A promising strontium and cobalt-free air electrode Pr1-xCaxFeO3-δ for solid oxide electrolysis cell

A promising strontium and cobalt-free ferrite Pr_1-xCa_xFeO_3-δ (PCF, x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) has been synthesized successfully by glycine-nitrate combustion method and used as the air electrode of solid oxide electrolysis cell (SOEC) for steam electrolysis. The crystal structure and electricity conductivity of PCF are investigated in detail. According to the conductivity test, Pr_0.6Ca_0.4FeO_3-δ (PCF64) with higher conductivity is selected as the air electrode to preparing the single cell with structure of PCF64|GDC|SSZ|YSZ-NiO. Under SOFC mode, the maximum power density of the single cell is 462.93 mW cm⁻² at 800 °C with hydrogen as fuel. Under SOEC mode, the current density reaches 277.14 mA cm⁻² and the corresponding hydrogen production rates is 115.84 mL cm⁻² h⁻¹ at 800 °C at 1.3 V. In the 10 h short-term stability test, the cell shows good electrolysis stability.     

Steam electrolysis performance of intermediate-temperature solid oxide electrolysis cell and efficiency of hydrogen production system at 300 Nm h

The steam electrolysis performance of an intermediate-temperature solid oxide electrolysis cell (SOEC) was measured at 650 °C at various steam concentrations. The cell voltage decreased with increasing steam concentration, which was attributed to a decrease in the steam electrode polarization. The highest performance of the SOEC was 1.32 V at 0.57 A cm⁻². On the basis of the electrolytic characteristics of this cell, the efficiency of a hydrogen production system operating at a capacity of 300 N m³ h⁻¹ was estimated. The system efficiency reached a higher heating value (HHV standard) of 98% due to the effective recovery of thermal energy from exhaust gas.     

A new anode material for solid oxide electrolyser: The neodymium nickelate Nd2NiO4+δ

Neodymium nickelate, with composition Nd₂NiO_4+δ is integrated as oxygen electrode in a solid oxide electrolyte supported cell made of a TZ3Y electrolyte and a Ni-CGO hydrogen electrode. This cell is tested in both fuel cell (SOFC) and electrolysis (SOEC) mode and the reversible operation is proven, ASR values being slightly lower in electrolysis mode. Performances in SOEC mode are compared with a commercial cell based on the same electrolyte and cathode, but with lanthanum strontium manganite (LSM) as anode. For a voltage of 1.3 V, current densities of 0.40, 0.64 and 0.87 A cm⁻² are measured at 750, 800 and 850 °C, respectively; they are much higher than the ones measured in the same conditions for the LSM-containing cell. Indeed, for a voltage of 1.3 V, current densities are respectively 1.7, 3 and 4.2 times higher for the Nd₂NiO_4+δ cell than for the LSM one at 850, 800 and 750 °C, respectively. Consequently, Nd₂NiO_4+δ can be considered as a good candidate for operating below 800 °C as oxygen electrode for high temperature steam electrolysis.     

Application of CuNi–CeO2 fuel electrode in oxygen electrode supported reversible solid oxide cell

The oxygen electrode-supported reversible solid oxide cell (RSOC) has demonstrated distinguishing advantages of fuel flexibility, shorter gas diffusion path and more choices for fuel electrode materials. However, there are serious drawbacks including the difficulty of co-firing the oxygen electrode and electrolyte, and the inefficient electrochemical performance. In this study, a (La_0.8Sr_0.2)_0.95MnO_3-δ (LSM) supported RSOC with the configuration of La_0.6Sr_0.4Fe_0.9Sc_0.1O_3-δ (LSFSc)-YSZ/YSZ/CuNi–CeO₂-YSZ is fabricated by tape casting, co-sintering and impregnation technologies. The single cell is evaluated at both fuel cell (FC) and electrolysis cell (EC) mode. Significant maximum power density of 436.0 and 377 mW cm^?2 is obtained at 750 °C in H₂ and CH₄ fuel atmospheres, respectively. At electrolysis voltage of 1.3 V and 50% steam content, current density of ?0.718, ?0.397, ?0.198 and ?0.081 A cm^?2 is obtained at 750, 700, 650 and 600 °C respectively. Much higher electrolysis performance than FC mode is exhibited probably due to the optimized electrodes with increased triple phase boundary (TPB) area and faster gas diffusion (oxygen and steam) and electrochemical reactions for water splitting. Additionally, the short-term stability of single cell in H₂ and CH₄ are also studied.     

