OXYGEN REDUCTION REACTION CATALYST

Abstract

A method for the manufacture of an oxygen reduction reaction (ORR) catalyst, the method comprising; providing a metal organic framework (MOF) material having a specific internal pore volume of 0.7 cm.sup.3g.sup.−1 or greater; providing a source of iron and/or cobalt; pyrolysing the MOF material together with the source of iron and/or cobalt to form the catalyst, wherein the MOF material comprises nitrogen and/or the MOF material is pyrolysed together with a source of nitrogen and the source of iron and/or cobalt is disclosed.

Claims

1. A method for the manufacture of an oxygen reduction reaction (ORR) catalyst, the method comprising; providing a metal organic framework (MOF) material having an isotropic cavity shape with a largest cavity size of 12 Å or greater; providing a source of iron and/or cobalt; pyrolysing the MOF material together with the source of iron and/or cobalt to form the catalyst, wherein the MOF material comprises nitrogen and/or the MOF material is pyrolysed together with a source of nitrogen and the source of iron and/or cobalt.

2. A method according to claim 1, wherein the MOF material comprises a transition metal selected from Zn, Mg, Cu, Ag, and Ni, or a combination of two or more thereof.

3. A method according to claim 1, wherein the transition metal comprises zinc.

4. A method according to claim 1, wherein the MOF material is a Zeolitic Imidazolate Framework (ZIF) material.

5. A method according to claim 1, wherein the source of iron and/or cobalt is a salt of iron and/or cobalt.

6. A method according to claim 1, wherein the pyrolysis of the MOF material is conducted at a temperature from 700 to 1500° C.

7. A method according to claim 1, wherein the source of nitrogen comprises a nitrogen-containing ligand, preferably 1,10-phenanthroline.

8. A method according to claim 1, wherein the pyrolysis is conducted under an atmosphere comprising, argon, nitrogen, ammonia, or hydrogen, or mixtures thereof.

9. A method according to claim 1, wherein the pyrolysis is conducted in two steps, a first step under an inert atmosphere and a second step under an atmosphere comprising ammonia, hydrogen, carbon dioxide and/or carbon monoxide.

10. A method according to claim 1, wherein the MOF material has an average crystal size with a longest size of 200 nm or less.

11. A method according to claim 1, wherein the MOF material is provided on an electrically conducting support.

12. A method according to claim 1, wherein the MOF material has a specific internal pore volume of 0.7 cm.sup.3g.sup.−1 or greater.

13. A method for the manufacture of an oxygen reduction reaction (ORR) catalyst, the method comprising: providing a metal organic framework (MOF) ligand and MOF metal source; providing a source of iron and/or cobalt; optionally providing a source of nitrogen; providing a source of energy sufficient to provide a catalyst precursor comprising a MOF material having an isotropic cavity shape with a largest cavity size of 12 Å or greater; and pyrolysing the catalyst precursor to provide the ORR catalyst.

14. An ORR catalyst that is made according to the method of claim 1.

15. An ORR catalyst that is made according to the method of claim 13.

16. A method according to claim 1, wherein the method further comprises forming an ink composition comprising the catalyst and a polymer.

17. A method according to claim 13, wherein the method further comprises forming an ink composition comprising the catalyst and a polymer.

18. An ink composition that is made according to the method of claim 16.

19. An ink composition that is made according to the method of claim 17.

20. A cathode electrode for a fuel cell comprising the ORR catalyst of claim 14.

21. A cathode electrode for a fuel cell comprising the ORR catalyst of claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] FIGS. 1 to 6 show non-limiting figures according to the present invention.

[0079] FIG. 1 shows the PEM fuel cell polarization curves recorded for MEAs comprising different catalysts at the cathode, in the high potential region where the fuel cell performance is controlled by the cathode ORR kinetics.

[0080] FIG. 2 shows ORR activity of Fe—N—C catalysts after pyrolysis at optimum temperature for each MOF against the specific internal pore volume of the pristine MOFs.

