WO2019186480A1

WO2019186480A1 - An electrochemical catalyst composition

Info

Publication number: WO2019186480A1
Application number: PCT/IB2019/052586
Authority: WO
Inventors: Sai Siddhardha R S; Sri Harsha AKELLA; Ebenezer D; Alkesh Ramdas AHIRE; Anirban Ghosh; Nawal Kishor Mal
Original assignee: Tata Chemicals Limited
Priority date: 2018-03-29
Filing date: 2019-03-29
Publication date: 2019-10-03

Abstract

An electrochemical catalyst comprising a metallic nanoparticle is disclosed. An electrochemical catalyst comprises; a metallic nanoparticle; a carbon support; and a tannic acid. The electrochemical catalyst is having at least 15% increased electro catalytic surface area as compared to the catalyst not having the tannic acid.

Description

AN ELECTROCHEMICAL CATALYST COMPOSITION

Field of Invention

The present disclosure relates to an electrochemical catalyst composition and a process of preparing same.

Background

Noble metals are widely utilized in various electro catalysis reactions and Fuel cell is a classic example this. Among these catalysts, platinum and ruthenium based materials are the most preferential candidates as they hold high activity towards hydrogen oxidation and oxygen reduction reactions. Abundant research has been carried out on these lines, in the sphere of energy conversion and storage. During the screening of these catalysts comparison is often sought by normalizing kinetic currents to their corresponding surface areas. Hence, the measurement of electrochemical active surface area (ECS A) is quintessential to decipher an insight into the relationship between structure and activity of the catalyst. The charge densities associated with the hydrogen adsorption - desorption curves in the cyclic voltammograms are conventionally used to estimate the real surface area of these catalysts. A high value of ECSA (of the nanocatalysts) figuratively corroborates to high catalytic activity of the nanocatalysts.

Very often unwantedly synthetic procedures end up in producing catalyst that yields low ECSA. This translates to reduced activity subsequently the synthesized batch becomes less useful. This causes undesired losses of Pt catalysts that are otherwise precious.

Therefore, there is a need to develop a simple and cost effective approach to maximize the ECSA and prevent losses of noble metals. Summary

An electrochemical catalyst comprising a metallic nanoparticle is disclosed. An electrochemical catalyst comprises; a metallic nanoparticle; a carbon support; and a tannic acid. The electrochemical catalyst is having at least 15% increased electro catalytic surface area as compared to the catalyst not having the tannic acid. Preferably, the electrochemical catalyst is having at least 15% increased electro catalytic surface area as compared to the catalyst not having the tannic acid.

In accordance with an embodiment of the invention, the metallic nanoparticle is selected from Pt nanoparticle, Au nanoparticle, Pd nanoparticle, bimetallic Pt nanoparticles, bimetallic Au nanoparticles, bimetallic Pd nanoparticles, trimetallic Pt nanoparticles, trimetallic Au nanoparticles, trimetallic Pd nanoparticles, and combination thereof.

A process of preparing the said catalyst is also disclosed.

Brief Description of Drawings Figure 1 Cyclic Voltammogram depicting the ECSA of a commercially available catalyst.

Figures 2 CVs depicting the ECSA of electrochemical catalyst for a control sample (a) and for examples prepared according to the example of the invention (b to d).

Figure 3 CVs depicting the ECSA of electrochemical catalyst for a control sample (a) and for examples prepared according to the example of the invention (b to d). Detailed Description

For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alterations and further modifications in the disclosed composition and method, and such further applications of the principles of the disclosure therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the disclosure and are not intended to be restrictive thereof.

Reference throughout this specification to“one embodiment”“an embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrase“in one embodiment”,“in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The disclosure relates to an electrochemical catalyst and a process for producing same. The electrochemical catalyst comprises a metallic nanoparticle, a carbon support, and a tannic acid. The electrochemical catalyst has increased electrochemical activity compared to catalyst having not having the tannic acid.

