Catalytic activity PEMFC

Beyond human-made Pt- or noble-metal-based ORR electrocatalysts, there exist very active biomimetic carbon-nitrogen-iron ORR electrocatalysts that show great potential for use in PEMFC cathodes, even rivaling the catalytic activity of pure Pt. 

The presence of micropores in carbon materials usually facilitates metal dispersion, which does not, however, translate into an increase in the catalytic activity. This may be explained by the blockage of metal nanoparticles in micropores, which are not accessible to reagents [98]. As a consequence, PEMFC specifications require that carbon materials used in PEMFCs comprise a high contribution level of mesopores, but no micropores. The ratio of the surface area of mesopores (the external surface area) to the overall pore area may be characterized by the Actab bet ratio, where Actab is the surface area measured using cetyltrimethyl-ammonium bromide. It has been shown that carbon blacks with a Actab/Abet... 

Finally, we addressed the complex problem of carbon corrosion, which is particularly relevant for PEMFC durability and thus commercialization of PEMFC technology. Carbon supports with an ordered crystalline structure, such as graphi-tized carbons, CNTs, and CNFs, as well as pyrolytic carbons of the Sibunit family hold out hope for the development of CLs with higher durability. More systematic studies are required to unveil the complex influence of the structure and morphology of carbon supports on the performance of the CLs and eventually, to develop a new generation of structurally ordered tailored materials for PEMFC applications with enhanced catalytic activities, low noble metal contents, and high dmabilities. 

Another system, where catalytic active oxide particles might lead to a new development, is the polymer electrolyte membrane fuel cell (PEMFC). This t5rpe of fuel cell preferentially works with platinum and platinum alloy catalysts. The development of an effective oxide catalyst could solve some of the problems connected with the application of these systems. 

CNTs, such as nitrogen-doped CNT, have been reported having high catalytic activity for PEMFC cathode reaction [163-165]. 

The standard PEMFC is built up in a sandwich-like manner with the membrane as a mirror plane. The proton-conducting membrane is at the core of the device, covered on both sides by the catalytically active porous electrodes, next to them GDLs for fine gas distribution, heat and water management, and flow fields for the coarse distribution of the feeds. The electrodes can then be bound to either the membrane (so-called catalyst-coated membrane = CCM) or the GDL (so-called gas diffusion electrode = GDE). 

We have reviewed the family of dealloyed Pt-based nanoparticle electrocatalysts for the electroreduction of oxygen at PEMFC cathodes, which were synthesized by selective dissolution of less-noble atoms from Pt alloy nanoparticle precursors. The dealloyed PtCua catalyst showed a promising improvement factor of 4-6 times on the Pt-mass ORR activity compared to a state-of-the-art Pt catalyst. The highly active dealloyed Pt catalysts can be implemented inside a realistic MEA of PEMFCs, where an in situ voltammetric dealloying procedure was used to constructed catalytically active nanoparticles. The core-shell structural character of the dealloyed nanoparticles was cmifirmed by advanced STEM and elemental line profile analysis. The lattice-contracted transition-metal-rich core resulted in a compressive lattice strain in the Pt-rich shell, which, in turn, favorably modified the chemisorption energies and resulted in improved ORR kinetics. 

Carbon is generally used as catalyst support material because of its high electric and thermal conductivity, chemical stability, and porous structure [11]. The catalytic activity of the catalyst layer increases with increasing carbon surface area due to better platinum dispersion. High surface area carbon blacks such as Ketjenblack and Vulcan are therefore preferred in PEMFC application. However, carbon is thermodynamically unstable at normal cathode potentials between 0.5 and 1V. As shown in Figure 20.1a, carbon is oxidized to carbon dioxide (CO2) or carbon monoxide (CO) at high electrode potentials whereas it is reduced to methane (CH4) at low electrode potentials. The following reactions are relevant for fuel-cell operation ... 

Koh S, Hahn N, Yu CF, Strasser P (2008) Effects of composition and annealing conditions on catalytic activities of dealloyed Pt-Cu nanoparticle electrocatalysts for PEMFC. J Electrochem Soc 155(12) B1281-B1288... 

The slow kinetics of the cathode oxygen reduction reaction (ORR) plays the key role in limiting PEMFC performance when pristine hydrogen is used as the fuel. Therefore, improving the catalytic activity for the ORR has drawn most of the research attention in catalysis studies. Cathode contamination has attracted less attention compared with anode contamination, and only a limited number of papers have been published. Pollutants in air include NOx (NO2 and NO), SOx (SO2 and... 

Catalyst durability. Pt is the key to fuel cells because of its unusually high catalytic activity. Unfortunately, Pt is expensive and has limited availability. The cost of the PEMFC is too high for practical applications. One of the strategies to... 

As the carbon-supported PtRu alloy catalyst seems the most practical for use in PEMFCs, many studies have investigated PtRu alloying with a third element, e.g., W, Mo, Sn, Mb, Au, Ag, Ir, Ni, in an attempt to further improve catalytic activity [21-25]. Some of the elements, such as W, Mo, Sn, Ir, andNi clearly show further improvement in activity due to flie co-catalytic effect, while other elements, such as Mb, Au, and Ag show no improvement but rather a negative effect CO/H2 electrooxidation activity. Furthermore, catalysts with different metal atomic ratios show unparallel activity toward CO/H2 electrooxidation. In summary, the... 

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