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A running steam electrolysis test with a 10Sc1CeSZ electrolyte supported cell with Ni/GDC hydrogen and LSCF oxygen electrodes has passed 34,000 h with -0.6 Acm-2 current density. The high specific ionic electrolyte conductivity allowed a moderate initial cell temperature (~780°C) for operation with a cell voltage close to the thermal neutral voltage (U th ~1.29 V), i.e. with highest energy-conversion efficiency. Part of the wide margin for temperature increase was used for compensation of the voltage degradation, done after 20 and 31 kh, respectively. The resulting compensation rate of 0.9°C/kh extrapolates to a degradation compensated lifetime beyond current system requirements (>50,000 h). Compensation was traced with voltage-current curves and with impedance. Voltage degradation for constant temperature amounted, after several kh initial stabilisation, to (4 - 5) mV/kh (0.3 - 0.4 %/kh). Degradation was mainly ohmic, without clearly detectable non-ohmic electrode contributions. Non-ohmic terms therefore added to the compensation of ohmic degradation via temperature increase.

The current ON/OFF switching of a solid oxide electrolysis cell is treated as elementary step for power variation in a steam electrolyser system. If the cell voltage in the ON mode is adjusted to the thermal neutral voltage, heat generation remains zero in both modes, which largely facilitates the thermal management. To verify whether the cells withstand the switching, an electrolysis durability test with an electrolyte supported solid oxide cell was performed during one year at about 850 °C. The cell consisted of a 3YSZ electrolyte, CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. The test included two operation blocks with each 40,000 cycles of 2 min duration and a current density of −0.7 Acm−2 in the ON mode (−0.07 Acm−2 in OFF mode), as well as steady-state ON periods with 5800 h duration. Voltage degradation was 5 mV/1000 h (0.4%/1000 h) and the increase in the area specific resistance 7 mΩcm2/1000 h, without notable dependence on current cycling. Impedance spectroscopic results were in agreement with the only small switching transients seen in the cell voltage; moreover, they confirmed a dominating ohmic degradation together with minor contributions from gas conversion and reaction, respectively. No electrode delamination was detectable after scheduled test completion.

Hydrogen production with reduced electricity consumption is achieved with high temperature steam electrolysis. The technology is based on solid oxide cells that are able to operate reversibly in a fuel cell for the production of electricity and heat. This chapter gives a general overview of the technology with details about cell materials and architecture as well as manufacturing processes from cells to stacks. The chapter then reviews performance and durability from singles cells to larger stacks operated under steam electrolysis for long-term steady-state operation as well as flexible operation. Structural analysis results after electrolysis operation are presented and main sources of degradation discussed. Finally, system’s designs and technical and economical analysis give a prospective view of the technology cost in comparison with the other technologies of water electrolysis.

In this review paper, a critical assessment of the main degradation processes in three key components of solid oxide fuel cells and electrolysers (negative and positive electrodes and the interconnect) is undertaken, attempting prioritization of respective degradation effects and recommendation of the best approaches in their experimental ascertainment and numerical modeling. Besides different approaches to quantifying the degradation rate of an operating solid oxide cell (SOC), the latest advancements in microstructural representation (3D imaging and reconstruction) of SOC electrodes are reviewed, applied to the quantification of triple-phase boundary (TPB) lengths and morphology evolution over time. The intrinsic degradation processes in the negative (fuel) electrode and the positive (oxygen) electrode are discussed, covering first the composition and governing mechanisms of the respective electrodes, followed by a comprehensive evaluation of the most important factors of degradation during operation. By this systematic identification of dominant degradation processes, measurement techniques, and modeling approaches, the foundations are laid for the definition of meaningful accelerated stress testing of SOC cells and stacks, which will help the technology achieve the constantly more demanding durability targets in market applications.

This paper addresses the design of the hydrogen supply chain (HSC) with a focus on the its decentralization degree. For this purpose, the superstructure used for HSC modelling is split into six echelons, i.e., primary energy source, H2 production, conditioning and centralized storage, distribution, conditioning and decentralized storage, final usage. The originality of this work is also the diversification of H2 production, conditioning and distribution methods for the H2 supply both for the industry and mobility sectors. The problem is formulated in a mono-optimization framework, where the total cost of the HSC is minimized. The model involves a multi-period (2020-2040) mixed-integer linear programming (MILP) formulation that is implemented in the General Algebraic Modelling System (GAMS) environment, using CPLEX as a solver. The optimization results highlight some significant applications of the developed methodological framework. The assessment of the decentralization degree of the system, the diversification of the final H2 demand, the share between liquid and compressed gaseous form are some of the Key Performance Indicators (KPIs) that can characterize a scenario. The scalability and replicability of the tool indeed allow to make forecasts on different geographical and temporal scales.

