Shaohua Chen

Shu Fen Tan

Harpreet Singh

Liang Liu

Mathieu Etienne

Pooi See Lee
email: pslee@ntu.edu.sg

Functionalized MXene Films with Substantially Improved Low-voltage Actuation

Keywords: MXene, tetrabutylammonium, ionic actuators, artificial muscles, liquid cell TEM

Ti3C2Tx MXene film is promising for low-voltage electrochemical actuators due to its excellent electrical conductivity, volumetric capacitance, and mechanical properties. However, its in-plane actuation is limited to little intralayer strain of MXene sheets under polarization. Here we demonstrate that a simple tetrabutylammonium (TBA) functionalization of MXene improves the in-plane actuation strain by 337% and also enhances the mechanical property and stability in air and the electrolyte. Various in situ characterizations reveal that the improved actuation is ascribed to the coinsertion/desertion of TBA and Li ions into/from MXene interlayer galleries and inter-edge gaps that causes a large in-plane sliding of MXene sheets under negative/positive polarizations. The assembled bending actuator has a high strength and modulus and generates a peak-to-peak strain difference of 0.771% and a blocking force up to 51.5 times its own weight under 1 V. The designed soft robotic tweezer can grasp an object under 1 V and hold it firmly under 0 V. The novel sheet sliding mechanism resembling the filament sliding theory in skeletal muscles may inspire the design of highperformance actuators with other nanomaterials.

Introduction

Electrochemical actuators (ECAs, also called ionic actuators), consisting of three/two electrodes and an electrolyte, can produce a deformation/force when a low voltage (usually < 5 V) is applied. Compared to many other actuators (e.g., pneumatic, moisture, chemical, dielectric, or thermal ones) [1- 8] , ECAs have many advantages such as high safety (low-voltage and low-temperature operation), lightweight, small system size, good softness, and easy control [9][10] . Hence, they are promising for a variety of applications such as tactile displays [11] , bionic devices [12] , and soft grippers [13] . The actuation of ECAs originates from electrode volume change induced by the electron/hole injection and ion insertion/desertion as well as possible deformation of the polymer electrolyte membrane [14- 17] . The key component is the active material in the electrodes, which requires high volumetric capacitance and electrical conductivity [10] . Various materials have been explored, including conductive polymers [9,14,[18][19] , noble metal nanoparticles [20][21] , and carbon nanomaterials (e.g., carbon nanotubes-CNTs). [15,[22][23] Recently, there is a growing interest in two-dimensional (2D) nanomaterials as ECA electrodes due to their ultrahigh specific surface area, strong mechanical property, and good electrical conductivity, such as graphene [24][25][26][27][28] , graphdiyne [29] , carbon nitride, [30] black phosphorene [13] , metallic MoS2 [31] , and MXene [12,[32][33][34][35][36] . Among these, Ti3C2Tx (T is the terminal group) MXene is promising for ECAs because of its high volumetric capacitance (900/450 F cm -3 in an acid/base electrolyte) [37][38] , excellent electrical conductivity (over 10000 S cm -1 ) [39] , and favorable polar surface for wet processing and electrolyte wetting [38,[40][41][42] .

Most MXene-based ECAs have a sandwich structure for bending and force output, relying on the in-plane strain difference of two electrodes and the device modulus. Although the Ti3C2Tx film exhibits considerable interlayer strain (between sheets) during the cation intercalation [34,37,43] , the intralayer strain (along single sheets) is significantly lower [33] , resulting in a small macroscale inplane strain of the film. For example, the c-lattice parameter of a Ti3C2Tx film changes by around -0.50 Å, -2.1%, and +1.42% during the cathodic scan in a 1 M MgSO4 [37] , 1M Li2SO4 [33][34] , and 1 M H2SO4 [35] solution, respectively. In contrast, the change in intralayer strain is only ~ +0.14% in the 1 M Li2SO4 solution, which aligns with the minimal change in macroscale in-plane strain of the film (~ +0.064%) [33] . Likely due to this reason, the first neat MXene-based bending ECA, [35] assembled from two aligned Ti3C2Tx films sandwiching a poly(vinyl alcohol)/H2SO4 electrolyte, generated a small peak-to-peak strain difference (only 0.26% under ±0.6 V, 1 mHz) compared to many other ECAs. Hybridization of MXene with specific polymers has been proven to enhance the strain difference of the bending ECAs [33] . For instance, a methylcellulose/Ti3C2Tx (15/85 in weight) hybrid film shows much larger in-plane actuation strain than the neat MXene film due to the possible sliding of MXene sheets, and the assembled ECA generates a peak-to-peak strain difference of 0.541% under ±1 V (1 mHz) [33,36] . However, the electrical conductivity is relatively low (36.4 S cm -1 ). The introduction of polystyrene microspheres into the Ti3C2Tx film improves the ion diffusion, and the resulting ECA produces a peak-to-peak strain difference of 1.18% under ±1.5 V (0.1 Hz). [36] However, the sacrificed electrode modulus leads to a low device modulus (0.246 GPa), limiting certain applications. Additionally, the shelf and service life of MXene-based ECAs is constrained by the poor stability of MXene in humid air and aqueous electrolytes. Therefore, it is urgent to enhance the in-plane actuation strain of MXene electrodes while maintaining good conductivity, modulus, and stability for developing high-performance bending ECAs.

Here, we demonstrate that the in-plane actuation strain of a Ti3C2Tx MXene film can be substantially improved by surface functionalization with tetrabutylammonium (TBA) groups. This functionalization reduces the number of hydroxyl groups on MXene and significantly reduces moisture absorption in ambient conditions. The resulting film also shows enhanced mechanical performance as well as improved stability in water and humid air. Based on comprehensive in situ and ex situ characterizations, we proposed a novel sheet sliding mechanism that is induced by the co-(de)insertion of TBA and Li ions for improved actuation in functionalized MXene film. Furthermore, a high-modulus ECA operating in air is assembled to produce a peak-to-peak strain difference of 0.771% under ±1 V (triangular wave, 0.1 Hz) and a blocking force up to 51.5 times its own weight under 1 V. Additionally, a soft robotic tweezer is demonstrated to be capable of grasping a flower under -1V and maintaining a tight grip even when the voltage is reduced to 0 V.

Results & Discussion 2.1 Fabrication and structure of the TBA-MXene film

The X-ray diffraction data (Figure S1, Supporting Information) indicates the successful preparation of Ti3C2Tx MXene from the Ti3AlC2 precursor. Previous works show that TBA hydroxide (TBAOH) has been successfully used to intercalate and exfoliate multilayered Ti3CNTx, V2CTx, or MoC2Tx MXene, except for multilayered Ti3C2Tx [44][45][46] . Here we found that exfoliated Ti3C2Tx sheets can react with TBAOH to produce TBA-functionalized MXene (denoted as TBA-MXene in this paper) via a neutralization reaction between acidic Ti-OH groups and basic hydroxide anions. [47] This is achieved by simply mixing a MXene aqueous dispersion with a TBA hydroxide aqueous solution at a TBAOH/MXene mass ratio of 20% (Figure 1a). After 3-min bath sonication, many small particles of aggregated TBA-MXene sheets are present (Figure 1d) due to the attachment of hydrophobic butyl chains onto MXene sheets. To evaluate the thickness change of MXene sheets, a small portion of the aggregates was re-dispersed in the mixture of water and ethanol (1:1 in weight) under ultrasonication. Atomic force microscope (AFM) images (Figure 1 b andc) show that the thickness increased from 1.71 to 2.82 nm after TBAOH treatment, confirming the attachment of functional groups on MXene sheets. Transmission electron microscope (TEM) images (Figure S2, Supporting Information) show that the aggregate is composed of crumpled/folded TBA-MXene sheets randomly stacked together. Despite this, an intact and flexible TBA-MXene film (Figure 1e) can be obtained by vacuum-assisted filtration due to water-flow-induced pressure applied to the aggregated particles. The weight increase after functionalization gives an estimated TBA/MXene mass ratio of (7.29±1.16)% in the TBA-MXene film, corresponding to a TBA/Ti3C2(OH)2 molar ratio of (6.06±0.96)% assuming all terminal groups as -OH. Thermal gravimetric analysis (TGA, Figure S3, Supporting Information ) indicates a similar mass ratio of TBA/MXene (6.69%).

SEM images (Figure 1f, g) show a coarser surface in TBA-MXene film due to the presence of aggregated particles during functionalization. However, the TBA-MXene sheets exhibit slightly improved alignment and stacking in the film, which can be ascribed to reduced repulsion force between MXene sheets after TBA-functionalization during the filtration assembly. Furthermore, the thickness of the TBA-MXene film is also greater, explaining the mass increase after functionalization. Energy dispersive X-ray spectroscopy (EDS) result (Figure S4, Supporting Information) shows the presence of N element, confirming the introduction of TBA groups in the functionalized film. The Fourier transform infrared spectra (FTIR, Figure 1h) show that the TBA-MXene film exhibits two new peaks at 2964 and 1486 cm -1 compared to the neat MXene film, indicating the introduction of butyl groups on MXene. X-ray photoelectron spectra (XPS) analyses (Figure 1i and Figure S5, Supporting Information) confirm a decrease in -OH groups and an increase in quaternary ammonium groups, verifying the successful grafting of TBA groups onto MXene sheets. Besides, slight oxidation of Ti atoms and fluorination of C atoms are observed in the XPS spectra, likely due to the partial degradation of Ti-C bonds during the functionalization. X-ray diffraction (XRD) data (Figure 1j) demonstrates that the TBA-MXene film shows a slightly larger (1.52 nm) interlayer distance (d(002)) than the neat MXene film (1.46 nm) under ambient conditions. However, after heat drying the d(002) of neat MXene decreased to 1.12 nm, significantly smaller than that of dry TBA-MXene (1.48 nm). This indicates the introduction of TBA groups between MXene sheets, which remarkably reduces the moisture uptake compared to the neat MXene (4.47 vs. 10.8 wt%). Besides, the TBA-MXene film exhibits sharper (002) and (006) peaks, implying a higher-ordered stacking of MXene sheets, agreeing well with the observation in the cross-sectional SEM image.

