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.     

NiO/YSZ nanocomposite particles synthesized via co-precipitation method for electrochemically active Ni/YSZ anode

NiO/YSZ composite particles were synthesized via a co-precipitation of hydroxides. We investigated the effect of pH on the morphology of the composite particles, as well as on the microstructure and the electrochemical property of the Ni/YSZ anode. The particles synthesized at pH 10 involved aggregated composites and large NiO. The particles resulted in coarse and inhomogeneous anode microstructure and moderate area specific resistance (ASR) as 0.57 Ω cm² at 800 °C under open circuit voltage (OCV). Contrarily, nano-sized composite particles were successfully synthesized at pH 13. The particles provided fine as well as homogeneous porous structure with the grain size in the range 200-400 nm and low ASR as 0.36 Ω cm² at 800 °C under OCV.     

Stability and robustness of metal-supported SOFCs

Tubular metal-supported SOFCs with YSZ electrolyte and electrodes comprising porous YSZ backbone and infiltrated Ni and LSM catalysts are operated at 700 °C. Tolerance to five complete anode redox cycles and five very rapid thermal cycles is demonstrated. The power output of a cell with as-infiltrated Ni anode degrades rapidly over 15 h operation. This degradation can be attributed primarily to coarsening of the fine infiltrated Ni particles. A cell in which the infiltrated Ni anode is precoarsened at 800 °C before operation at 700 °C shows dramatically improved stability. Stable operation over 350 h is demonstrated.     

Preparation of Pt/NiO-C electrocatalyst and heat-treatment effect on its electrocatalytic performance for methanol oxidation

Pt catalyst supported on Vulcan XC-72R containing 5 wt% NiO (Pt/NiO–C) showed larger electrochemical active surface area and higher electrochemical activity for methanol oxidation than Pt catalyst supported on Vulcan XC-72R using polyol method without NiO addition. Prepared Pt/NiO–C electrocatalyst was heat-treated at four temperatures (200, 400, 600, and 800 °C) in flowing N₂. X-ray diffraction and temperature-programmed desorption results indicated that NiO was reduced to Ni in inert N₂ during heat-treatments at temperatures above or equal to 400 °C, while oxygen from NiO reacted with carbon support due to the catalytic effect of Pt. The reduced Ni formed an alloy with Pt, which, according to the X-ray photoelectron spectroscopy data, resulted in a shift to a lower binding energy of Pt 4f electrons. The Pt/NiO–C electrocatalyst heat-treated at 400 °C showed the best activity in methanol oxidation due to the change in Pt electronic structure by Ni and the minimal aggregation of Pt particles.     

Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning

Tars in biomass gasification systems need to be removed to avoid damaging and clogging downstream pipes or equipment. In this study, Ni-based catalysts were made by mechanically mixing NiO and char particles at various ratios. Catalytic performance of the Ni/char catalysts was studied and compared with performance of wood char and coal char without Ni for syngas cleanup in a laboratory-scale updraft biomass gasifier. Reforming parameters investigated were reaction temperature (650–850 °C), NiO loading (5–20% of the weight of char support), and gas residence time (0.1–1.2 s). The Ni/coalchar and Ni/woodchar catalysts removed more than 97% of tars in syngas at 800 °C reforming temperature, 15% NiO loading, and 0.3 s gas residence time. Analysis of syngas composition indicated that concentrations of H₂ and CO in syngas significantly. Furthermore, performance of the Ni/coalchar catalyst was continuously tested for 8 h. There was slight deactivation of the catalyst in the early stage of tar/syngas reforming; however, the catalyst was able to stabilize soon after. It was concluded that chars especially coal char can be an effective and inexpensive support of NiO for biomass gasification tar removal and syngas conditioning.     

A possible solution to the mechanical degradation of Ni-yttria stabilized zirconia anode-supported solid oxide fuel cells due to redox cycling

The viability of cooling a solid oxide fuel cell (SOFC) during air exposure is investigated as a possible solution to the mechanical damage caused by inadvertent ‘redox cycling’ (cyclic exposure to air and then to H₂) of Ni-based anode-supported cells at high temperatures. In order to prevent electrolyte (and cell) cracking, it is shown that cooling the anode-supported Ni-YSZ samples during air exposure from 800 °C to <600 °C at rates >3 °C min⁻¹ significantly slows down the oxidation of Ni. This, in turn, minimizes the volume expansion due to NiO formation. Cell cooling rates of <3 °C min⁻¹ result in the cracking of the thin electrolyte layer, as sufficient time is then available for substantial NiO formation. It is also shown that partial oxidation during cool-down results in more extensive Ni oxidation in the outer regions of the anode layer compared to regions closer to the electrolyte. For the anode-supported cells investigated here, the electrolyte resists cracking when the nearby Ni particles (within 10-20 μm of the electrolyte) are prevented from oxidizing to an extent of more than 65%.     

