A Free-standing Nanoporous NiCoFeMoMn High Entropy Alloy as An E�cient Electrocatalyst Fast Driving Water Splittling

15 The development of low-cost non-noble metal-based electrocatalysts that can 16 work stably at high current densities for the application of Hydrogen evolution 17 reaction (HER) and oxygen evolution reaction (OER) in electrolyzed water is 18 paramount crucial. Herein, we report a free-standing nanoporous high entropy 19 alloy foil as dual-functional electrocatalyst via a combination of one-step 20 dealloying and polarization, which exhibits excellent electrocatalytic activity in 21 alkaline electrolyte with an extremely small overpotential of 150 mV at 1000 22


Introduction
As a kind of clean energy, hydrogen energy has a huge demand in all over the world, and the annual demand is about 70 million tons.However, the main raw materials for its production are traditional fossil fuels such as natural gas and coal, which not only consume a lot of non-renewable resources, but also bring serious carbon emissions 1,2,3 .Electrolyzed water is an environmentally friendly method for hydrogen production, which is regarded as the best choice for hydrogen production in the future 4,5,6 .However, due to the lack of low-cost electrocatalysts with low overpotential at high current density and still good durability, the practical application of electrolyzed water has been greatly limited 7,8,9 .
Electrolyzed water consists of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).The reaction kinetics of HER/OER are determined by the binding energy between the active site with reaction intermediate and the water dissociation barrier 10,11 .According to the volcanic curve, Pt has the optimized H binding energy and thus has been regarded as the best choice for electrolyzing water.However, the scarcity and high cost of Pt seriously limit its application in electrolytic water 12,13 .Therefore, many low-cost transition metal based catalysts have been developed 14 , such as transition metal based multicomponent alloys 15,16 , carbides 17 ， phosphide 14,18 , nitride 19 and sulfides 20 , or utilizes synergistic effect by stacking the two materials together to construct heterojunction structure to improve the catalytic activity 21 .However, the stable working lifetimes of these catalysts are generally short 22,23,24 .The nanoporous structure of multicomponent alloy is more stable than others material, which is promising to be used as a candidate material for long-term stable work under high current density 15,16 .
As a new kind of material with unusual microstructure, high entropy alloy (HEA) has mechanical and chemical properties which are incomparable with traditional materials, and has gradually attracted the interest of research in various fields, especially in electrocatalysis 4,25,26 .For example, bulk Ni20Fe20Mo10Co35Cr15 and Pt18Ni26Fe15Co14Cu27/C show high activities for hydrogen evolution reaction, but their cycle life is only a few hours 27,28 .
Consequently, it is very important to improve the intrinsic catalytic activity and cycle life of high entropy alloy catalyst for its practical application.HEAs containing two solid solution phases can be formed by spinodal decomposition.Its heterojunction-like structure with controlled electronic structure can be used to improve the intrinsic catalytic activity of the material and inherit the excellent mechanical properties of HEAs to extend work life 29,30,31 .
Here, a nanoporous NiCoFeMoMn high entropy alloy (np-HEA), as a bifunctional free-standing catalyst with high HER and OER activities, was prepared by one-step dealloying of Ni14Co14Fe14Mo6Mn52 multicomponent alloy that produces spinodal decomposition.Scheme 1 shows the HER process for the nanoporous NiCoFeMoMn HEA.The segregation area by spinodal decomposition is not easy to be dealloyed while the un-segregation area is prone to dealloying, which forms a unique heterojunction structure.Proofed by DFT results, the unique structures of two areas optimize the Gibbs free energies of hydrogen adsorption and H2O adsorption energy synergistically on active sites, and reduce the dissociation barrier of H2O so as to enhance the intrinsic catalytic activity of the material.In HER, the current density of 1000 mA cm -2 can be achieved only with an overpotential of 150 mV in 1 M KOH solution, and the Tafel slope is as low as 29 mV dec -1 .OER catalyst was synthesized by polarization of np-HEA, and the overpotential was only 350 mV at the current density of 1000 mA cm -2 .Using them as cathode and anode respectively, the current density of 10 mA cm -2 can be achieved with only 1.47 V cell voltage, and the excellent stability can be ensured for more than 375 h.Scheme 1.The HER process for the nanoporous NiCoFeMoMn HEA.

