Shape-Dependent Electrocatalytic Activity of Iridium Oxide Decorated

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Research Article Cite This: ACS Catal. 2018, 8, 8830−8843

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Shape-Dependent Electrocatalytic Activity of Iridium Oxide Decorated Erbium Pyrosilicate toward the Hydrogen Evolution Reaction over the Entire pH Range Paramita Karfa,*,† Kartick C. Majhi,† and Rashmi Madhuri† †

Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826 004, India

ACS Catal. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/24/18. For personal use only.

S Supporting Information *

ABSTRACT: The synthesis of iridium oxide decorated erbium pyrosilicate (Er2Si2O7:IrO2) was performed by a simple sol−gel technique. By simple alteration of the reaction parameters, different shapes (i.e., cubes, rods, large spheres, small spheres, and sheets) of Er2Si2O7:IrO2 were obtained and their catalytic activities were tested toward the hydrogen evolution reaction (HER). Among the different morphologies of Er2Si2O7:IrO2, cube-shaped nanoparticles (Er2Si2O7:IrO2−5) with sharp edges provided promising HER activity over a wide pH range from 0 to 14 in acidic, neutral, and basic media. Er2Si2O7:IrO2−5 exhibited an onset potential of −0.076 V with a very high current density of 252 mA cm−2 (at −0.54 V). The overpotential and Tafel slope for the HER using Er2Si2O7:IrO2−5 were found to be 130 mV and 49 mV/dec, 170 mV and 59 mV/dec, and 190 mV and 67 mV/dec in 0.5 M H2SO4, 1.0 M KOH, and 2.0 M PBS, respectively. The low cost, highly active electrocatalyst shows robust durability over acidic medium for nearly 250 min. Such a superior catalytic activity can be attributed to the synergistic effect between IrO2 and erbium pyrosilicate, in which the 4f orbital (not fully occupied) of rare-earth elements may have occupied the 5d orbital and become valence electrons, resulting in the improved HER activity. KEYWORDS: iridium oxide, erbium pyrosilicate, anisotropic electrocatalysts, hydrogen evolution reaction, entire pH range

1. INTRODUCTION Replacing fossil fuel energy with clean, green, and renewable energy could be a solution to the energy crisis and related global warming. Among the various possible alternatives, hydrogen energy having high energy density with the lowest possible or nil CO2 emission could be the most promising, reliable, convenient, and clean way out of the problems of global warming, fossil fuel depletion, and energy safety related issues.1,2 In a move toward the production of hydrogen energy, in recent times, electrochemical water splitting is considered as one of the most efficient and promising methods to produce sustainable energy at an economical rate.3 However, due to the internal sluggish kinetics of the hydrogen evolution reaction (HER), a catalyst is always needed that with minimal overpotential can initiate proton reduction and promote hydrogen gas production with towering Faradaic efficiency in spite of consumption of extra energy.4,5 In the present scenario, platinum (Pt)-group metals are the most effectual HER catalysts, but their poor abundance and high cost make them unsuitable for large-scale practical application in hydrogen production.6 Recently, transitionmetal sulfides,6,7 phosphides,8,9 selenides,10,11 and carbides12,13 have been exploited as suitable alternatives to the noble-metalbased catalysts for the HER. In addition, some rare earth element (REE) based catalysts have also been developed for the HER. According to the literature, the main reason behind © XXXX American Chemical Society

the enhanced electrocatalytic activity toward the HER of the rare earth elements is that there are vacant 4f orbitals, and the electrons of the 4f orbital could dwell in the 5d orbital and thus turn into valence electrons, which promotes electronic conduction in the rare earth.14 On the basis of this theory, Rosalbino et al. have prepared ternary Co-Ni-RE electrocatalysts (RE = Y, Ce, Pr, Er) and explored them for the HER in alkaline medium.15 Similarly, Dominguez-Crespo has synthesized Ni-RE (RE = La, Ce) materials by a solid-state reaction technique with an Ni source and explored its activity toward the HER in alkaline medium.16 Santos et al. have synthesized a combination of Pt and REE and used it for hydrogen evolution in alkaline water electrolysis.17 However, the existing studies on the HER are mainly limited to extremely acidic or alkaline conditions. Electrocatalysts working over a wide range of pH have several advantageous and wide applications: for example, acidic conditions are required for proton exchange membranes (PEMs)18 and neutral conditions are required for microbial electrolysis cells (MEC),19 while overall water splitting is usually performed in strongly basic media.20 Therefore, the foremost step toward the development of hydrogen energy is the search for Pt-free Received: April 8, 2018 Revised: August 9, 2018

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for enhanced Faradaic efficiency and current density.33 In this study, five different shapes of Er2Si2O7:IrO2 have been synthesized and, among the different shapes, a cube-shaped pyrosilicate nanocomposite shows the best performance with low onset potential (−0.076 V), small Tafel slope (49 mV/ dec), and small overpotential value (130 mV) to deliver an HER current density of 10 mA cm−2.

electrocatalysts, which could be prepared with sparing use of the precious-metal component and satisfy both good catalytic performance and reasonable economy over a wide pH range. Keeping in view the requirements and problem, herein, we have tried to develop different shapes (sheets, small spheres, large spheres, rods and cubes) of iridium oxide (IrO2) decorated erbium pyrosilicate (Er2Si2O7:IrO2) by a simple sol−gel technique with variation of the reaction conditions. On the basis of the different proportions of REE, Si, and O atoms, the RE-Si-O compounds can be divided into three different classes: oxy-orthosilicate (RE 2 SiO 5 ), apatite type (RE4.67Si3O13), and dislocate or pyrosilicate (RE2Si2O7). These compounds are basically popular as good host lattices for phosphors,21 in which pyrosilicates are mainly potential scintillator hosts.22 However, the use of pyrosilicate in energy applications such as the oxygen evolution reaction and hydrogen evolution reaction has still not been studied. In general account, 60% of the Earth’s crust is made up of silicate minerals, which make silicon the second most plentiful material.23 Therefore, if we can use silicate compounds in the energy field, the price could be lowered to some extent. Lithium iron orthosilicate (Li2FeSiO4) paved the way toward the synthesis of potential cathode materials due to its high theoretical capacity (332 mAh g−1) in addition to its high thermal stability due to strong inner Si−O bonding, low cost, and environmental friendliness.24 Silica and silicate-based polyanions are the main skeletons behind the direct synthesis of various silicates, and currently they are attractive research topics for Li and Na ion batteries. Unfortunately, these silicatebased cathode materials have certain disadvantages such as low rate capabilities and consequent low electronic and ionic conductivities of the material.25 To improve the electrical properties of these compounds, a large amount of effort is still needed such as nanoarchitecturing, easy synthesis process, and cation doping, as part of the ongoing research interest. Herein, we have incorporated Ir to improve the electrocatalytic properties of erbium pyrosilicate. According to the literature, Ir-based catalysts are very popular in the oxygen evolution reaction (OER) and several compounds such as CuIr nanocages,26 IrO2-TiO2,27 and IrO2-polymer nanocomposites28 have been reported in support of their extraordinary OER behavior. However, not much literature has been cited which leads toward the HER behavior of Ir-based catalysts and that can show behavior comparable with that of Pt.29,30 For example, Lim et al. have reported 3d and 4d metal decorated reduced graphene and studied their activity toward the HER.31 According to them, iridium-doped graphene with early onset potential and low Tafel plots proved to be an outstanding material despite the smallest amount of energy required to effect evolution of hydrogen gas or the reaction kinetics. Similarly, Kuttiyiel et al. have prepared hollow core−shell iridium-nickel nitride nanoparticles for the HER and showed that incorporation of Ir can enhance the HER activity of nanomaterials to a level comparable to that attained by Pt.32 Similar to the reported results, we have also found that incorporation of IrO2 in erbium pyrosilicate is able to enhance the HER activity and the resulting catalyst is efficient, stable, and durable under acidic, neutral, and basic pH conditions. Among the various literature reports for HER/OER or water splitting, there have been limited studies on the influence of nanomaterial morphology or shape. It is also found that a change in morphology of the nanomaterial results in a change in the edge to corner ratio, which is a very important parameter

