Carbon-air fuel cell without a reforming process

This paper describes a direct carbon-air fuel cell (DCFC) which uses a molten hydroxide electrolyte. In DCFCs, carbon is electrochemically directly oxidized to generate the power without a reforming process. Despite its compelling cost and performance advantages, the use of molten metal hydroxide electrolytes has been ignored by DCFC researches, primarily due to the potential lack of invariance of the molten hydroxide electrolyte caused by its reaction with carbon dioxide. This paper describes the electrochemistry of a patented medium-temperature DCFC based on molten hydroxide electrolyte, which overcomes the historical carbonate formation.To date, four successive generations of DCFC prototypes have been built and tested to demonstrate the technology, all using graphite rods as their fuel source. These cells all used a simple design in which the cell containers served as the air cathodes and successfully demonstrated delivering more than 40 A with the current density exceeding 250 mA/cm². The cathode is of non-porous structure made of an inexpensive Fe-Ti alloy, and gaseous oxygen is introduced into the cell by bubbling humid air through the electrolyte. Results obtained indicated that the cell operation was under a mixed Ohmic-mass transfer control. Anode and cathode reaction mechanisms are also discussed.     

A study of the Co-coated Ni cathode prepared by electroless deposition for MCFCs

In this study, we made a novel alternative cathode material of molten carbonate fuel cell (MCFC) using electroless deposition of Co on the surface of the Ni powder. Using this manufacturing method, we expect that economical and large-scale cathode can be made easily. The cathode prepared by this method formed LiCo_1−yNi_yO₂ phase in molten (Li_0.62K_0.38)₂CO₃ at 650 °C under CO₂:O₂=66.7:33.3% atmosphere, its solubility is 40% lower than that of NiO cathode. In the unit cell test, the performance of the cell composed of Co-coated Ni cathode was same as that of the cell composed of NiO cathode. Thus the cathode made by Co-coated Ni powder used as an alternative cathode can maintain the advantages of NiO cathode and lengthen the lifetime of MCFC.     

The recovery of chlorine from by-product hydrogen chloride Part 2: Molten metal cathode method

A new method of recovering chlorine from by-product hydrogen chloride is proposed and developed. According to the reaction Me+2HC1 = MeCl₂+H_o (Me = Metal) hydrogen chloride is reduced to give hydrogen and metal chloride. Gaseous hydrogen was drawn out from the reaction system and the metal chloride dissolved in the electrolyte, where it was electrolysed to give chlorine and metal using molten metal as a cathode. The metal was recovered on the cathode in a molten state and reused for the former reaction. Bench scale tests were also carried out, where zinc was used as a molten metal cathode and the cell capacity was about 50 A. The cell voltage was 6.5 V at 50 A (working temperature 560°C, distance between anode and cathode 5 mm) and in this case, the ohmic loss was about 70%. The current efficiency was about 90% (anodic current density 200 A/dm²) when the working temperature was 500°C and electrode distance between anode and cathode was 18 mm.This method seems very promising on the basis of the above-mentioned data.     

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.     

Cobalt and cerium coated Ni powder as a new candidate cathode material for MCFC

The dissolution of nickel oxide cathode in the electrolyte is one of the major technical obstacles to the commercialization of molten carbonate fuel cell (MCFC). To improve the MCFC cathode stability, the alternative cathode material for MCFC was prepared, which was made of Co/Ce-coated on the surface of Ni powder using a polymeric precursor based on the Pechini method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX) were employed in characterization of the alternative cathode materials. The Co/Ce-coated Ni cathode prepared by the tape-casting technique. The solubility of the Co/Ce-coated Ni cathode was about 80% lower when compare to that of pure Ni cathode under CO₂:O₂ (66.7:33.3%) atmosphere at 650 °C. Consequently, the fine Co/Ce-coated Ni powder could be confirmed as a new alternative cathode material for MCFC.     

