Ni-Cr-based catalyst based on citric acid co-calcination and preparation method of Ni-Cr-based catalyst

Abstract

The invention discloses a Ni-Cr-based catalyst based on citric acid co-calcination and a preparation method thereof.The preparation method comprises the steps that a nickel source, a chromium source and urea are dissolved in water for a hydrothermal reaction, and an obtained precipitation product is Ni-Cr hydroxide; and carrying out high-temperature calcination reaction on the Ni-Cr hydroxide and citric acid in an inert atmosphere to obtain the Ni-Cr-based catalyst. According to the Ni-Cr-based catalyst based on citric acid co-calcination and the preparation method of the Ni-Cr-based catalyst, Ni and Cr serve as metal sources, Ni-Cr hydroxide and citric acid are co-calcined, and the novel nickel-based catalyst with an unconventional crystal phase and good electro-catalysis urea oxidation activity and reaction stability is obtained.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic materials technology, and particularly relates to a Ni-Cr based catalyst based on co-calcination of citric acid and its preparation method. Background Technology

[0002] The urea oxidation reaction (UOR) has significant potential for energy and environmental applications, particularly for hydrogen production. Replacing the oxygen evolution reaction (OER) with UOR can reduce energy consumption by approximately 70%. This significant saving is due to the lower oxidation potential of UOR (0.37V vs. RHE) compared to OER (1.23V vs. RHE). However, the UOR reaction requires 6e oxidation states... — The transfer process, including the dehydrogenation of NH bonds, the breaking of CN bonds, the coupling of NN bonds, the oxidation of carbonyl groups, and the desorption of CO2, inevitably suffers from slow reaction kinetics. Furthermore, the reaction has an excessively high initial oxidation potential, requiring highly active catalysts to promote the reaction rate. These issues greatly limit the development of new energy technologies involving UOR.

[0003] Although noble metal-based catalysts (such as Pt, IrO2, and RuO2) have been shown to have high UOR activity, their high cost and resource scarcity limit their large-scale application. In fact, many nickel-based catalysts have exhibited catalytic performance comparable to their noble metal-based counterparts, thus nickel-based catalysts also show great promise for UOR. Considering their higher stability in alkaline media, the UOR reaction typically proceeds as follows: CO(NH2)2 + 6OH- - →CO2 + N2 + 5H2O + 6e - The key here is that using a nickel-based catalyst requires Ni reaction before UOR. 2+ To Ni 3+ The initial anodizing of Ni 3+ It serves as the active site. UOR typically occurs via two pathways using nickel-based catalysts: the indirect urea oxidation pathway, indicating that urea reacts with Ni... 3+ The reaction in which Ni 3+ Ni acts as an oxidizing agent, while urea acts as a reducing agent; the direct urea oxidation pathway indicates that Ni... 3+ It acts only as a catalyst and is not reduced by urea, thus exhibiting a greater tendency to produce Ni. 3+ Catalysts typically exhibit excellent UOR performance. Furthermore, Ni produced during the UOR process... 3+ The strong binding energy between the active site and the CO2 intermediate (1242.2 kJ / mol) -1 This makes the active sites highly susceptible to poisoning by this intermediate. Therefore, enhancing Ni... 3+The generation of [a specific substance] and the improvement of the catalyst's resistance to CO2 toxicity are two key challenges for Ni-based UOR catalysts. Currently, it is difficult to meet these two requirements using single-metal Ni-based catalysts. Summary of the Invention

[0004] To address the aforementioned technical issues, this invention provides a Ni-Cr based catalyst based on co-calcination with citric acid and its preparation method. Using Ni and Cr as metal sources, a novel unconventional crystalline phase nickel-based catalyst with good electrocatalytic urea oxidation activity and reaction stability is obtained by co-calcining Ni-Cr hydroxide with citric acid.

[0005] Preferably, a method for preparing a Ni-Cr-based catalyst based on citric acid co-calcination is characterized by comprising the following steps:

[0006] S1. Dissolve nickel source, chromium source and urea in water and carry out hydrothermal reaction. The resulting precipitate is Ni-Cr hydroxide.

[0007] S2. The Ni-Cr hydroxide and citric acid are subjected to high-temperature calcination reaction in an inert atmosphere to obtain the Ni-Cr-based catalyst.

[0008] Preferably, in step S1, the nickel source is at least one of nickel nitrate, nickel acetate, nickel chloride and their hydrates, and the chromium source is at least one of chromium nitrate, chromium chloride, chromium sulfate, chromium acetate and their hydrates.

[0009] Preferably, in step S1, the molar ratio of the nickel source, chromium source and urea is 1:0.01-0.05:1-5.

[0010] Preferably, in step S1, the temperature of the hydrothermal reaction is 100-150℃ and the time is 6-24h.

[0011] Preferably, in step S2, the mass ratio of the Ni-Cr hydroxide to citric acid is 1:1-5.

[0012] Preferably, in step S2, the inert atmosphere is at least one of nitrogen, argon, helium, or neon.

[0013] Preferably, the high-temperature calcination reaction is carried out at a temperature of 450-550°C for 1-3 hours.

[0014] The present invention also proposes a Ni-Cr based catalyst prepared by the above preparation method.

[0015] This invention also proposes an application of the above-mentioned Ni-Cr based catalyst in the electrocatalytic urea oxidation reaction.

[0016] Preferably, the Ni-Cr-based catalyst is used as the catalyst electrode for the urea oxidation reaction.

