The semiconductor SrFe0.2Ti0.8O3-δ-ZnO heterostructure electrolyte fuel cells

Highly ion-conducting properties in heterostructure composites and semiconductors have drawn significant attention in recent years for developing new electrolytes in low-temperature solid oxide fuel cells (LT-SOFCs). In this work, a new semiconductor heterostructure composite SrFe_0.2Ti_0.8O_3-δ (SFT)-ZnO consisting of p-type SFT and n-type ZnO is proposed and evaluated as an electrolyte in LT-SOFCs. Electrochemical studies reveal that the prepared SFT-ZnO is a mixed ion-electron conductor possessing a high ionic conductivity of 0.21 S cm⁻¹ at 520 °C and the assembled SFT-ZnO fuel cell can achieve a favorable peak power output of 650 mW cm⁻² along with high open-circuit voltage (OCV) of 1.06 V at 520 °C. By referring the semiconductor conduction types and energy band parameters of SFT and ZnO, a p-n bulk-heterojunction effect is proposed to describe the electronic blocking and ionic promotion processes of SFT-ZnO electrolyte in a fuel cell. Our work suggests a new insight into the design of effective LT-SOFC electrolytes by using semiconductor heterostructure material.     

Intermediate temperature fuel cells based on doped ceria–LiCl–SrCl2 composite electrolyte

A new type of oxide-salt composite electrolyte, gadolinium-doped ceria (GDC)–LiCl–SrCl₂, was developed and demonstrated its promising use for intermediate temperature (400–700 °C) fuel cells (ITFCs). The dc electrical conductivity of this composite electrolyte (0.09–0.13 S cm⁻¹ at 500–650 °C) was 3–10 times higher than that of the pure GDC electrolyte, indicating remarkable proton or oxygen ion conduction existing in the LiCl–SrCl₂ chloride salts or at the interface between GDC and the chloride salts. Using this composite electrolyte, peak power densities of 260 and 510 mW cm⁻², with current densities of 650 and 1250 mA cm⁻² were achieved at 550 and 625 °C, respectively. This makes the new material a good candidate electrolyte for future low-cost ITFCs.     

High-performance NdSrCo2O5+δ–Ce0.8Gd0.2O2-δ composite cathodes for electrolyte-supported microtubular solid oxide fuel cells

NdSrCo₂O_5+δ (NSCO) is a perovskite with an electrical conductivity of 1551.3 S cm⁻¹ at 500 °C and 921.7 S cm⁻¹ at 800 °C and has a metal-like temperature dependence. This perovskite is used as the cathode material for Ce_0.8Gd_0.2O_2-δ (GDC)-supported microtubular solid oxide fuel cells (MT-SOFCs). The MT-SOFCs fabricated in this study consist of a bilayer anode, comprising a NiO–GDC composite layer and a NiO layer, and a NSCO–GDC composite cathode. Three cell designs with different outer tube diameters, GDC thicknesses, and NSCO/GDC ratios are designed. The MT-SOFC with an outer tube diameter of 1.86 mm, an electrolyte thickness of 180 μm, and a 5NSCO–5GDC composite cathode presents the best performance. The flexural strength of the aforementioned cell is 177 MPa, which is sufficient to confer mechanical integrity to the cell. Moreover, the ohmic and polarization resistance values of the cell are 0.22 and 0.09 Ω cm² at 700 °C, respectively, and 0.15 and 0.03 Ω cm² at 800 °C, respectively. These results indicate that the NSCO-GDC composite exhibits high electrochemical activity. The maximum power densities of the cell at 700 and 800 °C are 0.46 and 0.67 W cm⁻², respectively, exceeding those of existing electrolyte-supported MT-SOFCs with similar electrolyte thicknesses.     

