Electrocatalytic devices using MOx/ICP composite thin films and methods of making the same

Abstract

The present disclosure is directed to electrocatalytic devices having an electrodeposited MO.sub.x/ICP composite thin films for performing oxygen reduction reactions.

Claims

1. An oxygen reduction reaction device, comprising: a composite film, comprising: a homogeneously dispersed composite film consisting essentially of a plurality of manganese oxide particles dispersed in an intrinsically conductive polymer that is a component of a cathode of an electrocatalytic device; wherein the intrinsically conductive polymer is selected from a group consisting of PEDOT and ProDOT; wherein the homogeneously dispersed composite film is formed by electrodeposition of the manganese oxide particles with the PEDOT or ProDOT to homogeneously disperse the manganese oxide particles in the intrinsically conductive polymer to form a composite film consisting of MnO.sub.2 and PEDOT or ProDOT; and wherein the composite film performs an oxygen reduction reaction.

2. The device of claim 1, further comprising a substrate.

3. The device of claim 1, wherein the device is an electrode.

4. The device of claim 3, wherein the electrode is a cathode.

5. The device of claim 1, wherein the composite film is a cathode that is part of a system selected from the group consisting of batteries, fuel cells, electrolyzers, hydrogen evolution reaction devices.

6. The device of claim 1, wherein the manganese oxide particles are nanoparticles.

7. The device of claim 1, wherein the manganese oxide particles are MnO.sub.2 particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1.0 shows a two-step preparation of a hybrid PEDOT/MnO2 nanoparticle film on a glassy carbon substrate to form an RDE working electrode or fused silica tab electrode according to an embodiment of the invention.

(2) FIG. 1A shows a schematic of the electrodeposition of MnO.sub.x/PEDOT composite thin films.

(3) FIG. 1B shows an XRD spectra of the MnO.sub.x/PEDOT film grown on ITO, with indices for MnO.sub.2 (dotted, PDF #053-0633) and background ITO (solid, PDF #039-1058).

(4) FIG. 1C shows CV scans in electrodeposition solutions for MnO.sub.x (dashed), PEDOT (solid), and MnO.sub.x/PEDOT (short dashed), vertical dashed line represents the optimum deposition potential, 0.75 V vs. SCE.

(5) FIG. 2A shows a Volcano plot showing the trends in onset potential (solid squares) and half-wave potential (open squares) as the deposition potential is changed.

(6) FIG. 2B shows a Volcano plot showing the trends in terminal current density (solid squares) and n value (open squares) as the Mn.sup.2+ deposition concentration is changed.

(7) FIG. 2C shows a QCM plot showing the mass change vs. time of the MnO.sub.x, PEDOT, and MnO.sub.x/PEDOT depositions, individual (dotted lines) and average (solid lines) runs.

(8) FIG. 2D shows an SEM image of a MnO.sub.x/PEDOT film grown for 40 seconds.

(9) FIG. 2E shows an SEM image of a MnO.sub.x/PEDOT film grown for 120 seconds.

(10) FIG. 3A shows disk current density LSVs for MnO.sub.x (dashed), PEDOT (thick solid), MnO-x/PEDOT (dotted) and 20% Pt/C (thin solid).

(11) FIG. 3B shows ring current density LSVs for MnO.sub.x, PEDOT, MnO.sub.x/PEDOT and 20% Pt/C (same color legend as FIG. 3A).

(12) FIG. 3C shows n value LSVs for MnO.sub.x, PEDOT, MnO.sub.x/PEDOT and 20% Pt/C (same color legend as FIG. 3A).

(13) FIG. 3D shows peroxide percentage LSVs for MnO.sub.x, PEDOT, MnO.sub.x/PEDOT and 20% Pt/C (same color legend as FIG. 3A).

(14) FIG. 3E shows EIS spectra for MnO.sub.x (three point circles), PEDOT (dark squares), MnO.sub.x/PEDOT (five point circles) and 20% Pt/C (light squares).

(15) FIG. 3F shows stability and methanol experiments on MnO.sub.x/PEDOT (broken squares) and 20% Pt/C (open circles), the arrow represents the addition of methanol in to the cell at two hours.

(16) Wherever possible, the same reference numbers will be used throughout the drawings to represent the same parts.

