Non-noble metal electrocatalysts for oxygen depolarized cathodes and their application in chlor-alkali electrolysis cells

Abstract

A simplified and efficient method for preparing non-noble metal catalysts for oxygen reduction reaction (ORR) based on nitrogen containing metal organic framework (MOF) is provided. The method includes formation of a first MOF product through a mechano-chemical reaction between a first transition metal compound and a first organic ligand in the presence of a catalyst. It further includes formation of a second MOF product incorporating a second transition metal and a second organic ligand into the first-MOF product. The second MOF product is converted into an electrocatalyst via pyrolysis, and optionally post-treatment. The electrocatalysts are applicable in various electrochemical systems, including oxygen depolarized cathodes (ODC) for chlorine evolution.

Claims

1. A method of synthesizing an electrocatalyst for an oxygen reduction reaction, the method comprising: (a) reacting, using a mechano-chemical reaction, a first organic ligand, a first transition metal or an oxide or a salt thereof, and a catalyst, thereby generating a partially or fully formed first metal organic framework (MOF) product containing the first transition metal, wherein the reaction is performed in the absence of solvent or in the presence of a trace amount of a solvent, and the catalyst is an acid or an inorganic salt; (b) mixing the first MOF product with a second organic ligand and a second transition metal or a salt thereof, whereby the second organic ligand and the second transition metal or the salt thereof coat the surface of and/or incorporate into pores of the first MOF product to generate a second MOF product; and (c) subjecting the second MOF product to pyrolysis, whereby most of the first transition metal evaporates, yielding the electrocatalyst.

2. The method according to claim 1, wherein the mechano-chemical reaction of step (a) comprises ball milling.

3. The method according to claim 1, wherein step (b) comprises a mechano-chemical reaction.

4. The method according to claim 3, wherein the mechano-chemical reaction comprises ball milling.

5. The method according to claim 1, wherein the first and second MOF products are not separated from other reaction components.

6. The method according to claim 1, wherein the first MOF product contains at least two different transition metals.

7. The method according to claim 1, wherein the second MOF product contains at least two different transition metals.

8. The method according to claim 1, wherein at least one of the first organic ligand and the second organic ligand is a heteroatom-containing organic molecule, the heteroatom being capable of catalyzing an oxygen reduction reaction.

9. The method according to claim 8, wherein the heteroatom-containing organic molecule comprises one or more heteroatoms selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur.

10. The method according to claim 1, wherein the pyrolysis is carried out at a temperature from about 600 C. to about 1100 C.

11. The method according to claim 1, wherein the pyrolysis is carried out in the presence of an inert gas carrier selected from the group consisting of argon, helium and nitrogen, or in the presence of a reductive gas carrier selected from the group consisting of ammonia, pyridine, and acetonitrile.

12. The method according to claim 1, further comprising, after step (c), subjecting the electrocatalyst to one or more treatments selected from the group consisting of acid washing, ball milling, and heating in an inert or reductive gas.

13. The method according to claim 12, wherein the electrocatalyst is heated in an inert gas and the inert gas is selected from the group consisting of argon, helium, and nitrogen, or wherein the electrocatalyst is heated in a reductive gas and the reductive gas is selected from the group consisting of ammonia, pyridine, and acetonitrile.

14. The method according to claim 1, further comprising: (d) etching the product of the pyrolysis in (c) with a 1-5 M acid solution; and (e) performing a second pyrolysis on the produce of (d) in a nitrogen-containing atmosphere, such as NH.sub.3, at a temperature in the range from about 850 C. to about 1100 C.

15. The method according to claim 1, wherein the second transition metal is selected from the group consisting of iron, cobalt, manganese, nickel, copper, zinc, chromium, and combinations thereof.

16. The method according to claim 1, wherein the first transition metal is selected from the group consisting of zinc, molybdenum, cobalt, iron, nickel, copper, manganese, and combinations thereof, and wherein the first and the second transition metals have oxidation states selected from the group consisting of all known oxidation states for the respective transition metal.

17. The method according to claim 1, wherein the first and the second transition metals have oxidation states selected from the group consisting of all known oxidation states for the respective transition metal.

