H2 Gas Evolution Research Articles

Lithium Carbonate as a Corrosion Inhibitor for Mg Alloy Sheet Metal Product

Reducing the overall mass of transportation vehicles by the use of strong lightweight materials is arguably the best enabling means to improve fuels efficiency and, thus reduce harmful emissions. Wrought Mg alloys with high specific strength, stiffness and warm formability (ZE alloys) provide enabling opportunities as an integral part of lightweight multi-material structural assemblies to achieve this goal. One key technical issue preventing widespread implementation of Mg alloy components is the high rate of dissolution (corrosion) in aqueous environments. This is driven in large part by the inherent reactivity of Mg (lowest standard reduction potential among industrial engineering metals), the high rate of the hydrogen evolution reaction that dominates the cathodic process and the tendency of the native oxide surface film to breakdown and be replaced with a significantly thicker and much less protective corrosion product film. Dissolved Li2CO3 (aq) is being evaluated as an effective corrosion inhibitor for bare Mg alloy sheet metal product (AZ31B and ZEK100) when immersed in 0.1 M NaCl (aq) as a precursor to the development of a conversion coating technology. The extent of corrosion protection is being determined at various Li2CO3 levels (mM) by making volumetric H2 gas evolution and mass loss measurements as a function of immersion time. Complementary potentiodynamic polarization measurements are being conducted to determine the manner in which dissolved Li2CO3 (aq) affects the global anodic and/or cathodic processes that control corrosion. SVP measurements are being made to resolve and track changes in the localized filament corrosion and associated cathodic activation resulting from effective inhibition of dissolved Li2CO3 (aq). Both surface analysis by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) examination of FIB-prepared cross-sections are being utilized to determine the film chemistry and local fine-scale structure involved in the filament corrosion and associated cathodic activation if and when it develops with time. A similar set of measurements is being made with Li(OH)2 (aq) additions (at equivalent pHs) to determine the specific role of Li+ (aq) and CO3 2− (aq) ions in providing improved corrosion control. This paper presents and discusses the results within the context of developing a self-healing conversion coating layer for inclusion in automotive paint schemes.

Fabrication, characterisation and corrosion of HA coated AZ31B Mg implant material: Effect of electrodeposition current density

Due to its biodegradability, Mg has recently been chosen as a potential temporary load bearing implant material. However the corrosion resistance of Mg must be improved to control its degradation rate matching with the bone healing process. One way to solve the issue is to deposit another bioactive and biocompatible hydroxyapatite (HA) coating on Mg implant via electrodeposition. With this motivation, the current paper explores how the current density, a crucial process parameter, influences the formation, characteristics and degradation of HA-coating. Electrodeposition at three different current densities (3, 6 and 13 mA/cm2) was performed on AZ31B Mg substrate followed by an alkali treatment to ensure the complete HA deposition. Surface morphology, chemical composition, crystallinity and texture of the coating were evaluated, followed by potentiodynamic corrosion tests to assess the degradation resistance of the coated samples. Characterisation results show that low current density of 3 mA/cm2 results in a compact, dense and uniform coating layer as compared to 6 and 13 mA/cm2. Accelerated H2 gas evolution at Mg cathode and faster deposition at higher current density (of 13 mA/cm2) would cause the lack of integration and bonding of crystals, thus resulting in relatively porous, inhomogeneous and rough HA coating. HA coating deposited at low current density is shown to exhibit the best corrosion protection compared to those at higher current density. The findings clearly indicate that the integrity of the coating layer is a crucial quality indicator. Higher current density can produce the coating thickness of as high as 50 μm with appropriate application of the current density followed by alkaline treatment. It is also demonstrated that the electrodeposition could be a promising viable method to fabricate HA-coated AZ31B implants with better and prolonged corrosion protection.