A phase inversion/sintering process to fabricate nickel/yttria-stabilized zirconia hollow fibers as the anode support for micro-tubular solid oxide fuel cells

NiO/YSZ hollow fibers were fabricated via a combined phase inversion and sintering technique, where polyethersulfone (PESf) was employed as the polymeric binder, N-methyl-2-pyrrolidone (NMP) as the solvent and polyvinylpyrrolidone (PVP) as the additive, respectively. After reduction with hydrogen at 750 °C for 5 h, the porous Ni/YSZ hollow fibers with an asymmetric structure comprising of a microporous layer integrated with a finger-like porous layer were obtained, which can be served as the anode support of micro-tubular solid oxide fuel cells (SOFCs). As the sintering temperature was increased from 1200 to 1400 °C, the mechanical strength and the electrical conductivity of the Ni/YSZ hollow fibers increased from 35 to 178 MPa and from 30 to 772 S cm⁻¹, respectively but the porosity decreased from 64.2% to 37.0%. The optimum sintering temperature was found to be between 1350 and 1400 °C for Ni/YSZ hollow fibers applied as the anode support for micro-tubular SOFCs.     

Studies on the carbon reactions in the anode of deposited carbon fuel cells

The carbon reactions in the anode of deposited carbon fuel cells were studied experimentally and theoretically. Deposition experiments were conducted by decomposing methane in a thermogravimetric analyzer at 800 °C, with both NiO or YSZ powders and small chips of an unused anode-supported SOFC button cell used separately as bed materials. The carbon tended to deposit on the Ni surfaces with the NiO or YSZ powders, while with the anode chips, the deposited carbon formed particles comparable in size to the Ni or YSZ particles with little carbon deposited near the electrolyte where the electrochemical reactions occur. Thus, the results infer that the deposited carbon has little opportunity to participate in the electrochemical reactions. A two-dimensional isothermal model was then developed to examine the influence of the deposited carbon on the cell performance. The results show the diffusion coefficient of CO has the largest influence, followed by the gasifying reactivity and the electrochemical reactivity of the carbon. Finally, a short deposition time and a high methane concentration are favored to improve the performance of deposited carbon fuel cells.     

A cost-effective process for fabrication of metal-supported solid oxide fuel cells

Metal-supported SOFC cells with Y₂O₃ stabilized ZrO₂ as the electrolyte were prepared by a low cost and simple process involving tape casting, screen printing and co-firing. The interfaces were well bonded after the reduction of NiO to Ni in the support and the anode. AC impedance was employed to estimate the cell polarizations under open circuit conditions. It was found that the electrode polarization resistance was high at low temperatures and became equivalent to the ohmic resistance at higher temperatures near 800°°C. The cell performance was evaluated with H₂ as the fuel and air as the oxidant, and maximum power density between 0.23 and 0.80  W/cm² was achieved in the temperature range of 650–800°C, which confirms the applicability of the cost-effective process in fabrication of metal-supported SOFC cells.     

Plasma sprayed metal supported YSZ/Ni–LSGM–LSCF ITSOFC with nanostructured anode

Intermediate temperature solid oxide fuel cells (ITSOFCs) supported by a porous Ni-substrate and based on Sr and Mg doped lanthanum gallate (LSGM) electrolyte, lanthanum strontium cobalt ferrite (LSCF) cathode and nanostructured yttria stabilized zirconia–nickel (YSZ/Ni) cermet anode have been fabricated successfully by atmospheric plasma spraying (APS). From ac impedance analysis, the sprayed YSZ/Ni cermet anode with a novel nanostructure and advantageous triple phase boundaries after hydrogen reduction has a low resistance. It shows a good electrocatalytic activity for hydrogen oxidation reactions. The sprayed LSGM electrolyte with ∼60 μm in thickness and ∼0.054 S cm⁻¹ conductivity at 800 °C shows a good gas tightness and gives an open circuit voltage (OCV) larger than 1 V. The sprayed LSCF cathode with ∼30 μm in thickness and ∼30% porosity has a minimum resistance after being heated at 1000 °C for 2 h. This cathode keeps right phase structure and good porous network microstructure for conducting electrons and negative oxygen ions. The APS sprayed cell after being heated at 1000 °C for 2 h has a minimum inherent resistance and achieves output power densities of ∼440 mW cm⁻² at 800 °C, ∼275 mW cm⁻² at 750 °C and ∼170 mW cm⁻² at 700 °C. Results from SEM, XRD, ac impedance analysis and I–V–P measurements are presented here.     