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Analytical grade chemicals were used. Tetraethyl orthosilicate (TEOS), potassium hexachloroiridate(IV), and citric acid were purchased from Sigma-Aldrich, while solvents such as ethanol, dimethylformamide (DMF), nitric acid, and sulfuric acid were purchased from Merck (India) and Spectrochem Pvt. Ltd. (India). Cetyltrimethylammonium bromide (CTAB) and potassium hydroxide were purchased from TCI Chemicals (India). Erbium(III) nitrate pentahydrate (Eu(NO3)3·5H2O) and sodium hydrogen citrate were purchased from Alfa Aesar. 2.2. Synthesis of IrO2-Decorated Different-Shaped Erbium Pyrosilicate (Er2Si2O7:IrO2). At first a silica sol was prepared by the acid-catalyzed hydrolysis of tetraethoxysilane (TEOS) with deionized water.34 For this, 30.0 mL of ethanol and 34.82 mL of water were mixed together in a round-bottom flask and the reaction mixture was kept at 40 °C for 1/2 h. Then, to the reaction mixture was further added 6.2 mL of TEOS in a dropwise manner followed by addition of different amounts of CTAB. The solution was stirred at different rotation speeds (change in rpm) for 1 h at room temperature, and the pH of the reaction mixture was adjusted to 2 using dilute nitric acid. After this, a mixture of 0.1 mM of the erbium nitrate and 0.3 mM of citric acid (as a chelating agent) was added to the silica sol and the mixture was stirred for 15 min at various speeds. A gel was formed, which was first dried for 84 h at 100 °C; then to remove alcohol, water, and citric acid, samples were heated at 400 °C for 2 h. The resulting dried gel material was ground and annealed at 900 °C for 2 h, to give a brown powder sample. A 0.5 g portion of the prepared powder sample was then placed in a round-bottom flask containing K2IrCl6 solution (0.08 mM in 10 mL of DMF) and 5 mM sodium hydrogen citrate. The pH of the reaction mixture was maintained at 7, and at the fixed pH refluxing was carried out for 2 h. The resultant product was centrifuged at 10000 rpm, dried, and stored in a vacuum desiccator. During the synthesis, the amounts of CTAB and rotation speed were varied to obtain different shapes of nanomaterials (Table S1). 2.3. Instrumentation. A field-emission scanning electron microscope (FE-SEM, Zeiss Model Supra 55) was used to capture the morphological study, elemental imaging, and EDAX spectra of the prepared Er2Si2O7:IrO2 nanocomposites. Powder X-ray diffraction (XRD) patterns were recorded on Xpert Pro MPD diffractometer using a Cu radiation source (λ = 30 mA). BET analysis was done by a Micromeritics 3 Flex Surface Characterization Analyzer. ESCA+ (Omicron Nanotechnology, Oxford Instrument Germany) was the instrument used for X-ray photoelectron spectroscopy (XPS) equipped with an aluminum Source (Al K radiation, hν = 1486.7 eV). A JEM-1400 transmission electron microscope (TEM) was used for TEM and SAED analysis. ICP-MS data were recorded with an Agilent ICP-MS 7900 instrument. More information related to instrumentation is given in the Supporting Information. All of the electrochemical experiments were conducted at room temperature (25 ± 1 °C). 8831

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Figure 1. FE-SEM analysis of differently shaped Er2Si2O7:IrO2: (A) Er2Si2O7:IrO2−1 (sheetlike morphology); (B) Er2Si2O7:IrO2−2 (small spheres); (C) Er2Si2O7:IrO2−3 (large spheres); (D) Er2Si2O7:IrO2−4 (rods); (E) magnified image of Er2Si2O7:IrO2−4 (rods); (F) Er2Si2O7:IrO2−5 (cubes); (G) magnified image of Er2Si2O7:IrO2−5 showing one full cube with sharp edges. XRD spectra of (H) Er2Si2O7:IrO2−3 (large spheres), Er2Si2O7:IrO2−2 (small spheres), and Er2Si2O7:IrO2−1 (sheet) and (I) Er2Si2O7:IrO2−5 (cubes) and Er2Si2O7:IrO2−4 (rods).

2.4. Turnover Frequency (TOF) Calculations. To calculate the TOF value, first the number of active sites (n) has to be calculated, and it is obtained by recording the CV in 0.5 M H2SO4 in the region of −0.2 to +0.6 V vs RHE at a scan rate of 50 mV/s. Using the formula given below, the total charge and number of active sites of the nanocomposites were calculated from CV runs.35

Electrochemical measurements were performed on a CHI instrument (USA, Model No. 660C) in a three-electrode onecompartment airtight cell organized with nanomaterialmodified free-standing carbon tape as the working electrode, a Pt-wire electrode as the counter electrode, and Ag/AgCl (in 3.0 M KCl electrolyte) as the reference electrode. For the fabrication of the working electrode, a mixture was prepared by evenly grinding 50.0 mg of nanomaterial and 0.5 mL of Nafion solution (5 wt %), to which we added 2 mL of a 1:1 volume ratio of water−ethanol solution. The mixture was then transferred into a glass vial and ultrasonicated for 1 h to form a homogeneous ink. The carbon tape was cut into 1 cm2 pieces, modified by the prepared ink, and dried at 100 °C for 20 min. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), controlled-potential electrolysis (CPE), and multistep chronoamperometry analysis were performed in an airtight sealed cell with N2 (99.9%) purging throughout the electrochemical reaction, to evaluate the electrocatalytic activity of the prepared nanocomposite toward the HER. The electrochemical analysis was performed in three different pH solutions: i.e., 0.5 H2SO4, 1.0 KOH, and 2.0 M phosphate buffered saline (PBS). For comparison of the catalytic activities of the prepared materials, a 0 wt % Pt/C (Alfa Aesar) modified electrode was also used and examined under the same conditions. All potentials were recorded with reference to the Ag/AgCl electrode in the CHI instrument and calculated and presented in terms of the reversible hydrogen electrode (RHE), using the equation ERHE = EAg/AgCl + 0.059 × pH + EθAg/AgCl. Here, EAg/AgCl is considered as the experimental potential calculated by the Ag/AgCl reference electrode and EθAg/AgCl is the customary potential of Ag/AgCl at 25 °C.

