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

Abstract

The present research explores the potential of manganese oxide waste ore in energy storage applications, focusing on supercapacitors. The investigation assesses the electrochemical capabilities of natural manganese oxides obtained from the Drama region, which has been the main mining center of Greece for manganese ore, especially that of battery-grade quality. Samples were collected from abandoned mining sites in the Kato Nevrokopi area, Drama. The structure and composition of the manganese minerals were determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Electrochemical tests involved the preparation of electrodes using natural nsutite and heat-treated nsutite (hausmannite). Then, the designed electrodes were subjected to cyclic voltammetry tests and charge–discharge measurements. The hausmannite electrode exhibited a higher specific capacitance of 667 F/g at a current density of 0.2 A/g, and the electrode material retained 98.3% of its initial capacitance after 1000 cycles. This study provides new perspectives on simple and efficient methods for transforming natural nsutite material from mining waste to hausmannite with greater structural homogeneity and better electrochemical behavior.

1. Introduction

Manganese oxides (Mn_xO_y) are renowned for their diverse range of niche applications in energy production and wastewater treatment [1]. These applications include heavy metal adsorption [2,3], serving as inorganic catalysts for oxygen reactions [4], and playing a crucial role in energy storage applications, notably in rechargeable lithium batteries [5]. More specifically, electrochemical-based storage devices such as batteries, capacitors, and supercapacitors provide a clean, low-cost, sustainable energy conversion and storage for renewable energy sources [6].

Identifying manganese oxide minerals poses a challenge due to their nano-crystalline nature, complex intergrowth of different phases, and structural and oxidation-state variations (Mn²⁺ to Mn⁷⁺) [7]. The atomic frameworks of these minerals are constructed from Mn-O octahedra that share edges and link corners to yield an array of phases with tunnel or layer structures [8,9]. The tunnel-structured Mn oxides are assembled from single, double, or even triple chains of edge-sharing MnO₆ octahedra, and the chains share corners to manufacture frameworks that consist of tunnels with square or rectangular cross-sections. In contrast, the layer-structured Mn oxides comprise stacks of sheets of edge-sharing MnO₆ octahedra [7,8,9].

Manganese oxides have several crystallographic polymorphs. γ-MnO₂ is the synthetic analog of the mineral nsutite, and both of them are among the most critical manganese dioxides due to their application as an electrode material for the battery industry [10,11]. It consists of an intergrowth structure of β-MnO₂ type tunnels (1 × 1) and ramsdellite–MnO₂ type tunnels (2 × 1), first described by De Wolff in 1959 [12]. Later, in 1995, Chabre and Pannetier proposed an additional structural defect, aside from the De Wolff defect, which is called micro-twinning and is accountable for the poor quality of X-ray powder diffraction patterns of most γ-MnO₂ and is correlated to the structure with their use as battery-grade material [13].

The development of manganese oxide materials for storing and converting energy has become increasingly important, prompting extensive research efforts. MnO₂ is particularly attractive in the battery sector due to its high theoretical capacitance, affordability, and low toxicity. Since its inception as a depolarizing agent in zinc alkaline cells, significant progress has been achieved by utilizing both naturally occurring and synthesized variants of MnO₂, including those prepared via electrochemical (EMD), chemical (CMD), and heat treatment (HTMD) methods [14,15].

Current research focuses mainly on the potential uses of synthetic manganese oxides in various battery cell configurations, either as cathode or anode materials. On the contrary, in the present paper, we examine the electrochemical performance of natural manganese oxides from the manganese ore deposit of Drama area, Greece. The greater area of Drama County was, in the past (1950–1994), the main mining center of Greece for manganese oxide ore, especially of battery-grade quality. Due to extensive mining and ore processing, there are voluminous abandoned mine tailings, low-grade ore, and heaps of ore concentrate at the main mines (25th Km and Mavro Xylo) adjacent to Kato Nevrokopi area, which seek environmental management [16]. Manganese ore ranks among the most extensively processed materials globally, holding crucial significance mainly in the manufacturing of steel alloys, chemicals, and batteries [17]. Nevertheless, the mining processing of this ore results in the accumulation of substantial waste in open-air landfills, with a high environmental impact, contaminating the soil and plants but, most importantly, affecting human neural health [18,19]. Hence, this paper also explores the valorization potential of manganese oxide mining wastes. 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 exploitations 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.

