Phase and Microstructural Stability of Electrolyte Matrix Materials for Molten Carbonate Fuel Cells

LiAlO₂ powder is used as a material for molten‐carbonate fuel cell (MCFC) matrices. The physical and chemical stabilities of LiAlO₂ powder during MCFC operation determine the performance and lifetimes of the cells. Change to the phase and particle size in the allotropic phase of LiAlO₂ was examined with long‐term stability tests on pure α‐LiAlO₂ matrix, Al‐reinforced α‐LiAlO₂ matrix, Al‐reinforced γ‐LiAlO₂ matrix, aqueous γ‐LiAlO₂ matrix and an α‐/β‐LiAlO₂ mixture powder in molten carbonate at 650 °C in air. In the γ‐LiAlO₂ and α‐/β‐LiAlO₂ mixture, the particle growth was continuous from the early stages of heat‐treatment to 20,000 h. Crystalline phase transformation (γ‐LiAlO₂ and β‐LiAlO₂ to α‐LiAlO₂ and γ‐LiAlO₂, respectively) of these powders and matrices also occurred, and γ‐LiAlO₂ made the third phase like LiAl₅O₈. By contrast, the sizes of these particles and the crystalline phase of α‐LiAlO₂ did not change during immersion tests. These results show that, among α‐/β‐ and γ‐LiAlO₂, α‐LiAlO₂ is the most stable phase in molten carbonate.     

Electrolytic synthesis of TiC/SiC nanocomposites from high titanium slag in molten salt

The molten salt electrolytic method for the preparation of titanium carbide and silicon carbide composites has been subjected to a systematic investigation by experimental analyses and thermodynamic calculations. It has been confirmed that the electrolysis of high titanium slag in the presence of mixed graphite powders generates intermediates CaTiO₃, Ti₂O₃, TiO, Fe₃Si and objective carbonous products TiC/SiC. It has been furthermore found that the deoxidization process depends critically on a number of process parameters, namely, electrolyte composition, graphitic regime, reaction temperature, cell voltage and reaction time. After careful optimization of these parameters, TiC/SiC nanocomposites with particle sizes of 10–174 nm has been produced by electrolysis of high titanium slag and graphite mixtures in molar ratio of 1:2 referred to Ti:C under 3.2 V at 900 °C for 6 h in 1 mol%CaO-CaCl₂-NaCl molten salt and with particle sizes of 12 nm~207 nm in 1 mol%CaO-CaCl₂ electrolyte.     

Absorption enhanced reforming of lignite integrated with molten carbonate fuel cell

A technical-economic assessment of an innovative system which integrates absorption enhanced reforming (AER) of lignite with molten carbonate fuel cell (MCFC) for electricity generation is investigated using the ECLIPSE process simulator.The simulation results show that the proposed system of combining AER with MCFC has the electricity output of 206 kW, with the electrical efficiency of 44.7% (low heating value – LHV) and CO₂ emissions of 751 g/kW h, when fuelled with lignite. The system has a specific investment (SI) of £11 642 and a break even electricity selling price (BESP) 21 p/kW e, compared to the SI of £10 477 and the BESP of 19 p/kW e for the basic case of MCFC fuelled with natural gas.A sensitivity analysis of the break even selling price (BESP) of electricity and the specific investment (SI) versus the capital cost show that capital costs have a significant effect on BESP and SI. Based on the basic case of capital cost of £2 398 000, when the capital cost of the system reduces 50%, the relevant BESP lowers down to 10.8 p/kW e, the SI also reduces by 50%, to £5864/kW e.A sensitivity analysis of fuel cost versus BESP show that the fuel cost has a little effect on BESP. For the basic case of the system with the cost of lignite £20/ton, the BESP is 21.1 p/kW e. While the fuel cost reduces by 50%, to £10/ton, the BESP lower down to 20.9 p/kW e, only reduces 0.2 p/kW e, the change is 0.9%.Although the BESP and SI are high for the AER + MCFC system, there are no nitrogen oxides (NO_x) and sulphur oxides (SO_x) emissions from the system; the CO₂ gas stream produced in the AER process is suitable for subsequent sequestration. Thus the combination system may become a power generation with zero greenhouse gas emissions.     

