Application of Chromium Sesquioxide-Modified CoOx Catalyst in Direct Electrolysis of Natural Seawater for Hydrogen Production

Abstract

The present invention relates to the application of a Cr₂O₃ - modified CoOx catalyst in direct electrolysis of natural seawater for hydrogen production. The Cr₂O₃ - modified CoOx catalyst grows on a titanium felt substrate, and the loading and area of the cathode and anode are the same. The loading is 2.0 - 6.0 mg cm⁻², and the catalyst area is 4 - 12 cm². The titanium felt loaded with the cathode and anode catalysts and the Nafion 115 / 117 proton membrane are heat - pressed to obtain a membrane electrode. The heat - pressing temperature is 90 - 120 °C, the heat - pressing pressure is 4 - 8 MPa, and the heat - pressing time is 0.5 - 1.5 min. By using the Cr₂O₃ - modified CoOx catalyst in the present invention, hydroxide ions can be preferentially enriched around the anode and chloride ions can be excluded, effectively avoiding harmful chlorine chemical reactions. The present invention uses natural seawater without pretreatment and without adding strong base, greatly reducing the production cost and simplifying the production process.

Description

Technical Field

[0001] The present invention belongs to the field of electrocatalysis, and particularly relates to a CoO modified with chromium sesquioxide (Cr2O3) x catalyst for direct electrolysis of natural seawater to produce hydrogen. Background Art

[0002] As the most promising "green hydrogen" technology, the invention of electrolyzing water has been around for over a century. However, large-scale freshwater electrolysis will impose a heavy burden on freshwater resources. The United Nations Sustainable Development Goals point out that more than 80% of the world's population currently faces a high risk of water security. Seawater is one of the most abundant natural resources on earth (accounting for 96.5% of the world's total water resources), and direct electrolysis of natural seawater to produce hydrogen is an effective strategy to solve problems such as the global shortage of freshwater resources. Therefore, hydrogen production by electrolyzing seawater has received extensive attention in the academic and industrial communities. However, the implementation of direct electrolysis of natural seawater to produce hydrogen still faces huge challenges: (1) The electrocatalyst for hydrogen production has poor activity (slow reaction rate and low current) in near-neutral natural seawater; (2) At the anode, the harmful side reaction of chlorine production (Cl - +H2O→ClO - +2H + ) will compete with the oxygen evolution reaction, thus reducing the efficiency of the oxygen evolution reaction; (3) At the cathode, with the consumption of hydrogen ions in the hydrogen evolution reaction (2H2O→H2+2OH - ), cations such as metal Mg and Ca will form precipitates (such as Mg(OH)2 or Ca(OH)2) with the excess hydroxide ions, clogging the cathode catalyst and resulting in reduced activity.

[0003] Currently, internationally, seawater pretreatment processes such as purification / desalination are mainly used to remove Cl - and cations such as metal Mg and Ca to avoid chlorine side reactions and precipitate formation, and strong bases are added to the purified seawater to improve the catalyst activity to solve the above problems. However, the cost of investing in seawater purification / desalination system facilities is very high (>1.25 US$m -3 seawater), and a large amount of energy input is required during the operation process. In addition, although the addition of strong bases improves the electrolysis efficiency, it also increases the cost of hydrogen production by electrolyzing seawater (>800 US$t -1 KOH). These have all hindered the widespread application of seawater electrolysis for hydrogen production. Summary of the Invention

[0004] The present invention provides a CoO modified with Cr2O3 x catalyst for direct electrolysis of natural seawater to produce hydrogen, which realizes large-scale, low-energy consumption and high-stability hydrogen production in natural seawater without pretreatment and without adding strong bases.

[0005] In the present invention, both the anode and cathode catalysts use CoO modified with Cr2O3 x catalyst, which can effectively avoid harmful chlorine chemical reactions at the anode and the formation of a large amount of precipitation at the cathode. Note that in CoO x the x it indicates that the valence state of Co will change during the reaction process, and x = 2 to 4.

