Robust and Elastic Bioinspired MXene‐Coated Foams with Enhanced Energy Storage and Conversion Capabilities

Constructing highly porous structures using Ti3C2Tx MXene provides a promising strategy toward achieving low density, high specific surface area, and shorter ion/molecule transport paths. However, the weak MXene‐MXene or MXene‐substrate interactions hinder the development of ultra‐robust and elastic MXene‐based architectures. To address this issue, a bio‐inspired strategy is developed to effectively adhere the MXene nanosheets onto melamine foam via covalent and hydrogen bonding interactions through polyethyleneimine/polydopamine‐modification. The enhanced interactions contribute to high MXene loading (≈94 wt.%) and reversible compressibility even after 10 000 compression/release cycles at 80% strain. The compressible supercapacitor device assembled from this foam exhibits high energy storage capability (119 F g−1 at 2 mV s−1) with capacitance retention of ≈93% after 1000 compression/release cycles at 50% strain. Moreover, the presence of polydopamine and MXene enable the absorption of light in the UV–Vis and near‐IR regions, respectively, inducing photothermal conversion functionality, with an evaporation rate of ≈1.5 kg m−2 h−1 and ≈89% solar evaporation efficiency under one sun illumination. It is envisaged that this bio‐inspired chemical modification offers a versatile strategy for the assembly of MXene nanosheets onto different substrates for various applications, such as electromagnetic interference shielding, energy storage, and conversion.

an example, super elastic MXene composite aerogels using reduced graphene oxide (rGO) as a gelation agent were assembled through a multi-step reduction, freeze-casting, and annealing processes. [17] These MXene/rGO composite aerogels found further use as electrodes for compressible supercapacitors and piezoresistive sensors. In another work, Yang and co-workers used aqueous inks with high concentrations (>50 mg mL −1 ) of large MXene (≈8 µm) to 3D print freestanding architectures with high specific surface area. [27] The resulting 3D device exhibited good electrochemical performance with high areal capacitance (2.1 F cm −2 ), and excellent capacitance retention of ≈90% for 10 000 cycles. However, the majority of these strategies rely on complex processes or require sophisticated facilities, thus facile preparation of low-cost MXene-based 3D monoliths remains challenging.
In contrast, methods that utilize microstructural templates such as polystyrene (PS), [28] and poly(methyl methacrylate) [7] spheres as fillers between MXene nanosheets, or the direct coating of MXene nanosheets onto the skeletons of porous 3D monoliths, [29] demonstrate promise in resolving the typical large-scale production issues. To this end, PS spheres with varying diameters were used as sacrificial templates to fabricate MXene foams with different pore structures. [28] The resulting foams exhibit excellent energy storage capability up to ≈474 F g −1 at 2 mV s −1 . However, the mechanical properties of these 3D MXene based monoliths are rather brittle and can be easily damaged under large mechanical strains or after many compression cycles. Moreover, the removal of these polymer templates usually needs high temperature and energy consumption, which affect the stability of MXene. 3D porous scaffolds, such as melamine foam, have been widely used as templates for loading of MXene to achieve 3D MXene monoliths. These have shown great potential in applications including supercapacitors, [30] solar-to-thermal conversion, [29] organics adsorption, [18] and piezoresistive sensors. [32,33] Unfortunately, the weak interactions between MXenes and these templates resulted in inadequate MXenes loading (<10 wt.%) into these 3D porous templates. [34,35] These structures are also mechanically and electrically unstable under deformation. These challenges remain significant bottlenecks for developing robust and elastic 3D MXene monoliths in practical applications.
In order to address these weak interaction issues, a bioinspired strategy utilizing natural materials such as the adhesive components from mussels (e.g., polydopamine (PDA) and other catechol-containing compounds) have been explored as binding agents for coating onto a wide range of material surfaces. [36] The versatility of mussels' adhesive components is attributed to their rich functionalities, which can form strong covalent and noncovalent interactions with most substrates. These groups also interact well with MXene's hydroxyl-and fluorine-terminated surface, resulting in an effective bridging interaction with the substrate. [37] For example, highly ordered MXene/PDA composite films fabricated through in situ polymerization of dopamine on MXene surface promoted covalent and hydrogen binding in between MXene nanosheets, [38] resulting in films with simultaneously improved mechanical strength and ambient stability.