[0017] The urea oxidation reaction (UOR) has attracted widespread attention in the energy and environmental protection fields, driving the demand for high-performance catalysts. To meet this demand, this invention involves the simple co-calcination of Ni-Cr hydroxide and citric acid. During calcination, Ni-Cr hydroxide is converted to Ni-Cr oxide, while citric acid undergoes carbon deposition, simultaneously reducing some Ni species to metallic Ni and depositing carbon in the catalyst structure, thus obtaining a Ni-Cr-based catalyst based on citric acid co-calcination. When this Ni-Cr-based catalyst was evaluated as a UOR catalyst, its catalytic activity was significantly enhanced compared to that of a precursor Ni-Cr hydroxide and a Ni-Cr-based catalyst obtained solely by calcining Ni-Cr hydroxide. Notably, the Ni-Cr-based catalyst of this invention achieves a specific catalytic activity of 1294 mA cm⁻¹. -2 mg -1 (Relative to Hg / HgO, 0.7V), its excellent catalytic performance is mainly attributed to its large electrochemical surface area (ECSA), low charge transfer resistance, and small urea adsorption resistance. Furthermore, the good hydrophilicity of the catalyst facilitates contact with urea solution, and its weak basicity enhances resistance to the poisoning of CO2 intermediates generated during the UOR process. Therefore, this invention provides an effective strategy for adjusting the phase and composition of Ni-based catalysts to achieve efficient urea electro-oxidation. Attached Figure Description

[0018] Figure 1 The following are XRD, SEM, and TEM images of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples: (a) XRD images of NiCr-CA, NiCr-OH, and NiCr-c; (bc) SEM image of NiCr-OH; (de) SEM image of NiCr-c; (fg) SEM image of NiCr-CA; (h) TEM image of NiCr-CA; (i) HRTEM image of NiCr-CA; (j) SEM image and elemental distribution map of NiCr-CA.

[0019] Figure 2 Raman spectra, XPS plots, and N2 adsorption-desorption isotherms of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples are shown below: (a) Raman spectra of NiCr-CA, NiCr-OH, and NiCr-c; (b) Ni 2p... 3 / 2XPS spectra; (c) Cr 2p XPS spectra of NiCr-CA, NiCr-OH, and NiCr-c; (d) O1s XPS spectra of NiCr-CA, NiCr-OH, and NiCr-c; (e) N2 adsorption-desorption isotherms of NiCr-CA, NiCr-OH, and NiCr-c; (f) Pore size distribution curves of NiCr-CA, NiCr-OH, and NiCr-c.

[0020] Figure 3 The Ni-Cr based catalysts (NiCr-CA, NiCr-OH, NiCr-c, NiCr-Glu, NiCr-Mel, NiCr-Urea, NiCr-Thi and NiCr-CA) described in the examples and comparative examples are as follows. 1 CV curves and specific current density comparisons for UOR: (a) CV curve for NiCr-OH; (b) CV curve for NiCr-c; (c) CV curve for NiCr-CA; (d) Specific current density diagrams for UOR catalyzed by NiCr-CA, NiCr-OH, and NiCr-c; (e) CV curves for NiCr-CA, NiCr-Glu, NiCr-Mel, NiCr-Urea, and NiCr-Thi; (f) NiCr-CA 1 The CV curve;

[0021] Figure 4 CV curves of the Ni-Cr based catalysts (NiCr-CA-urea 10%, NiCr-CA-urea 30%, NiCr-CA-urea 50%, and NiCr-CA-urea 70%) described in Comparative Example 2 for use in UOR;

[0022] Figure 5 The following are Tafel plots of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples: (a) Tafel plots of NiCr-CA, NiCr-OH, and NiCr-c; (b) Current density changes of NiCr-CA, NiCr-OH, and NiCr-c at 0.1 V (vs. Hg / HgO); (c) Nyquist plots of NiCr-CA, NiCr-OH, and NiCr-c at 0.5 V (vs. Hg / HgO); (d) CA curves of NiCr-CA, NiCr-OH, and NiCr-c at 0.6 V (vs. Hg / HgO).

[0023] Figure 6 The SEM image, Raman spectrum, and Ni2p spectrum of the Ni-Cr based catalyst (NiCr-CA) described in the examples after UOR are shown. 3 / 2XPS spectra and Cr 2p XPS spectra: (a) SEM image of NiCr-CA; (b) Raman spectrum of NiCr-CA; (c) Ni 2p spectrum of NiCr-CA. 3 / 2 XPS spectrum; (d) is the Cr 2p XPS spectrum of NiCr-CA;

[0024] Figure 7 The contact angles of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples are as follows: (a) is the contact angle of NiCr-OH; (b) is the contact angle of NiCr-c; and (c) is the contact angle of NiCr-CA.

[0025] Figure 8 The following are Nyquist and Bode plots of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples: (a), (c), and (e) are Nyquist plots of NiCr-CA, NiCr-OH, and NiCr-c in 1M KOH containing 0.33M NH3, respectively; (b), (d), and (f) are Bode plots of NiCr-CA, NiCr-OH, and NiCr-c in 1M KOH containing 0.33M NH3, respectively.

[0026] Figure 9 Bode diagrams of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples are as follows: (a), (e), and (i) are Bode diagrams of NiCr-OH, NiCr-c, and NiCr-CA in 1M KOH containing 0.33M NH3, respectively; (b), (f), and (j) are Bode diagrams of NiCr-OH, NiCr-c, and NiCr-CA in 1M KOH containing 0.33M NaNO2, respectively; (c), (g), and (k) are Bode diagrams of NiCr-OH, NiCr-c, and NiCr-CA in 1M KOH containing 0.33M NaOCN, respectively; (d), (h), and (l) are Bode diagrams of NiCr-OH, NiCr-c, and NiCr-CA in 1M KOH containing 0.33M Na2CO3, respectively.

[0027] Figure 10 The CO2-TPD spectra of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples are shown. Detailed Implementation

[0028] The present invention will now be described in detail through specific embodiments. However, these examples are clearly provided for illustrative purposes and are not intended to limit the scope of the invention.

[0029] In this invention, the raw material NiNO3·6H2O was purchased from Tianjin Damao Chemical Reagent Factory; the raw material Cr(NO3)3·6H2O was purchased from Saen Chemical Technology (Shanghai) Co., Ltd.; the raw material urea was purchased from Sinopharm Chemical Reagent Co., Ltd.; and the raw material citric acid was purchased from Tianjin Guangfu Fine Chemical Research Institute.

