Self-powered and speed-adjustable sensor for abyssal ocean current measurements based on triboelectric nanogenerators

Domain: earth_science
Published: 2024-07-20
Editorial outcome: major_revision

Abstract
The monitoring of currents in the abyssal ocean is an essential foundation of deep-sea research. The state-of-the-art current meter has limitations such as the requirement of a power supply for signal transduction, low pressure resistance, and an narrow measurement range. Here, we report a fully integrated, self-powered, highly sensitive deep-sea current measurement system in which the ultra-sensitive triboelectric nanogenerator harvests ocean current energy for the self-powered sensing of tiny current motions down to 0.02 m/s. Through an unconventional magnetic coupling structure, the system withstands immense hydrostatic pressure exceeding 45 MPa. A variable-spacing structure broadens the measuring range to 0.02–6.69 m/s, which is 67% wider than that of commercial alternatives. The system successfully operates at a depth of 4531 m in the South China Sea, demonstrating the record-deep operations of triboelectric nanogenerator-based sensors in deep-sea environments. Our results show promise for sustainable ocean current monitoring with higher spatiotemporal resolution.

Introduction
Deep ocean exploration and exploitation are essential to mapping the Earth’s internal tectonic and evolutionary history, searching for marine minerals, obtaining evidence for Earth science, and interpreting trends in deep-sea ecosystems1–3. Metrics of current are essential components of deep-sea oceanographic parameters and are important to understanding changes in the seabed geological structure, trends in the global climate, and the distribution of biological communities4–6. However, exploring the deep ocean, especially to the abyssal zone (at depths of 4000–6000 m), presents a complex challenge to the designers and manufacturers of instrumentation for deep-sea deployment, primarily owing to the extreme environment of the abyssal ocean. Furthermore, the tiny ocean flow (typically on the order of centimeters per second) means that sensors need to be ultra-sensitive. At present, mainly electromagnetic, acoustic, and mechanical instrumentation is used for ocean current monitoring7. The measurement accuracy of an electromagnetic current meter is reduced by external magnetic field interference. Although the acoustic current meter has the highest precision and can perform profiling measurements8, it is unsuitable for turbid or ultra-clean water and highly reflective near-bottom environments9. Moreover, its cost hinders its widespread use.

The mechanical current meter has the advantages of a simple structure, affordable price, and no blind measurement area. Despite its potential to be widely deployed for distributed ocean flow field observation, the conventional mechanical current meter has limited pressure resistance and an narrow measurement range. Moreover, all the aforementioned ocean current meters require an external power supply (e.g., batteries) for signal transduction. Limited battery power inevitably leads to the under-sampling of ocean states. Observations may not reflect actual trends at any given location and time because so few measurements are collected. These problems limit the potential of state-of-the-art current meters for long-term, large-scale, and distributed ocean observations with high spatiotemporal resolution and long duration.

Distributed renewable energy sources provide an impetus for developing distributed sensing systems10. Triboelectric nanogenerators (TENGs) are cost effective11,12, low weight13–15, and highly efficient at harvesting low-frequency mechanical energy16,17 and have been applied as micro and nano energy sources18–21, blue energy harvesters22–26, and self-powered sensors27–30. For self-powered sensors, the transformation from mechanical signals to recognizable electrical sensing signals does not need an additional power supply, and the TENG can perform this function well. For example, TENG-based self-powered sensing has been extensively studied in speed, vibration, biomedicine, and other sensing fields31–35. Recently, TENGs have demonstrated great potential in the development of self-powered sensors for ocean observations36–38. In fluid dynamics sensing, TENGs have been widely studied for wind speed39–41, pipeline velocity42,43, and raindrop sensing44. However, research on the use of TENGs in ocean current sensing in the deep sea is still lacking. Realizing a highly sensitive design that is suitable for monitoring subtle ocean current flow and that has a simple and effective structure capable of withstanding a high-pressure, highly corrosive deep-sea environment is a challenging task. Completing this task is important to building a low-cost deep-sea current observation network and promoting the deep-sea application of TENGs.

Herein, we report a fully integrated, self-powered, high-pressure-resistant, and ultra-sensitive ocean current monitoring system (DS-TENG) capable of measuring flows at depths down to 4531 m. The system employs a flexible TENG with biological fur and flexible foils for current energy harvesting and sensing, which reduces the friction resistance of the structure and effectively increases the sensitivity and durability of the structure. Through a magnetic coupling design, the DS-TENG achieves the highly efficient non-contact transmission of ocean current energy, generating a flow-velocity-dependent electrical signal (without the need for an external power supply). Notably, we further design a variable-spacing mechanism, which effectively modifies the distance of the magnetic coupling and adjusts the speed measurement range. The measurement range of the DS-TENG is 0.02–6.69 m/s, which is 67% wider than that of the traditional vertical-axis current sensor. Furthermore, this paper reports a field experiment conducted on the DS-TENG at depths down to 4531 m in the South China Sea. To the best of our knowledge, this is the first demonstration of a TENG-based sensor in the abyssal ocean. The DS-TENG operated continuously for 6 h at a flow velocity of 0.16 to 0.76 m/s with a linear correlation between the output signal and flow velocity up to R2=0.97. The DS-TENG provides a reliable and excellent solution for deep-sea current observations and has great potential in the construction of future ocean stereoscopic observation networks.

Results
Structures and working principle of the DS-TENG
The development of sustainable and efficient ocean energy harvesting technology is important to the construction of a distributed ocean observation network. Here, we design a TENG-based ocean current sensor for energy harvesting and self-powered ocean current measurements.

Figure 1a shows the exploded view of the DS-TENG, which mainly comprises a pressure-resistant and waterproof sealing and tank; printed circuit board for data acquisition, processing, and communication; TENG and magnetically coupled and variable-spacing structure; and rotating cup structure for energy harvesting and ocean current sensing. The rotating cup structure comprises six conical cups symmetrically arranged around the center of a rotating disk. When the sensor is exposed to ocean currents, the differential water pressure (relating to the varying shapes of the cups) on the two sides of the rotating axis rotates the cups. The harvested rotational energy is transferred to the TENG via the transmission structure. Here, we adopt a contactless magnetic coupling structure instead of a standard contact structure (Fig. 1b), as the former is conducive to the design of a sealed pressure-resistant structure for the harsh deep-sea environment.

We also propose a variable-spacing structure (Fig. 1c) for expanding the measurement range. By configuring the convexity in chute 1 (state 1) or chute 2 (state 2), two magnetic-coupling transmission clearances corresponding to different velocity measurement ranges, are realized. The variable-spacing structure means that the system has a wider measurement range and better pressure resistance than commercial current meters and can thus measure flow in a deep-sea high-pressure environment.

Finally, a highly flexible TENG (Fig. 1d) is designed with low-friction-resistance flexible fluorinated ethylene propylene (FEP) and fur as tribomaterials. Even for tiny ocean currents in the deep-sea environment, the rotor rotates smoothly and harvests current energy with a linear correlation between the current speed and frequency of the output electrical signal.

The working mechanism of the TENG in converting ocean current energy into electrical energy is shown in Fig. 1e and is based on the triboelectrification and electrostatic induction effect26. First, the fur and FEP film are electrically uncharged. When the fur and FEP film rub against each other, they generate an electrostatic charge through triboelectrification. Specifically, when the fur comes into contact with the FEP film, the triboelectric effect causes the fur and FEP film to take on equal amounts of positive and negative charge. Owing to the different contact areas (i.e., the area of the fur is half that of the FEP film), the Cu+ and Cu− electrodes generate equal amounts of positive and negative charges through electrostatic induction (Fig. 1e–I). As the fur slides counterclockwise on the FEP to balance the non-mobile triboelectric charges in the FEP layer, the negative charge is transferred from the Cu− electrode to the Cu+ electrode through the external circuit (Fig. 1e–II), generating a current from bottom to top. When the fur entirely coincides with the Cu+ electrode, all the negative charge on the Cu− electrode is transferred to the Cu+ electrode (Fig. 1e–III). Similarly, as the fur continues to rotate, the negative charge on the Cu+ electrode returns to the Cu− electrode (Fig. 1e–IV), producing a current in the external circuit in the opposite direction. According to previous work45, the formulas for calculating the open-circuit voltage (VOC), short-circuit current (ISC), and transferred charge (Q) are as follows.

Q = \int_{x=0}^{x=g+l} \frac{\sigma \omega dk}{1 + \frac{h}{c_i(k)}} (1)

VOC = Q / C

ISC = dQ / dt

Here, σ is the tribo-chargedensity, ω is the width of the fur, dk is a small region at the bottom of the fur containing tribo-charges, and the total charges on the Cu+ and Cu− electrodes are thus σωdk. ci(k) is the capacitance between the small region dk and the metal electrode.

The distribution of electric potential in the fur and FEP film at different stages (Fig. 1f) is simulated using COMSOL (COMSOL 5.6, COMSOL Inc., USA). The initial triboelectric charge density on the surfaces of the fur and FEP film is set at ±70 μC/m2, and the gap between the two surfaces is set at 1 mm. The potential difference is a maximum when the fur is directly above the Cu+/Cu− electrodes. The results of the potential distribution are consistent with theoretical predictions.

Material and structural optimization of the DS-TENG
To maximize the sensitivity, measurement range, and output stability of the DS-TENG, the materials and structure of the DS-TENG are optimized. This includes optimization of the treatment and length of the fur, selection, and thickness of the negative tribomaterials, number of electrodes of the TENG, and structure of the magnetic coupling and variable-spacing structure. The preparation of the fur as the positive tribomaterial for the TENG is shown in Fig. 2a. The surface of the fur is roughened by treatment with biological enzymes and plasma etching to create a greater contact area46, which is conducive to generating more electrical charge during the friction process.

