Platinum-based electro-catalysts

With regard to the electro-catalyst the main research issue is to identify a platinum-based catalyst, i.e. a binary, ternary or quaternary catalyst composed of platinum and one or more transition metals that will be more active (and thereby further reducing the applied potential), exhibit an improved lifespan, and have reduced platinum loadings to reduce the cost. The NWU, located in the North-West province of South Africa where the majority of the world s platinum is mined and produced, is currently setting itself up for the synthesis, characterisation and testing of platinum-based electro-catalysts specifically for normal water electrolysis as well as for S02 electrolysis. 

The oxygen reduction reaction (ORR) is the primary electrochemical reaction occurring at the cathode of a PEMFC, and is central to this promising technology for efficient and clean energy generation. The ORR is a multi-electron reaction that follows the direct four-electron mechanism on platinum-based electro-catalysts. It appears to occur in two pathways in acid electrolytes (Adzic and Lima, 2009) ... 

While it has been known for many years that CO poisons Pt-based electro-catalysts, it has only recently been discovered that CO2 also acts as a poisoning species in the PEMFC. To certain degree, CO2 itself is recounted inert on platinum. Therefore, deviafions of cell volfage from Nernst behavior is expecfed wifh CO2 as fhe fuel diluter [60]. For dilution effect, the impact of CO2 is limited. We have tested the polarization curves for a PEMFC single cell exposed to H2 containing various levels of CO2 (see Figure 4.15). 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

The electro-catalytic oxidation of hydrogen, and reduction of oxygen, at carbon supported platinum based catalysts remain essential surface processes on which the hydrogen PEM fuel cell relies. The particle size (surface structure) and promoting component (as adsorbate or alloy phases) influence the activity and tolerance of the catalyst. The surface chemical behavior of platinum for hydrogen, oxygen, and CO adsorption is considered, in particular with respect to the influence of metal adsorbate and alloy components on close packed and stepped (defect) platinum surfaces. Dynamical measurements (employing supersonic molecular beams) of the... 

The electro-oxidation of ethanol at platinum-based catalysts dispersed in an electron-conducting polymer was relatively sparsely investigated. It seems that... 

The presence of trace CO and sulfur and large quantities of CO2 affects the performance of the anode electro-catalyst. As a consequence, more platinum must be used (typically 0.4 to 1 mg/cm more), and even then the power density is typically 30 to 40 percent lower than with hydrogen-based systems. 

He C, Kunz HR, Fenton JM. Evaluation of platinum-based catalysts for methanol electro-oxidation in phosphoric acid electroljde. J Electrochem Soc 1997 144(3) 970-979. 

Okada T, Suzuki Y, Hirose T, Ozawa T. Novel system of electro-catalyst for methanol oxidation based on platinum and organic metal complexes. Electrochim Acta... 

Re search over several decades has found that platinum and platinum-eontaining catalysts are the most effective catalytic materials for PEMFCs nonetheless, platinum-based catalysts are expensive and have low durability (Borup et al, 2007 Thompsett, 2003 Rabis et al, 2012). Studies have been intensively conducted to significantly reduce the Pt loading in both electrodes and/or enhance the Pt electro-catalytic efficiency. Alternatively, Pt-free catalysts have been particularly studied to replace Pt (Chen, 2011). 

It is the aim of this chapter to review recent progress on nano-structured electro-catalysts for application to PEMFCs and direct methanol fuel cells (DMFCs). As one of the most important PEMFCs, DMFCs have been recognized as a potential future power source for portable electronic devices (Arico etal, 2001 Basri etal, 2010 Guo etal, 2008 Hamnett, 2003 Kamarudinef a/., 2009 Zhao et al, 2011). In particular, the relationships between nano-structure and electrocatalysis, and the catalytic mechanisms are discussed. Special attention has been paid in the latest avenues for making low-cost and effective catalysts such as novel nano-structures of Pt, binary and ternary platinum-based catalysts, new cost-effective synthesis routes, and new catalyst... 

As previously discussed, platinum and platinum-based materials are still the most effective electro-catalysts for ammonia electro-oxidation. In order to... 

Pt-based electrocatalysts are usually employed in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMSC). In direct-methanol fuel cells (DMFCs), aqueous methanol is electro-oxidized to produce COj and electrical current. To achieve enhanced DMFC performance, it is important to develop electrocatalysts with higher activity for methanol oxidation. Pt-based catalysts are currently favored for methanol electro-oxidation. In particular, Pt-Ru catalysts, which gave the best results, seem to be very promising catalysts for this application. Indeed, since Pt activates the C-H bounds of methanol (producing a Pt-CO and other surface species which induces platinum poisoning), an oxophilic metal, such as Ru, associated to platinum activates water to accelerate oxidation of surface-adsorbed CO to... 

In this work, a concept of the modification of platinum catalysts is presented. According to our proposal, the monolayer oxide systems should be the appropriate carriers that can influence the noble metal properties. The monolayer carrier effect depends on its acid-base or electro-nucleophilic properties and the number and capacity of monolayers. This is of great importance in the selective hydrogenation of unsaturated carbonyl compounds. The... 

Thirdly, in order to improve the dispersion of platinum catalysts deposited on carbon materials, the effects of surface plasma treatment of carbon blacks (CBs) were investigated. The surface characteristics of the CBs were determined by fourier transformed-infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), and Boehm s titration method. The electrochemical properties of the plasma-treated CBs-supported Pt (Pt/CBs) catalysts were analyzed by linear sweep voltammetry (LSV) experiments. From the results of FT-IR and acid-base values, N2-plasma treatment of the CBs at 300 W intensity led to a formation of a free radical on the CBs. The peak intensity increased with increase of the treatment time, due to the formation of new basic functional groups (such as C-N, C=N, -NHs, -NH, and =NH) by the free radical on the CBs. Accordingly, the basic values were enhanced by the basic functional groups. However, after a specific reaction time, Nz-plasma treatment could hardly influence on change of the surface functional groups of CBs, due to the disappearance of free radical. Consequently, it was found that optimal treatment time was 30 second for the best electro activity of Pt/CBs catalysts and the N2-plasma treated Pt/CBs possessed the better electrochemical properties than the pristine Pt/CBs. 

The mechanism of the electro-oxidation of methanol on platinum was thoroughly established, mainly after the identification of both reactive intermediates and adsorbed poisoning species [4]. In the first step, methanol is dissociatively adsorbed at Pt-based catalysts by cleavage of C - H bonds, leading to the so-called formyl-like species -(CHO)ads- From this species, different steps can occur, but with platinum, the dissociation of -(CHO)ads gives rapidly adsorbed CO, which is responsible for the electrode poisoning. This is the explanation of the rather poor performance of Pt catalysts, due to the relatively high potential necessary to oxidize such CO species. 

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