Nanoporous Nonprecious High‐Entropy Alloys as Multisite Electrocatalysts for Ampere‐Level Current‐Density Hydrogen Evolution

Developing robust nonprecious metal‐based electrocatalysts toward hydrogen evolution reaction is crucial for large‐scale hydrogen production via electrochemical water splitting. Herein, surface high‐entropy NiFeCoCuTi alloy on column‐nanostructured nanoporous Ni skeleton is reported as multisite electrocatalyst for highly efficient hydrogen evolution in nonacidic environments by making use of surface heterogeneous atoms with distinct hydrogen and hydroxyl adsorption behaviors to accelerate water dissociation and mediate adsorption of hydrogen intermediates for combination into molecules. Associated with the column‐nanostructured nanoporous Ni skeleton that facilitates electron transfer/mass transportation and enables highly accessible and abundant electroactive sites, self‐supported monolithic nanoporous high‐entropy NiFeCoCuTi alloy electrode exhibits superior nonacidic hydrogen evolution reaction (HER) electrocatalysis, with low onset overpotentials and Tafel slopes. It only takes overpotential of as low as ≈209 mV to deliver ultrahigh current density of 2 A cm−2, along with exceptional stability for more than 240 h, in 1 m KOH electrolyte. These outstanding properties make nanoporous NiFeCoCuTi high‐entropy alloy (HEA) electrode attractive candidate as cathode material in the water electrolysis for large‐scale hydrogen production and suggest HEAs as ideal platform to develop multisite electrocatalysts.


Introduction
[22] Furthermore, the use of Pt, even in various nanostructures such as commercially available carbon-supported Pt nanoparticles (Pt/C), always encounters cost and stability issues, essentially persecuting its practical application for large-scale hydrogen production. [10,11,16,23]In this regard, there has raised urgent demand for developing more efficient, cost-effective, and robust HER electrocatalytic materials as alternatives to Pt/C for industrial electrolyzers.][30][33][34][35] This is due to either improper electroactive sites with too strong DOI: 10.1002/sstr.202300042Developing robust nonprecious metal-based electrocatalysts toward hydrogen evolution reaction is crucial for large-scale hydrogen production via electrochemical water splitting.Herein, surface high-entropy NiFeCoCuTi alloy on column-nanostructured nanoporous Ni skeleton is reported as multisite electrocatalyst for highly efficient hydrogen evolution in nonacidic environments by making use of surface heterogeneous atoms with distinct hydrogen and hydroxyl adsorption behaviors to accelerate water dissociation and mediate adsorption of hydrogen intermediates for combination into molecules.Associated with the column-nanostructured nanoporous Ni skeleton that facilitates electron transfer/ mass transportation and enables highly accessible and abundant electroactive sites, self-supported monolithic nanoporous high-entropy NiFeCoCuTi alloy electrode exhibits superior nonacidic hydrogen evolution reaction (HER) electrocatalysis, with low onset overpotentials and Tafel slopes.It only takes overpotential of as low as %209 mV to deliver ultrahigh current density of 2 A cm À2 , along with exceptional stability for more than 240 h, in 1 M KOH electrolyte.These outstanding properties make nanoporous NiFeCoCuTi high-entropy alloy (HEA) electrode attractive candidate as cathode material in the water electrolysis for large-scale hydrogen production and suggest HEAs as ideal platform to develop multisite electrocatalysts.
][29][30]36] Therefore, it is highly desirable to design steady HEA electrocatalytic electrodes comprising multisite electroactive centers with near-optimal *H and *OH adsorption energies, in addition to a rational nanoarchitecture to enable sufficient accessibility of electroactive sites and facilitate electron transfer and mass transportation of electrolyte and gas molecules.
