Oxygen electrode degradation in solid oxide cells operating in electrolysis and fuel cell modes: LSCF destabilization and interdiffusion at the electrode/electrolyte interface

Three long-term experiments have been performed in SOEC and SOFC modes at different operating temperatures. The durability tests confirm a higher degradation in electrolysis mode with respect to fuel cell operation. In addition, a larger increase of the ohmic resistance is observed for the cell operated at higher temperature in electrolysis mode. The oxygen electrodes of the pristine and tested cells have been characterized by synchrotron X-ray micro-diffraction and micro-fluorescence to assess the relation between the material destabilization and the formation of insulating phases due to interlayer diffusion. The analyses of the pristine cell confirm the presence after the electrode sintering of strontium zirconate and a Gd-rich interdiffusional layer in the electrolyte just below the zirconates. Moreover, evolutions in the LSCF unit cell volume reveal strontium segregation after aging. The associated material destabilization is linked to the accumulation of SrZrO₃ at the barrier layer/interdiffusional layer interface in operation and both phenomena are found to be thermally-activated and promoted in electrolysis mode. Finally, the crystallographic evolution of the interdiffusional layer in electrolysis mode has been investigated by X-ray diffraction. A slight increase of the phase peaks intensity detected at the highest temperature is correlated to the largest formation of SrZrO₃ observed in this condition. Based on these preliminary results, it is proposed that the loss of Zr⁴⁺ from the electrolyte due to the zirconates formation could facilitate the interdiffusion of Gd, reducing the local ionic conductivity and thus significantly contributing to the largest increase in the ohmic resistance observed in this case.     

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La_0.7Sr_0.3MnO₃ (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm_0.2Ce_0.8O_1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm⁻² at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm⁻² was derived from the cell with LSM/YSZ cathode, using 100 ml min⁻¹ oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.     

Effects of sulfur poisoning on degradation phenomena in oxygen electrodes of solid oxide electrolysis cells and solid oxide fuel cells

The durabilities of a single solid oxide electrolysis cell (SOEC) and a solid oxide fuel cell (SOFC) operating at 0.3 A cm^?2 and 973 K under different air supply conditions were investigated. In the SOEC, S penetration was observed mainly at the gadolinium-doped ceria (CGO) electrolyte/lanthanum strontium cobalt oxide (LSC) oxygen electrode interface. In contrast, during SOFC operation, S was distributed widely within the LSC. The reaction governing S penetration into the LSC is an oxidizing one. Thus, it is likely that the high oxygen partial pressure at the CGO electrolyte/LSC oxygen electrode interface accelerated the penetration of S. When air was supplied using an activated carbon filter during SOEC operation, the degradation rate decreased to 0.6% kh^?1 within 3000 h. Finally, the results of accelerated tests performed using air containing 0.2 ppm SO₂ suggested that the effect of S poisoning was greater during SOEC operation than during SOFC operation.     

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.     

Effects of a YSZ porous layer between electrolyte and oxygen electrode in solid oxide electrolysis cells on the electrochemical performance and stability

The delamination of (La,Sr)MnO₃ (LSM) oxygen electrode is considered as a key reason for the degradation of solid oxide electrolysis cells (SOEC). In this study, a YSZ porous layer prepared by spinning coating has been introduced to inhibit significant degradation of LSM oxygen electrode during anodic polarization for 100 h under constant 500 mA cm⁻². Impedance spectra of LSM oxygen electrode are recorded before and after anodic polarization. By performing distribution of relaxation time (DRT) processing on the impedance spectra, it is indicated that the introduction of YSZ porous layer provides more active sites or three phase boundary (TPB) for oxygen oxidation reaction. The potential relaxation process of LSM oxygen electrode is measured by three-sequence chronopotentiometry. The result proves that the sample with a YSZ porous layer has lower the oxygen activity and faster the oxygen ion diffusion at the solid-solid two-phase (oxygen electrode and electrolyte) interface (SSTPI) due to more TPB and shorter oxygen ion diffusion paths.     

