Efficiently enhance the proton conductivity of YSZ-based electrolyte for low temperature solid oxide fuel cell

Yttrium stabilized zirconia (YSZ) as a typical oxygen ionic conductor has been widely used as the electrolyte for solid oxide fuel cell (SOFC) at the temperature higher than 1000 °C, but its poor ionic conductivity at lower temperature (500–800 °C) limits SOFC commercialization. Compared with oxide ionic transport, protons conduction are more transportable at low temperatures due to lower activation energy, which delivered enormous potential in the low-temperature SOFC application. In order to increase the proton conductivity of YSZ-based electrolyte, we introduced semiconductor ZnO into YSZ electrolyte layer to construct heterointerface between semiconductor and ionic conductor. Study results revealed that the heterointerface between ZnO and YSZ provided a large number of oxygen vacancies. When the mass ratio of YSZ to ZnO was 5:5, the fuel cell achieved the best performance. The maximum power density (P_max) of this fuel cell achieved 721 mW cm^?2 at 550 °C, whereas the P_max of the fuel cell with pure YSZ electrolyte was only 290 mW cm^?2. Further investigation revealed that this composite electrolyte possessed poor O^2? conductivity but good proton conductivity of 0.047 S cm^?1 at 550 °C. The ionic conduction activation energy of 5YSZ-5ZnO composite in fuel cell atmosphere was only 0.62 eV. This work provides an alternative way to improve the ionic conductivity of YSZ-based electrolytes at low operating temperatures.     

Semiconductor ionic based Sm0.2Ce0.8O2−δ-La0.6Sr0.4Fe0.8Cu0.2O3-δ heterostructure for low temperature ceramic fuel cells

Structural design/doping strategy is an efficient method to prepare electrolytes with high oxygen ionic conductivity, but there is still hindrance for solid oxide fuel cell (SOFC) commercialization. Recent advances in semiconductor ionic materials have developed a novel strategy in designing low-temperature electrolyte materials. Here, a heterostructure composite of LSFC (La_0.6Sr_0.4Fe_0.8Cu_0.2O_3-δ) and SDC (Sm_0.2Ce_0.8O_2?δ) is developed. The LSFC-SDC composite exhibits a high ionic conductivity, >0.1S/cm at 550 °C. With symmetrical NCAL (Ni_0.8Co_0.15Al_0.05LiO_2-δ)-coated electrode, cells with SDC-LSFC electrolyte exhibit high open-circuit voltage (OCV), and achieve a significant power improvement (>1000 mW/cm²) compared with pure SDC electrolyte at 550 °C. The short-term stability result has proven the operating ability of SDC-LSFC electrolyte under fuel cell environment (H₂/air). This work demonstrates a new developing route of low-temperature solid oxide fuel cell (LTSOFC), which is different from the conventional SOFC.     

Wet Atomisation of Gd‐doped CeO2 Electrolyte Slurries for Intermediate Temperatures' Microtubular SOFC Applications

A wet atomising system has been employed as a novel method to prepare ultrafine Gd‐doped CeO₂ (GDC) electrolyte slurries. By changing the fluid flow pressure and repeating the atomisation process several times for the same atomised slurries, we have obtained optimised ultrafine GDC slurry with high‐dispersed and homogeneous distribution. The sizes of the particles of GDC were in the range of tens of nanometres. A highly dense electrolyte layer (membrane) was prepared using the ultrafine GDC slurries for intermediate temperatures microtubular solid oxide fuel cell (SOFC) applications. The SOFC was fabricated by using supporting porous anode tubes of NiO and GDC, and the cathode consisted of La_0.6Sr_0.4Co_0.2Fe_0.8O_3–y and GDC. A dense 10 μm GDC electrolyte layer was obtained at a lower sintering temperature of 1,250 °C for 1 h. The SOFC was tested with humidified (3% H₂O) hydrogen as a fuel and the static air as an oxidant, and the tubular cell maintained its high performance even at 500 °C.     

