De-ionized water. Nickel target. Supplementary Figure S1. A schematic illustration of the experimental setup.

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1 Graphite Electrode Graphite Electrode De-ionized water Nickel target Supplementary Figure S1. A schematic illustration of the experimental setup.

2 Intensity ( a.u.) Ni(OH) 2 deposited on the graphite blank graphite ( degree) Supplementary Figure S2. X-ray diffraction (XRD) measurement of the as-synthesized sample.

3 Current (A g -1 ) mV s -1 10mV s -1 15mV s -1 20mV s Potential (V vs. SCE) Supplementary Figure S3. Typical CV curves of amorphous Ni(OH) 2 samples.

4 Specific capacitance ( F g -1 ) Potential (V vs. SCE) a 4 Current (A g -1 ) mv s mv s mv s mv s b Potiential (V vs. SCE) 2.6 A g A g A g A g A g A g c Time (s) Current density (A g -1 ) Supplementary Figure S4. Electrochemical Characterization of amorphous Ni(OH) 2 electrode. (a) CV curves, (b) Charge-discharge curves and (c) Specific capacitance of the amorphous Ni(OH) 2 nanospheres.

5 Anode Cathod e Ni 2+ Ni(OH) 2 e - H 2 O 2 4OH - + 4e - O 2 + 2H 2 O 2H + + 2e - H 2 Ni 2+ + OH - Ni(OH) 2 2H + + 2e - H 2 Supplementary Figure S5. Formation mechanism of amorphous nickel hydroxide nanospheres.

6 nickel hydroxide Ni 2+ ion graphite electrode 100nm 100nm 100nm 0.5 h 1.0 h 2.0 h Supplementary Figure S6. Formation of the amorphous nickel hydroxide nanospheres with the wrinkle surface.

7 Supplementary Methods 1. Synthesis of amorphous nickel hydroxide nanospheres The amorphous nickel hydroxide nanospheres are synthesized by a unique electrochemistry technique which is simple, green and low-cost [18], as shown in Supplementary Figure S1. Compared to the conventional electrochemistry method, the developed technique in this study has the following merits. Firstly, the highly pure de-ionized water, as the electrolyte instead of electrolytic solution having well electric conductivity, provides a chemically clean reaction environment without any chemical additives, which ensures highly pure and clean surface of productions. Secondly, the reaction speed of the electrochemical deposition is about 0.05 ma that is much lower than that of the conventional electrochemistry method (above 5 ma), which implies that the growth rate of nanostructures is very slow in this case. Well known, the slow growth benefits forming well-defined microstructure. Finally, the electrodes don t participate in the electrochemical reaction in our case. In this study, two parallel cleaned graphite flakes are used as cathode and anode electrodes immersed in the quartz chamber, both with a working area of about cm 2, and the distance between the electrodes is about 5.0 cm. A target of 99.99% pure nickel, with a diameter of 25.0 mm, is fixed at the center of the bottom of a quartz chamber and immersed in highly pure de-ionized water (18.2 MΩcm -1 ) without any chemical additives. The total experiment is carried out under a constant potential mode using a constant potential voltage at 75 V. 2. Characterization of amorphous nickel hydroxide Scanning electron microscopy (SEM, Quanta 400F) and transmission electron

8 microscopy (TEM, JEM-2010HR) are employed to identify the morphology and structure of the as-synthesized samples. Note that the x-ray diffraction (XRD) measurement of the as-synthesized sample as shown in Supplementary Figure S2 shows that, besides the peaks from the graphite substrate, there are no other peaks in the XRD pattern, which further confirms the amorphous nature of the sample. In order to understand the surface information of the as-synthesized samples, x-ray photoelectron spectroscopy (XPS, ESCALab250) is carried out to analyze the composition of the sample surface. Additionally, the infrared (IR) spectra and Raman spectra of the samples are operated to confirm the results of XPS analysis. IR spectroscopy is carried out on a Fourier transformation infrared spectrometer coupled with infra-red microscope (EQUINOX 55) in a range of cm -1, and Raman spectra is obtained on a Laser Micro-Raman Spectrometer (Renishaw invia) which is excited by an argon-ion laser with incident wavelength of nm. The mass of the as-synthesized amorphous nickel hydroxide is measured by an Inductively Coupled Plasma-atomic Emission Spectrometry (ICP, IRIS(HR)). The as-synthesized samples deposited on the graphite electrode are firstly soaked in the nitric acid solution (10%) for 48 h to make nickel hydroxide react with nitric acid absolutely. Then, we can measure the gravimetric nickel by ICP method. 3. Electrochemical characterization of pseudocapacitors 3.1. Electrochemical test of the single electrode In this case, the electrochemical measurements are conducted in a three-electrode electrochemical cell with a Pt counter electrode and a saturated calomel reference electrode in 1 M KOH solution. The as-synthesized amorphous nickel hydroxide materials deposited on the graphite electrode is used for the working electrode. Cyclic

