CN115537872B - Double-doped efficient electrolytic water catalyst and preparation method and application thereof - Google Patents

Abstract

The invention discloses a double-doped high-efficiency water electrolysis catalyst and its preparation method and application. The catalyst is doped with two transition metal elements on the surface of a nickel nitride-based material; wherein the nickel nitride-based material is in the form of nanosheets. Structure, transition metal elements are attached to the surface of nickel nitride-based nanosheets in the form of nanoparticles. In atomic-level resolution electron microscopy, Co and V exist in the form of unique single atoms and clusters, and the transition metal elements Including any two of V, Cr, and Co.

Description

A double-doped high-efficiency water electrolysis catalyst and its preparation method and application

Technical field

The invention relates to the technical field of electrolytic water catalysts, and in particular to a double-doped high-efficiency electrolytic water catalyst and its preparation method and application.

Background technique

Hydrogen has become a clean energy source with great potential because it only emits water and does not emit other harmful gases or solid particles during use. Currently, one of the challenges facing hydrogen energy research is to explore abundant, long-term availability, corrosion-resistant, and efficient electrocatalysts.

Commercial noble metals (such as Pt and Ru) have become the most prominent HER electrocatalysts due to their unique electronic properties, low Gibbs free energy of hydrogen desorption and adsorption, and low water molecule splitting energy barrier. However, its high cost, rarity, and non-durability limit its practical application in the electrolyzed water industry.

Transition metal nitride (TMN) is one of the excellent electrocatalysts for the preparation of HER due to its excellent conductivity and corrosion resistance. However, most reported bulk TMNs suffer from two drawbacks: slow kinetics and the need for additional energy to promote water dissociation. Transition metal doping (TMD) is one of the simplest solutions because it can adjust the physical and chemical properties and lattice structure of each electrocatalyst to improve HER catalytic performance and stability. However, there are problems with the doping of existing transition metals. If a single transition metal element is doped, the catalyst obtained will have poor performance; while the double-doped catalyst has problems such as difficulty in doping and low doping amount, and the catalyst obtained The performance is not ideal; moreover, the transition metal-doped catalyst prepared by the existing technology has poor corrosion resistance during the electrolysis of hydrogen production. Especially when electrolysis of seawater for hydrogen production, it cannot be used continuously for a long time.

Contents of the invention

In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a double-doped high-efficiency water electrolysis catalyst to solve the problems of difficulty in doping transition metal elements and poor corrosion resistance in the prior art.

In order to solve the above technical problems, the present invention adopts the following technical solutions:

A double-doped high-efficiency water electrolysis catalyst, the catalyst is doped with two transition metal elements on the surface of a nickel nitride-based material; wherein, the nickel nitride-based material has a nanosheet structure, and the transition metal elements are in the form of nanoparticles Attached to the surface of the nickel nitride-based nanosheet, the transition metal element includes any two of V, Cr, and Co. Among them, when Co and V are doped with two elements, Co and V exist in the form of unique single atoms and clusters in atomic-level resolution electron microscopy.

The invention also provides a method for preparing a double-doped high-efficiency water electrolysis catalyst for preparing the above catalyst, which specifically includes the following steps:

Step 1: Clean and pre-treat the nickel foam-based material to remove the surface oxide layer and impurities;

Step 2: Select any two transition metal sources from vanadium source, chromium source, and cobalt source, dissolve them in water with Ni(NO ₃ ) ₂ , NH ₄ F, and urea, and mix them to form a homogeneous solution; mix the homogeneous solution The nickel foam-based material treated in step 1 is transferred to an autoclave for hydrothermal reaction to obtain double-doped nickel hydroxide nanosheets, which are washed and dried;

Step 3: The nickel hydroxide nanosheets obtained in Step 2 are subjected to a nitridation reaction in a high-purity NH ₃ atmosphere to obtain the double-doped high-efficiency water electrolysis catalyst.

The invention also provides the application of a double-doped high-efficiency water electrolysis catalyst. The catalyst prepared by the above preparation method is used for electrolysis of water or hydrogen production from seawater.

