Influence of deposition temperature on the microstructure of thin-film electrolyte for SOFCs with a nanoporous AAO support structure

In this paper we investigate the influence of deposition temperature on the microstructure of yttria stabilized zirconia thin-films based on an anodized aluminum oxide (AAO) support structure. The yttria-stabilized zirconia (YSZ) electrolytes were deposited on a Pt anode/AAO support using DC magnetron reactive sputtering under deposition temperatures of RT and 500 °C. Elevating the deposition temperature led to enhanced surface mobility in the sputtered adatoms, which helped prevent pinhole generation and minimized the thickness of the electrolyte. A thin-film fuel cell with a YSZ electrolyte only 300 nm thick was successfully fabricated by elevating the deposition temperature. This cell exhibited an open circuit voltage (OCV) of 0.97 V, which is significantly higher than the OCV values of 0.3 V for a cell deposited at RT. However, in spite of the thin electrolyte, the performance of the cell deposited at the higher temperature showed limited value due to its relatively high polarization resistance. Through further investigation into the grain morphology, we verify that the increasing deposition temperature can affect not only the film density but also increase the grain size of the electrolyte, which is related to oxygen incorporation for ORR kinetics. Electrochemical impedance spectroscopy (EIS) results indicate that the grain size change caused by the elevated deposition temperature adversely affected the polarization resistance and the cell performance. These results indicate that careful adoption of elevated electrolyte deposition temperatures are required to optimize fuel cell performance.     

Comparative study of doped ceria thin-film electrolytes prepared by wet powder spraying with powder synthesized via two techniques

Fabrication of dense Sm_0.2Ce_0.8O_1.9 (SDC) thin-film electrolytes by wet powder spraying in combination with high-temperature sintering is investigated. Two powder synthesis techniques, i.e., a hydrothermal synthesis and an EDTA-citrate complexing sol-gel process, were investigated. X-ray diffraction, BET surface area and laser particle size analysis demonstrate there is certain level of aggregation in both powders. However, it is more pronounced in powders obtained by the complexing process, and only the colloidal suspensions of powders prepared by hydrothermal synthesis are stable. SEM analysis of the green and sintered thin-film electrolytes demonstrate that the SDC electrolyte with powders prepared via the hydrothermal synthesis is denser. By optimizing the fabrication conditions, dense SDC electrolytes with a thickness of ∼12 μm are successfully fabricated. The cells with SDC prepared from hydrothermal synthesis demonstrate open circuit voltages and power outputs similar to those of similar cells fabricated from other advanced techniques. Because of its simplicity and flexibility for anode substrate geometric shape, it turns out to be a promising technology to fabricate thin-film SDC electrolyte for solid-oxide fuel cell application.     

Advances in the development of thin-film cells for high temperature electrolysis

An apparatus for the deposition of thin ZrO₇/Y₂O₃ electrolyte layers on porous support structures has been developed. These electrolytes can be used for advanced high-temperature water vapour electrolysis HTE-cells. The electrodes on both sides of the electrolyte were prepared by a simple spraying and sintering technique. Preliminary electrolysis experiments with single cells demonstrated the advantages of the thin-film technology in comparison with sintered electrolyte cells due to drastically reduced ohmic losses of the electrolyte. Therefore, thin-film cells have the potential to be operated with significantly increased current densities at the same operating temperature and electrolysis voltage. For the technical realization a tubular design of the HTE-cell was chosen, because this geometry allows a simple gas inlet system and a simple electrical series connection of the single cells (in order to assemble electrolysis units). Preliminary tests with such cells have been undertaken successfully.     

Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells

A comparison of three solid oxide electrolyte fabrication processes, namely dip coating, screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs) is presented in this study. The effect of sintering temperature (1325–1400 °C) is also examined. The anode and cathode layers of the anode-supported cells, on the other hand, are fabricated by tape casting and screen printing, respectively. The quality of the electrolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases with the sintering temperatures for all fabrication methods studied. The results also show that with the process and fabrication parameters considered in this study, both dip coating and screen printing do not yield a desired dense electrolyte structure as proven by open circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the other hand, provide the highest performances regardless of the electrolyte sintering and cell operating temperatures. The best peak performance of 0.924 W/cm² is obtained from the cell with tape cast electrolyte sintered at 1400 °C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 °C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion.     