The effect of the fur length on the VOC of the TENG is shown in Fig. 2b. At a rotational speed of 100 rpm, VOC first increases and then decreases with an increase in the fur length. A maximum value of 121.6 V is reached when the fur length is 16 mm. The results are mainly due to the fur length affecting the effective contact area (Supplementary Fig. 1). When the fur is too short, it fails to make sufficient contact with the FEP surface. When the fur is too long, the fur makes contact with the positive and negative electrodes simultaneously, resulting in the cancellation of the induced potential at the outer part of the electrode pair. Moreover, longer fur leads to increased frictional resistance and, thus, reduced sensor sensitivity. The optimal fur length of 16 mm was thus obtained.

The effects of the materials and thickness of the negative tribomaterial on the output performance of the TENG are next investigated. Here, polyimide (PI), polytetrafluoroethylene (PTFE), and FEP as negative tribomaterials are tested. As depicted in Fig. 2c, the FEP-Cu combination has the best output performance. Furthermore, the voltage decreases as the thickness of the FEP film increases from 0.02 to 0.1 mm (Fig. 2d), indicating that a thinner triboelectric layer results in more induced charge during the friction process. However, a 0.02-mm film is prone to damage during use, inconvenient for integration, and not suited to long-term use. A 0.05-mm FEP film is thus adopted as the dielectric layer.

The effect of the number of electrodes on the output performance is shown in Fig. 2e. The output voltage decreases slightly with an increase in the number of electrode pairs, mainly because the area of a single electrode decreases as the number of electrodes increases. The area of the fur, thus needs to be reduced with an increase in the number of electrodes. Furthermore, a greater number of electrodes results in the fur touching the positive and negative electrodes simultaneously, which reduces the output slightly. Finally, the long-term stability of the designed TENG is tested. After 30 days of operation, the reduction in the induced charge of the TENG remains within 6% (Supplementary Fig. 2). The cause of the slight reduction in TENG performance is mainly attributed to the material wear or contact deformation of the rabbit fur. Nevertheless, the deformation of the rabbit fur and the attenuation of the induced charge will slow down, and the output of the TENG tends to fluctuate within a certain range. In addition, after more than 30 days of use, the surface of the rabbit fur shows no appreciable wear or tear (where detailed analysis and SEM charts are presented in Supplementary Fig. 3), demonstrating the excellent long-term stability of the TENG.

Magnetic coupling realizes non-contact energy transmission through the interaction of the north and south poles of active and passive magnets, and the transmission clearance directly affects the design of the pressure-resistant tank and the maximum velocity measurement range. The effect of the transmission clearance (i.e., 0, 12.5, 15.5, 18.5, 21.5, or 24.5 mm) of the magnetic transmission structure on the maximum effective transmission rotational speed is shown in Fig. 2f. An increase in the transmission clearance reduces the maximum effective transmission rotational speed. When the clearance is less than 12.5 mm, the magnetic coupling drives continuously and stably in the speed region of 1000 rpm. When the clearance is 15.5, 18.5, and 21.5 mm, the maximum rotational speed of the passive magnet that can be achieved before transmission failure is 600, 160, and 100 rpm, respectively. When the clearance exceeds 24.5 mm, the magnetic coupling fails, irrespective of the rotational speed of the passive magnet. Detailed transmission curves for different transmission clearances are presented in Supplementary Fig. 4.

To investigate the synchronicity of the active and passive magnets in the transmission process, we record the response time of the passive magnet when the rotational speed of the active magnet is set from 30 to 120 rpm using a tachometer. The results (Fig. 2g) show an inevitable delay for the passive magnet due to the transmission clearance, with the response time increasing with the clearance. As the clearance increases from 12.5 to 18.5 mm, the response time increases from 0.48 to 0.96 s. Such a response time is perfectly adequate for measuring upper-ocean flow (which varies over the course of hours or days)47 or abyssal flow (which varies over the course of months or seasons)3.

In summary, the starting torque and magnetic coupling are negatively correlated to the magnetic transmission clearance within the effective transmission range. Challenges associated with the development of the DS-TENG include realizing a broad measurement range with sensitivity to low flow velocities and robustness against high flow velocities. We thus design a variable-spacing magnetic-coupling structure that has a large transmission clearance (18.5 mm) at a low flow velocity to ensure the sensitivity of the ocean current sensor during startup, and that has a small transmission clearance (12.5 mm) at a high flow velocity to ensure the strength of the vertical rotor and the stability of the rotation process (where more detailed explanations and diagrams are presented in Supplementary Fig. 5).

Performance characterization of the DS-TENG
To characterize the performance of the DS-TENG at different current speeds and different ocean depths, the output performance of the DS-TENG with a transmission clearance of 12.5 mm is tested using a stepper motor in the laboratory. The output voltage, short-circuit current, and transferred charge of the DS-TENG under different rotational speeds are shown in Fig. 3a–c. The DS-TENG experiences slight increases in both the output voltage and transferred charge with rotational speed, which are attributed to the enhanced centrifugal effect produced by the fur as the rotational speed continues to rise. This leads to closer contact between the fur and dielectric layer and, thus, an increase in the friction charge and induced charge and, ultimately, a slight increase in the output. The peak current is positively correlated with the rate of charge transfer. As the rotational speed increases, the frequency at which the fur slides on the positive and negative electrodes increases, which accelerates the transfer of the triboelectric charge in the external circuit, such that the peak current clearly corresponds with the rotational speed. At a rotational speed of 1000 rpm, Vp-p reaches 157.61 V, Ip-p reaches 14.35 μA, and the transfer charge reaches 74.17 nC. When the rotational speed is 2000 rpm, Vp-p, Ip-p, and the transferred charge reach 161.69 V, 19.25 μA, and 74.47 nC, respectively, as shown in Supplementary Fig. 6. Figure 3d shows the relationship between the speed of the stepper motor and the output frequency of the voltage (fvoc) of the DS-TENG at a transmission clearance of 12.5 mm. In the range of 0–1000 rpm, the relationship between the speed of the motor and fvoc shows excellent linearity (R2=0.99). This indicates that the signal frequency of the DS-TENG has excellent linearity with the external trigger frequency and can be used as an ideal sensing signal.

For simulating the operating conditions of deep-sea exploration equipment at low speed, the charging capacity of the DS-TENG is tested at a lower speed of 120 rpm, as shown in Fig. 3e. The DS-TENG charges a 6 μF capacitor to 8.1 V within 100 s. We next investigate the load current and voltage of the DS-TENG under different resistance conditions, as shown in Fig. 3f. We find that the load current of the DS-TENG decreases with an increase in the external load resistance, whereas the load voltage of the DS-TENG system increases with the external load resistance. Figure 3g shows the peak power of the DS-TENG system as a function of the external load resistance at 120 rpm. It is seen that a peak power of 34.86 μW is generated when the external load resistance is 2 × 108 Ω. Furthermore, we investigate the effect of the flow direction on the output performance of the DS-TENG (Fig. 3h). In the horizontal direction, for the same velocity stimulation from different directions, the ratio between fvoc in other directions and fvoc in the initial direction (0°) remains above 95%. This indicates that the DS-TENG maintains accurate and stable output performance for water motions from all directions. Furthermore, the pressure-resistance test results of the DS-TENG in a pressure chamber are shown in Fig. 3i. The sensor remains intact as the applied pressure is increased to 49.5 MPa, indicating that the DS-TENG can withstand deep-sea hydrostatic pressure to a depth of at least 4500 m.

Flow velocity sensing of the DS-TENG
According to the working principle of the DS-TENG, a voltage is generated only when the fur inside the device slides against the surface of the FEP under the action of the ocean current, and the voltage is used as the velocity-sensing signal. As the peak current may be affected by the external environment (for example, the rotating speed), a more stable voltage frequency is chosen as the sensing signal. On this basis, a linear correlation between the DS-TENG’s output electrical signal and the velocity can be established to monitor the velocity.

To test the DS-TENG's output performance in the natural ocean environment, we conducted a field experiment in the waters near Sanya Bay, China. As shown in Fig. 4a, calibration is carried out using the commonly applied still water tower method48. The DS-TENG and a commercial current meter are fixed side by side at a depth of 1 m in front of a speed boat. This arrangement places both instruments in a relatively consistent flow environment, which is convenient for testing. A DS-TENG with two transmission clearances is designed to characterize the effect of the transmission clearance on the speed measurement range. For the small transmission clearance in state 1 (Fig. 4b), the magnetic transmission structure shows stronger coupling and transmission stability, leading to better performance under the condition of a higher flow velocity (Fig. 4c and Supplementary Movie 1). The DS-TENG stably generates voltages at a flow velocity between 0.22 and 6.69 m/s with a linear correlation between the frequency of the voltage (f) (or rotating cup speed) and relative velocity as high as 0.99 (Fig. 4d and Supplementary Fig. 7a).

Similarly, for the large clearance in state 2, the magnetic transmission structure has a lower starting torque and is more sensitive to the low flow velocity, having a starting velocity of 0.02 m/s. Within the flow velocity range of 0.02 to 0.26 m/s (Supplementary Movie 2), the coefficient of correlation between f (or the rotating cup speed) and the relative velocity reaches 0.99 (Fig. 4e, f and Supplementary Fig. 7b). Combining the measurement capabilities of the two transmission clearances, the measurement range of the DS-TENG becomes 0.02 to 6.69 m/s, which is 67% wider than that of the traditional commercial current sensor (Fig. 4g). Such a wide measurement range makes the system useful across a wider range of applications, demonstrating the superiority of our design.

Deep-sea applications of the DS-TENG
On the basis of the excellent performance of the DS-TENG in flow velocity sensing, we further integrate the DS-TENG on a remotely operated vehicle (ROV) to evaluate its operational performance in the harsh environment of the deep sea, especially that of the abyssal zone. The main differences in water between laboratory conditions and the deep sea are the ambient pressure and density, which are factors that directly affect sensor performance. To offer a comprehensive view of our system in the deep sea, we present a schematic illustration and photograph of the ROV in operation (Fig. 5a, b). Closer images of the DS-TENG on the ROV are presented in Supplementary Fig. 8.

