Energy-saving and environmentally friendly electrodeposition of γ-MnO₂

Abstract

The paper reports a new energy-saving and environmentally friendly method for synthesis of γ-MnO₂ using a Pt/C gas diffusion electrode instead of the traditional cathode. The Pt/C gas diffusion electrode has high activity in a 120 g dm⁻³ MnSO₄·H₂O + 30 g dm⁻³ H₂SO₄ electrolyte at 80 °C. At the same time, the cell voltage and electrode potential of the electrolytic cell with the Pt/C gas diffusion electrode as cathode are measured at 100 A m⁻² for 12 h. The results show that the Pt/C gas diffusion electrode demonstrated good stability. The effects of the current density on the synthesis of γ-MnO₂ (EMD) is studied, and the results show that the electric energy consumption of the GDE as cathode drops 61.06–57.75%, 65.6–62.9% and 61.4–55% compared with that of the Cu, Graphite, and Pt cathode at 80 A m⁻², 100 A m⁻², and 120 A m⁻² respectively in 12 hours. The γ-MnO₂ (EMD) shows a better discharge performance as a cathode material of Zn–MnO₂ than the commercial battery grade manganese dioxide.

---

Introduction

Energy and the environment are among the most important issues in the twenty-first century.¹ The batteries which are used as green energy have caused wide public concern. The γ-MnO₂, which is an environmentally friendly, safe, low cost material and has a relatively high discharge voltage compared to the α-, β-, and δ-MnO₂² has been reported to be as cathode active material^3,4 in batteries.

The γ-MnO₂ used in batteries can be categorized as three groups: natural manganese dioxide (NMD), chemical manganese dioxide (CMD) and electrolytic manganese dioxide (EMD), according to the production method.⁵ The CMD is synthesized by chemical method (oxidation method). The synthesis process of γ-MnO₂ (EMD) is that the cathodes and anode are placed in an aqueous bath of MnSO₄ and H₂SO₄ solution with a constant current under atmospheric conditions and then the high purity and high density γ-MnO₂ (EMD) is directly deposited on the anode surface. The electrodeposition process of γ-MnO₂ has better control over the properties of the deposited material than the chemical synthesis⁶ and higher yield densities of γ-MnO₂ (EMD), comparing with that of γ-MnO₂ (CMD).⁷ Hill⁸ found that γ-MnO₂ (EMD) materials are higher in energy than γ-MnO₂ (CMD) variants, and this is because the γ-MnO₂ (EMD) is characterized by the larger proportion of ramsdellite and microtwinning. In recent years, the nano γ-MnO₂ (coprecipitation,⁹ reflux¹⁰ and hydrothermal synthesis¹¹) used in supercapacitor,¹² battery¹³ and catalyst¹⁴ has been synthesized. But the γ-MnO₂ (EMD) has a much lower price comparing nano γ-MnO₂. As a result EMD (electrolytic γ-manganese dioxide) has become the most widely used materials in battery, such as alkaline¹⁵ and lithium cells.¹⁶

With increasing electrically powered consumer products, the demand for γ-MnO₂ is also ever-growing. The γ-MnO₂ (EMD) demand is more than 349 000 mt in 2011 and grows at a 5.3% CAGR from 2011 to 2016. Enormous research has been done about γ-MnO₂ (EMD). For example, Ghaemi et al.¹⁷ have reported the electrodeposition of manganese dioxide in acidic MnSO₄ solution and the cell voltage usually floated between 2.3 V and 2.4 V using graphite as cathode. Biswal et al.¹⁸ have reported energy consumption of EMD at 100 A m⁻² was 1660 kW h per ton. But the synthesis of γ-MnO₂ (EMD) is a very energy demanding and large volumes of hydrogen gas is generated at the cathode. However, the hydrogen gas carries acidic mist into the air, of which some spreads throughout the workplace. Not only is this acidic mist hazardous to health but also it damage the environment.¹⁹ It is necessary, therefore, to reduce the energy consumption and hydrogen gas emission in view of the issue of environmental pollution. Replacing the traditional cathode with a Pt/C gas diffusion electrode (GDE) significantly reduces the cell voltage, because of the potential difference between the oxygen reduction reaction of Pt/C GDE and the hydrogen evolution reaction of the traditional cathode (Cu, graphite, Pt). It is well known that electrodeposition reaction of manganese dioxide using traditional cathodes is:

