Size and Electronic Modulation of Iridium Nanoparticles on Nitrogen

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Energy, Environmental, and Catalysis Applications

Size and Electronic Modulation of Iridium Nanoparticles on Nitrogen Functionalized Carbon toward Advanced Electrocatalysts for Alkaline Water Splitting Hua Wang, Mei Ming, Min Hu, Caili Xu, Yi Wang, Yun Zhang, Daojiang Gao, Jian Bi, Guangyin Fan, and Jin-Song Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07639 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 17, 2018

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ACS Applied Materials & Interfaces

Size

and

Electronic

Modulation

of

Iridium

Nanoparticles on Nitrogen Functionalized Carbon toward Advanced Electrocatalysts for Alkaline Water Splitting Hua Wang†, Mei Ming†, Min Hu†, Caili Xu†, Yi Wang†, Yun Zhang†,‡,*, Daojiang Gao†,*, Jian Bi†, Guangyin Fan†,*, and Jin-Song Hu‡ †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068,

China. ‡

CAS

Key

Laboratory

of

Molecular

Nanostructure

and

Nanotechnology,

CAS

Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. KEYWORDS: HER, OER, electrocatalysis, iridium catalyst, N-doping

ACS Paragon Plus Environment

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ABSTRACT

Developing efficient catalytic materials for electrochemical water splitting is important. Herein, uniformly dispersed and size-controllable iridium (Ir) nanoparticles (NPs) were prepared using a nitrogen-functionalized carbon (Ir/CN) as the support. We found that nitrogen function can simultaneously modulate the size of Ir NPs to substantially enhance the catalytically active sites and adjust the electronic structure of Ir, thereby promoting electrocatalytic activity for water splitting. Consequently, the as-synthesized Ir/CN shows excellent electrocatalytic performance with overpotentials of 12 and 265 mV for hydrogen and oxygen evolution reactions in basic medium, respectively. These findings may pave a way for designing and synthesizing other similar materials as efficient catalysts for electrochemical water splitting.


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INTRODUCTION Hydrogen is regarded as the greenest fuel considering its clean burning nature and environmental friendliness. Compared with traditional techniques such as stream methane reforming, coal gasification, and chemical hydride hydrolysis to produce hydrogen, electrochemical water splitting is considered as the green and sustainable method.1-4 During water splitting, hydrogen and oxygen are released at the cathode and anode, respectively. However, serious electrode polarization effects lead to the consumption of additional energy in driving electrocatalytic water splitting, which greatly restricts its practical applications.5-6 Accordingly, various materials based on noble and non-noble metals are prepared and explored to act as the catalyst and reduce the high overpotentials on both anode and cathode sides. Between these two types of metals, the former is considered more effective in electrocatalyzing water splitting. Pt has been reported as the advanced electrocatalytic material toward hydrogen evolution reaction (HER), whereas Ir or Ru are the benchmark electrocatalytic materials to drive oxygen evolution reaction (OER). Considering the high prices and limited reserves of noble metals, boosting their catalytic performance and reducing their loading are among the most efficient ways for enabling electrochemical water splitting although several methods have been developed.5, 7-10 Nevertheless, most studies have focused on advancing the electrocatalytic properties toward only HER or OER, whereas research on electrocatalyst that can show both high HER and OER activities is limited.11-12 Among various developed catalysts, Ir nanoparticles (NPs)-based materials have attracted numerous interests particularly in electrocatalysis for OER or HER.12-14 Considerable attentions have been exerted in boosting the catalytic performance of these materials, but several disadvantages persist.15-16 First, the as-obtained NPs with relatively large size and polydispersity