[0006] In the present invention, the CoO modified with Cr2O3 x catalyst has a simple preparation process. Specifically:

[0007] (1) Prepare the CoO x seed solution: Weigh cobalt chloride and urea and dissolve them in absolute ethanol. The concentrations of cobalt chloride and urea are 0.1 to 0.3 M and 0.5 to 1.5 M respectively; stir at a constant temperature for 2 h, let it stand overnight, and then take the supernatant;

[0008] (2) Film pulling: Hang the titanium felt vertically on the fixture under the film pulling machine, turn on the film pulling machine to make the titanium felt descend uniformly. After it is completely immersed in the film pulling solution, let it stand for 0.5 to 1.5 min, and then raise it uniformly and take it out; after the titanium felt completely leaves the liquid surface, let it stand for 3 min and then take down the titanium felt and put it into an oven at 90 °C for aging for 5 to 20 min; repeat this 3 to 4 times;

[0009] (3) Annealing: Put the titanium felt after film pulling into a vacuum tube furnace and carry out vacuum annealing under the protection of nitrogen / argon; set the heating rate to 5 to 10 °C / min, keep it at 200 to 400 °C for 15 min to obtain a uniform CoO x seed layer;

[0010] (4) Prepare the CoO x growth solution: Weigh cobalt chloride and urea and dissolve them in deionized water. The concentrations of cobalt chloride and urea are 0.02 to 0.1 M and 0.08 to 0.4 M respectively;

[0011] (5) Grow CoO x : Place the side of the titanium felt with the CoO x seed layer facing downwards and obliquely into the inner liner of a 200 mL reaction kettle, and add about 180 mL of CoO x growth solution; put the reaction kettle into a constant temperature drying oven at 90 °C and react for 4 to 8 h;

[0012] (6) Annealing: Put the titanium felt into a vacuum tube furnace and carry out vacuum annealing under the protection of nitrogen: heat it at a heating rate of 5 to 10 °C / min to 350 to 450 °C, then keep it warm for 3 h, and then cool it to room temperature with the furnace; that is, obtain CoO grown on the titanium felt x ;

[0013] (7) Preparation of Cr2O3 precursor solution: Dissolve chromium nitrate with a concentration of 0.02 - 0.2 M in ethanol, immerse the titanium felt with CoO x grown on it in the precursor solution, and then dry it at room temperature;

[0014] (8) Annealing: Put the titanium felt into a vacuum tube furnace and carry out vacuum annealing under nitrogen protection: Heat it to 300 - 500 °C at a heating rate of 5 - 10 °C / min, then keep it warm for 2 - 4 h, and then cool it to room temperature with the furnace; namely, obtain the Cr2O3 - modified CoO x catalyst grown on the titanium felt.

[0015] In the present invention, the Cr2O3 - modified CoO x catalyst grows on the titanium felt substrate, and the loading amounts and areas of the cathode and anode are the same. The loading amount is 2.0 - 6.0 mg cm -2 , and the catalyst area is 4 - 12 cm 2 .

[0016] In the present invention, the titanium felt loaded with the cathode and anode catalysts and the Nafion 115 / 117 proton membrane are thermally pressed to obtain a membrane electrode. The thermal pressing temperature is 90 - 120 °C, the thermal pressing pressure is 4 - 8 MPa, and the thermal pressing time is 0.5 - 1.5 min.

[0017] In the present invention, the electrolytic cell is assembled in the order of cell body | bipolar plate | membrane electrode | bipolar plate | cell body, and is connected with a conductive rod and fixed with bolts.

[0018] In the present invention, natural seawater without pre - desalination treatment and without adding acid / alkali is used as the cathode and anode electrolytes. Under the control of a peristaltic pump, the natural seawater circulates in the electrolytic cell at a flow rate of 60 - 400 mL min -1 .

