Low-temperature ceria-electrolyte solid oxide fuel cells for efficient methanol oxidation

Low temperature anode-supported solid oxide fuel cells with thin films of samarium-doped ceria (SDC) as electrolytes, graded porous Ni-SDC anodes and composite La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF)-SDC cathodes are fabricated and tested with both hydrogen and methanol fuels. Power densities achieved with hydrogen are between 0.56 W cm⁻² at 500 °C and 1.09 W cm⁻² at 600 °C, and with methanol between 0.26 W cm⁻² at 500 °C and 0.82 W cm⁻² at 600 °C. The difference in the cell performance can be attributed to variation in the interfacial polarization resistance due to different fuel oxidation kinetics, e.g., 0.21 Ω cm² for methanol versus 0.10 Ω cm² for hydrogen at 600 °C. Further analysis suggests that the leakage current densities as high as 0.80 A cm⁻² at 600 °C and 0.11 A cm⁻² at 500 °C, resulting from the mixed electronic and ionic conductivity in the SDC electrolyte and thus reducing the fuel efficiency, can nonetheless help remove any carbon deposit and thereby ensure stable and coking-free operation of low temperature SOFCs in methanol fuels.     

Effect of hydrogen on the microstructure and mechanical properties of high temperature deformation of Ti6Al4V additive manufactured

The additive manufactured Ti6Al4V-xH titanium alloy was compressed at 600°C–750 °C on a Gleeble 3800 testing machine, and the compression rates were 1s⁻¹ and 0.01s⁻¹, respectively. The experimental results show that with the increase of hydrogen content, the flow stress of the alloy decreases firstly and then increases gradually. When the hydrogen content is 0.27 wt%, the flow stress of titanium alloy is the smallest. EBSD and TEM analysis were carried out and show that the α lamellar microstructure became larger at 0.27H, the corresponding flow stress also decreased, and slip bands appeared in the alloy. Dislocation slip was an important deformation mechanism of the alloy. When the hydrogen content continued to increase, the α phase in the alloy gradually decreased, and α″ appeared at 0.81H. Therefore, adding appropriate hydrogen can reduce the alloy flow stress and improve the performance of titanium alloy during hot deformation.     

Hydrogen production from glycerol on Ni/Al2O3 catalyst

The growing demand of hydrogen needs renewable sources of raw materials to produce it. Glycerol, by-product of biodiesel synthesis, could be a bio-renewable substrate to obtain hydrogen. A Ni(5.8%)-alumina catalyst was evaluated in the steam reforming of glycerol at 600–700 °C, atmospheric pressure, 16:1 water:glycerol molar ratio, and 3.4–10.0 h⁻¹ WHSV. A glycerol aqueous solution was fed, while a nitrogen stream was co-fed. After 4 h-on-stream, conversion was 96.8% at 600 °C increasing to 99.4% at 700 °C, reaching the largest hydrogen selectivity (99.7%) at 650 °C. After 8 h, conversion decreases more significantly at 600 °C, while the hydrogen selectivity does not significantly change with temperature and increases by decreasing WHSV. After 4 h, the main by-product was methane (76–97%), increasing at higher temperature, followed by ethene, ethane, propene, and propane. At 700 °C and 10.0 h⁻¹ WHSV, the main by-products were ethene (47%) and methane (37%); it could be associated to catalyst deactivation.     

Improvement of hydrogen storage properties of the AB2 Laves phase alloys for automotive application

Hydrogen storage properties of the Ti_1.1CrMn AB₂-type Laves phase alloys, for both low (−30 °C) and high (80 °C) temperature applications, are improved by substituting Zr at Ti site. In agreement with the larger radius of Zr than Ti, the lattice volume of (Ti_1−xZr_x)_1.1CrMn (x=0, 0.05, 0.06 and 0.1) alloys, prepared by arc melting, increases with x. The increase in the Zr content leads to a decrease in the equilibrium hydrogen sorption pressure plateau and faster absorption kinetics, associated with an increase in the hydrogen storage capacity from 1.9 to 2.2 wt% for Ti_1.1CrMn and (Ti_0.9Zr_0.1)_1.1CrMn alloys, respectively. At −5 °C, (Ti_0.9Zr_0.1)_1.1CrMn alloy reversibly absorbs and desorbs 2.2 wt% at 160 bar within 250 s. Based on thermodynamic calculated values, the optimized Zr substituted alloy (Ti_0.9Zr_0.1)_1.1CrMn desorbs hydrogen at 3.2 bar at −30 °C and 135 bar at 80 °C. This is a significant reduction of the sorption pressure plateau as compared with the current technology for mobile applications based on Ti_1.1CrMn alloy with hydrogen desorption plateau above 400 bar at 80 °C. Finally, the mechanism of improved hydrogen storage properties is discussed based on the radius and the hydrogen affinity of the substituting element.     

