WO2020115758A1

WO2020115758A1 - Fe/Fe3C ENCAPSULATED N-CNT ELECTRODE FOR ELECTROCHEMICAL APPLICATIONS AND METHOD OF PREPARATION THEREOF

Info

Publication number: WO2020115758A1
Application number: PCT/IN2019/050766
Authority: WO
Inventors: Sundara Ramaprabhu; Sreetama GHOSH; Meenakshi Seshadhri GARAPATI
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2018-12-05
Filing date: 2019-10-15
Publication date: 2020-06-11

Abstract

The invention relates to an electrode material, synthesis of an electrode material, a polymer electrolyte membrane (PEM) assembly with the electrode material. The electrode material includes a transition metal enriched nitrogen doped carbon nanotubes (ME-NCNTs). The ME-CNT exhibits a high surface area in the range of 180-220 m² g^-1 and porosity in the range of 2-3 nm which improves the electrochemical efficiency. The PEM cell includes a membrane electrode assembly with ME-NCNT in the range of 0.1 to 2 mg/cm² loaded as the anode or cathode or both. The PEM cell includes an anolyte and CO₂ containing catholyte. The CO₂ at the catholyte is reduced to formic acid. The ME-NCNT exhibits a very stable three dimensional structure which enables high conversion of CO₂ to formic acid of up to 97%. The formic acid exhibits 99.99% purity.

Description

Fe/Fe₃C ENCAPSULATED N-CNT ELECTRODE FOR ELECTROCHEMICAL APPLICATIONS AND METHOD OF PREPARATION THEREOF

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to Indian Patent Application No. 201841045891 entitled Fe/Fe₃C ENCAPSULATED N-CNT ELECTRODE FOR ELECTROCHEMICAL APPLICATIONS AND METHOD OF PREPARATION THEREOF filed on December 5, 2018.

FIELD OF THE INVENTION

[0002] The invention generally relates to materials for electrodes and in particular to catalyst materials for use in polymer electrolyte membrane conversion cells.

DESCRIPTION OF THE RELATED ART

[0003] Polymer electrolyte membranes (PEM) are extensively used in fuel cells. In order to carry out the electrochemical reduction of CO2 to fuels, just the reverse principle of fuel cell can be employed. PEM CO2 conversion cells have the main advantage of preventing any product formed from reduction of CO2 from re-oxidation at the cathode. Further, in comparison with stationary electrolytes, the continuous circulation of electrolyte helps in maintaining adequate pH near the cathode surface that is necessary for CO2 reduction. In this regard, electrochemical CO2 reduction using PEM cells has been studied, though not much extensively.

[0004] Formic acid is an important intermediate obtained during chemical reactions. It has significant use in variety of areas such as fertilizers, pharmaceuticals, as chemical feedstock, in hydrogen storage and fuel cells. The development of catalysts with high efficiency and selectivity towards formic acid is thereby challenging. At present, different metal catalysts such as Ag, Au, Pt, Cu, Sn etc. and metal complexes have been studied for electrochemical reduction of CO2. Precious metal based catalysts show relatively high activity towards electrochemical CO reduction but their high cost and low abundance limit their usage. Heteroatom doped carbon materials possess large accessible surface area and adjustable active sites while metal-based catalysts have good catalytic activity but poor stability. Enhancement in electrochemical performance has been seen inducing the synergistic effects of both.

[0005] The US patent application US20170354953A1 discloses a method of fabricating a porous structure by growing graphitic carbon in an environment which includes a transition metal and compound containing a transition metal and any nitrogen-bearing compound. The disclosed compound does not yield a high formic acid production.

[0006] The Chinese patent application CN106972180A discloses synthesis of a nitrogen- doped carbon nanotube enriched with a transition metal. But a dispersant polyvinylpyrrolidone is used to bind Fe with the NCNT. There is a need to eliminate use of a polymeric dispersant for the fabrication of an electrode.

[0007] The Chinese application CN105540590A discloses production of a Fe3C nanowire enriched in N doped carbon nanotube. The application uses a zinc precursor, which requires further removal of zinc, thereby resulting in a complex process.

[0008] There is therefore a need for an electrode material that is cost efficient when compared to the precious metals. There further a need to obtain a catalyst that matches the electrochemical performance of a precious metal based electrode. Also, there is a need for an easy and efficient method for production of an electrode material.

