Bilayer Electrolyte Research Articles

(Digital Presentation) GDC/YSZ Bilayer Electrolyte Fabrication by In-situ Hydrothermal Growth

In this work, we report a highly dense GDC/YSZ bilayer electrolyte prepared via cost-competitive method, at a relatively low sintering temperature. An ultra-thin and dense GDC barrier layer is grown on the surface of as-sintered YSZ electrolyte by twice successive in-situ hydrothermal growth at 180 oC. The GDC/YSZ bilayer electrolyte is successfully fabricated under low sintering temperature below 1200 oC, with overall layer as thick as ~540 nm and ultra-high density as the YSZ electrolyte. The anode supported single cell with LSCF cathode shows maximum power density of ~0.961 W/cm2 at 780 oC. Moreover, the cell runs stably at 720 oC for 300 hours, showing decent durability.

All-Solid-State Garnet Type Sulfurized Polyacrylonitrile/Lithium-Metal Battery Enabled by an Inorganic Lithium Conductive Salt and a Bilayer Electrolyte Architecture

All-Solid-State Garnet Type Sulfurized Polyacrylonitrile/Lithium-Metal Battery Enabled by an Inorganic Lithium Conductive Salt and a Bilayer Electrolyte Architecture

Visualizing the Chemical Incompatibility of Halide and Sulfide‐Based Electrolytes in Solid‐State Batteries

AbstractHalide‐based solid electrolytes are currently growing in interest in solid‐state batteries due to their high electrochemical stability window compared to sulfide electrolytes. However, often a bilayer separator of a sulfide and a halide is used and it is unclear why such setup is necessary, besides the instability of the halides against lithium metal. It is shown that an electrolyte bilayer improves the capacity retention as it suppresses interfacial resistance growth monitored by impedance spectroscopy. By using in‐depth analytical characterization of buried interphases by time‐of‐flight secondary ion mass spectrometry and focused ion beam scanning electron microscopy analyses, an indium‐sulfide rich region is detected at the halide and sulfide contact area, visualizing the chemical incompatibility of these two electrolytes. The results highlight the need to consider more than just the electrochemical stability of electrolyte materials, showing that chemical compatibility of all components may be paramount when using halide‐based solid electrolytes in solid‐state batteries.

Realizing compatibility of high voltage cathode and poly (ethylene oxide) electrolyte in all-solid-state lithium batteries by bilayer electrolyte design

Poly (ethylene oxide) (PEO) is easily oxidized at the cathode interface when coupled with high voltage cathodes (such as LiCoO2), leading to rapid capacity fade, limiting its application in high energy density all-solid-state battery. In this work, a bilayer concept is applied to design two PEO electrolyte layers composited with lithium difluoro (oxalato) borate (LiDFOB) and lithium bis(trifluoromethane sulfonyl) imide (LiTFSI), respectively. The thin PEO/LiDFOB layer is introduced by directly dropping the PEO/LiDFOB solution on the LiCoO2 cathode surface to construct a closely contact interphase. Meanwhile, a stable cathode electrolyte interphase (CEI) containing LixBxOy and LiF formed during electrochemical cycling realizes the LiCoO2/PEO interfacial compatibility. The self-generated PEO/LiTFSI layer towards the anode side provides high ionic conductivity and stabilizes the Li/electrolyte interface. As a result, the assembled cell using the bilayer PEO electrolyte achieves good cycling stability, the capacity retention increases from 15% to 75% after 100 cycles at 0.2C. The enhanced electrochemical performance is also achieved in LiNi0.6Co0.2Mn0.2O2/Li cell using this bilayer PEO electrolyte architecture. This work provides a simple strategy to make high-voltage cathode compatible with PEO electrolyte.

