Electrochemical performance and microstructure characterization of nickel yttrium‐stabilized zirconia anode

A nickel and yttrium‐stabilized zirconia (Ni‐YSZ) composite is one of the most commonly used anode materials in solid oxide fuel cells (SOFCs). One of the drawbacks of the Ni‐YSZ anode is its susceptibility to deactivation due to the formation of carbonaceous species when hydrocarbons are used as fuel supplies. We therefore initiated an electrochemical study of the influence of methane (CH₄) on the performance of Ni‐YSZ anodes by examining the kinetics of the oxidation of CH₄ and H₂ over operating temperatures of 600–800°C. Anode performance deterioration was then correlated with the degree of carbonization observed on the anode using ex‐situ X‐ray powder diffraction and scanning electron microscopy techniques. Results showed that carbonaceous species led to a significant deactivation of Ni‐YSZ anode toward methane oxidation. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

One‐pot synthesis of silver‐modified sulfur‐tolerant anode for SOFCs with an expanded operation temperature window

To develop solid oxide fuel cells (SOFCs) capable of operating on sulfur‐containing practical fuels at intermediate temperatures, further improvement of the sulfur tolerance of a Ni + BaZr_0.4Ce_0.4Y_0.2O_3‐δ (BZCY) anode is attempted through the addition of some metal modifiers (Fe, Co, and Ag) by a one‐pot synthesis approach. The effects of these modifiers on the electrical conductivity, morphology, sulfur tolerance, and electrochemical activity of the anode are systematically studied. As a result, the cell with Ag‐modified Ni + BZCY anode demonstrates highest power output when operated on 1000 ppm H₂S‐H₂ fuel. Furthermore, the Ag‐modified anode displays much better stability than Ni + BZCY with 1000 ppm H₂S‐H₂ fuel at 600°C. These results suggest that the addition of Ag modifier into Ni + BZCY is a promising and efficient method for improving the sulfur tolerance of SOFCs. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4287–4295, 2017     

Carbon-tolerance effects of Sm0.2Ce0.8O2−δ modified Ni/YSZ anode for solid oxide fuel cells under methane fuel conditions

Samarium-doped ceria (SDC) is coated onto a Ni/yttria-stabilized zirconia (Ni/YSZ) anode for the direct use of methane in solid-oxide fuel cells. Porous SDC thin layer is applied to the anode using the sol–gel coating method. The experiment was performed in H₂ and CH₄ conditions at 800 °C. The cell performance was improved by approximately 20 % in H₂ conditions by the SDC coating, due to the high ionic conductivity, the mixed ionic and electronic conductive property of the SDC, and the increased triple phase boundary area by the SDC coating in the anode. Carbon was hardly deposited in the SDC-coated Ni/YSZ anode. The cell performance of the SDC-coated Ni/YSZ anode did not show any significant degradation for up to 90 h under 0.1 A cm^?2 at 800 °C. The porous thin SDC coating on the Ni/YSZ anode provided the electrochemical oxidation of CH₄ over the whole anode, and minimized the carbon deposition by electrochemical carbon oxidation.     

Experimental Study of the Carbon Deposition from CH4 onto the Ni/YSZ Anode of SOFCs

A challenge in the operation of solid oxide fuel cells (SOFCs) with hydrocarbon fuels is the carbon deposition on the nickel/yttria‐stabilized zirconia (Ni/YSZ) anode. The Grabke‐type kinetic model has been proposed for the carbon formation based upon the assumption of elementary steps, which consist of a rate‐limiting dissociative chemisorption step and a stepwise dehydrogenation of the chemisorbed methyl group. This work experimentally studied the carbon formation on a SOFC Ni/YSZ anode exposed to CH₄+H₂ gas mixtures. Experiments were conducted with various gas compositions of CH₄/H₂ and temperatures in the range from 873 K to 1,123 K. The experimental results were used to determine a kinetic model that was applied to the SOFC operating environments. Based on the experimental data, the formula for the carbon formation rate that is dependent on the operating temperature and the gas compositions of CH₄/H₂ was established.     

