Molten Carbonate Fuel Cell Cathode Research Articles

Effects of minor alloying elements (Si, Mn and Al) on the corrosion behavior of stainless steels in molten carbonate fuel cell cathode environment

Abstract Effects of minor alloying elements (Si, Mn, and Al) on the corrosion resistance behaviors of stainless steel (SS) modified 310S used as a cathode current collector (CCC) material for molten carbonate fuel cells (MCFC) were examined in a mixture of 62 mol% Li2CO3–38 mol% K2CO3 at 650 °C by measuring the change in corrosion potential and the potentio-dynamic, potentio-static polarization responses. The corrosion potential of modified 310S gradually increased after 9 h of immersion due to an active to passive transition and that of SSs added with minor alloying elements drastically increased before 6 h of immersion due to the reactive alloys. Si, Mn, and Al addition to base SS led to a decrease in corrosion resistance due to the rapid corrosion rate at the cathode operation potential, −40 mV, of the MCFC. The steady state current densities of SSs added with minor alloying elements were higher than that of 310S and modified 310S. Addition of Si, Mn, and Al induced a decrease in corrosion resistance of CCC materials in molten carbonate fuel cell operating temperatures, 650 °C.

Performance and Stability of Molten Carbonate Fuel Cell Cathode Electrode Under Carbon Capture Endurance Tests

Introduction The molten carbonate fuel cell (MCFC) has emerged as one of the promising alternatives for carbon capture from industrial and power generation CO2 sources. MCFC technology is less energy-intensive compared to conventional amine scrubbing that requires a significant amount of energy (steam) for solvent regeneration process.MCFC has been commercially used for distributed power generation, however its use in carbon capture applications has been limited to development activities and a few demonstrations during the past decades. The electrochemical reactions taking place during cell operation involve the CO2 transfer from cathode to anode in the form of carbonate ions resulting in a gas stream with high CO2 concentration on the anode exhaust. Therefore, MCFC stack systems can be combined with conventional combustion-powered generators (coal and/or natural gas-based power plants) for simultaneous CO2 separation, power and hydrogen generation.CO2 capture applications will typically take in flue gas from industrial furnaces, coal and gas fired combined cycle plants as the cathode inlet gas stream having low CO2 concentrations ranging from 4% to 10% by volume and high CO2 utilization (>90%). These CO2-lean conditions may pose a challenge for attaining good mass-transfer performance and life stability, therefore innovative approaches are needed in terms of hardware design and cathode structure to achieve long-term stability.FCE has tested numerous single cells (250 cm2) and technology stacks (30 kW) under carbon capture operating conditions (4-8% CO2 in the cathode inlet as opposed to >12% in baseline MCFC systems) to understand impact of design parameters on performance, life, and to investigate design solutions for further enhancement. Figure 1 highlights the effect of cathode water content on CO2 utilization. It appears that differences exist between the ideal and measured CO2 utilizations, indicating that less CO2 is transferring from the cathode to anode than expected. This CO2 transference reduction becomes more pronounced with higher water concentration, suggesting that a species other than CO3 2- ion, likely hydroxide (OH- ion), may be generated and transported through the electrolyte matrix.This paper will review the cathode material stability and electrochemical performance under long-term carbon-capture operation. The effect of parameters such as water content, gas composition, utilization, and electrolyte chemistry, as well as approaches to enhance the CO2 capture efficiency and life, will be discussed. Figure 1

Performance evaluation of a novel CCHP system integrated with MCFC, ISCC and LiBr refrigeration system

This paper proposes a novel combined cooling, heating, and power (CCHP) system integrated with molten carbonate fuel cell (MCFC), integrated solar gas-steam combined cycle (ISCC), and double-effect absorption lithium bromide refrigeration (DEALBR) system. According to the principle of energy cascade utilization, part of the high-temperature waste gas discharged by MCFC is led to the heat recovery steam generator (HRSG) for further waste heat utilization, and the other part of the high-temperature waste gas is led to the MCFC cathode to produce CO32−, and solar energy is used to replace part of the heating load of a high-pressure economizer in HRSG. Aspen Plus software is used for modeling, and the effects of key factors on the system performances are analyzed and evaluated by using the exergy analysis method. The results show that the new CCHP system can produce 494.1 MW of electric power, 7557.09 kW of cooling load and 57,956.25 kW of heating load. Both the exergy efficiency and the energy efficiency of the new system are 61.69% and 61.64%, respectively. Comparing the research results of new system with similar systems, it is found that the new CCHP system has better ability to do work, lower CO2 emission, and can meet the cooling load, heating load and electric power requirements of the user side at the same time.

