High resolution EDS study of Ba2Y1+xNb1‐xO6‐δ sintered compounds

Electrochemical performance optimization of a conventional solid oxide fuel cell (SOFC) has been massively performed with computational fluid dynamic (CFD) modelling but it’s usage in a proton conducting fuel cell (PCFC) is still minimal. PCFC is a category of SOFC but with proton conductor as electrolyte instead of oxygen-ion conductor in a conventional SOFC. The fabrication of high electrochemical performance of PCFC is desirable because of its ability to operate at lower temperature. The objective of this short review study is to explore the possibilities of CFD modelling application to improve electrochemical performance of a PCFC system. Some CFD study that have been done to SOFC and PCFC were reviewed. One main finding from this short review is that the application of CFD modelling in PCFC design optimization is still minimal. There is a lack of studies that focus on the impact of PCFC anode microstructure on transport phenomena of the PCFC; for example on gas diffusion. It was also found that CFD modelling software Ansys Fluent with add-on Fluent SOFC module that is widely applied to conventional oxygen-ion SOFC need to be modify using User Defined Function (UDF) in order to be used in PCFC system.

Ceramic high-temperature fuel cells and electrolysers are efficient energy conversion systems for electrical power generation and hydrogen production. Their core components are constituted by a stack of electroactive Solid Oxide Cells (SOCs) in which the electrochemical reactions take place. Thanks to their flexibility, the same stack can be alternatively operated in both fuel cell and electrolysis modes. However, the insufficient durability of SOCs still constitutes a major limitation for the technology. The present study addresses this issue and aims to bring some new insights on the effect of the Solid Oxide Fuel Cell (SOFC) versus Solid Oxide Electrolysis Cell (SOEC) operating modes on degradation of a typical Ni-YSZ//YSZ//CGO//LSCF-CGO cell. The electrochemical degradations are generally attributed to several underlying phenomena such as electrode microstructural evolution, material chemical decomposition or electroactive sites poisoning by contaminants. Among them, it is generally considered that Ni agglomeration in the Ni-YSZ cermet and Lanthanum Strontium Cobalt Ferrite (LSCF) material destabilization are two prevalent mechanisms involved in the cell performance deterioration. Therefore, these two mechanisms have been specifically investigated by a coupled approach of long-term testing in both SOFC and SOEC modes (1000 ≤ t (h) ≤9000) and post-test characterizations. The experimental results have then been analyzed in the frame of an in-house multi-scale model with the purpose to interpret them and to quantify the effect of material ageing on cell performances. The extent of Ni agglomeration has been characterized by three-dimensional electrode reconstructions obtained by X-ray nano-holotomography at European Synchrotron Radiation Facility (ESRF) (on the new Nano-Imaging beamline ID16A-NI). The new set-up and protocol enable the reconstructions of valuable 3D volumes (Fig. 1) with a large field of view (~50 µm) along with a high spatial resolution (~50 nm). The electrode morphological properties, which have been measured on the 3D volumes, have revealed a substantial Ni coarsening over time at 850°C and 750°C, whereas no Ni depletion was detected at the electrolyte interface. The increase of the Ni particle size is found to induce a decrease in both (i) the density of Triple Phase Boundary (TPBs) lines and (ii) the interfacial surface area between Ni and gas. Moreover, it was found that the Ni/YSZ interfacial surface area does not evolve during the experiments. This statement indicates that the ceramic backbone in the cermet prevents a massive Ni agglomeration at the SOFC/SOEC operating temperature. The compilation of all experimental data have allowed fitting the parameters of a physically-based law for Ni coarsening that was introduced in the modelling framework. The simulations have revealed that Ni agglomeration explains around 20-25% of the electrochemical degradation at 850°C after 1000 hrs of operation. However, the electrode microstructural evolution is found not to be affected by the cell polarizations. Therefore, the mechanism cannot explain the higher degradation rates recorded in electrolysis mode compared to the fuel cell ones. To explain the impact of the operating modes (SOFC or SOEC) on the degradation rates, several post-test analyses (i.e. Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray µfluorescence and µdiffraction techniques) have been employed to investigate the phase reactivity in the region of the CGO barrier layer. The characterizations have revealed that Sr diffusion across the barrier layer and formation of SrZrO3 secondary phase occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode (Fig. 2). As a consequence, LSCF destabilization is found not to be involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. The post-test analyses have also revealed a diffusion and an accumulation of Co in the region of the barrier layer which is concomitant with the formation of SrZrO3. The formation of these Co-rich segregates in contact with SrZrO3 grains have been identified as cobalt-ferrite type compound. The in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF destabilization mechanism. The cell polarization curves and the local quantities within the O2 electrode have been computed in both fuel cell and electrolysis modes. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in the LSCF material. It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation has been detailed. Figure 1

