Summary Report on Solid-oxide Electrolysis Cell Testing and Development

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An ongoing project at Idaho National Laboratory involves generating hydrogen from steam using solid oxide electrolysis cells (SOEC). This report describes background information about SOECs, the Integrated Laboratory Scale (ILS) testing of solid-oxide electrolysis stacks, ILS performance degradation, and post-test examination of SOECs by various researchers. The ILS test was a 720- cell, three-module test comprised of 12 stacks of 60 cells each. A peak H2 production rate of 5.7 Nm3/hr was achieved. Initially, the module area-specific resistance ranged from 1.25 Ocm2 to just over 2 Ocm2. Total H2 production rate decreased from 5.7 Nm3/hr to a steady state value of 0.7 Nm3/hr. The decrease was primarily due to cell degradation. Post test examination by Ceramatec showed that the hydrogen electrode appeared to be in good condition. The oxygen evolution electrode does show delamination in operation and an apparent foreign layer deposited at the electrolyte interface. Post test examination by Argonne National Laboratory showed that the O2-electrode delaminated from the electrolyte near the edge. One possible reason for this delamination is excessive pressure buildup with high O2 flow in the over-sintered region. According to post test examination at the Massachusetts Institute of Technology, the electrochemical reactions have been recognized as one of the prevalent causes of their degradation. Specifically, two important degradation mechanisms were examined: (1) transport of Crcontaining species from steel interconnects into the oxygen electrode and LSC bond layers in SOECs, and (2) cation segregation and phase separation in the bond layer. INL conducted a workshop October 27, 2008 to discuss possible causes of degradation in a SOEC stack. Generally, it was agreed that the following are major degradation issues relating to SOECs: • Delamination of the O2-electrode and bond layer on the steam/O2-electrode side • Contaminants (Ni, Cr, Si, etc.) on reaction sites (triple phase boundary) • Loss of electrical/ionic conductivity of electrolyte.

Technology readiness levels (TRLs) of electrolysis systems have dramatically increased in recent years as the interest in clean hydrogen production and decarbonization of transportation, industrial and other sectors increases across the globe. This is especially true of high temperature steam electrolysis (HTSE) / solid oxide electrolysis cell (SOEC) systems which show promise of much higher system efficiencies than other more developed electrolysis technologies. This possibility of higher efficiencies of HTSE / SOEC systems has been previously assumed to be theoretically possible but in recent years it has become less theoretical and more realistic as an increasing amount of suppliers complete lab and pilot tests showing very promising results. Research in the areas of manufacturing techniques, material selection, electrode and electrolyte compositions, and balance of plant size and integration continues at a fast pace as an increasing number of suppliers both internationally and domestically become involved. The advantages of HTSE become more pronounced when HTSE is coupled with nuclear power plants (NPPs). This is because thermal energy produced by the nuclear reactor can be used in a series of heat transfer loops and heat exchangers to vaporize HTSE feedwater, which drastically improves the economics of the process. Idaho National Laboratory (INL) has been very involved in the research and modeling of HTSE systems for a number of years, in collaboration with other national laboratories, academia, and industry stakeholders both on the hydrogen production as well as the hydrogen demand side. The modeling completed over the years on a large variety of projects has led to a wealth of knowledge at INL including in the area of the technoeconomic assessment (TEA) of HTSE systems. TEAs include process modeling of the HTSE systems to calculate system energy requirements and equipment sizing, followed by estimation of capital and operating costs to enable calculation of the levelized cost of hydrogen (LCOH). The TEA work performed has produced incremental improvements and tuning of the methods, assumptions, models, and results of the analyses as well as providing some opportunities for validating these results. The purpose of this document is to record the current baseline HTSE analyses led by INL to show the current status of assumptions and costs of these systems. Given the rapid development of this technology, the variety of suppliers entering the space, and the increasing attention government and industry are giving to such systems, this document may be updated on a periodic basis with updated analysis and assumptions. This document compiles various analyses results and approaches completed over a period of years into a single document to be used as a baseline going forward. It represents what the INL HTSE analysis group assumes to be the internal best estimate of the current operation, costs, and landscape of the HTSE industry state of the art capability for current SOEC technology in an Nth-of-a-Kind (NOAK) plant, which in this study is defined as existence of the manufacturing capacity to support previous deployment of N = 100 count of 25 MWe modular HTSE blocks (with modular equipment component cost reductions specified as following a 95% learning curve). That said, Tthis is a public document and as such so no proprietary data was used or included in this report. There may be HTSE suppliers that have performance specifications, and cost estimates, and test data that differ from the analysisose presented in this document. This document is meant to be a best conservative estimate of the technology and not an absolute reference.

