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.     

DIRECT ELECTROCHEMICAL POWER GENERATION FROM CARBON IN FUEL CELLS WITH MOLTEN HYDROXIDE ELECTROLYTE

Historically, despite its compelling cost and performance advantages, the use of a molten metal hydroxide electrolyte has been ignored by direct carbon fuel cell (DCFC) researchers, primarily due to the potential for formation of carbonate salt in the cell. This article describes the electrochemistry of a patented medium-temperature DCFC based on a molten hydroxide electrolyte, which overcomes the historical carbonate formation.

An important technique discovered for significantly reducing carbonate formation in the DCFC is to ensure a high water content of the electrolyte. 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 the ability to deliver more than 40 A with the current density exceeding 250 mA/cm². Conversion efficiency greater than 60% was achieved.     

Predicting chemical reaction equilibria in molten carbonate fuel cells via molecular simulations

It has been recently suggested that hydroxide ions can be formed in the electrolyte of molten carbonate fuel cells when water vapor is present. The hydroxide can replace carbonate in transporting electrons across the electrolyte, thereby reducing the CO₂ separation efficiency of the fuel cell although still producing electricity. In this work, we obtain the equilibrium concentration of hydroxide in five molten alkali carbonate salts from molecular simulations. The results reveal that there can be a substantial amount of hydroxide in the electrolyte at low partial pressures of CO₂ . In addition, we find that the equilibrium concentration of molecular water dissolved in the electrolyte is over two orders of magnitude higher than that of CO₂ . Increasing the size and polarizability (or in other words reducing the “hardness”) of the cations present in the electrolyte can reduce the hydroxide fraction, but at the cost of lowering ionic conductivity.     

Effect of temperature on the electrochemical oxidation of ash free coal and carbon in a direct carbon fuel cell

The present study proposes the application of ash-free coal (AFC) as a primary fuel in a direct carbon fuel cell (DCFC) based on a molten carbonate fuel cell (MCFC). AFC was produced by solvent extraction using microwave irradiation. The influence of AFC-to-carbonate ratio (3: 3, 3: 1, 3: 0 and 1: 3 g/g) on the DCFC performance at different temperatures (650, 750 and 850 oC) was systematically investigated with a coin-type cell. The performance of AFC was also compared with carbon and conventional hydrogen fuels. AFC without carbonate (AFC-to-carbonate ratio=3: 0 g/g) gave a comparable performance to other compositions, indicating that the gasification of AFC readily occurred without a carbonate catalyst at 850 oC. The ease of gasification of AFC led to a much higher performance than for carbon fuel, even at 650 oC, where carbon cannot be gasified with a carbonate catalyst.     

Studies on the modeling of a molten carbonate fuel cell (MCFC) 5 kW class stack

A mathematical model was developed to simulate the performance of a molten carbonate fuel cell (MCFC) 5 kW class stack. In the modeling calculations, the average current densities of each cell were adjusted to be same for all cells in the stack. In this procedure the operating voltages of each cell were decided. Temperatures of matrixes with an electrolyte increased to a maximum value at the 7 ^th cell. Because the temperatures of the 1 ^st and 9 ^th cells were lower than those of the other cells, the operating voltage of these cells was lower than those of the other cells. Compared to the measured temperature distributions, the calculated results were quite low near the gas entrance. The measured data of the temperature of the matrixes with an electrolyte and the power were estimated well with the modeling calculations. The current density distributions in all cells from the model calculations were similar.     

Study on the preparation of activated carbon for direct carbon fuel cell with oak sawdust

Direct carbon fuel cell (DCFC) is a device, which converts chemical energy of carbon into electrical energy through electrochemical oxidisation directly and its performance enormously depends on the characteristics of the fuel used. In this study, oak sawdust is used to prepare the activated carbon for the DCFC, with K₂CO₃ as the activating agent. Nickel catalyst is applied to improve the electrical conductivity, while HNO₃ treatment is used for the purpose of surface modification and ash removal. The performance of the prepared activated carbon in DCFC is evaluated in a self‐built DCFC anode apparatus. The results show that the BET surface area of activated carbon reaches 1240 m²/g under the following conditions: activation temperature, 1173 K; activation time, 2 h; and impregnation ratio, 1. Electrical conductivity is well improved through the nickel catalyst while the amount of surface oxygen functional groups is increased and ash content is decreased through the HNO₃ treatment. When used as the fuel in the DCFC anode, the self‐made activated carbon exhibits predominant performance among all tested carbon fuels, including graphite, activated carbon fibre, etc. © 2011 Canadian Society for Chemical Engineering     

The stability of LiAlO2 powders and electrolyte matrices in molten carbonate fuel cell environment

The stability of LiAlO₂ electrolyte matrix is a key issue for the development of molten carbonate fuel cells (MCFCs). The phase transformation and particle growth of LiAlO₂ particles, observed after a long period of cell operation, is a serious problem and must be overcome in order to attain more than 40,000 h of MCFC life. This process is accompanied by pore size increase of the matrix, leading to a loss of capillary retention for electrolyte in the matrix, causing redistribution of electrolyte and finally resulting in the cross-over of gas. Therefore, efforts have been addressed to obtain a stable matrix with an appropriate pore structure and mechanical strength to provide effective gas-sealing properties without cracks formation during MCFC operation. This review deals on the chemical stability of LiAlO₂ powders in molten carbonates and the structural stability of LiAlO₂ matrices in MCFCs.     

