Three phase interface

Figure 4.1 Schematic of the atomic structure of the active three-phase interface between the metal particle that catalyzes the reaction, the carbon support necessary to conduct electrons, and the polymer electrolyte and solution necessary to conduct protons for electrocatalytic systems.

The functions of porous electrodes in fuel cells are 1) to provide a surface site where gas/liquid ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the three-phase interface once they are formed (so an electrode must be made of materials that have good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions, the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in high-temperature fuel cells because ionization reaction rates increase with temperature. It is also a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a one-sided manner (see latter part of next section). 

In MCFCs, which operate at relatively high temperature, no materials are known that wet-proof a porous structure against ingress by molten carbonates. Consequently, the technology used to obtain a stable three-phase interface in MCFC porous electrodes is different from that used in PAFCs. In the MCFC, the stable interface is achieved in the electrodes by carefully tailoring the pore structures of the electrodes and the electrolyte matrix (LiA102) so that the capillary forces establish a dynamic equilibrium in the different porous structures. Pigeaud et al. (4) provide a discussion of porous electrodes for MCFCs. 

In a SOFC, there is no liquid electrolyte present that is susceptible to movement in the porous electrode structure, and electrode flooding is not a problem. Consequently, the three-phase interface that is necessary for efficient electrochemical reaction involves two solid phases (solid electrolyte/electrode) and a gas phase. A critical requirement of porous electrodes for SOFC is that they are sufficiently thin and porous to provide an extensive electrode/electrolyte interfacial region for electrochemical reaction. 

Mixed conducting (i.e., electronic and ionic) materials for anodes may be advantageous if H2 oxidation can occur over the entire surface of the electrode to enhance current production, instead of only in the region of the three-phase interface (gas/solid electrolyte/electrode). Similarly, mixed conductors also may be advantageous for cathodes. 

A schematic of a typical fuel-cell catalyst layer is shown in Figure 9, where the electrochemical reactions occur at the two-phase interface between the electrocatalyst (in the electronically conducting phase) and the electrolyte (i.e., membrane). Although a three-phase interface between gas, electrolyte, and electrocatalyst has been proposed as the reaction site, it is now not believed to be as plausible as the two-phase interface, with the gas species dissolved in the electrolyte. This idea is backed up by various experimental evidence, such as microscopy, and a detailed description is beyond the scope of this review. Experimental evidence also supports the picture in Figure 9 of an agglomerate-type structure where the electrocatalyst is supported on a carbon clump and is covered by a thin layer of membrane. Sometimes a layer of liquid water is assumed to exist on top of the membrane layer, and this is discussed in section 4.4.6. Figure 9 is an idealized picture, and... 

The process is characterised by the electrofluorination of volatile organic substrates within the matrix of pores of a carbon anode immersed in molten KF 2HF as electrolyte (as in a mid-temperature fluorine generator cell), and depends on the phenomenon that the anodically charged porous carbon is not wetted by the electrolyte. The fluorination probably takes place at the three phase interface of organic vapour, solid carbon, and liquid electrolyte in close proximity to, or at the sites where fluorine is being evolved. 

Figure 23 Air-breathing microfluidic fuel cells showing the colaminar flow principle, in combination with oxygen capture via gas diffusion through a porous cathode A three-phase interface is established between gas, electrolyte, and catalyst/solid electrode (reprinted with permission from Jayashree et al., 2005. Copyright 2005 American Chemical Society).

An SOFC cathode normally consists of a porous matrix cast onto an oxide ion-conducting electrolyte substrate (see Figure 8.24), where the cathode porosities are typically 25-40 vol% [66,123,137], Besides, the cathode must be an electron conductor and catalytically active for the oxygen reduction reaction. However, because it is not an oxygen conductor, it must be porous with an optimized three-phase interface at which the reduction reaction takes place [33],... 

In the quest to improve fuel cell performance, the concept of fuel cell reactions requiring a three-phase interface was first proposed by Grove. In his initial experiment, he noticed that the reaction sped up when the three-phase area was large. In 1923, Schmid [7] developed the first gas diffusion electrode, which significantly increased the electrode active surface area and revolutionized fuel cell electrodes. The electrode contained a coarse-pore graphite gas-side layer and a fine porous platinum electrolyte layer. 

