Thin Film Electrolyte Research Articles

Highly Conducting Sc and Y co-doped Zirconia Solid Electrolyte Thin Films Prepared via Pulsed Laser Deposition for Solid Oxide Electrochemical Cells Applications

Scandium (Sc) and yttrium (Y) co-doped ZrO2 thin films were fabricated via pulsed laser deposition (PLD). The target material, 4 mol% Sc2O3 and 4 mol% Y2O3 co-doped ZrO2 (ScYSZ), was first synthesized via a solid-state reaction. The ScYSZ thin film was deposited on SiO2-Si substrate by laser ablation, using a Q-switched Nd3+:YAG laser (λ = 266 nm) and substrate temperatures of 600 and 800 oC. Micro-structural and conductivity properties of the resulting as-deposited thin films were investigated. X-ray diffraction (XRD) sample patterns revealed that the as-deposited polycrystalline films can be indexed to a cubic structure. In addition, varying the oxygen partial pressures during the deposition was studied. FE-SEM images revealed dense and crack-free surface morphologies achieved at lower oxygen partial pressures (0.01 Pa and 0.1 Pa) and rough surface with porous morphology at high oxygen partial pressures (1.0 Pa and 10 Pa). The thickness of the film is about 120 nm for the thin film deposited at 800 oC substrate temperature (TS). The total conductivity of the thin film was measured using electrochemical impedance spectroscopy (EIS). Interestingly, the calculated total conductivity of the deposited ScYSZ thin film, 0.1 Pa and TS at 800 oC, revealed about 2.8x10-1 S/cmat 700 oC and with activation energy (Ea) of 0.78 eV from 500 oC to 700 oC. The fabricated as-deposited ScYSZ thin film is one of the highest reported highly conducting solid electrolytes that is a promising material for fuel cell or electrolysis cell applications.

(Invited) Nanostructured Solid State Battery Architectures Enabled by Atomic Layer Deposition

Solid state batteries (SSBs) are considered the most promising next-generation energy storage technology both due to their increased safety over conventional wet Li-ion cells and their ability to fabricate novel architectures with simultaneous high energy and power density. However, the ionic conductivities of many solid electrolyte materials are orders of magnitude lower than liquid carbonate-based electrolytes. In order to offer SSBs with competitive charge/discharge rates, SSB active layers need to be thin (for fast ion diffusion) but have a large surface area (to enable large ion flux from anode to cathode). These geometric constraints establish the need for 3D electrode (and solid electrolyte) structures for a high performance battery architecture. Atomic layer deposition (ALD) is ideally suited to fabricate such devices due to its highly conformal nature and Angstrom-scale thickness control, however many challenges remain with materials fabrication and process integration.I will discuss development of both plasma-enhanced ALD (PEALD) and thermal ALD processes for the Li, Na, and K alkali polyphosphazene (A*PON) electrolytes1,2,3,4. The PEALD A*PON processes, by virtue of their 4-precursor synthesis route, allow some degree of chemical composition tuning. The thermal A*PON processes, unlike their PEALD counterparts, have a predetermined P-N ratio fixed by use of the precursors, however there is a tradeoff between the low temperature deposition enabled using plasma-based A*PON processes, and the conformality offered using the thermal A*PON processes. I will discuss and compare the reaction mechanisms and growth behavior among the entire family of A*PON solid electrolyte thin films, as well as discuss the integration challenges on the path to developing all-ALD planar and 3D SSBs. Pearse, A.; Schmitt, T.; Sahadeo, E.; Stewart, D. M.; Kozen, A.; Gerasopoulos, K.; Talin, A. A.; Lee, S. B.; Rubloff, G. W.; Gregorczyk, K. E., Three-Dimensional Solid-State Lithium-Ion Batteries Fabricated by Conformal Vapor-Phase Chemistry. ACS Nano 2018, 12 (5), 4286-4294. Pearse, A. J.; Schmitt, T. E.; Fuller, E. J.; El-Gabaly, F.; Lin, C.-F.; Gerasopoulos, K.; Kozen, A. C.; Talin, A. A.; Rubloff, G.; Gregorczyk, K. E., Nanoscale Solid State Batteries Enabled by Thermal Atomic Layer Deposition of a Lithium Polyphosphazene Solid State Electrolyte. Chemistry of Materials 2017, 29 (8), 3740-3753. Nuwayhid, R. B.; Jarry, A.; Rubloff, G. W.; Gregorczyk, K. E., Atomic Layer Deposition of Sodium Phosphorus Oxynitride: A Conformal Solid-State Sodium-Ion Conductor. ACS Applied Materials & Interfaces 2020, 12 (19), 21641-21650. Kozen, A. C.; Pearse, A. J.; Lin, C-F.; Noked, M.; Rubloff, G. W.; Atomic Layer Deposition of the Solid Electrolyte LiPON. Chemistry of Materials 2015, 27 (15), 5324-5331.

