Electrochemical Co-Polymerization of Neutral Red and Aniline for Electrocatalytic Hydrogen Evolution Reaction
Authors: Harada Yuya , Kaiwa Haruka , Huang Xin , Stadler Philipp , Konno Ryuto , Shiroishi Hidenobu , Yoshida Tsukasa 
Published: December 2020

Electrocatalytic hydrogen evolution reaction (HER) is promising to achieve conversion of renewable electricity into storable chemical energy. For practical installation of such technology, it is important to develop active and stable electrocatalysts without relying on rare metal elements. Recently, some metal-free conductive organic polymers such as poly-dopamine (PDA) and poly-guanine (PG) have been found to exhibit stably high catalytic activity towards HER1-3. Also, we have achieved polymerization of neutral red (NR) dye, which is chemically analogous to aniline and bears amino groups at a high density. Oxidative chemical vapor deposition (oCVD) or electrodepolymerization deposition (EPD) were the techniques to achieve formation of PNR. PNR exhibited a good catalytic activity for CO2 reduction reaction (CO2RR), although the one prepared by oCVD was about twice as active as that by EPD. Structural disordering of the oCVD sample was supposed to contribute to the catalysis. It is likely that there is an optimum for the balance between conductivity and the density of hydrogen bonding sites. In this study, we have studied electrochemical co-polymerization of NR and aniline to find the optimum design of the catalyst.
Electro-polymerization of poly-aniline (PANI), PNR or co-polymer PANI-PNR was carried out at an F-doped tin oxide (FTO) coated conductive glass (Asahi-DU) or a carbon felt (CF, Jing Long Te Tan) substrate by potential cycling between -0.2 and +1.0V vs. Ag/AgCl in aqueous solutions containing ANI, NR or their mixtures at appropriate ratios at a total monomer concentration of 50 mM and 0.1 M H2SO4 for 50 (FTO) or 20 cycles (CF) under N2. Linear sweep voltammetry (LSV) was performed on polymer coated CF electrodes in a 0.5 M H2SO4 under N2 for evaluation of HER catalysis.
PANI appeared dark green, PNR red, whereas PANI-PNR was black. These color differences already speak for successful loading of NR into PANI to achieve PANI-PNR copolymer. Cyclic voltammograms (CVs) during EPD of PANI and PNR show the characteristic redox peaks3 (Fig. 1 a, b). While redox peaks of PANI continue to grow, the reversible redox peaks of NR almost stay the same, since formation of cation radical of NR under the present acidic condition is inefficient.3 When ANI and NR are blended, however, CV that differ from those for homopolymers of PANI and PNR is obtained (Fig. 1c). Although this CV is not the exact superimpose of the two for PANI and PNR, it suggests the presence of NR in the film and its contribution to the redox behavior.
HER current in LSV almost remains the same when CF is coated with PANI, indicating its disability of electrocatalysis (Fig. 1d). Although PANI should be the most conductive among the polymers we tested here, PANI does not possess hydrogen bonding amino groups. PNR only showed moderate catalysis partly due to the small film thickness we could achieve under the current preparation condition. The PANI-PNR copolymer indeed exhibited very high catalytic activity to reduce the overvoltage as small as 250 mV. It is likely that NR provides hydrogen bonding catalytic centers while PANI assures a high conductivity. Thus, a superior catalysis has been achieved by the co-polymer strategy than using the homopolymers. The optimal compositional balance between PANI and PNR should be found by further experimental elaboration.

References:

Coskun et al., Mater., 2020, 1902177.

Coskun et al., Mater. interfaces, 2019, 1901364.

Xin Huang et al., 237th ECS meeting abstract, 2019, 135704.

Figure 1