Electrochemical properties of Cu6Sn5 alloy powders directly prepared by spray pyrolysis

Spherical shape Cu–Sn alloy powders with fine size for lithium secondary battery were directly prepared by spray pyrolysis. The mean size and geometric standard deviation of the Cu–Sn alloy powders prepared at a temperature of 1100 °C were 0.8 μm and 1.2, respectively. The powders prepared at a temperature of 1100 °C with low flow rate of carrier gas as 5 l min⁻¹ had main XRD peaks of Cu₆Sn₅ alloy and copper-rich Cu₃Sn alloy phases. Cu and Sn components were well dispersed inside the submicron-sized alloy powders. The discharge capacities of the Cu₆Sn₅ alloy powders prepared at a flow rate of 5 l min⁻¹ dropped from 485 to 313 mAh g⁻¹ by the 20th cycle at a current density of 0.1 C. On the other hand, the discharge capacities of the Cu–Sn alloy powder prepared at a flow rate of 20 l min⁻¹ dropped from 498 to 169 mAh g⁻¹ by the 20th cycle at a current density of 0.1 C.     

Preparation of supported SrCeO3-based membrane by spin coating method

SrCe_0.9Y_0.1O_3−δ (SCY10) powder with a pure perovskite phase is prepared by solid-state reaction method. NiO is dispersed uniformly in SCY10 powder to fabricate NiO-SCY10 anode substrate. The starting powder, the mixture of SrCO₃, CeO₂ and Y₂O₃, is deposited directly on the green substrate instead of SCY10 powder by spin coating. After co-firing at 1300 °C for 3 h, the starting powder reacts to form SCY10 top layer on the substrate. SEM micrographs show that the top layer is defect-free and adheres well with the anode substrate without any delamination. A single fuel cell is assembled with anode-supported SCY10 membrane as electrolyte membrane and Ag as cathode. The electrochemical property of the fuel cell is tested with hydrogen as fuel in the temperature range of 600-800 °C. The open circuit voltage (OCV) reaches 1.05 V at 800 °C, and the maximum power density is 50 mW cm⁻², 155 mW cm⁻², 200 mW cm⁻² at 600 °C, 700 °C, 800 °C, respectively.     

Characterisation of electrical performance of anode supported micro-tubular solid oxide fuel cell with methane fuel

The reduction and operation of Ni–YSZ anode-supported tubular cells on methane fuel is described. Cells were reduced on pure methane from 650 °C to 850 °C, varying reduction time and methane flow rate. The effect on electrochemical performance with methane fuel was then investigated at 850 °C after which temperature-programmed oxidation (TPO) was employed to measure carbon deposition. Results showed that carbon deposition was minimized after certain reduction conditions. The conclusion was that 30 min reduction at 650 °C with 10 ml min⁻¹ methane reduction flow rate led to the highest current output over 1.2 A cm⁻² at 0.5 V when the cell operated at 850 °C between 10 ml min⁻¹ and 12.5 ml min⁻¹ methane running flow rate. From these results, it is evident that solid oxide fuel cell (SOFC) performance can be substantially improved by optimising preparation, reduction and operating conditions without the need for hydrogen.     

