WO2018179005A1

WO2018179005A1 - Shape tailored ordered pdcu3 nanoparticle surpassing the activity of state-of-the-art fuel cell catalyst

Info

Publication number: WO2018179005A1
Application number: PCT/IN2018/050167
Authority: WO
Inventors: Sebastian Chirambatte PETER; Rajkumar JANA; Saurav Chandra SARMA
Original assignee: Jawaharlal Nehru Centre For Advanced Scientific Research
Priority date: 2017-03-25
Filing date: 2018-03-26
Publication date: 2018-10-04

Abstract

The present disclosure relates to a method of synthesizing shape tailored ordered intermetallic nanomaterials, PdCu3 and its application as electrocatayst in fuel cells. More particularly, the present disclosure relates to a synthesis of ordered PdCu3 nanomaterial starting from a spherical nanocrystal with crystallographically exposed (111) plane to a cubic nanocrystal with (100) plane, and its excellent efficiency as both cathode and anode material, surpassing the activity and stability of the state-of-the-art Pt/C electrocatalyst. The present disclosusre further relates to the unique, cost-effective way of synthesizing ordered Pd based nanocrystal in different shapes with different exposed crystallographic planes for catalysis. The present disclosure also relates to fuel cell device fabrication and applications of said nanomaterial as bi-functional electrode material (active as both anodic and cathodic electrode material).

Description

Shape Tailored Ordered PdCu₃ Nanoparticle Surpassing the Activity of State-of-the-

Art Fuel Cell Catalyst

FIELD OF THE INVENTION

The present invention relates to a method to synthesize shape tailored ordered intermetallic nanomaterials, PdCu₃ and its application as electrocatayst in the fuel cell. More particularly, the present invention relates to a synthesis of ordered PdCu₃ nanomaterial starting from a spherical nanocrystal with crystallographically exposed (111) plane to a cubic nanocrystal with (100) plane, and its excellent efficiency as both cathode and anode material, surpassing the activity and stability of the state-of-the-art Pt/C electrocatalyst. The present invention further relates to unique, cost-effective way of synthesizing ordered Pd based nanocrystal in different shapes with different exposed crystallographic planes for catalysis. The present disclosure also relates to a fuel cell device fabrication and applications of said nanomaterial as bi-functional electrode material (active as both anodic and cathodic electrode material).

BACKGROUND OF THE INVENTION

An electrocatalyst is a catalyst that takes part in electrochemical reactions and modify (increase) the rate of chemical reactions without being consumed in the process. It functions at the electrode surface and assists to transfer the electrons between the electrode and reactants facilitating an intermediated chemical transformation. An electrocatalyst can be homogeneous like pyridinium ion or heterogeneous such as platinum nanoparticles. Nanoparticles are very useful as electrocatalysts in fuel cell devices. A fuel cell device consists of two major parts cathodes (reduction occurs) and anode (oxidation occurs). Mainly, hydrogen is used as the green fuel, which reacts with oxygen, produces water and electricity. However, several major problems still need to be considered for the wide spread usage of these hydrogen based fuel cells in very competitive and dynamic market. Oxygen reduction reaction (ORR) occurred at the cathode of the fuel cell suffers several technical and scientific issues. Mainly, due to sluggish reaction kinetics, ORR has been considered as the kinetically limiting step in fuel cells and even metal air batteries. The efficiency of a full cell is mainly driven by the cathodic ORR kinetics. The state-of-the-art electrode material for both cathodic and anodic part are based on the highly expensive, earth scarce Pt metal. It has already been well established that the overpotential for ORR in alkaline medium is substantially reduced and hence offers the possibilities of lower usage of Pt and broader selection of electrocatalysts other than Pt towards electrocatalytic ORR. Besides, growing interest of developing anion exchange membrane fuel cell (AEMFC) adds further incentive towards the development of low Pt content or Pt free novel nanostructured electrocatalysts for ORR in alkaline medium. Besides, the cost and stability issue, development of efficient catalyst to produce water directly via four electrons transfer still remains a challenge. Though a plethora of Pt based compounds (PtNi, PtBi₂, PtCu₃) have been reported to efficiently reduce oxygen to form water, they are far beyond commercialization because of their low durability and high cost.

Though polymer electrolyte membrane fuel cells (PEMFCs) is being mainly used commercially for energy production purpose, but use of highly flammable hydrogen gas hinders its future application due to safety issue. Direct liquid fuel cells such as direct formic acid fuel cells (DFAFCs), direct ethanol fuel cells (DEFCs) have attracted a considerable attention as new generation power sources. Again, Pt and Pt based compounds (PtBi, PtRu, PtSn, PtAu) are considered as the best electrode materials for the oxidation of small organic molecules. Among the available direct liquid fuel cells, DEFCs have drawn great attention because they are considered as promising future power sources for electric vehicles and small portable electronics. Based on the electrolyte membrane used, DAFCs can be divided into two types: acid- and alkaline-membrane DAFCs. Over the past few decades, extensive attention has been paid to the acid-type DAFCs and significant progress has been made in their development. However, the commercialization of acid-type DAFCs still remains a challenge because in acidic media, the expensive Pt or Pt-based electrocatalysts are usually required. In addition, the kinetics of alcohol oxidation on Pt- based electrocatalysts is rather sluggish in acidic media. Recently, increasing attention has been paid to the alkaline-type DAFCs because in alkaline media less expensive Pd-based catalysts have comparable or even better electrocatalytic activities than Pt-based catalysts for alcohol oxidation, especially for ethanol oxidation. Along with the cost effectiveness the long-term durability of the Pt based catalyst is very less due to surface poisoning with the "CO" intermediate. Pd based catalysts have also been widely studied in recent times as an alternative to Pt because of its good resistance to CO poisoning as well. Many alloys, bimetallics and oxide supported Pd-M (M = Ru, Cu, Au, Ni, Ag, Sn, Ir, Co and Pb) binary electrocatalysts have exhibited improved electrocatalytic activity for ethanol oxidation. Shape tailored Pd and Pd based alloy nanocrystals are also found to exhibit enhanced electrocatalytic activity.

