Synthesis of non-PGM porous catalysts and ink manufacturing processes therefor

Description

Technical field of the invention

[0001] The present invention relates to the domain of industrial scale water electrolysers, in particular in the field of anion exchange membrane water electrolysis (AEMWE). More particularly, it relates to the synthesis of catalysts which do not involve metals belonging to the group of Platinum (non PGM metals) but which exhibit similar or better performances relating to, but not limited to, their catalyst activity and / or lifetime.Technical background of the invention

[0002] The performances of a catalyst depend on several parameters, such as the chemical formulation, the microstructure and the distribution of the catalyst within the electrochemical reactor. The interaction of the catalyst with the ionomer which is used to supply the ions required for the electrochemical reaction, is also critical in the context of water electrolysis.

[0003] Platinum group metals (PGM) such as Ruthenium (Ru) and Iridium (Ir) and their oxides, are considered as state-of-the-art catalysts for the Oxygen Evolution Reaction (OER) which occurs at the anode of electrolysis cells (acidic or alkaline) (1)< . However, large-scale use of these metals, or their oxides, pose several major issues due to their low availability on earth, strategic character and high price.

[0004] Non PGM transition metal-based catalysts demonstrated high performance and also satisfactory stability, either chemical or physical, in alkaline media (2)< . From the comparison of the electrochemical behaviour of 3,500 non-PGM ternary catalysts, Cobalt (Co) and Nickel (Ni) were determined to be essential elements of the catalysts for the OER in alkaline media, while others metals such as Iron (Fe), Gallium (Ga) or Chromium (Cr) were shown to increase the catalytic activity (3)< .

[0005] Numerous synthetic methods have been developed to produce NiCo catalysts for the OER (4)< . Spinel-type materials such as cobaltites, have exhibited very interesting performances for the OER on account of their high catalytic ability and thermodynamic stability in alkaline medium (5)< . It has been shown, that 2D ordered mesoporous structures possess a high surface area and enable an easier dispersion and formation of a steady layer on the working electrode compared to nanoparticles (6)< .

[0006] Such structures can be obtained using the nanocasting method, which enables the preparation of 2D mesoporous oxides of different compositions which exhibits large surface areas and highly connected mesoporous frameworks with a uniform pore size distribution (7)< . This method, which relies on the use of a hard template to structure the 2D oxide materials, has been used in numerous applications (8)< . Mesoporous silica templates, such as SBA 15 ™< (hexagonal structure), have been used for the replication of mesoporous Ni x Co y spinel structures 5,7< .

[0007] To include non-PGM electrocatalysts in the anode layer of an anion exchange membrane water electrolysis cell (AEMWEC), anion exchange ionomers (AEI) must be added to bind the catalyst and conduct hydroxide ions towards the active sites. Membrane electrode assemblies containing commercial or home-made non-PGM anode catalytic particles (NiCoFeO x , Ce 0.2 MnFe 1.8 O 4 , NiMn 2 O 4 , CuCoO x ) have been prepared using FAA ™< anion exchange membranes and FAA3 ™< as the ionomer in electrodes (both from Fumatech, Germany) (9,10,11)< or using a Tokuyama AEM A201 ™< and I 2 as the AEI ™< ionomer (Acta Spa, Italy) in electrodes (12)< .

[0008] Sustainion ™< (Dioxide materials, USA) and Aemion ™< (Ionomer Innovations, Inc., Vancouver, BC, Canada) AEM have also been used in AMEWE cells combined with non-PGM anode catalysts, but generally with Nafion ™< acting only as a binder (13)< .

[0009] Baranova and et al. studied the effects of the Aemion ™< ionomer in an anode catalytic layer (Ni 90 Fe 10 nanoparticles) in both three electrode and electrolysis (Aemion AF1-HNN8-50-X membrane, 1 M KOH, 50 °C) cell configurations; the results were compared to the commercial Fumion ™< and Nafion ™< ionomers, revealing a strong interaction between Aemion ™< ionomer and the NiFe catalyst which limits the Ni(OH) 2 / NiOOH transition and decreases the OER activity. This study highlights the importance of the catalytic ink (catalysts + ionomer + solvents) formulation and of the deposition method, either on a backing (catalyst coated backing - CCB) or on the membrane (catalyst coated membrane - CCM), towards the performance of the AEMWEC (14)< .

