Hexafluorozirconic acid-etched MXene for ammonium-ion capacitors

MXenes have gained significant attention for their exceptional electrochemical properties in applications such as supercapacitors and metal-ion batteries. However, the development of mild and efficient liquid-phase methods for synthesizing high-quality MXene nanosheets remains a considerable challenge, primarily due to the limitations of conventional etching technique. Herein, we report the successful synthesis of Ti3C2Tx MXene through the selective etching of the aluminum layer from the Ti3AlC2 MAX phase using hexafluorozirconic acid, a safer and more environmentally friendly alternative to hazardous hydrofluoric acid. Both theoretical calculations and experimental results confirm that Ti3C2Tx MXene exhibits excellent ammonium-ion storage capabilities, positioning it as a highly efficient electrode material. An asymmetric ammonium-ion capacitor, employing Ti3C2Tx MXene as the negative electrode and activated carbon as the positive electrode, achieved outstanding energy and power densities, along with a capacitance retention of 86 % after 10000 cycles. This work not only demonstrates the effectiveness of hexafluorozirconic acid as a versatile etching agent but also highlights the significant potential of Ti3C2Tx MXene in next-generation energy storage devices.