Evaluation of La2Ce2O7 (LCO) Protection Layer for Proton Conducting Electrolyte

Due to the susceptible reaction BaCeO3-based proton-conductors toward steam and carbon dioxide, La2Ce2O7(LCO) is proposed as a protect layer on top of solid electrolyte since LCO has shown high proton conductivity with adequate stability against steam. In the study, La(NO3)3·6H₂O and Ce(NO3)3·6H₂O were used as precursors to obtain La2Ce2O7(LCO). Particle size analyzer and XRD are applied to analysis this result. From the XRD analysis, it can be seen at the temperature range above 900°C, the reflections representing fluorite-structured LCO appeared with other reflections from BaCe0.5Zr0.3Y0.2O3-δ(BCZY) perovskite substrate. The result can also demonstrate that there is no reaction between LCO layer and BCZY substrate. It is suggested that LCO exhibits good chemical compatibility with BCZY. This synergic bilayer electrolyte design may be a feasible design to overcome the instability of Zr- or Ce-rich Ba(CeZr)O3−δ electrolysis cells against steam and CO2 for better long-term performance. The stability and protection of La2Ce2O7(LCO) on BaCe0.5Zr0.3Y0.2O3-δ(BCZY) will be evaluated based on structural and electrical properties measurement under humid and CO2 containing environment as function of time and temperatures.

Optimization of a ScCeSZ/GDC bi-layer electrolyte fabrication process for intermediate temperature solid oxide fuel cells

Cost-effective wet ceramic coating techniques for fabricating ScCeSZ/GDC bi-layer electrolyte anode-supported button cells were investigated in this study. Aqueous ceramic slurries were prepared by ball milling and then used for Ni/ScCeSZ half cell fabrication by tape casting and spin coating. Prepared cells were tested at operating temperature between 700 and 800°C with a fuel composition of hydrogen:nitrogen 3:1 and air at the cathode. The cell with a spin coated GDC film showed the maximum power density of 1.142, 1.012, 0.813 W•cm−2 at 800, 750, and 700°C, respectively. It was also able to produce power output around 0.7 W•cm−2 for 500 h at 750°C, which confirms the cell operational stability. More importantly, the GDC film prepared by spin coating effectively avoided the formation of the (Zr,Ce)O2−based solid solution at the ceria/zirconia interface compared with the other cells with the co-casted and sintered GDC film.

(Digital Presentation) Additional Voltage Loss in Terms of Electromagnetic Potential Related to Jarzynski’s Equality Using Sm-Doped Ceria Electrolytes in Wagner’s Equation for SOFCs

