High-performance NdSrCo2O5+δ–Ce0.8Gd0.2O2-δ composite cathodes for electrolyte-supported microtubular solid oxide fuel cells

NdSrCo₂O_5+δ (NSCO) is a perovskite with an electrical conductivity of 1551.3 S cm⁻¹ at 500 °C and 921.7 S cm⁻¹ at 800 °C and has a metal-like temperature dependence. This perovskite is used as the cathode material for Ce_0.8Gd_0.2O_2-δ (GDC)-supported microtubular solid oxide fuel cells (MT-SOFCs). The MT-SOFCs fabricated in this study consist of a bilayer anode, comprising a NiO–GDC composite layer and a NiO layer, and a NSCO–GDC composite cathode. Three cell designs with different outer tube diameters, GDC thicknesses, and NSCO/GDC ratios are designed. The MT-SOFC with an outer tube diameter of 1.86 mm, an electrolyte thickness of 180 μm, and a 5NSCO–5GDC composite cathode presents the best performance. The flexural strength of the aforementioned cell is 177 MPa, which is sufficient to confer mechanical integrity to the cell. Moreover, the ohmic and polarization resistance values of the cell are 0.22 and 0.09 Ω cm² at 700 °C, respectively, and 0.15 and 0.03 Ω cm² at 800 °C, respectively. These results indicate that the NSCO-GDC composite exhibits high electrochemical activity. The maximum power densities of the cell at 700 and 800 °C are 0.46 and 0.67 W cm⁻², respectively, exceeding those of existing electrolyte-supported MT-SOFCs with similar electrolyte thicknesses.     

High performance of direct methane-fuelled solid oxide fuel cell with samarium modified nickel-based anode

Natural gas is one of the most attractive fuels for solid oxide fuel cell (SOFC), while the anode activity for methane fuel has a great influence on the performance and stability of SOFC. Samarium is a good catalyst promoter for methane reforming. In this work, samarium is used to modify nickel catalyst, which results in small nickel oxide particles. The SmNi-YSZ (yttria-stabilized zirconia) anode has smaller particles and better interfacial contact between nickel and YSZ compared with conventional Ni-YSZ anode. The fine structure of SmNi-YSZ anode results in high activity for electrochemical oxidation of hydrogen and low polarization resistance of the cell. The performance of SmNi-YSZ anode cell with humidified methane as fuel is greatly improved, which is similar to that with hydrogen as fuel. The maximum power densities of SmNi-YSZ anode cell are 1.56 W cm⁻² for humidified hydrogen fuel and 1.54 W cm⁻² for humidified methane fuel at 800 °C. The maximum power density is increased by 221% when samarium is used to modify Ni-YSZ anode for humidified methane fuel at 650 °C. High cell performance results in good stability of SmNi-YSZ anode cell and the cell runs stably for more than 600 min for humidified methane fuel.     

Carbon-resistant Ni-YSZ/Cu–CeO2-YSZ dual-layer hollow fiber anode for micro tubular solid oxide fuel cell

A NiO-YSZ/porous YSZ dual-layer hollow fiber with an asymmetric structure was fabricated by a co-spinning-sintering method. A dense YSZ electrolyte film was prepared on NiO-YSZ layer by dip-coating method and calcined at 1450 °C; subsequently a porous cathode was dip-coated on the dense YSZ electrolyte film using LSM-YSZ (in the weight ratio 4:1) ink to fabricate a micro tubular solid oxide fuel cell (MT-SOFC). Cu–CeO₂ catalyst was impregnated into the porous YSZ layer to form the second anode composition. The power output of the MT-SOFC with Ni-YSZ/Cu–CeO₂-YSZ graded anode was up to 242 mW cm⁻² operated at 850 °C using CH₄ as fuel and air as oxidant. Little carbon deposition was observed on the double anode using methane as the fuel after 60 h' stable operation.     

