Electrolyte unsupported

Carbon-supported platinum (Pt) and platinum-rathenium (Pt-Ru) alloy are one of the most popular electrocatalysts in polymer electrolyte fuel cells (PEFC). Pt supported on electrically conducting carbons, preferably carbon black, is being increasingly used as an electrocatalyst in fuel cell applications (Parker et al., 2004). Carbon-supported Pt could be prepared at loadings as high as 70 wt.% without a noticeable increase of particle size. Unsupported and carbon-supported nanoparticle Pt-Ru, ,t m catalysts prepared using the surface reductive deposition... 

Figure 1. Compilation of platinum mass activities as a function of platinum B.E.T. surface area [ ] Watanabe et alJ [0] Buchanan et al.s [ ] Buchanan et al. and [0] Bregoli6. The solid line is 0.6A.m 2 constant specific activity platinum. The broad arrow on the abscissa denotes the maximum surface area for a platinum crystallite when all of the atoms are located at the surface (275 m2 g 1 Pt). Phosphoric acid at 190 °C and 0.9 V vs. hydrogen in the same electrolyte, (a) Data up 210 m2g" Pt. (b) Data below 100m2g Pt m Bregoli6 results on unsupported platinum black.

Free-flow electrophoresis is accomplished by a laminar flow of unsupported electrolyte between glass plates the absence of a supporting medium nullifies adsorption and filtration interactions, and the free flow enables relatively large quantities of sample to be processed. Free-flow electrophoresis not only fractionates dissolved charged materials, but also has the capability to fractionate suspended particulate material on the basis of charge as described by Strickler (1967). 

Most important in these processes is an active cathode. In most cases finely divided platinum is used for this purpose. Unsupported platinum, particularly the one reduced from Adams catalyst, and carbon-supported platinum electrocatalysts are effective in reducing the amount of platinum required for a given amount of power from a hydrocarbon fuel cell. A propane-oxygen fuel cell, which uses 10 mg Pt per cm supported on carbon can deliver about 35 mW cm using a hydrofluoric acid electrolyte at 105... 

Macdonald and coworkers [158-164] obtained an exact solution of the finite-length diffusion impedance in unsupported conditions where the Nemst-Planck and continuity equations for both negative and positive mobile charges were solved and involved full satisfaction of Poisson s equation. One can expect such conditions in diluted electrolytic solutions and in poorly conductive solids. 

It is thus unfortunate that there has been a tendency among some workers in the solid electrolyte field to take over many of the relatively simple theoretical results derived for supported conditions and use them uncritically in unsupported situations, situations where the snpported models and formulas rarely apply adequately. For example the expression for the Warburg impedance for a redox reaction in a supported situation is often employed in the analysis of data on unsupported situations where the parameters involved are quite different (e.g. Sections 2.2.3.2 and 2.2.3.3). 

Consider a system consisting of a binary, unsupported electrolyte between electrodes which are reversible only to the cation. The cell is initially at equilibrium (no net currents are passing). At very short times after the imposition of a potential difference, the concentrations of all species in the bulk of the electrolyte are uniform and the ions move in response to the applied field. The current is determined by the uniform electrolyte conductivity. 

In solid electrolytes, however, the unsupported electroactive species is often the sole charge carrier. It is thus impossible to change the interfacial potential difference without changing the concentration of ions in the double-layer region. This means of course, that the normal Warburg impedance is not seen, but it also means that there is a coupling between the faradic current and the double-layer charging. 

Solutions for unbiased, flat-band situations (i.e. where there are no intrinsic space charge layers at the boundaries) are simplest, and only these will be discussed below except when otherwise noted. We shall present a brief discussion of the supported situation, primarily appropriate for liquids and mixed conductors, and devote more space to results for unsupported materials, since most solid electrolytes involve unsupported ionic conduction under conditions of primary interest. For simplicity, theoretical results for flux, currents, impedances, and other circuit elements will be given in specific form, per unit of electrode area A so this area will not appear directly in the formulas. 

