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Development of Functionalized Graphene Supported Highly Durable Pt-Free Bi-metallic Electrocatalysts for PEMFC
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ScienceDirect Materials Today: Proceedings 18 (2019) 660–670
www.materialstoday.com/proceedings
ICN3I-2017
Development of Functionalized Graphene Supported Highly Durable Pt-Free Bi-metallic Electrocatalysts for PEMFC Avijit Ghosha,*, Biswajit Mandalb, Sunil Baran Kuilab a
Department of Chemical Engineering, Heritage Institute of Technology, Kolkata, 700107, India b Department of Chemical Engineering, Haldia Institute of Technology, Kolkata, 721657, India
Abstract
Functionalized graphene supported Pt-free single metallic (Pd/FGP or Au/FGP) and bimetallic (PdAu/FGP) electrocatalyst were synthesized using NaBH4 reduction techniques. The electro-catalytic activity of developed Pd/FGP, Au/FGP, and PdAu/FGP were investigated x-ray powder diffraction (XRD), transmission electron microscopy (TEM), oxygen reduction reaction (ORR), and cyclic voltammetry (CV). The performance of the developed bimetallic electrocatalyst was compared with the commercial Pt/C. The oxygen reduction of PdAu/FGP was started gradually at a potential around 1.0 V and reached a limiting plateau at a potential around 0.75 V. Though the 50% Pd was replaced by Au but the ORR performance of the PdAu/FGP is almost maintained. The similar electro-catalytic activity of the Pd and/or PdAu can also be explained based on the synergetic effect of the Au and Pd in ORR. The active electrochemical surface area (ESA) is measured using CV analysis. The ESA for the Pd/FGP, PdAu/FGP, and Pt/C were found to be around 63.32, 51.27, and 43.4 m2·g-1Pt, respectively. The single cell polymer electrolyte membrane fuel cell (PEMFC) power density of the Pd/FGP, PdAu/FGP, and Pt/C were found to be 439, 414, 373 mW.cm-2, respectively. The developed bimetallic (PdAu/FGP) electrocatalys shows significantly higher stability under ORR test as compared to commercial Pt/C. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords:PEMFC; Bimetallic Electrocatalyst; Oxygen Reduction Reaction
* Corresponding author. Tel.: +91- 33 6627 0600 (Extn. 783); fax: +91- 33 2443 0455. E-mail address:[email protected], [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
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1. Introduction Polymer electrolyte membrane fuel cell (PEMFC) is one of the challenging energy conversion devices for transportation and distributed power generation systems due to its attractive features such as high power density, low operating temperature, minimal emissions, negligible noise, and high efficiency. Kani et al [1] and Liu et al [2]. The electrocatalyst plays a vital role in the performance of PEMFC. Therefore, developing high-performing, cost effective low cost and/or Pt-free electrocatalyst is key to successful fuel cell commercialization. Wanga et al [3] and Li et al [4]. Platinum (Pt) is the state-of-the-art electrocatalyst for PEMFC to catalyze the anodic oxidation of hydrogen and the cathodic reduction of oxygen. Gasteiger at al [5], Lim et al [6], Norskov at al [7]. However, bulk platinum (noble metal) does not yield good performance of the PEMFC due to low surface area per unit weight of platinum. Moreover, due to the limited natural resource, the cost of Pt noble metal is gradually increasing with the time. Zhang et al [8]. Many efforts have been used to reduce the amount of platinum used in fuel cells. Generally, there are two ways to reduce the use of Pt in PEMFCs, which are (1) Pt electrodes with low Pt content and (2) total or partial substitution of Pt with other metals. Cho et al [9], Antolini at al [10], antolini at al [11]. In the former case, the use of Pt can be reduced by dispersing on an active support material. Wee at al [12]. The most commonly used carbon black (C) as an electrocatalyst support suffers for low surface area, low electrical conductivity, less durability, and start corrosion at higher potential (>0.8 V versus standard hydrogen electrode). Kangasniemi at al [13], Wang et al [15], Kulikoysky at al [15]. Owing to above started problem, graphene (GP) and functionalized graphene (FGP) is being exploited by many researchers and scientists as a promising candidate for the fuel cell electrocatalyst support due to their extraordinary salient properties. Novoselov et al [16], Geim at al [17], Liu at al [18], Zhang at al [19], Ghosh at al [20], Shan al al [21], Kuila at al [22], Antolini at al [23]. On the other hand, the partial or total substitution of platinum with palladium seems a promising way to reduce Pt content as Pd have very similar properties. Altolini et al [24]. Moreover, the cost of palladium (Pd) is about three times lower than that of platinum and it is also fifty times abundant on the earth than Pt. Moreover, Pd shows similar electro-catalytic activity for both the hydrogen oxidation and the oxygen reduction reaction (ORR). Maximov