Reversible Solid Oxide Fuel Cell Research Articles

A Novel Layout for Combined Heat and Power Production for a Hospital Based on a Solid Oxide Fuel Cell

This paper addresses the problem of the reduction in the huge energy demand of hospitals and health care facilities. The sharp increase in the natural gas price, due to the Ukrainian–Russian war, has significantly reduced economic savings achieved by combined heat and power (CHP) units, especially for hospitals. In this framework, this research proposes a novel system based on the integration of a reversible CHP solid oxide fuel cell (SOFC) and a photovoltaic field (PV). The PV power is mainly used for balancing the hospital load. The excess power production is exploited to produce renewable hydrogen. The SOFC operates in electrical tracking mode. The cogenerative heat produced by the SOFC is exploited to partially meet the thermal load of the hospital. The SOFC is driven by the renewable hydrogen produced by the plant. When this hydrogen is not available, the SOFC is driven by natural gas. In fact, the SOFC is coupled with an external reformer. The simulation model of the whole plant, including the reversible SOFC, PV, and hospital, is developed in the TRNSYS18 environment and MATLAB. The model of the hospital is calibrated by means of measured data. The proposed system achieves very interesting results, with a primary energy-saving index of 33% and a payback period of 6.7 years. Therefore, this energy measure results in a promising solution for reducing the environmental impact of hospital and health care facilities.

Investigation on SmBa0.5Sr0.5Co1.5Fe0.5O5+δ double perovskite as an oxygen electrode for reversible solid oxide fuel cell

The conventional oxygen electrode degrades when operated in reversible solid oxide fuel cell (RSOFC) mode. To address this issue, double perovskites have been explored as promising cathode electrodes in RSOFC with proton conducting electrolyte as the electrolyte. In the present study, SmBa0.5Sr0.5Co1.5Fe0.5O5+δ (SBSCF) double perovskite is explored as the oxygen electrode using YSZ as the electrolyte. The SBSCF has been successfully synthesized using a simple and cost-effective solution combustion technique as the oxygen electrode for RSOFC. The structural, morphological and electrical conductivity of SBSCF powder and the electrochemical performance of SOFC containing SBSCF as cathode material are discussed. The performance of the cell in the electrolyzer mode is also discussed.

Recent Development in Reversible Solid Oxide Fuel Cells: Theory, Integration and Prospective

AbstractReversible solid oxide fuel cell (RSOC) has gained widespread attention due to their potential for high efficiency in implementing multi‐energy distributed systems. When high power demand is required, RSOC can operate in the solid oxide fuel cell (SOFC) mode, directly converting the chemical energy from hydrogen or other renewable fuels into electricity. When excess electricity is available, RSOC can operate in the solid oxide electrolysis cell (SOEC) mode, producing fuels through the electrolysis of water or co‐electrolysis of water and carbon dioxide. The reversible operation of RSOC enables the direct conversion between chemical energy and electricity, offering a promising solution for clean and sustainable energy with low cost and high round‐trip efficiency. This paper introduces the research background and working principles of RSOC, provides a detailed overview of the current research status of electrolyte, fuel electrode, and oxygen electrode materials, discusses the optimization design of RSOC stacks and energy utilization strategies in the system. In addition, the future development directions of RSOC were also explored, which is of significant importance for the commercialization of RSOC.

Solid Oxide Cell Technology for Power Generation, Hydrogen Production and Energy Storage

