Degradation Behavior of Ni–ScSZ Cermet Anode under Various Humidified Conditions for Solid Oxide Fuel Cells

Abstract

Nickel-scandia-stabilized zirconia ͑Ni-ScSZ͒ cermet is one of the promising anodes for use in intermediate-temperature solid oxide fuel cells. Although ScSZ exhibits higher oxide-ion conductivity than yttria stabilized zirconia, the aging effects depending on the doping-concentration results in a decrease in conductivity. However, recent extensive research has clarified that the electrolyte with Ͼ10 mol % Sc 2 O 3 shows no decrease in conductivity. In this study, therefore, 10 mol % Sc 2 O 3 -ZrO 2 ͑10ScSZ͒ was used as the ionic conductor in the cermet anode. The degradation behaviors under various humidified conditions were investigated at 800°C on the basis of partial pressure of oxygen under both open-circuit and discharge conditions. The degradation was judged from the impedance spectra before and after oxidative treatments, such as a continuous discharge or an exposure to highly humidified fuel under the open-circuit condition. The Ni-10ScSZ cermet exhibited both reversible and irreversible changes in impedance spectra, depending on the steam concentration and passed current densities. It is noted that, even under the open-circuit condition, the irreversible degradation has occurred with supplying a gaseous mixture of 10% H 2 -20% H 2 O-N 2 balance ͑p O 2 = 1.72 ϫ 10 −18 atm͒, whereas the reversible change in interfacial resistance was observed in the case of 10% steam concentration ͑p O 2 = 4.29 ϫ 10 −19 atm͒.Solid oxide fuel cells ͑SOFCs͒ have a great potential as an energy conversion device due to their high efficiency and low emissions. This technology is in an advanced stage of development at present. However, the long-term stability of stacked cells is the most important requirement for practical use and needs to be improved with priority. Degradation of the fuel cell is characterized by a steady decrease in performance, which occurs over a period of time. The performance deterioration can be regarded as the sum of the aging process in each material, although the extent to which the process contributes has not been sufficiently clarified. 1 Many studies have been conducted to elucidate and resolve this issue with focusing on the material stability, i.e., the interfacial reactions between different components. [2][3][4][5] Nickel-zirconia cermets are the most widely and commonly used anodes in SOFCs. The addition of zirconia is necessary to support the Ni particles, to prevent the coarsening, and to make the thermal expansion coefficient of the anode compatible with the electrolyte. 6 The optimization of the cermet structure is a key factor to achieve better performance. Generally, SOFCs are operated at high fuel utilizations, and the downstream part of the cell is exposed to the severe conditions because of the depletion of fuel and the high steam concentration. In such actual conditions, the main problems observed are the oxidization and agglomeration of Ni during the long-term operation, which leads to a decrease in the triple-phase boundary and electronic conductivity. Moreover, several groups have ...

Effect of Au and/or Mo Doping on the Development of Carbon and Sulfur Tolerant Anodes for SOFCs—A Short Review

“…Consequently, the degradation of the operating potential in the cell with 3Au-3Mo-Ni/GDC is mainly related to the increase of the ohmic resistance. This trend can be attributed (Koh et al, 2002;Matsui et al, 2007Matsui et al, , 2010Mogensen et al, 2011;Niakolas, 2014) to the possible gradual re-oxidation and/or agglomeration of nickel, due to the high partial pressure of H 2 O in the reaction mixture.…”

Solid Oxide Electrolysis Cells: Microstructure and Degradation of the Ni/Yttria-Stabilized Zirconia Electrode

“…There is no tendency for delamination between the electrolyte and the O 2 electrode, even though such problems with delamination of the O 2 electrode from the electrolyte has been reported and has been suggested to be caused by pressure differences occurring upon O 2 evolution in closed pores in the electrode/electrolyte interface. 17,[39][40][41] Such delamination must be considered a question of optimizing processing techniques to preserve an optimal contact between the electrolyte and the LSM based electrode upon electrolysis testing. From a technological point of view it is important to notice that inexpensive production methods such as spraying and screen printing can be applied and still provide LSM based O 2 electrodes with microstructures able to sustain thousands of hours as O 2 evolution electrodes with no delamination from the electrolyte ͑Fig.…”

Development of SOFC Performance and Durability Evaluations Technology

“…Since it is assumed that magnitude of internal resistance is uniform in the cell and only depends on temperature, the internal resistance is represented by the Arrhenius formula as shown in formula [5]. Polarization resistances R a and R c are also represented by the Arrhenius formula as a function of temperature and partial gas pressure as shown in formula [6] and [7]. [ The composition of reactant gases is change along from an inlet to outlet of Cell.…”

“…In this calculation, distribution of E, η ne , R a , R c and J by the change of gas composition are taken into consideration. The a 0, c 0 , α and β in formula [6] and [7] are determined by using the optimum method to minimize the objective function between the experimental output voltages obtained from the several gas conditions and the calculated output voltages of formula [1]. In the calculation for minimizing the objective function, the final value of the average error between experimental and calculated output voltages is converged within 5 mV.…”

“…In order to determine the combination of each parameter of formula [6] and [7], the output voltage and the internal resistance of cell were systematically measured while changing, the operation temperature, the supplied fuel and oxidant gas composition. The supplied gas composition is controlled as shown in tables II and III.…”