Structure and electrocatalytical properties of electrodeposited M-Ir (M=Co, Ni) bimetallic alloy catalysts with low Ir loading obtained on copper foams for hydrogen evolution reaction

M-iridium (M = cobalt (Co), nickel (Ni)) bimetallic alloy catalysts with low iridium (Ir) loading of 0.3 ~ 2.0 mg·cm − 2 were prepared on copper foam (CF) supports by electrodeposition. The top surface of as-deposited M-Ir catalysts was mainly composed of metallic state and oxides states, such as metallic Ir, Ni(OH) 2 or Co(OH) 2 , Co(Ir) and Ni(Ir) solid solution, Ir oxides. M-Ir catalysts with low Ir loading exhibited excellent catalytic performance. Ni 67.4 Ir 32.6 /CF catalyst with low Ir loading of 2.0 mg·cm − 2 achieved a current density of 10 mA·cm 2 at an overpotential of 52 mV and a Tafel slope of 36 mV·dec − 1 . Co 64.2 Ir 35.8 /CF catalyst with low Ir loading of 0.7 mg·cm − 2 was uniformly scattered with small ellipsoidal particles, looking like �ne �uff, requiring an overpotential of 51 mV for hydrogen evolution reaction to reach a current density of 10 mA·cm − 2 , having a Tafel slope of 38 mV·dec − 1 . After long-term hydrogen evolution testing, M-Ir/CF catalysts exhibited excellent electrocatalytic stability for water splitting in alkaline solution.


Introduction
Developing safety and clean energy is highly signi cant for alleviating the dependence on the traditional fossil fuels and associated energy crisis [1,2].
Hydrogen production for water splitting, however, is hindered by the slow reaction kinetics due to multiple electron transfer [3,4].To initiate electrochemical water splitting, a theoretical thermodynamic potential of 1.23 V is required [5].In order to boost water splitting to produce hydrogen, advanced electrocatalysts are essential [6,7].Nanomaterials from the Pt-group metals, such as Pt, Ir, and their compounds, remain the benchmark electrocatalysts for hydrogen evolution reaction (HER) [8][9][10].
Iridium (Ir) has signi cant catalytic activity for HER [11][12][13].There is a prospective approach for coupling noble metals with ion-group metals to form bimetallic alloy catalysts, in order to reduce price and boost the availability of noble metals [14].Ir-doped electrocatalysts (i.e., changing the composition of catalysts) is put forward to enhance the utilization e ciency, and to improve electrocatalytic activity because of the synergistic effect and the modi ed electronic structure [15].According to the calculation of Density Functional Theory (DFT), Ir doping in ion-group metals such as Ni, Co can reduce the free energy of H * species (ΔG H * ) to optimal intensity, resulting in a fast rate of catalysts to adsorb and desorb H 2 , thus exhibiting the superior HER activity [16][17][18].
Up to date, Ir-based nanomaterials including Ir, Ir alloys and oxides as well as other Ir-containing compounds have been extensively applied for electrochemical HER [19], which have been synthesized through various techniques, such as electrodeposition [20], hydrothermal reduction [21], and pyrolysis [22].Electrodeposition is a convenient and exible process that is used to deposit metal and alloy catalysts [23][24][25], which can offer signi cant low-cost, reliability and environmental bene ts [26,27].Papaderakis et al. [28] synthesized one Ir-Ni layer with a spontaneous electrical displacement reaction, consisting of Ir shell and Ir-Ni core.Compared with Ir, the electrocatalytic activity of Ir-Ni electrocatalyst was signi cantly improved.Kim et al. [29] reported the Ir-Co-W alloy nanoparticles through colloidal synthesis exhibited the low overpotential of 35.82 mV to drive the current density of 10 mA•cm − 2 and the highest mass activity of 3.98 A/mg Ir with super-low loading of 40 µg•cm − 2 .Zhang et al. [30] synthesized a series of Ir-Fe/nitrogen-doped carbon nanotubes (Ir-Fe/NCNT) electrocatalysts with low Ir-content of 2 ~ 6 wt%.Ir-Fe/NCNT catalysts had excellent catalytic activity in both acidic and alkaline media.Ma et al. [31] prepared the self-supported Fe-Ir alloy nanoparticles by a solid-state synthesis method, which exhibited superior HER performance with an overpotential of 19 mV at a current density of 10 mV•cm − 2 and had a Tafel slope of 32 mV•dec − 1 , superior to pure Ir and Pt/C catalysts.
In the previous works [32][33][34][35], there is a high loading-level (about 4.03 mg•cm − 2 ) of Ir in M-Ir (M = Ni and Co) bimetallic alloy catalysts by electrodeposition that have been studied and reported.Co-Ir/Cu foam (Co-Ir/CF) catalysts with high Ir loading of 4.0 mg•cm − 2 were electrodeposited for water splitting in an alkaline medium, which required an overpotential of 108 mV for HER to reach a current density of 30 mA•cm − 2 , having a low Tafel slope of 36 mV•dec − 1 [35].
Ni 19 Ir 81 /CF catalysts with Ir loading of about 4.03 mg•cm − 2 achieved a current density of 10 mA•cm − 2 at an overpotential of 60 mV and a Tafel slope of 40 mV•dec − 1 [34].In this study, the bimetallic alloy electrocatalysts with low Ir loading were prepared to reduce the overpotential of hydrogen evolution reaction and to produce high-performance alkaline catalysts at a reduced cost, which is suitable for Ir applications.M-Ir bimetallic alloy catalysts with low Ir loading are electrodeposited onto low-cost porous CF supports.The structure, phase identi cation and chemical composition of M-Ir/CF bimetallic alloy catalysts with different Ir loadings are studied.At the same time, the electrocatalytic properties and long-term stability of M-Ir/CF bimetallic alloy catalysts are investigated.

