Compositional Evaluation of Coreduced FeâPt Metal Acetylacetonates

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Compositional Evaluation of Co-reduced Fe-Pt metal Acetylacetonates as PEM Fuel Cell Cathode Catalyst Robin Sandström, Guangzhi Hu, and Thomas Wagberg ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01536 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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ACS Applied Energy Materials

Compositional Evaluation of Co-reduced Fe-Pt metal Acetylacetonates as PEM Fuel Cell Cathode Catalyst Robin Sandström †, Guangzhi Hu†,‡, Thomas Wågberg*,† †

Department of Physics, Umeå University, Umeå 90187, Sweden

‡

Key Laboratory of Chemistry of Plant Resources in Arid Regions, State Key Laboratory Basis of Xinjiang Indigenous Medicinal Plants Resource Utilization, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China *Corresponding author: E-mail address: [email protected] Keywords: Platinum Iron nanoparticles Proton exchange membrane fuel cell Oxygen reduction reaction Solvothermal co-reduction Membrane electrode assembly Hydrogen energy

Abstract

Platinum-Iron nanoparticles were produced by solvothermal co-reduction of organic Fe and Pt precursor compounds and supported on conventional Vulcan XC 72. Evaluation of oxygen reduction performance reveals a highly active surface with up-to 5 times the specific activity of commercial Pt-Vulcan measured in O2 saturated 0.1 M HClO4. A particle size of 5.5 nm for the best performing sample, produced from an initial metal ratio of 1:1, provided 20% higher mass activity than the commercial reference. Membrane electrode assemblies, optimized for both H2/O2 and direct formic acid fuel cells were produced and the PEM fuel cell cathodic performance displayed results with similar enhancements as its ex situ measured mass activity, although a delamination of the catalyst layer from the membrane could be observed even when employing a hot-pressing procedure during MEA fabrication. Physical characterizations including X-ray photoelectron spectroscopy and in situ X-ray diffraction reveal oxidized states of Fe incorporated

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into the disordered face centered cubic Pt nanoparticles, supported by composition-dependent morphological changes as observed by transmission electron microscopy. The provided insight into fuel cell performance as well as CO-oxidation attributes are expected to assist in selecting suitable applications and operating conditions for such FePt type nanoparticles.

1. Introduction

Development of efficient catalysts for the oxygen reduction reaction (ORR), greatly limiting the performance of proton exchange membrane fuel cells (PEMFCs), has been a major research focus during the recent decade and remains a challenging task due to the reactions sluggish nature 1. Reduction of oxygen into water is a key reaction in PEMFCs, which are regarded as an important component in achieving a sustainable and environmentally sound source of energy in applications such as vehicles2 and portable electronics3-5. The anodic hydrogen oxidation reaction contributes with relatively low overpotential6, meaning that hydrogen based PEMFCs would benefit greatly from improved cathode catalysts facilitating ORR. Unfortunately, the dependence of a high platinum content makes commercialization problematic due to its high cost and low earth abundance. Challenges within the field of catalysis therefore include the development of efficient catalysts free from7-8, or with low content of9 precious metals. Platinum based catalysts still remain the most preferable choice in low temperature PEMFCs owing to its low overpotential and high operational stability in combination with a high selectivity and a full four electron reduction pathway10. Although a common approach in significantly decreasing Pt content include optimization of the membrane electrode assembly (MEA) fabrication process11, high electrocatalytic activity of the catalyst layer itself remain a crucial parameter for fuel cell performance.


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Alloying Pt with a transition metal such as Ni, Co or Fe, producing bimetallic nanoparticles (NPs) in a controlled synthesis procedure for a more favored crystal surface may lead to greater performance in terms of specific activity1, 12-13. Among the many attempts to increase Pt utilization for ORR, the concept of core-shell Pt based bimetallic nanoparticles has shown promising performances as well as high Pt utilization. Besides the advantageous principle of placing Pt at the surface of nanoparticles for better utilization, theoretical insight into the mechanism of ORR taking place on monolayer Pt has shown beneficial synergy effects between Pt and the underlying core transition metal13-14 where the nonprecious 3d type metals cause downshifts on the d-band electrons of the Pt shell, ultimately weakening the adsorption of intermediate and spectator species14-16. Hu et al. showed further that also other geometrical configurations than a core-shell structure could lead to similar synergy effects17.

In another perspective the addition of secondary metals can however display durability issues in strong acidic conditions, and stability issues for bimetallic catalyst certainly need consideration. Iron is good candidate in this respect for platinum modification, since it is known to form stable alloys with Pt. In addition, electrochemical studies demonstrate that alloy NPs can exhibit beneficial synergistic effects for ORR18-20. For example, Guo et al.21 developed a synthesis route for preparation of FePt NPs supported on reduced Graphene Oxide (rGO). Chen et al.22 further demonstrated a simple one-pot synthesis and decoration, without surfactants, of FePt composites on rGO. The catalytic activities toward ORR showed superior performance as compared to commercial pure Pt alternatives in both studies, and it was clear that the effect of support plays a crucial role in overall performance and also had a significant affect on durability23. Similar catalysts, as shown by Jiayuan et al.

24

, has also shown highly active and stable performance in

hydrogen PEM fuel cell conditions where it was hypothesized that Fe species in close proximity to



the surface Pt atoms is directly responsible for promoting favorable sites enabling high ORR activity. FePt alloying have further offered potential applications in oxidation of small organic molecules such as formic acid (FA)25-26 and methanol27-28, showing a degree of tolerance toward catalytic poisoning.

Various solvothermal approaches for the synthesis of FePt nanoparticles has been proposed in literature29-31. Co-reduction procedures of metal salts tend to result in a disordered face centered cubic (fcc) structure, while there are a few examples of synthesis of particles which can be transformed into a complete or partially face centered tetragonal (fct) phase at lowered temperatures, which is the structurally ordered phase that in addition display useful magnetic properties32-35. Whilst the nanoparticulate form of disordered and ordered FePt show promising ORR specific and mass activities, there are however limited studies detailing PEMFC operational performance.

Here we evaluate FePt NPs of a disordered fcc phase derived from a simple bottom-up synthesis based on co-reduction of platinum and iron metal acetylacetonates (acac), followed by a characterization of NPs prepared with varied stoichiometric compositions of Fe and Pt, decorated on commercial Vulcan XC 72 support for easy and trustworthy comparisonng with a commercial Pt-Vulcan catalyst with similar metal loading. The importance and effects of precursor ratio on the particle formation is highlighted and apparent differences, in both ORR performance and NP characteristics, are observed. The FePt NPs show high specific activities, which is attributed to the unique particle surface properties such as successful incorporation with Fe and indications of distinct truncated particle shapes and sizes revealing active surface terraces. The commercially available catalyst was subjected to similar electrochemical characterizations confirming the superiority of FePt NPs, with an optimum initial precursor composition of 1:1, in terms of both


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specific and mass activities. The produced catalyst exhibit improved cathodic performance in PEM fuel cells fueled by both FA and H2 where potential MEA fabrication issues were emphasized on the latter configuration. Superior performance in the DFAFC could also be attributed to a more CO-tolerant Pt surface at ORR potentials.

