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Non-Noble Intertransition Binary Metal Alloy Electrocatalyst for Hydrogen Oxidation and Hydrogen Evolution Alexandros Anastasopoulos, John Blake, and Brian E. Hayden* School of Chemistry, The University of Southampton, Southampton SO17 1BJ, United Kingdom ABSTRACT: Metastable and amorphous intertransition metal alloys of CuW are shown to catalyze both the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR). The constituent metals exhibit poor activity. The results are consistent with ab initio calculations predicting HER activity for Cu overlayers on W and with the observed changes of the density of states (DOS) at the Fermi level associated with CuW alloy formation. Two maxima in the HER activity are observed as a function of composition corresponding a bulk metastable phase at 80 at % Cu and a second at 50 at % Cu. The alloy at 50 at % also corresponds to a maximum in the HOR activity, whereas the phase at 80 at % Cu is not HOR active. The latter phase is shown to be oxygen-covered at the HOR potential, explaining its inactivity. These results highlight the possibilities of developing non-noble metal alloy catalysts for hydrogen fuel cells.
’ INTRODUCTION Platinum-based catalysts are ubiquitous in low-temperature fuel cell technology as they exhibit the best activity at both the anode (the hydrogen oxidation reaction, HOR) and cathode (the oxygen reduction reaction, ORR).1 Nevertheless, platinum is scarce and expensive,2 and any hydrogen-based economy relies on effective and cheap catalysts for the oxidation and reduction of hydrogen.3 The result is a significant effort to reduce the platinum loading in fuel cells by using highly dispersed Pt particles and alloying Pt with other metals4 6 and an intense search for nonplatinum7,8 or nonprecious metal fuel cell catalyst.2 Metal alloys have been exploited to modify the surface reactivity of the constituent elements, promoting increased activity and selectivity in heterogeneous catalysis through both electronic and geometric effects in the alloy surface.9 Ab initio calculations of the density of electronic states at the Fermi level of embedded atom and pseudomorphic overlayers of noble and non-noble metals have provided a powerful tool in modeling adsorption behavior and surface reactivity.10 Recent extensions to allow the modeling of reactivity at the surface/electrochemical double-layer interface have been successful in predicting the hydrogen evolution reaction (HER) activity of metals11 13 and embedded atom and pseudomorphic overlayers14 and the HOR on metals.13 The free energy of hydrogen adsorption (ΔGH*) is used as a simple descriptor for HER activity within the context of the Sabatier principle, providing a rationalization of the “Volcano” behavior characterizing the relationship of the hydrogen adsorption energy and the HER current densities of both the pure metal and pseudomorphic metal on metal overlayers.14 16 Within this framework, both W and Cu exhibit very low HER activity because ΔGH* is far from zero (negative and positive, respectively). It has recently been shown that W-supported Cu overlayer should exhibit HER activity, and some experimental r 2011 American Chemical Society
evidence is presented to support this assertion.17 HER also has been shown to be a good descriptor of HOR activity in binary alloy catalysts.18,19 There is also direct evidence from photoemission of amorphous WCu alloy phases20 that dilution of W with Cu results in a lowering of the density of states near the Fermi level: This should lead to a concomitant increase in ΔGH* on the alloy over W (a lower adsorption energy) and an increase in HER activity. The high-throughput modified molecular beam epitaxy methodology21 provides access to wide ranges of bulk compositions with similar surface compositions in unannealed thin films18,19 and when combined with microfabricated screening arrays, fast electrochemical screening methods allows the measurement of the electrocatalytic activity of both continuous thin films22 and supported particles23,24 under identical conditions. Here we provide direct evidence that the CuW alloy system is indeed more active than the constituent metals for HER over the entire alloy composition range and exhibits maxima in activity in two regions of composition: One of these alloy surfaces is also active in HOR. We also show from the surface redox behavior of the alloys why only one is active for HOR.
