TW202414869A - Bifunctional electrode and preparation method thereof and its application for zinc-air battery - Google Patents

Abstract

A bifunctional electrode, a preparation method thereof, and its application for a zinc-air battery are provided, wherein the preparation method of the bifunctional electrode includes the following steps. Supplied with carbon cloth. The carbon cloth is placed in the electroplating bath. An electroplating process is performed to form doped oxide on the carbon cloth. An annealing process is performed to obtain the bifunctional electrodes. The doped oxide includes tricobalt tetroxide doped with a first metal element or nickel oxide doped with a second metal element. The first metal element is selected from one of nickel, gallium, iron, lanthanum, magnesium, copper and molybdenum. The second metal element is selected from one of cobalt, lanthanum and manganese.

Description

Bifunctional electrode and preparation method thereof and application in zinc-air battery

The present invention relates to a bifunctional electrode, and in particular to a bifunctional electrode and a preparation method thereof and an application in a zinc-air battery.

As environmental and energy issues become increasingly prominent, the development of clean and renewable electrochemical energy storage technology has received widespread attention. The main energy conversion components are batteries, supercapacitors, and fuel cells. Among them, lithium-ion batteries have high gravimetric energy density and are one of the most common electrochemical energy sources. However, lithium-ion batteries still have problems such as safety, strict manufacturing environment requirements, the use of cobalt and nickel resource metals, high-temperature cathode processes, the use of hazardous solvents, and battery recycling issues, which limit their application scope.

Metal-air battery is an electrochemical battery driven by oxidizing metal and reducing oxygen. It has great advantages in theoretical energy density, including zinc-air battery (ZAB) (the zinc-air battery of the present invention is a rechargeable zinc-air fuel cell). If the positive electrode (cathode) of the zinc-air battery can perform oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the same time, the electrode has the dual functions of oxidation and reduction, and the resulting zinc-air battery can perform reversible reactions, which is conducive to long-term stable operation.

At present, the research on bifunctional electrodes (i.e. electrode materials with ORR/OER catalytic activity) for zinc-air batteries is mostly based on expensive metal-organic scaffolds to make cobalt-based materials. For example, using expensive zeolite imidazolate framework material (ZIF) and cobalt (Co) source (2021, Nano Energy), after long-term polymerization for 12 hours (h) and 1000 ^o C/3h to obtain CoO _x /Porous N-carbon (PNC), the battery performance is the best. At a current density of 10 mA/cm ² , the specific capacitance value is 887 mAh g ^-1 and the energy density is 1020 Wh kg ^-1 . After 200 h of operation, there is no degradation, but a relatively high voltage gap of 1.25 V is maintained (according to its long-term test chart, the charging and discharging is not stable, which means that the nanoparticles of this electrode material have the problem of loosening and falling off). Some studies have used expensive CoNi metal organic scaffold templates that require long-term polymerization (2019, Adv. Mater.), and then combined with 500 ^o C/NH ₃ atmosphere decomposition to obtain a bimetallic positive electrode (CoNi-SAC/NC). The assembled battery has a specific capacitance of 750.9 mAh g ^-1 , an energy density of 886.1 Wh kg ^-1 , a voltage gap of 0.82 V, and no degradation after 95 h of cyclic charge and discharge. Some studies have formed a La ₂ O ₃ /Co ₃ O ₄ /MnO ₂ –CNTs hybrid electrode (2016, Appl. Energy) by annealing at 150 ^o C/6h in a pressure autoclave and 300 ^o C/1h. The assembled battery has a specific capacitance of 810 mAh g ^-1 and an energy density of 970 Wh kg ^-1 . Although it remains stable after 90h (543 cycles) of charge and discharge, the voltage gap increases from 0.7 V to 0.8 V, an increase of 14.3%. Some studies have also used hydrothermal and annealing processes for Co ₃ O ₄ /MnO ₂ -CNT (2019, Nano Energy). The battery has a specific capacitance of 770 mAh g ^-1 and an energy density of > 842 Wh kg ^-1 . It remains stable after 135 h (810 cycles) of charge and discharge, but the voltage gap increases from 0.89 V to 1.25 V, a 40% increase. Some studies have also used hydrothermal and annealing processes to produce the required powder for Co-doped NiO, and then coated the carbon fiber paper with a conductive polymer to complete the air cathode (2019, Appl. Catal. B). At 5 mA/cm ² , the battery has a specific capacitance of 830 mAh g ^-1 and an energy density of 962 Wh kg ^-1 . Although it remains stable after charging and discharging for 110 h (charge 3h/discharge 3h), the voltage gap increases from 0.72 V to 0.82 V, an increase of 13.8%. The increased voltage gap may indicate an adverse condition of electrode material detachment.

As can be seen from the above, the current research on bifunctional electrode materials for zinc-air batteries involves time-consuming and complex processes, and the formula is complex, but it is difficult to achieve low voltage gap (for example, less than 0.86 V), long-term stable characteristics (i.e., multiple charge and discharge cycles), and extremely small voltage gap fluctuations. Therefore, the development of new bifunctional electrodes is the main challenge at present.

The present invention provides a bifunctional electrode and a preparation method thereof and an application in a zinc-air battery. The preparation process is simple and the preparation time is short. The bifunctional electrode has the advantages of excellent oxygen reduction (ORR)/oxygen evolution (OER) catalytic ability, low voltage gap and long-term stability, and extremely small voltage gap fluctuation.

An embodiment of the present invention provides a method for preparing a bifunctional electrode, comprising the following steps: providing carbon cloth; arranging carbon in an electroplating solution; performing an electroplating process to form a doped oxide on the carbon cloth; and performing an annealing process to obtain a bifunctional electrode. The doped oxide includes cobalt tetraoxide doped with a first metal element or nickel oxide doped with a second metal element, and the doped oxide has catalytic activity for oxygen reduction reaction and oxygen evolution reaction. The first metal element is selected from one of nickel, gallium, iron, tantalum, magnesium, copper and molybdenum. The second metal element is selected from one of cobalt, tantalum and manganese.