Fabrication and characterization of anode-supported single chamber solid oxide fuel cell based on La0.6Sr0.4Co0.2Fe0.8O3−δ–Ce0.9Gd0.1O1.95 composite cathode

Anode-supported solid oxide fuel cells consisting of nickel–gadolinium doped ceria (NiO–CGO, 60:40 wt%) anode, gadolinium doped ceria (CGO) electrolyte and lanthanum strontium cobaltite ferrite–gadolinium doped ceria (LSCF–CGO) cathode are developed and operated under single-chamber conditions, utilizing methane/air mixture. The cell performance is optimized regarding the electrolyte microstructure, cathode composition and testing conditions. The performance of the cell improves with the decrease of the thickness of the electrolyte and the increase of the ratio of methane to oxygen. The test cell with LSCF–CGO cathode (70:30 wt%) that was sintered at 1100 °C for 2 h and 150 μm dense electrolyte exhibits the maximum power output of ∼260 mW cm⁻² at 600 °C in CH₄/O₂ = 2 atmosphere.     

Stabilisation of composite LSFCO–CGO based anodes for methane oxidation in solid oxide fuel cells

A La_0.6Sr_0.4Fe_0.8Co_0.2O₃–Ce_0.8Gd_0.2O_1.9 (LSFCO–CGO) composite anode material was investigated for the direct electrochemical oxidation of methane in intermediate temperature solid oxide fuel cells (IT-SOFCs). A maximum power density of 0.17 W cm⁻² at 800 °C was obtained with a methane-fed ceria electrolyte-supported SOFC. A progressive increase of performance was recorded during 140 h operation with dry methane. The anode did not show any structure degradation after the electrochemical testing. Furthermore, no formation of carbon deposits was detected by electron microscopy and elemental analysis. Alternatively, this perovskite material showed significant chemical and structural modifications after high temperature treatment in a dry methane stream in a packed-bed reactor. It is derived that the continuous supply of mobile oxygen anions from the electrolyte to the LSFCO anode, promoted by the mixed conductivity of CGO electrolyte at 800 °C, stabilises the perovskite structure near the surface under SOFC operation and open circuit conditions.     

Electrochemical performances of LiNiCoMn bifunctional electrode for low temperature solid oxide fuel cells

Lithium transition metal oxides LiNi_0.83Co_0.11Mn_0.06O₂ (NCM-83) and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) are prepared and acted as cathodes and bifunctional electrodes for low temperature solid oxide fuel cells with H₂ and CH₄ fuels. The Ni anode-supported cell with NCM-83 cathode exhibits maximum power density (P_max) of 0.72 W cm⁻² with H₂ fuel at 600 °C. The symmetric cell with NCM-83 electrodes shows high P_max of 0.465 W cm⁻² with H₂ fuel and 0.354 W cm⁻² with CH₄ fuel at 600 °C. And the P_max of the cell with NCM-811 as anode and NCM-83 as cathode is 0.204W cm⁻² with H₂ fuel at 600 °C. The oxygen vacancies in NCM materials are conducive to the rapid oxygen ion conduction of the cathode, and in the anodic reduction atmosphere, the NCM materials will generate Ni/Co active particles in situ, proving the NCM materials can be advanced bifunctional electrode materials for hydrogen oxidation reaction and oxygen reduction reaction at low temperature.     

Novel fabrication technique of hollow fibre support for micro-tubular solid oxide fuel cells

In this work, a cerium-gadolinium oxide (CGO)/nickel (Ni)-CGO hollow fibre (HF) for micro-tubular solid oxide fuel cells (SOFCs), which consists of a fully gas-tight outer electrolyte layer supported on a porous inner composite anode layer, has been developed via a novel single-step co-extrusion/co-sintering technique, followed by an easy reduction process. After depositing a multi-layers cathode layer and applying current collectors on both anode and cathode, a micro-tubular SOFC is developed with the maximum power densities of 440-1000 W m⁻² at 450-580 °C. Efforts have been made in enhancing the performance of the cell by reducing the co-sintering temperature and improving the cathode layer and current collection from inner (anode) wall. The improved cell produces maximum power densities of 3400-6800 W m⁻² at 550-600 °C, almost fivefold higher than the previous cell. Further improvement has been carried out by reducing thickness of the electrolyte layer. Uniform and defect-free outer electrolyte layer as thin as 10 μm can be achieved when the extrusion rate of the outer layer is controlled. The highest power output of 11,100 W m⁻² is obtained for the cell of 10 μm electrolyte layer at 600 °C. This result further highlights the potential of co-extrusion technique in producing high quality dual-layer HF support for micro-tubular SOFC.     

