Preparation and application research of transition metal phosphides

Due to the serious environmental pollution in the world, the shortcut to harmonious development is to improve traditional disposable energy and identify new energy sources that can be utilized. The current problem that human society must solve is to discover and use efficient and inexhaustible new energy sources, among which electrolytic water technology is an effective way to develop clean energy. In recent years, researchers have focused on producing hydrogen by electrolyzed water, but the intermediate hydrogen evolution reaction and the oxygen evolution reaction are relatively slow kinetic processes, and they cannot give the factory a large range of electrolytic water hydrogen production work and push down the commercialization process, so improving the rate of electrolytic water process must be an efficient catalyst. Transition metal phosphide is one of the non-precious metal catalysts in the electrochemical hydrogen evolution reaction and oxygen evolution reaction. Transition metal phosphide can promote rapid change of chemical reaction rate, and it has the characteristics of being cheap and easy to operate so it has a great prospect in the electrolysis of water. In this experiment, the oxygen evolution performance was optimized based on the crystal surface regulation of cobalt-doped transition metal nickel phosphide. The optimal reduction time and a small overpotential (60 mV) were obtained by using sodium borohydride to reduce the sample.


Introduction
The global economy's fast-paced growth has resulted in a surge in demand for fossil fuels, coal being one of the primary sources.With industrialization and population growth accelerating in recent years, the world's energy needs have been on a constant rise.Consequently, environmental pollution and resource scarcity are becoming increasingly complex and posing serious threats to human society [1] .The issue of energy has long been a concern for humanity and remains a pressing problem in today's society, including issues such as energy resource depletion and soil pollution.As a result, researchers must explore a series of resources that not only meet societal needs but also reduce environmental pollution.Currently, the enormous consumption of fossil fuels results in significant energy expenditure, and the reserves of these fuels in nature are extremely limited.Therefore, the combustion of these primary energy sources will have a tremendous impact on the environment, making it urgent to develop new energy sources that are environmentally friendly, affordable, and reusable.
Solar energy and wind energy show promising prospects in the field of new energy [2] .Utilizing them for power generation can help solve the energy problem.Electrolyzing water, oxygen evolution reactions, and hydrogen evolution reactions are important directions in current energy research.Electrolyzing water will be one of the key points of future water-splitting kinetics research, as it can produce carbon-neutral fuel.Researchers are showing growing interest in electrolysis catalysts because of their exceptional catalytic properties.Transition metal phosphides have a promising future in the market economy as a new material.Following the discovery of transition metal carbides and nitrides, transition metal phosphides have gradually emerged in the public eye.They are new materials composed of transition metal elements and phosphorus elements, exhibiting multiple properties.These compounds have high efficiency, excellent conductivity, and good stability, thus attracting significant attention from researchers.
The mechanism of electrolyzing water involves the application of an electric current that induces the decomposition of water molecules into hydrogen and oxygen ions.This process forms hydroxide ions by combining the oxygen atoms in water molecules with negative ions, while the hydrogen atoms in water molecules combine with positive ions to form hydrogen ions.This process is known as ionization.In the process of water electrolysis, water molecules undergo dissociation into two distinctions: positively charged hydrogen ions and negatively charged hydroxide ions.By separating these two different ions and guiding them to different locations, they can be employed in the synthesis of hydrogen gas and oxygen gas for production purposes.The apparatus for the electrolysis of water typically consists of a power source, an anode, a cathode, and an electrolyte solution.
Under standard gas conditions, the water electrolysis reaction has a standard Gibbs free energy change of -237.2 kJ mol -1 .According to the relationship between the water electrolysis potential and the Gibbs free energy equation, it can be inferred that the theoretical voltage required for the decomposition of water is 1.23 V.However, in actual conditions, factors such as electrochemical polarization, concentration polarization, and solution resistance can cause the electrode potential to be higher than the theoretical equilibrium potential.Many of the costs of industrial alkaline hydrogen production through water electrolysis come from electricity expenses.It is estimated that approximately 4.5 to 5.5 kWh of electric energy is needed for about 1 m 3 H2.Therefore, reducing the overpotential of the electrolysis water reaction requires the addition of efficient electrocatalysts to improve the mutual conversion of energy and reduce energy consumption to lower the cost of hydrogen production.Up to now, precious metals such as Pt, RuO2, and IrO2 have exhibited the best catalytic activity and high current density for water electrolysis [3][4] .However, due to their low content and high cost, their large-scale use in various industries is limited.Therefore, it is necessary to promote the development of non-precious metal catalysts to replace the position of precious metals.
Researchers discovered the excellent performance of transition metal phosphides in the late 20th century.In the early 1970s, Levy R. B. and Boudart M. [5] found that tungsten exhibited good catalytic properties, gradually replacing other metal materials in catalytic applications.Oyama S. T. [6] identified the significant potential of molybdenum phosphide in catalyst applications.In the early 21st century, Stephanie J. Sawhill et al. synthesized NixPy/SiO2 and NixPy/Al2O3 catalysts through temperature-programmed reduction for hydrodesulfurization reactions [7] .Sun Xuping's research group successfully prepared a series of self-supported hydrogen evolution catalysts using a low-temperature phosphidation strategy, such as carbon cloth-supported Ni2P nanosheets [8] , foam copper-supported Cu3P nanowires [9] , and titanium sheet-supported CoP nanosheets [10] , all exhibiting excellent hydrogen evolution catalytic performance.In 2005, José Rodriguez et al. demonstrated through density functional theory (DFT) calculations that the Ni and P atoms on the Ni2P (001) crystal surface act as hydride and proton acceptors, respectively, with the P atom weakening the interaction between Ni and H, indicating that the (001) crystal surface of Ni2P should exhibit hydrogen evolution catalytic behavior similar to NiFe hydrogenases [11] .Additionally, Professor Schaak's research group at the University of Pennsylvania in the United States used an organic phosphorus source method to synthesize cobalt phosphides, including CoP and Co2P.Subsequently, Professor Schaak designed and prepared CoP and Co2P catalysts with similar sizes and morphologies to investigate the differences in their catalytic activity [12] .Transition metal phosphides play important roles in various fields, particularly in the field of catalysis, where they have gained significant attention from researchers.In the field of photocatalysis, transition metal phosphides can be used for organic pollutant degradation and hydrogen production from water.The research field of transition metal phosphide is also expanding [13][14] .
Based on the research progress mentioned above, transition metal phosphides have achieved significant results as catalysts for the OER.As the scientific level of research continues to advance, the focus has shifted from characterizing the basic properties of catalysts to studying catalytic mechanisms.Simultaneously, it has been discovered that their use as catalysts can accelerate catalysis.