Scalable fabrication process of thin-film solid oxide fuel cells with an anode functional layer design and a sputtered electrolyte

The fabrication process for anode-supported thin-film solid oxide fuel cells (SOFCs) was investigated by using scalable and cost-effective methods. The anode functional layer (AFL) was introduced on the surface of the substrate to stably deposit the thin-film electrolyte. In previous studies, the AFL has been generally designed to increase the catalytic activity; however, in this study, additional design parameters including the roughness and density were controlled to achieve a pinhole-free thin-film electrolyte and structural stability. Through the developed process, button and large-sized cells were fabricated, and the electrochemical performance evaluation showed potential power density and impedance values at relatively low operating temperature. Microstructural analyses showed that each layer of the AFL, electrolyte, and cathode was uniformly coated on the substrate. The thin-film electrolyte was densely deposited without cracks or pinholes. The electrochemical performance and microstructure confirmed that the developed thin-film SOFCs are reliable and reproducible without costly processes or materials.     

Intermediate-temperature fuel cell employing thin-film solid acid/phosphate composite electrolyte fabricated by electrostatic spray deposition

An effective resistance of solid acid/phosphate composites was reduced by fabricating their thin-film electrolyte membranes for fuel cells operating at 100-300 °C. Solid acid and phosphate serve as an ionic conductor and supporting matrix, respectively, in these composites. Three-types of porous matrices were synthesized on a Pd film substrate by the electrostatic spray deposition technique, and then the solid acid was soaked under reduced pressure. The thin-film composite electrolytes showed almost the same conductivity in a wide temperature range of 100-200 °C, regardless of the difference in matrix microstructure. Above 200 °C, however, the microstructure of matrix significantly affected the thermal stability of the thin-film composite. The composite consisting of the matrix with the reticular structure, characterized by a three-dimensional interconnected porous network, achieved high thermal stability as well as low area specific resistance. Fuel cells employing thin-film membrane electrode assemblies were successfully operated at 200 °C, and the electrochemical measurements clarified the improvements.     

Evaluation of coated metallic bipolar plates for polymer electrolyte membrane fuel cells

Metallic bipolar plates for polymer electrolyte membrane (PEM) fuel cells typically require coatings for corrosion protection. Other requirements for the corrosion protective coatings include low electrical contact resistance, good mechanical robustness, low material and fabrication cost. The authors have evaluated a number of protective coatings deposited on stainless steel substrates by electroplating and physical vapor deposition (PVD) methods. The coatings are screened with an electrochemical polarization test for corrosion resistance; then the contact resistance test was performed on selected coatings. The coating investigated include Gold with various thicknesses (2 nm, 10 nm, and 1 μm), Titanium, Zirconium, Zirconium Nitride (ZrN), Zirconium Niobium (ZrNb), and Zirconium Nitride with a Gold top layer (ZrNAu). The substrates include three types of stainless steel: 304, 310, and 316. The results show that Zr-coated samples satisfy the DOE target for corrosion resistance at both anode and cathode sides in typical PEM fuel cell environments in the short-term, but they do not meet the DOE contact resistance goal. Very thin gold coating (2 nm) can significantly decrease the electrical contact resistance, however a relatively thick gold coating (>10 nm) with our deposition method is necessary for adequate corrosion resistance, particularly for the cathode side of the bipolar plate.     

Analysis of SEI formed with cyano-containing imidazolium-based ionic liquid electrolyte in lithium secondary batteries

Two kinds of cyano-containing imidazolium-based ionic liquid, 1-cyanopropyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CpMI-TFSI) and 1-cyanomethyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CmMI-TFSI), each of which contained 20 wt% dissolved LiTFSI, were used as electrolytes for lithium secondary batteries. Compared with 1-ethyl-3-methylimidazolium-bis(trifluoromethane-sulfonyl)imide (EMI-TFSI) electrolyte, a reversible lithium deposition/dissolution on a stainless-steel working electrode was observed during CV measurements in these cyano-containing electrolytes, which indicated that a passivation layer (solid electrolyte interphase, SEI) was formed during potential scanning. The morphology of the working electrode with each electrolyte system was studied by SEM. Different dentrite forms were found on the electrodes with each electrolyte. The SEI that formed in CpMI-TFSI electrolyte showed the best passivating effect, while the deposited film formed in EMI-TFSI electrolyte showed no passivating effect. The chemical characteristics of the deposited films on the working electrodes were compared by XPS measurements. A component with a cyano group was found in SEIs in CpMI-TFSI and CmMI-TFSI electrolytes. The introduction of a cyano functional group suppressed the decomposition of electrolyte and improved the cathodic stability of the imidazolium-based ionic liquid. The reduction reaction route of imidazolium-based ionic liquid was considered to be different depending on whether or not the molecular structure contained a cyano functional group.     