[0030] Example 1

[0031] This embodiment presents a Ni-Cr based catalyst based on co-calcination of citric acid, which is prepared by the following method:

[0032] (1) NiNO3·6H2O (1.5 mmol), Cr(NO3)3·6H2O (0.045 mmol) and urea (5 mmol) were added to distilled water and stirred for 5 min to dissolve completely. The resulting mixed solution was transferred to a 50 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene and subjected to hydrothermal treatment at 120 °C for 12 h in an oven. After naturally cooling to room temperature, the product was collected by centrifugation and filtration. The product was then washed repeatedly with anhydrous ethanol and deionized water, and dried to obtain Ni-Cr hydroxide (abbreviated as NiCr-OH).

[0033] (2) Mix the above Ni-Cr hydroxide (0.1g) and citric acid (0.33g), place them in a tube furnace, and calcine them at 500°C under Ar protection for 2 hours to obtain the Ni-Cr based catalyst (abbreviated as NiCr-CA).

[0034] In this embodiment, the above-mentioned Ni-Cr hydroxide (0.1g) is placed directly in a tube furnace, heated to 500°C under Ar protection, and then kept for 2 hours for calcination to obtain a Ni-Cr based catalyst (referred to as NiCr-c).

[0035] Comparative Example 1

[0036] This comparative example presents a Ni-Cr based catalyst based on the co-calcination of glucose, melamine, urea or thiourea, which is prepared by the method described in Example 1. Except in step (2), the same mass of glucose, melamine, urea or thiourea is used instead of citric acid. The corresponding Ni-Cr based catalysts are referred to as NiCr-Glu, NiCr-Mel, NiCr-Urea and NiCr-Thi, respectively.

[0037] Comparative Example 2

[0038] This comparative example presents a Ni-Cr based catalyst based on the co-calcination of citric acid and urea, which is prepared by the method described in Example 1. Except for step (2), citric acid and urea in proportions of 10%, 30%, 50% and 70% are used to replace citric acid, respectively. The corresponding Ni-Cr based catalysts are named NiCr-CA-urea10%, NiCr-CA-urea30%, NiCr-CA-urea50%, and NiCr-CA-urea70% according to the different proportions of urea.

[0039] Comparative Example 3

[0040] This comparative example presents a Ni-Cr-based catalyst based on the hydrothermal reaction of citric acid, which is prepared by the following method:

[0041] (1) NiNO3·6H2O (1.5mmol), Cr(NO3)3·6H2O (0.045mmol), urea (5mmol) and citric acid (0.33g) were added to distilled water and stirred for 5min to dissolve completely. The resulting mixed solution was transferred to a 50mL stainless steel high-pressure reactor lined with polytetrafluoroethylene and subjected to hydrothermal treatment at 120℃ for 12h in an oven. After naturally cooling to room temperature, the product was collected by centrifugation and filtration. The product was then washed repeatedly with anhydrous ethanol and deionized water, and dried to obtain Ni-Cr hydroxide.

[0042] (2) The above-mentioned Ni-Cr hydroxide was placed in a tube furnace and calcined at 500°C under Ar protection for 2 hours to obtain the Ni-Cr based catalyst (referred to as NiCr-CA). 1 ).

[0043] To reveal the structure and morphology of the Ni-Cr based catalyst, the Ni-Cr based catalyst described in Example 1 was characterized by X-ray diffraction (XRD, Rigaku D / Max-2500), scanning electron microscopy (SEM, JSM-7500F), and transmission electron microscopy (TEM, JEM-F200). The results are as follows: Figure 1 As shown.

[0044] Figure 1 The images show the XRD patterns, SEM images, and TEM images of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples. (Refer to...) Figure 1As can be seen from the diagram, the precursor NiCr-OH of the Ni-Cr-based catalyst before calcination exhibits a typical Ni(OH)2 X-ray diffraction pattern (PDF#38-0715), and the crystallinity of NiCr-OH is not very good, which may be due to the introduction of Cr species. When NiCr-OH is directly calcined without co-calcination with citric acid, the NiCr oxide (NiCr-c) obtained exhibits a typical NiO X-ray diffraction pattern (PDF#65-5745), and no diffraction peaks representing Cr species can be found in NiCr-c, which may be due to the presence of Cr species. The high dispersion of Ni species in NiO is the cause. When co-calcined with citric acid, due to the reducing properties of citric acid, the Ni species in NiCr-OH will be partially reduced and converted into metallic Ni. Therefore, the X-ray diffraction pattern of metallic Ni was found in the obtained NiCr-CA (PDF#65-2865), and a very weak diffraction peak representing NiO was also found (marked with black arrows). In addition to metallic Ni and NiO, a broad but weak peak was found at about 22.6°, which may be caused by carbon deposition of citric acid, indicating the presence of carbon in NiCr-CA.

[0045] Reference Figure 1 As shown in b and 1c, NiCr-OH is composed of irregular nanoparticles, which aggregate to form larger particles. (Refer to...) Figure 1 As shown in d and 1e, NiCr-c inherits the morphological characteristics of NiCr-OH, and compared with NiCr-OH, the small nanoparticles tend to aggregate into larger particles. (Refer to...) Figure 1 As can be seen from f and 1g, co-calcination with citric acid can prevent further growth of nanoparticles, and the particle size of NiCr-CA is comparable to that of NiCr-OH.

[0046] Reference Figure 1 As can be seen from h, NiCr-CA is indeed composed of small nanoparticles with a size of approximately 20 nm. Figure 1 The high-resolution TEM image of NiCr-CA shown in image i reveals the lattice edges of the Cr2O3(024), Ni(111), and NiO(200) crystal planes, with lattice spacings of 0.18, 0.20, and 0.21 nm, respectively. The lattice edges of carbon are not observable, which may be due to the amorphous properties of carbon resulting from citric acid deposition. (Refer to...) Figure 1 As can be seen, Ni, Cr, C and O elements are evenly distributed; however, C and O elements are not present throughout the entire region, which may be due to the reduction of NiO.

