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Hydrogen produced from H2O electrolysis works as an energy carrier and helps to overcome the challenges of intermittent renewable energy sources. At present, no comprehensive environmental impact assessment is available for three commercially H2O electrolysis technologies, namely solid oxide electrolysis cell (SOEC), polymer electrolyte membrane electrolysis cell (PEMEC), and alkaline electrolysis cell (AEC). The study aimed to provide potential environmental impacts of the electrolysis technologies based on life cycle assessment. Among the investigated 16 impact categories, the stage of critical material use of three H2O electrolysis stacks was identified as the hotspot of environmental impacts. The critical materials were stainless steel and nickel from SOEC, platinum and iridium from PEMEC, and nickel from AEC. Life cycle impact results from PEMEC stack were much higher than these from SOEC and AEC stacks, while electricity played a more important role in the life cycle impact of hydrogen production. The sensitivity analysis indicated that the most effective approach to reducing potential impacts would be to reduce critical materials use on the current status of electrolysis technologies.

Chromium poisoning of the air electrode is a primary degradation mechanism for solid oxide cells (SOCs) operating under fuel cell mode. Recent experimental findings show that reversed pulse operation for SOCs operated as electrolyser cells can reverse this degradation and extend the lifetime. Here, we use a multiphysics model of an SOC to investigate the effects of reversed pulse operation for alleviating chromium poisoning of the air electrode. We study the effects of time fraction of the operation under fuel cell and electrolysis modes, cyclic operation starting after a certain duration, and fuel cell and electrolysis current densities on the cell lifetime, total power, and hydrogen production. Our modeling shows that reversed pulse operation enhances cell lifetime and total power for all different cases considered in this study. Moreover, results suggest that the cell lifetime, total power, and hydrogen production can be increased by reversed pulse operation at longer operation times under electrolysis mode, cyclic operation starting from the beginning, and lower electrolysis current densities. All in all, this paper documents and establishes a computational framework that can serve as a platform to assess and quantify the increased profitability of SOCs operating under a co-production operation through reversed pulse operation.

The ongoing green energy transition is increasing the need for dynamic and efficient Power-to-X (PtX) systems to convert surplus wind and solar power to high-value products. The solid oxide electrolysis cell (SOEC) technology offers the highest energy conversion efficiency. However, high degradation and thermal variations that cause thermomechanical stress hinders up-scaling of the SOEC technology. Here we present a novel operation method that alleviates temperature variations and minimize degradation caused by impurities and nickel migration. By rapidly switching between electrolysis mode and brief periods in fuel cell mode, a flat thermal profile is obtained. Our results thus establish a new, simple way to achieve increased SOEC stack and module size and extended lifetime. The new operation method enables dynamic operation of large SOEC modules for renewable energy powered PtX systems which could drastically decrease costs associated with production of high-value green fuels and chemicals from wind and solar power.

Severe degradation of Ni-YSZ (yttria stabilized zirconia) electrodes of solid oxide cells (SOCs) due to Ni migration is well known, but the literature contains apparent contradictions. The mechanisms are still under debate. Fine structured Ni-YSZ composite electrodes often degrade at operation temperature (700–950°C), because Ni particles lose electrical contact with each other as larger Ni-particles grow on the expense of smaller ones. Another type of Ni migration, which may be very damaging, is the relocation of Ni in the most active part of the Ni-YSZ cermet electrode next to the dense YSZ electrolyte. Emphasis is put on the migration of Ni away from the YSZ electrolyte in solid oxide electrolysis cells (SOECs). This is seen as an important obstacle to the commercialization of SOC systems. Apart from temperature, degradation of Ni-YSZ electrodes in SOCs is related to Ni-YSZ electrode overpotential and the local redox potential of the gas mixture inside the porous Ni-YSZ electrode. A unifying Ni migration mechanism is proposed, and methods of alleviating the electrode degradation are discussed. The hypothesis is that Ni migrates via surface diffusion of Ni(OH)x species below ca. 800°C and via Ni(OH)x species in gas phase above ca. 900°C.

Degradation of nickel-stabilized zirconia (Ni-SZ) electrodes is predominantly due to four features 1) high mobility of Ni, 2) fragile nature of SZ ceramics, 3) narrow three phase boundary (3PB), and 4) effects of impurities: i) in blocking (poisoning of 3PB), and ii) on mobility of Ni. Impurities may be contaminants in reactant gases and influenced by the reactants H2O, CO2 and CO, or impurities in cell and stack materials. Examples of important degradation types and hypotheses of the degradation mechanisms are described. Examples are: a) loss of electrochemical contact between Ni and YSZ (yttria stabilized zirconia) particles and loss of contact between Ni-Ni particles followed by Ni-migration away from the YSZ electrolyte - one reason is hypothesized being a result of huge potential and thermal gradients at the 3PB; b) growth of Ni-particles; c) redoxing; d) blocking of 3PB and reaction sites by impurities like Si and S. Mitigation methods are discussed.

Solid Oxide Cells (SOC), as a key energy-conversion technology, require sintering at temperatures exceeding 1200 °C, which tend to coarsen the structure of the fuel electrode. Nanostructured electrodes with high surface areas can help to decrease the electrode resistance and facilitate the operation of the SOC stacks at low temperatures with current collectors made from cheaper steel types. In this paper, we demonstrate and carefully evaluate a novel low-temperature manufacturing method for nanostructured Strontium Iron Molybdenum Oxide (SFM)/Gadolinium Doped Ceria (CGO) composite electrodes. The composite electrodes are applied on both sides of a Zr-based electrolyte with CGO barrier layers and sintered at 800 °C for 3 h in a 5% H2/N2 atmosphere. The preparation parameters, thermal behavior, and electrode microstructure are studied to improve electrochemical performance. Based on the fitting of Nyquist plots, the electrochemical performance is mainly limited by two reactions in series at low frequency, in the 0.08–1 Hz and 1–50 Hz ranges. The electrode polarization resistance is almost constant at 1.24 Ω cm2 for 110 h at 750 °C in 60 vol% CO/CO2.