Structured electrolytes to suppress dendrite growth in high energy density batteries

Dendrite formation on the anode surface of high energy density lithium batteries is closely related to the safety and capacity of batteries; therefore, the suppression of dendrite growth could significantly improve battery performance and lifetime. Many reports demonstrate the close relation between local mass transport and dendrite growth, and most of the research focuses on improving the transport properties of isotropic electrolytes (electrolytes with a uniform diffusion coefficient). Recent research reveals strong dendrite suppression effects with anisotropic electrolytes which have a directional diffusivity; however applying anisotropic electrolytes to existing battery systems is challenging. In this paper we propose several hybrid structured electrolyte designs which can generate local non‐uniform mass transport properties and induce dendrite suppression effects while still using conventional isotropic electrolytes. A numerical study is done to consider three hybrid electrolyte designs and shows that using a columnized solid structure with a typical isotropic liquid electrolyte can significantly suppress dendrite growth without sacrificing battery performance. The effects of the columnized hybrid electrolyte compare well with experimental data and suggest that through careful design of a columnar structure the benefits of an anisotropic electrolyte can be achieved without the need for developing new anisotropic liquid electrolytes. Copyright © 2016 John Wiley & Sons, Ltd.     

A novel fabrication of yttria-stabilized-zirconia dense electrolyte for solid oxide fuel cells by 3D printing technique

Three-dimensional (3D) printing technique represents a revolutionary advancement in the manufacturing sector due to its unique capabilities to process the shape complexity. This work is focusing on dense 8 mol.% yttria-stabilized-zirconia (8YSZ) electrolyte fabrication via digital light processing (DLP)-stereolithography-based 3D printing technique. Multiple 8YSZ electrolyte green bodies are printed simultaneously in a batch using ceramic-resin suspension made of 30 vol% 8YSZ powder loading in a photo-curable resin. Together with an optimized debinding and sintering procedure, the 8YSZ green body changes into a dense electrolyte, and the density of the sintered electrolyte was measured as 99.96% by Archimedes' water displacement method. The symmetric cell fabricated of silver-Ce_0.8Gd_0.2O_1.9 (Ag-GDC) as cathode/anode and dense 8YSZ electrolyte printed by DLP-stereolithography delivers a high open circuit voltage of approximately 1.04 V and a peak power density up to 176 mW·cm⁻² at 850 °C by using hydrogen as the fuel and air as the oxidant. The electrochemical performance of the symmetric cell Ag-GDC|YSZ|Ag-GDC with 8YSZ electrolyte fabricated via DLP-stereolithography is comparable to that of the same cell with 8YSZ electrolyte fabricated by conventional dry pressing method. This 3D printing technique provides a novel method to prepare dense electrolytes for solid oxide fuel cell (SOFC) with good performance, suggesting a potential application for one-step fabrication of complex structure SOFC stack.     

A promising CuOx/WO3 p-n heterojunction thin-film photocathode fabricated by magnetron reactive sputtering

A CuO_x/WO₃ thin-film based on p-n heterojunction proposed as a highly performance and stable photocathode. The CuO_x/WO₃ thin-film was deposited by magnetron reactive sputtering layer by layer, followed with slow rate annealing in O₂ ambient. This is an excellent method for high-quality and uniform composite thin-film deposition with large areas at a high growth rate. The optimized CuO_x/WO₃ thin-film photocathode after slow rate annealing at 500 °C in O₂ provides an obviously enhanced photoinduced current density of −3.8 mA cm⁻² at a bias potential of −0.5 V (vs. Ag/AgCl), which value is 1.5 times higher than that of bared CuO_x thin-film. This highly enhanced photoelectrochemical performance is attributed to p-n heterojunction, which accelerates the photogenerated electrons and holes transfer to n-WO₃ and p-CuO_x, thereby accelerate the separation of photogenerated carries. In addition, WO₃ layer covered on the surface of CuO_x thin film can improve the stability of Cu₂O in electrolytes.     

