Understanding Piezoionic Effects in Chemo–Mechanical Energy Harvesting by Carbon Nanotube Yarn Twists

Strategies for converting mechanical energy into electrical energy hold significant importance in diverse battery‐free and battery‐supported applications. Recent studies have demonstrated promising approaches involving the twisting of carbon nanotube yarns, which alter the intrinsic electrochemical capacitance during mechanical motion, thereby generating electrical energy in various aqueous environments. However, the fundamental mechanism of chemo–mechanical energy harvesters based on the nanoscale piezoionic effect, as well as the kinetics of both cations and anions within the system, remains to be clarified. In this study, experimental and computational approaches aimed at fundamentally understanding the piezoionic effect in nanoscale chemo–mechanical dynamics are presented. This phenomenon is analyzed using in situ Raman scattering, piezoelectrochemical impedance spectroscopy, and molecular dynamics simulations. The findings elucidate the collective contributions of cations and anions under mechanical energy inputs and demonstrate the impact of piezoionic kinetics on electrical energy outputs. By gaining a comprehensive understanding of the fundamental piezoionic effect in chemo–mechanical energy harvesting systems, significant advancements in energy sustainability across numerous practical applications are anticipated.


Introduction
Chemo-mechanical energy harvesters provide a novel means of converting surplus mechanical energy, encompassing human motion, water and airflow, pressure, and vibration, into electrical energy.[3][4][5] The concept of chemo-mechanical energy harvesters, specifically those based on the twisting of carbon nanotube (CNT) yarns, commonly known as "twistron", was introduced in 2017.This technology demonstrated the generation of electrical energy through the mechanical stretching and release of CNT coils within an electrolyte. [6]Chemo-mechanical energy harvesters have exhibited substantial promise owing to their operation in diverse aqueous environments, including acidic, alkaline, human sweat, and ocean waves, [6][7][8][9][10][11][12][13][14][15][16][17][18] as well as their ability to achieve the highest frequencynormalized peak output power, with 0.73 kW kg −1 at 1 Hz and 3.19 kW kg −1 at 30 Hz. [17] These findings underscore the considerable potential for sustainable energy applications, particularly in the low-frequency range, which represents the range in which most natural mechanical energy abounds (<10 Hz). [19,20]espite recent advancements in chemo-mechanical harvesters based on twisted CNT yarns, comprehending the fundamental piezoionic mechanism within the overall system remains a significant challenge.Thus far, the mechanism has been explained primarily by changes in capacitance resulting from the twisting of the CNT yarn.In brief, stretching the coiled CNT harvesters leads to additional twisting, increasing yarn density, which alters capacitance and generates a potential difference.Although this explains the generation of electric energy simplistically, the behavior of the electrical double layer (EDL) in chemo-mechanical energy-harvesting systems remains largely unexplored.Two possible explanations have been proposed to account for changes in capacitance and the resultant electrical power: 1) ion densification (involving cations packed inside the CNT) or 2) ion depletion (in which anions are released from the CNT into the electrolyte).Additionally, the impact of the physical properties of ions in the surrounding electrolytes on chemo-mechanical harvesting performance remains unclear.
To address these challenges, we present experimental and computational approaches for understanding the piezoionic effect within chemo-mechanical energy-harvesting systems.Our in situ Raman spectroscopy analysis supports the ion depletion hypothesis, providing compelling evidence for ion release during mechanical deformation.A novel approach combining piezoelectrochemical impedance spectroscopy (PIS) and electrochemical impedance spectroscopy (EIS) facilitates the analysis of ionic species-and frequency-dependent changes in stretch-induced current.Our findings confirm two critical aspects: 1) mass transport significantly influences piezoionic behavior at low stretching frequencies, and 2) charge transfer resistance affects har-vesting performance at higher frequencies.Furthermore, our assessment of the structure of solvated cation clusters and ligand bond strength through molecular dynamics (MD) simulations supports our experimental results, emphasizing the influence of physical properties such as ionic conductivity and the rigidity of the ionic shell on piezoionic kinetics and resulting electrical outputs.We anticipate that our findings will contribute to the design of high-performance chemo-mechanical energy harvesting systems.