SUMMARY OF THE INVENTION

(17) The present disclosure is directed to oxygen reduction reaction (ORR) devices having electrodeposited MO.sub.x/Intrinsically conductive polymers (ICP) composite thin films and their method of making. In an embodiment, the metal may be, but is not limited to a manganese oxide and manganese dioxide (collectively referred to as manganese dioxide), ruthenium oxide or other metal oxide capable of facilitating an oxygen reduction reaction. The metal oxide may include a metal additive, such as gold and platinum, for example, in the form of a Au(metal)-MnO.sub.x/ICP composite. In an embodiment, the metal oxide may be a mixed metal ORR oxide, with a second metal oxide selected from a group including Fe, Nb and TA, oxides and perovskites. In an embodiment, the manganese oxide may be Mn.sub.yO.sub.x or MnO.sub.x, from the group MnO, MnO.sub.2, MnO.sub.3, Mn.sub.3O.sub.4, Mn.sub.2O.sub.3, Mn.sub.2O.sub.7 and combinations thereof which are exemplified by MnO.sub.x or MnO.sub.2 for the remainder of the disclosure. The ICP may be, but is not limited to polythiophenes such as poly(3,4-ethylenedioxythiophene (PEDOT), propylenedioxythiophene (ProDOT)-phenylene polymers, and polyanilines, and polypyrolles, polyacetylenes, polyphenylenevinylenes and mixtures and unit combinations thereof.

(18) The present disclosure is directed to oxygen reduction reaction (ORR) devices having electrodeposited MnO.sub.x/ICP composite thin films and their method of making. In an embodiment, the ORR device may be a cathode. In an embodiment, the composite films may be produced by co-electrodeposition, (electrodeposition, followed by chemical modification) or other (chemical modification followed by a general physical deposition on a substrate). The ORR device may be part of an electrocatalytic device, such as, but not limited to a fuel cell, battery or electrolyzer.

(19) The present disclosure is further directed to methods of forming the composite thin film ORR devices by depositing the ORR thin film electrocatalyst upon a substrate. In an embodiment, the electrocatalyst may be a coating upon or within a cathode substrate, thereby forming an ORR cathode device. In an embodiment, the cathode may be part of, but not limited to a battery, fuel cell, electrolyzer, hydrogen evolution reaction (HER) device, or other device employing an ORR reaction.

(20) In an embodiment, ORR devices having MnO.sub.x/ICP composite thin films may be formed by electrodeposition of MnO.sub.x/ICP to form a homogeneous film upon a substrate. In another embodiment, the ORR devices having MnO.sub.x/ICP composite thin films may be formed by sequentially electrochemically depositing PEDOT and MnO.sub.2 to form hybrid films of PEDOT and MnO.sub.2 nanoparticles. In an embodiment, the sequential depositing is performed by electropolymerization of EDOT to form PEDOT, followed by aqueous, room-temperature growth of MnO.sub.2 nanoparticles by MnO.sub.4 reduction. In another embodiment, the ORR devices having MnO.sub.x/ICP composite thin films may be formed by sequentially physically depositing PEDOT and MnO.sub.2 to form hybrid films of PEDOT and MnO.sub.2 nanoparticles where physically depositing includes but is not limited to drop casting or spraying.

(21) In an embodiment, a method of forming a cathode is disclosed that includes co-depositing a MO.sub.x/ICP composite thin film upon an electrode substrate.

(22) In another embodiment, a method of forming a cathode is disclosed that includes depositing a MO.sub.x/ICP composite thin film upon an electrode substrate, wherein the thin film is formed by depositing sequential layers of MO.sub.x and ICP.

(23) In another embodiment, a method of forming a cathode is disclosed that includes depositing a pre-formed MO.sub.x/ICP hybrid that was formed by the chemical reaction of MnO.sub.4 ion on an existing ICP polymer.

(24) In another embodiment, a cathode is disclosed that includes an electrodeposited MO.sub.x/ICP composite thin film upon a cathode substrate.

(25) In another embodiment, a cathode is disclosed that includes an electrodeposited ICP thin film that is then chemically reacted in order to deposit MO.sub.x nanoparticles, thereby forming a hybrid MO.sub.x/ICP structure.