In this work, a bio-inspired chemical modification method was exploited and extended to functionalize a porous template with MXene nanosheets, producing an ultra-robust and elastic MXene-coated foam. For this approach, polyethyleneimine/ polydopamine (PEI/PDA) composite was utilized to modify melamine (MF) foam to which MXene nanosheets adhere strongly through synergistic interfacial interactions involving covalent and hydrogen bonding, as well as reduced electrostatic repulsion caused by cationic PEI. As a result, MX/PEI/PDA/ MF delivers high MXene loading (≈94 wt.%), good conductivity (0.78 S m −1 ), and reversible compressibility after 10 000 cycles at high strain of 80%. These desirable attributes render excellent performance as electrodes for fabricating compressible supercapacitor devices in terms of high energy storage capability, and outstanding capacitance retention of ≈93% after 1000 compression/release cycles at 50% strain. Moreover, the light absorbing capabilities of PDA and MXene at the UV-vis and near-IR regions respectively, resulted in composite foams with good light-to-thermal conversion ability (≈88% energy efficiency) and an evaporation rate of ≈1.5 kg m −2 h −1 under one sun illumination. These excellent properties demonstrate broad potential in energy storage and conversion applications.

Results and Discussion
A 3D MX/PEI/PDA/MF composite was fabricated through a bio-inspired method to adhere MXene onto a modified MF skeleton (Figure 1). Briefly, MF was first soaked in a mixture of PEI and dopamine (DA) solution at pH 8.5, accompanied with moderate stirring. The colorless MF gradually turned brown, indicating spontaneous deposition of PEI/PDA layer onto the MF skeleton ( Figure S1a, Supporting Information), as a result of simultaneous DA oxidation and catechol-amino Michael Addition/Schiff-base reactions ( Figure S1b, Supporting Information). [39] The dried PEI/PDA/MF was then stirred in a MXene suspension (1.0 mg mL −1 ), followed by vacuum drying at room temperature to obtain the MX/PEI/PDA/MF composite. Unlike typical 3D printed or freeze-dried monoliths, the simple processibility of this MF template allowed the easy fabrication of any desired dimension, cost effectively ( Figure S2, Supporting Information).
Scanning electron microscopy (SEM) was then employed to determine the morphology and microstructure of the MF, MX/PEI/PDA/MF, and its intermediates. At the micron scale, the MF scaffold possessed a smooth surface and sufficiently large porous structure for loading MXene (pore size ≈200 µm, Figure 1b). After PDA coating, the resulting PDA/MF skeleton exhibited a rough surface and with increased thickness ( Figure S3a, Supporting Information). The magnified SEM image showed that the rough surface consisted of considerable aggregated PDA on the PDA/MF surface ( Figure S3b, Supporting Information), which was unfavorable for attaching MXene on the skeleton due to incomplete mutual surface-surface interaction ( Figure S3c, Supporting Information). Instead, when PEI was co-polymerized with PDA, a smooth coating layer was produced on the MF surface, which was confirmed by imaging a cross-section of the skeleton (Figure 1c).
MXene was then coated onto the surface modified MF by simply soaking the latter into a MXene dispersion followed by vacuum drying (Figure 1a). Here, small MXene (≈300 nm, Figure S4, Supporting Information) was chosen for surface coating, as a small sheet size was crucial for easy infiltration and attachment into the MF's interior to maximize loading ( Figure S5, Supporting Information). After drying, the nanosheets could be seen distributed across the PEI/PDA/MF skeleton (Figure 1d), with homogeneous distribution confirmed through Ti maps acquired via energy-dispersive X-ray spectroscopy (EDX) (Figure 1e). The PEI/PDA modified MF resulted in a significantly higher MXene loading of ≈93.6%, against ≈9.4% for pristine foams (Figure 2a). This loading was also higher than that of MF treated with PEI or PDA alone, which only resulted in 19.6% and 21.3% of MXene loading, respectively. Furthermore, no MXene was observed flaking off the MX/PEI/PDA/ MF after compression compared with the controls ( Figure S6, Supporting Information), demonstrating that the bio-inspired MF surface modification facilitates superior MXene adhesion.
