Oxygen vacancy engineering in MXenes for sustainable electrochemical energy conversion and storage applications

Abstract

Ever-increasing global energy requirements and environmental pollution have directed major research focus on developing sustainable energy conversion and energy storage technologies. Ti₃C₂T_X MXenes are widely considered a potential electrode material for electrochemical hydrogen energy generation through water splitting and electrochemical energy storage supercapacitor applications. Herein, multifunctional Ti₃C₂T_X MXene-based nanocomposites with varying Eu₂O₃ concentrations were synthesized and systematically investigated for electro/photocatalytic water splitting and two-electrode supercapacitor properties. Ti₃C₂T_X MXenes exhibit high electrical conductivity, tunable surface functionalities and multivalent Ti oxidation states. Meanwhile, the incorporation of Eu₂O₃ supplements the electrochemical performance by altering the physio-chemical structure and overcoming the intersheet restacking and oxidative degradation issues of pristine Ti₃C₂T_X MXenes. More importantly, the interfacial charge transfer synergism between Ti₃C₂T_X MXenes and Eu₂O₃ creates oxygen vacancies that modulate the electronic structure of nanocomposites, aiding in the formation of abundant hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) sites, photo-generated charge trapping centres and pseudocapacitive sites. The key findings of the present study showed that the Eu₂O₃/MXene nanocomposite with an optimum oxygen vacancy content exhibited excellent performance with a small overpotential of 63 mV and 169 mV and a high faradaic efficiency of 96.2% and 95.23% to drive the HER and OER, respectively. Additionally, upon combination with CdS as the photoabsorber, the optimized nanocomposite achieved a high photocurrent density of 4.86 mA cm⁻², leading to a H₂ evolution rate of 56.67 μL min⁻¹. Considering supercapacitor characteristics, the optimized nanocomposite exhibited a high specific capacitance of 374.98 F g⁻¹ with an energy density and power density of 13.02 Wh kg⁻¹ and 300 W kg⁻¹, respectively. Thus, the results of the present study establish a facile approach to develop high-performance multifunctional electrodes for advancing electrochemical energy conversion and storage technologies.