Background

Chapter 1 Introduction

1.1 Background

Fuel cells are of considerable interests as alternative energy-generating systems for sustainable future with reduced emission. To render the fuel cells commercially viable, it is necessary to reduce system cost and operation life time simultaneously[1, 2]. Unfortunately, conventional electrocatalysts are primarily based on precious metals and they tend to aggregate or break off during cell operations. Therefore, one particular aspect to overcome these obstacles is to fabricate electrocatalysts in desirable core-shell structures with reduced particle size, and establish uniform dispersion and solid anchoring onto suitable carbonaceous supports. In a core-shell arrangement, the inexpensive element can constitute the core while the expensive one can reside on the surface instead. In such way, the electrocatalytic activity for the shell element remains intact but the catalyst cost is expected to be reduced substantially[3-5]. Alternatively, it is suggested that the carbon surface can be deliberately functionalized so the anchoring sites for depositing ions can be increased, leading to larger catalyst loading and stronger bonding between the carbon support and active metal[6]. It is surmised that interaction like this could relieve catalyst loss or aggregation.

Among many materials investigated for fuel cell applications, the development of bimetallic PtRu nanoparticles has attracted substantial attention recently because the PtRu is not only an effective electrocatalyst for methanol oxidation reaction in direct methanol fuel cells (DMFCs) but also demonstrates impressive CO oxidation ability for reformate hydrogen fuel cells[7, 8]. In DMFCs, methanol electro-oxidation entails consecutive removal of hydrogen that leaves a CO strongly bonded to the Pt, resulting in a gradual loss of catalytic activity known as CO poisoning.

For the reformate hydrogen fuel cells, there is often minute presence of residual CO in the hydrogen feeds so it becomes a concern once the Pt is employed for hydrogen oxidation at the anode. To alleviate the CO poisoning effect, the Ru is purposely alloyed with Pt because the Ru can either

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provide the oxygenated species for CO oxidation to CO₂ (known as bifunctional model) or alter the electronic structure of Pt so the CO-Pt bond is weakened significantly (known as ligand effect)[3, 9, 10].

To prepare PtRu nanoparticle, it is established that the electrochemical pulse electroplating method allows interrupted time for mass transport so better control over composition and morphology is possible over conventional galvanostatic or potentiostatic counterparts. For example, Tsai et al. (蔡春鴻教授) has employed the pulse deposition, with the addition of chemical additives, to prepare fine PtRu on carbon nanotube surface for enhanced catalytic actions[11]. In general, many electrochemical variables can be adjusted to attain desirable deposit properties (See Fig. 1.1 as follows). However, one of the drawbacks is that there are nucleation and growth occurring in each pulse so the deposits are known to reveal a wide size distribution of particle sizes. In addition, for PtRu the replenishment of individual cations depends on their respective concentrations and diffusion coefficients. As a result, this is likely to produce unnecessary variation in deposit composition in each pulse.

Figure 1.1 Possible operation modes for electrodeposition.

Recently, X-ray absorption spectroscopy (XAS) has been established as a powerful tool to

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elucidate the detailed arrangement for Pt and Ru in nanoparticulate forms[12, 13]. It is because with light source from NSRRC (National Synchrotron Radiation Research Center), any minute variation in the absorption coefficient can be diagnosed. For example, spectra of XANES (X-ray Absorption Near Edge Structure Spectroscopy) and EXAFS (Extended X-ray Absorption Fine Structure) are routinely used to determine the oxidation state, fractional d-electron density, atomic environment of the absorbing atom, as well as its short-range ordering and geometric arrangement. In this regard, with XAS, we can follow the formation mechanism of PtRu nanoparticles and analyze Pt and Ru for both deposit and solution states. Moreover, recent studies have adopted the in-situ XAS to characterize the surface rearrangement in PtRu nanoparticles during fuel cell operation (under polarizations) so better understanding over life time performance can be established[14, 15].

Another important factor affecting the performance of electrocatalysts is the catalyst supports.

Amount many conductive materials, carbonaceous materials have been widely employed as the substrates for catalyst impregnations in room tempeaturare fuel cells[11, 16, 17]. It has been found that nanoparticulate PtRu are able to distribute uniformly, leading to reduced loading and better catalyst utilization[6]. Untreated carbon is usually hydrophobic that allows poor adsorption of catalyst precursors and active metals. After proper surface functionalizations to render a hydrophilic surface, the carbon is expected to adsorb more catalyst precursors for a larger amount of catalyst deposition. In literature, carbon functionalization involves anodization treatments in corrosive acids at moderate temperature. For example, Kangasniemi et al. imposed potentiostatic treatments on Vulcan XC72 (XC72) in 1 M H2SO4 solution, and observed a signficant oxidation for the anodizing voltage of 1.2 V for 16 h. The degree of surface functionalization also depends on the type of carbon because its surface area and microstructure differ considerably. After functionalization, surface oxidized groups such as phenols, carbonyls, carboxylic acids, ethers, quinones, and lactones have been identified¹⁷. The exact mechanism responsible for the formation of selective functional groups is contingent on the processing steps employed and the type of carbon.

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Another route to functionalize carbons is via the chemical changes of polymeric binders. In electrode fabrications, Nafion ionomer is often added in mixture with carbon, serving not only as a binder but also conductive channels for proton transports. Therefore, it is expected that the Nafion ionomer would suffer from structural alteration and loss of sulfonic acid side chains if deliberate electrochemical treatments are imposed. Previously, extensive efforts have been devoted to understand the responsible mechanism for Nafion membrane degradation in different environments and factors including humidity, temperature, and oxygen concentration are found to be critical[18, 19]. According to Bruijn et al., hydroxyl (‧OH) and peroxy (‧OOH) radicals formed during fuel cell operations are able to attack polymer end groups that still contain residual terminal H-groups[20]. Further studies also indicate that the sulfonic acid side groups are more susceptible to radical attacks than poly(tetrafluoroethylene) backbone[21]. The broken species of Nafion ionomer contain free radicals that attach to the carbon which catalyze further carbon oxidation[22]. Presence of functionalized groups has been established to catalyze additional oxidized groups. Therefore, we realize that the intentional degradation of Nafion ionomer provides a facile route for carbon support functionalization. Fig. 1.2 depicts a schematic showing the formation of functionalized groups and adsorption of selective cations.

Figure 1.2 A schematic demonstration for the functional group formation and physical adsorption of selective cations.

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An alternative route to manipulate the surface composition of PtRu is to take advantage of the displacement reaction. The displacement reaction is also known as redox-transmetallation reaction or spontaneous deposition, and it often occurs in multi-component systems with constituents revealing distinct values of redox potentials[23, 24]. In principle, when a binary deposit is in contact with their respective cations in electrolyte, the constituent of lower redox potential is dissolved from the deposit while the one with a higher redox potential is reduced from the electrolyte.

Consequently, the deposit on the surface can be tailored for a desirable makeup which is different from that of bulk if the displacement reaction is carefully controlled. For PtRu, once the Ru is immersed in the electrolyte containing Pt cations, the Ru would undergo an oxidation reaction in conjunction with the reduction of Pt cations. Previously, Adzic et al. and Huang et al. (黃炳照教授) have adopted the displacement reaction to tailor core-shell nanoparticles with impressive results[12, 25].

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