[0047] To reveal the structural evolution and chemical state of Ni-Cr based catalysts, Raman spectroscopy (Horiba Jobin Yvon) and X-ray photoelectron spectroscopy (XPS, Thermo K-Alpha) were employed.+ The Ni-Cr based catalyst described in Example 1 was characterized. To reveal the BET surface area and pore characteristics of the Ni-Cr based catalyst, its N2 adsorption-desorption isotherm was measured at -196°C using a Micrometrics 3Flex device. All results are as follows: Figure 2 As shown.

[0048] Figure 2 The images show the Raman spectra, XPS plots, and N2 adsorption-desorption isotherms of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples. (Refer to...) Figure 2 From a, we can see that NiCr-OH at 453 and 535 cm⁻¹ -1 A peak can be observed at this point, which is attributed to A. 1g The vibration modes represent Ni(OH)2 and Cr(OH)3, respectively; additionally, at 863 cm⁻¹ -1 The peaks at 1041 and 1072 cm represent the ν2 bending mode of hydrotalcite. -1 The peaks at 778 cm⁻¹ represent the symmetric stretching mode of the intercalated ions, which are characteristic peaks of LDH, proving the successful preparation of NiCr-LDH. Compared to NiCr-OH, NiCr-c exhibits a completely different Raman spectrum: for NiCr-c, the peaks characteristic of LDH do not appear, but instead appear at 778 cm⁻¹. -1 A very strong peak appeared at 358 and 673 cm⁻¹, which should be the local vibrational peak of Cr₂O₃; in addition, there were peaks at 358 and 673 cm⁻¹. -1 The peaks at 588 and 991 cm⁻¹ also indicate the presence of Cr₂O₃; additionally, the peaks at 588 and 991 cm⁻¹ also indicate the presence of Cr₂O₃. -1 The two peaks at 551 cm⁻¹ can be attributed to the LO and 2LO modes of NiO, indicating that NiCr-LDH will transform into NiO-Cr₂O₃ compound after co-calcination without citric acid. In contrast, the Raman spectrum of NiCr-CA is only at 551 cm⁻¹. -1 A very weak peak is observed at 1335 and 1572 cm⁻¹, indicating the presence of NiO, and the typical peaks representing the presence of Cr₂O₃ cannot be identified; furthermore, at 1335 and 1572 cm⁻¹... -1 A weak peak can be observed at the point, which is attributed to the D and G bands of the carbon material, indicating that carbon should be present in NiCr-CA. The presence of citric acid can significantly modulate the structure of the final catalyst.

[0049] Reference Figure 2 b shows that NiCr-OH has a distinct peak at 855.76 eV, while the satellite peak is at 861.56 eV, confirming that Ni(OH)2-derived Ni 2+The presence of Ni; for NiCr-c, after peak splitting, two peaks at 854.50 and 856.07 eV and its satellite peak at 861.62 eV are obtained, indicating the presence of Ni. 2+ (854.50eV) and Ni 3+ (856.07 eV); For NiCr-CA, it represents Ni 2+ The peak at (855.76 eV) did not change compared to the peak of NiCr-OH, and a new peak was observed at 852.23 eV, which is a characteristic peak of metallic Ni. This shows that citric acid can indeed partially reduce the types of Ni, which is consistent with the XRD results.

[0050] Reference Figure 2 As shown in c, for NiCr-OH, after peak splitting, two peaks at 577.05 and 586.74 eV can be obtained, which are characteristic peaks of Cr(OH)3; for NiCr-c, four peaks can be obtained, at 576.01, 577.53, 585.75 and 587.28 eV respectively, indicating that Cr 3+ The presence of Cr; for NiCr-CA, the peak positions can be separated at 576.78 and 586.53 eV, which should also be attributed to Cr. 3+ .

[0051] Reference Figure 2 As shown in d, for NiCr-OH, it can be reasonably divided into three peaks at 529.39, 531.19, and 532.67 eV, corresponding to lattice oxygen (O1), hydroxyl group (O2), and adsorbed oxygen (O3), respectively. It is certain that the O2 content is very high, indicating the successful synthesis of Ni-Cr hydroxide. Similarly, for NiCr-c, it can also be divided into three peaks at 539.88 (O1), 531.04 (O2), and 533.61 eV (O3), respectively. The O1 content is significantly increased, proving that the calcination process enhances the lattice oxygen species in NiCr-c. For NiCr-CA, only two peaks at 530.34 (O1) and 531.53 eV (O2) can be separated. Compared with NiCr-c, the O1 content is further increased, indicating that the addition of citric acid by calcination is of great significance in regulating the chemical state of the catalyst.

[0052] Reference Figure 2As can be seen from the data, NiCr-OH, NiCr-c, and NiCr-CA exhibit typical Type IV adsorption isotherms with H3 hysteresis. A slight increase is observed when P / P0 is close to 0; a significant increase is observed when P / P0 is in the range of 0.2 to 0.8; and a sharp increase is observed when P / P0 is close to 1.0, indicating the presence of micropores, mesopores, and macropores in NiCr-OH, NiCr-c, and NiCr-CA. However, when P / P0 is in the range of 0.2 to 0.8, the adsorption isotherms of NiCr-c and NiCr-CA show a stronger increase than that of NiCr-OH, suggesting that the calcination process modulates the pore characteristics, favoring the formation of mesopores.

[0053] Reference Figure 2 As shown in f, the pore size distribution curve of NiCr-OH exhibits significant differences compared to NiCr-C and NiCr-CA. Clearly, the number of mesopores increases regardless of whether citric acid is present or absent during co-calcination. Furthermore, the presence of citric acid during calcination further modulates the catalyst's microstructure, as the main mesopore size of NiCr-CA (6.13 nm) is smaller than that of NiCr-C (7.34 nm). The BET surface areas of NiCr-OH, NiCr-C, and NiCr-CA are measured to be 44.92, 75.05, and 70.72 m², respectively. 2 g -1 The large BET surface area of ​​the catalyst provides a prerequisite for the exposure of active sites, thus the calcination process improves the catalytic activity of the NiCr-based catalyst.

