Preparation and performance of Na-doped La2Ce2O7 electrolytes for protonic ceramic fuel cells

The protonic material La₂Ce₂O₇ exhibits good tolerance to H₂O and CO₂ compared to BaCeO₃-based materials and has become increasingly popular for operation at low-to-intermediate temperatures in protonic ceramic fuel cells. In this work, doping La₂Ce₂O₇ with Na in a series with varying compositions is studied. All of the precursors are prepared by a common citrate-nitrate combustion method. X-ray diffraction images reveal that all of the La_2-xNa_xCe₂O_7-δ samples have a cubic structure. The La_2-xNa_xCe₂O_7-δ pellets are characterized by scanning electron microscopy and are observed to be dense without holes. The effects of Na-doping on the La₂Ce₂O₇ electrical conductivity are carefully investigated in air at 350–800 °C and 5%H₂-95% Ar environments at 350–700 °C. It is found that different levels of Na doping in La₂Ce₂O₇ are conducive to improving the electrical conductivity and sinterability. Among the pellets, La_1.85Na_0.15Ce₂O_7-δ exhibited the highest electrical conductivity in air and 5% H₂-95% Ar atmospheres. Anode-supported half cells with La_1.85Na_0.15Ce₂O_7-δ electrolyte are also fabricated via a dry-pressing process, and the corresponding single cell exhibited a desirable power performance of 501 mW cm⁻² at 700 °C. The results demonstrate that La_1.85Na_0.15Ce₂O_7-δ is a promising proton electrolyte with high conductivity and sufficient sinterability for use in protonic ceramic fuel cells operating at reduced temperatures.     

Electrochemical studies of ceramic carbon electrodes for fuel cell systems: A catalyst layer without sulfonic acid groups

Ceramic carbon electrodes (CCEs) have been produced via the sol-gel process using 20% Pt on Vulcan XC72 carbon black and tetra ethyl orthosilicate (TEOS) as the organosilane precursor. This process produces a homogenous distribution of SiO₂ and carbon supported Pt catalyst. Electrochemical experiments (cyclic voltammetry, electrochemical impedance spectroscopy) were performed to determine the effect of SiO₂ loading on the active area of Pt in the catalyst layer. A volcano-type dependence was observed with the maximum active area of Pt occurring with an SiO₂ loading of 45% by mass. Pt utilization was lower than that achieved with Nafion-based catalyst layers and was explained in terms of the lower proton conductivity of SiO₂ compared to Nafion. These CCE structures may be useful for high temperature fuel cell systems.     

A high-performance fuel electrode-supported tubular protonic ceramic electrochemical cell

Protonic ceramic electrochemical cells (PCECs) have received extensive attention for encouraging energy conversion and storage efficiencies. One of the key obstacles hindering the development of PCECs is the time-consuming and costly fabrication processes. In this study, we have fabricated fuel electrode-supported tubular PCECs via a facile and well-controlled phase inversion methhod combined with a dip-coating process. When operated in a fuel cell (FC) mode, single cells exhibit excellent peak power densities of 0.48, 0.91, 1.50, and 2.62 W cm^-2 at 550–700 ℃, respectively. While in electrolysis cell (EC) mode, these tubular cells achieve high electrolysis current densities of − 0.61, − 1.59, − 3.03, and − 4.67 A cm^-2 with reasonable Faradaic efficiencies at 550–700 ℃ and an applied voltage of 1.3 V, respectively. Additionally, cells can be stably operated at 0.5 A cm^-2 for 120 h in FC mode and − 0.5 A cm^-2 for 110 h in EC mode at 650 °C.     

The effect of room temperature and high temperature exposure on the elastic modulus, hardness and fracture toughness of glass ceramic sealants for solid oxide fuel cells

The reliable operation of solid oxide fuel cell stacks (SOFCs) depends strongly on the structural integrity of the sealing material. Indentation testing is used to determine the mechanical properties of glass-ceramic sealants typically used for solid oxide fuel cell stacks, in particular for the evaluation of elastic modulus, hardness and fracture toughness. Different sealing materials partly with reinforcement by metallic or ceramic filler (particles or short fibers) are tested. The materials are tested after the joining procedure and after additional annealing at operation temperatures to test the effect of further crystallization that might take place. Furthermore, the effect of environmentally enhanced slow crack growth at low temperatures in water saturated atmosphere is investigated. Finally, self-healing effects of the glass ceramic materials with and without pre-annealing at typical operation temperatures are considered.     