2. Experimental 2.1.

Synthesis

FePt alloy NPs were synthesized through a co-reduction process of common metal precursors as follows. Suitable amounts of Pt(acac)2 (Acros Organics) and Fe(acac)3 (Sigma-Aldrich) corresponding to metal molar ratios of 1:1, 1:3 and 3:1 (henceforth named FePt(1:1), FePt(1:3) and FePt(3:1) respectively) were dissolved in anhydrous N,N-Dimethylformamide (DMF, Aldrich) resulting in a fixed precursor concentration of 15mg/ml. The mixtures were then placed in a Teflon lined stainless steel autoclave with an open quartz vessel and maintained at 140˚C for 24h. While still warm, the colloidal dispersions of NPs were added into a slurry consisting of an appropriate amount of commercial Vulcan XC 72 and DMF (~10mg/ml), providing a final metal loading of 20wt.%. After proper mixing, the finished products were finally washed repeatedly in distilled water (H2ODI), ethanol and H2ODI respectively, followed by freeze-drying to finally obtain a fine, easily dispersible catalyst powder. The powder was finally briefly annealed in air at 200 ˚C for 10 min to remove most of the remaining organic decomposition products resulting from the acac salts. The loading of each catalyst was verified to about 20 wt.% by thermogravimetric analysis (TGA) (METTLER Toledo TGA/DSC 1) before further characterization, presented in supporting information Fig. S1 showing typical data of the three samples. All TGA measurements were conducted in airflow of 30ml/min with a heating rate of 10 ˚K/min.



2.2.

Physical Characterization

High resolution TEM imaging was performed on a JEOL-2100F microscope (JEOL, Japan) and in situ XRD measurements was performed with Rigaku D/Max-2500/pc using Cu Kα radiation (40 kV and 100 mA) over the 2θ range from 10 o to 80 o at the rate of 4o min-1. The sample was treated under flowing H2 of 30 ml min-1 from 25 oC to 500 oC at 5 oC min-1. Prior to collecting data at each temperature interval, the samples were treated at respective temperature for 20 min to achieve a steady state. In addition, the in situ XRD data was collected under N2 atmosphere at the same procedure for comparison. The recording of the standard XRD reflections took place between 2θ values of 20o and 90o, where the Pt(220) reflection was selected for average crystallite size calculation according to the Scherrer relation after Voight profile peak-fitting with PeakFit v4.12 software. Surface elements were investigated by X-ray photoelectron spectroscopy (XPS) and all spectra were acquired with a Kratos Axis Ultra DLD spectrometer using monochromated AlKα source operated at 120 W and an analyzer pass energy of 160 eV and 20 eV for acquiring wide spectra and individual photoelectron lines were used respectively. Additional compositional analysis was performed with energy dispersive x-ray (EDX) in a Zeiss Merlin FEG-SEM (Operation: 5 kV and 300 pA) equipped with an Oxford Instruments X-MAX 80 mm2 X-ray Detector with AZTEC 2.2 software for data management and evaluation. Four different spots on each sample were measured for an averaged result.

2.3.

Electrochemical characterization and MEA preparation

For preparation of glassy carbon (GC) electrodes, samples were first dispersed in a mixture of 2propanol and H2ODI (2:1 volume ratio) such that a catalyst concentration of 2.5 mg ml-1 with 20 wt.% Nafion (DuPont, Ion-Power) was achieved. 8 μl portions of the dispersions were then drop


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cast with a micropipette on a glassy carbon surface that had been carefully pre-polished with 50 nm alumina powder followed by subsequent sonication in acetone, H2ODI and ethanol respectively. The drying procedure took place in ambient conditions under a 700 rpm rotation36. The catalyst layers produced thus had approximately the same thickness owing to the identical loading of Vulcan XC 72. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed on the prepared GC electrode in 0.1 M HClO4 and recorded with an Autolab PGSTAT-302 N potentiostat connected to a rotating disk electrode (RDE) setup (Pine Instruments) and an RHE reference electrode (Gaskatel, hydroflex®). Before any electrochemical measurements, a break-in procedure was performed by potential cycling (0.0 – 1.2 V vs. RHE) in Ar saturated electrolyte. LSVs used for calculating specific activity (SA) and mass activity (MA) were recorded at 0.9 V vs. RHE from anodic linear sweeps between 0.05 – 1.05 vs. RHE with a scan rate of 20 mV s-1 and a constant rotation of 1600 rpm. The background was recorded in identical conditions under an Ar saturated electrolyte and post correction of the iR-drop was performed assuming a resistance of 21 ohm as confirmed by an i-interrupt technique, which is in close correspondence to literature values. For the purpose of approximating the electron transfer number (n), the diffusion limited currents (jl) were obtained by varying the rotational speed to 400, 600, 900, 1200, 1600 and 2000 rpm and utilizing the Koutecky – Levich equation (see supporting information eq. S1 for more details). Accelerated degradation was employed on the best performing sample and Pt/C by 6000 potential cycles (0.6 – 1.0 V vs. RHE) by a scan rate of 400 mV/s. Moreover, both hydrogen underpotential deposition (HUPD) measurements and CO-stripping was performed for measurement of electrochemically active surface area (ECSA). For HUPD, the potential was swept in the 0.05 – 1.0 V vs. RHE range in Ar saturated electrolyte 3 times at 50 mV/s to ensure stable ECSA. CO stripping was accomplished by maintaining the potential at 0.1 V vs. RHE while bubbling the electrolyte in CO for 20 min at 100 ml min-1 followed by 30 min in Ar to remove the remaining dissolved CO.



The potential was finally cycled, starting in the anodic direction with a scan rate of 20 mV min-1. Two cycles were recorded to achieve a CO stripping charge on the first, and a capacitive chargingcurrent baseline on the second sweep.

The ORR performance of each material was evaluated in low-temperature fuel cell conditions. The gas diffusion electrodes (GDEs) were prepared by first mixing certain amount of desired catalyst, Nafion dispersion (5 wt.% Ion-power) and 2-propanol + DI H2O mixture of a 1:1 volume ratio, by sonication for 30 min. The resulting ink, with a concentration of 4 mg ml-1, was then sprayed in an N2 atmosphere onto a carbon paper with teflonized microporous layer (Sigracet GDL 10 BC) until a loading of 0.1 mgPt cm-2 was applied, followed by drying on a hotplate (60 ˚C) for at least 1h. Commercially available Pt-Vulcan (20 wt.%, Premtek) was used for anode as well as a reference cathode. Both anode and cathode electrodes were produced with a final metal loading of 0.1 mg cm-2 and a Nafion content of 33 wt.%. Additionally, the cathode performance was also evaluated in a direct formic acid fuel cell (DFAFC) but where the anode instead was prepared by dispersing commercially available Pd-Vulcan (60 wt.% Premtek) with an amount of Nafion dispersion (5 wt.% Ion-power) and ethanol, by 30min sonication, achieving 20 wt.% Nafion loading. The resulting ink was then painted on carbon paper (Sigracet GDL 10 AA) until a metal loading of 4 mgPd cm-2 was achieved.

