Preparation and electrocatalytic oxygen evolution of bimetallic phosphates (NiFe) 2 P/NF

: The energy and environmental crisis pose a great challenge to human development in the 21st century. The design and development of clean and renewable energy and the solution for environmental pollution have become a hotspot in the current research. Based on the preparation of transition metal phosphates, transition metals were used as raw materials, Prussian blue-like NiFe(CN) 6 as a precursor, which was in situ grown on nickel foam (NF) substrate. After low temperature phosphating treatment, a bimetallic phosphide electrocatalyst (NiFe) 2 P/NF was prepared on NF substrate. Using 1 mol·L − 1 KOH solution as a basic electrolyte, based on the electrochemical workstation of a three-electrode system, the electrochemical catalytic oxygen evolution performance of the material was tested and evaluated. Experiments show that (NiFe) 2 P/NF catalyst has excellent oxygen evolution performance. In an alkaline medium, the overpotential required to obtain the catalytic current density of 10 mA·cm − 2 is only 220 mV, and the Tafel slope is 67 mV·dec − 1 . This is largely due to: (1) (NiFe)2p/NF nanocatalysts were well dispersed on NF substrates, which increased the number of active sites exposed; (2) the hollow heterostructure of bimetallic phosphates promotes the electron interaction between (NiFe) 2 P and NF, increased the rate of charge transfer, and the electrical conductivity of the material is improved; and (3) theoretical calculations show that (NiFe) 2 P/NF hollow heterostructure can e ﬀ ectively reduce the dissociation barrier of water, promote the dissociation of water; furthermore, the kinetic reaction rate of electrocatalytic oxygen evolution is accelerated. Meanwhile, the catalyst still has high activity and high stability in 30 wt% concentrated alkali solution. Therefore, the construction of (NiFe) 2 P/NF electrocatalysts enriches the application of non-noble metal nanomaterials in the ﬁ eld of oxygen production from electrolytic water.


Introduction
As a kind of traditional energy, in the combustion process that produces a variety of pollutants (CO 2 , SO x , NO x , etc.) [1], the problem of environmental pollution is becoming more and more serious.In addition, after the 2020 goal of "Carbon neutrality and peak carbon", reducing the use of fossil fuels, the development of green, clean, non-polluting new energy has become the focus of research.Hydrogen energy is a kind of environment-friendly green energy; the utility model has the advantages of high energy density, wide source, convenient transportation and storage, and the ultimate alternative to fossil fuels [2][3][4].As an effective means of hydrogen production, electrolyzed water provides a new way to solve the global energy crisis and environmental pollution.Hydrogen production from water electrolysis, which can produce "Green hydrogen", has become the most popular hydrogen production technology.The semi-reactive anodic oxygen evolution reaction (OER) in water electrolysis is a four-electron reaction process, its chemical kinetics is slow and requires a high overpotential, and the high energy barrier of OER severely limits the hydrogen production efficiency of water electrolysis.Therefore, there is an urgent need to develop catalysts with high activity to reduce OER overpotential.
Although noble metal catalysts such as IrO 2 and RuO 2 have high OER catalytic activity.However, its high price and low OER catalytic stability limit its wide application [5].How to design and develop cheap transition metal electrocatalysts with high activity and stability to replace noble metals is very important.In addition, industrial electrolysis uses 30 wt% concentrated alkali solution, most reported non-noble metal electrocatalysts have poor catalytic activity and stability in concentrated alkaline solutions, and it is difficult to achieve industrial promotion.
At present, transition metal compounds have the advantages of low cost and good catalytic performance and have become a hot spot in the current research field.For example, metal nitrides, phosphates, oxides, and hydroxides have been studied and achieved certain results [6][7][8][9][10].However, during pyrolysis at high temperatures, in most transition metal compounds, large-scale particle aggregation and skeleton collapse occur, and the specific surface area and the active site density of the catalyst are reduced, affecting the catalytic activity.In this context, transition metal phosphates (TMPs) have been shown to be one of the most promising OER catalysts [11].However, the reported TMP is limited by its single metal composition, bulk morphology, limited specific surface area, and poor conductivity, which severely limit its further application.An effective strategy is to design and synthesize nanomaterials consisting of polymetals with large specific surface area and further grow in situ on a conductive substrate, this method has been proven to be effective in improving the activity and stability of the catalyst.Moreover, bimetallic phosphates have been reported extensively and have some practical significance [12][13][14][15].
Metal-cyanogen compounds, Prussian blue, rich in metallic elements, including nickel, cobalt, iron, copper, manganese, molybdenum, tungsten, and precious metals; it can be etched or calcined to form a variety of derivative structures, for example, hollow cage structure and core-shell structure.This rich and adjustable composition of elements and a variety of high specific surface area nano-derived structures, Prussian blue-like in electrocatalysis, batteries, capacitors, and other energy-related research fields attracted extensive attention [16,17].Prussian blue-like elements make it an excellent precursor for the preparation of bimetallic and polymetallic catalysts [18].In addition, porous nanostructured materials with various morphologies and high specific surface areas can be obtained by controlling the preparation conditions of Prussian blue-like derivatives [19][20][21][22].Therefore, the Prussian blue is an excellent precursor for the preparation of bimetallic phosphates with hollow morphology.In addition, most of the electrocatalysts reported so far are powders; there are many disadvantages in the powder state, including poor conductivity, complex electrode preparation process, easily covered active sites of catalyst by added polymer binder, easy shedding of catalyst powder, and excessive dead volume [23].An effective solution is to grow the catalyst in situ on a conductive substrate; this can not only improve the conductivity of the catalyst but also effectively solve the problems of the powder catalyst.
In this study, Prussian blue-like NiFe(CN) 6 /nickel foam (NF) grown in situ on NF was used as a precursor treated with phosphating.The bimetallic phosphates (NiFe) 2 P/NF electrocatalysts are promising for the conversion and storage of renewable energy.In 1 mol•L −1 KOH solution, a current density of 10 mA•cm −2 can be achieved with only an overpotential of 220 mV.In addition, the catalyst exhibits high OER performance and stability in industrial electrolyte (30 wt% KOH); this makes it possible to replace noble metals as catalysts for oxygen production at high current densities.

