High Temperature Co-electrolysis Research Articles

Thermal integration of a high-temperature co-electrolyzer and experimental methanator for Power-to-Gas energy storage system

Performance of an innovative storage system for renewable energy, based on the Power-to-Gas concept are numerically predicted. The investigated system is composed by a high temperature co-electrolyzer of Solid Oxide Electrolyte Cell technology and an experimental methanation section, based on structured catalyst, suitable for high temperature operation. With the aim to thermally integrate high temperature co-electrolysis and methanation, a parametric thermodynamic analysis of the Power-to-Gas system is carried-out with a lumped-parameters approach, including all the thermal and electric energy consumptions. In particular, in order to optimize the system thermal balance of plant, various configurations involving internal heat recovery and pressurization of components are also considered. Numerical results are provided in terms of different performance indicators, such as electric-to-fuel conversion index, first law efficiency and second law efficiency and output-fuel quality indicators. The study demonstrates the possibility to thermally integrate the co-electrolyzer and the high-temperature methanation section achieving significant energy savings. Moreover, the calculated results show that the system set-up providing higher quality of the produced synthetic natural gas do not always lead to larger values in energy conversion efficiency. Eventually, advanced configurations of the Power-to-Gas system including heat recovery allow to achieve first-law efficiency up to values around 80–85% and second-law efficiency around 70–78%; a second methanation section based on conventional low-temperature reactors is included in the system and pressurization of the methanation section, or pressurization of the co-electrolysis section, is mandatory, in order to achieve large fraction of methane (up to 95–99%) in the produced synthetic fuel.

Production of syngas with controllable H2/CO ratio by high temperature co-electrolysis of CO2 and H2O over Ni and Co- doped lanthanum strontium ferrite perovskite cathodes

Conversion of CO2 and H2O into synthesis gas (a mixture of H2 and CO) in a high temperature solid oxide electrolysis cell (SOEC) is an attractive route for CO2 utilization. Depending on the composition of the syngas, it can be directly used as a fuel or fed to an oxo process or a Fischer-Tropsch process to produce value added chemicals. Designing an efficient and stable cathode for an SOEC that can yield syngas with controllable H2/CO ratio is of fundamental interest. In the current study, Ni and Co- doped A-site deficient perovskite materials of the form La0.7Sr0.2NixCoyFe1-x-yO3 (x,y = 0; x = 0, y = 0.2; x = 0.1, y = 0.1; x = 0.2, y = 0) are studied as SOEC cathodes for co-electrolysis of CO2 and H2O at 800 °C. Modifications of the surface and bulk properties of these materials due to doping were investigated using in-situ XRD, XPS, Raman spectroscopy, electronic conductivity measurements, oxygen mobility test, in-situ DRIFTS and XANES. The Co- doped perovskite La0.7Sr0.2Co0.2Fe0.8O3 showed the lowest Faradaic efficiency and the ones doped with Ni showed nearly 100% Faradaic efficiency for total production of H2 and CO where the H2/CO ratio in the produced syngas increased with increasing Ni content in the cathode material. This ratio could be controlled by tuning the B-site dopant levels, cell voltage and H2O/CO2 ratio in the cathode feed stream. In-situ XANES studies showed that during CO2 and H2O co-electrolysis, the Co ions in La0.7Sr0.2Co0.2Fe0.8O3 may get oxidized, thus possibly reducing the number of oxygen vacancies in the material and hence lowering the electrochemical activity. Moreover, post-electrolysis analyses show that the Co- doped cathode forms graphitic carbon, which lowers the Faradaic efficiency for syngas production. No graphitic carbon formation was observed on the Ni- doped cathodes. A long-term co-electrolysis test performed for approximately 110 h on a La0.7Sr0.2Ni0.1Co0.1Fe0.8O3 cathode shows good stability of these materials in terms of electrochemical performance and coke resistance.

