Nanoelectrode design from microminiaturized honeycomb monolith with ultrathin and stiff nanoscaffold for high-energy micro-supercapacitors

Downsizing the cell size of honeycomb monoliths to nanoscale would offer high freedom of nanostructure design beyond their capability for broad applications in different fields. However, the microminiaturization of honeycomb monoliths remains a challenge. Here, we report the fabrication of microminiaturized honeycomb monoliths—honeycomb alumina nanoscaffold—and thus as a robust nanostructuring platform to assemble active materials for micro-supercapacitors. The representative honeycomb alumina nanoscaffold with hexagonal cell arrangement and 400 nm inter-cell spacing has an ultrathin but stiff nanoscaffold with only 16 ± 2 nm cell-wall-thickness, resulting in a cell density of 4.65 × 109 cells per square inch, a surface area enhancement factor of 240, and a relative density of 0.0784. These features allow nanoelectrodes based on honeycomb alumina nanoscaffold synergizing both effective ion migration and ample electroactive surface area within limited footprint. A micro-supercapacitor is finally constructed and exhibits record high performance, suggesting the feasibility of the current design for energy storage devices.

This manuscript presented an interesting design strategy of micro-supercapacitor nanoelectrodes based on honeycomb alumina nanoscaffolds. Ultrathin alumina honeycomb nanoscaffold has been successfully used as the framework, using which the designed nanoelectrode has stable and verticallyaligned nanopores with large ion-accessible surface area and low ion transport resistance, as well as a large scaffold/electrode proportion due to the ultrathin walls. The device was shown an outstanding electrochemical performance in terms of peak energy and power density. The work should be worthwhile publishing after addressing the main concerns as listed below: 1. Mechanical stability was less discussed, needs more elaboration. For example, more discussions should be provided about the electrochemical performance of HAN@SnO2 as current collectors under mechanical pressing, say in Figure 2g. What are the residues in Figure 2i? Is it the residual glass fibers? 2. Why were MnO2 and PPy selected as the electrode materials, any particular justifications or reasons? 3. How about the performance cycling stability of the symmetric micro-supercapacitors? Beside morphological changes of electrode after long-termed cycling (Supplementary Figure 9), anything else? 4. More detailed discussion could be provided about the effect of the charge transport resistance variations in HAN@SnO2@MnO2 and HAN@SnO2@PPy electrodes on the electrochemical energy storage performance. 5. In Figures 5b-c, the CV curves of the asymmetric micro-supercapacitors have retained the rectangular shape, meaning the good electrode capacity matching, which should be pointed out in detail. 6. In Figure 5g, a distorted CV and GCD curves was observed on the asymmetric microsupercapacitors with ionic liquid electrolyte, as compared to those with Na2SO4 aqueous electrolyte, any explanations?

Comments to Authors
Microminiaturized honeycomb monoliths have been widely exploited as catalyst supporters in different gaseous reactor applications such as chemical and refining processes, catalytic combustion, ozone abatement, and photocatalytic air purification. This article reports the fabrication of microminiaturized honeycomb alumina nano scaffolds as a robust nanostructuring platform to assemble active materials for micro-supercapacitors. It's a smart and interesting strategy to fabricate the microminiaturized HANs. However, the insulating HANs are not preferable for assembling active materials of microsupercapacitors but could be better for some catalysis. Moreover, the electrochemical performances of those devices in this paper are not impressive. As such, I feel that this work would find a more suitable outlet in another journal. Some suggestions for authors to improve the quality of this paper: 1. What's the electrolyte in HAN@SnO2//HAN@SnO2 cell? SnO2 is not a good active material for supercapacitor. 2. How to get the discharge density in Fig. 2c? The current density isn't linear at a scan rate of 20 V/s. 3. What's the weight-specific capacitance of HAN@SnO2@PPy on the HAN? The pore depth of HAN in both two nanoelectrodes was 25 μm, and the mass loading of MnO2 and PPy would be high. 4. It's double-layer but not pseudocapacitive mechanism in the HAN@SnO2@PPy according to the CVs in Fig. 3 since the polypyrrole (PPy) electrode could not react with Na2SO4 electrolyte. 5. Very confusing, why replace aqueous electrolyte with the ionic liquid electrolyte EMIM-TFSI for 1