et al [25] and Rao at el. [26]. Recently, the supported Pt and/or Pd alloy electrocatalysts were found to be more electrochemically active due to their enhanced catalytic properties relative to individual carbon supported Pt or Pd electrocalatyst. Yucong et al [27], Adnan et al [28], Tan et al [29], Unni et al [30], Zhang et al [31], Rao et al [32], Lobato et al [33], Dutta et al [34], Zhang et al [35]. Generally metal nanoparticle gets agglomerated due to their surface energy. Xie et al [36] and Guilminot et al [37]. As a result the electrochemical durability (hydrogen oxidation and ORR) decreases of anode and cathode catalyst over time. This can be improved by alloying of Pt or Pd with other transition metals. Wang et al [38]. In addition, the supported (on carbon black or FGP) bimetallic electrocatalyst enhance the CO tolerance as compared to commercial Pt/C electrocatalyst, which is one of the most important desired properties of PEMFC. Gold (Au) nanomaterial is a special metal in view of its inertness nature and its high catalytic activity for oxygen reduction reaction. Carbon supported Au-based nanostructure materials have been extensively evaluated for the electrocatalyst oxidation and reduction. Park et al [39]. The enhanced ORR activity was observed for the carbon supported bimetallic (PtAu) electrocatalyst. This can be attributed to the modification of Pt nanoparticles by Au cluster, which lead to an increase in Pt oxidation potential and thus to significant enhancement in catalytic durability for ORR. Zhang et al [40]. Lee at al. [41] reported that the functionalized graphene supported Pt and/or Pt-alloy electrocatalyst show superior electro-catalytic activities as compared to Pt/C and Pt/MWCNT for PEMFC applications. However, from the practical point of view, the fuel cell test is the ultimate evaluation criterion for the supported bimetallic electrocatalyst. For example, Seger and Kamat reported a single-cell PEMFC performance using Pt/GP or Pt electrocatalyst at cathode whereas at anode commercial Pt/C was used. They have reported that the Pt/GP as a cathode electrocatalyst showed a maximum power density of 0.161 W·cm-2 as compared to 0.091 W·cm-2 for an unsupported Pt electrocatalyst. Lee et al [42]. The similar kind of support effect was reported, where functionalize graphene supported electrocatalyst was evaluated only as the cathode electrocatalyst for PEMFC. Jafri et al [43] and Ha et al [44]. It is found that the research work is being conducted on functionalized graphene but the work on FGP supported bimetallic electrocatalyst for fuel cell is feeble. Moreover, the potential of the FGP in
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PEMFC is still under exploited. There is hardly any report with FGP supported PdAu bimetallic electrocalatyst replacing the both anode and cathode catalysts of PEMFC. This paper reports the synthesis of Pd/FGP, Au/FGP and bimetallic PdAu/FGP electrocatalysts by NaBH4 reduction techniques. The synthesis and characterization of FGP is reported elsewhere. Ghosh et al [20]. The synthesized electrocatalysts were characterized thoroughly and evaluated in single cell PEMFC using both anode and cathode catalyst. Moreover, the performances of the as synthesized electrocatalyst were compared with the commercial Pt/C. The electrochemical stability test was performed for ORR using PdAu/FGP and compared with commercial Pt/C. 2. Experimental 2.1. Material Industrial grade natural graphite (NG) was procuredlocally. NaOH, NaBH4, dimethylformamide (DMF), and citricacid (CA) were procured from Merck, India. PdCl2(38-40wt.%) and aqueous solution of HAuCl4 (1 mM) were purchased from Spectrochem and Merck, India, respectively. NaOH and NaBH4, were procured from Merck, India. Nafion solution 5wt.% and Nafion-117 membrane was procured from DuPont, USA. Toray carbon paper (TGP-H120) and 20 wt.% Pt/C (Vulcan XC-72) were purchased from ElectroChem, USA. The purity of gases usedwas 99.99% and Millipore water was used whereverrequired. The in-house synthesized FGP is used as an electrocatalyst support. The details synthesis and characterization of FGP is described elsewhere. Ghosh et al [20]. 2.2. Methods 2.2.1
Synthesis of Pd/FGP, Au/FGP, and PdAu/FGPelectrocatalysts
The Pd/FGP (20wt.%) is prepared using precipitation method via NaBH4 reduction method using FGP, 20wt.% Palladium chloride (II) (PdCl2), and H2O. Initially, as-synthesized FGP was dispersed in ultra-pure millipore water by ultasonication for 1 h to make FGP slurry. Then, the Pd metal precursor was separately dissolved in water using ultrasonication. The metal precursor was added drop-wise to the FGP slurry under continuous sonication. The pH of the mixture was adjusted at around 12 using 1 M NaOH solution. The 1 M NaBH4 was added into the above mixture as a reducing agent under continuous stirring for 5 h and slow heating upto 90°C. The resulting mixture was treated with N2 for 30 min. Then the treated solution was filtered, washed copiously with water followed by ethanol, and dried in vacuum oven at 80°C for overnight to get Pd/FGP (20wt.%) electrocatalyst. The similar process was followed to synthesize Au/FGP and PdAu/FGP using aqueous HAuCl4 as metal precursor by similar NaBH4 reduction techniques as discussed earlier. The 20wt.% metal (Pd and/or Au) loading was kept for all the electrocatalysts. In case of PdAu electrocatalyst, the metal loading was calculated 1:1 (Pt:Au) ratios. 2.3. Characterization 2.3.1
