MnO Recovered from Alkaline Batteries Functionalized with Ruthenium and Carbon Nanofibers for Supercapacitor Applications ^†

Abstract

MnO is an attractive material due to its high specific capacitance and thermal and chemical activity. It can be recycled from alkaline batteries with a good yield and can be used for supercapacitor applications after enhancing its poor conductivity. In this study, Ru-MnO-Carbon nanofibers(Ru-MnO-CNFs) were prepared by the impregnation of Ru (1 wt%) into MnO recovered from used alkaline batteries, followed by their incorporation into polyacrylnitrile (PAN) nanofibers by electrospinning and carbonization. The prepared materials, Ru-MnO and Ru-MnO-CNFs, were characterized by scanning electron microscopy and Fourier infrared spectroscopy. The electrochemical characterization was performed, comparing the characteristics of Ru-MnO and Ru-MnO-CNFs samples. It was found that the capacitance of MnO recovered from used alkaline batteries could be enhanced by combining it with Ru and CNFs. The hybrid Ru-MnO/CNFs composite could be used as stable electrode material for high performance supercapacitors.

1. Introduction

Integrating supercapacitors into aviation and space applications represents a key advancement in sustainable energy storage. With high power density, supercapacitors deliver immediate energy for critical systems like satellite radar and thrust control, while their long cycle life suits space missions, in which maintenance is limited [1,2]. Their lightweight design also reduces fuel costs, enhancing efficiency. Carbon-based supercapacitors offer thermal stability, operating well under extreme conditions, though lower energy density compared to lithium-ion batteries limits their use in long-term missions. Research to improved energy capacity remains essential for the future role of supercapacitors in aerospace systems [3,4]. In recent years, hybrid supercapacitors, a combination of the conventional electric double-layer capacitors (EDLCs) and pseudocapacitors, have attracted much attention [5]. In general, such supercapacitors can store charges via both Faradaic and non-Faradaic processes, usually showing higher energy densities than EDLCs and higher power densities than pseudo capacitors [6]. So far, several pseudocapacitive materials, such as metal oxides, nitrides, hydrates, sulfides, and bimetallic oxides, have been functionalized with other metals or incorporated into carbon nanomaterials for hybrid supercapacitor electrodes [7,8,9,10]. Considering in particular MnO₂-based electrodes, their combination with conductive carbon nanomaterials might greatly increase their electronic conductance (rate capability) and might largely increase their utilization efficiency (specific capacitance) and cycling stability [11,12,13,14].

Currently, waste zinc–manganese (Zn-Mn) batteries are often discarded and finally sent to incineration, posing environmental and health issues [15]. Landfilling batteries alongside municipal waste quickly fills limited landfill space, driving up costs [16]. Heavy metals from waste batteries can leach into soil and water systems, where they persist for centuries, unlike organic pollutants [17]. These toxic substances can then enter the food chain, endangering ecosystems and human health [18]. Therefore, eco-friendly recycling methods are essential for managing waste Zn–Mn batteries [19].

End-of-life Zn–Mn batteries can be a resource for secondary raw materials. Recovered MnO from spent batteries can be repurposed for electronics due to its thermal stability and high capacitance, though its low conductivity requires improvement through functionalization with conductive materials [20]. Ru is a high-capacitance, conductive noble metal, and several studies report the potential of Ru-based materials to improve supercapacitor performance. RuO₂ coatings on carbon nanotubes (CNTs), through atomic layer deposition and electrochemical activation, demonstrated a 170-fold increase in specific capacitance compared to pure CNT electrodes [21]. Ru nanohybrid compounds combining hydrous ruthenium oxide and Ru nanoparticles capped by cysteine showed improved specific capacitance, rate capability, and electrochemical stability due to synergistic effects [22]. Symmetric supercapacitors with NiRu/RuO₂ electrodes exhibit a remarkably fast frequency response, high power density, and excellent capacitance retention [23]. The performance of Ru-based supercapacitors can be attributed to the fast kinetics of the redox reactions at the active layers. However, Ru’s high cost could limit usage; therefore, the use of small amounts could compromise the performance and cost of the material [24]. In this work, MnO has been recovered from used alkaline batteries and functionalized with 1% wt of Ru; the prepared MnO-Ru has been further incorporated into polyacrylonitrile (PAN) nanofibers and followed by carbonization to obtain a MnO-Ru@CNFs composite material.

