Microstructural effects on the electrical and mechanical properties of Ni–YSZ cermet for SOFC anode

The electrical and mechanical properties of Ni–YSZ cermet as the anode support of solid oxide fuel cell (SOFC) are determined by the metallic and ceramic components, respectively. We used YSZ and NiO commercial powders of the average particle size from 1 to 10 μm to fabricate Ni–YSZ cermets with different microstructures. The porosity of the cermets was also modified by the amount of carbon black addition. The distribution of each phase of cermets was analyzed with scanning electron microscopy combined with energy dispersive spectroscopy. The electrical conductivity and fracture strength of the Ni–YSZ cermets were investigated and interpreted in a view of percolation phenomena. The finer particles, either NiO or YSZ, were interlinked well by sintering and the electrical and mechanical properties of Ni–YSZ cermets were enhanced by the percolation of Ni and YSZ, respectively.     

Fabrication of tubular NiO/YSZ anode-support of solid oxide fuel cell by gelcasting

A gelcasting process has been developed to fabricate tubular NiO/YSZ anode-support for solid oxide fuel cells (SOFCs) successfully. The rheological behaviors of the ceramic particle suspensions for gelcasting were investigated as a function of the process parameters, such as the amount of pore former, pH value, dispersant concentration, monomer concentration, ball-mill time and solid loading. The sintering shrinkage, microstructure, bending strength and electrical conductivity of the sintered specimens were examined. The tubular Ni/YSZ anode-support obtained under the optimized preparation conditions exhibited a porosity of 39.6%, mean pore size of below 0.9 μm, 482 s cm⁻¹ in electrical conductivity at 700 °C, and the bending strength of 112.8 MPa, which can well meet the requirements for SOFCs.     

Engineered anode structure for enhanced electrochemical performance of anode-supported planar solid oxide fuel cell

A novel approach of fabricating SOFC anode comprising graded compositions in constituent phases having layer wise microstructural variation is reported. Such anode encompasses conventional NiO–YSZ (40 vol% Ni) with higher porosity at the fuel inlet side and Ni–YSZ electroless cermet (28–32 vol% Ni) with less porosity toward the electrolyte. Microstructures and thicknesses of the bilayer anodes (BLA) are varied sequentially from 50 to 250 μm for better thermal compatibility and cell performance. Significant augmentation in performance (3.5 A cm⁻² at 800 °C, 0.7 V) is obtained with engineered trilayer anode (TLA) having conventional anode support in conjunction with layers of electroless cermet each of 50 μm having 28 and 32 vol% Ni. Engineered TLA accounts for substantial reduction both in cell polarization (ohmic ASR: 78 mΩ cm² versus 2835 mΩ cm²; cell impedance: 0.35 Ω cm² versus 0.9 Ω cm²) and degradation rate (76 μV h⁻¹ versus 219 μV h⁻¹) compared to cells fabricated with conventional cermet.     

Fabrication,microstructure and properties of a YSZ electrolyte for SOFCs

Yttria stabilized zirconia (YSZ) has widely been used as an electrolyte in solid oxide fuel cell (SOFC) stacks. The microstructure and properties of YSZ related to the fabrication process are discussed in this paper. For the named two-step sintering process, uniform and hexagonal grains with a size of 1–4 μm were obtained from the adobe following tape calendaring (TCL). Elliptical and hexagonal grains with a size of 0.4–3 μm were obtained from the adobe of tape casting (TCS) using the three-step process. The electrical conductivities of YSZ with different grain sizes were measured via the four-probe DC technique and grain conductivities and grain boundary conductivities of YSZ were investigated by impedance spectroscopy. YSZ electrolytes with a grain size of 0.1–0.4 μm had the highest electrical conductivity in the range of 500–1000 °C, especially at medium and low temperatures 550–800 °C. As the YSZ grain size becomes small, the thickness of the intergranular region decreased greatly. The YSZ electrolytes with sub-micrometer grain sizes, high ion conductivity and low sintering temperatures are important to the electrode-supported SOFC, on which the dense YSZ electrolyte films are optimized at 10 μm.     

