Two-dimensional transition metal hydroxide-carbon nanotube heterojunction catalyst based on interlayer hydroxyl modification and regulation and control method for hydrogen production by alkaline electrolysis of water

Abstract

The invention relates to the technical field of chemical catalysts, in particular to a two-dimensional transition metal hydroxide-carbon nano tube heterojunction catalyst based on interlayer hydroxyl modification and an alkaline water electrolysis hydrogen production regulation and control method, and the catalyst comprises interlayer hydroxyl modified two-dimensional transition metal hydroxide LDH-OH and functionalized carbon nano tube CNT-OH; the LDH-OH is of a layered structure formed by Ni < 2 + > and Fe < 2 + > through hydroxyl coordination, and hydroxyl is contained between layers; the surface of the CNT-OH contains hydroxyl and / or carboxyl; the LDH-OH and the CNT-OH are in covalent binding through an-O-bond to form a heterojunction. The interlayer hydroxyl modification enables the interlayer spacing of the two-dimensional transition metal hydroxide to be enlarged, structure swelling and collapse in an alkaline environment are effectively inhibited, and the problems that a traditional non-noble metal catalyst is easy to corrode and poor in stability under a strong alkaline condition are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chemical catalysts, in particular to a two-dimensional transition metal hydroxide-carbon nanotube heterojunction catalyst based on interlayer hydroxyl modification and an alkaline electrolytic water hydrogen production regulation method. BACKGROUND

[0002] Alkaline electrolytic water hydrogen production (ALK) is a key technology for realizing efficient conversion of renewable energy, and the core challenge is to develop non-noble metal catalysts with high activity and stability. Traditional nickel-iron-based hydroxides (such as NiFe-LDH) have high oxygen evolution reaction (OER) activity, but have the following defects: interlayer hydroxyl (-OH) is easily removed in a strong alkaline environment, leading to structural collapse; poor intrinsic conductivity, which requires carbon material composite to improve charge transport efficiency; insufficient heterojunction interface synergy, limiting further improvement of catalytic activity. Carbon nanotubes (CNT) are often introduced into the catalyst system as carriers or conductive frameworks due to their high electrical conductivity and mechanical strength. However, simple physical mixing cannot achieve strong interface coupling between metal hydroxide and CNT, resulting in low utilization of active sites. In addition, existing research lacks systematic study on the directional modification of interlayer hydroxyl and the electronic regulation mechanism of the heterojunction interface, which restricts the breakthrough of catalyst performance. SUMMARY

[0003] The primary object of the present application is to provide an alkaline-stable interlayer hydroxyl-modified two-dimensional transition metal hydroxide-carbon nanotube heterojunction catalyst.

[0004] A further object of the present application is to provide an alkaline-stable interlayer hydroxyl-modified two-dimensional transition metal hydroxide-carbon nanotube heterojunction catalyst, comprising an interlayer hydroxyl-modified two-dimensional transition metal hydroxide LDH-OH and a functionalized carbon nanotube CNT-OH; the LDH-OH is prepared by hydroxyl coordination to form a layered structure, and the interlayer contains hydroxyl groups; the surface of the CNT-OH contains hydroxyl and / or carboxyl groups; the LDH-OH and the CNT-OH are covalently bonded to form a heterojunction through -O- bonds. 2+ and Fe 2+ form a layered structure through hydroxyl coordination, and the interlayer contains hydroxyl groups; the surface of the CNT-OH contains hydroxyl and / or carboxyl groups; the LDH-OH and the CNT-OH are covalently bonded to form a heterojunction through -O- bonds.

[0005] Preferably, the Fe 2+ in the LDH-OH is partially or completely replaced by Co 2+ to form an interlayer hydroxyl-modified two-dimensional transition metal hydroxide containing Ni 2+ and Co 2+ .

[0006] Preferably, it further comprises a doping element, and the doping element is Mo or W; the Mo partially substitutes Fe in the LDH-OH layer in the form of Mo 6+ , and the W partially substitutes Fe in the LDH-OH layer in the form of W 6+ .2+ Into a layered structure.