The progress in maturation of solid oxide cell technology has led to development of new applications that would explode its presence in many areas, far beyond power generation. The solid oxide cell technology has the potential to have a formidable presence in production of hydrogen and, eventually, long duration storage of electric power.Following the successful operation of a 200 kW SOFC system under a project supported by Department Energy/NETL, FuelCell Energy (FCE) is pursing the development of SOC based plant configurations from subMW to electrolysis, and further, to energy storage.Multi-faceted evolution of the technology has been underway since the earlier demonstration of SOFC power plant. FCE has developed state-of-the-art lightweight Compact SOFC Architecture (CSA) stacks that are packaged in compact modules with adaptability for use in a variety of configurations and capacities. The CSA stacks can operate directly on a variety of fuels - natural gas, biogas, and hydrogen- without any modification. FCE’s existing fuel cell pilot manufacturing line for CSA cells and stacks includes robotics and automation such as cell screen printing, interconnect subassembly, seal application, QC, as well as stack assembly and conditioning. Via a design for manufacturing approach, as well as focus on minimization of raw material, recent detailed cost studies show a path to low factory stack production cost (<100/kW) at high volumes (1,000 MW/year). The large market existing for power generation equipment, in the range of 200-300kW, is a significant driver for development of high efficiency SOFC products that would easily cater to early-adopters prior to wide-spread acceptance. The accelerated interest in hydrogen as the fuel source will widen the market for SOFC deployment even more. Under a project supported by DOE, FCE is working on design of MW-class SOFC power plants as future extension of the subMW SOFC plant products.FCE is developing a first-of-a-kind 250kW Solid Oxide Electrolyzer Cell (SOEC) system with the hydrogen production capacity of 150kg/day. The overarching goal of the project is to verify that the integration of Solid Oxide Electrolysis Cell (SOEC) systems within nuclear plants will maximize the plants’ efficiency and flexibility and will increase their revenue by switching between electric power generation and hydrogen production. Hybrid nuclear-hydrogen production operations are expected to help the present and future nuclear plants diversify and increase profitability.The 250kW SOEC system is planned to be demonstrated and operated at Idaho National Laboratory (INL). The project will culminate in verification and validation testing and solidify SOEC technology as a low cost and efficient means for hydrogen production integrated within the nuclear power plant environment. The SOEC system will be interfaced with a High-Level front-end Controller (HLC) simulating communications from a nuclear plant and the electric grid. The HLC will determine an optimized hydrogen production schedule to meet all contractual obligations, while maximizing revenue from the integrated operations.FCE is also developing energy storage systems based on the Company’s Solid Oxide Fuel Cell (SOFC) technology. Reversible Solid Oxide Fuel Cell (RSOFC) technology is suitable for medium to long-duration energy storage achieving high round trip electric efficiencies near 70% (electricity-in to electricity-out) at an expected levelized cycle cost of ≤ $0.05 / kWh-cycle.FCE is currently conducting operational tests of an RSOFC prototype system to accomplish the of validation and verification of engineering/pilot-scale RSOFC technology in a relevant environment. A bread-board pilot demonstration system is being utilized to verify cell materials and stack design improvements as well as to validate power electronics and system control strategies that will be utilized for optimization of efficiency, transient response, and lifetime characteristics.

A review on new cobalt-free cathode materials for reversible solid oxide fuel cells

The exponential growth in the requirement of fuel cells and batteries leads to increased demand for cobalt due to its common use in high-performance Li-ion batteries and high-temperature fuel cells/electrolyzers. This sharp increment in demand raises concern about the availability of limited reserves of cobalt which can impact the price of cobalt. Moreover, the geographic limitations of cobalt resources may endanger the whole supply chain. In addition to all those, huge moral issues of cobalt mining are also another problem. Hence, leading battery, fuel cells and electrolyzer manufacturers are looking for sustainable alternatives to reduce cobalt dependency. A more specific limitation is shown in Solid Oxide Fuel Cells (SOFCs) cathode materials that contain cobalt. Incompatibilities have already been observed between the cathode materials containing cobalt and the electrolytes in terms of the thermal expansion coefficient mismatch during the transition of the operating temperature from high to low. An advantage of low operating temperatures is the reduction of material costs compared to high temperature. Increasing the electrochemical performance of the cell and eliminating thermal expansion coefficient difference problems are in concert aimed at the development of cobalt-free cathode materials. Therefore, cobalt-free cathode materials are vital for the sustainability of SOFCs and green transition of the energy sector since they can be used as cathode and anode material in symmetrical SOFCs which is also known as reversible SOFC (RSOFC). In this review, we comprehensively summarize the recent advances of cobalt-free perovskite cathode materials for intermediate temperature RSOFCs.

(Invited) The SOFC Program at the DOE’s Office of Fossil Energy and Carbon Management (FECM) and National Energy Technology Laboratory (NETL)

The Solid Oxide Fuel Cell (SOFC) Program, managed by the U.S. Department of Energy (DOE) Office of Fossil Energy and Carbon Management (FECM) and administered by the FECM’s National Energy Technology Laboratory (NETL) is currently developing low-cost SOFC’s and reversible SOFC systems (rSOCFs). SOFC power systems have the potential to achieve greater than 60 percent efficiency. The SOFC’s operating temperature is lower than combustion-based processes and precludes NOx formation. In addition, there are near-zero emissions of CO2, criteria pollutants, and particulates. Furthermore, SOFC power systems require approximately one-third the amount of water relative to conventional combustion-based power systems. The Program includes the testing of SOFC with natural gas in distributed applications, including data centers. In addition to SOFC, there are several projects on various aspects of high-temperature solid oxide electrolysis cell (SOEC) for electrolysis of water. Development of reversible SOFC systems, rSOCFs, is the new central focus of FECM. The development of efficient systems for hydrogen production is very interesting, especially when supplementary heat is provided from solar or other waste heat sources such as nuclear power plants.

