Platinum Nanostructure Tailoring for Fuel Cell Applications Using

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22398−22407

Platinum Nanostructure Tailoring for Fuel Cell Applications Using Levitated Water Droplets as Green Chemical Reactors Gumaa A. El-Nagar,*,†,‡ Ö znur Delikaya,‡ Iver Lauermann,§ and Christina Roth*,‡ †

Chemistry Department, Faculty of Science, Cairo University, 12613 Cairo, Egypt Institute for Chemistry and Biochemistry, Freie Universität Berlin, 14195 Berlin, Germany § PvcomB, Helmholtz-Zentrum Berlin für Materialien und Energie, 12489 Berlin, Germany ‡

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S Supporting Information *

ABSTRACT: Tailoring of nanostructured materials with well-controlled morphologies and their integration into valuable applications in a facile, cheap, and green way remain a key challenge. Herein, platinum nanoparticles as well as Pt− polymer nanocomposites with unique shapes, including flower-, needle-, porous-, and worm-like structures, were synthesized and simultaneously deposited on a three-dimensional carbon substrate and carbon nanofibers in one step using a levitated, overheated water drop as a green, rotating chemical reactor. Sprinkling of a metal aqueous solution on a hot surface results in its sudden evaporation and creates an overheated zone along with the water self-ionization (i.e., charge separation) at the hot interface. These generated Leidenfrost conditions are believed to induce a series of chemical reactions involving the used solvent and counterions, resulting in the nanoparticles formation. Besides, the in situ generated basic conditions in the vicinity of the liquid−vapor interface due to the loss of hydronium ions into the vapor layer could also play a role in the mechanism of the nanoparticles formation, e.g., by discharging. The as-prepared Pt nanostructures exhibited a superior catalytic activity and stability toward the desired direct formic acid oxidation (essential anodic reaction in fuel cells) into CO2 without generating CO poisoning intermediates compared to the state-of-the-art commercial PtC electrode. The addressed nanotailoring technique is believed to be a promising, inexpensive, and scalable way for the sustainable manufacture of well-designed nanomaterials for future applications. KEYWORDS: nanostructures, fuel cells, electrocatalysis, Leidenfrost, green chemistry controlled shape (e.g., dendrites, flowers, porous particles, etc.) is difficult to achieve with traditional synthetic approaches such as wet-chemical routes, electrochemical techniques, and top-down synthesis strategies (e.g., chemical etching and templating). All of these aforementioned approaches have their own drawbacks in terms of cost, complicated procedures and devices, post-treatment, time and energy consumption, high temperature, environmental hazardous chemicals, and wastes.9−17 For example, most chemical synthesis approaches use reducing agents (e.g., hydrazine, sodium borohydride, etc.), of which the majority is toxic having harmful impact on both the environment and human organs.12,18 Additionally, various capping agents including carboxylic acids and hydrophobic solvents are normally used to control the morphology of the created nanoparticles together with preventing their agglomeration.19 These preparation approaches have many shortcomings; for instance, post-treatment is required to eliminate the excess organic ligands, which strongly adsorb

1. INTRODUCTION Direct formic acid fuel cells (DFAFCs) have recently received significant attention as promising power sources for small devices and portable applications, thanks to the environmental benignity of formic acid (FA) and its low crossover enabling the use of very high fuel concentrations.1−6 However, the high cost of the currently used commercial electrodes (i.e., Pt-based materials) and their insufficient stability are the major roadblocks in their worldwide commercialization. The poor stability of the commercial Pt-based electrodes is attributed to the agglomeration and/or dissolution of Pt active sites leading to a significant and permanent reduction in the electrochemically active surface area associated with inferior performance. In addition, CO intermediates that get generated in situ from “nonfaradic” dissociation of FA poison the active Pt surface sites already after a short time via a strong irreversible metal− ligand backbonding.1,2,7,8 The development of novel techniques that enable the facile tailoring of nanomaterials, their assembly, and incorporation into useful devices in a cheap and simple way is one of the most urgent challenges in the field of nanochemistry. In general, the synthesis of metal nanoparticles with well© 2019 American Chemical Society

Received: March 22, 2019 Accepted: May 31, 2019 Published: May 31, 2019 22398

DOI: 10.1021/acsami.9b05156 ACS Appl. Mater. Interfaces 2019, 11, 22398−22407

Research Article

ACS Applied Materials & Interfaces

affected by the chemical properties (e.g., composition and surface energy) and thermophysical features, including thermal conductivity and density of the formed liquid/solid interface. Besides, the solid surface properties, such as roughness, wettability, topography, etc., also have an essential impact on the levitated Leidenfrost droplet.21,22 Also, Leidenfrost20 found that a remnant solid was left on the hot plate after complete evaporation of the water droplet. Recently, Elbahri et al.18,23 have found that a small droplet loaded with a precursor salt (metal ions) left a residual material with nanoparticulate shapes, such as wires and rings, which are created via the so-called Leidenfrost effect. Bain et al.24 used levitated Leidenfrost droplets to accelerate some chemical reactions and organic syntheses, such as base-catalyzed Claisen−Schmidt condensation, hydrazone formation, and Katritzky conversion of pyrylium into pyridinium. Moreover, Lee et al.25 used the inverse setup of the traditional Leidenfrost droplets to fabricate hierarchical porous Pt and Pd structures. This interesting discovery opened a new avenue for the green tailoring of nanomaterials. Herein, the levitated water droplet was used as a green, overheated chemical reactor to design platinum nanostructures and Pt−polymer hybrid materials with different shapes, which were simultaneously and homogeneously decorated on 3D carbon substrates (e.g., commercial carbon felts, commercial gas diffusion layers and electrospun carbon nanofiber networks). The performance of the as-prepared Pt nanostructures toward the anodic reaction in direct formic acid fuel cells (DFAFCs) is also reported. The created Pt nanostructures using the above-described technique (Leidenfrost method) exhibited a superior activity and stability compared to the state-of-the-art commercial carbon-supported PtC electrode.

to the metal particle surfaces and thus reduce the number of accessible active sites. Furthermore, the carefully synthesized catalyst may agglomerate and its structure will be destroyed during the electrode preparation process, for example, during the sonication step. However, wet-chemical synthesis approaches, which are considered as simple and inexpensive alternatives, have as well serious shortcomings, as they may be time-consuming, using hazardous chemicals and many processing steps. Accordingly, finding a facile, eco-friendly, one-pot, and low-cost synthesis route for the reproducible synthesis of nanoparticles with precisely controlled shapes together with their simultaneous deposition on three-dimensional (3D) substrates (directly applied as electrodes in the fuel cell) is still a challenge. Consequently, it is hard to believe that a hot frying pan and water droplets could be enough to create nanoparticles with well-controlled shapes and simultaneously coat them on a 3D substrate in one step. Sprinkling a water drop on top of a hot skillet is a daily event that was first examined by Leidenfrost.20 He observed that when a cold water droplet touches a heated surface at temperatures much higher than the boiling point of water, its contact layer with the hot surface will vaporize instantaneously resulting in levitation of the droplet on its own vapor sheet11,21 (see Scheme 1). This formed Leidenfrost state is significantly Scheme 1. Three-Dimensional Sketch of the Leidenfrost State