Q = 1/V

∫E

E2

I dE

1

n = Q /2F

where E is the potential, Q is the charge, and V is the scan rate. When the value of n is placed in the following equation, persite turnover frequencies (TOFs in s−1) can be calculated:35 TOF = I (A)/2Fn

where I is the current (in A) recorded during the linear sweep voltammetry measurement at certain overpotential, F is the Faraday constant (C mol−1), and n is the number of active sites (mol) present in the electrode. The factor 1/2 present in the equation is due to the two electrons necessary for the formation of one hydrogen molecule starting from two protons (2H+ + 2e → H2).

3. RESULTS AND DISCUSSION 3.1. Characterization of the Prepared Nanocomposite. 3.1.1. Morphological Characterization of the Nanocomposites. For the synthesis of differently shaped Er2Si2O7:IrO2, the amount of CTAB and rotation speed were varied. It was found that, by changing the concentration of 8832

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process. However, the sharp peaks at 28.9, 40.0, 53.8, 55.4, and 57° can be assigned to the (110), (200), (211), (220), and (201) planes of IrO2 corresponding to JCPDS Card No. 860330. The assignment of peaks for Er2Si2O7 and IrO2 in the prepared nanocomposites clearly supports the successful synthesis of IrO2-decorated erbium pyrosilicate. The presence of similar peaks in the XRD pattern of each nanocomposite also suggests that their composition was not altered with changes in their morphology during synthesis. According to the JCPDS database, the prepared nanocomposite shows a monoclinic structure with a primitive lattice, which was confirmed with a = 4.683 Å, b = 5.556 Å, and c = 10.790 Å. The XRD pattern was also found to be clearly different from the standard JCPDS pattern of erbium oxide, which confirms the formation of pyrosilicate material. The XRD patterns of different shapes of nanomaterials, i.e. cubes, rods, large spheres, small spheres, and sheets, shows that all the diffraction peaks are at nearly the same 2θ values and morphology variations had little effect on the XRD pattern apart from a minor variation of ±1° shift in the 2θ positions. It was also noticed that the majority of the diffraction peaks at different 2θ values became sharper in the order nanocube > nanorod > large sphere > small sphere > sheet, owing to the increase in crystallite length. As we can see, the XRD pattern of sheets shows broad peaks in comparison to the other shapes of nanomaterials, which may possibly be due to their poor crystallinity. Additionally, the XPS spectrum of cube-shaped Er2Si2O7:IrO2−5 was also recorded and is shown in Figure S2 in the Supporting Information. The deconvuluted XPS spectra of Er 4d show a sharp peak at the binding energy of 172.2 eV, indicating the trivalent state of Er in the nanocomposite (Figure S2A in the Supporting Information).38 The deconvuluted XPS spectra of Si 2p show a single peak at 105.2 eV, which suggests that Si is in the +4 oxidation state in the nanocomposite, tetrahedrally coordinated to oxygen (Figure S2B in the Supporting Information).38 Other than these, Ir 4f shows a doublet (7/2 and 5/2) in the binding range of 58−70 eV. Two binding states of Ir are identified as 4f7/2 and 4f5/2 and are found at binding energies of 63.8 and 65.7 eV, which are very close to the values reported for a single crystal of IrO2 (Figure S2C in the Supporting Information). The peaks clearly suggest the presence of the +4 oxidation state of Ir in the present nanocomposite.39,40 The single peak at 534.2 eV for O 1s suggests the −2 oxidation state of oxygen (Figure S2D in the Supporting Information).39,40 For further compositional study, elemental mapping and the EDAX spectrum of the nanocomposite was also recorded. The EDAX spectra and corresponding elemental mapping analysis clearly confirm the presence of O, Si, Er, and Ir in the nanocomposite. In the elemental mapping a large amount of oxygen is seen, which confirms that oxygen is the major element in the nanocomposite. Additionally, it was also observed that the ratio Er:Si:O in the final sample was about 2:2:8 and the concentration of Ir was about 2%, which also confirms the successful synthesis of Er2Si2O7:IrO2. The amount of Ir in the cube-shaped nanocomposite was also calculated by inductively coupled plasma-mass spectrometry (ICP-MS) and found as 3.68 mg g−1. The EDX spectra of differently shaped nanocomposites were also recorded, and the obtained elemental percentage table is shown in Figure S3A−F in the Supporting Information. It was found that the percentage of Ir is almost constant: i.e., ∼2% in the entire prepared nanocomposite, irrespective of their different shapes.

CTAB and reaction stirring rate, different shaped/morphologies of Er2Si2O7:IrO2 could be easily synthesized. For morphological characterization, an FE-SEM analysis of differently shaped Er2Si2O7:IrO2 was performed and is shown in Figure 1A−G. As is evident from the figure, with an increase in CTAB amount and decrease in rotation speed the morphology of the nanomaterial was drastically changed and transformed to a sharp cube like material from a plane sheet like structure (Table S1 in the Supporting Information). On the basis of the obtained shape of Er2Si2O7:IrO2 during synthesis and some of the reported literature,36,37 we have sketched a probable mechanism to explain their formation process (Scheme S1 in the Supporting Information). First, a sheetlike morphology of the nanocomposite (Er2Si2O7:IrO2−1) was obtained, which has a layered and folded structure. On the surface of the sheets, some small nanoparticles are also visible. Afterward, the sheet might become broken with an increase in the concentration of CTAB and small spheres of Er2Si2O7:IrO2−2 having a diameter of ∼5 nm were obtained. With a further increase in CTAB concentration, the agglomeration of small nanoparticles started, resulting in the formation of large spheres of Er 2 Si 2 O 7 :IrO 2 − 3 of diameter ∼50 nm, rod-shaped Er2Si2O7:IrO2−4 having a diameter of ∼50−100 nm, and cubelike Er2Si2O7:IrO2−5.36,37 The cube-shaped Er2Si2O7:IrO2−5 with spectacular sides and edges has a size of ∼100−200 nm. A further increase in CTAB concentration was also studied but did not reveal any particular shape of nanomaterial (Er2Si2O7:IrO2−6, Table S1 in the Supporting Information). In addition, the synthesis was also performed in the absence of CTAB, which led to a bulk material having a size in the micrometer range (Er2Si2O7:IrO2−7, Table S1 in the Supporting Information). Further, to explore the nature of IrO2 in the prepared nanocomposite Er2Si2O7:IrO2, the TEM image of cube-shaped Er2Si2O7:IrO2−5 was also recorded and is shown in Figure S1 in the Supporting Information). As shown in the TEM images, the nanocomposite has a perfect cube shape with a size of 100−200 nm. The homogeneity in the shape of the cubes is also clearly visible in the TEM images (Figure S1A,B in the Supporting Information). The magnified TEM image of a single cube was also recorded and is shown in Figure S1C,D in the Supporting Information, where the contrast difference inside the cube is clearly seen. The homogeneous distribution of darker contrast inside the cube clearly suggests the presence of IrO2 in the nanocomposite. The same can be supported by the selected area electron diffraction (SAED) pattern of the nanocomposite, shown in Figure S1E in the Supporting Information, where the cube-shaped diffraction is clearly visible. In addition, the presence of the 110 plane of IrO2 also confirms the successful synthesis of cube-shaped Er2Si2O7:IrO2−5. 3.1.2. Surface Group and Compositional Characterization of the Nanocomposite. The powder XRD patterns of different shapes of Er2Si2O7:IrO2 were recorded and are shown in Figure 1H,I. In all of the prepared samples, similar XRD patterns were obtained; however, changes in their peak intensity were also observed. The observed peaks of Er2Si2O7:IrO2 at 21.4, 23.07, 26.1, 27.5, 29.6, 37.40, 38.5, 43.4, 45.8, 46.9, 50.8, and 51.8° can be assigned to the (101), (012), (111), (112), (013), (121), (201), (211), (212), (024), (220), and (115) planes of Er2Si2O7 from JCPDS Card No. 742143. The absence of a broad peak in the range of 15−30° excludes the formation of amorphous silica nanoparticles in the 8833

