Performance of Single Cells with Anode Functional Layer based on Ni-YSZ, ScSZ and SDC for IT-SOFCs

In the present study, solution combustion synthesis (SCS) is employed to prepare a hydrocarbon-compatible composite anode oxide powder containing nickel (Ni), copper (Cu), yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) (Ni0.9–Cu0.1–YSZ0.95–GDC0.05) through the single-step solution combustion method using two different fuels. The effect of fuel on the nature of the synthesized anode powder is established. Carburization studies are carried out on the synthesized anode powder to study the carbon-retarding ability. Phase stability and interface reactivity are studied by X-ray diffractometry and field emission scanning electron microscopy. The SCS-synthesized anode powder is used for the fabrication of electrolyte-supported solid oxide fuel cells (SOFCs). The fabricated SOFC single cells are characterized for their electrical and electrochemical performance. Based on their performance, it is realized that they can be used as an anode functional layer instead of the anode support. The SOFC containing the SCS-synthesized anode as the functional layer yields a high power density of 884 mW/cm2 in methane fuel at 800°C, which is higher than the reported power density of such cells. The electrode impedance and exchange current density are also derived for the SOFC.

In this 3D numerical study, the effects of cathode and anode functional layer thicknesses on the performance of solid oxide fuel cells under different operating parameters are analysed and elucidated. The performance of the model is evaluated using direct ammonia and an equivalent amount of hydrogen fuel with functional layers ranging from 0 to 50 μm. The findings unveiled that the cathode functional layer has a stronger influence on cell performance than the anode functional layer. The parametric analysis also showed that the effect of temperature gradually weakened with the increase of the cathode functional layer which is the exact opposite of what anode functional layers do. This gives more promising result for the cathode functional layer than the anode functional layer, especially for hydrogen-powered cells. However, the anode functional layer works best when green ammonia is supplied directly to the solid oxide fuel cell. In this regard, the 10 μm functional layers of the anode and cathode layers achieved the highest possible power density using ammonia and hydrogen fuels under similar operating conditions. This suggests that the anode layer has outstanding performance than the cathode counterparts for ammonia-fuelled cells and the layers of the cathode have better performance for hydrogen-fuelled cells. It is also observed that the performance of the cell is decreasing while the thickness of the functional layer is increasing particularly after 10 μm. The study also confirmed that solid oxide fuel cells with functional layers have outstanding performance over corresponding cells without functional layers. Thus, the finding of this study concluded that the functional layer thickness is strongly dependent on the types of fuel and electrodes used in the cells.

Anode supported solid oxide fuel cells (SOFC) were fabricated by addition of various metal oxides such as Fe2O3, Co3O4 and TiO2 to thin anode functional layers between the electrolyte (yttria-stabilized zirconia, YSZ) and electrode materials (anode support: YSZ-NiO). Effect of the additives on the power density and impedance spectra of SOFC was studied. It was found that addition of Co3O4 to anode functional layer was most effective towards improvement of power densities and reduction of the total ohmic resistance as well as the area specific resistance of the cells, while addition of TiO2 to anode functional layer resulted in lower power densities. Possible mechanisms on the relationship between the additives in electrode functional layers and the cell performance were briefly discussed.

Tubular solid oxide fuel cells (SOFCs) with one closed-end configuration have high tolerance of thermal cycling owing to the capability of expanding and shrinking freely in response to thermal stresses. Additionally, tubular cells with one closed-end have simple sealing requirements, and enable making the stacks with uniform temperature distribution. In this study, anode supported micro-tubular SOFCs with one closed-end were manufactured successfully by dip-coating and co-firing methods using carbon rods with round end as a template. A dense electrolyte layer was obtained at a co-firing temperature of 1400 °C, followed by cathode deposition and sintering. The prepared micro-tubular SOFCs comprised of Ni-yttria-stabilized zirconia (YSZ), Ni–scandia-stabilized zirconia (SSZ), strontium-doped lanthanum manganite (LSM)-SSZ, and LSM as the anode support, anode functional layer, electrolyte, cathode functional layer, and cathode current collector, respectively. The electrochemical performance and microstructure of fabricated closed-end micro-tubular SOFC was evaluated by electrochemical characterization and scanning electron microscopy. The developed dip-coating method is fast in turnaround time and enables fabricating the one closed-end tubes of various lengths and diameters by co-firing.

In order to develop solid oxide fuel cells (SOFCs) running on hydrocarbon fuels, we have focused on a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria (SDC) within the pores of electrode by a sol-gel coating technique. The SDC coating on the pores of anode made it possible to have a good stability for long-term operation due to low carbon deposition and nickel sintering. In this study, we demonstrated a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria and applied the modification technique to two different types of fuel cell structures, anode-supported SOFC and comb-shaped SOFC. From our results, the maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30μm, and cathode; La0.85Sr0.15MnO3) with the modified anode was ∼300mW∕cm2 at 700°C in the mixture of methane (25%) and air (75%) as the fuel, and air as the oxidant. The cell was operated for 500hr without significant degradation of cell performance. For the comb-shaped SOFCs operated in the mixed-fuels fuel cell conditions, the cell performance was 40mW∕cm2 at 700°C in the CH4∕O2 ratio of 1.