Exploring Piezoionic Effects in Different Types of Electrolytes
The primary objective of this study was to investigate the piezoionic effects driven by the physical properties of ionic clusters within electrolytes in chemo-mechanical energy harvesting systems.Figure 1a illustrates the experimental setup of a threeelectrode system, which incorporates solvated ions to assess the energy-harvesting performance of coiled CNT yarns (Figure S1, Supporting Information).The system consists of a working electrode composed of coiled CNT yarn, a counter electrode comprising Pt mesh/CNT buckypaper, and an Ag/AgCl reference electrode.Three neutral aqueous electrolytes, namely 0.1 m LiCl, NaCl, and KCl, were chosen to evaluate the impact of cations on energy harvesting while maintaining constant thermodynamic conditions, including pH, concentration, and temperature.Notably, during ionization, cations and anions are surrounded by water molecules.The size of the solvated cation clusters follows the order Li + < Na + < K + , which aligns with the order of atomic size. [21]In 0.1 m LiCl, NaCl, and KCl solutions, the coiled CNT yarns exhibit baseline open-circuit voltages (OCV) of 99.4,227.3, and 292.3 mV, short-circuit currents (SCC) of 73.4,86.5, and 144.7 A kg −1 , and peak power outputs of 28.5, 31.2, and 34.8 W kg −1 , respectively (Figure 1b,c).These measurements, conducted at 1 Hz with a 60% sinusoidal tensile strain, reveal significantly different energy harvesting performances dependent on the electrolyte employed.Notably, the electrical output performance metrics (e.g., OCV, SCC, and peak power) are directly proportional to the size of solvated cation clusters, as observed in this study.These performance discrepancies motivate our comprehensive investigation into the piezoionic mechanism, focusing on the controlled physical properties of surrounding ions within the harvester systems.Traditionally, changes in capacitance have been identified as the sole critical factor in chemo-mechanical harvesting mechanisms. [6]However, in our study, in contrast to the output performance differences observed through OCV, SCC, and peak power measurements, similar gravimetric capacitances of ≈3.9 F g −1 at 0% strain and ≈2.7 F g −1 at 60% strain were measured, indicating similar capacitance changes of ≈32% for all electrolytes (Figure 1d).Consequently, the results suggest the presence of other undisclosed factors that must be uncovered to comprehend the observed variations in energy harvesting performance across different electrolytes.
To gain insights into the unexplored principles of chemomechanical harvesters, we investigated the charge storage mechanism in CNT yarn harvesters through anodic and cathodic linear voltammetry scans in the three types of electrolytes (Figure 1e,f) to determine the dominant charge storage mechanism.The charge storage mechanism can be categorized into Faradaic contributions, characterized by diffusion-controlled effects, and non-Faradaic contributions, defined as capacitive effects, as expressed by the equation (i = C 1 v + C 2 v 1/2 = av b ), where i represents the current; v the sweep rate; C 1 v the capacitive current, and C 2 v 1/2 the diffusion-controlled current. [22,23]To calculate the b value, linear sweep voltammetry was conducted in the range 0.1-0.4V, as the OCV outputs of the harvesters fall within this range.The b value within the range of 0.7 ≤ b ≤1 and 0.35 ≤ b < 0.7 indicates that capacitive effects (non-Faraday) and diffusioncontrolled effects (Faraday) dominate the charge storage mechanism, respectively. [24]Given that the measured b values in all three types of electrolytes are close to 1, these findings imply that capacitive effects prevail over diffusion-controlled effects in the overall charge storage of coiled CNT yarn harvesters.Remarkably, considering the collective data discussed in Figure 1, including rectangular-like cyclic voltammetry (CV) curve and b values in the range 0.9-1, we categorize CNT-based chemo-mechanical energy harvesters as non-Faraday capacitive systems.This conclusion, highlighting the dominance of capacitive effects, guides the subsequent portion of the study toward investigating physical factors influencing piezoionic behaviors, such as ionic movements, ionic conductivity, and the rigidity of solvated ionic clusters, rather than emphasizing chemical reactions, including redox processes.

