Journal of Colloid and Interface Science 623 (2022) 947–961 Contents lists available at ScienceDirect Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis Rational design of MXene/activated carbon/polyoxometalate triple hybrid electrodes with enhanced capacitance for organic-electrolyte supercapacitors Jun-Jie Zhu a,⇑, Avireddy Hemesh b, Jordi Jacas Biendicho b, Luis Martinez-Soria a, Daniel Rueda-Garcia a, Joan Ramon Morante b,c, Belen Ballesteros a, Pedro Gomez-Romero a,d,⇑ a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain b Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930, Sant Adrià de Besòs, Spain c Faculty of Physics, University of Barcelona, Barcelona, Spain d Consejo Superior de Investigaciones Científicas (CSIC), Spain h i g h l i g h t s g r a p h i c a l a b s t r a c t  POMs were nano-dispersed within MXene with assistance of AC matrix.  The hybrid combined the merits and compensate for the demerits of each component.  Gravimetric capacitance of the hybrid was 2.2 times that of MXene.  POMs remained anchored in the long- term leading to great cycle stability.  Hybrid’s asymmetric cells outperformed AC//MXene cells by weight and volume. a r t i c l e i n f o a b s t r a c t Article history: We report a triple hybrid electrode (MXene/activated carbon (AC)/polyoxometalates (POMs)) combining Received 13 January 2022 the merits of three materials: MXene (high volumetric capacitance), AC (high gravimetric capacitance) Revised 26 April 2022 and Phosphotungstate (fast redox). Phosphotungstic acid (HPW12) and tetraethylammonium phospho- Accepted 28 April 2022 tungstate (TEAPW12) were the two POMs used to prepare MXene/AC/POMs triple hybrids. MXene/AC/ Available online 5 May 2022 TEAPW12 outperformed MXene/AC/HPW12 in 1 M tetraethylammonium tetrafluoroborate (TEABF4)/ acetonitrile. Nano-dispersion of POMs facilitates charge storage through surface capacitive processes Keywords: (91% at 2 mV s1). MXene/AC/TEAPW12 delivered significantly higher gravimetric capacitance (87F g1 Hybrid electroactive materials MXene at 1 mV s1) than MXene (40F g1 at 1 mV s1) in the same organic electrolyte, without sacrificing much Polyoxometalates volumetric capacitance (less than 10%). The gravimetric capacitance of the triple hybrid was similar to Organic that of MXene/AC, whereas its volumetric capacitance was 1.5 times higher. Replacing TEA cations with Supercapacitors 1-ethyl-3-methylimidazolium cations (EMIM+), the capacitance improved by 21%. Coupled with AC pos- itive electrodes in an asymmetric cell, MXene/AC/TEAPW12 delivered 4.6 times higher gravimetric energy density and 3.5 times higher volumetric energy density than a similar MXene asymmetric cell ⇑ Corresponding authors at: ICN2, Campus UAB, 08193 Bellaterra (Barcelona), Spain. E-mail addresses: zhu.junjie@icn2.cat (J.-J. Zhu), pedro.gomez@icn2.cat (P. Gomez-Romero). https://doi.org/10.1016/j.jcis.2022.04.170 0021-9797/Ó 2022 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 at relatively high-power densities. This study proves that MXene/AC/TEAPW12 combines the merits and compensates for the demerits of each component and is a promising electrode material for organic- electrolyte supercapacitors. Ó 2022 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction storage devices. In these devices, their proper dispersion at the nanoscale and long-term anchoring onto a conducting substrate Energy storage devices with high energy density, power density are key issues for fully harnessing the fast reversible multi- and long lifespan are in urgent demand for a growing number of electron redox activity characteristic of POMs. Several porous applications. Supercapacitors are a good starting point for this materials, such as activated carbon[39,41,42], and 2D materials, challenge since they feature intrinsically high power density and for example, reduced graphene oxide (rGO)[38,40], have been long cycle life. However, they run short of energy density. There shown to effectively anchor POMs[39,40]. The resulting hybrid are two main ways to increase the energy density of supercapaci- materials have shown enhanced performance, including capaci- tors: i) increasing working voltage through the use of organic elec- tance[38–42], and even increased potential window[39]. trolytes, and ii) increasing storage capacity by a proper choice of The synthesis of hybrid materials made of MXenes and POMs electrode materials or through hybridization[1]. In this study, we has been tackled by various authors. For example, Chen et al.[25] followed both approaches based on the design of complex but modified Ti3C2Tx MXene with Keggin-type POMs by ionic liquid unique hybrid materials leading to an optimal combination of polymer linker but used it in low voltage aqueous electrolytes. charge storage capacity and high conductivity together with the Chao et al.[32] proposed the in-situ growth of Keplerate-type high voltage range typical of organic electrolytes. POMs on Ti3C2Tx MXene, leading to a hybrid material with interest- We first needed a material with a very high volumetric capaci- ing performance in lithium-ion capacitors and sodium-ion capaci- tance, and MXenes excel in this respect as they constitute a family tors. However, in their study, the first report needed the use of a of two-dimensional materials, usually containing one layer of tran- linker to bond the acid form of a POM to MXene and its use was sition metal and one layer of carbide. MXenes have a general for- restricted to aqueous electrolytes. And while the second used mula Mn+1XnTx, where M is a transition metal, X is carbon and/or organic electrolytes they failed to disperse POMs, leading to a nitrogen, n is an integer between 1 and 3, and Tx represents surface material made of a mixture of phases, namely, MXene and crystal- functional groups[2]. A conductive inner transition metal carbide lized POM. Thus, in both cases there was a failure to harness the layer enables fast electron supply to electrochemically active sites, full potential of POMs combined with MXene while working in a which makes of MXenes promising electrode materials for super- high-voltage organic electrolyte. capacitors[3]. Accordingly, this was our goal, and in order to achieve it, we MXenes stand out from their competitors because of the high made use of activated carbon (AC) as a ‘‘transporting” phase for volumetric capacitance[4–6], the key parameter for the industry POMs, since, in a previous study, we had shown that this dual when compactness is required. In this scenario, titanium carbides hybrid material can work