Cathode catalyst materials

It is very important to develop a high performance cathode catalyst, because a sluggish ORR causes a large overpotential at low temperatures. With respect to the total performance of activity and stability, the cathode catalyst material is limited to Pt or its alloys at present. In acidic media such as Nation electrolyte or aqueous acid solutions, four-electron reduction is dominant at Pt-based electrodes ... 

Many cathode catalyst materials have been used. For noble metal catalysts, platinum was mainly used in fuel cells for space applications. For terrestrial use, one has to use less expensive materials, and non-noble metal catalysts are therefore mainly employed. Bacon used lithium-doped nickel oxide as a cathode catalyst for high-temperature AFCs. Lithium-doped nickel oxide has a sufficient electrical conductivity at temperatures above 150 °C. Currendy, mainly Raney silver and pure silver catalysts are favored. Developments of silver-supported materials containing PTFE are sometimes successful. Silver catalysts are usually prepared from silver oxide, Raney silver, and supported silver. Typically, the catalysts on the cathode are supported by PTFE because it is highly stable under basic and acidic conditions. In contrast, carbon is oxidized at the cathode in contact with oxygen, when carbon is used as an inexpensive support material. In the past, the silver catalysts frequentiy contained mercury as part of an amalgam to increase the stability and the lifetime of the cathode. Because mercury is partially dissolved during the activation procedure (see below) and during the fuel-cell operation, some electrolyte contamination can be observed. Because of the environmental hazard of mercury, this metal is currently not used in silver catalysts. 

Cathodic catalyst materials are typically nickel based [17, pp. 257-260]. A study of the electrocatalytic properties of nickel-based electrocatalysts showed that increasing the molybdenum content on the electrode surface reduces the overpotentials for hydrogen [42]. An investigation of materials for hydrogen evolution under real... 

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

In the following chapter examples of XPS investigations of practical electrode materials will be presented. Most of these examples originate from research on advanced solid polymer electrolyte cells performed in the author s laboratory concerning the performance of Ru/Ir mixed oxide anode and cathode catalysts for 02 and H2 evolution. In addition the application of XPS investigations in other important fields of electrochemistry like metal underpotential deposition on Pt and oxide formation on noble metals will be discussed. 

Figure 2.1 Schematic diagram of a DMFC, its electrode reactions and material transport involved, where (b) is the anode backing, (f) the cathode backing, (c) the Pt-Ru anode catalyst layer, (d) the Nafion 117 membrane and (e) the Pt cathode catalyst layer.

The structure of a SPE cell is shown in Fig. 2.3. The basic unit of a SPE electrolyzer is an electrode membrane electrode (EME) structure that consists of the polymer membrane coated on either side with layers (typically several microns thick) of suitable catalyst materials acting as electrodes [43,49,50], with an electrolyzer module consisting of several such cells connected in series. The polymer membrane is highly acidic and hence acid resistant materials must be used in the structure fabrication noble metals like Pt, Ir, Rh, Ru or their oxides or alloys are generally used as electrode materials. Generally Pt and other noble metal alloys are used as cathodes, and Ir, Ir02, Rh, Pt, Rh-Pt, Pt-Ru etc. are used as anodes [43,46]. The EME is pressed from either side by porous, gas permeable plates that provide support to the EME and ensure... 

Hydrogen evolution has been studied intensively using DFT [9, 10, 11], and new catalyst materials have been suggested [1, 12], However, the hydrogen evolution catalysis is very efficient on many noble metal surfaces, and in most cases, it is not the surface catalysis that limits the cathode reaction. 

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 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). 

In addition to the slow methanol oxidation kinetics, methanol that crosses over from the anode to the cathode side through the membrane can react with 02 at the cathode catalyst, leading to a mixed potential at the cathode side and thereby reducing cell performance. To solve this problem, methanol-tolerant catalysts as well as membranes with low methanol permeability have been investigated. However, these materials are still in the research stages and commercial applications have not been developed. 

The anode and cathode electrodes currently consist of Pt or Pt alloys on a carbon support. Two low-cost, nonprecious metal alternative materials for anode catalysts are WC and WO. Pt alloyed with W, Sn, or Mo has also been evaluated for anode catalyst materials. Some non-Pt cathode catalysts that are being evaluated include TaOo.92> Nj osZrO, pyrolyzed metal porphyrins such as Ee- or Co-NJC and... 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

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