Study on GDC-KZnAl composite electrolytes for low-temperature solid oxide fuel cells

Development of low-temperature solid oxide fuel cells (LTSOFC) is now becoming a mainstream research direction worldwide. The advancement in the effective electrolyte materials has been one of the major challenges for LTSOFC development. To further improve the performance of electrolyte, composite approaches are considered as common strategies. The enhancement on ionic conductivity or sintering behavior ceria-based electrolyte can either be done by adding a carbonate phase to facilitate the utilization of the ionic-conducting interfaces, or by addition of alumina as insulator to reduce the electronic conduction of ceria. Thus the present report aims to design a composite electrolyte materials by combining the above two composite approaches, in order to enhance the ionic conductivity and to improve the long-term stability simultaneously. Here we report the preparation and investigation of GDC-KAlZn materials with composition of Gd doped ceria, K₂CO₃, ZnO and Al₂O₃. The structure and morphology of the samples were characterized by XRD, SEM, etc. The ionic conductivity of GDC-KAlZn sample was determined by impedance spectroscopy. The composite samples with various weight ratio of GDC and KAlZn were used as electrolyte material to fabricate and evaluate fuel cells as well as investigate the composition dependent properties. The good ionic conductivity and notable fuel cell performance of 480 mW cm⁻² at 550 °C has demonstrated that GDC-KAlZn composite electrolyte can be regarded as a potential electrolyte material for LTSOFCs.     

Investigation of the sudden drop of electrolyte conductivity at low temperature in ceramic fuel cell with Ni0·8Co0·15Al0·05LiO2 electrode

The sudden drop of ionic conductivity of GDC (Gd_0.1Ce_0·9O_1.95) electrolyte in ceramic fuel cells with NCAL (Ni_0·8Co_0·15Al_0·05LiO₂) as electrode at low temperature was studied. It is found that the peak power density (PPD) of the cell with GDC electrolyte decreases linearly with the decreasing of the operation temperature above 400 °C. However, when the operation temperature drops to 400 °C, the cell PPD decreases significantly. EIS results show that the ionic conductivity of the electrolyte decreases linearly with the decrease of cell operating temperature. When the temperature decreases to approximately 400 °C, the ionic conductivity of the electrolyte decreases from 0.251 S cm^?1 at 425 °C to 0.026 S cm^?1 at 400 °C. The rapid decrease of the electrolyte ionic conductivity is considered to be the direct cause of the sudden decrease of the PPD. According to the results of XPS, FTIR and TG-DSC, LiOH/Li₂CO₃ formed in the NCAL anode diffuses into the electrolyte and melts at 419 °C or above, which is the reason for the high ionic conductivity of the electrolyte. The reason for the sudden drop of ionic conductivity is that LiOH/Li₂CO₃ and other compounds solidify in molten salts below 419 °C.     

Li2TiO3–LaSrCoFeO3 semiconductor heterostructure for low temperature ceramic fuel cell electrolyte

Semiconductor-based electrolytes have significant advantages than conventional ionic electrolyte fuel cells, especially for high ionic conductivity and power outputs at low temperatures (<600 °C). This work reports a p-n heterojunction composite electrolyte developed by a p-type La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ (LSCF) and n-type Li₂TiO₃ (LTO). It achieved a power output of 350 mW cm^?2 at 550 °C using LSCF-LTO heterostructure as the electrolyte. On the other hand, pure LSCF and Li₂TiO₃ were made as the fuel cell electrolyte as well. The former resulted immediately a short circuiting problem and exhibited no device voltage because of high electron (hole) conductivity. While the Li₂TiO₃ can reach an open circuit voltage (OCV) but deliver too low power output, 37 mW cm^?2 at 550 °C. Scanning Electron Microscope (SEM) combined with High-Resolution Transmission Electron Microscope (HR-TEM) clearly proved the formation of heterogeneous interface. Also, Fourier Transform Infrared Spectroscopy (FTIR) was performed to demonstrate the functional group of the synthesized materials. The results demonstrate clearly the semiconductor heterostructure effect. By adjusting apriority composition of the n-type and p-type components, electronic conduction is well suppressed in the membrane electrolyte. Meanwhile, by constructing p-n heterostructure and build-in field, we have succeeded in high ionic conductivity, high current and power outputs for the low temperature fuel cells. The results are interesting in general that to construct a p-n heterostructure electrolyte can be an effective and common way in developing low temperature ceramic fuel cells.     