Experimental
The bath chemistry for Ni-Ir catalysts consisted of trisodium hexabromoiridate (     Note: T = deposition temperature, t = deposition time, j = current density, ΔW = weight gain. The crystal structure and phase of M-Ir bimetallic alloy catalysts were determined by an X-ray diffractometer (XRD, Scintag, USA) equipped with an area detector and Cu-Kα radiation source (λ = 0.154 nm).The microstructure and morphology of the catalysts were observed by scanning electron microscopy (SEM, Supra55 Zeiss Sigma, DEU).The chemical composition of the catalyst was measured at three positions by an energy spectrometer (EDS, X-act, UK).By X-ray photoelectron spectrometer (XPS, ThermoFisherScienti cEscalab250Xi, USA), the chemical composition and elemental state of the top-surface of Co-Ir/CF and Ni-Ir/CF bimetallic alloy catalysts were tested under the vacuum condition of 3.3×10 − 8 Pa.The top-surfaces of M-Ir/CF bimetallic alloy catalysts were irradiated with 1486.6eVAl-Kα monochromatic source, and the resulting electrons were analyzed by a spherical capacitor analyzer using a 0.8 mm slit aperture.The C-1s carbon signal at 285 eV is used as the energy reference for the measured peak.To identify the elements on the top-surface of catalysts, a low-resolution measured energy spectrum was obtained in a wide energy range of 0-350 eV, with a passing energy of 150.0 eV and an increment of 1.0 eV•step − 1 .In increments of 0.05 eV•step − 1 , a high-resolution energy spectrum was obtained with an energizing energy of 30.0 eV, to determine the position and shape of the peak.XPS PEAK software was used to t the curves by the Gauss-Lorentz function.The two tting parameters-the positions of the peak and its full width at half maximum (FWHM) were xed in a range of less than ± 0.2 eV.
The electrochemical performance of the bimetallic alloy catalysts was characterized in 1.0 M potassium hydroxide (KOH) solution using an electrochemical workstation at room temperature.The M-Ir/CF electrode served as the working electrode.The graphite rod and Hg/Hg 2 Cl 2 (SCE) electrode were used as the counter and reference electrodes, respectively.3 Results and discussion