A signal processing system (Fig. 5c) is integrated within the DS-TENG to enable real-time signal transmission from the deep sea. Under the stimulation of ocean current, the DS-TENG generates voltages related to the flow velocity, which are then filtered and amplified through a signal processing circuit. The analog data are then converted into digital format and transmitted to a ship-based platform through an optical communication cable via the RS-232 protocol. The ship-based platform equipped with a data acquisition card reads and processes the data. Finally, the flow velocity is analyzed and visualized in real-time through a self-designed graphical user interface (where detailed circuit diagrams and analysis are presented in Supplementary Fig. 9).

A photograph and movie of the DS-TENG operating at a depth of 4531 m in the abyssal zone of the South China Sea (110°46’ E and 16°73’ N) are captured by a waterproof, deep-sea camera installed on the ROV (Fig. 5d, Supplementary Movies 3 and 4). As the ROV travels at different speeds, the onboard Doppler velocity log (DVL) records the relative flow velocity between the ROV and seawater in real-time. Meanwhile, the velocity data recorded by the DS-TENG are transmitted to the surface platform, ensuring a constant flow of real-time information (Supplementary Fig. 10). As shown in Fig. 5e, the DS-TENG effectively captures subtle changes in the flow velocity from 0.33 to 0.50 and 0.55 m/s. These changes are further evidenced by the corresponding waveform frequencies (Fig. 5f–h) of the voltages of 1.12, 1.22, and 1.31 Hz, respectively. In addition, this robust response to small velocity changes under abyssal zone conditions is supported by the linear correlation between the different relative flow velocities (between the ROV and seawater) and the output frequency of the DS-TENG (Fig. 5i). The output frequency of the DS-TENG shows good linear dependence (R2=0.97) on the flow velocity from 0.16 to 0.76 m/s. Furthermore, validation tests conducted under various depth conditions confirm that the velocity measured by the DS-TENG aligns closely with the data from the DVL (Fig. 5j), and the DS-TENG thus has the potential not only to make sea current velocity measurements but also to determine the speed of submersibles, expanding its utility beyond conventional applications. Impressively, as demonstrated in Fig. 5k, after 240 min of continuous operation, both the DS-TENG and DVL consistently deliver results with an accuracy exceeding 88.5%, indicating that the DS-TENG has excellent stability. In addition, the maximum depth in the field experiment is 4531 m (Supplementary Movie 5). Throughout the field expedition, the surface platform accurately receives data transmitted by the DS-TENG, and the equipment remains fully operational after recovery, which verifies the technical feasibility of the DS-TENG velocity measurement in actual high-pressure environments of the abyssal ocean. This marks an advance in the ability of a TENG to monitor ocean currents in the challenging domain of the abyssal ocean.

Methods
Fabrication of the TENG
The TENG comprises a rotor on the inside and a stator on the outside, with both components coaxially installed. The outer wall of the cylindrical rotor is symmetrically mounted with two brushes made of rabbit fur, each having a brush area of 18 mm × 37 mm. The inner wall of the stator is mounted with four pieces of Cu film, having dimensions of 15 mm × 37 mm × 0.065 mm. There is a gap of 2.5 mm between adjacent Cu films. FEP film with dimensions of 113 mm × 40 mm × 0.05 mm is adhered to the surface of the Cu film. The outside of the FEP film is attached to the Cu film, whereas the inside is in contact with the brush. The brush is coaxially installed with the magnetic coupling and transmits power through the rotating shaft.

Fabrication of the ocean current sensor
The structure of the ocean current sensor mainly includes a rotating cup, magnetic transmission, and variable-spacing structure, the TENG, a printed circuit board, and a sealing tank. The structural design of the DS-TENG is modeled using mechanical engineering design software and initially validated for three-dimensional printed objects using a three-dimensional printer (CR5060 Pro, CREALITY, China). Finally, we use 316 stainless steel with equal parameters to replace the exposed parts. The pressure tank is made of 316 stainless steel, which is a high-strength austenitic stainless steel with high yield strength (310 MPa) and tensile strength (620 MPa). Therefore, this material is beneficial in terms of reducing the thickness of the isolation sleeve while meeting the requirements of deep-sea pressure resistance. The sealing end cover is designed on top of the DS-TENG, and the two sealing grooves between the end cover and pressure tank are sealed by an O-ring made of nitrile material, which is a commonly used sealing material. As the mechanical current meter cup has a mature commercial design and usage, we opted to adhere directly to the national standard of rotating-element current meters (GB/T11826-2019, China) in our design process. Six evenly distributed rotating cups are three-dimensionally printed from aluminum alloy. Each cup has a radius of 20 mm. The details of the DS-TENG rotating cup are presented in Supplementary Fig. 11. The transmission shaft is made from aluminum alloy and has a diameter of 8 mm. The shaft mainly acts as a coaxial connection between the rotating cups and the active magnet. The magnetic coupling is a standard part (model HSF06-D35-P6-d8S, HSF Magnets Ltd., China). The outer diameter, inner hole diameter, and height of the coupling are 35, 8, and 20 mm, respectively. The passive magnet is installed coaxially with the TENG rotor. The variable-spacing structure mainly comprises two rotating bodies. Rotating body 1 has an inner diameter of 22.75 mm and an outer diameter of 29 mm. The body has a semicircular chute with a diameter of 3 mm as an axial opening along its outer surface, and two semicircular chutes with diameters of 2.5 mm as radial openings along its outer surface. Rotating body 2 has an inner diameter of 29.5 mm and an outer diameter of 36 mm. The body has a convex protrusion with a diameter of 2.5 mm along the radial direction of its inner surface, which slides in the chute of rotating body 1. Rotating body 2 is fixed relative to the active magnet, whereas rotating body 1 is fixed relative to the passive magnet. The sealing tank system mainly comprises a sealing tank, lid, and O-ring seal. The inner diameter, outer diameter, and bottom thickness of the sealing tank are 54, 78, and 11.5 mm, respectively. The sealing lid has a diameter and thickness of 78 and 20 mm, respectively, and axial and radial sealing grooves. The width and depth of the axial sealing grooves are 2.6 and 1.28 mm, respectively, the width and depth of the radial sealing grooves are 2.4 and 1.32 mm, respectively, and the wire diameter and outer diameter of the O-ring seal are 1.8 and 56 mm, respectively. In addition, a YMCBH6 water-tight connector connects the signal line from the top end of the sealed tank.

Performance characterization
The transferred charges and current are measured by an electrometer (Keithley 6514, Tektronix, USA). The data are collected with a data acquisition card (NI USB-6008, National Instruments, USA) and the LabVIEW software platform. A home-designed ocean current simulation system is built to systematically characterize the DS-TENG’s output performance. A diagram is shown in Supplementary Fig. 12. Resistors for impedance matching are calibrated using a precision impedance analyzer (E4980A, Keysight, USA).