A solid oxide fuel cell (SOFC) converts chemical energy from a fuel gas, such as hydrogen or methane, to electrical energy. Yttria-stabilized zirconia (YSZ) films are often used as electrolytes in which only oxygen ions are carriers. In such cases, the open circuit voltage (OCV = 1.15 V at 1073 K) is equal to the Nernst voltage (Vth = 1.15 V at 1073 K).Samaria-doped ceria (SDC) have higher ionic conductivity. Therefore, SDC films are possible electrolyte candidates. However, when SDC electrolytes are used in SOFCs, the OCV is only 0.80 V at 1073 K. The voltage loss has been explained by Wagner’s equation.Numerous subsequent models have been created based on Wagner’s equation. Using the Riess model, the current-voltage relationship can be calculated with the cathode and anode polarization voltage losses [1]. Furthermore, a nonlinear model was created by Duncan and Wachsman [2]. This model can also be used to explain the equilibrium process. According to the Riess model, the OCV should decrease during electrode degradation. However, experimentally, the OCV does not change during electrode degradation [3]. The change in the equilibration of thick SDC electrolytes in response to a change in the anode gas has never been explained using the model defined by Duncan and Wachsman. Experimentally, when a very thick (6.6 mm) SDC electrolyte is used, the OCV can reach 0.80 V in only 5 minutes [4]. According to Weppner, the corresponding delay in the electron diffusion current should be more than 2080 minutes [5].We proposed a current-independent anode voltage loss (0.35 V = 1.15 V- 0.80 V) [6]. Since we disproved the existence of large leakage currents in SDC electrolytes, our explanations seem to disregard and disprove Wagner’s equation. However, we noticed that the dismissal of this idea regarding large leakage currents in SDC electrolytes is nothing more than a side effect. When SDC electrolytes experience a large anode voltage loss (0.35 V), the leakage currents are very small. This is called the “anode shielding effect” [7].The ionic activation energy (Ea) of SDC electrolytes is 0.7 eV (= 0.35 V ×2e). During ion hopping processes in SDC electrolytes, the work done by ions on the surrounding lattice structure is 0.7 eV. The ions should regain the 0.7 eV after hopping. However, when there are many electrons in the hopping path, hopping processes are very complex, and calculating the loss of work is challenging. Jarzynski’s equality is a very useful method of calculating the loss of work only from the first equilibrium state to the second equilibrium state. Because the distribution of hopping ions always should be a canonical ensemble, the loss of work can be calculated. When there are enough electrons, the ions cannot regain the 0.7 eV, and voltage loss (0.35 V) occurs [8].To assist the above explanation, we use the electromagnetic potential to explain the voltage loss that occurs during ion hopping. We noticed that the potential of ions during hopping can be calculated from the Lorenz gauge condition.Reference:[1] I. Riess, J. Phys. Chem. Solids., 47(2), 129 (1986). doi: 10.1016/0022-3697(86)90121-6.[2] K. L. Duncan and E. D. Wachsman, J. Electrochem. Soc., 156, B1030 (2009). doi:10.1149/1.315851.[3] T. Miyashita, J. Mater. Sci., 41(10), 3183 (2006). doi: 10.1007/s10853-006-6371-8.[4] T. Miyashita, J. Electrochem. Soc., 164(11) 3190 (2017). doi: 10.1149/2.0251711jes[5] J. Liu and W. Weppner, Ionics, 5(1–2), 115 (1999). doi: 10.1007/BF02375914.[6] T. Miyashita, J. Mater. Sci., 40(22), 6027 (2005). doi: 10.1007/s10855-005-4560-2.[7] B. Dalslet, P. Blennow, P. V. Hendriksen, N. Bonanos, D. Lybye, and M. Mogensen, J. Solid State Electrochem., 10(8), 547 (2006). doi: 10.1007/s10008-006-0135-x.[8] T. Miyashita, Miyashita, ECSarXiv (2020) “Open-Circuit Voltage Anomalies in Yttria-Stabilized Zirconia and Samaria-Doped Ceria Bilayered Electrolytes”, https://ecsarxiv.org/xhn73/

A review on the preparation of thin-film YSZ electrolyte of SOFCs by magnetron sputtering technology

Solid oxide fuel cells (SOFCs) have become one of the most attractive energy conversion devices due to their fuel flexibility, environmental friendliness and promising energy conversion efficiency. However, the high operating temperature (typically above 800 °C) of SOFCs poses some challenges concerning stability, sealing integrity and safety. Reducing the operating temperature of SOFCs to 600 °C or lower is currently an important research topic in the field. One effective strategy is to reduce the thickness of the electrolyte with high stability and high performance at lower temperatures. Among the thin-film preparation technologies, magnetron sputtering is gaining attention due to the advantages in scaling up and commercialization. Here we compare and analyze various electrolyte preparation methods and present a comprehensive review of the preparation of oxygen-ion-conducting electrolyte thin films by magnetron sputtering technology. The yttria-stabilized zirconia (YSZ) electrolyte and YSZ-based bilayer or multilayer electrolyte are employed as examples due to their high stability and high ionic conductivity. The effects of processing parameters on the microstructures and properties of the electrolyte thin films, the relevant interfaces and full SOFCs are reviewed and discussed. Finally, we present some perspectives and directions for developing high-performance SOFCs with thin-film electrolytes prepared by magnetron sputtering technology.

