High Temperature Electrolysis at EIFER, Main Achievements at Cell and Stack Level

AbstractThe 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 9000hours, 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 45cm2 active area under a high current density of 1 A/cm2 indicates an extrapolated cell lifetime of at least 20 000h. 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

In Situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Titan ETEM

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°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

D.3.1 Test Matrix Document

The present document defines the test matrix, i.e. a list of test modules relevant for different applications. According to the project objectives the applications are SOFC (stationary and mobile), SOEC (H2- production) and combined SOFC/SOEC (electricity storage via H2). These test modules can be combined to form test programs in order to realize application-oriented testing. This test matrix has been created based on a brief review of results from the precedent project - FCTESQA dealing with cell/stack/system testing procedures for three types of fuel cells (PEMFC, SOFC and MCFC) and the on-going project STACKTEST dealing with testing procedures for PEMFC stacks. Industrial stake holders who are developing SOFC/SOEC products have been contacted to gather information regarding the required operation modes during the lifecycle of the product for each application. Feedbacks from industrial stake holders have also been integrated

In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

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°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

Quantitative study of anode microstructure related to SOFC stack degradation

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 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

Caractérisation électrochimique de matériaux céramiques à microstructure contrôlée pour Piles à Combustible SOFC fonctionnant à température réduite

This study deals with the elaboration and the characterisation of the three components of a SOFC cell working at intermediate temperature (650-750°C). A pure ionic conductivity of 9x10-3 S.cm-1 was found at 700°C for the retained electrolyte of composition La9Sr1Si6O26.5. This material with an apatite structure is chemically compatible with the mixed conducting cathode material Nd1.95NiO4+Δ. The study of the oxygen reduction process showed that the reaction is limited by the oxide ions transfer at the interface cathode/electrolyte. The anodic cermet Ni-apatite was elaborated by impregnation of the porous matrix with a nickel salt. This method allowed a better stabilisation of the nickel phase in the host structure. Thus this work provides evidence for the interest of a new ITSOFC based on a silicate electrolyte.Ce travail est consacré à l'élaboration et à la caractérisation des trois éléments d'une cellule SOFC fonctionnant à température intermédiaire (650-750°C). Une conductivité purement ionique de 9x10-3 S.cm-1 a été mesurée à 700°C sur l'électrolyte retenu de composition La9Sr1Si6O26,5. Cette céramique de structure apatite est stable chimiquement au contact du matériau de cathode conducteur mixte Nd1,95NiO4+Δ. L'étude de la réduction de l'oxygène a montré que l'étape limitante est le transfert des ions oxyde à l'interface cathode/électrolyte. Le cermet anodique Ni-apatite a été élaboré par imprégnation de la matrice poreuse d'apatite par un sel de nickel. Cette méthode permet une meilleure stabilisation du nickel dans la structure hôte. Ces travaux mettent en évidence l'intérêt d'une nouvelle cellule SOFC fonctionnant à température intermédiaire composée d'électrolyte de structure apatite

Modelled Behaviour of a High Temperature Electrolyser System Coupled with a Solar Farm

International audienceA system model was developed in Simulink® in order to describe the dynamic behaviour of HTE (High Temperature Electrolysis) systems fed with variable power. This model is composed of three coupled submodels focusing on the electrolyser, on the Balance of Plant and on the control strategies, respectively. The implemented control strategies ensure optimised operation in terms of both electrolysis material restrictions, such as thermal gradients, and system efficiency. The presented results were simulated for a standalone system composed of a four-unit electrolyser fed with electrical power from a virtual 1.35 MW solar farm in Marignane (France) and producing compressed hydrogen (3 MPa). A thermal insulation of 30 cm around each unit limits thermal losses, so the temperature drops by only 10 K overnight. Over the simulated year, the solar farm produced 2.76 GWh which were converted into 64.5 t of compressed hydrogen. The results show that HTE intermittent operation is technically feasible without an external heat source. Moreover, the system efficiency remains as high as 92 % based on the hydrogen Higher Heating Value (HHV) if suitable control strategies are employed

Model-based behaviour of a high temperature electrolyser system operated at various loads

International audienceThe 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

Benefits of external heat sources for high temperature electrolyser systems

International audienceHigh 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