Myeong Jae Lee

Ordered macroporous platinum electrode and enhanced mass transfer in fuel cells using inverse opal structure

Three-dimensional, ordered macroporous materials such as inverse opal structures are attractive materials for various applications in electrochemical devices because of the benefits derived from their periodic structures: relatively large surface areas, large voidage, low tortuosity and interconnected macropores. However, a direct application of an inverse opal structure in membrane electrode assemblies has been considered impractical because of the limitations in fabrication routes including an unsuitable substrate. Here we report the demonstration of a single cell that maintains an inverse opal structure entirely within a membrane electrode assembly. Compared with the conventional catalyst slurry, an ink-based assembly, this modified assembly has a robust and integrated configuration of catalyst layers; therefore, the loss of catalyst particles can be minimized. Furthermore, the inverse-opal-structure electrode maintains an effective porosity, an enhanced performance, as well as an improved mass transfer and more effective water management, owing to its morphological advantages.

Tuning the oxygen reduction activity of the Pt–Ni nanoparticles upon specific anion adsorption by varying heat treatment atmospheres

Heat treatment of Pt based nanoparticles under various conditions is one of the conventional ways to modify the electrocatalytic properties for enhancement of the oxygen reduction reaction (ORR). However, the effect of the heat treatment atmosphere on the ORR activity especially upon specific anion adsorption still remains unclear. This paper investigates the Pt-Ni bimetallic nanoparticles (Pt2Ni1), under various heat treatment atmospheres, as enhanced cathodic electrocatalysts for the high temperature-proton exchange membrane fuel cell (HT-PEMFC) using a phosphoric acid doped polybenzimidazole (p-PBI) membrane. The X-ray spectroscopic measurement showed the variations of the electronic structures of Pt-Ni nanoparticles under the heat treatment condition. In the half-cell measurement, the argon treated electrocatalyst demonstrated the highest catalytic activity owing to the appropriate electronic interaction between Pt and Ni. The single cell test with a p-PBI membrane, at 160 °C, also confirmed the excellent oxygen reduction reactivity and durability of the argon-treated Pt-Ni nanoparticles. This result suggested that the alteration of the electronic structure by a proper heat treatment atmosphere upon specific anion adsorption decisively influenced the ORR activity both at half-cell and single-cell scales.

Electrochemically Synthesized Nanoporous Molybdenum Carbide as a Durable Electrocatalyst for Hydrogen Evolution Reaction

Abstract Demands for sustainable production of hydrogen are rapidly increasing because of environmental considerations for fossil fuel consumption and development of fuel cell technologies. Thus, the development of high‐performance and economical catalysts has been extensively investigated. In this study, a nanoporous Mo carbide electrode is prepared using a top‐down electrochemical process and it is applied as an electrocatalyst for the hydrogen evolution reaction (HER). Anodic oxidation of Mo foil followed by heat treatment in a carbon monoxide (CO) atmosphere forms a nanostructured Mo carbide with excellent interconnections, and these structural characteristics lead to high activity and durability when applied to the HER. Additionally, characteristic behavior of Mo is observed; metallic Mo nanosheets form during electrochemical anodization by exfoliation along the (110) planes. These nanosheets are viable for chemical modification, indicating their feasibility in various applications. Moreover, the role of carbon shells is investigated on the surface of the electrocatalysts, whereby it is suggested that carbon shells serve as a mechanical barrier against the oxidative degradation of catalysts that accompanies unavoidable volume expansion.

Highly Efficient Bifacial Dye-Sensitized Solar Cells Employing Polymeric Counter Electrodes

Dye-sensitized solar cells (DSCs) are promising solar energy conversion devices with aesthetically favorable properties such as being colorful and having transparent features. They are also well-known for high and reliable performance even under ambient lighting, and these advantages distinguish DSCs for applications in window-type building-integrated photovoltaics (BIPVs) that utilize photons from both lamplight and sunlight. Therefore, investigations on bifacial DSCs have been done intensively, but further enhancement in performance under back-illumination is essential for practical window-BIPV applications. In this research, highly efficient bifacial DSCs were prepared by a combination of electropolymerized poly(3,4-ethylenedioxythiphene) (PEDOT) counter electrodes (CEs) and cobalt bipyridine redox ([Co(bpy)3]3+/2+) electrolyte, both of which manifested superior transparency when compared with conventional Pt and iodide counterparts, respectively. Keen electrochemical analyses of PEDOT films verified that superior electrical properties were achievable when the thickness of the film was reduced, while their high electrocatalytic activities were unchanged. The combination of the PEDOT thin film and [Co(bpy)3]3+/2+ electrolyte led to an unprecedented power conversion efficiency among bifacial DSCs under back-illumination, which was also over 85% of that obtained under front-illumination. Furthermore, the advantage of the electropolymerization process, which does not require an elevation of temperature, was demonstrated by flexible bifacial DSC applications.