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Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance
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Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance Elif Das‚ a, Selmiye Alkan Gu¨rsel b,c, Lale Is‚ıkel S‚anlı b, Ays‚e Bayrakc¸eken Yurtcan a,d,* a
Department of Nanoscience and Nanoengineering, Atatu¨rk University, Erzurum, 25240, Turkey Sabanci University Nanotechnology Research & Application Center (SUNUM), Istanbul, 34956, Turkey c Faculty of Engineering &Natural Sciences, Sabanci University, Istanbul, 34956, Turkey d Department of Chemical Engineering, Atatu¨rk University, Erzurum, 25240, Turkey b
article info
abstract
Article history:
Graphene is considered as a promising catalyst support for polymer electrolyte membrane
Received 14 February 2017
(PEM) fuel cells because of its excellent properties. Recent studies showed that electro-
Received in revised form
catalytic activity, fuel cell performance, catalyst utilization efficiency and long term
1 June 2017
durability can be improved by using graphene as the catalyst support. In here, we report
Accepted 12 June 2017
the synthesis of graphene nanoplatelets (GNPs) supported platinum catalysts using su-
Available online xxx
percritical carbon dioxide (scCO2) deposition technique. The prepared catalysts were characterized in terms of their structure, morphology and thermal behavior by using X-ray
Keywords:
diffraction (XRD) analysis, transmission electron microscopy (TEM), scanning electron
PEM fuel cell
microscopy (SEM), and thermogravimetric analysis (TGA). The electrocatalytic and half-cell
Platinum
performances of the as-prepared catalysts were investigated by cyclic voltammetry (CV)
Graphene nanoplatelets
and PEM fuel cell performance testing. Small size and well distribution of Pt nanoparticles
Supercritical carbon dioxide
on GNPs surfaces were achieved by using scCO2 deposition which improves the PEM fuel
deposition
cell performance. To the best of our knowledge decoration of Pt nanoparticles on GNPs via scCO2 deposition and their detailed fuel cell performances were presented for the first time in the literature. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the twenty-first century, special attention gained by fuel cells for being clean sources of energy [1,2]. Polymer Electrolyte Membrane (PEM) fuel cells gain more attention than other types of fuel cells because of their high efficiency, high power density, low cost, low weight and low working temperature
[3,4]. In spite of remarkable progress in PEM fuel cells, both durability and stability of catalyst still need to be enhanced [5,6]. Up to now, there are various studies on development of alternative catalyst support materials for PEM fuel cells. In this regards, various carbon materials, including activated carbon, mesoporous carbon, carbon nanotubes, carbon fiber or carbon aerogel have been utilized as catalyst support materials due to
* Corresponding author. Department of Chemical Engineering, Atatu¨rk University, Erzurum, 25240, Turkey. E-mail address: [email protected] (A. Bayrakc¸eken Yurtcan). http://dx.doi.org/10.1016/j.ijhydene.2017.06.108 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 1 e Adsorption isotherm of CODPtMe2 on GNPs in scCO2 (solid line represents Langmuir fit).
Fig. 2 e XRD patterns of plain GNPs and Pt/GNPs electrocatalysts.
their high surface area, low cost, high ORR activity and durability [7e10]. In addition to those carbon materials, graphene can serve as a promising catalyst support owing to its high surface area, excellent electrical conductivity, high surface area and strong affinity to metal particles [11,12]. Platinum (Pt)-based catalysts are significant in practical applications of PEM fuel cells because of their high efficiency stability and other factors affecting the catalytic activity of PEM fuel cells electrocatalyst, such as catalyst preparation conditions, the composition of the electrocatalyst and the metal loading over the support [13,14]. In the literature, various catalyst preparation methods, such as chemical reduction, impregnation, sputter deposition, electrochemical method and so on are used [15e20]. It is very critical to choose a proper method in order to achieve the targeted catalyst properties and also higher catalytic activities [21]. Supercritical carbon dioxide deposition (scCO2) is also an alternative method for preparation of nanoparticles [13,22e24]. There has been a growing interest in exploiting advantages of scCO2 as a “green” solvent with peculiar wetting abilities in the catalyst preparation for fuel cells applications [23]. ScCO2 environment provides an excellent medium to decorate nanoparticles onto the porous support materials. Moreover, scCO2 offers several advantages compared to conventional techniques. The high diffusivity, low viscosity and surface tension properties of scCO2 allows it and its mixtures to penetrate into the nanostructures easily [24,25]. Furthermore, the support structure is preserved upon the CO2 treatment. There are various studies about the deposition of Pt nanoparticles on carbon black,
carbon nanotubes and carbon aerogels via scCO2 [26e28]. However, there are only three studies in literature about the preparation of graphene supported Pt nanoparticles as the catalyst support for fuel cell reactions by scCO2. In this regard, Zhao et al. showed enhanced stability and electrocatalytic activity of Pt nanoparticles on GO for methanol oxidation [24]. In their recent study, Zhao et al. compared Pt/ graphene with Pt/carbon black and Pt/carbon nanotube prepared by sCO2 deposition and reported better stability of Pt/graphene compared to other two supports [25]. In both studies authors employed GO prepared by Hummers method [29] and expanded thermally. In another recent study Pt nanoparticles on functionalized graphene were synthesized by scCO2 [30]. In all these studies about graphene (GO or functionalized GO) supported Pt, authors investigated exsitu electrocatalytic activity of catalysts yet fuel cell performances of these catalysts were not shown. In this study, different Pt loadings over graphene nanoplatelets (GNPs) support were succesfully achieved with thermodynamic control via one-pot scCO2 method using dimethyl (1,5-cyclooctadiene) platinum (II) (PtMe2COD) as organometallic Pt precursor. The physical, structural and electrochemical properties of the catalysts were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), cyclic voltammetry (CV) and fuel cell tests. To the best of our knowledge, decoration of Pt nanoparticles on GNPs and their detailed fuel cell performances were presented for the first time in the literature.
Table 1 e Langmuir equilibrium constants for CODPtMe2, scCO2 and GNPs compared with the literature. Catalyst Pt/GNPs Pt/BP2000 Pt/MWCNT Pt/Vulcan
Support surface area [m2/g]
K1
Qo
K1 Qo
R2
SPt [m2/g]
Reference
759 1450 300 232
0.4357 0.54 0.29 0.87
924.34 1481 335 180
403 801 100 156
0.99 0.99 0.99 0.98
329 525 119 64
In this study [32] [32] [28]
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Experimental Materials Dimethyl (cyclooctadiene) platinum (II) was purchased from STREM. Graphene nanoplatelets (GNPs) support material purchased from XG Sciences (xGnP® Grade C, surface area of 759 m2/g) with particle diameter of less than 2 mm and particle thickness of a few nanometers was used [31]. Carbon dioxide, nitrogen, oxygen and hydrogen gases having high purity (>99.98%) were purchased from Habas‚. Nafion solution (15%) (Ion Solutions Inc.), 2 propanol (Sigma), 1,2 propandiol (Sigma), H2SO4 (Sigma) were used as received. Nafion 212 membrane was used as the electrolyte. GDL 34 BC (Sigracet) was used as gas diffusion layer. All the chemicals were used as-received.