in organic solvents, with properly dis- (Ti3C2Tx) is the most widely studied and developed MXene, which persed POMs with their electroactivity not limited by diffusion has been found to deliver a volumetric capacitance as high as 900F [33]. cm3 in an aqueous electrolyte[3]. Our working hypothesis was supported by our earlier work on Conventional aqueous electrolytes allow for a stable potential AC/POM hybrids, which showed proper dispersion of POM clusters window of less than 1.5 V. Since energy density is proportional through physical adsorption in micropores, leading to materials to the square of the voltage, other electrolytes like ‘‘water-in- able to operate both in aqueous or organic electrolytes[39,42,43]. salt” electrolytes (with V = 2.2 V)[7], organic electrolytes (with Likewise, ‘‘hybrid” MXene/AC flexible electrodes have also been V = 2.7 V)[8] or ionic liquids (with V > 3 V)[9,10] have been pre- reported, with activated carbon facilitating the charge transfer ferred for a rational optimized design. Indeed, several groups have from the electrode[16]. already attempted to use MXene in organic electrolytes. Some With these considerations in mind, we decided that a triple examples include MXenes for lithium-ion capacitors [11,12], hybrid material formed by MXene, AC and POM could provide a sodium-ion capacitors [13], or just supercapacitors with conven- complex but versatile system, in a rational design to produce a tional organic electrolytes[8,13–15]. Some experimental draw- multifunctional electrode for energy storage[44]. Herein, we report backs have already been described in these types of applications. the results of our work, which demonstrate that the MXene/AC/ For example, contrary to the previous hypothesis, Ti3C2Tx MXene POM system can be made to optimize the functionality of each of provides much lower specific capacitance in conventional organic the three components, leading to enhanced volumetric and gravi- electrolytes (less than 40 F g1 or 70 F cm3 in 1 M TEABF4 in ace- metric capacitances in an organic electrolyte symmetric tonitrile[16]) than in aqueous electrolytes. Various approaches supercapacitor. have been followed to enhance the energy storage capabilities of MXenes, including expanding interlayer distance by other nano- 2. Experimental materials or functional groups[5,17–22], increasing active sites by making porous MXenes[23,24], or trying to introduce active 2.1. Synthesis of MXene (Ti3C2Tx) materials[25–31], including polyoxometalates (POMs)[19,32]. However, these attempts have failed to intercalate POMs into MXene (Ti3C2Tx) was synthesized through a LiF/HCl etching MXenes and have led to mixed phases instead[32]. method[45]. Namely, 0.5 g MAX phase (Ti3AlC2, 99%, Y-Carbon POMs constitute a family of nanometric metal oxide clusters, ltd.) was added slowly into a mixture of 0.5 g LiF (>99.99%, able to provide fast reversible multi-electron redox reactions not Sigma-Aldrich) and 10 mL 9 M HCl solution (diluted from 37% limited by diffusion[33–35]. As such, POMs have been used as HCl, Sigma-Aldrich). The etching process was carried out in a active materials, just by themselves[35,36] or as active compo- capped Teflon-vessel for 24 h at 35°C. For washing and delamina- nents in hybrid materials[33,35,37–41] in many kinds of energy tion, the mixture was poured into a 50 mL centrifugation tube and 948 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 washed with deionized water by centrifugation at 3500 rpm for (SAED) patterns were obtained from an FEI Tecnai G2 F20 micro- 5 min. The supernatant was discarded. This step was repeated for scope. Volumetric N2 sorption isotherms were collected at 77 K several cycles until the pH of the dark-green supernatant (N2) using an ASAP 2020 HD (Micromeritics). Pore size distribution was > 6. The sediments at the bottom expanded upon washing, was estimated using a density functional theory (DFT) model (N2- which indicated delamination. Finally, the sediments were dis- cylindrical pores-oxide surface) implemented in the Microactive persed in water by hand-shaking and deaerated with Ar for 4.00 software with a regularization factor of 0.01. X-ray photoelec- 20 min and stored in the fridge for future use. A certain amount tron spectra were collected on Phoibos 150 probe from SPECS. Peak of the MXene colloid was filtered-off on a Celgard 3501 membrane fitting for the high-resolution spectra was performed using to determine the concentration and for further use. The as- CasaXPS Version 2.3.1. Before the peak fitting, the background prepared electrode had a thickness of 11 ± 1 lm and an areal den- was subtracted using a Shirley function. sity of 3 ± 0.3 mgcm2. 2.6. Electrochemical characterization 2.2. Synthesis of MXene/HPW12 T-type Swagelok cells were applied to fabricate three-electrode 20 mL MXene colloid (around 1 mg mL1) was mixed with configuration. The working electrodes were cut into 10 mm round 20 mL 10 mM phosphotungstic acid (H3PW12O40(HPW12), reagent pieces and the overloaded activated carbon counter electrodes grade, Sigma-Aldrich) solution, and probe-sonicated for 1 h under were cut into 12 mm round pieces. 0.01 M Ag/Ag+ served as the ref- circulating cooling water at 10 °C. The mixture was filtered-off and erence electrode. 1 M TEABF4 in acetonitrile or 1 M EMIMTFSI in washed with Milli-Q water until the filtrate was neutral. The pro- acetonitrile served as the electrolyte. CR2032 coin cells were used duct was peeled off from the membrane, dried in a vacuum oven at to fabricate asymmetric devices. The electrochemical tests were 60 °C and stored in an argon-filled glovebox. The as-prepared sam- carried out in an Argon-filled glove box with the oxygen and water ple (MXene/HPW12) could serve as a free-standing electrode. The lever under 5 ppm. as-prepared electrode had a thickness of 13 ± 1 lm and an areal The gravimetric capacitance (Cm) and the volumetric capaci- density of 3.3 ± 0.3 mg cm2. tance (Cv) were calculated from cyclic voltammograms (CVs) according to the following equations: 2.3. Synthesis of MXene/AC/HPW12 R IðV Þ  dV C¼ ð2Þ 0.05 g activated carbon (DLC Super 30 from Norit Chemical) and 2v DV 1.44 g phosphotungstic acid (HPW12) were added into 50 mL MXene colloid (around 1 mg mL1) and kept in probe-sonication C Cv ¼ ð3Þ for 1 h under circulating cooling water to keep the temperature Ve below 10 °C. Subsequently, the mixture was filtered-off onto a Cel- gard 3501 membrane to get a free-standing electrode. The as- C Cm ¼ ð4Þ prepared electrode had a thickness of 24 ± 2 lm and an areal den- m sity of 2.8 ± 0.2 mg cm2. R where IðV Þ  dV is the integral area of CVs, v is the scan rate, DV is voltage window, Ve is the volume (cm3) of the electrode and m is the 2.4. Synthesis of MXene/AC/TEAPW12 mass (g) of the electrode. The gravimetric energy density Em (Wh kg1) and volumetric Ev AC/TEAPW12 was prepared following our previously reported energy density Ev (Wh cm3), gravimetric power density Pm (W method[33]. Then, 0.05 g AC/TEAPW12 was added into 50 mL kg1) and volumetric power density P v (W cm3) were calculated MXene colloid (around 1 mg mL1) and kept in probe-sonication from the two-electrode cell discharge data using the following for 1 h under circulating cooling water to keep the temperature equations: below 10 °C. The mixture was filtered-off onto Celgard 3501 mem- brane to obtain a free-standing electrode. The as-prepared elec- C DV 2 trode had a thickness of 21 ± 2 lm and an areal density of 2. Em ¼ ð5Þ 3:6  2mtotal 4 ± 0.2 mg cm2. For comparison, MXene/TEAPW12 and MXene/AC were also C DV 2 prepared and characterized (Figs. S1, S2 and S10). Ev ¼ ð6Þ 3600  2V total 2.5. Material characterization 1000  IDV Pm ¼ ð7Þ Powder X-ray diffraction (XRD) patterns were collected on a mtotal PANalytical X’pert Pro-MRD diffractometer with Cu Ka radiationðk = 1.5406 Å) and PIXel detector. The interlayer distance (d) of the I DV Pv ¼ ð8Þ (0 0 2) planes was determined by Bragg’s law: V total k where mtotal is the total mass (g) of the two electrodes, V total is the d¼ ð1Þ 2sinh total volume (cm3) of the two electrodes, and I is the discharging current (A). where k is the X-ray wavelength,h is the the diffraction angle. Scanning electron microscopy (SEM) images were taken on a Quante 650 FEG microscope. For energy-dispersion X-ray spectra 3. Results and discussion (EDX) semi-quantitative analysis, we averaged the elemental anal- ysis of five spectra to present the final results. High resolution The crystal structure of the solid phases was characterized by transmission electron microscopy (HR-TEM) images, high angle XRD. Fig. 1 presents the XRD patterns of the pristine MXene and annular dark field scanning transmission electron microscopy derived materials. Table 1 shows the positions of (0 0 2) diffraction (HAADF-STEM) images, and selected area electron diffraction peaks and the interlayer distance. All the XRD patterns confirm the 949 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Fig. 1. (a) XRD patterns of pristine MXene, MXene/HPW12, MXene/AC/TEAPW12 and MXene/AC/HPW12. (b) Zoom-ins of (a) in the 5–10° 2h range showing (0 0 2) peaks. layered structure of the pristine MXene and the hybrid materials lowing EDX analyses in Table 2, confirm the existence of tungsten but with different interlayer distances and various crystallinity. (PW12). As can be observed, the pristine MXene has its (0 0 2) peak cen- In order to observe the morphology and evaluate the loading tred at 7.44°, and an interlayer distance of 11.87 Å, which is in the mass of PW12 clusters, the samples were characterized under typical range of LiF/HCl-etched Ti3C2Tx MXene[45]. The (0 0 2) peak the scanning electron microscope equipped with energy- of MXene/HPW12 is broader, revealing a poorer crystallinity, prob- dispersive X-ray spectroscopy. The pristine MXene (Fig. 2a) exhi- ably associated with the powerful probe-sonication that broke bits an accordion-like structure, which remains even after suffering down large nanosheets. In this case, the peak shifts towards smal- high-power probe-sonication (MXene/HPW12, Fig. 2b). For the ler two theta angles. On the other hand, the interlayer distance of MXene/HPW12 material, we cannot detect or discern any other 12.42 Å, is only slightly larger than that of the pristine MXene. heterogeneous aggregates, but the EDX spectrum confirms the Arup Chakraborty et al. have recently pointed out that even small presence of PW12 clusters, not apparent most likely due to the anions such as Cl– and Br– are difficult to insert into MXenes[46]. small nanosize of the PW12 clusters (1 nm). MXene/AC/HPW12 Since PW12 anions are 1 nm in diameter, significantly larger than (Fig. 2c) and MXene/AC/TEAPW12 (Fig. 2d) exhibit a similar mor- pre-intercalated Li+, we conclude that the PW12 clusters are not phology, different from pristine MXene with the particles spread- intercalated into MXene but are mainly anchored on the surface. ing on or surrounded by nanosheets. Since in the XRD patterns of The XRD pattern of MXene/AC/HPW12 presents a broad peak at MXene/AC/HPW12 and MXene/AC/TEAPW12 we cannot discern around 6.91°, corresponding to a slightly larger interlayer distance any diffraction peaks except those from MXene, we confirmed that (12.80 Å) compared with MXene. The AC particles are micron scale. the particles were amorphous AC, with dispersed, not well- Thus, neither AC nor PW12 clusters can insert MXene layers. The crystallized PW12 clusters. We have previously shown that AC is slight variation of interlayer distance is supposed to be associated an ideal matrix for absorbing PW12 and spreading them in nano- with the fact that high-power sonication slightly increases inter- pores[39]. We therefore propose that most PW12 clusters are layer distance[47]. Furthermore, the sonication during the prepara- absorbed on AC in these triple hybrid materials, as proven in the tion and the amorphous AC reduced the overall crystallinity of the EDX spectra and HAADF-STEM images, shown in the following final material. For the same reasons, the XRD pattern of MXene/AC/ Table 2 and Fig. 4. TEAPW12 presents a similarly broad peak, with minor shifting Table 2 lists the weight percentage of tungsten and titanium with respect to the pristine MXene. The XRD patterns of the hybrid derived from EDX spectra. MXene/HPW12 only contains 6 wt% materials (MXene/HPW12, MXene/AC/HPW12 and MXene/AC/ tungsten even with the assistance of high-power sonication, TEAPW12) do not exhibit any other peaks corresponding to revealing that MXene is not a good substrate for direct-anchoring HPW12 nor TEAPW12. This is very significant since it rules out of POMs. That is most likely the reason why most previous studies the inclusion of the POMs as crystallized extended phases. Instead, had to resort to linking groups[48,49] or in-situ synthesis[32]. XRD results are consistent either with the absence of POMs in the Indeed, through the in-situ synthesis (synthesis of MXene/ hybrid or with POM clusters being dispersed individually or at TEAPW12, in supplementary materials), we could load a large most in groups small enough to prevent bulk diffraction. The