Compared with the prior art, the present invention has the following beneficial effects:

1. The catalyst of the present invention is a diatom-doped catalyst, which can further adjust the electronic structure of the catalyst, provide more catalytic sites and high electron transfer rate, and increase the catalyst interface electron transfer rate and active sites. quantity, improving mass transfer kinetics.

2. In the catalyst of the present invention, the doped transition metal elements have a great contribution to the alkaline HER activity, among which V can increase the adsorption of water molecules and thereby enhance the hydrophilicity of the electrode surface, and the Co element increases the surface active sites of the catalyst. , Cr element promotes the adsorption of oxygen atoms by the catalyst; Co,V-Ni ₃ N shows the largest density of states near the Fermi level, indicating that the Co,V-Ni ₃ N surface has good conductivity and interfacial electron transfer dynamics.

3. The double-doped high-efficiency water electrolysis catalysts of the present invention have excellent catalytic performance, extremely low charge transfer resistance, and rich electrochemical active sites; Co,V-Ni ₃ N nanosheets exhibit excellent electrochemical properties. Catalytic performance and durability. At a current density of 10mA cm ^-2 , the overpotential of the Co,V-Ni ₃ N electrode is only 10mV (exceeding commercial Pt), and the Tafel slope is only 43mV dec ^-1 . It is a noble metal and non-noble metal catalyst that has been disclosed so far. HER _{overpotential} in The overpotentials of Cr, V-Ni ₃ N are 70 and 80 mV respectively, and the Tafel slopes are 75 and 94 mVdec ^-1 respectively. It can be seen that the catalysts of the present invention have good catalytic activity in seawater electrolysis, especially Co, V-Ni3N The electrode showed even better performance, with an overpotential of only 41mV at a current density of 10mA cm ^-2 .

Description of drawings

Figure 1 is a morphological photograph of Example 1 of the present invention; Figures 1a to 1c are scanning electron microscope images of Example 1 (Co, V-Ni ₃ N); Figures 1d to 1e are transmission images of Example 1 Electron microscope image; Figure 1f is a high-resolution transmission electron microscope image of the nanosheet of Example 1; Figure 1g is an electron diffraction pattern of Example 1; Figures 1h to 1l are HAADF-STEM images of Example 1 and Ni, Co, V and The element mapping image of N.

Figure 2 is a morphological photograph of Example 2 and Example 3 of the present invention: Figure 2a to Figure 2b are SEM pictures of Example 2 (Co, Cr-Ni ₃ N); Figure 2c to Figure 2d are Examples SEM picture of 3(Cr,V-Ni ₃ N).

Figure 3 is a graph showing the fresh water hydrogen evolution performance of Examples, Comparative Examples and Commercial Catalyst Pt/C of the present invention; Figure 3a is the LSV curve of Examples, Comparative Examples and Commercial Catalyst Pt/C in 1 mol KOH; Figure 3b is Tafel plots of the polarization curves of Examples, Comparative Examples and Commercial Catalyst Pt/C; Figure 3c is the Nyquist plot of Examples, Comparative Examples and Commercial Catalyst Pt/C, where the inset is an enlarged image; Figure 3d is the Example , the Cdl value of the control example and the commercial catalyst Pt/C; Figure 3e is the overpotential comparison of the example, the control example and the commercial catalyst Pt/C at 10mA cm ^-2 and 100mA cm ^-2 ; Figure 3f is the example, control Volcano plots of examples and commercial catalyst Pt/C; Figure 3g is the It curve of Example 1 at different current densities for 100 hours; Figure 3h is the durability test of Example 1 before and after 3000CV cycles, in which the inset is the durability test SEM image of Example 1.

Figure 4 is a graph showing the hydrogen evolution performance of alkaline seawater of Examples, Comparative Examples and Commercial Catalyst Pt/C of the present invention; Figure 4a is a LSV curve graph of Examples, Comparative Examples and Commercial Catalyst Pt/C in alkaline seawater; Figure 4b is the Tafel diagram obtained from the polarization curve of the example, control example and commercial catalyst Pt/C; Figure 4c is the example, control example and commercial catalyst Pt/C at 10mA cm ^-2 and 100mA cm ^-2 The overpotential comparison chart below; Figure 4d is the It curve of Example 1 and Comparative Example 1 at 10mA cm ^-2 for 6h; Figure 4e is the durability test of Example 1 before and after 3000CV cycles, in which the inset is the durability test. SEM images of Example 1; Figure 4f is XPS images of Example 1 before and after 3000CV cycles; Figures 4g to 4i are TEM images of Example 1 after HER.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and examples.