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adsorption of the precursor on GNPs. Metallic Pt nanoparticles over GNPs were achieved with the heat treatment of precursor adsorbed on GNPs at 400 C for 4 h in nitrogen atmosphere. To thermodynamically control the Pt loading on GNPs, the precursor amounts dissolved in the high pressure vessel were changed. Adsorption isotherm data were collected by taking into account the weight change of GNPs measured using an analytical balance before and after the adsorption of the precursor for each precursor amount inserted into the high pressure vessel. The adsorption data were collected until the saturation was reached. For all isotherms, temperature and the pressure were set to 308 K and 12 MPa. The catalysts were called as Pt/G1, Pt/G2, Pt/G3, Pt/G4 and Pt/G5 according to the Pt precursor amounts used in the experiments as in the order of 0.05, 0.1, 0.2, 0.3 and 0.4 g, respectively.
Catalyst characterization
Catalyst preparation In this study, Pt nanoparticles on GNPs were achieved by using scCO2 deposition method. 1,5-dimethyl platinum cyclooctadiene (CODPtMe2) (Strem) was used as the Pt organometallic precursor. A custom-made 54 ml stainless steel vessel equipped was used with two sapphire windows having 2.5 cm diameter as the deposition apparatus. For each catalyst, different Pt precursor amounts were placed in the vessel along with a stirring bar, and 0.1 g of GNPs was placed in a pouch made of filter paper. A stainless steel screen was positioned in the middle of the vessel in order to separate the pouch and the stirring bar. The catalyst preparation method includes dissolution of the Pt precursor in scCO2 (at 308 K and 12 MPa) and
Characterization of physical properties X-ray diffraction technique XRD (Rigaku Miniflex X-ray diffractometer) was employed for the investigation of the crystalline structure of Pt nanoparticles, The Rigaku Miniflex diffractometer with a CuKa (l ¼ 1,5406 A) radiation source was utilized for the these analyses. The scanning range of 2q was set in between 10 to 90 . FEI Inspect S50 model scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) was conducted to investigate the morphology and the chemical composition of GNPs. The structural changes, particle size and distribution of the Pt nanoparticles on GNPs were characterized by JEOL 2100 JEM high resolution transmission electron microscopy (HRTEM). The Pt loadings of the catalysts over GNPs were
Fig. 3 e SEM images and EDX spectra of plain GNPs. Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 4 e TEM images and particle size distribution patterns of catalysts prepared by different amount precursor (a) plain GNPs (b) Pt/G1 (c) Pt/G2 (d) Pt/G3 (e) Pt/G4 (f) Pt/G5. Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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analyzed by a Netzsch thermal thermogravimetric analyzer (TGA) from room temperature to 1000 C at a heating rate of 10 ºC/min under air atmosphere.
Electrochemical measurements The electrochemical characterization of all the prepared catalysts were performed at 25 C in a three-electrode cell connected to a rotating disk electrode system (Pine Instrument). A glassy carbon (GC) disk with an area of 0.1963 cm2 was used as working electrode. Ag/AgCl and a platinum wire served as reference and counter electrodes, respectively. Electrodes for each catalyst was prepared by using the same procedure. Briefly, a suspension was prepared by mixing the required amounts of the prepared catalysts with deionized water, 1,2propandiol, and 15% Nafion® solution. Then, the resulting suspension was ultrasonicated for a while and then, 5 mL of this suspension was dropped on 5 mm diameter GC electrode (the metal loading per unit area was set to 22 mg Pt cm2) and dried in air overnight. Cyclic voltammograms for the hydrogen oxidation reaction (HOR) were recorded in 1 M H2SO4 electrolyte solution that was saturated with nitrogen for 30 min to remove oxygen. The stability of the catalysts was investigated by runs up to 50 voltammetric cycles at a scan rate of 50 mV s1 between 0 and 1.2 V. Cyclic voltammograms for oxygen reduction reaction (ORR) were recorded in 1 M H2SO4 electrolyte solution that was saturated with oxygen by purging the electrolyte with oxygen for half an hour and then rotating the disk electrode in between 100 and 2500 rpm at a scan rate of 5 mV s1. An accelerated degradation test, carbon corrosion test, was applied to the fresh electrodes. Then, carbon corrosion of the prepared catalysts was investigated by subjecting them to a constant potential of 1.2 V for 24 h. The effect of carbon corrosion test on the activities of the prepared catalysts were evaluated by taking into account the change in the cyclic voltammograms before and after carbon corrosion test. In order to characterize the electrochemical activities of the prepared catalysts in situ PEM fuel cell performance tests were performed. For the cell performance test, the membrane electrode assemblies (MEAs) were prepared by spraying catalyst ink (including definite amounts of catalyst, 2 propanol and Nafion solution) onto GDLs. The platinum loading over the electrode was set to 0.3 mg Pt cm2. Then, a five-layer MEA was prepared by pressing these GDLs onto the Nafion® 212 membrane at 130 C, 400 psi for 3 min in order to create a good interfacial contact between the GDL and the catalyst layer. The geometric area of the electrode was 4.41 cm2. A commercially available PEM fuel cell hardware (Electrochem) was used for the experiments. During the experiments, the temperature of the single cell was maintained at 70 C, and fully humidified hydrogen/oxygen gases were fed into the anode/cathode at 70 C at a flow rate of 250 ml/min.
Table 2 e Pt loadings and particle sizes of Pt/GNPs catalysts according to the Pt precursor amount. Pt precursor amount [g] 0.05 0.10 0.20 0.30 0.40
Catalyst
Pt [wt.%]
Particle size [nm]
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
12.2 17.8 22.8 22.1 25.7
1.5 1.5 1.7 1.9 2.0
higher uptakes and so higher metal loadings support properties and accessibilities of the pores are very critical. Adsorption isotherm was obtained at 308 K and 12 MPa. The adsorption isotherm for CODPtMe2, scCO2 and GNPs is given in Fig. 1. The uptake of the precursor increased with an increase in the amount of the precursor dissolved up to some extent. The adsorption isotherm showed the partitioning of the Pt precursor between the GNPs and scCO2 phases. The adsorption isotherm was fitted to Langmuir model with the formula given below: q¼
K 1 Q0 c 1 þ K1 c
(1)
where K1 (g scCO2 mg1precursor) is the Langmuir adsorption constant, Qo (mgprecursor g1support) is the adsorption capacity, c (mgprecursor g1scCO2) is the solution concentration and q (mgprecursor g1support) is the uptake by the adsorbent. The Langmuir equilibrium constants obtained for this system is calculated with regression analysis and given in Table 1. Qo represents the maximum number of the available sorption sites that GNPs can supply for the Pt precursor molecules in case of the Langmuir isotherm which is increased with an increase in surface area [32]. K1 Qo value shows the relative affinity of the solute towards the surface of the adsorbent. Higher K1Qo values indicate that the CODPtMe2 has more affinity for GNPs than scCO2. The structure of the pores of the adsorbents and the molecular size of the adsorbate have significant effects on accessibility of the sorption sites. If it is assumed that the CODPtMe2 precursor molecules are spherical and in 0.5 nm
Results and discussion Support-precursor interactions and adsorption conditions are significantly affect the adsorption process. Adsorption of the Pt precursor over the porous support material will lead the thermodynamically control of the Pt loading. In order to get
Fig. 5 e TGA curves of synthesized Pt/GNPs catalysts.