Fabrication of electrolyte-impregnated cathode by dry casting method for molten carbonate fuel cells

A dry casting method for fabricating a porous Ni plate, which was used as the cathode for molten carbonate fuel cells, was proposed, and the basic characteristics of the as-prepared cathode were examined and compared with those of a conventional cathode fabricated by using the tape casting method. Through several investigations, we confirmed that the cathode fabricated by using the dry casting method has properties identical to those of the conventional cathode. Electrolyte-impregnated cathodes were also successfully fabricated by using the dry casting method. Several characteristics of the as-prepared electrolyte-impregnated cathodes including their electrical performance were investigated by using tests such as the single cell test. The cell performances of a single cell using a 25-wt% electrolyte-impregnated cathode and not the electrolyte-impregnated cathode were 0.867 V and 0.819 V at a current density of 150 mAcm^?2 and 650 °C, respectively. The single cell using a 25-wt% electrolyte-impregnated cathode was also operated stably for 2,000 h. The cell performance was enhanced, and the internal resistance and the charge transfer resistance were reduced after electrolyte impregnation in the cathode. Moreover, the increase in the surface area of the cathode and the further lithiation of the NiO cathode after the electrolyte impregnation in the cathode enhance the area of the three-phase boundary and the electrical conductivity, respectively. However, the cell performance of the single cell using the 35-wt% electrolyte-impregnated cathode was reduced, and the cell could not be operated for a long time because of the rapid increase in the N₂ crossover caused by the poor formation of a wet seal.     

In-situ Raman spectroscopic studies on the oxide species in molten Li/K2CO3

Many concerns have been raised about the mechanism of cathode reaction in molten carbonate fuel cell (MCFC). The chemical behavior of oxide species at cathode in molten carbonate is a key for understanding the process of cathode reactions. In this paper, the variety and role of the oxide species in both bulk and thin-film of basic molten carbonates were investigated by using a novel in-situ Raman spectroscopy. The results indicated that the dominant oxide species under basic conditions was peroxide ion, and it was possible to transform into the oxygen of crystalline lattice during the lithium-doped process. It was demonstrated that in-situ Raman spectroscopic technique was a promising tool to elucidate the mechanism of electrode reaction in molecular level in the MCFC condition.     

Behavior of diffusing elements from an integrated cathode of an electrochemical reduction process

The electrochemical reduction process for spent oxide fuel is operated in a molten salt bath and adopts an integrated cathode in which the oxides to be reduced act as a reactive cathode in the molten salt electrolyte cell. Heat-generating radioisotopes in the spent oxide fuel such as cesium and strontium are dissolved in the molten salt and diffuse from the integrated cathode. However, the behavior of the dissolved cations has not been clarified under an electrochemical reduction condition. In this work, the reduction potentials of cesium, strontium, and barium were measured in a molten LiCl-3 wt% Li₂O salt and their mass transfer behavior was compared with two current conditions on the cell. The concentration changes of the cations in the molten salt phase were measured and no significant differences on the dissolution behavior were found with respect to the current. However, under a continued current condition, the removal of the high heat-generating elements requires more time than the complete reduction of metal oxide due to the slow rate of diffusion.     

Oxygen depolarized electrolysis of sodium chloride by the use of a β-alumina-molten salt system

This paper is concerned with oxygen depolarized electrolysis of NaCl by the use of-alumina solid electrolyte and molten salts. In the electrolysis, dry chlorine gas and pure molten sodium hydroxide are produced from sodium chloride, oxygen and water. Because the oxygen reduction proceeds very smoothly in the molten sodium hydroxide, this process is very promising for the future. The theoretical decomposition voltage of this process is estimated to be 1.5 V, which is lower than that of the process without an oxygen cathode by 1.1 V. The model cell study shows that a terminal voltage of 3 V at 50 A dm^–2 is attainable.     

A molten salt electrolytic process for recovering chlorine and ammonia from ammonium chloride. II

This paper describes the electrolysis in detail and presents the results of bench-scale tests of the molten metal cathode process for recovering chlorine and ammonia from ammonium chloride, reported in the preceding paper (Part I). A zinc chloride electrolyte, 100 A bench-scale electrolytic cell was found to work well. The electrolytic power consumption for 1 kg zinc chloride electrolysis was calculated to be 1120Whkg^–1.     

Rechargeable sodium batteries—VI. Cycling behavior of VS2, ‘VCl3 + nS’ and NbS2Cl2 cathodes in molten NaAlCl4

The electrochemistry of VS₂ in molten NaAlCl₄ has been investigated in relation to its usefulness as a reversible positive electrode for sodium batteries operating in the moderate temperature range of 160–200°C. VS₂ reacted with NaAlCl₄ during early stages of cell cycling, and became VS₂Cl showing a reversible capacity of 2.8 electrons/vanadium. The electrochemistry of mixtures of VCl₃ and S in molten NaAlCl₄ showed similarities to that of VS₂Cl. The VS_xCl_y cathodes obtained via the two different routes exhibited excellent rechargeability as evidenced by the long-term cycling behavior of cells of the configuration, Na_(l)/β″-Al₂O₃/NaAlCl_4(l), cathode material. When NbS₂Cl₂ was used as the cathode material it underwent a structural change in the first cycle to form what appeared to be NbS₂Cl₂, the Nb analog of VS_xCl_y. The structurally modified niobium sulfur chloride cathode exhibited excellent rechargeability.     