[0019] In the present invention, an oscilloscope is used to measure the performance of the Cr2O3 - CoO x seawater electrolytic cell for electrolyzing natural seawater at 60 °C. The current test range is 0.225 - 9 A, and the voltage reading corresponding to each current value is taken as the average value after the voltage is stable. The electrolytic stability is evaluated by chronoamperometry. Reaction current: 500 mA cm -2 , reaction temperature: 25 °C

[0020] In the present invention, hydrogen and oxygen generated in the cathode chamber and the anode chamber are respectively collected through a gas - liquid separation tank, and their output amounts are recorded with a gas flowmeter.

[0021] In the present invention, the hydrogen production rate of the flow - type seawater electrolytic cell is estimated: Hydrogen production rate = V H2 / m 催化剂 , where V H2is the theoretical yield of H2 per unit area within 1 hour, m 催化剂 is the total loading of the cathode and anode catalysts.

[0022] The present invention uses CoO modified with Cr2O3 x as the catalyst. Cr2O3 is a Lewis acid that can preferentially enrich hydroxide ions around the anode and repel chloride ions, effectively avoiding harmful chlorine chemical reactions.

[0023] In the present invention, the locally generated hydroxide ions bind strongly to the Lewis acid Cr2O3, which can slow down the movement of hydroxide ions towards seawater under the driving of the electric field, enabling the hydroxide ions to be neutralized by buffer ions (such as carbonate ions) in natural seawater. Therefore, it can prevent the rapid increase of pH value and avoid the formation of a large amount of precipitation at the cathode.

[0024] The present invention uses natural seawater without pretreatment and without adding strong alkali, which greatly reduces the production cost (>800 US$ t -1 KOH) and simplifies the production process.

[0025] In the present invention, when the loading of the cathode and anode catalysts is 4.0 mg cm -2 and the catalyst area is 9 cm 2 , in a flow-type proton exchange membrane electrolyzer at 60 °C, the voltage required to reach a current density of 1.0 A cm -2 is only 1.869 V, which is much higher than that of existing natural seawater electrolyzers and even close to that of the RuO2||Pt / C proton exchange membrane electrolyzer in high-purity water. The H2 yield at 60 °C is as high as 40 L h –1 g –1 . And the stability at a current density >500 mA cm -2 exceeds 100 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : Transmission image of the CoO x catalyst modified with Cr2O3

[0027] Figure 2 : Picture of the flow-type natural seawater electrolyzer

[0028] Figure 3 : Performance test of the direct electrolysis system of natural seawater in a flow-type proton exchange membrane electrolyzer

[0029] Figure 4 : Stability test of the direct electrolysis system of natural seawater in a flow-type proton exchange membrane electrolyzer DETAILED DESCRIPTION OF THE INVENTION

[0030] The following further elaborates on the present invention in conjunction with the drawings and specific embodiments:

[0031] Example 1: Cr2O3-Modified CoO x Preparation of the catalyst (loading amount: 2 mg / cm -2 ):

[0032] (1) Preparation of the CoO x seed solution: Weigh a certain amount of cobalt chloride and urea and dissolve them in absolute ethanol. The concentrations of cobalt chloride and urea are 0.1 M and 0.5 M respectively. Stir at a constant temperature for 2 h, and take the supernatant after standing overnight.

[0033] (2) Film pulling: Vertically hang the titanium felt on the fixture under the film pulling machine, turn on the film pulling machine to make the titanium felt descend at a constant speed. After it is completely immersed in the film pulling solution, let it stand for 0.5 min, and then raise it at a constant speed and take it out; after the titanium felt completely leaves the liquid surface, let it stand for 3 min and then take down the titanium felt and put it into an oven at 90 °C for aging for 5 min. Repeat this 3 - 4 times.

[0034] (3) Annealing: Put the titanium felt after film pulling into a vacuum tube furnace and carry out vacuum annealing under the protection of nitrogen / argon; set the heating rate to 5 °C / min, keep the temperature at 200 °C for 15 min to obtain a uniform CoO x seed layer.

[0035] (4) Preparation of the CoO x growth solution: Weigh a certain amount of cobalt chloride and urea and dissolve them in deionized water. The concentrations of cobalt chloride and urea are 0.02 M and 0.08 M respectively.

