Fluoride-free etched MXene enhances cobalt MOF supercapacitor performance

Metal–organic frameworks (MOFs) offer significant potential for supercapacitors due to their vast surface area and adaptable redox chemistry. However, their application in high-performance devices has been limited by poor electrical conductivity and slow charge transport. A research team led by Tanveer Kaur, Renu Dhahiya, Pargati Vijayrania, and Ashok Kumar has developed a composite electrode that addresses these issues by integrating a cobalt-based MOF with a partially etched MXene, synthesized through a fluoride-free process. This study, published in Discover Chemical Engineering, details the synthesis of a [Co(BDC)DMF]n framework via a solvothermal method, combined with alkali-etched Ti3C2Tx MXene, where the surface terminations T include oxygen and hydroxyl groups, and x ranges from 1 to 4.

The etching process is as crucial as the materials themselves. Traditional MXene synthesis uses hydrofluoric acid, a hazardous and corrosive substance requiring careful handling. The researchers opted for an alkali-assisted partial etching method, which is safer and more environmentally friendly. This alkali treatment not only enhances safety but also increases the interlayer spacing of the MXene sheets, facilitating ion access and strengthening the interaction between the layered carbide and the MOF particles.

Analyses of the composite confirmed its successful formation, with X-ray photoelectron and Raman spectroscopies indicating strong interfacial coupling and improved electronic properties. This coupling is central to the design, as the redox-active Co2+ centers in the framework provide faradaic charge storage, while the conductive MXene network offers a continuous electron pathway that the MOF alone cannot. This results in a synergistic architecture where each material compensates for the other's weaknesses.

The composite electrode achieved a specific capacitance of 704 F g−1 at a current density of 1 A g−1, significantly surpassing the 417 F g−1 of the pristine [Co(BDC)DMF]n MOF under the same conditions. It also demonstrated enhanced power capability, indicating efficient charge delivery at useful rates.

Impedance measurements provided insight into these improvements. Electrochemical impedance spectroscopy showed a reduction in charge transfer resistance from 12.58 Ω for the pristine MOF to 8.47 Ω for the composite, confirming enhanced charge transport. Lower resistance at the electrode–electrolyte interface reduces energy loss as heat during rapid cycling, explaining the higher capacitance and better rate performance of the composite.

Durability, a common issue for MOF electrodes, also improved significantly. The composite electrode retained 91% of its capacitance after 5000 charge–discharge cycles, compared to 79% for the [Co(BDC)DMF]n MOF alone. The conductive MXene scaffold helps buffer the framework against degradation and loss of active material over repeated cycles.

To evaluate the material under practical conditions, the researchers constructed an asymmetric supercapacitor using the composite. The full cell operated across a 1.6 V voltage window and achieved an energy density of 22.7 W h kg−1. Remarkably, the device maintained 114% capacitance retention after 5000 cycles, suggesting performance slightly above its initial value under the reported conditions.

The authors attribute the superior performance to the synergy between the redox-active Co2+ centers and the conductive MXene network. The fluoride-free alkali etching not only eliminates the safety and environmental concerns of hydrofluoric acid but also creates a partially etched, expanded-layer MXene that promotes ion accessibility and interfacial bonding with the MOF.

This study presents a promising strategy for advanced supercapacitor applications by pairing redox-rich but poorly conducting frameworks with highly conductive two-dimensional carbides, prepared through greener chemistry. If scalable, this alkali etching route could position fluoride-free MXene–MOF composites as a viable platform for next-generation electrochemical energy storage devices.