Reviewer #1
This manuscript presented an interesting design strategy of micro-supercapacitor nanoelectrodes based on honeycomb alumina nanoscaffolds. Ultrathin alumina honeycomb nanoscaffold has been successfully used as the framework, using which the designed nanoelectrode has stable and vertically-aligned nanopores with large ion-accessible surface area and low ion transport resistance, as well as a large scaffold/electrode proportion due to the ultrathin walls. The device was shown an outstanding electrochemical performance in terms of peak energy and power density. The work should be worthwhile publishing after addressing the main concerns as listed below.
Reply: We greatly appreciate the reviewer for the positive feedbacks on the contents presented in our manuscript and the support on the publication of this work. Reply: We thank the reviewer for the useful comment and suggestion. Following your suggestion, we have provided more discussion on the electrochemical characterizations of HAN@SnO 2 as nanostructured current collectors under mechanical pressing in the revised manuscript (please see the revised manuscript, Pages 10-11). The residues in Figure 2i of the manuscript is the residual glass fibers from a glass microfiber filter (Whatman, GF/B) that was used as the separator to assemble the HAN@SnO 2 //HAN@SnO 2 device. Partials of the glass microfiber filter were damaged and fallen off during the disassembly of the HAN@SnO 2 //HAN@SnO 2 device for SEM characterizations.   Reply: We appreciate the reviewer for giving this useful suggestion. Following your suggestion, we have provided some more discussions about the electrochemical impedance spectroscopy (EIS) properties of MSCs to identify the kinetics of electron and ion transport within both HAN@SnO 2 @MnO 2 //HAN@SnO 2 @MnO 2 and HAN@SnO 2 @PPy//HAN@SnO 2 @PPy MSCs as well as the effect on the electrochemical energy storage performance (please see the revised manuscript, Pages 14-16).

Question 5: In Figures 5b-c, the CV curves of the asymmetric micro-supercapacitors have
retained the rectangular shape, meaning the good electrode capacity matching, which should be pointed out in detail.

Reply:
We highly appreciate the suggestion from the reviewer. The balance of the electrode masses or charges is crucial for constructing asymmetric supercapacitors. The charges passed through the positive and negative electrodes in an asymmetric supercapacitor must be the same. In this case, the masses of the positive and negative electrodes are fixed according to the charges passed through as demonstrated in equation where Q is the charge of the electrode, m is the mass of the electrode, C sp is the specific capacitance of the electrode, ΔE is the potential range, and the superscript + and -represent the positive and negative electrodes, respectively. In our work, it is worth noting that the C sp of both the positive and negative electrodes were calculated based on a two-electrode symmetric MSCs rather than a conventional three-electrode configuration cell, which helps to reach the good electrode capacity matching in an asymmetric MSCs.
Question 6: In Figure 5g,  For the comment "the insulating HANs are not preferable for assembling active materials of micro-supercapacitors", we agree that the as-prepared HANs are insulating (we also mentioned it in our manuscript). However, in order to enable our HANs to be preferable for supercapacitor applications, we coated the HANs with a thin layer of SnO 2 and hence converting the insulating HANs to nanostructured current collectors (denoted as HAN@SnO 2 in our manuscript) for assembling electrode active materials of micro-supercapacitors. SnO 2 was coated by atomic layer deposition (ALD), by which a high quality (conformally and uniformly) coated layer is ensured. This ALD-coated SnO 2 has a much higher electrical conductivity than those of many other metallic oxides. Thereafter, the HAN@SnO 2 hybrid structure has been further verified to be feasible as a nanostructured current collector to assemble MnO 2 and PPy (active supercapacitor materials) as electrodes. In another word, the robust nanostructuring platform in our work to assemble active materials for microsupercapacitors is HAN@SnO 2 rather than HAN, which is an important aspect of our work.
Regarding the comment on the performance of micro-supercapacitors "the electrochemical performances of those devices in this paper are not impressive", we are sorry that we could not agree with this comment based on the following: MSCs still needs to be further improved. In the present wok, we demonstrate the feasibility of the HAN-based nanoelectrode design strategy for achieving MSCs with high areal energy and power performance. To the best of our knowledge, the MSCs in this work is among the best comprehensive energetic performance of the reported MSCs (please see Figure 6b in the revised manuscript and Table S1 in the revised supplementary information). Particularly, the peak energy density of our MSCs reaches 160 μWh cm -2 , which is about fourfold that of the representative MSCs based on carbide-derived-carbons (of about 40 μWh cm -2 ) but with a similar peak power density (Science 2016, 351, 691.). Moreover, the areal energy density of our MSCs is even comparable with those of the state-of-the-art three-dimensional microbatteries (Nat. Commun. 2013, 4, 1732; Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 6573) but with much higher areal power density. Up to now (about six months after we submitted this manuscript on April 23 rd ), the comprehensive energetic performance of our MSCs is still the best one (as shown in Figures R2 and R3, and all the corresponding data are included in the revised Supplementary Table 1). Not limited to MSCs, the HAN-based nanoelectrode concept shown in this work shall be applicable to assemble different catalysts for various catalysis applications, as pointed out by the reviewer. Therefore, we believe that the very good performance of our MSCs will be of high interest to the scientists working in the field of electrochemical energy storage and to the broad readership of Nature Communications.  Reply: We appreciate the reviewer for this comment. The electrolyte in the HAN@SnO 2 //HAN@SnO 2 cell is 1.0 M Na 2 SO 4 aqueous solution.
We totally agree with the reviewer that SnO 2 is not a good active material for supercapacitor. Actually in this work, we used SnO 2 only as the conductive layer to convert the insulating HAN to current collector (as mentioned above), rather than as an electrode active material for supercapacitors. The obtained HAN@SnO 2 was served as a nanostructured current collector for assembling electrode active materials (i.e., MnO 2 and PPy in this work) of supercapacitors.
Question 2: How to get the discharge density in Fig. 2c? The current density isn't linear at a scan rate of 20 V/s.