2. METHODS 2.1. Chemicals. All of the chemicals used in this study were ultrapure purchased from Sigma-Aldrich and used as received without any further purification. All of the solutions were prepared using Milli-

Figure 1. Electronic images showing the fabrication of carbon felt pieces decorated with Pt nanostructures using the levitated Leidenfrost drop approach and displaying the change of the levitating drop of 1 mL of 2 mM H2PtCl6 aqueous solution containing 50 μL of 0.5 M formic acid (framed with green color, a−d) and 1 mL of 2 mM H2PtCl6 aqueous solution (e−h, framed with red color) with time. 22399


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Figure 2. (a−k) SEM images of bare carbon felt (a) and carbon nanofibers (d), and coated carbon felt (b, c) and coated carbon nanofibers (e, f) with Pt dandelion-like flowers created using 1 mL drop of 2 mM H2PtCl6 + 50 μL 0.5 M HCOOH. (g−i) SEM images of coated carbon felts with Pt particles using 1 mL drop of 2 mM H2PtCl6 in the absence (g) and presence of 200 μL (h) and 100 μL (i) of 1 M NaOH. (j, k) SEM images of coated carbon felts with Pt−chitosan nanocomposite obtained via using 1.5 mL drop composed of 100 μL of 0.5 M HCOOH, 200 μL of 0.1 M chitosan, and 1.2 mL of 2 mM H2PtCl6. (I) Raman spectra of commercial carbon felt before (1) and after (2−9) insertion in a levitated drop of (2) water, (3) 0.1 M NaOH, and (4) 0.1 M HCOOH; (5−9) are the Raman spectra of images (g), (b), (h), (I), and (k), respectively. Q water with resistivity of ∼18.2 MΩ cm. Dihydrogen hexachloroplatinate(IV) hydrate (purity, 99.99%), formic acid (purity, ca. 98−100 %), and chitosan (mol. wt, 50 000−190 000) were obtained from Sigma-Aldrich. Commercial carbon felt (GFA6 EA SGL Company) was used as substrate and decorated with the created Pt nanostructures. 2.2. Instruments. The electrochemical experiments were performed in a three-electrode electrochemical setup using GAMRY potentiostat/galvanostat (Reference 600), in which a piece of carbon felt (8 mm diameter) modified with Pt nanostructures deposited via the Leidenfrost technique, connected with a glassy carbon electrode (1 mm diameter and 50 mm length), was used as working electrode. A piece of carbon felt with a thickness of 6 mm and a size of 1 × 10 cm served as a counter electrode, and a saturated calomel electrode (SCE) with a potential of +0.261 V vs standard hydrogen electrode was used as the reference electrode. The performance of the asprepared catalyst materials for the anode reaction in DFAFCs (i.e., HCOOH oxidation) is investigated in 0.5 M H2SO4 containing 0.3 M HCOOH. A scanning electron microscope (HITACHI UHR FE-SEM SU8030) equipped with an energy-dispersive X-ray spectrometer (JEOL JSM-6510) was used to evaluate the obtained Pt particles morphology and chemical composition. X-ray diffraction (XRD) in

transmission geometry using a STOE STADI-P operated with Cu Kα (λ = 1.54 Å) radiation and position-sensitive detector was used to detect the crystallographic structure of the as-prepared catalysts. X-ray photoelectron spectroscopy (XPS), using a CLAM4 electron analyzer from Thermo VG Scientific and a Mg Kα X-ray source (1253.6 eV) XR 50 from SPECS was used to determine the samples’ chemical (surface) composition. For evaluation, a linear background was subtracted and O 1s peaks were fitted using Gaussian functions with identical full width at half-maximum (FWHM) for each component of the same element, while C 1s peaks were fitted using Lorentizan for sp2 carbon and Gaussian for other carbon species. Additionally, Pt 4f peaks were fitted using Lorentizan shape with identical FWHM.26 Raman spectra were recorded on a RENISHAW inVia Raman microscope with an Ar laser source of 633 nm. 2.3. Catalyst Preparation. Pt nanostructures were synthesized in a levitating and rotating water drop reactor in one step without using additional reducing agents or sophisticated and expensive instruments. By this approach, Pt nanoparticles with various geometries, including flowers, needles, and fibers, were fabricated and instantaneously deposited onto 3D substrates (i.e., commercial carbon felts) via only placing an aqueous solution containing Pt ions gently on a preheated surface at 300 °C. For example, Pt nanoparticles with dandelion-flower-like structures were tailored and 22400


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ACS Applied Materials & Interfaces directly loaded on carbon felts as follows (see Figure 1): first, 1 mL of Milli-Q water was gently dripped onto a preheated surface (with temperature 300 °C) to create the levitating water droplet utilized as eco-friendly, charged chemical reactor to fabricate nanoparticles. Then, a piece of a commercial carbon felt was inserted inside this levitating, rotating water drop (Figure 1a) and kept for 60 s, before adding the aqueous platinum salt solution, to allow the water to wet the entire fibers of the hydrophobic carbon felts. Next, a certain volume of 1 mM dihydrogen hexachloroplatinate containing 50 μL of 0.5 M formic acid (HCOOH and H2PtCl6 volumes can be changed to tune the Pt loading and structures) was added to this precreated levitating water droplet. The transparent color of the levitating drop directly turned into faint yellow, which is the color of the Pt salt. A naked eye observation of the levitating droplet of platinum salt solution shows that its faint yellow color rapidly turned into golden yellow, then into black simultaneously with shrinking of the levitating drop size (Figure 1b−d). Then, the obtained carbon felt loaded with Pt nanostructures (after complete evaporation of the levitated drop) was cleaned five times using 1 mL of Milli-Q water. The color of the levitating droplet changes with the composition of the platinum salt solution. For instance, in the presence of traces of formic acid, the color of the levitating Pt salt solution quickly turned from faint yellow into golden yellow and black, while in the absence of formic acid, its color changed from faint yellow into plasmonic red (Figure 1e−h). The same technique (levitating drop) is also used for the synthesis of Pt−polymer hybrid nanostructured materials, and their respective solution compositions are given in the Results and Discussion section. Commercial PtC catalyst and modified commercial carbon felt with electrodeposited Pt nanoparticles were prepared and used for comparing the performances of our prepared Pt nanostructuremodified carbon felts created by Leidenfrost technique toward the anodic reaction of the direct formic acid fuel cells. The synthesis procedures of these two catalysts, as well as the nanofibers production were mentioned in detail in the Supporting Information.