Electrochemical Performance of a Ni and YSZ Composite Synthesised by Ultrasonic Spray Pyrolysis as an Anode for SOFCs

The electrochemical performance of an anode material for a solid oxide fuel cell (SOFC) depends highly on microstructure in addition to composition. In this study, a NiO–yttria‐stabilised zirconia (NiO–YSZ) composite with a highly dispersed microstructure and large pore volume/surface area has been synthesised by ultrasonic spray pyrolysis (USP) and its electrochemical characteristics has been investigated. For comparison, the electrochemical performance of a conventional NiO–YSZ is also evaluated. The power density of the zirconia electrolyte‐supported SOFC with the synthesised anode is ∼392 mW cm^–2 at 900 °C and that of the SOFC with the conventional NiO–YSZ anode is ∼315 mW cm^–2. The improvement is ∼24%. This result demonstrates that the synthesised NiO–YSZ is a potential alternative anode material for SOFCs fabricated with a zirconia solid electrolyte.     

Comparison Between Anode‐Supported and Electrolyte‐Supported Ni‐CGO‐LSCF Micro‐tubular Solid Oxide Fuel Cells

Two types of micro‐tubular hollow fiber SOFCs (MT‐HF‐SOFCs) were prepared using phase inversion and sintering; electrolyte‐supported, based on highly asymmetric Ce_0.9Gd_0.1O_1.95(CGO) HFs and anode‐supported based on co‐extruded NiO‐CGO(CGO)/CGO HFs. Electroless plating was used to deposit Ni onto the inner surfaces of the electrolyte‐supported MT‐HF‐SOFCs to form Ni‐CGO anodes. LSCF‐CGO cathodes were deposited on the outer surface of both these MT‐HF‐SOFCs before their electrochemical performances were compared at similar operating conditions. The performance of the anode‐supported MT‐HF‐SOFCs which delivered ca. 480 mW cm^–2 at 600 °C was superior to the electrolyte‐supported MT‐HF‐SOFCs which delivered ca. six times lower power. The contribution of ohmic and electrode polarization losses of both FCs was investigated using electrochemical impedance spectroscopy. The electrolyte‐supported MT‐HF‐SOFCs had significantly higher ohmic and electrode polarization ASR values; this has been attributed to the thicker electrolyte and the difficulties associated with forming quality anodes inside the small (<1 mm) lumen of the electrolyte tubes. Further development on co‐extruded anode‐supported MT‐HF‐SOFCs led to the fabrication of a thinner electrolyte layer and improved electrode microstructures which delivered a world leading 2,400 mW cm^–2. The newly made cell was investigated at different H₂ flow rates and the effect of fuel utilization on current densities was analyzed.     

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.     

Electrochemical Analysis of a System Based on W and Ni Combined with CeO2 as Potential Sulfur‐tolerant SOFC Anode

A formulation of tungsten and nickel combined with CeO₂ (WNi‐Ce) was prepared and evaluated as sulfur‐tolerant anode for SOFC at intermediate temperature. Structural and morphological changes that take place in the system upon interactions with hydrogen sulfide were analyzed. The electrochemical performance was tested in a single cell, WNi‐Ce/LDC/LSGM/LSFC, varying H₂S concentration (0–500 ppm) at 750 °C using I–V curves, impedance spectroscopy and load demands. The highest cell performance was reached in H₂ and decrease with H₂S content increase in the fuel from 226 mW cm⁻² in pure H₂ to 108 mW cm⁻² in 500 ppm H₂S/H₂. Essentially, no decay in the cell performance was observed in the several short‐term load tests studied under several H₂S concentration (0–500 ppm) during 1h, and even in 500 ppm H₂S/H₂ during 70 h, indicating that this material could be a potential sulfur‐tolerant anode.     

Effects of Manganese Oxide Addition on Coking Behavior of Ni/YSZ Anodes for SOFCs

Solid oxide fuel cells with Ni‐MnO/yttria‐stabilized‐zirconia (YSZ) tricomposite anode supports were fabricated with different MnO concentrations, and the coking tolerances and catalytic activities were investigated in wet CH₄ atmosphere. Ni_0.9(MnO)_0.1/YSZ (10MnO) anode support cell exhibited a maximum power density of 210, 354, 505, and 620 mWcm⁻² at 700, 750, 800, and 850 °C, respectively, in H₂. Moreover, a maximum power density in wet CH₄ reaches 504 mWcm⁻² at 800 °C; while the Ni/YSZ cell showed poorer performances. The coking tolerance improved with an increase in their MnO content, and the 10MnO anode showed the highest tolerance. 10MnO exhibited stable performance for more than 40 h in wet CH₄ without undergoing deactivation. Furthermore, it showed negligible coke formation of 0.0045 g of coke per catalyst, during testing under steam reforming‐like conditions at a steam‐to‐carbon (S/C) ratio of 1. Outlet gas chromatography analysis indicated that MnO suppresses CH₄ cracking, while only minimally lowering the catalytic activity of steam reforming. Thus, it can be inferred that MnO promotes the adsorption of steam and oxygen on the reaction sites, owing to its high basicity and oxygen storage capacity. The increase in the local S/C and oxygen‐to‐carbon ratios suppresses CH₄ cracking and promotes coke gasification.     

