Study on Zinc Oxide-Based Electrolytes in Low-Temperature Solid Oxide Fuel Cells

A novel yttrium stabilized zirconia and ceria composite electrolyte lowering solid oxide fuel cells working temperature to 400 °C

Reducing the working temperature and improving the ionic conductivity of electrolytes have been the critical challenges for the gradual development of solid oxide fuel cells (SOFCs) in practical applications. The researchers all over the world attempt to develop alternative electrolyte materials with sufficient ionic conductivity. In this work, YSZ-CeO2 composite material was used as electrolytes in the construction of symmetrical SOFCs. The maximum power densities (Pmax) of YSZ-CeO2 based fuel cell can reach 680 mW cm-2 at 450 °C, 510 mW cm-2 at 430 °C, 330 mW cm-2 at 410 °C and even 200 mW cm-2 as the operational temperature was reduced to 390 °C. A series of characterizations indicates that the activation energy of the YSZ-CeO2 composite is significantly decreased, and the enhancement effect for ion conduction comes from interface transport. Our findings indicate the YSZ-CeO2 composite material can be a highly promising candidate for advanced low-temperature SOFC.

Investigating the Performance of a Zinc Oxide Impregnated Polyvinyl Alcohol-Based Low-Cost Cation Exchange Membrane in Microbial Fuel Cells

The current study investigated the development and application of lithium (Li)-doped zinc oxide (ZnO)-impregnated polyvinyl alcohol (PVA) proton exchange membrane separator in a single chambered microbial fuel cell (MFC). Physiochemical analysis was performed via FT-IR, XRD, TEM, and AC impedance analysis to characterize thus synthesized Li-doped ZnO. PVA-ZnO-Li with 2.0% Li incorporation showed higher power generation in MFC. Using coulombic efficiency and current density, the impact of oxygen crossing on the membrane cathode assembly (MCA) area was evaluated. Different amounts of Li were incorporated into the membrane to optimize its electrochemical behavior and to increase proton conductivity while reducing biofouling. When acetate wastewater was treated in MFC using a PVA-ZnO-Li-based MCA, the maximum power density of 6.3 W/m³ was achieved. These observations strongly support our hypothesis that PVA-ZnO-Li can be an efficient and affordable separator for MFC.

Designing High Interfacial Conduction beyond Bulk via Engineering the Semiconductor-Ionic Heterostructure CeO2-δ/BaZr0.8Y0.2O3 for Superior Proton Conductive Fuel Cell and Water Electrolysis Applications

Proton ceramic fuel cells (PCFCs) are an emerging clean energy technology; however, a key challenge persists in improving the electrolyte proton conductivity, e.g., around 10^-3-10^-2 S cm^-1 at 600 °C for the well-known BaZr_0.8Y_0.2O₃ (BZY), that is far below the required 0.1 S cm^-1. Herein, we report an approach for tuning BZY from low bulk to high interfacial conduction by introducing a semiconductor CeO_2-δ forming a semiconductor-ionic heterostructure CeO_2-δ/BZY. The interfacial conduction was identified by a significantly higher conductivity obtained from the BZY grain boundary than that of the bulk and a further improvement from the CeO_2-δ/BZY which achieved a remarkably high proton conductivity of 0.23 S cm^-1. This enabled a high peak power of 845 mW cm^-2 at 520 °C from a PCFC using the CeO_2-δ/BZY as the electrolyte, in strong contrast to the BZY bulk conduction electrolyte with only 229 mW cm^-2. Furthermore, the CeO_2-δ/BZY fuel cell was operated under water electrolysis mode, exhibiting a very high current density output of 3.2 A cm^-2 corresponding to a high H₂ production rate, under 2.0 V at 520 °C. The band structure and a built-in-field-assisted proton transport mechanism have been proposed and explained. This work demonstrates an efficient way of tuning the electrolyte from low bulk to high interfacial proton conduction to attain sufficient conductivity required for PCFCs, electrolyzers, and other advanced electrochemical energy technologies.

Materials Research Directions Toward a Green Hydrogen Economy: A Review

A constellation of technologies has been researched with an eye toward enabling a hydrogen economy. Within the research fields of hydrogen production, storage, and utilization in fuel cells, various classes of materials have been developed that target higher efficiencies and utility. This Review examines recent progress in these research fields from the years 2011-2021, exploring the most commonly occurring concepts and the materials directions important to each field. Particular attention has been given to catalyst materials that enable the green production of hydrogen from water, chemical and physical storage systems, and materials used in technical capacities within fuel cells. The quantification of publication and materials trends provides a picture of the current state of development within each node of the hydrogen economy.

