KR101926354B1 - Single atomic platinum catalysts for direct formic acid fuel cell and a method for preparing thereof - Google Patents

Abstract

The present invention relates to a single atom Pt catalyst for fuel cells and a method of manufacturing the same. The single atom Pt catalyst for fuel cells of the present invention has different selectivity from existing nanoparticle catalysts because there are no ensemble sites (because the atoms are gathered together) because Pt atoms are separated from each other. In the present invention, platinum is synthesized in a single atom form on Sb-doped SnO ₂ (antimony-doped tin oxide (ATO)) using incipient wetness impregnation method. DFT calculation shows that the platinum atom is SnSb or ATO It was confirmed that the substituted form of the antimony atom was thermodynamically stable. In the DFAFC complete cell using the Pt1 / ATO catalyst of the present invention as a positive electrode, the amount of Pt is 1/10 of that of the conventional catalyst, but it is equivalent to that of commercial Pt / C. The platinum single atom catalyst of the present invention can increase the energy efficiency of the direct formic acid fuel cell and lower the production cost of the direct formic acid fuel cell production catalyst or the direct formic acid fuel cell production cost.

Description

TECHNICAL FIELD The present invention relates to a single atomic platinum catalyst for direct formic acid fuel cells and a method for preparing the same,

The present invention relates to a single atomic platinum catalyst for direct formic acid fuel cells and a method of making the same. More particularly, the present invention relates to a single atom platinum catalyst having improved stability and activity for the electrocatalytic formic acid oxidation and a method for producing the same.

Single-atom catalysts (SACs) are being actively studied because they can maximize the use of expensive precious metals and exhibit unique selectivity due to lack of an ensemble site. For example, the single atom Pt immobilized on FeOx showed high activity for the preferential oxidation of CO and exhibited high chemoselectivity for nitroarene hydrogenation (HS Wei et al , Nat. Commun., 2014, 5; BT Qiao et al., Nat. Chem., 2011, 3, 634-641). A single platinum Pt atom on θ-Al ₂ O ₃ shows high activity for CO oxidation and a single atom Pt supported on TiN and TiC shows a high selectivity for the production of H ₂ O ₂ in an electrochemical catalytic oxidation-reduction reaction . However, single atom catalysts (SACs) have demonstrated applicability to a variety of heterogeneous reactions, but there are still issues to be addressed. SACs have been used with very low loading, usually less than 1 wt%, and are often poorly stable due to inherently unstable nature of the single atomic structure.

Single metal atoms are thermodynamically very unstable and prone to aggregation. For this reason, single metal atoms must be stabilized in supports such as metal oxide / carbide / nitride, graphene, zeolites or single crystal metals. For electrochemical applications, the support must have conductivity. SACs are generally composed of noble metal atoms immobilized on a non-metallic support, but noble metal atoms may be located on different metal surfaces. For example, a single atom of Pt or Pd is fixed on another metal such as Cu, Au, and Ag. However, such a structure often exhibits dissolution into the surface separation or reaction solution. A noble metal single atom structure on a conductive support with high load and high stability will be suitable for maximizing the activity and selectivity of the electrochemical reaction.

Direct formic acid fuel cells (DFAFC) are expected to be a next generation power source for portable devices due to easy system integration and high energy density of fuel. Formic acid is a liquid fuel that is convenient for storage and transportation and is safer than highly compressed hydrogen. Methanol has been extensively studied as a liquid fuel, but its use is limited due to toxicity, CO generation to poison the Pt catalyst, and crossover problems of methanol. Formic acid is much less of a problem (XW Yu and PG Pickup, J. Power Sources, 2008, 182, 124-132). Formic acid oxidation reaction (FAOR) occurs at the anode of DFAFC, which is known to have two different reaction mechanisms; The direct dehydrogenation route (HCOOH → HCOO _ads + H + + e- → CO ₂ + 2H + + 2e-) and indirect dehydration pathway (HCOOH → CO _ads + H ₂ O → CO ₂ + 2H + + 2e-). Due to catalyst poisoning by intermediate CO _ads , catalysts with high selectivity for direct pathways without a CO intermediate such as Pt-Au alloys, Pt-around-Au, Sb-doped Pt, and PtBi intermetallic compounds have been developed. The high activity and selectivity for the direct path is due to the control of Pt electrons or geometry. Among the various strategies, the single atom Pt structure has high activity and selectivity in the formic acid oxidation reaction (FAOR) because 1-2 Pt atoms are required for the direct path while the indirect pathway requires the Pt ensemble site.

In the present invention, antimony-doped tin oxide (ATO) was selected as a support for a single-atom Pt electrocatalyst. The strong interaction with Pt, high electrical conductivity, and high It is due to corrosion resistance. The single atom Pt on ATO (Pt1 / ATO) was prepared by substituting Pt atoms for Sb. The Pt1 / ATO catalyst is superior in electrocatalytic activity, selectivity and stability to the direct path of FAOR compared to commercial Pt / C. Other electrochemical reactions including the methanol oxidation reaction (MOR), the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) were also tested. In addition, a complete battery, DFAFC, was prepared using Pt1 / ATO as the anode catalyst, and its performance was compared with the Pt / C catalyst.

Numerous papers and patent documents are referenced and cited throughout this specification. The disclosures of the cited papers and patent documents are incorporated herein by reference in their entirety to better understand the state of the art to which the present invention pertains and the content of the present invention.

The present inventors have made extensive efforts to develop a single-atom catalyst capable of minimizing the amount of noble metal used in manufacturing a fuel cell. As a result, the present inventors made Pt1 / ATO containing single atom type Pt by using antimony-doped tin oxide (ATO) as a support. Pt1 / ATO showed better activity and stability for formic acid oxidation (FAOR) than commercial Pt / C catalysts, and the single atom Pt structure was retained after the severe stability test. The present inventors have made a complete battery for a direct formic acid fuel cell (DFAFC) using Pt1 / ATO as a positive electrode catalyst, confirming that the fuel cell catalyst exhibits excellent performance while minimizing the amount of Pt used, It was completed.

Accordingly, an object of the present invention is to provide a method for producing a single atomic platinum catalyst for a fuel cell.

Another object of the present invention is to provide an electrode for a fuel cell.

It is still another object of the present invention to provide a fuel cell.

Other objects and advantages of the present invention will become more apparent from the following detailed description of the invention, claims and drawings.