SEPARATION OF MOLYBDENUM SPECIES BY ELECTRODIALYSIS

Lab-scale tests have been carried out in order to assess the possibility of separating molybdenum from aqueous solutions by means of electrodialysis (ED). Mo-containing sodium hydroxide, sulfuric acid, ammonium hydroxide, and hydrochloric acid solutions were tested at 25°C in a five-compartment ED cell. Cell voltages were markedly lower in ammonium hydroxide and chloride solutions, but separation in the latter electrolyte was limited by the low solubility of Mo species. Best results were achieved for Mo separation by ED from an aqueous alkaline solution with 21.8 g L^?1 Mo and 3.4 M ammonium hydroxide and a cell current density of 120 A m^?2. Under these conditions, the Mo transport rate from the working solution was 5.6 mol m^?2 h^?1 and the specific energy consumption for Mo separation was 2.2 kWh kg^?1. These results suggest that this operation should be further studied at pilot scale.     

Effect of grain boundary diffusion on electrolyte stability in direct carbon fuel cells with antimony anodes

Direct carbon fuel cells (DCFC) that employ solid oxide electrolytes and molten Sb anodes are promising for the efficient generation of electricity using a range of carbonaceous fuels. The present study examined the etching of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) electrolytes by Sb₂O₃ produced during fuel-cell operation. Migration of Sb along grain boundaries and electrolyte corrosion were observed for both polycrystalline YSZ and GDC electrolytes; however, corrosion and electrolyte thinning were not observed for a single-crystal YSZ electrolyte, even after long-term operation. These results indicate that Sb migration along grain boundaries plays a significant role in electrolyte corrosion in DCFCs with molten Sb anodes. Several strategies for avoiding this problem are also discussed.     

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.     

Oxidation of ash-free coal from sub-bituminous and bituminous coals in a direct carbon fuel cell

The present study proposes the production of ash-free coal (AFC) and its oxidation as a primary fuel in direct carbon fuel cells (DCFCs). The AFC was produced by the extraction of Arutmin sub-bituminous coal (AFC1) and Berau bituminous coal (AFC2) using polar solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). It was carried out at a temperature of around 202 °C under atmospheric conditions and using a microwave irradiation method. Using NMP as the solvent showed the highest extraction yield, and the values of 23.53% for Arutmin coal and 33.80% for Berau coal were obtained. When NMP was added to DMSO, DMA and DMF, the extraction yield in the solvents was greatly increased. The yield of AFC from a sub-bituminous coal was slightly lower than that from a bituminous coal. The AFC was evaluated in a coin-type DCFC with a mixture of AFC and carbonate electrolyte (3 g/3 g) at 850 °C. The AFC and gaseous H₂ fuels were compared using the electrochemical methods of steady-state polarisation and step chronopotentiometry. The DCFC ran successfully with the AFCs at 850 °C. The open-circuit voltages were about 1.35 V (AFC1) and 1.27 V (AFC2), and the voltages at 150 mA cm^?2 were 0.61 V (AFC1) and 0.74 V (AFC2).     

Proton-conducting membranes from phosphotungstic acid-doped sulfonated polyimide for direct methanol fuel cell applications

A series of novel hybrid proton conducting membranes based on sulfonated naphthalimides and phosphotungstic acid (PTA) were prepared from N-Methyl Pyrrolidone (NMP) solutions. These hybrid organic-inorganic materials, composed of two proton-conducting components, have high ionic conductivities (9.3 × 10^?2 S cm^?1 at 60 °C, 15% PTA), and show good performance in H₂|O₂ polymer electrolyte membrane fuel cells (PEMFC), previously reported by us. Moreover, they have low methanol permeability compared to Nafion^®112. In this paper we describe, for the first time, the behaviour of these hybrid membranes as electrolyte in a direct methanol fuel cell (DMFC). The maximum power densities achieved with PTA doped sulfonated naphthalimide membrane, operating with oxygen and air, were 34.0 and 12.2 mW cm^?2, respectively; about the double and triple higher than those showed by the non-doped membrane at 60 °C.     

Electrolytic Reduction of Spent Nuclear Oxide Fuel as Part of an Integral Process to Separate and Recover Actinides from Fission Products

Abstract

Bench‐scale tests were performed to study an electrolytic reduction process that converts metal oxides in spent nuclear fuel to metal. Crushed spent oxide fuel was loaded into a permeable stainless steel basket and submerged in a molten salt electrolyte of LiCl–1 wt% Li₂O at 650°C. An electrical current was passed through the fuel basket and a submerged platinum wire, effecting the reduction of metal oxides in the fuel and the formation of oxygen gas on the platinum wire surface. Salt and fuel samples were analyzed, and the extent of fission product separation and metal oxide reduction was determined.     