Characterization
The SEM images of the surface of M-Ir/CF bimetallic alloy catalysts are shown in Fig. 1.In Fig. 1(a), some small cracks appear, and the particles with the size less than 200 nm are uniformly and densely covered on the CF support.The formation of the microcrack is attributed to the evolution of hydrogen bubbles during deposition [37].In Figs.1(b) and 1(c), Ni-Ir/CF catalyst is visible on the surface, the grain boundaries of the CF are clearly visible, and the ne nanosized particles do not completely cover on the surface of CF support.With the increase of deposition time, the thickness of the catalyst also increases.The Ni-Ir catalyst is attached to the surface of CF support.The surface of Co-Ir/CF catalyst appears a lot of small white particles and there are some ne microcracks with nano scale, as shown in Fig. 1(d).As shown in Fig. 1(e), the surface of sample #5 was uniformly scattered with small ellipsoidal particles with the diameter of 0.5 ~ 1.5 µm, looking like ne uffs on the top-surface.This morphological characterization of the catalyst can provide more exposed active sites to facilitate the catalytic reaction, which is favorable for HER performance.In Fig. 1(f), no specially shaped alloy particles were observed, the catalyst was uniform and tight, with the absence of obvious grain boundaries of Cu substrate, indicating that the catalyst coverage was relatively dense, in addition to the presence of some small microcracks.The rough surface of Co-Ir catalyst would be bene cial to the catalytic reaction by providing more exposed active sites, which is advantageous for the electrocatalytic activity in HER.The chemical composition of M-Ir/CF catalysts is shown in Table 2.The atomic content of Ir in M-Ir/CF catalysts ranges from 31.6-39.6%.As the deposition time continuously increased in the same bath, the proportion of Ir in the bimetallic alloy catalysts decreased gradually, which was attributed to the consumption of Ir species in the bath.4. Figure 3 shows the XPS curves of the top-surface of M-Ir/CF catalysts.The Br signal is derived from the hexabromoiridate chemical, which is present in low amount.The high contents of C and O elements are attributed to ordinary adsorption from the environment and oxides or hydroxides (see Table 4), signi cant amounts of oxides formed on the surface of the working electrode during electrodeposition.Unfortunately, the signal Ni is spurious, but the Ni content is proved to be abundant by EDS pattern, so the possible reasons for the messy peak signal are: the surface of CF support is not smooth, and there are some uneven local areas on the surface, and the distance between the surface of these areas and the reading device of the instrument may vary greatly, some electrons cannot be captured accurately, which leads to messy [45,46].The characteristic peak of Ir shifts in the direction of low binding energy, leading to the downshifting of the -d-band [49].The -d band center is directly related to the nature of adsorption in the catalytic process, the downshifting of the -d-band weakens the H adsorption strength, reducing the strong binding energy of Ni to H * (ΔG H* <0), accelerating the desorption process of the catalyst and increasing the catalytic activity [50,51].
In the Co-2p spectrum (see Fig.  To determine the charge delivery characteristics of Ir/CF, Ni/CF, and M-Ir/CF catalysts can be obtained by calculating the CV curves (Fig. 5).The anode charge transfer capacity Q a is determined by the following Eq.( 2) [62].

Electrochemical behavior and HER performance
2 where E is the electrode potential from − 0.  5.An increased speci c surface increases the injected charge.Therefore, the real active surface area needs to be further calculated and studied.
The determination of the real active surface area is challenging for many systems because it often depends on the applied potential, electrolyte, and the nature of the reaction involved, making it somewhat too complicated to estimate.Hence, electrochemically active surface area (ECSA), is often used.The ECSA is a good way to quantify active-sites of electrocatalysts.Herein, double-layer capacitances (C dl ) are alternatively measured as they are proportional to the ECSA [63].Via plotting the current density variation in cyclic voltammograms (CVs, Fig. S1) versus scanning rates, the C dl values can be estimated by the slopes of the tting lines (Fig. 6).The Ir/CF presents a C dl of 24.59 mF•cm − 2  The ECSA value can be estimated by the following Eq.( 3) [64]: where C s is the speci c capacitance, for a at electrode ranged from 20 to 60 µF•cm − 2 [65], and a moderated value of 40 µF•cm − 2 is herein adopted to calculate the ECSA values and make clear comparison.Table 5 shows the electrochemical active surface area, ECSA value of the catalysts.
where E corrected , E measured , I and R s represent the iR-compensated voltage, the measured voltage, the current owing in the system, and the solution resistance, respectively.
To clarify the kinetics of the HER, a Tafel slope of the electrocatalyst is performed (Fig. 7     Hydrogen evolution performance and stability of the M-Ir catalysts with low Ir loading are not inferior to the catalysts with high Ir loading in our previous works [32][33][34][35].Therefore, the M-Ir/CF catalysts with low Ir loading could maintain good catalytic performance while being cost-effective. Na 3 [Ir(III)Br 6 ]), and nickel sulphate hexahydrate (NiSO 4 •6H 2 O).The bath chemistry for Co-Ir catalysts consisted of Na 3 [Ir(III)Br 6 ], cobalt sulfate heptahydrate (CoSO 4 •7H 2 O).All chemical reagents were analytical grade.The electrolyte was purged by nitrogen (N 2 ) for 10 min before the deposition process.An N 2 blanket was passed over the solution during deposition.In all experiments, a magnetic stir bar was used to stir the solution.A conventional three-electrode cell was used to electrodeposit bimetallic alloy catalysts on the working electrode.The three electrodes consisted of a CF (10mm × 10mm × 3mm, porosity > 98%, Shenzhen Win y New Material Co., LTD., Shenzhen, CN) as the working electrode, platinum foil as the counter-electrode, and a reference Ag/AgCl 3M KCl electrode.The galvanostatic electrodeposition and electrochemical process were carried out in the cell, using an electrochemical workstation (CHI 660E, Chenhua Instrument, Shanghai, CN) coupled to a computer with speci c data acquisition software installed.The solution pH was analyzed by a pH meter (PHS-3C, INESA instrument, Shanghai, CN) at 70 o C and adjusted to the desired value by adding 5.0 M NaOH while being stirred.The volume of the electrolyte in the cell was ~ 15 mL.The bath temperature was controlled by a thermostatic bath (HH-501, JintanBaitaXinbao Instrument Factory, Changzhou, CN).The mass change was recorded by an analytical balance (FA2004B, resolution 0.1 mg, YoKe Instrument, Shanghai, CN).The bath chemistry and the process parameters for the catalysts are shown in