Performance of mix‐impregnated CeO2‐Ni/YSZ Anodes for Direct Oxidation of Methane in Solid Oxide Fuel Cells

CeO₂‐Ni/YSZ anodes for methane direct oxidation were prepared by the vacuum mix‐impregnation method. By this method, NiO and CeO₂ are obtained from nitrate decomposition and high temperature sintering is avoided, which is different from the preparation of conventional Ni‐yttria‐stabilised zirconia(YSZ) anodes. Impregnating CeO₂ into the anode can improve the cell performance, especially, when CH₄ is used as fuel_. The investigation indicated that CeO₂‐Ni/YSZ anodes calcined at higher temperature exhibited better stability than those calcined at lower temperature. Under the testing temperature of 1,073 K, the anode calcined at 1,073 K exhibited the best performance. The maximum power density of a cell with a 10 wt.‐%CeO₂‐25 wt.‐%Ni anode calcined at 1,073 K reached 480 mW cm^–2 after running on CH₄ for 5 h. At the same time, high discharge current favoured cell operation on CH₄ when using these anodes. No obvious carbon was found on the CeO₂‐Ni anode after testing in CH₄ as revealed from SEM and corresponding linear EDS analysis. In addition, cell performance decreased at the beginning of discharge testing which was attributed to the anode microstructure change observed with SEM.     

Modeling of Direct H2S Fueled Solid Oxide Fuel Cells with Reforming Chemical Kinetics

A direct H₂S fueled SOFC model is developed based on Ni‐YSZ/YSZ/YSZ‐LSM button cell test stand. The model considers the detailed reforming chemical processes of H₂S and multi‐physics transport processes in the fuel cell and fuel supply tubes. The model is validated using experimental data. Extensive simulations are performed to study the complicated interactions between multi‐physics transport processes and chemical/electrochemical reactions. The results elucidate the fundamental mechanisms of direct H₂S fueled SOFCs. It is found that suitably increasing the H₂O content in the supplied H₂S fuel can improve SOFC electrochemical performance; high operating temperature may facilitate the reforming of H₂S and improve the electrochemical performance. The sulfur poisoning effect may be mitigated by increasing the H₂O content in the fuel, increasing the operating temperature, decreasing the flow rate, and/or making the cell work at low voltage (or high current) conditions.     

Preparation of BaZr0.1Ce0.7Y0.2O3–δ Based Solid Oxide Fuel Cells with Anode Functional Layers by Tape Casting

Solid oxide fuel cells (SOFCs) based on the proton conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte were prepared and tested in 500–700 °C using humidified H₂ as fuel (100 cm³ min^–1 with 3% H₂O) and dry O₂ (50 cm³ min^–1) as oxidant. Thin NiO‐BZCY anode functional layers (AFL) with 0, 5, 10 and 15 wt.% carbon pore former were inserted between the NiO‐BZCY anode and BZCY electrolyte to enhance the cell performance. The anode/AFL/BZCY half cells were prepared by tape casting and co‐sintering (1,300 °C/8 h), while the Sm_0.5Sr_0.5CoO_3–δ (SSC) cathodes were prepared by thermal spray deposition. Well adhered planar SOFCs were obtained and the test results indicated that the SOFC with an AFL containing 10 wt.% pore former content showed the best performance: area specific resistance as low as 0.39 Ω cm² and peak power density as high as 0.863 W cm^–2 were obtained at 700 °C. High open circuit voltages ranging from 1.00 to 1.12 V in 700–500 °C also indicated negligible leakage of fuel gas through the electrolyte.     

