Experimental Study of Internal Reforming on Large‐area Anode Supported Solid Oxide Fuel Cells

The effect of endothermic internal steam reformation of methane and exothermic fuel cell reaction on the temperature of a planar‐type anode‐supported solid oxide fuel cell was experimentally investigated as a function of current density and fuel utilization. We fabricated a large‐area (22 × 33 cm²) cell and compared temperature profiles along the cell using 30 thermocouples inserted through the cathode end plate at 750 °C under various conditions (Uf ∼50% at 0.4 A cm⁻²; Uf ∼70% at 0.4 A cm⁻²; Uf ∼50% at 0.2 A cm⁻²) with hydrogen fuel and methane‐steam internal reforming. The endothermic effect due to internal reforming mainly occurs at the gas inlet region, so this process is not very effective to cool down the hot spot created by the exothermic fuel cell reaction. This eventually results in a larger temperature difference on the cell. The most moderate condition with regards to thermal gradient on the cell corresponds to high fuel utilization (Uf ∼70%) and low current density (∼0.2 A cm⁻²). The electrochemical performance was also measured, and it was found that the current–voltage characteristics are comparable for the cell operated under hydrogen fuel and internal steam reforming of methane because of lower polarization resistance with high partial pressure of water vapor.     

Start‐Up and Load‐Change Behavior of a Catalytic Burner for a Fuel‐Cell‐Based APU for Diesel Fuel

The catalytic burner CAB 4 was developed for a fuel‐cell‐based diesel‐APU (auxiliary power unit) with a capacity of 14.5 kW_th,APU. In order to operate a catalytic burner in such an APU, several requirements must be met. Normal operation involves combustion of anode off‐gas from the fuel cell. If the fuel cell malfunctions or if the gas quality is insufficient, the burner must also be able to fully convert the reformate while by‐passing the fuel cell. It must be possible to catalytically ignite the burner using a reformate with increased CO‐concentration. The burner must fully convert all combustible components in the fuel‐gas at all operating points. The energy contained in the fuel gas is utilized in the CAB to generate superheated steam with no oscillations and to supply this steam to the autothermal reformer. When the fuel processing system is being shut down, the burner should be able to continue providing steam for sweeping the downstream reactors for a limited period of time. Catalytic ignition of the CAB 4 was demonstrated with a reformate containing up to 5 mol.% CO. The behavior of the burner was characterized in steady‐state operation, during load changes, during transitions in the operating mode, and during shut‐down.     

Co‐Generation of Electric Power and Carbon Nanotubes from Dimethyl Ether (DME)

Coking occurs easily and would significantly degrade the electrochemical performance for hydrocarbon‐fueled solid oxide fuel cells (SOFCs). Here, we report an integrated device combining a SOFC and a stainless steel tubing as catalyst for hydrocarbon pyrolysis in the upstream of the fuel cell. Considerable carbon nanotubes (CNTs) are grown around the outside of the stainless steel tubing when dimethyl ether (DME) is fed to the device, which dramatically reduces the C:O ratio in the fuel reaching the cell anode. Correspondingly, the carbon‐removal reforming significantly prolongs the performance stability of the fuel cell compared to that directly fueled by DME. The present results suggest that hydrocarbons can be utilized more efficiently and economically by combining a SOFC and a fix‐bed reactor containing catalysts for CNTs formation, accompanying with the co‐generation of electric power and CNTs.     

Tuning the Performance of Ni‐Based Catalyst by Doping Coinage Metal on Steam Reforming of Methane and Carbon‐Tolerance

Density functional theory (DFT) calculations are employed to investigate the key reactions in steam reforming of methane (SRM) on Ni‐based bimetallic surface alloys, including the dissociation of CH₄ and H₂O, the oxidation of CH by oxygen atom to form formyl (CHO), and the dehydrogenation of CHO to form carbon monoxide (CO). The aim of this investigation is to hunt for an optimal catalyst for SRM, which can inhibit carbon formation while maintaining high activity to the SRM. Coinage metal impurity (Au, Ag, and Cu) doped Ni catalysts have been proven to inhibit carbon deposition. In this work, we focus on investigating the doping effects on some leading processes in SRM. It is found that the coinage metal doping has a little effect on the two‐step dissociation of H₂O, which has a linear correlation between the dissociation barriers and the OH–H coadsorption energies. In addition, the dehydrogenation of CHO is kinetically favorable on all alloy surfaces. However, for the CH oxidation to CHO, only the Ni–Cu surface remains high activity. These results suggest that Ni–Cu bimetallic material is an excellent active carbon‐tolerance SRM catalyst for solid‐oxide fuel cells.     