Nafion 212 and 115 membranes were used for H2/O2 fuel cells and DFAFCs respectively. Membranes were subjected to pretreating by heating for 1 h at 80 ˚C in 3 % H2O2, 0.5 M H2SO4 and distilled water (H2ODI) respectively. The membranes were rinsed in DI H2O between each step and finally stored in H2ODI until usage. MEAs were then prepared by sandwiching the catalyst coated GDL electrodes with the membrane into a 5 cm2 cell without hot-pressing. Evaluation of MEA performances were made by an 850e fuel cell test station (Scribner Associates) with enabled


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background current-interrupter for estimation of cell resistance. For the H2/O2 configuration, humidified reactants with a constant gas flow rate of 100 cc/min, for both anode and cathode, was fed to the cell operating at 60 ˚C and 95 % RH. The run-in phase consisted of an operation at a constant potential of 550mV for about 12 hours, ensuring stable performance. At least two polarization curves were then recorded to ensure reproducibility where data was acquired after a delay of 1 min/step with a step size of 100 mA. The DFAFC configuration was supplied with nonhumidified O2 at the cathode with a flow rate of 1 l/min and an anode flow of 0.7 + 0.7*I cc/min (3 M p.a. grade formic acid, Sigma-Aldrich). Here, the current increment was 50 mA and acquired after 5 s per data point. Break-in of the DFAFC MEAs were performed by first ensuring stable open-circuit potential by supplying a flow of fuel, without any applied load, for several hours. The cell was then operated by recording a number of polarization curves until minimal fluctuations in performance was achieved followed by storage in deionized H2O overnight (anode flow-fields). Thereafter the cell was once more operated, by following the same procedure described above until reaching stable performance, displaying slightly improved maximum power output compared to the previous day.

3. Results 3.1.

Characterization

FePt nanoparticles of three different reactant molar compositions were synthesized through coreduction of respective metal acetylacetonate. TEM and HRTEM images shown in Fig. 1 demonstrate

crystalline,

regularly

shaped

nanoparticles

(NPs)

successfully

attached

homogeneously on the Vulcan XC 72 support for all three samples, although with some few occasional aggregate clusters of NPs. The decoration method simply involving direct mixing of the colloidal NPs with non-functionalized support dispersed in DMF is likely the cause of regions



where non-optimal decoration is observed. However, since the target loading of 20 +/- 1 wt.% was achieved (see Fig. S1), it is clear that the precursors underwent full co-reduction and the vast majority of the resulting NPs were anchored to the support. Minimal weight loss in the temperature range surrounding the boiling point of DMF (~150 oC) indicate that no significant amounts of residual solvent remains in the sample. The successful reduction is attributed to the DMF solvents ability to act as both stabilizer and reducing agent, as previously shown for bimetallic Pt alloys37. It should nevertheless be noted that the molecular weight of Fe is significantly lower than Pt, and potentially un-alloyed Fe species could remain undetected in the TGA measurements. At low Feratio we see no signs of this, but for the FePt(3:1) sample a distinct red tint could be seen in the washing procedure. As discussed later in manuscript we believe that the Fe-content in the FePt alloys saturates at a certain ratio and that additional Fe do not further increase the Fe content after this saturation. We also note that when DMF not labeled as anhydrous was employed, the yield of NP formation was dramatically lowered, signified by loading ranging from 10-15 wt.%.

TEM imaging further reveal that higher Fe content tend to result in more spherically shaped particles while higher Pt proportion show a higher fraction of particles with truncated morphology, along with an increase in particle size. The ability to reduce NP size by addition of Fe(acac)3 is in line with earlier results where ethylene glycol (EG) was used as solvent and reducing agent38. In the extreme case where only Fe(acac)3 is present, tiny nanoparticles consisting exclusively of magnetite (Fe3O4) can be observed

39-40

, while the complete absence of Fe(acac)3 suppresses the

growth of Pt NPs. These results support that the lower reduction temperature of Fe(acac)3 allows the initial formation of iron-rich seeds followed by gradual deposition of Pt, possibly favouring a Pt enriched surface.


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Fig. 1: TEM and HRTEM images of FePt NPs on Vulcan XC 72 support showing samples FePt(3:1) (a,b), FePt(1:1) (c,d) and FePt(1:3) (e,f). Scale-bars of TEM (a,c,e) and HRTEM (b,d,f) images are 40 and 4 nm respectively, where all HRTEM image highlights the Pt(111) lattice fringes. FePt(3:1) NPs are estimated to be mostly 5.0-5.5 nm sized spherical particles while FePt(1:1) and FePt(1:3) show truncated morphology with sizes ranging from 5.5-6.5 nm and 6.09.0 nm respectively.



Metal surface compositions of the bimetallic nanoparticles were investigated by X-ray photoelectron spectroscopy (XPS). Representative XPS spectra originating from FePt(1:1) are depicted in Fig. S2a, where signals of both metals are observed, together with small amounts of additional impurities above the detection limit visible in the O, C and N core-level regions (not shown). These impurities can be ascribed to miniscule amounts of residual solvent and acetylacetone residue adsorbed to the catalyst and support surfaces in line with previous work using similar reaction conditions41. Comparison of the synthesis ratio with the derived atomic ratios (Table 1), indicate a clear increasing trend in Fe content with respect to relative Fe-precursor amount. However, FePt(1:3) show lower Pt content than the amount used in the synthesis while the other two catalysts are composed of significantly larger Pt-ratio. As previously mentioned this suggest that there is a saturation limit in the amounts of incorporated Fe that can be reached.

For investigation of the chemical states of each NP component, high resolution XPS was evaluated around the Fe 2p and Pt 4f core-level regions as illustrated in Fig. S2b-c. The Pt 4f 7/2 region display predominant metallic doublets at 71.1 eV, but also signs of likely Pt-C (72.0 eV) and PtO (73.2 eV) formation. The Fe 2p

3/2

region show that Fe species exist primarily in an

oxidized state (710.1 eV) with smaller presence of a metallic phase (707.2 eV). However, similar binding energies between Fe 2p 1/2 and Pt 4s which causes a partial signal overlap in combination with satellite peaks (see gray regions in Fig. S2c), make atomic ratio derivation through XPS somewhat problematic, motivating a complementary compositional determination by energy dispersive X-ray spectroscopy (EDS), shown in Fig. S3. As shown in Table 1, the actual Pt content in the bulk sample was either underestimated by XPS or since EDS is a technique causing a large e- interaction-volume, differences in atomic ratio determined by XPS and EDS could also indicate a non-isotropic core-to-shell configuration of the Pt and Fe atoms.


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Fig. 2: X-ray diffractogram of (a) the synthesized FePt fcc type NPs where the dashed vertical lines are fixed to FePt(1:3). Approximated NP average sizes (b) and Position of the d(220) reflection (c) vs. temperature, derived from In situ XRD experiments (see Fig. S4 and table S2-S4).