In accordance with an embodiment of the invention, the metallic nanoparticle is selected from Pt nanoparticle, Au nanoparticle, Pd nanoparticle, bimetallic Pt nanoparticles, bimetallic Au nanoparticles, bimetallic Pd nanoparticles, trimetallic Pt nanoparticles, trimetallic Au nanoparticles, trimetallic Pd nanoparticles, and combination thereof. The bimetallic and trimetallic nanoparticles are well known in the art and prepared by any known method. The Auxiliary metals for the bimetallic and trimetallic combinations are selected from but not limited to Ni, Co, Ti, Sn, W, Ag, Rh, Pt, Pd, Au, Ru and Fe. The bimetallic and process of synthesizing the same has been disclosed but not limited to the Indian patent application number 02841/MUM/2013.

In accordance with a preferred embodiment of the invention, the metallic nanoparticle is selected from Pt nanoparticle, bimetallic Pt nanoparticles, trimetallic Pt nanoparticles, and combination thereof.

In accordance with an embodiment of the invention, the carbon is selected from Vulcan carbon, graphene, graphite, fullerenes, and carbon nanotubes.

In accordance with an exemplary embodiment of the invention, the metallic nanoparticle is supported on the carbon. The synthesis of carbon supported metal nanoparticle is well known in the art. By way of an example, the synthesis of the carbon supported metal nanoparticle has been disclosed but not limited to Indian patent application number 00096/MUM/2011.

In accordance with an aspect of the invention, the ratio of metallic nanoparticle and tannic acid ranges from 500 to 1 to 5 to 1 by weight. The tannic was obtained from the commercial source.

In accordance with an aspect of the invention, the ratio of metallic nanoparticle and carbon ranges from 1 : 20 to 1.5 : 1 by weight. In accordance with an embodiment of the invention, the electrochemical catalyst is having at least 15% increase in electro catalytic surface area as compared to the catalyst not having the tannic acid. Preferably, the electrochemical catalyst is having at least 20% increase in electro catalytic surface area as compared to the catalyst not having the tannic acid.

In accordance with an embodiment of the invention, the electrochemical catalyst is having at least 15% increase in Faradaic charge as compared to the catalyst not having the tannic acid. Preferably, the electrochemical catalyst is having at least 20% increase in Faradaic charge as compared to the catalyst not having the tannic acid. In accordance with an embodiment of the invention, the electrochemical catalyst of the present invention is used in Fuel cell.

In accordance with an embodiment of the invention, the electrochemical catalyst of the present invention is mixed with a solvent, binder or combination of both for use as electrochemical catalyst. The solvent may be selected from any know solvent but not limited to water and isopropyl alcohol. The binder may be selected from any know binder but not limited to nafion.

ECSA calculation method:

The ECSA was obtained by electrochemical cycling between -0.3V to 1.3V in 0.5M H2SO4 electrolyte. The typical cyclic voltammetry (CV) of HiSPEC® 4000 catalyst in acid electrolyte is depicted in Figure 1. It comprises of 3 regions at different potential windows namely hydrogen region, double layer region and the oxygen region. In the central double layer region, a low but constant current density is observed in the both anodic and cathodic sweeps. This region is attributed to the charge that has been accumulated on the electrode owing to the double layer formation. This charge accumulation is associated to the non-Faradaic current density and varies with the scan rate proportionately. The oxygen region is found at more positive potentials. This region is associated with platinum monoxide formation in the forward sweep and its corresponding desorption in the reverse sweep to give rise to monometallic platinum.

In the Hydrogen region, the proton reduction do take place by the H⁺ adsorption from the acid electrolyte in the reverse sweep 0V to -0.3V vs. Ag/AgCl reference electrode. The subsequent forward sweep infers an oxidation (-0.3 V to 0 V vs. Ag/AgCl reference electrode), wherein the adsorbed hydrogen desorbs with the release of electrons. The charge (QH) associated with these curves in this region is used to calculate the active area of the catalyst. The electrochemical reaction follows the following equation (Equation 1):

2 H a,_|„_t.o„ + 2e + site ÷÷ H_{2 ad onsite} (1)

The number of electrons that are released during the low potential oxidation of hydrogen indicates, the number of H atoms that are desorbed from the catalyst. Furthermore, these desorbed atoms furnish data that pertains to the number of adsorption sites present on the electrode’s surface. Since the binding of H atom to Pt atoms is 1:1, this in turn has been utilized to decipher the active surface area of the catalyst.

