Porous matrix electrode layer

Kinetics of Reactant Transport Near and within Porous Matrix Electrode Layer ... 

It is worth pointing out that Eqns (4.13)—(4.20) are for the ORR on a smooth planar electrode or catalyst surface rather than in a porous matrix catalyst layer. It is expected that the situation in the catalyst layer may be more complicated than on the planar surface. However, it is believed that with modification using the apparent parameters as well as the real electrochemical active surface, the equations are still valid for quantitative treatment of experimental data. 

The most common electrode material used in LC-EC is carbon, either as solid glassy carbon disks in thin-layer cells, or as a high-surface-area porous matrix through which the mobile phase can flow. Gold electrodes are useful to support a mercury film and these are primarily used to determine thiols and disulfides, and also for carbohydrates using pulsed electrochemical detection... 

Hydrogen gas fuel and air (O2) are fed to anode and cathode Pt catalyst powder layers, respectively. The Pt catalysts is Teflon-bonded to porous carbon sheets to form gas-diffusion electrodes, with a catalyst loading of about 1.0 mg/cm. The Pt anode and cathode are separated by a thin inert porous matrix that is filled with concentrated phosphoric acid. The cell operates at 200°C (to improve the electrode kinetics), with a cell voltage of about 0.67 V at a current density of 0.150 A/cm. Most voltage losses occur at the air cathode. The hydrogen gas must be pure because sulfur and carbon monoxide poison the Pt anode catalyst. This type of fuel cell is commercially available today, with more than 200 systems installed all over the world in hospitals, hotels, office buildings, and utility power plants. 

Luminescence from Porous Electrodes As in bulk systems, photogeneration of charge carriers in a porous electrode or injection of minority carriers from solution can lead to light emission [25]. In a macroporous system in which the depletion layer can follow the contours of the porous matrix, one does not expect significant differences between bulk and porous electrodes with regard to the potential dependence of the emission. If, however, the porosity is high and the dimensions of the structures become very small (e.g. <5 nm) then special effects may be expected. These are indeed found as, for example, with nanoporous silicon. 

In the above sections, we have presented the electrode kinetics of electron-transfer reaction and reactant transport on planar electrode. However, for practical application, the electrode is normally the porous electrode matrix layer rather thtin a planner electrode siuface because of the inherent advantage of large interfacial area per unit volume. For example, the fuel cell catalyst layers are composed of conductive carbon particles on which the catalyst particles with several nanometers of diameter are attached. On the catalyst particles, some proton or hydroxide ion-conductive ionomer are attached to form a solid electrolyte, which is uniformly distributed within the whole matrix layer. Due to the electrode layer being immersed into the electrolyte solution, this kind of electrode layer is called the flooded electrode layer . 

Figure 2.12 (A) Schematic of the electrode/electrolyte interface of the porous matrix layer, and (B) equivalent electrode/electrolyte interfaces of the porous matrix layer and the oxidant distribution within the interfaces. (For color version of this figure, the reader is referred to the online version of this book.)...

In first approaches, the electrodes were covered with a polymeric membrane of macroscopic size, the enzymes being embedded within the porous matrix [1, 7, 8]. Such thick films are often unduly referred to as multilayered films. In fact, there are no distinct layers in these films, whose thickness is usually larger than 10 pm. The... 

Figure 2.12 shows an electrode layer in a double-layer supercapacitor. This electrode layer is composed of carbon particles and a binder. In the layer, the sources of the capacitance are the pores on the carbon particles and the porous channels within the matrix layer. It is obvious that only locations that are accessible by the electrolyte ions can form the electrode-electrolyte double-layer for capacitance generation. 

Based on Figure 2.12, particle size can also affect the exposed areas of carbons particle to the electrolyte solution. In general, the more porous the matrix layer, the larger the exposed area. Therefore, the carbon particle size should be optimized to yield the best porosity. However, if the porosity of the electrode matrix layer is too high, the electric conductivity of the matrix layer will be reduced, leading to high resistance of the electrode layer and lower power density of the supercapacitor. Therefore, there is a trade-off between the porosities and conductivities of the electrode materials. 

In electrochemical supercapacitors, the ESR is a real series resistance that involves the contact resistance between the current collector and the electrode layer, the resistance of the electrode layer interparticles due to the porous and particulate nature of the electrode matrix, the resistance of the external lead contact, the resistance of the electrolyte, and the resistance caused by the dielectric loss of the interphasal solvent and ions when the AC frequency is higher than hundreds of megahertz (MHz). Figure 2.13 displays a simple equivalent circuit of a supercapacitor in the presence of ESR. 

Natural convection can be eliminated entirely when electrolytes held in a matrix or porous support are used instead of free liquids. Natural convection will not develop in a pore space when the individual pores are sufficiently narrow. When such electrolytes are used, the diffusion layer propagates across the entire matrix (i.e., across the full electrode gap). 

Figure 18 Various models proposed for the surface films that cover Li electrodes in nonaqueous solutions. The relevant equivalent circuit analog and the expected (theoretical) impedance spectrum (presented as a Nyquist plot) are also shown [77]. (a) A simple, single layer, solid electrolyte interphase (SEI) (b) solid polymer interphase (SPI). Different types of insoluble Li salt products of solution reduction processes are embedded in a polymeric matrix (c) polymeric electrolyte interphase (PEI). The polymer matrix is porous and also contains solution. Note that the PEI and the SPI may be described by a similar equivalent analog. However, the time constants related to SPI film are expected to be poorly separated (compared with a film that behaves like a PEI) [77]. (With copyrights from The Electrochemical Society Inc., 1998.)...

Phosphoric-acid fuel cell (PAFC) — In PAFCs the -> electrolyte consists of concentrated phosphoric acid (85-100%) retained in a silicon carbide matrix while the -> porous electrodes contain a mixture of Pt electrocatalyst (or its alloys) (-> electrocatalysis) supported on -> carbon black and a polymeric binder forming an integral structure. A porous carbon paper substrate serves as a structural support for the electrocatalyst layer and as the current collector. The operating temperature is maintained between 150 to 220 °C. At lower temperatures, phosphoric acid tends to be a poor ionic conductor and poisoning of the electrocatalyst at the anode by CO becomes severe. 

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

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