[0036] (5) Growth of CoO x : Place the side of the titanium felt with the CoO x seed layer facing downwards and obliquely into the inner liner of a 200 mL autoclave, and add about 180 mL of CoO x growth solution. Place the autoclave in a constant temperature drying oven at 90 °C and react for 4 h.

[0037] (6) Annealing: Put the titanium felt into a vacuum tube furnace and carry out vacuum annealing under nitrogen protection: Heat it at a heating rate of 5 °C / min to 350 °C, then keep the temperature for 3 h, and then cool it to room temperature with the furnace; thus obtain CoO x grown on the titanium felt.

[0038] (7) Preparation of the Cr2O3 precursor solution: Dissolve chromium nitrate with a concentration of 0.02 M in ethanol, immerse the titanium felt with CoO x grown on it in the precursor solution, and then dry it at room temperature.

[0039] (8) Annealing: Place the titanium felt into a vacuum tube furnace and conduct vacuum annealing under nitrogen protection: Heat it to 300 °C at a heating rate of 5 °C / min, then hold for 3 h, and then cool it to room temperature in the furnace; thus, CoO modified by Cr2O3 grown on the titanium felt is obtained. x Catalyst.

[0040] The loadings of CoO modified by Cr2O3 on the cathode and anode of the titanium felt x are 2.0 mg cm -2 , and the catalyst area is 4 cm 2 .

[0041] Example 2: Preparation of CoO modified by Cr2O3 x catalyst (with a loading of 4 mg cm -2 ):

[0042] (1) Prepare CoO x seed solution: Weigh a certain amount of cobalt chloride and urea, and dissolve them in absolute ethanol. The concentrations of cobalt chloride and urea are 0.2 M and 1 M, respectively. Stir at a constant temperature for 2 h, and take the supernatant after standing overnight.

[0043] (2) Film pulling: Vertically suspend the titanium felt on the fixture under the film pulling machine, turn on the film pulling machine to make the titanium felt descend at a constant speed. After it is completely immersed in the film pulling solution, let it stand for 1 min, and then raise it at a constant speed and take it out; after the titanium felt completely leaves the liquid surface, let it stand for 3 min and then take down the titanium felt and place it in an oven at 90 °C for aging for 10 min. Repeat this 3 - 4 times.

[0044] (3) Annealing: Place the titanium felt after film pulling into a vacuum tube furnace and conduct vacuum annealing under nitrogen / argon protection; set the heating rate to 10 °C / min, hold at 400 °C for 15 min to obtain a uniform CoO x seed layer.

[0045] (4) Prepare CoO x growth solution: Weigh a certain amount of cobalt chloride and urea, and dissolve them in deionized water. The concentrations of cobalt chloride and urea are 0.05 M and 0.2 M, respectively.

[0046] (5) Grow CoO x : Place the side of the titanium felt with the CoO x seed layer facing down and obliquely into the inner liner of a 200 mL reaction kettle, and add about 180 mL of CoO x growth solution. Place the reaction kettle in a constant temperature drying oven at 90 °C and react for 6 h.

[0047] (6) Annealing: Place the titanium felt into a vacuum tube furnace and conduct vacuum annealing under nitrogen protection: Heat it to 400 °C at a heating rate of 10 °C / min, then hold for 3 h, and then cool it to room temperature with the furnace; thus, CoO grown on the titanium felt is obtained. x .

[0048] (7) Prepare the Cr2O3 precursor solution: Dissolve chromium nitrate with a concentration of 0.1 M in ethanol, immerse the titanium felt with CoO x grown on it in the precursor solution, and then dry it at room temperature.

[0049] (8) Annealing: Place the titanium felt into a vacuum tube furnace and conduct vacuum annealing under nitrogen protection: Heat it to 400 °C at a heating rate of 10 °C / min, then hold for 3 h, and then cool it to room temperature with the furnace; thus, the Cr2O3-modified CoO x catalyst is obtained. The morphological characterization of Cr2O3-CoO x is shown in the transmission pictures in Figure 1 .