The 1 . 0 M
KOH electrolyte was purged by N 2 gas for 4 min before electrochemical measurements.Voltammetric curves were recorded in a narrow potential range (a few tens of mV) at different sweep rates, which was performed in a small potential window (normally ± 50 mV around the open-circuit potential (OCP)) in which non-Faradaic processes occurred.The current in the middle of the potential range was then plotted as a function of the sweep rate.The linear sweep voltammograms (LSV) of the electrodes were tested in alkaline solution at a scanning rate of 5 mV•s − 1 using a three-electrode system.Cyclic voltammetry (CV) measurements were performed at a scanning rate of 10 mV•s − 1 .The electrocatalytic stability and durability of fresh M-Ir/CF catalysts were determined by LSV testing from − 1.8 V to -1.0 V vs. SCE at a scanning rate of 10 mV•s − 1 for 400 times, and the long-term stability of the M-Ir/CF catalyst was assessed by chronopotentiometric testing in 1.0 M KOH solution at a constant current density of 10 mA•cm − 2 for 36,000 s.The measured currents were normalized to the geometric surface area of the electrodes and converted to the current density (j).Furthermore, the potential was converted to the reversible hydrogen electrode (RHE) potential scale.The potential (versus SCE) was converted into the potential of RHE value in 1.0 M KOH solution at 25 o C according to the following Nernst Eq. (1) [36]: E(vs.RHE) = E(vs.SCE) + 0.0592*pH + E SCE (0.241V)(1)

Figure 5
Figure 5 displays the electrochemical redox processes for CV curves of M-Ir/CF catalysts in the alkaline solution.In the oxidation region (Fig. 5(a)), Ni in Ni 61.8 Ir 38.2 /CF catalyst is oxidized to Ni(OH) 2 at a potential of 0 to 0.1 V [58].At the positive potential of 0.6 V, the oxidation peak I of Ni/CF catalyst is caused by the initial oxidation state of Cu (Cu(I)).The observed oxidation peak II is mainly attributed to Cu oxidation (Cu (I) and Cu (II)), but it is also possible that Ir (IV) is caused by the potential in the range of 0.63-0.72V[34].In the reduction region, the reduction peak III should be the reduction of Cu (II) → Cu(I) ions at the potential of 0.73-0.75V.Moreover, the reduction peak V is primarily caused by the combined reactions of Cu (II) to Cu and Cu(I) to Cu, with the peak at a potential range from 0.25 to 0.34 V[59].However, there are other reduction peaks IV for Ir/CF and Ni 61.8 Ir 38.2 /CF catalysts at the potential of 0.64 V, which is possible due to the reduction of Ir(IV) to Ir(III) species.Ir oxides are electrochemically formed at high positive potential on the surface of Ir/CF and Ni 61.8 Ir 38.2 /CF catalysts during a positive scanning direction, but the formation of Ir oxides cannot be reduced to the metal state [60].In Fig.5(b), Co patches for Co and Co 61.2 Ir 38.8 deposits are oxidized to Co(OH) 2 at the potential of 0-0.12 V in the oxidation region[61].At the positive potential of 0.6-0.8V, the oxidation peak I is caused by the initial oxidation state of Cu to Cu(I) and Cu (II).The reduction peak II corresponds to the reduction of Cu (II) to Cu(I) ions at a potential of 0.73-0.75Vduring reverse scanning direction.The reduction peak IV results from the two processes of Cu (II) to Cu, and Cu(I) to Cu[59].At the potential of 0.64 V, there are other reduction peaks IV for Ir/CF and Co 61.2 Ir 38.8 /CF catalysts, which is possible attributed to the reduction of Ir(IV) to Ir(III) species.A passive layer formed on a Co electrode in 1.0 M KOH solution can be reduced only during potential arrest below − 0.9 V vs. SCE[61].The potential of -0.9 V vs. SCE was converted into the potential of RHE value in 1.0 M KOH solution at room temperature according to the Eq.(1).Therefore, the reduction peak V at the potential of 0.21V should be the reduction peak for Co(OH) 2 reduced to Co.
(b)).The Tafel slope indicates the change in overpotential required for a tenfold change in current density.The smaller the Tafel slope, the smaller the voltage required for a tenfold increase in current density, and the lower the energy consumption.HER is a multi-step reaction, including adsorption, reduction and desorption processes.The mechanism of HER occurred can be described as the following three reactions [67, 68], where the Tafel slope (b) indicates the rate-determining step."M" and the M-H ads represent a vacant surface site of the catalyst, and the absorbed hydrogen atoms, respectively.In alkaline electrolyte, where hydroxide ions are abundant, the HER can proceed via two possible steps: Volmer and Tafel reactions.The Volmer step involves the adsorption of a water molecule and an electron on the catalyst surface to form H ads atom and OH − ion.