Long-term tests of a Jülich planar short stack with reversible solid oxide cells in both fuel cell and electrolysis modes

A two-cell planar stack in the Jülich F-design with solid oxide cells has been built and the reversible operation between fuel cell and electrolysis modes has been demonstrated. The cells were anode supported cells (ASC) with yttria-stabilized zirconia (YSZ) electrolytes, Ni/YSZ hydrogen electrodes and perovskite oxygen electrodes with lanthanum strontium cobalt ferrite (LSCF). This paper summarizes and discusses the preliminary experimental results on the long-term aging tests of the reversible solid oxide planar short stack for fuel cell operation (4000 h) at a current density of 0.5 A cm⁻² which shows a degradation of 0.6% per 1000 h and for steam electrolysis operation (3450 h) and co-electrolysis operation of CO₂ and H₂O (640 h) under different current densities from −0.3 to −0.875 A cm⁻² which show different degradation rates depending on current density and on steam or co-electrolysis.     

Enhanced steam electrolysis with exsolved iron nanoparticles in perovskite cathode

The sustainable supply of clean energy may depend on the hydrogen, which usually derives from steam electrolysis for SOEC. However, the current steam electrolysis using SOEC still faces many challenges, such as low catalytic efficiency, poor structural stability. We synthesize a series of La_0.6Sr_0.4Fe_xO_3-δ (LSF_x, x = 0.8–1.2) materials, utilizing in situ exsolved metal (Fe) nanoparticles to construct a metal-oxide interface to enhance the performance of steam electrolysis and coking resistance. The active metal-oxide interface can effectively improve the performance of steam electrolysis. The H₂ production reaches 4.52 mL min^?1 cm^?2 with the current efficiency of 97.81% at 1.6 V and 850 °C for the cell with LSF_1.1-Ce_0.8Sm_0.2O_3-δ cathode and anode. It shows excellent long-term stability and redox cycling capability after dozens of hours of operation. This research is of great significance for efficient hydrogen production.     

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.     

Efficient and stable heterostructured air electrode for solid oxide steam electrolysis

High performance and excellent durability are essential for the practical application of solid oxide electrolysis cell (SOEC). Here we have demonstrated efficient and durable solid oxide steam electrolysis by constructing active La_0.8Sr_0.2CoO_3-δ/Gd_0.2Ce_0.8O_2-δ (LSC/GDC) heterointerface in air electrode using a simple co-impregnation method. The heterostructured air electrode exhibits the outstanding activity for oxygen evolution reaction, and its exchange current density (557 mA cm^?2) is 69 times higher than that of the traditional LSM-YSZ. The resulting cell reaches ?1.86 A cm^?2 @1.3 V and ?2.30 A cm^?2 @1.5 V at 800 °C and 50% absolute humidity (A.H), and the polarization resistance from the oxygen electrode only is 0.02 Ω cm². Most importantly, the heterostructured cell presents excellent long-term stability for the 1035 h steam electrolysis operation and excellent durability for 100 times charge-discharge cycles. In the heterostructured air electrode, the problem of electrode delamination is avoided due to the reduced oxygen partial pressure at anode/electrolyte resulting from easy diffusion of O^2? at the interphase, and the coarsening of LSC and GDC nanoparticles is limited because of the LSC/GDC percolative interfaces from phase segregation process. This work proposes a simple and effective strategy to design heterointerface for efficient and durable solid oxide steam electrolysis.     

Reversible operation performance of microtubular solid oxide cells with a nickelate-based oxygen electrode

This paper describes the reversible operation of a highly efficient microtubular solid oxide cell (SOC) with a nickelate-based oxygen electrode. The fuel cell was composed of a microtubular support of nickel and yttria stabilized zirconia (Ni-YSZ), an YSZ dense electrolyte, and a double oxygen electrode formed by a first composite layer of praseodymium nickelate (PNO) and gadolinium-doped ceria (CGO) and a second one of PNO. A good performance of the cell was obtained at temperatures up to 800 °C for both fuel cell (SOFC) and electrolysis (SOEC) operation modes, specially promising in electrolysis mode. The current density in SOEC mode at 800 °C is about −980 mA cm⁻² at 1.2V with 50% steam. Current density versus voltage curves (j-V) present a linear behavior in the electrolysis mode, with a specific cell area resistance (ASR) of 0.32 Ω cm⁻². Durability experiments were carried out switching the voltage from 0.7V to 1.2V. No apparent degradation was observed in fuel cell mode and SOEC mode up to a period of about 100 h. However, after this period especially in electrolysis mode there is an accumulated degradation associated to nickel coarsening, as confirmed by SEM and EIS experiments. Those results confirm that nickelate based oxygen electrodes are excellent candidates for reversible SOCs.     

