A cheap and non-destructive approach to increase coverage/loading of hydrophilic hydroxide on hydrophobic carbon for lightweight and high-performance supercapacitors

Carbon-based substrates offer unprecedented advantages in lightweight supercapacitors. However, it is still challenging to achieve high coverage or loading. Different from the traditional belief that a lack of defects or functional groups is the cause of poor growth on carbon-based substrates, we reckon that the major cause is the discrepancy between the hydrophilic nature of the metal oxide/hydroxide and the hydrophobic nature of carbon. To solve this incompatibility, we introduced ethanol into the precursor solution. The method to synthesize nickel copper hydroxide on carbon fiber paper employs only water and ethanol, in addition to nickel acetate and copper acetate. The results revealed good growth and tight adhesion of active materials on carbon fiber paper substrates. The specific capacitance and energy density per total weight of the active material plus substrate (carbon fiber paper, current collector) reached 770 F g−1 and 33 Wh kg−1 (1798 F g−1 and 54 Wh kg−1 per weight of the active materials), owing to the high loading of active material and the light weight of carbon fiber paper. These results signified the achievability of light, cheap and high-performance supercapacitors by an environmental-friendly approach.

S2 capacitance than β-phase. The second ring may be due to the intercalation of some anions into the (009) layer of nickel hydroxide. It is interesting that the copper hydroxide is not formed.
There is no inhomogeneity in the morphology as well as in crystal structures. A uniform distribution of metal cations on the atomic level without segregation is achieved.
Based on our previous studies, the ratio of nickel to copper has been optimized to be 4 to 1 in the precursors to achieve the highest supercapacitor performance. To determine the ratio of nickel to copper and distribution in the final product, ion chromatography (IC) analysis and energy-dispersive X-ray spectra mapping have been conducted. Figure S2 elucidates the uniform distribution of nickel, copper and oxygen. The IC analysis result reveals the ratio of nickel to copper is 699.3 to 181.4, which is quite close to that of 4:1 in the precursors.
To assess the chemical composition evolution of the nickel copper hydroxide, X-ray photoelectron spectroscopy (XPS) were measured. The XPS survey spectra show that there is only nickel, copper and oxygen existing on the surface, revealing the impurity level of our material is low. The surface information of the NCO/Cu could be found from the XPS results displayed in Figure S3.

Calculations
The mass and area specific capacitances can be calculated from the CV curve using the following equations 3,4 S3 where I (A) and U (V) are the current and potential in the CV, v (V s -1 ) is the scan rate, A (cm 2 ) is the area of the current collector together with the active material, U is the potential window of discharge (0.5 V here), I is the constant discharge current and t (s) is the discharge time.
The mass and area specific capacitances, power density and energy density are calculated based on the galvanic charging-discharging curves using the equation as follows 5,6 : where C ( F g −1 ) is specific capacitance, A is the area of the current collector, E (Wh kg −1 ) is energy density, P (W kg −1 ) is power density, is potential window (here 1.7 V), I (A) is discharge current, (s) is discharge time, m (g) is the sum of the masses of the positive electrode (NCH, here 8 mg) and negative electrode(reduced graphene oxide, here 20 mg), thus here m is 28 mg.
The areal energy density could be estimated by E a =E g ×m / (2 cm×2 cm) (The area of two pieces of carbon fiber paper) The volumetric energy density could be estimated by E v =E g ×m / (2 cm×2 cm×0.049 cm) (The volume of two pieces of carbon fiber paper)

S4
P v =P g ×m/ (2 cm×2 cm×0.049 cm) All the above parameters are based on the weight of active material, we also calculate and report all the parameters based on the weight of total electrode, i.e. active material and the weight of carbon fiber paper.
In the three electrode configuration, the weight of carbon fiber paper which is 12 mg is added.
In the two electrode configuration, the weight of two pieces of carbon fiber paper, which is 24 mg, is added.