Life Cycle Assessment of biogas upgrading routes

The upgrading of biogas to biomethane, by removing contaminants and carbon dioxide, is a treatment that allows this gaseous fuel to reach the specifications suitable for the injection in the natural gas grid and the use as vehicle fuel. This pathway enables the conversion of wet biomass into a perfect substitute of natural gas. Biogas upgrading is usually performed through CO2 removal and the most diffused method is water scrubbing. However, the embedded CO2 could be directly recycled into methane through a high-temperature co-electrolysis process followed by a methanation step, thus increasing the yield of biomethane for the same biogas inlet. In this paper the environmental impacts of two routes for biomethane production are compared in the framework of the Life Cycle Assessment (LCA) methodology. A sensitivity analysis for different shares of renewable content in electricity has been carried out. The results show that the large need for electrical energy penalizes the electrolytic process for renewable contents of the electricity which are not close to 100%.

One Giant Step for Synthetic Fuels

High-temperature co-electrolysis system demonstrated in Germany

High-Temperature Co-Electrolysis: A Versatile Method to Sustainably Produce Tailored Syngas Compositions

High-temperature co-electrolysis of carbon dioxide and steam is a promising method to produce ‘white’ syngas by making use of renewable energy and carbon dioxide as sustainable feedstock. The technological key advantage is the possibility to tailor syngas compositions over a broad range. This paper presents a systematic investigation of the syngas tailoring process by establishing relationships between feed gas compositions and flow rates to the syngas ratio. A linear dependence between the H2O:CO2 ratio in the feed gas and the H2:CO ratio in the output gas was observed. Furthermore, the syngas ratio remains mostly invariant upon variations in electrochemical potential and fluctuating gas utilizations/flow rates during operation of a co-electrolysis cell. Most importantly, the co-electrolysis performance was demonstrated to operate at high current densities of up to 3.2 A·cm−2 over a broad range of feed gas compositions with faradaic efficiencies of nearly 100%. The possibility to operate co-electrolysis under transient load conditions renders this method particularly suitable in future scenarios of intermittent availability of renewables. The results described here illustrate the versatility of co-electrolysis, which can produce all relevant syngas compositions in a single-step process at constantly high performance.

Spinel-based oxide cathode used for high temperature CO2/H2O co-electrolysis

Spinel oxides are applied as cathodes (fuel electrode) for the high temperature co-electrolysis of CO2/H2O mixtures. The spinel oxide is composed of two transition metals, which may improve activity for the reduction of CO2 and H2O. A series of spinel oxides have been synthesized and tested in co-electrolysis mode. A good candidate, CuFe2O4, was found and achieved a current density higher than 1 A/cm2 in CO2/H2O co-electrolysis with a fuel composition of 30%CO2/30%H2O-Ar at 800 °C, although the product was enriched in H2. This material presents superior Coulombic (Faradaic) efficiency for CO + H2 gas products, reaching more than 95%. It shows stable and reversible I-V curves, indicating good electrochemical cell performance in the co-electrolysis operation. In addition, a uniform porous structure with fine particles was observed after co-electrolysis, which may be associated with the stable and reversible electrochemical cell performance observed.

Electrocatalysts with Increased Activity for Coelectrolysis of Steam and Carbon Dioxide in Solid Oxide Electrolyzer Cells

The high-temperature coelectrolysis system can be helpful to solve environmental issues by reducing carbon dioxide emissions. The technology is highly promising because of its high selectivity and conversion efficiency toward the products. In addition, the produced syngas can also be further converted into very useful synthetic fuels. In this study, we investigated the series of reactions on a wide range of transition metals to evaluate their ability to increase the activity of the conventional Ni catalysts used in the fuel electrode of solid oxide electrolyzer cells. We theoretically identified that the adsorption energies of O and H are the common descriptors of coelectrolysis of steam and carbon dioxide. We then combined microkinetic analysis with density functional theory calculations to derive a volcano plot to predict the activity of coelectrolysis on a variety of transition metals. We could successfully suggest good candidates of Ni-based bimetallic alloy catalysts with excellent activities in the ...

Infiltrated mesoporous oxygen electrodes for high temperature co-electrolysis of H2O and CO2 in solid oxide electrolysis cells

In the last years high temperature Solid Oxide Electrolysis Cells (SOECs) have emerged as a promising solution for energy conversion and storage. Oxygen electrodes based on mesoporous materials are proposed for enhancing the performance and durability of SOEC devices.