Importance of pressure gradient in solid oxide fuel cell electrodes for modeling study

The pressure gradients in the electrodes of a solid oxide fuel cell (SOFC) are frequently neglected without any justification in calculating the concentration overpotentials of the SOFC electrodes in modeling studies. In this short communication, a comparative study has been conducted to study the effect of pressure gradients on mass transfer and the resulting concentration overpotentials of an SOFC running on methane (CH₄) fuel. It is found that the pressure gradients in both anode and cathode are significant in the fuel cell electrochemical activities. Neglecting the anode pressure gradient in the calculation can lead to underestimation of the concentration overpotential by about 20% at a typical current density of 5000 A m⁻² and at a temperature of 1073 K. The deviation can be even larger at a higher temperature. At the cathode, neglecting the pressure gradient can result in overestimation of the concentration overpotential by about 10% under typical working conditions.     

Solid oxide cells with cermet of silver and gadolinium-doped-ceria symmetrical electrodes for high-performance power generation and water electrolysis

A cermet of silver and gadolinium-doped-ceria (Ag-GDC) is investigated as novel symmetrical electrode material for (ZrO₂)_0.92(Y₂O₃)_0.08 (YSZ) electrolyte-supported solid oxide cells (SOCs) operated in fuel cell (SOFC) and electrolysis (SOEC) modes. The electrochemical performances are evaluated by measuring the current density-voltage characteristics and impedance spectra of the SOCs. The activity of hydrogen and air electrodes is investigated by recording overpotential versus current density in symmetrical electrode cells, respectively in hydrogen and air, using a three-electrode method. Conventional hydrogen electrode, Ni-YSZ, and oxygen electrode, LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ)-GDC, are tested as comparison. The results show that, as an oxygen electrode, Ag-GDC is more active than LSCF-GDC in catalyzing both oxygen reduction reaction (ORR) in an SOFC and oxygen evolution reaction (OER) in an SOEC. As a hydrogen electrode, Ag-GDC is more active than Ni-YSZ in catalyzing hydrogen oxidation reaction (HOR) in an SOFC and hydrogen evolution reaction (HER) in an SOEC, especially in high steam concentration. An SOC with symmetrical Ag-GDC electrodes operated in a fuel cell mode, with 3% H₂O humidified H₂ as the fuel, displays a peak power density of 395 mWcm^?2 at 800 °C. Its polarization resistance at open circuit voltage is 0.21 Ω cm². Ag-GDC electrode can be operated even at pure steam. An SOEC operated for electrolyzing 100% H₂O, the current density reaches 720 mA cm^?2 under 1.3 V at 800 °C.     

Sr2Fe1.5+xMo0.5O6-δ cathode with exsolved Fe nanoparticles for enhanced CO2 electrolysis

Solid oxide electrolysis cell (SOEC) can perform CO₂ electrolysis to produce CO feedstock. In this work, we show Sr₂Fe_1.5+xMo_0.5O_6-δ with exsolved Fe nanoparticles to enhance the activity to CO₂ electrolysis. A single SOEC with a configuration of SF_1.5+xM-SDC/LSGM/LSM-SDC shows a current density of 1.16 A cm⁻² at 1.8 V, which presents the CO production rate of 6.85 mL min⁻¹ cm⁻² and the current efficiency of up to 96.3% at 850 °C. We further demonstrate a stable electrolysis performance without obvious degradation being observed even after a long-time operation of 100 h. The exsolved metal-oxide interfaces function as three phase boundary which transports gas molecules, oxygen ions and electrons and therefore accommodate CO₂ splitting in electrochemical process.     

Electrode performance and analysis of reversible solid oxide fuel cells with proton conducting electrolyte of BaCe0.5Zr0.3Y0.2O3−δ