Although alloys and bimetallics are promising electrode materials for both oxidation and reduction reactions, due to disordered nature of the Pd-M electrocatalysts, the atomic positions are randomly occupied by Pt/Pd and M, creating irregular surface composition and random distribution of active sites leading to surface poisoning and hence decrease in activity and stability. Unlike disordered phases, structurally ordered counterparts have definite composition, structure and uniform distribution of the active sites, providing much better control over electronic and geometric structure leading to remarkable enhancement in the activity, stability and poison resistance. However, it is extremely difficult to synthesize ordered materials based on Pd and 3d transition metals due to large differences in reduction potential as the co-reduction of the precursor salts is one of the key step for ordered intermetallic formation by solution phase method. To reduce the appropriate transition metal precursors and incorporate them into Pd lattice requires strong and ultrafast reducing agent like organoborohydrides (superhydrides) or Na naphthalide. Moreover, most of the cases post synthetic high temperature thermal annealing has proved to be a necessary step for the formation of ordered intermetallic phases. However, such high temperature annealing process often results in agglomeration, particle size growth and producing carbonaceous surface deposits. Therefore, control over uniform particle size during ordered phase formation still remains a great challenge. On the other hand, shape control synthesis of nanomaterials has strong influence on their intrinsic properties. The shape control of Pt nanocrystals has drawn great attention due to the achievements in tuning electrocatalytic activity and selectivity by engineering exposed crystallographic facets. Though recently it has been recognized that synthesis of shape tailored Pt based bimetallic nanocrystals is an efficient way to obtain highly active and durable electrocatalysts with less consumption of Pt metal, it is still a significant challenge to realize the shape control process of Pt based nanocrystals with multimetal composition. Generally, the shape control of metal nanoparticles are triggered via the regulation of the relative growth rates of different crystallographic facets governed by three factors; (a) selective adsorption of different chemical species on specific crystal planes, (b) tuning of the nanocrystals growth (thermodynamic vs kinetic), (c) seed mediated epitaxial growth. Among many bimetallic or alloy systems, Pd-Cu is one of the most attractive electrocatalysts as it has the potential to act as both cathode and anode electrode material. Over the last two decades, many research activities have been devoted towards the synthesis of Pd-Cu random alloys and or/ bimetallics. Even, core-shell structure of PdCu alloy has been investigated towards the ORR and small molecule (especially ethanol) oxidation activity. In spite of that, the catalyst does not fulfill the demand of high activity and durability. However, shape-controlled synthesis of Pd-Cu nanocrystals has not been well-explored in the literature as the preference of Pd for segregation due to the considerable difference in reduction potentials (Pd /Pd=0.83V; Cuⁿ/Cu=0.34V vs HE) and diffusion rates of Pd and Cu. A very few reports are there on the shape-control synthesis of Pd-Cu nanocrystals and their electrocatalytic activities. But interestingly, less effort has been made towards the synthesis of ordered structure of PdCu. Along with that the effect of change of crystallographic facets of the ordered PdCu systems on electrocatalysis has never been looked upon by the scientific community. Besides, all the synthetic procedures adopted until now to synthesize highly active PdCu cube surface requires the use of highly expensive external reducing agent along with the expensive solvent.

Accordingly, there is a requirement of cost-effective route towards the synthesis of shape-controlled ordered PdCu₃ nanostructures as a bi-functional (active as both cathodic and anodic material)

SUMMARY OF INVENTION

Accordingly the present invention provides an electrocatalyst for a fuel cell comprising a shape-tailored ordered PdCu₃ nano structure.

In an embodiment of the present invention, the shape-tailored ordered PdCu₃ nanostructure is bifunctional electrode material and active both as anode and cathode.

In yet another embodiment of the present invention, the shape-tailored ordered PdCu₃ nanostructure is having spherical or cubic morphology.

In still another embodiment of the resent invention, the shape-tailored ordered PdCu₃ nanostructure comprises uniformly distributed Pd and Cu on the surface of nanoparticles.

In yet another embodiment of the present invention, the electrocatalyst comprising cubic PdCu₃ has half wave potential of 0.81V compared to 0.79 V half wave potential of Pt/C.

In yet another embodiment of the present invention, the electrocatalyst comprising cubic PdCu₃ has mass activity twice as higher than that observed on Pt/C, at 0.8V.

In yet another embodiment of the present invention, the electrocatalyst is ~3 times better for ORR and - 150 times better for EOR in comparison with Pt/C.

In still another embodiment of the resent invention, the electrocatalyst comprising cubic PdCu₃ shows lower tafel in comparison with Pt/C.

