Multi-Physics Modeling of Solid Oxide Fuel Cell Fueled by Methane and Analysis of Carbon Deposition

Internal reformation of low steam methane fuel is important for the high e ciency and low cost operation of solid oxide fuel cell. Understanding and overcoming carbon deposition is crucial for the technology development. Here a multi-physics model is established for the relevant experimental cells. Balance of electrochemical potentials for the electrochemical reactions, generic rate expression for the methane steam reforming, dusty gas model in a form of Fick's model for anode gas transport are used in the model. Excellent agreement between the theoretical and experimental current-voltage relations is obtained, demonstrating the validity of the proposed theoretical model. The steam reaction order in low steam methane reforming reaction is found to be 1. Detailed information about the distributions of physical quantities is obtained by the numerical simulation. Carbon deposition is analyzed in detail and the mechanism for the coking inhibition by operating current is illustrated clearly. Two expressions of carbon activity are analyzed and found to be correct qualitatively, but not quantitatively. The role of anode diffusion layer on reducing the current threshold for carbon removal is also explained. It is noted that the current threshold reduction may be explained quantitatively with the carbon activity models that are only qualitatively correct.     

Methane-fueled SOFC with traditional nickel-based anode by applying Ni/Al2O3 as a dual-functional layer

Novel γ-Al₂O₃ supported nickel (Ni/Al₂O₃) catalyst was developed as a functional layer for Ni–ScSZ cermet anode operating on methane fuel. Catalytic tests demonstrated Ni/Al₂O₃ had high and comparable activity to Ru–CeO₂ and much higher activity than the Ni–ScSZ cermet anode for partial oxidation, steam and CO₂ reforming of methane to syngas between 750 and 850 °C. By adopting Ni/Al₂O₃ as a catalyst layer, the fuel cell demonstrated a peak power density of 382 mW cm^?2 at 850 °C, more than two times that without the catalyst layer. The Ni/Al₂O₃ also functioned as a diffusion barrier layer to reduce the methane concentration within the anode; consequently, the operation stability was also greatly improved without coke deposition.     

固体氧化物燃料电池LSCM-CeO_2/Ni-ScSZ复合阳极制备及性能表征

采用双层流延法制备Ni-ScSZ阳极支撑层-ScSZ电解质复合膜.在烧结的Ni-ScSZ阳极支撑层表面丝网印刷一层LSCM-CeO2阳极催化层,得到LSCM-CeO2/Ni-ScSZ功能梯度层阳极.研究表明,LSCM/CeO2比为1:3(bymass)的功能梯度层阳极Ni-ScSZ13具有较佳的性能.单电池在850℃以H2和乙醇蒸气作燃料的最大功率密度分别为710和669mW/cm2,而LSCM/CeO2为1:0(bymass)的功能梯度层Ni-ScSZ10作阳极的单电池,最大功率密度分别为521和486m W/cm2.两种阳极单电池,分别在700℃于乙醇蒸气中作长时间运行实验,X-射线能量散射分析表明Ni-ScSZ13阳极比Ni-ScSZ10阳极具有较好的抗碳沉积性能.     

Single solid-oxide fuel cells with supporting ni-cermet anode

Single solid-oxide fuel cells (SOFCs) with a porous (36-41%) supporting Ni-cermet anode are manufactured and tested. The effect of the thickness of the supporting Ni-cermet anode on the electrochemical characteristics of single SOFCs is studied. It is shown that polarization losses on electrodes at the current density of 1.2 A/cm² increase by about 2 times from 0.13 to 0.25 V at an increase in the thickness of the supporting Ni-cermet anode from 0.40 to 1.27 mm. The impedance spectroscopy method is used to identify relaxation processes responsible for the behavior of the fuel cell anode and cathode. It is found that a significant percentage of polarization losses on the anode is due to transport limitations in fuel supply to the three-phase nickel/electrolyte/gas phase interface and removal of the reaction products away from it.     

