Carbon-resistant Ni-YSZ/Cu–CeO2-YSZ dual-layer hollow fiber anode for micro tubular solid oxide fuel cell

A NiO-YSZ/porous YSZ dual-layer hollow fiber with an asymmetric structure was fabricated by a co-spinning-sintering method. A dense YSZ electrolyte film was prepared on NiO-YSZ layer by dip-coating method and calcined at 1450 °C; subsequently a porous cathode was dip-coated on the dense YSZ electrolyte film using LSM-YSZ (in the weight ratio 4:1) ink to fabricate a micro tubular solid oxide fuel cell (MT-SOFC). Cu–CeO₂ catalyst was impregnated into the porous YSZ layer to form the second anode composition. The power output of the MT-SOFC with Ni-YSZ/Cu–CeO₂-YSZ graded anode was up to 242 mW cm⁻² operated at 850 °C using CH₄ as fuel and air as oxidant. Little carbon deposition was observed on the double anode using methane as the fuel after 60 h' stable operation.     

Performance analysis of hollow fibre-based micro-tubular solid oxide fuel cell utilising methane fuel

Solid oxide fuel cell (SOFC) has been studied as one of the most amazing development in energy production that could work directly with hydrocarbon fuel without reforming procedure. This study was conducted to analyse the micro-tubular solid oxide fuel cell (MT-SOFC) in terms of its performance by utilising methane as the fuel, subsequently compared with hydrogen. MT-SOFC that was investigated in this work consisted of thin cathode layer, coated onto co-extruded anode/electrolyte dual-layer hollow fibre (HF); in which its anode was made of nickel (Ni), coupled with cerium-gadolinium oxide (CGO) as an electrolyte, whereas the cathode was lanthanum strontium cobalt ferrite (LSCF) and CGO. The physical analyses carried out were three-point bending test and scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis was further conducted to examine the carbon deposition in HFs. In evaluating the performance of HFs, current-voltage (IV) measurement, as well as impedance analysis of various temperatures range from 500 °C to 700 °C were performed. Based on the results, the OCV, maximum power density and ohmic ASR of MT-SOFC exposed to methane fuel, were at 0.79 V, 0.22 W cm⁻² and 0.31 Ω cm²; compared to the other that was exposed to hydrogen fuel, recorded at 0.89 V, 0.67 W cm⁻² and 0.19 Ω cm² respectively. This indicates that there was a significant reduction in cell performance when methane was used as the fuel, due to the carbon deposition as proven by SEM, three-point bending and XRD.     

Anode supported micro-tubular SOFC fabricated with mixed particle size electrolyte via phase-inversion technique

Cerium-gadolinium oxide is a promising material for electrolytes of intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its high electrical conductivity at relatively lower temperatures of 400–700 °C. However, a high sintering temperature of up to 1550 °C is typically required to produce dense CGO electrolyte, eventually leading to an interfacial interdiffusion between the electrolyte and electrode components as well as generate a highly resistive interface which reduces ionic conductivity. Lowering the sintering temperature of the electrolyte will greatly benefit the fabrication of SOFCs. This study examines the effectiveness of introducing nano size CGO particles as an approach to get dense CGO electrolyte at lower sintering temperature. A series of dope suspensions with 0–50% nano size loading were prepared to observe rheology and measure viscosity. Then, 30% loading was selected and casting into flat sheet via phase-inversion technique. The flat sheet was characterized by morphology, surface roughness and mechanical strength tests. The suspension was extruded into dual-layer hollow fiber (DLHF) as well. The electrolyte/anode dual-layer hollow fibers (DLHFs) half-cell of micro-tubular solid oxide fuel cells (MT-SOFCs) were prepared via phase inversion based co-extrusion/co-sintering technique. The developed half-cell was characterized by morphological and gas tightness tests which further compared them with fully micron ones. The results show that the incorporation of 30% nanoparticle yielded to dense and tight CGO layers sintered at temperature 1450 °C, which about 50 °C lower than those reported previously for 100% micron particles. The I–V measurements demonstrated the maximum power density of 0.66 Wcm^?2 at temperatures 500 °C using 100% H₂ as fuel. Therefore, this approach is able to reduce the energy cost for the microstructural control of the prepared fiber and thus is recommended for the fabrication of low-cost dual-layer hollow fiber micro tubular SOFCs.     