In one embodiment of the present invention, the electroplating process includes: applying a constant voltage of -0.8V for 5 minutes. The annealing process includes: using air as the atmosphere gas and maintaining the temperature at 350 ^° C for 2 hours.

In one embodiment of the present invention, after the electroplating process and before the annealing process, the doped oxide is further cleaned and dried.

In one embodiment of the present invention, the electroplating solution comprises a precursor containing cobalt and a first metal precursor containing a first metal element, and the molar ratio of the cobalt precursor to the first metal precursor is 1:0.025 to 1:0.05.

In one embodiment of the present invention, the cobalt-containing precursor comprises cobalt nitrate. The first metal precursor comprises one selected from nickel nitrate, gallium nitrate, iron nitrate, onium nitrate, magnesium chloride, copper chloride, and molybdenum chloride.

In one embodiment of the present invention, the electroplating solution further includes a third metal precursor containing a third metal element. The third metal element is different from the first metal element. The third metal element is selected from one of magnesium, manganese, nickel and vanadium. The molar ratio of the first metal precursor to the third precursor is 1:0.5.

In one embodiment of the present invention, the third metal precursor is selected from one of magnesium chloride, manganese chloride, nickel nitrate and vanadium chloride.

In one embodiment of the present invention, the electroplating solution includes a precursor containing nickel and a second metal precursor containing a second metal element, and the molar ratio of the nickel precursor to the second metal precursor is 1:0.05 to 1:0.075.

In one embodiment of the present invention, the nickel-containing precursor comprises nickel nitrate, and the second metal precursor is selected from one of cobalt nitrate, nickel nitrate and manganese chloride.

An embodiment of the present invention provides a bifunctional electrode, including a doped oxide. The doped oxide is disposed on a carbon cloth. The doped oxide includes cobalt tetraoxide doped with a first metal element or nickel oxide doped with a second metal element. The first metal element is selected from one of nickel, gallium, iron, tantalum, magnesium, copper and molybdenum, and the second metal element is selected from one of cobalt, tantalum and manganese.

In one embodiment of the present invention, the first metal element is lumen, and in the cobalt tetroxide doped with the first metal element, the molar ratio of cobalt to lumen is 1:0.025 to 1:0.05.

In one embodiment of the present invention, the above-mentioned vanadium-doped cobalt tetraoxide includes a third metal element doped therein. The third metal element is selected from one of magnesium, manganese, nickel and vanadium.

In one embodiment of the present invention, in the above-mentioned cobalt tetraoxide doped with a third metal element and titanium, the molar ratio of titanium to the third metal element is 1:0.5.

In one embodiment of the present invention, the second metal element is cobalt, and in the nickel oxide doped with the second metal element, the molar ratio of nickel to cobalt is 1:0.05 to 1:0.075.

One embodiment of the present invention provides a zinc air battery, wherein the anode is metallic zinc or a zinc alloy, and the cathode is a bifunctional electrode as described in any one of the above items.

Based on the above, the present invention forms a bifunctional electrode comprising a doped oxide through a two-stage process of electroplating for 5 minutes and an annealing process for 2 hours, which has the advantages of short process time, simple process, low energy consumption and clean process. In addition, the bifunctional electrode comprising a doped (i.e., doped with other metal elements) oxide of the present invention has excellent oxygen reduction (ORR) and oxygen evolution (OER) catalytic ability, low voltage gap and long-term stability, and at the same time has the characteristics of extremely small voltage gap fluctuation, and is suitable as an electrocatalytic electrode material for zinc-air batteries.

In order to make the above features and advantages of the present invention more clearly understood, embodiments are given below and described in detail with reference to the accompanying drawings.

The present invention is more fully described with reference to the drawings of the present embodiment. However, the present invention may be embodied in various forms and should not be limited to the embodiments described herein. The thickness of layers and regions in the drawings are exaggerated for clarity. The same or similar reference numbers represent the same or similar elements, and the following paragraphs will not be repeated one by one.

It should be understood that when an element is referred to as being "on" or "connected to" another element, it can be directly on or connected to another element, or there can be intervening elements. If an element is referred to as being "directly on" or "directly connected to" another element, there are no intervening elements. As used herein, "connected" can refer to physical and/or electrical connections, and "electrically connected" or "coupled" can refer to the presence of other elements between two elements.

As used herein, "about", "approximately" or "substantially" includes the referenced value and the average value within an acceptable deviation range of a specific value that can be determined by a person of ordinary skill in the art, taking into account the measurement in question and the specific amount of error associated with the measurement (i.e., the limitations of the measurement system). For example, "about" can mean within one or more standard deviations of the stated value, or within ±30%, ±20%, ±10%, ±5%. Furthermore, as used herein, "about", "approximately" or "substantially" can select a more acceptable deviation range or standard deviation depending on the optical properties, etching properties or other properties, and can apply to all properties without a single standard deviation.

The terms used herein are used to illustrate exemplary embodiments only, rather than to limit the present disclosure. In this case, unless otherwise explained in the context, the singular form includes the plural form.

FIG. 1 is a flow chart of a method for preparing a bifunctional electrode according to an embodiment of the present invention.