Recycling electronic scrap to make molten carbonate fuel cell cathodes

The paper aims to examine the possibility of improving the manufacturing process for MCFC cathodes. using noble, semi-precious, and rare earth metals sourced from waste electric and electronic equipment (WEEE). As MCFC components are not particularly sensitive to ceramic and metal impurities. The addition of noble metals recovered from WEEE as catalysts are economically justifiable. The reported experimental research revealed the positive impact of MCFC cathode fabricated with 20% recycled electronic scrap. Especially the cell with powder marked as 4/1 enjoyed much better performance operating at 550 °C than the reference cell. During the operation at a temperature of 650 °C, the cell with powder marked as 4/1 has almost the same performance as the reference cell, i.e., 1.01 V OCV and power density of 0.13 A/cm2. The cell with cathode with 4/2 powder has the worst performance – current density of 0.09 A/cm2 and OCV of 0.97 V.

Microstructure and electrochemical behavior of layered cathodes for molten carbonate fuel cell

In the present paper, we demonstrate how modifications of the microstructure and the chemical composition can influence the electrochemical behavior of cathodes for molten carbonate fuel cells (MCFCs). Based on our experience, we designed new MCFC cathode microstructures combining layers made of porous silver, nickel oxide or nickel foam to overcome common issues with the internal resistance of the cell. The microstructures of the standard NiO cathode and manufactured cathodes were extensively investigated using scanning electron microscopy (SEM) and porosity measurements. The electrochemical behavior and overall cell performance were examined by means of electrochemical impedance spectroscopy and single-cell tests in operation conditions. The results show that a porous silver layer tape cast onto standard NiO cathode and nickel foam used as a support layer for tape cast NiO porous layer substantially decrease resistance components representing charge transfer and mass transport phenomena, respectively. Therefore, it is beneficial to combine them into a three-layer cathode since it facilitates separation of predominant physio-chemical processes of gas and ions transport in respective layers ensuring high efficiency. The superiority of the three-layer cathode has been proven by low impedance and high power density as compared to standard NiO cathode.

Decarbonizing cement plants via a fully integrated calcium looping-molten carbonate fuel cell process: Assessment of a model for fuel cell performance predictions under different operating conditions

This study is part of a comprehensive research devoted to the integration of a Calcium Looping (CaL) process with a Molten Carbonate Fuel Cell (MCFC) for the decarbonisation of a full-scale cement plant. In the proposed process, where the energy intensive oxy-combustion occurring in the CaL calciner is replaced with a conventional combustion in air. The CO2-rich gas leaving the calciner is injected into the MCFC cathode while the anode side is fuelled by H2-rich gases produced by a sorption-enhanced reforming (SER) process. The high CO2-concentrated gas leaving the anode will be sent to valorisation processes and/or the CO2 final disposal.Here we focus on modelling, simulation and characterization of the MCFC used as a device for CO2 separation as well as electricity production, here considered as a process by-product. Polarization curves (I–V curves) and Electrochemical Impedance Spectroscopy (EIS) were measured to support the development and the calibration of a semi-empirical model obtained by theoretical consideration.The experimental campaign demonstrated that the fitted model is able to reproduce the real cell performance when varying the temperature, H2 concentration, CO2 concentration at anode and cathode respectively as well as CO2 CaL capture rate.Indeed, the average difference between numerical and experimental results is always below 2%.Results also demonstrated that the MCFC can be usefully considered as an efficient CO2 concentrator, with a CO2 fraction at the anode outlet that is greater than 51% on a dry basis.