’ EXPERIMENTAL SECTION CuW binary alloy thin films have been prepared via a highthroughput physical vapor deposition (HT-PVD) technique described in detail elsewhere.21 A compositional gradient of the alloy was synthesized by simultaneously depositing the two elements from molecular beam epitaxy sources with the substrate at 298 K. This allows the complete mixing of elements and prevents the formation of thermodynamically stable phases Received: June 6, 2011 Revised: July 26, 2011 Published: August 19, 2011 19226
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The Journal of Physical Chemistry C caused by surface segregation and eliminating any need for postdeposition annealing. Cu was deposited from a Knudsen cell evaporation source (k-cell) (Cu slugs 99.99%, Alfa Aesar), whereas an electron beam evaporation source (e-gun) was employed for W deposition (W slugs 99.97%, Alfa Aesar). All depositions took place in a cryopumped UHV chamber (base pressure 1 10 10 Torr). The two evaporation sources were arranged at a 180° geometry to allow the widest possible compositional range to be achieved. Ex situ composition analysis of the CuW thin films was performed by electron dispersive X-ray spectroscopy (EDS) using a JEOL JSM5910 and Oxford Instruments INCA 300. We used 32 mm2 generally on silicon substrates (Nova Electronic Materials) or directly on substrates for other measurements. In situ spot X-ray photoelectron spectroscopy (XPS) was carried out by employing a PSP vacuum technology resolve multichannel analyzer and VG twin Al Mg X-ray source. Measurements were made with Al Kα anode X-ray (1486.6 eV) energy at an incident angle of 45°, and the average analysis depth (with Cu (2p 3/2) kinetic energy of ca. 555 eV) was ca. 3 to 4 ML. Structural analysis was undertaken using X-ray diffraction (XRD) using a Bruker D8 powder diffractometer with a C2 area detector. In this case, 32 mm2 silicon/silicon nitride substrates (Nova Electronic Materials) were used as substrate. Ten 10 silicon microfabricated arrays (Quedos) of independently addressable working electrodes were used as the substrate for the highthroughput electrochemical screening. Details regarding the design and fabrication of these arrays and the multichannel current follower/potentiostat system have been reported elsewhere.22,25 All prepared samples were vacuum-sealed immediately after preparation and kept this way until being loaded into the electrochemical cell to minimize any surface perturbation resulting from exposure to air. Electrochemical measurements were performed in a threecompartment glass cell, specially designed to accommodate the high-throughput array.22 A Pt mesh (Alfa Aesar, 99.99%) was used as counterelectrode inside a compartment separated from the working electrode by a glass sinter. The reference electrode was a mercury/mercuric sulfate electrode (MSE, 0.5 M H2SO4, Sentec) mounted inside a Luggin capillary. Although, all potentials reported here are converted to the reversible hydrogen electrode (RHE) by calibrating the MMS electrode against an RHE in the working electrolyte (0.5 M HClO4) at 298 K. All electrochemical measurements were performed at 298 K in a 0.5 M HClO4 working electrolyte. The latter was prepared from concentrated HClO4 (double-distilled 70%, GFS chemicals) and ultra-pure water (18.2 MΩ cm, ELGA). Prior to all experiments, the electrolyte solution was purged with Ar (N5 grade, Air Products) for 20 min to remove dissolved oxygen. During this process, all working electrodes were under potential control at 0 VRHE. Cyclic voltammograms were recorded in a potential region defined by a lower limit of 0.2 VRHE, an anodic limit of 0.4 VRHE, and an initial (idle) potential of 0 VRHE. A maximum of five cyclic voltammograms were recorded (scan rate 50 mV s 1), and those presented here were collected from the second cycle. The electrocatalytic properties of the CuW alloys toward HER and HOR were assessed by performing potential step experiments in the hydrogen evolution and the hydrogen oxidation regions of potential. The potential was altered in 0.05 V steps starting from 0 V to 0.25 VRHE and back for the HER and from 0 to 0.25 VRHE and back for the HOR. For HOR measurements, before
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Figure 1. XRD profiles collected from CuW alloy thin films of various compositions (indicated). 2Θ values of the main Bragg peaks are shown for Tungsten ((200), (210), and (211) in black) and the Cu40W60 metastable alloy phase ((110) gray).25,26
applying the positive potentials, the electrolyte was purged with H2 (5 min, BOC 99.995%), whereas during the experiment, H2 was allowed to bubble through the electrolyte solution. Note that the sequence in which the experiments were carried out was chosen to minimize any possible modification of the surface composition by oxidative modification at the higher potentials.
’ RESULTS AND DISCUSSION An HT-PVD methodology producing compositional gradients from MBE sources21 has been used to produce thin film alloys over a wide compositional range over a (10 10) element microfabricated electrochemical screening array.22 A number of arrays were prepared, typically alloy catalysts over wide compositional ranges such as 0 < Cu at % < 80 with alloy film thicknesses (AFMs) in the range of 60 80 nm. Figure 1 shows a selection of diffractograms for representative thin film compositions, with the reflections to which the Bragg peaks correspond indicated. The compositions have been measured directly by EDS on the samples. At 18 at % Cu, three peaks are observable at 2Θ = 35.5, 40, and 43.8°, which can be attributed to W (Pm3n) (200), (210), and (211) indices, respectively.26 Upon increasing the Cu content, the diffractograms are characterized by a shift of the main W(210) peak and a concomitant decrease in intensity of the (200) and (211). At 43 at % Cu, only a single peak at 2Θ = 40.5° can be distinguished, whereas a small shoulder is also present at 43.8°. The main peak at 2Θ = 40.5° is attributed to the (110) Bragg peak of the metastable phase Cu40W60.27 Diffractograms of more Cu-rich films indicate the presence of the amorphous alloys,20 and there was no direct evidence of the formation of a second metastable phase at 84 at % Cu.27 29 Further increase in Cu content to 90 at % resulted in the appearance of a single peak at 43.2 and a small shoulder at 45°, which can be attributed to Cu (Fm3m) (111) and (200) indices, respectively, thus indicating that for these alloys the crystal ordering has been restored. XRD carried out on the various substrates shows that there appears to be no influence of the substrates used in these studies on the phases observed. Figure 2 shows the integrated intensity of the Cu(2p3/2) XPS peak as a function of the bulk composition of a number of the range of 100 CuW alloy samples. The results show that within experimental error, the surface composition of the surface (measured by XPS) is the same as the bulk. The results are consistent with previous observations for metal alloys 19227
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Figure 2. Comparison of the surface composition measured by XPS of seven compositions in a CuW alloy array to the bulk composition. The Cu(2p3/2) peak area is plotted as a function of the Cu bulk composition measured by EDS.
synthesized using the HT-PVD method, where the substrate is at room temperature18,19 and where surface segregation would be expected in the material at equilibrium: The surface and bulk compositions are, within experimental error, the same.
