RutheniumâTungsten Composite Catalyst for the Efficient and

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Ruthenium−Tungsten Composite Catalyst for the Efficient and Contamination-Resistant Electrochemical Evolution of Hydrogen Ubisha Joshi,†,∥ Souradip Malkhandi,†,∥ Yi Ren,‡ Teck Leong Tan,§ Sing Yang Chiam,‡ and Boon Siang Yeo*,† †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 Institute of Materials Research and Engineering, Agency for Science Technology and Research, 2 Fusionopolis Way, Singapore 138634 § Institute of High Performance Computing, Agency for Science, Technology and Research, 1 Fusionopolis Way, Singapore 138632 ‡

S Supporting Information *

ABSTRACT: A new catalyst, prepared by a simple physical mixing of ruthenium (Ru) and tungsten (W) powders, has been discovered to interact synergistically to enhance the electrochemical hydrogen evolution reaction (HER). In an aqueous 0.5 M H2SO4 electrolyte, this catalyst, which contained a miniscule loading of 2−5 nm sized Ru nanoparticles (5.6 μg Ru per cm2 of geometric surface area of the working electrode), required an overpotential of only 85 mV to drive 10 mA/cm2 of H2 evolution. Interestingly, our catalyst also exhibited good immunity against deactivation during HER from ionic contaminants, such as Cu2+ (over 24 h). We unravel the mechanism of synergy between W and Ru for catalyzing H2 evolution using Cu underpotential deposition, photoelectron spectroscopy, and density functional theory (DFT) calculations. We found a decrease in the d-band and an increase in the electron work function of Ru in the mixed composite, which made it bind to H more weakly (more Pt-like). The H-adsorption energy on Ru deposited on W was found, by DFT, to be very close to that of Pt(111), explaining the improved HER activity. KEYWORDS: hydrogen evolution reaction, Ru−W, water electrolysis, electrocatalysts, bimetallic catalysts

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INTRODUCTION The electrolysis of water to generate hydrogen is potentially a major component in the implementation of a global renewable energy generation system.1 This process can be used to convert intermittent electrical energy generated from solar/wind farms to chemical energy, as well as to supply H2 for many applications. During the hydrogen evolution reaction (HER), protons from water are first discharged at the cathode (Volmer step: H+ + e− → Had), which then formed H2 gas by either two H atoms (Tafel step: Had + Had → H2) or a H atom + ion (Heyrovsky step: Had + H+ + e− → H2) recombination step.2 The energetic efficiency of the HER depends primarily on the electrocatalytic property of the cathode. Among the various types of catalysts, platinum is one of the highest performing in acidic electrolytes.3 However, because platinum is as expensive as gold, an intensive search for less expensive HER electrocatalysts has been ongoing.4−11 In addition to HER activity, immunity of a catalyst from poisoning is another important figure-of-merit that should be considered when assessing its applicability in the industries. The activity of a cathode typically decreases over time during HER because of interferences by contaminants in the electrolyte.12−14 These contaminants act by depositing on the cathode and blocking its active sites. Alternatively, they © XXXX American Chemical Society

deactivate the catalyst by altering its electronic properties. Most of the time, the effects of cathode poisoning from contamination go unnoticed in a laboratory because of the short time scale of measurement and use of ultrapure water. However, these practices are not practical in real-life applications and hence the eventual poisoning of the electrode becomes a key challenge that needs to be overcome. We shall demonstrate in the Results and Discussion section that platinum, in spite of being an excellent HER catalyst, is not immune to deactivation through contamination. Among the various approaches for synthesizing better electrocatalysts for HER, bimetallic or two-component-based catalysts have been gaining attention.15−19 For example, small amounts of Pt supported on the carbides of tungsten, titanium, and vanadium have worked well for H2 evolution, requiring overpotentials of 50−100 mV to drive a current density of 10 mA/cm2.20 Nonetheless, many significant areas still remain unexplored, in particular, developing and scaling up efficient and durable non-Pt-based catalysts and understanding the fundamental reasons that underlie their workings. Here, we Received: November 25, 2017 Accepted: January 24, 2018