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ACS Catalysis Table 1. Role of Morphology on Different Parameters of Prepared Electrocatalystsa entry

material

1 2 3

Pt/C Er2Si2O7:IrO2−1 Er2Si2O7:IrO2−2

4 5 6

Er2Si2O7:IrO2−3 Er2Si2O7:IrO2−4 Er2Si2O7:IrO2−5

morphology sheet small sphere large sphere rod cube

BET surface area (m2/g)

pore size (nm)

pore volume (cm3/g)

ESA (cm2)

Cdl (mF/cm ) 8.0 9.0 14.0 16.5 20.8

3.2697 5.5748

4 9

0.000831 0.002526

0.89 0.06 0.11

17.1946 45.7358 101.2330

11 36 45

0.005738 0.011735 0.022022

0.32 0.43 0.74

2

j0 mA/cm

Tafel slope (mV/dec)

no. of active sites (mol)

TOF (s−1) at η=0

0.19 3.0 × 10−3 7.07 × 10−3

31 70 62

1.0 × 10−9 2.0 × 10−10 4.2 × 10−9

0.77 0.03 0.08

0.01 0.026 0.06

57 53 49

3.73 × 10−9 1.47 × 10−9 1.06 × 10−8

0.22 0.38 1.07

2

a

Definitions: ESA = electroactive surface area; j0 = dxchange current density; TOF = turnover frequency.

In addition to the above characterization, BET specific surface areas of all the nanocomposites were measured by N2 adsorption desorption isotherms (Figure S5 in the Supporting Information and Table 1). The corresponding surface areas of the nanocomposites are Er2Si2O7:IrO2−5 (101.23 m2/g), Er2Si2O7:IrO2−4 (45.73 m2/g), Er2Si2O7:IrO2−3 (17.19 m2/g), Er2Si2O7:IrO2−2 (5.57 m2/g), and Er2Si2O7:IrO2−1 (3.26 m2/ g). The high surface area of the nanocomposite Er2Si2O7:IrO2−5 indicates the high surface roughness of the nanocomposite to a certain extent. The pore size distribution of the nanocomposite estimated by the desorption portion of the nitrogen isotherm by BJH method is in the order Er2Si2O7:IrO2−5 (45 nm) < Er2Si2O7:IrO2−4 (36 nm) < Er2Si2O7:IrO2−3 (11 nm) < Er2Si2O7:IrO2−2 (9 nm), < Er2Si2O7:IrO2−1 (4 nm). The high porosity of the nanocomposite led the way toward greater numbers of active sites and thus was favorable for good electrocatalytic activity. The single point total pore volumes of the nanocomposites are Er2Si2O7:IrO2−5 (0.022022 cm3/g), Er2Si2O7:IrO2−4 (0.011735 cm3/g), Er2Si2O7:IrO2−3 (0.005738 cm3/g), Er2Si2O7:IrO2−2 (0.002526 cm3/g), and Er2Si2O7:IrO2−1 (0.000831 cm3/g). 3.2. Electrochemical Hydrogen Evolution Reaction (HER). Prior to the investigation of HER activity of prepared nanocomposites, important analytical parameters such as scan rate, catalyst loading amount, and concentration of supporting electrolyte were optimized and are shown in Figure S4. From the LSV run, it is seen that noticeable HER activity, that is, smaller onset potential and the highest current density, is obtained with 15.0 mg of the cube-shaped pyrosilicate nanocomposite at 5.0 mV s−1 scan rate. Additionally, the concentration of each supporting electrolyte was also optimized, and it was found that 0.5 M H2SO4, 1.0 M KOH, and 2.0 M PBS is suitable for further studies. Figure 2A shows the linear sweep voltammetry (LSV) runs of prepared nanocomposites, recorded in acidic medium (0.5 M H2SO4), to explore their HER activity. Simultaneously, for comparison, the HER activities of the commercially available Pt/C (the composition is 20 wt % Pt on carbon black) was also measured and plotted under identical conditions. As shown in the figure, the cube-shaped nanocomposite (i.e., Er2Si2O7:IrO2−5) exhibits good performance toward the HER with low onset potential (−76 mV) and a high current density of 252 mA cm−2 at −540 mV. The values are found near to those of the Pt/C catalyst in terms of both current density as well as onset potential.41 The HER activity of the prepared catalyst was also compared with respect to the behavior shown by the Ir/C catalyst. The plot of Ir/C shows a steep decrease in the cathodic current beyond the zero potential, having an onset potential at −110 mV. The electrocatalyst Er2Si2O7:IrO2−5

shows better performance in comparison to Ir/C when the value of the onset potential is considered.32 Interestingly, the other shapes of nanocomposites show dramatically poor performance in comparison with the cubeshaped nanocomposite. The onset potential for other nanocomposites is as follows: Er2Si2O7:IrO2−4 (rods, −120 mV), Er2Si2O7:IrO2−3 (large spheres, −200 mV), Er2Si2O7:IrO2−2 (small spheres, −300 mV), and Er2Si2O7:IrO2−1 (sheets, −360 mV). Here, the onset potential means the potential where the current starts increasing from zero value or the potential from which the HER is initiated. In addition, the difference in onset potential values at zero current density (i.e., η0) and 10 mA cm−2 current density (i.e., η10) were also calculated and represented as overpotential (Δη). The value ofthe overpotential (Δη) for the cube-shaped nanocomposite is found to be lower than that observed for the other shapes of samples and for most of the recently reported Ir-based metal HER electrocatalysts.42 The respective overpotential values (Δη) are found to be Er2Si2O7:IrO2−5 (130 mV), Er2Si2O7:IrO2−4 (169 mV), Er2Si2O7:IrO2−3 (200 mV), Er2Si2O7:IrO2−2 (230 mV), and Er2Si2O7:IrO2−1 (260 mV). According to the Butler−Volmer equation43 the electrode kinetics in the HER can be totally explained, which is made easy with the postulation of no mass-transfer effect and can be denoted as j = j0 [e(1 − β)nF / RT − e−βnF / RT ]

where F = Faraday constant, R = gas constant, and β = symmetry factor (β ≈ 0.5). The symmetry factor designates the symmetry of the energy barrier for the HER reaction. Another factor here is the exchange current density (j0), which signifies the quantitative measurement of the rate of reaction at equilibrium (i.e., η = 0). Herein, the exchange current densities of prepared nanocomposites were also estimated and it was found that cubeshaped Er2Si2O7:IrO2−5 shows a very high exchange current density ,i.e. 0.06 mA cm−2, which is almost comparable to the value of Pt/C of 0.19 mA cm−2 (Figure S6 in the Supporting Information). According to the literature, on the basis of the exchange current density Pt/C is considered to be the best catalyst for HER activity.43 Therefore, the high exchange current density clearly suggests the superior HER activity of the proposed catalyst. The Tafel equation can be explained with the linear portion of the polarization curve having largely negative overpotential and the current density can be estimated with the second term (cathodic current density):43 j = −j0 e−βηF / RT 8834