2. Materials and Methods

2.1. Sample Selection

The samples collected represent ore from waste piles which are disposed of 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 [23,24]. The mineralogy of the studied samples is verified prior to the experiments by XRD analysis showing that the MLX3 is mostly comprised of 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.

2.2. Mineralogical Characterization

Mineralogical analysis was conducted by X-ray diffraction (Bruker D8 ADVANCE, Bruker Corporation, Billerica, MA, USA). XRD patterns were collected among 2θ angles of 5–75° (at Cu Κα radiation of 40 kV, 25 mA, and λ = 1.5406 Å). Mineral phases were identified using the Joint Committee for Power Diffraction Standards (JCPDS) file and the software of DIFFRAC.EVA (V6.1, Bruker Corporation, Billerica, MA, USA) provided by Bruker. The crystallite size of the powders was calculated from the broadening of the Bragg reflections with the application of the Debye–Scherrer equation (Equation (1)) [25]. The samples were also analyzed with transmission electron microscopy (Jeol 2100 HR, Tokyo, Japan, 200 kV). Elemental analyses were carried out using an Oxford X-Max 100 Silicon Drift Energy Dispersive X-ray spectrometer (EDS) connected to TEM.

In 1918, P. Scherrer described the phenomenon of a beam’s expansion when parallel monochromatic radiation falls on a randomly oriented mass of crystals due to the small size of the crystallite [26,27].

$$D=\frac{\mathrm{K}\times \lambda }{\mathsf{\beta }\times cos\theta }$$

(1)

where D is the size of the crystallite (nm), K is known as the Scherer’s constant (K = 0.89), λ is the X-ray wavelength (1.5406 Å), β is full width at half maximum (FWHM) of the diffraction peak, and θ (^o) is the angle of diffraction.

Moreover, specifically for γ-MnO₂, the percentage of micro-twinning and the de Wolff defect were quantified by using the equations of Chabre and Pannetier (Equation (2)) and P.M. De Wolff (Equation (3)) [12,13].

$$Tw\left(in\%\right)=100-25.20\Delta 2\theta [forreflections002and061,CuK\alpha radiation]$$

(2)

where Tw = 0% equals a defectless ramsdellite structure.

$$P_r=0.602\times \delta \left(\mathrm{D}\mathrm{W}\right)-0.198{\times \delta }^2\left(\mathrm{D}\mathrm{W}\right)+0.026\times {\delta }^3\left(\mathrm{D}\mathrm{W}\right)$$

(3)

where $\delta \left(\mathrm{D}\mathrm{W}\right)=2\theta {100}_{exp}-\delta \left(Tw\right)-21.808\left(indegrees2\theta \right)$

$$\begin{array}{l}\mathrm{When}Tw<55\%,\delta \left(Tw\right)=-0.0054\times {\rm T}\mathrm{w}-8.9\times {10}^{-5}\times {Tw}^2.\\ \mathrm{When}Tw>55\%,\left(Tw\right)=-0.56.\end{array}$$

where 0 ≤ P_r ≤ 1, 0 being the ideal ramsdellite structure with no intergrowth of pyrolusite 1 × 1 tunnels and 1 being the routile-like structure of pyrolusite.

Sinse nsutite (γ-MnO₂), as a natural manganese oxide, might not be a homogenous material, thus affecting the electrochemical properties. An investigation was carried out to find an additional route. Recent studies have directed their attention toward different manganese oxides to identify a fitting contender for electrochemical capacitors. Within this group of oxides, hausmannite (Mn₃O₄) stands out as a potential electrode material for supercapacitors due to its numerous advantages compared to alternative solutions. It is the most stable phase of manganese oxide, exhibiting strong redox capabilities and high theoretical specific capacitance [28].

In 1948, McMurdie and Golovato studied the modifications of Manganese oxides at different temperatures [29]. Following their work, Bish and Post studied the behavior under thermal treatment of the complex, tunneled-structured manganese oxides [30]. Both concluded that all of the different phases of manganese oxides transform to Mn₃O₄ at temperatures over 700–800 °C. Therefore, as part of this investigation, the specimens underwent heat treatment using a chamber furnace (Carbolite CWF 1200) at a temperature of 1000 °C for two hours to assess the thermal changes in our samples and their possible transition into hausmannite which was utilized as an electrode in the electrochemical experiments.