18. The method according to claim 1, wherein the first organic ligand is selected from the group consisting of imidazole, methylimidazole, pyridine, pyridine derivatives, pyrimidine, triazole, tetrazole, napthylene, and napthyridine.

19. The method according to claim 1, wherein the second organic ligand is selected from the group consisting of phenanthroline, porphyrin, imidazole, pyridine, pyrimidine, and triazole.

20. The method according to claim 1, wherein a trace of solvent is present at less than 1 wt %, and the solvent is selected from the group consisting of dimethylformamide, tetrahydrofuran, diethyl ether, dimethylsulfoxide, ethanol, isopropanol, methanol, and water.

21. The method according to claim 1, wherein the molar ratio of the organic ligand to the first transition metal oxide or salt is from about 2:1 to about 4:1.

22. The method according to claim 1, wherein steps (a) and (b) are carried out in a single reaction vessel.

23. The method according to claim 1, wherein the second transition metal is in the form of nanoparticles or a colloid accommodated within pores of the MOF.

24. The method according to claim 1, wherein the electrocatalyst is cross-linked as a result of the pyrolysis in step (c).

25. The method according to claim 1, wherein a trace of solvent is present in and a molar ratio of the first transition metal oxide to the solvent is from about 50:1 to about 3000:1.

26. The method according to claim 1, wherein the molar ratio of the first transition metal to the second transition metal is from about 161:1 to about 10:1.

27. The method according to claim 1, wherein the molar ratio of the first transition metal to the second organic ligand is from about 4:1 to about 0.17:1.

28. The method according to claim 1, wherein the amount of first transition metal incorporated into the electrocatalyst is from about 1 wt % to about 3 wt %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram for synthesizing non-noble metal catalysts (electrocatalysts) for ORR using a solid state reaction such as ball milling. Metal organic frameworks (MOFs) are used as precursors in the synthesis.

(2) FIG. 2 is a plot comparing the X-ray diffraction pattern of MOFs synthesized through solid state reactions with different ball milling times to a simulated X-ray diffraction pattern based on ZIF-8 crystal structure. The reaction products acquire the characteristic ZIF-8 crystal structure after 30 minutes of ball milling. The structure endures even with an additional 150 minutes of ball milling.

(3) FIG. 3 is a plot showing the X-ray diffraction pattern of the MOF and FePhenMOF precursors synthesized through solid state reactions compared to a simulated X-ray diffraction pattern based on ZIF-8 crystal structure. Ball milling phenanthroline and iron acetate with as-prepared MOF (i.e., MOF prepared in the previous step) doesn't disturb the ZIF-8 crystal structure of the MOF.

(4) FIG. 4 is a plot showing the X-ray diffraction pattern of a FePhenMOF catalyst obtained after a first pyrolysis in argon and that obtained after a second pyrolysis in NH.sub.3.

(5) FIG. 5 is a graph showing oxygen reduction reaction polarizations of electrolytic reactions obtained using a rotating ring disk electrode (RRDE) with FePhenMOF catalysts synthesized through (i) solid state reaction (SSR) and (ii) traditional solution reaction (SR) alongside those obtained with platinum as benchmark in 0.1 M HClO.sub.4 at room temperature. The non-noble metal catalysts were loaded at 0.6 mg/cm.sup.2.

(6) FIG. 6 is a graph of oxygen reduction reaction polarizations of electrolytic reactions obtained using rotating ring disk electrodes with FePhenMOF catalysts synthesized through the solid state reaction in 1M HCl or 1M HClO.sub.4 at room temperature. The state of the art Pt/C and Rh.sub.xS.sub.y/C catalysts are applied as benchmarks.

(7) FIG. 7 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF catalysts synthesized through the solid state reaction with different loadings at 0.1M HClO.sub.4 at room temperature.

(8) FIG. 8 is a set of Koutechy-Levich plots of rotating ring disk electrode measurements with FePhenMOF catalysts synthesized through the solid state reaction with different loadings at 0.1M HClO.sub.4 at room temperature.

(9) FIG. 9 is a set of Tafel plots of rotating ring disk electrode measurements with FePhenMOF catalysts synthesized through the solid state reaction with different loadings at 0.1M HClO.sub.4 at room temperature.