Characterization of Pd/FGP, Au/FGP, and PdAu/FGP electrocatalyst
Various physico-chemical characterization techniques were used to characterize the developed FGP supported metal (Pd and/or Au) electrocatalyst. The surface morphologies were studied using TEM (Transmission electron microscopy, JEOL, JEM 2100, Japan). The XRD (X-ray diffractometer, Bruker, USA) measurements were performed using CuKa source at a scan rate of 0.05°s-1. Ex-situ electrochemical performance of the developed electrocatalysts were evaluated using rotating disc electrode (RDE) for oxygen reduction reaction (ORR) and cyclic voltammetry (CV) for hydrogen adsorption/desorption (HAD) using three electrode assembly (CHI 600E, USA). All the potentials were referenced to the standard hydrogen electrode (SHE).
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2.3.1.1 The oxygen reduction reaction analyses using linear sweep voltammetry The oxygen reduction reaction (ORR) was carried out using rotating disc electrode (RDE) by linear sweep voltammetry (LSV). The LSV was conducted using three electrodes RDE cell assembly connected to a potentiostat/galvanostat. The Ag/AgCl and Pt were used as reference and counter electrodes, respectively. The glassy carbon disc electrode (3 mm diameter) was used as working electrode. The pretreatment of working electrode was done by polishing the electrode surface using 0.05 µm alumina paste and then washing with ethanol and ultrapure water under sonication. The electrocatalyst slurry was prepared by mixing around 5 mg of Pt/C (commercial) or Pd/FGP or Au/FGP or PtAu/FGP powder with 5wt.% nafion-isopropyl alcohol (1:4) solution using sonication for 30 min. The electrocatalyst slurry was then drop cast on the surface of the pretreated glassy carbon electrode and dried to obtain the working electrode. An aqueous solution of 0.5M H2SO4, saturated with O2, was used as electrolyte. The LSV analysis was conducted using RDE at a scan rate of 5 mV·s-1 at a constant rotation speed of 1600 rpm. 2.3.1.2 Hydrogen adsorption/desorption analyses using cyclic voltammetry The electrochemical hydrogen adsorption/desorption (HAD) activity of the developed electrocatalysts were conducted by cyclic voltammetry (CV) technique at room temperature. The conventional three electrode cell assembly was used for the HAD study. The Ag/AgCl and Pt electrodes were used as reference and counter electrodes, respectively. The working electrode was prepared by the developed electrocatalyst with metal (Pd and/or Au) loading of 0.4 mg·cm-2. The working electrode was prepared using toray carbon paper (TGP-H-120) and catalyst ink. The catalyst ink was prepared by mixing of required amount of catalyst, 5wt.% nafion solution, and isopropyl alcohol under continuous sonication for 15 min. Then the dispersed catalyst ink was sprayed on the Toray carbon paper. Subsequently, the catalyst coated carbon paper was dried in hot air oven for overnight. An aqueous solution of 0.5 M H2SO4, saturated with N2, was used as an electrolyte for CV analysis at a scan rate of 50 mV·s-1. Electrochemical surface area (ESA), one of the most important parameters for evaluating the activity of electrocatalysts, was measured from the CV plots. The active ESA is determined as per the procedure reported elsewhere Ghosh et al [20]. The charge associated with the hydrogen adsorption on Pt, which can be found out by integrating the hydrogen adsorption peak in the potential range of 0.04-0.4 V (SHE) for evaluating the electrochemical surface area. Chaparro et al [46]. 2.3.2
PEMFC performance evaluation
A PEMFC experimental setup was developed in the laboratory to study the performance of developed electrocatalyst in the real environment of the fuel cell. The membrane electrode assembly (MEA) was developed using Toray carbon paper (TGP-H-120) coated with catalyst layer and nafion 115 membrane using hot press technique at 110 oC under the pressure of 45 kg.cm-2 for 3 min. The prepared MEA was installed inside the in-house fabricated single cell PEMFC set-up, having active area of around 6.25 cm2. The catalyst loading on anode and cathode were kept as 0.2 mg·cm-2 and 0.4 mg·cm-2, respectively. The hydrogen and oxygen flow rates were maintained at 0.5 and 1.0 lpm, respectively. The output current and voltage were measure with the help of two digital multimeters. A rheostat was used as a variable load. I-V performance of the PEMFC was recorded at around 70 oC. All the parameters were kept same for analyzing different developed electrocatalysts.
