Nam-In Kim

Highly active and durable nitrogen doped-reduced graphene oxide/double perovskite bifunctional hybrid catalysts

A-site cation doping in perovskite-based materials with the general ABO3 formula has a significant effect on the bifunctional oxygen activity (oxygen evolution and reduction reactions) of chemically stable electrocatalysts, enabling the design of highly active, durable, and cost-effective catalysts. In particular, the oxygen activity of double perovskite-structured NdBa0.5Sr0.5Co1.5Fe0.5O5+δ (NBSCF) is 0.973 V, which is much greater than that of previously reported transition metal-based nanostructures. This result is verified by examination of the electronic structure, oxidation state, and electrical properties of the perovskite-based materials using density functional theory (DFT) calculations, the iodometric titration method, X-ray photon spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS) analysis. Further improvements of NBSCF for bifunctional oxygen activity are made by incorporating these synergistic hybrid structures with nitrogen doped-reduced graphene-based (N-rGO) nanostructures (NBSCF/N-rGO). The NBSCF/N-rGO has an oxygen electrode activity of 0.766 V, which is superior to that of other previously reported transition metal-based nanostructures and compares favorably to that of precious metal electrocatalysts. Furthermore, strong N-rGO provides considerably greater electrochemical long-term stability and integrity to NBSCF/N-rGO hybrid catalysts under continuous chronopotentiometric and long-term potential sweep testing conditions for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).

Oxygen-deficient triple perovskites as highly active and durable bifunctional electrocatalysts for oxygen electrode reactions

Triple perovskite, Nd1.5Ba1.5CoFeMnO9−δ, enriched with oxygen defects shows high activity and durability as a bifunctional oxygen electrocatalyst. Highly active and durable bifunctional oxygen electrocatalysts have been of pivotal importance for renewable energy conversion and storage devices, such as unitized regenerative fuel cells and metal-air batteries. Perovskite-based oxygen electrocatalysts have emerged as promising nonprecious metal bifunctional electrocatalysts, yet their catalytic activity and stability still remain to be improved. We report a high-performance oxygen electrocatalyst based on a triple perovskite, Nd1.5Ba1.5CoFeMnO9−δ (NBCFM), which shows superior activity and durability for oxygen electrode reactions to single and double perovskites. When hybridized with nitrogen-doped reduced graphene oxide (N-rGO), the resulting NBCFM/N-rGO catalyst shows further boosted bifunctional oxygen electrode activity (0.698 V), which surpasses that of Pt/C (0.801 V) and Ir/C (0.769 V) catalysts and which, among the perovskite-based electrocatalysts, is the best activity reported to date. The superior catalytic performances of NBCFM could be correlated to its oxygen defect–rich structure, lower charge transfer resistance, and smaller hybridization strength between O 2p and Co 3d orbitals.

B-site doping effects of NdBa0.75Ca0.25Co2O5+δ double perovskite catalysts for oxygen evolution and reduction reactions

As bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), we introduce an economical, particularly active, stable, and completely noble-metal-free catalyst based on CaO-doped NdBaCo2O5+δ with the aim of exploiting its high catalytic activity for the oxygen electrode. Further, the electrochemical performances of the NdBa0.75Ca0.25Co2O5+δ catalyst are improved by doping transition-metal oxides (Fe2+, Ni2+, Cu2+, and Mn2+) into the B-sites of double perovskite oxides, facilitating the movement of electrons and oxygen-ions through oxygen vacancies. Among these, NdBa0.75Ca0.25Co1.5Fe0.5O5+δ (NBCCFe) shows the highest electrocatalytic oxygen electrode activity for the OER and ORR, and NBCCFe thus is hybridized with nitrogen-reduced graphene oxide (N-rGO) to further improve its catalytic activity with excellent durability. Hybridization of NBCCFe with N-rGO further boosts bifunctional oxygen electrode activity (0.761 V) with better durability, which surpasses those of Pt/C (0.815 V) and Ir/C (0.768 V) catalysts. A rechargeable lithium–air battery assembled with the NBCCFe/N-rGO catalyst exhibits remarkably improved discharge capacity, reduced charge overpotential and an extended cycle life, corroborating the excellent bifunctional catalytic activity of NBCCFe/N-rGO. The NBCCFe/N-rGO catalyst also displays the most improved polarization during both discharge and charge, further confirming the highest bifunctional catalytic activity of NBCCFe/N-rGO.

Property Characterization and Analysis in Performance, Efficiency and Durability of the Membrane Electrode Assembly for Polymer Electrolyte Membrane Fuel Cell

인류는 오래 전부터 에너지를 이용해왔으며, 특 히 산업혁명을 거치면서 본격적으로 석유와 석탄 같은 화석연료를 에너지원으로 사용하게 되었다. 그 러나 많은 에너지 전문가들은 곧 화석연료의 매장 량이 바닥을 드러낼 것이며, 과도기적으로 현재는 원자력을 많이 사용하고 있지만, 이웃나라 일본의 예에서 보듯이 폐 핵연료 처리 및 방사능 유출의 가능성 때문에 화석연료 기반의 에너지 시대가 수 소 기반의 신재생 에너지 시대로 바뀔 것으로 예측 하고 있다[1]. 그 중에서 수소 연료전지는 기존 화 석연료의 치명적 문제인 환경오염 유발과 지구온난 화를 발생시키지 않는 청정 에너지 발전 고효율 시 스템이다[2]. 다양한 연료전지 중 고분자전해질 막을 전해질로 사용하는 고분자전해질 연료전지(Polymer electrolyte membrane fuel cell, PEMFC)는 수십 W급의 소규 모 발전 시스템부터 수백 KW급의 자동차 전원에 이 르기까지 다양한 적용분야로 많은 연구가 진행되고 있다[3]. 고분자전해질 연료전지는 애노드와 캐소드 에서 각각 수소가 산화되는 반응과 산소가 환원되는 전기화학적 반응을 통하여 부산물로 물과 전기가 생 산되는 친환경적인 고효율 발전시스템이다. 고분자전 해질 연료전지의 가장 큰 특징은 다른 연료전지에 비 해서 작동온도가 저온이기에 자동차를 비롯한 운송 부문에 적합하게 높은 운전-정지 순환특성을 보이며, 빠르게 초기성능을 얻을 수 있는 장점이 있다. 또한 매우 높은 전력밀도(300~1000 mW/cm)와 연료효율 (~60%)을 얻을 수 있다[3, 4]. 고분자전해질 연료전지의 막전극접합체는 크게 세 부분으로 구성되어 있다. 가장 핵심인 소재는 멤브 레인, 촉매층, 가스확산층(Gas diffusion layer, GDL) 이고 이를 합하여 막전극접합체(Membrane electrode assembly, MEA)라고 한다. 멤브레인은 애노드에서 공급된 수소가 이온화되고, 멤브레인을 통과하여 캐 소드로 가서 산소와 만나게 되는 발생장소이다. 따 라서, 무엇보다 높은 프로톤 전도도를 가져야 하며, 물리적, 기계적, 화학적 안정성이 좋아야 한다. 현재 는 술폰산기를 가지고 있는 Dupont 사의 Nafion계 열 막과 Gore사의 Prima막이 가장 많이 사용되며, 좋은 특성을 보이고 있다. 촉매층은 수소 기체가 쉽 게 이온이 되고, 프로톤이 빠르게 멤브레인을 통과 하여 캐소드로 넘어갈 수 있게 해주는 역할을 한다.