Carbon coated porous tin peroxide/carbon composite electrode for lithium-ion batteries with excellent electrochemical properties

A porous tin peroxide/carbon (SnO₂/C) composite electrode coated with an amorphous carbon layer is prepared using a facile method. In this electrode, spherical graphite particles act as supporter of electrode framework, and the interspace among particles is filled with porous amorphous carbon derived from decomposition of polyvinylidene fluoride and polyacrylonitrile. SnO₂ nanoparticles are uniformly embedded in the porous amorphous carbon matrix. The pores in amorphous carbon matrix are able to buffer the huge volume expansion of SnO₂ during charge/discharge cycling, and the carbon framework can prevent the SnO₂ particles from pulverization and re-aggregation. The carbon coating layer on the outermost surface of electrode can further prevent porous SnO₂/C electrode from contacting with electrolyte directly. As a result, the repeated formation of solid electrolyte interface is avoided and the cycling stability of electrode is improved. The obtained SnO₂/C electrode presents an initial coulombic efficiency of 77.3% and a reversible capacity of 742 mA h g⁻¹ after 130 cycles at a current density of 100 mA g⁻¹. Furthermore, a reversible capacity of 679 mA h g⁻¹ is obtained at 1 A g⁻¹.     

Thermal batteries with ceramic felt separators – Part 2: Ionic conductivity,electrochemical and mechanical properties

In this Part 2, the ionic conductivity of molten salt electrolytes, the electrochemical properties of single cells containing a ceramic separator infiltrated with an electrolyte, and the mechanical strength of the electrolyte layer are compared with those of the conventional pellet-pressed structure. The ionic conductivity for the molten electrolyte is higher than that of the previous report for both LiCl-KCl and LiF-LiCl-LiBr electrolytes, which is explained by the decrease in contact resistance using a graphite electrode instead of stainless steel. The electrochemical performance of the single cells containing a ceramic felt separator assembled with Li(Si)/FeS₂ electrodes shows longer operating time to a cut off voltage of 1.3 V compared to the conventional MgO-contained single cell. In addition, the flexural strength of the electrolyte layer with the ceramic felt separators is in the range of 2.80–6.29 kgf cm⁻², which is incomparable to that (=0.01 kgf cm⁻²) of the pellet-pressed conventional separator. These findings suggest that the ceramic felt separator can be an alternative to mitigate the current problems of pellet-pressed structure in thermal batteries, enhancing the mechanical strength and electrochemical properties.     

Hierarchically porous carbon by activation of shiitake mushroom for capacitive energy storage

We present a facile yet effective two-step activation method to prepare a hierarchically porous carbon with natural shiitake mushroom as the starting materials. The first step involves the activation of shiitake mushroom with H₃PO₄, while the second step is to further activate the product with KOH. The resulting carbon is comprised of abundant micro-, mesopores and interconnected macropores that has a specific surface area up to 2988 m² g⁻¹ and pore volume of 1.76 cm³ g⁻¹. With the unique porous nature, the carbon exhibited a specific capacitance of 306 and 149 F g⁻¹ in aqueous and organic electrolyte, respectively. Moreover, this carbon also shows a high capacitance retention of 77% at large current density of 30 A g⁻¹ and exhibited an outstanding cycling stability with 95.7% capacitance preservation after 15,000 cycles in 6 M KOH electrolyte. The far superior performance as compared with those of the commercially most used activated carbon RP20 in both aqueous and non-aqueous electrolyte demonstrates its great potential as high-performance supercapacitor electrode. The two-step method developed herein also represents a very attractive approach for scalable production of various functional carbon materials using diverse biomasses as starting materials.     

<Emphasis Type="Italic">In situ</Emphasis> microobservation of the cathode in molten carbonate fuel cells

In molten carbonate fuel cells (MCFC), the wettability of the electrode and the electrolyte distribution are very important factors influencing the active reaction area. We have observed the molten carbonate behaviour directly on the cathode (porous NiO) and the electrolyte plate (LiAlO₂) under various gas conditions and at controlled potentials using an environmental scanning electron microscope (ESEM) equipped with a hot stage. We estimated the liquid electrolyte distribution in the cathode and measured the contact angles on NiO and LiAlO₂ in the electrolyte. Moreover, the electrolyte movement in the reaction CO₂ + O₂ + 2e^– = CO₃ ^2– was observed on the surface of the porous NiO in a CO₂/O₂ atmosphere. The reaction CO₃ ^2– + 2e^– = CO + 2 O^2– of the gas generation was observed in a H₂O atmosphere. The active reaction points on the electrode are the areas where the electrolyte film is thin.     