A Control‐oriented Dynamic Model Adapted to Variant Steam‐to‐carbon Ratios for an SOFC with Exhaust Fuel Recirculation

The steam‐to‐carbon ratio (S/C) is a typical disturbance parameter in the operation of solid oxide fuel cell (SOFC) power generation system. A planar SOFC with a pre‐reformer and exhaust fuel recirculation system is investigated in this work. A lumped, nonlinear dynamic model is developed for the SOFC with consideration both of the spatial effect and the variant S/Cs. The dynamic model is deduced based on a fitting function so‐called Exponential Association Function, which is employed to describe the spatial distribution of state variables in SOFC. Three parameters of the fitting function are identified to integrate the spatial effect and S/C effect in the model. The parameters of Exponential Association Function are determined by function fitting on three‐dimensional numerical data at the sample operation points. Carbon formation activity is analysed using the simulation results and thermodynamic data. Dynamic simulation is implemented with the help of software MATLAB/SIMULINK. The results show that the developed model has good performance in predicting the responses of the state variables and catching the changes of S/C.     

SOFC Power Plants,the Siemens‐Westinghouse Approach

In 1998 Siemens AG (SAG) took over the tubular S olid O xide F uel C ell (SOFC) technology from Westinghouse. This merger placed SAG under the responsibility of Siemens Westinghouse Power Corporation (SWPC), and gave them the opportunity to concentrate on this technology as the most promising fuel cell option for the emerging distributed power market. A first full scale 100 kW demonstration plant in a “district heating” cycle, without integrated micro gas turbines, accumulated more than 15000 hours and is still working. All the expectations with respect to the design parameters and the operation behavior have been met or even exceeded. The first 220 kW “all electric” cycle demonstration plant with integrated micro gas turbine has successfully passed the factory tests. Additional field units are under contract which together with the above two pioneering demonstration plants will accumulate all the necessary learning experience to design a standardized SOFC product. In addition to the demonstration program, a research and development (R&D) program is being pursued. The outcome of both programs, the demonstration as well as the R&D, is expected to close the gap between the current high cost and the market price in order to enable substantial market penetration.     

Catalytic Influence of Oxide Component in Ni‐Based Cermet Anodes for Ammonia‐Fueled Solid Oxide Fuel Cells

Recently, the promising prospect of ammonia as a hydrogen carrier for solid oxide fuel cells (SOFCs) has attracted significant interests. In this work, the effects of temperature, fuel content, and total flow rate of anode gas on the performance of Ni/yttria‐stabilized zirconia (Ni/YSZ) anode for ammonia‐fueled SOFCs were investigated. Based on obtained results, the utilization route of ammonia on Ni/YSZ anode was discussed; the results of electrochemical experiments were related with the catalytic decomposition bahavior of ammonia over Ni/YSZ. Moreover, the catalytic activity for ammonia decomposition and anode performance of Ni/samarium‐doped ceria (Ni/SDC) and Ni/yttrium‐doped barium cerate (Ni/BCY) were also investigated. Among these anode materials, Ni/BCY exhibited the highest ammonia decomposition activity and anode performance for ammonia‐fueled SOFCs at intermediate temperatures.     

