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.     

Recent anode advances in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical reactors that can directly convert the chemical energy of a fuel gas into electrical energy with high efficiency and in an environment-friendly way. The recent trends in the research of solid oxide fuel cells concern the use of available hydrocarbon fuels, such as natural gas. The most commonly used anode material Ni/YSZ cermet exhibits some disadvantages when hydrocarbons were used as fuels. Thus it is necessary to develop alternative anode materials which display mixed conductivity under fuel conditions. This article reviews the recent developments of anode in SOFCs with principal emphasis on the material aspects. In addition, the mechanism and kinetics of fuel oxidation reactions are also addressed. Various processes used for the cost-effective fabrication of anode have also been summarized. Finally, this review will be concluded with personal perspectives on the future research directions of this area.     

Highly active lanthanum doped nickel anode for solid oxide fuel cells directly fuelled with methane

Lanthanum doped nickel and YSZ composite anode (LaNi–YSZ) exhibited a greatly reduced polarization resistance and high performance for electrochemical oxidation of hydrogen and methane, which resulted from a fine anode structure with a high dispersion of nickel catalyst and a high catalytic activity towards methane.     

Fabrication of solid oxide fuel cell anode electrode by spray pyrolysis

Large triple phase boundaries (TPBs) and high gas diffusion capability are critical in enhancing the performance of a solid oxide fuel cell (SOFC). In this study, ultrasonic spray pyrolysis has been investigated to assess its capability in controlling the anode microstructure. Deposition of porous anode film of nickel and Ce_0.9Gd_0.1O_1.95 on a dense 8 mol.% yttria stabilized zirconia (YSZ) substrate was carried out. First, an ultrasonic atomization model was utilized to predict the deposited particle size. The model accurately estimated the deposited particle size based on the feed solution condition. Second, effects of various process parameters, which included the precursor solution feed rate, precursor solution concentration and deposition temperature, on the TPB formation and porosity were investigated. The deposition temperature and precursor solution concentration were the most critical parameters that influenced the morphology, porosity and particle size of the anode electrode. Ultrasonic spray pyrolysis achieved homogeneous distribution of constitutive elements within the deposited particles and demonstrated capability to control the particle size and porosity in the range of 2-17 μm and 21-52%, respectively.     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.     

Combined Cu‐CeO2 /YSZ and Ni/YSZ dual layer anode structures for direct methane solid oxide fuel cells

In this study, a conventional Ni/yttria‐stabilized zirconia (YSZ) anode and a new Cu‐CeO₂‐YSZ anode structure were assembled in an attempt to combine the advantages of both structures for use in direct methane solid oxide fuel cells. For this purpose, only a limited region (≤20 μm) of NiO/YSZ was deposited at the boundary of the electrolyte to benefit from the superior catalytic activity of Ni in the cells, while the rest of the cell benefited from the Cu‐CeO₂‐YSZ anode structure, which does not cause cracking reactions. First, the effects of different pore formers on the anode skeleton, as well as the interactions of the Ni‐Cu species in the anode skeleton, are discussed. Then, the NiO/YSZ‐interlayer‐containing button cells with different thicknesses (≤20) and different ratios of NiO (40 wt%, 50 wt%, and 60 wt%) were studied. After the examination of the cells, 2 model cells with outstanding performance and 2 additional internal reference cells, conventional Ni/YSZ and Cu‐CeO₂‐YSZ, were scaled up, and performance analysis and long‐term stability studies were carried out. As a result, for solid oxide fuel cells with increased carbonization resistance (around 6% performance loss due to carbonization after 100‐hour stability testing) and 86.1% of the initial performance of the conventional Ni/YSZ anode structure, a 15‐μm‐thick 40 wt% NiO/60 wt% YSZ interlayer with a dual layer anode structure is proposed.     

An efficient ceramic-based anode for solid oxide fuel cells

A new ceramic-based multi-component material, containing one electronic conductor (Y-substituted SrTiO₃, SYT), one ionic conductor (YSZ) and a small amount (∼5 vol.%) of Ni catalyst, was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The ceramic framework SYT/YSZ shows good dimensional stability upon redox cycling and has an electrical conductivity of ∼10 S cm⁻¹ under typical anode conditions. Owing to the substantial electrocatalytic activity of the fine and well-dispersed Ni particles on the surface of the ceramic framework, the electrode polarization resistance of 5 vol.% Ni-impregnated SYT/YSZ anode reached 0.21 Ω cm² at 800 °C in wet Ar/5%H₂. Based on these results, a redox-stable anode-supported planar SOFC is expected using this anode material, thus offering great advantages over the current generation of Ni/YSZ-based SOFCs.     

