A highly active and stable hybrid oxygen electrode for reversible solid oxide cells

Reversible solid oxide cells (RSOCs) have attracted increasing attention due to the potential realizing the deep coupling between hydrogen and electricity. An efficient and stable oxygen electrode is needed for developing RSOCs. Herein, we report a nanostructured hybrid with a nominal composition of BaZr_0.2Co_0.8O_3-δ as oxygen electrode. The chemical composition, crystal structure and physicochemical properties of the BaZr_0.2Co_0.8O_3-δ hybrid have been characterized. The results show that the sintered BaCo_0.8Zr_0.2O_3-δ has grown into a hybrid, consisting of cubic perovskite BaZr_0.82Co_0.18O_3-δ, hexagonal perovskite BaCo_0.96Zr_0.04O_2.6+δ and a small quantity of hexagonal perovskite BaCoO₃. The cell with BaZr_0.2Co_0.8O_3-δ hybrid oxygen electrode delivers a current density of 2.09 A cm⁻² @0.7 V in SOFC mode, and gives −1.43 A cm⁻² @1.3 V in SOEC mode for steam electrolysis. The BaCo_0.8Zr_0.2O_3-δ cell shows a smooth transition from SOEC to SOFC, and gives no obvious degradation after 100 times SOEC↔SOFC cycle operation.     

Comparative study of Pd and PdO as cathodes for oxygen reduction reaction in intermediate temperature solid oxide fuel cells

Metallic Pd and PdO electrodes were prepared by using Pd and PdCl₂ slurries, respectively, and their electrochemical performance as a cathode for oxygen reduction reaction in intermediate temperature solid oxide fuel cells was evaluated by electrochemical impedance spectroscopy (EIS) and direct current polarization (DC polarization). The electrochemical activity of metallic Pd was much higher than that of PdO for the reaction of oxygen reduction; below the decomposition temperature, a thin layer of PdO formed on the surface of metallic Pd electrode, which increased its polarization resistance. The decomposition temperature of PdO decreased from 810 to 750 °C as oxygen partial pressure decreased from 20 to 5 kPa, and was further lowered under the influence of the applied current during DC polarization test. The charge transfer resistance of PdO increased by decreasing oxygen partial pressure, while that of metallic Pd was less sensitive to it.     

Cobalt-doped BaZrO3: A single phase air electrode material for reversible solid oxide cells

BaZr_1−xCo_xO_3−δ (BZC-x, x = 0.1, 0.2, 0.3, 0.4, 0.5) are prepared and identified as single phase air electrode materials for reversible solid oxide cells. BZC-x shows a typical perovskite structure with x ≤ 0.4, and slight BaCoO₃ second phase is observed with x = 0.5. Conductivity measurements suggest that BZC-x is an oxygen ion and electron mixed conductor in dry oxygen atmosphere. The electronic conductivity of BZC-x increases when increasing Co content in BZC-x specimen, while the ionic conductivity reduces. For BaZr_0.6Co_0.4O_3−δ, the electronic and ionic conductivity are 5.24 S cm⁻¹ and 1.20 × 10⁻³ S cm⁻¹, respectively, measured at 700 °C in dry oxygen. With BZC-0.4 as a single phase air electrode, the discharging and electrolysis current densities of a single cell are 299 mA cm⁻² (at 0.7 V) and −935 mA cm⁻² (at 1.5 V), respectively, measured at 700 °C. The polarization resistance of cells using this new air electrode is only 0.19 Ω cm², approximately 65% lower than that using a traditional Sm_0.5Sr_0.5CoO_3−δ/BaCe_0.5Zr_0.3Y_0.2O_3−δ composite air electrode.     

Performance and durability of metal-supported solid oxide electrolysis cells at intermediate temperatures

