Photobiological H2 Production: Theoretical Maximum Light Conversion Efficiency and Strategies to Achieve It

Photobiological H2 production depends on charge separation by reaction centers coupled to Chlorophyll a, Chlorophyll b and carotenoid light-absorbing antennae, with well-defined spectral and redox characteristics. The initial charge-separated state in the reaction centers is stabilized by electron transfer to carriers within two protein complexes, Photosystem I and Photosystem II that act in series. Two photons are required to transfer each electron from the PSII electron donor, water, to the final PSI electron acceptor, ferredoxin. Unlike most tandem photoelectrochemical designs, biological photosystems are coupled to water oxidizing and H₂- producing enzymes and these reactions occur within the same cell compartment, the chloroplast. The above-described physical parameters set the theoretical maximum solar-conversion efficiency of biological systems to 12-13%. However, due to a large number of structural and regulatory processes in vivo, the actual conversion efficiency of biological systems to H₂ is of the order of 1%. This paper addresses these limitations.