Influence of Au, Pt, and C Seed Layers on Lithium Nucleation Dynamics for Anode-Free Solid-State Batteries.

In the concept of anode-free lithium-ion batteries, cells are manufactured with a bare anode current collector where the lithium metal anode is electrochemically formed from the lithium-containing cathode during the first charge cycle. While this concept has many attractive aspects from a manufacturing and energy density standpoint, stable plating and stripping remain challenging. We have investigated gold, platinum, and amorphous carbon as seed layers placed between the copper current collector and the lithium phosphorus oxynitride thin-film solid electrolyte. These layers guide lithium nucleation and improve the plating and stripping dynamics. All seed layers facilitate reversible lithium plating and stripping even at high current densities up to 8 mA cm-2. Of particular note is the amorphous carbon seed layer, which allowed a significant reduction in plating potential from 300 mV to as low as 50 mV. These results underscore the critical role of seed layers in improving the efficiency of anode-free solid-state batteries and open the door to simplified manufacturing of anode-free battery designs.

High-performance lithium metal batteries enabled by a nano-sized garnet solid-state electrolyte modified separator

Lithium metal batteries, as one of high-energy-density batteries, have become the hot spot for numerous researchers. However, serious safety problems caused by lithium dendrites need to be urgently addressed. In this work, we introduce nano-sized garnet solid-state electrolyte Li6.75La3Zr1.75Ta0.25O12 (LLZTO) to fabricate a high-safety thin composite solid electrolyte membrane (20 μm) that is simple in process and facile in preparation conditions. The thin composite solid electrolyte film has better wrinkle resistance than ordinary polyethylene (PE) films and can guide the uniform deposition of lithium ions and suppress the growth of lithium dendrites. The thermal stability of the PE separator modified by double-coated nano-sized LLZTO is enhanced, which exhibits only 30 % shrinking percentage at 140 °C in comparison with 90 % of ordinary PE. The lithium symmetric cell with the nano-sized garnet solid-state electrolyte modified separator is relatively stable after 1000 h cycling at the current density of 1 mA cm−2. More importantly, after adding the adaptable electrolyte, the lithium metal battery assembled with modified separator can be maintained for 150 cycles (0.2C) stably with a high cathode active material loading of NCM811 (18 mg cm−2), and can maintain above 95 % Coulombic efficiency on average. Our work provides a pathway for the preparation of superior thermal stability and high safety garnet-based composite membranes towards lithium metal batteries.

Investigation of the electrochemical performance of Mg-ion batteries based on PVDF-HFP thin polymer electrolyte films