Reagents and instruments
The raw materials for preparing electrocatalyst (NiFe)  During the process, the solution gradually became turbid, and the color of the foam turned yellow, indicating that the growth of Prussian Blue-like had taken place on the foam nickel.

Synthesized (NiFe) 2 P/NF electrocatalyst
The dried NiFe(CN) 6 /NF and sodium hypophosphite were placed at the rearward and forward position along the internal airflow direction within the furnace, respectively.0.2 g of NaH 2 PO 2 was put in the combustion boat and calcined for 2 h under the protection of 350°C nitrogen, After cooling, the metal phosphide catalyst was obtained with a thickness of about 0.1 mm.

OER test
The oxygen evolution performance of electrocatalyst was tested by using Chenhua 660 electrochemical workstation.The IrO 2 working electrode is prepared as follows: 5 mg of IrO 2 powder was dispersed in 50 μL of 0.5% Nafion solution and then in 30 μL of the IrO 2 -Nafion mixture to the NF.The test area is 0.1 cm 2 .
Before the test, using adhesive to adhere to the surface of the catalyst, keep an area of 0.1 cm 2 in the bottom as the test area.
The LSV has a test speed of 1 mV•s −1 .The electrochemical active area was compared by testing the cyclic voltammetry curves of the catalyst at a fixed potential (1.2 vs RHE) in the non-Faraday region at different sweep velocities.
The electrocatalyst was tested for CV curves in the non-Faraday region, and the difference between the anodic and cathodic current densities was calculated at 1.2 V.The difference in current densities was plotted as the vertical axis and the sweep velocity of the CV curves as the horizontal axis.1/2 of the slope of the curves was the double-layer capacitance value (C dl ).
The electrochemical impedance at potential 1.4 V υs.RHE is very sensitive to determination.The test frequency is 100 kHz to 0.1 Hz.

XRD analysis
To study the phase structure information of new catalysts, it was tested by an X-ray diffraction analyser.The XRD patterns of NiFe(CN) 6 and (NiFe) 2 P/NF are shown in Figure 1.The XRD pattern of (NiFe) 2 P/NF after phosphating is shown in Figure 1(b).From the analysis in Figure 1(b), the diffraction peak of (NiFe) 2 P/NF was 43.5°at 2θ, which was a mixed phases of Ni 2 P(JCPDS:03-0953) and Fe 2 P(JCPDS: 27-1171).The characteristic peak of Prussian blue-like material disappeared completely, while the characteristic peak of metal phosphates appeared, indicating that (NiFe) 2 P/NF has been successfully prepared.