Alkaline Corrosion-Resistant Anodic Aluminum Oxide Formed By Etidronic Acid Anodizing

A newly discovered electrolyte for anodizing aluminum, etidronic acid (CH3C(OH)[PO(OH)2]2), operates at high voltages of more than 200 V, and ordered porous alumina measuring 400-670 nm in cell diameter (interpore distance) can be fabricated via constant voltage anodizing. Because such porous alumina film formed at high voltages possesses a thick barrier layer, etidronic acid anodizing may be useful for corrosion protection of aluminum. Here, we describe the alkaline corrosion-resistant porous alumina formed by high voltage anodizing in an etidronic acid solution, and its hydration behavior in boiling water was investigated. Highly pure aluminum plates (99.999 wt%) were anodized in 0.3 M sulfuric acid, 0.3 M oxalic acid, 0.3 M citric acid, and 0.3 M etidronic acid solutions (293 K) at a constant current density of 30 Am-2 or a constant voltage of 250 V. The anodized specimens were immersed in a 2.5 M NaOH solution (293 K). The nanostructural changes were examined by electrochemical impedance spectroscopy (EIS). The anodized specimens were immersed in boiling water to seal the pores in the porous alumina film. The surface and cross-section of the specimens were characterized by field-emission scanning electron microscopy (FE-SEM) The plateau voltages during anodizing aluminum at a constant current density of 30 Am-2 in sulfuric and etidronic acid solutions exhibited approximately 16 V and 197 V, respectively. As the specimen anodized in sulfuric acid was immersed in a 2.5 M sodium hydroxide solution, H2 gas evolution was observed from the surface at the beginning of immersion, and the amount increased with the immersion time. In contrast, the specimen anodized in etidronic acid did not result in hydrogen gas evolution during immersion until 8 min. Here, the capacitance (Cb ) is inversely proportional to the thickness of the barrier layer. Figure 1 shows the changes in the reciprocal Cb with the immersion time, ti . Complete dissolution of porous alumina formed in etidronic acid takes 15 times longer than dissolution of the alumina formed by sulfuric acid anodizing due to the formation of a thick barrier layer. As the porous alumina formed by etidronic acid anodizing is immersed in boiling water, plate-like hydroxides were formed at the bottom of the pores in the initial immersion stage. The thickness of the hydroxide layer increased with the immersion time, and the nanopores are completely sealed by long-term immersion. Figure 1

In-depth structural understanding of zinc oxide addition to alkaline electrolytes to protect aluminum against corrosion and gassing

Several approaches have been tried to reduce Al corrosion in primary and secondary alkaline batteries. The most successful to date has been the addition of ZnO to the electrolyte, which results in the spontaneous deposition of a protective metallic Zn film on the Al surface. However, there is a limited understanding in the literature of the structure of this Zn film and how that structure (i) influences the protection of Al, and (ii) is influenced by the electrolyte composition. This work aims to develop a fundamental understanding of the protection mechanism against Al corrosion when zincate is added to KOH electrolytes. This was accomplished through morphological studies of the resulting Zn film using SEM, XRD, N2 adsorption, and measurements of H2 gas evolution as well as electrochemical characterization during discharge. It was found that adding saturated ZnO to KOH electrolytes provides the most protective Zn film with good adherence and high density, which can lower the Al corrosion rate by 2 orders of magnitude. Moreover, configurations with a capacity near the Al theoretical capacity were achieved.

