Peer review of: An organic/inorganic electrode-based hydronium-ion battery (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

This work reports the design of a proton-based aqueous rechargeable battery using inorganic 
MnO2 as the cathode and organic PTO as the anode. While the reported performance is improved 
compared to previous reports in this area, I have some major concerns regarding the reaction 
mechanisms which are currently not clear. Therefore, I cannot recommend publication in NC. 
Detailed comments are as following: 
1. The authors discuss lots of merits of H+-based battery, one of which is the smallest radius of 
H+ compared to other cations such as Li+ and Na+ and H3O+. However, it seems that the 
actively inserted ions here in this work are still H3O+, not H+. So this leads the reviewer, and 
possibly the future readers to wonder how the mentioned drawbacks of H3O+ in the introduction 
are addressed in this work. Apparently to the reviewer, this is not addressed, which results in my 
doubt about the novelty of this work. At least, the authors should avoid discussing the H+-based 
battery, which is quite misleading. Using the term H3O+ instead of H+ (or proton) in both the title 
and the main text (including Scheme 1) will be more appropriate. 
2. Regarding the Equation 1, it seems that the authors provide enough evidence to confirm the 
anode side reaction mechanisms. However, there is no evidence showing that the cathode MnO2 
phase can be reversely taking part in the reaction. For example, there is no study confirming 
MnO2 can disappear from the cathode during the ion insertion reaction. More importantly, MnO2 
has been known to possess large opening framework for ion intercalation, and it experiences 
framework-drive ion intercalation reaction followed by conversion reaction with a phase transition, 
and such phase transition is typically not reversible. Saying that, the authors need to provide extra 
evidences to clarify the MnO2 cathode reaction mechanisms as described in the discussion. 
3. In Figure 2, please show more STEM-EDS results for gradual MnO2 loading experiments under 
different current. Can the authors please also provide the mass loading for MnO2 on the current 
collector? The reviewer believes this is very important to relate this to the overall capacity of the 
battery during cycling. 

4. The low-temperature performance is really interesting. Can the authors compare their value to 
other reported state-of-the-art values of low-T battery studies to demonstrate how good is good? 

5. Provide the pH values of the electrolyte, which is claimed by the authors to be a constant value. 
And also please explain how the value being constant during cycling is confirmed. 



Reviewer #2 (Remarks to the Author): 

In this paper the authors present a secondary battery composed of a quinone-type anode (pyrene-
4,5,9,10-tetraone) (PTO) and a graphite felt cathode (pre-deposited with MnO2) serving as 
substrate for oxidative deposition of Mn2+ forming MnO2. A 2M H2SO4 water solution containing 
2M MnSO4 is used as electrolyte and the authors claim that this battery operates according to a 
rocking-chair principle. Both the PTO anode and the carbon felt cathode are characterized in 3-
electrode set-up and both electrodes show excellent rate capability and cycling stability. These 
promising properties are also retained when the electrodes are combined into complete cells. In 
addition, the battery seems to perform well also at very low temperatures (-40 centigrade and -70 
centigrade) as well as under mechanical stress. The presented results are thus spectacular and will 
likely have a profound impact on future research in the field of organic-, water-based- and flexible 
batteries. However there are several points that need to be clarified before publication and this 
reviewer therefor recommend publication after major revision. Specific points are listed below with 
major issues presented first and minor issues later: 

Main concerns: 
1) The redox reaction occurring at the PTO anode is depicted in scheme 1. From the figure it is 
clear that the authors claim the complete reaction to be a 4e4H process. There is, however, 
insufficient proof of this over-all reaction in the manuscript. Some data rather indicate that the 
complete reaction is a 2e2H reaction: Based on the assumed 4e process and the molecular mass of 
PTO the theoretical capacity should be around 400 mAh/g. In both 3-electrode set-up and in 2 
electrode set-up the evaluated capacity is slightly more than half the theoretical value. In addition, 
the IR spectroscopy performed at different state of charge shows that even in the fully reduced 
form there is absorption corresponding to carbonyl vibration remaining. The claimed redox 
reactions occurring at the PTO anode thus need further support. 
2) In figure 3C the authors use a Ragone plot to benchmark the presented battery against existing 
technologies. Although this is often used in the literature it is instrumental for such comparison 
that the figures of merit are compared using the same normalization procedure. It is likely that the 
data from existing technologies used for comparison are on device level and hence that the 
normalization is against the devise mass. The capacity and power of the material presented in this 
report, on the other hand, is normalized against the weight of active material alone. (Using the 
energy density provided, 170.5 Wh/kg (line 160), the specific capacity can be estimated to 213 
mAh/g (C/m = Esp*1000/U) using an average output voltage (U) of 0.8 V. This is close to the 
capacity of the PTO electrode and hence it is clear that the authors use only active material 
weights for normalization). In doing so the mass of carbon additives and binders that make up for 
40% of the weight of the PTO membrane is ignored and so is 1) the mass of the electrolyte 
(containing the Mn2+ reservoir), 2) the titanium mesh used as current collector, 3) the MnO2 
coated carbon mesh, 4) the separator and 5) packaging. This makes the comparison with existing 
technologies misleading. 
3) The IR-spectra presented in Supplementary figure 8 looks highly distorted with unsymmetrical 
peaks suggesting that these spectra have been manipulated. Such manipulation must be described 
somewhere in the manuscript or provided as supporting information. In addition the interpretation 
of the IR-data could be improved in support of the claimed redox processes occurring. It would, for 
instance, be helpful to see how key transitions/absorptions evolve as function of potential. 
4) The most spectacular feature of the presented battery is the low temperature behavior. As 
briefly discussed in the presented manuscript, proton diffusion in frozen water is known to be 
relatively fast and, hence, as indicated in line 187-194 a proton rocking chair battery may function 
also in frozen media. However, the presented battery is not a proton rocking chair battery (see 
below). Instead the redox reaction at the cathode side require diffusion of both water molecules 
(and hydronium ions) and Mn2+ ions. The feasibility of such mass transport should be investigated 
in addition to the low-temperature battery tests or precise references be added. I.e. what is the 
diffusion coefficient of Mn2+ in the frozen electrolyte used? 