Tunning tin-based perovskite as an electrolyte for semiconductor protonic fuel cells

The use of ceramic semiconductors to serve as an efficient proton conductor is an evolving approach in the novel emerging field of semiconductor protonic fuel cells (SPFCs). One of the most critical challenges in SPFCs is to design a sufficient proton-conductivity of 0.1 S cm^?1 below <600 °C. Here we report to tune the perovskite BaSnO₃ (BSO), a semiconductor single-phase material, to be applied as a proton-conducting electrolyte for SPFC. It was found that the oxygen vacancies play a vital role to promote proton transport while the electronic short-circuiting issue of BSO semiconductor has been justified by the Schottky junction mechanism at the anode/electrolyte interface. We have demonstrated a SPFC device to deliver a maximum power density of 843 mW cm^?2 with an ionic conductivity of 0.23 S cm^?1 for BSO at 550 °C. The oxygen vacancy formation by increasing the annealing temperature helps to understand the proton transport mechanism in BSO and such novel low-temperature SPFC (LT-SPFC).     

Designing Gadolinium-doped ceria electrolyte for low temperature electrochemical energy conversion

Reducing the operational temperature of solid oxide fuel cells (SOFC) is vital to improving their durability and lifetime. However, a traditional SOFC suffers from high ohmic and polarization losses at low temperatures, leading to poor performance. Gadolinium-doped ceria is the best ionic conductor for SOFC at lower temperatures. The present work envisages the GDC as an electrolyte for applying low-temperature solid oxide fuel cells (LT-SOFCs). So, in this regard, herein, GDC is synthesized through a wet chemical co-precipitation technique as a functional electrolyte layer fixed between two symmetrical porous electrodes NCAL (Ni_0.8Co_0.15Al_0.05LiO₂). Due to the improved surface properties of the synthesized GDC, particles perform better than commercially available GDC. The synthesized GDC electrolyte shows an impressive fuel cell performance of 569 mW/cm² and a high ionic conductivity of 0.1 S/cm at a shallow temperature of 450 °C. Moreover, the fuel cell device utilizing the synthesized GDC remained stable for 150 h of operation at a high current density of 110 mA/cm² at 450 °C. The high conduction mechanism has been proposed in detail. The results show that excellent fuel cell performance, high ionic conductivity, and better stability can be achieved at exceptionally low enough temperatures. Also, the proposed work suggests that new electrolytes can be designed for developing advanced low-temperature fuel cell technology.     

Tuning an ionic-electronic mixed conductor NdBa0.5Sr0.5Co1.5Fe0.5O5+δ for electrolyte functions of advanced fuel cells

An ionic-conducting electrolyte mainly governs the solid oxide fuel cell performance. In this work, a mixed conductor NdBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ was tuned as an electrolyte via compositing with a proton conductor BaZr_0.3Ce_0.6Y_0.1O_{3- δ} (BZCY), which realizes an ionic conductivity of 0.16 S cm^?1 at 550 °C along with fuel cell power density of 470 mW cm^?2. The 10 wt.% proton conducting BZCY can not only effectively block the electronic conductivity of NBSCF, but also greatly improve its ionic conductivity and the corresponding device's power output. The interfacial conduction could take a crucial role in the ion transporting process of BZCY-NBSCF composite. These interfaces or nanoscale grain boundaries formed amongst two phases keep excellent capability for designing and creating high performance electrochemical devices along with high-power density.     

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 SrCo_0.3Sn_0.7O_3-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 SrCo_0.3Sn_0.7O_3-δ and CeO_2-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.     