The effects of CuO additives as the dendrite suppression and anti-corrosion of the Zn anode in Zn-air batteries

The new copper oxide-zinc composites (CuO–Zn) can be used to relieve the dendrite formation, corrosion reaction, and increase the reversibility of Zn anode. CuO–Zn composites were produced by mixing Zn and additive powders to prevent dendrites and the Zn corrosion and can be easily manufactured technically by simply adding it. To confirm the dendrite suppression, an electrodeposition method was conducted. Surface of Zn anode was analyzed by FE-SEM images and CuO showed a more controlled dendrite morphology than other Cu compounds. After the CuO–Zn electrodes were produced using 0.1, 0.5, 1.0, and 3.0wt.% CuO in Zn, an H2 gas evolution test was adapted to examine the anti-corrosion. The 0.5wt.% CuO electrode exhibited the lowest H2 gas evolution. To assess the improved performances, several electrochemical measurements like cathodic overpotential, tafel polarization, cyclic voltammetry, and DC-cycling test (full cell test) were conducted and the 0.5wt.% CuO mixed Zn electrode showed the best reversibility of all samples and longer cycle life until 19 cycles than 13 cycles of pristine Zn.

Comparative Study of Chloride and Fluoride Induced Al Pad Corrosion in Wire Bonded Packaging Assembly

Microelectronics are used in virtually all electronic equipment’s nowadays ranging from medical instruments, automobiles, computers, cell phones and home appliances. Wirebonding process which connects the chip to the lead frame in IC packaging plays a very important role in the commercial production of IC’s. Chip usually consists of Al bond pads with Cu/Au wires. Thin film Al bond pads are easily vulnerable to corrosion in presence of contaminants and moisture. Bond pad corrosion is one of the common modes of corrosion failure in wire-bonded devices IC packaging. Improper rinsing, packaging and passivation defects allows moisture and contaminants into bondpads resulting in severe corrosion. Most common source of this corrosion comes from the presence of halide ions contamination as (Cl-, F-, Br- etc.) However, the corrosion morphology and the severity varies from different ions contamination as the chemistry governing the corrosion differs from one ion to another. Our research is focused on key findings on corrosion of Aluminum bond pad due to chloride (Cl-) and fluoride (F-) contamination. Our central motivation is to identify this F- corrosion mechanism, so that it will help to develop a strategy to prevent it and study the comparison between Cl- and F- Corrosion using Micro-pattern corrosion screening technique, Wire-bonded device (WBD) and other characterization technique such as GC-MS, SEM and tafel. GC-MS reveals that H2 gas evolution is observed during Cl- corrosion of the WBD, whereas little or no H2 evolution is observed during F- corrosion. SEM and EDX reveals no presence of Cl- in the corroded areas of its corrosion, whereas strong F- signal is observed in F- corrosion of the bondpads revealing, that the corrosion products formed on the surface has very poor solubility showing that Cl- and F- has a complete different corrosion morphology. Striking contrasts in corrosion morphology observed during the Cl- and F- corrosion were explained in the present work and will lead to better understanding of factors influencing the bond pad corrosion leading to the wire bond failure in IC packaging devices. [Fig. 1] Figure 1

Understanding How Structure Influences Electrochemical Behavior When ZnO Is Used to Suppress Aluminum Corrosion in Alkaline Electrolytes

The severe corrosion rate of aluminum (Al), coupled with hydrogen evolution (H2 gassing), is the major roadblock preventing the widespread deployment of Al as the anode material in primary and secondary alkaline batteries. One of the most successful approaches to reduce this corrosion has been the addition of ZnO to the alkaline KOH electrolyte, which results in the spontaneous electroless deposition of a protective metallic Zn film on the Al surface. However, the fundamental structural and electrochemical characterization of Zn on Al as well as the influence of ZnO/KOH composition need to be investigated. This study aims to develop a novel fundamental understanding of the protection mechanism against Al corrosion when zincate (from dissolved ZnO) is added to KOH electrolytes. The effects of both ZnO concentration and KOH concentration are probed. Morphological studies of the spontaneous Zn film were realized using SEM, XRD, N2 adsorption and measurements of real time H2 gas evolution. These were coupled with discharge experiments to create a new structure-function understanding for ZnO protection of Al. It should be noted that electrolytes enabling protective Zn films with good adherence and density were found, and capacities near the Al theoretical capacity were achieved.

