TRANSITION METAL-DOPED NICKEL OXYHYDROXIDE CATALYST, PREPARATION METHOD THEREOF, AND USE THEREOF

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Chinese Patent Application No. 202410892456.7, filed on Jul. 4, 2024, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present disclosure pertains to the technical field of hydrogen production by seawater electrolysis, and more specifically, relates to a transition metal-doped nickel oxyhydroxide catalyst, a preparation method thereof, and an application thereof.

BACKGROUND OF THE INVENTION

Hydrogen, as a green energy source with high energy density and non-polluting combustion products, is considered an ideal alternative to traditional fossil fuels. Direct seawater electrolysis for hydrogen production has long been regarded as a major challenge in scientific research. Seawater has a complex composition (approximately 92 chemical elements), a salinity of about 3.5 wt. %, and a pH between 7.5 and 8.4, with sodium chloride constituting 90% of the dissolved salts. The chloride ion concentration in seawater can reach 0.5 M. Firstly, the abundance of chloride ions leads to a competitive chlorine evolution reaction at the anode during electrolysis, alongside the desired oxygen evolution reaction. Chlorine gas dissolving in water forms hypochlorous acid, which corrodes the electrode. Secondly, chloride ions in seawater can cause chemical corrosion of the metal catalyst substrate. Therefore, the primary difficulties in seawater electrolysis for hydrogen production are concentrated in the design of the anode catalyst. Furthermore, under the high current density conditions required for industrial applications, more stringent requirements are placed on the selectivity and corrosion resistance of the anode catalyst for the oxygen evolution reaction.

In the prior art, researchers have been dedicated to developing catalysts for seawater electrolysis. For example, Chinese Patent No. CN113046782A discloses a method for preparing a cuprous oxide octahedron catalyst supported on nickel foam via an in-situ process. This catalyst exhibits enhanced interfacial contact and binder-free characteristics, thereby improving electron transfer and showing good hydrogen evolution performance as a cathode catalyst in simulated seawater. Chinese Patent No. CN116497387A discloses an anode water oxidation catalyst for seawater electrolysis and its preparation method. This method involves preparing a mixed transition metal salt solution and treating nickel foam, which serves as a growth substrate. The substrate and the salt solution are subjected to a hydrothermal reaction to generate a precursor catalyst. Subsequently, the precursor catalyst is rapidly reconstructed into a NiFeOOH catalyst via electro-oxidation under a specific voltage in an alkaline solution. While this NiFeOOH catalyst shows high catalytic performance, its performance degrades rapidly during long-term operation in seawater electrolyte, failing to ensure long-term, high-efficiency hydrogen production.

Additionally, some research on seawater electrolysis catalysts aims to prevent chlorine corrosion by applying carbon-based anti-corrosion coatings on the catalyst surface or by forming anti-chlorine functional groups, such as sulfates or phosphates, on the catalyst surface. However, these methods have drawbacks. For instance, an anti-corrosion coating significantly increases the catalyst's electrical resistance, which is detrimental to the efficiency of the water electrolysis reaction. Moreover, the operating current density of existing anode catalysts for seawater electrolysis is typically below 500 mA/cm², with an operational lifetime of less than 500 hours. These studies often do not account for the exacerbated corrosion that may result from an increased chloride ion concentration after prolonged operation. It is evident that overcoming the technical bottleneck in seawater electrolysis for hydrogen production lies in developing catalyst materials that are suitable for the seawater environment and can operate efficiently and stably under high current density conditions.

SUMMARY OF THE INVENTION

The present disclosure provides a method for preparing a transition metal-doped nickel oxyhydroxide catalyst. The process is simple and efficient, and the resulting catalyst exhibits high catalytic activity, high selectivity, and high stability. The catalyst can operate stably for extended periods under high current density conditions, demonstrating significant application potential in direct seawater electrolysis systems.

The technical solution is as follows.

