Electrocatalyst anode loadings

Investigations at Siemens in Erlangen, Germany, have used unsupported platinum-ruthenium anodes (4 mg/cm ) and platinum black cathodes (4 mg/cm ). Their best performances were 0.52 V at 400 mA/cml At Los Alamos National Laboratory in New MexicoJ the electrocatalyst was unsupported R-RuOx at the anode and unsupported R black at the cathode (R loading about 2 mg/cm ). In a subsequent study, the thinner Nafion 112 membrane was used to reduce the ohmic drop. Under pressure at 400 mA/cm cell potentials of 0.57 V with Oj and 0.52... 

Pt/Ru electrocatalysts are currently used in DMFC stacks of a few watts to a few kilowatts. The atomic ratio between Pt and Ru, the particle si2 e and the metal loading of carbon-supported anodes play a key role in their electrocatalytic behavior. Commercial electrocatalysts (e.g. from E-Tek) consist of 1 1 Pt/Ru catalysts dispersed on an electron-conducting substrate, for example carbon powder such as Vulcan XC72 (specific surface area of 200-250 m g ). However, fundamental studies carried out in our laboratory [13] showed that a 4 1 Pt/Ru ratio gives higher current and power densities (Figure 1.6). 

The activities of CNTs have been evaluated by Girishkumar et al. [7] using ex situ EIS. Their study was conducted in a three-compartment electrochemical cell using a GDE electrode (a carbon fibre paper coated with SWCNTs and Pt black as an anode or cathode). Electrophoretic deposition was used to deposit both the commercially available carbon black (CB) for comparison and the SWCNT onto the carbon Toray paper. Commercially available Pt black from Johnson Matthey was used as the catalyst. In both cases, the loading of the electrocatalyst (Pt), the carbon support, and the geometric area of the electrode were kept the same. EIS was conducted in a potentiostatic mode at either an open circuit potential or controlled potentials. 

Adjacent the ionomeric membrane on both sides are the catalyst layers (Fig. 1). As described above, these are platinum black/PTFE composites with high platinum loadings (typically 4 mg Pt/cm on each electrode) or composites of carbon-supported platinum and recast ionomer, with or without added PTFE, of much lower platinum loading (as low as 0.1 mg Pt/cm on each electrode). The electrochemical processes in the fuel cell take place at these electrocatalysts. In the hydrogen (or methanol reformate)/air fuel cell, the processes at the anode and cathode, respectively, are ... 

Figure 7 Steady-state iR corrected Tafel plots of cathodic ORR performance of several binary Pt alloy electrocatalysts at 90 °C and 5-atm pressure. Performance for a Pt/C electrocatalyst is shown for comparison. The electrodes had 0.3mg/cm metal loading and the loading of the metal on carbon support was 20%. The humidifaction temperature for the anode and cathode gas streams were kept at 10 and 5°C above the cell temperature. 

An approach described most recently for maximizing CO tolerance at relatively low anode catalyst loadings involves a combination of a CO-tolerant electrocatalyst, for example, a PtRu alloy and a separate chemical oxidation catalyst layer of zero... 

Fuel-cell tests offer the ultimate verification of the usefulness of an electrocatalyst by determining its long-term stability under real operating conditions. They were performed on single cells using electrodes of 50 cm and an anode catalyst loading of 0.2 mg... 

Figure 21. Long-term test of the performance stability of the PtRu2o electrocatalyst in an operating fuel cell. The fuel cell voltage at constant current of 0.4 A cm is given as a function of time for the electrode of 50 cm with an anode containing to 0.18 mg Ru cm and 0.018 mg Pt cm / (approximately 1/10 of the standard Pt loading) and a standard air cathode with a Pt/C electrocatalyst. The fuel was clean H2 or H2 with 50 ppm of CO and 3% air temperature 80 C.

Much better performances, especially in terms of peak power densities, have been recently obtained with DEFCs containing anodes coated with the Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C electrocatalysts. " As shown in Fig. 25, these catafysts are extremely active also at room temperature and at low Pd loading (1 mg cm ), providing up to 60 mW cm at 0.3 V, which, to the best of our knowledge, is the highest value ever observed for a DEFC at room temperature. Just to better appreciate the result obtained with the DEFCs equipped with the Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C anodes, a peak power density of 27 mW cm at 25 °C has been recently obtained with an allmline fuel cell (AFC) containing a 3 M KOH solution as electrolyte and an anode coated with 1 mg cm of Pt-black. ... 

This chapter summarized the fundamental aspects and recent advances in electrocatalysts for the oxidation reactions of methanol and ethanol occurring at fuel cell anodes. Pt-based electrocatalysts are still considered to be the most viable for the anodic reactions in acidic media. The major drawback, however, is the price and limited reserves of Pt. To lower the Pt loading, the core-shell structure comprising Pt shells is more beneficial than the alloy structure, since aU the Pt atoms on the nanoparticle surfaces can participate in the catalytic reactions (and those in cores do not) particularly, the Pt submonolayer/monolayer approach would be an ultimate measure to minimize the Pt content. The architectures in nanoscale also have a significant effect on the reactivity and durability [119, 120] and thus should be explored continuously in a future. As for the ethanol oxidation, Rh addition is shown to enhance the selectivity towards C-C bond splitting however, Rh is even more expensive than Pt, and thus, the Rh content has to be very low, or less expensive constituents replacing Rh are necessary to be found. 

Supported nanoparticles are the main catalysts used in current fuel cell devices. The combined DFT, single crystal, polycrystalline, and electrochemical experiments demonstrated that WC and Pt/WC have catalytic properties that are promising for use as anode DMFC electrocatalysts. These fundamental results still left questions unanswered as to how these materials could be incorporated into a realistic device. These questions led to studies of WC and Pt/WC nanoparticles in a fuel cell test station [23]. The WC nanoparticles were obtained from Japan New Metals Company. The Pt/WC nanoparticles were prepared with a 10 wt% Pt loading using incipient... 

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. 

Electrocatalytic Reduction of Oxygen. Oxygen reduction reaction (ORR) occurs on the cathode side of low temperature fuel cells and heavily loaded Pt/C is the most common electrocatalyst. Replacement of ORR catalysts with less expensive materials would have higher technical impact than for anode catalysts. Transition metals loaded carbides and carbide-metal codeposited carbon have been investigated for ORR application. For example, 40 wt% Pt/WC electrocatalyst prepared with RDE electrode showed a cathodic current (-5 x 10 A) similar to that of 40 wt% Pt/C with 0.5 M H2SO4, 100 mv/s and 2000 rpm (160). Also, 40 wt% Pt/WC exhibited electrochemical stability after 100 cycles of cyclic voltammetry from 0 to 1.4 V (vs RHE), whereas the cathodic current of 40 wt% Pt/C disappeared after 100 cycles. 

The development of cheap and efficient electrocatalysts, especially the oxygen reduction electrocatalyst, is another task for the large-scale commer-ciahzation of PEMFCs. In any PEMFCs, there are two types of electrocatalysts one catalyzes the anodic hydrogen oxidation reaction and the other the cathodic oxygen reduction reaction. Both electrocatalysts rely heavily on the use of precious metals, especially the Pt-based electrocatalysts. The tasks in the development of electrocatalysts include the improvement of electrocatalytic activity, the reduction of the loading of precious metals, or even the replacement of precious metals with cheap metals, and the... 

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