Metal-organic framework derived Ni/NiO micro-particles with subtle lattice distortions for high-performance electrocatalyst and supercapacitor

Herein, an effective and novel strategy has been designed to increase electrochemical reaction active sites in Ni/NiO nanoparticles prepared by optimizing the annealing temperature of nickel-based metal organic framework (Ni-MOF). Note that the incorporation of Ni3+ into the primitive lattice of Ni/NiO nanoparticles will cause a subtle atomic rearrangement, providing a large number of electrochemical reaction active sites. The density functional theory (DFT) simulation results also exhibit that the introduction of Ni3+ significantly lowers the gibbs free energy of atomic hydrogen absorption on the surface of Ni/NiO, benefiting H2 gas evolution. On the basis of the above advantages, the as-synthesized Ni/NiO nanoparticles exhibit excellent performance for supercapacitors (SCs, the areal specific capacitance of 684.4 m F cm−2 at 1 mA cm−2) and Hydrogen evolution reaction (HER, ultralow overpotential of 41 mV at 10 mA cm−2). Significantly, the all-solid-state flexible asymmetric SCs based on Ni/NiO as anode and CNTs-COOH as cathode displays a high energy density of 61.3 W h kg−1 at the power density of 900 W kg−1. This work highlights the crucial role of the lattice distortion of materials for future applications in the sustainable energy storage and conversion.

Chemical and electronic structure analysis of a SrTiO3 (001)/p-Ge (001) hydrogen evolution photocathode

Abstract

Fe3O4@SiO2 magnetic nanoparticles for bulk scale synthesis of 4′-chloro-2,2′:6′,2″-terpyridine

Terpyridines are unique class of functional compounds that is extensively spotlighted in diverse fields like synthesis of supramolecular chemistry, nanomaterials, medicinal chemistry intermediates, drugs and active pharmaceutical ingredients and so on. The key challenges for the production of terpyridine lie in the bulk scale synthesis of intermediates. The expansively used synthon for terpyridine synthesis is 4′-chloro-2,2′:6′,2″-terpyridine and their bulk scale synthesis under the ambient conditions using a Fe3O4@SiO2 magnetic nanomaterial catalyst is investigated in the present work. In the protocol stabilized, ethyl-2-pinacolate and acetone were reacted in the presence of NaH to obtain 1,5-bis(2-pyridinyl) pentane-1,3,5-trione. The enolate of acetone is difficult to generate even with NaH and we used Fe3O4@SiO2 to increase the rate of H2 gas evolution. The triketone is cyclized with CH3COONH4 to obtain 2,6-bis(2-pyridinyl)-4-pyridine. This reaction proceeds quantitatively and the off-white solid was easy to isolate from the reaction medium. The subsequent aromatization was observed with PCl5/POCl3 and acidic silica gel promoted the product yield to reach ~78%. The crux of the present protocol is that it does not involve any column purification and significant yield of 4′-chloro-2,2′:6′,2″-terpyridine can be conveniently attained. The Fe3O4@SiO2 aids in the stabilization of carbonyl on the solid support and abstraction of hydrogen from methyl group of acetone. The 40 nm sized Fe3O4@SiO2 favored the maximum yield attributed to the density of active sites to promote the reactions. Due to high value nature of 4′-chloro-2,2′:6′,2″-terpyridine, the nominal 30% yield improvement achieved at the bulk scale gauges significant at the industrial scale.

Enhanced catalytic behavior of Ni alloys in steam methane reforming

Abstract The dissociation process of methane on Ni and Ni alloys are investigated by density functional theory (DFT) in terms of catalytic efficiency and carbon deposition. Examining the dissociation to CH3, CH2, CH, C, and H is not sufficient to properly predict the catalytic efficiency and carbon deposition, and further investigation of the CO gas-evolving reaction is required to completely understand methane dissociation in steam. The location of alloying element in Ni alloy needed be addressed from the results of ab-inito molecular dynamics (MD). The reaction pathway of methane dissociation associated with CO gas evolution is traced by performing first-principles calculations of the adsorption and activation energies of each dissociation step. During the dissociation process, two alternative reaction steps producing adsorbed C and H or adsorbed CO are critically important in determining coking inhibition as well as H2 gas evolution (i.e., the catalytic efficiency). The theoretical calculations presented here suggest that alloying Ni with Ru is an effective way to reduce carbon deposition and enhance the catalytic efficiency of H2 fueling in solid oxide fuel cells (SOFCs).

