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Approaching the Volcano Top: Iridium/Silicon Nanocomposites as Efficient Electrocatalysts for the Hydrogen Evolution Reaction Minqi Sheng, Binbin Jiang, Bin Wu, Fan Liao, Xing Fan, Haiping Lin, Youyong Li, Yeshayahu Lifshitz, Shuit-Tong Lee, and Mingwang Shao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07572 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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Approaching the Volcano Top: Iridium/Silicon Nanocomposites as Efficient Electrocatalysts for the Hydrogen Evolution Reaction Minqi Sheng,†,₴ Binbin Jiang,†,₴ Bin Wu,† Fan Liao,† Xing Fan,† Haiping Lin,*,† Youyong Li,† Yeshayahu Lifshitz*,‡ Shuit-Tong Lee,† Mingwang Shao*,† †Institute

of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-

Based Functional Materials & Devices, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, 199 Ren'ai Road, Suzhou, 215123, Jiangsu, PR China ‡Materials

Science and Engineering Department, Technion, Israel Institute of Technology, Haifa 3200003, Israel.

KEYWORDS: hydrogen evolution reaction; atomization enthalpy; Si; nanowires; Ir; electrocatalysis.

ABSTRACT Electrolysis of water to generate hydrogen is an important issue for the industrial production of green and sustainable energy. The best electrocatalyst currently available for the hydrogen evolution reaction (HER) is platinum. We herein show that iridium can be manipulated to achieve a record high HER activity surpassing platinum in every aspect: a lower overpotential

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at any given current density, a higher current density and mass activity for all bias potentials applied and a catalyst cost reduction of 50% for the same hydrogen generation rate. The superior HER activity was achieved by a binary Ir/Si nanowire catalyst design in which (as density functional theory calculations show) two distinct strategies act in synergy: (i) decreasing the size of the iridium nanoparticles to ~2.2 nm; (ii) dividing the H2 generation process to three steps occurring on two different catalysts: H adsorption on iridium, H diffusion to silicon and H2 desorption from silicon.


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Hydrogen is a pollution-free energy source with a high energy density,1-8 and has, therefore, been regarded as an important green energy fuel to replace oil and coals. Nevertheless, at present, the world hydrogen production is still dominated by the thermal reformation of fossil fuels. The electrolytic water splitting is considered as one of the most promising strategies for sustainable hydrogen production. Consequently, the conversion efficiency of electric power to chemical energy (H2 gas) needs to be improved to reduce the production cost of the electrogenerated hydrogen. Tremendous efforts have been devoted to the development of highly active electrocatalysts for the hydrogen evolution reaction (HER) process.9,

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Mechanistically, the HER process is

composed of two steps:11 (i) the formation and adsorption of hydrogen atoms on the catalysts (the Volmer step), and (ii) the production and desorption of hydrogen molecules (the Tafel step or Heyrovskey step) from the catalysts. Analysis of experimental measurements and theoretical calculations of various electrocatalysts revealed a crucial correlation between the activity of catalysis and the adsorption energy known as the Sabatier principle. The adsorption energy should be neither too high (endothermic, then the adsorption is slow) nor too low (exothermic, then the desorption is slow). The volcano plot presents the HER activity at the onset potential vs. the change of Gibbs free energy in the HER process (ΔGH).11 The volcano plot thus has maximum activity values for near-zero ΔGH. The activity values decrease for either more negative or more positive ΔGH values. The Sabatier principle means that the most active electrocatalysts should have near-zero values of ΔGH in the volcano plot.11-13 Pt has the maximum activity values of the volcano plot, with a near-zero ΔGH. It has thus been widely used as a commercial HER electrocatalyst for decades.14-19 Iridium (Ir), a neighbor of Pt, has a ΔGH (0.03 eV) which is even closer to zero than Pt (-0.09 eV).20 The reasons why Ir does


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not possess better HER activity than Pt in view of its smaller absolute value of ΔGH were not studied until now, to the best of our knowledge. We have noticed that the enthalpy of atomization (ΔHat), of Ir (669.4 kJ·mol-1) is much larger than that of Pt (564.8 kJ·mol-1). This implies that the Ir atoms/clusters have a stronger tendency to aggregation and coarsening than Pt.21, 22 The lower HER activities of Ir-based catalysts compared to those of Pt-based catalysts might thus be ascribed to the poor dispersion and stronger aggregation since, as is well known, the catalytic activity of catalysts is sensitive to their particle size.23, 24 Decrease of the Ir particle size and suppression of the coarsening of Ir particles may thus enhance the HER activity of Ir beyond that of Pt, as expected from the volcano plots. Attempting to achieve a better HER activity than that of Pt, we introduced the concept of a binary component metal/silicon nanowire (SiNW) catalyst.25 The adsorption and desorption of the H atom on two different surfaces enabled to overcome the limitations of the Sabatier principle and to increase the HER activity of metal/SiNW catalysts with respect to their activity as single component catalysts. Rh/SiNW, the best binary catalyst we have studied, possesses a higher HER activity than Pt in the range of high overpotentials. Still, the electrocatalytic performance of Pt at low overpotentials is better than that of Rh/SiNW.25 This work aims at the optimization of HER catalysts to surpass the HER properties of Pt in every aspect (including onset overpotential, activity at all overpotentials and stability). We achieve this in three steps. First, we compare the Pt and the Ir systems to understand why Ir (the ΔGH of which is the closest to zero) is not a better HER catalyst than Pt. We find that both have similar HER properties and what determines their actual activity is their particle size and agglomeration. A better HER performance requires a smaller particle size and smaller agglomeration. Then we apply the binary component metal/SiNW approach to further increase