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Figure 2. (A) Linear sweep voltammetry runs of all the Er2Si2O7:IrO2 nonocomposites with Pt/C in acidic medium: i.e., 0.5 M H2SO4. (B) Tafel plot obtained for all the Er2Si2O7:IrO2 nanocomposites with Pt/C in acidic medium. (C) EIS spectra of all Er2Si2O7:IrO2 nanocomposites. (D) Multiple cycles of CV runs of Er2Si2O7:IrO2−5 to explore stability. (E) Polarization curve of the cube-shaped nanocomposite after 2000 cycles (inset: controlled potential electrolysis for 250 min). (F) Calculated and experimentally generated H2 versus time (inset: picture of bubbles generated at the electrode surface).

The Tafel slope is a unique parameter of an electrocatalyst determined from its mechanism. The Tafel slope can be obtained from linear fitting between the linear portion of the polarization curve to the semilogarithmic equation η = a + b log j. The electrocatalytic activity can be determined from the slope and intercept of the Tafel plot, which is shown in Figure 2B. A small Tafel slope leads to good reaction kinetics for the HER and indicates a slower increase in overpotential with increasing current density.43 Herein, the 49 mV/dec Tafel slope value was obtained with the cube-shaped nanocomposite, which is the lowest among the various shapes of catalysts

prepared in this work. The other nanocomposites have the following Tafel slopes: Er2Si2O7:IrO 2−4 (53 mV/dec), Er2Si2O7:IrO2−3 (57 mV/dec), Er2Si2O7:IrO2−2 (62 mV/ dec), and Er2Si2O7:IrO2−1 (70 mV/dec). However, a Tafel slope of 31 mV/dec was obtained for Pt/C. Depending on the value of the Tafel slope, three possible reaction steps can be portrayed for the HER in acidic media.44 Volmer reaction step: 8835

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ACS Catalysis H+ + e− + Cat. → Cat.Hads 2.3RT b= (Tafel slope = 120 mV dec−1) αF

on the electrode during electrochemical reactions. The frequency of the ac voltage was in the range of 100 kHz to 1 Hz, and to calculate the Rct values the impedance data were fitted to the semicircle (Figure 2C). Among the prepared nanocomposites, the cube-shaped Er2Si2O7:IrO2−5 shows much a lower charge transfer resistance of 20 Ω, indicating a smaller reaction resistance and faster mass transfer for the HER. The turnover frequency (TOF) for each active site is the number of hydrogen molecules evolved from an active site in a particular time period (e.g., in 1 s). It is an important figure of merit to compare the catalytic activity of catalysts.35 Herein also, the TOF value was calculated for each catalyst at η = 0 and is portrayed in Figure S7 in the Supporting Information and Table 1. As shown in Table 1, the TOF value of cubeshaped Er2Si2O7:IrO2−5 is much higher than those for other shapes of nanocomposites. Similarly, the number of active sites for Er2Si2O7:IrO2−5 has also been calculated and compared with those for other nanocomposites and Pt/C. The cubeshaped nanocomposite possesses a larger active site value of 1.06 × 10−8 mol in comparison to the Pt/C value: i.e., 1.0 × 10−9 mol. The larger value of TOF and active sites confirms that the cube-shaped nanocomposite has a greater number of free active sites for hydrogen evolution, which is consistent with the polarization graph showing lower onset potential and higher current density for Er2Si2O7:IrO2−5 in comparison to Pt/C. Additionally, the TOF values of the cube-shaped nanocomposite have been plotted against the applied potential and it was found that a linear relationship was obtained between potential and TOF within the Tafel region (Figure S8 in the Supporting Information), which implies that the HER devoid of any other effects (e.g., mass transfer) is controlled by electrode kinetics.35 3.3. Stability Study for the Hydrogen Evolution Reaction (HER). In order to use the prepared nanocomposite in industrial or real-time applications, its physical−chemical stability, physical form, cost, and lifetime are very important. Herein, the stability of the cube-shaped nanocomposite was extensively studied by CV, LSV, controlled-potential electrolysis (CPE), and multistep chronoamperometry. At first multiple cycles of CV were taken in the potential range of −0.7 to +0.5 V at a constant scan rate of 5 mV s−1 (Figure 2D). The current densities at the 1st, 50th, 100th, 500th, 1000th, and 2000th cycles are found to be 0.289, 0.287, 0.281, 0.270, 0.264, and 0.252 A cm−2, respectively. A very small change in the current density was noted at a higher number of cycles with negligible change in the onset potential. The polarization curve of the cube-shaped nanocomposite was checked after 2000 cycles, and negligible degradation in the current density was observed, which implies the high stability of the prepared material (Figure 2E). After that, a long-term controlledpotential electrolysis was performed with the cube-shaped nanocomposite at an overpotential of −1.5 V in 0.5 M H2SO4 for 250 min, which shows a constant behavior and maintenance of the original catalytic activity after 250 min (Figure 2E, inset). CPE was also used to measure the quantity of H2 gas produced and to estimate the Faradaic efficiency. A water displacement method in an inverted buret was used to quantitatively measure the evolved H2 during the electrolysis. The volume of H2 collected in the buret varies linearly with time, demonstrating a stable rate of hydrogen evolution symbolizing high performance of the electrocatalyst prepared. The Faradaic efficiency was calculated with the help of the

(1)

Heyrovsky reaction step: Cat. Hads + e− + H+ → Cat. + H 2 2.3RT b= (Tafel slope 40 mV dec−1) (1 + α)F

(2)

Tafel reaction step: 2 Cat. Hads → 2 Cat. + H 2 2.3RT b= (Tafel slope 30 mV dec−1) (3) 2F Here, Cat. Hads represents the hydrogen adsorption on the catalytic sites of the catalyst surface. In HER, the first step is the adsorption of hydrogen ion over the surface of the nanocomposite-modified electrode followed by release of proton from the electrolytic solution, which then combines with one electron from the electrode surface, and this full step is denoted the Volmer reaction. After the Volmer reaction, there is two possibilities where the reaction can go through: one reaction step is the Heyrovsky reaction step, where the adsorbed hydrogen atom (Hads) reacts or combines with one electron and another proton each for the formation of one H2 molecule, and the other step is through a Tafel reaction pathway in which two neighboring Hads come together to form one H2 molecule. On the basis of these three reaction steps, two reaction pathways can be drawn for the HER:45 (A) Volmer−Heyrovsky pathway (V-H): i.e., a combination of reactions 1 and 2. (B) Volmer−Tafel pathway (V-T): i.e., a combination ofreactions 1 and 3. Applying the equation given below, the transfer coefficient (α) can be determined by the Tafel slope. The transfer coefficient has a particular value depending on the path of the mechanism and may be used to decide the rate-determining step (RDS): b = ∂η /∂ log j = −

