Co／Ce-coated Ni powder as a new candidate cathode material for Molten Carbonate Fuel Cell (MCFC)

국문 초록 (Abstract)

용융탄산염 연료전지(MCFC)의 양극으로 사용되는 산화 니켈 (NIO)은 탄산염과 산소 분위기에서 높은 안정성과 전기 전도도 를 갖는다. 하지만 전해질에서 산화 니켈(NiO) 전극의 용해 및 재 석출은 용융탄산염 연료전지(MCFC)의 상용화를 이루기 위한 큰 문제점으로 제기되고 있다. 그 침전된 니켈은 양극으로부터 니켈의 침전을 더욱 더 촉진시켜 니켈 이온(Ni2+)들을 감소시키게 하는 결과를 낳는다.
본 연구에서는 용융탄산염 연료전지(MCFC) 양극의 안정성을 향상시키기 위해 대체 양극 물질을 제조하였다. 이 양극 물질은 니켈 분말의 표면에 코발트(Co)와 세륨(Ce)을 Pechini method를 이용하여 균일하게 코팅하였다. X선 회절 분석 법(X-ray diffraction)과 주사 전자 현미경(Scanning Electron Microscopy)을 이용하여 니켈 분말의 표면에 코발트(Co)와 세륨(Ce)이 균일하게 코팅된 것을 확인하였다. 제조한 이 분말은 테이프 캐스팅 방법에 의해서 코발트(Co)와 세륨(Ce)이 코팅된 니켈 Sheet을 얻었으며, 이 Sheet을 수소 분위기에서 각 온도 조건 (750, 850 그리고 950oC)들에서 열처리하여 양극을 제조하였다. 각 조건들에서 제조한 양극들의 다공성(Porosity)은 아르키메데스의 원리(Archimedes’ principle)를 통해서 용융탄산염 연료전지의 양극 물질로 사용하기에 적합한 다공성(Porosity)을 확인하였다. 대체 양극 물질로 제조한 코발트 (Co)와 세륨(Ce)이 코팅된 니켈 전극은 용융탄산염 연료전지의 작동 조건(650oC, CO2: O2 = 66.7: 33.3%)에서 기존의 물질인 니켈 전극보다 용융탄산염에서 80%이상의 낮은 용해도를 나타내었으며, 코발트(Co)만 코팅된 니켈 전극보다도 낮은 용해도를 나타내었다. 그리고 300시간 동안 단위 전지를 운전하는 동안, 상용화된 NiO와 Co-coated Ni 그리고 Co/Ce-coated Ni 전극의 CCV의 평균 전압은 각각 0.80V, 0.82V 그리고 0.85V을 나타내어 기존의 전극들에 비해 Co/Ce-coated Ni 전극의 CCV의 평균 전압이 더 우수한 것을 확인하였다. 결론적으로, 코발트 (Co)와 세륨(Ce)이 코팅 된 니켈 분말은 용융탄산염 연료전지의 대체 양극 물질로서 가능성을 확인하였다.

다국어 초록 (Multilingual Abstract)

Nickel oxide (NiO) is commonly used as the cathode for the molten carbonate fuel cell (MCFC) due to its high stability in carbonate melt and in oxygen atmosphere. Also it has high electrical conductivity for obtaining a high performance of MCFC. But the dissolution of nickel oxide cathode in the electrolyte is one of the major technical obstacles to the commercialization of molten carbonate fuel cell (MCFC). The precipitated nickel can act as a sink for nickel ions, which promotes the further dissolution of nickel from the cathode. In this paper, to improve the MCFC cathode stability, the alternative cathode material for MCFC was prepared, which was made of Co/Ce-coated on the surface of Ni powder using a polymeric precursor based on the Pechini method. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) were employed in characterization of the alternative cathode materials. The Co/Ce-coated Ni sheet was prepared by the tape-casting technique. The cathodes sintered under H2 atmosphere at different temperature (750, 850 and 950oC). The cathodes prepared by sintering at different temperatures evaluated porosity through Archimedes’ principle. The solubility of the Co/Ce-coated Ni cathode was about 80% lower compare to that of pure Ni cathode under CO2: O2 (66.7: 33.3%) atmosphere at 650oC. During 300h cells operation, the closed-circuit voltage (CCV) of unit cells using the NiO, the Co-coated Ni and the Co/Ce-coated Ni cathodes are 0.80V, 0.82V and 0.85V at a current density of the 150mA/cm2, respectively. Consequently, the Co/Ce-coated Ni powder could be confirmed as a new alternative cathode material for MCFC.