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 6 e The CVs were measured in N2-purged 1 M H2SO4 electrolyte solution at 25 C. diameter, the area covered by the precursor is given as follows: S ¼ Q0 Apr2
(2)
where S is the area covered by precursor, Qo is the capacity, A is the Avogadro's number, and r is the radius of Pt precursor
Table 3 e ECSA loss of the catalysts after carbon corrosion. ECSA [m2/g]
Catalyst
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
ECSA loss [%]
Before aging
After aging
43.6 87.2 55.6 56.3 71.1
28.1 51.4 35.7 32.9 44.3
35.4 41.1 35.8 41.6 37.7
[33]. The area covered by Pt organometallic precursor is calculated as 329 m2 g1 as given in Table 1. In Table 1, the areas covered by Pt organometallic precursor for different carbon supports were also summarized. The XRD data obtained from the Pt/GNPs catalysts are shown in Fig. 2. The characteristic diffraction peaks at 39.8 , 46.3 , 68.2 , and 81.6 , correspond to the (111), (200), (220), and (311) facets of the face-centered cubic structures of Pt crystal, respectively. The XRD pattern show a sharper and narrower peak at ~26.5 attributed to the (002) plane of hexagonal graphite structure, meaning the formation of graphene nanoplatelets. The presence of the C (004) diffraction peak at 53 in GNPs is also indicative of the high crystallinity of its carbon structure [34]. Fig. 3 shows the SEM images and corresponding EDS spectra of plain GNPs which has wrinkled and porous nature. This nature of GNPs not only provides a large surface area for
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 7 e Hydrodynamic voltammograms of Pt/G1 catalyst before (a) and after (b) carbon corrosion test (c) Tafel slope losses of the catalysts and GNP after carbon corrosion test at 100 rpm.
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 7 e (continued).
Table 4 e Number of electrons transferred per oxygen molecule for all catalysts. Catalysts
n
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
3.9 3.7 3.5 3.6 2.8
Pt nanoparticles but also prevent GNPs from aggregating [35]. EDS also used to get the information about the chemical composition of the structure. According to the EDS results of graphene, results confirm the presence of C and O elements in the structure. ScCO2 environment provides homogeneous distribution of nanoparticles. Utilization of nanosized catalyst particles create higher surface area and improve the efficiency of the
Table 5 e Tafel slope losses of the prepared catalysts. Catalysts
Tafel slope losses [V] 100 rpm 400 rpm 900 rpm 1600 rpm 2500 rpm
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
0.04 0.02 0.07 0.03 0.05
NA: Not applicable.
0.03 0.01 0.06 0.02 0.004
0.02 0.01 0.05 0 0
0.02 0.007 0.05 0 0
0.02 0 0.05 NA 0
catalysts. More catalytic sites can be achieved by nanoparticles and increase the catalytic activity of the catalysts. The TEM images of as-prepared Pt/GNPs catalysts are presented in Fig. 4. It can be clearly seen that the Pt nanoparticles are succesfully anchored on the surface of the GNPs and average Pt nanoparticle size was change in between 1.5 and 2 nm (Table 2). Moreover, TGA was carried out to identify the Pt content achieved on GNPs. The Pt loadings over GNPs were determined by taking into account the difference between plain GNPs and GNPs supported Pt catalysts. Plain GNPs were also treated in scCO2 environment before the TGA experiments because untreated GNPs showed a different trend. It was observed that Pt content on GNPs was changed between 12.2 and 25.7 wt% (Table 2) according to the precursor amount loaded to the high pressure vessel as shown in Fig. 5. Cyclic voltammetry (CV) is an efficient tool for determination of the electrochemical active surface areas (ECSA) of catalyst on an electrode. The ECSA of an electrocatalyst both provides the number of electrochemically active sites per gram of the catalyst, and also used to compare different electrocatalytic materials. CVs of Pt/GNPs were conducted to estimate ECSAs of Pt on GNPs. The resulting CV curves are presented in Fig. 6. CV of plain GNPs showed an oxidationreduction peaks in between 0.4 and 0.6 V which represent the carbon oxidation due to carbon corrosion via quinonehydroquinone reaction [36]. Lower carbon corrosion in the support material is favorable for long term durable PEM fuel cells. The CV indicates typical electro-adsorption and desorption of H2 on the Pt site, Pt oxidation and reduction
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 8 e PEM fuel cell performance curves for Pt/GNPs catalysts a) at different Pt loadings over GNPs b) at different Pt loadings over electrode active areas.
peaks. The ECSA and ECSA losses of the prepared Pt catalysts (Table 3) were calculated by taking the average of the areas under the curve under hydrogen adsorption/desorption peaks before and after carbon corrosion test. When we look at before corrosion values we can see that the Pt/G2 catalysts have much higher ECSA value (87 m2/g) than the other catalysts. This can be attributed to the smaller size of the uniform ultrafine Pt nanoparticles on the large surface of GNPs, which generates more available active Pt sites for catalytic activity compared with the others. Although, Pt/G1 catalyst was found to have the lowest ECSA loss similar trend was observed for all the catalysts.
