Study on Surface-Modified Alloys and Core-Shell Catalysts for PEM Fuel Cells

Abstract

Fuel cells are an attractive device that directly converts from chemical energy to electric energy. Depending on the fuels and operating temperature, fuel cells can be divided in solid oxide fuel cell, molten carbonate fuel cell, proton exchange membrane fuel cell (PEMFC), alkaline fuel cell, and micro fuel cell. Among of these, PEMFCs are used in widely commercial field from stationary generator to vehicle and small size mobile device. However, there are many problems to be solved for the commercialization of PEMFCs. These problems can be divided in chemical and mechanical part. The chemical part is to maintain the efficiency and long-term durability of electrochemical reaction occurring at the catalyst surface. In previous studies, the study of the electrochemical activity and long-term durability through changes of catalyst surface composition and structure was conducted. The composition and stability effects of modified-surface on the formic acid oxidation reaction (FOR) at anode are investigated using the carbon-supported alloy catalysts through CO-induced segregation method. An electrochemical catalyst for the oxygen reduction reaction (ORR) occurring at cathode is discussed about the activity and long-term stability in core-shell structure. 1) To expose a selective material on modified-surface of Pd-based alloy catalyst, and 2) to control the Pt shell layer on Pd-based alloy core through chemical reduction method. Carbon supported Pd3Au nanoparticle catalysts were synthesized via chemical reduction. Surface segregation of Pd in Pd3Au catalyst was achieved via heat treatment under air, Ar, CO-Ar and CO atmospheres, in order to obtain surface with changed structures and composition. The surface composition was analyzed by electrochemical methods, and the Pd surface composition was observed to increase from 67.2 % (air (asp) sample) to 80.6 % (CO sample) after heat treatment under a CO atmosphere. The CO induced surface segregation of the Pd3Au electrocatalyst resulted a significantly improved formic acid oxidation and stability compared to the sample prepared in air, as well as increased mass activity, electrochemical surface area (ESAPd) and specific activity. The electrochemical activities were significantly increased because of changes in the structure and composition of the surface due to the increased surface ratio of Pd promoted by CO heat treatment. The exchange of Pd and Au atoms between the surface and the bulk material was observed to influence formic acid oxidation and stable performance. The CO induced surface segregation has the potential to greatly enhance the electrochemical activities and surface control of Pd-Au alloy nanoparticle with lower Pd contents. Various Pt shell layers on prepared Pd-based alloy core were synthesized via chemical reduction method. Pt shell layers were accomplished to control on Pt amounts for 1, 2, and 3 layers, respectively. The shape and elements distribution of core-shell structure were demonstrated through the line profile and element mapping using Cs-STEM, and then Pt shell layer thickness was calculated by formula of the changed particle size between core and core-shell on TEM images. The structural and electronic property of core-shell nanocatalysts depending on shell layers were studied by high resolution XRD and XPS analysis. The activity and durability of core-shell nanocatalysts was analyzed by electrochemical method. The accelerate durability test (ADT) was conducted from 0.6 V to 1.0 V during 10,000 cycles, and the mass and specific activity after ADT was resulted in stable performance on core@Pt[2]/C sample. In addition, the great performance before and after ADT was shown by core@Pt[2]/C sample compared with commercial and other samples. Importantly, these optimized Pt usages will be significantly contributed to PEMFC commercialization.