Disorder Beats Order: Carbon Microstructure Steers Radical Versus Cationic Pathways in Kolbe Electrolysis

Carbon electrodes are ubiquitous in organic electrochemistry, yet the structural origin of their selectivity in radical-mediated reactions remains poorly understood. Here, we show that carbon microstructure is a primary determinant of product selectivity in Kolbe electrolysis, a prototypical electrochemical C─C coupling reaction central to fuel, fine chemical, and pharmaceutical production. We investigated eight carbon electrodes in two complementary libraries: five commercial carbons spanning diverse macroarchitectures, and carbon nanofibers with controlled graphitization. Both reveal a consistent structure-selectivity trend for various carboxylic acids and a mixed-acid feed. Counterintuitively, the most disordered electrodes, despite the highest charge-transfer resistance, give about an order of magnitude more Kolbe dimer than highly graphitized ones, which give little or none. Disordered, sp³-rich surfaces bearing oxygen functionalities strongly favor both Kolbe coupling (alkane) and Hofer-Moest products (alcohol/aldehyde), whereas graphitized, sp²-rich surfaces favor further oxidation to carbocations, yielding mainly alkenes and esters. Density functional theory calculations suggest that oxygen-functionalized sites bind the alkyl radical about three times more strongly than pristine graphene, consistent with longer-lived surface-bound radicals that can couple. These findings help rationalize the widely used but poorly understood performance of reticulated vitreous carbon in organic electrosynthesis and offer guiding principles for rational carbon electrode design.