Silver coated cathode for molten carbonate fuel cells

Within this study, a layered cathode for use in a Molten Carbonate Fuel Cell (MCFC) has been developed. The substrate layer and reference MCFC cathode made of porous nickel was covered by a porous silver film with defined porosity and pore size. Both layers were fabricated using the tape casting method and further fired in a reductive atmosphere. The new cathode was assembled with other reference cell components to form a single MCFC, which was subjected to performance and durability tests. Scanning electron microscopy was used to analyze the microstructure of the materials before and after tests. The reference cathode was also studied for the comparison.The results show that the porous silver layer was able to enhance the electron transport between the cathode and current collector. It was also found that oxygen reduction is enhanced due to the presence of silver in the gas supply. As a result, the power density of the cell was increased by 50%. On the other hand, due to the separation from electrolyte by the NiO layer, no significant degradation of the silver layer, identified by SEM or electrochemical tests, was found after 1000 h.

Effect of Carbon Capture Endurance Conditions on Performance and Stability of Molten Carbonate Fuel Cell Cathode Electrode

IntroductionThe molten carbonate fuel cell (MCFC) is one of the most advanced high-temperature and clean power generating devices. To date >8 billion kWh of electricity has been produced commercially using this new technology. The high operating temperature of MCFC (550-650 °C) dramatically improves the reaction kinetics and eliminates the need for a noble metal catalyst. The electrochemical reactions taking place during cell operation involve the CO2 transfer from cathode to anode in the form of carbonate ions (Figure 1). Therefore, MCFC stack technology can be used for power generation and simultaneously, as an effective CO2 separator. MCFC systems can be combined with conventional combustion-powered generators (coal and/or natural gas based power plants) to efficiently separate CO2 in the exhaust gas and produce additional electricity.The state of the art MCFC cathode is porous lithiated nickel oxide. The cathode stability and performance are affected by several factors such as gas composition, temperature, electrode structure and electrolyte chemistry. To ensure long-term performance and material stability against NiO dissolution, polarization loss, and carbon capture efficiency, major considerations need to be taken into account in material selection, operating conditions, and electrode structure design.FCE has tested many single cells (250 cm2) and technology stacks (30 kW) under carbon capture operating conditions (4-5% CO2 in the cathode inlet as opposed to >15% in baseline MCFC systems) to understand parameters affecting performance, life, and to investigate design solutions for further enhancement. Figure 2 highlights the effect of CO2 concentration on the cathode lithiation. These measurements were conducted using an out-of-cell test (OCT) where cathode samples were exposed to different oxidizing environments. It appears that lean-CO2 gas atmospheres (<5%CO2) lead to increased lithium content in the NiO cathode. This may impact the electrolyte distribution and the cathode microstructure. Therefore, the electrolyte inventory (fill level), the lithium content and the basicity need to be considered to achieving optimal operating conditions.This paper will review the cathode material stability, microstructure, and durability under long-term carbon-capture operations. The effect of parameters such as electrolyte fill, gas composition, and electrolyte chemistry, as well as approaches to enhance the CO2 capture efficiency and life, will be discussed. Figure 1

Molten Carbonate Fuel Cell Performance for CO2 Capture from Natural Gas Combined Cycle Flue Gas

CO2 capture and sequestration (CCS) from stationary flue gas sources is one of the critical technologies needed as the future energy landscape shifts to low carbon intensity energy systems. Molten carbonate fuel cells (MCFCs) have the potential to capture CO2 from flue gas at higher thermal efficiency than traditional CCS technologies while simultaneously producing electricity. Herein, we present an investigation of molten carbonate fuel cell behavior at carbon capture conditions using simulated natural gas combined cycle flue gas. Measurements at these low CO2 and high current conditions reveal a lower than expected cathodic consumption of CO2 Based on the strong dependence of this deviation on water partial pressure as well as mass balances revealing a net consumption of water at the cathode, a parallel oxygen reduction mechanism is proposed. In this mechanism, water and oxygen are consumed at the cathode to produce hydroxide ions which migrate through the electrolyte to the anode. This parallel mechanism contributes to power generation but not to CO2 capture. Mass transport limitations in the molten carbonate fuel cell cathode were identified as the primary driver for this alternative mechanism which were heavily influenced by the design of the current collector and cathode interface.