In the present work, poly(vinylidene fluoride-hexafluoropropylene):magnesium trifluoromethanesulfonate (PVDF-HFP:MgTf) polymer electrolytic films with a porous structure were developed using the solution-casting method. With its high performance and versatility, PVDF-HFP has been widely employed in the development of solid polymer electrolyte films (SPEFs). In this study, metal salt particles are successfully included as an inorganic filler into solid PVDF-HFP films, reducing pore size and boosting the amorphous character of the films. Fourier transform infrared spectroscopy is used to examine the chemical structure of the produced SPEFs, and scanning electron microscopy is used to evaluate the surface morphology and locate pores in the SPEFs. The surface nature of the PVDF-HFP films and the PVDF-HFP:MgTf is confirmed by X-ray diffraction. The electrochemical stability of PVDF-HFP and PVDF-HFP:MgTf films is studied through a differential scanning calorimetry experiment. Results revealed that PVDF-HFP:MgT films could be stabilized when filled with electrolytes and electrodes and made less porous by adding inorganic salt particles. Further, electrochemical impedance spectroscopy is used to measure electrical conductivity between 303 and 423 K. At room temperature, PVDF-HFP and PVDF-HFP/Mg2+ SPEFs have optimal ionic conductivities of 3.38 × 10−7 and 1.08 × 10−6 SS ccmm−1−1 respectively. Ionic conductivity was found to increase with temperature. The modulus of the imaginary part of the loss tangent (M′ and M″) was found to be related to dielectric and conductivity relaxation.

Features of forming a low-temperature cubic Li7La3Zr2O12 film by tape casting

Currently, interest to lithium and lithium-ion all-solid-state power sources is rapidly growing all over the world. However, several issues should be addressed before all-solid-state batteries production: high resistance values of the solid electrolyte membrane and poor contact between electrolyte and electrode materials. The transition to thin-film technologies is one of the promising ways to solve these problems. Tape casting can be proposed to obtain thin-film solid electrolytes. In this research, the features of the structure formation, morphology and lithium-ion conductivity of Li7La3Zr2O12 films were investigated. Li7La3Zr2O12 films with the thickness of 35 µm were obtained by tape casting on Ni substrate. The influence of organic components’ content on homogeneous coatings formation was established. Heat treatment conditions for dried films were chosen based on differential scanning calorimetry and optical dilatometry. Phase change from tetragonal to low-temperature cubic modification occurs after annealing the Li7La3Zr2O12 films at 700 °C and higher. The annealed Li7La3Zr2O12 films have developed surface, which can lead to improved contact between the solid electrolyte and an electrode in an electrochemical cell. Li7La3Zr2O12 films annealed at 800 °C have the highest lithium-ion conductivity values (2.5·10–7 and 1.5·10–5 S·cm–1 at 90 and 215 °С, respectively). The technology of Li7La3Zr2O12 films formation with the thickness of ~23 µm by tape casting was developed.

Feasibility evaluation of low-temperature deposited thin-film electrolyte with successive post-annealing for solid oxide fuel cells

During the past decade, thin-film-based solid oxide fuel cells (TF-SOFCs) have demonstrated remarkable performance at a low operating temperature of ∼500 °C by reducing ohmic resistance with vacuum-deposited thin electrolytes. However, a high-temperature deposition process at approximately 700 °C is employed in TF-SOFCs, which is only available at a laboratory scale and is not appropriate for commercial deposition equipment. To address this issue, we investigate the feasibility of depositing electrolytes at relatively low temperatures (≤300 °C) accompanied with post-annealing. A TF-SOFC comprising a trilayer thin-film electrolyte, i.e., yttria-stabilized zirconia (YSZ) sandwiched between gadolinia-doped ceria (GDC), fabricated at low-temperature deposition with subsequent post-annealing is selected and tested. Based on a thorough examination using X-ray diffraction and scanning electron microscopy, the appropriate growth conditions for the YSZ and GDC thin-film layers are selected. Through the optimization, we successfully create a low-temperature deposited 750 nm-thick GDC/350 nm-thick YSZ/150 nm-thick GDC (CZC) trilayer electrolyte exhibiting comparable performances with that deposited at 700 °C. High performance of a single cell, a peak power density of 890 mW/cm2 at 500 °C, is achieved. This result suggests the potential of fabricating high-quality TF-SOFCs with commercial equipment.