Reversible solid oxide fuel cells (R-SOFCs) are regarded as a promising solution to the discontinuity in electric energy, since they can generate electric powder as solid oxide fuel cells (SOFCs) at the time of electricity shortage, and store the electrical power as solid oxide electrolysis cells (SOECs) at the time of electricity over-plus. In this work, R-SOFCs with thin proton conducting electrolyte films of BaCe_0.5Zr_0.3Y_0.2O_3−δ were fabricated and their electro-performance was characterized with various reacting atmospheres. At 700 °C, the charging current (in SOFC mode) is 251 mA cm⁻² at 0.7 V, and the electrolysis current densities (in SOEC mode) reaches −830 mA cm⁻² at 1.5 V with 50% H₂O-air and H₂ as reacting gases, respectively. Their electrode performance was investigated by impedance spectra in discharging mode (SOFC mode), electrolysis mode (SOEC mode) and open circuit mode (OCV mode). The results show that impedance spectra have different shapes in all the three modes, implying different rate-limiting steps. In SOFC mode, the high frequency resistance (R_H) is 0.07 Ωcm² and low frequency resistances (R_L) are 0.37 Ωcm². While in SOEC mode, R_H is 0.15 Ωcm², twice of that in SOFC mode, and R_L is only 0.07 Ωcm², about 19% of that in SOFC mode. Moreover, the spectra under OCV conditions seems like a combination of those in SOEC mode and SOFC mode, since that R_H in OCV mode is about 0.13 Ωcm², close to R_H in SOEC mode, while R_L in OCV mode is 0.39 Ωcm², close to R_L in SOFC mode. The elementary steps for SOEC with proton conducting electrolyte were proposed to account for this phenomenon.     

Modelling of anode delamination in solid oxide electrolysis cell and analysis of its effects on electrochemical performance

Degradation of a solid oxide electrolysis cell (SOEC) during long-term operation remains to be the key obstacle to their massive production and commercialization. One of degradation processes within SOEC is anode delamination. The anode of SOEC splits at the interface with solid electrolyte due to elevated pressure of oxygen that is produced by electrochemical reactions. The main assumption that anode delamination starts at the fuel inlet is based on post-mortem analysis of SOEC. This paper addresses numerical modelling of a single, electrolyte supported, SOEC. The anode delamination is modelled by implementing the modifications of SOEC's geometry. A brief overview of the model is also given. Verification of the implemented model relies on the measurement data from literature. The simulation results show that increasing the area of delaminated anode (A_delaminated) increases the operating voltage of the SOEC if a constant electrolysis current is applied. This strongly influences the conversion efficiency (η) of the SOEC. Indeed, if linear growth of A_delaminated over time is assumed, the η of SOEC degrades very fast at the beginning of SOEC's operation. The presented model also helps analyze the hot spots of current density, where high pressure of oxygen appears.     

High-performing electrolyte-supported symmetrical solid oxide electrolysis cells operating under steam electrolysis and co-electrolysis modes

Symmetrical solid oxide cells (s-SOC) present several advantages compared to typical configuration, as a reduction of sintering steps or a better thermomechanical compatibility between the electrodes and the electrolyte. Different mixed ionic-electronic conductors (MIEC) have been reported as suitable candidates for symmetrical configuration, allowing operations under steam electrolysis (SOEC) or co-electrolysis (co-SOEC) without the use of reducing safe gas (typically employed in SoA nickel based cells). In the present study, Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) electrodes are deposited on both sides of YbScSZ tapes previously coated with a Ce_1-xGd_xO_1.9 (GDC) barrier layer grown by PLD. Electrode sintering temperature is optimized and fixed at 1200 °C by means of electrochemical impedance spectroscopy (EIS) measurements in symmetrical atmosphere. The cell is then characterized at 900 °C in SOEC and co-SOEC modes without the use of any safe gas obtaining high current densities of 1.4 and 1.1 A cm⁻² at 1.3 V respectively. Short-term reversibility is finally proven by switching the gas atmosphere between the cathode and anode sides while keeping the electrolysis conditions. Similar performances are obtained in both configurations.     

Methane internal steam reforming in solid oxide fuel cells at intermediate temperatures

The internal steam reforming of methane (CH₄) on conventional solid oxide fuel cell (SOFC) anode (nickel-yttria stabilized zirconia or Ni-YSZ) offers significant advantages compared to the external reforming process. However, the technology is currently facing some major issues such as coking and oxidation of anode during operation. Here we report a low-temperature sinterable catalyst, Ce_0·77Ni_0·2Mn_0·03O_2-δ (CNMnO), applied on top of Ni-YSZ to perform the steam reforming reaction. A single cell with CNMnO/Ni-YSZ/YSZ/GDC/LSC configuration produces a peak power density of 492 mW cm^?2 in wet hydrogen and 371 mW cm^?2 in wet methane, at 600 °C. The cell also shows exceptional durability against Ni oxidation when tested in wet methane under 0.2 A cm^?2 for 100 h. The improved performance and durability of the catalyst layer has been attributed to the nanosized precipitated Ni and Mn particles distributed on the surface of individual CNMnO particles.     