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are unfavorable for practical applications, especially for catalytic reactions.9 Second, capping agents or carbon layers coated with well-defined NPs limit their surface accessibility by blocking catalytic sites, causing a deterioration in catalytic property.17-18 Third, to enhance the conductivity and dispersity of as-synthesized NPs, an additional deposition step is usually required.12 Fourth and last, despite several advantages in HER or OER, recent studies on Ir-based nanomaterials have focused on only HER or OER. Thus, obtaining small and uncapped Ir NPs is ideal but challenging, especially highly dispersed Ir NPs on suitable supports, to augment accessible active sites for boosting the electrocatalytic activity of HER and OER. Catalytic performance can be improved by modulating the electronic structures of supported catalysts by tuning the interaction between metals and supports.19-22 Nitrogen (N)-decorated carbon materials have shown potential applications in terms of finely loading metal NPs for various catalytic reactions.23-26 N-functionalization enables the decoration of carbon materials with a massive amount of N species, functioning as coordination sites for controlling NP nucleation and growth and then guaranteeing the synthesis of ultrafine and monodisperse supported catalysts.27-29 Consequently, numerous accessible active sites are provided for catalysis reaction. These N-containing groups can also finely regulate the electronic properties of metal NPs,30 and this can benefit electron transfer-related reactions, improve the adsorption and/or desorption of reaction participates, and endow the initial active sites with enhanced catalytic ability.31-32 Therefore, the delicate loading of Ir NPs on suitable N-doped carbon is highly significant in further improving HER and OER. Herein, a convenient strategy for preparing supported ultrafine and monodisperse Ir NPs supported on N-functioned carbon (CN) was developed. Premodulating carbon with N is the key for controllable preparation of such Ir NPs, which endows massively accessible active areas for


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HER and OER. Moreover, these N-containing groups on carbon support finely modulate the electronic property of Ir NPs, improving the intrinsic activity of Ir NPs. As expected, the asprepared Ir/CN shows excellent electrocatalytic performance with 12 mV overpotential at 10 mA cm-2 for HER in basic medium. Interestingly, Ir/CN displays outstanding performance for OER with 265 mV overpotential at the benchmark current density. This research supplies an effective and easy route for synthesizing highly active electrocatalyst materials for alkaline water splitting. EXPERIMENTAL SECTION Chemical and materials. Hexachloroiridium acid hydrate (H2IrCl6.xH2O, Ir≥36%), ruthenium (III) chloride hydrate, iridium oxide (IrO2, Ir≥84.5%), ethylene glycol, ascorbic acid, urea, and ethanol were provided by Aladdin Industrial Inc. Platinum supported on carbon, normally 20 wt% loading, was provided by Alfa Aesar. Nafion (5 wt%) and carbon black (Carbot Corp.) were supplied by Sigma–Aldrich. No further treatments were performed before the use of the available chemicals and materials. Ultrapure water was used to prepare samples and electrochemical measurements. Synthesis of CN. CN was prepared using the following procedure. First, 1.0 g of carbon and 1.5 g of urea were thoroughly mixed. The mixture was then transferred to a ceramic crucible and calcined at 300 oC for 2 h. After rinsing by ethanol thrice, resultant black solid was transferred to an oven at 60 oC overnight. After mixing 1.0 g of the dried sample with 2.0 g of urea again and calcining at a low temperature of 175 oC for 4 h, the washing and drying steps were repeated. The obtained black solid was stored in a glass vial for further use. Synthesis of Ir/CN and Ir/C. Ir/CN was prepared through a one-pot convenient solvothermal strategy. Typically, 75 mg of CN, 150 mg of ascorbic acid, 20 mL of ethylene glycol, and 16 mg


