Carbonate Fuel Cell Research Articles

No.2-14 直接炭素燃料電池 (DCFC) のスケールアップのための発電特性の検討

No.2-14 直接炭素燃料電池 (DCFC) のスケールアップのための発電特性の検討

Performance Analysis with Various Amounts of Electrolyte in a Molten Carbonate Fuel Cell

Performance Analysis with Various Amounts of Electrolyte in a Molten Carbonate Fuel Cell

Performance Analysis with Various Amounts of Electrolyte in a Molten Carbonate Fuel Cell

The effect of initial electrolyte loading (IEL) on cell performance in a coin-type molten carbonate fuel cell (MCFC) was investigated in this work. Since the material of MCFC depends on the manufacturer, optimisation requires experimental investigation. In total, four IEL values, 1.5, 2.0, 3.0, and 4.0 g, were used, corresponding to a pore filling ratio (PFR) of 38, 51, 77, and 102%, respectively. The cell performance with respect to the PFR was analysed via steady-state polarisation, step-chronopotentiomtery, and impedance methods. The electrochemical analyses revealed that internal resistance and overpotential of the cell decreased with increasing PFR, and a large overpotential was observed when the PFR was 102%, probably due to the flooding phenomenon. After operation, cross-section of the cell was analysed via surface analysis of SEM and EDS methods, and the remaining electrolyte was estimated by dissolution of the cell in 10 wt% acetic acid. A linear relationship between IEL and the weight reduction ratio by dissolution was obtained. Thus, the remaining amount of electrolyte could be measured after operation. The results of SEM and EDS showed that a PFR of 38 and 102% showed a lack and flooding of electrolytes at the cell, respectively, which led to a large overpotential. This work reports that MCFC performance is allowed only in the narrow range of PFR. Keywords: Molten carbonate fuel cell, Pore filling ratio, Electrolyte distribution, Overpotential, Post analysis

Spontaneous Hydrogen and Electricity Production in a Nested Carbon Fuel Cell

Introduction Hydrogen is a desirable fuel because it is an effective energy carrier and because it has minimal impact on the environment. However, there are no natural reservoirs of hydrogen, and the majority of hydrogen currently used needs to be produced either from steam reforming of hydrocarbons or electrolysis of water. Both of these methods are energy intensive and have significant drawbacks. Hydrogen from steam reforming of hydrocarbons contains trace amounts of CO that acts as a poison for the Pt catalyst in PEM fuel cells, and hence limits the usability of the hydrogen produced. Similarly, electrolysis of water is an electrochemically uphill process, which requires significant energy input. The steam-carbon-air fuel cell (SCAFC) concept is a viable alternative that is capable of producing carbon-free hydrogen and electricity spontaneously and at high primary efficiency. The SCAFC couples an air-carbon fuel cell [1] with a steam-carbon fuel cell [2] through a shared carbon bed at the anode, as shown in Figure 1. Both cells employ an ionically conducting and electrically insulating ceramic electrolyte membrane for selective transport of oxide ions from the cathode to the anode. While both the air-carbon and the steam-carbon cells are spontaneous (i.e., downhill in electrochemical potential), the steam-carbon cell is endothermic, while the air-carbon is highly exothermic. By coupling the air-carbon with the steam-carbon cell, the SCAFC achieves a novel device that produces both hydrogen and electricity spontaneously without the need for external heat or power input. This presentation builds upon modeling and experimental studies of both types of carbon fuel cells conducted in our laboratory [1-3] and describes how this novel idea can achieve efficient and clean production of hydrogen from solid carbonaceous fuels. Modeling of the Steam-Carbon-Air Fuel Cell A model of a SCAFC has been developed that takes into account reaction chemistry in the carbon bed, electrochemistry at the electrodes, mass transport of gases, and heat transfer. In the anode chamber, the carbonaceous fuel undergoes dry gasification via the reverse Boudouard reaction, forming CO that gets oxidized at the anode surfaces. The CO2 formed on the anode surfaces can then once again gasify the adjacent carbon, thus forming a self-sustained “CO shuttle” mechanism [4]. Practical carbonaceous fuels not only contain carbon, but also small amounts of hydrogen, which when released, will lead to an analogous “H2 shuttle” mechanism. Hydrogen gets oxidized at the anode surface forming steam that will then diffuse back to the carbon bed and react to form more CO and H2. How hydrogen in the fuel stock affects the fuel cell operation and performance will be discussed. A detailed mechanism for dry and wet gasification of the carbon bed has previously been developed in the lab and is used in the model [5]. The kinetic parameters needed to describe the half-cell reactions are derived experimentally. Electrochemical impedance spectroscopy (EIS) measurements are taken as a function of temperature, gas composition and cell voltage. The EIS data is used to extract polarization resistances for each half-cell reaction and then fitted to a detailed electrochemical reaction mechanism. Mass transport is important in determining the local gas composition, which affect carbon bed and electrode kinetics, as well as to determine fuel utilization. Any CO or H2 in the exhaust is fuel that is not fully oxidized, and is still capable of undergoing the oxidation reaction to produce electricity in the cell. Thus, fuel utilization improves with an increase in the ratio of (CO2 + H2O)/(CO+H2) in the exit gas, and this influences the overall efficiency of the cell. Since both the Boudouard gasification and the half-cell reactions are temperature dependent, it is important to understand the coupled relationship between reaction rates, heat release, and local temperature. Thus, consideration of heat transfer effects is necessary to develop a better understanding of the cell operation. The model is used to map out the operating space for hydrogen production, power output and cell efficiency. Furthermore, geometrical parameters are tuned to minimize the mole fraction of CO and H2in the exhaust and maximize the overall efficiency. Optimal operating conditions are identified for maintaining high efficiencies while also achieving realistic hydrogen production rates.