The scale-up from single cell, to stack and integrated stack module induces an increased degradation that needs to be assessed for improving the performance and the lifetime of the high-temperature electrolyzer system. Within this work, the degradation rate of a 30-cell stack and a 90-cell module (based on similar 3YSZ electrolyte supported solid oxide cell) is evaluated with operation time of 4,224 h (−0.5 Acm−2) and 2,200 h (from −0.5 to – 0.3 Acm−2), respectively. The voltage degradation rate of the 30-cell stack is 10.5 mV/kh (0.8%/kh referring to the thermoneutral voltage of 1.3 V) including the temperature correction factor for an increase of 2 °C/kh. It is at present/currently the lowest degradation value reported for a stack of 30 electrolyte supported cells considering the thermal factor correction. This implies an increase of 5.5 mV/kh for the stack compared to the single cell. Along the steady-state phase two dynamic phases of 8,500 and 7,500 ON/OFF cycles were performed every 2 min. It has been shown that these significant load cycles do not induce additional degradation. Moving to the 90-cell module, irreversible damage occurred due to incidences related to the balance of plant; based on their analysis, propositions are made to counteract them.

An electrolyte supported solid oxide cell of 45 cm2 area was operated in the steam-electrolysis mode during more than 23,000 h before scheduled shutdown, of which 20,000 h with a current density of j = −0.9 A cm−2. The cell consisted of a scandia/ceria doped zirconia electrolyte (6Sc1CeSZ), CGO diffusion-barrier/adhesion layers between electrolyte and electrodes, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. Voltage degradation in the operation period with j = −0.9 A cm−2 was 7.4 mV/1000 h (0.57%/1000 h) and the increase in the area specific resistance 8 mΩ cm2/1000 h. The final cell voltage was 1.33 V (at 851 °C cell temperature). After dismantling, the cell showed no mechanical damage at electrolyte and H2/H2O electrode; a small fraction of the oxygen electrode was delaminated. Impedance spectroscopy applied at the steady state DC current density confirmed a degradation dominated by an increasing ohmic term, mainly due to ionic conductivity decay in the electrolyte. In addition, a small non-ohmic and at least partly reversible O2 electrode contribution to degradation was identified, affected by a pollution from the (compressor) purge air.

A solid oxide short stack composed of 6 Ni-YSZ supported cells, YSZ electrolyte and GDC-LSC oxygen electrode has been tested for 10,700 hours in steam electrolysis. Initial degradation was followed by a global stabilization of the performance after lowering the current density, with a degradation rate below 0.5% kh(-1). Post-test analysis has been conducted on two repeating units (RUs) to highlight the most significant microstructure alterations. Nickel depletion was observed in the hydrogen electrode close to the interface with the electrolyte. Formation of small pores in the electrolyte was detected along the grain boundaries. A consequent detachment related to this phenomenon was observed in proximity of the GDC compatibility layer. At the oxygen electrode side, the formation of a approximate to 1 mu m dense mixed layer of GDC and YSZ was observed. Strontium from the LSC electrode migrated through GDC pores and reacted with YSZ, forming evident SrZrO3 inclusions. Distinct accumulation of silicon at the Ni/YSZ interface and chromium on the GDC barrier layer have been observed in both RUs. Despite this range of alterations observed, the stack degradation remained limited, testified from the fact that performance decay between 4,000 and 10,000 hours of operation was virtually nil.

This work presents a 790 h durability test performed on a solid oxide cell. An innovative Ce0.9Gd0.1O2-δ-La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen electrode with coral microstructure was deposited on a solid oxide fuel cell (SOFC) anode supported with 8 mol.% yttria-stabilized zirconia (8YSZ) electrolyte. An initial cell potential was found to be 817 mV at 773 °C under synthetic air on the oxygen electrode side, and pure hydrogen (500 NmL.min−1) on the hydrogen electrode side, under 0.5 A.cm−2 current density in SOFC. The initial cell potential in the solid oxide electrolysis cell (SOEC) was 1200 mV at 773 °C under synthetic air on the oxygen electrode side and a mixture of steam/hydrogen (80 %/20 %, respectively) on the hydrogen electrode side under −0.75 and −0.5 A.cm−2 current density. The cell resistance progressively increased during the durability test. The increase at the high frequency range was mainly responsible for an increase in total cell resistance. This frequency range was associated with the blocking effect on the oxygen electrode/electrolyte interface. U-j curves showed reversibility of the current loading from −0.75 to 0.5 A.cm−2. SEM observations indicated cracks in the 8YSZ electrolyte and the detachment of the oxygen electrode.

