Pd-catalysts for DFAFC prepared by magnetron sputtering

Applied Surface Science 419 (2017) 838–846

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Pd-catalysts for DFAFC prepared by magnetron sputtering I. Bieloshapka a,b,∗ , P. Jiricek b , M. Vorokhta a , E. Tomsik d , A. Rednyk a , R. Perekrestov a , K. Jurek b , E. Ukraintsev b , K. Hruska b , O. Romanyuk b , B. Lesiak c a

Charles University in Prague, Faculty of Mathematics and Physics, Prague 2, Ke Karlovu 3, Czech Republic Institute of Physics, Academy of Sciences of the Czech Republic, Prague 6, Cukrovarnicka 10, Czech Republic Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warszawa, Poland d Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Prague 6, Czech Republic b c

a r t i c l e

i n f o

Article history: Received 27 October 2016 Received in revised form 7 February 2017 Accepted 3 May 2017 Available online 8 May 2017 Keywords: Pd catalyst Formic acid fuel cell Magnetron sputtering DFAFC Surface morphology

a b s t r a c t Samples of a palladium catalyst for direct formic acid fuel cell (DFAFC) applications were prepared on the ® Elat carbon cloth by magnetron sputtering. The quantity of Pd was equal to 3.6, 120 and 720 ␮g/cm2 . The samples were tested in a fuel cell for electro-oxidation of formic acid, and were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). ® The XPS measurements revealed a high contribution of PdCx phase formed at the Pd/Elat surface interface, with carbon concentration in PdCx from x = 9.9–14.6 at.%, resulting from the C substrate and CO residual gases. Oxygen groups, e.g. hydroxyl (–OH), carbonyl (C O) and carboxyl (COOH), resulted from the synthesis conditions due to the presence of residual gases, electro-oxidation during the reaction and oxidation in the atmosphere. Because of the formation of CO and CO2 on the catalysts during the reaction, or because of poisoning by impurities containing the –CH3 group, together with the risk of Pd losses due to dissolution in formic acid, there was a negative effect of catalyst degradation on the active area surface. The effect of different loadings of Pd layers led to increasing catalyst efficiency. Current–voltage curves showed that different amounts of catalyst did not increase the DFAFC power to a great extent. One reason for this was the catalyst structure formed on the carbon cloth. AFM and SEM measurements showed a layer-by-layer growth with no significant variations in morphology. The results for electric power recalculated for the Pd loading per 1 mg of catalyst layers in comparison to carbon substrates decorated by Pd nanoparticles showed that there is potential for applying anodes for formic acid fuel cells prepared by magnetron sputtering. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Direct formic acid fuel cells (DFAFCs) operating at low temperatures (below 70 ◦ C) have the following advantages over other proton exchange membrane fuel cells (PEMFCs): higher electromotive force (theoretical open circuit potential 1.48 V), higher kinetics at room temperature, limited fuel crossover, and reasonable power densities at low temperatures [1,2]. As a consequence, they are promising candidates for generating electric power for a wide range of applications [3]. It has been shown that formic acid (FA) can be a product of many biomass processes [4]. FA has also attracted the interest of researchers for use in H2 storage and production sys-

tems, because of its high energy density (2086 Wh/L) [5], excellent stability and non-toxicity at room temperature [6]. Methanol has also been recognized as a liquid that can be used under ambient conditions in a storage-production system for H2 . Methanol has a higher energy density than FA (4690 Wh/L). However, it is currently a problem to achieve low methanol crossover through proton ® exchange Nafion membrane. FA exhibits low crossover through the membrane, allowing high concentrations (up to 15 M) that can be used with no critical drops in performance [7]. FA can be catalytically decomposed to H2 and carbon dioxide (CO2 ) via a dehydrogenation reaction using a catalyst via the reaction [8,9]:

∗ Corresponding author at: Institute of Physics, Academy of Sciences of the Czech Republic, Prague 6, Cukrovarnicka 10, Czech Republic. E-mail address: [email protected] (I. Bieloshapka).

HCOOH(l) = H2 (g) + CO2 (g)

http://dx.doi.org/10.1016/j.apsusc.2017.05.035 0169-4332/© 2017 Elsevier B.V. All rights reserved.

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I. Bieloshapka et al. / Applied Surface Science 419 (2017) 838–846

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Fig. 1. Dependence of the maximum specific power and voltage on current in DFAFC for catalyst of Pd different loading with amount of the catalyst 3.6 ␮g (P1), 120 ␮g (P2) and 720 ␮g/cm2 (P3).

It has been documented that the intermediate reactions that take place on the active centers of an anode have the following form [10]: HCOOH(l) = H2 O(l) + COads (g)

(2a)

COads (g) + H2 O(l) → CO2 + 2H+ + 2e− .