Here, we report self-supported monolithic hybrid electrode composed of surface NiFeCoCuTi HEA on columnnanostructured nanoporous Ni skeleton as a cost-effective and robust nonacidic HER electrocatalyst.Therein, surface multicomponent NiFeCoCuTi alloy serves as multisite electroactive center to accelerate water dissociation and mediate combination of *H into H 2 by virtue of distinct adsorption behaviors of *H and *OH on NiFeCoCu and Ti components.The specific activity of surface NiFeCoCuTi alloy is as high as %529 μA cm À2 at overpotential of 200 mV in 1 M KOH electrolyte, nearly one order of magnitude higher than that of NiFe alloy (%56 μA cm À2 ).Owing to the column-nanostructured nanoporous architecture that enlists electroactive NiFeCoCuTi sites to be highly accessible while facilitating electron transfer and mass transportation, the self-supported nanoporous high-entropy NiFeCoCuTi alloy exhibits superior alkaline HER electrocatalysis, with the low Tafel slope of %43 mV dec À1 and exceptional stability for 240 h, in 1 M KOH electrolyte.It reaches current density of 2 A cm À2 at a low overpotential of 209 mV.While in 1 M buffer electrolyte (pH = 6.9), it only takes the overpotential of as low as %159 mV to deliver 100 mA cm À2 .These impressive HER electrochemical properties make it promising candidate electrode material in nonacidic water electrolysis for large-scale hydrogen production.

Results and Discussion
The self-supported nanoporous HEA electrode is prepared by a facile and scalable alloying/dealloying strategy.Briefly, precursor alloy of quasi-eutectic-composition Ni 14 Fe 1.2 Co 1.2 Cu 1.2 Ti 2.4 Al 80 is mass-produced by arc-melting pure Ni, Fe, Co, Cu, Ti, and Al metals in atomic ratio of 14:1.2:1.2:1.2:2.4:80(Figure S1a, Supporting Information), followed by a procedure of meltspinning into alloy ribbons in an argon atmosphere.X-ray diffraction (XRD) characterization demonstrates that the precursor alloy of Ni  S2, Supporting Information).As illustrated by scanning transmission electron microscope energy-dispersive X-ray spectroscopy (STEM-EDS) elemental mapping images (Figure S3, Supporting Information), these intermetallic Al 13 Fe 4 , Al 9 Co 2 , and Al 2 Cu mix uniformly in column-nanostructured Al 3 Ni phase in addition to the phase-separated Al 3 Ti one from the α-Al phase.[39] Therein, the residual Ni, Fe, Co, Cu, and Ti atoms in situ form surface high-entropy NiFeCoCuTi alloy on Ni network. [40]As demonstrated by XRD patterns of nanoporous NiFeCoCuTi HEA electrode (Figure 1a), there displays one set of diffraction peaks at 2θ = 44.2°,50.8°, 75.7°, which are assigned to the (111), ( 200 200) and ( 220) intensity is ascribed to the reduction in crystal orderedness caused by heterogeneous atoms. [41]Figure 1b presents a typical scanning electron microscope (SEM) image of as-prepared nanoporous NiFeCoCuTi HEA electrode, displaying a hierarchical nanoporous structure consisting of %100 nm large channels and columns with ultrasmall nanopores (Figure 1c).Low-magnitude TEM image of nanoporous NiFeCoCuTi column illustrates the interconnective ligaments with characteristic length of %5 nm (Figure 1d), offering a specific surface area of as high as %44.3 m 2 g À1 , as demonstrated by the nitrogen adsorption/desorption isotherm (Figure S4, Supporting Information).Figure 1e  Figure 2 shows XPS spectra of nanoporous NiFeCoCuTi electrode, wherein Ni, Co, Cu, and Fe elements are primarily in metallic states in addition to their oxidized states caused by inevitable surface oxidation or adsorption of hydroxyl groups during dealloying process.[44] The shifts of Ni 0 2p peak in nanoporous NiFeCoCuTi, NiFeCoCu, NiFeCo, and NiFe electrodes relative to that in nanoporous Ni reflect the different influence of Fe, Co, Cu, and Ti on the chemical state of Ni, respectively (Figure 2b).Evidently, the incorporation of Ti enlists the Ni 0 2p peak of nanoporous NiFeCoCuTi to shift to a lower binding energy compared with nanoporous NiFeCoCu.Similar phenomena are also observed in Co 2p (Figure 2c) and Fe 2p (Figure 2d) except for Cu 2p (Figure 2e).As shown in Figure 2c, the Co 2p XPS spectrum of nanoporous NiFeCoCuTi electrode can be indexed to Co 0 , Co 2þ , and Co 3þ at 777.9/793.3,779.7/795.4,and 781.6/797.4eV, respectively, [37,42] which shift toward lower binding energies compared with the ones of NiFeCoCu electrode.As for the Fe 2p XPS spectrum of NiFeCoCuTi electrode (Figure 2d), it can be deconvoluted into two pairs of Fe 0 and Fe 2þ at 705.7/719.5 and 