Search

This work aims at developing an innovative renewable energy storage solution, based on reversible Solid Oxide Cell (rSOC) technology. That is to say, one system optimized to operate either in electrolysis mode (SOEC) to store excess electricity to produce H2, or in fuel cell mode (SOFC) when energy needs exceed local production, to produce electricity and heat again from H2 or any other fuel locally available. Firstly, work focused on optimization of the different layers constituting the single SOC cell to reach high initial performance applying state-of-the-art materials as previously reported [1]. Secondly, the initially highest performing cells were selected for long-term reversible SOFC/SOEC single cell tests. Thirdly, these cells were integrated in a stack design optimized for reversible operation at high degrees of H2 and H2O utilization. The long-term single cell tests showed significant degradation in galvanostatic test periods during electrolysis but not in fuel cell mode prior to starting the reversible test operation while the degradation diminished during the subsequent rSOC operation of the cells operating at 700 C, +0.6 and -1.2 A/cm2 in SOFC and SOEC modes respectively, at fuel utilization (FU) up to 80% in both modes. Electrochemical impedance spectroscopy analyses and post-mortem SEM investigations of tested single cells reveal that the fuel electrodes degraded significantly during the long-term single cell tests. Furthermore, long-term stack tests were conducted on 5-cell stacks, integrating both reference cells and optimised cells. The long-term stack tests were conducted applying different switches between SOFC and SOEC modes. Initially long duration tests (100h each mode) were performed in a mixture of 50% H2O and 50% H2 to see the effect of the polarisation only. The alternating cycle SOEC/SOFC was repeated over a 1800 h testing period. Then stack switched daily from SOEC mode (8h in 90% H2O and 10% H2 @ 0.84 A/cm2 FU 54%) to SOFC (16 h in 100% H2 @ 0.26A/cm2@ FU 60%) for full cycles over a testing period of 500 h. Stack tests were operated in reversible mode for up to several thousands of hours and results on the stack performance and lifetime will be presented.

A set of long-term tests (t ≥ 1000 h) have been carried out in fuel cell and electrolysis modes on typical Ni-YSZ//YSZ//LSCF-CGO cells. The degradation rates were found to be higher in electrolysis than in fuel cell operation. Post-test analyses have revealed that Sr diffusion and formation of SrZrO3 at YSZ/CGO interface occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode. As a consequence, LSCF destabilization is found to be not involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. An in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF demixing mechanism. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in LSCF material (while the fuel cell condition results in an increase in the concentration of oxygen vacancies). It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation is detailed.

