Covalent organic frameworks as porous supports for non-noble metal based water splitting electrocatalysts

Abstract

The present invention discloses porous covalent organic frameworks (COF) supported noble metal-free nanoparticles which are useful as electrocatalysts for a water splitting system, and to the process for preparation of such electrocatalysts. The covalent organic frameworks (COF) supported noble metal-free nanoparticles have general formula (I):
COF_AxBy(M)n(Formula I) wherein COF is selected from a Tris (4-formylphenyl)amine terephthaldehyde polymer or a benzimidazole-phloroglucinol polymer; A and B each independently represent a transition metal selected from the group consisting of Ni, Co, Fe, Mn, Zn, and mixtures thereof; or A and B together represent a transition metal selected from the group consisting of Ni, Co, Fe, Mn, Zn, and mixtures thereof; M represents hydroxide or a nitride ion; x and y represent the weight % of the metal loadings; or a ratio of x:y is between 0:1 and 1:0; and n is an integer 1 or 2 or 3.

Claims

1. A stable covalent organic framework (COF) supported noble metal free nanoparticle composite of the general formula I as electro catalysts for water splitting with low over potential comprising a compound of Formula I:
COF_AxBy(M)n(Formula I) wherein COF is a Tris (4-formylphenyl)amine terephthaldehyde polymer or a benzimidazole-phloroglucinol polymer; A and B each independently represent a transition metal selected from the group consisting of Ni, Co, Fe, Mn, and Zn; and M represents a hydroxide ion or a nitride ion; wherein, when M represents the hydroxide ion; x and y represent the weight % of the metal loadings in the range 16-18 wt % of total wt % of COF; or x=0 to 16 wt % or 0 to 18 wt % and y=0 to 16 wt % or 0 to 18 wt %; or x and y together represents 16 to 18 wt %; n is an integer of 2 or 3; and COF represents Tris(4-formylphenyl)amine terephthaldehyde polymer; or wherein, when M represents a nitride ion; x=0 to 8 wt % and y=0 to 8 wt %; or x and y together represent 8 wt %; n is 1 or 2 or 3; and COF represents benzimidazole-phloroglucinol polymer.

2. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 1, comprising a composite A of the formula;
COF2_AxBy(OH).sub.2 wherein COF2 represents Tris(4-formylphenyl)amine terephthaldehyde polymer.

3. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 2, wherein said composite comprises:
COF2_Co/Ni(OH).sub.2 wherein the Co:Ni ratio is 10 mg:30 mg for total 100 mg of COF2.

4. A stable covalent organic framework (COF) supported noble metal free nanoparticle composite, wherein said composite comprises:
COF2_16Ni(OH).sub.2;i.
COF2_16Co(OH).sub.2;ii.
COF2_8Co(OH).sub.2+8Ni(OH).sub.2;iii.
COF2_12Co(OH).sub.2+4Ni(OH).sub.2; oriv.
COF2_4Co(OH).sub.2+12Ni(OH).sub.2;v. wherein COF2 represents Tris(4-formylphenyl)amine terephthaldehyde polymer.

5. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 1, comprising a composite B of the formula;
COF3_AxBy(M).sub.n wherein: M represents a nitride ion; and COF3 represent benzimidazole-phloroglucinol polymer.

6. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 5, comprising
COF3_Ni.sub.3N wherein COF3 comprises benzimidazole-phloroglucinol polymer.

7. The process for synthesis of the stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 2, wherein the transition metal nanoparticle composite has the general formula:
COF2_AxBy(OH).sub.2; said process comprising: i. heating a mixture of Tris(4-formylphenyl)amine and 1,4-diaminobenzene in polar protic or non-polar solvents either alone or in combination thereof and aq. acetic acid under the conditions suitable to form solvated COF2 in -phase; ii. desolvating COF2- to obtain stable COF2 in phase; and iii. adding a solution of a hydrated metal salt to a suspension of COF2- in a solvent followed by reduction of metal ion (M.sup.2+) in aqueous medium to obtain the desired composite, wherein said hydrated metal salt is a hydrated salt of A, B, or a mixture thereof.

8. The process for synthesis of the stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 1, wherein the transition metal nanoparticle composite has the general formula COF3_Ni.sub.3N, said method comprising: i. reacting 3,3-diaminobenzidine and 1,3,5-triformyl phloroglucinol in a mixture of solvents at a temperature ranging from r.t to 130 C. to obtain COF3; ii. adding a mixture of grounded Nickel acetate tetra hydrate and urea or hexamethylenetetramine to COF3 powder followed by annealing to obtain the composite COF3_Ni.sub.3N.

9. A method for oxygen generation during splitting of water with low over potential, comprising contacting the water containing electrolyte with the covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 1.

10. A method for oxygen generation during splitting of water with low overpotential comprising contacting the water containing electrolyte with the covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 2.

11. The method as claimed in claim 10, wherein the electrolyte is de-aerated 0.1 M KOH.

12. The method for oxygen generation during splitting of water with low overpotential comprising contacting the water containing electrolyte with the covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 5.

