dc.description.abstract | Direct ethanol fuel cells (DEFCs are considered more and more important on mobile and portable applications. Ethanol is environmental friendly and can be massively produced by biomass or agriculture. Moreover, the byproduct of the chemical reaction in DEFCs is pure water. Therefore, DEFCs are regarded as the potential green technology for future energy systems. However, significant challenges in the development of DEFC technology are limited by the sluggish kinetic of ethanol oxidation reaction (EOR) and high price of noble metal catalysts. Therefore, in order to enhance EOR, development of high performance catalysts by means of manipulation of reducing temperatures, dealloying, heat treatments, and oxide promoter have been elucidated systematically.
In this study, Pd-Cu/C (with an atomic ratio of 3:1) electrocatalysts are prepared by the deposition–precipitation (DP) method using H2 as the reducing agent. SnO2 modified Pd-Cu/C catalysts are prepared by the two-steps reduction method. On the other hand, dealloying process and heat treatments have been applied for Pd-Cu/C catalysts to alternate structure and/or surface oxidation states and modify the EOR performance systematically. The activity-structure, morphology, surface species and electroactivity of the Pd-Cu/C and SnO2-modified Pd-Cu/C catalysts are characterized by the thermal gravimetric analysis (TGA), inductively coupled plasma-atomic emission spectrometer (ICP-AES), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS) measurements, and cyclic voltammetry (CV) measurements, respectively. For the Pd-Cu/C catalysts, it is observed that, If/Ib ratio, degree of alloying (Dalloy), and grain size increases as the reducing temperature increases. However, severe sintering deteriorates the EOR performance. Therefore, 570 K is a suitable reducing temperature for preparation of Pd-Cu/C catalysts with fine alloy structure, small crystalline size, good anti-poisoning ability and excellent EOR performance.
During dealloying route, the electrochemical surface area (ECSA) of as-reduced Pd-Cu/C catalysts can be enhanced after 5 cycles, however, their EOR activity is only slightly promoted. After dealloying of 25 cycles, the crystallinity of PdCu alloy becomes severely weak, thus degrading the EOR performance. For the heat-treated Pd-Cu/C catalysts, H2 treatment deconstructs the Pd-Cu alloy phase, and thereby decreasing the Dalloy and surface PdOx composition. On the contrary, O2 treatment causes the significant enrichment of amorphous PdOx on the surface and crystalline PdO in the bulk, which do not benefit EOR. The CO treatment optimizes the Dalloy and surface PdOx composition of the catalysts, thereby enhancing their CO oxidation and EOR performance.
For the SnO2 modified Pd-Cu/C catalysts, 10 wt % of Sn addition promotes the chronoamperometric (CA) performance, attributed to the significant amount of PdSn phase in the bulk and SnO2 species on the surface, which enhance their CO oxidation and ECSA. On the other hand, 20 wt % Sn additions may cause a great amount of SnO2 on the surface and block the active site of the catalysts, thus retarding the EOR performance. Consequently, the Dalloy, particle size, surface oxidation state, surface compositions, and structures of catalysts are significant conditions for EOR.
In this research, the CO heat-treated Pd-Cu/C catalyst has the best EOR activity which is enhanced about 190 % when compared with the as-reduced sample. In terms of SnO2 modified Pd-Cu/C catalysts, 10 wt % of Sn modified Pd-Cu/C catalysts containing moderate PdSn phase and surface SnO2 display high ECSA, CO-oxidation and chronoamperometric activity, which is enhanced about 230 % when compared with the as-reduced sample.
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