This invention relates to a system comprising one or more electrolysis cell(s) and at least one power electronic unit that supplies the cell(s) with a fluctuating voltage, and to a method for operating one or more electrolysis cell(s), comprising providing one or more voltage fluctuations to the electrolysis cell(s) by at least one power electronic unit, enabling the provision of a low-cost electrolysis system which simultaneously allows for fast-response dynamic operation, improved electrolysis efficiency, increased lifetime and high impurity tolerance.

The ongoing reduction of greenhouse gas emissions entails increased use of intermittent renewable energy technologies such as wind and solar. This raises the need for cost-effective and efficient electricity storage. In particular seasonal variations in supply and demand will require tremendous storage capacity. In this paper we present a truly large-scale electricity storage system which uses pressurized reversible solid oxide cells combined with catalytic reactors to store electricity as synthetic natural gas. By storing the produced gas in existing natural gas grids the system can create a strong and efficient link between the electricity and gas markets. In addition, the system is able to operate reversibly using gas from the grid to satisfy the electric power demand.The system performance is analyzed with a component-based thermodynamic modeling tool which shows that electricity can be stored as synthetic natural gas with an energy efficiency of 89%. The gas to electricity efficiency is equally high, resulting in a round-trip storage efficiency of 80% (DC-to-DC).

A critical issue in state-of-the-art solid oxide cell stacks is the contacting of the oxygen electrode. The commonly used ceramic contact layers are applied in a green state and cannot be sintered properly, due to compliance limitations arising from other stack components like sealing glasses and steels. The consequence is a low layer and interface strength. A metallic copper manganese foam, which is oxidized under operation conditions into a conductive Cu1+xMn2–xO4 spinel, is presented in this work as a viable contact solution. The foam has been electrochemically tested in a single repeating unit setup for 350 hours of constant operation, followed by dynamic conditions with thermal cycles. After operation, a micro structural analysis using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction was carried out. It was confirmed that after oxidation/operation the manganese solely formed a CuMn-spinel phase, mixed with a CuO phase. A separate Mn-oxide phase was not found. The conductivity and contacting of the foam was sufficient for > 350 h of SOFC operation. With an initial serial resistance comparable to single cell tests using gold foil as contact material and moderate degradation rates, the CuMn foam presented itself as an interesting cathode contact solution.

Synthetic fuels produced from carbonaceous sources and renewable electricity can play an important role in phasing out fossil fuels. By integrating electricity in fuel production, an indirect electrification of long-distance sea, air and road transport becomes feasible. This paper presents the modeling analysis of a flexible system for the conversion of biomass and electricity to methanol. The system integrates an efficient TwoStage biomass gasifier and solid oxide cells (SOC). The SOC can operate in both electrolysis mode and fuel cell mode. In this way, the system can store electricity in the form of methanol in electrolysis mode or have co-production of electricity in fuel cell mode. Additional operational modes are also presented, making it feasible to operate the system no matter the electricity price. The heat for the endothermic gasification reactions is provided either through electric heating or partial oxidation directly in the char fluid bed gasifier. Five operating modes have been modeled and analyzed, showing a promising input–output efficiency ranging between 71% with maximum electricity consumption, and 37% when producing only electricity (LHVdry). The presented system outperforms most of the more conventional state-of-the-art systems using biomass gasification as route to produce methanol. Most importantly, its flexibility enables continuous operation no matter the electricity price.

In this report, we present our work to characterize the electrochemical performance of solid-oxide fuel cell stacks under elevated-pressure operation. The effort is part of a five-year U.S. DoE ARPA-E INTEGRATE program to combine the high efficiency of solid-oxide fuel cells with the low-cost of reciprocating engines to demonstrate an 70%-efficient hybrid electric generator. Within this hybrid system, solid-oxide fuel cells (SOFCs) convert natural gas into electricity. Unconverted fuel leaving the SOFC stacks feeds a downstream reciprocating engine that drives an electric generator. In this architecture, the stacks are hydraulically placed upstream of the engine, necessitating stack operation at elevated pressures of 2-5 barg. SOFC stack performance at elevated pressure has witnessed limited study. The team at the Colorado School of Mines is characterizing SOFC stack performance at elevated pressure through design and commissioning of the pressurized stack electrochemical test stand shown in the figure. The facility is used to explore the performance of 5-kW SOFC stack test modules (Ceres Power, Ltd., Horsham, UK) at the unusual operating conditions presented by the hybrid system, including pressurized operation at high anode tail-gas recycle. A simulant of steam-reformed natural gas with anode tail-gas recycle is synthesized using mass flow controllers. A fuel processor will be incorporated into the stand at a later stage of the research program. Reactants flow through a preheat furnace and into a pressure vessel to reach the insulated solid-oxide fuel cell stack test module. The pressure within this vessel is matched to the operating pressure of the SOFC stack, minimizing pressure differentials between the internal flow channels of the stack and the surrounding environment. Following electrochemical conversion, exhaust gases exit the stack and the pressure vessel, and are cooled through a series of heat exchangers. Back-pressure regulators near the end of the flow stream regulate stack and vessel operating pressure. The stand has been used to explore the impacts of elevated pressure operation on the electrochemical performance of Ceres SteelCell stacks. Additional parameters being explored include fuel composition, anode recycle, and internal fuel reforming. In this talk, we will present our findings on the effects of these variables on stack performance, and their impact in achieving the aggressive performance targets of the SOFC-engine hybrid electric generator. A principal concern during performance testing at elevated pressures is minimizing pressure differentials across the anode and cathode gas-flow streams. Such differentials can bring substantial mechanical stress to the metal-supported membrane-electrode assemblies. Pressure differentials in this work are below +/- 25 mbar at 5 barg operating pressure, well below manufacturer’s specifications. The stacks show steady, stable performance over all conditions tested. In addition to exploring the electrochemical-performance impacts brought by pressurization and anode recycle, experiments explore the roles of methane internal reforming and cathode air flow on stack cooling. The endothermicity of methane steam reforming can bring meaningful reductions in cooling-air flow rate, raising system efficiency. The results of these studies and the trajectory of the research will be presented. Figure 1

Solid oxide electrolysis cells (SOECs) can efficiently convert electrical energy (e.g. surplus wind power) to energy stored in fuels such as hydrogen or other synthetic fuels. Performance and durability of the SOEC has increased orders of magnitudes within the last decade. This paper presents a short review of the R&D work on SOEC single cells conducted at DTU Energy from 2005 to 2015. The SOEC improvements have involved increasing the of the oxygen electrode performance, elimination of impurities in the feed streams, optimization of processing routes, and fuel electrode structure optimization. All together, these improvements have led to a decrease in long-term degradation rate from ~40 %/kh to ~0.4 %/kh for steam electrolysis at -1 A/cm2, while the initial area specific resistance has been decreased from 0.44 Wcm2 to 0.15 Wcm2 at -0.5 A/cm2 and 750 °C.

