Review on microfabricated micro-solid oxide fuel cell membranes

Micro-solid oxide fuel cells (μ-SOFC) are promising power sources for portable electronic devices. This review presents the current status of development of microfabricated micro-solid oxide fuel cell membranes for power delivery. The μ-SOFC membranes are developed using micro-electro-mechanical system (MEMS) fabrication and machining techniques. The different designs of free-standing μ-SOFC membranes and μ-SOFCs deposited on porous substrates are presented. The materials used in the μ-SOFC anode, electrolyte and cathode are discussed and compared along with their microstructures. The electrical performance data of the different μ-SOFC designs are compared and discussed. High μ-SOFC performances of 677 mW cm⁻² were demonstrated at temperatures as low as 400 °C.     

The impact of etching during microfabrication on the microstructure and the electrical conductivity of gadolinia-doped ceria thin films

Gadolinia-doped ceria, Ce_0.8Gd_0.2O_1.9−x (CGO), thin films deposited by spray pyrolysis and annealed to different degree of crystallinity between 0% and 95% are exposed to different etchants and etching methods. The attack of the etchants on the CGO thin films is analyzed with respect to changes in microstructure and in-plane electrical conductivity. It is found that amorphous CGO films are dissolved in hydrochloric acid after elongated etching times. Hydrofluoric acid severely attacks CGO thin films after already short times of exposure (1 min), more intense the less crystalline the thin film is. Ar ion etching smoothens the surface of the CGO thin films without considerable removal of material. No microstructural attack of NaOH, CHF₃/O₂ and SF₆/Ar is found. The electrical conductivity is in general only affected when microstructural changes are severe. Therefore, it is concluded that CGO thin films can be well used as functional layers in micro-fabricated devices and that micro-fabrication is, with the exception of hydrofluoric, not harmful for the electrical properties of crystalline CGO thin films.     

Reactive Spray Deposition Technology - An one-step deposition technique for Solid Oxide Fuel Cell barrier layers

In order to reduce the cost of the manufacturing of Solid Oxide Fuel Cells (SOFC), and to enable metal supported cell fabrication, a new fabrication method called Reactive Spray Deposition Technology (RSDT) for direct deposition of the material onto ceramic or metal support for low temperature SOFC is currently being developed. The present work describes the effect on the performance of a SOFC when a Gd_0.2Ce_0.8O_1.9 (GDC) layer has been introduced as diffusion barrier layer between the yttria stabilized zirconia (YSZ) electrolyte and the La_0.6Sr_0.4CoO_3−δ (LSC) cathode. The dense, thin and fully crystalline GDC films were directly applied by RSDT, without any post-deposition heating or sintering step. The quality of the film and performance of the cell prepared by RSDT was compared to a GDC blocking layer deposited by screen printing (SP) and then sintered. The observed ohmic resistance of the ASC with a GDC layer deposited by RSDT is 0.24 Ω cm², which is close to the expected theoretical value of 0.17 Ω cm² for a 5-μm thick 8 mol% yttria YSZ (8YSZ) electrolyte at 873 K.     

Synthesis and sintering of Gd-doped CeO2 electrolytes with and without 1 at.% CuO dopping for solid oxide fuel cell applications

Nano-sized Ce_0.8Gd_0.2O_2−δ and Ce_0.79Gd_0.2Cu_0.01O_2−δ electrolyte powders were synthesized by the polyvinyl alcohol assisted combustion method, and then characterized by powder characteristics, sintering behaviors and electrical properties. The results demonstrate that the as-synthesized Ce_0.8Gd_0.2O_2−δ and Ce_0.79Gd_0.2Cu_0.01O_2−δ possessed similar powder characteristics, including cubic fluorite crystalline structure, porous foamy morphology and agglomerated secondary particles composed of gas cavities and primary nano crystals. Nevertheless, after ball-milling these two powders exhibited quite different sintering abilities. A significant reduction of about 400 °C in densification temperature of Ce_0.79Gd_0.2Cu_0.01O_2−δ was obtained when compared with Ce_0.8Gd_0.2O_2−δ. The Ce_0.79Gd_0.2Cu_0.01O_2−δ pellets sintered at 1000 °C and the Ce_0.8Gd_0.2O_2−δ sintered at 1400 °C exhibited relative densities of 96.33% and 95.7%, respectively. The sintering of Ce_0.79Gd_0.2Cu_0.01O_2−δ was dominated by the liquid phase process, followed by the evaporation-condensation process, Moreover, Ce_0.79Gd_0.2Cu_0.01O_2−δ shows much higher conductivity of 0.026 S cm⁻¹ than Ce_0.8Gd_0.2O_2−δ (0.0065 S cm⁻¹) at a testing temperature of 600 °C.     

