Integrated System of Solar Cells with Hierarchical NiCo2O4 Battery-Supercapacitor Hybrid Devices for Self-Driving Light-Emitting Diodes
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Integration of solar cells, BSHs, and LEDs was developed for energy conversion, storage, and utilization in one system.
NiCo2O4//AC BSHs were charged by a-Si/H solar cells for stably driving LEDs showing high performances.
KeywordsIntegrated system NiCo2O4 Battery-supercapacitor hybrid devices Self-driving LED
Combination of energy conversion, storage, and utilization devices has been widely investigated due to its possibilities in practical applications [1, 2, 3]. Among the three devices, electrochemical energy storage devices serve as the core component for both energy storage and power output . As is well known, clean energies, including solar energy, wind energy, and tidal energy that are expected to be used in large scale in the near future, are intermittent and even irregular owing to the changes in weather and thus unsuitable for all day power supply . For instance, solar energy can only work under sunlight illumination, which is affected by not only the cycle of day and night, but also the variation of sunlight intensity during daytime such as clear and cloudy. Therefore, the energy devices are indispensible to serve as a buffer pool to mitigate the mismatch between solar energy supply and demand.
Currently, electrochemical energy storage devices cover batteries and supercapacitors primarily [6, 7, 8, 9, 10, 11, 12]. Batteries are really a big family, including conventional lead–acid [13, 14], nickel–cadmium [15, 16], nickel–metal hydride [17, 18, 19], and lithium-ion batteries [20, 21, 22, 23, 24, 25], as well as newly developed batteries such as lithium–sulfur [26, 27, 28], lithium–air [29, 30, 31], lithium–CO2 [32, 33, 34], sodium/potassium/magnesium/aluminum/zinc ion [35, 36, 37, 38, 39, 40, 41, 42, 43, 44], and aqueous metal ion batteries [45, 46]. However, the batteries are unsuitable as energy storage devices in the frequently charge and discharge area, such as solar energy, wind energy, and tidal energy, due to the shortcomings of low cycling performance (usually in the order of hundreds of times) [47, 48]. Conventional supercapacitors possess the virtual of long cycling lifetime, but they are restricted to the low energy density and high self-discharge. The giant volume is inevitable to match the energy output, which prevents supercapacitors from practical use. Recently, a new analogous device combined with faradic electrode and capacitive electrode, named hybrid supercapacitor (HSC) or battery-supercapacitor hybrid (BSH) [49, 50, 51, 52], provides a reliable approach to make the hybrid devices with advantages of both batteries and supercapacitors. This combination is promising to satisfy the requirements of both high energy density and high power density to drive different kinds of electrical equipments. With the merit of both long cycling characteristic and high energy density, the BSH devices are particularly suitable for the periodically changed energy generation sources.
In this work, we developed an integrated system with a-Si/H solar cells, three-dimensional (3D) hierarchical NiCo2O4 arrays//active carbon (AC) BSHs, and light-emitting diodes (LEDs) to realize energy conversion, storage, and utilization in one system. Benefitting from the unique hierarchical structure, the 3D hierarchical NiCo2O4 arrays electrode exhibits a high specific capacity of 130 mAh g−1. The BSHs assembled with NiCo2O4 faradic electrode and AC capacitive electrode display an energy density of 16.6 Wh kg−1, power density of 7,285 W kg−1, long cycling stability (100% retention after 15,000 cycles), and rather low self-discharge. Acting as the energy storage device in the integrated system, the BSHs can store solar energy and output power for driving LEDs. The NiCo2O4//AC BSHs are charged to 1.6 V in 1 s by the solar cells under light illumination, and then, the stored energy rationally powers the LEDs for light emitting. The self-driven integrated system has an overall efficiency of 8.1% with the storage efficiency of 74.24%. This study is expected to pave a road for commercial applications of BSHs in self-driven integrated systems. Moreover, the design of integrated system is expected to provide a potential strategy for clean energy applications, which is of great importance in the situation of today’s growing shortage of fossil energy.
