Hybrid photosynthesis aims to combine the flexibility of biological metabolism with the efficiency of photovoltaics to store sunlight as hydrocarbons with efficiencies much greater than photosynthesis. We have constructed theoretical models of four hybrid photosynthetic schemes that combine either all biological \(CO_2\) fixation or abiotic initial \(CO_2\) fixation with electron uptake through hydrogen-oxidation or direct cathodic uptake. These models incorporate internal cellular parameters including a reverse electron transfer chain believed to exist in electroactive microbes, external thermodynamic, and external kinetic parameters. We use these models to evaluate the maximum conversion efficiencies of sunlight to electricity through a silicon photovoltaic and subsequent storage as the biofuel butanol by these schemes.
The maximum efficiency of these schemes is for any realistic set of parameters >20%. This is 2.5x the maximum theoretical efficiency of natural algal photosynthesis, and 4.3x that of \(C_3\) photosynthetic conversion to biomass.
For most sets of parameters, the \(H_2\)-mediated schemes have a higher thermionic efficiencies than the direct cathodic uptake schemes. The \(H_2\)mediated scheme efficiency reaches up to 27% under internal parameters, with no external electrochemical losses. The efficiency of direct cathodic uptake schemes can be modulated significantly by changes to the internal machinery of the cell. Under the range of internal cellular parameters investigated here, the efficiency of this system (with no other electrochemical losses) can be changed from as low as 16% to as high as ≈ 26%.