High temperature thermo-photocatalysis driven carbon removal in direct biogas fueled solid oxide fuel cells

Solid oxide fuel cells (SOFCs) can directly convert renewable biogas into electricity with high efficiency at high temperature, however the long-term stability of SOFCs is significantly affected by the carbon deposition on the anode during cell operation. Herein, we report a novel carbon removal approach by high temperature infrared light driven photocatalytic oxidation. Upon the comparison of electrochemical performance of Ni-YSZ anode and TiO₂ modified Ni-YSZ anode in the state-of-the-art single cell (Ni-YSZ/YSZ/LSCM), the modified anodes exhibit markedly improved peak powder density with simulated biogas fuel (70% CH₄+ 30% CO₂) at 850 °C with less coking after 40 h operation. The high activity and carbon deposition resistance of the modified anode is possibly attributed to the in situ generated hydroxyl radical from the reduced TiO_x powder under high temperature infrared light excitation, which is supported by detailed analysis of microstructural information of anodes and the powder-based thermo-photocatalytic experiments.     

Anode substrate with continuous porosity gradient for tubular solid oxide fuel cells

Tubular solid oxide fuel cells (SOFCs) are fabricated using a modified phase inversion process to obtain anode structure with graded pore distribution. The novel structure is achieved using an additional graphite layer to control the phase separation reaction in the ceramic layer and to remove the skin layer, which always presents in phase inversion process. The graded anode can effectively eliminate the concentration polarization loss at high current density as observed for the anode with the skin layer. In addition, improved peak power density is obtained with the graded-anode based cell, demonstrating that the modified process is promise in fabricating tubular SOFCs.     

固体氧化物燃料电池Cu-LSCM-CeO2/LSCM-YSZ/Ni-ScSZ复合阳极制备及性能

采用流延法制得LSCM-YSZ阳极支撑层/Ni-ScSZ阳极活性层/ScSZ电解质层复合膜,在LSCM-YSZ支撑层上印刷一层Cu-LSCM-CeO₂阳极催化层,即Cu-LSCM-CeO₂/LSCM-YSZ/Ni-ScSZ功能梯度层阳极. 研究表明,Cu/LSCM/CeO₂质量配比为2:7:1功能梯度阳极（LSCM-YSZ2010）有较好的性能,单电池以氢气和乙醇为燃料（750 ^oC）最大功率密度分别为511和390 mW?cm^-2,单电池稳定性实验表明,LSCM-YSZ2010阳极单电池以乙醇为燃料750 ^oC长时间运行218 h,性能稳定. X-射线能量散射分析表明该阳极具有较好的抗碳沉积性能.     

A verification of the reaction mechanism of direct carbon solid oxide fuel cells

Anode-supported tubular solid oxide fuel cells (SOFCs) with Cu–CeO₂–yttria-stabilized zirconia (YSZ) anode, YSZ electrolyte film, and silver cathode were fabricated. The cells were tested with 5 wt% Fe-loaded activated carbon and dry CO, respectively, and their performances were compared to verify the reaction mechanism of direct carbon SOFCs (DC-SOFCs). The corresponding current–voltage curves and impedance characteristics of the cells operating on these two different fuels were found to be almost the same at high temperatures, demonstrating the presumed mechanism that the anode reaction of a DC-SOFC is the electrochemical oxidation of CO, just as in a SOFC operated directly on CO. Some experimental evidences including the difference in open circuit voltage at different temperatures and the operating stability of the cells were analyzed in detail.     

中温固体氧化物燃料电池优势和挑战的简要评述

燃料电池是一种将燃料的化学能直接转化为电能的电化学发电装置. 在各种类型的燃料电池中,固体氧化物燃料电池（SOFC）在600~800 ^oC的中温区运行,因此与质子交换膜燃料电池等低温燃料电池相比,它的燃料选择范围更广,具有更广泛的应用前景. 然而,SOFC的商业应用面临着两大挑战：成本和稳定性. 这两种挑战与阳极、阴极、电解质、连接体和密封材料等组件的加工、制备、性能、化学和微结构稳定性密切相关. 电池堆的导管连接材料也需要经过仔细地筛选,以最大限度地降低有毒害的挥发性成分,从而确保电池结构的稳定和完整. 本文旨在简要评述SOFC的材料和组分的研究现状,并提出展望. 本文也对新一代SOFC技术面临的机遇和挑战进行了探讨.     