Proton conduction of novel calcium phosphate nanocomposite membranes for high temperature PEM fuel cells applications

This work describes the synthesis and evaluation of nanocomposite membranes based on calcium phosphate (CP)/ionic liquids (ILs) for high-temperature proton exchange membrane (PEM) fuel cells. Several composite membranes were synthesized by varying the mass ratios of ILs with respect to the CP and all supported on porous polytetrafluoroethylene (PTFE). The membranes exhibit high proton conductivities. Two ionic liquids were investigated in this study, namely, 1-Hexyl-3- methylimidazolium tricyanomethanide, [HMIM][C₄N₃⁻], and 1-Ethyl-3-methylimidazolium methanesulfonate, [EMIM][CH₃O₃S⁻]. At room temperature, the CP/PTFE/[HMIM][C₄N₃⁻] composite membrane possessed a high proton conductivity of 0.1 S cm⁻¹. When processed at 200 °C, and fully anhydrous conditions, the membrane showed a conductivity of 3.14 × 10⁻³ S cm⁻¹. Membranes based on CP/PTFE/[EMIM][CH₃O₃S⁻] on the other hand, had a maximum proton conductivity of 2.06 × 10⁻³ S cm⁻¹ at room temperature. The proton conductivities reported in this work appear promising for the application in high-temperature PEMFCs operated above the boiling point of water.     

Branched Glucan from Leuconostoc Mesenteroides as the channel for ionic migration in the fabrication of protonic (H+) battery

This research paper reports the characteristics of dextran-glycerol-ammonium hexafluorophosphate (NH₄PF₆) electrolyte system and application in proton batteries. The solid polymer electrolyte films are obtained by solution cast method. Interaction between dextran, glycerol and NH₄PF₆ salt is examined using Fourier transform infrared spectroscopy (FTIR) analysis. Electrolyte with 18 wt.% NH₄PF₆ exhibits the highest room temperature conductivity of (1.43 ± 0.16) × 10^?4 S cm^?1. It is inferred that the conductivity is mainly controlled by the changes in ionic mobility and diffusion coefficient. Field emission scanning electron microscopy (FESEM) analysis shows the difference in the morphology with respect to NH₄PF₆ content and verifies the conductivity result. Differential scanning calorimetry (DSC) analysis confirms that the presence of plasticizer and 18 wt.% salt has decreased the glass transition temperature (T_g). The temperature dependence of conductivity for the highest conducting electrolyte shows a typical Vogel-Tamman-Fulcher (VTF) behavior. The proton batteries have been discharged at different constant currents.     

Enhanced proton conduction in zirconium phosphate/ionic liquids materials for high-temperature fuel cells

This work reports the synthesis of high temperature proton conductors based on zirconium phosphate and imidazolium-based ionic liquids. This material is evaluated for high temperature proton exchange membrane fuel cells applications operating at 200 °C. The characterization results show high proton conductivity, enhanced water uptake properties, changes in structure, and exfoliation in zirconium phosphates crystal layers upon the introduction of the ionic liquid. The proton conductivity results demonstrate that there is an optimum amount of ionic liquid that can be introduced into zirconium phosphates to enhance its conductivity beyond which, the conductivity starts to decrease. At the optimum conditions, the addition of ionic liquids enhances the proton conductivity of the zirconium phosphates material by orders of magnitude. The results show a high proton conductivity the order of 10^?2 S cm^?1 at room temperature and high anhydrous proton conductivity of 10^?4 S cm^?1 at 200 °C. These findings indicate that the zirconium phosphate-ionic liquid material has a great potential as solid proton conductors for fuel cells applications operating at elevated temperatures.     

Synthesis and characterization of NiO/GDC–GDC dual nano-composite powders for high-performance methane fueled solid oxide fuel cells

GDC (gadolinium-doped ceria) is well known as a high oxygen ionic conductor and is a catalyst for the electrochemical reaction with methane fuel leading to the oxidation of deposited carbon that can clog the pores of the anode and break the microstructure of the anode. NiO/GDC–GDC dual nano-composite powders were synthesized by the Pechini process, which were used as an AFL (anode functional layer) or anode substrates along with a GDC electrolyte and LSCF–GDC cathode. The anodes, AFL, and electrolyte were fabricated by a tape-casting/lamination/co-firing. NiO–GDC anode and NiO/GDC–GDC anode-supported unit cells were evaluated in terms of their power density and durability. As a result, the NiO/GDC–GDC dual nano-composite demonstrated an improved power density from 0.4 W/cm² to 0.56 W/cm² with H₂ fuel/air and from 0.3 W/cm² to 0.56 W/cm² with CH₄ fuel/air at 650 °C. In addition, it could be operated for over 500 h without any degradation with CH₄ fuel.     