2. Materials and Methods

2.1. Materials

Polyacrylonitrile (PAN, Mw = 150,000), ruthenium chloride (RuCl₂), ethanol, acetone, N, and N-dimethylformamide (DMF, AR) of analytical grade were purchased from Aldrich Chemical Co. (Milan, Italy). The electrode material fabrication was carried out using acetylene black and polyvinylidene (PVDF) as well as N-methyl pyrrolidinone (NMP) bought from Sigma Aldrich (St. Louis, MO, USA). Used alkaline batteries were collected from domestic use.

2.2. Preparation of Composite Samples

MnO was extracted from a spent alkaline battery, thoroughly washed, sonicated, and then calcined at 900 °C for 2 h. The MnO was then impregnated with Ru by preparing an aqueous mixture of MnO and RuCl₂ (with Ru constituting 1 wt% of MnO) in 20 mL of solution. This mixture was stirred at room temperature for 120 min, vacuum-dried in an oven, and subsequently calcined in air at 500 °C for 1 h.

The prepared MnO-Ru composite was mixed into a 12% PAN polymer solution in DMF and stirred for 1 h, and then electrospun at an applied voltage of 12 kV with a flow rate of 1 mL/h. The resulting nanofibers were collected on aluminum foil and oven-dried at 60 °C for 12 h. The dried nanofibers were then heat-treated at 250 °C for 2 h at a heating rate of 2 °C/min, followed by carbonization at 900 °C in a nitrogen atmosphere for 1 h with a heating rate of 5 °C/min (Figure 1).

3. Characterization

MnO, MnO-Ru, and MnO-Ru@CNFs were characterized using scanning electron microscopy (SEM) with a TESCAN-VEGA LMH (230 V) coupled with an energy dispersive X-ray (EDX) probe for elemental analysis (Assing S.p.A., Monterotondo, Rome, Italy), while the chemical composition was studied using Fourier transform infrared (FTIR) spectroscopy using a Vertex 70 apparatus (Bruker Corporation, Milan, Italy).

Electrochemical Measurement

Electrochemical testing was conducted in a three-electrode setup containing a 3 M KOH aqueous solution using an Autolab PGSTAT302N Potentiostat (Metrohm, Herisau, Switzerland). Platinum served as the counter electrode, while a saturated calomel electrode was used as the reference. Cyclic Voltammetry (CV) was performed over a 0.0 to 0.5 V range, and the specific capacitance (C_S) was calculated as per Equation (1) [25]:

$$C_s(CV)={\displaystyle \frac{1}{2\mathsf{\upsilon }∆Vm}}\int I\left(V\right)dV$$

(1)

A Galvanostatic Charge–Discharge (GCD) test was performed in the same potential range as was used for CV at 1, 3, 5, and 10 A/g current densities. The C_S from the GCD curves was calculated from Equation (2) [25]:

$$C_s(GCD)={\displaystyle \frac{It}{∆Vm}}$$

(2)

where a ∫ b I(V)dV is the area under the CV curve representing charge (C), I is the current (A), V is the potential window (V), m is the mass of the active material (g), υ is the scan rate (V/s), and t is the discharging time (s). The energy density (E, Wh/kg) and power density (P, W/kg) of the material were evaluated according to Equations (3) and (4) [26]. The electrochemical impedance spectroscopy (EIS) technique was carried out at open circuit potential in a frequency range from 0.01 Hz to 100 kHz.