A functional layer and a porous support that together constitute an anode for a solid oxide fuel cell were simultaneously formed by the phase‐inversion tape casting method. Two slurries, one composed of NiO and yttria‐stabilized zirconia (YSZ) powders and the other of NiO, YSZ, and graphite were cocasted and solidified by immersion in a water bath via the phase‐inversion mechanism. The as‐formed green tape consisted of a sponge‐like thin layer and a fingerlike thick porous layer, derived from the first slurry and the second slurry, respectively. The former acted as the anode functional layer (AFL), while the latter was used as the anode substrate. The AFL thickness was varied between 20 and 60 μm by adjusting the blade gap for the tape casting. Single cells based on such NiO‐YSZ anodes were prepared with thin YSZ electrolytes and YSZ‐(La 0.8 Sr 0.2 ) 0.95 MnO 3−δ (LSM) cathodes, and their electrochemical performance was measured using air as oxidant and hydrogen as fuel. The maximum power densities obtained at 750°C were 720, 821, and 988 mW cm −2 with the AFL thickness at 60, 40, and 20 μm, respectively. The satisfactory electrochemical performance was attributed to the dual‐layer structure of the anode, where the sponge‐like AFL layer provided plenty of triple‐phase boundaries for hydrogen oxidation, and the fingerlike thick porous substrate allowed for facile fuel transport. The phase‐inversion tape casting developed in this study is applicable to the preparation of other planar ceramic electrodes with dual‐layer asymmetric structure.

The most commonly used solid oxide fuel cell (SOFC) anode material is a two phase nickel and yttria stabilized zirconia (Ni/YSZ) cermet. During typical fuel cell operation, this material remains a cermet; however, in a commercial SOFC system seal leakage, fuel supply interruption, or other situations may cause the anode to reoxidize. The cyclic reduction and oxidation (redox) of nickel will result in large bulk volume changes, which may have a significant effect on the integrity of interfaces within the fuel cell and thus may cause significant performance degradation. Following initial study of the degradation mechanisms and redox kinetics, a number of solutions have been devised. These can be broadly categorised into system solutions and unit cell or materials solutions. The former includes purge gas, other means to produce a reducing gas, cell reversal and passive solutions such as hydrides, oxygen getters and steam purging. The latter includes alternative anode materials, microstructural modification of anode functional layers (AFLs), graded porosity and composition of AFL and redox barrier layers. This paper discusses the pros and cons of each approach and some initial feasibility testing in short stack repeat units.

The anode-supported solid oxide fuel cells (SOFCs) with an yttria-stabilized zirconia (YSZ) electrolyte have been successfully prepared by the sequential steps of fabricating technique including tape casting, spin coating, and screen-printing. Two types of the SOFCs are manufactured and their cell performances are characterized via the detail analysis of the electrochemical impedance spectroscopy (EIS). The maximum power densities are 411 and for the high and low performance cells at , respectively. According to the EIS analysis, the electrochemical performance of the poor cell is dominated by the anodic non-charge-transfer resistance, which is mainly resulted by the gas diffusion polarization due to the low porosity of the anode. The high performance cell is controlled by the simultaneous contributions of the ohmic resistance , the anodic polarization , and the cathodic polarization at the operation temperature of . However, the will dominate the performance of the cell as the temperature is lowered down to .

Influence of characteristics of stabilized zirconia electrolyte on performance of cermet supported tubular SOFCs

Precise porosity control is highly desirable for improving the electrochemical performance of solid oxide fuel cells (SOFCs). Freeze casting is an established method for enabling high bulk porosity in structures and controlling pore orientation. In this study, freeze casting was used to fabricate tubular, anode-supported SOFCs with aligned and varying amounts of porosity by controlling the solids/water ratio in different casting slurries. SOFCs were prepared with a Ni/yttria and scandia stabilized zirconia (ScYSZ) anode support (AS), an anode functional layer (AFL), a ScYSZ electrolyte, a lanthanum strontium manganite (LSM)/ScYSZ cathode interlayer (CIL), and an LSM cathode. The permeability of the anode support was found to increase from 1.4 × 10−2 to 1.8 × 10−2 m2 as porosity was increased from 57 to 64 vol%, while the total cell resistance decreased by 35% from 0.93 to 0.60 Ohm cm2. When evaluated with 30 vol% H2 as the fuel at 800 °C, the decrease of concentration polarization enabled an increase in electrochemical performance by 42% from 0.35 to 0.50 W/cm2 as the porosity in the anode support was increased. Mechanical strength characterization using a three-point method showed there is a practical upper limit of the amount of porosity that can be designed into the anode support. This work paves a way for controlling porosity by freeze casting and understanding the correlation between porosity and concentration polarization losses in SOFCs.

Development of proton conducting fuel cells using nickel metal support

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

Novel structured anode-supported solid oxide fuel cells with porous GDC interlayers

A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells (SOFCs)

Microtubular anode-supported solid oxide fuel cells (μt-SOFC) were created with a metallic coil embedded in the anode to act as current collector. The electrochemical performance was experimentally examined by comparing the power density (PD) of μt-SOFC with embedded coils of different turns per unit length and composition (nickel and palladium). It is shown that an increase in the turns per unit length results in a proportional current density increase and in a quadratic increment of PD. Additional performance improvement is found for the cell with palladium current collector due to the higher catalytic activity for hydrogen oxidation.