2.1 Synthesis of 3D Hierarchical NiCo2O4 Arrays
Hierarchical NiCo2O4 arrays were prepared by a continuous synthesis process. As the beginning step, a slice of Ni foam (20 × 40 × 1.8 mm3) was weighed for experiments. Nickel chloride hexahydrate (2.5 mmol), cobalt chloride hexahydrate (5 mmol), carbamide (9 mmol), and hexadecyl trimethyl ammonium bromide (CTAB, 2 mmol) were dissolved in ultrapure water (50 mL). After being stirred by magnetic stirrer to form homogeneous solution, the solution was transferred to a high-pressure autoclave with fill factor of 60%. The prepared Ni foam acted as substrate, and the reaction temperature was 100 °C for 6 h in an oven. Afterward, the Ni foam with products was rinsed with ultrapure water to remove any impurities and dried. After an annealing process at 350 °C for 3 h, the NiCo2O4 nanowire arrays were formed on the Ni foam, which are the intermediate product to obtain the final product.
A simple chemical bath deposition method was used for the further growth of NiCo2O4 nanoflakes. Nickel sulfate (1.0 M in 10 mL), cobaltous sulfate (2 M in 10 mL), and potassium peroxydisulfate (0.25 M in 16 mL) were mixed to form a solution, and then, the ammonia (25 − 28%, 4 mL) was added. The NiCo2O4 nanowire arrays grown on Ni foam were immersed into the obtained solution for 8 min to load NiCo2O4 nanoflakes. After the reaction, an annealed process (350 °C, 2 h) was carried out on the sample to achieve the final product of 3D hierarchical NiCo2O4 arrays. The mass loading of hierarchical NiCo2O4 arrays is ~ 4 mg cm−2.
2.2 Assembly of NiCo2O4//AC BSHs
The BSH devices were assembled with NiCo2O4 hierarchical arrays as the faradic electrode and AC as the capacitive electrode. The filter paper and 3 M KOH solution were used as membrane and electrolyte, respectively. The AC capacitive electrode was prepared by a generic method. Specifically, the AC, acetylene black, and polyvinylidene fluoride were mixed with a mass ratio of 8:1:1 under vigorous stirring. Then, the N-methyl pyrrolidone was dropped into the mixture under stirring. The paste was coated on the Ni foam after homogeneously mixed. Finally, the faradic electrode, filter paper membrane, and capacitive electrode were encapsulated in a coin cell.
2.3 Integration of Self-Powered System
2.4 Measurements and Evaluations
The characterizations of materials and performance evaluations of each device and whole integrated system are described in detail in Note S1 in the Supporting Information.
3 Results and Discussion
Figure 1a displays the diagram of integrated system. In a typical workflow, the BSHs were charged by the a-Si/H solar cells and then powered the LEDs for light emitting. Figure 1b, c gives the equivalent circuit describing the circuit connection of solar cells, BSHs, and LED in parallel. Figure 1b shows the charge or energy storage process of BSHs by solar cells under light illumination without LEDs in circuit. Theoretically, the BSHs can be charged to the open-circuit voltage of solar cells. Figure 1c exhibits the discharge process of BSHs by output electric power for LEDs when light source is removed.
3.1 Properties of 3D Hierarchical NiCo2O4 Arrays
Detailed structure and morphology of as-prepared 3D hierarchical NiCo2O4 arrays were further investigated by transmission electron microscope (TEM) as illustrated in Fig. 2d–f. Figure 2d displays the TEM image of an individual NiCo2O4 hierarchical nanostructure. The NiCo2O4 nanowire is elegantly wrapped with leaf-like NiCo2O4 nanoflakes, forming a hierarchical homostructure with enhanced surface area. Figure 2e exhibits the selected area electron diffraction (SAED) pattern of NiCo2O4 hierarchical structure. The concentric circles with a large amount of diffraction spots in SAED patterns can be obviously observed, which demonstrate that the NiCo2O4 hierarchical arrays are polycrystalline in nature. Figure 2f shows the high-resolution TEM (HRTEM) image of the NiCo2O4 nanoflake shell in one NiCo2O4 hierarchical structure, with planar spacings of 0.20 and 0.24 nm corresponding to (400) and (311) faces of cubic NiCo2O4.