Advances in novel SDC@Al2O3 core−shell electrolyte for low-temperature solid oxide fuel cell

The preparation of electrolyte with excellent ionic conduction is an important development direction in the practical application of solid oxide fuel cell (SOFC). Traditional methods to improve ion conduction was structure doping to develop electrolyte materials. In this work, the ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) was modified by insulator Al₂O₃ to enhance ion conduction and apply as electrolytes for the SOFC. The transmission electron microscopy (TEM) characterization clearly clarified that a thin Al₂O₃ layer in the amorphous state coated on SDC to form the SDC@Al₂O₃ core−shell structure. The SDC@Al₂O₃ electrolyte with the core−shell structure possesses a super ionic conductivity of 0.096 S cm⁻¹ and results in advanced cell performance of 1190 mW cm⁻² at 550°C. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the concentration of oxygen vacancies in the SDC@Al₂O₃ core–shell structure significantly improved in comparison with pure SDC, the newly produced oxygen vacancies can promote the oxygen ion transport. Moreover, the interface between SDC and Al₂O₃ provides a fast channel for the proton transport. In addition, the SDC-based SOFC was usually suffered from the reduction of the SDC electrolyte and the accompanying generated electron conduction should deteriorate the cell performance, this is the main challenge for the SDC electrolyte application. In our case, the Al₂O₃ shell on the SDC surface not only can avoid the contact between SDC and hydrogen to eliminate the reduction of SDC but also can restrain electron conduction due to the electron insulation characteristic of the Al₂O₃ shell. This work demonstrates an efficient approach to develop the advanced low-temperature SOFC technology from material fundamentals.     

Processing of high-performance Co doped Y2O3 as a single-phase electrolyte for low temperature solid oxide fuel cell (LT-SOFC)

The high-performance single-phase semiconductor materials with higher ionic conductivity have drawn substantial attention in fuel cell applications. Semiconductor materials play a key role to enhance ionic conductivity subsequently promoting low temperature solid oxide fuel cell (LT-SOFC) research. Herein, we proposed a semiconductor Co doped Y₂O₃ (YCO) samples with different molar ratios, which may easily access the high ionic conductivity and electrochemical performances at low operating temperatures. The resulting fabricated fuel cell 10% Co doped Y₂O₃ (YCO-10) device exhibits high ionic conductivity of ∼0.16 S cm⁻¹ and a feasible peak power density of 856 mW cm⁻² along with 1.09 OCV at 530 °C under H₂/air conditions. The electrochemical impedance spectroscopy (EIS) reveals that YCO-10 electrolyte based SOFC device delivers the least ohmic resistance of 0.11–0.16 Ω cm² at 530-450 °C. Electrode polarization resistance of the constructed fuel cell device noticed from 0.59 Ω cm² to 0.28 Ω cm² in H₂/air environment at different elevated temperatures (450 °C to 530 °C). This work suggests that YCO-10 can be a promising alternative electrolyte, owing to its high fuel cell performance and enhanced ionic conductivity for LT-SOFC.     

Electrochemical impedance spectra of solid-oxide fuel cells and polymer membrane fuel cells

Electrochemical impedance spectroscopy (EIS) is a very useful method for the characterization of fuel cells. The anode and cathode transfer functions have been determined independently without a reference electrode using symmetric gas supply of hydrogen or oxygen on both electrodes of the fuel cell at open circuit potential (OCP). EIS are given for both polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) at current densities up to 0.76 A cm⁻² (PEFC) and 0.22 A cm⁻² (SOFC). With increasing current density the PEFC-impedance decreases significantly in the low frequency range reaching a minimum at 0.4 A cm⁻². At even higher current densities an increasing contribution of water diffusion is observed: the cell impedance increases again. From EIS of SOFC a finite diffusion behavior is observed even at OCP, depending on water partial pressure of the anodic gas supply. This additional element reflects the influence of water partial pressure on the cell potential. The simulation of the measured EIS with an equivalent circuit enables the calculation of the individual voltage losses in the fuel cell.     

