For the first time, we report the electrochemical activity of anatase TiO2 nanorods in a Na cell. The anatase TiO2 nanorods were synthesized by a hydrothermal method, and their surfaces were coated ...by carbon to improve the electric conductivity through carbonization of pitch at 700 °C for 2 h in Ar flow. The resulting structure does not change before and after the carbon coating, as confirmed by X-ray diffraction (XRD). Transmission electron microscopic images confirm the presence of a carbon coating on the anatase TiO2 nanorods. In cell tests, anodes of bare and carbon-coated anatase TiO2 nanorods exhibit stable cycling performance and attain a capacity of about 172 and 193 mAh g–1 on the first charge, respectively, in the voltage range of 3–0 V. With the help of the conductive carbon layers, the carbon-coated anatase TiO2 delivers more capacity at high rates, 104 mAh g–1 at the 10 C-rate (3.3 A g–1), 82 mAh g–1 at the 30 C-rate (10 A g–1), and 53 mAh g–1 at the 100 C-rate (33 A g–1). By contrast, the anode of bare anatase TiO2 nanorods delivers only about 38 mAh g–1 at the 10 C-rate (3.3 A g–1). The excellent cyclability and high-rate capability are the result of a Na+ insertion and extraction reaction into the host structure coupled with Ti4+/3+ redox reaction, as revealed by X-ray absorption spectroscopy.
A clear understanding of the electrochemical stability of metal components is needed to ensure the sustainability of potassium-ion battery (KIB) materials. Herein, the effect of highly concentrated ...7 M potassium-bis(fluorosulfonyl)imide (KFSI) in dimethoxyethane (DME) electrolyte is investigated to determine the electrochemical stability of the Cu, Al, and 316L stainless steel components. Dynamic- and transient-mode polarization reveal that the Cu is passivated with a Cu–O (CuO or Cu2O) layer below 3.55 V vs. K+/K. Above this threshold potential, dissolution of Cu2+ ions is inevitable, causing general corrosion of the Cu metal. Strikingly, Al and 316L stainless steel are passivated even at 5 V vs. K+/K in the highly concentrated electrolyte, with two double layers; namely, an outermost M − F (MF3, M: Al, Fe, or Cr) layer, below which inner M − O (M2O3) layers sit on the metal bulk. In contrast, progressive dissolution of metal ions occurs for these metals in diluted electrolyte with 0.5 M KFSI salt. This finding suggests the electrochemical availability of Al and 316L stainless steel, indicating the potential application of these metals as both current collectors and cell cases for high-voltage KIBs.
Display omitted
•Highly concentrated potassium electrolyte is investigated for passivation study.•Electrochemical stability of coin cell parts is investigated in the solution.•Cu2+ ion dissolution from Cu electrode is inevitable above 3.55 V vs. K+/K.•Al and 316L stainless steel are stable up to 5 V vs. K+/K.•The stability is due to presence of outer fluoride layer at high potential.
•Nanoporous TiO2-based composite films are fabricated via hybrid anodization.•Films form by concurrent Ti anodization and electrophoretic SnO2/MoO3 deposition.•The inclusion of SnO2 or MoO3 enhance ...the discharge capacity for ~5 and ~3 fold.•Enhancement is due to improved charge-transfer and Li+ transfer paths.•A self-modification effect of TiO2-based films upon cycling is firstly reported.
Although modification with SnO2 or MoO3 is known to improve the properties of TiO2-based anode materials for lithium-ion batteries (LIBs), simple fabrication methods are required to realize practical applications. Herein, we report a novel approach to fabricate SnO2- or MoO3-modified nanoporous TiO2–TiO–TiN composite films by a hybrid anodization process in nitric-based aqueous solutions containing SnO32− or Mo7O246− ions. Concurrent anodic reactions resulted in Ti anodization to produce a composite film and the electrophoretic deposition of SnO2 or MoO3 colloids in the nanopores of the matrix film. Both TiO2–TiO–TiN@SnO2 and TiO2–TiO–TiN@MoO3 composite films exhibited enhanced discharge capacities (~5- and 3-fold higher than that of the bare matrix film). This enhanced performance was attributed to the synergic effect of improved charge-transfer and additional capacity by depositing nanocrystalline SnO2 and MoO3 nanoparticles in the TiO2–TiO–TiN films, and the presence of nanoporous structures, which provided suitable Li+ transfer paths and reaction sites. The hybridization of nanoporous TiO2 films with SnO2 and MoO3 nanoparticles can simultaneously enhance the discharge capacity and address structural degradation issues in Sn- and MoO3-based electrodes. This simple hybrid anodization approach provides a promising strategy for designing high-power-density and high-safety anode materials for future LIBs.
