Methane In Solid Oxide Fuel Cells Research Articles

Ni and Ru bimetal nanoparticles-anchored porous Mo2C nanorods used as an internal reforming catalyst layer for hydrocarbon-fueled solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are considered the most efficient technology for the direct conversion of hydrocarbons to electrical energy. Although the commonly used Ni/YSZ cermet materials in SOFCs show excellent electrocatalytic properties in H2 fuel, they suffer from carbon deposition problem when hydrocarbon fuels are used. In this work, we study the use of Ni0·05Ru0.05-Mo2C with a porous nanorod-shaped morphology as an internal reforming catalyst layer for methane-fueled SOFCs to solve the degradation of cell performance due to carbon deposition. The electrochemical performance and coking tolerance of two types of cells with and without the catalyst layer were systematically studied using methane and hydrogen as fuels. With the catalyst layer, the single cell using methane as fuel exhibits an improvement in peak power density by 24%, increasing from 297 to 367 mW cm−2 and there was no diffusion barrier for fuel gases within the cell. Furthermore, the cell can stably run for 100 h in methane at 700 °C. The improvement of electrochemical performance is attributed to the synergistic effect of Ni and Ru sites anchored on the porous Mo2C nanorods catalyst based on strong metal-support interactions, promoting the effective catalytic conversion of CH4 to CO and H2. This study demonstrates that the integration of a catalyst layer for internal reforming is an effective strategy for the direct use of methane in SOFCs.

Direct-methane anode-supported solid oxide fuel cells fabricated by aqueous gel-casting

Direct methane Solid Oxide Fuel Cells (SOFCs) operated under catalytic partial oxidation (CPOX) conditions are investigated, focusing on the processing of the anode support and the anode deactivation caused by carbon deposition. Anode-supported SOFCs based on gadolinium-doped ceria (GDC) electrolyte, and NiO-GDC anode support were fabricated by the gel-casting method. Suitable aqueous slurries formulations of NiO–GDC were prepared, starting NiO-GDC nanocomposite powders, agarose as gelling agent and rice starch as pore former. Electrochemical and mechanical tests evidenced that the support of 550 ± 50 µm thickness and 10 wt% pore former is a good candidate for direct-methane SOFCs. The cells operating under stoichiometric conditions of CPOX reached a performance of 0.64 W·cm−2 at 650 ºC, a very close value to that measured under humidified hydrogen (0.71 W·cm−2). The best electrochemical stability of the cell is achieved at a CH4/O2 ratio of 2.5, showing no evidence of carbon deposition and reducing nickel re-oxidation significantly.

Ni-doped Ba0.9Zr0.8Y0.2O3-δ as a methane dry reforming catalyst for direct CH4–CO2 solid oxide fuel cells

Fuel flexibility is one of the significant advantages of solid oxide fuel cells (SOFCs). The utilization of methane in SOFCs can not only reduce fuel costs, but also greatly expand its application scenarios, which is of great significance to the commercial development of SOFCs. However, when methane is directly used, Ni-based cermet anode suffers from coking, which seriously affects the durability of the cell. To alleviate the coking issue, a reforming layer outside the Ni-based anode-supporter was proposed in this study, and Ba0.9(Zr0.8Y0.2)1-xNixO3-δ (BZYNix, x = 0.05, 0.1, 0.15 and 0.2) was used as reforming layer material. Among BZYNix catalysts, BZYNi0.2 exhibited excellent catalytic activity toward dry reforming of methane, and methane conversion was as high as 85% at 750 °C. The excellent catalytic durability and coking-resistance of BZYNi0.2 were also confirmed. When BZYNi0.2 reforming layer was applied, the single cell fueled with CH4–CO2 fuel showed significantly improved electrochemical performance, durability and coking-resistance. The utilization of BZYNi0.2 reforming layer provides guidance for solving the coking issue of SOFC cermet anodes when fueled with hydrocarbon.

