Low Areal Mass Research Articles

High Mass Loading, Binder-Free Carbon Nanotubes Matrix on Ni Foam for High Areal Capacitance Supercapacitors

Supercapacitors (SCs) have excellent merits of high power density, rapid charge/discharge rates, and long cycle stability [1]. However, their relatively low energy density hampers the practical applications [2]. In general, the energy density of SCs strongly depends on their intrinsic properties of active materials and related inactive indispensable components such as current collector, electrolyte, separator, tabs, and package. Usually, the weight of these inactive components can reach around 50~70% of total mass [3]. In most literature, active materials are evaluated at a relatively low areal mass loading (<5 mg/cm2) to pursue the maximum performance by fully exposing the active sites and shortening the charge and ion transport pathways as much as possible [4]. However, these evaluation methods show limited guidance to design commercial SCs based on such active materials. Low areal mass loading of active materials results in excessive utilization of the current collector. In general, the density of current collector (Al foil: 2.7 g/cm3) is several times larger than that of active materials (compacted density of activated carbons (ACs): 0.4~0.6 g/cm3), hindering the development for high-energy, lightweight and low-cost SCs [5]. ACs are the most promising material for commercial SCs owing to their high surface area, robust electrical conductivity, good chemical stability, and low cost. In commercial SCs, the ACs electrode is fabricated with an optimized mass loading of >10 mg/cm2 and a thickness of 100-200 µm [6]. However, the capacitance of SCs is not linear with the mass loading or electrode thickness. Simultaneously, the increased mass loading or electrode thickness results in inferior performances due to the decreased accessible surface area, enlarged electrical resistance, prolonged ion transport channels, and poor electrolyte wetting. Therefore, it is still needed to design a novel active materials-current collector system with high mass loading for the practical applications of SCs.In this work, we report using carbon nanotubes (CNTs) matrix with high mass loading embedded in Ni foam (NF) to develop high areal capacitance electrodes for SCs. The CNTs matrix was directly grown on the NF (the size of 1×1×0.05 cm) by CVD method at the temperature of 650-800 °C without any other catalysts. Acetylene (C2H2) was used as the carbon source at a flow rate of 5 sccm, and the balance gases were H2 and Ar with flow rates of 100 and 300 sccm, respectively. The mass loading of the CNTs matrix reaches up to 60 mg/cm2. Figure 1 (a) and (c) show the top and bottom surface morphologies of CNT-NF electrode. Two electrodes with the mass loading of 60.2 and 59.5 mg/cm2 were packaged into a coin cell for evaluating the performances. The glass fiber and 1mol/L MeEt3NB4 in Acetonitrile were used as the separator and electrolyte, respectively.Figure 2 (a) shows the cyclic voltammetry (CV) of SCs at the scan rates of 100 mV/s and 2 mV/s. The calculated capacitance of SCs is 1214 mF. Given that, the corresponding areal, gravimetric, and volumetric capacitance of CNTs matrix active material are calculated as 2.428 F/cm2, 40.4 F/g, and 48.6 F/cm3, respectively. Figure b (a) presents the electrochemical impedance spectroscopy (EIS) of SCs in the frequency range from 100 kHz to 100 mHz. The impedance at high frequency is the contact resistance between the active material and current collector, indicating that the contact resistance of the CNT-NF electrode is 0.187 Ω·cm2 because the CNTs matrix grows on NF directly without using any binders. After 600 cycles, the capacitance of SCs reaches 1442 mF by an increase of 18.8% and because CNTs matrix has a poor wetting ability in the organic solvents, as shown in Figure 2(c). In figure 2(d), the corresponding contact resistance is 0.205 Ω·cm2 after 600 cycles, showing CNT-NF electrode has excellent rate stability.In conclusion, we have successfully used Ni foam as the scaffold structure and current collector for growing high mass loading CNTs matrix as the active material for high areal capacitance application. Importantly, this 3D CNTs matrix electrode system could offer a promising strategy to simultaneously improve the areal capacitance and energy density (gravimetric or volumetric) of SCs.

