Enhanced thermal properties of epoxy composites by constructing thermal conduction networks with low content of three-dimensional graphene.

Micro/nano electronic devices heat dissipation depends heavily on the thermal interface materials (TIMs). Despite notable progress, it is hard to efficaciously enhance the thermal properties of the hybrid TIMs with high-load additives due to an absence of effective heat transfer routes. Herein, the low content of three-dimensional (3D) graphene with interconnected networks is adopted as the additive to improve the thermal properties of epoxy composite TIMs. The thermal diffusivity and thermal conductivity of the as-prepared hybrids were dramatically improved by constructing thermal conduction networks after adding 3D graphene as fillers. The 3D graphene/epoxy hybrid's optimal thermal characteristics were observed at 1.5 wt% of 3D graphene content, corresponding to a maximum enhancement of 683%. Besides, heat transfer experiments were further performed to determine the superb heat dissipation potential of the 3D graphene/epoxy hybrids. Moreover, the 3D graphene/epoxy composite TIM was also applied to high-power LED to improve heat dissipation. It effectively reduced the maximum temperature from 79.8 °C to 74.3 °C. These results are beneficial for the better cooling performance of electronic devices and provide useful guidelines for advancing the next-generation TIMs.

Robust, Conductive, and High Loading Fiber-Shaped Electrodes Fabricated by 3D Active Coating for Flexible Energy Storage Devices.

Flexible power sources are critical to achieve the wide adoption of portable and wearable electronics. Herein, a facile and general strategy of fabricating a fibrous electrode was developed by 3D active coating technology, in which a stepping syringe with electrode paste was synchronously injected onto a rotating conductive wire, distinguished from the conventional direct-write 3D printing without a current collector. A series of such electrodes with different coating weight can be fabricated accurately and efficiently by adjusting critical process parameters following a set of derived equations. The demonstrated fibrous Zn-MnO2 battery with a high commercial ε-MnO2 loading of 14.9 mg cm-2 onto a stainless steel wire shows a reasonable energy density of 108 mWh cm-3, while the fiber-shaped supercapacitor with commercial porous graphene exhibits a high capacitance of 142.9 F g-1 and good durability for bending 10,000 cycles. This work constructs a bridge between materials and fiber-shaped electrodes for flexible energy storage devices.

Boosting Electrocatalytic Activity of Single Atom Catalysts Supported on Nitrogen-Doped Carbon through N Coordination Environment Engineering.

Nonprecious group metal (NPGM)-based single atom catalysts (SACs) hold a great potential in electrocatalysis and dopant engineering has been extensively exploited to boost their catalytic activity, while the coordination environment of dopant, which also significantly affects the electronic structure of SACs, and consequently their electrocatalytic performance, have been largely ignored. Here, by adopting a precursor modulation strategy, the authors successfully synthesize single cobalt atom catalysts embedded in nitrogen-doped carbon, Co-N/C, with similar overall Co and N concentrations but different N types, that is, pyridinic N (NP ), graphitic N (NG ), and pyrrolic N (NPY ). Co-N/C with the Co-N4 moieties coordinated with NG displays far superior activity for oxygen reduction (ORR) and evolution reactions, and superior activity and stability in both zinc-air batteries and proton exchange membrane fuel cells. Density functional theory calculation indicates that coordinated N species in particular NG functions as electron donors to the Co core of Co-N4 active sites, leading to the downshift of d-band center of Co-N4 and weakening the binding energies of the intermediates on Co-N4 sites, thus, significantly promoting catalytic kinetics and thermodynamics for ORR in a full pH range condition.

Electricity generation from ionic solution flowing through packed three-dimensional graphene powders.

Herein, we demonstrate a distinctive energy harvesting method that electricity can be generated from the ionic solution flowing through the interstices between packed three-dimensional graphene powders. A constructed electrokinetic nanogenerator with an effective flow area of ∼0.34 cm2can generate a large current of 91.33 nA under 10-6M NaCl solution with a flow rate of 0.4 ml min-1, corresponding to a maximum power density of 0.45µW m-2. Besides, it shows a good linear relationship between the streaming current and the flow rate, suggesting that it could be used as a self-powered micro-flowmeter. These results provide a convenient way for clean energy harvesting and show a bright future for self-powered systems.