Characterization of Ionic Kinetics by In Situ Raman Spectroscopy
To gain insights into the physical piezoionic behavior, we employed in situ Raman spectroscopy to examine the ionic motion within coiled CNT harvesters during mechanical stretching and release cycles.[27] Additionally, for clarity, the terms "stretching" and "releasing" refer to mechanical deformation input and output applied to the coiled CNT harvesters, respectively.Comparing the Raman peaks obtained in an air environment to those in aqueous environments with electrolytes and deionized (DI) water (Figure 2a), we observed an overall increase in intensity due to the high conductivities of the latter.When analyzing the Raman spectra, the G band emerged as a critical feature representing sp 2 carbon bonding.In the case of air, the G band peak exhibited separation, akin to stretched graphene, [28] with peak characteristics indicating tension on the sp 2 bonds owing to the highly twisted structure.However, no peak shift was observed in the spectrum of the coiled CNT yarn during stretching in air.In DI water, the separated G band peaks combined and shifted upward due to swelling (1585 cm −1 at 0% strain).However, no change was observed during stretching deformation.In the 0.1 m KCl electrolyte environment, the characteristic peak shifted leftward from 1585.6 to 1581.7 cm −1 during stretching (Figure 2b) and then reversibly returned to its original position upon release.Overall, the results from in situ Raman spectroscopy, particularly the leftward shift of the G-band peak, suggest that ions are released from the coiled CNT yarns into the electrolyte during stretching and re-inserted into the CNT yarns during release (Figure 2c).Given the rapid peak shifts, the OCV increases exponentially rather than linearly during the stretching cycle.Combining the OCV data and the in situ Raman spectroscopy results presented in Figure 1 and Figure 2, we can conclude that the removal of anions, leading to gap depletion between the CNT bundles, induces an OCV increase in the positive direction, as schematically depicted in Figure 2d.This confirms that anion depletion predominates during the stretching cycle and validates the direction of anionic movement in chemomechanical energy harvesters.However, the evidence remains insufficient to elucidate the impact of different cation types on the observed variations in energy harvesting performance, including OCV, SCC, and peak power, in 0.1 m LiCl, NaCl, and KCl.

Ionic Conductivity Effects Analysis by PIS and EIS
To comprehend the influence of cations on harvester performance, we developed a novel analytical approach termed PIS.PIS provides comprehensive insights into the variations in output current by manipulating applied voltage, deformation frequency, and the surrounding ionic environment.The primary parameter in PIS is the applied voltage, critical for determining the type of ions in the Stern and diffuse layers and the charge density of the EDL structure based on the Gouy-Chapman-Stern model.A positive or negative applied voltage magnitude is important in determining the type of ions in the Stern and diffuse layers, as well as the charge density of the EDL. [29]Additionally, the stretching frequency, influencing the rate of ionic movement, impacts the time-dependent electrochemical reactions at the electrodeelectrolyte interfaces. [29]IS involves two key steps: 1) determining the potential of zero charges (PZC) and 2) examining the response of a chemomechanical energy harvester to sinusoidal tensile strain at an applied voltage based on the PZC (V-PZC) across a wide range of frequencies.Initially, the PZC is defined as the potential at which the electrode surface (in this case, the coiled CNT yarns) becomes electrically neutral, implying no net charge within the EDL.To make valid performance comparisons based solely on cation type (Li+, Na+, and K+), rather than charge quantity, the PZC values for different electrolytes have to be identified.The PZC determination was accomplished using piezoelectrochemical spectroscopy (PECS), enabling simultaneous measurement of output current while cyclically varying voltage and mechanical deformation. [6]Figure 3a presents the PECS curves with and without 10% strain at 5 Hz.The intact stretch-induced currents were determined by subtracting the PECS curve without strain from that with 10% strain at 5 Hz in 0.1 m LiCl, NaCl, and KCl solutions (Figure 3b).The voltage at which the stretch-induced current was minimized and underwent a 180°phase shift was identified as the PZC.PZCs for the three electrolytes were distributed within a voltage range from −0.12 to −0.08 V (Figure 3c).