IrNi
-(10 mA•cm − 2 ), ^-(50 mA•cm − 2 ), j 0 =exchange current density, # -Electrodeposition The electrochemical stability of M-Ir/CF catalysts is assessed by the long-term hydrogen evolution testing.In Figs.8(a) and (c), M-Ir/CF catalysts are tested in 1.0 M KOH solution at a current density of 10 mA•cm − 2 for 10 h.There is almost no voltage decay within 10 h for Ni 63.4 Ir 36.6 /CF and Co 64.2 Ir 35.8 /CF catalysts.In Fig. 8(b), compared with the LSV curves of M-Ir/CF catalysts before and after the long hydrogen evolution cycle tests, the overpotential of Ni 63.4 Ir 36.6 /CF catalyst increases from 52 mV to 63.5 mV, and the overpotential of Co 64.2 Ir 35.8 /CF catalyst increases from 51 mV to 65.3 mV (Fig. 8(d)).After LSV testing from − 1.8V to -1.0V vs. SCE for 400 times, Co 64.2 Ir 35.8 /CF catalyst shows an increase of 10 mV in hydrogen evolution overpotential compared to Ni 63.4 Ir 36.6 /CF catalyst having an increase potential of 5 mV.The results show that Ni 63.4 Ir 36.6 /CF electrocatalyst has good stability in alkaline solution.

4
Conclusions M-Ir (M = Ni or Co) bimetallic alloy catalysts with low Ir loading are prepared on copper foam (CF) supports by galvanostatical electrodeposition.The top morphology, chemical composition and electrocatalytical properties of the M-Ir/CF bimetallic alloy catalysts are studied, simultaneously compared with Ir/CF, Ni/CF and Co/CF catalysts.The main conclusions are made as follows: M-Ir bimetallic alloy catalysts adhered to CF supports, and the surface of Co-Ir/CF catalyst is rougher than that of Ni-Ir/CF catalyst.The M-Ir catalysts with low Ir loading of 0.3-2.0mg•cm − 2 are composed of nanograins, Ni(Ir) and Co(Ir) solid solution.The surface of Co 64.2 Ir 35.8 /CF catalyst for sample #5 was uniformly

Figure 3 XPSFigure 4 High
Figure 3 XPS depth pro le for the top surface of M-Ir/CF catalysts (a) Ni 67.4 Ir 32.6 (b) Co 69.4 Ir 31.6

Figure 7 (
Figure 7 (a) LSV curves after an 85% iR-compensation of catalysts with a scanning rate of 5 mV•s -1 , (b) Tafel plots in 1.0 M KOH solution

Table 1
Bath chemistry and process parameters for M-Ir catalysts

Table 2
[43,44]l composition of M-Ir/CF catalysts Figure 2 depicts the XRD patterns of the M-Ir, Ni, Co and Ir catalysts on CF supports.From Ir/CF catalyst, four diffraction peaks at 2θ angles of 40.2 o , 46.8 o , 43.3 o and 50.5 o are separately indexed as face-centered cubic (FCC) structure Ir(111), Ir(200), Cu(111) and Cu(200) re ections, by comparison with JCPDS cards No. 87-0715 [38] and No. 04-0836 [39], respectively.The diffraction peaks at 2θ = 44.5 o and 51.6 o correspond to Ni(111) and Ni(200) re ections, respectively (JCPDS No. 04-0850) [39].In Fig. 2, for XRD analysis of Ni 61.8 Ir 38.2 /CF catalyst, there are two obvious peaks located at 43.9 o and 51.2 o .The diffraction peaks of Ni 61.8 Ir 38.2 /CF catalyst are slightly lower than those of Ni/CF catalyst, indicating the formation of Ni(Ir) solid solution.Therefore, Ni 61.8 Ir 38.2 /CF catalyst was composed of Ni(Ir) solid solution phase.The diffraction peaks of Co/CF catalyst at 2θ = 44.3oand51.8ocorrespondtoCo(111)andCo(200)reections,respectively(JCPDS No.15-0806)[40].For Co 61.2 Ir 38.8 /CF catalyst, there are two diffraction peaks located at 44.0 o and 50.9 o , which are also located between the standard diffraction peaks of Ir and Co.The atomic radius of Ir (0.141 nm) is larger than those of Ni (0.1246 nm) and Co (0.126 nm)[41].The diffraction peaks of M-Ir/CF catalysts show obvious shift toward lower diffraction angle degrees, which is attributed to the incorporation of Ir into metallic Ni, Co lattice to form a solid solution[42].Table3shows the lattice constants of M-Ir alloys calculated from XRD data by Vegard's law[43,44].The lattice constants of Ni 61.8 Ir 38.2 and Co 61.2 Ir 38.8 alloys are 3.65Å and 3.66 Å respectively, which are intermediate between the lattice constants of Ni (3.523 Å), Ir (3.839 Å) and Co (3.545 Å).The chemical composition and elemental states of Ni 67.4 Ir 32.6 /CF and Co 69.4 Ir 31.6 /CF bimetallic alloy catalysts are deeply analyzed by XPS technique.The atomic compositions of M-Ir/CF catalysts are listed in Table