Minor comments: 
Rocking-chair type batteries are characterized by the condition that the electrolyte does not have 
to accommodate excess ions necessary for the redox reactions occurring at the electrodes. In the 
present study the electrolyte serve as reservoir for electroactive Mn2+ ions and this reviewer 
therefor find it misleading to place the present battery under the rocking-chair category of 
batteries. 

Line 83-84: A linear increase of peak current with scan rate does not necessarily indicate fast 
transport. Also, in the corresponding supplementary figure 5 the use of “capacitive behavior” is 
misleading as the redox processes investigated are certainly not capacitive in nature nor is the 
current independent of potential as indicative of a capacitive response. Also the discussion in 
relation to figure ”Supplementary figure 9” is confusing. First of all, a slope of 0.61 is clearly far 
from what is expected for a Nernstian surface confined reaction (as is one proper way to describe 
a reaction with a linear ip = f(scan rate)). Second, a surface confined reaction cannot be expected 
as the reaction relies on diffusion of Mn+2 towards the electrode surface. Third, as the geometry is 
much more complicated than the authors imply a diffusion limited reaction cannot be expected to 
follow a simple square-root-dependence. 

Line 99, 100, and 110: The use of charge/discharge to describe oxidation/reduction of PTO is 
confusing as the opposite reaction corresponds to charge and discharge in the battery cell. 

Line 125: The statement that the coulombic efficiency is close to 100 % is clearly incorrect (as 
seen in figure 2b) 

Line 192-193: It is unclear what the authors mean by “Herein, the PTO//MnO2@GF proton cell 
shows a distinct characteristic in absence of desolvation process of H3O+, thus might be able to 
work well at low temperature”. 

Line 260: In addition to the areal mass loadings, please provide the electrode area used in the 
experiments. 

A general improvement of the language is recommended. 


Reviewer #3 (Remarks to the Author): 

In present work, Guo et al reported a rocking-chair-type proton battery based on an organic PTO 
anode and MnO2@graphite felt anode, which works upon quinone/hydroquinone redox reaction on 
anode and MnO2/Mn2+ conversion reaction on cathode. In consideration of electrode materials 
exploration for proton batteries, the intercalation-type inorganic electrode materials exhibit limited 
performance due to the large ion radii of hydronium ions, the existing form of proton in aqueous 
solution. In this work, it is a very ingenious breakthrough to utilize organic/inorganic battery 
system based on enolization and conversion reactions rather than intercalation mechanism, thus 
avoiding the inherent slow kinetics and inspiring the wide choices of hydronium ion host. Such 
proton batteries deliver high energy density and high power density comparable to 
supercapacitors, as well as good cycle stability. The fabricated flexible configuration also shows 
good performance with outstanding energy density compared to the state-of-art reports. While the 
most interesting and exciting result is that the developed proton battery could work normally 
under such low-temperature as minus 70 degrees, which performs as a powerful member for low-
temperature batteries, and might bring more inspiring research on development of low-
temperature power sources. Overall, this work is interesting and important, which can be 
recommended for the publication in Nature Communications after minor revision. The minor 
concerns are summarized as following: 
1. The theoretical specific capacity of PTO material should be calculated as about 400 mAh/g based 
on four carbonyl groups per PTO molecular. While the delivered specific capacity of PTO in this 
work is only 208 mAh/g. Why does PTO exhibit only a half of the theoretical capacity here? 
2. Both traditional and flexible configurations of the PTO-based proton batteries have been 
fabricated in this work. The mass loading of active material on cathode (i.e., PTO) for these two 
configurations should be given for better understanding. 
3. Although such battery exhibits some inherent advantages, its disadvantages, i.e., low electronic 
conductivity of organic electrode, i.e., PTO, should also be mentioned in this paper. 
4. In Figure 2, the authors show a lot of ex-situ characterization results of MnO2@GF cathode, e.g. 
ex-situ XPS spectra and XRD patterns. However, the test conditions of corresponding 
electrochemical measurements (such as applied currents) are not given. Please clarify this point in 
main-text or experimental section. 
5. When discussing the energy and power densities, the mass which calculation based on should 
be clearly defined, e.g., on both electrodes or not? This should be made clear when comparing to 
other reports, like in supplementary Table 1. 
6. The optical pictures of electrolyte under -40 and -70 degrees were given in Figure 4a. The real 
time temperature when the pictures were taken should be given clearly for precisely reflecting the 
actual state of electrolyte.