1. A double-doped high-efficiency water electrolysis catalyst

The catalyst of the present invention is doped with two or more transition metal elements on the surface of nickel nitride-based materials; wherein, the nickel nitride-based material has a nanosheet structure, and the transition metal elements are attached to the nickel nitride-based nanometers in the form of nanoparticles. The surface of the sheet, and the transition metal element includes any two of V, Cr, and Co. Calculated in terms of mass percentage, the mass percentage of transition metal elements in the catalyst is 5% to 10%. The thickness of the nickel nitride-based material is 20-30 nm, and the average particle size of the transition metal element nanoparticles is 100-300 nm.

After conducting in-depth research on the catalyst for electrolyzing water to produce hydrogen, the present invention found that various anions and cations rich in seawater are high-quality raw materials and can also improve the conductivity of the electrolyte. But unlike fresh water, the composition of seawater is very complex. High-concentration chloride ions will not only compete with the oxygen evolution reaction (OER) of electrolyzed water at the anode to release chlorine, but will also seriously corrode most catalysts containing metal elements. Hypochlorite, a highly corrosive by-product of the chlorine reaction (CER), can also block the active sites of precious metal catalysts. Therefore, if a precious metal catalyst is used, not only will the precious metal catalyst be consumed quickly and the cost high, but the precious metal catalyst will also be used in the electrolysis of seawater to produce hydrogen. , and cannot exert its proper catalytic effect. Therefore, the present invention improves the catalyst by introducing transition metal elements and combining the excellent conductivity and corrosion resistance of transition metal nitride (TMN). After doping many transition metal elements into TMN, it was found that some transition metal elements are difficult to dope into TMN, while the doping amount of some transition metal elements is too small, which does not contribute to the catalyst performance. almost none. In this series of studies, the present invention found that in atomic-level resolution electron microscopy, when the two elements Co and V are doped at the same time, Co and V exist in the form of unique single atoms and clusters, and part of Co and V They exist in the form of single atoms respectively. Some Co and V aggregate in the form of single atoms to form clusters. The synergistic effect of the two has a beneficial impact on the performance of the catalyst. At the same time, it is also found that the Co element can change the electronic structure of the catalyst and thereby It shows a low water decomposition overpotential. The V element can increase the adsorption of water molecules to enhance the hydrophilicity of the electrode surface. The positively valenced Cr atoms are more likely to attract negatively charged oxygen atoms in water molecules through strong electrostatic interactions, thereby promoting Subsequent HER reaction steps, therefore, the present invention selects these three elements for doping, and conducts in-depth research on the morphology and performance of the doped catalyst.

2. Examples and Comparative Examples

Table 1

Types and dosage of raw materials Vanadium source Chromium source Cobalt source Metal ion molar ratio Example 1 NH ₄ VO ₃ -- Co(NO ₃ ) ₂ ·6H ₂ O 1:1 Example 2 -- Cr(NO ₃ ) ₂ ·9H ₂ O Co(NO ₃ ) ₂ ·6H ₂ O 1:1 Example 3 NH ₄ VO ₃ Cr(NO ₃ ) ₂ ·9H ₂ O -- 1:1 Example 4 NH ₄ VO ₃ -- Co(NO ₃ ) ₂ ·6H ₂ O 1:0.5 Example 5 NH ₄ VO ₃ -- Co(NO ₃ ) ₂ ·6H ₂ O 1:2 Example 6 -- Cr(NO ₃ ) ₂ ·9H ₂ O Co(NO ₃ ) ₂ ·6H ₂ O 1:0.5 Example 7 -- Cr(NO ₃ ) ₂ ·9H ₂ O Co(NO ₃ ) ₂ ·6H ₂ O 1:2 Example 8 NH ₄ VO ₃ Cr(NO ₃ ) ₂ ·9H ₂ O -- 1:0.5 Example 9 NH ₄ VO ₃ Cr(NO ₃ ) ₂ ·9H ₂ O -- 1:2

Note: -- indicates that the raw material has not been added.