Evaluation of perovskite-type contact paste materials for reversible solid oxide fuel cell stacks

At the electrode/interconnect interface in reversible solid oxide fuel cells (RSOFC), a contact paste is applied during stack fabrication. Careful selection of a contact paste material (CPM) is essential for improving the robustness of the RSOFC stack. For the successful industrialization of RSOFC technology, development of optimized CPM is required. Herein, three lanthanum-based perovskite compositions, LaNi0.6Fe0.2Cu0.2O3-δ (LNFCu), LaNi0.6Fe0.3Cr0.1O3-δ (LNFCr), and LaNi0.5Co0.2Cu0.3O3-δ (LNCCu) were investigated as air electrode CPMs employed in between Crofer 22 APU interconnect (IC) and La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) electrode. More specifically, the three candidates were screened for thermal expansion coefficient (TEC), chemical compatibility with adjacent stack components, area specific resistance (ASR), and their performance as an air electrode CPM in a symmetrical LCFCr RSOFC. All three materials show good chemical compatibility with both the IC and LCFCr, excellent match of TEC with cell components, as well as markedly low ASR values. Most notably, from electrochemical impedance spectroscopy (EIS) study, LNFCr, and LNCCu CPMs exhibit polarization resistance values (Rp), at 800 °C, of ca. 0.83 Ω cm2, which is comparable to that of Au (0.63 Ω cm2), a material commonly regarded as the state-of-the-art. Of the three CPMs studied, it was concluded that LNFCr is the best candidate.

Optimal operation of RSOC integrated energy systems considering multi-state transitions

Reversible solid oxide fuel cells (RSOC) can realize energy storage and power generation in a compact structure, and their high-temperature operation characteristics provide the possibility for combined heat and power generation. Therefore, RSOC can be used as the node of the multi-energy flow intersection of the integrated energy system. The flexible operation mode of RSOC will greatly reduce the operation cost. However, the optimal operation of an integrated energy system including RSOC is very rare at present, so this paper establishes an integrated energy system of the electric gas network including RSOC. On this basis, this paper constructs an RSOC model, a DC power flow model and a linearized gas power flow model considering multiple operating states. Then, based on GAMS, the economic cost of a 24-20 electric gas composite network is optimized. After verifying the reliability of the model, the optimization results considering two working conditions and four working conditions are compared. It is concluded that the model considering multiple operating states of RSOC will reduce the operating cost of the system by 24%.

Exceptionally stable nanostructured air electrodes for reversible solid oxide fuel cells via crystallization-assisted infiltration

Nanostructures present favorable prospects of manufacturing high-performing air electrodes for reversible solid oxide fuel cells (RSOFCs) with the potential to reduce their operating temperature. Here, we present trichloroacetic acid as an original infiltration agent for the facile nanoengineering of RSOFC electrodes. The new process relies on the thermal decomposition of trichloroacetic acid in water at temperatures above 70 °C, which causes intense CO2 effervescence and crystallizes out the metal ions in the solution as slightly soluble carbonates. Essentially, this allows for the subsequent infiltration step to be performed immediately after drying, as opposed to conventional infiltration, which requires high-temperature calcination after each infiltration step. The anode-supported RSOFC consisting of a nanostructured LaCoO3 air electrode permitted smooth switching between fuel cell and electrolysis cell modes with no evidence of degradation. In addition, the RSOFC presented exceedingly durable performance during accelerated stability tests.

Crystal structure and electrical properties of LaNi0.6Fe0.2Cu0.2O3–δ and LaNi0.6Fe0.3Cr0.1O3–δ perovskites: Contact materials for reversible solid oxide fuel cell electrodes

LaNi0.6Fe0.2Cu0.2O3–δ and LaNi0.6Fe0.3Cr0.1O3–δ compositions were selected as air-side contact materials candidates for reversible solid oxide fuel cells (RSOFCs) electrodes based on their structural features and electrical properties. These perovskites were synthesized through a combustion method. Neutron powder diffraction was performed to examine structural modifications of the phases over a wide range of temperatures (from room temperature to 900 ​°C). Both materials exhibit a rhombohedra (R-3c) structure in the measured temperature range. A noticeable oxygen deficiency is observed at the application temperature in RSOFCs (i.e., 800 ​°C), with a refined stoichiometry of oxygen of 2.71(1) and 2.90(1) per formula unit in LaNi0.6Fe0.2Cu0.2O3–δ and LaNi0.6Fe0.3Cr0.1O3–δ at 900 ​°C, respectively. An electrical conductivity (σ) of 234 ​S ​cm−1 was measured for LaNi0.6Fe0.2Cu0.2O3–δ at 800 ​°C, while a slightly higher σ value was achieved for LaNi0.6Fe0.3Cr0.1O3–δ perovskite (277 ​S ​cm−1 at 800 ​°C). An average thermal expansion coefficient (TEC) of ≈13 ​× ​10−6 ​K−1 (25–900 ​°C, air) was measured for both compositions. Furthermore, we confirm compatibility of the synthesized materials with RSOFC stack components: the electrodes (La0.3Ca0.7Fe0.7Cr0.3O3-δ) and interconnects (Crofer 22 APU).