To explain this drastic increase in MXene loading, we conducted a series of experiments aimed to elucidate the interaction of MF withPEI/PDA, and ultimately with MXene. First, PEI and PDA was found to form a smooth surface when co-polymerized onto the MF skeleton at an optimized PEI/ PDA loading ratio ( Figure S7, Supporting Information). These amine-and hydroxyl-rich surface functionalized spots also tend to alter the surface charge of MF. [39,40] To confirm this, we carried out zeta potential measurements on different foams. Pure MF foam and MXene both showed negative surface zeta potentials of -50 and -33 mV, respectively (Figure 2b), resulting in strong mutual repulsion consequently causing the low MXene loading of unmodified MF (≈9.4 wt.%). The PDA modified MF exhibited a slight increase in zeta potential (-32 mV), leading to weaker repulsion which doubles the MXene loading. Coating MF with positively charged PEI resulted in a positive surface zeta potential (8.7 mV), which contributes to a higher MXene loading (≈21.3 wt.%). The surface zeta potential also increased to −17 mV when MF was coated with PEI/PDA, likewise suggesting a weaker electrostatic repulsion with MXene than uncoated or PDA coated MF. Although this value is still morenegative than PEI treated MF, we interestingly observed a four-fold increase in MXene loading (≈93.6 wt.%) for PEI/PDA modified MF. We suspect that besides enhanced electrostatic   (Figure 2e,f). However, compared with pristine MXene, a new peak at ≈286.8 eV was observed from MX/PEI/PDA/MF, which could be assigned as a catecholtitanium coordination bond. [38] This result was also confirmed by the presence of C-O-Ti bond in the deconvoluted Ti 2p core spectra ( Figure S9d (Figure 2i), however, the C-O-Ti bond at ≈531.6 eV might be overlapped by the existence of a quinone state (CO) from the PDA coating. [38,41] In addition, the decreased peak intensity of surface groups such as O-Ti-O, C-Ti-OH, and R-OH is suspected to be caused by covalent (the coordinated dehydration of catechol and the hydroxyl groups) and hydrogen bonding interactions between PEI/PDA and MXene nanosheet surfaces. [37,41] As such, we believe that higher MXene loading in PEI/PDA/MF relative to control samples is due to the following reasons. First, adding positively charged   (Figure 3a), a typical behavior for elastic foams. [17] Above this strain (ε > 80%), MX/PEI/PDA/ MF showed partial permanent deformation where it was unable to recover its original height ( Figure S10, Supporting Information). The stress-strain curves also exhibited two distinct domains, which are believed to correspond to different deformation behaviors. To elaborate, when the compressive strain was <60%, the stress increased slightly due to bucking or yielding of the skeleton. Between 60% < ε < 80% , the stress rapidly increased due to densification of the skeleton. The maximum stress of MX/PEI/PDA/MF was significantly higher (319 kPa) than pure MF (38 kPa, Figure 3b) or composite-coated MF (167 kPa, Figure 3c). The PEI/PDA coating was thought to be primarily responsible for the increased maximum stress by increasing MF densification at high compressive strains, at the slight expense of elasticity, the effect of which was further increased with addition of MXene.