Synthesis of Co–Ni and Cu–Ni based-catalysts for dry reforming of methane as potential components for SOFC anodes

In this work, we report a comparative study of Ni-based anode compositions, made of Cu and Co (40 and 80 mol%) and gadolinia-doped ceria (CGO) matrices, for application the dry reforming of methane (DRM) reaction using Solid Oxide Fuel Cells (SOFCs). The new compositions are synthesized by a one-step synthesis route, using citric acid as chelating agent, and characterized at three different stages: i) after synthesis, ii) after reduction, and iii) after DRM. X-ray diffraction (XRD) analysis combined with thermodynamic calculations is used to understand phase evolution along the different stages, revealing that complete solid solutions of NiCo- and NiCu-based alloys are formed after DRM reaction. Transmission Electron Microscopy (TEM) shows the formation of nanocrystalline powders, while surface area (SBET) measurements show higher values in the case of the NiCo-based samples. Moreover, the Co-containing compositions exhibit higher reducibility and stronger metal-ceramic interactions than the Cu-containing samples, according to the Temperature Programmed Reduction (TPR) results. Finally, DRM results demonstrate higher CO2 and CH4 conversions in the case of the Co-containing samples, as well as increased resistance towards carbon deposition, as confirmed by Thermogravimetric and Differential Scanning Calorimetry analyses (TG-DSC). Overall, the Co-based compositions are highly beneficial for their use as anodes for the CO2 reforming of methane in SOFCs.

Coking-resistant Ce0.8Ni0.2O2-δ internal reforming layer for direct methane solid oxide fuel cells

The development of direct methane solid oxide fuel cells (SOFCs) is severely hindered by the deactivation of conventional Ni-based anodes due to carbon fouling. Here, a Ce0.8Ni0.2O2-δ (CNO) internal reforming layer is imposed on conventional Ni-Sm0.2Ce0.8O2-x (SDC) anodes for direct methane SOFCs. In CNO, there are two types of Ni species which are segregated NiO dispersed over the CNO and incorporated Ni2+ in the ceria lattice, respectively. The Ni2+ dopants are stable in wet hydrogen at 650 °C; however, the segregated NiO is reduced into Ni under the same conditions. With the doping of Ni2+ into the ceria lattice, surface oxygen vacancies are generated in CNO. For the stability testing in wet methane (∼3 mol% H2O in methane) at 650 °C and 0.2 A cm−2, the voltage of the conventional Ni-SDC anode decreases by 43.1% in approximately 26 h, whereas the CNO internal reforming layer operates stably for 40 h. In wet methane at 650 °C, with the addition of the CNO internal reforming layer, the polarization resistance of the Ni-SDC anode reduces by 22.3% from 0.0917 to 0.0712 Ω cm2, and the maximum current density of it increases from 614 to 664 mW cm−2.

Proton-Conducting La-Doped Ceria-Based Internal Reforming Layer for Direct Methane Solid Oxide Fuel Cells.

Performance degradation caused by carbon deposition substantially restricts the development of direct methane solid oxide fuel cells (SOFCs). Here, an internal reforming layer composed of Ni supported on proton conducting La-doped ceria, such as La2Ce2O7 (LDC) and La1.95Sm0.05Ce2O7 (LSDC) is applied over conventional Ni-Ce0.8Sm0.2O2-x (SDC) anodes for direct methane SOFCs. The proton conducting layer can adsorb water for internal reforming thus significantly improving the performance of the direct methane SOFCs. In situ Raman and FTIR results confirm the water adsorption capacity of LDC and LSDC. They also exhibit excellent phase stability in wet CO2 at 650 °C for 10 h, which ensures that the additional catalyst layer maintains structure stability during the internal reforming. In wet methane at 650 °C, the peak power density of the conventional cell is only 580 ± 20 mW cm-2, and increases to 699 ± 20 and 639 ± 20 mW cm-2 with the addition of Ni-LDC and -LSDC layers, respectively. For the stability test in wet methane at 650 °C and 0.2 A cm-2, the voltage of the conventional cell starts to drop dramatically in 10 h, while the Ni-LDC and -LSDC catalyst layers operate stably in 26 h under the identical conditions. These catalyst layers even show comparable stability in dry and wet methane in 26 h, but for longer operation, the wet methane is still preferred for maintaining the stability of the cell.