Effect of Gd (Sm) doping on properties of ceria electrolyte for solid oxide fuel cells

Two systematic electrolytes of Ce_1−xGd_xO_2−x (GDC) and Ce_1−xSm_xO_2−x (SDC) (x=0–0.25) were synthesized using an oxalate coprecipitation process. Dependence of a, unit cell parameter versus dopant concentration, x, of Gd³⁺ and Sm³⁺ ions show that these solid solutions obey Vegard’s rule as a=5.4121+0.0525x for GDC and a=5.4117+0.1237x for SDC, respectively. Electrical conductivity reached maximum at x=0.15 in the temperature range of 400–850 °C for both kinds of doped-ceria electrolyte membranes. A single cell was made for the measurement of open circuit voltage. The results show that the open circuit voltages are greatly influenced by ionic transference number of the electrolyte, gaseous fuel composition and cathode membrane material.     

Production strategies of asymmetric BaCe0.65Zr0.20Y0.15O3-δ – Ce0.8Gd0.2O2-δ membrane for hydrogen separation

Mixed proton and electron conductor ceramic composites are among the most promising materials for hydrogen separation membrane technology especially if designed in an asymmetrical configuration (thin membrane supported onto a thicker porous substrate). However a precise processing optimization is needed to effectively obtain planar and crack free asymmetrical membranes with suitable microstructure and composition without affecting their hydrogen separation efficiency. This work highlights for the first time the most critical issues linked to the tape casting process used to obtain BaCe_0.65Zr_0.20Y_0.15O_3-δ – Ce_0.8Gd_0.2O_2-δ (BCZY-GDC) asymmetrical membranes for H₂ separation. The critical role of the co-firing process, sintering aid and atmosphere was critically investigated. The optimization of the production strategy allowed to obtain asymmetric membranes constituted by a dense 20 μm-thick ceramic-ceramic composite layer supported by a porous (36%) 750 μm-thick BCZY-GDC substrate. The asymmetric membranes here reported showed H₂ fluxes (0.47 mL min⁻¹ cm⁻² at 750 °C) among the highest obtained for an all-ceramic membrane.     

Ba1−xSrxCe0.8−yZryY0.2O3−δ protonic electrolytes synthesized by hetero-composition-exchange method for solid oxide fuel cells

This study reports the synthesis of proton-conducting Ba_0.8Sr_0.2Ce_0.6Zr_0.2Y_0.2O_3?δ oxides by using a combination of the sol–gel process and hetero-composition-exchange technique. The experimental results show that the sintered Ba_0.8Sr_0.2Ce_0.6Zr_0.2Y_0.2O_3?δ pellet synthesized by the hetero-composition-exchange method exhibits excellent sinterability, good relatively density, and high protonic conduction. Furthermore, the Pt/electrolyte/Pt single cell with such an electrolyte shows a significantly higher maximum power density as compared to those oxides prepared from conventional sol–gel powders. Based on the experimental results, we attempt to explain the improvement mechanism in terms of as-calcined particle characteristics and proton hopping distance. This work shows that the Ba_1?xSr_xCe_0.8?yZr_yY_0.2O_3?δ oxides synthesized by the sol–gel combined with hetero-composition-exchange method would be a promising electrolyte for H⁺-SOFC applications. More importantly, this new fabrication approach could be applied to other similar perovskite-type electrolyte systems.     

