Secondary reduction strategy synthesis of Pt–Co nanoparticle catalysts towards boosting the activity of proton exchange membrane fuel cells

Based on the volcanic relationship between catalytic activity and key adsorption energies, Pt–Co alloy materials have been widely studied as cathode oxygen reduction reaction (ORR) catalysts in proton exchange membrane fuel cells (PEMFCs) due to their higher active surface area and adjustable D-band energy levels compared to Pt/C. However, how to balance the alloying degree and ORR performance of Pt–Co catalyst remains a great challenge. Herein, we first synthesized a well-dispersed Pt/Co/C precursor by using a mild dimethylamine borane (DMAB) as the reducing agent. The precursor was calcined at high temperature under H₂/Ar mixed gas by a secondary reduction strategy to obtain an ordered Pt₃Co intermetallic compound nanoparticle catalyst with a high degree of alloying. The optimization of electronic structure due to Pt–Co alloying and the strong metal-carrier interaction ensure the high kinetic activity of the cell membrane electrode. Additionally, the high degree of graphitization increases the electrical conductivity during the reaction. As a result, the activity and stability of the catalyst were significantly improved, with a half-wave potential as high as 0.87 V, which decreased by only 20 mV after 10000 potential cycles. Single-cell tests further validate the high intrinsic activity of the ordered Pt₃Co catalyst with mass activity up to 0.67 A mg_pt⁻¹, exceeding the United States Department of Energy (US DOE) standard (0.44 A mg_pt⁻¹), and a rated power of 5.93 W mg_pt⁻¹.     

Mesoporous composite of LiFePO4 and carbon microspheres as positive-electrode materials for lithium-ion batteries

Mesoporous LiFePO₄/C microspheres consisting of LiFePO₄ nanoparticles are successfully fabricated by an eco-friendly hydrothermal approach combined with high-temperature calcinations using cost-effective LiOH and Fe³⁺ salts as raw materials. In this strategy, pure mesoporous LiFePO₄ microspheres, which are composed of LiFePO₄ nanoparticles, were uniformly coated with carbon (∼1.5 nm). Benefiting from this unique architecture, these mesoporous LiFePO₄/C microspheres can be closely packed, having high tap density. The initial discharge capacity of LiFePO₄/C microspheres as positive-electrode materials for lithium-ion batteries could reach 165.3 mAh/g at 0.1 C rate, which is notably close to the theoretical capacity of LiFePO₄ due to the large BET surface area, which provides for a large electrochemically available surface for the active material and electrolyte. The material also exhibits high rate capability (∼100 mAh/g at 8 C) and good cycling stability (capacity retention of 92.2% after 400 cycles at 8 C rate).     

Li-rich layered oxides: Structure,capacity and voltage fading mechanisms and solving strategies

Lithium rich layered oxides (LLOs) are attractive cathode materials for Li-ion batteries owing to their high capacity (>250 mA h g^–1) and suitable voltage (∼3.6 V). However, they suffer from serious voltage and capacity fading, which is focused in this review. First, an overview of crystal structure, band structure and electrochemical performances of LLOs is provided. After that, current understanding on oxygen loss, capacity fading and voltage fading is summarized. Finally, five strategies to mitigate capacity and voltage fading are reviewed. It is believed that these understandings can help solve the fading problems of LLOs.     

Solid-state synthesis of Li[Li0.2Mn0.56Ni0.16Co0.08]O2 cathode materials for lithium-ion batteries

Layered Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials were synthesized via a solid-state reaction for Li-ion batteries, in which lithium hydroxide monohydrate, manganese dioxide, nickel monoxide, and cobalt monoxide were employed as metal precursors. To uncover the relationship between the structure and electrochemical properties of the materials, synthesis conditions such as calcination temperature and time as well as quenching methods were investigated. For the synthesized Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ materials, the metal components were found to be in the form of Mn⁴⁺, Ni²⁺, and Co³⁺, and their molar ratio was in good agreement with stoichiometric ratio of 0.56:0.16:0.08. Among them, the one synthesized at 800 °C for 12 h and subsequently quenched in air showed the best electrochemical performances, which had an initial discharge specific capacity and coulombic efficiency of 265.6 mAh/g and 84.0%, respectively, and when cycled at 0.5, 1, and 2 C, the corresponding discharge specific capacities were 237.3, 212.6, and 178.6 mAh/g, respectively. After recovered to 0.1 C rate, the discharge specific capacity became 259.5 mAh/g and the capacity loss was only 2.3% of the initial value at 0.1 C. This work suggests that the solid-state synthesis route is easy for preparing high performance Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials for Li-ion batteries.     