[0054] To reveal the catalytic activity of the Ni-Cr-based catalysts described in Examples 1 and Comparative Examples 1, 2, and 3 for UOR, a three-electrode electrochemical system was established using a chemically modified glassy carbon electrode as the working electrode, a platinum wire electrode as the counter electrode, and Hg / HgO as the reference electrode. The working electrode consisted of the Ni-Cr-based catalyst and Nafion (DuPont, 0.5 wt%) introduced into an ethanol-water solution and uniformly dispersed under ultrasonic oscillation. The mixture was then dropped onto the glassy carbon electrode with an actual loading of 40 μg. To activate the Ni-Cr-based catalyst, an electrochemical workstation (CHI 660E) was used in 1 M KOH at 50 mV s⁻¹. -1 Cyclic voltammetry (CV) was performed at a scan rate of [missing value]. After activation, CV and chronoamperometry (CA) were used to study the catalytic performance of the Ni-Cr-based catalyst for UOR. Electrochemical impedance spectroscopy (EIS) was recorded in the frequency range of 0.1 Hz to 100 kHz. The results are as follows: Figure 3 , 4 As shown.

[0055] Figure 3 The Ni-Cr based catalysts (NiCr-CA, NiCr-OH, NiCr-c, NiCr-Glu, NiCr-Mel, NiCr-Urea, NiCr-Thi and NiCr-CA) described in the examples and comparative examples are as follows. 1 A comparison of CV curves and current densities for UOR. (Refer to...) Figure 3 As can be seen from this, in 1M KOH, oxidation peaks of 0.54V (vs. Hg / HgO) and 0.38V (vs. Hg / HgO) can be observed in the CV curve of NiCr-OH. This should be due to the Ni species (Ni) in NiCr-OH. 2+ / Ni 3+ The oxidation current density of NiCr-OH increases sharply and the reduction current density almost disappears in CV curves of 1M KOH containing 0.33M urea, indicating that NiCr-OH has catalytic activity for UOR. Similarly, referring to... Figure 3 As can be seen from b, in 1M KOH, the NiCr-c CV curve also contains a representation of Ni. 2+ / Ni 3+ The redox peaks were observed in 1M KOH containing 0.33M urea, with NiCr-c (20.94 mA cm⁻¹). -2 The oxidation current density of @0.7V vs. Hg / HgO is higher than that of NiCr-OH (8.17mA cm⁻¹). -2 The value of @0.7V (vs. Hg / HgO) is much higher. (Refer to...) Figure 3 c indicates that in 1M KOH, the NiCr-CA CV curve also contains elements representing Ni. 2+ / Ni 3+ The typical redox peaks were observed in NiCr-CA, and the redox current density of NiCr-CA in 1M KOH was greater than that of NiCr-OH and NiCr-C, which means that there should be more active sites in NiCr-CA. Therefore, it was observed that among the three catalysts containing 0.33M urea, NiCr-CA (63.46mA cm⁻¹) exhibited the highest redox current density. -2 The oxidation / reduction current density is highest at 0.7V vs. Hg / HgO.

[0056] Figure 3 d reveals the specific current activities of NiCr-OH, NiCr-c, and NiCr-CA to UOR after excluding the specific current activity generated by OER at 0.7V (vs. Hg / HgO). The specific current activities are as follows: NiCr-CA (1294mA cm⁻¹) -2 mg -1 )>NiCr-c(451mA cm -2 mg-1 )>NiCr-OH(143mAcm -2 mg -1 Therefore, it can be seen that adjusting the phase by co-calcining with citric acid can enhance the catalytic activity of UOR.

[0057] Figure 3 The CV curves of Ni-Cr based catalysts, obtained by e, were used to regulate the urea oxidation of four samples (NiCr-Glu, NiCr-Mel, NiCr-Urea, and NiCr-Thi) under 0.7 V (vs. Hg / HgO), showing current densities of 33.1, 51.2, 25.0, and 34.9 mA cm⁻¹, respectively. -2 Their catalytic performance is inferior to that of NiCr-CA.

[0058] Figure 3 f reveals the CV curve of the Ni-Cr based catalyst calcined after direct addition of citric acid during hydrothermal processing. Under the condition of 0.7V (vs. Hg / HgO), the catalyst (NiCr-CA) exhibits the following characteristics: 1 The current density is 22.7 mA cm⁻¹. -2 After deducting the specific current activity generated by the OER, the specific current activity potential of its UOR is 415 mA cm⁻¹. -2 mg -1 Its specific current activity is far lower than that of NiCr-CA.

[0059] Figure 4 CV curves for the Ni-Cr based catalysts (NiCr-CA-urea 10%, NiCr-CA-urea 30%, NiCr-CA-urea 50%, and NiCr-CA-urea 70%) described in Comparative Example 2 for use with UOR are shown below. Figure 4 It is evident that the unique properties of citric acid are crucial for improving the performance of Ni-Cr based catalysts. Urea, on the other hand, can effectively reduce the number of functional groups on the surface of carbon-containing materials, thereby increasing the graphitization degree of the final carbon material. Therefore, when citric acid and urea are mixed in different proportions and reacted with Ni-Cr hydroxide, it is observed that as the urea content increases, the oxidation / reduction current density of the formed Ni-Cr based catalyst gradually decreases, and the catalytic activity of the catalyst also gradually declines.

[0060] To investigate the kinetic characteristics of Ni-Cr based catalysts for UOR, Tafel plots were used, and the results are as follows: Figure 5 As shown. Figure 5 The image shows the Tafel diagrams of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the embodiments of the present invention.