H2PtCl6 containing 50 μL of 0.5 M HCOOH, as described and illustrated in detail in Figure 1a−d, resulted in the formation of Pt nanoparticles with dandelion-flower-like structures composed of nanospikes with average particle size of 31 nm as estimated from their XRD diffraction peaks using the Scherrer equation, which homogeneously coat the entire carbon fibers as shown in Figure 2a−c. However, insertion of a piece of electrospun nanofibers instead of commercial carbon felt resulted in decoration of their surfaces with Pt nanowires with smaller average particle sizes of ∼14 nm compared to the deposited Pt flowers on commercial carbon felt (Figure 2d−f). Thus, the features of the surface, such as wettability, roughness, etc., which the Leidenfrost droplet interacts with, are believed to play an essential role on the properties (e.g., size, shape, etc.) of the obtained nanostructured materials. Indeed, the morphology of the obtained Pt nanoparticles significantly depends on the composition of the levitated drop and the temperature of the hot plate. For instance, the insertion of a carbon felt piece into 1 mL of 2 mM H2PtCl6 levitated drop (without any other additives, as described and displayed in Figure 1e−h) showed Pt particles with nanoporous-like structures (∼23 nm, Figure 2g), while using a levitated droplet of 1 mL 2 mM H2PtCl6 containing 100 and 200 μL of 1 M NaOH coated the carbon felt fibers with nanoworm- (∼19 nm, Figure 2i) and nanoneedle-like Pt structures (∼16 nm, Figure 2h). Moreover, we succeeded in the synthesis of Pt−chitosan hybrid with chrysanthemum flowerlike structures and immediately loaded them on a carbon felt substrate only via using a 1.2 mL levitated droplet of 2 mM H2PtCl6 containing 100 μL of 0.5 M HCOOH and 200 μL of 0.1 M chitosan. Note that chitosan is a natural biopolymer that can be obtained from crustacean’s shells, such as crabs, shrimp, etc.,1 and it has been recently used for improving the performance of platinum and nickel nanoparticles for various electrochemical reaction related to fuel cells.1,38 Chitosan and formic acid are believed to act as stabilizing and reducing agents. Compared to the prepared modified carbon felt with electrodeposited Pt nanoparticles (PtNPs/CF), the synthesized Pt nanoparticles via using levitated Leidenfrost approach resulted in the formation of Pt nanostructures with wellcontrolled geometries, which are coated onto the entire carbon felt’s fibers. In contrast, the electrodeposition of Pt nanoparticles onto a carbon felt resulted in the formation of irregular Pt particles with different shapes, which coated only a small portion of the carbon felt’s surface fibers (see Figure S1). Raman spectroscopy was used next to examine whether the Leidenfrost synthesis approach affects the structure of the carbon fibers (data are displayed in Figure 2i). As seen in this figure, all of the carbon felts (untreated and treated in levitated Leidenfrost droplet reactor) exhibited the so-called D and G modes. These two peaks can be deconvoluted into five different bands, as shown and assigned in the inset of Figure 2i. The G band (∼1580 cm−1) is attributed to the stretching of all sp2 atoms (in-plane C−C bonds) in both rings and chains, while the other four peaks characterize D bands (D1−D4), which are assigned to the disorder in the carbon fibers structure (D1 ∼ 1350 cm−1 and D2 ∼ 1610 cm−1), amorphous sp2-bond carbon or interstitial defects, and sp2−sp3 bonds (D3 ∼ 1520 cm−1 and D4 ∼ 1200 cm−1).35,39 The ID1/IG + ID1 ratio increased from 1.5 for the untreated commercial carbon felt to ca. 2 and 2.1 for the treated carbon felts using a 1 mL water and 0.1 M NaOH levitating droplet, suggesting that more defects are formed in the process.

3. RESULTS AND DISCUSSION 3.1. Material Characterization. One-step tailoring of nanostructured materials and their immediate coating onto 3D substrates in an eco-friendly and facile manner is still a challenge. Here, platinum nanoparticles with different shapes were synthesized and instantaneously deposited on 3D carbon substrates (see Figure 2). This was achieved via sprinkling of a Pt aqueous solution atop a preheated surface at 300 °C, as described in experimental section. Once the cold water drop touches the hot surface, the adjacent layer of the droplet to the hot surface vaporizes and expands instantaneously, forcing the outer layers of the water drop to move away, resulting in the levitation of the droplet on its own vapor.18,27,28 Previous works18,29−32 demonstrated the presence of a significant temperature gradient over this levitating drop resulting in a thermoelectric effect and therefore a charge separation (water self-ionization) at the liquid−vapor interface. This results in an increase of the local concentration of both the metal (Pt4+) and hydroxyl (OH−) ions attributing to the loss of hydronium ions to the vapor. Accordingly, basic conditions are established, which are required for the syntheses of a variety of nanoparticles.18,33−35 Hydroxyl ions are believed to play a role as a reducing agent, for example, by discharging.18,36,37 As soon as the first metal clusters take shape, an autocatalytic path could be established, where more metal ions are adsorbed and reduced on the surface of these metal clusters. Figure 2a−f shows the SEM images of commercial carbon felt and carbon nanofibers before (a and d) and after (b, c and e, f) decorating them with Pt nanoparticles using the levitated Leidenfrost approach. As clearly seen in these images, the insertion of carbon felts in a levitated droplet of 1 mL of 2 mM 22401


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ACS Applied Materials & Interfaces The crystallographic orientation and the chemical composition of the as-prepared Pt nanostructure-modified carbon felts/carbon nanofibers were further investigated using XRD (Figure 3a) and EDS (Figure 3b, data are shown in Figure 3).

Figure 3. (a) XRD patterns and EDS analyses of Pt nanoporous- (1), Pt nanoneedle- (2), Pt dandelion-flower- (3), and Pt−chitosan chrysanthemum-flower (4)-modified commercial carbon felts, in addition to Pt dandelion-flower-modified carbon nanofibers. (b) The respective EDS analyses with the same color codes as used in Figure 2b.