[0050] The loading of the Cr2O3-modified CoO x catalyst on the cathode and anode of the titanium felt is 4.0 mg cm -2 , and the catalyst area is 9 cm 2 .

[0051] Example 3: Preparation of the Cr2O3-modified CoO x catalyst (with a loading of 6 mg cm -2 ):

[0052] (1) Prepare the CoO x seed solution: Weigh a certain amount of cobalt chloride and urea and dissolve them in absolute ethanol. The concentrations of cobalt chloride and urea are 0.3 M and 1.5 M, respectively. Stir at a constant temperature for 2 h, let it stand overnight, and then take the supernatant.

[0053] (2) Film pulling: Vertically hang the titanium felt on the fixture under the film pulling machine, turn on the film pulling machine to make the titanium felt descend at a constant speed. After it is completely immersed in the film pulling solution, let it stand for 1.5 min, and then raise it at a constant speed and take it out; after the titanium felt completely leaves the liquid surface, let it stand for 3 min and then remove the titanium felt and put it into an oven at 90 °C for aging for 10 min. Repeat this 3 - 4 times.

[0054] (3) Annealing: Place the titanium felt after film pulling into a vacuum tube furnace and conduct vacuum annealing under nitrogen / argon protection; set the heating rate to 8 °C / min, hold at 300 °C for 15 min to obtain a uniform CoO x seed layer.

[0055] (4) Prepare the CoO xGrowth solution: Weigh a certain amount of cobalt chloride and urea, and dissolve them in deionized water. The concentrations of cobalt chloride and urea are 0.1 M and 0.4 M respectively.

[0056] (5) Growth of CoO x : Place the side of the titanium felt with the CoO x seed layer facing downwards and obliquely into the inner liner of a 200 mL autoclave, and add approximately 180 mL of CoO x growth solution. Place the autoclave in a constant temperature drying oven at 90 °C and react for 8 h.

[0057] (6) Annealing: Place the titanium felt in a vacuum tube furnace and carry out vacuum annealing under nitrogen protection: Heat it to 450 °C at a heating rate of 8 °C / min, then hold for 3 h, and then cool it to room temperature with the furnace; thus obtaining CoO x .

[0058] (7) Preparation of Cr2O3 precursor solution: Dissolve chromium nitrate with a concentration of 0.2 M in ethanol, and soak the titanium felt with CoO x grown on it in the precursor solution, and then dry it at room temperature.

[0059] (8) Annealing: Place the titanium felt in a vacuum tube furnace and carry out vacuum annealing under nitrogen protection: Heat it to 500 °C at a heating rate of 8 °C / min, then hold for 3 h, and then cool it to room temperature with the furnace; thus obtaining the Cr2O3 - modified CoO x catalyst grown on the titanium felt.

[0060] The loading of the Cr2O3 - modified CoO x catalyst on the cathode and anode of the titanium felt is 6.0 mg cm -2 , and the catalyst area is 12 cm 2 .

[0061] Example 4: Fabricate the catalyst described in Examples 1 - 3 into a membrane electrode and assemble it into an electrolytic cell

[0062] (1) Treatment of Nafion 115 / 117 proton membrane: Pretreat it successively with 5 wt% H2O2, 1.0 M H2SO4 and deionized water at 80 °C

[0063] for 1 h, cool it to room temperature, and wash it with deionized water until the pH value of the washing water is neutral.

[0064] (2) Assembly of the membrane electrode: Place the Nafion 115 / 117 proton membrane in the middle of the titanium felt loaded with the cathode and anode catalysts as described in Examples 1 - 3, and hot - press it at 90 °C and 8 MPa for 1.5 min.

[0065] (3) Electrolyzer assembly: Assemble in the order of cell body | bipolar plate | membrane electrode | bipolar plate | cell body, connect with conducting rods and fix with bolts.