Basic performances of the TBA-MXene film

The mechanical property of MXene-based films directly impacts the actuation force and bending degree of the assembled ECA device. Tensile test results (Figure 2a) reveal that the TBA-MXene film assembled from the aggregated particles has a much higher tensile strength ( = 54.6 MPa) and elastic modulus (E = 9.49 GPa) than the neat MXene film ( = 29.2 MPa, E = 6.65 GPa). This improvement can be attributed to the superior alignment and stacking of TBA-MXene sheets as well as much lower moisture uptake in the film. The strength and modulus of the TBA-MXene film surpass those of previous materials used for ECAs, such as ionic liquid/polymer/SWCNT composite film ( = 17 MPa, E = 0.156 GPa) [48] , PEDOT-PSS/MXene hybrid film ( = 23.3 MPa, E = 0.668 GPa) [12] , and metallic MoS2 film ( ≈ 17 MPa, E ≈ 2.5 GPa) [31] , and are comparable to the recently reported methylcellulose/MXene hybrid film ( = 53.2 MPa and E = 12.4 GPa). The high modulus of the TBA-MXene film enables the ECA to exhibit sufficient stiffness for some applications such as lifting operations. [31] The electrical conductivity of the TBA-MXene film (179 S cm -1 ) is reduced compared to the neat MXene film (1500 S cm -1 ) due to the increased interlayer distance as well as the defects in partially degraded MXene sheets and the voids in the film formed from the aggregates. Nevertheless, it is much superior to the methylcellulose/MXene hybrid (15/85 in weight: 36.4 S cm -1 ) [33] and remains within acceptable range for ECA applications. It is found that after vacuum drying at 80 ℃, the conductivity is increased to 2800 and 242 S cm -1 for neat MXene and TBA-MXene, respectively, because of the removal of absorbed moisture. Notably, the TBA-MXene film demonstrates reduced moisture uptake (4.47 wt%) compared to the neat MXene film (10.8 wt%), thanks to the hydrophobicity of butyl groups in TBA. This leads to obviously higher retention of electrical conductivity in the TBA-MXene film (75.9%) compared to the neat MXene film (45.3%) after 28-day storage in ambient air (21 ℃, 65% relative humidity) (Figure 2c). Besides, the TBA-MXene film shows improved stability in water (Figure 2d), maintaining its shape and size under 1-hour ultrasonication treatment, while the neat MXene film quickly disintegrates after only 5 minutes. Moreover, the TBA-MXene film exhibits improved stability in a 5.6 M LiBr electrolyte solution, with minimal color change even after 15 days of immersion, unlike the neat MXene film that partially degrades to give a blue-colored product (Figure 2f). The improved stability of the TBA-MXene film in humid air, water, and the electrolyte can be attributed to the introduction of TBA groups, which increase the contact angle with water and the aqueous electrolyte (Figure 2e). These results highlight the improved stability of the TBA-MXene film in various environments, which is favorable for improving the shelf and service lives of practical actuator applications.

Electrochemical and actuation performances of the TBA-MXene film

The charge-storage and actuation behaviors of MXene-based films were evaluated in a sandwich ECA in a 5.6 M LiBr aqueous solution (Figure 3 a and b). The ECA was assembled by sandwiching a double-sided tape with two MXene-based strips as the working electrode (WE) and counter electrode (CE). The potential of the WE is reported versus the CE, i.e., the voltage between them. The neat MXene film shows a stable cycle voltammetry (CV) curve after 6 cycles at 4 mV s -1 , displaying a quasi-rectangular shape with two pairs of blunt peaks (Figure 3c and Figure S6a, Supporting Information). In contrast, the TBA-MXene film requires 15 cycles to achieve a stable CV curve at 4 mV s -1 , exhibiting a less rectangular shape with two obvious peaks (Figure 3d and Figure S6b, Supporting Information). The CV tests were also performed in a three-electrode cell using the recorded potential range from the two-electrode cell (Figure S7, Supporting Information). In the three-electrode cell, the neat MXene displays a quasi-rectangular CV curve with a pair of blunt peaks at -0.35 and -0.05 V vs. Ag and a raised anodic tail, while the TBA-MXene gives a CV curve with clearer but more distant peaks at -0.35 and +0.1 V vs. Ag. These results indicate a combination of capacitive and pseudocapacitive charge-storage mechanisms [37,49] in both neat and functionalized MXene films. The longer activation and less rectangular CV shape of the TBA-MXene electrode are probably due to its low affinity for the LiBr solution (Figure 2e). The stronger peaks might be ascribed to the enhanced redox activity of TBA-MXene due to the partial oxidation of MXene sheets during TBA-functionalization (Figure 1i), which exposes more Ti and C atoms for redox reactions. Note that the enhanced electrochemical activity of MXene due to partial oxidation has been observed in H2O2, CO2, plasma, or anodically treated Ti3C2Tx [49][50][51][52][53] . At higher scan rates (Figure 3 c andd), both neat MXene and TBA-MXene films display shuttle-shaped CV curves due to limited ion diffusion into the inner part of highly in-plane aligned films. The mass-specific capacitance is calculated based on the dry mass of MXene in one film electrode. Figure 3e shows that the TBA-MXene film has comparable specific capacitance to the neat MXene film at 4, 1000, and 4000 mV s -1 , but lower capacitance at moderate scan rates (10-400 mV s -1 ) due to the lower electrolyte affinity and lower electrical conductivity. Despite this, the TBA-MXene-based ECA generates significantly higher peak-to-peak (total) displacements at scan rates lower than 400 mV s -1 (0.1 Hz), and comparable values at higher scan rates. For example, at 4 and 10 mV s -1 , the total displacement of the TBA-MXene-based ECA is 2.07 and 2.17 times higher than that of the neat-MXene-based counterpart. Both ECAs show decreased displacements at higher scan rates because the in-plane aligned MXene sheets limits the ion diffusion and charge storage across the film electrode. The surprising improvement achieved by simple TBA functionalization demonstrates that the surface functional groups play a more vital role than the charge-storage capacity in the actuation performance of MXene-based electrodes. Considering that the microstructure (e.g., coarse surface and voids) of the TBA-MXene film may affect the actuation performance, we fabricated post-TBA-MXene films by treating neat MXene films with a TBAOH solution to avoid any aggregate particles in the final film. Their morphology, surface chemistry, and electrochemical and actuation performance are shown in Figure S8 (Supporting Information). SEM images show that the post-TBA-MXene film has a very similar microstructure to the neat MXene film except for the increased thickness. The XPS and XRD results confirm the successful TBA-functionalization of the neat MXene film by the post treatment. Interestingly, the post-TBA-MXene based ECAs also generate significantly higher displacements under low scan rates compared with neat MXene-based counterpart. However, under the scan rates of 10-100 mV s -1 they show much lower specific capacitance and displacement than the counterpart based on pre-mixing produced TBA-MXene films. This indicates that the microstructure formed from the TBA-MXene aggregates favors the actuation under moderate frequency, attributed to the improved ion diffusion.

Actuation mechanisms of the TBA-MXene film

Various in situ and ex situ characterizations were performed to investigate the actuation mechanisms in both neat and functionalized MXene films. Firstly, the macroscale in-plane strain of the film electrode was directly monitored in a lab-made two-electrode cell (Figure 4a and Figure S9a, Supporting Information) during the CV test at 4 mV s -1 after 5-h swelling. For the neat MXene film, the in-plane strain exhibits reversible changes after two cycles of activation, especially from the 18 th cycle (Figure S9c, Supporting Information), aligned with CV results (Figure S9d, Supporting Information). During the 24-26 th cycle (Figure 4b), the in-plane strain increases/decreases continuously when the potential is decreased/increased from 0 to -1/+0.6 V, likely due to the electron/hole-injection and ion intercalation/deintercalation. However, the in-plane strain increases when the potential goes from +0.6 to +1 V, probably due to the oxidation of surface Ti atoms. The change in in-plane strain of the neat MXene film is +0.165% from +1 to -1 V at the 26 th cycle. For the TBA-MXene film, the in-plane strain continuously increased to 0.705% during a further 2.5-h swelling before CV tests (Figure S9b, Supporting Information) due to the absorbed electrolyte. During the CV test, the baseline of the in-plane strain vs. time curve (Figure S9c, Supporting Information) increases sharply during the first 4 cycles, followed by gradual changes until the 20 th cycle, consistent with the variations in CV curves (Figure S9e, Supporting Information). These changes indicated the gradual activation of the TBA-MXene film during the cycling. Figure 4b shows that the in-plane strain of the TBA-MXene film undergoes a continuous sharp increase/decrease when the potential changes from 0 to -1/+1 V, likely due to the electron/hole injection and intercalation/deintercalation of Li and TBA ions. Despite lower specific capacitance and weaker electrolyte affinity, the TBA-MXene film achieves a remarkable 0.721% change in in-plane actuation strain when the potential varies from +1 to -1 V (at the 26 th cycle), 3.37 times higher than the neat MXene film. The continuous lengthening/shortening of the TBA-MXene film towards negative/positive potentials favors larger bending in the sandwich ECA due to the opposite actuation direction of the two electrodes. [33] These results highlight the advantage of TBA-functionalization and imply a novel actuation mechanism. Furthermore, assuming a constant modulus (E = 9.49 GPa) and a contraction strain ( ) of half the 0.721% under +1 V, the calculated contraction stress (E = 34.2 MPa) and work density (0.5E = 61.7 kJ g -1 or 159 kJ m -3 ) of the TBA-MXene film are significantly higher than those of a metallic MoS2 film in 0.5 M H2SO4 (17 MPa and 81 kJ m -3 ). [31] Secondly, the atomic-scale actuation strain of stacked MXene sheets in both films was probed by in situ XRD in lab-made two-electrode cells with the same electrolyte (Figure S10). As shown in Figure 4c-e and Figure S11 (Supporting Information), the change in (110) and (002) peaks under cyclic sweep-step potentials (Figure S11a, Supporting Information) was monitored in transmission and reflection modes, respectively, to assess the intralayer strain of single sheets and interlayer strain between sheets. [33] Both neat MXene and TBA-MXene films show comparable intensity in their (110) peaks during cycling (Figure S11 b-e, Supporting Information), with the d(110) values following a similar trend as the potential changes (Figure 4e and Figure S11j, Supporting Information). The change in d(110) indicates that under negative/positive potentials, single MXene sheets in both films undergo intralayer lengthening/shortening, resulting from the combined effects of electron/hole injection-induced elongation/contraction of the Ti-C bonds (quantum mechanical effect) and the counter-ion adsorption-induced intralayer expansion (electrostatic-double-layer effect). [15,33] In the neat MXene film, the change in microscale strain of single MXene sheets along the [110] direction (0.135%) is slightly lower than that in the macroscale in-plane strain of the film (0.165%), indicating minimal in-plane sliding of MXene sheets. However, in the TBA-MXene film, the change in microscale strain of single MXene sheets along the [110] direction (around 0.125%) is significantly smaller than that in the macroscale in-plane strain of the film (0.721%), implying a considerable inplane sliding of TBA-MXene sheets during polarization.

