The mechanism of electrolytic manganese dioxide anode and anode electrochemical process is complex, and the total reaction is MnSO 4 +2H 2 O===MnO 2 +H 2 SO 4 +H 2 (1)
1) a cathode manganese dioxide generally use an electrolytic process or a carbide purple copper tube as a cathode. Under negative electrodeposition, hydrogen evolution reaction mainly occurs.
2H + +2e - ===H 2 (g) (2)
When P H2 = 100Pa, φ 25 =0-0.0591 pH; φ 100 =0-0.071 pH
That is, the precipitation potential of the hydrogen evolution reaction (2) whose temperature is raised from 25 ° C to 100 ° C becomes negative as the pH value increases.
In view of the fact that there is no data on the average ion activity coefficient of high ionic strength acidic complex sulfuric acid solution (especially under high temperature conditions), Zhong Zhuqian and Mei Guanggui intend to calculate the pH value of high concentration acid sulfate by concentration instead of activity. And an approximate calculation is derived and calculated:
10 pH θ 10 -pH +{1+10 pH θ [(SO 4 2- ) T -(H + ) T ]}·10 -pH -[H + ] T =0 (3)
Where (SO 4 2- ) T is the total concentration of SO 4 2- component in solution, mol/L
(H + ) T - the total concentration of H + components in the concentrate, mol / L.
pH θ - SO 4 2- + H + = HSO 4 - The standard of the reaction [(SO 4 2- ) = (HSO 4 2- )] balances the pH.
The calculated pH θ values ​​for each temperature are as follows:
Temperature / °C | 25 | 40 | 60 | 80 | 100 |
pH θ | 1.91 | 2.093 | 2.4 | 2.738 | 3.091 |
Calculated according to formula (3):
(1) The pH value of each [H + ] T in the case of fixing [SO 4 2- ]T=2.3 mol/L at a temperature of 25 to 100 ° C is shown in Fig. 1 .
(2) The pH value of each [SO 4 2- ] T in the case of fixing [H + ] T = 0.4 mol/L and temperature 25 to 100 ° C, as shown in Fig. 2.
As is clear from Fig. 1 and Fig. 2, as the [SO - 4 ] T increases, especially the temperature rises, the pH in the solution increases. The increase in pH in the solution will show a significant effect on the efficiency of the anode current in the electrolysis. [next]
2) the process of electrolytic manganese dioxide, an anode of Ti are currently pattern plate glass or Ti-Mn alloy coating layer as an anode, from a front bell bamboo, Mei Guanggui making Ti-H 2 O-based φ-pH (Figure 3) It can be seen that under the oxidizing conditions of the anode, TiO 2 is formed on the surface of Ti, thereby exhibiting an insoluble passivation state.
The relevant reaction formula of the φ-pH diagram of the Ti-H 2 O system is as follows:
Ti 2+ +2e-===Ti (1)
φ θ 25 =-1.628
Ti 3+ +e - ===Ti 2+ (2)
φ θ 25 =-0.3686
TiO+2H + ===Ti 2+ +H 2 O (3)
pH θ 25 ==5.451
TiO+2H + +2e - ===Ti+H 2 O (4)
φ 25 =-1.3059-0.0591pH
Ti 2 O 3 +2H + +2e - ===2Ti+H 2 O+O 2 (5)
φ 25 =-1.2027-0.0591pH
Ti 2 O 3 +6H + +2e - ===2Ti 2+ +3H 2 O (6)
φ 25 =-0.5171-0.1183pH
TiO 2 +4H + +2e - ===Ti 2+ +2H 2 O (7)
φ 25 =-0.5171-0.1183pH
TiO 2 +4H + +e - ===Ti 3+ +2H 2 O (8)
φ 25 =-0.6657-0.2365pH
2TiO 2 +2H + +2e - ===Ti 2 O 3 +H 2 O (9)
φ 25 =-0.4714-0.0591pH
Titanium has excellent mechanical properties and corrosion resistance as an anode, has low density, high strength, and has good processability and is easy to mold. However, when titanium is used as an anode in an electrolysis process, it is easy to cause blunt phenomenon, and the conductivity of the passivation is seriously degraded. [next]
Titanium is between iron and zinc in the electrochemical sequence. It is a thermodynamically active metal with a standard equilibrium electrode potential of -1.63V, but the titanium surface is highly prone to form a protective oxide film (passivation film). The actual electrode potential is far from positive, and this passivation film with high resistance gives titanium excellent corrosion properties. When titanium is used as the anode, the passivation film on the surface of the titanium is continuously thickened due to the blunt action of the anode current, so that the cell voltage in the electrolysis process is sharply increased, and the power consumption is increased until the electrolysis process cannot be continued. Researchers have proposed many solutions around how to avoid titanium anode passivation and increase the current density in its application in order to reduce power consumption and increase productivity. Among them, the Ti plate blasting treatment or the Ti-Mn alloy layer anode is adopted, which is a better solution for the passivation of the Ti plate.