Performance and stability in H2S of SrFe0.75Mo0.25O3-δ as electrode in proton ceramic fuel cells

The H₂S-tolerance of SrFe_0.75Mo_0.25O_3-δ (SFM) electrodes has been investigated in symmetric proton ceramic fuel cells (PCFC) with BaZr_0.8Ce_0.1Y_0.1O_3-δ (BZCY81) electrolyte. The ionic conductivity of the electrolyte under wet reducing conditions was found to be insignificantly affected in the presence of up to 5000 ppm H₂S. The fuel cell exhibited an OCV of about 0.9 V at 700 °C, which dropped to about 0.6 V and 0.4 V upon exposure to 500 and 5000 ppm H₂S, respectively, on the fuel side. Post characterization of the fuel cell revealed significant degradation of the anode in terms of microstructure and chemical composition due to formation of sulfides such as SrS, MoS₂ and Fe₃S₄. Nevertheless, the fuel cell was still functional due to the sufficient electronic conductivity of some of these sulfides.     

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.     

Liquid-phase synthesis of SrCo0.9Nb0.1O3-δ cathode material for proton-conducting solid oxide fuel cells

A novel liquid-phase synthesis strategy is demonstrated for the preparation of the Nb-containing ceramic oxide SrCo_0.9Nb_0.1O_3-δ (SCN). In comparison with the traditional solid-state reaction (SSR) method, the liquid-phase synthesis route offers a couple of advantages, including a lower phase formation temperature and a smaller particle size of the SCN materials that are beneficial for applications as proton-conducting fuel cell cathode. With BaCe_0.4Zr_0.4Y_0.2O_3-δ (BCZY442) as the electrolyte and the SCN synthesized in this work as the cathode, a proton-conducting solid oxide fuel cell (SOFC) shows a peak power density of 348 mW cm^?2 at 700 °C, significantly higher than that of a SOFC fabricated with SCN cathode prepared using the SSR method, which can only deliver 204 mW cm^?2 at the same temperature. Additionally, this new synthesis strategy allows impregnation of Sr²⁺, Co³⁺and Nb⁵⁺ on the solid backbone in aqueous solution, further improving cell performance to reach a peak power density of 488 mW cm^?2 at 700 °C.     

Molten V2O5/Cs0.9K0.9Na0.2S2O7 and V2O5/K2S2O7 catalysts as electrolytesin an electrocatalytic membrane separation device for SO2 removal

Bench scale fuel cell tests have been carried out on the SO₂ oxidation catalyst systems V₂O₅/M₂S₂O₇ (M = alkali) used as electrolytes in a standard molten carbonate fuel cell (MCFC) fuel cell setup for removal of SO₂ from power plant flue gases. Porous Li _x Ni_(1–x)O electrodes were used both as anode and cathode. The cleaning cell removes SO₂ when a potential is applied across the membrane, potentially providing cheap and ecological viable means for regeneration of SO₂ from off-gases into high quality H₂SO₄. Results show that successful removal of up to 80% SO₂ at 450 °C can be achieved at approximately 5 mAcm^–2. However, the data obtained during the experiments explain the current limitations of the process, especially in terms of electrolyte wetting capability and acid/base chemistry of the electrolyte.     

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.     

Oxidative conversion of CH4 on Ni and Ag electrode-catalysts in molten carbonate fuel cell reactor

A fuel cell system using molten carbonates of potassium and lithium as electrolyte was applied to the oxidative conversion of methane over Ni and Ag electrodes. A possibility of cogeneration of valuable chemicals, like C₂-hydrocarbons, and electricity in such a system was demonstrated. With CO ₃ ^2– ions (oxygen) transported electrochemically, the rate of formation of C₂-hydrocarbons and selectivity for them on the Ag electrode were found to be greater than those with oxygen premixed to the gase phase.     

Operation of the Alkaline Direct Formate Fuel Cell in the Absence of Added Hydroxide

The direct formate fuel cell (DFFC) has recently been demonstrated as a viable alkaline direct liquid fuel cell (DLFC) that does not require addition of hydroxide to the fuel stream for operation. In this work, we report that the DFFC can produce significant power at low temperatures without added hydroxide, especially when compared with other alkaline DLFCs powered by alcohols. Using oxygen at the cathode, the DFFC powered by 1 M HCOOK achieves a maximum power density of 106 mW cm^–2 at 50 °C and 64 mW cm^–2 at 23 °C. Using air at the cathode, the same DFFC achieves a maximum power density of 76 mW cm^–2 at 50 °C and 27 mW cm^–2 at 23 °C. These power densities were achieved without addition of hydroxide to the fuel stream. Constant current operation demonstrates that the maximum power density can be maintained at least for several hours of operation. Finally, we use electrochemical analysis to demonstrate that the formate oxidation reaction is not dependent on pH between 9 and 14, which permits the use of formate fuel without added hydroxide in the DFFC. An alkaline DLFC that does not require added hydroxide is promising for safe and practical operation.