Steam (and co-) electrolysis demonstration projects with solid oxide cells (SOC) have reached a power range of several hundred kW (e.g., /1/). This generates an urgent demand for verification of the durability of the cells as core electrolyser elements. Cell lifetimes of several years are required, which largely exceeds the commonly reported testing periods. Apart from economic considerations, cell (or short stack) testing allows a more precise control of the cell temperature together with less temperature variation across the surface. This, in turn, facilitates quantification of degradation, in particular for the practically required low degradation rates of -2 with 7.3 mV/kh voltage degradation /2/). This result represents an example for SOFC/SOEC reversibility holding even for a current density magnitude exceeding the commonly applied values under SOFC operation (higher current densities are feasible in the electrolysis mode owing to the more favourable thermal conditions, but they will, in general, imply the development of appropriate cells). Comparably low degradation was confirmed in a one year long test coupling steady-state and ON/OFF switching operation (80,000 cycles) for a cell with a thin 3YSZ electrode instead of the 6Sc1CeSZ in the 23 kh test /3/. ON/OFF stack switching is part of an operation strategy for matching a fluctuating load (such as coupling with renewable energy sources or grid stabilisation). In this contribution, emphasis will be put on a cell test of even longer duration (>30 kh so far) with an electrolyte supported cell, maintaining the current density above the typical SOFC values (initial part of the test in ref. /4/). The operation conditions are chosen for a cell voltage close to the thermal neutral voltage (Uth ~1.3 V), in accord with the requirements for stack operation. The use of an electrolyte of high ionic conductivity, 10Sc1CeSZ, allows a moderate initial temperature below 800°C at the not degraded cell. Temperature is later increased for compensation of degradation. For an appropriate setting of the initial testing parameters together with a low cell degradation and a sufficiently wide temperature window, one should then obtain an operation close to the theoretical efficiency limit with zero voltage degradation during the entire envisaged lifetime. As in our previous work, the long-term cell behaviour is traced with in-situ impedance spectroscopy without interrupting the DC current flow. The main focus of the degradation analysis refers to the ohmic degradation, which typically dominates in the cells of concern /2-4/. References: /1/ . /2/ J. Schefold, A. Brisse, H. Poepke, Int. J. Hydrogen Energy, 42 (2017) 13415-26. /3/ J. Schefold, A. Brisse, A. Surrey, C. Walter, Int. J. Hydrogen Energy, (2019) available online. /4/ A. Brisse, J. Schefold, Proceedings 13th European SOFC & SOE Forum, B1502, 3 – 6th July 2018, Lucerne (Switzerland), pp. 91 - 100.

Steam electrolysis over 11000 h 1 with an electrolyte-supported solid oxide cell is discussed. The cell of 45 cm2 area consists of a scandia/ceria doped zirconia electrolyte (6Sc1CeSZ), CGO diffusion-barrier/adhesion layers, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel steam/hydrogen electrode. After initial 2500 h operation with lower current-density magnitude, the current density was set to j = -0.9 A cm−2 and the steam conversion rate to 51%. This led to a cell voltage of 1.185 V at 847 °C cell temperature. Average voltage degradation was 7.3 mV/1000 h ( 100% throughout the test (with an external heat source for evaporation). Impedance spectroscopic measurements revealed a degradation almost entirely due to increasing ohmic resistance. The rate of resistance increase was initially faster (up to 40 mΩ cm2/1000 h) and stabilised after several 1000 h operation. After 9000 h a small (non-ohmic) electrode degradation became detectable (

International audience; High Temperature ELectrolysers (HTEL) operate around 1073 K with steam at 1073 K. Water is previously heated up in the Balance of Plant by the hot outlet gases and optionally by electrical heaters. If liquid water is fed to the system, vaporisation needs are covered by electrical heating, which leads to a low system efficiency of 89% vs. HHV. Using steam instead of liquid water would suppress vaporisation needs, thus increase the efficiency. This work aims to analyse the potential benefits of a steam supply. Calculation is performed without considering the energy required to preheat water. Results show that feeding the system with low-temperature steam instead of liquid water enables a system efficiency jump of 18%. Further increasing the steam temperature to 933 K negligibly impacts the system efficiency and is therefore unnecessary. It is concluded that a low-temperature steam source is sufficient to increase significantly the HTEL system efficiency.