The particle phases were characterized by X-ray diffraction (XRD) illustrated in Fig. 2a. The broad peak at around 23˚ originate from C(002) crystallite regions in the carbonaceous Vulcan



XC 72 support

42

, while reflections arising due to the FePt fcc lattice include (111) ~40˚, (200)

~47˚, (220) ~68˚, (311) ~82˚ and (222) ~87˚

43-44

(see table S1). Absence of pure Fe and Fe3O4

support that the synthesized nanoparticles (after washing) consists exclusively of a Fe-doped Pt fcc structure. At low indices we observe no notable lattice contractions with increased Fe, as often observed in ordered FePt alloys45-46. Instead however, at higher order indices slight lattice expansions are indeed notable with increased Fe content (Table S1), owing to the non-linear nature of Bragg´s law. More importantly, a clear broadening, following a decrease in particle size, is also directly related to increased Fe content. Average particle sizes derived from the FWHM of the (220) peaks of the XRD pattern using the Scherrer formula gives 5.1 nm, 5.5 nm and 6.0 nm for FePt(3:1), FePt(1:1) and FePt(1:3) respectively, which is a trend that is also in line with observed TEM images. Interesting to note is that the C(002) peak is influenced by the sample produced with excess Fe(acac)3, signified by a downshift. We speculate that the residual Fe species (decomposition products) are responsible for partial intercalation into C crystallites, taking place in the warm DMF solution, ultimately swelling the C(002) plane and rendering the material difficult to wash completely. A modification of the support at high Fe content is further supported by a lower thermal stability as evidenced by a lowered carbon decomposition onset temperature for the FePt(3:1) sample (Fig. S1).

The internal crystal arrangement of fcc FePt NPs, and other transition metal Pt-alloys, are subject to change once annealed. Apart from annealing temperature, the Fe:Pt composition plays a role in such reorganization46-47. Thus, to further evaluate the FePt NPs containing oxidized chemical states of Fe, we performed In situ XRD experiments for temperatures up to 500 ˚C in both inert N2 and reductive H2 atmospheres. For this purpose, the catalysts were produced according to the procedure described in the experimental section but omitting the final brief


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annealing step (200 oC in air). Fig. 2b shows the resulting crystallite size derived from the Scherrer formula using the (220) reflection for respective catalyst composition and atmosphere. The increase in average size with increased sintering temperature is rationalized based on a combination of Ostwald ripening and NP migration and coalescence48. However, whilst all catalysts displayed similar crystallite growth under a reductive atmosphere, the FePt(1:1) and FePt(3:1) exhibited a dramatic growth when annealed in inert atmosphere with an onset of about 400 oC. We believe this effect can be attributed to a failure to efficiently remove O-groups from the NP lattices resulting in higher susceptibility toward inter-particle atomic transfer processes such as the Ostwald ripening effect, owing to the weaker lattice bonds in FePt at a larger fraction of oxygenated transition metal species. It must also be emphasized that larger NPs are more energetically favorable than smaller ones, which may partially contribute to the observed phenomena. However, the possible complete reduction of heavily oxidized Fe into Fe-O in pure hydrogen around 300-400 ˚C49-50, underpin the significance of this temperature range. Finally, we note that the onset of crystallite expansion was roughly 200 oC for all samples, implying that our brief annealing step for removal of solvent residue should not have any significant effects on the surface area in the bulk material.

FePt NPs can also exist in an ordered phase with characteristic fct lattices showing clear XRD features. To reach this phase, a high temperature treatment is required (typically above 600 ˚C) to promote the phase transformation from fcc into fct51-55. While certain synthesis methods reported in literature may promote the formation of a partially structurally ordered tetragonal phase at lower temperatures than 600 oC32-34, none of the NP compositions in this work showed any signs of ordering below 500 oC. As shown in the deconvoluted diffractograms of respective fcc reflection (Table S2-S4), most peaks suggest a lattice expansion caused by elevated temperatures where the trend in (220) reflections is the most pronounced, as illustrated in Fig. 2c. Here also, the FePt(1:1)



Page 16 of 27

and FePt(3:1) annealed under N2 demonstrate the most dramatic results with a significant lattice expansion, supporting the above assumption of less energetically favorable phases due to incorporated oxidized Fe as a likely cause of the observed significant crystallite growth. Lattices containing higher fraction of Fe-oxides in the inert annealing condition is thus a plausible reason for the superior stability under H2, further suggesting that Fe incorporation can be successfully tuned by initial precursor ratio.

Table 1: Summary of obtained compositional and electrochemical data. ORR activity was evaluated in 0.1 M HClO4 at a rotational speed of 1600 rpm where the composition determined through EDX analysis was chosen for mass activity calculation. Surface composition (XPS) [Fe:Pt]

Composition (EDS) [Fe:Pt]

NP avg. size (XRD) [nm]

ECSA (CO stripping) [m2/g_Pt]

e- Transfer number (n)

Specific activity [μA/cm2_Pt]

Mass activity [A/mg_Pt]

-

-

1.8

83.5

3.83

167

0.14

FePt(1:3)

1.0:2.2

1.0:2.7

6.0

27.8

4.03

354

0.10

FePt(1:1)

1.0:1.5

1.0:2.0

5.5

21.1

4.11

833

0.18

FePt(3:1)

1.0:1.1

1.0:1.4

5.1

11.6

4.09

483

0.06

Sample Pt/C*

*Commercial sample

3.2.

Oxygen reduction performance and fuel cell evaluation

Earlier observations in literature have shown active surface formation from Pt-FeOx systems toward ORR24. Likewise, here our LSV results (Fig. 3a) revealed similar enhanced surface activity. Since FePt comprise of significantly larger particle sizes and thereby lower ECSA (Fig. 4), the specific activities (SA) was calculated resulting in a trend of FePt(1:1) > FePt(3:1) > FePt(1:3) > Pt/C. Underestimation of ECSA measured by CO-stripping can be possible particularly among Pt-alloys56. However, the measured ECSA based on HUPD measurements yield similar results (see Table S5), supporting the calculated SA. In addition, FePt(1:1) still surpassed Pt/C in terms of mass activity (MA) by 22%, calculated from EDS compositional data (see Table 1).


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The activity after 6000 on-off cycles between 0.6-1.0 V vs. RHE, FePt(1:1) could retain 70% of its initial MA (Fig. 3b), compared to Pt/C which maintained 90% in an identical accelerated degradation test. Both catalysts end up with a similar MA of around 0.12-0.13 A mgPt-1. Dealloying due to leeching of Fe into the electrolyte is likely to be the main cause of the degradation leading to a re-installment of the pristine Pt surface after prolonged usage57. Furthermore, a Koutecky – Levich analysis on the diffusion limiting current region (Fig. 3c,d) show that an exclusive four electron pathway of O2 reduction took place for all samples.