Hydrogenation study of suction-cast Ti40Zr40Ni20 quasicrystal

Suction casting was predicted to be an usable method for improving the hydriding kinetics of Ti/Zr-based icosahedral quasicrystals (IQCs) in our previous work. To further determine it, a suction-cast Ti₄₀Zr₄₀Ni₂₀ IQC alloy was used for hydrogenation studies by Pressure Composition Isotherm (PCI) and Temperature Programmed Desorption (TPD) techniques. The results showed that, this alloy absorbed hydrogen rapidly with obvious hydrogen pressure plateau and some reversibility, however, displayed very limited hydrogen capacity (about 0.7 wt.%) and low equilibrium pressure. After several hydrogenation/dehydrogenation cycles, the IQC structure transformed into two hydride phases, ZrH_2−x and one unknown, both of which decomposed at above 600 °C, suggesting high thermo-stability for them. On the whole, indeed the suction-casting method can increase the hydrogen absorption rate of Ti/Zr-based IQCs, however, the hydrogenation properties of the Ti₄₀Zr₄₀Ni₂₀ IQC alloy still need a mighty advancement.     

Bi0.5Sr0.5MnO3 as cathode material for intermediate-temperature solid oxide fuel cells

Bi_0.5Sr_0.5MnO₃ (BSM), a manganite-based perovskite, has been investigated as a new cathode material for intermediate-temperature solid oxide fuel cells (SOFCs). The average thermal-expansion coefficient of BSM is 14 × 10⁻⁶ K⁻¹, close to that of the typical electrolyte material. Its electrical conductivity is 82-200 S cm⁻¹ over the temperature range of 600-800 °C, and the oxygen ionic conductivity is about 2.0 × 10⁻⁴ S cm⁻¹ at 800 °C. Although the cathodic polarization behavior of BSM is similar to that of lanthanum strontium manganite (LSM), the interfacial polarization resistance of BSM is substantially lower than that of LSM. The cathode polarization resistance of BSM is only 0.4 Ω cm² at 700 °C and it decreases to 0.17 Ω cm² when SDC is added to form a BSM-SDC composite cathode. Peak power densities of single cells using a pure BSM cathode and a BSM-SDC composite electrode are 277 and 349 mW cm² at 600 °C, respectively, which are much higher than those obtained with LSM-based cathode. The high electrochemical performance indicates that BSM can be a promising cathode material for intermediate-temperature SOFCs.     

The influence of melt hydrogenation on Ti600 alloy

In this paper, melt hydrogenation, which is a new hydrogen treatment method, was used to hydrogenate Ti600 alloy, the relationship between the hydrogen partial pressure and the hydrogen content was built by analyzing experimental data, and microstructure was observed and mechanical properties was tested. It was found that hydrogen addition made the thickness of the solidified shell thinner and a big temperature gradient existed from the top to bottom surface of the alloy melt. With increasing hydrogen partial pressure, the directional solidification structure can be formed in the Ti600 alloy ingots. Microstructure of Ti600 alloy was modified significantly and the amount of α′ and β phases after melt hydrogenation. When increasing the content of hydrogen to 7.2 at.%, γ hydride was obtained in Ti600 alloy. The flow stress and yield stress decrease with increasing the hydrogen content.     

Fabrication of micro-tubular solid oxide fuel cells with a single-grain-thick yttria stabilized zirconia electrolyte

This study discusses the fabrication and electrochemical performance of micro-tubular solid oxide fuel cells (SOFCs) with an electrolyte consisting a single-grain-thick yttria stabilized zirconia (YSZ) layer. It is found that a uniform coating of an electrolyte slurry and controlled shrinkage of the supported tube leads to a dense, crack-free, single-grain-thick (less than 1 μm) electrolyte on a porous anode tube. The SOFC has a power density of 0.39 W cm⁻² at an operating temperature as low as 600 °C, with YSZ and nickel/YSZ for the electrolyte and anode, respectively. An examination is made of the effect of hydrogen fuel flow rate and shown that a higher flow rate leads to better cell performance. Hence a YSZ cell can be used for low-temperature SOFC systems below 600 °C, simply by optimizing the cell structure and operating conditions.     

