Development of Fluorine-Free Tantalum Carbide MXene Hybrid Structure as a Biocompatible Material for Supercapacitor Electrodes

Abstract

The application of nontoxic 2D transition-metal carbides (MXenes) has recently gained ground in bioelectronics. In group-4 transition metals, tantalum possesses enhanced biological and physical properties compared to other MXene counterparts. However, the application of tantalum carbide for bioelectrodes has not yet been explored. Here, fluorine-free exfoliation and functionalization of tantalum carbide MAX-phase to synthesize a novel Ta₄C₃T_x MXene-tantalum oxide (TTO) hybrid structure through an innovative, facile, and inexpensive protocol is demonstrated. Additionally, the application of TTO composite as an efficient biocompatible material for supercapacitor electrodes is reported. The TTO electrode displays long-term stability over 10 000 cycles with capacitance retention of over 90% and volumetric capacitance of 447 F cm^-3 (194 F g^-1) at 1 mV s^-1. Furthermore, TTO shows excellent biocompatibility with human-induced pluripotent stem cells-derived cardiomyocytes, neural progenitor cells, fibroblasts, and mesenchymal stem cells. More importantly, the electrochemical data show that TTO outperforms most of the previously reported biomaterials-based supercapacitors in terms of gravimetric/volumetric energy and power densities. Therefore, TTO hybrid structure may open a gateway as a bioelectrode material with high energy-storage performance for size-sensitive applications.

Keywords: Ta 4C 3Tx MXene‐tantalum oxide; biocompatible electrode; fluorine‐free Ta 4C 3Tx MXene; human stem cells; hybrid structures; supercapacitors.

Figure 1

Schematic model and stoichiometry of TTO hybrid structure. Illustration of a) step‐by‐step schematic and b) mechanism of reaction for the fluorine‐free conversion of the Ta₄AlC₃ MAX phase to surface‐modified Ta₄C₃T _x MXene nanosheets decorated with tantalum oxide nanoparticles.

Figure 2

Morphology and microstructural characterization of the synthesized TTO hybrid structure. Scanning electron microscopic (SEM) images of the functionalized a) Ta₄C₃T _x MXene and b) TTO nanostructure samples after heat treatment at 220 °C for 2 h. Field‐emission SEM observations of TTO hybrid structure reveal effective delamination of layers, anchored by a tantalum oxide particle array. The thermal treatment further improved the oxidization of Ta₄C₃T _x MXene layers. TEM images of the oxidized c) Ta₄C₃T _x MXene (inset) and d) TTO (inset) composites. The images show successful organization of TTO hybrid structures. The thermal treatment led to further improvement and well‐defined distribution of Ta‐oxide nanoparticles. High‐resolution TEM images confirmed the presence of two different lattices with d‐spacing of 0.261 and 0.338 nm, which is attributed to Ta₄C₃T _x MXene layers and tantalum oxide composites. e) XPS narrow scan spectra of Ta 4f, O 1s, Al 2p corresponding to Ta₄AlC₃ MAX phase and oxidized TTO samples after thermal treatment at 220 °C for 2 h, confirming proper extraction of Al from the MAX phase structure with effective exfoliation of MXene nanosheets. The XPS fittings further demonstrate that exfoliated Ta₄C₃T _x MXene sheets were successfully composited with Ta₂O₅‐TaO₂ particles.

Figure 3

Specific surface area measurement using Brunauer–Emmett–Teller analysis. a) N₂ adsorption‐desorption isotherm curves of the Ta₄AlC₃ MAX phase, oxidized Ta₄C₃T _x MXene, and TTO hybrid structure. The BET data depicted that specific surface area of the materials was 1.29, 41.79, and 51.02 m² g⁻¹, respectively. b) Pore size distribution of the MAX phase, oxidized MXene, and TTO hybrid structure. As shown, the average pore diameter of the MAX phase was decreased about four‐fold in TTO nanostructure.

