High-performance proton exchange membrane fuel cell with ultra-low loading Pt on vertically aligned carbon nanotubes as integrated catalyst layer

Reducing a Pt loading with improved power output and durability is essential to promote the large-scale application of proton exchange membrane fuel cells(PEMFCs).To achieve this goal,constructing optimized structure of catalyst layers with efficient mass transportation channels plays a vital role.Herein,PEMFCs with order-structured cathodic electrodes were fabricated by depositing Pt nanoparticles by Ebeam onto vertically aligned carbon nanotubes(VACNTs)growth on Al foil via plasma-enhanced chemical vapor deposition.Results demonstrate that the proportion of hydrophilic Pt-deposited region along VACNTs and residual hydrophobic region of VANCTs without Pt strongly influences the cell performance,in particular at high current densities.When Pt nanoparticles deposit on the top depth of around 600 nm on VACNTs with a length of 4.6μm,the cell shows the highest performance,compared with others with various lengths of VACNTs.It delivers a maximum power output of 1.61 W cm^(-2)(H_(2)/O_(2),150 k Pa)and 0.79 W cm^(-2)(H_(2)/Air,150 k Pa)at Pt loading of 50μg cm^(-2),exceeding most of previously reported PEMFCs with Pt loading of<100μg cm^(-2).Even though the Pt loading is down to 30μg cm^(-2)(1.36 W cm^(-2)),the performance is also better than 100μg cm^(-2)(1.24 W cm^(-2))of commercial Pt/C,and presents better stability.This excellent performance is critical attributed to the ordered hydrophobic region providing sufficient mass passages to facilitate the fast water drainage at high current densities.This work gives a new understanding for oxygen reduction reaction occurred in VACNTs-based ordered electrodes,demonstrating the most possibility to achieve a substantial reduction in Pt loading<100μg cm^(-2) without sacrificing in performance.

Aligned carbon nanotubes for lithium-ion batteries:A review

Nanoscale materials are gaining massive attention in recent years due to their potential to alleviate the present electrochemical electrode constraints.Possessing high conductivity(both thermally and electrically),high chemical and electrochemical stability,exceptional mechanical strength and flexibility,high specific surface area,large charge storage capacity,and excellent ionadsorption,carbon nanotubes(CNTs)remain one of the most researched of other nanoscale materials for electrochemical energy storage.Rather than having them packed at random,CNTs perform better when packed/grown to order,vertically or horizontally aligned to a substrate.This study presents an overview of the impact of CNT alignment on the electrochemical performance of lithium-ion batteries(LIBs).The unique properties of vertically aligned CNTs(VACNTs)for LIB application were discussed.Furthermore,the mechanisms of charge storage and electrochemical performances in VACNT-based(pristine and composites)anodes and cathodes of LIBs were succinctly reviewed.In the end,the existing challenges and future directions in the field were also briefly discussed.

Synthesis and Device Applications of High-Density Aligned Carbon Nanotubes Using Low-Pressure Chemical Vapor Deposition and Stacked Multiple Transfer

For nanotube-based electronics to become a viable alternative to silicon technology,high-density aligned carbon nanotubes are essential.In this paper,we report the combined use of low-pressure chemical vapor deposition and stacked multiple transfer to achieve high-density aligned nanotubes.By using an optimized nanotube synthesis recipe,we have achieved high-density aligned carbon nanotubes with density as high as 30 tubes/μm.In addition,a facile stacked multiple transfer technique has been developed to further increase the nanotube density to 55 tubes/μm.Furthermore,high-performance submicron carbon nanotube field-effect transistors have been fabricated on the high-density aligned nanotubes.Before removing the metallic nanotubes by electrical breakdown,the devices exhibit on-current density of 92.4μA/μm and normalized transconductance of 13.3μS/μm.Moreover,benchmarking with the aligned carbon nanotube transistors in the literature indicates that our devices exhibit the best performance so far,which is attributed to both the increased nanotube density and scaling down of channel length.This study shows the great potential of using such high-density aligned nanotubes for high performance nanoelectronics and analog/RF applications.

Effects of Temperature and Catalyst Concentration on the Growth of Aligned Carbon Nanotubes

The effects of preheating and pyrolysis temperatures and catalyst concentration on the synthesis of aligned carbon nanotubes （CNTs） using ferrocene as the catalyst and xylene as the carbon source in chemical vapor deposition were experimentally studied. The as-grown aligned CNTs were characterized by field emission scanning electron microscopy, transmission electronic microscopy, high-resolution transmission electronic microscopy, and Raman spectroscopy. The growth rate, the diameters, and the degree of crystal structure of the aligned CNTs were all found to depend on the preheating and pyrolysis temperatures and the catalyst concentration. The optimized conditions for the growth of aligned CNTs resulted in a rapid growth rate of 20.4 um/min, with the CNTs having a good, uniform crystal structure, and clean surfaces with little amorphous carbon. The results also show that higher preheating temperatures and lower ferrocene concentrations favor the growth of single-walled CNTs.