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the HER activity of Ir-based catalysts. We find out to our surprise that SiNWs stabilize very small sized Ir particles which do not aggregate, enhancing the HER activity and the stability of the Ir/SiNW system. Further we realize that the separation of H adsorption/desorption to two distinct surfaces improves the HER properties of Ir/SiNW beyond those of Pt (commercial Pt tested in our laboratory and previously reported HER activities). Ir in Ir/SiNW is found out to be the most active HER atom. The advantages of the Ir/SiNW system over Pt-based HER catalysts will be beneficial for its extensive use for efficient, stable and cheap H2 production by electrolysis.

RESULTS First, we show that indeed 17.7 wt% Ir/SiNW surpasses Pt, the "holy grail" of HER catalyst, in all elecrocatalytic aspects and for all overpotentials. The Pt reference we use is a commercial 20 wt% Pt/C catalyst which has about the same weight concentration of metals as 17.7 wt% Ir/SiNW. The LSVs of 17.7 wt% Ir/SiNW show a better catalytic activity than those of 20 wt% Pt/C both for low overpotentials (Figure 1A) and high overpotentials (Figure 1B). In both regimes, the current density obtained for the same overpotential is higher for 17.7 wt% Ir/SiNW than for 20 wt% Pt/C. The overpotential of 17.7 wt% Ir/SiNW at -10 mA·cm-2 is smaller than that of 20 wt% Pt/C (Figure 1G).


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Figure 1. A comparison between the HER performance of 17.7 wt% Ir/SiNW and 20 wt% Pt/C. (A) LSV curves for a small potential window; (B) LSV curves for a large potential window; (C) LSV plots in terms of current per mole (representing activity per atom); (D) hydrogen evolution by 17.7 wt% Ir /SiNW and 20wt% Pt/C catalysts at -0.1 V vs. RHE. The hydrogen evolution using 17.7 wt% Ir/SiNW is larger by 40%; (E) stability measurements using the chronopotentiometric curve - the cathodic potential needed to obtain a current density of -10 mA·cm-2 vs. the operation time up to 50000 s; (F) Tafel plots and the corresponding Tafel slopes; and (G) the electrochemical parameters. The HER properties of 17.7 wt% Ir/SiNW are better than those of 20 wt% Pt/C in all aspects.

The Ir/SiNW catalysts are superior to conventional Pt/C electrodes for practical applications which demand high current densities. Industrial hydrogen production requires current densities


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of the order of -1 A·cm-2.26 The overpotential of the 17.7 wt% Ir/SiNW catalyst at -1 A·cm-2 is lower than that of 20 wt% Pt/C (Figure 1B) so that the electric energy to hydrogen conversion efficiency of 17.7 wt% Ir/SiNW is larger by 25.6% than that of 20 wt% Pt/C (for details see Supporting Materials). Figure 1C shows the LSV curves of 17.7 wt% Ir/SiNW and 20 wt% Pt/C catalysts in terms of current per metal catalyst mole. For an overpotential of 1 V the current per mole of 17.7 wt% Ir/SiNW is larger by 75% than that of 20 wt% Pt/C indicating that the activity per atom of Ir is much larger than that of Pt. To further substantiate the improved efficiency for hydrogen generation obtained by the Ir/SiNW catalysts, we compared the hydrogen evolution of the 20 wt% Pt/C and the 17.7 wt% Ir/SiNW catalysts at the same potential of -0.1 V vs. RHE (Figure 1D). The hydrogen evolution of the Ir/SiNW catalyst was larger by 40%. Figure 1E shows that the stability (an important property of catalysts) of the 17.7 wt% Ir/SiNW under electrochemical operation is also much better than that of 20 wt% Pt/C. The catalysts were tested in oxygen-free 0.5 M H2SO4 via the chronopotentiometry technique at room temperature under a constant current density of -10 mA·cm-2 for 50,000 s. The cathodic potential increase of 17.7 wt% Ir/SiNW needed to generate -10 mA·cm-2 for 50,000 s was four times smaller than the cathodic potential increase of 20 wt% Pt/C. The LSV curves in Figure 1A were used to derive the Tafel plots (Figure 1F) and the corresponding Tafel slopes, which are commonly used to quantitatively compare the HER electrocatalytic activities. A smaller Tafel slope represents a higher electrocatalytic activity. The Tafel equation, V = a + b·log (-j) (where V and j are the overpotential and the corresponding current density, respectively, and b is the Tafel slope) is derived from the electrochemical