Ultrathin Cobalt Phthalocyanine@Graphene Oxide Layer-Modified Separator for Stable Lithium-Sulfur Batteries.

Rechargeable lithium-sulfur (Li-S) batteries have aroused great attention due to their high energy density and low cost. However, Li-S batteries suffer from rapid capacity decay owing to the shuttle effect of the intermediate polysulfides. To tackle this issue, functional separators with the ability to absorb polysulfides play a vital role to block them from passing through the separator. Herein, an ultrathin and lightweight layer of graphene oxide (GO) loaded with Co phthalocyanine (CoPc) is fabricated on a polypropylene (PP) separator. The thickness of CoPc@GO is about 200 nm with a low areal mass of 22 μg cm-2. CoPc is uniformly dispersed on GO sheets through π-π interactions, which inhibits the shuttle effect and facilitates the conversion of the intermediate polysulfides. In consequence, the battery with a CoPc@GO-PP separator exhibits good cycling stability with a low-capacity decay rate of 0.076% per cycle at 1 C over 400 cycles and a high specific capacity of 919 mA h g-1 after 250 cycles at 0.5 C.

Tuning the Mechanical and Electrical Properties of Porous Electrodes for Architecting 3D Microsupercapacitors with Batteries-Level Energy.

Microsupercapacitors (MSCs) are vital power sources for internet of things (IoTs) and miniaturized electronics. The performance of MSCs is often restricted by its low areal energy density, which is due to the low areal mass loading of active materials. Constructing thick planar microelectrode with fine structure and high aspect ratio is an efficient way to increase mass loading, but limited by the breakable nature of porous electrode materials. Here, it is found that the mechanical and electrical properties of porous electrodes, as well as their surface area utilization and internal ion diffusion pathway, can be synergistically tuned by infilling gel electrolyte into internal pores of porous electrode films. The tuned thick porous electrode films are robust enough to enable laser ablation of three dimensional (3D) microelectrodes for high mass loading and high aspect ratio. The areal capacitance of 3D microelectrodes is able to increase linearly with mass loading (or thickness) up to at least 13 mg cm−2 (or 260 µm) for a value of up to 4640 mF cm−2 based on active carbon. The 3D MSCs deliver areal energy density of 1318 μWh cm−2, which is comparable to the best of Li‐ion 3D microbatteries while exhibiting superior electrochemical and mechanical stability.

Engineering Oxygen Vacancies in a Polysulfide-Blocking Layer with Enhanced Catalytic Ability.

The practical application of the lithium-sulfur (Li-S) battery is seriously restricted by its shuttle effect, low conductivity, and low sulfur loading. Herein, first-principles calculations are conducted to verify that the introduction of oxygen vacancies in TiO2 not only enhances polysulfide adsorption but also greatly improves the catalytic ability and both the ion and electron conductivities. A commercial polypropylene (PP) separator decorated with TiO2 nanosheets with oxygen vacancies (OVs-TiO2 @PP) is fabricated as a strong polysulfide barrier for the Li-S battery. The thickness of the OVs-TiO2 modification layer is only 500 nm with a low areal mass of around 0.12 mg cm-2 , which enhances the fast lithium-ion penetration and the high energy density of the whole cell. As a result, the cell with the OVs-TiO2 @PP separator exhibits a stable electrochemical behavior at 2.0 C over 500 cycles, even under a high sulfur loading of 7.1 mg cm-2 , and an areal capacity of 5.83 mAh cm-2 remains after 100 cycles. The proposed strategy of engineering oxygen vacancies is expected to have wide applications in Li-S batteries.

A flexible phased array system with low areal mass density

Phased arrays are multiple antenna systems capable of forming and steering beams electronically using constructive and destructive interference between sources. They are employed extensively in radar and communication systems but are typically rigid, bulky and heavy, which limits their use in compact or portable devices and systems. Here, we report a scalable phased array system that is both lightweight and flexible. The array architecture consists of a self-monitoring complementary metal–oxide–semiconductor-based integrated circuit, which is responsible for generating multiple independent phase- and amplitude-controlled signal channels, combined with flexible and collapsible radiating structures. The modular platform, which can be collapsed, rolled and folded, is capable of operating standalone or as a subarray in a larger-scale flexible phased array system. To illustrate the capabilities of the approach, we created a 4 × 4 flexible phased array tile operating at 9.4–10.4 GHz, with a low areal mass density of 0.1 g cm^(−2). We also created a flexible phased array prototype that is powered by photovoltaic cells and intended for use in a wireless space-based solar power transfer array.