RT ln e αF

For the V-H pathway, any one of the reactions could be the RDS. If the Tafel slope is 120 mV dec−1 and α = β = 0.5, the Volmer reaction is the RDS.45 With a Tafel slope of 40 mV/ dec and α = 1.5 and β = 0.5, the Heyrosky reaction will be the RDS. Similarly, in the V-T pathway, if the Tafel slope is 120 mV/dec and α = β = 0.5, the Volmer reaction is the RDS.45 However, if the Tafel slope is 30 mV/dec with α = 2 and β = 0.5, the Tafel reaction is the RDS. With the best HER performance and lowest Tafel slope of 49 mV/dec, the values of α and β for the cube-shaped Er2Si2O7:IrO2−5 nanocomposite based HER reaction have been calculated and found to be 1.48 and 0.48, respectively. Considering the Tafel slope and α and β values for Er2Si2O7:IrO2−5, the V-H mechanism with z Heyrovsky reaction as the RDS might be the main pathway for hydrogen evolution on our catalyst. Additionally, the electrode kinetics for HER was further measured by electrochemical impedance spectroscopy (EIS). The semicircle region of the Nyquist plots of all nanocomposites represents the charge transfer resistance (Rct) at high frequency and conveys the value of resistance occurring 8836

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Figure 3. (A) Multistep chronoamperometric curves for Er2Si2O7:IrO2−5 in 0.5 M H2SO4. (B) Polarization curves of the cube-shaped nanocomposite Er2Si2O7:IrO2−5: initial curve and curve after 2000 cycle in 1.0 M KOH (pH 14) (the inset shows controlled-potential electrolysis for 250 min). (C) Polarization curves of the cube-shaped nanocomposite Er2Si2O7:IrO2−5: initial curve and curve after 2000 cycles in 2.0 M PBS (pH 7) (the inset shows controlled-potential electrolysis for 250 min). (D) Tafel slope obtained from LSV plot of Er2Si2O7:IrO2−5 at pH 14 and pH 7. Multistep chronoamperometric curves for Er2Si2O7:IrO2−5 in (E) 1.0 M KOH (pH 14) and (F) 2.0 M PBS (pH 7).

The graph of theoretically calculated and experimentally generated H2 versus time is shown in Figure 2F, where almost 100% agreement was found between experimental and theoretical results. The Faradaic efficiency was calculated using the following formula:46 Faradaic efficiency = ((measured amount of hydrogen) × 100)/(theoretical amount of hydrogen). Herein, the Faradaic efficiency is calculated to be 98%. Moreover, we recorded a movie of H2 evolution (Video 1), in which vigorous bubbling was observed at the cubeshaped nanocomposite modified electrode (Figure 2F, inset). However, in clear contrast, nearly no bubbles were observed at

theoretical amount of hydrogen produced by the catalyst. The amount of hydrogen produced was determined by using the steady-state current obtained from long-term controlledpotential electrolysis done under an inert atmosphere in a two-chamber electrochemical cell. In a one-compartment cell, the O2 evolved on the anode can partially dissolve into the electrolyte and migrate to the cathode, which could result in an erroneous amount of H2 produced and would further affect the Faradaic efficiency. Herein, the working electrode and reference electrode were kept in one compartment and the platinum counter electrode was placed in another chamber. 8837

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Table 2. Comparison between HER Catalysts in Different pH Media on the Basis of Overpotential and Tafel Slope Values entry

materiala

Δηacidic (mV)

βacidic (mV/dec)

Δηbasic (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

rGO-few layer FePS3 Ni3S2@NPC MoP/Mo2C@C MoP/SN Co-Ni-B MoP2 NPs/Mo FePNPs@NPC NixP/CNT hollow Zn0.30Co2.70S4 WP2 NPs/W Co-NRCNTs Mo2C@NC MoS2 NAs/Ti Er2Si2O7:IrO2−5

108 ± 2 91.6 89 120 479 143 130 150 80 223 260 124 108 130

54 63.5 45 45 123

192 ± 2 60.8 75 150 133 193 214 109 85 214 370 60 190 170

67 61.8 47.5 66 69 60 52 49

βbasic (mV/dec)

58 59 121 82 119.8 92

100 59

Δηneutral (mV)

βneutral (mV/dec)

220 193.0b 136

67.5 93

170 211 386 105 90 210 500 156 200 190

51 136 100 95

152 67

ref 46 47 48 49 50 51 52 53 54 55 56 57 58 This work

Definitions: Ni3S2@NPC = Ni3S2 films grown on nanoporous copper; MoP/Mo2C@C = molybdenum phosphide and molybdenum carbide composite nanoparticles (NPs) coated by few-layer N-doped graphitic carbon shells; MoP/SN = sulfur and nitrogen dual-doped molybdenum phosphide nanocrystallites; Co-Ni-B = copper nickel boride nanocatalyst; MoP2 NPs/Mo = MoP2 nanoparticle films on a metal Mo plate; FePNPs@NPC = phytic acid derivative iron phosphides encapsulated in N,P-codoped carbon; NixP/CNT = spongelike nickel phosphide-carbon nanotube hybrid electrodes; hollow Zn0.30Co2.70S4 = hollow cobalt-based bimetallic sulfide polyhedra; WP2 NPs/W = Tungsten diphosphide nanoparticles on tungsten foil; rGO-few layer FePS3= Iron phosphochalcogenide in reduced graphene oxide; Co-NRCNTs = Cobalt-embedded nitrogen-rich carbon nanotubes; Mo2C@NC= molybdenum carbide nitrogen-rich nanocarbon; MoS2 NAs/Ti = MoS2 nanosheet arrays on the Ti plate. bCalculated at a current density of 2 mA cm−2. a

