Properties of Porous Electrodes

Practical units of fuel cells could not operate without porous electrode structures. Porous electrodes with their large electrochemically active surface allow reasonable currents to be supplied at acceptable losses due to polarization (see section 2 of chapter II). Although a few properties, like maximum available surface of electrocatalyst and hydrogenation and dehydrogenation of carbonaceous species for Teflon-bonded platinum black electrodes, and formation of oxygen layers for Raney nickel electrodes, have been discussed in preceding chapters, a discussion of the parameters that determine the operation of porous electrodes had to be offered in a separate chapter. While the empirical aspects concerning the operation of porous electrodes are covered in this chapter, theoretical aspects are dealt with in chapter XVI. 

According to the mode of operation the porous electrodes may be classified in two groups  

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Chapters I to III introduce the reader to the general problems of fuel cells. The nature and role of the electrode material which acts as a solid electrocatalyst for a specific reaction is considered in chapters IV to VI. Mechanisms of the anodic oxidation of different fuels and of the reduction of molecular oxygen are discussed in chapters VII to XII for the low-temperature fuel cells and the strong influence of chemisorhed species or oxide layers on the electrode reaction is outlined. Processes in molten carbonate fuel cells and solid electrolyte fuel cells are covered in chapters XIII and XIV. The important properties of porous electrodes and structures and models used in the mathematical analysis of the operation of these electrodes are discussed in chapters XV and XVI. 

Our experiments, as well as analysis of the proposed theoretical model for a generalized system of porous electrode "active material - carbon additive" proved that thermally exfoliated graphite (TEG) can be one of the most effective conductive additive and structural support for the different new and existing active materials. The reason for such wide application of TEG is a following unique complex of TEG properties low density, relatively high conductivity and stability to electrochemical oxidation. 

Silicon exhibits a diverse range of electrochemical phenomena, such as current oscillation, anisotropic etching, formation of porous silicon, etc. Each of these phenomena has extremely rich details that are governed by complex relationships between structures and properties of silicon electrodes on the one hand and between properties and experimental conditions on the other. The silicon/electrolyte interface is a complex system in which a great many variables are interacting with each other in a great many ways." ... 

The primary challenge in commercialization of MCFC remains in the proper selection of materials for the cathode. The life expectancy of the electrode structure is aimed toward 40,000 hr for successful commercialization of MCFC. The following cathode properties were recognized as of fundamental importance with respect to the cell performance 1) high electronic conductivity at 650°C (cr > 1 S/cm) 2) low chemical reactivity and solubility in the electrolyte 3) thermodynamic stability at 650°C in carbonate electrolyte at different partial pressures of O2/CO2 mixtures 4) high electrocatalytic activity for the oxygen reduction reaction and 5) suitability for the fabrication of porous electrodes. ... 

Electrochemical engineering Opportunities for improving the productivity from the U.S. investment in basic electrochemical research are described in areas of porous electrodes and extended interfacial regions, surface creation and destruction phenomena, process analysis and optimization, process invention, and the physical property data base. 

In short, the overall capacitive performance of a RTIL-based supercapacitor is determined by both RTIL properties and electrode nature, such as the electrolyte dynamics, electrolyte ion species, geometrical structure of porous electrode, working potential, and operating temperature. 

Yu XI et al (2013) Preparation and electrochemical properties of porous silicon/carbon composite as negative electrode materials. J Inorg Mater. doi 10.3724/SP.J.1077.2013.12672 Yue L et al (2013) Porous silicon coated with S-doped carbon as anode material for lithium ion batteries. J Solid State Electrochem doi 10.1007/s/10008-012-1944-8 Zhang Y, Huang J (2011) Hierarchical nanofibrous silicon as replica of natural cellulose substance. J Mater Chem 21 7161-7165... 

Endres HE, Hartinger R, Schwaiger M, Gmelch G, Roth M (1999) A capacitive CO sensor system with suppression of the hunudity interference. Sens Actuators B 57(l-3) 83-87 Erdamar O, BUen B, Skarlatos Y, Aktas G, Ind MN (2007) Effects of humidity and acetone on the optical and electrical properties of porous silicon nanostructures. Physica Status Sohdi C 4 601-603 Fiiijes P, KovScs A, Diicso Cs, Adam M, Muller B, Mescheder U (2003) Porous sihcon-based humidity sensor with interdigital electrodes and internal heaters. Sens Actuators B 95 140-144 Goeders KM, Colton JS, Bottomley LA (2008) Microcantilevers sensing chemical interactions via mechanical motion. Chem Rev 108 522-542... 

Recently, pore network modeling has been applied to simulate the accumulation of liquid water saturation within the porous electrodes of polymer electrolyte membrane fuel cells (PEMFCs). The impetus for this effort is the understanding that liquid water must reside in what would otherwise be reactant diffusion pathways. It therefore becomes important to be able to describe the effect that saturation levels have on reactant diffusion. Equally important is the understanding of how the properties of porous materials affect local saturation levels. This requirement is in contrast to most continuum modeling of the PEMFC, where porous materials are treated with volume-averaged properties. For example, the relationship between bulk liquid saturation and capillary pressures foimd through packed sand and other soil studies are often employed in continuum models. ... 

This chapter will cover major topics of CL research, focusing on (i) electrocatalysis of the ORR, (ii) porous electrode theory, (iii) structure and properties of nanoporous composite media, and (iv) modern aspects in understanding CL operation. Porous electrode theory is a classical subject of applied electrochemistry. It is central to all electrochemical energy conversion and storage technologies, including batteries, fuel cell, supercapacitors, electrolyzers, and photoelectrochemi-cal cells, to name a few examples. Discussions will be on generic concepts of porous electrodes and their percolation properties, hierarchical porous structure and flow phenomena, and rationalization of their impact on reaction penetration depth and effectiveness factor. 

The most extensive approach to the investigation of fuel cells numerically so far, applies continuum equations and global relations to predict the characteristics of fuel cells on a system level. For porous material, the homogeneous properties of porous material, like porosity and tortuosity, are used to calculate an equivalent result. This strategy reduces the complexity of modelling within the microstructure but sacrifices the precision of modeling. Moreover, detailed information of the transfer process at the electrode/electrotyte interface is missing. 

Cai K.F., Muller E., Drasar C., Mrotzek A. Preparation and thermoelectric properties of Al-doped ZnO ceramics. Mater. Sci. Eng. B 2003 104 45 8 Candy J.P., Fouilloux P., Keddam M., Takenouti H. The characterization of porous-electrodes by impedance measurements. Electrochimica Acta 1981 26 1029-1034 Choi Y.M., Pyun S.I., Moon S.I., Hyung Y.E. A study of the electrochemical lithium intercalation behaviour of porous LiNi02 electrodes prepared by solid-state reaction and sol-gel methods. J. Power Sources 1998 72 83-90... 

Simple models of porous structures involve straight cylindrical pores. An understanding of the processes which determine the operation of a single pore under certain conditions is important. The discussion of the properties of single pores represents the first step. It is followed by the consideration of more elaborate models of porous structures. A comprehensive review of the progress made in the theory of porous electrodes in the last three decades was given by Chismadzhev [1] recently. Transient responses of porous electrodes are not discussed in this chapter since steady-state conditions are chiefly of interest for the operation of fuel cells. The reader is referred to the review by de Levie [2]. 

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