Optimizing Na0.5Bi0.5TiO3 electrolyte fuel cell through constructing heterostructures

Heterostructure materials deliver special properties comparing with single phase materials. In this study, the performance of Na_0.5Bi_0.5TiO₃ (NBT) electrolyte fuel cell is proved to be optimized by constructing heterostructures with other materials. SOFC based on NBT single phase electrolyte exhibits poor stability and low power output. By mixing NBT with electronic conductor La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF), improved performance is obtained and power output is observed to be dependent on the weight ratio of NBT to LSCF. It is interesting that the best performance is obtained in the cell with an optimized weight ratio of 30 wt% NBT - 70 wt% LSCF, in which the amount of electronic conductor exceeds that of ionic conductor. However, the stability of SOFC based on NBT-LSCF composite electrolyte still needs to be improved. In addition, the composition of sodium carbonate and lithium carbonate is added to the NBT-LSCF composite electrolyte for the purpose of creating amorphous shells around the NBT-LSCF particles, which is expected to protect the NBT-LSCF particles from reducing by hydrogen. Improved stability of the cell is then observed. This study provides an effective way to enhance the ionic conductivity and stability of electrolyte by constructing heterostructures.     

Boosting intermediate temperature performance of solid oxide fuel cells via a tri-layer ceria–zirconia–ceria electrolyte

Using cost-effective fabrication methods to manufacture a high-performance solid oxide fuel cell (SOFC) is helpful to enhance the commercial viability. Here, we report an anode-supported SOFC with a three-layer Gd_0.1Ce_0.9O_1.95 (gadolinia-doped-ceria [GDC])/Y_0.148Zr_0.852O_1.926 (8YSZ)/GDC electrolyte system. The first dense GDC electrolyte is fabricated by co-sintering a thin, screen-printed GDC layer with the anode support (NiO–8YSZ substrate and NiO–GDC anode) at 1400°C for 5 h. Subsequently, two electrolyte layers are deposited via physical vapor deposition. The total electrolyte thickness is less than 5 μm in an area of 5 × 5 cm², enabling an area-specific ohmic resistance as low as 0.125 Ω cm⁻² at 500°C (under open circuit voltage), and contributing to a power density as high as 1.2 W cm⁻² at 650°C (at an operating cell voltage of 0.7 V, using humidified [10 vol.% H₂O] H₂ as fuel and air as oxidant). This work provides an effective strategy and shows the great potential of using GDC as an electrolyte for high-performance SOFC at intermediate temperature.     

Development of a Novel Ceramic Support Layer for Planar Solid Oxide Cells

The conventional solid oxide cell is based on a Ni–YSZ support layer, placed on the fuel side of the cell, also known as the anode supported SOFC. An alternative design, based on a support of porous 3YSZ (3 mol.% Y₂O₃–doped ZrO₂), placed on the oxygen electrode side of the cell, is proposed. Electronic conductivity in the 3YSZ support is obtained post sintering by infiltrating LSC (La_0.6Sr_0.4Co_1.05O₃). The potential advantages of the proposed design is a strong cell, due to the base of a strong ceramic material (3YSZ is a partially stabilized zirconia), and that the LSC infiltration of the support can be done simultaneously with forming the oxygen electrode, since some of the best performing oxygen electrodes are based on infiltrated LSC. The potential of the proposed structure was investigated by testing the mechanical and electrical properties of the support layer. Comparable strength properties to the conventional Ni/YSZ support were seen, and sufficient and fairly stable conductivity of LSC infiltrated 3YSZ was observed. The conductivity of 8–15 S cm^–1 at 850 °C seen for over 600 h, corresponds to a serial resistance of less than 3.5 m Ω cm² of a 300 μm thick support layer.     