New Stack Design of Micro‐tubular SOFCs for Portable Power Sources

Micro‐tubular solid oxide fuel cells (SOFCs) have high thermal stability and higher volumetric power density, which are considered to be ideal features for portable power sources and auxiliary power units for automobile. Here, we report a new stack design using anode supported micro‐tubular SOFCs with 2 mm diameter using Gd doped CeO₂ (GDC) electrolyte, NiO‐GDC anode and (La, Sr)(Co, Fe)O₃ (LSCF)‐GDC cathode. The new stack consists of three bundles with five tubular cells, sealing layers and interconnects and fuel manifolds. The performance of the stack whose volume is 1 cm³ was shown to be 2.8 V OCV and maximum power output of 1.5 W at 500 °C, applying air only by natural convection. The results also showed strong dependence of the fuel flow rates on the stack performance, which was correlated to the gas diffusion limitation.     

Slip casting combined with colloidal spray coating in fabrication of tubular anode-supported solid oxide fuel cells

A simple and cost-effective slip casting technique was successfully developed to fabricate NiO–YSZ anode substrates for tubular anode-supported single SOFCs. An YSZ electrolyte film was coated on the anode substrates by colloidal spray coating technique. A single cell, NiO–YSZ/YSZ (20 μm)/LSM–YSZ, using the tubular anode supports with YSZ coating, was assembled and tested to demonstrate the feasibility of the techniques applied. Using humidified hydrogen (75 ml/min) as fuel and ambient air as oxidant, the maximum power densities of the cell were 760 mW/cm² and 907 mW/cm² at 800 °C and 850 °C, respectively. The observed OCV was closed to the theoretical value and the SEM results revealed that the microstructure of the anode fabricated by slip casting is porous while the electrolyte film coated by colloidal spray coating is dense.     

Study of Carbon Deposition Behavior on Cu–Co/CeO2–YSZ Anodes for Direct Butane Solid Oxide Fuel Cells

Electro‐catalytic activity of Cu–Co/CeO₂–YSZ anodes towards oxidation of H₂ and n‐C₄H₁₀ fuels and carbon depositions are investigated using different Cu–Co loadings. Cu–Co/CeO₂–YSZ anode based SOFCs with YSZ as electrolyte and LSM/YSZ as cathode were prepared by tape casting and wet impregnation methods and performance was analyzed using I–V characteristics and impedance spectroscopy. The Cu–Co/CeO₂–YSZ anodes with Cu–Co loading of 10, 15, and 25 wt.% produced power density of 60, 197, and 400 mW cm^–2 in H₂ and 190, 225, and 275 mW cm^–2 in n‐C₄H₁₀ at 800 °C. The power density is increased with the increase in Cu–Co loading in Cu–Co/CeO₂–YSZ anodes. The electrochemical impedance spectra shows less ohmic and polarization resistance for 25 wt.% Cu–Co loading in comparison to 10 and 15 wt.% Cu–Co. Scanning electron microscopy and high resolution transmission electron microscopy shows that the carbon fibers formed are hollow in nature with 70 nm size, whereas, thermal gravimetric analysis and X‐ray diffraction points out that they are amorphous in nature. The performance degradation of Cu–Co/CeO₂–YSZ anodes in n‐C₄H₁₀ in 16 h is attributed to increasing amount of carbon deposition with time, which is contrary to our earlier observation in Cu‐Fe/CeO₂–YSZ anode.     

Development of a novel proton conducting fuel cell based on a Ni‐YSZ support

A new proton conducting fuel cell design based on the BZCYYb electrolyte is studied in this research. In high‐performance YSZ‐based SOFCs, the Ni‐YSZ support plays a key role in providing required electrical properties and robust mechanical behavior. In this study, this well‐established Ni‐YSZ support is used to maintain the proton conducting fuel cell integrity. The cell is in a Ni‐YSZ (375 μm support)/Ni‐BZCYYb (20 μm anode functional layer)/BZCYYb (10 μm electrolyte)/LSCF‐BZCYYb (25 μm cathode) configuration. Maximum power density values of 166, 218, and 285 mW/cm² have been obtained at 600°C, 650°C, and 700°C, respectively. AC impedance spectroscopy results show values of 2.17, 1.23, and 0.76 Ω·cm² at these temperatures where the main resistance contributor above 600°C is ohmic resistance. Very fine NiO and YSZ powders were used to achieve a suitable sintering shrinkage which can enhance the electrolyte sintering. During cosintering of the support and BZCYYb electrolyte layers, the higher shrinkage of the support layer led to compressive stress in the electrolyte, thereby enhancing its densification. The promising results of the current study show that a new generation of proton conducting fuel cells based on the chemically and mechanically robust Ni‐YSZ support can be developed which can improve long‐term performance and reduce fabrication costs of proton conducting fuel cells.     