Surface tuned La0.9Ca0.1Fe0.9Nb0.1O3-δ based anode for direct methane solid oxide fuel cells by infiltration method

It has been found in our previous study that La0.9Ca0.1Fe0.9Nb0.1O3-δ-Sm0.2Ce0.8O1.9 (LCFNb-SDC) anode shows high catalytic activity and stability for solid oxide fuel cells (SOFCs) under H2 and CO atmospheres. In this study, LCFNb-SDC is further tuned into a promising anode for direct methane SOFCs through surface modification with Ni. The LCFNb, SDC and Ni materials are found to be thermal-mechanically and chemically compatible with each other up to 850°C. Although LCFNb-SDC anode shows poor catalytic activity for methane oxidation, further modification of such anode with a small amount of Ni particles significantly improves the cell performance in CH4. The maximal power densities of 1031 and 729mWcm−2 in H2 and CH4 have been achieved for the electrolyte supported SOFC with LCFNb-SDC-Ni|SSZ (Sc0.2Zr0.8O2−δ)|LSM (La0.8Sr0.2MnO3) configuration at 850°C, respectively. Besides, the same cells also exhibit good stabilities in both H2 (500h) and CH4 (100h) with no obvious performance degradation and carbon deposition. Surface tuned LCFNb anode by impregnating SDC and a small amount of Ni is thus highly promising as an anode for direct methane SOFCs to deliver favorable performance with reasonable stability.

Controlled deposition and utilization of carbon on Ni-YSZ anodes of SOFCs operating on dry methane

Solid oxide fuel cells (SOFCs) are promising power-generation systems to utilize methane or methane-based fuels with a high energy efficiency and low environmental impact. A successive multi-stage process is performed to explore the operation of cells using dry methane or the deposited carbon from methane decomposition as fuel. Stable operation can be maintained by optimizing the fuel supply and current density parameters. An electrochemical impedance analysis suggests that the partial oxidization of Ni can occur at anodes when the carbon fuel is consumed. The stability of cells operated on pure methane is investigated in three operating modes. The cell can run in a comparatively stable state with continuous power output in an intermittent methane supply mode, where the deposition and utilization of carbon is controlled by balancing the fuel supply and consumption. The increase in the polarization resistance of the cell might originate from the small amount of NiO and residual carbon at the anode, which can be removed via an oxidation-and-reduction maintenance process. Based on the above strategy, this work provides an alternative operating mode to improve the stability of direct methane SOFCs and demonstrates the feasibility of its application.

Development of a Coking‐Resistant NiSn Anode for the Direct Methane SOFC

AbstractThe present work reports on the development of a coking‐resistant NiSn‐based membrane electrode assembly (MEA) for internal CH4 reforming in solid oxide fuel cells (SOFCs). Catalyst powder was prepared in a centrifugal casting oven by melting stoichiometric amounts of Ni and Sn under vacuum. The formation of Ni3Sn2 intermetallic phase was confirmed by XRD analysis. Catalytic activity for CH4 reforming and stability of the NiSn powder were first evaluated in a quartz glass reactor for 4 h at 600–1,000 °C. The main reaction products H2 and CO were detected by gas chromatography while no carbon formation was detected during the experiments. Then, 3YSZ electrolyte‐supported MEAs were fabricated with a Ni3Sn2/YSZ anode and LSM/YSZ cathode and characterized under SOFC conditions. The MEA showed an excellent stability under CH4 atmosphere (3% H2O) at 850 °C over more than 650 h. No substantial decrease in cell potential was observed during this period.

Composite Anodes with Ni Impregnated LST-SSZ for Direct Methane Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFCs) is one of the most attractive energy conversion devices because of their high efficiency, low pollution, and fuel flexibility. La0.2Sr0.8TiO3 (LST) anode material shows its potential utilization in direct oxidation of methane fuel without carbon formation. In this paper, LST powders were synthesized by traditional solid-state method, and then mixed with scandia-stabilized zirconia (SSZ) in mass ratio of 5: 5 to prepare the composite anode materials. Symmetrical cells with LST-SSZ composite anode were fabricated and measured. Their polarization resistances in hydrogen at 700°C, 750°C and 800°C were 5.3, 3.0 and 2.0 Ω·cm, respectively. Considering the insufficient conductivity of LST, 10wt%Ni was impregnated into the composite anode to improve the anode performance. The polarization resistance of the symmetrical cell with 10wt% Ni impregnation load is obviously reduced. The maximum power density of a cathode supported single cell with 10wt%Ni-LST-SSZ composite anode are 225 mW/cm and 175 mW/cm in hydrogen and in methane at 750°C, respectively, and the single cell shows its stability running in methane fuels.

