Counterion-Mediated Humidity Modulation of Electronic Transport in Pristine MWCNT/TOCNF Composites

Humidity-driven electrical responses in carbon-cellulose composites are commonly attributed to matrix swelling. However, in TEMPO-oxidized nanocellulose (TOCNF), water uptake also releases Na⁺ counterions from carboxylate residues, enabling interfacial polarization via image-charge interactions that can modulate junction-limited electronic transport. Here, we formulate aqueous inks in which TOCNF disperses pristine (nonfunctionalized) multiwalled carbon nanotubes (MWCNT) without covalent modification, yielding homogeneous, spray-processable composite films. Using DC electrical measurements under controlled relative humidity, impedance spectroscopy, scanning probe microscopy, and a mean-field electrostatic model for counterion-mediated polarization, we show that percolated TOCNF/MWCNT films exhibit reproducible resistance gains, reaching up to ∼270%, that are inconsistent with a swelling mechanism alone. To support this view, a control based on nonoxidized nanocellulose (with a negligible amount of dissociable counterions) was designed, which shows only modest changes at high humidity, thereby sustaining an important, counterion-mediated interfacial route. The impedance data indicate predominantly electronic transport and a humidity-dependent low-frequency reactive response near the percolation threshold. The DC measurements establish a reproducible steady-state behavior and within-session cycling, while intrinsic response times and long-term stability remain to be more explored. The model rationalizes how increased counterion concentration under humidification can induce polarizing charges at the nanotube surface, driving electrostatic repulsion and relative reorientation that weaken conductive CNT-CNT junctions across the network. Invoking the Derjaguin approximation, an effective, counterion-induced repulsive interaction between CNT surfaces is computed, which prevents the building-up of large connecting junctions across the network, thus reducing their ability to assemble percolating pathways. These findings establish an overlooked counterion-mediated design principle to tune mixed ionic-electronic transport in sustainable CNT/nanocellulose electronic composites operating under humid or electrolyte-adjacent conditions.