SUMMARY OF THE INVENTION

[0009] The present invention discloses a ME-NCNT. Particularly a ME-CNT for use as an electrode for a PEM cell for the production of a carboxylic acid.

[0010] In various embodiments provided herein is a polymer electrolyte membrane (PEM) electrode material ME-NCNT. The ME-NCNT includes metal nanoparticles encapsulated within nitrogenated carbon nanotubes (NCNTs). The metal nanoparticles comprise Fe C, and the NCNTs, are nitrogenated (N) in the range of 3-6 wt.% N. [0011] In some embodiments, the electrode material is configured to be loaded on an electrode surface for use in an electrochemical cell at 0.1 to 2 mg/cm².

[0012] In many embodiments, the metal nanoparticles further comprise one or more transition metals selected from Co, Ni, Ru, Os and Eu and the Fe in the Fe C particles is substituted with the one or more transition metals.

[0013] In various embodiments, the electrode material exhibits a surface area in the range of 180-220 m² g ¹ and porosity in the range of 2-3 nm in diameter. In various embodiments, the material produces an X-ray diffraction pattern with peaks of 26.4 °, 37.7 °, 39.8 °, 40.7 °, 43.0 °, 43.8 °, 44.7°, 45.9°, 49.1 °, 51.9 °, 54.5 ⁰ and 58.2° corresponding to Fe/Fe3C encapsulated within nitrogen-doped carbon nanotubes.

[0014] In some embodiments, a polymer electrolyte membrane (PEM) electrochemical conversion cell includes one or more membrane electrode assemblies (MEA). The MEA includes a membrane layer sandwiched between a cathode and an anode. At least one of the cathode or anode comprises an electrode material ME-NCNT loaded on to its surface for use in an electrochemical cell at 0.1 to 2 mg/cm2. The ME-NCNT includes metal nanoparticles encapsulated within nitrogenated carbon nanotubes (NCNTs). The metal nanoparticles comprise Fe C and the CNTs are nitrogenated (N) (103) in the range 3-6 wt.% N.

[0015] In some embodiments, the anode is a Pt anode or a CNT anode. In various embodiments each MEA further includes an an anolyte flow channel and a catholyte flow channel.

[0016] In various embodiments, a process for generating formic acid using the (PEM) electrochemical conversion cell. The cell includes at least one MEA. The process includes providing deionized water through the anolyte flow channel of the MEA. C02 saturated deionized water is provided through the catholyte flow channel of the MEA. Applying a voltage in the range of 1.9 to 2.2 Volts for a time period in the range of 115-125 mins through the MEA. Formic acid is obtained by the reduction of dissolved C02 in the catholyte at a cathode output collector.

[0017] In some embodiments, the yield of product by CO conversion is at least 90%. In many embodiments, the purity of the formic acid is at least 99.99%. [0018] In various embodiments a method of preparing an electrode material comprising metal nanoparticle encapsulated nitrogen doped carbon nanotubes (ME-NCNT) is provided. Providing a first precursor comprising melamine and a second precursor comprising a transition metal chloride comprising FeCfi at a ratio ranging from 2: 1 to 1:2 as raw materials. Mixing the raw materials thoroughly in a volatile solvent to form a first solution. Drying the first solution to obtain a first product. Heating the first product to a temperature in the range of 600 to 900°C at a predetermined heating rate under a predetermined nitrogen flow rate to form a second product. Drying the second product to obtain the nitrogen doped metal carbide nanoparticle encapsulated carbon nanotubes (ME-NCNT).

[0019] In some embodiments, the transition metal chloride comprises a transition metal selected from one or more of Co, Ni, Ru, Os and Eu. In many embodiments, drying step is preceded by purification comprising washing the second product with an acid.

[0020] This and other aspects are disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0022] FIG. 1A depicts a 2D image of a Fe/Fe3C-NCNT electrode material.

[0023] FIG. IB depicts a 3D image of a Fe/Fe3C-NCNT electrode material.

[0024] FIG. 1C depicts a blown image of N doped with Fe/Fe3C.

[0025] FIG. 2 A depicts an exploded view of an electrochemical setup.

[0026] FIG. 2B depicts a side view of an assembled electrochemical setup.

[0027] FIG. 3 illustrates a polymer electrolyte membrane (PEM) electrochemical conversion cell.

[0028] FIG. 4 depicts a method of forming Fe/Fe3C-NCNT electrode material.