NiO/YSZ,anode-supported,thin-electrolyte,solid oxide fuel cells fabricated by gel casting

A simple and cost-effective gel-casting technique is developed and optimized to fabricate NiO/stabilized yttria–zirconia (YSZ) anode-supported solid oxide fuel cells (SOFCs). The effect of ammonium poly-(methacrylate) (PMAA) dispersant and pH on the zeta potential of YSZ and NiO particles and the viscosity of the NiO/YSZ slurries is studied in detail. The results show that the absolute zeta potential of YSZ and NiO particles reaches a maximum value at pH value ∼10 and the viscosity of the NiO/YSZ slurry is lowest when the PMAA dispersant content is 1.5 wt.%. A gel-cast NiO/YSZ anode-supported button cell with a spin-coated, thin, YSZ electrolyte film (∼9 μm) and a La_0.72Sr_0.18MnO_3−δ (LSM)/YSZ composite cathode gives a peak power output of 1.07 and 0.65 W cm⁻² at 900 and 800 °C under humidified hydrogen and air. The effect of a graphite pore-former in the gelation and microstructure of NiO/YSZ anode substrates is investigated.     

Effect of various coal contaminants on the performance of solid oxide fuel cells: Part I. Accelerated testing

The contaminants that are potentially present in the coal-derived gas stream and their thermochemical nature are discussed. Accelerated testing was carried out on Ni-YSZ/YSZ/LSM solid oxide fuel cells (YSZ: yttria stabilized zirconia and LSM: lanthanum strontium manganese oxide) for eight main kind of contaminants: CH₃Cl, HCl, As, P, Zn, Hg, Cd and Sb at the temperature range of 750-850 °C. The As and P species, at 10 and 35 ppm, respectively, resulted in severe power density degradation at temperatures 800 °C and below. SEM and EDX analysis indicated that As attacked the Ni region of the anode surface and the Ni current collector, caused the break of the current collector and the eventual cell failure at 800 °C. The phosphorous containing species were found in the bulk of the anode, they were segregated and formed “grain boundary” like phases separating large Ni patches. These species are presumably nickel phosphide/phosphate and zirconia phosphate, which could break the Ni network for electron transport and inhibit the YSZ network for oxygen ion transport. The presence of 40 ppm CH₃Cl and 5 ppm Cd only affected the cell power density at above 800 °C and Cd caused significant performance loss. Whereas the presence of 9 ppm Zn, 7 ppm Hg and 8 ppm Sb only degraded the cell power density by less than 1% during the 100 h test in the temperature range of 750-850 °C.     

Improvement of the performances of tubular solid oxide fuel cells by optimizing co-sintering temperature of the NiO/YSZ anode-YSZ electrolyte double layers

The effects of co-sintering temperature on anode microstructure, electrolyte film microstructure, and final cell performance of tubular solid oxide fuel cells (SOFCs) were fully studied. The co-sintering of the NiO/YSZ anode-YSZ electrolyte double layers at temperature ranging from 1350 to 1400 °C for 5 h was carried out. Porosity and electrical conductivity were measured to examine the anodes microstructure, and the electrolyte films microstructure were characterized by scanning electronic microscope (SEM). A higher open current voltage (OCV) value of 0.99 V was achieved by co-sintering the cell at 1400 °C indicating denser electrolyte film, while the maximum power density of the cell co-sintered at 1380 °C was achieved with 322 mW cm⁻² at 800 °C, which was higher than that (241.3 mW cm⁻²) of the cell co-sintered at 1400 °C because of better anode microstructure.     

Enhanced sinterability of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ by addition of nickel oxide

The effect of nickel oxide addition on the sintering behavior and electrical properties of BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) as an electrolyte for solid oxide fuel cells was systematically studied. Results suggest that the addition of a small amount (∼1 wt%) of NiO to BZCYYb greatly promoted densification, achieving ∼96% of the theoretical density after sintering at 1350 °C in air for 3 h (reducing the sintering temperature by ∼200 °C). Further, a sample sintered at 1450 °C for 3 h showed high open circuit voltages (OCVs) when used as the electrolyte membrane to separate the two electrodes under typical SOFC operating conditions, indicating that the electrical conductivity of the electrical conductivity of the BZCYYb was not adversely affected by the addition of ∼1 wt% NiO.     

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.     

Preparation of supported Ni catalysts with a core/shell structure and their catalytic tests of partial oxidation of methane

Synthesis of supported Ni catalysts with a core/shell structure at the multibubble sonoluminescence (MBSL) condition and their catalytic tests for methane decomposition by partial oxidation were performed in this study. The catalysts prepared were analyzed by XRD, TEM and XPS. Without doping the third components, the supported catalyst of core/shell structure made with 10% Ni loading on Al₂O₃ yields 96% conversion efficiency of methane at reaction temperature of 800 °C and shows excellent thermal stability for the first 40 h. It turns out that coexistence of NiO and NiOx species on the surface of the catalysts play a very important role in the partial oxidation of methane. In addition, the uniform layer of Ni particles on the surface of support material hindered coke formation and sintering process, which enhances thermal stability for the catalysts.     