Fig. 3: Linear sweep voltammetry (LSV) recorded on a glassy carbon rotating disk electrode at 1600 rpm (a) and a stability test of the best performing sample (FePt(1:1)) consisting of an LSV



before and after 6000 potential cycles (b). Koutecky-Levich plot of respective catalysts (c) produced from a diffusion limited current region at rotational speeds of 400, 600, 900, 1200, 1600 and 2000 rpm (d). All LSVs were performed in O2 saturated 0.1 M HClO4 with GC electrodes loaded with identical amounts of Vulcan support.

Additional crystallographic information can be estimated by evaluating the ECSA CVprofiles while comparing them to previously reported peaks obtained in acidic electrolytes, owing to the structurally-sensitive reactions taking place on the surfaces. First, we note that all FePt samples show signs of a higher fraction of (100) sites due to clear presence of a more positively shifted H-desorption peak (~0.27 V vs. RHE) in contrast to Pt/C (Fig. 4)58-59. A slightly downshifted CO stripping peak as compared to the much smaller commercial Pt/C NPs, appearing slightly below 0.8 V vs. RHE, supports the emergence of such terraces in conjunction with a decrease in steps, kinks and edge sites60-61. Moreover, another CO stripping peak appearing at ~0.7 V vs. RHE indicates that the surface may contain a fraction of terrace Pt domains manifested as (111) sites62-64, without excluding the possibility of exposed lower coordinated (110) facets, which declines with increased Fe content. Likely, these observations can be explained by the NP size differences, where there is a significant decrease in edge and corner sites followed by an increase in terrace sites as the NPs become larger65-66, in line with the observed TEM and XRD results showing larger and visibly more truncated NPs with lowered Fe content. The possibility of downshifted CO oxidation potentials caused by Pt sites with nearby oxygenated transition metal surface species should however not be excluded56, 67.


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Fig. 4: ECSA measurements under Ar saturated 0.1 M HClO4, showing both CO-stripping and HUPD cyclic voltammetry curves of all FePt samples (a, b) and the Pt/C commercial reference (c,d) respectively. The metal loading of all catalysts were close to 20 wt.% and GC electrodes were loaded such that the same amount of support was deposited on the electrode surface.

The ORR performance in actual fuel cell conditions was investigated to determine whether our FePt catalysts remain functional in the cathodic PEMFC conditions. Both H2/O2 and DFAFC polarization profiles are shown in Fig. 5 (a,b) and (c) respectively. Similar to measured LSVs discussed above, the FePt(1:1) initially display similar, but marginally lower, overpotential compared to the commercial Pt/C catalyst in both FC types. Interestingly, the FePt(1:1) outperforms the Pt/C at elevated currents where the latter show signs of ohmic losses and more



severe mass transport limitations. While recording the polarization curve for the H2/O2 cell, it was however noted that there was a difference in average cell resistance obtained from simultaneous iinterrupt measurements between the two MEAs where FePt(1:1) fluctuated between 30-35 mOhm and Pt/C between 40-45 mOhm, meaning an iR compensation reduces the difference in ohmic drop, thus yielding similar polarization profiles. As the porous morphology of the catalyst layer (CL) and the CL membrane interface may play a significant role on the overall cell resistance68-69, the fuel cell performances in Fig. 5a-b may not necessarily represent the intrinsic properties of the ORR catalyst. To address this issue, the contribution of the ohmic drop originating from the adjacent interfaces of the CL was minimized by attempting a mild hot-pressing MEA fabrication procedure, commonly a requirement for commercialization of MEAs. By compressing the MEA for 2 min at 130oC with a pressure of ~60 kg cm-2, the commercial fuel cell grade Pt/C clearly benefited significantly from the process. The effects on the FePt(1:1) CL however was detrimental to the activity. We also observed irreversible behavior while recording multiple polarization curves. After prolonged operation, a complete delamination of the catalyst coated diffusion layer from the membrane could be observed, indicating that the resulting surface properties of the support after the NP decoration were suboptimal for MEA fabrication by hot-pressing, possibly due to the interaction of residual decomposition products and the support observed in the above XRD and TGA experiments. We believe however that this problem could potentially be remedied by proper surface functionalization of the support prior to decoration, making it more Nafion-compatible, such as sulfonic acid based grafting70.

While the H2/O2 MEAs exhibited dissimilar cell resistance values, the DFAFCs did not (~70 +/- 3 mOhm); meaning the above explanation does not merit the superior FePt(1:1) in Fig. 5c. The main changes in the cathodic reaction environments between H2 and formic acid based fuel


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cells are lower overall reaction rate and the relatively severe fuel crossover of formic acid through the N115 membrane occurring in DFAFCs71, ultimately forcing the cathode catalyst to perform ORR in the presence of small amounts of FA. To test how the FePt(1:1) fare against Pt/C in such environment, we first conducted an ORR performance evaluation similar to the above LSVs, but in an electrolyte of 0.5 M H2SO4 with an added amount of 0.1 M FA (see Fig. S5). However, once the FA was added, FePt(1:1) showed nearly identical ORR performance as Pt/C, suggesting that the explanation lies elsewhere. Reasonably, since we observed an additional downshifted peak in our CO-stripping voltammograms for all FePt samples (Fig. 4), we believe that the lowered onset of CO oxidation potentials aids the removal process of adsorbed CO formed as a reaction intermediate through the indirect dehydration pathway of FA oxidation72, as similarly observed for platinum – iron catalysts with FePt compositions of atomic ratio close to one26. As a result, the FePt(1:3) and FePt(3:1) also follows similar performance to Pt/C in DFAFC conditions in terms of maximum power output, compensating for their higher overpotentials.

Fig. 5: Fuel cell polarization curves where a) and b) shows the performance of FePt(1:1) as compared to Pt/C in an H2/O2 cell. Hollow squares and circles show performance for MEAs produced through relatively mild hot-pressing for 2 min at 130oC with a pressure of ~60 kg cm-2. Cathode performance in a DFAFC of each sample is shown in c). All cathodes were produced with a loading of 0.1 mgPt cm-2.



4. Conclusions An efficient Platinum – Iron based catalyst has been synthesized from a varied molar ratio of their acetylacetonate derivatives, resulting in an actual optimum measured bulk composition of FeOxPt2 originating from an M-acac molar ratio of 1:1, for the purpose of facilitating the oxygen reduction reaction. The catalyst show signs of highly favorable NP surface toward ORR as demonstrated by its nearly five times higher specific activity than a commercial Pt – Vulcan catalyst measured in an identical manner. Its higher performance in terms of mass activity is also shown to represent fuel cell performance in both H2/O2 and DFAFC cathode conditions. However a common MEA preparation procedure (hot pressing), known to significantly boost performance in most H2/O2 cells, did not benefit the performance of FePt(1:1), posing a challenge for future developments of catalysts developed in similar fashion. Enhanced relative performance of FePt(1:1) in the DFAFC cathode is likely attributed to a more CO tolerant Pt surface supported by the appearance of an additional low-voltage peak in the CO-stripping voltammograms, possibly making it suitable as a cathode catalyst for portable PEM fuel cells fueled by small organic molecules such as methanol, ethanol and formic acid.

