The Concentration Determines Reaction Efficiency of Hetero‐Coupling of Bio‐Based Medium Chain Carboxylic Acids by Kolbe Electrolysis

Mixtures of seven n‐ carboxylic acids ( n‐ CA), ranging from acetic acid to n‐ octanoic acid, that can be gained from the microbial conversion of waste biomass, were converted into liquid drop‐in fuel via Kolbe electrolysis. We demonstrate a strong, sigmoidal correlation between the total n ‐CA concentration and the Coulombic efficiency for fuel‐like liquid products (CE fuel ). Crucially, only above an apparent threshold concentration of 0.1 mol L −1 n‐ CA, Kolbe electrolysis takes place, whereas at lower concentrations the oxygen evolution reaction (OER) dominates. Optimal performance was achieved at the highest concentration tested, 1.5 mol L −1 , yielding a maximum CE fuel of 63.8 ± 1.4%, whereas for n‐ CA to 0.1 mol L −1 and 0.5 mol L −1 , the chain shortening reaction (CSR) strongly affects CE fuel . Furthermore, the study provides valuable insights into reaction selectivity, demonstrating that longer‐chain n ‐CA react preferentially compared to shorter‐chain n ‐CA, a phenomenon that is attributed to their higher hydrophobicity and hence accumulation in the hydrophobic layer formed on the electrode. Using a one‐chamber cell system at optimized concentration significantly lowered energy demand to 2.92 ± 0.04 kWhL −1 of fuel produced, representing a 40% reduction compared to similar two‐chamber setups.