Introduction
The development of advanced electrode materials capable of delivering high energy and power densities while maintaining long-term operational stability remains a central challenge in electrochemical energy storage. Organic pseudocapacitive materials have emerged as promising candidates in this context, as they offer fast redox kinetics, molecular-level tunability, mechanical flexibility, and compatibility with low-energy, solution-based fabrication processes. In particular, electropolymerized conjugated polymers enable direct, binder-free growth of electroactive films on conductive substrates, ensuring intimate electrical contact and precise control over film thickness and morphology, features that are highly desirable for advanced energy materials.
Porphyrins constitute a unique class of redox-active molecular scaffolds owing to their extended π-conjugation, multielectron redox chemistry, and exceptional chemical robustness. Fully fluorinated porphyrins are especially attractive building blocks, as their strongly electron-withdrawing substitution pattern confers outstanding electrochemical stability and enhanced resistance toward oxidative degradation. When functionalized with polymerizable conjugated substituents, porphyrins can be integrated into extended polymer networks that combine molecular redox activity with efficient charge transport and structural durability. However, the electropolymerization of porphyrin derivatives remains nontrivial, and subtle variations in molecular architecture (such as the number and nature of polymerizable units) can critically influence polymer growth, electrochemical stability, and long-term pseudocapacitive performance. Establishing clear structure–property–performance relationships is therefore essential for advancing porphyrin-based electropolymers toward practical energy storage applications.
Herein, we present a comparative study of two structurally related fluorinated porphyrin monomers, P-4E, bearing four 3,4-ethylenedioxythiophene (EDOT) units, and P-3E-BDT, incorporating three EDOT units and one benzothiadiazole substituent, which were used as precursors for the electropolymerized films p-P-4E and p-P-3E-BDT, respectively. These polymer systems provide an ideal platform to elucidate how the nature and distribution of conjugated substituents influence electrochemical stability, rate capability, and pseudocapacitive behavior.
Methods
Electropolymerization of the porphyrin monomers was performed by oxidative cyclic voltammetry using a conventional three-electrode configuration. Glassy carbon (GC), platinum, and indium tin oxide (ITO) electrodes were employed as working electrodes, with a platinum wire counter electrode and a silver wire quasi-reference electrode. Polymer growth was carried out in dichloromethane solutions containing 0.10 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte, applying consecutive potential cycles at a scan rate of 0.1 V s⁻¹ within the anodic window required to promote EDOT coupling. After deposition, the electrodes were rinsed and transferred to monomer-free electrolyte solutions for electrochemical characterization.
Spectroelectrochemical measurements were conducted using polymer-coated ITO electrodes in a UV–Vis transparent electrochemical cell. Film morphology and surface texture were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Electrochemical impedance spectroscopy (EIS) measurements were performed using a 5 mV AC perturbation amplitude over a frequency range of 100 kHz to 0.1 Hz. Galvanostatic charge–discharge (GCD) measurements at different current densities were used to evaluate gravimetric capacitance, rate capability, and cycling stability, including extended cycling tests.
Results
Both fluorinated porphyrin monomers were efficiently electropolymerized, yielding uniform and strongly adherent films on GC and ITO substrates. Cyclic voltammetry revealed reversible redox processes associated with both the porphyrin macrocycle and the conjugated EDOT backbone, indicating effective electronic communication within the polymer network.
p-P-4E exhibited a highly reproducible electrochemical response upon extended cycling, whereas p-P-3E-BDT displayed more pronounced changes over repeated cycles. GCD measurements confirmed pseudocapacitive behavior for both materials, with nearly linear discharge profiles within the selected potential windows. Spectroelectrochemical measurements revealed reversible, potential-dependent spectral changes consistent with the formation of radical species during oxidation, involving both the EDOT backbone and the porphyrin macrocycle and confirming preservation of the porphyrinic framework during cycling.
At a current density of 5 A g⁻¹, p-P-3E-BDT delivered a gravimetric capacitance of approximately 240 F g⁻¹, whereas p-P-4E reached higher values of around 320 F g⁻¹ under identical conditions. Moreover, p-P-4E showed improved capacitance retention with increasing current density and retained ~90% of its initial capacitance after 3000 charge–discharge cycles, demonstrating excellent durability. Capacitance values derived from GCD were independently corroborated by EIS. SEM and AFM analyses revealed more porous and textured morphologies for p-P-4E, consistent with its superior electrochemical performance.
Conclusions
This work demonstrates that molecular engineering of fluorinated porphyrin electropolymers provides an effective approach to access stable and high-performance pseudocapacitive materials. The investigated EDOT–porphyrin polymer networks exhibit reversible redox activity, efficient charge storage, and robust electrochemical behavior over extended cycling. While both systems display pseudocapacitive characteristics, the polymer incorporating a larger number of EDOT units (p-P-4E) exhibits enhanced gravimetric capacitance and cycle stability within the investigated current-density range. The combined analysis of galvanostatic charge–discharge and electrochemical impedance spectroscopy confirms a consistent pseudocapacitive response. These results highlight the importance of molecular architecture in porphyrin-based electropolymers and establish fluorinated EDOT–porphyrin systems as promising candidates for advanced organic electrode materials in electrochemical energy storage.