The present invention also provides a method of synthesizing shape tailored ordered intermetallic nanomaterial of PdCu₃, said method comprising:

(i) forming a reaction mixture in a reaction vessel, said reaction mixture comprising: (a) a palladium precursor,

(b) a copper precursor,

(c) a hexadecyltrimethylammonium bromide (CTAB), and

(d) a solvent;

(ii) heating said reaction mixture to a reaction temperature of about 180-220° C;

(iii) maintaining said temperature of reaction mixture of step (ii) for about 24-36 hours;

(iv) cooling said reaction mixture of step (iii) to room temperature to obtain a precipitate; and

(v) washing said precipitate of step (iv) with a 1: 1 (vol ratio) mixture of hexane and ethanol and drying at about 60-80°C for about 6-8 hrs to obtain a shape tailored ordered intermetallic nanomaterial of PdCu₃,

wherein the shape tailored ordered intermetallic nanomaterial of PdCu₃ is synthesized without using any external reducing agent.

According to the method of the present invention, the palladium precursor is palladium acetylacetonate.

In an embodiment of the present invention, the copper precursor is copper acetylacetonate.

In yet another embodiment of the present invention, the solvent is selected from the group consisting of oleylamine, oleic acid and combination thereof.

According to the method of the present invention, the shape tailored ordered intermetallic nanomaterial of PdCu₃ is Spherical ordered PdCu₃ nanocrystal, when the solvent is oleylamine. The present invention also provides a Spherical ordered PdCu₃ nanocrystal as obtained by this process, wherein said Spherical ordered PdCu₃ nanocrystal has a particle size of about 6-10 nm and with (l l l)-oriented faces or facets.

According to the method of the present invention, the shape tailored ordered intermetallic nanomaterial of PdCu₃ is Cubic ordered PdCu₃ nanocrystal, when the solvent is 8: 1 volume ratio mixture of oleylamine and oleic acid. The present invention also provides a Cubic ordered PdCu₃ nanocrystal as obtained by this process, wherein said Cubic ordered PdCu₃ nanocrystal has a particle size of about 7-10 nm with (lOO)-oriented faces or facets.

In yet another embodiment of the present invention, the shape tailored ordered intermetallic nanomaterial is synthesized without using high temperature and external strong reducing agent. In yet another embodiment of the present invention, the oleylamine is used as a solvent, stabilizer and reducing agent.

In an embodiment of the present invention, the CTAB is used as a surfactant as well as reduction potential modulating agent.

In an embodiment of the present invention, the shape tailored ordered intermetallic nanomaterial is used as electrocatalyst in a fuel cell.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

In order to understand the disclosure readily and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with a description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure wherein:

Figure 1: shows schematically, the synthesis technique of morphology controlled ordered PdCu₃ nanocrystals.

Figure 2: Figure 2a shows the comparison of PXRD patterns of as synthesized different shaped PdCu₃ intermetallic nanoparticles by solvothermal method with simulated powder patterns of PdCu₃ (ICSD no 103084) and Pd (ICSD no 52251), indicating 100% phase purity. The peaks that appear due to ordering of Pd and Cu are marked with *. Figure 2b shows Core-level X-ray photoelectron spectroscopy (XPS) spectra of spherical PdCu₃ (PdCu₃_S) and cubic PdCu₃ (PdCu₃_C) nanoparticles, confirms the presence of elemental Pd and Cu in both the intermetallic compounds.

Figure 3: shows EDAX spectra of (a) PdCu₃_S, (b) PdCu₃_C nanocrystals. The average chemical compositions of Pd and Cu are listed as the inset.

Figure 4: Figure 4 (a, b) show the Transmission Electron Microscopy (TEM) images confirming the morphologies of the as synthesized nanocrystals. Figure 4 (c, d) show High Resolution Transmission Electron Microscopy (HRTEM) images indicating the exposed crystallographic facets and formation of ordered structure, (e, f) Selected Area Diffraction Pattern (SAED) patterns of PdCu₃_S PdCu₃_C, respectively.

Figure 5: Figure 5 (a, b) show Linear Polarization curve of (a) PdCu₃_S, (b) PdCu₃_C at different rotation rates indicating oxygen reduction reaction (ORR) kinetics. Figure 5 (c, d) show Koutecky-Levich (K-L) plot obtained for (a) PdCu₃_S (b) PdCu₃_C. Figure 5(e) shows number of electrons calculated for PdCu₃_S and PdCu₃_C at different potentials, indicating almost four electron transfer throughout the potential which shows direct pathway for oxygen reduction reaction. Figure 5(f) shows Kinetic current densities calculated for PdCu₃_S and PdCu₃_C from the intercept of K-L plot.

Figure 6: Figure 6a shows Polarisation curves of PdCu₃_S, PdCu₃_C and commercial Pt/C at 1200 rpm indicating better activity of cube shaped nanocrystal compared to the sate-of- the-art material Pt/C. Figure 6b shows Tafel plot for Pt/C (green), PdCu₃ spheres (red), PdCu₃ cubes (blue) indicating faster reduction kinetics on cube shaped nanoparticles. Figure 6c shows Comparison of the activity of Pt (green), PdCu₃_S (red), and PdCu₃_C (blue) based on the number of electrons and kinetic current density. Figure 6d shows ORR mass activity summaries at 0.8V showing ~2 times better mass activity than Pt/C.