Electrophoretic Deposition of Zirconia Thin Film on Nonconducting Substrate for Solid Oxide Fuel Cell Application

Electrophoretic deposition (EPD) of 8 mol% yttria‐stabilized zirconia (YSZ) electrolyte thin film has been carried out onto nonconducting porous NiO‐YSZ cermet anode substrate using a fugitive and electrically conducting polymer interlayer for solid oxide fuel cell (SOFC) application. Such polymer interlayer burnt out during the high‐temperature sintering process (1400°C for 6 h) leaving behind a well adhered, dense, and uniform ceramic YSZ electrolyte film on the top of the porous anode substrate. The EPD kinetics have been studied in depth. It is found that homogeneous and uniform film could be obtained onto the polymer‐coated substrate at an applied voltage of 15 V for 1 min. After the half‐cell (anode + electrolyte) is co‐fired at 1400°C, a suitable cathode composition (La_0.65Sr_0.3MnO₃) thick film paste is screen printed on the top of the sintered YSZ electrolyte. A second stage of sintering of such cathode thick film at 1100°C for 2 h finally yield a single cell SOFC. Such single cell produced a power output of 0.91 W/cm² at 0.7 V when measured at 800°C using hydrogen and oxygen as fuel and oxidant, respectively.     

BaZr0.1Co0.4Fe0.4Y0.1O3-SDC composite as quasi-symmetrical electrode for proton conducting solid oxide fuel cells

Symmetric solid oxide fuel cells (SSOFCs) with the identical anode and cathode electrocatalysts show promise to reduce material and system cost while increasing the cell lifespan. In this work, BaZr_0.1Co_0.4Fe_0.4Y_0.1O₃ (BZCFY) oxide perovskite is proposed as a symmetric electrode for SSOFCs based on proton conducting electrolyte, with targets of reducing temperature and high-performance application. Active oxygen ionic conductor and catalyst, SDC, is composited to improve the cell performance and electrode durability. Those materials show good chemical compatibility while BZCFY is decomposed to alloy and mixed oxide composite, which significantly affects electrode activity. SDC-BZCFY composite gives an electrode polarization resistance of 1.35–13.7 Ω cm² and 0.32–1.59 Ω cm² for hydrogen oxidation reaction and oxygen reduction reaction on the proton conducting electrolyte, BZCY, at the temperature range of 700–550 °C, respectively. Moreover, it displays an excellent oxygen reduction kinetics with an impressive activation energy of 0.91 eV. The polarization resistances are significantly reduced in the fuel cell condition owning to the electrochemical promotion effect under open-circuit condition. Quasi-SSOFCs with BZCY electrolyte in a thickness of 480 μm and electrode thickness of 25 μm give a peak power density of 114.8 and 74.3 mW cm⁻² at 650 and 600 °C, respectively. In addition, SSOFC also displays acceptable durability under constant voltage operational condition for 25 h. This work highlights alternative active electrode material for symmetric solid oxide fuel cells for low temperature operation.     

Chemical Bulk Diffusion Coefficient of a La0.5Sr0.5CoO3−δ Cathode for Intermediate‐Temperature Solid Oxide Fuel Cells

In the first part of this study, the characteristics of a La_0.5Sr_0.5CoO_3?δ cathode are described, including its chemical bulk diffusion coefficient (D_chem), and electrical conductivity relaxation experiments are performed to obtain experimental D_chem measurements of this cathode. The second part of this study describes two methods to improve the single‐cell performance of solid oxide fuel cells. One method uses a composite cathode, i.e., a mix of 30 wt% electrolyte and 70 wt% cathode; the other method uses an electrolyte‐infiltrated cathode, i.e., an active ionic‐conductive electrolyte with nano‐sized particles is deposited onto a porous cathode surface using the infiltration method. In this work, 0.2M Ce_0.8Sm_0.2O_1.9 (SDC)‐infiltrated La_0.5Sr_0.5CoO_3?δ exhibits a maximum peak power density of 1221 mW/cm² at an operating temperature of 700°C with a thick‐film SDC electrolyte (30 μm), a NiO + SDC anode (1 mm) and a La_0.5Sr_0.5CoO_3?δ cathode (10 μm). The enhancement in electrochemical performances using the electrolyte‐infiltrated cathode is attributed to the creation of electrolyte/cathode phase boundaries, which considerably increases the number of electrochemical sites available for the oxygen reduction reaction.     

