While electric power supply by renewable sources such as solar and wind has become viable for their significant cost reduction, its intermittency demands urgent development of large scale storage technologies. Although conversion of electrical energy into storable hydrogen via electrolysis of water is ideal, noble metals and their oxides are used as electrocatalysts for their activities and stabilities. Alternative electrocatalysts out of abundant elements are needed for sustainable technological development.

Recently, some metal-free organic conductive polymers with hydrogen-bonding capabilities were found to exhibit high electrocatalytic activities towards hydrogen evolution reaction (HER) ^[1,2]. We have succeeded electropolymerization of neutral red (NR) resulting in a formation of conductive poly-NR (PNR) that shows a relatively high HER catalytic activity ^[3]. PNR possesses hydrogen-bonding N atoms as annelated in the phenazine aromatic system as well as those in the amino substituents. Electrochemical analysis combined with in situ spectroscopy as well as DFT calculation was performed to clarify the mechanism and kinetics of HER electrocatalysis by PNR.

This films of PNR were obtained by electropolymerization on F-doped SnO₂ coated conductive glass (FTO, Asahi Glass) and SIGRATHERM^® GFA5 Carbon felt with potential cycling between -0.2 and 1.2 V (vs. Ag/AgCl) at 50 mV s^-1 for 50 times in a 5 mM NR - 0.1 M H₂SO₄ aqueous solution under N₂. Polyaniline (PANI) was also obtained by the same method for comparison. The HER catalysis was evaluated by linear sweep voltammetry (LSV) in a 1 M trifluoromethanesulfonic acid (TfOH) under N₂. The film samples were characterized by Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy. In-situ UV-visible spectroelectrochemical monitoring of the reaction intermediate was carried out to determine the rate of HER.

PNR undergoes a pseudo-reversible reduction that shifts -61.8 mV/pH under acidic conditions and HER takes off right at this point (Fig. 1, a). Coupling of the same number of protons / electrons is expected as the electrochemical stoichometry from the observed Nernstian relationship, namely, resulting in a singly reduced PNR-H or doubly reduced PNR-H2 as depicted in Fig. 1 b. Protonation of N atoms was reasonably expected and also supported by the DFT calculation. The hydrogen atoms stabilized in the reaction intermediate can be associated to release H₂ (Tafel mechanism) to complete the cycle of electrocatalysis of HER.

Reduction of PNR in fact is associated with a color change. The broad reddish absorption of PNR peaking at around 500 nm attenuates to change the color to a pale yellow by constantly applying -0.15 V vs. RHE to produce PNR-H and/or PNR-H2 state as shown in Fig. 1c. Gradual recovery of the original red PNR was observed under open circuit under N₂, associated with the H₂ release. Thus, the rate of the spectral change is analyzed to determine the rate of the Tafel process. Pseudo-first order reaction rate law can be applied as proton is abundant and its concentration is constant, so that the rate of HER (r_H2) is simply described as the rate of consumption of PNR-H and/or PNR-H2 as,

r_H2=-dC_PNR-H2/dt =-kt (1)

The ratio of PNR-H and PNR-H2 as compared to their initial amount (x_PNR-red) was defined from the absorbance at 545 nm before and after reduction as,

x_PNR-red = C_PNR-red/C_0-PNR-red = exp (-kt) (2)

Good fitting of the experimental data was obtained to yield a pseudo-first order reaction rate constant of 9.98×10^-4 s^-1 for this rate-limiting step.

References

[1] H. Coskun et al. Advanced Materials 32, 1902177 (2020).

[2] H. Coskun et al. Advanced Materials Interfaces 7, 1901364 (2020).