Modifications of MXene layers for supercapacitors

Highlights

• Re-stacking of MXene nanosheets was worked out by modifying the layer morphology.

• Resulting foamy electrode materials show remarkable pseudocapacitive behavior.

• An asymmetric device was built on MXene foam and MnO2 electrodes.

• Attractive power and energy densities were obtained in aqueous KOH electrolyte.

Abstract

The re-stacking of Ti₃C₂T_x-MXene layers has been prevented by using two different approaches: a facile hard templating method and a pore-forming approach. The expanded MXene obtained by using MgO nanoparticles as hard templates displayed an open morphology based on crumpled layers. The corresponding electrode material delivered 180 F g⁻¹ of capacitance at 1 A g⁻¹ and maintained 99% of its initial capacitance at 5 A g⁻¹ over five thousand charge-discharge cycles. On the other hand, the MXene foam prepared after heating a MXene-urea composite at 550°C, showed numerous macropores on the surface layer and a complex open 3D inner-architecture. Thanks to this foamy porous structure, the binder-free electrode based on the resulting MXene foam displayed a great capacitance of 203 F g⁻¹ at 5 A g⁻¹ current density, 99% of which was retained after five thousand cycles. In comparison, the pristine MXene –based electrode delivered 82 F g⁻¹, only, in the same operating conditions. An asymmetric device built on a negative MXene foam electrode and a positive MnO₂ electrode exhibited an attractive energy density of 16.5 Wh kg⁻¹ (or 10 Wh L⁻¹) and 160 W kg⁻¹ (or 8.5 kW L⁻¹) power density. Altogether, the enhanced performances of these nano-engineered 2D materials are a clear demonstration of the efficiency of the chosen synthetic approaches to work out the re-stacking issue of MXene layers.

Introduction

In the wake of graphene [1], other two dimensional materials [2], such as, δ-MnO₂ [[3], [4], [5]], MoS₂ [6,7] and MXene [[8], [9], [10]], have received a surge of interest from the material science community, as they offer, together with their unique “planar” physical peculiarities, an infinite number of surface chemistry opportunities, especially when compared with their carbonaceous flagship [11]. These opportunities have been explored in many application fields including electrochemical energy storage [[12], [13], [14]]. In the corresponding devices, both basal planes and defected edges are usually contributing to charge storage [15]. As a result, one can take profit of the strong in-plane covalent bonding and weak out-of-plane Van der Waals interactions between layers [16], to develop chemical strategies to provide or tune further active sites. These proceed by many chemical and physical methods including exfoliation [17], intercalation [18] and hybridization [19]. The main idea is to prevent the natural tendency of the individual 2D layers to restack to minimize the surface energy just as in the pristine material(s). By chemical engineering at the nanoscale, associating layers of 2D materials of different chemical compositions and physical properties offers wonderful opportunities to take advantage of resulting synergistic effects [20]. Potential combinations are unnumbered, imagination is probably the limit. As such, in the resulting composite, expanded, hybrid … materials, the opened 2D space between the layers can, for example, provide a “path” strongly suited for ion adsorption and transport. These characteristics have made 2D materials as very attractive electrode materials in batteries [21] and supercapacitors [22]. Two different kinds of supercapacitors (SC), can be distinguished, on the basis of the corresponding charge storage mechanism: electrostatic interactions in electrical double layer capacitors (EDLCs), and fast redox reactions near the surface material in pseudocapacitors [23]. With 2D materials, this classification remains as graphene is probably mostly EDLC type while exfoliated MnO₂ birnessite is pseudocapacitive [[24], [25], [26], [27], [28], [29]]. Composites obtained by stacking of EDLC and pseudocapacitive, EDLC and battery type, or battery type and pseudocapative 2D materials have been recently reported [[30], [31], [32], [33]]. However, the anisotropy of material properties could also be a critical drawback for 2D materials, especially in energy storage electrode materials. Although, both in-plane ionic and electronic conductivities can be remarkable, through-plane conductivities can be fairly limited. Depending on the layer orientation toward the current collector surface, perpendicular or parallel, electrode overall performance can be down-graded.

MXene [34], as a remarkable 2D material, containing a conductive carbide core along with transition metal oxide-like surfaces and intercalated water molecules [35], triggered much attention and attracted worldwide researches in the field of energy storage and more specifically for supercapacitors [[36], [37], [38], [39], [40]]. MXene is prepared from the corresponding MAX phase by a chemical etching method, usually using a fluorine-containing solution. MAX phases are layered ternary carbides and nitrides with the formula M_n+1AX_n, where M is an early transition metal (such as Ti, V and Nb), A is an element from A-group (usually Al or Si) and X is carbon and/or nitrogen [8,41,42]. Ti₃C₂T_x-MXene could be obtained by etching Ti₃AlC₂ in a mixture of LiF and HCl. T_x stands for the termination moieties at the layer surface. Their chemical nature depends on the etching process. They have a strong impact on the electrolyte/electrode interface especially through the hydrophilic/hydrophobic surface balance. Ti₃C₂T_x-MXene also shows up to ≈6700 S cm⁻¹ of metallic conductivity which is highly favorable to fast electron transfer [[43], [44], [45]]. However, as for other 2D materials, the MXene flakes tend to restack during the preparation process, resulting in a drastic loss of the developed electroactive surface area, and hindering the electrolyte ion access into the electrode bulk [15]. To prevent this re-stacking issue and simultaneously enhance the through-plane ionic conductivities, alternative methods based, for instance, on the modification of the layer morphology and texture have to be considered.

In this study, exfoliated Ti₃C₂T_x-MXene was first prepared by a chemical etching method from Ti₃AlC₂ corresponding MAX phase. To prevent the re-stacking of the resulting individual layers, several routes were explored. First, nano-sized MgO particles were used as solid spacer. After removal of the particles adsorbed at the layer surface, electrolytic ions were able to be efficiently transported in between the layers of the resulting expanded MXene during the charge-discharge process. Therefore, prepared expanded MXene (EM) electrodes showed enhanced electrochemical performances when compared to regular MXene. Alternatively, urea was used as molecular spacer or template. After a thermal treatment under argon atmosphere, a Ti₃C₂T_x-MXene foam (MF) was obtained. The resulting MF electrode showed an attractive and seriously improved capacitance performance, especially at high rate. When associated to a MnO₂ positive-electrode in an MF//MnO₂ asymmetric device, an attractive energy density of 16.5 Wh kg⁻¹ (or 10 Wh L⁻¹) was obtained at 160 W kg⁻¹ (or 8.5 kW L⁻¹) power density.