Microbes can reduce CO2 into multicarbon chemicals with electrons acquired from the cathode of a bioelectrochemical reactor. This bioprocess is termed microbial electrosynthesis (MES). One of the main challenges for the development of highly productive MES reactors is achieving efficient electron transfer from the cathode to microbes. Here, carbon cloth cathodes modified with reduced graphene oxide functionalized with tetraethylene pentamine (rGO-TEPA) were readily self-assembled in the cathodic chamber of a MES reactor. Electroactive biofilms with unique spatial arrangement were subsequently formed with Sporomusa ovata at the surface of rGO-TEPA-modified electrodes resulting in a more performant MES process. The acetate production rate from CO2 was increased 3.6 fold with the formation of dense biofilms when wild type S. ovata was combined with rGO-TEPA. An improvement of 11.8 fold was observed with a highly structured biofilm including multiple spherical structures possibly consisting of bioinorganic networks of rGO-TEPA and bacterial cells from a novel strain of S. ovata adapted to reduce CO2 faster. The three dimensional biofilms observed in this study enabled highly effective electric interactions between S. ovata and the cathode, demonstrating that the development of dense cathode biofilms is an effective approach to improve MES productivity.