Evaluation of La0.8 Sr0·2MnO3 perovskite prepared by fast solution combustion

La0·8Sr0·2MnO3 (LSM) perovskite as oxygen electrode material for the reversible solid oxide fuel cells (ReSOFC) was synthesized by the fast solution combustion method and assessed for subsequent calcination influence. The microstructural, morphological, compositional and optical properties of the obtained material were analyzed with X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM) coupled with an energy-dispersive X-ray spectroscopy detector (EDS) and UV–visible spectroscopy techniques. The XRD results showed the coexistence of rhombohedral R-3c and Pm-3m polymorphs for the perovskite phase, with a decreased fraction of the cubic phase as the temperature and/or time used for the calcination were increased. The HR-TEM images confirmed the existence of the R-3c and Pm-3m polymorphs for the sample subjected to calcination at 1300 °C, showing that the rapid combustion method did not allow the pure formation of the La0·8Sr0·2MnO3 phase for the calcination temperatures below 1400 °C, due to the swiftness of the combustion synthesis 500 °C for 5 min. The average grain size was found to be increased with the calcination time. The EDS analysis depicted a better agreement in stoichiometry with the theoretical composition. The apparent porosity was decreased with the increase in the temperature and calcination time due to the coalescence of the sintering pores. The sample obtained after the calcination at 1400 °C for 8 h exhibited 1.6% of porosity. The hardness was improved with the higher calcination time and temperature and reached a maximum value of 5.7 GPa that merely matched the bulk density. A similar trend was observed in the temperature dependence resistivity studies and all the samples presented a low resistivity of ∼1.2 Ω cm in the temperature range of 600–700 °C. The optical characterization exhibited a broad absorption in 560–660 nm.

Structural and dielectric properties of ultra-fast microwave-processed La0.3Ca0.7Fe0.7Cr0.3O3−δ ceramics

Perovskite La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr) is a mixed ionic and electronic conductor (MIEC) that can be employed as an electrode material in reversible solid oxide fuel cells (RSOFCs). In this work, an ultra-fast (15 ​min) one-step microwave (MW)-assisted combustion synthesis route has been developed to obtain phase pure and highly crystalline LCFCr powder. The synthesized powders exhibited a sponge-like microstructure with increased electrochemical reaction sites. Neutron thermodiffraction analysis revealed a structural transition above 500 ​°C from the room temperature (RT) orthorhombic Pnma to a rhombohedral R3c perovskite phase. The oxygen vacancy concentration was found to increase from δ ​= ​0.272(7) at RT to δ ​= ​0.333(5) at 900 ​°C. Furthermore, a 3-dimensional G-type antiferromagnetic structure was detected at RT. MW-sintering of pressed green ceramic pellets was carried out at 950 ​°C for 1 ​h, using a MW-transparent quartz fiber crucible or alternatively a SiC crucible acting as a MW-absorber. Impedance spectroscopy data on sintered ceramic pellets revealed electronic inhomogeneity as demonstrated by the occurrence of three dielectric relaxation processes associated with two grain boundary (GB)-like contributions and one bulk. The dielectric inhomogeneity encountered may be restricted to the extrinsic GB areas, which may be rather thin. More homogeneous dielectric properties of the GBs were found in the pellet that was sintered in the SiC crucible.

Development of a dual digital signal processor based cyber-physical system model for a multi-carrier energy system

Multi-energy systems are under exploration for a variety of benefits that can provide multi-energy demands with combined electricity, heat and gas, the most widespread energy demands, to end-users. To satisfy such requirements, this paper integrates a multi-carrier energy system (MCES), including electricity, heat and hydrogen with the cyber-physical system (CPS) model to establish a novel energy CPS. Firstly, the finite state machine (FSM) and the time series method are used to establish a unified CPS architecture of energy nodes. Simultaneously, the embedded architecture of the dual digital signal processor (DSP) system at the energy node level is proposed to implement external control and internal self-control for the energy CPS. Secondly, the CPS modeling method in MCES is conducted by analyzing multi-class energy nodes including energy source nodes, energy conversion nodes, energy storage nodes and energy end-user nodes. Subsequently, the physical system, information system and control system of a reversible solid oxide fuel cell (RSOFC) node and the dynamic operation of MCES are analyzed and simulated by introducing the CPS model, where the controllability and interactivity of the designed energy CPS model are further demonstrated.

