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The flow field design is very essential to the material distribution uniformity and overall output performance of planar solid oxide fuel cells. In this paper, the structure of the external manifold and cathode side interconnect is optimized to solve the uneven flow field distribution problem. The effects of crucial geometric parameters on the internal mass transfer, component transport, electrochemical reaction, temperature field and overall output performance of the fuel cell are studied by three-dimensional multi-physics numerical simulation. It is found that the combination of the three-inlet-two-outlet external manifold and cylindrical ribbed interconnect significantly reduces the uneven gas distribution between different channels and greatly improves the diffusion of oxygen under the ribs, thus enhancing the overall output performance of SOFC. Increasing the number of cylindrical ribs is beneficial to the transport and uniform distribution of oxygen. However, the excessive number of cylindrical ribs will hinder oxygen diffusion to the end of the flow channel, resulting in increased concentration polarization along the flow channel. Moreover, reducing the total contact area of the ribs enhances oxygen diffusion beneath the ribs but also raises the contact resistance and may deteriorate the cell performance. • The structure of external manifold and cathode side interconnect is optimized. • The discrete cylindrical ribs significantly enhance the diffusion of oxygen beneath the ribs. • An excessive quantity of cylindrical ribs hinder the flow of oxygen to the end of the channel. • Reducing the contact area of the ribs leads to a rise in ASR and performance degradation. [ABSTRACT FROM AUTHOR]

The effects of height, length and angle of obstacle in the flow channel on the performance of solid oxide fuel cell (SOFC) are studied by computational fluid dynamics (CFD). The results show increasing the height of the obstacle can improve the cell performance, but the optimization effect is not significant when the height of the obstacle is too low. As the length of the obstacle increases, the cell performance gradually increases. But too long length will reduce the improvement of cell performance and eventually lead to performance degradation. As the obstacle angle increases, the cell performance also increases. The geometric parameters affecting the cell performance from large to small are height, angle and length. The power density of the cell with the optimal obstacle length, height and angle of 2.4 mm, 0.6 mm and 135° is 17% higher than that of the cell with the traditional rectangular channel. • The geometric parameters of obstacle channel's height, length and angle are studied. • The influence on cell performance from large to small is height, angle and length. • The optimization of obstacles channel structure greatly improve the cell performance. • The obstacles channel reduces the temperature gradient of the cell. [ABSTRACT FROM AUTHOR]

This study analyzed the influence of channel resistance, inlet flow rate, manifold depth and other related parameters on the gas distribution uniformity and pressure drop in an external 20-cell solid oxide fuel cell stack by computational fluid dynamics (CFD) technique. It provides a basis for determining the physical and geometric parameters of the stack. Furthermore, the flared inlet tube structure shows the effect of controlling the initial velocity of gas inflow and buffering; the flared outlet tube structure has the benefit of reducing the local pressure loss and the pressure drop. Combined with the perforated sheet structure, the gas in the inlet manifold is redistributed to improve the uniformity of gas flow. Compared with the basic model, the pressure drop of perforated sheet stack was reduced by 17.6% from 349.5 Pa to 288.5 Pa, and the gas flow uniformity index of that was reduced from 4.06% to 0.22%. • The physical and geometric parameters of a 20-cell SOFC stack are studied. • The diameter of inlet and outlet flared tubes are optimized. • A perforated sheet is designed to improve the flow uniformity and pressure drop. [ABSTRACT FROM AUTHOR]