Fabrication and Characterization of LSM/YSZ/GDC Tri-Composite Cathode-Supported Tubular Solid Oxide Direct Carbon Fuel Cell

The purpose of this study was to fabricate a porous and high mechanical strength cathode support for solid oxide direct carbon fuel cells. Therefore, the effect of Gd0.1Ce0.9O2-δ (GDC) addition on the phase stability, sintering behavior, thermal expansion, porosity, electrical conductivity, and mechanical strength of La0.65Sr0.3MnO3-δ/(Y2O3)0.08(ZrO2)0.92 (LSM/YSZ) composite was evaluated. The sintering temperature and the porosity of LSM/YSZ composite were observed to increase with an increase in the amount of GDC, on the contrary, the electrical conductivity and the mechanical strength was decreased. An LSM/YSZ/GDC tri-composite with optimized properties was selected to fabricate the tubular cathode-supported SOFCs (LSM/YSZ/GDC|YSZ|NiO/YSZ) through extrusion and co-firing. A special chamber was designed for the DCFC operation of the tubular cell. Electrochemical characterization was done by measuring the polarization curves and electrochemical impedance spectroscopy using dry hydrogen or the syngas produced by in situ steam gasification of carbon black.

Effect of gas composition produced by gasification, on the performance and durability of molten carbonate fuel cell (MCFC)

Utilization of biomass using gasification technology combined with Molten Carbonate Fuel Cell (MCFC), is an option to solve fossil fuels and environmental pollution problem. Various types of biomass give different composition of the gas produced by gasification process and indirectly affect the performance and durability of MCFC.This research is focused on determination of gas composition produced by gasification process (CO, CO2, H2 and N2) that gives optimum performance and durability of MCFC either in dry or wet condition. Using mixture design method, the gas composition supplied to MCFC anode is varied in the range of 14%–25% for CO, 9%–17% for H2, 10%–20% for CO2, and 51%–58% for N2. The effect of feed gas composition on the performance and durability of MCFC is characterized by using potentiodynamic method and accelerated load cycle method respectively. SEM and XRD is used in this study to evaluate gas composition effect on MCFC anode morphology.Based on experiments that are carried out, the optimum result is obtained by using syn-gas from coconut shell gasification with composition of 25% CO, 12% H2, 10% CO2 and 53% N2 in wet condition. Electrical power of 0.13 W is generated within 228 days of durability or it is decreased by 135 mV/1000 h or 16% (v/v)/1000 h.