Deciphering the Interaction of Single-Phase La0.3Sr0.7Fe0.7Cr0.3O3-δ with CO2/CO Environments for Application in Reversible Solid Oxide Cells.

A detailed study aimed at understanding and confirming the reported highly promising performance of a La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr) perovskite catalyst in CO2/CO mixtures, for use in reversible solid oxide fuel cells (RSOFCs), is reported in this work, with an emphasis on chemical and performance stability. This work includes an X-ray diffraction (XRD), thermogravimetric analysis (TGA), and electrochemical study in a range of pO2 atmospheres (pure CO2, CO alone (balance N2), and a 90-70% CO2/10-30% CO containing mixture), related to the different conditions that could be encountered during CO2 reduction at the cathode. Powdered LSFCr remains structurally stable in 20-100% CO2 (balance N2, pO2 = 10-11-10-12 atm) without any decomposition. However, in 30% CO (balance N2, pO2 ∼ 10-26 atm), a Ruddlesden-Popper phase, Fe nanoparticles, and potentially some coke are observed to form at 800 °C. However, this can be reversed and the original perovskite can be recovered by heat treatment in air at 800 °C. While no evidence for coke formation is obtained in 90-70% CO2/10-30% CO (pO2 = 10-17-10-18 atm) mixtures at 800 °C, in 70 CO2/30 CO, minor impurities of SrCO3 and Fe nanoparticles were observed, with the latter potentially beneficial to the electrochemical activity of the perovskite. Consistent with prior work, symmetrical two-electrode full cells (LSFCr used at both electrodes), fed with the various CO2/CO gas mixtures at one electrode and air at the other, showed excellent electrochemical performance at 800 °C, both in the SOFC and in SOEC modes. Also, LSFCr exhibits excellent stability during CO2 electrolysis in medium-term potentiostatic tests in all gas mixtures, indicative of its excellent promise as an electrode material for use in symmetrical solid oxide cells.

LaNi0.6Co0.4−xFexO3−δ as Air-Side Contact Material for La0.3Ca0.7Fe0.7Cr0.3O3−δ Reversible Solid Oxide Fuel Cell Electrodes

The goal of the current work was to identify an air-side-optimized contact material for La0.3Ca0.7Fe0.7Cr0.3O3−δ (LCFCr) electrodes and a Crofer22APU interconnect for use in reversible solid oxide fuel cells (RSOFCs). LaNi0.6Co0.4−xFexO3 (x = 0–0.3) perovskite-type oxides were investigated in this work. The partial substitution of Co by Fe decreased the thermal expansion coefficient values (TEC) closer to the values of the LCFCr and Crofer 22 APU interconnects. The oxides were synthesized using the glycine–nitrate method and were characterized using X-ray thermodiffraction and 4-probe DC electrical conductivity measurements. Based on the materials characterization results from the Fe-doped oxides investigated here, the LaNi0.6Co0.2Fe0.2O3−δ composition was selected as a good candidate for the contact material, as it exhibited an acceptable electrical conductivity value of 395 S·cm−1 at 800 °C in air and a TEC value of 14.98 × 10−6 K−1 (RT-900 °C).