The catalytic performance of Ni/MgSiO3 catalyst for methane steam reforming in operation of direct internal reforming MCFC

Active and tolerant Ni-based catalyst for methane steam reforming in direct internal reforming molten carbonate fuel cell (DIR-MCFC) was developed. Deactivation of reforming catalysts by alkali metals from electrolyte composed of Li₂CO₃ and K₂CO₃ is one of the major obstacles to be overcome in commercialization of DIR-MCFC. Newly developed Ni/MgSiO₃ reforming catalyst showed activities of ca. 82% methane conversion for 240 min in out-of-cell test. In duration test, the unit cell containing Ni foam impregnated with Ni/MgSiO₃ in anode gas channel did not give performance degradation for more than 2000 h, while the unit cell assembled with Ni/MgSiO₃-coated anode showed a significant performance loss after an operation of 1200 h. Results obtained from X-ray diffraction and Brunauer–Emmett–Teller technique revealed that Ni sintering and support deterioration were decisive factors in decreasing the catalytic activity.     

Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V₂O₅+44 wt% Na₂SO₄ molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.     

Hierarchical carbon nanotube membrane with high packing density and tunable porous structure for high voltage supercapacitors

We reported the fabrication of a hierarchical carbon nanotube (CNT) membrane by using the 90% granulated double- or triple-walled CNTs and 10% 100 μm long multiwalled CNTs as the linker. The membrane with packing density of 420 kg/m³, excellent electrical conductance and good mechanical strength, functioned as both the electrode and current collector and allowed the weight ratio of CNTs increased up to 45–50% based on the weight of CNT, electrolyte and separator. The granulated double or triple walled CNTs, by the aggregation at high temperature etching using CO₂, simultaneously exhibited high surface area and tunable pore structure and high pore volume, and were favorable for the ion transport of organic electrolyte, due to the effect of opening cap or side wall by the CO₂. The CNT membrane electrode, exhibited the capacitance of 57.9 F/g and the energy density of 35 W h/kg, as operated at 4 V.     

Structure and electrochemical performance of highly nanoporous carbons from benzoate–metal complexes by a template carbonization method for supercapacitor application

A series of highly nanoporous carbons have been prepared by converting benzoate–metal complexes, including zinc benzoate, magnesium benzoate and aluminium benzoate through a template carbonization process. The carbonization temperature plays a pivotal role in determining the carbon structures as well as the resultant electrochemical behaviors in supercapacitors. The carbon–Zn-900 sample derived from zinc benzoate complex has a high specific surface area (1466.4 m² g^–1), large pore volume (2.54 cm³ g^–1) and hierarchical pore size distribution. It can also deliver a large specific capacitance of 314.1 F g⁻¹ at a current density of 0.5 A g⁻¹, together with a large energy density of 67.2 Wh kg⁻¹ when measured in a three-electrode system using 6 mol L⁻¹ KOH as electrolyte. Besides, the carbon–Zn-900 sample has been tested in a two-electrode system using [EMIm]BF₄/AN as electrolyte at different operation temperatures of 25/50/80 °C.     

Influence of anode pore forming additives on the densification of supported BaCe0.7Ta0.1Y0.2O3−δ electrolyte membranes based on a solid state reaction

We describe a solid state reaction for the preparation of both NiO–BaCe_0.7Ta_0.1Y_0.2O_3?δ anode substrates and BaCe_0.7Ta_0.1Y_0.2O_3?δ (BCTY10) electrolyte membranes on porous NiO–BCTY10 anode substrates. The amounts of the pore forming additive in the substrates showed a significant influence on the densification of the BCTY10 membranes. After sintering at 1450 °C for 5 h, the BCTY10 membrane on a NiO–BCTY10 anode containing 30 wt.% starch achieved a high density and showed adequate chemical stability against H₂O and CO₂. The chemical stability of BCTY10 was even better than that of BaCe_0.7Zr_0.1Y_0.2O_3?δ. With a mixture of BaCe_0.7Zr_0.1Y_0.2O_3?δ (BZCY7) and La_0.7Sr_0.3FeO_3?δ (LSF) as a cathode, a single fuel cell with 12 μm thick BCTY10 electrolyte generated maximum power densities of 142, 93, 29 mW/cm² at 700, 600 and 500 °C, respectively. The electrolyte resistance and interfacial polarization resistance of the cell under open circuit conditions were also investigated.     