Revealing the correlation between structural evolution and long-term cyclability of the LiNi0.5Co0.2Mn0.3O2/artificial graphite pouch cells at various rates

In recent years, researches on improving high-voltage performance of lithium-ion batteries incorporating LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) and artificial graphite (AG) have been widely reported. However, limited attentions have been paid to understand the effects and influence mechanisms of charge and discharge rates and charge limit currents on cyclability of NCM523/AG cells. Herein, a ∼1.9 Ah NCM523/AG pouch cell is employed, whose electrochemical and structural evolutions after 800 cycles at various rates are comprehensively investigated. We find that cycling performances are strongly influenced by charge rate, followed by limit current and discharge rate. The cell charged at a high rate and cell charged until reaching a low limit current both exhibit low capacity retentions compared to the cell discharged at a high rate. Possible failure reasons are analyzed by advanced characterizations. Results reveal that NCM523 cathodes of the cells deteriorated early experience severe transition metal dissolution, lattice distortion, and partial phase transformation. Meanwhile, the deposited transition metals on AG anodes catalyze the electrolyte consumption, lithium plating and active area loss. Finally, these side reactions notably increase cell impedance and electrochemical polarization. Undoubtedly, these findings clearly outline the challenges and optimization direction for high-rate NCM523/AG cells.     

Carbon combustion synthesis of lithium cobalt oxide as cathode material for lithium ion battery

Lithium cobalt oxide （LiCoO2） was synthesized by carbon combustion synthesis （CCS） using carbon as fuel. X-ray diffraction （XRD） and scanning electron microscope （SEM） measurements showed that carbon combustion led to the formation of layered structure of LiCoO2 and the particle size could be controlled by carbon content. For the LiCoO2 sample prepared at 800℃ for 2 h, at molar ratio of C/Co = 0.5, the particle-size distribution fell in the narrow range of 3-5 μm. Electrochemical tests indicated this LiCoO2 sample delivered an initial discharge capacity of 148 mAh/g with capacity retention rate higher than 97% after 10 cycles.     

Composing atomic transition metal sites for high-performance bifunctional oxygen electrocatalysis in rechargeable zinc–air batteries

Rechargeable zinc–air batteries have attracted extensive attention as clean, safe, and high-efficient energy storage devices. However, the oxygen redox reactions at cathode are highly sluggish in kinetics and severely limit the actual battery performance. Atomic transition metal sites demonstrate high electrocatalytic activity towards respective oxygen reduction and evolution, while high bifunctional electrocatalytic activity is seldomly achieved. Herein a strategy of composing atomic transition metal sites is proposed to fabricate high active bifunctional oxygen electrocatalysts and high-performance rechargeable zinc–air batteries. Concretely, atomic Fe and Ni sites are composed based on their respective high electrocatalytic activity on oxygen reduction and evolution. The composite electrocatalyst demonstrates high bifunctional electrocatalytic activity (ΔE = 0.72 V) and exceeds noble-metal-based Pt/C + Ir/C (ΔE = 0.79 V). Accordingly, rechargeable zinc–air batteries with the composite electrocatalyst realize over 100 stable cycles at 25 mA cm⁻². This work affords an effective strategy to fabricate bifunctional oxygen electrocatalysts for high-performance rechargeable zinc–air batteries.     