A-site deficient semiconductor electrolyte Sr1−xCoxFeO3−δ for low-temperature (450–550 °C) solid oxide fuel cells

Fast ionic conduction at low operating temperatures is a key factor for the high electrochemical performance of solid oxide fuel cells (SOFCs). Here an A-site deficient semiconductor electrolyte Sr_1−xCo_xFeO_3−δ is proposed for low-temperature solid oxide fuel cells (LT-SOFCs). A fuel cell with a structure of Ni/NCAL-Sr_0.7Co_0.3FeO_3−δ–NCAL/Ni reached a promising performance of 771 mW cm⁻² at 550 °C. Moreover, appropriate doping of cobalt at the A-site resulted in enhanced charge carrier transportation yielding an ionic conductivity of >0.1 S cm⁻¹ at 550 °C. A high OCV of 1.05 V confirmed that neither short-circuiting nor power loss occurred during the operation of the prepared SOFC device. A modified composition of Sr_0.5Co_0.5FeO_3−δ and Sr_0.3Co_0.7FeO_3−δ also reached good fuel cell performance of 542 and 345 mW cm⁻², respectively. The energy bandgap analysis confirmed optimal cobalt doping into the A-site of the prepared perovskite structure improved the charge transportation effect. Moreover, XPS spectra showed how the Co-doping into the A-site enhanced O-vacancies, which improve the transport of oxide ions. The present work shows that Sr_0.7Co_0.3FeO_3−δ is a promising electrolyte for LT-SOFCs. Its performance can be boosted with Co-doping to tune the energy band structure.

Fast ionic conduction at low operating temperatures is a key factor for the high electrochemical performance of solid oxide fuel cells (SOFCs).

Remarkable Ionic Conductivity in a LZO-SDC Composite for Low-Temperature Solid Oxide Fuel Cells

Recently, appreciable ionic conduction has been frequently observed in multifunctional semiconductors, pointing out an unconventional way to develop electrolytes for solid oxide fuel cells (SOFCs). Among them, ZnO and Li-doped ZnO (LZO) have shown great potential. In this study, to further improve the electrolyte capability of LZO, a typical ionic conductor Sm_0.2Ce_0.8O_1.9 (SDC) is introduced to form semiconductor-ionic composites with LZO. The designed LZO-SDC composites with various mass ratios are successfully demonstrated in SOFCs at low operating temperatures, exhibiting a peak power density of 713 mW cm⁻² and high open circuit voltages (OCVs) of 1.04 V at 550 °C by the best-performing sample 5LZO-5SDC, which is superior to that of simplex LZO electrolyte SOFC. Our electrochemical and electrical analysis reveals that the composite samples have attained enhanced ionic conduction as compared to pure LZO and SDC, reaching a remarkable ionic conductivity of 0.16 S cm⁻¹ at 550 °C, and shows hybrid H⁺/O²⁻ conducting capability with predominant H⁺ conduction. Further investigation in terms of interface inspection manifests that oxygen vacancies are enriched at the hetero-interface between LZO and SDC, which gives rise to the high ionic conductivity of 5LZO-5SDC. Our study thus suggests the tremendous potentials of semiconductor ionic materials and indicates an effective way to develop fast ionic transport in electrolytes for low-temperature SOFCs.

Standardized Procedures Important for Improving Low-Temperature Ceramic Fuel Cell Technology: From Transient to Steady State Assessment

