Polyaniline electrode reveals its hidden structural choreography through coulovoltammetry

Researchers at the University of Calicut in Kerala, India, have opened an unusually intimate window onto one of the most technically important conducting polymers in modern electrochemistry, polyaniline, by applying a deceptively simple graphical tool to a material long prized for its ability to switch between dramatically different chemical states. In a study published in Discover Electrochemistry, the team, led by Madari P. Sidheekha and corresponding author Yahya A. Ismail, demonstrates that a technique known as coulovoltammetry can dissect the redox behaviour of chemically synthesized polyaniline into distinct structural components, quantify the charge consumed by each of them, and even show that the polymer reaction itself functions as a built-in sensor of its electrical, chemical and thermal surroundings.

Polyaniline belongs to a family of materials called conducting polymers, which behave in many respects like reactive macromolecular gels when wet. When an electric current is passed through them, their polymer chains undergo oxidation or reduction, gaining or losing electrons. This triggers the exchange of counterions and solvent molecules with the surrounding electrolyte, causing the chains to swell, shrink, relax or compact. The researchers draw a striking parallel with living cells: in both cases, electrical stimuli drive chemical reactions in macromolecular chains that produce conformational movement, ionic exchange and solvent exchange, in other words, a reversible change in the chemical composition of the entire polymer-ion-solvent system. It is precisely this biomimetic quality that makes polyaniline attractive for applications ranging from sensors and artificial muscles to supercapacitors, smart windows and drug delivery systems.

The chemical mechanism underlying the switching is formally described as an anion-driven process. In the reduced state, polyaniline chains carry sigma bonds between consecutive monomeric units, allowing free rotation and numerous conformations. Upon oxidation, radical cations form along the chain, introducing new pi bonds that restrict rotation and effectively turn the chain into a molecular motor. To maintain charge neutrality, chloride anions from the 1 M hydrochloric acid electrolyte diffuse into the polymer, and water molecules follow for osmotic balance. During reduction, the reverse occurs: protons are incorporated, and chloride ions and water are expelled. The multistep redox chemistry connects three canonical forms of the polymer, leucoemeraldine, emeraldine and pernigraniline, with anodic peaks observed at 0.30 V and 0.53 V and corresponding reduction peaks at 0.44 V and 0.078 V in the team’s cyclic voltammograms.

The central innovation of the study lies in coulovoltammetry, a technique in which the cyclic voltammogram is integrated to plot the consumed charge directly against the applied potential. The resulting closed or open loops encode an enormous amount of information. A perfectly closed loop indicates that the anodic and cathodic charges are equal, meaning only reversible redox processes are occurring. Between 0 and -0.3 V on the cathodic side, and up to 0.7 V on the anodic side, the team found just such a window of pure reversibility for polyaniline in hydrochloric acid. Beyond these limits, new irreversible processes appear, identified as hydrogen evolution on the cathodic side and overoxidation or oxygen evolution on the anodic side, whose associated charges grow exponentially with increasingly extreme potential limits.

More remarkably still, the coulovoltammogram allows the authors to separate four distinct structural faradaic processes, in line with the Electrochemically Stimulated Conformational Relaxation model developed by Toribio Otero and colleagues. Starting from the fully oxidized state, the fast reduction-shrinking stage proceeds under diffusion control of counterions leaving the swelling film. Once the shrinking chains close the gel structure at a characteristic closing potential, further reduction cannot stop but slows dramatically, because the departing anions must physically push polymer chains apart to open their exit pathways. This slow reduction-compaction stage even persists into the beginning of the subsequent anodic sweep, a phenomenon the team calls reduction electrochemical inertia, which they confirmed using chronocoulograms showing continuous faradaic charge decay. Conversely, oxidation begins with a slow relaxation process that opens the compact structure and generates free volume for incoming anions, followed by fast oxidation-swelling, and finally an oxidation inertia that continues past the anodic potential limit into the cathodic sweep.

The analysis also yielded an unexpected energetic finding. By integrating the coulovoltammetric areas, the researchers calculated the electrical energy consumed during oxidation and reduction across different potential windows. The reduction energies turned out to be almost twice the oxidation energies, revealing a pronounced charge symmetry but energy asymmetry in the reaction. The authors attribute this asymmetry to the inherently different structural processes on the two sides of the cycle, including conformational relaxation, swelling, shrinking, compaction, ion trapping and electrochemical inertia, compounded by osmotic solvent exchange. Intriguingly, they suggest this mirrors the asymmetry of reaction-driven biological functions, noting that muscles perform work during contraction but not relaxation, and that ionic channels in living systems pass ions preferentially in one direction.