The expression for the total charge of the H atom desorption can be presented by following equation (Equation 2):

Whereas, El and E2 represent the electrochemical potential range for desorption of H atom. The total charge that corresponds to the H atom desorption can be related to the integral of the curve for a certain duration of the potential interval, where H atoms are being desorbed.

The above value of the charge under the curve can be calculated directly from the Biologic Science Instruments with EC-Lab^® software that has an in built program to measure. The total charge measurements using EC-Lab^® is done by analyzing the selected portion of the graph with integral tool. This is marked as green crosslined total charge zone in Fig. 1 for readers understanding. The total charge comprises of both Faradaic and Capacitative (non-Faradaic) charge combinations. The Capacitative charge zone is manually identified by observing the flat zone in the total charge zone as labelled by red shade in Figure 1. The charge value of this zone can be obtained by multiplying voltage with current for the identified area (red shaded in Fig. 1) in the CV. The Faradaic part is measured by subtracting the non Faradaic component from the total charge evaluated using the software. During the measurements, the non Fardaic component does not reflect any value in measuring the active surface area of the Pt catalyst and is due to mere accumulation of charges and therby need to be nullified. Thus obtained Faradaic charge is further divided with the scan rate to nullify the scan rate based diffusion effects.

Using the charge value, the ECS A can be calculated using Equation 3. ECSA = Q_H / (m_p, x 210pC. cm ²) (3)

Where, 210 mC.cm-2 is the charge associated with monolayer H adsorption. mPt is the mass of the platinum in grams that has been deposited over the electrode. QH in pC units is the measured charge after subtracting Capacitative charge and dividing with scan rate. The ECSA value finally obtained is converted to m²/g across all catalyst sample testing.

Preparation of Electrode:

The electrochemical catalyst of the present invention is coated on the glassy carbon electrodes (GCEs) to study the ECSA by doing a potential cycling. All the catalyst ink dispersions are physically mixed well using stirrer and Spinix vortex shaker, prior to their casting on electrode.

The GCEs are polished over alumina slurries of particle sizes 1.0, 0.3 and 0.05 pm, until mirror like electrode surface has been obtained. Further, the electrodes are scrupulously rinsed with distilled water and sonicated in ethanol-water mixture for about 30 seconds. Followed by electrode’s surface was dried at room temperature. Finally carbon supported platinum catalyst plat GCE was fabricated by casting the ink dispersion on a clean GCE and subsequently dried the electrode at room temperature under inert gas condition. Example 1 lOmg of Johnson Matthey catalyst (HiSPEC® 4000) was weighed in l5ml culture tube. 50m1 water, 200mg of 20% w/w Nafion solution along with 1850 mΐ isopropanol was added in the tube. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. This slurry was devoid of any amounts of Tannic acid and served as a blank sample during subsequent experimentation. Furthermore, 10 pl of this slurry was casted over scrupulously cleaned GCE of 0.07 lcm2 geometric area. The ECSA was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H₂S0₄ electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 40% /C - JM.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 2 (a). The ECSA was measured from the average of 20 cycles and found to be 69.9 m²/g.

