Atomic Modulation of 3D Conductive Frameworks Boost Performance of MnO2 for Coaxial Fiber-Shaped Supercapacitors

Highlights 3D Zn-doped CuO framework was designed for aligned distributing high mass loading of MnO2 nanosheets. Zn could be introduced into the CuO crystal lattice to tune the covalency character and thus improve charge transport. A free-standing asymmetric coaxial fiber-shaped supercapacitor based on Zn–CuO@MnO2 core electrode possesses superior performance including higher capacity and better stability under deformation. Electronic supplementary material The online version of this article (10.1007/s40820-020-00529-8) contains supplementary material, which is available to authorized users.


S2 XPS data of CuO and Zn-CuO Samples
The binding energies of 934.5 eV and 954.2 eV are assigned to Cu 2p3/2 and Cu 2p1/2 for pristine CuO nanowires, in good agreement with Cu 2p state (Fig. S4b). The detailed XPS pattern of the O1s are fitted with two peaks (Fig. S4c). The binding energies at 529.8 eV can be ascribed to O 2ion present in CuO and higher binding energy (O2) at 531.5 eV can be attributed to O2and Oions in oxygen-deficient regions. High-resolution spectra of the Cu 2p for Zn-CuO sample in Fig. S4d displays similar results with Cu 2+ of CuO nanowires. The XPS of Zn 2p spectrum fitted with two Gaussian Lorentz peaks shows a peak centered at 1022 eV which is ascribed to Zn 2p3/2. The binding energy at 1044 eV can be fitted with two Gaussian Lorentz peaks that is attributed to Zn 2p1/2 (Fig. S4e). This demonstrated that the Zn phases were present in the samples. The intensity of O 1s of 531.5 eV binding energy of Zn-CuO is more than that of CuO sample, demonstrating oxygen vacancies increase after Zn doping into CuO [S1, S2].  after MnO2 deposition, exhibiting that internal nanowire structure with the diameter of ~200 nm is covered by the interconnected tiny nanosheets (Fig. S6g). The HRTEM image of consisted of crystals with a lattice fringe spacing of 0.69 nm and 0.239 nm, consistent with the (110) and (211) planes of the structure of MnO2. Moreover, the ring pattern spots observed in the SAED confirms polycrystalline nature of the prepared material.

S4 Electrochemical Performance Measurements
All related electrochemical data including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) curves were measured by utilizing an electrochemical workstation (CHI 760E, Chenhua). The CuO, Zn-CuO, CuO@MnO2 and Zn0.11CuO@ MnO2 and VN/CNT film were directly used as the working electrode. A saturated calomel electrode and a platinum electrode were used as the reference electrode and the counter electrode, respectively. The electrochemical properties of the as-fabricated electrodes were characterized by three-electrode system in a 1 M Na2SO4 aqueous electrolyte. The electrochemical characterization of all-solid asymmetric coaxial fibershaped supercapacitor (ACFSC) device was carried out in a two-electrode system. The specific capacitance was calculated by Eq. S1: where Ccell is the specific capacitance of the electrode, i is the discharge current during the charge and discharge process, t is the discharge time from high to low potential, S is the area/volume of the electrode, ΔV is the gap of high and low potential windows, respectively.
The energy density (E) and power density (P) were obtained based on Eq. S2 and S3, respectively: where Ccell is the specific capacitance of the electrode, ΔV is the voltage gap of high and low potential windows, t is the discharge time from high to low potential, respectively.
The galvanostatic charge-discharge (GCD) curves of CuO electrodes with the different content of Zn were performed in a three-electrode system in 1M Na2SO4 aqueous electrolyte to analyze the effect of the contents of Zn on the capacitive performance. As shown in Fig. S7a, Zn-doped CuO electrodes have a longer discharge time than that of CuO electrode, illustrating Zn doping into CuO have a limited improvement for the capacitance performance of CuO electrode. The specific capacitance was calculated and the corresponding results of these electrodes are plotted in Fig. S7b. It is observed that there is no obvious trend for capacitance change of Zn-doped CuO electrodes with increasing contents of Zn. But, the potential drop of Zn-doped CuO electrodes have a significant decrease, demonstrating Zn doping into CuO could effectively improve the conductivity of CuO electrode, as well as improve electron collection rates and the charge transport during electrochemical reaction.
The charge transfer mechanisms and electrode kinetics were further investigated by Dunn's method, which provides the quantitative separation of the capacitive charge process (electrical double-layer effect and faradaic charge contribution) and diffusion process (capacitance arising from slow ion de/intercalation in the active materials). To obtain the ratio of as-prepared electrodes capacitive contribution from the total capacity, the formula was divided into two parts quantitatively: i(v) = k1v + k2v 1/2 (k1 and k2 are scan rate independent constants; v is scan sweep; k1v represents capacity contribution; k2v 1/2 represents diffusion contribution) [S3, S4]. For our Zn-CuO electrodes, their capacitive-controlled capacitances are higher than that of CuO electrodes which illustrates a faster faradaic charge transfer process in Zn-CuO electrodes. Zn0.11CuO and Zn0.15CuO nanowires electrodes possess the larger capacitive-controlled capacitance of about 75% than CuO and other Zn-CuO electrodes (Fig. S7c). Furthermore, we also investigated the charge transfer mechanism of Zn0.11CuO@MnO2 electrode (Fig. S7d). The capacity contribution increases with the increasing of the scan rate. At a scan rate of 100 mV/s, the capacitive-controlled capacitance of the Zn0.11CuO@MnO2 electrode is estimated at 72.6%, indicating that the rapid faradaic charge transfer process onto the surface or near surface of electrodes.