Freestanding FeS2-Integrated Reduced Graphene Oxide Aerogels Enhance Cathode-to-Cell Electron Transfer in Three-Dimensional Living Cathodes

Three-dimensional (3D) living cathodes can retain dense biofilms, but cells within porous scaffolds may remain electronically underutilized. Here, we integrate FeS2 nanoparticles throughout a freestanding reduced graphene oxide (rGO) aerogel, combining interconnected macropores, continuous electron pathways, and spatially distributed FeS2 interfaces within a living cathode. This architecture provides three complementary functions: 3D biofilm retention, low-resistance cathode-to-cell electron delivery, and greater availability of intracellular reducing equivalents for fumarate reduction by Shewanella oneidensis MR-1. Compared to pristine rGO, FeS2-rGO generated a ~ 12-fold higher cathodic current density, accelerated complete fumarate depletion within 50 h, and reached 67% Faradaic efficiency to succinate. FeS2-rGO also exhibited a 2.4~4.1-fold higher biomass-normalized succinate partial current, the lowest fitted interfacial resistance, and the highest NADH/NAD+ ratio. Less than 2% of the loaded Fe dissolved, as measured by ICP-OES, while the characteristic FeS2-associated diffraction features were retained after operation. Mechanistically, mutant strain responses further showed that FeS2 strengthened electron transfer through the native outer membrane cytochrome-CymA network. By positioning electronically connected FeS2 interfaces throughout a freestanding porous electrode, this work combines long-range scaffold conduction with enhanced local, pathway-dependent electron uptake in a 3D living cathode.