Performance and Stability of Cathode Electrode Under Carbon Capture Operating Conditions

Introduction The high-temperature molten carbonate fuel cell (MCFC) is one of the most advanced clean power generating devices. To date >8 billion kWh of clean electricity has been produced commercially using this new technology. The high operating temperature of MCFC (550-650°C) dramatically improves the reaction kinetics and eliminates the need for a noble metal catalyst. The electrochemical reactions taking place during cell operation involve CO2 transfer from cathode to anode through electrolyte matrix in the form of carbonate ions (Figure 1). The transport of CO3 = is equivalent to that of O= and CO2 (CO3 = ⇔ O= + CO2). Therefore, MCFC stack technology can be utilized for efficient simultaneous power generation and CO2 separation/capture by integrating with conventional combustion-based coal and/or natural-gas power plants. The state of the art MCFC cathode is porous lithiated NiO. The cathode performance and stability are affected by several factors such as gas composition, temperature, electrode structure, and electrolyte chemistry. To ensure long-term performance and material stability (such as NiO cathode dissolution, polarization loss and CO2 capture efficiency), material selection, operating characteristics, and electrode design need to be carefully considered. FCE has operated numerous bench-scale single cells (100 W) and technology stacks (30 kW) under carbon capture operating conditions (4-5% CO2 in the cathode inlet as opposed to >15% in baseline MCFC systems) to understand parameters affecting performance, life, and to investigate design solutions for further enhancement. Figure 2 shows similar NiO cathode stability under standard as well as carbon capture conditions, projecting to useful life of >7-years under both operation conditions. More endurance carbon capture tests are being conducted for further verification. This paper will review the cathode material stability, microstructure, and durability under long-term carbon-capture operations. The effect of parameters such as electrolyte fill, gas composition and electrolyte chemistry, as well as approaches to enhance the CO2 capture efficiency and life, will be discussed. Figure 1

Characterization of Spatial Distribution of Electrolyte in Molten Carbonate Fuel Cell Cathodes

High‐resolution computed X‐ray tomography (XCT) is applied in this study to characterize the microstructure of Molten Carbonate Fuel Cell (MCFC) cathodes after operation, where a “virgin cathode” formed by porous nickel has been in situ oxidized and infiltrated by liquid electrolyte. This technique extends state‐of‐the art methodology to characterize pores such as porosity analysis using Mercury porosimetry. XCT enables 3D imaging of the internal structure of the electrodes including all components present at the cathode side, particularly NiO, pores, and electrolyte. The quantitative 3D microstructure analysis based on high‐resolution XCT provides topological information for each component and their mutual spatial distribution, which is significant for the enhancement of the MCFC performance. In particular, volume fraction, specific surface area, continuity of each phase, as well as the Triple Phase Boundary (TPB) density are calculated from the experimental data. The results indicate that superior properties of the multi‐modal pore sized cathode, used in this study, can be attributed to the formation of three self‐interpenetrating networks of pathways for the transport of gasses, electrons, as well as ions throughout pores, NiO, and electrolyte, respectively.

Multi-modal porous microstructure for high temperature fuel cell application

In this study, the effect of microstructure of porous nickel electrode on the performance of high temperature fuel cell is investigated and presented based on a molten carbonate fuel cell (MCFC) cathode. The cathode materials are fabricated from slurry consisting of nickel powder and polymeric binder/solvent mixture, using the tape casting method. The final pore structure is shaped through modifying the slurry composition - with or without the addition of porogen(s). The manufactured materials are extensively characterized by various techniques involving: micro-computed tomography (micro-XCT), scanning electron microscopy (SEM), mercury porosimetry, BET and Archimedes method. Tomographic images are also analyzed and quantified to reveal the evolution of pore space due to nickel in situ oxidation to NiO, and infiltration by the electrolyte. Single-cell performance tests are carried out under MCFC operation conditions to estimate the performance of the manufactured materials. It is found that the multi-modal microstructure of MCFC cathode results in a significant enhancement of the power density generated by the reference cell. To give greater insight into the understanding of the effect of microstructure on the properties of the cathode, a model based on 3D tomography image transformation is proposed.