(10) FIG. 10 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using rotating ring disk electrodes with FePhenMOF catalysts synthesized through the solid state reaction at 0.1 M NaOH at room temperature. The state of the art Pt/C and Ag catalysts are applied as benchmarks.

(11) FIG. 11 is a schematic diagram for the half-cell design for the chlor-alkali electrolysis cell. The cathode is oxygen reduction reaction, while the anode is the oxygen evolution reaction. 5 M NaOH electrolytes are pumped through the cell to simulate the actual chlor-alkali electrolysis cells in industry at the cathode side.

(12) FIG. 12 is a graph of voltage responses as a function of applied current density of a chlor-alkali electrolysis half-cell using FePhenMOF synthesized through the solid state reaction, Pt/C and Ag (Denora) catalysts with pure oxygen as feedstock at the cathode side. 5 M NaOH is utilized as electrolytes.

(13) FIG. 13 is a graph of voltage responses as a function of applied current density of a chlor-alkali electrolysis half-cell using FePhenMOF catalysts synthesized through the solid state reaction, Pt/C and Ag (Denora) catalysts with 95% oxygen as feedstock at the cathode side. 5 M NaOH is utilized as electrolytes.

(14) FIG. 14 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF and FePyridineMOF catalysts synthesized through the solid state reaction at 0.1M HClO.sub.4 at room temperature. The two catalysts have different organic ligands for the filling/coating compounds. The loading of the non-noble metal catalysts is 0.6 mg/cm.sup.2.

(15) FIG. 15 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF catalysts synthesized through the solid state reaction at 0.1M HClO.sub.4 at room temperature. The catalysts have different ratios of iron acetate in starting materials. The loading of the non-noble metal catalysts is 0.6 mg/cm.sup.2.

(16) FIG. 16 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF catalysts synthesized through the solid state reaction at 0.1M HClO.sub.4 at room temperature. The catalysts have different ratios of phenanthroline as filling/coating compounds in starting materials. The loading of the non-noble metal catalysts is 0.6 mg/cm.sup.2.

(17) FIG. 17 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF catalysts synthesized through the solid state reaction at 0.1M HClO.sub.4 at room temperature. The catalysts were ball milled for various periods during step (a) and step (b). The loading of the non-noble metal catalysts is 0.6 mg/cm.sup.2.

(18) FIG. 18 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF, PhenMOF and MOF catalysts synthesized through the solid state reaction at 0.1M HClO.sub.4 at room temperature. The loading of the non-noble metal catalysts is 0.6 mg/cm.sup.2. The presence of Fe as a second transition metal improves the performance of those as-prepared catalysts dramatically.

(19) FIG. 19 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF catalysts synthesized through the solid state reaction with and without acid wash as post treatment at 0.1M HClO.sub.4 at room temperature. The loading of the non-noble metal catalysts is 0.6 mg/cm.sup.2.

(20) FIG. 20 is a graph of oxygen reduction reaction polarizations of electrolytic reactions using a rotating ring disk electrode with FePhenMOF catalysts synthesized through the solid state reaction with different batch sizes at 0.1M HClO.sub.4 at room temperature. The loading of the non-noble metal catalysts is 0.6 mg/cm.sup.2.

(21) FIG. 21 is a Fourier transform of EXAFS (extended X-ray absorption fine structure) spectra of FePhenMOF electrocatalyst with only FeN.sub.4 as active sites.

DETAILED DESCRIPTION OF THE INVENTION

(22) The present invention provides a method for synthesizing metal organic frameworks using mechano-chemical reactions for use in the production of an electrocatalysts for oxygen reduction reactions. The method has several unique features. In contrast to the conventional solution reaction methods, no solvent or only trace amounts of a solvent is used in the method.

(23) As such, the method does not require a step for separating reaction intermediates/products from unused solvent. In addition, the method eliminates the need for using excessive amounts of reactants, thereby reducing cost. Further, all of the steps of the method are carried out in a single vessel, making the method more efficient and cost effective.