As the stress–strain curve of standardized metal samples provides the basic details about mechanical properties of structural materials, the polarization curve or current–voltage characteristics of fuel cells are vitally important to explore the scientific mechanism of various solid oxide cells aiming at low operational temperatures (below 600 °C), ranging from protonic conductor ceramic cells (PCFC) to emerging Semiconductor ionic fuel cell (SIFC)/Semiconductor membrane fuel cells (SMFC). Thus far, worldwide efforts to achieve higher nominal peak power density (PPD) at a low operational temperature of over 0.1 s/cm ionic conductivity of electrolyte and super catalyst electrode is the key challenge for SIFCs. Thus, we illustrate an alternative approach to the present PPD concept and current–voltage characteristic. Case studies reveal that the holy grail of 1 W/cm² from journal publications is expected to be reconsidered and normalized, since partial cells may still remain in a transient state (TS) to some extent, which means that they are unable to fulfill the prerequisite of a steady state (SS) characteristic of polarization curve measurement. Depending on the testing parameters, the reported PPD value can arbitrarily exist between higher transient power density (TPD) and lower stable power density (SPD). Herein, a standardized procedure has been proposed by modifying a quasi-steady state (QSS) characterization based on stabilized cell and time-prolonged measurements of common I–V plots. The present study indicates, when compared with steady state value, that QSS power density itself still provides a better approximation for the real performance of fuel cells, and concurrently recalls a novel paradigm transformation from a transient to steady state perspective in the oxide solid fuel cell community.

Progress in Material Development for Low-Temperature Solid Oxide Fuel Cells: A Review

Solid oxide fuel cells (SOFCs) have been considered as promising candidates to tackle the need for sustainable and efficient energy conversion devices. However, the current operating temperature of SOFCs poses critical challenges relating to the costs of fabrication and materials selection. To overcome these issues, many attempts have been made by the SOFC research and manufacturing communities for lowering the operating temperature to intermediate ranges (600–800 °C) and even lower temperatures (below 600 °C). Despite the interesting success and technical advantages obtained with the low-temperature SOFC, on the other hand, the cell operation at low temperature could noticeably increase the electrolyte ohmic loss and the polarization losses of the electrode that cause a decrease in the overall cell performance and energy conversion efficiency. In addition, the electrolyte ionic conductivity exponentially decreases with a decrease in operating temperature based on the Arrhenius conduction equation for semiconductors. To address these challenges, a variety of materials and fabrication methods have been developed in the past few years which are the subject of this critical review. Therefore, this paper focuses on the recent advances in the development of new low-temperature SOFCs materials, especially low-temperature electrolytes and electrodes with improved electrochemical properties, as well as summarizing the matching current collectors and sealants for the low-temperature region. Different strategies for improving the cell efficiency, the impact of operating variables on the performance of SOFCs, and the available choice of stack designs, as well as the costing factors, operational limits, and performance prospects, have been briefly summarized in this work.

Complex Impedance Analyses of Li doped ZnO Electrolyte Materials

The recent studies indicate that internal point defects in solid electrolytes modify the electronic and ionic conductivity and relaxation mechanism of solid oxide fuel cells. We focused on synthesis of Lithium (Li) doped Zn1-xCoxO (x = 0.00, and 0.10) nanoparticles employing chemical synthesis technique with a reflux setup under constant Argon gas flow. The structural characterizations were performed by x-ray powder diffractometer (XRD) and x-ray photoelectron spectroscopy (XPS). Then, Rietveld refinements were performed to investigate the replacement of Li atom amount in ZnO lattice. Moreover, the variations in ionic conduction dependent on 5, 10 and 20 mol% Li doped ZnO were analysed via ac impedance spectroscopy. The complex measurements were performed in an intermediate temperature range from 100 °C to 400 °C. Ac conductivity responses of each sample were disappeared at a certain temperature due to becoming electronic conductive oxides. However, this specific temperature was tuned to high temperature by Li doping amount in ZnO lattice. Furthermore, the activation energy change by Li dopant amount implied the tuneable ionic conduction mechanism.

Preparation, Characterization and Intermediate-Temperature Electrochemical Properties of Er3+-Doped Barium Cerate-Sulphate Composite Electrolyte

In this study, BaCe_0.9Er_0.1O_3−α was synthesized by a microemulsion method. Then, a BaCe_0.9Er_0.1O_3−α–K₂SO₄–BaSO₄ composite electrolyte was obtained by compounding it with a K₂SO₄–Li₂SO₄ solid solution. BaCe_0.9Er_0.1O_3−α and BaCe_0.9Er_0.1O_3−α–K₂SO₄–BaSO₄ were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry. AC impedance spectroscopy was measured in a nitrogen atmosphere at 400–700 °C. The logσ~log (p_O2) curves and fuel cell performances of BaCe_0.9Er_0.1O_3−α and BaCe_0.9Er_0.1O_3−α–K₂SO₄–BaSO₄ were tested at 700 °C. The maximum output power density of BaCe_0.9Er_0.1O_3−α–K₂SO₄–BaSO₄ was 115.9 mW·cm⁻² at 700 °C, which is ten times higher than that of BaCe_0.9Er_0.1O_3−α.