Summary LiFePO4-based cathodes suffer from various degradation mechanisms, which influences the battery performance. In this paper, morphological and structural degradation phenomena in laboratory cathodes made of LiFePO4 mixed with carbon black (CB) in a 1 mol/L LiPF6 in EC : DMC (1:1 by weight) electrolyte are investigated by transmission electron microscopy at various preparation, assembling, storage, and cycling stages. High-resolution transmission electron microscopy imaging shows that continuous SEI layers are formed on the LiFePO4 particles and that both storage and cycling affect the formation. Additionally, loss of CB crystallinity, CB aggregation, and agglomeration is observed. Charge–discharge curves and impedance spectra measured during cycling confirm that these degradation mechanisms reduce the cathode conductivity and capacity. Copyright © 2016 John Wiley & Sons, Ltd.

Denmark has the ambition to reduce CO2 emission by 70% by 2030 and become free of fossil energy by 2050. Today approximately 50% of Denmark’s electricity supply is produced by wind power with a fast pace of construction of further wind power capacity, and electricity production from solar photovoltaics is likewise increasing. Large parts of the world and in particular of Western Europe have similar ambitions for implementing large amounts of wind and solar power. However, until now only relatively minor efforts in conversion and storage technology development and implementation have taken place. During the recent year politicians have finally realized that electrochemical technologies in the form of batteries, electrolysers and fuel cells will be absolutely necessary in order to secure 100% renewable energy and an electricity supply that matches the customers’ needs. In spite of an increasing agreement on the necessity of electrochemistry in conversion and storage of renewable electricity, there is still an ongoing debate about what the main role and share of various electrochemical devices in the conversion and storage should be This presentation is an overview of the main types of electrochemical devices for conversion and storage, and it will discuss the most rational application of the various options based on simple technical criteria such as energy density, power density, and flexibility. For instance Li-batteries can convert electricity to chemical energy in the battery, but this chemical energy can only be converted back into electricity, whereas e.g. reversible solid oxide cells (RSOCs) can use electricity to convert steam and carbon dioxide into syngas, which - apart from possible conversion back into electricity - may be converted chemically into methane, methanol or other useful hydrocarbon or oxygenates. Nowadays such fuels are often called e-fuels, which is short for “electrochemically synthesized fuel”. Short terms of the processes are “power to fuel” (P2F), or “power to X” (P2X), where X stands for any chemical compound, which has been synthesized – partly or fully - by electrochemical processes. Cost-effectiveness will naturally be most decisive for how the different technologies will be applied and to which extent. This will be discussed qualitatively based on aspects such as the abundancy of the raw materials in the crust of the earth and the properties of the chemical compounds used for the energy storage. Another aspect is the demand of properties to the storage media depending on the application. Storage of energy in a zinc-bromine flow battery will probably become much cheaper per kWh than storage in a lithium battery, but as the gravimetric power density of the lithium battery is many times higher than that of the flow battery, the lithium battery will be preferred in a car while the flow battery may be preferred for stationary energy storage. The size of the energy system may also be determining for the choice of type of electrochemical technology. It is expected that in most renewable energy systems, seasonal storage will be necessary, i.e. storage of large amounts of energy through several months. For such a case it will be feasible to store the energy in form of hydrocarbons or ammonia using already existing infrastructure. If this further can be done using suitable reversible electrochemistry with formation of methane inside an RSOC in electrolysis mode then the round trip efficiency may become very high. Such a system is sketched in the Figure, which is taken from G. Butera, S.H. Jensen, L.R. Clausen, Energy, 166 (2019) 738. It illustrates an idea of using the natural gas grid for storage of CH4, and storage of CO2 in a salt cavern. The oxidant would come from the air, and the electrical source should be renewable energy and the recipient the power grid – all coupled together with a huge RSOC system. Other examples, like using solid oxide electrolysis cells and electricity for manufacturing of ammonia without any use of fossil fuel, and for direct conversion of CO2 into CO, will also be presented. Figure 1

Solar and wind power are now the lowest cost electricity sources, making an urgent need for low-cost long-duration energy storage technologies to turn these intermittent renewables into on-demand power. Long-duration energy storage is defined as an energy to power ratio of more than 10 h. For some applications, hundreds of hours storage capacity will be needed to balance supply and demand. Available storage systems are inadequate: conventional batteries are too expensive, water electrolysis with fuel-cell re-oxidation is too inefficient, and pumped hydro and compressed air are geographically constrained. Reversible solid oxide electrochemical cells operated like flow batteries have the potential to meet the needs of long-duration storage by using ultra-low-cost storage media. This presentation will include: (1) the advantages of such solid oxide batteries using different storage media than the classical H2/O2/H2O chemistry, (2) experimental demonstration and system design, (3) analysis of the storage requirements for reversible electricity storage applications, and (4) techno-economic analysis of novel solid oxide battery systems compared with conventional batteries and flow batteries. Figure 1

Solar and wind power are now the lowest cost electricity sources, making an urgent need for low-cost long-duration energy storage technologies to turn these intermittent renewables into on-demand power. Long-duration energy storage is defined as an energy to power ratio of more than 10 h. For some applications, hundreds of hours storage capacity will be needed to balance supply and demand. Available storage systems are inadequate: conventional batteries are too expensive, water electrolysis with fuel-cell re-oxidation is too inefficient, and pumped hydro and compressed air are geographically constrained. Reversible solid oxide electrochemical cells operated like flow batteries have the potential to meet the needs of long-duration storage by using ultra-low-cost storage media. This presentation will include: (1) the advantages of such solid oxide batteries using different storage media than the classical H2/O2/H2O chemistry, (2) experimental demonstration and system design, (3) analysis of the storage requirements for reversible electricity storage applications, and (4) techno-economic analysis of novel solid oxide battery systems compared with conventional batteries and flow batteries. Figure 1