Tuning semiconductor LaFe0.65Ti0.35O3-δ to fast ionic transport for advanced ceramics fuel cells

Heterostructure and their associated properties like band energy, band bending, and interface play a vital role in the conduction of charge carriers. Enhancement of ionic conductivity has been observed by the semiconductor SrTiO₃ and ionic conductor heterostructure formation, such insightful effect may be beneficial for electrolyte application in solid oxide fuel cells. Herein we report the formation of semiconductor and ionic materials heterostructure of LaFe_0.65Ti_0.35O_3-δ (LFT) and Sm and Ca co-doped cerium oxide Ce_0.8Sm_0.05Ca_0.15O_2-δ (SCDC) with three folds enhancement in the ionic conductivity. When LFT-SCDC heterostructure was applied in the fuel cell, LFT-SCDC work as a good electrolyte and achieve a maximum power output density of 0.98 W/cm². LFT-SCDC maintains the ionic and electronic conduction, the presence of electrons, their blockage and the fast promotion of ion transport play a key role in physical interpretation in realizing outstanding performance and understanding the mechanism of semiconductor electrolyte ceramics fuel cells. The constructed heterostructure between two different constituent phases of LFT and SCDC has established strong band bending at heterointerface, leading to the fast ionic transport in the interface. The combination of UV–visible spectroscopy and ultraviolet photoelectron spectroscopy (UPS) determine the band structure of both constituents, where the creation of oxygen vacancies are supported by X-ray photoelectron spectroscopy (XPS). It is revealed by the various investigation of electrical properties of LFT-SCDC heterostructure that it has both electronic and ionic behavior, where the built-in electric field formed by band energy alignment helps to enhance the transport of ions.     

Development of anion conducting zeolitic imidazolate framework bottle around ship incorporated with ionic liquids

Imidazolium based ionic liquids (denoted BMIM-ILs) with the altering anions (OH⁻, HCO₃⁻ and AcO⁻) as ionic carriers were synthesized and bottle around ship introduced into the cages of zeolite-type metal organic frameworks (ZIF-8) as porous host, resulting in a series of anion-containing composites (BMIM-ILs/ZIF-8) with anion conducting. Bottle around ship method could integrate with the superiorities of BMIM-ILs and ZIF-8 for more excelling conduction and quite eliminate IL leakage. Besides, the hybrid membranes combined BMIM-ILs/ZIF-8 as fillers with polymer blends consisting of polyvinylidene fluoride (PVDF) and polyvinylpyrrolidone (PVP) as matrix were assembled with varying weight percentages. The structures of the prepared composites and hybrid membranes were inspected. It should be noted that ZIF-8 encapsulating into BMIM-OH (6.8 × 10⁻⁴ S cm⁻¹) and BMIM-HCO₃ (1.08 × 10⁻³ S cm⁻¹) reveal two orders of magnitude increase in conductivity-values comparison to that of the parent framework (2.3× 10⁻⁵ S cm⁻¹) at 353 K and ∼98% relative humidity (RH). The hybrid membrane containing 30%BMIM-OH/ZIF-8 has the advantage in proton conductivity of 1.02 × 10⁻³ S cm⁻¹ at 353 K and ∼98% RH, which is 6.49 times higher than that of the ZIF-8/PVP/PVDF. The activation energies (E_a) of BMIM-OH/ZIF-8 and its hybrid membrane are calculated to be 0.15 eV and 0.23 eV, respectively. They could be regarded as a fast-ion conductor on the ground of high conduction and low activation energy, which make them bright outlooks and wonderful potential for electrochemical devices.     