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DOI: 10.1021/acsami.7b17970 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces report a new composite catalyst made of tungsten (W) and ruthenium (Ru) that behaved synergistically to give an activity for electrochemical H2 evolution that approached that of platinum. Ru, although a noble metal, is cheaper than Pt, and W is a low-cost metal. Therefore, a bimetallic catalyst based on Ru and W has the potential to become an efficient and economical HER catalyst. Our composite catalyst was easily prepared by simple physical mixing of small amounts of Ru and W nanopowders. In addition, we found that it exhibited excellent immunity against contamination. A detailed study of the working of this composite catalyst is provided using copper underpotential deposition (Cu upd), photoelectron spectroscopy (valence band measurements), and electronic structure density functional theory (DFT) calculations.

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RESULTS AND DISCUSSION Catalysts based on ruthenium particles supported on carbon (Ru/C) were synthesized (Supporting Information (SI) Section S1) and characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The TEM image of Ru/C indicated that the Ru particles were well dispersed in the carbon matrix and have sizes of 2−5 nm (Figure 1a). The Ru/C catalyst was then physically mixed with carbon black (BP2000) and 60−80 nm sized tungsten nanoparticles in the mass ratio of 1:5:5, respectively. The SEM image of this composite catalyst (W + Ru/C + BP2000) showed agglomerates of particles with diameters of hundreds of nanometers (Figure 1b). These were presumed to be from carbon black and tungsten. The Ru particles were not visible owing to their comparatively smaller dimensions. However, as the corresponding EDX mapping images suggest, the Ru particles were homogenously dispersed around the W particles (Figure 1c,d). XPS analysis of the composite catalyst showed the metallic Ru0 3d5/2 peak at ∼280.4 eV with a broad shoulder at ∼281 eV that represented oxidized Ru (Figure 1e).21 The composite catalyst was also analyzed by XRD, and its diffractogram was found to be dominated by peaks from the tungsten nanoparticles (Figure 1f). Ruthenium signals could not be discerned from either the composite catalyst or the as-synthesized Ru/C catalyst. In the latter catalyst, the Ru peaks were likely enveloped by the broader and more intense peaks of carbon. Electrochemical measurements with W + BP2000, Ru/C, W + Ru/C + BP2000, and Pt/C in an aqueous 0.5 M H2SO4 electrolyte are shown in Figure 2a,b (Table S1). Tungsten is not a good HER catalyst as evident from the large overpotential (465 ± 4 mV) required for it to drive 10 mA/cm2 (geometric current density) of H2 evolution and its high Tafel slope (84 ± 9 mV/dec). This observation is consistent with the high chemisorption energy of H on tungsten (∼−0.7 eV).3 However, when tungsten is mixed with Ru/C, it is able to significantly decrease the overpotential of the latter by 80 mV for 10 mA/cm2 H2 production and to reduce its Tafel slope from 93 ± 1 to 46 ± 2 mV/dec. The overpotential (85 ± 4 mV) required to drive 10 mA/cm2 of H2 evolution for our W + Ru/C + BP2000 catalyst is thus very close to that of Pt (33 ± 2 mV) (20% Pt on Vulcan XC-72; loading: 5.6 μg Pt/cm2). Our composite catalyst also outperforms many recently reported non-Pt-based HER catalysts (Table 1). To ascertain that the improvement in HER by the composite catalyst is intrinsic and not due to changes in surface area, we