Abstract A newly designed Fe-Cr-Mn alloy, considered as a candidate intermediate-temperature solid oxide fuel cell (SOFC) interconnect material, was cyclically oxidized at 650 °C, 750 °C, and 850 °C in a reducing atmosphere consisting of N 2 +3.75 vol% H 2 +25 vol% H 2 O(g) and corresponding to the SOFC anode environment. Results indicate that this alloy possesses good oxidation resistance and acceptable electrical conductivity at 750 °C in the reducing atmosphere. During the 1000 h oxidation, the oxide scale formed on the substrate at this temperature consists of an inner layer of Cr 2 O 3 doped with Ti, a Mn-rich middle layer of Mn-Cr spinel and Mn 2 TiO 4 , and a top layer of MnO, with an overall thickness of around 3 μm. The oxidation kinetics at 750 °C obey a parabolic law with rate constants of 1.37 × 10−13 g2 cm4 s−1 from 0 to 130 h, and of 8.84 × 10−14 g2 cm−4 s−1 from 130 to 1000 h. The area specific resistance (ASR) of the oxide scale measured at 750 °C is 16.3 mΩcm2, which is higher than that formed in the oxidizing atmosphere but is still within the maximum limit of 100 mΩcm2, required for metallic interconnects. The increase in oxidizing temperature from 650 °C to 850 °C promoted the outward diffusion of Mn and Ti ions, thus changing the composition and morphology of the formed oxides; in addition, the oxidation rate of the alloy increased dramatically, leading to an increase of both the oxide scale thickness and the ASR values. Highlights • The composition and morphology of oxides were influenced by the reducing atmosphere. • The outward diffusion of Ti was promoted by the reducing atmosphere. • Oxidation rate increases with the increase of temperature, resulting in thicker oxide scale and higher ASR. [ABSTRACT FROM AUTHOR]

Oxidation behavior of integrated interconnect with bipolar plate and corrugated sheet made by ferrite steel SUS430 is investigated and compared in simulated environment and in a realistic stack. Electrical current is found to have a direction-related impact on the thickness of the Cr 2 O 3 /MnCr 2 O 4 composite oxide scale. Oxide scale of the interconnect aged in the stack exhibits a dual-layered structure of a complex Mn-Cr oxide layer covered by iron oxide. The oxidation rates vary greatly depending on its local environment, with different thermal, electrical density, as well as gas composition conditions. By analyzing the thickness distribution of oxide scale and comparing them with the simulated test result, the oxidation behavior of interconnect in stack is described in high definition. ASR distribution is also conducted by calculation, which could help further understanding the behavior of stack degradation. [ABSTRACT FROM AUTHOR]

The oxidation resistance and electrical contact property of a low-Cr-containing alloy, named as FeCro, was examined as metallic interconnect at 750 °C in simulated solid oxide fuel cell (SOFC) anode environment. This alloy was proved to process excellent oxidation resistance at 750 °C in N 2 + 4.85% H 2 + 3% H 2 O (g) for up to 1000 h. The thermally grown oxide scale, around 1 μm thick, consisted of Mn doped (Mn, Cr) 3 O 4 spinel with Cr 2 O 3 sublayer. The oxidation kinetics obeyed the parabolic law with two rate constants, 5.23 × 10 −14 g 2 cm −4 s −1 (0–100 h) and 3.85 × 10 −15 g 2 cm −4 s −1 (100–900 h) respectively. After 581 h of exposure in N 2 + 3.75% H 2 + 25% H 2 O (g) at 750 °C, the oxidation behavior of FeCro alloy in contact with Ni mesh under an applied current of 500 mA cm −2 was quite different from that in air or anode gas containing 3% H 2 O (g). The internal oxidation was promoted in the presence of water vapor, and a Fe- or Cr-rich internal oxide scale with a network structure was formed. Owing to a poor supply of Cr, Fe-rich external oxide scale composed of Fe 3 O 4 and FeO was formed on the surface of FeCro alloy in which metallic compound Ni 3 Fe was embedded. Although FeCro alloy suffered from severe corrosion with non-protective Fe-rich oxide scale at 750 °C in N 2 + 3.75% H 2 + 25% H 2 O (g), the area specific resistance of Ni mesh/FeCro alloy was considerably low to offer sufficient metallic conduction. [ABSTRACT FROM AUTHOR]

Spinel oxides with the general formula of MnCu x Co 2−x O 4 (x = 0.1, 0.3, 0.5, 0.7 respectively) are used as coating materials on the metallic interconnect SUS 430 alloy to reduce the cathodic performance degradation by Cr poisoning and the interfacial resistance to improve the surface stability. The effect of Cu doping at the B position of spinel on the electrical conductivity and coefficient of temperature expansion (CTE) is investigated to determine the oxidation behavior and effectiveness as a protective layer. The result confirms that MnCu 0.5 Co 1.5 O 4 spinel shows highest electrical conductivity of 105.46 S cm −1 at 750 °C in air and an average CTE value of 12.27 × 10 −6 K −1 at temperature range of 20–960 °C. MnCu 0.5 Co 1.5 O 4 coating is stable enough after 2000 h cyclic oxidation at 750 °C. The growth of Cr 2 O 3 and formation of MnCr 2 O 4 in the scale layer are effectively suppressed by the presence of MnCu 0.5 Co 1.5 O 4 coating. The oxidation kinetics obeys the parabolic law with a rate constant as low as 2.76 × 10 −15 g 2 cm −4 ·s −1 and the ASR contributed by the oxide scale is 8.04 mΩ cm 2 at 750 °C. The research indicates that MnCu 0.5 Co 1.5 O 4 is a promising coating material for metallic interconnects of the intermediate temperature solid oxide fuel cells. [ABSTRACT FROM AUTHOR]

