Ni(OH)2 Nanosheet Ink for Wearable Energy Storage Devices

Flexible electronics have received considerable attention, particularly in energy storage devices. Among the various techniques available for their fabrication, solution based methods such as ink-jet printing, screen printing, and roll-to-roll printing assume significance in terms of their low-cost, high processing speed, and scalability. Nevertheless, formulation of inks containing suitable active materials with better dispersion and stability remains a challenge. Researchers at Nanjing Tech University, China, Nanyang Technological University, Singapore, Lanzhou University, China, Nanjing University of Posts and Telecommunications, China and Northwestern Polytechnical University, China have developed a facile method to prepare a highly concentrated ink comprised of 2D ultrathin Ni(OH)₂ nanosheets, which can be easily coated on commercial printing paper as well as carbon fiber yarns (CFYs). Using CFY@Ni(OH)₂ as a weavable electrode, they have fabricated wearable energy storage devices.

The Ni(OH)₂ nanosheets were prepared by co-precipitation method. Since the precipitated Ni(OH)₂ nanosheets tend to aggregate, they were exfoliated by ultrasonication. The Ni(OH)₂ nanosheets dispersed in water, ethanol, and DMF exhibit excellent stability. The Ni(OH)₂ ink (20 mg/mL in water) (Fig. 1(a)) was directly coated on printing paper using a brush. The desired thickness of the Ni(OH)₂ coating can be tuned by a careful choice of the number of cycles. The Ni(OH)₂ coated paper is highly flexible and it can be easily wound around a glass rod (Fig. 1(b)). Selective removal of Ni(OH)₂ from the coated paper (Fig. 1(c)) using 3M HCl enables patterning of suitable designs (Fig. 1(d)).

Fig. 1 (a) Ni(OH)₂ ink (20 mg/mL in water); (b, c) flexible and plain printing papers coated with Ni(OH)₂ nanosheets; and (d) selective removal of Ni(OH)₂ from the coated printing paper using 3M HCl for patterning of suitable designs

The Ni(OH)₂ nanosheet ink was also coated on carbon fiber yarn (CFY). Ethanol was used as the solvent to improve wettability of CFY with the ink. Nafion (Nf) was used as an ionic binder to promote the interaction between Ni(OH)₂ nanosheets and CFY. The CFY@Nf-Ni(OH)₂ electrode was prepared by impregnation-dyeing method (Fig. 2(a)) in which the CFY was repetitively impregnated in Nf-Ni(OH)₂ ink (20 mg/mL in ethanol) for 1 min for each cycle and dried at 80 °C. The CFY@Nf-Ni(OH)₂ maintains excellent flexibility (Fig. 2(b)). Morphological features reveal that the Ni(OH)₂ is uniformly coated on CFY (300 nm thick for 6 impregnations) (Figs. 2(c) and 2(d)).

Fig. 2 (a) Schematic illustration of the preparation of CFY@Nf-Ni(OH)₂ by impregnation-dyeing method; (b) Photograph of highly bended CFY@Nf-Ni(OH)₂; (c) SEM image of a single yarn CF@Nf-Ni(OH)₂; (d) Magnified image of (c) 300 nm thick Ni(OH)₂ coated layer obtained after 6 impregnations.

Galvanostatic charge–discharge curves of CFY@Nf-Ni(OH)₂ indicate that a maximum  specific volumetric capacitance (CV) is obtained after 6 times of impregnation whereas gravimetric capacitance (Cg) is decreased from 1010.0 to 640.0 F/g with an increase in impregnation times from 2 to 10. Repetitive impregnations though enable an increase in the mass loading of Ni(OH)₂ nanosheets, it leads to a reduction in the conductivity of CFY@Nf-Ni(OH)₂. The CFY@Nf-Ni(OH)₂ electrode exhibits good cycling stability, which is evidenced by its 89% capacitance retention after 5000 charge–discharge cycles.

A hybrid supercapacitor was fabricated using CFY@Nf-Ni(OH)₂ as positive electrode, CFY@CPs (carbon particles) as negative electrode and poly(vinyl alcohol-KOH gel as the solid-state electrolyte, enclosed in a thermal plastic tube to seal the device (Fig. 3). The hybrid supercapacitor exhibits a wide potential window of 1.5 V and delivers a high energy density of 11.3 mWh/cm³ at a power density of 0.3 W/cm³.

Fig. 3 Schematic of the fabrication of yarn-based hybrid supercapacitor

The CFY-based hybrid supercapacitor can be woven in the form of a glove (Fig. 4), which at different bending angles is capable of retaining 96% capacitance after 5000 bending–unbending cycles. This behaviour seems to be promising for their potential use as energy storage devices in e-textiles.

Fig. 4 (a) Photographs of yarn-based hybrid supercapacitor woven in the form of a glove at different bending states; (b) Capacitance retention of the hybrid supercapacitor at different bending angles.