Figure 1. (a) TEM image of the Ru/C catalyst; the inset shows the Ru particle size distribution. (b) SEM image of the W + Ru/C + BP2000 catalyst. Elemental distribution analysis of (c) Ru and (d) W using SEM-EDX for the composite catalyst seen in (b). (e) X-ray photoelectron spectrum of Ru 3d5/2 of the W + Ru/C + BP2000 catalyst (black solid line with circle: measured spectra, blue line with cross: envelope, red line: deconvoluted peak of metallic ruthenium, and purple line: deconvoluted peak of oxidized ruthenium). (f) X-ray diffractogram of the W + Ru/C + BP2000 catalyst; the inset shows XRD of (i) W and (ii) Ru/C. The brown, green, black, and red vertical lines indicate the expected patterns for α-W, β-W, graphitic carbon, and Ru, respectively.

normalized the currents exhibited by the catalysts to their real surface areas (SI Section S1). The normalized currents in the voltammograms revealed that with the addition of W, the HER activity of Ru increased by close to an order of magnitude (∼9) at an overpotential of 85 mV (Figure 2a, inset). The exchange current densities (i0) of the catalysts were also determined (SI Section S1). The i0 values for W + Ru/C + BP2000 and Ru/C were, respectively, estimated to be 48.8 and 26.2 μA/cm2. On the basis of these values, the HER activity for the composite catalyst increased by a factor of 1.86 relative to that of Ru/C. Furthermore, control experiments with W + RuO2 + BP2000 and WO3 + Ru/C + BP2000 indicated no significant improvements in HER (Figure S1). This shows that the synergy is not from the interaction of either tungsten with the oxides of Ru or Ru with the oxides of tungsten. Experiments were also performed with composites based on Mo, Ti, V, Zr, Nb, and Ta (Table S1). With the exception of Mo, which like W also lies in group B


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ACS Applied Materials & Interfaces

for H2 production. The detailed protocol for estimating the FE has been described in our earlier work.6 Interestingly, we discovered that the HER catalytic activity of our W + Ru/C + BP2000 catalyst was stable, even in the presence of contaminants (Figure 2c). In these tests, the HER activities of W + Ru/C + BP2000 (blue bar) and a Pt electrode (orange bar) were measured in 0.5 M H2SO4 electrolytes contaminated with Cl−, S−, Cu2+, and Fe2+ ions. These four ionic species were selected as model contaminants because chloride is abundant in sea water, sulfide is a known catalyst poison for platinum,13 and copper and iron tend to interfere negatively with cathodic reactions. The performance index was measured as the absolute change of overpotential, ΔE (at 10 mA/cm2), after 10 000 s of HER (SI Section S1). As compared with Pt, W + Ru/C + BP2000 was more immune toward deactivation by Cu2+ (ΔE for Pt is 3 times more than ΔE for W + Ru/C + BP2000), whereas for other contaminants, the immunity was comparable for both catalysts. A further durability study was performed on W + Ru/C + BP2000 held in a Cu2+-spiked H2SO4 electrolyte (Figure 2d). Here, this catalyst was found to be highly stable for 10 mA/cm2 H2 production for over 24 h compared with the Pt electrode kept in a similarly prepared electrolyte. Considerable effort has been devoted to relate the work function of an electrode with its HER activity,22,23 although such a correlation does not necessarily imply a causation.24 The work function of an electrode can be probed by the underpotential deposition of a metal. An essential condition for underpotential deposition to occur is a substrate with a work function higher than that of the metal for deposition.25−27 Here, we found that both W + Ru/C + BP2000 and Ru/C showed the Cu upd stripping peak (Figure 3), whereas RuO2 and W did not show any Cu upd stripping peak (Figure S3a,b). Interestingly, the Cu upd stripping peak on ruthenium in the presence of tungsten was shifted positively by 50 ± 1 mV in comparison to that of bare Ru. A shift in upd stripping peak suggests a change in work function, i.e., presence of an electronic interaction between tungsten and ruthenium. Specifically, a positive shift of the peak by 50 mV shows that tungsten increases the work function27 of the ruthenium surface by 0.1 eV (SI Section S2). Considering the position of ruthenium on the work function versus HER activity plot,22 an increase in work function by 0.1 eV should lead to an increase in HER activity by a factor of 1.8 (calculated factor is based on exchange current density; SI Section S2). This value is thus remarkably close to the value of 1.86 estimated earlier using our experimental data. However, note that the actual improvement