Anodic Reaction Mechanisms in Direct Carbon Fuel Cells Using Carbon Particles Dispersed in Molten Carbonate

Highly efficient technologies for the conversion of solid fuels, such as coal and biomass, to electricity are required for clean energy production and sustainable energy supply. Direct carbon fuel cells (DCFCs), which convert the chemical energy in solid carbon directly into electricity without gasification, have attracted much attention because of their theoretical ability to achieve 100 % efficiency. However, solid carbon particles have disadvantages in terms of fluidity when compared to gaseous fuels. Thus, carbon particles are dispersed into molten carbonate to form a carbon/carbonate slurry in which the carbon particles can move. Then, the anode is inserted into the carbon/carbonate slurry for discharge in the DCFC [1-5]. Carbon dispersed into the molten carbonate is easily transported to the anode, leading to an improvement of the formation of triple phase boundary (carbon, electrolyte, and anode). During discharge, carbon is supposed to be consumed through the complete electrochemical oxidation (C+2CO3 2-→3CO2+4e- (R1)) which releases four electrons per carbon. However, several studies have suggested a possible formation of CO from partial electrochemical oxidation (C+1/2CO3 2-→3/2CO+e- (R2)) which releases one electron per carbon, or even from the Boudouard reaction (C+CO2→2CO (R3)) where CO2 produced during discharge reacts with carbon particles [3,6]. Although the partial oxidation (R2) and the Boudouard reaction (R3) reduce the DCFC efficiency, little effort has been made to study the reaction mechanisms in the DCFC. The aim of this study is to investigate the anodic reaction mechanisms in the DCFC using carbon/carbonate slurry experimentally and theoretically. Figure shows setup of the DCFC using carbon/carbonate slurry stirred by gas bubbling. The working electrode (WE), counter electrode (CE), and reference electrode (RE) were made from gold sheets. Cell discharge was controlled with a potentio/galvano-stat. The reactor was heated using an electric furnace to 1073 K. Activated carbon particle with an average diameter of 44.1 μm was used as fuel. Ternary molten carbonate (Li2CO3/Na2CO3/K2CO3 (= 16.9/25/58.1 mol%)) was used as electrolyte. The carbon content in the carbonate was set to 1.0 wt% or 2.0 wt%. The carbon/carbonate slurry was contained in the porous alumina tube having an average pore diameter of 120 nm. This allowed the separation between the carbon/carbonate slurry and the molten carbonate electrolyte. In the setup, the carbon/carbonate slurry was able to be stirred by gas bubbling. CO and CO2 concentrations in off-gases were measured by FID with methanizer, and CO and CO2 production rates were determined at the open circuit mode or constant discharge mode at 20 mA/cm2. Moreover, to study the impact of Boudouard reaction (R3) on CO formation, CO2 bubbling with a flow rate of 1.0 ml/min, which was approximately 5 times of theoretical CO2 produced during discharge at 20 mA/cm2, was used in the carbon/carbonate slurry at the open circuit condition, and CO concentration in off gases was measured. As a result, an amount of CO was formed when the carbon/carbonate slurry was not stirred by Ar gas bubbling during the constant discharge at both the carbon contents. At the open circuit condition, CO was not formed by CO2 bubbling (1.0 ml/min) simulated CO2 produced during discharge. This indicated that the impact of Boudouard reaction (R3) was insignificant in the DCFC. Moreover, measured CO and CO2 production rates were well balanced on the calculation based on two electrochemical oxidations (R1 and R2). And, the partial electrochemical oxidation accounted for approximately 80 % in the carbon consumption at the carbon content of 2.0 wt%, whereas that accounted for approximately 40 % at the carbon content of 1.0 wt%. It was shown that the ratio of partial electrochemical oxidation depended on the carbon concentration near the anode. On the other hand, when the carbon/carbonate slurry was stirred by Ar bubbling with a flow rate of 50 ml/min, only CO2 was formed and the production rate of CO2 corresponded to the theoretical rate based on R1 at both the carbon contents. This indicated the carbon particles were consumed only by the complete electrochemical oxidation (R1). It was shown that stirring the carbon/carbonate slurry was effective to enhance the complete electrochemical oxidation. Reference [1] N. J. Cherepy et al., J. Electrochem. Soc., 152 (2005) A80-A87. [2] X. Li et al., Ind. Eng. Chem. Res., 47 (2008) 9670-9677. [3] H. Watanabe et al., J. Power Sources, 273 (2015) 340-350. [4] H. Watanabe et al., Energy and Fuels, 30 (2016) 1835-1840 [5] H. Watanabe et al., J. Chem. Eng. Jpn., 49 (2016) 237-242 [6] T.M. Gür, J. Electrochem. Soc., 157 (2010) B751-B759 Figure 1