The durability of MX/PEI/PDA/MF was demonstrated by carying out 10 000 compression relaxation cycles at high compression strain of 80% (Figure 3d). The foam showed remarkable durability and robustness, as the stress-strain curves are largely invariant, with stress remaining above zero at ε < 5% strain even after 10 000 compression/release cycles ( Figure  S11, Supporting Information). Moreover, MX/PEI/PDA/MF was able to maintain ≈85% of its maximum stress at ε = 80%  and recover to its original height when stress released after 10 000 cycles (Figure 3e; Figure S12, Supporting Information), which is better than previously reported 3D MXene monoliths (Table S1, Supporting Information). The observed superior reversible compressibility was largely due to the highly elastic MF skeleton. Furthermore, the exceptional stability of MX/PEI/PDA/MF demonstrated through consistently stable stress-strain curves at different applied compression rates (10-100 mm min −1 ), imply the independence of its mechanical properties to compression rate (Figure 3f). This is confirmed from the images presented in Figure 3g of the compressed and relaxed state, showing the excellent recovery of the foam. Finally, the electrical properties of MX/PEI/PDA/MF tested under different compressive strains revealed conductivity of 5.91 S m −1 at 80% strain, which is more than 7.5 times the conductivity at its original state (0.78 S m −1 , strain of 0% Figure 3h). The decreasing resistance of the foam under compression can be attributed to the increased contact area of MXene due to densification.
Due to its excellent conductivity, elasticity, and durability, combined with high energy storage capabilities ( Figure S13, Supporting Information), MX/PEI/PDA/MF was further studied as an electrode for compressible energy storage devices. As shown in Figure 4a, The MX/PEI/PDA/MF was fabricated into a symmetric all-solid-state compressible supercapacitor device. Briefly, two MX/PEI/PDA/MF electrodes were attached to separate two pieces of copper tape, which serve as current collectors in this experiment. The PVA-H 2 SO 4 gel electrolyte was then dropped over the electrodes' surfaces until saturated ( Figure S14, Supporting Information). The two components were then put back together and allowed to dry at room temperature for 24 h. To explore the cell voltage of the device, CV curves were run at various voltage windows, which showed a stable voltage of between 0 and 0.8 V (Figure 4b). Furthermore, the CV and GCD curves exhibited the expected quasirectangular and symmetrical forms, respectively (Figures 4c,d). Based on the CV curves, the gravimetric capacitance was 119 F g −1 at 2 mV s −1 , equivalent to a volumetric capacitance Adv. Mater. Technol. 2023, 8, 2201611   Figure 4. a) Photo image of the as-fabricated device. b) CV curves of the device at different voltage windows from 0-0.4 to 0.9 V. c) CV curves at scan rates from 2 to 20 mV s −1 . d) GCD curves of device at different current density. e) The changes of specific capacitance at increased scan rate. f) Capacitance retention of the device after 20 000 cycles at 20 mV s −1 . g) CV curves and h) capacitance retention of the device measured from 0% to 70% compressive strain. i) Capacitance retention of the device after 1000 compression cycles at 50% strain. of 6 F cm −3 (Figure 4e), which was higher or comparable to previously reported foam-based supercapacitor devices (Table S2, Supporting Information). In addition, the device exhibited ≈84% capacitance retention after 20 000 cycles at 20 mV s −1 with retention of the CV shape across 20 000 cycles demonstrating excellent long-term cycling stability in its relaxed-state (Figure 4f).
To evaluate the performance for practical applications, the electrochemical properties of the device was also tested under different compression states. Under compression rates up to 70%, the shape of the CV curve remained largely unchanged (Figure 4g), but the enclosed area of the CV curve increased with compression strain, leading up to 46% increased specific capacitance (Figure 4h). This enhanced capacitance can be attributed to the consequent increase in electrical conductivity of the MX/PEI/PDA/MF electrode under compression. [17] Finally, the device also exhibited high capacitance retention of ≈93% after 1000 compression-release cycles under strain of 50%, demonstrating its robust electro-mechanical durability needed for compressible energy storage devices (Figure 4i).