Display omitted
Titania nanorods and nanowires are synthesized via a hydrothermal reaction of amorphous TiO2 in alkaline NaOH, followed by ion exchange in HCl aqueous solution, and dehydration at 400 °C. Although ...the hydrothermal treatment produces three different particle morphologies depending on the reaction time (nanosheets, nanorods, and nanowires), the products exhibit the same crystal structure. Ion exchange of Na2Ti3O7 in HCl aqueous solution brings about a phase change to H2Ti3O7, but there is no change in the particle morphology. Dehydration of the nanostructured H2Ti3O7 leads to two types of crystal structure—anatase TiO2 for the nanorods, and TiO2–B for the nanowires—although no significant difference is found in the morphology of the products even after dehydration. The nanorods are 40–50 nm in length and 10 nm in diameter, whereas the nanowires are several micrometers in length and tens to hundreds of nanometers in thickness. In‐situ X‐ray diffraction revealed the formation of anatase TiO2 from the TiO2–B above 450 °C. This finding implies that the phase transformation occurs rather slowly for the TiO2–B nanowires due to the larger particle size and higher crystallinity of H2Ti3O7. Tests with Li‐metal half cells indicated that the anatase TiO2 nanorods are more favorable for the storage and release of Li ions because of their greater surface area than the TiO2–B nanowires.
Titania nanorods and nanowires produced by hydrothermal reaction of amorphous TiO2 are presented here. Ion exchange of the products gave protonated H2Ti3O7, while dehydration of the H2Ti3O7 nanorods and nanowires at 400 °C led to different crystal structures; namely, anatase TiO2 nanorods and TiO2–B nanowires. Particles with higher surface area (nanorods) displayed facile Li+ transport due to the shortened diffusion path in the anatase TiO2 nanorods.
The interface reaction between Al2O3-coated LiLi0.05Ni0.4Co0.15Mn0.4O2 and liquid electrolyte was investigated. The Al2O3-coated LiLi0.05Ni0.4Co0.15Mn0.4O2 showed no large difference in the bulk ...structure, comparing to bare LiLi0.05Ni0.4Co0.15Mn0.4O2. The coated Al2O3 was found to have an amorphous structure from X-ray diffraction study. A small amount of Al2O3 coating (0.25 wt % in the final composition) showed that a uniform mesoporous Al2O3-coating layer whose thickness is of about 5 nm covers LiLi0.05Ni0.4Co0.15Mn0.4O2 particles, as confirmed by transmission electron microscopy. At higher concentration (2.5 wt % in the final composition), the irregular tens of nanometer-sized Al2O3 powders were observed on the surface of the active material instead of the uniform coating layer. Despite the insulating nature of Al2O3, the thin coating was effective to improve the battery performances, depending on the thickness of the Al2O3-coating layer, and used electrolytic salt. The Al2O3 coating resulted in a higher capacity retention, especially at 60 °C. The alumina layer was significantly protective against HF attack into the electrolyte during cycling so that the decomposition of active material from HF attack would be greatly suppressed. The lower impedance would be ascribed to the positive effects on the electrode/electrolyte interface, the less amount of decomposition of active material by HF and/or scavenging of HF by Al2O3-coating layer into the electrolyte. These effects made it possible to maintain the morphology of active material during extensive cycling. Meanwhile, the bare particles were severely degraded by cycling due to HF.
Type 310S stainless steel bipolar plate is investigated by means of single cell operated at several current densities for 500h. During the cell operations, the voltage decay is drastic at a lower ...current density (139mV at 0Acm−2), while the fluctuation is mitigated at a higher current density (21mV at 0.5Acm−2). The operation results are highly related to the surface conditions of bipolar plates, in particular, the cathode part. At the lower current density, the thickened layer mainly composed of iron oxide layers appears. Meanwhile, the outermost surface is enriched with thin chromium oxide layers when the higher current is applied. It is likely that the thick passive layer on the type 310S stainless steel increases the interfacial contact resistance between the gas diffusion layer and the bipolar plates of the cathode, thereby resulting in progressive voltage decay during the operation. Interestingly, general corrosion is not involved throughout the cell operation, confirming the superiority of type 310S stainless steel under the cell operation environment.