Cooling of a Diesel Reformate Fuelled Solid Oxide Fuel Cell by Internal Reforming of Methane: A Modelling Study

In this paper a system combining a diesel reformer using catalytic partial oxidation (CPOX) with the Solid Oxide Fuel Cell (SOFC) for Auxiliary Power Unit (APU) applications is modeled with respect to the cooling effect provided by internal reforming of methane in anode gas channel. A model mixture consisting of 80%n-hexadecane and 20% 1-methylnaphthalin is used to simulate the commercial diesel. The modelling consists of several steps. First, equilibrium gas composition at the exit of CPOX reformer is modelled in terms oxygen to carbon (O/C) ratio, fuel utilization ratio and anode gas recirculation. Second, product composition, especially methane content, is determined for the methanation process at the operating temperatures ranging from 500 °C to 520 °C. Finally, the cooling power provided by internal reforming of methane in SOFC fuel channel is calculated for two concepts to increase the methane content of the diesel reformate. The results show that the first concept, operating the diesel reformer at low O/C ratio and/or recirculation ratio, is not realizable due to high probability of coke formation, whereas the second concept, combining a methanation process with CPOX, can provide a significant cooling effect in addition to the conventional cooling concept which needs higher levels of excess air.

The conversion among reactions at Ni-based anodes in solid oxide fuel cells with low concentrations of dry methane

Dry methane with different concentrations is directly fed to solid oxide fuel cells (SOFCs) with a Ni-yttria-stabilized zirconia (Ni-YSZ) anode and a Ni-scandia-stabilized zirconia (Ni-ScSZ) anode. The anode outlet gases are measured in situ by gas chromatography (GC) to study the reactions of dry methane at different current densities. The comparison between the measured open-circuit voltages (OCVs) and theoretical values, the quantitative analysis of components under different current densities and the activation energy analysis of elementary reactions of CH4 are investigated to identify the types of reactions occurring to methane in SOFCs. It is found that reactions of partial oxidation, CH4+2O2−→CO+H2O+H2+4e−, CH4+3O2−→CO+2H2O+6e−, and complete oxidation occur to CH4 at the Ni-based anodes in sequence while the current density increasing.

Pd-Promoted La0.75Sr0.25Cr0.5Mn0.5O3 / YSZ Composite Anodes for Direct Utilization of Methane in SOFCs

Palladium-impregnated zirconia (LSCM/YSZ) composite anode is investigated in detail for the direct utilization of methane in solid oxide fuel cells (SOFCs). Impregnation of Pd nanoparticles significantly promotes the electrocatalytic activity of LSCM/YSZ composite anodes for the methane electro-oxidation reaction in wet . At , the electrode polarization resistance for the methane oxidation is reduced by a factor of 3 after impregnation of 0.10–0.66 mg Pd. Pd impregnation primarily enhances the electrode processes at low frequencies, indicating the significant promotion effect on the oxygen transfer process and the decomposition for the reaction on the LSCM/YSZ composite anodes. No carbon deposition was observed for the reaction in wet on Pd-impregnated LSCM/YSZ composite anodes. In contrast, Pd impregnation has little effect on the hydrogen oxidation in wet . The results demonstrate that the Pd-impregnated LSCM/YSZ composite is a promising carbon-tolerant anode for natural gas fuel-based SOFCs.

Al-Doped (La,Sr)TiO3+δ Anodes for the Direct Oxidation of Methane in SOFCs

Perovskite (La,Sr)TiO3+δ compounds have been identified as suitable materials for solid oxide fuel cell (SOFC) anodes, due to their high conductivity and stability in reducing conditions. Additionally, when using hydrocarbon fuels, these materials have been proven resistant to carbon deposition. Previous research has shown that the fuel cell performance can be significantly improved by replacing the titanium on the perovskite B-site with other elements. (La,Sr)TiO3+δ compounds, doped with aluminium, were synthesised by solid state reaction. Structural analysis, by XRD and TEM, revealed the structure of these compounds varies with the aluminium content and the amount of oxygen excess (δ). The electrical properties of these materials were analysed using A.C. impedance and four point D.C. conductivity measurements. The conductivity of the compounds was shown to be dependent on the structure. The use of these materials as anodes, in preliminary fuel cell tests has revealed that some compositions have catalytic activity for the direct oxidation of methane. Open cell potentials as high as 1.14V were obtained at 950{degree sign}C.