Low-temperature SOFCs using biomass-produced gases as fuels

The electromotive force (e.m.f) is calculated for solid oxide fuel cells (SOFCs) based on doped ceria electrolytes using biomass-produced gases (BPG, 14.7% CO, 14.2% CO₂, 15.3% H₂, 4.2% CH₄, and 51% N₂) as fuels and air as oxidant. It reveals that the BPG derived e.m.f. is very close to hydrogen when doped ceria is used as the electrolyte. A 35-m-thick samaria-doped ceria based single cell was tested between 450 and 650°C using BPG as fuel. Maximum power density of about 700 mW cm^–2 was achieved at 650 °C. The open-circuit voltage at 450 °C was 0.96 V, close to the calculated value. However, the cell power density using BPG as fuel was relatively lower than that using humidified hydrogen (3% H₂O), and close to that using humidified methane (3% H₂O). Impedance measurements indicate that the relatively lower power output may be attributed to the high anode--electrolyte interfacial polarization resistance when BPG is used as fuel.     

A feasible strategy for tailoring stable spray-coated electrolyte layer in micro-tubular solid oxide fuel cells

By this work, the viability of the spray coating as a cost-effective and reliable technique for the coating of Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte layer on the mini-tubular NiO–GDC anodes based a solid oxide fuel cell (SOFC) fabrication was assessed. The compatibility of the anode and electrolyte was analyzed by using XRD. The variation in thickness and morphology of the electrolyte film as a function of the coating cycles was discussed with optical and scanning electron microscopes. By similar formulation, the coating of La_0.6Sr_0.4Fe_0.8Co_0.2O₃ –Ce_0.9Gd_0.1O_2–δ (LSCF–GDC) was performed to achieve porous cathode. An individual micro-tubular anode supported cell with configuration NiO–GDC/GDC/LSCF–GDC as anode/electrolyte/cathode was tested in the SOFC mode with humidified hydrogen as fuel and stationary air as oxidant. The fabricated mini-SOFC prototype that generated a maximum power density of 0.510 W/cm² at 600°C signifies the potential of this industrially scalable low-cost coating technique.     

A novel cobalt–free La0.5Ba0.5Fe0.95Mo0.05O3–δ electrode for symmetric solid oxide fuel cell

Cobalt − free perovskite oxide La_0.5Ba_0.5Fe_0.95Mo_0.05O_3−δ (LBFMo) was investigated as the electrode of symmetric solid oxide fuel cell (S − SOFC) based on 300−um − thick La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte. The electrochemical performance of the S − SOFC with LBFMo|LSGM|LBFMo configuration was evaluated using ambient air as oxidant and H₂ as fuel. The maximum power density (P_max) of the S − SOFC achieves as high as 0.96 W cm⁻² at 800 °C; meanwhile, the total polarization resistance (R_pt) of the S − SOFC (including the contributions of both cathode and anode) is only ∼0.12 Ω cm². Impedance spectra analysis indicates the polarization associated with anode plays a more rate − limiting role in the whole electrochemical reaction process of the S − SOFC. In addition, using LBFMo as symmetric electrode, the S − SOFC also exhibits good cell stability. All results indicate that the LBFMo is a very potential candidate for S − SOFC electrode.     

质子传导陶瓷电解质燃料电池特性分析

An electrolyte model for the solid oxide fuel cell (SOFC) with proton conducting perovskite electrolyte is developed in this study, in which four types of charge carriers including proton, oxygen vacancy (oxide ion), free electron and electron hole are taken into consideration. The electrochemical process within the SOFC with hydrogen as the fuel is theoretically analyzed. With the present model, the effects of some parameters, such as the thickness of electrolyte, operating temperature and gas composition, on the ionic transport (or gas permeation) through the electrolyte and the electrical performance, i.e., the electromotive force (EMF) and internal resistance of the cell, are investigated in detail. The theoretical results are tested partly by comparing with the experimental data obtained from SrCe0.05M0.05O3-α(M=Yb, Y) cells.     