Anode: Mn²⁺ + 2H₂O → MnO₂ + 4H⁺ + 2e⁻E⁰ = 1.23 V vs. SHE (1)

Cathode: 2H⁺ + 2e⁻ → H₂↑ E⁰ = 0 V vs. SHE (2)

Overall: Mn²⁺ + 2H₂O → MnO₂ + H₂↑ + 2H⁺ (3)

where E⁰ is the electrode potential vs. SHE. The decomposition voltage E⁰ of the total electrode reactions is 1.23 V.

If a gas diffusion electrode is employed in the electrolytic manganese dioxide cell instead of the traditional electrode, the electrochemical reaction is:

Anode: Mn²⁺ + 2H₂O → MnO₂ + 4H⁺ + 2e E⁰ = 1.23 V vs. SHE (4)

Cathode: O₂ + 4H⁺ + 4e → 2H₂O E⁰ = 1.229 V vs. SHE (5)

Overall: Mn²⁺ + 1/2O₂ + H₂O → MnO₂ + 2H⁺ (6)

The decomposition voltage E⁰ of the total electrode reactions is 0.001 V. The decomposition voltage drops by 1.229 V comparing to the traditional cathode. Fig. 1 shows the schematic diagrams of electrolysis with the Pt/C GDE. It is anticipated that the use of a Pt/C GDE can save the energy of 90% under standard conditions. Therefore, not only can energy savings be obtained through the introduction of the Pt/C GDE, but also this method protects environment.

Fig. 1 The schematic diagrams of the Pt/C GDE electrolysis.

With this context, we study the possibility of utilizing a Pt/C GDE as cathode in electrolysis process. The energy-saving mechanism of the Pt/C GDE is discussed using cyclic voltammetry (CV). The energy consumption of electrolysis bath with the Pt/C GDE is compared to the Cu, graphite, Pt cathode by electrolytic experiments. The discharge characteristic of γ-MnO₂ (EMD) is tested.

Results and discussion

Fig. 2A exhibits XRD patterns of the Pt/C samples. The first broad peak located at 25° which was associated with the Vulcan XC-72R support material. The Pt peaks appeared at 39.76°, 46.24°, 67.46°and 81.28°, which were attributed to the (111), (200), (220), and (311) planes of face-centered cubic structure^20,21 of Pt (JCPDS PDF#70-2057), respectively. The macroscopic properties of Pt/C such as surface morphology, particle size, and metal dispersion were investigated by TEM. TEM micrographs of 40 wt% Pt/C are shown in Fig. 2B. It is observed that the Pt particles were well dispersed on the carbon support and most of the particles were in the range of 2–3 nm. Fig. 3 shows CV of the Pt/C GDE surface at air. It is clearly seen that the reduction peak is at 0.6 to −0.05 V vs. Hg/HgSO₄ and the high reduction peak is at about 0.1 V vs. Hg/HgSO₄, According to redox potential, this reduction current peaks can be attributed to the formation of O₂ to H₂O. The current density increased linearly from 0.6 V to 0.1 V using Pt/C GDE in electrolyte at air indicating that the Pt/C GDE has high activity.²² The window voltage of ORR was so big, which could be attributed to the intrinsic properties of the gas diffusion electrode. The electrode/electrolyte/gas three-phase boundaries of GDE have higher solubility of oxygen in electrolyte.²³

Fig. 2 XRD patterns (A) and TEM image (B) of 40 wt% Pt/C catalysts.

Fig. 3 CV of the GDE in a 120 g dm⁻³ MnSO₄·H₂O + 30 g dm⁻³ H₂SO₄ electrolyte at 80 °C, scan rate: 20 mV s⁻¹ in air at the flow rate of 30 mL min⁻¹.