Microstructure tailoring of YSZ/Ni-YSZ dual-layer hollow fibers for micro-tubular solid oxide fuel cell application

YSZ/NiO-YSZ dual-layer hollow fibers with a thin YSZ top layer integrated on a porous NiO-YSZ (60:40 in weight) support, have been developed by one step method via a co-spinning-sintering process. Hydrogen reduction was performed to form YSZ/Ni-YSZ micro tube as the half solid oxide fuel cells (SOFCs). The microstructure of the dual-layer hollow fibers was tailored by adding ethanol as non-solvent in the initial mixture dopes for NiO-YSZ anode spinning. LSM cathode containing 20 wt%-YSZ was deposited on the electrolyte surface by dip-coating method to fabricate micro-tubular SOFCs. Experimental results indicate that the dual-layer hollow fibers from the anode dopes containing 15–20 wt% of ethanol possess the desired microstructure with optimized properties, such as the bending strength of 180 MPa, the porosity of 38–35% and the conductivity of 3000 S cm⁻¹ at room temperature. The micro-tubular SOFCs fabricated from such hollow fibers show a maximum power density up to 485 mW cm⁻² at 850 °C with 20 mL min⁻¹ of H₂ as fuel and 30 mL min⁻¹ air as oxidant, respectively.     

Anode-supported solid oxide fuel cell based on dense electrolyte membrane fabricated by filter-coating

A simple and cost-effective technique, filter-coating, has been developed to fabricate dense electrolyte membranes. Eight mole percent yttria-stabilized zirconia (YSZ) electrolyte membrane as thin as 7 μm was prepared by filter-coating on a porous substrate. The thickness of the YSZ film was uniform, and could be readily controlled by the concentration of the YSZ suspension and the rate of the suspension deposition. The YSZ electrolyte film was dense and was well bonded to the Ni-YSZ anode substrate. An anode-supported solid oxide fuel cell (SOFC) with a YSZ electrolyte film and a La_0.85Sr_0.15MnO₃ (LSM) + YSZ cathode was fabricated and its performance was evaluated between 700 and 850 °C with humidified hydrogen as the fuel and ambient air as the oxidant. An open circuit voltage (OCV) of 1.09 V was observed at 800 °C, which was close to the theoretical value, and the maximum power density measured was 1050 mW cm⁻². The results demonstrate that the dense YSZ film fabricated by filter-coating is suitable for application to SOFCs.     

Study of slurry spin coating technique parameters for the fabrication of anode-supported YSZ Films for SOFCs

A slurry spin coating method was developed to fabricate gas-tight anode-supported YSZ films for solid oxide fuel cells (SOFCs). Several technique parameters for slurry spin coating, such as the slurry viscosity, spinning speed, number of coating cycles, film thickness and their effects on YSZ electrolyte film were investigated. SEM results, open-circuit voltage (OCV) values and cell performance indicated that these parameters had crucial and obvious influences on YSZ film quality and fuel cell performance. Based on the optimized parameters, anode-supported YSZ films and several single fuel cells were successfully fabricated and tested. An OCV as high as 1.06 V was obtained at 800 °C and maximum power densities of 900, 1567, 2005 mW cm⁻² were achieved at 700, 750, 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

Novel fabrication technique of hollow fibre support for micro-tubular solid oxide fuel cells

In this work, a cerium-gadolinium oxide (CGO)/nickel (Ni)-CGO hollow fibre (HF) for micro-tubular solid oxide fuel cells (SOFCs), which consists of a fully gas-tight outer electrolyte layer supported on a porous inner composite anode layer, has been developed via a novel single-step co-extrusion/co-sintering technique, followed by an easy reduction process. After depositing a multi-layers cathode layer and applying current collectors on both anode and cathode, a micro-tubular SOFC is developed with the maximum power densities of 440-1000 W m⁻² at 450-580 °C. Efforts have been made in enhancing the performance of the cell by reducing the co-sintering temperature and improving the cathode layer and current collection from inner (anode) wall. The improved cell produces maximum power densities of 3400-6800 W m⁻² at 550-600 °C, almost fivefold higher than the previous cell. Further improvement has been carried out by reducing thickness of the electrolyte layer. Uniform and defect-free outer electrolyte layer as thin as 10 μm can be achieved when the extrusion rate of the outer layer is controlled. The highest power output of 11,100 W m⁻² is obtained for the cell of 10 μm electrolyte layer at 600 °C. This result further highlights the potential of co-extrusion technique in producing high quality dual-layer HF support for micro-tubular SOFC.     