Figure 4. XPS measurements: (a) survey spectra of bare commercial carbon felt (1) and carbon nanofibers (2), Pt dandelion-flowersmodified carbon nanofibers (3), Pt nanopores-modified carbon felt (4), Pt nanoneedles-modified carbon felt (5), Pt dandelion-flowersmodified carbon felt (6), and Pt−chitosan chrysanthemum-flowersmodified carbon felt (7). (b) Pt 4f spectra of Pt nanopores- (1), Pt nanoneedles- (2), Pt dandelion flowers- (3), and Pt−chitosan chrysanthemum flowers (4)-like structures-modified carbon felts, besides Pt dandelion-flowers-modified carbon nanofibers (5). The inset of (b) shows the fitting results of (b) curve 3. (c, d) C 1s spectra (c) and O 1s spectra (d) of the untreated commercial carbon felt (1) and treated carbon felts using 1 mL levitated Leidenfrost droplet of (2) NaOH and (3) water at 300 °C. The insets of (c) and (d) show the percentages of various carbon and oxygen functionalities for untreated (black) and Leidenfrost treated with H2O (green) and NaOH (red) carbon felts.

As clearly seen in Figure 3a, all of the as-prepared Pt nanostructures exhibited the diffraction peaks characteristic of the face-centered cubic Pt crystal lattice, assigned to (111), (200), (220), (311), and (222) planes. Moreover, the diffraction peaks of both Pt−chitosan nanocomposites (see curve 4 in Figure 2a) are sharper compared to the diffraction peaks of the other prepared Pt nanostructures including Pt dandelion flowers (see curves 1−3 and 5 in Figure 2a), suggesting larger crystallite sizes of the Pt−chitosan nanocomposites than nanoporous, nanoneedle, and dandelion flower Pt structures. Furthermore, their corresponding EDS analyses exhibited peaks assigned to platinum, carbon, and oxygen elements, as shown in Figure 3b. XPS measurements were conducted to further elucidate the chemical composition, identify oxidation states, and probe the electronic structure of the as-prepared catalyst materials (results are presented in Figure 4). Figure 4a shows XPS survey spectra of the bare carbon felts (curve 1) and carbon nanofibers (curve 2), in addition to their modification with Pt nanostructures using the levitated Leidenfrost drop technique (curves 3−7). As seen in this figure, both the unmodified carbon felts (curve 1) and carbon nanofibers (curve 2) showed only peaks attributed to the elements carbon, oxygen, and nitrogen, while additional peaks were observed for the modified carbon felts (curves 4−7) and carbon nanofibers (curve 3), which can be assigned to elemental Pt. Figure 5b displays high-resolution Pt 4f XPS images of Pt nanopores- (synthesized as described in Figure 2g caption, curve 1), Pt nanoneedles- (produced as described in Figure 2h caption, curve 2), Pt dandelion-flower- (fabricated as described in Figure 2b,c caption, curve 3), and Pt−chitosan chrysanthemum flowers (prepared as described in the caption of Figure 2j,k)-modified commercial carbon felts. Curve 5

illustrates the 4f XPS image of Pt dandelion flowers-modified carbon nanofibers (produced as described in Figure 2e,f caption). All of the catalysts exhibited a doublet peak assigned to Pt 4f5/2 and Pt 4f7/2, located around 74.9 and 71.5 eV, respectively. These two peaks could be deconvoluted into three pairs of doublets (as shown in the inset of Figure 4b) attributed to different oxidation states of Pt, where the most intense doublet could be assigned to metallic Pt, the weaker doublet could be attributed to Pt2+ oxidation state species such as PtO and Pt(OH)2, and the weakest doublet is most likely due to a small amount of Pt4+ residue on the surface.1,40 The excitation of Pt 4f signals of modified carbon nanofibers with Pt dandelion flowers (curve 5) and of modified carbon felt with chitosan−Pt chrysanthemum flowers are shifted to higher binding energy, suggesting an electronic interaction between chitosan and carbon nanofibers OH- and NH2-like functionalities, which is in a good agreement with a previously reported work.1 Figure 4c,d shows high-resolution XPS images (with their respective fitting analysis) of C 1s (c) and O 1s (d) of the untreated commercial carbon felt (curves 1) and carbon felts treated with levitated drops of 1 mL 0.5 M NaOH (curves 2) and 1 mL water (curves 3). 22402


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of sp2-hybridized carbon together with an increase in the oxygen-like functionalities. These oxygen-containing functionalities are believed to play a significant role in improving the activity and stability of the deposited Pt nanostructures. That is, the insertion of the carbon substrates inside the levitated Leidenfrost droplets is believed to activate them via increasing the amount of oxygen surface functionalities and/or increasing the density of defect sites. Generally, thermal treatment, acid treatment, and/or electrochemical treatment have been used to activate the commercial carbon felts and improve their performance for various applications in the past.45−47 For instance, redox flow battery researchers usually heat-treat the commercial carbon felts at 400 °C in air to activate them via adding oxygen surface groups.47 Herein, we introduce a novel, facile, fast, and efficient way to functionalize the carbon substrates via inserting them inside the levitated Leidenfrost droplets. Based on the above results, we succeeded in synthesizing Pt nanoparticles as well as Pt−polymer hybrids and controlled their morphologies using only water drops and a hot surface. This certainly will open new opportunities in the synthesis of 3D metal and metal−hybrid materials by a cheap and eco-friendly approach. Once the cold water drop touches the hot surface, the adjacent layer of the droplet to the hot surface vaporizes and expands instantaneously, forcing the outer layers of the water drop to move away, resulting in the levitation of the droplet on its own vapor.18,27,28 Previous works18,29−32 demonstrated the presence of a significant temperature gradient over this levitating drop resulting in a thermoelectric effect and therefore a charge separation (water self-ionization) at the liquid−vapor interface. This results in an increase of the local concentration of both the metal (Pt4+) and hydroxyl (OH−) ions attributing to the loss of hydronium ions to the vapor. Accordingly, basic conditions are established, which are required for the syntheses of a variety of nanoparticles.18,33−35 Hydroxyl ions are believed to play a role as a reducing agent, for example, by discharging.18,36,37 As soon as the first metal clusters take shape, an autocatalytic path could be established, where more metal ions are adsorbed and reduced on the surface of these metal clusters. How are the Pt nanoparticles formed inside the Leidenfrost water droplet? And what is the role of the hydroxyl ions? In the next paragraph, we will try to provide answers to these two interesting questions. As recently demonstrated by Elbahri,11 water droplets under Leidenfrost conditions exhibit various features, including temperature gradient-enhanced self-ionization together with charge separation. They succeeded in showing that the interior of the drop is negatively charged, while the underneath vapor layer is positively charged under the Leidenfrost conditions. These findings support our assumption that the local concentration of the hydroxyl ions increases at the liquid−vapor interface attributing to the loss of counterions (H3O+) to the underneath vapor layer. This results in establishing basic conditions inside the Leidenfrost drop. Additionally, the colorless Syringaldazine solution, which was placed on a glass slide directly above the PtCl62− Leidenfrost droplet, turned into red-purple, which is an indication of the evolution of chlorine gas from the PtCl62− levitated Leidenfrost droplet. Furthermore, XPS, XRD, and EDS show only peaks characteristic of the Pt nanoparticles, besides the electrochemistry measurements exhibited the typical characteristic CV of a clean polycrystalline Pt surface. Based on this information, the Pt nanoparticles are believed to

Figure 5. (a) Cyclic voltammetries (CVs) obtained at electrodeposited Pt nanoparticles (1) and various synthesized Pt structures using the levitated Leidenfrost technique, including nanoporous- (2), nanoneedles- (3), nanoworms- (4), and dandelion-flower (5)modified commercial carbon felts in N2-saturated 0.1 M H2SO4 with 50 mV/s potential scan. The inset shows the CV of the commercial PtC electrode. (b) CVs obtained at a commercial PtC electrode (1), electrodeposited Pt nanoparticles-modified carbon felt (2), and Pt dandelion-flowers-modified carbon felts created using the levitated Leidenfrost technique (3) in 0.1 M H2SO4 containing 0.2 M HCOOH with a potential scan of 20 mV/s.