13. The method as claimed in claim 12, wherein the electrolyte is de-aerated 1 M KOH.

14. A stable covalent organic framework (COF) supported noble metal free nanoparticle composite of the general formula I as electro catalysts for water splitting with low over potential comprising a compound of Formula I:
COF_AxBy(M)n(Formula I) wherein COF is a Tris (4-formylphenyl)amine terephthaldehyde polymer or a benzimidazole-phloroglucinol polymer; A and B each independently represent a transition metal selected from the group consisting of Ni, Co, Fe, Mn, and Zn; the ratio x:y is between 0:1 and 1:0; M represents a hydroxide ion or a nitride ion; and n is an integer ranging from 1 to 3.

15. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 14, wherein M is a hydroxide ion.

16. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 14, wherein M is a nitride ion.

17. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 15, wherein COF is a Tris (4-formylphenyl)amine terephthaldehyde polymer.

18. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 16, wherein COF is a benzimidazole-phloroglucinol polymer.

19. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 14, wherein COF is a Tris (4-formylphenyl)amine terephthaldehyde polymer.

20. The stable covalent organic framework (COF) supported noble metal free nanoparticle composite as claimed in claim 14, wherein COF is a benzimidazole-phloroglucinol polymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand various exemplary embodiments, reference is made to the accompanying drawings, wherein:

(2) FIG. 1A illustrates the Pawley fit for the solvated phase (Top); and for the desolvated phase (Bottom).

(3) FIG. 1B illustrates the variable temperature PXRD analysis showing the transformation of -phase to upon desolvation, the dotted lines indicate the shrinking of the a-axis and expansion of the c-axis during this irreversible phase transformation.

(4) FIG. 1C illustrates the TGA of the IISERP-COF2 in its and forms.

(5) FIG. 2 illustrates 77K Nitrogen adsorption isotherm of the COF2 suggesting micropores. The inset shows the pore size being bimodal with 18 and 22 pores.

(6) FIG. 3A illustrates the PXRD of the composite-1A showing the presence of peaks corresponding to Ni(OH).sub.2 nanoparticles.

(7) FIG. 3A illustrates the IR spectra of composite 1A (Top trace); the IR spectra of IISERP-COF2 in its form (Middle trace); and the IR spectra of IISERP-COF2 in its form (Bottom trace).

(8) FIGS. 4A through 4D illustrate the XPS spectra of the composite with the metal loading ratio of 4Co:12NiCOF or composite-1A. The binding energy (BE) values calculated using fitted profiles. Note: the numbers indicate the wt. % of metal loading and COF refers to IISERP-COF2.

(9) FIG. 5A illustrates a PXRD studies showing the peaks corresponding to the Co nanoparticles (Lower trace), Ni nanoparticles (Upper trace), and CoNi nanoparticles (Middle trace) obtained from their composites with IISERP-COF2.

(10) FIG. 5B illustrates the TGA of the IISERP-COF2 and its metal nanoparticles composites. Note: COF in this figure represents the IISERP-COF2.

(11) FIG. 5C illustrates the nitrogen adsorption isotherm at 77K of the IISERP-COF2 and its Co, Ni, CoNi composites showing the drop in porosity upon Nanoparticle loading.

(12) FIG. 6A illustrates PXRD studies showing scalability and stability of the IISERP-COF3.

(13) FIG. 6B illustrates the TGA of the IISERP-COF3 and the composite-2 synthesized from Urea and HMTA methods.

(14) FIG. 6C illustrates the IR spectra of the IISERP-COF3 (As synthesized; bottom trace) and the composite-B synthesized from the Urea method (Middle trace) and the HMTA method (Upper trace).

(15) FIG. 6D illustrates the TEM image of IISERP-COF3 showing well-dispersed nanoparticles of the as-made COF.

(16) FIG. 6E illustrates the Scalability and stability of IISERP-COF3 inferred from 77K N2 adsorption isotherm.

(17) FIG. 6F illustrates the Pore size fits modelled using DFT methods using the 77K N2 isotherms.

(18) FIG. 6G illustrates the High resolution XPS spectra of composite-B.

(19) FIG. 6H illustrates the HRTEM image of the composite-B showing homogeneous distribution of Ni.sub.3N nanoparticles on the COF surface.

(20) FIG. 6I illustrates the SAED diffraction image of the composite-B showing the peaks corresponding to Ni.sub.3N.

(21) FIGS. 7A through 7D illustrate High resolution XPS spectra of Composite-2. Linesfitted data, dotsexperimental data and red line-background.

(22) FIG. 8A illustrates the HRTEM image showing the dark regions from the Ni.sub.3N nanoparticles and its lattice fringes having sizes<5 nm. FIG. 8B illustrates the HRTEM image showing the growth of the Ni.sub.3N particles on the COF surface exposing different facets. Lattice fringes with d-spacings and angles corresponding to hexagonal Ni.sub.3N (Sp.gr.: P6.sub.322).

(23) FIG. 9A illustrates the LSV plots for the composites-A and B. FIG. 9B shows a zoom-in on the LSV plots showing the intercepts corresponding to over potentials of 250 and 310 mV at 20 mA/cm.sup.2 for composites A and B, respectively. FIG. 9C shows the Tafel plots for the different composites made using IISERP-COF2 and Ni/Co/NiCo nanoparticles (75% IR-compensated) showing the most favourable charge transfer kinetics for the composite-A (30:10 wt. % Ni:Co). FIG. 9D shows the Tafel plot for the composite-B (75% IR-compensated). FIGS. 9E and 9F show the Chronoampeperometry plots for composites A and B showing the stability in the current outputs over 13/20 hrs. Inset shows the Ni(III)-Ni(IV) redox couple being stable over 500 CV cycles.