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kinetics and reveals the relation between the electrochemical reaction rate and the overpotential. The Tafel slope indicates the increase of the electrochemical current (i.e. the reaction rate) upon an incremental increase of the overpotential. Small slopes represent a better electrocatalyst, since for small slopes the current gain with potential increase will be large. The Tafel slope of the 17.7 wt% Ir/SiNW catalyst is 20 mV·dec-1, much lower than 30 mV·dec-1, the Tafel slope of 20 wt% Pt/C (Figures 1F and 1G). The lower slope indicates that 17.7 wt% Ir/SiNW has a better HER kinetics. The apparent and real exchange current densities (see definition in the Supporting Information) of 17.7 wt% Ir/SiNW are larger than those of 20 wt% Pt/C (Figure 1G), indicating its better intrinsic activity. The HER catalytic data obtained in the present work was compared to the published data. Table S1 summarizes the present and past data of Ir-based catalysts, Table S2 of 20 wt% Pt/C catalysts. Table S3 compares the turnover frequencies (TOFs, see definition in the supplementary information) at zero potential of Ir/C, Ir/SiNW and Pt/C in the present and in previous works, and Table S4 compares the mass activity with the recently reported Pt- and Rhbased HER catalysts. The HER activity of 17.7 wt% Ir/SiNW was found better than all previously reported Ir based catalysts. The HER activity of the reference commercial 20 wt% Pt/C used in the present work was found in accord with that of the best previously reported Pt/C catalysts. This indicates that the use of the reference Pt/C electrode in the present work is valid and that the HER activity of the 17.7 wt% Ir/SiNW catalyst indeed surpasses those of all previously reported Ir based and Pt/C catalysts.


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Figure 2. The correlation between HER activity, catalyst particle size and aggregation. (A) LSV curves of pure Pt, pure Ir, 20 wt% Pt/C, and 20 wt% Ir/C; (B) chronopotentiometric curves (at 10 mA·cm-2) of 17.7 wt% Ir/SiNW, 20 wt% Pt/C, 20 wt% Ir/C, pure Pt and pure Ir. Note that only the 17.7 wt% Ir/SiNW catalyst is stable under HER operation; (C) LSV curves of 17.7 wt% Ir/SiNW catalysts with Ir average diameters of 2.17 nm and 18.1 wt% Ir/SiNW catalysts with Ir average diameter of 4.80 nm respectively. Note that the small Ir NPs lead to better HER properties; (D) the size distributions of 17.7 wt% Ir/SiNW and 18.1 wt% Ir/SiNW catalysts; (E)(H) TEM images of catalysts before ((E), (G)) and after ((F), (H)) the stability test showing aggregation following the stability test: (E) pristine 20 wt% Pt/C; (F) 20 wt% Pt/C after the stability test; (G) pristine 20 wt% Ir/C; (H) 20 wt% Ir/C after the stability test; (I) TEM image of 17.7 wt% Ir/SiNW catalysts; (J) HRTEM image of 17.7 wt% Ir/SiNW. The 0.21 nm spacing corresponds to Ir (111) and the 0.31 nm spacing corresponds to Si (111); and (K)-(O) elemental


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mapping of 17.7 wt% Ir/SiNW: (K) HAADF-STEM image of an Ir/SiNW; (L-O) its corresponding EDS mapping showing the Si, O, and Ir distributions.

We now study the correlation between the particle size of the Pt- and Ir-based catalysts and their HER activity trying to resolve the riddle of the worse HER catalytic activity of Ir with respect to Pt despite of its smaller ΔGH. This study was motivated by our observation that the enthalpy of atomization ΔHat of Ir is larger than that of Pt so that aggregation of the Ir particles might be the reason for its inferior activity. We thus checked the LSVs of four catalysts (Figure 2A): 20 wt% Pt/C, 20 wt% Ir/C, pure Ir nanoparticles (NPs) and pure Pt NPs. Additionally, we have also studied the electrochemical stability of these catalysts (Figure 2B). TEM analysis before and after the electrochemical tests detected their morphology (Figures 2E-2H and S1-S2). All catalysts exhibit obvious aggregation after the stability test (Figures 2E-2H and S1) except the 17.7 wt% Ir/SiNW catalyst which shows no aggregation (Figure S2). And there is no obvious shift for Ir 4f XPS core spectrum for 17.7 wt% Ir/SiNW catalyst after stability test (Figure S3), indicating its stability. A clear correlation between the catalysts' particle size (and amount of aggregation) and the HER activity was observed. The pristine commercial 20 wt% Ir/C catalyst was characterized by more aggregation than the 20 wt% Pt/C leading to a better HER activity of the latter. In contrast, the pure Ir NPs were smaller than the pure Pt NPs leading to a better HER activity of the Ir NPs. It is obvious that a continuous running of the catalysts leads to aggregation associated with the deterioration of the HER activity. To further check the particle size effect on the HER activity we have prepared two Ir/SiNWs with different Ir NPs sizes (their size distributions is shown in Figure 2D). Increasing the Ir NPs size from 2.17 nm to 4.8 nm significantly deteriorated the HER activity of the Ir/SiNW (Figure 2C). It might be concluded