In Situ S-Doping Strategy of Promoting Iron Coordinated by Nitrogen-Doped Carbon Nanosheets for Efficient Oxygen Reduction Reaction.

Improving transition metal-nitrogen-carbon (M-N-C) as a noble-metal-free catalyst for the oxygen reduction reaction (ORR) is critical to achieve low-cost electrochemical energy conversion. Herein, an in situ S doping strategy of enhancing Fe-N-C activity for ORR was developed by newly designed Fe(II) ion coordinated S-containing bis(imino)-pyridine-based polymers as precursors, which were synthesized through copolymerizing three monomers of 2, 6-diacetylpyridine (DAP), triamterene (TIT), and 2,5-dithiobiurea (DTB) as both N and S sources. All samples derived from various molar ratios of the three monomers possess a self-supporting structure of nanosheets. Additionally, incorporating DTB into the copolymer can not only strongly affect the derived coordinative species of N dopants to Fe atom but also effectively induce the synergistic effect between S dopants and FeNx moieties, resulting a significant improvement for ORR. The S-doped Fe-N-C nansheets with Fe coordinated by 4 pyrrolic N dopants exhibit the highest ORR activity and stability in alkaline media with a higher power output of Zn-air battery than that of the same loading of Pt/C. Theoretical calculation identifies that the thiophenic S dopant adjacent to Fe-pyrrolic N moiety can decrease the d band center of Fe atom, greatly weakening the energy profiles of oxygenated intermediates and thus enhancing ORR. In addition, because of the designability of transition metal coordinated S-containing bis(imino)-pyridine based polymers in the work, therefore, it is believable that this strategy would open a wide space to explore the structural relationship between precursors and MNx active sites with S dopants for the purpose of achieving highly efficient and robust M-N-C catalysts for energy-related electrocatalysis.

Flow boiling heat transfer enhancement under ultrasound field in minichannel heat sinks.

The enhancement of the heat transfer assisted by ultrasound is considered to be an interesting and highly efficient cooling technology, but the investigation and application of ultrasound in minichannel heat sinks to strengthen the flow boiling heat transfer are very limited. Herein, a novel installation of ultrasound transducers in the flow direction of a minichannel heat sink is designed to experimentally study the characteristics of heat transfer in flow boiling and the influence of operation parameters (e.g., heat flux, mass flux rate) and ultrasound parameters (e.g., frequency, power) on the flow boiling heat transfer in a minichannel heat sink with and without ultrasound field. Bubble motion and flow pattern in the minichannel are analyzed by high-speed flow visualization, revealing that the ultrasound field induces more bubbles at the same observation position and a forward shift of the onset of nucleation boiling along the flow direction, as ultrasonic cavitation produces a large number of bubbles. Moreover, bubbles hitting the channel wall on the left and right sides are found, and the motion speed of the bubbles is increased by 31.9% under the ultrasound field. Our results demonstrate that the heat transfer coefficient obtained under the ultrasound field is 53.9% higher than in the absence of the ultrasound field under the same conditions, and the enhancement ratio is decreased in the high heat flux region due to the change of the flow regime with increasing heat flux. This study provides a theoretical basis for the application of an ultrasound field in minichannel heat sinks for the enhancement of flow boiling heat transfer.

Ultrathin-shell IrCo hollow nanospheres as highly efficient electrocatalysts towards the oxygen evolution reaction in acidic media.

