The rising concerns about CO2 emissions have led to a growing realization that it is not possible to sustain the world's current development based on fossil fuels without a substitution of clean and renewable energy. Hydrogen, in addition to being an important chemical feedstock in global industry, is now firmly considered as one of the most likely future fuels. However, current hydrogen production primarily relies on the steam methane reforming process which is neither sustainable nor favored because the process requires high energy (heat) input and produces CO2 as a by-product. It is widely believed that room temperature electrochemical reduction of water to molecular hydrogen offers a significant 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 processes. All these applications require large-scale, commercial processes for water electrolysis, which in turn require breakthrough discoveries in at least two areas: (1) the availability of electricity derived from renewable energy sources, such as solar and wind, and (2) the discovery of low-cost electrocatalysts to replace precious metals that are currently the state-of-the-art HER catalysts.
HER in an acidic environment generally requires lower overpotentials than those in a basic environment. 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 a cathode, but also for oxygen evolution reaction at an anode. Regardless of acidic or basic conditions, Pt, along with its alloys, is the benchmark electrocatalyst that requires very small overpotentials to drive the reaction, while 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 finding and engineering low-cost alternative catalysts. For example, tungsten and molybdenum carbides and sulfides, nickel phosphides, and electrodeposited Ni—Cu alloy have been identified as potential electrocatalysts for HER, but 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 electrocatalysts. 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 activity. Monometallic catalysts have been studied extensively using the DFT method. However, monometallic non-precious metals show HBE values significantly different from that of Pt.
In some aspects, the invention provides a metallic alloy including Cu and one or more metals M selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Zn, wherein the alloy has a surface in the form of a vermiculated arrangement of irregular, nanoporous lands separated by troughs or channels.
In some aspects, the invention provides a method of making the foregoing metallic alloy. The method includes contacting a precursor alloy including Cu, M and Al with a caustic liquid under conditions sufficient to remove the Al from the precursor alloy.
In some aspects, the invention provides a water electrolyzer or a hydrogen fuel cell employing the foregoing alloy as a HER or HOR electrocatalyst, respectively.
In some aspects, the invention provides a metallic alloy including Cu and one or more metals M selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Zn, wherein the one or more metals M in total constitute in a range of 3 at. % to 7 at. %, relative to the total of Cu and M.
In some aspects, the invention provides a water electrolyzer or a hydrogen fuel cell employing the alloy immediately above as a HER or HOR electrocatalyst, respectively.
a shows possible bimetallic sites on a Ti modified Cu surface according to the invention.
b shows a volcano plot of HBEs corresponding to the sites shown in
c shows a comparison of HER activities of various bulk CuTi alloy surfaces according to the invention with HER values of the corresponding monometallic standards.
d shows a comparison of exchange current densities of various bulk CuTi alloy surfaces according to the invention with those of the corresponding monometallic standards.
a shows an SEM image of an Al80Cu19Ti1 pristine catalyst electrode according to the invention.
b shows the corresponding XRD pattern relating to
c-e show the corresponding EDX mapping of Cu (c), Al (d) and the composite Cu versus Al (e), respectively, relating to
f shows an SEM image of np-CuTi according to the invention.
g shows the corresponding higher magnification SEM image of the np-CuTi shown in
a shows the XRD patterns of np-CuTi according to the invention 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 shows the high angle annular dark field (HAADF) scanning (S)TEM image of a cross-sectioned np-CuTi sample according to the invention, using the FIB technique.
c shows an HAADF STEM image with a higher magnification. The box indicates the region selected for EELS study.
d-f show contrast images of the selected region for an EELS mapping study and the corresponding Cu (e) and Ti (f) maps, respectively.
g shows a high resolution TEM image with visible lattice fringes. Inset: The Fourier transform confirms that np-CuTi is composed of an extended crystalline network.
a shows HER activities for Pt/C, np-CuTi according to the invention, a Ti-free np-Cu control sample, and polycrystalline Cu standard in 0.1 M KOH electrolyte, respectively.
b shows the corresponding Tafel plots.
a shows full PXRD patterns for bulk CuTi alloys according to the invention.
b shows the enlarged Cu (111) peak region for bulk CuTi alloys according to the invention. All peaks were shifted towards to lower angular positions indicating a lattice expansion due to Ti modifications.
a shows XPS characterizations for Cu 2p spectra for bulk CuTi alloys according to the invention.
b shows XPS characterizations for Ti 2p spectra for bulk CuTi alloys according to the invention. Partial surface oxidation was observed in both Cu and Ti due to the handling of materials in atmospheric air.