Thin-film Li3InCl6 electrolyte prepared by solution casting method for all-solid-state batteries

Thin-film Li3InCl6 electrolyte prepared by solution casting method for all-solid-state batteries

In situ study the effects of bias and electric field intensity on electrochemical migration behavior of Sn96.5Ag3.0Cu0.5 solder alloy

In this work, the influence of bias and electric field intensity on the electrochemical migration of SAC305 solder alloy is investigated using a self-designed in-situ thin electrolyte film device. The results indicate that when the bias is 0.5 V, a compact passivation film forms on the sample surface, effectively inhibiting dendrite growth. When the applied bias reaches 0.75 V, it is observed that specific corrosion products are generated between the two plates, and the occurrence of dendritic growth phenomena is still absent. Dendrite development is observed when a bias voltage of 1 V is applied. Regarding the electrode reactions, when the applied bias falls below 1.5 V, a notable polarization resistance is encountered in the anode reaction. In this case, the cathode reaction primarily entails the process of oxygen reduction. As the applied voltage climbs to 1.5 V, the anodic potential shows an increase, leading to a reduction in the polarization resistance of the anodic process that determines the rate of the electrochemical reaction. At this time, the cathode potential exhibits a comparatively negative value, leading to the progressive occurrence of the hydrogen evolution process. Additionally, the influence mechanism of bias on ECM is also proposed.

(Digital Presentation) Fabrication and Characterization of 4Sc4YSZ/LSM-4Sc4YSZ Novel Electrode-Supported Half-Cell Composition for Solid Oxide Electrochemical Cells

Scandium and Yttrium co-doped ZrO2 (4Sc4YSZ) thin film solid electrolyte has exhibited improved conductivities and phase stability by considering advantages of both Y-doped zirconia (YSZ) and Sc-doped zirconia (ScSZ). In this study, a solid electrolyte thin film of 4Sc4YSZ deposited on a new LSM-4Sc4YSZ composite electrode was conducted. To obtain a dense electrolyte, the effect of pre-sintering temperature of the LSM-4Sc4YSZ electrode substrate in the deposition of the 4Sc4YSZ thin film was investigated. X-ray diffraction (XRD) analysis revealed distinct phases of rhombohedral peaks for LSM and cubic peaks for the composite 4Sc4YSZ component and deposited thin film electrolytes. It was observed from the morphological results via scanning electron microscopy (SEM) that pre-sintering of the electrode substrate affects the densification of the deposited solid electrolyte. A dense 4Sc4YSZ thin film electrolyte was obtained by pre-sintering the LSM-4Sc4YSZ electrode at 1300°C than those deposited on the substrates with lower pre-sintering temperatures.

Highly Conducting Sc and Y co-doped Zirconia Solid Electrolyte Thin Films Prepared via Pulsed Laser Deposition for Solid Oxide Electrochemical Cells Applications

Scandia and yttria-stabilized zirconia (ScYSZ) have been widely used as electrolyte materials for solid oxide electrochemical cells (SOEC) applications due to its high ionic conductivity and stable mechanical and thermal properties. In this study, ScYSZ thin films were fabricated via pulsed laser deposition (PLD) method at substrate deposition temperature (Ts ) of 800 °C with varying oxygen oxygen partial pressures (). The as-deposited ScYSZ thin films were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and electrochemical impedance spectroscopy (EIS). XRD analysis revealed that all fabricated films, even with varying , have a polycrystalline cubic-fluorite structure. Morphological FESEM images revealed a good-quality dense ScYSZ electrolyte thin film with no visible cracks. Notably, a high total conductivity thin film of about 2.8x10-1 S/cm at 700 °C and low activation energy (Ea ) of 0.78 eV, from 500°Cto 700 °C, were achieved. Interestingly, the PLD-prepared ScYSZ dense solid electrolyte showed a higher total conductivity than the reported conductivities of zirconia-based solid oxide electrolytes.