Thin-film solid oxide fuel cells are receiving increasing attention within the scientific community thanks to their high energy density and high efficiency in different temperature ranges. To be utilized as power supply systems for portable electronic devices, thin-film solid oxide fuel cells must demonstrate high power supply and low-temperature operation capabilities. Both capabilities are highly dependent on the function of the electrolyte layer, commonly comprised of ceramic mixed-ionic-electronic conductors [1]. The state-of-the-art thin-film solid oxide fuel cells utilize yttrium stabilized zirconia as an electrolyte material, despite its relatively low ionic conductivity within the typical working-temperature range of solid oxide fuel cells (up to 1000 °C). Moreover, the ionic conductivity of yttrium stabilized zirconia dramatically reduces with decreased temperature, limiting its performance as an electrolyte material for low-temperature (below 600 °C) solid oxide fuel cells [2].Gadolinia doped ceria is an alternative promising electrolyte material for low temperature solid oxide fuel cells. Its relatively high ionic conductivity below 600 °C makes it attractive as an electrolyte material although its partial electronic conductivity is a significant drawback [3]. This drawback is more pronounced in ultra-thin layers (sub-micron scale) solid oxide fuel cells devices. As a result, gadolinia doped ceria have been utilized only to a limited extent for thin-film solid oxide fuel cells.The issue of partial electronic conductivity might be resolved thanks to a non-conventional deposition technique of polycrystalline gadolinia doped ceria thin films. The proposed sputtering technique includes metallic alloy target and results in a granular morphology, as opposed to the columnar morphology obtained using the conventional sputtering method. As electrical conductivity of polycrystalline thin films is higher along the grain boundaries [4], gadolinia doped ceria layers with random crystallographic orientation are expected to be less electronically conductive. Here, we investigated the integration and the performance of granular gadolinia doped ceria as an electrolyte material for thin-film solid oxide fuel cells. The electronic-conduction properties along the grain boundaries of gadolinia doped ceria were studied, and a thin-film solid oxide fuel cell prototype was fabricated. Such devices with engineered grain boundaries hold the potential for low-temperature operation, high power output, and silicone-compatibility for novel integration opportunities and bulk-fabrication.

Proton conductors are attractive materials with a wide range of potential applications such as proton-conducting fuel cells (PCFCs). The conventional strategy to enhance the proton conductivity is acceptor doping into oxides without oxygen vacancies. However, the acceptor doping results in proton trapping near dopants, leading to the high apparent activation energy and low proton conductivity at intermediate and low temperatures. The hypothetical cubic perovskite BaScO2.5 may have intrinsic oxygen vacancies without the acceptor doping. Herein, we report that the cubic perovskite-type BaSc0.8Mo0.2O2.8 stabilized by Mo donor-doing into BaScO2.5 exhibits high proton conductivity within the ‘Norby gap’ (e.g., 0.01 S cm−1 at 320 °C) and high chemical stability under oxidizing, reducing and CO2 atmospheres. The high proton conductivity of BaSc0.8Mo0.2O2.8 at intermediate and low temperatures is attributable to high proton concentration, high proton mobility due to reduced proton trapping, and three-dimensional proton diffusion in the cubic perovskite stabilized by the Mo-doping into BaScO2.5. The donor doping into the perovskite with disordered intrinsic oxygen vacancies would be a viable strategy towards high proton conductivity at intermediate and low temperatures.