Please refer to FIG. 1 , first perform step S100: provide carbon cloth. Carbon cloth (CC) is used as an electrode support material. Since the quality of the film deposited by the subsequent electroplating process may be closely related to the hydrophilicity and cleanliness of the carbon cloth, the carbon cloth may first undergo multiple cleanings. In this embodiment, the method for cleaning the carbon cloth may, for example, include but is not limited to the following steps: First, place the carbon cloth in 1 molar concentration (M) of hydrochloric acid (HCl), shake it with an ultrasonic oscillator and soak it for about 12 hours. Then, place the carbon in ethanol, shake it with an ultrasonic oscillator and soak it for about 10 minutes. Then, place the carbon in deionized water, shake it with an ultrasonic oscillator and soak it for about 10 minutes. The carbon cloth is then placed in an oven to dry for use. The drying conditions may be, for example, drying at 60 ^° C. for about 2 hours.

Then, step S102 is performed: carbon is arranged in the electroplating solution. The preparation method of the electroplating solution is, for example, mixing a precursor containing cobalt (i.e., the cobalt-containing precursor in this article) or a precursor containing nickel (i.e., the nickel-containing precursor in this article) with a precursor solution containing different binary/ternary metal elements (i.e., the first metal precursor, the second metal precursor, and the third metal precursor in this article). In this embodiment, the cobalt-containing precursor is, for example, cobalt nitrate (Co(NO ₃ ) ₂ ), and the nickel-containing precursor is, for example, nickel nitrate (Ni(NO ₃ ) ₂ ), but is not limited thereto. In some embodiments, when the electroplating solution uses cobalt nitrate (Co(NO ₃ ) ₂ ) as the main precursor (main precursor means that the component has a high proportion in the electroplating solution), other mixed precursors may include nitrates and/or metal chlorides. Nitrates include, but are not limited to, nickel nitrate (Ni(NO ₃ ) ₂ ), gallium nitrate (Ga(NO ₃ ) ₃ ), iron nitrate (Fe(NO ₃ ) ₃ ), and vanadium nitrate (La(NO ₃ ) ₃ ). Metal chlorides include, but are not limited to, magnesium chloride (MgCl ₂ ), copper chloride (CuCl ₂ ), molybdenum chloride (MoCl ₂ ), manganese chloride (MnCl₂), and vanadium chloride (VCl ₃ ). In some other embodiments, when the electroplating solution uses nickel nitrate as the main precursor, other mixed precursors may be, for example but not limited to, cobalt nitrate, nickel nitrate or manganese chloride.

In detail, in some embodiments, the electroplating solution includes a cobalt-containing precursor containing cobalt and a first metal precursor containing a first metal element. The cobalt-containing precursor may be, for example, cobalt nitrate. The first metal element may be selected from one of nickel, gallium, iron, onium, magnesium, copper and molybdenum, and the first metal precursor corresponds to one selected from but not limited to nickel nitrate, gallium nitrate, iron nitrate, onium nitrate, magnesium chloride, copper chloride and molybdenum chloride. In some embodiments, the molar ratio of the cobalt-containing precursor to the first metal precursor is approximately 1:0.025 to 1:0.05.

In other embodiments, the plating solution further includes a third metal precursor containing a third metal element, so that the plating solution simultaneously has a cobalt-containing precursor containing cobalt, a first metal precursor containing the first metal element, and a third metal precursor containing the third metal element. The third metal element is different from the first metal element. The third metal element is one selected from magnesium, manganese, nickel, and vanadium, so the third metal precursor may correspond to one selected from but not limited to magnesium chloride, manganese chloride, nickel nitrate, and vanadium chloride.

In some embodiments, the molar ratio of the cobalt-containing precursor, the first metal precursor, and the third precursor is approximately 1:0.05:0.025.

In some other embodiments, the electroplating solution may include, for example, a nickel-containing precursor containing nickel and a second metal precursor containing a second metal element. The second precursor is nickel nitrate, and the second metal element is selected from one of cobalt, onium and manganese. Therefore, the second metal precursor may correspond to one selected from but not limited to cobalt nitrate, onium nitrate and manganese chloride. In some embodiments, the molar ratio of the nickel precursor to the second metal precursor may be approximately 1:0.005 to 1:0.075.

Next, place the carbon cloth into the prepared electroplating solution.

Then, step S104 is performed: an electroplating process is performed to form a doped oxide on the carbon cloth. In some embodiments, the electroplating process is performed under conditions such as applying a constant voltage of -0.8 volts for 5 minutes. At this time, the doped oxide is deposited on the carbon cloth in the form of a thin film.

According to the composition of the electroplating solution used, the doped oxide may include cobalt tetraoxide (Co ₃ O ₄ ) doped with a first metal element or nickel oxide (NiO) doped with a second metal element, wherein the first metal element is selected from one of nickel, gallium, iron, vanadium, magnesium, copper and molybdenum, and the second metal element is selected from one of cobalt, vanadium and manganese. In some other embodiments, the cobalt tetraoxide doped with the first metal element is further doped with a third metal element. The third metal element is different from the first metal element, and the third metal element is selected from one of magnesium, manganese, nickel and vanadium.

Next, after the electroplating process, the doped oxide may be cleaned and dried. Specifically, after the electroplating process is completed, it may be cleaned with deionized water and then dried in an oven, for example, at 75°C for 2 hours.

Then, step S106 is performed: an annealing process is performed to obtain a bifunctional electrode. In some embodiments, the annealing process may be performed under conditions such as air as an atmosphere gas, a heating rate of 5°C/min, and a temperature of 350 ^° C for 2 hours. Thus, the bifunctional electrode including the doped oxide of the present invention has been prepared.

In an embodiment of the present invention, the bifunctional electrode prepared by the above-mentioned preparation method may include a doped oxide. The doped oxide may be disposed on a carbon cloth. Specifically, the doped oxide includes cobalt tetraoxide doped with a first metal element or nickel oxide doped with a second metal element, and the doped oxide has catalytic activity for oxygen reduction reaction and oxygen evolution reaction. The first metal element is selected from one of nickel, gallium, iron, yttrium, magnesium, copper and molybdenum, and the second metal element is selected from one of cobalt, yttrium and manganese.

