pH-Dependent Oxygen Reduction on Metal Monoxide/Polypyrrole Hybrid Catalysts
Authors: Yoon Subhin , Lee Dong-gyu , Shin Seokmin , Lee Yongdae , Song Hyun-Kon 
Published: December 2024

The oxygen reduction reaction (ORR) is at the heart of many such fuel cell devices and holds a special place in the field of electrocatalysis, but it is notoriously slow. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells.
Theoretical approaches have been reported that dual-catalyst system in which two catalysts work simultaneously to decrease specific adsorption energies of intermediates, resultantly, linear scaling relationship were circumvented, could be an effective way to improve the catalytic activity. A calculation work suggested that combination of transition metal sites and hydrogen donor/acceptor sites were available to improve catalytic performances of ORR and OER beyond limitations originated from linear scaling relations. Polypyrrole, well known as a conductive polymer, is capable of deprotonation / deprotonation with adsorption and desorption of anions. It has been reported that pPy are easily and reversibly deprotonated and protonated in aqueous solution. The deprotonation / protonation of polypyrrole is reversible, suggesting that pPy can act as a proton donor in the ORR
Electrocatalytic ORR in aqueous media proceeds along a pathway having multiple steps in which multiple surface intermediates are involved (* →*OO → *OOH → *O → *OH where * = active site of catalyst). Proton supplied from media plays a crucial role on the ORR mechanism. Therefore, pH of media affects the kinetics of ORR significantly. In the same vein, it is expected that the ORR activity is improved in the presence of proton donor (HD) around active sites of catalysts.
In this work, we demonstrate a possibility that a series of proton donors are involved in protonation of the surface intermediates. we demonstrate the improvements of the ORR activities on various metal oxides through an additional organic compartment, polypyrrole (pPy), that can provide protons to oxygen intermediate. pPy has been well studied to modification of metal catalysts such as metal organic compounds, M-N-C for enhancing ORR kinetics. Herein, as a different role of pPy, it is studied that the interaction of polypyrrole with oxygen intermediate adsorbed on transition metal catalysts to provide protons. As an experimental result, it was confirmed that this change leads to an improvement of ORR kinetics.
Moreover, we present pH dependency of overpotential reduction by the pPy also supported that the proton transfer from pPy to the metal monoxide catalyst would affect ORR kinetics. The ORR overpotential was improved by 50mV at pH 13 on transition metal oxide catalysts. The overpotential improvement was intensified as the pH of media decreased. It is suspected that the pH affects deprotonation of the proton donors (HD à H+ + D-). High pH deprotonates the proton donors so that the effects of proton donors decrease. However, the proton of HD survives in acidic media so that the proton transfer from HD to active site is facilitated. The ORR overpotential gain was as large as 295 mV at pH 4 even if the oxide catalyst was not stable at the acidic pH. At pH 7, the overpotential improvement was estimated at 254 mV.

Figure 1