(26) In another embodiment, a device is disclosed that includes an electrodeposited MO.sub.x/ICP composite thin film upon a cathode substrate. The device may be, but is not limited to a battery, a fuel cell, or an electrolyzer.

DETAILED DESCRIPTION OF THE INVENTION

(27) The present disclosure is directed to composite films, electrodes including the composite films, electrocatalytic devices including the composite film electrodes, and methods of making the same. The composite films include metal oxide particles dispersed in an intrinsically conductive polymer (ICP). The metal oxide is an electrocatalytically active mater, such as but not limited to manganese oxides.

(28) The present disclosure is further directed to methods that improve MnO.sub.x conductivity and ORR activity by hybridizing manganese oxide (MnO.sub.x) nanoparticles with an intrinsically conductive polymer (ICP), such as, but not limited to poly(3,4-ethylenedioxythiophene) (PEDOT). According to an embodiment, hybrid PEDOT/MnO.sub.x nanoparticle films were prepared by organic electropolymerization of the 3,4-ethylenedioxythiophene (EDOT) monomer, followed by room temperature, aqueous growth of MnO.sub.x nanoparticles. This process is shown in FIG. 1.0. The PEDOT/MnO.sub.x film exhibits high activity toward the ORR in alkaline electrolyte, with an onset potential and half-wave potential. The PEDOT/MnO.sub.x film also shows predominantly a four-electron ORR mechanism and electrochemical selectivity in the presence of methanol.

(29) The present disclosure is further directed to MnO.sub.x/ICP composite thin films as a highly active catalyst for the oxygen reduction reaction in alkaline electrolyte. The composite films may be used as a coating upon a substrate to form a cathode to be used in electrocatalytic devices. The electrocatalytic device may be, but is not limited to fuel cells, batteries and electrolyzers.

(30) The present disclosure is further directed to methods of forming electrocatalytic devices having a MnO.sub.x/ICP composite thin film deposited thereupon.

(31) The present disclosure is further directed to ICP polymers that have been functionalized to include cationic, anionic functional groups or sites in order to aid ionic transport.

(32) The present disclosure is further directed to ICP polymers that have been functionalized to include fluorinated moieties, such as fluorocarbon chains to effect wettability and gas transport properties.

(33) In an embodiment, the electrocatalytic devices having MnO.sub.x/ICP composite thin films may be formed by electrodeposition of MnO.sub.x/ICP to form homogeneous films. In another embodiment, the electrocatalytic devices having MnO.sub.x/ICP composite thin films may be formed by sequentially depositing ICP and MnO.sub.2 to form hybrid films of ICP and MnO.sub.2 nanoparticles. In an embodiment, the sequential depositing is performed by organic electropolymerization of an ICP, followed by aqueous, room-temperature growth of MnO.sub.2 nanoparticles by MnO.sub.4 reduction.

(34) In another embodiment, a method of forming a cathode is disclosed that includes depositing a MO.sub.x/ICP composite thin film upon an electrode substrate, wherein the thin film is formed by depositing sequential layers of MO.sub.x and ICP.

(35) In another embodiment, a method of forming a cathode is disclosed that includes depositing a pre-formed MO.sub.x/ICP hybrid that was formed by the chemical reaction of MnO.sub.4.sup. ion with an existing ICP polymer.

(36) According to an embodiment of the disclosure, MnO.sub.x/ICP composite thin films were electrodeposited by the aqueous micellar route in the presence of Mn.sup.2+ (see FIG. 1A to form the electrocatalytic devices. Briefly, MnO.sub.x and PEDOT were co-electrodeposited from an aqueous solution of Mn(OAc).sub.2, EDOT, LiClO.sub.4, and sodium dodecyl sulfate (SDS) at an anodic potential. MnO.sub.x only films and PEDOT only films were also electrodeposited, as controls, in the absence of EDOT or Mn(OAc).sub.2, respectively.