A method for preparing a transition metal-doped nickel-iron oxidehydroxide catalyst, including:

• • (1) Preparing a mixed metal-salt solution; using a conductive substrate as a working electrode, a platinum electrode as a counter electrode, and a saturated calomel electrode as a reference electrode; employing the mixed metal-salt solution as an electrolyte; and electrodepositing, by chronoamperometry or chronopotentiometry means, a precursor catalyst onto the conductive substrate;
  • (2) Using the catalyst-bearing conductive substrate obtained in step (1) as the working electrode, a platinum electrode as the counter electrode, and a saturated calomel electrode as the reference electrode; using an alkaline solution as an electrolyte; and, by chronopotentiometry electrolysis, converting the precursor in situ into the transition-metal-doped nickel hydroxyoxide catalyst;
  • wherein the mixed metal-salt solution includes a first metal salt (nickel salt), a second metal salt (iron salt), and a third metal salt selected from cobalt, chromium, manganese, and molybdenum salts, alone or in combination.

The present invention first employs an electrodeposition method to load a transition metal-doped nickel-iron oxide (hydroxide) onto a substrate. Further, an anodic oxidation reaction is used to convert the transition metal-doped nickel-iron oxide (hydroxide) in situ into a transition metal-doped nickel oxyhydroxide catalyst. The lattice mismatch created by doping with a third metal element (a transition metal element) induces a structural distortion, transforming the originally loose, flaky nickel-iron oxide (hydroxide) into a dense nanosheet layer structure that protects the substrate. Concurrently, the amorphous transition metal oxides, such as those of chromium, cobalt, manganese, or molybdenum, provide resistance to chloride ion corrosion in seawater. This ensures that the resulting transition metal-doped nickel oxyhydroxide catalyst exhibits high catalytic activity, high selectivity, and stability in direct seawater electrolysis for hydrogen production applications.

Preferably, the first metal salt is nickel nitrate, the second metal salt is iron nitrate, and the third metal salt is one or two selected from the group consisting of cobalt nitrate, chromium nitrate, manganese nitrate, and molybdenum nitrate.

Optionally, in the mixed metal salt solution, the molar ratio of the first metal salt to the second metal salt to the third metal salt is 1-4:0.5-2:1. The molar concentration of the first metal salt in the mixed metal salt solution is 15-20 mM.

The said conductive substrate comprises materials such as carbon paper, carbon cloth, or nickel foam.

Preferably, in step (1), the conditions for electrodeposition by the chronoamperometry method are: a voltage of −1 to −0.9 V, a temperature of 20-30° C., and a duration of 50-70 min. The conditions for electrodeposition by the chronopotentiometry method are: a current of-5 to-10mA, a temperature of 20-30° C., and a duration of 50-70 min.

Optionally, the alkaline solution is deionized water, simulated seawater, or a natural seawater solution containing potassium hydroxide or sodium hydroxide, with the concentration of potassium hydroxide or sodium hydroxide being 0.1-3 M.

Preferably, in step (2), the conditions for converting the precatalyst in situ into the transition metal-doped nickel oxyhydroxide catalyst by the chronopotentiometry method are: a current density of 20-200 mA cm⁻², a temperature of 20-40° C., and a duration of 6-24 h.

The present invention also provides the transition metal-doped nickel oxyhydroxide catalyst prepared by the aforementioned method,

The present invention also provides for the use of the said transition metal-doped nickel oxyhydroxide catalyst in hydrogen production from seawater electrolysis.

The present invention also provides a method for hydrogen production from seawater electrolysis using the said transition metal-doped nickel oxyhydroxide catalyst.

Further, hydrogen production from seawater electrolysis is carried out in a two-electrode system via an electrochemical workstation, wherein the working electrode is the said transition metal-doped nickel oxyhydroxide catalyst, the counter electrode is a platinum mesh, and the electrolyte is natural seawater, a 0.5 M NaCl solution, or a 2 M NaCl solution, each prepared with 2 M KOH.

In comparison with the prior art, the beneficial effects of the present invention are:

(1) The present invention first loads a transition metal-doped nickel-iron oxide (hydroxide) onto a substrate via electrodeposition, which is then used as the anode (working electrode). Anodic oxidation is then employed to convert the transition metal-doped nickel-iron oxide (hydroxide) in situ into a transition metal-doped nickel oxyhydroxide catalyst. The lattice mismatch formed by the doping of transition metal elements induces the originally loose, flaky nickel-iron oxide (hydroxide) to distort into a dense nanosheet layer structure that protects the substrate. Simultaneously, the amorphous transition metal oxides, such as those of chromium, cobalt, or manganese, are resistant to corrosion from chloride ions in seawater. Consequently, this transition metal-doped nickel oxyhydroxide catalyst demonstrates high catalytic activity, high selectivity, and stability in direct seawater electrolysis applications.