Metal-supported solid oxide electrolysis cells (MS-SOECs) operating at 600–700 °C are attractive for storage of intermittent renewable electricity from solar and wind energy due to their advantages of easy sealing and fast startup. This paper reports on the fabrication of MS-SOECs consisting of dense scandium stabilized zirconia (SSZ) electrolytes, Ce_0.8Sm_0.2O_2−δ (SDC)/Ni impregnated 430L/SSZ cathodes and SmBa_0.5Sr_0.5Co₂O_5+δ (SBSCO) impregnated SSZ anodes supported on porous 430L alloys. Such cells demonstrated excellent electrolysis performance with current densities at 650 °C as high as 0.73 A⋅cm⁻² at 1.3 V in 50% H₂O-50% H₂ and 0.95 A⋅cm⁻² at 1.5 V in 90% CO₂-10% CO. Electrochemical impedance measurements indicated that the cell performance was largely limited by the ohmic losses for steam electrolysis and by the cathodic reduction reactions for CO₂ electrolysis, especially at reduced temperatures. Pronounced degradation was observed for both steam and CO₂ electrolysis over the preliminary 90-h stability measurements at 600 °C. SEM examination and EDS mapping of measured cells showed significant aggregation and coarsening of impregnated Ni particles, resulting in smaller activities for H₂O and CO₂ reduction reactions. As evidenced by the almost unaltered ohmic resistances over the measurement durations, the 430L stainless steel substrates demonstrate excellent resistances against corrosions from H₂O and CO₂ and thus show great promise for applications in reduced-temperature MS-SOECs.     

High performance and stability of nanocomposite oxygen electrode for solid oxide cells

Solid oxide electrochemical cell (SOC) is a highly promising alternative for fuel conversion and power-to-gas due to its high efficiency and low emission. However, degradation resulting from the electrolyte-electrode interface is a major challenge in both fuel cell mode and electrolysis mode. Here, a co-sintering tri-layer structure cell with nanocomposite oxygen electrode is developed to mitigate the interface issue. A 10 × 10 cm² NiO/YSZ||YSZ||YSZ-La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cell has been conducted under different fuels in SOFC mode. A power density output of 558 mW/cm² @0.7 V-800 °C in wet H₂ and a durability of 300 h in simulated syngas have been obtained. The performance of LSF, LSCF and SSC oxygen electrodes have been studied in both SOFC and SOEC modes. It suggests that three oxygen electrodes have an order of SSC > LSCF > LSF in electrochemical performance, and an opposite order in stability of SOEC. The degradation of the LSCF and SSC can be derived from the solid-state reactions at the interface between Co-containing perovskites and YSZ during operation. It demonstrates that GDC and Ag modification can enhance the oxygen electrode stability by impeding the solid-state reactions and the nanoparticles sintering. Results suggest that GDC has a negative effect on the cell performance and Ag has a positive effect, implying that enhancing the electric conductivity of YSZ-LSCF is the key to improve the cell performance. Moreover, cell with YSZ-SFM/GDC has been applied in CH₄ assisted SOEC process (CH₄-SOEC), in which a significant reduction of electricity consume can be realized.     

Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells

An electrochemical model for degradation of solid oxide electrolyzer cells is presented. The model is based on concepts in local thermodynamic equilibrium in systems otherwise in global thermodynamic non-equilibrium. It is shown that electronic conduction   through the electrolyte, however small, must be taken into account for determining local oxygen chemical potential, μ_O2${\mu }_{{\mathrm{O}}_2}$, within the electrolyte. The μ_O2${\mu }_{{\mathrm{O}}_2}$ within the electrolyte may lie out of bounds in relation to values at the electrodes in the electrolyzer mode. Under certain conditions, high pressures can develop in the electrolyte just near the oxygen electrode/electrolyte interface, leading to oxygen electrode delamination. These predictions are in accord with the reported literature on the subject. Development of high pressures may be avoided by introducing some electronic conduction in the electrolyte.     

Electrochemical performance and stability of nano-structured Co/PdO-co-impregnated Y2O3 stabilized ZrO2 cathode for intermediate temperature solid oxide fuel cells

Palladium (Pd) is an attractive cathode catalyst component for solid oxide fuel cells (SOFCs) that has high tendency to agglomerate during operation at around 800 °C. This work shows that such agglomeration can be inhibited by alloying Co into Pd. PdO, Pd_0.95Co_0.05O, Pd_0.90Co_0.10O, and Pd_0.80Co_0.20O were synthesized and characterized. Powder X-ray diffraction patterns at 750 and 900 °C confirmed that PdO decomposition to Pd which normally occurred at 840 °C was suppressed for Co containing Pd alloys while thermal gravimetric analyses indicated improved redox reversibility of PdO ? Pd conversion for alloys during the thermal cycling between 600 and 900 °C. Scanning electron microscopy images supported these arguments. Pd_0.90Co_0.10+yttria stabilized zirconia (YSZ) electrode (i.e., 10 mol % Co containing PdO-impregnated YSZ electrode) displayed the highest oxygen reduction reaction (ORR) performance and stability. The polarization resistance for ORR on Pd_0.90Co_0.10+YSZ cathode is only 0.088 Ω cm² at 750 °C. During polarization test at 750 °C, Pd_0.90Co_0.10+YSZ cathode showed stable performance for 30 h while the performance of Pd+YSZ cathode degraded after 10 h.     