(2b)

The CO poisoning effect depends on factors such as the type of catalyst and the reaction conditions, including FA purity, temperature, the pH value of the reaction system, and the pressure between the anode plate and the cathode. A wide range of bimetallic catalysts (including AgPd, AuPd, CoPd, FePd, NiPd, PdPt, PdSn, etc.) have been investigated in recent times for use as a DFAFC electro-oxidation catalyst. Binary noble metal alloy catalysts exhibit adventitious long-term stability at high formic acid concentrations, due to their greater resistance to acid corrosion [11]. An undesirable reaction that indirectly generates carbon monoxide (2a) can dramatically poison the catalytic surface of DFAFC [6,12]. Pd-based catalysts mainly oxidize formic acid via the direct pathway, but a small amount of formic acid is still oxidized via the indirect pathway. Consequently, CO accumulates over Pd-based catalysts and reduces the time-stability of the catalyst. It has been shown that Pd is currently one of the best of the widely-used catalysts for FA electrochemical oxidation in DFAFC [10,13]. Mainly for this reason this type of catalyst was selected in our experiments. One of the main disadvantages of the use of Pd catalysts at the anode is that the desirable activity and stability in DFAFC appears only when high-purity formic acid is used [14]. After a fuel cell using low-purity FA has been working for some time, it is important to apply a reactivation procedure due to the anode degradation. Applying reversed voltage up to −0.3 V at the anode can reduce the poisoning effect [15]. Unfortunately, however, this reactivation procedure has speed limitations, as it slows down the deactivation reaction at less negative voltages [16]. Impressive results in dealing with catalyst poisoning were achieved by cobalt phosphide (CoP) material during formic acid oxidation [17]. In our experiment we did not use a deactivation procedure, because our investigation focused on the power of the fuel cell, and not on its time stability. Many techniques have been found to be acceptable for preparing the anode part of FCs: fabricating layers using a mixing procedure with catalyst particles [18], ink-based catalyst layer fabrication [19], screen printing [20], spray coating [21], inkjet printing [22], sputter deposition [23], dual ion-beam-assisted deposition [24], electrodeposition [25], reactive spray deposition [26], pulsed laser deposition [27], and others [28]. However, it is necessary during the anode preparation process, and also when utilizing the catalysts, to take into account the relatively high cost of these techniques at the present time.

Magnetron sputtering has proved to be an effective technology for producing anodes with a predicted low amount of catalyst, due to the well-controlled sputtering parameters. For this reason, it has been used in hydrogen fuel cells [29]. In our study, we decided to use the magnetron sputtering technique for preparing anodes in other types of PEM fuel cells, e.g. for DFAFC. The typical loading of Pd for the chemically prepared anode part is around 0.5–8 mg/cm2 . In our paper, emphasis is placed on investigating and comparing the results for a Pd catalyst on a carbon cloth with various loadings (from a few ␮g up to hundreds of ␮g) after sputtering, in order to detect correlations in the maximum power characteristic and in minimal loading. Another aim of our present work has been to deepen our understanding of the structural and chemical properties of the Pd magnetron layers formed on the carbon cloth surface when the magnetron sputtering process is used. The structural and chemical characterization methods that we applied during our investigation were: atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). 2. Experimental 2.1. Pretreatment and preparation of the anode ®

Conductive carbon cloth (Elat , Fuel Cells Etc. Co, USA) was used as a support anode material for the DFAFC. Due to FA low viscosity, its flow resistance transport through the carbon cloth was rather low. The raw carbon cloth was cleaned with distilled water, dried and cut into rectangles with 2 cm2 of active surface to form a conductive anode layer. The non-reactive magnetron sputtering method was used to prepare Pd thin films on the surface of the carbon cloth. Sputtering was carried out in an Ar atmosphere with a residual gas pressure of less than 4 × 10−1 Pa. Before deposition was started, the sputtering chamber was evacuated up to 5 × 10−4 Pa. The palladium thin films were deposited using a dc sputtering source. A 99.99% purity palladium target was applied. The thickness of all deposited films was measured “ex-situ” using a drop of varnish, which was removed after the deposition process. This provided a growth rate of Pd films speed of about 1 nm/min. 2.2. Samples ®

The layers of Pd on an Elat carbon cloth 3 nm, 100 nm and 600 nm in thickness, corresponding to loadings of 3.6 ␮g, 120 ␮g and 720 ␮g/cm2 , were denoted as P1, P2 and P3, respectively. These samples named P1–P3 were investigated by AFM, SEM and XPS after ® a fuel cell test and aging. In addition, a sample of Pd layers on Elat after synthesis, denoted as the reference sample (Ref), and the same

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P1 P2 P3

40 32 24 16 3.9 2.6

Power density, mW mg -1

840

1.3 0.0 0

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2

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Current, mA Fig. 2. Comparison of the power density on current for catalyst of different Pd loading recalculated per mg of Pd with amount of the catalyst 3.6 ␮g (P1), 120 ␮g (P2) and 720 ␮g/cm2 (P3).

sample measured just after a fuel cell test, denoted as RefFC , were investigated by XPS. In order to determine the chemical state of the Pd in the samples correctly, the metallic Pd standard (Pdmet ) of 99.99% purity was used in the XPS studies. 2.3. DFAFC setup and measuring system The anodes with different loadings were investigated by a homemade FC work station and a direct FA electrochemical cell. ® The FA (85% HCOOH pure for analysis, Sigma–Aldrich , USA) solution of 1 M was obtained by dilution with deionized water. It was ® then delivered to the anode with a peristaltic pump (Longer Pump BT 300-2J, a Halma Company, USA) at 6 rpm. Voltage and cur® rent measurements were performed utilizing the B&K Precision 8502 system (USA). A commercial cathode with 4 mg/cm2 of PtB on carbon close (Fuel Cells Etc. Co, USA) was applied during all measurements. Higher oxygen flow rates were used when testing the ® Pd/Elat with the liquid fuel, in order to ensure that the cell would not be limited by oxygen transport on the cathode side. Oxygen was supplied to the cathode using a humidifier. The fuel cell was sandwiched between two stainless steel plates with a surface area of 12 cm2 within 4 bar of constant total pressure. This was achieved using a pneumatic system with N2 as the working gas. Humidified ® oxygen was supplied to the cathode using Nafion tubes. The flow ® −1 rate in the cathode inlet was set at 0.2 L min . Nafion 212 (Fuel Cells Etc. Co, USA) was used as an ion conductor. During FC operation, all tests were performed at room temperature. The working ® electrode was prepared without using the Nafion solution as a support top layer and hot-pressing process. 2.4. Methods for characterizing anodes The surface morphology of samples P1, P2 and P3 was observed using SEM (JXA-733 electron microprobe, JEOL, USA). The same setting parameters were applied for all samples during all measurements. Atomic force microscopy (AFM Bruker ICON, Germany) was applied to detect detailed changes in the surface area of samples P1, P2 and P3. AFM worked in the air using the Peak Force Quantitative Nano Mechanical characterization mode. Cantilevers were prepared in CF4 plasma treated Multi75Al [30]. This tip pretreatment significantly reduced contamination of the AFM tips during