Anodic Reaction Mechanisms in Direct Carbon Fuel Cells Using Carbon Particles Dispersed in Molten Carbonate

This study aims to investigate the anodic reaction mechanisms in direct carbon fuel cell using a carbon/carbonate slurry stirred by Ar bubbling. CO2 and CO production rates were measured during the constant discharge. Gas products were calculated based on complete (C+2CO3 2-→3CO2+4e-) and partial electrochemical oxidation (C+1/2CO3 2-→3/2CO+e-). The impact of Boudouard reaction (C+CO2→2CO) on CO production was also discussed. As a result, an amount of CO was formed without Ar bubbling, whereas CO was not formed with Ar bubbling. The impact of CO2 bubbling on CO formation was insignificant, and calculations of gas products almost agreed with experiments of those. These indicated that CO was formed through not Boudouard reaction but the partial electrochemical oxidation. Moreover, partial electrochemical oxidation helped to explain the trends of open circuit voltages. It was shown that the anodic reactions consisted of complete and partial electrochemical oxidations.

Direct Carbon Fuel Cells – Wetting behavior of graphitic carbon in molten carbonate

The wetting behavior of graphitic carbon in a eutectic melt of LiK molten carbonate was investigated at two different temperatures (650 and 750 °C) under two different CO2 partial pressures. The development of the meniscus and the formation of a super-meniscus film were followed optically over a period of 24 h after partial immersion of the graphite rods. Post-test EDX analyses of the graphite surfaces exposed to the melt, above and below the meniscus, were compared to those of surfaces not exposed to the melt. The most important feature found in the EDX analyses is the non-uniform atomic distribution of oxygen both along the length of the immersed surface and in the radial cross-section of the rods exposed to the melt. In addition, the effect of pre-test exposure to gas atmospheres at various temperatures and CO2 partial pressures was investigated. The wetting behavior is found to be influenced strongly by reactions occurring at the carbon surface before and after the start of wetting. In particular, CO produced by the reverse Boudouard reaction, which appears to be physisorbed, and chemically adsorbed oxide ion, O2−, are inferred to play a dominant role in the development of the meniscus leading to a super-meniscus film on the graphitic surface. Several reaction mechanisms are examined to explain the results observed in this work, which agree with results of earlier work from this laboratory, in terms of CO and oxide ion formation.

Determination of Optimum Production Conditions of Casting Slurry in the Manufacture of Molten Carbonate Fuel Cell Electrodes with the Tape Casting Technique

In this study, Ni anode and LiAlO2 electrolyte green sheets were manufactured for MCFC system. Experimental scale production of green sheets was carried out by using tape casting technique. The method of casting slurry, which is the first stage of preparation, has a direct impact on the final product. Therefore, cast slurries with different ratios of a solvent, a binder, a dispersant, plasticizer loading to the organic compounds were prepared to produce green sheets with the technique of tape casting in order to investigate the viscosity, shear stress, thixotropy and flow characteristics. After sheets were dried, physical properties (mechanical tests) were determined. Mixing method (balling mill and mechanical stirrer) and mixing duration of slurry, mixing weight ratio of binder, solvent, plasticizer, dispersant, Li/K carbonate, Ni, and LiAlO2 powder in the slurry was examined.

