Toward Flexible and Wearable Embroidered Supercapacitors from Cobalt Phosphides-Decorated Conductive Fibers
The conductive silver-plated nylon yarns fully fit the demand for wearable supercapacitor skeleton, owing to the advantages of high conductivity and flexibility.
The computerized programming embroidering technique was firstly applied for realizing standardized batch processing of the flexible supercapacitor skeleton in various patterns.
Cobalt phosphides were properly electrodeposited on the conductive embroidery as the pseudocapacitive materials, providing remarkable electrochemical performance.
KeywordsWearable supercapacitor Conductive fiber Computerized embroidering Interdigital pattern Cobalt phosphide
Wearable energy storage devices are receiving great attention and popularity for the growing demands from the modern portable electronics and smart textiles [1, 2, 3, 4]. In particular, supercapacitors (SCs), also known as electrochemical capacitors, are drawing attention for their irreplaceable advantages in high power density and long cycle life, leading to a worldwide trend seeking for the novel and superior electroactive materials [5, 6, 7, 8]. Among the hot-spot pseudocapacitive materials, the transition metal nickel- and cobalt-based compounds are widely studied owing to their high theoretical capacitance, such as Ni–Co oxides [9, 10, 11], Ni–Co hydroxides [12, 13], and Ni–Co sulfides [14, 15, 16]. However, for nickel and cobalt phosphides, more studies were carried out in the fields of hydrogen evolution and Na-ion batteries and interests casted on SCs are still far from enough from the perspective of electrochemical energy storage [17, 18, 19, 20]. Reviewing the recent work of Ni/Co-related phosphides in SCs [21, 22, 23], Co2P nanoflowers- and graphene-based asymmetric SCs showed impressive electrochemical properties, with the energy density of 8.8 Wh kg−1 (at a high power density of 6 kWh kg−1) . Also, a CoP nanowires-based carbon cloth displayed remarkable negative capacitive behavior in the neutral LiCl/PVA electrolyte, but it dissolved gradually due to the side reactions with Cl− system . In this regard, it is highly expected to further explore the potential of Ni/Co-based phosphides as the electrode material of SCs.
As a matter of fact, for flexible and wearable devices, the typical textile processing techniques [26, 27, 28, 29, 30], such as knitting, weaving, and embroidering, are rarely reported for integrating with the applications. Instead, more works paid their attention to the pure synthesis of the electroactive materials [31, 32, 33]. And the commonly reported flexible substrates mainly include carbon-derived materials (e.g., activated carbon (AC), carbon nanotubes (CNTs), and graphene) [34, 35, 36], polymer-based films (e.g., PET) [37, 38, 39], and metallic foils (e.g., Cu foil and Ti foil) [40, 41], which showed only limited flexibility when compared to the real garments. By contrast, the embroidery stitches, originating from the conventional hand sewing, can not only produce various patterns on the fabric, but also realize the standardized batch processing via the computer-aided programming techniques .
In this work, inspired by the design of the small in-plane SCs [43, 44, 45], a conductive silver-plated nylon yarns (SPNYs)-patterned embroidery was, for the first time, rationally designed and interdigitally constructed on the common fabric via the computerized programming. A thin layer of nickel nanothorn arrays (NTAs) was then electrodeposited on the conductive embroidery for restoring the silver surfaces and creating more porous active sites for loading CoP. Finally, the CoP microspheres were finely located on the Ni NTAs-covered embroidery skeleton, and the all-solid-state in-plane CoP@Ni NTAs@SE SCs were then assembled and measured, showing not only competitive capacitance but also extraordinary flexibility. Moreover, the computerized programming successfully embroidered a monogrammed conductive pattern on the laboratory gown, demonstrating a promising prospect in large-scale production of the wearable energy storage devices.
Chemicals of LiOH·H2O 99%, NaH2PO2 98%, CoCl2·6H2O 99%, NiCl2·6H2O 99%, NiSO4·6H2O 99%, CH3COONa 58%, H3BO3 99%, Na3C6H5O7·2H2O 99%, polyvinyl alcohol (PVA), pyrrole 98%, H2SO4 96%, HCl 37%, and absolute ethanol are all of analytical grade. Conductive silver-plated nylon yarns (20D, 15 Ω cm−1) were purchased from Qingdao Zhiyuanxiangyu Functional Fabric Co., Ltd.
