Ir nanoclusters on ZIF-8-derived nitrogen-doped carbon frameworks to give a highly efficient hydrogen evolution reaction

WANG Xi-ao ,GONG Yan-shang ,LIU Zhi-kun ,WU Pei-shan ,ZHANG Li-xue ,SUN Jian-kun,

（1.College of Chemistry and Chemical Engineering,Collaborative Innovation Center for Hydrogen Energy Key Materials and Technologies of Shandong Province,Qingdao University,Qingdao 266071, China;2.Wanhua Chemical Group Co.,Ltd.,Yantai 264000, China;3.Institute of Analysis,Guangdong Academy of Sciences,Guangdong Provincial Key Laboratory of Emergency Test for Dangerous Chemicals,Guangzhou 510070, China）

Abstract：The precise change of the electronic structure of active metals using low-active supports is an effective way of developing high-performance electrocatalysts.The electronic interaction of the metal and support provides a flexible way of optimizing the catalytic performance.We have fabricated an efficient hydrogen evolution reaction (HER) electrocatalyst,in which Ir nanoclusters are uniformly loaded on a nitrogen-doped carbon framework (Ir@NC).The synthesis process entails immersing an annealed zeolitic imidazolate framework-8 (ZIF-8),prepared at 900 °C as a carbon source,into an IrCl3 solution,followed by a calcination-reduction treatment at 400 °C under a H2/Ar atmosphere.The three-dimensional porous structure of the nitrogen-doped carbon framework exposes more active metal sites,and the combined effect of the Ir clusters and the N-doped carbon support efficiently changes the electronic structure of Ir,optimizing the HER process.In acidic media,Ir@NC has a remarkable HER electrocatalytic activity,with an overpotential of only 23 mV at 10 mA cm?2,an ultra-low Tafel slope (25.8 mV dec?1) and good stability for over 24 h at 10 mA cm?2.The high activity of the electrocatalyst with a simple and scalable synthesis method makes it a highly promising candidate for the industrial production of hydrogen by splitting acidic water.

Key words: Ir nanoclusters；Nitrogen-doped carbon support；Electronic interaction；Electrocatalysis；Hydrogen evolution reaction

1 Introduction

Hydrogen energy with high energy density is a clean and sustainable energy resource which can be easily transported and stored,allowing for flexibility in energy distribution[1–3].Additionally,hydrogen fuel cells have high energy conversion efficiency and produce only water as a byproduct,minimizing environmental impact[4–5].However,obtaining green hydrogen via water electrolysis is largely hindered by its low energy efficiency.Recently,acidic electrolyzers,generally operate at lower voltages and have higher energy efficiency than alkaline counterparts,have become interesting alternatives[6–8].Furthermore,acidic electrolyzers also exhibit faster reaction kinetics,enabling higher current densities and overall improved performance[9].However,one major challenge in acidic catalytic systems is the stability of the catalysts.Most of the catalysts that can be utilized in alkaline conditions,especially the non-noble metal catalysts,are severely degraded in acidic electrolytes[10–12].This instability can lead to decreased catalytic activity and shortened catalyst lifespan[13–14].Addressing catalyst stability is crucial for the development and commercialization of efficient and durable electrocatalytic hydrogen production in acidic conditions[15].

Despite the low abundance and high cost,precious metals like Pt,Ir and Ru are still the main electrocatalysts that are extensively utilized in acidic electrolytes[16–17].For instance,You et al.reported Ir nanoparticles anchored cucurbit [6] uril,which exhibited slightly worse HER performance (η10=54 mV) than Pt/C in 0.5 mol L?1H2SO4[18].Song et al.reported that a material with Ru dispersed on CoP nanoparticles exhibits superior HER catalytic activity,with a low overpotential of 49 mV to achieve 10 mA cm?2in 0.5 mol L?1H2SO4solution,by lowering the energy barrier of proton-coupled electron transfer[19].Drouet et al.reported a porous Ru nanomaterial,which needed an overpotential of 83 mV to deliver 10 mA cm?2in 0.5 mol L?1H2SO4solution,owing to the porous structure of the material[20].Although great progress has been made in this direction,methods for regulating the electronic structure while simultaenously increasing the utilization efficiency of precious metal atoms is still challenging[21].