According to one aspect of the present invention, the present invention provides a process for preparing a single atomic platinum catalyst for a fuel cell comprising the steps of:

(a) mixing a platinum precursor and an antimony-doped tin oxide (ATO) support in an organic solvent; And

(b) depositing a platinum single atom on the surface of the support by heating the result of step (a) to a temperature of 400 ° C or higher to reduce the platinum precursor.

The present inventors have made extensive efforts to develop a single-atom catalyst capable of minimizing the amount of noble metal used in the production of a fuel cell, and as a result, they have found that antimony-doped tin oxide (ATO) PtO / ATO containing single atomic Pt was prepared. Pt1 / ATO showed better activity and stability for formic acid oxidation (FAOR) than commercial Pt / C catalysts, and the single atom Pt structure was retained after the severe stability test. The present inventors have made a complete battery for a direct formic acid fuel cell (DFAFC) using Pt1 / ATO as a positive electrode catalyst, and confirmed that the fuel cell catalyst exhibits excellent performance while minimizing the amount of Pt used.

Unlike typical nanoparticle catalysts, single-atom catalysts (SACs) exhibit unique selectivity because they utilize noble metal atoms most efficiently and lack an ensemble site (where the atoms are gathered together). However, designing a SAC by solving metal loading and stability is still difficult. In the present invention, Pt SACs supported on antimony-doped tin oxide (Pt1 / ATO) were synthesized with less than 8 wt% of Pt by conventional incipient wetness impregnation. The single atom Pt structure was confirmed by HAADF-STEM (high angle annular dark field scanning tunneling electron microscopy) and EXAFS (extended X-ray absorption fine structure) analysis. Density functional theory (DFT) calculation results show that it is energetically advantageous to replace the Sb site with a Pt atom in the bulk phase, or PtSn or ATO surface. Pt1 / ATO catalysts were used for various electrochemical reactions. In particular, Pt1 / ATO showed superior activity and stability to formic acid oxidation (FAOR) compared to commercial Pt / C catalysts. The single atom Pt structure was retained after the harsh stability test, and cyclic voltammograms ranging from 0.05 to 1.4 V (vs RHE) over the 1800 cycles were repeated. As expected for the single atom Pt structure, Pt1 / ATO, unlike the FA nanoparticle catalyst, followed the two pathways of FAOR direct path and oxygen reduction reaction (ORR). The complete battery for direct formic acid fuel cell (DFAFC) was fabricated using Pt1 / ATO as the anode catalyst, and showed excellent performance while minimizing the amount of Pt used.

Many electrochemical devices require noble metal catalysts, and minimizing the use of these noble metals is essential for a wide range of applications of electrochemical devices. Precious metals are dispersed on a support at the atomic level and single atom catalyst (SAC) can be a revolutionary strategy to reduce the amount of precious metal. However, SAC has hardly been studied as an electrode catalyst since it is difficult to immobilize a metal single atom on a conductive support, SAC has a very low metal load, typically less than 1 wt%, and is generally low in stability. In this study, a single atomic catalyst in which platinum is dispersed on an antimony-doped tin oxide support (Pt1 / ATO) was prepared to contain up to 8 wt% Pt. The Pt1 / ATO catalyst showed high activity for the oxidation of formic acid along the direct path without surface poisoning. Pt nanoparticle catalysts generally follow an indirect pathway with much less activity. In particular, the Pt1 / ATO catalyst showed remarkable stability while maintaining single atomicity even after the stability test under very severe conditions. The full cell of the formic acid fuel cell was fabricated directly using the prepared Pt SAC as the anode catalyst. The fuel cell showed excellent performance while minimizing the amount of Pt. The present invention will thus facilitate the development of various SACs for electrochemical applications.

Hereinafter, a method for producing a single atomic platinum catalyst for a fuel cell of the present invention will be described in detail.

Step (a): Platinum precursor and Carrier  mix

First, a platinum precursor and an antimony-doped tin oxide (ATO) support are mixed in an organic solvent. For example, the mixture can be mixed by stirring at room temperature (about 25 degrees). In the present invention, the mixture is mixed at room temperature for 15 minutes as a glass film.

In the present invention, the platinum precursor is platinum salts. According to one embodiment, the platinum precursor is chloroplatinic acid hexahydrate _{(H 2 PtCl 6 · 6H 2} O), H 2 PtCl 6, platinum (Ⅱ) acetylacetonate (Pt (acac) _2), potassium-tetra chloro platinum carbonate (K ₂ PtCl _4), hydrogen hexachloro-platinum carbonate (H ₂ PtCl _4), platinum (ⅱ) cyanide (Pt (CN) _2), platinum (ⅱ) chloride (PtCl _2), platinum (II) bromide (PtBr ₂ ), K ₂ PtCl ₆ , Na ₂ PtCl ₄ .6H ₂ O, and Na ₂ PtCl ₆ .6H ₂ O and mixtures thereof. According to a particular embodiment of the invention, the platinum salt is chloroplatinic acid hexahydrate.

In the present invention, the support is a solid substrate capable of accommodating metal particles, and the support is antimony-doped tin oxide (ATO).

According to the present invention, the platinum precursor to be mixed with the support in step (a) can be used in an amount of 1% by weight or more based on the total mixed composition. According to an embodiment of the present invention, the platinum precursor may be used in an amount of 1 wt% to 20 wt% with respect to the total mixed composition. According to another embodiment of the present invention, the platinum precursor is used in an amount of 2 to 20% by weight, 4 to 20% by weight, 8 to 20% by weight or 10 to 20% by weight, Can be used. According to certain embodiments of the present invention, the platinum precursor may be used in an amount of 2.6 wt% to 18.8 wt% with respect to the total mixed composition. For the 2.6 wt% or 18.8 wt% platinum precursor, Pt 1 wt% or Pt 8 wt% Pt 1 / ATO, respectively, is produced.

Single-atom catalysts developed so far have a very low noble metal content of less than 1% by weight and a low long-term stability. However, in the present invention, Pt 1 / ATO containing 1% by weight of Pt, 4% by weight of Pt or 8% by weight of Pt was developed using 1 to 20% by weight of platinum precursor based on the total mixed composition, It was confirmed that, despite the high content of wt%, platinum exhibited the characteristics of a single atom.