709.6/723.4eV, respectively. [37,45]Because of the strong electronic interaction, the Fe 0 2p peaks of NiFeCoCuTi electrode show negative shifts relative to those of NiFeCoCu.Cu features metallic species without evident satellite peaks (Figure 2e), [46,47] and the characteristic peaks of Cu 2p spectra in nanoporous NiFeCoCuTi and NiFeCoCu electrode locate at the same binding energy.These observations are due to electron transfer from Ti to Ni, Co and Fe in NiFeCoCuTi, different from Fe and Co in NiFeCoCu, where they undergo electron transfer to Ni (Figure 2b).Owing to electron transfer from Ti to other components in NiFeCoCuTi electrode, the surface Ti atoms are primarily in oxidized states, with the characteristic peaks of Ti 2p XPS spectrum at the binding energies of 458.7 and 464.4 eV belonging to Ti 2þ 2p 3/2 and Ti 2þ 2p 1/2 (Figure 2f ). [47]These modulate near-optimal adsorption energies of *H and *OH on surface NiFeCoCuTi alloy for distinct adsorption behaviors for highly efficient water dissociation as well as adsorption of *H intermediates and their combination into molecules.
To evaluate electrocatalytic properties, all self-supported nanoporous electrocatalysts are directly employed as the working electrodes for electrochemical measurements in a classic threeelectrode configuration, where a graphite rod and an Ag/AgCl electrode are used as the counter electrode and the reference electrode, respectively.All potentials are iR-corrected and normalized with respect to the reversible hydrogen electrode (RHE) (Figure S7, Supporting Information).Figure 3a   Nafion as polymer binder (Pt/C/Ni), in Ar-saturated 1 M KOH electrolyte.Because of the enhanced electrocatalysis of surface NiFeCoCuTi HEA, the nanoporous NiFeCoCuTi electrode delivers a current density of 1 A cm À2 at the overpotential of %134 mV, in sharp contrast with nanoporous NiFeCoCu (%0.11A cm À2 ), NiFeCo (%0.10A cm À2 ), NiFe (%0.057A cm À2 ), and Ni (0.014 A cm À2 ) electrodes.Despite commercially available Pt/C is a benchmark electrocatalyst for acidic HER, nanoporous Pt/C/Ni electrode only exhibits the current density of %0.43A cm À2 , a value %2.3-fold lower than that of nanoporous NiFeCoCuTi.This is probably caused by insufficient activity of monometallic Pt for the multistep electrocatalytic processes of alkaline HER, in addition to insulative polymer binder preventing electron transfer and burying some Pt active sites.In contrast, the nanoporous NiFeCoCuTi electrode provides surface HEA alloy as multisite electroactive sites with distinct adsorption behaviors of *H and *OH to accelerate water dissociation and mediate simultaneously adsorption of *H intermediates for their combination into H 2 .Moreover, the column-nanostructured hierarchical nanoporous Ni network as current collector facilitates the electron transfer and mass transportation of H 2 O/OH À along their interconnective Ni ligaments and interpenetrative channels, enabling sufficient accessibility of surface NiFeCoCuTi HEA electroactive sites.Seven specimens of nanoporous NiFeCoCuTi that are prepared by the same alloying/dealloying procedures exhibit almost overlapping polarization curves (Figure S8, Supporting Information), demonstrating the excellent reproducibility.The remarkably enhanced kinetics of alkaline HER electrocatalysis of nanoporous NiFeCoCuTi electrode is attested by the low Tafel slope of %43 mV dec À1 (Figure 3b), the smallest value among the investigated electrodes including nanoporous NiFeCoCu (%90 mV dec À1 ), NiFeCo (%100 mV dec À1 ), NiFe (%101 mV dec À1 ), and Ni (%116 mV dec À1 ), as well as Pt/C/Ni (%44 mV dec À1 ).This is also demonstrated by electrochemical impedance spectroscopy (EIS) analysis of nanoporous electrodes.As shown in the Nyquist plot (Figure 3c), the EIS spectrum of nanoporous NiFeCoCuTi electrode exhibits two characteristic semicircles in middle-to low-frequency ranges, which correspond to the charge transfer resistance (R CT ) and the pore resistance (R P ) in parallel with the constant phase elements (CPEs), while the intercept on the real axis refers to the intrinsic resistance of electrode and electrolyte (R I ). [48,49]According to the equivalent circuit with these descriptors (inset of Figure 3c), the R CT and R P values of nanoporous NiFeCoCuTi alloy electrode are assessed to be %2.5 and %2.5 Ω, respectively, much smaller than those of nanoporous NiFeCoCu (%15.6, %2.6 Ω), NiFeCo (%21.7,%3.0 Ω), NiFe (%48.8, %4.5 Ω), and Ni (%73.8, %32.2 Ω) (Figure S9, Supporting Information).
