Abstract
The oxygen reduction reaction electrocatalyst is a Pt/N/C electrocatalyst that provides an efficient ORR catalyst suitable for use in polymer electrolyte membrane (PEM) fuel cells, for example. The oxygen reduction reaction electrocatalyst is in the form of platinum nanoparticles embedded in a nitrogen-enriched mesoporous carbon matrix, particularly a nitrogen-enriched graphite matrix. The nitrogen-enriched graphite matrix has an average surface area of 240.4 m.sup.2/g, and the platinum nanoparticles each have an average diameter between 10 nm and 12 nm.
Claims
1. An oxygen reduction reaction electrocatalyst, comprising platinum nanoparticles embedded in a nitrogen-enriched mesoporous graphite matrix having an average surface area of 240.4 m2/g.
2. The oxygen reduction reaction electrocatalyst as recited in claim 1, wherein the platinum nanoparticles each have an average diameter between 10 nm and 12 nm.
3. A method of making an oxygen reduction reaction electrocatalyst, comprising the steps of: reacting 2,4,6-trihydrazinyl-1,3,5-triazine with p-terephthaldehyde in an organic solvent to form a polymeric solution; refluxing the polymeric solution to produce a conductive polymer having the structure: ##STR00001## adding the conductive polymer to methanol to form a polymeric suspension; adding H2PtCl4 to the polymeric suspension to form a mixture; stirring and sonicating the mixture; drying the stirred and sonicated mixture to yield platinum nanoparticles suspended in the conductive polymer; and annealing the suspension of platinum nanoparticles in the conductive polymer to yield platinum nanoparticles embedded in a mesoporous, graphitic carbon matrix forming the oxygen reduction reaction electrocatalyst.
4. The method of making an oxygen reduction reaction electrocatalyst as recited in claim 3, further comprising the step of reacting hydrazine hydrate with cyanuric chloride in a polar organic solvent to form the 2,4,6-trihydrazinyl-1,3,5-triazine.
5. The method of making an oxygen reduction reaction electrocatalyst as recited in claim 3, wherein the step of refluxing the polymeric solution comprises refluxing the polymeric solution for 30 hours.
6. The method of making an oxygen reduction reaction electrocatalyst as recited in claim 5, wherein the step of stirring and sonicating the mixture comprises stirring the mixture for one hour at room temperature.
7. The method of making an oxygen reduction reaction electrocatalyst as recited in claim 6, wherein the step of stirring and sonicating the mixture further comprises sonicating the mixture at a temperature of 40 C. for four hours.
8. The method of making an oxygen reduction reaction electrocatalyst as recited in claim 7, wherein the annealing step comprises annealing the suspension of platinum nanoparticles in the conductive polymer in a tube furnace at a temperature of 800 C. at a heating rate of 5 C./min.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a reaction scheme for the synthesis of a conduction polymer that forms the mesoporous carbon matrix for an oxygen reduction reaction electrocatalyst.
(2) FIG. 2 is a pictorial diagram of a reaction scheme for the synthesis of an oxygen reduction reaction electrocatalyst from the conduction polymer of FIG. 1.
(3) FIG. 3 is a plot of N.sub.2 adsorption-desorption measurements of the oxygen reduction reaction electrocatalyst synthesized according to FIGS. 1 and 2.
(4) FIG. 4 is a tunneling electron microscope (TEM) micrograph of the oxygen reduction reaction electrocatalyst synthesized according to FIGS. 1 and 2.
(5) FIG. 5 is the X-ray diffraction (XRD) spectrum of the oxygen reduction reaction electrocatalyst synthesized according to FIGS. 1 and 2.
(6) FIG. 6 are cyclic voltammetry curves for the oxygen reduction reaction electrocatalyst in 0.1 M KOH, saturated with both N.sub.2 and O.sub.2.
(7) FIG. 7 are linear sweep voltammetry (LSV) curves for the oxygen reduction reaction electrocatalyst at different rotation rates in rotating disc electrode (RDE) measurements in O.sub.2-saturated 0.1 M KOH at 10 mV/s.
(8) FIG. 8 are Koutecky-Levich (K-L) plots corresponding to the LSV curves of FIG. 7.
(9) FIG. 9 is a graph of percent relative current as a function of time, showing the chronoamperometric response of the oxygen reduction reaction electrocatalyst.
(10) FIG. 10 is a graph showing Fourier-transform infrared spectroscopy (FTIR) transmittance results of the oxygen reduction reaction electrocatalyst.
(11) FIG. 11 is a graph showing thermogravimetric analysis and differential scanning calorimetry (TGA/DTA/DSC) results of the oxygen reduction reaction electrocatalyst.