Chapter 12 - Advanced Materials for Solid Oxide Fuel Cells and Membrane Catalytic Reactors

Solid Oxide Fuel Cells (SOFCs) are one of the most attractive energy conversion and storage devices, which can be applied for decentralized energy applications. However, the high operating temperature making SOFCs still unmarketable, which indicates that lowering the working temperature of SOFCs while still maintaining a high-power output is very crucial. For Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs), new cathode materials with high electrocatalytic activity and stability are necessary to provide a stable and high performance of cells. Perovskite-structured oxide (ABO3−δ) is one group of the most interesting material candidates for IT-SOFCs, presenting great potential in chemical composition modifications, yielding the design and gain of desired physicochemical and electrochemical properties [1]. The simple perovskite LaCuO3 is one of the well-studied Cu-content oxides with a high conductivity (106 S·cm−1), but it can be hardly obtained and suffers with stability issues in air [1, 2]. To stabilize the perovskite structure and to improve electrochemical properties of Cu-content oxides A-site and B-site modification strategy can be applied. It has been noted that Cu- and Ni-containing LaNi0.5Cu0.5O3−δ simple perovskite possesses low cathodic polarization of 0.056 Ω·cm2 at 800 °C, and a high power output of 870 mW·cm−2 at 900 °C [1, 3]. The generation of oxygen vacancies by strontium doping contributes to the increase of ionic conductivity component, and the Sr dopant can reduce the oxygen vacancy formation energy and increase the ionic conductivity of La1−xSrxMO3−δ (M = Fe, Mn) perovskites [1,4]. Therefore, in this work, Cu-content La1−xSrxNi1−yCuyO3−δ oxides with strontium doping at the A-site were evaluated as very promising Co-free cathode material candidates for IT-SOFCs.In this work, all La1−xSrxNi1−yCuyO3−δ compounds have been successfully obtained using soft chemistry. The room-temperature crystal structure of all oxides was assigned to the R-3c trigonal system. The substitution of La with Sr lowers the phase transition temperature from the R-3c space group to Pm-3m simple perovskite while increases the oxygen non-stoichiometry and the thermal expansion coefficient of studied materials. All investigated materials are chemically compatible with GDC-10, while unstable in contact with LSGM and 8YSZ. The manufactured anode-supported IT-SOFC with the La0.95Sr0.05Ni0.5Cu0.5O3−δ cathode presents an excellent power density of 445 mW·cm−2 at 650 °C in humidified H2. The results confirmed that La1−xSrxNi1−yCuyO3−δ perovskites can be potentially qualified as promising cathode candidates for IT-SOFCs, yielding promising electrochemical performance in the intermediate-temperature range [1].

Two perovskite materials with SrMo1−xAlxO3−δ (x = 0.1, 0.2) compositions have been synthesized by reduction from the corresponding scheelite phases, with SrMo1−xAlxO4−δ stoichiometry; the pertinent characterization shows that the defective perovskites can be used as anode materials in solid oxide fuel cells, providing maximum output power densities of 633 mW/cm2 for x = 0.2. To correlate structure and properties, a neutron powder diffraction investigation was carried out for both perovskite and scheelite phases. Both perovskites are cubic, defined in the Pm-3m space group, displaying a random distribution of Mo and Al cations over the 1a sites of the structure. The introduction of Al at Mo positions produced conspicuous amounts of oxygen vacancies in the perovskite, detected by neutrons. This is essential to induce ionic diffusion, providing a mixed ionic and electronic conduction (MIEC), since in MIEC electrodes, charge carriers are combined in one single phase and the ionic conductivity can be one order of magnitude higher than in a conventional material. The thermal expansion coefficients of the reduced and oxidized samples demonstrated that these materials perfectly match with the La0.8Sr0.2Ga0.83Mg0.17O3−δ electrolyte, La0.4Ce0.6O2−δ buffer layer and other components of the cell. Scanning electron microscopy after the test in a real solid oxide fuel cell showed a very dense electrolyte and porous electrodes, essential requirements for this type of fuel. SrMo1−xAlxO3−δ perovskites are, thus, a good replacement of conventional biphasic cermet anodes in solid oxide fuel cells.