Structural and electrochemical studies of Ba0.6Sr0.4Co1−yTiyO3−δ as a new cathode material for IT-SOFCs

The influence of titanium doping level in Ba_0.6Sr_0.4Co_1−yTi_yO_3−δ (BSCT) oxides on their phase structure, electrical conductivity, thermal expansion coefficient (TEC), and single-cell performance with BSCT cathodes has been investigated. The incorporation of Ti can lead to the phase transition of Ba_0.6Sr_0.4CoO_3−δ from hexagonal to cubic structure. The solid solution limitation of Ti in Ba_0.6Sr_0.4Co_1−yTi_yO_3−δ is 0.15–0.3 under 1100 °C. BSCT shows a small polaron conduction behavior and the electrical conductivity increases steadily in the testing temperature range (300–900 °C), leading to a relatively high conductivity at high temperatures. The electrical conductivity decreases with increasing Ti content. The addition of Ti deteriorates the cathode performance of BSCT slightly but decreases the TEC significantly. The TEC of BSCT is about 14 × 10⁻⁶ K⁻¹, which results in a good physical compatibility of BSCT with Gd_0.2Ce_0.8O_2−δ (GDC) electrolyte. BSCT also shows excellent thermal cyclic stability of electrical conductivity and good chemical stability with GDC. These properties make BSCT a promising cathode candidate for intermediate temperature solid oxide fuel cells (IT-SOFCs).     

Fabrication and evaluation of Ag-impregnated BaCe0.8Sm0.2O2.9 composite cathodes for proton conducting solid oxide fuel cells

Ag-BaCe_0.8Sm_0.2O_2.9 (BCS) composite cathodes are fabricated by an ion impregnation technique in this work, and the effect of fabrication details on their electro-performance is studied. The firing temperature of impregnated Ag has little effect on Ag loading but has a great impact on the polarization resistances. When fired at 400 °C, the minimum polarization resistance for symmetric cells reaches 0.11 Ω cm² measured at 600 °C with an Ag loading of 0.40 mg cm⁻². When fired at 600 °C, the minimum polarization resistance is 29.73 Ω cm² at 600 °C with 0.24 mg cm⁻² Ag-impregnated cathodes due to severe aggregation. The performance of Ag-impregnated cathodes is also compared with that of a Sm_0.5Sr_0.5CoO_3−δ (SSC) impregnated cathode. With the same volume ratio of 57%, the polarization resistance of an Ag-impregnated cathode is only about half of that for a SSC-impregnated cathode. Resistance simulation suggests that the reduction of low frequency resistances is the main reason for the decrease in polarization resistances in Ag-impregnated cathodes, which is consistent with its high oxygen diffusion coefficient. With a 57 vol.% Ag-impregnated cathode fired at 400 °C, the maximum power density of single cells is 283 mW cm⁻² at 600 °C, about 16% larger than that for a 57 vol.% SSC-impregnated cathode.     

Hydrogen production from ethanol over Co/ZnO catalyst in a multi-layered reformer

A Co/ZnO catalyst was prepared by coprecipitation method, and was applied for ethanol steam reforming. The effect of reaction conditions on the ethanol steam reforming performance was studied in the temperature ranges from 400 °C to 600 °C and the space velocity ranges from 10,000 h⁻¹ to 120,000 h⁻¹ in a fixed bed reactor. The Co/ZnO showed high activity with an ethanol conversion of 97% and a H₂ concentration of 73% at a gas hourly space velocity of 40,000 h⁻¹ and a moderately low temperature of 450 °C. EXAFS analysis for fresh and spent samples confirms that Co phase maintains during reaction. The catalyst was then loaded into a multi-layered reformer of which the design concept allows for integrating endothermic steam reforming, exothermic combustion and evaporation in a reactor. The performance of the compact reformer demonstrated that the hydrogen production rate satisfy a PEMFC stack power level of 540 W suitable for portable power supplies.     

Novel polymer electrolytes based on triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO2

A novel polymer electrolyte based on triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO₂ inclusions has been designed. The electrolyte shows favorable features for ion migration such as low glass transition temperature and high concentration of amorphous phase. Combined with the effect of active SiO₂, its ionic conductivity is about 8.0 × 10⁻⁵ S cm⁻¹ at 30 °C, which exceeds that for the PEO-based systems. As applying them to cells with LiFePO₄-type cathodes, a capacity of about 147.0 mAh g⁻¹ is obtained at 60 °C, which is retained by more than 90% after 40 charge/discharge cycles. Moreover, about 100 mAh g⁻¹ could still be delivered as temperature decreases to 30 °C.     