To further reveal the role of each element in the HEA electrode, Figure 3d compares the polarization curve of nanoporous NiFeCoCuTi electrode with those of nanoporous NiFeCoTi, NiFeCuTi, NiCoCuTi, and NiFeCoCu electrodes (Figure S5f-h S1f-h, Supporting Information), respectively.The nanoporous NiFeCoCuTi electrode reaches ultrahigh current density of 2 A cm À2 at the overpotential of %209 mV, outperforming nanoporous NiFeCoTi (%1.29 A cm À2 ), NiFeCuTi (%1.09A cm À2 ), NiCoCuTi (%0.79A cm À2 ), and NiFeCoCu (%0.57A cm À2 ).According to the electrochemical surface areas (ECSAs) of nanoporous electrodes assessed by their double-layer capacitances in a non-Faradaic voltage window (Figure S11 and S12, Supporting Information), the specific activity of high-entropy NiFeCoCuTi alloy is calculated to be as high as %529 μA cm À2 at the overpotential of 200 mV, %9.4-, %5.6-, %3.7-, %2.2-, and %2.0-fold higher than those of NiFe (%56 μA cm À2 ), NiFeCoCu (%94 μA cm À2 ), NiCoCuTi (%144 μA cm À2 ), NiFeCuTi (%237 μA cm À2 ), and NiFeCoTi (%268 μA cm À2 ), respectively (Figure 3e).Evidently, the presence of Ti atoms is crucial to improve electrocatalytic HER activity because of its proper adsorption of *OH, which enables the formation of the multisite electrocatalytic center associated with surface alloy of NiFeCoCu with near-optimal adsorption energy of *H.The superior intrinsic activity of high-entropy NiFeCoCuTi alloy is also reflected by the turnover frequency (TOF) of as high as %0.81 s À1 at the overpotential of 200 mV, %32-fold higher than that of bare Ni (Figure S13, Supporting Information).Figure 3f shows the electrochemical durability of nanoporous NiFeCoCuTi electrode, which is performed at À80 mV versus RHE for 240 h in 1 M KOH electrolyte.Obviously, there is not evident attenuation at the corresponding current density of %260 mA cm À2 due to exceptional stability of nanoporous electrode structure.As illustrated by SEM image of nanoporous after long-term durability test (inset of Figure 3f ), it always retains initial nanoporous structure for 240 h.Furthermore, there is only a trace amount of Ni, Fe, Ti, and Al ions to be detected by ICP-OES in the electrolyte after the durability test (Table S1, Supporting Information).Compared with state-of-the-art HER electrocatalysts reported previously, including HEAs and metallic compounds, the nanoporous NiFeCoCuTi electrode has the lowest values of the Tafel slope and the overpotential at 100 mA cm À2 (Figure 3g; Table S2, Supporting Information), [21,27,28,31,[50][51][52] demonstrating the outstanding electrocatalysis for alkaline HER.