Fig. 3. Scanning electron micrographs of the anode of different Pd loading. Amount of the catalyst was: (a) 3.6 ␮g (P1); (b) 120 ␮g (P2); (c) 720 ␮g/cm2 (P3). Magnification of 20 000 × can be found at the right upper corner of each image.

their interaction with the surface, and therefore optimal and reproducible measurements could be made. Images with dimensions of 10 × 10 ␮m were obtained. The force threshold in PFQNM mode was 5 nN. An evaluation of the elemental composition and the chemical state of the anode surface was made using the AXIS Supra photoelectron spectrometer (Kratos Analytical, UK). The incidence angle of the monochrome AlK␣ radiation was 54.4◦ and the photoelectron emission angle was ˛out = 0◦ , with respect to the surface normal. The hemispherical electron energy analyzer operated in the constant pass energy mode Ep = 20 eV. Surface cleaning was applied to Pdmet standard using an ion source prior to the XPS measurements.

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Fig. 4. AFM images of Pd/Elat with different Pd loading: (a) 3.6 ␮g (P1); (b) 120 ␮g (P2); (c) 720 ␮g/cm2 (P3).

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3. Results and discussion The electrical properties of the formic acid fuel cell using O2 at the cathode were measured at room temperature with different catalyst loadings at the anode (Fig. 1). Dear reader can see differences between the three samples with dissimilar catalyst loadings, which occurred when they were being prepared by sputtering. An increase in power due to increasing catalyst loading was observed (Fig. 1). Maximum power density per cm2 was achieved for 720 ␮g loading (sample P3). This power density is lower than the density described in the literature. Slight differences in the morphology of the Pd catalysts were probably the main reason for the nonsignificant increase in FC efficiency. The low FC productivity also can be explained in part by FA contamination and by fuel transport limitations due to the accumulation of CO2 gas bubbles on the anode catalyst layer. In addition, products of the fuel dissociation reaction can block the active surface part of the anode. A recalculation of the results to the mg weight of the Pd catalyst is presented in (Fig. 2). Surprisingly, catalyst P1 showed much higher power density per mg per cm2 than samples P2 and P3, and can be compared with some chemically prepared anodes for DFAFC [15][15 and references therein]. A slight difference in morphology after depositing Pd on the ® Elat cloth was observed in the SEM images of samples P1, P2, and P3 (Fig. 3). Formation of grained surface can be observed particularly at (Fig. 3c) with the increasing of the catalyst. Grains up to 1 ␮m started to appear on carbon cloth. The distinct failure to form large surface structures during catalyst loading reduces the functionality of FC. One reason for similarity of the formed Pd layers was that the same experimental parameters were applied. Different Pd loadings were obtained by varying only the time of magnetron sputtering. The AFM images used for obtaining detailed information about the structure of samples P1, P2 and P3 were measured along the carbon fibers (Fig. 4). The 3D images were shown without correction, and for this reason some artifacts can be found. The left side of the figures provides information about the roughness of the car-

Fig. 5. Comparison of the XPS survey spectra recorded from: (a) Pd/Elat of different Pd loading after a fuel cell test and few weeks aging (P1, P2 and P3) and Pdmet ® samples, (b) Pd/Elat reference sample after synthesis (Ref) and just after a fuel cell test (RefFC ).

bon cloth fiber after Pd catalyst deposition. The right side shows a slight growth of the surface area due to the formation of Pd islands. Third order flattening was applied to all images for the analysis of the surface area, and horizontal scans were removed. The surface area was calculated for flattened 2D images. The diameter of the fibers was less than 10 ␮m. The top and bottom parts of some AFM images were therefore obtained using the tip cone, not the AFM tip apex. The data used to create the figures includes values for the distorted surface area. However, the information obtained from the data showed that the surface area of the catalyst was only slightly greater after the thickness of the Pd layers had been increased 200-times (from 3 to 600 nm). For sample P3, the Pd surface area increased by 22% (Fig. 4c) in comparison with the surface for P1 (Fig. 4a), due to the formation of Pd islands on sample P3. The XPS characterization of samples P1, P2 and P3 was carried out after the fuel cell test and some weeks of aging. For a study of the influence of aging on the properties of the prepared anodes, we investigated an additional Pd magnetron layer sample after synthesis, denoted as the reference sample (Ref), and also a sample taken just after the fuel cell test, denoted as RefFC . The XPS technique was used to quantify the elemental content and to investigate changes in their chemical state. The XPS wide scans and detailed C 1s, O 1s and Pd 3d5/2–3/2 photoelectron spectra were investigated. The wide scans showed the contribution of palladium, carbon and oxygen, with O 1s spectra overlapping with the Pd 3d3/2 spectrum. Fig. 5 presents a comparison of the survey spectra for samples P1, P2, P3, clean metallic Pdmet and the reference samples after synthesis (Ref) and after a fuel cell test (RefFC ). The elemental content at the surface was quantified after subtracting the Shirley background [31], using MultiQuant software [32], which is described in [33], assuming a homogeneous distribution of atoms on the surface, Scofield subshell photoionization crosssections, a correction for analyzer transmission, contamination,

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Fig. 6. XPS Pd 3d5/2–3/2 , C 1s and O 1s spectra recorded from the Pd/Elat samples of different Pd loading (P1, P2 and P3) after a fuel cell test and few weeks aging fitted to the asymmetric Gaussian–Lorentzian functions after Tougaard background subtraction.

