Schematic overview of the experimental procedure, along with the electrolysis profile, and formate concentration during the IEMC experiments (50 mL, media I1 or I2 at pH 7.5 for cultures of <i>C</i>. <i>necator</i>, or at pH 6.8 for cultures of <i>M</i>. <i>extorquens</i>).

<p><b>Left figure:</b> Electrolysis profile prior to adding the microorganisms with a potential of −1.2 V vs. RHE (red bar, formate establishing), in CO₂ saturated medium with a constant CO₂ gas flow of 10 mL min⁻¹ (blue bar, CO₂ saturation) under stirring (140 rpm). At 0 h the microorganisms were added (green bar, adding microorganisms and resuming electrolysis) and the electrolysis was continued for another 2 h, where the formate concentration increased (red line, 2 h). After electrolysis (2 h), the microorganisms were grown under ambient air conditions (grey bar, stopping electrolysis) for 6 h (total of 8 h run). The growth, formate uptake (green line, 8 h) and PHB production were measured. <b>Right figure:</b> Establishment of formate concentrations in media I1 or I2 (blue bar up to 0 h) and the measured formate concentration after resuming electrolysis (red bar, from 0 to 2 h).</p

Metabolic pathways of <i>C</i>. <i>necator</i> compared to <i>M</i>. <i>extorquens</i> for PHB production from formate.

<p><i>C</i>. <i>necator</i> (left) reduces formate into CO₂, then captures it through the CBB cycle to produce PHB (M-FDH: membrane formate dehydrogenase, S-FDH: soluble formate dehydrogenase, CBB: Calvin Benson Cycle, 3GP: 3 glyceraldehyde phosphate). <i>M</i>. <i>extorquens</i> (right) however, condenses formate with tetrahydropterin (H₄F) and gives formyl-tetrahydrofolate that is further converted to methylenetetrahydrofolate (H₂ = H₄F). The later condenses with glycine in the serine cycle. The glycine is produced from glyoxylate that is regenerated from in the ethylmalonyl-CoA cycle (EMC) from acetyl-CoA (Ac-CoA) assimilation. From the EMC cycles, PHB branches out, using 3HB-CoA (3 hydroxybutyrate-CoA). The EMC cycle also directs the Ac-CoA for methylmalys-CoA production that is required by the TCA cycle.</p

How to go beyond C₁ products with electrochemical reduction of CO₂

The electrochemical reduction of CO2to produce fuels and value-added organic chemicals is of great potential, providing a mechanism to convert and store renewable energy within a carbon-neutral energy circle. Currently the majority of studies report C1products such as carbon monoxide and formate as the major CO2reduction products. A particularly challenging goal within CO2electrochemical reduction is the pursuit of multi-carbon (C2+) products which have been proposed to enable a more economically viable value chain. This review summaries recent development across electro-, photoelectro- and bioelectro-catalyst developments. It also explores the role of device design and operating conditions in enabling C-C bond generation