(24) FIG. 1 shows a schematic representation of one embodiment of the method to synthesize an electrocatalyst for an oxygen reduction reaction. In this embodiment the mechano-chemical reaction between a transition metal oxide and an organic ligand is carried out through ball milling in the presence of a catalyst. It was observed that certain acids and ammonium salts can facilitate the synthesis of MOF. Without intending to be limited by any theory or mechanism of action, it is believed that acid and base properties of H.sup.+ and NH.sub.4.sup.+ ions, respectively, aid in the reaction. Using this method, the reaction time for producing the electrocatalyst was significantly reduced relative to the time required using conventional wet chemistry approach. For example, while synthesis of ZIF-8 MOF using solution reaction requires over 24 hours, mechanically activated synthesis, i.e., synthesis using mechano-chemical reaction, allows the MOF to be obtained within 1 hour (FIG. 2). Any one of the inorganic salts selected from the group consisting of NaNO.sub.3, KNO.sub.3, NH.sub.4NO.sub.3, Na.sub.2SO.sub.4, and (NH.sub.4).sub.2SO.sub.4, can be used as a catalyst. Also, any one of the following acids, namely, formic acid, acetic acid, propionic acid and citric acid may be used as a catalyst to form the MOF. The MOF obtained is not used directly to prepare catalysts for ORR. Instead, a second transition metal (Fe, Co, etc.) and a second organic ligand are incorporated into the MOF. The second transition metal and organic ligand serve as part of the MOF or associate with MOF by incorporation into its pores or coating its surface. Transition metals that may be used as a second transition metal include Fe, Co, Ni, Cu, Mn, Cr, and Ta. In certain embodiments, phenanthroline and iron acetate are utilized as the second filling/coating organic ligand and the second transition metal source, respectively. The use of the second transition metal and the second organic ligand does not disturb the crystal structure of ZIF-8. See the X ray diffraction pattern in FIG. 3 demonstrating that the MOF structure (solid line) is retained even after additional two hours of ball milling.

(25) The step of pyrolysis leads to coordination of the evenly distributed non-noble transition metals with heteroatoms organized into graphene-like carbon sheets. The pyrolysis step may be carried out once or twice, each time in the presence of an inert or a reductive gas. Optionally, etching with strong acid (1-5M) can be performed between first and second pyrolysis steps. In certain instances, the structure of FePhenMOF precursors turned from ordered crystalline (ZIF-8) to amorphous carbon after the first heat treatment, but there was no significant change in the structure after the second heat treatment (FIG. 4). The amorphous material formed after first pyrolysis exhibited porous, alveolar, and interconnected hollow structures as seen by transmission electron microscopy, with abundant pores of 50-100 nm. The ORR polarization of electrolytic reactions measured using a rotating disk electrode (RRDE) with a FePhenMOF catalyst (prepared as described in Example 1 below and referred as sample 1) in 0.1M HClO.sub.4 at room temperature is shown in FIG. 5 (light dotted line). ORR polarizations obtained using with Pt/C (solid line) and FePhenMOF synthesized through a solution reaction as the catalyst (bold broken line) are shown for comparison.

(26) Anion poisoning is a common problem in electrocatalysis in aqueous media and results from strong interaction of catalytic metals (Pt, Rh, Ru, etc.) with impurities at potentials above potential of zero charge (PZC). The poisoning blocks access of the reactants (e.g., oxygen in ORR reactions) to the active centers on the metal surface, resulting in increased overpotential. In acidic environment water molecules act as weak anionic species and interact with the metallic surface through the oxygen atoms of hydroxide ions. More electronegative moieties such as chloride or bromide or other anions when present replace the hydroxide ions. The metal-anion interaction grows in strength with increased positive potentials, which is specifically challenging for oxygen reduction reactions as the ORR onset is preferred to occur at high potentials. Even small concentrations of anions result in significant losses in the activity of the catalyst. ORR polarization of electrolytic reactions (in 1M HCl or 1M HClO.sub.4), carried out using a FePhenMOF electrocatalyst (sample 1) prepared according to the method described above show the electrocatalyst to be superior compared to the state of the art catalysts in resisting anion poisoning, e.g., chloride ion poisoning (see FIG. 6).

