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While the global fuel utilization of solid oxide fuel cells (SOFCs) is limited by the stack aging rate, the fuel excess is typically used in a burner, and thus limiting the system electrical efficiency. Further, natural-gas-fueled SOFCs require treated water for the steam reforming process, which increases operational cost. Here, we introduce a novel micro anode off-gas recirculation fan that is driven by a partial-admission (21%) and low-reaction (15%) steam turbine with a tip diameter of 15 mm. The 30 W turbine is propelled by pressurized steam, which is generated from the excess stack heat. The shaft runs on dynamic steam-lubricated bearings and rotates up to 175 krpm. For a global fuel utilization of 75% and a constant fuel mass flow rate, the electrical gross DC efficiency based on the lower heating value was improved from 52 % to 57 % with the anode off-gas recirculation, while the local fuel utilization decreased from 75% to 61%, which is expected to significantly increase stack lifetime. For a global fuel utilization of 85%, gross efficiencies of 66% in part load (4.5 kWe) and 61% in full load (6.3 kWe) were achieved with the anode off-gas recirculation. The results suggest that the steam-driven anode off-gas recirculation can achieve a neutral water consumption.

This work presents the poisoning effect of organic oxo-silicon compounds (siloxanes) which are generally found in sewage biogas. Lifetime and durability associated with solid oxide fuel cell (SOFC) technology is strongly related to the amount of contaminants that reach the stack. Several experiments on Ni anode-supported (AS) single cells are performed in order to clarify the mechanism of degradation and also the possibility of cell performance recovery. Three experiments focus on the degradation and recovery of AS Ni-YSZ fed with H-2, co-feeding 5 ppm D4-siloxane (octamethylcyclotetrasiloxane, C8H24O4Si4) as representative compound for the organic silicon species, at 800 degrees C. A fourth experiment focuses on the durability of the AS Ni-YSZ cell with variable concentrations of the impurity (0-5ppm), during steady state polarization (0.25Acm(-2)) for 250 h, using simulated biogas-reformate fuel H-2/CO/CO2/H2O: irreversible degradation was observed with the D4-impurity feed in the anode gas. Post-test scanning electron microscopy (SEM) results indicate formation of SiO2(s) deposits, which block pores and reduce the TPB length.

This paper presents an experimental study of the impact of toluene on the performance of anode-supported (AS) Ni-YSZ SOFC operating at ~800 °C, 0.25 A/cm2 fed with H2 and reformed biosyngas. Toluene was added to the fuel stream and its concentration increased stepwise from tens to thousands of ppm. Each poisoning test was followed by a recovery step (no toluene in the fuel stream) for the duration of 25 hours. The main goal of this work is to define a concentration threshold of toluene as a model tar compound in the fuel stream and later to identify the degradation mechanism caused by this tar compound. Polarization behavior, current-voltage and electrochemical impedance spectroscopy (EIS) were analyzed to evaluate the cell performance. A linear degradation is observed at concentrations above 1000 ppmv for the cell which was fed with pure H2 (cf Figure) The degradation was due to local C(s) deposition. In contrast, toluene even up to 3500 ppm did not cause a severe degradation when the cell was fed with reformed biosyngas. Figure 1

Lifetime and durability issues connected with Solid Oxide Fuel Cell (SOFC) technology are strongly related to the amount of contaminants that reach the stack. In this study the focus is on organic silicon compounds (siloxanes) and their highly detrimental effects on the performance of SOFC Ni-YSZ anodes. The involved mechanism of degradation is clarified and quantified through several test runs and subsequent post-mortem analysis on tested samples. In particular, experiments on both Ni anode-supported single cells and 11-cell-stacks are performed, co-feeding D4-siloxane (octamethylcyclotetrasiloxane, C8H24O4Si4) as model compound for the organic silicon species which are generally found in sewage biogas. High degradation rates are observed already at ppb(v) level of contaminant in the fuel stream. Post-test analysis revealed that Si (as silica) is mostly deposited at the inlet of the fuel channel on both the interconnect and the anode side of the cell suggesting a relatively fast condensation-type process. Deposition of the Si was found on the interconnect and on the anode contact layer, throughout the anode support and the three phase boundary in the anode, correlating with the observed increase of polarization losses from the EIS analysis of tested cells. (C) 2015 Elsevier B.V. All rights reserved.