Thermodynamic Analysis of an Integrated SOFC,Solar ORC and Absorption Chiller for Tri‐generation Applications

Thermodynamic performance assessment of an integrated tri‐generation energy system for power, heating and cooling production is conducted through energy and exergy analyses. Sustainability assessment is performed and some parametric studies are undertaken to analyze the impact of system parameters and environmental conditions on the system performance. The tri–generation system consists of (a) an internal reforming tubular type solid oxide fuel cell (IR‐SOFC), which works at ambient pressure and fueled with syngas, (b) a combustor and a air heat exchanger, (c) a heat recovery and steam generation unit (HRSG), (d) a two‐ stage Organic Rankine cycle (ORC) driven by exhaust gases of SOFC, (e) parabolic trough solar collectors (PTSC), and (f) a lithium‐bromide absorption chiller (AC) cycle driven by exhaust gases from SOFC unit. The largest irreversibility occurs at the SOFC unit due to high temperature requirement for reactions. Fuel utilization factor, recirculation ratio, dead state conditions, and solar unit parameters have influential effects on the system efficiencies. Energy and exergy efficiencies of tri‐generation unit become 85.1% and 32.62%, respectively, for optimum SOFC stack and environmental conditions. The overall system energy and exergy efficiencies are 56.25% and 15.44% higher than that of conventional SOFC systems, respectively.     

Degradation Mechanism of Anode‐Supported Solid Oxide Fuel Cell In Planar‐Cell Channel‐Type Setup

The degradation mechanism of anode‐supported planar solid oxide fuel cells is investigated in the present work. We fabricate a large‐area (10 cm × 10 cm) cell and carry out a long‐term test with the assembly components. A constant current of ∼0.4 A cm^–2 is applied to the cell for ∼3,100 h, and the furnace temperature is controlled in the sequence 750–800–750 °C to investigate the effect of operating temperature and thermal cycling on the degradation rate. Impedance spectra and current–voltage characteristics are measured during the operation in order to trace any increase in Ohmic and non‐Ohmic resistance as a function of time. The degradation rate is rapid during the operation at the higher temperature of ∼800 °C compared to that during the operation at ∼750 °C. Even after cooling down to ∼750 °C, that rate is still accelerated. The main contribution to the cell degradation is from an increase in the Ohmic resistance. Postmaterial analyses indicate that the cathode is delaminated at the electrolyte/cathode interface, which is attributed to the difference in thermal expansion coefficient (TEC). Thus, the present results emphasize the importance of matching the TEC between cell layers, especially under severe operating conditions such as long duration and complex thermal cycling.     

Emulator Rig for SOFC Hybrid Systems: Temperature and Power Control with a Real‐Time Software

This work is based on the hybrid system emulator plant developed by the Thermochemical Power Group (TPG) of the University of Genoa. This rig is composed of a 100 kW microturbine coupled with high temperature fuel cell emulation devices. A real‐time model is used for components not physically present in the laboratory (solid oxide fuel cell (SOFC), reformer, anodic circuit, off‐gas burner, cathode blower). It is necessary to evaluate thermodynamic and electrochemical performance related to SOFC systems. Using an User Datagram Protocol (UDP) based connection with the control/acquisition software, it generates a hardware‐in‐the‐loop (HIL) facility for hybrid system emulation. Temperature, pressure, and mass flow rate at the recuperator outlet and machine rotational speed are measured in the plant and used as inputs for the model. The turbine outlet temperature (TOT) calculated by the model is fed into the machine control system and the turbine electric load is changed to match the model TOT values (effective plant/model coupling to avoid modifications on microturbine controller). Different tests were carried out to analyze hybrid system technology through the interaction between an experimental plant and a real‐time model. Double step and double ramp tests of current and fuel provided the system dynamic response.     

Effect of Ru/CGO versus Ni/CGO Co‐Infiltration on the Performance and Stability of STN‐Based SOFCs

Electrolyte supported cells (ESC), with Sc₂O₃‐stabilized ZrO₂ (ScSZ) electrolytes, Gd‐doped ceria (CGO) or M/CGO (M = Ni, Ru) infiltrated Sr_0.94Ti_0.9Nb_0.1O₃ (STN94) anodes and LSM/YSZ cathodes, were evaluated for their initial performance and long‐term stability. Power density for the Ru/CGO infiltrated cell reached ∼0.7 W cm^–2 at 850 °C, 4% H₂O/H₂, whereas the Ni/CGO infiltrated cell reached ∼0.3 W cm^–2, with the current morphologies and loadings. Operation at 0.125 A cm^–2, 850 °C, feeding 50% H₂O/H₂ to the anode and air to the cathode, for a period >300 h, showed superior stability for the Ru/CGO infiltrated cell, with ∼0.04 mV h^–1 degradation rate, when compared to the Ni/CGO infiltrated cell (∼0.5 mV h^–1). For the Ni/CGO case, the observed degradation has been tentatively linked to initial changes in the electrochemical active area and long‐term detrimental interactions between components.     