Electrochemical Carbon Separation in a SOFC–MCFC Polygeneration Plant With Near-Zero Emissions

The modularity and high efficiency at small-scale make high temperature (HT) fuel cells an interesting solution for carbon capture and utilization at the distributed generation (DG) scale when coupled to appropriate use of CO2 (i.e., for industrial uses, local production of chemicals, etc.). The present work explores fully electrochemical power systems capable of producing a highly pure CO2 stream and hydrogen. In particular, the proposed system is based upon integrating a solid oxide fuel cell (SOFC) with a molten carbonate fuel cell (MCFC). The use of these HT fuel cells has already been separately applied in the past for carbon capture and storage (CCS) applications. However, their combined use is yet unexplored. The reference configuration proposed envisions the direct supply of the SOFC anode outlet to a burner which, using the cathode depleted air outlet, completes the oxidation of the unconverted species. The outlet of the burner is then fed to the MCFC cathode inlet, which separates the CO2 from the stream. This layout has the significant advantage of achieving the required CO2 purity for liquefaction and long-range transportation without requiring the need of cryogenic or distillation plants. Furthermore, different configurations are considered with the final aim of increasing the carbon capture ratio (CCR) and maximizing the electrical efficiency. Moreover, the optimal power ratio between SOFC and MCFC stacks is also explored. Complete simulation results are presented, discussing the proposed plant mass and energy balances and showing the most attractive configurations from the point of view of total efficiency and CCR.

Preliminary design of a MW-class demo system for CO2 capture with MCFC in a university campus cogeneration plant

One of the most promising, short-term options for efficiently capturing CO2 from combustion exhaust gases – potentially from any combustion process source - is based on the operating principle of Molten Carbonate Fuel Cells (MCFC): their electrochemical reactions promote the transport of both CO2 and O2 molecules from the cathode side (which can be fed with combustion effluents) to the CO2-rich atmosphere of the anode side (fed with internally reformed natural gas), by means of a CO32--ion conducting electrolyte. In the present work, the preliminary design of a 1 MWel MCFC demo plant operating downstream a Combined Heat and Power (CHP) Internal Combustion Engine (ICE) installed at the Politecnico di Milano campus is investigated, with the aim of promoting a valid solution for high efficiency, de-carbonised heat and electricity production. The study envisages two purification strategies for the CO2-rich stream at the MCFC anode outlet: i) the CO2 is separated and compressed in a cryogenic unit and the unconverted fuel is either recycled at the anode inlet or burned and sent to the MCFC cathode inlet ii) the anode exhausts are burned in a catalytic oxy-combustor, increasing both the thermal energy available in the cogeneration unit and the CO2 concentration in the stream sent to the storage site. Subsequently to a thermodynamic analysis carried out with a 0D model calibrated upon experimental data available for a commercial MCFC unit, the main components are designed by taking into account all the operating constraints of the machines and the CO2 capture limitations associated to the size of the MCFC modules currently available on the market. Moreover, an economic analysis is performed in order to assess the feasibility of such an installation within the university campus cogeneration grid. As a main finding, the use of MCFCs to capture CO2 at a distributed generation scale allows reaching interesting energy and environmental performances, highlighted by promising values of the Specific Primary Energy Consumption for CO2 Avoided (SPECCA=0.9-1.9 MJ/kgCO2) and Carbon Capture Ratios (CCR=68-84%). Within a mid-term perspective for MCFC specific cost, the economic analyses reveal acceptable values for the cost of electricity and the cost of CO2 avoided, respectively close to 130 €/MWhel and 100 €/tCO2.