a shows HER polarization curves for bulk Cu and CuTi alloys according to the invention with Ti doping levels of 1, 3, and 5 at. %.
b shows HER polarization curves for bulk Ti and CuTi alloys according to the invention with Ti doping levels of 5, 7, and 9 at. %. All current densities are scaled with the surface roughness factor.
a shows N2 adsorption/desorption isotherms for as-prepared np-CuTi according to the invention and its Ti-free np-Cu counterpart.
b shows the corresponding pore size distributions derived from desorption isotherms using the BJH method. Both materials exhibited near-identical specific surface areas (46 m2 g−2 for np-CuTi; 45 m2 sg−2 for Ti-free np-Cu) and pore size distributions.
a shows an EDX spectrum of np-CuTi according to the invention, showing a Ti concentration of about 3 wt. % (ca. 5 at. %).
b shows an EDX spectrum of the acid-treated material, showing no detectable Ti content.
a shows Cu 2p XPS characterizations for np-CuTi according to the invention and Ti-free np-Cu.
b shows Ti 2p XPS characterizations for np-CuTi according to the invention and Ti-free np-Cu. The surface conditions and surface Ti composition of np-CuTi is similar to that of bulk Cu95Ti5 (12.7% for np-CuTi; 10.9% for Cu95Ti5). Partial surface oxidation was observed in both Cu and Ti due to the handling of materials in atmospheric air, similar to that seen in bulk materials.
a shows an SEM image of Ti-free np-Cu at low magnification.
b shows an SEM image of Ti-free np-Cu at higher magnification.
a shows XPS spectra in the Cu 2p region of the np-CuTi sample referred to in
b shows XPS spectra in the Ti 2p region of the np-CuTi sample referred to in
a-c shows typical AFM images of the surface of a bulk CuTi alloy according to the invention with respect to amplitude (a), phase (b), and height (c).
d shows line scans of the height fluctuations for the three lines indicated in
The invention provides robust and efficient non-precious metal catalysts for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR), and these catalysts can be used as electrocatalysts at the cathodes and anodes of water electrolyzers and hydrogen fuel cells, respectively. The catalysts are referred to herein as CuM catalysts or alloys, and are alloys of Cu and one or more metals M selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Zn. The CuM and np-CuM alloys are referred to herein as bimetallic because they include Cu and at least one other metal, but bimetallic alloys of the invention include ones in which more than one metal M is present. Some forms of the alloys, referred to as np-CuM alloys or catalysts, have a hierarchical nanoporous structure, and can be made by caustic leaching of Al from an Al-CuM precursor alloy.
The inventors have performed DFT calculations showing that CuTi bimetallic materials have similar hydrogen binding energy (HBE) values as Pt, and therefore are promising non-precious metal HER electrocatalysts. This has been experimentally verified on both bulk CuTi alloys and np-CuTi catalysts. Other CuM and np-CuM alloys can be made by the methods described herein for producing the CuTi and np-CuTi alloys, and these alloys also have utility as HER and/or HOR electrocatalysts in water electrolyzers and hydrogen fuel cells. Discussions of CuM and np-CuM alloys in the present disclosure relate largely to the preparation and evaluation of CuTi and np-CuTi, but it is to be understood that alloys using the other metals M may be used instead according to the invention.
The inventors have found that np-CuTi bimetallic electrocatalyst is able to produce hydrogen from water under a mild overpotential at a rate more than two times higher than that of the current state-of-the-art carbon-supported platinum catalyst. Although both Cu and Ti are known to be poor HER catalysts, their combination as described herein results in exceptional HER activity. Without wishing to be bound to any particular explanation, the inventors believe that the np-CuTi alloys comprise Cu—Cu—Ti hollow sites that have a hydrogen binding energy very similar to that of Pt, resulting in the exceptional HER activity. Additionally, the hierarchical porosity of the np-CuTi catalyst also contributes to its high HER activity, because it not only provides a large surface area for electrocatalytic HER, but also improves mass transport. Moreover, the np-CuTi catalyst is self-supported, eliminating the overpotential associated with the catalyst/support interface.