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of hexachloroiridium acid hydrate were added to a 50 mL vial. The vial was capped, ultrasonicated for 30 min to achieve a homogeneous mixture, transferred into a hydrothermal reactor, and heat treated at 180 oC for 5 h. Afterward, rinsing the colloidal product with ethanol thrice and placed in a vacuum-drying oven, the temperature was maintained at 60 oC for 24 h. As a control sample, Ir/C was synthesized through the same procedure except that carbon black was used instead of CN matrix. Characterization. All synthesized materials were characterized on a transmission electron microscopy system (JEM-2100F and JEOL). Moreover, the size distributions of Ir NPs were analyzed based on over 300 particles by measuring their diameters. Structural information of the as-synthesized materials was recorded on a Rigaku D/Max-2500 diffractometer with copper Kα radiation. The corresponding element compositions as well as chemical states were recorded by a Thermo ESCALAB 250 Axis Ultra analyzer. Electrochemical tests. All electrocatalytic measurements were conducted on a standard threeelectrode setup connected with CHI 760E setup (Shanghai, Chenhua Corp.). The reference and counter electrodes are Hg/HgO electrode and graphite rod. The working electrode is the glassy carbon electrode (GCE, diameter = 4 mm) loaded with catalyst. In a typical procedure, 2.0 mg of each sample, 800 µL of ethanol together with 8 µL of nafion solution were ultrasonically dispersed to form a homogenous suspension. A 25 µL suspension was then distributed on the GCE surface. After that, the work electrode was successfully obtained (loading = 497.6 ug cm-2). For reference, Pt/C was deposited onto the GCE surface with the same mass loading using same procedure. Linear sweep voltammetry (LSV) was performed to determine the overpotentials of samples. The scan rate was controlled at 5 mV per second. Unless specifically mentioned, the potentials obtained in this study were calibrated to RHE.


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RESULTS AND DISCUSSION In a typical preparation, N-doped carbon together with Ir precursor was initially dispersed in ethylene glycol. In situ chemical reduction of Ir precursor was then performed by solvothermal method. For reference, Ir/C was synthesized using carbon black instead of CN matrix as support. The structure and morphology of Ir/CN and Ir/C were investigated by TEM. As shown in TEM image of Ir/C (Figure 1a), Ir NPs onto carbon without N-functionalization are in large size (2.1 nm diameter) and severe aggregation (Figure 1b) due to few nucleating sites. By contrast, homogeneously dispersed Ir NPs in Ir/CN with a diameter of 1.3 nm without obvious aggregation are achieved, as illustrated in Figures 1c and d. The successful synthesis of Ir nanocrystals can be confirmed by observing the typical (111) plane of metallic Ir with a lattice spacing of 0.22 nm (Figure 1e). Furthermore, the STEM-EDS images (Figures 1f-i) suggest the homogeneous dispersion of element Ir, O, and N in the Ir/CN. X-ray diffraction (XRD) was performed for analyzing crystal structures of these samples. As can be seen from Figure S1, a peak at 41o is observed in the XRD pattern of Ir/C, well corresponding to Ir (111) plane, whereas a wider peak is achieved at the same position for Ir/CN, indicating smaller Ir NPs in Ir/CN than that in Ir/C. These findings demonstrate that the carbon substrate modified with N species is highly beneficial for Ir nucleation and distribution, resulting in the formation of highly-dispersed ultrasmall Ir NPs. Reasonably, a highly active surface area may be expected.


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Figure 1. TEM images and corresponding Ir NP distribution for (a, b) Ir/C and (c, d) Ir/CN. (e) HRTEM image of Ir/CN. (f) STEM image and EDS mapping images of Ir/CN: (g) Ir, (h) O, and (i) N. X-ray photoelectron spectroscopy (XPS) was also performed to characterize these catalysts. As shown in Figure 2a, the elements C, O, and Ir are detected on the near-surface of carbon in both samples based on XPS survey scans. Conversely, only the survey spectrum of Ir/CN presents the N peak, indicating the successful N-functionalization of the carbon matrix. Furthermore, the N1s spectrum in Ir/CN sample is divided into three major peaks, which can be indexed to pyridine N,33-34 amine and amide groups (C-NH and C-NH2), and quaternary N18 (Figure S2). Highresolution Ir 4f-orbital spectra are also recorded to examine the chemical state of Ir in both samples (Figure 2b). The Ir 4f spectra of both samples can be divided into two sets of doublets due to the presence of different states of iridium. Regarding Ir/C, the doublet attributed to the 4f7/2 and 4f5/2 components located at the binding energies of 61.38 and 64.38 eV, corresponds to


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Ir0, whereas the doublet with components located at 62.45 and 65.40 eV, is assigned to Ir4+, respectively. However, the binding energy of Ir0 4f7/2 in Ir/CN positively shifted 0.27 eV compared with the Ir/C. The shift confirms the electron interaction between Ir NPs and N-doped carbon substrate.