A study of the process parameters for yttria-stabilized zirconia electrolyte films prepared by screen-printing

Screen-printing technology was developed to fabricate gas-tight yttria-stabilized zirconia (YSZ) electrolyte films on porous NiO–YSZ anode substrates for use in solid oxide fuel cells (SOFCs). Several key process parameters such as the starting YSZ powder, printing ink composition, printing time and sintering temperature were studied and reported in detail. SEM results revealed that the selected process parameters exerted obvious influences on the microstructure of the screen-printed YSZ films. Open-circuit voltages (OCVs) were used to evaluate the usage feasibility of screen-printed YSZ films in SOFCs. Cell performance test results showed that the above-mentioned parameters had crucial effects on the OCVs and power density of the prepared cells. Based on appropriate parameters, an OCV value of 1.081 V and a power density of 0.96 Wcm⁻² were obtained at 800 °C using hydrogen as fuel and ambient air as oxidant.     

Fabrication of integrated BZY electrolyte matrices for protonic ceramic membrane fuel cells by tape-casting and solid-state reactive sintering

Integrated porous/dense/porous tri-layer BaZr_0.8Y_0.2O_3-δ (BZY) electrolyte asymmetrical matrices were designed for protonic ceramic membrane fuel cells (PCMFCs) and fabricated by multilayer tape-casting and solid-state reactive sintering. The effects of pore-former, sintering aid and sintering program on the microstructure of integrated electrolyte matrices (IEMs) were studied. Graphite and NiO were appropriate pore-former and sintering aid, respectively, and an accelerated heating program was more desirable. The conductivities of the IEM with designed microstructure in different atmospheres were measured by AC impedance spectroscopy at 400–600 °C. The highest conductivity of 6.9 × 10^?3 S cm^?1 at 600 °C was obtained in wet air atmosphere, and the corresponding activation energy was 0.602 eV. Gas-tightness of the IEM was confirmed by a stable open circuit voltage (OCV) of 0.97 V at 600 °C from a button fuel cell with impregnated NiO anode and BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) cathode. These indicate that the fabricated BZY-based IEM has great potential for PCMFC application.     

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La_0.8Sr_0.2Co_0.2Fe_0.8O₃–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm⁻² at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.     

Influence of deposition temperature on the microstructure of thin-film electrolyte for SOFCs with a nanoporous AAO support structure

In this paper we investigate the influence of deposition temperature on the microstructure of yttria stabilized zirconia thin-films based on an anodized aluminum oxide (AAO) support structure. The yttria-stabilized zirconia (YSZ) electrolytes were deposited on a Pt anode/AAO support using DC magnetron reactive sputtering under deposition temperatures of RT and 500 °C. Elevating the deposition temperature led to enhanced surface mobility in the sputtered adatoms, which helped prevent pinhole generation and minimized the thickness of the electrolyte. A thin-film fuel cell with a YSZ electrolyte only 300 nm thick was successfully fabricated by elevating the deposition temperature. This cell exhibited an open circuit voltage (OCV) of 0.97 V, which is significantly higher than the OCV values of 0.3 V for a cell deposited at RT. However, in spite of the thin electrolyte, the performance of the cell deposited at the higher temperature showed limited value due to its relatively high polarization resistance. Through further investigation into the grain morphology, we verify that the increasing deposition temperature can affect not only the film density but also increase the grain size of the electrolyte, which is related to oxygen incorporation for ORR kinetics. Electrochemical impedance spectroscopy (EIS) results indicate that the grain size change caused by the elevated deposition temperature adversely affected the polarization resistance and the cell performance. These results indicate that careful adoption of elevated electrolyte deposition temperatures are required to optimize fuel cell performance.     

Evaluation of hybrid and enzymatic nanofluidic fuel cells using 3D carbon structures

Membraneless nanofluidic fuel cells are devices that utilize fluid flow through nanoporous media which serve as three-dimensional electrodes. In the case of hybrid fuel cells (HFC) an enzymatic and an abiotic catalyst are incorporated on the electrodes. Here we compared two different HFC. In the first one (HFC-1), glucose oxidase- and Pt-based electrodes were used as bioanode and cathode respectively. This cell reached an open circuit voltage (OCV) of 0.55 V and a maximum power density of 5.7 mWcm^?2. In the second one (HFC-2), AuAg- and laccase-based electrodes were used as anode and biocathode respectively. This cell exhibited an OCV of 0.91 V and a maximum power density of 17 mWcm^?2. Finally, enzymatic electrodes were used to develop a high performance biofuel cell (3.2 mWcm^?2) that exhibited high stability over 4 days. These preliminary results indicate that the incorporation of enzymes into the 3D carbon structures is an efficient alternative for miniaturized nanofluidic power sources.     