Experimental research on short-term feeding of dust contaminated gas to a molten carbonate fuel cell cathode

Abstract The paper presents initial research on processes present on the cathode side of Molten Carbonate Fuel Cells (MCFC), when the supplied gas is an aerosol containing solid particulate matter. The research is based on experiments conducted at the Institute of Heat Engineering of Warsaw University of Technology. The main task is to determine whether and to what extent solid particles disable or hinder the operation of a molten carbonate fuel cell. It is thought that they might change the penetrability of porous layers by clogging their void volumes. Under investigation are the sizes and amount of solid particles required to significantly affect the processes occurring on the triple phase. Experimental investigation was conducted that determined the change in operational parameters due to dust contamination. Surprisingly, there is no sudden drop in the electric parameters of the fuel cell subject to dust poisoning. Supposedly, the dust creates a porous, permeable to gases, structure on the electrode. The only varying parameter was the pressure difference between the inlet and the outlet to the cathode.

Effect of Electrolyte Amount on the Performance in a Molten Carbonate Fuel Cell

This work has been focused on the effect of electrolyte amount in a coin type molten carbonate fuel cell (MCFC). The MCFC uses molten carbonate as an electrolyte. The carbonates are distributed at the anode and cathode electrodes to provide three phase boundaries where the electrochemical reactions occur. One factor which controls polarization is the degree of filling in electrodes. MCFC cathodes consist of in-situ oxidized NiO. The cathode is wetted completely by molten carbonate, so the cathode is entirely covered with a thin film of electrolyte. This causes some diffusion resistance even under optimal operating conditions, i.e., at 15-30% filling of the total pore volume by electrolyte. However, when the pores become filled with excess amount of electrolyte, calling as flooding, diffusion increases steeply. On the other hand, the anode is much less sensitive to the filling than the cathode. The reason is that electrode kinetics and mass transport are relatively rapid at the anode. Moreover, the level of polarization is not high because of the rapid kinetics. Therefore the anode can serve as an electrolyte reservoir. Thus, electrolyte amount in the electrodes are very important factor in the cell performance. Four different amounts of electrolyte, 1.5, 2, 3, and 4 g, were used. In general, the carbonate amount is expressed by pore filling ratio (pfr) which is the ratio of electrolyte filling in the void volume of electrodes. The electrolyte amounts of 1.5, 2, 3, and 4 g are corresponding to 0, 10, 60, and 110% of pfr, respectively. A coin type molten carbonate fuel cell was used in this work. The diameter of electrodes was ca. 3 cm. The matrix was made of LiAlO2. The anode gas was a mixture of 69 mol% H2, 17 mol% CO2 and 14 mol% H2O. A gas mixture of 70 mol% air and 30 mol% CO2 served as the cathode gas. The electrolyte was 62 mol% Li2CO3 and 38 mol% K2CO3. All the cells were operated at 923K under atmospheric condition. The cell performance of the electrolyte was analyzed via steady state polarization, step- chronopotentiometry, and impedance method. First is steady state polarization. The current density was applied from 0 to 150 mA/cm2. In general, the voltage of MCFC is the difference between the open circuit voltage (OCV) and total voltage loss. At pfr 60%, the voltage decreases linearly and slightly from the OCV, indicating that pfr 60% has negligible activation polarization. The reason is that pfr 60% has a large active surface area. When the electrolyte amounts were increase, total voltage loss decreased. However, the pfr of 110% has a severe voltage loss at high current density, representing large mass transfer resistance takes place at the electrode due to the flooding phenomenon. Second is step-chronopotentiometry. The current steps were applied at every 60 seconds and following voltage relaxation was observed. All the cells showed step-voltage relaxation according to the current step. In particular, voltage relaxation was reduced by the increase of electrolyte amount up to pfr 60%. However, voltage relaxation tended to increase at pfr 0% and 110%. The reason is that too much or too small amounts of electrolyte reduced surface area and it resulted in large mass transfer resistance and voltage relaxations. Third is impedance behavior with various amounts of electrolyte at open circuit state. The AC signal of 5 mV RMS was applied from the high frequency (10 kHz) to low frequency (10 mHz). Similar to the bench scale cell, two half circles at high and low frequency regions are observed. The high-frequency half circle reflected cathodic overpotential, and low frequency one represented anodic overpotential [2]. The starting point of half circuit represented internal resistance. In this work, internal resistance and electrode overpotential are decreased by enlarging electrolyte amount up to pfr 60%. The enlarged amount of electrolyte provides larger gas-liq-solid three phase boundary, thus the active surface would increase. The pfr 0% and 110% showed larger high frequency half circle than other pfrs. It is plausible that the large mass transfer resistance at the electrodes is resulted from high or lower electrolyte amounts. Moreover, a large resistance gap was observed between pfr 0% and 10%. In the case the difference of electrolyte amount was only 0.5 g. It is estimated that a permissible range of electrolyte is limited and electrode is very sensitive to the degree of filling.

