Highly Active and Stable Alkaline Hydrogen Evolution Electrocatalyst Based on Ir‐Incorporated Partially Oxidized Ru Aerogel under Industrial‐Level Current Density

Abstract The realization of large‐scale industrial application of alkaline water electrolysis for hydrogen generation is severely hampered by the cost of electricity. Therefore, it is currently necessary to synthesize highly efficient electrocatalysts with excellent stability and low overpotential under an industrial‐level current density. Herein, Ir‐incorporated in partially oxidized Ru aerogel has been designed and synthesized via a simple in situ reduction strategy and subsequent oxidation process. The electrochemical measurements demonstrate that the optimized Ru98Ir2‐350 electrocatalyst exhibits outstanding hydrogen evolution reaction (HER) performance in an alkaline environment (1 M KOH). Especially, at the large current density of 1000 mA cm−2, the overpotential is as low as 121 mV, far exceeding the benchmark Pt/C catalyst. Moreover, the Ru98Ir2‐350 catalyst also displays excellent stability over 1500 h at 1000 mA cm−2, denoting its industrial applicability. This work provides an efficient route for developing highly active and ultra‐stable electrocatalysts for hydrogen generation under industrial‐level current density.


Introduction
[3][4][5] Directly generating H 2 through water electrolysis is DOI: 10.1002/advs.202307061[13] However, the state-of-the-art Pt/C suffers durability issues at high current density.Although a considerable number of transition metal-based HER electrocatalysts are designed and prepared, they are far from the expectations to achieve the standard of industrial application (current density up to 1000 mA cm −2 ) owing to their slow water activation kinetics caused by their active electronic states near the Fermi-level.Therefore, exploiting highly active and stable electrocatalysts for the HER catalytic process at high current density is highly desirable but remains a challenge.
[16][17][18] For instance, Alexander and co-workers initially investigate the influence of different polymetallic components of noble metal-based aerogels toward HER. [19]The resulting bimetallic Au-Rh alloy aerogel shows outstanding HER activities under pH-universal conditions and also surpasses the benchmark Pt/C catalyst.Interestingly, boron -doped osmium aerogel has also been synthesized via a simple and effective NaBH 4 reduction method toward HER. [20]Benefiting from the large porosity, numerous catalytic active sites, and the B doping to optimize the electronic structure and stabilize Os as active sites in an electron-deficient state under realistic working conditions, the resultant B-Os aerogel catalyst delivers better HER performances than Pt/C in three different electrolytes (acidic, alkaline, and neutral conditions).Recently, our group has designed a heterostructure consisting of Ru/RuO 2 metal aerogel by partially oxidizing the pre-fabricated single Ru metal aerogel. [21]To produce a current density of 10 mA cm −2 , the optimized Ru-30 sample requires overpotentials of only 36 and 24 mV in alkaline and acidic media, respectively.Although those studies present exceptional HER activity, their long-term stability is usually not satisfactory.Specifically, up to now, there is still no report on metal aerogel-based HER electrocatalyst with a long-term stability over 500 h at an industrial current density of 1000 mA cm −2 .
Metal-doping engineering has proven to be an effective strategy to promote the electrocatalytic activity and/or stability of catalysts for several electrochemical reactions by modifying their electronic structure to modulate the adsorption free energy of the intermediates and improve the electron transfer property. [22,23]As a typical example, nanosheets made of Fe-doped Ni 5 P 4 and Fedoped Ni(OH) 2 have been fabricated, they deliver excellent oxygen evolution reaction (OER) and HER catalytic properties in alkaline media. [24][27][28] However, it is still challenging to develop highly active and ultra-stable HER electrocatalyst based on the RuO 2 nanomaterials under industrial-level current density.