electrochemical study. For this, LSV runs of fresh and used catalyst (i.e., before and after all the electrochemical studies) were compared and almost no change in the onset potential and current density value was observed (Figure S11 in the Supporting Information). Moreover, the nanocompositemodified electrode was stored at room temperature for 6 months and LSV runs were taken at intervals of 1 month. Approximately 1% change in current density with insignificant change in the onset potential was observed (Figure S12 in the Supporting Information). The change in morphology of the cube-shaped nanocomposite was also tested by recording its FE-SEM image after storage for 6 months (Figure S13 in the Supporting Information). No change in the surface morphology or shape was noticed in comparison to the fresh catalyst, which clearly supports the robustness of the proposed catalyst in all aspects. 3.4. HER Performance of Nanocomposite in Different pH Ranges. To evaluate the performance of proposed cubeshaped catalysts toward the HER over the entire pH range, their electrocatalytic activity was also recorded in 1 M KOH (pH 14) and 2.0 PBS (pH 7). As shown in the polarization curve (Figure 3B,C), overpotentials (Δη) of 170 and 190 mV were required to reach the current density of 10 mA cm−2 in 1 M KOH (pH 14) and 0.1 PBS, respectively. The proposed values are among the best results for currently reported metalbased catalysts in alkaline and neutral media in terms of overpotential, Tafel slope, and onset potential value (Table 246−58). In addition, after multiple LSV runs (2000 cycles), no change in current density or onset potential was observed in both media (Figure 3B,C, inset). More interestingly, the Tafel slope of the cube-shaped nanocomposite is found to be 49 mV dec−1 in pH 14 and 59 mV dec−1 in pH 7 aqueous media (Figure 3D), suggesting that the electrocatalyst follows the Volmer−Heyrovsky (RDS) mechanism. The study gives a clear idea about the similar behavior of the proposed catalyst over the entire pH range, starting from 0 to 14. Furthermore, Er2Si2O7:IrO2−5 retains a stable electrocatalytic property in pH 14 and pH 7 electrolytes during the long-term electrochemical

the other electrode. This suggests that the Er2Si2O7:IrO2−5 can be directly used as a device for electrochemical hydrogen production, which is highly desirable for practical applications. In order to explore the role of prepared catalysts in an overall water splitting unit, their behavior toward the oxygen evolution reaction (OER) was also studied (Figure S9 in the Supporting Information) under optimized reaction conditions (i.e., loading amount of catalyst 15.0 mg, scan rate 5.0 mV s−1, and 2.0 M KOH as supporting electrolyte). The LSV run of the nanocomposite toward the OER was recorded in the potential range of +0.8 to +2.7 V vs RHE. As depicted in Figure S9, the catalyst showed a trend similar to that obtained for the HER: i.e., cube-shaped catalysts showed superior current density at particular potential values. The onset potential for OER with overpotential (in parentheses) is found as follows: Er2Si2O7:IrO2−5, +1.29 V (360 mV); Er2Si2O7:IrO2−4, +1.40 V (470 mV); Er2Si2O7:IrO2−3, +1.52 V (670 mV); Er 2 Si 2 O 7 :IrO 2−2 , +1.66 V (740 mV); Er2Si2O7:IrO2−1, +1.84 V (860 mV). The results clearly demonstrated the practical utility of the proposed catalysts in overall water splitting. Consecutive multistep chronoamperometric tests for the cube-shaped nanocomposite were performed at various overpotential values, ranging from −100 to −800 mV with a continuous increase in potential value at each stage (1000 s) (Figure 3A). With a change in the potential, an increase in current was observed, which remains constant for the entire step time: i.e., 1000 s. In addition, multistep chronoamperometric tests were also performed at a constant potential of −50 mV for various cycles, and the results are portrayed in Figure S10A in the Supporting Information. In addition, the LSV run was taken after every cycle (Figure S10B in the Supporting Information) and negligible change in the current density was observed for each step. The study clearly indicates promising mass transport performance conductivity and mechanical robustness of the prepared nanocomposite. The higher stability can also be concluded from the unchanged behavior of the nanocomposite, after the whole 8838

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Figure 4. CV runs at different scan rates from 0.0 to 0.8 V for (A) Er2Si2O7:IrO2−2, (B) Er2Si2O7:IrO2−1, (C) Er2Si2O7:IrO2−3, (D) Er2Si2O7:IrO2−4, and (E) Er2Si2O7:IrO2−5. (F) Graph of current density versus scan rate for all Er2Si2O7:IrO2 nanocomposites at 0.50 V.

3.5. Reason for Higher HER Activity of Cube-Shaped Nanocomposite. In order to explore the reason behind the extraordinary behavior of the cube-shaped nanocomposite toward the HER, its electroactive surface area and double-ayer capacitance were studied. The electroactive surface area was calculated via CV techniques in order to judge the efficacy of prepared nanocomposites using a 5.0 mM solution of K4Fe[(CN)6] with 1.0 M KCl as the supporting electrolyte. The Randles−Sevcik equation was used to estimate the electroactive surface area:59

process and multistep chronoamperometric studies (Figure 3E,F). All of the results prove that the cube-shaped nanocomposite possesses superior HER activity over the entire pH range. Finally, the overpotentials (Δη = η0 − η10), Tafel slopes, and exchange current densities (Figure S14 in the Supporting Information) of Er2Si2O7:IrO2−5 in electrolytes with different pH values (0−14) were determined one by one and all the corresponding results are portrayed in Table S2 in the Supporting Information. The performance of proposed catalyst with comparatively small overpotentials, high current densities (basic, ∼201 mA cm−2 @ −0.89 V; neutral, ∼82 mA cm−2 @ −0.68 V), and low Tafel slopes demonstrate the extraordinary behavior of the electrocatalyst over the whole pH range.

Ip (μ A) = (2.69 × 105)n3/2AD1/2V1/2C

where Ip = peak current, A = electroactive surface area, n = number of electrons involved in the reaction, D = diffusion 8839

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ACS Catalysis coefficient of the analyte, V = scan rate, and C = concentration of the analyte. In common, the redox reaction of K4Fe(CN)6/K3Fe(CN)6 involves a one-electron-redox reaction (n = 1) and the value of thre diffusion coefficient (D) is 0.76 × 10−5 cm2 s−1.60 As shown in the CV runs, after modification of the nanocomposite materials, a higher current was observed in comparison to the bare electrode (Figure S15 in the Supporting Information). From the Randles−Sevcik equation, the electroactive surface area of each nanocomposite was calculated and is portrayed in Table 1. The electroactive surface area of cube-shaped Er2Si2O7:IrO2 is found to be around 15 times higher than that of the bare electrode (i.e., 0.05 cm2). The electrochemical surface area of other nanomaterials were found in the order Er2Si2O7:IrO2−5 > Er2Si2O7:IrO2−4 > Er2Si2O7:IrO2−3 > Er2Si2O7:IrO2−2 > Er2Si2O7:IrO2−1. In addition, the surface area of the Pt/C-modified electrode was also calculated and found to be lower than that of Er2Si2O7:IrO2−5 but better than those of the others. After comparison of the structural behavior of differently shaped nanomaterials, the role of IrO2 in the electrochemical performance of the proposed nanocomposite was also studied. For this, CV runs for ferrocyanide and LSV runs toward the HER were recorded using Er2Si2O7 (i.e., nanocomposite without IrO2) modified PGE and are shown in Figure S16 in the Supporting Information). On comparison of the current response for ferrocyanide using Er2Si2O7:IrO2 and Er2Si2O7 modified PGEs, it was found that Er2Si2O7:IrO2 shows better electrochemical response toward the ferrocyanide redox reaction. Similarly, Er2Si2O7:IrO2 modified PGEs showed better HER response in comparison to the Er2Si2O7 modified PGEs. In addition to this, the HER responses recorded for all catalysts (shown in Figure 2A) were normalized with respect to the electrochemical surface area of IrO2 modified PGE and are shown in Figure S17A in the Supporting Information. While comparing the data, we found that there is only a change in terms of current density and no other changes were observed. The result supports that the IrO2 decoration is required for the enhanced electrochemical performance of a prepared nanocomposite, i.e. Er2Si2O7:IrO2, but the different behaviors of the prepared catalyst are dependent on their shape alone. In addition to this, the HER behavior of commercially available IrO2 modified PGE was also recorded and is shown in Figure S17B in the Supporting Information. In general, in all previous articles published IrO2 proved to be the most dynamic catalyst for the oxygen evolution reaction (OER); it exhibits high electrochemical properties, electronic conductivity, and stability.30 However, the HER activity of IrO2 has not been well explored/studied, owing to the strong Ir−H chemical bond, which could hinder the hydrogen desorption and decrease the reaction rate of the HER.30 Similar results, i.e. poor current and high overpotential, for the IrO2 modified PGE were obtained toward the HER in the present study as well (Figure S17 in the Supporting Information), which are improved after IrO2 incorporation with pyrosilicate. The study suggests that incorporation of IrO2 with other materials could facilitate hydrogen desorption and as a result can improve the HER activity in the composite material.30 To further support the results obtained from the above study, we have also measured the electrochemical double-layer capacitance (Cdl) of the studied catalysts by determining the non-Faradaic capacitive current in association with the double-