Deep-sea experiments
From May 2 to 4, 2023, we carried out a deep-sea validation of the DS-TENG in the waters near 110°46‘ E and 16°73’ N. The mothership in this experiment was the 6000-tonne integrated research vessel TAN SUO ER HAO, designated TS2-24 for this voyage. A deep-sea experiment was carried out using the DS-TENG aboard an ROV. Two sea trials were conducted, with each sea test lasting ~6 h. The maximum test depth was 4531 m. The test process was as follows. (1) The ROV was kept stationary at depths of 100, 250, 500, 750, and 1000 m during its descent. At the same time, the DS-TENG obtained flow velocity information at different depths for a collection time of 10 s. (2) In the 4000-m abyssal ocean area, the ROV ran at a constant speed of 0.5, 1, 1.5, and 2 knots. During these runs, the DS-TENG continued its data collection routine for 10 s. This process was conducted twice for robust validation. (3) In the 4000-m abyssal ocean area, the ROV accelerated in the direction opposing the ocean current (with speed reaching a maximum within 20 s) and slowed after reaching the maximum speed (with the ROV power output decreasing to zero within 20 s), and the DS-TENG recorded and uploaded the data in real-time. This process was conducted twice. (4) Data were transmitted with a data acquisition frequency of 200 Hz and a transmission frequency of 20 Hz, and in RS-232 data communication mode.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper proposed a self-powered and speed-adjustable sensor for abyssal ocean-current measurements. The sensor shows some outstanding characteristics, such as fully integrated, self-powered, highly sensitive and withstand immense hydrostatic pressure. Impressively, this paper reported an underwater sensing experiment at depths down to 4531 m, showing a practical potential in the abyssal ocean. I read this paper with great interest. However, there are still some issues that need to be clarified. To be published in nature communications, following revisions must be conducted.\n1. The author needs to explain how the DS-TENG can be “self-powered”. And clearly defined it in the paper.\n2. In Section Introduction, the author claimed that “The measurement range of the DS-TENG is 67% wider than the traditional vertical-axis current meter.” However, the author only showed the maximum velocity of different flow velocity sensors in Fig. 4g and compared them with the sensor proposed in this paper. The minimum detected flow velocity of this paper also needs to be compared with that of the commercial sensors. Meanwhile, the wide flow range of the proposed sensor is achieved by two gears (i.e. transmission clearances), is it also necessary for traditional sensors?\n3. Line 180: Is “rabbit fur” a natural animal material or a synthetic material? How durable is it? The loss after 30 days of operation needs to be quantitatively measured and demonstrated.\n4. The DS-TENG can be driven by collecting ocean current with the rotating cup. This design used six conical cups symmetrically arranged around the center of a rotating disk. It seems that the design of rotating cup is directly related to the detected flow range of the sensor. At present, the author only mentioned the diameter of each rotating cup in Section Method, but does not show how they are arranged (e.g. diameter of the circular arrangement), so the author needs to draw the detailed design in Supplementary. In addition, it is mentioned in Method that the multiple blades were made using 3D printing. The performance of different rotating cups also needs to be shown in Supplementary, and explains why the rotating cup shown in the paper was finally selected.\n5. What is the material used to constitute the sealing and tank? Does their thickness affect the maximum torque of the magnetic coupling? Please elaborate.\n6. Line 210: The relationship between response time and flow velocity should be further clarified. Specifically, why claimed “the large transmission clearance at a low flow velocity”? A large transmission clearance corresponds to an extended response time, right? How to ensure the start-up sensitivity claimed by the author with the large transmission clearance? And how to balance response time and starting torque?\n7. In Fig. 3b, the short-circuit current and rotational speeds also show a distinct correspondence. The reason for these changes needs to be analyzed and supplemented in the paper, such as that of output voltage and transferred charge in Line 225.\n8. Line 234: The author claimed “the range of rotational speed is 0 ~ 1000”, but there were only 8 frequency points to calibrate the speed in Fig. 3d. Please add more frequency measurement points.\n9. Line 239: Why choose 120rpm as the lower speed to simulate the charging capacity under the deep-sea condition of a low flow velocity? It needs to be explained and supported by the literature or some other authoritative means.\n10. Line 251: The mentioned pressure-resistance test lasted less than 20 minutes at 45 MPa. Is there any evidence or literature for this test and duration time? How to judge “the sensor remains intact”, can the sensing test be repeated under high pressure?\n11. Line 265: The author proposed that “the peak current may be affected by the external environment”, but a sealed and pressure-resistant environment is provided, right? Please explain this seemingly superfluous concern and provide relevant literature. At the same time, it is proved that the selected voltage signal will not be affected.\n12. Line 288: The measurement range of 0.02 to 6.69 m/s is realized by combining two transmission clearances. In the practical application of this paper, is the clearance pre-set by manual or can be real-time adjusted during the procedure? As the author claimed, this will affect the width of the measurement.\n13. Writing needs improvement, existing some formatting problems. For example, the capacitance size in 240 Line, the caption of Fig. 3d, etc.\n14. Before Fig. 5, the voltage were tens of volts, but all of them were 3V in Fig. 5. The reason needs to be explained.\n15. Is the circuit on the ROV in Fig. 5c the same as that in the sensor shown in Fig. 1a? The author needs to introduce these two circuits in more detail and show the circuit diagram.\n16. It should be pointed out that the velocity shown in Fig. 5i refers to the ocean current velocity or the relative flow velocity between the ROV and seawater. Is it calibrated by the DVL?", "reviewer_2": "This paper presents a triboelectric nanogenerator (TENG) based self-powered ocean current measurement sensor. This system can distinguish the ocean current from 0.02 m/s to 6.69 m/s with systematic adaptation, which is a 67 % larger range than the commercial ocean current sensors. Along with this significant sensitivity, this system can be operated under the ocean even at a depth of 4500 m. In this regard, I think this paper has a distinct novel point to be published in Nature Communications after the revision process. Our comments to which the author can refer is suggested below:\n1) Given that the magnetic coupling structure is utilized for the rotational energy transmission, the spacing between the two magnetic coupling can be easily adjusted, which allows the alternation of the sensing range. However, it seems that the spacing needs to be adjusted manually. Given that the author suggesting the TENG operation in deep sea, I think self-adjustment of the sensing range must be possible due to the low accessibility of the deep-sea environment. I think the author should consider about it for its novelty.\n2) Also, I guess the magnet must be strong enough to affect each other. In this regard, if the electromagnetic generator is hybridized with the suggested TENG, it can be helpful to use this module as an energy harvesting source, not only for the ocean current sensing module. Given that energy harvesting on the inaccessible environment is always getting spotlight, it can enhance the applicability. The author should consider about it.\n3) In Figure 2f, the transmission curve of the active and passive magnets at different transmission clearances is shown. However, it is hard to distinguish the data, because several data are duplicated. The author should refurbish the figure for the readers’ understanding.\n4) In Figures 4b to 4f, the electrical output generated from the two different state is shown. However, although the state 1 is well explained in Figure 4b, details about state 2 are not explained in the main figures. I think this figure set does not reveal the correlation between the operational state and the sensitivity range. In this regard, I think author should add about the detail about state 2 in Figure 4. It will enhance the readability of the figure.\n5) In Figure 5e, the raw voltage output generated underwater is depicted. However, it seems that there are unclear data which can potentially decrease the accuracy of the current sensing. Where these abnormal peaks are coming from, and how these peaks will affect the sensing?", "reviewer_3": "This is a well-written manuscript and I think it is appropriate for this journal. The manuscript is well referenced with appropriate citations and is well illustrated with high quality figures. The design of the current meter has the potential to provide increased capabilities to the oceanographic community. I would recommend this paper for publication if these issues can be addressed:\n1. It is not clear to me from the description in the paper that the power generated by the instrument is sufficient to power a self-contained acquisition and logging system, which would need to be integrated if the instrument were to be deployed as a stand-alone device that provides data on recovery from deployment. If this were the case then this would be a very significant advance as it would enable long-term monitoring.\n2. I would like some clarification on this sentence: ‘After 30 days of operation, the reduction in the induced charge of the TENG remains within 6%’. What is the cause of this reduction and would it continue beyond the 30 day period? What causes it and what effect does it have on the lifespan of the instrument? Maybe I have misinterpreted the sentence, but it would be good to have further details in the manuscript to ensure that others accurately interpret what is being said here.\nIn addition to these two points I have made various comments in an attached word document. I think the majority of these comments can be easily addressed and it would be good to see a revised manuscript."}, "author_response": "April 16th, 2024\nRE: Manuscript resubmission to Nature Communications\nDear Reviewers:\nThank you for your valuable time in reviewing our manuscript “A self-powered and\nspeed-adjustable sensor for abyssal ocean-current measurements based on a\nnanogenerator”. The original work (No.: NCOMMS-23-56530) has been reviewed\nfor publication in Nature Communications and obtained constructive comments from\nthe reviewers. To give a clear explanation, the comments and suggestions have been\ncarefully considered and replied point-by-point. In the following, we quote the\nreviewers’ reports in full, and our responses are interspersed in blue. Action taken is\nindicated in red.\nResponse to Reviewer #1:\nReviewer #1:\nThis paper proposed a self-powered and speed-adjustable sensor for abyssal ocean-\ncurrent measurements. The sensor shows some outstanding characteristics, such as\nfully integrated, self-powered, highly sensitive and withstand immense hydrostatic\npressure. Impressively, this paper reported an underwater sensing experiment at\ndepths down to 4531 m, showing a practical potential in the abyssal ocean. I read this\npaper with great interest. However, there are still some issues that need to be clarified.\nTo be published in nature communications, following revisions must be conducted.\nResponse: We greatly appreciate the generally positive feedback and thank the editors\nand reviewers for carefully reading our text and providing comments to improve our\nmanuscript.