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that the HER activity and stability of the Ir-based catalysts depend on the Ir NPs size and amount of aggregation. Keeping a small Ir NPs size and prevention of aggregation improve both the HER activity and the stability of the Ir-based catalysts. Figure 2I shows the TEM image of the 17.7 wt% Ir/SiNW catalyst indicating 2.17 nm sized Ir NPs well dispersed on the SiNW, which is also beneficial for the conductance of the catalysts. The high-resolution TEM (HRTEM) image (Figure 2J) of the Ir/SiNW shows that Ir NPs are deposited on the SiNW. The lattice spacings of 0.21 nm and 0.31 nm correspond to the (111) interplanar spacing of the cubic Ir and Si, respectively. High angle annular dark field scanning TEM (HAADF-STEM) image of Ir NPs deposited on a SiNW (Figure 2K) and energy dispersive spectroscopy (EDS) mapping (Figures 2L-2O) show the elemental distribution of silicon (green), oxygen (blue), and iridium (red), respectively. We have determined the effect of particle size and aggregation on the HER activity and attributed the inferior performance of commercial Ir/C (despite of its lower ΔGH) with respect to Pt/C to aggregation. Now we apply our binary catalyst concept in which metal NPs are grown on SiNWs.25 We first study the effect of the Ir particles concentration (wt Ir/wt (Ir+SiNW)) on the HER activity. We follow by comparing the HER activity of binary catalysts with different metals keeping the most active concentration found for Ir (i.e. ~20 wt% metal/SiNW). We further compare the activity of the 17.7 wt% Ir/SiNW to that of commercial 20 wt% Ir/C and 20 wt% Pt/C as well as 20 wt% Rh/SiNW (a catalyst found by us better than Pt for high overpotentials in a previous work).25 We support our findings by deriving the Tafel slopes of these catalysts showing that 17.7 wt% Ir/SiNW is indeed the best HER catalyst. Last in this section we present the LSVs in terms of activity per mass, per mole and per $ showing that in every category 17.7 wt% Ir/SiNW is the most efficient HER catalyst.


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Figure 3. Electrochemical properties of 17.7 wt% Ir/SiNW compared to those of other catalysts. (A) LSV plots of pure Ir, SiNWs and Ir/SiNW catalysts with different Ir contents. 17.7 wt% Ir/SiNW shows the best HER properties; (B) LSV plots of Ag, Ru, Au, Pd, Os, Pt and Ir/SiNW catalysts with a metal weight concentration of ~20%. 17.7 wt% Ir/SiNW has the best HER activity among all metal/SiNW catalysts with about 20 wt% metal; (C) LSV plots of 17.7 wt% Ir/SiNW, 20 wt% Ir/C, 20 wt% and 40 wt% Pt/C, and 20.6 wt% Rh/SiNW; (D) Tafel plots and Tafel slopes of 17.7 wt% Ir/SiNW, pure Ir NPs, 20 wt% and 40 wt% Pt/C; (E) Tafel plots and Tafel slopes of Ru, Au, Pd, Pt, Os, Pt and Rh/SiNW derived from (B) and (C); and (F) EIS plots of 17.7 wt% Ir/SiNW at different overpotentials, the inset being the equivalent circuit model.

Figure 3A presents LSV curves of Ir/SiNW catalysts with different Ir concentrations as well as pure Ir NPs and bare SiNWs. Bare SiNWs are not active at all. The HER activity of Ir/SiNW clearly depends on the Ir concentration and is the highest for 17.7 wt%, surpassing the activity of pure Ir NPs.