Nanoscaling of material dimensions plays a critical role in enhancing the specific surface area of catalysts to provide more active sites[22].The nano-catalysts often exhibit distinct and impressive properties compared to bulk materials.In particular,the metal nanoclusters with extremely high specific surface area and a lower surface metal-metal coordination number,improve the surface-to-volume ratio as well as the atomic efficiency of catalyst[23].However,as the size decreases,the catalysts with much higher surface energy become fragile and unstable,inducing degradation and collapse of the active components.The metal-support interaction has been considered as a promising approach to regulate the electronic structure of the active sites and simultaneously prevent side reactions that destroy their structures[24–26].For instance,Xiao and co-workers reported an IrMo nanoclusterembedded N-rich electrocatalyst under alkaline conditions,which possesses ultrasmall bimetal nanoclusters and distinctive porous structures,enhancing the activity and stability of metal nanoclusters[27].In addition,Zhang et al.reported a catalyst with Ir clusters loaded on Pd nanosheets,in which the charge redistribution results in an optimum hydrogen adsorption at the interface[28].Apparently,loading precious metal nanocluster catalysts on stable supports will enable the combination of optimized electronic structure and enhanced stability in acidic electrolytes,but challenging.

Herein,we utilized annealed ZIF-8 as a carbon source to achieve uniform loading of Ir nanoclusters with an average diameter of 1.78 nm onto a three-dimensional porous N-doped carbon scaffold.This was accomplished by a simple impregnation and calcination-reduction method.The formation of strong covalent Ir-N bonds effectively suppressed the corrosion and agglomeration of Ir clusters in acidic environments.Moreover,the iridium element in Ir@NC exhibited a lower valence state compared to the Ir@C sample,which is conducive to the HER process.This is attributed to the abundant N doped in the carbon support,which regulates the electronic structure of Ir through a strong electronic effect[29].As a result,the electrocatalyst exhibits superior HER performance than Pt/C under acidic conditions.This work demonstrates the importance of selecting appropriate catalyst supports to improve the intrinsic activity of metals and highlights the potential of N-doped carbon materials in enhancing the HER performance of Ir-based catalysts under acidic conditions.

2 Experimental section

2.1 Synthesis of NC

To prepare the ZIF-8 precursor,2-methylimidazole (5.677 g) and hexadecyl trimethyl ammonium bromide (CTAB) (0.018 g) were dissolved in 87 mL of deionized water.Then the 13 mL of deionized water containing 0.367 g of Zn(NO3)2·6H2O was mixed with the above solution.The solution was stirred and aged for 6 h.Then the product was collected and dried at 60 °C.The dried ZIF-8 was subsequently annealed in a 10% H2/Ar atmosphere at 900 °C for 2 h.This process resulted in the formation of a black nitrogendoped carbon (NC) powder.

2.2 Synthesis of Ir@NC

To prepare Ir@NC,NC (0.025 g) and IrCl3·nH2O(0.005 g) were dispersed in 1 mL of deionized water.Then the solution was kept at 60 °C for 6 h.The resulting product was collected,washed and dried at 60 °C under vacuum conditions.Next,the dried product was annealed at 400 °C for 4 h in a H2/Ar atmosphere.After cooling down,the black colored Ir@NC powder was obtained.

For comparison,Ir@C was prepared using a similar process,but instead of NC,Ketjenblack ECP-600JD was used as the carbon support.