1-Allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as an effective organic additive in aluminum-air battery

In Al-air batteries, the self-corrosion of Al electrodes and the parasitic hydrogen evolution during battery discharge are the major defects that lead to the degradation in the performance of the battery. Here, the ionic liquid (1-Allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) (IL) is used as an organic additive to minimize the H2 gas evolution and corrosion rate in the alkaline electrolyte (4.0M NaOH) for Al-air battery. The efficiency of IL compound is investigated using H2 gas evolution, polarization, electrochemical frequency modulation (EFM) and battery performance measurements. The surface morphology of Al electrode was characterized by scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) analysis. The results presented below show that the IL compound inhibits both the H2 gas evolution and the self-corrosion of Al electrode. In addition to this, the presence of IL compound enhances the capacity density of battery reaching to the maximum value (2554 mAhg-1) at 1.5mM of IL.

Elucidating the impact of A-site cation change on photocatalytic H2 and O2 evolution activities of perovskite-type LnTaON2 (Ln = La and Pr).

Transition metal (oxy)nitrides with perovskite-type structures have been regarded as one of the promising classes of inorganic semiconductor materials that can be used in solar water splitting systems for the production of hydrogen as a renewable and storable energy carrier. The performance of transition metal (oxy)nitrides in solar water splitting is strongly influenced by the crystal structure-related dynamics of photogenerated charge carriers. Here, we have systematically assessed the influence of A-site cation exchange on the visible-light-induced photocatalytic H2 and O2 evolution activities, photoanodic response, and dynamics of photogenerated charge carriers of perovskite-type LnTaON2 (Ln = La and Pr). The structural refinement results reveal the orthorhombic Imma and Pnma structures for LaTaON2 and PrTaON2, respectively; the latter has a more distorted crystal structure from the ideal cubic perovskite due to the smaller size of Pr3+ cations. Compared with LaTaON2, PrTaON2 exhibits lower photocatalytic H2 and O2 gas evolution activities and photoanodic response owing to an excessive amount of intrinsic defects associated with anionic vacancies and reduced tantalum species stemming from a long high-temperature nitridation process under reductive NH3 atmosphere. Transient absorption signals evidence the faster decay of photogenerated electrons (holes) in Pt (CoOx)-loaded LaTaON2 than that in Pt (CoOx)-loaded PrTaON2, consistent with the photocatalytic and photoelectrochemical performance of the two photocatalysts. This study suggests that in addition to selecting a suitable A-site cation, it is prerequisite to synthesize LnTaON2 (Ln = La and Pr) crystals with a low defect density to improve their photo-conversion efficiency for solar water splitting.

Li2O–ZnO–Co3O4–TiO2 composite thin-film electrocatalyst for efficient water oxidation catalysis

ABSTRACTThin films of different Li2O–ZnO–Co3O4–TiO2 (LZCT) compositions were prepared and employed as electrocatalysts (i.e., anodes) to perform water oxidation reaction (WOR). The electrocatalytic activities of these thin films were compared with those exhibited by the sodium salt of cobalt phosphate (Na2CoP2O7) (CP) thin-film electrocatalyst, which is a well-known water oxidation catalyst (WOC). These results suggest that the 10Li2O–10ZnO–40Co3O4–40TiO2 composition exhibits a better catalytic activity in terms of higher faradaic efficiency (>98%), lower over potentials (<400 mV), higher reaction stability (up to 30 continuous cyclic voltammetry (CV) cycles), and the rate of O2 and H2 gas evolution in terms of current density (about 1 mA/cm2) in comparison with those exhibited by CP thin-film electrocatalyst. Furthermore, these LZCT thin films exhibited very high specific surface area values and due to the unique microstructure of ZnCo2O4 phase evolved out of these LZCT compositions at a calcination temperature of 550°C for 30 min it has been found to be responsible for the higher specific surface area values measured for these thin-film compositions.