[0007] Preferably, the interlayer hydroxyl modification is formed with the aid of citric acid; the total molar ratio of the citric acid to the metal ions in the LDH-OH is 1.1-1.2:1.

[0008] Preferably, the CNT-OH is prepared by dispersing carbon nanotubes in concentrated nitric acid, refluxing at 80°C for 3-5h, and centrifuging and washing to neutral.

[0009] Preferably, the mass ratio of the LDH-OH to the CNT-OH is 2:1; the -O- bond is formed by mixing the LDH-OH and the CNT-OH and freeze-drying, and then annealing at 280-320°C for 1.5-2.5h.

[0010] Preferably, the structural formula of the LDH-OH is [Ni 1.5 Co 0.5 (OH)2] 0.5+ (OH - ) 0.5 ·mH2O wherein M is Fe 2 + , Co 2+ or a combination thereof, and m is the number of crystal water.

[0011] Preferably, the catalyst is used in the application of alkaline electrolysis water hydrogen evolution or oxygen evolution.

[0012] Compared with the prior art, the present application has the following remarkable advantages:

[0013] 1. The interlayer hydroxyl modification expands the interlayer spacing of the two-dimensional transition metal hydroxide, effectively inhibits the structural swelling and collapse in the alkaline environment, and solves the problems of easy corrosion and poor stability of the traditional non-noble metal catalyst in strong alkaline conditions. For example, the activity attenuation of the hydroxyl-modified catalyst is only 8% after 2000 cycles, which is much better than the 35% attenuation rate of the unmodified catalyst; the carbon nanotubes and the LDH are covalently combined through the -O- bond to form a stable heterojunction, avoiding the defects of weak interface bonding force and easy falling caused by physical mixing, and further enhancing the structural stability of the catalyst.

[0014] 2. Oxygen evolution reaction performance: through element substitution and doping regulation, the OER overpotential is significantly reduced. For example, after Co replaces Fe, the OER current density at 1.62V reaches 50mA / cm 2, the overpotential of OER is reduced by 30 mV compared with the NiFe system; after W doping, the OER overpotential is further reduced to 210 mV, which is better than the Mo-doped and undoped systems, which is due to the strong hybridization of the 5d orbit of W and the 2p orbit of O, which promotes the adsorption-desorption balance of the reaction intermediate; hydrogen evolution reaction performance: transition metal substitution and doping effectively improve the HER activity, and the current density of the W-doped catalyst reaches 100 mA / cm 2 , which is better than the Co-substituted and undoped systems, and exhibits better reaction kinetics.

[0015] 3. The introduction of functionalized CNTs greatly improves the conductivity of the catalyst, for example, the conductivity of the W-doped system reaches 0.8 S / cm, which is 16 times that of single LDH-OH, solving the problem of poor intrinsic conductivity of traditional hydroxides and accelerating the transfer of charges inside the catalyst; the formation of a covalent heterojunction enhances the interface electron coupling between LDH and CNT, avoiding the interface resistance loss of physical mixing, and further optimizing the electron transport path.

[0016] 4. Non-noble metals such as Ni, Fe, and Co are used as active components, replacing expensive noble metal catalysts such as ruthenium-based catalysts, significantly reducing the preparation cost and being suitable for large-scale industrial application. The preparation process of the catalyst is simple and controllable, and the catalyst can exhibit excellent performance in 1.0M KOH and other conventional alkaline electrolytes, and has practical application feasibility; by adjusting the amount of citric acid, the annealing temperature, the type and ratio of the doping element, the activity and stability of the catalyst can be directionally controlled, providing a clear design idea for subsequent performance optimization.