[0010] The scientifc articles "Three dimensionally ordered mesoporous hydroxylated NixCo3-xO4 spinels for the oxygen evolution reaction: on the hydroxyl-induced surface restructuring effect" by I. Abidat et al,, JOURNAL OF MATERIALS CHEMISTRY A, vol. 5, no. 15, 17 March 2017, pages 7173-7183 and "Mesoporous NiCo2O4 Spinel: Influence of Calcination Temperature over Phase Purity and Thermal Stability" by Moisés Cabo et al., CRYSTAL GROWTH & DESIGN, vol. 9, no. 11, 4 November 2009, pages 4814-4821 are both directed the preparation of a mesoporous NiCo 2 O 4 spinel by a silica template method.

[0011] A solution which is able to provide a new cheaper and highly active non-PGM OER catalyst, would be beneficial to produce oxygen and hydrogen, through the electrolysis of water using an anion exchange membrane water electrolysis cell, without harming the environment.Description of the invention

[0012] A material having a 2D-ordered mesoporous physical structure, in the context of the present invention, is a material which consists of atomically thin sheets exhibiting covalent in-plane bonding and weak interlayer and layer-substrate bonding. Such a definition was published in 2017 by Andrew J. Mannix et al (15)< . Reference is also made to Yan Ai et al (16)< .

[0013] The present invention relates to a synthesis method of a material belonging to the spinel nickel cobaltite family, represented by the formula Ni Co (2-x) Fe x O 4 in which x is greater or equal to 0 and less than 1, and having a 2D-ordered mesoporous physical structure which does not comprise any mesoporous template, comprising the following steps: A step a) of mixing a 2D-ordered mesoporous template, a Ni(II) inorganic salt, a Co(II) inorganic salt, optionally a Fe(III) inorganic salt and an alcohol, to form a first alcoholic suspension of said 2D-ordered mesoporous template and said inorganic salts and; A step b) during which said first alcoholic suspension of said 2D-ordered mesoporous template and of said inorganic salts, obtained at step a), is first homogenised and subsequently dried, to form a powder of a mixture of said 2D-ordered mesoporous template and said inorganic salts; A step c) of heating under air, the powder obtained at step b) to form a powder of a mixture of said 2D-ordered mesoporous template, the Ni(II) oxide, the Co(II) oxide and optionally the Fe(III) oxide; A step d) of mixing a 2D-ordered mesoporous template, a Ni(II) inorganic salt, a Co(II) inorganic salt, optionally a Fe(III) inorganic salt and an alcohol, to form a second alcoholic suspension of said 2D-ordered mesoporous template and said inorganic salts; A step e) during which said second alcoholic suspension of said 2D-ordered mesoporous template and said inorganic salts, obtained at step d), is mixed with said powder of a mixture of said 2D-ordered mesoporous template, the Ni(II) oxide, the Co(II) oxide and optionally the Fe(III) oxide, obtained at step c), to form a third suspension; A step f) of first homogenising, and subsequently drying, the third suspension, obtained at step e), to form a powder; A step g) of heating under air, the powder obtained at step f) to obtain a material belonging to the spinel nickel cobaltite family represented by the formula Ni Co (2-x) Fe x O 4 in which x is greater or equal to 0 and less than 1 and having a 2D-ordered mesoporous physical structure; A step h) of mixing heated aqueous alkaline solution and the material obtained at step g), to form an aqueous alkaline suspension of said material; and A step i) of filtrating the aqueous alkaline suspension obtained at step h), to separate a retentate, which is further rinsed with water and dried to form, by removing the 2D-ordered mesoporous template still present, said expected material belonging to the spinel nickel cobaltite family represented by the formula Ni Co (2-x) Fe x O 4 in which x is greater or equal to 0 and less than 1 and having a 2D-ordered mesoporous physical structure, which does not comprise any mesoporous template.

[0014] The Ni(II) Co(II) and Fe(III) inorganic salts, in the context of the invention, are for example nitrate, sulfite, sulfate or phosphate salts.

[0015] According to a particular embodiment, the invention relates to the synthesis method, as defined above, characterized in that x is equal to 0 in the formula Ni Co (2-x) Fe x O 4 .

[0016] According to another particular embodiment, the invention relates to the synthesis method as defined above, in which said Ni(II) inorganic salt, Co(II) inorganic salt and optional Fe(III) inorganic salt, implemented at steps a) and d), are all nitrate salts.