Hydrodynamic voltammograms obtained from the oxygen purged H2SO4 electrolyte before and after carbon corrosion test are shown in Fig. 7 for Pt/G1 catalyst. The results for other catalysts are provided in supplementary material (Fig. S1). Koutecky-Levich plots were obtained by using the voltammogram data and also Eq. (3) given below by plotting i1 versus w1/2; 1 1 1 ¼ þ i Bw1=2 ik
(3)
where i, ik, w and B represents the current, kinetic current, rotation speed and Levich constant, respectively. B is obtained
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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from the Koutecky-Levich plots and by using it in the following Eq. (4) one can calculate the number of electrons transferred per oxygen molecule when w is in rad s1; B ¼ 0:62nFc0 D2=3 n1=6
(4)
where n represents the number of electrons per oxygen molecule, F is the Faraday constant (96485 C mol1), co shows the oxygen bulk concentration (1.02 106 mol cm3) [37], D shows the diffusion coefficient of oxygen (1.04 105 cm2 s1) [38] and ѵ shows the electrolyte viscosity (0.017 cm2 s1) [39]. Levich constant and Eq. (4) gives us the number of electrons transferred per oxygen molecule for all the catalysts as tabulated in Table 4. As seen from table that the number of electrons transferred per oxygen molecule for Pt/G1, Pt/G2, Pt/ G3 and Pt/G4 catalysts are all very close to 4 which depicts the catalyst follows 4e pathway which represents the complete water formation and only negligible H2O2 formation. Carbon corrosion test showed that HOR affected significantly by applying 1.2 V potential to the fresh electrode. Effect of carbon corrosion on ORR activity of the prepared catalysts were also investigated and Tafel slope losses of the prepared catalysts at 100 rpm rotation speed is given in Fig. 7(c). Tafel slope loss plots for other rotation speeds are given in Fig. S2 (supplementary material). The corresponding Tafel slope losses are tabulated in Table 5. Table 5 shows that the Tafel slope losses are getting less when the rotation speed is increased and applying 1.2 V harsh condition did not change the ORR activity of the GNPs supported catalysts significantly. Single PEM fuel cell performances of the MEAs assembled with the catalysts having different Pt loadings were taken in order to determine the effect of increase in Pt loadings over GNPs (Fig. 8(a)) and increase in Pt loadings over electrode active area (Fig. 8(b)) on PEM fuel cell performance. PEM fuel cell performances of the catalysts showed an increasing trend upto Pt/G3 catalyst and then started to decrease from this point. Although the Pt loadings over GNPs and also the particle sizes of the catalysts were so close to each other, the performances of the catalysts significantly different far from each other. Considering these results, Pt/G3 catalyst showed the best performance compared to other catalysts and further increase in the Pt loadings over GNPs may affect the dispersion and also the homogeneity of the nanoparticles which may induce a decrease in the PEM fuel cell performance. As can be seen from figure that the Pt loadings over electrode active area also significantly affect the performance and further increase will also develop a better performance.
Conclusions The MEA is the most important component where hydrogen and oxygen react electrochemically to generate electrical power in PEM fuel cells. In the MEA mostly Pt based electrocatalysts were used for reduction of oxygen at the cathode and the oxidation of hydrogen at the anode. Pt based electrocatalysts are required to be durable in the corrosive environment of the PEM fuel cell. The high quality electrocatalysts depend strongly on their synthesis technique. In here, we report a facile and effective approach for the synthesis Pt
nanoparticles on GNPs. The presence of scCO2 leads to highly dense and homogeneous deposition of Pt nanoparticles on GNPs surfaces. In total, 5 different catalysts were synthesized by the thermodynamic control of the Pt loadings via adsorption isotherm and denoted as Pt/G1, Pt/G2, Pt/G3, Pt/G4 and Pt/ G5 where the numbers of the samples implying the different amounts (in grams) of Pt used in the catalysts and the Pt/GNPs prepared by our approach seem to be very promising electrocatalysts for PEM fuel cells. Pt nanoparticles can be successfully decorated on GNPs via scCO2 deposition and the Pt loadings can be thermodynamically controlled. Superior PEM fuel cell performances can be achieved by controlling the properties of the catalysts.
Acknowledgements The research leading to these results has received funding from the Scientific and Technological Research Council of _ ¨ BITAK) Turkey (TU (Grant no. 114F506 and 114F029) and the European Union's Horizon 2020 research and innovation programme under grant agreement No 696656 (Graphene Flagship).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.06.108.
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[9] Cheng K, He D, Peng T, Lv H, Pan M, Mu S. Porous graphene supported Pt catalysts for proton exchange membrane fuel cells. Electrochim Acta 2014;132:356e63. [10] Liu R, Wu D, Feng X, Mullen K. Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction. Angew Chem 2010;49:2565e9. [11] Sahoo NG, Pan Y, Li L, Chan SH. Graphene-based materials for energy conversion. Adv Mater 2012;24:4203e10. [12] Quesnel E, Roux F, Emieux F, Faucherand P, Kymakis E, Volonakis G, et al. Graphene-based technologies for energy applications, challenges and perspectives. 2D Mater 2015;2:030204. [13] Bayrakc¸eken A, Smirnova A, Kitkamthorn U, Aindow M, _ et al. Pt-based electrocatalysts for polymer lu I, Tu¨rker L, Erog electrolyte membrane fuel cells prepared by supercritical deposition technique. J Power Sources 2008;179:532e40. [14] Zheng J, Fu R, Tian T, Wang X, Ma J. Effect of the microwave thermal treatment condition on PteFe/C alloy catalyst performance. Int J Hydrogen Energy 2012;37:12994e3000. [15] Qiu J-D, Wang G-C, Liang R-P, Xia X-H, Yu H-W. Controllable deposition of platinum nanoparticles on graphene as an electrocatalyst for direct methanol fuel cells. J Phys Chem C 2011;115:15639e45. [16] Li Y, Gao W, Ci L, Wang C, Ajayan PM. Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation. Carbon 2010;48:1124e30. [17] Liu S, Wang J, Zeng J, Ou J, Li Z, Liu X, et al. “Green” electrochemical synthesis of Pt/graphene sheet nanocomposite film and its electrocatalytic property. J Power Sources 2010;195:4628e33. [18] Shafiei M, Spizzirri PG, Arsat R, Yu J, du Plessis J, Dubin S, et al. Platinum/graphene nanosheet/SiC contacts and their application for hydrogen gas sensing. J Phys Chem C 2010;114:13796e801. [19] Sun S, Jaouen F, Dodelet J-P. Controlled growth of Pt nanowires on carbon nanospheres and their enhanced performance as electrocatalysts in PEM fuel cells. Adv Mater 2008;20:3900e4. [20] Si Y, Samulski ET. Exfoliated graphene separated by platinum nanoparticles. Chem Mater 2008;20:6792e7. [21] S‚anlı LI, Bayram V, Yarar B, Ghobadi S, Gu¨rsel SA. Development of graphene supported platinum nanoparticles for polymer electrolyte membrane fuel cells: effect of support type and impregnationereduction methods. Int J Hydrogen Energy 2016;41:3414e27. [22] Bayrakc¸eken A, Cangu¨l B, Zhang LC, Aindow M, Erkey C. PtPd/BP2000 electrocatalysts prepared by sequential supercritical carbon dioxide deposition. Int J Hydrogen Energy 2010;35:11669e80. SE, Gu¨mu¨s‚og lu T, Yılmaztu¨rk S, Ayala CJ, Aindow M, [23] Bozbag € z H, et al. Electrochemical performance of fuel cell Deligo catalysts prepared by supercritical deposition: effect of different precursor conversion routes. J Supercrit Fluids 2015;97:154e64.