(37) FIG. 1B shows that the X-ray diffraction (XRD) spectra of a MnO.sub.x/PEDOT film deposited on indium tin oxide coated glass (ITO, PDF #039-1058) resembles MnO.sub.2 (PDF #053-0633), but with weak intensity. Similarly electrodeposited films of MnO.sub.x have been characterized as MnO.sub.2 and MnO.sub.x in the literature. Here we will refer to the films as hydrous MnO.sub.x. Cyclic voltammetry (CV) was examined in the deposition solutions in order to monitor the redox processes of manganese and EDOT. FIG. 1C shows the CV curves, between 1.0 and 1.0 V vs. SCE at 100 mV s.sup.1, of the MnO.sub.x, PEDOT, and MnO.sub.x/PEDOT deposition solutions. Two peaks in the anodic scan of MnO.sub.x deposition appear around 0.45 V and 0.8 V vs. SCE, attributing to a change in Mn valance from 2+ to a mix of 3+ and 4+. Generation of MnO.sub.2 in potentiostatic electrodeposition is proposed to proceed through the generation of bulk Mn.sup.3+, which decomposes in to a mix of Mn.sup.2+ and Mn.sup.4+, forming MnO.sub.2 through hydrolysis..sup.30 Oxidation current of the PEDOT deposition solution begins around 0.25 V vs. SCE, corresponding to the oligomerization and polymerization processes at the electrode surface resulting in a thin film, while the rapid current growth beginning at 0.75 V vs. SCE is consistent with the oxidation potential of EDOT in aqueous solution. In the combined MnO.sub.x/PEDOT deposition solution, the anodic current of PEDOT coincides with the oxidation processes of manganese.

(38) Because the CV experiments were unclear in providing the optimum oxidation potential for co-deposition of MnO.sub.x and PEDOT, films were prepared at varying potentials (0.6 V-0.85 V, in 0.5 V increments) and their electrocatalytic performance compared utilizing rotating disk electrode (RDE) experiments. FIG. 2A shows the trends in ORR metrics of onset potential and half wave potential, as the deposition potential is changed. With the highest onset (0.84 V) and half-wave (0.75 V) potential values, 0.75 V vs. SCE was identified as the deposition potential resulting in the most active MnO.sub.x/PEDOT films and all future composite films were prepared at this potential. The possibility of effecting the MnO.sub.x film growth and performance by changing the concentration of Mn.sup.2+ in the deposition solution was also investigated. FIG. 2B shows the trends in ORR metrics of terminal current density and reaction order (n value), as determined from the Koutecky-Levich equation, when the Mn.sup.2+ concentration was halved (5 mM) and doubled (20 mM). With the highest terminal current density (1.62 mA cm.sup.2) and n value (3.74), 10 mM Mn.sup.2+ was identified as producing the most active film, and all future composite films were prepared at this concentration.

(39) A quartz crystal microbalance (QCM) was utilized to monitor the changes in film mass during the electrodepositions. FIG. 2C shows the resulting mass change over time for the electrodeposition of MnO.sub.x, PEDOT, and MnO.sub.x/PEDOT thin films. PEDOT exhibits a near-linear growth rate between 0 and 240 seconds, while MnO.sub.x and MnO.sub.x/PEDOT show an accelerated rate between 0 and 60 seconds followed by a near-linear region between 60 and 240 seconds. The resemblance of the MnO.sub.x/PEDOT growth to MnO.sub.x is consistent with the qualitative visual observation that the first 60 seconds of co-electrodeposition is almost completely MnO.sub.x film (gold color) growth, followed by PEDOT polymerization after this initiation period (blue color). QCM was also used to quantify film growth rates, and all of the catalyst films were prepared at an identical mass loading of 40 g cm.sup.2.

(40) Scanning electron microscopy (SEM) images were taken to examine the morphology of the films and provide further evidence of the co-electrodeposition growth process. FIGS. 2D and 2E show SEM images of MnO.sub.x/PEDOT films grown for 40 and 120 seconds, respectively. The images show the nanotextured morphology of the films, and that there is no appreciable difference in the overall morphology between 40 seconds and 120 seconds of deposition. Energy dispersive spectroscopy (EDS) elemental analysis shows the presence of S and Mn and that their intensity is growing at a linear rate with respect to time. The S present in the film is likely due to both polymer growth and SDS incorporation in to the structure, which is known to occur with the aqueous micellar electropolymerization of PEDOT. Furthermore, cross-sectional analysis indicates that the MnO.sub.x/PEDOT films have thicknesses of 58, 84, and 111 nm for deposition times of 40, 80, and 120 seconds, respectively. These values suggest that the films become slightly denser and less porous as the deposition time increases.