The C 1s peak of both the untreated and treated carbon felts using the levitated Leidenfrost drops synthesis can be separated into six different peaks, which, by comparison with the literature, could be attributed to sp2-hybridized species, oxidized carbon species (e.g., COH, CO, C−O−C, COOH), and intrinsic π−π* shake-up processes, as shown in Figure 4c.38,41,42 Note that similar peaks are also detected for the various carbon felts and carbon nanofibers modified with Pt nanoparticles with various shapes (data are not shown). However, the intensity of the sp2-hybridized carbon peaks of the treated carbon felts via the levitated Leidenfrost droplet approach is significantly decreased in favor of oxidized carbon species (COH, CO, COOH; see the insets of Figure 4c,d). This is in good agreement with our obtained Raman results (Figure 1i), wherein the insertion of carbon felts in levitated water/NaOH drop increases the number of oxygen-containing and surface interstitial defects. Furthermore, O 1s spectra could be deconvoluted into three different peaks assigned to carbonyl (CO), carboxyl (COOH), and alcohol (C−OH) groups, as shown in Figure 4d.41,43,44 As seen in this figure, the contribution of both CO and C−O groups seems to be more prominent for the carbon felts treated using either NaOH or H2O levitated Leidenfrost droplet, as shown in the inset of Figure 4d. In summary, the treatment of commercial carbon felts using the levitated Leidenfrost drop approach results in a decrease in the amount 22403


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Pt surface. Both of them showed an oxidation peak (∼0.4 V) attributed to the desired direct (dehydrogenation path) formic acid oxidation to CO2, as well as another larger peak (∼0.75 V) assigned to the undesired indirect (i.e., dehydration path) formic acid oxidation, in which strongly adsorbed CO intermediates are in situ generated (at low potential) from nonfaradic dissociation of formic acid atop the Pt surface and oxidized to CO2.1,2,7,48,491,2,7,48,49 Two functions are normally used to evaluate the electrocatalytic activity of the electrocatalyst materials toward formic acid oxidation. The first function is the ratio of the intensities of the direct (Ipdirect) and indirect (Ipdirect) oxidation peaks, Ipdirect/Ipindirect, and the second parameter is the ratio of the intensities Ipdirect and Ibackward (Ipdirect/Ibackward), where Ibackward is the obtained current density in the backward potential scan direction. The former probes the catalyst activity and the favorable oxidation pathway, where higher Ipdirect/Ipindirect demonstrates that this direct oxidation pathway is the favorable path for formic acid oxidation. The latter one (Ipdirect/Ibackward) probes the tolerance of the electrocatalyst materials against CO poisoning, where the catalyst tolerance increases as this ratio increases. As seen in Figure 5b, the electrodeposited platinum nanoparticles (PtNPs)-modified carbon felt (assigned as PtNPs/CF, curve 2) exhibited superior electrocatalytic activity together with higher CO tolerance compared to that of the commercial PtC electrode (curve 1) as indicated from its higher Ipdirect/Ipindirect and Ipdirect/Ibackward ratios. Remarkably, Pt dandelion-flower-like structures-modified carbon felt (assigned as Ptdandelion/CF, curve 3) synthesized via the levitated Leidenfrost droplet approach exhibited much higher electrocatalytic activity compared to both the commercial PtC and PtNPs/CF electrodes. This is demonstrated by the significant and reproducible negative shift of the HCOOH oxidation associated with a significant improvement of the desired direct HCOOH oxidation path (Ipdirect). Furthermore, the undesired, poisoning HCOOH indirect oxidation route (Ipindirect) is observed to vanish. Formic acid is exclusively oxidized via the desired dehydrogenation pathway at the Ptdandelion/CF electrode. Additionally, Ipdirect/Ibackward increased from 0.17 for the commercial PtC electrode and 0.29 for PtNPs/CF to 0.82 for Ptdandelion/CF electrode (∼5 times higher), suggesting a much higher tolerance against CO-related poisoning intermediates. It is worth mentioning here that all of the aforementioned synthesized Pt nanostructures (e.g., nanoneedles, nanopores, nanoworms, etc.) exhibited superior electrocatalytic activity and CO poisoning tolerance compared to the commercial PtC electrode, where all of them showed significant enhancement of the direct HCOOH oxidation peak at the expense of the undesired indirect HCOOH oxidation peak together with a large negative shift of the HCOOH oxidation onset potential (data are not shown). Additionally, the as-prepared Pt nanoparticles with dandelion flowers exhibited better performance compared to most of the recently reported Pt nanostructures (see Table S1). Moreover, Ptdandelion/CF electrode exhibited 10 times higher turnover frequency compared to commercial PtC and PtNPs/ CF electrodes. The insufficient stability of the state-of-the-art commercial electrodes of DFAFCs (PtC) so far restricted their worldwide commercialization. Catalyst materials, which also satisfy criteria of higher stability, are much better than highly active and low-stability catalyst materials from an economic point of view. Consequently, the stability of the as-prepared Pt nanostructures-modified carbon felts was further investigated

form inside the levitated Leidenfrost water droplet according to the following equations: [PtCl 6]2 − = Pt + 3Cl 2 + 2e− −

4OH = O2 + 2H 2O + 4e

−

(1) (2)

Or 2H 2O = O2 + 4H+ + 4e−

(3)