Y-substituted SrTiO3-YSZ composites as anode materials for solid oxide fuel cells: Interaction between SYT and YSZ

Donor-substituted SrTiO₃ ceramic materials were investigated as the anodes of solid oxide fuel cells (SOFCs). Sr_0.89Y_0.07TiO_3−δ (SYT) samples with good electrical conductivity and redox stability were prepared. The thermal and chemical expansions of SYT are both compatible with YSZ electrolyte. Half cells consisting of a flat anode substrate and an electrolyte layer with outer dimensions of 5 cm × 5 cm were fabricated, Ni particles were infiltrated on the pore walls within the ceramic anode framework, and the redox stability of the half cell was tested by He leakage tests after redox cycling. Ti diffusion but no Sr migration was found between the anode and electrolyte layers. Sr_0.895Y_0.07Ti_xO_3±δ (x = 1.00-1.20)-YSZ composites with a volume ratio 2:1 were prepared to investigate the influences of this interdiffusion between YSZ and SYT materials. The results indicate that the conductivity of SYT decreases because of the Ti diffusion, and a small amount of Ti excess can solve this problem.     

Alternative anode materials for solid oxide fuel cells

The electrolyte of a solid oxide fuel cell (SOFC) is an O²⁻-ion conductor. The anode must oxidize the fuel with O²⁻ ions received from the electrolyte and it must deliver electrons of the fuel chemisorption reaction to a current collector. Cells operating on H₂ and CO generally use a porous Ni/electrolyte cermet that supports a thin, dense electrolyte. Ni acts as both the electronic conductor and the catalyst for splitting the H₂ bond; the oxidation of H₂ to H₂O occurs at the Ni/electrolyte/H₂ triple-phase boundary (TPB). The CO is oxidized at the oxide component of the cermet, which may be the electrolyte, yttria-stabilized zirconia, or a mixed oxide-ion/electron conductor (MIEC). The MIEC is commonly a Gd-doped ceria. The design and fabrication of these anodes are evaluated. Use of natural gas as the fuel requires another strategy, and MIECs are being explored for this application. The several constraints on these MIECs are outlined, and preliminary results of this on-going investigation are reviewed.     

Investigation of the electrochemical active thickness of solid oxide fuel cell anode

Determination of the electrochemical active thickness (EAT) is of paramount importance for optimizing the solid oxide fuel cell (SOFC) electrode. However, very different EAT values are reported in the previous literatures. This paper aims to systematically study the EAT of SOFC anode numerically. An SOFC model coupling electrochemical reactions with transport of gas, electron and ion is developed. The microstructure features of the electrode are modeled based on the percolation theory and coordinate number theory. Parametric analysis is performed to examine the effects of various operating conditions and microstructures on EAT. Results indicate that EAT increases with decreasing exchange current density (or decreasing TPB length) and increasing effective ionic conductivity. In addition to the numerical simulations, theoretical analysis is conducted including various losses in the electrode, which clearly shows that the EAT highly depends on the ratio of concentration related activation loss R_act,con to ohmic loss R_ohmic. The theoretical analysis explains very well the different EATs reported in the literature and is different from the common understanding that the EAT is controlled mainly by the ionic conductivity of electrode.     

NiO/YSZ nanocomposite particles synthesized via co-precipitation method for electrochemically active Ni/YSZ anode

NiO/YSZ composite particles were synthesized via a co-precipitation of hydroxides. We investigated the effect of pH on the morphology of the composite particles, as well as on the microstructure and the electrochemical property of the Ni/YSZ anode. The particles synthesized at pH 10 involved aggregated composites and large NiO. The particles resulted in coarse and inhomogeneous anode microstructure and moderate area specific resistance (ASR) as 0.57 Ω cm² at 800 °C under open circuit voltage (OCV). Contrarily, nano-sized composite particles were successfully synthesized at pH 13. The particles provided fine as well as homogeneous porous structure with the grain size in the range 200-400 nm and low ASR as 0.36 Ω cm² at 800 °C under OCV.     

A review on anode on-cell catalyst reforming layer for direct methane solid oxide fuel cells

The commercialization of solid oxide fuel cells (SOFCs) can be significantly promoted with the direct utilization of methane, which is the primary component in natural gas and the second most abundant anthropogenic greenhouse gas. However, carbon deposition on most commonly used Ni-based anode is the bottle-necking issue inhibiting long-term stability of direct methane SOFCs. To avoid such a problem, methane is typically reformed (internally or externally) in SOFCs. Considering the cost, system simplification, coking resistance, and material selection, the on-cell catalytic reforming layer (OCRL) is one of the most promising designs for direct methane SOFCs. Reforming catalytic materials are typically consisted of active component, substrate and catalytic promoter, all of which have a significant impact on the catalytic activity, sintering resistance and coking resistance of methane reforming catalysts. This review summarizes the influence of the various components, some common OCRL materials and their applications in direct methane SOFCs, reforming and coking resistance mechanism, as well as the remaining challenges. The effective utilization of OCRL plays a pivotal role in promoting the development of direct methane SOFCs and the commercialization of SOFCs.     

