Electrochemical Behavior of Natural Manganese Oxides: Transforming Mining Waste into Energy Storage Materials

Round 1

Reviewer 1 Report

Overall the authors have presented a good manuscript on the performance of hausmannite- and nsutite-based electrodes as supercapacitors. I recommend the publication of this manuscript after minor revision as per the following queries:

1. Could the authors comment on how consistent is the composition of the mining waste collected? Was there any separation process or characterization on the total concentration of nsutile-todorokite in these waste samples?

2. Please discuss and provide literature references on the electrochemical performance differences between the hausmannite-based electrode and its nsutite counterpart? How readily available is the hausmannite compared to nsutite? 

3. Supercapacitors have the ability to withstand thousands of charge-discharge cycles before degradation. The authors only performed 40 cycles. Why is this? Can this be repeated with higher number of charge-discharge cycles?

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below:

1. Could the authors comment on how consistent is the composition of the mining waste collected? Was there any separation process or characterization on the total concentration of nsutile-todorokite in these waste samples?

We changed 2.1 (sample selection) to explain better the composition of the waste ore.

“The samples collected represent ore from waste piles which are disposed adjacent to the closed mine sites of Mavro Xylo and “25th Km”, Kato Nevrokopi, Drama, Greece. The ore paragenesis in these sites mainly consists of pyrolusite, nsutite (MnO₂) and todorokite [Ca,K,Na,Mg,Ba,Mn)(Mn,Mg,Al)₆O₁₂*3H₂O] at various abundances [20, 21]. The mineralogy of the studied samples is verified prior to the experiments by XRD analysis (Fig. 2), showing that the MLX3 is mostly nsutite and sample MLX1 is a mixture of nsutite and todorokite with minor matrix calcite. Therefore the selected samples are representative of the typical waste ore in the area of interest.”

2. Please discuss and provide literature references on the electrochemical performance differences between the hausmannite-based electrode and its nsutite counterpart? How readily available is the hausmannite compared to nsutite?

More discussion was added regarding the results of the electrochemical experiments (lines 355-364).

"Many studies have shown that pure and synthetic nsutite (or γ-MnO₂) is the best candidate among the manganese oxides for energy applications [10,11]. Nsutite's high electrochemical performance correlate with its structure and the presence of Mn⁴⁺, while hausmannite’s spinel-like structure and presence of Mn²⁺ and Mn³⁺ make it an inferior choice [49]. However, in this study, nsutite is not a homogenous material (Fig.5), while hausmannite, as shown in Fig. 4, is well-crystallized due to heat treatment. Hence, hausmannite emerges as the optimal choice for repurposing waste ores in energy storage applications, not only due to its superior performance but also owing to its potential for greater abundance compared to nsutite. This is because all manganese oxides can convert into hausmannite through heat treatment [29, 30]."

3. Supercapacitors have the ability to withstand thousands of charge-discharge cycles before degradation. The authors only performed 40 cycles. Why is this? Can this be repeated with higher number of charge-discharge cycles?

We conducted new experiments of 1000 cycles of charge-discharge and the results are added in the text and in the Supplementary document (Figure S1).

Reviewer 2 Report

Please find comments in the attached file.

Comments for author File: Comments.pdf

No comments

Author Response

The manuscript contains vague words and expressions (such as “intricate
intergrowth”). Please consider proofreading the document to ensure concise,
straightforward word structure avoiding personal nouns, and vague expressions.

Changed to complex intergrowth.

2. The “Introduction” is poor in providing information related to the analyses performed
after and the followings suggestions are provided:
a. Manganese oxide structure (Ln36 to Ln44) is rather confusing and does not
relate to either the electrochemical performance (main objective of the
study) or the results of the tests performed (so to be used as reference for
validation).

As shown in the manuscript and in the reference [13], structure plays a significant role in the electrochemical performance of nsutite which is one of the two manganese oxides present in this paper. 

b. Given the objective of the study it is suggested to provide more literary
information in terms of the current treatments of the specific mining wastes
and the characterisation methods reported from previous studies

more information was added: 

"Manganese ore tailings have been studied extensively through the years revealing various applications such as the recovery of soluble manganese and their use in ceramic materials [20, 21]. However, one of the most optimal utilization is their use as cathode material for Zinc Ion Batteries [22]. This study endeavors to discover an even more cost-effective and simplified approach to harnessing these tailings, as well as aiding the transition to green energy"

3. In the “Materials and Methods” section:
a. The units of parameters can be included for equation 1.

units added 

4. The description of Figure 3 may be included in the text instead of the Figure title.

description included in the text instead of the figure 

5. A “Discussion” section may be added discussing further the alternative routes and the potential valorization of manganese-oxide mining wastes.

more "duscussion" was added

"Many studies have shown that pure and synthetic nsutite (or γ-MnO₂) is the best candidate among the manganese oxides for energy applications [10,11]. Nsutite's high electrochemical performance correlate with its structure and the presence of Mn⁴⁺, while hausmannite’s spinel-like structure and presence of Mn²⁺ and Mn³⁺ make it an inferior choice [49]. However, in this study, nsutite is not a homogenous material (Fig.5), while hausmannite, as shown in Fig. 4, is well-crystallized due to heat treatment. Hence, hausmannite emerges as the optimal choice for repurposing waste ores in energy storage applications, not only due to its superior performance but also owing to its potential for greater abundance compared to nsutite. This is because all manganese oxides can convert into hausmannite through heat treatment [29, 30]."