Low Temperature Growth of Vertically Aligned Carbon Nanotubes via Floating Catalyst Chemical Vapor Deposition Method

Synthesis of carbon nanotubes （CNTs） below 600℃ using supporting catalyst chemical vapor deposition method was reported by many research groups. However, the floating catalyst chemical vapor deposition received less attention due to imperfect nanotubes produced. In this work, the effects of varying the preheating temperature on the synthesis of CNT were investigated. The reaction temperature was set at 570℃. The preheating set temperature was varied from 150 to 400℃ at 50℃ interval. Three O-ring shape heating mantels were used as heating source for the preheater. In situ monitoring device was used to observe the temperature profile in the reactor. Benzene and ferrocene were used as the carbon source and catalyst precursor, respectively. Vertically aligned CNTs were synthesized when the preheating temperature was set at 400℃. When the preheating temperature was increased up to 400℃, both the length and the alignment of CNTs produced were improved.

Effect of Particle Density on the Aligned Growth of Carbon Nanotubes

Aligned carbon nanotubes (CNTs) were prepared on Ni-coated Ni substrate by microwave plasma chemical vapor deposition (MWPCVD) with a mixture of methane and hydrogen gases at temperature of 550℃.The experimental results show a direct correlation between the alignment of CNTs and the density of the catalyst particles at low temperature.When the particle density is high enough,among CNTs there are strong interactions that can inhibit CNTs from growing randomly.The crowding effect among dense CNTs results in the aligned growth of CNTs at low temperature.

Investigation on the Formation Mechanism of Double-Layer Vertically Aligned Carbon Nanotube Arrays via Single-Step Chemical Vapour Deposition

The mechanism for the formation of double-layer vertically aligned carbon nanotube arrays(VACNTs) through single-step CVD growth is investigated. The evolution of the structures and defect concentration of the VACNTs are tracked by scanning electron microscopy(SEM) and Raman spectroscopy. During the growth, the catalyst particles are stayed constantly on the substrate. The precipitation of the second CNT layer happens at around 30 min as proved by SEM.During the growth of the first layer, catalyst nanoparticles are deactivated with the accumulation of amorphous carbon coatings on their surfaces, which leads to the termination of the growth of the first layer CNTs. Then, the catalyst particles are reactivated by the hydrogen in the gas flow, leading to the precipitation of the second CNT layer. The growth of the second CNT layer lifts the amorphous carbon coatings on catalyst particles and substrates. The release of mechanical energy by CNTs provides big enough energy to lift up amorphous carbon flakes on catalyst particles and substrates which finally stay at the interfaces of the two layers simulated by finite element analysis. This study sheds light on the termination mechanism of CNTs during CVD process.

Nonpolar cross-stacked super-aligned carbon nanotube membrane for efficient wastewater treatment

Membrane separation technology has made great progress in various practical applications,but the unsatisfactory separation performance of prevailing membrane materials hampers its further sustainable growth.This study proposed a novel nonpolar super-aligned carbon nanotube(SACNT)membrane,which was prepared with a layer-by-layer cross-stacking method.Through controlling the number of stacked SACNT layers,three kinds of SACNT membranes(SACNT_200,SACNT 300,and SACNT_400)were prepared.Systematic characterizations and filtration tests were performed to investigate their physico-chemical properties,surface wetting behavior,and filtration performance.Compared with two commercial membranes(Com_0.22 and Com_0.45),all the SACNT membranes achieved smoother and more uniform structures.Due to the hexagonal graphene structure of CNTs,the surface chemistry of the SACNT membranes is simple and inert,thereby potentially eliminating the covalent-bonding-induced membrane fouling.Besides,the SACNT membranes exhibited a typical nonpolar wetting behavior,with high contact angles for polar liquids(water:~124.9°-126.5°;formamide:~80.0°-83.99)but low contact angles for nonpolar diodomethane(~18.8°-20.99).This unique nonpolar feature potentially leads to weak interactions with polar substances.Furthermore,compared with the commercial membranes,the SACNT membranes obtained a significantly higher selectivity while achieving a comparable or higher permeability(depending on the number of stacked layers).Moreover,the SACNT membranes exhibited superior separation performance in various application scenarios,including municipal wastewater treatment(2.3 times higher cleaning efficiency),electro-assistant fouling inhibition(or even self-cleaning),and oil/water separation(>99.2%of separation efficiency),suggesting promising application prospects in various fields.

Pave the way to the batch production of SWNT arrays for carbonbased electronic devices

Single-walled carbon nanotubes(SWNTs)have been regarded as one of the most promising candidates to supplement or even replace silicon in the post-Moore era.The requirement is to prepare the horizontally aligned SWNTs arrays(HASAs)with multiple indicators,including high density,high semiconducting purity,and wafer-scale uniformity.However,after all the fevered works being done in controlled synthesis,we still have a long way to go before realizing the application of SWNTs in highly performed electronic devices.The methods of batch production and high-throughput characterization techniques of the HASAs are the two main challenges.In this outlook,we first summarized the progresses in synthesis of HASAs with either high density or high semiconducting purity.Then the methods adopted in characterizing SWNTs and HASAs were discussed according to the different principles of characterization techniques.Afterwards,the development of carbon nanotube based electronic devices,specifically,the field effect transistors(FETs),was reviewed from three perspectives.The problems involved in electronic applications bring forward the higher request to the HASAs itself.Therefore,in the end of this outlook,we prospected the future of the synthesis and corresponding characterization of HASAs,and tried to provide our ideas about how to pave the way to the batch production of HASAs for carbon based electronic devices.