2.3. Electrochemical Characterization

Both nsutite (MXL3) and hausmannite (MXL3HT) powders were used separately for the electrode preparations. These electrodes were prepared by combining hausmannite (or nsutite) and carbon black (Merck) with a binder mixture containing poly(vinylidene) fluoride (-CH₂CF₂-) and 1-methyl-2-pyrrolidone (C₅H₉NO). The solid components, which included mineral powder, carbon black, and poly(vinylidene) fluoride, were mixed in a ratio of 7:2:1, while the ratio of mineral powder to solvent (C₅H₉NO) was 70 mg:1.4 g [31]. The mixture was subjected to 30 min of sonication and was then deposited onto carbon paper (Toray carbon black, TGP-H-60, Thermo Scientific, Waltham, MA, USA) measuring 1 cm × 1 cm. Subsequently, it was dried in a dry oven for approximately 12 h at 80 °C. The electrode area mass loading was 1.2 mg/cm² for hausmannite and 4.3 mg/cm² for nsutite.

A VersaSTAT 3 Potentiostat Galvanostat (AMETEK, Inc., Berwyn, Pennsylvania, USA) and the VersaSTAT software (version 2.63.3.0) were used for the cyclic voltammetry (CV) and charge–discharge (CDC) measurements. The measurements were extracted using a three-electrode single-compartment cell, including an Ag/AgCl electrode (reference electrode), a Pt wire (counter-electrode), and the prepared materials of hausmannite and nsutite (working electrodes) at room temperature under Ar conditions (Figure 1).

The electrochemical properties of these working electrodes were examined in 0.1 M Potassium Hydroxide, KOH, supporting electrolyte solution. (Potassium Hydroxide Pellets, Merck (Sigma-Aldrich, Saint Louis, MO, USA). All the potentials reported for the cyclic voltammetry measurements are expressed vs. NHE after correction.

The electrodes’ electrochemical properties, such as specific capacitance (F/g), energy density (Wh/Kg), and power density (W/Kg), were calculated from cyclic voltammetry and charge–discharge measurements using the following equations [32,33]

$$Cp=\frac{Q}{\Delta V}=\frac{\left(I\times t\right)}{\Delta V}$$

(4)

where Cp (F/g), Q (C), I (A/g), t (s), and ΔV (V) are the specific capacitance, charge, specific current, discharge time, and potential window, respectively.

$$E=\frac{\left(\mathrm{C}p\times {\Delta V}^2\right)}{2}$$

(5)

where E (Wh/Kg), Cp (F/g), and ΔV (V) are the energy density, specific capacitance, and potential window, respectively.

$$P=\frac{\mathrm{E}}{\Delta t}$$

(6)

where P (W/Kg), E (Wh/Kg), and Δt (hr) are the power density, energy density, and discharge time, respectively.

Then, the calculations based on Equations (5) and (6) were graphed on a Ragone plot. This plot was used to compare the energy density and power density across different energy-storage systems to find the best-fitting application of our electrodes [34,35].

3. Results and Discussion

3.1. X-ray Diffraction Analysis

A structural analysis of the samples was carried out using pXRD and is given in Figure 1 and

Table 1. XRD data for the nsutite, todorokite, and calcite together with the miller indices (hkl).

Nsutite [13]		Todorokite [37]		Calcite [38]
2θ	hkl	2θ	hkl	2θ	hkl
22.14	110	9.23	100	23.1	012
34.75	130	12.2	110	36	110
36.86	021	18.2	200	39.4	113
38.8	040	18.55	120	43.16	202
40.36	200	26.21	300	47.13	024
42.26	121	37.52	201	47.51	018
43.3	140	41.23	031	48.51	116
55.95	221	49.86	440
56.66	240	60.28	521

. The XRD of MXL1 (Figure 2a) shows a predominant peak at a 2θ value of 29.5° which, together with the peaks at 23.1°, 36°, 39.4°, 43.16°, 47.13°, 47.51°, 48.51°, corresponds to the mineral phase of calcite (CaCO₃). Apart from calcite, strong peaks appear at 2θ values of 9.23°, 12.2°, 18.2°, 37.52°, and 60.28°, which are attributed to the crystal planes of todorokite. Lastly, the 2θ values of 22.14°, 34.75°, 36.86°, 38.8°, 40.36°, 42.26°, 43.3°, 55.95°, 56.66°, and 62.59° were attributed to the mineral phase of nsutite [6,36,37,38].