Table 4
OH) 2 , respectively.Among them, the peaks at 862.8 eV and 856.4 eV belong to Ni 2p 3/2 corresponding to NiO, and the peaks at 879.7 eV and 873.3 eV belong to Ni 2p 1/2 corresponding to Ni(OH) 2 .It is consistent with the rule that the as-deposited catalysts are composed of mostly hydroxides on the topsurface.The Ir-4f spectrum is divided into two sets of doublets due to the presence of different states of Ir.There are two chemical states, Ir 0 4f and Ir 4+ 4f, corresponding to metallic Ir and IrO 2 , respectively, and it is assumed from their area ratio of 2:1 that metallic Ir is mainly present on the top-surface of the catalyst.The existence of Ir 4+ can be ascribed to the partial oxidation of Ir under air atmosphere.The binding energies of the two sets of doublets are different because of the different oxidation states and electronic con gurations of Ir 0 4f and Ir 4+ 4f.The higher the oxidation state, the higher the binding energy.The peaks located at 60.68 eV and 63.73 eV are assigned to Ir 4f 7/2 and Ir 4f 5/2 of Ir, and the peaks located at 61.7 eV and 64.5 eV are assigned to Ir 4f 7/2 and Ir 4f 5/2 of Ir 4+ (Fig.4(b)).The binding energies of Ir 0 4f 5/2 and Ir 0 4f 7/2 in the catalyst have a weaker variation in Ni-Ir catalysts compared to pure Ir (63.8 eV and 60.8 eV)[48].The reason for this negative shift of electron binding energy is related to the formation of Ni-Ir alloy and the interaction between Ir and Ni atoms.
Figure 4 shows the high-resolution XPS spectra of Ni 67.4 Ir 32.6 /CF and Co 69.4 Ir 31.6 /CF bimetallic alloy catalysts.In Fig. 4(a), 2p 3/2 and 2p 1/2 peaks of Ni represent the 2p shell energy level.In the XPS spectrum of Ni-2p, the peaks of 873.3 eV and 856.4 eV with satellite peaks at 879.7 eV, and 862.8 eV, have energy splitting of 17.1 eV and 16.9 eV, which are consistent with Ni 2+ [47].There are four peaks for the Ni-2p, corresponding to the multiple structures of NiO or Ni( [55]53]there are four peaks for Co 3+ 2p 3/2 , Co 2+ 2p 3/2 , Co 3+ 2p 1/2 , and Co 2+ 2p 1/2 of Co, corresponding to binding energies of 780.5 eV, 782.8 eV, 796.6 eV, and 798.1 eV, respectively.The two peaks with binding energies of 796.6 eV and 780.5 eV are the signi cant characteristic peaks of Co 3 O 4 , the orbital splitting energy is 15.2 eV, and the existence of both Co 2+ and Co 3+ is proved[52,53].The two spin-orbit doublets in the Co-2p spectrum correspond to Co 2+ and Co 3+ , and there are two shake-up satellites.Figure4(d) displays that the high-resolution XPS spectrum of Ir for Co 69.4 Ir 31.6 /CF catalyst.The peaks located at 60.7 eV and 63.7 eV are assigned to Ir 4f 7/2 and Ir 4f 5/2 of Ir, and the peaks located at 61.8 eV and 65.1 eV are assigned to Ir 4f 7/2 and Ir 4f 5/2 of Ir 4+ .According to the area of high-resolution XPS spectrum, the content of Ir oxide in Co 69.4 Ir 31.6 /CF catalyst is less than that of Ni 67.4 Ir 32.6 /CF catalyst.Compared with metallic Ir, Ir oxides are less active in HER because of its higher -d-band center energy (− 1.77 eV) compared with Ir (− 2.57 eV) and Pt (− 2.47 eV)[54].According to the review of Watanabe et al.