Example 2 lOmg of Johnson Matthey catalyst was weighed in l5ml culture tube. 50m1 water,

O.lmg of Tannic acid (Sigma-Aldrich), 200mg of 20% w/w Nafion solution and 1850 mΐ isopropanol was added in the culture tube. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. 10 mΐ of this slurry was casted over scrupulously cleaned GCE of 0.07 lcm2 geometric area. The ECSA was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H₂SO₄ electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 40% /C - JM -TA 0.01.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 2 (b). The ECSA was measured from the average of 20 cycles and found to be 66.7 m²/g. Example 3 lOmg of Johnson Matthey catalyst was weighed in l5ml culture tube. 50m1 water, O.Olmg of Tannic acid, 200mg of 20% w/w Nafion solution and 1850 mΐ isopropanol was added in the culture tube. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. 10 pl of this slurry was casted over scrupulously cleaned GCE of 0.07 lcm2 geometric area. The ECSA was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H₂S0₄ electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 40% /C - JM -TA 0.1.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 2 (c). The ECSA was measured from the average of 20 cycles and found to be 83.1 m²/g.

Example 4 lOmg of Johnson Matthey catalyst was weighed in l5ml culture tube. To this 50m1 water, 0.5mg of Tannic acid, 200mg of 20% w/w Nafion solution and 1850 mΐ isopropanol was added. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. 10 mΐ of this slurry was casted over scrupulously cleaned GCE of 0.07 lcm2 geometric area. The ECSA was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H₂S0₄ electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 40%

/C - JM -TA 0.5.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 2 (d). The ECSA was measured from the average of 20 cycles and found to be 62.8 m²/g. Example 5

In a 1 L conical flask, 0.74 g of K₂PtCl₆, 0.26mg RuCE xH20 and 25 mL of 5 mM MPSA solution were taken, and 400 mL of H₂0 was added to it, subsequently the solution was cooled to 0 °C. 0.60 gm of Vulcan carbon was added to the reaction mixture and stirred at 0°C for getting homogeneous dispersion. 20 mL of 0.4 M NaBH4 solution was added to the reaction mixture drop wise within a span of 1 min. The stirring was continued for 30 min under ice cold conditions and then filtered, washed with copious amounts of water. The material was vacuum dried at room temperature. The organic ligand of the material was removed by oxidizing -SH group with 400 mL of 15% H₂0₂ at room temperature for 1 hour. The oxidized samples were again filtered, washed with copious amounts of water and dried under vacuum. Final yield of the sample was 1 g. This sample was termed as Pt 25% - Ru 15% /C. 10 mg of Pt 25% - Ru 15% /C catalyst was weighed in l5ml culture tube. To this

50pl water, 200mg of 20% w/w Nafion solution along with 1850 mΐ isopropanol was added. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. This slurry was devoid of any amounts of Tannic acid and served as a blank sample during subsequent experimentation. 10 mΐ of this slurry was casted over scrupulously cleaned GCE of 0.07 lcm² geometric area. The ECSA was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H₂S0₄ electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 25% - Ru 15% /C.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 3 (a). The ECSA was measured from the average of 20 cycles and found to be 27.0 m²/g.

Example 6:

10 mg of Pt 25% - Ru 15% /C catalyst was weighed in l5ml culture tube. 50m1 water, O.Olmg of Tannic acid, 200 mg of 20% w/w Nafion solution and 1850 mΐ isopropanol was added in the culture tube. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. 10 pl of this slurry was casted over scrupulously cleaned GCE of 0.07 lcm² geometric area. The ECS A was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H₂S0₄ electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 25% - Ru 15% /C -TA 0.01 hereupon.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 3 (b). The ECSA was measured from the average of 20 cycles and found to be 43.8 m²/g.

Example 7: 10 mg of Pt 25% - Ru 15% /C catalyst was weighed in l5ml culture tube. 50m1 water, O.lmg of Tannic acid, 200 mg of 20% w/w Nafion solution and 1850 mΐ isopropanol was added in the culture tube. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. 10 mΐ of this slurry was casted over scrupulously cleaned GCE of 0.07 lcm2 geometric area. The ECSA was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H2SO4 electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 25% - Ru 15% /C -TA 0.1.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 3 (c). The ECSA was measured from the average of 20 cycles and found to be 39.46 m²/g. Example 8:

