Developing hydrogen storage materials from Cigarette butts – Stepping towards the reality of achieving hydrogen economy

Hydrogen possesses a high gravimetric energy capacity and it is used as a green energy source in automobiles since it eliminates CO₂ emissions. A variety of hydrogen storage materials such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), highly porous carbonaceous materials are currently emerging. Researchers at the University of Nottingham, U. K., have developed a process to prepare porous carbon using fresh and smoked cigarette butts (filters) by a sequence of treatments – hydrothermal carbonisation followed by activation.

Unused (F group) and smoked (S group) filters containing cellulose acetate as the main ingredient were used as the starting materials. The F and S group cigarette filters were ground to form a fluffy white or yellow-brown mass, mixed with water (at a ratio of 1 g filter to 10 mL water), hydrothermally carbonized in a stainless steel autoclave to 250 °C for 2 h and the resultant carbonaceous matter (hydrochar) was dried at 112 °C. The hydrochars derived from F and S group cigarette filters were denoted as FF-hydrochar and SF-hydrochar, respectively.

The hydrochars were ground with KOH (KOH/hydrochar ratio = 4), activated at 600, 700 and 800 °C for 1 h and allowed to cool under N₂. The resultant activated carbons were washed initially using 2M HCl followed by deionized water to remove the residual acidity and dried at 112 °C. The activated carbons were designated as FF-4T (from FF-hydrochar) and SF-4T (from SF-hydrochar, respectively in which 4 represents the KOH/hydrochar ratio and T refers to the activation temperature.

Fig. 1 Schematic of the conversion of cigarette butts (filters) to activated carbon

The porosity and pore size distribution of FF and SF series activated carbons is found to increase with an increase in activation temperature from 600 to 800 °C. For FF-4T activated carbons, both the apparent surface area and pore volume are increased with an increase in activation temperature with a maximum apparent surface area of 4113 m²/g and pore volume of 1.87 cm³/g are obtained for FF-4800. In contrast, the trend is reversed for SF-4T activated carbons in which a maximum apparent surface area of 4310 m²/g and pore volume of 2.09 cm³/g are obtained for SF-4600. Among all the samples evaluated, SF-4600 has the highest apparent surface area of 4310 m²/g with a micropore surface area of 3867 m²/g, which is 90% of the total surface area ever reported for activated carbons. This is due to the presence of metal additives such as K, Ca, Na, Mg, etc., in the smoked filters which could have acted as activating agent besides KOH. The high surface area and high microporosity with a significant proportion of pores are < 1 nm in size of SF-4600 are the important attributes needed for hydrogen storage materials.

Assessment of hydrogen uptake properties of FF-4T and SF-4T series activated carbons at -196 ºC and 0 – 40 bar (cryo-storage conditions required for low pressure vehicular hydrogen storage) indicates that SF-4600 contributes to the highest hydrogen uptake. A combination of high apparent surface area, high microporosity and high oxygen content (16 – 31 wt% with oxygen functional groups such as COOH, C-OH and O-C=O) enables SF-4600 to achieve a high hydrogen uptake.

Fig. 2 Excess and total hydrogen uptake at -196 °C of activated carbons derived from (a) fresh cigarette filters and (b) smoked cigarette filters/butts; (c) Bench marking of hydrogen uptake of SF-4600 with high surface area MOFs

T.S.N. Sankara Narayanan

For more details, the reader may kindly refer: T.S. Blankenship and R. Mokaya, Cigarette butt-derived carbons have ultra-high surface area and unprecedented hydrogen storage capacity, Energy Environ. Sci., 2017, DOI: 10.1039/C7EE02616A

Capturing CO2 using metal-organic framework (MOF)

The steady increase in concentration of CO₂ in the atmosphere (from 310 ppm to > 380 ppm during the past five decades) and its continuous increasing trend until this moment, is really a matter of concern. Power plants contribute to ~ 60% of the total CO₂ emission worldwide. Hence, development of effective CO₂ capture systems that could selectively remove CO₂ from the exhaust gas is warranted. Porous metal-organic frameworks (MOFs) are promising for CO₂ capture. Nevertheless, development of MOFs for CO₂ capture directly from the exhaust gas of power plants is indeed challenging.

Researchers at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, University of Science and Technology of China, Hefei National Laboratory for Physical Sciences at the Microscale and Wenzhou University, China have designed and synthesized a Cu(II)-MOF (FJI-H14) with high density of open metal sites (OMS) and Lewis basic sites (LBS) in which both OMS and LBS interact synergistically with CO₂ and help to capture it.

A mixture of 2,5-di(1H-1,2,4-triazol-1-yl)terephthalic acid (H₂BTTA) (0.05 mM) and Cu(NO₃)₂·3H₂O (0.05 mM) in H₂O (4 ml) in a sealed Teflon vial under hydrothermal conditions at 120 °C for 3 days has lead to the formation of rod-shaped blue crystals of FJI-H14 ([Cu(BTTA)H₂O]_n·6_nH₂O) with 73% yield based on the organic ligand H₂BTTA (Fig. 1).

Fig. 1 Structural illustration of FJI-H14: (a) ligand H₂BTTA; (b) co-ordination environment of Cu(II) ions with BTTA; (c) one-dimensional nano-porous channels; and (d) topology of MOF (Cu atom, cyan; C atom, gray; O atom, red; N atom, blue; H atom, white)

The FJI-H14 is stable in boiling water as well as in acidic and basic environments (pH: 2 to 12) at temperatures as high as 373 K. It is also thermally stable up to 230 °C. The Brunauer–Emmett–Teller (BET) specific surface area of FJIH14 is 904 m²/g and its Langmuir-specific surface area is 1004 m²/g. The total pore volume of FJIH14 estimated from CO₂ isotherm is 0.45 cm³/g. The high porosity and high concentration of open active sites in the framework has lead to an increase in the extent of CO₂ uptake up to 279 cm³/g (Fig. 2(a)). The strong absorption bands at 2,340 cm⁻¹ and 2,328 cm⁻¹ in the IR spectra indicate that the CO₂ molecules tend to stack around the open Cu(II) sites, which is also in line with the theoretical calculations. Besides high adsorption capacity, reusability is an important property for any adsorbent. FJI-H14 maintains 100% adsorption capacity even after five cycles of adsorption, suggesting its suitability as a reusable adsorbent for CO₂ capture (Fig. 2(b)).

Since the flue gas from power plants contains a large amount of N₂ (73–77 %) than CO₂ (15–16 %), CO₂/N₂ selectivity is a crucial parameter in CO₂ capture applications. The CO₂/N₂ selectivity FJI-H14 (for the 15/85 CO₂/N₂ mixture at 298 K and at 1 atm) is 51. The high selectivity for  adsorption of CO₂ over N₂ suggests that the densely populated open active sites in the framework have a positive effect on CO₂ adsorption. The relatively narrow pores in FJIH14 could have easily blocked the relatively large N₂ molecules thus favouring selectivity for CO₂ (Figs. 2(c) and 2(d)). FJI-H14 is also capable of catalyzing chemical transformation of CO₂ into value-added chemicals, such as dimethyl carbonate, cyclic carbonates, N,N’-disubstituted ureas or formic acid.

Fig. 2 Experimental CO₂ adsorption by FJI-H14: (a) CO₂ adsorption isotherm for FJI-H14 at 195 K; (b) Cycles of CO₂ adsorption for FJI-H14 at 298 K; (c) N₂ and CO₂ adsorption isotherms for FJI-H14 at 298 K; and (d) CO₂/N₂ selectivity for 15/85 CO₂/N₂ mixture at 298 K.

