PtSn carbon-supported

CHAPTER 15. Carbon-Supported Core-Shell PtSn Nanopartieles ... 

Carbon-Supported Core-Shell PtSn Nanoparticles Synthesis, Characterization and Performance as Anode Catalysts for Direct Ethanol Fuel Cell... 

In this chapter, two carbon-supported PtSn catalysts with core-shell nanostructure were designed and prepared to explore the effect of the nanostructure of PtSn nanoparticles on the performance of ethanol electro-oxidation. The physical (XRD, TEM, EDX, XPS) characterization was carried out to clarify the microstructure, the composition, and the chemical environment of nanoparticles. The electrochemical characterization, including cyclic voltammetry, chronoamperometry, of the two PtSn/C catalysts was conducted to characterize the electrochemical activities to ethanol oxidation. Finally, the performances of DEFCs with PtSn/C anode catalysts were tested. The microstmc-ture and composition of PtSn catalysts were correlated with their performance for ethanol electrooxidation. 

CARBON-SUPPORTED CORE-SHELL PtSn NANOPARTICLES... 

FIGURE 15.9. Performance comparison of RSn anode based direct ethanol fuel cells at 90°C. Anode catalysts Carbon supported PtSn with a R loading of 1.5 mg/cm, ethanol concentration 1.0 mol/L, flow rate 1.0 mL/min. Cathode (20 Pt wt.%, Johnson Matthey Inc.) with a R loading of 1.0 mg/cm, Pq2 = 2 bar. Electrolyte Naflon -115 membrane. 

The impregnation-reduction method has been frequently used for the synthesis of PtSn supported on inorganic carriers such as SiOg, AlgOs, or SAPO, but this approach has rarely been employed for the synthesis of carbon supported electrocatalysts. ° In general, the metal content in those samples is ca. 1-2 wt%, well below the demands of a state of the art fuel-cell electrocatalyst. A number of routes have been explored for the synthesis of carbon supported bimetallic PtSn samples. In general, they lead to materials composed of a wide range of phases, such as metallic and/or oxide Pt, Sn oxides, or PtSn solid solutions of different stoichiometry. 

Typical results were obtained with a membrane-electrode assembly (MBA) consisting of a Nafion 117 membrane on which are pressed the anodic electrocatalysts (Pt or Pt-based catalysts dispersed on a high surface area carbon support) and the cathodic catalyst (usually Pt/C with metal loading from 40 % to 60 %). An example of the electrical characteristics of a DEFC with Pt/C, PtSn/C or PtSnRu/C anode catalysts is given in Fig. 2. 

Figure 4.10. DMFC polarization curves using PtSn anode catalysts produced by a microwave-assisted polyol method. Anode catalyst load 4 mg cm , 2 M CH3OH with 2 ml min cathode Pt/C 3 mg cm , O2 pressure not specified in the original source, O2 flow rate 500 cm min 353 K [85]. (Reproduced from Electrochemistry Communications, 8(1), Liu Z, Guo B, Hong L, Lim TH, Microwave heated polyol synthesis of carbon-supported PtSn nanoparticles for methanol electrooxidation, 83-90, 2006, with permission from Elsevier.)...

Recent results with PtSn/C obtained by the Tekion Inc. group demonstrated stable operation over extended periods of time at various temperatures [165], confirming previous results in this area by Adzic and co-workers [36]. Figure 4.31 exemplifies the half-cell performance of Pt/Sn at 100 mA cm". Furthermore, it shows that for the same metal and ionomer load in the catalyst layer (3 mg cm and 0.75 mg cm Nafion, respectively) and an identical atomic ratio of Pt Sn = 4 1, the catalyst with 25 wt% metal dispersion on the support gave about 25 mV lower anode potential [165]. Thus, in this case it is likely that the interface between PtSn/Nafion/carbon support controlled the overall anode performance, affecting the catalyst utilization. These aspects are further detailed in Section 4.3 of this chapter. 

In the case of a direct formic acid fuel cell equipped with Ti mesh anode support, Chetty and Scott carried out a comprehensive comparative investigation of Pd and PtSn catalysts prepared by either thermal or electrochemical deposition [309]. Generally, PtSn/Ti mesh performed better than Pd/Ti mesh the maximum power output for each at 333 K using 1 M HCOOH was about 20 mW cm and 37 mW cm, respectively. It is noteworthy that according to this study the Ti mesh-supported PtSn gave about three times higher peak power density than the GDE with commercial carbon supported PtSn [309]. Furthermore, the performance of the three-dimensional anode improved with formic acid concentration up to 7 M, and excellent catalyst stability was observed during 72 h of continuous operation. 

The PEDOT-PSS composite support was employed in conjunction with Pt, PtSn, and PtPb for ethanol electrooxidation catalysis, showing the expected beneficial effect of Sn [329]. No comparison was provided with conventional carbon-supported or unsupported catalyst layers therefore, the effectivness of the support cannot be judiciously analyzed. 

Liu Z, Guo B, Hong L, Lim TH. Microwave heated polyol synthesis of carbon supported PtSn nanoparticles for methanol electrooxidation. Electrochem Commun 2006 8 83-90. 

Colmati F, Antolini E, Gonzalez ER. Ethanol oxidation on a carbon-supported PtSn electrocatalyst prepared by reduction with formic acid. J Electrochem Soc 2007 154 B39 7. 

Crabb EM, Marshall R, Thompsett D. Carbon monoxide electro-oxidation properties of carbon-supported PtSn catalysts prepared using surface organometallic chemistry. J Electrochem Soc 2000 147 4440-7. 

In the case of PtSn catalysts, no evidence of a ligand effect was observed from an in situ FTIR study on Pt3Sn(l 10) bulk alloy and PtSn nanoparticles supported on carbon. It was proposed that the bifunctional mechaiusm was mainly involved in the oxidation process. The fact that the transition from positive to negative Stark shift of infrared v(CO) frequency during CO oxidation was much more pronoimced on a PtSn/C catalyst than on the Pt/C catalyst was interpreted in terms of the different ways in which OHads (necessary to oxidize CO) nucleates on each catalyst. 

Oliveira Neto et al. [28] have investigated a series of PtRu/C, PtSn/C, and PtSnRu/C catalysts prepared by the alcohol-reduction method using an ethylene glycol/water solution. Particle sizes in the order of 2.7nm were achieved for the PtSn/C composition, which displayed catalytic activity close to 8.0 A gpt . Spinace et al. [60] have used the same method for the production of PtSn/C, PtRh, and PtSnRh/C catalysts. Their results were similar to those described in [28], i.e., small particle size (2.0nm), and uniform particle distribution on the carbon support, which culminated in significant catalytic activity for ethanol electro-oxidation. 

Colmati and co-workers [106] have examined how experimental conditions affect Pt75Sn2s/C catalysts prepared by the formic acid method. Prior to heat treatment, particle sizes of 4.5 nm and experimental compositions close to the nominal one were achieved, but these particles presented low power density, i.e., 20 mW cm" for ethanol oxidation in a PEM-DEFC. Antolini et al. [24] have looked into the effect of introducing ruthenium into PtSn/C catalysts. Again, the desired catalytic composition and homogeneously distribution small particles (3.5 nm) on the carbon support were obtained, but the power density reported for DEFC was relatively low (28mWcm ) for a PEM-DEFC. 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

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