Denmark has the ambition to reduce CO2 emission by 70% by 2030 and become free of fossil energy by 2050. Today approximately 50% of Denmark’s electricity supply is produced by wind power with a fast pace of construction of further wind power capacity, and electricity production from solar photovoltaics is likewise increasing. Large parts of the world and in particular of Western Europe have similar ambitions for implementing large amounts of wind and solar power. However, until now only relatively minor efforts in conversion and storage technology development and implementation have taken place. During the recent year politicians have finally realized that electrochemical technologies in the form of batteries, electrolysers and fuel cells will be absolutely necessary in order to secure 100% renewable energy and an electricity supply that matches the customers’ needs. In spite of an increasing agreement on the necessity of electrochemistry in conversion and storage of renewable electricity, there is still an ongoing debate about what the main role and share of various electrochemical devices in the conversion and storage should be This presentation is an overview of the main types of electrochemical devices for conversion and storage, and it will discuss the most rational application of the various options based on simple technical criteria such as energy density, power density, and flexibility. For instance Li-batteries can convert electricity to chemical energy in the battery, but this chemical energy can only be converted back into electricity, whereas e.g. reversible solid oxide cells (RSOCs) can use electricity to convert steam and carbon dioxide into syngas, which - apart from possible conversion back into electricity - may be converted chemically into methane, methanol or other useful hydrocarbon or oxygenates. Nowadays such fuels are often called e-fuels, which is short for “electrochemically synthesized fuel”. Short terms of the processes are “power to fuel” (P2F), or “power to X” (P2X), where X stands for any chemical compound, which has been synthesized – partly or fully - by electrochemical processes. Cost-effectiveness will naturally be most decisive for how the different technologies will be applied and to which extent. This will be discussed qualitatively based on aspects such as the abundancy of the raw materials in the crust of the earth and the properties of the chemical compounds used for the energy storage. Another aspect is the demand of properties to the storage media depending on the application. Storage of energy in a zinc-bromine flow battery will probably become much cheaper per kWh than storage in a lithium battery, but as the gravimetric power density of the lithium battery is many times higher than that of the flow battery, the lithium battery will be preferred in a car while the flow battery may be preferred for stationary energy storage. The size of the energy system may also be determining for the choice of type of electrochemical technology. It is expected that in most renewable energy systems, seasonal storage will be necessary, i.e. storage of large amounts of energy through several months. For such a case it will be feasible to store the energy in form of hydrocarbons or ammonia using already existing infrastructure. If this further can be done using suitable reversible electrochemistry with formation of methane inside an RSOC in electrolysis mode then the round trip efficiency may become very high. Such a system is sketched in the Figure, which is taken from G. Butera, S.H. Jensen, L.R. Clausen, Energy, 166 (2019) 738. It illustrates an idea of using the natural gas grid for storage of CH4, and storage of CO2 in a salt cavern. The oxidant would come from the air, and the electrical source should be renewable energy and the recipient the power grid – all coupled together with a huge RSOC system. Other examples, like using solid oxide electrolysis cells and electricity for manufacturing of ammonia without any use of fossil fuel, and for direct conversion of CO2 into CO, will also be presented. Figure 1

Solid oxide cells (SOC) are reversible electrochemical cells that can be operated as solid oxide fuel cells (SOFC) or as solid oxide electrolysis cells (SOEC). SOFC convert fuels such as H2 and natural gas into electric power and heat; while SOEC can be used e.g. for storing surplus renewable electrical energy via electrolysis of H2O and/or CO2 to produce fuels like H2, CO or synthesis gas (CO+H2). Synthesis gas can subsequently be catalyzed into a variety of synthetic fuels. In addition to the fact that SOC are made of abundant materials (no precious metals, expensive IrO2 or the like), the SOEC are also highlighted as superior when it comes to flexibility in comparison to well-known electrolysis technologies such as alkaline and PEM. The flexibility of the SOCs lies in: 1) reversible operation mode (rSOC) 2) high efficiency in both SOFC and SOEC mode, 3) operating at high current density with lower internal resistance than other electrolysis technologies and 4) high fuel flexibility e.g. the ability to operate the cell in CO2 electrolysis or co-electrolysis mode or with direct reforming of ammonia in SOFC mode. However; operating an SOC as rSOC is not necessarily simple and for the operation to match fluctuating supply from renewable energy sources, harsh rSOC operating conditions may be required [1,2]. Operating the SOEC at high current density is possible, but can lead to severe and irreversible degradation caused by migration of Ni in the innermost part of the fuel electrode as previously reported [3,4] and illustrated in Figure 1. Lastly, even though SOC offer large fuel flexibility, special attention should be paid to the effect of impurities in the gasses. For CO2 electrolysis operation significant degradation has been observed to be caused by traces (down to ppb level) of impurities. The effect becomes increasingly critical at operation conditions close to the carbon deposition threshold [5]. This talk will therefore touch upon the three issues: 1) Load cycling operation of SOC, 2) Ni/YSZ electrode degradation caused by Ni migration and 3) the puzzling interplay between impurities in the fuel and carbon deposition during CO2 electrolysis. In the context of load cycling operation of SOC results from a European project, REFLEX, will be presented. This project aims to develop an innovative renewable energy storage solution based on rSOC (“Smart Energy Hub”) for decentralized storage of electrical energy and to produce electrical energy and heat locally when needed. Tests in this project have included both single cells and stacks. Moreover; test operation schemes have been designed to investigate harsh load cycling operation. In relation to Ni migration; this talk will discuss and seek to answer questions such as: what is the critical parameter for onset of Ni migration away from the electrolyte/electrode interface and what is the driving mechanism for the transport? Can we use phase-field modelling to describe not only Ni coarsening but also Ni migration? Will Ni migration only occur for electrolysis when steam is present? Or will it also take place for CO2 electrolysis? And can we find means to minimize or fully hinder Ni migration? With respect to sensitivity towards impurities in the fuel and the interplay with carbon deposition, this talk will exemplify the effect of impurities – even in the ppb range - and seek to provide solutions for handling of gas stream impurities. Furthermore, this topic provides an excellent opportunity to showcase complementary characterization techniques, such as impedance spectroscopy, scanning electron microscopy (SEM) and Raman spectroscopy for investigation of the degrading Ni/YSZ fuel electrode. References [1] X. Sun, B.R. Sudireddy, X. Tong, M. Chen, K. Brodersen, A. Hauch, ECS Trans. . 91 (2019) 2631–2639. [2] A. Ploner, A. Hauch, S. Pylypko, S. Di Iorio, G. Cubizolles, J. Mougin, ECS Trans. 91 (2019) 2517–2526. [3] M.B. Mogensen, A. Hauch, X. Sun, M. Chen, Y. Tao, S.D. Ebbesen, K. V. Hansen, P. V. Hendriksen, Fuel Cells, 17(4), (2017), 434-441. [4] A. Hauch, K. Brodersen, M. Chen, M.B. Mogensen, Solid State Ionics. 293 (2016). [5] Hauch, A.; Skafte, T.L.; Küngas, R.; Traulsen, M.L.; Jensen, S.H. in: A. Weber (Ed.), 13th Eur. SOFC SOE Forum 2018, Lucerne, Switzerland, 2018: p. B1501. Figure 1: Low-voltage in-lens SEM image of a cross section of a fuel inlet piece of a long-term (½ year) tested SOEC. Bright appearing particles are Ni particles constituting a percolating network. Figure 1