10 mg of Pt 25% - Ru 15% /C catalyst was weighed in l5ml culture tube. 50m1 water, 0.5mg of tannic acid, 200 mg of 20% w/w Nafion solution and 1850 mΐ isopropanol was added in the culture tube. The slurry was subjected to a series of stirring and mechanical shaking for 1 hour. Black syrupy catalyst slurry was obtained. 10 mΐ of this slurry was casted over scrupulously cleaned GCE of 0.071 cm² geometric area. The ECS A was calculated from the Faradaic curve in CV between -0.3V to 1.1V. The operating conditions were 0.5M H₂S0₄ electrolyte, 50mV/sec scan rate, and 20 cycles. This sample was termed as Pt 25% - Ru 15% /C -TA 0.5.

CV depicting the ECSA of electrochemical catalyst has been shown in figure 3 (d). The ECSA was measured from the average of 20 cycles and found to be 32.26 m²/g.

Results and interpretation: The ECSA values along with the Fardaic and Capacitative areas are tabulated in

Table 1.

The composition of the present invention has resulted in an invariable increase in the ECSA. The benezene framework of tannic acid possibly binds non covalently to the carbon support, whereas, the phenolic/quinones functionalities of tannic acid has an inherent affinity to widespread in the water solvent. This causes an efficient drag of the catalyst into the water solvent therby efficient exposure of active metal nanoparticle sites. Possibly, this is the main reason for the enhancements in the ECSA of metal nanoparticle catalyst. To substantiate this further, the Faradaic area is only increasing but no the Capacitative area with tannic acid addition across samples. This corroborates that the entire enhancement phenomenon is solely due to H adsorptions but not due to any kind of charge accumulations or bindings.

Tannic acid alone was tested in an acid electrolyte, without any catalyst and did not led to any polarization peaks (-1.3V-0V) with appreciable current density in the hydrogen region of the Cyclic Voltammogram.

Industrial Applicability Nobel metal electrocatalyst plays an important role in fuel cells, catalytic converters and many chemical processes used in industry. Nobel metals are very expensive materials and therefore it becomes bottleneck in use as catalyst in electrolysers. The present disclosed invention enhances the performance of the catalyst by increasing the ECSA. This allows industry to use such electrochemical catalyst at industrial scale by minimizing the cost bottleneck. The process of making the proposed ECSA is also very simple and does not require very much specialized equipment.

Claims

We Claim:

1. An electrochemical catalyst comprises; a metallic nanoparticle; a carbon support; and a tannic acid.

2. An electrochemical catalyst of claim 1, wherein the electrochemical catalyst is having at least 15% increased electro catalytic surface area as compared to the catalyst not having the tannic acid.

3. An electrochemical catalyst of claim 1, wherein the metallic nanoparticle is selected from Pt nanoparticle, Au nanoparticle, Pd nanoparticle, bimetallic Pt nanoparticles, bimetallic Au nanoparticles, bimetallic Pd nanoparticles, trimetallic Pt nanoparticles, trimetallic Au nanoparticles, trimetallic Pd nanoparticles and combination thereof.

4. An electrochemical catalyst of claim 1, wherein the carbon is selected from Vulcan carbon, graphene, graphite, fullerenes, and carbon nanotubes.

5. An electrochemical catalyst of claim 1, wherein the electrochemical catalyst is having at least 15% increased electro catalytic surface area as compared to the catalyst not having the tannic acid.

6. An electrochemical catalyst of claim 1, wherein the tannic acid ranges from the ratio of metallic nanoparticle and tannic acid ranges from 500 to 1 to 10 to 1 by weight.

7. An electrochemical catalyst of claim 1, wherein the tannic acid ranges from the ratio of metallic nanoparticle and carbon ranges from the ratio of metallic nanoparticle and carbon ranges from 1 : 20 to 1.5 : 1 by weight.

8. An electrochemical catalyst of claim 1, wherein further comprises a solvent, a binder or combination of both.

9. A Fuel cell comprising the electrochemical catalyst of claim 1.