Development of disk-type solid oxide electrolysis cell for CO2 reduction in an active carbon recycling energy system

CO2 recycling method characterized by the application of an Active Carbon Recycling Energy System (ACRES) to an iron- making process (iACRES) was studied. In this system, pure iron is produced from the reduction of iron oxide using regenerated CO by high temperature CO2 electrolysis with the use of a disk-type solid oxide electrolysis cell (SOEC) and powered by a high temperature gas-cooled reactor (HTGR) generating electricity and thermal energy.Thus, the focus of this research is to further improve the performance of disk-type SOEC by developing its structure and determining the most suitable electrode and electrolyte materials, and the appropriate fabrication method. In this present study, new cathode material such as Ni-SDC (SDC: samarium-doped ceria) was checked and tested for the SOEC. The electrochemical performance of the cells were evaluated at 800 ~ 900 °C by analyzing current-voltage characteristics, AC impedance spectra, and CO production data. Results showed CO production rates close to theoretical values corresponding to current density values observed. The performances of the SOEC samples were investigated for their potential use in iACRES, and the SOEC with Ni- SDC cathode showed the most enhanced electrochemical performance for iACRES.The results showed that iACRES is available for utilization of the high temperature heat at about 850 ~ 950 °C provided by HTGR in this system. This would mean utilization of CO2 emission-free thermal energy, and eventual expansion of use of nuclear energy beyond electrical power generation.

Surface Chemistry of Perovskite-Type Electrodes During High Temperature CO2 Electrolysis Investigated by Operando Photoelectron Spectroscopy.

Any substantial move of energy sources from fossil fuels to renewable resources requires large scale storage of excess energy, for example, via power to fuel processes. In this respect electrochemical reduction of CO2 may become very important, since it offers a method of sustainable CO production, which is a crucial prerequisite for synthesis of sustainable fuels. Carbon dioxide reduction in solid oxide electrolysis cells (SOECs) is particularly promising owing to the high operating temperature, which leads to both improved thermodynamics and fast kinetics. Additionally, compared to purely chemical CO formation on oxide catalysts, SOECs have the outstanding advantage that the catalytically active oxygen vacancies are continuously formed at the counter electrode, and move to the working electrode where they reactivate the oxide surface without the need of a preceding chemical (e.g., by H2) or thermal reduction step. In the present work, the surface chemistry of (La,Sr)FeO3−δ and (La,Sr)CrO3−δ based perovskite-type electrodes was studied during electrochemical CO2 reduction by means of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at SOEC operating temperatures. These measurements revealed the formation of a carbonate intermediate, which develops on the oxide surface only upon cathodic polarization (i.e., under sufficiently reducing conditions). The amount of this adsorbate increases with increasing oxygen vacancy concentration of the electrode material, thus suggesting vacant oxygen lattice sites as the predominant adsorption sites for carbon dioxide. The correlation of carbonate coverage and cathodic polarization indicates that an electron transfer is required to form the carbonate and thus to activate CO2 on the oxide surface. The results also suggest that acceptor doped oxides with high electron concentration and high oxygen vacancy concentration may be particularly suited for CO2 reduction. In contrast to water splitting, the CO2 electrolysis reaction was not significantly affected by metallic particles, which were exsolved from the perovskite electrodes upon cathodic polarization. Carbon formation on the electrode surface was only observed under very strong cathodic conditions, and the carbon could be easily removed by retracting the applied voltage without damaging the electrode, which is particularly promising from an application point of view.

Making synthetic fuels for the road transportation sector via solid oxide electrolysis and catalytic upgrade using recovered carbon dioxide and residual biomass