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[24] Zhao J, Zhang L, Chen T, Yu H, Zhang L, Xue H, et al. Supercritical carbon-dioxide-assisted deposition of Pt nanoparticles on graphene sheets and their application as an electrocatalyst for direct methanol fuel cells. J Phys Chem C 2012;116:21374e81. [25] Zhao J, Yu H, Liu Z, Ji M, Zhang L, Sun G. Supercritical deposition route of preparing Pt/Graphene composites and their catalytic performance toward methanol electrooxidation. J Phys Chem C 2014;118:1182e90. SE, Erkey C. Supercritical deposition: current status [26] Bozbag and perspectives for the preparation of supported metal nanostructures. J Supercrit Fluids 2015;96:298e312. [27] Bozbag SE, Sanli D, Erkey C. Synthesis of nanostructured materials using supercritical CO2: Part II. Chemical transformations. J Mater Sci 2011;47:3469e92. [28] Bayrakc¸eken A, Smirnova A, Kitkamthorn U, Aindow M, _ et al. Vulcan supported Pt electrocatalysts lu I, Tu¨rker L, Erog for PEMFCs prepared using supercritical carbon dioxide deposition. Chem Eng Commun 2008;196:194e203. [29] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [30] Ma J, Wang L, Mu X, Cao Y. Enhanced electrocatalytic activity of Pt nanoparticles supported on functionalized graphene for methanol oxidation and oxygen reduction. J colloid interface Sci 2015;457:102e7. [31] Das‚ E, Alkan Gu¨rsel S, Is‚ikel S‚anli L, Bayrakc¸eken Yurtcan A. Comparison of two different catalyst preparation methods for graphene nanoplatelets supported platinum catalysts. Int J Hydrogen Energy 2016;41:9755e61. [32] Bayrakc¸eken A. Platinum and Platinum-Ruthenium based catalysts on various carbon supports prepared by different methods for PEM fuel cell applications. Ankara/Turkey: METU; 2008 [PhD Thesis]. [33] Zhang Y, Kang D, Aindow M, Erkey C. Preparation and characterization of ruthenium/carbon aerogel nanocomposites via a supercritical fluid route. J Phys Chem B 2005;109:2617e24. [34] Thakur S, Karak N. Green reduction of graphene oxide by aqueous phytoextracts. Carbon 2012;50:5331e9. [35] Kou R, Shao Y, Wang D, Engelhard MH, Kwak JH, Wang J, et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochem Commun 2009;11:954e7. [36] Wang J, Yin G, Shao Y, Zhang S, Wang Z, Gao Y. Effect of carbon black support corrosion on the durability of Pt/C catalyst. J Power Sources 2007;171:331e9. [37] Itoe RN, Wesson GD, Kalu EE. Evaluation of oxygen transport parameters in H2SO4-CH3OH mixtures using electrochemical methods. J Electrochem Soc 2000;147. 2445. [38] Gubbins K, Walker R. The solubility and diffusivity of oxygen in electrolytic solutions. J Electrochem Soc 1965;112:469. [39] Cruz RG, Feria OS. Oxygen reduction in acid media by a RuxFeySez(CO) (n) cluster catalyst dispersed on a glassy carbon supported nafion film. J Solid State Electrochem 2003;7:289.
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Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance Elif Das‚ a, Selmiye Alkan Gu¨rsel b,c, Lale Is‚ıkel S‚anlı b, Ays‚e Bayrakc¸eken Yurtcan a,d,* a
Department of Nanoscience and Nanoengineering, Atatu¨rk University, Erzurum, 25240, Turkey Sabanci University Nanotechnology Research & Application Center (SUNUM), Istanbul, 34956, Turkey c Faculty of Engineering &Natural Sciences, Sabanci University, Istanbul, 34956, Turkey d Department of Chemical Engineering, Atatu¨rk University, Erzurum, 25240, Turkey b
article info
abstract
Article history:
Graphene is considered as a promising catalyst support for polymer electrolyte membrane
Received 14 February 2017
(PEM) fuel cells because of its excellent properties. Recent studies showed that electro-
Received in revised form
catalytic activity, fuel cell performance, catalyst utilization efficiency and long term
1 June 2017
durability can be improved by using graphene as the catalyst support. In here, we report
Accepted 12 June 2017
the synthesis of graphene nanoplatelets (GNPs) supported platinum catalysts using su-
Available online xxx
percritical carbon dioxide (scCO2) deposition technique. The prepared catalysts were characterized in terms of their structure, morphology and thermal behavior by using X-ray
Keywords:
diffraction (XRD) analysis, transmission electron microscopy (TEM), scanning electron
PEM fuel cell
microscopy (SEM), and thermogravimetric analysis (TGA). The electrocatalytic and half-cell
Platinum
performances of the as-prepared catalysts were investigated by cyclic voltammetry (CV)
Graphene nanoplatelets
and PEM fuel cell performance testing. Small size and well distribution of Pt nanoparticles
Supercritical carbon dioxide
on GNPs surfaces were achieved by using scCO2 deposition which improves the PEM fuel
deposition
cell performance. To the best of our knowledge decoration of Pt nanoparticles on GNPs via scCO2 deposition and their detailed fuel cell performances were presented for the first time in the literature. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the twenty-first century, special attention gained by fuel cells for being clean sources of energy [1,2]. Polymer Electrolyte Membrane (PEM) fuel cells gain more attention than other types of fuel cells because of their high efficiency, high power density, low cost, low weight and low working temperature
[3,4]. In spite of remarkable progress in PEM fuel cells, both durability and stability of catalyst still need to be enhanced [5,6]. Up to now, there are various studies on development of alternative catalyst support materials for PEM fuel cells. In this regards, various carbon materials, including activated carbon, mesoporous carbon, carbon nanotubes, carbon fiber or carbon aerogel have been utilized as catalyst support materials due to
* Corresponding author. Department of Chemical Engineering, Atatu¨rk University, Erzurum, 25240, Turkey. E-mail address: [email protected] (A. Bayrakc¸eken Yurtcan). http://dx.doi.org/10.1016/j.ijhydene.2017.06.108 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 1 e Adsorption isotherm of CODPtMe2 on GNPs in scCO2 (solid line represents Langmuir fit).