FJI-H14 possesses the characteristics of an ideal MOF in terms of high CO₂ uptake at ambient conditions, excellent chemical and thermal stabilities, selectivity for CO₂ over N₂, reusability, direct and smooth conversion of CO₂ into corresponding cyclic carbonates and ease of preparation at large scale.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Liang et al., Carbon dioxide capture and conversion by an acid-base resistant metal-organic framework, Nature Communications, 8 (2017) 1233, DOI: 10.1038/s41467-017-01166-3

Development of a hierarchical micro/nano-porous structure on acupuncture needles for the treatment of colorectal cancer

Acupuncture is considered to be an effective therapy for treating functional disorders, pain relief, drug abuse and psychiatric disorders. The treatment mechanism involves the release of endogenous opiates and neurotransmitters, which are mediated through electrical stimulation of the central nervous system. In order to increase the intensity of the stimuli, it is necessary to use thicker needles and/or deeper insertion of the needles. However, a similar effect could be achieved by increasing the surface area of the needles. Researchers at the Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu Haany University, Republic of Korea and Flux Photon Corporation, USA, have fabricated a porous structure with hierarchical micro/nano-scale conical pores on the surface of conventional stainless steel acupuncture needles.

Conventional stainless steel acupuncture needles (CN) (length: 8 cm; diameter: 0.18 mm) were anodized in ethylene glycol medium modified with the addition of 0.2 wt. % NH₄F + 2.0 vol. % deionized water at 20 V for 30 min to form a porous structure with hierarchical micro/nano-scale conical pores (PN). The morphological features of CN and PN are shown in Fig. 1. The CN possess a smooth surface (Fig. 1(a)) whereas a hierarchical micro/nano-scale porous surface topology is developed after anodization (Figs. 1(b), 1(c) and 1(d)). The surface area of PN (1.03 m²∙g⁻¹) is ~ 25 times higher than CN (0.04 m²∙g⁻¹).

Fig. 1 Morphological features of (a) conventional acupuncture needle (CN);  and (b, c, d) nanoporous acupuncture needle (PN); (c, d) high resolution images.

The surface modified stainless steel acupuncture needles (PN) enhanced the therapeutic effects of colorectal cancer (CRC) treatment in rats.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Bo Ram Lee et al., Enhanced Therapeutic Treatment of Colorectal Cancer Using Surface-Modified Nanoporous Acupuncture Needles, Scientific Reports, 7: 12900,  DOI:10.1038/s41598-017-11213-0

 

Self-folding 3D graphene

Graphene finds widespread application in energy storage, sensors and flexible electronics. For most of the applications, a planar geometry of graphene would suffice while applications such as wearable electronics, biological or dispersible sensors, and actuators demand a curved and folded architectures. Since pristine graphene is highly chemically inert, achieving controlled self-folding in response to external environmental stimuli is very difficult.

Researchers at the Johns Hopkins University and Massachusetts Institute of Technology, USA have developed a method to fold and unfold monolayer graphene into ordered 3D structures so that they can be designed and fabricated in accordance with a predictable shape (Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084). The processing involves various stages including surface functionalization of graphene, transfer of the functionalized graphene on patterned Al coated on Si, shape design by photolithography, removal of unwanted graphene by oxygen plasma etching, removal of Al by chemical etching and folding of the functionalized graphene by an increase in solution temperature.

Surface functionalization of graphene

Immersion of monolayer graphene in a dilute aqueous solution of dopamine (2.0 mg/ml) at pH of 8.5 (10 mM tris-HCl) for 2 – 4 h promoted self-polymerization of dopamine, that lead to the formation of a thin layer (~5 nm) of polydopamine (PD) on the surface of graphene. Subsequently, the PD-coated graphene was immersed in a dilute aqueous solution containing amine-terminated poly(N-isopropylacrylamide) (PNIPAM) (2.0 mg/ml) at pH 8.5 (10 mM tris-HCl) at 60 °C for 3 h (Fig. 1(a)). The PD served as an intermediate active layer to graft PNIPAM on PD-coated graphene. The thermoresponsive properties of PNIPAM enables the surface functionalized graphene to behave as an ultrathin shape-changing material.

Fabrication of self-folding microstructures

A patterned sacrificial Al layer was deposited on Si. Subsequently, the PD-PNIPAM functionalized graphene was transferred onto the substrate. The functionalized graphene was patterned into various shapes by photolithography, and the graphene in unwanted areas was removed by oxygen plasma etching. The underlying Al layer was dissolved using 5 mM NaOH + 3 mM sodium dodecyl sulphate. Folding of the functionalized graphene was induced by heating the solution to 45 °C using a hot plate. Selective pinning prevented the folded structures from being washed away (Fig. 1(b)).

Fig. 1 Schematic illustration of surface functionalization, patterning, fabrication and folding process of graphene microstructures

Grafting of the thermoresponsive PNIPAM to the surface of graphene is necessary for folding; neither the pristine graphene nor the PD-graphene exhibit the self-folding behavior with an increase in temperature. Different 3D shapes, including flower, dumbbell, and box can be obtained after folding (Fig. 2(a-c)). The reversible switching behavior of PNIPAM also helps to unfold the structure by reversing the temperature from 45 °C to 25 °C (Fig. 2(d-f). Addition of a rigid polymer layer to the petals (increase in thickness up to 100 nm) reduces the adhesion between the petals and favours easy reversibility (Fig. 2(g-i)).

The flower shaped folding tends to fold its free petals toward the center and go from an open to a closed state, it is possible to encapsulate live cells within the self-folding flower. The temperature employed for cell culture (37 °C) is sufficient to induce folding of the functionalized graphene flowers to encapsulate the cells inside its petals. It is confirmed that the cells are alive after encapsulation, which suggests that the self-folding process is biocompatible and can be used to capture biological cargo.

The process is highly tunable and offer control over the self-folding nature of graphene. The extent of folding is increased with an increase in temperature from 25 °C to 45 °C while reversing the temperature enabled unfolding of the structure. Different 3D shapes such as flower, dumbbell, box, etc., can be achieved. A variety of applications such as encapsulation and delivery of cells, design and fabrication of novel electrical devices and field-effect transistors, formation of a variety of Origami and Kirigami shape-changing structures, etc. are envisioned.

Fig. 2 (a-c) Snapshots of temperature-induced self-folding of ultrathin graphene microstructures with a flower geometry; (d-i) Reversibility of the temperature-induced self-folding. The sequence of images (d, e f) shows folding and unfolding of functionalized graphene flower; The sequence of images (g, h, i) shows folding and unfolding of a flower with rigid SU8 polymer petals, with better stability and reversibility but with increased thickness (100 nm). Scale bars: (a-c) 100 µm; and (d-i) 50 µm.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084

Super-formable pure magnesium at room temperature

Magnesium is a promising material that can substitute for steel and aluminium alloys to achieve weight reduction in automobile, aerospace and allied industries, which is considered to contribute for energy efficiency and eco-friendly. However, one of the major impediment is the limited formability of magnesium.

Researchers from the Department of Materials Science and Engineering and Department of Mechanical and Aerospace Engineering, Monash University, Australia and Automotive Steel Research Institute, China have reported a breakthrough in the development of polycrystalline pure magnesium that can be tailored to be super-formable at room temperature by conventional processes.

The study reveals that polycrystalline pure magnesium becomes super-formable at room temperature after it is extruded ≤ 80 °C, exhibit no work hardening and shows no sign of fracture during compression at room temperature and at a strain rate of 10⁻³ s^-1. In contrast, those extruded at 150 to 400 °C exhibit poor formability at room temperature, high work hardening and show clear signs of fracture when compressed by 20–30% reduction in height (Fig. 1).

Fig. 1 Room temperature compression of specimens extruded at 80 and 400 °C. Photographs in the inset show the specimens before and after compression test. Specimens extruded at 400 °C fractures after ~20% height reduction while those extruded at 80 °C can be compressed from 10 to 1.5 mm without fracture.