Batch fabrication of micro-tubular protonic ceramic fuel cells via a phase inversion-based co-spinning/co-sintering technique

Micro-tubular protonic ceramic fuel cells (MT-PCFCs) are prominently advantageous in energy conversion and storage, such as intermediate operating temperature, large volumetric power density, and low sealing requirements. Formation of thin and dense electrolyte film and efficient current collection, particularly from the micro lumen, are the primary challenges for preparing high-performance MT-PCFCs. The present work carries out a one-step fabrication of triple-layer hollow fibers (TLHFs) with anodic current collector/graded anode/electrolyte using BaCe0·7Zr0·1Y0·2O3-δ via the phase-inversion based co-spinning/co-sintering technique. A BaCo0·4Fe0·4Zr0·1Y0·1O3-δ based porous cathode is then applied on the TLHF's outer surface to create a complete fuel cell. The resultant single MT-PCFC yields a peak power density of 601.2 mW cm−2 at 700 °C. Furthermore, an MT-PCFC stack is made by bundling seven TLHFs using the porous cathode. With an effective area exceeding 12 cm2, the MT-PCFC stack generates a power output of 2.5 W at 700 °C. The stability test lasts for 100 h at 600 °C under 100 mA cm−2 and for next 60 h at varied temperatures from 700 to 600 °C under 200 mA cm−2. The inventive design of TLHFs facilitates the scale-up of fuel cells, promoting the development of a large-scale MT-PCFC stack or systems.

Influence of pore size on mass transport: Bifunctional MnOx-coated nickel foam vs carbon corrosion prone commercial GDE in alkaline electrolyte

A promising step towards a carbon-free gas diffusion electrode (CF-GDE) design for alkaline applications is shown in this work. MnOx´s are electrodeposited on nickel foam by varying the settings, and then they were annealed at 300 °C. These CF-GDEs were characterized and compared with a commercial carbon-based GDE (GDEref). Samples were analyzed by XRD, SEM, X-ray tomography, contact angle method, nitrogen sorption, and electrochemical methods. Electrochemical impedance spectroscopy confirmed the higher reactant transport of CF-GDEs during OER and ORR. The large pore size of the CF-GDE substrate (nickel foam) results in a high supply of catalyst sites deep inside the CF-GDE. The transport distance of gaseous O2 is also reduced since the thin electrolyte film is at the gas bulk of the CF-GDE. The catalytic bifunctionality was additionally examined by galvanostatic measurements at 5 and 10 mA cm− 2. A promising CF-GDE exhibited 0.43 V ηOER and −2.43 V ηORR at 10 mA cm−2. Lastly, long-term stability measurements of 2100 cycles at 1.5 V (OER) and −0.75 V (ORR) vs. Hg/HgO in 1 M NaOH show the carbon corrosion of GDEref (60% current density loss) and a stable current density of CF-GDE.

Nanocrystal Engineering of Thin-Film Yttria-Stabilized Zirconia Electrolytes for Low-Temperature Solid-Oxide Fuel Cells.

To overcome significantly sluggish oxygen-ion conduction in the electrolytes of low-temperature solid-oxide fuel cells (SOFCs), numerous researchers have devoted considerable effort to fabricating the electrolytes as thin as possible. However, thickness is not the only factor that affects the electrolyte performance; roughness, grain size, and internal film stress also play a role. In this study, yttria-stabilized zirconia (YSZ) was deposited via a reactive sputtering process to fabricate high-performance thin-film electrolytes. Various sputtering chamber pressures (5, 10, and 15 mTorr) were investigated to improve the electrolytes. As a result, high surface area, large grain size, and residual tensile stress were successfully obtained by increasing the sputtering pressure. To clarify the correlation between the microstructure and electrolyte performance, a YSZ thin-film electrolyte was applied to anodized aluminum oxide-supported SOFCs composed of conventional electrode materials which are Ni and Pt as the anode and the cathode, respectively. The thin-film SOFC with YSZ deposited at 15 mTorr exhibited the lowest ohmic resistance and, consequently, the highest maximum power density (493 mW/cm2) at 500 °C whose performance is more than five times higher than that of the cell with YSZ deposited at 5 mTorr (94.1 mW/cm2). Despite the basic electrode materials, exceptionally high performance at low operating temperature was achieved via controlling the single fabrication condition for the electrolyte.