(27) ORR polarizations obtained through the use of a FePhenMOF electrocatalyst (sample 1) loaded onto an electrode in different amounts, measured using a RRDE in 0.1M HClO.sub.4 at room temperature are shown in FIG. 7. The corresponding Koutechy-Levich plots and Tafel plots are shown in FIG. 8 and FIG. 9, respectively. Table 1 lists the kinetic current densities and exchange current densities derived from the Koutechy-Levich plots and Tafel plots at various loadings obtained from RRDE in acid. The Tafel slopes and intercepts of the ORR polarization curves under RRDE conditions using the FePhenMOF catalyst in acid are listed in Table 2.

(28) TABLE-US-00001 TABLE 1 Kinetic current densities and exchange current densities for FePhenMOF catalyst (sample 1) at various loadings obtained from RRDE in acid. Loading jk J.sup.0 (mA/cm.sup.2) k.sub.e (cm/s) (mg/cm.sup.2) n (mA/cm.sup.2) 1E6 1E9 100 3.89 6.01 0.93 0.11 0.25 200 3.92 37.45 0.81 1.30 2.91 400 3.90 7.67 0.85 1.26 2.83 600 3.82 5.71 0.92 0.51 1.17

(29) TABLE-US-00002 TABLE 2 Tafel slopes and intercepts of the polarization curves for ORR under RRDE conditions with FePhenMOF catalyst (sample 1) in acid. Loading Tafel Slope Y.sub.int (g/cm.sup.2) (mV/dec) (V) R.sup.2 Fit 100 62.3 0.7969 0.99695 200 71.4 0.8079 0.99812 400 67.9 0.8293 0.99890 600 63.0 0.8336 0.99947

(30) The ORR polarization curves of electrolytic reactions measured using a RRDE with a FePhenMOF catalyst (sample 1) in 0.1M NaOH at room temperature with Pt/C and the Ag (from DE NORA) catalyst as benchmarks are shown in FIG. 10. Remarkably, the FePhenMOF catalyst prepared according to the method of the present disclosure has a superior performance when compared to the state of the art Pt/C and Ag based catalysts in alkali media.

(31) Use of ODCs is expected to reduce total energy consumption for chlorine production at a typical current density of 4 kA/m.sup.2 by 30%. The catalysts prepared according to the method described herein were tested in various advanced applications including preparation of oxygen depolarized cathodes for chlorine evolution using chlor-alkali electrolysis cells. FIG. 11 is a schematic diagram of a half-cell design for a chlor-alkali electrolysis cell. While oxygen reduction reaction takes place at the cathode, at the anode oxygen evolution reaction occurs. On the cathode side, 5 M NaOH was pumped to simulate industrial chlor-alkali electrolysis cells. A graph of voltage response as a function of applied current density of a chlor-alkali electrolysis half-cell containing FePhenMOF electrocatalyst synthesized according to the solid state reaction described herein (sample 1) is shown in FIG. 12. For comparison, data obtained using state of the art Pt/C and Ag (Denora) as catalysts and pure oxygen as feedstock at the cathode side are also shown. Clearly, as to chlorine evolution in chlor-alkali electrolysis cells, FePhenMOF electrocatalyst prepared according to the solid state reaction method of the present disclosure has a superior performance compared to the state of the art catalysts such as Pt/C and Ag prepared using noble metals. Additionally, as is clear from FIG. 13, performance of an electrolysis cell containing an ODC having FePhenMOF as a catalyst does not deteriorate when the feedstock at the cathode side changes from pure oxygen to an atmosphere of less than 100% oxygen, e.g., 95% oxygen and 5% nitrogen.

EXAMPLES

Example 1

(32) A ZIF-8 Metal organic framework (MOF) structure was formed through liquid assisted grinding (LAG) using ammonium sulfate as a catalyst. Zinc oxide and 2-methylimidazole were used as raw materials to form the ZIF-8 structure and iron(II) acetate and phenanthroline were used as the source, respectively, of the second transition metal and the second organic ligand for coating the MOF or filing its pores. First, zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to form the MOF structure. Then, iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate, and phenanthroline was 1:2:0.04:0.025:2 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 14 (solid line).

Example 2

(33) Zinc oxide and 2-methylimidazole were used as raw materials to form the ZIF-8 structure. Iron(II) acetate and pyridine were used as the source of the second transition metal and the second filling/coating organic ligand to be encapsulated in the MOF structure. First, zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to form the MOF structure. Iron(II) acetate and pyridine were next added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and pyridine was 1:2:0.04:0.025:2 with ZnO being used at a scale of 10 mmol. The FePyridineMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 14 (broken line).

