Highly porous non-precious bimetallic electrocatalysts for efficient hydrogen evolution

Abstract

A robust and efficient non-precious metal catalyst for hydrogen evolution reaction is one of the key components for carbon dioxide-free hydrogen production. Here we report that a hierarchical nanoporous copper-titanium bimetallic electrocatalyst is able to produce hydrogen from water under a mild overpotential at more than twice the rate of state-of-the-art carbon-supported platinum catalyst. Although both copper and titanium are known to be poor hydrogen evolution catalysts, the combination of these two elements creates unique copper-copper-titanium hollow sites, which have a hydrogen-binding energy very similar to that of platinum, resulting in an exceptional hydrogen evolution activity. In addition, the hierarchical porosity of the nanoporous copper-titanium catalyst also contributes to its high hydrogen evolution activity, because it provides a large-surface area for electrocatalytic hydrogen evolution, and improves the mass transport properties. Moreover, the catalyst is self-supported, eliminating the overpotential associated with the catalyst/support interface.

ARTICLE

Received 4 Nov 2014 | Accepted 9 Feb 2015 | Published 16 Mar 2015

Qi Lu1,2, Gregory S. Hutchings1, Weiting Yu2, Yang Zhou3, Robert V. Forest1, Runzhe Tao4, Jonathan Rosen1, Bryan T. Yonemoto1, Zeyuan Cao5, Haimei Zheng4, John Q. Xiao3, Feng Jiao1 & Jingguang G. Chen2

A robust and efcient non-precious metal catalyst for hydrogen evolution reaction is one of the key components for carbon dioxide-free hydrogen production. Here we report that a hierarchical nanoporous copper-titanium bimetallic electrocatalyst is able to produce hydrogen from water under a mild overpotential at more than twice the rate of state-ofthe-art carbon-supported platinum catalyst. Although both copper and titanium are known to be poor hydrogen evolution catalysts, the combination of these two elements creates unique copper-copper-titanium hollow sites, which have a hydrogen-binding energy very similar to that of platinum, resulting in an exceptional hydrogen evolution activity. In addition, the hierarchical porosity of the nanoporous copper-titanium catalyst also contributes to its high hydrogen evolution activity, because it provides a large-surface area for electrocatalytic hydrogen evolution, and improves the mass transport properties. Moreover, the catalyst is self-supported, eliminating the overpotential associated with the catalyst/support interface.

DOI: 10.1038/ncomms7567 OPEN

Highly porous non-precious bimetallic electrocatalysts for efcient hydrogen evolution

1 Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA.

2 Department of Chemical Engineering, Columbia University, New York, New York 10027, USA. 3 Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA. 4 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

5 Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, USA. Correspondence and requests for materials should be addressed to F.J. (email: mailto:[email protected]

Web End [email protected] ) or to J.G.C. (email: mailto:[email protected]

Web End [email protected] ).

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The rising concerns about CO2 emissions have led to a growing realization that it is not possible to sustain the worlds current development based on fossil fuels without a

substitution of clean and renewable energy13. Hydrogen, other than being an important chemical feedstock in global industry, is now rmly considered as one of the most likely future fuels4,5. However, current hydrogen production primarily relies on the steam methane reforming process which is neither sustainable nor favoured because the process requires high-energy (heat) input and produces CO2 as a by-product57. It is widely believed that room temperature electrochemical reduction of water to molecular hydrogen offers a signicant promise for supplying CO2-free hydrogen, which can be used directly as a fuel or as reactant to convert CO2 and to upgrade petroleum and biomass feedstocks to value-added chemicals and fuels through hydrotreating processes810. All these applications require large-scale, commercial processes for water electrolysis, which in turn require breakthrough discoveries in at least two areas: (i) the availability of electricity derived from renewable energy sources, such as solar and wind, and (ii) the discovery of low-cost electrocatalysts to replace precious metals that are currently the state-of-the-art hydrogen evolution reaction (HER) catalysts.