(2) The present invention employs a two-step electrochemical method to prepare the transition metal-doped nickel oxyhydroxide catalyst. The process is simple, easy to implement, and utilizes readily available raw materials, facilitating large-scale production and showing excellent application prospects in direct seawater electrolysis systems.

(3) The transition metal-doped nickel oxyhydroxide catalyst provided by the present invention operates stably for 6000 h at a current density of 500 mA cm⁻² in a simulated alkaline seawater electrolyte with a high chloride ion concentration (2.0 M KOH+2.0M NaCl). It operates stably for 6500 h at a current density of 500 mA cm⁻² in a simulated alkaline seawater electrolyte with a conventional chloride ion concentration (2.0 M KOH+0.5M NaCl), and operates stably for over 2800 h at a current density of 500 mA cm⁻² in a natural alkaline seawater electrolyte (2.0 M KOH+East China Sea water). During electrolysis, the Faradaic efficiency for the oxygen evolution reaction of this transition metal-doped nickel oxyhydroxide catalyst reaches over 99%. The catalyst features high catalytic activity, high selectivity, and high stability, and it can operate stably for long durations under high current density conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope (SEM) image of the iron-cobalt-chromium doped nickel oxyhydroxide catalyst prepared according to Embodiment 1.

FIG. 2 is an X-ray diffraction (XRD) spectrum of the NiFeCoCr precatalyst and the iron-cobalt-chromium doped nickel oxyhydroxide catalyst of Embodiment 1.

FIG. 3 is a linear sweep voltammetry (LSV) curve showing the electrochemical oxidation process for the catalyst of Embodiment 1 in a 2.0 M KOH+0.5 M NaCl electrolyte.

FIG. 4 is an LSV curve showing the electrochemical oxidation process for the catalyst of Embodiment 1 in a 2.0 M KOH+2.0 M NaCl electrolyte.

FIG. 5 is an LSV curve showing the electrochemical oxidation process for the catalyst of Embodiment 1 in a 2.0 M KOH+natural seawater electrolyte.

FIG. 6 is a diagram showing the Faradaic efficiency for the oxygen evolution reaction of the catalyst of Embodiment 1 in an alkaline anion exchange membrane electrolyzer using a 2.0 M KOH+0.5 M NaCl electrolyte.

FIG. 7 is a diagram showing the Faradaic efficiency for the oxygen evolution reaction of the catalyst of Embodiment 1 in an alkaline anion exchange membrane electrolyzer using a 2.0 M KOH+2.0 M NaCl electrolyte.

FIG. 8 is a stability test plot obtained via the chronopotentiometry method for the catalyst of Embodiment 1 in a 2.0 M KOH+0.5 M NaCl electrolyte at a current density of 500 mA/cm².

FIG. 9 is a stability test plot obtained via the chronopotentiometry method for the catalyst of Embodiment 1 in a 2.0 M KOH+2.0 M NaCl electrolyte at a current density of 500 mA/cm².

FIG. 10 is a stability test plot obtained via the chronopotentiometry method for the catalyst of Embodiment 1 in a 2.0 M KOH+natural seawater electrolyte at a current density of 500 mA/cm².

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure will be further elucidated below in conjunction with embodiments and the accompanying drawings. It should be understood that these embodiments are provided for illustrative purposes only and are not intended to limit the scope of the disclosure. Unless otherwise specified, operational methods in the following embodiments are carried out under conventional conditions or as recommended by the manufacturer.