Table 1 Atomic and weight concentrations of elements for Pd standard and Pd magnetron layers surfaces. Sample

Pdmet P1 P2 P3 Ref RefFC

at.%

wt.%

Pd

C

O

Pd

C

O

87.7 27.8 41.8 54.2 22.9 16.6

10.7 54.1 39.3 20.8 61.5 56.3

1.6 18.1 18.9 25.0 15.6 27.1

98.4 75.9 85.2 89.9 71.1 61.4

1.3 16.7 9.0 3.9 21.6 23.5

0.3 7.4 5.8 6.2 7.3 15.1

asymmetry parameters of the angular distribution of photoelectrons and elastic scattering. The content of oxygen was evaluated after correcting the O 1s spectra for overlapping Pd 3d3/2 spectra, accounting for the ratio of Pd 3d5/2–3/2 and Pd 3p3/2 spectra intensities recorded from the clean Pdmet . The surface quantification results are listed in Table 1. The content of carbon on the surface of the Pd catalysts exceeded the content of metallic Pd standard, i.e. it was between 20.8 and 61.5 at.%. The content of oxygen ranged from 18.1 to 27.1 at.%. The lowest content of oxygen was observed

for a sample after synthesis (Ref), i.e. 15.6 at.%, in contrast to this sample after a fuel cell test only (RefFC ), i.e. 27.1 at.%, with a lower oxygen content for samples P1–P3, i.e. 18.1–25.0 at.%. For an examination of the detailed spectra, all the binding energy (BE) values were referenced to the C 1s peak at 284.4 eV. The Pd 3d, C 1s and O 1s spectra, submitted to Shirley background subtraction [31], were fitted using the Gaussian–Lorentzian asymmetric functions to the metallic components with the use of XPS Peak4.1 software [34]. The values of Pd 3d binding energy (BE) for metal-

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Fig. 7. XPS Pd 3d5/2–3/2 , C 1s and O 1s spectra recorded from the Pd/Elat sample after synthesis (Ref) and just after a fuel cell test (RefFC ) fitted to the asymmetric Gaussian–Lorentzian functions after Tougaard background subtraction.

Table 2 Atomic content of Pd chemical state resulting from the fitting of the XPS Pd3d5/2–3/2 spectra and binding energy (BE) values. Sample

Chemical state of Pd – Pd 3d5/2 Pdmet (at.%)BE = 335.0 eV

PdCx /C O (at.%)BE = 335.65 ± 0.05 eV

PdO (at.%)BE = 336.3 eV

PdO2 (at.%)BE = 338.0 eV

P1 P2 P3 Ref RefFC

2.99 17.65 24.76 5.77 4.74

21.85 19.38 12.54 12.75 10.37

0.25 1.68 11.29 2.40 0.63

2.71 3.09 5.61 1.98 0.86

Table 3 Atomic content of C chemical states resulting from the fitting of the XPS C 1s spectra and binding energy (BE) values. Sample

P1 P2 P3 Ref RefFC

Chemical state of C – C 1s PdCx (at.%) BE = 283.8 eV

C sp2 (at.%) BE = 284.4 eV

C sp3 (at.%) BE = 285.3 eV

C–OH (at.%) BE = 286.3 eV

C O (at.%) BE = 287.4 eV

COOH (at.%) BE = 288.6 eV

2.65 1.92 1.83 1.82 0.27

35.71 27.93 12.21 41.70 21.75

6.88 3.32 2.66 10.13 13.95

3.68 1.70 0.39 3.95 10.88

2.55 3.03 2.43 2.38 4.01

2.63 1.40 1.28 1.52 5.44

Table 4 Atomic content of O chemical states resulting from the fitting of the XPS O 1s spectra and binding energy (BE) values. Sample

P1 P2 P3 Ref RefFC

Chemical state of O – O 1s PdO (at%) BE = 529.1 eV

PdO2 (at.%) BE = 529.8 eV

C O (at.%) BE = 531.05 ± 0.05 eV

C–OH (at.%) BE = 532.9 eV

COOH (at.%) BE = 531.9 eV, 534.2 eV

H2 O (at.%) BE = 535.2 eV, 538.2 ± 0.2 eV

0.25 1.73 9.14 1.99 0.59

2.70 3.21 4.55 1.64 0.81

2.54 3.11 1.92 1.97 3.78

3.65 1.75 0.31 3.27 10.25

2.46; 2.46 1.44; 1.44 1.01; 1.01 1.25; 1.25 5.13; 5.13

0.06–3.98 0–6.22 0.75–6.31 1.14–3.09 0–1.41

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PdCx

Pd 3d5/2-3/2

25

Pd chemical state [at%]

