Pt催化水解为氢气和氧气

DOI: 10.1126/science.1211934 , 1256 (2011);334 Science , et al.Ram Subbaraman -Pt Interfaces2-Ni(OH)+Tailoring Li Enhancing Hydrogen Evolution Activity in Water Splitting by This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles ): December 3, 2011 www.sciencemag.org (this infomation is current as of The following resources related to this article are available online at http://www.sciencemag.org/content/334/6060/1256.full.html version of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/content/suppl/2011/11/30/334.6060.1256.DC1.html can be found at: Supporting Online Material http://www.sciencemag.org/content/334/6060/1256.full.html#related found at: can berelated to this article A list of selected additional articles on the Science Web sites http://www.sciencemag.org/content/334/6060/1256.full.html#ref-list-1 , 3 of which can be accessed free:cites 38 articlesThis article http://www.sciencemag.org/cgi/collection/chemistry Chemistry subject collections:This article appears in the following registered trademark of AAAS. is aScience2011 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience o n D ec em be r 3 , 2 01 1 w w w .s ci en ce m ag .o rg D ow nl oa de d fro m (aL †aR †|vac〉) given an expected mean number of events m0 = d 2Nps consistent with zero concur- rence, where ps = 1.0 × 10 −3 is the probability of generating a Stokes heralding photon and N is the number of experimental runs. Our results (X = 3, N = 1.9 × 1014, and m0 = 9.1 T 0.9) indicate positive concurrence at a 98 T 1%confidence level. Therefore, based on this detection of entangle- ment between Stokes and anti-Stokes modes, we can infer entanglement between the phononmodes of two macroscopic solids at room temperature. Finally, we examine the quality of entangle- ment generated between the diamonds by ne- glecting the vacuum component in Fig. 3, which is only caused by inefficiencies in coupling, de- tection, and readout of the anti-Stokes mode. To do this, we performed quantum state tomog- raphy (25) on the joint polarization state of the Stokes and anti-Stokes modes, postselecting on the detection of both photons. The reconstructed state is shown in Fig. 4, and we have subtracted accidental coincidences calculated from the Stokes and anti-Stokes singles rates. These results provide a more complete estimate of the coherence be- tween the two modes than the interference fringes in Fig. 2. The concurrence of this subspace, 0.85, provides an estimate of the achievable entangle- ment between the two diamond phonon modes as the readout efficiency, coupling, and detector efficiencies approach unity (i.e., p00→ 0). Fur- ther, the fidelity to the nearest Bell state ½jHV 〉þ jVH〉�= ffiffiffi2p is 0.91. However, in the presence of real coupling and detection losses, the existence of entanglement can only be inferred from the density matrix in Fig. 3 (22). In our experiment, short-lived quantum cor- relations were revealed by combining an ul- trafast interferometric pump-probe scheme with photon-counting techniques. The large optical bandwidth enabled the resolution of extremely fast dynamics in the solids, and also operation at high data rates, providing sufficient statistics to establish entanglement even in the presence of losses. This approach lays the foundation for future studies of quantum phenomena in many- body, strongly interacting systems coupled to strong- ly decohering environments and points toward a novel platform for ultrafast quantum information processing at room temperature. References and Notes 1. R. Penrose, in Mathematical Physics, A. Fokas, T. W. B. Kibble, A. Grigoriou, B. Zegarlinski, Eds. (Imperial College Press, London, 2000), pp. 266–282. 2. D. P. 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Fairchild, F. Hossain, S. Prawer, Mater. Today 11, 22 (2008). 21. L. Prechtel et al., Nano Lett. 11, 269 (2011). 22. S. J. van Enk, Phys. Rev. A 75, 052318 (2007). 23. W. Wootters, Phys. Rev. Lett. 80, 2245 (1998). 24. G. J. Feldman, R. D. Cousins, Phys. Rev. D Part. Fields 57, 3873 (1998). 