Heterogeneous Spin States in Ultrathin Nanosheets Induce Subtle Lattice Distortion To Trigger Efficient Hydrogen Evolution.

The exploration of efficient nonprecious metal eletrocatalysis of the hydrogen evolution reaction (HER) is an extraordinary challenge for future applications in sustainable energy conversion. The family of first-row-transition-metal dichalcogenides has received a small amount of research, including the active site and dynamics, relative to their extraordinary potential. In response, we developed a strategy to achieve synergistically active sites and dynamic regulation in first-row-transition-metal dichalcogenides by the heterogeneous spin states incorporated in this work. Specifically, taking the metallic Mn-doped pyrite CoSe2 as a self-adaptived, subtle atomic arrangement distortion to provide additional active edge sites for HER will occur in the CoSe2 atomic layers with Mn incorporated into the primitive lattice, which is visually verified by HRTEM. Synergistically, the density functional theory simulation results reveal that the Mn incorporation lowers the kinetic energy barrier by promoting H-H bond formation on two adjacently adsorbed H atoms, benefiting H2 gas evolution. As a result, the Mn-doped CoSe2 ultrathin nanosheets possess useful HER properties with a low overpotential of 174 mV, an unexpectedly small Tafel slope of 36 mV/dec, and a larger exchange current density of 68.3 μA cm(-2). Moreover, the original concept of coordinated regulation presented in this work can broaden horizons and provide new dimensions in the design of newly highly efficient catalysts for hydrogen evolution.

Chemical Stability of Graphite-Polypropylene Bipolar Plates for the Vanadium Redox Flow Battery at Resting State

Current collectors called bipolar plates (BPP) are important elements within the conversion unit of the vanadium redox flow battery (VRFB). They are in direct contact with acidic electrolytes, containing vanadium species in different oxidation states. The influence of the state of charge (SOC) on the calendar aging of BPPs was examined. Graphite-polypropylene BPPs were immersed in positive and negative vanadium electrolytes at 0%, 20%, 80% and 100% SOC for 30, 90 and 190 days. H2 gas evolution was observed as side reaction on the surface of the BPPs in the negative electrolyte. After electroless aging, scanning electron (SEM) and confocal microscopy measurements showed no significant changes in the surface morphology. The electrical conductivities of the BPPs were not affected significantly. However, contact angle (θ) measurements revealed that the positive electrolyte influenced the wettability of the BPPs. X-ray photoelectron (XP) spectroscopy showed progressing oxidation of the BPP surfaces in the positive electrolyte and adsorption or entrapment of vanadium ions in the pores at high SOC. Cyclic voltammograms (CV) provided evidence that the graphite was oxidized combined with an increase in effective surface area. ATR-FTIR measurements showed slight oxidation of pure polypropylene granulate in the positive electrolyte with 100% SOC.