Figure 7: shows Chronoamperometric responses for PdCu₃_C obtained in an 0₂-saturated 0.5M KOH solution at 0.67V versus RHE at a rotation rate of 1200 rpm.

Figure 8: shows Cyclic Voltammetry (CV) measurements obtained for PdCu₃_S, PdCu₃_C, commercial Pd/C and Pt/C and Pd/C in alkaline (0.5M KOH) solution at a scan rate of 50 mV/sec.

Figure 9: shows CV curves measured for the catalysts PdCu₃_S, PdCu₃_C, commercial Pd/C and commercial Pt/C with respect to cycle in 0.5 M KOH containing 1 M ethanol solution at a scan rate of 50mV/sec, Figure 9 (a) shows comparison of specific activities, Figure 9 (b) shows comparison of mass activities for the 1^st cycle, Figure 9 (c) shows comparison of specific activities, Figure 9 (d) shows comparison of mass activities for the 250^th cycle, Figure 9 (e) shows comparison of specific activities, Figure 9 (f) shows comparison of mass activities for the 500^th cycle.

Figure 10: shows Chronoamperometric measurements (CA) of PdCu₃_S, PdCu₃_C, commercial Pd/C and commercial Pt/C catalysts in 1 M KOH + 1M ethanol solution for 1000 sec at electrode potential of -0.2V vs Hg/HgO.

Figure 11: shows main panels show Fourier transform magnitudes of the Pd K-edge EXAFS oscillations (weighted by k²) from the PdCu₃ nanoparticles. Insets in (a) and (b) are the corresponding EXAFS oscillations. In all the cases, Solid lines are the results of a single shell fit considering the nearest neighbours (Cu atoms in this case) from the excited atom.

DETAILED DESCRIPTION OF THE INVENTION

The present invention reveals a unique, cost-effective route towards the synthesis of shape-controlled ordered PdCu₃ nanostructures as a bi-functional (active as both cathodic and anodic material). Therefore, the present invention ends the search of novel highly active and stable electrocatalysts by developing crystallographically engineered ordered PdCu₃ nanocubes (with the 100 exposed facets) with remarkable activity and stability even better than existing state-of-the-art Pt/C catalysts.

The main issue with the development of highly active and durable electrocatalyst is the use of earth-scarce and highly expensive Pt metal. All the commercially available fuel cell devices are based on Pt catalyst. Though Pt based electrocatalysts possess considerable activity but their stability is very less. Even, in terms of activity also Pt based catalysts are not up to the mark. Therefore, development of low-cost Pt free highly stable and durable electrocatalysts is essential. The present invention addresses this issue with the development of low-cost, structurally ordered PdCu₃ material with earth abundant and less expensive elements. More interestingly, the major component of the material, Cu (75 at.%) is easily available and extremely low-cost. Though there are a few reports on PdCu electrocatalysts for fuel cell application, the activity and stability of those catalysts are considerably less than our catalysts. Not only that, most of the reported catalysts are less active than the state- of-the-art commercial Pt/C. But, the developed catalyst of the present invention is ~3 times better for ORR and - 150 times better for Ethanol Oxidation Reaction (EOR) in comparison with Pt/C. Along with that, all the PdCu catalysts reported till now is synthesized using more Pd content which enhances the cost of the synthesis of the catalysts. But, our catalyst is composed of only (25 at.%) Pd.

Another drawback for the synthesis of PdCu catalysts is the use of harsh reaction condition (high temperature i.e. 200-500°C) along with use of expensive reducing agents. But, in the present invention, catalyst has been synthesized using very mild low temperature condition without using any external, expensive reducing agent. In fact, the present invention is considered as the most cost-effective way to synthesize PdCu₃ nanoparticle in different shapes. Along with that, this is first finding on the synthesis of any type of shape tailored Pd based ordered structure without using high temperature and external strong reducing agent.

This invention is useful in the fabrication of highly efficient and robust fuel cell, which is a renewable and green source of energy. Two major problems that the whole world is facing today: depletion of fossil fuel leading to energy crisis and global warming due to high rate of C0₂ emission to the atmosphere. Both the issues need to be addressed in no time. Therefore, use of fuel cell as a source solves the above-mentioned issue. Already in different countries all over the world, fuel cell is being used as green source of energy not only in portable devices but also in other stationary systems. The major challenge in the development of the highly efficient fuel cell is the development of highly active and stable electrocatalysts. At the current state, the active catalysts are made of expensive and earth scarce Pt metal with very low stability. Not only stability issue but also activity of the compounds is not up to the mark. This limits its use in large scale. This invention mainly discusses about the design and development of highly active and robust Pt free low-cost electrocatalysts, which are superior to commercial Pt catalysts. This finding not only solves the stability issue of fuel cell but also makes the availability of the fuel cell more convenient.

The present invention relates to a method to synthesize shape tailored ordered intermetallic nanomaterials, PdCu₃ and its application as electrocatayst in the fuel cell. More particularly, the present invention relates to a synthesis of ordered PdCu₃ nanomaterial starting from a spherical nanocrystal with crystallographically exposed (111) plane to a cubic nanocrystal with 100 plane, and its excellent efficiency as both cathode and anode material, surpassing the activity and stability of the state-of-the-art Pt/C electrocatalyst. The present invention further relates to the unique, cost-effective way of synthesizing ordered Pd based nanocrystal in different shapes with different exposed crystallographic planes for catalysis. The present invention also relates to fuel cell device fabrication and applications of said nanomaterial as bi-functional electrode material (as both anodic and cathodic electrode material).