In the present invention, the organic solvent is selected from the group consisting of methanol, ethanol, butanol, isopropyl alcohol, pentane, hexane, heptane, cyclohexane, toluene, acetone, ethyl acetate, methylene chloride, chloroform, ether, petroleum ether, acetonitrile, benzene, , Propylene glycol, butylene glycol, and mixtures thereof. According to an embodiment of the present invention, the organic solvent is a lower alcohol, and in an embodiment of the present invention, the organic solvent is methanol.

Meanwhile, the step (a) may further include a step of vacuum-drying the resultant of step (a). According to one embodiment of the present invention, the result of mixing (a) is dried in a vacuum oven at 50 DEG C overnight.

Step (b): Preparation of a single atomic platinum catalyst

Next, the result of step (a) is heated to 400 ° C. or higher to reduce the platinum precursor, thereby depositing a platinum single atom on the surface of the support.

According to the present invention, the step (b) is performed by heating to 400 ° C or higher, and platinum is not deposited as a single atom in a temperature range of 400 ° C or lower. The present inventors have experimentally confirmed that a platinum single atom catalyst is not formed at a temperature of 100 ° C to 300 ° C (data not shown).

In the step (b), Sn or Sb atoms located in the lattice arrangement on the surfaces of SnSb and SnO ₂ are substituted with platinum atoms through an oxidation-reduction reaction. That is, Pt can replace Sn or Sb on the ATO support after the reduction process. According to one embodiment of the present invention, it is thermodynamically most stable to replace the Sb site with Pt.

On the other hand, in step (b), a reducing agent may be additionally used. The reducing agent may be used a variety of known reducing agents, e.g., hydrogen 10 vol% (nitrogen 90 vol%), NaBH _4, formic acid (formic acid), ethylene glycol (ethylene glycol), oleyl amine (oleyl amine), The reaction efficiency can be improved by using a reducing agent such as oleic acid or tetraethylene glycol.

The single atom platinum catalyst of the present invention exhibited different electrochemical properties from the conventional platinum catalyst. In particular, Pt1 / ATO had a high direct path selectivity in the formic acid oxidation reaction, and in the oxygen reduction reaction, The selectivity was high. This is in contrast to the high selectivity of formic acid to the indirect path and the high selectivity to the oxygen 4-electron path, which are characteristic of conventional platinum nanoparticle catalysts

According to another aspect of the present invention, there is provided an electrode for a fuel cell comprising a single atomic platinum catalyst for a fuel cell manufactured by the method of the present invention.

According to an embodiment of the present invention, the electrode is a cathode or an anode for a fuel cell including an electrode catalyst.

The electrode for a fuel cell of the present invention uses a single atom platinum catalyst according to the method of the present invention described above, and the description common to both of them is omitted in order to avoid the excessive complexity of the present specification.

According to another aspect of the present invention, there is provided a fuel cell comprising a single atomic platinum catalyst for a fuel cell manufactured by the method of the present invention.

The fuel cell of the present invention uses a single atomic platinum catalyst or an electrode for a fuel cell according to the method of the present invention described above, and the description common to both of them is omitted in order to avoid the excessive complexity of the present specification.

According to an embodiment of the present invention, the fuel cell is a direct formic acid fuel cell (DFAFC). In addition, various methanol fuel cells have a 'cross over' phenomenon in which methanol, which is a fuel, diffuses from the anode (anode) through the membrane to the cathode (cathode), thereby reducing the catalytic activity of the cathode. Also, in the methanol oxidation reaction, CO, which is a byproduct of the reaction, strongly adsorbs on the surface of the catalyst to lower its activity, and methanol itself is very harmful to the human body. In the case of a hydrogen fuel cell, it is difficult to transport and store hydrogen, which is a fuel, and there is a safety problem.

However, formic acid, which is a fuel for direct formic acid fuel cells, is advantageous to be applied as a portable device because it has a low cross-over phenomenon, is environmentally friendly, and is in a liquid state.

According to another aspect of the present invention, there is provided a single atomic platinum catalyst for a fuel cell, wherein a single atomic platinum is deposited on an antimony-doped tin oxide (ATO) support .

According to another aspect of the present invention, there is provided a single atomic platinum catalyst for a fuel cell on which a single atomic platinum is deposited on the above-mentioned antimony-doped tin oxide (ATO) support. The present invention provides a direct formic acid fuel cell comprising

The features and advantages of the present invention are summarized as follows:

(a) The present invention relates to a single atom Pt catalyst for fuel cells and a method of manufacturing the same.

(b) The single atom Pt catalyst for fuel cell of the present invention has different selectivity from existing nanoparticle catalysts because there are no ensemble sites (because the atoms are gathered together) because Pt atoms are separated from each other.

(c) In the present invention, platinum is synthesized in a single atom form on Sb-doped SnO ₂ (antimony-doped tin oxide (ATO)) using incipient wetness impregnation method. DFT calculation shows that the platinum atom is SnSb or ATO surface was found to be thermodynamically stable.

(d) In the DFAFC complete cell using the Pt1 / ATO catalyst of the present invention as a positive electrode (oxidation electrode), the Pt content is comparable to that of commercial Pt / C despite being very low at 1/10 of the existing catalyst.

(e) The platinum single atom catalyst of the present invention can directly increase the energy efficiency of the formic acid fuel cell, and can lower the production cost of the direct formic acid fuel cell production catalyst or the direct formic acid fuel cell production cost.