Meanwhile, the (002) peak in both films exhibits more complex changes. For the neat MXene film, after 4-h swelling in the liquid cell, the (002) peak shifts to a lower 2 angle and becomes weaker as compared to that in ambient air (Figure S11f, Supporting Information). This indicates a swellinginduced interlayer expansion and possible in-plane sliding of MXene sheets. After CV cycling and 4 sweep-step cycles, the change in the (002) peak becomes relatively reversible (Figure 4d, e and Figure S11k, Supporting Information). During the 5 th and 6 th sweep-step cycles, the interlayer distance (d(002)) increases/decreases when the potential varies from 0 to +1/-1 V, which could be attributed to the weakened/enhanced interlayer interaction through Li-ion deintercalation/intercalation and partial oxidation/reduction of surface Ti atoms [33][34]54] . The resulting interlayer strain change is 2.86% and 2.33% from +1 to -1 V at the 5 th and 6 th cycles, respectively, over 16 times higher than that along the [110] direction (0.135%), demonstrating anisotropic actuation behavior of neat MXene sheets.

In contrast, the (002) peak of the TBA-MXene film shows negligible 2 shift but significantly decreased intensity after 8 hours of swelling in the liquid cell compared to that in ambient air (Figure S11h, Supporting Information). This implies that the 5.6 M LiBr electrolyte has limited intercalation between TBA-MXene sheets due to poor affinity, but it can fill the hydrophilic inter-edge gaps and cause the sheets to slide apart, resulting in greatly reduced diffraction intensity. This is supported by the considerable increase in in-plain strain of the TBA-MXene film during swelling (Figure S9b, Supporting Information). After CV activation, the (002) peak shifts to a lower 2 angle and becomes weaker and broader (Figure S11h, Supporting Information), indicating a slightly increased interlayer distance and further sliding of TBA-MXene sheets. This observation aligns with the increase in-plane strain of the TBA-MXene film during initial CV cycles (Figure S9c, Supporting Information). After 4 sweep-step cycles, the (002) peak presents relatively reversible changes (Figure 4d-e and Figure S11 h and k, Supporting Information). In contrast to the neat MXene film, the TBA-MXene film undergoes interlayer contraction/expansion when the potential varies from 0 to +1/-1 V, ascribed to the co-deintercalation/intercalation of Li ions and large-sized TBA ions. The contraction from -0.5 to -1 V may result from increased interlayer interaction due to intercalated Li ions or partial chemical reduction of surface groups. [33,54] The generated actuation-strain changes along the [002] direction are 1.86% and 1.78% from +1 to -1 V at the 4 th and 5 th cycles, respectively, over 13 times greater than that along the [110] direction (0.125%). This demonstrates that the TBA-MXene film also shows an anisotropic actuation behavior. Meanwhile, the (002) peak intensity of the TBA-MXene film is obviously decreased/increased when the potential varies from 0 to -0.5/+0.5 V, implying possible intercalation/deintercalation-induced sliding of MXene sheets away from/close to each other. This contributes to significant macroscale in-plane lengthening/shortening of the film. Additionally, the TBA-MXene film exhibits a lower interlayer distance than the neat MXene film under all potentials (Figure S11k, Supporting Information), suggesting that hydrated Li ions may preferably insert into the hydrophilic inter-edge gaps rather than the electrolyte-phobic galleries between TBA-MXene sheets, causing them to slide away from/close to each other under negative/positive potentials.

Thirdly, the surface chemistry of neat MXene and TBA-MXene films during the sweep-step potential treatment in a two-electrode cell was studied by in situ FTIR spectroscopy (Figure 4f) and ex situ XPS analysis (Figure 4g and Figure S12, Supporting Information) after CV cycling activation. Unlike the neat MXene electrode, the TBA-MXene film shows obvious changes in infrared absorption from 3000 to 2800 cm -1 during the polarization. When the potential varies from 0 V to +1/-1 V, the peaks at 2960 and 2874 cm -1 (ascribed to the butyl groups) become much weaker/stronger, implying the desertion/insertion of TBA cations from/into the TBA-MXene electrode. The O 1s XPS results show that the peak around 532.3 eV (attributed to Ti-OH) in the fresh MXene film (Figure 1e) become significantly weaker after cycling activation (Figure 4g, under 0 V), indicating that many protons on fresh MXene sheets are exchanged by Li ions in the 5.6 M LiBr electrolyte. For the cycled neat MXene film, the main O 1s peak around 529.9 eV under 0 V (attributed to Ti-O-Ti/Li) becomes stronger/weaker at + 1 V/-1V, while the peak around 532 eV under 0 V (attributed to Ti-OH and Li-OH) shows an opposite behavior. Simultaneously, the main Ti 2p2/3 peak at 455.4 eV under 0 V (attributed to C-Ti-C/O/F) shifts slightly to 455.6/455.2 eV under +1/-1 V (Figure S12a, Supporting Information). These findings indicate that some surface Ti atoms are oxidized to produce Ti-O-Ti under +1 V and reduced to generate low-valent Ti atoms (connected with O-vacancy and -OH) under -1 V. For the cycled TBA-MXene film, the Ti 2p peaks (Figure S12b, Supporting Information) show similar changes to those o neat MXene, indicating partial oxidation/reduction of surface Ti atoms on TBA-MXene sheets under +1/-1 V. Notably, the O 1s spectrum (Figure 4g and Figure S12d, Supporting Information) exhibits more obvious changes: the peak at 529.8 eV under 0 V (attributed to Ti-O-Ti/Li) increases/decreases obviously under +1/-1 V, while the peak at 532.1 eV (attributed to Ti-OH, Li-OH, and Ti-O-TBA) shows the opposite trend. This indicates that large-sized TBA cations could be deintercalated/intercalated together with Li ions under +1/-1 V, agreeing well with the in situ FTIR results and the large interlayer expansion/contraction of the TBA-MXene film under these potentials.

Finally, the actuation of an ultrathin TBA-MXene film (around 20 nm thick, Figure S13b, Supporting Information) was observed by in situ liquid cell TEM with a 5.6 M LiBr solution as the electrolyte. As shown in Figure 4h and i, the transferred TBA-MXene thin film consists of stacked TBA-MXene sheets. Figure S14 (Supporting Information) shows that TBA-MXene sheets are stable and almost immobile in electrolyte solution under irradiation without applied potential. The actuation of this cluster during a CV test at 20 mV s -1 is presented in Supplemental Movie 1 and the snapshots under different potentials vs. Pt are shown in Figure 4i-k. The top portion of the cluster profile (yellow dotted lines in Figure 4i-k) extends significantly when the potential is decreased from 0 to -0.4 V and then to -0.8 V vs. Pt. This large lateral actuation displacement corresponds well with the increased inplane strain of the macroscale TBA-MXene film (Figure 4b), which is likely due to the cointercalation of TBA and Li ions under negative potentials. (b) Illustration of the filament sliding model for the contraction and relaxation of skeletal muscles [55] .

