The Role of Carbonate and Phosphate in the Electro-Catalytic Water Oxidation by NiIIL2+

Ariela Burg, Chemical Engineering Department, , SCE, Beer Sheva, Israel
Yaniv Wolfer , Chemistry Department, Bgu, Beer Sheva, Israel
Dror Shamir, Chemistry Department, Nuclear Research Center Negev, Beer Sheva, Israel
Haya Kornweitz , Sciences Department, Ariel University, Ariel, Israel
Yael Albo, Chemical Engineering Department, Ariel University, Ariel, Israel
Eric Maimon, Chemistry Department, Nuclear Research Center Negev, Beer Sheva, Israel
Dan Meyerstein, Sciences Department, Ariel University, Ariel, Israel


Water splitting photolytically or electrochemically is of major importance in developing green energy sources. These processes consist of two halves: the reduction of water and the oxidation of water, the latter is the more difficult one. All such processes require a catalyst, usually consisting of a transition-metal compound, if possible using a low cost metal. Nickel complexes are often used as catalysts for this goal, by homogenous and heterogeneous catalysis ^[1]. Recently it was shown that Ni^IIL²⁺, where L = a tetraazamacrocyclic ligand, are good electro-catalysts for water oxidation via the formation of Ni^IVL complexes, in neutral phosphate buffered solutions^[2]. The role of phosphate in the process was not elucidated.  Cyclic voltammetry experiments and DFT calculations point out that Ni^IIL²⁺ is oxidized in the presence of an axial ligand X^n-, X = PO₄^3- or CO₃^2-, to form Ni^IVLX₂. This is followed by a ligand exchange reaction:

(1) Ni^IVLX₂^4-2n  + H₂O  →  L(X)Ni^IVOH^4-(n+1)  +  HX^(n-1)-

This reaction is followed either by:

                                      H₂O        

(2) 2L(X)Ni^IVOH^4-(n+1)  → 2L(X)Ni^IIIOH^3-(n+1) + H₂O₂ + 2H₃O⁺

or by:

>
(3) L(X)Ni^IVOH^4-(n+1) + H₂O → Ni^IIL²⁺ + H₂O₂ + X^n- + H₃O⁺

The results prefer reaction (2) to reaction (3). The results point out that in the presence of carbonate also reaction (4) or (5) contribute to the oxidation process.

(4) L(CO₃)Ni^IVOH⁺ + HCO₃^-  → Ni^IIL²⁺ + HCO₄^- + HCO₃^-

(5) L(CO₃)Ni^IVOH⁺ + HCO₃^-  → Ni^IIL²⁺ + C₂O₆^2- + H₂O

Thus carbonate acts not only as an axial ligand that lowers the redox potential of the central cation but is also involved in the water oxidation process.   

[1]        aM. Gao, W. Sheng, Z. Zhuang, Q. Fang, S. Gu, J. Jiang, Y. Yan, Journal of the American Chemical Society 2014, 136, 7077-7084; bY. Han, Y. Wu, W. Lai, R. Cao, Inorganic Chemistry 2015, 54, 5604-5613.

[2]        M. Zhang, M.-T. Zhang, C. Hou, Z.-F. Ke, T.-B. Lu, Angewandte Chemie 2014, 53, 13042-13048.