Integrated System of Solar Cells with Hierarchical NiCo2O4 Battery-Supercapacitor Hybrid Devices for Self-Driving Light-Emitting Diodes

Highlights 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. Electronic supplementary material The online version of this article (10.1007/s40820-019-0274-0) contains supplementary material, which is available to authorized users.


S1.1 Characterizations of 3D hierarchical NiCo2O4 arrays
Morphologies and structures of the products were characterized by field emission scanning electron microscope (FESEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI F20). Crystal structures were characterized by X-ray diffraction (PANalytical B.V. Empyrean 200895) with a Cu Kα radiation. The chemical valence states of the products were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) with a monochromatic Al Kα (1486.6 eV) radiation source.

S1.2 Electrochemical measurements of hierarchical NiCo2O4 electrodes
Electrochemical measurements of 3D hierarchical NiCo2O4 arrays were carried out in three-electrode configuration with platinum (Pt) foil (1×2 cm 2 ) as counter electrode and saturated calomel electrode (SCE) as reference electrode in 3 M KOH aqueous electrolyte. The nickel foam with NiCo2O4 active materials was used as working electrode directly. The cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) measurements were conducted as both qualitative and quantitative analysis of electrochemical properties. Electrochemical impedance spectroscopy (EIS) measurements were carried out in a frequency range from 1 Hz to 100 KHz under a sinusoidal disturbance voltage of 5 mV at open circuit potential. The specific capacity (C1) of working electrode was calculated based on galvanostatic discharge measurements: where Idis is the discharge current, Δt is the discharging time corresponding to the specified potential change ΔV, and m is the mass of active materials.

S1.3 Evaluations of NiCo2O4//AC battery-supercapacitor hybrid devices
Electrochemical measurements of 3D hierarchical NiCo2O4 arrays BSH were carried out in two-electrode system, where cathode (NiCo2O4), membrane, and anode (AC) are packed. The CV and GCD were conducted in a potential range from 0-1.6 V. The specific capacity (C2) of BSH was calculated from GCD measurements: Energy density and power density of BSH were obtained as: where v represents the potential.
The constant voltage hold test was carried out as followings. The BSH was charged to 1.6 V at current density of 20 mA cm -2 and constantly hold for 5 hours. Then, the device was discharged to 0 V at current density of 20 mA cm -2 . The above cycles were repeated up to 100 hours.

S1.4 Evaluations of a-Si:H solar cells
The current density-voltage (J-V) characteristics of a-Si:H solar cells were analyzed using a solar simulator (Wacom WXS-140S) at standard test conditions (AM 1.5, 100 mW cm -2 , 25°C) with an area of 1.00 cm 2 . The external quantum efficiency (EQE) of a-Si:H solar cells was calculated from the spectral response measured at zero bias.

