Dissolution Of NiO Cathode Research Articles

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

The effect of La oxide additive on the solubility of NiO in molten carbonates

For practical use of molten carbonate fuel cells (MCFCs), Ni shorting due to NiO cathode dissolution during high-pressure operation is still a serious problem. The molten carbonate electrolyte should be more basic in order to reduce the NiO cathode solubility. In order to obtain a lower NiO solubility, the solubility of La oxide was investigated in the Li/Na/K and the Li/Na eutectic carbonates and the effect of the La oxide addition to the molten carbonate on the NiO solubility has also been investigated. The stable phase of La oxide in molten carbonate was found to be La 2O 2CO 3 under P C O 2 / P O 2 = 0.7/0.3 atm at 923 K. The solubility of NiO significantly decreased due to La additive in the acid dissolution region that is the operating condition for the MCFCs. The order of the base comparative index on the NiO solubility in the molten carbonate was La ≫ Ca ≒ Sr > Li > Na > K.

Flowsheet calculation of a combined heat and power fuel cell plant with a conceptual molten carbonate fuel cell with separate CO2 supply

Abstract A new type of MCFC with a separate CO 2 supply (improved or i-MCFC) is previously presented, which has the potential for reducing NiO cathode dissolution and system enhancement by CO 2 removal from fuel gas. This article presents the first flowsheet calculations of an i-MCFC system that utilizes the potential of reducing NiO dissolution. A submodel that simulates energy and massflows of the i-MCFC is built using standard flowsheeting components. The performance of the i-MCFC is assumed to be equal to the MCFC and differences in Nernst potentials and irreversible losses are neglected. To compare the differences in concept, a MCFC combined heat and power (CHP) system flowsheet is modified and the MCFC model substituted by the i-MCFC submodel. The overall efficiencies of both fuel cell systems are calculated using a flowsheeting program. The calculated results are compared and the differences analyzed. The overall system performance of this i-MCFC CHP system is slightly lower than the MCFC CHP reference system (about 0.1% point in average). The difference in performance is ascribed to the change in gas composition and heat capacity of the cathode gas. The change in heat capacity increases the total massflow through the i-MCFC resulting to an increase in overall auxiliary power consumption. The low CO 2 content of the cathode gas should reduce the NiO cathode dissolution to a negligible level.

The formation of LiCoO2 on a NiO cathode for a molten carbonate fuel cell using electroplating

Lithiated NiO cathode dissolution has been a major problem for the development of molten carbonate fuel cells (MCFCs). Many studies have been contributed to find new alternative cathodes; here, lithium cobalt oxide, LiCoO2, was coated onto the commonly used NiO cathode by the electroplating method and the resulting cathode showed much reduced solubility compared with that of the common nickel oxide cathode. Thin film lithium cobalt oxide was prepared by the oxidation of Co metal deposited on a nickel plate in molten (Li,K)2CO3 at 650 °C under a CO2–O2 (2 ∶ 1 vol%) atmosphere. When this coated nickel plate was oxidized, the open circuit potential (OCP) decayed gradually, indicating two well-defined potential plateaux; the oxide films produced at each potential plateau were identified by X-ray diffraction methods. The surface product at the first plateau was CoO. LiCoO2 was formed at the second OCP plateau [around −0.47 V vs. CO2–O2 (2 ∶ 1 vol%) reference electrode]. By the same method, LiCoO2 was coated onto a porous nickel cathode in order to produce a MCFC. For 300 h steady operation of the cells, the mean voltages of the cells were 0.80 V using a NiO cathode and 0.85 V for a LC-NiO(EP) cathode at a current density of 150 mA cm−2. In addition to the cell efficiency improvement, the solubility of the LC-NiO(EP) cathode was much lower than that of the NiO cathode.

Performance and life of 10-kW molten-carbonate fuel cell stack using Li/K and Li/Na carbonates as the electrolyte

NiO cathode dissolution is a serious problem with molten carbonate fuel cells (MCFC). The target life-time of such cells is 40,000 h, but shorting by NiO cathode dissolution markedly decreases cell performance. NiO cathode dissolution depends on the composition of the molten carbonate electrolyte. The electrolyte generally comprises a mixture of lithium carbonate and potassium carbonate. Since the solubility of NiO in a mixture of lithium carbonate and sodium carbonate is lower than in lithium and potassium carbonate, it is expected that shorting by NiO cathode dissolution will take longer in a mixture of lithium and sodium. Therefore, a mixture of lithium carbonate and sodium carbonate is a strong candidate electrolyte. A unique 10-kW class stack, which uses a mixture of lithium and potassium carbonate and mixture of lithium and sodium carbonate as the electrolyte, has been developed and tested. The basic performance and life time of both electrolyte cells of the stack are reported. In particular, the change in cathode polarization caused by NiO cathode dissolution is evaluated quantitatively.

酸化ニッケルカソード短絡による溶融炭酸塩燃料電池寿命予測モデル

酸化ニッケルカソード短絡による溶融炭酸塩燃料電池寿命予測モデル