3 Results and discussion

The synthesis of Ir@NC sample involves a simple three-step method (Fig.1).First,ZIF-8 was obtained by solvothermal treatment and the structure was confirmed by X-ray diffraction (XRD) patterns with the observed diffraction peaks consistent with the simulated ones (Fig.S1).Then,a porous NC skeleton was fabricated by pyrolyzing ZIF-8 at 900 °C.The diffraction peaks of ZIF-8 disappeared and 2 broad peaks at approximately 26° and 44° that belong to the graphitic carbon structure (Fig.2a) were observed,confirming the formation of the NC skeleton[30].Subsequently,the NC sample was immersed in an Ir3+solution to obtain Ir precursor@NC.Finally,the reduction of Ir3+to Ir0clusters was carried out under H2/Ar conditions,resulting in the formation of Ir@NC.Notably,no diffraction peak of Ir was observed probably due to the small size.

Fig.1 Schematic illustration of the formation of Ir@NC electrocatalyst

Fig.2 (a) XRD patterns of NC and Ir@NC sample.SEM images of (b) ZIF-8,(c) NC and (d) Ir@NC.(e-f) HRTEM images of Ir@NC.(g) Size distribution of Ir nanoclusters.(h) HAADF-STEM and (i) the corresponding EDS elemental mapping images of Ir@NC

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and structure of the prepared samples.SEM image of ZIF-8 in Fig.2b exhibits a uniform cubic shape with a size of 140 nm.After the pyrolysis treatment,the NC sample maintains its initial cubic morphology,but a reduced particle size of 75 nm due to the evaporation of In[31–32](Fig.2c).Upon the incorporation of Ir clusters,the size of Ir@NC is further reduced,presenting a shrunken cubic shape with a smaller size of 60 nm (Fig.2d).TEM images of Ir@NC (Fig.2e,f) reveal that ultra-small Ir clusters are uniformly distributed on the cubic Ndoped carbon framework.This uniform dispersion can be attributed to the abundance of N atoms on the NC substrate,which act as coordinating atoms and provide nucleation sites for the formation of Ir clusters[33].The average size of the Ir clusters is approximately 1.78 nm (Fig.2g and Fig.S2),explaining the absence of the Ir diffraction peaks in the XRD pattern.As shown in Fig.S3,a weak diffuse ring pattern for the Ir@NC sample is found from selectedarea electron diffraction (SAED) images,consistent with the results of TEM and XRD.Moreover,highangle annular dark-field scanning TEM (HAADFSTEM) images further demonstrate the uniform distribution of Ir clusters supported on the NC substrate(Fig.2h),and X-ray energy dispersive spectroscopy(EDS) elemental mapping images confirm the coexistence of C,N and Ir elements in the Ir@NC sample(Fig.2i).The Ir content in Ir@NC,determined by inductively coupled plasma optical emission spectrometry (ICP-OES),was found to be 8.02%,which is in good agreement with the EDS result (Table S1).For comparison,Ir@C sample was prepared using a similar process,but with carbon black instead of ZIF-8 as the carbon source.From the XRD pattern shown in Fig.S4,no diffraction peaks corresponding to Ir were detected in the Ir@C sample.Instead,only two broad peaks attributed to the graphitic carbon structure were observed,which is similar to the Ir@NC sample.

The specific surface area and pore structure of NC and Ir@NC were determined by nitrogen adsorption/desorption analysis.The Brunauer-Emmett-Teller (BET) surface area of NC and Ir@NC was calculated to be 1 060 and 1 163 m2g?1,respectively(Fig.3a and Table S2).The higher surface area of Ir@NC can be due to the incorporation of Ir clusters.Both the NC and Ir@NC samples exhibit a hierarchical pore structure with micropores and mesopores,as indicated by the hysteresis curves and hysteresis loop,which is verified by the pore size distribution curves(Fig.3b).This presence of micropores facilitates the ion diffusion in the electrolyte,while the mesoporous structure enhances the mass transport of active species and enables the exposure of more active sites.Therefore,the synergistic effect of pore structure promotes electrochemical reaction kinetics[34].