Proton Transport in Hydrated GdOx Measured with GdCo Magnetic Sensing Layer

Solid state ionic materials such as metal oxide ceramics are of interest due to the wide variety of properties that can be controlled through voltage control of ion concentration and transport in these materials, e.g. electrical resistivity, optical properties, and magnetic properties. Ion movement through a solid oxide electrolyte film can be controlled very simply by electrical gating of the oxide, i.e. applying a voltage across the film. The ease of processing and CMOS compatibility of metal oxides makes them strong candidate materials for next-generation devices for information storage and computing, sensing, energy generation (fuel cells), and energy storage (batteries).Hydrogen ions (protons) take the form of (OH)O • defects in oxides and move by hopping between oxygen sites (Grotthuss mechanism); their small size allows for high conductivity.8 Protonic conduction in oxides has been studied in solid oxide fuel cells (SOFCs)1; proton incorporation has also been shown to change the optical4, electrical2,3, and magnetic properties of the oxide5, or a combination of multiple properties e.g. through inducing phase changes in strontium cobalt oxide (SCO)6. However, current work focuses on materials that require high-temperature processing and operation (>200C, or up to 600-1000C for SOFCs), and bulk or microscale films. This work aims to take solid oxide protonic devices down to the ultrathin (10-100nm) scale, to introduce simpler fabrication techniques (sputtering), and to achieve room-temperature operation of ionic devices. We investigate gadolinium oxide (GdOx) as a model material due to its hygroscopic nature, which allows for fast room-temperature proton conduction.7,8 Using variations of a Pt/Co/GdOx/Au device structure, a gate voltage of 2-5V is applied, initiating a hydrolysis reaction at the top electrode and allowing water to be incorporated into the film. The electric field within the GdOx drives protons to the bottom interface, where it can diffuse into the adjacent magnetic layer and affect a variety of properties. Thin film ferromagnetic materials such as cobalt have been shown to undergo dramatic changes in their magnetic anisotropy with voltage gating, due to the incorporation and removal of hydrogen at the interface with the electrolyte and within the magnetic layer.6 Control of the compensation temperature of a thin film ferrimagnet and control of exchange bias in an antiferromagnet have also been demonstrated with this magneto-ionic proton pump mechanism.9,10 Using a gadolinium cobalt (GdCo) magnetic layer inserted into the MOM device stack to detect the presence of hydrogen, we have developed a sensor for use in a novel time-domain proton transport measurement. We apply short voltage pulses to the device and use magneto-optic Kerr effect (MOKE) to observe the resulting changes in the compensation temperature and magnetic state of the sensor layer. This can be used as a more direct measure of the presence of protons at the bottom interface, as compared to frequency domain measurements such as electrochemical impedance spectroscopy (EIS). We also show that humidity affects the proton transport in the oxide layer, using neutron and x-ray reflectometry to observe the growth of a gadolinium hydroxide (Gd(OH)3) layer with high hydrogen concentration upon exposing the film to a high humidity environment.11 References Q. Minh, M. B. Mogensen. Reversible Solid Oxide Fuel Cell Technology for Green Fuel and Power Production. Electrochem. Soc. Interface, 55–62 (2013).Huang et al. Three-terminal resistive switch based on metal/metal oxide redox reactions, Scientific Reports, 7 (2017).Messerschmitt, F. Kubicek, M., Rupp, J. L. M. How Does Moisture Affect the Physical Property of Memristance for Anionic–Electronic Resistive Switching Memories?, Advanced Functional Materials, 25, 5117-5125 (2015).Huang, M., Jun Tan, A., Büttner, F. et al. Voltage-gated optics and plasmonics enabled by solid-state proton pumping. Nat Commun 10, 5030 (2019).Tan et al. Magneto-ionic control of magnetism using a solid-state proton pump, Nature Materials, 17 (2018).Lu, N. et al. Electric-field control of tri-state phase transformation with a selective dual-ion switch, Nature 546, 124–128, (2017)Norbya, T. Proton conduction in oxides, Solid State lonics, 40/41, 857-862 (1990).Larring and Norby. Protons in rare earth oxides, Solid State Ionics, 77 (1995).Huang, M., Hasan, M.U., Klyukin, K. et al. Voltage control of ferrimagnetic order and voltage-assisted writing of ferrimagnetic spin textures. Nanotechnol. 16, 981–988 (2021).Zehner, J. et al. Magnetoionic control of perpendicular exchange bias. Phys. Rev. Materials 5, L061401 (2021).Tan, A.J., Huang, M., Sheffels, S., et al. Hydration of gadolinium oxide (GdOx) and its effect on voltage-induced Co oxidation in a Pt/Co/GdOx/Au heterostructure. Phys. Rev. Materials 3, 064408 (2019). Figure 1

Towards in Operando XAS Investigation of CO2 Electrolysis in Solid Oxide Cells: Synchrotron Studies of La0.3Ca0.7Fe0.7Cr0.3O3-δ Perovskites