A newly developed low-Cr containing Fe–Cr–Co alloy, named as FeCro, is evaluated as a candidate material of metallic interconnects for intermediate temperature solid oxide fuel cells (IT-SOFCs). This alloy possesses excellent oxidation resistance and adequate electrical conductivity at 750 °C in air, and shows slight Cr deposition in/around La 0.72 Sr 0.18 MnO 3 (LSM) electrode under a harsh accelerating condition of 400 mA cm −2 and 850 °C. The thickness of the oxide scale thermally grown at 750 °C in air for 1000 his less than 1 μm, presenting a double-layered structure with dense (Mn, Cr) 3 O 4 on the top of Cr 2 O 3 . The oxidation kinetics at 750 °C obeys the parabolic law with a low rate constant of1.42 × 10 −15 g 2 cm −4 s −1 . The Cr deposition in/around the LSM electrode in the presence of the FeCro alloy is remarkably reduced, compared to the commercial Crofer 22H alloy. The measured area specific resistance (ASR) at 750 °C in air after 1000 h isothermal oxidation is 14 mΩ cm 2 . It is the unique microstructure of the formed oxide scale that significantly enhances the resistances of the FeCro alloy to oxidation and Cr volatilization. [ABSTRACT FROM AUTHOR]

Three kinds of anodes prepared by NiO impregnation, (Ni, Mg)O impregnation and conventional sintering methods are investigated under the conditions of anodic current polarization and redox cycling. The optimized NiO loading in the NiO-impregnated anode is 40 wt%; and the minimum polarization resistance is 1.40, 0.71 and 0.60 Ω cm2 at 700, 750 and 800 °C, respectively, due to the increased triple phase boundary and conductivity that promote the charge-transfer process of H2 oxidation reaction. The conventional Ni–YSZ cermet anode is less sensitive to the current polarization at 200 mA cm−2; however, its polarization resistance is much higher than those of the impregnated anodes. (Ni, Mg)O impregnation improves the performance durability and redox-ability at 800 °C, with a low polarization resistance of 0.93 Ω cm2 after 48 h of current polarization and of 0.71 Ω cm2 after 10 redox cycles. The addition of Mg lowers the reducibility of (Ni, Mg)O particles; and its improved electrochemical performance and redox cycling resistance are attributed to its stabilized microstructure consisting of nano-scale Ni particles distributed on the surface of the pre-sintered YSZ scaffold. The agglomeration of fine Ni particles is suppressed by the unreduced (Ni, Mg)O in the anode. [ABSTRACT FROM AUTHOR]

Abstract: The contact resistance and chemical compatibility of LaCo0.6Ni0.4O3−δ (LCN) coated Ni–Mo–Cr alloy are investigated at 750 °C in air for more than 530 h to simulate the contact situation of cathode/contact layer/interconnect in SOFC stacks. With La0.72Sr0.18MnO3 (LSM) as the cathode, the area specific resistance (ASR) of LSM/LCN/Ni–Mo–Cr alloy assembly increases to a maximum of 240 mΩ cm2 during the early stage of the test, and decreases after 55 h to a steady level of ∼220 mΩ cm2 till the end of the test. The contribution of formed oxide scale on the alloy to the measured ASR is negligible, compared to that of big AgLaMo2O8 particles sporadically distributed in LCN matrix. AgLaMo2O8 is formed of evaporated Mo from the alloy, Ag from the testing lead and La from the LCN before the oxide scale on the alloy is well developed. This reaction is expected to cease once the oxide scale is fully established. [Copyright &y& Elsevier]