XPS analysis
The electrochemical reaction takes place mainly on the surface of the catalyst, the chemical composition and electronic structure of the catalyst surface can be analyzed by XPS, and the results of the analysis are shown in Figure 2. The full spectrum of XPS photoelectron energy is shown in Figure 2(a), The peaks of Ni, Fe, P, and O are obvious, it was proved that Ni, Fe, P, and O existed in the catalyst.
As shown in Figure 2(b), in the high-resolution HR-XPS map of Ni 2p, there are six characteristic peaks after fitting.Among them, the binding energy peaks in 856.1 and 873.7 eV correspond to the positively charged nickel species (Ni δ+ ) in Ni 2p 3/2 and Ni 2p 1/2 orbits, the peaks with binding energies 862.3 and 879.8 eV belong to the orbit characteristic satellite peak [24,25], and the peak of binding energy at 852.5 and 870.6 eV belongs to Ni 2+ peak after oxidation on Ni x P y surface.
As shown in Figure 3(c), in the Fe 2p high-resolution HR-XPS map, there are six characteristic peaks after fitting.Among them, the peaks of the binding energies at 707.1 eV and 720.1 eV correspond to the Fe 2p 3/2 and Fe 2p 1/2 orbits, and those at 715.0 eV and 728.8 eV correspond to the orbit characteristic satellite peaks, while those at 711.0 eV and 723.6 eV correspond to the Fe-O bonds [26].
In Figure 3(d), it was shown in the HR-XPS map of P 2p, the peak of a binding energy at 129.1 eV was attributed to the negatively charged phosphorus species (P δ− ), while the peak at 133.6 eV was attributed to the water molecules adsorbed on the surface and the phosphorous compounds formed after partial oxidation by air [27].
Therefore, there is an obvious electron interaction between (NiFe) 2 P and NF components; (NiFe) 2 P/NF hollow heterostructure catalyst promotes electron transfer, so there is a lot of negative charge around the P atom.This is helpful to further improve the overall electrocatalytic performance of the material in the process of oxygen evolution from electrolytic water.

SEM analysis
To further study the surface distribution of the new catalyst, scanning electron microscopy and transmission electron microscope were used to characterize its morphology.Figure 3 shows the characterization characteristics of the new catalyst.
Figure 3(a) shows a SEM image of NiFe(CN) 6 nanoparticles grown on an NF substrate, the particle size is about 300 nm. Figure 3(b) shows a (NiFe) 2 P/NF SEM image, and the particle morphology is well maintained.When phosphating at high temperatures, the particles will collapse and shrink; the particle size is about 200 nm.From the analysis of Figure 3(a) and (b), NiFe(CN) 6 nanoparticles were uniformly grown on NF, and the morphology of NiFe(CN) 6 nanoparticles was maintained by phosphating (NiFe) 2 P/NF.This nanoparticle structure is conducive to increasing the specific surface area of the catalyst to fully expose the active site, and it is good for accelerating mass transfer and charge transfer; thus, the electrocatalytic activity is enhanced.
Figure 3(c) shows a TEM image of (NiFe) 2 P; the hollow heterostructure was observed.Figure 3(d) shows an HR-TEM image of (NiFe) 2 P. From the analysis of Figure 3(c) and (d), the lattice diffraction fringes show that the interplanar spacing of 0.192 and 0.223 nm corresponds to the (210) and ( 111) crystal planes of Ni 2 P and Fe 2 P.This result is consistent with the XRD result.At the same time, the existence of (NiFe) 2 P phase and NF phase was also confirmed.
Figure 3(e)-(i) is a mapping image of (NiFe)2p/NF.The results further confirmed the presence of Ni, Fe, P, and O in the catalyst.

OER evaluation
Electrochemical performance using Chenghua 660 model electrochemical workstation, Linear voltammetric curve, TAFIR curve, Nyquist curve, and chronopotentiometric curve were used to evaluate the activity and stability of electrocatalytic oxygen evolution.

