Analysis of pressurized operation of 10 layer solid oxide electrolysis stacks

High temperature steam electrolysis using solid oxide electrolysis cell (SOEC) technology can provide hydrogen as fuel for transport or as base chemical for chemical or pharmaceutical industry. SOECs offer a great potential for high efficiencies due to low overpotentials and the possibility for waste heat use for water evaporation. For many industrial applications hydrogen has to be pressurized before being used or stored. Pressurized operation of SOECs can provide benefits on both cell and system level, due to enhanced electrode kinetics and downstream process requirements. Experimental results of water electrolysis in a pressurized SOEC stack consisting of 10 electrolyte supported cells are presented in this paper. The pressure ranges from 1.4 to 8 bar. Steady-state and dynamically recorded U(i)-curves as well as electrochemical impedance spectroscopy (EIS) were carried out to evaluate the performance of the stack under pressurized conditions. Furthermore a long-term test over 1000 h at 1.4 bar was performed to evaluate the degradation in exothermic steam electrolysis mode. It was observed that the open circuit voltage increases with higher pressure due to well-known thermodynamic relations. No increase of the limiting current density was observed with elevated pressure for the ESC-stacks (electrolyte supported cell) that were investigated in this study. The overall and the activation impedance were found to decrease slightly with higher pressure. Within the impedance studies, the ohmic resistance was found to be the most dominant part of the entire cell resistance of the studied electrolyte supported cells of the stack. A constant current degradation test over 1000 h at 1.4 bar with a second stack showed a voltage degradation rate of 0.56%/kh.     

Improved durability of SOEC stacks for high temperature electrolysis

An experimental study has been conducted at Idaho National Laboratory to demonstrate recent improvements in long-term durability of solid oxide electrolysis cells (SOEC) and stacks. Results of five stack tests are presented. Electrolyte-supported SOEC stacks were provided by Ceramatec Inc. and electrode-supported SOEC stacks were provided by Materials and Systems Research Inc. (MSRI), for these tests. Long-term durability tests were generally operated for durations of 1000 h or more. Stack tests based on technologies developed at Ceramatec and MSRI have shown significant improvement in durability in the electrolysis mode. Long-term degradation rates of 3.2%/khr and 4.6%/khr were observed for MSRI and Ceramatec stacks, respectively. One recent Ceramatec stack even showed negative degradation (performance improvement) over 1900 h of operation. Optimization of electrode and electrolyte materials, interconnect coatings, and electrolyte–electrode interface microstructures contribute to improve the durability of SOEC stacks.     

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.     

Chemically-induced mechanical unstability of samaria-doped ceria electrolyte for solid oxide electrolysis cells

Doped-ceria is an attractive electrolyte material for solid oxide electrolysis cells (SOECs) operated at intermediate temperatures. However, ceria is highly prone to break down under high applied voltages and low oxygen partial pressures at the fuel side. This phenomenon is analyzed for the typical Sm_0.2Ce_0.8O_1.9−δ electrolyte based on the chemically-induced stress, which is caused by the inhomogeneous distribution of oxygen non-stoichiometry throughout the thickness of electrolyte plate. The sensitivities of the maximum tensile stresses are explored under typical SOEC operating parameters such as temperature, applied voltage and oxygen partial pressure. Varying from short-circuit of solid oxide fuel cell (SOFC) mode to high voltage of SOEC conditions, the applied voltage sharpens the maximum tensile stress by seven times and raises the minimum permitted oxygen partial pressure at the cathode-electrolyte interface by a factor of 10^4.5 at most. The analysis results indicate that a ceria-based electrolyte under SOEC conditions denotes a definite trend of collapse at 700 °C even 600 °C, suggesting the inapplicability of doped-ceria electrolyte in SOEC mode.     

Achieving high-efficiency hydrogen production using planar solid-oxide electrolysis stacks

Steam electrolysis in solid oxide electrolysis cells (SOECs) is considered as an effective method to achieve high-efficiency hydrogen production. In the present investigation, samples of 1-cell, 2-cell and 30-cell SOEC stacks were tested under electrolysis of steam to give a practical evaluation of the SOEC system efficiency of hydrogen production. The samples were tested at 800 °C under various operating conditions up to 500 h without significant degradation, and obtained steam conversion rates of 12.4%, 23% and 82.2% for the 1-cell, 2-cell and 30-cell stacks, respectively. System efficiencies of hydrogen production were calculated for the samples based on their real performance. A maximum efficiency value of 52.7% was achieved in the 30-cell stack.     

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.     

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.     

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.     