A 5-cell solid oxide electrolyzer cell (SOEC) stack has been operated at ca. 700 °C for 2,000 h under static conditions followed by a short-term transient operation. The best repeating unit (RU) showed an area specific resistance of 0.4 Ω cm2 and an initial voltage of 1.125 V at 705 °C and –0.6 A cm–2 when fed with 90% H2O/10% H2 at the hydrogen-electrode. One instability factor has been observed, which led to dramatic voltage fluctuations for most RUs in the stack. Such kind of voltage fluctuation is correlated predominantly to the variation of ohmic resistances of RUs, pointing out most probably unstable electrical contact in the stack under the present testing conditions. Disregarding the worst RU (RU1), the average degradation rate (voltage increase rate) of all RUs in the stack has been determined to be around 2% kh–1 (24 mV kh–1) for the last 500 h of static operation free of voltage fluctuations. The stack showed no noticeable degradation during the short-term transient operation (totally 756 ON/OFF current cycles), indicating preliminarily that it could be possible to couple intermittent power sources with high-temperature steam electrolyzers.

Degradation is one of the most important issues in solid oxide electrolysis cells (SOECs). The lowest long-term degradation reported in the literature for SOEC under practical operation is about 1.7%/1000 h during 3600 h at −1 A/cm2 which is twice the degradation of similar cells under fuel cell operation. To reduce the degradation, a clearer identification of the polarization and degradation mechanisms of SOEC is important in order to adapt the operating conditions, optimize the cell architecture to the purpose of electrolysis by modifying and/or replacing one or both of the electrodes with alternative materials. This paper tends to review the cell degradation phenomena observed under high temperature electrolysis. A description of reactions potentially involved in the degradation mechanisms associated with the thermodynamic and kinetic considerations are used to build a fault tree in order to classify the different defaults and better understand the relationship between the causes and symptoms.

Water electrolysis is a well-established process for hydrogen production but requires efficiency improvements to reduce costs. High temperature electrolysis (HTE) as a means to higher efficiency was advanced in the EU project RelHY. Through Life Cycle Assessment (LCA), also the environmental performance of five HTE-based hydrogen production systems was evaluated: operation with power and steam from a nuclear plant, continuous and intermittent operation with wind power and water, intermittent operation with natural gas or biogas reforming as back-up. Large scale natural gas reforming (NGR) was used as a reference. The LCA aims to identifying environmental hotspots of HTE plants and comparing their operation. The results show that stack manufacturing has the strongest impact during construction of the HTE plant while the impacts during H2 production are largely due to power supply. All HTE variants studied lead to less life cycle CO2-equivalent emissions than NGR. However, only the wind powered HTE variants without back-up use less energy than NGR. The other impacts and flows show different patterns. The results and limitations of the study are discussed.

An anode-supported solid oxide fuel cell (SOFC) was operated for 9000 h as a solid oxide electrolyser cell (SOEC) with a current density of −1 Acm−2 showing a voltage increase of about 40 mV kh−1, i.e. an overall voltage degradation rate of 3.8% kh−1. The cell consisted of yttria-stabilised zirconia (YSZ) combined with a gadolinia-substituted ceria layer as electrolyte. The hydrogen electrode was a Ni/YSZ cermet, and the oxygen electrode a La0.58Sr0.4Co0.2Fe0.8O3 perovskite (LSCF). After electrolysis operation, the cell was metallographically analysed to identify degradation processes. Compared with the initial state, the most pronounced change was found at the electrolyte layer, which showed trans-granular longitudinal porosity as well as severe pore formation along grain boundaries. In addition, material transport was observed into the ceria-based diffusion barrier layer. The oxygen electrode showed compositional variations in the sub-μm range and different degrees of recrystallisation across the electrode thickness. The microstructural observations agree with the already reported impedance spectroscopic results, which indicated (i) an increase in ohmic electrolyte resistance as the dominant degradation feature, which is specific to SOEC operation, and (ii) an increase in the electrode reaction overpotentials.

The European Institute for Energy Research is working on the application of the solid oxide cell technology for high temperature electrolysis with the aim to produce hydrogen and syngas. Since 2004, numerous tests of single cells and stacks with 5 to 25 cells have been conducted. Test durations were rather long, ranging from 1000 to 9000 hours, with current densities between 0.4 and 1 A/cm2. A summary of the experimental results is presented with a focus on the observation of cell and stack degradation. Long term operation of cells with 45 cm2 active area under a high current density of 1 A/cm2 indicates an extrapolated cell lifetime of at least 20 000 h. Cell integration into short stacks shows additional constraints such as non-homogeneous cell behavior, electrical contacting resistances of the cell interconnects which are more critical under operation at high current density, and increased degradation rates.Techno-economical analysis have been realized in parallel to establish the hydrogen production cost by high temperature electrolysis as function of the electrolyser environment (availability of an external heat source, electricity source, hydrogen compression stages.). Finally, the hydrogen production costs using high temperature electrolysis are discussed and the high temperature electrolysis is positioned on the roadmap of development and deployment of the electrolysis technologies for hydrogen and syngas production.