Comparative study on the performance of tubular and button cells with YSZ membrane fabricated by a refined particle suspension coating technique

Both tubular and button solid oxide fuel cells (SOFCs) with configuration NiO–YSZ/YSZ/PNSM–YSZ were assembled and compared in their performance. A refined particle suspension coating technique was used for preparing thin dense YSZ electrolyte layer on the two types of anode supports, and the thickness of YSZ membrane was controlled by the time of tubular anode dipped into YSZ suspension and the suspension volume dropped onto the button anode, respectively. Current–voltage tests and AC impedance measurements were carried out to characterize the performance and ohmic resistances in the two cells. Compared with tubular cell, higher peak power density values of 933 mW cm⁻² at 850 °C was achieved, which is 2.2 times higher than the value of tubular cell. AC impedance indicated that lower performance of tubular cell was restricted by the ohmic loss at the operating temperatures.     

A novel fabrication of yttria-stabilized-zirconia dense electrolyte for solid oxide fuel cells by 3D printing technique

Three-dimensional (3D) printing technique represents a revolutionary advancement in the manufacturing sector due to its unique capabilities to process the shape complexity. This work is focusing on dense 8 mol.% yttria-stabilized-zirconia (8YSZ) electrolyte fabrication via digital light processing (DLP)-stereolithography-based 3D printing technique. Multiple 8YSZ electrolyte green bodies are printed simultaneously in a batch using ceramic-resin suspension made of 30 vol% 8YSZ powder loading in a photo-curable resin. Together with an optimized debinding and sintering procedure, the 8YSZ green body changes into a dense electrolyte, and the density of the sintered electrolyte was measured as 99.96% by Archimedes' water displacement method. The symmetric cell fabricated of silver-Ce_0.8Gd_0.2O_1.9 (Ag-GDC) as cathode/anode and dense 8YSZ electrolyte printed by DLP-stereolithography delivers a high open circuit voltage of approximately 1.04 V and a peak power density up to 176 mW·cm⁻² at 850 °C by using hydrogen as the fuel and air as the oxidant. The electrochemical performance of the symmetric cell Ag-GDC|YSZ|Ag-GDC with 8YSZ electrolyte fabricated via DLP-stereolithography is comparable to that of the same cell with 8YSZ electrolyte fabricated by conventional dry pressing method. This 3D printing technique provides a novel method to prepare dense electrolytes for solid oxide fuel cell (SOFC) with good performance, suggesting a potential application for one-step fabrication of complex structure SOFC stack.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

A solid oxide carbon fuel cell operating on pomelo peel char with high power output

The pomelo peel char (PC) was prepared and used as fuel for solid oxide electrolyte direct carbon fuel cells with nickel‐yttrium stabilized zirconia anode, thin‐film YSZ electrolyte, and La_0.8Sr_0.2MnO₃ cathode. The power densities of fuel cells operating on PC and catalyst‐loaded PC (PCC) fuels achieved 309 and 518 mW cm^?2 at 850°C, respectively, which are among the highest power densities reported in the literature on DCFCs. The PC exhibited superior gasification reactivity than coal char due to its unique reticulated foam carbon structure with a homogeneously distributed inherent catalyst. The stability tests at a current density of 50 mA cm^?2 and 825°C indicate that the cell using PC fuel operated in a more stable manner than that using PCC, and the fuel availabilities for PC and PCC were 47.25% and 34.71%, respectively. The results suggest that PC is a promising solid carbonaceous fuel for solid oxide electrolyte direct carbon fuel cells based on its adequate gasification reactivity and high compatibility with the fuel cells.     

Screen-printed thin YSZ films used as electrolytes for solid oxide fuel cells

A thin yttria-stabilized zirconia (8 mol% YSZ) film was successfully fabricated on a NiO-YSZ anode substrate by a screen-printing technique. The scanning electron microscope (SEM) results suggested that the YSZ film thickness was about 31 μm after sintering at 1400 °C for 4 h in air. A 60 wt% La_0.7Sr_0.3MnO₃ + 40 wt% YSZ was screen-printed onto the YSZ film surface as cathode. A single cell was tested from 650 to 850 °C using hydrogen as fuel and ambient air as oxidant, which showed an open circuit voltage (OCV) of 1.02 V and a maximum power density of 1.30 W cm⁻² at 850 °C. The OCV was higher than 1.0 V, which suggested that the YSZ film was quite dense and that the fuel gas leakage through the YSZ film was negligible. Screen-printing can be a promising method for manufacturing YSZ films for solid oxide fuel cells (SOFCs).     