Example 3

(34) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.0125:2 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 15 (solid line).

Example 4

(35) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.05:2 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 15 (bold broken line).

Example 5

(36) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.025:0.5 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 16 (solid line).

Example 6

(37) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.025:1 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 16 (light dotted line).

Example 7

(38) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.025:3 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 16 (solid broken line; uppermost curve).

Example 8

(39) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for 30 minutes to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two and half hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.025:2 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 17 (solid line).

Example 9

(40) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for two hours to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.025:2 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 17 (solid short broken line).

Example 10

(41) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for one hour. The molar ratio of zinc oxide, 2-methylimidazole, ammonia sulfate, iron(II) acetate and phenanthroline was 1:2:0.04:0.025:2 with ZnO being used at a scale of 10 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 17 (solid long broken line).

Example 11

(42) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. The molar ratio of zinc oxide, 2-methylimidazole, and ammonia sulfate was 1:2:0.04 with ZnO being used at a scale of 10 mmol. The MOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 18 (solid line). For comparison, the ORR polarization curve of the electrocatalyst generated in Example 1, i.e., FePhenMOF, is also shown (solid broken line).

Example 12

(43) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then phenanthroline was added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, and ammonia sulfate was 1:2:0.04 with ZnO being used at a scale of 10 mmol. The MOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 18 (light dotted line). For comparison, the ORR polarization curve of the electrocatalyst generated in Example 1, i.e., FePhenMOF, is also shown (solid broken line).

Example 13

(44) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, and ammonium sulfate, iron(II) acetate, and phenanthroline is 1:2:0.04:0.025:2 with ZnO being used at a scale of 10 mmol. The MOF structure was not destroyed after adding iron(II) acetate and phenanthroline. Next, the FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. Subsequently, FePhenMOF catalysts generated were acid washed in 1 M HCl at 80 C. for 2 hrs. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 19 (light dotted line). For comparison, the ORR polarization curve of the electrocatalyst generated in Example 1, i.e., FePhenMOF, is also shown (solid line).

Example 14

(45) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, and ammonium sulfate, iron(II) acetate, and phenanthroline was 1:2:0.04:0.025:2 with ZnO being used at a scale of 20 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 20 (light dotted line). For comparison, the ORR polarization curve of the electrocatalyst generated in Example 1, i.e., FePhenMOF (ZnO used at a 10 nmol scale), is also shown (solid line).

Example 15

(46) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, and ammonium sulfate, iron(II) acetate, and phenanthroline was 1:2:0.04:0.025:2 with ZnO being used at a scale of 40 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 20 (short broken line). For comparison, the ORR polarization curve of the electrocatalyst generated in Example 1, i.e., FePhenMOF (ZnO used at a 10 nmol scale), is also shown (solid line).

Example 16

(47) Zinc oxide, 2-methylimidazole, and ammonium sulfate were ball milled with 400 L MeOH for one hour to completely form the MOF structure. Then iron(II) acetate and phenanthroline were added to the as-prepared MOF and ball milled for two hours. The molar ratio of zinc oxide, 2-methylimidazole, and ammonium sulfate, iron(II) acetate, and phenanthroline was 1:2:0.04:0.025:2 with ZnO being used at a scale of 60 mmol. The FePhenMOF precursor obtained was pyrolyzed under argon and NH.sub.3 at 1050 C. for 1 hour and 15 minutes, respectively, with a ramping rate of 15 C./min. The ORR polarization curve of the electrocatalyst generated, obtained using a RRDE in 0.1M HClO.sub.4 at room temperature, is shown in FIG. 20 (long broken line). For comparison, the ORR polarization curve of the electrocatalyst generated in Example 1, i.e., FePhenMOF (ZnO used at a 10 nmol scale), is also shown (solid line).

(48) All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

(49) As used herein, consisting essentially of allows the inclusion of materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term comprising, particularly in a description of components of a composition or in a description of elements of a device, can be exchanged with consisting essentially of or consisting of.

(50) From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the scope of the following claims.