Tafel slope analysis
In addition to current density and overpotential, the Tafel slope can reflect the kinetic process of the electrocatalytic Preparation and electrocatalytic properties of phosphates  5  process, the lower the slope, the faster the current density increases as the overpotential decreases.Therefore, the Tafel slope can provide reference information for the reaction path and can be used to evaluate the chemical kinetics speed.Figure 4(b) shows the Tafel slope of (NiFe)2p/NF, NiFe(CN) 6 /NF, IrO 2 , and NF of 67, 115, 74, and 125 mV•dec −1 , respectively.(NiFe) 2 P/NF is obviously lower than other catalysts.Therefore, (NiFe) 2 P/NF has a rapid electrocatalytic oxygen evolution chemical kinetics, and the Tafel slope can reflect the rate control step from the side.The entire OER process can be represented as the following four intermediate steps: where * indicates the active site, *OH, *O, and *OOH are the intermediates adsorbed on the active sites.When the Tafel slope is 120, 60, and 40 mV•dec −1 , the rate control steps are 1, 2, and 3. Therefore, it can be inferred that the rate control step of (NiFe) 2 P/NF is the formation of *O(*OH The rate control step of NiFe(CN) 6 /NF is the formation of *OH(* + OH − → *OH + e − ).It is shown that the rate control step can be changed after the preparation of (NiFe) 2 P/NF; thus, the performance of electrocatalytic oxygen evolution can be improved.

Analysis of active area and catalytic mechanism
The double-layer capacitance (C dl ) value can represent the size of electrochemical activity-specific surface area (ECSA); so, the C dl value can be calculated to compare the activity area of the samples.Figure 4(c) shows the current density difference-sweep velocity curves for (NiFe) 2 P/NF, NiFe(CN) 6 /NF, IrO 2 , and NF; the C dl values of (NiFe) 2 P/NF, NiFe(CN) 6 /NF, IrO 2 , and NF double-layer capacitors were calculated at potential 1.2 V (15.9, 3.6, 10, and 1.7 mF•cm −2 , respectively).Known from the C dl data, (NiFe) 2 P/NF has the largest C dl value, which is about 8 times higher than that of NF, indicating that (NiFe) 2 p/NF has large ECSAs and abundant catalytic activity.These characteristics could be attributed to the hollow heterogeneous structure of the material, which is advantageous to the exposure of more active sites, thus accelerating mass transfer and charge transfer, thereby enhancing the electrocatalytic oxygen evolution activity of the material.

Electrochemical impedance analysis
To further analyze the oxygen evolution reaction activity of the catalyst, electrochemical impedance spectroscopy (EIS) was used to evaluate the charge transfer ability of catalysts.Figure 4(d) shows the electrochemical impedance Nyquist plot of (NiFe) 2 P/NF, NiFe(CN) 6 /NF, and NF.From graph analysis, (NiFe) 2 P/NF shows a smaller diameter arc than NiFe(CN) 6 /NF and NF, with the smallest resistance.The Rct of charge transfer resistance is 1.3859 Ω, much lower than NiFe(CN) 6 /NF and NF.This means that (NiFe) 2 P/NF has lower charge transfer impedance, higher conductivity, and higher charge transfer rate.At the same time, the coupling of (NiFe) 2 and P also endows the (NiFe) 2 P/NF with fast electron transfer ability and good OER kinetic performance, thus accelerating the oxygen evolution reaction rates.

Analysis of catalytic activity of strong base solution with high concentration
Industrial electrolysis of aquatic oxygen requires a catalyst with high activity and stability at high current density in a high concentration of strong alkaline solution; therefore, the oxygen production performance of the catalyst in 30 wt% potassium hydroxide of concentrated alkali solution was further studied.Figure 4(e) shows an LSV plot of (NiFe) 2 P/NF, NiFe(CN) 6 /NF, IrO 2 , and NF in 30 wt% potassium hydroxide solution.From the analysis of the Figure 4(e) curve, (NiFe) 2 P/NF still has the highest catalytic efficiency.When the current densities of 100, 500, and 1,500 mA•cm −2 are reached, (NiFe) 2 P/NF only requires overpotentials of 200, 280, and 480 mV.