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Figure 3B shows the LSV curves of different metal/SiNW catalysts containing about 20 wt% metal. The Ir/SiNW catalyst definitely shows the best HER activities among the metal/SiNW catalysts. Furthermore, 17.7 wt% Ir/SiNW exhibits higher catalytic activity at all overpotentials than 20 wt% Ir/C, 20 wt% Pt/C, 40 wt% Pt/C and surpasses the activity of 20.6 wt% Rh/SiNW (Figure 3C) as well as that of 29.1 wt% Rh/SiNW (Figure S4), which was found in our previous work an excellent HER electrocatalyst (better than Pt/C) at high overpotentials.25 Figures S5-S7 are LSV curves of different metal/SiNW catalysts containing about 20 wt% metal in terms of current per mass, per mole (i.e. per atom) and per $. The Ir/SiNW catalyst is the best one among all these metal/SiNW catalysts and it possesses the highest HER activity per atom among all the metals investigated. The current density of 17.7 wt% Ir/SiNW reaches -10 mA·cm-2 at the overpotential of 22 mV and its corresponding TOF is determined to be 0.166 s-1, which is larger than those of other catalysts at the same overpotential (22 mV) (Table S5). The overpotentials at the current density of -10 mA·cm-2 and TOFs at an overpotential of 22 mV for 20 wt% Pt/C, 40 wt% Pt/C and 20 wt% Ir/C are 32, 30, 44 mV and 0.103, 0.053, 0.027 s-1, respectively. The lower overpotential at the current density of -10 mA·cm-2 and the larger TOF value conclusively proves the superior HER activity of 17.7wt% Ir/SiNW with respect to all the catalysts studied. Additionally, the Tafel slope of 17.7 wt% Ir/SiNW (20 mV·dec-1) is smaller than those of 20 and 40 wt% Pt/C and 20 wt% Ir/C as well as pure Ir and Pt NPs (Figures 1F and 3D) and all other 20 wt% metal/SiNW catalysts (Figure 3E). This is an additional proof to the superior HER activity of 17.7 wt% Ir/SiNW. The apparent exchange current density of catalysts (Table S6) is an additional key parameter for evaluation of the catalysts’ HER activity. The 17.7 wt% Ir/SiNW catalysts exhibits the largest apparent exchange current density among all catalysts studied. Note


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that iR correction of the electrochemical data of Figures 1-3 was performed for completion and the associated Tafel plots were calculated (Figures S8 and S9). The electrode kinetics of 17.7 wt% Ir/SiNW in HER process was investigated by electrochemical impendence spectroscopy (EIS) analysis, as shown in Figure 3F. The equivalent circuit model (inset in Figure 3F) consists of solution resistance Rs, a R1||CPE1 and a Rct||L||CPE2 elements connected in series, each component is fully described in Supporting Materials. The semicircle at high frequencies corresponds to the electrode surface pores (R1) and that at low frequencies is associated with the electrocatalytic kinetics (Rct). Figure 3F indicates that R1 is nearly independent of the overpotential, while the values of Rct strongly decrease with the increase of the overpotential. The calculated values of the various circuit elements are listed in Table S7. The EIS was employed to calculate the real surface area and real exchange current density (Table S8) of the catalysts.25, 27 The pH dependent relation of the HER provides a further understanding of the reaction mechanism. The reaction order is 1.92 (Figure S10). The HER activation energies for 17.7 wt% Ir/SiNW and 20 wt% Pt/C catalysts were obtained as well from the catalysts’ LSVs at different temperatures. The LSV curves were recorded applying an oxygen-free 0.5 M H2SO4 solution at temperatures ranging from 298 to 338 K at a sweep rate of 5 mV·s-1. The Tafel plots (at different temperatures) of 17.7 wt% Ir/SiNW (Figure S11A) and 20 wt% Pt/C (Figure S11C) were derived from their corresponding LSV curves. The increase of temperature resulted with an enhanced HER activity, i.e. enhancing current densities and decreasing overpotentials. The Arrhenius plots for 17.7 wt% Ir/SiNW and 20 wt% Pt/C catalysts obtained from the Tafel plots are presented in Figures S11B and S11D. The electrochemical activation energies could be calculated according to the Arrhenius equation:


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log j 0  log(FKc) - G 0/2.303RT , where R is the Rydberg gas constant and  G is the apparent 0

thermal activation energy. The calculated values of  G0 for 17.7 wt% Ir/SiNW and 20 wt% Pt/C catalysts are 16.7 and 29.4 kJ·mol-1 respectively, which indicates that 17.7 wt% Ir/SiNW has the smaller thermal activation energy.

DISCUSSION The HER of a one component metal catalyst (M) system is occurring in two consecutive steps: (i) H adsorption by the catalyst, (ii) H2 desorption from the catalyst. The corresponding reactions are:   The Volmer mechanism: H  M  e  M  H , and

  The Heyrovsky mechanism: H  M  H  e  H 2  M , or

the Tafel mechanism:

M  H  M  H  H 2  2M

The Tafel slopes of the one component system can be calculated from the kinetics of reactions (i)-(ii). The Tafel slope assuming the Volmer reaction is the rate limiting process is 120 mV·dec1.

The slope assuming the desorption is the rate limiting process and Heyrovsky is faster than

Tafel is 40 mV·dec-1. The slope assuming the desorption is the rate limiting process and Tafel is faster than Heyrovsky is 30 mV·dec-1. The Tafel step has the lowest slope, i.e. it has the best HER activity for a single component catalyst. This is indeed the Tafel slope of Pt/C. Our 17.7 wt% Ir/SiNW catalyst has a Tafel slope of 20 mV·dec-1, i.e. the HER process should be different than that of a single component catalyst.


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Figure 4. A schematic representation of the “Volmer – H diffusion – Tafel” or the “Volmer – H diffusion – Heyrovsky” reaction pathway: (i) the protons are reduced to hydrogen atoms on the Ir surface. (ii) the hydrogen atoms diffuse from Ir atoms to the Si atoms at the Ir-Si interfaces. Hydrogen atoms on Si atoms are combined (Tafel, iiia) or react with H+ from the solution (Heyrovsky, iiib) to form hydrogen molecules.