We have attempted to give supported results in a form appropriate for comparison with unsupported ones by considering full-cell conditions. The transition problems discussed above only occur for unstirred (liquid) electrolytes or for solid electrolytes. When a stirred solution or rotating electrode with lanfinar flow is employed, the I which appears in Z/> and Zw expressions is replaced by 5n, where 5n is the thickness of the Nemst diffusion layer. It decreases as the frequency of rotation of a rotating electrode increases and the experiment is always carried out for conditions where 5 1. 

Figure 2. 8. (a) Transmission line representation of Nernst-Planck Poisson equation system for a binary electrolyte. Rp and R are charge transfer resistances for positive and negative charge species at the electrode, respectively, (b) General approximate equivalent circuit (full-cell, unsupported) for the [0. Ai> cases applying to a homogeneous liquid or solid material. 

Sah [1970] introduced the use of networks of electrical elements of infinitesimal size to describe charge carrier motion and generation/recombination in semiconductors. Barker [1975] noted that the Nemst-Planck-Poisson equation system for an unsupported binary electrolyte could be represented by a three-rail transmission line (Figure 2.2.8fl), in which a central conductor with a fixed capacitive reactance per unit length is connected by shunt capacitances to two resistive rails representing the individual ion conductivities. Electrical potentials measured between points on the central rail correspond to electrostatic potential differences between the corresponding points in the cell while potentials computed for the resistive rails correspond to differences in electrochemical potential. This idea was further developed by Brumleve and Buck [1978], and by Franceschetti [1994] who noted that nothing in principle prevents extension of the model to two or three dimensional systems. 

Nafion 117 (purchased from DuPont) was used as electrolyte membrane for the DMFC single cell, which was pretreated in mildly boiling water with 3% H2O2 for 2 hours, then boiled in 2 M H2SO4 for 2 hours. For each treatment the membrane was washed in de-ionized water several times. After these treatments it was stored in water for preparation of membrane electrode assembly (MEA). Johnson Matthey s unsupported Pt black (2mg/cm ) and Pt-Ru... 

Flooded type CCE are most suited for applications involving mass transport from/to the electrolyte and from/to the gas side, while channeled electrodes are best for situations where the gas transport is the sole rate determining step (Lev et al. 2005). The electrodes can be used as supported films spread on a porous substrate (where the gas flows through this substrate into the bulk electrode and the other side of the electrode is immersed in an electrolyte) or as an unsupported film, with one side immersed in the electrolyte and its other side connected to an external lead and exposed to the gas. 

RALEIGH Could you comment on the relative nature of Warburg impedance in supported and unsupported electrolytic media The concept of a Warburg impedance in an unsupported medium may be confusing, since it was originally derived for field-free diffusion in supported electrolytes. 

It is essential that a solubilized form of an alkaline anion-exchange polymer be developed to improve the interface between the electrodes and the AEM electrolyte. Success in this effort will decrease MEA resistances. A water-based soluble form which can be rendered water insoluble when cast would be preferred, as there are safety concerns (primarily with industrial scale production) about using organic solvents near finely dispersed (pyrophoric) metal catalysts (unsupported or supported on carbon). 

Arico AS, Shukla AK, El-Khatib KM, Creti P, Antonucci V. Effect of carbon-supported and unsupported Pt-Ru anodes on the performance of sohd-pol5uner-electrolyte direct methanol fuel cells. J Appl Electrochem 1999 29 671-6. 

Aquivion E87-12S short-side chain perfluorosulfonic acid (SSC-PFSA) membrane with equivalent weight (EW) of 870 g eq and 120 pm thickness produced by Solvay Specialty Polymers was tested in a polymer electrolyte membrane water electrolyser (PEMWE) and compared to a benchmark Nation N115 membrane (EW 1100 g eq ) of similar thickness [27]. Both membranes were tested in conjunction with in-house prepared unsupported Ir02 anode and carbon-supported Pt cathode electrocatalyst. The electrocatalysts consisted of nanosized Ir02 and Pt particles (particle size 2-4 nm). The electrochemical tests showed better water splitting performance for the Aquivion membrane and ionomer-based membrane-electrode assembly (MEA) as compared to Nafion (Fig. 2.21). Lower ohmic drop constraints and smaller polarization resistance were observed for the electrocatalyst-Aquivion ionomer interface indicating a better catalyst-electrolyte interface. A current density of 3.2 A cm for water... 

---