A universal standardized method for output capability assessment of nanogenerators

To quantitatively evaluate the output performance of triboelectric nanogenerators, figures of merit have been developed. However, the current figures of merit, without considering the breakdown effect that seriously affects the effective maximized energy output, are limited for application. Meanwhile, a method to evaluate output capability of nanogenerators is needed. Here, a standardized method that considers the breakdown effect is proposed for output capability assessment of nanogenerators. Contact separation and contact freestanding-triboelectric-layer modes triboelectric nanogenerators are used to demonstrate this method, and the effective maximized energy output and revised figures of merit are calculated based on the experimental results. These results are consistent with those theoretically calculated based on Paschen’s law. This method is also conducted to evaluate a film-based piezoelectric nanogenerator, demonstrating its universal applicability for nanogenerators. This study proposes a standardized method for evaluating the effective output capability of nanogenerators, which is crucial for standardized evaluation and application of nanogenerator technologies.


(NCOMMS-18-35238) Point to Point Response to the referees' reports (comments in black and responses are in blue):
REVIEWER REPORT(S): Reviewer #1 (Remarks to the Author): In this paper, a standardized assessment method is firstly proposed for output capability assessment of nanogenerators. Contact separation and contact freestandingtriboelectric-layer modes TENGs are used to demonstrate this method. To further demonstrate the broad applicability of this method on various nanogenerators, a PVDFfilm based piezoelectric nanogenerator is utilized to understand their output capability as well. Although the work seems interesting, some explanations and figures are not clear. I recommend publication of the manuscript after major clarifications. The comments to the author are given as below:

Answer:
We would like to express our sincere thanks to the reviewer for clearly understanding the significance, innovation and broad impact of this work.
1. In the experiment, the position between the dielectric layer and the electrode is timevarying or static? What is the distance between two triboelectric layers in Fig. 2b?

Answer:
Thank the reviewer for the concerning. The position between the dielectric layer and the electrode is static. The gap of the Q-V plot in Fig. 2b is 2mm with visible sparks as shown in supporting video S1. The gap of the original inset photo with obvious spark is 5mm. We have changed this inset photo in Fig. 2b to be a screenshot of spark at x = 2mm, which is consistent with the Q-V plot. The original photo of x=5mm has been put in the supporting information as Fig. S8, to demonstrate the existence of breakdown effect in TENGs with a larger gap.
2. In the experiment of Q-V curves with no-breakdown and breakdown states, the target TENG and PENG are equivalent to a capacitor? However, the distance between two triboelectric layers are changed during the working process. Author should add the experimental data of real-time Q-V curves considering the distance change during this process.

Answer:
Thank the reviewer for the suggestion. Yes, in the experimental measurement of Q-V curves to determine non-breakdown and breakdown states, the target TENG and PENG are considered as equivalent to capacitors, and there is no energy dissipation considered during the operation process other than the breakdown effect. The slope of the Q-V plot at non-breakdown region equals to the capacitance.
As an alternative method, the real-time Q-V plot can be measured by directly connecting the TENG with the external resistance R, as shown in Fig. R1(a). The real-time current I can be measured, and then Q and V are obtained based on the equations shown below: The real-time Q-t plot is as shown in Fig. R1(b). Fig. R1(c) shows the real-time Q-V plot. Similar to the static measurement results in the manuscript, when breakdown happens, there are turning points observed, which can be considered as the breakdown points. However, in fact we can only mark a few suspected breakdown points by the green arrows in Fig. R1(c) since sparks can be only observed sometimes to confirm the breakdown. The video record of the real-time sparks is added in the supporting video S5. Therefore, the reasons that we cannot use this real-time Q-V measurement method are as below: Firstly, these turning points are quite hard to be identified by the measured curves only, since the real-time curves in non-breakdown areas are usually not straight lines, as demonstrated in our previous work 1 . Secondly, this method cannot guarantee the measurement of breakdown points at all positions since the displacement is keeping varying. Thirdly, we cannot extract the real capacitance of the TENG from the curves to validate our measurement due to the time-varied capacitance. Hence, the static Q-V plot based method as developed in the manuscript is more suitable to work as the universal method. 3. In order to increase the readability of Figure, the X-axis and Y-axis should be aligned for Fig. 3d, 4e, 5c.