25. D. F. V. James, P. G. Kwiat, W. J. Munro, A. G. White, Phys. Rev. A 64, 052312 (2001). Acknowledgments: We thank V. Vedral, A. Datta, and L. Zhang for valuable insights. This work was supported by the Royal Society, Engineering and Physical Sciences Research Council (grant GR/S82176/01), EU IP Q-ESSENCE (grant 248095), EU ITN FASTQUAST, U.S. European Office of Aerospace Research and Development (grant 093020), Clarendon Fund, St. Edmund Hall, and Natural Sciences and Engineering Research Council of Canada. Supporting Online Material www.sciencemag.org/cgi/content/full/334/6060/1253/DC1 Materials and Methods SOM Text References (26–36) 29 July 2011; accepted 27 October 2011 10.1126/science.1211914 Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces Ram Subbaraman,1,2 Dusan Tripkovic,1 Dusan Strmcnik,1 Kee-Chul Chang,1 Masanobu Uchimura,1,3 Arvydas P. Paulikas,1 Vojislav Stamenkovic,1 Nenad M. Markovic1* Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in alkaline environments is one key to reducing the high overpotentials and associated energy losses in water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni (OH)2 clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)2 clusters promoted the dissociation of water and the production of hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li+-induced destabilization of the HO–H bond, resulting in a factor of 10 total increase in activity. Electrocatalysis of the hydrogen evolutionreaction (HER) is critical to the operationofwater-alkali electrolyzers (1–6), inwhich hydrogen is the main product, and chlor-alkali electrolyzers (5, 6), in which it is a side product. These two technologies are highly energy-intensive and are known to account for ~25 to 30% (87,600 to 92,000GWh/year) of the total electrical energy consumption by industrial processes in the United States (3, 7). The HER is also an electrochemical reaction of fundamental scientific importance; the basic laws of electrode kinetics, as well as many modern concepts in electrocatalysis, were devel- oped and verified by examining the reactionmech- anisms related to the charge transfer–induced conversion of protons (in acid solutions) and wa- ter (in alkaline solutions) to molecular hydrogen. Although previous studies have helped to ra- tionalize which surface properties govern the variations in reactivity among catalysts (8–12), many key questions concerning the HER remain unanswered. For example, it is not clear why the rate of the HER is ~2 to 3 orders of magnitude lower at pH = 13 than at pH = 1, nor why the reaction is sensitive to the catalyst surface structure in alkaline media but largely insensitive in acids (13–17).A practical implication of the slowkinetics in alkaline solution is the lower energy efficiency for both water-alkali and chlor-alkali electrolyzers. For water-alkali electrolyzers, the high overpoten- tials for the oxygen evolution reaction (OER) at the anode also contribute significantly overall energy losses (18). This has led to various ap- proaches to identify catalysts for both the OER and HER. However, such design strategies have rarely been based on molecular-level understanding of the reaction pathways. In addition, the influence of noncovalent (van der Waals–type) interactions on the overall kinetics of the HER has been underexplored, particu- larly in light of recent studies highlighting the impact of noncovalent interactions on the rates of many electrochemical reactions such as the oxygen reduction reaction, together with CO and methanol oxidation reactions (19–22). Currently, various combinations ofmetals (Pt, Pd, Ir, Ru, Ag, Ni), metal alloys (Ni-Co, Ni-Mn, Ni-Mo), metal oxides (RuO2), and Ni sulfides and phosphides are used to catalyze the conver- 1Materials Science Division, Argonne National Laboratory, Lemont, IL 60559, USA. 2Nuclear Engineering Division, Argonne National Laboratory, Lemont, IL 60559, USA. 3Ad- vanced Materials Laboratory, Nissan Research Center, Kana- gawa 237-8523, Japan. *To whom correspondence should be addressed. E-mail: nmmarkovic@anl.gov 2 DECEMBER 2011 VOL 334 SCIENCE www.sciencemag.org1256 REPORTS o n D ec em be r 3 , 2 01 1 w w w .s ci en ce m ag .o rg D ow nl oa de d fro m sion of H2O to H2 (2, 10–12, 23–27). Although Pt and Pt-based systems offer the highest activ- ity and stability of all materials used for the HER, the benefits have not, to date, warranted the high cost associated with these materials. As a result, conventional electrolyzers generally use high– surface area Raney Ni and Ni alloys as the HER catalysts (2, 23, 24, 28). Several engineering ap- proaches have been used to improve these non- noble catalyst materials, such as enhancing the surface area, changing the alloy composition, and using higher catalyst loadings (~25 to 40 times the equivalent for Pt) (2). Although