Novel polymer–ceramic nanocomposite based on high dielectric constant epoxy formula for embedded capacitor application

Embedded capacitor technology can increase silicon packing efficiency, improve electrical performance, and reduce assembly cost compared with traditional discrete capacitor technology. Developing a suitable material that satisfies electrical, reliability, and processing requirements is one of the major challenges of incorporating capacitors into a printed wiring board (PWB). Polymer–ceramic composites have been of great interest as embedded capacitor material because they combine the processability of polymers with the high dielectric constant of ceramics. A novel nanostructure polymer–ceramic composite with a very high dielectric constant (ε_r ～110, a new record for the highest reported ε_r value of a nanocomposite) was developed in this work. A high dielectric constant is obtained by increasing the dielectric constant of the epoxy matrix (ε_r >6) and using the combination of lead magnesium niobate–lead titanate (PMN–PT)/BaTiO₃ as the ceramic filler. This nanocomposite has a low curing temperature (<200°C); thus, it is multichip‐module laminate (MCM‐L) process‐compatible. An embedded capacitor prototype with a capacitance density of 50 nF/cm² was manufactured using this nanocomposite and spin‐coating technology. The effect of the composite microstructure on the effective dielectric constant was studied. This novel nanocomposite can be used for integral capacitors in PWBs. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1084–1090, 2002     

Electrophoretic deposition of ceramic materials for fuel cell applications

La_0.8Sr_0.2Ga_0.875Mg_0.125O_3-x (LSGM), La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ (LSCF), yttria stabilized zirconia (YSZ) and (Ce_0.8Gd_0.2)O_1.9 (CGO) were electrophoretically deposited on Ni foils and Ni-yttria stabilized zirconia substrates prepared by tape casting. It was demonstrated that the ethyl alcohol–phosphate ester–polyvinyl butyral system is an effective solvent–dispersant–binder system for electrophoretic deposition of these materials. The influence of dispersant, binder and current density on deposition efficiency and deposit morphology was studied. The microstructure of the deposits was examined by electron microscopy. The proposed solvent–dispersant–binder medium for electrophoretic deposition of LSGM, LSCF, YSZ and CGO has important advantages and implications in fuel cell design.     

Effects of conductive ceramic on the electrochemical performance of ZnO for Ni/Zn rechargeable battery

A novel ZnO/conductive-ceramic nanocomposite was prepared by a homogeneous precipitation method. The conductive ceramic with the nominal chemical composition of (ZnO)_0.92(Bi₂O₃)_0.054(Co₂O₃)_0.025(Nb₂O₅)_0.00075(Y₂O₃)_0.00025, as the nucleation sites of ZnO, was prepared by ball milling and surface modification process and its effects on electrochemical performance of ZnO were investigated by charge/discharge cycling, slow rate cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared with pure ZnO, the ZnO/conductive-ceramic electrode exhibited improved electrochemical properties, such as superior cycle stability, higher discharge capacity and utilization ratio. When the conductive ceramic content reached 14 wt.%, the discharge capacity of the ZnO/conductive-ceramic nanocomposite hardly declined over 50 cycling test, the average utilization ratio could reach 99.5%, and the electrodes had no obvious weight loss after cycling tests.     

An electrochemical impedance spectroscopy study of fuel cell electrodes

Electrochemical impedance spectroscopy (EIS) was used to study the capacitance and ion transport properties of fuel cell catalyst layers. It was found that limiting capacitance correlates with active area. The capacitance per gram of catalyst was calculated and is proposed as a measure of catalyst utilization. Results obtained with catalyst layers immobilized on glassy carbon electrodes agree very well with results obtained with gas diffusion electrodes. EIS was also used to study ion conductivity and active area in fuel cell electrodes that contain the electroactive probe Os(bpy)₃²⁺. Together, these results validate the hypothesis that the non-ideal behavior of fuel cell electrodes is due to variations of conductivity across the layer, rather than variations in capacitance.     