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3. Result and discussions 3.1. XRD analyses of Pd/FGP, Au/FGP, and PdAu/FGP The XRD diffraction patterns of synthesized Pd/FGP, Au/FGP, and PdAu/FGP are shown in the fig. 1, and compared with commercial Pt/C diffraction pattern. The broad diffraction peak at 2θ value of around 26.5o is associated with the graphitized carbon C(002) for all the catalysts. The synthesized Pd/FGP electrocatalyst shows face center cubic (fcc) crystalline Pd diffraction of (111), (200), and (220) at 2θ of 39.95, 46.05, and 68.01, respectively (JCPDS 65-2867) [3]. The peak at 2θ of 38.11, 44.51, and 66.31 corresponds to Au (111), (200), and (220) crystal planes, respectively (JCPDS 65-2870). The diffraction peaks of PdAu/graphene were located between the expected positions of pure Pd (no. 65-2867) and Au (no. 65-2870). This XRD phenomenon has also been observed in other bimetallic alloy nanostructures. Huang et al [47] and Lu et al [48]. The shift of each peak of PdAu/FGP electrocatalyst to lower 2θ value compared to Pd/FGP and to higher 2θ value compared to Au/FGP. It indicates that a single phase bimetallic electrocatalyst is formed on the FGP support due to the incorporation of Au atoms into the Pd lattice, which leads to the formation of PdAu alloy. Wang et al [3]. The separate diffraction peaks of Pd and Au were not observed in the XRD diffraction of PdAu/FGP due to the similar crystalline structure of Pd and Au. The Pd, Au, and PdAu average crystallite sizes were calculated using broad diffraction peak of (111) by Scherrer’s equation. Ghosh et al [20]. The average crystallite sizes of Pd and Au, and PdAu particle for Pd/FGP, Au/FGP, and PdAu/FGP were about 4.1, 5.4 and 4.4 nm, respectively, which can be further supported by TEM analyses. Moreover, the XRD crystallite size is a mass average, which is inherently larger than the TEM average counted by number.
Fig. 1. XRD patterns of Pd/FGP, Au/FGP, and PdAu/FGP
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3.2. TEM analyses of Pd/FGP, Au/FGP, and PdAu/FGP The TEM images of the synthesized electrocatalysts are shown in the fig. 2 and compared with micrograph of commercial Pt/C electrocatalyst. The fig. 2 (a, b. and c) shows that the Pd and/or Au nanoparticles are dispersed on the funtionalized graphene (FGP) support material. It may be noted that the carbon and FGP support were having same loading (20 wt.%) of metal electrocatalyst (Pt or Pd and/or Au). The hogh surface area support material is favourable for better dispersion of metal nanoparticles.The TEM images of Pd/FGP, Au/FGP, and PdAu/FGP shows that the Pd and/or PdAu nanoparticles are well distributed as compared to Pt/C. It may be due to the very less surface area of carbon (249.46 m2.g-1) as compared to FGP (910.32 m2.g-1). Ghosh et al [20]. The distribution of Pt on the carbon black shows large agglomeration of Pt particles at many places compared to Pd and/or Au particles on functionalized graphene support. Moreover, it can be seen that the Pd and PdAu alloy nanoparticles are comparatively well dispersed on FGP as compared to Au nanoparticles.
Fig 2. TEM surface morphology of (a) Pd/FGP; (b) Au/FGP; (c) PdAu/FGP; and (d) Pt/C.
