Status and Outlook of Solid Oxide Cells for Hydrocarbon Fuel Conversion.

Solid oxide electrolysis cells (SOECs) for converting CO2 to more useful chemical species are attracting considerable attention because of their high electrolytic efficiency, offering the possibility of direct conversion of CO2 to CO (CO2 → CO + 1/2O2). In addition, recently, high temperature electrolysis of steam to H2 is also attracing much interest. Heat energy supplied to SOECs can maximize electrolysis efficiency, offering both thermodynamic and kinetic advantages. The generated CO can be used as a fuel gas, converted into syngas via reaction with H2, or to reduce iron oxide to pure iron in the iron-making process. The application of SOECs and related technologies can contribute to a more sustainable economy. For the electrolyte in SOECs, ZrO2 doped with 8 mol% of Y2O3 (YSZ) has always been used because of its chemical stability and mechanical strength. However, an electrolyte for CO2 or H2O electrolysis with lower internal resistance and increased efficiency is highly desirable. LaGaO3-based electrolytes have higher oxide-ion conductivities and transport numbers, which can lead to higher energy conversion efficiency; hence, a doped-LaGaO3 electrolyte (La0.9Sr0.1Ga0.8Mg0.2O3, LSGM) has been tested in this study. For CO2 and H2O electrolysis, another drawbacks is durability since electrode is easily aggregated and deactivated. Although metals, particularly Ni, are widely used as cathodes in CO2 electrolysis cells, we found that a LaFeO3−δ-based perovskite cathode (La0.6Sr0.4Fe0.8Mn0.2O3−δ, LSFM6482) showed the highest activity for CO2 and H2O electrolysis among the examined oxides Among the cells using a La0.6Sr0.4Fe0.9M0.1O3−δ-based oxide cathode (M = Mn, Co, Ni, or Cu) for CO2 electrolysis at 1073K. Mn was the most effective Fe replacement on the B site, and the La0.6Sr0.4Fe0.9Mn0.1O3−δ cathode showed the highest activity and current density for CO2 electrolysis of this set of cathodes. Among the tested dopants, Mn and Co have ionic sizes most similar to Fe and a smaller size mismatch between the host and dopant cations in the perovskite structure, which should suppress dopant segregation and minimize detrimental effects of cation segregation. Overall, among the examined cations, Mn was the best dopant for the B site of the LaFeO3 cathode. A cell consisting of BLC64/LSGM/LSFM6482 exhibited the highest CO2 electrolysis activity (a current density of 0.52 A/cm2 at 1.6 V and 1173 K) of all cathodes investigated in this study and reduced CO2 at a rate of 153 μmol/cm2•min at 1173 K and 1.6 V with negligible carbon formation. In this study, change in internal resistance during electrolysis is also discussed based on impedance measurement.

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.

The effects of the introduction of tetrachloromethane into the feedstream for the partial oxidation and oxidative coupling of methane

A direct method of converting natural gas into ethylene is the heterogeneously catalyzed oxidative coupling of methane (OCM), however, only with hydrocarbon yields limited to 30-35% despite enormous efforts to optimize the catalysts. By combining the exothermic OCM with a secondary process, namely steam reforming of methane (SRM), the methane conversion can be increased significantly while improving temperature control and simultaneously producing valuable synthesis gas. In this thesis, two different reactor concepts were developed to integrate the OCM and SRM reactions in an overall autothermal process, so that the OCM process is effectively cooled and the generated reaction energy is efficiently used to produce synthesis gas. The integration is most optimally achieved on the catalyst particle scale, which would eliminate the need for external heat exchange and opens up the possibility to use distributive oxygen dosing with which much higher product yields can be achieved. It is proposed to use a dual function catalyst particle in which the two chemical processes are physically separated by an inert, porous layer, such that additional diffusional resistances are intentionally created to control the reaction rates. This concept was studied with numerical simulations on the scale of a single catalyst particle and on reactor scale. It was found that the SRM and OCM reaction rates could be effectively tuned to achieve autothermal operation at the reactor scale, while the methane conversion was enhanced from 44% to 55%. An alternative integrated process can be achieved by combining OCM and SRM in a heat exchange reactor comprising of two separate reaction chambers which are thermally coupled. The OCM is carried out in packed bed reverse flow membrane reactor tubes submerged into a fluidized bed where the unconverted methane and byproducts from OCM are reformed, thus producing synthesis gas and consuming the reaction heat liberated by OCM. The feasibility of this concept is supported by experiments of OCM on a Mn/Na2WO4/SiO2 catalyst in a packed bed (porous Al2O3) membrane reactor. The results demonstrated that a C2 yield of 25-30 % can be achieved and that distributed feed of oxygen is optimal for the combined OCM/SRM reactor concept.