The super-formability of polycrystalline pure magnesium specimens extruded at 80 °C was demonstrated by rolling them at room temperature (cold rolling) without any intermediate annealing stage. Continuous reduction in their thickness from 3 to 1 mm fails to display any edge cracking (Fig. 2(a)). The ability of 1 mm-thick sheet to bent through 180° (hemming) without any failure (Fig. 2(b)), suggests its suitability for the fabrication of automotive panels. The 1 mm thick sheet can be further cold rolled to 0.5 mm, and even 0.12 mm strips (96 % reduction in total thickness equivalent to a true strain of 3.2). The 0.12 mm strips are amenable for cutting and shaping in the form of letters “m” and “g” (Fig. 2(a)). Folding twice followed by complete unfolding fails to display any visible cracks (Fig.2 (c)), which refute the conventional belief that magnesium would fracture after heavy cold work or bending.

Fig. 2 Cold rolling of extruded specimens: (a) Photograph of 3 mm thick magnesium plate extruded at 80 °C, and after 67 and 96% cold rolling without any trimming of specimen edges along the rolling direction. The strip cold rolled by 96% was cut and shaped in the form of letters “m” and g”; (b) Photograph of cold-rolled 1 mm thick strip bent by ~180° at room temperature; (c) Photographs showing folding and unfolding of 0.12 mm strip without any visible cracks. (Scale bars in a –c:  20, 3 and 5 mm, respectively)

Specimens extruded at 80 and 400 °C posses strong basal texture and contain predominantly equiaxed grains with an average grain size of ~1.3 and ~82 μm, respectively. For the specimen extruded at 400 °C followed by 20% cold compression or rolling, the average grain size is decreased to 56–61 μm. In contrast, there observed to be very little change in the size and shape of grains for the specimen extruded at 80 °C followed by 50% cold compression or rolling. Microstructural evolution reveals the presence of a large number of deformation twins (Fig. 3(a)) and slip traces (Fig. 3 (b)) for the specimen extruded at 400 °C followed by 20% cold compression or rolling. In contrast, these features are not detected in the specimen extruded at 80 °C.

Fig. 3 Secondary electron micrographs showing: (a) deformation twins (T); and (b) slip traces (S) in the specimen extruded at 400 °C and compressed by 20%.

The results of the study reveals the occurrence of significantly different deformation modes during cold forming of specimens extruded at 80 °C, even though they also possess a strong basal texture. The superformability behaviour is due to the occurrence of dynamic recrystallisation during extrusion of pure magnesium specimens at room temperature. In addition, dynamic recrystallisation is also possible during compression/rolling at room temperature, either to accommodate grain boundary sliding or to act as an independent softening mechanism. The study demonstrates that extruded pure magnesium remains superformable, even after substantial plastic deformation at room temperature. The findings of the study provide a new avenue for the design and development of highly formable magnesium products by conventional thermomechanical processes that are cost-effective, efficient and industrially scalable. This attribute assumes significance for the development of light weight materials in automobile, aerospace and allied industries.

T.S.N. Sankara Narayanan

For more detailed information, the reader may kindly refer: Zeng et al., Super-formable pure magnesium at room temperature, NATURE COMMUNICATIONS | 8: 972 | DOI: 10.1038/s41467-017-01330-9

Growth-accommodating implants – laying the foundation for a new paradigm of paediatric device development

Medical implants are often available in fixed size. The inability of such devices to accommodate with normal tissue growth remains a challenge, particularly in case of children. Hence, development of implant devices that could correct themselves in accordance with anatomical deformities and easily accommodate with tissue growth is highly warranted.

Researchers at the Harvard Medical School, USA and University College Dublin, Ireland have developed a growth-accommodating device that consists of a tubular braided sleeve and a biodegradable polymer core (Eric N. Feins et al., A growth-accommodating implant for paediatric applications, Nature Biomedical Engineering, 1 (2017) 818–825).

Fig. 1 (a) Schematic of a degradable polymer core (dark blue) placed inside a braided sleeve to control sleeve diameter, coupling inner polymer degradation to braided sleeve (and overall device) elongation; (b) A dissolvable spherical sucrose core (red) inside a nitinol biaxial braid acts as a degradable polymer surrogate. Upon immersion in water, the sucrose core gradually dissolves leading to a gradual decrease in the braided sleeve diameter along with a concomitant autonomous elongation; (c) Variation in length and diameter of the braided sleeve during core degradation.

A hydrophobic surface-eroding, biodegradable and biocompatible polymer poly(glycerol sebacate) (PGS) was used as the base material. The rationale behind the choice of PGS was justified based on its minimal swelling in water, ability to offer the requisite mechanical properties to resist compressive forces from the braided sleeve and capability to maintain structural integrity throughout degradation. To minimize the stretching of PGS to less than 5%, it was treated at 155 °C for 86 h in vacuum, which maximizes its cross-linking leading to the formation of extra-stiff PGS (ESPGS). The ESPGS was used as the polymer core while the braided sleeve was made of nitinol alloy.

The concept behind the development involves coupling the degradation of a surface-eroding polymer core to the braid length and overall device elongation. After implantation, once the polymer core starts to degrade, the braided sleeve begins to thin out and elongates in response to surrounding tissue growth (Figs. 1(a) and 1(b)), without the necessity for any additional interventions. Since the sleeve length and diameter of the braid are inversely related,  thinning of the sleeve results in its elongation (Fig. 1(c)).

The flexible nature of the braided sleeve and polymer contributes to the durability of the device and no evidence of fatigue failure of either the braided sleeve or the ESPGS core could be observed during in vivo studies using animal models.

By altering the number and thickness of braid fibres, the braid geometry and rate of degradation of the polymer core, it would be possible to modify the device elongation profile to match a wide spectrum of clinical applications.

Variability in the polymer erosion rate is the current limitation of the proposed device and achieving uniform degradation of polymer core will be the focus of future work.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer Eric N. Feins et al., A growth-accommodating implant for paediatric applications, Nature Biomedical Engineering, 1 (2017) 818–825.

Manipulating the growth mode of ice crystals by changing the surface wettability could help design better anti-icing surfaces

Design of anti-icing surfaces assumed significance in aerospace, power systems, marine vessels and automotive sectors. Easy removal of ice from solid surfaces has economic, energy and safety implications. A group of researchers from China and USA have described wettability-dependent ice morphology on the surface of aluminium that had been covered with a hydrophobic, or water-repellent, coating under atmospheric conditions and published their findings recently (Liu et al., Distinct ice patterns on solid surfaces with various wettabilities, www.pnas.org/cgi/doi/10.1073/pnas.1712829114).

The researchers have established a correlation between surface wettability and growth mode of ice crystals and suggested that surface wettabilities dictate the ice growth mode. Accordingly, below a critical value of contact angle, the growth of ice crystals follow along-surface growth mode whereas above this critical value of contact angle, the growth of ice crystals follow off-surface growth mode. It has been demonstrated that the ice crystals grown with off-surface growth mode, having a single point attachment with the surface, can be easily blown away by a breeze whereas those grown with along-surface growth mode, having multiple attachment points, stuck to the solid surface.

The discovery of different ice growth modes on solid surfaces and the feasibility of achieving easy removal of ice crystals grown with off-surface growth mode can be exploited to design better anti-icing surfaces.

The schematic illustrations, snap shots acquired using optical microscopy and video clips will give a better insight about their findings.

Fig. 1 Schematic illustration of the effect of solid surfaces on ice growth; (A) introduction of AgI nanoparticles on solid surfaces to achieve ice nucleation over the entire solid surfaces in the same environment.