Owing to the multisite electrocatalytic centers of surface HEA alloy, nanoporous NiFeCoCuTi electrode also exhibits exceptional electrocatalysis for hydrogen evolution in a neutral electrolyte.As shown in Figure 4a, the nanoporous NiFeCoCuTi electrode achieves the current density of 100 mA cm À2 at the overpotential of %159 mV in 1 M PBS electrolyte, outperforming nanoporous NiFeCoCu (37 mA cm À2 ), NiFeCo (26 mA cm À2 ), NiFe (17 mA cm À2 ), and Ni (10 mA cm À2 ) electrodes, as well as Pt/C/Ni (65 mA cm À2 ) (Figure 4b).Its fast reaction kinetics is also manifested by the lowest Tafel slope of %50 mV dec À1 , compared with those of nanoporous NiFeCoCu (150 mV dec À1 ), NiFeCo (154 mV dec À1 ), NiFe (155 mV dec À1 ), and Ni (159 mV dec À1 ) electrodes (Figure 4c).Compared with the neutral electrocatalysts recently reported (Figure 4d; Table S3, Supporting Information), [53,54] the nanoporous NiFeCoCuTi electrode has the lowest values in both the overpotentials at 50 mA cm À2 and the Tafel slopes.Furthermore, it can deliver stable current density of %50 mA cm À2 for 40 h (Figure S14, Supporting Information), signifying good electrochemical durability for applicability in neutral medium.Along with these outstanding electrochemical properties in both alkaline and neutral electrolytes, the nanoporous NiFeCoCuTi electrode satisfies the industrial requirements of hydrogen production and holds genuine potential as robust and cost-effective cathodic electrocatalytic material in electrochemical nonacidic water splitting.

Conclusion
In summary, we have developed self-supported hierarchical nanoporous high-entropy NiFeCoCuTi alloy electrocatalysts for ampere-level current-density hydrogen evolution.In view of the surface high-entropy NiFeCoCuTi alloy that in situ forms on hierarchical nanoporous Ni skeleton, nanoporous NiFeCoCuTi electrode enables highly accessible multisite electroactive centers to accelerate water dissociation and mediate adsorption of *H intermediates for their combination into H 2 by making use of distinct adsorption behaviors of *H and *OH on NiFeCoCu and Ti.Associated with columnnanostructured nanoporous architecture to expose abundant electroactive sites and facilitate electron transfer and H 2 O/ HO À transport, the self-supported NiFeCoCuTi electrode delivers current densities of 2000 and 100 mA cm À2 at low overpotentials of %209 and %159 mV, along with the small Tafel slopes of %43 and %50 mV dec À1 , in 1 M KOH and 1 M PBS electrolyte, respectively.Moreover, it exhibits impressive durability at as high as %260 mA cm À2 for more than 240 h in alkaline medium due to steady nanoporous electrode structure and thermodynamically stable HEA covered on the surface.These outstanding properties make nanoporous NiFeCoCuTi electrode as a promising electrode candidate for industrial hydrogen production.(at%) were first made by arc melting pure Ni and Al metals with/without the addition of Fe, Co, Cu, or/and Ti under the protection of Ar atmosphere and cooled to room temperature in the water cycle-assisted furnace.Their ribbons with thickness of %80 μm were further produced by melt-spinning method and then chemically dealloyed in a N 2 -purged 6 M KOH electrolyte at 70 °C until there does not generate any bubbles.These as-dealloyed specimens were rinsed thoroughly in ultrapure water (18 MΩ) to remove residual chemical substance in nanopores and directly employed as self-supported electrodes for physicochemical characterizations and electrochemical measurements.Nanoporous Ni-supported Pt/C electrode was prepared by mixing commercially available Pt/C nanocatalyst (20 wt%, Johnson Matthey) and Nafion (0.05 wt%, Sigma-Aldrich) in the solution containing isopropanol (20%) and water (80%) and casting onto nanoporous Ni current collector.