Abstract: In an effort to improve the performance of SUS 430 alloy as a metallic interconnect material, a low cost and Cr-free spinel coating of NiMn2O4 is prepared on SUS 430 alloy substrate by the sol–gel method and evaluated in terms of the microstructure, oxidation resistance and electrical conductivity. A oxide scale of 3–4μm thick is formed during cyclic oxidation at 750°C in air for 1000h, consisting of an inner layer of doped Cr2O3 and an outer layer of doped NiMn2O4 and Mn2O3; and the growth of Cr2O3 and formation of MnCr2O4 are depressed. The oxidation kinetics obeys the parabolic law with a rate constant as low as 4.59×10−15 g2 cm−4 s−1. The area specific resistance at temperatures between 600 and 800°C is in the range of 6 and 17mΩcm2. The above results indicate that NiMn2O4 is a promising coating material for metallic interconnects of the intermediate temperature solid oxide fuel cells. [Copyright &y& Elsevier]

Abstract: The NiCo2O4 spinel coating is applied onto the surfaces of the SUS 430 ferritic stainless steel by the sol–gel process; and the coated alloy, together with the uncoated as a comparison, is cyclically oxidized in air at 800°C for 200h. The oxidation behavior and oxide scale microstructure as well as the electrical property are characterized. The results indicate that the oxidation resistance is significantly enhanced by the protective coating with a parabolic rate constant of 8.1×10−15 g2 cm−4 s−1, while the electrical conductivity is considerably improved due to inhibited growth of resistive Cr2O3 and the formation of conductive spinel phases in the oxide scale. [Copyright &y& Elsevier]

The influence of operating temperature on Cr poisoning effect on solid oxide fuel cells (SOFCs) cathode was comprehensive studied. The La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ -Ce 0.90 Gd 0.10 O 1.95 (LSCF-GDC) symmetric cells were investigated at 550, 600, 650, 700, 750 °C respectively in the presence of Crofer 22 APU. After aging in the air for 200 h, the cell operated at 650 °C expressed the most serious degradation. At the operating temperature lower than 650 °C, the degradation rate was accelerated with the increased operating temperature. At the operating temperature higher than 650 °C, the degradation rate slowed down with the increased operating temperature. Combined the experimental results with first principles calculation, the mechanism could be put forward that competition existed between the oxygen reduction reaction (ORR) activity and the formation reaction of SrCrO 4. The activity of ORR and the formation of SrCrO 4 at different operating temperatures codetermined the fluctuating degradation rate of LSCF-GDC cathode. This result could offer a valuable guidance for the rational application of LSCF-GDC cathode in SOFCs. [ABSTRACT FROM AUTHOR]

The impact of current density on Cr-poisoning of Sr-doped lanthanum manganite cathode is studied at 750 °C. The presences of SUS430 interconnect alloys cause rapid degradation in LSM cathode performance. The Cr deposits can be found not only on the LSM surface close to the electrode/electrolyte interface, but also on the YSZ surface. The deposition area is reach to 4.1 μm from the electrode/electrolyte interface after cathodic polarization with a current density of 400 mA cm −2 for 1200 min. TEM results clearly demonstrate that the particles on LSM surface are MnCr 2 O 4 spinels, which means that the Cr species would react with Mn from LSM and then lead to the structural damages of LSM cathode. A high polarization current of 800 mA cm −2 significantly accelerates Cr deposition process, resulting in a wider deposition area of 5.9 μm on LSM surface. The present results show that the Cr deposition occurs beyond the electrolyte/electrode interface, which reveals that the deposition of Cr species is driven by chemical reaction, not electrochemical reduction. Cr species are induced to deposit on LSM cathode surface by chemical reaction with Mn 2+ ions, and extend to YSZ surface under cathodic current. [ABSTRACT FROM AUTHOR]

As a new energy technology with distributed generation prospects, the load-following capability of solid oxide fuel cell (SOFC) systems is one of the main obstacles for commercial operation. This paper introduces a time delay control with an observer (TDO) in the fuel supply system to enhance the load-following capability to realize rapid load-following without fuel starvation. A dynamic model reflecting a 5-kW SOFC system with components such as a cell stack, combustor, heat exchanger and gas supply is developed in MATLAB/Simulink to implement time delay control. TDO is improved to TDOF (time delay control with an observer and filter) to avoid the undesirable impact of external disturbances (such as fuel disturbances) on the stability of the SOFC system. The fuel supply system can be driven by the TDOF controller to closely follow the reference trajectory based on the past responses and the input variables of the system during load-following operation. Simulation results reveal that the SOFC system is able to meet the requirement of load-following due to the satisfactory dynamic performance when the TDOF controller is applied in the fuel supply system. The system is appropriate for complicated power environments. [ABSTRACT FROM AUTHOR]

