Power-to-X with biological step

Power-to-X with a biological step uses renewable electricity to drive living microbes that reduce CO2 into fuels, chemicals and protein. The flagship mode, microbial electrosynthesis (MES), runs at a cathode immersed in an anaerobic broth of electroactive acetogens such as Sporomusa ovata or Clostridium ljungdahlii: electrons travel from the electrode into the cell — directly through outer-membrane cytochromes and conductive nanowires, or indirectly via electrolytically generated hydrogen — and the microbe uses them, together with CO2, to build acetyl-CoA through the Wood-Ljungdahl pathway. From that central intermediate it releases acetate, ethanol or butanol, or grows as single-cell protein. A closely related mode, power-to-methane, uses hydrogenotrophic archaea such as Methanothermobacter to convert CO2 and green hydrogen into grid-quality biomethane. Unlike abiotic Power-to-Gas (water electrolysis plus a high-temperature Sabatier reactor), the biological route runs at ambient pressure and around 30-37C, which is its main engineering appeal and its main scale-up hurdle.

The key directions of power-to-X with a biological step are:

1. Microbial electrosynthesis to acetate and chemicals (MES to Acetate & Chemicals): electroactive acetogens at a cathode fix CO2 into C2-C6 building blocks (acetate, ethanol, butanol) for the green-chemistry and polymer industries.
2. Power-to-methane / biological methanation (Power-to-Methane): hydrogenotrophic archaea convert CO2 and green hydrogen into pipeline-quality biomethane for gas-grid injection.
3. CO2-to-protein (CO2-to-Protein): hydrogen-oxidizing microbes turn electricity-derived hydrogen and CO2 into single-cell protein for feed and food.
4. Electrode and biofilm engineering (Electrode & Biofilm Engineering): high-surface-area 3D porous cathodes and conductive biofilms raise the electron-transfer rate that ultimately sets reactor productivity.

Sectoral value chain#

[renewable power + CO2] ──> [MES bioelectrochemical reactor] ──> [cathode electron transfer to microbes]
                                                                                    │
                                                                      (Wood-Ljungdahl CO2 fixation)
                                                                                    │
            [target product: acetate / methane / protein] <─── [membrane separation] <─── [metabolite-rich broth]

Value chain levels#

Cross-cutting technologies of the sector:

• Cathode-to-microbe electron transfer (Electron Transfer Interface): direct transfer via cytochromes and conductive pili, plus H2-mediated transfer via hydrogenases, together set the coulombic efficiency of the cell.
• Wood-Ljungdahl pathway (Wood-Ljungdahl Pathway): the most energy-efficient biological CO2-fixation route, converting two CO2 molecules into one acetyl-CoA that branches into acetate, alcohols and biomass.
• Pressurized gas dissolution (Hollow-Fiber Membrane Biofilm Reactors): hollow-fiber membranes deliver CO2 and hydrogen bubble-free straight into the biofilm, lifting gas availability into the cells.

The stack sits at the intersection of electrocatalysis, strict-anaerobe physiology and high-pressure gas mass transfer.

1. Cathode-to-microbe electron transfer (Electron Transfer Interface):
   • direct transfer: bacteria dock on the cathode through c-type cytochromes and conductive nanowires; 3D porous carbon-felt cathodes, often loaded with nickel or magnetite nanoparticles, enlarge the contact area.
   • H2-mediated transfer: the cathode electrolyzes water to hydrogen at mild overpotential (molybdenum-disulfide or low-platinum coatings), and bacterial hydrogenases split the hydrogen to feed reduction.
2. Wood-Ljungdahl pathway biochemistry (Wood-Ljungdahl Pathway):
   • the most energy-efficient CO2-fixation route: a methyl branch reduces one CO2 to a methyl group (via formate and tetrahydrofolate) while a carbonyl branch reduces a second CO2 to CO (via CODH); acetyl-CoA synthase condenses them into acetyl-CoA, which branches to acetate, alcohols and biomass.
3. Pressurized gas dissolution (Hollow-Fiber Membrane Biofilm Reactors):
   • hydrogen and CO2 are poorly soluble in water; hollow-fiber membrane biofilm reactors (HfMBR) deliver the gases bubble-free at mild pressure straight into the biofilm growing on the fiber outer wall, lifting gas availability into the cells.