HER in an acidic environment generally requires lower overpotentials than those in a basic environment1113. However, a hydrogen production system in a basic environment is still more promising, because of the possibility to consider non-precious-metal-based catalysts that cannot be used in acidic conditions, not only for HER at cathode, but also for oxygen evolution reaction at anode14,15. Regardless of acidic or basic conditions, Pt, along with its alloys, is the benchmark electrocatalyst that requires very small overpotentials to drive the reaction, whereas the scarcity and high cost of Pt hinder its large-scale use for H2 production. As a result, enormous research efforts have been devoted to nding and engineering low-cost alternative catalysts. For example, tungsten and molybdenum carbides and suldes1625, nickel

phosphides26 and electrodeposited Ni-Cu alloy27 have been identied as potential electrocatalysts for HER, unfortunately most of these catalysts exhibit poor intrinsic activity and/or stability in strong bases.

Over the past decade, density functional theory (DFT) predictions, in conjunction with experimental efforts, have played a pivotal role in providing design principles of electrocatalysts2833. For hydrogen evolution, the activities (in terms of exchange current density) of different catalytic surfaces can be correlated with their hydrogen-binding energy (HBE) via a volcano-type relationship, revealing that an optimal HBE would lead to the highest activity34,35. Monometallic catalysts have been studied extensively using the DFT method. However, monometallic non-precious metals show HBE values signicantly different from that of Pt. In the present paper, DFT calculations show that Cu-Ti bimetallic materials have similar HBE values as Pt, and therefore are promising non-precious metal HER electrocatalysts. These predictions are experimentally veried on both bulk Cu-Ti alloys and highly porous catalysts.

ResultsDFT prediction of Cu-Ti bimetallic catalyst. As monometallic catalysts, Cu and Ti are known to be poor HER catalysts because their HBE values are either too small or too large, respectively36. Using DFT calculations, we have demonstrated that the Cu-Cu-Ti hollow site on a Cu-Ti bimetallic surface exhibits an optimal HBE for HER. As shown in Fig. 1a, on a Ti-modied Cu surface, three distinct adsorption sites can be identied. Their corresponding HBE values were calculated using DFT and were incorporated into a volcano plot constructed from previously studied monometallic surfaces (Fig. 1b)34. It can be seen that the two types of Cu-Cu-Ti hollow sites exhibit HBE values very close to that of Pt. In contrast, the Cu-Ti-Ti hollow site containing two Ti atoms binds hydrogen too strongly. Therefore, replacement of

3

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Figure 1 | Modelling studies. (a) The possible bimetallic sites on a Ti-modied Cu surface. (b) The corresponding HBEs incorporated in a volcano plot. The error bar stands for the variation of exchange current density in different experimental measurement. A comparison of (c) HER activities and(d) exchange current densities of various bulk Cu-Ti alloy surfaces and the corresponding monometallic standards.

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one surface Cu atom with Ti on every 3 3 Cu unit cell would

result in an optimal surface, in principle achieving a maximum density of the Cu-Cu-Ti sites without introducing the Cu-Ti-Ti-inactive sites. A lower or higher Ti content will decrease the HER activity because of insufcient number of active sites or creation of inactive sites. It should be noted that it was proposed recently that other than HBE, the binding of surface hydroxyl groups could be another descriptor of the catalytic activity for HER in base13,37. Although this nding may open a new horizon for designing HER catalyst, it also raised some different opinions regarding the effect of pH values38,39. The ndings presented in this paper, however, suggest that the HBE appears to be an appropriate descriptor to predict HER activity for Cu-Ti in basic environment.