Fig. 2 Snapshots acquired at different time periods using an optical microscope coupled with a high-speed camera: (B, D) top-view images; and (C, E) side-view images; (B) growth process of six-leaf clover-like ice on a hydrophobic surface (θ = 107.3°); (C) Off-side growth mode; (D) growth process of sunflower-like ice on a hydrophilic surface (θ = 14.5°); (E) Along-surface growth mode (growth environment: surface temperature is −15 °C; and supersaturation is 5.16)

Video clip demonstrating the growth process of six-leaf clover-like ice on a hydrophobic surface (θ = 107.3°)
http://movie-usa.glencoesoftware.com/video/10.1073/pnas.1712829114/video-1

Video clip demonstrating the growth process of sunflower-like ice on a hydrophilic surface (θ = 14.5°)
http://movie-usa.glencoesoftware.com/video/10.1073/pnas.1712829114/video-2

Fig. 3 Schematic illustration depicting that the ice crystals grown with off-surface growth mode can be easily blown away by a breeze whereas those grown with along-surface growth mode stuck to the solid surface.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Liu et al., Distinct ice patterns on solid surfaces with various wettabilities,  
www.pnas.org/cgi/doi/10.1073/pnas.1712829114).

Rapid charging of your smart phones – Are we getting closer to reality?

Supercapacitors are used as an alternative power source for rechargeable batteries due to their efficient operation at high power density, long cycle life and improved safety. Nevertheless, the limited energy density, typically of the order of 5-8 Wh/L, limits their widespread use for many practical applications. Boosting capacitance and extending window of cell voltage are the available options to impart further improvement in their energy density. Researchers at University of Waterloo, Canada and Jain University, Bangalore, India have proposed a novel approach towards the development of high voltage super capacitors with high energy density (ACS Nano, 2017, 11 (10), pp 10077–10087).

Ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) and Tween 20 (nonionic surfactant) were mixed together to obtain a stable microemulsion with nanometer sized particles. Upon mixing it with graphene oxide (GO), the surfactant stabilized microemulsion spontaneously adsorbs on the surface of GO. This dispersion was directly casted onto copper with the formation of a dense nanocomposite film of GO/IL/Tween 20. Subsequent thermal treatment leads to the removal of IL by evaporation and reduction of GO to reduced graphene oxide (rGO). The resultant electrode is referred as IL-mediated reduced graphene oxide (IM-rGO).

Fig. 1 Schematic of fabrication of IM-rGO electrode assembly: (a) spontaneous adsorption of surfactant stabilized microemulsion particles on the surface of GO; (b) enlarged view of EMImTFSI/Tween 20/H₂O microemulsion particle; (c) film structure after drop-casting and water evaporation; and (d) film structure after evaporation of Tween 20 following thermal reduction

The surface morphology of the nanocomposite film reveals the presence of macropores (Fig. 2(a)) due to evaporation of water and Tween 20 during thermal treatment. Morphology at the cross-section indicates a layered structure (Fig. 2(b)), in which the sheets lay parallel to the current collector, thus providing a relatively high bulk density.

Fig. 2 Morphology of the IM-rGO film fabricated using 60% IL: (a) at the surface; and (b) at the cross-section

The electrochemical performance of the IM-rGO electrode fabricated using 60 wt% of IL at RT is depicted in Fig. 3. The formation of a dense film enabled a CV of 218 F/cm³. This electrode offered a maximum energy density of 45 Wh/L at a power density of 571.4 W/L and maintained a high energy density of 21.7 Wh/L at a power density as high as 6.04 kW/L at RT.

Fig. 3 Electrochemical performance of 60% IL electrodes at RT: (a) CVs and (b) GCDs for IM-rGO at RT; (c) specific capacitance at varying current density

Eliminating the macropores of the film still remains a challenge and elimination of macropores would help to achieve even higher bulk density. The easy adoptability of the proposed methodology provides new avenues for the manufacturing of large-scale supercapacitors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Zimin She et al., ACS Nano, 2017, 11 (10), pp 10077–10087

Smart plasma copolymer coatings with tunable wettability from superhydrophobicity to superhydrophilicity

Stimuli-responsive surfaces with switchable wettability assumed significance  in drug delivery, biomedical engineering, sensors and bio-fuel cells. Most of the responsive surfaces show intrinsic responsive wettability and it is difficult to tune chemical structures with controlled wetting characteristics.

Researchers at the Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, China have demonstrated that it would be possible to fabricate smart surfaces with tunable wettability and reversibly switchable pH-responsiveness using plasma copolymerization technique (Reference: Iqbal Muzammil et al., Plasma Processes and Polymers, 14 (10) (2017), DOI: 10.1002/ppap.201700053).

Plasma copolymerization is an efficient one-step process to fabricate new surfaces. It is a clean, dry, and environmentally benign process that enables conformal deposition of coatings over surfaces with complex geometries. In this perspective, the Chinese researchers have deposited a series of plasma copolymer coatings with various carboxylic acid and fluorocarbon group ratio on nanotextured low-density polyethylene (LDPE) surfaces via capacitively coupled radio frequency plasma (CCP) polymerization technique.

Acrylic acid (AA) and octafluorocyclobutane (C₄F₈) were used as monomers. Low density polyethylene (LDPE) was chosen as the substrate and it was oxygen plasma etched at 200W for 30 min with an oxygen flow rate of 50 sccm to develop a nanotextured surface. The C₄F₈-co-AA plasma polymer coatings was deposited on flat nanotextured LDPE surfaces by radio frequency (RF) capacitively coupled plasma reactor (CCP) mode at 50W for 1 min. The C₄F₈ monomer flow rate was fixed for 40 sccm while the AA monomer flow rates was changed from 5 to 40 sccm.

C₄F₈ plasma polymer coating deposited on LDPE surface shows a static water contact angle (SWCA) of 119°. As the carboxylic acid group concentration increases, the SWCA of C₄F₈-co-AA plasma polymer coatings is decreased. For C₄F₈-co-AA (40:5) plasma polymer coatings, the SWCA is decreased to 97°. C₄F₈-co-AA (40:40) plasma polymer coatings show a rapid decrease in SWCA leading to a lower hysteresis. An increase in AA feed ratio increases the ratio of carboxylic acid group to CF_x group, leading to a lower SWCA. The hydrophilic carboxylic acid group controls the wetting state since the polar carboxylic acid group allows permeation of polar water molecules into plasma copolymer coatings. In contrast, the hydrophobic fluorocarbon group controls the dewetting state since the nonpolar groups like CF₂ and CF₃ of low surface energy repel water.

The oxygen plasma etching treatment enables the formation forest like nano-filaments on the surface of LDPE (Fig. 1(a)). Subsequent deposition of either C₄F₈ (Fig. 1(b)) as well as C₄F₈-co-AA plasma polymer coatings (Figs. 1 (c) and 1(d)) has no significant effect on the surface nanotexture. Nevertheless, the surface nanotextures amplify the surface wettability.

Fig. 1 Scanning electron micrographs (a) nanotextured surface of LDPE after oxygen plasma etching treatment; (b) C₄F₈ plasma polymer coating; (c) C₄F₈-co-AA (40:15) plasma polymer coating; and (d) C₄F₈-co-AA (40:25) plasma polymer coating on nanotextured LDPE surfaces

C₄F₈ plasma polymer coating deposited over nanotextured LDPE surface became superhydrophobic with a SWCA of ~163° and low apparent hysteresis of < 1°. This high SWCA with low hysteresis developed over the nanotextured surface can be explained by the Cassie model. Accordingly, water droplet cannot penetrate into cavities of the nanotextured surface as air is trapped at the interface between the water droplet and sharp corners of nanotextured surface.