Solid oxide cells (SOCs) are poised to provide a significant contribution to the emerging field of green technology. SOCs could form the basis for a closed loop, sustainable energy system in which fuel cells generate power efficiently and any waste products are converted back into fuel or useful products.Early solid oxide fuel cell (SOFC) systems utilized ceramic-metal composites, such as Ni-YSZ, as the fuel electrodes. Ni metal provides electronic conduction while YSZ plays the role of oxygen anion conductor. This requires the catalyst target (e.g., H2) to come into contact with both Ni and YSZ, forming a so-called triple phase boundary.Cermet systems perform well but are typically unstable under oxidizing conditions and are susceptible to passivating phenomena, such as sulfur poisoning. Since then, mixed ionic and electronic conductors (MIECs) have been developed, where a single material such as a perovskite replaces both cermet components. The gas-MIEC interface acts as a double phase boundary, theoretically providing electrochemical reaction sites on the entire surface of the electrode. This makes them less susceptible to poisoning by impurities, and the oxide composition can be tailored to improve stability in reducing or oxidizing environments. SOFCs based on mixed-conducting catalysts have shown marked performance increases over room-temperature fuel cells in ORR catalysis, often with an order of magnitude advantage in current response. Further work led to MIEC compositions being used at SOFC fuel electrodes, and more recently, as active catalysts for the carbon dioxide reduction (CO2RR) reaction in electrolysis cells (SOECs).Our research group recently demonstrated that the perovskite material, La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr), shows excellent stability and activity during the OER and ORR. This single composition is also highly active for the CO2RR, opening the door for reversible and symmetrical SOC devices (RSOFCs) that promise to greatly reduce device complexity. This should ease the difficulty of scaling up these technologies for industrial applications. While we have observed performance gains in both SOC modes of operation using LCFCr, we sought to better understand the underlying mechanism of catalysis in CO2 electrolysis. This has led us to begin a thorough investigation of LCFCr-based reversible SOFCs using synchrotron x-ray absorption spectroscopy (XAS), culminating with in operando studies, where CO2 electrolysis can be tracked as it happens in fully operational electrolysis cells. Harsh operating conditions have made in situ or in operando SOC studies more challenging than those for RT cells, which also share the barriers of access to electrode surfaces and the attenuation effect of feedstocks on incident energy used for spectroscopy.This presentation will introduce our initial efforts in synchrotron XAS studies of the as-synthesized LCFCr material, which was prepared by combustion synthesis of precursor metal nitrates. Synchrotron energy provided by the Canadian Light Source (CLS) was used to study powder samples of this composition; experiments were carried out at both hard (SXRMB) and soft (SGM) x-ray sources at the CLS. Preliminary experiments consisted of ex situ XAS testing of samples heat treated using conditions similar to those encountered in operation (800 °C in air or CO2). When compared to spectra of as-synthesized compositions, this provides a conventional “before and after” comparison and has demonstrated that overall changes in oxidation state and coordination environment could be observed. In situ testing was also carried out at SXRMB on La0.3Ca0.7Fe0.7Cr0.3O3-δ powder samples. Samples were placed in a “six-shot” apparatus that provided a versatile test bed for semi-realistic conditions. In these studies, materials were heated in air to 800 °C, while XANES spectra (Fe K-edge) were collected simultaneously. This process was repeated as the gas was changed to CO2, and then upon cooling. These transitionary XAS measurements allowed us to observe both structure and valence state of the material responding in real time. The results showed changes in the pre-edge feature and edge intensity that we have attributed to a change in local vacancy concentration around Fe metal centers. At each stable set of conditions, full EXAFS spectra were collected for all four metals; initial analysis suggests that chromium undergoes redox changes while the iron valence state remains largely unchanged. Together, this set of experiments is allowing us to assemble a dynamic picture of the chemical and structural state of the catalyst during operation.This study underscores the value of in situ and in operando investigation of SOC catalysts compared to traditional ex situ analysis. Insights into efficient SOC electrolysis of CO2 could give us the ability to reduce carbon emissions. It is both urgent and vital that the underlying mechanisms are understood more completely.

A Simplified Model for Reversible Solid Oxide Fuel Cells

: Butler-Volmer Equations based on a first order approximation of global redox reaction kinetics have been used in the past successfully for design analysis of solid oxide fuel cells. Such a model forms the backbone of the widely used multidimensional code, DREAM-SOFC originally developed at West Virginia University and currently being used by the fuel cell research team at the US DOE lab, National Energy Technology Laboratory (NETL). Recent interest in using solid oxide cells (SOCs) for hydrogen production has prompted the question as to whether the Butler-Vomer Model (BVM) empirically calibrated and used in DREAM-SOFC would be appropriate for simulation of reversible fuel cells as well. To this end we have simulated anode supported Ni/YSZ/LSCF button cells in reversible mode and found that the BVM is not capable of reproducing the trends observed in experiments performed at various applied steam, hydrogen and oxygen concentrations, even if some key parameters such as effective bulk diffusivity and exchange current density are treated as calibration parameters and adjusted freely. To bridge this gap, we modified the oxygen electrode model using a simplified version of the single pathway oxygen reduction reaction equations originally put forth by Svensson et al. [1]. In the simplified model, it is assumed that the adsorbed oxygen concentration remains close to its equilibrium value in most of the oxygen electrode active layer. This assumption eliminates the need to solve the more difficult, stiff surface coverage transport equation leaving only the oxygen ion vacancy concentration to be solved for, thus simplifying the model greatly. This paper presents the mathematical model, and an in-depth analysis of the results from this model applied to button cells with LSCF and LSM electrodes. Figure 1 shows the comparison between the simulated and experimental polarization curves. As shown, good agreement is achieved between simulations and experiments for fuel cell and electrolysis modes. We conclude that by extending the BVM only by adding the vacancy concentration transport equation, leads to a simple yet sufficient model with which one can perform very fast calibration and design analysis of reversible SOCs with minimal computation time.Reference:[1] A.M. Svensson, et al., J. Electrochem. Soc., 145(4), 1390 (1998) doi: 10.1149/1.1838471 Figure 1