Perhaps the most consequential demonstration is that polyaniline’s electrochemical reaction is self-sensing, meaning it can report on its own working conditions without any additional sensor hardware. The team derived theoretical sensing equations from the reaction kinetics and Faraday’s laws, then verified them experimentally. The charge consumed by the reversible reaction showed a clean double-logarithmic linear dependence on the scan rate, with an excellent correlation coefficient of 0.993, and the same held true when frequency was used as the electrical variable. At slower scan rates, the reaction has more time to drive deep conformational changes, exchange large numbers of ions and solvent molecules, and consume correspondingly more charge and energy; at faster rates, the polymer only partially switches. The consumed electrical energy likewise varied linearly in semi-logarithmic plots against scan rate, with distinct sensitivity slopes of -6.77 J g-1 per logarithmic unit for reduction and -2.15 for oxidation.

The same logic extended to chemical and thermal conditions. When the electrolyte concentration was varied from 1 M down to 0.075 M hydrochloric acid, the consumed charge increased in a double-logarithmic fashion with concentration, because higher concentrations provide more chemical energy for deeper oxidation and greater counterion uptake. The consumed energy per gram of polymer also rose linearly with concentration, with sensitivity slopes of 2.948 J g-1 per logarithmic unit for reduction and 0.608 for oxidation. Temperature produced analogous behaviour: as the working temperature increased, thermal energy pushed the reaction toward deeper redox states, and the charge obeyed the semilogarithmic Arrhenius-derived sensing equation, with a temperature sensitivity slope of -601.28, while consumed energy rose linearly by 0.152 J g-1 per degree Celsius for reduction and 0.061 for oxidation.

Because the sensing parameters here are simply the consumed charge and the consumed electrical energy during the reaction, any electrochemical device built on polyaniline, whether a supercapacitor electrode, an actuator, an electrochromic window or a drug delivery platform, could in principle monitor its own instantaneous operating state without separate instrumentation. The authors emphasize that this differs fundamentally from conventional chemical sensing, which aims to identify or quantify specific analytes; the goal instead is a material whose reaction is intrinsically aware of the energetic and chemical ambient in which it operates, much as biological macromolecular motors simultaneously generate actuation and sense their surroundings through the same underlying reaction.

The study represents the first systematic coulovoltammetric analysis of chemically synthesized polyaniline, extending to this polymer a methodology previously applied mainly to polypyrrole, and tackling the added complexity of polyaniline’s multistep proton-coupled redox transitions. The researchers synthesized the material by chemical oxidative polymerization of aniline with ammonium persulphate in hydrochloric acid, characterized it by FTIR spectroscopy, dielectric spectroscopy, thermogravimetric analysis and electron microscopy, and coated it onto glassy carbon electrodes for electrochemical testing with a Zennium Pro workstation. Supported by fellowships from the University Grants Commission and the Kerala State Council for Science, Technology and Environment, the work contributes both to the fundamental structural electrochemistry of conducting polymers and, the authors argue, to the development of soft, wet, reactive multifunctional biomimetic devices that could reshape soft robotics, bioelectronics and biomedicine in the years ahead.

Subject of Research: Coulovoltammetric analysis of structural electrochemistry, charge-energy asymmetry and reactive self-sensing capabilities of polyaniline

Article Title: Understanding the electrochemistry of polyaniline structural processes, charge, energy and reactive sensing capabilities through coulovoltammetry

Article References: Sidheekha, M. P., Rajan, L., Prakash, S., Shabeeba, A., & Ismail, Y. A. (2026). Understanding the electrochemistry of polyaniline structural processes, charge, energy and reactive sensing capabilities through coulovoltammetry. Discover Electrochemistry, 3(1), Article 84. https://doi.org/10.1007/s44373-026-00172-9

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00172-9

Keywords: polyaniline, coulovoltammetry, conducting polymers, structural electrochemistry, redox switching, ESCR model, energy asymmetry, self-sensing materials, biomimetic devices, electrochemical sensors, charge trapping, conformational relaxation