텅스텐카바이드(WCs) 기반의 전기화학적 촉매에 관한 연구

Abstract

Transition metal carbides (TMCs) have been attracted as catalysts because of their noticeable catalytic properties. In addition, their refractory properties can give resistance against sintering and corrosion under reaction such as acidic and basic condition. They have outstanding catalysis in many reactions, particularly those hydrogen involved reactions such as hydrotreating, hydrogenation, dehydrogenation, isomerization, hydrogenolysis, reforming, and aromatization so on. Recently they are also receiving attention as elecrocatalysts. In many of these reactions, metal carbides behave like Group 8？10 metals (noble metals such as Pt) of high cost and limited supply and have potential to reduce or replace them. Especially, in low temperature fuel cell system, tungsten carbide is most attractive material due to their Pt-like behavior, high electric conductivity and CO resistance so on. Thus tungsten carbide based materials have been applied to low temperature fuel cells (DMFC, PEMFC) as anodic and cathodic electrode materials. However, the intrinsic catalysis of tungsten carbide itself is too much lower than that of platinum. In many catalytic system, small amount of metals including noble species have been used with tungsten carbides for synergistic effect between tungsten carbides and metals for high electrochemical activities related with low temperature fuel cells.Tungsten carbides have been specified by various phases such as tungsten mono-carbide (WC), tungsten di-carbide (W2C) and tungsten partial oxidized form (WC1-x). Among them, WC has been reported as high electrochemical activity and stability in acidic condition like low temperature fuel cells than other tungsten carbides. In this point of view, various tungsten carbides have been synthesized and analyzed including the electrochemical characterization concentrated tungsten mono-carbide (WC). In this work, tungsten carbides (WCs) which preserved the outstanding physicochemical properties were fabricated via new economic strategy specified by polymer induced carburization. The phases of tungsten carbides were controlled throughout the various synthetic pathways in polymer formation step such as refluxing and hydrothermal reaction. The polymer intermediates were calcined under inert and reducing atmosphere specified by using gases at high temperature. Conventionally, metal oxides have been utilized by the precursor for metal carbides fabricated by temperature programmed reaction (TPR) process. The physicochemical properties of metal oxides could affect to the final product, metal carbides. For 1-D nanowire types tungsten oxides were synthesized by microwave-assisted hydrothermal method, and these 1-D structures would be employed by precursor for 1-D tungsten carbides. For this unique structural tungsten oxides were made by effective strategy, and the fundamental electrochemical properties focused on the hydrogen evolution reaction (HER) were included due to the pivotal possibility to tungsten carbides.In methanol oxidation reaction (MOR), the objective was a reduction of Pt amount with tungsten carbides. Generally, WCs have played a role as booster species for methanol oxidation to generate the methoxy group not easily shown in Pt metal due to the outstanding dehydrogenation property. The generated methoxy groups were spilled over on Pt metals, and the sequent reaction was actively occurred. And also, the CO tolerance via bifunctional effect was one of the pivotal roles of WCs shown in Ru containing electrocatalysts. Thus the amount of Pt could be reduced, and Ru could be almost replaced by WCs. For these information, Pt and PtRu alloy were loaded on fabricated tungsten carbides (WCs) via chemical reduction method (CR) and polyol method (Polyol), and metal loaded WCs were evaluated for application to the methanol oxidation reaction (MOR) in initial stage. Among the same metal loaded WCs, mesoporous tungsten mono-carbide (WC) nanoparticles indicated the outstanding electroactivity in MOR analyzed by half and single cell conditions. Therefore, mesoporous WC nanoparticles were focused on the best electrocatalysts and support materials for other metals. Firstly, the single cell performances were embodied with small amount of PtRu loaded WC nanoparticles, and the mass activity was higher than that of same amount of PtRu loaded carbon electrocatalysts.For hydrogen oxidation reaction (HOR), the efforts to find out the non-Pt electrocatalysts based on the WC nanoparticles via the combination with non-Pt metals. In HOR, WC nanoparticles have shown the electroactivity. However, when Pd based metals were loaded on WC nanoparticles, the electroactivity and stability could show the comparable grades to overcome the each foible of Pd and WC with Pt/C commercial electrocatalyst which has been in state-of-the-art. The synergistic phenomenon was observed by various techniques, and the unique and grand synergistic effects for HOR were introduced with the originations. For hydrogen evolution reaction (HER), the electrolysis of water has been a strategy of hydrogen production from renewable. The efficiency of water electrolysis has depended critically on cathode materials. The most popular cathode material for HER in acidic media was Pt metal. However, high loadings of these expensive noble metals have been needed, which makes the cost of water electrolysis prohibitively high. New cathodic materials have been urgently needed to replace these noble metals with less expensive materials, or reduce their usage. Therefore, cathodic electrocatalysts would be introduced by tungsten di-carbides and 1-D tungsten oxide nanowires which could be precursors for 1-D tungsten carbides.Synthesis, material characterization, various metal loading (including the characterizations) and electrochemical investigations will be treated in regular sequence.