Influences of diverse Fe(acac)3:Pt(acac)2 ratios on the solvothermally produced NPs were investigated by various physical characterization methods. Besides showing strong signs of successful Fe incorporation, it was clear that reduced particle sizes were accompanied by an increase in Fe content in addition to a lowered structural stability of the fcc lattice, highlighting a possible compromise that has to be considered while developing such NPs. The high ORR activity of FePt(1:1) can thus be attributed to a combination of a well-balanced compromise between size controlling, surface morphology and the internal fraction of Fe within individual


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NPs. The synthesis method of colloidal Pt NPs is simple and free from surfactants or cappingagents utilizing only Fe(acac)3 as a size-controller, enabling further studies of FePt – based catalysts with additional modifications.

ASSOCIATED CONTENT Supporting Information Available: **[General electrochemical experimental details, TGA analysis, XPS spectra, Ex situ and in situ XRD data and additional electrochemical data]**

Acknowledgements The authors acknowledge Umeå Core Facility for Electron Microscopy (UCEM) and the vibrational spectroscopy core facility (VISP) at Umeå University T. W acknowledges support from

Vetenskapsrådet (2017-04862) Energimyndigheten (45419-1), and Ångpanneföreningen (15-483).

References (1) Wu, J. B.; Yang, H. Platinum-Based Oxygen Reduction Electrocatalysts. Acc. Chem. Res. 2013, 46, 1848-1857. (2) Hwang, J.-J. Sustainability Study of Hydrogen Pathways for Fuel Cell Vehicle Applications. Renewable Sustainable Energy Rev. 2013, 19, 220-229. (3) Fujiwara, N.; Yamazaki, S.; Yasuda, K. Research and Development on Direct Polymer Electrolyte Fuel Cells. J. Jpn. Pet. Inst. 2011, 54, 237-247. (4) Hu, G. Z.; Nitze, F.; Sharifi, T.; Barzegar, H. R.; Wagberg, T. Self-Assembled Palladium Nanocrystals on Helical Carbon Nanofibers as Enhanced Electrocatalysts for Electro-Oxidation of Small Molecules. J. Mater. Chem. 2012, 22, 8541-8548. (5) Nitze, F.; Sandstrom, R.; Barzegar, H. R.; Hu, G. Z.; Mazurkiewicz, M.; Malolepszy, A.; Stobinski, L.; Wagberg, T. Direct Support Mixture Painting, Using Pd(0) Organo-Metallic Compounds - an Easy and Environmentally Sound Approach to Combine Decoration and Electrode Preparation for Fuel Cells. J. Mater. Chem. A 2014, 2, 20973-20979. (6) Ralph, T. R.; Hogarth, M. P. Catalysis for Low Temperature Fuel Cells. Platinum Met. Rev. 2002, 46, 117-135. (7) Higgins, D. C.; Chen, Z. W. Recent Progress in Non-Precious Metal Catalysts for Pem Fuel Cell Applications. Can. J. Chem. Eng. 2013, 91, 1881-1895. (8) Othman, R.; Dicks, A. L.; Zhu, Z. H. Non Precious Metal Catalysts for the Pem Fuel Cell Cathode. Int. J. Hydrogen Energy 2012, 37, 357-372.



(9) Zhou, Y.; Neyerlin, K.; Olson, T. S.; Pylypenko, S.; Bult, J.; Dinh, H. N.; Gennett, T.; Shao, Z.; O'Hayre, R. Enhancement of Pt and Pt-Alloy Fuel Cell Catalyst Activity and Durability Via NitrogenModified Carbon Supports. Energy Environ. Sci. 2010, 3, 1437-1446. (10) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for Pemfcs. Appl. Catal., B 2005, 56, 9-35. (11) Tang, H. L.; Wang, S. L.; Jiang, S. P.; Pan, M. A Comparative Study of Ccm and Hot-Pressed Meas for Pem Fuel Cells. J. Power Sources 2007, 170, 140-144. (12) Guo, S. J.; Zhang, S.; Sun, S. H. Tuning Nanoparticle Catalysis for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 8526-8544. (13) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241-247. (14) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Norskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552-556. (15) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3ni(111) Via Increased Surface Site Availability. Science 2007, 315, 493-497. (16) Jennings, P. C.; Aleksandrov, H. A.; Neyman, K. M.; Johnston, R. L. A Dft Study of Oxygen Dissociation on Platinum Based Nanoparticles. Nanoscale 2014, 6, 1153-1165. (17) Hu, G. Z.; Nitze, F.; Gracia-Espino, E.; Ma, J. Y.; Barzegar, H. R.; Sharifi, T.; Jia, X. E.; Shchukarev, A.; Lu, L.; Ma, C. S.; Yang, G.; Wagberg, T. Small Palladium Islands Embedded in Palladium-Tungsten Bimetallic Nanoparticles Form Catalytic Hotspots for Oxygen Reduction. Nat. Commun. 2014, 5. (18) Kim, J.; Lee, Y.; Sun, S. H. Structurally Ordered Fept Nanoparticles and Their Enhanced Catalysis for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 4996-+. (19) Mazumder, V.; Chi, M. F.; More, K. L.; Sun, S. H. Core/Shell Pd/Fept Nanoparticles as an Active and Durable Catalyst for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 7848-+. (20) Wang, C.; van der Vliet, D.; More, K. L.; Zaluzec, N. J.; Peng, S.; Sun, S. H.; Daimon, H.; Wang, G. F.; Greeley, J.; Pearson, J.; Paulikas, A. P.; Karapetrov, G.; Strmcnik, D.; Markovic, N. M.; Stamenkovic, V. R. Multimetallic Au/Fept3 Nanoparticles as Highly Durable Electrocatalyst. Nano Lett. 2011, 11, 919926. (21) Guo, S. J.; Sun, S. H. Fept Nanoparticles Assembled on Graphene as Enhanced Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 2492-2495. (22) Chen, D.; Zhao, X.; Chen, S. S.; Li, H. F.; Fu, X. N.; Wu, Q. Z.; Li, S. P.; Li, Y.; Su, B. L.; Ruoff, R. S. One-Pot Fabrication of Fept/Reduced Graphene Oxide Composites as Highly Active and Stable Electrocatalysts for the Oxygen Reduction Reaction. Carbon 2014, 68, 755-762. (23) Kannan, R.; Silva, A. A.; Cardoso, F. M.; Gupta, G.; Aslam, Z.; Sharma, S.; Steinberger-Wilckens, R. Study of Fept Deposited Reduced Graphene Oxide's Utility as a Catalyst Towards Oxygen Reduction and Methanol Oxidation Reactions. RSC Adv. 2015, 5, 36993-36998. (24) Li, J.; Wang, G.; Wang, J.; Miao, S.; Wei, M.; Yang, F.; Yu, L.; Bao, X. Architecture of Ptfe/C Catalyst with High Activity and Durability for Oxygen Reduction Reaction. Nano Res. 2014, 7, 1519-1527. (25) Chen, W.; Kim, J.; Sun, S. H.; Chen, S. W. Electro-Oxidation of Formic Acid Catalyzed by Fept Nanoparticles. PCCP 2006, 8, 2779-2786. (26) Chen, W.; Kim, J. M.; Sun, S. H.; Chen, S. W. Composition Effects of Fept Alloy Nanoparticles on the Electro-Oxidation of Formic Acid. Langmuir 2007, 23, 11303-11310. (27) Guo, J. X.; Sun, Y. F.; Zhang, X.; Tang, L.; Liu, H. T. Fept Nanoalloys Anchored Reduced Graphene Oxide as High-Performance Electrocatalysts for Formic Acid and Methanol Oxidation. J. Alloys Compd. 2014, 604, 286-291.