Interface modification of Ru-CeO2 co-infiltrated SFM electrode and construction of SDC/YSZ bilayer electrolyte for direct CO2 electrolysis

A novel electrolysis cell with SDC/YSZ bilayer electrolyte structure and Ru-CeO2 co-infiltrated SFM-SDC electrode has been fabricated by facile drop-coating method and vacuum infiltration process. The drop-coated YSZ thin film facilitates SDC electrolyte to be employed in direct CO2 electrolysis. The results of TPD analysis and AC impedance spectroscopy measurement indicate that the infiltration of Ru and CeO2 nanocatalysts of SFM-SDC electrode significantly enhance the chemical adsorption of CO2 and electrochemical performance of the electrode. With LSM-SDC as the anode, LSM-SDC/YSZ-SDC/Ru-CeO2-SFM-SDC single cell exhibits good coking-resistant and stability for CO2 reduction electrolysis, reaching a peak current density of 1.82 A·cm−2 under an applied voltage of 1.5 V. The combined strategy offers a promising approach to cell structure construction and interface modification for solid oxide electrolysis cells.

A review on materials, advantages, and challenges in thin film based solid oxide fuel cells

In accomplishment of massive energy demand, the solid oxide fuel cells (SOFCs) have proven as apotheosis candidates due to their high energy conversion efficiencies, low pollution exhaust, fuel flexibility, and environmental friendliness which make these the most promising alternatives to conventional electricity generators. Despite numerous advantages of SOFCs, large-scale industrial applications are yet to be realized owing to their high operating temperatures (800°C-1000°C). To lower the operating temperature, it is crucial to minimize ionic resistances which can be accomplished by reducing the electrolyte thickness to lesser than 5 μm. Therefore, present article embraces a concise review on conventional materials, designs, and generations of SOFCs with their compatibility and drawbacks. The strengths and limitations of conventional and advanced tools for fabricating thin film based SOFCs (TF-SOFCs) as well as the associated properties of electrolytes and electrodes are thoroughly examined, and further route cast is provided to attain better performance. The present review encourages the development of TF-SOFCs using advanced techniques employing ultrathin single and bilayer electrolytes which can have potential to reduce the operating temperature of SOFCs from 450°C to 600°C region and pave the way for global commercialization.

Fabrication and characterization of micro-tubular solid oxide fuel cells with yttria stabilized zirconia and Hafnia-Erbia co-doped bismuth oxide bilayer electrolyte

Current solid oxide fuel cell (SOFC) technology relies on YSZ electrolyte material due to its high stability under SOFC operation conditions. However, the ionic conductivity of YSZ decays as the operation temperature decreases to intermediate or low-temperature range.Commercialization of SOFC systems is hindered by the high manufacturing cost, which is directly related to the use of special and expensive system components because of the high operating temperatures of SOFCs. It is possible to use low-cost materials in system production if the operating temperature of SOFCs is reduced below 600 °C.In this study, we investigated the possibility of using 8% mol Yttria stabilized Zirconia (YSZ) and Hafnia-Erbia Co-doped Bismuth Oxide (HEB, ((Er2O3)0.20(Bi2O3)0.80)0.99-(HfO2)0.01) bilayer electrolyte configuration to lower the operating temperatures of SOFC to 600 °C and below. The cell performances at low-temperature region (T < 600 °C) are aimed to be increased with the additional HEB layer by the increased oxygen reduction kinetics on the air electrolyte side.Using a combination of thermoplastic extrusion and dip coating processes, four distinct cell types were created. Cells with monolayer and bilayer electrolyte were generated for each cathode type using two different cathode types, Lanthanum Strontium Manganate (LSM) and LSM-HEB. On the cell performance and operating temperatures, the impacts of electrolyte configuration and cathode type were examined. It was also explored whether the HEB material would be more effective as a second electrolyte or as a composite cathode if it was added to the LSM. Under various cell operating circumstances (different oxygen partial pressure values), several cell parameters such as the ohmic resistance, exchange current density, and anodic and cathodic saturation current densities were derived by fitting the experimentally recorded current density–voltage curves into the polarization model. The polarization resistances were calculated using cell parameters and four different cells were compared with each other. The performances measurements of both cell configurations showed that the peak power density of the bi-layered electrolyte configuration improved by a factor of 1.45 and reached 0.94 W/cm2 at 600 °C, while it was only 0.51 W/cm2 for the monolayered YSZ electrolyte configuration. Additionally, the effect of HEB addition to LSM cathode resulted in a performance increase of %84 compared to the cell with LSM cathode only. The presence of HEB either as a second electrolyte or ionic conductive phase in the composite cathode is lowered activation polarization of the cells both bi- or monolayered electrolyte cells and resulted in increased performance.