Effect of Cs2CO3 in Li/K and Li/Na Carbonate on the Oxygen Reduction Reaction for MCFC

Electrolyte properties are significantly important for the long-term stability and performance of molten carbonate fuel cells (MCFCs). The Li/Na carbonate is considered an attractive electrolyte for MCFCs due to its high ionic conductivity and low volatility. However, low oxygen solubility is a serious drawback in Li/Na carbonate electrolytes, which hinders the oxygen reduction reaction (ORR) at the MCFC cathode. Several additives have been found to enhance the oxygen solubility in alkali carbonate electrolytes. In this study, the effect of Cs2CO3 in Li/Na and Li/K carbonate mixtures was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The oxygen reduction activity improved with the increasing Cs concentration (1, 3, and 5 mol%) in all the studied carbonate melts. The addition of Cs2CO3 (5 mol%) to the Li/Na carbonate (68.0/32.0) resulted in an approximately 30% increase of the mass transfer parameter ) of the dissolved oxygen species from 0.99 × 10−9 to 1.29 × 10−9 mol/cm2 s1/2 at 620°C. The changes in the ionic conductivity of the molten electrolytes with the Cs additive were also studied.

Determination of Charge and Mass Transfer for Molten Carbonate Fuel Cell Cathode Using NiO Coated Ring Electrode