Catalyst Preparation
Add 2.619 g of cobalt (II) nitrate hexahydrate and 1.197 g of L-aspartic acid into a 500 mL beaker.Next, combine 45 mL of deionized water with 54 mL of ethylene glycol to create a mixture.Place the beaker on a hotplate with a magnetic stirrer and stir vigorously.Wait for complete dissolution and then slowly add 9 mL of 2 M NaOH solution dropwise.Continue stirring for 0.5 hours until the solution turns pink.Next, transfer three 36 mL portions of the resulting mixture into three 50 mL stainless steel autoclaves using graduated cylinders.Place one piece of dried nickel foam into each autoclave.React the autoclaves at 180℃ in a high-temperature drying oven for 5 hours.After the reaction is complete, pour out the solution.There will be pink precipitates on the nickel foam.Wash the foam with deionized water and anhydrous ethanol three times.Transfer the cleaned foam into an evaporating dish and dry it in the high-temperature drying oven at 60℃ for three hours.Let it cool and keep it for further use.Place the obtained samples in a double-temperature zone tube furnace.Raise the temperature from ambient to 300℃ and heat for 3 hours.Then allow it to cool back to room temperature.Finally, collect the resulting black phosphide product.Weigh 5.7 g of sodium borohydride using an electronic balance and dissolve it in 50 mL of water in a beaker.At this point, the solution will bubble, which will disappear after a certain time.Transfer the black product obtained above into three separate 25 mL beakers labeled 1, 2, and 3. Then pour the dissolved sodium borohydride solution into each of the three beakers.Let the first reaction proceed for 1 hour, the second reaction for 2 hours, and the third reaction for 3 hours.After the respective time intervals, rinse each set with deionized water and anhydrous ethanol.Once cleaned, transfer them to an evaporating dish.Vacuum-dry them at 60℃ in a high-temperature drying oven for 5 hours to collect the black product.The samples are designated as Co1-NixPy, Co2-NixPy, and Co3-NixPy, respectively.