[0061] Reference Figure 5As can be seen from this, the UOR kinetics of NiCr-OH are relatively slow (Tafel slope: 279mV dec). -1 NiCr-c slightly improved UOR kinetics (Tafel slope: 266 mV dec). -1 NiCr-CA significantly enhances UOR kinetics (Tafel slope: 73mV dec). -1 Therefore, the accelerated UOR kinetics ensure the excellent catalytic activity of NiCr-CA. The number of active sites in NiCr-OH, NiCr-c, and NiCr-CA was estimated using the electrochemical surface area (ECSA), which is related to the electrochemical double-layer capacitance (C). dl Directly related. C dl Evaluation was performed using CV testing at different scan rates to assess the non-Radida potentials. (Refer to...) Figure 5 b indicates that the C of NiCr-OH, NiCr-c, and NiCr-CA dl The values ​​were 168, 179, and 233 μF cm, respectively. -2 Clearly, co-calcination with citric acid significantly increases the number of active sites in Ni-Cr based catalysts. (Refer to...) Figure 5 As shown in c, typical semicircles can be observed in the Nyquist plots of NiCr-OH and NiCr-c, while two semicircles can be observed in the Nyquist plot of NiCr-CA. Using the equivalent circuit diagram R0(CPE1(R1(CPE2R2)) to fit the original data of the Nyquist plot, the corresponding resistance parameters (R0: solution resistance; R1: charge transfer resistance; R2: urea adsorption resistance) can be specified. For NiCr-OH, NiCr-c, and NiCr-CA, the R1 values ​​are 461.2, 127.0, and 53.4 Ω, respectively, indicating that the charge transfer ability of the Ni-Cr based catalyst is definitely promoted after calcination, and the introduction of citric acid can further improve the charge transfer ability. For NiCr-OH, NiCr-C, and NiCr-CA, the R2 values ​​are 1423.0, 234.7, and 41.0 Ω, respectively, indicating that NiCr-CA has the lowest urea adsorption resistance. Therefore, the unique phase composition of NiCr-CA should be more suitable for the electro-oxidation of urea.

[0062] The stability of NiCr-OH, NiCr-c, and NiCr-CA was studied using the CA test. (Refer to...) Figure 5 As can be seen from d, all three catalysts are relatively stable for UOR; all three catalysts experience fluctuations in current density, especially NiCr-CA, which should be caused by the effects of the generated CO2 and N2 products.

[0063] Figure 6Image a shows the SEM image of NiCr-CA after UOR. No obvious changes were observed, proving that the morphology of NiCr-CA can be maintained, which should be the reason for the catalytic stability of NiCr-CA towards UOR. Figure 6 b shows the Raman spectrum of NiCr-CA after UOR, at 529 cm⁻¹. -1 A peak representing the Ni-O bond can be observed at 1317 cm⁻¹, and also at 1317 cm⁻¹. -1 (Section D) and 1571cm -1 Peaks representing the presence of carbon species can also be observed in (G segment). It can be seen that the Raman results of NiCr-CA after UOR are similar to those of NiCr-CA before UOR, indicating that UOR does not have a significant effect on the structure of NiCr-CA. However, the chemical state of the metal species on the surface of NiCr-CA has changed. Figure 6 c shows the Ni 2p on the NiCr-CA surface after UOR. 3 / 2 The XPS spectrum shows that the peak indicating the presence of metallic Ni has disappeared, and in its place, a new peak is observed at 854.06 eV, which is Ni's... 2+ The typical signal is observed at 856.07 eV, and a peak can also be observed at 856.07 eV, indicating that Ni... 3+ The presence of this indicates that the Ni species on the NiCr-CA surface should be partially oxidized after UOR. Figure 6 Figure d shows the Cr 2p XPS spectrum of NiCr-CA after UOR, where two peaks at 576.67 and 586.50 eV can be observed, confirming that Cr 3+ The presence of Cr species in NiCr-CA after UOR indicates that the chemical valence of Cr species on the NiCr-CA surface should also be adjusted, compared with that before UOR. Therefore, although the morphology of NiCr-CA is preserved, the chemical state of the metal species on the NiCr-CA surface should be changed to make it more suitable for catalyzing UOR.

[0064] To investigate the enhancing effect of Ni-Cr based catalysts on UOR catalytic performance, the contact angle of the Ni-Cr based catalyst described in Example 1 was measured using a contact angle meter (JC 2000D1). The results are as follows: Figure 7 As shown.

[0065] Figure 7 The contact angles are those of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples. (Refer to...) Figure 7It is evident that Ni-Cr based catalysts all exhibit good hydrophilicity. The contact angles of NiCr-OH, NiCr-c, and NiCr-CA are 14.8°, 11.3°, and 8.5°, respectively. NiCr-CA demonstrates the best hydrophilicity among these three catalysts, which should provide a favorable prerequisite for its sufficient contact with urea in the electrolyte. Furthermore, the CV results for NiCr-OH, NiCr-c, and NiCr-CA in 1M KOH with and without 0.33M urea show that the onset potential of UOR is almost identical to that of the Ni oxide species, indicating that Ni... 3+ The species should be the active site of UOR.

[0066] To verify the catalytic mechanism, in-situ EIS measurements were performed on the Ni-Cr based catalyst described in Example 1, and the results are as follows: Figure 8 As shown.

[0067] Figure 8 The Nyquist and Bode plots are shown for the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples. (Refer to...) Figure 8 As can be seen from the Nyquist plots of NiCr-OH at different potentials, only a typical semicircle is observed. The radius of the semicircle decreases with increasing potential, and there is almost no change when the potential exceeds 0.56V (vs. Hg / HgO). The R1 value decreases with increasing potential, indicating that higher potentials should favor charge transfer. However, when the potential increases and exceeds 0.52V (vs. Hg / HgO), the R2 value shows a significant decrease, indicating a fluctuation. The Bode plots of NiCr-OH at different potentials are shown below. Figure 8 As shown in b, it is indeed at 10. 2.4 A typical peak can be observed around Hz, which is Ni 2+ with Ni 3+ The characteristics of the reaction indicate the existence of an indirect urea oxidation pathway using NiCr-OH as a catalyst; as the potential increases, this peak gradually shifts to higher frequencies, and the phase height gradually decreases, indicating that higher potentials will promote Ni oxidation. 2+ Oxidation to Ni 3+ However, when the potential exceeds 0.48V (vs. Hg / HgO), low frequencies (<10) 0 The curves (Hz) overlapped, and even shifted to a high phase angle when the potential exceeded 0.52V (vs. Hg / HgO), indicating that the direct urea oxidation pathway would be inhibited at high potentials.