[55], the peaks located at 529.6 eV and 530.5 eV were assigned to O ad and OH ad respectively, the peak located at 531.1 eV was assigned to a second type of H 2 O ad1 , and the peak at 532.6 eV was assigned to H 2 O ad2 .In the O 1s spectrum (see Fig.4(e)), the main peaks at 529.6 eV and 530.5 eV peaks were assigned to O ad and OH ad , and the peaks at 531.1 eV, 532.5 eV are assigned to H 2 O ad1 , H 2 O ad2 , respectively.
In Fig.4(f), the peaks located at 529.6 eV and 530.5 eV were assigned to O ad and OH ad , the peak at 531.9 eV was assigned to a second type of H 2 O ad1 , and the peak at 533.6 eV was assigned to H 2 O ad2 , which is consistent with the assignments of Michael et al. [56].The Ni 67.4 Ir 32.6 /CF catalyst contains 5.3% OH ad , whereas Co 69.4 Ir 31.6 /CF catalyst has 13.1% OH ad .In general, the OH ad coverage on the surface of a catalyst affects the HER performance in alkaline medium by in uencing the rate of the Volmer step, which is the rst step of the HER.As OH ad coverage increases, the catalytic site becomes less available and the HER overpotential increases.However, McCrum et al. [57] examined the kinetics of hydrogen evolution on a Pt(111) single-crystal electrode decorated with nanometer-sized transition metal hydroxide clusters and/or islands (Ni, Co), and found that Ni(OH) 2 and Co(OH) 2 increased the rate of HER in alkaline solution.Therefore, it is not su cient to only consider the OH ad coverage of the catalysts and the inherent adsorption capacity for H * .Electrochemical interface solutions, catalyst structures, and catalytic mechanisms are also important factors to be taken into account.Consequently, Co 69.4 Ir 31.6 /CF catalyst with high OH ad coverage performs the good HER performance.Calculating from the XPS analysis, the atomic ratios of Ni 67.4 Ir 32.6 /CF and Co 69.4 Ir 31.6 /CF catalysts are about 8.45:4.32 and 10.44:5.65 respectively, which is in agreement with the EDS elemental analysis (see Table4).
3 to 0.8V vs. RHE, i is the measured current density, ν is the scanning rate, and E c and E a are the cathode and anode potential limits, respectively.The Q a values of Ni/CF, Ir/CF, Ni 61.8 Ir 38.2 , Co/CF, and Co 61.2 Ir 38.8 catalysts are 104, 242, 122, 104, and 151 mC•cm − 2 , respectively.The delivered charges of Ni 61.8 Ir 38.2 and Co 61.2 Ir 38.8 alloy catalysts are considerably higher than single Ni/CF, Co/CF catalysts, except Ir/CF catalyst.The catalyst con guration in uences the number of surface atoms available for charge transfer during electrochemical experiments.This is illustrated by the CV curves displayed in Fig.