Fig.3 (a) N2 adsorption-desorption isotherms and corresponding (b) pore diameter distribution curves of NC and Ir@NC

The chemical composition and valence states of Ir@NC,NC and Ir@C samples were examined using X-ray photoelectron spectroscopy (XPS).The existence of the corresponding elements is confirmed by the XPS survey spectra of each sample (Fig.4a and Fig.S5).The weak peak of Zn 2p appears in both Ir@NC and NC samples due to the incomplete removal of Zn from ZIF-8.The residual Zn does not significantly contribute to the catalytic activity[26],as will be further verified by the following electrochemical characterization.The O element detected in the spectra originates from inevitable surface oxidation when exposed to air.In the C 1s spectra (Fig.4b),the fitted peaks located at 284.8 and 286.3 eV belong to C―C and C―N coordination,respectively.

Fig.4 XPS spectra of NC and Ir@NC.(a) Survey scan spectra of NC and Ir@NC.High-resolution spectra of (b) C 1s and (c) N 1s for Ir@NC and NC.(d) High-resolution spectra of Ir 4f for Ir@NC and Ir@C

In the N 1s spectra of NC and Ir@NC samples(Fig.4c),the signal can be well fitted with 5 peaks corresponding to pyridinic nitrogen (398.4 eV),metal―nitrogen bond (399.7 eV),pyrrolic nitrogen(400.8 eV),graphitic nitrogen (401.9 eV),and oxidic nitrogen (404.1 eV) species,respectively.The presence of metal―nitrogen bond in the NC sample mainly originates from residual Zn,while Ir@NC possesses both Zn―N and Ir―N bonds.Apart from pyridinic N and metal―N,the other nitrogen species in both samples have nearly the same content.The pyridinic N and metal―N account for 35% and 9% of the total N atoms in the NC sample,while in Ir@NC,these 2 species account for 30% and 14%,respectively.This difference indicates that a portion of pyridinic N was converted into metal-nitrogen bonds owing to the formation of Ir―N bonds with the incorporation of Ir clusters.The electron-donating properties of pyridinic N enable it to serve as metal-coordination sites to immobilize the Ir atoms[33–34].Additionally,the peak of metal―N in Ir@NC was shifted to a higher binding energy,suggesting the significant electronic interaction between pyridinic N and Ir atoms.In the Ir 4f spectra (Fig.4d),doublet peaks of Ir 4f7/2and Ir 4f5/2with 2 satellite peaks at 62.9 and 66.2 eV were observed.Compared to Ir@C,the binding energies of Ir 4f7/2and Ir 4f5/2in Ir@NC sample are negatively shifted from 62.1 and 65.1 eV to 61.7 and 64.7 eV,manifesting the significant interaction between Ir and N,consistent with the results of the N 1s.The corresponding data and valence states of C 1s,N 1s and Ir 4f in XPS spectra have been listed in Table S3-5.The synergistic effect between Ir clusters and NC support allows to effectively regulate electronic structure of Ir and optimize electrocatalytic HER process[35–39].