Characterizations
The powder X-ray diffraction (XRD) measurement was conducted using Cu Kα radiation with a wavelength of 0.1541 nm on the Ultima IV X-ray diffractometer with a current of 40 mA and a respective voltage of 40 kV.The 2θ scans covered the range of 25 o -65 o with a speed of 10 o min -1 .

Catalytic Activity Testing
Electrochemical testing is conducted using an electrochemical analyzer instrument (CHI660E) in a standard three-electrode system.The sodium hydroxide solution prepared as described earlier is used as the electrolyte.In the three-electrode system used, the reference electrode is a graphite carbon rod, and the working electrode is a platinum electrode clip.The testing is carried out under these conditions.

X-ray diffraction analysis (XRD)
XRD characterization was performed on the three sets of samples, and the resulting XRD patterns are shown in the following three figures.Phase analysis was conducted using XRD.From the three graphs, it is evident that all measured samples exhibit distinct diffraction peaks, indicating the excellent crystalline structure of the catalyst prepared in the previous steps.In Figure 1a, the peak positions of Co1-NixPy are observed at 40.8°, 44.6°, 47.3°, and 54.2°, corresponding to the (111), ( 201), (210), and (300) crystal planes of Co1-NixPy, respectively (PDF card number: 03-0953).In Figure 1a, the peak positions of Co2-NixPy are observed at 41.6°, 46.9°, and 48.9°, corresponding to the (321), (420), and (312) crystal planes of Co2-NixPy, respectively (PDF card number: 22-1190).Last, Figure 1b shows a similar pattern to Figure 1c, with comparable peak positions and crystal planes.In conclusion, it can be observed that after reduction by sodium borohydride, all samples have completely transformed into nickel phosphide, indicating pure nickel phosphide.The treatment with sodium borohydride leads to structural damage, resulting in defects.These defects may potentially have an impact on the subsequent oxygen evolution reaction.The increase in treated defect sites is primarily due to the disruption of the crystal structure of NixPy, and it is not significantly related to the doping of the Co element.This ultimately results in significant changes in peak position in the image of Figure 1c compared to Figures 1a and 1b, leading to a lower hydrolysis potential due to structural modifications.

Cyclic voltammetry (CV)
This electrochemical test allows for the assessment of the reversibility of electrode reactions.Next, three sets of samples were subjected to scanning tests at different scan rates for observation and comparison.From these three graphs, it can be observed that as the scan rate increases, the difference between the two ends of the curve becomes larger, indicating excellent stability of the catalyst material.This portion refers to the pre-activation of the LSV test before conducting the actual test.The CV images obtained at different scan rates in Figure 2c exhibit good separation and significant current changes, indicating that Co3-NixPy has better electronic transport properties.This assures a lower hydrolysis potential at the microscopic level.

Linear sweep voltammetry (LSV).
This is the primary method in electrochemical testing used for quantitatively analyzing the OER performance.Next, three sets of samples were tested and compared.From

Conclusion
In this study, transition metal phosphide nickel underwent sodium borohydride treatment, which had a certain effect on the crystal structure of nickel phosphide.Synthesis of nickel phosphide was achieved through the solvothermal method, and the above images show that cobalt compounds were loaded onto nickel phosphide.The results indicate that different crystal structures of nickel phosphide have varying effects on OER performance, with Co1-NixPy > Co2-NixPy > Co3-NixPy, demonstrating that Co3-NixPy has the best catalytic performance.