$$E={\displaystyle \frac{1}{2}}{C}_s.∆V^2{\displaystyle \frac{1000}{3600}}$$

(3)

$$P={\displaystyle \frac{E}{∆t}}$$

(4)

4. Results and Discussion

The morphology of the produced composite was examined using SEM, as presented in Figure 2a–c. The SEM images show that the MnO nanoparticles exhibit a rough diameter distribution in the range of 100 to 600 nm. After impregnation with Ru, the MnO-Ru composite maintains a similar diameter range, with a uniform distribution of particles. Upon incorporation into CNFs, the MnO-Ru@CNFs composite displays a narrower diameter range from 200 to 400 nm, with a smoother surface and uniform particle distribution. The observed reduction in diameter may be attributed to contraction effects during high-temperature processing after the removal of hydrogen and oxygen from the polymer structure [27]. The EDX spectra and elemental composition tables (Figure 2d–f) provide clear evidence of Ru, Mn, O, and C within the MnO-Ru composite. In the MnO-Ru@CNFs there is a notable increase in the C content, indicating the successful carbonization and integration of Ru into MnO and the subsequent incorporation of the MnO-Ru composite within the CNF matrix.

Figure 3 reports the FTIR spectra, and a peak at 601 cm⁻¹ in both the MnO and MnO-Ru samples can be observed, characteristic of Mn–O and Ru-O bonding [28]. In the MnO-Ru@CNFs spectrum, C-N bands can be identified at ~2170 cm⁻¹ and ~1570 cm⁻¹. The peaks at ~600 cm⁻¹ are allocated at the characteristic peaks of Mn-O and Ru-O. Additional peaks are observed at 1983cm⁻¹ and 2245 cm⁻¹ which can be attributed to C=C and C=N bonding, indicating the successful incorporation of CNFs [29,30].

Electrochemical Measurement

Figure 4a–c displays the CV curves of MnO, MnO-Ru, and MnO-Ru@CNFs at different scan rates, ranging from 2 to 50 mV/s, in a 3 M KOH electrolyte. The CV curves of MnO exhibit redox peaks at 0.35 V and 0.25 V, indicative of the material’s pseudocapacitive behavior. After Ru impregnation, the intensity of these redox peaks increases, reflecting enhanced pseudocapacitive properties due to the transition metal oxide characteristics of MnO and Ru, such as variable oxidation states and a high surface area with a substantial pore volume [31]. These findings indicate that the capacitance of MnO and MnO-Ru electrodes is driven by a charge-storage mechanism in KOH, facilitated by the rapid intercalation of K⁺ ions during redox reactions [32]. In contrast, the CV curves of the MnO-Ru@CNFs reveal that the redox peaks are nearly absent, exhibiting a combination of pseudocapacitive and electrical double-layer capacitance (EDLC) behaviors, which suggests a hybrid charge-storage mechanism involving surface insertion/extraction and the adsorption/desorption of ions at the electrode–electrolyte interface [33,34].

Figure 4d shows the specific capacitance (SC) values for MnO, MnO-Ru, and MnO-Ru@CNFs at different scan rates, revealing a decrease in specific capacitance with increasing scan rate, as reported in

Table 1. Specific capacitance of samples calculated at different scan rates.

Specific Capacitance (F/g)
Sample	Scan Rate (mV/s)							Current Density (A/g)
	2	5	10	20	40	50	0.5	1	2	3	5
MnO	210	85	72	65	59	51	166	160	149	138	128
MnO-Ru	350	200	120	67	61	53	366	320	300	280	196
MnO-Ru@CNFs	688	413	376	157	89	81	586	565	539	521	384

. The MnO electrode achieves an SC of 210 F/g at 2 mV/s. To enhance its electrochemical performance, MnO was impregnated with Ru, resulting in an SC of 350 F/g, an improvement attributed to the higher capacitance properties of Ru [35]. Further enhancement was achieved by incorporating MnO-Ru into CNFs, yielding an SC of 688 F/g. This significant increase in capacitance is attributed to the improved electrical conductivity, increased specific surface area, and porous structure provided by the CNFs [33].