Low thermal expansion material Bi0·5Ba0·5FeO3-δ in application for proton-conducting ceramic fuel cells cathode

The cobalt-free material of Bi³⁺-doped BaFeO_3-δ (BBFO) is synthesized and applied as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) with proton conducting electrolyte Ba(Zr_0·1Ce_0·7Y_0.2)O₃ (BZCY). The as-prepared BBFO demonstrates a tetragonal structure with sufficient chemical compatibility, high thermal stability and low thermal expansion coefficient. BBFO exhibits higher electrical conductivity of 4.1 S cm⁻¹compared to the parent material BaFeO_3-δ (BFO) of 3.5 S cm⁻¹. The composite cathode BBFO-BZCY with the mass ratio of 7:3 presents a relatively low R_p of 0.128 Ω cm² at 700 °C in air. According to the oxygen reduction reaction process, the rate-determining step is transformed from charge transfer to oxygen gas (O₂) adsorption-dissociation with the rising temperature. In addition, the performance of the anode-supported cell NiO-BZCY∣BZCY∣BBFO-BZCY is 120 mW cm⁻² as the thickness of electrolyte is 150 μm. It thus promises BBFO as a novel cathode for proton-conducting ceramic fuel cells.     

Semiconductor Fe-doped SrTiO3-δ perovskite electrolyte for low-temperature solid oxide fuel cell (LT-SOFC) operating below 520 °C

High-temperature operation of solid oxide fuel cells causes several degradation and material issues. Lowering the operating temperature results in reduced fuel cell performance primarily due to the limited ionic conductivity of the electrolyte. Here we introduce the Fe-doped SrTiO_3-δ (SFT) pure perovskite material as an electrolyte, which shows good ionic conduction even at lower temperatures, but has low electronic conduction avoiding short-circuiting. Fuel cell fabricated using this electrolyte exhibits a maximum power density of 540 mW/cm² at 520 °C with Ni-NCAL electrodes. It was found that the Fe-doping into the SrTiO_3-δ facilitates the creation of oxygen vacancies enhancing ionic conductivity and transport of oxygen ions. Such high performance can be attributed to band-bending at the interface of electrolyte/electrode, which suppresses electron flow, but enhances ionic flow.     

Performance analysis of LiAl0.5Co0.5O2 nanosheets for intermediate-temperature fuel cells

Exploring high conductive materials is still a challenge for high performance intermediate-temperature fuel cells. In this study, two-dimensional LiAl_0.5Co_0.5O₂ (LACO) nanosheets coated by a compatible amorphous LiAlO₂ (LAO) layer are evaluated as proton/Li⁺ conductor electrolyte. The fuel cell in which the LAO-LACO is used as electrolyte could deliver the maximum power output of 1120 mW cm^?2 at 550 °C. The LAO coating enhances not only the ionic conductivity by modifying the space-charge regions, but also improves the LACO's chemical stability and device performance. Kelvin probe force microscopy further detected a local electric field (LEF) built in the LAO-LACO coating confines protons at the interface to transport fast. Such heterostructure with the LEF accelerating mechanism presents a novel approach for developing high-performance intermediate-temperature fuel cells.     

Effect of added Ni on defect structure and proton transport properties of indium-doped barium zirconate

We investigated the influence of Ni on protonic ceramic fuel cells based on indium-doped barium zirconate. A tubular fuel cell was fabricated and evaluated with BaZr_0.8In_0.2O_3−δ as an electrolyte. The maximum power density was 0.143 W cm⁻² and the ohmic resistance of the electrolyte was 0.91 Ω cm² at 873 K. We used secondary ion mass spectrometry to measure the dissolution of Ni in the electrolyte N to be 0.015. To clarify the effect of Ni on proton transport properties of BaZr_0.8In_0.2O_3−δ, electrical conductivity and proton concentration were measured by AC impedance analysis and thermogravimetric analysis. Electrical conductivity decreased as the NiO content increased. Conversely, proton concentration was independent of the NiO content and proton diffusivity decreased. The sample density also depended on the NiO content. The density decreased as NiO content increased. These results were consistent with the density calculated based on a model describing formation of oxygen vacancies.