Micro Tubular Solid Oxide Reversible Cell Using LaGaO3 Electrolyte Film Prepared on Channel Size Controlled NiO-YSZ Substrate

Reversible operation of SOFC is now important subjects from energy storage of renewable electric power from solar or wind power. At present, planer type cell design is widely used for this purpose, however, because of tight gas sealing, tubular type cell design is more suitable. In this study, micro tubular solid oxide cell using La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) thin electrolyte film was prepared by dip-coating and co-sintering process.NiO-YSZ porous substrate with 10 mm diameter and 30 mm length was used for preparation of the cell. LaGaO3 based oxide film was deposited with dip coating method. Two kinds of NiO and YSZ particles with different particle size (0.91 and 0.65 mm for NiO and 0.62 and 0.25 mm for YSZ) were mixed and extruded for tubular substrate. 4 types of NiO-YSZ anode porous substrate were Average porosity of each 4 tubular substrate was ca. 70% after reduction. However, from SEM observation, channel size in NiO-YSZ substrate was changed and NiO-YSZ using larger NiO and YSZ particles shows larger channel diameter.Power generation property of 4 tubular cell using different NiO and YSZ particle size for substrate. It was found that the cell deposited on NiO-YSZ using larger particles shows lager power density, in particular, the cell using both NiO and YSZ larger particles shows the largest power density (0.37 W/cm2 at 873 K), which is reasonably large power density. On the other hand, in case of electrolysis, channel size is also strongly influenced on the electrolysis current. As similar manner with fuel cell operation, the LSGM cell prepared on larger particles size of NiO and YSZ shows larger steam electrolysis current. Impedance analysis suggests that the effects of particle size of NiO and YSZ was explained by small concentration overpotential, and so larger channel size is effective for improving the initial performance of the SORC performance. Long term stability of the steam electrolysis was performed and it was found that the LSGM tubular cell prepared on NiO-YSZ with larger particles shows the reasonable stable electrolysis performance in long term stability (-4%/1000h average degradation rate). The main reason for degradation was assigned to the increased IR loss however overpotential was hardly changed over 600 h and so it seems most likely that the reoxidation of NiO or aggregation of NiO is the main reason for degradation of the cell.This study reveals that the control of channel size in NiO-YSZ porous substrate is highly important for initial performance and long-term stability of SORC.

Understanding Polymer Structure and Conductivity in Composite Electrolytes

Lithium ion batteries are a crucial technologies for electric vehicles and portable electronics due to their high energy density. However, flammable liquid electrolytes create a safety concern and transitioning to all solid state batteries allows for the potential of higher energy density by utilizing lithium metal anodes. Two classes of solid materials have been explored to replace liquid electrolytes: ceramic and polymer electrolytes. Composite electrolytes, typically in the form of dispersed ceramic particles or nanowires in polymer matrix, combine these two materials to have good ionic conductivity while maintaining the mechanical properties of the polymer. While they have achieved fairly high conductivity (typically 10-4 S/cm), the mechanism for such ion conduction is not well understood.The polymer likely plays a crucial role as the ceramic volume fractions are typically under 20%.1,2 Four possible ways that polymers could increase in conductivity in the composite have been identified: decreased crystallinity, increased free volume, disruption of polymer alignment, and increased lithium ion dissolution due to acidic surface groups on the ceramic.1,3,4 We have developed thin film model systems to probe the interface of the polymer with and ceramic surfaces. We found that when confined to thin films, the conductivity of the polymer electrolyte increased compared to the bulk. We utilized differential scanning calorimetry to identify the crystallinity and found that the small crystallinity variation could not account for the variation in conductivity. Subsequently, we utilized surface conductivity and transference number measurements to understand the effects of acidic surface groups. X-ray Absorption measurements will be similarly utilized to probe the variation in lithium ion environments within these materials to understand any variation in the lithium salt dissolution. It was found that the variation in conductivity based on this effect is small.In order to probe the possible variation in free volume, Small Angle Neutron Scattering will be utilized to quantify the free volume within thin films, bulk polymer, and composite electrolytes. This can quantify the volume associated with each monomer and therefore how much space is between polymer chains for lithium ion movement.5 Lastly, while the polymer chain alignment has not been extensively discussed composite literature, it has been identified that when polymer electrolytes are cast on a surface, polymer electrolytes typically show anisotropy with measurements parallel to the surface having higher conductivity due to the alignment of the chain.6 Therefore, ceramic fillers in the composites may disrupt this packing and allow for higher conductivity perpendicular to the cast surface, the direction in which they are most commonly measured. In order to probe this effect, we measure the anisotropy of both our thin film electrolytes as well as composite electrolytes to compare the significance of the anisotropy and utilize small angle x-ray scattering to measure any anisotropy in the structure.