Introduction Molten carbonate fuel cell (MCFC) systems have very high energy conversion for distributed generation systems, and the largest electric power generation capacity in stationary fuel cell systems. In order to improve energy conversion efficiency and durability, cathode and electrolyte materials are key factors. We found that the solubility of LaNiO3 in the La2O3 saturated molten alkaline metal carbonates is significantly smaller than that of NiO in the La2O3 free molten carbonates1), and should know the kinetics on the LaNiO3 in the carbonates for oxygen reduction reaction (ORR). However, even the kinetics of the ORR on the NiO is not clarified enough2). Furthermore, the performance of gas diffusion electrode for practical system should be affected by both charge and mass transfer.In this study, charge transfer on a sintered NiO electrode and mass transfer through a meniscus on the electrode have been investigated with chronoamperometry using a meniscus ring electrode to establish electrochemical method for the evaluation of charge and mass transfer separately. Experimental Li/Na (=52/48 mol %) or Li/K (=62/38 mol %) molten carbonate were used as an electrolyte. The working electrode was a sintered NiO on a gold ring electrode. The height of the working electrode from surface of molten carbonate was controlled by a micrometer. Reference electrode was a reversible oxygen electrode (ROE), and counter electrode was a gold coil electrode. The surface area of the sintered NiO was BET method.The oxygen reduction reaction current was determined with chronoamperometry (CA) from 10 to -10 mV vs. ROE. The initial current density was evaluated by extrapolation to t = 0 with the linear relation between current and the square root of time to determine mass transfer free polarization at 2mm of the height. The steady state current density was evaluated for 100 s after 500 s of CA at 4mm of the height with mass transfer resistance of the meniscus.Symmetry factor: a was determined with Allen-Hickling equation for the mass transfer free polarization curve, and the exchange current density: i 0 was determined with mass transfer free Butler-Volmer equation, the a and polarization curve. The limiting current density: i l was determined with Butler-Volmer equation with the i l, i 0, a, the steady state polarization curve. Results and discussion Figure 1 shows the i 0 (Fig. 1-a)) and the i lc (Fig. 1-b)) as a function of CO2 or O2 partial pressure in CO2-O2-Ar. The CO2 or O2 partial pressure was fixed at 0.1 atm for p O2 or p O2 function, respectively. The i 0 the i lc were determined at 2 mm height to minimize the influence of ohmic drop of the electrolyte and at 4 mm height to minimize mass transfer resistance of reaction gas through the meniscus. Here, the i 0 and the i lc are normalized the BET surface area and geometrical surface area, respectively. The BET surface area of the sintered NiO was 2.5 m2g-1. Therefore, the apparent exchange current density per geometrical surface area was 1000 times larger than the limiting current density. The dependence of the i 0s on the partial pressures were smaller than that of the i lcs. All slopes could not explained by a simple reaction mechanism model of peroxide, superoxide, or parcarbonate path2). The polarization of practical electrode depends on liquid – gas interface surface area rather than real surface area of the electrode3). Therefore, performance of the practical electrode is controlled by the combination of the charge and mass transfer, and the i 0 and the i lc, which is a function of gas solubility, diffusion coefficient, and morphology of molten carbonate film, should be an effective index to determine activity of electrode and property of electrolyte. References 1) K. Matsuzawa, Y. Akinaga, S. Mitsushima, and K. Ota, J. Power Sources, 196, 5007 (2011).2) T. Nishina, Y. Masuda and I. Uchida, Molten Salt Chem., 93, 424 (1993).S. Kuroe, M. Takeuchi, S. Nishimura, K. Ohtsuka, DENKIKAGAKU, 58, 928 (1990).

Electrochemical properties of Atomic layer deposition processed CeO2 as a protective layer for the molten carbonate fuel cell cathode

In order to increase the lifetime and performance of the molten carbonate fuel cell, it is compulsory to control the corrosion and dissolution of the state of the art porous nickel oxide cathode. A protective coating constituted by more stable oxides appears to be the best approach. Previous research on CeO2 coatings obtained by DC reactive magnetron sputtering to protect the Molten carbonate fuel cell cathode gave promising results but it was necessary to improve the coating adhesion. In this paper Atomic Layer Deposition, producing high quality, homogeneous and conformal layers, was used to obtain thin layers of CeO2 (20nm and 120nm) deposited over porous nickel. CeO2-Ni coated samples were tested as cathodes in Li2CO3-K2CO3 (62-38mol %) eutectic mixture under standard cathode atmosphere (CO2/air 30:70 vol. %). Structural and morphological characterizations of the nickel coated cathode before and after electrochemical tests in the molten carbonate melt are reported together with the Open Circuit Potential evolution all over 230h for both the bare porous nickel and the CeO2-coated samples.

Electrochemical behavior of Mx−1Ox (M = Ti, Ce and Co) ultra-thin protective layers for MCFC cathode

In the world of alternative energy sources, the Molten Carbonate Fuel Cell (MCFC) is one of the promising technologies for the efficient conversion of hydrogen or hydrocarbons to power and heat. One of the main issues for optimizing this device is the control of the dissolution of the state-of-the-art porous nickel oxide cathode. A protective coating by more stable metal oxides seems to be one of the best solutions. In this paper, ultra-thin layers of TiO2 (50 nm), Co3O4 (50 nm) and CeO2 (20 nm) were deposited on porous nickel substrates, by a sequential CVD technique, known as Atomic Layer Deposition (ALD), producing high quality, homogeneous and conformal layers. The electrochemical behavior and morphological features of the three coated samples were compared in a Li2CO3–K2CO3 (62–38 mol%) eutectic melt under a standard cathode atmosphere (CO2/air 30:70 vol%) for 230 h. Finally, the respective advantages and drawbacks of Co3O4, TiO2 and CeO2 coatings are pointed out.