Classical solid oxide fuel cell anode (Ni–cermet) could be employed as solid oxide electrolysis cell cathode. Ni–cermet has been synthesized and tested as solid oxide electrolyzer cathode using three-electrode techniques between 700 °C and 900 °C. yttria stabilized zirconia was used as the electrolyte and Pt as the counter electrode. Polarization curves and impedance spectra have been analyzed for two gas compositions. The presented results demonstrated an influence of Ni–cermet electrode behavior upon gas composition and temperature. The present results highlight a mechanism changing on Ni–cermet electrode upon gas composition. In a second part, a one-dimensional steady state model is developed to predict the cathodic behavior of Ni–cermet. This model takes into account mass and charge conservation, transport of species and reaction kinetics. It considers the porous electrode to be a homogeneous medium characterized. The influence of varying chemical and electrochemical steps kinetic on the shape of polarization curves is discussed. At high overpotential values the model with two rate-limiting steps has been validated using numerical optimization method.

Results from a 4000 h test of a solid oxide fuel cell (SOFC) stack operated in the H2O electrolyser mode are reported. The stack, which consisted of 5 cells with electrolyte supported cells of each 100 cm2 active area, was operated at 820°C under a current density of -0.4 Acm-2, an absolute feed gas humidity of 90%, and a steam conversion of 40%. Faradaic efficiencies close to 100% and initial cell voltages of ~1.2 V were achieved, which translates to high electrical-to-chemical energy conversion efficiency. Average (cell + interconnect) voltage degradation under constant current operation amounted to 5.6% per 1000 h. The cell specific, partly reversible, voltage evolution with time together with impedance spectroscopic results indicates that a significant part of degradation arises from ohmic losses at the cell interconnects. The response of the stack to a loss of steam supply under current flow is described and post-mortem analysis is reported.

A one-dimensional model has been developed to model the mass and charge transfer occurring in the water reduction on nickel-cermet. An artificial neural network (ANN) was identified and validated from this model. The ANN, in equation form, was used in a three-dimensional model of a serial repeat unit of a Solid Oxide Electrolysis Cell stack. The electrochemical processes can be modeled with acceptable accuracy with the one-dimensional model based on a two-step mechanism for the water reduction. The presented results show that the ANN could be employed to model electrochemical reactions and be implemented in a macro-scale model.

A fairly broad experimental basis exists for solid oxide fuel cells reversibly operated as H2O electrolyser cells (SOECs), notably for classical cells with Ni based anodes, YSZ electrolytes, and LSM cathodes. High electrical-to-chemical energy conversion efficiencies are achieved at such cells. Results from the long-term SOEC testing of two 5-cell stacks are compared, stack I with (H2) electrode and stack II with electrolyte supported cells. Operation times were 2600 h and 2000 h, respectively, under current densities between -0.4 and -0.6 Acm-2, 90% feed gas humidity, and 40 to 60 % steam conversion. The voltage degradation under constant current operation amounts to 3% (stack I) and 4.6% (stack II) per 1000 h. The evaluation is complicated by instabilities arising from poor endplate contacts and also by initial passivation/activation at stack I. Faradaic efficiencies are close to 100%. Electronic conduction in the electrolyte limits the cell voltage under current flow without steam supply.

Anode supported solid oxide fuel cells (SOFCs) with yttria stabilised zircona (YSZ) electrolytes were operated at ~800{degree sign}C as solid oxide H2O electrolyser cells (SOECs). Typical operation voltages were 1.0 to 1.3 V. When operation was extended to current densities j corresponding to a steam conversion rate above 100%, or when the steam supply was interrupted under constant current conditions, cell voltages saturated at about 1.8 to 2.0 V. Cells survived long-term current polarisation with j ~ -0.3 to -0.5 Acm-2 without steam supply. The mechanism limiting the cell voltage is attributed to electronic conduction in the YSZ electrolyte. Electrolyte conduction in the normal SOEC mode (with reasonable steam conversion rates) remains predominantly ionic in the investigated temperature range. Depending on the cell characteristics and the operation conditions (notably the temperature), however, the ionic transfer number may be significantly lower compared to the SOFC operation.

The common technology for solid oxide fuel cells (SOFC) is based on a cermet anode of nickel with yttrium stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this anode technology is the redox tolerance. In this study, two techniques are used to quantify the anode expansion after one redox cycle at different temperatures. First, the increase of porosity is measured using scanning electron microscopy (SEM) image quantification. The second method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. An influence of redox cycling temperature on anode volume expansion was already observed by Ettler et al. but not fully understood. Here a model based on nickel oxidation mechanisms is proposed to explain the relation between redox temperature and volume expansion. The same methods are used to quantify anode expansion after consecutive redox cycles at constant temperature. In particular, small cracks caused at low oxidation temperature (600°C) were closed again after a subsequent reduction. The quantifying technique is then applied to cells tested in real stack conditions. The cell corners can undergo redox cycles depending on stack design and fuel utilization. The study of this location can give information on the number of cycles experienced by the anode support. Moreover, cracks in the electrolyte show infiltration of NiO that close the openings. Important infiltration can produce a short circuit between anode and cathode. The presence of strontium chromate on the infiltrated NiO was observed.

