Carbon-supported platinum-based cells

The electro-catalytic oxidation of hydrogen, and reduction of oxygen, at carbon supported platinum based catalysts remain essential surface processes on which the hydrogen PEM fuel cell relies. The particle size (surface structure) and promoting component (as adsorbate or alloy phases) influence the activity and tolerance of the catalyst. The surface chemical behavior of platinum for hydrogen, oxygen, and CO adsorption is considered, in particular with respect to the influence of metal adsorbate and alloy components on close packed and stepped (defect) platinum surfaces. Dynamical measurements (employing supersonic molecular beams) of the... 

Approaches to Synthesize Carbon-Supported Platinum-Based Electrocatalysts for Proton-Exchange Membrane Fuel Cells... 

Platinum electrocatalysts are dispersed as small particles on high surface area conductive supports for effective use of costly Pt. The size of platinum particles, therefore, plays an important role in the oxygen reduction kinetics for fuel cell applications, in terms of both electrocatalytic activity and practical application of catalysts. Carbon-supported platinum shows a large surface area and increased catalytic activity. Alloy catalysts with various transition metals have been employed to increase the catalytic activity and reduce the cost. Various Pt-based alloy catalysts (binary, ternary, and quaternary alloy) have been tested over the last two decades. Many researchers have reported that Pt-based alloy catalysts show not only higher activity than Pt alone, but also exhibit good performance in the ORRs in PEFCs and DMFCs [100-108]. 

Kaiser J, Simonov PA, Zaikovskii VI, Hartnig C, Joerissen L, Savinova ER. 2007. Influence of the carbon support on the performance of the platinum based oxygen reduction catalysts in a pol3mier electrol3fte fuel cell. J Appl Electrochem 37 1429-1437. 

The recent study by Lakshmi et al. [63] is just one example of the very extensive research efforts devoted to the improvement of performance of fuel cells by increasing the dispersion of the electrocatalyst on the carbon support by virtue of carbon surface functionalization [64], Without acknowledging their familiarity with the most relevant prior studies, they did note that the point of neutral charge evaluation helps in identifying the platinum complex to be used for electrocatalyst synthesis based on their charge, in the sense that, for example, if the carbon surface is positively charged an anionic platinum complex is needed (see Section 5.2.1). 

The PE MFC has a solid ionomer membrane as the electrolyte, and a platinum, carbon-supported Pt or Pt-based alloy as the electrocatalyst. Within the cell, the fuel is oxidized at the anode and the oxidant reduced at the cathode. As the solid proton-exchange membrane (PEM) functions as both the cell electrolyte and separator, and the cell operates at a relatively low temperature, issues such as sealing, assembly, and handling are less complex than with other fuel cells. The P EM FC has also a number of other advantages, such as a high power density, a rapid low-temperature start-up, and zero emission. With highly promising prospects in both civil and military applications, PEMFCs represent an ideal future altemative power source for electric vehicles and submarines [6]. 

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

Fundamental anode catalyst research is imperative for improved direct formic acid fuel cell (DFAFC) performance and stability such that an intimate understanding of the interplay between structural, morphological, and physicochemical properties is needed. The primary base catalysts found to be active for formic acid electrooxidation are either platinum (Pt) or palladium (Pd). The cyclic voltammograms in Fig. 4.1 compare the activity of carbon-supported Pt to Pd towards formic acid electrooxidation. The anodic (forward) scan, relevant to DFAFC performance, is relatively inactive on Pt/C until the applied potential... 

Platinum-Based Cathode Catalysts for Polymer Electrolyte Fuel Cells, Table 1 Brunauer-Errunett-Teller (BET) surface area of the most used carbon supports in Pt/C catalysts [4]... 

To date, the catalysts for low-temperature fuel cell electrodes (phosphoric acid and alkaline cells) have been the precious metal blacks and, more recently, precious metals on carbon supports. Development of fuel cell catalysts using precious metals remains very active. Also, some work is being done on systems that may be substituted for the noble metals. For example, tungsten carbide based anode catalysts have been shown to have good durability over long periods, but they are not as active as platinum. 

The electrochemical active surface area (EASA) of fuel cell Pt-based catalysts could be measured by the electrochemical hydrogen adsorption/desorption method. For carbon supported Pt, Pt alloy, and other noble metals catalysts, the real surface area can be measured by the cyclic voltammetry method [55-59], which is based on the formation of a hydrogen monolayer electrochemically adsorbed on the catalyst s surface. Generally, the electrode for measurement is prepared by dropping catalyst ink on the surface of smooth platinum or glassy carbon substrate (e.g, a glassy carbon disk electrode or platinum disk electrode), followed by drying to form a catalyst film on the substrate. The catalyst ink is composed of catalyst powder, adhesive material (e.g., Nafion solution), and solvent. 

The high cost of platinum means that the car industry strives to reduce the amount of catalyst required in a PEM fuel cell. Current strategies focus on the use of PtM nanoparticles (M = another ri-block metal), nanoparticles with a PtM core encased in Pt atoms, and thin Pt films dispersed on nanostructured supports. Iron-based catalysts would be much cheaper, but their performance is usually poor. A promising advance (still at the research stage) is in the application of microporous carbon-sup-ported iron-based catalysts in which the iron cations are thought to be coordinated in Fe(phen)2p sites, the phenanthroline-units being incorporated into graphitic sheets. ... 

This section starts with a short overview of state-of-the-art catalysts, all of which are based on platinum, followed by an explanation of the carbon monoxide issue when designing a catalyst for fuel cell applications. As the oxygen reduction reaction is the main source of kinetic performance losses of the cell, the reaction is discussed in more detail to set guidehnes for new catalyst concepts. New approaches cover platinum-containing core shell catalysis as well as de-alloyed approaches, where both classes aim for a severely reduced overall loading, and platinum-free alternatives are discussed in more detail. Finally, the influence of carbon supports on performance is discussed and alternative catalyst supports are presented. 

Nanofibre for use in proton exchange membrane fuel cells has been a focus of research during the last 5 years. These fuel cells have the potential for high thermodynamic efficiency and almost zero emissions, but are currently hindered by high cost of the platinum-based catalyst and low durability. Carbon nanofibre webs as a supporting medium for platinum nanoparticles have been employed [46]. 

An appreciable increase in the utilization efficiency of platinum catalysts in fuel cells was attained when the highly dispersed platinum was deposited not directly onto the conductive electtode base but onto carbon black or other carbon materials serving as an intermediate base for the nanodispersed catalyst. On carbon supports, the nanosized platinum crystallites were less subject to recrystallization and coarsening. In addition, new technical devices such as adding Nafion ionomer to the active mass have helped to considerably improve the contact between catalyst and solid electrolyte (a Nafion-lype membrane). Carbon black was found to be a very convenient support for platinum catalysts. It is readily available and not expensive. Certain blacks (such as furnace black Vulcan XC-72) have special surface properties that have a favorable effect on catalyst activity. 

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