A more efficient anode microstructure for SOFCs based on proton conductors

While the desired microstructure of the state-of-the-art Ni-YSZ anode for a solid oxide fuel cell (SOFC) based on YSZ is well known, the anode microstructure for a SOFC based on a proton conductor is yet to be optimized. In this study, we examined the effect of anode porosity on the performance of a SOFC based on BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb), a mixed ion (proton and oxygen anion) conductor with high ionic conductivity at intermediate temperatures. Three cells with Ni-BZCYYb cermet anodes of different porosities (37%, 42%, and 50%) and identical electrolytes and cathode components were fabricated and tested. Under typical fuel cell operating conditions, the cell with anode of the lowest porosity (37%), prepared without pore former, achieved the highest performance, demonstrating a peak power density of 1.2 W/cm² at 750 °C. This is radically different from the results of Ni-YSZ anodes for YSZ based cells, where high anode porosity (∼55%) is necessary to achieve high performance. The observed increase in performance (or electrocatalytic activity for anode reactions) is attributed primarily to the unique microstructure of the anode fabricated without the use of pore forming precursors.     

Electrochemical characteristics and performance of anode-supported SOFCs fabricated using carbon microspheres as a pore-former

This paper evaluates the influence of carbon microspheres (CMSs) as an electrode pore-former on the fabrication and electrochemical properties of the anode-supported solid oxide fuel cells (SOFCs). The anode supports are fabricated by dry-pressing of CMS and NiO/YSZ (nickel-oxide/yttria-stabilized zirconia) composite powder, and the YSZ electrolyte layer is prepared by the electrophoretic deposition technique. The ohmic and polarization resistances for NiO/YSZ–YSZ half cells at different testing temperatures (650–850 °C) are analyzed by electrochemical impedance spectroscopy (EIS). The polarization ASR (area specific resistance) for the fabricated half cells increases from 0.583 Ω cm² to 3.047 Ω cm² when the temperature decreases from 850 °C to 650 °C. The electrochemical performance of single cells is measured at different temperatures (700–850 °C) and the results indicate that the cells fabricated using CMS as the pore-former exhibit much higher electrochemical performance than those without using CMS. A maximum power density of 207.7 mW cm⁻², 431.2 mW cm⁻², and 571.6 mW cm⁻² is recorded at 850 °C for the cells fabricated by adding 0 wt. %, 2.5 wt. % and 5 wt. % of CMS, respectively. The maximum fuel utilization efficiency is also found to increase from 26.5% for the cell prepared without CMS to 47.0% and 59.6% for the cells prepared with 2.5 wt. % and 5 wt. % of CMS, respectively. The increase in the electrochemical performance by adding CMS as pore-former to anode-supports is attributed to higher porosity and pore size of the electrode.     

Ni infiltrated YSZ anode stabilization by inducing strong metal support interaction between nickel and titania in solid oxide fuel cell under accelerated testing

Sintering of Ni particles in Ni infiltrated porous YSZ anodes and decrease in triple phase boundary is the reason for performance loss in SOFC. In the present work, the idea of strong metal support interaction (SMSI) has been used to prevent the sintering of Ni particles by introducing TiO₂ as support with Ni catalyst. Electrical conductivity variation of porous YSZ matrix impregnated with Ni and Ni/TiO₂ have been investigated. Single button cells (anode supported) with and without TiO₂ impregnated Ni–YSZ anode were fabricated and characterized through current–voltage measurement at different loads. It is shown that the conductivity of porous Ni–YSZ anode and the performance of SOFC button cell with the same anode decreased with the increase in temperature and redox cycling at different time intervals. The power density of 12% Ni–YSZ anode was 116 mW/cm² and it increased to 180 mW/cm² for 12% Ni–4% TiO₂–YSZ based anodes at 800 °C. This increase was interpreted by strong attachment of Ni particles on TiO₂ preventing Ni coarsening during prolonged reduction in H₂ at 800 °C as observed by SEM. The power density increased with further increase in Ni loading and it reached to 400 mW/cm² for 16% Ni–4% TiO₂–YSZ based anodes. The performance increases with addition of TiO₂ support in Ni–YSZ based anodes corroborates with the impedance spectroscopy analyses.     

Design of experiment approach applied to reducing and oxidizing tolerance of anode supported solid oxide fuel cell. Part I: Microstructure optimization

The main drawback of Ni/YSZ anode supports for solid oxide fuel cell application is their low tolerance to reducing and oxidizing (RedOx) atmosphere changes, owing to the Ni/NiO volume variation. This work describes a structured approach based on design of experiments for optimizing the microstructure for RedOx stability enhancement. A full factorial hypercube design and the response surface methodology are applied with the variables and their variation range defined as: (1) NiO proportion (40-60 wt% of the ceramic powders), (2) pore-former proportion (0-30 wt% corresponding to 0-64 vol.%), (3) NiO particle size (0.5-8 μm) and (4) 8YSZ particle size (0.6-9 μm).To obtain quadratic response models, 25 different compositions were prepared forming a central composite design. The measured responses are (i) shrinkage during firing, (ii) surface quality, (iii) as-sintered porosity, (iv) electrical conductivity after reduction and (v) expansion after re-oxidation. This approach quantifies the effect of all factors and their interactions. From the quadratic models, optimal compositions for high surface quality, electrical conductivity (>500 S cm⁻¹ at room temperature) and RedOx expansion (<0.2% upon re-oxidation) are defined. Results show that expansion after re-oxidation is directly influenced by the sample porosity whereas, surprisingly, the NiO content, varied between 40 and 60 wt%, does not show any impact on this response.     

