SOFC Durability against Standby and Shutdown Cycling

Abstract

To simulate realistic operating conditions in SOFC systems, we investigate the influence of thermal cycling on the performance of electrolyte-supported planar SOFCs. Thermal cycling is often associated with interruption of fuel supply, with three main modes; hot standby, cold standby, and shutdown. Cell performance degradation is most significant during shutdown cycles. Nickel oxidation and agglomeration are more pronounced when SOFCs are subjected to lower temperatures for longer periods of time, leading to significant performance degradation. Ostwald ripening at the anode leads to degradation as Ni grains increase in size with cycling. Ni particle precipitation on the anode zirconia grains and along electrolyte grain boundaries is found for the first time in shutdown cycling tests. When H 2 S is mixed with the fuel, the internal reforming reactions and electrode reactions are inhibited by sulfur poisoning of the Ni anodes, accelerating degradation. The SOFC cycling degradation mechanisms are discussed in detail. Solid oxide fuel cells (SOFCs) have several advantages including high efficiency, fuel flexibility, and utilization of non-noble metal, Pt-free catalysts, due to their relatively high operation temperature. Commercialization of SOFC systems for residential electric power applications began in Japan in 2011. Such systems are frequently stopped and restarted in normal operation, e.g. when power is not required, or in an emergency. Such start-stop operation results in thermal cycling, and is often associated with an interruption in fuel supply. Although it is well known that thermal and redox cycling under startstop operation deteriorates SOFC electrochemical performance, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] there are only a limited number of studies that systematically focus on this technologically relevant issue.Continuous SOFC stack operation with constant power output generally results in a gradual degradation in performance during long-term operation (SOFCs should run for up to a decade). However, SOFCs can suffer to a greater degree from changes in operation conditions. Due to thermal expansion mismatch between the different components, the cells can suffer from mechanical degradation mechanisms, such as delamination and crack formation with simple thermal cycling. Changes in atmosphere can result in more serious degradation. The influences of thermal cycling and current density cycling on cell degradation have been previously investigated.1-4 The change in volume causes Ni agglomeration. [5][6][7] Redox cycles are typically associated with oxidation of Ni particles at the anode. [8][9][10][11][12] Furthermore, redox cycling results in the formation of Ni hydroxides at a certain vapor pressure in oxidizing atmosphere, with a high water vapor concentration. 13,14 In real residential SOFC power units, cells and stacks are not subjected to these different conditions independently; the changes occur much more dynamically. Therefore, cycle durability studies should be performed using reali...

Alternative Ni-Impregnated Mixed Ionic-Electronic Conducting Anode for SOFC Operation at High Fuel Utilization

“…By separating the different overvoltages, Figs. 4b and 4c show that the anode IR losses and non-ohmic overvoltage of conventional anodes gradually increased, the reason for which has been previously determined by Hanasaki et al 4 In addition, the anode IR losses for the Ni-impregnated anodes, remained stable even after redox cycling. Whilst the conducting path of conventional anodes can be destroyed due to Ni oxidation, the network structure of the MIEC oxide in Ni-impregnated anodes remains intact for electron transport, as confirmed by Shen et al 23,24 In addition, agglomeration of the Ni catalyst nanoparticles on the anode backbone was prevented by optimizing the Ni loadings, to avoid increases in non-ohmic overvoltage.…”

“…Scandia-stabilized zirconia (ScSZ) plates (10 4 Porous anode layers were screen-printed onto the ScSZ electrolyte plates, followed by sintering in air at 1300…”

“…The anode-side voltage losses (anode IR losses, and anode non-ohmic overvoltage) were divided using the well-known current interruption technique. 4 Redox cycling test.-In this study, redox cycling tests were conducted following a cycle protocol shown in Fig. 1c.…”

In-situ diagnosis and assessment of longitudinal current variation by electrode-segmentation method in anode-supported microtubular solid oxide fuel cells

“…This evidence reveals the importance of the spatial characterization of the cells to comprehend the mechanisms involving in the current variation phenomenon.From the durability point of view, so-called Nickel RedOx cycling possibly occur during the local fuel starvation. Due to the high volumetric expansion/contraction ratio of Nickel (1.66), micro-cracks can form in the anode[7,8].…”

Thermally Driven SOFC Degradation in 4D: Part I. Microscale

“…1 However, an inability to withstand thermal cycling due to the mismatch in the thermal expansion coefficients (TECs) of the constituent materials remains a prominent source of cell degradation. [2][3][4] The reaction sites within the electrodes are named the triple-phase boundaries (TPBs), due to the three transport networks which are essential in the production of current from an SOFC: the transport of electrons through the metal, oxide ions (O 2− ) through the ceramic and gaseous reactants/products through the pores. The location where these three networks meet is where the TPB is located and is characterized by a one-dimensional reaction site length (L TPB ).…”