Platinum -solution interface deposition

An illustration of the role of the oxidation state in the distribution of the metal particles across the CP layer was presented in single-step potential deposition experiments on platinum in sulfonated PANl that were carried out at two values of the potential (-0.2 and -1-0.2 V vs. Ag/AgCl reference electrode), corresponding to the reduced and oxidized states of PANI [ 101 ]. In the first case a homogeneous distribution of Pt across the entire CP layer up to the underlying carrying substrate was observed, whereas in the second case (oxidized PANI) the Pt content decreased steeply within a narrow region close to the polymer/solution interface [101]. 

Reactions involving charge transfer through the interface, and hence the flow of a current, are called electrochemical reactions. Two types of such reactions are indicated in Fig. 1.1. The upper one is an instance of metal deposition. It involves the transfer of a metal ion from the solution onto the metal surface, where it is discharged by taking up electrons. Metal deposition takes place at specific sites in the case shown it is a hollow site between the atoms of the metal electrode. The deposited metal ion may belong to the same species as those on the metal electrode, as in the deposition of a Ag+ ion on a silver electrode, or it can be different as in the deposition of a Ag+ ion on platinum. In any case the reaction is formally written as ... 

Recent studies performed with deactivated anodes show [55] that electroless or electrolytic platinum deposition on failed anodes, not only lowered the polarisation behaviour of these anodes (see Fig. 5.20), but also demonstrated an equivalent lifetime as that of a new anode in accelerated life tests in the sulphuric acid solution (see Fig. 5.21). These results unequivocally demonstrate that the deactivation of anodes, for which the Ru loading is still high, is a direct consequence of the depletion of Ru from the outer region of the anode coating. Note that this process of surface enrichment by conducting electroactive species will not lead to reactivating a failed anode, if there is a TiC>2 build-up at the Ti substrate/coating interface. 

Directly applying Gubkin s concept of a plasma cathode, Koo et al. produced isolated metal nanoparticles by reduction of a platinum salt at the free surface of its aqueous solution [39]. The authors used an AC discharge as cathode over the surface of an aqueous solution of ITPtCk. Platinum particles with a diameter of about 2 nm were deposited at the plasma liquid electrolyte interface by reduction with free electrons from the discharge. 

A thin film of a membrane-forming material is deposited on a metal wire (silver, platinum, nickel, etc.) or on carbon, the membrane being, for example, a liquid exchanger immobilized in PVC (Fig. 13.12). This electrode is thus similar to an ion-exchange selective electrode, but without solution and without internal reference. The detailed mechanism of its operation is not clear but it is certain, and logical, that the interface... 

Pt, which is not trivial over the projected lifetime for a PAFC. Migration of the platinum from the cathode towards the anode is due to the platinum being deposited, not on the anode catalyst, but rather on the silicon carbide matrix, adjacent to the anode, as a consequence of the small solubility of hydrogen in solution at the electrode/matrix interface. 

When a very thin film (e.g., thicknesses of less than 1 pm) of a polymer is applied to a smooth surface of platinum, most polymers peel off within minutes upon immersion in liquid Lf20. This is also true for most plasma polymers applied to platinum surfaces. However, when an ultrathin film of CH4 LCVD was deposited under the conditions that provide plasma energy density sufficiently high to sputter aluminum from the electrodes, tenacious adhesion that survived over 10 h of boiling in saline solution was obtained, probably due to the incorporation of electrode metal at the interface. 

In Section 3, the slow rate of the ORR at the Pt/ionomer interface was described as a central performance limitation in PEFCs. The most effective solution to this limitation is to employ dispersed platinum catalysts and to maximize catalyst utilization by an effective design of the cathode catalyst layer and by the effective mode of incorporation of the catalyst layer between the polymeric membrane electrolyte and the gas distributor/current collector. The combination of catalyst layer and polymeric membrane has been referred to as the membrane/electrode (M E) assembly. However, in several recent modes of preparation of the catalyst layer in PEFCs, the catalyst layer is deposited onto the carbon cloth, or paper, in much the same way as in phosphoric acid fuel cell electrodes, and this catalyzed carbon paper is hot-pressed, in turn, to the polymeric membrane. Thus, two modes of application of the catalyst layer - to the polymeric membrane or to a carbon support - can be distinguished and the specific mode of preparation of the catalyst layer could further vary within these two general application approaches, as summarized in Table 4. 

Taylor et al.8 were the first to report an electrochemical method for preparation of MEAs for PEMFCs. In their technique, Pt was electrochemically reduced and deposited at the electrode membrane interface, where it was actually utilized as an electrocatalyst. Nation, which is an ion exchange polymer membrane, is first coated on a noncatalyzed carbon support. The Nafion-coated carbon support is then immersed into a commercial acidic Pt plating solution for electrodeposition. Application of a cathodic potential results in diffusion of platinum cations through the active Nation layer. The migrated platinum species are reduced and form Pt particle at the electrode/membrane interface only on the sites which are both electronically and ionically conductive. The deposition of Pt particles merely at the electrode/membrane interface maximizes the Pt utilization. The Pt particles of 2-3.5 nm and a Pt loading of less than 0.05 mg cm-2 were obtained employing this technique.8 The limitation of this method is the difficulty of the diffusion of platinum... 

A Millipore Milll-Q purification system was used for subphase preparation, and a constant temperature bath was used to control the subphase temperature. The mixed spreading solutions were dispersed at the air-water Interface and then slowly compressed at speeds of about 5 A2 mol min-" to surface pressures of 10-15 mN/m prior to deposition. Monolayers were transferred onto electron microscope grids for transmission electron microscopy and electron diffraction, using both the horizontal and vertical dipping techniques. Multilayer assemblies were prepared onto platinum-coated substrates using the vertical dipping technique for Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). 

A petri dish of diameter 20 cm and depth 10 cm is filled with an aqueous electrolyte solution. The platinum cathode is then set at the centre of the petri dish so that the flat tip is placed just on the interface. The electro-deposition is initiated by applying a DC voltage between the cathode and the anode. The anode is immersed in the solution a little below the interface. The metal leaf grows two dimensionally at the interface. The temperature of the system is maintained constant. Depending on the experimental... 

The experimental setup of a batch reactor employed by Rastogi et al. [52] to monitor potential changes at the cathode during electrochemical deposition of lead metal at an air-water interface is shown in Fig. 13.23. In this experiment the micro-slide is put in a dish containing an aqueous solution of lead acetate in such a manner that a small volume of the solution is just above the slide. Two platinum electrodes Pj and Pj are inserted in the solution. The anode Pj is inserted below the surface of the solution while the lower end of the cathode is put just at the surface of the solution. Potential changes during electrochemical deposition of lead metal were monitored with the help of platinum electrode P2 coupled with a calomel electrode C. 

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