Perovskite Sr2Fe1.5Mo0.5O6−δ as electrode materials for symmetrical solid oxide electrolysis cells

Perovskite Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) has been successfully prepared by a microwave-assisted combustion method in air and employed as both anode and cathode in symmetrical solid oxide electrolysis cells (SOECs) for hydrogen production for the first time in this work. Influence of cell operating temperature, absolute humidity (AH) as well as applied direct current (DC) on the impedance of single cells with the configuration of SFM|La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM)|SFM has been evaluated. Under open circuit conditions and 60 vol.% AH, the cell polarization resistance, R_P is as low as 0.26 Ω cm² at 900 °C. An electrolysis current of 0.88 A cm⁻² and a hydrogen production rate as high as 380 mL cm⁻² h have been achieved at 900 °C with an electrolysis voltage of 1.3 V and 60 vol.% AH. Further, the cell has demonstrated good stability in the long-term steam electrolysis test. The results showed that the cell electrolysis performance was even better than that of the reported strontium doped lanthanum manganite (LSM) – yttria stabilized zirconia (YSZ)|YSZ|Ni–YSZ cell, indicating that SFM could be a very promising electrode material for the practical application of SOEC technology.     

A study on solid oxide electrolyzer stack and system performance based on alternative mapping models

In this study, a solid oxide electrolyzer cell (SOEC) stack model is developed based on an alternative mapping concept. The SOEC stack performance in a commercial hot box is systematically studied under different operating currents, flow rates, and flow directions. The results revealed that the SOEC stack operated in a hot box has thoroughly different temperature distributions, resulting in additional efficiency losses and an increase in thermal neutral voltage. The SOEC stack model computation results are summarized into stack performance diagrams and used in the system design. A 6-Nm³/h SOEC system with preheaters and recycling cathode materials is designed, and its performance is studied. The system efficiency is greatly influenced by the steam generator, and an external steam source can help increase the total efficiency of the system to more than 83%. Even the current increase may deteriorate the stack performance. It can increase the SOEC system efficiency by saving energy in the steam generator and preheaters. An increase in the flow rate around anode and cathode can improve the system capacity and efficiency. The system's maximum capacity is limited by the preheater heat balance and the stack output temperature. The feasible maximum system capacity is 33.4 kW electrolysis electric power input and 9.93 Nm³/h hydrogen production rate. At a constant system capacity, decreasing the air flow rate can minimize the heat losses in anode off-gas and achieve more than 87% nonsteam system efficiency.     

Performance and long-term durability of direct-methane flat-tube solid oxide fuel cells with symmetric double-sided cathodes

In this work, we investigated the performance and stability of a large flat-tube SOFC with symmetric double-sided cathodes (DSC), which was directly fueled with methane. The effect of steam/carbon (S/C) ratio, temperature, and current density on the performance, and long-term stability of the DSC as well as the catalytic behavior of the anode was investigated in details. The thick anode support and inner channels of the DSC formed an efficient microreactor for steam-reforming of methane, resulting in high conversion rate of methane and CO selectivity. In particular, when the S/C was 2, the conversion of CH₄ at 750 °C achieved 100% in the DSC and no carbon deposition was observed. Moreover, the voltage of DSC with was stable throughout 190 h under a discharge current density of 0.257 A cm⁻².     

Demonstration of hydrogen production in a hybrid lignite-assisted solid oxide electrolysis cell

A Ni-YSZ/YSZ/Ag–Pt cell was used to demonstrate the concept of high temperature lignite-assisted electrolysis in hybrid (combined solid oxide and molten carbonate) operation. To assess the performance of the hybrid concept, the same cell was also used for lignite-assisted electrolysis in absence of an anodic carbonate load, as well as for fuel cell measurements using H₂ and lignite as fuels, the latter both with and free of molten carbonates. In fuel cell operation, the hybrid direct lignite fuel cell obtained 45% higher maximum power density than the H₂ fed SOFC and 160% higher power density than the lignite fuel cell without carbonates. For high temperature electrolysis, the hybrid concept of admixed lignite and carbonates at the anode led to a 350% higher current density (up to 508 mA∙cm⁻², at 1.95 V), compared to the lignite-assisted operation in absence of carbonates (145 mA∙cm⁻², at 1.95 V). Thus, the anodic addition of carbonates within the same cell, increased H₂ production 3.5 times. This was accompanied by an equivalent increase of the anodic fuel consumption, and the cell's efficiency was essentially unaffected. Nonetheless, significant anodic and cathodic resistances at low overpotentials restricted electrolysis performance and efficiency, in either the absence or the presence of carbonates. These resistances, most likely due to both the cathodic steam activation and the anodic “shuttle” of the CO intermediate, were drastically alleviated at higher overpotentials. The presence of carbonates caused an earlier and more rapid decrease of the anodic area specific resistance, to much lower values at high overpotentials, resulting in the considerably higher performance of the hybrid mode.