Hybrid direct carbon fuel cells and their reaction mechanisms—a review

As coal is expected to continue to dominate power generation demands worldwide, it is advisable to pursue the development of more efficient coal power generation technologies. Fuel cells show a much higher fuel utilization efficiency, emit fewer pollutants (NO _x , SO _x ), and are more easily combined with carbon capture and storage (CCS) due to the high purity of CO₂ emitted in the exhaust gas. Direct carbon (or coal) fuel cells (DCFCs) are directly fed with solid carbon to the anode chamber. The fuel cell converts the carbon at the anode and the oxygen at the cathode into electricity, heat and reaction products. The use of an external gasifier and a fuel cell operating on syngas (e.g. integrated gasification fuel cells) is briefly discussed for comparative purposes. A wide array of DCFC types have been investigated over the last 20 years. Here, the diversity of pre-commercialization DCFC research efforts is discussed on the fuel cell stack and system levels. The range of DCFC types can be roughly broken down into four fuel cell types: aqueous hydroxide, molten hydroxide, molten carbonate and solid oxide fuel cells. Emphasis is placed on the electrochemical reactions occurring at the anode and the proposed mechanism(s) of these reactions for molten carbonate, solid oxide and hybrid direct carbon fuel cells. Additionally, the criteria of choosing the ‘best’ DCFC technology is explored, including system design (continuous supply of solid fuel), performance (power density, efficiency), environmental burden (fresh water consumed, solid waste produced, CO₂ emitted, ease of combination with CCS) and economics (levelized cost of electricity).     

An investigation on the kinetics of direct carbon solid oxide fuel cells

A direct carbon solid oxide fuel cell (DC-SOFC) is an all-solid-state electricity generation device that operates directly with solid carbon as fuel, without any liquid medium and feeding gas. Tubular electrolyte-supported solid oxide fuel cells (SOFCs), with silver-gadolinium doped ceria (Ag-GDC) as both anode and cathode materials, are fabricated and operated directly with activated carbon as fuel. The kinetics of the DC-SOFCs is carried out through analyzing the correlations of the cell reaction rates to the emitting rates of CO and CO₂. It turns out that higher operating current corresponds to higher rates of consuming and producing CO, through electrochemical oxidation at the anode and the Boudouard reaction at the carbon fuel, respectively. The rate of consuming CO can be maintained constant by controlling the operating current while the rate of producing CO decreases with time because of carbon consumption. When the CO producing rate becomes smaller than the CO consuming rate, the operation will be terminated. Compared to the rates of the chemical reactions, the diffusion rates of CO and CO₂ are so fast that their impeding effect on the cell performance can be neglected.     

Fabrication of 10%Gd-doped ceria (GDC)/NiO–GDC half cell for low or intermediate temperature solid oxide fuel cells using spray pyrolysis

Solid oxide fuel cells (SOFCs) with comparably low operating temperature play a critical role in its commercialization and reliability by allowing low-cost fabrication and a promised longer life. Recently, 10%Gd-doped ceria (GDC) has revealed its importance as solid electrolytes for intermediate temperature SOFCs. Additionally, if GDC is employed in thin film form, rather higher ionic conductivity at further lower temperatures can be obtained and thereby allowing its use in low temperature SOFC. In the present investigation, the preparative parameters of spray pyrolysis technique (SPT) were optimized to deposit dense and adherent films of GDC on ceramic substrate. NiO–GDC was used as ceramic substrate, which also acts as a precursor composite anode for GDC-based SOFCs. Prepared half cells (GDC/NiO–GDC) were characterized using XRD, SEM, and electrochemical impedance spectroscopy. The surface and fractal SEM observations of post heat-treated (at 1,000 °C) GDC/NiO–GDC structure revealed that GDC films were uniform in thickness with improved adherence to substrate. The relative density of post heat-treated films was of the order of 96%, which was attributed to the presence of nano-granules in the thin films. Maximum thickness of the GDC film prepared with optimized preparative parameters (in single run) was of the order of 13 μm. Fractal SEM of post heat-treated GDC/NiO–GDC system showed homogenous interface, which was further analyzed by electrochemical impedance spectra and found that it does not affect electrical properties of structure significantly.     