Result Material synthesis and characterization.
Firstly, a novel Mn-rich master alloy (Ni14Co14Fe14Mo6Mn52) was synthesized by arc melting and single-roller melt spinning.As shown in Fig. 1a, X-ray diffraction (XRD) shows four peaks at 43.14, 50.24, 73.51, and 88.98°, which are characteristics of (111), (200), ( 220) and (311) planes of face centered cubic (FCC) phase.The scanning electron microscopy (SEM) image (Fig. 1b) indicates that the spinodal decomposition seems occur due to the appearance of nanoscale (the diameter is 200~300 nm) precipitates at grain boundaries.By Energy-dispersive X-ray spectroscopy (EDS) of SEM proved that the atoms ratio quantified to be Ni, Co, Fe, Mo and Mn is 14.1, 14.4, 14.S1, Supporting Information).In addition, according to the EDS image, segregation of Mo atoms was found in the NiCoFeMoMn HEA, the composition change will also provide more abundant atomic chemical environment and more active sites, which may improve the performance of electrolyzed water 4 .In order to distinguish, the element segregation area is denoted SA and the other areas are denoted un-SA.Subsequently, the nanoporous NiCoFeMoMn HEA were prepared by one-step electrochemical dealloying method in 1.0 M (NH4)2SO4 solution at -0.5 V (vs Ag/AgCl).After dealloying 6 hours, the SEM images in Fig. 2a and 2b show that the cracks with a width of 300~500 nm were formed on the surface of the np-NiCoFeMoMn HEA.In the Fig. 2b, it displays a hierarchical nanoporous skeleton with ~5 nm small nanopores on the surface and ~40 nm large nanopores around the SA.N2 adsorption-desorption measurements further verified that np-NiCoFeMoMn shows a large Brunauer-Emmett-Teller (BET) surface area of 45.35 m 2 g -1 with an average nanopore size of 4.76 nm using the Barrett-Joyner-Halenda (BJH) method (Fig. S2).Fig. 2c and Fig. S3 are the HADDF-STEM image of nanoporous NiCoFeMoMn alloy after six hours of dealloying (np-NiCoFeMoMn 6h alloy), which are evidenced the diameter of nanopores around the SA is larger than that of other area.This is due to the potential difference in 1 M (NH4)2SO4, which is discussed in detail in the Supporting Information.The XRD pattern of np-NiCoFeMoMn ribbons displays a similar crystal structure with the Ni14Co14Fe14Mo6Mn52 excepting for a slight shift, inferring that the distance between crystal planes increases with the removal of a part of atoms (in Fig. 1a and Fig. S4, Supporting Information).This change was also observed by HR-TEM of Ni14Co14Fe14Mo6Mn52 in Fig. S5 and np-NiCoFeMoMn in Fig. 2d, the interplanar spacing of (111) planes increased from 2.06 to 2.17 Å after dealloyed.Fig. 2e shows the HADDF-STEM-EDS image of np-NiCoFeMoMn 6h, which reveals that Mn, Fe and a small amount of Co are removed after dealloying.Through HADDF-STEM-EDS before and after dealloying, the element atom proportion of each area is summarized in Fig. 2f.By comparing the changes of element contents in different regions before and after different dealloying time, it is shown that the dealloying process mainly occurs in un-SA (de-un-SA).The elements composition of Mn, Fe and Co are decreasing, especially the content of Mn decreased from 51.46% to 18.55%.And the composition of SA barely changes when dealloying (de-SA) time is less than six hours (in Fig. S6).After that, the SA begins to corrode obviously.Through the quantitative analysis of SEM-EDS, HADDF-STEM-EDS and XPS, it is proved that the surface composition of np-NiCoFeMoMn 6h is consistent (in Fig. 2e, Fig. S7 and Table S1 and S2).To determine the np-NiCoFeMoMn HEA surface chemical states, we also tested X-ray photoelectron spectrometer (XPS) and analyzed the elements of Ni, Co, Fe, Mo, Mn and O (in Fig. S8 and Table S3 in the Supporting Information).
Fig. 3a shows the deconvolution of the Ni 2p core level spectra, which exhibits three peaks of Ni 2p 1/2 with binding energies of 852.62, 855.90 and 861.49eV assigned to metallic Ni, Ni(II) and relevant satellite peak, respectively 32 .
Among them, Ni(II) is the main valence state with a peak area of 87.3%.Due to the active nature of transition metals, the metallic composition is much lower than that of other states, which is also obvious in other metal elements.In Co 2p region (Fig. 3b), Co(II) is the main valence state, and its peak area is 89.3% at 781.64 eV (2p 3/2) and 796.84 eV (2p 1/2); the peaks of metallic Co at 778.06 (2p 3/2) and 793.03 eV (2p 1/2); the other two peaks (785.54 and 802.77eV) are satellite peaks.The Fe 2p spectrum in Fig. 3c shows seven peaks, two main peaks at 710.30 and 724.30eV, which can be ascribed to Fe 2p 3/2 and Fe 2p 1/2 of Fe(II), respectively; The two smaller peaks (713.23 and 726.83 eV) next to the two main peaks are satellite peaks; The peaks of metallic Fe at 707.10(Fe 2p 3/2) and 720.30eV(Fe 2p 1/2) 11 .The peak-fitting analysis of Mo 3d spectrum (Fig. 3d) shows that the peak areas of metallic Mo, Mo(II), Mo(IV) and Mo(VI) are 10.0%, 7.9%, 20.9% and 51.2%, respectively 15 .From the Mn 2p spectrum (Fig. 3e), according to the characteristic peak at 646.99 eV, we can judge that the peak of 641.37 and 653.07 eV can be assigned to Mn 2p 3/2 and Mn 2p 1/2 of Mn(II); The peaks of metallic Mn at 638.43 and 649.48 eV 33 .The O 1s XPS spectrum in Fig. 3f can be simulated by the combination of three features at 530.37, 531.51 and 533.26 eV, corresponding to the lattice oxygen (O 2-), M-OH group and adsorbed oxygen on surface of nanoporous NiCoFeMoMn alloy 34 .It is interesting to find that the main oxygen state is M-OH groups absorbed on the catalyzer surface, which has been reported that it is beneficial to the adsorption and desorption of H intermediates to accelerate HER kinetics 15 .