Mg Corrosion Control By Biopolymer-Polyelectrolyte Membranes

Mg and Mg alloys offer a great potential for use as a biomedical material for temporary implant applications: its lack of toxicity, ability to easily dissolve in aqueous environments, and its bone-like bulk properties. One of the key drawbacks however, that prevents the more widespread use of magnesium-based materials is their fast and often uncontrollable corrosion within the human body that impedes its utilisation in a wide range of orthopaedic components1,2. The possibility to control this dissolution is vital if Mg alloys are to be considered as a practical, temporary biomedical implant materials. In this study we detail how this aim can be attained by the use of biocompatible polymeric membrane materials. For example, by using a polyelectrolyte like poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) to modify cellulose acetate (CA) membranes it is possible to achieve Mg dissolution control. Commercially pure magnesium surfaces were spin-coated with differing concentrations of PDMAEMA to create a number of cellulose based surfaces that were subsequently exposed to an aqueous solution. On immersion it was observed that the CA film distends and begins to function like a membrane that can control both ion flow and H2 gas evolution. Electrochemical measurements (open circuit potential, polarisation and linear sweep voltammograms) demonstrate that by varying the CA:PDMAEMA ratio, the rate of Mg corrosion can be controlled. Additional measurements of pH and with ICP-OES clearly demonstrate that the accumulation of corrosion products between the membrane and the sample also reduces the undesirable effects of H2 formation and high local pH that takes place during the corrosion process3. Furthermore, SECM studies (with ferrocenemethanol as a redox mediator and using a custom-made cell set-up) were performed in order to determine membrane permeability: knowledge over the permeability of the membrane is crucial when designing the membrane for dissolution control. Overall, these findings demonstrate that the presence of polyelectrolyte modified cellulose membranes have the potential to control Mg corrosion in physiological environments and that the permeability of the membrane can be controlled by additions of a cationic polyelectrolyte like PDMAEMA. (1) F. Witte, N. Hort, C. Vogt, S. Cohen, K. U. Kainer, R. Willumeit, F. Feyerabend, Current Opinion in Solid State and Materials Science, 2008, 12, 63-72. (2) S. Virtanen, Mater. Sci. Eng. B, 2011, 176, 1600-1608. (3) K. Yliniemi, B. P. Wilson, F. Singer, S. Höhn, E. Kontturi S. Virtanen, ACS Applied Materials and Interfaces, 2014, 6, 22393-22399.

Mechanism of Gas Generation in Lithium Ion Batteries By Overdischarge

To ensure safety, lithium ion batteries (LIBs) are used with control circuits that restrict their operation voltages within an appropriate range. However, because of the capacity variation among LIB cells and the potential failure of battery control circuits, the behavior of LIBs at voltages higher or lower than the safe range should be fully clarified. Previous studies have focused primarily on over-charge and less on over-discharge (ODC), but the risk of ODC seems to be growing due to the wide spread and long-term use of LIBs. ODC has been reported to generate gas, but the gas species detected have varied. It has also been reported that ODC causes Cu to dissolve from the anode current collector, and that deep ODC would destroy SEI films. We examined the structural change of the SEI film by ODC as well as the gas species generated to elucidate how ODC generates gas. Furthermore, we performed isotopic labeling of the electrolyte to find the origin of the gas. We prepared LiMn2O4/carbon-based LIBs with aluminum-laminate packaging by using LiPF6 and an electrolyte consisting of EC and DEC. A fresh cell and a cycled cell with a capacity that had degraded to 40% were discharged to 0.5 V. The amount of ODC-generated gas was estimated from the cell-volume change. The gas volume was 2.1 cc in the fresh cell and 33.3 cc in the degraded cell, indicating that degraded cells suffer from a higher amount of gas evolution. The generated gas was analyzed by GC-TCD (carrier gas: Ar) and QMS. The generated gas was dominantly H2 (>75%), including a small amount of CxHy and COz molecules, which was in contrast to previous studies. Moreover, QMS detected a trace of CuCO, which might be produced by a reaction between the dissolved Cu atoms and the electrolyte. The difference in the major gas species may be due to the sample materials but could also be due to the gas analysis conditions such as the carrier gas species and mass scan ranges. The thickness of the SEI films in the anode was evaluated by AES with Ar+ sputtering. The SEI thickness, which we define as the depth at which the carbon proportion reaches 50%, was about 40 nm before ODC and 25 nm after ODC. Additionally, XPS measurements on the anodes revealed that ODC reduced the Li-related SEI components. These results indicate that ODC causes SEI to decompose. We also confirmed with cross-sectional SEM-EDX that ODC roughened the Cu anode current collector, thus causing Cu atoms to diffuse into the carbon active materials. To investigate the origin of the hydrogen involved in the generated gas, we prepared two types of cells with deuterated EC (series A) and standard hydrogenated EC (series B). We cycled them at 45oC until their capacity faded to 50%. Since EC is known to decompose more easily than DEC, the series-A samples were expected to have SEI films containing D atoms after the cycling, which we confirmed by cross-sectional TOF-SIMS. We also prepared a third type of cells (series C) by replacing the electrolyte of the cycled series-B cells with that containing deuterated EC. The three types of cells were discharged to 0.5 V, and the evolved gas was analyzed by QMS. In the series-A samples, a small amount of HD and D2 molecules were detected, but the majority consisted of H2 molecules. In the series-B samples, only H2 molecules were detected as expected. The molecules generated in the series-C samples were primarily H2. These results indicate that the origin of the generated H2 is not the SEI components derived from the electrolyte (not from EC, at least), and that the H2-gas evolution is not a result of direct decomposition of the electrolyte. The origin of the H2 is presumably hydrogen that is derived from the cathode and incorporated into the SEI on the anode. In summary, we found that ODC destroys SEI films on the anode and generates H2 gas. The H2 does not seem to be derived from the electrolyte (EC); rather, H-impurities in the cathode active material are likely to be the H2 source. The volume of the ODC-induced gas increases with cycles. This tendency is attributable to a thicker SEI with H atoms and a higher anode potential upon ODC of the degraded cells. Suppressing the residual H-concentration in the cathode and SEI growth seems to be an effective way to eliminate the gas generation upon ODC.