Three dimensional stress analysis of solid oxide fuel cell anode micro structure

One of the most common problems in solid oxide fuel cells (SOFCs) is the delamination and thus the degradation of electrode/electrolyte interface which occurs in the consequences of the stresses generated within the different layers of the cell. Nowadays, the modeling of this problem under certain conditions is one of the main issues for the researchers. The structural and thermo-physical properties of the cell materials (i.e. porosity, density, Young's modulus etc.) are usually assumed to be homogenous in the mathematical modeling of solid oxide fuel cells at macro-scale. However, during the real operation, the stresses created in the multiphase porous layers might be very different than those at macro-scale. Therefore, micro-level modeling is required for an accurate estimation of the real stresses and the performance of SOFCs. This study presents a microstructural characterization and a finite element analysis of the delamination and the degradation of porous solid oxide fuel cell anode and electrode/electrolyte interface under various operating temperatures, compressing forces and material compositions by using the synthetically generated microstructures. A multi physics computational package (COMSOL) is employed to calculate the Von Misses stresses in the anode microstructures. The maximum thermal stress in the electrode/electrolyte interface and three phase boundaries is found to exceed the yield strength at 900 °C while 800 °C is estimated as a critical temperature for the delamination and micro cracks due to thermal stress generated. The thermal stress decreases in the grain boundaries with increasing content of one of the phases (either Ni or YSZ) and the porosity of the electrode. A clamping load higher than 5 kg cm⁻² is also found to exceed the shear stress limit.     

Ni–Fe + SDC composite as anode material for intermediate temperature solid oxide fuel cell

Composite materials of Sm_0.2Ce_0.8O_1.9 (SDC) with various Ni–Fe alloys were synthesized and evaluated as the anode for intermediate temperature solid oxide fuel cell. The performance of single cells consisting of the Ni–Fe + SDC anode, SDC buffer layer, La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) electrolyte, and SrCo_0.8Fe_0.2O_3 − δ (SCF) cathode were measured in the temperature range of 600–800 °C with wet H₂ as fuel. It was found that the anodic overpotentials of the different Fe–Ni compositions at 800 °C were in the following order: Ni_0.8Fe_0.2 < Ni_0.75Fe_0.25 < Ni < Ni_0.7Fe_0.3 < Ni_0.9Fe_0.1 < Ni_0.95Fe_0.05 < Ni_0.33Fe_0.67. The single cell with the Ni_0.8Fe_0.2 + SDC anode exhibited a maximum power density of 1.43 W cm⁻² at 800 °C and 0.62 W cm⁻² at 700 °C. The polarization resistance of the Ni_0.8Fe_0.2 + SDC anode was as low as 0.105 Ω cm² at 800 °C under open circuit condition. A stable performance with essentially negligible increase in anode overpotential was observed during about 160 h operation of the cell with the Ni_0.8Fe_0.2 + SDC anode at 800 °C with a fixed current density of 1875 mA cm⁻². The possible mechanism responsible for the improved electrochemical properties of the composite anodes with the Ni_0.8Fe_0.2 and Ni₀₇₅Fe_0.25 alloys was discussed.     

Fabrication of tubular NiO/YSZ anode-support of solid oxide fuel cell by gelcasting

A gelcasting process has been developed to fabricate tubular NiO/YSZ anode-support for solid oxide fuel cells (SOFCs) successfully. The rheological behaviors of the ceramic particle suspensions for gelcasting were investigated as a function of the process parameters, such as the amount of pore former, pH value, dispersant concentration, monomer concentration, ball-mill time and solid loading. The sintering shrinkage, microstructure, bending strength and electrical conductivity of the sintered specimens were examined. The tubular Ni/YSZ anode-support obtained under the optimized preparation conditions exhibited a porosity of 39.6%, mean pore size of below 0.9 μm, 482 s cm⁻¹ in electrical conductivity at 700 °C, and the bending strength of 112.8 MPa, which can well meet the requirements for SOFCs.     

Improvement of output performance of solid oxide fuel cell by optimizing Ni/samaria-doped ceria anode functional layer

Anode functional layers (AFLs) were fabricated using slurry spin coating method on anode substrates to improve the performance of cells based on samaria-doped ceria (SDC) films. The effects of the chemical compositions of AFL and AFL thickness on the performance of solid oxide fuel cell anodes were investigated by studying their effect on the ohmic loss, electrode overpotential, and output performance of cells in different atmospheres. With humidified hydrogen used as fuel and oxygen as oxidant, the cell with an 8-μm-thick AFL (NiO:SDC = 6:4) exhibited excellent maximum power densities of 3.41, 2.89, 1.46 and 0.80 W cm⁻² at 650, 600, 550 and 500 °C, respectively.     