The search for beneficial biofuels with positive energy gains and reduced carbon dioxide emissions is a key target of the road transportation sector. This work focuses on alternative fuels for fossil fuel substitution in the transportation sector and we have examined the production of three different types of synthetic fuels: Dimethyl Ether, Synthetic Natural Gas and the diesel fraction of the Fischer-Tropsch synthesis products. We analyze different pathways to make green synthetic fuels starting from waste carbon dioxide recovered from various sources and residual biomass. Processes for biofuel production that combine high temperature co-electrolysis in a solid oxide electrolysis cell (supplied with renewable electricity) to a downstream catalytic reactor that upgrades further the electrolysis-derived syngas are investigated. The integrated co-electrolysis catalytic upgrade is a way to “recycle” CO2 to either a gas or liquid fuel depending on the selected catalytic route. We have considered two different sources for waste carbon dioxide recovery: the CO2-rich off-gas of biogas-to-biomethane plants and CO2 that could be available from fossil fuel power plants with carbon capture. Residual biomass is the other CO2 source analyzed in this work. Fuel demand and, the CO2 and renewable electricity resources available on a regional scale have been assessed to develop a case study of synthetic fuels production in Italy. Results show how synfuel production from biogas off-gas and residual biomass allow to cover a limited percentage of the overall fuel demand (ranging from 1.5% to 37% depending on the synfuel), while using carbon dioxide recovered from fossil power plants could potentially provide enough synfuel to cover completely the fuel demand using Dimethyl Ether or Synthetic Natural Gas. However, an increase of renewable electricity would be needed (ranging from 50% to a three-fold increase depending on the synfuel). Finally, the impact of fuel substitution on the overall carbon dioxide emissions has been investigated showing that a complete substitution of fossil fuels using synfuels produced from waste recovered CO2 could lead to an emission reduction of almost 40% with respect to the current fossil fuel dominated scenario.

Electrochemical characterization and mechanism analysis of high temperature Co-electrolysis of CO2 and H2O in a solid oxide electrolysis cell

Flexible nuclear power for synthetic fuel production through high temperature co-electrolysis technology (HTCE) using solid oxide electrolysis cell (SOEC) has recently received increasing international interest in the large-scale, highly efficient and carbon-neutral energy storage field. It is of great importance to enhancing the understanding of co-electrolysis process and the related mechanism. In this paper, CO2 behavior and its effect on the performance of SOEC were examined by the electrochemical characterization and impedance analysis to determine the proper operating conditions, such as H2O, CO2, H2, CO, operation temperature and electrolysis current. The polarization mechanism is also investigated by the experimental and modeling results. It was found that the electrolysis of CO2 is much harder than that of H2O, and the ASR of pure CO2 electrolysis is about three times that of H2O. When the CO2 content decreases from 50% to 10%, the ASR decreases from 1.59 to 0.90 Ω cm2. Increasing the H2O content could also improve the electrolysis efficiency to some degree, while the CO addition in the inlet gas was not favorable for the process. Mechanism study shows that the diffusion impedance of CO2 should be the restricted step for the polarization energy loss.

Electrochemical Performance of Solid Oxide Electrolysis Cells with LSCF6428-SDC/SDC Electrode for H2O/CO2 High Temperature Co-Electrolysis

High temperature co-electrolysis of H2O/CO2on solid oxide electrolysis cell (SOEC) is one technology to produce syngas using carbon dioxide. In this study, Ni-8YSZ/8YSZ/LSCF6428-SDC button cells were fabricated and the effect of different heat treatment temperatures on the performance of SOEC was investigated. The SOEC with LSCF6428-SDC air electrode, SDC buffer layer, and YSZ-(LSCF- SDC) adhesion layer gave better performance in overall results and the voltage obtained for this SOEC at current density of 0.8 A.cm-1 was from ~ 1.2 V to ~ 1.8 V. Co-electrolysis and the electrochemical performance of the cells were dependent on heat-treatment temperature of the electrode and the buffer layers. To identify the electrochemical processes occurring at the electrodes of SOEC, analysis of the electrochemical impedance (EIS) data was carried out.

