A cobalt-phosphorus-cobalt oxide composite electrode material and application thereof

Abstract

The application discloses a cobalt-phosphorus-cobalt oxide composite electrode material and application thereof as an anode of a direct borohydride fuel cell. First, a conductive substrate is placed in an aqueous solution containing a cobalt source, ammonium fluoride and urea to perform a hydrothermal reaction; after being reduced to room temperature, the conductive substrate is repeatedly cleaned with deionized water, and then is placed in a solution containing sodium hypophosphite to perform immersion; finally, the conductive substrate is calcined at high temperature in a vacuum environment to obtain the cobalt-phosphorus-cobalt oxide loaded on the conductive substrate in situ. The electrode material preparation method provided by the application converts the original contact between a carrier gas and an electrode surface in a phosphorization process into the in-situ adhesion of a phosphorus solution on the electrode surface, realizes a uniform phosphorization process, is low in cost and easy to be industrialized, and the material can be used as an anode material of the direct borohydride fuel cell.

Description

Technical Field

[0001] This invention relates to a cobalt-phosphorus-cobalt oxide composite electrode material prepared by impregnation-thermal reduction phosphating and its application as an anode in a direct borohydride fuel cell, belonging to the field of direct borohydride fuel cell anode material preparation technology. Background Technology

[0002] Compared to proton exchange membrane fuel cells (PEMFCs), which are widely researched and commercially applied in the fuel cell field today, direct borohydride fuel cells (DBFCs) use an alkaline solution of borohydrides as fuel, which solves the problems of hydrogen storage and transportation. Simultaneously, the alkaline anode environment allows the use of transition metal-based catalysts, significantly reducing battery costs. Furthermore, DBFCs possess extremely high theoretical open-circuit potentials (up to 2.11 V with hydrogen peroxide as the oxidant) and specific capacities (5668 A·h·kg⁻¹) in the liquid fuel cell field. -1 Furthermore, the reaction system is environmentally friendly, thus DBFC shows great application potential. One of the challenges facing DBFC is the difficulty in simultaneously achieving high anode activity and low cost. In other words, to obtain ideal performance, noble metal-based catalysts or carefully designed transition metal-based catalysts are typically used. The high cost of anode catalysts significantly reduces the competitiveness of DBFC. Therefore, the development of low-cost, high-activity electrodes for DBFC anodes is of great significance.

[0003] Cobalt phosphide has been proven to have extremely high activity for the anodic reaction of DBFC (see Zhang Junjun; Zhang Dongming; Cui Can; Wang Haoyu; Jiao Weizhou; Gao Jing; Liu Youzhi, A three-dimensional porous Co–P alloy supported on a copper foam as a new catalyst for sodium borohydride electrooxidation, Dalton Transactions, 2019, 48(35):13248–13259.). However, its electrode material is synthesized by electrodeposition, which is easily affected by many factors (such as reaction solution temperature, electric field line distribution, etc.), making it unfavorable for industrial production. Thermal reduction-phosphating is one of the main methods for synthesizing transition metal phosphides (see Chen Fanpeng; Zhao Bohang; Sun Mengyao; Liu Cuibo; Shi Yanmei; Yu Yifu; Zhang Bin, Mechanistic insight into the controlled synthesis of metal phosphide catalysts from annealing of metal oxides with sodium hypophosphite, Nano Research, 2022, 15(12): 10134–10141.). It uses an inert gas as a carrier gas to introduce the thermal decomposition products of the phosphorus source (such as sodium hypophosphite) into the material, completing the phosphating process in a high-temperature environment. The disadvantages of this method are: (1) the phosphating process is highly dependent on the contact between the carrier gas and the electrode surface. Phosphating is difficult in the "dead volume" area where the material contacts the carrier gas, thus making it difficult to guarantee the uniformity of the phosphating process; (2) the process is difficult to scale up, which is not conducive to industrial production. Therefore, this phosphating method needs to be improved to address the above problems. Summary of the Invention

[0004] This invention aims to provide a cobalt-phosphorus-cobalt oxide composite electrode material prepared by impregnation-thermal reduction phosphating and its application as an anode in direct borohydride fuel cells. This invention solves the problem of achieving both low cost and high performance in direct borohydride fuel cell anodes. Furthermore, this invention improves upon the classic thermal reduction phosphating method, making it more suitable for industrial application.