The effect of a Ti-V-based BCC alloy as a catalyst on the hydrogen storage properties of MgH2

The effect of Ti_0.4Cr_0.15Mn_0.15V_0.3 (termed BCC due to the body centered cubic structure) alloy on the hydrogen storage properties of MgH₂ was investigated. It was found that the hydrogenated BCC alloy showed superior catalysis properties compared to the quenched and ingot samples. As an example, the 1 h milled MgH₂ + 20 wt.% hydrogenated BCC shows a peak temperature of dehydrogenation of about 294 °C. This is 16, 27 and 74 °C lower than those of MgH₂ ball milled with quenched BCC, ingot BCC and an uncatalysed MgH₂ sample, respectively. The hydrogenated BCC alloy is much easier to crush into small particles, and embed in MgH₂ aggregates as revealed by X-ray diffraction and scanning electron microscope results. The BCC not only increases the hydrogen atomic diffusivity in the bulk Mg but also promotes the dissociation and recombination of hydrogen. The activation energy, E_a, for the dehydrogenation of the MgH₂/hydrogenated BCC mixture was found to be 71.2 ± 5 kJ mol H₂⁻¹ using the Kissinger method. This represents a significant decrease compared to the pure MgH₂ (179.7 ± 5 kJ mol H₂⁻¹), suggesting that the catalytic effect of the BCC alloy significantly decreases the activation energy of MgH₂ for dehydrogenation by surface activation.     

High performance metal-supported solid oxide fuel cells fabricated by thermal spray

Metal-supported solid oxide fuel cells (SOFCs) have been fabricated and characterized in this work. The cells consist of porous NiO-SDC as anode, thin SDC as electrolyte, and SSCo as cathode on porous stainless steel substrate. The anode and electrolyte layers were consecutively deposited onto porous metal substrate by thermal spray, using standard industrial thermal spray equipment, operated in an open-air atmosphere. The cathode materials were applied to the as-sprayed half-cells by screen-printing and heat-treated at 800 °C for 2 h. The cell components and performance were examined by scanning electron microscopy (SEM), X-ray diffraction, leakage test, ac impedance and electrochemical polarization at temperatures between 500 and 700 °C. The half-inch button cells exhibit a maximum power density in excess of 0.50 W cm⁻² at 600 °C and 0.92 W cm⁻² at 700 °C operated with humidified hydrogen fuel, respectively. The half-inch button cell was run at 0.5 A cm⁻² at 603 °C for 100 h. The cell voltage decreased from 0.701 to 0.698 V, giving a cell degradation rate of 4.3% kh⁻¹. Impedance analysis indicated that the cell degradation included 4.5% contribution from ohmic loss and 1.4% contribution from electrode polarization. The 5 cm × 5 cm cells were also fabricated under the same conditions and showed a maximum power density of 0.26 W cm⁻² at 600 °C and 0.56 W cm⁻² at 700 °C with dry hydrogen as fuel, respectively. The impedance analysis showed that the ohmic resistance of the cells was the major polarization loss for all the cells, while both ohmic and electrode polarizations were significantly increased when the operating temperature decreased from 700 to 500 °C. This work demonstrated the feasibility for the fabrication of metal-supported SOFCs with relatively high performance using industrially available deposition techniques. Further optimization of the metal support, electrode materials and microstructure, and deposition process is ongoing.     

Correlation of electrical and physical properties of photoanode with hydrogen evolution in enzymatic photo-electrochemical cell

