Cathode functional layer

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

In addition to bilayered anode and cathode functional layer and current collector/sup-port layer combinations, bilayered electrolyte structures are commonly fabricated, particularly for low-temperature operation below 700°C, by a variety of processing methods. Bilayered electrolytes are used for several purposes ... 

Figure 9.5 Current density at 700 mV of ASCs with an LSM cathode between 700 and 800 °C as function of the 8YSZ LSM ratio of the cathode functional layer (CFL).

Cathode functional layer Catho de functional layer - paste Applying catho de functional layer Cathode... 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

FIG U RE 6.4 SEM cross-sectional micrograph of an SOFC, showing an anode support layer, anode functional layer, electrolyte, and cathode [24]. Reprinted from [24] with permission from Elsevier. 

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

In an SOFC, the electrochemical reactions take place in the electrodes in the functional layer, that is, a zone within a distance of less than 10-20 pm from the electrolyte surface [5,136-138], The portion of the electrode beyond this width is principally a current collector structure, which has to be porous to permit the admission of gas to the functional layer where the oxidation and reduction reactions occur. Besides, the electrolyte has to be gas impermeable to avoid direct combination and combustion of the gases [137], The essential parts of the SOFC, that is, the electrolyte, the anode, and the cathode, are made of ceramic materials produced with appropriate electrical conducting properties, chemical and structural stabilities, similar expansion coefficients, and negligible reactivity properties [135],... 

Figure 5.30. Schematic of the catalyst layer geometry and its composition, exhibiting the different functional parts, a A sketch of the layer, used to construct a continuous model, b A one-dimensional transmission-line equivalent circuit where the elementary unit with protonic resistivity Rp, the charge transfer resistivity Rch and the double-layer capacitance Cj are highlighted [34], (Reprinted from Journal of Electroanalytical Chemistry, 475, Eikerling M, Komyshev AA. Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells, 107-23, 1999, with permission from Elsevier.)...

Larring, Y. and Norby, T., Spinel and perovskite functional layers between Plansee metallic interconnect (Cr-5 wt% Fe-1 wt% Y2O3) in ceramic (Lao 85Sro.i5)o.9iMn03 cathode materials for solid oxide fuel cells, J. Electrochem. Soc., 147, 3251-3256 (2000). 

Xie, Z. et ah. Functionally graded cathode catalyst layers for polymer electrolyte fuel cells, J. Electrochem. Soc., 152, A1171, 2005. 

Fig. 11 Schematic picture of the cathode catalyst layer and its composition, exhibiting the different functional parts. The typical catalyst layer thickness is l 10-20 pm.

The role of the porous structure and partial liquid-water saturation in the catalyst layer in performance and fuel cell water balance has been studied in Ref. 241. As demonstrated, the cathode catalyst layer fulfills key functions in vaporizing liquid water and in directing liquid-water fluxes in the cell toward the membrane and cathode outlet. At relevant current densities, the accumulation of water in the cathode catalyst layer could lead to the failure of the complete cell. The porous structure controls these functions. 

The three components of the fuel cell, anode, cathode, and electrolyte form a membrane-electrolyte assembly, as, by analogy with polymer electrolyte fuel cells, one may regard the thin layer of solid electrolyte as a membrane. Any one of the three membrane-electrode assembly components can be selected as the entire fuel cell s support and made relatively thick (up to 2 mm) in order to provide mechanical stability. The other two components are then applied to this support in a different way as thin layers (tenths of a millimeter). Accordingly, one has anode-supported, electrolyte-supported, and cathode-supported fuel cells. Sometimes though an independent metal or ceramic substrate is used to which, then, the three functional layers are applied. 

In summary, coating the surface of Li jMOy cathode materials by appropriate thin functional layers that form buffer zone on their surface may provide the following advantages [91] (as exhibited in a recent review article) ... 

Figure 23.6 Experimental data of Konysheva et al. [17], Cathode polarization voltage versus time for the functional layers of a thickness 13, 20, and 50 [im (FL13, FL20, and FL50, respectively). The cell current (A cm )...

Conventional organic bulk heterojunction solar cells do not have hole-blocking electron transport layers (HBETL) at the cathode/active layer interface. Metals with reasonably low-work function (e.g., calcium, aluminium, magnesium, or barium) are typically evaporated directly onto fullerene/polymer blend forming electronic contacts with both donor and acceptor materials under optimal conditions. This situation leads to significant recombination of positive and negative charge carriers at the active layer/cathode interface. 

Solid Oxide Fuel Cells, Direct Hydrocarbon Type, Fig. 3 F iedicted gas constitution versus position in the SOFC anode support, for a current density of 1 A/cm. The top shows a cell with a conventional NiYSZ support and the bottom a Sro.8Lao.2Ti03 support Both cell types had a NiYSZ anode functional layer, YSZ electrolyte, and LSMYSZ (LSM = Lao.8Sro.2Mn03) cathode (From Ref [34])... 

Zh. Xie, T. Navessin, K. Shi, R. Chow, Q. Wang, D. Song, B. Andreaus, M. Eikerling, Zh. Liu, and S. Holdcroft. Functionally graded cathode catalyst layers for polymer electrolyte fuel cells. J. Electrochem. Soc., 152 A1171—A1179, 2005. 

A fuel cell constructed from sequential functional layers of anode, electrolyte, and cathode in a planar geometry. 

In physical terms, an SOFC fuel cell is a very thin, planar sheet of flexible, ceramic material consisting of three functional layers - the anode, the cathode, and the electrolyte (Figs. 20.1 and 20.2). 

The mechanical strength of the thin-film cell as well as the electrical and thermal conductivity are provided by an open porous metalfic substrate which also serves as a fuel gas distributor. The functional layers of the cell - anode, electrolyte, and cathode - are consecutively deposited onto this substrate by a multistep vacuum plasma spray process. The contact from the cathode to the ferritic steel interconnect (bipolar plate) is provided by a flexible and ductile perovskite coating. An... 

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