John T. S. Irvine

Disruption of extended defects in solid oxide fuel cell anodes for methane oxidation

Point defects largely govern the electrochemical properties of oxides: at low defect concentrations, conductivity increases with concentration; however, at higher concentrations, defect–defect interactions start to dominate. Thus, in searching for electrochemically active materials for fuel cell anodes, high defect concentration is generally avoided. Here we describe an oxide anode formed from lanthanum-substituted strontium titanate (La-SrTiO3) in which we control the oxygen stoichiometry in order to break down the extended defect intergrowth regions and create phases with considerable disordered oxygen defects. We substitute Ti in these phases with Ga and Mn to induce redox activity and allow more flexible coordination. The material demonstrates impressive fuel cell performance using wet hydrogen at 950 °C. It is also important for fuel cell technology to achieve efficient electrode operation with different hydrocarbon fuels, although such fuels are more demanding than pure hydrogen. The best anode materials to date—Ni-YSZ (yttria-stabilized zirconia) cermets—suffer some disadvantages related to low tolerance to sulphur, carbon build-up when using hydrocarbon fuels (though device modifications and lower temperature operation can avoid this) and volume instability on redox cycling. Our anode material is very active for methane oxidation at high temperatures, with open circuit voltages in excess of 1.2 V. The materials design concept that we use here could lead to devices that enable more-efficient energy extraction from fossil fuels and carbon-neutral fuels.

Synthesis and Characterization of ( La0.75Sr0.25 ) Cr0.5Mn0.5 O 3 − δ , a Redox-Stable, Efficient Perovskite Anode for SOFCs

Perovskite-related materials, (La 0.75 Sr 0.25 ) 1-x Cr 0.5 Mn 0.5 O 3-δ (0 ≤ x ≤ 0.1) (LSCM), have been synthesised and examined as potential anode materials for solid oxide fuel cells (SOFCs). La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3 exhibits a rhombohedral structure. It appears to be chemically compatible with yttria-stabilized zirconia (YSZ) to at least 1300°C. At 900°C, its electrical conductivity is about 38 S/cm in air and 1.5 S/cm in 5% H 2 (p O , 10 -21 atm). Good performance was achieved using LSCM as anode with a polarization resistance 0.9 and 0.47 Ω cm 2 in wet 5% H 2 /Ar and wet H 2 , respectively. The anode polarization was further reduced to 0.6 and 0.25 Ω cm 2 in wet 5% H 2 /Ar and wet H 2 when a thin layer of Ce 0.8 Gd 0.2 O 2-δ (CGO) layer was coated between YSZ and LSCM anode. Stable performance was sustained for at least for 4 h operating in wet methane. By improving the electrode microstructure the electrode polarization resistance approaches 0.2 Ω cm 2 at 900°C in 97% H 2 /3% H 2 O for LSCM containing a small amount of YSZ to improve adherence but without CGO. Very good performance is achieved for methane without using excess steam. Using ambient humidification (i.e., 3% H 2 O), the same performance is achieved with methane at 950°C as for hydrogen at 850°C. The anode is stable in both fuel and air conditions and shows stable electrode performance in methane. Thus, both redox stability and operation in low-steam hydrocarbons have been demonstrated, overcoming two of the major limitations of the current generation of nickel zirconia cermet SOFC anodes. LSCM and other complex perovskites are promising anode materials for SOFCs.

A red metallic oxide photocatalyst

Light absorption across the bandgap in semiconductors is exploited in many important applications such as photovoltaics, light emitting diodes and photocatalytic conversion. Metals differ from semiconductors in that there is no energy gap separating occupied and unoccupied levels; however, it is still possible to excite electrons between bands. This is evidenced by materials with metallic properties that are also strongly coloured. An important question is whether such coloured metals could be used in light harvesting or similar applications. The high conductivity of a metal would preclude sufficient electric field being available to separate photocarriers; however, the high carrier mobility in a metal might also facilitate kinetic charge separation. Here we clearly demonstrate for the first time the use of a red metallic oxide, Sr(1-x)NbO(3) as an effective photocatalyst. The material has been used under visible light to photocatalyse the oxidation of methylene blue and both the oxidation and reduction of water assisted by appropriate sacrificial elements.

Layered oxygen-deficient double perovskite as an efficient and stable anode for direct hydrocarbon solid oxide fuel cells

Different layered perovskite-related oxides are known to exhibit important electronic, magnetic and electrochemical properties. Owing to their excellent mixed-ionic and electronic conductivity and fast oxygen kinetics, cation layered double perovskite oxides such as PrBaCo2O5 in particular have exhibited excellent properties as solid oxide fuel cell oxygen electrodes. Here, we show for the first time that related layered materials can be used as high-performance fuel electrodes. Good redox stability with tolerance to coking and sulphur contamination from hydrocarbon fuels is demonstrated for the layered perovskite anode PrBaMn2O5+δ (PBMO). The PBMO anode is fabricated by in situ annealing of Pr0.5Ba0.5MnO3-δ in fuel conditions and actual fuel cell operation is demonstrated. At 800 °C, layered PBMO shows high electrical conductivity of 8.16 S cm(-1) in 5% H2 and demonstrates peak power densities of 1.7 and 1.3 W cm(-2) at 850 °C using humidified hydrogen and propane fuels, respectively.