For a one component metal catalyst system the change of Gibbs free energy of the HER reaction is: ΔGH = 0.03 eV for Ir and ΔGH = - 0.09 eV for Pt, both are near the top of the volcano plot. Using the metal/SiNW system we split the HER to three parts (Figure 4): (i) the protons are adsorbed on the Ir surfaces and reduced to hydrogen atoms (Volmer), (ii) the hydrogen atoms migrate from Ir atoms to Si atoms at the Ir-Si interfaces (diffusion), (iii) absorbed H atoms on the Si surface combine with each other (Tafel, iiia) or with H+ from the solution (Heyrovsky, iiib) to generate the hydrogen molecules which are desorbed. The corresponding reactions are:


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(i)

Ir  Si  H   e   H  Ir  Si 

(Volmer)



(ii) H  Ir  Si  H  e  H  Ir  Si  H (diffusion) (iiia)

2 H  Ir  Si  H  H 2  2 H  Ir  Si

(iiib)

H  Ir  Si  H  H   e   H 2  H  Ir  Si

(Tafel) or (Heyrovsky)

The feasibility of the diffusion step was investigated using DFT calculations. The Ir/SiNW catalyst was modeled with a hydrogen-terminated Si(111) surface and a Ir(111) surface (Figures 5A-5D). The calculations show that the H migration is an exothermic process (the energy of final state is 1.01 eV below that of the initial state) with a small activation barrier of 0.13 eV, indicating that the H migration is a spontaneous and kinetically favorable process.28 The H atoms on Si atom may combine together to produce H2 molecules. Figure S12 shows that H-SiNWs release H2 gas easily when they immersed in water. DFT calculations (Figures S13-S15) show that the formation and desorption of H2 molecules from the Si surface by either Tafel (step iiia) or Heyrovsky (step iiib) are feasible. The Tafel slopes can be calculated from the kinetics of reactions (i)-(iii). For the (i)-(ii)-(iiib) process the Tafel slope is 24 mV·dec-1.22 For the (i)-(ii)-(iiia) process the Tafel slope is:

2.303RT 2(1   ) F = 20 mV·dec-1 (assuming   0.5 , F is the Faraday constant, R the Rydberg gas constant, T the absolute temperature). The detailed calculations are given in the Supporting Materials. Both cases (reactions (i)-(ii)-(iiia) and (i)-(ii)-(iiib)) initiate a significant improvement of the HER activity when switching from a single component Ir catalyst to a binary component Ir/SiNW catalyst. The experimental Tafel slopes indicate that the HER activation of the 17.7 wt% Ir/SiNW catalyst proceeds via the (i)-(ii)-(iiia) process (Tafel slope = 20 mV·dec-1).


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Figure 5. Theoretical analysis of the HER process in the Ir/Si system. (A)-(D) The migration of a H atom (marked with a red circle) from Ir surfaces to the Si surfaces: (A) IS, initial state; (B) TS, transition state; and (C) FS, final state. (D) The energetic diagram of the three states (A)-(C). The activation barrier is 0.13 eV. The energy of final state is 1.01 eV lower than that of the initial state. (E)-(G) The calculated adsorption energies and corresponding Bader charges of the adsorbed (E) Ir, (F) Pt and (G) Rh atom on the OH terminated Si(111) surface. The adsorbed Ir atom carries more negative charge (-1.26|e|) than the adsorbed Pt (-1.00|e|) and Rh(-0.80|e|) atoms. The adsorption energy of a single Ir, Pt and Rh atom is -5.98, -5.29 and -5.21 eV, respectively.


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The splitting of the HER reaction to two surfaces (metal-Si) does reduce the obtainable Tafel slope with respect to the single metal catalyst system. The additional H diffusion step necessary in the binary catalyst system occurs very fast so that the binary system indeed enables to bypass the limitations of the Sabatier principle. The experimental data shows that indeed a significantly reduced Tafel slope is obtainable applying the Ir/SiNW system as well as superior HER characteristics compared to Pt in every aspect. The worse catalytic behavior of commercial Ir/C with respect to commercial Pt/C despite the smaller ΔGH of Ir was attributed to the larger particle size and stronger aggregation of Ir/C with respect to Pt/C. This is associated with the larger ∆Hat of Ir compared to Pt. Surprisingly, the introduction of the SiNW system was also found helpful in fabricating small Ir NPs (2.17 nm in size) and in suppressing the aggregation of the Ir particles enhancing the HER activity and extending the stability of the Ir/SiNW system. DFT calculations show that the absorption of metal atoms on the Si surface is associated with a charge transfer from the Si atoms to the metal atoms (Figures 5E-5G). The electrostatic repulsion assists in having a better dispersion and less aggregation. Note that the negative charge carried by the Ir atom on Si (-1.26|e|) is larger than that carried by the Pt atom (-1.00|e|) and by the Rh atom (0.80|e|). Consequently, the dispersion of Ir NPs on Si and the suppression of its aggregation would be better than those of Pt and Rh. DFT calculations also reveal that the Ir/SiNW catalyst is able to self-detoxify from hydroxyle poisoning (Figure S16). When a Si atom is terminated with a hydroxyl, the adsorbed H on the Ir surface will combine with the hydroxyl to regenerate the Si atom by producing water molecules. The energy barrier of the self-detoxifying process is 0.43 eV and the difference of the Gibbs energy is -0.68 V so that the reaction is spontaneous. Indeed, the Ir/SiNW stability is better than those of Pt/C, Ir/C, Pt NPs and Ir NPs.