lic Pd [35,36], PdCx [36,37], Pd–C O [38], PdO and PdO2 [39], C 1s for oxygen groups [40] and PdCx [36,37], O 1s for oxygen groups [41], water at 535.2 eV and 538.2 ± 0.2 eV [42–44] Pd 3p for metallic Pd and Pd oxides [45] overlapping with O 1s were considered from the literature data. The Pd 3d, C 1s and O 1s spectra recorded from the P1, P2 and P3 catalyst layers are shown in Fig. 6, while the respective spectra from the samples after synthesis (Ref) and after a fuel test only (RefFC ) are shown in Fig. 7. The atomic concentrations of Pd chemical states (metallic Pd, PdCx and/or Pd–C O, PdO and PdO2 ), carbon chemical state and oxygen groups resulting from the fitting of C 1s and O 1s spectra, including the respective BE of photoelectrons taking part in the transition, are listed in Table 2 (Pd 3d), Table 3 (C 1s) and Table 4 (O 1s). The Pd 3d spectra recorded for three Pd catalysts indicated metallic Pd, a large amount of PdCx phase separated by 0.6–0.7 eV from metallic Pd [36,37] and/or C O adsorbed [38], PdO at BE of 336.3 eV and PdO2 at 338.0 eV [39]. Since O 1s including Pd 3p3/2 spectra did not indicate a large amount of C O [38], the predominant contribution of Pd 3d5/2 spectra at 335.6–335.7 eV (Table 2, Fig. 6) resulted from the PdCx phase. This phase was partly formed during the magnetron sputtering reaction, due to the presence of carbon cloth support and/or CO residuals in the vacuum chamber. The amount of Pd oxides resulting from Pd 3d spectra fitting (Table 2) was in agreement with the results from O 1s spectrum fitting (Table 4), as was the amount of oxygen groups resulting from C 1s spectrum fitting (Table 3) and O 1s spectrum fitting (Table 4). Samples P1, P2 and P3 after aging and fuel cell tests differed in the amount of PdCx , Pd oxides and oxygen groups (Tables 2–4, Fig. 7). The greatest amount of PdCx observed in sample P1 resulted from the surface and the interface of the Pd/carbon cloth. The information depth (ID) of a specified signal percentage (P), e.g. P = 99%, obtained from XPS may be evaluated assuming a straight line approximation (SLA) model, neglecting electron elastic scattering according to the formula [46]: ID(P) =  cos ˛out ln(1/(1 − P/100)), where  is the inelastic mean free path (IMFP) of the electrons dependent on the material and the kinetic energy (KE) of the investigated photoelectron [47]. In Pd the value of ID (P = 99%) is 6.8 nm. For sample P1 3 nm in layer thickness the interface of the Pd/carbon cloth is therefore also accounted for in a measured signal. Since the composition of PdCx evaluated from a comparison of its content resulting from fitting the Pd 3d (Table 2) and C 1s spectra (Table 3) was similar for all Pd layers 3 nm, 100 nm and 600 nm in thickness, i.e. x = 9.9–14.6 at.%. The residual contaminations of CO also participate in the formation of the PdCx phase. With increasing thickness of the Pd layers the following results were observed: (i) a decreasing content of PdCx phase resulting from attenuation of the signal from the Pd/carbon cloth interface (Fig. 8a), (ii) a decreasing content of C–OH, COOH and C O groups (Fig. 8b) accompanied by an increasing content of Pd oxides (PdO > PdO2 ) (Fig. 8a), indicating the highest surface reactivity for oxidation in the aged sample P3. The sample that was investigated just after a fuel cell measurement (RefFC ) indicated a decreasing amount of Pd oxides and composition of the PdCx phase (x = 2.6 at.%) accompanied by an increasing number of oxygen groups (C–OH > COOH > C O) (Fig. 8b). This resulted from the reduction of Pd oxides to metallic Pd by FA, formation of PdCx Hy phase, confirmed elsewhere [48], due to the presence of hydrogen provided in the electro-oxidation reaction in FA. This could finally lead to a decrease in the composition of PdCx phase and the attachment of other oxygen-hydrogen species to the surface of the catalyst. Water at the surface shows BE at 535.2 eV and 538.2 ± 0.2 eV, what can be attributed to adsorbed water and water in a liquid and gas phase [35,42–44]. Aged samples (P2, P3) indicate more substantial amount of water at the surface in contrary to samples after synthesis (P1, Ref) and fuel cell test (RefFC ) (Table 4). The most important factors influencing the decreasing catalytic efficiency were: the surface morphology related to the surface

a)

PdO PdO2

20 15 10 5 0 0

1

P1

2

P2

C 1s

12

C chemical state [at%]

844

3

P3

4

5

Ref RefFC

6

b)

PdCx C-OH C=O COOH

10 8 6 4 2 0 0

1

P1

2

P2

3

P3

4

Ref

5

RefFC

6

Fig. 8. Comparison of amount (at.%) of: (a) Pd chemical states resulting from the fitting of Pd 3d5/2–3/2 spectra and (b) oxygen groups resulting from the fitting of C 1s spectra in the investigated samples.