A review on process design and bilayer electrolyte materials of bipolar membrane fuel cell

A review on process design and bilayer electrolyte materials of bipolar membrane fuel cell

Lithium Nafion–Modified Li6.05Ga0.25La3Zr2O11.8F0.2 Trilayer Hybrid Solid Electrolyte for High-Voltage Cathodes in All-Solid-State Lithium-Metal Batteries

All-solid-state batteries containing ceramic-polymer solid electrolytes are possible alternatives to lithium-metal batteries containing liquid electrolytes in terms of their safety, energy storage, and stability at elevated temperatures. In this study we prepared a garnet-type Li6.05Ga0.25La3Zr2O11.8F0.2 (LGLZOF) solid electrolyte modified with lithium Nafion (LiNf) and incorporated it into poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrixes. We used a solution-casting method to obtain bilayer (Bi-HSE) and trilayer (Tri-HSE) hybrid solid electrolytes. A layer of functionalized multiwalled carbon nanotubes (f-MWCNTs) coated with LiNf (LiNf@f-MWCNT) in the Tri-HSE led to good compatibility with the polymer slurry and adhered well to the Li anode, thereby improving the interfacial contact at the electrode-solid electrolyte interface and suppressing dendrite growth. The Tri-HSE membrane displayed high ionic conductivity (5.6 × 10-4 S cm-1 at 30 °C), a superior Li+ transference number (0.87), and a wide electrochemical window (0-5.0 V vs Li/Li+). In addition, Li symmetrical cells incorporating this hybrid electrolyte possessed excellent interfacial stability over 600 h at 0.1 mA cm-2 and a high critical current density (1.5 mA cm-2). Solid-state lithium batteries having the structure LiNf@LiNi0.8Co0.1Mn0.1O2/Tri-HSE/Li delivered excellent room-temperature stable cycling performance at 0.5C, with a capacity retention of 85.1% after 450 cycles.

A Bilayer Electrolyte Design to Enable High-Areal-Capacity Composite Cathodes in Polymer Electrolytes Based Solid-State Lithium Metal Batteries

High-areal-capacity cathodes are needed for energy-dense solid-state batteries. Here, we demonstrate a bilayer polymer electrolyte design for cycling 3–6 mAh/cm2 NMC811 composite cathodes. The bilayer electrolyte comprises a cross-linked poly(ethylene oxide) (PEO)-based electrolyte layer and a linear-PEO-based electrolyte layer. The former provides dendritic resistance, and the latter provides a seamless interface with the cathode during cycling. Using a single layer of either membrane led to severe shorting or extremely low Coulombic efficiency (CE) in the first cycle. The general concept of a rigid dendrites-inhibiting electrolyte facing Li anode and a softer, cathode-integrated electrolyte that ensures contact with the cathodes during cycling may present a pattern for enabling high-energy-density cathodes.

Solid oxide fuel cell with a spin-coated yttria stabilized zirconia/gadolinia doped ceria bi-layer electrolyte.

A thin yttria stabilized zirconia (YSZ)/gadolinia doped ceria (GDC) bi-layer membrane is fabricated through the slurry spin coating technique and used as an electrolyte of a solid oxide fuel cell with La0.6Sr0.4Co0.2Fe0.8O3−δ as the cathode. The viscosity of the YSZ slurry is controlled by adding ethanol in the terpineol solvent, which shows a negligible effect on the thickness but a remarkable influence on the porosity of the YSZ film. The thickness of the YSZ layer increases with the YSZ content in the slurry. The YSZ films are pre-sintered at various temperatures, and the one sintered at 1200 °C has a moderate interaction with the GDC slurry, forming a 10 μm-thick YSZ/GDC bilayer with a low porosity and a low ohmic resistance. The corresponding single cell shows a maximum power density of 1480 mW cm−2 at 750 °C.