’ SURFACE REDOX BEHAVIOR The surface redox behavior of the CuW alloy films was assessed by performing cyclic voltammograms over a relatively wide potential window (from 0.2 to 0.4 VRHE). Figure 3a compares the second cycle of the voltammogram for Cu, W, and a CuW alloy. Typically five or more cycles were carried out, and there was little change in the observed voltammetry. For Cu and W surfaces, the cycling to anodic potentials result in some oxidation and dissolution or passivation, respectively, as one may expect at high anodic potentials.30 In the case of the alloy, a reversible surface redox couple is evident (surface oxidation above 0.2 VRHE, surface reduction at 0.18 VRHE), and the alloys are much more stable, with little change from first to subsequent cycles. At potentials below 0.1 VRHE, the small cathodic current, most evident on the alloy surface, is associated with hydrogen evolution. Note that subsequent cycling of the W results in further passivation, and a further diminution of the small cathodic hydrogen evolution current is observed. The surface redox behavior and the extent of hydrogen evolution on the alloys were dependent on the alloy composition. This is shown in Figure 3b, which compares the voltammetry on a selected number of the 100 alloy compositions. The most extensive surface oxidation takes place for the Cu85W15 alloy, as evident in both the anodic and cathodic charges of the forward and reverse sweeps, respectively. Figure 3c shows a plot of the cathodic charge associated with the surface reduction peak centered at 0.18 VRHE as a function of alloy composition. The surface oxidation increases with Cu content slowly to produce a broad plateau at ca. Cu50W50, and at higher Cu concentrations, the charge increases sharply to produce a distinct maximum centered at ca. Cu80W20. We associate these alloy redox behaviors with the two metastable bulk alloy phases of the system expected at Cu40W60 and Cu85W15,27 29 the former which we also observe in the XRD (above). The trend in the hydrogen evolution activity as a function of alloy composition is evident in the expanded plot of the cathodic currents of the negative going sweep shown in the inset of Figure 3b. The largest HER activity is evident for alloys with compositions of Cu above 50 at %. There is some underlying contribution to the currents from a surface reduction process for
Figure 3. (a) Cyclic voltammograms for Cu (black), W (red), and Cu50W50 (blue). The reversible redox couple of the alloy is evident for Cu50W50. (b) Cyclic voltammograms for five CuW alloy films demonstrating the different redox behavior observed among compositions. Inset: expanded cathodic sweeps of these alloys in the hydrogen evolution region. (c) Cathodic charge (0.18 VRHE surface reduction peak) as a function of alloy composition. The dashed line is added to guide the eye.
the two highest Cu compositions, and steady-state HER (and HOR) measurements have been made to assess the underlying catalytic activity of the alloys.
’ HYDROGEN EVOLUTION REACTION The HER is not only an important catalytic reaction in its own right but also appears to be a useful descriptor for the HOR reaction on binary alloys.18 It is also the HER for which the ab initio calculations have been made on the metal overlayer and metal “defect” surfaces.17 The activity toward HER was determined by performing potential step experiments. The electrode potential was decreased stepwise from 0 V to 0.25 VRHE in 0.05 V steps with averaging of the respective steady-state current at each step over 20 s. Figure 4 shows the steady-state HER current densities as a function of Cu at % collected at 0.25 VRHE. The HER activity of the alloys is always higher than that of the constituent metals, and two distinct maxima in the activities are observed at 50 and 84 at % Cu. These two compositions correspond to the two reported metastable alloy phases for CuW, 19228
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Figure 4. Steady-state HER current density measured at 0.25 VRHE as a function of CuW alloy composition. Data points are calculated by averaging the HER current collected at the last 20 s of the step. Values for W and Cu are also shown in gray. The dotted line is added to guide the eye.
Figure 6. Steady-state HOR current density as a function of CuW alloy composition. Data points are calculated by subtracting surface oxidation currents from mixed HOR and surface oxidation currents (Figure 5). Values for W and Cu are also shown in gray. The dotted line is added to guide the eye.
Figure 5. Mixed steady-state HOR and surface oxidation current density measured at +0.25 VRHE as a function of CuW alloy composition. Data points are calculated by averaging the HOR current collected at the last 20 s of the step. Values for W and Cu are also shown in gray. The dotted line is added to guide the eye.
one of which at 50 at % Cu was directly observed here (Figure 1). They also correspond to the two alloy compositions associated with distinctive surface redox behaviors on these catalysts (Figure 3). Although the overpotential is quite large ( 0.25 VRHE), these results demonstrate unambiguously that the CuW alloy exhibits HER catalytic activity significantly above the constituent metals and that the most active phases appear to be associated with the two known bulk metastable phases for the system. This appears to be in agreement with the ab initio calculations17 for CuW, although at present, these describe the CuW overlayer rather than the alloy.