Study on Sm1.8Ce0.2CuO4-Ce0.9Gd0.1O1.95 composite cathode materials for intermediate temperature solid oxide fuel cell

Sm_1.8Ce_0.2CuO₄-xCe_0.9Gd_0.1O_1.95 (SCC-xCGO, x = 0-12 vol.%) composite cathodes supported on Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte are studied for applications in IT-SOFCs. Results show that Sm_1.8Ce_0.2CuO₄ material is chemically compatible with Ce_0.9Gd_0.1O_1.95 at 1000 °C. The composite electrode exhibits optimum microstructure and forms good contact with the electrolyte after sintering at 1000 °C for 4 h. The polarization resistance (R_p) reduces to the minimum value of 0.17 Ω cm² at 750 °C in air for SCC-CGO06 composite cathode. The relationship between R_p and oxygen partial pressure indicates that the reaction rate-limiting step is the surface diffusion of the dissociative adsorbed oxygen on the composite cathode.     

All oxide solid-state lithium-ion cells

Solid-state lithium-ion cells have been prepared using thin film Li₄Ti₅O₁₂ as the anode, thin film LiCoO₂ as the cathode and Li_0.33La_0.56TiO₃ as the electrolyte. The electrolyte was prepared as a relatively thick ceramic with a thickness close to 1 mm. This type of cell develops a voltage of slightly greater than 2 V and is stable to cycling. Perhaps the most interesting aspect of this cell, is that even with a relatively thick, poor quality ceramic electrolyte, this cell has been able to develop current densities as great as 40 μA/cm².     

Cobalt-free composite cathode for SOFCs: Brownmillerite-type calcium ferrite and gadolinium-doped ceria

A cobalt-free composite Ca₂Fe₂O₅ (CFO) – Ce_0.9Gd_0.1O_1.95 (GDC) is investigated as a new cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on a Gd_0.1Ce_0.9O_1.95 (GDC) electrolyte. The cathodes had brownmillerite structure with x wt.% Gd_0.1Ce_0.9O_1.95 (GDC) – (100−x) wt.% Ca₂Fe₂O₅ (CFO), where x = 0, 10, 20, 30 and 40. The effect of GDC incorporation on the thermal expansion coefficient (TEC), electrochemical properties and thermal stability of the CFO–GDC composites is investigated. The composite cathode of 30 wt.% GDC – 70 wt.% CFO (CG30) coated on Gd_0.1Ce_0.9O_1.95 electrolyte showed the lowest area specific resistance (ASR), 0.294 Ω cm² at 700 °C and 0.122 Ω cm² at 750 °C. The TEC of the CG30 cathode was 13.1 × 10⁻⁶ °C⁻¹ up to 900 °C, which is a lower value than for CFO alone (13.8 × 10⁻⁶ °C⁻¹). Long-term thermal stability and thermal cycle testing of CG30 cathodes were performed. Stable ARS values were observed during both tests without delamination at the cathode–electrolyte interface. An electrolyte-supported single cell with a 300-μm-thick GDC electrolyte and an anode-supported single cell with ∼10-μm-thick yttria-stabilized zirconia (YSZ) with a GDC buffer layer attained maximum power densities of 395 mW cm⁻² at 750 °C and 842 mW cm⁻² at 800 °C, respectively. The unique composite composition of CG30 demonstrates enhanced electrochemical performance and good thermal stability for IT-SOFCs.     

Interface channels accelerating ion transport through alkaline earth metal carbonate - Gd0.1Ce0.9O1.95 heterostructure

Designing the interface proton channel between different phases to accelerate the proton ion transport is an effective way to realize the high proton conduction for the low-temperature ceramic fuel cell (CFC). Cerium based materials coated with molten carbonate has been widely demonstrated for high performance CFCs. Here, we prepared alkaline earth metal carbonate - Gd_0.1Ce_0.9O_1.95 (GDC) heterostructure composites in various compositions by precipitation method using NH₄HCO₃ and NaHCO₃ as the deposit. The samples prepared using NH₄HCO₃ as the electrolyte, the cell can deliver an even higher power output of 811 mW cm⁻². The results are much higher than that reported in the literature for the GDC electrolyte fuel cells. The ion conduction on the interface between GDC and solid carbonate particles is proposed. The ionic conductivity is determined to be 0.13 S cm⁻¹ at 500 °C; while GDC as reported in literature is 0.005 S cm⁻¹ at the same temperature. This proposed solid carbonate-GDC heterostructure method has succeeded in enhancing ionic conductivity and the CFC performance, which presents a new way to develop high proton conducting materials and advanced ceramic fuel cells at low temperatures (<550 °C).     