C₄F₈ plasma polymer coatings show no significant SWCA change with a change in pH, suggesting the absence of pH-responsive behaviour. In contrast, the C₄F₈-co-AA plasma polymer coatings start to a exhibit pH-responsive behaviour with sufficient increase in pH-sensitive carboxylic acid groups. C₄F₈-co-AA (40:5) plasma polymer coatings in different pH solutions of 1, 4, 9, and 13 for 10 min shows a SWCA of 80, 76, 70, and 65°, respectively. C₄F₈-co-AA (40:10) and (40:15) plasma polymer coatings show a SWCA 72 and 70° at pH 1, 67 and 64° at pH 4, 61 and 58° at pH 9, 54 and 49° at pH 13, respectively.

An increase in concentration of carboxylic acid groups as well as pH leads to a decrease in SWCA. This phenomenon can be explained by the protonation and deprotonation of dangling carboxylic acid groups. These carboxylic acid groups become uncharged due to protonation at low pH. It shrinks and gives an additional surface to fluorinated part of the copolymer thus higher SWCA is observed. Correspondingly, at higher pH’s due to deprotonation of the carboxylic acid groups, it becomes charged and the charged state increases the polarity of the polymeric coatings leading to a lower SWCA.

Fig. 2 Schematic illustration of the change in static water contact angle with pH demonstrating the development of C₄F₈-co-AA plasma polymer coatings with tunable wettability and reversibly switchable pH-responsiveness using plasma copolymerization technique

This methodology opens up a potential door for the fabrication of smart surfaces.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Iqbal Muzammil et al., Tunable wettability and pH-responsiveness of plasma copolymers of acrylic acid and octafluorocyclobutane, Plasma Processes and Polymers, 14 (10) (2017), DOI: 10.1002/ppap.201700053

Fabrication of electrochemical paper based analytical devices (ePADs) by direct laser scribing of paperboard

Researchers at the Institute of Chemistry, University of São Paulo, São Paulo, Brazil have developed a method for the fabrication of electrochemical paper based analytical devices by direct laser scribing (LS-e-PAD) of a paperboard surface without the need for any chemical reagents or controlled atmospheric conditions. Pyrolysis of the paperboard using a CO₂ laser enables the formation of a conductive, porous, non-graphitizing carbon material, which is composed of graphene sheets and aluminosilicate nanoparticles. The high conductivity and enhanced active/geometric area ratio suggest that this material is highly promising for the development of portable electrochemical devices.

Comparison of the performance of the LS-ePAD system with that of the conventional glassy carbon electrode and a commercial screen-printed (DropSens^®) electrode using 5 mM mixture of potassium ferricyanide/ ferrocyanide solution as a redox probe reveals superior performance of the LS-ePAD system. Following their higher sensitivity and better reversibility, the utility of LS-ePAD as portable electrochemical sensors is explored for the detection of ascorbic acid and caffeic acid (important antioxidants present in food and dietary supplements) as well as in the forensic detection of picric acid (a military explosive).

The methodology of fabrication of LS-ePAD is simple, easily automated, and scalable for mass production. It is highly promising for the development of portable electrochemical devices with good reproducibility at low-cost.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer William R. de Araujo et al., Single-step Reagentless Laser Scribing Fabrication of Electrochemical Paper-based Analytical Devices, Angew. Chem. Int. Ed. 10.1002/anie.201708527

Programmable liquid materials

Liquid metals such as eutectic gallium indium alloy (EGaIn), due to their unique attributes such as voltage controlled surface tension, high liquid-state conductivity and liquid-solid phase transition at room temperature, open new avenues towards the development of programmable liquid materials.

Researchers at the University of Sussex and Swansea University, UK have exploited the self-locomotion, self-rotation, voltage controlled surface tension, high liquid-state conductivity and deformation characteristics of EGaIn and modulated its stiffness and density so that it would be possible to program the liquid metal in to a desired shape.

A liquid metal blob (certain quantity) in an electrolyte solution is highly conductive. In the absence of contact with any of the electrodes, external force and an applied voltage, the electrolyte induces a uniform charge distribution on the blob’s surface (Fig. 1(a)). However, when the blob is in contact with the anode (Fig. 1(b)) and a suitable voltage is applied to one or more of the other electrodes, the difference in the conductivity between the electrolyte and the liquid metal alters the charge distribution on the blob’ surface. The formation of an electric double-layer (EDL) at the blob’s interface enables deformation of liquid metal in the direction of the electric field (Fig. 1(b)). Hence, it would be possible to deform the liquid metal from anode (high voltage electrode) to cathode (low voltage electrode) to any desired shape (Fig. 1(c)).

Fig. 1 Influence of electric field on the deformation of liquid metal: (a) A liquid metal blob far from the electrodes and in absence of field is subject to no force; (b) Deformation of the blob upon contact with anode and an electric voltage is applied across it; and (c) Deformation of liquid metal to a desired shape.

The basic electrode array control algorithm to deform liquid metal in to a desired shape is shown in Fig. 2. In this system arrangement, the cathode attracts the liquid metal and the anode keeps the liquid metal in a wet and flat state (having the lowest surface tension). Hence, by switching only one selected electrode as cathode (low voltage) and setting all other electrodes as anodes (high voltage), the movement of liquid metal can be controlled. The relative voltage difference decides the speed of liquid metal deformation.

Fig. 2 Basic electrode array control algorithm to make alphabet letter “S”.

Three main problems that are inherent to liquid metal deformation still remains to be solved:

• H₂ evolution at the cathode causes the liquid metal to branch out as multiple trees
• Higher surface tension of liquid metals at smaller size leads to splitting
• Liquid metal body interference stops its movement towards the cathode

The programmable liquid materials will find applications in soft robotics and shape changing, reconfigurable electronic circuits and display domains.

T.S.N. Sankara Narayanan

For a more detailed information, the reader may kindly refer: Yutaka Tokuda et al., Programmable Liquid Matter: 2D Shape Deformation of Highly Conductive Liquid Metals in a Dynamic Electric Field, Proceedings of the Interactive Surfaces and Spaces on ZZZ -ISS ’17 (2017). DOI: 10.1145/3132272.3134132  

Synthesis of atomically thin metal oxides at room temperature using liquid metals – A novel approach to expand the realm of 2D materials

Metals when exposed to air under ambient conditions leads to the formation of self-limiting atomically thin oxide layer at the metal-air interface, which is considered to be a naturally occurring two-dimensional (2D) material. However, isolation of 2D metal oxides from the metal surface poses considerable challenges.

Researchers at RMIT University Australia, Queensland University of Technology,  Australia and California NanoSystems Institute, University of California, USA have shown that it would be possible to synthesis atomically thin metal oxides (2D metal oxide) at room-temperature using liquid metals as reaction environment (Reference: Ali Zavabeti et al., A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249)

In this study galinstan (liquid metal alloy containing gallium, indium and tin) was used as a reaction environment. Galinstan alloyed ~1 wt % of elemental hafnium, aluminum, or gadolinium served as the precursors for the formation of their respective oxides (HfO₂, Al₂O₃ and Gd₂O₃). The choice of these alloying elements were made on the basis of thermodynamic considerations (Gibbs free energy (ΔG_f) value).

Two different methods were proposed for isolating the surface oxides; (i) van der Waals (vdW) exfoliation technique; and (ii) gas injection method.

The van der Waals (vdW) exfoliation technique is quite similar to the method for obtaining monolayer of graphene which involves touching the liquid metal droplet with a solid substrate. The liquid nature of the parent metal allows a clean delamination of the oxide layer (Fig. 1). This technique is suitable for the production of high-quality thin oxide sheets on substrates.

The second technique relies on the injection of pressurized air into the liquid metal, in which the metal oxide forms rapidly on the inside of air bubbles and rose through the liquid metal. When the released air bubbles pass through deionized water placed above the liquid metal, allows dispersion of the oxide sheets in the aqueous suspension. Subsequently, the suspension can be subjected to drop casting to prepare 2D metal oxide films on suitable substrates (Fig. 2). This technique is highly scalable and hence suitable for the synthesis of the target oxide nanosheets with high yield.