Dual-membrane reactor for methane oxidative coupling and dry methane reforming: Reactor integration and process intensification

Influence of nanocatalyst on oxidative coupling, steam and dry reforming of methane: A short review

Hydrogen is at the periphery of breaking through to become the main source of fuel for energy production and storage. However, the cost of hydrogen remains a major challenge for the implementation of hydrogen economy. The recent DOE’s Hydrogen Shot has a target to reduce the price of clean hydrogen to $1 per kilogram by 2031.1 In the meantime, methane remains the fuel of choice and recent discoveries of new natural gas resources have made methane an inexpensive and readily available source of energy that is being used in various industries such as energy production and chemical manufacturing.2 In a typical high-temperature solid oxide electrolyzer (SOE) cell, steam is converted to hydrogen at the cathode while oxygen is evolved at the anode. We have recently developed a series of aliovalent doped barium niobate electrodes for SOEC that produces syngas (CO + H2) at conditions that are prevalent in an SOEC anode.3–5 Here, we present our results on coproduction of hydrogen and syngas in a SOE cell utilizing typical Ni-YSZ cathodes and a wide variety of anodes including Ni-YSZ, LSM, BCNF, BMNF and their composites. Characterization of the electrode materials was done using TGA, SEM, and XRD measurements while their utility as an electrode for cogeneration was carried out using CV, impedance, chronoamperometry, and polarization measurements. The role of electrode material properties such as its mixed ionic electronic conductivity, Smith basicity and chemical stability are analyzed to correlate the results on cogeneration of syngas and hydrogen and faradaic efficiency. We show a unique design of SOE cells for generating hydrogen that can help reduce its production cost by more than half and move us towards meeting the US-DOE cost targets. References (1) Hydrogen Shot. Energy.gov. https://www.energy.gov/topics/hydrogen-shot (accessed 2025-01-21).(2) United States Natural Gas Industrial Price (Dollars per Thousand Cubic Feet). https://www.eia.gov/dnav/ng/hist/n3035us3m.htm (accessed 2025-01-21).(3) Ramaiyan, K.; Benavidez, A.; Garzon, F. Durable Electrocatalysts Based on Barium Niobates Doped with Ca, Fe, and Y: Enhancing Catalytic Activity and Selectivity for Oxidative Coupling of Methane. Chem. Eng. J. 2024, 500, 156976. https://doi.org/10.1016/j.cej.2024.156976.(4) Denoyer, L. H.; Benavidez, A.; Garzon, F. H.; Ramaiyan, K. P. Highly Stable Doped Barium Niobate Based Electrocatalysts for Effective Electrochemical Coupling of Methane to Ethylene. Adv. Mater. Interfaces 2022, 9 (27), 2200796. https://doi.org/10.1002/admi.202200796.(5) Denoyer, L. H.; Benavidez, A.; Brearley, A.; Ramaiyan, K. P.; Garzon, F. H. Electrochemical Oxidative Coupling of Methane: Deciphering the Exceptional Properties of BaMg0.33Nb0.67–XFexO3−δ for Enhanced Electrocatalysis and Durable Operation. Energy Fuels 2024, 38 (9), 8134–8144. https://doi.org/10.1021/acs.energyfuels.4c00720.