3.3. Oxygen reduction reaction (ORR) analyses of Pd/FGP, and (b) PdAu/FGP Figure 3 shows the oxygen reduction reaction (ORR) analyses of the synthesized electrocatalysts (Pd/FGP, Au/FGP, and PdAu/FGP) and results were compared with commercial Pt/C electrocatalyst. The LSV was conducted in the potential range 1.1 V to 0.1 V at a scan rate 5 mV.s-1 and at 1600 r.p.m. The initial region is the kinetic control region and the oxygen reduction current density is not affected by the rate of mass transfer. The mixed control region can be seen in the plots (0.746-0.897 V for Pt/C; 0.754-0.933 V for Pd/FGP; 0.633-0.866 V for Au/FGP; and 0.751-0.921 V for PdAu/FGP), where the reduction current depends upon the rare of mass transport and kinetic of electron transport. The diffusion control region showing defined current plateau towards lower potential from beyond 0.746 V for Pt/C, 0.754 V for Pd/FGP, 0.633 V for Au/FGP, and 0.751 V for PdAu/FGP. Rao et al [26] and Trongchuankil et al [49]. The ORR of PdAu/FGP was started gradually at a potential around 1.1 V and reached a limiting plateau at a potential around 0.75 V. The mixed control region can be seen in the voltammograms between 0.75 and 0.92, the diffusion control is started beyond 0.75 V towards lower potential. The ORR diffusion current density at around 0.5 V for PtAu/FGP electrocatalyst is observed around 5.8 mA.cm-2, which is marginally higher
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that Pd/FGP current density (5.5 mA.cm-2) and significantly higher that Au/FGP (4.6 mA.cm-2) and Pt/C ( 5.3 mA.cm-2) electrocatalysts (fig.3). It may be noted that the diffusion controlled region is slightly deviated from the standard trend due to solution impurity and noise in the instrument. Leon et al [50]. It can be seen that though the 50% Pd was replaced by Au but the ORR performance of the PdAu/FGP is almost maintained. The similar electrocatalytic activity of the Pd and/or PdAu can also be explained based on the synergetic effect of the Pd and Au in ORR. It can be seen that the Au/FGP electrocatalyst is less active for ORR as compared to Pd/FGP, PdAu/FGP, and Pt/C electrocatalyst, because Au is not capable of providing adsorptyion sites for the formation of Au-OH. Teliska et al [51]. Moreover, the Au in PtAu electrocatalyst can facilitate the breaking of Pd-OH thus allowing Pd to act efficiently in the ORR electrocatalyst activity. Therefore, in comparision with Pd/FGP, Au/FGP, and Pt/C, the higher ORR activity was observed for the PdAu/FGP electrocatalyst (fig.3). Ma et al [52].
Fig.3. LSV for ORR of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP at a scan rate of 5 mV.s-1 using RDE at 1600 rpm (O2 saturated 0.5 M H2SO4).
3.4. Hydrogen adsorption/desorption (HAD) analyses of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP
Fig.4. Cyclic voltammetry for HAD of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP at a scan rate of 50 mV.s-1 (H2 saturated 0.5 M H2SO4).
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The cyclic voltammetry analyses of the synthesized electrocatalysts (Pd/FGP, Au/FGP, and PdAu/FGP) are shown in fig.4 and compared with the commercial Pt/C electrocatalyst. Hydrogen adsorption/desorption can clearly be seen in the fig.4 for Pt/C, Pd/FGP, and PdAu/FGP in the potential range of 0.04 V to 0.4 V. Chaparro et al [46]. The high electrochemical surface area is favourable for the PEMFC hydrogen oxidation and oxygen reduction reaction. Therefore, the charge associated with the peak of hydrogen adsorption on Pt or Pd and/or Au was found out by integrating the hydrogen adsorption peak of cyclic voltamograms for the determining the electrochemical surface area (ESA) as per the procedure explained in the section 2.3.1.2. The ESA for hydrogen adsorption of Pt/C and Pd/FGP were determined as 60.32 m2.g-1pt and 43.4 m2.g-1pt, respectively. The ESA of Pd/FGP was found to be higher than 45.8 % as compared to commercial Pt/C. This can be attributed to the high surface area and very good electrical conductivity of the functionalized graphene (FGP) support as compared to carbon black support. The CV plots display characteristics of a polycrystalline Au electrode, showing the formation of Au surface oxide in the anodic region and a cathodic peak around 1.1 V corresponding to the subsequent reduction of Au oxides. However, no significant hydrogen adsorption/desorption peaks were observed for the Au/FGP electrocatalyst. Selthil et al [53] and Khosravi et al [54]. However, for PdAu/FGP electrocatalyst, the characteristic peak of Au is significantly reduced and the characteristic peaks of Pd are clearly observed (fig.4). Therefore, it can be suggested that the surface of the bimetallic nanoparticles has a Pd-rich composition and Au is preferentially distributed. The ESA of PdAu/FGP electrocatalyst was calculated (51.27 m2.g-1pt) for hydrogen adsorption on Pd polycrystalline, assuming that hydrogen is adsorbed on the Pd surface not on Au. The ESA of the synthesized electrocatalyst was in increasing order of Pd/FGP (60.32 m2.g-1pt) > PdAu/FGP (51.27 m2.g-1pt) > commercial Pt/C (43.4 m2.g-1pt). It can be seen that though 50% of the Pd loading was replaced by Au in PdAu/FGP electrocatalyst, the ESA was found to be higher than Pt/C electrocatalyst. Therefore, it can be used for the evaluation in real PEMFC environment for the better understanding of the electrocatalyst activity, which is discussed in the subsequent section. 3.5. Fuel Cell Performance evaluation using synthesized electrocatalysts
Fig.5. PEMFC performance of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP (filled symbol for current density and cell voltage; hollow symbol for current density and power density).