The following method was used to prepare Example 1:

1) Pretreatment of nickel foam substrate

Pre-treat the nickel foam substrate with dilute hydrochloric acid solution to remove the oxide layer and impurities on the surface, and then clean it multiple times with deionized water and ethanol;

2) Preparation of double-doped nickel hydroxide (Co,V-Ni(OH) ₂ ) nanosheet array

Dissolve 0.05mmol NH ₄ VO ₃ , 0.05mmol Co(NO ₃ ) ₂ ·6H ₂ O, 0.90mmol Ni(NO ₃ ) ₂ ·6H ₂ O, 5mmol Co(NH ₂ ) ₂ and 2mmol NH ₄ F in 20mL deionized water. In water, stir to form a homogeneous solution. Transfer the solution and a piece of nickel foam (NF, 2cm×3cm) to a 50mL autoclave, seal it, and heat it with water at 120°C for 8 hours to obtain a double-doped nickel hydroxide (Co,V-Ni(OH) ₂ ) nanosheet array. The obtained products were washed with water and ethanol respectively. V-Ni(OH) ₂ was synthesized using a similar method without adding Co(NO ₃ ) ₂ ·6H ₂ O;

3) Preparation of Co,V-Ni ₃ N

Put the prepared nickel hydroxide (Co,V-Ni(OH) ₂ ) nanosheets into a tube furnace, and nitride the nickel hydroxide nanosheets at 450°C for 2 hours in a high-purity NH ₃ atmosphere. The products obtained were named Co, V-Ni ₃ N respectively.

Prepare materials according to Table 1 and Table 2, and use the process conditions in Table 2 and the preparation method of Example 1 to prepare Examples 2 to 9. At the same time, Ni ₃ N without transition elements was used as Comparative Example 1.

3. Catalyst morphology and performance testing

Taking Examples 1 to 3 and Comparative Example 1 as representatives, their performance is analyzed and compared. The morphology of the catalyst of Example 1 (Co,V-Ni ₃ N) was detected, and its hydrogen evolution performance was tested. Figures 1a-1b show typical low-magnification scanning electron microscope (SEM) images of Example 1 (Co, V-Ni ₃ N), with thin (usually around 20-30nm) nanosheet morphology, and even thinner nanosheets. The sheet structure has a higher surface area, exposing more active sites. The transmission electron microscope (TEM) images shown in Figures 1d-1e can fully demonstrate the typical nanosheet structure of Example 1. The high-resolution transmission electron microscope (HR-TEM) image of Example 1 (Fig. 1f) shows that the 0.18 nm lattice fringe corresponds to the (201) plane of Ni ₃ N, which is consistent with the XRD card Ni ₃ N (JCPDS No. 89- 5144) consistent. After detection, it was found that the incorporation of Co and V did not cause lattice distortion, but existed in the form of unique single atoms and clusters. In addition, based on selected area electron diffraction (SAED) and elemental mapping, it was further determined that Ni, Co, V and N elements were evenly distributed in this area in Figures 1g-1l, providing clearer evidence for the successful preparation of Example 1. Figures 2a-2b show that Example 2 (Co, Cr-Ni ₃ N) exhibits a typical nanoparticle structure. After annealing, the nanosheets in Example 2 completely collapsed and were difficult to directly observe in the electron microscope image. This was due to the unstable structure of the Co, Cr-Ni(OH) ₂ nanosheets in Example 2. At the same time, the thickness of the electrode surfaces of Example 3 (Cr, V-Ni ₃ N) and Example 1 (Co, V-Ni ₃ N) was compared and found (Figure 2c-2d) that the surface of Example 3 was more scattered and messy. , this is caused by the incoordination of Cr and V, and this messy nanosheet structure is not conducive to the generation of redox active sites and charge storage, resulting in a reduction in the electrochemical performance of the material. These results confirm the microscopic morphology of the catalyst It has a lot to do with the type of doping elements.