layer charging obtained from the scan-rate dependent CV runs in a potential window of 0.0 to 0.8 V (Figure 4A−E). The measured current in this non-Faradaic potential region is assumed to be due to double-layer charging. The CV runs were performed at various scan rates (2, 10, 50, 100, and 200 mV s−1). The value of Cdl is approximated by plotting the ΔJ value (= Ja − Jc) at 0.50 V (vs RHE) against the scan rate (Figure 4F), where the slope is equal to Cdl.61 The Cdl value for each nanocomposite was calculated and is portrayed in Table 1. The cube-shaped nanocomposite showed a maximum Cdl value of 20.8 mF cm−2, which indicates that the highly exposed active sites in Er2Si2O7:IrO2−5 help to promote the electrochemical process. Thus, it can be concluded from the surface area and Cdl calculation that cube-shaped Er2Si2O7:IrO2−5 has a much higher electrochemically active surface area with more exposed edges and proliferative active sites in comparison to other differently shaped nanocomposites, which is associated with its large exchange current density and outstanding HER activity over the total pH range. These types of noteworthy electrocatalytic properties can be attributed to the following reasons. (i) Shape-specific properties of nanocomposites: the superior electrocatalytic activity of differently shaped Er2Si2O7:IrO2 is mainly due to an increased number of active sites. It was confirmed from different studies that the catalytic performance is dependent on the surface atom arrangement or morphological characteristics of the materials.62 It has been reported in the literature that nanocubes have gained a great deal of attention as electrocatalysts in comparison to other morphologies63 because of the selective and high-index crystallographic facets of nanocubes which ease electrolyte transport and facilitate electrolysis. (ii) Synergistic effect between IrO2 and erbium pyrosilicate: IrO2 is known to be the most active catalyst for the oxygen evolution reaction (OER) and is known to exhibit high electronic conductivity and stability in electrochemical systems.30 According to the literature, metal−H2O interactions are very important in favoring the splitting of water molecules; on the other hand, the M−H interactions must not be too strong to interrupt hydrogen desorption.30 However, in comparison to iridium, the interaction of hydrogen with a rare-earth metal is much stronger, due to the presence of a vacant 4f orbital in rare earths; therefore, the addition of iridium oxide to the erbium pyrosilicate eases the desorption of hydrogen and thus facilitates hydrogen evolution. (iii) Presence of a rare-earth metal in the nanocomposite: Brewer/Engel valence bond theory postulated that the hyperelectronic d orbitals of rareearth metals produce a definite synergy in the hydrogen evolution reaction electrocatalysis. For this reason they create an electronic structure which shows superior electrocatalytic activity because of the appropriate arrangement of orbitals.64

4. CONCLUSION Hydrogen can be produced from several sources such as natural gas and biomass, but water is the only sustainable resource to produce hydrogen in the future, as it is inexhaustible on the earth. In this work, the shape-dependent catalytic activities of Er2Si2O7:IrO2 were investigated for the HER. The study supports the idea that the HER activity of the catalyst depends not only on size and chemical composition but also on the morphology of the material. The cube-shaped pyrosilicate with high electroactive surface area dominates other shapes brilliantly in HER activity and shows high stability and durability in acidic/neutral and basic media. Overall, this 8840

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laboratory facility at the Functional Nanomaterials Research Laboratory, Department of Applied Physics, IIT (ISM), Dhanbad.

study suggests that the rational control of catalyst morphology is important for enhancing HER activity. The tricky use of a low concentration of iridium leads to the synthesis of highly efficient electrocatalysts having properties of precious metal at lower cost, which could be an effective option toward their commercialization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b01363. Instrumentation details, different synthetic conditions used for the preparation of various shapes of Er2Si2O7:IrO2, synthesis of differently shaped nanocomposites, TEM and SAED patterns of Er 2 Si 2 O 7 :IrO 2 −5 , XPS spectra of cube-shaped Er2Si2O 7:IrO2−5, EDX spectra and corresponding elemental table of different nanocomposites and elemental mapping of Er2Si2O7:IrO2−5, optimization of loading mass, scan rate, and supporting electrolytes for the HER, calculation of overpotential value for all nanocomposites in 0.5 M H2SO4, BET surface area N2 adsorption desorption isotherm of all the nanocomposites, exchange current density graph of all the nanocomposites in acidic medium, CV run for all the nanocomposites and Pt/C for the calculation of TOF values, turnover frequency (TOF) of Er2Si2O7:IrO2−5 catalyst, OER activity of all the nanocomposites, multistep chronoamperometric runs and corresponding LSV runs of Er2Si2O7:IrO2−5 catalyst, electrocatalytic activity between the fresh and used cube-shaped nanocomposites, storage stability of the Er2Si2O7:IrO2−5 catalyst, FE-SEM image of the Er2Si2O7:IrO2−5 catalyst after several electrochemical studies, exchange current density plot of the cube-shaped catalyst at different pHs, performance of cube-shaped electrocatalyst for various parameters at diverse pHs, electrocatalytic activity study using potassium ferrocyanide for all the catalysts, CV runs for ferrocyanide and LSV run toward HER using Er2Si2O7 modified PGE, HER performance of all the catalysts normalized with the electrochemical surface area of IrO2 modified PGE, and LSV run for IrO2 modified PGE (PDF) Video showing H2 evolution (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*P.K.: e-mail, [email protected]; tel, +91 8250897637. ORCID

Paramita Karfa: 0000-0002-5684-1517 Rashmi Madhuri: 0000-0003-3600-2924 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the DST for sponsoring the research project of R.M. (Ref. No.: SERB/F/2798/2016-17). The experimental work has been carried out by P.K., and she is responsible for all the data presented in this work. She has compiled and revised the paper with the help of K.C.M. P.K. is also thankful to Dr. P. K. Sharma for providing some of the 8841

DOI: 10.1021/acscatal.8b01363 ACS Catal. 2018, 8, 8830−8843

Research Article

ACS Catalysis

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DOI: 10.1021/acscatal.8b01363 ACS Catal. 2018, 8, 8830−8843