Tuning Kolbe electrolysis selectivity by chain length of fatty acids: Mechanistic insights and a synergistic mixed-feed strategy in an aqueous electrolyte

Electrochemical decarboxylation (Kolbe electrolysis) is an important process for converting fatty acids into hydrocarbon biofuels under mild conditions. However, the low selectivity generally observed for short-chain fatty acids, such as valeric acid (C5), severely limits their practical application efficiency. This study systematically investigates the decisive role of fatty acid chain length (C5, C7, C8 and C9) in governing decarboxylation efficiency and pathway selectivity in alkaline aqueous media. A striking chain length dependence was uncovered that while medium-chain acids (C8 and C9) exhibit high decarboxylation efficiency, the performance of the short-chain acid C5 is paradoxically suppressed at elevated potentials. This challenges the conventional belief that high potential universally favors the Kolbe reaction. Through a combination of LSV, contact angle and EIS analyses, this suppression was found to be due to the inability of C5 to form a protective hydrocarbon layer. In contrast, the medium-chain acids readily generate a protective layer to effectively suppressing OH − oxidation side reactions. Capitalizing on this mechanism, a co-electrolysis strategy was devised. By using C8 as an in situ-formed interfacial modifier, the hydrocarbon selectivity of C5 was dramatically boosted from 16.6% to 59.0% at an optimal C8-to-C5 ratio of 4:1, while the mixed system achieved a hydrocarbon Faradaic efficiency of 63%. This work provides fundamental insights into chain-length-mediated electrochemistry and presents a straightforward, cost-effective, and efficient approach to the value-added conversion of complex fatty acid feedstocks.