[0081] FIG. 3 shows ORR activity of Fe—N—C catalysts against the isotropic cavity size in the pristine MOFs. For each pristine MOF, three pyrolysis temperatures were investigated.

[0082] FIG. 4 shows ORR activity of Fe—N—C catalysts after pyrolysis at optimum temperature for each MOF against the isotropic cavity size in the pristine MOFs.

[0083] FIG. 5 shows ORR activity of Fe—N—C catalysts after milling at 100 rpm against the specific internal pore volume of the pristine ZIF-based MOFs.

[0084] FIG. 6 shows ORR activity of Fe—N—C catalysts prepared using the ‘one-pot’ synthesis method.

EXAMPLES

[0085] The invention will now be described in relation to the following non-limiting examples.

[0086] Measurement Techniques

[0087] Specific Internal Pore Volume

[0088] The specific internal pore volume was calculated using crystallographic structures for each MOF. For that purpose, the crystal structure was first built following the single crystal data given in the literature for each solid. The geometry was optimised using Lennard Jones parameters and electrical charges to determine the positions of the atoms in the structure. In this case, the Universal Force Field (UFF) for Lennard Jones parameters was considered. Within the entire volume of optimized structures and following the strategy previously reported by Düren et al. (T. Duren, F. Millange, G. Férey, K. S. Walton, R. Q. Snurr, J. Phys. Chem. C, 2007, 111, 15350), a theoretical probe size of 0 Å was then used to determine the entire volume of the unit crystallographic cell. The unit cell is the smallest volume of a crystalline solid determined by its repetition in three dimensions that can predict the macroscopic structure of the solid. The volume of the unit cell was determined by moving the 0 Å theoretical probe inside the entire unit cell. This determined whether the probe was localized in the space occupied by atoms or in the free volume, i.e. in pores, using a Monte Carlo algorithm. Such a strategy allowed the determination of the specific internal pore volume of the macroscopic porous solid by dividing the free pore volume of the unit cell by the mass of the atoms present in the unit cell.

[0089] Pore Size Distribution and Isotropic Cavity Size

[0090] Using the same parameters for the structure atoms (UFF), the methodology of Gelb and Gubbins (L. D. Gelb, K. E. Gubbins, Pore size distributions in porous glasses: a computer simulation study, Langmuir, 1999, 15, 305-308) was used to calculate the pore size distribution (PSD). It consists of trying to position spheres of increasing diameter into the free volume of the unit cell in order to determine the largest sphere able to fit in the structure, using Monte Carlo calculations. Evidently, the sphere occupies the free pore volume of the unit cell and cannot be superposed with the space occupied by atoms of the structure. Using this methodology, it is possible to determine the pore size distribution (PSD), i.e. the probability to find pores of a given size in the structure. Using the PSD curve, it is then possible to estimate the isotropic cavity size, as well as the size of the windows allowing species to pass from one cavity to another in the structure.

[0091] Exemplary Synthesis Method 1

[0092] Catalyst precursors were prepared via a dry ball-milling approach from a given MOF powder, Fe(II) acetate and 1,10-phenanthroline.

[0093] Weighed amounts of the dry powders of Fe(II)Ac, phenanthroline and ZIF-8 were poured into a ZrO.sub.2 crucible. 100 zirconium-oxide balls of 5 mm diameter were added and the crucible was sealed under air, and placed in a planetary ball-miller. Generally, the ball-to-catalyst precursor ratio and/or milling speed can be adjusted in order to keep the crystalline structure of the pristine MOF intact after the milling, as demonstrated by XRD patterns. With the milling conditions and equipment employed, the XRD of the MOFs were shown to be unmodified after the milling step when using a milling speed of 100 rpm.

[0094] The resulting catalyst precursor was then pyrolyzed at a given temperature (900° C. or more for zinc-based MOFs). The pyrolysis temperature was optimized for each MOF, by steps of e.g. 50° C. In this first method, the catalyst precursor was directly pyrolyzed in flowing NH.sub.3 for 15 minutes via a flash pyrolysis mode (see Jaouen et al, J. Phys. Chem. B 110 (2006) 5553). All catalyst precursors contained 1 wt % of iron and the mass ratio of phenanthroline to ZIF-8 was 20/80. The obtained powder was finally ground in an agate mortar.

