<p>Cellulose surface chemistry controls the Seebeck coefficient of SWCNT buckypaper composites independently of filler loading, enabling partial replacement of the conductive phase without loss of thermoelectric performance. Microfibrillated cellulose was covalently functionalized with citric acid (Cell-CA), 3-trimethoxysilylpropyl methacrylate (Cell-TMSPMA), and 4,4’-diphenylmethane diisocyanate (Cell-MDI), and physically modified with polyethylene glycol (Cell-PEG), then combined with single-wall carbon nanotubes (SWCNTs) at 0.1 to 90 wt% loading. Cell-MDI composites achieved power factors of 101 to 111 µW m^− 1 K^− 2 at 70 to 90 wt% SWCNT, matching pure SWCNT buckypaper (106 µW m^− 1 K^− 2) despite 10 to 30 wt% of the conductive phase being replaced by cellulose. The Cell-MDI Seebeck coefficient remained at 46 to 52 µV K^− 1 from 10 to 90 wt% SWCNT regardless of loading, while neat cellulose composites ranged from 23 to 45 µV K^− 1 with no trend. Solvent-exchange controls confirmed that this stability originates from the covalent urethane functionalization; vacuum drying further increased the Seebeck coefficient to 70.4 µV K^− 1 at 1 wt% SWCNT. Cell-CA reached the highest conductivity (62 kS m^− 1) with lower Seebeck coefficients (19 to 30 µV K^− 1) consistent with additional p-doping; Cell-TMSPMA showed no electronic effect, consistent with phase separation; Cell-PEG achieved N-type conversion (− 27 µV K^− 1 at 60 wt%) with decreased conductivity. Composites above the percolation threshold provided electromagnetic shielding effectiveness up to 30 dB across 8 to 37&#xa0;GHz. A proof-of-concept 8-couple Cell-MDI/Cell-PEG module delivered an open-circuit voltage of 10 mV across a ~ 30&#xa0;K gradient, demonstrating thermoelectric harvesting in a thin-film format.</p>