Shape tailored ordered intermetallic nanomaterial of PdCu₃ means crystallographically ordered PdCu₃ nanomaterial having various shapes like spherical, cubic, etc. "Ordered Structure" means in the cubic unit cell of PdCu₃, the atomic position of Pd and Cu are different and well-defined i.e. Wyckoff sites of each type of atoms (Pd, Cu) are distinct (Cu: 3c; Pd: la). "Shape tailored" mean synthesizing the same nanomaterials in different shapes or morphologies (spherical, cubic). In terms of "composition" both the materials (Spherical ordered PdCu₃ and Cubic ordered PdCu₃) are having the same atomic composition i.e. 1:3 (25 and 75 at.%)

In an embodiment of the present invention, structurally ordered PdCu₃ intermetallic nanoparticles were synthesized in different shapes by solvothermal method. In a typical solvothermal procedure, 0.1 mmol Pd(acac)₂, 0.3 mmol Cu(acac)₂ and 75 mg CTAB were mixed together in 18 ml oleylamine with vigorous stirring and loaded in 23 ml Teflon lined autoclave. The autoclave was kept at 180°C for 24 hrs. This leads to the formation of spherical PdCu₃. Decrease in reduction time and temperature leads to the formation of disordered Pdo_.2sCuo.75 alloy. PdCu₃ nanocubes were obtained by changing the solvent from oleylamine to 8:1 ratio of oleylamine and oleic acid. The product was repeatedly washed several times with a 1 : 1 mixture of hexane and ethanol and dried in vacuum oven at 60°C for 6 hrs.

In another embodiment of the present invention, based on the characterization studies, to confirm the formed nanocrystals are structurally ordered or not, PXRD were measured on the as synthesized nanoparticles. Comparison of the experimental PXRD with the simulated pattern of the bulk compounds clearly shows pure phase of PdCu₃ without any impurity (as shown in Figure 2a). Compared to Pd, evolution of five additional peaks of miller indices 100, 110, 210, 211, 300 indicates the ordering into the systems. The core-level XPS spectra of both the spherical and cube shaped nanocrystals clearly indicate the elemental state of both Pd and Cu in PdCu₃ nanoparticles (as shown in Figure 2b). The near surface composition analysis of both PdCu₃_S and PdCu₃_C by XPS is also consistent with the stoichiometric ratio of 1 :3 for Pd and Cu. The compositions of the nanoparticles were further confirmed from inductively coupled plasma atomic emission spectroscopy (ICP- OES) analysis, which are consistent with the ED AX analysis (as shown in Figure 3).

In still another embodiment of the present invention, TEM images of PdCu₃_S and

PdCu₃_C clearly show the synthesized nanocrystals are spherical and cubic and the average particle sizes in PdCu₃_S and PdCu₃_C are ~8 and 10 nm, respectively (as shown in Figure 4a, 4b). To better understand the order nature of the PdCu₃ nanoparticles, URTEM and SAED pattern have been studied. The d-spacing (inter-fringe distance) calculated from HRTEM images are 0.225 nm and 0.389 nm, respectively for PdCu₃_S and PdCu₃_C nanoparticles which is consistent with the lattice spacing of the (111) and (100) planes respectively of the ordered PdCu₃ intermetallic nanoparticle. This clearly indicates that the exposed crystallographic facets for PdCu₃_S and PdCu₃_C nanoparticles are (111) and (100), respectively (as shown in Figure 4c, 4d). Formation of intermetallic PdCu₃ was further confirmed by SAED patterns containing (111), (200), (300), (311) planes of the ordered structure (as shown in Figure 4e, 4f).

In yet another embodiment of the present disclosure, in order to obtain more insight into the mechanism of the formation of spherical and cubic nanocrystals with changing solvent, we have studied some controlled experiments. Though, co-reduction is essential for ordered intermetallic nanoparticle formation, a mild reducing agent like oleylamine is never expected to co-reduce Pdⁿ/Pd (0.915V vs SHE), Cu VCu (0.34V vs SHE). But, the presence of CTAB probably affects the reduction rates of Pd and Cu leading to co-reduction of the corresponding salts to form ordered intermetallic nanostructure. In absence of CTAB, there is a formation of disordered Pdo.25Cuo_.75 alloy as has been observed. Apart in controlling the reduction rates, the tailoring of shape from sphere to cube is dependent on both oleic acid and CTAB. Only oleylamine favored the formation of spherical nanoparticle (PdCu₃_S), but the mixture of oleylamine and oleic acid (8: 1) lead to cube shaped nanoparticles (PdCu₃_C). Without being bound by theory, a probable explanation can be based on the specific binding of the surfactant/capping agent to a particular crystal facet. It is well known that oleylamine with - H₂ group binds onto the (100) crystal facet and hinders the growth along (100). On the other hand, oleic acid with a carboxylic group, -COOH has a selective binding on the (111) facet, thus facilitating the growth along (100) direction. Increase in the amount of oleic acid leads to the more condensation reaction forming mainly amide which leads to the uncontrolled growth of the nanoparticles. Additionally, CTAB also plays an important role in cube formation as no cube shaped particle was formed in the absence of CTAB. Firstly, CTAB preferentially binds parallel to the (110) planes of the crystals and favours the growth along (100) direction. Secondly, the presence of CTAB can affect the reduction rates of the Pd and Cu salt by the pre-formation of Cu nanocrystal although the standard reduction potential (E) for Pdⁿ/Pd (0.98V) is more positive than that of Cu VCu (0.34V). Finally, galvanic replacement of Cu nanocrystals with Pd species in the solution leads to the formation of cube shaped nanoparticles.