Figure 4d–f presents the GCD profiles of MnO, MnO-Ru, and MnO-Ru@CNFs at various current densities from 0.5 A/g to 5 A/g. The charge–discharge curves exhibit a combination of linear and curved regions, indicative of both pseudocapacitive and EDLC behavior in the samples. Maximum discharge times of 80 s, 150 s, and 280 s were achieved for MnO, MnO-Ru, and MnO-Ru@CNFs, respectively. The specific capacitance values were calculated from the GCD curves using Equation (3) across different current densities, as described in Table 1. At 0.5 A/g, the specific capacitances for MnO, MnO-Ru, and MnO-Ru@CNFs were 166 F/g, 368 F/g, and 586 F/g, respectively, consistent with the values obtained from CV analysis. Zhao and co-workers reported an MnOx/CNFs material with a specific capacitance of 120 F/g [36].

The higher values of MnO-Ru and MnO-Ru@CNFs compared to MnO could be attributed to the capacitance contribution of Ru and their unique hollow structures combined with the high specific surface area of the CNFs [32]. As shown in Figure 4a, there is an inverse relationship between specific capacitance and current density, with specific capacitance decreasing as current density increases. The energy storage performances of MnO, MnO-Ru, and MnO-Ru@CNFs were evaluated in terms of specific energy density and power density. The energy densities were found to be 5.4 Wh/kg, 10.4 Wh/kg, and 16.7 Wh/kg for MnO, MnO-Ru, and MnO-Ru@CNFs, respectively, while the corresponding power densities were 2479 W/kg, 2527 W/kg, and 2527 W/kg. EIS measurements were conducted to assess both internal resistance (R_i) and charge transfer resistance (R_ct), as depicted in Figure 5b. In the Nyquist plot (Figure 5b), MnO, MnO-Ru, and MnO-Ru@CNFs show a semicircle at low frequencies, indicating R_ct, while the intercept at high frequencies represents R_i. The 45° line in the Nyquist plot corresponds to the Warburg element (Zw) [37]. The prepared MnO, MnO-Ru, and MnO-Ru@CNFs composites exhibited R_i values of 0.81, 0.8, and 0.3 Ohm, respectively, suggesting rapid ion and electron mobility. The differences in resistance could be related to the conductivity differences in the electrode materials, and the low value of resistance is attributed to the higher carbon and Ru content on the electrode surface, as verified by EDX, providing more sites for electron and ion transport and enhancing conductivity [38]. These electrochemical evaluations, with low R_i and R_ct, demonstrated excellent redox performance (CV and GCD), confirming that the MnO-Ru@CNFs composite is a promising material for supercapacitor applications.

5. Conclusions

MnO can be recovered from discarded alkaline batteries, then functionalized with 1 wt% Ru through impregnation, and incorporated into PAN nanofibers by electrospinning followed by carbonization at 900 °C to form MnO-Ru@CNFs. We found that fine MnO-Ru particles with a diameter of approximately 400 nm and smooth, bead-free carbon nanofibers with an average diameter of about 250 nm can be obtained, as shown by SEM, EDX, and FTIR characterization. The obtained products have specific capacitances of 168 F/g, 368 F/g, and 586 F/g at a current density of 0.5 A/g, along with energy densities of 5.4 Wh/kg, 10.4 Wh/kg, and 16.7 Wh/kg and power densities of 2479 W/kg, 2527 W/kg, and 2527 W/kg, respectively. The integration of recycled MnO into supercapacitor technology supports sustainable energy storage solutions, particularly for high-demand applications. This work underscores the potential of MnO-Ru@CNFs as a high-performance, environmentally friendly supercapacitor material with promising applications in the aerospace sector.