Solid Oxide Electrolysis Cells (SOEC) can be used for efficient power-to-X (P2X) conversion. Here X symbolizes the myriad of end products that can be made using green power, H2O and CO2. The SOEC converts H2O and CO2 into gas (most often H2 and/or CO) which can be catalyzed into a broad range of fine- and speciality-chemicals, as well as bulk transportation fuels. Dynamic operation of the SOECs can help balance the electricity consumption with the electricity production from intermittent energy sources, such as wind and solar. P2X can therefore assist the on-going transition to a non-fossil energy infrastructure by substituting fossil-based chemicals and fuels with sustainable alternatives, while simultaneously providing energy balancing services. It is important to realize that the requirement for electricity balancing ranges in time from milliseconds to months. Batteries offer quick response times and are increasingly being used to provide fast ancillary services to the electrical grid. Unfortunately battery-based electricity storage costs hundreds of $ per kWh of capacity, which renders them impractical for large-scale storage exceeding time-scales of about one hour. This is where dynamic electrolysis operation with part load enters the scene. During operation of SOECs at part load, i.e. below the thermo-neutral point of operation, the endothermic reaction heat will dominate the internal heat balance and generate thermal gradients in the SOEC stack. The gas exiting the stack will therefore be colder than the gas entering the stack, such that an external (electric) heater is required to pre-heat the gas before the gas can enter the stack. The thermal gradients cause temperature variations inside the stack that increases with increasing stack size. It is an advantage to avoid thermal gradients, as these cause thermo-mechanical stresses and variations in the cells’ resistance across the stack. Thermo-mechanical stresses may lead to cracks in the brittle ceramic cells and delamination at interfaces. Varying cell resistance causes variations in current density and electrode overpotential across the stack, which increases the risk of accelerated degradation. Thus, thermal gradients hinder an increase of the footprint (i.e. the size) of SOEC stacks. Consequently, upscaling to MW-systems faces considerable complexity related to wiring, piping and heat management of hundreds of smaller kW-sized stacks. This limits cost reductions related to economy-of-scale, in particular if the SOEC MW-systems are to be used for dynamic operation and electricity balancing. In this work, we demonstrate theoretically and experimentally how pseudo thermo-neutral operation can be achieved at part load. The uniform temperature profile is established by overlaying the DC voltage in electrolysis mode with a rapid alternating voltage, such that the Joule heat balances the endothermic reaction heat. The alternating voltage causes the stack to be operated a few milliseconds in SOFC (or SOEC) mode before it returns to SOEC (or SOFC) mode. The frequency of the alternating voltage is around 20 – 100 Hz. This means the stack temperature and outlet gas composition is not affected by the alternating voltage. During SOFC mode, excessive heat is generated to balance the endothermic SOEC operation. As the SOFC operation produces more heat than the SOEC operation consumes per time interval, a net-production of fuel is realized. By controlling the time and cell voltage in respectively SOEC and SOFC mode the overall load can be continuously changed from 0% (corresponding to OCV) to 100% (corresponding to DC operation at thermo-neutral voltage) while keeping the gas outlet temperature equal to the gas inlet temperature. The operation method therefore enables very fast load variations without having to change the stack temperature or gas compositions. This enables dynamic operation of MW SOEC stacks to enter the market for fast electricity balancing, where batteries are currently applied. Importantly, the operation method seems to decrease the stack degradation rate, which we speculate is caused by desorption of impurities that otherwise adsorb on the surface of the electrodes. The potential for the novel operation method to combine chemicals and fuel production with electricity balancing at time scales spanning from milliseconds to months, can hopefully help commercialize and upscale the SOEC technology towards MW-systems.

Low energy conversion efficiency and high storage costs still hamper a successful implementation of sustainable energy systems. Recent theoretical studies show that reversible electrochemical conversion of H2O and CO2 to CH4 inside pressurized solid oxide cells combined with subsurface storage of the produced gases can facilitate seasonal electricity storage with a round-trip efficiency reaching 80% and a storage cost below 3 ¢/kWh. Here we present test results with a 30-cell SOFCMAN 301 stack operated with carbonaceous gases at 18.7 bar and 700 °C in both electrolysis and fuel cell mode. In electrolysis mode the CH4 content in the stack outlet gas increased from 0.22% at open circuit voltage to 18% at a current density of −0.17 A cm−2. The degradation observed by scanning electron microscopy studies correlate well with the observed electrochemical stack degradation. The degradation rates in both fuel cell and electrolysis mode were comparable to previously reported SOFCMAN stack degradation rates measured at ambient pressure operation with H2/H2O gas mixtures.