The electrolysis MnO 2 anode process mainly occurs as follows: two competitive reactions of O 2 and precipitation of MnO 2 :
O 2 +4H + +4e - ===2H 2 O (1)
When P o2 =100Pa,
φ 25 =1.229-0.0591pH
φ 40 =1.2163-0.062pH
Φ 60 =1.200-0.066 pH
φ 80 =1.1834-0.07005pH
φ 100 =1.167-0.074 pH
MnO 2 +4H + +2e - ===Mn 2+ +2H 2 O (2)
When [Mn 2+ ]=1mol/L,
φ 25 =1.229-0.01182pH
φ 40 =1.219-0.1241pH
Φ 60 =1.206-0.132 pH
φ 80 =1.1943-0.1401pH
φ 100 =1.1824-0.148pH
It can be seen from the φ values ​​of the reaction (1) and the reaction (2) that the difference between the standard φ Ө of the above two reactions is not significant, and the effect of the pH on the difference is very large. obviously.
Substituting the pH of the corresponding conditions in Figs. 1 and 2 into the equilibrium potential φ of the above (1) and (2) reaction temperatures, we fabricated the φ-[H + ] map (see Fig. 4), φ-[SO 4 2- ] T map (Figure 5) and φ-temperature map (Figure 6).
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It can be seen from Fig. 5 and Fig. 6 that the temperature rises and increases [SO 4 2- ] T , both φ 2 and φ 1 decrease, and the difference of φ 1 - φ 2 increases. It shows that the use of high concentration [SO 4 2- ] T solution and high temperature electrolysis is beneficial to the preferential precipitation of MnO 2 .
From the MeSO 4 -H 2 SO 4 -H 2 O system φ-[H + ] T diagram (Fig. 4), for a given [SO 4 2- ] T concentration solution, at the beginning of electrolysis (ie When [H + ] T =0, the difference between φ 1 and φ 2 is the largest, and MnO 2 is preferentially precipitated at this time. At the later stage of electrolysis (i.e., [H + ] T increases, φ 1 - φ 2 has a difference of zero), and MnO 2 and O 2 precipitate simultaneously. It is shown that to obtain high anode current efficiency, the increase in [H + ] T is limited. It shows that high temperature and high concentration [SO 4 2- ]T concentrated liquid electrolysis is beneficial to the preferential precipitation of MnO 2 . The above points are confirmed by the results of our MnO 2 electrolysis test. [next]
Zhong Shaolin studied the reaction mechanism of electrolytic MnO 2 electrode in detail, and the results are as follows:
Steady-state polarization curve method and rotating disk electrode were carried out in a neutral solution of Na 2 SO 4 0.5 mol · L -1 , MnSO 4 0.01 mol · L -1 and temperature 25~35 °C using a glassy carbon electrode. Seven kinds of modern electrochemical test tests, such as steady-state polarization curve method, linear potential sweep voltammetry, convolutional sweep potential voltammetry, cyclic voltammetry, current step method and potential step method, systematically studied MnO 2 Electrochemical behavior of anodic deposition, electrode reaction kinetic parameters were measured, as shown in Table 1. At the same time, the apparent activation energy ΔE=50.2kJ/mol of the electrode reaction when the overpotential is 0.39V is measured by the steady-state polarization curve method, and the Mn 2+ participates in the reaction by the steady-state polarization curve method of the rotating disk electrode. The number of stages is level 1. These data have practical significance for strengthening the process of electrolytic MnO 2 . Table 2 describes the study of the electrolytic MnO 2 Yanghu deposition reaction.