DETAILED DESCRIPTION

(24) Various embodiments will now be described with reference to certain preferred and optional features, so that the various aspects therein will be more clearly understood and appreciated.

(25) The catalytic activity of covalent organic framework (COF) can be improved by modulation of the support by periodic replacement of carbon atoms of a phenyl group with more electronegative organic elements which could reduce the activation energy associated with the anodic evolution reaction during electrolysis of water and also improve the charge/mass transfer kinetics. Further, introducing specific characteristics in the COF framework also enhances the interaction of guest nanoparticles with the host support which is useful to lower the over potential during electro catalytic water splitting or to be with par to the acceptable value of the art.

(26) In a preferred embodiment, the present disclosure describes porous covalent organic framework (COF) supported transition metal nanoparticle, in the form of metal or its hydroxides or nitrides, composites that show both structural and electronic synergism useful for electro catalytic water splitting.

(27) Accordingly, the present invention discloses novel covalent organic framework (COF) supported transition metal nanoparticle composites of the general formula I;
COF_AxBy(M)nI

(28) wherein COF is selected from Tris(4-formylphenyl)amine terephthaldehyde polymer or benzimidazole-phloroglucinol polymer;

(29) A and B represents independently transition metals selected from Ni, Co, Fe, Mn or Zn;

(30) M represents hydroxide or a nitride ion;

(31) x and y represent the weight % of the metal loadings in the range 16-18 wt % of total wt % of COF; or x and y together represent the valency of the ion; or x=0 to 16 wt % or 0 to 18 wt % and y=0 to 16 wt % or 0 to 18 wt %; or x and y together represents 16 to 18 wt %;

(32) n is an integer 1 or 2 or 3;

(33) with the proviso that when A and B independently represent transition metals selected from Ni, Co, Fe, Mn and Zn:

(34) M represents hydroxide ion;

(35) x and y represent the weight % of the metal loadings in the range 16-18 wt % of total wt % of COF; x=0 to 16 wt % or 0 to 18 wt % and y=0 to 16 wt % or 0 to 18 wt %; or x and y together represents 16 to 18 wt %;

(36) n is an integer 2 or 3; and

(37) COF represent Tris(4-formylphenyl)amine terephthaldehyde polymer; or

(38) with the proviso that when A and B together represent a transition metal selected from Ni, Co, Fe, Mn and Zn:

(39) M represents a nitride ion;

(40) x and y together represent the valency of the ion; x=0 to 8 wt % and y=0 to 8 wt %; or x and y together represents 8 wt %;

(41) n is 1 or 2 or 3; and

(42) COF represent benzimidazole-phloroglucinol polymer.

(43) In an embodiment, the electro catalyst of the present invention is noble metal free.

(44) In an embodiment, the present invention discloses covalent organic framework (COF) supported transition metal nanoparticle composite A of the general formula A;
COF2_A.sub.xB.sub.y(OH).sub.2A

(45) wherein A and B represent independently transition metals selected from Ni, Co, Fe, Mn and Zn;

(46) COF2 represents Tris(4-formylphenyl)amine terephthaldehyde polymer; and

(47) x and y represent the weight % of the metal loadings in the range 16-18 wt % of total wt % of COF; or x=0 to 16 wt % or 0 to 18 wt % and y=0 to 16 wt % or 0 to 18 wt %; or x and y together represents 16-18 wt % of total wt % of COF2. In an embodiment, the COF2 support material comprises the stable -phase configuration which is stable up to 450 C.

(48) The IISERP-COF2_AxBy(OH).sub.2 composite A of the present disclosure may be loaded with homometallic or heterometallic nanoparticles. In an embodiment, the IISERP-COF2 forms a composite with 16 wt % Co (1) or 16 wt % of Ni (2) or 8 wt % Co+8 wt % Ni (3) or 12 wt % Co+4 wt % Ni (4) or 4 wt % Co+12 wt % Ni (5 or 1A).

(49) In a preferred embodiment, the present invention discloses covalent organic framework (COF) supported transition metal nanoparticle composite 1A comprising IISERP-COF2_Co/Ni(OH).sub.2 wherein the Co:Ni ratio is 10 mg:30 mg for 100 mg of COF2.

(50) In another preferred embodiment, the present invention discloses covalent organic framework (COF) supported transition metal nanoparticle composite B of the formula COF3_Ni.sub.3N wherein COF3 comprises benzimidazole units connected to a phloroglucinol core to form a polymeric conjugated framework.