Low-dielectric-constant LiAlO2 ceramics combined with LBSCA glass for LTCC applications

This study used Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) glass to reduce the sintering temperature of LiAlO₂ ceramics and to realise the low dielectric constants (ɛ_r<5) of low-temperature co-fired ceramic (LTCC) materials. LBSCA glass remarkably enhanced the densification of LiAlO₂ ceramics. X-ray diffraction patterns indicated that only the γ-LiAlO₂ phase occurred within the doping range of 1 wt% to 3.5 wt%. Scanning electron microscopy images showed dense and uniform grains in samples with 3.0 wt% LBSCA glass. These samples also exhibited low dielectric constants and low dielectric loss when sintered at 900 °C and 950 °C (i.e., ɛ_r=4.48, Qf=35,540 GHz and τ_f=−53 ppm/°C at 900 °C; ɛ_r=4.50, Qf=38,979 GHz and τ_f=−55 ppm/°C at 950 °C, respectively). The material prepared was chemically compatible with silver and showed potential in applications of high-frequency LTCC microwave substrates.     

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V₂O₅ reaction and melt infiltration in plasma-sprayed 7 wt% Y₂O₃–ZrO₂ (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V₂O₅ with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV₂O₇ (for reactions at 750 °C and below), m-ZrO₂ and YVO₄. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V₂O₅ was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V₂O₅ at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V₂O₅.     

Evaluation of CaO–Al2O3 adhesive bonding properties for β″-Al2O3 solid electrolyte sealing for alkali metal thermal electric converter

The β″-Al₂O₃ solid electrolyte (BASE) tube is an essential component of the alkali metal thermal electric converter (AMTEC) system for inducing the conduction of Na ions and generating electricity. Maintaining gas-tightness to produce Na vapor pressure deference and providing insulation to prevent the loss of generated current from BASE are important factors for the AMTEC system. The Na–sulfur (NAS) battery has a similar driving system and uses glass adhesives, which are not adequate for operation temperatures higher than 800 °C or in a Na atmosphere. In this study, CaO–Al₂O₃ was used as the adhesive to resolve such bonding issues. The bonding strength changes were evaluated as the adhesive bonding process temperature varied and also the results showed that CaO–Al₂O₃ maintained bonding shear strength of 400 MPa for more than 1000 h in a molten Na environment. This study also proposes an experimental technique based on tube-type impedance measurement to assess the bonding between the BASE and the α-Al₂O₃ insulator and to detect Na leakage. After conducting the experiment for 500 h, CaO–Al₂O₃ adhesive can offer higher reliability than glass adhesives.     

Capillary rise properties of porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method to investigate their capillary rise properties. Rayon fibers 16.5 μm in diameter and 800 μm long were used as the pore formers by kneading with alumina powder, kaolin clay, China earthen clay and binder with varying Fe₂O₃ contents of 0, 5 and 7 mass%. The resulting pastes were extruded into cylindrical tubes (outer diameter (OD) 30–50 mm and inner diameter (ID) 20–30 mm), dried at room temperature and fired at 1500 °C for 4 h. The bulk densities of the resulting porous ceramics ranged from 1.31 to 1.67 g/cm³, with apparent porosities of 43.2–59.3%. The pore size distributions measured by Hg porosimetry showed a sharp peak at 10.0 μm in the sample without Fe₂O₃ and at 15.6 μm in the samples containing Fe₂O₃; these pores, which arose from the burnt-out rayon fibers, corresponded to total pore volumes ranging from 0.24 to 0.34 ml/g. SEM showed a microstructure consisting of unidirectionally oriented pores in a porous mullite matrix. Prismatic mullite crystals were well developed on the surfaces of the pore walls owing to the liquid phase formed by the Fe₂O₃ component added to color the samples. The bending strengths of the tubular samples ranged from 15.6 to 26.3 MPa. The height of capillary rise, measured under controlled relative humidities (RH) of 50, 65 and 85%, was greater in the ceramics containing Fe₂O₃ than in those without Fe₂O₃, especially in the thinner samples. The maximum capillary rise reached about 1300 mm, much higher than previously reported. This excellent capillary rise ability is thought to be due to the controlled pore size, pore distribution and pore orientation in these porous mullite ceramics.     