Bulk CuM and np-CuM Alloys
The bulk CuM alloys of the invention comprise M at a level of at least 1 at. % (atomic percent), or at least 2, 3 or 4 at. %, relative to the total of Cu and M, where M includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni and Zn and the at. % refers to the total of these present in the bulk material. The level of M is at most 15 at. %, or at most 10, 9, 8, 7 or 6 at. %. A preferred level is 5 at. %, especially when M consists of Ti alone. The balance is Cu. The bulk alloys can be produced by arc-melting followed by melt-spinning, so that they are in the form of a solid solution of M in Cu, or a phase of a compound of Cu and M, or a mixture of these. The bulk CuM alloys may consist of, or consist essentially of, Cu and Ti, or Cu and Ni, or Cu and Co.
The np-CuM alloys may be of the same elemental compositions on an atomic percent basis as the bulk alloys, based on the total of Cu and M, i.e., not counting any residual Al that might be present after caustic treatment of the Al-CuM precursor alloy. The caustic treatment is performed with a caustic liquid under conditions suitable to remove the Al. Suitable caustic liquids are aqueous bases, for example NaOH and KOH. The np-CuM alloys are in the form of a solid solution of M in Cu, or a phase of a compound of Cu and M, or a mixture of these.
The Al-CuM precursor alloys are multiphase alloys having an overall Al content of at least 50 at. %, or at least 55, 60, 65, 70 or 75 at. %. The Al content is at most 99 at. %, or at most 95, 90, 87 85 or 83 at. %. The balance is Cu and M, in any of the proportions described above with respect to the bulk CuM and np-CuM alloys. The Al-CuM precursor alloys may be prepared by arc-melting Al, Cu and M together.
As seen in
As monometallic catalysts, Cu and Ti are known to be poor HER catalysts because their HBE values are either too small or too large, respectively. Using DFT calculations, we have demonstrated that the Cu—Cu—Ti hollow site on a CuTi bimetallic surface exhibits an optimal HBE for HER. As shown in
To verify the DFT predictions, a series of Cu100-xTix (x=1, 3, 5, 7, and 9) alloys with homogeneously distributed atoms was 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 (Table 1). Powder X-ray diffraction (PXRD) analysis suggests all alloys retain the fcc structure of crystalline Cu with a lattice expansion due to Ti doping (
The HER activities of all CuTi alloys as well as pure Cu and Ti standards were compared by plotting their polarization curves in 0.1 M KOH electrolyte (
To extend the DFT predictions and bulk alloy results to practical high-performance catalysts, the inventors designed a nano-architecture for the catalytic material. The inventors wished to be able to operate electrolyzers at high reaction rates (i.e. high currents), a task made difficult by the possibility of the produced hydrogen bubbles building up inside the porous network and blocking the active sites. The inventors have addressed this by providing a CuTi electrocatalyst with a highly hierarchical porous structure (denoted as np-CuTi) by dealloying a multi-phase Al—CuTi precursor alloy (atomic ratio Al:Cu:Ti=80:19:1) to remove Al via caustic leaching. The atomic ratio of Ti to (Cu+Ti) was chosen to be the optimal value (5 at. %) from bulk CuTi studies. The nano-sized pores of the resulting np-CuTi are responsible for high surface areas, whereas the micron-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.
Structural Analysis of np-CuTi
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
The PXRD pattern in
The PXRD data for np-CuTi (
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-field (HAADF) TEM image again confirmed the bimodal porous nature of np-CuTi (
The np-CuTi catalyst was also examined using electron energy loss spectroscopy (EELS). While
Electrocatalytic Activity of np-CuTi
The electrocatalytic performances of np-CuTi were evaluated and compared to a commercial state-of-the-art Pt/C electrocatalyst.