[0029] FIG. 5A depicts the XRD patterns of Fe/Fe3C. [0030] FIG. 5A depicts the XRD patterns of Fe/Fe3C formed at various pyrolysis temperatures.

[0031] FIG. 6A depicts XPS pattern of Fe/Fe3C-NCNT electrode material

[0032] FIG. 6B depicts the composition of various elements in Fe/Fe3C-NCNT.

[0033] FIG. 7A TEM images of Fe/Fe₃C-NCNT at 600° C.

[0034] FIG. 7B TEM images of Fe/Fe₃C-NCNT at 650° C.

[0035] FIG. 7C TEM images of Fe/Fe₃C-NCNT at 700° C.

[0036] FIG. 7D TEM images of Fe/Fe₃C-NCNT at 800° C.

[0037] FIG. 8A depicts working potential for PEM CO2 conversion cell after 120 min conversion.

[0038] FIG. 8B depicts UV absorption spectra of cathode reservoir outlet solution of Anode: NCNT, Cathode: NCNT.

[0039] FIG. 8C depicts UV absorption spectra of cathode reservoir outlet solution of Anode: Fe/Fe₃C-NCNT, Cathode: Fe/Fe₃C-NCNT.

[0040] FIG. 9 depicts Concentration of formic acid formed with reference to time.

[0041] FIG. 10 depicts chromatograms with standard formic acid solutions.

[0042] FIG. 11 depicts HPLC product analysis results of liquid phase samples generated from electrochemical CO2 reduction with Fe/Fe3C-NCNT on both anode as well as cathode.

DETAIFED DESCRIPTION OF THE EMBODIMENTS

[0043] While the invention was disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0044] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0045] In the following detailed description, references are made to the accompanying drawings that form a part hereof, and that show, by way of illustration, specific embodiments or examples. Although various aspects of the disclosure will be described using with regard to illustrative examples and embodiments, those disclosed embodiments and examples should not be construed as limiting.

[0046] In various embodiments, the invention discloses a polymer electrolyte membrane (PEM) electrode material 100 designated ME-NCNT. The electrode material as illustrated in FIG. 1A, is constituted of nitrogenated carbon nanotubes 103 (NCNT) within which metal nanoparticles 105 are encapsulated. In some embodiments the metal nanoparticles are Fe/Fe C nanoparticles. In some embodiments the NCNTs 103, are nitrogenated in the range of 3-6 wt.% N.

[0047] In various embodiments the NCNTs 103 may either be single walled nanotubes or multi-walled nanotubes. The nanotubes of the NCNTs 103 as shown in FIG. IB, may predominantly be formed of carbon atoms 110, while a few of the carbon atom sites may include nitrogen atoms 111. In some embodiments as illustrated in FIG. 1C, the metal nanoparticles 105 may include a shell of Fe C 120 surrounding a core of Fe 121. In various embodiments the electrode material 100, the metal nanoparticles may further comprise one or more transition metals other than Fe. The transition metals may be selected from Co, Ni, Ru, Os and Eu. The one or more transition metals may substitute for Fe in the Fe C particles. In some embodiments, the core of the metal nanoparticles 121 may fully react to form Fe C layer 120 and only a trace of transition metal may be present.

[0048] In some embodiments the electrode material 100 is configured to be loaded on an electrode surface for use in an electrochemical cell. The loading may in some embodiments be done at a coverage 0.1 to 2 mg/cm². In one embodiment the loading may be done at 1 mg/cm². In some embodiments the electrode material exhibits a surface area in the range of 180-220 m² g ¹ and porosity in the range of 2-3 nm in size.

[0049] In various embodiments the electrode material ME-NCNT is characterized in

X-ray diffraction as exhibiting peaks corresponding to 26.4 °, 37.7 °, 39.8 °, 40.7 °, 43.0 °, 43.8°, 44.7°, 45.9°, 49.1 °, 51.9 °, 54.5 ⁰ and 58.2° corresponding to Fe/Fe₃C encapsulated within nitrogen-doped carbon nanotubes. The XRD pattern characterizes the crystalline nature ME-NCNT (Fe/Fe3C@NCNT). At 26.4 ⁰ a broad peak appears corresponding to the (002) peak of graphitized carbon. A less conspicuous peak for NCNT appears at 42.6 ⁰ that is indexed as C (101). The diffraction peaks located at 37.7 °, 39.8 °, 40.7 °, 43.0 °, 43.8 °, 44.7 °, 45.9 °, 49.1 °, 51.9 °, 54.5 ⁰ and 58.2 ⁰ may be assigned to (112), (200), (120), (121), (210), (022), (211), (122), (212), (004) and (104) crystalline planes of Fe C respectively (JCPDS fde no. 89-2867).