[0005] This invention provides a uniform phosphating method that is easy to scale up industrially; and based on this method, a high-performance cobalt-phosphorus-cobalt oxide composite electrode material is prepared, which can be used as the anode of a direct borohydride fuel cell. The main innovations are: ① The electrode material preparation method is an improvement on the traditional phosphating method; the traditional phosphating method (thermal reduction phosphating) is almost impossible to obtain a uniformly phosphated electrode. The impregnation method used in this invention transforms the phosphating process, which originally relied on the contact between the carrier gas and the electrode surface, into the in-situ adhesion of phosphorus solution to the electrode surface, making uniform phosphating possible; more importantly, under a fairly wide range of phosphorus source concentrations (0.1~10 mol / L), the electrode performance is not only extremely high but also shows little variation, thus having great potential for process scale-up; ② The cobalt-phosphorus-cobalt oxide electrode prepared by this invention can be used as the anode of a direct borohydride fuel cell; and the electrode performance is excellent, superior to most transition metals and surpassing most noble metals.

[0006] This invention provides a cobalt-phosphorus-cobalt oxide composite electrode material, prepared by an impregnation-thermal reduction phosphating method. First, a conductive substrate is selected, and a hydrothermal method is used to thermally decompose urea to create an alkaline environment and precipitate cobalt salts. The morphology is then adjusted using ammonium fluoride to obtain a precursor material. Next, the precursor is impregnated in a sodium hypophosphite aqueous solution of a certain concentration. Phosphating is completed by high-temperature calcination in a tube furnace under vacuum, relying on the trace amounts of phosphorus source solution adhering to the material surface.

[0007] The preparation method of the above-mentioned cobalt-phosphorus-cobalt oxide composite electrode material specifically includes the following steps:

[0008] a1: Add cobalt source, urea, and ammonium fluoride to the aqueous solution in sequence and stir thoroughly until completely dissolved; wherein the molar ratio of cobalt source, urea, and ammonium fluoride is 1:5:4;

[0009] a2: Place the conductive substrate in the solution described in a1, then transfer it to a reaction vessel and heat it at 110~130 ℃ for 11~13 h; after heating, cool it to room temperature and rinse the conductive substrate loaded with the precursor repeatedly with deionized water;

[0010] a3: Immerse the conductive substrate loaded with the precursor obtained in a2 in an aqueous solution of sodium hypophosphite for 20-40 min;

[0011] a4: Place the material impregnated with a3 in a tube furnace, heat it to 330~370℃ at 8~12℃ / min in an environment with a pressure of less than 0.05 MPa, hold it at that temperature for 1.8~2.2 h, and then let it cool naturally to room temperature;

[0012] a5: After the a4 tube furnace cools to room temperature, the material is taken out and repeatedly rinsed with deionized water to obtain a cobalt phosphide-cobalt oxide composite electrode material with an in-situ conductive substrate.

[0013] Furthermore, the cobalt source described in a1 includes one of cobalt nitrate, cobalt chloride, cobalt sulfate, and cobalt acetate.

[0014] Furthermore, the conductive substrate described in a2 includes one of nickel foam, copper foam, carbon cloth, and carbon paper.

[0015] Furthermore, the concentration of the sodium hypophosphite aqueous solution described in a3 is 0.1~10 mol / L.

[0016] The present invention also provides the application of the above-mentioned cobalt phosphorus-cobalt oxide composite electrode material as an anode in a direct borohydride fuel cell.

[0017] The beneficial effects of this invention are:

[0018] (1) The cobalt-phosphorus-cobalt oxide composite electrode material provided by the present invention has a relatively low preparation cost, and the battery exhibits excellent performance when used as the anode of a direct borohydride fuel cell; the present invention is expected to further enhance the cost competitiveness of direct borohydride fuel cells.

