Assessment of Artificial Photosynthetic Systems for Integrated Carbon Capture and Conversion

Abstract

Sustainable and continuous operation of an artificial photosynthetic (AP) system requires a constant supply of CO2 captured from the dilute sources such as the flue gas and the air to make fuels and chemicals. Although the architecture of AP systems resembles that of the natural leaves, they lack an important component like stomata to capture CO2 directly from the dilute sources. Here we design and evaluate the solar-to-fuel (STF) efficiency of the integrated AP system that captures CO2 directly from the air/flue gas and converts it to fuels using sunlight. The thermodynamic limit to the STF efficiency of such integrated AP system range from 34% to 40% for various products such as CO, HCOOH, CH4, CH3OH, C2H4, and C2H5OH using ideal multijunction light absorbers and reversible carbon capture. The performance limits of real, integrated AP systems are obtained here for two different integration schemes such as integrated cascade systems and fully integrated systems that use technology-ready materials and components. The fully integrated AP systems can be >66% more efficient than the integrated cascade systems as they do not need additional energy for compression, separation, and recycling of CO2. While the integrated cascade systems show highest STF efficiency with the adsorption-based carbon capture process, the fully integrated AP systems are only compatible with the membrane-based carbon capture process. We also show that the synthesis of higher-electron products such as CH4, CH3OH, C2H4, and C2H5OH can be more favorable for the robust operation of an integrated AP system. A design of the fully integrated AP system is proposed that uses moisture-gradient across the anion-exchange membrane to capture CO2 from the air, which is then converted directly to fuels using water, and sunlight. Such a fully integrated AP system can produce ∼0.4 ton/day of CO at a cost of ∼185/ton and STF efficiency of ∼14% while reducing the CO2 level of the surrounding air by 10% at steady-state operation. The fully integrated AP systems are modular, scalable, and ∼14 times more efficient than natural leaves.