An Investigation On Effects Of Indium On Electrochemical Performances Of Zinc Electrode In KOH Solution

The effects of indium on the electrochemical performances of zinc in KOH solution are investigated by electrochemical and nonelectrochemical methods, including linear potential sweep, cyclic voltammogram, chronocoulometry, chronopotiometry, AC impedance, hydrogen evolution measurement, XRD, SEM and ICP. The investigation involves: (1) the inhibition of zinc corrosion by organic and inorganic inhibitors, (2) the electrochemical performances of indium and zinc in 3mol/L KOH solution, (3) the influences of electroless-plated indium on zinc and in zinc-indium alloys on the electrochemical performances of zinc in 3mol/L KOH. Following results are obtained:1. There is a significant synergism for zinc corrosion when some kinds of organic and inorganic inhibitors are used together. The reaction between Zinc and In2O3 can take place kinetically, resulting in the deposition of indium on zinc..2. Hydrogen evolution process on indium in 3mol/L KOH solution is controlled by electrochemical reaction step. Anodic oxidation of indium takes place in a dissolution-precipitation model. The anodic oxidation of indium proceeds by two steps at active region, forming [In(OH)x]1-x (x≥3) from Indium and [m(OH)3+y]y'from [In(OH)x]1-xIn = In+ + e- (1)In+ + xOH- --> [In(OH)x]1-x (x≥3) (2)[In(OH)x]1-x+ (3+y-x)OH- = [In(OH)3+y]y-(y≥1) + 2e- (3)At lower potential, the formation of [In(OH)x]1-x is fast but the formation of [In(OH)3+y]y-is slow, so the reaction behaves as one electron transfer. At higher potential, both formations of [In(OH)x]1-x and [In(OH)3+y]y- are fast, so the reaction behaves as three electrons transfer. [In(OH)3]ad deposits when the concentration of [In(OH)3+y]y- near the electrode surface is satuated. The reactions at active region are controlled by mass transport step. [In(OH)3]ad is not stable and can dissolve into the solution:[In(OH)3]ad + yOH- = [In(OH)3+y]y-(4)At higher potential, indium is oxidized directly to compact indium oxide under thedeposited [In(OH)3]ad film. The stability of the indium oxide depends on the formation potential.3. Hydrogen evolution process on zinc in 3mol/L KOH solution is also controlled by electrochemical reaction step. Zinc corrosion takes place at open circuit potential in 3mol/L KOH solution. The anodic oxidation of zinc also follows a dissolution-precipitation model. Zinc is oxidized to ZnO22~ or Zn(OH)42- at active region:Zn + 4OH-=Zn(OH)42- + 2e- (5)Zn + 4OH-=ZnO22- + 2H2O + 2e (6)When the concentration of these anions reaches to saturation, Zn(OH)2 is deposited on the surface of zinc resulting in the first anodic oxidation peak.Then the second anodic peak appears due to the formation of compact zinc oxide under the deposited Zn(OH)2- Both Zn(OH)2 and ZnO are unstable in 3mol/L KOH solution and can dissolve into solution.ZnO + H2O = Zn(OH)2 (7)Zn(OH)2 + 2 OH* = Zn(OH)42- (8)4. Hydrogen evolution process on zinc with electroless-deposited indium and Zn-In alloys in 3mol/L KOH solution is controlled by electrochemical reaction step. Both electroless-deposited indium and indium inside Zn-In alloys can increase hydrogen evolution overpotential and reaction resistance on zinc electrode in 3mol/L KOH solution, which inhibits zinc corrosion. The inhibition increase with the increase of indium layer thickness and indium content in Zn-In alloys.Electroless-deposited indium with suitable thickness can improve the activity of zinc. Electroless-deposited indium can also improve the rechargeability of zinc, which is valuable to the application of zinc in secondary batteries.The anodic dissolution of zinc with deposited indium is different to that of zinc in Zn-In alloys. The former takes place in solid indium solution. The latter in alloys, which is the same as pure metal Zinc.