The cell voltage, cathode potential and anode potential time curves of electrolysis with Pt/C GDE and Cu cathode were measured at the same time, at 100 A m⁻² for 12 h. As shown in Fig. 4A, the cathode potential versus Hg/HgSO₄ of the gas diffusion electrode slightly vibrated with time and range of the voltage fluctuation was only ±0.007 V, suggesting that the Pt/C GDE was almost steady. The average cathode potential of gas diffusion electrode was −0.02 V versus Hg/HgSO₄ at 100 A m⁻². However, the average cathode potential of Cu electrolysis was as high as −1.10 V. This indicates that the cell voltage could reduce from −1.10 V to −0.02 V due to the potential of oxygen reduction is higher than that of hydrogen evolution.^24,25 The cell voltage drop (1.08 V) is less than theoretical value (1.23 V) which is due to overpotential of the oxygen reduction reaction.²⁶ Not all oxygen molecule is reduced to H₂O (direct 4-electron route), and part of oxygen molecule may be reduced to H₂O₂ (ref. 27 and 28) and then be reduced to H₂O.

Fig. 4 Electrolytic experiments of the GDE cathode and Cu cathode: (A) electrode potential (B) cell voltage (C) the product of electrolysis.

Fig. 4B exhibits the cell voltages of the electrolysis bath with Pt/C GDE and Cu cathode corresponding to cathode potential with Pt/C GDE cathode and Cu cathode shown in Fig. 3A. The voltage kept fluctuation range within ±0.014 mV and the average cell voltages of the electrolysis bath with Pt/C GDE was 0.748 V. However, standard cell potential was 0.64 V, and that is because the iR_cell causes an increase in cell voltages as shown in eqn (7).²⁹

E_cell = E_anode − E_cathode + ∑η + iR_cell (7)

The average cell voltage of the electrolysis bath with Cu electrolysis was 1.798 V, indicating that the cell voltage with Pt/C GDE decreased by 1.05 V when compared with that of Cu cathode. Fig. 4C exhibits XRD patterns of the product of electrolysis. The first broad peak appeared at 22.3°and four sharper peaks appeared at 37.3°, 42.7°, 56.5°and 67.3° indicating that the product was γ-MnO₂. The result agrees with γ-MnO₂ models reported by Chabre and Parmetier.³⁰ The batch electrolyses using Pt/C GDE as cathode and Cu 2 × 2.5 cm² (Beijing Cuibolin Non-Ferrous Technology Developing Co., Ltd), graphite 2 × 2.5 cm² (Beijing Jixing Sheng an Industry & Trade Co., Ltd), Pt 2 × 2.5 cm² (Beijing Cuibolin Non-Ferrous Technology Developing Co., Ltd) cathode as cathode were carried out to study the effects of the current density on the electrical energy consumption of electrolytic manganese dioxide. Table 1 displays the variation of electrical energy consumption and cell voltages with Pt/C GDE and Cu, graphite, Pt cathode for same time (12 h) at different current density. As expected, current efficiency decreased from 98.6% to 96.7% and the cell voltage of electrolysis with Pt/C GDE rose gradually from 0.70 V to 0.81 V with increasing the current density from 80 to 120 A m⁻². But the cell voltage of electrolysis with Pt/C GDE was lower than the cell voltage of electrolysis with Cu, graphite, Pt cathode at same current density for same electrolysis time due to the low cathode potential of Pt/C GDE compared to the Cu, graphite, Pt cathode. The cell voltage of electrolysis with Pt/C GDE reduced less than 1.0 V from 80 to 120 A m⁻² compared to the Cu, graphite, Pt cathode. According to Pan et al.³¹ the high cell voltage will cause a high energy consumption. Therefore, the electric energy consumption of electrolysis with Pt/C GDE decreased due to a decrease in cell voltage when compared with that of the Pt/C GDE. The decrease in electric energy consumption of electrolysis with Pt/C GDE are 686.4–716.2 kW h per ton and 833.4–875.1 kW h per ton and 659.4–695.5 kW h per ton, comparing with that of the Cu, graphite, Pt cathode, respectively at 80 A m⁻², 100 A m⁻² and 120 A m⁻². It can be concluded that electrolysis with Pt/C gas-diffusion cathode can cut electric energy consumption by 61.06–57.75%, 65.6–62.9% and 61.4–55% compared to Cu, graphite, Pt cathode at 80 A m⁻², 100 A m⁻² and 120 A m⁻² respectively in 12 hours.

