Water oxidation catalyst having low overpotential for oxygen evolution reaction

Abstract

The present invention discloses a water oxidation catalyst having composition Zn.sub.xCo.sub.(3-x)O.sub.4 for splitting water into oxygen and hydrogen gas and a process for the preparation thereof.

Claims

1. A process for the synthesis of a water oxidation catalyst having composition Zn.sub.xCo.sub.(3-x)O.sub.4, for splitting water into oxygen and hydrogen gases, wherein x is selected from 0, 0.2, 0.4, 0.6, 0.8 and 1.0, the process comprising the steps of: a) dissolving cobalt nitrate hexahydrate and zinc nitrate hexahydrate separately in solvent to afford solution of both; b) adding said both solutions to the solution of glycine in solvent followed by stirring to obtain a uniform solution, wherein the ratio of cobalt plus zinc nitrate or metal nitrate:glycine is in the range of 1:0.3 to 1:0.5; and c) evaporating the solution of step (b) at temperature in the range of 180 to 220 C. followed by burning the resulting thick mass to obtain a water oxidation catalyst.

2. The process as claimed in claim 1, wherein said solvent of step (a) is water.

3. The process as claimed in claim 1, wherein said water oxidation catalyst is Zn.sub.0.8Co.sub.2.2O.sub.4.

4. The process as claimed in claim 1, wherein the ratio of cobalt plus zinc nitrate or metal nitrate:glycine is in the range of 1:0.3 to 1:0.45.

5. The process as claimed in claim 1, wherein said water oxidation catalyst shows electrocatalytic activity with low overpotential in the range of 0.25 V to 0.27 V, at 10 mA cm.sup.2.

6. The process as claimed in claim 1, wherein said catalyst shows electrocatalytic activity with low overpotential of 0.254 V, at 10 mA cm.sup.2.

7. The process as claimed in claim 1, wherein said catalyst is comprised of spinel structure and spherical in shape with the size range of 7-10 nm.

8. The process as claimed in claim 1, wherein surface area of said water oxidation catalyst is in the range of 70-120 m.sup.2/g.

9. The process as claimed in claim 1, wherein said catalyst is used as oxygen evolution catalyst.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: Tafel plots of ZC0 (Co.sub.3O.sub.4), ZC2 (Zn.sub.0.2Co.sub.2.8O.sub.4) and ZC4 (Zn.sub.0.4Co.sub.2.6O.sub.4)

(2) FIG. 2: Tafel plots of ZC6 (Zn.sub.0.6Co.sub.2.4O.sub.4), ZC8 (Zn.sub.0.8Co.sub.2.2O.sub.4) and ZC10 (ZnCo.sub.2O.sub.4)

(3) FIG. 3: Variation of overpotential with Zn content in Zn.sub.xCo.sub.3-xO.sub.4

(4) FIG. 4: Variation of Co.sup.3+ spin contribution with Zn content in Zn.sub.xCo.sub.3-xO.sub.4

(5) FIG. 5: Quantitative oxygen evolution of ZC 8 at the applied current density of 5 mA/cm.sup.2

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

(6) The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

(7) The present invention provides a water oxidation catalyst having composition Zn.sub.xCo.sub.(3-x)O.sub.4, for splitting water into oxygen and hydrogen gases

(8) Wherein x is selected from 0, 0.2, 0.4, 0.6, 0.8 and 1.0, characterized in that said catalyst is prepared by the process comprising the steps of: a) dissolving cobalt nitrate hexahydrate and zinc nitrate hexahydrate separately in solvent to afford solution of both; b) adding said both solutions to the solution of glycine in solvent followed by stirring to obtain a uniform solution; c) evaporating the solution of step (b) at temperature in the range of 180 to 220 C. followed by burning the resulting thick mass to obtain a zinc cobalt oxide.

(9) In preferred embodiment, said solvent of step (a) is water.

(10) In another preferred embodiment, said water oxidation catalyst is Zn.sub.0.8Co.sub.2.2O.sub.4.

(11) In yet another preferred embodiment, said water oxidation catalyst shows electrocatalytic activity with low overpotential in the range of 0.25 V to 0.27 V, at 10 mA/cm.sup.2.

(12) In still another preferred embodiment, said water oxidation catalyst shows electrocatalytic activity with low overpotential of 0.254 V, at 10 mA/cm.sup.2.