fol- amount of TEAPW12 on MXene (Table S1). As for MXene/AC/ HPW12, improvement in the loading mass of PW12 must be ascribed to the inclusion of AC in the material providing a microp- orous matrix for anchoring POMs. MXene/AC/TEAPW12 contains Table 1 less W than Mxene/AC/HPW12 because we carried out an ex-situ The positions of (0 0 2) peaks and their corresponding interlayer space derived from synthesis, in which TEAPW12 clusters had been already immobi- the XRD patterns. lized in AC accounting for 37.5 wt% in AC/TEAPW12 precursor. Sample 2h values for (0 0 2) diffraction Interlayer peaks/° distance/Å Table 2 MXene 7.44 11.87 Semi-quantitative analysis result from EDX spectra. MXene/HPW12 7.09 12.42 Sample Ti/ wt. % W/ wt. % MXene/AC/ 6.91 12.80 TEAPW12 Mxene/HPW12 51.2 ± 3.6 6.1 ± 1.1 MXene/AC/ 6.95 12.71 Mxene/AC/HPW12 31.4 ± 2.1 17.5 ± 2.0 HPW12 Mxene/AC/TEAPW12 39.7 ± 2.8 13.5 ± 1.8 950 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Fig. 2. SEM images of (a) MXene, (b) MXene/HPW12, (c) MXene/AC/HPW12 and (d) MXene/AC/TEAPW12. (e) N2 sorption isotherms and (f) pore size distribution curves of MXene, MXene/HPW12 and MXene/AC/TEAPW12. Since the SEM images reveal a remarkable expansion of the addition of HPW12 leads to a decrease of specific surface area space among MXene nanosheets due to the AC particles, we car- (from 15 m2 g1 to 7.8 m2 g1), the pore structure remains the ried out volumetric N2 sorption experiments to investigate the same (Fig. 2f). This can be explained by the heavy molecular influence of the inclusion of HPW12 or AC/TEAPW12 on the por- weight of HPW12 as well as the fact that HPW12 clusters occupy ous properties of MXene. Fig. 2e presents the nitrogen sorption some mesopores. isotherms of MXene, MXene/HPW12 and MXene/AC/TEAPW12. In order to compare the surface composition, bonding forming MXene/AC/TEAPW12 exhibits a significantly larger absorbed vol- and element valence, we carried out XPS analysis on the pristine ume. The specific surface area of MXene/AC/TEAPW12 is 499 m2 MXene, the double hybrid (MXene/HPW12) and the triple hybrid g1, 30 times larger than pristine MXene (15 m2 g1), which we material (MXene/AC/TEAPW12). The low-resolution survey spectra ascribe to the microporous nature of AC/TEAPW12 (specific sur- (Fig. 3a) show that in addition to elements Ti, C, F, Cl and O that face area = 1007 m2 g1)[33]. The pore-size distribution (Fig. 2f) compose MXene, the hybrid materials (MXene/HPW12 and reveals that MXene/AC/TEAPW12 exhibits a hierarchical porous MXene/AC/TEAPW12) contain element W (Fig. 3a inset, W 4d structure characterized by the dominance of AC micropores and peaks). The elemental compositions calculated by XPS are pre- the development of the mesopores inherited from MXene. While sented in Table S2, which shows that MXene/AC/TEAPW12 has a the inclusion of AC/TEAPW12 promotes mesopore formation, the higher W atomic ratio than MXene/HPW12. 951 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Fig. 3. (a) XPS full spectrum of MXene, MXene/HPW12 and MXene/AC/TEAPW12 (inset is the zoom-in area to discern W 4d peaks). C 1 s spectra of (b) MXene, (c) MXene/ HPW12 and (d) MXene/AC/TEAPW12. Ti 2p spectra of (e) MXene, (f) MXene/HPW12 and (g) MXene/AC/TEAPW12. (g) W 4f spectra of MXene/HPW12 and MXene/AC/ TEAPW12 (Ti 3p overlap). 952 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Fig. 3b-d illustrate the C 1 s spectra of MXene, MXene/HPW12 In some areas, the nanosheets are covered by a thin, bright layer, and MXene/AC/TEAPW12. The pristine MXene and MXene/ corresponding to PW12 confirmed by EDX (Fig. S4). We conclude HPW12 show three peaks located at 281.85, 284.8 and 288.1 eV, that MXene is not an effective substrate for dispersing PW12 clus- which are matched to C–Ti, CAC and CAO, respectively. The ters because aggregation happens even in low loading mass. On the MXene/AC/TEAPW12 shows an extra peak at 285.3 eV, which we other hand, the dispersion states of PW12 clusters in triple hybrid attribute to the CAC bonds in AC. Fig. 3e-g depict the Ti 2p spectra materials MXene/AC/HPW12 (Fig. 4c) and MXene/AC/TEAPW12 of MXene, MXene/HPW12 and MXene/AC/TEAPW12, in which the (Fig. 4d) are quite different from the MXene/HPW12. The bright Ti–C (2p3/2 455.0 eV, 2p1/2 461.1 eV), Ti(II) (2p3/2 455.8 eV, 2p1/2 dots, around 1 nm (as confirmed by EDX element mapping and line 461.5 eV), Ti(III) (2p3/2 457.0 eV, 2p1/2 462.9 eV) and Ti(IV)(2p3/2 scan in Fig. S5), spread homogeneously on the substrate, indicating 458.6 eV, 2p1/2 464.4 eV) peaks agree well with results from previ- that the PW12 clusters are well-dispersed at the nanoscale. Fig. 4e- ous research[50,51]. Ti(II) and Ti(III) are associated with surface f present the STEM image and the EDX element mapping in an terminating groups, while Ti(IV) is associated with TiO2. MXene/ extended area, in which we can discern the MXene nanosheets sur- HPW12 presents less intensive Ti(II) peaks and more intensive Ti rounding the AC particles loaded with PW12 clusters. The SAED (III) peaks than MXene, implying the inclusion of the small amount patterns (Fig. S6) prove the triple hybrid materials have mixed of HPW12 changed the valence of Ti. HPW12 is anchored on phases: crystallized MXene, amorphous AC and uncrystallized MXene through chemical bonds. By contrast, the Ti 2p spectrum PW12 clusters. This is in good agreement with the fact that of MXene/AC/TEAPW12 does not show such significant change MXene/AC/HPW12 and MXene/AC/TEAPW12 do not show any except for stronger Ti(IV) peaks, which should be ascribed to the diffraction peaks other than those of MXene in XRD. fact that AC is just physically constrained among MXene With the images of SEM and STEM, we illustrate how the three nanosheets and TEAPW12 are pre-immobilized on AC. The stronger components combine in MXene/AC/TEAPW12 (Fig. 5). First, the Ti(IV) peaks in MXene/HPW12 and MXene/AC/TEAPW12 must be PW12 nanoclusters are anchored on microporous AC matrix and associated with the slight oxidation due to the high-power dispersed at nanoscale. Then, the PW12-anchored AC particles probe-sonication. We also collected high-resolution spectra from mix with flexible MXene nanosheets and stack together under 25 to 45 eV to investigate the W 4f orbital in hybrid materials. vacuum-assisted filtration. Finally, we obtain the PW12-anchored However, since the binding energy of Ti 3p is very close to W 4f, AC wrapped by MXene nanosheets. the W 4f7/2 and 4f5/2 peaks are deformed. Nonetheless, it is still Cyclic voltammograms were carried out to characterize the possible to discern the two