(51) In an embodiment, the present invention relates to a process for synthesis of covalent organic framework (COF) supported transition metal nanoparticle composite A of the general formula;
COF2_A.sub.xB.sub.y(OH).sub.2

(52) wherein A and B represents independently transition metals selected from Ni, Co, Fe, Mn and Zn;

(53) COF2 represents Tris(4-formylphenyl)amine terephthaldehyde polymer;

(54) x and y represent the weight % of the metal loadings in the range 16-18 wt % of total wt % of COF2; or x=0 to 16 wt % or 0 to 18 wt % and y=0 to 16 wt % or 0 to 18 wt %; or x and y together represents 16-18 wt % of total wt % of COF2. The process comprises: i. heating a mixture of Tris(4-formylphenyl)amine, 1,4-diaminobenzene in polar protic or non-polar solvents either alone or in combination thereof and aq. acetic acid under the conditions suitable to form solvated IISERP-COF2 in -phase; ii. desolvating IISERP-COF2- to obtain stable IISERP-COF2 in phase; and iii. adding hydrated metal salt solution (MCl.sub.2.6H.sub.2O) to a suspension of IISERP-COF2-3 in a solvent followed by reduction of metal ion (M.sup.2+) in aqueous medium to obtain the desired composite.

(55) In another preferred embodiment, the present invention discloses covalent organic framework (COF) supported transition metal nanoparticle composite B of the formula COF3_Ni.sub.3N wherein COF3 comprises benzimidazole units connected to a phloroglucinol core to form a polymeric conjugated framework.

(56) The solvent is selected from polar protic solvents such as lower alcohols, formic acid, acetic acid and the like; organic solvent such as Dioxan, DMF, THF, halogenated hydrocarbons, 1,4 dioxane, mesitylene and the like; either alone or in combination thereof.

(57) Suitable reaction conditions include heating the reaction mixture of step (i) to form solvated IISERP-COF2 in -phase at a temperature in the range of 110-130 C. for about 70-80 hours. The desolvation is achieved by drying the contents in an oven at about 120 C. for about 3-4 days. The aqueous phase reduction preferably results in the formation of composites as the respective hydroxides suspended in the IISERP-COF2- matrix.

(58) The transformation of -phase to -phase of IISERP-COF2 during desolvation is an irreversible process. The IISERP-COF2- obtained in step (ii) is stable up to temperature of 450 C.

(59) In an embodiment, the IISERP-COF2- is loaded with homometallic or heterometallic nano particles. Accordingly, IISERP-COF2 is loaded with 16 wt % Co (composite 1) or 16 wt % of Ni (composite 2) or 8 wt % Co+8 wt % Ni (composite 3) or 12 wt % Co+4 wt % Ni (composite 4) or 4 wt % Co+12 wt % Ni (composite 5 or 1A).

(60) The IISERP-COF2- formed extremely thin wafers of COF2 with a bimodal pore size distribution with pore sizes 19 and 21 and has BET and Langmuir surface areas 557 and 866 m.sup.2/g, respectively. The composite contain the nanoparticles with sizes<3 nm.

(61) In another embodiment, the present invention discloses a process for preparing IISERP COF3_Ni.sub.3N composite B comprising: i. reacting 3,3-diaminobenzidine and 1,3,5-triformyl phloroglucinol in a mixture of solvents at a temperature ranging from r.t to 130 C. to obtain IISERP-COF3; and ii. adding a mixture of grounded Nickel acetate tetra hydrate and urea or hexamethylenetetramine to IISERP-COF3 powder; and iii. annealing the mixture of step (ii) to obtain composite B.

(62) The milligram scale solution reaction for synthesis of IISERP-COF3 has been carried out in 10 gram scale with a facile scale-up procedure.

(63) The transformation of -phase to -phase of IISERP-COF2 during desolvation is an irreversible process. The IISERP-COF2- obtained in step (ii) is stable up to temperature of 450 C.

(64) The solvents for the process are selected from non-polar solvents such as mesitylene and polar solvents such as 1,4-dioxane, THF, DMF and the like. Suitable reaction conditions include maintaining the temperature of step (i) in the range of 110-130 C. for about 70-75 hours. The nanoparticles are loaded on to the COF via a solid state synthesis comprising grinding the mixture of COF3 with Nickel acetate tetra hydrate and urea or hexamethylenetetramine and heating the mixture to a temperature in the range of 300-400 C. for about 6 hrs.

(65) The composite B obtained is further characterized by FE-SEM, HRTEM, PXRD as shown in FIG. 6. The IISERP-COF3 is two-dimensional structure built from hexagonal layers separated by a distance of 3.69 and are held together by -stacking interactions between the aromatic rings and has BET surface area of 403 m.sup.2/g. The particle size of Ni.sub.3N is approx. 2 nm.

(66) In yet another embodiment, the present invention relates to a method for splitting water at low overpotential comprising contacting the aqueous solution containing electrolyte with the electrocatalyst of formula (I).

(67) The present invention can produce not only oxygen but also electrons and protons that can be further reduced to produce hydrogen.

(68) In an embodiment, the present invention relates to a method for splitting of water using the electro catalysts of composite A or composite B. The electro catalysts of the present invention are coated on a glassy carbon electrode and serve as a working electrode in a three electrode test cell. The catalyst mass loading is maintained at 20 g for all the electrochemical studies of IISERP-COF2_nanoparticle composites (composite A) and 5 g for IISERP-COF3 nanoparticle composite-B. The water which is subjected to oxidative splitting is in the form of aqueous solution containing an electrolyte.

(69) In an embodiment, the electrolyte is de-aerated 0.1 M KOH when the catalyst is IISERP-COF2_nanoparticle composite A.

(70) In an embodiment, the electrolyte is de-aerated 1M KOH when the catalyst is IISERP-COF2_nanoparticle composite B.