Bimetallic (Ni–Fe) anode-supported solid oxide fuel cells with gadolinia-doped ceria electrolyte

Anode-supported solid oxide fuel cells (SOFC) comprising nickel + iron anode support and gadolinia-doped ceria (GDC) of composition Gd_0.1Ce_0.9O_2−δ thin film electrolyte were fabricated, and their performance was evaluated. The ratio of Fe₂O₃ to NiO in the anode support was 3 to 7 on a molar basis. Fe₂O₃ and NiO powders were mixed in the desired proportions and discs were die-pressed. All other layers were sequentially applied on the anode support. The cell structure consisted of five distinct layers: anode support – Ni + Fe; anode functional layer – Ni + GDC; electrolyte – GDC; cathode functional layer – LSC (La_0.6Sr_0.4CoO_3−δ) + GDC; and cathode current collector – LSC. Cells with three different variations of the electrolyte were made: (1) thin GDC electrolyte (∼15 μm); (2) thick GDC electrolyte (∼25 μm); and (3) tri-layer GDC/thin yttria-stabilized zirconia (YSZ)/GDC electrolyte (∼25 μm). Cells were tested with hydrogen as fuel and air as oxidant up to 650 °C. The maximum open circuit voltage measured at 650 °C was ∼0.83 V and maximum power density measured was ∼0.68 W cm⁻². The present work shows that cells with Fe + Ni containing anode support can be successfully made.     

Anode-supported ScSZ-electrolyte SOFC with whole cell materials from combined EDTA–citrate complexing synthesis process

The potential application of combined EDTA–citrate complexing process (ECCP) in intermediate-temperature solid-oxide fuel cells (IT-SOFCs) processing was investigated. ECCP-derived scandia-stabilized-zirconia (ScSZ) powder displayed low packing density, high surface area and nano-crystalline, which was ideal material for thin-film electrolyte fabrication based on dual dry pressing. A co-synthesis of NiO + ScSZ anode based on ECCP was developed, which showed reduced NiO(Ni) and ScSZ grain sizes and improved homogeneity of the particle size distribution, as compared with the mechanically mixed NiO + ScSZ anode. Anode-supported ScSZ electrolyte fuel cell with the whole cell materials synthesized from ECCP was successfully prepared. The porous anode and cathode exhibited excellent adhesion to the electrolyte layer. Fuel cell with 30 μm thick ScSZ electrolyte and La_0.8Sr_0.2MnO₃ cathode showed a promising maximum peak power density of 350 mW cm⁻² at 800 °C.     

Preparation and characterization of Ni-based positive electrodes for use in aqueous electrochemical capacitors

We have prepared NiO particles on Ni sheet and Ni foam substrates by chemical bath deposition and the following heat-treatment, and assembled a hybrid capacitor (HC) cell with the NiO-loaded Ni sheet or Ni foam positive electrode and activated carbon negative electrode. The deposited NiO particles had flower-like porous morphology which was composed of aggregated nanosheets. The maximum operating voltage of both HC cells was 1.5 V, which was much higher than theoretical decomposition voltage of water (1.23 V). The HC cell with NiO/Ni foam (HC_foam) had higher discharge capacitance and high-rate dischargeability and lower IR drop than the HC cell with NiO/Ni sheet (HC_sheet) because of the increase in the utilization of NiO active material. Both energy and power densities per mass of active materials, were much higher than those for the HC_sheet. Both HC_foam and HC_sheet showed excellent cycle stability for 2000 cycles.     

Experimental study on effect of compaction pressure on performance of SOFC anodes

NiO/yttria-stabilized zirconia (YSZ) anode substrates were fabricated at two compaction pressures of 200 and 1000 MPa, the particle size distributions of NiO and YSZ were investigated with powders treated under different conditions using a laser scattering technique (Mastersizer 2000, Malvern Instruments) and the effect of compaction pressure on the performance of solid oxide fuel cell (SOFC) anodes was investigated by studying the effect of compaction pressure on compaction density, sintered density, sintering shrinkage behavior, electronic and ionic conductivities. The results of investigation indicated that the SOFC with the anode compacted at a higher pressure exhibited a superior output performance, for example, a single cell with hydrogen as fuel and oxygen as oxidant exhibited excellent maximum power densities of 2.77 and 0.90 W cm⁻² at 800 and 650 °C, respectively, which suggested the development of an intermediate temperature SOFC through optimization of anode fabrication parameters.