Commercial Pt/C was used as a control example to compare performance with the embodiments of the present invention. As shown in Figures 3a and 3e, the linear sweep voltammetry (LSV) curves show Example 1 (Co, V-Ni ₃ N), Example 2 (Co, Cr-Ni ₃ N), Example 3 (Cr, V-Ni ₃ N), control example 1 (Ni ₃ N) and commercial Pt/C in 1 M KOH solution with a current density of 10 mAcm ^-2 have overpotentials of 10 mV, 70 mV, 82 mV, 141 mV and 38 mV, respectively. The results show that Example 1 (Co,V-Ni ₃ N) has the best alkaline electrochemical hydrogen evolution performance among DTMD Ni ₃ N, and its activity even significantly exceeds commercial Pt/C. The overpotential of Example 2 (Co, Cr-Ni3N) is 75mVdec ^-1 , the overpotential of Example 3 (Cr, V-Ni ₃ N) is 94mV dec ^-1 and the overpotential of Comparative Example 1 ( _Ni3N ) is 75mVdec -1. Compared with the potential of 123mV dec ^-1 , the Tafel slope (43mV mV dec ^-1 ) of Example 1 (Co,V-Ni ₃ N) is smaller, which directly shows that Example 1 (Co,V-Ni ₃ N) is more Favorable HER kinetics (Figure 3b). As shown in Figure 3c, the charge transfer resistance (Rct) was studied by electrochemical impedance spectroscopy (EIS). The Rct value of Example 1 (Co, V-Ni ₃ N) is small, about 0.98Ω, which is significantly lower than other examples (Example 2 is 2.4Ω, Example 3 is 4.3Ω), and is also lower than the comparative example. 1 (Ni ₃ N is 7.0Ω) and the commercial product Pt/C (Pt/C is 1.3Ω). This confirms that the type of doping element is crucial to again produce efficient electrocatalysts. Example 1 (Co, V-Ni ₃ N) has fast electron transfer speed, good conductivity and good reaction kinetics, which helps to enhance their catalytic activity for HER. Next, cyclic voltammograms (CV) in the non-Faradaic region were recorded at different scan rates (10-60mV s-1) to further study the intrinsic activity of Ni ₃ N. The electrochemically active specific surface area (ECSA) was determined from the Cdl curve obtained by CV. As shown in Figure 3d, the Cdl value of Example 1 (Co, V-Ni ₃ N) (11.8mF cm ^-2 ) is higher than that of Example 2 (Co, Cr-Ni ₃ N's Cdl value is 10.1mF cm ^-2 ). Example 3 (Cr, V-Ni ₃ N has a Cdl value of 5.5mF cm ^-2 ), Comparative Example 1 (Ni ₃ N has a Cdl value of 2.6mF cm ^-2 ) and commercial Pt/C (Pt/C Cdl The value is 11.7mF cm ^-2 ). At the same time, Example 1 shows excellent performance at large current density, and the overpotential of Example 1 at 500 mA cm ^-2 is only 203 mV. The operating durability of electrocatalysis is another indispensable indicator. The overpotential of the Example 1 (Co, V-Ni3N) catalyst after 3000CV cycles is negligibly higher than the overpotential before testing. The results of chronopotentiometric measurement Its excellent stability was also demonstrated (Figure 3h). As shown in Figure 3h, neither the electrocatalytic performance nor the macroscopic morphology changed after long-term cycling.