Experimental verication of Cu-Ti bimetallic catalyst. To verify the DFT predictions, a series of Cu100 xTix (x 1, 3, 5, 7 and 9)

alloys with homogeneously distributed atoms were fabricated using an arc-melting technique followed by a melt-spinning process in order to retain their solid solution phase formed at high temperatures. After polishing, the resulting materials have smooth surfaces with roughness factor smaller than 1.1 (Supplementary Table 1). Powder X-ray diffraction (PXRD) analysis suggests all alloys retain the fcc structure of crystalline Cu with a lattice expansion because of Ti doping (Supplementary Fig. 1). It is well known that the elemental compositions of a bimetallic system can be different on the surface and in the bulk because the surface composition is determined as a result of minimization of alloy surface free energy with respect to atom exchange between surface and bulk40. X-ray photoelectron spectroscopy (XPS) characterization was therefore conducted and the results (Supplementary Fig. 2) conrmed that the surface Ti content is about twice as large as the bulk stoichiometry (Supplementary Table 1). The HER activities of all Cu-Ti alloys as well as pure Cu and Ti standards were compared by plotting their polarization curves in 0.1 M KOH electrolyte (Supplementary Fig. 3). As shown in Fig. 1c, a signicant increase in HER activity can be achieved after modifying the Cu surface with as little as 1 at. % of Ti, and a maximum enhancement was observed for a bulk stoichiometry of Cu95Ti5. The surface Ti composition of

Cu95Ti5 is found to be 10.9 at. %, which is in good agreement with the optimal value predicted by DFT calculations of 1 Ti atom in a 3 3 cell (11.1 at. %). A further increase in Ti concentration leads

to a decrease of HER activity, which is likely due to the rapid formation of Cu-Ti-Ti sites resulting from the large cohesive energy of Ti.

Design of nanostructured Cu-Ti bimetallic catalyst. To extend the DFT predictions and bulk alloy results to practical high-performance catalysts, it is important to design a nano-architecture for the catalytic material. Nanoporous materials4143, especially recently reported nanoporous bi- and tri-metallic materials44,45 have attracted signicant research interests for their enhanced electrocatalytic activities primarily due to the enhanced surface to bulk ratio. For example, a recent work reported by Kibsgaard et al. demonstrated that nanoporous MoS2 catalyst exhibited high HER activities because of its higher density of active surface sites compared with the aligned MoS2 nanowire counterpart46,47. However, its material utilization became worse at high reaction rates (that is, high currents), because the produced hydrogen bubbles built up inside the porous network and blocked the active sites. Here, we designed and synthesized a Cu-Ti bimetallic electrocatalyst with a highly hierarchical porous structure (denoted as np-CuTi) by making a multi-phase Al-Cu-Ti precursor (atomic ratio Al:Cu:Ti 80:19:1), followed by a

dealloying process. The atomic ratio of Ti to (Cu Ti) was

chosen to be the optimal value (5 at. %) from bulk Cu-Ti studies. The nano-sized pores of the resulting np-CuTi are responsible for high surface areas, whereas the micrometre-sized pores served as gas diffusion channels to enhance mass transport properties. This catalyst is monolithic and self-supported, which enhances the electric transportation and eliminates the necessity of using a supporting conductive substrate.

Although the formation and catalytic application of nanoporous metals have been studied previously4851, a highly hierarchical nanoporous bimetallic material with well-dened bimodal pore size distribution has not been explored to date. In the present study, the origin of the hierarchical porosity was explored using various structural characterizations. A typical scanning electron microscopy (SEM) image of an Al80Cu19Ti1

plate is presented in Fig. 2a, in which two distinctly contrasted phases were observed. The dimension of each phase, either bright or dark, is about several hundred nanometers in width and extends to several micrometers in length. Energy-dispersive X-ray spectroscopy (EDX) analysis (Fig. 2ce) clearly shows that the bright region is a Cu-rich phase, whereas the darker region is mainly composed of Al. The location of Ti atoms cannot be determined by EDX measurements because of its low atomic concentration (1%). The PXRD pattern in Fig. 2b shows two sets of distinct diffraction proles, corresponding to Al2Cu and Al. The angular positions of the indexed Al2Cu peaks matched the calculated values of the standardized crystal structure, whereas the Al peaks were found to be slightly shifted towards lower angles, indicating a possible crystal lattice expansion because of Ti doping. In addition, a weak Al3Ti peak (112; 2y 39) was also