\n1. The author needs to explain how the DS-TENG can be “self-powered”. And clearly\ndefined it in the paper.\nResponse: To elaborate the concept of the DS-TENG being “self-powered”, we begin\nby examining a conventional sensor. In general, conventional mechanical sensors used\nto measure the ocean current velocity have measurement, signal processing, and\npower supply modules. As the measurement module cannot generate electricity on its\nown, the power supply module must continuously power the measurement module\nthroughout the measurement process. Taking the LS1206B mechanical flow meter as\nan example (Fig. R1a), in addition to the power consumed by the signal processing\nmodule, the generation of the pulsed electrical signal by the measurement module\nrequires power to activate and deactivate the reed switch associated with the flow rate.\nHence, both the measurement and signal processing modules rely on an external\npower supply.\nFig. R1. Schematic diagram of the working principles of (a) LS1206B and (b) the DS-\nTENG.\nUnlike conventional sensors, the DS-TENG utilizes a triboelectric nanogenerator\nto self-generate power (without the need for an external power supply). It does this by\ngenerating electrical signals that correlate with the flow rate when triggered by water\nflow. These signals are then directly transmitted to the signal processing module.\nAlthough the signal processing module still requires a power supply, our work is an\nimportant development beyond the conventional sensor.\nMoreover, by further enhancing the output power of the DS-TENG, the energy\nharvested by the TENG can be utilized to recharge the power supply module, thereby\nprolonging the device’s lifespan. Studies have widely reported that the advancement\nof TENG technology has facilitated the development of various self-powered sensors\n(Joule 2017, 1 (3), 480-521; Nat. Commun. 2020, 11 (1); Adv. Energy Mater. 2017, 7\n(12); Nat. Commun. 2015, 6 (1)). Hence, our upcoming research will focus on\noptimizing the output performance of the TENG and developing efficient energy\nharvesting and management circuits to achieve fully self-powered operation of the\ncurrent meter.\nAction: The interpretation of the DS-TENG being “self-powered” has been clarified\nin the revised manuscript (Line 81).\n2. In Section Introduction, the author claimed that “The measurement range of the\nDS-TENG is 67% wider than the traditional vertical-axis current meter.” However,\nthe author only showed the maximum velocity of different flow velocity sensors in\nFig. 4g and compared them with the sensor proposed in this paper. The minimum\ndetected flow velocity of this paper also needs to be compared with that of the\ncommercial sensors. Meanwhile, the wide flow range of the proposed sensor is\nachieved by two gears (i.e. transmission clearances), is it also necessary for traditional\nsensors?\nResponse: We appreciate your suggestions. Indeed, the maximum and minimum\nvelocity measurements should be compared simultaneously, as in Fig. 4g and Fig. R2.\nHere, our DS-TENG has a velocity measurement range of 0.02–6.67 m/s whereas a\ncommercial mechanical current sensor (such as LS-45 or LB-70) has a measurement\nrange of 0.01–4 m/s. Therefore, the measurement range of the DS-TENG is 67%\nwider than that of similar commercial products.\nLimited by the fixed mechanical structure, conventional current meters are\nconstrained to a specific measurement range. To obtain a broader velocity\nmeasurement range, the combination of multiple measurement ranges is necessary.\nInterestingly, the concept of multi-measurement range superposition is adopted in\nother instrumentation design. For instance, we consider an impedance analyzer like\nthe Keysight E4980A, which offers five measurement ranges to measure impedance\nacross a wide range of values through adjusting the gain multiplier of the vector ratio\ndetector.\nEmploying this design strategy, we introduced the variable-transmission\nclearance structure, capable of switching between different measurement ranges.\nOur experiments demonstrated that this design strategy expanded the measurement range\nfor ocean currents. Furthermore, we believe that the approach of achieving a wider\nvelocity measurement range using the variable magnetic-coupling transmission\nclearances structure serves as a reference for the traditional flow velocity sensor.\nAction: We have updated Fig. 4g to include both the maximum and minimum\nvelocity measurement capabilities of the DS-TENG in comparison with similar\ncommercial products.\nFig. R2. Comparison of the flow velocity measuring range between the DS-TENG\nand commercial current meters.\n3. Line 180: Is “rabbit fur” a natural animal material or a synthetic material? How\ndurable is it? The loss after 30 days of operation needs to be quantitatively measured\nand demonstrated.\nResponse: Rabbit fur is a natural animal material, which is extensively used in the\nclothing and various other industries after undergoing industrial standard treatments\nsuch as cleaning and bleaching to remove grease and impurities.\nAs rabbit fur exhibits excellent toughness and elasticity at the right length, using\nrabbit fur as the friction material is advantageous to the DS-TENG in terms of\nreducing material wear. Elastic and soft rabbit fur has been widely adopted as a\ntriboelectric material (Pang, Hao, et al. “Segmented swing‐structured fur‐based\ntriboelectric nanogenerator for harvesting blue energy toward marine environmental\napplications.” Advanced Functional Materials 31.47 (2021): 2106398; Chen, Pengfei,\net al. “Super‐durable, low‐wear, and high‐performance fur‐brush triboelectric\nnanogenerator for wind and water energy harvesting for smart agriculture.” Advanced\nEnergy Materials 11.9 (2021): 2003066; Han, Jiajia, et al. “Achieving a large driving\nforce on triboelectric nanogenerator by wave‐driven linkage mechanism for\nharvesting blue energy toward marine environment monitoring.” Advanced Energy\nMaterials 13.5 (2023): 2203219.). It has been reported that a wind-driven soft-contact\nrotary triboelectric nanogenerator based on rabbit fur demonstrated stable\nperformance. Remarkably, the device exhibited no obvious wear even after 480,000\ncycles (Han, Jiajia, et al. “Wind‐driven soft‐contact rotary triboelectric nanogenerator\nbased on rabbit fur with high performance and durability for smart farming.”\nAdvanced Functional Materials 32.2 (2022): 2108580), showing that rabbit fur is\neffective in the simultaneous generation of high-density charges, reduction of\nfrictional resistance, and extension of the lifetime of the device.\nTo quantitatively demonstrate the durability of the rabbit fur, after 30 days of use,\nwe visually inspected the fur and found no noticeable wear on its surface.\nFurthermore, we used SEM to observe the surface topography of the rabbit fur before\nand after 30 days of use. As shown in Fig. R3, there is almost no wear on the fur\nsurface, which indicates that the combination of rabbit fur and flexible polymer film is\nreasonable as the friction material of the TENG.\nFig. R3. SEM image of rabbit fur after running for more than 30 days.\nAction: We have added SEM images of rabbit fur before and after 30 days of use in\nthe Supplementary Information (Supplementary Fig. 3).\n4. The DS-TENG can be driven by collecting ocean current with the rotating cup.\nThis design used six conical cups symmetrically arranged around the center of a\nrotating disk. It seems that the design of rotating cup is directly related to the detected\nflow range of the sensor. At present, the author only mentioned the diameter of each\nrotating cup in Section Method, but does not show how they are arranged (e.g.\ndiameter of the circular arrangement), so the author needs to draw the detailed design\nin Supplementary Information. In addition, it is mentioned in Method that the multiple\nblades were made using 3D printing. The performance of different rotating cups also\nneeds to be shown in Supplementary Information, and explains why the rotating cup\nshown in the paper was finally selected.\nResponse: We thank the reviewer for highlighting the importance of the rotating cup\ndesign. The performance of the mechanical current meter hinges on several key\nfactors, such as the inertia of the rotor, friction in the bearings, and the rotor’s\nresponsiveness to water flow (American Society of Civil Engineers New York, 1994:\n376-385; Proceedings of the National Conference on Hydraulic Engineering. 1993 (pt\n2): 1452-1457). As the mechanical current meter cup has a mature commercial design\nand usage, we opted to adhere directly the national standard of rotating-element\ncurrent meters (GB/T11826-2019, China) in our design process. The standard dictates\na range of three–six rotating cups with a rotational diameter between 50 and 150 mm.\nAs previous studies have indicated that a current meter has an improved start-up\nperformance when equipped with six rotating cups (Journal of Hydraulic Research,\n2007, 45(6): 752-762), we opted for the standard six rotating cups and a moderate\nrotation diameter of 110 mm. The detailed design parameters of the DS-TENG\nrotating cup module are shown in Fig. R4. The opening diameter of the six rotating\ncups is 40 mm, and the length of the bus bar is 35 mm. The rotating cups are evenly\ndistributed in the same plane and have a rotation diameter of 110 mm.\nAction: We have explained the design principles and references of the rotating cups in\nthe revised manuscript (Line 412) and included the detailed design of the DS-TENG\nrotating cup in the revised Supplementary Information (Supplementary Fig. 11).\nFig. R4. The detailed design parameters of the rotating cups of DS-TENG.\n5. What is the material used to constitute the sealing and tank? Does their thickness\naffect the maximum torque of the magnetic coupling? Please elaborate.\nResponse: The pressure tank is made of 316 stainless steels, which is a high-strength\naustenitic stainless steel with high yield strength (310 MPa) and tensile strength (620\nMPa). Therefore, this material is beneficial in terms of reducing the thickness of the\nisolation sleeve while meeting the requirements of deep-sea pressure resistance, and it\nis commonly used in deep-sea equipment (Oceans’ 97. MTS/IEEE Conference\nProceedings. IEEE, 1997, 2). In addition, the non-magnetic nature of 316 stainless\nsteel ensures that its effect on the magnetic coupling is negligible.\nThe sealing end cover is designed on top of the DS-TENG, and the two grooves\nbetween the end cover and the pressure tank are sealed by an O-ring made of nitrile\nmaterial, which is a commonly used sealing material in deep-sea equipment. Detailed\ninformation has been added to the materials section of the manuscript.\n(1) The sealing structure is on the top of the DS-TENG and does not make\ncontact with the magnetic coupling, such that it does not affect the transmission.\n(2) The thickness of the sidewall of the pressure-resistant tank mainly affects the\ncompressive strength and weight of the equipment and does not affect the magnetic\ntransmission in this work, and we thus reduce the thickness of the sidewall as much as\npossible to reduce the weight while ensuring the compressive strength.\n(3) For the thickness of the tank between the magnetic couplings, it is most\nimportant to consider both the pressure resistance and magnetic transmission\nperformance. Theoretically, a smaller thickness corresponds to a smaller magnetic\ndrive clearance. The magnetic drive clearance is negatively correlated with the torque\ntransferred and positively correlated with the start-up time. In this work, experiments\nand calculations showed that the minimum thickness of the bottom of the tank that\nensured sufficient pressure resistance in the deep sea was 11 mm.