Graphitic carbon nitride/activated carbon composite inks for screen-printed electrodes: Influence of composition on rheology, microstructure and electrochemical performance

Composition of conductive inks plays a decisive role in determining both printability and electrochemical performance of screen-printed electrochemical sensors. In this work, composite conductive inks based on graphitic carbon nitride (g-C₃N₄) and activated carbon (AC) were prepared with systematically varied filler ratios and evaluated by rheological measurements, scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. All inks exhibited shear-thinning behavior suitable for screen printing, while increasing the g-C₃N₄ content enhanced the low-shear viscosity and promoted rapid structural recovery after shear. Scanning electron microscopy revealed that nanosized g-C₃N₄ filled the interparticle voids between larger AC particles, producing a denser and more homogeneous composite microstructure. Electrochemical performance was evaluated using a [Fe(CN)₆]^3−/4− redox probe. Increasing the AC fraction reduced the peak-to-peak separation from 130 mV for pure g-C₃N₄ electrode to 100 mV for the 50:50 and 25:75 composites, indicating more facile interfacial charge-transfer kinetics, although the associated Faradaic peak currents decreased with the increasing AC content due to lower density of g-C₃N₄-associated redox-active sites. Electrochemical impedance spectroscopy showed a substantial decrease in charge-transfer resistance from 148.5 Ω cm² for pure g-C₃N₄ to 7.2 Ω・cm² for g-C₃N₄:AC (25:75) composite, outperforming even pure AC electrode (16.9 Ω・cm²). The results demonstrate synergistic interaction between both conductive components. Both materials, g-C₃N₄ and AC, provide electrochemically active surface sites that promote the Faradaic response, whereas AC establishes an efficient conductive network that facilitates charge transport. The optimized g-C₃N₄:AC (25:75) represents a promising conductive ink for the fabrication of low-cost, reliable, screen-printed electrodes that can be readily modified through further surface or bulk functionalization for diverse electrochemical sensing applications.