High Temperature Co-Electrolysis of CO2 and Water on Doped Lanthanum Chromites

High temperature Solid Oxide Electrolysis Cells (SOEC) have recently attracted global research interest towards syngas (CO and H2) production [1-3]. Co-electrolysis of CO2 and water is a key element of power-to-chemicals and power-to-fuel strategies for green chemicals, CO2 recovery and electricity storage at large scale. High temperature solid oxide co-electrolysis of CO2 and H2O is highly efficient to CO and hydrogen production with high reaction rates. Additionally, this technology has the advantage of controlling syngas composition at the outlet by applying the proper potential. This is extremely important when the produced syngas will be fed in conventional Fischer-Tropsch catalytic fuel synthesis reactors [4]. The commercialization of co-electrolysis could provide a simpler and cost efficient way for the production of light-fuels compared to other conventional technologies, whereas it is advantageous in that the expected technology development can be based and use the progress of the existing more mature Solid Oxide Fuel Cells technology. At the same time, strategies to achieve technical targets and technology development are expected to lead to significant improvement of the system performance, e.g. thermal and mass integration with other units and components of a complete system (e.g. H2O vaporizer, CO2recovery and compressor, methanation reactor etc.). Several recent studies focus on technical development and operation of Solid Oxide Electrolysers, primarily for water electrolysis but also for the co-electrolysis and fuel conversion process. On the other hand, conventional materials used to construct SOFC cells are not offering the required performance to achieve high efficiency in SOEC devices. Therefore, studies concerning electrode materials and materials of cell structure or electrolytes, as well as cell design are on the top of the research agenda. This paper summarizes progress in materials aspects for the development and optimization of a solid oxide electrolyser based on perovskitic cathode electrodes able to efficiently perform co-electrolysis of CO2 and H2O for syngas production. Laboratory synthesized lanthanum strontium chromites doped with Fe [5] have shown adequate efficiency and remarkable stability during co-electrolysis compared to the commercial and state-of-the-art Ni-YSZ material. The study is conducted in the framework of the four-years EU Research project funded by the Fuel Cells and Hydrogen 2 Joint Undertaking, entitled SElySOs: “Development of new electrode materials & understanding of degradation mechanisms on Solid Oxide Electrolysis Cells”. Acknowledgements The authors would like to acknowledge financial support from the European Union’s Fuel Cells and Hydrogen 2 Joint Undertaking (SElySOs project, Grant Agreement No. 671481).

Co-Electrolysis, Quo Vadis?

Co-electrolysis, the electrochemical conversion of steam and carbon dioxide with the use of renewably generated electricity to produce syngas, can be regarded as the enabling step to the sustainable production of synfuels and high-value chemicals. This advantageous method of beneficial CO2-valorization is predetermined to couple energy-intense sectors such as “traffic and transportation” and “chemical industry”. The needed products for these markets presuppose different syngas compositions through their production routes and according to that determine the applied technology of syngas generation by its achievable H2:CO ratio. The concept of high-temperature co-electrolysis offers both an efficient electrochemical conversion and accessibility to H2:CO distributions from 1:1 to 3:1 by a given set of parameters (temperature, current density, material,…). Though, this is yet to be shown as research has been focused on electrochemical analysis of materials, cells and stacks rather than on product distribution depending on operating parameters and an all-embracing quantitative product analysis[1]. Herein, we examine the possibilities and variety on syngas production with focus on (electrochemical) performance and syngas yield and composition under different conditions, e.g. temperature, current density, substrate composition. The experiments were executed on a cathode supported cell (CeramTec) composed of an LSCF anode, a CGO barrier-layer, an 8YSZ electrolyte and a Ni/8YSZ cathode. The testing took place in the temperature range of 750 to 900 °C with preset cell voltages up to 1.4 V. The ratios of water vapor to carbon dioxide have been set to 3:1, 2:1 and 1:1. Furthermore, the cells have been tested with a 50% water – 50% hydrogen mixture for comparison. I/V-measurements and electrochemical impedance spectroscopy have been conducted under the combinations of mentioned conditions. In addition, a full spectrum analysis of the product gases has been performed, i. e. water was not condensed from the product stream prior to the mass spectrometer. Current densities of up to over 1.7 A·cm-2 with polarization resistances of 24 mΩ·cm-2 have been obtained at 900 °C and a cell voltage of 1.4 V with a 9 nl·h-1 flow of 40% water, 40% CO2 and 20% H2 used. I/V-curves decrease in slope and increase in current density with increasing temperature (figure 1). Further investigation is ongoing and the electrochemical performance and efficiency of syngas production will be presented and discussed in detail. [1] S. Foit, I. C. Vinke, L. G. J. de Haart, R.-A. Eichel, Angewandte Chemie International Edition 2016, DOI: 10.1002/anie.201607552 Figure 1: I/V characteristics of a Ni/YSZ//YSZ//CGO//LSCF single cell during co-electrolysis of different H2/CO2 mixtures in dependence of temperature Figure 1

Electrochemical Performance of Solid Oxide Electrolysis Cells with LSCF6428-SDC/SDC Electrode for H2O/CO2 High Temperature Co-Electrolysis

High temperature co-electrolysis of H2O/CO2 on solid oxide electrolysis cell (SOEC) is one of technology to produce syngas as an energy conversion and storage solution using carbon dioxide. Here Ni-8YSZ/8YSZ/LSCF6428-SDC button cells were fabricated and the effect of the different heat-treatment temperature on the performance of SOEC has been investigated. The SOEC with LSCF6428-SDC air electrode and SDC buffer layer as an addition of YSZ-(LSCF-SDC) adhesion layer gives the better performance in overall results and the voltage obtained for this SOEC at current density of 0.8 A·cm-1 is from ~ 1.2 V to ~ 1.8 V.