(71) The method for the oxygen evolution comprises bringing aqueous solution containing 0.1M KOH into contact with the water splitting catalyst comprising COF2_AxBy(OH).sub.2; wherein A and B represents independently transition metals selected from Ni, Co, Fe, Mn or Zn; COF2 represents Tris(4-formylphenyl)amine; x and y represent the weight % of the metal loadings in the range 16-18 wt % of total wt % of COF2; or x=0 to 16 wt % or 0 to 18 wt % and y=0 to 16 wt % or 0 to 18 wt %; or x and y together represents 16-18 wt % of total wt % of COF2.

(72) The IISERP-COF2 forming a composite with 16 wt % Co (1) or 16 wt % of Ni (2) or 8 wt % Co+8 wt % Ni (3) or 12 wt % Co+4 wt % Ni (4) or 4 wt % Co+12 wt % Ni (5 or 1A) are evaluated for oxygen evolution reaction (OER) and associated charge transfer kinetics. The results are provided in Table 1 below.

(73) In an embodiment, the over potential for IISERP-COF2 nanoparticle composite A ranged from 245 to 255 mV at current density of 10 mA/cm.sup.2. In a preferred embodiment, the composite 5 or 1A comprising IISERP-COF2 loaded with 4 wt % Co and 12 wt % Ni shows the OER characteristics with lowest onset potential at 1.43V and over potential of 250 mV at current density of 10 mA/cm.sup.2.

(74) In another embodiment, the method for quantifying the oxygen evolution comprises bringing aqueous solution containing 1M KOH into contact with the water splitting catalyst comprising IISERP-COF3_Ni.sub.3N wherein COF3 comprises benzimidazole units connected together to form a polymeric conjugated framework; to split the water. The Composite-B, IISERP-COF3_Ni.sub.3N, has an onset potential at 1.44V and an overpotential of 230 mV@10 mA/cm.sup.2. Further, the composite-B is capable of delivering current densities of 350 mA/cm.sup.2 in 1 M KOH.

(75) In yet another embodiment, the kinetics associated with the mass transfer at the electrode-electrolyte contacts were examined using a Tafel plot (with 75% IR-compensation). The values ranged from 38 mV/decade to 91 mV/decade of current for composites A and B, respectively. Specifically, IISERP-COF2 loaded with 4 wt % Co and 12 wt % Ni composite showed a low value of 38 mV/decade. The composite-B, IISERP-COF3_Ni.sub.3N showed a value of 79 mV/decade. The low values indicate that the electrocatalysts of composite A and composite B have good charge transfer activity.

(76) An important parameter that assesses the activity of metal nanoparticles during the oxygen evolution reaction (OER) is the Turn Over Frequency (TOF). The TOF estimated for composite A and composite B are 0.186170 s.sup.1@overpotential of 400 mV and 0.52 s.sup.1@overpotential of 300 mV, respectively. The values indicate that the electrocatalysts of the present invention is about six- and twenty-fold higher in their activity than that obtained with the benchmarked Ir/C (0.027 s.sup.1).

(77) In an embodiment, the electrocatalyst of the present invention comprising Composite A and Composite B are stable even after 500 cycles with minimal current loss, minimum surface passivity.

(78) In yet another embodiment, the present invention relates to the use of electro catalyst consisting of covalent organic framework (COF) supported transition metal nanoparticle composites of the general formula I for use in water splitting system.

(79) The water splitting electrocatalyst of the present invention is useful in productions of oxygen, protons, hydrogen, electrons, hydrocarbons, and the like using water as the raw material.

(80) Further details of the present invention will be apparent from the examples presented below. Examples presented are purely illustrative and are not limited to the particular embodiments illustrated herein but include the permutations, which are obvious as set forth in the description.

Example 1: Synthesis of IISERP-COF2-

(81) Tris(4-formylphenyl)amine (40 mg, 0.12 mmol) and 1,4-diaminobenzene (20 mg, 0.19 mmol) were weighed into a Pyrex tube and dissolved in ethanol (3.0 mL). To the mixture O-dichlorobenzene (3.0 mL) was added and stirred until a clear yellow solution was observed. This was followed by addition of 0.25 mL of aqueous acetic acid (3 M solution). The Pyrex tube was flash frozen in a liquid nitrogen bath and sealed. The Pyrex tube along with its contents was placed in an oven at 120 C. for 3 days to obtain about 48 mg of yellow coloured solid which was washed with DMF, dioxane, MeOH, Acetone and THF.

(82) Isolated yield: 86%; (Formula for COF: C.sub.240N.sub.32H.sub.168, Mol. wt. 3500.2 g/mol, CHN Obsd. C=77.35%; H=4.409%; N=10.85%. Calc. 82.36%; H=4.84%; N=12.81%).

Example 2: Characterization of IISERP-COF2- and IISERP-COF2-

(83) Pawley-Le Bail Method (in P6/m) reveal unit cell for IISERP-COF2-: a=b=31.71(3); c=10.25(1); IISERP-COF2-: a=b=29.43(3); c=28.48(4), The hexagonal layers of the IISERP-COF2-(3, show ABAB . . . arrangement with uniform triangular-shaped 18.5 pores along the ab-plane which agree well with the pore size determined from adsorption studies. The layers are held together by a six-point CH . . . Phenyl type interactions and their buckling results in uniform cavities (22 ) along the interlayer spaces. PXRD studies showing stability of the IISERP-COF2 are shown in (FIG. 1A). TGA of the IISERP-COF2 in its and forms are shown in (FIG. 1C).

