Why Is MnO2 Deposited at Positive Potentials in Electrochemical Reactions?

[angela2007]

Mn2+ + 2 H2O - MnO2 + 4 H+ + 2 e− ( −1.22 V), this reaction is acucurate. the standard potential is negative, then why all the work for the formation of MnO2 need to be done in anodic potential?

 

[symbolipoint]

According to what you wrote for the reaction (interpreting for one wrong "-" symbol, needs to be an arrow), the half-reaction is an oxidation and requires energy in order to occur. The negative half-reaction potential tells you that the half reaction is not spontaneous and so requires energy to be supplied.

 

[angela2007]

my question is in three electrode system: working electrode, Pt counter electrode, and Ag/AgCl reference electrode.
for example, Mn2+ + 2 H2O -MnO2 + 4 H+ + 2 e− ( −1.22 V), the deposition of Mno2 will be accomplished on positive potential. Cr3+ +3e- Cr ( −0.74 V), the depsotion of metallic Cr will be accomplished on negative potential.
I Know for both the half-reactions, they are non-spontaneous and need external electric energy. My point is why the deposition of Mno2 is not done in negative potential range.