2.2 Fabrication of CoP-Decorated Patternable Conductive Embroidery SCs
2.2.1 Creation of the Conductive SPNYs Embroidery
TAJIMA wearable embroidery machine with the model of TCMX-600 was used for automatically making the conductive embroidery stitches with a stitching speed up to 750 rpm. The embroidery machine has a working space of D × W by 460 × 550 mm2, and the stitch length ranges from 0.1 to 12.1 mm. With the aid of computerized programming, various patterns were properly embroidered on the base fabrics. Instead of using the common cotton or polyester yarns, the highly conductive SPNYs were selected for creating the embroidery serving as both the substrates and current collectors for the in-plane SCs. The detailed pattern dimensions are shown in Fig. S1.
2.2.2 Pre-deposition of Ni Nanothorn on SPNYs Embroidery
Before the fabrication of the electroactive material CoP, a thin layer of nickel nanothorn arrays (NTAs) was preferentially deposited on the SPNYs. That is, a piece of the embroidered SE was immersed into a 50-mL beaker with 10% (volume concentration) H2SO4 solution for the surface pretreatment of 1 h. The fabric was then removed and washed with the deionized (DI) water, ethanol, and acetone in order, followed by a drying process in the vacuum oven for 2 h under 60 °C. The electrodeposition of the Ni NTAs was conducted via a chronoamperometry mode, with the pretreated SE as the working electrode, a stainless steel plate (3.0 × 4.0 cm2) as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The base electrolytic solution was composed of 30 mM NiSO4, 0.4 M H3BO3, and 0.2 M C6H5NA3O7·2H2O, with the pH around 4 ~ 5. To optimize the synthetic conditions, the electrodeposition was performed at various potentials (1.0, 1.2, 1.4, and 1.6 V), and the resulted products were then rinsed thoroughly by the DI water and stored in the vacuum oven under 60 °C.
2.2.3 Decoration of the CoP Microspheres on the Ni NTAs@SE
The cobalt phosphide was electrodeposited via the similar electrochemical configuration as Ni NTAs, while the electrolytic solution consisted of 10 mM CoCl2, 0.1 M Na3C6H5O7, 0.6 M NaH2PO2, with nitrogen bubbled through for 30 min before using. The electrodeposition was performed at different potentials (0.9, 1.1, and 1.3 V) and durations (500, 750, 1000, and 1250 s) for further confirming the most suitable conditions. The electrolytic solutions with different Ni/Co ratios (1:0, 1:1) were also applied for a comparison.
2.3 Characterization and Electrochemical Measurements
The chemical phases of SE, Ni NTAs@SE, and CoP@Ni NTAs@SE were all determined by the X-ray diffraction (XRD: Cu target, Ka, λ = 0.15406 nm) with a Bruker D8 Advance X-ray diffractometer and a scanning transmission electron microscope (STEM: Jeol JEM-2100F). The morphologies were measured by an EDS detector equipped with scanning electron microscopy (SEM: TESCAN VEGA3).
Raman spectra were recorded via a micro-Raman spectroscope (JY-HR800) with the laser emitting at 532 nm. Fourier transform infrared spectroscopy (FTIR) was achieved by applying a Spectrum 100 spectrometer (PerkinElmer) in the range of 600–3500 cm−1 at a resolution of 4 cm−1.
2.4 Fabrication of All-Solid-State Embroidery SCs
The gel PVA/LiOH electrolyte was prepared by dissolving 5 g PVA and 1.2 g LiOH in 50 mL DI water at 85–90 °C for 1.5 h. The all-solid-state SCs were then assembled by pouring the as-prepared PVA/LiOH gel electrolyte on the electroactive materials-loaded embroidery and drying at room temperature for 2 h.