The Ni(OH)₂ nanosheet inks seem to be promising for large-scale production of high-performance energy storage devices for flexible electronics.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer to Peipei Shi et al., Highly Concentrated, Ultrathin Nickel Hydroxide Nanosheet Ink for Wearable Energy Storage Devices, Adv. Mater. 2017, 1703455, DOI: 10.1002/adma.201703455

Formation of Luminescent Carbon Nitride Nanosheets by Spontaneous Liquid Phase Dissolution

2D materials due to their unique physical and chemical properties assume significance in a wide variety of applications. Among the various strategies employed for the synthesis of 2D materials, liquid phase exfoliation from layered crystalline precursors (bottom-up route) is considered to be beneficial. Nevertheless, use of aggressive chemicals and formation of fragmented or chemically modified nanosheets limit the applicability of this methodology. Researchers at University College London, Imperial College London, University of Bristol, United Kingdom and École Polytechnique Fédérale de Lausanne, Switzerland have demonstrated a liquid phase dissolution route for the synthesis of 2D carbon nitride (CN) nanosheets using poly(triazine imide)-lithium bromide (PTI-LiBr) as the crystalline precursor and aprotic polar solvents as the liquid phase. The spontaneous dissolution of PTI-LiBr in organic solvents yield solutions containing defect-free, crystalline, 2D CN nanosheets.

Dicyandiamide (DCDA), lithium bromide (LiBr) and potassium bromide (KBr) were used as the starting materials. 2 g of DCDA was mixed with 10 g of the LiBr/KBr (52%:48%) and thoroughly ground. 7 g of the ground homogeneous powder was heated to 400 °C under flowing N₂ and soaked at 400 °C for 6 h. 4 g of this pretreated mixture was placed inside a quartz tube sealed at one end. The quartz tube was evacuated to < 10^-6 mbar and sealed. The quartz ampoule was heated to 600 °C for 12 h. The resultant brown coloured  material was removed from the ampoule, repeatedly washed with hot deionized water, centrifuged at 4000 rpm and the retrieved PTI-LiBr was washed with methanol. The structural and morphological properties of PTI-LiBr are shown in Fig. 1

Fig. 1 (a) XRD pattern of crystalline PTI·LiBr (Inset: one unit cell of a PTI·LiBr); (b) SEM image of an aggregate of hexagonal prismatic PTI·LiBr crystallites (Inset: TEM image of hexagonal PTI·LiBr crystallites).

Dissolution of as-synthesized PTI-LiBr crystals in N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) indicates a change in the color of the liquid over time (Fig. 2(a)). The extent of dissolution of PTI-LiBr crystals is enhanced under UV-light illumination (Fig. 2(b)).

Fig. 2 Time-lapse photographs depicting spontaneous dissolution of PTI-LiBr in DMSO up to 48 h under visible and UV illuminations.

The high-resolution TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP (Figs. 3(a)-3(c)) indicate that the CN nanosheets are atomically intact with well-defined edges and maintained the hexagonal shape with its lateral dimensions close to that of the precursor bulk crystals. No evidence of any dislocations or point defects could be observed.

Fig. 3 (a-c) HR-TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP

Both bulk and exfoliated CN exhibit luminescence in the UV/visible range. The normalized photoluminescence (PL) emission spectra of CN nanosheets dissolved in DMF exhibit a broad peak ∼380 nm, which slightly shift toward blue-green range with an increase in wavelength excitation from 260 to 330 nm (Fig. 4(a)). The PL spectra of stacked or aggregated films of CN nanosheets deposited from dissolved solution also exhibit a broad peak centered ∼480 nm (red-shift when compared to PL spectra of dissolved CN nanosheet) (Fig. 4(b)). The broadening of the PL spectra of CN nanosheets dissolved in DMF as well as the stacked or aggregated CN film deposited from dissolved solution indicates that they could be composed of 9 to 40 layers in thickness. These inferences indicate that depending on the thickness of CN nanosheets, it would be possible to tune the PL wavelength from narrow UV to broad-band white.

Fig. 4 PL spectra of CN nanosheets at varying excitation wavelength: (a) CN nanosheets dissolved in DMF; (b) stacked or aggregated CN film deposited from dissolved nanosheets

The methodology employed for the synthesis of 2D CN nanosheets is simple  and easily scalable. The spontaneous dissolution of PTI-LiBr crystals in NMP, DMF, and DMSO results in the formation of stable solutions of pristine, defect-free CN nanosheets with well-defined functional properties. The luminescence property of dissolved as well as stacked film of CN nanosheets indicate that they can be explored as potential next-generation materials for photocatalysis. The tunability of PL spectra depending on the stack thickness of CN nanosheets makes them as suitable candidate materials for UV-blue and white LED emitters. The CN nanosheets prepared by this method can be used for a wide range of optoelectronic devices.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Thomas S. Miller et al., Single Crystal, Luminescent Carbon Nitride Nanosheets Formed by Spontaneous Dissolution, Nano Lett. 2017, 17, 5891−5896.