[0017] In summary, through multi-dimensional structure and electronic state regulation, the non-noble metal catalyst successfully realizes the "high activity, high stability, and low cost" synergy in alkaline electrolytic water hydrogen production, providing an important technical reference for catalyst development in the field of renewable energy conversion. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present application will be described below. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0019] Example 1: Preparation of NiFe-LDH-OH / CNT catalyst

[0020] (1) Intercalated hydroxyl modification: dissolve 1.5 mmol of Ni(NO3)2·6H2O and 0.5 mmol of Fe(NO3)2·4H2O in 50 mL of deionized water, add 2.25 mmol of citric acid, and stir until clear;

[0021] (2) Hydrothermal synthesis: transferred to a 100 mL autoclave, 180 °C for 12 h, centrifugal washing and freeze-drying;

[0022] (3) CNT functionalization: 100 mg CNT was dispersed in 50 mL concentrated nitric acid, 80 °C for 4 h, centrifugal washing to neutral;

[0023] (4) Heterostructure construction: NiFe-LDH-OH and CNT-OH were mixed at 2:1, ultrasonic dispersion and freeze-drying, 300 °C for 2 h.

[0024] (5) Application test: in 1.0 M KOH, OER current density reached 50 mA / cm 2 at 1.65 V, HER reached 100 mA / cm 2 at -100 mV. The structural formula of NiFe-LDH-OH is as follows:

[0025]

[0026] Example 2: NiCo-LDH-OH / CNT catalyst with Co replacing Fe

[0027] (1) Composition

[0028] Metal salt: 1.5 mmol nickel nitrate hexahydrate (Ni(NO3)2·6H2O), 0.5 mmol cobalt nitrate hexahydrate (Co(NO3)2·6H2O)

[0029] Hydroxyl modifier: 2.25 mmol citric acid (C6H8O7)

[0030] Carrier: 100 mg carbon nanotubes (CNT, molecular formula can be represented as (C n )

[0031] Others: concentrated nitric acid (HNO3), deionized water (H2O)

[0032] Preparation process

[0033] (1) Interlayer hydroxyl modification:

[0034] Dissolve Ni(NO3)2·6H2O and Co(NO3)2·6H2O in deionized water, add citric acid and stir. Citric acid acts as a chelating agent to form a complex with Ni 2+ , Co 2+ , promoting the generation of interlayer hydroxyl (-OH),

[0035] (2) Hydrothermal synthesis: The above solution was transferred to an autoclave and reacted at 180°C for 12 h to form interlayer hydroxyl-modified NiCo layered double hydroxide (NiCo-LDH-OH) with a general structure of [Ni 1.5 Co 0.5 (OH)2] 0.5+ (OH - ) 0.5 ·mH2O (further stabilizing the structure of the interlayer hydroxyl).

[0036] (3) CNT functionalization: CNT was refluxed with concentrated nitric acid to form hydroxyl (-OH) and carboxyl (-COOH) groups on the surface,

[0037] (4) Heterostructure construction:

[0038] NiCo-LDH-OH and CNT-OH were mixed at a ratio of 2:1, ultrasonically dispersed, and freeze-dried, and then annealed at 300°C for 2 h. Annealing promoted the covalent bonding of LDH and CNT through -O- bonds, forming a stable heterojunction.

[0039] Performance test results

[0040] In 1.0 M KOH electrolyte:

[0041] OER: Current density reached 50 mA / cm 2 (30 mV lower than the original NiFe-LDH-OH / CNT of 1.65 V), attributed to the high activity of the Co 2+ / Co 3+ redox pair.

[0042] HER: Current density reached 100 mA / cm 2 at -95 mV.

[0043] Stability: After 2000 cycles, the activity decayed only by 8%, better than the 35% of Comparative Example 1.

[0044] Example 2: Mo-doped NiFe-LDH-OH / CNT catalyst

[0045] (1) Based on Example 1, 0.1 mmol Mo(CO)6 was added to the metal salt solution to prepare a Mo-doped catalyst (denoted as NiFeMo-LDH-OH / CNT).