A strontium-free and iron-based oxygen electrode for solid-oxide electrochemical cells (SOCs)

A solid-oxide electrochemical cell (SOC) that converts fuel to electricity back and forth is a possible large-scale storage technique for the renewable energy. Mixed ion-and-electron-conducting (MIEC) perovskites with Sr²⁺ as aliovalent dopants are proposed as the oxygen electrode to alleviate the degradation as in the conventional Sr-doped LaMnO₃. However, the high mobility of Sr²⁺ cation under an electric current, poor chemical compatibility with the yttria-stabilized zirconia (YSZ) of the MIEC perovskites could lead to the loss of long-term performance of the cell stacks. In this study, a novel Sr-free MgO-doped LaFeO₃ (La_0.95Fe_0.7Mg_0.3O_3-δ) showing an acceptable chemical compatibility with YSZ and high stability under anodic/cathodic current is proposed as the oxygen electrode for an SOC. A polarization resistance of 1.75 Ωcm² at 700 °C was observed for the single-phase electrode and the value could be decreased to 0.75 Ωcm² if a nanostructured composite electrode with YSZ was prepared using an infiltration technique.     

Perovskite-related intergrowth cathode materials with thin YSZ electrolytes for intermediate temperature solid oxide fuel cells

The electrochemical performances of solid oxide fuel cells with thin yttria-stabilized zirconia (YSZ) electrolytes and YSZ/Ni anodes were studied with two intergrowth oxides cathodes (Sr_2.7La_0.3Fe_1.4Co_0.6O_7−δ and LaSr₃Fe_1.5Co_1.5O_10−δ) and the results compared to a related perovskite cathode (La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ). It was found that cells produced with LaSr₃Fe_1.5Co_1.5O_10−δ exhibited peak power densities close to 0.75 W cm⁻², despite the relatively modest electrical conductivity of this compound. In contrast, cells produced with Sr_2.7La_0.3Fe_1.4Co_0.6O_7−δ and La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ cathodes both exhibited peak power densities of less than 0.4 W cm⁻². The greater performance for the cells produced with LaSr₃Fe_1.5Co_1.5O_10−δ may be attributed to a higher catalytic activity for this compound or to an improved adhesion of the cathode to the interlayer/electrolyte.     

Dynamic analysis of current overshoots in reversible solid oxide cells

Reversible solid oxide cell (RSOC) has attracted significant attention due to its multi-functionalities for energy conversion and storage. This device works as a fuel cell (FC) or an electrolyzer cell (EC) with superior efficiency. However, its dynamic response is complex and may cause a large overshoot of current operated in reversible cell mode, which impairs the long-term stability of RSOC. Although several works report the overshoot phenomenon, the mechanism and effects on cell performance are still unclear so far. In this work, we build a 3D dynamic planar RSOC model to investigate the detailed behavior of overshoot. The results show that the overshoot of current for RSOC can be caused under switching mode conditions. Especially a larger overshoot of current can be observed within switching mode from EC to FC. The overshoot of current is mainly caused by the load change, slower gas diffusion, and differentiated temperature distribution in RSOC.     

Enhanced ionic conductivity of yttria-stabilized ZrO2 with natural CuFe-oxide mineral heterogeneous composite for low temperature solid oxide fuel cells

We report for the first time that the commercial yttrium stabilized zirconia (YSZ) nanocomposite with a natural CuFe-oxide mineral (CF) exhibits a greatly enhanced ionic conductivity in the low temperature range (500–600 °C), e.g. 0.48 S/cm at 550 °C. The CF–YSZ composite was prepared via a nanocomposite approach. Fuel cells were fabricated by using a CF–YSZ electrolyte layer between the symmetric electrodes of the Ni_0.8Co_0.2Al_0.5Li (NCAL) coated Ni foam. The maximum power output of 562 mW/cm² has been achieved at 550 °C. Even the CF alone to replace the electrolyte the device reached the maximum power of 281 mW/cm² at the same temperature. Different ion-conduction mechanisms for YSZ and CF–YSZ are proposed. This work provides a new approach to develop natural mineral composites for advanced low temperature solid oxide fuel cells with a great marketability.     

Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.     