[0066] Example 5: Fabricate the membrane electrode from the catalysts described in Examples 1 - 3 and assemble it into an electrolyzer

[0067] (1) Treatment of Nafion 115 / 117 proton exchange membrane: Pretreat successively with 5 wt% H2O2, 1.0 M H2SO4, and deionized water at 80 °C

[0068] for 1 h, cool to room temperature, and wash with deionized water until the pH of the washing water is neutral.

[0069] (2) Membrane electrode assembly: Place the Nafion 115 / 117 proton exchange membrane between the titanium felts loaded with cathode and anode catalysts as described in Examples 1 - 3, and hot press at 100 °C and 6 MPa for 1 min.

[0070] (3) Electrolyzer assembly: Assemble in the order of cell body | bipolar plate | membrane electrode | bipolar plate | cell body, connect with conducting rods and fix with bolts. The electrolyzer and supporting devices such as peristaltic pumps and gas - liquid separation tanks are shown in Figure 2.

[0071] Example 6: Fabricate the membrane electrode from the catalysts described in Examples 1 - 3 and assemble it into an electrolyzer

[0072] (1) Treatment of Nafion 115 / 117 proton exchange membrane: Pretreat successively with 5 wt% H2O2, 1.0 M H2SO4, and deionized water at 80 °C for 1 h, cool to room temperature, and wash with deionized water until the pH of the washing water is neutral.

[0073] (2) Membrane electrode assembly: Place the Nafion 115 / 117 proton exchange membrane between the titanium felts loaded with cathode and anode catalysts as described in Examples 1 - 3, and hot press at 120 °C and 4 MPa for 0.5 min.

[0074] (3) Electrolyzer assembly: Assemble in the order of cell body | bipolar plate | membrane electrode | bipolar plate | cell body, connect with conducting rods and fix with bolts.

[0075] Example 7: Perform performance tests on the Cr2O3 - CoO x electrolyzers described in Examples 4 - 6

[0076] (1) Electrolyte: Natural seawater without pre - desalination treatment and without adding acid / alkali. Under the control of a peristaltic pump, the natural seawater in the electrolyzer flows at a flow rate of 60 mL / min -1

[0077] ​(2) Electrolytic performance test: Measure the performance of the Cr2O3-CoO x seawater electrolyzer at 60 °C using an oscilloscope. The current test range is 0.225 - 9 A, and the corresponding voltage readings at each current value are taken from the average value after the voltage stabilizes. The electrolytic stability is evaluated by chronoamperometry. Reaction current: 500 mA cm -2 , reaction temperature: 25 °C

[0078] (3) Hydrogen / oxygen collection: Collect the hydrogen and oxygen generated in the cathode chamber and anode chamber respectively through a gas-liquid separation tank, and record their output with a gas flow meter.

[0079] (4) Estimate the hydrogen production rate of the flow-through seawater electrolyzer: Hydrogen production rate = V H2 / m 催化剂 where V H2 is the theoretical production rate of H2 per unit area within 1 hour, and m 催化剂 is the total loading of the cathode and anode catalysts.

[0080] Example 8: Perform performance tests on the Cr2O3-CoO x electrolyzer described in Examples 4 - 6

[0081] (1) Electrolyte: Natural seawater without pre-desalination treatment and without adding acid / alkali. Under the control of a peristaltic pump, the natural seawater in the electrolyzer flows at a rate of 200 mL min -1 .

[0082] (2) Electrolytic performance test: Measure the performance of the Cr2O3-CoO x seawater electrolyzer at 60 °C using an oscilloscope. The current test range is 0.225 - 9 A, and the corresponding voltage readings at each current value are taken from the average value after the voltage stabilizes. The electrolytic stability is evaluated by chronoamperometry. Reaction current: 500 mA cm -2 , reaction temperature: 25 °C

[0083] (3) Hydrogen / oxygen collection: Collect the hydrogen and oxygen generated in the cathode chamber and anode chamber respectively through a gas-liquid separation tank, and record their output with a gas flow meter.