[0050] In various embodiments a polymer electrolyte membrane (PEM) electrochemical conversion cell 200 as illustrated in FIG. 2A and 2B is disclosed. The cell 200 includes one or more membrane electrode assemblies (MEA). Each MEA comprises a polymer electrolyte membrane layer 201 sandwiched between an anode catalyst layer 202 and a cathode catalyst layer 203. In some embodiments, the cathode 203 or anode 202 is made of the electrode material ME-NCNT 100. In various embodiments, the ME-NCNT 100 may be loaded on to the electrode at 0.1 to 2 mg/cm². In one embodiment the loading of material 100 may be done at 1 mg/cm². [0051] In some embodiments the PEM cell 200 may include a ME-NCNT cathode catalyst layer, whereas the anode catalyst layer 202 may include a Pt/carbon layer, or a CNT layer. The catalyst layer is further enclosed within porous graphite blocks 205, current collector plates 207 and end plates 210. Gas to operate the conversion cell may be conveyed by an anolyte flow channel 211 on the anode side and a catholyte flow channel 213 on the cathode side, as shown in FIG. 2A and 2B.

[0052] In various embodiments, the PEM cell 200 is used for a process of generating formic acid. The experimental setup is as shown in FIG. 3. The PEM cell may include one or a plurality of MEA assemblies. An anolyte 301 is supplied through an anolyte flow channel 211. A catholyte 303 is supplied through a catholyte flow channel 213. The anolyte 301 for the generation of formic acid may include but is not restricted to deionized (DI) water. The catholyte 303 for the generation of formic acid may include but is not restricted to deionized (DI) water saturated with CO .

[0053] A voltage up to 2.2 Volts is applied across the anode and cathode of the PEM cell 200. The voltage supply is maintained for a predetermined time period. Doping of nitrogen and Fe in the carbon lattice results in the redistribution of charge and spin density, thereby positively charged atoms for CO adsorption is created at the cathode. Oxygen is released at the anode outlet 305. Electrons pass through the external circuit under the applied potential. At the cathode side, the protons and electrons recombine with the CO saturated DI water to reduce it to formic acid. Formic acid is collected at cathode outlet 307. In some embodiments, the yield of formic acid by the reduction of CO is at least 90%. In some embodiments a yield of up to 97% is obtained using the PEM cell 200 of the invention. In many embodiments, the purity of the formic acid obtained is at least 99.99%.

[0054] In various embodiments provided herein is a method 400 of preparing an electrode material as shown in FIG. 4. In a first step 401, a first precursor comprising melamine and a second precursor comprising a transition metal chloride are provided at a ratio ranging from 2: 1 to 1:2. In some embodiments the precursor may comprise FeCE or other transition metal chloride. In some embodiments the other transition metal chloride may be a transition metal chloride such as Co, Ni, Ru, Os or Eu. In step 403, the raw materials are thoroughly mixed in a volatile solvent to form a first solution. In some embodiments, the volatile solvent is one of ethanol or acetone. The first solution is dried to obtain a first product. In step 405, the first product is heated at a temperature in the range of 600- 900 °C for 3 h inside a tubular furnace under a nitrogen flow of 20 ml min ¹ with a heating rate of 5 °C min ¹ to obtain a second product. In one embodiment, in step 405, the temperature is 800 °C. In step 407 the second product is washed in an acid. The acid washed product is filtered and further dried to obtain ME-NCNT electrode material.

[0055] Theoretical and computational studies reveal that the doping of nitrogen and Fe in the carbon lattice results in the redistribution of charge and spin density, thereby creating positively charged atoms for CO2 adsorption. These results, therefore, confirm the indispensable role of nitrogen and Fe dopants in transforming pristine CNT into active electrocatalyst for CO2 reduction. The invention therefore provides a catalytic electrode material for use in a fuel cell configured such that Fe/Fe3C centers encapsulated inside the NCNTs act as active CO2 reduction sites apart from the doped N sites in the NCNTs, thus providing enhanced number of active sites for CO2 reduction.

[0056] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope. Further, the examples to follow are not to be construed as limiting the scope of the invention which will be as delineated in the claims appended hereto.