Considering the multi-perspective characterizations, the actuation mechanisms of neat and functionalized MXene films are discussed, as illustrated in Figure 5a. Under negative/positive potentials, both neat MXene and TBA-MXene sheets undergo comparable intralayer lengthening/shortening due to the injection of a similar number of electrons/holes into the MXene framework and adsorption of similar number of counter ions on the MXene surface, accompanied by partial reduction/oxidation of surface Ti atoms. However, they differ significantly in terms of interlayer and inter-edge responses to the swelling and ion (de)intercalation. The neat MXene film shows pronounced interlayer swelling in the 5.6 M LiBr solution, evident from the largely increased interlayer distance. Its macroscale in-plane actuation strain is primarily attributed to the chargeinjection-induced atomic-scale intralayer strain of single MXene sheets, with a minor contribution from in-plane sliding. The intercalation/deintercalation of Li ions in the neat MXene film induces considerable interlayer contraction/expansion but slight sheet sliding under negative/positive potentials. In contrast, the TBA-MXene film in the same electrolyte exhibit significant inter-edge swelling, indicating substantial sliding of TBA-MXene sheets away from each other due to the presence of hydrophobic butyl groups between the sheets, which repel water and reduce the interlayer interaction. The large in-plane actuation strain of the TBA-MXene film under negative/positive potentials is mildly contributed by the intralayer strain of single MXene sheets but mainly by the inplane sheet sliding, likely caused by co-intercalation/deintercalation of large-sized TBA ions and Li ions into/from interlayer galleries and inter-edge gaps. The reduced interlayer interaction between TBA-MXene sheets could favor the sheet sliding when more TBA/Li ions are inserted into the film. The preferential insertion of hydrated Li ions into hydrophilic inter-edge gaps is expected to further enhance the sliding of TBA-MXene sheets under negative polarizations. Besides, the microstructure (e.g., coarse surface and inner voids) in the TBA-MXene film can favor the ion diffusion and charge storage to generate moderate actuation strain under mid-range frequency. Therefore, simple TBA functionalization endows the MXene film with a novel and more efficient in-plane actuation mechanism. Intestinally, the sheet sliding mechanism in the TBA-MXene film resembles the filament sliding theory governing the contraction/relaxation of mammalian skeletal muscles (Figure 5b). [55]

TBA-MXene-based ECAs working in air

An integrated ECA (IECA, Figure 6a) capable of working in air was assembled by sandwiching two TBA-MXene based strips with a thermoplastic polyurethane/cellulose microfiber (TPU/CMF) paper, followed by infiltration of a 5.6 M LiBr aqueous solution. This electrolyte has high wetting ability and stability in ambient conditions (21℃, 65% relative humidity) as well as higher ionic conductivity (> 150 mS cm -1 ) [56] and lower cost than typical organic ionic liquids (i.e., EMImBF4). With a TPU binder, two electrodes can be well bonded to the porous CMF paper (Figure S15, Supporting Information). Thanks to the high mechanical performance of the TBA-MXene strips and the TPU/CMF paper, combined with strong interface bonding, the resulting IECA has an impressive tensile strength () of 9.03 MPa and elastic modulus (E) of 1.69 GPa, surpassing most reported ECAs working in air (Table S1, Supporting information). These include ECAs based on neat MXene ( ≈ 0.65 MPa, E ≈ 0.04 GPa), [35] MXene/(polystyrene-MXene) composite ( ≈ 0.75 MPa, E ≈ 0.246 GPa), [36] graphdiyne/PVDF composite ( = 5.5 MPa, E = 0.42 GPa), [29] and (phosphorene-CNTs)/CNTs composite ( ≈ 7.7 MPa, E ≈ 0.246 GPa) [13] . Its mechanical property is also comparable to that of the recently reported methylcellulose/MXene-hybrid-based IECA ( = 8.27 MPa, E = 1.93 GP). [33] The high mechanical performance of the TBA-MXene-based IECA is favorable for lifting or grasping applications. TBA-MXene-based IECA shows comparable CV curves and specific capacitances (Figure 6 b andc) to the double-sided-tape-based sandwich ECA working in the same liquid (Figure 3 d ande). Under a triangular-wave potential of ±1 V, this IECA generates a peak-to-peak displacement of 12.7, 6.45, and 0.82 mm at 0.001, 0.01, and 0.1 Hz (Supplemental Movie 2), respectively, corresponding to a peak-to-peak (total) strain difference of 0.771, 0.431, and 0.0566% (Figure 6d-f). Notably, these displacements are 44%-206% higher than those of neat-MXene-based counterpart (Figure S16, Supporting Information), despite similar stored charges. The voltage-normalized peak-to-peak strain differences also surpass those of ECAs based on gold layers, [21] CNT/PVDF films, [29] neat MXene films, [35] or methylcellulose/MXene hybrid films [33] (Table S1, Supporting information). The IECA demonstrates excellent cyclic bending stability, with a displacement retention of 102.6% after 10,000 cycles under a square-wave potential of ±1 V and 0.1 Hz (Figure 6g). Under DC 1 V, it produces a blocking force of 1.01 mN (equivalent to 7.55 times its own weight and 16.6 times the weight of two electrodes) within 10.8 s, with 80% of the maximum achieved at 3.3 s (Figure 6h). The voltagenormalized force-to-weight ratio (7.55 V -1 ) exceeds that of previous ECAs based on methylcellulose/MXene hybrids (4.7 V -1 ) [33] or gold composite layers (~1 V -1 ). [21] The blocking force is further improved to 1.81 mN by reducing the free length of the IECA to 6 mm (Figure S17a, Supporting Information), giving an outstanding force/weight ratio of 51.5 due to the ultrahigh modulus of the device. Scaling up the IECA width to 18 mm also increases the blocking force to 5.02 mN (Figure S17 b and c, Supporting Information). After 35-day storage in ambient conditions (21 ℃, 65% relative humidity), the TBA-MXene-based IECA shows a capacitance retention of 90% at 4 mV s -1 (Figure 6i), thanks to the TBA functionalization. After the storage and re-activation, the displacement retention (compared at a scan rate of 40 mV s -1 ) is 70% (Figure S18, Supporting Information), indicating the partial degradation of TBA-MXene sheets that needs to be addressed in future studies.

Finally, a low-voltage soft tweezer robot is designed (Figure S19, Supporting Information) and fabricated by mounting two TBA-MXene-based IECAs onto a trapezoid holder, allowing their ends to contact tightly. As shown in Figure 6j and Supplemental Movie 3, this tweezer can open under 1 V, grab a flower weighing five times that of two IECAs under -1 V, and hold it securely under 0 V (WE vs. CE) without loosening and damaging the flower. The firm clamping achieved 0 V is attributed to the high modulus of the TBA-MXene IECAs, providing energy-efficient transport of the grabbed object. Furthermore, this tweezer can grasp an even heavier flower (7.9 times the weight of two IECAs) and hold it steadily when -1 V is being applied (Figure 6k). These results highlight the promising potential of TBA-based IECAs for robotic applications that require softness, low-voltage operation, and energy efficiency.

Conclusion

The simple mixing of Ti3C2Tx MXene with TBA hydroxide leads to TBA-functionalized MXene via a neutralization reaction. As compared to the neat MXene film, the TBA-MXene film exhibits obviously enhanced mechanical properties and improved stability in both ambient air and the aqueous electrolyte. Despite possessing a similar charge-storage capacity, the TBA-MXene film generates a substantially improved in-plane actuation strain under polarization. Based on comprehensive in situ and ex situ investigations, this enhancement is attributed to the co-insertion/desertion of TBA and Li ions into/from MXene interlayer galleries and inter-edge gaps, which induces a considerable in-plane sliding of TBA-MXene sheets with reduced interlayer interaction. The fabricated IECA displays impressive tensile strength and modulus and can operate in air. It generates a peak-to-peak strain difference of 0.771% under a triangular-wave potential of ±1 V and a blocking force up to 51.5 times its own weight under 1 V. The IECAs can be used to assemble low-voltage, energy-saving, and soft tweezers/grippers. This simple functionalization strategy and novel sliding-induced actuation mechanism could inspire the design of high-performance ECAs based on other active nanomaterials (e.g., CNTs and graphene). Further enhancement of the actuation performance could be achieved by incorporating nanofillers and inducing a random/vertical alignment of functionalized MXene sheets to facilitate faster response and generate higher strain. The stability of MXene-based actuators could be further improved by other analogous surface functionalization methods and the use of hydrophobic ionic liquid electrolytes.

Experimental Section

Preparation of MXene dispersion: Ti3C2Tx MXene was obtained by etching Ti3C2Al powder in a LiF-HCl solution using a modified "MILD" method. [33,45] In a 100-ml plastic container, 4.8 g LiF (Sigma-Aldrich), 3 g Ti3AlC2 (≥98%, Famouschem Technology), and 15 ml deionized water were mixed under magnetic stirring for 5 min. Then 45 ml concentrated HCl (37%, AR, Shedelco) was slowly added in around 5 minutes. The container was capped loosely and placed in a 35-℃ oil bath for 36 hours of reaction under stirring. The resulting mixture was centrifuged at 5000 rpm for 5 min, and most of the liquid was removed. The sediment was washed with 300 ml water followed by centrifugation at 5000 rpm for 5-10 min. This wash and separation process was repeated 5 times to obtain sediment of multilayered Ti3C2Tx. Half of the sediment was diluted with deionized water to obtain a 300 ml suspension, which was purged with Ar and ultrasonicated in a water-ice bath in a sonicator (37 kHz, output 330 W) for 1 h. After centrifugation at 3000 rpm for 0.5 h, around 80% of the upper liquid was collected, yielding a Ti3C2Tx MXene dispersion of 2.57 mg ml -1 . The MXene dispersion was stored in the fridge for later usage.

Fabrication of MXene-based films:

A neat MXene film was fabricated by vacuum-assisted filtration of a 23.34 g MXene dispersion (containing 60.0 mg MXene) through an O2-plasma-treated polypropylene membrane (Celgard 2500), followed by drying at 80 ℃ under vacuum for 12 h. Unless specified, the TBA-MXene films mentioned in the text were prepared by a pre-mixing method. Typically, a 23.34 g MXene dispersion was diluted with 36.66 g water and added with1.20 g TBAOH aqueous solution (1 wt%, Sigma-Aldrich), followed by stirring at 800 rpm for 15 min and bath sonication of 3 min. The resulting suspension with many small particles was filtrated and dried in the same way as the neat MXene film. The mass of TBA in the TBA-MXene film was estimated as the mass difference between the dry film and the raw MXene (60.0 mg). The average mass ratio of TBA to MXene was (7.29 ±1.16)% as calculated from 5 samples. Alternatively, a post-treatment method was used to prepare post-TBA-MXene films by immersing the neat MXene film in a 1% or 4% TBAOH solution in water-ethanol mixture (1:1 in weight) for 4 h followed by ethanol washing and vacuum drying. For a fair comparison of each performance, neat and functionalized MXene films were prepared at very close time gap (time difference ≤30 h) from the same batch of MXene stored for the same duration in the fridge. All films were stored in Ar-filled glass vials before characterizations.