Figure 2. (a) Linear sweep voltammograms of BP2000, W + BP2000, Ru/ C, W + Ru/C + BP2000, and Pt/C. The currents were normalized to the electrode geometric surface area; inset: voltammograms with the currents of W + BP2000 (green) normalized to the real surface area of W, Ru/C (red) normalized to the real surface area of Ru, and W + Ru/ C + BP2000 (blue) normalized to the real surface area of Ru. (b) Tafel plots of the catalysts W + BP2000, Ru/C, W + Ru/C + BP2000, and Pt/ C. (c) Absolute change in overpotentials in the presence of contaminants for W + Ru/C + BP2000 (blue bar) and Pt electrode (orange bar) after 10 000 s operation at 10 mA/cm2 current density. (d) Chronopotentiogram (−10 mA/cm2) of W + Ru/C + BP2000 and Pt in the presence of 10 μM Cu2+.

6 of the periodic table, no other metal showed synergy when mixed with Ru/C (Figure S2). Specificity of synergy with tungsten and molybdenum, along with the real surface-areanormalized voltammetry and exchange current density data, suggests strongly that the observed synergy cannot be simply attributed to the improved dispersion of the Ru nanoparticles in the composite catalyst. It is, as we shall demonstrate later, due to electronic interactions between these two metals. The Faradic efficiency (FE) of H2 evolution catalyzed by our W + Ru/C + BP2000 composite catalyst was also measured using online gas chromatography and ascertained to be 99% (Table S2). This demonstrates that all cathodic currents were utilized

Table 1. Comparison of HER Performances of Different Catalysts catalysts W + Ru/C + BP2000 Ru/C Pt/C Ni2P [Mo3S13]2− Mo2C/CNT-GRa CoP/carbonb WP CoSe2 NP/CPc

electrolyte 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

M M M M M M M M M

H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H2SO4

loading (μg/cm2)

current density (mA/cm2)

overpotential (mV)

Tafel slope (mV/dec)

reference

5.6 (Ru) 141 (W) 5.6 (Ru) 5.6 (Pt) 1 × 103 100 6.5 × 102 300 1 × 103 2.8 × 103

10 10 10 20 10 10 10 10 10

85 186 33 130 1.8 × 102 130 95.8 120 139

46 93 26 46 40 58 33 54 42

this work this work this work 43 44 45 7 46 47

a Mo2C/CNT-GR: Mo2C supported on carbon nanotube−graphene. bCoP/carbon: CoP on three-dimensional nanoporous carbon. cCoSe2 NP/CP: CoSe2 nanoparticles grown on carbon fiber paper.

C


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ruthenium. The reported value of the d-band center for Ru is around −2 eV. Therefore, a negative shift of the d-band center is expected to reduce the adsorption energy of hydrogen on Ru, which will consequently result in its higher HER activity (dband center for Pt ∼−2.5 eV).32 The peaks in the 5−9 eV range in the valence band spectra (Figure 4) are highly likely to be due to the O 2p-derived band, including hybridized bands from oxides of W and Ru.31,33,34 Tungsten in the W + Ru/C + BP2000 composite catalyst was mostly in the oxidized state. Its XPS spectra showed an oxide peak at ∼36.1 eV, which represented a binding energy close to that of the W6+ state (Figure S4). Metallic W was also present in a small amount at ∼31.6 eV, as shown by its core-level spectra. After HER, the core-level binding energy of the W oxidized state was lowered to 35.8 eV, indicating a partial reduction toward the W5+ state. The metallic state could no longer be observed after the HER. The dominance of the oxide peaks could be due to exposure of the sample to the ambient environment during its transfer for photoelectron spectroscopy measurements. Ultraviolet photoelectron spectroscopy (UPS) measurements showed a similar shift in the work function of Ru (Figure 5 and

