Anode catalyst PEMFC

Design parameters of the anode catalyst for the polymer electrolyte membrane fiiel cells were investigated in the aspect of active metal size and inter-metal distances. Various kinds of catalysts were prepared by using pretreated Ketjenblacks as support materials. The prepared electro-catalysts have the morphology such as the sizes of active metal are in the range from 2.0 to 2.8nm and the inter-metal distances are 5.0 to 14.2nm. The electro-catalysts were evaluated as an electrode of PEMFC. In Fig. 1, it looked as if there was a correlation between inter-metal distances and cell performance, i.e. the larger inter-metal distances are related to the inferior cell performance. 

Significant (and even spectacular) results were contributed by the group of Norskov to the field of electrocatalysis [102-105]. Theoretical calculations led to the design of novel nanoparticulate anode catalysts for proton exchange membrane fuel cells (PEMFC) which are composed of trimetallic systems where which PtRu is alloyed with a third, non-noble metal such as Co, Ni, or W. Remarkably, the activity trends observed experimentally when using Pt-, PtRu-, PtRuNi-, and PtRuCo electrocatalysts corresponded exactly with the theoretical predictions (cf. Figure 5(a) and (b)) [102]. 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

Would the preferential CO oxidation reaction be needed if the proton-exchange membrane fuel cell (PEMFC) with Pt anode catalyst were able to work at temperatures higher than about 403 K ... 

The theoretical cell voltage of a DMFC at standard conditions is 1.20 V. The materials used in DMFCs are similar to those in PEMFCs. Pt, PtRu, and Nafion membrane are used as cathode catalyst, anode catalyst, and proton transfer membranes, respectively. However, the catalyst loading in a DMFC is much higher than the loading used in H2/air fuel cells, because both side reactions are slow (Pt loadings 4 mg/cm2 for a DMFC, 0.8 mg/cm2 for a H2/air fuel cell). 

CO contamination is widely documented in the literature and recognized as a serious issue in the investigation of PEMFCs a decline in PEMFC performance is very often due to deactivation of the Pt anode catalyst, caused by traces of CO. Many attempts [14, 41, 42] have been made to use the EIS method to understand the mechanisms of CO poisoning and CO tolerance, by feeding an anode gas mixture of CO and H2. Examples of such studies will be discussed in detail in Chapter 6. 

This survey focuses on recent catalyst developments in phosphoric acid fuel cells (PAFC), proton exchange membrane fuel cells (PEMFC), and the previously mentioned direct methanol fuel cell (DMFC). A PAFC operating at 160-220 °C uses orthophosphoric acid as the electrolyte the anode catalyst is Pt and the cathode can... 

Figure 12 Scheme of the preparation of colloidal Pt/Ru/Al PEMFC anode catalysts (>20% metal on Vulcan XC72 ) via the precursor concept. (From Ref. 66.)... 

Chapters 7-10 cover the syngas purification and separation. When reforming and water-gas shift are applied to PEMFC systems, trace amounts of CO in the gas that poisons anode catalyst must be removed. This is achieved by preferential CO oxidation, which is covered in Chapter 7 by Marco J. Castaldi of Columbia... 

Gotz, M. Wendt, H. Binary and ternary anode catalyst formulations including the elements W, Sn and Mo for PEMFCs operated on methanol or reformate gas. Electrochim. Acta 1998, 43 (24), 3637-3644. 

More than a hundred articles have been published on the use of CNTs or CNFs as catalyst supports for DMFC and PEMFC. The most studied reaction is methanol oxidation (anode catalyst), followed by oxygen reduction (cathode catalyst) and to a lesser extent, hydrogen oxidation (anode catalyst). Platinum is... 

Nevertheless, the full commercialization of PEMFC systems needs a stable supply of hydrogen, which must be characterized by high purity for avoiding the CO poisoning of the PEMFC anodic catalyst [26]. 

Inspired by the observation, that binary Pt=Ru nanoparticles supported on CNT showed a promising performance as anode catalysts in PEMFCs, Harris et al. [65] used density functional theory calculations to study the anchoring of the nanoparticles to the nanofibers. They found a strong metal-carbon bond ( 3 eV) with covalent character between the graphene structure and the metal (111) crystal planes, which might be the reason for the higher stability found in these systems. 

Serov A, Kwak C (2009) Review of non-platinum anode catalysts for DMFC and PEMFC application. Appl Catal B Environ 90 313-320... 