Hydrogen production via high temperature steam electrolysis is a promising technology as it involves less electrical energy consumption compared to conventional low temperature water electrolysis, as consequence of the more favourable thermodynamic and electrochemical kinetic conditions for the reaction. This paper reports on the Solid Oxide Electrolyser Cell (SOEC) performance as function of the operating parameters temperature, humidity and current density. Current–voltage measurements are coupled with impedance spectroscopy, in order to identify the different loss terms in the cell behaviour coming from the electrolyte resistance and the electrode processes. Remarkably high electrical-to-hydrogen energy conversion efficiencies are achieved (e.g., cell voltages of 1.0 and 1.25 V at −1 A cm−2 and 900 and 800 °C, respectively). Results obtained, moreover, show that an important limitation for the electrolysis reaction, at least at moderate absolute humidity values below about 70 vol.% can be the steam diffusion in the hydrogen/steam electrode.

International audience; In this study, anodic cermets nickel/apatite were elaborated and deposited by spin-coating on dense pellets of the apatite electrolyte La9Sr1Si6O26+δ (δ ≈ 0.5). The sintering characteristics and microstructural properties of this cermet were investigated. The influence of the slurries compositions, of the spin-coating conditions and of the sintering temperatures on the deposit properties was studied. The particle sizes of NiO and of apatite material were respectively 300 nm and 1 μm as shown on the SEM micrographs. This difference in particle size resulted in different sintering rates and could explain the lack of adherence of the deposits after reduction of NiO. Agglomeration of the NiO particles occurred above a sintering temperature of 1,200 °C. This result was corroborated by impedance spectroscopy. The highest resistance was measured for the sample sintered at 1,200 °C.

Degradation is one of the most important issues in solid oxide electrolysis cells (SOECs). The lowest long-term degradation reported in the literature for SOEC under practical operation is about 1.7%/1000 h during 3600 h at −1 A/cm2 which is twice the degradation of similar cells under fuel cell operation. To reduce the degradation, a clearer identification of the polarization and degradation mechanisms of SOEC is important in order to adapt the operating conditions, optimize the cell architecture to the purpose of electrolysis by modifying and/or replacing one or both of the electrodes with alternative materials. This paper tends to review the cell degradation phenomena observed under high temperature electrolysis. A description of reactions potentially involved in the degradation mechanisms associated with the thermodynamic and kinetic considerations are used to build a fault tree in order to classify the different defaults and better understand the relationship between the causes and symptoms.

International audience; The objective of this study is to describe the steady-state behaviour of a Solid Oxide Electrolysis Cell (SOEC) system operated at different power loads without an external heat source and producing compressed hydrogen (3 MPa). A zero-dimensional model is proposed to describe the system, which is composed of an SOEC unit and a Balance of Plant. Results found that the system efficiency equals 91% vs. HHV and is slightly impacted by the operating load. However, due to the SOEC sensitivity to thermal gradients, the SOEC unit has to be operated close to the thermoneutral mode, which restricts the SOEC system power load to the range 60-100%. Control strategies, such as additional heating and independently operated SOEC units, should be employed below 60% load to enable the electrolyser operation across a wider load range.

International audience; In order to investigate the transient behaviour of a solid oxide electrolysis cell, tests were carried out on a single SOEC both under steady-state and transient conditions for 600 h. Steady-state operation was performed at the reference current of -20 A, corresponding to the thermoneutral voltage (1.28 V), whereas transient tests consisted in 1800 square waves applied over 140 cumulated hours, with a current of -1 A and -20 A as well as 0 A and -20 A. Independently from the operating conditions, the measured voltage degradation rate of 5% (70 mV)/1000 h was stable for the entire experiment. Consequently, it was concluded that an SOEC can be operated under on-off conditions with no increase of the degradation rate. This result paves the way for modular operation of a high temperature electrolyser fed with intermittent electrical energy.

International audience; Nickelate-based oxides are potential cathodes in Solid Oxide Fuel Cells operating at intermediate temperatures. The chemical compatibility between apatite electrolyte, i.e., La9Sr1Si6O26.5, and Nd-deficient nickelate, i.e., Nd1.95NiO4 + δ, has been characterized. The equilibrium between the nickelate material and the gas phase has been studied as functions of temperature and oxygen partial pressure, using potentiometric measurements with microelectrodes, i.e., metallic or ceramic point electrodes. Two solid electrolytes were used, i.e., apatite and yttria-stabilized zirconia. The response of the nickelate is discussed in terms of oxygen stoichiometry.