Assembly of MXene-based ECAs:

Two kinds of ECAs were assembled from MXene-based strip electrodes. The first type operates in a 5.6 M LiBr solution and is fabricated by sandwiching a doublesided tape (23 mm × 3.5 mm × 0.050 mm, polyethylene terephthalate as the base, Darit) between two MXene or TBA-MXene strips (22 mm × 3 mm). The second type is an integrated ECA (IECA) designed for air operation, which is fabricated as follows. A TBA-MXene strip (22 mm × 3 mm) is laid on a PTFE plate, covered with a porous cellulose microfiber (CMF) paper (23 mm × 3.6 mm × 0.040 mm), and then coated with an 18 uL thermoplastic polyurethane (TPU)/dimethylformamide solution (5 wt%). Another TBA-MXene strip of the same size was placed on the paper, and the multilayered structure was vacuum dried at 80 ℃ for 2 hours to ensure bonding. Subsequently, the resulting sandwich was immersed overnight in a 5.6 M LiBr aqueous solution and then taken out to remove the excess liquid by tissue paper. The thickness of the TBA-MXene-based IECA is around 0.100 mm. A large area IECA of 18 mm × 21.5 mm (Figure S17b, Supporting Information) was fabricated similarly. A soft tweezer robot was assembled using two IECAs and a trapezoid support, as illustrated in Figure S19 (Supporting Information).

Characterizations: To measure the thickness of neat and functionalized sheets by an AFM (Cypher S, Asylum Research), the dilute dispersion in the mixture of water and ethanol (1:1 in weight) was drop-casted onto a freshly cleaved mica plate followed by vacuum drying. The moisture content in the film was calculated as the difference between the dry mass (after vacuum drying at 80 ℃ for 12 h) and the ambient mass (after storage at 21℃, 65 RH% for 12 h).TGA measurements were performed on a thermogravimetric analyzer (Q500, TA Instrument) at a ramp rate of 5 min -1 under flowing N2. The morphology of TBA-MXene sheets was observed on a TEM (JEOL 2100F). The surface and cross-section of MXene-based films and IECAs were observed by a field-emission SEM (JEOL JSM-6340F). FTIR spectra were collected in the attenuated total reflection (ATR) mode on a PerkinElmer Frontier spectrometer. For XPS tests, the fresh films were secured to the metal holder by conductive silver paste and stored in the Ar-filled glove box until transferred to an X-ray photoelectron spectrometer (Quantera II with Al Kα X-ray). XRD data was collected on an X-ray diffractometer (Shimadzu XRD-6000 with Cu Kα X-ray). To compare the peak shift in ambient and dry MXenebased films, all films were adhered to a double-sided tape and sealed with a thin polyimide film before the XRD test. Tensile tests were performed on a dynamic mechanical analyzer (Q800, TA Instruments) at a strain speed of 1% and a gauge length of 10 mm. The resistance of strips was measured with a two-probe multimeter (Fluke 15B+) and was used to calculate the electrical conductivity based on the cross-sectional area observed by the SEM. Contact angles were measured with an optical contact angle measuring system (OCA 15Pro, DataPhysics).

The electrochemical performance was measured with an electrochemical potentiostat (PGSTAT128N, Metrohm Autolab). The sandwich ECAs were clamped by a pair of Pt sheets at one end, exposing a free part of 20 mm long for bending. CV tests were performed from -1 to 1 V (WE vs. CE) with scan rates from 4 to 4000 mV s -1 . The mass-specific capacitance ( ) and average massspecific capacitance ( ) of MXene in one electrode were calculated as follows:

= 2 ( ) ⁄ (1) = ∫ | | ( -) (2)
where I, , m , , and is the current, scan rate, dry mass of MXene in the working electrode, and lower and upper potentials, respectively. In another two-electrode cell with two identical MXene or TBA-MXene electrodes, the potential of the WE was recorded vs. a Ag wire while performing the CV test from -1 to 1 V (WE vs. CE) at 4 mV s -1 . Then the steady potential range of WE vs. Ag was used to perform the CV tests at 2 mV s -1 in a three-electrode configuration with the Ag wire as the reference electrode. The potential of the Ag wire in a 5.6 M LiBr solution was measured to be ~ -0.25 V vs. Ag/AgCl (saturated KCl solution).

The actuation displacement of the film or ECAs during CV measurements was recorded by a laser displacement sensor (ILD-2300-20, Micro Epsilon) or a smartphone camera (Huawei Mate20). The in-plane actuation strain of film electrodes was directly measured in a special two-electrode cell (Figure 4a and Figure S9a, Supporting Information) by recording the displacement of an L-shaped Al foil attached beneath the working electrode with the laser sensor. For ECAs working in the liquid electrolyte, the tip displacement was analyzed from the recorded video during the CV test. For IECAs working in air, the displacement was recorded by a laser sensor, with the beam pointing on the IECA around 17 mm away from the clamping point. The strain difference (∆ε) between two opposite sides of the ECA was estimated by the following equation: [14] ∆ = 2 /( + )

where , δ, and is the device thickness, displacement, and distance between the clamping point and the real or imaginary laser beam. The blocking force of the IECA is recorded by positioning its tip against a balance (ME204, Mettler Toledo) while applying a DC voltage of 1 V.

For in situ FTIR analyses, a two-electrode cell was constructed by sandwiching a porous PVDF membrane infiltrated with a 5.6 M LiBr solution between two MXene or TBA-MXene strips. After activation at 4 mV s -1 for 10 CV cycles (-1 to 1 V), the cell was pressed against the ATR diamond plate with a force of 25 units, and the FTIR spectra of the bottom electrode were collected during the polarization with a cyclic sweep-step potential (Figure S11a, Supporting Information). For ex situ XPS analyses, a two-electrode cell with two MXene or TBA-MXene strips (15 mm × 3 mm) was activated in the same electrolyte similarly. Then the cell was successively charged and maintained at 0, 0.5, and 1 V for 5 min each. After each step, the far ends (5 mm long) of the working and counter electrodes (with opposite potentials) were cut and respectively quenched in tetrahydrofuran (THF, 5 ml) for 10 min. After removal of THF, the cut portions were transferred to a glove box and attached to an XPS holder using a conductive silver paste, and the residual LiBr layer was scratched away to expose the MXene or TBA-MXene sheets. The samples were stored in an Ar-filled bottle until being quickly transferred to the XPS chamber for analysis. No calibration of the XPS data was performed because of the good grounding state.

In situ XRD measurements were performed in lab-made two-electrode cells (Figure S10) according to our previous report. [33] The electrodes were two strips of neat MXene or TBA-MXene of the same size, and the electrolyte was a 5.6 M LiBr aqueous solution. The (002) peak of the working electrode, which reflects the interlayer distance between MXene sheets, was monitored in a reflection mode while blocking the signal of the counter electrode with a porous membrane. In contrast, the (110) peak of the working electrode, which reflects the intralayer strain of single MXene sheets, was monitored in a transmission mode while blocking the signal of the counter electrode by a nickel plate. All cells were first activated for 5-10 CV cycles (-1 to 1 V, 4 mV s -1 ) until steady curves were observed, then biased under a sweep-step potential (Figure S11a, Supporting Information) for 6 cycles. The XRD data was smoothed by the Savitzky-Golay method and the average d(002) and d(110) were calculated according to Bragg's law using weight-averaged 2 values of these peaks.

For in situ liquid cell TEM experiments, we used an electrochemical liquid cell (Protochips Inc., USA) which consisted of a spacer chip and an electrochemical chip patterned with three Pt electrodes, both with a 50-nm-thick SiNx window (Figure S13a, Supporting Information). Both chips were treated with air plasma for ~40 seconds to make the SiNx surfaces hydrophilic. A thin TBA-MXene film (~20 nm thick) on the polypropylene membrane was obtained by vacuum filtration. The size of the TBA-MXene layer was reduced by using a single-sided tape to create a small rectangular pattern. A small polyvinyl acetate film (~20 m thick) was covered on the pattern, slightly pressed at 80 ℃ for 3 min, peeled off the membrane, and then transferred onto the Pt working electrode in the electrochemical chip. Finally, the chip was rinsed twice in 5 ml of acetone for 5 min each time, followed by drying under ambient conditions. A large MXene pattern (~10 nm thick, Figure S13b, Supporting Information) was transferred to the Pt counter electrode using a similar protocol to increase its capacitance. Then ~1 µl of 5.6 M LiBr aqueous solution was drop-casted onto the center of the electrochemical chip, which was then assembled with the spacer chip on the liquid cell TEM holder. After leak-checking, the liquid cell holder was inserted into a TEM (JEOL 2100-F) operated at 200 kV for in situ imaging under an incident flux below 10 e Å -2 s -1 . The image series were acquired at a frame rate of 0.5-1 frame per second using Orius camera (Gatan Inc, USA) while performing electrochemical tests using a portable potentiostat (Gamry Reference 600+, USA). CV tests were run at 20 mV s -1 for at least 3 cycles from -0.8 to 0 V vs. Pt, corresponding to -0.85 to -0.05 V vs. a Ag/AgCl (saturated KCl solution) reference electrode in the 5.6 M LiBr solution.

Figure 1 .

 1 Figure 1. Fabrication and structural characterizations of neat MXene and TBA-MXene. (a) Illustration of the TBA functionalization and film formation. (b, c) AFM images of neat MXene and TBA-MXene sheets. (d) Photos of diluted neat MXene dispersion and TBA-MXene suspension (0.05 mg ml -1 in water). (e) Photo of a TBA-MXene film. (f, g) SEM images of (f) the top surface and (g) the cross-section of the films. (h) FTIR spectra. (i) High-resolution Ti 2p, C 1s, O 1s, F 1s, and N 1s XPS spectra normalized to the area of Ti 2p peak. (j) XRD patterns of ambient-stored (21℃, 65 RH%) and dried (80 ℃ under vacuum overnight) MXene-based films.

Figure 2 .