[0095] Worked Examples—First Series

[0096] All catalysts in the first series of examples were prepared and tested in a similar manner, the sole difference being the nature and structure of the MOFs used to prepare the catalyst precursors.

[0097] The MOFs listed in Table 1 were synthesized beforehand according to previously reported methods, except for ZIF-8 which was purchased from Sigma Aldrich (trade name Basolite®, produced by BASF).

[0098] The catalyst precursors for the synthesis of Fe—N—C catalysts were prepared from fixed amounts of Fe(II)acetate (Fe(II)Ac), 1,10-phenanthroline (phen) and MOF. Catalysts were prepared through a dry ball milling approach. The dry powders of Fe(II)Ac, phen and a given MOF were weighed (31.4, 200 and 800 mg respectively) and poured into a ZrO.sub.2 crucible filled with 100 zirconium oxide balls of diameter 5 mm. The crucible was sealed under air and placed in a planetary ball-miller to undergo ball-milling at 400 rpm. The resulting catalyst precursor was then transferred into a quartz boat and inserted into a quartz tube and shock-heated within about 2 minutes to the temperature of pyrolysis (900, 950 or 1000° C.) in a flowing NH.sub.3 atmosphere and held at this temperature for 15 minutes. The pyrolysis was stopped by opening the split hinge oven and directly removing the quartz tube from the oven. The resulting catalyst was investigated as is. No acid wash was performed.

TABLE-US-00001 TABLE 1 Specific internal Isotropic pore Cavity volume, size, To- cal- Cal- Sample po- culated/ culated/ code Formula/Name Ligand logy cm.sup.3g.sup.−1 Å CAT-29 [Zn(Im)(mIm)]- Imidazole, zni 0.21 1.16 ZIF-61 2-methyl- imidazole CAT-37 [Zn(eIm).sub.2] 2-ethyl- qtz 0.17 1.5 qtz imidazole CAT-30 [Zn(Im).sub.2]/ZIF-4 Imidazole cag 0.43 4.76 CAT-38 [Zn (Im).sub.2]-zni Imidazole zni 0.27 3.16 CAT-14 [Zn(bzIm).sub.2] Benz- sod 0.37 3.5 ZIF-7 imidazole CAT-31 [Zn(eIm).sub.2] 2-ethyl- ana 0.49 5.0 ZIF-14 imidazole ZIF 8 [Zn(mIm).sub.2] 2-methyl- sod 0.66 11.6 ZIF-8 imidazole CAT-12 [Zn(bzIm).sub.2] Benz- rho 0.56 13.8 ZIF-11 imidazole CAT-28 [Zn(eIm).sub.2] 2-ethyl- rho 1.05 18.0 (inventive) rho imidazole CAT-19 [Zn.sub.2(bdc).sub.2(dabco)] 1,4- — 0.92 Aniso- (inventive) benzenedi- tropic carboxylate, cavity 1,4-diaza- bicyclo [2.2.2] octane MOF-5 [Zn.sub.4O(bdc).sub.3] 1,4- pcu 1.32 12.0/15.2 (inventive) benzenedi- carboxylate

[0099] Table 1 provides a summary of the imidazole-based MOFs and non-ZIF MOFs investigated. Im=imidazole, mim=methyl-Imidazole, elm=ethyl-imidazole, bzlm=benzimidazole, bdc=1,4-benzenedicarboxylate. The two last columns report the specific internal pore volume and isotropic cavity size calculated using density functional theory as described above.

[0100] Testing method—The activity for ORR of the catalysts was measured in a single fuel cell. For the membrane electrode assembly (MEA), cathode inks were prepared using the following formulation: 20 mg of Fe—N—C catalyst, 652 μl of a 5.0 wt % Nafion® solution, 326 μl of ethanol and 272 μl of de-ionized water.