Electrochemical performance of a new structured low temperature SOFC with BZY electrolyte

A symmetrical solid oxide fuel cell (SOFC) with a novel microstructure of BaZr_0.9Y_0.1O_3–δ (BZY) as the electrolyte is investigated in this study. The cell with the Ni_0.8Co_0.15Al_0.05LiO₂ (NCAL)-foam Ni/BZY/foam Ni-NCAL structure is prepared by a co-pressing method. The maximum obtained power density is 735.6 mW cm^?2 in H₂ at 550 °C, which is comparable to the results obtained using Ni cermet anode-supported SOFCs with an extremely thin electrolyte. The ionic conductivity of the BZY electrolyte prepared in this study is much higher than that of the conventional BZY electrolyte. The activation energy of ionic conduction is much lower than that of traditional oxygen ion or proton conduction. Electrochemical impedance spectra (EIS) results of the cell with the BZY electrolyte measured in different atmospheric conditions and the results of oxygen ion filtration experiments for the cell using the BZY/Ce_0.9Gd_0.1O₂ (GDC) bilayer electrolyte indicate that oxygen ion is one of the carriers in the BZY electrolyte prepared in this study. According to the results of X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR), an interfacial O^2? conduction mechanism at the interface of BZY particles in the electrolyte is discussed.     

Effect of thickness of Gd0.1Ce0.9O1.95 electrolyte films on electrical performance of anode-supported solid oxide fuel cells

In the present paper, we investigated the electrical performance of anode-supported solid oxide fuel cells (SOFCs) composed of Gd_0.1Ce_0.9O_1.95 (GDC) electrolyte films of 1-75 μm in thickness prepared by simple and cost-effective methods (dry co-pressing process and spray dry co-pressing process), and discussed the effect of thickness of the GDC electrolyte films on the electrical performance of the anode-supported SOFCs. It was shown that reducing the thickness of the GDC electrolyte films could increase the maximum power densities of the anode-supported SOFCs. The increase of the maximum power densities was attributed to the decrease of the electrolyte resistance with reducing the electrolyte thickness. However, when the thickness of the GDC electrolyte films was less than a certain value (approximately 5 μm in this study), the maximum power densities decreased with the decrease in the thickness of the GDC electrolyte films. The calculated electron fluxes through the GDC electrolyte films increased obviously with reducing the thickness of the GDC electrolyte films, which was the reason why the maximum power densities decreased. Therefore, for anode-supported SOFCs based on electrolytes with mixed electronic-ionic conductivity, there was an optimum electrolyte thickness for obtaining higher electrical performance.     

Fabrication using sequence wet-chemical coating and electrochemical performance of Ni–Fe-foam-supported solid oxide electrolysis cell for hydrogen production from steam

Ni–Fe-alloy-foam supported solid oxide electrolysis cell with an arrangement of nickle and Sc_0.1Ce_0.005Gd_0.005Zr_0.89O₂ (Ni-SCGZ) cathode, SCGZ electrolyte and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) anode is successfully fabricated by the sequence wet-chemical coating. The multi-layer cathode with a gradient of thermal expansion coefficient (TEC) is deposited on the alloy-foam support. Two-step firing processes are applied including cathode pre-firing (1373 K, 2 h) and electrolyte sintering (1623 K, 4 h) using slow heating rate enhanced with compressive loading. The fabricated cell shows current density of ?0.95 Acm^?2 at 1.1 V with H₂O:H₂ = 70:30 and 1073 K, providing hydrogen production rate at 4.95 × 10^?6 mol s^?1. However, performance degradation was observed with the rate of 0.08 V h^?1, which can be ascribed to the delamination of BSCF anode under operating at high current density.     