Mutual relationship of adsorption processes on the surface of a cathode and the processes taking place in the parts of a heavy-current plasma discharge close to the electrodes

The effect of alkali metal vapor on the work function of the cathode material in various MHD installations has as yet been little studied [1]. Nor has the mutual influence of adsorption processes on the cathode surface and processes taking place in the parts of a plasma discharge close to the electrodes, although this information is extremely vital in order to make a correct determination of the emission characteristics of cathodes in plasma. The manner in which the electrode becomes coated with the plasma material determines the work function of the electrode and thus the discharge current density and cathode potential drop _s. On the other hand, the degree of coverage of the cathode with the adsorbed particles depends substantially on the value of _s. In this paper we shall propose a method of calculating the emission characteristics of cathodes during a heavy-current plasma discharge allowing for the mutual influence of the processes in question. The problem is solved in a one-dimensional setting for an automatic thermionic-emission discharge (discharge of the spotless type).Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 29–32, September–October, 1972.     

Lithium-storage properties of SiO2 nanotubes@C using carbon nanotubes as templates

Silica-based anode material is the most concerned material at present, which has the advantages of good cycle stability, high theoretical specific capacity and abundant reserves. However, silica suffers from inherent low conductivity, severe volume expansion effect and low initial coulombic efficiency, which limits its application in lithium-ion batteries. Nanotubes structure can mitigate the volume expansion during lithiation/delithiation. In this article, silica nanotubes (SNTs) were prepared using carbon nanotubes (CNTs) as a template, and then the uniform carbon layer was coated on their surface by carbonization of citric acid. The hollow structure of nanotubes provides more sites for the insertion of Li⁺ during lithiation and additional channels for Li⁺ migration in the cycles, which improves the electrochemical performance. Conductivity can be enhanced by coating carbon layer. The specific capacity of the composite material is about 650 mAh g⁻¹ at 0.1 A g⁻¹ after 100 cycles. With a specific capacity of 400 mAh g⁻¹ even at 1 A g⁻¹ after 100 cycles. The silica-based material is a competitive anode material for lithium-ion batteries.     

CeO2 nanoparticles-decorated CoP nanocubes for accelerating alkaline electrocatalytic oxygen evolution reaction

Constructing heterojunction interface as an active catalyst is an effective strategy to boost electrocatalytic activity of oxygen evolution reaction (OER). Herein, we report an interfacial CoP/CeO₂ heterostructure catalyst constructed by interface engineering and selective phosphorization procedure. X-ray photoelectron spectroscopy (XPS) suggests that coupling CeO₂ nanoparticles on the surface of CoP will generate interfacial interaction at the two-phase interface, resulting in electron transfer between CoP and CeO₂ components at the interface. Benefitting from the interfacial interaction, large exposed interface area, and luxuriant mesopores structure, CoP/CeO₂ shows fascinating alkaline OER performance. At the current densities of 10 and 50 mA cm⁻², the optimal CoP/CeO₂ heterojunction exhibits lower overpotential (257 and 298 mV) than either CoP (288 and 354 mV) or RuO₂ (305 and 409 mV). This work provides a facile synthetic protocol for constructing heterostructure interfaces to improve OER performance.     

Multi-ion intercalated Ti3C2Tx MXene and the mutual modulation within interlayer

Intercalation of ions between the adjacent MXene layers can change the interlayer environment and influence the electrochemical ion storage capacity. In order to understand the effect of multi-ions confined by the MXene layers on the performance of electrochemical energy storage, Co²⁺, Mn²⁺ and Ni²⁺ intercalated into Ti₃C₂T_x MXene which already pre-intercalated Al³⁺ are obtained by spontaneous static action. Based on the monitor of (002) crystal orientation, intercalated multi-ions can regulate and control the interlayer environment of MXenes via stress, which induces lattice shrinkage occurring in the c axis. Limited by ion storage mechanism-performance, the multi-ion occupies the interspace of MXene and affects the electrochemical performance. This work would offer guidance to understand the relationship among the multi-ion and MXene by two-dimensional (2D) layered materials.     