Enhanced and stable strontium and cobalt free A-site deficient La1-xNi0.6Fe0.4O3 (x=0, 0.02, 0.04, 0.06, 0.08) cathodes for intermediate temperature solid oxide fuel cells

Perovskite oxides with cobalt and strontium element exhibit severe degradation during the operation for the solid oxide fuel cells (SOFC). Here, we report stable non-cobalt and non-strontium La_1-xNi_0.6Fe_0.4O₃ perovskite cathodes with improved oxygen reduction reaction (ORR) activity. A-site deficient La_1-xNi_0.6Fe_0.4O₃ cathodes within 8 at.% all exhibit the invariable phase structure with LaNi_0.6Fe_0.4O₃ (LNF), and the matched thermal expansion coefficient with that of the (Ce_0.90Gd_0.10)O_1.95 (GDC) electrolyte. The polarization resistance of the La_0.94Ni_0.6Fe_0.4O₃ (LNF94) cathode is 0.61 Ω cm² at 750 °C in air, which is 1/5 of the LaNi_0.6Fe_0.4O₃ (2.78 Ω cm²). The peak power density of the corresponding single cell with LNF94 cathode is 0.37 W cm⁻² at 750 °C, which is 2.36 times higher than that of the single cell with LNF cathode (0.11 W cm⁻²). We further study the long-term stability of LNF and LNF94 cathodes, the polarization resistance of the LNF94 electrode slightly fluctuates around 0.18 Ω cm² during 50 h operation at 800 °C, while the polarization resistance of the LNF increases by about 15%. This work highlights the A-site deficient LNF as an effective and stable non-cobalt and non-strontium cathode for the intermediate temperature solid oxide fuel cells.     

Silver modified cathode for a micro-tubular, single-chamber solid oxide fuel cell

Micro-tubular, solid oxide fuel cells consisting of nickel, yttria-stabilized zirconia (Ni-YSZ) anode, yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium cobaltite ferrite-gadolinium doped ceria (LSCF-GDC) cathode have been developed and operated under single-chamber conditions, utilizing methane/air mixture. The cell performance was compared with a silver modified cathode by the addition of 10wt% silver-paste in LSCF-GDC cathode. The cells with and without silver addition yielded maximum power densities of 118.75 mW cm⁻² and 61.53 mW cm⁻² at 700 °C, respectively. The results demonstrate that silver is a good candidate for enhancing the oxidation reduction kinetics via improved adsorption, desorption, dissociation and subsequent diffusion. However, long term performance of the silver modified cathode is not guaranteed under single-chamber conditions.     

Co-extruded dual-layer hollow fiber with different electrolyte structure for a high temperature micro-tubular solid oxide fuel cell