EXAMPLES

[0057] Example 1 - Synthesis of Fe/FesC -NCNT

[0058] A single step thermal decomposition technique was adopted using melamine as carbon and nitrogen source and ferric chloride hexahydrate (FeC13.6H20) as the metal precursor. Both the precursors were mixed thoroughly in ethanol and then the dried sample was collected and heated at 800 °C for 3 h inside a tubular furnace under a nitrogen flow of 20 ml min-1 with a heating rate of 5 °C min-1. The sample was washed with acid, filtered and dried and finally, nitrogen doped and metal encapsulated carbon nanotubes were obtained and designated as Fe/Fe3C-NCNT. The same synthesis method was followed to synthesize nitrogen-doped carbon nanotubes (NCNTs) without metal encapsulation when melamine: FeC13.6H20 was taken in 16: 1 ratio instead of 1 : 1.

[0059] Example 2 - C02 conversion analysis (full-cell measurement)

[0060] NCNT and Fe/Fe3C-NCNTs have been used as both anode and cathode catalyst material in Proton Exchange Membrane (PEM) CO2 conversion cell. Digital photographs of the entire full-cell experimental set-up and the PEM C02 conversion cell has been shown in Figure 2 and 3. A metal loading of 0.5 and 1 mg cm ² has been maintained at anode and cathode sides respectively for all the experiments. CO2 saturated DI water having a pH of 5-6 is used as the catholyte whereas only DI water is used as anolyte. The continuous circulation of electrolytes on both cathode as well as anode side provide moderate convective mixing and helps to maintain adequate pH necessary for CO2 reduction near the cathode catalyst. The anode was given a positive potential with respect to the cathode. The chemical reaction taking place at the anode due to water electrolysis is given by equation (1):

(1)

[0061] The protons are capable of passing to the anode through the proton exchange membrane to reach the cathode. Electrons pass through the external circuit under the applied potential. Oxygen is produced at the anode and is liberated to the atmosphere. At the cathode side, the protons and electrons recombine with the CO2 saturated DI water to reduce it to formic acid. The liquid product formed at the cathode outlet was analyzed after every 30 min by UV-Vis spectroscopy as well as by High Performance Liquid Chromatography (HPLC).

[0062] The cell potential plays a very vital role in the electrochemical CO2 reduction. The standard electrochemical reduction potential for the hydrogenation of CO2 to formic acid is -0.61 V vs. NHE. On the other hand, the theoretical oxidation potential to split water is +1.23 V vs. NHE. The (PEM) electrochemical conversion cell mimics a conventional proton exchange membrane fuel cell (PEMFC) with a reverse working principle. It is well reported that the practical open circuit potential of PEMFC is always less than 1.23 V due to the overpotential involved in the reaction mechanism. Similarly, the potential to electrolyze water in the anode side is expected to be higher than the theoretical potential. Now in order to determine an optimum potential needed for the maximum yield of formic acid after a certain interval of time, the cell was operated at various potentials keeping all other parameters constant. It has been demonstrated that formic acid formation rate is maximum when +2.1 V positive voltage is applied at the anode w.r.t. the cathode. No significant change in efficiency was observed at higher potentials.

[0063] Example 3: Formation of formic acid by varying the electrodes

[0064] Three different sets of anode and cathode catalysts have been demonstrated for CO2 conversion efficiency keeping all other parameters constant. Firstly, we tested with Pt/C (0.5 mg cm ²) as anode catalyst and Fe/Fe3C-NCNTs (1 mg cm ²) as the cathode catalyst. Secondly, NCNT (0.5 mg cm ²) as anode catalyst and NCNTs (1 mg cm ²) as the cathode catalyst. And, thirdly, Fe/Fe3C-NCNTs (0.5 mg cm ²) as anode catalyst and Fe/Fe3C-NCNTs (1 mg cm ²) as the cathode catalyst.

[0065] Example 4: Characterization

[0066] The XRD patterns FIG. 5B and TEM images FIG. 7A- 7D of Fe/Fe₃C-NCNT shows the mechanism of formation of Fe/Fe3C-NCNT at different pyrolysis temperatures.