Performance of microtubular SOFCs with infiltrated electrodes under thermal cycling

In this research, tubes consisting of a co-extruded dense YSZ electrolyte (∼10 μm) and porous NiO–YSZ anode (∼200 μm) were modified with different cathodes and anode infiltration to investigate the effects on both power and thermal cycling tolerance. Type of cathode (produced by infiltration of LSM into a porous YSZ matrix or by hand-painting of an LSM–YSZ ink), the type of pore former used in the cathode (graphite or poly (methyl methacrylate), PMMA) and the infiltration of the anode (no infiltration, or with infiltration steps using a co-precipitated SDC (Samaria doped ceria) mixture, or Ni–SDC mixture) were investigated as variables. The overall aim of this work is to produce cells that are more tolerant to thermal cycling, without sacrificing power density.     

A novel low-pressure injection molding technique for fabricating anode supported solid oxide fuel cells

A low pressure injection molding (LPIM) technique is successfully developed to fabricate porous NiO–YSZ anode substrates for cone-shaped tubular anode-supported solid oxide fuel cells (SOFCs). The porosity and microstructure of the anode samples prepared with different amount of pore formers are investigated through the Archimedes method and SEM analysis. Experimental results show that with 15 wt.% paraffin as plasticizer, porosity of the NiO–YSZ substrates sintered at 1400 °C is proportional to the amount of graphite as pore former, and proper porosities can be obtained with or without 5 wt.% graphite. NiO–YSZ/YSZ/LSM–YSZ single cells are assembled and tested to demonstrate the feasibility of the LPIM technique. At 800 °C, with moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant, the cell with the anode substrate fabricated with 5 wt.% pore former shows a maximum power density of 531 mW cm⁻², while the cell without any pore former, 491 mW cm⁻². Two of the single cells (without graphite) are applied to assemble a two-cell-stack which gives an open circuit voltage of 1.75 V and a maximum output power of 5.32 W, at operating temperature of 800 °C.     

Investigation on microstructures of NiO–YSZ composite and Ni–YSZ cermet for SOFCs

NiO–YSZ composites and Ni–YSZ cermets were successfully performed for solid oxide fuel cell applications. These composites must have enough porosity and appropriate microstructure for transferring the fuel gases. In this study, ball-milling was used as a simple, cost-effective method for the purpose of mixing the raw materials. The homogeneity of NiO–YSZ composites was examined by Map mode of SEM. NiO–YSZ composites were reduced at the high temperature under the controlled atmosphere to fabricate Ni–YSZ cermet. Variations in the anode phases were investigated by XRD and microstructure and porosity of composites were observed by SEM. Effective parameters like temperatures and the amount of pore former were investigated on open porosity, bulk density, electrical conductivity as well as electrochemical impedance of NiO–YSZ composites and Ni–YSZ cermet. A thin layer of YSZ was deposited by EPD as an electrolyte on NiO–YSZ composites which had various amount of open porosity, to study its effect on the performance of semi-cells by electrochemical impedance.     

An efficient ceramic-based anode for solid oxide fuel cells

A new ceramic-based multi-component material, containing one electronic conductor (Y-substituted SrTiO₃, SYT), one ionic conductor (YSZ) and a small amount (∼5 vol.%) of Ni catalyst, was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The ceramic framework SYT/YSZ shows good dimensional stability upon redox cycling and has an electrical conductivity of ∼10 S cm⁻¹ under typical anode conditions. Owing to the substantial electrocatalytic activity of the fine and well-dispersed Ni particles on the surface of the ceramic framework, the electrode polarization resistance of 5 vol.% Ni-impregnated SYT/YSZ anode reached 0.21 Ω cm² at 800 °C in wet Ar/5%H₂. Based on these results, a redox-stable anode-supported planar SOFC is expected using this anode material, thus offering great advantages over the current generation of Ni/YSZ-based SOFCs.     