In this study, the phase inversion-based co-extrusion method was employed to fabricate a structural-improved electrolyte/anode dual-layer hollow fiber (HF) precursor, which was then co-sintered at 1450 °C. The electrolyte structures were thoroughly investigated by varying the loading of electrolyte material (i.e. Yttria-stabilized zirconia, YSZ) with differing particle sizes (i.e. micron, sub-micron, and nano-sized) during suspension preparation. The results showed that the most promising electrolyte layer with thin, dense, gas-tight, and defect-free properties was obtained by mixing 70% submicron-YSZ and 30% nano-YSZ in electrolyte suspension (E-0.7sub0.3nano). This electrolyte formulation co-extruded with a thick nickel-oxide-YSZ (NiO-YSZ) anode layer yielded the highest bending strength of 85 MPa, providing major mechanical strength to the HF. Besides that, the nitrogen permeability value at 2.87 × 10^?6 mol m^?2 s^?1 Pa^?1 suggested that the electrolyte was gas-tight, preventing fuel and oxidant transport. The fiber was then reduced to nickel (Ni)-cermet anode. It was developed to be a complete micro-tubular solid oxide fuel cell (MT-SOFC) by depositing the lanthanum strontium cobalt ferrite (LSCF)/YSZ cathode via brush painting on the dual-layer HF. The cell was fed with hydrogen gas and yielded an open-circuit voltage (OCV) as high as 1.06 V with maximum power density of 0.243 W cm^?2, at 875 °C. Based on this test, it was found that the electrolyte structural-modified dual-layer hollow fiber-based MT-SOFC using mixed particle sizes may result in a promising OCV. However, the relatively low value for power density may be due to a less porous anode; thus, improvements in the anode's structure are required in future research.     

Effects of a current treatment for an in-situ sintered cathode in a Ni-supported solid oxide fuel cell

Metal-supported solid oxide fuel cells (SOFCs) are one of the most promising candidates for applications in power plants as well as in portable applications due to their good mechanical and thermal properties. A Ni-supported SOFC that consists of a metal support (Ni, ∼180 μm), an anode functional layer (Ni-yttrium stabilized zirconia YSZ, ∼15 μm), an electrolyte (YSZ, ∼5 μm), and a nanocrystalline La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode is prepared. A nanocrystalline LSCF synthesized with ethylenediaminetetraacetic acid, citric acid, and inorganic nanodispersants, is used as an in situ sinterable cathode. The Ni-supported SOFC with nanocrystalline LSCFs is operated without a high temperature treatment for cathode sintering. The cell exhibits the maximum power density of 580 mW cm⁻² at 780 °C. A current treatment for 8 h (0.5 A cm⁻² at 780 °C) enhances the interfacial contact between the cathode and the electrolyte. After the current treatment, the maximum power density at 730 °C increase by 1.6 times from 260 mW cm⁻² to 390 mW cm⁻². The ohmic resistance (R_ohm) at 730 °C decreases from 0.43 Ω cm² to 0.21 Ω cm² and the charge transfer polarization at 0.7 V decreases from 0.42 Ω cm² to 0.30 Ω cm² due to lowered interfacial resistance between the cathode and the electrolyte. However, the mass transfer polarization increases from 0.09 Ω cm² to 0.17 Ω cm², which may result from the morphological change in the porous microstructure of the Ni support. The current treatment of the Ni-supported SOFC with in situ sintered LSCFs exhibit an increment in fuel cell performance due to the lowered ohmic resistance, which is beneficial for simple and mechanically improved fabrication and operation of metal-supported SOFCs.     

Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells

The microstructure of anode has a significant influence on the whole electrochemical performance of solid oxide direct carbon fuel cells (SO-DCFCs). The tubular SO-DCFCs based on cathode supported solid oxide fuel cells was fabricated by dip-coating and co-sintering techniques. As the anode porosity mainly came from the pore former (graphite) in the dip-coating process, different contents of graphite were added into the anode slurries. When the graphite was 10.1% wt., the SO-DCFCs showed the best performance and stability. The peak power density reached 242 mW cm⁻² at 850 °C, with carbon black (located 5% Fe) as the fuel and air as the oxidant.     