active area, the PdCx content and the contaminations (that can appear during anode preparation and application procedures and may also change the electronic properties of the metal) [49,50]. The interstitial PdCx phase, which was formed on the surface of Pd ® layers from the residual gases in a chamber and at the Pd/Elat interface, may significantly alter the bulk and surface structure of Pd during the magnetron sputtering procedure. In addition, it can lead to catalytic deactivation in reactions involving hydrocarbons [51–53]. The highest catalytic activity obtained for sample P3 (Fig. 1) was related to the highest active surface area (Figs. 3 and 4) and the lowest PdCx content (Tables 2 and 3, Fig. 8). Despite, the high PdCx content and composition, the results for electric power recalculated for Pd loading per 1 mg for sample P1 (Fig. 2) in comparison with the Pd nanoparticles decorated carbon nanomaterial support, i.e. functional carbon nanotubes [50], indicated catalytic activity lower by only about 30%. This provided evidence that magnetron sputtering layers may be an option for a fuel cell catalyst in a formic acid electro-oxidation reaction. In the further development of new magnetron sputtering anodes for formic acid electro-oxidation, emphasis should be laid on increasing the surface area, and on decreasing the PdCx content, composition and contaminations. 4. Conclusions Anode catalysts with different Pd loadings (3.6 ␮g, 120 ␮g, and 720 ␮g/cm2 ) were prepared using a non-reactive magnetron sput® tering method on an Elat carbon cloth without support for further electro-oxidation of formic acid in DFAFC. Structural and chemical characterization of the catalysts by SEM, AFM and XPS analysis showed that the catalytic efficiency depended strongly on the sur-

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face morphology, active area surface, the amount of PdCx phase and oxygen (hydroxyl –OH, carbonyl C O and carboxyl COOH) groups. For 1 M of FA, poisoning due to the formation of PdCx during anode material preparation may be the issue that needs to be solved or limited in order to increase the working ability of the catalyst. It should also be pointed out that the occurrence of the PdCx phase can lead to a reduction in the DFAFC power working results. An electrochemical examination also showed that a catalyst loading of 720 ␮g produced slightly higher electrocatalytic activity than catalyst loadings of 3.6 ␮g and 120 ␮g/cm2 . However, for extrapolated results recalculated to 1 mg of Pd, the power density of the 3.6 ␮g sample was close to 40 mW. In general, our study has shown that further research work should be focused on increasing the surface area of the support/carbon layer and finding ways of decreasing the amount of PdCx phase generation at the anode. Acknowledgments Support for this research project was provided by the Grant Agency of Charles University under Project No. 259595; by the Grant Agency of the Czech Republic, under Project No. P108/12/G108; and by the Ministry of Education Youth and Sports of the Czech Republic, Project No. LM2015088. In addition, I.B. thanks J. Houdkova for her consultations. References [1] C.-H. Chem, W.-J. Liou, H.-M. Lin, S.-H. Wu, A. Mikolajczuk, L. Stobinski, A. Borodzinski, P. Kedzierzawski, K. Kurzydlowski, Carbon nanotube-supported bimetallic palladium-gold electrocatalysts for electro-oxidation of formic acid, Phys. Status Solidi A 5 (2010) 1160. [2] S. Ha, R. Larsen, Y. Zhu, R.I. Masel, Direct formic acid fuel cells with 600 mA cm−2 at 0.4 V and 22 degrees C, Fuel Cells 4 (2004) 337. [3] C.M. Miesse, W.-S. Jung, K.-J. Jeong, J.K. Lee, J.-Y. Lee, J.-H. Han, S.-P. Yoon, S.-W. Nam, T.-H. Lim, S.-A. Hong, Direct formic acid fuel cell portable power system for the operation of a laptop computer, J. Power Sources 162 (2006) 532. [4] J. Zhang, M. Sun, X. Liu, Y. Han, Catalytic oxidative conversion of cellulosic biomass to formic acid and acetic acid with exceptionally high yields, Catal. Today 233 (2014) 77. [5] J.F. Hull, Y. Himeda, W.-H. Wang, B. Hashiguchi, R. Periana, D.J. Szalda, J.T. Muckerman, E. Fujita, Reversible hydrogen storage using CO2 and a proton switchable iridium catalyst in aqueous media under mild temperatures and pressures, Nature Chem. 4 (2012) 383. [6] D.A. Bulushev, L.G. Bulusheva, S. Beloshapkin, T. O’Connor, A.V. Okotrub, K.M. Ryan, Pd clusters supported on amorphous, low-porosity carbon spheres for hydrogen production from formic acid, Appl. Mater. Interfaces 7 (16) (2015) 8719. [7] S. Ha, Z. Dumbar, R.I. Masel, Characterization of a high performing passive direct formic acid fuel cell, J. Power Sources 158 (2006) 129. [8] C. Hu, S.W. Ting, J. Tsui, K.Y. Chan, Formic acid dehydrogenation over PtRuBiOX/C catalyst for generation of CO-free hydrogen in a continuous-flow reactor, Int. J. Hydrogen Energy 37 (2012) 6372. [9] T.C. Johnson, D.J. Morris, M. Wills, Hydrogen generation from formic acid and alcohols using homogeneous catalysts, Chem. Soc. Rev. 39 (2010) 81. [10] X. Yu, P.G. Pickup, Recent advances in direct formic acid fuel cells (DFAFC), J. Power Sources 182 (2008) 124. [11] S.A. Lewis, J.P. Wilburn, M.S. Wellons, D.E. Cliffel, C.M. Lukehart, Carbon-supported AuPt and AuPd bimetallic nanocomposites as formic acid electrooxidation catalyst, Phys. Status Solidi A 212 (12) (2015) 2903. [12] W.L. Law, A.M. Platt, P.D.C. Wimalaratne, S.L. Blair, Effect of organic impurities on the performance of direct formic acid fuel cells, J. Electrochem. Soc. 156 (5) (2009) B553. [13] H. Miyake, T. Okada, G. Samjeske, M. Osawa, Formic acid electrooxidation on Pd in acidic solutions studied by surface enhanced infrared absorption spectroscopy, Phys. Chem. 12 (2008) 3662. [14] A. Mikolajczuk, A. Borodzinski, P. Kedzierzawski, L. Stobinski, B. Mierzwa, R. Dziura, Deactivation of carbon supported palladium catalyst in direct formic acid fuel cell, Appl. Surf. Sci. 257 (19) (2011) 8211. [15] X. Yu, P.G. Pickup, Mechanistic study of the deactivation of carbon supported Pd during formic acid oxidation, Electrochem. Commun. 11 (2009) 2012. [16] X. Yu, P.G. Pickup, Deactivation/reactivation of a Pd/C catalyst in a direct formic acid fuel cell (DFAFC): use of array membrane electrode assemblies, J. Power Sources 187 (2009) 493. [17] L. Feng, J. Chang, K. Jiang, H. Xue, C. Liu, W.-B. Cai, W. Xing, J. Zhang, Nanostructured palladium catalyst poisoning depressed by cobalt phosphide in the electro-oxidation of formic acid for fuel cells, Nano Energy 30 (2016) 355.