Figure 4

Electrical and electrochemical measurements of fabricated TTO hybrid structure electrode. a) Cyclic voltammetry curves of the oxidized Ta₄C₃T _x MXene electrode and b) TTO hybrid structure electrode at different scan rates in PVA/H₃PO₄ solid electrolyte after 10 000 cycles of the two‐electrode experiment. c) The galvanostatic charge/discharge (GCD) curves of the oxidized Ta₄C₃T _x MXene electrode and d) the TTO hybrid structure electrode. e) Specific capacitance for both electrodes and volumetric capacitance of TTO hybrid structure electrode at different scan rates and f) different specific currents. g) The Nyquist plot of the oxidized Ta₄C₃T _x MXene electrode and TTO hybrid structure electrode. The inset shows the electrical equivalent circuit. h) CV curve of the TTO hybrid structure electrode at 100 mV s⁻¹. The pink and blue areas show the direct contributions of the capacitive and diffusion mechanisms respectively. i) Capacity contribution from capacitive and diffusion‐controlled kinetic processes at different scan rates for the TTO hybrid structure electrodes.

Figure 5

Comparison of TTO supercapacitor with some of the previously reported organic and inorganic electrode materials. a) Ragone plots comparing the performance of TTO with electrical double layer (EDL) capacitors (35 and 50 mF, 300 µF/3 V), graphene oxide modified protein electrode supercapacitor, aluminum electrolytic capacitor (12 µA h/3.3 V) and lithium‐ion thin film battery (LiTF, 500 µA h/5 V).^{[ 45} , ⁴⁶ , ⁴⁷ , ⁴⁸ , ⁴⁹ , ⁵⁰ , ⁵¹ , ^{52 ]} Energy density/power density of the TTO is significantly higher than above‐mentioned electrodes. b) Ragone plots comparing energy and power densities of the TTO hybrid structure supercapacitor to PANI/Ti₃C₂T _{x ,} ^{[ 42 ]} RuO₂/MXene yarn,^{[ 43 ]} Mo_1.33C/PEDOT:PSS,^{[ 43 ]} Ti₃C₂Tx/RGO,^{[ 53 ]} and MXene/NiCo‐LDHs.^{[ 54 ]} c) Supercapacitor cycling stability, volumetric capacitance retention, and charge–discharge cycle at a current density of 1 A g⁻¹. d) The image shows an LED powered by the TTO supercapacitor electrodes. Zoom‐view panel shows the schematic view of TTO‐based solid‐state supercapacitor containing PVA/H₃PO₄ gel electrolyte. The picture demonstrates that TTO supercapacitor was able to successfully power the LED.

Figure 6

Assessment of biocompatibility of the Ta₄AlC₃ MAX phase and Ta₄C₃T _x MXene‐tantalum oxides materials with human cells. a) The MAX phase, oxidized Ta₄C₃T _x , and TTO materials were cocultured with human iPSC‐derived‐ fibroblasts, cardiomyocytes, and neural progenitor cells (NPCs) for 24 h. WST‐1 proliferation assay was performed to evaluate cytocompatibility of materials. Our data demonstrate that MXene was compatible with all three cell types, as coculture with biomaterial did not affect cellular proliferation compared to control group. b) Cytotoxicity evaluation of the MAX phase, oxidized Ta₄C₃T _x , and TTO hybrid structure was assessed by LDH release after coculturing with human MSC for 24 h. LDH data show no significant difference among different MXene groups and the control group. c) LIVE/DEAD assay was performed using the fluorescent dye to assess biocompatibility of human MSC with the material. After coculture with different forms of MXene, MSC were stained with Calcein (for live cells, green) and EthD‐1 (for dead cells, red). Images were captured using Nikon Ti‐2 fluorescent microscope. No significant difference in viability between different groups was detected. (n=3–4 per group). (“ns” = statistically no significant difference, * = p < 0.05 and ** = p < 0.01).