Current collector layers

Better performance in fuels with 5% H2S than in pure H2. Pt current collector layer. 

Ni-YSZ cermet infiltrated by Mo orW precursors [116] 2005 Still poisoned by sulfur (C4H4S) but to a lesser extent and showed gradual recovery. No Pt current collector layer. Pmax °f 300 mW/cm2 at 750°C in H2. 

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],... 

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]). 

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. 

The average grain size and grain size distribution of the CFL and cathode current collector layer (CCCL)... 

The complete process for the fabrication of the proton-conducting membranes, previously reported in [69,73], can be described as follows. A 4-inch 520 /itm thick n+ (100)-oriented double-side polished silicon wafer is first thermally oxidized in an oven at 1000 °C under O2 and water steam flows to obtain a 2 nm thick Si02 layer on both sides of the substrate. These layers will allow the electrical insulation between the two parts of the future fuel cell. Then these previous layers are covered with sputtered Cr-Au layers on both sides. The Cr layers are used as adherence layers for the Au layers and are relatively thin (30 nm). The Au layers are 800 nm thick and will serve as current collector layers for the fuel cells. They are also useful as masking layers for the next different etchings since Au is not etched neither by KOH solution nor by HF solution, the two wet etchants used in the next steps. 

Wet Powder Spraying Bilayer cathodes consisting of cathode and cathode current collector layer can be manufactured by wet powder spraying (WPS). The cathode contact layer is also applied on the metallic interconnects by spraying. Limitations of the technology are the overspray (the amount of suspension which is sprayed past the object to be coated) and the formation of a suspension mist which needs to be extracted by suction. The overspray can be recycled, though. 

Obviously, the design of the components and properties of materials must accommodate the above-listed processes with minimum obstruction and losses. Because in some of the components more than one process takes place, very often with conflicting requirements, the properties and the design must be optimized. For example, the gas diffusion layer must be optimized so that the reactant gas may easily diffuse, yet at the same time that water, which travels in the opposite direction, does not accumulate in the pores. On top of that, the diffusion layer (or current collector layer as it is sometimes called) must be both electrically and thermally conductive. Similar requirements may be established for almost every fuel cell component. Although a fuel cell seems to be a very simple device, numerous processes take place simultaneously. It is therefore important to understand those processes, their mutual interdependence, and their dependence on components design and materials properties [1]. 

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