Herein, we show that the incorporation of a small amount of Ir into partially oxidized Ru aerogel improves the alkaline HER activity at an industrial-level current density.Our material only requires an overpotential of 121 mV to achieve a current density of 1000 mA cm −2 in 1 M KOH, significantly lower than that of the commercial Pt/C catalyst.More importantly, the optimized Ru 98 Ir 2 -350 catalyst also shows outstanding long-term stability over 1500 h at 1000 mA cm −2 , which is better to the pure Ru 100 -350 sample, commercial Pt/C and most of previously reported HER catalysts.Our findings confirm that the incorporation of Ir in the catalyst plays a pivotal role in preventing further oxidation of Ru during the HER process, resulting in notably enhanced durability.Our study offers promising avenues for the development of advanced metal oxide-based electrocatalysts for HER.

Materials Preparation and Structural Characterization
The synthesis of the Ir-incorporated partially oxidized Ru metallic aerogel (Ru 98 Ir 2 -350) can be divided into two steps.Initially, a freshly prepared sodium borohydride (NaBH 4 ) aqueous solution is added to an aqueous solution of RuCl 3 and IrCl 3 with a molar ratio of 98:2 to produce Ru 98 Ir 2 through an in situ coreduction route.Subsequently, the as-prepared Ru 98 Ir 2 intermediate product is oxidized at 350 °C to obtain Ir-incorporated partially oxidized Ru aerogel (Ru 98 Ir 2 -350), the detailed synthesis procedure is described in Figure 1a.The other control samples with different molar ratios of Ru to Ir (Ru 100 , Ru 99 Ir 1 , Ru 95 Ir 5 , and Ru 90 Ir 10 ) and their relevant oxidized products (Ru 100 -350, Ru 99 Ir 1 -350, Ru 95 Ir 5 -350, and Ru 90 Ir 10 -350) are synthesized via a similar procedure with that of Ru 98 Ir 2 and Ru 98 Ir 2 -350, respectively (detailed information are reported in the experimental section).The morphological properties of all the as-prepared products are initially characterized by field-emission scanning electron microscope (FE-SEM).As presented in Figure S2 (Supporting Information), the as-prepared Ru 98 Ir 2 sample shows a conventional metal aerogel morphology composed of interconnected nanoparticles, and these nanoparticles randomly fuse at different angles to form a highly open and porous network. [15,29]After oxidation, the morphological characteristic of the corresponding Ru 98 Ir 2 -350 is wellretained compared to the original sample (Figure 1b).In addition, it can be also observed from the FE-SEM images that all the other control samples including Ru 100 , Ru 99 Ir 1 , Ru 95 Ir 5 , Ru 90 Ir 10 and their partially oxidized products possess a similar morphology to that of Ru 98 Ir 2 and Ru 98 Ir 2 -350 (Figure S3, Supporting Information).The transmission electron microscopy (TEM) image of Ru 98 Ir 2 -350 further reveals that the constituent units of Ru 98 Ir 2 -350 consist of branched aggregates (Figure 1c).