Layered double perovskite materials of type AA’B2O5+δ (where A is rare earth element and A’ is alkaline earth element) are suggested as potential candidate for a cathode in solid oxide fuel cells (SOFCs) because of their distinct oxygen-deficient perovskite-related structure, higher oxygen ion diffusion rate and surface exchange coefficient following a stacking sequence of /BO/AO/BO/A’O/BO/... [1,2]. In general, the layered double perovskites with rare earth metals at A-site, Ba at A’-site and Co at B-site have received much attention due to high oxygen vacancy concentration, high electronic conductivity and high catalytic activity. Presence of Pr, Gd and Nd at the A-site of the structure are observed to improve electronic conductivities, oxygen self-diffusion, oxygen exchange, resulting into reduced area specific resistances (ASRs) of the electrode [1-3]. In this study, Density Functional theory (DFT) calculations are performed for NdBaCo2O5+δ (NBCO) and Sr-doped at NBCO to study and compare oxygen vacancy formation energies in different planes and surface energies for different terminating surfaces of the NBCO material and access the role of Sr doping.Using DFT with Hubbard U correction of 3.3 eV for Co, oxygen vacancy formation energies in Ba/O, Co/O and Nd/O planes of bulk are observed as 245.88 KJ/mol, 144.93 KJ/mol and 72.13 KJ/mol respectively. DFT calculations predicted difficulty in creating oxygen vacancies in Ba/O plane as compared to Co/O and Nd/O plane of bulk NBCO. On doping one fourth and half of Ba with Sr resulted into a decrease in oxygen vacancy formation energies in BaSr/O plane of bulk i.e., less energy is needed to remove oxygen in BaSr/O plane. Further the Sr-doping at A’-site have shown decrease in lattice parameters. The energetics of surface with terminating surfaces Ba/Co, Nd/Co, Co/Ba and Co/Nd in (001) direction revealed Ba/Co and Co/Nd surface as the stable terminal surfaces. The trend of surface energies revealing Ba/Co surface as most stable in NBCO material is found to be similar as predicted in our previous studies on PrBaCo2O5+δ, GdBaCo2O5+δ[4,5]. Different energetics of the surfaces play an important role in the electrochemical performance of the cathode material. In conclusion, Sr-doping at A’-site in is likely to improve the energetics of oxygen vacancy formation in the BaSr/O planes as compared to undoped NBCO material.

Ti-doped Sr2Fe1.4-xTixMo0.6O6-δ double perovskites with improved stability as anode materials for Solid Oxide Fuel Cells

Improvement of ionic conductivity in A-site lithium doped sodium bismuth titanate

We report on a series of SrMo1−xMnxO3−δ perovskite oxides designed as potential anode materials for solid oxide fuel cells (SOFCs). These materials were synthesized using a citrate method, yielding scheelite-type precursors with nominal SrMo1−xMnxO4 compositions, which were further reduced to obtain the active perovskite oxides. Their structural evolution was examined through X-ray diffraction (XRD) and neutron powder diffraction (NPD). These techniques provided insights into the crystallographic changes upon Mn doping, revealing key factors influencing ionic conductivity. Whereas the oxidized scheelite precursors are tetragonal, space group I41/a, the reduced perovskite specimens are cubic, space group Pm-3m, and show the conspicuous absence of oxygen vacancies, even at the highest temperature of 800 °C. The transport properties were analyzed through electrical conductivity measurements, exhibiting a metallic-like behavior. Thermogravimetric analysis (TGA) and dilatometry give insights into the thermal stability and expansion behavior, essential for SOFC operation. Test single SOFCs were built in an electrolyte-supported configuration, on LSGM pellets of 300 μm thickness, assessing the performance of the title materials as anodes. This work emphasizes the critical relationship between the crystal structure and its electrochemical behavior, providing a deeper understanding of how doping strategies can optimize fuel cell performance.