Answer:
Thank the reviewer for the suggestion. Figures of the manuscript has already been modified.
4. In Fig. 2b, three times spark are happened. Which one is the breakdown voltage in Fig. 3b.

Answer:
Thank the reviewer for the question. The first one corresponds to the breakdown voltage. The measured breakdown voltage is always related to a certain surface charge density, and the surface charge density may change after each breakdown happens. The breakdown voltage of different gap distance should be compared at the same surface charge density, so only the voltage of the first spark or turning point is recorded as the breakdown voltage.

Answer:
Thank the reviewer for the suggestion. The FOMP is the performance FOM of TENG developed in the former research, which can be defined as: 1 = • So the revised FOMP can be defined as: The FOMP is directly proportional to the largest possible effective average output power and related to the highest achievable energy-conversion efficiency, regardless the mode and the size of the TENG.
6. Author should explain why the measured values are larger than the calculated values for the first three data points in Fig. 3b.

Answer:
Thank the reviewer for the question. In our experiment, the fully-contact status of zero displacement (x=0) may not be determined precisely due to surface roughness, which may impact the results. Specifically, at the very small displacement (< 10 -4 m), the real effective gap could be larger than the measured one, so that the corresponding breakdown voltage becomes larger due to Paschen's law, resulting in the enlarged first three points in Fig. 3b. 7. In the process of testing the target TENG and PENG, TENG1 is always used as the high voltage source?

Answer:
Thank the reviewer for the question. Yes, during the whole operation process of testing the target TENG and PENG, TENG1 is always used as the high voltage source, which is a SFT mode TENG with 7cm × 14cm size of electrodes and a 2cm maximal gap between the two electrodes.
Reviewer #2 (Remarks to the Author): A Universal Standardized Method for Output Capability Assessment of Nanogenerators ' takes the breakdown effect into consideration and builds a universal standardized method for assessing the output capability of different nanogenerators. This is a key step for evaluating the effective output capability of nanogenerators in-depth and in a standardized way with the breakdown effect considered, and developing the revised figure-of-merits of different modes. The manuscript is solid in its contents, accurately and precisely capturing the current state-of-the-art researches around TENGs and other nanogenerators. I highly recommend the manuscript for publication in Nature Communications. Here are some points that should pay attention.

Answer:
We would like to express our sincere thanks to the reviewer for clearly understanding the significance, innovation and broad impact of this work.
1. How to choose the value of QSC,max?

Answer:
Thank the reviewer for the concerns. The selection of QSC,max is mainly based on the measured value of the fabricated TENG2 (target TENG), which is determined by triboelectric and electrostatic induction effects. The value is chosen as the average value of measured QSC,max by several times. Here, the key of this universal method is to keep the value of QSC,max identical for each measurement process at different x of the same TENG2 device.
2. The dmax of different modes TENG in this manuscript is only 2mm. Is this universal method still usable for a higher dmax?

Answer:
Thank the reviewer for the concerns. We actually use x to represent gap distance instead of dmax or d. Yes, just as the inset photo of visible spark between the tribo-layers shown in Fig. S8 (originally inset of Fig. 2b), when x = 5mm, visible spark still exists, demonstrating the existence of the breakdown effect. And the turning points in Q-V curves can also be measured to confirm the breakdown points with x=5mm. Therefore, this universal method is applicable for a higher x.
3. Because the structure of TENG is becoming more and more complicated. Whether this universal method can be applied for complicated TENGs in future?

Answer:
Thank the reviewer for the concerning. Yes, we have demonstrated CS, CFT TENGs in the manuscript. And besides, since all the TENGs have capacitive behaviors and can be described by the V-Q plot 1 , we can always determine the breakdown points in a certain displacement x through this universal method. Following the process flow in Fig. 1d, the breakdown points at various x can be determined, forming the breakdown line, regardless of the structures. Therefore, this method is universally applicable for complicated TENGs in the future.
We would like to thank the reviewers again for the valuable comments and suggestions. We have made revisions in the manuscript accordingly.