main peaks at 36.1 and 38.2 eV, agree- electrochemical properties of the materials, as well as to determine ing well with W(Ⅵ) 4f7/2 and 4f5/2 peaks in PW12 (Fig. S2d). the capacitance of the various electrodes studied and to analyze HAADF-STEM is an effective method to investigate the disper- the energy storage mechanisms. The CV of MXene in 1 M TEABF4 sion of heavy atoms, particularly the PW12 clusters. Fig. 4a-d pre- in acetonitrile (Fig. 6a) is predominantly capacitive (ideally rectan- sent HAADF-STEM images of pure MXene and these hybrid gular shape) but superposed a pair of broad redox waves. The samples. The corresponding HR-TEM images are presented in reduction wave centred at –1.75 V vs. Ag/Ag+ must be ascribed to Fig. S3. Under HAADF-STEM, MXene (Fig. 4a) presents a homoge- the intercalation of TEA+. The corresponding oxidation peak, cen- neous texture, while MXene/HPW12 (Fig. 4b) is slightly different. tred at –1.48 V vs. Ag/Ag+, is broader and weaker, indicating the Fig. 4. HAADF-STEM images of (a) MXene, (b) MXene/HPW12, (c) MXene/AC/HPW12 and (d) MXene/AC/TEAPW12. (e) STEM image at low magnification and (f) the corresponding element mapping of MXene/AC/TEAPW12. 953 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Fig. 5. Schematic illustration of the combination of TEAPW12, AC and MXene. (b) Schematic illustration of the working mechanism of MXene/AC/TEAPW12. intercalation–deintercalation process is not 100% reversible. This iðVÞ ¼ k1 v 1=2 þ k2 ð9Þ agrees well with the XRD patterns of cycled MXene, in which we v 1=2 observe a significant shifting of (0 0 2) peak to low angle (Fig. S7). In the CV of MXene/HPW12 (Fig. 6a), besides the broad waves from where iðVÞ is the current at the given potential V, v is the scan rate. MXene, we can discern some new small peaks from PW12 clusters. k1 is a variable characterizing the contribution from surface capac- Since the loading mass of HPW12 is very low, these peaks are weak itive processes (e.g. pseudocapacitance, electric double layer capac- and their contribution to the capacitance is not very substantial. itance), and k2 is the other variable representing the contribution The CV of MXene/AC/TEAPW12 (Fig. 6b) mainly shows four pairs from diffusion-controlled processes (e.g. intercalation–deintercala of redox waves, similar to AC/TEAPW12[33]. Since TEAPW12 is tion, sluggish redox reactions) at a given potential. k1 and k2 can immobilized on AC in advance, MXene/AC/TEAPW12 presents be determined by a linear fitting of Eq9. redox waves at the same positions. Fig. 5(b) illustrates how the tri- Fig. 6c shows the contribution of the diffusion-controlled pro- ple hybrid MXene/AC/TEAPW12 works. The MXene nanosheets and cess versus the surface capacitive process to the overall capaci- porous AC particles construct the framework for electron flow. The tance of MXene/AC/TEAPW12 from 0.2 to 2 mV s1. The surface hierarchical porous structure facilitates ion diffusion. As a result, all capacitive process dominates the energy storage process in the three components contribute to electrochemical energy storage: whole range. At 0.2 mV s1, the surface capacitive process accounts MXene and TEAPW12 store charges through reversible redox reac- for 78.9% of total capacitance; when the scan rate rises to 2 mV s1, tions; AC store charges through electric double-layer capacitance. this ratio increases to 91%. With the rise of the scan rate, the con- On the other hand, the situation is more complicated for tribution of surface capacitance increases, revealing that the sur- MXene/AC/HPW12. The CV of MXene/AC/HPW12 (Fig. 6b) shows face capacitive process prevails at faster scan rates. This trend is more than six pairs of redox waves. In principle, PW12 clusters common in many other hybrid materials[53,54]. MXene/AC/ only present three or four pairs of redox waves in this potential TEAPW12 maintains a higher surface capacitive contribution in range (more redox pairs are possible in a wider potential range, the whole range, mainly benefiting from the excellent dispersion but they are not 100% reversible). The extra redox waves must be of PW12 clusters. associated with different anchoring states: PW12 clusters anchor- Fig. 6d presents the gravimetric capacitance of MXene, MXene/ ing on AC; and PW12 clusters anchoring on MXene (The classifica- HPW12, MXene/AC/HPW12 and MXene/AC/TEAPW12 in 1 M tion of these redox waves is presented in Fig. S8a-c). TEABF4 in acetonitrile at various scan rates. MXene and MXene/ To investigate the energy storage mechanism, we carried out CVs HPW12 almost deliver the exact equivalent gravimetric capaci- at various scan rates (Fig. S9). The dependence of the current tance because the loading of PW12 on MXene/HPW12 is very response on the scan rate can provide insights into the charge- low. MXene/AC/HPW12 and MXene/AC/TEAPW12 exhibit signifi- storage mechanism according to Eq9 as suggested by John Wang cantly higher gravimetric capacitance, thanks to AC with high et al. [52]: gravimetric capacitance. Compared with the hybrid material 954 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Fig. 6. (a) Cyclic voltammograms of MXene and MXeneHPW12 at 1 mV s1 in 1 M TEABF4 acetonitrile. (b) Cyclic voltammograms of MXene/AC/HPW12 and MXene/AC/ TEAPW12 at 1 mV s1 in 1 M TEABF4 acetonitrile. (c) Surface capacitive contribution and diffusion-controlled contribution at various scan rates. (d) Gravimetric capacitance of MXene, MXene/HPW12, MXene/AC/HPW12 and MXene/AC/TEAPW12 at various scan rates. (e) Electrochemical impedance spectra of MXene, MXene/HPW12, MXene/AC/ HPW12 and MXene/AC/TEAPW12. (f) Equivalent circuit for fitting impedance spectra. MXene/AC of the same weight ratio (MXene/AC 1:1, 88F g1 at 0.1 (Fig. 6f). The fitted values are presented in Table S4. The intercepts A g1[16], or 77F g1 at 1 mV s1(Fig. S10)), MXene/AC/TEAPW12 at the very high-frequency region represent equivalent series resis- could deliver the equivalent gravimetric capacitance. The addition tance (Rs), which is a combination of the ionic resistance of the of TEAPW12 does not improve the gravimetric capacitance due to electrolyte, the intrinsic resistance of the active materials, and the large molecular weight of PW12 clusters (>2800 g mol1). On the contact resistance at the active material/current collector inter- the other hand, as other studies show, the anchoring of TEAPW12 face. Since all the electrodes were characterized in the same setup, can improve the volumetric capacitance, instead of the gravimetric their Rs are similar. The arc at high frequency represents