After establishing the PZCs for various electrolytes, we employed constant-voltage piezoelectrochemical spectroscopy (C-PECS) to obtain precise results for the stretch-induced current over a wide frequency range, including extremely low frequencies.C-PECS, a one-variable-at-a-time technique, measures the stretch-induced current under sinusoidal tensile strain while maintaining a constant V-PZC.C-PECS results were obtained at different constant voltages (−0.6 to 0.6 V of V-PZC) and various stretching frequencies (0.1-10 Hz) with a sinusoidal tensile strain of 10% (Figures S2,S3,S4, Supporting Information).At constant negative voltages and low frequencies (0.1 Hz), the output current amplitudes increased in the order of 0.1 m LiCl, NaCl, and KCl, primarily due to differences in cation types (Figure 3d (left) and Figure 3e (left)).Conversely, no significant difference was observed in the stretch-induced current among the three electrolytes at constant positive voltages, owing to the presence of the same Cl − anions in all systems (Figure 3d (right) and Figure 3e (right)).However, above 0.5 Hz of stretching frequencies at positive voltages, the output current amplitudes increased in all electrolyte types.Figure 3f illustrates the difference in output current amplitudes as a function of stretching frequency at a constant positive voltage.To recapitulate the findings from Figure 3, we confirmed that the output current performance of the harvester follows the order KCl > NaCl > LiCl at a positive V-PZC.This reconfirms the results previously observed for OCV, SCC, and peak power performance differences.Systematically controlling the V-PZC conditions excludes the possibility of differences in total charge quantity in different ionic environments.
Finally, to address the fundamental question-why the performances vary under different electrolytes-we conducted impedance analysis during the harvesting process using electrochemical impedance spectroscopy (EIS) in 0.1 m LiCl, NaCl, and KCl solutions. [30]We hypothesized that physical ionic properties, such as ionic conductivity, influence the harvester systems' output performances.During EIS measurements, the harvester remains mechanically fixed (i.e., no stretching or releasing) and undergoes ionic charging and discharging cycles via frequency sweep.In brief, the EIS results aligned with the output current relationships observed via C-PECS analysis in Figure 3g,h.As shown in the Bode and Nyquist plots, impedance decreases more rapidly at high frequencies in the order of 0.1 m KCl, NaCl, and LiCl.This is crucial to note because a rapid decrease in impedance signifies higher ionic conductivity.Consequently, this trend correlates with ionic conductivity, resulting in charging/discharging rates and improved output current performance (K+ > Na+ > Li+).
In more detail, the Bode plots covering frequencies from 0.1 to 10 Hz reveal similar magnitude differences in impedance below 0.5 Hz, with distinctions becoming more apparent as the frequency surpasses 0.5 Hz.The reciprocals of the impedance magnitude closely correlate with the peak-to-peak current at the stretching frequency determined using C-PECS.The bottom panel of Figure 3g illustrates the phase angle of the impedance as a function of frequency.At 0.1 Hz, phase angles of ≈−80°w ere observed for each electrolyte, closely resembling the −90°c haracteristic of an ideal capacitor.As frequency increases, resistive behavior becomes more dominant than capacitive behavior.The values of the time constant (), calculated at a phase angle of −45°, were found to be 1.64, 1.43, and 1.30 s for 0.1 m LiCl, NaCl, and KCl, respectively (Figure S5, Supporting Information).This indicates that the EDL charging and discharging rate increases in the order of 0.1 m LiCl, NaCl, and KCl solutions.The Nyquist plots display the real part of the impedance, represented by the x-intercept at the right end of the depressed semicircle at high frequencies, signifying the sum of the solution and charge-transfer resistances.Consequently, impedance at high frequencies decreases in the order of 0.1 m LiCl, NaCl, and KCl solutions.The nearly vertical slopes observed at low frequencies indicate the capacitive behavior of each electrolyte.The EIS results not only strongly support the hypothesis that ionic conductivity affects harvester performance but also provide additional critical insights: higher ionic conductivity enhances harvester performance.
Further analysis was conducted on electrolytes featuring the same cations paired with different anions (Figure S6, Supporting Information).At low frequencies (0.1 Hz), similar stretchinduced currents were observed at negative voltages in 0.1 m HCl and HBr and 0.05 m H 2 SO 4 solutions, while the slopes of peakto-peak current clearly varied for different electrolytes at positive voltage ranges.In comparison to C-PECS analysis results obtained in 0.1 m LiCl, NaCl, and KCl solutions, the voltage direction showing clear current amplitude differences was the opposite.At a high frequency of 5 Hz, the stretch-induced currents were higher in the order of 0.05 m H 2 SO 4 , 0.1 m HCl, and 0.1 m HBr.To support the influence of anion species, the MD simulation will be revisited in the subsequent discussion.In conclusion, in the energy harvesting process, the influence of the Stern layer is crucial at low frequencies, whereas the charge-transfer resistance of the electrolyte plays a significant role at high frequencies.