From the high-resolution transmission electron micrographs (HRTEM), four family of lattice fringes with interplanar spacings of 0.205, 0.222, 0.256, and 0.321 nm can be clearly observed in Figure 1e-i X-ray photoelectron spectroscopy (XPS) analysis is carried out to study the surface chemical composition and electronic properties of the samples.The survey spectrum of Ru 98 Ir 2 -350 presents characteristic peaks of Ru, Ir, and O elements (Figure S5, Supporting Information), being consistent with the EDX results mentioned above.The Ru 98 Ir 2 sample exhibits doublets at 461.8 and 484.1 eV in the high-resolution Ru 3p spectrum, indexed to Ru 0 3p 3/2 and Ru 0 3p 1/2 , respectively (Figure 2c). [30,31]The peaks located at 61.2 and 64.1 eV are attributed to Ir 0 4f 7/2 and Ir 0 4f 5/2 , respectively (Figure 2d). [8,32,33]The characteristic signatures of zero valent Ru and Ir indicate that Ru 3+ and Ir 3+ species from the metal salts are fully reduced to metallic species (Ru 0 and Ir 0 ) during the NaBH 4 reduction procedure.It can be observed that the metallic Ru peaks of the Ru 98 Ir 2 -350 sample after annealing exhibits shift to a higher binding energy value in comparison with that of Ru 98 Ir 2 , demonstrating electron transfer from Ru to RuO 2 in the Ru/RuO 2 heterointerface, which is consistent with previous reports. [34]Additionally, two newly peaks centered at 464.6 and 487.3 eV appear in the Ru 3p core spectrum of Ru 98 Ir 2 -350, ascribed to Ru 4+ 3p 3/2 and Ru 4+ 3p 1/2 , respectively, further suggesting the partial oxidation of the sample. [35]This result is well consistent with HRTEM and FFT results discussed above.In the Ir 4f core spectrum, in addition to the weak Ir 0 peaks, a doublet at higher binding energies of 61.7 and 64.6 eV for Ru 98 Ir 2 -350 sample is dominated, which are assigned to Ir 4+ 4f 7/2 and Ir 4+ 4f 5/2 , respectively. [36]The formation of high valence Ir may be due to the incorporation of Ir into the lattice of RuO 2 during the annealing process.From the above structural and morphological characterizations, it can be affirmed that the as-prepared Ru 98 Ir 2 -350 product possesses a primary high valence state Irincorporated into the Ru/RuO 2 aerogel structure.Additionally, the Raman spectra of the as-prepared samples are measured to further investigate the changes in metal valence states of materials after oxidation process.There are no obvious peaks appearing in the Raman spectrum of Ru 100 , whereas Ru 100 -350 synthesized by subsequent oxidation process possesses two signal peaks located at 496 and 614 cm −1 , which are indexed to Ru─O bond, demonstrating the generation of RuO 2 (Figure S6, Supporting Information). [37]The newly peaks related to RuO 2 also appear in the Raman spectra of other products.It is worth noting that the two peaks indexed to RuO 2 show a slight positive shift as the Ir content increases in the oxidized samples, which can be attributed to the successfully incorporation of Ir into RuO 2 lattice to induce the lattice distortion and defects by nonstoichiometry according to previous literatures. [38]Furthermore, two very weak peaks of Ir─O bonds located at around 558 and 725 cm −1 appear in the Raman spectrum of Ru 90 Ir 10 -350, effectively indicating the presence of Ir 4+ in the oxidized sample. [39]

Electrocatalytic HER Performance Evaluation
The electrocatalytic behavior of all the as-prepared products toward alkaline HER is evaluated in 1 M KOH electrolyte (pH = 14) with a conventionally three-electrode electrochemical cell.In order to compare with commercial catalysts, the HER performance of Pt/C is also measured under the same conditions.Figure 3a displays the linear sweep voltammetry (LSV) curves with a 100% iR compensation.It can be observed that the as-prepared Ru 98 Ir 2 sample exhibits slightly lower HER electrocatalytic property than the benchmark Pt/C material.Notably, the Ru 98 Ir 2 -350 sample after annealing delivers a superior catalytic activity, especially at large current density.Remarkably, at current densities of 10, 500, and 