Restricted by their energy storage mechanism, current energy storage devices have certain drawbacks, such as low power density for batteries and low energy density for supercapacitors. Fortunately, the nearest ion capacitors, such as lithium-ion and sodium-ion capacitors containing battery-type and capacitor-type electrodes, may allow achieving both high energy and power densities. For the inspiration, a new zinc-ion capacitor (ZIC) has been designed and realized by assembling the free-standing manganese dioxide–carbon nanotubes (MnO₂–CNTs) battery-type cathode and MXene (Ti₃C₂T_x) capacitor-type anode in an aqueous electrolyte. The ZIC can avoid the insecurity issues that frequently occurred in lithium-ion and sodium-ion capacitors in organic electrolytes. As expected, the ZIC in an aqueous liquid electrolyte exhibits excellent electrochemical performance (based on the total weight of cathode and anode), such as a high specific capacitance of 115.1 F g⁻¹ (1 mV s⁻¹), high energy density of 98.6 Wh kg⁻¹ (77.5 W kg⁻¹), high power density of 2480.6 W kg⁻¹ (29.7 Wh kg⁻¹), and high capacitance retention of ~ 83.6% of its initial capacitance (15,000 cycles). Even in an aqueous gel electrolyte, the ZIC also exhibits excellent performance. This work provides an essential strategy for designing next-generation high-performance energy storage devices.

A multi-dimensional conductive heterojunction structure, composited by TiO₂, SnO₂, and Ti₃C₂T_X MXene, is facilely designed and applied as electron transport layer in efficient and stable planar perovskite solar cells. Based on an oxygen vacancy scramble effect, the zero-dimensional anatase TiO₂ quantum dots, surrounding on two-dimensional conductive Ti₃C₂T_X sheets, are in situ rooted on three-dimensional SnO₂ nanoparticles, constructing nanoscale TiO₂/SnO₂ heterojunctions. The fabrication is implemented in a controlled low-temperature anneal method in air and then in N₂ atmospheres. With the optimal MXene content, the optical property, the crystallinity of perovskite layer, and internal interfaces are all facilitated, contributing more amount of carrier with effective and rapid transferring in device. The champion power conversion efficiency of resultant perovskite solar cells achieves 19.14%, yet that of counterpart is just 16.83%. In addition, it can also maintain almost 85% of its initial performance for more than 45 days in 30–40% humidity air; comparatively, the counterpart declines to just below 75% of its initial performance.