Engineering Electro- and Photocatalytic Carbon Materials for CO ₂ Reduction by Formate Dehydrogenase

Abstract

Semiartificial approaches to renewable fuel synthesis exploit the integration of enzymes with synthetic materials for kinetically efficient fuel production. Here, a CO ₂ reductase, formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough, is interfaced with carbon nanotubes (CNTs) and amorphous carbon dots ( a-CDs). Each carbon substrate, tailored for electro- and photocatalysis, is functionalized with positive (−NHMe ₂ ⁺) and negative (−COO ^–) chemical surface groups to understand and optimize the electrostatic effect of protein association and orientation on CO ₂ reduction. Immobilization of FDH on positively charged CNT electrodes results in efficient and reversible electrochemical CO ₂ reduction via direct electron transfer with >90% Faradaic efficiency and −250 μA cm ^–2 at −0.6 V vs SHE (pH 6.7 and 25 °C) for formate production. In contrast, negatively charged CNTs only result in marginal currents with immobilized FDH. Quartz crystal microbalance analysis and attenuated total reflection infrared spectroscopy confirm the high binding affinity of active FDH to CNTs. FDH has subsequently been coupled to a-CDs, where the benefits of the positive charge (−NHMe ₂ ⁺-terminated a-CDs) were translated to a functional CD-FDH hybrid photocatalyst. High rates of photocatalytic CO ₂ reduction (turnover frequency: 3.5 × 10 ³ h ^–1; AM 1.5G) with dl-dithiothreitol as the sacrificial electron donor were obtained after 6 h, providing benchmark rates for homogeneous photocatalytic CO ₂ reduction with metal-free light absorbers. This work provides a rational basis to understand interfacial surface/enzyme interactions at electrodes and photosensitizers to guide improvements with catalytic biohybrid materials.