Microwave-assisted polyol synthesis of LiMnPO4/C and its use as a cathode material in lithium-ion batteries

We synthesized LiMnPO₄/C with an ordered olivine structure by using a microwave-assisted polyol process in 2:15 (v/v) water–diethylene glycol mixed solvents at 130 °C for 30 min. We also studied how three surfactants—hexadecyltrimethylammonium bromide, polyvinylpyrrolidone k30 (PVPk30), and polyvinylpyrrolidone k90 (PVPk90)—affected the structure, morphology, and performance of the prepared samples, characterizing them by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, charge/discharge tests, and electrochemical impedance spectroscopy. All the samples prepared with or without surfactant had orthorhombic structures with the Pnmb space group. Surfactant molecules may have acted as crystal-face inhibitors to adjust the oriented growth, morphology, and particle size of LiMnPO₄. The microwave effects could accelerate the reaction and nucleation rates of LiMnPO₄ at a lower reaction temperature. The LiMnPO₄/C sample prepared with PVPk30 exhibited a flaky structure coated with a carbon layer (∼2 nm thick), and it delivered a discharge capacity of 126 mAh/g with a capacity retention ratio of ∼99.9% after 50 cycles at 1C. Even at 5C, this sample still had a high discharge capacity of 110 mAh/g, demonstrating good rate performance and cycle performance. The improved performance of LiMnPO₄ likely came from its nanoflake structure and the thin carbon layer coating its LiMnPO₄ particles. Compared with the conventional polyol method, the microwave-assisted polyol method had a much lower reaction time.     

Gradient augmented reinitialization scheme for the level set method

A hybrid scheme for reinitializing the level set function and its gradient within the frame work of the augmented level set method is presented. It is based on first dividing the domain into an interfacial region (i.e. nodes close to the interface) and its complement. Within the interfacial region, the level set and its gradient are updated explicitly through a modified version of Newton's method (Chopp, 2001, SIAM J. Sci. Comput. 23 230‐244) and is implemented here within the context of Hermite polynomials. In the region away from the interface, the solution pertains to a semi‐Lagrangian implementation of the reinitialization equations, which are solved based on Hermite polynomials and are time marched with a single step and a multipoint scheme. It is shown that for various exercises, the present method predicts the signed distance function and its gradient to 4^th and 3^rd order (in space), respectively with regards to the L₁, L₂, and L _∞ norms, provided the level set field is sufficiently smooth. A range of test cases are also considered from the literature, where the present method is compared with existing methods and shown to be generally more accurate. Moreover, the well‐known issue of volume loss due to reinitialization is addressed successfully with the current implementation, even for objects that are of the size of one grid cell, and whose local radius of curvature falls below the local grid size. For both time marching schemes, it is shown that the L₂ and L _∞ errors decay to negligible levels, are smooth in space, and do not exhibit temporal oscillations. Finally the performance of the hybrid scheme is evaluated by applying it on various kinematic test cases. For solid body rotation problems (zero deformation flow field), the benefit stemming from hybrid reinitialization is marginal. When applied to kinematic cases involving severe deformation, such as the standard vortex flow, the reinitialization strategy helps maintain a smooth level set field, which prevents serious numerical errors from developing.Copyright © 2013 John Wiley & Sons, Ltd.     

Measurement of thermal expansion coefficients of composites using strain gages

The measurement of the coefficients of thermal expansion (CTEs) of composite materials using electrical resistance strain gages is addressed. Analytical expressions for the CTEs of an orthotropic lamina are derived, accounting for the effects of transverse sensitivity and possible misalignment of the gages. Experiments are performed for the characterization of the thermal expansion behavior of a fiber-glass-reinforced epoxy unidirectional lamina using an invar specimen as reference material. Preliminary training cycles are performed for the determination of an optimal heating rate for the measurements, which ensures thermal equilibrium conditions. Three measurement cycles yield the principal CTEs of the lamina α₁, α₂ and α₁₂ with repeatability within ±0.34×10⁻⁶, ±0.85×10⁻⁶ and ±2.8×10⁻⁶/°C, respectively. It is noted that inhomogeneity of the specimen and variation in thermomechanical properties of the gages can cause a noticeable spead in the measurements.     

