Anode catalyst

In this study, we investigated the effects of anode catalyst loading and oxidant on the performance of DFAFC at various temperatures to better understand the significance of anode catalyst in a DFAFC system. 

Fig. 4. Effect of anode catalyst loading on the DFAFC performance.

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

Development of Anode Catalyst for Internal Reforming of CH4 by CO2 in SOFC System... 

In this work, the catalytic reforming of CH4 by CO2 over Ni based catalysts was investigated to develop a high performance anode catalyst for application in an internal reforming SOFC system. The prepared catalysts were characterized by N2 physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). 

The Ni based anode catalysts were prepared by a physical mixing method. NiO (99.99%, Sigma-Aldrich Co.), YSZ (TZ-8Y, TOSOH Co.), MgO (98%, Nakarai Chemical Co.) and Ce02 (99.9%, Sigma-Aldrich Co.) were used as raw materials. The physically mixed catalyst... 

To reduce the formation of carbon deposited on the anode side [2], MgO and Ce02 were selected as a modification agent of Ni-YSZ anodic catalyst for the co-generation of syngas and electricity in the SOFC system. It was considered that Ni provides the catalytic activity for the catalytic reforming and electronic conductivity for electrode, and YSZ provides ionic conductivity and a thermal expansion matched with the YSZ electrolyte. 

Figure 3 and 4 show SEM images for the surface of anode catalyst and the cross section of ESC (NiO-YSZ-CeOa I YSZ I (LaSr)Mn03), respectively. The micro structure of the catalyst electrode was characterized by SEM (Hitachi Co., S-4200). The morphology of particles over Ni0-YSZ-Ce02 was uniformly distributed. 

Fig. 3. SEM image for the surface of anode catalyst in ESC (NiO-YSZ-CeOa I YSZ I (LaSrlMnOj).

The internal reforming of CH4 by CQzin SOFC system was performed over an ESC (electrolyte st rported cell) prepared with Ni based anode catalysts. Figure 5 diows the performance of voltage and power density with current density over various ESC (Ni based anodes I YSZ (LaSr)Mn03) at SOOC when CH4 and CO2 were used as reactants. To improve the contact between single cell and collector, different types of SOFC reactor were used [5]. In the optimized reactor (C), it was found fliat die opai-... 

Design parameters of the anode catalyst for the polymer electrolyte membrane fiiel cells were investigated in the aspect of active metal size and inter-metal distances. Various kinds of catalysts were prepared by using pretreated Ketjenblacks as support materials. The prepared electro-catalysts have the morphology such as the sizes of active metal are in the range from 2.0 to 2.8nm and the inter-metal distances are 5.0 to 14.2nm. The electro-catalysts were evaluated as an electrode of PEMFC. In Fig. 1, it looked as if there was a correlation between inter-metal distances and cell performance, i.e. the larger inter-metal distances are related to the inferior cell performance. 

Significant (and even spectacular) results were contributed by the group of Norskov to the field of electrocatalysis [102-105]. Theoretical calculations led to the design of novel nanoparticulate anode catalysts for proton exchange membrane fuel cells (PEMFC) which are composed of trimetallic systems where which PtRu is alloyed with a third, non-noble metal such as Co, Ni, or W. Remarkably, the activity trends observed experimentally when using Pt-, PtRu-, PtRuNi-, and PtRuCo electrocatalysts corresponded exactly with the theoretical predictions (cf. Figure 5(a) and (b)) [102]. 

However, the Pt anode is seriously poisoned by trace amounts of carbon monoxide in reformates (fuel gas reformed from hydrocarbon), because CO molecules strongly adsorb on the active sites and block the HOR [Lemons, 1990 Igarashi et ah, 1993]. Therefore, extensive efforts have been made to develop CO-tolerant anode catalysts and cell operating strategies to suppress CO poisoning, such as anode air-bleeding or pulsed discharging. 

This is the first experimental demonstration of changes in the strength of CO adsorption at Pt-based alloy electrodes. Nprskov and co-workers theoretically predicted a similar linear relation between changes in ads(CO) and shifts in the (i-band center [Hammer et al., 1996 Hammer and Nprskov, 2000 Ruban et al., 1997]. Because the Pt4/7/2 CL shift due to alloying can be more easily measured by XPS than the li-band center can, this should be one of the most important parameters to aid in discovering CO-tolerant anode catalysts among Pt-based alloys or composites. 

Fujino T. 1996. Development of anode catalysts for PEFCs. MEng Thesis, Yamanashi University. 

Wakisaka M, Mitsui S, Hirose H, Kawashima K, Uchida H, Watanabe M. 2006. Electronic structures of Pt-Co and Pt-Ru alloys for CO-tolerant anode catalysts in polymer electrol3de fuel cells studied by EC-XPS. J Phys Chem B 110 23489-23496. 

Watanabe M, Igarashi H, Fujino T. 1999. Design of CO tolerant anode catalysts for polymer electrolyte fuel cell. Electrochemistry 67 1194-1196. 

Dinh FIN, Ren X, Garzon FTF, Zelenay P, Gottesfeld S. 2000. Electrocatalysis in direct methanol fuel cells in-situ probing of FTRu anode catalyst surfaces. J Electroanal Chem 491 ... 

Lei HW, Suh S, Gurau B, Workie B, Liu R, Smotkin ES. 2002. Deuterium isotope analysis of methanol oxidation on mixed metal anode catalysts. Electrochim Acta 47 2913-2919. 

Vigier F, Coutanceau C, Perrard A, Belgsir EM, Lamy C. 2004b. Development of anode catalysts for a direct ethanol fuel cell. J Appl Electrochem 34 439-446. 

Zhou WJ, Li WZ, Song SQ, Zhou ZH, Jiang LH, Sun GQ, Xin Q, Poulianitis K, Kontou S, Tsiakaras P. 2004a. Bi- and tri-metaUic Pt-based anode catalysts for direct ethanol fuel cells. J Power Sources 131 217-223. 

Diemant T, Hager T, Hosier HE, Rauscher H, Behm RJ. 2003. Hydrogen adsorption and coadsorption with CO on well-defined himetallic PtRu surfaces—A model study on the CO tolerance of himetallic PtRu anode catalysts in low temperature polymer electrolyte fuel cells. Surf Sci 541 137. 

Rao V, Simonov PA, Savinova ER, Plaksin GV, Cherepanova S V, Kryukova GN, Stimming U. 2005. The influence of carbon support porosity on the activity of PtRu/Sibunit anode catalysts for methanol oxidation. J Power Sources 145 178-187. 

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