1a. HR-TEM image of (a) 4 wt% Pt NP / ATO and (b) 4 wt% Pt1 / ATO. Pt NP / ATO was prepared by reducing the platinum precursor at 100 ℃ and reducing Pt1 / ATO at 400 ℃ to other synthesis conditions. (c) EDS elemental mapping of Pt1 / ATO at the same position and (d) HAADF-STEM image. 1B-1C. (e), (g) HAADF-STEM images of Pt1 / ATO, and (f) and (h) are magnified images of some of (e) and (g). FIG. 1D is a graph showing the HAADF-STEM magnification image of Pt1 / ATO and the change in reactivity per mass during the long-term stability test of Pt / ATO, Pt NP / ATO, and Pt / C in formic acid oxidation.
2a-2d. EXAFS (line: measured, symbol: fitted) and (2b) XANES Pt L3 edge for Pt foil, reduced 20 wt% Pt / C, 4 wt% Pt NP / ATO and 4 wt% , (2c) XPS Pt 4f, and (2d) XPS Sb 3d and O1s data. Commercial 20 wt% Pt / C was reduced for 1 h at 100 ° C before measurement.
3a-3d. (3a) Results of CV measurements in Ar-purged 0.1 M HClO ₄ solution after catalyst activation for 20 wt% Pt / C, 4 wt% Pt NP / ATO and 4 wt% Pt1 / ATO. CV was measured at a scan rate of 50 mV / s. (3b) FAOR Forward scan. (3c) MOR forward scan. FAOR or MOR was performed by performing the CV on Ar-saturated 0.1 M HClO ₄ containing 0.5 M HCOOH or 0.5 M methanol solution at 1600 rpm and a scan rate of 50 mV / s. (3d) ORR current density and H ₂ O ₂ selectivity. The ORR was performed in a 0.1 M HClO ₄ solution saturated with O ₂ at a scan rate of 10 mV / s at 1600 rpm.
Figures 4a-4d. (4a) Changes in the mass activity of the direct pathway during the stability test by repeating CVs ranging from 0.05-1.4 V for 1800 cycles in 0.1 M HClO ₄ and 0.5 M formic acid solution at 1600 rpm at 50 mV / s scan rate. HR-TEM image of 20 wt% Pt / C, (4c) 4 wt% Pt NP / ATO and (4d) 4 wt% Pt1 / ATO after FAOR stability test.
Figures 5a-5b. (5a) FAOR performed forward scans according to various Pt contents in Pt1 / ATO samples. FAOR performed CVs in 0.1 M HClO ₄ Ar-saturated with 0.5 M HCOOH solution at a scan rate of 50 mV / s at 1600 rpm. (5b) During the stability test, in which the CV was repeated up to 1800 times in the range of 0.05 - 1.4 V under a solution of 0.1 M HClO ₄ and 0.5 M formic acid, which showed a scan rate of 50 mV / s at 1600 rpm, .
6a-6b. (a) Voltage curves and (6b) power density curves as a function of current density for a DFAFC single cell using 20 wt% Pt / C and 4 wt% Pt1 / ATO as an anode catalyst. Pt loading on the anode was 0.04 mgPt / cm ² for 0.4 mgPt / cm ² and Pt1 / ATO for the Pt / C. The cathodes were fabricated with Pt loading of 2 mgPt / cm ² using 40 wt% Pt / C in both cases.
Figure 7. Supercell (a) Bulk Sn: 1 x 1 x 1 supercell (a = b = 5.96 A, c = 3.18 A) contains four Sn atoms. k-points used 8 x 8 x 8. (b) Bulk SnSb: 1 x 1 x 1 supercell (a = b = c = 6.18A) contains 16 atoms (Sn-8, Sb-8). k-points used 8 x 8 x 8. (c) Bulk ATO: 2 × 2 × 2 supercells (a = b = 9.62 ÅA, c = 6.47 Å) contain 48 atoms (Sn-15, Sb-1, O-32). (d) Sn (100) surface: The surface is modeled as a 4-layer slab, with k-points = 5 × 5 ×× 1. One floor of the floor was fixed. The supercell (a = 5.96 A, b = 6.36 A) contains 48 atoms. (e) SnSb (100): The surface was modeled as a 4 layer slab. k-points = 5 x 5 x 1. One floor was fixed. The supercell (a = b = 8.74 ÅA) contains 40 atoms (Sn-20, Sb-20). ATO surface slab is SnO ₂ A layer-by-layer atomic layer was isolated from the bulk crystal structure. In the surface model of the present invention, at least five Sn layers were kept and three bottom layers were fixed. In the present invention, a 15 ÅA vacuum slab was used along the c-direction for all surface calculations. And the Sb atoms were doped to the surface and subsurface. k-points used 5 × 5 ×× 1. (f) ATO (100) _r : (100) surface was non-stoichiometric Sn-terminated when the highest and lowest oxygen layer was removed from the stoichiometric surface. As surface reduction, all surface Sn atoms formed three coordinates. (G) ATO (110) _r : This is a non-stoichiometric Sn-terminated reducing surface (a), which contains 52 atoms . The supercell (a = 6.46A, b = 6.78A) contains 56 atoms (Sn-19, Sb-1, O-36). (h) ATO (101) _r : All bridging oxygenated surfaces were Sn terminated. The supercell (a = 5.78 A, b = 9.60 A) contains 52 atoms (Sn-19, Sb-1, O-30). (i) ATO (211) _r : The supercell (a = 7.41 Å, b = 11.41 Å) contains 80 atoms (Sn-27, Sb-1, O-52).
8A-8D. (8a) STEM image of Pt NP / ATO and (8b) Pt nanoparticle size histogram. TEM image of ATO support before and after reduction at 400 ° C (8c) and after (8d).
Figure 9. XRD patterns on various crystals
Figures 10a-10d. (10a) FAOR and (10b) MOR in 20 wt% Pt / C, 4 wt% Pt NP / ATO, 4 wt% Pt1 / ATO, ATO 100, ATO 400, and 4 wt% Pt / SnO ₂ . Scan speed 50 mV / s at 1600 rpm. ATO was reduced at 100 ° C (ATO 100) and 400 ° C (ATO 400), respectively. 4 wt% Pt / SnO ₂ was prepared in the same manner as 4 wt% Pt 1 / ATO except that SnO ₂ without Sb was used as the support. (10c) HR-TEM images, and (10d) the HR-TEM image of close-up of Pt / SnO _2.
11a-11b. OER LSV curves normalized to (11a) Pt weight and (11b) electrode area at 20 wt% Pt / C, 4 wt% Pt NP / ATO and 4 wt% Pt1 / ATO. The reaction was carried out in Ar-purged 0.1 M HClO ₄ solution at a scan rate of 5 mV / s, 1600 rpm.
12A-12D. (12a) TEM image of 1 wt% Pt1 / ATO and (12b) 8 wt% Pt1 / ATO sample. (12c) XANES and (12d) EXAFS data with various Pt contents in Pt1 / ATO.
13a-13b. 1 wt% Pt1 / ATO (13a) STEM and (13b) corresponding EDS mapping image.
Figure 14. FAOR forward scan of 20 wt% Pt / C and various Pt contents of Pt1 / ATO after 1800 cycle stability test. The stability test was carried out by repeating the CVs in the Ar-saturated 0.1 M HClO ₄ and 0.5 M formic acid solution at a scan rate of 50 mV / s at 1600 rpm, ranging from 0.05 V to 1.4 V.