Fig. 1 Schematic representation of the van der Waals exfoliation technique. The pristine liquid metal droplet is first exposed to an oxygen-containing environment. Touching the liquid metal with a suitable substrate allows transfer of the interfacial oxide layer.

Fig. 2 Schematic representation of the gas injection method (left), photographs of the bubble bursting through the liquid metal (center), and an optical image of the resulting sheets drop-cast onto a SiO₂/Si wafer (right)

The findings of the study indicate that oxide layers formed on liquid metals can be manipulated by an appropriate choice of alloying elements based on Gibbs free energy. The two method proposed to isolate the 2D nanosheets require simple experimental set-up and allows either a direct deposition on solid surfaces or formation of an aqueous suspension that can be drop cast over a variety of substrates. The methodology outlined in this study provides a novel pathway for the synthesis and easy isolation of 2D materials that was previously inaccessible.

The 2D materials, viz., HfO₂, Al₂O₃ and Gd₂O₃, synthesized in this study hold promise for applications in energy storage, such as supercapacitors and batteries. HfO₂ can be used as an ultrathin insulator dielectric material for the fabrication of field-effect transistors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer Ali Zavabeti et al., Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249

 

 

B and N codoped nanodiamond – A novel electrocatalyst for the selective electrochemical reduction of CO2 to ethanol

Ethanol is a clean and renewable liquid fuel with high heating value. Use of ethanol as a fuel is a viable strategy in terms of resource utilization and mitigating problems with regard to global warming. Electrochemical reduction of CO₂ can be considered as a possible route to obtain ethanol. Transition metal oxides and chalcogenides, B doped diamond, N doped carbon nanotubes and graphene were hitherto explored as potential electrocatalysts for CO₂ reduction. In spite of their good activity and durability, they reduce CO₂ to CO, HCHO or HCOO^– as the major products with a Faradic efficiency of 74.0 to 87.0%. Nevertheless, development of durable electrocatalysts for selective conversion of CO₂ to CH₃CH₂OH with high Faradic efficiency remains a big challenge.

Researchers from Dalian University of Technology, China and California Institute of Technology, USA have reported that B and N codoped nanodiamond (BND) could function as an efficient and stable electrode for the selective reduction of CO₂ to ethanol (Yanming Liu et al., Angew. Chem. Int. Ed. 10.1002/anie.201706311)

The BND film was deposited on Si by hot filament chemical vapor deposition method using a gas mixture of CH₄/B₂H₆/N₂/H₂ that had 2.5% CH₄. Three different BNDs with same B₂H₆ content (12.5%) but different N₂ levels (2.5%, 5.0% and 10.0% denoted as BND1, BND2 and BND3) were prepared.

All the three BNDs possess a similar crystal structure, morphological features and B content (Fig. 1(a)). All of them are found to be active for electrocatalytic reduction of CO₂ and they preferentially convert CO₂ to CH₃CH₂OH. The BNDs present a more negative H₂ evolution potential, which is favorable for CO₂ reduction with higher Faradic efficiency. The production rate of CH₃CH₂OH is significantly increased with a negative shift in potential from -0.8 V to -1.1 V (Fig. 1(b)) while the extent of formation of CH₃OH and HCOO^– remains low. The synergistic effect of B and N codoping is considered responsible for the better activity and high selectivity for the conversion of CO₂ to CH₃CH₂OH. Among the BNDs, maximum Faradic efficiency for the conversion of CO₂ to CH₃CH₂OH is achieved on BND3 (93.2% at -1.0 V) (Fig. 1(c)), suggesting that the higher the N content, the greater the electrocatalytic effect. The high durability of BND3 for electrocatalytic reduction of CO₂ is evidenced by its ability to show a Faradaic efficiency of ~93.2% during 16 consecutive experiments (Fig. 1(d)). Since a higher N content is likely to promote H₂ evolution, a balance between N content and H₂ evolution must be maintained to achieve better results.

Fig. 1 (a) Surface morphology of BND3; (b, c) production rates of CH₃CH₂OH, CH₃OH and HCOO^– in CO₂ saturated 0.1 M NaHCO₃ and the corresponding Faradaic efficiencies on BDN3; and (d) Faradic efficiency for CO₂ reduction during 16 consecutive runs on BND3 at -1.0 V

Based on the experimental results and density function theory (DFT) calculations, the possible pathway for the multi-electron reduction of CO₂ to CH₃CH₂OH proceeds as follows: CO₂ → *COOH → *CO → *COCO → *COCOH → *COCHOH → *COCH₂OH → *CHOCH₂OH → *CH₂OCH₂OH → CH₃CH₂OH

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Yanming Liu et al., Selective Electrochemical Reduction of Carbon Dioxide to Ethanol on a Boron- and Nitrogen-Co-doped Nanodiamond, Angew. Chem. Int. Ed. 10.1002/anie.201706311

Cyro-electron microscopy reveals the secret of what is limiting the life time of lithium ion batteries

In lithium-ion batteries (LIBs), during charge-discharge cycles, both Li metal and organic electrolyte become unstable. The continuous deposition and stripping of the Li metal results in a large structural change while dendrite growth worsen the situation. Decomposition of the organic electrolyte at the anode leads to the formation of a solid electrolyte interphase (SEI) layer consisting of organic and inorganic components. The changes in the SEI layer as well as the growth directions of the dendrites could alter the efficiency of the system. Since the Li containing electrode material, organic electrolyte and the SEI layer are chemically reactive and sensitive to electron-beam irradiation, it is hard to characterize them using transmission electron microscopy. These attributes poses difficulty in identification of failure mechanism of LIB.

Researchers at the Stanford University, USA, ShanghaiTech University, China, Universität Erlangen–Nürnberg, Germany, National Accelerator Laboratory, USA have developed a cryo-transfer method (Fig. 1) based on cyro-electron microscopy (cyro-EM) and demonstrated that it would be possible to obtain  atomic-resolution images of sensitive battery materials in their native state (Yuzhang Li et al., Science, 358, Issue 6362, 2017, pp. 506-510).

Fig. 1 Preserving and stabilizing Li metal by cryo-transfer method: (a) Li metal dendrites are electrochemically deposited directly onto a Cu TEM grid and then plunged into liquid N₂ after battery disassembly; and (b) The specimen is then placed onto the cryo-TEM holder while still immersed in liquid nitrogen and isolated from the environment by a closed shutter. During insertion into the TEM column, temperature is not increased > –170 °C, and the shutter prevents air exposure to the Li metal.

The cryo-TEM and cyro-SEM images of the electrodeposited Li metal dendrites (Figs. 2(a) and 2(b)) reveal that the dendrite structure is preserved during the cryo-transfer method. Time time-lapse images obtained under constant electron-beam irradiation (~50 e Å^–2 s^–1) in cryogenic conditions (Figs. 2(c), 2(d) and 2(e)) show no signs of damage in the dendrite morphology even after 10 min. The lack of reactivity of the Li metal with liquid N₂, helps the dendrites to retain their electrochemical state so that the relevant structural and chemical information could be obtained. The inferences made in this study reveal that in carbonate-based electrolyte, Li metal dendrites grow as single-crystalline nanowires along three primary growth directions: <111>, <110>, and <211> (Figs. 2(f), 2(g) and 2(h)) with 49% growth along the <111> direction, followed by 32% along <211> and 19% along <110> direction. In spite of growing as single-crystalline nanowires along a linear direction, the Li metal dendrites often change their growth directions.

Fig. 2 (a) Cryo-TEM and (b) Cryo-SEM images of Li metal dendrites depicting that the morphology is preserved by the cryo-transfer method; (c to e) time-lapse images of  Li dendrite; (f to h) growth of Li metal dendrites along: (f) <111>; (g) <110>; and          (h) <211> directions.

