Electrocatalysts for DMFC

Strasser, P., Fan, Q., Devenney, M., Weinberg, W. H., Combinatorial Exploration of ternary fuel cell electrocatalysts for DMFC anodes — a comparative study of PtRuCo, PtRuNi and PtRuW systems, AIChE fall meeting, 2003, San Francisco. 

Lu Y, Reddy R (2007) The electrochemical behavior of cobalt phthalocyanine/platinum as methanol-resistant oxygen-reduction electrocatalysts for DMFC. Electrochim Acta 52(7) 2562-2569... 

Antolini E, Salgado IRC, Gonzalez ER (2006) The methanol oxidation reaction on platinum alloys with the first row transition metals the case of Pt-Co and -Ni alloy electrocatalysts for DMFCs a short review. Appl Catal Environ 63 137-149... 

An important factor affecting the performance of DMFCs is the kinetics of catalyst. Platinum (Pt/C) is the most effective catalyst for oxygen reduction reaction but it is not selective towards ORR in presence of methanol. The addition of yttrium to Pt increases the ORR activity and are promising ORR electrocatalyst [207]. Carbon supported PtY(OH)3 hybrid catalyst are developed with dynamic spillover of metal oxide [208]. Recently, catalyst for DMFC Pt Pd/C NP was prepared by the galvanic displacement reaction between Pt and Pd. A simple synthesis strategy was followed to prepare carbon based [209] and carbon-supported Pd nanostructure [190]. A higher methanol tolerance of Pt Pd/C with less Pt content than Pt/C suggests that it is potential alternative cathode electrocatalyst for DMFCs [190]. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

The mechanism of electrooxidation of methanol is now nearly well understood. From the considerable effort made during the past 20 years, it is now possible to propose electrocatalysts with acceptable activities for DMFCs, even though further improvement is still necessary. Despite considerable research efforts, R-Ru alloys are the only acceptable catalysts for the electrooxidation of methanol at low anode potentials. Two questions still remain unanswered ... 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

Table 4 and Fig. 18 illustrate the performance levels achieved by the active players in DMFC R D. The main goal in DMFC research in the U.S. and European programs is to achieve a stable performance level of 200 mW/cm at a cell potential of 0.5 to 0.6 V. It is because of the relatively low activity of the electrocatalyst for methanol electrooxidation that this power level is less than half that of a PEMFC with Hj as a fuel. A higher power level of the DMFC is essential for a transportation application, but the present power level goal is quite adequate for small portable power sources. 

This chapter presents the design and application of a two-stage combinatorial and high-throughput screening electrochemical workflow for the development of new fuel cell electrocatalysts. First, a brief description of combinatorial methodologies in electrocatalysis is presented. Then, the primary and secondary electrochemical workflows are described in detail. Finally, a case study on ternary methanol oxidation catalysts for DMFC anodes illustrates the application of the workflow to fuel cell research. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

Carbon aerogels and xerogels have been used as supports for Pt and Pt-based electrocatalysts for proton-exchange membrane fuel cells (PEMFCs), also known as polymer-electrolyte fuel cells [56,58,83-90], These fuel cells are convenient and environmentally acceptable power sources for portable and stationary devices and electric vehicle applications [91], These PEMFC systems can use H2 or methanol as fuel. This last type of fuel cell is sometimes called a DMFC (direct methanol fuel cell). 

Case Study 1 Pt/Ru Carbon Three types of 30 wt.% PtsoRuso/Vulcan XC 72 electrocatalysts for Direct Methanol Oxidation Fuel Cells (DMFC) were prepared using Nl BetsH (Cat. 1), LiBetsH (Cat 2.), and Al(Me)3 (Cat 3.) for the co-reduction of Pt- and Ru-salts [169] ... 

Wang CH, Du HY, Tsai YT, Chen CP, Huang CJ, Chen LC, Chen KH, Shih HC (2007) High performance of low electrocatalysts loading on CNT directly grown on carbon cloth for DMFC. J Power Sources 171 55-62... 

Franceschini EA, Bruno MM, Viva FA, Williams FJ, Jobbagy M, Corti HR (2012) Mesoporous Pt electrocatalyst for methanol tolerant cathodes of DMFC. Electrochim Acta 71 173-180... 

Palladium-alloy materials have been recently introduced as a promising ORR cathode electrocatalyst for replacing Pt [91-98], Pd alloys are considerably less expensive than Pt. Besides, experimental studies indicate that they have high methanol tolerance for direct methanol fuel cells (DMFCs) in which the methanol crossover to the cathode significantly decreases the cell s efficiency [99, 100]. 