Effect of temperature and molecular weight on the interfacial tension of PS/PDMS blends

The imbedded-fiber retraction (IFR) method was used to study the effect of temperature and PDMS molecular weight on the interfacial tension of PS/PDMS blends. The interfacial tension decreased with increasing temperature and analysis of the temperature dependence using a simple linear fit gave –dγ/dT value of 0.058±0.010 dyn/cm-deg. Reported –dγ/dT values of PS/PDMS blends are highly dependent on the molecular weights of the polymers and can have values that are <0, 0, or >0. Our interfacial tension values were independent of the molecular weight of PDMS and this was attributed to the molecular weights studied here being well above the entanglement values of both polymers. However, analysis of interfacial tension data from this work and the literature showed the following empirical relationship between apparent blend molecular weight, M_b, and interfacial tension of PS/PDMS blends with a correlation of 0.94: γ₁₂=γ₀+k₂M_b ^(–2/3), where γ₀=7.3±0.3 dyn/cm; k₂=–517±41 (dyn/cm)(g/mol)^2/3.     

Synthesis of layered cathode materials Li[CoxNiyMn1−x−y]O2 from layered double hydroxide precursors

Cathode materials Li[Co_xNi_yMn_1?x?y]O₂ for lithium secondary batteries have been prepared by a new route using layered double hydroxides (LDHs) as a precursor. The resulting layered phase with the α-NaFeO₂ structure crystallizes in the rhombohedral system, with space group R-3m having an interlayer spacing close to 0.47 nm. X-ray photoelectron spectroscopy (XPS) was used to measure the oxidation states of Co, Ni and Mn. The effects of varying the Co/Ni/Mn ratio on both the structure and electrochemical properties of Li[Co_xNi_yMn_1?x?y]O₂ have been investigated by X-ray diffraction and electrochemical tests. The products demonstrated a rather stable cycling behavior, with a reversible capacity of 118 mAh/g for the layered material with Co/Ni/Mn = 1/1/1.     

Utilizing the capacity below 0 V to maximize lithium storage of hard carbon anodes

Compared with conventional graphite anode, hard carbons have the potential to make reversible lithium storage below 0 V accessible due to the formation of dendrites is slow. However, under certain conditions of high currents and lithiation depths, the irreversible plated lithium occurs and then results in the capacity losses. Herein, we systematically explore the true reversibility of hard carbon anodes below 0 V. We identify the lithiation boundary parameters that control the reversible capacity of hard carbon anodes. When the boundary capacity is controlled below 400 mAh g⁻¹ with current density below 50 mA g⁻¹, no lithium dendrites are observed during the lithiation process. Compared with the discharge cut-off voltage to 0 V, this boundary provides a nearly twice reversible capacity with the capacity retention of 80% after 172 cycles. The results of characterization and finite element model reveal that the large reversible capacity below 0 V of hard carbon anodes is mainly benefited from the dual effect of lithium intercalation and reversible lithium film. After the lithium intercalation, the over-lithiation induces the quick growth of lithium dendrites, worsening the electrochemical irreversibility. This work enables insights of the potentially low-voltage performance of hard carbons in lithium-ion batteries.     

SiCH油/TiN薄膜复合体系的摩擦学性能研究

本文中采用多弧离子镀TiN薄膜对钢基体进行表面改性与SiCH润滑油相结合的方式,研究了SiCH油/TiN薄膜复合体系的真空摩擦学性能,并分析了该复合润滑体系的摩擦磨损机理.研究表明:在SiCH油/TiN薄膜复合体系中,摩擦副对偶双方表面均采用TiN薄膜进行改性后,由于TiN薄膜具有良好的稳定性和耐磨性,与SiCH润滑油构成的复合润滑体系在长寿命摩擦试验中表现出良好的减摩抗磨性能,平均摩擦系数约0.07,在经过1.8×10~6r的摩擦试验后,尽管SiCH油中形成了微量的多甲基基团的硅碳化合物Si-[R-(CH_3)_3]_3并未影响其良好的润滑性能,表明SiCH油/TiN薄膜复合体系耐磨寿命高达1.8×10~6r以上.