Validating the efficiency of γ-Al2O3/La0.6Sr0.4Co0.2Fe0.8O3-δ double-layer electrolyte for low temperature solid oxide fuel cell

Perovskite La0.6Sr0.4Co0.2Fe0.8O3+δ (LSCF) as a promising cathode material possessed overwhelming electronic conduction along with certain ionic conductivity. Its strong electron conduction capability hinder the application of pure-phase LSCF as electrolyte in semiconductor membrane fuel cell (SMFC). In order to constrain the electron transport and take advantage of the decent ion conduction of LSCF, a thin layer of γ-Al2O3 with insulating property was added as an electron barrier layer and combine with LSCF to form a two-layer structure electrolyte. Through adjusting the weight ratio of LSCF/γ-Al2O3 to optimize the thickness of double layers, an open circuit voltage of 0.98 V and a maximum power density of 690 mW/cm2 was received at 550 °C. At the same time, SEM, EIS and other characterization technology had proven that the LSCF/γ-Al2O3 bi-layer electrolyte can work efficiently at low temperature. The advantage of this work is the application of double-layer (γ-Al2O3/LSCF) structure electrolyte to instead of mixed material electrolyte in low-temperature solid oxide fuel cells. Structural innovation and the using of insulating materials provided clues for the further development of SMFC.

A Bi-Layer Dense/Porous Solid Electrolyte Interphase for Enhanced Lithium-Metal Stability

Due to its high theoretical capacity and low electrochemical potential, lithium metal is a highly investigated anode for next-generation high-energy batteries. However, the unstable chemical and topographical heterogeneous surface of lithium gives rise to safety and efficiency concerns that prevent it from being utilized in practical applications. Exposure to electrolyte leads to non-uniform, mixed organic-inorganic solid-electrolyte interphase (SEI) formation, starting a cascading to non-uniform flux distribution, consumption of active materials, and dendrite growth. The SEI is the key feature that will lead to harnessing the capabilities possible with lithium metal.In this work, the formation of a closed-host bi-layer solid electrolyte interphase (SEI) improves the stability of lithium anode. This is successfully realized by forming an interconnected porous LiF-rich artificial SEI in contact with Li metal, and a dense, stable in-situ formed upper layer SEI. The porous layer increases the number of Li/LiF interfaces, which reduces local volume fluctuations and improves Li+ diffusion along these interfaces. Additionally, the tortuous porous structure guides uniform Li+ flux distribution and mechanically suppresses dendrite propagation. The dense upper layer of the SEI accomplishes a closed-host design through reaction with LiNO3 additive and prevents continuous consumption of active materials seen without the additive included in the electrolyte. The duality of a dense top layer with porous bottom layer led to extended cycle life and improved rate performance, evidenced with symmetric cell testing, as well as full cell testing paired with sulfur and LiFePO4 (LFP) cathodes. This work is a good example of a rational design of the SEI, based on comprehensive consideration of various critical factors to improve Li-metal anode stability, and highlights a new pathway to improve cycling and rate performances of Li metal batteries. Figure 1

Naturally diffused sintering aid for highly conductive bilayer electrolytes in solid oxide cells.

Solid oxide cells (SOCs) are promising sustainable and efficient electrochemical energy conversion devices. The application of a bilayer electrolyte comprising wide electrolytic oxide and highly conductive oxide is essential to lower the operating temperatures while maintaining high performance. However, a structurally and chemically ideal bilayer has been unattainable through cost-effective conventional ceramic processes. Here, we describe a strategy of naturally diffused sintering aid allowing the fabrication of defect-free doped-zirconia/doped-ceria bilayer electrolyte with full density and reduced interdiffusion layer at lower sintering temperature owing to the supply of small but appropriate amount of sintering aid from doped zirconia to doped ceria that makes the thermal shrinkages of both layers perfectly congruent. The resulting SOCs exhibit a minimal ohmic loss of 0.09 ohm cm2 and remarkable performances in both fuel cell (power density exceeding 1.3 W cm−2) and electrolysis (current density of −1.27 A cm−2 at 1.3 V) operations at 700°C.