(41) Catalyst films were then grown directly on the disk electrode of a rotating ring disk electrode (RRDE) in order to test their electrocatalytic activity toward the ORR. RRDE was used to simultaneously monitor the ORR current at the disk and oxidation current from generated peroxide (if any) at the ring, while scanning from low to high ORR overpotential in O.sub.2.sup. purged and blanketed 0.1 M KOH. It was expected that the MnO.sub.x films would indicate a quasi-four-electron reduction as the intrinsic ability of MnO.sub.x to catalytically decompose peroxide is known..sup.32-33 PEDOT, while conductive, is generally known to catalyze the ORR by the two-electron mechanism, although there is one example of a vapor phase-polymerized PEDOT operating via a 4.sup. electron ORR process. Co-electrodeposited MnO.sub.x/PEDOT, while used for capacitors, until now has not yet been investigated for electrocatalytic ORR. FIG. 3A shows the relevant catalytic ORR data for the electrodeposited films and commercial 20% Pt/C, all at the total mass loading of 40 g cm.sup.2.

(42) FIG. 3A shows the increase in ORR activity from the MnO.sub.x and PEDOT films to the MnO.sub.x/PEDOT composite film. The synergistic effect of the co-electrodeposition can be seen in the improvement in onset potential (MnO.sub.x: 0.682 V vs. RHE, PEDOT: 0.622 V, MnO.sub.x/PEDOT: 0.877 V), half-wave potential (MnO.sub.x: 0.593 V vs. RHE, PEDOT: 0.481 V, MnO.sub.x/PEDOT: 0.825 V), and terminal current density (MnO.sub.x: 0.892 mA cm.sup.2, PEDOT: 0.971 mA cm.sup.2, MnO.sub.x/PEDOT: 1.617 mA cm.sup.2). Furthermore, the metric values of the MnO.sub.x/PEDOT films are equal to or better than those of commercial benchmark catalyst 20% Pt/C, with an onset potential of 0.875 V vs. RHE, a half-wave potential of 0.791 V vs. RHE, and a terminal current density of 1.667 mA cm.sup.2. While the onset potential and terminal current values are similar, the MnO.sub.x/PEDOT film (half-wave potential: 0.825 V vs. RHE) outperforms 20% Pt/C (half-wave potential: 0.791 V) in the half-wave regime. This distinction is of importance as the half-wave region is generally around the potential in which the maximum power could be extracted from a fuel cell..sup.34 Tafel slopes were also calculated to assess the kinetic effectiveness of each catalyst in the onset region. A low slope signifies a more effective catalyst; and the slopes trend with the overall ORR activity here, PEDOT (106 mV dec.sup.1)>MnO.sub.x (91 mV dec.sup.1)>20% Pt/C (87 mV dec.sup.1)>MnO.sub.x/PEDOT (39 mV dec.sup.1). An ORR metric comparison with all other PEDOT and PEDOT composite catalysts studied by RDE can be seen in Table Si, Supporting Information. This data shows the superior performance of the MnO.sub.x/PEDOT films prepared here when compared to other PEDOT-based ORR electrocatalysts.