Page 24 of 27

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Liu, Y. T.; Yuan, Q. B.; Duan, D. H.; Zhang, Z. L.; Hao, X. G.; Wei, G. Q.; Liu, S. B. Electrochemical Activity and Stability of Core Shell Fe2o3/Pt Nanoparticles for Methanol Oxidation. J. Power Sources 2013, 243, 622-629. (29) Sun, S. H. Recent Advances in Chemical Synthesis, Self-Assembly, and Applications of Fept Nanoparticles. Adv. Mater. 2006, 18, 393-403. (30) Harpeness, R.; Gedanken, A. The Microwave-Assisted Polyol Synthesis of Nanosized Hard Magnetic Material, Fept. J. Mater. Chem. 2005, 15, 698-702. (31) Kang, S. S.; Jia, Z. Y.; Shi, S. F.; Nikles, D. E.; Harrell, J. W. Easy Axis Alignment of Chemically Partially Ordered Fept Nanoparticles. Appl. Phys. Lett. 2005, 86. (32) Iwamoto, T.; Matsumoto, K.; Matsushita, T.; Inokuchi, M.; Toshima, N. Direct Synthesis and Characterizations of Fct-Structured Fept Nanoparticles Using Poly(N-Vinyl-2-Pyrrolidone) as a Protecting Agent. J. Colloid Interface Sci. 2009, 336, 879-888. (33) Howard, L. E. M.; Nguyen, H. L.; Giblin, S. R.; Tanner, B. K.; Terry, I.; Hughes, A. K.; Evans, J. S. O. A Synthetic Route to Size-Controlled Fcc and Fct Fept Nanoparticles. J. Am. Chem. Soc. 2005, 127, 1014010141. (34) Jeyadevan, B.; Hobo, A.; Urakawa, K.; Chinnasamy, C. N.; Shinoda, K.; Tohji, K. Towards Direct Synthesis of Fct-Fept Nanoparticles by Chemical Route. J. Appl. Phys. 2003, 93, 7574-7576. (35) Jeyadevan, B.; Urakawa, K.; Hobo, A.; Chinnasamy, N.; Shinoda, K.; Tohji, K.; Djayaprawira, D. D. J.; Tsunoda, M.; Takahashi, M. Direct Synthesis of Fct-Fept Nanoparticles by Chemical Route. Jpn. J. Appl. Phys., Part 2 2003, 42, L350-L352. (36) Shinozaki, K.; Zack, J. W.; Richards, R. M.; Pivovar, B. S.; Kocha, S. S. Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique I. Impact of Impurities, Measurement Protocols and Applied Corrections. J. Electrochem. Soc. 2015, 162, F1144F1158. (37) Carpenter, M. K.; Moylan, T. E.; Kukreja, R. S.; Atwan, M. H.; Tessema, M. M. Solvothermal Synthesis of Platinum Alloy Nanoparticles for Oxygen Reduction Electrocatalysis. J. Am. Chem. Soc. 2012, 134, 8535-8542. (38) Wang, H. L.; Krier, J. M.; Zhu, Z. W.; Melaet, G.; Wang, Y. H.; Kennedy, G.; Alayoglu, S.; An, K.; Somorjai, G. A. Promotion of Hydrogenation of Organic Molecules by Incorporating Iron into Platinum Nanoparticle Catalysts: Displacement of Inactive Reaction Intermediates. ACS Catal. 2013, 3, 2371-2375. (39) Sun, S. H.; Zeng, H. Size-Controlled Synthesis of Magnetite Nanoparticles. J. Am. Chem. Soc. 2002, 124, 8204-8205. (40) Liu, C.; Wu, X. W.; Klemmer, T.; Shukla, N.; Yang, X. M.; Weller, D.; Roy, A. G.; Tanase, M.; Laughlin, D. Polyol Process Synthesis of Monodispersed Fept Nanoparticles. J. Phys. Chem. B 2004, 108, 6121-6123. (41) Hu, G.; Gracia-Espino, E.; Sandstrom, R.; Sharifi, T.; Cheng, S.; Shen, H.; Wang, C.; Guo, S.; Yang, G.; Wagberg, T. Atomistic Understanding of the Origin of High Oxygen Reduction Electrocatalytic Activity of Cuboctahedral Pt3co-Pt Core-Shell Nanoparticles. Catal. Sci. Technol. 2016, 6, 1393-1401. (42) Liu, Z. L.; Ling, X. Y.; Su, X. D.; Lee, J. Y. Carbon-Supported Pt and Ptru Nanoparticles as Catalysts for a Direct Methanol Fuel Cell. J. Phys. Chem. B 2004, 108, 8234-8240. (43) Li, Y. J.; Gao, W.; Ci, L. J.; Wang, C. M.; Ajayan, P. M. Catalytic Performance of Pt Nanoparticles on Reduced Graphene Oxide for Methanol Electro-Oxidation. Carbon 2010, 48, 1124-1130. (44) Zhang, J.; Yang, H. Z.; Yang, K. K.; Fang, J.; Zou, S. Z.; Luo, Z. P.; Wang, H.; Bae, I. T.; Jung, D. Y. Monodisperse Pt3fe Nanocubes: Synthesis, Characterization, Self-Assembly, and Electrocatalytic Activity. Adv. Funct. Mater. 2010, 20, 3727-3733. (45) Klemmer, T. J.; Shukla, N.; Liu, C.; Wu, X. W.; Svedberg, E. B.; Mryasov, O.; Chantrell, R. W.; Weller, D.; Tanase, M.; Laughlin, D. E. Structural Studies of L1(0) Fept Nanoparticles. Appl. Phys. Lett. 2002, 81, 2220-2222.