The methodology described in this work is likely to provide a complete understanding of the failure mechanisms in high-energy batteries.

T.S.N. Sankara Narayanan

 For more information, the reader may kindly refer: Yuzhang Li et al., Atomic structure of sensitive battery materials and interfaces revealed by cryo–electron microscopy, Science, 358, Issue 6362, pp. 506-510.

Molten-salt electrodeposition of Si films – Will it open the gates for cost-effective manufacturing of Si solar cells?

Developments in photovoltaic (PV) technologies is believed to make a significant impact in realizing conversion of sunlight into electricity. Reducing the manufacturing costs of Si solar cells is likely to provide a competitive edge for solar energy conversion. Researchers at the University of Texas at Austin, USA, lead by Prof. Allen J. Bard have developed a simple method for electrodeposition of high quality Si films using a CaCl₂-based molten salt electrolyte (Angew. Chem. Int. Ed. 10.1002/anie.201707635)

The CaCl₂-based molten salt electrolyte was modified with CaO (4.8 mol %) and SiO₂ nanoparticles (NPs) (3.9 mol %). Electrodeposition of Si was performed on graphite at 15 mA/cm² for 1, 3 and 7 h. The mechanism of formation of Si films is depicted in Fig. 1. In the CaCl₂-based molten salt, solid SiO₂ NPs react with O^2- to form soluble Si^IV−O anions, which are reduced to Si atoms on the graphite cathode. Continuous generation of soluble Si^IV−O anions and elimination of suspended SiO₂ particles from the molten salt electrolyte are essential to produce good quality Si films.

Impurities in the Si film, particularly, B and P, which are considered to be the most problematic ones for Si used for solar cells, are limited to 0.9 ppm and 0.6 ppm, respectively. The morphological features of the Si film reveals that it is uniform, dense and crystalline (Fig. 1). Photoelectrochemical measurements indicate that the deposited Si films possess a p-type semiconductor character due to doping of Al during electrodeposition.

Fig. 1 Schematic illustration of the formation mechanism of Si film onto a graphite substrate by electrodeposition in molten CaCl₂−CaO−SiO₂ and SEM images of the Si films deposited on graphite substrates in molten CaCl₂−CaO−SiO₂ (CaO: 4.8 mol%; SiO₂: 3.9 mol%) at 1123 K by electrodeposition at 15 mA/cm² for 1 h

 

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer Xiao Yang et al., Angew. Chem. Int. Ed. 10.1002/anie.201707635

Reducing Charge Carrier Transport Barrier in Lithium Ion Batteries by Adopting a Functionally Layer-Graded Electrode Design Approach

High-performance lithium ion batteries (LIBs) with fast charging capability and high capacity are required from portable electronics to electric vehicles. In LIBs, during charging, Li-ions depleted at the cathode get accumulated at the anode. An increase in Li-ion concentration on the anode surface when compared to the bulk, results in premature discharge, which is termed as concentration polarization induced overpotential. The slow Li-ion diffusion and poor electronic conductivity within the electrode limit the performance of LIBs. Hence, it is imperative to decrease the ionic and electronic resistance at electrolyte/electrode and electrode/current collector interfaces, respectively. In addition, it is important to minimize Li-ion diffusion barrier and maximize the electronic conductivity along the charge carrier transport direction.

Researchers at Nanyang Technological University, Singapore and University of Oslo, Norway have employed a rational design approach and fabricated functionally layer-graded electrodes composing of TiO₂(B) nanotubes and reduced graphene oxide (RGO) to reduce charge carrier transport barrier within the electrode (Angew. Chem. Int. Ed. 10.1002/anie.201707883).

The functionally layer-graded electrodes, composed of cross-linking TiO₂(B) nanotubes and well-dispersed RGO nanosheets with different configurations, were fabricated by layer-by-layer coating (Fig. 1). Among the electrode designs explored, up-graded electrode (Fig. 1(a)) exhibits a remarkable capacity of 128 mAh/g at a high charging/discharging rate at 20 °C (6.7 A/g), which is much higher than that of the traditionally homogeneous electrode (74 mAh/g) (Fig. 1(b)) with a similar composition. The improved performance of up-graded electrode is due to the synergistic effect of decrease in Li-ion diffusion energy barrier and improvement of electronic properties within the electrodes.

Fig. 1 Schematic representation of the multilayered electrode design:  (a) up-graded electrode (proposed design); (b) conventionally homogeneous electrode; and (c) down-graded electrode (reference). Colour mapping scale (in the right side) indicates the weight ratio of RGO in RGO/TiO₂(B) nanotube

The remarkable performance of the up-graded electrode design when compared to the homogeneous electrode design is explained in Fig. 2. In the up-graded electrode design arrangement, a high concentration of RGO nanosheets are available at the bottom layer, which ensures excellent electric contact between RGO/TiO₂(B) films and the current collector. This arrangement could offer a considerable reduction in interface resistance of electrode/current collector and improve electronic conductivity of whole electrode.

Fig. 2 Proposed mechanistic pathway for Li-ion and electron transport in the up-graded electrode and homogeneous electrode design arrangements

It is believed that this concept of functionally graded material can be extended to other hybrid electrodes to minimize charge carrier transport barriers and other kinetically-limited electrochemical reactions.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Yanyan Zhang et al., Reducing the Charge Carrier Transport Barrier in Functionally Layer-Graded Electrodes, Angew. Chem. Int. Ed. 10.1002/anie.201707883

 

Meta-biomaterials: Combining rational design and additive manufacturing towards the development of next generation medical devices

Meta-biomaterials are part of the emerging concept of metamaterials that possess a desired combination of mechanical  (i.e. negative Poisson’s ratio), mass transport (e.g. permeability and diffusivity) and biological properties (e.g. tissue regeneration performance).

Total hip replacement (THR) implants often encounter mechanical failure at the implant-bone interface (aseptic loosening), which limits their lifetime. The femoral part of THR is repeatedly loaded under bending for ~2 million cycles per year, which creates tensile loading and compression on either side of the neutral axis of the implant. The implant–bone interface is more susceptible to failure when subjected to tension as compared to compression. Since bone exhibits higher mechanical strength in compression than in tension, the side of the THR that experiences tension (i.e. retracts from the bone) is more susceptible to interface failure. Hence, it is necessary to design THR implants in such as way to create compression on both sides of its neutral axis.

Researchers at Delft University of Technology, The Netherlands, 3D Systems, Leuven, Belgium and University Medical Centre Utrecht, The Netherlands have demonstrated a proof-of-concept of applying a combination of rational design and additive manufacturing in the design of meta-biomaterials to improve longevity of implants. (Reference: Helena M. A. Kolken et al., Rationally designed meta-implants: a combination of auxetic and conventional meta-biomaterials, Mater. Horiz., 2017, DOI: 10.1039/C7MH00699C)

Two types of meta-biomaterials, one with a negative Poisson’s ratio (i.e. auxetic) (‘A’ in Fig. 1) while the other one with a positive Poisson’s ratio (i.e. conventional) (‘B’ in Fig. 1) were designed. Subsequently, both types of meta-biomaterials were combined to create a hybrid meta-biomaterial with different values of the Poisson’s ratio (‘C’ in Fig. 1). The meta-implants were then designed using these combined meta-biomaterials, in which the Poisson’s ratio of the meta-biomaterials changed around the neutral axis to compress the implant against the bone on both sides. Totally, six different combinations were designed and they were manufactured by selective laser melting (SLM) using biomedical-grade titanium alloy Ti6Al4V-ELI powders.