High-performance solid-state metal-air batteries with an innovative dual-gel electrolyte

An innovative dual-gel electrolyte design is proposed in this work for solid-state metal-air batteries, which utilizes the acid-alkaline dual-gel for the Al-air and Zn-air batteries, and the acid-salt dual-gel for the Mg-air battery. During battery storage, a plastic thin-film can be used for dual-gel separation, which can be removed before battery usage. And during battery working, benefited from the higher ionic diffusion resistance of gel electrolytes, the mixing of different electrolytes is significantly reduced without using complex flowing system or expensive bipolar membrane separator, ensuring a pseudo-stable battery system. The acid-alkaline dual-gel Al-air and Zn-air batteries can provide ～0.5 V higher voltage output than the corresponding alkaline single-gel batteries, and the discharge lifetime is not compromised too much due to the effective suppression of acid-alkaline neutralization. As for the acid-salt dual-gel Mg-air battery, attributed to the Mg(OH)₂ dissolution by crossovered acid, both a 0.65 V higher discharge voltage and a 5.5 times longer discharge lifetime are achieved compared with the salt single-gel battery. By simulating the variation of ion concentration inside gels during battery discharge, it is found that the effective ion diffusivity in gel plays a vital role in the battery stability. In general, a moderate value is favored to avoid both the ion shortage at the electrode-electrolyte interface and the vigorous neutralization at the gel-gel contact surface. Furthermore, a flexible version of the dual-gel metal-air battery is demonstrated by using paper-based gel electrolytes.     

Zinc ion conducting polymer electrolytes based on oligomeric polyether/PVDF-HFP blends

Here we report novel zinc ion conducting polymer electrolytes based on oligomeric polyether/PVDF-HFP blends with or without the incorporation of a small amount of organic carbonates. Their thermal properties, ionic conductivity and electrochemical properties are characterized and the effect of different Zn salts and incorporation of a small amount of organic carbonates are investigated. These polymer electrolyte membranes exhibit essentially no or very low volatility, high thermal stability, high ionic conductivity, wide electrochemical stability window, acceptable interfacial resistance with zinc, and the capability for reversible Zn plating/stripping. Particularly promising are electrolyte systems based on the combination of low lattice energy zinc imide salt and a special co-solvent of oligomeric poly(ethylene glycol) dimethyl ether (PEGDME) mixed with a small amount of ethylene carbonate (EC), dimensionally stabilized with PVDF-HFP. Such novel polymer electrolyte membranes could lead to the development of new kinds of electrochemical energy storage devices based on zinc electrochemistry, including solid-state, thin-film rechargeable zinc/air cells envisaged.     

A novel structure of ceramics electrolyte for future lithium battery

In order to fabricate large scale all-solid-state Li battery, we suggested a novel structure of solid electrolyte, which is composed of porous electrolyte supported by honeycomb-type electrolyte. A possibility of fabrication of the honeycomb-supported porous electrolyte and a compatibility of this structure with all-solid-state battery were examined using LLT (Li_0.35La_0.55TiO₃) solid electrolyte which is one of the anticipated solid electrolytes due to its high Li ion conductivity. A porous layer membrane with 3 dimensionally ordered (3DOM) macroporous structure was prepared by a colloidal crystal templating method. The porous honeycomb was fabricated by pushing the membrane into holes of honycomb using a needle followed by calcination. The 3DOM membrane and honeycmb electrolyte were sintered well each other. After filling the 3DOM pores with LiMn₂O₄ cathode material, the compatibility of this novel porous honeycomb electrolyte with all-solid-state battery was examined. The LiMn₂O₄/porous honeycomb cell clearly demonstrated charge and discharge behaviors, indicating the porous honeycomb structure can be applied to the all-solid-state battery. The discharge capacity was 71 mA h g⁻¹ (1.3 mA h cm⁻²) at 30 °C.     

Graphene oxide membranes for electrochemical energy storage and conversion

Graphene oxide (GO) membranes have recently attracted considerable attention for various applications involving filtration. In electrochemical systems, GO membranes serve as a separator or solid-state electrolyte; the roles which have been played for over four decades by the commercial ionic polymers such as Nafion. Owing to the versatility of GO membranes, they have shown an incredible potential for electrochemical energy storage and conversion. In lithium-sulphur batteries and similar electrochemical systems in which the electrode redox system is based on a conversion mechanism and subject to the so-called shuttle effect, a selective membrane can facilitate the transport of the electroactive species such as Li ions while blocking the release of the electroactive material (e.g., polysulphides) into the other half-cell. The same requirement is essential for the fabrication of redox flow batteries. Owing to the growing interest in flexible supercapacitors, there is a desire to replace liquid electrolytes with solid electrolytes, and GO membrane provides an appropriate scaffold for trapping gel electrolytes. The performance of Nafion in fuel cells has also been improved by the preparation of GO/Nafion membranes. All these different applications reveal the practical potential of GO membranes for the future energy storage and conversion.     