Hereinafter, the present invention will be described in more detail with reference to Examples. It is to be understood by those skilled in the art that these embodiments are only for describing the present invention in more detail and that the scope of the present invention is not limited by these embodiments in accordance with the gist of the present invention .

Example

Materials and Methods

One. Catalyst synthesis

(H ₂ PtCl ₆ .6H ₂ O, Sigma-Aldrich; 1.3 mg in 1% by weight, 5.5 mg in 4% by weight) in 15 μl of anhydrous ethanol (≤0.005% water, Sigma-Aldrich) And 11.5 mg at 8 wt%) were mixed with 50 mg ATO (99.5%, Alfa Aesar) and dried overnight in a vacuum oven at 50 < 0 > C. In a tube furnace under a 10 vol% H ₂ (N ₂ balance) for 1 hour at a flow rate of 200 sccm, the Pt precursor was supported on PtO ATO (Pt NP / ATO) at 100 ° C. or Pt nanoparticles Lt; / RTI >

2. Electrochemical measurement

All electrochemical measurements for the half-cell test were performed at 298 K using a three-electrode electrochemical cell with bi-potentiostat (CHI 730E, CH Instruments, Inc.). 3M NaCl Ag / AgCl (RE-5B, BASi) and coiled Pt wires were used as the reference electrode and the counter electrode, respectively. Prior to each measurement, reversible hydrogen electrode (RHE) potentials were measured by hydrogen release and oxidation in a H ₂ -purified 0.1 M HClO ₄ solution using a rotating platinum electrode (70%, Sigma Aldrich) to correct potentials. All potentials represent potentials relative to RHE. All catalyst inks were prepared by sonication of 5 mg of catalyst dispersed in 5 ml of absolute ethanol for 15 minutes. 15 μg of the catalyst was deposited on a rotating ring disk electrode (RRDE), and 10 μl of the nepheline solution prepared by dispersing 10 μL of lysozyme (5 wt%, Aldrich) in 1 ml of anhydrous ethanol was deposited . Activation was performed by cyclic voltammetry (CV) from 0.05 V to 1.1 V for 50 cycles at a scan rate of 100 mV / s prior to the electrocatalytic reaction test. FAOR or MOR was performed by performing CV in 0.1 M HClO ₄ Ar-saturated with 0.5 M HCOOH or 0.5 M methanol solution at 1600 rpm at a scan rate of 50 mV / s. The stability of a single atom platinum catalyst was tested by repeating CV for 1800 cycles in the range of 0.05-1.4 V at a scan rate of 50 mV / s in argon purge 1600 rpm, 0.1 M HClO ₄ + 0.5 M HCOOH solution. The ORR was performed with a linear scan voltage method (LSV) from O ₂ -saturated 0.1 M HClO ₄ solution to 0.05 V-1.1 V at a scan rate of 10 mV / s at 1600 rpm. IR correction was performed prior to ORR measurement using CHI software. The OER was measured by performing LSV in Ar-saturated 0.1 M HClO ₄ solution at a scan rate of 5 mV / s at 1600 rpm.

3. Characterizations

The image of the catalyst was obtained using a transmission electron microscope (TEM, Tecnai TF30 ST, FEI) equipped with a Cs compensator and a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM, Titan cubed G2 60-300, FEI) 200 kV). The crystal structure was measured with an X-ray diffraction (XRD) pattern (Rigaku, Cu Kα radiation) at a scan rate of 2 ° / min. Electronic structures were observed with X-ray photoelectron spectroscopy (XPS, Kα, Thermo VG Scientific). The binding energy was measured by placing the peak intensity of C 1s peak at 284.8 eV as a reference. X-ray absorption near edge (XANES) structure and X-ray absorption microstructure (EXAFS) were measured in a Pohang Light Source (PLS) 8C nano XAFS beamline. The actual Pt content of the catalyst was determined by inductively coupled plasma-emission spectroscopy (ICP-OES 720, Agilent) after the catalyst was dissolved in aqua regia. The Pt content leached into the solution after the stability test was measured with an ICP-mass spectrometer (ICP-MS, Perkin-Elmer).

4. MEA machining and single cell test

Membrane electrode assemblies (MEAs) for DFAFC were fabricated using a catalyst coated diffusion-layer (CCD) method with a geometric area of 5 cm ² . For the anode, the prepared catalyst was deposited on the gas diffusion layer (CNL energy) by spraying. The catalyst ink was prepared by mixing a catalyst dispersed in a solvent (deionized water: isopropyl alcohol = 1: 3) with Nion's solution (5 wt%, Aldrich). Pt content was 0.4 mgPt / cm ² for 0.04 mgPt / cm2 and Pt / C for Pt1 / ATO in the positive electrode. The negative electrode was made of 40 wt% Pt / C and the Pt content was much higher than 2 mgPt / cm ² to avoid the effect from the reduction reaction. The content of the endion ionomer in both the anode and the cathode was adjusted to 10 wt% in the catalyst layer.

The electrode was then hot pressed with a Nafion 212 membrane at 135 DEG C and 430 psi for 135 seconds. The prepared MEA was tested under a single cell configuration consisting of a graphite flow channel, a gasket and a current collector. Activation was performed in two steps. First, the battery temperature was set at 60 占 폚, a 7 M HCOOH solution was supplied to the positive electrode at a flow rate of 2 sccm, and H ₂ (100% RH) gas was introduced at a flow rate of 100 sccm before humidification. Cyclic voltammetry was performed in a voltage range of 0.05 V - 1.1 V until the current density saturates. Second, the inlet to the cathode was changed to a humidified O ₂ (100% RH) gas with a flow rate of 200 sccm, and cyclic voltammetry was performed in a voltage range of 0.1 V to 0.7 V until the current density saturates. After activation, the IV polarization curves were measured by increasing the current density by 2 mA / cm ² per second.