But what is the role of the hydroxyl ions? Do they act as a reducing agent or do they play an additional role, e.g., a structure-directing agent? To answer these questions, we have synthesized Pt nanoparticles from nonaqueous solutions of H2PtCl6 (absolute ethanol and dimethylformamide). Interestingly, Pt particles with dendritic and fiberlike structures were obtained as shown in Figure S2. This indicates that nanoparticles could also be created in the absence of hydroxyl ions, where the solvent plays an essential role in the obtained nanoparticles morphology. We believe that the Leidenfrost conditions (high temperature, charge separation, etc.) induced the chemical reaction of the solvent and the metal ions (see, for example, eqs 1−3). However, the full mechanism of the nanoparticle’s formation inside the Leidenfrost droplets requires further investigation. At the moment, we design a system that will allow us to follow the changes in the composition of the Leidenfrost droplet online and track down the byproducts produced from the chemical reaction of the used solvent and salts. 3.2. Electrocatalytic Performance toward DFAFCs Anodic Reaction. Figure 5a shows the cyclic voltammograms of a commercial PtC electrocatalyst (inset of Figure 5a) and various created Pt nanostructures, including nanopores-, nanoneedles-, and dandelion-flowers-modified commercial carbon felt and carbon nanofibers synthesized using the levitated Leidenfrost water droplet technique, as described above. As clearly seen in this figure, all of the as-prepared Pt nanostructures-modified carbon fibers exhibited the characteristic response of a clean polycrystalline Pt surface, exhibiting well-defined peaks attributed to hydrogen adsorption/ desorption (E ≤ 0.2 V) and broad Pt oxide formation (broad peak, 0.6 V ≤ E ≤ 1.2 V) coupled with a Pt oxide reduction peak around 0.4 V. Additionally, the electrochemically active surface areas of the various synthesized Pt nanostructures were estimated from their respective CVs using hydrogen adsorption/desorption peaks and a standard value of 210 μC/cm2. Pt nanoparticles with dandelion-flowerlike structures exhibited the highest electrochemically active surface area (6.3 m2) among the other prepared Pt structures, including nanoporous (3.1 m2), nanoneedles (4.7 m2), and nanoworms (5.5 m2), besides the electrodeposited PtNPs on CF (1.5 m2) and commercial PtC (0.05 m2). The decorated commercial carbon felts with Pt dandelion flowers via the Leidenfrost approach exhibited ∼4.2 times higher electrochemically active surface area compared to the modified carbon felt with electrodeposited Pt nanoparticles. Figure 5b displays the obtained cyclic voltammetry at commercial PtC- (curve 1), electrodeposited Pt nanoparticles(curve 2), and Pt dandelion flowers (synthesized via the levitated Leidenfrost technique, curve 3)-modified carbon felts in formic acid solution. Both the commercial PtC and commercial carbon felt modified by electrodeposited Pt nanoparticles (PtNPs/CF) exhibited the typical cyclic voltammetry (CV) of formic acid electrooxidation at a clean 22404


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The electrochemically active surface area of the commercial PtC electrode (Figure 6b) is significantly decreased after 16 h of continuous formic acid oxidation even after the oxidative removal of adsorbed CO species, where a loss of 70% from its original active surface area (n.b., this is a permanent loss) compared to only ∼41 and ∼15% loss for the nanoporous and dandelion flowers Pt-modified carbon felt electrodes have been found (see Figures S3 and 6b,c). Consequently, the outstanding stability of the Ptdandelion/CF electrode can be attributed to its resistance to the accumulation of CO intermediates. This can be inferred from the indeed small peak of the oxidative removal of the CO accumulated on its surface (Figure 6d), highlighting the significant effect of morphology on the formic acid oxidation mechanism.

and compared to that of the commercial PtC electrodes (data are displayed in Figure 6). Figure 6a shows the chronoampero-

4. CONCLUSIONS This study introduced a novel and cheap synthesis method to design controlled Pt nanostructures/nanocomposites with unique morphologies, including nanoflowers, nanoporous structures, nanoneedles, and nanoworms, together with their simultaneous coating on suitable 3D substrates, such as commercial carbon felts in an eco-friendly, one-step manner without using reducing agent. This is achieved by the so-called levitated Leidenfrost approach, in which the aforementioned Pt nanostructures were created by placing its aqueous solution on a preheated surface (∼300 °C). That is, a frying pan and an overheated water droplet could be enough to reliably create fascinating metal and metal−hybrid nanostructures. The created Pt nanostructures exhibited a much higher performance with respect to both activity (HCOOH exclusively oxidized via desired direct path) and stability (14 times) toward the DFAFCs anodic reaction compared to that of the state-of-the-art commercial PtC electrode.

Figure 6. (a) Chronoamperometry test (i−t plots) obtained at commercial PtC- (1), PtNPs/CF- (2), nanoporous Pt- (3), and Pt dandelion-flowers (4)-modified carbon felts measured in 0.3 M HCOOH + 0.5 M H2SO4 at 0.3 V. (b−d) Their respective CVs measured in 0.5 M H2SO4 after continuous formic acid electrolysis for various times in 0.3 M HCOOH + 0.5 M H2SO4 at 0.3 V (typically, 0, 2, 8, and 16 h) with potential scan rate of 20 mV/s. (b−d) Commercial PtC-, nanoporous Pt-, and Pt dandelion-flowers-modified carbon felt, respectively. Dashed colored lines in (b)−(d) represent cycle number 10, while the solid lines represent the first measured cycle.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05156. Synthesis procedures for fabrication of commercial carbon felts with electrodeposited Pt nanoparticles and the preparation of core−shell electrospun nanofibers (PDF)

metric responses (current transients) of a state-of-the-art PtC electrode (curve 1)-, PtNPs/CF (PtNPs electrodeposited on CF, curve 2)-, Pt nanopores (created using Leidenfrost approach, curve 3)-, and Pt dandelion-flowers (synthesized via Leidenfrost approach, curve 4)-modified carbon felts. As clearly seen in this figure, Pt particles with dandelionflower-like structures (Ptdandelion/CF) exhibited the highest catalytic stability compared to the other investigated electrodes, where the catalytic activity of Ptdandelion/CF electrode was reduced by only ∼4% after 16 h of continuous formic acid electrolysis compared to ∼80% for the state-of-the-art PtC electrode-, PtNPs/CF-, and Pt nanopores-modified carbon felt electrode. However, what is the origin of the outstanding stability of Pt dandelion flowers? Cyclic voltammograms (CVs) of all of the above investigated electrodes were recorded before and after measuring the chronoamperometric test in 0.3 M HCOOH solution at 0.3 V for different times (i−t plot) to answer this question (data are displayed in Figure 6b−d). As evident in Figure 6b,c, both the state-of-the-art commercial PtC electrode and nanoporous Pt-modified carbon felts exhibited a well-defined peak ∼0.5 V, attributed to the stripping of the accumulated CO species in situ generated from the dehydration of formic acid on their surface. These in situ formed CO intermediates deactivate the majority of the Pt surface active sites as indicated from the significant decrease of the hydrogen adsorption/desorption peaks.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.A.E.-N.). *E-mail: [email protected] (C.R.). ORCID

Gumaa A. El-Nagar: 0000-0001-8209-4597 Notes

The authors declare no competing financial interest.