Figure 3. Copper underpotential-deposited monolayer stripping curve for Ru/C (red line) and W + Ru/C + BP2000 (blue line) (1 mM CuSO4 in 0.5 M H2SO4 solution, 50 mV/s scan rate).

at an overpotential of 85 mV is ∼9 (Figure 2a). This could be because our experimental results had shown a remarkable improvement of the Tafel slope, indicating a change in reaction mechanism in the presence of tungsten. The catalytic activity of a metallic surface toward HER is also related to its H-adsorption strength. This, in turn, is determined by the d-band structure of the surface,28,29 and from theoretical calculations, a bimetallic interaction will lead to a shift of the dband center from the Fermi level.30 Therefore, we use X-ray photoelectron spectroscopy to investigate the d-band structure of our catalysts before and after the electrochemical HER. The valence band structure of Ru/C alone showed the presence of d-band electrons expected between Ef and Ef − 4 eV and a sharp Fermi edge at 0.04 eV (Figure 4). The presence of

Figure 5. UPS spectra of Ru/C (red line) and W + Ru/C + BP2000 before (blue line) and after (black line) electrochemical measurements. Inset: secondary cutoff. The work function of the material was determined from ultraviolet photoelectron spectroscopy data. The secondary cutoff (Ec) was measured under an external applied bias of negative 10 V. The work function was then determined by subtracting the secondary cutoff and Fermi-level separation from the photon energy (He−I: 21.2 eV).

Table S3). The measured work function of Ru was ∼5.02 eV, which increased to ∼5.17 eV after W incorporation. After HER, this further increased to ∼5.65 eV. The increase in work function after W incorporation agrees with the interpretation of a decrease in the d-band center that can lead to better HER activities. Thus, a combination of Cu upd and valence band spectroscopy demonstrates the presence of an electronic interaction, namely, a modification (decrease) of the d-band of Ru due to its interaction with tungsten, to give a resultant catalyst with Pt-like sites. To ascertain the underlying reasons for the high HER activity of our catalysts, we examined the hydrogen (H) adsorption/ binding energy on them using theory. Nørskov et al. have shown that the adsorption energy of H should lie within an optimal energy range for a good HER catalyst. This results in a volcano curve when the electrocatalytic HER activity is plotted against the H-adsorption energy.3,35 From DFT calculations, the H-adsorption energies can be accurately predicted and the

Figure 4. Offset-corrected high-resolution valence band spectra of Ru/ C (red line) and W + Ru/C + BP2000 catalysts before (blue line) and after (black line) the electrochemical experiment. These spectra are obtained from X-ray photoelectron spectroscopy. Inset: magnified region showing the shift in valence band edge from the Fermi level.

oxidized Ru was shown by the small Ru−O σ-peak near −6.9 eV.31 The W + Ru/C + BP2000 catalysts showed a slight shift of the Fermi edge of Ru/C to ∼0.07 eV, which is further shifted to an even higher value of 0.19 eV after the HER. These shifts in the Fermi edge are due to the reduction in the density of dband states. The density of states for the d-band of Ru electrons was seemingly lower after the introduction of W and was further reduced after the electrochemical experiment. We did not attempt any calculation of the d-band center due to the complex superimposition of valence bands of tungsten and D


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Table 2. Calculated Adsorption Energies of H (EADS(H)) on the Various Surfaces, Average Hydrogen−Metal (H−M) and Metal−Metal (M−M) Bond Lengths at the Adsorption Site,a,b and Calculated Work Functions (Φ) of the Surfaces

a

surface

EADS(H) (eV)

relative EADS(H) (eV) (relative to Pt(111))

average H−M bond length (Å)

average M−M bond length (Å)

calculated Φ of surface (eV)