5. Computational Details

Density function theory (DFT) calculations were performed using the Vienna ab initio simulations (VASP) package to investigate the properties of single nuclei Pt in ATO. Exchange-correlation effects were tested by Perdew-Burke-Ernzerhof (PBE) based on GGA (generalized gradient approximation) function. The wave function consisted of an extension of the plane wave with an energy cutoff of 450 eV. Geometry relaxation was performed using the junction gradient method until the residual force for each atom was less than 0.02 eV / A. Based on experimental observations, and considering the Sn, SnSb, SnO _2, and ATO in the calculation. To model bulk antimony tin oxide (ATO), we first considered a 48 atom 2 × 2 × 2 supercell of rutile SnO ₂ (Sn ₁₆ O ₃₂ ). One of the 16 Sn atoms was replaced by one Sb atom. A 4 × 4 × 6 k-point mesh was generated for the Brillouin zone sampling by the Monkhorst-Pack method. For all ATO calculations, the lattice parameter was fixed to the theoretical value of undoped SnO2 bulk. Bulk Sn and SnSb were modeled as tetragonal (?) And NaCl phases, respectively. A periodic slab model was used to simulate the surface. The dipole correction is added perpendicular to the surface. In order to model the ATO nanoparticles after reduction, we considered the fully reduced (110), (100), (101) and (211) surfaces. Surface oxygen was decomposed and a surface layer of Sn ^{2 +} having a threefold coordination was generated. The present inventors considered (100) planes for the surfaces of? Sn and SnSb. All super cells used in the calculation are shown in Fig. The substitution energy was calculated as E _sub = (E _doped + E _atom ) - (E _pure + E _Pt ). Wherein E _sub substituted energy, E _doped Pt and with a substituent bulk / surface energy, E _atom is Pt atoms bulk / Mo atom (parent atom) in the surface energy, E _pure energy of the pure bulk / surface to be.

Actual results and discussion

Various synthetic methods have been reported for the manufacture of SACs, including incipient wetness impregnation, co-precipitation, and metal vacuum deposition. Incipient wetness impregnation and co-precipitation are the most widely used methods in the world for the production of heterogeneous catalysts, and the vacuum deposition method is not suitable because of high facility cost and low catalyst production. It is desirable to produce SAC using existing methods.

The present inventors used a typical initial wet impregnation method to make a single atom Pt on an antimony-doped tin oxide (ATO) support. Pt precursors with various Pt weight percentages (1, 4, and 8 wt%) were mixed with an ATO support and reduced at different temperatures of 100 캜 and 400 캜. When 4 wt% Pt was deposited on the ATO support, the characteristics of the deposited Pt were very different depending on the reduction temperature. 1a shows the formation of separate Pt nanoparticles when reduced at 100 < 0 > C. The size of the Pt nanoparticles was estimated to be 2.5 +/- 0.7 nm (Figure 8). When the Pt precursor was reduced at 400 ° C, no Pt nanoparticles were observed. Instead, only the ATO domain was observed as in Figure 1a (b). TEM images of the ATO support before and after reduction at 400 ° C are shown in Figures S2 (c) and (d), and no differences from Figure 1a (b) were found. In the HAADF-STEM image of Fig. 1A (d), Pt nanoparticles did not appear. However, the EDS image with the QMap net counts mode of FIG. 1A (c) clearly showed the existence of the Pt domain.

The geometry of the catalyst was confirmed by X-ray absorption spectroscopy. The EXAFS results in Figure 2a clearly show the difference between the reduced sample at 100 ° C and 400 ° C. The sample reduced at 100 ° C (denoted "Pt NP / ATO") showed similar data to commercially available Pt / C catalyst. The number of Pt-Pt coordination was estimated to be 7.15 for Pt / C and 7.29 for Pt NP / ATO as shown in Table 1. However, the sample reduced at 400 ℃ did not show a peak at 2.6A, unlike Pt NP / ATO or Pt / C, but a new peak at 2.4A. This peak is due to the Pt-Sn interaction. The number of Pt-Sn coordination was estimated to be 2.61. Since the Pt-Pt interaction was not observed in the reduced sample at 400 ° C, this sample was labeled "Pt1 / ATO" indicating that Pt was dispersed as a single atom. A weak Pt-Cl interaction was observed for Pt NP / ATO with a coordination number of about 0.38, but not for Pt1 / ATO, presumably because all of the Cl ligands were removed during the high temperature reduction process. The platinum single atom structure was confirmed by an expanded HAADF-STEM image. (F) and (h) in FIGS. 1B and 1C show the HAADF-STEM image enlarged in (e) and (g), showing that the Pt single atom (represented by a red circle) is on the SnSb and SnO ₂ surfaces . Since all Pt single atoms are located on the lattice constellation, it is assumed that the Pt atoms displace atoms on the lattice. A lattice pattern of SnSb 202 (d = 0.159 nm), SnO ₂ (101) (d = 0.271 nm) and SnO ₂ (200) (d = 0.219 nm) Clearly observed. The lattice spacing was similar to the previously reported values (DF Zhang et al., Adv. Mater., 2003, 15, 1022-; HG Yang and HC Zeng, Angew. 5930-5933).

Fitting values for EXAFS analysis of Pt foil, 20 wt% Pt / C, 4 wt% Pt NP / ATO and 4 wt% Pt1 / ATO. Sample Path N R (A) Debye-Waller factor
(σ ² / ÅA ² ) R-factor Pt foil Pt-Pt 12 2.76 0.005 0.005 Pt-Pt 6 3.91 0.007 Pt / C Pt-Pt 7.15 2.76 0.007 0.016 Pt-O 0.75 2.02 0.000 Pt NP / ATO Pt-Pt 7.29 2.76 0.006 0.020 Pt-O 0.13 2.07 0.003 * Pt-Cl 0.38 2.26 0.003 Pt1 / ATO Pt-Pt - - - 0.018 Pt-Sn 2.61 2.65 0.003 Pt-O 0.77 2.05 0.003 *

* This factor was fixed during the fitting

The electronic structure of Pt was measured by XANES as shown in FIG. 2B. Pt / C exhibited high white line strength as an oxide characteristic, but Pt NP / ATO exhibited very similar white line strength to Pt foil. Electron transfer from the ATO support to the Pt nanoparticles results in a high degree of metallicity of the Pt nanoparticles. Pt1 / ATO showed clear XANES data. As is common in Pt alloys, the position of the white line peak shifted to higher energy compared to other Pt catalysts. The surface properties were further characterized by the XPS shown in Figures 2c and 2d. Generally Pt 4f XPS peak may be as indicated by the gray dotted line in the Pt / C Pt ^0, convolution (deconvolution) to see Deacon as Pt ^2+, and Pt ^{+ 4.} Pt NP / ATO showed slightly lower binding energy for Pt ⁰ peak, and electron transfer from ATO to Pt was also confirmed. However, Pt1 / ATO showed only Pt ^{2 +} peak without Pt ⁰ or Pt ^{4 +} peak. In Pt1 / ATO surface, all Pt is present in the form of Pt ^{+ 2,} not metal Pt atom. Interestingly, surface Sb peaks were hardly observed in Pt1 / ATO and clearly observed in Pt NP / ATO.