For practical application, the present invention further studies the catalytic performance of the catalyst in seawater electrolysis. Pure seawater was collected from the coast of a certain place (pH≈8.1). Natural seawater has the following problems as an electrolyte: First, the chlorine evolution reaction (CER) occurs at the anode of seawater electrolysis and competes with OER; the generated chlorine and hypochlorite dissolved in the electrolyte will corrode the cathode material. Secondly, high concentrations of toxic cations (Ca ²⁺ and Mg ²⁺ ) in natural seawater will be electrodeposited on the cathode surface, blocking active sites. Third, CER is more competitive than OER in natural seawater and will produce more chlorine and hypochlorite, corroding electrodes. Therefore, the present invention further studies the HER performance of Ni ₃ N-based catalysts in alkaline seawater (pH=13.1). The overpotential of Example 1 (Co, V-Ni3N) at 10mA cm ^-2 is only 41mV, and the Tafel slope is 46mV dec ^-1 (Figure 4a-c), which is better than Example 2 (88mV), Example 3 (82mV) and Control Example 1 (197mV) performance. At the same time, Example 1 also has the fastest reaction kinetics. In addition to activity, the most critical catalytic performance indicator is stability. Therefore, Example 1 and Comparative Example 1 were subjected to continuous chronoamperometric testing of 6 h and 3000 CV cycles (Fig. 4d, 4e). After working for a long time, the catalyst of Example 1 still has excellent activity, which corresponds to the fact that the Ni peak in Figure 4f basically does not shift. HR-TEM images (Figure 4g-4i) show that the nanosheet morphology after HER reaction is very similar to the original morphology. The above tests show that since the doping of Co and V introduces more types of metal bonding (Co-Co), it can also provide higher corrosion resistance. Therefore, the continuous long-term hypertonic process has almost no corrosive effect on the surface structure of Example 1.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the technical solutions. Those of ordinary skill in the art should understand that those technical solutions of the present invention can be modified or equivalently substituted without departing from the present technology. The purpose and scope of the solution should be covered by the claims of the present invention.

Claims (8)

1. A double-doped high-efficiency water electrolysis catalyst, characterized in that the catalyst is doped with two transition metal elements on the surface of a nickel nitride-based material; wherein the nickel nitride-based material has a nanosheet structure, and the transition metal The elements are attached to the surface of the nickel nitride-based nanosheets in the form of nanoparticles, and the transition metal elements include V and Co; the thickness of the nickel nitride-based material is 20~30nm, and the transition metal element nanoparticles are The average particle size is 100~300nm; calculated in terms of mass percentage, the mass percentage of transition metal elements in the catalyst is 5%~10%. 2. A method for preparing a double-doped high-efficiency water electrolysis catalyst, characterized in that it is used to prepare the catalyst according to claim 1, specifically including the following steps: Step 1: Clean and pre-treat the nickel foam-based material to remove the surface oxide layer and impurities; Step 2: Select two transition metal sources, vanadium source and cobalt source, dissolve them in water with Ni (NO ₃ ) ₂ , NH ₄ F, and urea, and mix them to form a homogeneous solution; after processing the homogeneous solution in step 1 The foamed nickel-based material is transferred to an autoclave for hydrothermal reaction to obtain double-doped nickel hydroxide nanosheets, which are washed and dried; Step 3: The nickel hydroxide nanosheets obtained in Step 2 are subjected to a nitridation reaction in a high-purity NH ₃ atmosphere to obtain the double-doped high-efficiency water electrolysis catalyst. 3. The method for preparing a double-doped high-efficiency water electrolysis catalyst according to claim 2, wherein the vanadium source includes metavanadate; and the cobalt source includes one of nitrate or chloride salt. 4. The preparation method of a double-doped high-efficiency water electrolysis catalyst according to claim 3, characterized in that, in step 2, when the transition metal source is a vanadium source and a cobalt source, the metal ion molar ratio of V and Co is 1 : (0.5~2). 5. The preparation method of double-doped high-efficiency water electrolysis catalyst according to claim 4, characterized in that, in step 2, the concentration of Ni (NO ₃ ) ₂ in the homogeneous solution is 0.04~0.05mol/L, NH ₄ The concentration of F is 0.08~0.12mol/L, and the concentration of urea is 0.2~0.3mol/L. 6. The preparation method of a double-doped high-efficiency water electrolysis catalyst according to claim 2, characterized in that in step 2, the hydrothermal reaction is carried out at 100°C to 140°C for 6h to 10h. 7. The preparation method of a double-doped high-efficiency water electrolysis catalyst according to claim 2, characterized in that, in step 3, the nitriding reaction is: nitriding at 400°C to 500°C for 1h to 3h; Purity NH ₃ is ammonia gas with a mass fraction of at least 99%. 8. The application of a double-doped high-efficiency water electrolysis catalyst, characterized in that the catalyst prepared by the preparation method of any one of claims 2 to 7 is used for electrolysis of water or hydrogen production from seawater.