Latest development of double perovskite electrode materials for solid oxide fuel cells: a review

Recently, the development and fabrication of electrode component of the solid oxide fuel cell (SOFC) have gained a significant importance, especially after the advent of electrode supported SOFCs. The function of the electrode involves the facilitation of fuel gas diffusion, oxidation of the fuel, transport of electrons, and transport of the byproduct of the electrochemical reaction. Impressive progress has been made in the development of alternative electrode materials with mixed conducting properties and a few of the other composite cermets. During the operation of a SOFC, it is necessary to avoid carburization and sulfidation problems. The present review focuses on the various aspects pertaining to a potential electrode material, the double perovskite, as an anode and cathode in the SOFC. More than 150 SOFCs electrode compositions which had been investigated in the literature have been analyzed. An evaluation has been performed in terms of phase, structure, diffraction pattern, electrical conductivity, and power density. Various methods adopted to determine the quality of electrode component have been provided in detail. This review comprises the literature values to suggest possible direction for future research.     

Nanocomposite electrode materials for low temperature solid oxide fuel cells using the ceria-carbonate composite electrolytes

Low temperature solid oxide fuel cell (LTSOFC, 300–600 °C) is one of the hot areas in recent fuel cell developments. In order to develop high performance LTSOFCs, compatible electrodes are highly demanded. We used NANOCOFC (nanocomposites for advanced fuel cell technology) approach to develop nanocomposite electrodes based on metal oxides Ni–Cu–Zn-oxide and samarium doped ceria (SDC). It was found that the materials consist of individual metal oxide and SDC phase, indicating the material as a composite with a homogenous distribution for all constituent components. Highly homogenous distribution of the particles enhanced the catalyst function for electrode applications in LTSOFC devices. We constructed the devices using the SDC-carbonate nanocomposite (NSDC) as the electrolyte and above as prepared composite as electrodes in a symmetrical configuration. We found that the prepared composite electrodes had good catalytic function for both H₂ and O₂, to prove its anode and cathode functions. Based on the material properties, the LTSOFC devices have reached a power output more than 730 mW cm⁻² at 550 °C.     

A novel dual phase BaCe0.5Fe0.5O3-δ cathode with high oxygen electrocatalysis activity for intermediate temperature solid oxide fuel cells

A novel iron-based perovskite BaCe_0.5Fe_0.5O_3-δ (BCF) powders were successfully fabricated and the phase composition, lattice structure, oxygen surface exchange coefficient and electrochemical performance were investigated. The ultrafine BCF powder with grain size of about 200 nm consisted of dual phase BaCe_0.15Fe_0.85O_3-δ and BaCe_0.85Fe_0.15O_3-δ. Electrical conductivity relaxation measurement illustrates the low conductivity activation energy and the high oxygen surface exchange kinetics with oxygen surface exchange coefficient of 3.8 × 10⁻⁵ cms⁻¹ at 600 °C. BCF cathode exhibits 1.04 Ωcm² on doped ceria electrolyte and remains stable in 400 h long term test at 600 °C. A single cell based on doped ceria electrolyte with BCF cathode shows a maximum power density of 228 mW cm⁻² at 650 °C. The preliminary results indicate that the dual phase BCF can be applied as cathode material for oxygen ion conductive solid oxide fuel cells.     

Model validation and performance analysis of regenerative solid oxide cells: Electrolytic operation

The use of regenerative, high temperature solid oxide cells (SOCs) as energy storage devices has the potential for round-trip efficiencies that are competitive with other storage technologies. The focus of the current study is to investigate regenerative SOC operation (i.e., working in both fuel cell and electrolysis modes) through a combination of modeling and numerical simulation. As an intermediate step, this paper focuses on the electrolysis mode and presents a dynamic cell model that couples the reversible electrochemistry, reactant chemistry, and the thermo-fluidic phenomena inside a cell channel. The model is calibrated and validated using available experimental and numerical data for button cells, single cells, and multi-cell stacks supplied with either steam or syngas. Parametric studies are also performed to show how the investigated parameters affect model validity. The results show that the present model can accurately simulate the electrolytic cell behavior, especially in the low current range, which is a favored operating point in practical systems. It is observed that improvements in stack-level model precision require further investigation to better represent the contact resistance of the stack components and to improve the estimation of the activation polarization throughout the operating envelope. It is also concluded that the CO₂ electrochemical reaction can be neglected when the concentration of the steam supplied to the cell is high enough to support the water–gas shift reaction.     