[0101] The inks were alternatively sonicated and agitated with a vortex mixer every 15 min. The required aliquot of ink was then pipetted on to a 5.0 cm.sup.2 gas diffusion layer material (SGL Sigracet S10-BC) to result in a Fe—N—C loading of 1.0 mgcm.sup.−2. The cathode was then placed in a vacuum oven at 90° C. to dry for 2 h. The anode was 0.5 mgcm.sup.−2 Pt loading on Sigracet S10-BC gas diffusion layer. MEAs were prepared by hot-pressing 5.0 cm.sup.2 anode and cathode against either side of a Nafion™ NRE-117 membrane (Chemours Company) at 135° C. for 2 min.

[0102] PEMFC tests were performed with a single-cell fuel cell with serpentine flow field (Fuel Cell Technologies Inc.). For the tests, the fuel cell temperature was 80° C., the humidifiers were set at 100° C. (near 100% relative humidity of the incoming gases), and the inlet pressures were set to 1 bar gauge for both anode and cathode sides. The flow rates for humidified H.sub.2 and O.sub.2 were about 50-70 standard cubic centimetres per metre (sccm) downstream of the fuel cell.

[0103] FIG. 1 shows the PEM fuel cell polarization curves recorded for different catalysts, in the high potential region where the performance is controlled by the ORR kinetics. In order to present the results in a concise manner, the current density is read at 0.9 V iR-free potential, then divided by the catalyst loading (1.0 mgcm.sup.−2).

[0104] The scalar Ag.sup.−1 at 0.9 V iR-free potential represents the activity of a given catalyst in these fixed experimental conditions of O.sub.2 pressure, relative humidity and temperature. Since all catalysts were synthesized identically except for the pyrolysis temperature, the catalyst label only includes the sample code of the MOF used and the applied pyrolysis temperature in NH.sub.3 (900, 950 or 1000° C.). The three- or four-digit number used in the legend corresponds to the pyrolysis temperature in NH.sub.3, optimized for each MOF structure. The two-digit number following CAT corresponds to the internal code, and the corresponding structure can be found in Table 1. The figure shows a range of activities from about 1.0 to 5.6 Ag.sup.−1 at 0.9 V, highlighting the importance of selecting a proper MOF structure in order to obtain the highest optimized ORR activity after pyrolysis. Three MOFs (CAT 28, CAT 19, MOF 5) result in higher ORR activity than that obtained with ZIF-8, the prior state-of-the art.

[0105] FIG. 2 shows a correlation between the optimum mass activity of the catalyst (as dependent on the optimum pyrolysis temperature) and the specific internal pore volume in the pristine MOF.

[0106] FIG. 3 shows the correlation between the mass activity for ORR of this series of Fe—N—C catalysts and the calculated isotropic cavity size of the MOFs (for those MOFs that have isotropic cavities). For pristine MOF structures showing several cavity sizes (CAT-31, MOF-5), the largest cavity size was selected to produce FIG. 2.

[0107] While for a given MOF (fixed x-axis value in FIG. 3), the ORR activity after pyrolysis depends on the pyrolysis temperature, a clear correlation is observed between the ORR activity at the optimum temperature (MOF-dependent) and the calculated isotropic cavity size in the pristine MOF (FIG. 4). A code is applied to indicate the crystalline topologies in those various MOFs (see legend in the figures).

[0108] In this first series of examples, the milling rate used to mix Fe(II) acetate, 1,10-phenanthroline and a MOF was 400 rpm. In these conditions, this milling speed was able to amorphise the crystalline MOFs. This effect is particularly emphasized on MOFs with large cavity size that are probably less mechanically robust. There is nevertheless a memory effect of the cavity size in pristine MOFs on the final pyrolyzed products, as clearly demonstrated in FIGS. 2 to 4.