\nAction: We have added details of the tank and sealing materials in the “Fabrication of\nthe velocity sensor” section of the revised manuscript (Line 406).\n6. Line 210: The relationship between response time and flow velocity should be\nfurther clarified. Specifically, why claimed “the large transmission clearance at a low\nflow velocity”? A large transmission clearance corresponds to an extended response\ntime, right? How to ensure the start-up sensitivity claimed by the author with the large\ntransmission clearance? And how to balance response time and starting torque?\nResponse: According to the transmission principle of the magnetic coupling, the two\nmagnets of the magnetic coupling generate an axial force that attracts each magnet to\nthe other in addition to transmitting torque. As shown in Fig. R5, the axial force\nincreases the frictional resistance of the internal structure. In addition, as the\ntransmission clearance decreases, the axial force gradually increases, resulting in\ngreater internal frictional resistance and therefore worse start-up performance at low\nflow velocity. Therefore, at low flow velocity, we use a large clearance to reduce\nfrictional resistance and thus achieve better start-up. Moreover, according to the\nacceleration formula V = V + at and F = ma, we have F = mV/t when V = 0 (where\nt 0 t 0\nV is the final velocity, V is the initial velocity, a is the acceleration, m is the mass of\nt 0\nthe object, and t is the time required to reach V). This indicates that the time required\nt\nfor the cup to reach the rotating speed (V) at a certain flow velocity is inversely\nt\nproportional to the thrust of the flow (F). Therefore, owing to the small F at low flow\nvelocity, a long response time is required. Indeed, a large transmission clearance\ncorresponds to an extended response time.\nFig. R5. Diagram of the effect of the axial force of magnetic coupling on transmission\nresistance.\nIn this work, as the clearance increases from 12.5 to 18.5 mm, the response time\nincreases from 0.48 to 0.96 s. Even though the use of a large transmission clearance at\nlow flow velocity indeed sacrifices the response time to a certain extent, the response\ntime is sufficient for long-term observations of upper-ocean flow (which varies over\nthe course of hours or days) or abyssal flow (which varies over the course of months\nor seasons; Zhou et al., Geophysical Research Letters 49.3 (2022): e2021GL096530).\nTherefore, using a large transmission clearance fulfills the requirements for measuring\nslow currents with minimal variations in flow velocity.\nFinally, we agree with the reviewer that it is important to balance the response\ntime and starting torque. In the design phase, the response time of the DS-TENG was\noptimized by adjusting the transmission clearance, optimizing the length of rabbit fur,\nand selecting a bearing material in alignment with application requirements. To\nreduce the response time of the DS-TENG to a greater extent, on one hand, we can\nfurther optimize the material and structure of the TENG to reduce the friction brought\nby the TENG itself while ensuring sufficient signal output. On the other hand, the\ntransmission clearance can be tailored for different use scenarios to meet the required\nresponse time and starting torque, and the transmission clearance can be minimized to\nreduce the response time under the condition of the minimum detection speed.\nAction: We have included the above discussions in the Supplementary Information\n(Supplementary Fig. 5).\n7. In Fig. 3b, the short-circuit current and rotational speeds also show a distinct\ncorrespondence. The reason for these changes needs to be analyzed and supplemented\nin the paper, such as that of output voltage and transferred charge in Line 225.\nResponse: During the process of stable friction between the fur and fluorinated\nethylene propylene (FEP), the triboelectric charge density remains constant over time.\nThe short-circuit current of the TENG (I ) is the instantaneous current and is\nsc\ncalculated using Equation (1). I is the peak current of the TENG and is calculated\np-p\nusing Equation (2). If the frequency of rotor rotation is f, the relationship between the\ncharge transfer time (t) and (f) in the circuit is that given by Equation (3). From Eqs.\n(1) (2) (3), it is derived that I is positively related to 1/f2 and I is positively related\nsc p-p\nto f. As the rotational speed (n) is equal to the frequency of rotor rotation (f), the peak\ncurrent I is positively related to the rotational speed (n).\np-p\ndQ\nI = (1)\nsc dt\nd𝐼\n𝐼 = 𝑠𝑐 (2)\n𝑝−𝑝 dt\n1\n𝑡= (3)\n4f\nAction: We have included a discussion in the revised manuscript (Line 241).\n8. Line 234: The author claimed “the range of rotational speed is 0 ~ 1000”, but there\nwere only 8 frequency points to calibrate the speed in Fig. 3d. Please add more\nfrequency measurement points.\nResponse: Thank you for your comments. We added more frequency points in Fig. 3d.\nThe original and revised figures are as follows.\nAction: The curve in Fig. 3d of the manuscript has been replaced.\n9. Line 239: Why choose 120 rpm as the lower speed to simulate the charging\ncapacity under the deep-sea condition of a low flow velocity? It needs to be explained\nand supported by the literature or some other authoritative means.\nResponse: Thank you for your question. Here, the statement “To simulate the output\ncapacity of the DS-TENG under the deep-sea condition of a low flow velocity” is\ninaccurate. We have revised the text to “For simulating the operational conditions of\ndeep-sea exploration equipment at low speeds”. This correction is made because the\nDS-TENG is designed as a versatile flow velocity sensor applicable not only for\nmeasurement of the fixed-point flow rate (absolute flow velocity of seawater) but also\nfor speed detection in underwater vehicles (such as autonomous underwater vehicles)\nduring missions. Typically, autonomous underwater vehicles attain maximum speeds\nof 4 knots (approximately 2 m/s) whereas large submersibles (such as Orca XLUUV,\nUSA) reach speeds exceeding 8 knots (approximately 4 m/s). Consequently, the speed\nrange of the DS-TENG needs to cover the velocity range from the deep-sea flow\nvelocity (a few centimeters per second) to the cruising speed of an underwater vehicle\n(a few meters per second). As the relative flow rate measured by the DS-TENG at 120\nrpm is approximately 1 m/s, 120 rpm is selected as the lower speed in simulating the\ncharging capability of the deep-sea exploration equipment at a certain low speed.\nAction: Line 254: The text “To simulate the output capacity of the DS-TENG under\nthe deep-sea condition of a low flow velocity” has been replaced with “For simulating\nthe operating conditions of deep-sea exploration equipment at low speed”.\n10. Line 251: The mentioned pressure-resistance test lasted less than 20 minutes at 45\nMPa. Is there any evidence or literature for this test and duration time? How to judge\n“the sensor remains intact”, can the sensing test be repeated under high pressure?\nResponse: Thank you for your question. Most of the conditions in the hydrostatic\npressure test adhere to Environmental Test Methods for Oceanographic Instruments –\nPart 15: Hydrostatic Pressure Test (GB/T 32065.15-2019, China). The pressurization\nrate, depressurization rate, and maximum test pressure were set in full accordance\nwith the testing standards, and the only parameter that did not meet the standards was\nthe duration of the test. Given our original plan to carry out a sea trial at a depth of\n3600 m, we opted for a maximum test pressure of 45 MPa (calculated as 36 MPa ×\n1.25), paired with a compression rate and pressure reduction rate of 2 MPa/min. As\nmentioned, the selection of the pressure holding time (20 min) was not that of the\nstandard. (According to the standard, the pressure holding time should be set within\n1/2/4/8 h).\nIn the actual deep-sea test, after completing the originally planned 3600-m trial,\nthe DS-TENG’s on-board platform (or remotely operated vehicle, ROV) temporarily\nconducted a deep-sea test at 4500 m. We were pleased to find that the DS-TENG\ncontinued to operate normally after more than 6 hours of testing at 4500 m. After the\ncompletion of the test, the DS-TENG remains operational, which indicates that the\nDS-TENG can be used repeatedly for sensing tests under high pressure. The judgment\nthat “the sensor remains intact” is mainly based on the fact that the pressure tank had\nno deformation after the pressure test, the inside of the sealed tank remained dry, and\nthe DS-TENG still worked normally.\nFig. R6. The original (left) and revised (right) hydrostatic pressure tests.\nTo better understand the repeatability and pressure resistance of the DS-TENG at\n4500 m, we supplemented the standard pressure test of the DS-TENG (Fig. R6). With\nreference to the above standard (GB/T 32065.15-2019, China), the compression rate,\npressure reduction rate and pressure holding time were set at 1 MPa/min, 2 MPa/min\nand 4 h respectively, and the number of cycles was set as two. The maximum test\npressure was set at 49.5 MPa (considering the small size of the DS-TENG, the 4500\nm test depth in the field, and an appropriate safety factor of 1.1 (45 MPa × 1.1 = 49.5\nMPa). Our additional pressure test demonstrated that the DS-TENG maintained\nnormal operation in repeated high-pressure tests. During the actual sea trail, the DS-\nTENG worked well in the two tests totaling 8 hours at 4500 m. In addition, we\nconducted two further deep-sea field tests totaling 8 hours, and the DS-TENG\nmaintained normal operation after recovery, demonstrating that it performs stable\noperation under repeated high-pressure conditions.\nAction: The curve in Fig. 3i of the manuscript has been replaced.\n11. Line 265: The author proposed that “the peak current may be affected by the external\nenvironment”, but a sealed and pressure-resistant environment is provided, right?\nPlease explain this seemingly superfluous concern and provide relevant literature.\nAt the same time, it is proved that the selected voltage signal will not be\naffected.\nResponse: Thank you for your question. As you said, we have provided the TENG\npart with sealed and pressure-resistant conditions, such that the TENG has a stable\nworking environment. The TENG is inside the sealed tank and the rotation of the\nTENG rotor is triggered by the water flow. Therefore, the “external environment”\nmentioned here refers more to different scenarios of the flow velocity.\nAccording to the power generation principle of triboelectric nanogenerators,\nthere is an obvious correspondence between the short-circuit current and the external\ntrigger frequency (Nat. Commun. 2021, 12 (1)). For the TENG alone, according to I\n= dQ/dt and U = Q/C, the voltage signal is relatively stable because the quantity Q is\neasily saturated and stabilized, and the capacitance C is relatively stable, whereas the\ncurrent is heavily dependent on the rotational speed. As shown in Fig. 3b, a higher\nspeed of the TENG rotor corresponds to greater peak current output. Therefore, in the\ncase of low flow velocity, the short-circuit current of the TENG is low, and if the\ncurrent is used as a sensing signal, the output waveform may be submerged by the\npresence of noise. In contrast, the peak voltage of the TENG output is not directly\nrelated to the external trigger frequency, which makes the output by the TENG stable\nand easy to parse in both low and high-flow velocity scenarios. Meanwhile, the\nfrequency of voltage signals is positively correlated with the external trigger\nfrequency, and we thus choose the voltage signal as the sensing signal.\nAction: Line 284: We have corrected the text “the peak current may be affected by the\nexternal environment” to “the peak current may be affected by the external\nenvironment (for example, the rotating speed)”.