To improve the industry benchmark of solid oxide fuel cell (SOFC) systems, we consider anode off-gas recirculation (AOR) using a small-scale fan. Evolutionary algorithms compare different system design alternatives with hot or cold recirculation. The system performance is evaluated through multi-objective optimization (MOO) criteria, i.e., maximization of electrical efficiency and cogeneration efficiency. The aerodynamic efficiency and rotordynamic stability of the high-speed recirculation fan is investigated in detail. The results obtained suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable, including for small power scale (10 kWe).

One of the major limitations of the nickel (Ni) - yttria-stabilized zirconia (YSZ) anode support for solid oxide fuel cells (SOFC) is its low capability to withstand transients between reducing and oxidizing atmospheres ("RedOx" cycle), owing to the Ni-to-NiO volume expansion. This work presents results on different anode supports fabricated by tape casting. Three compositions are prepared, as the outcome of a preceding design of experiment approach. The NiO proportion is 40, 50 and 60 wt% of the anode composite.

The locally-resolved degradation behavior was studied during 1900 hours in a SOFC repeat-element. In-situ measurements of local electrochemical performance were made on 18 locations over a segmented anode-supported cell. The evolution of local current densities, overpotentials and area-specific resistances was studied, showing a reorganization of the electrochemical reaction with time. The extent and the spatial distribution of degradation were established for different electrochemical reactions steps using impedance spectroscopy. The low-frequency cathode contribution was the mostly altered process, followed by the charge transfer reaction on anode side. Post-experiment analyses allowed to identify three major pollutants on the cathode side (chromium, silicon and sulfur), whose spatial distributions corresponded to the observed local degradation. Chromium was present only near the air inlet, while silica was found to affect a larger area, with decreasing amounts along the flow. Sources of pollutants were identified in system components as well as within the stack repeat-element.

Planar SOFC stack technology based on a unique concept (SOFConnex™) uses structured gas distribution layers between unprofiled metal sheet interconnects and thin Ni-YSZ anode supported electrolyte cells. The layers are flexible both in material and design and allow to implement new configurations relatively simply; manifolding can be internal, external, or combined. Together with thin stack components, independent of the supplier, the SOFConnex™ stacking approach allows compact planar assembly with low cost potential and adequate power density. Different cell and flow designs have been realized. With a basic flow configuration, short stacks (50cm2 cell active area) were assembled and tested, power density at 800°C reaching 0.5W∕cm2 at 0.7V average cell voltage (1.5kWe∕L, 0.36Ωcm2 area specific resistance), for 65% fuel utilization and 35% lower heating value electrical efficiency. Short stacks were thermally cycled and operated with both hydrogen and syngas. Degradation was essentially Ohmic (confirmed from impedance spectroscopy on stacks) and at first mainly due to the cathode-electrolyte interfacial reaction, performance loss was subsequently strongly reduced after cathode replacement. Using multiple voltage probes with additional interconnects allowed to separately monitor current collection losses during polarization. With an improved design in terms of sealing, postcombustion control and flow field, stacks up to 1kWe have been operated.

International audience; The neodymium-deficient nickelate Nd1.95NiO4+δ, mixed conducting K2NiF4-type oxide, was evaluated as cathode for solid oxide fuel cells. The electrochemical properties were investigated on planar Ni–YSZ anode-supported SOFC based on co-tape casted and co-fired HTceramix® cells. Using a layer of strontium doped lanthanum cobaltite as current collector, a current density of 1.31 A cm−2 (at 0.70 V) was obtained at 800 °C using hydrogen fuel with small single cells, after optimizing the cathode sintering temperature. Impedance spectroscopy measurements were performed; the different resistive contributions and the values of the corresponding equivalent capacities are discussed.

The degradation of the electrochemical performance of solid oxide fuel cell (SOFC) devices is a major hurdle to overcome before commercialisation. The interplay between the phenomena and the long testing times complicate the research, which highlights the relevance of modelling to propose mitigation approaches. This study comprises two parts. This Part II proposes approaches for the simulation of the degradation induced by: (i) interconnect corrosion, (ii) loss of ionic conductivity of the ion-conducting materials, (iii) nickel particle growth in the anode, (iv) chromium contamination and (v) formation of insulating phases in the cathode. The literature survey highlights the lack of data for a completely consistent calibration of the models, despite the simplifications. The support for the implementation is the electrochemical model validated in Part I and a two-dimensional model of the cell and interconnection system. The cathode largely contributes to the degradation. The local overpotential predominantly governs chromium contamination, which can promote the formation of insulating phases, as operation proceeds. The local electronic current density has comparatively a weak direct influence on the degradation. Qualitative agreement with experimental data from the literature could be achieved, without dedicated adjustments of the parameters. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596435] All rights reserved.