[0109] Worked Examples—Second Series

[0110] To better demonstrate the correlation between the cavity size in pristine isotropic MOFs and the ORR activity in pyrolyzed products, the milling speed was reduced to 100 rpm in order to maintain the XRD patterns of the pristine MOFs (and hence their cavity size) after the milling of iron acetate, 1,10-phenanthroline and MOF. Unmodified XRD patterns after 100 rpm milling were observed on all MOFs in those conditions (not shown here). In this second series of examples, the catalyst precursors before pyrolysis are therefore characterized by the cavity size of the pristine MOFs. The synthesis conditions were otherwise identical to those indicated for the first series of examples. For each MOF, the optimum temperature (as shown in FIG. 4) was selected as the pyrolysis temperature.

[0111] This second series of catalysts demonstrated the ORR activity-specific internal pore volume correlation for catalyst precursors whose XRD patterns show the retained structure of pristine MOFs, even after the milling stage at 100 rpm (FIG. 5).

[0112] Exemplary Synthesis Method 2

[0113] The catalyst precursors prepared according to method 1 may be pyrolyzed first in inert gas such as N.sub.2, Ar, etc (ramp heating mode or flash heating mode) at a temperature sufficient to remove, together with volatile products, the first transition metal present in the MOF, and to effect the carbonization of the MOF, then pyrolyzed in an etching gas (NH.sub.3, CO.sub.2, CO, etc) that further increases the porosity of the catalysts and increase the number of Metal-NxCy sites present on the surface of the catalysts.

[0114] Exemplary Synthesis Method 3 (One-Pot Synthesis)

[0115] The catalyst precursors were prepared via a so-called one-pot approach. Typically, weighed amounts of the dry powder of Fe(II)Ac, 1,10-phenanthroline, MOF ligand and ZnO were mixed by grinding or ball-milling. The MOF formation then occurred under solvothermal or mechanical conditions. The catalyst precursors were then pyrolysed in flowing ammonia at the optimum temperature already identified for each MOF in the Exemplary Synthesis Method 1.

[0116] Worked Examples:

[0117] Cat-28 (Example of the Invention)

[0118] ZnO (3.0047 g, 37 mmol), elm (7.1495 g, 72 mmol), (NH.sub.4).sub.2SO.sub.4 (0.7541 g, 7 mmol), Fe(Ac).sub.2 (0.1188 g, 0.68 mmol) and 1,10-phenanthroline (2.377 g, 13 mmol) were placed in a zirconium mill pot with DMF (6 ml) and zirconia milling balls. The mixture was ground for 30 min in a Fritsch mill at 400 rpm. The light pink solid obtained was dried in air. The product was then pyrolysed in flowing ammonia at 950° C. according to the method disclosed in Exemplary Synthesis Method 1.

[0119] ZIF-8 (Comparative Example)

[0120] ZnO (2.2803 g, 28 mmol), mlm (5.0349 g, 61 mmol), Fe(Ac).sub.2 (0.0679 g) and 1,10-phenanthroline (1.2092 g, 6.7 mmol) were ground into a homogenous mixture then sealed in solvothermal bomb under Ar. The reaction mixture was heated to 180° C. for 18 hours. Upon cooling a damp red solid was obtained. The product was dried under vacuum at 100° C. for 3 hours and a pink solid product obtained. The product was then pyrolysed in flowing ammonia at 1000° C. according to the method disclosed in Exemplary Synthesis Method 1.

[0121] CAT-38 (Comparative Example)

[0122] ZnO (2.2709 g, 28 mmol), Im (4.1942 g, 62 mmol), Fe(Ac).sub.2 (0.0655 g, 0.35 mmol) and 1,10-phenanthroline (1.2330 g, 6.9 mmol) were ground into a homogenous mixture then sealed in solvothermal bomb under Ar. The reaction mixture was heated to 180° C. for 18 hours and a pink solid product obtained. The product was then pyrolysed in flowing ammonia at 1000° C. according to the method disclosed in Exemplary Synthesis Method 1.

[0123] The results are shown in FIG. 6 and it can be seen that the example of the invention shows superior activity to the comparable examples when made by the one-pot method (Exemplary Synthesis Method 3).

[0124] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents. It is particularly noted that although the examples were based on Fe—N—C active sites comparable results may be achieved using Co—N—C catalysts.