Performance assessment of a hybrid system integrating a molten carbonate fuel cell and a thermoelectric generator

A hybrid system consisting of an MCFC (Molten Carbonate Fuel Cell), a TEG (Thermoelectric Generator) and a regenerator is proposed. In this system, the MCFC produces electricity and heat from fuels and the TEG utilizes the heat for additional power generation. A numerical model is developed to evaluate the performance of the proposed system. The relationship between the operating current density of the MCFC and the dimensionless current of the TEG is theoretically derived and the operating current density region of the MCFC that allows the TEG to function is determined. Numerical expressions of the power output and efficiency for the hybrid system are specified under different operating conditions. The general performance characteristics and optimum criteria for the hybrid system are revealed. It is found that the hybrid system is superior to the stand-alone MCFC system as the bottoming TEG can effectively increase the maximum power density. In addition, the effects of the operating current density, operating temperature, operating pressure, heat conductivity, integrated parameters and dimensional figure of merit on the performance characteristics of the hybrid system are investigated. The results obtained are useful for the design and optimization of novel MCFC system to achieve a better performance.

A first combined electrochemical and modelling strategy on composite carbonate/oxide electrolytes for hybrid fuel cells

Carbonate/oxide composites are very promising electrolyte materials in hybrid fuel cells which could operate at lower temperature than the usual Molten Carbonate Fuel Cells (MCFC) or Solid Oxide Fuel Cells (SOFC), presenting 0.1 S cm−1 at 600 °C. This paper shows some significant and/or unexpected experimental data obtained by impedance spectroscopy, related to the impact of the carbonate/oxide ratios and the heating and cooling cycles in reducing atmospheres on the electrical behaviour of the composites. Based on this peculiar experimental behaviour, combined DFT calculations are selected for acquiring a deeper understanding of the transport mechanisms in such materials. A modelling strategy, in parallel with experimental results, is developed in the present paper.

Characteristics of Solid Fuel Oxidation in a Molten Carbonate Fuel Cell

Characteristics of Solid Fuel Oxidation in a Molten Carbonate Fuel Cell

Analysis of Vitrification Processes and Characteristics of Geochang Granite Stone Sludge for Desulfurization Sorbent of MCFC

The vitrification processes and characteristics of Geochang granite stone sludge were investigated for the application of desulfurization sorbent of Molten Carbonate Fuel Cell (MCFC). From the X-ray fluorescence analysis (XRF) results using Geochang granite stone sludge, it was confirmed that the network formers (SiO₂ and Al₂O3) in granite stone sludge were present between 83.2 and 87.2 wt% and the rest was comprised of network modifiers (K₂O, Na₂O, CaO, Fe₂O3 and MgO). When Borax (Na₂B₄OSUB7/SUB-10H₂O) was added in the granite stone sludge system for promoting the decomposition and reducing the melting viscosity of refractory SiO₂ and Al2O3 comprising of stone sludge, it was observed that the melting point of the granite stone sludge system was reduced by more than 400°C and the optimized melting temperature for granite stone and borax systems was around 1400°C when comparing X-ray diffraction analysis (XRD) results. Phase transitions and thermal reactions of granite stone sludge systems were also investigated from thermal analysis using Thermogravimetric Analysis (TGA)/ Differential Scanning Calorimeter (DSC).

Experimental investigation of SO2 poisoning in a Molten Carbonate Fuel Cell operating in CCS configuration

One of the most interesting innovations in the CCS (Carbon Capture and Storage) field is the use of MCFCs as carbon dioxide concentrators, feeding their cathode side (or air side) with the exhaust gas of a traditional power plant. The feasibility of this kind of application depends on the resistance of the MCFC to air-side contaminants, with particular attention to SO2. The aim of this work is to investigate the effects of poisoning when sulphur dioxide is added to the cathodic stream in various concentrations and in different operating conditions. This study was carried out operating single cells (80 cm2) with a cathodic feeding composition simulating typical flue gas conditions, i.e. N2, H2O, O2 and CO2 in 73:9:12:6 mol ratio as reference mixture. On the anodic side a base composition was chosen with H2, CO2 and H2O in 64:16:20 mol ratio. Starting from these reference mixtures, the effect of single species on cell poisoning was experimentally investigated considering, as main parameters chosen for the sensitivity analysis, SO2 (0–24 ppm) and CO2 (4–12%) content in the cathodic feeding mixture, H2 (40–64%) content in the anodic stream as well as the operating temperature (620–680 °C). Results showed that degradation caused by SO2 poisoning is strongly affected by the operating conditions. Data gathered during this experimental campaign will be used in a future work to model the poisoning mechanisms through the definition of MCFC electrochemical kinetics which take into account the SO2 effects.