Li X, Chen G, Xie J, Zhang L, Xia D, Wu Z (2010) An electrocatalyst for methanol oxidation in DMFC PtBi/XC-72 with Pt solid-solutitm structure. J Electrochem Soc 157(4) B580-B584... 

Direct methanol fuel cells (DMFCs) are attracting much more attention for their potential as clean and mobile power sources for the near future [1-8], Generally, platinum (Pt)- or platinum-alloy-hased nanocluster-impregnated carbon supports are the best electrocatalysts for anodic and cathodic fuel cell reactions. These materials are veiy expensive, and thus there is a need to minimize catalyst loading without sacrificing electro-catalytic activity. Because the catalytic reaction is performed by fuel gas or fuel solution, one way to maximize catalyst utilization is to enhance the external Pt surface area per unit mass of Pt. The most efficient way to achieve this goal is to reduce the size of the Pt clusters. 

The synergistic effect between Pt and WC was demonstrated with Pt/WC powder catalysts. Thus, 7.5 wt% PtA 2C led to high specific activity of 156 mA/cm at 0.75 V (vs Ag/AgCl) in half cell test for methanol oxidation without Ru, which was commonly used with Pt for DMFC anodes (33). This specific activity represents higher electroactivity than that of commercial Pt-Ru/C electrocatalyst (20 wt% PtRu/C, E-Tek) imder the same experimental condition. Further improvement in the activity of methanol oxidation was observed when W2C was prepared to contain mesopores (34). There is also a report that 20 wt% Pt(W2C electrocatalysts show a higher mass current density (mA/mgpt) than 20 wt% Pt(C at the same potential under 0.5 M H2SO4 electrolyte, SCE reference electrode (155). 

Figure 4.11. The effect of various binary anode catalyst formulations on DMFC polarization PtNi and Pt/macrocycles (Ni, Co, Mn, and Rh). Anode Pt load 0.4 mg cm 1 M CH3OH, 1 ml min cathode operated with dry O2 at atmospheric pressiure 368 K [87]. (With kind permission from Springer Science+Business Media Joimial of Applied Electtochemistry, Composite electrocatalysts for anodic methanol and methanol-reformate oxidation, 31, 2001, 811-17, Goetz M, Wendt H, figme 4.)...

Electrocatalysts for application in low-temperature fuel eells (ineluding PEMFCs and DMFCs) constitute a special type of heterogeneous eatalyst. The most important difference between an electrocatalyst and a normal heterogeneous catalyst is that the former should have good conductivity, whereas most typieal heterogeneous catalysts are insulators therefore, most characterization techniques for electrocatalysts are the same as for regular heterogeneous catalysts, but some special techniques are required for electrocatalysts because of their conductivity. ... 

Another obstacle for DMFC applications is the low catalytic activity of electrodes for both the oxygen reduction reaction and the methanol oxidation reaction. It is well known that the catalytic activity of an electrocatalyst is strongly dependent on the particle dispersion of the active components. Many doping techniques have been explored to widely distribute the active components on the catalyst supports [145-148]. In addition to the synthesis method, catalyst support also plays an important role in the dispersion of active components. Carbon materials with high surface areas (e.g., Vulcan XC72 carbon black) have been widely employed as electrocatalyst supports to enhance the dispersion of metal nanoparticles and thus to increase the utilization of the precious metal eatalyst... 

Joo SH, Pak C, You DJ, Lee SA, Lee HI, Kim JM, et al. Ordered mesoporous carbons (OMC) as supports of electrocatalysts for direct methanol fuel cells (DMFC) effect of carbon precursors of OMC on DMFC performances. Electrochim Acta 2006 52 1618-26. 

Combinatorial methods have also been applied to the area of exploring methanol-tolerant Pt and non-Pt alloy eatalysts for DMFC cathodes. Liu et al. investigated a series of Pt-based and non-Pt binary alloys as ORR electrocatalysts by employing the high-throughput optical screening method. The catalyst arrays were prepared... 

Due to the faeile poisoning effect of CO on Pt, many Pt-based binary alloys, such as Pt-Ru, Pt-Os, Pt-Sn, Pt-W, Pt-Mo, and so on, have been investigated as electrocatalysts for the methanol oxidation reaction (MOR) on flic DMFC anode. Among them, the Pt-Ru alloy has been found to be the most active binary alloy catalyst, and is commonly used in state-of-the-art DMFCs [32]. 

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