 2 Figure 2. Basic performances of neat MXene and TBA-MXene films. (a) Tensile stress-strain curves. (b) Summary of the tensile strength, Young's modulus, electrical conductivity, and moisture uptake (moisture/dry MXene) under ambient conditions (21 ℃, 65% relative humidity). (c) Retention of electrical conductivity during ambient storage. (d) Photo of the films in water before and after ultrasonication treatment (37 kHz, 330 W). (e) Contact angles of the films with water and a 5.6 M LiBr aqueous solution. (f) Photos of the films just immersed and after storage in a 5.6 M LiBr aqueous solution for 15 days.

Figure 3 .

 3 Figure 3. Electrochemical and actuation performances of neat MXene and TBA-MXene-based ECAs in a 5.6 M LiBr solution. (a, b) Photos of the ECAs at the largest bending states under different potentials. The red dashed lines in (a) indicate the position at 0 V. The largest bending of TBA-MXene-based ECA is not achieved at ±1 V because of phase delay. Insets illustrate the configuration of the ECAs. (c, d) CV curves of the ECAs under different scan rates. (e) Specific capacitance based on the mass of MXene in one electrode. (f) Dependence of the total displacement on the scan rate/frequency.

Figure 4 .

 4 Figure 4. Investigations on the actuation mechanisms of neat MXene and TBA-MXene films. (a) Illustration of the direct measurement of the in-plane actuation strain of the working electrode (WE) by recording the horizontal displacement of the L-shaped Al foil attached to the end of the WE. (b) Dependence of the in-plane strain of the WE on the applied potentials vs. CE at 4 mV s -1 . Arrows indicate the scanning direction. (c) Illustration of d(002) and d(110) in MXene-based films. (d) In situ XRD patterns of the WE film at different potentials vs. CE. The dotted lines indicate the (002) peak in ambient air. (e) Changes of d(002) and d(110) of the WE at different potentials vs. the CE. (f) In situ FTIR spectra of WE films after charging to different potentials vs. the CE. (g) Ex situ O 1s XPS spectra of the electrode films biased at different potentials vs. the CE. (h-k) In situ liquid cell TEM images of TBA-MXene sheets: (h) low magnification image under no bias potential (TBA-MXene clusters reside inside the yellow dotted circle); (i-k) high magnification images under different bias potentials vs. Pt during a CV test at 20 mV s -1 . The yellow dotted line in (i-k) indicates the position of the profile at 0 V.

Figure 5 .

 5 Figure 5. The proposed actuation mechanism. (a) Illustration of the proposed actuation mechanism for neat MXene and TBA-MXene films in a 5.6 M LiBr electrolyte solution. The t0V indicates the thickness of the stacked sheets under 0 V (vs. CE) after cycling. The negative surface charge on MXene can be due to the dissociation of Ti-O-Li/TBA into Ti-O -in the electrolyte. For clarity, the intralayer contraction/expansion of single MXene sheets under positive/negative polarization and the presence of hydrated Br -ions are not shown.

Figure 6 .

 6 Figure 6. Performance of TBA-MXene-based IECAs working in air. (a) Tensile stress-strain curve and illustration of the device. (b) CV curves under different scan rates. (c) Dependence of specific capacitance of MXene (in one electrode) on the scan rate. (d) Photos of the IECA with maximum displacements under the indicated bias voltages (instead of -1 and +1 V due to actuation delay). (e) Displacement curves at different scan rates. (f) Dependence of the peak-to-peak (total) displacement and strain difference on the scan rate/frequency. (g) Displacement retention under a square wave (±1 V, 0.1 Hz) during the cycling. (h) Blocking force under DC 1 V. (i) CV curves at 4 mV s -1 of the device before and after 35-day storage in ambient conditions. (k, l) Snapshots of a two-IECA-based soft tweezer during opening and grasping a flower of (k) 5.0 times and (l) 7.9 times its own weight (based on the free part). The bottom shows the corresponding voltage profiles.

Figure S3 .

 S3 Figure S3. TGA of (a) the neat MXene film and (b) the TBA-MXene film. The neat MXene film shows a sharp mass loss below 200 o C due to the evaporation of considerable amount of absorbed moisture. In contrast, the TBA-MXene film underwent only a slight mass loss below 200 o C indicating little moisture absorption under ambient condition. The mass loss between 179 to 500 o C can be ascribed to the degradation of TBA groups. The weight ratio of TBA/MXene is around 6.69 wt%, close to the value obtained by vacuum drying at 80 o C for 12 h.

Figure S4 .

 S4 Figure S4. (a) SEM image, (b-g) corresponding EDS mapping of different elements, and (h) EDS of cross-section of a TBA-MXene film.

Figure S5 .

 S5 Figure S5. Deconvoluted (a) Ti 2p (T denotes terminal groups such as -O, -OH, -OLi, -O-TBA, -F and -Cl), (b) O 1s, (c) F 1s, (d) C 1s, and (e) N 1s XPS spectra of the neat MXene and TBA-MXene films. The intensity of all spectra is normalized to the area of the Ti2p peak.

Figure S6 .

 S6 Figure S6. Supplemental data of the ECAs working in a 5.6 M LiBr solution electrolyte (as shown in Figure S3). (a,b) CV curves at 4 mV s -1 of (a) neat MXene and (b) TBA-MXene based ECAs. (c) Evolution of the specific capacitance and total displacement of TBA-based ECA during the CV measurement at 4 mV s -1 .

Figure S7 .

 S7 Figure S7. CV curves of (a-c) the neat MXene and (d-f) the TBA-MXene in (a, b, d, e) 2-electrode cells and (c, f) 3-electrode cells with a 5.6 M LiBr solution as the electrolyte. In all cells, the mass and size of working electrode (WE) and counter electrode (CE) were almost the same. For (b) and (e, the potential of the WE vs. a Ag wire in a 2-electrode cell was simultaneously recorded while running the CV measurements on a multichannel modular potentiostat. As shown in (b) and (e), the potential range of WE (either neat MXene or TBA-MXene) vs. Ag gradually shifts to around -0.5~+0.5 V, which was used for the CV scan in the 3-electrode cell.

Figure S8 .

 S8 Figure S8. Characterizations of the post-TBA-MXene films. (a-d) SEM images of (a, c) the surface and (b, d) the cross-section of (a, b) the neat MXene film and (c, d) the post-TBA-MXene film (from 1% TBAOH solution). This shows that both films have very similar microstructure except that the post-TBA-MXene film has a higher thickness due to the functionalization. (e) Ti 2p, C 1s, O 1s and N 1s XPS of a fresh neat MXene strip and post-TBA-MXene strips obtained by treating the neat MXene film with 1% and 4% TBAOH solution. These strips were originated from the same piece of

Figure S9 .

 S9 Figure S9. Supplemental data for the direct in-plane strain measurement. (a) Illustration of a twoelectrode cell for the measurement. The electrolyte (5.6 M LiBr aqueous solution) is not shown. (b) Evolution of in-plane strain of a pre-swollen TBA-MXene film during the further swelling in the cell. (c) Evolution of in-plane actuation strain and potential of neat MXene and TBA-MXene working electrode during CV tests at 4 mV s -1 for 26 cycles. (d, e) CV curves of neat MXene and TBA-MXene based cells.

Figure S10 .

 S10 Figure S10. Illustration of in-situ XRD tests of neat MXene and TBA-MXene films in two-electrode cells. (a) The transmission mode for the (110) peak. A Nickel plate (thickness: ~ 0.1 mm) was covered on the counter electrode (the lower one) to block its (110) signal. (b) The reflection mode for the (002) peak. The porous poly(vinylidene difluoride) separator (pore size: 0.22 mm; thickness: ~0.1 mm) was confirmed effective for blocking the (002) signal of the bottom electrode.

Figure S11 .

 S11 Figure S11. Supplemental data for in situ XRD investigations. (a) The sweep-step potential profile in one cycle. Each step potential was maintained for 5 min, and the sweep rate is 4 mV s -1 . (b, c) Evolution of the (110) peak of (b) neat MXene and (c) TBA-MXene working electrodes in the wet cell before and during CV activation and sweep-step cycling. The voltage is kept zero during these measurements. (d, e) Evolution of the (110) peak of (d) neat MXene and (e) TBA-MXene working electrodes under different step potentials vs. counter electrode in one half cycle. (f, h) Evolution of the (002) peak of (f) neat MXene and (h) TBA-MXene working electrodes in the wet cell after different CV and sweep-step cycles. The voltage is kept zero during these measurements. (g, i) Evolution of the (002) peak of (g) neat MXene and (i) TBA-MXene working electrodes under different step potentials vs. counter electrode in one half cycle. (j, k) Change of (j) d(110) and (k) d(002) of the working electrodes during the sweep-step cycles.

Figure S12 .

 S12 Figure S12. Ex-situ XPS spectra of the working electrode (WE) films after charging to different potentials vs. the counter electrode (CE): (a, b) Normalized Ti 2p survey; (c, d) Normalized O 1s survey.

Figure S13 .

 S13 Figure S13. Set-up for in situ electrochemical liquid cell TEM experiments. (a, b) Optical images of the electrochemical chip (a) before and (b) after sample transfer (red dotted rectangle). (c) Lowmagnification TEM image of the transferred sample (red dotted line).

Figure S14 .

 S14 Figure S14. Sequential TEM images of TBA-MXene sheets in the liquid cell before applying a potential. The yellow dotted line indicates the position of the profile at t s. The sheets are stable under irradiation for more than 5 min.

Figure S15 .

 S15 Figure S15. SEM images of the cross-section of a TBA-MXene based IECA before infiltration of the liquid electrolyte. Red dotted lines mark the interface between the TBA-MXene film and the TPU/CMF paper.

Figure S16 .

 S16 Figure S16. Comparison of the specific capacitance and total displacement of IECAs based on neat MXene and TBA-MXene under different scan rates. The free length was 17 mm.

Figure S17 .