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REFERENCES

(1) El-Nagar, G. A.; Hassan, M. A.; Lauermann, I.; Roth, C. Efficient Direct Formic Acid Fuel Cells (DFAFCs) Anode Derived from Seafood waste: Migration Mechanism. Sci. Rep. 2017, 7, No. 17818. (2) El-Nagar, G. A.; Mohammad, A. M.; El-Deab, M. S.; ElAnadouli, B. E. Propitious Dendritic Cu2O−Pt Nanostructured Anodes for Direct Formic Acid Fuel Cells. ACS Appl. Mater. Interfaces 2017, 9, 19766−19772. (3) Yu, X.; Pickup, P. G. Recent advances in direct formic acid fuel cells (DFAFC). J. Power Sources 2008, 182, 124−132. 22405


Research Article

ACS Applied Materials & Interfaces (4) Zhang, B.-W.; Yang, H.-L.; Wang, Y.-X.; Dou, S.-X.; Liu, H.-K. A Comprehensive Review on Controlling Surface Composition of PtBased Bimetallic Electrocatalysts. Adv. Energy Mater. 2018, 8, No. 1703597. (5) Jiang, X.; Yan, X.; Ren, W.; Jia, Y.; Chen, J.; Sun, D.; Xu, L.; Tang, Y. Porous AgPt@Pt Nanooctahedra as an Efficient Catalyst toward Formic Acid Oxidation with Predominant Dehydrogenation Pathway. ACS Appl. Mater. Interfaces 2016, 8, 31076−31082. (6) Rice, C.; Ha, S.; Masel, R. I.; Waszczuk, P.; Wieckowski, A.; Barnard, T. Direct formic acid fuel cells. J. Power Sources 2002, 111, 83−89. (7) El-Deab, M. S.; Mohammad, A. M.; El-Nagar, G. A.; ElAnadouli, B. E. Impurities Contributing to Catalysis: Enhanced Electro-Oxidation of Formic Acid at Pt/GC Electrodes in the Presence of Vinyl Acetate. J. Phys. Chem. C 2014, 118, 22457−22464. (8) Latsuzbaia, R.; Negro, E.; Koper, G. Synthesis, Stabilization and Activation of Pt Nanoparticles for PEMFC Applications. Fuel Cells 2015, 15, 628−638. (9) Tappan, B. C.; Steiner, S. A.; Luther, E. P. Nanoporous Metal Foams. Angew. Chem., Int. Ed. 2010, 49, 4544−4565. (10) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. New Approaches to Nanofabrication: Molding, Printing, and Other Techniques. Chem. Rev. 2005, 105, 1171−1196. (11) Abdelaziz, R.; Disci-Zayed, D.; Hedayati, M. K.; Pöhls, J.-H.; Zillohu, A. U.; Erkartal, B.; Chakravadhanula, V. S. K.; Duppel, V.; Kienle, L.; Elbahri, M. Green chemistry and nanofabrication in a levitated Leidenfrost drop. Nat. Commun. 2013, 4, No. 2400. (12) Lee, D.-W.; Jin, M.-H.; Lee, Y.-J.; Park, J.-H.; Lee, C.-B.; Park, J.-S. Reducing-Agent-Free Instant Synthesis of Carbon-Supported Pd Catalysts in a Green Leidenfrost Droplet Reactor and Catalytic Activity in Formic Acid Dehydrogenation. Sci. Rep. 2016, 6, No. 26474. (13) Krishna, K. S.; Sandeep, C. S. S.; Philip, R.; Eswaramoorthy, M. Mixing Does the Magic: A Rapid Synthesis of High Surface Area Noble Metal Nanosponges Showing Broadband Nonlinear Optical Response. ACS Nano 2010, 4, 2681−2688. (14) Saha, M. S.; Neburchilov, V.; Ghosh, D.; Zhang, J. Nanomaterials-Supported Pt Catalysts for Proton Exchange Membrane Fuel Cells. Wiley Interdiscip. Rev.: Energy Environ. 2013, 2, 31− 51. (15) Shim, K.; Kim, J.; Shahabuddin, M.; Yamauchi, Y.; Hossain, M. S. A.; Kim, J. H. Efficient Wide Range Electrochemical Bisphenol-A Sensor by Self-Supported Dendritic Platinum Nanoparticles on Screen-Printed Carbon Electrode. Sens. Actuators, B 2018, 255, 2800−2808. (16) Shim, K.; Lee, W.-C.; Heo, Y.-U.; Shahabuddin, M.; Park, M.S.; Hossain, M. S. A.; Kim, J. H. Rationally Designed Bimetallic Au@ Pt Nanoparticles for Glucose Oxidation. Sci. Rep. 2019, 9, No. 894. (17) Shim, K.; Wang, Z.-L.; Mou, T. H.; Bando, Y.; Alshehri, A. A.; Kim, J.; Hossain, M. S. A.; Yamauchi, Y.; Kim, J. H. Facile Synthesis of Palladium-Nanoparticle-Embedded N-Doped Carbon Fibers for Electrochemical Sensing. ChemPlusChem 2018, 83, 401−406. (18) Kou, J.; Bennett-Stamper, C.; Varma, R. S. Green Synthesis of Noble Nanometals (Au, Pt, Pd) Using Glycerol under Microwave Irradiation Conditions. ACS Sustainable Chem. Eng. 2013, 1, 810− 816. (19) El-Nagar, G. A.; Lauermann, I.; Sarhan, R. M.; Roth, C. Hierarchically Structured Iron-Doped Silver (Ag−Fe) Lotus Flowers for an Efficient Oxygen Reduction Reaction. Nanoscale 2018, 10, 7304−7310. (20) Leidenfrost, J. G. De Aquae Communis Nonnullis Qualitatibus Tractatus; Johann Straube: Duisburg, Germany, 1756. (21) Zhong, L.; Guo, Z. Effect of Surface Topography and Wettability on the Leidenfrost Effect. Nanoscale 2017, 9, 6219−6236. (22) Kruse, C.; Anderson, T.; Wilson, C.; Zuhlke, C.; Alexander, D.; Gogos, G.; Ndao, S. Extraordinary Shifts of the Leidenfrost Temperature from Multiscale Micro/Nanostructured Surfaces. Langmuir 2013, 29, 9798−9806.

