Thermodynamic and electrochemistry analysis of the zinc electrodeposition in NH4Cl-NH3 electrolytes on Ti, Glassy Carbon and 316L Stainless Steel

Thermodynamic diagrams, X-ray diffraction and electrochemical analysis are conducted to evaluate the solution chemistry of the Zn(II)-NH4Cl-NH3-H2O system as well as the feasibility of zinc electrorecovery at different pH values. Titanium, Glassy Carbon and 316L Stainless Steel substrates are used as cathode materials. At any region of pH, multiple Zn(II) complexes coexist in solution, but only one predominates. While chloro- (ZnCl42−), ternary (ZnNH3Cl3− and Zn(NH3)3Cl+) and amino-complexes (Zn(NH3)42+) dominate the region of low, neutral and alkaline pH, respectively, the solubility of the system is limited by the formation of two solids, Zn(NH3)1.6Cl0.4(s) and ZnO(s) at neutral and alkaline conditions, respectively. The thermodynamic and electrochemical evaluations reveal that the potential required to deposit Zn(s) becomes more negative as the pH value is increased, whereby the reduction of amino-complexes demand a larger amount of energy compared to the chloro-complexes. This effect is accounted for different ligand substitution mechanisms operating for the chloro- and amino-complexes of Zn(II). The onset of the zinc deposition relies on the cathode material and is accompanied by the HER regardless of the substrate utilized. The surface of Stainless Steel electrode exhibits the smallest overpotential, followed by the Glassy Carbon, and Ti cathodes where the TiO2(s) (native film) plays a determining role during the deposition. Higher current efficiencies are obtained on every substrate as the pH value is increased. Experimental conditions around neutral pHs (5.5 < pH < 8) are potentially suitable to perform the zinc electrodeposition for this system.