Fig. 2 e XRD patterns of plain GNPs and Pt/GNPs electrocatalysts.
their high surface area, low cost, high ORR activity and durability [7e10]. In addition to those carbon materials, graphene can serve as a promising catalyst support owing to its high surface area, excellent electrical conductivity, high surface area and strong affinity to metal particles [11,12]. Platinum (Pt)-based catalysts are significant in practical applications of PEM fuel cells because of their high efficiency stability and other factors affecting the catalytic activity of PEM fuel cells electrocatalyst, such as catalyst preparation conditions, the composition of the electrocatalyst and the metal loading over the support [13,14]. In the literature, various catalyst preparation methods, such as chemical reduction, impregnation, sputter deposition, electrochemical method and so on are used [15e20]. It is very critical to choose a proper method in order to achieve the targeted catalyst properties and also higher catalytic activities [21]. Supercritical carbon dioxide deposition (scCO2) is also an alternative method for preparation of nanoparticles [13,22e24]. There has been a growing interest in exploiting advantages of scCO2 as a “green” solvent with peculiar wetting abilities in the catalyst preparation for fuel cells applications [23]. ScCO2 environment provides an excellent medium to decorate nanoparticles onto the porous support materials. Moreover, scCO2 offers several advantages compared to conventional techniques. The high diffusivity, low viscosity and surface tension properties of scCO2 allows it and its mixtures to penetrate into the nanostructures easily [24,25]. Furthermore, the support structure is preserved upon the CO2 treatment. There are various studies about the deposition of Pt nanoparticles on carbon black,
carbon nanotubes and carbon aerogels via scCO2 [26e28]. However, there are only three studies in literature about the preparation of graphene supported Pt nanoparticles as the catalyst support for fuel cell reactions by scCO2. In this regard, Zhao et al. showed enhanced stability and electrocatalytic activity of Pt nanoparticles on GO for methanol oxidation [24]. In their recent study, Zhao et al. compared Pt/ graphene with Pt/carbon black and Pt/carbon nanotube prepared by sCO2 deposition and reported better stability of Pt/graphene compared to other two supports [25]. In both studies authors employed GO prepared by Hummers method [29] and expanded thermally. In another recent study Pt nanoparticles on functionalized graphene were synthesized by scCO2 [30]. In all these studies about graphene (GO or functionalized GO) supported Pt, authors investigated exsitu electrocatalytic activity of catalysts yet fuel cell performances of these catalysts were not shown. In this study, different Pt loadings over graphene nanoplatelets (GNPs) support were succesfully achieved with thermodynamic control via one-pot scCO2 method using dimethyl (1,5-cyclooctadiene) platinum (II) (PtMe2COD) as organometallic Pt precursor. The physical, structural and electrochemical properties of the catalysts were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), cyclic voltammetry (CV) and fuel cell tests. To the best of our knowledge, decoration of Pt nanoparticles on GNPs and their detailed fuel cell performances were presented for the first time in the literature.
Table 1 e Langmuir equilibrium constants for CODPtMe2, scCO2 and GNPs compared with the literature. Catalyst Pt/GNPs Pt/BP2000 Pt/MWCNT Pt/Vulcan
Support surface area [m2/g]
K1
Qo
K1 Qo
R2
SPt [m2/g]
Reference
759 1450 300 232
0.4357 0.54 0.29 0.87
924.34 1481 335 180
403 801 100 156
0.99 0.99 0.99 0.98
329 525 119 64
In this study [32] [32] [28]
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Experimental Materials Dimethyl (cyclooctadiene) platinum (II) was purchased from STREM. Graphene nanoplatelets (GNPs) support material purchased from XG Sciences (xGnP® Grade C, surface area of 759 m2/g) with particle diameter of less than 2 mm and particle thickness of a few nanometers was used [31]. Carbon dioxide, nitrogen, oxygen and hydrogen gases having high purity (>99.98%) were purchased from Habas‚. Nafion solution (15%) (Ion Solutions Inc.), 2 propanol (Sigma), 1,2 propandiol (Sigma), H2SO4 (Sigma) were used as received. Nafion 212 membrane was used as the electrolyte. GDL 34 BC (Sigracet) was used as gas diffusion layer. All the chemicals were used as-received.
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adsorption of the precursor on GNPs. Metallic Pt nanoparticles over GNPs were achieved with the heat treatment of precursor adsorbed on GNPs at 400 C for 4 h in nitrogen atmosphere. To thermodynamically control the Pt loading on GNPs, the precursor amounts dissolved in the high pressure vessel were changed. Adsorption isotherm data were collected by taking into account the weight change of GNPs measured using an analytical balance before and after the adsorption of the precursor for each precursor amount inserted into the high pressure vessel. The adsorption data were collected until the saturation was reached. For all isotherms, temperature and the pressure were set to 308 K and 12 MPa. The catalysts were called as Pt/G1, Pt/G2, Pt/G3, Pt/G4 and Pt/G5 according to the Pt precursor amounts used in the experiments as in the order of 0.05, 0.1, 0.2, 0.3 and 0.4 g, respectively.
Catalyst characterization
Catalyst preparation In this study, Pt nanoparticles on GNPs were achieved by using scCO2 deposition method. 1,5-dimethyl platinum cyclooctadiene (CODPtMe2) (Strem) was used as the Pt organometallic precursor. A custom-made 54 ml stainless steel vessel equipped was used with two sapphire windows having 2.5 cm diameter as the deposition apparatus. For each catalyst, different Pt precursor amounts were placed in the vessel along with a stirring bar, and 0.1 g of GNPs was placed in a pouch made of filter paper. A stainless steel screen was positioned in the middle of the vessel in order to separate the pouch and the stirring bar. The catalyst preparation method includes dissolution of the Pt precursor in scCO2 (at 308 K and 12 MPa) and
Characterization of physical properties X-ray diffraction technique XRD (Rigaku Miniflex X-ray diffractometer) was employed for the investigation of the crystalline structure of Pt nanoparticles, The Rigaku Miniflex diffractometer with a CuKa (l ¼ 1,5406 A) radiation source was utilized for the these analyses. The scanning range of 2q was set in between 10 to 90 . FEI Inspect S50 model scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) was conducted to investigate the morphology and the chemical composition of GNPs. The structural changes, particle size and distribution of the Pt nanoparticles on GNPs were characterized by JEOL 2100 JEM high resolution transmission electron microscopy (HRTEM). The Pt loadings of the catalysts over GNPs were
Fig. 3 e SEM images and EDX spectra of plain GNPs. Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 4 e TEM images and particle size distribution patterns of catalysts prepared by different amount precursor (a) plain GNPs (b) Pt/G1 (c) Pt/G2 (d) Pt/G3 (e) Pt/G4 (f) Pt/G5. Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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analyzed by a Netzsch thermal thermogravimetric analyzer (TGA) from room temperature to 1000 C at a heating rate of 10 ºC/min under air atmosphere.