Model Based Comparative Analysis of GDC and YSZ Based Solid Oxide Fuel Cells

Designing solid oxide fuel cell (SOFCs) stacks for transportation applications places new challenges on SOFC architecture and operation with respect to power density, dynamic response, and thermal management. Recent developments in SOFCs with either thin-film yttria stabilized zirconia (YSZ) electrolytes (thickness ≤ 5 μm) or highly conductive, gadolinia-doped ceria (GDC) electrolytes (thicknesses ≥10 μm) enable thigh-power-density stack operation at inlet temperatures well below 700ºC with the potential for using compliant seals and lower cost ferrous interconnect materials. Applications, such as hybridized aircraft engines, can use such SOFC stacks if they meet strict requirements with respect to specific power and thermal management. Highly conductive GDC-based electrolytes offer pathways to high power-density SOFCs at operating temperatures below 650ºC, but the rise in mixed ionic electronic conductivity (MIEC) above 650ºC reduces open circuit voltages and thus fuel cell efficiency [1,2]. This necessitates thermal management to maintain operating temperatures and mitigate thermo-mechanical stresses within an SOFC. Thin-film YSZ electrolytes offer lower power densities below 650ºC, but avoid the penalties associated with MIEC behavior at higher temperatures and therefore reduce the challenges associated with thermal management at high power densities. In this work, a model-based approach is used to compare SOFC stack operation with thin-film YSZ electrolytes and with GDC-based electrolytes for operation on hydrogen and reformate fuels at high-power densities relevant for transportation applications.To calibrate models to experimental cell performance, polarization curves of a commercially available YSZ-based cell operating between 600-750ºC and of a high-power-density GDC cell operating between 500-650ºC are used to fit electrochemical model parameters for both electrolytes. Modeling of the YSZ based cell can neglect electronic charge transfer through the membrane electrolyte, but GDC-cell modeling must account for the multiple charge carriers (ceria polarons and oxygen vacancies) in the electrolyte [3]. Electrochemical parameters for ionic conductivities and electrochemical reaction rate parameters for oxide cathodes and nickel-electrolyte-composite anodes are fit to the experimental polarization data over the broad range of operating temperatures. Global rate expressions to account for internal reforming on the active nickel surfaces of anode are adopted from literature [4,5]. The calibrated cell model for each cell type is adopted to a down-the-channel discretized cell simulation to predict heat loss and temperature gradients at high current densities.This presentation compares GDC- and YSZ-based SOFCs in high power-density applications from fundamental and practical perspectives. The open-circuit voltages for GDC-based cells drops with increasing temperature to about 0.85 V/cell at 650°C which is lower than the Nernst open-circuit voltage of YSZ-based cells (≥ 1.05 V/cell) at similar conditions. Both types of cells can sustain power densities around 1.0 W/cm2 with high excess cathode air ratios ≥ 5 to keep temperature gradients near 10°C/cm. These results show that GDC-based cells support higher power densities at lower cell flow inlet temperatures < 600°C, whereas thin-film YSZ cells provide superior performance at flow inlets that drive cell temperatures > 600°C. Higher stack power densities > 1.0 W/cm2 must be supported by stack designs that alleviate high thermal stresses and allow steeper gradients of temperature. A summary of stack design and operational strategies that meet the stringent requirements of high power-density operation will be presented in the light of the modeling resuls..