1000 mA cm −2 , the Ru 98 Ir 2 -350 only requires overpotentials of 26, 94, and 121 mV, respectively, which are much lower than those of the Ru 98 Ir 2 (35, 250, and 369 mV) and commercial Pt/C (29, 205, and 349 mV).The comparison of the overpotentials is illustrated in Figure 3b.We have also evaluated the HER performance of various Ru x Ir y -350 samples, it is found that the Ru 98 Ir 2 -350 exhibits the best activity in comparison with all the other samples (Figure 3c).Furthermore, compared to other Ru x Ir y samples, their corresponding oxidized products display a significantly enhanced HER electrocatalytic activity (Figure S7, Supporting Information).Part of the reason for the improved HER performance can be attributed to the generation of the Ru/RuO 2 heterostructure, which is beneficial to reducing hydrogen adsorption-free energy (ΔG H* ) according to our previous work. [21]It is well known that RuO 2 is inactive towards H* adsorption, while metallic Ru is active, thus the formation of Ru/RuO 2 can modulate the H* adsorption and H 2 desorption at its surface to optimize the HER activity.Herein, in our Ru 98 Ir 2 -350 system, the incorporation of Ir into the Ru/RuO 2 heterostructure can reduce the oxidation of metallic Ru into RuO 2 to maintain the Ru/RuO 2 interface, thus improving the HER activity compared to pure Ru-containing samples.This assumption is supported by the XPS results, which have been discussed in the latter section (Figure 5c,d).The remarkable HER electrocatalytic activity of Ru 98 Ir 2 -350 is further supported by the Tafel plots derived from the LSV curves.As shown in Figure 3d and Figure S8 (Supporting Information), Ru 98 Ir 2 -350 presents an ultra-low Tafel slope of 8.3 mV dec −1 , which is better than the commercial Pt/C catalyst (15.6 mV dec −1 ) and other as-prepared samples (9.8-31.8mV dec −1 ). Figure 3e and Table S1 (Supporting Information) present the comparison of the overpotential at 1000 mA cm −2 and Tafel slope value of the Ru 98 Ir 2 -350 product with other previously reported highly active HER electrocatalysts.The comparison points to the superior HER performance of our Ru 98 Ir 2 -350 sample, demonstrating the achievement of electrocatalytic H 2 production at industrially-compatible current densities.
The mass activity of various catalysts is further calculated to estimate their intrinsic activity.As displayed in Figure 3f, Ru 98 Ir 2 -350 delivers a higher mass activity value of 560.7 A g −1 at an overpotential of 100 mV, which is 4. , the detailed calculation method is presented in the experimental section. [40,41]As listed in Table S1 (Supporting Information), Ru 98 Ir 2 -350 presents the highest ECSA value of 954 cm 2 among all the other samples.The Faraday efficiency (FE) of Ru 98 Ir 2 -350 is also measured at 50 mA cm −2 with the water displacement method to further investigate the HER   4a,b,c).

Electrocatalytic HER Stability Evaluation
The resistance of HER materials at a high current density is another important parameter for industrial H 2 production applications.By comparing the LSV curves before and after 10 000 CV cycles (Figure 4d), it was found that the electrocatalytic activity of Ru 98 Ir 2 -350 showed only a little decrease, demonstrating excellent CV stability.In addition, the i-t curve at 1000 mA cm −2 is further used to estimate the long-term HER stability of the catalyst at large current density.As shown in Figure 4e, after continuous i-t testing for up to 1500 h, the current density of the catalyst only slightly decreases under constant applied potential.The stability of Ru 98 Ir 2 -350 is much better than that of commercial Pt/C and mostly previous reported literatures (Figure 4f), confirming its superior durability at high current density.Notably, the Ru 98 Ir 2 -350 sample also displays a greatly enhanced durability compared to the as-prepared Ru 100 -350 control sample.This result can be preliminarily attributed to the incorporation of Ir, which contributes in improving the stability of the Ru/RuO 2 heterostructured material.