To improve solid oxide electrolysis cell (SOEC) system designs and fabrication quality, endurance testing of SOEC systems provides a proving ground on which real-world usage can be simulated, and quality assurance issues can be identified early on. A test facility at Idaho National Laboratory (INL) was designed, built, and operated to facilitate endurance testing of a reversible system by Oxeon Energy. Designated Emerald, the test facility is currently the only test facility at INL designed to allow both SOEC and solid oxide fuel cell (SOFC) testing of the same module. To safely operate a reversible SOEC test facility, several safety features are required, and a hazard and operability analysis is of Emerald is discussed here. Design challenges due to differences in the endothermic SOEC and exothermic SOFC reactions were addressed in the process recuperators. Controls for the Emerald test facility were built in an open-source JavaScript runtime environment, Node.js and executed on a Linux-based industrial controller. The Oxeon Energy module has been successfully brought up to SOEC ready conditions. Initial operations of the Emerald test facility are being improved to eventually achieve non-stop operations and thousands of hours of testing of both SOEC and SOFC.

The use of High Temperature Electrolysis (HTE) for the efficient production of hydrogen without the greenhouse gas emissions associated with conventional fossil-fuel hydrogen production techniques has been under investigation at the Idaho National Engineering Laboratory (INL) for the last several years. The activities at the INL have included the development, testing and analysis of large numbers of solid oxide electrolysis cells, and the analyses of potential plant designs for large scale production of hydrogen using a high-temperature gas-cooled reactor (HTGR) to provide the process heat and electricity to drive the electrolysis process. The results of this research led to the selection in 2009 of HTE as the preferred concept in the U.S. Department of Energy (DOE) hydrogen technology down-selection process. However, the down-selection process, along with continued technical assessments at the INL, has resulted in a number of proposed modifications and refinements to improve the original INL reference HTE design. These modifications include changes in plant configuration, operating conditions and individual component designs. This report describes the resulting new INL reference design coupled to two alternative HTGR power conversion systems, a Steam Rankine Cycle and a Combined Cycle (a Helium Brayton Cycle with a Steam Rankine Bottoming Cycle). Results of system analyses performed to optimize the design and to determine required plant performance and operating conditions when coupled to the two different power cycles are also presented. A 600 MWt high temperature gas reactor coupled with a Rankine steam power cycle at a thermal efficiency of 44.4% can produce 1.85 kg/s of hydrogen and 14.6 kg/s of oxygen. The same capacity reactor coupled with a combined cycle at a thermal efficiency of 42.5% can produce 1.78 kg/s of hydrogen and 14.0 kg/s of oxygen.

As part of the Next Generation Nuclear Plant (NGNP) project, an evaluation of a low-temperature heat-pump interface design for a nuclear-driven high-temperature electrolysis (HTE) hydrogen production plant was performed using the UniSim process analysis software. The low-temperature interface design is intended to reduce the interface temperature between the reactor power conversion system and the hydrogen production plant by extracting process heat from the low temperature portion of the power cycle rather than from the high-temperature portion of the cycle as is done with the current Idaho National Laboratory (INL) reference design. The intent of this design change is to mitigate the potential for tritium migration from the reactor core to the hydrogen plant, and reduce the potential for high temperature creep in the interface structures. The UniSim model assumed a 600 MWt Very-High Temperature Reactor (VHTR) operating at a primary system pressure of 7.0 MPa and a reactor outlet temperature of 900°C. The low-temperature heat-pump loop is a water/steam loop that operates between 2.6 MPa and 5.0 MPa. The HTE hydrogen production loop operated at 5 MPa, with plant conditions optimized to maximize plant performance (i.e., 800°C electrolysis operating temperature, area specific resistance (ASR) = 0.4 ohm-cm2, and a current density of 0.25 amps/cm2). An air sweep gas system was used to remove oxygen from the anode side of the electrolyzer. Heat was also recovered from the hydrogen and oxygen product streams to maximize hydrogen production efficiencies. The results of the UniSim analysis showed that the low-temperature interface design was an effective heat-pump concept, transferring 31.5 MWt from the low-temperature leg of the gas turbine power cycle to the HTE process boiler, while consuming 16.0 MWe of compressor power. However, when this concept was compared with the current INL reference direct Brayton cycle design and with a modification of the reference design to simulate an indirect Brayton cycle (both with heat extracted from the high-temperature portion of the power cycle), the latter two concepts had higher overall hydrogen production rates and efficiencies compared to the low-temperature heat-pump concept, but at the expense of higher interface temperatures. Therefore, the ultimate decision on the viability of the low-temperature heat-pump concept involves a tradeoff between the benefits of a lower-temperature interface between the power conversion system and the hydrogen production plant, and the reduced hydrogen production efficiency of the low-temperature heat-pump concept compared to concepts using high-temperature process heat.