[0067] FIG. 5A shows the XRD pattern that confirms the crystalline nature of pure NCNT and Fe/Fe3C@NCNT. At 26.4 ⁰ a broad peak appears corresponding to the (002) peak of graphitized carbon. A less conspicuous peak for NCNT appears at 42.6 ⁰ that is indexed as C (101). The diffraction peaks located at 37.7 °, 39.8 °, 40.7 °, 43.0 °, 43.8 °, 44.7 °, 45.9 °, 49.1 °, 51.9 °, 54.5 ⁰ and 58.2 ⁰ can be assigned to (112), (200), (120), (121), (210), (022), (211), (122), (212), (004) and (104) crystalline planes of Fe3C respectively (JCPDS file no. 89-2867). The high-intensity peak at 44.7 ⁰ of Fe3C superimposes with a metallic Fe peak indicating the presence of some traces of metallic Fe also in the sample. This confirms that the nanoparticles encapsulated inside the nanotubes are predominantly Fe3C nanoparticles with some small amount of metallic Fe and hence we have represented it as Fe/Fe3C encapsulated nitrogen-doped carbon nanotubes. [0068] FIGs. 8 A- 8B shows a variation in cell potential with the concentration of formic acid formed after 120 min. So for all the experiments, a constant potential of 2.1 V has been applied at the anode w.r.t the cathode during the CO2 conversion experiment.

[0069] A gradual increase in the intensity of the formic acid peak with time was observed in both UV-Vis spectra and HPLC data signifying that the yield of formic acid getting converted from CO2 is increasing with time (FIGs 9A-11B). FIGs 9A- 9C show the UV absorption spectra of the cathode reservoir outlet solution for the different catalysts as stated above. FIG 9D shows the change in formic acid concentration obtained from the cathode reservoir outlet as a function of the reduction time. The cell with Pt/C as anode catalyst and Fe/Fe3C-NCNTs as cathode catalyst showed the highest yield of formic acid after 120 min (66 ± 3 mM). It has been well reported that noble metals such as platinum act as a good water-splitting catalyst but they are very expensive. So, Pt/C on the anode side was replaced with non-precious metal catalyst Fe/Fe3C-NCNTs, keeping the same catalyst loading as before (0.5 mg cm-2). Now, the yield of formic acid was plotted with time and it can be seen that Fe/Fe3C-NCNT as both anode and cathode catalyst gives appreciably comparable yield of formic acid after 120 min (58 ± 2 mM) when compared to the cell with Pt/C as anode catalyst and Fe/Fe3C-NCNT as cathode catalyst. Heterogeneous carbon materials have also been considered as promising metal-free electrocatalysts for CO2 reduction reactions. So, Fe/Fe3C-NCNTs were then replaced with pure NCNTs, with no metal encapsulation, as both anode and cathode electrocatalysts keeping loading constant.

[0070] It has been observed that the formic acid formation rate reduced by almost half (39 ± 2 mM) in this case. So, from the above discussion, it can be attributed that Fe/Fe3C- NCNTs serve as a multifunctional electrocatalyst for CO2 reduction devoid of any precious metal -based catalysts for a consistent period of time. At the anode side, the catalyst plays a vital role in electrochemical water splitting. It also helps in the Hydrogen Oxidation Reaction (HOR) where the hydrogen molecule (generated from water splitting) gets adsorbs on the surface of the catalyst and forms protons (H+) which passes through the proton exchange membrane to the cathode side with the subsequent loss of an electron that passes through the external circuit. On the cathode catalyst surface, a proton gets adsorbed along with an electron to produce an adsorbed hydrogen atom). This adsorbed hydrogen atom interact with the incoming CO2 gas at the three phase boundary on the cathode catalyst surface to form an adsorbed formate radical. This formate radical reacts with another adsorbed hydrogen atom to produce a formic acid molecule. Overall, 2 protons and 2 electrons convert a CO2 molecule to HCOOH In all the experiments, high purity (99.99 %) CO2 gas was continuously supplied to the cathode reservoir solution throughout the experiment. However, in the absorption spectra for all the samples, around 300 nm a very less conspicuous peak like structure is visible. So we cannot discard the formation of some minor product along with formic acid after CO2 reduction. But, the concentration of these products being less than 1 % is lower than the HPLC detection limit. So the major conversion product being obtained has been considered as formic acid with maximum selectivity. The yield of formic acid after 60 min of each reaction was calculated as follows:

[0071] Considering the solubility of CO2 in DI water at 25 ^°C and 1 atm pressure to be 33 mM, the yield of product was calculated after 1 hour to be 73 %, 97 %, 90 % for NCNT (both anode and cathode catalyst), Fe/Fe3C-NCNT (both anode and cathode catalyst) and Pt/C on anode, Fe/Fe3C-NCNT on cathode respectively.