Reviewer 3 Report

In this manuscript, authors explored the potential of Manganese-oxide spent ore as electrode material for supercapacitors. The authors should accomplish these minor revisions before the final manuscript can be accepted for publication.

1.     The authors selected only samples of manganese oxide spent ore, then characterized and investigated the electrochemical performance. How did you selected these two samples, it is suggested to explain optimization.

2.      In Fig. 6 (a) & (b), the redox processes are mentioned. What are these redox reactions? They should be include in the text.

3.     What is the difference between the specific capacitance and capacity, also confirm their units.

4.     For the conventional three electrode test in alkaline electrolyte, the reference electrode should be Hg/HgO, rather than the Ag/AgCl. The authors should pay attention to this issue.

5.     Figures should be clearer, also in Figs. (2) & (4), the respective indices should be added. In SAED pattern only 1,2,3 and 4 are written. Write plane properly and correlate with XRD.

6.     The writing of the manuscript should be improved. For examples equation (4) & (5). Write powers and superscript/subscript in proper format.

Comments for author File: Comments.pdf

Author Response

1. The authors selected only samples of manganese oxide spent ore, then characterized and investigated the electrochemical performance. How did you selected these two samples, it is suggested to explain optimization.

We rewrite 2.1 (sample selection) to explain better the composition of the waste ore.

“The samples collected represent ore from waste piles which are disposed adjacent to the closed mine sites of Mavro Xylo and “25th Km”, Kato Nevrokopi, Drama, Greece. The ore paragenesis in these sites mainly consists of pyrolusite, nsutite (MnO2) and todorokite [Ca,K,Na,Mg,Ba,Mn)(Mn,Mg,Al)6O12*3H2O] at various abundances [20, 21]. The mineralogy of the studied samples is verified prior to the experiments by XRD analysis (Fig. 2), showing that the MLX3 is mostly nsutite and sample MLX1 is a mixture of nsutite and todorokite with minor matrix calcite. Therefore the selected samples are representative of the typical waste ore in the area of interest.”

2. In Fig. 6 (a) & (b), the redox processes are mentioned. What are these redox reactions? They should be include in the text.

The redox reactions added in the text (lines 303-315)

"

Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction (ORR) are two important processes that occur in electrochemical systems, particularly in fuel cells and metal-air batteries. The oxygen evolution reaction is a key process in electrochemical water splitting or electrolysis, where water is split into oxygen and hydrogen gas and usually occurs at the anode electrode of an electrochemical cell. The oxygen reduction reaction is an important process in energy conversion devices such as fuel cells and metal-air batteries, which involves the reduction of oxygen molecules from the air, usually to water, by accepting electrons and protons. The ORR typically occurs at the cathode electrode of an electrochemical cell [43].

Both voltammograms' remaining anodic and cathodic peaks correlate with the manganese redox reactions. The cathodic peaks correspond to the reduction of the manganese oxidation state while the anodic peaks correspond to the oxidation of the manganese oxidation state [44-46]."

3. What is the difference between the specific capacitance and capacity, also confirm their units.

Specific capacitance is a measure of the ability of a material to store electrical energy (units : lines 162-163)

Capacity is the general term used to refer to a quantity, or amount of something that a material can hold.

4. For the conventional three electrode test in alkaline electrolyte, the reference electrode should be Hg/HgO, rather than the Ag/AgCl. The authors should pay attention to this issue.

It is true that the option of the reference electrode (R.E) depends on the system and the solution which are under examination. The Hg/HgO, electrοde as reference one, is more preferable for alkaline solution indeed. (https://doi.org/10.1016/j.checat.2021.07.001 ). However, Ag/AgCl or SCE reference electrodes provide a stable half-cell potentials and remain unalterable according to the external conditions. For the above reasons and regarding the literature which is referred in the main text of this work [42,44,45], Ag/AgCl had been chosen as an alternative reference electrode.

5. Figures should be clearer, also in Figs. (2) & (4), the respective indices should be added. In SAED pattern only 1,2,3 and 4 are written. Write plane properly and correlate with XRD.

Respective indices added (table 1).  In figure 5d) indices were added. In table 4 xrd and SAED data are correlated.

6. The writing of the manuscript should be improved. For examples equation (4) & (5). Write powers and superscript/subscript in proper format.

Equations (4), (5) were corrected accordingly.

Round 2

Reviewer 2 Report

I thank the authors for considering the comments and revising the manuscript accordingly.

I recommend the work to be accepted in the present form.

Author Response

Thank you for your comments and for accepting the paper!