Ru(0001) 2L-Ru(0001)@W(110) 4L-Ru(0001)@W(110) Pt(111) 2L-Ru(0001)@WO3 (001)

−0.610 −0.481 −0.482 −0.499 −0.598

−0.110 0.019 0.017

1.91 1.90 1.89 1.86 1.99

2.72 2.61 2.63 2.86 2.92

5.05 5.09 5.15 5.75

−0.099

The lowest-energy adsorption site is reported for each surface. bH is bonded to three metal sites (see Figure 6).

values can be used to identify promising HER catalysts produced from binary alloys and nanoparticles.36−40 We created a model of the Ru−W composite system by stacking Ru layers (to model the finite size effect of Ru nanoparticle) onto a W slab (to model the W support) and calculated the adsorption energy of H (SI Section S3). The H-adsorption energies of our Ru−W models were found to be very close to those of Pt(111) (Table 2). The latter is known to be the best HER catalyst among metals, although its H-adsorption energy is slightly too strong (∼0.08 eV) compared to that of the ideal HER catalyst (corresponding to the peak of the volcano curve).3 We observe that the Ru(0001) surface in itself is not a good HER catalyst as it overbinds to H (−0.61 eV). However, in the presence of a tungsten support, H adsorbs less strongly on the Ru(0001) layers and its adsorption energy becomes slightly more positive (∼0.02 eV) than that of Pt(111). This implies that the Ru−W composite system could theoretically outperform Pt(111). Although our slab model may not be fully representative of the experimental Ru−W nanocomposite, it captures the essence of electronic and strain effects arising from the W substrate and provides insights into their effects on H adsorption on Ru. Table 2 shows that the H-adsorption energy remained virtually unchanged as the number of Ru layers (L-Ru) goes from 2 to 4. The average metal−H bond length also remained unchanged (2L-Ru = 1.90 Å and 4L-Ru = 1.89 Å). This implies that the metal−H interaction could be short-ranged, i.e., between the H atom and the topmost layer of Ru. The charge density difference plots in Figure 6 support this assertion, where the adsorption of H is found to affect mostly the charge densities of the top two layers of the catalyst surface. The effect of strain induced by the W support on Ru, on the other hand, could possibly play a significant role. Comparing the metal−metal bond length in Table 2, the supported Ru layers (2.61−2.63 Å) are slightly compressed versus the unsupported Ru(0001) surface (2.72 Å). The compressive strain affects the electron distribution in the Ru layers, which in turn decreases the adsorption strength of H from −0.610 eV on unsupported Ru to −0.481 eV on supported Ru. Thus, strain effects tune the adsorption energy of H on Ru−W closer to that of Pt(111) (−0.499 eV), leading to an enhancement in HER catalytic activity. The calculated work functions of the catalysts also fully corroborate those measured from photoelectron spectroscopy (Table S3). Essentially, the W substrate increases the electron work function of the Ru layers, resulting in the latter’s weaker interaction with H. DFT calculations performed on a model structure of Ru(0001) layers deposited on the WO3(001) surface (Figure S5) revealed that the adsorption strength of H on the Ru layer is too strong, 0.1 eV stronger than that of Pt (Table 2). Hence, Ru@WO3 is not expected to be a good HER catalyst. This

Figure 6. Electron charge density difference resulting from H adsorption on (a) two layers of Ru(0001) on the W(110) substrate, (b) four layers of Ru(0001) on the W(110) substrate, (c) the Ru(0001) surface, and (d) Pt(111) surface slab models. (i) and (ii) Shows their respective top and side views. The isosurface of 0.0015 e/ b3 is plotted, with yellow (green) regions representing electron charge accumulation (depletion). Unit cells are delineated by dotted lines. The brown, gray, and silver spheres represent Ru, W, and Pt atoms, respectively, whereas smaller spheres represent H atoms.

