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Double activation of oxygen intermediates of oxygen reduction reaction by dual polymer/oxide electrocatalysts

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Title
Double activation of oxygen intermediates of oxygen reduction reaction by dual polymer/oxide electrocatalysts
Author
Lee, Dong-Gyu
Advisor
Song, Hyun-Kon
Keywords
Oxygen reduction reaction; Dual electrocatalysis; Secondary-amine-conjugated polymer
Issue Date
2020-02
Publisher
Graduate School of UNIST
Abstract
Oxygen reduction reaction (ORR) is one of the classical topics in electrochemistry which has been investigated for several decades. ORR, a notoriously sluggish cathodic process, requires an electrocatalyst to improve its kinetics. Platinum is well-known as the best catalyst for ORR. However, due to its high cost, many kinds of catalyst such as non-precious-metal-based catalysts (metal oxide, metal alloy and M-N-C) and metal-free catalysts (nitrogen doped carbon) have been studied as replacements for platinum. These studies on catalyst development commonly focus on significant generation of active sites. In this dissertation, dual polymer/oxide electrocatalytic system is presented, which further improves catalytic activity by adding the polymers that interact with the oxygen, not active site of oxides. The additional components act as co-catalysts by simultaneously participating in ORR with the original catalyst. The double activation of oxygen intermediates in the dual electrocatalytic system reduces adsorption energies of intermediates compared to a single catalytic system. Dual electrocatalysis achieved by simply mixing metal oxide particles (original catalyst) with polymers (additional catalyst) is one of the most efficient and easiest ways to improve the catalytic activity of cost-efficient catalysts to match the level of precious metal catalysts. Firstly, perovskite oxides were investigated as a catalyst for ORR. Perovskite is a good model for the basic investigation of electrocatalysis because their physical properties, especially electrical conductivities, can be dynamically controlled with their composition and stoichiometry. Controlling conductive environment surrounding active sites, achieved by more conductive catalysts (providing internal electric pathways) or higher carbon content (providing external electric pathways), contributes to presenting the conductivity-dependent trend of the number of electron transfer in ORR. Then, the perovskite catalysts were studied in presence of polypyrrole (pPy), which is the first additional catalyst considered for ORR. Nitrogen-containing electrocatalysts such as metal-nitrogen-carbon (M-N-C) composites and nitrogen-doped carbons are known to exhibit high activities for ORR. Even if the mechanism by which nitrogen improves the activities is not completely understood, strong electronic interaction between nitrogen and active sites has been found in these composites. In this work, I demonstrate a case in which nitrogen improves electroactivity, but in the absence of strong interaction with other components. The overpotentials of ORR and oxygen evolution reaction (OER) on perovskite oxide catalysts were significantly reduced by simply mixing the catalyst particles with polypyrrole/carbon composites (pPy/C). Any strong interactions between pPy (a nitrogen-containing compound) and active sites of the catalysts were not confirmed, but interaction between secondary amine(N-H) of pPy and oxygen was observed. To activate oxygen and other intermediates, a series of the secondary amine conjugated polymers (HN-CPs) was tested with three different cobalt-based electrocatalysts (CoO, Co3O4, and LiCoO2). In this work, the synergistic electrocatalysis of oxygen reduction reaction (ORR) was successfully demonstrated by showcasing the significantly improved kinetics of ORR. The electron donation number of the HN-CPs to diatomic oxygen (δ in O2δ-) described the order of activity improvement by polypyrrole (pPy) > polyaniline (pAni) > polyindole (pInd). The maximum overpotential gain at ~150 mV was achieved by using pPy, characterized by the highest δ. In the mechanism, the activated diatomic oxygen species (O2δ-) was transferred to the active site of electrocatalysts, while maintaining interaction with NH-CPs. As ORR proceeded along the mechanistic pathway following *OO → *OOH → *O → *OH → *OO (the surface intermediates on catalyst), the proton of the HN-CP was transferred to the single oxygen surface species (*O). The kinetic gains were obtained in the surface single oxygen formation step (*OOH to *O) at the equilibrium potential and the proton transfer step (*O to *OH) at a biased potential by the dual catalysis, when compared with the conventional electrocatalysis in the absence of OA (RDS = the surface peroxide formation step of *OO to *OOH).
Description
Department of Energy Engineering (Energy Engineering)
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