Table 1 The effect of the current density on electrolytic

Current density	Cathode	Average cathode potential (V vs. Hg/HgSO4)	Average anode potential (V vs. Hg/HgSO4)	Average cell potential (V)	Energy consumption (kW h per ton)
80 A m^−2	GDE	0	0.601	0.70	437.8
	Cu	−1.05	0.608	1.71	1124.2
	Graphite	−1.23	0.597	1.95	1271.2
	Pt	−1.03	0.614	1.70	1133.3
100 A m^−2	GDE	−0.02	0.621	0.75	471.0
	Cu	−1.10	0.614	1.80	1187.2
	Graphite	−1.32	0.627	2.02	1268.5
	Pt	−1.10	0.650	1.80	1130.4
120 A m^−2	GDE	−0.03	0.659	0.81	516.6
	Cu	−1.12	0.652	1.85	1222.8
	Graphite	−1.43	0.628	2.18	1391.7
	Pt	−1.14	0.657	1.81	1148.3

---

The SEM image of the surface of γ-MnO₂ (EMD) which was deposited on the Ti-based Ti–Mn alloy anode and was obtained by using Pt/C GDE as cathode at 80 °C in a 120 g dm⁻³ MnSO₄·H₂O + 30 g dm⁻³ H₂SO₄ at 100 A m⁻² for 12 h is shown in Fig. 5. It is clear that deposits which were piled on each other were porous and irregular shapes. Fig. 6 compares the discharge characteristics of EMD and commercial battery grade manganese dioxide as cathodes in 9 mol L⁻¹ KOH electrolyte at the discharge current density of 50 mA g⁻¹ and the temperature of 25 °C. The two cells were discharged to a cut-off voltage of 1.0 V. The open-circuit potential of EMD was higher than the commercial battery grade MnO₂. Note that the maximum discharge capacity γ-MnO₂ (EMD) is as high as 124.16 mA h g⁻¹, which is higher than that of commercial battery grade MnO₂, comparing with that of the commercial battery grade manganese dioxide.

Fig. 5 SEM image of γ-MnO₂ (EMD) using Pt/C GDE as cathode at 80 °C in a 120 g dm⁻³ MnSO₄·H₂O + 30 g dm⁻³ H₂SO₄ at 100 A m⁻² for 12 h.

Fig. 6 Discharge curves of γ-MnO₂ (EMD) and commercial battery grade MnO₂.

Conclusions

The Pt/C GDE is successfully employed in the preparation of electrolytic manganese dioxide in order to save energy and protect the environment. The Pt/C GDE has high activity in electrolyte and is almost steady. The electric energy consumption of the electrolysis with Pt/C GDE can save 61.06–57.75%, 65.6–62.9% and 61.4–55% comparing to Cu, graphite, Pt cathode at 80 A m⁻², 100 A m⁻² and 120 A m⁻² respectively in 12 hours. The product of electrolysis is highly reactive γ-MnO₂. The γ-MnO₂ (EMD) shows a better discharge performance as cathode material of Zn–MnO₂ than the commercial battery grade manganese dioxide. During the experiment process, there are no releases of the toxic substance. Therefore, the electrolytic process is high energy-saving and environmentally-friendly.