Chemical Stability and CO2 Electrolysis Performance of La0.3Sr0.7Fe0.7Cr0.3O3- δ in CO2/CO Environments

LMFCr (M = Sr, Ca) has emerged as a highly promising class of perovskite oxide materials for use as symmetrical electrodes in reversible solid oxide fuel cells (RSOFCs).1 While there are numerous studies on the efficacy of these materials as symmetrical electrodes for SOFCs,1-5 a detailed study on their chemical stability and electrochemical performance as fuel electrodes in CO2/CO environments has not yet been reported. This work aims to address the stability of the Sr analogue of LMFCr, namely, La0.3Sr0.7Fe0.7CrO3-δ (LSFCr), in low pO2 environments consisting of CO2/CO gas mixtures (10-11-10-18 atm) to establish the range of pO2 over which the electrode material is structurally and electrochemically stable. This is relevant under CO2 reduction conditions as the stability limit will define the allowable CO2 utilization in these cells in practice.The chemical and phase stability was studied on LSFCr powders and pellets via XRD, TGA, and SEM in different pO2 atmospheres ranging from pure CO2, CO (balance N2), and 90-70% CO2:10-30% CO containing mixtures. Powdered LSFCr remains structurally stable in 20-100% CO2 (balance N2, pO2 = 10-11 – 10-12 atm) from room temperature to 800 oC without any decomposition. On the other extreme, a reversible decomposition occurs at 800 oC in 30% CO (balance N2, pO2 ~ 10-26 atm) resulting in the formation of a Ruddlesden-Popper phase, Fe nanoparticles (NPs), and some carbon fibers, originating from the Fe NPs. However, this decomposition can be reversed by heat treatment in air at 800 oC. While no evidence for coke formation is obtained in 90-70% CO2:10-30% CO (pO2 = 10-17 – 10-18 atm) mixtures at 800 oC, in 70CO2:30CO, minor impurities of SrCO3 and Fe NPs are observed, with the latter potentially being beneficial to the electrochemical activity of the perovskite.Electrochemical characterization was carried out on symmetrical cells prepared by printing ~25 µm single phase LSFCr electrode layers, with an area of ca. 0.5 cm2, on both sides of a 1” diameter SDC buffered YSZ electrolyte, followed by painting on Au paste as the current collectors. The cells were mounted in an open flanges cell testing setup (Fiaxell, Switzerland) and supplied with 50 mL/min air at the oxygen electrode and 50 mL/min CO2:CO mixtures at the fuel electrode. The performance of full cells was determined in various CO2:CO ratios (Pure CO2, 90:10, and 70:30) at 800 oC via electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry. Consistent with prior work, symmetrical 2-electrode full cells showed very good electrochemical performance at 800 oC, both in the SOFC and SOEC modes. Furthermore, LSFCr full cells exhibited excellent short-term stability in 15-min potentiostatic stability tests at 1.2-1.5 V, while medium-term potentiostatic tests at 1.1 and 1.2 V showed that LSFCr exhibits exceptional stability during CO2 electrolysis in all gas atmospheres over several 100 hours, with the oxygen electrode facilitating the oxygen evolution reaction (OER) at the same time. These results suggest that LSFCr has the potential to be a very efficient and stable electrode material for symmetrical SOC applications. References (1) Chen, M.; Paulson, S.; Thangadurai, V.; Birss, V. Journal of Power Sources 2013, 236, 68-79.(2) Chen, M.; Paulson, S.; Thangadurai, V.; Birss, V. ECS Transactions 2012, 45, 343-348.(3) Chen, M.; Paulson, S.; Kan, W. H.; Thangadurai, V.; Birss, V. Journal of Materials Chemistry A 2015, 3, 22614-22626.(4) Molero-Sanchez, B.; Addo, P.; Buyukaksoy, A.; Paulson, S.; Birss, V. Faraday Discussions 2015, 182, 159-175.(5) Addo, P. K.; Molero-Sanchez, B.; Chen, M.; Paulson, S.; Birss, V. Fuel Cells 2015, 15, 689-696.

A Simplified Model for Reversible Solid Oxide Fuel Cells

A simplified reversible steady-state solid oxide cell model is formulated and solved analytically with only a few calibration parameters to match experimental performance data. A Butler-Volmer type model is used for the hydrogen electrode while a two-step single pathway reduced-order oxygen reduction reaction model is applied in the oxygen electrode. It is shown that this simple model is capable of accurately predicting the trends of V-I data produced experimentally at various gas concentrations in both fuel cell and electrolysis modes.