Electropolymerization of Neutral Red
Authors: Kono Daiki , Harada Yuya , Xinjie Dai , Yoshida Tsukasa 
Published: November 2022

Hydrogen bonding conductive polymers have attracted attention as metal-free electrocatalysts for energy conversion (1)(2). Neutral red (NR), a pH indicator in the family of phenazine dyes, can be polymerized into poly-NR (PNR) just like aniline (ANI) does into poly-ANI (PANI). Due to the presence of hydrogen bonding N atom, PNR has been found to act as a good catalyst for hydrogen evolution reaaction (HER) (3). Although electropolymerization to achieve these polymer films is believed to take off by oxidation of the monomers into their respective radicals, potential sweep and cycling in a wide potential range are almost always employed, in other words, the films are obtained in the course of cyclic voltammetry (CV) in the monomer solutions. A clear benefit of such a method is to facilitate monitoring of the film growth, as the electrodeposited polymers often show a redox behavior. However, it is not easy to elucidate electrochemical stoichiometry and its Faradic efficiency by charge integration from the CVs during the film growth. If the electrochemical oxidation of the monomers is the only important process, potentiostatic electrolysis and chronoamperometric (CA) monitoring should work as well and be beneficial to the understanding of the reaction mechanism. However, to our knowledge, no such studies have been reported so far.
In this study, several different protocols of electrolysis have been tested and compared for electropolymerization of NR, i.e., potential sweep/cycling (CV), potentiostatic electrolysis (CA), potential stepping pulsed electrolysis (PP), with variation of conditions and also by their combinations. While reddish black thin films of PNR were nicely obtained by the standard CV method up to +1.2 V vs. Ag/AgCl to oxidize NR and down to -0.2 V to observe reduction of NR as well as electrodeposited PNR in acidic aqueous solutions of NR + H2SO4, the CA method to apply a constant potential of +1.05 V, sufficiently positive to promote oxidation of NR resulted in in a significant slowdown of PNR growth. The CA method, however, did result in a formation of PNR. Comparison of anodic charge for monomer oxidation both in the CV and CA methods with the optical density (O.D.) of the resulting PNR proved proportionality of O.D. to charge with the same O.D./charge ratios, thus the same Faradic efficiencies. In fact, CA showed a drastic decrease of current down to a rather small steady-state value. This was not caused by depletion of NR at the vicinity of the electrode, but by slowdown of charge transfer kinetics, as the introduction of forced convection by employing a rotating disk electrode (RDE) did not result in any enhancement of current. On the other hand, the anodic peak current only slightly decreased over multiple potential cycling in the CV method. The peak current was approximately linearly proportional to the square root of the scan rate, so that fast charge transfer to hit the diffusion limit of NR was achieved in the CV method.
The cause of the difference of charge transfer kinetics was further studied. Anodic growth of PNR naturally results in doping of anions, in this case sulfate, to stabilize polarons to achieve conductivity of PNR. Careful observation of CVs found presence of a couple of small peaks which correspond to doping / dedoping at around +0.5 V. Electrodeposition of PNR by the CV method with variation of negative end potential revealed the importance of dedoping of sulfate to refresh the surface of PNR to be favorable to the oxidation of NR, not the reduction /re-oxidation of the existing PNR. Thus, the PP method to switch the potential between that for the oxidation of NR monomer and the one for dedoping of sulfate from PNR (not the reduction of PNR) achieved the fastest growth of PNR.
The above-documented understanding of the electropolymerization of NR will be explained in a quantitative manner along with the experimental data. It is also interesting to mention the difference of the absorption spectra of the PNR films between those obtained by CV and CA methods, indicating their differences in the electronic structure to let us anticipate their differences in the HER electrocatalysis.

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