Free Standing 3D-MXene Thin-Film Structures for Supercapacitor Applications

MXenes have been found to be promising electrode materials for supercapacitor applications. They achieve high power output and fast charging/discharging by fast faradaic redox reactions at the surface, which has made them the focus of supercapacitor research in recent years [1].<br/>The titanium carbide MXene (Ti₃C₂T_x), has been shown to hold significant interest, as it offers great pseudocapacitive properties combined with a high electronic conductivity. Thin film electrodes made up of Ti₃C₂T_x have shown exceptional capacitance of up to 450 F/g and a good rate performance even up to 100 V/s [2].<br/>Herein we advance the previously developed method for the fabrication of interconnected MXene (Ti₃C₂T_x) 2D thin films in a 3D macroscopic assembly [3][4][5], by coating the electrode structures with the co-polymer, poly(2-hydroxyethyl methacrylate-co-ethylene glycol dimethacrylate), (p(HEMA-co-EGDMA)) using initiated chemical vapour deposition (iCVD) [6]. The iCVD process, where a polymer thin film is polymerized directly on the substrate surface from the gas phase, enables the conformal coating of complex 3D structures, which would be difficult by other means [6][7]. The applied co-polymer p(HEMA-co-EGDMA) is a hydrogel with minimal swelling and great long-term stability in an aqueous environment, which makes it a promising material for supercapacitor applications.<br/>At a thickness of 50 µm and areal loadings of more than 5 mg/cm², the uncoated electrode networks demonstrated volumetric and specific capacitances of up 140 F/cm³ and 240 F/g respectively, at a scan rate of up to 200 mV/s [5]. At the same scan rate an areal capacitance of ~1.4 F/cm² is recorded, outperforming that of state-of-the-art MXene based electrodes by a factor of ~2 [5]. However, during long term cycling (&gt;10000 cycles) MXene based electrodes decrease in performance due to delamination and oxidation of the individual flakes. This negatively influences the overall conductivity and capacitance of the structure. To address this problem, the p(HEMA-co-EGDMA) hydrogel thin film is applied to the 3D MXene thin film structures., where it is supposed to act as a supportive outer layer, fixing the MXene flakes in place and inhibiting the performance loss during cycling, without the need for a binder in between the flakes.<br/>First experiments revealed, that with the hydrogel coatings, the cycling stability at 10 A g^-1 of these network electrodes showed a full capacitance retention of 100% across 30000 cycles (C₁ = 156 F g^-1, C₃₀₀₀₀ = 237 F g^-1), whereas the uncoated electrode lost ~30% of its maximum capacitance (C₁ = 245 F g^-1, C₃₀₀₀₀ = 189 F g^-1).<br/><br/><b>References</b><br/>[1] W. Raza et al.; Nano Energy; <b>2018</b>; vol. 52; pp. 441–473<br/>[2] M. R. Lukatskaya et al.; Nat. Energy;<b> 2017</b>; vol. 6; no.7; pp. 1–6<br/>[3] F. Rasch et al.; ACS Appl. Mater. Interfaces; <b>2019</b>; vol. 11; no. 47; pp. 44652–44663<br/>[4] F. Schütt et al.; Nat. Commun.; <b>2017</b>; vol. 8; no. 1; pp. 1–10<br/>[5] D.Spurling et al.; Energy Storage Materials <b>2024</b> vol. 65 103148<br/>[6] W. Reichstein, Polymers 2021, 13, 186]<br/>[7] I.Barg et al. Adv. Funct. Mater.<b>2023</b>, 33, 2212688