845

[18] C.H. Chen, W.J. Liou, H.M. Lin, S.H. Wu, A. Borodzinski, L. Stobinski, P. Kedzierzawski, Palladium and palladium gold catalysts supported on MWCNTs for electrooxidation of formic acid, Fuel Cells 10 (2) (2010) 227. [19] A. Malolepszy, M. Mazurkiewicz, L. Stobinski, B. Lesiak, L. Kövér, J. Tóth, B. Mierzwa, A. Borodzinski, F. Nitze, T. Wågberg, Deactivation resistant Pd–ZrO2 supported on multiwall carbon nanotubes catalyst for direct formic acid fuel cells, Int. J. Hydrogen Energy 40 (46) (2015) 16724. [20] N.-C. Cheng, R.A. Webster, M. Pan, S.-C. Mu, L. Rassaei, Sh-Ch. Tsang, F. Marken, One-step growth of 3–5 nm diameter palladium electrocatalyst in a carbon nanoparticle–chitosan host and characterization for formic acid oxidation, Electrochim. Acta 55 (2010) 6601. [21] S.M. Baika, J.-H. Han, J.-S. Kim, Y-Ch. Kwon, Effect of deactivation and reactivation of palladium anode catalyst on performance of direct formic acid fuel cell (DFAFC), Int. J. Hydrogen Energy 36 (22) (2011) 14719. [22] S. Shukla, K. Domican, K. Karan, S. Bhattacharjee, M. Secanell, Analysis of low platinum loading thin polymer electrolyte fuel cell electrodes prepared by inkjet printing, Electrochim. Acta 156 (2015) 289. [23] M.E. Tague, J.M. Gregoire, A. Legard, E. Smith, D. Dale, R. Hennig, F.J. DiSalvo, ˜ High throughput thin film Pt–M alloys for Fuel R.B. van Dover, H.D. Abruna, electrooxidation: low concentrations of M (M = Sn, Ta, W, Mo, Ru, Fe, In, Pd, Hf, Zn, Zr, Nb, Sc, Ni, Ti, V, Cr, Rh), J. Electrochem. Soc. 159 (2012) F880. [24] M.S. Saha, A.F. Gullá, R.J. Allen, S. Mukerjee, High performance polymer electrolyte fuel cells with ultra-low Pt loading electrodes prepared by dual ion-beam assisted deposition, Electrochim. Acta 51 (2006) 4680. [25] A.M. Mohammada, G.A. El-Nagara, I.M. Al-Akraab, M.S. El-Deaba, B.E. El-Anadoulia, Towards improving the catalytic activity and stability of platinum-based anodes in direct formic acid fuel cells, Int. J. Hydrogen Energy 40 (24) (2015) 7808. [26] H. Yua, J.M. Rollerb, W.E. Mustaina, R. Marica, Influence of the ionomer/carbon ratio for low-Pt loading catalyst layer prepared by reactive spray deposition technology, J. Power Sources 283 (2015) 84. [27] S. Cuynet, A. Caillard, S. Kaya-Boussougou, T. Lecas, N. Semmar, J. Bigarre, P. Buvat, P. Brault, Membrane patterned by pulsed laser micromachining for proton exchange membrane fuel cell with sputtered ultra-low catalyst loadings, J. Power Sources 298 (2015) 299. [28] D.P. Wilkinson, J. Zhang, R. Hui, J. Fergus, X. Li, Proton Exchange Membrane Fuel Cells: Materials Properties and Performance, CRC Press, 2009. [29] R. Fiala, M. Vaclavu, A. Rednyk, I. Khalakhan, M. Vorokhta, J. Lavkova, V. Potin, I. Matolinova, V. Matolin, Pt–CeOx thin film catalysts for PEMFC, Catal. Today 240 (2015) 236. [30] B. Rezek, E. Ukraintsev, M. Krátká, A. Taylor, F. Fendrych, V. Mandys, Epithelial cells morphology and adhesion on diamonds films deposited and chemically modified by plasma processes, Biointerphases 9 (2014) 031012. [31] S. Touggard, Background Analysis of XPS/AES QUASES Simple Backgrounds, Ver. 2.2, Tougaard Inc, http://www.quases.com/ (accessed 12.15.01). [32] M. Mohai, Multimodel X-ray photoelectron spectroscopy quantification program for 32-bit Windows, XPS Multiquant, ver. 1999–2001. [33] M. Mohai, XPS MultQuant: multimodel XPS quantification software, Surf. Interface Anal. 36 (2004) 828. [34] R.M. Kwok, XPS Peak Fitting Program XPSPEAK, ver.4.1, Department of Chemistry, The Chinese University of Hong Kong, [email protected], http://www.uksaf.org/software.html/. [35] C.D. Wagner, A.V. Naumkin, A.A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R. Rumble Jr., NIST Photoelectron Database, NIST SRD 20, ver. 4.1., online, PC. [36] A.V. Matveev, V.V. Kaichev, A.S. Saraev, V.V. Gorodetskii, A. Knop-Gericke, V.I. Bukhiyarov, B.E. Nieuwenhuys, Oxidation of propylene over Pd(551): temperature hysteresis induced by carbon deposition and oxygen adsorption, Catal. Today 244 (2015) 29. [37] O. Balmes, A. Resta, D. Wermeille, R. Felici, M.E. Messing, K. Deppert, Z. Liu, M.E. Grass, H. Bluhm, R. van Rijn, J.W.M. Frenken, R. Westerström, S. Blomberg, J. Gustafson, J.N. Andersen, E. Lundgren, Reversible formation of PdCx phase and Pd nanoparticles upon CO and O2 exposure, Phys. Chem. Chem. Phys. 14 (2012) 4796. [38] H. Kondoh, R. Toyoshima, Y. Monya, M. Yoshida, K. Mase, K. Amemiya, B.S. Mun, In situ analysis of catalytically active Pd surfaces for CO oxidation with near ambient pressure XPS, Catal. Today 260 (2016) 14. [39] L.S. Kibis, A.I. Titkov, A.I. Stadnichenko, S.V. Koscheev, A.I. Boronin, X-ray photoelectron spectroscopy of Pd oxidation by RF discharge in oxygen, Appl. Surf. Sci. 255 (2009) 9248. [40] Y.V. Butenko, S. Krishnamurthy, A.K. Chakraborty, V.L. Kuznetsov, V.R. ˇ Photoemission study of onionlike carbons Dhanak, M.C. Hunt, L. Siller, produced by annealing nanodiamonds, Phys. Rev. B71 (2005) 075420. [41] B. Lesiak, L. Stobinski, A. Malolepszy, M. Mazurkiewicz, L. Kövér, J. Tóth, Preparation of graphene oxide and characterisation using electron spectroscopy, J. Electron Spectrosc. Rel. Phenom. 193 (2014) 92. [42] B. Lesiak, P. Jiricek, I. Bieloshapka, Chemical and structural properties of Pd nanoparticle-decorated graphene – electron spectroscopic methods and QUASES, Appl. Surf. Sci. 404 (2017) 300. [43] S. Yamamoto, H. Bluhm, K. Andersson, K. Kettler, H. Ogasawara, M. Salmeron, A. Nilsen, In situ x-ray photoelectron spectroscopy of water on metals and oxides at ambient conditions, J. Phys.: Condens. Mater. 20 (2008) 184025. [44] B. Winter, E.F. Aziz, U. Hergenhahn, M. Faubel, I.V. Hertel, Hydrogen bonds in liquid water studied by electron spectroscopy, J. Chem. Phys. 126 (2007) 124503.