Multiobjective Genetic Algorithm-Based Optimization of PID Controller Parameters for Fuel Cell Voltage and Fuel Utilization

Nowadays, given the great deal of fossil fuel consumption and associated environmental pollution, solid oxide fuel cells (SOFCs) have shown their great merits in terms of high energy conversion efficiency and low emissions as a stationary power source. To ensure power quality and efficiency, both the output voltage and fuel utilization of an SOFC should be tightly controlled. However, these two control objectives usually conflict with each other, making the controller design of an SOFC quite challenging and sophisticated. To this end, a multi-objective genetic algorithm (MOGA) was employed to tune the proportional–integral–derivative (PID) controller parameters through the following steps: (1) Identifying the SOFC system through a least squares method; (2) designing the control based on a relative gain array (RGA) analysis; and (3) applying the MOGA to a simulation to search for a set of optimal solutions. By comparing the control performance of the Pareto solutions, satisfactory control parameters were determined. The simulation results demonstrated that the proposed method could reduce the impact of disturbances and regulate output voltage and fuel utilization simultaneously (with strong robustness).

The sintering temperature effect on electrochemical properties of Ce0.8Sm0.05Ca0.15O2-δ (SCDC)-La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) heterostructure pellet

Recently, semiconductor-ionic materials (SIMs) have emerged as new functional materials, which possessed high ionic conductivity with successful applications as the electrolyte in advanced low-temperature solid oxide fuel cells (LT-SOFCs). In order to reveal the ion-conducting mechanism in SIM, a typical SIM pellet consisted of semiconductor La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and ionic conductor Sm and Ca Co-doped ceria Ce_0.8Sm_0.05Ca_0.15O_2-δ (SCDC) are suffered from sintering at different temperatures. It has been found that the performance of LSCF-SCDC electrolyte fuel cell decreases with the sintering temperature, the cell assembled from LSCF-SCDC pellet sintered at 600 °C exhibits a peak power density (P_max) of 543 mW/cm² at 550 °C and also excellent performance of 312 mW/cm² even at LT (500 °C). On the contrary, devices based on 1000 °C pellet presented a poor P_max of 106 mW/cm². The performance difference may result from the diverse ionic conductivity of SIM pellet through different temperatures sintering. The high-temperature sintering could severely destroy the interface between SCDC and LSCF, which provide fast transport pathways for oxygen ions conduction. Such phenomenon provides direct and strong evidence for the interfacial conduction in LSCF-SCDC SIMs.

Yb-Doped BaCeO₃ and Its Composite Electrolyte for Intermediate-Temperature Solid Oxide Fuel Cells

BaCe_0.9Yb_0.1O_3−α was prepared via the sol-gel method using zirconium nitrate, ytterbium trioxide, cerium nitrate and barium acetate as raw materials. Subsequently, it reacted with the binary NaCl~KCl salt to obtain BaCe_0.9Yb_0.1O_3−α-NaCl~KCl composite electrolyte. The structure, morphology, conductivity and fuel cell performance of the obtained samples were investigated. Scanning electron microscope (SEM) images showed that BaCe_0.9Yb_0.1O_3−α and NaCl~KCl combined with each other to form a homogeneous 3-D reticulated structure. The highest power density and conductivity of BaCe_0.9Yb_0.1O_3−α-NaCl~KCl was 393 mW·cm⁻² and 3.0 × 10⁻¹ S·cm⁻¹ at 700 °C, respectively.

Yb₂O₃ Doped Zr0.92Y0.08O2-α(8YSZ) and Its Composite Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

Yb³⁺ and Y³⁺ double doped ZrO₂ (8YSZ+4Yb₂O₃) samples were synthesized by a solid state reaction method. Moreover, 8YSZ+4Yb₂O₃-NaCl/KCl composites were also successfully produced at different temperatures. The 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) samples were characterized by x–ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that a dense composite electrolyte was formed at a low temperature of 800 °C. The maximum conductivities of 4.7 × 10⁻² S·cm⁻¹, 6.1 × 10⁻¹ S·cm⁻¹, and 3.8 × 10⁻¹ S·cm⁻¹ were achieved for the 8YSZ+4Yb₂O₃, 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C), and 8YSZ+4Yb₂O₃-NaCl/KCl (1000 °C) samples at 700 °C, respectively. The logσ~log (pO₂) plot result showed that the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite electrolyte is a virtually pure ionic conductor. An excellent performance of the 8YSZ+4Yb₂O₃-NaCl/KCl (800 °C) composite was obtained with a maximum power density of 364 mW·cm⁻² at 700 °C.