Effectiveness of anode in a solid oxide fuel cell with hydrogen/oxygen mixed gases

A porous Ni/YSZ cermet in mixed hydrogen and oxygen was investigated for its ability to decrease oxygen activity as the anode of a single chamber SOFC. A cell with a dense 300 μm YSZ electrolyte was operated in a double chamber configuration. The Ni–YSZ anode was exposed to a mixture of hydrogen and oxygen of varying compositions while the cathode was exposed to oxygen. Double chamber tests with mixed gas on the anode revealed voltage oscillations linked to lowered power generation and increased resistance. Resistance measurements of the anode during operation revealed a Ni/NiO redox cycle causing the voltage oscillations. The results of these tests, and future tests of similar format, could be useful in the development of single chamber SOFC using hydrogen as fuel.     

Fabrication of micro-tubular solid oxide fuel cells with a single-grain-thick yttria stabilized zirconia electrolyte

This study discusses the fabrication and electrochemical performance of micro-tubular solid oxide fuel cells (SOFCs) with an electrolyte consisting a single-grain-thick yttria stabilized zirconia (YSZ) layer. It is found that a uniform coating of an electrolyte slurry and controlled shrinkage of the supported tube leads to a dense, crack-free, single-grain-thick (less than 1 μm) electrolyte on a porous anode tube. The SOFC has a power density of 0.39 W cm⁻² at an operating temperature as low as 600 °C, with YSZ and nickel/YSZ for the electrolyte and anode, respectively. An examination is made of the effect of hydrogen fuel flow rate and shown that a higher flow rate leads to better cell performance. Hence a YSZ cell can be used for low-temperature SOFC systems below 600 °C, simply by optimizing the cell structure and operating conditions.     

Comparison of solid oxide fuel cell anode coatings prepared from different feedstock powders by atmospheric plasma spray method

Atmospheric plasma spray (APS) deposition of a high-performance anode coating, which is essential for obtaining high power density from a solid oxide fuel cell (SOFC), is developed. A conventional, micron-sized, nickel-coated graphite – yttria stabilized zirconia (YSZ) – graphite blend feedstock leads to a non-uniform layered coating microstructure due to the difference in the physical and thermo-physical properties of the components. In this research, new types of feedstock material received from a spray-drying method, which includes nano-components of NiO and YSZ (300 nm), are used. The microstructure and mechanical properties of a coating containing a nano composite that is prepared from spray-dried powders are evaluated and compared with those of a coating prepared from blended powder feedstock. The coating microstructures are characterized for uniformity, mechanical properties and electrical conductivity. The coatings prepared from spray-dried powders are better as they provide larger three-phase boundaries for hydrogen oxidation and are expected to have lower polarization losses in SOFC anode applications than those of coatings prepared from blended feedstock.     

An anode-supported micro-tubular solid oxide fuel cell with redox stable composite cathode

A NiO–YSZ anode-supported hollow fiber solid oxide fuel cell (HF-SOFC) has been fabricated with redox stable (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ–Sm_0.2Ce_0.8O_1.9–YSZ (LSCM–SDC–YSZ) composite cathode. The characterization of NiO–YSZ hollow fibers prepared by the phase inversion method is focused on the microstructure, porosity, bending strength and electrical conductivity. A thin YSZ electrolyte membrane (about 10 μm) can be prepared by a vacuum-assisted dip-coating process and is characterized in terms of microstructure and gas-tightness. The performance of the as-prepared HF-SOFC is investigated at 750–850 °C with humidified H₂ as fuel and ambient air as the oxidant. The peak power densities of 513, 408 and 278 mW cm⁻² can be obtained at 850, 800 and 750 °C, respectively, and the corresponding interfacial polarization resistances are 0.14, 0.29 and 0.59 Ω cm². The high performance at intermediate-to-high temperatures could be attributed to thin electrolyte and proper composite cathode with low interfacial polarization resistance. The low interfacial polarization resistance suggests potential applications of LSCM–SDC–YSZ composite oxides as the redox stable cathode. This investigation indicates that the redox stable LSCM–SDC–YSZ is a promising cathode material system for the next generation YSZ-based HF-SOFC. The results will be expected to open up a new phase of the research on the micro-tubular SOFCs.     

The effect of electrode infiltration on the performance of tubular solid oxide fuel cells under electrolysis and fuel cell modes

The electrochemical performance of two different anode supported tubular cells (50:50 wt% NiO:YSZ (yttria stabilized zirconia) or 34:66 vol.% Ni:YSZ) as the fuel electrode and YSZ as the electrolyte) under SOFC (solid oxide fuel cell) and SOEC (solid oxide electrolysis cell) modes were studied in this research. LSM (La_0.80Sr_0.20MnO_3−δ) was infiltrated into a thin porous YSZ layer to form the oxygen electrode of both cells and, in addition, SDC (Sm_0.2Ce_0.8O_1.9) was infiltrated into the fuel electrode of one of the cells. The microstructure of the infiltrated fuel cells showed a suitable distribution of fine LSM and SDC particles (50–100 nm) near the interface of electrodes and electrolyte and throughout the bulk of the electrodes. The results show that SDC infiltration not only enhances the electrochemical reaction in SOFC mode but improves the performance even more in SOEC mode. In addition, LSM infiltrated electrodes also boost the SOEC performance in comparison with standard LSM–YSZ composite electrodes, due to the well-dispersed LSM nanoparticles (favouring the electrochemical reactions) within the YSZ porous matrix.     