Ceramic high-temperature fuel cells and electrolysers are efficient energy conversion systems for electrical power generation and hydrogen production. Their core components are constituted by a stack of electroactive Solid Oxide Cells (SOCs) in which the electrochemical reactions take place. Thanks to their flexibility, the same stack can be alternatively operated in both fuel cell and electrolysis modes. However, the insufficient durability of SOCs still constitutes a major limitation for the technology. The present study addresses this issue and aims to bring some new insights on the effect of the Solid Oxide Fuel Cell (SOFC) versus Solid Oxide Electrolysis Cell (SOEC) operating modes on degradation of a typical Ni-YSZ//YSZ//CGO//LSCF-CGO cell. The electrochemical degradations are generally attributed to several underlying phenomena such as electrode microstructural evolution, material chemical decomposition or electroactive sites poisoning by contaminants. Among them, it is generally considered that Ni agglomeration in the Ni-YSZ cermet and Lanthanum Strontium Cobalt Ferrite (LSCF) material destabilization are two prevalent mechanisms involved in the cell performance deterioration. Therefore, these two mechanisms have been specifically investigated by a coupled approach of long-term testing in both SOFC and SOEC modes (1000 ≤ t (h) ≤9000) and post-test characterizations. The experimental results have then been analyzed in the frame of an in-house multi-scale model with the purpose to interpret them and to quantify the effect of material ageing on cell performances. The extent of Ni agglomeration has been characterized by three-dimensional electrode reconstructions obtained by X-ray nano-holotomography at European Synchrotron Radiation Facility (ESRF) (on the new Nano-Imaging beamline ID16A-NI). The new set-up and protocol enable the reconstructions of valuable 3D volumes (Fig. 1) with a large field of view (~50 µm) along with a high spatial resolution (~50 nm). The electrode morphological properties, which have been measured on the 3D volumes, have revealed a substantial Ni coarsening over time at 850°C and 750°C, whereas no Ni depletion was detected at the electrolyte interface. The increase of the Ni particle size is found to induce a decrease in both (i) the density of Triple Phase Boundary (TPBs) lines and (ii) the interfacial surface area between Ni and gas. Moreover, it was found that the Ni/YSZ interfacial surface area does not evolve during the experiments. This statement indicates that the ceramic backbone in the cermet prevents a massive Ni agglomeration at the SOFC/SOEC operating temperature. The compilation of all experimental data have allowed fitting the parameters of a physically-based law for Ni coarsening that was introduced in the modelling framework. The simulations have revealed that Ni agglomeration explains around 20-25% of the electrochemical degradation at 850°C after 1000 hrs of operation. However, the electrode microstructural evolution is found not to be affected by the cell polarizations. Therefore, the mechanism cannot explain the higher degradation rates recorded in electrolysis mode compared to the fuel cell ones. To explain the impact of the operating modes (SOFC or SOEC) on the degradation rates, several post-test analyses (i.e. Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray µfluorescence and µdiffraction techniques) have been employed to investigate the phase reactivity in the region of the CGO barrier layer. The characterizations have revealed that Sr diffusion across the barrier layer and formation of SrZrO3 secondary phase occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode (Fig. 2). As a consequence, LSCF destabilization is found not to be involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. The post-test analyses have also revealed a diffusion and an accumulation of Co in the region of the barrier layer which is concomitant with the formation of SrZrO3. The formation of these Co-rich segregates in contact with SrZrO3 grains have been identified as cobalt-ferrite type compound. The in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF destabilization mechanism. The cell polarization curves and the local quantities within the O2 electrode have been computed in both fuel cell and electrolysis modes. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in the LSCF material. It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation has been detailed. Figure 1

Elcogen is one of the leading European manufacturers of solid oxide cell (SOC) technology. Elcogen products are unique combination of best in class performance and low-cost product structure. The unit cell and stack products are designed for low temperature operation enabling major cost reductions in the system level. Elcogen's operations are located in the Nordic-Baltic technology cluster of Finland and Estonia. This article summarizes the recent activities and development trends of Elcogen.

Solid oxide cells are among the best candidates for viable energy conversion and storage solutions. Their capability to operate reversibly as electrolyser (‘SOEC mode’) or as fuel cell (‘SOFC mode’), provides an opportunity to balance out electricity needs. Yet, reversible operation requires an optimized cell and stack design to withstand alternating conditions. In this study, more than ten different cells have been produced and their initial performance evaluated. For selected cells, durability tests were conducted and showed that primarily the oxygen electrode contributes to performance losses in SOFC mode whereas the fuel electrode is mainly affected under SOEC mode conditions. The stack design was optimized in order to withstand the envisaged operating conditions. An optimization of internal pressure drop (reduced by a factor of 2) and sealing resistances towards higher pressure has been achieved while maintaining the electrochemical performance.

In the present work, the behavior of hydrazine borane N 2 H 4 BH 3 in the presence of alkali/alkaline-earth hydrides is investigated. ( i ) Hydrazine borane N 2 H 4 BH 3 is readily destabilized by an alkali hydride MH (M=Li, Na, K). The electronic properties of M drive the reactivity of MH 1 towards N 2 H 4 BH 3 . KH is the most reactive (at 25 °C, Δ r H = −70.25 kJ mol −1 ) while K is the least electronegative and the biggest element. Hydrazinidoboranes MN 2 H 3 BH 3 form. ( ii ) Hydrazine borane N 2 H 4 BH 3 is destabilized by MH x ( x = 2, 3; M=Mg, Ca, Al). In comparison to pristine N 2 H 4 BH 3 , better dehydrogenation properties are found: MgH 2 has a catalytic effect; CaH 2 strongly destabilizes N 2 H 4 BH 3 ; and, unstable AlH 3 is able to destabilize N 2 H 4 BH 3 under heating. Though the synthesis of hydrazinidoboranes M(N 2 H 3 BH 3 ) x is difficult, the mixtures MH x –N 2 H 4 BH 3 leads to composites. The most efficient composite is CaH 2 –N 2 H 4 BH 3 . The aforementioned hydrazinidoboranes and composites may have potential as solid-state hydrogen storage materials. This is discussed herein.