3 Results and Discussion
3.1 Characterization of CoP@Ni NTAs@SE
Electrical resistances of different SPNYs
The electroactive material of CoP was then electrodeposited on the as-prepared Ni NTAs, as shown in Fig. 3c, d. Generally, studies on the nickel or cobalt composites are normally performed by investigating different Ni-to-Co ratios for gaining the optimal electrochemical performance [16, 49, 50]. In this regard, here, three distinct and empirical ratios (Ni/Co = 1:0, Ni/Co = 1:1, Ni/Co = 0:1) were carried out to make a comparison pending the capacitance between electrodes with different nickel-to-cobalt ratios. As shown in Fig. 4b, CV curves of both pure nickel phosphide and Ni/Co 1:1 phosphide showed much smaller loops than the pure cobalt phosphide, which means the mixing of nickel phosphide into the cobalt phosphide is almost in vain. Therefore, the electroactive material of the pure cobalt phosphide was used for the further study. Experiments with different CoP electrodepositing voltages and durations were conducted in order to gain the most suitable synthetic conditions. Figure 4c shows the CV curves of CoP formed at three different electrodepositing voltages (0.9, 1.1, and 1.3 V), where CoP obtained at 1.1 V displayed the highest electrochemical performance. Correspondingly, the SEM images of CoP obtained at 0.9 and 1.3 V are also shown in Fig. S3a, b for comparison. It can be seen that more dense and compact particles were formed at 0.9 V, and the microspheres covered by the irregular fakes or sheets (could be the Co(OH)2 formed by the hydrogen evolution [51, 52]) were obtained at 1.3 V, as shown in Fig. S3b. Notably, the unwanted impurities can negatively affect the CoP synthesis. By contrast, Fig. 2e shows that a layer of very uniform microspheres deposited on Ni NTAs@SE at a moderate voltage of 1.1 V, with a large number of macropores between each other, which could benefit the ion diffusion between the electrolyte and the electroactive materials. This is consistent with the above result from the CV curves, and it is reasonable to control the CoP electrodeposition at a moderate voltage of 1.1 V.
Subsequently, to investigate and control the optimum mass loading, Fig. 4d gives the information on CV curves based on CoP mass loadings at different electrodepositing durations, where the gravimetric capacitance enlarged gradually as the loading of CoP increased, from 0.95 mg cm−2 at 500 s to 2.38 mg cm−2 at 1000 s, but a degradation was observed at the mass loading of 2.86 mg cm−2 at 1250 s. Thus, it is better to control the CoP amount around 2.38 mg cm−2 obtained at the electrodepositing duration of 1000 s. Finally, the most suitable electrodepositing condition for fabricating the electrode is nickel layer 1.4 V at 2000 s and cobalt phosphide 1.1 V at 1000 s. The resulted electrode showed the weight percentages of silver-plated nylon yarns, Ni layer, and CoP layer were 67.63%, 21.97%, and 11.56%, respectively. The cross-section information of the resulted CoP@Ni NTAs@SE is also displayed in Fig. S5a, b, including the SEM image and real photograph, which shows that the interdigital fingers have the quasi-elliptical cross section, with the long and short axes around 1 and 0.7 mm.
EDS mapping images of the CoP microspheres are displayed in Fig. 2f, g, with the red and blue representing elements Co and P, respectively, and the spectrum in Fig. S4 clearly exhibits the existence of Co and P, with the atomic percentage ratio to be 53.28:46.72 (nearly 1:1). The SAED image in Fig. 2h also discloses a series of lattice planes, mainly including CoP (201) and CoP (220) from inside to outside. Figure 2i shows the high-resolution TEM image, and the lattice fringes have an interplane spacing of 0.233 nm corresponding to CoP (201) plane. The XRD curves in Fig. 2j show the strong peak situated at the 2θ of 38.18° in all three samples, representing the crystal plane of Ag (111). Interestingly, the peak strength became weaker in Ni NTAs@SE but stronger in CoP@Ni NTAs@SE, and this could be probably because of the coverage of nickel and CoP on the SPNYs, where the Ni NTAs decreased the silver exposure and the CoP reinforced the peak strength because of the crystal plane of CoP (111) in the same location. The strongest characteristic peak of CoP (101) was well located in 31.58°, and the other (200), (220), and (311) can be also properly observed, which is well in agreement with both the results of SEAD and XRD patterns. This further proved that CoP was successfully synthesized via the electrodeposition.
Raman spectra of SE in Fig. S6a showed two characteristic peaks at 1356 cm−1 (D-band) and 1590 cm−1 (G-band), indicating the structural defects and disorders of carbons in the nylon yarns. For the Ni NTAs@SE and CoP@Ni NTAs@SE, no more peaks were observed, and the D-band and G-band peaks were weakened disproportionally due to the encapsulation of Ni and CoP on the SPNYs, which is consistent with the previous studies [53, 54]. The FTIR result in Fig. S6b shows no significant difference among the three curves, and only weak peaks were observed in SE, which might be from the organic compositions in the thin silver particle-coated nylon yarns. This also laterally supports the aforementioned analysis.