[0068] The Nyquist plot of NiCr-C also shows a typical semicircle at different potentials. Figure 8c) The radius of the semicircle gradually decreases with increasing potential; the values ​​of R1 and R2 decrease with increasing potential, indicating that charge transfer and urea adsorption processes should be promoted at high temperatures. The Bode of NiCr-c at different potentials is as follows: Figure 8 As shown in d, it represents Ni. 2+ Oxidation to Ni 3+ and indirect urea oxidation pathway (>10) 2 The peak value of (Hz) gradually shifts to a higher value, accompanied by a significant decrease in phase angle with increasing potential; in addition, when the potential reaches 0.6V (vs. Hg / HgO), a new low-frequency peak (approximately 10 Hz) appears. 1.5 (Hz), which is an indicator of the occurrence of the direct urea oxidation pathway; therefore, when NiCr-c is used as a catalyst, both indirect and direct urea oxidation pathways should exist.

[0069] Figure 8 The Nyquist plots of NiCr-CA at different potentials were recorded. Unlike the Nyquist plots of NiCr-OH and NiCr-c, two semicircles can be seen in the Nyquist plot of NiCr-CA, and the R1 and R2 values ​​also decrease with increasing potential. In addition, at the same potential, the R1 and R2 values ​​of NiCr-CA are much smaller than those of NiCr-OH and NiCr-c, which should be the reason for its better catalytic activity for UOR. Figure 8 f shows the Bode plots of NiCr-CA at different potentials. Compared with NiCr-OH and NiCr-c, the high-frequency peaks of NiCr-CA (>10) are more pronounced. 2.8 It can also be noted that the smallest phase angle indicates that using NiCr-CA as a catalyst should be more favorable for the charge transfer process in UOR; in addition, when the potential exceeds 0.48V (vs. Hg / HgO), a weak low-frequency peak (approximately 10 Hz) can be observed in the Bode plot of NiCr-CA. 0.35 Hz); therefore, NiCr-CA should be used as a catalyst to promote the direct urea oxidation pathway, which is difficult to occur at low potentials (<0.55V vs. Hg / HgO); therefore Ni 3+ The species should be the active sites of NiCr-OH, NiCr-c, and NiCr-CA. Using NiCr-CA as a catalyst will promote both indirect and direct urea oxidation pathways.

[0070] Due to Ni 3+ The species were identified as active sites for NiCr-OH, NiCr-c, and NiCr-CA. Therefore, in-situ EIS measurements were used to investigate the effects of NiCr-OH, NiCr-c, and NiCr-CA on all identified intermediates (NH3, NO2) formed during urea oxidation. - OCN- and CO3 2- The catalytic behavior of ) and the results are as follows Figure 9 As shown.

[0071] Figure 9 This is a site map of NiCr-OH, NiCr-c, and NiCr-CA in 1M KOH at different potentials. For NiCr-OH, only one high-frequency (>10) site was observed in 1M KOH for different intermediates. 2.0 The Hz peak is characteristic of the charge transfer process and represents Ni. 3+ Speciation. In 1 M KOH containing 0.33 MNH3, the phase angle gradually decreases; when the potential is below 0.52 V (vs. Hg / HgO), the phase angle decreases, while when the potential exceeds 0.52 V (vs. Hg / HgO), the phase angle remains almost unchanged; when the potential is below 0.52 V, the phase angle decreases (vs. Hg / HgO), and then when the potential exceeds 0.52 V, the phase angle increases (vs. Hg / HgO); therefore, CO3 derived from CO2 intermediates... 2- It should be an obstacle to Ni 3+ The main reason for the formation of active sites is consistent with previous research results.

[0072] For NiCr-c, two distinct peaks can be observed in 1M KOH containing 0.33M NH3, with higher frequency (>10) compared to NiCr-OH. 3 The phase angle of the Hz peak is very small, and this peak decreases slightly with increasing potential, indicating that the NH3 intermediate in the electrolyte affects Ni. 3+ The charge transfer process at the active site was unaffected. Furthermore, at potentials exceeding 0.48V (vs. Hg / HgO), low frequencies (<10) showed no effect. 0 The appearance of the Hz peak indicates that NH3 passes directly through Ni. 3+ Catalytic oxidation was performed. High-frequency (>10) NiCr-c oxidation was also observed in 1 M KOH containing 0.33 M NaNO2. 2 The peak (Hz) shows a phase angle that decreases significantly with increasing potential, but this indicates the presence of Ni. 3+ Direct catalysis of NO2 - The oxidation peak appears at 0.44 V (relative to Hg / HgO), then disappears between 0.48 and 0.56 V (vs. Hg / HgO), and reappears at 0.6 V (vs. Hg / HgO); in 1 M KOH containing 0.33 M NaOCN, the high frequency (>10) of NiCr-c... 3 The phase angle of the Hz peak also decreases as the potential increases, while the phase angle of the low frequency (<10 Hz) peak decreases. 0The high-frequency (>10 Hz) peak appears at a potential of 0.6 V (vs. Hg / HgO). In 1 M KOH containing 0.33 M Na₂CO₃, the high-frequency (>10 Hz) peak appears at a potential of 0.6 V (vs. Hg / HgO). 3 The Hz peak still exists; however, the phase angle of this peak fluctuates with increasing potential, indicating that CO32-... 2- It also affects the charge transfer of the catalyst; due to Ni 3+ The generation of active sites will significantly affect catalytic activity; therefore, CO3 derived from CO2 intermediates... 2- This is likely the main reason affecting the catalytic performance of NiCr-C; therefore, CO2 intermediates do not have a significant poisoning effect on NiCr-CA.