목차 (Table of Contents)

• Contents
• Abstract
• List of Figures
• List of Tables
• List of Papers
• 1. Introduction
• 1.1 History of Fuel Cells
• 1.2 Principle and Characterization of Fuel Cells
• 1.3 The Types of Fuel Cells
• 1.4 Molten Carbonate Fuel Cells (MCFCs)
• 2. Experimental
• 2.1 Preparation of the Co/Ce-coated Ni powders
• 2.2 Lithiation of the Co/Ce-coated Ni cathodes
• 2.3 Preparation of the Co/Ce-coated Ni cathodes
• 2.4 Characterization of the cathode materials
• 2.5 Solubility test and unit cells performance
• 3. Results and Discussion
• 3.1 Preparation of the Co/Ce-coated Ni powders
• 3.2 Lithiation of the Co/Ce-coated Ni powders
• 3.3 Preparation of the Co/Ce-coated Ni cathodes
• 3.4 Solubility test
• 3.5 Unit cells performance
• 4. Conclusion
• References
• List of Figures
• Fig.1 The schematic diagram showing the operation of Molten
• Carbonate Fuel Cells (MCFC).
• Fig.2 Procedures for the preparation of Co/Ce-coated Ni powders
• using Pechini method.
• Fig.3 Device for lithiation measurement: 1 thermocouple, 2 blowing
• tube, 3 furnace cover, 4 furnace, 5 crucible cover, 6 crucible,
• samples and 8 insulator.
• Fig.4 The schematic diagram of tape casting method.
• Fig.5 The preparation of the Co/Ce-coated Ni cathodes.
• Fig.6 Device for solubility test: 1 thermocouple, 2 a ceramic pipette,
• 3 blowing tube, 4 furnace cover, 5 furnace, 6 crucible cover,
• crucible, 8 samples (cathodes) and 9 insulator.
• Fig.7 The schematic diagrams of a unit cell.
• Fig.8 Start-up condition of 1×2cm2 single cell.
• Fig.9 Thermal analysis data of the Co/Ce-coated Ni powder gel
• precursors: (a) TGA, (b) DSC.
• Fig.10 XRD patterns of the Co/Ce-coated Ni powders with different
• Co/Ce mole fractions from calcination at 350oC, Co/Ce
• Fig.11 SEM images of (a) the Co/Ce-coated Ni powder, (b) the Co
• mapping, (c) the Ni mapping and (d) the Ce mapping were obtained from calcination at 350oC for 3 h.
• Fig.12 XRD patterns of the Co/Ce-coated Ni powders obtained after
• immersion test at various temperatures for 96h under CO2: O
• (66.7: 33.3%), (a) 2θ
• Fig.13 SEM images of (a) the pure Ni cathode and the Co/Ce-coated
• Ni cathode obtained after sintering at 750oC under reduction atmosphere, in the molar ratios of Co/Ce
• 9.0/1.0 and (d) 8.0/2.0 (mol%).
• Fig.14 The pore size distribution curves of the pure Ni (○) and the
• Co/Ce-coated Ni cathodes (■).
• Fig.15 Time-dependence of NiO solubility in (Li0.62K0.38)2CO3 at
• 650oC under CO2: O2 (66.7: 33.3%), (a) the pure NiO cathode, (b) the 10 mol%-Co-coated NiO cathode, and the Co/Ce-coated NiO cathode at the molar ratio of Co/Ce
• Fig.16 SEM images of the Co/Ce (9.5/0.5 mol%)-coated NiO cathode
• obtained after sintering at 750oC under reduction atmosphere (H2), (a) before solubility test, (b) after solubility test.
• Fig.17 Performances of unit cells (CCV at 150mA/cm2): (a) the NiO
• cathode, (b) the Co-coated NiO cathode, (c) the Co/Ce-coated NiO cathode.
• Fig.18 Cell polarization curves using (a) the Co/Ce-coated NiO
• cathode, (b) the NiO cathode and the Co-coated NiO cathode at 650oC under CO2: O2 (66.7: 33.3%).
• List of Tables
• Table 1 Summary of Major Differences of the Fuel cell types.
• Table 2 Evolution of cell component technology for Molten
• Carbonate Fuel Cells (MCFC).
• Table 3 The components of the 1×2cm2 unit cells.
• Table 4 The porosity of the Ni cathode and the Co0.1-x/Cex-coated Ni