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. Although increasing the active specific surface area by providing smaller surface features can lead to enhanced HER activity, as seen by the much higher HER activity of np-Cu vs. that of bulk Cu (
Tafel Analysis of np-CuTi
Under alkaline conditions, HER proceeds in the following sequences of either Volmer-Heyrosky or Volmer-Tafel mechanisms:
Volmer step: H2O+e−+*Had+OH− (1)
Heyrosky step: Had+H2O+e−H2+OH−+* (2)
Tafel step: 2HadH2+2* (3)
where * represents the hydrogen adsorption sites. Literature reports indicate 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. Therefore a Tafel analysis of np-CuTi was performed in an attempt to gain insights into the kinetics of HER. The linear portions of the Tafel plots were fitted to the Tafel equation (η=b log|j|+a) and yielded the Tafel slope (b) (
It is worth noting that partial surface oxidation of both Cu and Ti was observed from np-CuTi (
Electrolyzers and Fuel Cells Using np-CuM Electrocatalysts
In some embodiments of the invention, np-CuM alloys or bulk CuM alloys may be used as the cathodic electrocatalyst to facilitate HER in a water electrolyzer, for example one using a hydroxide exchange membrane. Analogously, the np-CuM alloys or bulk CuM alloys may serve as anodic electrocatalysts to facilitate HOR in hydrogen fuel cells, for example those using hydroxide exchange membranes. Numerous configurations and methods of making water electrolyzers and hydrogen fuel cells are known to the skilled person, and the np-CuM alloys of the invention may be used as electrocatalysts in any of these. Schematic representations of an electrolyzer and a fuel cell are shown in
The DFT calculation of HBE was performed with the Vienna Ab initio Simulation Package (VASP). The PW-91 functional was used in the generalized gradient approximation (GGA) 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 Monkhorst-Pack k-point grid was used for all calculations. All surfaces were modeled by adding six equivalent layers of vacuum onto four layers of metal atoms corresponding to the most close-packed configurations. The two bottom layers of the slab were fixed at a distance of 2.59 Å while the top two layers were allowed to relax to reach the lowest energy configuration. Spin-polarization was included for both surfaces. The binding energy was calculated using the equation:
E
atomic
H
=E
H-slab
−E
slab−0.5×EH
where EatomicH is the binding energy of atomic hydrogen on the given slab, EH-slab is the energy of the surface with 1/9 ML hydrogen adsorbed, Eslab is the energy of the slab in a vacuum, and EH
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) to achieve a homogeneous Cu(Ti) solution phase (
To synthesize hierarchical porous CuTi catalyst, an Al—CuTi precursor alloy was first prepared by arc melting pure Al (Alfa Aesar, 99.99%), Cu (Alfa Aesar, 99.999%) and Ti (Alfa Aesar, 99.99%) at an atomic ratio of 80:19:1 Al:Cu:Ti under an argon atmosphere. After verifying the composition by energy dispersive X-ray spectroscopy, the resulting alloy ingot was cut into thin plates with dimensions of 10 mm×5 mm×0.20 mm using a precision wafering machine. Surface rust was removed using 240 Grit sandpaper, and the surface was further polished using finer grade sandpapers (600 Grit and 1200 Grit). A copper wire, which served as the current collector, was connected to one end of the alloy plate using spot welding. In a following step, the pristine electrodes were immersed in a 6 M KOH solution to remove Al, thereby forming hierarchical porous CuTi (np-CuTi). Ti was removed from a sample of the np-CuTi catalyst by immersing in a 0.05 M H2SO4 solution until gas bubbles stopped forming. All catalysts were rinsed in DI water multiple times and subjected to electrochemical evaluations directly without drying. The apparent electrode size of hierarchical porous catalysts used for the hydrogen evolution test was about 0.50 cm2.
Powder X-ray diffraction patterns were collected using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation. The porous material samples were assembled in an Ar filled glove box with a Mylar film (Chemplex, 2.5 μm thick) mounted on the surface for preventing severe oxidation. Refinement of the PXRD patterns was conducted using the Rietveld approach implemented in Rigaku's software package PDXL.
Scanning electron microscopy studies were performed with a ZEISS CrossBeam Auriga 60 FIB-SEM. The high resolution TEM (bright field) image and HAADF (high angle annular dark field) 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/channel and the acquisition time for each spectrum was set at 1.2 seconds to achieve a useful signal for Ti, Cu and O. In order to obtain high spatial resolution for EELS, the total acquisition time for each map was set to be at least 40 minutes which corresponds to around 1200 pixels with drift correction. The EELS mappings were extracted from each target element's peak independently, and do not reflect the relative proportion between Cu and Ti.
The cross-sectioned TEM sample was prepared with the ZEISS CrossBeam Auriga 60 FIB-SEM. The porous material was embedded in M-Bond 610 Adhesive System (SPI Supplies) for improving mechanical properties prior to the FIB preparation. The surface roughness factors of bulk CuTi alloys were characterized using an atomic force microscopy (Dimension 3100, Veeco instruments Inc.) in tapping mode (
An X-ray photoelectron spectroscopy system (Physical Electronics VersaProbe 5000) was used to analyze the surface. The system was equipped with a 16 channel hemispherical analyzer 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 analyzed using CasaXPS software, and peaks were fit 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.
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 CuTi 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 MΩ) 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 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.
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims without departing from the invention.
This application claims priority benefit of U.S. Provisional patent application No. 62/023,202, filed on 11 Jul. 2014, the entirety of which is incorporated herein by reference for all purposes.
Number | Date | Country | |
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62023202 | Jul 2014 | US |