Non-precious metal-based catalysts

Heat-Treated Non-precious-Metal-Based Catalysts for Oxygen Reduction... 

The current status of precious and non-precious metal-based catalysts employed at the PEMFC cathodes is discussed below. 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

CESI is developing an improved water gas shift catalyst formulation that targets transportation applications with catalyst characteristics such as 1) no pre-conditioning requirement, 2) no air sensitivity, 3) preferably non-precious metal based, and 4) low sensitivity to condensed water. For the data analysis, the integral fixed bed reactor design equation was used with an empirical rate expression... 

Significant work during this period was directed at developing a non-precious metal-based or base metal water gas shift catalyst (Figure 1). The initial activity, k , and the deactivation rate, kj, are determined from a fit of the measured conversion curve shown in Figure 1 after the first 5 to 10 hours. These kj and values for these catalysts are summarized in Table 1 with the Pt catalyst reported in Reference 2 included for comparison. 

In spite of a very significant progress achieved with heat-treated macrocyclic compounds as ORR catalysts since the early 1970s, the activity and durability of that family of catalysts are stiU insufficient for replacing platinum at the fuel cell cathode and in other applications. Furthermore, the complex structure of macrocyclic compounds makes their synthesis expensive and potentially noncompetitive with precious-metal-based catalysts also from the materials cost point of view. For those reasons, much effort has been invested by the electrocatalysis research community in recent years into finding less expensive and catalytically more active non-precious metal ORR catalysts that would not rely on macrocylic compounds as either catalysts or catalyst precursors. In the past decade, there has been a significant improvement both in the activity and of non-macrocyclic catalysts, expected to be manufactured at a fraction of the cost of their macrocyclic counterparts. In this section, we review the precursors, synthesis routes, and applications of this relatively new family of catalysts. 

Oxide-based cathode catalysts are entirely new non-precious metal cathode catalysts for low-temperature fuel cells such as jxtlymer electrolyte fuel cells (PEFCs). These catalysts were developed from a viewpoint that high chemical stability was essentially required for the cathode for PEFCs. The cathode catalysts for PEFCs are exposed to an acidic and oxidative atmosphere, that is, a strong corrosive environment, therefore, even platinum nanoparticles dissolved during a long-time operation. This instability of electrocatalysts is one of the factors which hindered the wide commercialization of PEFCs. 

Several processes based on non-precious metal also exist. Because of high catalyst deactivation rates with these catalyst systems, they all require some form of continuous regeneration. The Fluid Hydroforming process uses fluid solids techniques to move catalyst between reactor and regenerator TCR and Hyperforming use some form of a moving bed system. 

Aqueous phase reforming of glycerol in several studies by Dumesic and co-workers has been reported [270, 275, 277, 282, 289, 292, 294, 319]. The first catalysts that they reported were platinum-based materials which operate at relatively moderate temperatures (220-280 °C) and pressures that prevent steam formation. Catalyst performances are stable for a long period. The gas stream contains low levels of CO, while the major reaction intermediates detected in the liquid phase include ethanol, 1,2-pro-panediol, methanol, 1-propanol, propionic acid, acetone, propionaldehyde and lactic acid. Novel tin-promoted Raney nickel catalysts were subsequently developed. The catalytic performance of these non-precious metal catalysts is comparable to that of more costly platinum-based systems for the production of hydrogen from glycerol. 

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminium oxide (LiAI02) matrix. Since they operate at extremely high temperatures of 650°C and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs. 

Among non-precious metals, copper-chromium combinations seem to be the most effective catalysts for CO exhaust elimination. For example, monoliths and pellets impregnated with copper-chromite have been reported to have activities near those of precious metal-based auto-emission control catalysts (ref. 1,2). This comparison has also been extended to the oxidation of CO with NO (ref. 3,4), another important auto-emission pollutant. 

Other approaches have focused upon using non-precious metals and their oxides as alternatives to the platinum catalysts. For example, the mixed oxide catalysts of the binary and ternary alloys of noble metals and transition metals have been investigated for the oxygen evolution reaction in solid polymer electrolyte water electrolyzers. Binary, ternary, and quaternary platinum alloys with base metals of Cu, Ni, and Co have been used as electrocatalysts in liquid acid electrolyte cells. It was also reported that a R-Cu-Cr alloy displayed better activity to oxygen reduction than R and Pt-Cr in liquid electrolyte.The enhanced electrocatalytic activity of these types of alloys has been attributed to various factors, including the decrease of the nearest neighbor distance of platinum,the formation of Raney type... 

In general, non-noble metal alloy nanoparticles have shown some methanol tolerance effects, but their activity towards ORR is lower than that of Pt/C. Furthermore, it has been found that the non-precious metal catalysts do not present the required stability in the acidic environment, even in the case of Pd (at high potential). On the other hand, some works have shown that this instability (dissolution), mainly of the non-noble metal, can be overcome by the addition of small amounts of stabilizers like Au. Based on this, Mathiyarasu and Phani [30] examined the effect of the addition of Au, Ag and Pt on the activity and stability of several Pd-Co/C electrocatalysts. Results showed higher ORR activities for Pd-Co-Pt/C, equal to that of a commercial Pt/C electrocatalyst. 

K., Zhang, J., and Cao, X. (2013) Graphene-xerogel-based non-precious metal catalyst for oxygen reduction reaction. Electrochem. Commun., 28, 5-8. 

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