[0084] (4) Estimate the hydrogen production rate of the flow-through seawater electrolyzer: Hydrogen production rate = V H2 / m 催化剂 where V H2 is the theoretical production rate of H2 per unit area within 1 hour, and m 催化剂 is the total loading of the cathode and anode catalysts.

[0085] Example 9: Perform performance tests on the Cr2O3-CoO x electrolyzer described in Examples 4 - 6

[0086] (1) Electrolyte: Natural seawater without pre-dilution treatment and without adding acid / alkali, under the control of a peristaltic pump, the natural seawater in the electrolytic cell flows at a rate of 400 mL min -1 .

[0087] (2) Electrolysis performance test: Measure the performance of the Cr2O3-CoO x seawater electrolytic cell at 60 °C using an oscilloscope. The current test range is 0.225 - 9 A, and the corresponding voltage readings at each current value are taken from the average value after the voltage stabilizes. The electrolysis stability is evaluated by chronoamperometry. Reaction current: 500 mA cm -2 , reaction temperature: 25 °C

[0088] (3) Hydrogen / oxygen collection: Collect the hydrogen and oxygen generated in the cathode chamber and anode chamber respectively through a gas-liquid separation tank, and record their output with a gas flow meter.

[0089] (4) Estimate the hydrogen production rate of the flow-through seawater electrolytic cell: Hydrogen production rate = V H2 / m 催化剂 where V H2 is the theoretical yield of H2 per unit area within 1 hour, and m 催化剂 is the total loading of the cathode and anode catalysts.

[0090] Example 10: Results of the performance test of the Cr2O3-CoO x electrolytic cell described in Example 7

[0091] The present invention uses a CoO x catalyst modified with Cr2O3. Cr2O3 is a Lewis acid, which can preferentially enrich hydroxide ions around the anode and repel chloride ions, effectively avoiding harmful chlorine chemical reactions. The locally generated hydroxide ions bind strongly to the Lewis acid Cr2O3, which can slow down the movement of hydroxide ions towards seawater under the driving of the electric field, enabling the hydroxide ions to be neutralized by buffer ions (such as carbonate ions) in natural seawater. Therefore, the pH value can be prevented from rising rapidly, and a large amount of precipitation can be avoided at the cathode. Therefore, the present invention can use natural seawater without pretreatment and without adding strong alkali, greatly reducing the production cost (> 800 US$ t - 1 KOH), and simplifying the production process.

[0092] In Example 7, the performance curves of the Cr2O3-CoO x and RuO2||Pt / C proton exchange membrane electrolytic cells are as Figure 3 shown. For Cr2O3-CoO x , the loading of the cathode and anode catalysts is 4.0 mg cm -2 , and the catalyst area is 9 cm2 At that time, in a flow-type proton exchange membrane electrolyzer at 60 °C, the voltage required to reach a current density of 1.0 A cm -2 is only 1.869 V, much higher than existing natural seawater electrolyzers and even close to the RuO2||Pt / C proton exchange membrane electrolyzer in high-purity water. The H2 production rate at 60 °C is as high as 40 L h –1 g –1 . And the stability at a current density > 500 mA cm -2 exceeds 100 hours (the stability curve of the Cr2O3-CoO x electrolyzer is as shown in Figure 4 ).

[0093] For the technical solutions disclosed and proposed in the present invention, those skilled in the art can achieve them by referring to the content of this article and appropriately changing conditions, routes and other links. Although the methods and preparation techniques of the present invention have been described through preferred embodiments, it is obvious that relevant technical personnel can make changes or recombinations to the methods and technical routes described in this article without departing from the content, spirit and scope of the present invention to achieve the final preparation technique. It should be particularly noted that all similar substitutions and changes are obvious to those skilled in the art, and they are all regarded as being included in the spirit, scope and content of the present invention.

Claims

1. A Cr2O3-modified CoO x catalyst and its application method for directly electrolyzing natural seawater to produce hydrogen; characterized in that: Both the cathode and anode catalysts are CoO modified with Cr2O3 x catalysts, where x = 2 - 4; CoO modified with Cr2O3 x The catalysts are loaded on a titanium felt substrate, and the loading amount and area are the same for both the cathode and anode: the loading amount is 2.0 - 6.0 mg cm -2 , and the catalyst area is 4 - 12 cm 2 .