Direct-methane solid oxide fuel cells with Cu1.3Mn1.7O4 spinel internal reforming layer

Cu_1.3Mn_1.7O₄ spinel oxide has been synthesized and characterized as anode internal reforming layer for Ni–SDC anode-supported solid oxide fuel cells (SOFCs) directly operating on methane fuel. XRD and Cu mapping image results of Cu_1.3Mn_1.7O₄ oxide after in-situ reduction by methane show that a highly dispersed nano-Cu metal network has been obtained. By adopting Cu_1.3Mn_1.7O₄ as an internal reforming layer, the cell demonstrated maximum power densities of 242 and 311 mW cm^?2 at 650 and 700 °C, respectively using methane as fuel and ambient air as oxidant. More importantly, Cu_1.3Mn_1.7O₄ internal reforming layer has significantly improved the cell performance stability. The cell with the Cu_1.3Mn_1.7O₄ internal reforming layer has demonstrated reasonably stable performance while the cell without it degraded very rapidly.     

阳极负载型SOFC阳极基底厚度对性能的影响

制备不同厚度阳极负载型YSZ薄膜固体氧化物燃料电池 ,并对电池的极化、放电性能进行了测试 .结果表明 ,电池的性能明显受阳极性能的影响 ,阳极过电位大的原因之一是受多孔阳极气体扩散的影响 .降低阳极基底的厚度 ,阳极过电位明显减小 ,电池性能明显提高 .当阳极基底厚度为 0 .5mm时 ,在 80 0℃工作温度下 ,电池的功率密度达到 0 .1 9W·cm- 2 ,较之阳极厚度为 1 .0mm的电池性能提高近 1 .5倍 (0 .1 3W·cm- 2 ) .     

Methanol dissociation and oxidation on single Fe atom supported on graphitic carbon nitride

Considering environmental protection and sustainable development, fuel cells have always been an ideal choice for clean energy. For the performance of fuel cells, the anode catalysts are a key consideration. In the work reported, we designed a new type of anode catalyst, in which graphitic carbon nitride was used as the substrate and transition metal iron as the supported atom. The calculation results were characterized by adsorption energy, reaction energy barrier, potential energy surface and charge analysis. Based on density functional theory, the gradual decomposition of methanol molecules on the catalyst is realized; the decomposition products are oxidized to reduce the formation of CO, and the efficiency of anode catalysis is improved, which provides a new idea for the design of anode materials for methanol fuel cells.     

硬模板法制备固体氧化物燃料电池Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x包覆管状SDC立体阳极

为在固体氧化物燃料电池中有效利用干甲烷为燃料,需制作多孔立体阳极。采用硬模板法和浸渍法制备Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x包覆管状SDC阳极材料(Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x/SDC),为作对比,用溶胶凝胶法制备粉末状Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x,机械混合SDC粉末制备Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x-SDC。将这两种阳极材料分别制作电解质支撑的单电池Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x/SDC|YSZ|LSMYSZ与Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x-SDC|YSZ|LSM-YSZ,并进行发电性能测试以及长期稳定性实验。结果表明,800℃下,干甲烷环境中,Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x-SDC为阳极的单电池最大功率密度为324.99 m W/cm2,运行10 h后,电压下降5.60%;而以Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x/SDC为阳极的单电池最大功率密度达到384.54 m W/cm2,运行100 h后,电压未严重衰减。实验后阳极的SEM照片表明,Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x-SDC阳极内孔隙狭小,易被积炭堵塞;而Ni_(0.5)Cu_(0.5)Ba_(0.05)O_x/SDC阳极呈立体多孔结构,有利于燃料气体与反应后气体的扩散。催化剂颗粒均匀地包覆在SDC纤维管表面,有利于增加三相界面,提高电池的稳定性。     