 S17 Figure S17. Performance of short and wide IECAs: (a) Blocking force of a short IECA (free length = 6 mm) under DC 1 V for 30 s. (b, c) Photo and blocking force of a wide IECA under DC 1 V for 30 s.

Figure S18 .

 S18 Figure S18. Effect of aging in ambient condition (21 o C, 65% relative humidity) on the performance of TBA-MXene based ECA: (a) CV curves at 4 mV s -1 before and after 35-day aging; (b) CV curves at 40 mV s -1 before and after 35-day aging. The activation was done by running a CV at 4 mV s -1 for 10 cycles; (c) Actuation displacement of fresh and aged device at 40 mV s -1 under a triangle wave of ± 1V and 0.01 Hz.

Figure S19 .

 S19 Figure S19. Illustration of a soft tweezer composed of two TBA-MXene based IECAs with the ends touching each other.

Table S1 .

 S1 Summary of ECAs based on MXene and other inorganic materials.

		Electrode parameters				Device parameters
	Ref.	Component	Strength [MPa]	Elastic modulus [GPa]	Electrolyte membrane	Device modulus	Applied voltage [V]	Device Size a [mm 3 ]	Peak-to-peak ∆ /Voltage [% V -1 ]	Blocking force/ Voltage [mN V -1 ]	(Force/ Weight)/ Voltage [V -1 ]
	[1]	Deposited Au layer	-b	-	EMIBF4/ HNC/PAA-co-PAN	0.00035	±5	25×2.5× 0.35	0.208 (0.02 Hz)	~0.002 (0.02 Hz)	~1
	[2]	Chitosan/ MWCNT	-	-	EMIBF4/ glycerol/ chitosan	-	±3 square	30×5× 0.17	~0.05 (0.5 Hz)	-	-
	[3]	SWCNT/ BMIBF4/ PVDFHFP	-	-	BMIBF4/ PVDFHFP	-	±3.5 square	15×1× 0.28	0.257 (0.01 Hz)	-	-
		(Long
	[4]	SWCNT)/ EMITFSI/	17	0.156	EMITFSI/ PVDFHFP	<0.156	±2.5 square	15×1× ~0.06	~0.091 (10 Hz)	-	-
		PVDFHFP
	[5]	CNT/ PVDF	-	-	EMIBF4/ PVDF	-	±2.5	25×2.5× 0.08	~0.032 (0.1 Hz)	0.552 (0.1 Hz)	-
	[5]	Graphene /PVDF	-	-	EMIBF4/ PVDF	-	±2.5	25×2.5× 0.08	~0.096 (0.1 Hz)	0.768 (0.1 Hz)	-
	[5]	Graphdiyne /PVDF	-	-	EMIBF4/ PVDF	0.420	±2.5	25×2.5× 0.08	~0.624 (0.1 Hz)	1.35 (0.1 Hz)	-
	[6]	Carbon nitride/PVDF /EMImBF4	-	-	EMIBF4/ PEO/NBR	-	±3 (square)	20×2.5× ~0.085	~0.62 (0.001 Hz)	0.23 4 V) (0.01 Hz,	-
	[7]	(BP-CNT) /CNT	23	0.95	EMIBF4/ PVDF-HFP	0.246	±2.5 (square)	23×5× 0.115	0.668 (0.1 Hz)	2.4	-
	[8]	MXene	-	-	H2SO4/ PVA	0.04	±0.6 triangular	5×40× ~0.075	0.433/0.025 (0.001/0.1 Hz)	-	-
	[9]	MXene/3D (PS-MXene)	~15	3.3	EMIBF4/ PVDF	0.246	±1.5 (square)	20×5× 0.095	0.787 (0.1 Hz)	-	-
	[10]	Methyl-cellulose /MXene	53.2	12.4	5.6 M LiBr/ TPU/CMP	1.93	±1 triangular	22×3× 0.123	0.541/0.320/0 1 Hz) .0364 (0.001/0.01/0.	0.826 (DC 1 V)	4.7
	This work	TBA-MXene	54.6	9.49	5.6 M LiBr/ TPU/CMP	1.69	±1 triangular	22×3× 0.10 (Lf c = 20) 8×3×	0.771/0.431/0 .0566 (0.001/0.01/0. 1 Hz)	1.01 (DC 1 V in 10 s)	7.55
								0.10	-	1.81	51.5
								(Lf = 6)

Note: a length×width×thickness; b Not available; c free length (mm)

Acknowledgements

This work is funded by National Research Foundation of Singapore under project no. NRF2020-NRF-ANR102 MEACT. S.F.T. acknowledges support from NTU Presidential Postdoctoral Fellowship. The authors acknowledge Dr. Teddy Salim at FACTS, Nanyang Technological University, Singapore for the XPS measurements.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supporting Information

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

Author Contributions S. C. and P. S. L. designed the research. P. S. L. supervised the research and edited the manuscript. S.C. performed most of the experiments (except the XPS and TEM operations) and data analyses and wrote most of the manuscript. S. F. T. guided the transfer of MXenes to liquid cell chips, performed the TEM experiments and TEM image analysis, and wrote the corresponding experimental part. H. S., L. L. and M. E. participated in the discussion and gave some suggestions. All authors reviewed the manuscript.

Conflict of Interest

The authors declare no conflict of interest.

Supporting Information Functionalized MXene Films with Substantially Improved Low-voltage Actuation

Shaohua Chen 1 , Shu Fen Tan 1,2


Pierrick Martin
email: pierrick.martin@irap.omp.eu

Sylvie Brau-Nogué

Mickael Coriat

Philippe Garnier

Annie Hughes

Jürgen Knödlseder

Luigi Tibaldo

A comprehensive assessment of the carbon footprint of an astronomical institute

The development and use of research infrastructures accounts for more than 70% of the carbon footprint of IRAP. Our community needs to rethink this crucial facet of astronomical research to engage in effective and perennial reduction strategies.

It is a scientific fact that climate change -at a rate that is unprecedented over at least the last 2000 years -can be attributed to human influence. The disorder has reached a point where the impacts of climate change are now directly perceptible by a large fraction of mankind, notably in the intensity and frequency of climate and weather extremes, such as floods, heat waves and droughts. The phenomenon seems to be accelerating, and each of the last four decades has been successively warmer than any preceding decade since 1850 [START_REF] Masson-Delmotte | Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF].

The global mean surface temperature has already risen by approximately +1.0 • C above 1850-1900 pre-industrial levels. In an intermediate emission pathway with CO 2 emission rates remaining around current levels until the middle of the century before declining, a +2.0 • C increase would be reached sometime between 2041-2060 [START_REF] Masson-Delmotte | Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]. In such a world, adverse consequences on food security, water supply, habitat, or biodiversity are expected to affect several hundred million people, with a disproportionately higher risk for vulnerable populations [START_REF] Masson-Delmotte | Global Warming of 1.5 • C. An IPCC Special Report on the impacts of global warming of 1.5 • C above pre-industrial levels and related global greenhouse gas emission pathways[END_REF] that have the smallest responsibility for climate change [START_REF] Hickel | National responsibility for ecological breakdown: a fair-shares assessment of resource use, 1970-2017[END_REF].

Avoiding such a trajectory, and remaining under a +1.5 • C increase, requires reducing our greenhouse gases (GHG) emission rates by 40 -60% by 2030 relative to 2010 levels (implying an even 1 stronger reduction relative to 2022), and reaching net zero emissions by 2050 (when anthropogenic emissions are balanced globally by anthropogenic removals). This implies an average reduction rate of 7.6% every year over the next decade [START_REF]Cut global emissions by 7.6[END_REF]. The magnitude of the challenge is revealed by the recent Covid-19 pandemic, the drastoc reponse to which resulted in CO 2 emissions falling by 6.4% in 2020 before bouncing back [START_REF] Tollefson | Covid curbed carbon emissions in 2020 -but not by much[END_REF]. This is roughly the level of reduction we need to achieve every year, sustained over more than a decade. Because of the present-day structure of our societies and their deep dependence on fossil fuels, all sectors of human societies are concerned, and only fundamental transformations of our organizations will enable the necessary transition. Distributing the required transformation equitably across society is fundamental for their acceptance, and there is no a priori reason why scientific research should be exempted from this effort.

In the field of astronomy and astrophysics, several carbon footprint estimates have recently been published, including an assessment for the Max Planck Institute for Astronomy in Heidelberg (MPIA) [START_REF] Jahnke | An astronomical institute's perspective on meeting the challenges of the climate crisis[END_REF], and the Australian [START_REF] Stevens | The imperative to reduce carbon emissions in astronomy[END_REF] and Dutch [START_REF] Van Der Tak | The carbon footprint of astronomy research in the Netherlands[END_REF] astronomy communities. While these studies have identified professional air travel and supercomputing as significant sources of GHG emissions, potentially large sources of GHG emissions such as the consumption of goods and services and the use of space-and ground-based astronomical observatories were excluded from these analyses. A much wider scope of a research institute's activities was investigated for a comprehensive assessment of GHG emissions at the "Institute for Research in Astrophysics and Planetology (IRAP)" for the reference year of 2019 [START_REF] Martin | The carbon footprint of IRAP[END_REF]. IRAP is the largest astronomy research institute in France with 116 researchers, 28 postdocs, 78 engineers, technicians and administrative staff, and 41 PhD students employed over the full year of 2019. Scientists at IRAP conduct research on a variety of subjects: the geology of Earth and its ionized spatial environment, stars including the Sun and their planetary systems, physics and chemistry of the interstellar medium, the formation and evolution galaxies, compact objects like neutron stars and black holes, and cosmology. Research activities at IRAP include observations, modeling and theory, instrumentation and laboratory experiments. With technical personnel qualified in the field of design, construction, integration and operation of instruments on the ground and in space, IRAP is a major international center for the development of ground-and space-based as-tronomical instrumentation. The institute is spread over three different sites in the south of France: two buildings in the city of Toulouse, including one shared with other laboratories, and another shared building in the city of Tarbes.