The electrochemical reduction of CO2 was studied via solid oxide electrolysis cell (SOEC), a type of device that can work reversely into a solid oxide fuel cell (SOFC) to generate electricity. Application of SOEC for CO2 electrolysis possesses potential rewards both in energy and environmental aspects, as it offers a way to recycle CO2 into chemicals and value-added fuels, which helps to reduce the accumulation of atmospheric CO2 and realize the carbon neutral cycling of fuels. Secondly, SOEC techniques provide a means to utilise the intermittent renewable sources, such as wind, tide, etc., as energy input to store excess electricity in the form of H2, CO and hydrocarbons and use these chemicals when necessary. Yet it is a challenging task to realize an efficient reduction of CO2 by SOEC due to the non-polar nature of CO2 fuels which are hard to be chemically absorbed and activated in high temperature range. The CO product from CO2 reduction is also demanding for the choice of fuel electrode (i.e. cathode in SOEC and anode in SOFC) materials, which has been an issue for the CO/hydrocarbon-fuelled SOFCs. To date, the CO2 electrolysis by SOEC is still at the starting point, and the mechanisms on the electrochemical reduction of CO2through SOECs are not fully understood. Extensive efforts need to be dedicated to the material developments, mechanism study, and system designs etc. Effort was made in our lab to find a highly performed, long-term durable cathode material for electrochemical reduction of CO2 by SOEC and to obtain more understandings of the mechanism of CO2 electrochemical reduction process. Different cathode materials were employed, including Ni-8 mol% yttria stabilised zirconia (YSZ) cermet, (La,Sr)(Cr,Mn)O3 (LSCM)-YSZ composite, and LSCM-(Gd, Ce)O2 (GDC) composite. Focus was casted on LSCM based cathodes which were found to be carbon-resistive, and the microstructure of LSCM-based cathodes was tuned to high performance and efficient CO2 electrolysis by the strategy of applying a gradient composite cathode and of adopting wet impregnation as cell fabrication procedures. The electrochemical performance of CO2 electrolysis was characterized in various CO2-CO mixtures and applied potentials in 900-750oC with the aid of impedance spectroscopy, on YSZ electrolyte supported three-electrode SOECs. In this paper, impedance behaviour of the electrochemical reduction reaction of CO2 from different cathode SOECs was correlated with the variations in gas composition, operating temperature and loading potential. As both charge transfer and surface adsorption/desorption equilibration and surface diffusion of activated species were found to be dominant processes taking place in the scope of cathodes under examination, with the later being significant in CO2-rich fuels, areas of discussion will be the effects of operational conditions on the impedance arcs associated with these steps, and how the cathode microstructure impacts the impedance behaviour and the corresponding elementary steps from CO2electrochemical reduction. By introducing a LSCM-YSZ 30-70/LSCM-YSZ 60-40 graded cathode and by incorporating extra catalyst into LSCM-GDC composite, the cathode performance was greatly improved, and the surface activity was profoundly accelerated. However, the most effective way to promote cathode performance was to introduce the cathode components in separate steps via wet impregnation. A competitive CO2 electrolysis performance to Ni-YSZ cermet was obtained from the GDC impregnated LSCM-YSZ cathode with 0.5% Pd extra catalyst, which also showed a comparable performance between SOEC and SOFC when operating in CO2-CO 50-50 mixture.

In this study, a 2D model of Solid Oxide Electrolysis Cells (SOECs) was developed to evaluate their performance in CO2 and H2O co-electrolysis. The numerical results were rigorously validated against prior studies, demonstrating high consistency. The investigation focused on understanding the influence of various factors such as support type and operating temperature on SOEC performance.Analysis of polarization and performance curves revealed that anode-supported and cathode-supported SOECs exhibited similar characteristics, while electrolyte-supported SOECs displayed lower performance due to inadequate conductivity and increased electrolyte thickness. At 1.6 V, the average current density for cathode-supported SOEC was approximately 2.3679 A/cm², slightly lower than that of anode-supported SOEC, which was approximately 2.3879 A/cm². Moreover, at an average current density of around 5.30 A/cm², the cathode-supported SOEC yielded an average power density of 10 W/cm², while the anode-supported SOEC achieved 10.1 W/cm².Furthermore, increasing temperature was found to enhance SOEC performance by promoting more efficient chemical reactions, reducing resistance, and improving gas production rates during electrolysis of H2O and CO2. However, careful consideration of optimal operating temperatures is essential to ensure cell durability and material lifespan.Moreover, comparing co-flow and cross-flow configurations highlighted minor differences in performance, with co-flow demonstrating slightly lower average current density but comparable power density at 1.6 V. Co-flow configuration was favored for its homogeneous operation, facilitating efficient gas mixing and diffusion, while counter-flow configurations may introduce heterogeneity, potentially affecting overall performance.Overall, this study provides valuable insights into optimizing SOEC performance and efficiency, emphasizing the importance of support type, operating temperature, and flow configuration in achieving optimal performance for CO2 and H2O co-electrolysis applications.

Investigations of the selective partial oxidation of methanol and the oxidative coupling of methane over copper catalysts

Towards improved partial oxidation product yield in mixed ionic-electronic membrane reactors using CSTR and CFD modelling

CH4 valorisation reactions: A comparative thermodynamic analysis and their limitations

A new and very promising application of auto-thermal reactors is the coupling of endothermic and exothermic reactions where the product of the endothermic reaction is the desired one. Therefore, in this work, a reactor in which oxidative coupling of methane (OCM) and steam re-forming of methane (SRM) reactions take place simultaneously was modeled. The results were obtained in a wide range of different conditions such as inlet feed, inlet temperature, portions of OCM and SRM catalysts, and inlet velocity. In selection of the catalysts, more attention was drawn to prevent re-forming of OCM products. The main parameters of each reaction under different conditions such as conversion of the feed components, products selectivity and yield, temperature in the length of reactor, and component's concentration in the reactor were considered in course of this study. The results revealed that simultaneous OCM and SRM reactions in one reactor will tend to be auto-thermal, and the waste of energy will be reduced. The results also show that complete conversion of water and majority of methane and oxygen will decrease the amount of unwanted products at the reactor's discharge-a constraint that exists in single reactors of each reaction specially OCM.