As the performances of Solid Oxide Fuel Cells (SOFC) get attractive, long term degradation becomes the main issue for this technology. Therefore it is essential to localise the origin of degradation and to understand its processes in order to find solutions and improve SOFC durability. The electrode microstructure ageing, in particular nickel grain coarsening at the anode side, is known to be a major process to cause performance loss. The increase in nickel particle size will diminish the Triple Phase Boundary (TPB), where fuel oxidation takes place, and decrease the anode electronic conductivity. These two effects degrade the electrochemical performance of the fuel electrode. Degradation is defined as the decrease of potential at constant current density with time in %/1000h or mV/1000h. This study is based on HTceramix® anode supported cells tested in stack conditions from 100 to more than 1000 hours. The anode microstructure has been characterized by Scanning Electron Microscopy (SEM). As the back scattered electron yield coefficients of nickel and yttria stabilized zirconia (YSZ) are very close, the contrast of the different phases (Ni, YSZ and pores) is low. Various techniques are used to enhance the contrast. A new technique is presented here using impregnation and SEM observation based on secondary electron yield coefficients to separate the phases. Image treatment and analysis is done with an in-house Mathematica® code. Image treatment follows four steps: 1. inhomogeneous background correction, 2. double thresholding, 3. cleaning of the binary images and 4. reconstruction of a three-phase image. Image analysis gives information about phase proportion, particle size, particle size distribution, contiguity and finally a new procedure is developed to compute TPB density. A model to describe the coarsening of the nickel particles is also developed. The model assumes an exponential growth of the nickel particles. Using a particle population balance, it estimates the growth of the nickel particles and the concomitant drop in the TPB length. This model is in very good agreement with experimental data, especially for relatively low fuel cell operation times (up to 100-200 hours). This model can be used in the estimation of operational parameters of the anode electrode such as the degradation rate using fundamental parameters of the cermet anode like the anode overpotential and the work of adhesion of the nickel particles on the YSZ substrate. This model gives the portion of stack degradation that corresponds to anode performance decrease due to particle sintering. Finally this study gives the possibility to isolate the degradation coming from the anode sintering and compare to the full SOFC stack degradation.

International audience; The electrochemical properties of dense La9Sr1Si6O26.5 pellets were studied. High conductivity values were measured (8.8 × 10− 3 S cm− 1 at 700 °C) for this pure ionic conductor. Semipermeability measurements allowed measuring an electronic transport number of 5 × 10− 5 at 700 °C under air. These results prove that La9Sr1Si6O26.5 is a pure ionic conductor. Ageing tests of this compound during one month showed a stable behaviour of the conductivity under air and a slight decrease of it under humidified atmosphere.

Hydrogen production through water electrolysis is one of the key processes required in the conversion of electricity to fuels. Carbon free hydrogen produced through water electrolysis can be used as a fuel for hydrogen powered applications (such as fuel cell cars) or grid injection, or as a reactant in downstream processes to produce synthetic fuels such as SNG or methanol. Among available technologies, high temperature electrolysis potentially offers significantly higher electrical to chemical conversion efficiencies compared to alkaline and PEM electrolysers, with values in excess of 100% achievable if additional thermal energy is supplied to the system. High temperature electrolysers are essentially solid oxide fuel cells operated in reverse. Hence, the development of solid oxide electrolysers (SOE) has hugely benefited from the intensive research carried out for the development of Solid Oxide Fuel Cells (SOFCs) in the past decades. A 25 cells stack manufactured by Topsoe Fuel Cells has been tested at EIFER in high temperature electrolysis mode at about 750°C and current densities of 0.57 and 0.72 A/cm2. The applied steam conversion rate was 50%, with a 90 % absolute humidity gas feed. The total duration of the test was almost 9100 hours, split between 8300 current hours and 800 standby hours induced by incidents that occurred during the test. Incidents included losses of the steam and current supplies. After each incident, the stack performance was recovered, indicating a good robustness of the stack towards unexpected and potentially damaging conditions. The main issue during the test has been the considerable increase of the voltage drop between the positive end plate and the bottom stack repeat unit (SRU), before a sharp improvement occurred after about 2000 hours of operation, which has influenced the stack temperature distribution and in turn individual SRU degradation. The maximum observed voltage drop culminated at 2.73 V, before stabilising around 0.6V. The stack has shown an overall voltage degradation of 2% per thousand hours, as can be seen on Figure 1. The corresponding voltage losses were 613 mV and 678 mV per thousand hours, for 0.57 and 0.72 A/cm2 current densities, respectively. This in turn corresponds to an average degradation in the range 0.24 – 0.27 mV per thousand hours per SRU. Moreover, the stack temperature has been adjusted on a few occasion during the test, which highlighted the possibility of using the wide operating temperature range of the solid oxide technology to counter balance the stack degradation and increase the lifetime. Indeed, allowing the stack to increase at a rate of 3°C per thousand hours produced a reduced degradation rate of 1.80 % per thousand hours. Figure 1