Solid-oxide electrolyzer based power-to-methane is promising for large-scale energy storage as well as biogas upgrading by efficiently converting intermittent renewable power and CO2 (e.g., in biogas) into synthetic methane. Either air or pure oxygen can be employed in solid oxide electrolyzers for anode sweeping and thermal management. This work investigates the optimal conceptual design of a variety of power-to-methane layouts to (i) identify the effect of sweep-gas type on system performance and (ii) compare different concepts for biogas upgrading. Bi-objective optimization is performed to understand the trade-off between system efficiency and methane production with the effects of the key design variables. The results indicate that oxygen sweep only marginally affected the system performance (6% reduction in methane production at the same system efficiency). The methanation inside the electrolyzer helped achieve a higher system efficiency (over 90%) by maintaining the electrolyzer temperature as high as possible. Unlike most biogas-upgrading systems, which behaved similarly to the standalone power-to-methane system, with an efficiency range of 70–86%, the direct biogas-electrolysis performed within a wide efficiency range (52–88%) and a reduced methane yield (50% less than the other systems operating at 70% efficiency). The detrimental methane reforming inside the electrolyzer was limited by increasing the reactant conversion and the electrolysis pressure. The various solid-oxide electrolyzer based power-to-methane concepts showed promising results for biogas upgrading applications. The practical choice of biogas-upgrading concepts will depend on the requirement of operational flexibility to handle variable renewable power considering different gas storage and carbon capture technologies.

Use of intermediate temperature solid oxide fuel cells for power generation is attractive, due to the highest achievable electrical efficiencies in the low power generation range. Nonetheless, there still exists a potential to improve the industry benchmark SOFC systems, which use natural gas or biogas as fuel and consider steam methane reforming, with external steam supply, for syngas production and usage in the stack. Anode off-gas recirculation using a blower is the add-on to our next-generation SOFC system. Since the recirculating feed contains steam produced in the stack, no external steam supply is needed for reforming. This eliminates the use of expensive water de-ionisation sets. Further, it allows for high overall fuel utilization at low diffusion losses. System performance is evaluated through multi-objective optimization criteria, i.e. maximization of electrical efficiency and cogeneration efficiency. Evolutionary algorithms compare the different design alternatives, i.e. co-flow or counter-flow stack operation with hot or cold recirculation. The system flowsheet which includes models for BOP components and an inhouse experimentally validated SOFC stack model, is solved using the commercial software Belsim VALI. The design variables identified for this system are 1. oxygen to carbon ratio before the external reformer 2. external to internal methane reforming fraction 3. reducing fuel species molar fraction at anode outlet 4. air-fuel equivalent ratio in the burner and 5. blower inlet temperature. Successive generations of population are obtained by reproduction and mutation of the existing population. Following the ‘survival of the fittest’ rule, the iteration is stopped when a non-dominated solution set represented by a Pareto-optimal front is obtained. The results obtained (eg. figure 1) suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable. Figure 1

This paper reports on medium term tests of anode-supported five-cell short stacks, as well as on some separate anode development. Two stacks were operated under steady-state conditions: one with unprotected metal interconnects, H-2 fuel and 0.35 A cm(-2) (40% fuel utilisation) polarisation current showed an average cell voltage degradation of 56 mV per 1000 h for 2750 h; one with coated metal interconnects, synthetic reformate fuel and 0.5 A cm(-2) (60% fuel utilisation) polarisation current showed an averaged cell voltage degradation slope of 6.6 mV per 1000 h for 800 h before a power cut prematurely interrupted the test. A third stack was subjected to 13 complete thermal cycles over 1000 h, average cell voltage degradation was evaluated to -2 mV per cycle for operation at 0.3 A cm(-2), open circuit voltage (OCV) remained stable, whereas area specific resistance (ASR) increase amounted on average to 0.008 Omega cm(2) per cycle. To improve anode reoxidation tolerance, MgO addition was considered. (Mg, Ni)O solid solutions were synthesized from MgO and NiO powders at 1500 degrees C for 2 h in air. Samples were reduced at 1000 degrees C in 4 vol% H-2 in N-2, and their expansion behaviour was observed. XRD and SEM/EDX analysis suggested that the expansion was caused by the segregation of Ni particles between (Mg, Ni)O grains. (c) 2008 Elsevier B.V. All rights reserved.