Experiment

Preparation of Pt/C gas diffusion electrode

The GDE consists of a three-layer structure which are diffusive layer, substrate and catalyst layer. The diffusion layer was prepared as follows: 295 mg of acetylene black, 600 mg of 60% PTFE (Hesen, Shanghai, China), 307 mg of Na₂SO₄, 300 mg of activated carbon powder, and 200 mL of ethanol was used. The above blend was mixed and stirred at 80 °C using magnetic stirring apparatus and the resulting wet paste was rolled into a 0.4 mm thin diffusion layer. The substrate was nickel screens. A slurry for the catalyst layer was prepared by blending 50 mg of 40% Pt/C (VXC72R) catalyst (Hesen, Shanghai, China), 200 mL of ethanol as a dispersant, 198 mg of 60% PTFE (Hesen, Shanghai, China), as a wet-proofing agent and binder, the 155 mg of acetylene black and 600 mg of activated carbon powder. The process of 0.3 mm catalyst layer-making is same as the diffusion layer. The Pt loading was 0.36 ± 0.02 mg cm⁻². Catalyst layer and diffusive layer were dried at 150 °C by electric drying oven in order to remove surfactants of PTFE and then the diffusive layer was rinsed in deionized water and dried in electric oven. Furthermore, the diffusion layer was rolled on one side of the nickel screens to give a flat sheet of 0.7 mm thickness and then the catalyst layer were pressed onto the other sides of nickel screens to obtain about 0.8 mm flat sheet.

Electrolytic deposition process of γ-MnO₂

Electrolytic γ-MnO₂ experiments were carried out with three electrodes at 80 °C in a 120 g dm⁻³ MnSO₄·H₂O + 30 g dm⁻³ H₂SO₄ electrolyte in air, using a Ti-based Ti–Mn alloy as anode (Beijing nonferrous metal research institute), a GDE as cathode and Hg/HgSO₄ as reference electrode. The GDE was placed in a PTFE holder and the gas flow passed over the diffusive layer of the GDE cathode through a pipe inside the holder. The apparent area of the GDE electrode face in contact with the solution was 5 cm². A direct current power supply was employed to provide the direct current. During the electrolysis process the γ-MnO₂ is deposited directly on the anode immersed in the electrolysis cell, according to eqn (1). At the end of this process of the electrolysis process, the electrolytic γ-MnO₂ is removed from the Ti-based Ti–Mn alloy anode. And then the electrolytic γ-MnO₂ was crushed, ground, repeatedly washed in water, neutralized by washing with 10% NaOH and repeatedly washed in water again until neutral and then dried in a drying oven. The electrolytic deposition process of γ-MnO₂ has the advantages of environment friendliness, simple operation and easy control.

γ-MnO₂ (EMD) discharge performance

The γ-MnO₂ (480 mg) was first mixed with active carbon (120 mg), acetylene black (60 mg) and 60% PTFE (1000 mg) as a binder and then mixture was pressed on nickel sheet. The discharged experiments were carried out with two electrodes at 25 °C in 9 M KOH with the discharge current density of 50 mA g⁻¹, using a mixture on a nickel sheet as working electrode and Zn sheet as counter electrode, using cs350 electrochemical work station.

Characterization

X-ray diffraction analysis was carried out with a TTRIII X-ray power diffractometer using the Cu K_ (λ = 1.5405 Å) radiation source operating at 40 kV and 300 mA. The XRD patterns were measured in the range of 2θ = 10–90° scanning at the rate of 5° min⁻¹ with an angular resolution of 0.02°. The size and morphology distribution of Pt particles of catalyst were determined by TEM (JEM-2010F TEM microscope) using an accelerating voltage of 200 kV.

Electrochemical measurement

The CV spectra were recorded using a CHI660 electrochemistry station with a standard a three-electrode consisting of a Ti-based Ti–Mn alloy electrode (16 cm²) as a counter electrode, an Hg/HgSO₄ as reference electrode, and the GDE as working electrode in air. During the experiment process, the temperature was kept at 80 ± 2 °C. The scan rate was 20 mV s⁻¹.

Acknowledgements

The authors acknowledge the financial support of the Natural Science Foundation of China (51274027).