(12) Similar reference characters denote corresponding features consistently throughout the attached drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(13) The oxygen reduction reaction electrocatalyst is a Pt/N/C electrocatalyst that provides an efficient ORR catalyst suitable for use in polymer electrolyte membrane (PEM) fuel cells, for example. The oxygen reduction reaction electrocatalyst is in the form of platinum nanoparticles embedded in a nitrogen-enriched mesoporous carbon matrix, particularly a nitrogen-enriched graphite matrix. The nitrogen-enriched graphite matrix has an average surface area of 240.4 m.sup.2/g, and the platinum nanoparticles have an average diameter between 10 nm and 12 nm.
(14) The structure, synthesis, and characterization of the oxygen reduction reaction electrocatalyst is exemplified in the following examples.
Example 1
Synthesis of Conductive Polymer
(15) Referring to FIG. 1, the oxygen reduction reaction electrocatalyst is made by first preparing a hydrazine derivative. Hydrazine hydrate (20 mL, 80%) is reacted with cyanuric chloride (20 mmol) in 50 mL of acetonitrile at 0 C. to form the hydrazine derivative 2,4,6-trihydrazinyl-1,3,5-triazine. Then, the 2,4,6-trihydrazinyl-1,3,5-triazine (40 mmol) is reacted with p-terephthaldehyde (60 mmol) in 100 mL of dimethyl sulfoxide (DMSO) to form a polymeric solution, and this polymeric solution is refluxed for 30 hours under inert conditions to produce a conductive polymer, the monomer having the structure shown in FIG. 1. The conductive polymer contains azomethine networks, which appear as a yellow solid.
Example 2
Synthesis of Oxygen Reduction Reaction Electrocatalyst
(16) The conductive polymer is washed and dried before being added to methanol to form a polymeric suspension (0.1 g in 50 mL of methanol). H.sub.2PtCL.sub.4 (50 mL, 10.sup.3 M in acidic water) is then added to the polymeric suspension to form a mixture. The mixture is stirred for one hour at room temperature, and then sonicated at 40 C. for four hours. During the stirring and sonicating, the mixture changes color from light yellow to black. The stirred and sonicated mixture is left to stand at room temperature for six hours and then dried at room temperature to yield platinum nanoparticles contained in the conductive polymer. The conductive polymer containing the platinum nanoparticles is then washed and filtered off. The conductive polymer containing the platinum nanoparticles is then annealed in a tube furnace at 800 C. with a heating rate of 5 C./min under the flow of helium (100 mL/min) to yield the oxygen reduction reaction electrocatalyst, shown diagrammatically in FIG. 2. The oxygen reduction reaction electrocatalyst is black in color, and the final products are dispersed in 1 M HCl solution and stirred for two hours to partially remove impurities.
(17) FIG. 10 is a graph showing Fourier-transform infrared spectroscopy (FTIR) transmittance results of the oxygen reduction reaction electrocatalyst. The FTIR spectra show that the conducting polymer has CN and CC bonds, and the prepared Pt nanoparticles have no chemical bonding with the polymer matrix. For the electrocatalyst, the peaks around 1126 and 1400 cm.sup.1 belong to the CH vibrations, and the peaks around 1635 and 1535 cm.sup.1 correspond to CN and CC vibrations, respectively.
(18) FIG. 11 is a graph showing thermogravimetric analysis and differential scanning calorimetry (TGA/DTA/DSC) results of the oxygen reduction reaction electrocatalyst. Thermogravimetric analysis was conducted in an inert atmosphere from room temperature to 800 C. The TGA/DTA/DSC thermogram results reveal that the prepared electrocatalyst was decomposed in two stages. The first degradation stage was up to 200 C., due to the removal of moisture and adsorbed water. The second degradation stage was between 200-600 C., that is, the second degradation stage was the main degradation stage, and it may have been due to the degradation of the main organic continents. The residue weight 91.50% was observed at 800 C. as Pt and carbon residue.
Example 3
Nitrogen Adsorption-Desorption Isotherm
(19) FIG. 3 shows N.sub.2 adsorption-desorption measurements to measure the porosity and surface nature of the oxygen reduction reaction electrocatalyst, which is a natural adsorbent. The adsorption isotherms are type IV for both the platinum nanoparticles and the nanoporous carbon support. Specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method and the pore size distribution plot was derived based on the Barrett-Joyner-Halenda (BJH) method. The Brunauer-Emmett-Teller (BET) surface area of the natural adsorbent was found to be 240.4 m.sup.2/g.
Example 4
TEM Morphology
(20) Morphology and particles sizes were determined using both field emission scanning electron microscopy (FESEM) and transmission electron microscopy (FE-TEM). FIG. 4 is a tunneling electron microscope (TEM) micrograph of the electrocatalyst prepared according to Examples 1 and 2. The TEM image clearly shows the platinum nanoparticles embedded in the carbon matrix with irregular particles size of about 10-12 nm.
Example 5
X-Ray Powder Diffraction
(21) FIG. 5 shows the X-ray diffraction (XRD) spectrum of the electrocatalyst prepared according to Examples 1 and 2. The diffraction peaks are found at 20 values of about 39.8, 46.3, 67.6 and 81.5, which are ascribed to the facets (111), (200), (220) and (311), characteristic of face-centered cubic (fcc) crystalline Pt (JCPDS, Card No. 04-0802), suggesting that the Pt species were reduced to the metallic state by the conduction polymer. One other broad diffraction peak is found at a 20 value of about 26, which could be ascribed to graphitic carbon.