In-Situ Thermo-Mechanical Characterization and Strain Engineering of Lipon Thin Films

High temperature multi-optical-stress sensor (HTMOSS) has been used to characterize the coefficient of thermal expansion (CTE) and yield stress of 1-micron thick LiPON films. Six fully dense, amorphous films were deposited on glass and sapphire substrates. The films were then annealed at temperatures ranging from 80 to 200 C for 3 hours. The CTE of LiPON is found to be approximately 4.1e-6, and argued to be independent of the substrate type. Because of this intermediate CTE value, by varying the substrate, we could impose either tension and compression due to thermal mismatch to the film. We observed further that the yield stress of the film is approximately 60 MPa under compression and 100 MPa under tension. Using constant-load hold at and beyond yield point, the films were found to relieve the stress developed during heating with visco-plastic deformation, which led to permanent residual stress during cooling as high as 120 MPa in either tension or compression depending on substrate type. We also found that at annealing temperature higher than 140 C LiPON lost ductility which could be due to composition changes as indicated by XPS measurement. The stress-relief mechanism at constant-load indicates that LiPON may be beneficial as a protective layer against dendrites penetration. Moreover our experimental platform proves that it's an effective method to engineer strain into thin-film solid electrolytes that could be extended to other materials like LLZO or sulfide electrolyte.

Low temperature preparation of YSZ-based thin-film limiting current type oxygen sensor

Oxygen sensors based on YSZ solid electrolyte thin films have great application prospects due to their advantages in miniaturization and integration. Here, we report the gas sensing performance of Pt/YSZ/Pt oxygen gas sensors with low preparing temperatures. YSZ thin films with (200) preferred orientation have been prepared by room temperature RF magnetron sputtering combined with annealing at a temperature of 650 °C. The 700 nm-thick YSZ films exhibit an obvious limiting current platform, and the limiting current IL shows a linear dependence on oxygen concentration X(O2) (in the range of 6%–21%). Moreover, the devices with 700 nm-thick YSZ films worked well at the temperature range from 500 to 650 °C, and a linear relationship between logIL and 1000/T has been observed. These results demonstrate that YSZ thin films prepared by low temperature magnetron sputtering combined with the annealing process can reduce the working temperature.

A micro room temperature oxygen sensor based on LaF3 solid electrolyte thin film

PurposeDetecting O2 gas in a confined space at room temperature is particularly important to monitor the work process of precision equipment. This study aims to propose a miniaturized, low-cost, mass-scale produced O2 sensor operating around 30°C.Design/methodology/approachThe O2 sensor based on lanthanum fluoride (LaF3) solid electrolyte thin film was developed using MEMS technology. The principle of the sensor was a galvanic cell H2O, O2, Pt | LaF3 | Sn, SnF2 |, in which the Sn film was prepared by magnetron sputtering, and the LaF3 film was prepared by thermal resistance evaporation.FindingsThrough pretreatments, the sensor’s response signal to 40% oxygen concentration was enhanced from 1.9 mV to 46.0 mV at 30°C and 97.0% RH. Tests at temperatures from 30°C to 50°C and humidity from 32.4% RH to 97.0% RH indicated that the output electromotive force (EMF) has a linear relationship with the logarithm of the oxygen concentration. The sensitivity of the sensor increases with an increase in both humidity and temperature in the couple mode, and the EMF of the sensor follows well with the Nernst equation at different temperatures and humidity.Practical implicationsThis research could be applied to monitor the oxygen concentration below 25% in confined spaces at room temperature safely without a power supply.Originality/valueThe relationship between temperature and humidity coupling and the response of the sensor was obtained. The nano-film material was integrated with the MEMS process. It is expected to be practically applied in the future.