Experimental study of dry reforming of biogas in a tubular anode-supported solid oxide fuel cell

The performance of a tubular Ni/YSZ anode supported SOFC directly fed by an anaerobic digestion simulated biogas, with an extra addition of carbon dioxide to operate in conservative operating conditions to avoid coking on the anode support, was investigated. The fuel cell has been tested at a fixed oven temperature of 800 °C and under biogas/CO₂ mixtures with different volumetric ratios, fuel utilization (FU) and current densities. Polarization curves and performance maps were obtained to better understand the influence of the investigated operational parameters on the cell behavior. Furthermore, since the tubular geometry enables an easy separation of the anode and cathode exhaust gases, the anode off-gas has been collected and monitored through a gas-chromatograph under open circuit voltage to investigate on the catalytic behavior of a Ni-based state-of-the-art anode. For corresponding operative conditions, performances of the cell for biogas/CO₂ 1/1.5 (i.e. CH₄/CO₂ 30/70) and 1/2 (i.e. CH₄/CO₂ 24/76) were at least 2% and 4% lower than the case 1/1 (i.e. CH₄/CO₂ 20/80), respectively. The highest efficiency of 43.4% was reached at 17.5 A and FU = 70%.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.     

Complementary techniques for solid oxide electrolysis cell characterisation at the micro- and nano-scale

High-temperature steam electrolysis by solid oxide electrolysis cells (SOEC) is a method with great potential for transforming clean and renewable energy from non-fossil sources to synthetic fuels such as hydrogen, methane or dimethyl ether, which have been identified as promising alternative energy carriers. With the same technology, fuel gas can be used in a very efficient way to reconvert chemically stored energy into electrical energy, since SOECs also work in the reverse mode, operating as solid oxide fuel cells (SOFC). As solid oxide cells (SOC) perform at high-temperatures (700–900 °C), material degradation and evaporation can occur, e.g., from the cell-sealing material, leading to poisoning effects and aging mechanisms that decrease the cell efficiency and long-term durability. To investigate such cell degradation processes, thorough examination of SOCs often requires a chemical and structural characterisation at a microscopic and nanoscopic level. The combination of different microscopic techniques such as conventional scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and the focused ion beam (FIB) preparation technique for transmission electron microscopy (TEM) allows for post-mortem analysis at a multi-scale level. These complementary techniques can be used to characterise structural and chemical changes over a large and representative sample area (micro-scale) as well as at the nano-scale level for selected sample details. This article presents a methodical approach for the structural and chemical characterisation of changes in aged cathode-supported electrolysis cells produced at Risø DTU, Denmark. Additionally, we present results from the characterisation of impurities at the electrolyte/hydrogen interface caused by evaporation of sealing material.     

The properties of the fuel electrode of solid oxide cells under simulated seawater electrolysis

In this study, electrolysis of seawater in flat-tube nickel-yttria-stabilized zirconia (Ni-YSZ) electrode-supported solid oxide electrolysis cells (SOECs) were modeled and the effects of variations in electrical conductivity and microstructure of Ni-YSZ electrode support were investigated. When the current density was greater than 700 mA·cm⁻², the conductivity of the electrode support decreased slightly with an increase in current density at 800 °C in hydrogen reduction environment; the conductivity of the electrode support decreased with an increase in the current density when the current density was greater than 400 mA·cm⁻² at 800 °C in the seawater electrolysis environment. During long-term durability experiment of seawater electrolysis, the degradation rates in area specific resistance (ASR) were 0.096 mΩ·cm²/100 h and 0.207 mΩ·cm²/100 h with a current density of 300 mA·cm⁻² (i.e., ≤400 mA·cm⁻²) and 1000 mA·cm⁻² (i.e., ≥400 mA·cm⁻²), respectively. Besides, the various ions commonly present in seawater did not contaminate the Ni-YSZ support during the long-term durability test. The degradation mechanism of seawater electrolysis in flat-tube SOECs is discussed and clarified.     

