Display omitted •Electro-stimulation of microbial chain elongation for ethanol and CO2 conversion.•Caproate was produced at 2.41 ± 0.69 g L−1 d−1, and reached to 7.66 ± 1.38 g L−1.•Maximum SelectivityS was 91.47 ± 0.58% and SelectivityS+E was 80.28 ± 0.52%.•Carbon of 23.43 ± 0.69% in final products was from CO2.•Microbial electrochemical caproate was produced without base addition. The production of value added chemicals from CO2 is of critical importance for the practical application of microbial electrosynthesis (MES). Here, a binary electron donor (ED) design (using electrode and ethanol) was introduced to provide an efficient caproate production with the bioconversion of both CO2 and ethanol. A maximum caproate production rate of 2.41 ± 0.69 g L−1 d−1, and a final concentration of 7.66 ± 1.38 g L−1 was achieved. Caproate production selectivity based on the substrate increased to 91.47 ± 0.58% (Binary EDs) from 32.22 ± 32.58% (open circuit Electrode ED). An observed amount of 23.43 ± 0.69% of carbon within the final binary ED products originated from the CO2. This work proves for the first time the potential of caproate production from CO2 utilization and ethanol upgrading using solid electrodes to regulate the chain elongation process.