The XRD data of Figure 9 show that ATO and Pt NP / ATO only have SnO2 crystalline phase, while Pt1 / ATO additionally has SnSb and metal Sn phases. Pt can replace Sn or Sb on the ATO support after the reduction process. Table 2 shows the Pt displacement energy in various bulk or surface structures calculated by density function theory (DFT). The substitution energy is negative when Pt replaces Sn or Sb on metal Sn or SnSb, or Sb on ATO bulk or ATO surface. It was the most thermodynamically stable to replace the Sb site with Pt on SnSb. Also, as can be seen from the values in parentheses in Table 2, replacing the Sb site with Pt on the ATO surface was more energetically stable on all ATO facets.

Substitution energy (eV) of bulk and surface Pt atoms estimated by DFT calculation. The subscript 'r' indicates the reduced surface. Alternative energies in the presence of Sn or Sb below the surface are indicated in parentheses. Site Bulk (eV) Surface (eV) Sn SnSb ATO Sn
(100) SnSb
(100) ATO
(100) _r ATO
(110) _r ATO
(101) _r ATO
(211) _r Sn -1.84 -1.80 1.45 -2.31 -2.47 1.67
(1.29) 0.14
(1.39) 0.81
(1.12) 1.69
(0.97) Sb - -2.11 -0.07 - -2.89 0.57
(-0.62) 0.69
(-0.48) -0.35
(-0.63) 1.22
(-1.50) O - - 5.16 - - 1.41 1.21 1.21 1.57

The electrocatalytic activity of the prepared catalysts was tested in various electrochemical reactions. Figure 3a shows a cyclic voltammetry in Ar- purge 0.1M HClO ₄ aqueous solution for Pt / C, Pt NP / ATO and Pt1 / ATO (CV). Pt nanoparticles typically exhibit an under-potential deposition of hydrogen (Hupd) peak at a potential range of 0.02-0.3 V. These peaks were clearly observed in Pt / C and also broad peaks were observed in Pt NP / ATO. However, Pt1 / ATO did not show Hupd peaks in the same range. Hupd is known to form on steps, defects and Pt ensemble sites at terrace sites. Without the Hupd peak in Pt1 / ATO, there is stronger evidence that Pt nanoparticles are absent.

Figure 3b shows a forward scan of the CV obtained from an electrocatatitive formic acid oxidation reaction (FAOR). FAOR follows a direct or indirect path. The direct path requires one or two Pt atoms to activate the OH bond of formic acid, but the indirect path requires a Pt ensemble site to activate the CO bond. The oxidation peak is typically ~ 0.6V for the direct path and ~ 1.0V for the indirect path. Pt1 / ATO showed a large peak at 0.6 V without a peak for the indirect path. Pt / C followed the indirect path and there was a negligible peak for the direct path. Pt NP / ATO showed a peak in both direct and indirect pathways. When the platinum mass activity was compared at 0.6 V, it was 3.35 A / mgPt for Pt / ATO, 1.30 A / mgPt for Pt NP / ATO and 0.18 A / mgPt for Pt / C. The current density normalized by the electrode area can also be seen in FIG. The ATO support and Pt / SnO ₂ prepared by the same synthetic method showed no activity against FAOR. This indicates that Pt on the ATO is the active site and the conductivity of the support is important when used as an electrode catalyst. It is known that a Pt ensemble site is required for electrode catalyst methanol oxidation (MOR). As can be seen in FIG. 3c, Pt1 / ATO without Pt ensemble sites does not exhibit activity against MOR.

Instead, Pt / C showed the highest Pt mass activity. Pt1 / ATO exhibited exceptionally high activity for FAOR but no activity for MOR; This confirmed that there is a single atom Pt site without Pt nanoparticles.

The electrocatalytic oxygen reduction reaction (ORR) has two electron paths (O ₂ + 2H + + 2e- → H ₂ O ₂ ) and four electron paths (O ₂ + 4H + + 4e- → 2H ₂ O). Electrons (two electron paths) can occur on a single atom Pt, while the latter (four electron paths) require a Pt ensemble site that breaks the OO bond. Pt / C followed 4 electron paths with H ₂ O ₂ selectivity mostly less than 3%, whereas Pt 1 / ATO showed much higher H ₂ O ₂ selectivity, as shown in FIG. 3 d. The electrode catalyst oxygen evolution reaction (OER) was also tested as in FIG. Pt NP / ATO and Pt1 / ATO were almost inactive for OER.

The stability of the single atom platinum catalyst was tested by repeating the CV for 1800 cycles in the range 0.1 M HClO ₄ + 0.5 M formic acid solution, 0.05-1.4 V. Figure 4a shows the change in Pt mass activity in a forward scan of the CV after repeated cycles. The Pt mass activity for the direct pathway of FAOR after 1800 cycles was 0.59 A / mg Pt for Pt1 / ATO, 0.35 A / mg Pt for Pt NP / ATO and 0.003 A / mg Pt for Pt / C. While Pt / C lost most of the activity, Pt1 / ATO still maintained high activity against FAOR. Although the carbon support was severely degraded at high potentials, the ATO support was much more stable. The amount of Pt leached into the solution was measured by ICP-MS after the stability test. Compared with the original amount of Pt, Pt, C, Pt NP / ATO and Pt1 / ATO dissolve 82.3%, 20.3% and 12.3% Pt, respectively. Figures 4b-4d show TEM images of each catalyst after stability testing. Although many Pt aggregates were observed in Pt / C, Pt aggregates did not appear in Pt NP / ATO. Pt1 / ATO retained only a few Pt nanoparticles after the stability test.

Different weights of Pt were deposited on the ATO support. Figure 6 shows a TEM image of 1 wt% Pt1 / ATO and 8 wt% Pt1 / ATO. In both samples, the Pt nanoparticles were not noticeable and only images very similar to the ATO support were obtained. The XANES data for the 1, 4 and 8 wt% Pt1 / ATO samples are very similar to those shown in Figure 12C. The EXAFS data of 8 wt% Pt1 / ATO also did not show the Pt-Pt interaction, indicating the absence of Pt nanoparticles at high Pt content.