Experimental Section
Physicochemical Characterizations: A field-emission scanning electron microscope (JSM-7900 F, 5 kV) equipped with X-ray energy-dispersive spectroscopy (EDS) was used to characterize the microstructure and chemical composition features of precursor alloys and nanoporous electrocatalysts.HRTEM images were obtained by a field-emission transmission electron microscope (JEOL JEM-2100F, 200 keV) equipped with EDS.XRD measurements were performed on a Rigaku Smartlab diffractometer at a power of 9 kW with a monochromated Cu Kα radiation.Surface chemical states of nanoporous electrocatalysts were analyzed by XPS on a Thermo ECSALAB 250 with an Al anode, where the charging effect was corrected according to the binding energy of C 1s peak (284.8 eV).The metal ion concentrations were detected by ICP-OES (Thermo electron) analysis.Nitrogen adsorption/desorption isotherms were measured at 77 K with a Micromeritics ASAP 2020 analyzer (Micromeritics Instrument Corporation).Before analysis, specimens were degassed under vacuum (10 À3 Torr) at 120 °C for 4 h.Specific surface areas of nanoporous electrodes were calculated by the Brunauer-Emmett-Teller (BET) method using the adsorption branch.The pore size distribution was evaluated by density functional theoretic (DFT) models.
Electrochemical Characterizations: All electrochemical measurements were carried out in a classic three-electrode configuration, where selfsupported nanoporous electrodes were directly used as the working electrodes, a graphite rod as the counter electrode, and an Ag/AgCl electrode as the reference electrode.The polarization curves of these electrocatalysts were conducted at a scan rate of 0.5 mV s À1 in Ar-saturated 1 M KOH (pH = 13.8) and 1 M PBS (pH = 6.9) electrolyte.All potentials are iR-corrected and converted to RHE by the equation E RHE = E Ag/ AgCl þ 1.016 V -iR in 1 M KOH electrolyte and E RHE = E Ag/AgCl þ 0.607 V -iR in 1 M PBS electrolyte.Electrochemical impedance spectroscopy analysis was performed at the overpotential of 100 mV in frequency range from 10 mHz to 100 kHz with 5 mV amplitude.In order to assess the electrochemical surface area (ECSA) of these nanoporous electrocatalysts, cyclic voltammetry curves were tested from À0.85 to À0.75 V (vs Ag/ AgCl) at various scan rates to obtain their double-layer capacitance (C dl ) values.The HER stability test of nanoporous NiFeCoCuTi electrode was conducted at the overpotential of 80 mV in 1 M KOH electrolyte for 240 h.
), (220) planes of face-centered cubic (FCC) Ni (JCPDS 04-0850).In surface NiFeCoCuTi HEA, Ti and Cu elements have larger atomic radiuses than Ni; while for Fe and Co, their atomic radiuses are similar to that of Ni.As a result, Fe, Co, Ti, and Cu partially substituting Ni sites to form surface HEA lead to the lattice expansion, which gives rise to the shift of diffraction peaks of NiFeCoCuTi electrode toward lower angles compared with the characteristic ones of nanoporous Ni electrode.The decrease in the ( shows a high-resolution TEM (HRTEM) image of nanoporous NiFeCoCuTi electrode, of which the crystallographic structure is identified to be the same as that of Ni core by the fast Fourier transform (FFT) pattern.Viewed along the [011] zone axis, the interplanar spacing of %0.208 nm corresponds to the lattice plane of high-entropy NiFeCoCuTi(111), deviating from the value of Ni(111) (0.203 nm) due to the random substitution of Ni atoms by Fe, Co, Cu, and Ti heterogeneous atoms.X-ray photoelectron spectroscopy (XPS) survey demonstrates the presence of Fe, Co, Cu, and Ti on Ni skeleton, with the atomic ratio of 1:1:1:1 determined by EDS (Figure S5a, Supporting Information) and inductively coupled plasma optical emission spectroscopy (ICP-OES).Figure 1f shows STEM image and the corresponding EDS elemental mapping of Fe, Co, Cu, and Ti, illustrating their uniform distributions along the Ni skeleton via the formation of high-entropy NiFeCoCuTi surface alloy.Such unique hierarchical nanoporous architecture not only facilitates electron transfer via interconnected Ni skeleton but also ensures sufficient accessibility of electroactive sites of high-entropy NiFeCoCuTi alloy.In order to reveal high-entropy effect, nanoporous NiFeCoCu (Figure S5b,S6a, Supporting Information), NiFeCo (Figure S5c,S6b, Supporting Information) and NiFe (Figure S5d,S6c, Supporting Information) alloys with less components are also fabricated by the same procedure from their corresponding precursor alloys of Ni 16.4 Fe 1.2 Co 1.2 Cu 1.2 Al 80 , Ni 17.6 Fe 1.2 Co 1.2 Al 80 , Ni 18.8 Fe 1.2 Al 80 , and Ni 20 Al 80 , respectively (Figure S1b-e, Supporting Information).
compares the HER polarization curve of nanoporous NiFeCoCuTi electrode with those of nanoporous NiFeCoCu, NiFeCo, NiFe, and bare Ni electrodes, as well as that of commercially available Pt/C catalysts immobilized on nanoporous Ni current collector by dint of

Figure 1 .