\n12. Line 288: The measurement range of 0.02 to 6.69 m/s is realized by combining\ntwo transmission clearances. In the practical application of this paper, is the clearance\npre-set by manual or can be real-time adjusted during the procedure? As the author\nclaimed, this will affect the width of the measurement.\nResponse: As you state, the velocity measurement interval of 0.02–6.69 m/s is\nachieved by combining the velocity measurement intervals of two transmission\nclearances. In the current work, this adjustment of the speed range is realized\nmanually, by which the speed range is increased. The flexible design of this variable\nclearance structure allows for further increases in the minimum and maximum flow\nvelocities by optimizing the clearance and a further setup of multiple gears to handle\nmore flow velocity test scenarios.\nTherefore, in subsequent work, we will realize the automatic mechanical\nadjustment of the speed measurement interval by designing the scheme shown in Fig.\nR7. By designing an additional rotor, a chute-iron ball mechanism is integrated into\nthe blades of the rotor, with the iron ball connected via steel wire and a housing (green\npart) that controls the clearance of the magnetic coupling. Our structure is based on\nthe centrifugal force generated during the rotation of the small iron ball to generate\ntension in the axial direction, which moves the active magnet upward. In this way, the\nmechanical regulation of the transmission clearance by the rotational speed can be\nrealized. Specifically, in the initial state, the DS-TENG defaults to the low flow\nvelocity with the highest accuracy (when the transmission clearance is a maximum,\nFig. R7a). When the rotating speed of the rotor increases with the flow velocity, the\niron ball moves outward under the action of the centrifugal force and pulls the green\npart upward (Fig. R7b), such that the transmission clearance is reduced. Thus, the\nmeasurement range can be real-time adjusted with this structure.\nFig. R7. Schematic diagram of automatic adjustment system for velocity measuring\ninterval. (a) Small flow conditions and (b) large flow conditions.\n13. Writing needs improvement, existing some formatting problems. For example, the\ncapacitance size in 240 Line, the caption of Fig. 3d, etc.\nResponse: Thank you for your suggestion. We have made corresponding formatting\nchanges in the revised manuscript.\n14. Before Fig. 5, the voltage were tens of volts, but all of them were 3 V in Fig. 5.\nThe reason needs to be explained.\nResponse: The direct output voltage (tens of volts) of the TENG is beyond the input\nvoltage of the analog-to-digital converter. In safeguarding the signal processing circuit,\nthe output voltage of the TENG is reduced to several volts by circuit divides (10:1) to\nensure that the input signal is within the acceptable range. Consequently, the voltages\nin Fig. 5 are approximately 3 V.\n15. Is the circuit on the ROV in Fig. 5c the same as that in the sensor shown in Fig. 1a?\nThe author needs to introduce these two circuits in more detail and show the circuit\ndiagram.\nResponse: The circuit for the ROV in Fig. 5c is the same as that in Fig. 1a. As shown\nin Fig. R8, the original signal generated by the TENG is input into the circuit board,\nwhich includes an acquisition, processing, and communication circuit. The function of\nthe acquisition circuit is to acquire the high-resistance and high-voltage signals of the\nTENG. First, the acquisition circuit divides and filters the TENG signals. Then, the\nmicrocontroller in the processing circuit performs analog-to-digital conversion and\ndata encoding of analog signals through an analog-to-digital converter. Finally, the\ncommunications circuit further transmits the encapsulated data to the ground platform\nvia the ROV platform.\nIn addition, to avoid a misunderstanding, we modified Fig. 5c such that it is\nconsistent with the schematic in Fig. 1c. The modified figure is shown in Fig. R9.\nFig. R8. The circuit diagram of the DS-TENG.\nThe original (top) and revised (bottom) images are as follows.\nFig. R9. Corrected diagram of Fig. 5c\nAction: We corrected Fig. 5c in the revised manuscript. Furthermore, the details of\nthe circuit design are explained and included in the Supplementary Information\n(Supplementary Fig. 9).\n16. It should be pointed out that the velocity shown in Fig. 5i refers to the ocean\ncurrent velocity or the relative flow velocity between the ROV and seawater. Is it\ncalibrated by the DVL?\nResponse: The velocity shown in Fig. 5i refers to the relative flow velocity between\nthe ROV and seawater, which is calibrated by the DVL.\nAction: Line 346: The text “this robust response to small velocity changes under\nabyssal zone conditions is supported by the linear correlation between the different\nflow velocities” has been revised to “this robust response to small velocity changes\nunder abyssal zone conditions is supported by the linear correlation between the\ndifferent relative flow velocities (between the ROV and seawater)”.\nResponse to Reviewer #2:\nThis paper presents a triboelectric nanogenerator (TENG) based self-powered ocean\ncurrent measurement sensor. This system can distinguish the ocean current from 0.02\nm/s to 6.69 m/s with systematic adaptation, which is a 67 % larger range than the\ncommercial ocean current sensors. Along with this significant sensitivity, this system\ncan be operated under the ocean even at a depth of 4500 m. In this regard, I think this\npaper has a distinct novel point to be published in Nature Communications after the\nrevision process. Our comments to which the author can refer is suggested below:\nResponse: We greatly appreciate the positive feedback and thank the reviewer again\nfor carefully reviewing our manuscript.\n1) Given that the magnetic coupling structure is utilized for the rotational energy\ntransmission, the spacing between the two magnetic coupling can be easily adjusted,\nwhich allows the alternation of the sensing range. However, it seems that the spacing\nneeds to be adjusted manually. Given that the author suggesting the TENG operation\nin deep sea, I think self-adjustment of the sensing range must be possible due to the\nlow accessibility of the deep-sea environment. I think the author should consider\nabout it for its novelty.\nResponse: We strongly agree with the reviewer. Owing to the poor reachability in a\ndeep-sea environment, it is necessary (and possible) to realize the self-adjustment of\nthe sensing range when the actual flow velocity is out of the current range. For\nexample, we designed a structure (Fig. R10) to realize the adjustments of axial\nspacing. This structure uses the centrifugal force generated by removable iron ball in\nblades during the rotation of the rotors to form an axial tension that moves the active\nmagnet upward. In this way, the mechanical regulation of the transmission clearance\nby the rotational speed can be realized. Specifically, in the initial state, the DS-TENG\ndefaults to the low flow velocity with the highest accuracy (when the transmission\nclearance is a maximum, Fig. R10a). When the rotating speed of the rotor increases\nwith the flow velocity, the iron ball moves outward under the action of the centrifugal\nforce and pulls the green part upward (Fig. R10b), such that the transmission\nclearance is reduced. Thus, the measurement range can be real-time adjusted with this\nstructure.\nFig. R10. Schematic diagram of automatic adjustment system for velocity measuring\ninterval. (a) Small flow conditions and (b) large flow conditions.\n2) Also, I guess the magnet must be strong enough to affect each other. In this regard,\nif the electromagnetic generator is hybridized with the suggested TENG, it can be\nhelpful to use this module as an energy harvesting source, not only for the ocean\ncurrent sensing module. Given that energy harvesting on the inaccessible environment\nis always getting spotlight, it can enhance the applicability. The author should\nconsider about it.\nResponse: Thank you for your suggestion. The TENG has the advantage of high\nvoltage and the electromagnetic generator (EMG) has the advantage of high current.\nCoupling an EMG and TENG can improve the energy harvesting efficiency and\noutput power of the whole system. In addition, the energy harvesting technology for\nmarine environments based on the coupling of the TENG and EMG has been widely\nstudied (Adv. Energy Mater. 2021, 11(31): 2101116; Adv. Electron. Mater. 2021, 7(9):\n2100277; Small 2023, 19(25): 2300847), and we will further consider how to\nintegrate an EMG inside the DS-TENG in future work to improve the environmental\nadaptability of the DS-TENG.\n3) In Figure 2f, the transmission curve of the active and passive magnets at different\ntransmission clearances is shown. However, it is hard to distinguish the data, because\nseveral data are duplicated. The author should refurbish the figure for the readers’\nunderstanding.\nResponse: Thank you for the suggestion. Owing to a data overlap in Fig. 2f, detailed\ndata curves (Fig. R11) are presented in the Supplementary Information.\nThe figure in manuscript is as follows.\nThe figure in the Supplementary Information is as follows.\nFig. R11. Transmission curve at different transmission clearances. (a) 0 mm, (b) 12.5\nmm, (c) 15.5 mm, (d) 18.5 mm, (e) 21.5 mm, (f) 24.5 mm.\nAction: We have added Fig. R11 to the Supplementary Information (Supplementary\nFig. 4).\n4) In Figures 4b to 4f, the electrical output generated from the two different state is\nshown. However, although the state 1 is well explained in Figure 4b, details about\nstate 2 are not explained in the main figures. I think this figure set does not reveal the\ncorrelation between the operational state and the sensitivity range. In this regard, I\nthink author should add about the detail about state 2 in Figure 4. It will enhance the\nreadability of the figure.\nResponse: Thank you for your suggestion. We recognize that not showing the two\nstates might prevent the readers from understanding that the two states correspond to\nthe two sensitivities.\nAction: Figure 4 has been revised to include state 2 and thus enhance the reader’s\ncomprehension of the distinction between states 1 and 2.\nThe original (top) and revised (bottom) images are as follows.\n5) In Figure 5e, the raw voltage output generated underwater is depicted. However, it\nseems that there are unclear data which can potentially decrease the accuracy of the\ncurrent sensing. Where these abnormal peaks are coming from, and how these peaks\nwill affect the sensing?\nResponse: Thank you for your question. Measurement circuits for voltage can be\ncategorized into impedance-type circuits (e.g., oscilloscopes) and capacitive-type\ncircuits (e.g., electrostatic meters). In our circuit design, the impedance-type circuit\ndesign is used. When the rotor of the DS-TENG rotates slowly or does not rotate,\nas the TENG can be modeled as a capacitor, the impedance-type circuit generates\nabnormal peaks due to the generation of electrical current during the voltage\nacquisition. Therefore, to avoid this situation, future improvements of the signal\nprocessing circuit need to consider the matching of impedance and capacitance\ncharacteristics, and the combination of the two to realize a better signal processing\nmethod.\nResponse to Reviewer #3:\nThis is a well-written manuscript and I think it is appropriate for this journal. The\nmanuscript is well referenced with appropriate citations and is well illustrated with\nhigh quality figures. The design of the current meter has the potential to provide\nincreased capabilities to the oceanographic community. I would recommend this\npaper for publication if these issues can be addressed:\nResponse: We sincerely thank the reviewer for carefully reviewing our manuscript\nand making professional comments that are important to improving the quality of the\nmanuscript.