The three-phase interface is a well-known concept, notably in connection with the invention by Bacon of bi-porous electrodes (Bacon, 1969). At equilibrium, in such electrodes, gas, liquid electrolyte and catalytic solid are in contact at a convoluted meniscus at the coarse-pore, liquid-containing region interface with the fine-pore, gas-containing region. The picture changes with departure from equilibrium. The meniscus becomes mobile under the influence of surface tension gradients or Marangoni forces - an invisible complex. 

The three-phase interface can be favourably influenced by the introduction of a thin layer of material, capable of conducting both ions and electrons, between the electrolyte and the electrode structure. 

Electrochemical reactions in a PEMFC occur at the Three-phase interface, where the three necessary components for the reaction, i.e., reactant (e.g., gas), electron conductor (e.g., metal electrode), and ionic conductor (e.g., solid polymer electrolyte), meet. Fig. 6 schematically illustrates this in the PEMFC situation. The three components can actually meet, not only on the line defining the three-phase contact, but also in an area of two-phase contact, where gas molecules dissolve and diffuse through the electrolyte onto the electrode surface. As the electrochemical reaction depends on the concentrations of reacting species at the interface, the diffusion and solubility of the reactants and products in the electrolyte are important parameters in determining the overall rate of the electrochemical reactions. In practice, the polymer electrolyte, which is dissolved in solution, is mixed with electrode... 

As mentioned above, tests to determine the corrosion resistance of materials in industrial corrosive media can be performed in the laboratory, in pilot plants, and in existing production plants. With regard to the choice of locations for specimens, bear in mind that reactors and other apparatus may be attacked differentially by the liquid and vapor phase of the corrosive agent and at the three-phase interface of the liquid, vapor, and material. As a rule, therefore, material specimens must be exposed at each of these phases. Corrosion testing principles are described in well-known standards (ASTM G 4-84 1984 ASTM G 31-72 1985 ASTM G 78-83 1983 ASTM G 30-79 1964 ASTM G 38-73 1984). 

The proper construction of a stable, well-dispersed, three-dimensional catalyst layer is one of the most critical determinants of performance for a PEM fuel cell. The membrane isolates the reactants from one another and provides an ionic current path from one electrode to another, and the flow fields and gas-diffusion layers distribute the reactants to the catalyst layer, but all of the relevant electrochemical reactions are carried out in the catalyst layers themselves. It is the proper construction of the so-called three-phase interface that allows the reactants and products to be brought into intimate contact and makes possible the operation of the fuel cell. Indeed, it is the tailoring of this layer by Raistrick et al. [1] in 1991 that demonstrated the practical feasibility of lowering precious metal loadings by a faetor of 40 over previous designs and helped to usher in the past deeade of inereased activity and investment in fuel cell development. 

The internal resistance of the polymer electrolyte membrane depends on the water content of the membrane. The water ionizes add moieties providing mobile protons, like protons in water [1-3]. Absorbed water also swells the membrane, which may affect the interface between the polymer electrolyte and the electrodes. Nafion, a Teflon/perfluorosulfonic acid copolymer, is the most popular polymer electrolyte because it is chemically robust to oxidation and strongly acidic. The electrodes are commonly Pt nanoparticles supported on a nanoporous carbon support and coated onto a microporous carbon cloth or paper. These structures provide high three-phase interface between the electrolyte/catalyst/reactant gas at both the anode and cathode. 

Subbaraman R, StrumcnikD, Stamenkovic V, Markovic NM (2010) Three phase interfaces at electrified metal-solid electrolyte systems 1. Study of the Pt(hkl)-Nafion interface. J Phys Chem C 114 8414-8422... 

In Eq. 3, 0i indicates the contact angle at the three-phase interface. 

Yano, H., T. Tanaka, M. Nakamaya, and K. Ogura (2004). Selective electrochemical rednction of CO2 to ethylene at a three-phase interface on copper(I) halide-confined Cn-mesh electrodes in acidic solutions of potassium halides. J. Electroanal. Chem. 565(2), 287-293. 

The pressure drop across the anode for a cell operating isothermally at 0.7 V is shown in Fig. 7.9. The anode is 750 p.m thick and the inlet fuel consists of 40% CH4 and 60% H2O. The pressure drop principally follows the trend of current density. Higher current density regoins within the cell lead to higher pressure drop due to higher production of H2O at the three-phase interface. 

---