Improving the utilization of Ir electrocatalysts for the oxygen evolution reaction (OER) to significantly reduce their loading is essential for low-cost hydrogen production in proton exchange membrane water electrolysis. Herein, IrCo hollow nanospheres featuring a novel structure with ultrathin continuous shells which have only eleven atomic layers (2.26 nm) were synthesized by a facile sequential reduction route using NaBH4 as a reducing agent at room temperature. It is revealed that the key intermediate in the formation of hollow nanospheres is amorphous cobalt boride formed between Co2+ and NaHB4 in the first reducing step. The average diameter of the IrCo nanospheres was found to be 73.71 nm with the atomic ratio of 47.1% and 52.9% for Co and Ir, respectively. The IrCo hollow nanospheres exhibit highly efficient OER activity and long-term durability with a low overpotential of 284 mV at 10 mA cm-2 (32.5 µgIr cm-2) and a high mass activity of 8.49 A mg-1 (5.7 times higher than that of commercial IrO2 (1.49 A mg-1) at 1.7 V. The performance is also proved using an overall water splitting device with the overpotential of 318 mV to achieve 10 mA cm-2 as well as a 17 mV shift at 5 mA cm-2 after 14 h. This improvement is critically attributed to the advantages of the hollow structure, ultrathin continuous shells which are oxidized into IrOxin situ and strong lattice strain effects induced by the specific hollow structure and alloying Co into Ir crystal lattices (1.6% against metallic iridium). These characteristics endow the hollow nanospheres with great potential to minimize the Ir loading dramatically for practical applications, compared to other previously reported structures like nanoparticles, nanoneedles and nanowires.

Highly Efficient Multifunctional Co-N-C Electrocatalysts with Synergistic Effects of Co-N Moieties and Co Metallic Nanoparticles Encapsulated in a N-Doped Carbon Matrix for Water-Splitting and Oxygen Redox Reactions.

Electrochemical water-splitting reactions (hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)) and oxygen redox reactions (oxygen reduction reaction (ORR) and OER) are core processes for electrochemical water-splitting devices, rechargeable metal-air batteries, and regenerative fuel cells. Developing highly efficient non-noble multifunctional catalysts in the same electrolyte is an open challenge. Herein, efficient Co-N-C electrocatalysts with a mixed structure comprising Co-N moieties and Co nanoparticles encapsulated in a N-doped carbon layer were prepared via pyrolysis of a new structure of Co-coordinated bis(imino)pyridine polymer constructed by 2,6-diacetylpyridine and 3,3'-diaminobenzidine. Results demonstrate that Co ion sources have a remarkable impact on the final Co-N-C performance. The Co-N-C catalyst prepared using cobalt acetate as a precursor displays remarkable overall multifunctional performance. It needs only a cell voltage of 1.66 V (obtained from the half-cell test) for the water-splitting reaction (HER/OER) to reach 10 mA·cm-2 in 1.0 M KOH, and the overall oxygen redox activity (OER/ORR) is 0.72 V in 0.1 M KOH, outperforming the reported nonprecious metal catalysts. The excellent activity is attributable to the synergistic effects between active sites with encapsulated metallic Co for HER and OER and Co-N moieties for ORR.

Synthesis and characterization of platinum catalysts on multiwalled carbon nanotubes by intermittent microwave irradiation for fuel cell applications.

Pt electrocatalysts supported on multiwalled carbon nanotube (Pt/MWCNT) nanocomposites have been synthesized by a rapid intermittent microwave irradiation (IMI) technique for polymer electrolyte and direct methanol fuel cells (PEFCs and DMFCs), using H(2)PtCl(6) as Pt precursor. The Pt/MWCNT nanocomposites are characterized by XRD, XPS, and TEM. The results indicate that Pt particle size and distribution on the MWCNT support are affected significantly by the oxidation treatment of MWCNTs, the IMI procedure, and the MWCNT tube diameter or surface area. The PtO(x) (x = 1, 2) species was first deposited on the surface of MWCNTs by the IMI and subsequently reduced to Pt(0) with refluxing in the presence of HCOOH. Pt/MWCNT nanocomposites synthesized by this IMI method have achieved extremely uniform dispersed Pt nanoparticles with a particle size of approximately 3 nm. Electrochemical measurement indicates that Pt/MWCNT nanocomposites synthesized by the IMI method display a significantly higher electrochemically active area and higher catalytic activity for the methanol oxidation reaction in comparison to a commercial Pt/C catalyst.