In another embodiment of the present invention, oxygen reduction reaction in alkaline medium has been studied. The linear polarization curves for PdCu₃_S and PdCu₃_C and commercial Pt/C (in which current densities are normalized with geometric surface area of the glassy carbon electrode) show that the activity in terms of the parameters like onset potential as well as the half wave potential for PdCu₃_C catalyst is much better than PdCu₃_S and state-of-the-art material Pt/C (as shown in Figure 6a). As seen from the plots (Figure 6a), the half wave potential observed on the catalysts were 0.81V (PdCu₃_C), 0.79 V (Pt/C) and 0.75 V (PdCu₃_S). The mass activity observed on PdCu₃_C is twice as higher than that observed on Pt/C at 0.8V (as shown in Figure 6d). Also, when compared to spherical morphology, PdCu₃_C show an onset 50 mV positive shifted and 4 times higher mass activity. This illustrates that one can tune the activity of the catalyst by tuning the morphology. Our experiments as well as electronic structure calculations show that by tuning the morphology of the catalyst a favourable adsorption sites for oxygen molecule on the catalyst were available and it increase the kinetics as well as activity. The tafel slope (as shown in Figure 6b) observed on the catalyst for low current region follows the order PdCu _C< Pt/C< PdCu _S. On Pt/C a slope value close to 60mV/dec is observed which is due to Temkin like adsorption. On PdCu₃_S to PdCu₃_C the value is decreased from 71mV/ dec to 56mV/dec. This indicates that on PdCu₃_C, the adsorption of intermediates is more facile than on the other catalyst. This experimental observation also confirms that the tuning the morphology of the catalyst results in a better pathway for the reaction. It can be seen from the K-L plot, on PdCu₃_C, the number of electrons found to be 3.95 and which indicates 97% direct conversion of oxygen to water while that for PdCu₃_C, it is around 3.56 and this corresponds to only 78% direct pathway of reduction of oxygen. In other words, on PdCu₃_C, the amount of hydrogen peroxide formed during reduction of oxygen is only 3% while that on spheres this is around 22% which indicates four electron transfer to oxygen to form water is more favourable for cube compared to sphere. In all potential, the kinetic current density observed for the PdCu₃_C catalyst is better than PdCu₃_S. The number electrons involved in the reduction of one molecule of oxygen on the catalyst is in the order of PdCu₃_S< Pt/C< PdCu₃_C and that for kinetic current density also follows the same order. This clearly shows that on the PdCu₃_C, facile reaction kinetics is observed as compared to other catalysts. In addition to the high intrinsic and mass activities, PdCu₃_C shows remarkable durability towards the electrochemical operation (Figure 7). The durability of the PdCu₃_C was evaluated by using chronoamperometric method at 0.67V (vs RHE) (as shown in Figure 7). The PdCu₃_C retains almost similar current density (-80% retention) after a 30000s of chronoamperotric i-t test, indicating the long-term stability of the catalyst (as shown in Figure 7).

In another embodiment of the present invention, ethanol oxidation reaction in alkaline medium on the same catalysts has been studied. PdCu₃_C was found to have more electrochemically active surface area (ECS A) compared to PdCu₃_S, commercial Pd/C and Pt/C indicating better activity of the cube shaped catalysts. The ECS A of the PdCu₃_C catalyst is 2.4 times better than Pt/C. A significant increase in mass normalized current density is observed for the sample PdCu₃_C in comparison with commercial Pd/C (1.2 times), Pt/C (12 times) for the same loading for the 1^st cycle (as shown in Figure 8). The ethanol oxidation efficiency of PdCu₃_C was better than its counterpart, PdCu₃_S, Pd/C, Pt/C as observed in the cycling study. After 500^th cycles of EOR, specific activity of the PdCu₃_C catalysts is -68 times and mass activity is -150 times higher than Pt/C whereas for the Pd/C catalyst also there is a large degradation of activity after 500^th cycle (as shown in Figure 9). Therefore, the catalyst is highly active till 500^th cycle of EOR. The high stability of the catalyst in terms of current density up to such large cycle life infers that the catalyst is greatly resistant to surface poisoning. Interestingly, the current density of Pt/C and Pd/C has dropped by ~ 2.7 times and -2.4 times respectively at the 500^th cycle as compared to the 2^nd cycle. In addition, as expected, the efficient catalyst PdCu₃_C showed increase in mass activity by -2.1 times. The chronoamperometric (CA) measurement for PdCu₃_C, PdCu₃_S, commercial Pd/C and Pt/C at -0.2V vs. Hg/HgO for the catalytic ethanol oxidation of 1M ethanol in 1M KOH for 1000s was carried out to follow the electrochemical stability of the catalyst (as shown in Figure 10). The stability of PdCu₃_C is better than the catalysts. The order of decay observed was in the order of PdCu₃_C < PdCu₃_S< Pd/C<Pt/C, with Pt/C showing -157 times more decay profile than PdCu₃_C between 900-1000s.