High Temperature Co-Electrolysis of CO2 and Water on Doped Lanthanum Chromites

The high temperature Solid Oxide Electrolysis or Co-electrolysis (SOEC) technology has a huge potential for future mass production of hydrogen (and synthesis gas) and shows great dynamics to become commercially competitive against other electrolysis technologies, which are better established but more expensive and less efficient. Despite the fact that Ni-YSZ, a state-of-the-art (SoA) SOEC electrode, has been proven to be an efficient electrode for electrolysis applications in terms of current density, it suffers from various degradation issues. In this respect, the perovskitic material La0.75Sr0.25Cr0.9Fe0.1O3- δ which is a mixed ionic-electronic conductor is prepared and studied as cathode electrode in electrolysis. This electrode, in combination with a typical LSM anode electrode in electrolyte (YSZ) supported cells, shows high efficiency comparable to that of the-state-of-the-art Ni-YSZ, while it exhibits highly stable performance during both H2O electrolysis and H2O-CO2 co-electrolysis.

Co-Electrolysis, Quo Vadis?

This work focuses on the impact of reaction parameters of high-temperature co-electrolysis of carbon dioxide and water on the electrochemical performance and product gas distribution. Experiments have been performed with a conventional electrode (cathode) supported solid oxide cell. The electrochemical performance was investigated by I/V-curve measurements and impedance spectroscopy. The temperature was varied in the range of 750 to 900 °C. At 900 °C the impact of different feed gas compositions was investigated. The provided volumetric water vapor content was changed from 40 % with 20 % H2 up to 47.5 % with 5 % H2 in addition to the varied CO2 to H2O ratios 0, 1:1, 1:2 and 1:3. Parameter controlled syngas tailoring was shown by ramps of increasing current densities (at 0.001 mA·s-1) and coupled online product analysis at feed flow rates of 2, 3 and 6 nl·h-1, confirming its principle.

First-Principles Study of High Temperature CO2 Electrolysis on Transition Metal Electrocatalysts

Electrochemical reduction of CO2 using the electrical energy generated from renewable sources has attracted increasing interest as a potential route for producing high energy molecules from CO2. In this contribution, high temperature electrochemical reduction of CO2 to CO on a series of transition metal electrocatalysts is studied using DFT calculations combined with microkinetic modeling under solid oxide electrolysis cells (SOECs) operating conditions. We show that CO2 dissociation via a two-electron transfer process into adsorbed CO and O2– ions in the electrolyte is favorable on most of the metal electrocatalysts considered, with a dependence of the simulated electrolysis current density on the applied potential consistent with experimental observations. A “volcano”-type relation between the calculated electrolysis rates and the binding energies of atomic O is found, suggesting that the binding energy of O might be a good activity descriptor for high temperature CO2 electrolysis on transition metals. ...

Power-to-Syngas: An Enabling Technology for the Transition of the Energy System?

Power-to-X concepts promise a reduction of greenhouse gas emissions simultaneously guaranteeing a safe energy supply even at high share of renewable power generation, thus becoming a cornerstone of a sustainable energy system. Power-to-syngas, that is, the electrochemical conversion of steam and carbon dioxide with the use of renewably generated electricity to syngas for the production of synfuels and high-value chemicals, offers an efficient technology to couple different energy-intense sectors, such as "traffic and transportation" and "chemical industry". Syngas produced by co-electrolysis can thus be regarded as a key-enabling step for a transition of the energy system, which offers additionally features of CO2 -valorization and closed carbon cycles. Here, we discuss advantages and current limitations of low- and high-temperature co-electrolysis. Advances in both fundamental understanding of the basic reaction schemes and stable high-performance materials are essential to further promote co-electrolysis.