[0019] (2) Compared with the classic thermal reduction phosphating method, the phosphating process of the impregnation-thermal reduction phosphating method used in this invention is more uniform: The phosphating process of this method relies on a trace amount of phosphorus source solution to phosphate the material surface at high temperature. Since the impregnation method can easily achieve phosphorus source attachment on the surface of any volume of material, this method has the advantages of more uniform phosphating process and is conducive to industrial scale-up; at the same time, the amount of phosphorus doping in the material can be adjusted by changing the concentration of the impregnation sodium hypophosphite solution; in addition, the material can obtain extremely high activity in a wide range of sodium hypophosphite concentrations, which is conducive to the scale-up and reproducibility of the process.

[0020] (3) The material has a porous structure, and a large number of internal pores can provide abundant ion transport pathways, which is beneficial to expose more active sites and alleviate liquid phase diffusion control;

[0021] (4) Cobalt exists in an electron-rich state in cobalt phosphorus, which gives the material good conductivity and is conducive to the rapid capture and transport of reaction electrons; cobalt exists in an electron-poor state in cobalt oxide, which is conducive to the adsorption of electronegative reactants on the electrode surface and promotes the anodic reaction. Attached Figure Description

[0022] Figure 1 The CoP-0.3 / CoO prepared in Example 1 x Scanning electron microscope image of @NF;

[0023] Figure 2 The CoP-0.3 / CoO prepared in Example 1 xTransmission electron microscope image of @NF;

[0024] Figure 3 The CoP-0.3 / CoO prepared in Example 1 x X-ray diffraction pattern of @NF;

[0025] Figure 4 The CoP-0.3 / CoO prepared in Example 1 x Cyclic voltammetry curves of sodium borohydride electrooxidation catalyzed by @NF;

[0026] Figure 5 The CoP-0.2 / CoO prepared in Example 2 x Cyclic voltammetry curves of sodium borohydride electrooxidation catalyzed by @NF;

[0027] Figure 6 The CoP-0.5 / CoO prepared in Example 2 x Cyclic voltammetry curves of sodium borohydride electrooxidation catalyzed by @NF;

[0028] Figure 7 The CoP-2 / CoO prepared in Example 2 x Cyclic voltammetry curves of sodium borohydride electrooxidation catalyzed by @NF;

[0029] Figure 8 CoP-5 / CoO prepared in Example 2 x Cyclic voltammetry curves of sodium borohydride electrooxidation catalyzed by @NF;

[0030] Figure 9 CoP-10 / CoO prepared in Example 2 x Cyclic voltammetry curves of sodium borohydride electrooxidation catalyzed by @NF;

[0031] Figure 10 The CoP-10 / CoO prepared in Example 3 x Cyclic voltammetry curves of @CC-catalyzed sodium borohydride electrooxidation;

[0032] Figure 11 The CoP-0.3 / CoO prepared in Example 4. x The polarization curve and power density curve of the cell when @NF is used as the anode of a direct sodium borohydride-hydrogen peroxide fuel cell. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto. Example 1

[0034] a1. Accurately weigh 0.5093 g of cobalt nitrate hexahydrate, 0.5255 g of urea, and 0.2593 g of ammonium fluoride, dissolve them in 35 mL of deionized water, and stir thoroughly until completely dissolved;

[0035] a2, 1*1 cm 2 The nickel foam of different sizes was placed in solution a1 and transferred to a polytetrafluoroethylene liner. Then it was placed in a high-pressure reactor and kept at 120°C for 12 h. After cooling to room temperature, the nickel foam loaded with the precursor was repeatedly rinsed with deionized water.

[0036] a3, the nickel foam loaded with the precursor described in a2 is immersed in 10 mL of deionized water containing 0.3180 g of sodium hypophosphite monohydrate for 30 min.