In the present invention, a method for preparing a bifunctional electrode comprising oxides doped with different metal elements (i.e., doped oxides) is through two-stage steps: a 5-minute electroplating process and a 2-hour annealing process, which has the advantages of short process time, simple process steps, low energy consumption, and clean process. In addition, the bifunctional electrode comprising doped oxides of the present invention has excellent oxygen reduction (ORR) and oxygen evolution (OER) catalytic capabilities, low voltage gap and long-term stability, and at the same time has the characteristics of extremely small voltage gap fluctuation, and is suitable as an electrocatalytic electrode material for zinc-air batteries.

In the present invention, compared with the electrode containing cobalt tetraoxide or nickel oxide, the bifunctional electrode containing cobalt tetraoxide doped with the first metal element or nickel oxide doped with the second metal element can have better catalytic ability of oxygen reduction (ORR) and oxygen evolution (OER). In addition, a lower voltage gap can be obtained, and the advantages of maintaining long-term stability and extremely small voltage gap fluctuation can be obtained.

The following will describe the characteristics of the bifunctional electrode of the present invention in more detail through examples and comparative examples. Although the following embodiments are described, the materials used, their amounts and ratios, processing details, and processing procedures, etc. may be appropriately changed without exceeding the scope of the present invention. Therefore, the present invention should not be construed restrictively by the embodiments described below. < Experimental Methods >

Preparation of bifunctional electrode: It has been described in detail above and will not be repeated here.

Symbols represent: Taking Ni _0.05 -Co/CC-350 as an example, it means that the cobalt/nickel precursor is electroplated on the carbon cloth at a molar ratio of 1:0.05, and then annealed at 350°C to obtain a battery. The rest of the symbols are similar.

Preparation of the battery: Place the prepared carbon cloth on the device with a reaction area of 3.14 ^cm2 , and cover it with hydrophobic carbon paper as a diffusion layer to prevent electrolyte leakage. Then put the lid on and lock the device. Fill the electrolytic cell with 6 M potassium hydroxide and 0.2 M zinc acetate electrolyte, and finally put in the zinc sheet as the anode.

Triode electrochemical measurement of half-cell characteristics: The materials developed and adopted in the experiment are used as the working electrode, the saturated Hg/HgO electrode is used as the reference electrode, the Pt sheet is used as the counter electrode, and the electrolyte is 0.1 M KOH. Linear scanning voltammetry, cyclic voltammetry, and impedance spectroscopy are performed to find the best formula. Once the material selection is completed, the diode full-cell measurement is performed. The cathode is a self-made test piece, the anode is a zinc sheet, and the electrolyte includes 6 M potassium hydroxide and 0.2 M zinc acetate. This equipment is mainly used to perform constant current charge and discharge and linear scanning voltammetry. < Experimental results >

FIG2A is a diagram showing polarization curves of a bifunctional electrode comprising cobalt oxide doped with different metal elements measured in oxygen reduction reaction (ORR). FIG2B is a diagram showing polarization curves of a bifunctional electrode comprising cobalt oxide doped with different metal elements measured in oxygen evolution reaction (OER).

Please refer to FIG. 2A and FIG. 2B at the same time. This embodiment uses the electrochemical technique of linear voltammetry to measure the half-wave potential (V _1/2 ) of the ORR reaction, the V ₁₀ potential of the OER oxygen evolution reaction at 10 mA/cm ² , and the full battery reaction potential (V ₁₀ -V _1/2 ), thereby finding the material with the minimum reversible battery reaction voltage as the best material combination. When the cobalt-containing precursor (i.e., the main precursor) is cobalt nitrate, the first metal element is supplemented with nickel, gallium, magnesium, iron, copper, molybdenum, and tantalum, and is added to the plating solution to form a bifunctional electrode material (i.e., cobalt oxide doped with the first metal element). After annealing at 350°C, the polarization curves of the ORR and OER reactions are shown in Figures 2A and 2B, respectively. The ORR reaction half-wave potential and the V ₁₀ potential of the OER reaction at 10 mA/cm ² obtained from the curves are presented in Table 1, respectively. At the same time, the doping effect on the reversible full-cell reaction potential of electroplated Co ₃ O ₄ is also calculated in Table 1. Table 1 shows the effect of doping on the reversible electrode and full-cell potential of electroplated Co ₃ O ₄ . The first metal element includes nickel, gallium, magnesium, iron, copper, molybdenum, and rhenium. The obtained full battery potentials are 0.73, 0.75, 0.98, 0.76, 0.81, 0.82, 0.85, and 0.70 V, respectively. La _0.05 -Co/CC-350 has the lowest voltage value, which means it is most suitable as a reversible electrode for zinc air batteries.