The catalytic properties of different samples were evaluated in 0.5 mol L?1H2SO4and all linear sweep voltammetry (LSV) curves were corrected with 85%IR to eliminate the effect of internal resistance.Notably,the immersion concentration of Ir salt solution plays a crucial role in determining the HER activity due to the different loading amounts at different concentrations,and the optimized performance was obtained at 5 mmol L?1(Fig.S6).Promisingly,Ir@NC exhibited remarkable HER catalytic activity with an ultra-low overpotential of 23 mV to deliver 10 mA cm?2(η10=23 mV) in acidic solution,better than the original NC with negligible activity,Ir@C(η10=37 mV),and even the commercial Pt/C (η10=28 mV) (Fig.5b).Notably,Ir@NC outperforms the control sample,especially the sample of Ir@C,even more at higher current densities,highlighting the pivotal role of the electronic interaction between Ir and N in promoting the HER activity.Additionally,Ir@NC possesses a higher electrochemically active surface area (ECSA) of 141 m2g?1than commercial Pt/C (30.7 m2g?1) (Fig.S7),attributed to the well dispersed Ir nanoclusters enabled by the pyridinic N.Promisingly,the HER activity of Ir@NC outperforms most of the recently reported typical Ir-based electrocatalysts[6,9–10,18,29,33–34,40–44](Fig.5d).The data comparing various performance metrics,including overpotential and Tafel slope,have been list in Table S6.Furthermore,the Tafel slope,reflecting the kinetics of the HER,is only 25.8 mV dec?1for Ir@NC (Fig.5c),indicating the Volmer-Tafel mechanism and substantially lower than Ir@C (44.2 mV dec?1),even Pt/C(29.4 mV dec?1).To further investigate the charge transfer and reaction kinetics of the HER process,electrochemical impedance spectroscopy (EIS) was performed.In Fig.5e,all the Nyquist curves of different samples display a near-semicircle shape and the sample of Ir@NC exhibited the smallest diameter of the semicircle,indicating the lowest charge transfer impedance.These findings indicate that the loading of Ir on the NC support,particularly the electronic structure modification of Ir through metal-support interaction (as verified by XPS results),can expedite the charge and electron transfer process,thereby contributing to the significantly enhanced HER performance[37–38].

Fig.5 HER catalytic performance of different electrocatalysts in 0.5 mol L?1 H2SO4.(a) LSV polarization curves of HER.(b) Overpotentials of different catalyst to achieve 10 and 50 mA cm?2.(c) Corresponding Tafel slope.(d) Comparison of the overpotential at 10 mA cm?2 and Tafel slope of Ir@NC with the recently reported Ir-based HER catalysts in 0.5 mol L?1 H2SO4.(e) Nyquist plots.(f) Chronopotentiometric curves of Ir@NC,Ir@C and Pt/C at 10 mA cm?2 without IR correction

Ir@NC exhibits good stability at 10 mA cm?2under acidic conditions for more than 24 h (Fig.5f),apparently better than the commercial Pt/C sample and Ir@C.Additionally,the structure of the NC and the cubic morphology of the Ir@NC sample were well preserved after the stability test (Fig.S8a and S8b).More importantly,Ir nanoclusters were still well-distributed on the NC framework without any significant agglomeration and dissolution (Fig.S8c).The average size of Ir clusters after the stability test is about 1.72 nm,which is close to the original size of 1.78 nm(Fig.S8d).These results indicate that the metal-support electron interactions between the Ir clusters and the NC support inhibit the aggregation and dissolution of the Ir clusters,unambiguously contributing to the good stability of Ir@NC during HER under acidic conditions.

4 Conclusion

In conclusion,our study demonstrates the successful synthesis of uniformly dispersed ultra-small Ir nanoclusters on the porous nitrogen-doped carbon derived from ZIF-8.These Ir@NC catalysts exhibit remarkable electrocatalytic activity for HER in acidic electrolytes.With a low overpotential of only 23 mV,Ir@NC achieves 10 mA cm?2,showcasing its high efficiency.The Ir nanoclusters are uniformly dispersed thanks to the rich porous structure of the N-doped carbon support.Furthermore,the existence of pyridinic N on the surface of the NC support plays a crucial role in immobilizing the Ir nanoclusters and establishing strong electron interactions between the support and the Ir clusters.This synergistic effect enhances the overall catalytic performance of Ir@NC.Additionally,the Ir@NC catalyst exhibits excellent electrochemical and structural stability,thanks to the coupling effect between the Ir and pyridinic N atoms.The facile synthesis method of Ir@NC further enhances its potential for widespread use in commercial proton exchange membrane electrolysis.These findings highlight the significance of precisely modulating the electronic structure using low-active supports in the design of high-performance electrocatalysts for efficient hydrogen production.

Acknowledgements

This research was funded by the China Postdoctoral Science Foundation (2020M671990) and the Qingdao Applied Fundamental Research Project.