Catalysts by pyrolysis: Direct observation of transformations during re-pyrolysis of transition metal-nitrogen-carbon materials leading to state-of-the-art platinum group metal-free electrocatalyst

Transition metal-nitrogen-carbon (M-N-C) materials have been the focus of scientists’ efforts to address the rising need for earth-abundant materials solutions for energy technology and decarbonization of the economy. They are viewed as one of the most promising candidates to replace platinum group metal (PGM) catalysts in the fuel cell and energy conversion fields, including the application of oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction. In the effort to improve M-N-C materials properties and achieve atomic dispersity of the transition metal in the carbonaceous matrix, a re-pyrolysis process has been proposed. This secondary heat treatment process of already obtained primary pyrolysis-derived M-N-C materials has been widely reported to substantially improve the electrochemical performance and operational stability of the catalysts. Here, we report a systematic investigation of this process used on samples of templated M-N-C catalysts to obtain state-of-the-art catalysts via in situ heating X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), and X-ray computed tomography (CT) characterization methods. It is found that the re-pyrolysis of M-N-C materials could result in the partial amorphization of the carbonaceous substrate. It causes the rearrangement and transformation of multitudinous N moieties, leading to optimization of their morphological display and association with atomically dispersed transition metal dopants. Ultimately, the re-pyrolysis results in an increase in uniformity of the active Fe-N_x sites distribution without the formation of nano-crystalline phases (metallic or carbide) and with overall preservation of the morphology of the carbonaceous framework achieved during the first formative pyrolysis step of the templated synthesis. These observations provide confirmation that empirically established re-pyrolysis is recommended to be used on all M-N-C materials despite the different synthesis routes to obtain a practical advanced catalytic material.     

Anodic corrosion of heteroatom doped graphene oxide supports and its influence on the electrocatalytic oxygen evolution reaction

Transition metal-based electrocatalysts supported on carbon substrates face the challenges of anodic corrosion of carbon during oxygen evolution reaction at high oxidation potential. The role of electrophilic functional groups (carbonyl, pyridinic, thiol, etc.) incorporated in graphene oxide has been studied towards the anodic corrosion resistance. Heteroatom functionalized carbon supports possess modified electronic properties, surface oxygen content, and hydrophilicity, which are crucial in governing electrochemical corrosion in the alkaline oxidative environment. Evidently, electron-withdrawing groups in NGO support (pyridinic, cyano, nitro, etc) and its lower oxygen content impart maximum corrosion resistance and anodic stability in comparison to the other sulfur-doped and co-doped graphene oxide support. In this report, we establish the baseline evaluation of carbon-supported OER electrocatalysts by a systematic analysis of activity and substrate corrosion resistance. The result of this study establishes the role of surface composition of the doped supports while for designing a stable, corrosion-resistant OER electrocatalyst.     

Synergistic effect of Fe and Ag co-doping on the persistent photoconductivity of vertical ZnO nanorods

Undoped, doped and co-doped vertically aligned ZnO nanorods (NRs) are synthesized using sonicated sol-gel immersion method. A significant variation in structural, morphological, optical and photoconductivity properties of ZnO NRs after incorporation of transition metal ions (Fe or/and Ag) is obtained. XRD analysis revealed that incorporation of Fe ameliorates while that of Ag deteriorates the c-axis growth of NRs. The diameter of the NRs is tuned from 236 nm to 103 nm. The Fe-doped ZnO NRs exhibit significantly thinner diameter, longer length, and highest aspect ratio. The doping and co-doping reduces the optical band gap of ZnO by 20 meV and 10 meV respectively. A reduction in near band edge emission whereas enhancement in defect-related-green-emission is obtained. Noticeable enhancement in the light harvesting efficiency and significant quenching of the persistent photoconductivity is obtained by co-doping.     

Preparation of carbon fibers@NiS/Ni3S4@MoS2 with core-sheath structure for high efficiency and wide-bandwidth microwave absorption

Numerous studies have focused on the preparation of carbon fibers (CFs)-based high-efficiency microwave absorbers with reasonable structural design and surface morphology control, which simultaneously meet the required impedance matching and loss ability. Here, CFs@NiS/Ni₃S₄@MoS₂ (CNNM) with core-sheath structure was prepared through several simple hydrothermal reactions. The morphology of the as-prepared CNNM nanocomposite is controlled by the amount of added sodium molybdate dihydrate, which causes the difference in minimum reflection loss (RL) and effective attenuation bandwidth among the samples. For the microwave absorbing performance, the minimum RL is ?18 dB and the effective attenuation bandwidth is 8.7 GHz, which appear at the thickness of 2.8 mm and cover most of the X- and Ku-bands. The excellent absorbing performance is attributed to optimized impedance matching and enhanced polarization loss. These results originate from the transition metal sulfides, which not only effectively prevent the skin effect by decreasing the conductivity of CFs but also increase interfaces and flaws, leading to interface polarization and dipole polarization losses.     