Co-electrolysis of CO2 and H2O in pressurized solid oxide electrolysis cells (SOEC) results in internal methanation when the fuel electrode contains nickel, as nickel catalyzes the methanation reaction. Recent SOEC stack experiments operated at 19 bar and 700 °C produced a gas with a methane content of 18 vol% (dry). The exothermic methanation reaction is a perfect match for the endothermic electrolysis reactions, enabling an overall slightly exothermic stack operation at a moderate polarization voltage. When using a pressurized SOEC for biomass syngas upgrading to synthetic natural gas, it is possible to achieve very high energy efficiency because a high share of the exothermic methane formation can occur inside the SOEC. The production of waste heat from the downstream methanation reactor is therefore reduced significantly. In this paper, such an integrated system design is proposed and evaluated by thermodynamic modelling and analysis. The analysis shows that the proposed system can reach 84% energy efficiency from wood pellets and electricity to synthetic natural gas. This is substantially higher than the ~70% efficiency than can be achieved with steam electrolysis based systems. If steam drying isintegrated to allow the use of wet wood chips, the efficiency drops to 82%.

This work describes manufacture, analysis and test of a new well conducting corrosion-protection coating that can be applied on steel types with high chromium content. Electrolytic deposition of yttrium salts is used to form thin (2 with 90% H2O. The corrosion rate at 750 °C of the coated Crofer 22 APU sheets is compared with that of uncoated samples and of samples with a commercial magneton sputtered CGO coating revealing that the coatings reduce the parabolic rate constant characteristic of the corrosion by a factor of 10 and 20 for the CGO and the Y, respectively.The coated meshes also exhibit high corrosion resistance and long-term-stable low electrical resistance at 650 °C, both in 90% H2O in H2, and in air. In addition to the experimental work, Y–Cr–O2 phase diagrams are constructed illustrating the high stability of the YCrO3 phase (that forms during corrosion) over a wide temperature and pO2 range.

In this work we investigate the electrode degradation mechanisms in a commercial 2.5 Ah LiFePO4/graphite 26650 cylindrical cell. Aged and fresh electrode samples were prepared by cycling two cells respectively five and 22 k times. Subsequently the cells were disassembled in a glovebox and the electrode samples were prepared for electrochemical testing in a 3-electrode setup, and for characterization with XRD, XPS and low-kV FIB/SEM tomography. A 1 μm thick CEI (cathode electrolyte interface) layer was observed at the electrode/electrolyte interface of the aged LiFePO4 electrode. Relative to the fresh LiFePO4 electrode, the aged electrode exhibited a larger series resistance which indicates the observed degradation layer increases the ionic resistance. In addition, micron-sized agglomerates, probably a mixture of carbonaceous material and decomposition products from the electrolyte, were observed at the electrode/electrolyte interface of the aged graphite electrode. These layers may contribute significantly to the loss of lithium inventory (LLI) in the cell, and to the loss of active material (LAM) in the graphite electrode. Low-voltage FIB/SEM tomography was used to detect local charging effects of graphite particles in the carbon electrode, an effect of poor dissipation of the electric charge to the ground after the sample interaction with the electron beam. The charging effects were primarily observed in the aged electrode and most of the locally charged particles were found to be close to the electrode/electrolyte interface, indicating a poorly percolating graphite network near this interface.

As global and local energy production becomes more dependent on intermittent renewable sources like wind and solar, efficient and reversible conversion of electricity to storable fuels becomes increasingly important. Solid oxide cells (SOCs) are interesting in this context since they can be operated either as electrolyzers (SOEC) to convert electricity to fuels such as hydrogen or methane, and as fuel cells (SOFC) to convert fuels to electricity. Both productivity and conversion efficiency can be improved if the SOC operation pressure can be increased from ambient pressure to 10–30 bar. In this paper we characterize an SOC stack operated at pressures from ambient pressure to 10 bar. The pressure dependency of stack temperature, cell area specific resistance (ASR), current-voltage (iV) curves, stack impedance spectra and pressure drop across the stack and heat exchangers is analyzed in this paper. Additionally, the expected impact on the hydrogen production efficiency and cost is discussed.

This work presents an investigation of the degradation mechanisms that occur in LiFePO4/C battery electrodes during charge/discharge cycling. Impedance spectra were measured on a fresh electrode and an electrode aged by cycling. The spectra were modeled with an equivalent circuit which indicates that both the ionic and electronic pathways in the electrode were negatively affected by the cycling. Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) tomography of both electrodes shows that cycling causes agglomerations of Carbon black (CB). In addition to this, Low-voltage FIB/SEM revealed non-conductive CB in the aged electrode.

GdxCe1-xO2-δ (GDC) nanoparticles were synthesized using continuous hydrothermal flow synthesis. By varying the synthesis conditions, particle size and morphology could be tailored. Here, particle sizes between 6 to 40 nm with polyhedral or octahedral shape could be obtained. Gd0.2Ce0.8O2-δ nanoparticles were further processed into inks for inkjet printing. Despite the small particle size/large surface area, inks with excellent printing behavior were formulated. For proof-of-concept, thin GDC layers were printed on a) green NiO-GDC substrates, and on b) pre-sintered NiO-YSZ substrates. While no dense layers could be obtained on the green NiO-GDC substrates, GDC nanoparticles printed on NiO-YSZ substrates formed a dense continuous layer after firing at 1300 °C.

Electricity storage is needed on an unprecedented scale to sustain the ongoing transition of electricity generation from fossil fuels to intermittent renewable energy sources like wind and solar power.

The integration of anaerobic digestion and thermal gasification can ensure a high biomass utilization, as the unconverted biomass from digesters can be converted by thermal gasification. When integrating steam electrolysis or co-electrolysis, all the CO and CO2 in the biogas and gasification gas can be upgraded to synthetic natural gas (SNG), achieving a very high bio-SNG yield per biomass input. In this paper, a highly integrated system combining anaerobic digestion, thermal gasification, and pressurized solid oxide cells for bio-SNG production from manure is presented and analyzed by thermodynamic modeling. The system is compared to a similar system without anaerobic digestion. The analysis finds that the energy yield of bio-SNG can reach 138% in relation to the manure input (LHV-dry), while the yield drops to 107% without anaerobic digestion. The total energy efficiency from manure and electricity to bio-SNG can reach 79% with anaerobic digestion and 64% without it. By combining thermal gasification and anaerobic digestion, it is thereby almost possible to reach the same efficiency as a thermal gasification system operating on wood (up to 84%).