Fabrication and characterization of micro-tubular cathode-supported SOFC for intermediate temperature operation

We report the fabrication and characterization of a micro-tubular cathode-supported cell consisting of a Ce_0.9Gd_0.1O_1.95 electrolyte with a Ni–cermet anode on a porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ/Ce_0.9Gd_0.1O_1.95 (60:40 volume) tube (460 μm wall thickness and 2.26 mm diameter). The cells were fabricated by a cost-effective technique involving extrusion molding and slurry coating through a co-firing process. Densification of the ceria film (thickness < 15 μm) was successful by co-firing the laminated electrolyte with the porous cathode at 1200 °C. NiO–Ce_0.9Gd_0.1O_1.95 (Ni: Ce_0.9Gd_0.1O_1.95 = 50:50 in volume after reduction) was subsequently sintered on the electrolyte at 1100 °C to construct a 10 μm thick, porous and well-adherent anode. The cell having 1.5 cm tube length fed with humidified 30 vol.% H₂–Ar (3% H₂O) yielded the maximum power densities of 0.16, 0.13 and 0.11 W cm⁻², at 600, 550 and 500 °C, respectively. It was found that the cell performance is strongly dominated by the tube length, due to a high substrate resistance from the cathode current collections.     

BaZr0.2Ce0.8−xYxO3−δ solid oxide fuel cell electrolyte synthesized by sol–gel combined with composition-exchange method

This study reports the synthesis of proton-conducting BaZr_0.2Ce_0.8−xY_xO_3−δ (x = 0–0.4) oxides by using a combination of citrate-EDTA complexing sol–gel process and composition-exchange method. Compared to those oxides prepared from conventional sol–gel powders, the sintered BaZr_0.2Ce_0.8−xY_xO_3−δ pellets synthesized by sol–gel combined with composition-exchange method are found to exhibit improved sinterability, a higher relative density, higher conduction, and excellent thermodynamic stability against CO₂. Moreover, the Pt/electrolyte/Pt single cell using such a BaZr_0.2Ce_0.6Y_0.2O_3−δ electrolyte shows an obviously higher maximum powder density in the hydrogen-air fuel cell experiments. Based on the experimental results, we discuss the improvement mechanism in terms of calcined particle characteristics. This work demonstrates that the BaZr_0.2Ce_0.8−xY_xO_3−δ oxides synthesized by sol–gel combined with composition-exchange method would be a promising electrolyte for the use in H⁺-SOFC applications. More importantly, this new fabrication approach could be applied to other similar ABO₃-perovskite material systems.     

Solid oxide fuel cells with Sm0.2Ce0.8O2−δ electrolyte film deposited by novel aerosol deposition method

In this study, dense electrolyte ceramic Sm_0.2Ce_0.8O_2−δ (SDC) thin films are successfully deposited on NiO-SDC anode substrate by aerosol deposition (AD) with oxygen as the carrier gas at the substrate temperature ranging from room temperature to 300 °C. To remove the effect of humidity on the starting powders, this study found that, in depositing SDC films, having the starting powders preheat-treated at 200 °C helped generate a smooth and dense layer, though a lower deposition rate was achieved. At a deposition time of 22 min, SDC films with a uniform thickness of 1.5 μm and grain sizes of ≈67 nm are obtained. SOFC single cells are then built by screen printing a LSCF cathode on the anode-supported substrates with SDC electrolyte. The cross-sectional SEM micrographs exhibit highly dense, granular, and crack-free microstructures. The open circuit voltages (OCV) of the single cells decrease with the rise in temperature, dropping from 0.81 V at 500 °C to 0.59 V at 700 °C. Maximum power densities (MPD) decline with decreasing operating temperature from 0.34 to 0.01 W cm⁻² due to the increase of the R₀ and R_P of the single cells. The electrochemical results testify to the fine quality of SDC films as well as illustrate the electrolyte thickness effect and the effect of mixed ionic and electronic conduction of the SDC electrolyte in the reducing atmosphere.     