[0017] The mesoporous template, in the context of the invention, is a chemical compound which is used in the synthesis method of the material, as defined above, to generate the formation of its physical mesoporous structure. More particularly, said mesoporous template, is a 2D-ordered mesoporous silica template. An example of such a template is SBA-15 ™< .

[0018] According to another particular embodiment, the invention relates to the synthesis method, as defined above, in which the 2D-ordered mesoporous template, implemented at steps a) and d), is a 2D-ordered mesoporous silica template.

[0019] The alcohol implemented at steps a) and d), in the context of the invention, are namely a C 1 to C 4 alkanol, such as ethanol, propanol, isopropanol, butanol, isobutanol, sec-butanol or tert-butanol.

[0020] According to another particular embodiment, the invention relates to the synthesis method, as defined above, in which said alcohol, implemented at steps a) and d), is ethanol.

[0021] The alcoholic suspensions made at steps a) and e), in the context of the invention, are homogenized by known methods.

[0022] According to another particular embodiment, the invention relates to the synthesis method, as defined above, in which the alcoholic suspensions made at steps a) and e) are sonicated in an ultrasonic bath at steps b) and f).

[0023] According to another particular embodiment, the invention relates to the synthesis method, as defined above, in which said step c) comprises: a sub-step c 1 )step b a sub-step c 2 )step c 1 ), is maintained under air at said working temperature for approximately 5 hours to 10 hours.

[0024] According to a more particular embodiment, step c) of the synthesis method, as defined above, is achieved by heating under air the powder obtained at step b) such that the temperature gradually increases, at a rate of 2°C per minute, until a working temperature of approximately 250°C is reached, and by subsequently maintaining said heated powder for approximately 6 hours at this working temperature.

[0025] According to a particular embodiment, said step g) of the synthesis method, as defined above, comprises: a sub-step g 1 )step f a sub-step g 2 )step g 1 ) is maintained under air at said working temperature for approximately 5 hours to 10 hours.

[0026] According to a more particular embodiment, step g) of the synthesis method, as defined above, is achieved by heating under air the powder obtained at step f) such that the temperature gradually increases, at a rate of 2°C per minute, until a working temperature of approximately 450°C is reached, and by subsequently maintaining said heated powder for approximately 6 hours at this working temperature.

[0027] According to another embodiment, the present invention relates to a preparation method of the formulation, as defined above, comprising the following steps: A step j) of mixing a liquid medium with an anion exchange ionomer to obtain a ionomer suspension, A step k) of dispersing the material obtained at step j), in an aqueous solution of an aliphatic alcohol, A step l) of homogenising the alcoholic dispersion obtained at step k), A step m) of mixing the homogenised alcoholic dispersion obtained at step l), with said ionomer suspension obtained at step j), and A step n) of homogenising the mixture obtained at step m), to obtain the expected formulation.

[0028] More particularly, in the preparation method of the formulation, as defined above, said liquid medium, implemented at step j), is a mixture of ethanol and water or a mixture of acetone and ethanol, said anion exchange ionomer implemented at step j) is AP3-HNN6-00-X ™< and said aliphatic alcohol implemented at step k) is ethanol.

[0029] An embodiment not according to the invention relates to the use of the material belonging to the spinel nickel cobaltite family, represented by the formula Ni Co (2-x) Fe x O 4 in which x is greater or equal to 0 and less than 1, and having a 2D-ordered mesoporous physical structure which does not comprise any mesoporous templates, as defined above, as a non-PGM electrocatalyst on the anode layer of an anion exchange membrane water electrolysis cell.

[0030] Another embodiment not according to the invention relates to an electrode assembly for electrolysis, in particular an anion exchange membrane electrolysis, comprising as an electrocatalyst, the material belonging to the spinel nickel cobaltite family, represented by the formula Ni Co (2-x) Fe x O 4 in which x is greater or equal to 0 and less than 1, and having a 2D-ordered mesoporous physical structure which does not comprise any mesoporous template, as defined above.

[0031] A further embodiment not according to the invention relates to an anion exchange membrane water electrolysis cell, which comprises on its anode layer, as the sole electrocatalyst, the material belonging to the spinel nickel cobaltite family, represented by the formula Ni Co (2-x) Fe x O 4 , in which x is greater or equal to 0 and less than 1, and having a 2D-ordered mesoporous physical structure which does not comprise any mesoporous template, as defined above.