Cathodic Activity of Corrosion Filaments Formed on Mg Alloy AM30

The filiform-like corrosion of extruded Mg alloy AM30 immersed in a dilute near-neutral NaCl solution was investigated using electrochemical techniques coupled with transmission electron microscopy (TEM). Scanning vibrating electrode technique (SVET) measurements showed that the filament-like corrosion consisted of an intensely anodic propagation front supported by a cathodically-activated filament behind. The TEM examination of the corroded and intact surface films in cross-section using thin foils prepared by focused-ion-beam (FIB) milling indicated that the cathodic activity was likely a combined result of the formation of a thick, highly-defective MgO film and the ability of Al-Mn intermetallic particles to catalyze the cathodic H2 gas evolution reaction. The formation of an Al-rich layer at the film/alloy interface with time, due to the incongruent dissolution of the alloy, was not able to stop the initiation and propagation of the filaments.

The same etchant produces both near-atomically flat and microfaceted Si(100) surfaces: The effects of gas evolution on etch morphology

The effects of H2 gas evolution during the etching of silicon surfaces by aqueous ammonium fluoride (NH4F) solutions were investigated by scanning tunneling microscopy, atomic force microscopy, optical microscopy, and noncontact profilometry. If H2 bubbles, a reaction product, were removed from the etching surface or if their coalescence was suppressed, near-atomically flat surfaces were produced. Otherwise, the etched surface developed significant roughening on many length scales with several characteristic morphological features, including nested, nearly-concentric circular etch pillars, circular etch pits, and faceted micropits. Mechanisms for the production of all three types of features are proposed. Chemical and physical means of suppressing bubble-induced surface roughening are presented. These results explain the conventional wisdom that aqueous fluoride etchants roughen Si(100) surfaces, in part by promoting the formation of Si{111} microfacets. Although some conditions promote the formation of a high density of {111}-faceted micropits (areal densities of 30%–50% were observed), microfacet formation is not inherent to the atomic-scale reactions. Instead, the microfacets are a direct result of gas evolution during the etching reaction.