3.2 Electrochemical Properties of the All-Solid-State Embroidery Supercapacitor
Based on the charge and discharge curves, both the areal and gravimetric capacitances were derived and calculated, as shown in Fig. 5d. The highest specific capacitance of 156.6 mF cm−2 (65.72 F g−1) was obtained at the current density of 0.6 mA cm−2 (0.25 A g−1). As the current density increased, both the areal and gravimetric capacitances dropped accordingly, and the lowest specific capacitance of 33.5 mF cm−2 (14.06 F g−1) was acquired at the highest current density of 6 mA cm−2 (2.5 A g−1). This is because the insufficient pseudocapacitive reactions occurred between the electrolytic ions and the electroactive materials at the very fast electrons embedding and disembedding processes. To examine the flexibility of the embroidery SC, the device was bent from 0° to 180° up to 300 times, with a bending radius of 1.5 mm. The corresponding curves to the 50th, 100th, 200th, and 300th GCD are displayed in Fig. 5e, respectively. It showed that within 200 times, the original curve shape was retained very well without significant changes, demonstrating a decent electrochemical stability under various deformations. However, the charging/discharging duration decreased greatly, when it cycled to the 300th time. This might be the result of structural damages from both the Ni layers and the CoP particles . Figure 5f displays the cycling stability of the CoP@Ni NTAs@SE by charging and discharging up to 5000 times at various current densities. It was noted that the specific capacitance dropped more dramatically at the current density of 0.25 A g−1 in the first 1000 cycles, from 67.82 to 57.34 F g−1, and the corresponding capacitance retention declined to 86.11% at the same time. This is probably because the electrolyte ions diffuse adequately to the electroactive material surfaces at a low current density, where the redox was carried out more effectively than that at higher current densities, leading to more structural defects and much faster capacitance degradation [58, 59]. After increasing the charging and discharging currents, the capacitance was retained much better, with the capacitance loss less than 5% after the 5000th time.
To investigate the overall resistance, EIS was performed from 100 kHz to 0.01 Hz with a voltage amplitude of 5 mV as shown in Fig. 5g. As the inset displays, the intercept on the high-frequency region shows a low solution resistance, with about 18.42 Ω, reflecting the decent movement of the solid-state electrolyte ions. The moderate semicircle region represents the charge transfer resistance, which can be determined to be around 160 Ω. The linear part at the low-frequency region shows the typical Warburg impedance curve, with the phase angle greater than 45°. The moderate slope reflects that the diffusing condition within the electrode is one of the factors posing negative effects on the capacitive behavior of the device [60, 61]. The Ragone plot (energy density vs. power density) of the device is properly drawn for comparisons. As shown in Fig. 5h, the areal and gravimetric values are plotted in violet and orange, respectively, and it shows that a high energy density of 0.013 mWh cm−2 (5.55 Wh kg−1) was achieved at a power density of 0.24 mW cm−2 (100 W kg−1), which is comparable and even superior to the recently reported studies with the interdigital configuration based on MoS2@rGO (0.0019 mWh cm−2 at 0.08 mW cm−2) , 3D graphene (0.00042 mWh cm−2 at 1 mW cm−2) , polyaniline (0.00092 mWh cm−2 at 1 mW cm−2) , graphene film (0.0025 mWh cm−2 at 1.2 mW cm−2) , cellular graphene film (0.00022 mWh cm−2 at 0.98 mW cm−2) , graphene/CNTs film (0.00034 mWh cm−2 at 0.68 mW cm−2) , MnO2 CNTs (0.00088 mWh cm−2 at 0.16 mW cm−2) .
On the whole, the high performance of the CoP@Ni NTAs@SE SC can be summarized as follows: (1) The creative and advanced computerized embroidering techniques realized an interdigitally patterned conductive embroidery, (2) the conductive SPNYs-based embroidery substrate offered both fast electron transporting passage and excellent flexibility, (3) the subsequent Ni NTAs conductive layers provided not only improved conductivities, but also porous structures for loading the electroactive materials, and (4) the cobalt phosphides possess not only outstanding pseudocapacitance, but also a wide voltage window from negative to positive ranges.
3.3 Application of the Embroidery SCs
By implementing the strategy of loading novel pseudocapacitive material on the flexible textiles, the embroidered in-plane supercapacitors were successfully created via the computer-aided textile technology and the electrochemical synthesis approaches. Great flexibility and remarkable electrochemical performances were accordingly achieved owing to the porous nickel nanothorns-anchored CoP microspheres. Finally, a monogrammed SC was properly embroidered in series on the laboratory gown, inspiring a promising perspective for integrating more wearable technologies into the next generation of wearable energy storage devices.
The authors acknowledge The Hong Kong Polytechnic University for the funding support (Nos. 1-YW1B, G-YBV2, and G-UACC) of this work. Wen Jianfeng would also like to thank The Hong Kong Polytechnic University for providing him with a postgraduate scholarship.
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