(13) In still another preferred embodiment, said catalyst Zn.sub.xCo.sub.(3-x)O.sub.4 is used as oxygen evolution catalyst.

(14) The power diffraction and TEM studies shows that all the catalysts are comprised of spinel structure and spherical in shape with the size range of 7-10 nm. Gas absorption studies shows that all the catalysts have the surface area in the range of 70-120 m.sup.2/g. All the catalysts have been tested for their electrocatalytic oxygen evolution.

(15) From the Tafel plots of FIG. 1 and FIG. 2 the overpotential values are noted at 10 mA/cm.sup.2 (table 1). Table 1 shows overpotential of the respective catalysts obtained from Tafel plots.

(16) TABLE-US-00001 TABLE 1 Catalyst Overpotential(V) at 10 mA/cm.sup.2 Co.sub.3O.sub.4 0.358 Zn.sub.0.2Co.sub.2.8O.sub.4 0.325 Zn.sub.0.4Co.sub.2.6O.sub.4 0.306 Zn.sub.0.6Co.sub.2.4O.sub.4 0.287 Zn.sub.0.8Co.sub.2.2O.sub.4 0.254 ZnCo.sub.2O.sub.4 0.300

(17) From FIG. 3 it is observed that when the Zn content(x) in Zn.sub.xCo.sub.3-xO.sub.4 increases, the overpotential (at 10 mA/cm.sup.2) for oxygen evolution reaction decreases and achieves a minimum overpotential of 0.254 V when x=0.8 (Zn.sub.0.8Co.sub.2.2O.sub.4).

(18) FIG. 4 shows that when the Zn content in Zn.sub.xCo.sub.3-xO.sub.4 increases, the Co.sup.3+ spin contribution also increases. This can be due to conversion of low spin Co.sup.3+ to high spin Co.sup.3+. The increased population high spin Co.sup.3+ will increase the e.sub.g electron occupancy. Usually, in Cobalt oxides (Co.sub.3O.sub.4), Co.sup.3+ is in the low spin state and the magnetic moment is only from the Co.sup.2+. But, in the present invention, detailed magnetic analysis reveals the spin contribution from the Co.sup.3+.

(19) Quantitative oxygen evolution of catalyst is carried out by applying constant current to calculate the Faradaic efficiency of the reaction. Catalyst (ZC 8) coated on SS316 mesh is used as the anode and platinum foil is used as the cathode. The area of the SS316 mesh is 2 cm.sup.2 and the catalyst loading is 1 mg/cm.sup.2. A current of 5 mA/cm.sup.2 is applied for 6 hours and the gas mixture is analyzed by gas chromatography by every 2 hours. 91.4% Faradaic efficiency is obtained as shown in FIG. 5.

(20) The comparison of overpotential of precious metal oxides and the present catalyst is summarized in table 2. As shown in table 2, Zn.sub.0.8Co.sub.2.2O.sub.4 shows the lowest overpotential (0.254 V) for the oxygen evolution reaction which is comparable to the overpotential values reported for the precious metal oxides RuO.sub.2 and IrO.sub.2.

(21) TABLE-US-00002 TABLE 2 Catalyst Overpotential (V) at 10 mA/cm.sup.2 Commercial RuO.sub.2 0.366 RuO.sub.2 0.256 IrO.sub.2 0.320 Zn.sub.0.8Co.sub.2.2O.sub.4 0.254

(22) For the comparison of apparent overpotential of the different catalysts, the loading of the catalyst, electrolyte and electrode support (like Glassy carbon electrode, Ni foam, Ti substrate, etc) should be same. Even though they are different in different reports, to some extend one can roughly compare the overpotential values from the literature reports.

(23) In an embodiment, the present invention provides Magnetic moments study of said water oxidation catalyst Zn.sub.xCo.sub.(3-x)O.sub.4.

EXAMPLES

(24) Following examples are given by way of illustration therefore should not be construed to limit the scope of the invention.