Fabrication of ceramic composite anode at low temperature for high performance protonic ceramic fuel cells

A ceramic composite anode composed of (La_0.8Sr_0.2) (Cr_0.5Mn_0.5)O_3-δ (LSCM), Ba(Zr_0.75Y_0.15)O_3-δ (BZY), and catalysts was applied in hydrocarbon fuels for protonic ceramics fuel cells. LSCM and BZY served as an electronic conductor and a protonic ceramic, respectively. The single phase of LSCM, a promising electronically conductive ceramic, could be obtained by performing calcination when exposed to air and hydrogen reduction at 973 K, which was much lower than the conventional calcination temperature (approximately 1273 K). The LSCM-BZY composite anode was fabricated successfully at such a low temperature using the infiltration method. By testing the composite electrodes at different temperatures, namely 973, 873, 1223, and 1373 K, the effect of the calcination temperature of LSCM on anode performance in hydrogen and methane fuels was successfully investigated. The composite anode with LSCM calcined at 973 and 1073 K showed three-fold improved performance in H₂ fuel and two-fold in CH₄ fuel than that of the composite anode with LSCM calcined at 1373 K.     

PEM fuel cell current regulation by fuel feed control

We demonstrate that the power output from a PEM fuel cell can be directly regulated by limiting the hydrogen feed to the fuel cell. Regulation is accomplished by varying the internal resistance of the membrane-electrode assembly in a self-draining fuel cell with the effluents connected to water reservoirs. The fuel cell functionally operates as a dead-end design where no gas flows out of the cell and water is permitted to flow in and out of the gas flow channel. The variable water level in the flow channel regulates the internal resistance of the fuel cell. The hydrogen and oxygen (or air) feeds are set directly to stoichiometrically match the current, which then control the water level internal to the fuel cell. Standard PID feedback control of the reactant feeds has been incorporated to speed up the system response to changes in load. With dry feeds of hydrogen and oxygen, 100% hydrogen utilization is achieved with 130% stoichiometric feed on the oxygen. When air was substituted for oxygen, 100% hydrogen utilization was achieved with stoichiometric air feed. Current regulation is limited by the size of the fuel cell (which sets a minimum internal impedance), and the dynamic range of the mass flow controllers. This type of regulation could be beneficial for small fuel cell systems where recycling unreacted hydrogen may be impractical.     

Microstructure optimization of fuel electrodes for high-efficiency reversible proton ceramic cells

Reversible protonic ceramic cells (R-PCCs) are efficient energy storage and conversion devices that can operate in two modes, namely, in the fuel cell mode for the conversion of fuel to electricity, and in the electrolysis (EC) mode for the EC of water into hydrogen and oxygen. Fuel electrode is a critical component of fuel-electrode-supported R-PCCs, and its pore structure directly affects the electrochemical performance of the R-PCCs, but it has not been fully studied yet. Herein, the pore structure of Ni–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (Ni–BZCYYb) fuel electrodes was systematically modulated by varying the weight ratio (0, 5, 10, and 15 wt.%) of the pore-former added to Ni–BZCYYb, and the electrochemical performance characteristics in the fuel cell and EC modes were investigated. The cell with 10 wt.% pore-former in the Ni–BZCYYb electrode achieved a remarkable peak power density of 540.7 mW cm⁻² and a high current density of –2.28 A cm⁻² at 1.3 V at 700°C in the fuel cell and EC modes, respectively, and showing excellent durability for over 100 h. These results further highlight the critical role of the microstructure of fuel electrodes, which can be modified to achieve exceptional performance, particularly in EC operations.     

Platinum-based ternary catalysts for low temperature fuel cells: Part II. Electrochemical properties

The development of high performance electrode materials is currently one of the main activities in the field of the low temperature fuel cells, fuelled with H₂/CO or low molecular weight alcohols. A promising way to attain higher catalytic performance is to add a third element to the best binary catalysts actually used as anode and cathode materials. In Part I of this review an overview of the preparation and structural characteristics of Pt-based ternary catalysts was presented. This part of the review deals with the electrochemical properties of these catalysts regarding their CO tolerance and electrocatalytic activity for methanol and ethanol oxidation in the case of anode materials, and their activity for oxygen reduction and stability in fuel cell conditions when used as cathode materials.     

