Anode catalysts for DMFC

Over the decades, researchers have been eagerly searching for alternative energy sources for solving the energy crisis. The urgent need originates from high-energy demands, fossil fuel depletion, environmental pollution, etc. Among manoy technologies under development, fuel cells have become one of the candidates that provide an efficient and clean energy source. There are various sorts of fuel cells. In particular, a promising type known as direct methanol fuel cell (DMFC) has received considerable attention due to its favorable characteristics with respect to fuel usage and feed strategies [1-2].

It is well known that the key materials that greatly affect the performance of a DMFC are the membrane and electro-catalyst. Unfortunately, severe challenges are present for the electro-catalyst such as the mass activity, reliability, durability, and cost reduction. The improvement of anode catalyst for methanol oxidation reaction (MOR) has been accomplished with the exploration of noble and non-noble electrochemical active materials. Compared with non-noble catalysts, noble metal catalysts such as Pt, demonstrates reasonable electrochemical activity and stability. For the objective of cost reduction and performance improvement, the strategy has shifted to identifying alloys with comparable performance. To date, PtRu alloys have emerged as the leading material that seem to be the state-of-the-art binary anode catalyst.

Pure Pt atom goes through a series of reactions for MOR [3], including (1) methanol adsorption; (2) C-H bond activation (methanol dissociation); (3) water adsorption; (4) water activation; and (5) CO oxidation. Though out the reaction process, high potential is required to form OH by activating water on the Pt surface which is necessary for the oxidative removal of adsorbed CO. If not, the CO is considered to be a poisoning substance among the intermediates in MOR that occupy the active sites of Pt. However, undesirable high potential limits the performance of Pt for

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MOR. Therefore, the introduction of complementary metal which provides the oxidative removal of adsorbed CO at lower potential becomes necessary and which leads to the development of Pt-based alloys. To date, PtRu was found to be the outstanding binary alloy among PtOs, PtSn, PtW, PtMo, etc. The enhancement of PtRu catalyst compared with pure Pt for MOR can be attributed to both bi-functional mechanism [4] and ligand effect [5]. Ru induces influences in the electronic structure of Pt which weakens the Pt－CO bond [5]. The bi-functional mechanism implies that the oxygen containing species are adsorbed on the Ru atoms at a lower potential. Therefore, the mechanism of oxidative removal of CO to CO2 is summarized below [4]:

Pt + CH3OH  PtCO_ads + 4H⁺ + 4e^- (1)

Ru + H₂O  Ru(OH)_ads + H⁺ + e^- (2)

PtCO_ads + Ru(OH)_ads  CO₂ + Pt + Ru + H⁺ + e^- (3)

The catalytic activity of PtRu is greatly influenced by the composition, atomic structure, the degree of alloying, morphology, and particle size. Also, the fabrication methods for PtRu catalysts show specific manipulation of the properties based on the characteristics of each method. There are three principal methods for preparing carbon supported PtRu catalysts. They are impregnation method, the colloidal method, and the mrcroemulsion method. Generally, all methods involve chemical procedures for forming nanoparticles, followed by deposition on the carbon supports with uniform dispersion. The schematic diagram in Figure 1 presents the flow chart for each fabrication methods.

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Figure 1. The synthesis methods for carbon supported PtRu catalysts: (1) impregnation method, (2) colloidal method, and (3) microemulsion method [6].

The impregnation method is the most widely used method among the rest due to its simplicity and efficiency [5, 7-8]. First, the Pt and Ru precursors are mixed together with carbon black in aqueous solution for form a homogeneous mixture. Then, the chemical reduction step is carried out by addition of reducing agents such as Na2S2O3, Na4S2O5, NaBH4, N2H4, or formic acid insolution.

Alternatively, the reduction process can be conducted with hydrogen at elevated temperature.

Synthetic conditions including the nature of the metal precursors, reduction methods, and heating temperature all play important roles in impregnation process [9-11]. The major weakness of

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impregnation method is the difficulty of controlling nanoparticle size and distribution. Moreover, particle segregation on the carbon supports is another frequently encountered issue.

The colloidal method is another preparation method for carbon supported PtRu catalyst [12-14].

Common steps are: (1) preparation of PtRu colloids; (2) deposition of such colloids onto the carbon support; (3) and chemical reduction of the mixture. For example, a well-established colloidal method was proposed by Watanabe et al. in which co-deposition of colloidal Pt and Ru oxides on carbon in aqueous media was conducted, followed by a reduction with bubbling hydrogen [15]. The metal oxide colloid route offers better specific surface area of PtRu compared with that of conventional impregnation method. However, the control over particle growth and agglomeration are regarded as the weakness of this method.

The microemulsion method is a relatively new route compared with previous ones [16-18]. In this method, the PtRu nanoparticles are formed through a water-in-oil microemulsion reaction, followed by a reduction step. The microemulsion is a nano grade droplet of aqueous noble metal precursor. The droplets are capsuled by surfactant molecules and dispersed uniformly in an immiscible organic solvent. The reduction step can be performed by adding a reducing agent into the microemulsion system or mixing it with another microemulsion system containing suitable reducing agents. Therefore, the reduction reaction is confined in the nano-grade droplet, so that the particle size can be easily controlled by the magnitude of the microemulsion size. The surfactant around the droplet serves as a protection from the agglomeration of PtRu nanoparticles. The

removal of the surfactant molecules can be achieved by proper heat treatment. The main advantage of microemulsion method is the ease to control metallic composition and particle size within a narrow distribution by optimized fabrication conditions.