Performance and structural stability of Gd0.2Ce0.8O1.9 infiltrated La0.8Sr0.2MnO3 nano-structured oxygen electrodes of solid oxide electrolysis cells

Effect of Gd_0.2Ce_0.8O_1.9 (GDC) infiltration on the performance and stability of La_0.8Sr_0.2MnO₃ (LSM) oxygen electrodes on Y₂O₃-stabilized ZrO₂ (YSZ) electrolyte has been studied in detail under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The incorporation of GDC nanoparticles significantly enhances the electrocatalytic activity for oxygen oxidation reaction on LSM electrodes. Electrode polarization resistance of pristine LSM electrode is 8.2 Ω cm² at 800 °C and decreases to 0.39 and 0.09 Ω cm² after the infiltration of 0.5 and 1.5 mg cm⁻² GDC, respectively. The stability of LSM oxygen electrodes under the SOEC operating conditions is also significantly increased by the GDC infiltration. A 2.0 mg cm⁻² GDC infiltrated LSM electrode shows an excellent stability under the anodic current passage at 500 mA cm⁻² and 800 °C for 100 h. The infiltrated GDC nanoparticles effectively shift the reaction sites from the LSM electrode/YSZ electrolyte interface to the LSM grains/GDC nanoparticle interface in the bulk of the electrode, effectively mitigating the delamination at the LSM/YSZ interface. The results demonstrate that the GDC infiltration is an effective approach to enhance the structural integrity and thus to achieve the high activity and excellent stability of LSM-based oxygen electrode under the SOEC operating conditions.     

Ageing of anode-supported solid oxide fuel cell stacks including thermal cycling,and expansion behaviour of MgO–NiO anodes

This paper reports on medium term tests of anode-supported five-cell short stacks, as well as on some separate anode development. Two stacks were operated under steady-state conditions: one with unprotected metal interconnects, H₂ fuel and 0.35 A cm⁻² (40% fuel utilisation) polarisation current showed an average cell voltage degradation of 56 mV per 1000 h for 2750 h; one with coated metal interconnects, synthetic reformate fuel and 0.5 A cm⁻² (60% fuel utilisation) polarisation current showed an averaged cell voltage degradation slope of 6.6 mV per 1000 h for 800 h before a power cut prematurely interrupted the test. A third stack was subjected to 13 complete thermal cycles over 1000 h, average cell voltage degradation was evaluated to −2 mV per cycle for operation at 0.3 A cm⁻², open circuit voltage (OCV) remained stable, whereas area specific resistance (ASR) increase amounted on average to 0.008 Ω cm² per cycle.     

Role of initial microstructure on nickel-YSZ cathode degradation in solid oxide electrolysis cells

Electrical performance degradation relating to materials interactions at elevated temperatures is one of the primary technical limitations to the near term successful commercialization of solid oxide electrolysis cells (SOECs). Electrochemical performance and structural degradation of nickel-YSZ fuel electrodes (SOEC cathodes) were studied as functions of materials, preparation techniques, and operating conditions. Significant electrochemical degradation was observed in cells operated at 840 °C with and without voltage bias. Carbon deposition, Ni evaporation, and impurity poisoning were not observed to negatively affect performance. However, the high Ni content and large initial Ni particle size caused rapid Ni migration and agglomeration. The agglomeration reduced Ni electrical interconnectivity as well as contact area with the current collector.     

Study of catalyst sprayed membrane under irradiation method to prepare high performance membrane electrode assemblies for solid polymer electrolyte water electrolysis

In this work, a catalyst sprayed membrane under irradiation (CSMUI) method was investigated to develop high performance membrane electrode assembly (MEA) for solid polymer electrolyte (SPE) water electrolysis. The water electrolysis performance and properties of the prepared MEA were evaluated and analyzed by polarization curves, electrochemistry impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The characterizations revealed that the CSMUI method is very effective for preparing high performance MEA for SPE water electrolysis: the cell voltage can be as low as 1.564 V at 1 A cm⁻² and the terminal voltage is only 1.669 V at 2 A cm⁻², which are among the best results yet reported for SPE water electrolysis with IrO₂ catalyst. Also, it is found that the noble metal catalysts loadings of the MEA prepared by this method can be greatly decreased without significant performance degradation. At a current density of 1 A cm⁻², the MEA showed good stability for water electrolysis operating: the cell voltage remained at 1.60 V without obvious deterioration after 105 h operation under atmosphere pressure and 80 °C.     

Microstructural modification of the anode/electrolyte interface of SOEC for hydrogen production

The microstructure of the anode/electrolyte interface is one of key factors to affect the hydrogen production performance of solid oxide electrolysis cells (SOECs). In this paper, a novel interfacial modification method was developed to enhance the active electrode area and the electrolysis performance via preparing a porous YSZ layer on the surface of the dense electrolyte. The effects of YSZ electrolyte pre-sintering temperature, the thickness of dense YSZ electrolyte film and preparation of porous YSZ on the microscopic morphology and the performance of single button cells were investigated. After optimization, a 9 μm porous YSZ layer was successfully prepared on the surface of a 4 μm dense YSZ electrolyte film with an OCV of 1.072 V at the temperature of 850 °C. The results of electrochemical tests showed that the current densities could elevate from −0.681 A cm⁻² to −1.118 A cm⁻² when electrolyzed at 1.5 V under SOEC mode after microstructural modification.     

