저온형 연료전지용 전극 촉매 개발

Abstract

Low temperature fuel cells (LTFCs), DMFCs and PEMFCs, are attracting tremendous attention as new power sources for portable devices and transportations. Still, there exist many technical barriers for wide-spread of LTFCs. To solve the problems of DMFCs including low performance in spite of heavy amount of noble metals and methanol crossover, methyl formate was introduced as an alternative fuel instead of methanol. And, efficient catalysts for elecrooxidation of methyl formate were investigated. In case of PEMFCs, many efforts were devoted to develop noble metal-free catalysts for cathode of PEMFCs, because cathode reaction in PEMFCs (oxygen reduction reaction) is sluggish and corrosive, causing heavy usage of Pt and durability problem.Carbon-supported PtSn, PtPd, and PtRuPd catalysts were prepared by chemical reduction with hydrothermal treatment, and their electrochemical properties for methyl formate electrooxidation were investigated by cyclic voltammetry, chronoamperometry, and CO stripping voltammetry in comparison with commercial Pt/C and PtRu/C catalysts. At low potentials below 0.4V, the dominant reaction is the oxidation of formic acid produced by acid-catalyzed hydrolysis of methyl formate, and PtPd/C exhibits the highest activity. At higher potentials, methanol joins the oxidation process of methyl formate and PtRu/C, PtSn/C, and PtRuPd/C show the high activity. In single cell, direct methyl formate fuel cell operation, PtRuPd/C containing both active component for formic acid oxidation (Pd) and CO-tolerant component (Ru) shows the highest performance for electrooxidation of methyl formate.Carbon-supported PtPb intermetallic compound (PtPb/C) was prepared by modified polyol method, and its electrochemical performance for methyl formate electrooxidation was investigated by half cell and single cell tests in comparison with commercial Pt/C and PtRu/C catalysts. In half cell operation at low potentials below 0.4V, the dominant reaction is the oxidation of formic acid produced by acid-catalyzed hydrolysis of methyl formate, and PtPb/C exhibits the highest activity. At higher potentials, methanol joins the oxidation process of methyl formate and PtRu/C shows the high activity. In single cell operation, PtPb/C shows the highest performance for electrooxidation of methyl formate and could become a practical anode electrocatalyst for direct methyl formate fuel cells.Much attention has been paid to transition metal nitrides as a possible replacement of Pt-group metal catalysts. Recently, early transition metal nitrides, oxynitrides, and carbonitrides have been used as oxygen reduction reaction (ORR) catalysts. Here, five different transition metal nitrides (Mo2N, W2N, NbN, Ta3N5, and TiN) were prepared by simple urea glass route and screened for ORR based on their electrochemical activity and stability. Through cyclic voltammetry (CV) and linear sweep voltammetry (LSV), TiN was determined as the most promising electrocatalyst for ORR due to its high activity and stability in the potential range of ORR compared to the others. To increase the activity of TiN for ORR, TiN was modified with nanostructured carbon supports including graphene (GR), CNT, and CNT and GR composite (CNT-GR). The resultant TiN nanoparticles (TiN NPs) on CNT-GR hybrid (TiN/CNT-GR) exhibited much higher activity for ORR compared to state-of-the-art nitride-based materials as well as bare TiN, TiN/GR, and TiN/CNT. The reason for enhanced activity was ascribed to synergistic effect between TiN NPs and CNT-GR hybrid, whose roles were to provide active sites for ORR and eletron pathways to TiN NPs, respectively. Besides, TiN/CNT-GR exhibited an increased large mesopores that can be easily accessed by electrolyte due to the formation of 3-D like CNT-GR structure assembled between 2-D graphene and 1-D CNT, which results in the increased number of TiN sites that can contribute to ORR current density. Further, it showed strong metanol tolerance for DMFCs compared to commercial Pt/C catalyst. Thus, our TiN/CNT-GR could be a promising ORR electrocatalysts for PEMFCs and DMFCs. Finally, as an effort to broaden the application field of synthesized nitride materials except for fuel cell, TiN/CNT-GR was investigated as a counter electrode material for quantum dot-sensitized solar cells (QDSSCs). QDSSCs are promising low-cost alternatives to conventional silicon-based photovoltaic technologies. Our TiN/CNT-GR possesses an extremely high surface roughness on the FTO substrate, a good metal-support interaction, and less aggregation compared to bare TiN. The novel TiN/CNT-GR counter electrode exhibits a superior solar power conversion efficiency of 4.13% with applying a metal mask, which is much higher than 3.35% of the state-of-the-art Au electrode. Thus, TiN/CNT-GR could be a efficient noble metal-free counter electrode for QDSSCs.