Manganese dioxide is widely used in the production of Zn-Mn batteries. It is the positive active material of Zn-Mn batteries. Compared with natural manganese dioxide, manganese dioxide produced by electrolysis can greatly increase the capacity of Zn-Mn batteries. Therefore, manganese dioxide produced by electrolysis is also called active manganese dioxide.
Manganese dioxide is prepared by electrolysis in sulfuric acid solution of manganese sulfate; lead is used for the cathode; and lead-lead dioxide is used for the anode. The cathodic reaction during electrolysis is:
2H++2e-→H2
While the mechanism by which the anodic process generates manganese dioxide is unclear, one possible anodic reaction and subsequent secondary reactions are:
Mn2+→Mn4++2e-
Mn4++2H2O→MnO2+4H+
Current efficiency is a major economic and technical index in electrolysis and electroplating production. The current efficiency of the cathode process or the anode process is usually expressed by η, and its meaning is:
(1)
According to Faraday's law, the electricity required for 1mol Mn4+ is 2x96500 C. The electricity passing through the battery during electrolysis is:
Q=electrolysis current (A) x electrolysis time (s)
The molar mass of manganese dioxide is 86.93. If Q electricity passes through the anode, the quality of manganese dioxide that should be obtained in theory is:
(2)
When equation (2) is introduced into equation (1), the current efficiency of manganese dioxide generated by the anode process can be expressed as:
(3)
Factors affecting the current efficiency of the anodic process product manganese dioxide are as follows:
(1) Oxygen precipitation reaction on the anode
This electrode reaction is catalyzed by the formation of manganese dioxide attached to the anode.
(2) The anodic Mn4+ is reduced on the cathode due to diffusion and electromigration.
(3) The Mn4+ generated by the anode is not completely transformed according to the above formula and remains in the solution.
(4) The acidity of the electrolyte decreases, the concentration of manganese sulfate decreases, the temperature of the electrolyte increases, the anode current density increases, and the presence of various impurities, especially iron, copper, and other variable ions, will significantly reduce the current efficiency of the anode process.
The most suitable conditions for electrolysis were obtained through experimental research, as follows:
- The initial composition of the electrolyte: MnSO4 300~500 g/L, H2SO4 180~200 g/L.
- Electrolyte temperature: 20~25 oC.
- Current density: 5 A/dm2 for the anode and 10 A/dm2 for the cathode.
- Distance between electrodes: 30 mm.
Instruments and Reagents
DC power supply (transistor stabilizer), ammeter, oven, platform scale, 500 mL beaker, lead sheet, manganese sulfate, sulfuric acid.
Experimental Procedure
(1) Prepare an electrolyte. The composition of the electrolyte is MnSO4 300 g/L, H2SO4 180~200 g/L, and the sulfuric acid solution is first prepared and stirred continuously.
(2) Put the anode, cathode, and 300 mL electrolyte into a 500 mL beaker, connect the electrolysis circuit, and start electrolysis after inspection, with an electrolysis current of 5 A.
(3) Electrolyze for 2.5 hours, and record the electrolysis voltage every 20 minutes. After stopping electrolysis, remove the electrode and rinse it with distilled water.
(4) Put the electrolyzed solution in an oven and keep it at 80 oC for 3 hours to precipitate manganese dioxide. Pour off the upper solution, filter the precipitate with suction, and wash the precipitate several times (weigh the filter paper used in advance). Put the obtained manganese dioxide into an oven, dry it at 110 oC, weigh the obtained manganese dioxide, and calculate the current efficiency.