(84) The Field emission SEM patterns of the IISERP-COF2-(3 showed the growth of uniform hexagonal flakes. These hexagonal flakes stack among themselves to form large sheet structures which seem to wrap in-wards around their corners particularly once they reach sizes>500 nm as revealed by HRTEM. A 77K Nitrogen adsorption isotherm of the COF disclose that the IISERP-COF2- possess a bimodal pore size distribution with pores of the sizes 18 and 22 pores (FIG. 2).

Example 3: Synthesis of IISERP-COF2-_Co/Ni(OH)2 composite (1A)

(85) IISER-COF2- (100 mg) was suspended in 20 mL of n-hexane and the mixture was sonicated for about 30 min until it became yellow color homogeneous suspension. To this, a solution containing NiCl.sub.2.6H.sub.2O (20 mg) and CoCl.sub.2.6H.sub.2O (20 mg) dissolved in 1 mL of MeOH was added drop by drop over period of 3 h with vigorous stirring until the color changed from yellow to wine. Stirring was continued for about 12 h at room temperature. The solid particles settled down and the solvent was decanted. Resulting solid was dried at room temperature for 12 hrs and the wine colored powder was maintained at 150 C. for 12 h. To this was added about 20 mL of Millipore water followed by addition of 25 mL of freshly prepared 0.6 M aqueous NaBH.sub.4 solution. The product was a dark green suspension and the solid was collected by centrifuging, drying under vacuum. CHN % Obsd. (Calc. % within brackets): C=60.73 (62.47); H=4.45 (4.19); N=8.92 (9.71).

(86) The as prepared IISERP-COF2-3_Co/Ni(OH).sub.2 composite 1A was characterized by PXRD (FIG. 3A) and IR spectra (FIG. 3B). The loading of metal nanoparticles on to COF2 was determined by XPS spectra (FIG. 4A to 4D).

(87) The heterometallic composites with varying metal ratios were prepared by the procedure disclosed in example 3. For preparation of heterometallic composites 100 mg of COF was used and 40 mg of the metal salt.

(88) To prepare heterometallic composites comprising of Co and Ni, the Co:Ni ratio employed were: 20 mg Co:20 mg Ni, 10 mg Co:30 mg Ni, 30 mg Co:10 mg Ni. In all the cases approx. 16 to 18% metal loading was observed. The as-prepared composites were further characterized (FIG. 5A to 5C).

Example 4: Synthesis of Homometallic-IISER-COF2- Composites

(89) The COF (100 mg) was dispersed in 20 mL of n-hexane and the mixture was sonicated for about 30 min resulting in a yellow color suspension. To this, a clear methanolic solution of NiCl.sub.2.6H.sub.2O or CoCl.sub.2.6H.sub.2O (40 mg in 0.5 mL of MeOH) was added drop by drop over period of 3 h with vigorous stirring until the color of the suspension changed from yellow to deep orange color. The contents were further stirred for 12 h at room temperature and the solid particles were extracted by decanting the solvent and dried at room temperature. They were further heated at 150 C. for 12 h and then cooled to room temperature. The resulting orange solid was then washed with copious amounts of Millipore water (50 mL) and ethanol (25 mL). The deep-orange solid was further suspended in 20 mL of water and reduction was carried out by adding 25 mL freshly prepared 0.6 M aqueous NaBH.sub.4 solution under vigorous stirring to obtain COF supported Nickel or Cobalt catalysts as a dark green solid. The synthesized samples were collected by centrifuging, dried under vacuum and used for the catalytic studies. Also, the final green solid even upon sonicating in water did not seem to produce any colored solution, suggesting lack of any unreacted or unloaded Ni.sup.2+ salts that could be leaching out. The EDAX analyses of solid extracted from aqueous supernatant confirmed the absence of any metal. In addition, the EDAX analysis carried out on the extracted metal-COF composite showed no trace of chloride ions. (CHN % Obsd. NiCOF with 15-16% Ni loading (Calc. % within brackets): C=61.60 (62.49); H=4.35 (4.20); N=9.13 (9.72) and CHN Obsd. Co COF with 15-16% Co loading: C=62.32 (62.45); H=4.45 (4.19); N=9.24 (9.71).

Example 5: Synthesis of IISERP-COF3 at High Temperature (HT)

(90) In a 20 ml pyrex tube, 1,3,5-tri formyl phloroglucinol (0.053 g, 0.25 mmol) was dissolved in a mixture of 2 ml mesitylene and 4 ml 1,4-dioxane. To this solution, 3,3-diaminobenzidine (0.082 g, 0.38 mmol) was added under vigorous stirring and contents were stirred at room temperature for 30 mins. A deep red colored slurry was formed. 0.5 ml of aqueous acetic acid was added to the mixture and stirred for another 30 mins. The tube was then purged with N.sub.2 and flash frozen in liquid nitrogen. The tube was sealed under nitrogen flow and heated to 120 C. for 3 days. After the reaction mixture was cooled to room temperature bright red color fluffy powder was isolated by vacuum filtration and was subjected to vigorous washings with DMF, DMA, acetone and finally with copious amount of THF. The bright red color product was dried in an oven before further characterization.