A symmetrical solid oxide fuel cell demonstrating redox stable perovskite electrodes

The perovskite (La0.75Sr0.25)Cr0.5Mn0.5O3 (LSCM) is shown to be an effective, redox-stable electrode that can be used for both cathode and anode SOFC operation, to provide a symmetrical fuel cell system with good performance characteristics.

Preparation and characterisation of apatite-type lanthanum silicates by a sol-gel process

Abstract Recent reports have indicated good fast oxide ion conductivity in apatite silicates. In this article we report on the successful low temperature synthesis of the apatite-type lanthanum silicates, La 10 (SiO 4 ) 6 O 3 and La 9.33 (SiO 4 ) 6 O 2, via a sol-gel process. The properties of the resulting apatite phases have been characterised by thermal analysis (TGA-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and conductivity measured by both a.c. impedance spectroscopy (IS) and d.c. methods. The apatite phases may be obtained at 800°C. On further reaction at 1400°C, crystallinity improved, with the apatite structure retained. The room-temperature structure is hexagonal, space group P6 3 or P6 3 /m, with a = 9.722(1), c = 7.182(1)A for La 10 (SiO 4 ) 6 O 3 and a = 9.717(2), c = 7.177(1)A for La 9.33 (SiO 4 ) 6 O 2, i.e., the cell volume of La 10 (SiO 4 ) 6 O 3 is a little greater than that of La 9.33 (SiO 4 ) 6 O 2. Both compositions exhibit high ionic conductivity, although the grain boundary resistance is the dominant feature in the impedance spectrum of both. At 1000K the total conductivity is 10 -3 Scm -1. In general, the conductivity of La 10 (SiO 4 ) 6 O 3 is higher than La 9.33 (SiO 4 ) 6 O 2 indicating that oxygen interstitials may be introduced into the apatite lattice for La 10 (SiO 4 ) 6 O 3, which may benefit the oxygen ion transportation. The a.c. and d.c. conductivitis are comparable for La 10 (SiO 4 ) 6 O 3 but not for La 9.33 (SiO 4 ) 6 O 2.

In situ growth of nanoparticles through control of non-stoichiometry

Surfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields, including renewable energy and catalysis. Typically, these structures are prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO3 stoichiometry. This non-stoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (that is, metallic, oxides or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be influenced strongly by surface reorganization characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application.

Effect of alumina additions upon electrical properties of 8 mol.% yttria-stabilised zirconia

Solid oxide fuel cells (SOFCs) are energy converters that directly transform the chemical energy of combustible gases into electrochemical energy by oxidation. The design of SOFC, which has the highest volumetric power density, is a planar one in which the electrolyte, yttria-stabilised zirconia (YSZ), can be optimised by strengthening with small additions of α-Al2O3. Different commercial powders have different impurity contents and thus show different changes in ionic conductivity when α-Al2O3 is added. We describe the changes in oxide ion conductivity of Tosoh 8 mol.% YSZ that has been added to with α-alumina. AC impedance measurements show that small additions (∼1 wt.%) of Al2O3 can cause the ionic conductivity of Tosoh 8YSZ to increase due to a decrease in the grain boundary impedance which is observable at low to medium temperatures. Small wt.% additions of α-Al2O3 also cause an overall decrease in the high temperature impedance of 8YSZ. We have found that 10 wt.% alumina can be added to 8YSZ without any significant decrease in ionic conducting properties. Further additions of alumina cause a rapid decrease in conductivity due to the large volume percent of insulating alumina phases, which are present, and also due to the cracking of pellets that occurs on firing. We also report the improved stability of added-alumina 8YSZ to hydrothermal ageing. Hydrothermal ageing of unadded to 8YSZ, in an autoclave at 180°C, can lead to a decrease in the conductivity at 1000°C by as much as 40%. The drop in conductivity of 8YSZ can be limited to a decrease of only 10% by the addition of greater than 5 wt.% Al2O3.

Novel tin oxide-based anodes for Li-ion batteries

Abstract Three new possible Li-ion battery negative electrode materials, ZnO, ZnO:SnO 2 ball-milled mixture and Zn 2 SnO 4 , were prepared, and tested electrochemically. These new materials have smaller capacities than SnO 2 , but still show reversible capacities around 500 mA h g −1 . The charge and discharge profiles of all the materials are similar, with potentials close to that of the Li + /Li(m) couple. A large loss of capacity between the initial and the later cycles, is observed similar to the tin oxides, due to the required reduction of the tin and zinc ions to the bulk metal. These materials do show some promise as Li-ion battery anodes due to their large reversible lithium capacity at low potentials.

Synthesis of ammonia directly from air and water at ambient temperature and pressure

The N≡N bond (225 kcal mol−1) in dinitrogen is one of the strongest bonds in chemistry therefore artificial synthesis of ammonia under mild conditions is a significant challenge. Based on current knowledge, only bacteria and some plants can synthesise ammonia from air and water at ambient temperature and pressure. Here, for the first time, we report artificial ammonia synthesis bypassing N2 separation and H2 production stages. A maximum ammonia production rate of 1.14 × 10−5 mol m−2 s−1 has been achieved when a voltage of 1.6 V was applied. Potentially this can provide an alternative route for the mass production of the basic chemical ammonia under mild conditions. Considering climate change and the depletion of fossil fuels used for synthesis of ammonia by conventional methods, this is a renewable and sustainable chemical synthesis process for future.