observed. The PXRD results indicate the existence of Ti in the Al-rich region in two phases: a Al-Ti solid solution phase and a metallic Al3Ti compound phase. The subsequent selective dealloying process conducted in strong alkaline media resulted in two different sets of pores in np-CuTi (Fig. 2f,g). The micrometre-size pores were resulted from a complete removal of the Al-rich region; the nano-sized pores were obtained by the removal of Al atoms in the Al2Cu compound. N2 adsorption-desorption measurement further conrmed that the resulting np-CuTi exhibits a relatively large BrunauerEmmettTeller surface area of about 46 m2 g 1 with an average nanopore size of ca 15 nm using the BarrettJoynerHalenda method (Supplementary Fig. 4). Note that the micrometre-sized pores in np-CuTi are too large to be measured in the gas adsorption experiments.

The PXRD data for np-CuTi (Fig. 3a) suggested a similar crystal structure with that of the bulk Cu-Ti alloys. A small unit cell expansion (Supplementary Table 2) was also observed because ofTi substitution. The atomic ratio of Ti to (Cu Ti)

of np-CuTi was veried to be about 5 at. % by EDX analysis (Supplementary Fig. 5), mimicking the optimal composition of Cu95Ti5 from the bulk Cu-Ti alloy study. The consistency in their surface conditions was also conrmed using XPS characterizations (Supplementary Fig. 6). To further study the structure of np-CuTi, transmission electron microscopy (TEM) characterization was performed on a cross-sectioned specimen prepared using a focused ion beam (FIB) technique. High-angle annular dark-eld (HAADF)-TEM image again conrmed the bimodal porous nature of np-CuTi (Fig. 3b). The high-magnication image, Fig. 3c, clearly shows that the material ligaments and nanopores are similar in size (ca 15 nm), in good agreement with the grain size estimated from PXRD data using the Scherrers method (Supplementary Table 2) and the pore size observed in N2 adsorption-desorption analysis (Supplementary Fig. 4). The np-CuTi catalyst was also examined using electron energy loss

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Al(Ti) (111)

Al(Ti) (200)

Observed Refined

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Intensity (a.u.)

Al 2Cu (110)

Al 2Cu (200)

Al 2Cu (222)

Al 2Cu (211)

Al 2Cu (112)

Al 2Cu (220)

Al 2Cu (310)

Al 2Cu (202)

Al 2Cu (312)

Al(Ti) (311) Al 2Cu (420)

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20 40 602 ()

Cu Cu versus Al

Al

Figure 2 | XRD and SEM characterization. (a) SEM image of a Al80Cu19Ti1 pristine catalyst electrode. Scale bar, 1 mm. (b) The corresponding XRD pattern. (ce) The corresponding EDX mapping of Cu (c), Al (d) and the composite Cu versus Al (e). (f) SEM image of np-CuTi after selective dealloying.

Scale bar, 1 mm. (g) The corresponding higher magnication SEM image. Scale bar, 200 nm.

spectroscopy (EELS). Although Fig. 3d shows the contrast image of a selected region for spectroscopic evaluation, Fig. 3e,f shows the associated Cu and Ti EELS mappings using the L2,3 edge of

Cu and Ti, respectively. It is evident that Cu and Ti atoms are homogeneously distributed along the material ligaments, consistent with the conclusion of a solid solution from the PXRD results. Moreover, near-edge ne structure analysis conrmed the metallic nature of Cu and Ti. No oxygen K-edge signal was detected in the EELS spectra. High-resolution TEM image exhibits uniform lattice fringes (Fig. 3g), further conrming the highly crystalline nature of np-CuTi.