Semiconductor electrolyte for low-operating-temperature solid oxide fuel cell: Li-doped ZnO

Semiconductors have been successfully demonstrated as the electrolytes for solid oxide fuel cells (SOFCs) in recent years. Many such semiconductors have shown their potentials as a competent ionic conductor for fuel cell operation, indicated by the appreciable ionic conduction and electrochemical performance. In the present study, we depart from traditional electrolyte concept to introduce a new semiconductor electrolyte, Li-doped ZnO to low-operating-temperature SOFCs. The used material was synthesized via a co-precipitation method and investigated from phase structure, morphology and UV–vis absorption perspectives. Utilizing Li-doped ZnO as electrolyte layer, we found the corresponding fuel cell exhibited a remarkable maximum power density of 443 mW cm^?2 along with open circuit voltage (OCV) of 1.07 V at 550 °C, and represented a lower-temperature operation feasibility with power outputs of 138–165 mW cm^?2 at 425–450 °C. Besides, high ionic conductivities of 0.028–0.087 S cm^?1 and low activation energy of 0.5 eV were also found in the synthesized Li-doped ZnO at 425–550 °C. Our investigation in terms of electrochemical impedance spectra (EIS) analysis manifested that Li-doped ZnO as the electrolyte layer boosted the electrode reactions of the device, which resulted in rather small polarization resistances and eventually realized good low-temperature performances. Further study based on the rectification characteristic of Ni/Li-doped ZnO contact verified the Schottky junction formation of Li-doped ZnO with anodic Ni, which can avoid the underlying electronic short-circuiting problem. These findings show a profound significance of using doped semiconductor for the future exploitation of SOFC electrolytes.     

A study of Ni + 8YSZ/8YSZ/La0.6Sr0.4CoO3−δ ITSOFC fabricated by atmospheric plasma spraying

An intermediate temperature solid oxide fuel cell (ITSOFC) based on 8YSZ electrolyte, La_0.6Sr_0.4CoO_3−δ (LSCo) cathode, and Ni − 8YSZ anode coatings were consecutively deposited onto a porous Ni-plate substrate by atmospheric plasma spraying (APS). The spray parameters including current, argon and hydrogen flow rate, and powder feed rate were investigated by an orthogonal experiment to fabricate a thin gas-tight 8YSZ electrolyte coating (80 μm). By proper selection of the spray parameters to decrease the particles velocity and temperature, the sprayed NiO + 8YSZ coating after reducing with hydrogen shows a good electrocatalytic activity for H₂ oxidation. With the same treatment, 100–170 μm dimensions LSCo particle could keep phase structure after spraying. And the deposited LSCo cathode shows a good cathode performance and chemical compatibility with 8YSZ electrolyte after operating at 800 °C for 50 h. Output power density of the sprayed cell achieved 410 mW cm⁻² at 850 °C and 260 mW cm⁻² at 800 °C. Electrochemical characterization indicated that IR drop of 8YSZ electrolyte, cathodic polarization, and the contact resistance at LSCo/8YSZ interface were the main factors restricting the cell performance. The results suggested that the use of APS cell allowed the reduction of the operating temperature of the SOFC to below 850 °C with lower production costs.     

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

The performance of solid oxide fuel cells (SOFCs) is affected by various polarization losses, namely, ohmic polarization, activation polarization and concentration polarization. Under given operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometric parameters. Solid oxide fuel cells (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte, Ni–YSZ anode support, Ni–YSZ anode interlayer, strontium doped lanthanum manganate (LSM)–YSZ cathode interlayer, and LSM current collector, were fabricated. The effect of various parameters on cell performance was evaluated. The parameters investigated were: (1) YSZ electrolyte thickness, (2) cathode interlayer thickness, (3) anode support thickness, and (4) anode support porosity. Cells were tested over a range of temperatures between 600 and 800 °C with hydrogen as fuel, and air as oxidant. Ohmic contribution was determined using the current interruption technique. The effect of these cell parameters on ohmic polarization and on cell performance was experimentally measured. Dependence of cell performance on various parameters was rationalized on the basis of a simple analytical model. Based on the results of the cell parameter study, a cell with optimized parameters was fabricated and tested. The corresponding maximum power density at 800 °C was ∼1.8 W cm⁻².