Ni-YSZ anode

Lee J-H, Moon H, Lee H-W, Kim J, Kim J-D, and Yoon K-H. Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni-YSZ cermet. Solid State Ionics 2002 148 15-26. 

Anode Ni/YSZ Cu/doped CeOi, several conducting oxides (e g., doped SrTiOj)... 

Currently, electrolyte-supported, cathode-supported, anode-supported, and metallic substrate-supported planar SOFCs are tmder development. In electrolyte-supported cells, the thickness of the electrolyte, typically YSZ, is 50-150 pm, making then-ohmic resistance high, and such cells are suitable only for operation at 1,000°C. In electrode-supported designs, the electrolyte thickness can be much lower, typically 5-20 pm, which decreases their ohmic resistance and makes them better suited for operation at lower temperatures. The anode (Ni/YSZ cermet) is selected as the supporting electrode, because it provides superior thermal and electrical conductivity, superior mechanical strength, and minimal chemical interaction with the electrolyte. Kim et al. [83] have reported power densities as high as 1.8 W/cm at 800°C for such anode-supported SOFCs. At Pacific Northwest National Laboratory [84, 85], similar anode-supported cells have been developed using 10 pm... 

One can only admire the insight of the first researchers who used Ni as the active electrode material in the Ni/YSZ cermet anodes In addition to being a good electrocatalyst for the charge transfer reaction (3.8), Ni is also an excellent catalyst for the steam or C02-reforming of methane ... 

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

I.V. Yentekakis, Y. Jiang, S. Neophytides, S. Bebelis, and C.G. Vayenas, Catalysis, Electrocatalysis and Electrochemical Promotion of the Steam Reforming of Methane over Ni Film and Ni-YSZ cermet Anodes, Ionics 1, 491-498 (1995). 

Figure 5.32. Double layer capacitance as a function of overpotential of the system a) Pt/YSZ, b) Au/YSZ, c) Ni/YSZ and d) Au/YSZ before ( ) and after (O) prolonged anodic overpotential application.55 Reprinted with permission from the National Institute of Chemistry, Ljubljana, Slovenia.

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

In the search of high-performance SOFC anode, doped ceria have been evaluated as possible anode materials [9,10]. Comparing Ni-samaria-doped ceria (SDC) with Ni-YSZ, the Ni-SDC anode exhibits higher open-circuit voltages and a lower degree of polarization with either methanol as the fuel, as shown in Fig. 5, or methane as the fuel, as shown in Fig. 6. It was found that the depolarization ability of the anode is associated with the catalytic activity, the electrical conductivity, and the oxygen ionic conductivity of the anode materials [9]. It was also found that the anodic polarization and electro-catalytic activity strongly depend on the Ni content in the anode, and the optimum result for the Ni-SDC anode is achieved with 60... 

Fig. 6. Variations of the open-circuit voltage (OCV) with nickel content of Ni-SDC and Ni-YSZ anodes. Operating conditions 600 °C, 1 atm feed composition CH4 Ar = 10 90 total flow rate 100 ml/min [10].

To reduce the formation of carbon deposited on the anode side [2], MgO and Ce02 were selected as a modification agent of Ni-YSZ anodic catalyst for the co-generation of syngas and electricity in the SOFC system. It was considered that Ni provides the catalytic activity for the catalytic reforming and electronic conductivity for electrode, and YSZ provides ionic conductivity and a thermal expansion matched with the YSZ electrolyte. 

The internal reforming of CH4 by CQzin SOFC system was performed over an ESC (electrolyte st rported cell) prepared with Ni based anode catalysts. Figure 5 diows the performance of voltage and power density with current density over various ESC (Ni based anodes I YSZ (LaSr)Mn03) at SOOC when CH4 and CO2 were used as reactants. To improve the contact between single cell and collector, different types of SOFC reactor were used [5]. In the optimized reactor (C), it was found fliat die opai-... 

FIGURE 1.12 Cross-sectional image of a five-layer cell with the anodic substrate Ni-YSZ, anodic functional layer Ni-ScSZ, electrolyte ScSZ, and interlayer GDC prepared with a tapecasting technique [79]. 

There are a number of informative reviews on anodes for SOFCs [1-5], providing details on processing, fabrication, characterization, and electrochemical behavior of anode materials, especially the nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes. There are also several reviews dedicated to specific topics such as oxide anode materials [6], carbon-tolerant anode materials [7-9], sulfur-tolerant anode materials [10], and the redox cycling behavior of Ni-YSZ cermet anodes [11], In this chapter, we do not attempt to offer a comprehensive survey of the literature on SOFC anode research instead, we focus primarily on some critical issues in the preparation and testing of SOFC anodes, including the processing-property relationships that are well accepted in the SOFC community as well as some apparently contradictory observations reported in the literature. We will also briefly review some recent advancement in the development of alternative anode materials for improved tolerance to sulfur poisoning and carbon deposition. 

The microstructure, properties, and performance of Ni-YSZ anodes depend sensitively on the microscopic characteristics of the raw materials (e.g., particles size and morphology of NiO and YSZ powders). The particle sizes of the starting YSZ powders vary usually from 0.2 to 0.3 pm, whereas those for the NiO powders are 1 pm. The Ni to YSZ volume ratio usually varies from 35 65 to 55 45. For example, the reported Ni to YSZ volume ratios include 34 66 [20, 21], 40 60 [24], 43 57 [22], and 55 45 [23], For a bilayer anode, the functional anode layer in contact with the electrolyte contains 45 to 50 vol% Ni, whereas the anode support layer has 35 to 40 vol% Ni [25, 26], A pore former is usually added to tailor the shrinkage (for the cofiring) and to achieve sufficient porosity (>30 vol%) in the anode or the anode support layer. 

In the following sections, the electrical conductivity, electrochemical activity toward hydrogen oxidation, and the sulfur poisoning behavior of Ni-YSZ cermet anodes will be discussed in detail, together with the effects of various processing procedures and testing conditions. 

The electrical conductivity of a Ni-YSZ cermet anode depends on the composition (i.e., Ni to YSZ volume ratio), the microscopic features of the starting materials (e.g., particle size and distribution of NiO and YSZ powders), and the sintering and reduction conditions (e.g., temperature and atmosphere), as will be discussed in detail in the following sections. 

Since the conductivity of Ni is more than 5 orders of magnitude greater than that of YSZ under the fuel cell operating conditions, the electrical conductivity of a porous Ni-YSZ cermet anode changes several orders of magnitude, usually from -0.1 S/cm... 

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