Apart from high energy storage capability, MXene with layered structure and metallic nature possesses excellent light absorption. To elaborate, absorbed waves can pass through the MXene lattice structure and undergo internal reflections between the layers, and are eventually dissipated in the form of heat within the material. [18,42] Moreover, the presence of aromatic PDA groups synergistically shifts the system's electron excitation to visible light irradiation, and has shown great potential in photo-thermal conversion. [42] The presence of strong light absorbing species (e.g., MXene and PDA), combined with a highly porous structure inducing multiple internal reflections of the trapped light in MX/PEI/PDA/MF, makes the composite attractive toward photothermal conversion applications. To demonstrate such, the steam generating properties of as-prepared MX/PEI/PDA/MF were also investigated. While all these samples showed high absorption (>90%) across the sunlight wavelengths (200-2500 nm), the absorption of MXene/ MF was higher than PDA/MF or PEI/PDA/MF in the NIR region (700-2500 nm) but lower than in the UV-Vis region (200-700 nm) (Figure 5a). Additionally, MX/PEI/PDA/MF exhibited superior full-spectrum absorption of ≈96% leading to the better solar-to-thermal conversion efficiency. Moreover, the porous structure of MX/PEI/PDA/MF induces multiple internal reflections enhancing light absorption efficiency, as well as enabling continuous escape of water to the evaporating surface critical for steam generators. The water absorption time of the MX/PEI/PDA/MF was evaluated using contact angle measurements, which showed that only 200 ms was needed for water droplet impregnation indicating fast water supply and transportation for water evaporation under solar illumination (Figure 5b). This was a function of both the porous nature of the network and the great affinity of MXene toward water. [43] The temperature change of MX/PEI/PDA/MF under one sun solar illumination (1 kW m −2 ), displayed a sharp increase within 6 min, indicating its excellent photothermal conversion efficiency (Figure 5c). The increased temperature of the foam led to a concomitant increase in water temperature near its surface (up to ≈46°C), facilitating faster water evaporation. The evaporation rate of the samples (6 cm 2 ) was quantified by mass loss (Figure S15, Supporting Information), with MX/ PEI/PDA/MF showing the largest mass change, corresponding to an evaporation rate of 1.47 kg m −2 h −1 (Figure 5d). This evaporation efficiency was calculated to be 88.7% (Figure 5e), which is comparable or higher than the controlled samples or previously reported MXene based materials, respectively (Figure 5f). [8,42,31,[44][45][46][47][48] The evaporation rate and efficiency was also evaluated at different irradiation intensities (1, 3, and 5 kW m −2 ), which showed that evaporation rate was correlated with light intensity (Figure 5g; Figure S16, Supporting Information). However, this increased rate was at the slight expense of efficiency. To understand the influence of MXene size to photothermal performance, large MXene sheets as coating material (L-MX/PEI/PDA/MF) was also investigated. The evaporation rate of L-MX/PEI/PDA/MF is ≈1.35 kg m −2 h −1 , which is lower than that of the composite foam loaded with small MXene (Figure S17, Supporting Information), suggesting the importance of high MXene loading for photothermal efficient conversion. In summary, the superior efficiency and durability of MX/PEI/PDA/MF for solar steam generation, compared to other materials, is believed to be caused by these properties (Figure 5i); 1) the combination of MXene and PDA adds broadband light absorption capability; 2) the porous structure of leads to multiple reflections and desirable light localization within the architecture; 3) rapid adsorption ensures continuous flow of water to the evaporation surface. These properties suggest that MX/PEI/PDA/MF is a promising material for solar steam generation and related applications.

Conclusion
In summary, we have demonstrated that an ultra-robust and elastic MXene based foam (MX/PEI/PDA/MF) can be fabricated through addition of a bio-inspired PEI/PDA composite to promote the interaction with MXene nanosheets through reduced electrostatic repulsion, and the formation of covalent and hydrogen bonds. By optimizing the PEI/PDA coating layer, MXene loading can be maximized, resulting in a MX/PEI/PDA/ MF foam incorporating large amounts of MXene (≈94 wt.%), which is higher than the values reported for MXene loading in porous templates. The foam exhibited good conductivity and reversible compressibility after 10 000 cycles at high compressive strain, due to the high loading of MXene on a flexible and robust MF skeleton. Given these superior attributes, compressible supercapacitors were assembled using MX/PEI/PDA/ MF electrodes, which displayed high energy storage capability, and a capacitance retention of 93% after 1000 compressionreleasing cycles at strain of 50%. Moreover, the MX/PEI/PDA/ MF with porous structure, broadband light absorption, multiple reflection, and rapid water transport channels, is favorable for use in applications such as a light absorber for steam generation. It demonstrated high light-to-thermal conversion ability, efficiency, and evaporation rate. As such, the bio-inspired composite approach is an effective and facile method to fabricate ultra-robust and elastic 3D MXene based devices for different applications such as wearable sensors, EMI shielding, energy storage, and conversion.