charge- capacitance in the same electrolyte[33]. Due to the porous nature transfer resistance Rct. The following transitory parts between of AC, MXene/AC delivers 33% lower volumetric capacitance than the semicircles and the final linear parts represent the Warburg that of MXene[16]. In contrast, in our study, the volumetric capac- impedance, which is associated with mass transfer (diffusion). itance of MXene/AC/TEAPW12 is only around 10% lower than pris- The Rct of MXene and MXene/HPW12 are smaller (around 50%) tine MXene (Fig. S10), but much higher than MXene/AC[16], AC/ than MXene/AC/HPW12 and MXene/AC/TEAPW12, but the diffu- TEAPW12[33] and AC[33]. sion impedance values are significantly larger (5 times), revealing To investigate the electrochemical response as well as diffusion that different steps restrict the charge store process. In MXene properties of the triple hybrids, we carried out impedance tests and MXene/HPW12, due to the high electronic conductivity of (Fig. 6e). All the spectra are fitted by a modified Randles circuit MXene nanosheets, the active sites which have already been 955 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 exposed to the electrolyte can carry out the charge transfer process whole range, but the diffusion-controlled process starts to play a quickly, leading to a relatively smaller Rct. After that, ions have to bigger role. At 0.2 mV s1 ,the diffusion-controlled process diffuse through the slits between nanosheets, or even insert layers accounts for 43.5%, more than double that in 1 M TEABF4 in ace- to reach the active sites. As a result, their diffusion impedance val- tonitrile. The remarkably increased ratio of the diffusion- ues are extremely large. By contrast, MXene/AC/HPW12 and controlled contribution proves the deeper involvement of cation MXene/AC/TEAPW12 have larger Rct due to the inferior electronic intercalation–deintercalation in MXene, thus, MXene and MXene/ conductivity of AC, but the diffusion is greatly promoted thanks AC/TEAPW12 could deliver more capacitance in 1 M EMIMTFSI in to the hierarchical porous structure. acetonitrile. In general, the electrochemical performance of any electrode is We charge-discharged MXene/AC/TEAPW12 for 10,000 cycles in related to its thickness, especially for MXenes. Normally, thicker both electrolytes (Fig. 7d) and ran CVs before and after cycling MXene electrodes lead to remarkably lower volumetric capaci- (Fig. 7e and f). MXene/AC/TEAPW12 shows excellent cycle stability tance[4]. In our case, on the contrary, the increased porosity of in 1 M TEABF4 acetonitrile. Its capacitance almost remains the our hybrid material leads to a thicker electrode (as shown by same after 10,000 cycles. Furthermore, the CV after 10,000 cycles SEM) but with improved capacitance. We chose to compare pris- exhibits four pairs of redox waves. Their current densities even tine MXene with hybrid electrodes keeping their masses (areal stay the same, indicating the anchoring and redox activities of densities) comparable, rather than their thicknesses. PW12 clusters do not change. The excellent cycle stability and The combination of the three components has a synergistic long-term anchoring of PW12 clusters must be ascribed to the effect in that several merits are merged in the MXene/AC/ AC matrix, which has been proved to be an ideal substrate for TEAPW12 triple hybrid. We attempted to estimate the overall anchoring POMs[35,39,43]. In contrast, the in-situ growth capacitance contributed from each component according to the MXene/TEAPW12 show much worse cycle stability (Fig. S8d) in composition and individual specific capacitance. For instance, at the same electrolyte. The redox waves almost disappear after 1 mV s1 the gravimetric capacitance of MXene/AC/TEAPW12, 1000 cycles (Fig. S8e). Other previously reported MXene/POM MXene and AC/TEAPW12 is 76F g1, 40F g1 and 90F g1, respec- hybrid materials cannot give very long-term cycle stability[32]. tively. Since MXene and AC/TEAPW12 each account for half the Thus, AC is the key to the excellent cyclic performance of the triple mass of MXene/AC/TEAPW12, a MXene:AC/TEAPW12 = 1:1 mix- hybrid materials. In 1 M EMIMTFSI in acetonitrile, the capacitance ture would have a gravimetric capacitance of 65F g1, lower than of MXene/AC/TEAPW12 increases by 7% after 10,000 cycles, which the MXene/AC/TEAPW12 hybrid material. The extra capacitance could be associated with the expanded interlayer distance due to must be ascribed to the fact that AC/TEAPW12 promotes the por- cation intercalation during cycling[11,55]. However, the deep ous properties of MXene, which is a part of the synergistic effect. cation intercalation is not problem-free. Firstly, this cation Furthermore, the MXene provides a conductive substrate with high intercalation–deintercalation is not 100% reversible, which is volumetric capacitance. AC provides a microporous matrix for reflected by the fact that in 1 M EMIMTFSI in acetonitrile the TEAPW12 anchoring, and also contributes to high gravimetric Coulombic efficiency is lower because more cations involve in capacitance. TEAPW12 clusters could be nanodispersed and dur- intercalation. Moreover, the long-term, repeating intercalation–d ably anchored on AC, providing extra-fast reversible redox reac- eintercalation could damage the microstructure and worsen the tions not limited by diffusion, and compensating the sacrificed conductivity, which is revealed by CVs (Fig. 7f). The electrode after volumetric capacitance due to the addition of AC. Finally, we were 10,000 cycles suffers larger resistance. The electrochemical impe- able to obtain a well-balanced triple hybrid material with both dance spectra further confirm it. high gravimetric and volumetric capacitances in a conventional Fig. 7g compares the impedance spectra of MXene/AC/ organic electrolyte. TEAPW12 before and after cycled in 1 M TEABF4 in acetonitrile. The capacitance of MXene in organic electrolytes can be The the spectrum was fitted by the same equivalent circuit improved by fine tuning of the ions used in the electrolyte. Thus, (Fig. 6f) and the fitting values are presented in Table S5. The two EMIMTFSI has flatter cations than TEABF4, and the cations could spectra almost overlap. The zoom-in figure (Fig. 7g inset) presents intercalate into MXene more easily. Accordingly, MXene can deli- the slight different, in which we can only discern the slightly larger ver higher capacitance in the EMIMTFSI/acetonitrile electrolyte arc, corresponding to Rct. By contrast, the impedance spectrum of [8]. We compared the electrochemical