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Solid oxide fuel cells (SOFCs) are promising electrochemical devices that enable the highest fuel-to-electricity conversion efficiencies under high operating temperatures. The concept of multi-stage electrochemical oxidation using SOFCs has been proposed and studied over the past several decades for further improving the electrical efficiency. However, the improvement is limited by fuel dilution downstream of the fuel flow. Therefore, evolved technologies are required to achieve considerably higher electrical efficiencies. Here we present an innovative concept for a critically-high fuel-to-electricity conversion efficiency of up to 85% based on the lower heating value (LHV), in which a high-temperature multi-stage electrochemical oxidation is combined with a proton-conducting solid electrolyte. Switching a solid electrolyte material from a conventional oxide-ion conducting material to a proton-conducting material under the high-temperature multi-stage electrochemical oxidation mechanism has proven to be highly advantageous for the electrical efficiency. The DC efficiency of 85% (LHV) corresponds to a net AC efficiency of approximately 76% (LHV), where the net AC efficiency refers to the transmission-end AC efficiency. This evolved concept will yield a considerably higher efficiency with a much smaller generation capacity than the state-of-the-art several tens-of-MW-class most advanced combined cycle (MACC).

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...

Long-term performance testes by CRIEPI (Central Research Institute for Electric Power Industry) on six industrial stacks have revealed an interesting correlation between cathode polarization loss and ohmic loss. To make clear the physicochemical meaning of this correlation, detailed analyses were made on the conductivity degradation of YSZ electrolyte in button cells and then on the ohmic losses in the industrial cells in terms of time constants which are determined from speed of the tetragonal transformation through the Y diffusion from the cubic phase to the tetragonal phase. In some cases, shorter time constants (faster degradations) were detected than those expected from the two-time-constant (with and without NiO reduction effects) model, suggesting that additional ohmic losses after subtracting the contribution from the tetragonal transformation must be caused from other sources such as cathode-degradation inducing effects. Main cathode degradations can be ascribed to sulfur poisoning due to contamination in air in the CRIEPI test site. An important feature was extracted as this cathode degradations became more severe when the gadolinium-doped ceria (GDC) interlayers were fabricated into dense film. Plausible mechanisms for cathode degradations were proposed based on the Sr/Co depletion on surface of lanthanum strontium cobalt ferrite (LSFC) in the active area. Peculiar cathode degradations found in stacks are interpreted in term of changes in surface concentration by reactions with sulfur oxide, electrochemical side reactions for water vapor emission or Sr volatilization, and diffusion of Sr/Co from inside LSCF.

Sulfur is one of the major contaminants for SOFC anodes, and can cause performance degradation of SOFC systems. In the present study, H 2 S poisoning phenomena are analyzed at around 800 • C, using electrolyte-supported single cells to understand Ni anode degradation mechanisms, especially under relatively oxidizing conditions. Anode performance degradation caused by sulfur in partially or fully pre-reformed CH 4 -based fuels has been evaluated as a function of various operational parameters, including; operational temperature, pre-reforming ratio, steam-to-carbon ratio, fuel utilization, and current density, for different fuel compositions. Besides the well-known change in cell voltage by the reversible surface adsorption/desorption mechanism, sulfur poisoning phenomena associated with voltage oscillation and Ni oxidation are newly found at high fuel utilizations and at high current densities, respectively. Such degradation is associated with the increase in the O/Ni ratio, and grain growth of Ni-based particles in the anodes.

Previously, most studies of proton-conductive electrolytes for SOFCs were conducted to achieve lower-temperature operation. In this study, we investigate a proton-conductive electrolyte to realize high-efficiency SOFCs. To this end, the dependencies of the total conductivity of Ba(Zr 0.1 Ce 0.7 Y 0.1 Yb 0.1)O 3-δ on the oxygen partial pressure and temperature under wet and dry conditions were measured. Based on the measurement data, we analyzed the ratio of ionic current density to electronic current density in the temperature range of 550-900°C. Assuming that the area-specific resistance of the electrolyte and the external current density were 0.383 Ωcm 2 and 0.25 Acm-2 , respectively, the leakage current densities caused by the minority carriers were calculated to be 5.7% and 10.2% of the external current density at 550°C and 600°C, respectively. This study developed a method to evaluate proton-conductive electrolyte materials and established guidelines for the development of new materials for high-efficiency SOFCs.

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