Electrochemical characterizations.
In order to evaluate the electrochemical performance of np-NiCoFeMoMn HEA, a three electrodes configuration was used to perform all electrochemical related tests in 1 M KOH solution.The np-NiCoFeMoMn HEA was applied as the working electrode, graphite rod as the counter electrode and Ag/AgCl as the reference electrode.All linear scan voltammogram (LSV) curves were calibrated with reversible hydrogen electrode and IR compensated.The LSV curves of commercial Pt/C and np-NiCoFeMoMn HEA catalysts are exhibited in Fig. 4a, the dealloying time of np-NiCoFeMoMn HEA is 0, 3, 4, 5, 6 and 7 hours, respectively.The results show that the overpotential of NiCoFeMoMn 0h is 322 mV at 10 mA cm -2 current density.After dealloying for 6 hours, the best HER properties were obtained, the overpotential was only ~14 mV at the current density of 10 mA cm -2 , while, the overpotential of commercial Pt/C electrode is 32 mV at 10 mA cm -2 .The Tafel slope is used as the descriptors of the intrinsic activity in HER, calculated by Tafel formula,  = a + blog |j| (Where  is the over potential, j is the current density, a is the Tafel constant, and b is the Tafel slope).The results in Fig. 4b show that the Tafel slope of np-NiCoFeMoMn HEA 6h has an ultra-low Tafel slope of 29 mV dec -1 that corresponds to the Volmer-Tafel mechanism and the Tafel step (2Had↔H2+2*, Had is adsorbed hydrogen, * is active site) is the rate determining step of HER, instead of the common Volmer reaction, which is lower than the commercial Pt/C electrode (33 mV dec -1 ).This mechanism determines that HER kinetics of np-NiCoFeMoMn HEA 6h is faster, and the current density can reach 500 and 1000 mA cm  S14 in the Supporting Information, it believes that the hierarchical porous structure and the increased electrochemical active area of the np-NiCoFeMoMn 6h catalyst supply a significant influence on the excellent electrochemical performance.The HER performance of np-NiCoFeMoMn 6h is 160 times higher than that of NiCoFeMoMn 0h, which may be due to the formation of larger nanopores near the element SA with the increase of corrosion depth after dealloying, which leads to the exposure of more element segregation area and provides more active sites.In order to confirm this inference, we prepared two ribbons according to the composition of SA and un-SA, respectively.The LSV curves are shown in Fig. S15.The results show that the HER performance of the electrodes with SA is higher than un-SA.Fig. 4d shows the relationship between the dealloying time and HER performance.The HER performance is improved with increasing dealloying time, the highest of which is obtained when the dealloying time is 6 hours.
Then the catalyst performance began to decline with increasing dealloying time as same as that of ECSA, which would be the primary cause for the corresponding declined performance.This infers that the synergistic effect of np-NiCoFeMoMn HEA also appears to have a major contribution to promoted HER performance.
As shown in Fig. 4e, the np-NiCoFeMoMn can be stably worked in 1 M KOH solution for 180 and 100 h at the current density of 100 and 500 mA cm -2 .After a long-term durability test, the performance of the catalyst did not decline significantly.
The SEM images show that the surface structure of catalyst has not changed obviously (Fig. S16).Additionally, the np-NiCoFeMoMn 6h electrode shows the onset potential (potential required to reach -1 mA cm -2 ) for HER at 5 mV (Fig. 4f), which can only be observed for commercial Pt/C catalyst.Furthermore, by comparing the price activity of np-NiCoFeMoMn 6h electrode and commercial Pt/C electrode at an overpotiential of -100 mV (in Fig. 4g), the price activity of np-NiCoFeMoMn alloy electrode is as high as 3642 A dollar -1 , which is more than 792 times of that of Pt/C electrode, showing that np-NiCoFeMoMn alloy can greatly reduce the cost of HER.This is the mark great advantages for the practical industrial application of nanoporous HEA catalyst.Furthermore, the multi-step chronoamperometric curve in Fig. S17 and multi-step voltage curve in Fig. S18 was performed in 1.0 M KOH solution.The multi-step chronoamperometric curve at overpotential starting at 0 mV and ending at 690 mV with an increment of 80 or 300 mV per 2 h.The multi-step voltage curve at current density ranges from 100 to 1400 mA cm -2 .The results reveal that the current remains very stable at each potential in the entire range, which further validates that the material has high catalytic stability in a wide current range (0~1400 mA cm -2 ).The low Tafel slope and overpotential of nanoporous HEA are further confirmed by comparison with other catalysts.As shown in Fig. 4h and Table S4, it is further proved that the catalyst has excellent HER performance.The Tafel slope and the overpotential at 100 mA cm -2 of 42 kinds of HER catalysts reported recently are compared.The np-NiCoFeMoMn 6h electrode is state-of-the-art non noble metal HER catalyst, and even surpasses the performance of noble metal catalysts.The np-NiCoFeMoMn with perfect corrision resistance can also be used as the OER catalyst, which is activated by a electrochemical polarization strategy at 1.2 V (vs Ag/AgCl).XPS analysis (Fig. S19) indicated that HEA was fully covered by a layer of oxy-hydroxide during polarization.However, the XRD analysis (Fig. S20) show no obvious oxide phases form, inferring only a very thin layer of oxy-hydroxide was formed and self-passivated to prevent further oxidation, similar to the previous reports 35 .Fig. 5a and 5b show the OER performance and the corresponding Tafel slopes of the polarized np-HEA electrode.Obviously, the polarized np-HEA 6h obtained the pick of OER catalysts.The overpotential of polarized np-HEA 6h is 243 mV at the current density of 10 mA cm -2 and the achieved Tafel slope is 37 mV dec -1 , respectively.Compared to the Tafel slope of HEA with the value of 80 mV dec -1 , the decrease of this value indicates that the polarized np-HEA greatly promoted the four electrons progress in OER, reaction rate of each step was optimized.In Fig. 5c, the overpotential of all catalysts at different current densities is compared, which shows that this trend is very close to their HER performance and present a similar volcano shape.In addition to the specific surface area, the OER performance may be affected by the synergistic effect of different dealloying time catalysts.Meanwhile, the excellent OER durability of catalyst comes from its stable structure, it can operate stably for more than 230 h when the current density is 100 mA cm -2 in 1M KOH solution (Fig. 5d)，and even at 500 mA cm -2 it can still work over 100 h (Fig. S21).
The structure of polarized np-HEA remains in its original state after durability test by SEM (Fig. S22), which indicates that polarized np-HEA also has good stability in OER.Due to the formation of oxy-hydroxide on the surface of polarized np-HEA, the material is more stable at high current density.By comparing the OER data with others literature, the Tafel slope and overpotential at large current density of polarized np-HEA are very attractive in OER performance in metal-oxide based catalysts and have great application prospects (Table S5).Due to the excellent HER and OER properties of HEA, nanoporous alloy and polarized np-HEA were used as cathode and anode respectively for full water splitting test.Fig. 6a shows the LSV curve measured in an electrolytic cell with two electrodes in 1 M KOH solution.It requires a very low cell voltage of 1.47 V to reach 10 mA cm -2 .Even at the current density of 500 and 1000 mA cm -2 , the cell voltage is 1.7 and 1.8 V.This performance is better than many bifunctional catalysts in alkaline electrolytes by compared in Table S6.The alkaline electrolyzer using bifunctional electrodes of np-HEA|| polarized np-HEA shows excellent durability for practical use.
It can steadily work for continuously full water splitting for as long as 375 h at 10 mA cm -2 (Fig. 6b).Furthermore, the durability of np-HEA|| polarized np-HEA electrode as a bifunctional catalyst has higher constancy than previously reported in the literature.

