Are Polyaniline and Polypyrrole Electrocatalysts for Oxygen (O2) Reduction to Hydrogen Peroxide (H2O2)?

In this report, we present results on the electrocatalytic activity of conducting polymers [polyaniline (PANI) and polypyrrole (PPy)] toward the electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2). The electropolymerization of the polymers and electrolysis conditions were optimized for H2O2 production. On flat glassy carbon (GC) electrodes, the faradaic efficiency (FE) for H2O2 production was significantly improved by the polymers. Rotating disc electrode (RDE) studies revealed that this is mainly a result of blocking further H2O2 to the water reduction pathway by the polymers. PPy on carbon paper (CP) significantly increased the molar production of H2O2 by over 250% at an average FE of above 95% compared to bare CP with a FE of 25%. Thus, the polymers are acting as catalysts on the electrode for the ORR, although their catalytic mechanisms differ from other electrocatalysts.


Optimizing the polyaniline polymerization conditions
In order to produce fully metal-free polyaniline (PANI) films on electrodes, electrochemical oxidative polymerization, initially discovered by Letheby 1 , was performed based on the conditions described by Nunziante and Pistoia 2 . The goal was, to obtain thin films of PANI which are in ideal case fully covering the whole surface, starting with the comparison of number of CV cycles performed in FigureS 1:

S2
As a next step, the concentration of aniline in the electrolyte solution was varied in order to find the best conditions: In the following FigureS 3 the influence of scan rate upon film morphology was investigated: In a final test, the importance of the applied potentiodynamic polymerization over constant potential polymerization was investigated. Therefore, in the following FigureS 4 the results of polymerizations kept at + 800 mV for 30 min are compared:

Quantification of hydrogen peroxide (H 2 O 2 ) produced
The quantification of H 2 O 2 produced during chronoamperometry was done according to recent reports of a colorimetric detection by Su et al. 3 and Apaydin et al. 4 . To start with, a stock solution of 4 mM p-nitrophenyl boronic acid (pNBA, Sigma Aldrich) in DMSO (VWR) and a 150 mM Na 2 CO 3 /NaHCO 3 (Fluka; Sigma Aldrich) buffer solution at pH 9 in a 1:1 ratio was prepared. This solution was mixed with the samples and after 36 min reaction time under dark conditions, the absorbance values at 411 nm were recorded using a Thermo Fischer Multiskan Go Microplate Spectrophotometer. Upon calibration with H 2 O 2 standard solutions (Merck) resulting in the calibration curve in FigureS 5, the amount of H 2 O 2 was calculated. In order to get the required change in absorbance, the absorbance of a blank sample was subtracted.

Spectroscopic materials characterization
The spectroscopic results of ATR-FTIR and Raman spectroscopy of PANI on Cr-Au electrodes are shown in the following FigureS 6:

FigureS 6: a) ATR-FTIR spectrum and b) Raman spectrum of PANI.
The characteristic bands of the polyaniline base were detected and are in accordance to literature reports 5,6 .
The spectroscopic results of ATR-FTIR and Raman spectroscopy of PPy on Cr-Au electrodes is shown in the following FigureS 7: From the FTIR, the characteristic bands of the polypyrrole base were detected and are in well accordance to literature reports 6 . The Raman spectrum of PPy only showed faint features due to instability during the laser treatment.

Carbon paper -SEM image characterization
In the following FigureS 8, SEM images of bare carbon paper are shown: Similar to Figure 2c+d, the carbon fibres are visible, but they have a smooth surface structure.

Blank CV experiments
The following shows the CV curves of blank GC as well as blank CP:

FigureS 9: CV of GC and CP w/o O 2 .
As can be seen from FigureS 9, CP possessed a distinct reduction peak for ORR compared to GC, which only showed a reductive feature. Under N 2 saturated conditions, both GC and CP show nearly no electrochemical feature in the regime studied. S6

H 2 O 2 production -Polyaniline
In order to determine the optimum pH medium as well as reduction potential for H 2  The results for H 2 O 2 production using CP/PANI as well as blank CP are summarized in the following FigureS 13:

H 2 O 2 production -Polypyrrole
In order to investigate polypyrrole towards electrochemical H 2 O 2 production, GC/PPy and CP/PPy were investigated at pH 2, as shown in FigureS 14:

Hydrodynamic voltammetry
Hydrodynamic experiments like rotating disc electrode (RDE) or rotating ring-disc electrode (RRDE) are frequently used to compare electrocatalysts and determine the number of transferred electrons for the ORR step. The exchange current I K and the number of transferred electrons n can be calculated by performing linear sweep voltammetry (LSV) at various rotation speed ω by applying the Koutecki-Levich-Equation 7 shown in eq. 1: By investigation of RRDE, the number of transferred electrons n (eq. 2) as well as the corresponding faradaic efficiency (eq. 3) can be calculated by performing LSV at various rotation speeds from the disc current I D and the ring current I R 8,9 : Both formulas in eq. 2 and eq. 3 contain the value for the collection efficiency N, which is a quantity determined by the RRDE geometry and can be determined experimentally using the following eq. 4 (1)