Table 1. Effect of doping on the reversible electrode and full cell potential of electroplated Co ₃ O ₄ System Co: electroplated Co ₃ O CC: carbon cloth 250, 350: annealing temperature OER V ₁₀ voltage (V vs RHE) ORR Half Wave (V _1/2 ) Voltage (V vs RHE) Voltage gap V ₁₀ - V _1/2 (V vs RHE) Co/CC-250 1.57 0.82 0.75 Doped cobalt tetraoxide (Co ₃ O ₄ ) first metal element added M _0.05 -Co/CC-350 system Ni _0.05 -Co/CC-250 1.58 0.85 0.73 Ni _0.05 -Co/CC-350 1.59 0.84 0.75 Ga _0.05 -Co/CC-350 1.66 0.68 0.98 Mg _0.05 -Co/CC-350 1.60 0.84 0.76 Fe _0.05 -Co/CC-350 1.57 0.76 0.81 Cu _0.05 -Co/CC-350 1.59 0.77 0.82 Mo _0.05 -Co/CC-350 1.58 0.73 0.85 La _0.05 -Co/CC-350 1.55 0.85 0.70 The third metal element is added to the La _0.05 -Co/CC-350 system Mg _0.025 -La _0.05 -Co/CC-350 1.56 0.85 0.71 Mn _0.025 -La _0.05 -Co/CC-350 1.55 0.82 0.73 Ni _0.025 -La _0.05 -Co/CC-350 1.57 0.85 0.72 V _0.025 -La _0.05 -Co/CC-350 1.62 0.75 0.87 Additional coating treatment for La _0.05 - Co/CC-350 CTAB-La _0.05 -Co/CC-350 1.57 0.85 0.72 PVP-La _0.05 -Co/CC-350 1.56 0.83 0.73 Thiourea-La _0.05 -Co/CC-350 1.57 0.80 0.77 La _0.05 -Co/CC-350@Ag 1.64 0.85 0.79 La _0.05 -Co/CC-350@rGO 1.57 0.81 0.76 La _0.05 -Co/CC-350@CNT 1.55 0.83 0.72 La _0.05 -Co/CC-350@Ni 1.53 0.82 0.71 La _0.05 -Co/Ni/CC-350@CNT 1.57 0.84 0.73 Doped nickel oxide (NiO) second metal element added to M-Ni/CC-350 system Ni/CC-350 1.67 0.82 0.85 Co _0.05 -Ni/CC-350 1.57 0.85 0.72 La _0.05 -Ni/CC-350 1.62 0.74 0.88 Mn _0.05 -Ni/CC-350 1.60 0.84 0.76 Co _0.025 -Ni/CC-350 1.57 0.84 0.75 Co _0.075 -Ni/CC-350 1.58 0.86 0.72

Please continue to refer to Table 1. When the third metal precursor is added to the plating solution for making La _0.05 -Co/CC-350, electroplating of the ternary metal element (i.e., cobalt oxide doped with the first metal element and the third metal element) is carried out. The resulting electrode is also subjected to measurement and evaluation of three electrical properties: ORR reaction half-wave potential, OER reaction V ₁₀ potential at 10 mA/cm ² , and full cell potential. The third metal element includes magnesium, manganese, nickel, and vanadium, and the obtained full battery potential is 0.71, 0.73, 0.72, and 0.87 V, respectively. As shown in Table 1, it is found that _My - _La0.05 -Co/CC-350 has no additional benefit compared to _La0.05 -Co/CC-350 without the third metal element. Therefore, the third metal element is not important to add.

Please continue to refer to Table 1. In this embodiment, La _0.05 -Co/CC-350 is subjected to additional coating treatment, such as adding surfactant CTAB to the electroplating solution, adding PVP polymer for dispersion, adding thiourea (Thiourea) as a sulfur source, external coating with reduced silver, external coating with reduced graphene oxide (rGO), external coating with CNT, external coating with electroplated nickel, first coating and then coating La _0.025 -Co/CC-350, etc. The full battery potential is 0.72, 0.73, 0.77, 0.79, 0.76, 0.72, 0.71, and 0.73 V, respectively, as shown in Table 1. This result shows that the above-mentioned additional coating treatment has no additional benefit.

Therefore, it can be seen from the above results that La addition is very important for the battery properties of cobalt tetroxide.

Please continue to refer to Table 1. When the main precursor is nickel nitrate, the second metal element is supplemented with cobalt, rhenium, and manganese. The catalytic electrode obtained by adding 5% or 7.5% of the second metal element precursor to the electroplating solution is annealed at 350°C, and the polarization curves of the ORR and OER reactions are measured. The V ₁₀ potential of the OER reaction at 10 mA/cm ² and the ORR reaction half-wave potential obtained from the curve are presented in Table 1, respectively. At the same time, the doping effect on the reversible full-cell reaction potential of electroplated Co ₃ O ₄ is also calculated in Table 1. This Table 1 also shows the effect of doping on the reversible electrode and full-cell potential of electroplated NiO. When 5% of the second metal element including cobalt, rhenium and manganese is added, the full battery potential is 0.72, 0.88 and 0.76 V respectively, and Co _0.05 -Ni/CC-350 has the lowest voltage value. When the cobalt content is changed to 2.5%, 5% and 7.5%, the full battery potential is 0.75, 0.72 and 0.72 V respectively. Both Co _0.05 -Ni/CC-350 and Co _0.075 -Ni/CC-350 are suitable as reversible electrodes for zinc-air batteries.

Figure 3 shows the polarization curves of the ORR (as shown in Figure 3A) and OER (as shown in Figure 3B) reactions of La _Z -Co/CC-350 (Z = _0.025 , 0.05, _0.075 , 0.1) electrocatalytic electrodes deposited in the electroplating solution with different doping amounts. After annealing at 350 ^° C, the obtained electrocatalytic electrodes have the following reactions:

Table 2. The measured potential values of Co/CC-250 and La _z -Co/CC-350 (z = 0.025, 0.05, 0.075, 0.1) films with different indium doping concentrations in oxygen evolution reaction, oxygen reduction reaction, and full cell reaction in 0.1 M KOH oxygen environment. Electrocatalyst OER voltage (V) vs RHE OER overpotential(V) ORR half-wave potential (V) Full battery reaction potential (V) V ₁₀ V _1/2 V ₁₀ -V _1/2 Co/CC-250 1.57 0.34 0.82 0.75 La _0.025 -Co/CC-350 1.56 0.33 0.85 0.71 La _0.05 -Co/CC-350 1.55 0.32 0.85 0.70 La _0.075 -Co/CC-350 1.57 0.34 0.82 0.75 La _0.1 -Co/CC-350 1.59 0.36 0.78 0.81

Figure 4 is an electron microscope microstructure image of La _0.025 -Co/CC-350 film and its composition Co, O, La element distribution diagram. The film has a nanosheet structure morphology, and the element distribution proves that La, Co, and O are all coated on the electrode. In this embodiment, appropriate doping with indium can replace part of Co ³⁺ , resulting in more lattice substitution defects, and thus more catalytic active sites, improving electronic efficiency and enhancing conductivity. XPS analysis is used to explore the influence of indium doping concentration and the coexistence of divalent cobalt and trivalent cobalt caused by replacing Co on properties.