In other words, piezoionic mechanisms and resulting energy harvesting performances are determined by ionic conductivities and mechanical input frequencies.

Computational Multiscale Modeling
To gain a deeper understanding of the transport kinetics of hydrated alkali-ion clusters in the EDL on multi-walled CNT (MWCNT) surfaces, we conducted first-principles density functional theory (DFT) calculations and classical MD simulations.Initially, we determined the charge distribution of each nanotube shell comprising the MWCNT using DFT calculations.Owing to the cylindrical shape and wall-to-wall interactions of a single MWCNT, the intrinsic charge was redistributed with the beta spin density at the innermost wall and the alpha spin density at the outermost wall (Figure 4a).This spin density distribution results from the effect of the CNT radius on the energy level of the band structure, [31,32] with the highest electron affinity [31] and low band energy levels [32,33] of the innermost wall causing charge transfer from the outer wall to the inner wall.Conversely, when the MWCNTs were charged by injecting holes or electrons, we observed a significant contribution from the outermost wall (Figure 4b).This surface-dominant behavior in the charged state is plausible because a valence band is formed in the alpha spin state.
The response of alkali ions to changes in the spatial electrostatic potential field was further explored using all-atom MD simulations.Multiphase systems consisting of ionic solutions and MWCNT bundles were modeled.The charge distributions of MWCNTs in neutral and charged states obtained from DFT calculations were implemented (Figure S7, Supporting Information).The results show that the electrochemical characteristics of each alkali ion are distinct in the cation distribution profiles, while no significant deviation is observed in the Cl − profiles according to the cation pair (Figure S8, Supporting Information).Specifically, K+ exhibited the most intense peak among the alkali ions in the neutral state and responded sensitively to the charged state (Figures S8b,S9, Supporting Information), whereas Li+ showed relatively insensitive transportation kinetics on the MWCNT surfaces, aligning with our experimental observations.These results indicate that the excellent mobility of potassium at the EDL is identified by its significant population change.
We further explored the effect of changes in the electrostatic potential field of the MWCNTs on the binding properties of ions and hydrated clusters. [34,35]The structural properties of hydrated alkali ions were quantified following a specific methodology (Figure 4c; Figure S10, Supporting Information). [36]The most frequent number of water ligands found in the hydration clusters of the bulk solution was four for Li+ ions, six for Na+ ions, and seven for K+ ions, all exhibiting a highly symmetric structure.][39] In contrast, Li+ ions did not alter the number of water ligands during adsorption, while K+ ions underwent significant dehydration.This suggests that hydrated Li+ and K+ clusters tend to favor inner-and outer-sphere adsorption, respectively.Outer-sphere adsorption typically results in a relatively lower level of interfacial charge transfer compared to inner-sphere adsorption, and the remaining water ligands act as a shield, reducing the electrostatic interaction between the cations and the MWCNT electrode. [34]otably, changes in the spatial electrostatic potential field due to electron and hole injection further shifted the coordination peak of the hydrated K+ cluster.The cleavage of water ligands during this process produced excess charge in the incomplete primary shell, leading to the observed highly agile electrochemical potential change during K+ ion adsorption.
The results of the charge simulation indicate that the adsorption kinetics of water ligands comprising hydrated ion clusters play a pivotal role in shaping the electrostatic potentials on the surface of the MWCNTs.To delve deeper into the origin of the structural properties of these hydrated ion clusters, we conducted DFT calculations (Figure 4c).The adsorption characteristics of each alkali ion can be elucidated by considering the rigidity of the hydrated clusters, which essentially relates to their mechanical properties, including ligand bond strength and interligand repulsion. [36]During the process of inner-sphere adsorption, the primary shell experiences structural deformation.The degree of structural rigidity in hydrated ion clusters is directly proportional to their resistance against angular deformation and stretching of the water ligands.This high rigidity in ion clusters can be achieved when both the interligand repulsion force and ligand bonding force are particularly strong.Depending on the energy levels of the electrochemical potential, [40] Li+ clusters exhibit the highest level of rigidity among the alkali ions, while hydrated K+ clusters possess the least rigidity.This finding also clarifies why hydrated K+ clusters lose most of their water ligands compared to other alkali ions during the inner-sphere adsorption process.This chemo-mechanical interplay among ion clusters offers valuable theoretical insights into the optimal harvesting performance observed under high-frequency conditions, revealing the exceptional conductivity of K+ ions within the EDL.
