Electrocatalyst cathode loadings

Figure 20.5. Comparison of hydrogen/air polarization curves for spray-based and conventional electrocatalysts at 0. 2 mgPt/cm cathode loadings anode catalyst is identical at 0.05 mgPt/cm. Single 50 cm cell PEM MEA performance measured at 50 °C and atmospheric pressure at constant flow, corresponding to stoichiometry of 1.2 for hydrogen and 2.2 for air at 1 A/cm, and with 100% humidification of the gases, Nafion 112 membrane. (Reproduced with permission of Cabot Corporation.)...

Guild AF, Gancs L, Allen RJ, Mukerjee S (2007) Carbon-supported low-loading rhodium sulfide electrocatalysts for oxygen depolarized cathode applications. Appl Catal A 326 227-235... 

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... 

Fang, B., et al., High Pt loading on functionalized multiwall carbon nanotubes as a highly efficient cathode electrocatalyst for proton exchange membrane fuel cells. Journal of Materials Chemistry, 2011. 21(22) p. 8066-8073. 

Subsequent deployment of the new catalyst in the cathode layer of small-area MEAs first, then large-area MEAs, and finally fuel cell stacks represents the typical series of performance tests to check the practical viability of novel ORR electrocatalyst materials. Figure 3.3.15A shows the experimental cell voltage current density characteristics (compare to Figure 3.3.7) of three dealloyed Pt-M (M = Cu, Co, Ni) nanoparticle ORR cathode electrocatalysts compared to a state-of-the-art pure-Pt catalyst. At current densities above 0.25 A/cm2, the Co- and Ni-containing cathode catalysts perform comparably to the pure-Pt standard catalyst, even though the amount of noble metal inside the catalysts is lower than that of the pure-Pt catalyst by a factor of two to three. The dealloyed Pt-Cu catalyst is even superior to Pt at reduced metal loading. 

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. 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

Taylor et al.8 were the first to report an electrochemical method for preparation of MEAs for PEMFCs. In their technique, Pt was electrochemically reduced and deposited at the electrode membrane interface, where it was actually utilized as an electrocatalyst. Nation, which is an ion exchange polymer membrane, is first coated on a noncatalyzed carbon support. The Nafion-coated carbon support is then immersed into a commercial acidic Pt plating solution for electrodeposition. Application of a cathodic potential results in diffusion of platinum cations through the active Nation layer. The migrated platinum species are reduced and form Pt particle at the electrode/membrane interface only on the sites which are both electronically and ionically conductive. The deposition of Pt particles merely at the electrode/membrane interface maximizes the Pt utilization. The Pt particles of 2-3.5 nm and a Pt loading of less than 0.05 mg cm-2 were obtained employing this technique.8 The limitation of this method is the difficulty of the diffusion of platinum... 

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.

Also corrosion problems of the carbon support have been considered as a cause of electrocatalyst durabihty loss [32], in particular carbon oxidation can occur through electrochemical oxidation at the cathode, with formation of CO2 (C -I- 2H2O = CO2 -I- 4H -F 4e ), or through water gas shift reaction, with the production of CO (C H2O = CO H2). Both these routes are catalyzed by Pt [56, 57] and subtract caibon useful for platinum loading, with consequent metal sintering and decrease of the electrochemical surface area [58]. 

Zeis R, Mathur A, Fritz G, Lee J, Erlebacher J (2007) Platinum-plated nanoporous gold an efficient, low Pt loading electrocatalyst for PEM fuel cells. J Power Sources 165 65-72 Wu H, Wexler D, Wang G (2009) PtjNi alloy nanoparticles as cathode catalyst for PEM fuel cells with enhanced catalytic activity. J Alloy Compd 488 195-198... 

The project goals are to significantly improve both the kinetic performance of the electrocatalyst powder at low noble metal loading and its utilization in the cathode layers through layer structure development. Limitations in the catalyst performance will be addressed through combinatorial discovery of supported catalyst compositions and microstructures. The discovery of these new catalyst formulations will be carried out under conditions that have been scaled for commercial powder production. A large variation of binary, ternary and quaternary noble metal -transition metal alloys and mixed metal-metal oxide catalyst compositions will be screened. To improve the utilization/performance of the catalyst in MEAs,... 

MEA structure deyelopment. The deyelopment of the electrode structure was focused on finding optimal combinations of electrocatalyst loadings, layer thicknesses and ionomer-to-catalyst ratios. Figure 5a shows the polarization curyes for MEAs with 60 wt.% Pt/Carbon used as cathode catalyst at Pt loadings in the range of 0.25 to 0.65 g Pt/cm. Figure 5b compares the performance of these MEAs in terms of g Pt/kW at 0.8 V. For all these benchmark Pt loadings, a performance around 2 g Pt/ kW was demonstrated. 

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