Example 1: Synthesis of Water Oxidation Catalyst

(25) Stoichiometric amounts of Cobalt nitrate (Co(NO.sub.3).sub.2.6H.sub.2O) and Zinc nitrate Zn(NO.sub.3).sub.2.6H.sub.2O), separately dissolved in minimum amount of distilled water. And added to the particular molar solution of glycine in a minimum amount of distilled water and stirred to obtain a uniform solution. The mixed solution is evaporated at 200 C., on a hot plate. After the evaporation of water, the resulting thick mass is burnt spontaneously to obtain a zinc cobalt oxide. Table 3 shows the mole ratio of metal ions and glycine used in the synthesis of catalyst [ZC0 (Co.sub.3O.sub.4), ZC2 (Zn.sub.0.2Co.sub.2.8O.sub.4), ZC4 (Zn.sub.0.4Co.sub.2.6O.sub.4), ZC6 (Zn.sub.0.6Co.sub.2.4O.sub.4), ZC8 (Zn.sub.0.8Co.sub.2.2O.sub.4) and ZC10 (ZnCo.sub.2O.sub.4)].

(26) Co(NO.sub.3).sub.2.6H.sub.2O and Zn(NO.sub.3).sub.2.6H.sub.2O, taken in the required molar ratio, ranging from 0:3 to 1:2, are dissolved in minimum amount of water. Glycine is dissolved in minimum amount of water where the molar ratio of metal nitrate to glycine is in the range of 1:0.45 to 1:0.30. (For example, for the synthesis of Co.sub.3O.sub.4(ZC 0), 5 g of cobaltous nitrate was dissolved in 10 ml water and 0.58 g glycine dissolved in 5 ml water and then mixed together. Similarly, for ZnCO.sub.2O.sub.4(ZC 10), 1.70 g of zinc nitrate in 5 ml water, 3.33 g cobalt nitrate in 10 ml water, 0.39 g glycine in 5 ml water and mixed together).

(27) Table 3 shows details of composition and mole ratio of metal ions and glycine used.

(28) TABLE-US-00003 TABLE 3 Mole ratio of Metal Sample Mole ratio of Mole ratio of nitrate to code Composition Zn(NO.sub.3).sub.26H.sub.2O Co(NO.sub.3).sub.26H.sub.2O Glycine ZC 0 Co.sub.3O.sub.4 0 3 1:0.45 ZC 2 Zn.sub.0.2Co.sub.2.8O.sub.4 0.2 2.8 1:0.42 ZC 4 Zn.sub.0.4Co.sub.2.6O.sub.4 0.4 2.6 1:0.39 ZC 6 Zn.sub.0.6Co.sub.2.4O.sub.4 0.6 2.4 1:0.36 ZC 8 Zn.sub.0.8Co.sub.2.2O.sub.4 0.8 2.2 1:0.345 ZC 10 ZnCo.sub.2O.sub.4 1 2 1:0.30

(29) The powder X-ray diffraction data is summarized in table 4.

(30) TABLE-US-00004 TABLE 4 Sample Code Crystallite size (nm) ZC 0 7 ZC 2 6 ZC 4 11 ZC 6 12 ZC 8 10 ZC 10 11

(31) The BET surface area of catalysts from gas adsorption measurement is summarized in table 5.

(32) TABLE-US-00005 TABLE 5 Sample Code BET surface area (m.sup.2/g) ZC 0 70 ZC 2 76 ZC 4 106 ZC 8 119 ZC 10 97

Example 2: Synthesis of Water Oxidation Catalyst

(33) Stoichiometric amounts of Cobalt nitrate (Co(NO.sub.3).sub.2.6H.sub.2O) and Zinc nitrate Zn(NO.sub.3).sub.2.6H.sub.2O), separately dissolved in minimum amount of distilled water. And added to the particular molar solution of glycine in a minimum amount of distilled water and stirred to obtain a uniform solution. The mixed solution is evaporated at 200 C., on a hot plate. After the evaporation of water, the resulting thick mass is burnt spontaneously to obtain a zinc cobalt oxide.

