Effect of Doping Inorganic Acid Radical Ions on Electrochemical Properties of Polyaniline/Graphite Carbon Paper Electrodes

The inorganic proton acid-doped polyaniline (H-PANI-X) is synthesized directly on a graphite carbon paper electrode. The polyaniline doped with hydrochloric acid (yielding H-PANI-Cl), sulfuric acid (yielding H-PANI-HSO4), and nitric acid (yielding H-PANI-NO3) is employed to construct both finite molecule and periodic molecule computational models. Theoretical calculation and experimental measurement of a polyaniline/graphite carbon paper electrode are adopted to reveal the doping effect of inorganic acid radical ions (Cl−, HSO4−, NO3−) on electrical and electrochemical properties of H-PANI-X. H-PANI-X shows a lower electronic band gap structure, indicating more feasible electron transfer than PANI. H-PANI-X shows a lower HOMO-LUMO orbital energy gap, indicating lower excitation energy than PANI. H-PANI-X also shows a higher electronic density of states level, indicating higher electrical conductivity than PANI. The charge density difference of H-PANI-X reveals a more delocalized electrostatic potential distribution, indicating an enhanced electrostatic interaction between protonated PANI and charge-balancing anions. Furthermore, H-PANI-HSO4 and H-PANI-NO3 exhibit hydrogen bonding between the protonated PANI and charge-balancing anions, resulting in reduced electronic band gaps and enhanced electronic density of states compared with H-PANI-Cl. H-PANI-NO3 with higher electronic states at the Fermi level and higher anionic electronegativity exhibits higher electrical conductivity than H-PANI-Cl and H-PANI-HSO4. The experimental measurement is conducted to investigate the electrochemical properties of H-PANI-X. The electrochemical impedance spectroscopy measurement indicates H-PANI-NO3 maintains lower charge transfer resistance (0.357 Ω) than H-PANI-HSO4 (3.003 Ω) and H-PANI-Cl (10.571 Ω). The cyclic voltammetry measurement indicates that H-PANI-NO3 has much higher redox current and mean current density responses, accordingly exhibiting superior capacitance (208.0 mF cm−2) performance in comparison with H-PANI-Cl (129.5 mF cm−2) and H-PANI-HSO4 (157.9 mF cm−2). Theoretical calculation and experimental investigation confirm H-PANI-NO3 presents superior electroactivity to H-PANI-Cl and H-PANI-HSO4 for promoting its electrochemical capacitance performance.