High-temperature CO2 and H2O co-electrolysis using solid oxide cells can produce efficiently syngas that in turn can be converted to synfuel through technologically mature technologies like the Fischer-Tropsch (F-T) process. It can be used to convert renewable electrical energy to chemical energy stored in liquid fuels to support a sustainable transport in the future. In the present paper a commercial electrolyte-supported solid oxide cell has been operated under CO2/H2O-electrolysis conditions. It has been found that the product gas composition can be well predicted by assuming that the reverse water-gas shift (RWGS) reaction reaches a thermodynamic equilibrium. In comparison to wet CO2 electrolysis (CO2 with H2O or H2), the cell ASR is only slightly increased under dry CO2 electrolysis conditions, indicating that, besides the RWGS reaction, electrochemical reduction of CO2 could also play an important role for CO formation under CO2/H2O co-electrolysis conditions.

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Although it is not yet technologically mature, the high-temperature steam/CO2 co-electrolysis process offers potentially a feasible and environmentally benign way to convert carbon-free or low-carbon electrical energy into chemical energy stored in syngas with a desired H2 to CO ratio for further processing. An attractive application is to convert the as-produced syngas further into synthetic liquid fuels through the Fischer–Tropsch (F-T) process. The synfuel can be used as alternative fuels in the transportation sector while keeping the existing infrastructure and motor engine technology unchanged. The combination of the high-temperature steam/CO2 co-electrolysis process and the F-T process thus offers an efficient way to store electricity in transportation fuels. The implementation of such a quasi carbon-neutral process depends on its economic competitiveness. In the present paper, an economic assessment of this process is performed through process modelling and sensitivity analysis. As an energy-intensive process, the availability of cost-effective electricity is crucial for its economic competitiveness. Preferred electricity sources are probably nuclear power and surplus wind power, with which synthetic fuels could be produced at a cost comparable to BTL (Biomass to Liquid) process. The present process is biomass-independent, and can also be located in regions where solar energy is abundant.

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Anode-supported solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolytes, both commercial and research, are used at 800-900°C as solid oxide H 2 O electrolyzer cells (SOECs). When the operation is extended to current densities j corresponding to a steam-conversion rate above 100%, or when the steam supply is interrupted under constant current conditions, cell voltages saturate at ~ 1.9 V at 810°C. A cell survives 64 h polarization at j = -0.34 A cm -2 without any steam supply. The mechanism limiting the cell voltage is attributed to electronic conduction in the YSZ electrolyte. No indications are found for electrolyte decomposition. Because the saturation voltage exceeds typical operation voltages by several hundred millivolts, electrolyte conduction in the normal SOEC mode remains predominantly ionic. The rise in the cell voltage to the value determined by electronic conduction occurs when steam transport in the hydrogen/steam electrode becomes limiting. As a consequence of H 2 O back-diffusion from the cell exhaust in the used unsealed test configuration, the rise occurs at nonzero current density under zero steam supply. Impedance spectroscopic data are compared with and without steam supply and are qualitatively interpreted with an equivalent circuit model.

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Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500{degree sign}C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle.

A new SOFC cell based on a silicate electrolyte with apatite structure is investigated for use at intermediate temperatures (IT). A dense pellet of lanthanum silicate (La 9 Sr 1 Si 6 O 26,5 ) exhibits at 700°C a conductivity of 9x10 -3 S/cm -1 that is very stable down to 10 -24 atm of oxygen at any temperature. A new cermet of mixed silicate electrolyte and nickel powders adheres to a dense substrate of lanthanum silicate electrolyte at a sintering temperature of 1100°C. Electrochemical characterization of symmetrical cells (cermet/dense silicate electrolyte/cermet) showed the lowest electrode resistance at 1100°C. However, microstructural characterizations showed a bad interconnect between cermet particles after reduction that could result from incomplete sintering of the silicate electrolyte particles. The association of silicate electrolyte with a cathode of nickelate (Nd 1.95 NiO 4+δ ) was also examined. Electrode resistance on a symmetrical cell (nickelate cathode/dense silicate electrolyte/nickelate cathode) was much less at 700°C than LSM cathode on a traditional YSZ electrolyte. Chemical compatibility of nickelate material with the silicate electrolyte was also checked.