Performance durability is one of the essential requirements for solid oxide fuel cell materials operating in the intermediate temperature range (500–700 °C). The trade‐off between desirable catalytic activity and long‐term stability challenges the development and commercialization of electrode materials. Here an oxygen cathode material, Ba0.5Sr0.5(Co0.7Fe0.3)0.69−xMgxW0.31O3−δ (BSCFW‐xMg), that exhibits excellent electrocatalytic performance through the addition of an optimized amount of Mg to the self‐assembled nanocomposite Ba0.5Sr0.5(Co0.7Fe0.3)0.69W0.31O3−δ (BSCFW) by simple solid‐state reaction is reported. Distinct from the bulk and surface approaches to introduce vacancies and defects in materials design, here the Mg2+ ions concentrate at the single perovskite/double perovskite interface of BSCFW with dislocations and Mg2+‐rich nanolayers, resulting in stressed and compositionally inhomogeneous interface regions. The interfacial chemistry within these nanocomposites provides an additional degree of freedom to enable performance optimization over single phase materials and promotes the durability of alkaline‐earth based fuel cell materials.

The National Energy Technology Laboratory (NETL) has undertaken a review of coal gasification technologies that integrate with solid oxide fuel cells (SOFC) to achieve system efficiencies near 60% while capturing and sequestering >90% of the carbon dioxide [1–2]. One way to achieve an overall system efficiency of greater than 60% is in a power plant in which a catalytic coal gasifier produces a syngas with a methane composition of roughly 25% on a dry volume basis and this is sent to a SOFC, with CO2 capture occurring either before or after the SOFC. Integration of a catalytic gasifier with a SOFC, as opposed to a conventional entrained flow gasifier, is improved due to (a) decreased exergy destruction inside a catalytic, steam-coal gasifier producing a high-methane content syngas, and (b) decreased exergy destruction in the SOFC due to the ability to operate at lower air stoichiometric flow ratios. For example, thermal management of the SOFC is greatly improved due to the steam-methane reforming in the anode of the fuel cell. This paper has two main goals. First, we converted the levelized cost of electricity (LCOE) estimates of various research groups into an average internal rate of return on investment (IRR) in order to make comparisons between their results, and to underscore the increased rate of return on investment for advanced integrated gasification fuel cell systems with carbon capture & sequestration (IGFC-CCS) compared with conventional integrated gasification combined cycle (IGCC-CCS) systems and pulverized coal combustion (PCC-CCS) systems. Using capital, labor, and fuel costs from previous researchers and using an average price of baseload electricity generation of $61.50 / MW-hr, we calculated inflation-adjusted IRR values of up to 13%/yr for catalytic gasification with pressurized fuel cell and carbon dioxide capture and storage (CCS), whereas we calculate an IRR of ∼4%/yr and ∼2%/yr for new, conventional IGCC-CCS and PCC-CCS, respectively. If the carbon dioxide is used for enhanced oil recovery rather than for saline aquifer storage, then the IRR values improve to 16%/yr, 10%/yr, and 8%/yr, respectively. For comparison, the IRR of a new conventional IGCC or PCC power plant without CO2 capture are estimated to be 11%/yr and 15.0%/yr, respectively. Second, we conducted an exergy analysis of two different configurations in which syngas from a catalytic gasifier fuels a SOFC. In the first case, the CO2 is captured before the SOFC, and the anode tail gas is sent back to the catalytic gasifier. In the second case, the anode tail gas is oxy-combusted using oxygen ion ceramic membranes and then CO2 is captured for sequestration. In both cases, we find that the system efficiency is greater than 60%. These values compare well with previous system analysis [1–4]. In future work, we plan to calculate the IRR of these two cases and compare with previous economic analyses conducted at NETL [1–2].