Characterization of Post-HER Ru98Ir2-350 and Mechanism of Ultra-High Stability
The morphological, compositional, and structural characterizations of Ru 98 Ir 2 -350 after HER stability measurement in alkaline media is further estimated by FE-SEM, XRD, and XPS.As presented in Figure S16 (Supporting Information), Ru 98 Ir 2 -350 still exhibits an unchanged 3D porous morphology after long-term operation.Moreover, all reflections in the powder XRD pattern exhibit no variation in comparison to the freshly prepared sample, indicating that the structure is not affected by the HER electrocatalytic process (Figure 5a).The Ir 4f and Ru 3p XPS spectra of Ru 98 Ir 2 -350 before and after HER durability measurement (Figure 5b,c) are also unchanged.The above results confirm that Ru 98 Ir 2 -350 possesses an excellent structural and compositional stability during the HER electrocatalytic process.In order to better understand the stability of Ru 98 Ir 2 -350, the Ru 3p XPS signal peaks of Ru 100 -350 after the same durability test is investigated and compared.The binding energies corresponding to the Ru species (Ru 0 and Ru 4+ ) shows a positive shift to the higher binding energy values, illustrating that Ru species in the Ru 100 -350 material (without Ir incorporation) are inclined to be oxidized to form a higher valence state during the HER process, which might be related to the etching of the Ru-based materials by the alkaline electrolyte (Figure 5d).Whereas, the Ru oxidation state in Ru 98 Ir 2 -350 is well maintained, thus the molar ratio of Ru and RuO 2 in the catalyst remains constant, which results in an improvement of the long-term stability.This result demonstrates that the introduction of Ir in the Ru/RuO 2 heterostructure plays a key role in achieving an improvement of the long-term stability.

Conclusion
In summary, we have demonstrated a systematic approach to design and fabricate a novel aerogel material consisting of Irincorporated in partially oxidized Ru toward alkaline HER application.By combining the advantages of the unique 3D porous structure and the incorporation of Ir into Ru/RuO 2 inhibits the further oxidation of metallic Ru during the HER process in an alkaline solution.The as-derived optimized Ru 98 Ir 2 -350 sample presents a remarkable electrocatalytic performance, requiring an ultralow overpotential of 121 mV to reach an industrial-level current density of 1000 mA cm −2 , accompanied by a low Tafel slope of 8.3 mV dec −1 and a high mass activity of 560.7 A g −1 .These properties are significantly better than that of commercial Pt/C catalyst.More importantly, the Ru 98 Ir 2 -350 also delivers an outstanding long-term stability over 1500 h at 1000 mA cm −2 , further reinforcing its potential for industrial-scale hydrogen production applications.This study provides valuable insights for the development of HER electrocatalysts with both high activity and longterm stability.Material Characterizations: The crystallographic structures of the asprepared samples were characterized by XRD (PANalytical B.V. Empyrean, Cu K  ,  = 1.5406Å, 40 mA, and 40 kV).The morphological images of the as-prepared samples were collected on the FE-SEM (FEI Nova NanoSEM, 15 kV).The chemical valence state and surface electronic properties of the as-synthesized samples were confirmed with XPS (Thermo Scientific ES-CALAB250).The composition, element information, and atomic structural characterization of the samples were all acquired from HRTEM equipped with a EDX spectrometer (FEI Tecnai G2 F20).The electrochemical data of the samples were obtained from an electrochemical analyzer (CHENHUA, CHI660E).The Raman spectrum of as-synthesized sample was tested by a LabRAM ARAMIS Smart Raman Spectrometer.

Experimental Section
Electrocatalytic Performance Measurements: The evaluation of the electrochemical properties of all the as-prepared products toward HER in 1 M KOH electrolyte (pH = 14) were performed with a typical three-electrode electrochemical cell at room temperature.Herein, catalyst-coated carbon paper (loading area: 0.4×0.49cm 2 ), Hg/HgO electrode, and graphite rod were served as the working electrode, reference electrode, and counter electrode, respectively.The Hg/HgO electrode was calibrated before electrochemical tests as follows: two Pt plates polished with sandpapers were served as both the counter and working electrode, and then CV with two sweep segments was performed in the H 2 -saturated 1 M KOH solution (1 mV s −1 ).The average value (−0.914V) of the two potentials at the zero-crossing point of the current was considered as the thermodynamic potential of the hydrogen electrode reaction, and the calibration result was illustrated in Figure S1 (Supporting Information).Therefore, the potential from Hg/HgO electrode can be converted to a reversible hydrogen electrode (RHE) using the following equation: E RHE = E Hg/HgO + 0.914 V.