The effect of solids loading on the screen-printing and properties of nickel/scandia-stabilized-zirconia anodes for solid oxide fuel cells

Screen-printing is a commonly used technique to fabricate thick film solid oxide fuel cell (SOFC) electrodes, followed by drying and high temperature sintering, and this study focuses on the rheology of such screen-printing inks for SOFC anode fabrication, and in particular on the effect of solids content. Commercially available powders of NiO and scandia stabilized zirconia were used, with average particle sizes of 0.714 and 2.151 μm, respectively. The thixotropy and viscosity of inks increased with increasing solid content. The particle network strength, important to the production of films with good particle connectivity and mechanical strength, also increased as the solids content increased. The complex modulus, G*, (indicative of the overall elasticity and tackiness of the inks) ranged from 500 to 4000 Pa, a range found acceptable for screen-printing. The minimum and maximum acceptable solids content were determined to be 25 and 30 vol% respectively. Inks having solids content outside this range were too liquid like and tacky, respectively, for effective screen-printing. The percentage of ink recovery, indicative of the print quality of the resultant films, also increased with solid content. The relevant properties of the anode films, including mechanical strength, electronic conductivity and electrochemical performance, all improved with increasing solids content as a result of improved particle connectivity, consistent with the conclusions drawn from the rheological study. In summary, from the perspective of ink rheology, screen-printability and performance, inks having 28–30 vol% solids were determined as the most suitable for the production of high quality SOFC anode films with a thickness of around 10 μm.     

Performance of SOFC coupled with n-C4H10 autothermal reformer: Carbon deposition and development of anode structure

The performance deterioration of solid oxide fuel cells (SOFCs, Nickel-Yttria stabilized zirconia (Ni-YSZ)/YSZ/lanthanum doped strontium manganite-YSZ (LSM–YSZ)) coupled with n–C₄H₁₀ steam reformers (SR), autothermal reformers (ATR), or catalytic partial oxidation reformers (CPOX) was examined using an integrated system of a micro-reactor reformer and SOFC unit. The terminal voltage rapidly degraded in CPOX-driven SOFC (oxygen to carbon ratio (OCR) = 0.5) while it was fairly stable for SR-driven SOFC (steam to carbon ratio (SCR) = 2) over 250 h. For ATR-driven SOFC at near the thermoneutral point (OCR = 0.5 and steam to carbon ration (SCR) = 1.3), significant deterioration of the terminal voltage was observed in 100 h of operation. The main precursors of carbon deposition on the SOFC were identified by reformate gas analysis during the tests. In this study, we reveal that the carbon deposition on the SOFC anode can be affected by not only lower-order hydrocarbons (C₁∼C₄), but also by the CO/H₂ gas mixture. The change in electrical conductivity of the Ni-YSZ cermet used for the SOFC anode was investigated under different gas mixtures. To investigate the propensity for carbon deposition by each carbon-containing gas mixture, we defined the ratios of steam to specific carbon (C₁∼C₄ lower-order hydrocarbons and CO) in the reformate gas (SSCR, steam to specific carbon ratio). To inhibit carbon deposition on SOFC anode, the SSCR must be sufficiently high. However, the reformer operates near its maximum efficiency at low SSCR value and the higher the SSCR value, the lower the open circuit voltage and operating power density due to Nernst potential. In this study, a metal-foam supported SOFC single cell (Ni-YSZ/YSZ/Gd-doped ceria (CGO) buffer layer/lanthanum strontium cobalt ferrite-samarium doped ceria (LSCF-SDC)), impregnated with catalyst was designed; this novel SOFC was then examined for operation at a low SSCR value of the autothermal reformer conditions (near maximum efficiency of n-C₄H₁₀ reformer and thermal neutral point, SSCR = 0.5, OCR = 0.5 and SCR = 1.3). The voltage for the metal-foam supported SOFC impregnated with 0.5 wt% Rh/CGO remained at a nearly constant value, around 0.8 V, for 200 h under a constant temperature of 750 °C and current load of 250 mA cm⁻².     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.