Figure 1.Microstructure properties of nanoporous NiFeCoCuTi electrodes.a) XRD patterns of nanoporous NiFeCoCuTi, NiFeCoCu, NiFeCo, NiFe, and bare Ni electrodes.The line patterns show reference cards 04-0850 for Ni according to JCPDS.b,c) Representative low-(b) and high-magnification SEM images (c) of column-structured nanoporous high-entropy NiFeCoCuTi electrode that is fabricated by chemically dealloying Ni 14 Fe 1.2 Co 1.2 Cu 1.2 Ti 2.4 Al 80 precursor alloy in 6 M KOH solution.d) TEM image of nanoporous structure of high-entropy NiFeCoCuTi alloy.e) HRTEM image of nanoporous high-entropy NiFeCoCuTi alloy.Inset: FFT pattern of selected area.f ) STEM and the corresponding EDS elemental mapping images of Ni, Fe, Co, Cu, and Ti in nanoporous NiFeCoCuTi electrode, where Ni is in pink, Fe in light magenta, Co in orange, Cu in cyan, and Ti in blue, respectively.

Figure 3 .
Figure 3. Electrochemical properties of nanoporous HEAs for alkaline HER.a) Typical HER polarization curves for nanoporous NiFeCoCuTi, NiFeCoCu, NiFeCo, NiFe, and bare Ni electrodes, as well as commercial Pt/C nanocatalysts immobilized on nanoporous Ni (Pt/C/Ni).Electrolyte: 1 M KOH solution.Scan rate: 0.5 mV s À1 .b) The corresponding Tafel plots for these electrocatalysts obtained from their polarization curves in (a).c) EIS spectra of nanoporous NiFeCoCuTi, NiFeCoCu, NiFeCo, NiFe, and bare Ni electrodes.Inset: The electrical equivalent circuit, where R I and R CT denote the intrinsic electrode and electrolyte resistance and the charge transfer resistance, respectively, CPE represents the constant phase element, and R P is the resistance of nanopores.d) HER polarization curves of Ti-contained nanoporous high-entropy NiFeCoCuTi, NiFeCoTi, NiFeCuTi, NiCoCuTi electrodes, compared with the one without Ti component, i.e., nanoporous high-entropy NiFeCoCu.Scan rate: 0.5 mV s À1 .e) Comparison of specific activities for nanoporous NiFeCoCuTi, NiFeCoTi, NiFeCuTi, NiCoCuTi, NiFeCoCu, NiFeCo, NiFe alloys, and bare Ni sites at the overpotential of 200 mV.f ) Long-term stability of nanoporous high-entropy NiFeCoCuTi electrode at overpotential of 80 mV for 240 h in 1 M KOH electrolyte.g) Overpotential at 100 mA cm À2 and Tafel slope of nanoporous high-entropy NiFeCoCuTi electrode, comparing with the values of representative alkaline HER electrocatalysts reported previously.

Figure 4 .
Figure 4. Electrochemical characterization of nanoporous HEAs for neutral HER.a) Polarization curves for nanoporous NiFeCoCuTi, NiFeCoCu, NiFeCo, NiFe, bare Ni, and Pt/C/Ni electrodes.Electrolyte: 1 M PBS electrolyte.Scan rate: 0.5 mV s À1 .b) Comparison of current densities of nanoporous electrodes at the overpotential of 159 mV.c) The Tafel plots of nanoporous electrodes obtained from the polarization curves in (a).d) Tafel slope and overpotential at 50 mA cm À2 of nanoporous NiFeCoCuTi electrode, comparing with the values of neutral HER electrocatalysts reported previously.