Activating Nonenzymatic Hemoglobin for Highly Selective CO₂-to-Formate Photoreduction in Water through Supramolecular Phenolic Mesocrystal Encapsulation.

Abstract

Photocatalytic CO₂ reduction offers a promising solar-to-chemical route, but most systems struggle to meet economic and scalability benchmarks. Current approaches fall short of the U.S. Department of Energy's Carbon Negative Shot target of < $100/t CO₂ removed. Here, we develop an all-biomolecular nanophotosystem (<i>Hb</i>-EA) without the involvement of synthetic metal catalysts, pairing a nonenzymatic protein with a plant-derived photosensitizer. Hemoglobin (<i>Hb</i>), a ubiquitous oxygen-carrier protein with reversible CO₂-binding and redox-active heme sites that serve as biologically embedded metal centers, is supramolecularly encapsulated by crystalline ellagic acid (EA), forming an <i>Hb</i>-EA nanohybrid where π-π stacking and hydrogen bonding create an organic-semiconductor-like shell with a 2.44 eV bandgap and broad visible absorption. Upon illumination, the EA shell efficiently harvests light and funnels electrons into <i>Hb</i> heme centers, selectively capturing and reducing CO₂ to formic acid. <i>Hb</i>-EA achieves a benchmark-leading CO₂-to-formate production rate of 397.61 μmol h^-1 USD^-1 with ∼93% selectivity, significantly surpassing prior systems in cost-normalized performance. This biobased photocatalyst uses no noble metals or external cocatalysts and is assembled from inexpensive biobuilding blocks. Scalability is demonstrated by a 100 × 60 cm <i>Hb</i>-EA membrane photoreactor under outdoor sunlight, where it maintained high activity and stability, establishing a paradigm for artificial photosynthesis linking structural biochemistry with sustainable photonic materials for green, economically viable CO₂ reduction.