Although the effects of impurities on the Ni-YSZ electrode of a solid oxide cell in fuel cell operation have been studied, reports on their effects during electrolysis operation are limited. Here, short-term experiments at various current densities and a durability test (1400 h) are performed to investigate the effects of HCl on the Ni-YSZ electrode of a solid oxide electrolysis cell (SOEC) operated in co-electrolysis mode. Without current bias, exposure to 10 ppmv of HCl was tolerated whereas significant degradation was observed when the SOEC was exposed to 5 ppmv of HCl under polarization, mostly related to an increase of the resistance associated with the charge-transfer processes. Prolonged exposure to HCl revealed an initial steady degradation before an increase by a factor of > 10 caused by polarization resistance increases, thereby emphasizing the importance of prolonged poisoning tests. Stopping the HCl supply and operating the SOEC in an HCl -free atmosphere allowed for voltage stabilization but no performance recovery was observed. An increased serial resistance indicates that the desorption of Cl can also be detrimental. A low-frequency pseudo-inductive hook appeared in the electrochemical impedance spectra, possibly related to the increase of the charge-transfer processes’ resistance and indicating modification to the electrochemical pathways.

A short overview of recent work on electroceramic materials relevant to methane-fueled SOFC systems is given. Various fuel feed options are considered, such as pure methane, biogas, and the addition of reforming agents. The principle and implementation into SOFC systems of mixed conducting ceramics for oxygen separation is described, as well as their characterisation by electrochemical methods. Ceramic anodes capable of operating with methane-rich fuel injection are presented. The document concludes with electrochemical results on planar anode supported ceramic fuel cells operating at reduced temperature.

The Laboratory for Applied Mechanical Design (LAMD) designed, manufactured, and experimentally tested a novel steam-driven anode off-gas recirculation (AOR) fan for 10 kWel solid oxide fuel cell (SOFC) systems. Due to the dynamic steam-lubricated bearings, the AOR unit is expected to have a high lifetime, even at elevated rotational speeds and temperatures. Additional, the unit is oil-free, explosion-proof, very compact, and cheap to manufacture. The AOR fan diameter is at 19.2 mm and the nominal rotational speed is 175 krpm. The unit was coupled to a 6 kWel SOFC system, reaching electrical gross DC efficiencies, based on the fuel LHV, of 66 % in part load (4.5 kWel gross DC) and 61 % in full load (6.3 kWel) for a global fuel utilization of 85 %. To the best of the authors' knowledge, this was the first time that a steam-driven AOR fan was demonstrated in-situ with an SOFC system.

Internal steam reforming (IR) of methane was investigated on Ni-YSZ anode supported cells, looking in particular at the effect of the steam to carbon (S/C) ratio on the degradation rate. The cells were fed with different H2O/CH4 mixtures during 100 hours sequences, alternating with sequences of dry H2 feeding. V-I characterization was performed before and after each sequence, and EIS measurements were performed regularly. A marked degradation was observed during the IR sequences while it was negligible under dry H2 feed. The observed degradation, attributed to carbon deposition on the anode active sites, was partially reversible for S/C >1.5, whereas it became irreversible at lower S/C.

Our planar SOFC stacking technology uses unprofiled metallic interconnects (MIC) and thin cells of tape cast anode supported YSZ. The key element is the gas diffusion layer (GDL) between cell and MIC, which consists of so-called SOFConnex™. Using square cells with internal manifolds, 0.5 W/cm2 stack power density (800°C) can be obtained on short stacks. However, this open design configuration limits the assembly of large stacks and the durability of operation, owing to postcombustion and redox cycling occurring at unprotected cell edges. A new design, inspired from modeling work and the adaptability of the SOFConnex™ GDL, led to oblong-shaped cells, assembled in a closed stack casing with external air manifolding and fuel recovery manifolding, avoiding postcombustion. While stack power density in both designs remains similar, the operation at increased fuel utilization has become more stable in the 2nd design. Furthermore, a correlation of performance homogeneity during stack testing was drawn to assembly quality control. A 36-cell stack in dilute H2 at 800°C achieved 625 Wel (28% LHV efficiency, 0.35 W/cm2) under continuous polarisation, with all 6 clusters of 6 cells showing coincident i-V-output.