采用甲烷燃料的管状固体氧化物燃料电池浸渍阳极电化学性能

以NiO和8％（摩尔分数）氧化钇稳定的氧化锆为原料,采用注凝成型工艺制备了管状同体氧化物燃料电池阳极支撑体．用离子浸渍法对阳极支撑体进行表面修饰．用电化学工作站测单电池交流阻抗和输出性能并且用化学气相色谱仪对电池尾气进行分析．测试结果表明修饰后的阳极在通甲烷的情况下出现了一定程度的积炭,但是积炭现象在一定的测试时间内达到平衡,没有对电池造成破坏,并且显著地提高了电池阳极的电化学性能．单电池存通入氯气和甲烷的情况下最大输出功率密度分别达到了225和400mW／cm^2．     

Formation of NiO/YSZ functional anode layers of solid oxide fuel cells by magnetron sputtering

The decrease in the polarization resistance of the anode of solid-oxide fuel cells (SOFCs) due to the formation of an additional NiO/(ZrO₂ + 10 mol % Y₂O₃) (YSZ) functional layer was studied. NiO/YSZ films with different NiO contents were deposited by reactive magnetron sputtering of Ni and Zr–Y targets. The elemental and phase composition of the films was adjusted by regulating oxygen flow rate during the sputtering. The resulting films were studied by scanning electron microscopy and X-ray diffractometry. Comparative tests of planar SOFCs with a NiO/YSZ anode support, NiO/YSZ functional nanostructured anode layer, YSZ electrolyte, and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were performed. It was shown that the formation of a NiO/YSZ functional nanostructured anode leads to a 15–25% increase in the maximum power density of fuel cells in the working temperature range 500–800°C. The NiO/YSZ nanostructured anode layers lead not only to a reduction of the polarization resistance of the anode, but also to the formation of denser electrolyte films during subsequent magnetron sputtering of electrolyte.     

固体氧化物燃料电池（SOFC）电池稳定性分析

固体氧化物燃料电池(SOFC)要长期可靠运行,必须具有较高的稳定性。本文从SOFC内阻的主要来源出发,详细分析了影响电池长期稳定性、特别是引起性能衰减的主要因素,并研究其衰减机理。通过对电解质、阴极、阳极及连接材料等关键材料的选择及性能稳定性进行分析,系统论述了阴极与其它材料的相互反应、阳极性能变化以及连接材料表面氧化层等诸多引起SOFC性能衰减的不利因素。在氧化、还原气氛和密封效果等方面对电池长期稳定性的影响也进行了阐述。通过对电池性能衰减的原因及其衰减机理进行分析,对于SOFC长期运行稳定性、进而商业化应用具有一定的理论和实际意义。     

Improvement of multi-layer anode for direct ethanol Solid Oxide Fuel Cells

Anodes for Solid Oxide Fuel Cells (SOFCs) that are capable of directly using hydrocarbon fuels without external reforming have been of great interest in the recent years. We have fabricated a two-layer structure containing a Cu–CeO₂ catalyst layer by tape casting and screen printing technology. The interface combination between the Cu–CeO₂ catalyst layer and the Ni–YSZ-supported anode issues importantly for the stability of the anode structure. In this paper, a Ni–CeO₂ interlayer was added between these two layers to improve the long-term stability of the multi-layer anode. Meanwhile, the exit gas of the single cell was also analyzed by a gas chromatographer (GC) to determine the reaction mechanism of the ethanol fuel in the anode.