Fast polydopamine coating on reverse osmosis membrane: Process investigation and membrane performance study

We report a novel membrane surface modification method using a fast polydopamine coating (fPDAc) strategy. Specifically, NaIO4 was introduced in the coating process to accelerate the polydopamine deposition rate. Surface properties and separation performances of fPDAc-coated reverse osmosis membranes were characterized and compared to those obtained using the conventional slow polydopamine coating (sPDAc) strategy. Quartz crystal microbalance measurements showed greatly increased polydopamine deposition rate using the fPDAc method, resulting in a reduction of 97% coating time to reach an areal mass of 2000 ng/cm2. Both fPDAc and sPDAc enhanced the surface hydrophilicity and reduced the membrane surface charge. At relatively low areal mass deposition (<1000 ng/cm2), fPDAc-coated membranes showed improved NaCl rejection together with only mild loss of pure water flux. Nevertheless, this rejection enhancement effect was not noticeable when extensive polydopamine coating was applied due to the undesirable cake-enhanced concentration polarization effect. The extensive polydopamine coating was further accompanied with severe loss of membrane permeability, suggesting that shorter coating time (e.g., 4 min) is preferred using the fPDAc method. Our study provides a more rapid and effective membrane surface coating method compared to the conventional sPDAc method.

RuO2/MnO2 Bifunctional Electrocatalysts Supported on CNT with Cathode-Separator monolith for Pragmatic Li-O2 Batteries: High-Rate and High-Areal-Capacity with Improved Cycle Life

Though lithium-oxygen (Li-O2) batteries have very high energy density (about 3.5 kWh kg-1), many obstacles such as low energy efficiency, poor cycle performance and poor rate capability are exist to make Li-O2 batteries widely used. And these problems become severe especially at high areal mass loading. Now, researches were widely done in Li-O2 batteries field but most researches use low areal mass loading cathode. For practical usage of Li-O2 batteries, high areal capacity and mass loading are essential so many researches were unsuitable for commercialization. In this research, air cathode with high areal mass loading was fabricated and this cathode shows unprecedentedly high capacity at high rate along with long cycle life. A separator-carbon nanotube (CNT) monolith-type cathode was fabricated and high areal capacity was achieved, but the high overpotential and poor cycle performance make CNT-only separator monolith cathode unsuitable for pragmatic usage. So we adopted RuO2/MnO2 solid catalysts on the CNT (RMCNT), and sufficient cycle performance, low overpotential and high energy efficiency were obtained. RMCNT have catalytic effects in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and completely decompose Li2CO3 and LiOH which were byproducts that were made at discharge process and clogged surface of cathode. High mass loading, binder-free, intimate contact with separator, and most importantly, catalysts for reversibility were obtained by this separator-RMCNT monolith. Together, these features provide a remarkably long cycle life at unprecedentedly high capacity and high rate: 315, 45, and 40 cycles, with areal capacity limits of 1.5, 3.0, and 4.5 mAh cm− 2, respectively. At a rate of 1.5 mA cm− 2, Cycling is possible even at the curtailing capacity of 10 mAh cm− 2.

Extremely broadband ultralight thermally-emissive optical coatings.

We report the design, fabrication, and characterization of ultralight highly emissive structures with a record-low mass per area that emit thermal radiation efficiently over a broad spectral (2 to 30 microns) and angular (0-60°) range. The structures comprise one to three pairs of alternating metallic and dielectric thin films and have measured effective 300 K hemispherical emissivity of 0.7 to 0.9 (inferred from angular measurements which cover a bandwidth corresponding to 88% of 300K blackbody power). To our knowledge, these micron-scale-thickness structures, are the lightest reported optical coatings with comparable infrared emissivity. The superior optical properties, together with their mechanical flexibility, low outgassing, and low areal mass, suggest that these coatings are candidates for thermal management in applications demanding of ultralight flexible structures, including aerospace applications, ultralight photovoltaics, lightweight flexible electronics, and textiles for thermal insulation.