To fabricate the catalyst ink-modified working electrode, 2 mg of the assynthesized electrocatalyst and 10 μL of Nafion D-117 dispersion (5 wt%) were dispersed into 490 μL of a mixed solvent with a volume ratio of V water : V isopropanol = 1:1.Then the above mixture was vigorously sonicated for 1 h to generate a homogeneous electrocatalyst ink.50 μL of as-prepared electrocatalyst ink (4 mg mL −1 ) was subsequently cast on the carbon paper electrode surface and dried in the air at room temperature, resulting in a mass loading density of 1 mg cm −2 .For comparison, the working electrode fabrication procedure with the commercial 20% Pt/C catalyst was similar to that of the above samples and the corresponding mass loading density was 5 mg cm −2 .All LSV curves of the samples were tested by scanning LSV with a scan rate of 1 mV s −1 and the CV curves for the estimation of the stability were measured with a scan rate of 100 mV s −1 .All the LSV curves for all HER electrocatalytic measurements were obtained with a 100% of ohmic potential drop (iR) compensation.The EIS measurements were performed from 1-10 5 Hz under an AC voltage of −0.98 V (vs Hg/HgO).The C dl value of sample was obtained from CV measurements at various scan rates (50, 40, 30, 20, 10 mV s −1 ) under a non-Faraday range of 0.19-0.29 V (vs RHE).The C dl values were obtained by plotting the Δj/2 (Δj = j aj c, where j c and j a correspond to the negative and positive current, respectively) at 0.24 V (vs RHE) versus scan rates.The ECSA value of sample was calculated from the equation: ECSA = C dl × S/C s , where S was the geometric area of catalyst decorated-electrode (0.196 cm 2 ) and C s was the value of the specific capacitance value (40 μF cm −2 ). [40,41]The mass activity values were evaluated with the equation: mass activity = j/m, where j was the current density (mA cm −2 ) at an overpotential of 100 mV and m was the mass loading of the catalysts.The applied voltage for i-t stability test of Ru 98 Ir 2 -350, Ru 100 -350, and commercial Pt/C in 1 M KOH electrolyte was −1.62, −2.11, and −2.22 V (vs Hg/HgO), respectively.The FE calculation for H 2 production was based on the following equation: FE = (z × F × n/I × t) × 100%, where z was 2 (number of electrons needed to generate one H 2 molecule), F was the Faraday constant (96 485.3 C mol −1 ), n was the moles of gas evolved, and I was the constant current applied for t min.The drainage method was performed to measure the volume of the generated H 2 during HER process.The change of the liquid volume in the pipette before and after the HER reaction was taken as the volume of the generated H 2 .
, which can be assigned to the (101) crystal plane of metallic Ru, (101) crystal plane of RuO 2 , (111) crystal plane of RuO 2 , and (110) crystal plane of RuO 2 , respectively.Additionally, their corresponding Debye-Scherrer rings can be observed in the fast Fourier transform (FFT) patterns calculated from the Ru 98 Ir 2 -350 HRTEM micrograph (Figure 1d).Through the analysis of HRTEM and FFT results, it is preliminarily confirmed that metallic Ru and RuO 2 are present in the Ru 98 Ir 2 -350 sample, further illustrating that a portion of metallic Ru is oxidized into RuO 2 upon heat treatment.Owing to the extremely low content of Ir, no identified structural information of Ir species (Ir or IrO 2 ) is found in the above characterizations.To further characterize the local composition and the presence of Ir, high-angle annular dark-field scanning (HAADF-STEM) and relevant energy dispersive X-Ray (EDX) elemental mapping are measured and shown in Figure 1j-m.Apart from Ru and O uniformly distributed in the aerogel structure, Ir can also be found in the EDX mapping of the Ru 98 Ir 2 -350 sample.Moreover, the characteristic signals of Ru, Ir, and O elements are clearly observed in the EDX spectra further confirming the composition of the as-prepared Ru 98 Ir 2 -350 sample (Figure 2a).Powder X-ray diffraction (XRD) is also employed to shed light on the crystallographic structure of asprepared products.In the XRD patterns of the pre-oxidized samples, only a broad peak at around 2 = 43°appears, which