The solid oxide fuel cells (SOFCs) have been considered as important alternative energy technology, due to a variety of advantages, such as their high energy conversion efficiency, free noxious emissions, and direct use of a wide range of hydrocarbon fuels. Especially, the reverse operation of SOFCs, which is called solid oxide electrolysis cell (SOEC), is currently one of the most promising technologies for fuel production. However, their high-temperature operation (typically >700°C) leads to the rapid degradation of electrodes and necessitates a high balance of plant costs. To make SOFCs more economically feasible and viable, further reduction of operating temperature is inevitable. To this end, the utilization of proton-conducting oxides, which exhibit relatively high ionic conductivity with the low activation energy (<0.5eV), enables protonic ceramic fuel cells (PCFCs) to operate at an intermediate temperature regime (IT, 400–600°C), where the aforementioned benefits of SOFCs can also be accomplished. Moreover, PCFCs prevents the dilution of the fuel during the operation and provides higher resistance of carbon cocking on hydrocarbon fuels.Although the PCFCs have distinct advantages against traditional oxygen conducting SOFCs, the lower-than-predicted performance of PCFCs and scalability issues caused the considerable skepticism of the successful applicability of PCFCs. Most importantly, unlike traditional SOFCs, the development of mixed oxide ion and electron conductors (MIECs) for oxygen reduction reaction (ORR) at the cathode of PCFCs has shown marginal success due to the absence of protonic conduction and still needs to be improved. At the cathode, the ORR is limited to triple phase boundary (TPB), where the electrolyte, gas, and cathode mutually contact, and its geometrical enhancement at the interface area between cathode and electrolyte layers is crucial to improve the performance of PCFCs. Therefore, electrode-electrolyte interface architecting via micro-patterning is one of the most attractive approaches, since it significantly expands the catalytic active sites of TPB near the interface, where ORR preferentially occur. Moreover, this architecting approach also facilitates the use of currently viable state-of-art cathode materials of SOFCs; Lanthanum Strontium Cobalt Ferrite (LSCF), Barium Strontium Cobalt Ferrite (BSCF). Therefore, patterns with a high aspect ratio, which maximize the interface area, are required for significant enhancement of electrochemical performance.Here, for the first time, we describe a three-dimensionally micro-architecting process for anode-supported PCFCs, demonstrating dramatic performance enhancement from the expanded interface area between cathode and electrolyte. The various patterns with high ratios were exactly transferred from the original PUA (polyurethane acrylate) molds to the thin and dense protonic conducting electrolyte layer. The patterns maintained the same aspect ratios during the sintering process and finally increased the interface area of more than 30%. The morphology of the patterned surface was characterized by scanning electron microscope (SEM) and atomic force microscopy (AFM). The AC electrochemical impedance spectroscopy was utilized to examine the electrochemical performances of patterned cells. Our unique patterning method opens the new pathway for further performance enhancement of PCFCs. Figure 1

Acceptor-doped barium zirconate is one of the most promising proton conducting materials for stationary hydrogen fuel cells. Dopant-defect proton traps shape the proton conduction landscape. Inspired by findings that oxygen vacancies may decrease trapping near some dopant defects ( Chem. Mater. 2018, 30, 4919−4925), the effect of oxygen vacancies on the proton conduction landscape of barium zirconate is explored at 12.5% doping with aluminum, scandium, and yttrium dopant at the zirconium site. Density functional theory (DFT) with the PBE functional in the Vienna ab initio simulation package (VASP) was used to find the electronic energy for barium zirconate configurations. The conjugate-gradient minimization method is used to find the lowest energy structures and the climbing nudged elastic band (cNEB) method is used to find transition states. As the dopant (D) ion radius increases, the lattice size expands from 4.24 to 4.29 Å and increases DO6 octahedral tilting in barium zirconate. Inclusion of an oxygen vacancy broadens ZrOD and ZrOZr angle distributions. While there are three distinct minima for oxygen vacancy locations, relative energies of minima and transition states show that only the dopant nearest neighbor oxygen vacancy is significant for aluminum- and scandium-doped barium zirconate. In contrast, the yttrium-doped system shows 67% and 33% probabilities at 800 K for the dopant nearest and second nearest neighbor oxygen vacancies, respectively. The dopant nearest oxygen vacancy leaves the dopant polyhedral center charge exposed. The small size of the aluminum dopant allows oxygen ions to shift and partially screen the dopant charge. This coupled with strong hydrogen bonds increases proton trapping. For the larger scandium and yttrium ions, there is no significant oxygen ion rearrangement around the dopant with a nearest neighbor oxygen vacancy. Instead, the positive dopant charge exposed by the vacancy raises the energy of several dopant nearest neighbor proton sites decreasing trapping locally.