(Invited) Imperceptible Graphene Based Electronic Tattoo Sensors

During last decade, there have been advances in wearables with applications in mobile healthcare, athletic, assistive technology, human machine interface and internet of things. However, current wearable sensors and systems have at least one or more of the following drawbacks: they are bulky, expensive, distractive, unsuitable for long term measurement or prone to motion artifact. We present ultrathin graphene based electronic tattoo (GET) sensors with the overall thickness of less than 500 nm. Presented e-tattoo sensors can be laminated on skin like as temporary tattoo and form conformal contact to microscopic texture of the skin. GET is a set of dry sensors with electrode-skin interface impedance comparable to wet gel electrodes that are gold standard in medical applications. Low electrode-skin interface impedance of these e-tattoo sensors results in high signal to noise ratio and high sensing quality. Due to low areal mass density of GET, it attaches to skin without any adhesive or tape, just by Van der Waals force. Presented e-tattoos are both optically and mechanically imperceptible, therefore, they are desirable for applications in prominent areas of appearance such as face. We present the successful demonstration of GET applications for electrocardiography (EEG), electroencephalography (EEG), electromyography (EMG), electrooculography (EOG), skin temperature sensing, skin hydration sensing and human robot interface.

Development and characterization of sub-monolayer coatings as novel calibration samples for X-ray spectroscopy

With the advent of both modern X-ray fluorescence (XRF) methods and improved analytical reliability requirements the demand for suitable reference samples has increased. Especially in nanotechnology with the very low areal mass depositions, quantification becomes considerably more difficult. However, the availability of suited reference samples is drastically lower than the demand. Physical vapor deposition techniques have been enhanced significantly in the last decade driven by the need for extremely precise film parameters in multilayer production. We have applied those techniques for the development of layer-like reference samples with mass depositions in the ng-range and well below for Ca, Cu, Pb, Mo, Pd, Pb, La, Fe and Ni. Numerous other elements would also be possible. Several types of reference samples were fabricated: multi-elemental layer and extremely low (sub-monolayer) samples for various applications in XRF and total-reflection XRF analysis. Those samples were characterized and compared at three different synchrotron radiation beamlines at the BESSY II electron storage ring employing the reference-free XRF approach based on physically calibrated instrumentation. In addition, the homogeneity of the multi-elemental coatings was checked at the P04 beamline at DESY. The measurements demonstrate the high precision achieved in the manufacturing process as well as the versatility of application fields for the presented reference samples.

Ionic Liquid-Derived Nitrogen-Enriched Carbon/Sulfur Composite Cathodes with Hierarchical Microstructure—A Step Toward Durable High-Energy and High-Performance Lithium–Sulfur Batteries

We describe the preparation of turbostratic carbon in monolithic form by silica template method and its use as host matrix to trap polysulfides in Li–S batteries. The synthesized ionic liquid-derived carbon is hierarchically structured with pore size maxima around 6 nm and 750 nm, and it has a room temperature electrical conductivity of 7–8 S cm–1, owing to a high content of pyridinic and graphitic nitrogen. Further, this carbon can accommodate up to ∼80% sulfur by weight, and the resulting nanocomposite demonstrates promising performance as novel electrode material for Li–S batteries. Cathodes with low areal mass loading (1 mgsulfur cm–2) can be cycled at C/5 and 1C over several hundreds of cycles with low polarization and very little capacity fading (4% between the fifth and 1000th cycles at 1C). Highly loaded cathodes (4 mgsulfur cm–2) also exhibit good cyclability at C/5 with areal capacities of 2.6 mAh cm–2 on average over 200 cycles. Yet the high capacities make the cells more prone to electrolyte d...