by CO₂ flow through the membrane normalized by membrane area and transmembrane pressure) and CO₂/N₂ separation selectivity.

Our group has developed scalable methodologies to fabricate membranes hosting CO₂-selective layer that is just one atom thick. This approach is motivated by the fact that CO₂ permeance is inversely proportional to the thickness of the selective layer. As a result, atom-thick materials such as single-layer graphene constitutes the ultimate limit of this concept. This concept is extremely relevant to the postcombustion carbon capture as technoeconomic analyses have demonstrated that membranes with a large CO₂ permeance and a moderate CO₂/N₂ selectivity minimize the needed membrane area (a major part of the capital cost) as well as energy (operating cost, e.g., by utilizing vacuum pumps) needed for the capture.[1][2][3]

Conventionally, the membranes for postcombustion capture are dominated by polymeric materials attributing to the fact that polymers can be rapidly processed into a thin film morphology. However, the capture performance especially the CO₂ permeance from the polymeric membranes cannot be improved beyond a certain limit which is imposed by the intrinsic properties of the polymer (free volume between the polymeric chains, chain stiffness, binding properties) which determines gas sorption and diffusion. As a result, the state-of-the-art carbon capture membranes based on polymeric membranes tend to reduce the thickness of selective layer to 20-100 nm by adopting thin film composite morphology to maximize permeance. Yet, the CO₂ permeance from the state-of-the-art membranes is limited to 1000-2000 gas permeation units or GPU (1 GPU = 3.35 × 10^-10 mol m^-2 s^-1 Pa^-1).

High-performance membrane based on porous graphene hosting an inorganic porous lattice has an intrinsic advantage over conventional (polymeric) membranes because the porous inorganic structure of graphene yields a large flux and is chemically and thermally robust against degradation and aging (change in the size of the pore). This offers an attractive regime for membranes. However, it is still challenging to synthesize such membranes in a scalable and cost-effective manner to justify the added complexity of synthesis. In recent year, there has been extraordinary development in the field of single-layer graphene. It is now commercially available at extremely high quality (by chemical vapor deposition) with production capacity reaching 100,000 m² per year. Yet, the challenge in preparing graphene membrane lies in incorporating CO₂-selective pores in graphene, and preparing membranes in scalable manner.

Our research group has addressed both the fundamental and engineering challenges mentioned above. We approached the problem in incorporating nanopores in graphene for CO₂/N₂ separation by developing (i) scalable pore incorporation in graphene,[4–7] (ii) nitrogen functionalization of the pore to improve CO₂ uptake,[8,9] and (iii) bottom-up synthesis routes to simply manufacturing of porous graphene [10,11]. This was possible by understanding fundamental insights in oxidation chemistry of graphene, which led to simplification in which pores are incorporated in graphene, involving simply flowing ozone over graphene at room temperature. Nitrogen functionalization of graphene pore led to extremely competitive CO₂ uptake leading to record-high capture performance with unprecedent values of CO₂ permeance and CO₂/N₂selectivity, equal to 13500 GPU and 48.8, respectively at CO₂ concentration of 20%. This also leads to very attractive performance under dilute CO₂ conditions with CO₂/N₂ selectivity surpassing 1000. A techno-economic analysis for carbon capture from these sources using a double-stage membrane process with CO₂ recovery and purity of 90 and 95%, respectively, predicts a capture cost of 21 $/ton_CO2 for concentrated feed (20% CO₂), and 68 $/ton_CO2 for dilute feed (1% CO₂). The attractive capture penalty for extremely dilute feed can be used to improve the CO₂ recovery (e.g., to 99%) which if otherwise emitted to the atmosphere increases the burden on the negative emission technologies.

Finally, motivated by the high potential of these membranes in postcombustion carbon capture, we have successfully developed simple and scalable pathways to produce large-area (250 cm²) membrane coupons. A pilot-plant demonstration project sponsored by industry and Swiss Federal Office of Energy is underway.

References

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