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

RR3
Reviewers' comments:
Reviewer #2 (Remarks to the Author):
  I appreciate the clarification and thorough response on my remaining concern. In my opinion there
 are, however, critical questions regarding the low temperature behavior that should be addressed
 prior to publication, but I will (of course) leave the final decision to the editor. Two questions still
 need to be clarified 1) how is it possible, at all, to operate the presented battery at freezing
 conditions and 2) why are the effect of lowering the temperature below the electrolyte freezing
 point so small (close to negligible)?
 The first question relate to that Mn2+ diffusion should be frozen out and any electron transfer
 event are expected to become energetically unfavorable as the electrolyte can no longer rearrange
 to counter balance the new electronic configuration. Hence, the electron transfer reaction must
 occur without solvent rearrangement. In water solution the outer reorganization energy (the
 energy required to transfer an electron without rearranging the nucleis) is usually around 1 eV.
 The effect of freezing out electron transfer reactions is well documented in the electron transfer
 literature. It is possible that the high salt/acid concentration lessen this effect and that electron
 transfers in this particular electrolyte is possible, but perhaps not very likely based on current
 knowledge. The results provided in ref 46 indeed suggest that Mn2+ diffusion is possible in frozen
 media and hence provide support that idea. So, there might be ways to account for a working
 battery with frozen electrolyte although consensus in the scientific literature (known to me) point
 in another direction.
 The second question is more critical and I pointed this out in my last review report. Why are the
 effects of freezing the electrolyte so small? You state that the conductivity is reduced by 2 orders
 of magnitude (more on this below). Why does the cell resistance decrease only by a factor of 1.2
 and 2 at -40 centigrades and -70 centigrades, respectively? From the decrease in electrolyte
 conductivity alone a much larger increase in resistance is expected. And, in addition to electrolyte
 resistance the charge transfer resistance should give additional contributions to the cell resistance
 due (partially) to the effects above.
 As I stated in my first response it is likely that proton diffusion dominate the ion transport in
 frozen media. When comparing the resistance in 1M MnSO4 (from literature) with 2M MnSO4+2M
 H2SO4 (present work) the authors state that the difference in conductivity seen in frozen solution
 between these electrolytes is due to a decreased freezing point in the acidic solution. An
 alternative explanation, and a more likely explanation in my opinion, is that the increased
 conductivity in the acidic electrolyte is due to the diffusion of protons. The assumption that the
 diffusion coefficient for H+, Mn2+ and SO42- are equal is a gross over-simplification known to be
 wrong, in particular in frozen solution. Hence, even though the electrolyte resistance is reduced by
 two orders of magnitude it is likely that the Mn2+ resistivity is much higher. Since Mn2+ diffusion
 is required, based on the cell reactions, an even larger effect, than the two orders of magnitude
 drop in electrolyte resistance would suggest, is expected.