Figure 5 shows the XRD diffraction analysis of Co/CC-250 and La-doped La _z -Co/CC-350 (z= 0.025, 0.05, 0.075, 0.1) films. The carbon cloth contributes low-angle diffraction peaks. The electroplated film is spinel-structured Co ₃ O ₄ ( PDF#65-3103 ), and its peaks are at 31.4°, 36.9°, 44.9°, 59.5°, and 65.5°, which are contributed by the diffraction of the (220), (311), (400), (511), and (440) lattice planes, respectively. After annealing at 350°C and doping with 0.025 M La and 0.05 M La, the peak becomes wider, and the strength and crystallinity decrease due to doping. When the doping is increased to 0.075 M La and 0.1 M La, no obvious peak exists. This result shows that the electroplated film is mainly La-doped cobalt tetraoxide (La-Co ₃ O ₄ ).

Figure 6 shows the charge-discharge polarization curves of Co/CC-250, La _z -Co/CC-350 (z=0.025, 0.05, 0.075, 0.1), and PtC+RuO ₂ /CC. As shown in Table 3, at a current density of 10 mA/cm ² , the voltage gaps are 1.64, 1.01, 1.01, 1.70, 1.60, and 0.99 V, respectively. The smaller the voltage gap, the better the charge-discharge capability. Therefore, it can be seen that the voltage gaps of La _0.025 -Co/CC-350 and La _0.05 -Co/CC-350 are similar to those of the ideal air battery catalyst PtC+RuO ₂ /CC, which means that they have excellent charge-discharge performance.

Table 3. Charge-discharge polarization curves of Co/CC-250, La _z -Co/CC-350 (z= 0.025, 0.05, 0.075, 0.1), and PtC+RuO ₂ /CC, and the voltage gap values obtained at a current density of 10 mA cm ^-2 Electrocatalyst Voltage gap at 10 mA/ ^cm2 Co/CC-250 1.64 La _0.025 -Co/CC-350 1.01 La _0.05 -Co/CC-350 1.01 La _0.075 -Co/CC-350 1.70 La _0.1 -Co/CC-350 1.60 PtC+RuO ₂ /CC 0.99

It is not possible to tell which of La _0.025 -Co/CC-350 and La _0.05 -Co/CC-350 is better from the charge and discharge curves, so long-term discharge will be used for evaluation. Figure 7 shows the long-term discharge diagram of La _0.025 -Co/CC-350, La _0.05 -Co/CC-350, and PtC+RuO ₂ /CC at a current density of 2 mA cm ^-2 . The specific capacities are 789, 635, and 736 mAh g _Zn ^-1 , respectively, and the energy densities are 903, 723, and 910 Wh kg ^-1 , respectively. It can be seen that La _0.025 -Co/CC-350 has excellent performance and has similar characteristics to the ideal reversible air electrode PtC+RuO ₂ /CC.

Figure 8 shows the constant current discharge experiment of La _0.025 -Co/CC-350 at different current densities of 1 mA cm ^-2 , 2 mA cm ^-2 , 5 mA cm ^-2 , 10 mA cm ^-2 , 20 mA cm ^-2 , and finally back to 2 mA cm ^-2. The average discharge voltages are 1.17, 1.13, 1.05, 0.93, 0.74, and 1.14 V, respectively. It can be seen that with the increase of current density, the average discharge voltage decreases more but remains stable, with a recoverable characteristic, which shows the discharge stability of the battery.

During the galvanostatic charge-discharge test, the current density was 2 mA cm ^-2 , the charge was 10 minutes, the discharge was 10 minutes, and the cycle was repeated 300 times. La _0.025 -Co/CC-350 and Pt/C+RuO ₂ /CC were charged and discharged for a long time (100 h) to further evaluate their charge and discharge stability. Figure 9 shows the long-term cyclic charge and discharge stability test data of La-doped Co ₃ O ₄ and Pt/C+RuO ₂ /CC at a current density of 2 mA cm ^-2 . The results show that the initial voltage gap of Pt/C+RuO ₂ /CC is only 0.64 V, while the initial voltage gap of La _0.025 -Co/CC-350 is 0.86 V. However, after 300 cycles, the voltage gap of La _0.025 -Co/CC-350 is 0.88 V. It can be seen that the voltage gap has only increased by 0.02 V, a 2.3% increase. However, Pt/C+RuO ₂ /CC cannot complete 300 cycles, and the voltage gap has increased significantly. Therefore, it can be seen that La _0.025 -Co/CC-350 has excellent long-term charge and discharge stability.

After 300 cycles of charge and discharge testing, the samples before and after the test were analyzed by Raman spectroscopy for bonding and X-ray photoelectron spectrometer (XPS) for composition. Figure 10 shows the Raman graph of the La _0.025 -Co/CC-350 full battery before and after the test. Table 4 shows the XPS element content of the La _0.025 -Co/CC-350 full battery before and after the test. There is no obvious change in the analysis results of both, and the XPS element content analysis is almost unchanged, indicating that the electrode performs reversible ORR and OER reactions.