Experimental research method | Temperature t/°C | Transfer coefficient βn a | Exchange current Io/(A·cm -2 ) | Standard speed constant K o f /(cm·s) | Diffusion coefficient D/(cm 2 ·s -1 ) |
Steady state polarization | 30 | 0.458 | 4.83×10 -8 | 2.56×10 -10 | |
Rotating electrode method | 29 | 0.468 | 1.13×10 -7 | 5.35×10 -10 | 3.78×10 -6 |
Scanning potential method | 31 | 0.501 | |||
Convolutional scanning potential method | 31 | 0.475 | 4.55×10 -8 | 1.99×10 -10 | |
Current step method | 35 | 0.505 | 7.48×10 -9 | 2.31×10 -10 | 6.82×10 -6 |
Potential step method | 30 | 0.515 | 4.00×10 -8 | 1.10×10 -10 | 4.28×10 -6 |
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Through the analysis of steady-state polarization curve and cyclic voltammetry curve, the history of MnO 2 deposition reaction is Mn 2+ ===Mn 4+ +2e -
Mn 4+ +2H 2 O===MnO 2 +4H +
Some of them Mn 3+ +2H 2 O===MnOOH+3H +
MnOOH===MnO 2 +H + +e -
Analysis conclusion:
1 The process of anodic deposition of electrolytic manganese dioxide from neutral manganese sulfate solution is Mn 2+ ===Mn 3+ +e - (1)
Mn 3+ ===Mn4 + +e - (2)
Mn 4+ +2H 2 O===MnO 2 +4H + (3)
Part of Mn 3+ +2H 2 O===MnOOH+3H + (4)
MnOOH=MnO 2 +H + +e - (5)
Before the electrode potential is 1.2V, the reactions of (1), (2), (3), and (4) are mainly performed, and when the potential is >1.2V, the reaction of (5) occurs simultaneously.
The electrochemical reaction of 2Mn 2+ oxidation to MnO 2 is not to reverse the electrode process, and its irreversibility is caused by the first electron transfer retardation, that is, the electrode reaction Mn 2+ = Mn 3+ + e - is the control step of the reaction.
3 Various electrochemical test experiments measured the transfer coefficient of MnO 2 anode deposition reaction βn a = 0.46, exchange current density i 0 = 4.83 × 10 -8 A / cm 2 , standard reaction rate constant K ƒ ° = 1.98 × 10 - 10 cm / s, the liquid phase diffusion number of manganese ions D = 4.28 × 10 -6 cm 2 / s, at the overpotential η = 0.39V, the apparent reaction activation energy ΔE = 51.8kj / mol, and manganese ions participate in the electrode The number of stages of the reaction is grade 1.
4 In the MnO 2 anode deposition reaction, the acidity of the solution, ie the pH value, has a greater influence on the reaction, the acidity increases, and the reversibility of the reaction increases.
5 Increasing the temperature and lowering the acidity are beneficial to increase the current efficiency of the electrolytic MnO 2 .
In industrial production, the main technical conditions of MnO 2 electrolysis process are: bath temperature 95~100°C, bath MnSO 4 concentration 90~110g/L, bath H 2 SO 4 acidity 35~40g/L, current density 50~80A/ m 2 , the tank voltage is 2.5~4.0V, and the electrolysis period, the troughing period, and the clear cathode period are all about 15, depending on the specific conditions.
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