Due to the low Pt content, reliable EXAFS data could not be obtained even at 1 wt% Pt1 / ATO. Single atom Pt structure can be obtained up to 8 wt% Pt content. The activity and stability according to various Pt contents in the Pt1 / ATO sample were tested as in FIG. All samples followed the FAOR direct path, indicating a single peak at ~ 0.6V. In comparison of Pt mass - normalized activity, 1 wt% Pt1 / ATO showed the highest value of 9.16A / mgPt, while 8 wt% Pt1 / ATO showed a much lower value of 0.89 A / mgPt. This difference arises because in a sample with a high Pt content, a significant portion of the single atom Pt is located inside the support without being exposed to the surface. The EDS image of the 1 wt% Pt / ATO sample of FIG. 13 shows a uniform Pt distribution on the ATO support without a separate Pt domain, unlike the 4 wt% Pt1 / ATO sample. Both Pt1 / ATO samples with various Pt contents showed high stability over FAOR repeat cycles at 0.05-1.4V. After 1800 cycles, 48.7% and 8.8% of the original Pt amount was leached into the solution in the 1 and 8 wt% Pt1 / ATO samples. Figure 14 shows a forward scan of FAOR for Pt / C and Pt1 / ATO catalysts after a stability test. Pt / C showed little activity against FAOR. All Pt1 / ATO samples with different Pt contents showed only the peak for the direct path and the peak for the indirect path was indistinguishable. This indicates that the Pt1 / ATO sample retains the Pt structure as a single atom even after severe stability testing.

A 4% wt Pt1 / ATO sample was used as a cathode catalyst to prepare a full battery for DFAFC. Pt content was 0.04 mgPt / cm ² and Pt / C in 0.4 mgPt / cm ² in Pt1 / ATO of the positive electrode. The polarization curve of FIG. 6A shows that Pt1 / ATO catalysts have higher catalytic activity and higher voltages in the low current density region, while the actual Pt content is only one tenth of the Pt / C catalyst. Maximum power density is Pt1 / ATO For the case of 35.3mW / cm ² and the Pt / C catalyst was 38.1mW / cm ^2. The power density normalized to Pt mass is 0.882W / mgPt in Pt1 / ATO and 0.095W / mgPt in Pt / C. The Pt1 / ATO catalyst showed excellent performance in DFAFC while minimizing the amount of Pt used.

conclusion

Single atom Pt catalysts were successfully prepared on an ATO support (Pt1 / ATO) by an initial wet impregnation method. When the Pt precursor mixed with the ATO support was reduced at 400 ° C, Pt replaced the Sb position on the surface of the bulk phase or SbSn or ATO. This substitution was confirmed by DFT calculation showing energy preference. HAADF-STEM images and EXAFS analysis showed that Pt was present as a single atom without Pt nanoparticles. Single atom Pt structures were obtained for various Pt contents up to 8 wt%. Pt1 / ATO catalysts were used for various electrochemical reactions of FAOR, MOR, ORR and OER. Single atom Pt follows a direct path to FAOR and two electron paths to ORR, while Pt nanoparticles follow an indirect path to FAOR and four electron paths to ORR.

In particular, Pt1 / ATO showed excellent activity and stability against FAOR. When the stability of Pt1 / ATO was tested by CV repetition in the range of 0.5-1.4V, Pt1 / ATO still maintained high activity for FAOR following direct pathway after 1800 cycles. Commercial Pt / C catalysts lost activity after the same stability test. This remarkable stability appears as an energy-stable structure of ATO's corrosion resistance and Pt1 / ATO. A DFAFC complete battery was fabricated using Pt1 / ATO catalyst as an anode. The amount of Pt was only 1/10, but the performance of the battery was equivalent to that of commercial Pt / C. The Pt1 / ATO catalyst showed high activity, selectivity and stability as an electrocatalyst. The present invention suggests the possibility of practical application of single atom catalysts in many other heterogeneous catalytic reactions.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the same is by way of illustration and example only and is not to be construed as limiting the scope of the present invention. Accordingly, the actual scope of the present invention will be defined by the appended claims and their equivalents.

Claims (11)

A method for preparing a single atomic platinum catalyst for direct formic acid fuel cells (DFAFCs) comprising the steps of:
(a) mixing 2 wt% to 20 wt% of a platinum precursor, and antimony-doped tin oxide (ATO) support on an entire mixed composition in an organic solvent; And
(b) depositing a platinum single atom on the surface of the support by replacing the Sn and Sb atoms on the surface of the support by a platinum single atom by heating the result of the step (a) to 400 ° C or higher to reduce the platinum precursor :
The resultant single atom platinum catalyst of step (b) comprises 1 wt% to 8 wt% of platinum based on the total weight of the support and the single atom platinum.
The method of claim 1, wherein the platinum precursor is a platinum salt.
The method of claim 1, wherein the platinum precursor is selected from the group consisting of chloroplatinic acid hexahydrate (H ₂ PtCl ₆ .6H ₂ O), H ₂ PtCl ₆ , platinum (II) acetylacetonate (Pt (acac) ₂ ), potassium tetrachloroplatinate carbonate (K ₂ PtCl _4), hydrogen hexachloro-platinum carbonate (H ₂ PtCl _4), platinum (ⅱ) cyanide (Pt (CN) _2), platinum (ⅱ) chloride (PtCl _2), platinum (ⅱ) Wherein the catalyst is selected from the group consisting of bromide (PtBr ₂ ), K ₂ PtCl ₆ , Na ₂ PtCl ₄ .6H ₂ O, and Na ₂ PtCl ₆ .6H ₂ O and mixtures thereof.
The method of claim 1, wherein the organic solvent is selected from the group consisting of methanol, ethanol, butanol, isopropyl alcohol, pentane, hexane, heptane, cyclohexane, toluene, acetone, ethyl acetate, methylene chloride, chloroform, ether, petroleum ether, acetonitrile, , Ethylene glycol, propylene glycol, butylene glycol, and mixtures thereof.
The method of claim 1, further comprising vacuum drying the resultant of step (a) after step (a).
delete 6. An electrode for a direct formic acid fuel cell (DFAFCs) comprising a single atomic platinum catalyst for a fuel cell produced by the method of any one of claims 1 to 5.
A direct formic acid fuel cell (DFAFCs) comprising a single atomic platinum catalyst prepared by the process of any one of claims 1 to 5. delete delete delete

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