can be indexed to the (101) plane of metallic Ru according to the standard PDF card No.06-0663 (Figure 2b).After the heat treatment at 350 °C, a sharper XRD diffraction peak of 2 = 43°and two additional peaks at 2 = 38°and 42°appear, which correspond to the (100) and (002) planes of metallic Ru.Moreover, several low intensity peaks centered at 2 = 28°, 35°, 40°, 54°, and 70°c haracteristic of RuO 2 appear in the XRD pattern of Ru 98 Ir 2 -350, which are assigned to the (110), (101), (111), (211), and (301) reflections of RuO 2 (JCPDS Card No. 40-1290), respectively, further confirming the partial oxidation of Ru to RuO 2 .The other RuIr aerogels samples exhibit similar crystalline structures as the Ru 98 Ir 2 and Ru 98 Ir 2 -350 samples before and after annealing, respectively (Figure S4, Supporting Information).

Figure 3 .
Figure 3. a) LSV curves of the as-synthesized Ru 98 Ir 2 -350, Ru 98 Ir 2 samples, and commercial Pt/C catalyst.b) The overpotential comparison at the current density of 10, 500, and 1000 mA cm −2 .c) LSV curves of the Ru 100 -350, Ru 99 Ir 1 -350, Ru 98 Ir 2 -350, Ru 95 Ir 5 -350, and Ru 90 Ir 10 -350 samples.d) Tafel plots of the as-synthesized Ru 98 Ir 2 -350, Ru 98 Ir 2 samples, and commercial Pt/C catalyst.e) Comparison of overpotential at 1000 mA cm −2 and Tafel slope value of the Ru 98 Ir 2 -350 product with other highly active HER electrocatalysts.f) Mass activity value at an overpotential of 100 mV.g) The comparison of theoretically calculated and experimentally measured H 2 evolution of Ru 98 Ir 2 -350.h) Photos of recording changes in liquid level scale in Faraday testing.
7 and 15.2 times as high as the one of Ru 98 Ir 2 (119.4A g −1 ) and Pt/C (36.8A g −1 ), respectively.The price activity of Ru 98 Ir 2 -350 also present at least 4.5 times as high as that of the benchmark Pt/C catalyst (Figure S9, Supporting Information), indicating the cost-effectiveness of Ru 98 Ir 2 -350.The charge transfer kinetics of all the catalysts are evaluated by electrochemical impedance spectroscopy (EIS).As depicted in Figure S10 (Supporting Information), the Ru 98 Ir 2 -350 sample displays the smallest charge transfer resistance (R ct ) value among all the samples.The double-layer capacitance (C dl ) extracted from cyclic voltammetry (CV) at different scan rates under non-Faraday region is closely related to the electrochemical active surface area (ECSA) of the electrocatalyst (Figure S12, Supporting Information).As depicted in Figure S13 (Supporting Information), Ru 98 Ir 2 -350 displays the largest C dl value of 194.7 mF cm −2 in comparison to other samples (22.8-127.5 mF cm −2 ).According to the C dl results, the ECSA values of all materials are further obtained based on the following equation: ECSA = C dl ×S/C s , where C s = 40 μF cm −2
selectivity of the material.As shown in Figure 3g,h, the calculated FE value of Ru 98 Ir 2 -350 within 90 min is ≈100%, indicating the high selectivity of Ru 98 Ir 2 -350 toward H 2 production.To estimate the influence of the oxidation temperature on the HER performance of Ru 98 Ir 2 , the Ru 98 Ir 2 -200 and Ru 98 Ir 2 -500 control samples are also fabricated through a similar synthesis procedure to that of Ru 98 Ir 2 -350, but under different annealing temperature (200 and 500 °C, respectively).FE-SEM images of Ru 98 Ir 2 -200 and Ru 98 Ir 2 -500 indicate similar morphologies compared to Ru 98 Ir 2 -350, showing that the oxidation process does not have a significant impact on the morphology of the samples (Figure S14, Supporting Information).From the XRD pattern, it can be observed that as the annealing temperature increases, the XRD reflections assigned to RuO 2 becomes stronger, revealing that more Ru in the aerogel is oxidized to RuO 2 (Figure S15, Supporting Information).Ru 98 Ir 2 -350 displays significantly lower overpotentials at 10, 100, and 500 mA cm −2 as well as lower Tafel slope values than those of Ru 98 Ir 2 -500 and Ru 98 Ir 2 -200.These results prove that the partially oxidized Ru 98 Ir 2 -350 samples provides the best compromise leading to excellent HER electrocatalytic activity and reaction kinetics (Figure

Figure 5 .