[0046] (2) Performance improvement: The OER overpotential was further reduced to 230 mV, attributed to the d-electron regulation effect of Mo. The structural chemical formula of the functionalized CNT (CNT-OH) is as follows:

[0047]

[0048] Example 3: W-doped NiFeW-LDH-OH / CNT catalyst

[0049] 1. Composition and molecular formula

[0050] Metal salt: 1.5 mmol Ni(NO3)2·6H2O, 0.5 mmol Fe(NO3)2·4H2O, 0.1 mmol sodium tungstate (Na2WO4)

[0051] Hydroxyl modifier: 2.25 mmol C6H8O7

[0052] Support: 100 mg CNT ((C) n )

[0053] Others: HNO3, H2O

[0054] 2. Preparation process and reaction equation

[0055] (1) Interlayer hydroxyl modification and W doping:

[0056] Add Na2WO4 in the original NiFe system, W 6+ Substitute part of Fe 3+ Into the LDH layer, citric acid assisted hydroxyl coordination, the reaction is as follows:

[0057] (2) Subsequent hydrothermal synthesis, CNT functionalization and heterostructure building steps are the same as Example 1, W 6+ Form a d-electron conjugation effect with surrounding metal ions, optimize electron transport.

[0058] Performance test results

[0059] In 1.0 M KOH electrolyte:

[0060] OER overpotential is reduced to 210 mV, because the 5d orbital of W and the 2p orbital of O have stronger hybridization, which promotes the adsorption-desorption balance of OER intermediates (*OH, *O, *OOH).

[0061] HER: Current density reaches 100 mA / cm 2 ( better than Mo-doped -100 mV).

[0062] Conductivity is improved to 0.8 S / cm, because W doping enhances interlayer electron delocalization.

[0063] Comparative Example 1: NiFe-LDH / CNT without hydroxyl modification

[0064] Omit the addition of citric acid in step 1, directly hydrothermally synthesize NiFe-LDH, and then composite with CNT.

[0065] Performance comparison: OER overpotential increased to 270 mV, activity decayed by 35% after 1000 cycles.

[0066] Comparative Example 2: Single NiFe-LDH-OH

[0067] Step 3 of CNT compounding was omitted, and only interlayer hydroxyl modified NiFe-LDH was used.

[0068] Performance comparison: conductivity decreased to 0.05 S / cm, OER current density was only 15 mA / cm 2 at 1.65 V.

[0069] Comparative Example 3: Traditional physical mixing method

[0070] NiFe-LDH-OH was directly mixed with CNT by grinding, without annealing treatment.

[0071] Interface defect: the heterojunction interface bonding force was weak, and TEM observation showed that there was a clear gap between CNT and the interlayer of LDH.

[0072] The chemical formula of the heterojunction NiFe-LDH-OH / CNT is as follows in Formula 3:

[0073]

[0074] The structure of the doped type such as NiFeW-LDH-OH is as follows in Formula 4:

[0075]

[0076] The performance test of the embodiments of the present application is as follows in Table 1:

[0077]

[0078]

[0079]

[0080] From the above Table 1, the influence of catalyst structure optimization on performance is as follows:

[0081] (1) Example 1 (NiFe-LDH-OH / CNT) generated interlayer hydroxyl with the help of citric acid, and its OER performance was significantly better than that of Comparative Example 1 (OER overpotential 270 mV) without hydroxyl modification, and the stability was better (Comparative Example 1 decayed by 35% after 1000 cycles, while Example 2 only decayed by 8% after 2000 cycles). This shows that the interlayer hydroxyl can enhance the structural stability, reduce the overpotential, and improve the catalytic activity.

[0082] (2) Example 1 and single NiFe-LDH-OH comparison shows that the conductivity is greatly improved after CNT compounding, and the OER current density is significantly improved. It is proved that CNT as a carrier can optimize the efficiency of electron transmission and promote the dispersion of active components

[0083] (3) Example 1 makes LDH and CNT form -O- bond covalent combination through annealing treatment, while Comparative Example 3 has a gap due to weak interfacial bonding force, which limits the performance. This proves that the covalent heterojunction can enhance the interfacial interaction and improve the efficiency of electron transfer.