[0037] a4. The impregnated material described in a3 is placed in a quartz boat, then placed in a tube furnace, and heated to 350°C in a vacuum environment at a heating rate of 10°C / min, and held for 2 hours. After cooling to room temperature, it is removed and repeatedly rinsed with deionized water. The obtained material is labeled as CoP-0.3 / CoO. x @NF. Phosphating is a reduction process; some oxygen atoms escape spontaneously during calcination (corresponding to the dehydration process of cobalt hydroxide), while others are replaced by phosphorus atoms (the reduction process itself). Therefore, CoO... x The oxygen content is unknown, so using 'x' to represent it is more scientific and rigorous.

[0038] Appendix Figure 1 These are scanning electron microscope images of the material prepared by the above method. The images show that the material exhibits a sheet-like structure. (Attached) Figure 2 These are transmission electron microscope images of the material prepared by the above method, showing that the material has a porous structure; (Attached) Figure 3 The above is the X-ray diffraction pattern of the material prepared by the above method. The peaks at 36.8°, 44.8° and 65.2° correspond to the (311), (400) and (440) crystal planes of Co3O4 (PDF#73-1701), respectively, and the peaks at 31.6°, 35.3° and 46.2° correspond to the (011), (200) and (112) crystal planes of CoP (PDF#29-0497).

[0039] The CoP-0.3 / CoO prepared by the above method was selected. x @NF material as working electrode, 1*1 cm 2A three-electrode system was formed, consisting of a platinum sheet as the counter electrode and an Ag / AgCl (saturated KCl) electrode with a salt bridge as the reference electrode. The reaction solution was prepared as 0.1 mol / L NaBH4 + 1 mol / L NaOH. Cyclic voltammetry curves of the prepared material catalyzing the electro-oxidation of sodium borohydride were measured using an electrochemical workstation at a scan rate of 25 mV / s. (See attached image) Figure 4 As shown, the prepared electrode achieves a maximum oxidation current density of 780 mA cm⁻¹ at 0 V. -2 This indicates that the prepared electrode has extremely high activity for the electro-oxidation of sodium borohydride. Example 2

[0040] b1. Repeat steps a1 and a2 in Example 1 to obtain 5 portions of nickel foam loaded with the precursor.

[0041] b2, the five nickel foams loaded with precursors described in b1 were immersed in 10 mL of deionized water containing 0.2120 g, 0.5300 g, 2.1198 g, 5.2599 g, and 10.5990 g of sodium hypophosphite monohydrate for 30 min respectively.

[0042] b3. The impregnated material described in b2 is placed in a quartz boat, then placed in a tube furnace, and heated to 350°C in a vacuum environment at a heating rate of 10 K / min, and held for 2 h. After cooling to room temperature, it is removed and repeatedly rinsed with deionized water. The obtained materials are labeled as CoP-0.2 / CoO according to the sodium hypophosphite concentration in the impregnation solution. x @NF、CoP-0.5 / CoO x @NF、CoP-2 / CoO x @NF、CoP-5 / CoO x @NF、CoP-10 / CoO x @NF.

[0043] Five materials prepared by the above method were selected as working electrodes, each 1*1 cm. 2 A three-electrode system was formed, consisting of a platinum sheet as the counter electrode and an Ag / AgCl (saturated KCl) electrode with a salt bridge as the reference electrode. The reaction solution was prepared as 0.1 mol / L NaBH4 + 1 mol / L NaOH. Cyclic voltammetry curves of the prepared material catalyzing the electro-oxidation of sodium borohydride were measured using an electrochemical workstation at a scan rate of 25 mV / s. (See attached image) Figure 5 Appendix Figure 6 Appendix Figure 7 Appendix Figure 8 Appendix Figure 9 As shown, CoP-0.2 / CoO x @NF、CoP-0.5 / CoO x@NF、CoP-2 / CoO x @NF、CoP-5 / CoO x @NF、CoP-10 / CoO x @NF exhibits maximum oxidation current densities of 627, 739, 733, 728, and 698 mA cm⁻¹ at 0 V. -2 The prepared electrode exhibits high catalytic activity for the electro-oxidation of sodium borohydride across a wide range of sodium hypophosphite solution concentrations, indicating that the impregnation-thermal reduction phosphating method employed in this invention is easily scaled up and reproducible.