Example 6
Voltammetry
(22) FIG. 6 shows the cyclic voltammetry curves of the electrocatalyst prepared according to Examples 1 and 2 in 0.1 M KOH saturated with N.sub.2 and O.sub.2. The cyclic voltammetry results reveal that there is a cathodic (i.e., oxygen reduction) peak in O.sub.2-saturated 0.1 M KOH solution at 0.887 V. However, no redox peaks were observed with N.sub.2-saturated 0.1 M KOH solution. Prominent catalytic activity was observed for the oxygen reduction reaction (ORR) in an alkaline medium. For the cyclic voltammetry, 5 mg of the present electrocatalyst and 30 l of 5 wt % Nafion solution were dispersed in 1 mL of ethanol, followed by at least 30 minutes of sonication to form a homogeneous ink. 2.4 L of the catalyst ink (containing 12 g of electrocatalyst) was loaded onto a glassy carbon electrode having a diameter of 3 mm (loading 0.17 mg/cm.sup.2). Cyclic voltammetry (CV) (using a potentiostat from CH660 Instruments) was conducted in an electrochemical cell using Ag/AgCl as the reference electrode, platinum as the counter electrode, and the glassy carbon electrode as the working electrode. The KOH electrolyte was saturated with oxygen and nitrogen by bubbling prior to the start of each experiment. A flow of O.sub.2 was maintained over the electrolyte during the recording of CVs in order to ensure continued O.sub.2 saturation. The working electrode was cycled at least five times before data were recorded at a scan rate of 10 mV/s.
(23) FIG. 7 shows the linear sweep voltammetry (LSV) curves for the present electrocatalyst at different rotation rates in rotating disc electrode (RDE) measurements in O.sub.2-saturated 0.1 M KOH at 10 mV/s. For the RDE measurements, catalyst inks were prepared by the same method as for cyclic voltammetry. About 4 pt of ink (containing 20 g of catalyst) was loaded on a glassy carbon rotating disk electrode of 5 mm in diameter (ALS Instruments +CH1600), giving a loading of 0.1 mg/cm.sup.2, The working electrode was scanned cathodically at a rate of 10 mVs.sup.1 with varying rotation speed from 225 rpm to 2500 rpm. The present electrocatalyst shows a high onset potential (E.sub.o) and half-wave potential (E.sub.1/2) of 0.996 V and 0.875 V, respectively. It was observed that the limit diffusion current density was increased with an increase in rotation rate due to the diffusion distance of the O.sub.2-saturation.
(24) FIG. 8 shows the Koutecky-Levich (K-L) plots corresponding to FIG. 5, which are used to determine the kinetic parameters and the number of electron transfers per molecule of oxygen. The present electrocatalyst shows good linearity and nearly parallel fitted lines between j.sup.1 and .sup.1/2 of the K-L plots, indicating a first-order reaction towards the oxygen reduction. The number of electrons transferred (n) was calculated to be in the range of 3.97 for a potential range of 0.0-0.90 V, which indicates that it favors a 4e.sup. oxygen reduction.
(25) The Koutecky-Levich plots (j.sup.1 vs. .sup.1/2) were analyzed at various electrode potentials. The slopes of their best linear fit lines were used to calculate the number of electrons transferred (n) on the basis of the Koutecky-Levich equation:
(26)
where B=0.62 nFC.sub.o(D.sub.o).sup.2/3v.sup.1/6=1.15210.sup.4 A cm.sup.2s.sup.1/2 rad.sup.1/2 and J.sub.k=nFkC.sub.o. Here, J is the current density (A.Math.cm.sup.2), J.sub.k is the kinetic current density (A.Math.cm.sup.2), J.sub.L is the diffusion-limiting current density (A.Math.cm.sup.2), F is Faraday's constant (96485, C.Math.mol.sup.1), D.sub.o is the diffusion coefficient of O.sub.2 in 0.1 M KOH (1.9310.sup.5, cm.sup.2/s), v is the kinematic viscosity of the electrolyte (0.1 M KOH) (1.0910.sup.2 cm.sup.2/s), C.sub.o is the saturation concentration of O.sub.2 in 0.1 M KOH at 1 atm O.sub.2 pressure (1.2610.sup.6 mol.Math.cm.sup.3), and is the rotation rate (rad/s).
(27) FIG. 9 shows the durability of the present electrocatalyst. The chronoamperometric response of the electrocatalyst was carried out in 0.1M KOH solution (O.sub.2-saturated) with a rotation rate of 1,600 rpm at 0.6V for 15,000 s. The results revealed that about 90% of the original current density was retained.
(28) It is to be understood that the oxygen reduction reaction electrocatalyst and method of making the same is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.