(46) Sun, S. H.; Fullerton, E. E.; Weller, D.; Murray, C. B. Compositionally Controlled Fept Nanoparticle Materials. IEEE Trans. Magn. 2001, 37, 1239-1243. (47) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Monodisperse Fept Nanoparticles and Ferromagnetic Fept Nanocrystal Superlattices. Science 2000, 287, 1989-1992. (48) Hansen, T. W.; Delariva, A. T.; Challa, S. R.; Datye, A. K. Sintering of Catalytic Nanoparticles: Particle Migration or Ostwald Ripening? Acc. Chem. Res. 2013, 46, 1720-1730. (49) Jozwiak, W. K.; Kaczmarek, E.; Maniecki, T. P.; Ignaczak, W.; Maniukiewicz, W. Reduction Behavior of Iron Oxides in Hydrogen and Carbon Monoxide Atmospheres. Appl. Catal., A 2007, 326, 1727. (50) Khader, M. M.; Elanadouli, B. E.; Elnagar, E.; Ateya, B. G. Kinetics of the Reduction of Fe2o3 with Hydrogen. J. Solid State Chem. 1991, 93, 283-290. (51) Suber, L.; Marchegiani, G.; Olivetti, E. S.; Celegato, F.; Coisson, M.; Tiberto, P.; Allia, P.; Barrera, G.; Pilloni, L.; Barba, L.; Padella, F.; Cossari, P.; Chiolerio, A. Pure Magnetic Hard Fct Fept Nanoparticles: Chemical Synthesis, Structural and Magnetic Properties Correlations. Mater. Chem. Phys. 2014, 144, 186-193. (52) Liu, C.; Wu, X.; Klemmer, T.; Shukla, N.; Yang, X.; Weller, D.; Roy, A. G.; Tanase, M.; Laughlin, D. Polyol Process Synthesis of Monodispersed Fept Nanoparticles. The Journal of Physical Chemistry B 2004, 108, 6121-6123. (53) Rutledge, R. D.; Morris, W. H.; Wellons, M. S.; Gai, Z.; Shen, J.; Bentley, J.; Wittig, J. E.; Lukehart, C. M. Formation of Fept Nanoparticles Having High Coercivity. J. Am. Chem. Soc. 2006, 128, 14210-14211. (54) Li, Q.; Wu, L. H.; Wu, G.; Su, D.; Lv, H. F.; Zhang, S.; Zhu, W. L.; Casimir, A.; Zhu, H. Y.; MendozaGarcia, A.; Sun, S. H. New Approach to Fully Ordered Fct-Fept Nanoparticles for Much Enhanced Electrocatalysis in Acid. Nano Lett. 2015, 15, 2468-2473. (55) Hong, Y.; Kim, H. J.; Yang, D.; Lee, G.; Nam, K. M.; Jung, M. H.; Kim, Y. M.; Choi, S. I.; Seo, W. S. Facile Synthesis of Fully Ordered L1(0)-Fept Nanoparticles with Controlled Pt-Shell Thicknesses for Electrocatalysis. Nano Res. 2017, 10, 2866-2880. (56) Rudi, S.; Cui, C. H.; Gan, L.; Strasser, P. Comparative Study of the Electrocatalytically Active Surface Areas (Ecsas) of Pt Alloy Nanoparticles Evaluated by H-Upd and Co-Stripping Voltammetry. Electrocatalysis 2014, 5, 408-418. (57) Jayasayee, K.; Van Veen, J. A. R.; Manivasagam, T. G.; Celebi, S.; Hensen, E. J. M.; de Bruijn, F. A. Oxygen Reduction Reaction (Orr) Activity and Durability of Carbon Supported Ptm (Co, Ni, Cu) Alloys: Influence of Particle Size and Non-Noble Metals. Appl. Catal., B 2012, 111, 515-526. (58) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J. Characterization of High-Surface Area Electrocatalysts Using a Rotating Disk Electrode Configuration. J. Electrochem. Soc. 1998, 145, 2354-2358. (59) Devivaraprasad, R.; Ramesh, R.; Naresh, N.; Kar, T.; Singh, R. K.; Neergat, M. Oxygen Reduction Reaction and Peroxide Generation on Shape-Controlled and Polycrystalline Platinum Nanoparticles in Acidic and Alkaline Electrolytes. Langmuir 2014, 30, 8995-9006. (60) Urchaga, P.; Baranton, S.; Coutanceau, C. Changes in Cochem Oxidative Stripping Activity Induced by Reconstruction of Pt (111) and (100) Surface Nanodomains. Electrochim. Acta 2013, 92, 438445. (61) Antoniassi, R. M.; Silva, J. C. M.; Lopes, T.; Neto, A. O.; Spinace, E. V. Carbon-Supported Pt Nanoparticles with (100) Preferential Orientation with Enhanced Electrocatalytic Properties for Carbon Monoxide, Methanol and Ethanol Oxidation in Acidic Medium. Int. J. Hydrogen Energy 2017, 42, 2878628796. (62) Urchaga, P.; Baranton, S.; Coutanceau, C.; Jerkiewicz, G. Electro-Oxidation of Cochem on Pt Nanosurfaces: Solution of the Peak Multiplicity Puzzle. Langmuir 2012, 28, 3658-3663.


Page 26 of 27

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(63) Ciapina, E. G.; Santos, S. F.; Gonzalez, E. R. Electrochemical Co Stripping on Nanosized Pt Surfaces in Acid Media: A Review on the Issue of Peak Multiplicity. J. Electroanal. Chem. 2018, 815, 47-60. (64) Guillen-Villafuerte, O.; Garcia, G.; Orive, A. G.; Anula, B.; Creus, A. H.; Pastor, E. Electrochemical Characterization of 2d Pt Nanoislands. Electrocatalysis 2011, 2, 231-241. (65) Guerin, S.; Hayden, B. E.; Lee, C. E.; Mormiche, C.; Owen, J. R.; Russell, A. E.; Theobald, B.; Thompsett, D. Combinatorial Electrochemical Screening of Fuel Cell Electrocatalysts. J. Comb. Chem. 2004, 6, 149-158. (66) Kinoshita, K. Particle-Size Effects for Oxygen Reduction on Highly Dispersed Platinum in Acid Electrolytes. J. Electrochem. Soc. 1990, 137, 845-848. (67) Ahmad, R.; Singh, A. K. Simultaneous Site Adsorption Shift and Efficient Co Oxidation Induced by V and Co in Pt Catalyst. J. Phys. Chem. C 2017, 121, 12807-12816. (68) Mench, M. M. Fuel Cell Engines. Wiley: 2008. (69) Zhang, J. Pem Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications. Springer London: 2008. (70) Gharibi, H.; Yasi, F.; Kazemeini, M.; Heydari, A.; Golmohammadi, F. Fabrication of Mea Based on Sulfonic Acid Functionalized Carbon Supported Platinum Nanoparticles for Oxygen Reduction Reaction in Pemfcs. RSC Adv. 2015, 5, 85775-85784. (71) Jeong, K. J.; Miesse, C. A.; Choi, J. H.; Lee, J.; Han, J.; Yoon, S. P.; Nam, S. W.; Lim, T. H.; Lee, T. G. Fuel Crossover in Direct Formic Acid Fuel Cells. J. Power Sources 2007, 168, 119-125. (72) Yu, X. W.; Pickup, P. G. Recent Advances in Direct Formic Acid Fuel Cells (Dfafc). J. Power Sources 2008, 182, 124-132.


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