(43) Ring current linear scanning voltammetry (LSV) scans in FIG. 3B represent the current from the oxidation of peroxide being generated by the catalysts performing ORR on the disk. The PEDOT and MnO.sub.x films exhibit significantly higher ring current during the ORR, especially when considered as a ratio of ring/disk current density (PEDOT j.sub.R/j.sub.D=0.177, MnO.sub.x j.sub.R/j.sub.D=0.045, MnO.sub.x/PEDOT j.sub.R/j.sub.D=0.012, 20% Pt/C jRj.sub.D=0.003). The quantification of ring and disk current by RRDE allows an accurate calculation of the n value and percent of peroxide generated for each catalyst, based on the collection efficiency of the instrument (see Supporting Information). n values, n=4(I.sub.D)/(I.sub.D+I.sub.R/N) where I.sub.D is the disk current, I.sub.R is the ring current, and N is the collection efficiency, are shown in FIG. 3C. 20% Pt/C, as expected, showed an n value of 4 from low (onset, n=3.98) to high overpotential (steady-state, n=3.98) as it generally proceeds by an efficient four-electron ORR mechanism. MnO.sub.x/PEDOT exhibited a quasi-four-electron ORR mechanism, perhaps two rapid two electron transfers, with an essentially constant n value from low (onset, n=3.86) to high overpotential (steady-state, n=3.92). These values demonstrate the remarkable synergism between MnO.sub.x and PEDOT as the maximum attained n values for these catalysts individually were 3.68 and 2.96, respectively. Hence, the peroxide generation (FIG. 3D), % (H.sub.2O.sub.2)=200(I.sub.R/N)/(I.sub.R/N+I.sub.D), of the MnO.sub.x/PEDOT film was significantly lower (<5%) at all overpotentials than the MnO.sub.x (>15%) and PEDOT (>50%) films. 20% Pt/C generated<1% peroxide, further verifying its efficient reduction of oxygen.

(44) Considering the similarity of the MnO.sub.x/PEDOT film and 20% Pt/C in ORR activity, further testing was done to assess their charge transfer and stability characteristics. Electrochemical Impedance Spectroscopy (EIS) experiments were carried out at constant half-wave current in O.sub.2-purged and blanketed 0.1 M KOH, FIG. 3E. Modeling to the equivalent Randles circuit yielded charge transfer resistance (R.sub.CT) values of 361 (MnO.sub.x/PEDOT), 394 (20% Pt/C), 478 (MnO.sub.x), and 3117 (PEDOT). R.sub.CT values calculated in the onset region of the ORR LSV yielded the same trend: 806 (MnO.sub.x/PEDOT), 1181 (20% Pt/C), 2215 (MnO.sub.x), and 2997 (PEDOT). Co-electrodeposition of MnO.sub.x and PEDOT clearly helps facilitate electron transfer during the ORR, perhaps even more efficiently than the 20% Pt/C benchmark catalyst. The intimate electrode contact realized by electrodeposition could explain the low resistances, another fundamental and practical advantage over a commercial powder electrocatalyst. Galvanostatic stability experiments were also carried out at constant half-wave current in O.sub.2.sup. purged and blanketed 0.1 M KOH for 8400 seconds, FIG. 3F. The MnO.sub.x/PEDOT and 20% Pt/C catalysts were extremely stable over a two hour period, both retaining >97% of their activity. After two hours, methanol (5 wt. %) was introduced in order to test the catalysts' electrocatalytic selectivity for ORR in the presence of methanol (arrowFIG. 3F). The MnO.sub.x/PEDOT film displayed higher tolerance for methanol than 20% Pt/C suggesting its compatibility with methanol fuel cells. Combined with the fact that PEDOT has been used in fuel cells to limit methanol crossover and as a catalyst support, the development of MnO.sub.x/PEDOT could find real application as an electrocatalyst in fuel cells.

(45) In summary, MnO.sub.x/ICP composite thin films were anodically electrodeposited by an aqueous micellar route and used as electrocatalysts for the ORR. The composite MnO.sub.x/ICP thin film showed significant improvements over the MnO.sub.x and ICP control films for the ORR: 0.2-0.25 V more positive onset potential, 0.23-0.34 V more positive half-wave potential, 0.6-0.7 mA cm.sup.2 increase in terminal current density, and 100-2700 decrease in R.sub.CT, as studied here. The activity of the MnO.sub.x/PEDOT proved competitive with the commercial benchmark catalyst 20% Pt/C in terms of onset potential (MnO.sub.x/PEDOT: 0.877 V vs. RHE, 20% Pt/C: 0.875 V), half-wave potential (MnO.sub.x/PEDOT: 0.825 V vs. RHE, 20% Pt/C: 0.791 V), R.sub.CT (MnO.sub.x/PEDOT: 361 , 20% Pt/C: 394), and exhibited superior electrocatalytic selectivity for ORR when exposed to methanol. The synergism and high activity of the MnO.sub.x/PEDOT film is attributed to the facilitated electron transport, realized by co-electrodepositing MnO.sub.x and PEDOT.

(46) While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.