Transition metal nanoparticles doped carbon paper as a cost-effective anode in a microbial fuel cell powered by pure and mixed biocatalyst cultures

The poor wettability and high cost of the carbonaceous electrodes materials prohibited the practical applications of microbial fuel cells (MFCs) on large scale. Here, a novel nanoparticles of metal sheathed with metal oxide is electrodeposited on carbon paper (CP) to introduce as high-performance anodes of microbial fuel cell (MFC). This thin layer of metal/metal oxide significantly enhance the microbial adhesion, the wettability of the anode surface and decrease the electron transfer resistance. The investigation of the modified CP anodes in an air-cathode MFCs fed by various biocatalyst cultures shows a significant improving in the MFC performance. Where, the generated power and current density was 140% and 210% higher as compared to the pristine CP. Mixed culture of exoelectrogenic microorganism in wastewater exhibited good performance and generated higher power and current density compared to yeast as pure culture. The excellent capacitance with a distinctive nanostructure morphology of the modified-CP open an avenues for practical applications of MFCs.     

Ultra-wideband balun and power divider using coplanar waveguide to microstrip transitions

An ultra-wideband transition from coplanar waveguide (CPW) to a pair of microstrip lines is presented in this letter. It is shown that the proposed transition can be used to devise an ultra-wideband T-junction power divider with equally split in-phase output signals. More importantly, it is demonstrated that the out-of-phase magnetic currents in the slots of the CPW can be utilized to achieve an out-of-phase power divider or a balun. Both the T-junction power divider and the balun benefit from compact size and good performance over an ultra wide frequency band from 3 to 16 GHz. Prototypes of both proposed devices are fabricated and measured to provide an experimental verification on the concept and numerically predicted features.     

A first-principles investigation on mechanical and metallic properties of titanium carbides under pressure

The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase diagram of the Ti-C system at 0 K was obtained by elucidating formation enthalpies as a function of compositions, and their mechanical and metallic properties of titanium carbides were investigated systematically. We also discussed the relation of titanium concentration to the both mechanical and metallic properties of titanium carbides. It has been found that the average valence electron density and tractility improved at higher concentrations of titanium, while the degree of covalent bonding directionality decreased. To this effect, the hardness of titanium carbide decreases as the content of titanium increases. Our results indicated that the titanium content significantly affected the metallic properties of the Ti-C system.     

纳米抛光碳化硅压力对相变影响的分子动力学模拟

为了研究纳米抛光碳化硅时压力变化对表面的影响规律,建立了金刚石磨粒纳米抛光碳化硅的分子动力学模型,数值模拟了纳米尺度下的碳化硅抛光过程,具体分析了抛光压力线性增大过程中的配位数为1至6的原子数量的变化规律,揭示了线性改变抛光压力对被加工表面相变的影响规律,仿真结果表明:压力是诱导碳化硅相变的主要因素,当抛光压力增大时,发生相变的原子数增多,碳化硅的相变深度增加,其中配位数为1、2和4的原子数减少,配位数为3、5和6的原子数增多。     

Trimetallic NiFeCr-LDH/MoS2composites as novel electrocatalyst for OER

The development of efficient and stable oxygen evolution reaction (OER) catalysts is an ongoing challenge. In order to solve the problem of low oxygen evolution efficiency of the current OER catalysts, a novel material was synthesized by the incorporation of NiFeCr-LDH and MoS₂, and its structural and electrochemical properties were also investigated. The introduction of MoS₂ improves the electrochemical performance of NiFeCr-LDH. The polarization curve shows that the potential of composite material is only 1.50 V at a current density of 10 mA cm^?2, which is far superior to commercial precious metal catalysts. In addition, the stability experiment shows that the composite material has excellent stability, and the current density has little change after 500 cycles. Furthermore, we found that some metal ions, such as Ni, Cr and Mo, exist in the form of high valence on the surface of NiFeCr-LDH@MoS₂, which is also conducive to the occurrence of oxygen evolution reaction.     

Template confined construction of Fe–NiCoP/NiCoP/NF heterostructures for highly efficient electrocatalytic oxygen evolution reaction

Rational design of oxygen evolution reaction (OER) electrocatalysts with advance nanostructures and composition superiority is an urgent need to promote electrocatalytic property. In this research, we fabricate Fe–NiCoP/NiCoP/NF electrocatalyst for OER via the interfacial scaffolding strategy with Prussian-blue-analogue (PBA) followed by low-temperature phosphating. The cube-on-sheet multimetallic-TMPs-based nanoarchitecture of Fe–NiCoP/NiCoP/NF exhibits outstanding OER performance, which only requires the overpotential of 201 mV to achieve a current density of 10 mA cm⁻² and possesses good durability up to 50 h in 1.0 M KOH solution. The superior OER property of Fe–NiCoP/NiCoP/NF is mainly characteristic to the rich composition that optimizes the electronic structure and the cube-on-sheet multimetallic-TMPs-based nanoarchitecture which can facilitate more effective active sites exposure and ultimately promote charge transfer at the same time. This research provides a new strategy for the construction of advanced nanoarrays structure and the improvement of the electrocatalytic performance of polymetallic phosphides, which offers its promising applications especially in energy storage and conversion technology.