In this study, the electrical and physical properties, including the current density, open-circuit voltage, morphology and crystalline structure, of an anodized TiO₂ electrode on a titanium foil are correlated with the hydrogen production rate in an enzymatic photo-electrochemical system. The effect of light intensity at ca. 74 and ca. 146 mW cm⁻² on the properties is also examined. Anodizing (20 V; bath temperature 5 °C; anodizing time 45 min) and subsequent annealing (350–850 °C for 5 h) of the Ti foils in an O₂ atmosphere led to the formation of a tube-shaped, or a compact layered, TiO₂ film on the Ti substrate depending on the annealing temperature. The annealing temperature has a similar effect on the properties of the sample and the hydrogen evolution rate. The generated electrical value, the chronoamperometry (CA), is +13 to −229 and +13 to −247 μA for light intensities of ca. 74 and ca. 146 mW cm⁻², while the corresponding open-circuit voltage (OCV) is in the range of −41 to −687 and −144 to 738 mV, respectively. In the absence of light (dark), the CA is 13–29 μA and the OCV is +258 to −126 mW cm⁻². The trend in the electrical properties for the different samples is well matched with the rate of hydrogen evolution. The samples with higher activities (450, 550, and 650 °C) have similar X-ray diffraction (XRD) patterns, which clearly indicates that the samples showing the highest evolution rate are composed of both anatase and rutile, while those showing a lower evolution rate are made of either anatase or rutile. Increasing the intensity of the irradiated light causes a remarkable enhancement in the rate of hydrogen production from 71 to 153 μmol h⁻¹ cm⁻².     

Chemical kinetics of Ru-catalyzed ammonia borane hydrolysis

Ammonia borane (AB) is a candidate material for on-board hydrogen storage, and hydrolysis is one of the potential processes by which the hydrogen may be released. This paper presents hydrogen generation measurements from the hydrolysis of dilute AB aqueous solutions catalyzed by ruthenium supported on carbon. Reaction kinetics necessary for the design of hydrolysis reactors were derived from the measurements. The hydrolysis had reaction orders greater than zero but less than unity in the temperature range from 16 °C to 55 °C. A Langmuir–Hinshelwood kinetic model was adopted to interpret the data with parameters determined by a non-linear conjugate-gradient minimization algorithm. The ruthenium-catalyzed AB hydrolysis was found to have activation energy of 76 ± 0.1 kJ mol⁻¹ and adsorption energy of −42.3 ± 0.33 kJ mol⁻¹. The observed hydrogen release rates were 843 ml H₂ min⁻¹ (g catalyst)⁻¹ and 8327 ml H₂ min⁻¹ (g catalyst)⁻¹ at 25 °C and 55 °C, respectively. The hydrogen release from AB catalyzed by ruthenium supported on carbon is significantly faster than that catalyzed by cobalt supported on alumina. Finally, the kinetic rate of hydrogen release by AB hydrolysis is much faster than that of hydrogen release by base-stabilized sodium borohydride hydrolysis.     

(Titanium,chromium) nitride coatings for bipolar plate of polymer electrolyte membrane fuel cell

(Titanium, chromium) nitride [(Ti,Cr)N] coatings are synthesized on a 316L stainless-steel substrate by inductively-coupled, plasma-assisted, reactive direct current magnetron sputtering. The chemical and electrical properties of the coating are investigated from the viewpoint of it application to bipolar plates. Nanocrystallized Cr–Ti films are formed in the absence of nitrogen gas, while a hexagonal β-(Ti,Cr)₂N phase is observed at N₂ = 1.2 sccm. Well-crystallized (Ti,Cr)N films are obtained at N₂ > 2.0 sccm. The corrosion resistance of the coating is examined by potentiodynamic and potentiostatic tests in 0.05 M H₂SO₄ + 0.2 ppm HF solution at 80 °C, which simulates the operation conditions of a polymer electrolyte membrane fuel cell. The Davies method is used to measure the interfacial contact resistance between the sample and carbon paper. The (Ti,Cr)N coating exhibits the highest corrosion potential and lowest current density. In a cathode environment, the corrosion potential and current density are 0.33 V (vs. SCE) and <5 × 10⁻⁷ A cm⁻² (at 0.6 V), respectively. In an anode environment the corresponding values are 0.16 V and <−5 × 10⁻⁸ A cm⁻² at −0.1 V. The (Ti,Cr)N coatings exhibit excellent stability during potentiostatic polarization tests in both anode and cathode environments. The interfacial contact resistance decreases with deposition of the (Ti,Cr)N film, and a minimum value of 4.5 mΩ cm² is obtained at a compaction force of 150 N cm⁻², which indicates that the formation of oxide films can be successfully prevented by the (Ti,Cr)N film. Analysis with Auger electron spectroscopy reveals that the oxygen content at the surface decreases with increase in the nitrogen content.