One should note that poisoning of PEMFC anode catalysts by CO is also a severe problem as CO is found to some extent in most H2 gas supplies, as H2 is usually produced by steam reforming of CH4 (and CO is a by-product). It has been reported that a CO content as low as 10 ppm in H2 fuel will result in the poisoning of Pt electrocatalysts [74], As shown in Eqs. 17.8 and 17.9, the formation of OHads by water oxidation at the Pt surface is necessary for the oxidative removal of adsorbed CO. However, the formation of Pt-OH only occurs appreciably above 0.8 V vs. RHE [75]. This factor is considered to be the origin of the high overpotentials for the MOR and COOR and, often, a second metal that can provide oxide species at low potentials is added to Pt electrocatalysts to reduce such overpotentials. For example, Pt-based alloys containing elements such as Ru, Mo, W, and Sn have been used in attempts to speed up the electrocatalysis of methanol [70,76,77]. The Pt-Ru alloy (1 1 atomic ratio) is the most active binary catalyst and is most frequently used as the anode catalyst in DMFCs [78]. Ru is more easily oxidised than Pt and is able to form Ru-oxide adsorbates at 0.2 V vs. RHE, thereby promoting the oxidation of CO to CO2, as summarised in Eqs. 17.11-17.13 ... 

Tungsten carbide based non-Pt electrocatalyst have been evaluated for single cell performance of PEMFC. The power densities of WC/C (1 mg/cm ) and Ni-WC/C (1 mg/cm ) anodes fabricated by TPRe were 6.5 and 8.9 mW/cm, respectively, cathode of 20 wt% Pt and membrane of Naflon 117 (151). W2C (0.48 mg/cm ) anode catalyst was prepared by ball-milling of WO3, Mg, and C mixture and tested in a single cell mode at 80°C with Pt/C cathode (0.3 mg/cm ), Naflon 112 membrane, and H2/Air = 5. This electrocatalyst produced high current densities of 240 mA/cm (at 2 atm) and 280 mA/cm (at 3 atm) at 0.5 V, and an open circuit voltage of 0.95 V(152). These values represent the highest current density reported for WC-based anode in the literature. In the analysis, the material... 

The reformate composition that enters the anode compartment of the PEMFC stack is dependent on the hydrocarbon feed. For natural gas feed, the typical reformate composition is, in mol% H2 50%, CH4 <1%, H2O 1.5%, CO2 15 %, N2 32% (for Dutch natural gas), CO < 10 ppm (ECN, internal communication. The CO content in the reformate is the most critical parameter, because the anode catalyst in the PEMFC, a Pt-based catalyst, will be poisoned by CO. 

Anodic catalyst Cathodic catalyst Figure 1 Schematic diagram of a PEMFC... 

DMFCs are PEMFCs fed with methanol as fuel. The technologies required by DMFCs are similar to those of PEMFCs. What differ between DMFCs and PEMFCs are in the following two aspects The proton exchange membrane used for DMFCs must possess low methanol permeability or crossover, and the anode catalyst must possess high activity toward the oxidation of methanol and high tolerance to CO and other intermediates from methanol oxidation. In this section, the applications of NMR techniques for the development of DMCFs as well as essential materials are going to be briefly reviewed. 

Since the proton exchange membrane fuel cell (PEMFC) anode catalyst can be poisoned by CO at ppm levels of concentration, it is necessary to estimate how much CO will exist at each of the fuel processing steps. Let us use the steam reforming As an example to do some analyses. Table 3.2 lists the molar Gibbs free energy of formation of the species involved, plus values for CO2 and O2 for later analysis. 

A DMFC is quife similar to a proton exchange membrane fuel (PEMFC) in stack structure and components. They both use a PEM for transporting the protons and Pt-based catalysts at the cathode. The anode catalyst for a DMPC is typically a Pt-Ru alloy that has higher CO tolerance than Pt alone, and this is similar to the PEMFC when H2 contains trace amounts of CO. In fhe infer-mediate sfeps during methanol oxidation, some CO-like species will form, which can seriously poison the anode catalyst. The presence of Ru helps fhe removal of fhe CO-like species from fhe Pt surface trough Reaction 7.6. 

Amraig various anode catalysts developed, ft-Ru alloys are generally cmisidered as the best candidates for H2/CO and alcohol oxidation these alloy catalysts show high CO tolerance and acceptable durability under FC operating conditions. Several commercial ft-Ru alloy nanoparticles supported on carbon black have been available for applications in PEMFCs, DMFCs, and DEFCs. Efforts to improve the activity and stability of ft-Ru alloy catalysts continuously are being made. Recently, the nanocapsule method has been successfully employed to synthesize ft-Ru nanoparticles with... 

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