Figure 5 shows the single cell PMFC performance evaluation of the synthesized Pd/FGP, Au/FGP, and PdAu/FGP electrocatalyst at both anode and cathode electrodes and compared with commercial Pt/C. It can be seen that the PEMFC performance of Pd/FGP and PdAu/FGP is better than the Pt/C and Au/FGP. Au/FGP is not suitable for the hydrogen adsorption on Au polycrystalline surface as discussed in the section 3.4. Therefore, Au/FGP is giving very
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less power output as compared to Pt/C electrocatalyst. However, the maximum power densities of the Pt/C, Pd/FGP, and PdAu/FGP were obtained as 373, 439, and 414 mW.cm-2, respectively. Furthermore, the probable reason for the better performance of Pd/FGP and PdAu/FGP as compared to Pt/C can be understand by the results of ORR and HAD analysis discussed earlier. The Pt mass specific activity was determined as 0.62, 0.73, and 1.38 W.mg-1 for Pt/C, Pd/FGP, and PdAu/FGP, respectively. It can be seen that the Pt mass specific activity of PdAu/FGP electrocatalyst at the maximum power density is around 1.89 times higher than Pd/FGP and around 2.2 times higher than commercial Pt/C. This may be attributed to the improvement of the utilizations of Pd-Au metal nanostructure. The interaction between the Pt and Au may also contribute to its high performance. Moreover, the electrochemical stability of PdAu/FGP was conducted using ORR and results compared with commercial Pt/C, which are discussed in the subsequent section. 3.6. Electrochemical stability analyses of different electrocatalyst An electrochemical stability test of the bimetallic PdAu/FGP and Pt/C electrocalatyst towards ORR was conducted by continuously potential cycling between 1 V to 0.1 V at a scan rate of 5 mV.s-1 and in an O2 saturated 0.5 M H2SO4 at 1600 rpm. The Linear sweep voltammetry of PdAu/FGP and Pt/C was conducted upto 5000 cycles and the results are reported for 1st, 2500th, and 5000th cycles (fig.6). Table 1 shows that the comparison of decreasing of the mass specific activity at 0.6 V with respect to 1st scan. At a given potential of 0.6 V, the ORR mass activity on PdAu/FGP decreases 0.93% and 5.86% for 2500th scan and 5000th scan, respectively. Whereas, for the Pt/C the mass specific activity is decreases 2.79% and 13.66% after 2500th and 5000th cycles, respectively. It can be seen (table 1) that the electrocatalyst performances of the PdAu/FGP and Pt/C are more or less similar for 2500th scan cycle. However, for a long run (after 5000th cycle) the PdAu/FGP outperformed as compared to Pt/C. It is evident that the interaction between metal (Pd and/or Au) and support (FGP) was improved due to the presence of functional groups on FGP. Moreover, metal nanoparticles inherently show a strong tendency to agglomerate due to their high specific surface energy. The bimetallic electrocatalysts (PdAu/FGP) prevents metal aggregation due to their lower specific surface energy. In turn, the lower specific surface energy of bimetallic electrocatalysts also prohibits the catalysts from aggregation to form larger nanoparticles during the electrochemical measurements, thus leading to an enhanced durability. Zhang et al [35].
Fig.6. LSVs of (a) bimetallic PtAu/FGP catalysts; and (b) Pt/C in 0.5 M H2SO4 saturated with O2 using RDE (5 mV. s-1 and 1600 rpm).