Durability test and degradation behavior of a 2.5 kW SOFC stack with internal reforming of LNG

Forschungszentrum Jülich has demonstrated SOFC stacks and systems ranging from 50 W to 20 kW. Previous studies have shown the reproducible stable long-term performance of the F10-design short stacks developed in Forschungszentrum Jülich. Within this work, a 2.5 kW F20-stack consisting of eighteen cells was assembled, and tested at a furnace temperature of 700 °C mainly with the simulated reformate gas, which corresponds to 10% pre-reforming of liquefied natural gas (LNG). The current density and fuel utilization were mostly kept at 0.5 A cm⁻² and 70%, respectively. The purpose was to investigate the behavior of the stack in the kW-range for at least 5000 h with internal reforming of LNG or methane at a fuel utilization of at least 60%. A voltage degradation rate of around 0.3%/1000 h was obtained during the operation with pre-reformed LNG. The stack performance under normal working conditions and an unplanned redox cycle, as well as the results from post mortem analysis are discussed.     

Performance and stability of (La,Sr)MnO3–Y2O3–ZrO2 composite oxygen electrodes under solid oxide electrolysis cell operation conditions

The electrochemical performance and stability of (La,Sr)MnO₃–Y₂O₃–ZrO₂ (LSM-YSZ) composite oxygen electrodes is studied in detail under solid oxide electrolysis cells (SOECs) operation conditions. The introduction of YSZ electrolyte phase to form an LSM-YSZ composite oxygen electrode substantially enhances the electrocatalytic activity for oxygen oxidation reaction. However, the composite electrode degrades significantly under SOEC mode tested at 500 mA cm⁻² and 800 °C. The electrode degradation is characterized by deteriorated surface diffusion and oxygen ion exchange and migration processes. The degradation in electrode performance and stability is most likely associated with the breakup of LSM grains and formation of LSM nanoparticles at the electrode/electrolyte interface, and the formation of nano-patterns on YSZ electrolyte surface under the electrolysis polarization conditions. The results indicate that it is important to minimize the direct contact of LSM particles and YSZ electrolyte at the interface in order to prevent the detrimental effect of the LSM nanoparticle formation on the performance and stability of LSM-based composite oxygen electrodes.     

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.     

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.     

Elementary reaction modeling and experimental characterization of solid oxide fuel-assisted steam electrolysis cells

A one-dimensional elementary reaction kinetic model for solid oxide fuel-assisted steam electrolysis cell (SOFEC) is developed coupling heterogeneous elementary reactions, electrochemical reaction kinetics, electrode microstructure and transport processes of charge and mass. This model is calibrated and validated by experimental data from a button cell with anode gases of H₂, CO and CH₄ at 800 °C. After comparisons with solid oxide electrolysis cell (SOEC), the energy demands, performance and efficiency of CO-assisted SOFEC and CH₄-assisted SOFEC are investigated numerically. One important finding is that over 80% of electricity can be saved by SOFEC at a current density of 3000 A m⁻². SOFEC assisted by CO or CH₄ for steam electrolysis has better performance than SOEC, especially by CH₄. The efficiencies of 12% CO-SOFEC and 12% CH₄-SOFEC are at least, respectively, 7% and 30% higher than that of SOEC at 800 °C with the current density of below 2500 A m⁻². Finally, the effects of type of assisting-fuel, fuel composition and applied voltage are studied. It is found that CO-SOFEC shows higher anode polarization and thus lower performance than CH₄-SOFEC with the same molar fraction of fuel. It is also found that the performance of SOFEC increases with increasing proportion of assisted fuel in anode at high current density.     

Durable SOC stacks for production of hydrogen and synthesis gas by high temperature electrolysis

Electrolysis of steam and co-electrolysis of steam and carbon dioxide was studied in Solid Oxide Electrolysis Cell (SOEC) stacks composed of Ni/YSZ electrode supported SOECs. The results of this study show that long-term electrolysis is feasible without notable degradation in these SOEC stacks. The degradation of the electrolysis cells was found to be influenced by the adsorption of impurities from the applied inlet gases, whereas the application of chromium containing interconnect plates and glass sealings do not seem to influence the durability when operated at 850 °C. Cleaning the inlet gases to the Ni/YSZ electrode resulted in operation without long-term degradation, and may therefore be a solution for operating these Ni/YSZ based SOEC stacks without degradation.