\n1. It is not clear to me from the description in the paper that the power generated by\nthe instrument is sufficient to power a self-contained acquisition and logging system,\nwhich would need to be integrated if the instrument were to be deployed as a stand-\nalone device that provides data on recovery from deployment. If this were the case\nthen this would be a very significant advance as it would enable long-term monitoring.\nResponse: We appreciate and agree with your views. In general, conventional\nmechanical sensors used to measure the ocean current velocity have measurement,\nsignal processing, and power supply modules. As the measurement module cannot\ngenerate electricity on its own, the power supply module must continuously power the\nmeasurement module throughout the measurement process (Fig. R12a). Hence, both\nthe measurement and signal processing modules rely on an external power supply.\nUnlike conventional sensors, the DS-TENG utilizes a triboelectric nanogenerator\nto self-generate power (without the need for an external power supply, Fig. R12b) by\ngenerating electrical signals correlated to the flow rate when triggered by water flow.\nThese signals are then directly transmitted to the signal processing module.\nAt present, the power generated by the DS-TENG under conditions of low flow\nvelocity is approximately 30 μW, while the signal processing and storage module are\npowered by a 3-V DC power (Li-ion battery) with an operating current of 3 mA. Thus,\nthe DS-TENG in its present state is insufficient to power a self-contained acquisition\nand logging system as a stand-alone device. Although the signal processing module\nstill requires a power supply, our work is an important development beyond the\nconventional sensor.\nTo realize the self-supply of the entire flow velocity measurement system and\nenable the long-term monitoring of the deep-sea flow velocity, improvements can be\nmade: (1) optimizing the friction material of the TENG to increase surface charge\ndensity for improved output power; (2) enhancing the rotational and friction grid\nstructure of the TENG to expand the effective friction area and optimize the induced\ncharge flow velocity, thereby boosting the output voltage and current; and (3) refining\nthe low-power signal processing circuit and strategizing the division of tasks between\nperiodic signal acquisition and electrical energy harvesting to reduce the overall\nsystem power consumption (Fig. R12c). Through the above improvements and\noptimization, we expect to realize an ocean current sensor with a self-contained\nacquisition and logging system for long-term monitoring.\nFig. R12. Schematic diagram of the working principles of current sensors: (a)\ntraditional system, (b) present DS-TENG, and (c) next DS-TENG.\n2. I would like some clarification on this sentence: “After 30 days of operation, the\nreduction in the induced charge of the TENG remains within 6%”. What is the cause\nof this reduction and would it continue beyond the 30 days period? What causes it and\nwhat effect does it have on the lifespan of the instrument? Maybe I have\nmisinterpreted the sentence, but it would be good to have further details in the\nmanuscript to ensure that others accurately interpret what is being said here.\nResponse: The output performance of the TENG is affected by external\nenvironmental factors and TENG’s inherent factors. Environmental factors primarily\ncomprise temperature, humidity, radiation, and electromagnetic interference. This\nmanuscript does not consider external environmental factors (e.g., humidity, pressure,\nand corrosiveness) because the TENG is isolated in a sealed tank.\nTENG’s inherent factors are described as follows.\n(1) Material and wear factors: First, the two friction materials (FEP and fur) have\ngood chemical stability such that they will generally last for many years. Second, the\nmaterials have good abrasive resistance. In principle, during the long-term friction\nprocess, the two materials rubbing against each other leave material debris on one\nanother, which may reduce the surface charge density of the tribomaterials and\nthereby affect the TENG output. Given that the rabbit fur and FEP used in this study\nhave high wear resistance and the contact friction between them is slight, the material\nand wear factors do not greatly affect the material lifetimes.\n(2) Structural factors: The amount of triboelectric charge is related to the degree\nof contact between the two friction materials. The rabbit fur used in this study makes\ngood contact with the FEP in the initial stage. With an increase in the running time,\nthe contact force between the rabbit fur and FEP decreases because of the poor elastic\nrestoring force, which reduces the area of contact and thus attenuates the output\ncharge. Theoretically, the lifetime of the TENG might be over 10 years if the\nperformance decay due to contact is avoided. Therefore, to increase the lifetime of the\nDS-TENG, the rabbit fur structure can be further optimized (1) by surface treatment\nof the rabbit fur, or by further reducing the fur length to increase the fur’s elasticity\nfor long-term stable contact with the FEP, or (2) by replacing the rabbit fur with other\npolymer brush structures with better elasticity to ensure a stable contact in the long-\nterm.\nIn this work, the cause of the slight reduction in the TENG performance beyond\nthe 30-day period was mainly attributed to the TENG’s inherent factors (i.e., material\nwear or contact deformation of the rabbit fur). Nevertheless, the deformation of the\nrabbit fur and the attenuation of the induced charge will slow down, and the output of\nthe TENG tends to fluctuate within in a certain range. In quantitatively demonstrating\nthe durability of the rabbit fur, through visual inspections and SEM observations of\nthe rabbit fur after 30 days of use (Fig. R12), we found no notable wear on the surface,\nindicating that it is appropriate to combine rabbit fur and FEP as the tribomaterials of\nthe TENG. Furthermore, given that the sensor data are derived from the frequency of\nthe output voltage rather than the amplitude, the signal-to-noise ratio remains\nsubstantial even if the charge continues to attenuate following the observed trend for\nover a year. Consequently, we believe that the reduction in charge would have\nminimal impact on the lifespan of the instrument. In future work, the lifespan of the\nDS-TENG will be further improved by optimizing the materials and structure.\nFig. R12. SEM image of rabbit fur after running for more than 30 days.\nAction: We have revised the manuscript (Line 188) and included a detailed discussion\nin the Supplementary Information (Supplementary Fig. 3).\nOnce again, the authors would like to thank the reviewers for their constructive\ncomments concerning our manuscript entitled “A self-powered and speed-\nadjustable sensor for abyssal ocean-current measurements based on a\nnanogenerator” (No.: NCOMMS-23-56530).\nSincerely yours,\nYang Yang, Prof. Dr. Chang Bao Han, Prof. Dr.\nInstitute of Deep-Sea Science and Engineering Faculty of Materials and Manufacturing\nChinese Academy of Sciences Beijing University of Technology\nLuhuitou Road 28, Sanya, Hainan 572000 Pingleyuan 100, Chaoyang, Beijing 100124\nP.R. China P.R. China"}, {"round": 2, "reviewer_comments": {"reviewer_1": "All concerns from the reviewer have been addressed well, this manuscript is recommended to bu\npulished on Nature Communications in current version.", "reviewer_2": "First of all, I appreciate of the authors’ efforts to improve the quality of the manuscript. The author\nkindly responded to the comments which are given. With the responses, the whole manuscript\nbecomes more straightforward for readers to understand. So, I think this paper can be published\non nature communications.", "reviewer_3": "Thank you for you comprehensive responses to myself and the other two reviewers. I would be\nhappy for the paper to be published if further consideration could be given to clearly explaining\nwhat is meant by self-powered. This point was also raised by Reviewer 1 and I don't think the\namendments made (Line 81) sufficiently address this point to fully explain to the reader. This is\ncritical to be clear here as it is in the title of the paper."}, "author_response": "May 16th, 2024\nRE: Manuscript resubmission to Nature Communications\nDear Reviewers:\nThank you for your valuable time in reviewing our manuscript “A self-powered and\nspeed-adjustable sensor for abyssal ocean-current measurements based on a\nnanogenerator”. We appreciate your consideration of our revisions to the manuscript\n(No.: NCOMMS-23-56530A). Here, we have further revised the manuscript in\nresponse to your comments. In the following, we quote the reviewers’ reports in full,\nand our responses are interspersed in blue. Action taken is indicated in red.\nResponse to Reviewer #1:\nReviewer #1:\nAll concerns from the reviewer have been addressed well, this manuscript is\nrecommended to be published on Nature Communications in current version.\nResponse: We greatly appreciate the positive feedback and thank the reviewer for\ncarefully reading our text and providing comments to improve our manuscript.\nResponse to Reviewer #2:\nReviewer #2:\nFirst of all, I appreciate of the authors’ efforts to improve the quality of the manuscript.\nThe author kindly responded to the comments which are given. With the responses, the\nwhole manuscript becomes more straightforward for readers to understand. So, I think\nthis paper can be published on nature communications.\nResponse: We greatly appreciate the positive feedback and thank the reviewer again\nfor carefully reviewing our manuscript.\nResponse to Reviewer #3:\nReviewer #3:\nThank you for you comprehensive responses to myself and the other two reviewers. I\nwould be happy for the paper to be published if further consideration could be given to\nclearly explaining what is meant by self-powered. This point was also raised by\nReviewer 1 and I don't think the amendments made (Line 81) sufficiently address this\npoint to fully explain to the reader. This is critical to be clear here as it is in the title of\nthe paper.\nResponse: We sincerely thank the reviewer for carefully reviewing our manuscript and\nmaking professional comments that are important to improving the quality of the\nmanuscript.\nTo further explain what is meant by the term “self-powered”, we have added a\ndescription of the definition of “self-powered” in the Introduction of the revised\nmanuscript (Line 67), with references to the relevant literature (Nat. Commun. 2020, 6,\n1-8; Joule 2017, 1, 480-521; Nat. Rev. Cardiol. 2020, 18, 7-21; Sci. Adv. 2017, 3, 1-7;\nSci. Adv. 2020, 6, 1-10).\nAction: “For self-powered sensors, the transform from mechanical signals to\nrecognizable electrical sensing signals does not need an additional power supply, and\nthe TENG can perform this function well. For example, TENG-based self-powered\nsensing has been extensively studied in speed, vibration, biomedicine and other sensing\nfields” was added to the Introduction of the manuscript (Line 67), with the addition of\nthe corresponding references (References 31-35).\nOnce again, the authors would like to thank the reviewers for their constructive\ncomments concerning our manuscript entitled “A self-powered and speed-adjustable\nsensor for abyssal ocean-current measurements based on a nanogenerator” (No.:\nNCOMMS-23-56530A).\nSincerely yours,\nYang Yang, Prof. Dr. Chang Bao Han, Prof. Dr.\nInstitute of Deep-Sea Science and Engineering Faculty of Materials and Manufacturing\nChinese Academy of Sciences Beijing University of Technology\nLuhuitou Road 28, Sanya, Hainan 572000 Pingleyuan 100, Chaoyang, Beijing 100124\nP.R. China P.R. China"}, {"round": 3, "reviewer_comments": {"reviewer_3": "Thank you for the modifications. I would be happy for the manuscript to be published in its current\nform."}, "author_response": ""}]

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