Mn‐Stabilised Microstructure and Performance of Pd‐impregnated YSZ Cathode for Intermediate Temperature Solid Oxide Fuel Cells

The effect of Mn alloying on PdO powder and Pd‐impregnated Pd + YSZ cathode for the O₂ reduction reaction in intermediate temperature solid oxide fuel cells has been studied in detail. The microstructure, thermal stability, electrochemical activity and performance stability of the powder and cathode were analysed using thermal gravimetric analysis, X‐ray diffraction, scanning electron microscopy/energy dispersive spectroscopy and electrochemical impedance spectroscopy. The results indicate that an addition of 5 mol.‐% Mn effectively inhibits the growth and coalescence of Pd and PdO particles at high temperatures and stabilises the microstructure of the powders and the electrode; as a consequence, the electrochemical performance and stability of the cathode are significantly improved. The electrochemical performance of the Pd + YSZ and Pd_0.95Mn_0.05 + YSZ cathodes so prepared is much better than that of the conventional LSM‐based cathodes and is also comparable with the mixed ionic and electronic conducting oxide cathodes such as LSCF.     

Tailoring Electrochemical Property of Layered Perovskite Cathode by Cu‐doping for Proton‐Conducting IT‐SOFCs

Layered perovskite oxide YBaCuCoO_5+x (YBCC) was synthesized by an EDTA‐citrate complexation process and was investigated as a novel cathode for proton‐conducting intermediate temperature solid oxide fuel cells (IT‐SOFCs). The thermal expansion coefficient (TEC) of YBCC was 15.3 × 10⁻⁶ K⁻¹ and the electrical conductivity presented a semiconductor‐like behavior with the maximum value of 93.03 Scm⁻¹ at 800 °C. Based on the defect chemistry analysis, the electrical conductivity gradually decreases by the introduction of Cu into Co sites of YBaCo₂O_5+x and the conductor mechanism can transform from the metallic‐like behavior to the semiconductor‐like behavior. Thin proton‐conducting (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3–δ) BZCYYb electrolyte and NiO–BZCYYb anode functional layer were prepared over porous anode substrates composed of NiO–BZCYYb by a one‐step dry‐pressing/co‐firing process. Laboratory‐sized quad‐layer cells of NiO‐BZCYYb / NiO‐BZCYYb / BZCYYb / YBCC with a 20 μm‐thick BZCYYb electrolyte membrane exhibited the maximum power density as high as 435 mW cm⁻² with an open‐circuit potential (OCV) of 0.99 V and a low interfacial polarization resistance of 0.151 Ωcm² at 700 °C. The experimental results have indicated that the layered perovskite oxide YBCC can be a cathode candidate for utilization as proton‐conducting IT‐SOFCs.     

Evaluation of a Cu/YSZ and Ni/YSZ Bilayer Anode for the Direct Utilization of Methane in a Solid‐Oxide Fuel Cell

Carbon deposition is an issue when operating solid oxide fuel cells (SOFC) on fuels other than hydrogen, and so a variety of strategies have been used to prevent carbon accumulation on the anodes. In this paper, we describe a bilayer anode that contains a functional layer consisting of Ni/YSZ and a conduction layer consisting of Cu/YSZ. The anode‐supported button cells were fabricated using a uni‐axially pressing technique to produce the anode, followed by impregnation with Cu. The cells were tested at 1,023 K in dry CH₄ and their performance compared to that of a typical Ni/YSZ anode. The Cu does not catalyze the cracking of methane and as such less carbon deposits in the conduction layer resulting in anode stability for over 100 h. The limitation with using Cu in the anode is the temperature of operation.     

