Ohhun Gwon

A Highly Efficient and Robust Cation Ordered Perovskite Oxide as a Bifunctional Catalyst for Rechargeable Zinc-Air Batteries

Of the various catalysts that have been developed to date for high performance and low cost, perovskite oxides have attracted attention due to their inherent catalytic activity as well as structural flexibility. In particular, high amounts of Pr substitution of the cation ordered perovskite oxide originating from the state-of-the-art Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) electrode could be a good electrode or catalyst because of its high oxygen kinetics, electrical conductivity, oxygen capacity, and structural stability. However, even though it has many favorable intrinsic properties, the conventional high-temperature treatment for perovskite synthesis, such as solid-state reaction and combustion process, leads to the particle size increase which gives rise to the decrease in surface area and the mass activity. Therefore, we prepared mesoporous nanofibers of various cation-ordered PrBa0.5Sr0.5Co2-xFexO5+δ (x = 0, 0.5, 1, 1.5, and 2) perovskites via electrospinning. The well-controlled B-site metal ratio and large surface area (∼20 m2 g-1) of mesoporous nanofiber result in high performance of the oxygen reduction reaction and oxygen evolution reaction and stability in zinc-air battery.

Conductivity-Dependent Completion of Oxygen Reduction on Oxide Catalysts

The electric conductivity-dependence of the number of electrons transferred during the oxygen reduction reaction is presented. Intensive properties, such as the number of electrons transferred, are difficult to be considered conductivity-dependent. Four different perovskite oxide catalysts of different conductivities were investigated with varying carbon contents. More conductive environments surrounding active sites, achieved by more conductive catalysts (providing internal electric pathways) or higher carbon content (providing external electric pathways), resulted in higher number of electrons transferred toward more complete 4e reduction of oxygen, and also changed the rate-determining steps from two-step 2e process to a single-step 1e process. Experimental evidence of the conductivity dependency was described by a microscopic ohmic polarization model based on effective potential localized nearby the active sites.

Cloud-like graphene nanoplatelets on Nd0.5Sr0.5CoO3−δ nanorods as an efficient bifunctional electrocatalyst for hybrid Li–air batteries

With the recognition of metal–air batteries as promising candidates for clean and efficient energy storage, the development of inexpensive and effective bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) has become one of the most important topics in this field. Herein, we designed new composite catalysts consisting of a nanorod type Nd0.5Sr0.5CoO3−δ (NSC) perovskite and edge-iodinated graphene nanoplatelets (IGnPs) as bifunctional catalysts for ORR and OER. Interestingly, the simple application of ultrasonication endowed the catalyst with a fascinating morphology comprising cloud-like IGnPs on NSC nanorods (NSC@IGnP). Benefiting from the unique morphological features, NSC@IGnP provides superior bifunctional activities toward both ORR and OER, remarkable cell performance, and exceptionally high stability for hybrid Li–air batteries.

One-pot surface engineering of battery electrode materials with metallic SWCNT-enriched, ivy-like conductive nanonets

A longstanding challenge facing energy conversion/storage materials is their low electrical conductivity, which often results in unwanted sluggish electrochemical reactions. Here, we demonstrate a new class of one-pot surface engineering strategy based on metallic single-walled carbon nanotube (mSWCNT)-enriched, ivy-like conductive nanonets (mSC nanonets). The mSC nanonets are formed on the surface of electrode materials through a poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO)-assisted sonication/filtration process. PFO is known as a dispersant for SWCNTs that shows a higher affinity for semiconducting SWCNTs (sSWCNTs) than for mSWCNTs. Driven by this preferential affinity of PFO, sSWCNTs are separated from mSWCNTs in the form of sSWCNT/PFO hybrids, and the resulting enriched mSWCNTs are uniformly deposited on electrode materials in the form of ivy-like nanonets. Various electrode materials, including lithium-ion battery cathodes/anodes and perovskite catalysts, are chosen to explore the feasibility of the proposed concept. Due to their ivy-like conductive network, the mSC nanonets increase the electronic conductivity of the electrode materials without hindering their ionic transport, eventually enabling significant improvements in their redox reaction rates, charge/discharge cyclability, and bifunctional electrocatalytic activities. These exceptional physicochemical advantages of the mSC nanonets, in conjunction with the simplicity/versatility of the one-pot surface engineering process, offer a new and facile route to develop advanced electrode materials with faster electrochemical reaction kinetics.