Interface engineering of bi-layer semiconductor SrCoSnO3-δ-CeO2-δ heterojunction electrolyte for boosting the electrochemical performance of low-temperature ceramic fuel cell

A comparative study is performed to investigate the electrochemical performance of the low-temperature ceramic fuel cells (CFCs) utilizing two different novel electrolytes. First, a perovskite semiconductor SrCo0.3Sn0.7O3-δ was used as an electrolyte in CFCs due to its modest ionic conductivity (0.1 S/cm) and demonstrated an acceptable power density of 360 mW/cm2 at 520 °C. The performance of the cell was primarily limited due to the moderate ionic transport in the electrolyte. In order to improve the ionic conductivity, a new strategy of using a novel bi-layer electrolyte concept consist of SrCo0.3Sn0.7O3-δ and CeO2-δ in CFCs. These bi-layers of two electrolytes have successfully established heterojunction which considerably improved the ionic conductivity (0.2 S/cm) and enhance the open-circuit voltage of the cell from 0.98 V to 1.001 V. Moreover, the CFCs utilizing bi-layer electrolyte have produced a remarkable power density of 672 mW/cm2 at 520 °C. This enhancement of ionic conduction, power density and blockage of electron conduction in the bi-layer electrolyte was studied via band alignment mechanism based on proposed p-n heterojunction. Our work presents a promising methodology for developing advanced low-temperature CFC electrolytes.

Design and Modeling of Proton-Conducting Bilayer Electrolytes Using a Nernst-Planck-Poisson Formulation

Proton-conducting oxides have attracted much attention in recent years due to their good ion conductivity at intermediate temperatures (500-700°C), as opposed to conventional oxide ion-conducting oxides that require high temperatures (usually, >900°C). Lowering the operating temperature of reversible protonic ceramic electrochemical cells (RePCECs) is desirable as it reduces material, heating, and insulation costs, while also increasing the operating life of the cell. Nevertheless, along with the desired proton conductivity, typical proton-conducting solid electrolytes also allow the transport of other charged defects like oxygen-vacancies, electrons, and electron holes. The presence of these other defects, especially the electron holes, causes leakage current that significantly reduces the cell’s electrical efficiency. Therefore, an electrolyte capable of reducing leakage current is necessary. Unfortunately, there is usually a trade-off between high ionic conductivity and low hole conduction. For example, while lanthanum tungstate (LWO) has a low electron conductivity and is capable of suppressing hole conduction below 800°C, its proton conductivity is lower than that of commonly used perovskite materials like yttria-doped barium cerate (BCY) or yttria-doped barium zirconate (BZY). Nonetheless, previous studies showed that LWO67 can act as a leakage current-blocking layer when deposited on a BZY82 base electrolyte and the estimated leakage current was almost 200 times lower than that of a BZY82 single layer electrolyte under open-circuit conditions. This suggests that under certain operating conditions, bilayer electrolytes can achieve better performance when compared to their corresponding single-layer electrolytes. Therefore, in this study, we design proton-conducting bilayer electrolytes by investigating the incorporation and transport of charged defects using a steady-state Nernst-Planck-Poisson (NPP) model.The main goals of this work are twofold. Firstly, to find the combination of electrolytes that better suppresses leakage current. For this purpose, we calculate the defect concentration, flux, and electrostatic profiles inside common materials like BCY, BZY, BZCYYb, BZCY, LWO, and different combinations of bilayer electrolytes to better understand how the addition of a leakage current-blocking layer affects proton conduction and overall efficiency. We consider three mobile defects when solving the NPP equation, protons (OHO •), oxygen vacancies (vO ••), and electron holes that are treated as small polarons localized at oxygen sites (OO •). Afterward, the I-V curve and relevant theoretical efficiency are predicted within and A cm-2 (fuel cell and electrolyzer operation) at 500, 600 and 700°C, in order to identify the most promising one. Secondly, to design a cell by finding the optimal thickness of the better performing bilayer electrolyte that maximizes efficiency.