Fig. 1. Overview of the MSCs fabrication process and rheological characteristics of NiHCF ink. (a) Schematic illustration of the fabrication process for 3D-printed NiHCF//AC MSCs. (b) Apparent viscosity as a function of shear rate, (c) viscosity evolution under alternating step shear rates, and (d) storage modulus (G′) and loss modulus (G′′) as functions of oscillation stress for NiHCF ink. (e)-(h) Photographs of various printed geometric patterns.

Fig. 2. Materials characterization of NiHCF nanoparticles and the 3D-printed NiHCF/rGO/CNTs electrode. (a) SEM image, (b) XRD pattern, (c) FTIR spectrum, and (d) N2 adsorption-desorption isotherm of NiHCF nanoparticles. High-resolution XPS spectra of NiHCF nanoparticles in the (e) N 1s, (f) Fe 2p, and (g) Ni 2p regions. (h) SEM image and (i) corresponding elemental mappings of the 3D-printed NiHCF/rGO/CNTs electrode.

Fig. 3. Electrochemical performance of the NiHCF electrode and NiHCF//AC MSCs. (a) CV curves of the NiHCF electrode at scan rates ranging from 5 to 30 mV s−1, and (b) GCD curves at current densities from 1 to 10 A g−1 in a three-electrode system. (c) CV curves of AC and NiHCF electrodes at a scan rate of 10 mV s−1 in a three-electrode system. (d) CV curves of the NiHCF//AC MSCs at scan rates ranging from 5 to 50 mV s−1, (e) plots of log (peak current) versus log (scan rate), (f) capacitive and diffusion-controlled contribution ratios versus scan rate, and (g) Nyquist plot with an inset showing the high-frequency region. (h) Cycling performance of the NiHCF//AC MSCs at a current density of 20 mA cm−2, with GCD curves at cycles 1-5, 1001-1005, and 1996-2000 shown in the inset.

Fig. 4. Electrochemical performance of NiHCF//AC MSCs with 1-, 2-, and 3-layer printed electrodes. (a) CV curves of MSC−3 at scan rates ranging from 5 to 50 mV s−1, and (b) the relationships between log (scan rate) and log (peak current). (c) CV curves of MSCs with different numbers of printed electrode layers at a scan rate of 10 mV s−1, (d) Nyquist plots with an inset showing the high-frequency region, (e) normalized capacitive contribution ratios, and (f) areal capacitances at various current densities calculated from GCD curves. (g) Comparison of areal capacitances, and (h) Ragone plot for this work and previously reported quasi-solid-state MSCs.

Fig. 5. Flexibility and integration performance of the NiHCF//AC MSCs. (a) CV curves at bending angles of 30°, 50°, and 70° at a scan rate of 10 mV s−1, and (b) relative capacitance at different bending angles of the MSCs, with a schematic illustration inserted in the lower middle of the figure. (c) Illustrations of two MSCs connected in series and parallel. (d, f) CV curves at 10 mV s−1, and (e, g) GCD curves at 5 mA cm−2 for MSCs connected in series and parallel, respectively. (h) Optical image of FZU LEDs powered by the tandem MSCs.