(91) The yield was 83% with respect to triformylphlorogucinol; CHN values (calculated values in % within brackets): C: 70.23(68.86); H: 4.12 (3.26); N: 16.42 (17.80).

Example 7: Synthesis of IISERP-COF3 at Room Temperature (RT)

(92) To a 20 ml conical flask, 1,3,5-tri formyl phloroguicinol (0.053 g, 0.25 mmol) was dissolved in a mixture of 2 ml Mesitylene and 4 ml 1,4-dioxane. Following this 3,3-diaminobenzidine (0.082 g, 0.38 mmol) was added to the mixture under vigorous stirring. Contents were stirred at room temperature for 30 mins. A deep red color slurry was formed. 0.5 ml of aqueous acetic acid was added to the mixture and stirred for another 30 min. Bright red color fluffy powder was isolated by vacuum filtration and was washed with DMF, DMA, acetone and finally with copious amounts of THF. The bright red color product was dried in hot air oven before further characterization.

(93) Yield was 78% with respect to triformylphlorogucinol. CHN values (calculated values in % within brackets): C: 69.23(68.86); H: 4.86 (3.26); N: 16.12 (17.80).

Example 8: Synthesis of IISERP-COF3 at Room Temperature (RT) (Gram Scale)

(94) To a 100 ml conical flask, 1,3,5-tri formyl phloroguicinol (0.53 g, 2.5 mmol) was dissolved in a mixture of 10 ml Mesitylene and 30 ml 1,4-dioxane. Following this 3,3-diaminobenzidine (0.82 g, 3.8 mmol) was added to the mixture under vigorous stirring and the stirring was continued at room temperature for 1 hr. Homogeneous deep red color slurry was formed. 0.5 ml of aqueous acetic acid was added to the mixture and stirred for another 1 hr. The final product, a bright red color fluffy powder was isolated by vacuum filtration and was subjected to vigorous washings with DMF, DMA, acetone and finally with copious amount of THF and was dried in an oven before further characterization.

(95) Yield was 78% with respect to triformylphlorogucinol. CHN values (calculated values in % within brackets): C: 69.78(68.86); H: 4.67 (3.26); N: 16.02 (17.80)

Example 9: Structural Characterization of IISERP-COF3

(96) Pawley and Le Bail Method (in P6/m) reveal two-dimensional structure built from hexagonal layers for IISERP-COF3 with unit cell dimensions: a=b=55.87(5); c=3.53(7); Pawley refinement statistics: Rp=4.52%; wRp=5.20%. The layers of the IISERP-COF3, had an AAA . . . stacking with hexagonal shaped 29 pores along the ab-plane. The layers were separated by a distance of 3.53 and held together by -stacking interactions between the aromatic rings. The energy estimated for this configuration using Tight Binding Density Functional Theory (DFTB) method was observed to be the lowest of all the attempted space groups and could be attributed to the close interlayer separation favoring strong - interactions between the layers. These interactions along with the additional hydrogen bond stability provided by the keto-enol tautomerized OH groups account for the high chemical and thermal stabilities of the COF3. The short interlayer separations could be attributed to hydrogen bond interactions around the phloroglucinol cores which enhance the - interactions indicating the crystalline nature of IISERP-COF3.

(97) The IISERP-COF3 was further characterized by PXRD, TGA, IR, Adsorption isotherm, HRTEM (FIG. 6A to 6I)

Example 10: Structural Characterization of IISERP-COF3_Ni3N

(98) XPS spectra (FIG. 7A to 7D) and HRTEM (FIGS. 8A and 8B) images of the Ni.sub.3N loaded phase, IISERP-COF3_Ni.sub.3N suggests uniform distribution of metal nanoparticles within the COF3. The nanocrystallites have sizes in the range of <5 nm. The uniform distribution of the nanoparticles significantly reduces the porosity of the sample. A 72% drop in porosity was observed upon nanoparticle loading.

Example 11: Electrochemical Measurements

(99) The test cell was fabricated in traditional fashion of three-electrode test cell by using Hg/HgO and platinum flag as the reference and counter electrode, respectively. The catalyst coated glassy carbon electrode dried under an IR-lamp was used as the working electrode. The catalyst mass loading was maintained at 20 g for all the electrochemical studies of IISERP-COF2 composites-A and it was 5 g for IISERP-COF2_Ni.sub.3N composite B. In all the IISERP-COF2 composites-A, de-aerated 0.1M KOH was used as an electrolyte, while in the case of Composite-B de-aerated 1M KOH was used as an electrolyte.

(100) Linear sweep voltammograms (LSVs) of composites 1-5 were investigated to examine the anodic reaction by scanning in a potential window of 1.1 to 1.7 V (vs. RHE) at 1600 rotation per minute (rpm) of working electrode.