In another embodiment of the present invention, local structural data from EXAFS (as shown in Figure 11) evidenced an enhancement in the atomic disorder in the spherical nanoparticles compared to the cubic. This can be readily appreciated from the data Fourier transform magnitude of the EXAFS oscillation from the Pd K-edges (reduced amplitude for the spherical nanoparticles). In general in nanoparticles, the size reduction will introduce an increase in the surface to volume ratio. Such an effect will appear as a hugely decreased average near-neighbour co-ordination number in an EXAFS model fit. Relatively smaller average particle-size of the spherical nanoparticles compared to cube, evidenced from the TEM results, is expected to result in an increased atomic disorder. The cube and sphere nanoparticle show similar Pd-Cu local bond distance and the mean square relative displacement, however, in both cases, EXAFS fit provides a reduced near neighbour coordination number in agreement with an increased atomic disorder. Besides, enhancement in the Pd co-ordination number leads to more charge-transfer interaction with Cu in cubic PdCu₃ which results in the enhancement of the activity. A more complete understanding can be obtained by reference to the following specific examples, which are provided for purposes of illustration only and are not intended to limit the scope of the disclosure. Nonetheless, the examples provided herein, form part of the detailed description of the instant disclosure.

EXAMPLES

Experimental details

Materials Used: Palladium acetylacetonate (Pd(acac)₂), oleylamine and nafion binder (5 wt%) were purchased from Sigma-Aldrich, hexadecyltrimethylammonium bromide (CTAB) were purchased from SDFCL and copper acetylacetonate (Cu(acac)₂) was purchased from Alfa Aesar. All the chemicals (more than 99% purity) were used as purchased without further purification. Millipore water of conductivity 18.2 MQcm was used for the synthesis and all other studies. Example 1:

Spherical ordered PdCu^ nanocrystals Synthesis: In a typical solvothermal procedure, 0.1 mmol Pd(acac)₂, 0.3 mmol Cu(acac)₂ and 75 mg CTAB were mixed together in 18 ml oleylamine with vigorous stirring and loaded in 23 ml Teflon lined autoclave. The autoclave was kept at 180°C for 24 hrs. After that, the autoclave was cooled to room temperature naturally. The obtained precipitate was repeatedly washed several times with a 1: 1 mixture of hexane and ethanol and dried in vacuum oven at 60°C for 6 hrs.

Cubic ordered PdCu^ nanocrystals Synthesis: In a typical solvothermal procedure, 0.1 mmol Pd(acac)₂, 0.3 mmol Cu(acac)₂ and 75 mg CTAB were mixed together in 8: 1 ratio of oleylamine and oleic acid (16 ml oleylamine, 2ml oleic acid) with vigorous stirring and loaded in 23 ml Teflon lined autoclave. The autoclave was kept at 180°C for 24 hrs. After that, the autoclave was cooled to room temperature naturally. The obtained precipitate was repeatedly washed several times with a 1: 1 mixture of hexane and ethanol and dried in vacuum oven at 60°C for 6 hrs.

Spherical disordered PdCu^ nanocrystals Synthesis: In a typical solvothermal procedure, 0.1 mmol Pd(acac)₂ and 0.3 mmol Cu(acac)₂ were mixed together in 18 ml oleylamine with vigorous stirring and loaded in 23 ml Teflon lined autoclave. The autoclave was kept at 180°C for 24 hrs. After that, the autoclave was cooled to room temperature naturally. The obtained precipitate was repeatedly washed several times with a 1: 1 mixture of hexane and ethanol and dried in vacuum oven at 60°C for 6 hrs.

Example 2:

Characterization :

PXRD measurements were done at room temperature on a Rigaku Miniflex X-ray diffractometer with Cu-K_a X-ray source (λ = 1.5406 A), equipped with a position sensitive detector in the angular range 20° < 2Θ < 80° with the step size 0.02° and scan rate of 0.5 s/step calibrated against corundum standards. The experimental patterns were compared to the pattern simulated from the data reported in the literature (Figure 2).

Quantitative microanalysis on all the samples were performed with a FEI NOVA NANOSEM 600 instrument equipped with an ED AX^® instrument. Data were acquired with an accelerating voltage of 20 kV and a 100 s accumulation time. The EDAX analysis was performed using P/B-ZAF standardless method (where, Z = atomic no. correction factor, A = absorption correction factor, F = fluorescence factor, P/B = peak to background model) on selected spots and points (Figure 3). TEM and high resolution TEM (HRTEM) images, selected area electron diffraction (SAED) patterns were collected using a JEOL 200 TEM instrument. Samples for these measurements were prepared by dropping a small volume of sonicated nanocrystalline powders in ethanol onto a carbon-coated copper grid (Figure 4). Pd and Cu K-edge EXAFS measurements were carried out at 20-BM-B beamline of the APS synchrotron. EXAFS measurements were repeated at the BM-26A beamline of the ESRF. In both cases, measurements were carried out in transmission mode, by homogenously mixing the sample with an inert cellulose matrix to have an absorption edge jump close to one. Standard data analysis procedure was used to extract the EXAFS signal from the measured absorption spectra (Figure 11).