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Table 1. Comparison in Mass Activity of the PdAu/FGP and Pt/C during the stability test. 1st scan Mass activity at 0.6 V
2500th Scan
5000th scan
PdAu/FGP
Pt/C
PdAu/FGP
Pt/C
PdAu/FGP
113.12
107.5
112.08
104.58
106.86
94.58
Pt/C
0.93%
2.79%
5.86%
13.66%
(mA. mg-1 of metal) % decrease in mass activity with respect to 1st scan
4. Conclusions Functionalized graphene (FGP) supported bimetallic (PdAu) electrocatalyst was developed using NaBH4 reduction techniques successfully. The average crystallite sizes of Pd and/or Au particle for Pd/FGP, Au/FGP, and PdAu/FGP electrocatalysts were found to be 4.1, 5.4 and 4.4 nm, respectively. The TEM morphology analyses confirmed the better dispersion of metal nanoparticles (Pd and/or Au) over FGP support as compared Pt nanoparticles on carbon black support (fig.2). The better electrochemical activity was observed for the developed Pd/FGP and PdAu/FGP electriocatalysts as compared to commercial Pt/C electrocatalyst. The ORR diffusion current density (at around 0.5 V) of PtAu/FGP electrocatalyst was found to be around 10% higher than to that of commercial Pt/C. The cyclic voltammetry ESA of the synthesized electrocatalyst was in increasing order of Pd/FGP (60.32 m2.g-1pt) > PdAu/FGP (51.27 m2.g-1pt) > commercial Pt/C (43.4 m2.g-1pt). It can be seen that though 50% of the Pd loading was replaced by Au in PdAu/FGP electrocatalyst, the ESA was found to be higher than Pt/C electrocatalyst. The single cell PEMFC maximum power density of the Pt/C, Pd/FGP, and PdAu/FGP electrocatalysts were obtained as 373, 439, and 414 mW.cm-2, respectively. It can be seen that the PEMFC maximum power density increased around 11% and 6 % for PdAu/FGP and Pd/FGP, respectively as compared to commercial Pt/C. The Pt mass specific activity was determined as 0.62, 0.73, and 1.38 W.mg-1 for Pt/C, Pd/FGP, and PdAu/FGP, respectively. It can be seen that the Pt mass specific activity of PdAu/FGP electrocatalyst at the maximum power density is around 1.89 times higher than Pd/FGP and around 2.2 times higher than commercial Pt/C. The developed bimetallic electrocatalyst (PdAu/FGP) shows higher stability under stimulated electrochemical environment (upto 5000th cycle in ORR fig.6.). The percentage degradation of mass specific activity for PdAu/FGP was more than two times lower than commercial Pt/C for ORR (Table1). It can be attributed that PdAu/FGP can be an alternative low cost electrocatalyst as compared to the commercial Pt/C electrocatalys. References [1] M. Kani et al. J. Chen, R. Wang, J. Energy. Chem. 27(4) (2018) 1124-1139. [2] C.-W. Liu, H.-S. Chen, C.-M. Lai, J.-N. Lin, L.-D. Tsai, K.-W. Wang, ACS Appl. Mater. Interfaces 6 (2014) 1589−1594. [3] M. Wanga, X. Qin, K. Jiang, Y. Dong, M. Shao, W.-B. Cai, J. Phys. Chem. C 121 (2017) 3416-3423. [4] X. Li, L. An, X. Chen, N. Zhang, D. Xia, W. Huang,W. Chu, Z. Wu, Sci. Rep. 3 (2013) 1-5. [5] H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Appl. Catal., B 56 (2005) 9-35. [6] B. Lim, M. J. Jiang, P. H. C. Camargo, E. C. Cho, J. Tao, X. M. Lu,Y. M. Zhu and Y. N. Xia, Science 324 (2009) 1302-1305. [7] J. K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard and H. Jonsson, J. Phys. Chem. B 108 (2004) 17886-17892. [8] S. Zhang, Y. Shao, G. Yin, Y. Lin, J. Mater. Chem. A. 1 (2013) 4631-4641. [9] Y.-H. Cho, B. Choi, Y.-H. Cho, H.-S. Park, Y.-E. Sung, Electrochem. Commun. 9 (2007) 378–381 [10] E. Antolini, S. C. Zignani, S. F. Santos, E. R. Gonzalez, Electrochim. Acta 56 (2011) 2299–2305 [11] E. Antolini, Energy Environ. Sci. 2 (2009) 915-931. [12] J.-H.Wee, K.-Y. Lee, S.H. Kim, J. Power Sources 165 (2007) 667-677. [13] K. H. Kangasniemi, D. A. Condt, T. D. Jarvi, J. Electrochem. Soc. 151 (2004) E125-132. [14] M. X. Wang, F. Xu, H. F. Sun, Q. Liu, K. Artyushkova, E. A. Stach, J. Xie, Electrochim. Acta 56 (2011) 2566-1573. [15] A. A. Kulikovsky, J. Electrochem. Soc. 158 (2011) B957-B962. [16] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 306 (2004) 666-669. [17] A. K. Geim, Science 324 (2009) 1530-1534. [18] J. Liu, Y. Xue, M. Zhang, L. Dai, MRS Bulletin. 37 (2012) 1265-1272. [19] M. Zhang, Z. Yan, Q. Sun, J. Xie, J. Jing, New J. Chem. 36 (2012) 2533–2540. [20] A. Ghosh, S. Basu, A.Verma, Fuel Cells 13 ( 2013) 355-363. [21] C. Shan, H. Yang, D. Han, Q. Zhang, A. Ivaska, L. Niu, Langmuir 25 (2009 12030-12033. [22] T. Kuila, S. Bose, A. K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Prog. Mater. Sci. 57 (2012) 1061-1105.