Galvanostatic charge discharge (GCD) properties of hierarchical NiCo2O4 electrodes were recorded at current densities from 0.63 to 12.56 A g−1, and the results are shown in Fig. 3b. The tilted platform during discharge illustrates a redox reaction during this process, in accordance with a redox reaction energy storage process. The specific capacity of hierarchical NiCo2O4 electrodes is calculated by using GCD curves. Figure 3c illustrates the derived specific capacity values. As the current density increases from 0.63 to 12.56 A g−1, the specific capacity of 3D hierarchical NiCo2O4 arrays decreases from 130 to 67 mAh g−1, with 51.4% capacity retained suggesting the high rate capability. All the values are remarkable for use in BSHs. The excellent electrochemical properties can be attributed to the sophisticated 3D hierarchical nanostructure with elevated mass loading, plenty of diffusion channels, and abundant pore structures, which increases the mass of active materials and provide enough active sites for interfacial reactions.
3.2 Behaviors of Hierarchical NiCo2O4//AC BSHs
Cycle performance is conducted for further evaluating BSHs (Fig. 4c). The BSH exhibits an excellent cycle performance with 100% retention of capacity after 15,000 cycles, demonstrating that the 3D hierarchical homostructure can meet the requirement of long cycle ability. The two insets in Fig. 4c show the GCD curves of the first six cycles and last six cycles. The unchanged shape of GCD curves from the beginning to the end confirms the long-term stability of our BSH. Also, an electrochemical kinetic analysis was carried out to provide a detail insight on energy storage mechanisms (Fig. S4). Both the double-layer capacity and faradic capacity contributed to the whole capacity.
Electrochemical impedance spectroscopy (EIS) of BSHs was further acquired under sinusoidal disturbance voltage from 1 Hz to 100 kHz (Fig. 4d). The equivalent circuit diagram is displayed in Fig. S5, where Rel represents solution resistance, L represents inductance, CPEdl represents double-layer capacitance, RF represents charge transfer resistance, and CPEpse represents pseudocapacitance. The depressed semicircle is observed in the high-frequency area and the sloping line in the low-frequency area, revealing the combination of charge transfer resistance with capacitive features. The equivalent serial resistance (Rel) obtained from the fitted data is only 0.74 Ω, illustrating a very low internal resistance of the BSHs device. The charge transfer resistance (RF), caused by the faradic reactions at the surface of electrode, is around 0.97 Ω. Bode graph of EIS plot is provided in Supplementary Material (Fig. S6), where the fitted curve matches well with the experimental one, confirming the rationality of fitting procedure. The impedance data verify that our BSHs possess both low internal resistance and low charge transfer resistance.
3.3 Performances of Self-Driven Integrated System
The a-Si/H solar cells serve as the energy production device in the integrated system under sunlight illumination. In the experiment of integrated system, the NiCo2O4//AC BSH was charged by two solar cells in series under AM 1.5 G irradiation. In Fig. 5b, the BSH can be charged readily by the solar cells (via solar cell charging controller) with a saturation voltage of 1.601 V due to the Voc restriction. The charging process can be accomplished within 1 s. The NiCo2O4//AC BSHs was constantly discharged at 100 mA cm−2. Figure 5b provides five charge and discharge curves of the BSHs supplied by solar cells. All the five curves show exactly similar shape, illustrating the stable and reliable nature of energy production and storage.