The title compound, N2H4(BH3)2, is prepared from a mixture of freshly ground [N2H5][HSO4] and NaBH4 in THF (Ar-filled glovebox, round-bottom flask, 40 °C, 42—90 h, 87% yield).

International audience; Hydrazine bisborane N2H4(BH3)2 (HBB; 16.8 wt %) recently re-emerged as a potential hydrogen storage material. However, such potential is controversial: HBB was seen as a hazardous compound up to 2010, but now it would be suitable for hydrogen storage. In this context, we focused on fundamentals of HBB because they are missing in the literature and should help to shed light on its effective potential while taking into consideration any risk. Experimental/computational methods were used to get a complete characterization data sheet, including, e.g., XRD, NMR, FTIR, Raman, TGA, and DSC. From the reported results and discussion, it is concluded that HBB has potential in the field of chemical hydrogen storage given that both thermolytic and hydrolytic dehydrogenations were analyzed. In solid-state chemical hydrogen storage, it cannot be used in the pristine state (risk of explosion during dehydrogenation) but can be used for the synthesis of derivatives with improved dehydrogenation properties. In liquid-state chemical hydrogen storage, it can be studied for room-temperature dehydrogenation, but this requires the development of an active and selective metal-based catalyst. HBB is a thus a candidate for chemical hydrogen storage.

As energy storage becomes increasingly more important in contemporary and future energy solutions, the ability of a solid oxide cell (SOC) to work reversibly as an electrolyzer/fuel cell gives this technology great potential for use in large energy storage projects1,2. In theory, it is possible to use the state-of-the-art solid oxide cells in electrolysis operation to produce fuel (i.e. split water into oxygen and hydrogen) or generate electricity and heat using the so-called fuel cell regime. In addition, it is possible to produce syngas (H2 + CO)1,2 by co-electrolyzing water and CO2. While the performance and stability of the solid oxide fuel cell (SOFC) are in focus of most research regarding this technology, the technological possibilities of reversible solid oxide cells (RSOC) are far less studied. Although both technologies mostly share their device composition and main architecture, using the SOC in electrolysis mode creates new degradation mechanisms and technological problems3,4. In this work the authors focus on the electrolysis performance of the RSOC. The authors analyzed the influence of active layer thickness on SOC performance in electrolysis mode by using a commercial solid oxide cell (Ni-3YSZ|Ni-8YSZ|8YSZ|GDC|LSC). The study has been performed to establish the optimal active layer thickness and therefore to maximize the SOC performance in electrolysis mode. Electrochemical characterization of SOC button cells has been conducted using electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic methods at several operating conditions including different temperatures and different water vapor contents in the gas stream. The gas compositions have been exchanged to simulate working conditions at different locations in the RSOC stack during electrolysis work. The results of the electrochemical studies have been analyzed using equivalent circuit fitting method of Nyquist plots. Complex analysis of different porous architectures and chemical compositions at different operating conditions gave more information on how the structure of the SOC determines the electrochemical performance of the water electrolysis process. References: Giulio Vialetto, Marco Noro, Paolo Colbertaldo, Masoud Rokni, International Journal of Hydrogen Energy, 44, 9608-9620 (2019) M. B. Mogensen, M. Chen, H.L. Frandsen, C. Graves, J.B. Hansen, K.V. Hansen, A. Hauch, T. Jacobsen, S.H. Jensen, T.L. Skafte and X. Sun, Clean Energy, 3, 175–201 (2019) P. Mocoteguy and A. Brisse, International Journal of Hydrogen Energy, 38 (36), 15887-15902 (2013) Y. Tao, S. D. Ebbesen, M. B. Mogensen, Journal of Power Sources, 328, 452-162 (2016)