The assessment of the institute's carbon footprint was conducted by an environmental commission officially established at IRAP in 2018. To acquire the necessary skills, eight IRAP staff members, including all co-authors of this paper, followed a 40 h training course on the Bilan Carbone c , a carbon accounting methodology and set of tools that have been developed and used in France for more than 20 years.

The overall philosophy of the methodology is to identify the most powerful lever arms to achieve significant GHG emission reductions globally, rather than compiling a list of the emissions that are either the most visible, or for which an organization recognises direct responsability. An exhaustive approach is key to developing a perennial and effective reduction plan since it reveals the deep changes that may be required to achieve significant permanent reductions. Carbon accounting over a highly restricted scope risks excluding an institute's dominant sources of emissions and hiding the reasons behind certain sources of carbon emissions.

To do so, a first step is to make a census of all activities performed at IRAP and then capture all the input flows that these activities critically depends upon as well as all output flows that the institute produces and delivers to external partners. IRAP's core activities include: (a) instrument development, including hardware and software, (b) astronomical observations, laboratory experiments and data analysis, (c) analytical and numerical modeling of natural phenomena, (d) teaching, training and public outreach, and (e) animation and participation in the scientific community. Category (d) extends significantly beyond the perimeter of the institute, and was mostly excluded from the reporting since most of the impact of teaching and training, including the commuting of students, is more appropriately assessed at the level of universities and schools. What remains inside our scope from category (d) are expenses connected to students directly using our resources and facilities (e.g. electricity used during their internships in the institute), the commuting of students affiliated to IRAP, and regular commuting of the staff between the institute and teaching sites.

According to the ISO14069 standard, we included the following emission categories in our as-sessment (usually termed scope): direct emissions from owned or controlled sources (scope 1), indirect emissions from the generation of purchased energy (scope 2), and all other indirect emissions (scope 3). In our case, the latter category dominates IRAP's carbon footprint. The list of sources we considered is given in Table 1. We emphasise that some GHG emissions sources were not included in our assessment, or only partially, because of difficulties in obtaining all the relevant data (e.g. the end use of some products delivered by IRAP, or support activities such as administration, maintenance, and financial and insurance services provided by the hosting institutions).

Formally, the carbon footprint reported below is thus a lower limit.

In practice, the assessment of GHG emissions is performed by multiplying activity data that quantify the usage of a given source (e.g. kWh of electricity used, km travelled etc.) with emission factors that quantify its unitary carbon footprint (e.g. g CO 2 e/kWh of electricity or g CO 2 e/km for a given transportation mode). The purchase of goods and services is included in a similar way, following a so-called cost-based approach that converts the economic value of the purchased goods and services into an estimate for the associated GHG emissions using economic sector-average emission factors (typically equating a ke or Me of expenditure on a given family of products, e.g. electronic equipment, to kg CO 2 e or tCO 2 ).

Activity data were obtained from a variety of sources, e.g. travel or purchase listings provided by the institute's administration, power or water consumption measurements by our hosting university, or an online survey of the staff. Emission factors were taken from the "Base Carbone" database of the French Environment and Energy Agency (ADEME) [START_REF]Base carbone[END_REF]. Uncertainties on both activity data and emission factors were adopted following the Bilan Carbone c recommendations, or adapted to our specific case when possible.

Results

Table 1 presents our estimates of IRAP's GHG emission by source, while Fig. 1 provides a graphical representation of the data. In total, we estimate that IRAP had a carbon footprint of 7,400 ± 900 tCO 2 e in 2019. About 60% of the footprint is attributed to the use of observational data from space missions and ground-based observatories, and the use of these infrastructures is clearly the primary source of IRAP's GHG emissions. The second major contributor (18% of emissions) is related to the fraction of the staff responsible for 20% (50%) of the emissions has an average age of 46.9 yr (47.3 yr), to be compared with an average of 44.3 yr for the whole staff. There seems to be a much more pronounced effect of gender: 92% (87%) of the persons responsible for 20% (50%) of the emissions are male, to be compared with an average for the whole staff of 75%. Finally, there is a non-negligible fraction of engineers and technicians among the people responsible for most of the GHG emissions from professional travel, and they are primarily involved in the development of future space missions, demonstrating that this activity is an important driver of travel-related emissions at IRAP.

Our results clearly points to the main driver of IRAP's carbon footprint: astronomical research infrastructures. In total, use of data from astronomical facilities and the purchase of goods and services for instrument development account for about 70% of IRAP's carbon footprint. This is a considerable contribution, and one omitted in previous estimates of the carbon footprint of astronomical institutes. In addition, it is likely that a significant fraction of professional trips at IRAP are also connected to instrument development projects, which only strengthens this conclusion.

Whether such a repartition can be considered as generic for the astronomy community remains to be confirmed by performing comprehensive carbon footprint assessments at other institutes. IRAP has a long history in instrument development and observational data analysis, which is no doubt reflected in the present result, which already exhibits interesting similarities and differences to other published assessments. At IRAP, flights account for 8.1 tCO 2 e per researcher with a PhD degree, which is comparable to the estimates of 8.5 tCO 2 e being found for MPIA and 12.0 tCO 2 e for the Australian community [START_REF] Jahnke | An astronomical institute's perspective on meeting the challenges of the climate crisis[END_REF], but much larger than the 2.0 tCO 2 e per researcher for the Dutch community Ref. [START_REF] Van Der Tak | The carbon footprint of astronomy research in the Netherlands[END_REF]. Conversely, the emissions from supercomputing at IRAP amount to 0.2 t CO 2 e/yr per researcher, on average, which covers the impact from electricity consumption, equipment, and operations of the computing centers; at MPIA, supercomputing generates an average 4.6 t CO 2 e/yr per researcher, from electricity consumption only. Only a small part of the difference can be explained by the carbon intensity of electricity, which is a factor ∼ 4 higher at MPIA with respect to

IRAP.

An important factor affecting the final repartition of GHG emissions is that the operation of local infrastructure -heating, electricity, commuting, food, waste, etc. -makes a relatively small contribution of about 800 tCO 2 e to IRAP's carbon footprint. The energy sources from which electricity and heating are produced have a relatively low carbon footprint, with electricity being predominantly of nuclear origin in France and heating of our largest building arising from biomass burning, with related emission factors being 60-70 gCO 2 e kWh -1 . Assuming a worst-case carbon intensity of ∼800 gCO 2 e kWh -1 instead -representative of countries like Australia, Poland, China, India, or South Africa -the related sources would increase to about 2700 tCO 2 e yr -1 , comparable to the sum of professional travels and purchase of goods and services at IRAP. A higher carbon intensity of electricity would also affect other sources such as external computing, and to some extent the purchase of goods and services. If IRAP were situated in a country that relies significantly on fossil fuels for electricity production and heating, we estimate that our total 2019 footprint would be at least 10000 tCO 2 e.

Discussion

This assessment shows that performing research in astronomy and astrophysics at IRAP according to the standards of 2019 stimulated GHG emissions equivalent to 28 t CO 2 e/yr per person involved in that activity, on average. These emissions are spread across a variety of social and economic sectors and dividing GHG emissions of an activity by the number of people working on it is a standard metric that enables comparison between entities of the activity sector and between different activity sectors. It is a measure of the carbon cost of what this or that activity provides, which is relevant in anticipation of trajectories towards a sustainable future.

The global average target of 2 t CO 2 e/yr per capita by 2050 is a budget within which societies should fit what they deem necessary to human life. How emissions could be distributed across activity sectors is a political question that needs to be addressed by a wide-ranging democratic decision process. The place of scientific research should naturally be part of this discussion, informed by a quantitative estimate of its environmental footprint and social benefits. The discussion should not be restricted to the research community alone.

We estimated that the total carbon footprint of IRAP could be reduced by up to 10% by changing our traveling and commuting habits. Since the impact of professional travel is very unevenly distributed across IRAP staff, significant reductions can be achieved by enforcing very reasonable limits that would largely preserve the possibility to meet and exchange in-person with international colleagues. Refurbishing the local infrastructure in line with current standards or goals would only provide an improvement at the percent level. Achieving stronger reductions to meet France's national reduction targets of 50% by 2030 and 80% by 2050, requires acting on the main shares of our footprint that relate to research infrastructures. Efficient measures have to be taken at a level that essentially reaches beyond the perimeter over which IRAP has some operational control. Revising the criteria for the purchase of goods and services offers some potential for carbon footprint reduction. Since most of the instrumental projects IRAP is involved in are multilateral, however, this solution involves a progressive shift of standards that should be promoted and supported at the institutional level. We further note that establishing both new purchase procedures for research organizations and the environmental footprint information by all suppliers will take years.

The magnitude of the challenge and the necessity to quickly engage in an effective transition calls for acting on all levers: lowering the carbon intensity of our activities, reducing their pace, and shifting our work practices towards less emission-intensive options. In doing so, we should not disregard the second option, especially since it is directly under our control as a community and can have quick and direct effects, as opposed to the uncertain decarbonization trajectories of suppliers and partner organizations. Ultimately, our recommendation for a community-based reduction strategy would be to divert a growing fraction of our budgets to fund the decarbonization of existing operational infrastructures, enhance the research and development of low-carbon technologies on which future projects will be based, and to reduce the cadence and scale of the deployment of new research infrastructures. The latter point cannot be left out of the equation, otherwise any benefit in decarbonizing existing facilities will be promptly wiped out by an increase in the number of facilities. The timescales involved in the development of astronomical research infrastructures lock in our emissions for the next decades, and the problem will only be exacerbated if we continue to postpone the implementation of a far-reaching emissions reduction strategy.

Figure 1 :

 1 Figure 1: Distribution of IRAP's GHG emissions by emission source in 2019

Table 1 :

 1 Summary of IRAP's GHG missions in 2019.

	Source

about 260 employees. Interestingly, there is a very limited effect of seniority in this distribution:

Competing interests

The authors declare no competing interests.