High temperature lithium/sulphur batteries: a preliminary investigation of a zeolite—sulphur cathode

A reversible lithium/sulphur high temperature cell is described in which the cathode consists of elemental sulphur absorbed in a zeolite 4A matrix. A lithium—aluminium alloy is used for the anode and the electrolyte is a molten ionic salt consisting of a LiI-KI eutectic (m.p. 260°). The cell was operated at 300° and underwent more than 70 continuous charge/discharge cycles (800 h) without significant loss in efficiency. The coulombic efficiency was more than 90% and the energy density 404 Wh/kg (based on the masses of active electrode materials, viz lithium metal and zeolite 4A—sulphur). The cell showed an open circuit voltage of 1.80 V, a short-circuit cd of about 1.1 A/cm² and a maximum power of about 0.5 W/cm². The internal resistance was 0.54 Ω (cathode surface area ～ 3 cm²).     

Copper(II) formate cathodes for seawater batteries

The preparation of copper(II) formate cathodes, their discharge in magnesium seawater cells, and the discharge reactions are described. This soluble active material could be discharged at high voltages (1.4–1.0 V) with high efficiencies (80–95%) in magnesium cells, when polystyrene solutions were used as the binder to make the cathode plates. For single cells energy densities in the range 70–120 Whkg^–1 (based on the dry state) were obtained. The Cu(HCOO)₂/Mg cell would meet many applications at low or moderate discharge rates as a substitute for the AgCl/Mg cell.     

Electrochemical Performance of Na3V2(PO4)2F3 Electrode Material in a Symmetric Cell

A NASICON-based Na₃V₂(PO₄)₂F₃ (NVPF) cathode material is reported herein as a potential symmetric cell electrode material. The symmetric cell was active from 0 to 3.5 V and showed a capacity of 85 mAh/g at 0.1 C. With cycling, the NVPF symmetric cell showed a very long and stable cycle life, having a capacity retention of 61% after 1000 cycles at 1 C. The diffusion coefficient calculated from cyclic voltammetry (CV) and the galvanostatic intermittent titration technique (GITT) was found to be ~10⁻⁹–10⁻¹¹, suggesting a smooth diffusion of Na⁺ in the NVPF symmetric cell. The electrochemical impedance spectroscopy (EIS) carried out during cycling showed increases in bulk resistance, solid electrolyte interphase (SEI) resistance, and charge transfer resistance with the number of cycles, explaining the origin of capacity fade in the NVPF symmetric cell. Finally, the postmortem analysis of the symmetric cell after 1000 cycles at a 1 C rate indicated that the intercalation/de-intercalation of sodium into/from the host structure occurred without any major structural destabilization in both the cathode and anode. However, there was slight distortion in the cathode structure observed, which resulted in capacity loss of the symmetric cell. The promising electrochemical performance of NVPF in the symmetric cell makes it attractive for developing long-life and cost-effective batteries.     

Lithium-polymer battery based on an ionic liquid-polymer electrolyte composite for room temperature applications

A lithium-polymer battery based on an ionic liquid-polymer electrolyte (IL-PE) composite membrane operating at room temperature is described. Utilizing a polypyrrole coated LiV₃O₈ cathode material, the cell delivers >200 mAh g⁻¹ with respect to the mass of the cathode material. Discharge capacity is slightly higher than those observed for this cathode material in standard aprotic electrolytes; it is thought that this is the result of a lower solubility of the LiV₃O₈ material in the IL-PE composite membrane.     

Novel electrochemical measurements on direct electro-deoxidation of solid TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> in molten calcium chloride medium