Enhanced degradation of reactive dyes using a novel carbon ceramic electrode based on copper nanoparticles and multiwall carbon nanotubes

A novel carbon ceramic electrode consisting of CuNPs and MWCNT was developed to treat reactive orange 84 (RO84) wastewater using ultrasound-assisted electrochemical degradation. The proposed electrode generated more hydroxyl radicals than non-nanoparticle electrodes did. In addition, a new electrochemical sensor was applied to determine residue RO84 in an aqueous medium during discoloration. This sensor is based on a glassy carbon electrode modified with gold nanourchins and graphene oxide and can detect RO84 concentration in the range of 1.0-1200 μmol·L^-1 with the detection limit of 0.03 μmol·L^-1. The degradation effects of the modified electrode on RO84 were evaluated systematically with different initial pH values, time durations, and amounts of CuNPs and MWCNT. The results suggested that the removal efficiency of RO84 was approximately 83% after 120 min of electrolysis in a phosphate buffer with pH 8.0 using a carbon ceramic electrode made with 4.0 wt% CuNPs and 4.0 wt% MWCNT. The possible mechanism of RO84 degradation was monitored by gas chromatography-mass spectrometry, and degradation pathways were proposed.     

Fine-woven punctured felt preform advanced ceramic composite for long duration hybrid fuel firing testing

A three-dimensional fine-woven punctured felt green body was used as a preform to generate an advanced ceramic. After chemical vapor infiltration and several cycles of precursor infiltration and pyrolysis, the pyrolysis carbon, as well as the silicon carbide and zirconium carbide ceramic matrix were formed. The advanced ceramic composite was tested under a hybrid fuel firing environment (hydrogen peroxide/ hydroxyl-terminated polybutadiene, H₂O₂-HTPB) for a relatively long time (240 seconds) as the puncture fibers head to the firing direction. After the test, beaded and bald fibers could be identified, and the sphere particles of firing residuals sizing from 10 μm to 1 mm were found. The test results show that the material is capable of withstanding a severe firing environment up to 2 MPa with an average linear erosion rate of 0.082 mm/s.     

Effects of Ni content on microstructure,mechanical properties and Inconel 718 cutting performance of AlTiN-Ni nanocomposite coatings

AlTiN-Ni coatings with various Ni contents (0–3?at%) were deposited using cathodic arc evaporation. X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, a nanohardness tester, scratch-adhesion tester, and cutting tester were used to examine the microstructure, mechanical properties, and cutting performance of the coatings. The AlTiN coatings exhibited a columnar structure, while the AlTiN-Ni coatings exhibited a nanocrystal structure due to the formation of nc-AlTiN/Ni nanocomposite coatings. The nanohardness of the AlTiN-Ni coatings decreased from 26.2?GPa to 20.9?GPa as the Ni content increased from 0 to 3?at%. At an Ni content of 1.5?at%, the coating possessed a high toughness and sufficient adhesion strength; however, these dropped drastically for the AlTiN-Ni coating with 3?at% Ni owing to the presence of amorphous Ni. The results for the Inconel 718 turning indicated that the wear mode is adhesion at the rake face, abrasion and adhesion (built-up edge) at the flank face, and chipping at the cutting edge. Compared to AlTiN-Ni₃ and AlTiN-coated tools, the lifetime of the AlTiN-Ni_1.5 coated tool increased to 160% at a cutting speed of 40?m/min. This was attributed to less adhesion at the rake face and chipping at the cutting edge, due to the nanocrystal structure and higher toughness of the AlTiN-Ni_1.5 coating.     