Yield issues on the fabrication of 30 cm×30 cm-sized Cu(In,Ga)Se2-based thin-film modules

The approaches to establish a more robust and reproducible baseline process for 30cm×30 cm-sized CIGS-based thin-film circuits with a Zn(O,S,OH)_x buffer layer are reported, which also lead to an achievement of 12.93% efficiency on an aperture area of 864 cm². Monitoring the transparency or transmittance (%T) of dip solution as a process control parameter in the chemical bath deposition (CBD)-buffer deposition step and setting the end point of dipping the CIGS-based absorbers in the solution as the %T of 60% remarkably contribute to make our CBD-buffer deposition process more reproducible. By considering carefully the growth process of metal-organic chemical vapor deposition (MOCVD)-ZnO:B window, a thin layer of high-resistivity, intrinsic ZnO is deposited on the Zn(O,S,OH)_x buffer layer to simulate the film structure of MOCVD-ZnO:B window in the case of sputtered-5.7 GZO window. Achievement of the reproducibility of 85% for the CIGS-based thin-film circuits with a sputtered-5.7 GZO window confirms that the yield goal of 85% is surely attainable independent of window-layer deposition techniques, such as MOCVD and sputtering. In this study, it is emphasized how important to eliminate unknown factors in the fabrication process for CIGS-based thin-film modules to improve both reproducibility and efficiency.     

Enhanced conductivity of SDC based nanocomposite electrolyte by spark plasma sintering

Recently, ceria-based nanocomposites have been considered as promising electrolyte candidates for low-temperature solid oxide fuel cells (LTSOFC) due to their dual-ion conduction and excellent performance. However, the densification of these composites remains a great concern since the relative low density of the composite electrolyte is suspected to deteriorate the durability of fuel cell. In the present study, the ionic conductivity of two kinds of SDC-based nanocomposite electrolytes processed by spark plasma sintering (SPS) method was investigated, and compared to that made by conventional cold pressing followed by sintering (normal processing way). The density of solid electrolyte can reach higher than 95% of the theoretical value after SPS processing, while the relative density of the electrolyte pellets by normal processing way can hardly approach 75%. The structure and morphology of the sintered pellets were characterized by XRD and SEM. The ionic conductivity of samples was measured by electrochemical impedance spectroscopy (EIS). The results showed that the ionic conductivity of the two kinds of electrolytes treated with SPS was significantly enhanced, compared with the electrolyte pellets processed through the conventional method. The profile of impedance curve of the electrolytes was altered as well. This study demonstrates that the conductivity of SDC based nanocomposite electrolyte can be further improved by adequate densification process.     

Surface layer formed on silicon thin-film electrode in lithium bis(oxalato) borate-based electrolyte

A Si thin-film electrode of 200 nm is prepared using E-beam evaporation and deposition on copper foil. The use of a lithium bis(oxalato) borate (LiBOB)-based electrolyte markedly improves the discharge capacity retention of a Si thin-film electrode/Li half-cell during cycling. The surface layer formed on Si thin-film electrode in ethylene carbonate/diethyl carbonate (3/7) with 1.3 M LiPF₆ or 0.7 M LiBOB is characterized by means of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopic analysis. The surface morphology of the electrode after cycling is investigated using scanning electron microscopy. The relationship between the physical morphology and the electrochemical performance of Si thin-film electrode is discussed.     

Effect of fluoroethylene carbonate additive on interfacial properties of silicon thin-film electrode

A silicon thin-film electrode (thickness = 200 nm) is prepared by E-beam evaporation and deposition on copper foil. The electrochemical performance of a lithium/silicon thin-film cell is investigated in ethylene carbonate/diethyl carbonate/1.3 M LiPF₆ with and without 3 wt.% fluoroethylene carbonate (FEC). The addition of FEC remarkably improves discharge capacity retention and coulombic efficiency. The surface morphology and chemical composition of the solid electrolyte interphase (SEI) formed on the surface of the silicon thin-film electrode after cycling are studied through scanning electron microscopy and X-ray photoelectron spectroscopy analysis. A smoother and more stable SEI layer structure is generated by the introduction of the FEC additive to the electrolyte.