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CONCLUSIONS This work focuses on the fabrication of an Ir-based HER catalysis with a world record activity surpassing that of Pt in every aspect. First, we compared the Ir/C and Pt/C catalysts and have found that the inferior performance of Ir/C despite its smaller ΔGH (which should make Ir a better catalyst according to the volcano plot) results from the poor dispersion and the aggregation of the Ir NPs. This aggregation decreases the initial HER activity and further deteriorates it under operation hampering the HER stability. We followed by applying the binary component catalyst concept on Ir studying the Ir/SiNW system. Ir on Si is charged significantly improving the Ir dispersion, enabling the growth of small 2.17 nm sized particles and suppressing aggregation. This feature enhances both the activity and stability of the catalyst system. The application of the binary metal/SiNW catalyst system was found to overcome the limitations of the Sabatier principle by splitting the H adsorption and desorption to two different surfaces. The best Ir/SiNW catalyst had an Ir concentration of 17.7 wt%. The 17.7 wt% Ir/SiNW catalyst was found the best of all 20 wt% metal/SiNW catalysts. Moreover, its HER properties surpassed those of Pt/C (commercial Pt/C used as a reference in the present work as well as previously reported Pt/C electrodes) and those of previously reported Ir-based catalysts in every aspect: (i) lower onset potential, (ii) higher apparent and real exchange current densities, (iii) higher current density (i.e. activity) for both small and large overpotentials, (iv) larger activity per mass, per mole (atom) and per $, (iv) lower Tafel slope, (v) higher TOFs, (vi) better stability. The experimental data was substantiated by theoretical calculations and by DFT simulations. Such 17.7 wt% Ir/SiNW is expected to make a major impact on the industry not only by


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improving the efficiency of H2 generation by electrolysis, but on other important catalytic industrial processes as well.

EXPERIMENTAL SECTION Materials. Hexachloroiridium acid hydrate (H2IrCl6·xH2O, Ir 35% in HCl), RuCl3, H2PtCl6·6H2O, RhCl3·3H2O and PdCl2 were purchased from Aladdin Industrial Co. The Nafion (5 wt %) was obtained from Sigma-Aldrich Co. The HAuCl4·4H2O and AgNO3 were purchased from Sinopharm Chemical Reagent Co., Ltd. The commercial Pt/C catalysts were supplied by Alfa Aesar Co. (40 wt% or 20 wt% Pt in carbon black). The commercial 20 wt % Ir/C was obtained from Premetek Co. Other reagents were of analytical grade without further purification. Doubly-distilled water was used throughout the experiment. Fabrication of Ir and other metal NPs modified SiNWs. SiNWs were obtained via the oxide-assisted growth method.29 The fabrication of Ir NPs deposited SiNWs with Ir diameter of 2.17 nm was as following: The obtained SiWNs (60 mg), with diameters ranging from 15 nm to 90 nm, were dispersed in 30 mL doubly-distilled water, and then 20 μL H2IrCl6·xH2O was added and stirred for 30 min. After that, 1.2 mL HF aqueous solution (5%) was drop-wisely added into the above solution with continuously stirring. Finally, the mixture was transferred to a Teflonlined stainless steel autoclave, which was heated to 120 °C and held at this temperature for 2 h. The Ir/SiNW composite was obtained after centrifugation and washing with doubly-distilled water and ethanol for three times. The content of Ir in the composites was measured to be 17.7 wt% by X-ray fluorescence.