Prediction of macroscopic effective elastic modulus of micro-nano-composite ceramic tool materials based on microstructure model

In order to investigate the effect of microstructure parameters on the elastic modulus, based on the microstructure model represented by Voronoi tessellation, the reasonable macro-micro connection and boundary conditions were proved through the Hill’s lemma, and the uniaxial tensile process of composite ceramic tool materials was simulated. The influences of grain size, second phase volume fraction and nanoparticle volume fraction on elastic properties were studied through numerical simulation, and the elastic modulus of composite ceramic tool materials was predicted. The results showed the elastic modulus of Al₂O₃-based ceramic tool materials could be increased effectively with the contents of the second phase TiB₂ and nano-particle TiC being 20% and 10%, respectively. The numerical simulation results were in good agreement with the experimental results.     

Improvement of Ni-Cermet performance of protonic ceramic fuel cells by catalyst

Generally, the NiO composite anode becomes porous after reduction. To infiltrate additional catalysts such as Pd into the NiO-composite anode before reducing NiO to Ni, a porous NiO composite anode for protonic ceramic fuel cells (PCFCs) was fabricated in this study. The porous NiO composite was fabricated by adding graphite as a pore former along with CuO as a sintering agent. The addition of graphite increased the porosity of the NiO composite anode but resulted in poor sinterability, which was addressed by adding CuO as a sintering agent to the NiO composite anode. The Pd catalyst was added to the NiO-composite anode before reducing NiO to Ni. The composite anode for PCFC with three components, namely Ni, protonic ceramics, and a Pd catalyst, was obtained by reducing NiO to Ni during the measurement. The addition of the Pd catalyst improved the anode performance in methane fuel and hydrogen fuel by enhancing the catalytic activity for the electrochemical reaction on the surface.     

A high-performance intermediate-to-low temperature protonic ceramic fuel cell with in-situ exsolved nickel nanoparticles in the anode

Protonic ceramic fuel cells (PCFCs) show great potential in terms of lowering the operation temperature and overall cost of solid oxide fuel cells based on the high ionic conductivity and low activation energy of proton-conducting electrolytes in intermediate or low temperature environments. However, a significant reduction in anode activity with decreasing temperature hinders the broad application of PCFCs. In this study, a novel anode material Ni–Ba_0.96(Ce_0.66Zr_0.1Y_0.2Ni_0.04)O_3-δ (Ni-BCZNY) with in-situ exsolved Ni nanoparticles is developed. This material exhibits extremely high activity in PCFCs in intermediate and low temperatures. A cell fabricated with this anode material achieves a power density of 912 mW cm⁻² and polarization resistance of 0.04 Ω cm² in wet H₂/air at 700 °C. Additionally, the microstructure, electrochemical performance, electrochemical impedance, and electrode processes of a Ni-BCZNY cell are analyzed in detail. The results indicate that performance enhancements can be attributed to the Ni nanoparticle exsolution promoting charge transfer and hydrogen dissociative adsorption.     

Mass transportation in diethylmethylammonium trifluoromethanesulfonate for fuel cell applications

To use the protonic mesothermal fuel cell without humidification, mass transportation in diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), trifluoromethanesulfuric acid (TfOH)-added [dema][TfO], and phosphoric acid (H₃PO₄)-added [dema][TfO] was investigated by electrochemical measurements. The diffusion coefficient and the solubility of oxygen were ca. 10⁻⁵ cm² s⁻¹ and ca. 10⁻³ M (=mol dm⁻³), respectively. Those of hydrogen were a factor of 10 and one-tenth compared to oxygen, respectively. The permeability, which is a product of the diffusion coefficient and solubility, of oxygen and hydrogen were almost the same for the perfluoroethylenesulfuric acid membrane and the sulfuric acid solution; therefore, these values are suitable for fuel cell applications. On the other hand, a diffusion limiting current was observed for the hydrogen evolution reaction. The current corresponded to ca. 10⁻¹⁰ mol cm⁻¹ s⁻¹ of the permeability, and the diffusion limiting species was the hydrogen carrier species. The TfOH addition enhanced the diffusion limiting current of [dema][TfO], and the H₃PO₄ addition eliminated the diffusion limit. The hydrogen bonds of H₃PO₄ or water-added H₃PO₄ might significantly enhance the transport of the hydrogen carrier species. Therefore, [dema][TfO] based materials are candidates for non-humidified mesothermal fuel cell electrolytes.