Supporting Information. High-Performance Supercapacitor Electrodes

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1 Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is The Royal Society of Chemistry 2017 Supporting Information NiCo 2 O Hybrid Nanostructures on Ni Foam as High-Performance Supercapacitor Electrodes Cheng Zhang, Xinpei Geng, Shaolong Tang, Mingsen Deng, and Youwei Du tangsl@nju.edu.cn; deng@gznc.edu.cn Morphological characterization of rgo, NiCo 2 O 4 nanosheets and NiCo 2 O As shown in Figure1:(a-b), the TEM images clearly show that the reduced graphene oxide consists of a few sheets with micrometre lateral size. The NiCo 2 O 4 nanosheets are uniformly grown on Ni foam to form a conformal coating on the surface, and the nanosheets are interconnected with each other to form a wall-like structure, as shown in Figure1:(c-d). Raman spectra of GO and rgo on Ni foam The local electronic density of state (LDOS) of rgo on NiCo 2 O 4 (400) Surface Local electronic density of state (LDOS) of rgo on NiCo 2 O 4 (400) surface (Figure2) also shows that the adsorption involves hybridization between the rgo orbitals and the NiCo 2 O 4 orbitals. The 3d states of Co and Ni are split into lots of peaks, and hybridize with the 2p states of C. An important common character in the LDOS for Co and Ni 3d-transition-metal adatoms is partial occupied and has strong hybridization at 1.75 ev, 1.30 ev, 0.80 ev and 0.5 ev below the Fermi energy. To whom correspondence should be addressed 1

2 Figure 1: (a-b) TEM images of reduced graphene oxide; (c-d) SEM images of NiCo2 O4 nanosheets on Ni foam; (e-f) TEM images of the NiCo2 sample. D Intensity (a.u.) G rgo GO Raman shift (cm ) Figure 2: Raman spectra of GO and rgo on Ni foam (D and G are corresponding to the welldocumented G and D bands, respectively) 2

3 Ni 3d O 2p Co 3d C 2p LDOS Energy (ev) Figure 3: Local electronic density of state (LDOS) of rgo on NiCo 2 O 4 (400) surface. The energy is relative to E F. The occupation of the spin-up and spin-down states is very uneven, suggesting large magnetic moments. The electronic properties of rgo on NiCo 2 O 4 (311) Surface Figure 4: (a)atomic model for NiCo 2 O 4 (311) substrate interfacing with the rgo; (b) Differential charge density by first-principles simulations illustrates the increase (green color) and decrease (blue color) of electron distributions. The optimized configurations of rgo on NiCo 2 O 4 (311) surface as shown in Figure3:(a), the rgo is corrugated due to the mismatch with the lattice parameters. The nearest distance between rgo and NiCo 2 O 4 (311) surface is 1.85 Å. The work function of NiCo 2 O 4 (311) and rgo are 5.42 and 4.60 ev, respectively, suggesting the free electrons will flow from rgo to the NiCo 2 O 4 until 3

4 the Fermi levels are aligned. To better study this interfacial effect, we have analyzed the differential charge density for NiCo 2 O interface. As shown in Figure3:(b), a significant increase in electron density (indicated by green color) has been observed at the NiCo 2 O 4 (311) surface, while the reduction of electron density has been found at the rgo surface (blue color). Under equilibrium, the NiCo 2 O 4 is negatively charged and the rgo is positively charged near its surface due to electrostatic induction. Meanwhile, the electric field between the NiCo 2 O 4 and rgo cannot be effectively screened due to the low concentration of carrier density in the NiCo 2 O 4 semiconductor. This causes the free charge carrier to be accumulated near the NiCo 2 O 4 surface. In addition, the energy bands bend downward toward the interface by thinking about the electrostatic energy induced by an electron as it moves through the interface. Both the space charge accumulation and band bending in NiCo 2 O 4 surface exhibit superior electron collection efficiency, which surely improves the redox activity of the NiCo 2 O 4 leading to a synergic combination featuring increased specific capacitance. Furthermore, we find a significant hybridization between the rgo π orbitals and Co 3d orbitals near the Fermi levels, as shown as Figure4:(c), thus acting as an electron source to ensure the redox reactions, as well as the strong hybridization will largely enhanced the stability of the NiCo 2 O hybrid electrodes. 4

5 Figure 5: (a-b) Potential lineup diagram of NiCo 2 O 4 (311) surface and NiCo 2 O 4 (311) substrate interfacing with the rgo. (c) Local electronic density of state (LDOS) of rgo on NiCo 2 O 4 (311) surface. The energy is relative to E F. 5