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MXenes have emerged as promising high-volumetric-capacitance supercapacitor electrode materials, whereas their voltage windows are not wide. This disadvantage prevents MXenes from being made into aqueous symmetric supercapacitors with high energy density. To attain high energy density, constructing asymmetric supercapacitors is a reliable design choice. Here, we propose a strategy to achieve high energy density of hydrogen ion aqueous-based hybrid supercapacitors by integrating a negative electrode of Ti3C2 T x MXene and a positive electrode of redox-active hydroquinone (HQ)/carbon nanotubes. The two electrodes are separated by a Nafion film that is proton permeable in H2SO4 electrolyte. Upon charging/discharging, hydrogen ions shuttle back and forth between the cathode and anode for charge compensation. The proton-induced high capacitance of MXene and HQ, along with complementary working voltage windows, simultaneously enhance the electrochemical performance of the device. Specifically, the hybrid supercapacitors operate in a 1.6 V voltage window and deliver a high energy density of 62 Wh kg–1, which substantially exceeds those of the state-of-the-art aqueous asymmetric supercapacitors reported so far. Additionally, the device exhibits excellent cycling stability and the all-solid-state planar hybrid supercapacitor displays exceptional flexibility and integration for bipolar cells to boost the capacitance and voltage output. These encouraging results provide the possibility of designing high-energy-density noble-metal-free asymmetric supercapacitors for practical applications.

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Li 4 SiO 4 was obtained by using quartz powder of different particle sizes (75−180 μm, 45−75 μm, 38−45 μm, and <38 μm) and Li 2 CO 3 as raw materials through a solid-state reaction at 720 °C. X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential thermal analysis and thermogravimetry (DTA/TG) were used to examine the sintering behavior and properties of the samples. The results indicated that when the particle size of the quartz powder decreased, the solid-state reaction performed more completely, the content of the Li 4 SiO 4 phase increased, and the size of the grain agglomerates decreased gradually. The enhanced chemical reactivity of the quartz powder with Li 2 CO 3 and the shortened diffusion distance as the quartz size decreases are helpful to the formation of the Li 4 SiO 4 phase. The sorption analysis revealed that the samples synthesized using the quartz powder with smaller particle sizes experienced a more rapid absorption−desorption process with a higher absorption efficiency.

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