This report provides documentation of experimental research activities performed at the Idaho National Laboratory and at Ceramatec, Inc. during FY07 under the DOE Nuclear Hydrogen Initiative, High Temperature Electrolysis Program. The activities discussed in this report include tests on single (button) cells, short planar stacks and tubular cells. The objectives of these small-scale tests are to evaluate advanced electrode, electrolyte, and interconnect materials, alternate modes of operation (e.g., coelectrolysis), and alternate cell geometries over a broad range of operating conditions, with the aim of identifying the most promising material et, cell and stack geometry, and operating conditions for the high-temperature electrolysis application. Cell performance is characterized in erms of initial area-specific resistance and long-term stability in the electrolysis mode. Some of the tests were run in the coelectrolysis mode. Research into coelectrolysis was funded by Laboratory Directed Research and Development (LDRD). Coelectrolysis simultaneously converts steam to hydrogen and carbon dioxide to carbon monoxide. This process is complicated by the reverse shift reaction. An equilibrium model was developed to predict outlet compositions of steam, hydrogen, carbon dioxide, and carbon monoxide resulting from coelectrolysis. Predicted ompositions were compared to measurements obtained with a precision micro-channel gas chromatograph.

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

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A three-dimensional computational fluid dynamics (CFD) model has been created to model high-temperature steam electrolysis in a planar solid oxide electrolysis cell (SOEC). The model represents a single cell, as it would exist in an electrolysis stack. Details of the model geometry are specific to a stack that was fabricated by Ceramatec, Inc. and tested at the Idaho National Laboratory. Mass, momentum, energy, and species conservation and transport are provided via the core features of the commercial CFD code FLUENT. A solid-oxide fuel cell (SOFC) model adds the electrochemical reactions and loss mechanisms and computation of the electric field throughout the cell. The FLUENT SOFC user-defined subroutine was modified for this work to allow for operation in the SOEC mode. Model results provide detailed profiles of temperature, Nernst potential, operating potential, anode-side gas composition, cathode-side gas composition, current density and hydrogen production over a range of stack operating conditions. Mean model results are shown to compare favorably with experimental results obtained from an actual ten-cell stack tested at INL.

Electrolyte degradation in anode supported microtubular yttria stabilized zirconia-based solid oxide steam electrolysis cells at high voltages of operation

High temperature electrolysis is a promising option for carbon-free hydrogen production and huge energy storage with high energy conversion efficiencies from renewable and nuclear resources. Over the past few decades, yttria-stabilized zirconia (YSZ) based ion conductor has been widely used as a solid electrolyte in solid oxide electrolysis cells (SOECs). However, its high operation temperature and lower conductivity in the appropriate temperature range for solid electrochemical devices were major drawbacks. Regarding improving ionic-conducting electrolytes, several groups have contributed significantly to developing and applying LaGaO3 based perovskite as a superior ionic conductor. La(Sr)Ga(Mg)O3 (LSGM) electrolyte was successfully validated for intermediate-temperature solid oxide fuel cells (SOFCs) but was rarely conducted on SOECs for its high efficient electrolysis performance. Their lower mechanical strengths or higher reactivity with electrode compared with the YSZ electrolysis cells, which make it difficult to choose compatible materials, remain major challenges. In this field, SOECs have attracted a great attention in the last few years, as they offer significant power and higher efficiencies compared to conventional YSZ based electrolysers. Herein, SOECs using LSGM based electrolyte, their applications, high performance, and their issues will be reviewed.

To support the development of hydrogen production by high temperature electrolysis using solid oxide electrolysis cells (SOECs), the effects of operating conditions on the performance of the SOECs were investigated using a one‐dimensional model of a cathode‐supported planar SOEC stack. Among all the operating parameters, temperature is the most influential factor on the performance of an SOEC, in terms of both cell voltage and operation mode (i.e. endothermic, thermoneutral and exothermic). Current density is another influential factor, in terms of both cell voltage and operation mode. For the conditions used in this study it is recommended that the SOEC be operated at 1,073 K and with an average current density of 10,000 A m–2, as this results in the stack operating at almost constant temperature along the cell length. Both the steam molar fraction at the inlet and the steam utilisation factor have little influence on the cell voltage of the SOEC but their influence on the temperature distribution cannot be neglected. Changes in the operating parameters of the SOEC can result in a transition between endothermic and exothermic operation modes, calling for careful temperature control. The introduction of air into the anode stream appears to be a promising approach to ensure small temperature variations along the cell.