Electrochemical measurements The electrochemical characterization of all the prepared catalysts were performed at 25 C in a three-electrode cell connected to a rotating disk electrode system (Pine Instrument). A glassy carbon (GC) disk with an area of 0.1963 cm2 was used as working electrode. Ag/AgCl and a platinum wire served as reference and counter electrodes, respectively. Electrodes for each catalyst was prepared by using the same procedure. Briefly, a suspension was prepared by mixing the required amounts of the prepared catalysts with deionized water, 1,2propandiol, and 15% Nafion® solution. Then, the resulting suspension was ultrasonicated for a while and then, 5 mL of this suspension was dropped on 5 mm diameter GC electrode (the metal loading per unit area was set to 22 mg Pt cm2) and dried in air overnight. Cyclic voltammograms for the hydrogen oxidation reaction (HOR) were recorded in 1 M H2SO4 electrolyte solution that was saturated with nitrogen for 30 min to remove oxygen. The stability of the catalysts was investigated by runs up to 50 voltammetric cycles at a scan rate of 50 mV s1 between 0 and 1.2 V. Cyclic voltammograms for oxygen reduction reaction (ORR) were recorded in 1 M H2SO4 electrolyte solution that was saturated with oxygen by purging the electrolyte with oxygen for half an hour and then rotating the disk electrode in between 100 and 2500 rpm at a scan rate of 5 mV s1. An accelerated degradation test, carbon corrosion test, was applied to the fresh electrodes. Then, carbon corrosion of the prepared catalysts was investigated by subjecting them to a constant potential of 1.2 V for 24 h. The effect of carbon corrosion test on the activities of the prepared catalysts were evaluated by taking into account the change in the cyclic voltammograms before and after carbon corrosion test. In order to characterize the electrochemical activities of the prepared catalysts in situ PEM fuel cell performance tests were performed. For the cell performance test, the membrane electrode assemblies (MEAs) were prepared by spraying catalyst ink (including definite amounts of catalyst, 2 propanol and Nafion solution) onto GDLs. The platinum loading over the electrode was set to 0.3 mg Pt cm2. Then, a five-layer MEA was prepared by pressing these GDLs onto the Nafion® 212 membrane at 130 C, 400 psi for 3 min in order to create a good interfacial contact between the GDL and the catalyst layer. The geometric area of the electrode was 4.41 cm2. A commercially available PEM fuel cell hardware (Electrochem) was used for the experiments. During the experiments, the temperature of the single cell was maintained at 70 C, and fully humidified hydrogen/oxygen gases were fed into the anode/cathode at 70 C at a flow rate of 250 ml/min.
Table 2 e Pt loadings and particle sizes of Pt/GNPs catalysts according to the Pt precursor amount. Pt precursor amount [g] 0.05 0.10 0.20 0.30 0.40
Catalyst
Pt [wt.%]
Particle size [nm]
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
12.2 17.8 22.8 22.1 25.7
1.5 1.5 1.7 1.9 2.0
higher uptakes and so higher metal loadings support properties and accessibilities of the pores are very critical. Adsorption isotherm was obtained at 308 K and 12 MPa. The adsorption isotherm for CODPtMe2, scCO2 and GNPs is given in Fig. 1. The uptake of the precursor increased with an increase in the amount of the precursor dissolved up to some extent. The adsorption isotherm showed the partitioning of the Pt precursor between the GNPs and scCO2 phases. The adsorption isotherm was fitted to Langmuir model with the formula given below: q¼
K 1 Q0 c 1 þ K1 c
(1)
where K1 (g scCO2 mg1precursor) is the Langmuir adsorption constant, Qo (mgprecursor g1support) is the adsorption capacity, c (mgprecursor g1scCO2) is the solution concentration and q (mgprecursor g1support) is the uptake by the adsorbent. The Langmuir equilibrium constants obtained for this system is calculated with regression analysis and given in Table 1. Qo represents the maximum number of the available sorption sites that GNPs can supply for the Pt precursor molecules in case of the Langmuir isotherm which is increased with an increase in surface area [32]. K1 Qo value shows the relative affinity of the solute towards the surface of the adsorbent. Higher K1Qo values indicate that the CODPtMe2 has more affinity for GNPs than scCO2. The structure of the pores of the adsorbents and the molecular size of the adsorbate have significant effects on accessibility of the sorption sites. If it is assumed that the CODPtMe2 precursor molecules are spherical and in 0.5 nm
Results and discussion Support-precursor interactions and adsorption conditions are significantly affect the adsorption process. Adsorption of the Pt precursor over the porous support material will lead the thermodynamically control of the Pt loading. In order to get
Fig. 5 e TGA curves of synthesized Pt/GNPs catalysts.
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Fig. 6 e The CVs were measured in N2-purged 1 M H2SO4 electrolyte solution at 25 C. diameter, the area covered by the precursor is given as follows: S ¼ Q0 Apr2
(2)
where S is the area covered by precursor, Qo is the capacity, A is the Avogadro's number, and r is the radius of Pt precursor
Table 3 e ECSA loss of the catalysts after carbon corrosion. ECSA [m2/g]
Catalyst
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
ECSA loss [%]
Before aging
After aging
43.6 87.2 55.6 56.3 71.1
28.1 51.4 35.7 32.9 44.3
35.4 41.1 35.8 41.6 37.7
[33]. The area covered by Pt organometallic precursor is calculated as 329 m2 g1 as given in Table 1. In Table 1, the areas covered by Pt organometallic precursor for different carbon supports were also summarized. The XRD data obtained from the Pt/GNPs catalysts are shown in Fig. 2. The characteristic diffraction peaks at 39.8 , 46.3 , 68.2 , and 81.6 , correspond to the (111), (200), (220), and (311) facets of the face-centered cubic structures of Pt crystal, respectively. The XRD pattern show a sharper and narrower peak at ~26.5 attributed to the (002) plane of hexagonal graphite structure, meaning the formation of graphene nanoplatelets. The presence of the C (004) diffraction peak at 53 in GNPs is also indicative of the high crystallinity of its carbon structure [34]. Fig. 3 shows the SEM images and corresponding EDS spectra of plain GNPs which has wrinkled and porous nature. This nature of GNPs not only provides a large surface area for
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 7 e Hydrodynamic voltammograms of Pt/G1 catalyst before (a) and after (b) carbon corrosion test (c) Tafel slope losses of the catalysts and GNP after carbon corrosion test at 100 rpm.
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Fig. 7 e (continued).
Table 4 e Number of electrons transferred per oxygen molecule for all catalysts. Catalysts
n
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
3.9 3.7 3.5 3.6 2.8
Pt nanoparticles but also prevent GNPs from aggregating [35]. EDS also used to get the information about the chemical composition of the structure. According to the EDS results of graphene, results confirm the presence of C and O elements in the structure. ScCO2 environment provides homogeneous distribution of nanoparticles. Utilization of nanosized catalyst particles create higher surface area and improve the efficiency of the
Table 5 e Tafel slope losses of the prepared catalysts. Catalysts
Tafel slope losses [V] 100 rpm 400 rpm 900 rpm 1600 rpm 2500 rpm
Pt/G1 Pt/G2 Pt/G3 Pt/G4 Pt/G5
0.04 0.02 0.07 0.03 0.05
NA: Not applicable.