Table 5
As indicated, the Ir/CF catalyst has a highest ECSA value of 614.7 cm2and the ECSA values of Ni/CF, Ni 61.8 Ir 38.2 /CF, Co/CF, Co 61.2 Ir 38.8 /CF and bare CF catalysts are equal to 133.5, 327.4,200.7, 409.7, 101.7 cm 2 , respectively.This indicates the active sites of the M-Ir catalysts were remarkably enriched after Ir decoration.Due to the plush-like morphology (Figure S2), the ECSA value of Co/CF catalyst is larger than that of Ni/CF catalyst.What's more, the Co 61.2 Ir 38.8 /CF catalyst morphologically resembles the Co/CF catalyst, although it has a low Ir loading and a large electrochemical active surface area.The catalysts with a large ECSA value may provide much more reaction sites, but the composition, structure, and reaction conditions of the catalysts must be taken into account, some comprehensive factors in uence the catalytic activity of the catalysts.The electrocatalytic activities of M-Ir/CF bimetallic alloy catalysts are evaluated in 1.0 M KOH solution, simultaneously compared with those of Ir/CF, Ni/CF and Co/CF catalysts.Compared with the LSV curves after iR correction in Fig. 7(a), Co 64.2 Ir 35.8 /CF catalyst shows the best electrocatalytic activity in M-Ir bimetallic alloy catalysts with requiring an overpotential of 51 mV to achieve the current density of 10 mA•cm − 2 (η 10 ), which is lower than those of Ni 67.4 Ir 32.6 /CF, Ni 61.8 Ir 38.2 /CF and Co 69.4 Ir 31.6 /CF catalysts required for the overpotential values of 52 mV, 66 mV and 56 mV, respectively.In addition, the overpotentials at 10 mA•cm − 2 (η 10 ) of pure Ni/CF, Co/CF, bare CF, and Ir/CF catalysts are 177 mV, 274.3 mV, 502.5 mV and 26 mV respectively.Due to high catalytic activity of Ir metal, pure Ir catalyst has the lowest overpotential.The formula (4) for iR compensation is [66]: [70]79]][72][73][74][75]the [77]mbination of two adsorbed H ads atoms to form H 2 .The Tafel slopes of Ni 63.4 Ir 36.6 /CF and Ni 67.4 Ir 32.6 /CF catalysts are both equal to 36 mV•dec − 1 , which is lower than those of Co 64.2 Ir 35.8 /CF (38 mV•dec − 1 ), Co 69.4 Ir 31.6 /CF (37 mV•dec − 1 ), Co 61.2 Ir 38.8 /CF (40 mV•dec − 1 ), Ir/CF (47 mV•dec − 1 ), Ni 61.8 Ir 38.2 (58 mV•dec − 1 ), Ni/CF (120 mV•dec − 1 ) and Co/CF (127mV•dec − 1 ).Hence, HER may undertake the Volmer-Tafel pathway on M-Ir/CF catalysts.The result con rms that Ni 63.4 Ir 36.6 /CF and Ni 67.4 Ir 32.6 /CF catalysts have faster reaction kinetics than other catalysts.However, the Tafel slopes for the Co 69.4 Ir 31.6 /CF and Co 64.2 Ir 35.8 /CF catalyst are also only 37 mV•dec − 1 and 38 mV•dec − 1 respectively.Although the Tafel slope of Co 64.2 Ir 35.8 /CF catalyst is slightly higher than that of Ni 67.4 Ir 32.6 /CF catalyst, the overpotential value of Co 64.2 Ir 35.8 /CF catalyst at 10 mA•cm − 2 and 51 mA•cm − 2 is slightly lower than that of Ni 67.4 Ir 32.6 /CF catalyst.In Table 6, the exchange current densities (j 0 ) of the catalysts are followed in the order of Ir/CF (0.517 mA•cm − 2 ) > Co 64.2 Ir 35.8 /CF (0.491 mA•cm − 2 ) > Ni 63.4 Ir 36.6 /CF (0.489 mA•cm − 2 ) > Ni 67.4 Ir 32.6 /CF (0.483 mA•cm − 2 ) > Co 69.4 Ir 31.6 /CF (0.467 mA•cm − 2 ) > Co 61.2 Ir 38.8 = Ni 61.8 Ir 38.2 /CF (0.448 mA•cm − 2 ) > Co/CF (0.352 mA•cm − 2 ) > Ni/CF (0.347 mA•cm − 2 ) > CF (0.122 mA•cm − 2 ), which directly re ect the reaction rate of reactants at the active site [69].In addition, the HER performance of Ni, Co, Ir and M-Ir bimetallic alloy electrocatalysts is summarized in Table 6[70][71][72][73][74][75][76][77].Although the ESCA values of M-Ir/CF catalysts were second only to pure Ir/CF catalyst (see the above discussion).However, the electrocatalytic activity of the low Ir loading M-Ir/CF alloy catalysts was signi cantly improved, which was only slightly lower than that of the pure Ir catalyst compared to the single-element (Ni, Co, Cu) catalysts, illustrating that the M-Ir/CF catalysts in HER has a synergistic effect for M and Ir.In both alkaline and acidic solutions (see Table6), the HER performance of metallic Ni, Co catalysts doped with Ir is greatly improved, but the catalytic performance does not increase proportionally as the Ir loading increases.Thus, the high loading of Ir in iron-group catalysts are not a guarantee of catalytic activity.There are many factors that affect the performance of electrocatalytic hydrogen evolution, such as metal type, support type and morphology[78,79].The supports such as graphene, carbon nanotubes, and nitrogen-doped carbon, are promising candidates for HER application due to the high electrical conductivity, large surface area, and tunable properties.The surface morphology of the electrocatalyst affects its exposure of active sites, mass transport, and charge transfer.Nanostructured electrocatalysts, such as nanoparticles, nanowires, nanosheets, and even single-atom catalysts, can offer high surface-to-volume ratios, improved kinetics, and reduced overpotentials for HER[80].These factors can be optimized to design e cient and cost-effective hydrogen evolution electrocatalysts for HER applications.

Table 6
Comparison of HER catalytic performances of the iridium-containing electrocatalysts