846

I. Bieloshapka et al. / Applied Surface Science 419 (2017) 838–846

[45] R. Toyoshima, M. Yoshida, Y. Monya, K. Suzuki, H. Abe, K. Mase, K. Amemiya, H. Kondoh, In situ ambient pressure XPS study of CO oxidation reaction on Pd(111) surfaces, J. Phys. Chem. C 166 (2012) 18692. [46] A. Jablonski, C.J. Powell, Practical expressions for the mean escape depth, the information depth and the effective attenuation length in Auger-electron spectroscopy and X-ray photoelectron spectroscopy, J. Vac. Sci. Technol. A 27 (2009) 253. [47] S. Tanuma, C.J. Powell, D.R. Penn, Calculations of electron inelastic mean free paths. IX. Data for 41 elemental solids over the 50 eV to 30 keV range, Surf. Interface Anal. 43 (2011) 689. [48] W. Vogel, Interaction of a nanosized Pd catalyst with active C from the carbon support: an advanced in situ XRD study, J. Phys. Chem. C 115 (2011) 1506. [49] W.P. Zhou, A. Lewera, R. Larsen, R.I. Masel, P.S. Bagus, A. Wieckowski, Size effects in electronic and catalytic properties of unsupported palladium nanoparticles in electrooxidation of formic acid, J. Phys. Chem. B 110 (2006) 13393.

[50] M. Mazurkiewicz, A. Malolepszy, A. Mikolajczuk, L. Stobinski, A. Borodzinski, B. Lesiak, J. Zemek, P. Jiricek, Pd/MWCNTs catalytic activity in the formic acid electrooxidation dependent on catalyst surface treatment, Phys. Status Solidi 248 (11) (2011) 2516. [51] M. Bonarowska, J. Pielaszek, V.A. Semikolenov, Z. Karpinski, Pd-Au/sibunit carbon catalysts: characterization and catalytic activity in chydrodechlorination dichlorodifluoromethane (CFC-12), J. Catal. 209 (2002) 528. [52] M. Bonarowska, A. Malinowski, W. Juszczyk, Z. Karpinski, Hydrodechlorination of CCl2 F2 (CFC-12) over silica-supported palladium–gold catalysts, Appl. Catal. B: Environ. 30 (1–2) (2001) 187. [53] A. Sarkany, Hydrocarbonaceous deposit assisted n-butane formation in hydrogenation of 1,3-butadiene over Pd catalysts, Appl. Catal. A: Gen. 175 (1998) 245.