Scaling up self-stratifying supercapacitive microbial fuel cell

Self-stratifying microbial fuel cells with three different electrodes sizes and volumes were operated in supercapacitive mode. As the electrodes size increased, the equivalent series resistance decreased, and the overall power was enhanced (small: ESR = 7.2 Ω and P_max = 13 mW; large: ESR = 4.2 Ω and P_max = 22 mW). Power density referred to cathode geometric surface area and displacement volume of the electrolyte in the reactors. With regards to the electrode wet surface area, the large size electrodes (L-MFC) displayed the lowest power density (460 μW cm⁻²) whilst the small and medium size electrodes (S-MFC, M-MFC) showed higher densities (668 μW cm⁻² and 633 μW cm⁻², respectively). With regard to the volumetric power densities the S-MFC, the M-MFC and the L-MFC had similar values (264 μW mL⁻¹, 265 μW mL⁻¹ and 249 μW cm⁻¹, respectively). Power density normalised in terms of carbon weight utilised for fabricating MFC cathodes-electrodes showed high output for smaller electrode size MFC (5811 μW g⁻¹-C- and 3270 μW g⁻¹-C- for the S-MFC and L-MFC, respectively) due to the fact that electrodes were optimised for MFC operations and not supercapacitive discharges. Apparent capacitance was high at lower current pulses suggesting high faradaic contribution. The electrostatic contribution detected at high current pulses was quite low. The results obtained give rise to important possibilities of performance improvements by optimising the device design and the electrode fabrication.     

Experimental optimization of the fabrication parameters for anode-supported micro-tubular solid oxide fuel cells

A systematic optimization of several parameters significant in the fabrication of anode-supported micro-tubular solid oxide fuel cell via extrusion and dip coating is presented in this study. Co-sintering temperature of anode-support and electrolyte, the vehicle type and solid powder content used in electrolyte dip-coating slurry, electrolyte submersion time, cathode sintering temperature, powder ratio in the cathode functional layer, submersion time for the cathode functional layer and, submersion time and coating number of the anode functional layer are studied in this respect and optimized in the given order according to the performance tests and microstructural analyses. The performance of the micro-tubular cell is significantly improved to 0.49 Wcm⁻² at 800 °C after the optimizations, while that of the base cell is only 0.136 Wcm⁻². 12-cell micro-tubular stack is also constructed with the optimized cells and the stack is tested. Each cell in the stack is found to show very close performance to the single-cell performance and the stack with a maximum power of ~26 W at an operating temperature of 800 °C is therefore evaluated to be successful.     

Solid oxide fuel cells with bi-layered electrolyte structure

In this work, we have developed solid oxide fuel cells with a bi-layered electrolyte of 2 μm SSZ and 4 μm SDC using tape casting, screen printing, and co-firing processes. The cell reached power densities of 0.54 W cm⁻² at 650 °C and 0.85 W cm⁻² at 700 °C, with open circuit voltage (OCV) values larger than 1.02 V. The electrical leaking between anode and cathode through an SDC electrolyte has been blocked in the bi-layered electrolyte structure. However, both the electrolyte resistance (R_el) and electrode polarization resistance (R_p,a+c) increased in comparison to cells with single-layered SDC electrolytes. The formation of a solid solution of (Ce, Zr)O_2−x during sintering process and the flaws in the bi-layered electrolyte structure seem to be the main causes for the increase in the R_el value (0.32 Ω cm²) at 650 °C, which is almost one order of magnitude higher than the calculated value.     

Using rhodium as a cathode catalyst for enhancing performance of microbial fuel cell

Rhodium with activated carbon as carbon base layer (Rh/AC) was exploited as an oxygen reduction reaction (ORR) catalyst to explore its applicability in microbial fuel cell (MFC). Four MFCs were fabricated using the Rh/AC catalyst, adopting varying Rh loadings of 0.5, 1.0 and 2.0 mg cm⁻² and without Rh on carbon felt cathode in order to understand the optimum loading of this catalyst to enhance the performance of MFC. The participation of Rh/AC in ORR was confirmed by cyclic voltammetry and electron impedance spectroscopy analysis, which supported the enhanced charge transfer capacity of the cathode modified with the prepared catalysts. Volumetric power density of MFC was found to be improved by 2.6 times when Rh/AC was used as cathode catalyst (9.36 W m⁻³) at a loading of 2.0 mg cm⁻² in comparison to the control MFC (3.65 W m⁻³) without Rh on the cathode. It was thus inferred that the increase in the Rh loading up to 2 mg cm⁻² can improve the performance of MFC significantly.     