Table 4. XPS analysis of element contents before and after La _0.025 -Co/CC-350 full battery test La _0.025 -Co/CC-350 Co (at.%) La (at.%) O _L (at.%) O _v % Co ²⁺ Co ³⁺ Before cycle charge and discharge 40.49 5.99 53.51 13.66 15.49 25.00 After cyclic charge and discharge 40.99 5.91 53.10 15.75 15.75 25.24

Figure 11 shows the polarization curves of (A) ORR and (B) OER reactions of the undoped NiO electrocatalyst electrode obtained by adding 5% cobalt, lithium, and manganese precursors to the electroplating solution after annealing at 350 ^° C. The ORR reaction half-wave potential and the V ₁₀ potential of the OER reaction at 10 mA/cm ² obtained from the curves are also presented in Table 1. The ORR half-wave potentials of undoped M _0.05 -Ni/CC-350 (M= Co, La, Mn) are 0.82, 0.85, 0.74, and 0.84 V, respectively; the V10 potentials of the OER reaction at 10 mA/cm ² are 1.67, 1.57, 1.62, and 1.60 V, respectively; and the full battery potentials are 0.85, 0.72, 0.88, and 0.76 V, respectively. Co _0.05 -Ni/CC-350 has the lowest voltage value and is most suitable as a reversible electrode for zinc-air batteries, so it will be used for full battery testing.

Figures 12A and 12B are low-power and high-power electron microscope microstructure images of Co _0.05 -Ni/CC-350 film. The film has a nanosheet structure morphology similar to La _0.05 -Co/CC-350. Figure 13 is an XRD diffraction analysis of the doped M _0.05 -Ni/CC-350 (M _0.05 = Co, La, Mn) film. In addition to the low-angle diffraction peaks contributed by the carbon cloth, the electroplated film is verified to be NiO phase, which is consistent with the rhombohedral NiO standard card PDF#44-1159. Its two obvious peaks are contributed by the diffraction of the (101) and (012) lattice planes.

Figure 14 shows the long-term discharge graphs of Co _0.05 -Ni/CC-350 and PtC+RuO ₂ /CC at a current density of 2 mA cm ^-2 . The specific capacities are 714 and 740 mAh g _Zn ^-1 , respectively, and the energy densities are 806 and 908 Wh kg ^{-1 ,} respectively. It can be seen that Co-doped NiO has excellent performance and has similar characteristics to the ideal reversible air electrode PtC+RuO ₂ /CC.

The current density was 2 mA cm ^-2 , the charge time was 10 minutes, the discharge time was 10 minutes, and the cycle was repeated 240 times. Co _0.05 -Ni/CC-350 (Cobalt-doped NiO) and Pt/C+RuO ₂ /CC were charged and discharged for a long time (80 h) to further evaluate their long-term charge and discharge stability. Figure 15 shows the long-term cyclic charge and discharge stability test data of Co _0.05 -Ni/CC-350 and PtC+RuO ₂ /CC at a current density of 2 mA cm ^-2 . The results show that the initial voltage gap of Pt/C+ _RuO2 /CC is only 0.63 V, while the initial voltage gap of Co-doped NiO is 0.87 V. However, after 240 cycles, the voltage gap of Co-doped NiO is reduced to 0.85 V. It can be seen that the voltage gap is reduced by 0.02 V, only 2.3%. Pt/C+ _RuO2 /CC begins to deteriorate after 40h (120 cycles of charge and discharge). Therefore, it can be seen that Co-doped NiO has excellent long-term charge and discharge stability.

In summary, the present invention not only produces excellent modified doped oxides, such as Co ₃ O ₄ and NiO, but also forms dual-functional electrodes on carbon cloth to form La-doped Co ₃ O ₄ and Co-doped NiO, which have excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) capabilities, and are suitable as electrocatalytic electrode materials for reversible zinc-air batteries and for making their components. Furthermore, the preparation method disclosed in the present invention also has application advantages such as simple process, extremely short process time, low energy consumption and clean process, which is conducive to the rapid manufacture of competitive reversible zinc-air batteries.

On the other hand, in the embodiment of the present invention, the full battery obtained by La doping Co ₃ O ₄ in alkaline electrolyte has an open circuit voltage of 1.42 V, ^a specific capacitance of 786 mAh g _Zn ^-1 , an energy density of 903 Wh kg ^-1 , and can be stably cycled for 300 times within 100 hours, with the voltage gap increasing by only 0.02 V to 0.88 V, a change of only 2.3%. The specific capacitance of Co doped NiO is 714 mAh g _Zn ^-1 , an energy density of 806 Wh kg ^-1 , and can be stably cycled for 240 times within 80 hours, with the voltage gap slightly decreasing from 0.87 V to 0.85 V, a change of only 2.3%. This achieves the advantages of a small voltage gap (0.86 V), long-term stable characteristics, and a very small voltage gap fluctuation (2.3%, 100h/300 cycles) for a long time.

Although the present invention has been disclosed as above by the embodiments, they are not intended to limit the present invention. Any person with ordinary knowledge in the relevant technical field can make some changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be defined by the scope of the attached patent application.