Performance and methane production characteristics of H2O–CO2 co-electrolysis in solid oxide electrolysis cells

The increasing contribution of renewable energy sources (RES) to the power generation sector has motivated researchers to devise new methods for reliable energy storage systems. The significance of mechanisms for storing large amounts of electricity for longer time-scales is accentuated by the dynamics of renewable electricity generation that depend upon the season of the year, time of day, and the location. Conventional large-scale electricity storage technologies such as pumped-hydro and compressed-air storage systems are limited by geographical features (e.g., access to a cavern or high elevations). Traditional rechargeable batteries are limited by the fact that energy and power capacities do not scale independently. One of the promising and multipurpose techniques for storing electricity from these fluctuating renewable sources such as wind and solar, is conversion of the electric power to sustainable and carbon free gaseous fuel like hydrogen (power-to-gas (P2G)). In fact, owing to its diverse applications and worthwhile features such as high gravimetric energy density, hydrogen has become an attractive means of delivering renewable energy to many heavy-duty transport and industrial processes that are difficult to electrify. The renewable hydrogen produced from these zero emission electricity sources could be stored in caverns or injected into existing natural gas grids, initially at low concentrations as mixed with natural gas and ultimately piecewise replacing natural gas in the system. The stored hydrogen could be utilized in transportation for fuel cell electric vehicles (FCEV), in power generation for stationary fuel cells or in chemical industries as a feedstock for chemical synthesis of low-carbon fuels or other products, enabling the eventual decarbonization of all energy conversion. One of the efficient and cost-effective techniques for electrochemical conversion of electricity to hydrogen is steam electrolysis using solid oxide electrolysis cells (SOEC). The steam electrolysis reaction is intrinsically an endothermic reaction for which the required energy is to be supplied as electricity or heat or a combination of both. High temperature electrolyzers, such as SOEC, benefit from both lower energy required for the electrochemical reactions and an ability to provide part of the endothermic energy demand with the heat content of the inlet streams. At high current densities, SOEC can operate exothermically, meaning that the heat generation due to the losses and electricity supply are greater than the endothermic energy demand. When the SOEC systems are connected to intermittent renewable sources like solar or wind, the varying electricity profile will cause the SOEC operation to dynamically switch between endothermic to exothermic modes and all levels of endothermicity and exothermicity in between. This transient behavior of the electrolyzer could lead to highly dynamic thermal response that could have detrimental effects on thermal stress and degradation. In this work, an energy storage system (ESS) is designed in Matlab/Simulink which simulates the dynamic operation of an integrated SOEC system applied to an intermittent RES. The main focus of the research is the thermal management of the SOEC stack during highly dynamic operation throughout the entire operating regime from highly endothermic to thermo-neutral and to highly exothermic operation. An additional thermal management challenge is to keep the SOEC system hot during periods of non-operation (hot standby). These challenges are addressed in the current work by a novel system design that integrates a solid oxide fuel cell (SOFC) with the SOEC and combines several control strategies within the balance of plant (BoP). For thermal balance of the system the SOFC exothermicity is used to reduce the dependence of the system on electric heaters. The SOFC system oxidizes a fraction of the renewable hydrogen produced by SOEC and generates heat and electricity. A heat exchanger network is designed to convey the SOFC heat to the SOEC for a wide range of endothermic conditions. The main function of SOFC is to keep the SOEC warm when it is not operating or when it is operating in highly endothermic regions. The proposed configuration is integrated with 2 different patterns of electricity production from RES: one representing measured wind dominant RES such as that in Germany, and one representing solar dominant RES such as that in California. Dynamic analysis of the system in terms of the electric power generated by RES for these 2 conditions is presented. The response of different parameters in the system to keep the integrated system warm, self-sustaining, and within desired limits for lower thermal stress and degradation are presented. These parameters include required SOFC power, voltage and temperature profiles of both SOEC and SOFC, BoP parasitic consumption, system efficiencies, hydrogen production amounts, consumption, and storage levels during transient operation.