these ap- proaches have offered small performance gains, key problems with the use of such non-noble ma- terials remain, including the decrease in activity arising from both the formation of hydrides and the oxidative dissolution of the catalyst during intermittent operation (2). These materials limi- tations suggest that superior performance might be achieved if lower-cost Pt-based cathode ma- terials could be developed. Indeed, by substan- tially increasing the activity of Pt and by also decreasing the Pt loading through the use of Pt-shell nanomaterials with non-noble cores (29, 30), it would be possible to envision the use of highly active, durable, and low-cost Pt-based HER electrodes for alkaline electrolyzers. Limitations in the catalytic activities of Pt and Pt-group metals arise from the fact that although most of these materials are good catalysts for the adsorption and recombination of the reactive hy- drogen intermediates (Had), they are generally inefficient in the prior step of water dissocia- tion. Conversely, metal oxides (and in some cases other compounds such as sulfides), although ef- fective for cleaving the HO–H bond, are poor at converting the resulting Had intermediates to H2 (31–33). Hence, optimal HER catalyst design will depend on combining the catalytic profi- ciencies of metals and metal oxides by creating new bifunctional metal oxide–metal systems (metal oxides deposited on metal substrates) (34–37). Here, we report the design and performance of composite materials to facilitate different parts of the overall multistep HER process in alkaline environments: an oxide to provide the active sites for dissociation of water, and a metal to facilitate adsorption of the atomic hydrogen produced and its subsequent association to form H2 from these intermediates. This involved growth of conduc- tive ultrathin Ni(OH)2 clusters (height 0.7 nm, width 8 to 10 nm) on both pristine Pt single- crystal surfaces and Pt surfaces modified by two- dimensional (2D) Pt ad-islands [Pt-islands/Pt(111)]. We found that, relative to the corresponding Pt single-crystal surfaces, themost activeNi(OH)2/Pt- islands/Pt(111) electrodes in KOH solutions are more active for the HER by a factor of ~8 at an overpotential of –0.1 V. Further enhancement of water dissociation is achieved by the intro- duction of solvated Li+ ions into the compact portion of the double layer, resulting in a factor of 10 total increase in activity. Finally, we demon- strate that the knowledge attained by studying single-crystal surfaces can be used for the design of prospective commercial nanocatalysts for al- kaline electrolyzers. As a starting point, to develop more complete structure-function relationships for the HER, we used scanning tunneling microscopy (STM) to compare the atomic structures of Pt(111) and Pt(111) modified by electrochemically deposited Pt islands, referred as Pt-islands/Pt(111), (38). In agreement with prior reports (39, 40), the image of Pt(111) in Fig. 1A displays the presence of a few randomly distributed mono-atomic steps and 2DPt islandswith diameters of 1 to 2 nm and monoatomic height. Considering that the shape of the current-potential curve in both the under- potentially deposited hydrogen (Hupd) region [defined as the state of hydrogen adsorbed at a potential that is positive of the Nernst poten- tial for the hydrogen reaction (33)], between 0.05 to 0.35 V, and the region of reversible ad- sorption of hydroxyl (OHad) species, above 0.6 V, is consistent with earlier reports for a perfect Pt(111) surface, we conclude that these defects are invisible in cyclic voltammetry (CV) traces. In the STM image of the islands shown in Fig. 1B, however, the Pt adatoms can be clearly resolved as 2D features with diameters of ~1 to 3 nm and a height of 1 atomic layer. The CV trace of such a surface encompasses two sharp Hupd peaks centered at 0.23 V and 0.4 V (Fig. 1B). On the basis of prior studies (39, 40), we associate these peaks with hydrogen adsorption at the (111)-(111) and (111)-(100) terrace-step sites, respectively. Consistent with the higher oxophilicity of low-coordinated Pt sites, the onset of OH adsorption starts at more nega- tive potentials on the Pt island–covered elec- trode than on pristine Pt(111), whereas the OHad peaks are less reversible on the former surface. After 50 potential cycles between –0.3 V and +0.3 V, the STM images and the CV traces remain the same, indicating