Furthermore, we conducted an investigation into the behavior of hydrated ions in response to interstitial space contraction through all-atom MD simulations.Our simulation setup involved two MWCNTs positioned side by side, with a center-to-center distance of 3.8 nm.The simulation box was filled with water and ions (as depicted in Figure 4d).To create the initial morphology, we removed four MWCNTs from the bilayer model that had been considered in the charging simulations.Subsequently, we set the two MWCNTs in motion, making them approach each other rapidly at a velocity of 20 m s −1 .Throughout the movement of the MWCNTs, we ensured that the thermodynamic equilibrium of the entire system was maintained under the NVT ensemble, with a target temperature of 300 K.
As the two MWCNTs approach each other within a certain distance, the EDLs associated with each MWCNT start to overlap and eventually collapse, as illustrated in Figure 4e.Simultaneously, we examined the structural properties of the water ligands by monitoring the coordination number of hydrated ions located between the MWCNTs (Figure 4f).In the case of Li, the hydrated shell structure with four water ligands remains intact until the MWCNTs come into full contact (i.e., the distance between the centers of the MWCNTs is reduced to 2.3 nm).The strong ligand bonds in the hydrated Li structure do not break even during EDL collapse.Instead, they move away from between the MWC-NTs due to their structural flexibility.Consequently, there is no direct interaction between the Li-ion core and the MWCNTs during this mechanical deformation.This phenomenon provides an explanation for the observed lowest energy harvesting performance of the LiCl solution in our experiments.In the case of Na ions, the most frequently observed coordination number for the hydration shell is 5 under the initial configuration conditions of the MWCNTs.Considering that the original hydration shell of Na consists of six water ligands, the observed coordination number in the interstitial space suggests inner-sphere adsorption of the ion. [36]Furthermore, as the MWCNTs approach each other, the peak of the coordination distribution shifts to lower values.This shift is caused by interligand repulsion, which contributes to the structural resilience of the ion clusters, pushing them away from the MWCNTs.This, in turn, results in a hydration shell with a highly symmetric ligand structure.Therefore, the strong structural rigidity of the ion clusters with highly symmetrical ligand structures sharpens the peak of the coordination distribution.From this perspective, it is straightforward to understand the observation that the K cluster, with low ligand repulsion, shifts the peak to the left without changing its height.The softness of the hydration shell of K ions leads to the highest cation adsorption under the same applied voltage conditions, providing strong support for our experimental findings.Additionally, we conducted all-atom MD simulations to determine the hydrated ion cluster structures of the anions Br, Cl, and SO (Table S1, and Figure S11, Supporting Information).Figure S12 (Supporting Information) demonstrates that the difference in energy harvesting performance between Cl and Br ions is closely related to the structural mechanics of the ligands, similar to what was observed for cations.

Understanding the Piezoionic Energy Harvesting Mechanism
Finally, a piezoionic energy harvesting mechanism, involving the EDL change and ion kinetics due to the mechanical deformation of CNT bundles, was proposed based on both experimental and computational results (Figure 5a-d).We summarize our understanding of chemo-mechanical energy harvesters in terms of the gap distance between the CNTs (d gap ) in four aspects: 1) sufficient d gap without EDL overlapping; 2) contacting diffuse layer; 3) overlapping diffuse layer and contacting Stern layer; 4) overlapping Stern layer.