Figure 5. a) XRD patterns, b) Ir 4f XPS spectra, c) Ru 3p XPS spectra of Ru 98 Ir 2 -350 before and after HER stability test in 1 M KOH.d) Ru 3p XPS spectra of the Ru 100 -350 sample before and after HER durability measurement.
Chemicals and Materials: Iridium chloride (IrCl 3 •xH 2 O), ruthenium chloride (RuCl 3 •xH 2 O), and Nafion D-117 dispersion were all purchased from Sigma-Aldrich.Commercial Pt/C (20 wt%) material was provided by Johnson Matthey.Sodium borohydride (NaBH 4 ) was acquired from Xilong Scientific Co. Ltd.Synthesis of RuIr Aerogel: In this work, five RuIr aerogel samples (Ru 100 , Ru 99 Ir 1 , Ru 98 Ir 2 , Ru 95 Ir 5 , and Ru 90 Ir 10 ) with different Ru to Ir ratios were synthesized via a simple one-step in situ NaBH 4 reduction method.In a typical synthesis of Ru 98 Ir 2 , 0.098 mmol of RuCl 3 •xH 2 O and 0.002 mmol of IrCl 3 •xH 2 O (total metal salt was 0.1 mmol) were dissolved into 2 mL of deionized water in a 20 mL glass bottle, and sonicated for 1 min to form a homogenous solution.Next, the as-fabricated fresh NaBH 4 aqueous solution (0.3 mmol of NaBH 4 dissolved in 3 mL of deionized water) was added to the above solution under 60 °C.After 60 min, a black precipitate was formed at the bottom of the flask.Afterward, the black precipitate was further washed with ultrapure water for 5 times to remove the residual impurities and then freeze-dried overnight to obtain the Ru 98 Ir 2 product.The synthesis of other various Ir-doping Ru metal aerogels (Ru 100 , Ru 99 Ir 1 , Ru 95 Ir 5 , and Ru 90 Ir 10 ) followed the same preparation procedure as that of the Ru 98 Ir 2 sample with the appropriate stoichiometry.Synthesis of Ir-Incorporated Partially Oxidized Ru Aerogel: The prefabricated Ru 100 , Ru 99 Ir 1 , Ru 98 Ir 2 , Ru 95 Ir 5 , and Ru 90 Ir 10 samples were further transferred into a muffle furnace and heated for 30 min at 350 °C to obtain Ir-incorporated partially oxidized Ru aerogels, which were named as Ru 100 -350, Ru 99 Ir 1 -350, Ru 98 Ir 2 -350, Ru 95 Ir 5 -350, and Ru 90 Ir 10 -350, respectively.Similar to the procedure to prepare Ru 98 Ir 2 -350, the pre-fabricated Ru 98 Ir 2 sample was transferred into a muffle furnace and heated at 200 and 500 °C for 30 min to obtain Ru 98 Ir 2 -200 and Ru 98 Ir 2 -500, respectively.