Study on energy consumption of Al2O3 coating prepared by cathode plasma electrolytic deposition

Al₂O₃ coatings with large specific surface were prepared on cast nickel-based superalloy K418 by cathode plasma electrolytic deposition (CPED) in aqueous solutions at different concentrations. The significance of energy consumption and a simple calculation method during CPED were proposed, and the influence of electrolyte concentration on current density-voltage curve, energy consumption, and microstructure of coatings were studied. It was found that increasing the concentration of electrolyte can effectively reduce the current density at the initial stage while prolonging the deposition time and stepping up the energy consumption of whole coating preparation. The morphology observation results showed that the pore size of Al₂O₃ coatings enlarges with the increase of the concentration, and the optimum electrolyte concentration is 0.5–1 mol L^?1. Under this condition, the new method of oxidation pretreatment at 950 ℃ on samples for 30 min can efficiently decrease the current density during the early stage of preparation, which is beneficial to the deposition of complex-shaped samples with large size.     

Microporous carbons finely-tuned by cyclic high-pressure low-temperature oxidation and their use in electrochemical capacitors

Microporous carbons with a finely controlled porosity have been prepared from non-porous chars by cyclic oxidation/thermal desorption and further used in supercapacitor electrodes working in organic medium. The described activation method is shown to be effective for at least two types of non-porous carbons derived from sucrose and cellulose. The low temperature oxidation is realized by H₂O₂ at 200 °C and followed by thermal desorption of the surface functional groups at 900 °C under nitrogen flow. The porosity-forming procedure involves 4–5 oxidation/decomposition cycles, thus allowing a gradual adjustment of average pore size to that of ions making up the standard organic electrolyte ?1 mol L^?1 TEA⁺ BF₄^? in acetonitrile. The build-up of pore volume during the initial cycles proceeds essentially through the opening/formation and deepening of narrow micropores (L₀ ≈ 0.8 nm), whereas a slight pore widening appears to be the main outcome of further cycles. Due to the low burn-off of the overall process, the carbons are shown to form much denser coatings (0.71 g cm^?3) than a steam-activated carbon used in industrial supercapacitors (0.52 g cm^?3).     

The effects of N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide–based electrolyte on the electrochemical performance of high capacity cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2

The effects of ionic liquid (IL) N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Py₁₄TFSI) based electrolyte on the electrochemical performance of cathode material Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ have been investigated. The results of thermogravimetric analysis (TGA), flammability and differential scanning calorimetry (DSC) tests indicate that Py₁₄TFSI addition enhances thermal stability of the electrolyte and reduces the safety concern of Li-ion battery. Electrochemical measurements demonstrate that the cathode material shows good electrochemical performance in Py₁₄TFSI-added electrolyte. The cathode material is able to deliver high initial discharge capacity of 250 mAh g^?1 in electrolyte with Py₁₄TFSI content up to 80% at 0.1 C. In addition, the cathode material delivers less initial irreversible capacity loss and higher initial coulombic efficiency in electrolyte with higher Py₁₄TFSI content. However, increasing Py₁₄TFSI content in the electrolyte affects rate capability of the cathode material distinctively. With 60% Py₁₄TFSI-added electrolyte, Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ shows better cycling stability with a capacity retention of 84.4% after 150 cycles at 1.0 C than that in IL free electrolyte. The superior cycling performance of the cathode material cycled in Py₁₄TFSI-added electrolyte is mainly ascribed to the formation of stable electrode/electrolyte interfaces, based on the results of scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS) and electrochemical impedance spectroscopy (EIS) investigations.     

Fabrication and electrochemical properties of SOFC single cells using porous yttria-stabilized zirconia ceramic support layer coated with Ni

A porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce_0.8Sm_0.2O_1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ cathode was fabricated. The porosity, mechanical strength, and microstructure of porous YSZ ceramics were investigated with respect to the amount of poly(methyl methacrylate) (PMMA) used as a pore former. Porous YSZ ceramics with 56 vol.% PMMA showed a mechanical strength of 24 ± 3 MPa and a porosity of 37 ± 1%. The electrochemical properties of the single cell employing the porous YSZ support layer were measured using hydrogen and methane fuels, respectively. The single cell exhibited maximum power densities of 421 mW/cm² in hydrogen and 399 mW/cm² in methane at 800 °C. Moreover, at a current density of 550 mA/cm², the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 °C.