[0084] (4) In Example 2 (NiCo-LDH-OH / CNT), the OER overpotential is reduced by 30mV compared with Example 1 after Co replaces Fe, and the HER performance is improved, which is attributed to the high activity of Co 2+ / Co 3+ redox pair, indicating that transition metal substitution can adjust the electronic structure and optimize the catalytic kinetics.

[0085] (5) Mo doping reduces the OER overpotential to 230mV, which benefits from the d-electron regulation effect of Mo; W doping further reduces the OER overpotential to 210mV, and the HER performance is optimal, because the 5d orbital of W and the 2p orbital of O are strongly hybridized, promoting the adsorption-desorption balance of intermediates, and enhancing the interlayer electron delocalization and improving the conductivity. Both kinds of doping optimize the catalytic activity through electronic effect, and the effect of W doping is better than that of Mo.

[0086] (6) The W-doped NiFeW-LDH-OH / CNT has the best comprehensive performance, with the lowest OER overpotential, the highest HER activity, and the conductivity reaching 0.8S / cm, which reflects the effectiveness of the synergistic strategy of "interlayer hydroxyl modification + CNT covalent compounding + high valence metal doping".

[0087] The present application successfully prepares an alkaline stable catalyst with low cost, high activity and high stability by the combined design of interlayer hydroxyl stabilized structure, CNT enhanced electron transmission and transition metal substitution / doping regulated electronic state, which solves the problems of low activity and poor stability of traditional non-noble metal catalysts in alkaline conditions, and provides an efficient solution for alkaline water electrolysis hydrogen production

[0088] The preferred embodiments of the present application disclosed above are only used to help explain the present application. The preferred embodiments do not describe all the details and limit the application to the specific embodiments described. Obviously, many modifications and changes can be made according to the content of the present application. The present application selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present application, so that those skilled in the art can well understand and utilize the present application. The present application is only subject to the claims and the entire scope and equivalents thereof.

Claims

1. An alkali-stable interlayer hydroxyl-modified two-dimensional transition metal hydroxide-carbon nanotube heterojunction catalyst, characterized in that, comprise two-dimensional transition metal hydroxide LDH-OH and functionalized carbon nanotube CNT-OH with interlayer hydroxyl modification; the LDH-OH is made of Ni 2+ and Fe 2+ form a layered structure through hydroxyl coordination, containing hydroxyl between layers; the CNT-OH surface contains hydroxyl and / or carboxyl; the LDH-OH and CNT-OH are covalently bonded through -O- bond to form a heterojunction.

2. The catalyst according to claim 1, characterized in that, Fe in the LDH-OH 2+ partly or completely replaced by Co 2+ to form a two-dimensional transition metal hydroxide interlayer modified with Ni 2+ and Co 2+ .

3. The catalyst of claim 1, wherein Also included are doping elements, which are Mo or W; the Mo is partially substituted for Fe in the LDH-OH layers in the form of Mo 6+ 6+ 2+ into the layered structure.​​ 4. Catalyst according to any one of claims 1 to 3, characterized in that The interlayer hydroxyl group is modified by citric acid assisted formation; the total mole ratio of citric acid to metal ions in LDH-OH is 1.1-1.2:

1.

5. The catalyst of claim 1, wherein The CNT-OH is prepared by dispersing carbon nanotubes in concentrated nitric acid, refluxing at 80℃ for 3-5h, and centrifuging and washing to neutral.

6. The catalyst of claim 1, wherein The mass ratio of LDH-OH to CNT-OH is 2:1; the -O- bond is formed by mixing LDH-OH and CNT-OH and freeze-drying, and then annealing at 280-320℃ for 1.5-2.5h.

7. The catalyst of claim 1, wherein The structural formula of the LDH-OH is [Ni 1.5 Co 0.5 (OH)2] 0.5+ (OH - ) 0.5 ·mH2O wherein M is Fe 2+ , Co 2+ or a combination thereof, and m is the number of crystallization water.

8. Use of the catalyst according to any one of claims 1-7 in alkaline electrolysis of water for hydrogen evolution or oxygen evolution.

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