that within this potential range, the morphologies of the Pt(111) and Pt-islands/Pt(111) surfaces are stable. There- fore, such well-defined surfaces offer a unique opportunity to correlate the kinetic rates of the HER with a truly atomically resolved surface structure. The mechanism of the HER in alkaline media is typically treated as a combination of three ele- mentary steps: the Volmer step—water dissocia- tion and formation of a reactive intermediate Had (2H2O +M+ 2e –⇆ 2M-Had + 2OH –)—followed by either the Heyrovsky step (H2O +Had-M+ e –⇆ M + H2 + OH –) or the Tafel recombination step (2M-Had⇆ 2M + H2). Adsorbed hydrogen species Had formed at potentials negative of the Nernst reversible potential for the HER are also referred to as overpotentially deposited hydrogen (Hopd). To distinguish the different states of adsorbed hydrogen, we use a thermodynamic notation, Fig. 1. (A to C) STM images (60 nm by 60 nm) and CV traces for (A) Pt(111), (B) Pt(111) with 2D Pt islands, and (C) Pt(111) modified with 3D Ni(OH)2 clusters in 0.1 M KOH electrolyte. (D) HER activities for Pt(111), Pt-islands/Pt(111), and Ni(OH)2/Pt(111) surfaces in alkaline solution (a, b, and c, respectively). Corresponding HER activities for Pt(111) and Pt-islands/Pt(111) electrodes in acid solutions (a´ and b´) are shown to emphasize large initial differences between kinetics of the HER in alkaline versus acid solutions. The inset shows XANES spectra for Ni(OH)2 on Pt(111) shown for three different potentials: HER (–0.1 V), Hupd (0.1 V), and near OER (1.2 V). Also shown is the reference spectrum for Ni(OH)2. No shift in the edge energy in XANES spectra between HER and Hupd regions is observed. www.sciencemag.org SCIENCE VOL 334 2 DECEMBER 2011 1257 REPORTS o n D ec em be r 3 , 2 01 1 w w w .s ci en ce m ag .o rg D ow nl oa de d fro m referring to Hupd as the strongly adsorbed state and Had (i.e., Hopd) as a weakly adsorbed state. Any rigorous kinetic analysis of the HER lies beyond the scope of the present discussion. Rath- er, we focus mainly on the design of interfaces for efficient electrochemical conversion of H2O to H2. For example, Fig. 1D shows that in alka- line solution, relative to the corresponding pris- tine Pt(111) surface, the Pt-islands/Pt(111) surface is ~5 to 6 times as active for the HER. Figure 1D also shows that in acid solution, the HER on the Pt-islands/Pt(111) electrode is im- proved by a factor of only ~1.5. In turn, this strong pH effect indicates that the low-coordinated Pt atoms may have a major effect on the rate- determining step of the HER in alkaline sol- utions. Because the major difference between the reaction pathways in alkaline and acid solutions is that the hydrogen in alkaline solutions is dis- charged from water instead of from hydronium ions (H3O +) (13–15), we propose that the large promoting effect of low-coordinated Pt atoms in alkaline solution is due to more facile dissocia- tive adsorption of water. In turn, this would be consistent with the Volmer reaction being the rate-determining step for the HER in alkaline electrolytes. The role of edge-step sites in accel- erating dissociative adsorption of water on metal surfaces is well documented in ultrahigh-vacuum (UHV) environments (33).We therefore conclude that for materials with near-optimal M-Had en- ergetics (such as Pt), surface reactivity for the HER can be further improved by tailoring the ac- tive sites for more efficient dissociative adsorp- tion of water molecules. To fulfill this requirement, wemodified Pt(111) and Pt-island/Pt(111) surfaces by depositing Ni-(hydr)oxide clusters (38), as the 3d-transition metal oxides might be even more active for wa- ter dissociation than Pt defect sites (31). The fac- ile water dissociation properties of Ni(hydroxy) oxides relative to other transition metal oxides have been well established (26–28), motivating the use of Ni(OH)2 for this work. The local sym- metry, the oxidation state of Ni atoms, and the number and identities of nearest-neighbor atoms and the distances between them were determined by in situ x-ray absorption spectroscopy (XAS) measurements (41, 42). For example, from the analysis of the x-ray absorption near-edge struc- ture (XANES) and extended x-ray absorption fine structure (EXAFS) of the XAS spectra (inset of Fig. 1D; see also fig. S3), we found Ni-O and Ni-Ni bond distances of 2.05 T 0.01 Å and 3.08 T 0.01 Å, and we also determined that Ni remains