Conductivity contact surface area

A possible explanation for the sharp increase in conductivity of the ceria electrolytes that occurs in contact with the proton-conducting alkali-metal carbonates or with nonconducting oxides was offered in a paper by Zhu et al. (2003). Adsorption of an excess number of oxygen ions (0 ) is possible at the contact surface areas between the nanoparticles of the two phases. In this way a new, additional interfacial conduction pathway is opened up for the ionic current. 

Certain aspects of the operation of mini-fuel cells with a large contact surface area between cathode and ambient air were examined by Schmitz et al. (2004) for a methanol-air mini-fuel cell. They studied the elimination of product water when the open contact surface area was in different spatial orientations pointing up horizontally, and vertical. The tests were conducted while ambient temperature varied between 25 and 31°C. It was found that cell operation was more stable with a vertical orientation of the contact surface area. In this orientation, the values of relative humidity at points close to the contact surface area were lower than in the horizontal orientation. As an explanation, the authors suggested that the water vapor produced will partially condense, yielding liquid droplets, which, under the effect of gravity, will slip down the vertical surface. 

Thermal Conductance through Pressed Contacts Previously, we discussed the thermal impedance Z associated with a bar of length / and cross-sectional area A. Now consider the joint between two surfaces. Although it has essentially zero thickness, it causes a thermal impedance between the two materials. The heat transfer can be calculated from the conductance formulas in the form P = KM = AT/Z. As before, the conductance K is proportional to the contact surface area A, but in this case there is no length involved. 

Tape adhesives can be made thermally conductive by the dispersion of small articles of a conductive filler such as Saint-Gobain boron nitride (BN) PCTH3MHF and spherical aluminum oxide (AI2O3) available from Denka Corp. [42]. For example, 3M Corp offers pressure-sensitive adhesive (PSA) tapes filled with thermally conductive ceramic particles and flame retardant fillers. This product is designed with a thin polyester (PET) film and a soft acrylic polymer. It conforms to surfaces to which it adheres thus providing contact surface area for heat transfer [43]. 

A considerable decrease in platinum consumption without performance loss was attained when a certain amount (30 to 40% by mass) of the proton-conducting polymer was introduced into the catalytically active layer of the electrode. To this end a mixture of platinized carbon black and a solution of (low-equivalent-weight ionomeric ) Nafion is homogenized by ultrasonic treatment, applied to the diffusion layer, and freed of its solvent by exposure to a temperature of about 100°C. The part of the catalyst s surface area that is in contact with the electrolyte (which in the case of solid electrolytes is always quite small) increases considerably, due to the ionomer present in the active layer. 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

In addition to bilayered electrodes with a functional layer and a support layer, electrodes have also been produced with multilayered or graded structures in which the composition, microstructure, or both are varied either continuously or in a series of steps across the electrode thickness to improve the cell performance compared to that of a single- or bilayered electrode. For example, triple-layer electrodes commonly utilize a functional layer with high surface area and small particle size, a second functional layer (e.g., reference [26]) or diffusion layer with high porosity and coarse structure, and a current collector layer with coarse porosity and only the electronically conductive phase (e.g., reference [27]) to improve the contact with the interconnect. 

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

The Cottrell equation, as written here, relates to an electrode in the form of a cylindrical wire. One end of the wire will be embedded in a non-conductive sleeve (e.g. glass), so that only one end of the wire will ever be in contact with the analyte solution. If the wire has a length h and a diameter r, then the surface area A of the wire is given by ... 

In addition to the criticisms from Anderman, a further challenge to the application of SPEs comes from their interfacial contact with the electrode materials, which presents a far more severe problem to the ion transport than the bulk ion conduction does. In liquid electrolytes, the electrodes are well wetted and soaked, so that the electrode/electrolyte interface is well extended into the porosity structure of the electrode hence, the ion path is little affected by the tortuosity of the electrode materials. However, the solid nature of the polymer would make it impossible to fill these voids with SPEs that would have been accessible to the liquid electrolytes, even if the polymer film is cast on the electrode surface from a solution. Hence, the actual area of the interface could be close to the geometric area of the electrode, that is, only a fraction of the actual surface area. The high interfacial impedance frequently encountered in the electrochemical characterization of SPEs should originate at least partially from this reduced surface contact between electrode and electrolyte. Since the porous structure is present in both electrodes in a lithium ion cell, the effect of interfacial impedances associated with SPEs would become more pronounced as compared with the case of lithium cells in which only the cathode material is porous. 

Graphites with larger surface areas or greater porosities have a distinctly lower percolation threshold. It is assumed that the conductivity of a compound depends upon the structured agglomerates being sufficiently close to each other, or in direct contact above the percolation point, and on the continuous current pathways created thereby (14-15). 

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