XPS measurement has been performed with Mg-Ka (1253.6 eV) X-ray source with relative composition detection better than 0.1% on an Omicron Nanotechnology spectrometer (Figure 2).

All electrochemical experiments were carried out using three-electrode cell with CH660c potentiostat/galvanostat workstation at ambient temperature. Catalyst modified glassy carbon electrode of 3 mm diameter used as working electrode. The electrocatalyst inks were prepared by adding 3 mg of material in isopropanol water mixture (1: 1) along with 5 μΐ_^ of 5 wt% Nafion as a binder. Resultant mixture is ultra- sonicated for 30 min. to get a fine dispersion. The dispersion was drop casted on glassy carbon surface to get a catalyst loading of 0.2 mg/cm . The large area Pt plate and mercury/mercuric oxide (MMO) electrode were used as counter and reference electrode respectively. The MMO electrode was calibrated and all the potentials were reported with respect to RHE. All the experiments were carried out in 0.5 M KOH solution. The kinetics of the oxygen reduction reaction on the catalysts was studied by rotating disk electrode (RDE) experiments. The rotation rate of the electrodes for RDE measurements were precisely controlled by pine instruments which is co-operated with the CH660C potentiostat/ galvanostat. Linear scan technique is used to get the polarization curve for the catalyst at a scan rate of 5 mV/sec in oxygen saturated 0.5 M KOH solution at different rotation rate (Figure 5-10).

Claims

Claims:

1. An electrocatalyst for a fuel cell comprising a shape-tailored ordered PdCu₃ nanostructure.

2. The electrocatalyst as claimed in claim 1, wherein the shape-tailored ordered PdCu₃ nanostructure is bifunctional electrode material and active both as anode and cathode.

3. The electrocatalyst as claimed in claim 1, wherein the shape-tailored ordered PdCu₃ nanostructure is having spherical or cubic morphology.

4. The electrocatalyst as claimed in claim 1, wherein the shape-tailored ordered PdCu₃ nanostructure comprises uniformly distributed Pd and Cu on the surface of nanoparticles.

5. The electrocatalyst as claimed in claim 1, wherein the electrocatalyst comprising cubic PdCu₃ has half wave potential of 0.81V compared to 0.79 V half wave potential of Pt/C.

6. The electrocatalyst as claimed in claim 1, wherein the electrocatalyst comprising cubic PdCu₃ has mass activity twice as higher than that observed on Pt/C, at 0.8V.

7. The electrocatalyst as claimed in claim 1, wherein the electrocatalyst is ~3 times better for ORR and - 150 times better for EOR in comparison with Pt/C.

8. The electrocatalyst as claimed in claim 1, wherein the electrocatalyst comprising cubic PdCu₃ shows lower tafel in comparison with Pt/C.

9. A method of synthesizing shape tailored ordered intermetallic nanomaterial of PdCu₃, said method comprising:

(i) forming a reaction mixture in a reaction vessel, said reaction mixture comprising:

(a) a palladium precursor,

(b) a copper precursor,

(c) a hexadecyltrimethylammonium bromide (CTAB), and

(d) a solvent;

(v) washing said precipitate of step (iv) with a 1: 1 (vol. ratio) mixture of hexane and ethanol and drying at about 60-80°C for about 6-8 hrs to obtain a shape tailored ordered intermetallic nanomaterial of PdCu₃, wherein the shape tailored ordered intermetallic nanomaterial of PdCu₃ is synthesized without using any external reducing agent.

10. The method as claimed in claim 9, wherein the palladium precursor is palladium acetylacetonate.

11. The method as claimed in claim 9, wherein the copper precursor is copper acetylacetonate.

12. The method as claimed in claim 9, wherein the solvent is selected from the group consisting of oleylamine, oleic acid and combination thereof.

13. The method as claimed in claim 9, wherein the shape tailored ordered intermetallic nanomaterial of PdCu₃ is Spherical ordered PdCu₃ nanocrystal, when the solvent is oleylamine.

14. The method as claimed in claim 9, wherein the shape tailored ordered intermetallic nanomaterial of PdCu₃ is Cubic ordered PdCu₃ nanocrystal, when the solvent is 8: 1 volume ratio mixture of oleylamine and oleic acid.

15. The method as claimed in claim 9, wherein the shape tailored ordered intermetallic nanomaterial is synthesized without using high temperature and external strong reducing agent.

16. The method as claimed in claim 12, wherein the oleylamine is used as a solvent, stabilizer and reducing agent.

17. The method as claimed in claim 9, wherein the CTAB is used as a surfactant as well as reduction potential modulating agent.

18. The method as claimed in claim 9, wherein the shape tailored ordered intermetallic nanomaterial is used as electrocatalyst in a fuel cell.

19. A Spherical ordered PdCu₃ nanocrystal as obtained by the process as claimed in claim 13, wherein said Spherical ordered PdCu₃ nanocrystal has a particle size of about 6-10 nm and with (11 l)-oriented faces or facets.

20. A Cubic ordered PdCu₃ nanocrystal as obtained by the process as claimed in claim 14, wherein said Cubic ordered PdCu₃ nanocrystal has a particle size of about 7-10 nm with (lOO)-oriented faces or facets.