Structural and electrical properties of grain boundaries in Ce0.85Gd0.15O1.925 solid electrolyte modified by addition of transition metal ions

In this work we present studies on applicability of transition metal additives as sintering and electrical conductivity aids for cerium gadolinium oxide electrolyte. The nanosized Ce_0.85Gd_0.15O_1.925 powder obtained by coprecipitation method was modified with Cr³⁺, Fe³⁺, Ni²⁺ or Cu²⁺ ions. Using high-intensity high-resolution X-ray powder diffraction data we have determined that Cr, Fe and Ni ions do not incorporate into the cerium gadolinium oxide surface or bulk when sintered at 1300 °C, but react with Gd ions to form Cr_0.9Gd_0.1O, GdFeO₃ and GdNiO₃ phases, while Cu incorporates in the material up to 0.7 mol% with a significant fraction of remaining material showing poorly crystalline CuO phase. The nanosized Ce_0.85Gd_0.15O_1.925 material shows already improved sintering properties than previous reports but full sintering is not achieved below 1300 °C, however Cr, Fe and mainly Cu impregnation allows full sintering at 1300 °C. 0.5 mol% Ni impregnated material sintered at 1500 °C shows enhanced grain boundary conductivity that probably indicates that Ni incorporates into Ce_0.84Gd_0.15O_1.925 above 1300 °C. The global results indicate, however, that optimization of ceria microstructure is at least of equal importance for sinterability and grain boundary conductivity than impregnation of the material with transition metal ions.     

Synthesis and characterization of scandia ceria stabilized zirconia powders prepared by polymeric precursor method for integration into anode-supported solid oxide fuel cells

Scandia ceria stabilized zirconia (10Sc1CeSZ) powders are synthesized by polymeric precursor method for use as the electrolyte of anode-supported solid oxide fuel cell (SOFC). The synthesized powders are characterized in terms of crystalline structure, particle shape and size distribution by X-ray diffraction (XRD), transmission electron microscopy (TEM) and photon correlation spectroscopy (PCS). 10Sc1CeSZ electrolyte films are deposited on green anode substrate by screen-printing method. Effects of 10Sc1CeSZ powder characteristics on sintered films are investigated regarding the integration process for application as the electrolytes in anode-supported SOFCs. It is found that the 10Sc1CeSZ films made from nano-sized powders with average size of 655 nm are very porous with many open pores. In comparison, the 10Sc1CeSZ films made from micron-sized powders with average size of 2.5 μm, which are obtained by calcination of nano-sized powders at higher temperatures, are much denser with a few closed pinholes. The cell performances are 911 mW cm⁻² at the current density of 1.25 A cm⁻² and 800 °C by application of Ce_0.8Gd_0.2O₂ (CGO) barrier layer and La_0.6Sr_0.4CoO₃ (LSC) cathode.     

A high performance intermediate temperature fuel cell based on a thick oxide–carbonate electrolyte

A high performance intermediate temperature fuel cell (ITFC) with composite electrolyte composed of co-doped ceria Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) and a binary carbonate-based (52 mol% Li₂CO₃/48 mol% Na₂CO₃), 1.2 mm thick electrolyte layer has been developed. Co-doped Ce_0.8Gd_0.05Y_0.15O_1.9 was synthesized by a glycine–nitrate process and used as solid support matrix for the composite electrolyte. The conductivity of both composite electrolyte and GYDC supporting substrate were measured by AC impedance spectroscopy. It showed a sharp conductivity jump at about 500 °C when the carbonates melted. Single cells with thick electrolyte layer were fabricated by a dry-pressing technique using NiO as anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ or lithiated NiO as cathode. The cell was tested at 450–550 °C using hydrogen as the fuel and air as the oxidant. Excellent performance with high power density of 670 mW cm⁻² at 550 °C was achieved for a 1.2 mm thick composite electrolyte containing 40 wt% carbonates which is much higher than that of a cell based on pure GYDC with a 70 μm thick electrolyte layer.