Characteristics of Solid Fuel Oxidation in a Molten Carbonate Fuel Cell

Characteristics of Solid Fuel Oxidation in a Molten Carbonate Fuel Cell

Performance and Durability of the Molten Carbonate Electrolysis Cell and the Reversible Molten Carbonate Fuel Cell

The molten carbonate electrolysis cell (MCEC) provides the opportunity for producing fuel gases, e.g., hydrogen or syngas, in an environmentally friendly way, especially when in combination with renewable electricity resources such as solar, wind, and/or hydropower. The evaluation of the performance and durability of the molten carbonate cell is a key for developing the electrolysis technology. In this study, we report that the electrochemical performance of the cell and electrodes somewhat decreases during the long-term test of the MCEC. The degradation is not permanent, though, and the cell performance could be partially recovered. Since conventional fuel cell materials consisting of nickel-based porous catalysts and carbonate electrolyte are used in the MCEC durability test, it is also shown that the cell can alternatingly operate as an electrolysis cell for fuel gas production and as a fuel cell for electricity generation, i.e., as a so-called reversible molten carbonate fuel cell (RMCFC). This study reveals that the cell performance improves after a long period of RMCFC operation. The stability and durability of the cell in long-term tests evidence the feasibility of the electrolysis and reversible operations in carbonate melts using a conventional fuel cell setup, at least in lab scale.

A thermo-economic analysis of repowering of a 250 MW coal fired power plant through integration of Molten Carbonate Fuel Cell with carbon capture

This paper presents a thermo-economic assessment of a repowering configuration of a 250MW old existing coal fired power plant through integration of Molten Carbonate Fuel Cells (MCFC) at the downstream of the existing boiler. Hydrogen-rich syngas generated from natural gas in an external reformer used as fuel in MCFC unit. In the downstream of MCFC the residual combustible species from anode side is burnt with 98% pure oxygen, followed by heat recovery, cooling and moisture separation, CO2 compression and storage. The proposed repowering scheme helps to increase the plant capacity by about 27%, while capturing 67% of emitted CO2. Contrary to the conventional CO2 capture process, this MCFC repowering does not eat up the plant's efficiency, rather, it increases the net efficiency by 1.1%-points. The effect of repowering on cost of electricity, CO2 avoided cost (CCA), specific primary energy consumption for CO2 avoided (SPECCA) are estimated. The study shows that this repowering scheme yields a lesser unit cost of electricity (COE) compared to the commercially available monoethanolamine (MEA) based carbon capture retrofit.

Development of Solid Oxide Fuel Cells at Versa Power Systems & FuelCell Energy

Versa Power Systems (VPS) is a developer of solid oxide fuel cells (SOFCs) for clean power generation and is a wholly owned subsidiary of FuelCell Energy (FCE). FCE is a global leader in the design, manufacture and distribution of Molten Carbonate Fuel Cell (MCFC) power plants. From an economic perspective, MCFCs scale-up very well and as a result FCE’s MCFC products are in the multi-megawatt size range. SOFCs are complementary because they scale-down well and hence are suited to submegawatt applications. This paper highlights the status of VPS and FCE’s SOFC technology in the areas of cell, stack and system development.

Molten carbonate fuel cell operation under high concentrations of SO2 on the cathode side

Molten Carbonate Fuel Cells (MCFCs) can be used as CO2 separators in fossil fuel power plants. Fossil fuel power plants emit flue gases with SO2 contaminants, thus the influence of SO2 on the operation of MCFC is investigated both theoretically and experimentally. The influence of SO2 contaminants (up 680 ppm) on MCFC performance was examined. The experimental investigation revealed there is a boundary limit where SO2 contamination could increase the MCFC voltage.