Solid oxide fuel cell (SOFC) is a promising energy conversion technology with high efficiency, low emission and fuel flexibility. In practical and dynamic operation, the thermal stress at the interface of different layers can cause cracks or even thermal mechanical failure. This study investigates the transient thermomechanical behavior of SOFC under changing operating conditions. A three-dimensional dynamical model is developed by considering the electrochemical reaction, electron and ion transport, gas diffusion, heat and momentum transfer, and thermal-mechanical interaction. When the operating conditions vary, the time it takes for the thermal stress to reach a steady state is consistent with the time required for heat transfer to stabilize. The electrolyte layer is subjected to the maximum gradient of thermal stress change after the voltage is switched. With an increase in the inlet gas temperature, the change in the cell temperature can be divided into two stages: a rapid rise and a slow rise. Appropriate prolongation of the temperature rise time is beneficial to the thermal stability of SOFC. The increase in inlet gas temperature changes the distribution trend of cell temperature and thermal stress, while the 50 % reduction in cathode and anode inlet velocity changes its magnitude. • An electric-thermal-mass-mechanical 3D transient model of SOFC is developed. • The transient thermomechanical behavior of SOFC is studied. • The temperature field is obtained by multiphysics models. • The distribution and variation of thermal stress are clarified. • The electrolyte is most prone to failure when the operation conditions change. [ABSTRACT FROM AUTHOR]

Abstract: LaCo0.6Ni0.4O3−δ (LCN64) was prepared through the polymeric steric entrapment precursor route with Polyvinyl alcohol (PVA) as the entrapment agent and was evaluated as a contact material between the metallic interconnect and the cathode in planar intermediate temperature solid oxide fuel cell stacks (IT-SOFC). The ratio of PVA to metal nitrates and the calcination temperature of the precursor were optimized for the process. The electrical conductivity and thermal expansion coefficient (TEC) of the synthesized LCN64 and its chemical compatibility with SUS 430 were also characterized. The results indicate that 1:4 is a proper ratio of PVA to metal nitrates for process control and safety management; and calcination of the precursor at temperatures above 650 °C leads to formation of single perovskite phase LCN64. The conductivity of fully sintered LCN64 is above 1150 S cm−1 in the temperature range between 100 °C and 800 °C, which is higher than those of conventional contact materials La1−x Sr x MnO3 (LSM) and LaNi y Fe1−y O3 (LNF). The average TEC is 17.22 × 10−6 K−1 at temperatures below 900 °C, which is higher than those of the metallic interconnect and cell components. Mn and Cr elements contained in SUS 430 migrated into the porous LCN64 layer at 800 °C without chemically forming resistive phases. [Copyright &y& Elsevier]

Abstract: Four Fe–17Cr alloys with various Mn contents between 0.0 and 3.0wt.% are prepared for investigation of the effect of Mn content on the oxidation behavior and electrical conductivity of the Fe–Cr alloys for the application of metallic interconnects in solid oxide fuel cells (SOFCs). During the initial oxidation stage (within 1min) at 750°C in air, Cr is preferentially oxidized to form a layer of Cr2O3 type oxide in all the alloys, regardless the Mn content, with similar oxidation rate and oxide morphology. The subsequent oxidation of the Mn containing alloys is accelerated caused by the fast outward diffusion of Mn ions across the Cr2O3 type oxide layer to form Mn-rich (Mn, Cr)3O4 and Mn2O3 oxides on the top. After 700h oxidation a multi-layered oxide scale is observed in the Mn containing alloys, which corresponds to a multi-stage oxidation kinetics in the alloys containing 0.5 and 1.0wt.% of Mn. The oxidation rate and ASR of the oxide scale increase with the Mn content in the alloy changes from 0.0 to 3.0wt.%. For the application of metallic interconnects in SOFCs, Mn-free Fe–17Cr alloy with conducting Cr free spinel coatings is preferred. [Copyright &y& Elsevier]