Example 3: Electrocatalytic Oxygen Evolution Measurement Study

(34) The power diffraction and TEM studies shows that all the catalysts are comprised of spinel structure and spherical in shape with the size range of 7-10 nm. Gas absorption studies shows that all the catalysts have the surface area in the range of 70-120 m.sup.2/g. All the catalysts have been tested for their electrocatalytic oxygen evolution. The catalyst ink has been made by mixing the catalyst with carbon black and nafion (binder) in the ratio of 75:20:5 and dispersed in ethanol water mixture. And further it is sonicated for 30 minutes. Then the catalyst ink is coated on the surface of glassy carbon electrode with the loading of 1 mg/cm.sup.2. And it is dried under table lamp for few minutes. For all the electrochemical measurements, 3 mm diameter glassy carbon electrode, Hg/HgO electrode and platinum foil are used as working, reference and counter electrode, respectively. Freshly prepared 0.1 M KOH is used as electrolyte. For Tafel plot data collection, a constant anodic current was applied for 5 min. After that, the steady-state potential was noted. Likewise, ten different currents are applied per decade from 10.sup.4 to 10.sup.2 A/cm.sup.2. The electrolyte solution was stirred at 400 rpm throughout the experiment to reach the steady state. Before starting the measurement, the solution resistance was measured using iR test function. The resistance value was used to correct the uncompensated resistance manually. Table 6 shows the overpotential values obtained by the use of different catalyst composition at 10 mA/cm.sup.2.

(35) TABLE-US-00006 TABLE 6 Sample code Overpotential(V) at 10 mA/cm.sup.2 ZC 0 0.358 ZC 2 0.325 ZC 4 0.306 ZC 6 0.287 ZC 8 0.254 ZC 10 0.300

(36) Usually, in Cobalt oxides (Co.sub.3O.sub.4), Co.sup.3+ is in the low spin state and the magnetic moment is only from the Co.sup.2+. But, in the present invention, detailed magnetic analysis reveals the spin contribution from the Co.sup.3+. FIG. 4 shows that when the Zn content in Zn.sub.xCo.sub.3-xO.sub.4 increases, the Co.sup.3+ spin contribution also increases. This can be due to conversion of low spin Co.sup.3+ to high spin Co.sup.3+. The increased population high spin Co.sup.3+ will increase the e.sub.g electron occupancy. And the direct comparison of e.sub.g occupancy and overpotential is reported in the perovskite systems. But, in spinels, no such report found.

Example 4: Quantitative Oxygen Evolution

(37) Quantitative oxygen evolution has been done by applying constant current to calculate the Faradaic efficiency of the reaction. Catalyst (ZC 8) coated on SS316 mesh is used as the anode and platinum foil is used as the cathode. The area of the SS316 mesh is 2 cm.sup.2 and the catalyst loading is 1 mg/cm.sup.2. A current of 5 mA/cm.sup.2 is applied for 6 hours and the gas mixture is analyzed by gas chromatography in every 2 hours. 91.4% Faradaic efficiency is obtained as shown in FIG. 5.

Example 5: Magnetic Moment Study

(38) Table 7 shows that the Magnetic moment study of said water oxidation catalyst Zn.sub.xCo.sub.3-xO.sub.4.

(39) TABLE-US-00007 TABLE 7 Sample code Curie constant (emu K mol.sup.1) Magnetic moment (.sub.B) ZC 0 2.87 4.79 ZC 2 2.48 4.45 ZC 4 2.2 4.20 ZC 6 2.05 4.05 ZC 8 1.55 3.52 ZC 10 1.58 3.56

(40) The catalyst also can be considered as novel with respect to the magnetic moment. Because, Co.sub.3O.sub.4 and ZnCo.sub.2O.sub.4 synthesized by auto combustion method in the present invention possess higher magnetic moment than the reported magnetic moments.

(41) Table 8 shows comparison Magnetic moment study of Co.sub.3O.sub.4, ZnCo.sub.2O.sub.4 and Zn.sub.xCo.sub.3-xO.sub.4.

(42) TABLE-US-00008 TABLE 8 magnetic moment Reported magnetic in the present Name moment (in .sub.B) invention (in .sub.B) Co.sub.3O.sub.4 4.27 (ref 1) 4.79 ZnCo.sub.2O.sub.4 2.79 (ref 2) 3.56 1. P. Dutta, M. S. Seehra, S. Thota, J. Kumar, Journal of Physics-Condensed Matter 2008, 20, 015218. 2. P. Cossee, Recueil Des Travaux Chimiques Des Pays-Bas-Journal of the Royal Netherlands Chemical Society 1956, 75, 1089-1096.

ADVANTAGES OF THE INVENTION

(43) 1. Comparable electrocatalytic activity with that reported for precious metal oxide catalysts, but with low overpotential for oxygen evolution 2. Easy method of synthesis and can be applicable for larger scale 3. Catalyst are based on non-precious and earth abundant elements 4. Process of preparation of catalyst is autocombustion, quick process, half-one hour process, and technique is simple.