Catalytic Behavior of Pd–Ni/Composite Anode for Direct Oxidation of Methane in SOFCs

Electrocatalysis of a Pd–Ni/composite anode (small amounts of Pd–Ni catalyst on a lanthanum chromite-based porous composite anode) for direct oxidation of dry methane in solid oxide fuel cells (SOFCs) was studied at . Synergy of Pd and Ni catalysts was observed for the direct oxidation of dry methane. Maximum power densities with the Pd–Ni/composite anode were 150 and at 1073 and , respectively. Carbon deposition on the Pd–Ni/composite anode was quite low under both open- and closed-circuit conditions. Therefore, stable power generation was performed under the closed-circuit condition, and no physical damage was observed for the cell under the open-circuit condition with dry methane. XRD analysis suggested the formation of Pd–Ni alloy on the composite anode. A suitable reaction scheme for the direct oxidation of dry methane over the Pd–Ni/composite anode was proposed on the basis of the kinetic experiment, the gasification experiment of the deposited carbon by steam, and impedance spectra. The decomposition of to and C, and the gasification of C by to and CO, are key reactions for the effective direct oxidation of dry over the Pd–Ni/composite anode.

Cu-Co Bimetallic Anodes for Direct Utilization of Methane in SOFCs

Cu-Co bimetallic mixtures with ceria and YSZ have been investigated as solid oxide fuel cell (SOFC) anodes. While pure Co is unstable in dry CH 4 at 1073 K due to severe carbon formation, Cu-Co mixtures with even 90 wt % Co showed only small amounts of carbon following exposure to dry CH 4 for 2 h at 1073 K. Cells based on Cu-Co mixtures showed improved performance in both H 2 and CH 4 compared to cells prepared with only Cu. Finally, a cell with a 50:50 ratio of Cu and Co was shown to be stable for 500 h in humidified (3 mol % H 2 O) CH 4 at 1073 K.

Operation of anode-supported solid oxide fuel cells on methane and natural gas

Solid oxide fuel cells (SOFCs) with thin yttria-stabilized zirconia (YSZ) electrolytes on porous Ni-YSZ anodes were successfully operated with humidified methane and natural gas. For both fuels, single-cell tests were carried out with low fuel utilizations in order to approximate the conditions expected near the fuel inlet in a direct methane SOFC stack. Open circuit voltages (OCV) increased from ≈1.1 to 1.17 V, and power densities increased from ≈0.1 to 1.0 W/cm 2, as the operating temperature was increased from 600 to 800 °C. Comparisons of open circuit voltages with Nernst potentials suggested partial oxidation of solid C as the main anode reaction. SOFCs were operated in methane for >90 h at 700 °C and 0.6 V, yielding a stable power density of ≈0.35 W/cm 2. Very little carbon was detected on the anodes, suggesting that carbon deposition was limited during cell operation.

Cu-Ni Cermet Anodes for Direct Oxidation of Methane in Solid-Oxide Fuel Cells

We have examined the use of Cu-Ni alloys as anodes for the direct oxidation of methane in solid-oxide fuel cells (SOFC) at 1073 K. Ceramic-metal (cermet) composites having alloy compositions of 0, 10, 20, 50 and 100% Ni were exposed to dry methane at 1073 K for 1.5 h to demonstrate that carbon formation is greatly suppressed on the Cu-Ni alloys compared to that of pure Ni. Increased reduction temperatures also reduced the carbon formation on the alloys. The performance of a fuel cell made with a Cu(80%)-Ni(20%) cermet was tested in dry methane for 500 h and showed a significant increase in power density with time. Impedance spectra of similar fuel cells suggest that small carbon deposits are formed with time and that the increase in performance is due to enhanced electronic conductivity in the anode. Finally, the implications of the use of metal alloys for SOFC applications are discussed. © 2002 The Electrochemical Society. All rights reserved.