Fig. 1 Schematic drawings showing the topological designs of (A) auxetic and (B) conventional meta-biomaterials, (C) hybrid meta-biomaterials (left); and design of meta-implants (right): (C1) control type 1 with conventional hexagonal honeycombs. (H1) Hybrid type 1 with a 50/50 cell ratio. (C2) Control type 2 with re-entrant hexagonal honeycombs, showing the different parts of the implant: (1) top, (2) porous region and (3) bottom. (H2) Hybrid type 2 with a 50/50 cell ratio and a solid core. (H1) Hybrid type 1 showing the different parts of the implant: (1) top-middle-bottom and (2) porous region. (H3) Hybrid type 3 with a 70/30 cell ratio

Fig. 2 shows the photographs of the selective laser melted Ti6Al4V-ELI THR meta-implants (Fig. 2(a)); the test set-up in which the THR implant was loaded including bone-mimicking materials (Fig. 2(b)); and the horizontal strains in the bone-mimicking materials surrounding the meta-implants at t = 0 and t = 180 s at 1.5 mm displacement for C1, C2, H1, H2 and H3 (Fig. 2(c)).

Fig. 2 (a) Additively manufactured (selective laser melting) Ti6Al4V-ELI THR meta-implants; (b) test set-up in which the THR implant was loaded including bone-mimicking materials; and (c) Horizontal strains in the bone-mimicking materials surrounding the meta-implants at t = 0 and t = 180 s at 1.5 mm displacement for C1, C2, H1, H2 and H3.

The findings of the study clearly reveal that meta-implant with design H2 compress against the bone under repetitive loads that are applied during gait and other daily activities. According to the Hoffman’s failure criterion, this combination of compression and shear is less deleterious than tension and shear.

The current proof-of-concept study demonstrated the feasibility of applying rational design and metamaterials for the development of the next generation of medical devices. Nevertheless, the performance of these materials has to be evaluated using animal models and clinical trials.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Helena M. A. Kolken et al., Mater. Horiz., 2017, DOI: 10.1039/C7MH00699C)

 

Smart Dental Braces – Pushing the boundary of personalized health care electronics to the next level

Batteries contribute to the overall weight and size of implantable devices such as cardiac peacemakers and neuro-stimulators. The rigid encapsulation and requirement for proper insulation from corrosive materials limit the widespread utility of batteries for implantable devices. Besides weight and design aspects, the batteries have to be biocompatible and offer a high performance. Hence, development of light weight, physically flexible, biocompatible, high performance batteries are highly warranted to meet the demands of advanced personalized health care applications.

Researchers at King Abdullah University of Science and Technology (KAUST), Saudi Arabia, lead by Prof. Muhammad M. Hussain, have demonstrated a transfer-less method to develop a flexible, light-weight, biocompatible, high performance lithium ion batteries (LIBs) for implantable devices. They have also proposed a strategy for integrating the LIBs with flexible electronics and embedding them in a three-dimensional (3D) printed dental brace for orthodontics application (npj Flexible Electronics 1, Article No:7 (2017) doi:10.1038/s41528-017-0008-7)

Polydimethylsiloxane (PDMS) was used as the carrier substrate. A bulk LIB (thickness: 130 μm) was flipped on to the PDMS substrate. The Si (at the base of the LIB) was etched using xenon difluoride (XeF₂) at a rate of 67 nm/s for 36 min (50 cycles). The surface roughness was monitored using atomic force microscopy (AFM) to optimize the conditions of XeF₂ etching. The thinned LIB was removed from the PDMS (Fig. 1(a)). Complete removal of the Si substrate from the thinned LIB results in a free standing and physically flexible active stack (thickness: 30 μm). It consists of SiO₂ (insulation layer), Al (cathode current collector), lithium cobalt oxide (cathode), lithium phosphorous oxynitride (electrolyte), Ti (anode current collector) and protective layers at the top surface (Fig. 1(b)). In spite of a large reduction in thickness (from 130 μm to 30 μm), the flexible LIB experienced only a lower strain (five times less) when compared to the bulk, for a 10 mm bending. Cell cultures grown on LIB for 3 to 5 days exhibited a healthy proliferation of the cells, which confirmed the biocompatibility of LIB. The flexible LIB possess a light weight (236 μg for each microcell of 2.25 × 1.7 mm) and exhibits a very high energy density (200 mWh/cm³) with a capacity retention of up to 70% after 120 cycles.

Fig. 1 (a) Schematic of the fabrication of flexible LIBs; and (b) cross sectional morphology of the flexible thinned battery and its components

Polyethylene terephthalate (PET) was used as the base for the flexible electronic device. It was metallized with Al (thickness: 0.023 mm). Interconnections were patterned using 1.06 μm ytterbium-doped fiber laser. Flip-chip technology and stencil printing allowed placement of the components. A conductive silver epoxy was used to bond the LIBs and LEDs. The LIB in the flexible electronic device exhibits minimal strain since most of the stress is experienced by the PET film. Transparent orthodontic brace was prepared by 3D printing using a clear resin. Finally, the flexible electronic device is integrated with the 3D printed orthodontic brace. The schematic of the integration of the flexible thinned LIBs with the flexible electronics and 3D printed dental braces is shown in Fig. 2(a). A pictorial representation of how the device fits in conformably onto the human dental arch is shown in Fig. 2(b).

Fig. 2 (a) Schematic of the integration of the flexible thinned LIBs with flexible electronics and 3D printed dental braces; and (b) Pictorial representation of how the device fits in conformable manner onto the human dental arch.

T.S.N. Sankara Narayanan

 

 

For more information, the reader may kindly refer: Arwa T. Kutbee et al., npj Flexible Electronics 1, Article No:7 (2017) doi:10.1038/s41528-017-0008-7

Upcycling waste polyethylene to high performance graphitic carbon – A new avenue for utilization of plastic waste

Linear low density polyethylene (LLDPE) is one of the most widely used plastics, mainly as a packaging film, contributes to the generation of a huge volume of plastic waste. Only 5.8 % of waste LLDPE was recycled in 2014, while the remaining wastes were buried in landfills, posing a huge challenge for solid waste management.

Researchers from Korea Institute of Science and Technology, Korea University of Science and Technology, Konkuk University, Republic of Korea and Georgia Institute of Technology, USA have developed a process to LLDPE to graphitic carbon and demonstrated that it would be possible to convert typical household LLDPE waste products such as cling wrap and poly-gloves into high quality carbon materials. (Choi et al., High performance graphitic carbon from waste polyethylene: thermal oxidation as a stabilization pathway revisited,
Chem. Mater., DOI: 10.1021/acs.chemmater.7b03737).

Thermal oxidation was used as a pre-treatment to modulate the chemical structure of LLDPE. During the thermal oxidative pre-treatment (~330 °C) the ‘non carbonizable’ LLDPE was successfully transformed into an ordered carbon (50% yield). The aliphatic LLPDE chain is reorganized into thermally stable cross-linked polyaromatic moieties with a cyclized ladder structure (Scheme 1), which makes them suitable for carbonization.

Scheme 1 Proposed chemical structural transformation of aliphatic LLDPE chains into cyclized polyaromatic moieties through thermal oxidation

The thermally stable cross-linked polyaromatic moieties, in turn, guides a spontaneous transition into well-stacked polyaromatic carbon structures with a further increase in temperature from 330 to 2400 °C (Scheme 2). The resultant product possesses a high quality graphitic structure that exhibits superior degree of ordering and electrical performance.

Scheme 2 Growth of basic structural unit (BSU) of thermally oxidized LLDPE samples during carbonization and graphitization processes

It is a low-cost production route for graphitic carbon materials, which would find application in energy storage and flexible, printed electronics.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Choi et al., High performance graphitic carbon from waste polyethylene: thermal oxidation as a stabilization pathway revisited, Chem. Mater., DOI: 10.1021/acs.chemmater.7b03737