Anode Current Collecting Efficiency of Tubular Anode‐supported Solid Oxide Fuel Cells

Anode current collection points (ACCPs) were fabricated on the outside surface of a tubular anode‐supported solid oxide fuel cell (SOFC). The ACCPs were distributed axially along the SOFC tube with the distance between every adjacent two ACCPs the same. The effect of collecting current with different number of ACCPs on the performance of the SOFC was studied. It was found that with the same effective area, using more ACCPs to collect the current leads to better performance, while with a SOFC with a determined total surface area, there is an optimum number of ACCPs to be made and used considering the area occupied by the ACCPs themselves.     

Evaluation of a Non‐sealed Solid Oxide Fuel Cell Stack with Cells Embedded in Plane Configuration

A non‐sealed solid oxide fuel cell stack with cells embedded in plane configuration was fabricated and operated successfully in a box‐like stainless‐steel chamber. For a two‐cell stack, it demonstrated an open circuit voltage (OCV) of 2.13 V and a maximum power output of 569 mW at the flow rate of 67 sccm CH₄ and 33 sccm O₂. A fuel utilization of 4.16% was obtained. The cell performance was dominated by two different mechanisms, the polarization of the cathode at low current and the concentration polarization of the anode at high current. Finally, a scaled‐up stack with six cells in series generated an OCV of 6.4 V and a maximum power output of 8.18 W.     

Fabrication and Power Densities of Anode‐Supported Solid Oxide Fuel Cells with Different Ni‐YSZ Anode Functional Layer Thicknesses

We investigated an appropriate preparation condition for anode‐supported SOFCs: (La,Sr)MnO₃/cathode functional layer/YSZ/Ni‐YSZ were fabricated with and without a Ni‐YSZ anode functional layer (AFL) via the tape‐casting method, where the AFL thicknesses were controlled from approximately 20 to 80 μm. The warpage depended on the co‐sintering temperature of the electrolyte/AFL/anode‐support half‐cells, indicating that similar shrinkage of the electrolyte/AFL/anode support is significant for lower warpages. The electrical properties of SOFCs with AFLs were compared to those of SOFCs without AFLs. In this regard, the use of an AFL decreased the ohmic and activation polarization resistances due to both the decrease in contact resistance between the electrolyte and the AFL and the increase in three‐phase boundaries. However, the polarization diffusion increased when an AFL was employed, because AFL layers are denser than the anode support. The maximum power densities of samples with AFL were higher than those of SOFCs without AFLs, indicating that the decrease in both ohmic and activation‐polarization resistances is more significant for improving the power densities, as compared to the concentration polarization resistance.     

Start‐Up Characteristics of Segmented‐In‐Series Tubular SOFC Power Modules Improved by Catalytic Combustion at Cathodes

For large‐scale SOFC power generation systems, a shorter start‐up time of SOFC cell stacks with relatively large heat capacity is one of the most important technological issues to determine the flexibility in SOFC system operation. In this study, start‐up procedures have been examined to shorten the start‐up time period. The conventional heating procedure using pre‐heated hot air and self‐heating by SOFC operation at low temperatures had a difficulty to shorten the start‐up time period because of the limitation in power generation at lower temperatures. In this study, as an alternative start‐up procedure, catalytic combustion at the SOFC cathodes is, for the first time, demonstrated to be useful on the system level. The applicability of the catalytic combustion to shorten the start‐up time period has been verified numerically as well as experimentally by using a large‐scale cell stack cartridge. This unique start‐up procedure enables to operate SOFC‐based large‐scale power generation systems.     

Performance Evaluation of Solid Oxide Fuel Cells with Thin Film Electrolyte Fabricated by Binder‐Assisted Slurry Casting

A gas‐tight yttria‐stabilized zirconia (YSZ) electrolyte film was fabricated on porous NiO–YSZ anode substrates by a binder‐assisted slurry casting technique. The scanning electron microscope (SEM) results showed that the YSZ film was relatively dense with a thickness of 10 μm. La_0.8Sr_0.2MnO₃ (LSM)–YSZ was applied to cathode using a screen‐print technique and the single fuel cells were tested in a temperature range from 600 to 800 °C. An open circuit voltage (OCV) of over 1.0 V was observed. The maximum power densities at 600, 700, and 800 °C were 0.13, 0.44, and 1.1 W cm^–2, respectively.