Powders from sample MXL3 gave similar peaks (Figure 2b), which resemble the pattern of the mineral phase of nsutite but with some of their peaks shifted [39]. The alterations of the 2θ values are due to the aforementioned structural defects (micro-twinning, De Wolff defect) of γ-MnO₂. Despite having many similarities, recent studies indicate a high degree of structural heterogeneity among γ-MnO₂; therefore, for the specimens from Kato Nevrokopi (Drama, Greece), an attempt was made to calculate these structural defects.

According to the research of Chabre and Pannetier (1995) [13], the intensity profile of γ-MnO₂ diffraction patterns is governed by two kinds of defects that modify both the position and shape of most diffraction lines:

(a)

The more or less random stacking of the ramsdellite and pyrolusite structures;

(b)

The micro-twinning of the ramsdellite lattice in 021/061 planes.

For example, if the 2θ peak of 21.8° (1 1 0) is shifted to lower 2θ values, that is a consequence of the micro-twinning of the structure. On the other hand, if the same peak is shifted to higher values, then that is due to De Wolff disorder.

As a result, the powder diffraction patterns of manganese dioxides should be analyzed carefully. The line shifts due to micro-twinning and De Wolff defect are given in

Table 2. Shift of 2theta positions due to micro-twinning and De Wolff disorder on sample MXL3.

Miller Indices			Ideal Ramsdellite	MXL3
h	k	l	2θ (CuKa)	2θ (CuKa) Measured	Δ2θ Ideal	Micro-Twinning	De Wolff
1	1	0	21.8	22.14	0.34	−	+
1	3	0	35.1	34.75	−0.35	+	-
0	2	1	36.8	36.86	0.06	+
1	1	1	38.4	-		−	−
0	4	0	38.8	38.4	−0.4	−
2	0	0	39.7	40.36	0.66
1	2	1	42	42.26	0.26	+
1	4	0	43.8	43.3	−0.5	−
1	3	1	47.7	47.933	0.233	-	+
2	2	1	55.3	55.95	0.65	+
2	4	0	56.8	56.66	−0.14	−
2	3	1	60	-	-	−	+
1	5	1	63.1	62.59	−0.51	+	−
0	0	2	65	66.035	1.035	+
0	6	1	69	68.09	−0.91	−
3	3	0	69.2	68.73	−0.47	+	−
1	1	2	69.5	69.76	0.26	−	+
3	0	1	70.4	-		−
3	1	1	71.2	-		−	−

, alongside the line positions of our samples compared to those of a defectless ramsdellite.

Micro-twinning (Tw):

Based on the powder diffraction patterns analysis, it has been determined that both defects are present in the samples. The defects were quantified by applying the abovementioned Equations (2) and (3).

$$Tw\left(in\%\right)=100-25.20\Delta 2\theta [forreflections002and061,CuK\alpha radiation]$$

TwMLX3a = 51.36%

De Wolff disorder:

$$P_r=0.602\times \delta \left(\mathrm{D}\mathrm{W}\right)-0.198{\times \delta }^2\left(\mathrm{D}\mathrm{W}\right)+0.026\times {\delta }^3\left(\mathrm{D}\mathrm{W}\right)$$

Pr = 0.42

After calculating these parameters, we can project them in a structural diagram (Figure 3) [13] where the following apply:

• A (0,0) is the ideal-defectless ramsdellite;
• B (1,0) represents the pyrolusite;
• C (0,100) describes the fully twinned structure of ramsdellite;
• D (100,0) represents the fully twinned pyrolusite.

By projecting quantified disorders in this plot from several γ-MnO₂ samples, which included chemically and electrochemically made manganese dioxides (CMD and EMD, respectively), the researchers observed that EMDs, which are widely known for their battery applications, have analog structures [13]. The percentage of micro-twinning is mainly above 60%; as for the Pr value, it seems that EMD samples tend to group along a vertical line at $0.40\le Pr\le 0.45$ values.