Conventional supercapacitors generally have a tendency for relatively fast self-discharge rate due to the thermodynamic favorable characteristic. The self-discharge mechanisms mainly include overcharging, side reaction, and ohmic leakage. As a similar structure of BSHs with conventional supercapacitors, the self-discharge behavior of the BSHs was studied in our work after it was charged by the solar cells. After charging the BSHs up to specified potential, it was allowed to self-discharge in an open-circuit state, which was monitored with a voltmeter. The BSH self-discharge timescales and shapes in Fig. 5c illustrate an open-circuit voltage variation curve. From the curve, it can be seen that the voltage of the BSHs is first charged to 1.601 V by the solar cells and then decreases to 1.594 V in the first 5 s; afterward, the voltage kept unchanged in the rest time (in 24 h during measurements). These investigations verify the property of integrated hybrid system with a rather low self-discharge.
Cycling stability of the integrated system was also investigated by charging the BSHs with the solar cells, and then discharging galvanostatically for 10,000 cycles, as shown in Fig. 5d. After 10,000 charging and discharging cycles, the capacity of the BSH is decreased only 0.3%, suggesting the excellent cycling stability. The high cycling stability of BSHs driven by solar cells is comparable with that driven by electrochemical workstation. Despite the participation of faradic electrode during charge and discharge process, the high cycling stability is almost the same with double-layer supercapacitors. In the long cycling stability test up to 10,000 cycles, no obvious deterioration is observed, which is really important for BSHs in practical applications. To further confirm the stability of our BSHs, the voltage holding test was investigated at 1.6 V, as shown in Fig. S7. During a procedure for 100 h, the capacity retention was around 90.7% for the NiCo2O4//AC BSH, which can be comparable with previously reported in carbon-based electric double-layer capacitors [55, 56]. The long-lasting cycling stability of NiCo2O4//AC BSHs in our work is rarely obtained, which is believed to be attributed to the well-designed 3D hierarchical structure. The superior cycling performance of the NiCo2O4 array-based BSHs makes it suitable for energy storage of intermittent energies, including solar energy, wind energy, and tidal energy.
In conclusion, we have proposed an integration of energy conversion (a-Si/H solar cells), storage (NiCo2O4//AC BSHs), and utilization (LEDs) in one system. Each of the component functions well, and the whole system works independently. As the key of the integrated system, NiCo2O4//AC BSHs are elaborately designed, with lengthened carambola-like 3D hierarchical NiCo2O4 arrays as faradic electrodes and AC as capacitive electrodes. The BSHs exhibit excellent overall performances with high energy density of 16.6 Wh kg−1, high power density of 7,285 W kg−1, long cycling stability of 100% retention after 15,000 cycles, and rather low self-discharge. During the operation, the NiCo2O4//AC BSHs are charged to 1.6 V within 1 s by a-Si/H solar cells, serving as a stable voltage supply for powering LEDs. The self-driven integrated system has an overall efficiency of 8.1% with the storage efficiency of 74.24%, presenting stable and reliable behaviors in photovoltaic conversion, efficient energy storage, and functions as light emitting. This work is supposed to prove the potential applications of BSHs in self-driven systems and provide an evaluation method for such kind of integrated systems.
Y. Hou thanks the support of National Natural Science Foundation of China (Nos. 51702284 and 21878270), Zhejiang Provincial Natural Science Foundation of China (LR19B060002), and the Startup Foundation for Hundred-Talent Program of Zhejiang University (112100-193820101/001/022). Y. J. Zeng thanks the support of Shenzhen Science and Technology Project of China (JCYJ20170412105400428). J. G. Lu thanks the support of Zhejiang Provincial Natural Science Foundation of China (LR16F040001) and Open Project of Laboratory for Biomedical Engineering of Ministry of Education, Zhejiang University. Y. Hou, Q.H. Zhang, Q.G. He, and J.G. Lu thank the support of Innovation Platform of Energy Storage Engineering and New Material in Zhejiang University (K19-534202-002) and Provincial Innovation Team on Hydrogen Electric Hybrid Power Systems in Zhejiang Province.
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