Anisotropic,Wrinkled,and Crack-Bridging Structure for Ultrasensitive,Highly Selective Multidirectional Strain Sensors

Flexible multidirectional strain sensors are crucial to accurately determining the complex strain states involved in emerging sensing applications.Although considerable efforts have been made to construct anisotropic structures for improved selective sensing capabilities,existing anisotropic sensors suffer from a trade-off between high sensitivity and high stretchability with acceptable linearity.Here,an ultrasensitive,highly selective multidirectional sensor is developed by rational design of functionally different anisotropic layers.The bilayer sensor consists of an aligned carbon nanotube(CNT)array assembled on top of a periodically wrinkled and cracked CNT-graphene oxide film.The transversely aligned CNT layer bridge the underlying longitudinal microcracks to effectively discourage their propagation even when highly stretched,leading to superior sensitivity with a gauge factor of 287.6 across a broad linear working range of up to 100%strain.The wrinkles generated through a pre-straining/releasing routine in the direction transverse to CNT alignment is responsible for exceptional selectivity of 6.3,to the benefit of accurate detection of loading directions by the multidirectional sensor.This work proposes a unique approach to leveraging the inherent merits of two cross-influential anisotropic structures to resolve the trade-off among sensitivity,selectivity,and stretchability,demonstrating promising applications in full-range,multi-axis human motion detection for wearable electronics and smart robotics.

A Highly Ordered Hydrophilic–Hydrophobic Janus Bi-Functional Layer with Ultralow Pt Loading and Fast Gas/Water Transport for Fuel Cells

One of the critical challenges that limit broad commercialization of proton exchange membrane fuel cells(PEMFC)is to reduce the usage of Pt while maintaining high power output and sufficient durability.Herein,a novel bifunctional layer consisting of vertically aligned carbon nanotubes(VACNTs)and nanoparticles of Pt-Co catalysts(Pt-Co/VACNTs)is reported for highperformance PEMFCs.Readily prepared by a two-step process,the Pt-Co/VACNTs layer with a hydrophilic catalyst-loaded side and a hydrophobic gas diffusion side enables a PTFE-free electrode structure with fully exposed catalyst active sites and superior gas–water diffusion capability.When tested in a PEMFC,the bi-functional Pt-Co/VACNTs layer with ultralow Pt loading(~65μgcathodecm-2)demonstrates a power density of 19.5 kW gPt cathode-1 at 0.6 V,more than seven times that of a cell with commercial Pt/C catalyst(2.7 kW gPt cathode-1 at 0.6 V)at a loading of 400μgcathodecm-2 tested under similar conditions.This remarkable design of VACNTs-based catalyst with dual functionalities enables much lower Pt loading,faster mass transport,and higher electrochemical performance and stability.Further,the preparation procedure can be easily scaled up for low-cost fabrication and commercialization.

Growth of Serpentine Carbon Nanotubes on Quartz Substrates and Their Electrical Properties

A simple method for high-yield,chemical vapor deposition(CVD)synthesis of serpentine carbon nanotubes,employing quartz substrates and a molecular cluster catalyst,is described.The growth mechanism is analyzed by controlled addition of nanoscale barriers,and by mechanical analysis of the curved sections.The serpentine structures are used to study the electrical transport properties of parallel arrays of identical nanotubes,which show three-terminal conductance that scales linearly with the number of nanotube segments.

Encapsulated carbon nanotube array as a thermal interface material compatible with standard electronics packaging

Vertically aligned carbon nanotubes arrays(VACNTs)are a promising candidate for the thermal interface material(TIM)of next-generation electronic devices due to their attractive thermal and mechanical properties.However,the environment required for synthesizing VACNTs is harsh and severely incompatible with standard device packaging processes.VACNTs’extremely low in-plane thermal conductivity also limits its performance for cooling hot spots.Here,using a transfer-and-encapsulate strategy,a two-step soldering method is developed to cap both ends of the VACNTs with copper microfoils,forming a standalone Cu-VACNTs-Cu sandwich TIM and avoiding the need to directly grow VACNTs on chip die.This new TIM is fully compatible with standard packaging,with excellent flexibility and high thermal conductivities in both in-plane and through-plane directions.The mechanical compliance behavior and mechanism,which are critical for TIM applications,are investigated in depth using in situ nanoindentation.The thermal performance is further verified in an actual light emitting diode(LED)cooling experiment,demonstrating low thermal resistance,good reliability,and achieving a 17℃ temperature reduction compared with state-of-the-art commercial TIMs.This study provides a viable solution to VACNTs’longstanding problem in device integration and free-end contact resistance,bringing it much closer to application and solving the critical thermal bottleneck in next-generation electronics.