[0044] This embodiment presents five electrode preparation methods and performance data involving sodium hypophosphite solutions of different concentrations. This embodiment demonstrates that the electrodes prepared by this invention exhibit good performance across a fairly wide range of sodium hypophosphite solution concentrations, with minimal differences, indicating that the preparation method of this invention has good potential for industrial scale-up. Example 3

[0045] c1. Accurately weigh 0.5093 g of cobalt nitrate hexahydrate, 0.5255 g of urea, and 0.2593 g of ammonium fluoride, dissolve them in 35 mL of deionized water, and stir thoroughly until completely dissolved.

[0046] c2, 1*1 cm 2 Carbon cloth (CC) of various sizes was placed in solution c1 and transferred to a polytetrafluoroethylene liner. Then it was placed in a high-pressure reactor and kept at 120°C for 12 h. After cooling to room temperature, the carbon cloth loaded with precursor was repeatedly rinsed with deionized water.

[0047] c3, the carbon array with precursor described in c2 is immersed in 10 mL of deionized water containing 0.3180 g of sodium hypophosphite monohydrate for 30 min.

[0048] c4. The impregnated material described in c3 is placed in a quartz boat, then placed in a tube furnace, and heated to 350°C in a vacuum environment at a heating rate of 10 K / min, and held for 2 h. After cooling to room temperature, it is removed and repeatedly rinsed with deionized water. The obtained material is labeled as CoP-0.3 / CoO. x @CC.

[0049] The CoP-0.3 / CoO prepared by the above method was selected. x @CC material is used as the working electrode, 1*1 cm 2A three-electrode system was formed, consisting of a platinum sheet as the counter electrode and an Ag / AgCl (saturated KCl) electrode with a salt bridge as the reference electrode. The reaction solution was prepared as 0.1 mol / L NaBH4 + 1 mol / L NaOH. Cyclic voltammetry curves of the prepared material catalyzing the electro-oxidation of sodium borohydride were measured using an electrochemical workstation at a scan rate of 25 mV / s. (See attached image) Figure 10 As shown, the prepared electrode achieves a maximum oxidation current density of 612 mA cm⁻¹ at 0 V. -2 This indicates that the electrode can still exhibit good catalytic activity when carbon cloth is used as a conductive substrate. Example 4

[0050] d1, Repeat steps a1, a2, a3, and a4 in Example 1 to obtain CoP-0.3 / CoO x @NF;

[0051] d2, Cut the carbon fiber cloth into 1*1 cm pieces. 2 The carbon cloth was placed in a polytetrafluoroethylene liner, 10 mL of 98% concentrated nitric acid was added, and then it was placed in a high-pressure reactor and kept at 180℃ for 3 h. After cooling to room temperature, the carbon cloth was removed and rinsed repeatedly with deionized water.

[0052] d3, using the carbon cloth described in d2 as the working electrode, 1*1 cm 2 A three-electrode system was constructed, consisting of a platinum sheet as the counter electrode and an Ag / AgCl (saturated KCl) electrode with a salt bridge as the reference electrode. The electrodeposition solution was prepared as follows: 0.3 mol / L H3BO3 + 1 mol / L NH4Cl + 1.5 mM PdCl2. An electrochemical workstation with a current density of -50 mA cm⁻¹ was used. -2 A carbon cloth-loaded palladium electrode was obtained by electrodeposition for 30 min.