Characterisation of electrical performance of anode supported micro-tubular solid oxide fuel cell with methane fuel

The reduction and operation of Ni–YSZ anode-supported tubular cells on methane fuel is described. Cells were reduced on pure methane from 650 °C to 850 °C, varying reduction time and methane flow rate. The effect on electrochemical performance with methane fuel was then investigated at 850 °C after which temperature-programmed oxidation (TPO) was employed to measure carbon deposition. Results showed that carbon deposition was minimized after certain reduction conditions. The conclusion was that 30 min reduction at 650 °C with 10 ml min⁻¹ methane reduction flow rate led to the highest current output over 1.2 A cm⁻² at 0.5 V when the cell operated at 850 °C between 10 ml min⁻¹ and 12.5 ml min⁻¹ methane running flow rate. From these results, it is evident that solid oxide fuel cell (SOFC) performance can be substantially improved by optimising preparation, reduction and operating conditions without the need for hydrogen.     

Physically mixed LiLaNi-Al2O3 and copper as conductive anode catalysts in a solid oxide fuel cell for methane internal reforming and partial oxidation

Different concentrations of copper are added to LiLaNi-Al₂O₃ to improve the electronic conductivity property for application as the materials of the anode catalyst layer for solid oxide fuel cells operating on methane. Their catalytic activity for the methane partial oxidation, steam and CO₂ reforming reactions at 600-850 °C is systematically investigated. Among the three catalysts, the LiLaNi-Al₂O₃/Cu (50:50, by weight) catalyst presents the best catalytic activity. Thus, the catalytic stability, carbon deposition and surface conductivity of the LiLaNi-Al₂O₃/Cu catalyst are further studied in detail. O₂-TPO results indicate that the coking resistance of LiLaNi-Al₂O₃/Cu is satisfactory and comparable to that of LiLaNi-Al₂O₃. The surface conductivity tests demonstrate it is extremely improved for LiLaNi-Al₂O₃ catalyst due to the addition of 50 wt.% copper. A cell with LiLaNi-Al₂O₃/Cu (50:50) catalyst layer is operated on mixtures of methane-O₂, methane-H₂O and methane-CO₂, and peak power densities of 1081, 1036 and 988 mW cm⁻² are obtained at 850 °C, respectively, comparable to the cell with LiLaNi-Al₂O₃ catalyst layer. In summary, the results of the present study indicate that LiLaNi-Al₂O₃/Cu (50:50) catalysts are highly coking resistant and conductive catalyst layers for solid oxide fuel cells.     

Solid oxide fuel cell bi-layer anode with gadolinia-doped ceria for utilization of solid carbon fuel

Pyrolytic carbon was used as fuel in a solid oxide fuel cell (SOFC) with a yttria-stabilized zirconia (YSZ) electrolyte and a bi-layer anode composed of nickel oxide gadolinia-doped ceria (NiO-GDC) and NiO-YSZ. The common problems of bulk shrinkage and emergent porosity in the YSZ layer adjacent to the GDC/YSZ interface were avoided by using an interlayer of porous NiO-YSZ as a buffer anode layer between the electrolyte and the NiO-GDC primary anode. Cells were fabricated from commercially available component powders so that unconventional production methods suggested in the literature were avoided, that is, the necessity of glycine-nitrate combustion synthesis, specialty multicomponent oxide powders, sputtering, or chemical vapor deposition. The easily-fabricated cell was successfully utilized with hydrogen and propane fuels as well as carbon deposited on the anode during the cyclic operation with the propane. A cell of similar construction could be used in the exhaust stream of a diesel engine to capture and utilize soot for secondary power generation and decreased particulate pollution without the need for filter regeneration.     

Direct operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell stack with methane

Operation of cone-shaped anode-supported segmented-in-series solid oxide fuel cell (SIS-SOFC) stack directly on methane is studied. A cone-shaped solid oxide fuel cell stack is assembled by connecting 11 cone-shaped anode-supported single cells in series. The 11-cell-stack provides a maximum power output of about 8 W (421.4 mW cm⁻² calculated using active cathode area) at 800 °C and 6 W (310.8 mW cm⁻²) at 700 °C, when operated with humidified methane fuel. The maximum volumetric power density of the stack is 0.9 W cm⁻³ at 800 °C. Good stability is observed during 10 periods of thermal cycling test. SEM-EDX measurements are taken for analyzing the microstructures and the coking degrees.