(23) Elbahri, M.; Adelung, R.; Paretkar, D. Method for Producing Nanostructures on a Substrate. U.S. Patent US7914850, 2011. (24) Bain, R. M.; Pulliam, C. J.; Thery, F.; Cooks, R. G. Accelerated Chemical Reactions and Organic Synthesis in Leidenfrost Droplets. Angew. Chem., Int. Ed. 2016, 55, 10478−10482. (25) Lee, D.-W.; Jin, M.-H.; Lee, C.-B.; Lee, S.-W.; Park, J.-W.; Oh, D.; Park, J. C.; Park, J.-S. Straightforward Synthesis of Metal Nanoparticles and Hierarchical Porous Metals Assisted by Partial Film Boiling Phenomena. Chem. Mater. 2015, 27, 5151−5160. (26) Lauermann, I.; Steigert, A. CISSY: A Station for Preparation and Surface/Interface Analysis of Thin Film Materials and Devices. J. Large-Scale Res. Facil. 2016, 2, 67. (27) Bernardin, J. D.; Mudawar, I. A Cavity Activation and Bubble Growth Model of the Leidenfrost Point. J. Heat Transfer 2002, 124, 864−874. (28) Nikolayev, G.; Bychenkov, V.; Skripov, V. Saturated Heat Transfer to Evaporating Droplets from a Hot Wall at Different Pressures. Heat Transfer - Sov. Res. 1974, 6, 128−132. (29) Blanchard, D. C. Positive Space Charge from the Sea. J. Atmos. Sci. 1966, 23, 507−515. (30) Andreev, S. N.; Il’ichev, N. N.; Firsov, K. N.; Kazantsev, S. Y.; Kononov, I. G.; Kulevskii, L. A.; Pashinin, P. P. Generation of an Electrical Signal upon the Interaction of Laser Radiation with Water Surface. Laser Phys. 2007, 17, 1041−1052. (31) Anderson, R.; Gathman, S.; Hughes, J.; Björnsson, S.; Jónasson, S.; Blanchard, D. C.; Moore, C. B.; Survilas, H. J.; Vonnegut, B. Electricity in Volcanic Clouds. Science 1965, 148, 1179. (32) Gilbert, H. W.; Shaw, P. E. Electrical Charges Arising at a Liquid-Gas Interface. Proc. Phys. Soc., London 1924, 37, 195. (33) Liu, B.; Zeng, H. C. Hydrothermal Synthesis of ZnO Nanorods in the Diameter Regime of 50 nm. J. Am. Chem. Soc. 2003, 125, 4430−4431. (34) Kan, C. X.; Cai, W. P.; Fu, G. H.; Li, C. C.; Zhang, L. D. Synthesis and Thermal Stability of Gold Nanowires Within Monolithic Mesoporous Silica. Appl. Phys. A 2004, 78, 1187−1191. (35) Sue, K.; Kimura, K.; Yamamoto, M.; Arai, K. Rapid Hydrothermal Synthesis of ZnO Nanorods Without Organics. Mater. Lett. 2004, 58, 3350−3352. (36) Sawyer, D. T.; Roberts, J. L. Hydroxide Ion: An Effective OneElectron Reducing Agent? Acc. Chem. Res. 1988, 21, 469−476. (37) Gitis, D.; Mukhopadhyay, S.; Rothenberg, G.; Sasson, Y. Solid/ Liquid Palladium-Catalyzed Coupling of Haloaryls Using Alcohols as Reducing Agents: Kinetics and Process Optimization. Org. Process Res. Dev. 2003, 7, 109−114. (38) El-Nagar, G. A.; Derr, I.; Fetyan, A.; Roth, C. One-pot Synthesis of a High Performance Chitosan-Nickel Oxyhydroxide Nanocomposite for Glucose Fuel Cell and Electro-sensing Applications. Appl. Catal., B 2017, 204, 185−199. (39) Fetyan, A.; Derr, I.; Kayarkatte, M. K.; Langner, J.; Bernsmeier, D.; Kraehnert, R.; Roth, C. Electrospun Carbon Nanofibers as Alternative Electrode Materials for Vanadium Redox Flow Batteries. ChemElectroChem 2015, 2, 2055−2060. (40) Xia, B. Y.; Wang, B.; Wu, H. B.; Liu, Z.; Wang, X.; Lou, X. W. Sandwich-Structured TiO2−Pt−Graphene Ternary Hybrid Electrocatalysts with High Efficiency and Stability. J. Mater. Chem. 2012, 22, 16499−16505. (41) Fetyan, A.; El-Nagar, G. A.; Lauermann, I.; Schnucklake, M.; Schneider, J.; Roth, C. Detrimental role of Hydrogen Evolution and its Temperature-dependent Impact on the Performance of Vanadium Redox Flow Batteries. J. Energy Chem. 2019, 57−62. (42) Derr, I.; Przyrembel, D.; Schweer, J.; Fetyan, A.; Langner, J.; Melke, J.; Weinelt, M.; Roth, C. Electroless Chemical Aging of Carbon Felt Electrodes for the All-vanadium Redox Flow Battery (VRFB) Investigated by Electrochemical Impedance and X-ray Photoelectron Spectroscopy. Electrochim. Acta 2017, 246, 783−793. (43) Oh, Y. J.; Yoo, J. J.; Kim, Y. I.; Yoon, J. K.; Yoon, H. N.; Kim, J.H.; Park, S. B. Oxygen Functional Groups and Electrochemical Capacitive Behavior of Incompletely Reduced Graphene Oxides as a 22406


Research Article

ACS Applied Materials & Interfaces Thin-film Electrode of Supercapacitor. Electrochim. Acta 2014, 116, 118−128. (44) Bortz, D. R.; Heras, E. G.; Martin-Gullon, I. Impressive Fatigue Life and Fracture Toughness Improvements in Graphene Oxide/ Epoxy Composites. Macromolecules 2012, 45, 238−245. (45) Fetyan, A.; El-Nagar, G. A.; Derr, I.; Kubella, P.; Dau, H.; Roth, C. A Neodymium Oxide Nanoparticle-Doped Carbon Felt as Promising Electrode for Vanadium Redox Flow Batteries. Electrochim. Acta 2018, 268, 59−65. (46) Flox, C.; Skoumal, M.; Rubio-Garcia, J.; Andreu, T.; Morante, J. R. Strategies for Enhancing Electrochemical Activity of Carbon-Based Electrodes for All-vanadium Redox Flow Batteries. Appl. Energy 2013, 109, 344−351. (47) Sun, B.; Skyllas-Kazacos, M. Modification of Graphite Electrode Materials for Vanadium Redox Flow Battery ApplicationI. Thermal Treatment. Electrochim. Acta 1992, 37, 1253−1260. (48) El-Deab, M. S.; Kibler, L. A.; Kolb, D. M. Enhanced Electrooxidation of Formic acid at Manganese Oxide Single Crystalline Nanorod-Modified Pt Electrodes. Electrochem. Commun. 2009, 11, 776−778. (49) Joo, J.; Uchida, T.; Cuesta, A.; Koper, M. T. M.; Osawa, M. Importance of Acid−Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum. J. Am. Chem. Soc. 2013, 135, 9991−9994.

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