S1.5 Evaluations of integrated system
In the integrated system, the a-Si:H solar cells were the energy conversion devices, the hierarchical NiCo2O4 hierarchical arrays BSHs were the energy storage devices, and LEDs were the energy utilization devices. The NiCo2O4 hierarchical arrays BSHs were charged by the a-Si:H solar cells. The LEDs were driven by the NiCo2O4 hierarchical arrays BSHs. The integrated system was evaluated carefully by the above-mentioned methods. Figure S1a illustrates the experimental scheme of the two-step method for synthesis of 3D hierarchical NiCo2O4 arrays. SEM was conducted to obtain the morphology of NiCo2O4 nanowire arrays and finally obtained 3D NiCo2O4 hierarchical arrays, as shown in Figures S1b and S1c, respectively. The intermediate product of NiCo2O4 nanowire arrays was successfully grown on the nickel foam, with a shrunk diameter from the bottom to top, and a porous structure with small particles assembled, forming an ideal backbone for the further growth of NiCo2O4 nanoflakes. After the chemical bath deposition, the final product of 3D hierarchical NiCo2O4 arrays appears clearly. The 3D hierarchical structure makes all nanowires and nanoflakes highly accessible to electrolyte for energy storage.
To obtain detailed information on the crystal structure, XRD patterns of 3D NiCo2O4 hierarchical arrays was first conducted as shown in Figure S1d.  [S1,S2]. Similarly, the Co spectrum is best fitted with two spin-orbit doublets of Co 2p3/2 (780.7 eV) and Co 2p1/2 (795.9 eV), corresponding to characteristic of Co 2+ and Co 3+ . The O 1s spectrum show two oxygen contributions: the peak positioned at 529.5 eV denoted as O1 is associated with metal-oxygen bonds in metal oxide, whereas the peak positioned at 531.0 eV (O2) is corresponding to defect sites with low oxygen coordination in the material with small particle size [S3].
These results show that the as-sythesized sample has a composition of Ni 2+ , Ni 3+ , Co 2+ , Co 3+ , and metal-bonded O, which is in good agreement with the results in the literature for NiCo2O4 [S1-S3]. As mentioned in the main text, the NiCo2O4 hierarchical arrays//AC BSHs exhibit a long cycle lifetime, with 100% retention of capacity after 15000 cycles. No loss is observed in our experiment, which is the evident merit for our BSHs. To the best of our knowledge, this is a pretty long lifetime for supercapacitors with metal oxides and hydroxides as electrodes. To clarify the origin of this stability and the charge storage mechanism as well, an electrochemical kinetic analysis of NiCo2O4 hierarchical arrays will be investigated in what follows. In a CV measurement in three electrode configuration, the total charge can be separated into two components: the contributions from surface charge accumulation and the contributions from diffusion controlled intercalation. Therefore, the current response of CV test can be described as the sum of two contributions originated from surface double layer effects and the diffusion controlled intercalation process: For analytical purposes, the formula can be rearranged to: In the formula above, k1v and k2v corresponds to the current contributions from surface double layer effect and the diffusion controlled intercalation, respectively. By determining k1 and k2, we are able to quantify the fraction of current contributions from each mechanism.
Figure S4a enables us to determine the fraction of double layer current from the total current response. By comparing the double layer current with the whole current response, we find that the double layer effect contributes only one part to the whole charge storage. Figure S4b shows the total capacity, as well as double layer contribution and intercalation contribution as a function of scan rate. As the scan rate increases, the total capacity of NiCo2O4 hierarchical arrays decreases. This phenomenon is attributed to the fact that intercalation process can't keep up with the high scan rate due to the slow diffusion of ions. Therefore, only the intercalation part decreases with the scan rate, whereas the double layer contribution keeps almost constant. The data shows that the double layer contribution does not change with the increase of scan rate. The double layer contribution proportions of NiCo2O4 hierarchical arrays are 52.9%, 60.4%, 66.7%, and 80.9% at the scan rate of 2, 5, 10, and 20 mV s −1 , respectively. Clearly, the double layer contribution plays a more and more important role in the whole capacity with the increasing scan rate. Thanks to the high double layer contribution of NiCo2O4 hierarchical arrays, the whole capacity can keep at a high level under fast charge-discharge conditions. It is believed that the dominant double layer contribution in the whole capacity may be also the reason for the long cycling lifetime. Specifically, a voltage of 1.6 V was applied to the BSH and charge-discharge was applied every 5 hours between 0 V and 1.6 V with a constant current density of 20 mA cm -2 . During a test procedure for 100 hours, the capacity retention of NiCo2O4//AC was 90.7%. The results further confirm the high stability of the NiCo2O4//AC BSHs. We propose two "volume" here. The "effective volume" denotes the real volume of active materials, including cathode, anode and membrane. The coin cell volume is the volume of cell we used to package our active materials. It should be noted that the volume of coin cell is much larger than the effective volume in this study; that's why we propose two "volume" here with two different definitions. Coin cell volume is the actual volume in our study, but effective volume reflects the real volume of active material with reference significance and represents real volumetric performance if the package process is optimized.