Figure 2. (a) XPS full spectra and (b) Ir 4f spectra of Ir/CN and Ir/C. The electrocatalytic property of Ir/CN was studied for HER catalysis in basic medium (1.0 M KOH). For references, CN and Ir/C were also explored as electrocatalysts for HER under identical conditions. The corresponding polarization curves with iR correction were recorded and displayed in Figure 3a. CN without Ir displays negligible HER electrocatalytic activity, indicating that CN support is not very active for HER. After loading Ir NPs, Ir/CN and Ir/C samples possess significantly enhanced HER electrocatalytic activities. Specifically, Ir/C delivers a 32 mV overpotential at the benchmark current density, whereas Ir/CN sample with same Ir loading displays a lower overpotential of 12 mV (Figure 3b). These results indicate that N modification of carbon support is indispensable to the enhancement of HER activity. Notably, the HER activity of Ir/CN is better than commercial Pt/C (η10 =16 mV) (Figure S3a) and many


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other reported Ir- and Pt-based HER electrocatalysts at 10 mA cm-2, such as IrNi/NCs (η10 =15 mV),12 Ir/CC (η10 =28 mV),13 Pt/CC (η10 =52 mV),13 Ru@C2N (η10 =22 mV),35 Pt/MoS2 (η10 =60 mV),36 Pt3Ni3 (η10 =30 mV),37 and PtFeCo (η10 =50 mV)38 (Figure 3c). To study HER kinetics of Ir/CN and Ir/C, the Tafel plots of potential vs. current density in logarithm scale were constructed. Specifically, the Tafel slope of Ir/CN is 28 mV dec-1, which is smaller than commercial Pt/C, Ir/C as well as CN, demonstrating the excellent HER kinetics of Ir/CN in alkaline medium (Figures 3b and S4). According to the Tafel slope range, HER catalysis over Ir/CN is presumably underwent the Volmer-Tafel mechanism.13 Moreover, the cyclic voltammetry (CV) range adopted was -0.03 to 0.01 V to measure the HER durability of Ir/CN in alkaline medium. As displayed in Figure 3d, no noticeable changes in overpotential are observed before and after 1000 CV cycles, suggesting the high HER durability of Ir/CN in alkaline medium.


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Figure 3. (a) HER LSV curves and (b) Tafel slopes and overpotentials at the benchmark current density with Ir/CN, Ir/C, and CN. (c) Comparison of the overpotential of Ir/CN with other reported catalysts. (d) HER polarization curves over Ir/CN before and after 1000 CV cycles.


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Figure 4. (a, b) CV curves at different sweep rates and (c) capacitive current at 0.075 V versus scan rates for Ir/CN and Ir/C. The superior electrocatalytic property for HER over Ir/CN can be explained as follows. First, the structure sensitivity of electrocatalytic water splitting over the two samples is analyzed. Increased metal dispersion is known to result in increased number of exposed reactive sites, thereby leading to higher activity for electrocatalytic water splitting. Metal dispersion, reportedly used to roughly estimate the average size of NPs, was adopted to measure the fraction of exposed reactive sites in catalyst.39 Basis on the results of TEM analysis, the dispersion of Ir NPs in Ir/CN is calculated to be 84.9%, much higher than that of Ir/C (52.6%), again demonstrating the high dispersion of Ir NPs in Ir/CN. According to above results, the catalytic activities of Ir/CN are expected to be good, which can be confirmed by the characterization results of electrochemical surface area (ECSA) by testing capacitance (Cdl) in non-faradaic region. As demonstrated in