Notes and references

1. Z. L. Wang, D. Xu, J. J. Xu and X. B. Zhang, Chem. Soc. Rev., 2014 10.1039/C3CS60248F.
2. W. I. Jung, M. Nagao, C. Pitteloud, K. Itoh, A. Yamada and R. Kanno, J. Mater. Chem., 2009, 19, 800.
3. F. Y Cheng, J. Chen, X. L Gou and P. W. Shen, Adv. Mater, 2005, 17, 2753.
4. S. Sarciaux, A. L. G. L. Salle, A. Verbaere, Y. Piffard and D. Guyomard, J. Power Sources, 1999, 81–82, 661.
5. T. X. T. Sayle, C. R. A. Catlow, R. R. Maphanga, P. E. Ngoepe and D. C. Sayle, J. Cryst. Growth, 2006, 294, 118.
6. K. R. Prasad and N. Miura, J. Power Sources, 2004, 135, 354.
7. E. I. Wang, L. Lin and W. L. Bowden, US Pat. 5348726A, 1994.
8. J. R. Hill, C. M. Freeman and M. H. Rossouw, J. Solid State Chem., 2004, 177, 165.
9. S. T. Xing, R. R. Han, Z. C. Ma, Y. S. Wu and Z. C. Zhou, CrystEngComm, 2011, 13, 6033.
10. X. Zhang, X. Z. Sun, H. T. Zhang, D. C. Zhang and Y. W. Ma, Electrochim. Acta, 2013, 87, 637.
11. X. H. Zhang, B. X. Li, C. Y. Liu, Q. X. Chu, F. Y. Liu, X. F. Wang, H. W. Chen and X. Y. Liu, Mater. Res. Bull., 2013, 48, 2696.
12. A. B. Yuan, X. L. Wang, Y. Q. Wang and J. Hu, Energy Convers. Manage., 2010, 51, 2588.
13. F. Y. Tu, T. H. Wu, S. Q. Liu, G. H. Jin and C. Y. Pan, Electrochim. Acta, 2013, 106, 406.
14. F. Y. Cheng, Y. Su, J. Liang, Z. L. Tao and J. Chen, Chem. Mater., 2010, 22, 898.
15. C. Mondoloni, M. Laborde, J. Rioux, E. Andoni and C. Lévy-Clément, J. Electrochem. Soc., 1992, 139, 954.
16. M. Manickam, P. Singh, T. B. Issa, S. Thurgate and R. D. Marco, J. Power Sources, 2004, 130, 254.
17. M. Ghaemi, Z. Biglari and L. Binder, J. Power Sources, 2001, 102, 29.
18. A. Biswal, K. Sanjay, M. K. Ghosh, T. Subbaiah and B. K. Mishra, Hydrometallurgy, 2011, 110, 44.
19. R. A. Shakarji, Y. H. He and S. Gregory, Hydrometallurgy, 2013, 131–132, 76.
20. L. Johnson, W. Thielemans and D. A. Walsh, Green Chem., 2011, 13, 1686.
21. S. C. Zignani, E. Antolini and E. R. Gonzalez, J. Power Sources, 2008, 182, 83.
22. L. J. Li and A. Manthiram, J. Mater. Chem. A, 2013, 1, 5121.
23. H. Cheng and K. Scott, J. Electroanal. Chem., 2006, 596, 117–123.
24. Y. Tang, Y. J. Li, Y. Z. Sun, J. X. Wang, Y. M. Chen, X. J. Yang and P. Y. Wan, Green Chem., 2012, 14, 334.
25. Y. Tang, Y. J. Li, Y. Z. Sun, J. X. Wang, Y. M. Chen, X. J. Yang and P. Y. Wan, Electrochem. Commun., 2013, 27, 108.
26. I. E. L. Stephens, A. S. Bondarenko, U. Grønbjerg, J. Rossmeisl and I. Chorkendorff, Energy Environ. Sci., 2012, 5, 6744.
27. M. Inaba, H. Yamada, J. Tokunaga and A. Tasaka, Electrochem. Solid-State Lett., 2004, 7, A474.
28. A. Kishi, S. Shironita and M. Umeda, J. Power Sources, 2012, 197, 88.
29. K. Zeng and D. K. Zhang, Prog. Energy Combust. Sci., 2010, 36, 307.
30. Y. Chabre and J. Parmetier, Prog. Solid State Chem., 1995, 1, 23.
31. J. Q. Pan, C. Zhang, Y. Z. Sun, Z. H. Wang and Y. S. Yang, Chem. Commun., 2012, 19, 70.

---