Experimental Section
Synthesis of Ti 3 C 2 T x MXene: Ti 3 C 2 T x MXene was synthesized by selectively etching Ti 3 AlC 2 MAX phase using the "MILD" method. [17] Briefly, 1g of Ti 3 AlC 2 powder was slowly added to the mixture of LiF (1 g) and HCl (20 mL of 9 M), followed by magnetic stirring for 24 h at 35 °C. The resulting solid residue was washed repeatedly with deionized water (DI) and centrifuged until the pH of the supernatant reached ≈6. Afterward, 100 mL of DI water was added to the above sediment, which was sonicated for 10 min under an Ar atmosphere to obtain the single-/ few-layer MXene. After a centrifugation process (1500 rpm for 20 min) to remove the incompletely etched MAX phase and multilayer MXene, the small sized MXene was finally obtained after probe sonication the single-/few-layer MXene in iced water for 20 min.
Fabrication of MX/PEI/PDA Coated MF: For the preparation of PEI/ PDA/MF, commercially available MF was cut to desired shape and size (typically, but not restricted to, 0.5 cm × 0.5 cm × 2 cm), followed by washing with DI water and dried at 60 °C. The MF was then fixed in a 20 mL beaker containing a mixture of DA (40 mg), PEI (4 mg) and Tris buffer (12 mg) dissolved in DI water (20 mL) with a pH 8.5. After mechanical stirring for 24 h, the resultant PEI/PDA/MF composite was washed with DI water and dried at 60 °C for 12 h. MX/PEI/PDA/MF was fabricated by placing the PEI/PDA/MF in a beaker filled with 20 mL of MXene solution (1 mg mL −1 ), followed by stirring for 12 h, after which the solution turned from black to nearly clear. Finally, the MX/PEI/PDA/ MF was obtained after being dried under vacuum for 24 h. The control PDA/MF and PEI/MF was fabricated using the same procedure without the addition of PEI and PDA, respectively.
Fabrication of All-Solid-State Supercapacitor: The PVA-H 2 SO 4 gel electrolyte was prepared following the method detailed in our previous report. [17] For the supercapacitor fabrication, two MX/PEI/PDA/MF electrodes were fixed on two pieces of copper tape which were used as the current collector, followed by drop-casting the PVA-H 2 SO 4 gel electrolyte on their surface until saturated ( Figure S14, Supporting Information). Lastly, the two separate parts were assembled and dried at room temperature for 24 h to remove excess water.
Characterization: The morphologies of the foams were characterized by using a field emission SEM (Zeiss SUPRA 55-VP at 3-5 kV), and elemental mapping images were observed at 20 kV by the EDX detector. The surface zeta potentials of the foams and MXene were  analyzed by using a SurPASSTM 3 Zeta Potential Analyzer. The chemical compositions and bond binding energy of the samples were characterized by XPS using a Kα X-ray source (Kratos Axis Nova, hν = 1486.6 eV). The mechanical properties of the foams were tested using an Instron Tensile Tester with a 100 N load cell at different rates from 10 to 100 mm min −1 . The electrical conductivity of the MX/PEI/PDA/MF was acquired using a Digital Multimeter (Keysight 34461A) with a two-point probe setup. Electrochemical properties including CV, GCD, and EIS of the MX/PEI/PDA/MF electrode and supercapacitor device were tested using an electrochemical station (Biologic SP-300). Water evaporation tests of the cylindrical foams (diameter of 2.8 cm and thickness of 1 cm) were carried out using simulated sunlight (CEL-HXF300, China) at room temperature (≈23 °C). The water weight loss was recorded in real-time by an electrical balance (FA 2004) connected to a computer, as illustrated in Figure S15 (Supporting Information). Temperature changes of the samples measured by an IR thermal camera (Fluke). The light absorption of the samples was measured using a UV-vis-NIR spectrophotometer (Cary 5000).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.