The electrocatalytic performances of np-CuTi were evaluated and compared with a commercial state-of-the-art Pt/C electrocatalyst. Figure 4a shows the HER polarization curves of normalized current densities versus applied potential (iR corrected). The activity of the np-CuTi catalyst exceeded Pt/C steadily with a more than twofold enhancement, most likely owing to its highly active surface, large surface area and enhanced mass transport properties. To prove the high activity of Cu-Cu-Ti sites on the internal np-CuTi surface, a Ti-free nanoporous Cu with identical hierarchical porous structure (denoted as np-Cu) was synthesized by introducing one additional dealloying process using aqueous H2SO4 solution to remove the Ti content from np-CuTi.

Structural characterization results conrmed that the resulting material exhibited a near-identical structure as np-

CuTi in terms of morphology (Supplementary Fig. 7), pore size distribution and specic surface area (Supplementary Fig. 1), but no Ti was detected by EDX (Supplementary Fig. 5) or XPS (Supplementary Fig. 6) analysis. More importantly, the X-ray diffraction peaks of np-Cu were shifted towards higher angles compared with those of np-CuTi and aligned precisely with the peak positions of pure Cu (Fig. 3a), indicating a successful removal of Ti atoms with a concomitant lattice contraction (Supplementary Table 2). As expected, the HER activity of the Ti-free np-Cu sample, although sharing a similar hierarchical porous structure, decreased by a factor of more than 50. It should be noted that decreasing the size of copper can lead to enhanced HER activity as can be seen that the HER activity of np-Cu is much higher than that of bulk Cu (Fig. 4a), but such an enhancement is not the dominant reason of the unique HER activity of np-CuTi. Based on the electrocatalysis results for both np-Cu and bulk polycrystalline Cu, it can be concluded that the exceptional HER activity observed in the np-CuTi sample (Fig. 4a) is due to the combination of the active Cu-Cu-Ti surface sites and the hierarchical porous structure. The long-term stability of np-CuTi catalyst was also examined with an extended reaction period of 5,000 potential cycles, in which both the electrochemical performance and material structure remained remarkably stable (Supplementary Figs 8 and 9).

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np-CuTi refined Ti-free np-Cu refined Observed

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Figure 3 | XRD and TEM characterization. (a) The XRD patterns of np-CuTi and Ti-free np-Cu. Inset: the enlarged region of Cu (111) diffraction peaks, with the dotted line indicating the peak position of pure Cu. (b) High-angle annular dark-eld (HAADF) scanning (S)TEM image of a cross-sectioned np-CuTi sample prepared using FIB technique. Scale bar, 1 mm. (c), HAADF STEM image with a higher magnication. The box indicates the region selected for EELS study. Scale bar, 50 nm. (df) The contrast image of the selected region for EELS mapping study and its corresponding Cu (e) and Ti (f) maps. Scale bar, 50 nm. (g) High-resolution TEM image with visible lattice fringes. Inset: The Fourier transform conrms that np-CuTi is composed of an extended crystalline network. Scale bar, 2 nm.

DiscussionIn alkaline conditions, HER proceeds in the following sequences of either VolmerHeyrosky or VolmerTafel mechanisms:

Volmer step : H2O e $ Had OH 1

Heyrosky step : Had H2O e $ H2 OH 2

Tafel step : 2Had $ H2 2 3 where * represents the hydrogen adsorption sites. In a recent study conducted by Durst et al., it is reported that the Volmer step is the rate-limiting step for HER on Pt/C in alkaline conditions, leading to a Tafel slope of about 120 mV dec 1 (ref. 38). Although a systematic kinetic study is beyond the scope of the current work, the Tafel analysis of np-CuTi was performed with an attempt to gain insights into the kinetics of HER. The linear portions of the Tafel plots were tted to the Tafel equation (Z b log|j| a) and yielded the Tafel slope (b; Fig. 4b).