Mathematical Modelling of Proton‐Conducting Solid Oxide Fuel Cells and Comparison with Oxygen‐Ion‐Conducting Counterpart

Proton‐conducting solid oxide fuel cells (H‐SOFC), using a proton‐conducting electrolyte, potentially have higher maximum energy efficiency than conventional oxygen‐ion‐conducting solid oxide fuel cells (O‐SOFC). It is important to theoretically study the current–voltage (J–V) characteristics in detail in order to facilitate advanced development of H‐SOFC. In this investigation, a parametric modelling analysis was conducted. An electrochemical H‐SOFC model was developed and it was validated as the simulation results agreed well with experimental data published in the literature. Subsequently, the analytical comparison between H‐SOFC and O‐SOFC was made to evaluate how the use of different electrolytes could affect the SOFC performance. In addition to different ohmic overpotentials at the electrolyte, the concentration overpotentials of an H‐SOFC were prominently different from those of an O‐SOFC. H‐SOFC had very low anode concentration overpotential but suffered seriously from high cathode concentration overpotential. The differences found indicated that H‐SOFC possessed fuel cell characteristics different from conventional O‐SOFC. Particular H‐SOFC electrochemical modelling and parametric microstructural analysis are essential for the enhancement of H‐SOFC performance. Further analysis of this investigation showed that the H‐SOFC performance could be enhanced by increasing the gas transport in the cathode with high porosity, large pore size and low tortuosity.     

A Novel Molten Oxide Fuel Cell Concept

We suggest a novel molten oxide fuel cell (MOFC) concept. The MOFC is based on the oxygen‐ion‐conducting solid/molten oxide electrolyte (so‐called liquid‐channel‐grain‐boundary‐structure, LGBS, material) consisting of TeO₂ solid grains and chemically compatible TeO₂+Te₄Bi₂O₁₁ liquid electrolyte at the grain boundaries. The intergranular liquid channels provide the LGBS mechanical plasticity (ductility), which makes it easy to shape and alleviates problems due to thermal incompatibility with electrodes (CTE), and high ionic conductivity. The volume fraction of liquid varied from 0.15 to 0.17 at 600–640 °C. The cell performance has been examined by standard electrochemical methods. With air used as a cathode gas, the single cell showed the power 11.5 mW cm⁻² at the current density 90 mA cm⁻², electrolyte thickness 2.5 mm, and temperature 640 °C.     

Deficiency of O2 molecules enhances ionic conductivity in Cr-doped O-Ce-O for solid oxide fuel cell applications

Production of high performance low-energy loss solid oxide fuel cells (SOFCs) is a challenge and is the global demand of the current market. We have focused on to develop SOFCs that can be operated at 600–800 °C with better ionic conductivity when compared to the conventional SOFCs functioning at 1000–800 °C. Bulk cerium oxide (CeO₂)-based solid electrolyte lessens ionic conductivity at room temperature, thus nanocrystalline CeO₂ has been used to improve the conductivity and to control the temperature. The transition metal-doped CeO₂ (Ce_(1−X)Cr_(X)O₂) nanocrystalline is used to increases the deficiency of oxygen molecules which in turn enhances ionic conductivity in electrolyte material for SOFC applications. The structural and morphological characterization have been done using XRD, RAMAN and FESEM, while electrical and magnetic characterization at room temperature was analysed using vibrating sample magnetometer, impedance spectroscopy and cyclic voltammetery shows better ionic conductivity in Cr doped CeO₂ in comparison with pure nanocrystalline CeO_2.     

Analysis of a proton-conducting SOFC with direct internal reforming

This paper presents a performance analysis of a planar SOFC (solid oxide fuel cell) with proton-conducting electrolyte (SOFC-H⁺). The SOFC-H⁺ is fueled by methane and operated under direct internal reforming and isothermal conditions. A one-dimensional steady-state model coupled with a detailed electrochemical model is employed to investigate the distribution of gas composition within fuel and air channels and all the electrochemical-related variables. The current–voltage characteristics of SOFC-H⁺ are analyzed and the result shows that the operation of SOFC-H⁺ at 0.7 V gives a good compromise on power density and fuel utilization. However, high CO content at fuel channel is observed at this condition and this may hinder the SOFC-H⁺ performance by reducing catalyst activity. The effect of key cell operating parameters, i.e., steam to carbon ratio, temperature, pressure, and water content in oxidant, on the performance of SOFC-H⁺ and the content of CO is also presented in this study.