Embodiment 1

This embodiment provides a method for preparing an iron-cobalt-chromium-doped nickel oxyhydroxide catalyst, including the following steps:

(1) At room temperature, 2.7 mmol of nickel nitrate (Ni(NO₃)₂), 0.9 mmol of iron nitrate (Fe(NO₃)₃), 0.9 mmol of cobalt nitrate (Co(NO₃)₂), and 0.9 mmol of chromium nitrate (Cr(NO₃)₃) are added to a round glass deposition cell. Deionized water is added to a total volume of 150 mL and stirred to obtain a mixed metal salt solution. A three-electrode system is constructed with a 1 cm×1 cm piece of nickel foam as the working electrode (anode), a platinum electrode as the counter electrode, and a saturated calomel electrode as the reference electrode. The mixed metal salt solution serves as the electrolyte. The three-electrode system is connected to an electrochemical workstation to form a complete electrical circuit. Electrodeposition is performed using a chronoamperometry method at a constant voltage of −0.9 V and a temperature of 25° C. for 1 hour. The nickel foam loaded with the NiFeCoCr precatalyst is then removed.

(2) The NiFeCoCr-loaded nickel foam was rinsed thoroughly with deionized water. A three-electrode system was re-established with the precatalyst-loaded nickel foam as the working electrode, a platinum electrode as the counter electrode, and a saturated calomel electrode as the reference electrode. A 2 M KOH aqueous solution is used as the electrolyte. The system is connected to an electrochemical workstation. An electrochemical oxidationmethod is used to convert the precatalyst in situ into the iron-cobalt-chromium-doped nickel oxyhydroxide catalyst by applying a constant current density of 100 mA/cm² at a temperature of 25° C. for 12 hours. Through this process, the NiFeCoCr precursor was converted in situ into the Fe-Co-Cr-doped nickel hydroxyoxide catalyst, which included passivation layers composed of cobalt-based and chromium-based oxides.

Embodiment 2

This embodiment provides a method for preparing an iron-cobalt-doped nickel oxyhydroxide catalyst, including the following steps:

(1) At room temperature, 2.7 mmol of nickel nitrate (Ni(NO₃)₂), 0.9 mmol of iron nitrate (Fe(NO₃)₃), and 0.9 mmol of cobalt nitrate (Co(NO₃)₂) are added to a round glass deposition cell, with deionized water added to 150 mL and stirred to form a mixed metal salt solution. A three-electrode system was constructed as in Embodiment 1. Electrodeposition was performed using a chronoamperometry method at a constant voltage of −1 V and a temperature of 30° C. for 1 hour. The nickel foam loaded with the NiFeCo precatalyst was removed.

(2) The nickel foam loaded with the NiFeCo precatalyst was rinsed and used as the working electrode in a 2 M KOH aqueous solution. An electrochemical oxidation chronopotentiometry method was used for in situ conversion by applying a constant current density of 200 mA/cm² at 30° C. for 24 hours. As a result, the NiFeCo precursor was converted into a Fe-Co-doped nickel hydroxyoxide catalyst featuring a cobalt-based oxide passivation layer.

Embodiment 3

This embodiment provides a method for preparing an iron-chromium-doped nickel oxyhydroxide catalyst, comprising the following steps:

(1) At room temperature, 2.7 mmol of nickel nitrate (Ni(NO₃)₂), 0.9 mmol of iron nitrate (Fe(NO₃)₃), and 0.9 mmol of chromium nitrate (Cr(NO₃)₃) are added to a round glass deposition cell, with deionized water added to 150 mL and stirred. A three-electrode system is constructed. Electrodeposition is performed using a chronopotentiometry method at a constant current of −7.5 mA and a temperature of 25° C. for 1 hour. The nickel foam loaded with the NiFeCr precatalyst is removed.

(2) The nickel foam loaded with the NiFeCr precatalyst is rinsed and used as the working electrode in a 2 M KOH aqueous solution. In situ conversion is performed via a electrochemical oxidation chronopotentiometry method at a constant current density of 100 mA/cm² at 25° C. for 12 hours. The resulting iron-chromium-doped nickel oxyhydroxide catalyst possesses a chromium-based oxide passivation layer.

Embodiment 4

This embodiment provides a method for preparing an iron-manganese-doped nickel oxyhydroxide catalyst, comprising the following steps:

(1) At room temperature, 3 mmol of nickel nitrate (Ni(NO₃)₂), 1.8 mmol of iron nitrate (Fe(NO₃)₃), and 1.2 mmol of manganese nitrate (Mn(NO₃)₂) are added to a round glass deposition cell, with deionized water added to 150 mL and stirred. A three-electrode system is constructed. Electrodeposition is performed using a chronopotentiometry method at a constant current of −10 mA and a temperature of 20° C. for 70 minutes. The nickel foam loaded with the NiFeMn precatalyst is removed.

