Pt-free electrocatalysts

This explains the higher methanol tolerance of the alloy material in relation to that of pure Pt/C. For Pt-free electrocatalysts, PCI4C01/C showed to be very active for the ORR even at a high concentration of methanol. The addition of noble metal such as Au, Ag and Pt onto the PdCo material, in order to increase their stability in acid electrolyte, conducts to a lowered MOR activity and high ORR kinetics. For the RuSe/C and RhS/C materials, the former presents a considerable tolerance to the presence of methanol. However, the observed loss of selenium from the surface, observed upon exposure to potentials greater than 0.85 V, indicates a detrimental effect on the implementation of RuSe/C as a cathode material in fuel cell applications. The commercially available rhodium sulphide underperforms and exhibits higher susceptibility to methanol compared to RuSe/C, but it is more stable under similar testing conditions. 

Reducing Pt content by developing metallic alloy or Pt-free electrocatalysts. 

As in many other systems, the nature and the structure of the electrode material play an important role in the adsorption and electrooxidation of most organic fuels commonly used in DAFC. Many groups attempted to develop Pt-free electrocatalysts as anode materials for alcohol... 

In Pt-free electrocatalysts, the surfaces of PdAu/C electrocatalysts are less strongly poisoned by CO than those of PtRu at temperatures of 60 °C. In 2001, Schmidt et al. [65] reported the CO tolerance of PdAu/C (Vulcan XC72) that was prepared via bimetallic colloidal precursors. This work was based on an earlier study by Fishman [375] in which PdAu-black alloys provided a highly active medium for the hydrogen oxidation reaction and a second metal (Au) produced... 

The development of Pt-based binary or ternary metallic electrocatalysts and Pt-free electrocatalysts is an intuitional idea to mitigate the problem caused by the strong adsorption of impurities on Pt. Therefore, numerous efforts have been made in this area. However, almost all of these works exclusively... 

The dependence of the Gibbs free energy pathway on electrode potential (Figure 3.3.10A) manifests itself directly in the experimental current potential characteristic illustrated in Figure 3.3.10B. At 1.23 V, no ORR current is measureable, while with decreasing electrode potentials the ORR current increases exponentially until at +0.81 V, processes other than surface kinetics (e.g. mass transport) begin to limit the overall reaction rate. Figure 3.3.10B represents a typical performance characteristic of a Pt or Pt-alloy electrocatalyst for the ORR. 

One-step facile hydrothermal methods can synthesize novel nanoporous Ptdr and Pt on RuO2(110) catalysts for ORR. Pt-free catalysts such as Pd-Co, Pd-Ni, Pd-Cr, Pd-Ta, Co-Ni, and CoPds metal alloys"" " as well as the chalcogenide Ru-S and Ru-Se catalysts" " are believed to be promising candidates. Modem studies on ORR electrocatalysts deal with nano-porous gold particles (10-20 nm) with large specific surface area that reduce... 

Since Pt is a scarce and expensive metal, in 2002, the US Department of Energy (DOE) set targets for the maximum quantity of Pt to be used in H2/O2 (Air) PEM fuel cells. Initially, the 2015 target was set to 0.2 g Pt/kWe (with kWe for the rated electric power) for total anode and cathode Pt content combined. However, due to the considerable rise in Pt price, a new target—0.125 g Pt/kWe—was recently set for 2017 [5]. At around 1,700-1,800 per Pt Troy ounce (31.1 g), 0.125 g Pt/kWe would represent a Pt raw material cost of around 7 Pt/kWe. Meanwhile, the DOE also sets the cost target for the entire membrane electrode assembly to 9/kWe There is, therefore, a strong case for replacing Pt with a lower cost non-noble metal-based electrocatalyst (or a metal-free electrocatalyst). 

Thus far, aU N-doped carbons appear to only be able to reduce oxygen oti graphitic-type nitrogen atoms, and this reduction only leads to the production of H2O2. However, experimentally, it is known that metal-free N-doped carbons are capable of reducing O2 to water with an apparent transfer of 4e, as reported by various research groups covered in this chapter. It is also clear from Fig. 3 in ref. [116], for instance, that similar limiting current densities are reached at the same rotation rate (from 100 to 1,600 rpm) for ORR on a metal-fi ee N-doped carbon, a Fe/N/C or a Pt/C electrocatalyst. 

Figure 7.5. (a) ORR on different catalysts in O2 saturated 1 mol dm KOH solution, and (b) ethanol oxidation on Pd-NiO/C catalyst in 1.0 mol dm ethanol/1.0 mol dm KOH [55, 56]. (Graph (a) reprinted from Electrochemistry Commununications, 8, Meng H, Shen PK. Novel Pt-free catalyst for oxygen electroreduction, 588-94, 2006 with permission from Elsevier. Graph (b) reprinted from Electrochemistry Commununications, 8, Alcohol oxidation on nanocrytaUine oxide Pd/C promoted electrocatalyst, 184-8, 2006 with permission from Elsevier.)... 

Many papers in recent years have presented details of new preparation methods and the performance of Pt-based eleetroeatalysts such as PtSn/C, PtMo/C, PtRuMo/C, PtRu-HxMoOa/C, and PtRu/(carbon nanotubes). In addition, efforts to develop R-free electrocatalysts such as PdAu/C have been undertaken. 

As such, a major limitation ofthe current PEMFC is that the Pt anode electrocatalyst is poisoned by CO at the 5-10 ppm level in the state-of-the-art fuel cells operating at about 80°C (Lee, 1999 Denis, 1999). One approach to solving the CO poisoning problem is to operate at higher temperature where the free energy of adsorption of CO on Pt has a larger positive temperature dependence than that ofH2, which means that the CO tolerance level increases with temperature (Kreuer, 1997). 

Considerable improvement in the platinum tolerance of the electrocatalysts to carbon monoxide poisoning. Carbon monoxide is a major problem because trace amounts of CO in the H2 feed gas (more than 10 ppm) will poison the Pt anode electrocatalyst in PEMFCs operating at 80 C. A quantitative analysis of the free energy for H2 and CO adsorption as a function of temperature suggests that, by elevating the operating temperature of the cell to 145 °C, CO tolerance at the anode should increase by a factor of 20 (from 5-10 to 100-200 ppm). 

Apart from platinum s intermediate nature on bonding, another point in platinum s favor is availability platinum can be purchased in various suitable forms at a reasonable price some noble metals are difficult to find and purchase. The word noble means here stable and of course that is a first point one wants in an electrocatalyst. It must be a catalyst, not enter into the reaction. It is meant to accelerate the reaction. It must itself be stable, thermally and electrochemically. On the last point, platinum is only fairly good because oxide-free platinum does start itself to dissolve around 1.0 V on the normal hydrogen scale. By using it in anodic reactions in a potential range anodic to 1.0 V, Pt(II) is likely to get into the solution and may be deposited on the cathode. 

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