(101) The peaks at 1.30 V for Composite-A and at 1.36V for Composite-B, before the onset of oxygen evolution reaction (OER) corresponds to the formation of Ni (III) or Ni (IV) species, that are characteristics of the active nickel site. This was followed by a sharp augment in current due to the oxygen evolution as evident from FIG. 9A. The composites 1-5 show the characteristic property of oxygen evolution at potential>1.40V (vs. RHE) (FIG. 9B). Among the composites A, composite 1A with 4 wt % Co+12 wt % Ni showed the best OER characteristics with lowest onset (1.43V) and overpotential (250 mV@10 mA/cm.sup.2). The Composite-B, IISERP-COF3_Ni.sub.3N, has an onset potential at 1.43V and an overpotential of 230 mV@10 mA/cm.sup.2, which we report as one of the lowest overpotential stated for any water splitting electrocatalyst. Also, composite-B was capable of delivering current densities of 350 mA/cm.sup.2 in 1 M KOH.

(102) It was further observed that although the gas evolution at the catalyst coated electrode during the electrolysis was very vigorous, gas bubbles dissipated rapidly into the solution with no bubble accumulation observed on the electrode surface. This was evident from the kinetics associated with the mass transfer at the electrode-electrolyte contacts which were examined using a Tafel plot (with 75% IR-compensation). The Tafel slopes (FIGS. 9C and 9D) from a log I vs. potential showed a low value of 38 mV per decade of current for Composite-A and 79 mV/decade for current for Composite-B. Thus, both the composites of the present invention possess good kinetics at the electrode-electrolyte interface which indicates poor rate of corrosion of the catalyst and enhanced stability.

(103) To evaluate the Faradaic efficiency of the composites of the present invention, Rotating Ring Disk Electrode (RRDE) experiment was carried out by applying the series of current density steps from 2 to 10 mAcm.sup.2, Faradaic efficiency was calculated to be 0.9 and 0.98@1 mA/cm.sup.2 for composite-A and B, respectively. The electrochemical surface area (ECSA) and roughness factor (RF) for composite-A were 4.7 cm.sup.2 and 644, while these were 1.21 cm.sup.2 and 172 for composite-B. Composites-A and B had double layer capacitance of 3.9810.sup.4 and 3.6310.sup.5 F/cm.sup.2, respectively.

(104) Another factor that quantifies the oxygen evolution is the Turn Over Frequency (TOF), which was estimated for both the composites. TOF was measured to be 0.186170 s.sup.1@overpotential of 400 mV for composite A and 0.52 s.sup.1@overpotential of 300 mV (assuming that all the metal sites are involved in OER) for composite B, which was six- and twenty-fold higher than that obtained with the benchmarked Ir/C (0.027 s.sup.1).

(105) The chronoampeperometry measurements on the composites indicated the stable current that could be generated over several hours (FIGS. 9E and 9F). The cyclic stability was confirmed by potential scanning for 500 cycles in Faradaic region (0.9 to 1.45 V, Insets of FIGS. 9E and 9F). All the samples displayed good stability with minimal current loss, which indicate the COF backbone was assisting the particles to be more stable in a compacted area. The XPS analysis of the post OER samples showed the presence of Ni and Co in their +2 oxidation states. In order to verify the stability of the catalyst under the operating conditions, metal-loaded COFs were suspended in 0.1M KOH solution for 24 hrs and subjected the dried solid extracted from the supernatant to an EDAX analysis. No metal components were observed in the extract, except for the potassium from the electrolyte, which confirmed the lack of catalyst leaching. The electrochemical stability was further substantiated from the highly reproducible redox activity of the catalyst even after 500 cycles and the onset and overpotential of the sample before and after such cycles remained unchanged. These results confirmed lack of any kind of surface passivation.

(106) The Electrochemical measurements for composite A and composite B are provided in Table 1 below.

(107) As has been described above, the present disclosure makes it possible to provide electro catalysts comprising covalent organic framework (COF) as a porous support for noble metal free nanoparticles in water splitting with low overpotential, with minimal current loss, minimum surface passivity and stability up to 500 cycles. The electrocatalyst of the present invention reduces the activation energy associated with the anodic evolution reaction during electrolysis of water and also improves the charge/mass transfer kinetics.

(108) Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be affected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.

(109) TABLE-US-00001 TABLE 1 Important electrochemical parameters quantifying the OER efficiency Over- potential Tafel Onset at 10 Slope Sr. Name of the potential Electro- mA/cm.sup.2 (mV/ No Compound Name or code (V) lyte (mV) dec) 1 IISERP-COF2 Tris(4-formyl phenyl)amine- terephaldehyde polymer 2 IISERP-COF2_ 1 1.5523 0.1M 480 91 16Ni(OH).sub.2 KOH 3 IISERP-COF2_ 2 1.5383 0.1M 394 89 16Co(OH).sub.2 KOH 4 IISERP-COF2_ 3 1.4683 0.1M 308 64 8Co(OH).sub.2 + KOH 8Ni(OH).sub.2 5 IISERP-COF2_ 4 1.4683 0.1M 392 83 12Co(OH).sub.2 + KOH 4Ni(OH).sub.2 6 IISERP-COF2_ 5 or 1.43 0.1M 250 38 4Co(OH).sub.2 + Composite-1A KOH 12Ni(OH).sub.2 7 IISERP-COF3 BzImidazole- phloroglucinol polymer 8 IISERP-COF3_ Composite-B 1.4300 1M 230 79 8Ni.sub.3N KOH