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Figure 4, Ir/CN possesses a considerably higher Cdl of 22.61 mF cm-2 than Ir/C (17.32 mF cm-2), indicating a higher ECSA for Ir/CN. Considering the similar BET surface areas of both samples as listed in Figure S5, the Ir/CN with a high ECSA implies that it has more surface active site available for electrocatalytic water splitting than Ir/C. Moreover, the XPS demonstrated interaction between Ir NPs and N-doped carbon would modulate the electronic structure of Ir NPs, thus the adsorption/desorption of water/hydrogen intermediate species, therefore, contributing to the improvement of electrocatalytic water splitting. Additionally, the catalytically active components for HER over Ir-based catalysts are reportedly sensitive to valence of Ir species and zero-valent Ir can deliver the best catalytic activity. For demonstrating this, we initially performed OER for an oxidation of Ir NPs and subsequent taken a HER test over Ir/CN. As shown in Figure S6a, Ir/CN after OER test to get the benchmark current density should need an overpotential of 111 mV, 99 mV overpotential larger than itself before OER test. This significantly decreased electrocatalytic activity for HER should ascribed to the oxidation of Ir NPs and again demonstrates the Ir0 in Ir/CN is the main active specie for HER. As calculated from XPS results, the fraction of Ir0 in Ir/CN was 0.64, which is much higher than that of Ir/C (0.50). The numerous Ir0 active specie means Ir/CN with abundant active sites for HER. Furthermore, it was reported that pyridinic N can reduce the adsorption energy barrier of reactants on adjacent carbon atoms and promote the electron transferring, resulting in a prominent reactivity improvement of HER catalytic activity.40-42 Recent works have found that quaternary N presence in graphene matrix can boost heterogeneous electron distribution, corresponding to the enhanced electrocatalytic reactivity of carbon matrix.42-43 As shown in Figure S6b, the electrocatalytic activity of CN material is higher than carbon support without N modification, verifying the vital role of nitrogen dopants in carbon support to boost HER activity.


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Overall, the improvement in catalytic activity for HER after N-doping over Ir/CN should be ascribed to the high dispersion and ultrafine Ir NPs, electron interaction between Ir NPs and CN, increased zero-valent Ir specie, and N dopants. Notably, Ir/CN also displays excellent electrochemical activity toward OER in basic solution. Figure 5a displays the LSV curves of CN, Ir/C, and Ir/CN samples. It can be clearly seen that the CN is nearly inactive for OER whereas Ir/CN exhibits excellent OER reactivity with a 265 mV overpotential at benchmark current density. The OER catalytic activity of Ir/CN outperforms the commercial IrO2 (η10 = 314 mV), commercial Pt/C (η10 = 487 mV), and Ir/C (η10 =289 mV), solidly demonstrating the improved electrocatalytic activity of Ir/CN by pre-modification of carbon substrate via N doping (Figure 5b and S3b). The OER electrocatalytic activity of asprepared Ir/CN is also better than those of reported Ir-based catalysts, such as IrNi NCs (η10 =270 mV),12 Ir/GF (η10 =290 mV),13 IrOx/SrIrO3 (η10 =290 mV),44 Ir/C (η10 =289 mV), iridium oxide (η10 =300 mV),45 IrNi oxide (η10 =310 mV),9 IrOx (η10 =320 mV),46 mesoporous IrOx (η10 =320),47 and IrNiOx/Meso-ATO (η10 =321 mV)10 (Figure 5c). Calculating from Figures 5b and S7, the Tafel slope of Ir/CN is 35 mV dec-1, smaller than commercial Pt/C, commercial IrO2, Ir/C as well as CN, indicating kinetic superiority for OER over Ir/CN. The stability of Ir/CN for OER catalysis was tested using chronoamperometric measurement. As displayed in Figure 5d, the degradation of V-t curve is negligible during test, indicating the high OER stability of Ir/CN in alkaline medium.