Reply:
Thanks for the comment. The discharge current densities in Fig. 2c of the manuscript were calculated by normalizing the discharge currents (taken from the cyclic voltammetry profiles at 0.5 V for discharge segments) to the footprint area of HAN@SnO 2 //HAN@SnO 2 cell (Adv. Energy Mater. 2015, 5, 1500003.). The nonlinear current density at a scan rate of 20 V s -1 for the HAN@SnO 2 //HAN@SnO 2 cell should be due to the limited ion diffusion at higher scan rates (Adv. Energy Mater. 2015, 5, 1500003) and the relatively low electrical conductivity of SnO 2 compared to metals (Adv. Sci. 2015, 3, 1500299;Adv. Energy Mater. 2019, 9, 1901061). for MnO 2 at a scan rate of 0.2 mA cm -2 (or 0.31 A g -1 ) and 239.4 F g -1 for PPy at a scan rate of 0.2 mA cm -2 (or 0.15 A g -1 ), respectively. worth noting that all the electrochemical measurements (i.e., cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy) were carried out in a two-electrode configuration rather than a three-electrode configuration. The shape of cyclic voltammetry profiles measured from a two-electrode configuration with well-matched capacity of positive and negative electrodes would be more symmetric.
Reply: Thank for this comment. Most of the recent research efforts are devoted to improving the energy density of supercapacitors, and the replacement of aqueous electrolytes with ionic liquid electrolytes is regarded as an effective strategy to extend the working potential window of supercapacitors and thus improve the energy density of supercapacitors. The main purpose to replace aqueous electrolyte with the ionic liquid electrolyte is to further extend the working potential window of HAN@SnO 2 @MnO 2 //HAN@SnO 2 @PPy MSCs, and subsequently to improve the energy density of devices. With 1.0 M Na 2 SO 4 aqueous electrolyte, the maximum potential window of HAN@SnO 2 @MnO 2 //HAN@SnO 2 @PPy MSCs is 1.6 V, and it can be further extended to 3.0 V when using the EMIM-TFSI ionic liquid electrolyte.
Although the device capacity was slightly reduced from 144 mF cm -2 to 128 mF cm -2 at a same current density of 0.5 mA cm -2 , the overall device energy density of HAN@SnO 2 @MnO 2 //HAN@SnO 2 @PPy MSCs with the EMIM-TFSI ionic liquid electrolyte is significantly enhanced attributing to the extended working potential window (i.e., from 1.6 V to 3.0 V) according to the following equation: