Method of synthesizing high-efficiency bifunctional electrocatalysts

Abstract

The design of bifunctional catalysts for water splitting by modifying the electronic structure of the catalyst. That bifunctional catalyst that is synthesized is a quaternary FeNiPSe nanoporous film (FeNiPSe NF). A self-supported FeNiPSE NF is synthesized and used as an anode and a cathode in a two-electrode electrolytic cell. The cell is subjected to a water source, and the FeNiPSe NFs split the water molecules to produce hydrogen fuel. The slightly oxidized FeNiPSe surface serves as an active site for oxygen evolution reactions, making hydrogen evolution reactions and oxygen evolution reactions well-balanced, thereby improving electrolysis efficiency.

Claims

1. A method of synthesizing a bifunctional catalyst for water splitting applications, the method comprising the steps of: forming a self-supported quaternary iron-doped nickel phosphoselenide nanoporous film by: performing a top-down anodic conversion of an iron-doped nickel alloy film to form an iron-doped nickel-oxygen nanofilm; thermally treating, via a tube furnace, the iron-doped nickel-oxygen nanofilm via a phosphorization treatment using a bottom-up chemical vapor deposition, forming an iron-doped nickel-phosphorus nanofilm; subsequent to forming the iron-doped nickel-phosphorus nanofilm, cooling the tube furnace to room temperature for a predetermined period of time; and thermally treating, via the tube furnace, the iron-doped nickel-phosphorus nanofilm with selenium vapor, using the bottom-up chemical vapor deposition, to partially substitute selenium for phosphorus, forming a quaternary iron-doped nickel phosphoselenide nanoporous film bifunctional catalyst; wherein the quaternary iron-doped nickel phosphoselenide nanoporous film bifunctional catalyst comprises high valence nickel; wherein the selenium stabilizes the bifunctional catalyst and improves an electrical conductivity of the bifunctional catalyst; wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes an oxidized surface as an active site for oxygen evolution reactions, such that the quaternary iron-doped nickel phosphoselenide nanoporous film is capable of both hydrogen evolution reactions and oxygen evolution reactions during a water splitting application, thereby improving electrolysis efficiency; and wherein the quaternary iron-doped nickel phosphoselenide nanoporous film exhibits a rate-determining final electron transfer process (hereinafter RDS) for hydrogen evolution reactions, whereby the RDS is dominated by the Heyrovsky step.

2. The method of claim 1, further comprising the step of forming a plurality of pores disposed through the quaternary iron-doped nickel phosphoselenide nanoporous film, such that the plurality of pores improve a transportation of mass through the nanoporous film.

3. The method of claim 1, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes a thickness of 5 m.

4. The method of claim 3, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film is disposed on a surface of an unreacted iron-nickel alloy matrix.

5. The method of claim 1, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes at least 10 wt % iron, at least 65 wt % nickel, at least 0.5 wt % phosphorus, and at least 23 wt % selenium.

6. The method of claim 1, further comprising the step of subjecting the quaternary iron-doped nickel phosphoselenide nanoporous film to an amount of water.

7. The method of claim 6, further comprising the step of converting, via the hydrogen evolution reactions, the amount of water into hydrogen fuel that is usable as a renewable energy source.

8. The method of claim 6, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film exhibits a turnover frequency of about 3.48 s.sup.1 for oxygen evolution reactions.

9. A method of improving electrolysis efficiency during water splitting applications for improved hydrogen fuel production, the method comprising the steps of: forming a self-supported quaternary iron-doped nickel phosphoselenide nanoporous film by: performing a top-down anodic conversion of an iron-doped nickel alloy film to form an iron-doped nickel-oxygen nanofilm; thermally treating, via a tube furnace, the iron-doped nickel-oxygen nanofilm via a phosphorization treatment using a bottom-up chemical vapor deposition, forming an iron-doped nickel-phosphorus nanofilm; subsequent to forming the iron-doped nickel-phosphorus nanofilm, cooling the tube furnace to room temperature for a predetermined period of time; thermally treating, via the tube furnace, the iron-doped nickel-phosphorus nanofilm with selenium vapor, using the bottom-up chemical vapor deposition, to partially substitute selenium for phosphorus, forming a quaternary iron-doped nickel phosphoselenide nanoporous film bifunctional catalyst; forming a plurality of pores disposed through the quaternary iron-doped nickel phosphoselenide nanoporous film; wherein the quaternary iron-doped nickel phosphoselenide nanoporous film bifunctional catalyst comprises high valence nickel; and wherein the quaternary iron-doped nickel phosphoselenide nanoporous film exhibits a rate-determining final electron transfer process (hereinafter RDS) for hydrogen evolution reactions, whereby the RDS is dominated by the Heyrovsky step; subjecting the quaternary iron-doped nickel phosphoselenide nanoporous film to an amount of water and flowing the amount of water through the plurality of pores disposed through the quaternary iron-doped nickel phosphoselenide nanoporous film; and converting the amount of water into hydrogen fuel that is usable as a renewable energy source by splitting the amount of water into hydrogen molecules and oxygen molecules by capturing the oxygen molecules on an oxidized surface of the quaternary iron-doped nickel phosphoselenide nanoporous film to perform oxygen evolution reactions, such that the hydrogen molecules are recovered via hydrogen evolution reactions.

10. The method of claim 9, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes a thickness of 5 m.

11. The method of claim 10, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film is disposed on a surface of an unreacted iron-nickel alloy matrix.

12. The method of claim 9, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes at least 10 wt % iron, at least 65 wt % nickel, at least 0.5 wt % phosphorus, and at least 23 wt % selenium.

13. The method of claim 9, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film exhibits a turnover frequency of about 3.48 s.sup.1 for oxygen evolution reactions.

14. A method of synthesizing a high-efficiency bifunctional electrocatalyst, the method comprising the steps of: forming a self-supported quaternary iron-doped nickel phosphoselenide nanoporous film by: performing a top-down anodic conversion of an iron-doped nickel alloy film to form an iron-doped nickel-oxygen nanofilm; thermally treating, via a tube furnace, the iron-doped nickel-oxygen nanofilm via a phosphorization treatment using a bottom-up chemical vapor deposition, forming an iron-doped nickel-phosphorus nanofilm; subsequent to forming the iron-doped nickel-phosphorus nanofilm, cooling the tube furnace to room temperature for a predetermined period of time; thermally treating, via the tube furnace, the iron-doped nickel-phosphorus nanofilm with selenium vapor, using the bottom-up chemical vapor deposition, to partially substitute selenium for phosphorus, forming a quaternary iron-doped nickel phosphoselenide nanoporous film bifunctional catalyst; forming a plurality of pores disposed through the quaternary iron-doped nickel phosphoselenide nanoporous film; and wherein the quaternary iron-doped nickel phosphoselenide nanoporous film bifunctional catalyst comprises high valence nickel; wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes at least 10 wt % iron, at least 65 wt % nickel, at least 0.5 wt % phosphorus, and at least 23 wt % selenium; wherein the selenium stabilizes the bifunctional catalyst and improves an electrical conductivity of the bifunctional catalyst; wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes an oxidized surface as an active site for oxygen evolution reactions, such that the quaternary iron-doped nickel phosphoselenide nanoporous film is capable of both hydrogen evolution reactions and oxygen evolution reactions during a water splitting application, thereby improving electrolysis efficiency; wherein the quaternary iron-doped nickel phosphoselenide nanoporous film exhibits a rate-determining final electron transfer process (hereinafter RDS) for hydrogen evolution reactions, whereby the RDS is dominated by the Heyrovsky step; and wherein the iron-doped nickel phosphoselenide nanoporous film is disposed upon at least one portion of a surface of an unreacted iron-nickel alloy.

15. The method of claim 14, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film includes a thickness of 5 m.

16. The method of claim 14, further comprising the step of subjecting the quaternary iron-doped nickel phosphoselenide nanoporous film to an amount of water.

17. The method of claim 16, further comprising the step of flowing the amount of water through the plurality of pores disposed through the quaternary iron-doped nickel phosphoselenide nanoporous film.

18. The method of claim 16, further comprising the step of converting, via the hydrogen evolution reactions, the amount of water into hydrogen fuel that is usable as a renewable energy source.

19. The method of claim 16, wherein the quaternary iron-doped nickel phosphoselenide nanoporous film exhibits a turnover frequency of about 3.48 s.sup.1 for oxygen evolution reactions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:

(2) FIG. 1 depicts x-ray diffraction patterns, comparing FeNi films, FeNiO films, FeNiP films, FeNiSe films, and FeNiPSe nanofilms.

(3) FIG. 2 is a scanning electron microscopy image of a FeNiPSe nanofilm.

(4) FIG. 3 is a graphical representation of high-resolution XPS profiles for a FeNiPSe nanofilm.

(5) FIG. 4 depicts polarization curves, Tafel plots, turnover frequencies, and reaction pathways for a FeNiPSe nanofilm.

(6) FIG. 5 is a graphical representation of electrochemical performance of FeNiPSe nanofilms for overall water splitting implementations.

(7) FIG. 6 graphically compares NiPSe NFs with FeNiPSe NFs by comparing hydrogen evolution reactions and oxygen evolution reactions thereof.

(8) FIG. 7 depicts scan-rate dependent cyclic voltammetry values for FeNiPSE NFs, FeNiP NFs, and FeNiSe NFs, at scan rates of 10, 20, 40, 60, 80, and 100 mVs.sup.1.

(9) FIG. 8 is a graphical representation of high-resolution XPS profiles for a FeNiPSe nanofilm after hydrogen evolution reaction testing.

(10) FIG. 9 is a graphical representation of high-resolution XPS profiles for a FeNiPSe nanofilm after oxygen evolution reaction testing.

(11) FIG. 10 is a graphical representation of water electrolyzer performance using FeNiPSe nanofilms.

DETAILED DESCRIPTION OF THE INVENTION

(12) In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part thereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention.

(13) As used in this specification and the appended claims, the singular forms a, an, and the include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term or is generally employed in its sense including and/or unless the context clearly dictates otherwise.

(14) The present invention includes the design of bifunctional catalysts for water splitting by modifying the electronic structure of the catalyst. That catalyst used herein is a quaternary FeNiPSe nanoporous film (FeNiPSe NF). Metal phosphoselenides are used due to the weaker bond strength of SeH (276 kJ/mol) as compared with PH (322 kJ/mol), leading to a better capability for the selenides to capture the reactants and accelerate a subsequent discharge step. Meanwhile, the slightly oxidized FeNiPSe surface serves as an active site for OER, making HER and OER well-balanced. Furthermore, Fe-doping was used to further improve the OER activities and conductivities of NiPSe under alkaline media by forming high valence nickel. The designed FeNiPSe NFs are self-supported and can be directly used as bifunctional catalysts without adding any additives, allowing the direct investigation of the synergistic effects among the quaternary elements (Ni, Fe, Se, and P) for overall water splitting without interference from carbon and other additives.

(15) The quaternary FeNiPSe NFs were synthesized by anodically converting the electrodeposited FeNi alloy films (atomic ratio of Fe:Ni=15:85) to FeNiO NFs followed by thermal treatments (firstly phosphorization, followed by selenylation) using a chemical vapor deposition (CVD) apparatus. Due to the oxygen/moisture-sensitivity of TMPs, the FeNiP NFs were further thermally treated under selenium vapor in order to partially substitute P by Se. The incorporation of Se in the quaternary FeNiPSe NFs plays dual roles of stabilizing the catalysts in the air and improving the electrical conductivity of the catalysts. The methods of synthesizing the Fe-NiPSe NFs are described in greater detail herein below.

(16) Synthesis of FeNiPSe Nanofilms

(17) FeNi alloys were synthesized in an electrolyte bath prepared in an aqueous plating solution by dissolving Ni.sub.2SO.sub.4.Math.6H.sub.2O, NiCl.sub.2.Math.6H.sub.2O, FeSO.sub.4.Math.7H.sub.2O, H.sub.3BO.sub.3, Na.sub.3C.sub.6H.sub.5O.sub.7.Math.2H.sub.2O and saccharin with a certain amount in distilled water and then stirring for 30 min at room temperature. A bottom-up electrochemical deposition of FeNi alloy films was performed in a home-made two-electrode cell with stainless steel substrate as the cathode and a Pt mesh as the anode at a current density of 25 mA cm.sup.2 for 20 min. FeNiO NFs were then synthesized via a top-down anodic treatment at a constant voltage of 20 V for 20 min in an electrolyte of 0.2 M NH.sub.4F and 2 M H.sub.2O in ethylene glycol.

(18) The obtained FeNiO films were placed at the downstream side while NaH.sub.2PO.sub.2 was placed at the upstream side in a tube furnace. The tube was evacuated to 50 mTorr for at least 10 min and then purged with Ar to remove the residual air. Then, the furnace upstream and downstream of the tube furnace was maintained at 250 C. and 300 C. for 15 min with a heating rate of 5 C. min.sup.1. During the reaction, Ar (100 sccm) was used as a carrier gas; after cooling to room temperature, Se powder was placed at the upstream to replace the residual NaH.sub.2PO.sub.2, and the furnace upstream and downstream of the tube furnace were both kept at 300 C. for another 15 min to obtain FeNiPSe NFs. As control experiments, FeNiP NFs and FeNiSe NFs were prepared without using Se and P sources, respectively.

(19) As shown in FIG. 1, only the characteristic peaks of the metallic FeNi films could be found from the XRD patterns due to the much higher contents and stronger intensities for the diffraction peaks of FeNi films. The cross-sectional scanning electron microscopic (SEM) images and elemental mapping, shown in FIG. 2, indicate the formation of a porous structure throughout the entire film with a total thickness of 5 m, including a thin layer of FeNiPSe NFs (600 nm) on the surface of unreacted FeNi alloy matrix. The pores help to accelerate the mass transportation through the film. The contents of Fe, Ni, P, and Se in the FeNiPSe NFs were estimated to be 10.5 wt %, 65.1 wt %, 0.7 wt %, and 23.7 wt %. The much higher content of Se than that of P is due to the substitution of P with Se during the CVD treatments.

(20) FIG. 3 shows the high-resolution XPS Ni 2p profile for the FeNiPSe NFs (in section A) having three sets of peaks at 853.0 eV/870.3 eV, 854.5 eV/872.8 eV, and 859.0 eV/876.3 eV corresponding to the metallic Ni.sup.0, Ni.sup.2+, and the Ni satellite peaks, respectively. The XPS Fe 2p profile (in section B) shows peaks located at 706.1 eV and 712.4 eV, which are attributed to the Fe.sup.0 and Fe.sup.3+, respectively. The XPS P 2p profile (in section C) shows that the peaks located at 129.0 eV and 133.4 eV are attributed to the phosphide and phosphate species, respectively. The XPS Se 3d profile (in section D) shows the peaks at 54.2 eV and 55.0 eV corresponding to Se 3d.sub.5/2 and Se 3d.sub.3/2, respectively, which are the core level bands of Se 3d in NiSe.sub.2/FeSe.sub.2. In addition, the intermediate fitting lines at 55.9 eV and 58.3 eV are associated with the slightly oxidized surface.

(21) Performance of the Catalysts

(22) The electrochemical HER and OER performance of the catalysts was firstly studied in a three-electrode system using Ar-saturated 1 M KOH solution as an electrolyte to make a comparison with the commercial Pt/C (platinum decorated carbon) (20 wt %) and IrO.sub.2 benchmark catalysts. The onset potential for the FeNiPSe NFs (shown in section A of FIG. 4) is 29 mV, which is much lower than those of FeNiP NFs (230 mV) and FeNiSe NFs (135 mV), and very close to the commercial Pt/C (0 mV). To reach a current density of 10 mA cm.sup.2, the FeNiPSe NFs, FeNiP NFs, and FeNiSe NFs require of 172 mV, 337 mV, and 295 mV, respectively. Tafel slopes are usually used to detect the rate-determining step (RDS) for HER through the following pathways:
*+H.sub.2O+e.sup..fwdarw.OH.sup.+*H.sub.ads Volmer step(1)
*H.sub.ads+H.sub.2O+e.sup..fwdarw.*+OH.sup.+H.sub.2 Heyrovsky step(2)
2*H.sub.ads.fwdarw.2*+H.sub.2 Tafel step(3)
where * denotes the surface active site. As shown in section C of FIG. 4, the FeNiPSe NFs have a much smaller Tafel slope (101 mV dec.sup.1) than FeNiP NFs (125 mV dec.sup.1) and FeNiSe NFs (148 mV dec.sup.1), indicating the intrinsically favorable kinetics of FeNiPSe NFs for HER. Hence, the RDS for FeNiPSe NFs is dominated by the Heyrovsky step, whereas the RDS for FeNiP NFs and FeNiSe NFs is controlled by the Volmer step (section E of FIG. 4).

(23) The electrochemical OER performance of the catalysts was also examined by linear sweep voltammograms (LSV, as shown in section B of FIG. 4) and Tafel plots (section D of FIG. 4). The oxidation peaks located in the potential range of 1.4-1.45 V are ascribed to the oxidation of Ni.sup.2+ to Ni.sup.3+. The FeNiPSe NFs show much lower overpotentials to reach the current densities of 10 mAcm.sup.2 and 20 mAcm.sup.2 ( of 254 mV and 267 mV, respectively) than those of FeNiP NFs ( of 279 mV and 332 mV, respectively) and FeNiSe NFs ( of 290 mV and 317 mV, respectively). It should be noted that the OER performance of FeNiP NFs is better than the FeNiSe NFs because it is easier to form the oxidized surface on the FeNiP NFs for OER. More importantly, all the nanoporous film catalysts show better OER performance than the commercial IrO.sub.2. The Tafel slope of FeNiPSe NFs (48.1 mV dec.sup.1) is much lower than those of FeNiP NFs (57.9 mV dec.sup.1) and FeNiSe NFs (96.3 mV dec.sup.1), indicating different RDS and reaction kinetics for OER (shown in section E of FIG. 4) according to the most accepted four-electron reaction steps in alkaline solution:
*+OH.sup..fwdarw.*OH.sub.ads+e.sup.(4)
*OH.sub.ads+OH.sup..fwdarw.*OOH+e.sup.+H.sub.2O(5)
*O+OH.sup..fwdarw.*OOH+e.sup.(6)
*OOH++OH.sup..fwdarw.*+O.sub.2+e.sup.+H.sub.2O(7)

(24) Typically, in a multi-electron involved OER process, the Tafel slopes of 24 mV dec.sup.1, 40 mV dec.sup.1, and 60 mV dec.sup.1 imply that the third-electron transfer, the second-electron transfer, and the chemical step following the first-electron transfer are the RDS, respectively. Distinctly, the FeNiPSe NFs have a Tafel slope of 48.1 mV dec.sup.1, indicating that the second-electron transfer process is the RDS (shown in section E of FIG. 4). However, the RDS for FeNiP NFs is the first-electron transfer process. The large Tafel slope of FeNiSe NFs indicates that the RDS is limited by the initial step during which the catalyst surface is strongly bonded with OH groups. In contrast, the small Tafel slope of FeNiPSe NFs suggests that the RDS is at the final step of the multiple-electron transfer processes, proving an excellent OER activity. As compared to HER, OER is kinetically sluggish and eventually determines the efficiency of the overall water splitting. In order to further examine the OER activities of the catalysts, turnover frequency (TOF) was calculated as shown in section F in FIG. 4. The TOF at the overpotential of 0.3 V for FeNiPSe NFs is about 3.48 s.sup.1, which surpasses FeNiP NFs (0.85 s.sup.1), FeNiSe NFs (0.59 s.sup.'), and IrO.sub.2 (1.06 s.sup.1) by 4.1 times, 5.9 times, and 3.3 times, respectively. The high TOF of FeNiPSe NFs undisputedly suggests a supreme activity towards OER, contributing to the improved electrolysis efficiency. The bifunctional HER/OER activities of FeNiPSe NFs were further investigated in a three-electrode system, as shown in section G of FIG. 6. A potential of 1.897 V is required to deliver a high current density of 100 mA.Math.cm.sup.2 for overall water splitting.

(25) Water Splitting Performance

(26) A two-electrode electrolytic cell using the FeNiPSe NFs as both anode and cathode was employed to study the practical water splitting performance. The potentials of 1.59 V and 1.93 V were required to deliver current densities of 10 mA cm.sup.2 and 100 mA cm.sup.2, respectively (as shown in section A of FIG. 5), superior to the state-of-the-art bifunctional catalysts for overall water splitting. A further I-t curve testing (shown in section B of FIG. 5) shows that approximately 66% current can be well maintained at 1.8 V for more than 100000 s (27.7 hours).

(27) Electrochemical impedance spectroscopy (EIS) was used to probe the reaction kinetics for the catalysts. The Nyquist plots (shown in section C of FIG. 5) and the corresponding equivalent circuit at an overpotential of 0.3 V shows that the system resistance (R.sub.S) is approximately equal for all the catalysts (1.6, shown in Table 1 below). The charge transfer resistance (R.sub.CT) of FeNiPSe NFs is much smaller than those of two control catalysts, indicating a greatly enhanced conductivity, facilitated electron transfer, and thus improved catalytic activity for water splitting by forming quaternary alloy phosphoselenide. In addition, the NiPSe NFs without Fe-doping (shown in section A-C of FIG. 6) show a much inferior HER/OER performance than the FeNiPSe NFs due to the low conductivity (shown in section D of FIG. 6). The double-layer capacitance (C.sub.DL) calculated from the CV curves (shown in FIG. 7) was used to estimate the electrochemically active surface areas of the nanoporous film catalysts. The C.sub.DL of FeNiPSe NFs, FeNiP NFs, and FeNiSe NFs was estimated to be 1.29 mF cm.sup.2, 0.98 mF cm.sup.2, and 0.42 mF cm.sup.2 (shown in FIG. 7), respectively, indicating that the FeNiPSe NFs have a much higher surface area than the other two counterparts for the catalytic reactions.

(28) TABLE-US-00001 TABLE 1 System resistance (R.sub.S) and charge resistance (R.sub.CT) for three samples Sample R.sub.S () R.sub.CT () FeNiPSe NFs 1.61 1.13 FeNiP NFs 1.60 1.58 FeNiSe NFs 1.62 2.15

(29) XPS was also performed on the catalysts after long-term HER (as shown in FIG. 8) and OER (as shown in FIG. 9) stability tests, which shows the disappearance of Ni.sup.0 and Fe.sup.0 due to the surface reconstruction under harsh conditions (high pH and high polarization) used for water splitting. Similarly, in the XPS P 2p profiles, the metal phosphide phase on the catalyst surface was well preserved during HER (shown in section C of FIG. 8); however, the phosphide was oxidized to the high valence state P during OER (shown in section C of FIG. 9). The XPS Se 3d profiles show a similar characteristic to the P 2p, where the core level bands of Se 3d still presents after HER test shown in section D of FIG. 8) but was oxidized to SeO.sub.x peaks after OER (section D of FIG. 9) due to the surface reconstruction.