Internal shorting and fuel loss of a low temperature solid oxide fuel cell with SDC electrolyte

A solid oxide fuel cell with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte of 10 μm in thickness and Ni–SDC anode of 15 μm in thickness on a 0.8 mm thick Ni–YSZ cermet substrate was fabricated by tape casting, screen printing and co-firing. A composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSCo) + 25 wt.% SDC, approximately 50 μm in thickness, was printed on the co-fired half-cell, and sintered at 950 °C. The cell showed a high electrochemical performance at temperatures ranging from 500 to 650 °C. Peak power density of 545 mW cm⁻² at 600 °C was obtained. However, the cell exhibited severe internal shorting due to the mixed conductivity of the SDC electrolyte. Both the amount of water collected from the anode outlet and the open circuit voltage (OCV) indicated that the internal shorting current could reach 0.85 A cm⁻² or more at 600 °C. Zr content inclusions were found at the surface and in the cross-section of the SDC electrolyte, which could be one of the reasons for reduced OCV and oxygen ionic conductivity. Fuel loss due to internal shorting of the thin SDC electrolyte cell becomes a significant concern when it is used in applications requiring high fuel utilization and electrical efficiency.     

Stability study of cermet-supported solid oxide fuel cells with bi-layered electrolyte

Performance and stability of five cermet-supported button-type solid oxide fuel cells featuring a bi-layered electrolyte (SSZ/SDC), an SSC cathode, and a Ni-SSZ anode, were analyzed using polarization curves, impedance spectroscopy, and post-mortem SEM observation. The cell performance degradation at 650 °C in H₂/air both with and without DC bias conditions was manifested primarily as an increase in polarization resistance, approximately at a rate of 2.3 mΩ cm² h⁻¹ at OCV, suggesting a decrease in electrochemical kinetics as the main phenomenon responsible for the performance decay. In addition, the initial series resistance was about ten times higher than the calculated resistance corresponding to the electrolyte, reflecting a possible inter-reaction between the electrolyte layers that occurred during the sintering stage. In situ and ex situ sintered cathodes showed no obvious difference in cell performance or decay rate. The stability of the cells with and without electrical load was also investigated and no significant influence of DC bias was recorded. Based on the experimental results presented, we preliminarily attribute the performance degradation to electrochemical and microstructural degradation of the cathode.     

Theoretical study on pressurized operation of solid oxide electrolysis cells

In this paper the influence of pressure on the performance of solid oxide electrolysis cells is theoretically analyzed in a pressure range between 0.05 and 2 MPa. A previously validated electrochemical model of a solid oxide fuel cell stack is used to predict electrolysis behavior. The effect of pressure on thermodynamics, kinetics and gas diffusion is discussed. It is shown that thermodynamics are negatively influenced by an increase in pressure whereas kinetics and gas transport are improved. Overall pressure effects are therefore only small. At low current density the electrolysis cell shows better performance at low pressure whereas performance improves with pressure at high current densities.     

Oxidation failure modes of anode-supported solid oxide fuel cells

Investigations on anode-supported solid oxide fuel cells (SOFCs) using Ni-based anode supports are presented aiming at understanding how much oxidation such a cell can tolerate before incurring irreversible mechanical damage. The cells were oxidised both directly in air and electrochemically. The different oxidation procedures performed exhibited different damage modes. For free-standing cells oxidised in air, the main damage mode was electrolyte cracking after oxidation of approximately 50% of the Ni in the substrate. However, cells oxidised electrochemically failed by substrate cracking after only ca. 5% of the Ni was oxidised, mainly due to the non-uniform nature of oxidation in the SOFC. Models of the stress generation and fracture processes were developed for interpretation of the results.     

Design of industrially scalable microtubular solid oxide fuel cells based on an extruded support

The current work describes the adaptation of an existing lab-scale cell production method for an anode supported microtubular solid oxide fuel cell to an industrially ready and easily scalable method using extruded supports. For this purpose, Ni–YSZ (yttria stabilized zirconia) anode is firstly manufactured by Powder Extrusion Moulding (PEM). Feedstock composition, extruding parameters and binder removal procedure are adapted to obtain the tubular supports. The final conditions for this process were: feedstock solid load of 65 vol%; a combination of solvent debinding in heptane and thermal debinding at 600 °C. Subsequently, the YSZ electrolyte layer is deposited by dip coating and the sintering parameters are optimized to achieve a dense layer at 1500 °C during 2 h. For the cathode, an LSM (lanthanum strontium manganite)–YSZ layer with an active area of ∼1 cm² is deposited by dip coating. Finally, the electrochemical performance of the cell is measured using pure humidified hydrogen as fuel. The measured power density of the cell at 0.5 V was 0.7 W cm⁻² at 850 °C.     