[0053] The CoP-0.3 / CoO mentioned in d1 x @NF was used as the anode, and the palladium electrode supported on the carbon cloth described in d3 was used as the cathode. A direct sodium borohydride-hydrogen peroxide fuel cell was assembled using Nafion 117 as the membrane. The anode reaction solution was prepared as 0.4 mol / L NaBH4 + 1.5 mol / L NaOH, and the cathode reaction solution was prepared as 1.4 mol / L H2O2 + 2 mol / L H2SO4. The polarization curve of the assembled fuel cell was tested at room temperature using a battery testing system, and its power density was calculated, as shown in the attached figure. Figure 11 As shown. The assembled fuel cell can achieve an open-circuit potential of 1.8 V and a maximum power density of 280 mW / cm². -2This indicates that when the cobalt-phosphorus-cobalt oxide composite electrode material prepared in this invention is used as the anode of a direct borohydride fuel cell, the cell can exhibit excellent performance.

[0054] It is worth noting that the above examples are merely a few specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a cobalt-phosphorus-cobalt oxide composite electrode material, characterized in that: It was prepared by impregnation-thermal reduction phosphating method; firstly, a conductive substrate was selected, and a hydrothermal method was used to thermally decompose urea to generate an alkaline environment and precipitate cobalt salt, and the morphology was adjusted by ammonium fluoride to obtain the precursor material. Then the precursor is impregnated in a sodium hypophosphite aqueous solution of a certain concentration, and uniform phosphating is completed by high-temperature calcination in a tube furnace under vacuum, relying on the trace amount of phosphorus source solution attached to the surface of the material. Obtaining CoP / CoO with in-situ loading on conductive substrate x Composite electrode materials.

2. The preparation method of the cobalt-phosphorus-cobalt oxide composite electrode material according to claim 1, characterized in that... Includes the following steps: a1: Add cobalt source, urea, and ammonium fluoride to the aqueous solution in sequence and stir thoroughly until completely dissolved; wherein the molar ratio of cobalt source, urea, and ammonium fluoride is 1:5:4; a2: Place the conductive substrate in the solution described in a1, then transfer it to a reaction vessel and heat it at 110~130 ℃ for 11~13 h; after heating, cool it to room temperature and rinse the conductive substrate loaded with the precursor repeatedly with deionized water; a3: Immerse the conductive substrate loaded with the precursor obtained in a2 in an aqueous solution of sodium hypophosphite for 20-40 min; a4: Place the material impregnated with a3 in a tube furnace, heat it to 330~370℃ at 8~12℃ / min in an environment with a pressure of less than 0.05 MPa, hold it at that temperature for 1.8~2.2 h, and then let it cool naturally to room temperature; a5: After the a4 tube furnace cools to room temperature, the material is taken out and repeatedly rinsed with deionized water to obtain a cobalt phosphide-cobalt oxide composite electrode material with an in-situ conductive substrate.

3. The method for preparing the cobalt-phosphorus-cobalt oxide composite electrode material according to claim 2, characterized in that: The cobalt source described in a1 includes one of cobalt nitrate, cobalt chloride, cobalt sulfate, and cobalt acetate.

4. The preparation method of the cobalt-phosphorus-cobalt oxide composite electrode material according to claim 2, characterized in that: The conductive substrate described in a2 includes one of nickel foam, copper foam, carbon cloth, and carbon paper.

5. The method for preparing the cobalt-phosphorus-cobalt oxide composite electrode material according to claim 2, characterized in that: The concentration of the sodium hypophosphite aqueous solution described in a3 is 0.1~10 mol / L.

6. A cobalt-phosphorus-cobalt oxide composite electrode material prepared by the preparation method according to any one of claims 1 to 5.

7. The application of the cobalt-phosphorus-cobalt oxide composite electrode material of claim 6 as an anode in a direct borohydride fuel cell.

8. An application according to claim 7, characterized in that: Using cobalt-phosphorus-cobalt oxide composite electrode material as the working electrode, 1*1 cm 2 A three-electrode system was formed, consisting of a platinum sheet as the counter electrode and an Ag / AgCl electrode with a salt bridge as the reference electrode; the reaction solution was prepared as 0.1 mol / L NaBH4 + 1 mol / L NaOH; the cyclic voltammetry scan rate was 25 mV / s.

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