The selective and high-rate microbial electrosynthesis of a valuable molecule is favoured from an economic perspective. We demonstrate here that increasing selectivity towards n-butyrate and n-caproate over acetate, while maintaining high production rate and electron recovery, is achievable by adjusting the CO2 feeding strategy and hydraulic retention time. We show that high CO2 loading rate (173 L d−1) and long hydraulic retention time (14 days) triggers bioelectrochemical chain elongation to n-butyrate and n-caproate
(53.6 ± 4.1%C), whereas lower CO2 loading rate (8.6 L d−1) does not, even with increasing HRT (96.4 ± 2.4%C into acetate). Moreover, temporarily detaching and re-attaching the biofilm allowed to obtain high-rate n-caproate production of 2.0 ± 0.1 g L−1 d−1 (max 3.1 g L−1), while producing 3.3 ± 0.2 g L−1 d−1n-butyrate (max 9.3 g L−1), at an increased 63.6 ± 5.0%C into both. This was achieved with 69.8 ± 2.8% total electron recovery at −100.8 Am−2.@en