plotted in Figure 6 and is associated only with the HOR reaction. The result is very similar, with a maximum in the activity observed now at ca. 50 at %: There appear to have been very small surface oxidation currents associated with the alloys rich in W in Figure 5: This is likely to be the continued underlying passivation of W at 0.25 VRHE to form WO3.30 The active alloy catalyst for HOR appears to be associated with the CuW metastable phase identified here and elsewhere27 at this composition. Note that this also corresponds to one of the alloys exhibiting HER activity (Figure 3). Residual oxidation currents for the HOR active 50 at % alloy are not significant, indicating that this catalyst is stable at 0.25 VRHE. Conspicuous by its absence in Figure 6 is any HOR activity of the alloy 84 at % Cu. This alloy exhibited the highest HER activity (Figure 3). One may have expected that this alloy (84 at % Cu) would also be active for HOR on the basis that HER is a good descriptor for HOR,18 and indeed the HER and HOR activity of the 50 at % alloy is consistent with this observation. Reference to the surface redox behavior in Figure 3, however, helps us understand why this CuW alloy may be active for HER and not HOR. At 0.25 VRHE, the oxidation of the 84 at % Cu alloy is taking place, and the charge associated with the oxidation is large; that is, the surface oxidation is quite extensive. Therefore, the surface on which the HER is taking place (a reduced CuW alloy) at 0.25 VRHE is simply not the same as that for HOR (an oxidized CuW alloy) at 0.25 VRHE. The reduced CuW alloy at ca. 84 at % Cu may be active for HOR but is inaccessible at the overpotential at which the HOR activity was measured.
’ HYDROGEN OXIDATION REACTION The steady-state HOR activity of the CuW alloys was also evaluated using potential step experiments. The potential of the electrodes was stepped from 0 to 0.25 VRHE in 0.05 V steps in a hydrogen saturated (continuous bubbling) electrolyte measuring the steady-state current at each potential for 90 s. Results are shown in Figure 5 for a potential of 0.25 VRHE for which the HOR activity was understandably the highest. All of the alloys exhibit higher HOR activities than the constituent metals, with a single broad maximum in the activity observed at ca. 40 at % Cu. It was important to establish that the observed currents were associated with the HOR reaction and not associated with any residual steady-state surface oxidation currents (Figure 3) at this potential. The steady-state measurements were therefore repeated at each potential step in the absence of hydrogen, and the residual currents were subtracted from the currents measured in the presence of hydrogen (Figure 5). The difference in these values is
’ CONCLUSIONS A high-throughput synthetic methodology has been used to produce compositional gradients of CuW alloys. The HER and HOR activity of all alloys is higher than the pure component elements, with the two most active alloys catalyzing the HER at 0.25 VRHE corresponding to Cu50W50 and Cu84W16. Both of these alloys correspond to the two metastable alloy phases expected for this binary system, one of which is evident in XRD. Only one of these alloys exhibits HOR activity at 0.25 VRHE, that at Cu50W50. At first, this may appear to show that HER is not always a good descriptor of HOR activity in an alloy system, as suggested for surfaces exhibiting low overpotentials for the reactions.18 The redox behavior of the alloys, however, provides an explanation for the difference in the case of CuW: The Cu84W16 surface exhibits a more extensive surface oxidation at 0.25 VRHE, and the surfaces of the alloys for this composition at 19229
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The Journal of Physical Chemistry C 0.25 VRHE (HER) and 0.25 VRHE (HOR) are not the same. Whereas the CuW alloy catalyst activity for HOR is not likely to be sufficient for it to replace Pt at the anode of PEM fuel cells, these results provide direct evidence that alloys may well provide a route to non-noble metal catalysts for fuel cells. These results also confirm the possibility of using HER as a descriptor for HOR activity even for higher overpotentials for the reaction, the importance being that ab initio calculations of HER are accessible already for overlayer and imbedded “defect” systems.17 The results are consistent with the expectation that the decrease in the density of states at near the Fermi level20 in the alloy over pure W will decrease the adsorption energy of hydrogen and increase the HER activity by bringing ΔGH* closer to zero. The compositional (and concomitant structural) dependent variation in the activities is more difficult to predict theoretically directly.