As shown in Fig. 4b, the np-CuTi catalyst exhibits a Tafel slope of 110 mV dec 1 (Z 70115 mV), and is very close to the value of

Pt/C (111 mV dec 1, Z 85120 mV). Ideally, the Tafel slope is

an inherent property of electrocatalytic materials and is a useful indicator of the rate-limiting step for reactions involving electron transfer. In reality, however, the Tafel slope can be dependent on many factors other than the kinetic exponent of the electrons, such as the presence of adsorbates and the mass transport effect in porous structure. Although the results in Fig. 4 showed nearly identical Tafel slopes for np-CuTi and Pt/C, future mechanistic studies are desired in order to determine whether the HER kinetics of np-CuTi is similar to that of Pt/C.

It is also worthnoting that partial surface oxidation of both Cu and Ti were observed from np-CuTi (Supplementary Fig. 6). Although the HER is known to provide a highly reductive environment, the exact surface chemical nature of np-CuTi under

reaction conditions was not clear because of the lack of in situ methods capable of resolving the valence state of surface atoms in the potential region of HER11 and the limitation of ex situ measurement (that is, extensive exposure of samples to air). It is likely that the electrode surface is not completely metallic and the Ti atoms may also be in an oxidized state of Ti3 or Ti4 irrespective of contact of oxygen. It is reported recently that oxophilic groups, such as non-precious metal hydroxides, are able to aid metallic catalysts during their HER processes in alkaline because of their ability to break the OH bond11,13,52,53. Therefore, there is a plausible scenario that un-reduced surface oxides (if there is any) of np-CuTi would facilitate the HER in a similar manner, and therefore could explain the enhanced HER activity in addition to the optimal HBE suggested by the DFT results.

In summary, a new non-precious bimetallic material, Cu-Ti alloy, was identied to be highly active for hydrogen evolution reaction under basic conditions through DFT predictions and experimental verication. A hierarchical porous Cu95Ti5 catalyst

with a bimodal pore size distribution was designed and synthesized for the rst time, which exhibited an HER activity more than twice that of the state-of-the-art Pt/C electrocatalyst. The achieved high activity, high stability and cost-effectiveness make np-CuTi a most promising electrocatalyst with overall HER performance. An important area of future studies is to determine the surface valence state using in situ/operando surface-sensitive techniques. Also, a scale-up test of np-CuTi using practical electrolysers, such as hydroxide exchange membrane-based electrolysers, is highly desired for the implementation to commercial processes.

Methods

DFTcalculations. The DFT calculation of HBE was performed with the Vienna Ab initio Simulation Package. The PW-91 function was used in the generalized

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0

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Figure 4 | Electrochemical characterization. (a) HER activities for Pt/C, np-CuTi, Ti-free np-Cu control sample and polycrystalline Cu standard in0.1 M KOH electrolyte. (b) The corresponding Tafel plots.

gradient approximation calculation, and a kinetic cutoff energy of 396 eV was used for the plane wave truncation. A periodic 3 3 unit cell with a 3 3 1 Mon

khorst-Pack k-point grid was used for all calculations. All surfaces were modelled by adding six equivalent layers of vacuum onto four layers of metal atoms corresponding to the most close-packed congurations. The two bottom layers of the slab were xed at a distance of 2.59 , whereas the top two layers were allowed to relax to reach the lowest energy conguration. Spin-polarization was included for both surfaces. The binding energy was calculated using the equation:

EHatomic EH slab Eslab 0:5 EH g 4

Where EHatomic is the binding energy of atomic hydrogen on the given slab, EHslab is the energy of the surface with 1/9 ML hydrogen adsorbed, Eslab is the energy of the slab in a vacuum and EH is the energy of hydrogen in the gas phase. Different adsorption sites, such as atop, bridge, fcc and hcp, were calculated. The hcp sites always yield the lowest adsorption energy, indicating they are the most stable binding site and their values are included in Fig. 1b. The inclusion of a Ti atom in the sublayer of the model only slightly affect the calculated HBE value and does not inuence the HBE trend (Supplementary Table 3).