SrCe0.9Sm0.1O3-α Compounded with NaCl-KCl as a Composite Electrolyte for Intermediate Temperature Fuel Cell

SrCeO₃ and SrCe_0.9Sm_0.1O_3-α were synthesized using a high-temperature solid-state reaction method using Sm₂O₃, SrCO₃, CeO₂ as precursors, then the SrCe_0.9Sm_0.1O_3-α-NaCl-KCl composite electrolyte was fabricated by compounding SrCe_0.9Sm_0.1O_3-α with NaCl-KCl and sintering it at a lower temperature (750 °C) than that of a single SrCeO₃ material (1540 °C). The phase and microstructure of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The conductivities of the samples were measured in dry nitrogen atmosphere using electrochemical analyzer. The conductivities of the SrCeO₃, SrCe_0.9Sm_0.1O_3-α and SrCe_0.9Sm_0.1O_3-α-NaCl-KCl at 700 °C were 2.09 × 10^-5 S·cm^-1, 1.82 × 10^-3 S·cm^-1 and 1.43 × 10^-1 S·cm^-1 respectively. The conductivities of SrCe_0.9Sm_0.1O_3-α-NaCl-KCl composite electrolyte are four orders of magnitude higher than those of SrCeO₃ and two orders of magnitude higher than those of SrCe_0.9Sm_0.1O_3-α. The result of logσ ~ logpO₂ plot indicates that SrCe_0.9Sm_0.1O_3-α-NaCl-KCl is almost a pure ionic conductor. The electrolyte resistance and the polarization resistance of the H₂/O₂ fuel cell based on SrCe_0.9Sm_0.1O_3-α-NaCl-KCl composite electrolyte under open-circuit condition were 1.0 Ω·cm² and 0.2 Ω·cm² respectively. Further, the obtained maximum power density at 700 °C was 182 mW·cm^-2.

Novel Composite Electrolytes of Zr0.92Y0.08O2-α(8YSZ)-Low Melting Point Glass Powder for Intermediate Temperature Solid Oxide Fuel Cells

In this study, Zr_0.92Y_0.08O_2-α(8YSZ) powders were synthesized by the sol-gel method. The chemical physics changes and phase formation temperature of 8YSZ crystal were determined by thermogravimetry analysis and differential scanning calorimetry (TGA-DSC). 8YSZ-low melting point glass powder (8YSZ-glass) composite electrolytes with various weight ratios were prepared and calcined at different temperatures. The X-ray diffraction (XRD) patterns of the composite electrolytes were tested. The effects of synthesis temperature, weight ratio, test temperature, and oxygen partial pressure on the conductivities of 8YSZ-glass composite electrolytes, were also investigated at 400–800 °C. The result of the logσ ~ log(pO₂) plot indicates that the 8YSZ-20% glass (700 °C) is almost a pure ionic conductor. The oxygen concentration discharge cell illustrates that the 8YSZ-20% glass (700 °C) composite electrolyte is a good oxygen ion conductor.

Special Issue: Zinc Oxide Nanostructures: Synthesis and Characterization

Zinc oxide (ZnO) is a wide band gap semiconductor with an energy gap of 3.37 eV at room temperature. It has been used considerably for its catalytic, electrical, optoelectronic, and photochemical properties. ZnO nanomaterials, such as quantum dots, nanorods, and nanowires, have been intensively investigated for their important properties. Many methods have been described in the literature for the production of ZnO nanostructures, such as laser ablation, hydrothermal methods, electrochemical deposition, sol⁻gel methods, Chemical Vapour Deposition, molecular beam epitaxy, the common thermal evaporation method, and the soft chemical solution method. The present Special Issue is devoted to the Synthesis and Characterization of ZnO nanostructures with novel technological applications.