A solid metal oxide cathode undergoes significant chemical changes during the molten salt electro-deoxidation process. The changes in the chemical composition lead to changes in the electrical resistivity and potential of the electrode. Two novel electrochemical techniques, based on these two parameters, have been employed to study the electro-deoxidation of solid TiO₂ and ZrO₂ in molten calcium chloride at 900 °C. The in situ resistance measurements carried out by the IR drop method conclusively proved that TiO₂ electrode remains highly conducting throughout the electro-deoxidation process and hence is amenable for reduction. The ZrO₂ electrode, on the other hand, developed very high resistance midway in the electro-deoxidation, and could not be reduced completely. The resistance measurements give strong indication that the electron-transfer reactions taking place at the cathode determine the rate and efficiency of the electro-deoxidation process to a great extent. The low-current galvanostatic electro-deoxidation of TiO₂ electrodes, in conjunction with a graphite pseudo reference electrode to monitor the half cell potentials, showed that the metal oxide passes through two stages during the electrolysis; a high current, low resistant stage 1, where Ca²⁺ ions are inserted to the metal oxide cathode to produce different intermediate compounds and stage 2 where electro-deoxidation of the cathode take place continuously. Removal of oxygen, from the cathode, in stage 1 of the electro-deoxidation is considered to be insignificant. The anodic and cathodic voltages in this stage remained more or less stable at ~1.4 V and ~−1 V, respectively. When the oxygen ions in the melt were depleted at the end of this stage, both the anode and cathode potentials were increased in the anodic direction and this behaviour suggested that the graphite pseudo reference electrode was changed from a C/CO electrode in stage 1 to a Ca²⁺/Ca electrode in stage 2.     

电脱氧法工艺研究用熔盐电解材料

电脱氧法是在熔盐体系中由金属氧化物直接电解制备金属或合金的工艺。本文介绍了电脱氧法及其发展,并对电脱氧法中熔盐电解材料的特性及应用进行了详细阐述和比较。包括:Ti、刚玉、石墨、不锈钢等坩埚材料的选用;阳极材料和阴极材料的应用以及阴极制备方法的改进;CaCl2、K2TiF6等熔盐体系和导线材料的选择。     

Material problems in electrowinning of aluminium by the Hall-Heroult process

An overview is given for the process and constructional materials used in the molten salt electrolysis of aluminium by the Hall-Héroult process. Carbon materials, which constitute the anode, the cathode and the side lining, have been and are still the universal front or hot face materials in the electrolytic cell. A comment is made on the various types of anode and on the attempts to substitute the anode carbon by non-consumable ceramic oxide artefacts. The metallic electrical conductors and their joints play an important role in the design and operation of the cells. In this context particular attention is paid to the properties and behaviour of steel as a material for passing the electrolytic current into the anode and cathode up to temperatures of about 950°C.Paper presented at the meeting on Materials Problems and Material Sciences in Electrochemical Engineering Practice organised by the Working Party on Electrochemical Engineering of the European Federation of Chemical Engineers held at Maastricht, The Netherlands, September 17th and 18th 1987.     

A low temperature NaS battery incorporating A soluble S cathode

A NaS battery having the configuration

is described. This cell, operating just above the melting point of Na, utilizes sodium polysulfides (Na₂S_n) dissolved in an organic solvent as the cathode. Use of this cathode system allows operation at a lower temperature than in the Na/molten Na polysulfide battery. In order to find a suitable solvent for the cathode, solubility and stability data for Na₂S and Na₂S₄ were obtained in 26 organic solvents at 150°C. The solvent meeting the optimum requirements of high boiling point, polysulfide solubility, thermal stability and chemical compatibility with Na₂S_n is N,N-dmethylacetamide (DMAC). Charge—discharge data for cells incorporating an Na₂S₄/DMAC cathode are presented. Discharge to at least S^?2₂ appears possible. On recharge, a long chain, soluble Na polysulfide is formed rather than elemental S. This avoids possible problems due to separation of two phases as in the high temperature NaS battery.     

High-temperature ultrafast welding creating favorable V2O5 and conductive agents interface for high specific energy thermal batteries

The high interfacial resistance between V₂O₅ cathode materials and conductive agents (molten salt and super carbon) is one of the biggest issues that hinder the development of high specific energy thermal batteries. Designing fast Li⁺ and e^– transport channels in cathode electrodes is considered as an effect method to improve electrochemical performance. Hence, a high-temperature ultrafast welding is proposed to reduce V₂O₅/conductive agents interfacial resistance by reconstructing the transmission channels of Li⁺ and e^– in this paper. The experimental studies reveal the optimum ultrafast welding of 700 °C for 10 s, eliminating gap resistance of cathode electrodes induced by the melt of solid molten salt and rebuilding the more plentiful Li⁺ and e^– transport channels, further reducing the contact resistance and gap resistance. Therefore, the electrodes deliver a high specific capacity of 270.69 mAh g^?1 and a high specific energy of 610.60 Wh kg^?1 at 0.1 A cm^?2 and 500 °C with a cut-off voltage of 1.6 V. The high-temperature ultrafast welding provides guidance to build Li⁺ and e^– transport channels of other cathode materials in thermal batteries.