9 voltammetry (CV) measurements are carried out using an electrochemical workstation in the scan range of 0 to 0.5 V Electrochemical test of asymmetric supercapacitor The as-synthesized amorphous nickel hydroxide materials deposited on the graphite electrode is used for positive electrode and the active carbon (AC) is used for the positive electrode prepared according to the following steps: The mixture containing 80 wt % AC, 10 wt % carbon black, and 10 wt % polyvinylidene fluoride (PVDF) is well mixed and dissolved with N-methyl kelpyrrolidide solution, then pressed onto a graphite sheet (current collector). At last, the electrode is dried at 55 for 24 h. The mass of electrodes are measured by XP2U Ultra-microbalance (d = 0.1 μg). Supplementary Figure S3 shows the typical CV curves of the amorphous Ni(OH) 2 samples in 1 M KOH electrolyte at different scan rates between 0.0 and 0.5 V. The specific capacitances of the amorphous Ni(OH) 2 samples are calculated to be 1587 F g -1 at scan rates of 5 mv s 1. Supplementary Figure S4a shows the typical CV curves of the AC electrode in 1 M KOH electrolyte at different scan rates between 0.0 and -0.8 V and CV curves appear nearly rectangular. The specific capacitances of the amorphous Ni(OH) 2 samples are calculated to be 175 F g -1 at scan rates of 5 mv s 1. Supplementary Figure S4b shows the charge-discharge curves at different current densities (2.6 to 2.8 A g -1 ) between 0.0 and 0.8 V display linearity that are the typical pure double-layer capacitors behaviour. In order to obtain a well electrochemical performance for supercapacitor, the charge balance between the two electrodes should be follow the relationship q + = q -, the q is is calculated by the formula

10 (S1) where the q is the charge stored by electrode, C; C s is the specific capacitance, F g -1, m is the mass of electrode, g, and the is the potential range for the charge/discharge process, V. According to equation (1), the ratio of m + /m - can be express as follows (S2) Base on the analysis above, the mass ratio between positive and negative electrodes of the Ni(OH) 2 AC based asymmetric capacitor should be Formation of amorphous nickel hydroxide nanospheres with wrinkle surface Now we discuss the basic physics and chemistry involved in the synthesis of the amorphous nickel hydroxide nanospheres with the wrinkle surface upon the unique electrochemistry. We propose a formation mechanism of the amorphous nickel hydroxide nanospheres in our case as shown in Supplementary Figure S5, which can be divided into two processes below. (i) When the nickel target is immersed in pure de-ionized water with an extra electric field, it can be polarized and releases a small quantity of nickel ion (Ni 2+ ) and e -, adhered on the surface of nickel target. (ii) Ni 2+ moves toward to the cathode under an extra electric field, and then the electrochemical reaction takes place on the cathode as follows: Ni 2+ + OH - Ni(OH)2 2H + + 2e - H 2 (S3) (S4) At the same time, H + moves toward to the surface of the nickel target and OH - moves toward to the anode under an extra electric field, finally, the electrochemical reaction takes place as follows: 2H + + 2e - H 2 (S5)

11 4OH - + 4e - O 2 + 2H 2 O (S6) Supplementary Figure S6 illustrates the formation of the amorphous nickel hydroxide nanospheres with the wrinkle surface. Firstly, the smooth nickel hydroxide nanospheres form on the substrate surface. Then, the nanospheres grow up with the rough surface with the reaction operating. Along with the reaction further continuing, the amorphous nickel hydroxide nanospheres with the wrinkle surface are fabricated at last. The relevant experimental observations support the formation above as shown in Supplementary Figure S6. Note that, H 2 gas can be continuously produced on the cathode under such a high voltage, which maybe also lead to the formation of the wrinkle morphology of the nanospheres.