’ AUTHOR INFORMATION Corresponding Author
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(20) Engelhardt, M. A.; Jaswal, S. S.; Sellmyer, D. J. Phys. Rev. B 1991, 44, 12671. (21) Guerin, S.; Hayden, B. E. J. Comb. Chem. 2006, 8, 66. (22) Guerin, S.; Hayden, B. E.; Lee, C. E.; Mormiche, C.; Russell, A. E. J. Phys. Chem. B 2006, 110, 14355. (23) Guerin, S.; Hayden, B. E.; Pletcher, D.; Rendall, M. E.; Suchsland, J. P.; Williams, L. J. J. Comb. Chem. 2006, 8, 791. (24) Hayden, B. E.; Pletcher, D.; Suchsland, J. P. Angew. Chem., Int. Edit. 2007, 46, 3530. (25) Guerin, S.; Hayden, B. E.; Lee, C. E.; Mormiche, C.; Owen, J. R.; Russell, A. E. J. Comb. Chem. 2004, 6, 149. (26) International Centre for Diffraction Data W Card. (27) Mashimo, T.; Huang, X. S.; Tashiro, S. J. Mater. Sci. Lett. 1997, 16, 1051. (28) Nastasi, M.; Saris, F. W.; Hung, L. S.; Mayer, J. W. J. Appl. Phys. 1985, 58, 3052. (29) Chen, Y. G.; Liu, B. X. J. Alloys Compd. 1997, 261, 217. (30) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; National Association of Corrosion Engineers: Houston, TX, 1974.
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was funded by the European Union under the FCANODE NMP3-CT-2007-032175 project and a CASE studentship from EPSRC. We acknowledge James Barnett of Ilika plc for performing the XRD measurements. ’ REFERENCES (1) Vielstich, W.; Gasteiger, H. A. Handbook of Fuel Cells: Fundamentals, Technology, and Applications; Wiley: West Sussex, U.K., 2003. (2) Bashyam, R.; Zelenay, P. Nature 2006, 442, 63. (3) Hydrogen as a Future Energy Carrier; Zuttel, A., Borgschulte, A., Schlapback, L., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (4) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241. (5) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (6) Lin, S. D.; Hsiao, T. C.; Chang, J. R.; Lin, A. S. J. Phys. Chem. B 1999, 103, 97. (7) Wang, B. J. Power Sources 2005, 152, 1. (8) Serov, A.; Kwak, C. Appl. Catal., B 2009, 90, 313. (9) Sinfelt, J. H. Bimetallic Catalysts: Discoveries, Concepts, and Applications; Wiley: New York, 1983. (10) Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nat. Chem 2009, 1, 37. (11) Santos, E.; Lundin, A.; Potting, K.; Quaino, P.; Schmickler, W. Phys. Rev. B 2009, 79, 235436. (12) Skulason, E.; Karlberg, G. S.; Rossmeisl, J.; Bligaard, T.; Greeley, J.; Jonsson, H.; Norskov, J. K. Phys. Chem. Chem. Phys. 2007, 9, 3241. (13) Skulason, E.; Tripkovie, V.; Bjorketun, M. E.; Gudmundsdottir, S.; Karlberg, G.; Rossmeisl, J.; Bligaard, T.; Jonsson, H.; Norskov, J. K. J. Phys. Chem. C 2010, 114, 18182. (14) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I. B.; Norskov, J. K. Nat. Mater. 2006, 5, 909. (15) Parsons, R. Trans. Faraday Soc. 1958, 54, 1053. (16) Kibler, L. A. ChemPhysChem 2006, 7, 985. (17) Bjorketun, M. E.; Bondarenko, A. S.; Abrams, B. L.; Chorkendorff, I.; Rossmeisl, J. Phys. Chem. Chem. Phys. 2010, 12, 10536. (18) Al-Odail, F. A.; Anastasopoulos, A.; Hayden, B. E. Phys. Chem. Chem. Phys. 2010, 12, 11398. (19) Al-Odail, F. A.; Anastasopoulos, A.; Hayden, B. E. Top. Catal. 2011, 54, 77. 19230
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