(30) Traditionally, strongly acidic and alkaline solutions are widely used for water splitting because of the increased ionic conductivities, thus making the dissociation of water quickly and efficiently. According to the pH of the feedstock solutions, water electrolysis is usually categorized into proton and anion exchange membrane (PEM and AEM) electrolyzer. So far, PEM and AEM electrolyzers are still limited by the high cost and low efficiency of PGM catalysts. An ideal and ultimate strategy to replace the traditional electrolyzers operated under harsh conditions (either strongly acidic or alkaline) is to use pure water or even natural seawater as feedstock solutions because they have low corrosion to the electrolyzers and catalysts. Especially, seawater covers 70% surface of the earth crust, which is naturally available for the mass production of H.sub.2 at low cost.

(31) In order to demonstrate the possibility seawater splitting, a practical AEM electrolyzer was employed to explore the performance of the rationally designed FeNiPSe NFs using four different independent electrolyte feed ways, namely (I-IV) as shown in section A of FIG. 10. The current densities of 0.305 A cm.sup.2, 0.44 A cm.sup.2, 0.559 A cm.sup.2, and 0.815 A cm.sup.2 were achieved at an electrolysis voltage of 1.7 V when using (I-IV) feeding modes (Sections B and C of FIG. 10), respectively. The current densities further increased to 0.421 A cm.sup.2, 0.6 A cm.sup.2, 0.769 A cm.sup.2, and 1.144 A cm.sup.2 for (I-IV), respectively, when the voltage was set at 1.8 V. Noticeably, the seawater electrolysis performance achieved by the FeNiPSe NFs meet the demanding requirements (0.4-1 A cm.sup.2) for the practical application in the industries. Furthermore, the O.sub.2 Faraday efficiency (FE) at 1.8 V using device III was examined by gas chromatography (GC). The O.sub.2 FE over 97% (Section D of FIG. 10, solid histograms) was detected when using natural seawater as the cathode feedstock solution (Section A of FIG. 10, device I). If the conventional symmetric seawater feeding (Section A of FIG. 10, device III) was employed, the O.sub.2 FE above 92% was achieved (Section D of FIG. 10, histograms with inclined pattern). A 200 hour continuous and stable operation of electrolyzer was further demonstrated using the asymmetric (device I in FIG. 10) and symmetric (device III in FIG. 10a) models of natural seawater feeding (Section E of FIG. 10). During the continuous testing at a cell voltage (E.sub.cell) of 1.8 V, well maintained current densities at 0.41 A cm.sup.2 and 0.8 A cm.sup.2 with the O.sub.2 FE.sub.O2 above 95% and 92%, respectively, were obtained. More importantly, the electrolysis efficiency of 78.4% at 1.6V was gained, which is superior to the Department of Energy (DOE) 2020 target (77%).

(32) Conclusion

(33) The FeNiPSe NFs show greatly improved activities towards overall water splitting in alkaline solution with overpotentials () of 0.17 V and 0.25 V to reach a current density of 10 mA cm.sup.2 for HER and OER, respectively. Moreover, the turnover frequency (TOF) for OER at of 0.3 V is 3.48 s.sup.1, which is 2.3 times higher than that of IrO.sub.2. When used as bifunctional catalysts in an actual water electrolyzer using pure water and even seawater as feedstock solutions, an electrolysis efficiency of 78.4% was obtained, higher than those of the state-of-the-art electrolyzers.

(34) The advantages set forth above, and those made apparent from the foregoing description, are efficiently attained. Since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

(35) It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.