Facile synthesis of rhombohedral type FeF3 introduced via two consecutive steps is introduced: i) acidic treatment of Fe2O3 followed by thermal evaporation at 80 °C resulting in hydrated β-FeF3·3H2O ...and ii) a simple thermal decomposition of the as-received β-FeF3·3H2O at 400 °C under an Ar atmosphere. A Rietveld refinement of x-ray diffraction data for the as-synthesized FeF3 indicates the formation of a highly crystalline FeF3 structure with a R3¯c space group. To overcome the high ionicity and improve the diffusivity, FeF3 is ball-milled with the aid of carbon (acetylene black). The electrochemical performance of nanosized FeF3 is not favored in voltage range of 1.5–4.5 V because the repetitive intercalation–conversion reaction accelerates the structural disruption within a few cycles, although a high capacity (518 mAh (g-fluoride)−1 at 20 mA g−1) is observed, assisted by the three-electron redox of Fe3+/0. Raising the lower cut-off voltage to 2 V, which allows only intercalation reaction, the FeF3 delivers a high capacity of 224 mAh g−1 with significantly improved capacity retention (71% at 100th cycle).
► FeF3 is readily synthesized via a direct reaction of Fe2O3 and HF. ► The resulting β-FeF3·3H2O transforms to FeF3 at 400 °C in an Ar atmosphere. ► The FeF3 delivers a high capacity of 224 mAh g−1 with good capacity retention. ► This results from the maintenance of the crystal structure by topotactic reaction.
Aluminum and aluminum alloys are light materials with some of the preferences such as lightweight, high specific strength, good elasticity, and good workability that play an important role in today's ...modern and industrial world. The economic loss and environmental and safety problems are also the most concerning aspects of these materials. As a result, numerous studies are performed by the researchers to improve the overall environment throughout the materials world. That is why, in this study, the surface morphology and corrosion behavior of pure aluminum and its alloys, such as 7S10 and 7003H, were investigated in aqueous sulfuric acid medium through immersion process at different temperatures. Open‐circuit potential and potentiodynamic polarization techniques were used to evaluate the resistance to corrosion of pure aluminum and its alloys 7S10 and 7003H. The current density increased with increasing temperature in the case of the alloys, and pure aluminum showed the highest corrosion resistance properties. The surface roughness measurement was performed using atomic force microscope to find out the amount of roughness of the used materials before and after the immersion process. Surface roughness was higher on the alloys than in pure Al, which indicates that less corrosion was formed in pure Al than in the alloys. The surface morphology analysis was also carried out using scanning electron microscopic data. The results revealed that the alloy 7003H undergoes more corrosion than pure aluminum and 7S10 in sulfuric acid medium, which clearly indicates that pure aluminum has higher corrosion resistance than the alloys 7S10 and 7003H. The corrosion rate of the test materials decreased with increasing immersion time.
The surface morphology and corrosion behavior of pure aluminum and its alloys such as 7S10 and 7003H were investigated in aqueous sulfuric acid at different temperatures with the help of open circuit potential and potentiodynamic polarizations. The microscopic images, atomic force microscope images, and scanning electron microscopic data were also analyzed.
Stainless steels (types 304 and 310S) were employed as bipolar plates for polymer electrolyte membrane fuel cells. For the cell operation, the decayed cell voltage was approximately 22
mV for the ...type 310S stainless steel after 1000
h operation, while that for type 304 stainless steel was about 46
mV. Corrosion products appeared on the cathode side bipolar plate for the type 304 stainless steel, while trace of corrosion was barely detected for type 310S stainless steel. In order to follow the pH on the bipolar plates during fuel cell operation, polarization tests were carried out for the type 310S stainless steel in synthetic solutions (0.05
M SO
4
2− (pH 1.2–5.5)
+
2
ppm F
−) as a function of pH (1.2–5.5) at 353
K. We also examined the contact resistance between the stainless steel and carbon diffusion layer before and after polarization. X-ray photoelectron spectroscopic (XPS) analyses were carried out for comparison of the surface states of the steels after the polarization tests and cell operation. In the synthetic solutions with lower pHs (≤3.3), the films were thinner and were mainly composed by enriched with chromium oxide. Whereas, they mainly consisted of relatively thick iron oxide when the solution pH was higher (≥4.3). XPS analyses for the bipolar plate of type 310S stainless steel on cathode side after cell operation demonstrated pH gradient on the plate, that is, the thicker iron-rich surfaces presented relatively higher pH from the gas inlet to center area, and the thinner chromium-rich surface appeared with lower pH around the gas outlet.
