Photosynthetic semiconductor biohybrids integrate the best attributes of biological whole-cell catalysts and semiconducting nanomaterials. Enzymatic machinery enveloped in its native cellular environment offers exquisite product selectivity and low substrate activation barriers while semiconducting nanomaterials harvest light energy stably and efficiently. In this Review Article, we illustrate the evolution and advances of photosynthetic semiconductor biohybrids focusing on the conversion of CO2 to value-added chemicals. We begin by considering the potential of this nascent field to meet global energy challenges while comparing it to alternate approaches. This is followed by a discussion of the advantageous coupling of electrotrophic organisms with light-active electrodes for solar-to-chemical conversion. We detail the dynamic investigation of photosensitized microorganisms creating direct light harvesting within unicellular organisms while describing complementary developments in the understanding of charge transfer mechanisms and cytoprotection. Lastly, we focus on trends and improvements needed in photosynthetic semiconductor biohybrids in order to address future challenges and enhance their widespread adoption for the production of solar chemicals.

Catalytic CO₂ conversion to renewable fuel is of utmost importance to establish a carbon-neutral society. Bioelectrochemical CO₂ reduction, in which a solid cathode interfaces with CO₂-reducing bacteria, represents a promising approach for renewable and sustainable fuel production. The rational design of biocatalysts in the biohybrid system is imperative to effectively reduce CO₂ into valuable chemicals. Here, we introduce methanol adapted Sporomusa ovata (S. ovata) to enhance the slow metabolic activity of wild-type microorganisms to our semiconductive silicon nanowires (Si NWs) array for efficient CO₂ reduction. The adapted whole-cell catalysts enable an enhancement of CO₂ fixation with a superior faradaic efficiency on the poised Si NWs cathode. The synergy of the high-surface-area cathode and the adapted strain achieves a CO₂-reducing current density of 0.88 ± 0.11 mA/cm², which is 2.4-fold higher than the wild-type strain. This new generation of biohybrids using adapted S. ovata also decreases the charge transfer resistance at the cathodic interface and facilitates the faster charge transfer from the solid electrode to bacteria.

Bioelectrochemical CO2-reducing rate at high potential was limited by poor bacteria-nanowire interface resulting from an inhospitable alkaline local environment. Tuning the bulk electrolyte pH and increasing its buffering capacity mitigated this issue and supported the formation of a close-packed nanowire-bacteria cathode. The resulting close-packed biohybrid operated with a CO2-reducing current density of 0.65 ± 0.11 mA cm−2 at ∼−1.2 V versus standard hydrogen electrode and enabled solar-powered CO2 fixation with solar-to-acetate efficiency of ∼3.6% over 1 week.

Solar-driven bioelectrosynthesis represents a promising approach for converting abundant resources into value-added chemicals with renewable energy. Microorganisms powered by electrochemical reducing equivalents assimilate CO₂, H₂O, and N₂ building blocks. However, products from autotrophic whole-cell biocatalysts are limited. Furthermore, biocatalysts tasked with N₂ reduction are constrained by simultaneous energy-intensive autotrophy. To overcome these challenges, we designed a biohybrid coculture for tandem and tunable CO₂ and N₂ fixation to value-added products, allowing the different species to distribute bioconversion steps and reduce the individual metabolic burden. This consortium involves acetogen Sporomusa ovata, which reduces CO₂ to acetate, and diazotrophic Rhodopseudomonas palustris, which uses the acetate both to fuel N₂ fixation and for the generation of a biopolyester. We demonstrate that the coculture platform provides a robust ecosystem for continuous CO₂ and N₂ fixation, and its outputs are directed by substrate gas composition. Moreover, we show the ability to support the coculture on a high-surface area silicon nanowire cathodic platform. The biohybrid coculture achieved peak faradaic efficiencies of 100, 19.1, and 6.3% for acetate, nitrogen in biomass, and ammonia, respectively, while maintaining product tunability. Finally, we established full solar to chemical conversion driven by a photovoltaic device, resulting in solar to chemical efficiencies of 1.78, 0.51, and 0.08% for acetate, nitrogenous biomass, and ammonia, correspondingly. Ultimately, our work demonstrates the ability to employ and electrochemically manipulate bacterial communities on demand to expand the suite of CO₂ and N₂ bioelectrosynthesis products.