[0072] A probable reason might be a good interaction of CO2 with MT- NCNTs. A comparison of the present work with reported literature has been shown in Table 1.

Table 1: Comparison of Fe/Fe3C-NCNT with respect to other electrodes in a PEM cell

Anode Cathode Efficiency (%)

Pt/C (0.5 mg cm ²) Fe/Fe₃C@NCNT 90

( 1 mg cm ²)

Fe/Fe₃C@NCNT (0.5 Fe/Fe₃C@NCNT 97

mg cm ²) ( 1 mg cm ²)

NCNT (0.5 mg cm ²) NCNT ( 1 mg cm ²) 73

Claims

WE CLAIM:

1. A polymer electrolyte membrane (PEM) electrode material ME-NCNT (100), comprising:

metal nanoparticles (105) encapsulated within nitrogenated carbon nanotubes (NCNTs) (101),

wherein the metal nanoparticles comprise Fe C, and the NCNTs, are nitrogenated (N) (103) in the range of 3-6 wt.% N.

2. The electrode material (100) of claim 1, wherein the material is configured to be loaded on an electrode surface for use in an electrochemical cell at 0.1 to 2 mg/cm².

3. The electrode material (100) of claim 1, wherein the metal nanoparticles further comprise one or more transition metals selected from Co, Ni, Ru, Os and Eu and the Fe in the Fe C particles is substituted with the one or more transition metals.

4. The electrode material ME-NCNT of claim 1, wherein the electrode material exhibits a surface area in the range of 180-220 m² g ¹ and porosity in the range of 2-3 nm in diameter.

5. The electrode material ME-NCNT of claim 1, wherein the material produces an X-ray diffraction pattern with peaks of 26.4 °, 37.7 °, 39.8 °, 40.7 °, 43.0 °, 43.8 °, 44.7°, 45.9°, 49.1 °, 51.9 °, 54.5 ⁰ and 58.2° corresponding to Fe/Fe C encapsulated within nitrogen- doped carbon nanotubes.

6. A polymer electrolyte membrane (PEM) electrochemical conversion cell (200), comprising:

one or more membrane electrode assemblies (MEA) (201), wherein an MEA comprises a membrane layer sandwiched between a cathode and an anode, at least one of the cathode or anode comprises an electrode material ME-NCNT (100) loaded on to its surface for use in an electrochemical cell at 0.1 to 2 mg/cm², comprising:

wherein the metal nanoparticles comprise Fe C and the CNTs are nitrogenated (N) (103) in the range 3-6 wt.% N.

7. The PEM cell of claim 6, wherein the anode is a Pt anode or a CNT anode.

8. The PEM cell of claim 6, each MEA further comprising:

an anolyte flow channel (211) for; and

a catholyte flow channel (213) for.

9. A process for generating formic acid using the (PEM) electrochemical conversion cell of claim 6 comprising at least one MEA, comprising:

providing deionized water through the anolyte flow channel of the MEA; providing CO saturated deionized water through the catholyte flow channel of the MEA;

applying a voltage in the range of 1.9 to 2.2 Volts for a time period in the range of 115-125 mins through the MEA; and

obtaining formic acid by the reduction of dissolved CO in the catholyte at a cathode output collector.

10. The process of claim 9, wherein the yield of product by CO conversion is at least 90%.

11. The process of claim 9, wherein the purity of the formic acid is at least 99.99%.

12. A method of preparing an electrode material comprising metal nanoparticle encapsulated nitrogen doped carbon nanotubes (ME-NCNT), the method comprising the steps of:

providing a first precursor comprising melamine and a second precursor comprising a transition metal chloride comprising FcCfi at a ratio ranging from 2: 1 to 1 :2 as raw materials;

mixing the raw materials thoroughly in a volatile solvent to form a first solution;

drying the first solution to obtain a first product;

heating the first product to a temperature in the range of 600 to 900°C at a predetermined heating rate under a predetermined nitrogen flow rate to form a second product; and

drying the second product to obtain the nitrogen doped metal carbide nanoparticle encapsulated carbon nanotubes (ME-NCNT).

13. The method of claim 12, wherein the transition metal chloride comprises a transition metal selected from one or more of Co, Ni, Ru, Os and Eu.

14. The method of claim 12, wherein the drying step is preceded by purification comprising washing the second product with an acid.