Through the carbon combustion process, the oxygen produced by solid oxide electrolysis cell (SOEC) in the purging gas can be consumed. And the purging gas with low oxygen partial pressure can be recycled into the anode of SOEC, which achieves ultra-low power consumption for hydrogen production. In this study, the I-V curve of steam electrolysis in SOEC with CO2 anode gas is analyzed and discussed. The results show that the cell voltage for hydrogen production can be reduced to 0.62 V at 400 mA/cm2 under CO2 atmosphere. A combined process coupled with carbon fuel combustion and SOEC hydrogen production is proposed. By comparing three different processes of single SOEC, SOEC with heat supply directly by combustion and SOEC coupled with combustion, the advantages of SOEC hydrogen production coupled with carbon fuel combustion are analyzed. The results show that the ultra-low power consumption of SOEC can be achieved by matching the anode circulating gas flow rate, carbon fuel usage and external oxygen amount. When considering the energy efficiency of electricity to hydrogen, carbon fuel combustion can significantly reduce the electrolysis voltage and improve the energy efficiency of the system, but it will lead to an increase in the amount of anode circulating gas.

In order to address the issue of the anthropogenic greenhouse effect, the supply with renewable energy will play a major role in the future energy systems. A particularly promising approach to store or even use renewable energy in sectors where electricity cannot be used effectively yet (for example in aviation) are synthetic fuels. Within the synthetic fuel generation process, known as Power-to-X, syngas (H2+CO) can be synthesized into liquid hydrocarbons, such as gasoline or jet fuel, via the Fischer-Tropsch process. In comparison with alternative processes, such as a steam electrolyzer in combination with a reverse water gas shift reactor, solid oxide electrolysis cells (SOEC) offer great potential for producing the required syngas in a single, less complex, and more efficient step within Power-to-X utilizing only steam (H2O), carbon dioxide (CO2), and electricity from renewable sources.This so-called co-electrolysis, which involves the simultaneous reduction of H2O and CO2 in SOECs, is of particular importance. It enables direct and efficient syngas production with a tailored H2/CO ratio, thereby reducing the need for additional downstream processing steps. This integrated approach enhances overall system efficiency, minimizes energy losses, and supports carbon utilization efforts, making it a crucial technology for sustainable fuel and chemical production. However, different syngas applications and associated SOE operating conditions can cause performance deterioration of SOECs. Consequently, a comprehensive understanding of losses incurred by diverse inlet mixtures, in conjunction with other operational conditions, such as temperature or the total flow rate, during electrolysis is essential.To the best of the authors’ knowledge, there is currently a lack of literature on operating cells with CO at the inlet, especially in CO2-electrolysis, where H2 is often added to avoid oxidation of Nickel in the fuel electrode. In order to investigate the influence of different gas components (H2O, H2, CO2, CO) of inlet compositions on performance and losses of SOECs, a series of experiments was performed on a commercial fuel electrode-supported cell with an active area of 81 cm2. To maintain a total flow rate of 2.2 slpm nitrogen was added, and the amount of H2O, H2, CO2, CO was varied, depending on the desired mixture. It is noteworthy that the air flow rate applied to the air electrode remained constant at 2.2 slpm throughout the experimental series. Initially, H2O electrolysis was conducted as a reference, followed by various mixtures for co-electrolysis and CO2 electrolysis, each with distinct H2O/CO2-, H2/CO-, and product/reactant-ratios. In the final experiment, pure CO2 was utilized as the feedstock for CO2-electrolysis, aiming to determine the losses in electrolysis in the absence of product species (CO).During the experiments polarization curves and electrochemical impedance spectroscopy (EIS) in 50 mA/cm2 steps were used for characterizing the performance of the cell at current densities up to 350 mA/cm2. The monitoring of the temperature distribution over the active area was realized through thermocouples integrated in the cell-housing near the gas flow channels on both the anode and cathode side. This approach enables further analysis of the reaction mechanism over the cell area, correlation of impedance data with thermal effects, and detection of hotspots. The outlet gas composition was monitored via a gas analyzer to determine the dry composition.This study investigates the impact of gas components at the inlet in co- and CO2-electrolysis on the performance of fuel electrode-supported SOECs, emphasizing losses in specific operating conditions. Furthermore, it offers a distinctive perspective on CO2 electrolysis with pure CO2 in an industrially sized cell, emphasizing substantial losses with no products at the inlet. The results show that the resistance in co-electrolysis is close to H2O-electolysis and way lower than in CO2-electrolysis which indicates that the main electrochemical reaction is the H2O-electolysis and CO2 is mostly converted to CO via the reverse water gas shift reaction. Figure 1