0.03 0.01 0.06 0.02 0.004
0.02 0.01 0.05 0 0
0.02 0.007 0.05 0 0
0.02 0 0.05 NA 0
catalysts. More catalytic sites can be achieved by nanoparticles and increase the catalytic activity of the catalysts. The TEM images of as-prepared Pt/GNPs catalysts are presented in Fig. 4. It can be clearly seen that the Pt nanoparticles are succesfully anchored on the surface of the GNPs and average Pt nanoparticle size was change in between 1.5 and 2 nm (Table 2). Moreover, TGA was carried out to identify the Pt content achieved on GNPs. The Pt loadings over GNPs were determined by taking into account the difference between plain GNPs and GNPs supported Pt catalysts. Plain GNPs were also treated in scCO2 environment before the TGA experiments because untreated GNPs showed a different trend. It was observed that Pt content on GNPs was changed between 12.2 and 25.7 wt% (Table 2) according to the precursor amount loaded to the high pressure vessel as shown in Fig. 5. Cyclic voltammetry (CV) is an efficient tool for determination of the electrochemical active surface areas (ECSA) of catalyst on an electrode. The ECSA of an electrocatalyst both provides the number of electrochemically active sites per gram of the catalyst, and also used to compare different electrocatalytic materials. CVs of Pt/GNPs were conducted to estimate ECSAs of Pt on GNPs. The resulting CV curves are presented in Fig. 6. CV of plain GNPs showed an oxidationreduction peaks in between 0.4 and 0.6 V which represent the carbon oxidation due to carbon corrosion via quinonehydroquinone reaction [36]. Lower carbon corrosion in the support material is favorable for long term durable PEM fuel cells. The CV indicates typical electro-adsorption and desorption of H2 on the Pt site, Pt oxidation and reduction
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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Fig. 8 e PEM fuel cell performance curves for Pt/GNPs catalysts a) at different Pt loadings over GNPs b) at different Pt loadings over electrode active areas.
peaks. The ECSA and ECSA losses of the prepared Pt catalysts (Table 3) were calculated by taking the average of the areas under the curve under hydrogen adsorption/desorption peaks before and after carbon corrosion test. When we look at before corrosion values we can see that the Pt/G2 catalysts have much higher ECSA value (87 m2/g) than the other catalysts. This can be attributed to the smaller size of the uniform ultrafine Pt nanoparticles on the large surface of GNPs, which generates more available active Pt sites for catalytic activity compared with the others. Although, Pt/G1 catalyst was found to have the lowest ECSA loss similar trend was observed for all the catalysts.
Hydrodynamic voltammograms obtained from the oxygen purged H2SO4 electrolyte before and after carbon corrosion test are shown in Fig. 7 for Pt/G1 catalyst. The results for other catalysts are provided in supplementary material (Fig. S1). Koutecky-Levich plots were obtained by using the voltammogram data and also Eq. (3) given below by plotting i1 versus w1/2; 1 1 1 ¼ þ i Bw1=2 ik
(3)
where i, ik, w and B represents the current, kinetic current, rotation speed and Levich constant, respectively. B is obtained
Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
from the Koutecky-Levich plots and by using it in the following Eq. (4) one can calculate the number of electrons transferred per oxygen molecule when w is in rad s1; B ¼ 0:62nFc0 D2=3 n1=6
(4)
where n represents the number of electrons per oxygen molecule, F is the Faraday constant (96485 C mol1), co shows the oxygen bulk concentration (1.02 106 mol cm3) [37], D shows the diffusion coefficient of oxygen (1.04 105 cm2 s1) [38] and ѵ shows the electrolyte viscosity (0.017 cm2 s1) [39]. Levich constant and Eq. (4) gives us the number of electrons transferred per oxygen molecule for all the catalysts as tabulated in Table 4. As seen from table that the number of electrons transferred per oxygen molecule for Pt/G1, Pt/G2, Pt/ G3 and Pt/G4 catalysts are all very close to 4 which depicts the catalyst follows 4e pathway which represents the complete water formation and only negligible H2O2 formation. Carbon corrosion test showed that HOR affected significantly by applying 1.2 V potential to the fresh electrode. Effect of carbon corrosion on ORR activity of the prepared catalysts were also investigated and Tafel slope losses of the prepared catalysts at 100 rpm rotation speed is given in Fig. 7(c). Tafel slope loss plots for other rotation speeds are given in Fig. S2 (supplementary material). The corresponding Tafel slope losses are tabulated in Table 5. Table 5 shows that the Tafel slope losses are getting less when the rotation speed is increased and applying 1.2 V harsh condition did not change the ORR activity of the GNPs supported catalysts significantly. Single PEM fuel cell performances of the MEAs assembled with the catalysts having different Pt loadings were taken in order to determine the effect of increase in Pt loadings over GNPs (Fig. 8(a)) and increase in Pt loadings over electrode active area (Fig. 8(b)) on PEM fuel cell performance. PEM fuel cell performances of the catalysts showed an increasing trend upto Pt/G3 catalyst and then started to decrease from this point. Although the Pt loadings over GNPs and also the particle sizes of the catalysts were so close to each other, the performances of the catalysts significantly different far from each other. Considering these results, Pt/G3 catalyst showed the best performance compared to other catalysts and further increase in the Pt loadings over GNPs may affect the dispersion and also the homogeneity of the nanoparticles which may induce a decrease in the PEM fuel cell performance. As can be seen from figure that the Pt loadings over electrode active area also significantly affect the performance and further increase will also develop a better performance.
Conclusions The MEA is the most important component where hydrogen and oxygen react electrochemically to generate electrical power in PEM fuel cells. In the MEA mostly Pt based electrocatalysts were used for reduction of oxygen at the cathode and the oxidation of hydrogen at the anode. Pt based electrocatalysts are required to be durable in the corrosive environment of the PEM fuel cell. The high quality electrocatalysts depend strongly on their synthesis technique. In here, we report a facile and effective approach for the synthesis Pt
nanoparticles on GNPs. The presence of scCO2 leads to highly dense and homogeneous deposition of Pt nanoparticles on GNPs surfaces. In total, 5 different catalysts were synthesized by the thermodynamic control of the Pt loadings via adsorption isotherm and denoted as Pt/G1, Pt/G2, Pt/G3, Pt/G4 and Pt/ G5 where the numbers of the samples implying the different amounts (in grams) of Pt used in the catalysts and the Pt/GNPs prepared by our approach seem to be very promising electrocatalysts for PEM fuel cells. Pt nanoparticles can be successfully decorated on GNPs via scCO2 deposition and the Pt loadings can be thermodynamically controlled. Superior PEM fuel cell performances can be achieved by controlling the properties of the catalysts.
Acknowledgements The research leading to these results has received funding from the Scientific and Technological Research Council of _ ¨ BITAK) Turkey (TU (Grant no. 114F506 and 114F029) and the European Union's Horizon 2020 research and innovation programme under grant agreement No 696656 (Graphene Flagship).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.06.108.
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Please cite this article in press as: Das‚ E, et al., Thermodynamically controlled Pt deposition over graphene nanoplatelets: Effect of Pt loading on PEM fuel cell performance, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.108