[0032] Finally, an embodiment not according to the invention relates to the use of the formulation, as defined above, to coat the material belonging to the spinel nickel cobaltite family, represented by the formula Ni Co (2-x) Fe x O 4 in which x is greater or equal to 0 and less than 1, of which the physical structure is a 2D-ordered mesoporous structure, and which does not comprise any mesoporous template, on the anode layer of an anion exchange membrane water electrolysis cell.Experimental data Preparation of a catalyst according to the invention

[0033] Ni(NO 3 ) 2 ·6H 2 O and Co(NO 3 ) 2.6 H 2 O (Ni:Co atomic ratio of 1:2) are suspended in 5 cm 3< ethanol. 0.5 g of the SBA-15 template is subsequently added. The mixture is sonicated in an ultrasonic bath for one hour, and is then dried in an oven overnight at 50°C. The obtained powder is calcined at 250°C for six hours under air, following a temperature ramp up from room temperature to 250°C at 2°C / min.

[0034] A second solution of the Ni and Co salts is prepared as above and the calcined powder previously obtained is added. The mixture is sonicated in an ultrasonic bath during one hour, and is then dried in an oven overnight at 50°C. The obtained powder is calcined at 450°C for six hours under air, following a temperature ramp up from room temperature to 450°C at 2°C / min.

[0035] A NaOH 2M aqueous solution is prepared and heated to 80°C. The obtained calcined powder is subsequently added to the solution. The mixture is allowed to rest for one hour and subsequently filtered. The retentate is thoroughly rinsed with ultrapure water, and dried at 80°C overnight in an oven to obtain the expected compound of the formula NiCo 2 having a 2D-ordered mesoporous structure.Preparation of a catalytic ink formulationPreparation of the Aemion suspension

[0036] Method 1: The Aemion solution is prepared by weighing 0.35 g of the ionomer AP3-HNN6-00-X ™< (Ionomer Innovations, Inc., Vancouver, BC, Canada) in a 30 cm 3< flask. Ethanol (5.98 g) is first added to the ionomer, followed by water (0.66 g). The mixture is kept under magnetic stirring at 45°C, before using it.

[0037] Method 2: The Aemion solution is prepared by weighing 1 g of AP3-HNN6-00-X ™< (Ionomer Innovations, Inc., Vancouver, BC, Canada) in a 30cm 3< flask. Acetone (10.5 g) is added first, followed by ethanol (4.5 g) dropwise. The mixture is kept under magnetic stirring at room temperature before using it.Preparation of the catalytic ink

[0038] The catalytic ink is prepared by dispersing 150 mg of the catalytic powder in a solution of 1 cm 3< water and 1.5 cm 3< ethanol. The mixture is sonicated in an ultrasonic bath for thirty minutes. 215 mg of an Aemion solution (5 wt%) are subsequently added, and the mixture is again sonicated in an ultrasonic bath for three minutes.Performance and activity

[0039] The performances and stability of the catalyst are evaluated using a 25 cm 2< active area single cell and several anion exchange membranes. The test consists of operating the cell at a constant current density for a minimum of one thousand hours. A 1M KOH aqueous solution is used as electrolyte, flowing on both the anode and cathode sides of the cell at a constant temperature of 60°C. Figure 2 shows the evolution of the cell voltage over time (over 1,000 hours) resulting from the degradation of cell components, but also due to reversible losses mostly associated to bubble trapping inside the cell. Degradation of components are monitored by a "post mortem" analysis and analysis of the KOH flowing through the cell.

[0040] Samples of KOH are taken periodically to identify if chemical elements are released during the operation (catalyst, corrosion products,). No traces of catalyst loss or dissolution were detected during the entire test.

[0041] The excellent and stable performances of the cell, over more than thousand hours, indicate that the non-PGM catalyst exhibits similar, or even slightly better, performances compared to the same cell using PGM catalysts, such IrO 2 , at the anode. The graphic in Figure 1 illustrates the catalytic activity of different catalyst formulations measured using a rotating disk electrode.