calculation corroborates with our experimental results on the WO3 + Ru/C + BP2000 catalyst (Figure S1b), where the addition of WO3 did not improve the HER activity of Ru/C. We have also considered if the two-site mechanistic model, which we proposed in our previous work using Mo and Au composite catalysts, could be operating here.41,42 This model required one site to facilitate the proton discharge step and consequently acting as a reservoir of Had and the other site to play the role of recombination center. However, the adsorption energies of hydrogen on Ru and W are strong, −0.61 (Table 2) and −0.7 eV, respectively.3 According to the volcano plot from Nørskov’s work, both metals should thus serve only as H adsorbers.3 Moreover, we had observed Tafel slopes of 63 ± 3 mV/dec in our earlier work,41 whereas the Tafel slope for the W + Ru/C + BP2000 catalyst is smaller, 46 ± 2 mV/dec. On the basis of these evidence, we rule out the applicability of the twosite model for the present work. The practical impact of this work is the discovery of a highperformance and easy-to-synthesize HER catalyst, which has the potential to become a more economical substitute for Pt. Another positive aspect of our catalyst is its immunity against contaminants, which is often neglected by many works on HER. The improved tolerance toward contaminants implies that tainted water could be used without extensive pretreatment and will result in overall energy savings. In addition to practical applications, our bimetallic composite catalyst can open up new E



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ACKNOWLEDGMENTS This work is supported by an academic research fund (R-143000-631-112) from the National University of Singapore. U.J. acknowledges a Ph.D. scholarship from SINGA. We thank Ren Dan (Department of Chemistry, NUS) for assisting with the gas chromatography experiments.

vistas for catalyst research and provide an interesting model system for the advancement of knowledge on the HER activity via d-band structure engineering.

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CONCLUSIONS In summary, we report that tungsten and ruthenium interacted synergistically to enhance the activity of the electrochemical HER. At an overpotential of 85 mV, our composite catalyst, with a small Ru loading of 5.6 μg/cm2, exhibited HER activities close to an order of magnitude higher than those of ruthenium alone. In comparison to Pt, our catalyst was also more immune toward common contaminants, such as Cu2+. We observed that among most of the group 4−6 metals, only tungsten and molybdenum showed synergy with ruthenium, whereas titanium, vanadium, zirconium, niobium, and tantalum have no synergistic influence. Copper upd experiments as well as XPS and UPS showed an increase in the work function of Ru upon W incorporation, whereas the experimentally measured valence band structure of Ru showed a shift of its band edge by 0.15 eV in the presence of tungsten. These analyses demonstrate that an electronic effect was felt by Ru in the presence of the W substrate. From DFT calculations, we showed the possibility of compressive lattice strain modifying the electronic structure of the Ru layers, leading to an increase in their work function and decrease in their d-band. This, in turn, tunes the H-binding energy of the Ru surface layer, such that its value is closer to that of Pt(111). Our timely discovery of a highly efficacious HER catalyst, made by physically mixing small quantities of Ru and W, represents a significant step forward in the development of effective non-Pt-based catalysts ideal for industrial-scale water splitting.

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EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17970. Experimental details (catalyst synthesis, electrode preparation, FE measurements, real surface area measurements using Cu upd, exchange current density calculations); details of characterizations (XRD, SEM, EDX, XPS, and UPS); electrochemical measurements on oxides and various transitional metals; XPS spectra of tungsten; calculations of work functions; DFT calculations (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Teck Leong Tan: 0000-0002-7089-8966 Sing Yang Chiam: 0000-0002-5157-3533 Boon Siang Yeo: 0000-0003-1609-0867 Author Contributions ∥

REFERENCES

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Details of the experiments and DFT calculations are presented, respectively, in Sections S1 and S3 of the Supporting Information.

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Research Article

U.J. and S.M. contributed equally to this work.

Notes

The authors declare no competing financial interest. F


Research Article

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