Comparative Performance Analysis of Anode‐Supported Micro‐Tubular SOFCs with Different Current‐Collection Architectures

In this study, the performances of single micro‐tubular solid oxide fuel cells based on the NiO–YSZ/YSZ/LSM system with two different current‐collection architectures were compared. In the first case, a straight Ni wire was inserted within the hole of the cell before the electrochemical testing, and in the second case, a coil integrated‐current collector within the anode layer was already arranged for electrical connections during cell processing. The current produced in each case was collected from double terminal and the performance of the cells was estimated by electrochemical I–V characterization. The maximum power outputs generated in the cells with the integrated‐current collector and the common current‐collection architectures were of ∼200 and ∼55 mW cm^–2, respectively at 800 °C under a wet H₂ fuel flow.     

Dense 8 mol% yttria-stabilized zirconia electrolyte by DLP stereolithography

Solid Oxide Fuel Cells (SOFCs) are environmentally efficient energy conversion devices, but are partially limited by the complicated fabrication procedure. In this work, dense 8 mol% yttria-stabilized zirconia (8YSZ) ceramics were successfully realized through a DLP (digital light processing) stereolithography method and the electrolyte self-supported fuel cell was also tested at 800 °C. The sintering behavior of the as-printed planar samples were investigated and a fully dense ceramic can be achieved at 1450 °C. The total conductivity of the sintered 8YSZ can reach 2.18 × 10⁻² S cm⁻¹ at a test temperature of 800 °C, which is acceptable for practical application. For the electrolyte self-supported fuel cell test, a power density of 114.3 mW cm⁻² can be achieved when Ni-8YSZ cermet and La_0.8Sr_0.2MnO₃ (LSM) were used as anode and cathode. It was demonstrated that 3D printing is a promising processing technique to build up electrolyte self-supported SOFCs with desired structure for the future development.     

A comparison of La0.75Sr0.25Cr0.5Mn0.5O3−δ and Ni impregnated porous YSZ anodes fabricated in two different ways for SOFCs

In this paper, La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM) and Ni impregnated porous yttria-stabilized zirconia (YSZ) anodes have been fabricated in two different ways. The testing results demonstrated the excellent performance of the anode made by infiltrating a mixture of LSCrM and Ni(NO₃)₂ solutions into porous YSZ matrix. After reduction of the anode with hydrogen, an inner nano-network structure with mixed ionic-electronic conducting path has been formed within and between these added particles. A single cell with the anode at 800 °C exhibited the maximum power densities of 1151 and 704 mW cm⁻² when dry H₂ and CH₄ were used as the fuels, respectively; under the same conditions, the cell performances for LSCrM and Ni impregnated YSZ anode separately were 810 and 508 mW cm⁻². A cavity model was proposed to simulate the impregnating process and the loading was calculated. No carbon deposition was detected in the anode, even with the presence of Ni, after operation in dry CH₄ for about 6 h under open-circuit condition.     

Intermediate Temperature Anode‐Supported Fuel Cell Based on BaCe0.9Y0.1O3 Electrolyte with Novel Pr2NiO4 Cathode

A proton conducting ceramic fuel cell (PCFC) operating at intermediate temperature has been developed that incorporates electrolyte and electrode materials prepared by flash combustion (yttrium‐doped barium cerate) and auto‐ignition (praseodymium nickelate) methods. The fuel cell components were assembled using an anode‐support approach, with the anode and proton ceramic layers prepared by co‐pressing and co‐firing, and subsequent deposition of the cathode by screen‐printing onto the proton ceramic surface. When the fuel cell was fed with moist hydrogen and air, a high Open Circuit Voltage (OCV > 1.1 V) was observed at T > 550 °C, which was stable for 300 h (end of test), indicating excellent gas‐tightness of the proton ceramic layer. The power density of the fuel cell increased with temperature of operation, providing more than 130 mW cm^–2 at 650 °C. Symmetric cells incorporating Ni‐BCY10 cermet and BCY10 electrolyte on the one hand, and Pr₂NiO_{4 + δ} and BCY10 electrolyte on the other hand, were also characterised and area specific resistances of 0.06 Ω cm² for the anode material and 1–2 Ω cm² for the cathode material were obtained at 600 °C.