Discussion
To further understand the HER mechanism, DFT calculations were performed to investigate the modulation essence of catalytic activity.The np-HEA model of SA and un-SA were established, which can be found in Supporting Information.As a well-known descriptor for HER catalysis in basic condition, the calculated Gibbs free energies of hydrogen adsorption (∆GH*) of all possible active sites on (111) surface were comprehensively evaluated in Fig. S23 (SA) and S24 (un-SA) respectively, seen in Supporting Information.Remarkably, the ∆GH* value of SA is much closer to the thermo neutral position than that of un-SA.On some active sites of SA in Fig. S23, the ∆GH* on 7, 8 and 16 sites (-0.05, -0.03 and 0.01 eV) exhibit platinum-like catalytic properties (-0.09 eV).In comparison, the enhanced ∆GH* of un-SA in Fig. S24 will sluggish the kinetic process of hydrogen production.The colored ∆GH* comparison of SA and un-SA is presented in Fig. 7a.Therefore, the SA in np-HEA plays a crucial role in the hydrogen adsorption characters, which is in good agreement with the experimental results.Moreover, the reasons of proton being most likely to be adsorbed on the hollow position and ∆GH* of SA being thermo neutral were further explained by the electronic structure analysis including the electron localization functions (ELF) map in Fig. 7b and charge density difference in Fig. S25 of (111) surface on SA and un-SA 36 .It demonstrates the metal bonding in the compound and the valance electrons in d orbital localizing strongly in the interstices of the nearest three atoms.Specifically, for the two electrons involved HER process, the valance electron localization will guarantee the proton adsorption.Furthermore, the ∆GH* difference between SA and un-SA can be explained by the density of states in Fig. 7c.
In comparison with the electronic structure of un-SA, more surface occupied states of SA are delocalized near the Fermi level, which is associated with the thermal neutral ∆GH* and faster proton transfer kinetics.
In addition to the hydrogen adsorption, the H2O adsorption effect was further unveiled as well.The optimized atomic structures of SA and un-SA can be found in