Materials. The Cu100 xTix (x 1, 3, 5, 7 and 9) alloys with nominal compositions

were prepared by arc melting pure Cu (Alfa Aesar, 99.999%) and Ti (Alfa Aesar,99.99%) under an argon atmosphere. In a subsequent step, a melt spinning technique was introduced to re-melt the alloy ingot and rapidly quench on the surface of a spinning metal roller (50 m s 1) to achieve a homogeneous Cu(Ti) solution phase (Supplementary Fig. 1). The resulting alloy materials were in a ribbon form with a dimension about 3 mm wide and 0.5 m long. After their compositions were veried using energy dispersive X-ray spectroscopy, the alloy ribbon surfaces were polished using 0.1, 0.05 and 0.01 mm size alumina particles (Buehler) in sequence.

Electrodes for electrochemical testing were fabricated by attaching one end of those alloy ribbons with copper wire (Alfa Aesar, 99.999%) as the current collector using silver paint (SPI Supplies). The apparent electrode size used for hydrogen evolution

test is about 0.30 cm2. For the preparation of Pt/C catalyst electrode, 10 mg commercial Pt/C catalyst (TKK, 48.8 wt%) was dispensed in 20 ml deionized water. After a rigorous sonication of 30 min in a water/ice mixture bath, 15 ml of the suspension was deposited onto a glassy carbon electrode (0.20 cm2) and was dried in air to form a uniform thin lm for electrochemical characterizations.

To synthesize hierarchical porous Cu-Ti catalyst, an Al-Cu-Ti precursor was rst prepared by arc melting pure Al (Alfa Aesar, 99.99%), Cu (Alfa Aesar,99.999%) and Ti (Alfa Aesar, 99.99%) with desired atomic ratio (80:19:1) under an argon atmosphere. After the verication of composition by energy dispersive X-ray spectroscopy, the resulting alloy ingot was cut into thin plates with dimensions of 10 5 0.20 mm3 using a precision wafering machine. The surface rust was

removed using 240 Grit sandpaper, and the surface was further polished using ner grade sandpapers (600 Grit and 1200 Grit). A copper wire, which served as the current collector, was connected to the one end of the alloy plate using spot welding. In a following step, the pristine electrodes were immersed into a 6 M KOH solution to remove Al with a free corrosion-forming hierarchical porous Cu-Ti. For further removing Ti, the Cu-Ti catalyst was dealloyed for a second time in a 0.05 M H2SO4 solution for about 10 min until no gas bubbles were produced from the materials. Those resulting catalysts were rinsed in DI water for multiple times and applied to the electrochemical evaluations directly without drying. The apparent electrode size of hierarchical porous catalysts used for hydrogen evolution test is about 0.50 cm2.

Structural characterization. PXRD patterns were collected using a Rigaku Ultima IV X-ray diffractometer with Cu Ka radiation. The porous material samples were assembled in an Ar-lled glove box with a Mylar lm (Chemplex, 2.5 mm thick) mounted on the surface for preventing severe oxidation. Renement of the PXRD patterns was conducted using the Rietveld approach implemented in Rigakus software package PDXL. SEM studies were performed with a ZEISS CrossBeam Auriga 60 FIB-SEM. The high-resolution TEM (bright eld) image and HAADF image were taken by 200 kV FEI F20 UT Tecnai with spatial resolution of 0.14 nm and an energy resolution of EELS of 0.6 eV without a monochromator. The energy dispersion for EELS mapping was set to 0.3 eV per channel and the acquisition time for each spectrum was set at 1.2 s to achieve decent signal for Ti, Cu and O. To obtain high-spatial resolution for EELS, the total acquisition time for each map was set to be at least 40 min, which corresponds to around 1,200 pixels with drift correction. The EELS mappings are extracted from target elements peak independently and do not reect the relative proportion between Cu and Ti. The cross-sectioned TEM sample was prepared with the ZEISS CrossBeam Auriga 60 FIBSEM. The porous material was embedded in M-Bond 610 Adhesive System (SPI Supplies) for improving mechanical properties before the FIB preparation. The surface roughness factors of bulk Cu-Ti alloys were characterized using an atomic force microscopy (Dimension 3100, Veeco instruments Inc.) in tapping mode (Supplementary Fig. 10). Six different areas (25 25 mm2) were randomly selected

