Tungsten carbide fuel cell catalysts

Shao M, Merzougui B, Shoemaker K, Stolar L, Protsailo L, Mellinger ZJ, Hsu IJ, Chen JG (2011) Tungsten carbide modified high surface area carbon as fuel cell catalyst support. J Power Sources 196(18) 7426-7434... 

To date, the catalysts for low-temperature fuel cell electrodes (phosphoric acid and alkaline cells) have been the precious metal blacks and, more recently, precious metals on carbon supports. Development of fuel cell catalysts using precious metals remains very active. Also, some work is being done on systems that may be substituted for the noble metals. For example, tungsten carbide based anode catalysts have been shown to have good durability over long periods, but they are not as active as platinum. 

Transition metal carbides, such as tungsten carbide and its alloys, tantalum carbide, titanium carbide, and molybdenum carbide (Cowling et al, 1970,1971 Voorhies et al., 1972 Scholl et ah, 1992,1994 Borup et al., 2007), have been studied as catalysts for electrochemical reactions. However, it has been found that these transition metal carbides are unstable under high potentials and in acid solution, and this limits their application as PEM fuel cell catalysts (Borup et al., 2007). Transition metal nitrides have been studied as electrochemical catalysts in PEM fuel cell environments, and Zhong et al. (2006) showed that molybdenum nitride supported on carbon powder resulted in a cell performance of about 0.3 V at 0.2 A cm, and the catalyst was stable for 60 h of cell operation. However, the long-term performance durability is still questionable. 

Carbides have been studied for ORR in acidic media.219,220 Tungsten carbide was shown to be promising for ORR in acidic media,219 though WC has a corrosion problem in acidic systems.221 To increase the stability of the catalyst in PEM fuel cell conditions, tantalum was added to tungsten carbide.220 The Ta-WC catalyst was tested under fuel cell conditions and compared to WC. The corrosion resistance was markedly improved as well as the activity for ORR. It is thought that a Ta-W alloy acted as a stabilizer for the catalyst while WC remained the active site for ORR.220... 

New catalysts of hydrogen oxidation for low-temperature fuel cells are molybdenum and tungsten carbides [2, 3], For solid polymeric fuel cells the novel catalysts by plasma treatment of polymer membrane have been developed. The radicals at surface are generated. These radicals are catalysts of anodic reactions [4]... 

Yang, X.G. and Wang, C.Y., Nano structured tungsten carbide catalysts for polymer electrolyte fuel cells, Appl. Phys. Lett., 86, 224104, 2005. 

The utility of carbide and nitride catalysts has prompted numerous studies of their reactivity that use carbide and nitride overlayers as the catalyst rather than bulk carbides or nitrides. This approach permits careful manipulation of the surface metal/nonmetal stoichiometry, which is crucial to probing reactivity. These studies consistently reveal the catalytic activity of carbide and nitride overlayers and, in several cases, the similarities between their behavior and that of noble metal catalysts. For example, the same benzene yield and reaction pathway for the dehydrogenation of cyclohexane was observed for both p(4x4)-C/Mo(110) and Pt(l 11) surfaces. Furthermore, carbon-modified tungsten may be a more desirable catalyst for direct methanol fuel cells than Pt or Ru surfaces because the transition metal carbide exhibits higher activity toward methanol and water dissociation and is more CO-tolerant. ... 

However, the important criteria for WC to be implemented in fuel cells are its surface area, phase, and porosity. Ganesan and Lee reported that WC with a surface area of 170 m /g was obtained by thermal method, but the product tuned to be containing more sub-tungsten carbide (W2C) [70]. The latter was used to support Pt catalyst for methanol oxidation reaction. No test was done for ORR. Nevertheless, authors believed that oxide layer formed on carbide support is the key player in promoting alcohol oxidation by providing oxygen species as indicated by the decrease in desorption temperature of CO. In a different study carried out by the same group, mesoporous WC was synthesized and used as a support for Pt [71]. The mesoporosity was introduced by addition of surfactant like cetyltrimethylammonium bromide (CTABr). Catalyst performance was evaluated under identical conditions as previously stated however, no statement has been reported on ORR activity and electrochemical stability in both cases [70, 71]. 

Zhang S, Zhu H, Yu H, Hou J, Yi B, Ming P (2007) The oxidation resistance of tungsten carbide as catalyst support for proton exchange membrane fuel cells. Chin J Catal 28(2) 109-111... 

Hara Y, Minami N, Matsumoto H, Itagaki H (2007) New synthesis of tungsten carbide particles and the synergistic effect with Pt metal as a hydrogen oxidation catalyst for fuel cell applications. Appl Catal A 332(2) 289-296... 

Chapter 1 discusses the current status of electrocatalysts development for methanol and ethanol oxidation. Chapter 2 presents a systematic study of electrocatalysis of methanol oxidation on pure and Pt or Pd overlayer-modified tungsten carbide, which has similar catalytic behavior to Pt. Chapters 3 and 4 outline the understanding of formic acid oxidation mechanisms on Pt and non-Pt catalysts and recent development of advanced electrocatalysts for this reaction. The faster kinetics of the alcohol oxidation reaction in alkaline compared to acidic medium opens up the possibility of using less expensive metal catalysts. Chapters 5 and 6 discuss the applications of Pt and non-Pt-based catalysts for direct alcohol alkaline fuel cells. 

Figure 4.45. Cyclic voltammogram of 1 M formate (A) and lactate (B) on WC/graphite foil electrode at pH 5 in 0.1 M KCl at 293 K. Catalyst load 20 mg cm. The electrode potential is given vs. Ag/AgCl (SSCE.) Inset figures show the eurrent density measured at 0.2 V vs. SSCE as a function of anolyte concentration [217]. Dashed curve = blank electrolyte, solid curve = electrolyte with formate (A) or lactate (B). (Reproduced from Applied Catalysis B Environmental, 74(3-4), Rosenbaum M, Zhao F, Quaas M, WulffH, Schroder U, Scholz F, Evaluation of catalytic properties of tungsten carbide for the anode of microbial fuel cells, 261-9, 2007, with permission from Elsevier.)...

Non-Pt metal-based catalysts can be used as both anode and cathode components. The addition of tungsten carbide has been reported to improve the catalytic ability of some non-Pt catalysts. Izhar and coworkers (2009) investigated carbon-supported cobalt-tungsten and molybdenum-tungsten carbides and their activities as anode catalysts, using a single fuel cell and half-cell RDE. The maximum power densities of their 873 K-carburized CoWC/KB and MoWC/KB were 15.7 and 12.0 mW cm 2, respectively, which were 14% and 11%, compared to a 20 wt% Pt/C catalyst. 

Tungsten carbide, mentioned in Section 12.5, is a good catalyst for hydrogen oxidation, but in the presence of oxygen, oxide layers are formed, blocking the surface, and therefore it cannot be used in mixed-reactant-supply fuel cells. 

The catalysts tungsten carbide (WC) [272-274] and carbon [275-279] for electrodes working in acidic electrolyte were studied in flie topics of fuel cells. 

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