2. The CoO modified by Cr2O3 according to Claim 1 x The method for applying the catalyst to directly electrolyze natural seawater to produce hydrogen; characterized in that: The described CoO modified by Cr2O3 x catalyst, the method comprising the following steps: (1)Prepare CoO x Seed solution: Weigh cobalt chloride and urea and dissolve them in absolute ethanol; the concentrations of cobalt chloride and urea are 0.1 - 0.3 M and 0.5 - 1.5 M respectively; stir at a constant temperature for 2 h, take the supernatant after standing overnight; (2)Film pulling: Vertically suspend the titanium felt on the fixture under the film pulling machine, start the film pulling machine to make the titanium felt descend at a constant speed, and wait until it is completely immersed in the CoO x seed solution, let it stand for 0.5 - 1.5 min, and then raise it at a constant speed and take it out; after the titanium felt completely leaves the liquid surface, let it stand for 3 min, then remove the titanium felt and put it into an oven at 90 °C for aging for 5 - 20 min; repeat this 3 - 4 times; (3) Annealing: Put the titanium felt after film drawing into a vacuum tube furnace and carry out vacuum annealing under the protection of nitrogen / argon; set the heating rate to 5-10 °C / min, hold at 200-400 °C for 15 min to obtain a uniform CoO x seed layer; (4)Prepare CoO x Growth solution: Weigh a certain amount of cobalt chloride and urea and dissolve them in deionized water. The concentrations of cobalt chloride and urea are 0.02 - 0.1 M and 0.08 - 0.4 M, respectively; (5)Growth of CoO x : Place the side of the titanium felt with the CoO x seed layer facing downwards and obliquely into the inner liner of a 200 mL autoclave, and add 180 mL of CoO x growth solution; Place the autoclave in a constant temperature drying oven at 90 °C and react for 4 - 8 h; (6) Annealing: Put the titanium felt into a vacuum tube furnace and carry out vacuum annealing under nitrogen protection: Heat it to 350 - 450 °C at a heating rate of 5 - 10 °C / min, then hold for 3 h, and then cool it to room temperature with the furnace; thus CoO grown on the titanium felt is obtained. x ; (7) Prepare the Cr2O3 precursor solution: Dissolve chromium nitrate with a concentration of 0.02 - 0.2 M in ethanol, immerse the titanium felt with CoO x grown on it in the precursor solution, and then dry it at room temperature; (8) Annealing: Put the titanium felt into a vacuum tube furnace and carry out vacuum annealing under nitrogen protection: heat it to 300 - 500 °C at a heating rate of 5 - 10 °C / min, then keep it warm for 2 - 4 h, and then cool it to room temperature with the furnace; thus, the CoO modified by Cr2O3 grown on the titanium felt is obtained. x catalyst.

3. The Cr2O3-modified CoO according to claim 1 x A method for applying the catalyst to directly electrolyze natural seawater to produce hydrogen, characterized in that: The titanium felt loaded with cathode and anode catalysts and the Nafion 115 / 117 proton membrane are subjected to hot pressing treatment to obtain a membrane electrode. The hot pressing temperature is 90 - 120 °C, the hot pressing pressure is 4 - 8 MPa, and the hot pressing time is 0.5 - 1.5 min.

4. The Cr2O3-modified CoO according to claim 3 x The method for applying the catalyst to directly electrolyze natural seawater to produce hydrogen, characterized in that: The electrolytic cell is assembled in the order of the cell body, bipolar plate, membrane electrode, bipolar plate and cell body, and is connected by a conductive rod and fixed with bolts; both the anode and cathode electrolytes use natural seawater without pre-desalination treatment and without adding acid / alkali; under the control of a peristaltic pump, the natural seawater circulates in the electrolytic cell at a flow rate of 60~400 mL min −1 .

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