behaviors of MXene/AC/ MXene/AC/TEAPW12 cycled in 1 M EMIMTFSI in acetonitrile HPW12 and MXene/AC/TEAPW12 in 1 M EMIMTFSI in acetonitrile (Fig. 7h) is significantly different from the fresh one. The intercept and 1 M TEABF4 in acetonitrile (Fig. 7a). The current densities of on X-axis (representing Rs) stays the same, but the arc (Rct) is the CVs in 1 M EMIMTFSI in acetonitrile are remarkably higher remarkably larger and the followed linear transition zoom (related than those in 1 M TEABF4 in acetonitrile, indicating a larger capac- to diffusion impedance) is remarkably longer, revealing that the itance. The CVs of MXene/AC/HPW12 and MXene/AC/TEAPW12 charge-transfer and diffusion processes are impeded in the cycled show more than one pair of redox waves in 1 M EMIMTFSI in ace- electrode. The impedance spectra confirm that long-term tonitrile, implying the PW12 clusters are still redox active in this intercalation–deintercalation in 1 M EMIMTFSI in acetonitrile electrolyte. Fig. 7b presents the gravimetric capacitance of MXene, could damage the microstructure, hindering ion diffusion and lead- MXene/AC/HPW12 and MXene/AC/TEAPW12 at various scan rates. ing to high resistance. MXene/AC/TEAPW12 still delivers the largest gravimetric capaci- Table 3 compares the capacitive performance of the triple tance (102 F g1 at 1 mV s1) among the three, almost double that hybrid material MXene/AC/TEAPW12 and the double hybrid mate- of MXene (52 F g1 at 1 mV s1), and also higher than the gravimet- rials MXene/AC, AC/TEAPW12 and MXene/TEAPW12 in 1 M ric capacitance of MXene/AC/TEAPW12 (76 F g1 at 1 mV s1) in TEABF4 in acetonitrile, which demonstrates the superiority of the 1 M TEABF4 in acetonitrile. The enhancement of capacitance triple hybrid material over double hybrid materials. Compared derives from the cation intercalation–deintercalation in the energy with MXene/AC and AC/TEAPW12, MXene/AC/TEAPW12 shows storage process. Since the cation intercalation–deintercalation is enhanced volumetric capacitance without sacrificing gravimetric usually diffusion-controlled, we carried out the CVs at various scan capacitance and cycle stability. Meanwhile, the defects of MXene/ rates to analyze the contribution from these two mechanisms. TEAPW12, i.e. poor cycle stability and low gravimetric capacitance, Fig. 7c presents the contribution of the diffusion-controlled and are overwhelmed in MXene/AC/TEAPW12. surface capacitive processes from 0.2 mV s1 to 2 mV s1. The sur- We assembled asymmetric supercapacitors to compare the face capacitive process still dominates the energy storage in the potential application of the MXene/AC/TEAPW12 triple hybrid 956 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Fig. 7. (a) CVs of MXene/AC/HPW12 and MXene/AC/TEAPW12 at 1 mV s1 in 1 M TEABF4 in acetonitrile or 1 M EMIMTFSI in acetonitrile; (b) Gravimetric capacitance of MXene, MXene/AC/HPW12 and MXene/AC/TEAPW12 in 1 M EMIMTFSI in acetonitrile; (c) Diffusion-controlled contribution and surface capacitive contribution at various scan rates; (d) Cycling stability test: capacitance retention and Coulombic efficiency of MXene/AC/TEAPW12 in the two electrolytes (current density: 2 A g1). (e) CVs of MXene/AC/TEAPW12 in 1 M TEABF4 in acetonitrile before and after cycling; (f) CVs of MXene/AC/TEAPW12 in 1 M EMIMTFSI in acetonitrile before and after cycling. Electrochemical impedance spectra of MXene/AC/TEAPW12 before and after cycling in (g) 1 M TEABF4 in acetonitrile and (h) 1 M EMIMTFSI in acetonitrile. materials (AC//MXene/AC/TEAPW12) and pristine MXene (AC// Fig. 8a presents the CVs of the positive (AC) and negative MXene) in full-cell. The conventional AC electrodes served as the (MXene/AC/TEAPW12) electrodes in the respective potential range, positive electrode. 1 M TEABF4 in acetonitrile served as the elec- in which the negative electrode is charged to –2.2 V, and the pos- trolyte. The details about preparing AC electrodes and charge bal- itive electrode is charged to + 0.5 V. As a result, a voltage window ance calculation are presented in supplementary materials. of 2.7 V can be achieved in the full cell. Cyclic polarization (CP) was 957 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 Table 3 Comparison of the capacitive performance of the triple hybrid material MXene/AC/TEAPW12 and the double hybrid materials MXene/AC, AC/TEAPW12 and MXene/TEAPW12. Gravimetric capacitance Volumetric capacitance Capacitance retention Reference MXene/AC/TEAPW12 87F g1 at1 mV s1 76F cm3 at 1 mV s1 102% after 10,000 cycles this work MXene/ACa 88F g1 at 0.1 A g1 30F cm3 at 0.1 A g1 92% after 10,000 cycles [16] AC/TEAPW12b 92F g1 at1 mV s1 64F cm3 at 1 mV s1 93% after 10,000 cycles [33] MXene/TEAPW12 36F g1 at1 mV s1 120F cm3 at 1 mV s1 80% after 1,000 cycles this work a, MXene account for 50 wt% in MXene/AC. b, it is not a free-standing electrode, the mass of the binder, conductive additive and current collector are not included in calculating the gravimetric capacitance. Fig. 8. (a) CVs of the positive and negative electrodes in AC//MXene/AC/TEAPW12 asymmetric cell. Galvanostatic charge–discharge curves of (b) AC//MXene and (c) AC// MXene/AC/TEAPW12 cells at various current densities. (d) Gravimetric and (e) volumetric Ragone plot of AC//MXene and AC//MXene/AC/TEAPW12 cell. (f) Cycling stability test: capacitance retention and Coulombic efficiency of AC//MXene and AC//MXene/AC/TEAPW12 cell during charge–discharge at 1 A g1 for 10,000 cycles. carried out on both asymmetric cells at various scan rates (from MXene/AC/TEAPW12 cell. The specific capacitance normalized by 2 mV s1 to 1 V s1, Fig. S11a-d). The CP curves of AC//MXene/ the total mass of two electrodes at various scan rates is presented AC/TEAPW12 present several weak waves during the charging in Fig. S11e. and discharging processes, deriving from the redox reaction of Galvanostatic charge–discharge (GCD) was carried out to evalu- TEAPW12. At high scan rates (>100 mV s1), the CP curves of ate the performance of the asymmetric cells in practical applica- AC//MXene/AC/TEAPW12 remain rectangular while those of AC// tion. The GCD curves of AC//MXene (Fig. 8b) diverge from the MXene do not, indicating the better rate capability of AC// ideal triangular shape due to the sluggish intercalation–deintercala 958 Jun-Jie Zhu, A. Hemesh, Jordi Jacas Biendicho et al. Journal of Colloid and Interface Science 623 (2022) 947–961 tion of cations in MXene. In contrast, the GCD curves of AC// prepared MXene/AC/TEAPW12 triple hybrid material delivered MXene/AC/TEAPW12 (Fig. 8c) are triangular, which