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The volume of H2IrCl6·xH2O was regulated to obtain different contents of Ir in Ir/SiNW nanocomposites. The corresponding data are listed in Table S9. The pure Ir NPs were obtained through HF etching of the 17.7 wt% Ir/SiNW. For the fabrication of Ir/SiNW with Ir particle diameter of 4.80 nm, a two-step method was used. The obtained SiWNs (60 mg) were dispersed in 30 mL doubly-distilled water, and then 10 μL H2IrCl6·xH2O was added and stirred for 30 min. After that, 0.6 mL HF aqueous solution (5%) was drop-wisely added into the above solution while continuously stirring. Finally, the mixture was transferred to a Teflon-lined stainless steel autoclave, which was heated to 120 °C and held at this temperature for 2 h. After the autoclave was cooled down naturally, another 10 μL H2IrCl6·xH2O and 0.6 mL HF aqueous solution (5%) were added with continuous stirring. The autoclave was heated to 120 °C again and held at this temperature for 2 h. The Ir/SiNW composite was obtained after centrifugation and washing with doubly-distilled water and ethanol for three times. The content of Ir in the composites was measured to be 18.1 wt% by X-ray fluorescence. Rh/SiNW, Pt/SiNW, Pd/SiNW, Ru/SiNW, Au/SiNW and Ag/SiNW nanocomposites with different metal/Si ratio were obtained in a similar manner with different amounts of the corresponding starting materials. The contents of metals in catalysts were determined by inductively coupled plasma mass spectrometry (ICP-MS). The pure Pt NPs were obtained through HF etching of 21.2 wt% Pt/SiNW, respectively. Characterization. The structure of the samples was characterized by XRD (Philips X'pert PRO MPD diffractometer) applying Cu Kα radiation (λ = 0.15406 nm) (Figure S17). The chemical states of the catalysts were studied by X-ray photoelectron spectroscopy (XPS) (Figure S3) using a Kratos AXIS UltraDLD ultrahigh vacuum surface analysis system with Al Kα


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radiation (1486 eV) as probe. The X-ray fluorescence (XRF) was conducted with an XRF-1800 spectrometer. The ICP-MS was conducted with an Aurora M90 spectrometer. ). TEM and HRTEM images were recorded using a FEI Tecnai F20 transmission electron microscope with the accelerating voltage of 200 kV (e.g. Figure S18). The TEM inages were used to derive the size distribution of the metallic NPs deposited on the SiNWs (Figure S19). Electrochemical measurements. The conventional three-electrode system connected to a Princeton VersaSTAT4 electrochemistry workstation was used to carry out all the electrochemical measurements. A carbon rod was used as the counter electrode, a saturated calomel electrode (SCE) was used as the reference electrode and a modified glassy carbon electrode with a diameter of 0.3 cm was used as the working electrode. The potentials in this work were calibrated to a reversible hydrogen electrode for the tests of HER (in 0.5 M H2SO4 solution, Evs.RHE= Evs.SCE + 0.245 + 0.0591 × pH). The geometrical area of the glassy carbon electrode Sgeo, on which the catalysts were loaded, was calculated to be 0.0707 cm2. This value was used to determine the current density in the linear scan voltammetry. The working electrode was fabricated via the following procedure: 4 mg Ir/SiNW catalysts or other catalysts were dispersed in 900 μL of 5 : 1 v/v water-isopropanol mixed solvent with 100 μL 0.5 wt% Nafion solution, and ultrasonically stirred for at least 30 min to reach a homogeneous suspension. Then 6 μL of the suspension was drop-wisely added onto a glassy carbon electrode (loading ~0.339 mgcatalyst∙cm-2). Finally, the modified electrode was dried at room temperature. The working electrodes were activated by running them via the CV method in the range of 0 to -0.1 V for 50 cycles with scanning rate of 50 mV·s-1. All data were reported without iR compensation unless otherwise mentioned.


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Calculations. The theoretical study of the composite catalysts was performed in the framework of DFT with the Vienna ab initio Simulation Package (VASP).30, 31 The electron-ion interactions were described using the projected augmented wave (PAW) method.32 The exchange-correlation energy was calculated with the general gradient approximation (GGA) functionals of Perdew-Burke-Ernzerhof (PBE).33 An energy cutoff of 400 eV was selected for the plane-wave expansion. The dispersion corrections of the hydrogen and surface interactions were included by the van der waals density functional (vdw-DF) proposed by Dion.34 The catalysts were modeled with periodic slabs consisting of at least five atomic layers. A vacuum of 15 Å was adopted to avoid the periodic image interactions normal to the surface. In all cases, the top three layers of atoms were allowed to relax in three dimensions. The transition states were determined with the Climbing Image Nudge-Elastic-Band (CI-NEB) calculations, in which nine structural images were inserted between the initial and the final states.35, 36 The water solvent environment was simulated with an implicit solvation model implemented in VASPsol.37, 38 The convergence threshold for structural relaxations was set to be 0.01 eV/Å. The first Irreducible Brillouin zone (IBZ) was sampled using Monkhorst-Pack grid of 1 x 2 x 1 for binary catalysts and 3 x 3 x 1 for hydrogen terminated Si (111) surfaces, respectively.39

ASSOCIATED CONTENT Supporting Information. Structural characterization, electrochemical data, DFT data, the comparison of recently reported HER. The authors declare no competing financial interests.


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AUTHOR INFORMATION Corresponding Author [email protected] (HP Lin); [email protected] (Y Lifshitz); [email protected] (MW Shao). Author Contributions ₴MQ

Sheng and BB Jiang contributed equally to this work.

ACKNOWLEDGMENTS The project was supported by The National Key Research and Development Program of China (2017YFA0204800), National MCF Energy R&D Program (2018YFE0306105), the National Natural Science Foundation of China (No. 21771134), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project.

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SYNOPSIS


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Silicon Nanocomposites as

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