Inkjet printing of 8% Y2O3-stabilized ZrO2 (YSZ) thin films is achieved by designing a novel water-based reactive ink for Drop-on-Demand (DoD) inkjet printing. The ink formulation is based on a novel chemical strategy that consists of a combination of metal oxide precursors (zirconium alkoxide and yttrium salt), water and a nucleophilic agent, i.e. n-methyldiethanolamine (MDEA). This chemistry leads to metal-organic complexes with long term ink stability and high precision printability. Ink rheology and chemical reactivity are analyzed and controlled in terms of metal-organic interactions in the solutions. Thin dense nanocrystalline YSZ film below 150 nm are obtained by low temperature calcination treatments (400-500 {\deg}C), making the deposition suitable for a large variety of substrates, including silicon, glass and metals. Thin films and printed patterns achieve full densification with no lateral shrinkage and high ionic conductivity., Comment: 30 pages,6 figures, 2 tables, open access

Recent theoretical studies show that reversible electrochemical conversion of H2O and CO2 to CH4 inside pressurized solid oxide cells (SOCs) combined with subsurface storage of the produced gases can facilitate seasonal electricity storage with a round-trip efficiency reaching 70-80% and a storage cost below 3 ¢/kWh. Here we show test results with a 30-cell SOFCMAN 301 stack operated with carbonaceous gases at 18.7 bar and 700 ˚C in both electrolysis and fuel cell mode. The CH4 content in the stack outlet gas increased from 0.22% at open circuit voltage (OCV) to 18% at -0.17 A cm-2 in electrolysis mode. The degradation rates in both fuel cell and electrolysis mode were comparable to previously reported SOFCMAN stack degradation rates measured at ambient pressure operation with H2/H2O gas mixtures.

In this work an Equivalent Circuit Model (ECM) is developed and used to model impedance spectra measured on a commercial 26650 LiFePO4/Graphite cylindrical cell. The ECM is based on measurements and modeling of impedance spectra recorded separately on cathode (LiFePO4) and anode (Graphite) samples, harvested from the commercial cell. Modeling of the single-electrode impedance spectra provided information about the electronic and ionic resistance in the porous composite electrodes, as well as the solid state diffusion. Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) of anode and cathode samples was used to make 3-D maps of the electrode microstructures and to obtain microstructural data for the ECM. The complementary analysis was crucial for the resolution of the single electrode impedance parameters and the proposal and validation of a new equivalent circuit used to model the full commercial battery impedance.

), both the Ni-YSZ and the LSM-YSZ electrode degraded. The initial degradation(0 - 200 h) is mostly due to increasing of polarization resistance, while the long term degradation is mainly caused by increasing ofserial resistance.© 2013 The Electrochemical Society. [DOI: 10.1149/2.106309jes] All rights reserved.Manuscript submitted May 8, 2013; revised manuscript received July 8, 2013. Published July 16, 2013. This was Paper 1553presented at the Boston, Massachusetts, Meeting of the Society, October 9–14, 2011.

Solid oxide cell (SOC) performance is limited by various processes. One way to investigate these processes is by electrochemical impedance spectroscopy. In order to quantify and characterize the processes, an equivalent circuit can be used to model the SOC impedance spectra (IS). Unfortunately, the optimal equivalent circuit is often unknown and to complicate matters further, several processes contribute to the SOC impedance - making detailed process characterization difficult. In this work we analyze and model a series of IS measured during steam electrolysis operation of an SOC. During testing, degradation is only observed in the Ni/YSZ electrode and not in the electrolyte or the LSM/YSZ electrode. A batch fit of the differences between the IS shows that a modified Gerischer element provides a better fit to the Ni/YSZ electrode impedance than the frequently used RQ element - albeit neither equivalent circuit provides a perfect fit. However, modeling with the Gerischer element indicates that the Ni/YSZ electrode performance decrease, relates to an electrochemical reaction resistance increase at the electrode triple phase boundaries.

A promising way to store wind and solar electricity is by electrolysis of H2O and CO2 using solid oxide electrolysis cells (SOECs) to produce synthetic hydrocarbon fuels that can be used in existing fuel infrastructure. Pressurized operation decreases the cell internal resistance and enables improved system efficiency, potentially lowering the fuel production cost significantly. In this paper, we present a thermodynamic analysis of synthetic methane and dimethyl ether (DME) production using pressurized SOECs, in order to determine feasible operating conditions for producing the desired hydrocarbon fuel and avoiding damage to the cells. The main parameters of cell operating temperature, pressure, inlet gas composition and reactant utilization are varied to examine how they influence cell thermoneutral and reversible potentials, in situ formation of methane and carbon at the Ni–YSZ electrode, and outlet gas composition. For methane production, low temperature and high pressure operation could improve the system efficiency, but might lead to a higher capital cost. For DME production, high pressure SOEC operation necessitates higher operating temperature in order to avoid carbon formation at higher reactant utilization. Optimal operating conditions are dependent on the total system design.

Wind and solar power is troubled by large fluctuations in delivery due to changing weather. The surplus electricity can be used in a Solid Oxide Electrolyzer Cell (SOEC) to split CO2 + H2O into CO + H2 (+O2). The synthesis gas (CO + H2) can subsequently be catalyzed into various types of synthetic fuels using a suitable catalyst. As the catalyst operates at elevated pressure the fuel production system can be simplified by operating the SOEC at elevated pressure. Here we present the results of a cell test with pressures ranging from 0.4 bar to 10 bar. The cell was tested both as an SOEC and as a Solid Oxide Fuel Cell (SOFC). In agreement with previous reports, the SOFC performance increases with pressure. The SOEC performance, at 750 °C, was found to be weakly affected by the pressure range in this study, however the internal resistance decreased significantly with increasing pressure.

In a future power systems with additional wind power capacity there will be an increased need for large scale power management as well as reliable balancing and reserve capabilities. Different technologies for large scale electricity storage provide solutions to the different challenges arising with high wind power penetration. This paper presents a review of the electricity storage technologies relevant for large power systems. The paper also presents an estimation of the economic feasibility of electricity storage using the west Danish power market area as a case.

In Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolyser Cells (SOECs) the change of reactant gasses to product gasses is one of the factors that limit the performance of the cells. Therefore it is important to have a detailed understanding of the impedance associated with this change in the gas concentrations and how it relates to various operating conditions. In this paper we describe a SOEC tested as a steam electrolyser in a plug-flow setup. The measured gas concentration impedance increases during the test, even though the current, the inlet gas concentrations and the flow rates are kept constant. From the impedance arcs we conclude that the increase must be due to a build up of an uneven resistance distribution perpendicular to (and not parallel to) the gas flow direction in the Ni-electrode.

In this work we study the structural degradation of a laboratory Li-ion battery LiFePO4/Carbon Black (LFP/CB) cathode by various electron microscopy techniques including low kV Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) 3D tomography. Several changes are observed in FIB/SEM images of fresh and degraded cathodes, including cracks in the LFP particles, secondary disconnected particles, and agglomeration of CB. Low voltage (1 kV) SEM images show that the CB agglomerates have a different brightness than the fresh CB, due to charging effects. This suggests that the electronic conductivity of the CB agglomerates is low compared to that of the fresh CB particles. HRTEM analysis shows that fresh CB particles are quasi crystalline, whereas the LFP/CB interface in the degraded electrode shows amorphous carbon surrounding the LFP particles. The presence of the amorphous carbon is known to impede the electronic conductivity and thereby decreasing percolation in the cathode and reducing the electrode capacity.

Solid oxide cells (SOCs) can be operated either as fuel cells (SOFC) to convert fuels to electricity or as electrolyzers (SOEC) to convert electricity to fuels such as hydrogen or methane. Pressurized operation of SOCs provide several benefits on both cell and system level. If successfully matured, pressurized SOEC based electrolyzers can become more efficient both energy- and cost-wise than PEM and Alkaline systems. Pressurization of SOFCs can significantly increase the cell power density and reduce the size of auxiliary components. In the present study, a SOC stack was successfully operated at pressures up to 25 bar. The pressure dependency of the measured current-voltage (I–V) curves and impedance spectra on the SOC stack are analyzed and the relation between various system parameters and pressure is derived. With increasing pressure the open circuit voltage (OCV) and the reaction kinetics (electrode performance) increases for thermodynamic and kinetic reasons, respectively. Further, the summit frequency of the gas concentration impedance arc and the pressure difference across the stack and heat exchangers is seen to decrease with increasing pressure following a power-law expression. Finally a durability test was conducted at 10 bar.

Wind and solar power are troubled by large fluctuations in delivery due to changing weather. The surplus electricity can be used in a Solid Oxide Electrolyser Cell (SOEC) to split CO 2 + H 2 O into CO + H 2 ( + O 2 ) which can be catalyzed into various types of synthetic fuel using a suitable catalyst. H 2 O electrolysis with a new SOEC resulted in a record breaking current density of - 3.6 A / cm 2 at a cell voltage of 1.48 V. Assuming the surplus electricity to cost 3.6 US$/GJ, the H 2 production price is estimated to 5 US$/GJ equivalent to 30 US$/barrel crude oil or to 0.6 US$/gge (gallon of gasoline equivalent).

In a solid oxide fuel cell the cell performance is largely determined by the cathode properties, in particular at low operating temperature (

Lithium-ion batteries are a promising technology for automotive application, but limited performance and lifetime is still a big issue. The aim of this work is to study and address degradation processes which affect LiFePO4 (LFP) cathodes - one of the most common cathodes in commercial Li-ion batteries. In order to evaluate how the LFP cathode is affected by C-rate a LFP working electrode, Lithium metal foil counter electrode and Lithium metal reference electrode was tested in a 3-electrode setup with a standard 1M LiPF6 in 1:1 EC/DMC electrolyte and glass fiber separator. The working electrode/counter electrode was subjected to several charge/discharge cycles between 3.0 V and 4.0 V at different discharge rates. Figure 1 shows the voltage profile of the LFP electrode (solid line) and full battery (dotted line) during charge/discharge process. It is seen that the higher the C-rate, the higher is the polarization furnished by the counter electrode which reduces the capacity. In Figure 2, the discharge capacity [mAh/g] is plotted vs the number of charge/discharge cycles. Series of 10 cycles at a given C-rate was applied to the battery. Each series was followed by a C/10 cycle (green points). A linear fit has been applied to the first series (omitting first two cycles where instability of the system is observed), in order to calculate the degradation rates. High C-rates are seen to affect the discharge capacity, but the capacity is almost completely recovered (green points) and only a limited degradation occurs. Impedance spectroscopy has been also applied to investigate the LFP cathode degradation. Figure 3 shows the imaginary part of the impedance measured at 50% State-of-Charge after each series of cycles. The relative increase in the impedance arc around 1 KHz (assumed to be associated with charge transfer resistance at the LFP particle surfaces) is seen to gradually decrease with increasing number of series. This indicates that more cycles per series is needed to establish a convincing relation between C-rate and degradation. The degradation studies will be coupled with FIB/SEM analysis in order to observe changes in the pore structure or micro cracks that would affect electronic percolation. Figure 4 displays an example of a fresh LFP cathode after FIB cutting. White particles are LFP grains while the black area contains carbon particles and pores, which are difficult to distinguish from each other. Substitution of the epoxy resin with a silicon resin increases the contrast between pores and carbon particles [1] and this will be used in the forthcoming FIB/SEM analysis. References [1] M. Ender et al, Journal of The Electrochemical Society, 159 (7) A972-A980 (2012)

One promising energy storage technology is the solid oxide electrochemical cell (SOC), which can both store electricity as chemical fuels (electrolysis mode) and convert fuels to electricity (fuel-cell mode). The widespread use of SOCs has been hindered by insufficient long-term stability, in particular at high current densities. Here we demonstrate that severe electrolysis-induced degradation, which was previously believed to be irreversible, can be completely eliminated by reversibly cycling between electrolysis and fuel-cell modes, similar to a rechargeable battery. Performing steam electrolysis continuously at high current density (1 A cm(-2)), initially at 1.33 V (97% energy efficiency), led to severe microstructure deterioration near the oxygen-electrode/electrolyte interface and a corresponding large increase in ohmic resistance. After 4,000 h of reversible cycling, however, no microstructural damage was observed and the ohmic resistance even slightly improved. The results demonstrate the viability of applying SOCs for renewable electricity storage at previously unattainable reaction rates, and have implications for our fundamental understanding of degradation mechanisms that are usually assumed to be irreversible.

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