[0073] To reveal the enhanced anti-poisoning ability of NiCr-CA against CO2 intermediates, CO2-TPD experiments were conducted using a MicrotracBEL BELCAT II instrument. The experimental procedure was as follows: 0.079 g of Ni-Cr-based catalyst was heated from room temperature to 250 °C at a heating rate of 10 °C / min. -1 The catalyst was kept at room temperature for 1 hour, cooled to room temperature and kept at room temperature for 20 minutes, then CO2 was injected into the Ni-Cr based catalyst and kept at room temperature for 40 minutes. He was then injected into the reactor to completely remove CO2 from the surface of the Ni-Cr based catalyst. Finally, a programmed temperature rise from room temperature to 700°C was performed at a rate of 10°C / min. -1 Heating removes CO2, and the results are as follows: Figure 8 As shown.

[0074] Figure 10 The CO2-TPD spectra of the Ni-Cr based catalysts (NiCr-CA, NiCr-OH, and NiCr-c) described in the examples are shown below. (Refer to...) Figure 10It can be seen that for NiCr-OH, a significant desorption peak appears at 108℃, which should be attributed to physical adsorption or weakly basic sites. Another peak is observed at 328℃, with a shoulder at 363℃. The peak is relatively large, appearing above 220℃, indicating the presence of abundant moderately to strongly basic sites in NiCr-OH. CO2 can be easily adsorbed by NiCr-OH. Therefore, due to the very strong binding ability of CO2 intermediates with NiCr-OH, it is easy to poison NiCr-OH. For NiCr-c, two peaks are mainly observed at 357℃ and 609℃, indicating the presence of moderately to strongly basic sites in NiCr-c. In addition, the amount of CO2 adsorbed by NiCr-c is much less than that of NiCr-OH, indicating a decrease in the basicity of the catalyst. For NiCr-CA, the CO2 desorption peaks mainly appear at 467℃ and 591℃, and the CO2 adsorption capacity is the lowest among the prepared catalysts. Therefore, by co-calcining Ni-Cr hydroxide with citric acid to adjust the phase, the basicity of Ni-Cr catalyst can be effectively reduced, weakening the catalyst's ability to bind with CO2. Due to the weak interaction with CO2, this will promote the anti-toxicity of NiCr-CA, thereby accelerating the catalytic performance of NiCr-CA for UOR.

[0075] In summary, the phase composition of the Ni-Cr based catalyst was successfully regulated by co-calcination of Ni-Cr hydroxide and citric acid. Experimental results show that Ni-Cr hydroxide can be completely decomposed into Ni-Cr oxide, and after calcination with citric acid, Ni can be partially reduced to metallic Ni. Furthermore, carbon can be generated due to the carbonization effect of citric acid at high temperatures. By regulating the phase composition of NiCr-CA, the chemical state of the metal species on the catalyst surface can be adjusted, and the microstructure of the catalyst can be optimized, thus providing the preconditions for promoting the formation and exposure of active sites. As a catalyst for UOR, NiCr-CA exhibits excellent catalytic activity, with a specific current activity reaching 1294 mA cm⁻¹. -2 mg -1The electrochemical oxidation state (ECSA) of NiCr-CA (0.7V vs. Hg / HgO) is approximately 3 to 9 times that of NiCr-c or NiCr-OH. Larger ECSA, better hydrophilicity, lower charge transfer, and reduced urea adsorption resistance all contribute to the easier formation and exposure of Ni active sites during urea oxidation. In addition to the indirect urea oxidation pathway, using NiCr-CA as a catalyst also promotes the formation of the direct urea oxidation pathway. The influence of intermediates was investigated using in-situ EIS measurements. Intermediates generated during the UOR process showed no significant toxicity to NiCr-CA, while CO2 intermediates exhibited significant toxicity to NiCr-OH and NiCr-c. CO2-TPD testing indicated that NiCr-CA is less basic than NiCr-OH and NiCr-c, which should lead to a weakened binding between CO2 and the active sites in NiCr-CA. This work provides new insights into tuning phase composition to improve electrocatalytic performance and enhance the anti-poisoning properties of Ni-based electrocatalysts.

[0076] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a Ni-Cr-based catalyst based on co-calcination of citric acid, characterized in that, Includes the following steps: S1. Dissolve nickel source, chromium source and urea in water and carry out hydrothermal reaction. The resulting precipitate is Ni-Cr hydroxide. S2. The Ni-Cr hydroxide and citric acid are subjected to high-temperature calcination reaction in an inert atmosphere to obtain the Ni-Cr-based catalyst.

2. The method for preparing Ni-Cr-based catalyst based on citric acid co-calcination according to claim 1, characterized in that, In step S1, the nickel source is at least one of nickel nitrate, nickel acetate, nickel chloride and their hydrates, and the chromium source is at least one of chromium nitrate, chromium chloride, chromium sulfate, chromium acetate and their hydrates.

3. The method for preparing Ni-Cr-based catalysts based on citric acid co-calcination according to claim 1 or 2, characterized in that, In step S1, the molar ratio of the nickel source, chromium source and urea is 1:0.01-0.05:1-5.

4. The method for preparing the Ni-Cr-based catalyst based on co-calcination of citric acid according to any one of claims 1-3, characterized in that, In step S1, the hydrothermal reaction is carried out at a temperature of 100-150℃ for 6-24 hours.

5. The method for preparing the Ni-Cr-based catalyst based on citric acid co-calcination according to claims 1-4 via hydrothermal reaction, characterized in that, In step S2, the mass ratio of the Ni-Cr hydroxide to citric acid is 1:1-5.

6. The method for preparing the Ni-Cr-based catalyst based on co-calcination of citric acid according to any one of claims 1-5, characterized in that, In step S2, the inert atmosphere is at least one of nitrogen, argon, helium, or neon.

7. The method for preparing the Ni-Cr-based catalyst based on co-calcination of citric acid according to any one of claims 1-6, characterized in that, The high-temperature calcination reaction is carried out at a temperature of 450-550℃ for 1-3 hours.

8. A Ni-Cr-based catalyst prepared by the preparation method according to any one of claims 1-7.

9. The application of the Ni-Cr based catalyst of claim 8 in the electrocatalytic urea oxidation reaction.

10. The application of the Ni-Cr based catalyst according to claim 9 in the electrocatalytic urea oxidation reaction, characterized in that, The Ni-Cr-based catalyst was used as the catalyst electrode for the urea oxidation reaction.

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