agrees with much larger gravimetric capacitance ((87 F g1 at 1 mV s1)) than the previous single electrode analysis: MXene/AC/TEAPW12 MXene (40 F g1 at 1 mV s1) in conventional organic electrolyte mainly stores charges through surface capacitive processes. The without sacrificing much volumetric capacitance (less than 10%). gravimetric capacitance of the asymmetric cells at various current Even compared with MXene/AC of the same mass ratio, the gravi- densities or scan rates is presented in Fig. S11f. The AC//MXene/AC/ metric capacitance of MXene/AC/TEAPW12 was almost equivalent TEAPW12 surpasses AC//MXene, especially at high rates. and the volumetric capacitance was 1.5 times higher. The capaci- Fig. 8d-e present Ragone plots normalized by the total mass or tive performance can be further improved by replacing spherical volume of two electrodes, respectively. The data in these plots TEA with flat EMIM cations in the electrolyte. The asymmetric cell demonstrate that the hybrid AC//MXene/AC/TEAPW12 features AC//MXene/AC/TEAPW12 outperformed AC//MXene cell concern- higher gravimetric energy densities as well as higher volumetric ing both gravimetric (4.6 times higher at 13.5 kW kg1) and volu- energy densities compared with AC//MXene. In the gravimetric metric energy densities (3.5 times higher at around 13 W cm3) Ragone plot, AC//MXene/AC/TEAPW12 delivers an energy density and featured very good cycle stability (92% after 10,000 cycles). of 14.2 Wh kg1 at a power density of 540 W kg1, 25% higher than The work shed light on the synergic effects in the triple hybrid AC//MXene. At high rates, the superiority of AC//MXene/AC/ materials. We can further improve gravimetric and/or volumetric TEAPW12 is remarkable. The energy density of AC//MXene/AC/ capacitance by using other POMs, fine-tuning composition ratios, TEAPW12 is 5.6 times that of AC//MXene at 13.5 kW kg1. Mean- etc. while, even normalized by volume, AC//MXene/AC/TEAPW12 does not fall behind. At low power densities, AC//MXene/AC/TEAPW12 CRediT authorship contribution statement delivers almost equivalent volumetric energy densities (0.012 Wh cm3 at 0.23 W cm3) compared with AC//MXene (0.013 Wh Jun-Jie Zhu: Methodology, Conceptualization, Formal analysis, cm3 at 0.29 W cm3). At high power densities, AC//MXene/AC/ Investigation, Writing – original draft. Avireddy Hemesh: TEAPW12 outperforms AC//MXene significantly as well (8.5 Resources, Conceptualization, Formal analysis, Project administra- mWh cm3 at 11.5 W cm3 for AC//MXene/AC/TEAPW12, 1.9 tion. Jordi Jacas Biendicho: Software, Writing – review & editing. mWh cm3 at 14.4 W cm3 for AC//MXene). The advantage of Luis Martinez-Soria: Resources, Data curation. Daniel Rueda- AC//MXene/AC/TEAPW12 lies in its excellent rate capability of Garcia: Formal analysis, Resources. Joan Ramon Morante: Writing the asymmetric cell that must be associated with the merits of – review & editing, Funding acquisition. Belen Ballesteros: Formal the hybrid MXene/AC/TEAPW12 electrode. That is, AC particles analysis, Data curation. Pedro Gomez-Romero: Writing – review & suppress restacking of MXene flakes, promoting ion diffusion and editing, Supervision, Funding acquisition. TEAPW12 clusters are dispersed homogeneously on AC, able to conduct fast reversible redox reaction. Both of them lead to excel- Declaration of Competing Interest lent rate capability. We also charge-discharged both asymmetric cells at 1 A g1 for The authors declare that they have no known competing finan- 10,000 cycles to evaluate their cycle stability (Fig. 8f). As we have cial interests or personal relationships that could have appeared proved the excellent cycle stability of MXene/AC/TEAPW12 single to influence the work reported in this paper. electrode in a wide potential range (Fig. 5d-e), the AC//MXene/ AC/TEAPW12 cell is expected to have good cycle stability. After 10,000 cycles, 92% of its initial capacitance remained and although Acknowledgements the AC//MXene cell also exhibits good cyclability, its cycle stability decreased faster (88% remaining after 10,000 cycles). The Coulom- Partial funding from Ministry of Science, Innovation and bic efficiencies of both cells increased at the first few cycles, then Universities (MCIU), the State Research Agency (AEI) and the Euro- stayed over 99.5%. We performed post-mortem analyses pean Regional Development Fund (FEDER) (grant RTI2018-099826- (Fig. S12) on the cycled MXene/AC/TEAPW12 electrode. The mor- B-I00) and AGAUR (2017 SGR 00870, 2017 SGR 1246) are gratefully phology of the cycled MXene/AC/TEAPW12 is as same as the fresh acknowledged. ICN2 and IREC are funded by the CERCA pro- one, and the EDX spectrum indicates the existence of W. The long- gramme / Generalitat de Catalunya, and ICN2 is also supported term cycling did not damage the porous structure and the anchor- by the Severo Ochoa Centres of Excellence programme, funded ing of POMs. The good cycle stability can be ascribed to both the by the Spanish Research Agency (AEI, grant no. SEV-2017-0706). stability of active materials and the proper charge balance and J.J Z. acknowledges his scholarship (No. 201806370211) under the fact that the potential of the two electrodes is not taken beyond China Scholarship Council. We thank Haizea Rodriguez Sagasti the stable potential window of the electrolyte. for help with the programming to calculate the contribution from different energy storage mechanisms. This work has been carried out within the framework of doctoral program (PhD) of Material 4. Conclusion Science (Department of Physics) of Universitat Autònoma de Barce- lona (UAB). In this work, we did design and prepared for the first time a MXene/activated carbon (AC)/polyoxometalate(POM) hybrid Appendix A. Supplementary material material which shows synergic effects, making it possible an effec- tive, functional and long-lasting incorporation of POMs to MXene. Supplementary data to this article can be found online at Our work also has shown how to modulate the properties of a final https://doi.org/10.1016/j.jcis.2022.04.170. electrode by choosing wisely the properties of the components and the final composition. Thus, MXene plays the role of a conductive References scaffold, provides high volumetric capacitance and allows for fab- ricating free-standing electrodes. AC serves as the matrix for [1] D.P. Dubal, O. Ayyad, V. Ruiz, P. Gomez-Romero, Hybrid energy storage: the anchoring POMs, provides high gravimetric capacitance and merging of battery and supercapacitor chemistries, Chem. Soc. 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