[0042] Figure 3 is an electron microscope image of the 2D-tubular structure of the catalyst synthesized using the SBA15 template.Cited references

[0043] (1) S. Cherevko et al. / Catalysis Today (2016), 262, 170-180. (2) L. Trotochaud et al. / J. Am. Chem. Soc. (2012), 134(41), 17253-17261. (3) J.B. Gerken et al. / Energy Environ. Sci. (2014), 7(7), 2376-2382. (4) C.X. Zhao et al. / Chem. Soc. Rev. (2021), 50, 7745-7778. (5) I. Abidat et al. / J. Mater. Chem. A (2015), 3(33), 17433-17444. (6) T. Grewe et al. / Chem. Mater. (2013), 25, 4926-4935. (7) I. Abidat et al. / J. Mater. Chem. A (2017), 5(15), 7173-7183. (8) D. Gu et al. / Chem. Soc. Rev. (2014), 43, 313-344. (9) T. Pandiarajan et al. / RSC Adv. (2015), 5, 34100-34108. (10) D. Xu et al. / ACS Catal. (2019), 9, 7-15. (11) A. Carbone et al. / Int. J. Hydr. Energy (2020), 45, 9285-9292. (12) I. Vincent et al. / Int. J. Hydr. Energy (2017), 42, 10752-10761. (13) D. Henkensmeier et al. / J. Electrochem. En. Conv. Stor. (2021), 18, 024001-1 / 18. (14) E. Cossar et al. / ACS Appl. Energy Mater. (2022), 5, 9938-9951. (15) A.J. Mannix et al. / Nature Reviews Chemistry (2017), 1, 0014. (16) Y. Ai et al. / Natl. Sci. Rev., 2022, Vol. 9, nwab 108

Claims

1. A synthesis method of a material belonging to the spinel nickel cobaltite family, represented by the formula Ni Co(2-x) FexO4 in which x is greater or equal to 0 and less than 1 and having a 2D-ordered mesoporous physical structure which does not comprise any mesoporous templates in it, comprising the following steps: - A step a) of mixing a 2D-ordered mesoporous template, a Ni(II) inorganic salt, a Co(II) inorganic salt, optionally a Fe(III) inorganic salt and an alcohol, to form a first alcoholic suspension of said 2D-ordered mesoporous template and said inorganic salts and; - A step b) during which said first alcoholic suspension of said 2D-ordered mesoporous template and of said inorganic salts, obtained at step a), is first homogenised and subsequently dried, to form a powder of a mixture of said 2D-ordered mesoporous template and said inorganic salts; - A step c) of heating under air, the powder obtained at step b) to form a powder of a mixture of said 2D-ordered mesoporous template, the Ni(II) oxide, the Co(II) oxide and optionally the Fe(III) oxide; - A step d) of mixing a 2D-ordered mesoporous template, a Ni(II) inorganic salt, a Co(II) inorganic salt, optionally a Fe(III) inorganic salt and an alcohol, to form a second alcoholic suspension of said 2D-ordered mesoporous template and said inorganic salts; - A step e) during which said second alcoholic suspension of said 2D-ordered mesoporous template and said inorganic salts, obtained at step d), is mixed with said powder of a mixture of said 2D-ordered mesoporous template, the Ni(II) oxide, the Co(II) oxide and optionally the Fe(III) oxide, obtained at step c), to form a third suspension; - A step f) of first homogenising, and subsequently drying, the third suspension, obtained at step e), to form a powder; - A step g) of heating under air, the powder obtained at step f) to obtain a material belonging to the spinel nickel cobaltite family represented by the formula Ni Co(2-x) FexO4 in which x is greater or equal to 0 and less than 1 and having a 2D-ordered mesoporous physical structure; - A step h) of mixing heated aqueous alkaline solution and the material obtained at step g), to form an aqueous alkaline suspension of said material; and - A step i) of filtrating the aqueous alkaline suspension obtained at step h), to separate a retentate, which is further rinsed with water and dried to form, by removing the 2D-ordered mesoporous template still present, said material belonging to the spinel nickel cobaltite family represented by the formula Ni Co(2-x) FexO4 in which x is greater or equal to 0 and less than 1 and having a 2D-ordered mesoporous physical structure, which does not comprise any mesoporous template.

2. A synthesis method according to claim 1 in which x is equal to 0 in the formula Ni Co(2-x) FexO4.

3. A synthesis method according to any one of claims 1 or 2, in which said Ni(II) inorganic salt, said Co(II) inorganic salt and said optional Fe(III) inorganic salt implemented at steps a) and d) are all nitrate salts.

4. A synthesis method according to any one of claims 1 to 3, in which said 2D-ordered mesoporous template implemented at steps a) and d), is a 2D-ordered-mesoporous silica template.