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Figure 5. (a) OER LSV curves and (b) Tafel slopes and overpotentials at the benchmark current density with Ir/CN, Ir/C, and CN. (c) Comparison of the overpotential of Ir/CN with other reported catalysts. (d) Long-term OER stability test of Ir/CN. Inspired by the excellent HER and OER activity of Ir/CN, a water-alkali electrolyzer was assembled using Ir/CN as the both anode and cathode to assess its activity for actual application (Figure S8). The test result displays that the developed electrolyzer can achieve a benchmark


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current density with only 1.50 V cell voltage, better than most of reported efficient materials toward overall water splitting at the same current density, including NiCoP (1.58 V),48 NiCo2O4 (1.65 V),49 FeB2 (1.57 V),50 Cu@CoFe (1.68 V).51 Moreover, the strategy for preparation of Ir/CN also can be used to synthesize Ru NPs supported on N-doped carbon (Ru/CN). TEM images and statistical analysis show that Ru NPs in a diameter of 2.1 nm are highly distributed on the N-doped carbon (Figure S9). This result suggests that our developed method can be applied to prepare other small noble metal NPs except for Ir. CONCLUSION In summary, highly dispersed ultrasmall Ir NPs (1.3 nm) as efficient HER and OER electrocatalysts were synthesized using N-functionalized carbon as supports. The modification of carbon support by N results in the formation of highly dispersed ultrafine Ir NPs with enriched active sites and in the modulation of electronic structure of Ir NPs with improved intrinsic activity. Above merits promise Ir/CN with an outstanding electrochemical catalytic performance toward HER and OER in basic medium. Thus, an easy one-pot approach exploring N-doped carbon as a matrix can offer new opportunities for synthesizing highly dispersed and ultrafine metal NPs for widespread catalysis applications. ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge via the Internet at http://pubs.acs.org: Additional XRD patterns, XPS spectrum, Tafel slopes, N2 adsorption-desorption isotherm, and control experiments.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected]; *Email: [email protected]; *Email: [email protected]. ACKNOWLEDGMENT We acknowledge the financial supports from the Sichuan Youth Science and Technology Foundation (2016JQ0052) and the National Natural Science Foundation of China (21777109). REFERENCES (1) Zhou, W.; Jia, J.; Lu, J.; Yang, L.; Hou, D.; Li, G.; Chen, S. Recent Developments of Carbon-Based Eectrocatalysts for Hydrogen Evolution Reaction, Nano Energy, 2016, 28, 29-43. (2) Zhong, F.; Wang, Q.; Xu, C.; Yang, Y.; Wang, Y.; Zhang, Y.; Gao, D.; Bi, J.; Fan, G. Ultrafine and Highly Dispersed Ru Nanoparticles Supported on Nitrogen-Doped Carbon Nanosheets: Efficient Catalysts for Ammonia Borane Hydrolysis, Appl. Surf. Sci., 2018, 455, 326-332. (3) Fan, G.; Liu, Q.; Tang, D.; Li, X.; Bi, J.; Gao, D. Nanodiamond Supported Ru Nanoparticles as an Effective Catalyst for Hydrogen Evolution from Hydrolysis of Ammonia Borane, Int. J. Hydrog. Energy, 2016, 41, 1542-1549. (4) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting, Chem. Soc. Rev., 2015, 44, 5148-5180. (5) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions, Chem. Soc. Rev., 2015, 44, 2060-2086.


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A Table of Contents

Well-defined Ir nanoparticles loaded on the nitrogen-doped carbon are explored as new efficient electrocatalysts for HER and OER in alkaline solution due to the size and electronic modulation of iridium nanoparticles on nitrogen functionalized carbon.


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Size and Electronic Modulation of Iridium Nanoparticles on Nitrogen

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