Supplementary Files
This is a list of supplementary les associated with this preprint.Click to download. SupportingInformation.pdf

3 , 6
and 51.2%, which is consistent with the composition ratio of designed alloys in Fig. S1 (Supporting Information).In Fig 1c, the transmission electron microscopy (TEM) image and the corresponding selected area electron diff raction (SAED) patterns suggested the dual-phase both are FCC phase, confirming that the formation of nanoscale precipitates is due to the spinodal decomposition.The element distribution in the NiCoFeMoMn alloys was characterized by high-angle annual-dark-field scanning TEM (HAADF-STEM) and EDS in Fig. 1d.It revealed that the element contents are same as that in SEM-EDS(Table
-2 with 104 and 150 mV overpotential, respectively.However, the overpotential of commercial Pt/C reaches np-NiCoFeMoMn catalyst.Commercial Pt/C catalyst may have high overpotential due to the addition of Nafion.The electrochemical impedance spectroscopy (EIS) was used to further analyze the reason why HER kinetics was greatly improved.The EIS curves of np-NiCoFeMoMn HEA 6h with different dealloying time and commercial Pt/C electrode are shown in Fig. S9 (Supporting Information).Fig. S10 shows the equivalent alternative circuits of the electrodes.The semicircle in the curve represents the charge transfer resistance, and the np-NiCoFeMoMn 6h electrode displays a smallest semicircle that represent a lowest charge transfer resistance (Rct is only 22.86 Ohm), which is far lower than NiCoFeMoMn 0h (292.8Ohm).Fig. S11 shows the EIS curve of np-NiCoFeMoMn 6h, the Rct value decreases rapidly as the overpotential increases, which represents a rapid HER kinetics 5 .The double-layer capacitance (Cdl) reflects the electrochemical surface area (ECSA) of the catalyst.In Fig. S12, the double-layer capacitance curves of the nanoporous high entropy electrode with different dealloying time are tested by cyclic voltammogram (CV) measurements.The Cdl value of the catalyst can be calculated by the curve in Fig. 4c and Fig. S13 with the voltage of 0.15V (vs RHE) at different sweep rates, results reveal a larger Cdl of the np-NiCoFeMoMn 6h (261 mF cm -2 ), which is 70 times of NiCoFeMoMn 0h (3.71 mF cm -2 ).As further confirmed by the ECSA results in Fig.

Figure 6 .
Figure 6.Electrocatalytic over water splittling performances of the nanoporous alloy

Figures Figure 1
Figures