Electrochemical stability analysis
Stability is another key parameter for the performance evaluation of electrocatalysts.The electrochemical stability of (NiFe) 2 P/NF catalyst electrode was studied at the current density of 1,000 mA•cm −2 for 10 h. Figure 4(f) shows the (NiFe) 2 P/NF stability current density-time curve.As shown in Figure 4(f), in a 10 h potentiostatic test, (NiFe) 2 P/NF catalyst has no obvious change.The results indicate that the catalytic stability of (NiFe) 2 P/NF is very good in both 1 mol•L −1 KOH and 30 wt% potassium hydroxide.
The above results show that (NiFe) 2 P/NF is a highly efficient and stable electrocatalyst.It is expected to replace precious metals in the field of water decomposition and oxygen production.

Conclusions
(1) Transition metal phosphates are regarded as effective substitutes for the activity of electrocatalytic oxygen evolution of noble metals.By in situ generation of Prussian blue-like NiFe(CN) 6 /NF precursor on NF substrate, after low temperature phosphating treatment, the electrocatalyst (NiFe) 2 P/NF, which was grown on the NF substrate, was prepared, and the preparation and development of a non-powdered bimetallic phosphates electrocatalyst material were completed.(2) Using 1 mol•L −1 KOH solution as a basic electrolyte, the electrocatalytic oxygen evolution performance of (NiFe) 2 P/ NF was evaluated.In an alkaline medium, the overpotential required to obtain the catalytic current density of 10 mA•cm −2 is only 220 mV, the Tafel slope is 67 mV•dec −1 , and the catalytic activity of OER can be maintained for at least 10 h.(3) Representation-based and theoretical computation: (1) (NiFe) 2 P/NF nanoparticles were uniformly distributed on the surface of NF substrate, the number of active sites exposed was increased, C dl = 15.9 mF•cm −2 , and it is beneficial to improve its electrocatalytic hydrogen evolution activity.(2) Combined with the XPS results, the existence of hollow heterostructure promotes the electron interaction between (NiFe) 2 P and NF, the charge transfer rate is increased, and the electrical conductivity of the material is improved.(3) (NiFe) 2 P/NF hollow heterogeneous structure catalyst effectively reduces the dissociation barrier of water, the process of water dissociation is greatly promoted, the process of OER kinetic reaction is accelerated, and the oxygen evolution reaction performance of the catalyst is optimized.(4) In this study, the (NiFe) 2 P/NF electrocatalyst was constructed and its performance was evaluated, and it provides a theoretical reference for the design of highly active OER electrocatalysts.The research results have a positive guiding significance for the development of self-supporting electrode catalytic materials for clean energy development in the future; at the same time, it also enriches the application of non-precious metal nanomaterials in the field of oxygen production from electrolytic water.

( 3 )
Ultra-pure water from Pall PURELAB TM Plus, USA.The apparatus and optimum application conditions for electrocatalytic performance test in this study are as follows: (1) X-ray diffraction (XRD), Rigaku D/max-II B, Cu target (λ = 0.1541 nm), scanning range 5°-90°, step length 10°•min −1 , tube pressure 40 KV, tube current 40 mA.(2) X-ray photoelectron spectroscopy (XPS), ESCALABMKII.The test condition is a monochromatic Al Kα radiation source, and the radiation voltage is 1,480.4eV.(3) Scanning electron microscope (SEM), model JEOL JSM 4800F, SEM 25-35 kV, magnification 30-205 K, and resolution 1.3 nm.(4) Electrochemical workstation, model Chenhua 660, using a three-electrode system for electrochemical testing.The NF electrode, the carbon rod electrode, and the mercury oxide electrode (Hg/HgO) are the working electrode, the pair electrode, and the reference electrode, respectively.The electrochemical measurements were performed in 1 mol•L −1 KOH solution (pH = 14) saturated with O 2 .2.2 Synthesized NiFe(CN) 6 /NF nanoparticles 2.646 g of sodium citrate and 1.57 g of nickel sulfate were dissolved in 200 ml of distilled water to obtain Solution A. Separately, 1.316 g of potassium ferricyanide was dissolved in another 200 ml of distilled water to obtain Solution B. Solution B was then slowly poured into Solution A with continuous stirring for 15 min, resulting in a clarified Solution C. Immerse the cleaned NF into Solution C and leave it for 1 h in the 40°C environment.Then, cool down the temperature and leave it for 10 h at room temperature.