This work aims at developing an innovative renewable energy storage solution, based on reversible Solid Oxide Cell (rSOC) technology. That is to say, one system optimized to operate either in electrolysis mode (SOEC) for steam electrolysis to store excess electricity and to produce H2, or in fuel cell mode (SOFC) when energy needs exceed local production, to produce electricity and heat again from H2. In this work rSOC tests were conducted at single cell, in order respectively to finely understand performance and degradation mechanisms and to perform tests in representative conditions; and single cell rSOC test are finally compared to stack test results. Single cell rSOC tests were conducted to investigate the effect of current density, inlet steam content and degree of steam utilization both upon galvanostatic SOFC and SOEC testing, and upon load cycling operation. Cells having high initial electrochemical performance from Elcogen were used for these studies (1). Via four different long-term tests, we have investigated degradation at 700°C during constant galvanostatic SOFC (250 h) and SOEC (250 h) conditions followed by load cycling operation (up to 1000 h) in cycles of 16 h SOFC and 8 h SOEC. The applied current densities were either severe (0.6 A/cm2 in SOFC and -1.2 A/cm2 in SOEC) or moderate (0.4 A/cm2 and -0.6 A/cm2); applying different inlet gas composition ie. p(H2O)/p(H2) of 90/10 in SOEC and dry H2 in SOFC, or applying p(H2O)/p(H2) of 50/50 for both modes. Furthermore, two different steam and H2 utilization degrees of 80% and 40% were used. Figure 1 depicts examples of recorded cell voltage curves over time for two examples of the long-term rSOC testing where the inlet gas compositions were varied between p(H2O)/p(H2) of 50/50 for one test and p(H2O)/p(H2) of 90/10 and dry H2 for the other test. The electrochemical durability was assessed recording voltage over time and via electrochemical impedance spectroscopy (EIS). The characterization of the cells was supplemented by post-mortem scanning electron microscopy studies. Key findings include: 1) The cells degraded to a significantly larger degree in SOEC mode than in SOFC mode during the initial galvanostatic testing sequences. 2) At the given conditions and applying a current density of -1.2 A/cm2, the cells operate at a cell voltage above thermoneutral potential during the constant SOEC operation while this is not the case for the low-current density tests applying an electrolysis current of -0.6 A/cm2. 3) Load cycling does not increase the rate of degradation; rather the cell voltage stabilized for parts of the load cycling test sequences. 4) The major contribution to the resistance increase during long-term operation can be attributed to the fuel electrode and for the test, having the highest fuel electrode resistance increase (high current density test) a ~60% increase of the ohmic resistance was also observed during SOEC testing. 5) For the harshest tested (high current density) cells, Ni migration have been observed in parts of the fuel electrodes. 6) Nano-scaled detachment between Ni and YSZ particles were observed in several locations in several cells together with non-percolating Ni particles ie. where Ni-Ni connection has been lost; and a general trend was that the higher the fuel electrode overpotential during testing, the more pronounced was the effect of Ni-network changes upon microscopy post-mortem analysis in line with previous studies (2). 7) Inclusions of silica-containing impurities were found in the Ni/YSZ fuel electrodes as also observed in previous work (3, 4). Based on the single cell tests the recommendation for stack and system operation will, at 700 °C be, to operate in the current density regime of 0.4 A/cm2 and -0.6 A/cm2 rather than 0.6 A/cm2 and -1.2 A/cm2 to ensure long-term durability. Single cell test results were supplemented and trends confirmed by long-term rSOC test on 5-cell stacks at similar conditions. Figure 1: Cell voltage curves over time for two long-term rSOC test at 700°C. Inlet gas compositions of p(H2O)/p(H2):50/50 (blue curve) for one test and p(H2O)/p(H2):90/10 and dry H2 for the other test (red curve). Current densities of 0.4 A/cm2 and -0.6 A/cm2, respectively and a H2O (SOEC) and H2 (SOFC), utilization of 40%. References A. Ploner, A. Hauch, S. Pylypko, S. D. Iorio, G. Cubizolles, J. Mougin, in ECS Transactions (2019), vol. 91, pp. 2517–2526. A. Hauch, K. Brodersen, M. Chen, M. B. Mogensen, Solid State Ionics. 293, 27–36 (2016). A. Hauch, S. H. Jensen, J. B. Bilde-Sørensen, M. Mogensen, J. Electrochem. Soc. 154 (2007), doi:10.1149/1.2733861. M. Riedel, M. P. Heddrich, K. A. Friedrich, Fuel Cells. 20, 592–607 (2020). Figure 1