A high-performance ceramic composite anode for protonic ceramic fuel cells based on lanthanum strontium vanadate

The composite electrodes for protonic ceramic fuel cells (PCFC) were fabricated by infiltration of (La_0.8Sr_0.2)FeO_3−δ (LSF) cathode and (La_0.7Sr_0.3)V_0.90O_3−δ (LSV) anode into a porous protonic ceramic, Ba(Ce_0.51Zr_0.30Y_0.15Zn_0.04)O_3−δ (BCZY-Zn), respectively. Further, Pd-ceria catalysts were added into the composite anode. In the same method, the oxygen ion conducting fuel cells with the yttria-stabilized zirconia as an electrolyte (YSZ cell) were also fabricated. At 973 K, the non-ohmic area specific resistance (ASR) of PCFC (0.09 Ω cm²) was much smaller than that of the YSZ cell (0.28 Ω cm²) although the protonic conductivity of BCZY-Zn was slightly smaller than the oxygen ion conductivity of YSZ. According to the analysis of the symmetric cells with BCZY-Zn as an electrolyte, the LSV-composite anode showed better performance than the LSF-composite cathode at low temperatures.     

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

The performance of solid oxide fuel cells (SOFCs) is affected by various polarization losses, namely, ohmic polarization, activation polarization and concentration polarization. Under given operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometric parameters. Solid oxide fuel cells (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte, Ni–YSZ anode support, Ni–YSZ anode interlayer, strontium doped lanthanum manganate (LSM)–YSZ cathode interlayer, and LSM current collector, were fabricated. The effect of various parameters on cell performance was evaluated. The parameters investigated were: (1) YSZ electrolyte thickness, (2) cathode interlayer thickness, (3) anode support thickness, and (4) anode support porosity. Cells were tested over a range of temperatures between 600 and 800 °C with hydrogen as fuel, and air as oxidant. Ohmic contribution was determined using the current interruption technique. The effect of these cell parameters on ohmic polarization and on cell performance was experimentally measured. Dependence of cell performance on various parameters was rationalized on the basis of a simple analytical model. Based on the results of the cell parameter study, a cell with optimized parameters was fabricated and tested. The corresponding maximum power density at 800 °C was ∼1.8 W cm⁻².     

Effects of anode and electrolyte microstructures on performance of solid oxide fuel cells

To improve the performance of anode-supported solid oxide fuel cells (SOFCs), various types of single cells are manufactured using a thin-film electrolyte of Yttria stabilized zirconia (YSZ) and an anode functional layer composed of a NiO–YSZ nano-composite powder. Microstructural/electrochemical analyses are conducted. Single-cell performances are highly dependent on electrolyte thickness, to the degree that the maximum power density increases from 0.74 to 1.12 W cm⁻² according to a decrease in electrolyte thickness from 10.5 to 6.5 μm at 800 °C. The anode functional layer reduced the polarization resistance of a single cell from 1.07 to 0.48 Ω cm² at 800 °C. This is attributed to the provision by the anode layer of a highly reactive and uniform electrode microstructure. It is concluded that optimization of the thickness and homogeneity of component microstructure improves single-cell performances.     

Co-sintering anode and Y2O3 stabilized ZrO2 thin electrolyte film for solid oxide fuel cell fabricated by co-tape casting

Fabricating a large-area unit cell is very important for the development of solid oxide fuel cell (SOFC) stack. In this study, details of sintering process of half cell with NiO-yttria stabilized zirconia (YSZ) anode-supported YSZ thin electrolyte film fabricated by co-tape casting have been discussed. The results demonstrates that the shrinkages and shrinking rates mismatches between the electrolyte and the anode can be controlled by the organic additive content in the anode slurry composition and heating rate. Low heating rate suppresses the cracks formation in the electrolyte films. A warp-free unit cell with size of 100 × 100 mm² and dense electrolyte has been successfully fabricated. A power of 22.2 W, with a power density of 0.27 W cm⁻² has been achieved at 0.7 V and 750 °C in O₂/humidified H₂ atmosphere. The area specific resistance of the cell is 1.20 Ω cm² at 0.7 V and 750 °C.