for each samples and were analysed with same scan parameters and same scan rate of 1 Hz. The roughness factors listed in Supplementary Table 1 were the average value from six different measurements. An XPS system (Physical Electronics VersaProbe 5000) was used to analyse the surface property. The system is equipped with a 16-channel hemispherical analyser and Al anode monochromatic X-ray source. The binding energy scale was calibrated by comparing the position of the primary photoelectron peaks in Cu, Au and Ag reference foils to values in literature. Data were analysed using CasaXPS software, and peaks were t using a Gaussian/Lorentzian product line shape and Shirley background. N2 adsorption/

desorption isotherms were collected at 77 K by using a Micromeritics ASAP 2020.

Electrochemical evaluation. A typical three-electrode cell equipped with an Ag/ AgCl reference electrode (3.0 M KCl, BASi) was used for hydrogen evolution reaction studies. A graphite rod (Sigma-Aldrich, 99.999%) was used as counter electrode for testing Cu-Ti samples. A piece of Pt wire was used as counter electrode for testing Pt/C samples. The electrolyte was 0.1 M KOH (Sigma-Aldrich,99.99%) made with MilliQ water (18.2 MO) and was continuously purged with N2 (Keen, 99.999%). The reference electrode was calibrated to the reversible hydrogen potential using platinum wires for both working and counter electrodes in the same electrolyte purged with H2 (Keen, 99.999%). The calibration resulted in a shift of

0.974 V versus the reversible hydrogen electrode (RHE). The sweep rates used in the cyclic voltammetry studies were 5 mV s 1 for bulk materials and Pt/C;

0.5 mV s 1 for porous materials in order to suppress the capacitive current due to their high surface area. All experiments were conducted using a Princeton Applied

Research VersaSTAT 3 potentiostat and were performed at room temperature.

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Acknowledgements

Q.L. acknowledges the support from Fondazione Oronzio e Niccol De Nora Fellowship. Authors at the Columbia University acknowledge support from the US Department of Energy (DE-FG02-13ER16381). Authors at the University of Delaware acknowledge the University of Delaware Research Foundation and National Science Foundation CAREER Program (Award No. CBET-1350911) for nancial support. Y.Z. and J.Q.X. thanks nancial support from National Aeronautics and Space Administration (NASA) under Grant No. NNX11AQ29A. Authors at Lawrence Berkeley National Laboratory thank the US Department of Energy, Ofce of Science Early Career Research Program. We thank Professor Karl M. Unruh for the assistance in data analysis and Dr Huolin Xin for the help in microscopic studies.

Author contributions

Q.L. and Y.Z. fabricated the material and performed the structural characterization; Q.L. and J.R. performed the catalytic investigations; G.S.H., R.T. and Q.L. performed electron microscopy analysis; R.V.F. and Q.L. did the XPS measurement; B.T.Y. did the N2 adsorption/desorption measurement; Z.C. did the AFM measurement; W.Y. did the DFT calculation; Q.L., F.J. and J.G.C. designed the project; Q.L., J.Q.X., H.Z., F.J. and J.G.C.

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How to cite this article: Lu, Q. et al. Highly porous non-precious bimetallic electrocatalysts for efcient hydrogen evolution. Nat. Commun. 6:6567 doi: 10.1038/ ncomms7567 (2015).

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analysed and interpreted the data; Q.L., F.J. and J.G.C. wrote the manuscript; F.J. and J.G.C. supervised the whole project.

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