Development and modelling of a novel electricity-hydrogen energy system based on reversible solid oxide cells and power to gas technology

Compared to the conventional thermal units and electrolytic devices, reversible fuel cells have very high efficiencies on both fuel cell mode of generating electricity and electrolysis mode of producing hydrogen or CH_x. However, previous studies about fuel cells and its benefits of power to gas are not fully investigated in the electricity-gas energy system. Moreover, state-of-art studies indicate that hydrogen could be directly injected to the existing natural gas (NG) pipeline within an amount of 5%–20%, which are considered to make a slight influence on the natural gas technologies. This work proposes a novel electricity-hydrogen energy system based on reversible solid oxide cells (RSOCs) to demonstrate the future vision of multi-energy systems on integrating multiple energy carriers such as electricity, pure hydrogen, synthetic natural gas (SNG) and mixed gas of H₂-natural gas. The P2G processes of RSOC are sub-divided modelled by power to H₂ (P2H) and power to SNG (P2SNG). The co-electrolysis/generation processes and time-dependent start-up costs are considered within a unit commitment model of RSOC. The proposed electricity-hydrogen energy system optimization model is formulated as mixed-integer linear programming (MILP), where the H₂-blended mixed gas flow is linearized by an incremental linearize relaxation technic. The aim of the optimization is to reduce the energy cost and enhance the system's ability to integrate sufficient renewables through NG networks. Besides quantified the benefits of renewable level and H₂ injection limit on the P2G process, the numerical results show that RSOC combined with H₂/SNG injection results in productive economic and environmental benefits through the energy system.     

Pr2NiO4-Ag composite as cathode for low temperature solid oxide fuel cells: Effects of silver loading methods and amounts

Active cathode materials for low temperature solid oxide fuel cells (SOFC) below 600 °C are urgently required due to the sluggish oxygen reduction reaction (ORR) kinetics at reduced temperature. In this work, a detailed experimental fabrication and characterization of silver modified Pr₂NiO₄ composite material for low temperature SOFC cathode catalyst with superior ionic conducting ceria-carbonate composite electrolyte was carried out. Pr₂NiO₄ was prepared by a co-precipitation method with NaOH as precipitant, and it was composited with silver to improve the electrode activity toward ORR through three various methods of impregnation, solid-state mixing and freeze-drying, respectively. Effects of Ag loading on the electrochemical activity were systematically investigated. It was found that composite materials originated from impregnation method presented the optimal material microstructure, and 15% Ag loaded composite gave the lowest area specific resistance of 0.45 Ω cm² at 600 °C, which is reduced by around 300% compared with previous work, indicating that impregnated Pr₂NiO₄-15Ag composite is a promising cathode catalyst for low temperature SOFC with ceria-carbonate composite electrolyte.     

Performance and degradation of metal-supported solid oxide fuel cells with impregnated electrodes

Metal-supported solid oxide fuel cells (MS-SOFCs) containing porous 430L stainless steel supports, YSZ electrolytes and porous YSZ cathode backbones are fabricated by tape casting, laminating and co-firing in a reducing atmosphere. Nano-scale Ni and La_0.6Sr_0.4Fe_0.9Sc_0.1O_3−δ (LSFSc) coatings are impregnated onto the internal surfaces of porous 430L and YSZ, acting as the anode and the cathode catalysts, respectively. The resulting MS-SOFCs exhibit maximum power densities of 193, 418, 636 and 907 mW cm⁻² at 650, 700, 750 and 800 °C, respectively. Nevertheless, a continuous degradation in the fuel cell performance is observed at 650 °C and 0.7 V during a 200-h durability measurement. Possible degradation mechanisms were discussed in detail.     

A micro/macroscale model for intermediate temperature solid oxide fuel cells with prescribed fully-developed axial velocity profiles in gas channels

A two-dimensional micro/macroscale model is proposed as an efficient numerical framework for simulating intermediate temperature solid oxide fuel cells (IT-SOFCs). This model employs a comprehensive microscale model that describes the detailed electrochemical reactions in Ni/YSZ cermet anodes and LSM/YSZ composite cathodes based on the three-phase boundary length (TPBL). A simplified macroscale model has been combined with the microscale model to consider the heat and mass transport processes in IT-SOFCs with prescribed fully-developed laminar velocity profiles in gas channels. A hydrogen-fed IT-SOFC is simulated at various operating conditions in order to demonstrate the capabilities of the proposed micro/macroscale model. The results elucidate the effects of co- and counter-flow configurations, inlet temperature, and air and fuel flow rates on the performance of the IT-SOFC.