In the absence of external bias voltages and mechanical stress, we observed a positive OCV baseline based on the PZC ( a ) and EDL capacitance (C a ) of the chemo-mechanical harvesters in 0.1 m LiCl, NaCl, and KCl solutions.The positive OCV indi- cates the accumulation of chloride ions as a Stern layer on the hole-injected CNTs.Subsequently, cations and anions form a diffuse layer through electrical attraction.When a compressive force is applied to the CNT bundle, d gap decreases, resulting in sequential layer overlap.However, the potential of the CNT bundle will not increase unless d gap is sufficiently close for the diffuse layers to overlap ( b =  a and C b = C a ).As d gap decreases, the overlap of the diffuse layers causes ion desorption due to repulsive forces.Consequently, capacitance slightly diminishes (C c < C b ), leading to a minor potential increase ( c >  b ).Ion desorption induces a potential difference and electron transport from the working electrode to the counter electrode.Subsequently, Stern layers start to overlap, depleting anions, which strongly adhere to the electrode surface and are responsible for potential screening.This results in a significant capacitance reduction (C d << C c ), a rapid electrode potential increase ( d >>  c ), and subsequent massive electron transport.During the process of restoring the stretched coiled CNT yarn to its initial length, d gap increases stepwise, leading to ion readsorption onto the electrode, with electrons moving from the counter electrode to the working electrode.
Our findings suggest that lower ionic concentrations yield better performance at higher frequencies, while at higher ionic concentrations, lower frequencies are more effective (Figure 5e; Figure S13, Supporting Information).This insight enables the tailored design of energy harvesters to optimize their performance for potential applications in various aqueous environments.

Conclusion
In summary, this study provides a comprehensive understanding of the piezoionic harvesting mechanism in twisted CNT-based harvesters, covering piezoionic kinetics and EDL distribution.Although the mechanism of chemo-mechanical energy harvesters has traditionally been explained by capacitance changes within the EDL during mechanical deformation, our study delves into other previously undisclosed factors that influence harvester performance.We have presented a combination of experimental and computational approaches, including in situ Raman spectroscopy, PIS and EIS analysis, and MD simulation, to unveil previously unknown relationships involving ionic movement, ionic conductivity, and the rigidity of ionic clusters in energy harvesting performances.Future advancements in high-performance piezoionic chemo-mechanical energy harvesters will necessitate optimizing ion mobility, ion conductivity, solvated ion cluster structure, ligand bonding force, interligand repulsive force, and rigidity within the harvester electrolyte system design to facilitate rapid mass-ion absorption/desorption between the electrode and electrolyte while maintaining low impedance.Additionally, extensive research into electrode materials and structures that enhance efficient piezoionic kinetics will be of utmost importance.

Experimental Section
Fabrication of Coiled CNT Yarn Harvesters: CNT forests, consisting of vertically aligned CNT nanofibers and serving as precursors for the CNT yarn, were produced through the chemical vapor deposition method. [41]he CNT nanofibers extracted from the CNT forest are referred to as CNT sheets. [42]Subsequently, a CNT yarn with a diameter of ≈94 μm was manufactured by homochirally twisting two-layer-stacked CNT sheets while maintaining a constant load.
Characterization: The CNT yarn harvesters were twisted using a custom-made fiber-twisting machine, while a sinusoidal tensile strain was applied using a custom-made stretching machine.To assess the electrochemical properties of the harvesters, an electrochemical analysis device (Zive SM6, WonA Tech.) was used.In addition, vibrational spectra of the coiled CNT yarns immersed in aqueous media containing KCl were obtained using a RAMAN force dispersive Raman spectrometer (Nanophoton, Osaka, Japan).The spectrometer had a 532 nm green laser source and a diffraction grating with a groove density of 300 lines per mm.Raman scatterings were recorded using an air-cooled front-illuminated chargecoupled device detector, with the spectra being calibrated against a silicon standard sample having a representative peak position of 520 cm −1 .For the Raman spectrum collection, a coiled CNT yarn was immersed in KCl solutions or DI water, and an average of 50 successive 1-s frames were acquired.
Calculations: The gravimetric capacitance (C) was calculated from the CV curves as follows: where I is the average discharge current; dV/dt is the scan rate; and m is the mass of the CNT yarn harvester.The charge density (Q) was calculated from the generated current using: where I is the sinusoidal output current; t is the time; and m is the mass of the CNT yarn harvester.The peak power (P) was calculated as follows: where V is the peak voltage; R is the impedance-matched resistance; and m is the mass of the CNT yarn harvester.

Figure 1 .