Within the diagram (Figure 3), the grey circles depict the standard structural attributes corresponding to each γ-manganese dioxide category. The line-shaded area corresponds to the field of structural imperfections observed in Electrochemical Manganese Dioxides. These measurements were conducted on different EMDs by Chabre and Pannetier [13], defining the extent of structural defects. The black square represents the sample of MXL3, which has structural characteristics similar to the EMDs. Comparing these data to ours, it can be presumed that their structure is very close to those of EMDs and that they are acceptable candidates for dry-cell batteries and supercapacitor applications.

Another characteristic that can be determined from the powder X-ray diffraction pattern is the size of the crystallite of the manganese oxides. By calculating the broadening of the Bragg reflection for the characteristic peak (highest intensity peak) of the mineral phase with the application of the Debye–Scherrer equation (Equation (1)), it is possible to determine the crystallite size D (

Table 3. The calculated crystallite sizes (Equation (1)) of the different mineral phases that are present in the ore samples.

Sample Name	Mineral Phase	D-Crystallite Size (nm)
MXL1	Todorokite	25
MXL3	Nsutite (γ-MnO2)	11

) [40,41].

These calculations show that the samples of nsutite have very small crystallite sizes that can help elevate their electrochemical properties.

X-ray diffraction analysis was carried out after the heat treatment of both samples (Figure 4). The results indicated that the sample containing only nsutite (MXL3) successfully transformed into a well-crystallized hausmannite (Figure 4a) [39]. In contrast, the MXL1 sample (Figure 4b) transitioned not only into hausmannite but also into mineral phases like braunite ([Mn²⁺Mn³⁺]₆[SiO₄]O₈), tephroite ([Mn²⁺]₂SiO₄), and Manganosite (MnO). This occurrence is likely due to the presence of manganese oxides (todorokite and nsutite) in conjunction with calcite (CaCO₃) and quartz (SiO₂) in the specific sample, which may act as catalysts in the development of manganese silicates, meaning the samples of manganese oxides that contain matrix calcite and quartz need to be separated first and then undergo heat treatment in order to transform into hausmannite successfully.

3.2. Transmission Electron Microscope Analysis

The samples of MLX3 have been analyzed by an Transmission Electron Microscope to verify and characterize the structure of nsutite (Figure 5). TEM results and the selected area electron diffraction patterns (

Table 4. Selected area electron diffraction patterns solution and its correlation with corresponding d spacing calculated from powder X-ray diffraction (pXRD) signify nsutite’s structural characteristics.

	d Spacing Calculated from SAED (nm)	d Spacing Calculated from pXRD (nm)	Hkl Lattices	2theta Peak (pXRD)
1	0.2396	0.2336	0 2 1	36.89
2	0.2424	0.2434	0 4 0	38.4
3	0.1385	0.1375	0 0 2	66.035
4	0.1404	0.1413	0 6 1	68.09

) confirm that the structure of the mineral is that of nsutite; even so, the X-ray energy dispersive spectra (EDS) indicate that, although the structure of nsutite is present, its chemical composition resembles that of todorokite (Table S1) [42]. The process of hydrothermal fluid alteration can explain this phenomenon. It has been reported by Thackeray (1997) that when a manganese oxide with a structure of a-MnO₂ (hollandite, todorokite, etc.) comes in contact with a low-pH fluid for prolonged periods, the primarily formed a-MnO₂ converts to a ramsdellite–MnO₂ structure [5]. In this case, todorokite was the first mineral that occurred with a chemical formula of [Ca,K,Na,Mg,Ba,Mn)(Mn,Mg,Al)₆O₁₂*3H₂O], and after the process of hydrothermal alteration, nsutite was formed. This is also confirmed by the impurities of sulfur (S), zinc (Zn), and lead (Pb) in the mineral phase of nsutite (Table S1). Additionally, different phases of manganese oxide formed on a nanoscale, with a high Pb concentration, which probably represents coronadite (Pb[(Mn⁴⁺, Mn²⁺)₈]O₁₆).