S100, S102, S104, S106: Steps

FIG1 is a flow chart of a method for preparing a bifunctional electrode according to an embodiment of the present invention. FIG2A is a polarization curve diagram of a bifunctional electrode formed of cobalt tetroxide doped with different metal elements, measured in oxygen reduction reaction (ORR). FIG2B is a polarization curve diagram of a bifunctional electrode deposited with cobalt tetroxide doped with different metal elements, measured in oxygen evolution reaction (OER). FIG3A is a polarization curve diagram of an ORR reaction obtained by annealing an electrocatalytic electrode at 350 ^° C after depositing different amounts of formic acid doping in an electroplating solution. Figure 3B shows the polarization curves of the OER reaction obtained after annealing at 350 ^° C for the electrocatalytic electrode deposited in the electroplating solution with different amounts of titanium doping. Figure 4 shows the electron microscope microstructure image of La _0.025 -Co/CC-350 film and the distribution of Co, O, and La elements in its composition. Figure 5 shows the XRD diffraction analysis of Co/CC-250 and La-doped La _z -Co/CC-350 (z= 0.025, 0.05, 0.075, 0.1) films. Figure 6 shows the charge-discharge polarization curves of Co/CC-250, La _z -Co/CC-350 (z= 0.025, 0.05, 0.075, 0.1), and PtC+RuO ₂ /CC. Figure 7 shows the long-term discharge graph of La _0.025 -Co/CC-350, La _0.050 -Co/CC-350, and PtC+RuO ₂ /CC at a current density ^of 2 mA cm ^-2 . Figure 8 shows the constant current discharge test graph of La _0.025 -Co/CC-350 at different current densities. Figure 9 shows the long-term cycle charge-discharge stability test data of La doped Co ₃ O ₄ and Pt/C+ RuO ₂ /CC at a current density of 2 mA cm -2. Figure 10 shows the Raman graph of La _0.025 -Co/CC-350 before and after the full battery test. Figure 11A is a doped NiO electrocatalytic electrode obtained by adding cobalt, titanium and manganese precursors to the electroplating solution. After annealing at 350 ^o C, the polarization curve of the ORR reaction is obtained. Figure 11B is a doped NiO electrocatalytic electrode obtained by adding cobalt, titanium and manganese precursors to the electroplating solution. After annealing at 350 ^o C, the polarization curve of the OER reaction is obtained. Figures 12A and 12B are low-power and high-power electron microscope microstructure images of Co _0.05 -Ni/CC-350 (cobalt-doped NiO) thin films. Figure 13 is an XRD diffraction analysis diagram of the doped MN-Ni/CC-350 (MN= Co, La, Mn) thin film. Figure 14 shows the long-term discharge graph of Co _0.05 -Ni/CC-350 (cobalt-doped NiO) and PtC+RuO ₂ /CC at a current density ^{of 2 mA cm -2 .} Figure 15 shows the long-term cyclic charge-discharge stability test data of Co _0.05 -Ni/CC-350 (cobalt-doped NiO) and PtC+RuO ₂ /CC at a current density of 2 mA cm -2.

S100, S102, S104, S106: Steps

Claims (15)

A method for preparing a bifunctional electrode, comprising: providing a carbon cloth; placing the carbon in an electroplating solution; performing an electroplating process to form a doped oxide on the carbon cloth; and performing an annealing process to obtain the bifunctional electrode, wherein the doped oxide comprises cobalt tetraoxide doped with a first metal element or nickel oxide doped with a second metal element, and has catalytic activity for oxygen reduction reaction and oxygen evolution reaction, wherein the first metal element is selected from one of nickel, gallium, iron, yttrium, magnesium, copper and molybdenum, and the second metal element is selected from one of cobalt, yttrium and manganese. The preparation method as described in claim 1, wherein the electroplating process includes: applying a constant voltage of -0.8 volts for 5 minutes; wherein the annealing process includes: using air as the atmosphere gas and maintaining the temperature at 350 ^o C for 2 hours. The preparation method as described in claim 1, further comprising: Cleaning and drying the doped oxide after the electroplating process and before the annealing process. The preparation method as described in claim 1, wherein the electroplating solution comprises: a cobalt-containing precursor containing cobalt; and a first metal precursor containing the first metal element, wherein the molar ratio of the cobalt-containing precursor to the first metal precursor is 1:0.025 to 1:0.05. The preparation method as described in claim 3, wherein the cobalt-containing precursor includes cobalt nitrate, and the first metal precursor includes one selected from nickel nitrate, gallium nitrate, iron nitrate, onium nitrate, magnesium chloride, copper chloride, and molybdenum chloride. The preparation method as described in claim 3, wherein the plating solution further comprises: A third metal precursor containing a third metal element, wherein the third metal element is different from the first metal element, and the third metal element is selected from one of magnesium, manganese, nickel and vanadium, wherein the molar ratio of the first metal precursor to the third precursor is 1:0.5. The preparation method as described in claim 5, wherein the third metal precursor is selected from one of magnesium chloride, manganese chloride, nickel nitrate and vanadium chloride. The preparation method as described in claim 1, wherein the electroplating solution comprises: a nickel-containing precursor containing nickel; and a second metal precursor containing the second metal element, and the molar ratio of the nickel-containing precursor to the second metal precursor is 1:0.025 to 1:0.075. The preparation method as described in claim 7, wherein the nickel-containing precursor includes nickel nitrate, and the second metal precursor is selected from one of cobalt nitrate, nickel nitrate and manganese chloride. A bifunctional electrode, comprising: A doped oxide disposed on a carbon cloth, wherein the doped oxide comprises cobalt tetraoxide doped with a first metal element or nickel oxide doped with a second metal element, and has catalytic activity for oxygen reduction reaction and oxygen evolution reaction, wherein the first metal element is selected from one of nickel, gallium, iron, yttrium, magnesium, copper and molybdenum, and the second metal element is selected from one of cobalt, yttrium and manganese. A bifunctional electrode as described in claim 10, wherein the first metal element is titanium, and in the cobalt tetroxide doped with the first metal element, the molar ratio of cobalt to titanium is 1:0.025 to 1:0.05. A bifunctional electrode as described in claim 11, wherein the indium-doped cobalt tetroxide includes a third metal element doping, and the third metal element is selected from one of magnesium, manganese, nickel and vanadium. A bifunctional electrode as described in claim 12, wherein in the cobalt tetroxide doped with the third metal element and the titanium doped, the molar ratio of titanium to the third metal element is 1:0.5. A bifunctional electrode as described in claim 10, wherein the second metal element is cobalt, and in the nickel oxide doped with the second metal element, the molar ratio of nickel to cobalt is 1:0.025 to 1:0.075. A zinc air battery, wherein the anode is metallic zinc or a zinc alloy, and the cathode is a bifunctional electrode as described in any one of claims 10 to 14.