Figure 1.Harvester performance with different chloride electrolytes.a) Schematic of the experimental setup to measure the chemo-mechanical energy harvesting and the hydrated alkali ions.All experiments were conducted using the three-electrode system in 0.1 m LiCl, NaCl, and KCl solution.b) OCV and SCC and c) peak power of the coiled CNT yarn under a sinusoidal tensile strain of 60% at 1 Hz (black: 0.1 m LiCl, red: 0.1 m NaCl, and blue: 0.1 m KCl).d) Cyclic voltammetry curves at 0% strain (straight line) and 60% strain (dash line).e) Linear sweep voltammetry curves and f) calculated b-value for anodic and cathodic scans.

Figure 2 .
Figure 2. In situ Raman spectroscopy for piezoionic kinetics.a) Raman spectra of the coiled CNT yarn during 60% stretching in 0.1 m KCl solution (top), DI water (middle), and air (bottom).b) Magnified in situ Raman spectra at the G band from 0% to 60% strain in 0.1 m KCl solution.c) OCV and the Raman shift of G peak versus strain in DI water and 0.1 m KCl solution.d) Illustration of the process of ion depletion in the coiled CNT yarn harvesters during mechanical stretching.

Figure 3 .
Figure 3. PIS and EIS for ionic conductivity effects for energy harvesting.a) Cyclic voltammetry curves of the coiled CNT yarn without stretching (green) and during 10% stretching at 0.2 (orange) and 5 Hz (gray).b) Stretch-induced current measured via PECS in 0.1 m LiCl (black), 0.1 m NaCl (red), and 0.1 m KCl (blue).c) Peak-to-peak current profiles near the PZC.d) C-PECS results with applied constant voltages and sinusoidal tensile strain resulting in the stretch-induced current at the stretching frequency of 0.1 Hz. (Left: 0-30 s, Right: 40-70 s) e) Peak-to-peak current versus applied voltage based on the PZC at the stretching frequency of 0.1 Hz. (left: constant negative voltage (−0.6-0V), right: constant positive voltage (0-0.6 V) f) Peak-to-peak current versus the stretching frequency at the V-PZC of 0.6 V. g) Bode plots and h) Nyquist curves in different electrolytes.

Figure 4 .
Figure 4. Multiscale models for piezoionic energy harvesting.a) Spin density distribution of the neutral state MWCNT.Positive and negative values indicate alpha and beta spin, respectively.b) Charge distribution for each shell of (6, 0), (15, 0), and (24, 0) constituting the MWCNT.c) Ligand bond strength ( Ēp ), cluster binding energy ( Ēb ), and interligand repulsion ( Ēr ) for water ligands in the hydrated ion clusters.Each value is the average obtained for all water ligands in the hydrated ion cluster.d) Molecular system to evaluate EDL collapsing under interstitial space shrinkage.For visibility, the water molecules are illustrated as a blue spatial mesh.Alkali cations and chlorine anions are represented by yellow and red beads, respectively.e) Structural changes in EDL while MWCNTs approach each other.Ions were only visualized within 0.6 nm from the MWCNT surface.f) Coordination number distribution of ions present in the sampling region depicted in (d).

Figure 5 .
Figure 5. Understanding of piezoionic harvesting model and potential application environments.a) Illustration of the harvesting cell, including the working electrode of the coiled CNT yarn and the counter electrode of the pt mesh/CNT buckypaper in aqueous solution.From left to right, the illustration displays the circuit configuration, microscale electrode structures, nanoscale cross-sectional view of the CNT bundle spacing, and EDL formation on the surfaces.Initial potential ( a ) is measured without the external voltage and mechanical deformation.b) Slightly stretching the coiled CNT yarn, the CNTs move closer but not sufficiently close to form overlapping diffuse layers ( b =  a ).c) With the further stretching of the coiled CNT yarn and compression of CNT bundles, diffuse layers overlap, and Stern layers make contact.Simultaneously, adsorbed ions are released into the electrolyte, leading to a potential difference ( c >  b ) and consequent electron transport from the counter electrode to the working electrode.d) When stretching the coiled CNT yarn until overlapping Stern layers, massive ion desorption due to gap depletion leads to a rapid increase in the potential of the working electrode ( d >>  c ) and resulting substantial electron transport.e) Potential application environments of the piezoionic energy harvesters.