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.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

Development of redox stable fuel electrode supported solid oxide cells

A novel solid oxide cell concept, named as redox solid oxide cell, is proposed in this work. To demonstrate the concept, solid oxide cells with doped-SrTiO₃ fuel electrodes and modified NiO-3YSZ fuel electrode support were developed to realize the redox-stable solid oxide cells. By modifying the particle characteristics of NiO, 3YSZ, slurry composition and sintering profile, a redox stable and multifunctional NiO-3YSZ fuel electrode support was successfully developed. Furthermore, two different types of doped-SrTiO₃ (Sr_0.94Ti_0.9Nb_0.1O₃ and La_0.49Sr_0.31Fe_0.03Ni_0.03Ti_0.94O₃) fuel electrode materials were successfully integrated in to the half-cells with redox stable NiO-3YSZ support. Defect free solid oxide cells of 12 cm × 12 cm size were fabricated. The redox stability of these cells was evaluated and compared with the state-of-the-art NiO-3YSZ solid oxide cells at 850 °C. It was clearly demonstrated that the newly developed redox solid oxide cells have superior stability compared to the state-of-the-art cells. In order to establish the potential of the newly developed redox solid oxide cells, the evaluation of the electrochemical performance is required.     

Optimization of the electrode-supported tubular solid oxide cells for application on fuel cell and steam electrolysis

Hydrogen electrode-supported tubular solid oxide cells (SOCs) were fabricated by dip-coating and co-sintering method. The electrochemical properties of tubular SOCs were investigated both in fuel cell and electrolysis modes. Ni-YSZ was employed as hydrogen electrode support. The pore ratio of Ni-YSZ support strongly affected the performance of tubular SOCs, especially in steam electrolysis mode. The pore ratio was adjusted by the content of pore-former in support slurry. The results showed that 3 wt.% pore former content is the proper value to produce high performance both in fuel cell and electrolysis modes. In fuel cell mode, the maximum power density reached 743.1 mW cm⁻² with H₂ (105 sccm) and O₂ (70 sccm) as working gases at 850 °C. In electrolysis mode, as the electrolysis voltage was 1.3 V, the electrolysis current density reached 425 mA cm⁻² with H₂ (35 sccm) and N₂ (70 sccm) adsorbed 47% steam as working gases in hydrogen electrode at 850 °C. The stability of tubular SOCs was related to the ratio of NiO/YSZ in the support. The sample with NiO/YSZ = 60/40 shows a better performance than the sample with NiO/YSZ = 50/50.     

Ni/YSZ and Ni–CeO2/YSZ anodes prepared by impregnation for solid oxide fuel cells

In this paper, Ni/YSZ and Ni–CeO₂/YSZ anodes for a solid oxide fuel cell (SOFC) were prepared by tape casting and vacuum impregnation. By this method, the Ni content in the anode could be reduced compared to the traditional tape casting method. It was found that adding CeO₂ into the Ni/YSZ anode by a Ni(NO₃)₂ and Ce(NO₃)₃ mixed impregnation could further enhance cell performance. This was investigated in H₂ at 1073 K. XRD patterns indicated that CeO₂ and Ni were separate phases, and the CeO₂ addition could enhance the Ni dispersion on the YSZ framework surface which was observed by SEM images. It was shown that adding CeO₂ into the Ni anodes could decrease the cell polarization resistance. The maximum power density for cells with 25 wt.% Ni, 5 wt.% CeO₂–25 wt.% Ni/YSZ, or 10 wt.% CeO₂–25 wt.% Ni/YSZ anode was 230 mW cm⁻², 420 mW cm⁻² and 530 mW cm⁻², respectively, in H₂ at 1073 K. The OCV for these cells was 1.05–1.09 V, indicating that a dense electrolyte film was obtained by co-firing porous YSZ layer and dense YSZ layer.