(2) The nickel foam loaded with the NiFeMn precatalyst is rinsed and used as the working electrode in a 2 M KOH aqueous solution. In situ conversion is performed via a electrochemical oxidation chronopotentiometry method at a constant current density of 100 mA/cm² at 20° C. for 18 hours. The resulting iron-manganese-doped nickel oxyhydroxide catalyst possesses a manganese-based oxide passivation layer.

Sample Analysis

The precatalyst and the final iron-cobalt-chromium-doped nickel oxyhydroxide catalyst prepared in Embodiment 1 were characterized and tested.

FIG. 1 shows an SEM image of the catalyst from Embodiment 1. As seen in the figure, the catalyst consists of a dense nanosheet catalyst layer tightly adhering to the nickel foam substrate, with additional flake-like catalyst particles attached to this primary layer.

FIG. 2 shows the XRD spectra of the NiFeCoCr precatalyst and the final catalyst from Embodiment 1. The diffraction peaks of the NiFeCoCr precatalyst correspond to the JCPDS card #40-0215 for nickel-iron layered double hydroxide. The diffraction peaks of the final catalyst correspond to the JCPDS card #06-0141 for nickel oxyhydroxide. These confirms that after the chronopotentiometry electrochemical oxidation process, the nickel-based oxide (hydroxide) in the precatalyst was converted into the more electrochemically active nickel oxyhydroxide. The absence of distinct diffraction peaks for iron, cobalt, chromium, or their oxides, along with a prominent amorphous phase, indicates that these elements are doped into the catalyst as amorphous oxides.

FIG. 3 shows the LSV curve for the catalyst of Embodiment 1 in a 2.0 M KOH+0.5 M NaCl electrolyte. The oxygen evolution reaction (OER) activity is superior to that of a pure nickel-iron layered double hydroxide catalyst, demonstrating the catalyst's excellent electrocatalytic activity in an alkaline simulated seawater electrolyte with a conventional chloride ion concentration.

FIG. 4 shows the LSV curve in a 2.0 M KOH+2.0 M NaCl electrolyte. The catalyst's OER activity remains superior to the pure nickel-iron layered double hydroxide catalyst, indicating excellent electrocatalytic activity even in a highly corrosive, ultra-high chloride concentration environment,

FIG. 5 shows the LSV curve in a 2.0 M KOH+natural seawater electrolyte. The catalyst again shows superior OER activity, confirming its excellent performance in a real-world alkaline natural seawater electrolyte.

FIG. 6 shows the Faradaic efficiency for OER in a 2.0 M KOH+0.5 M NaCl electrolyte. At current densities of 100, 250, and 500 mA/cm², the Faradaic efficiency is consistently above 96%, demonstrating excellent selectivity for the oxygen evolution reaction.

FIG. 7 shows the Faradaic efficiency for OER in a 2.0 M KOH +2.0 M NaCl electrolyte. At the same current densities, the Faradaic efficiency remains above 96%, confirming excellent selectivity even under ultra-high chloride conditions.

FIG. 8 shows the results of a long-term stability test conducted at a constant current density of 500 mA/cm² in a 2.0 M KOH+0.5 M NaCl electrolyte. The catalyst operated stably for over 6500 hours, with the test still ongoing, demonstrating its outstanding stability.

FIG. 9 shows the stability test results at 500 mA/cm² in a 2.0 M KOH+2.0 M NaCl electrolyte. The catalyst operated stably for over 6000 hours, with the test ongoing, highlighting its excellent stability in a highly corrosive environment.

FIG. 10 shows the stability test results at 500 mA/cm² in a 2.0 M KOH+natural seawater electrolyte. The catalyst operated stably for over 2800 hours, with the test ongoing, confirming its excellent stability in natural seawater.

The embodiments described above provide a detailed explanation of the technical solution of the present disclosure. It should be understood that these descriptions are merely specific embodiments and are not intended to limit the disclosure. Any modifications, additions, or similar substitutions made within the principles of the present disclosure shall be included within its scope of protection.