3.3. Electrochemical Experiments

Cyclic voltammetry and charge–discharge tests were conducted to characterize the electrochemical performance of nsutite and hausmannite composites. Figure 6 shows the cyclic voltammograms of MXL3 (nsutite) and MXL3HT (hausmannite) electrodes with a scan rate of 50 mV/s in a potential window of (−0.8, 1.2) and (−0.8, 1.2), respectively, using as reference electrode Ag/AgCl and Pt as the counter electrode. The two electrodes show a noticeable deviation from each other due to the different nature of the material. Apropos the cyclic voltammetry, experiments were carried out to investigate the redox processes at the electrodes. In MXL3 and MXL3HT voltammograms, the anodic peaks of +0.83 V (MXL3) and +0.85 (MLX3HT) and the cathodic peaks of +0.48 V (MXL3) and +0.35 V (MXL3HT) correspond to the oxygen evolution (OER) and oxygen reduction (ORR) reactions [40].

The 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 [44,45,46].

Charge–discharge (CDC) experiments can provide helpful information about the electrochemical characteristics of the electrodes and their capabilities in energy-storage applications. For the CDC tests (Figure 7), three different current densities (A/g) were used for each electrode, which was calculated by dividing the constant current of the experiments (0.2 mA, 2 mA, 200 mA) with the active mass of the electrodes (4.3 mg for nsutite and 1.2 mg for hausmannite). Current densities were more elevated in the MXL3HT electrode, leading to its superior performance compared to the MXL3 electrode. This difference can be traced back to the non-uniform nature of the MXL3 sample in contrast to the homogeneity achieved through the heat treatment in the case of hausmannite.

Subsequently, the specific capacitance, energy density, and power density of both electrodes were determined using Equations (4)–(6). The results are presented in

Table 5. Electrochemical calculations using the abovementioned equations for the MXL3 (nsutite) and MXL3HT (hausmannite) electrodes.

Electrodes	Current Density (A/g)	Specific Capacitance (F/g)	Energy Density (Wh/Kg)	Power Density (W/Kg)
MXL3	0.043	92.55	0.04	1.27
	0.43	434.8	2.79	46.77
	43	17.39	60.38	107,684
MXL3HT	0.167	191	0.45	10.82
	1.67	667.3	5.93	213.78
	167	38.65	234.5	551,765

.

The energy and power densities were then graphed on a Ragone plot (Figure 8) to identify the most suitable application for these electrodes [47].

The Ragone plot shows that the hausmannite-based electrode exhibits a higher potential to function effectively as a supercapacitor than the nsutite counterpart. The nsutite electrode also holds supercapacitor potential, albeit under certain limited conditions. This probably arises from the heterogeneity of the natural manganese oxides, as observed with the TEM analysis. Moreover, the hausmannite electrode (MXL3HT) shares attributes comparable to a combustion engine, implying its ability to deliver substantial power output within a brief timeframe. Two additional hausmannite electrodes were created and subjected to identical electrochemical experiments, which confirmed these findings.

The cyclic stability of the MXL3HT electrode was analyzed by performing a 1000-cycle charge–discharge test at a current density of 167 A/g (Figure S1) [48]. The capacitance retention shows no noteworthy alteration, indicating the necessity for additional examination with even longer cycles to observe the electrode’s maximum cycle count.

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 correlates with its structure and the presence of Mn⁴⁺, while hausmannite’s spinel-like structure and the presence of Mn²⁺ and Mn³⁺ make it an inferior choice [49]. However, in this study, nsutite is not a homogenous material (Figure 5), while hausmannite, as shown in Figure 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].

4. Conclusions

This research highlights the structural and electrochemical characteristics of certain manganese oxides found in the waste piles of abandoned mines located at Mavro Xylo in the Drama region, Greece. Although the chemical composition resembles the manganese oxide phase of todorokite, nsutite emerges as the sample’s dominant phase due to hydrothermal alterations in the area of interest. Even though nsutite (γ-MnO₂) is one of the most promising candidates for energy applications, the samples’ heterogeneity necessitates exploring an alternative approach in this particular case. Therefore, hausmannite was produced by subjecting nsutite and todorokite samples to heat treatment in a dry oven. Two separate electrodes were created, utilizing nsutite and hausmannite as the primary components. The results of electrochemical experiments on these electrodes suggest that further investigation into the structural aspects of hausmannite is a promising avenue. Furthermore, they suggest that the mining wastes located in Greece and around the world can hopefully be utilized to manufacture energy storage electrodes, which can bring economic and environmental benefits in view of their valorization and could support the swift towards green energy.