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ICN3I-2017
Development of Functionalized Graphene Supported Highly Durable Pt-Free Bi-metallic Electrocatalysts for PEMFC Avijit Ghosha,*, Biswajit Mandalb, Sunil Baran Kuilab a
Department of Chemical Engineering, Heritage Institute of Technology, Kolkata, 700107, India b Department of Chemical Engineering, Haldia Institute of Technology, Kolkata, 721657, India
Abstract
Functionalized graphene supported Pt-free single metallic (Pd/FGP or Au/FGP) and bimetallic (PdAu/FGP) electrocatalyst were synthesized using NaBH4 reduction techniques. The electro-catalytic activity of developed Pd/FGP, Au/FGP, and PdAu/FGP were investigated x-ray powder diffraction (XRD), transmission electron microscopy (TEM), oxygen reduction reaction (ORR), and cyclic voltammetry (CV). The performance of the developed bimetallic electrocatalyst was compared with the commercial Pt/C. The oxygen reduction of PdAu/FGP was started gradually at a potential around 1.0 V and reached a limiting plateau at a potential around 0.75 V. Though the 50% Pd was replaced by Au but the ORR performance of the PdAu/FGP is almost maintained. The similar electro-catalytic activity of the Pd and/or PdAu can also be explained based on the synergetic effect of the Au and Pd in ORR. The active electrochemical surface area (ESA) is measured using CV analysis. The ESA for the Pd/FGP, PdAu/FGP, and Pt/C were found to be around 63.32, 51.27, and 43.4 m2·g-1Pt, respectively. The single cell polymer electrolyte membrane fuel cell (PEMFC) power density of the Pd/FGP, PdAu/FGP, and Pt/C were found to be 439, 414, 373 mW.cm-2, respectively. The developed bimetallic (PdAu/FGP) electrocatalys shows significantly higher stability under ORR test as compared to commercial Pt/C. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords:PEMFC; Bimetallic Electrocatalyst; Oxygen Reduction Reaction
* Corresponding author. Tel.: +91- 33 6627 0600 (Extn. 783); fax: +91- 33 2443 0455. E-mail address:[email protected], [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).
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1. Introduction Polymer electrolyte membrane fuel cell (PEMFC) is one of the challenging energy conversion devices for transportation and distributed power generation systems due to its attractive features such as high power density, low operating temperature, minimal emissions, negligible noise, and high efficiency. Kani et al [1] and Liu et al [2]. The electrocatalyst plays a vital role in the performance of PEMFC. Therefore, developing high-performing, cost effective low cost and/or Pt-free electrocatalyst is key to successful fuel cell commercialization. Wanga et al [3] and Li et al [4]. Platinum (Pt) is the state-of-the-art electrocatalyst for PEMFC to catalyze the anodic oxidation of hydrogen and the cathodic reduction of oxygen. Gasteiger at al [5], Lim et al [6], Norskov at al [7]. However, bulk platinum (noble metal) does not yield good performance of the PEMFC due to low surface area per unit weight of platinum. Moreover, due to the limited natural resource, the cost of Pt noble metal is gradually increasing with the time. Zhang et al [8]. Many efforts have been used to reduce the amount of platinum used in fuel cells. Generally, there are two ways to reduce the use of Pt in PEMFCs, which are (1) Pt electrodes with low Pt content and (2) total or partial substitution of Pt with other metals. Cho et al [9], Antolini at al [10], antolini at al [11]. In the former case, the use of Pt can be reduced by dispersing on an active support material. Wee at al [12]. The most commonly used carbon black (C) as an electrocatalyst support suffers for low surface area, low electrical conductivity, less durability, and start corrosion at higher potential (>0.8 V versus standard hydrogen electrode). Kangasniemi at al [13], Wang et al [15], Kulikoysky at al [15]. Owing to above started problem, graphene (GP) and functionalized graphene (FGP) is being exploited by many researchers and scientists as a promising candidate for the fuel cell electrocatalyst support due to their extraordinary salient properties. Novoselov et al [16], Geim at al [17], Liu at al [18], Zhang at al [19], Ghosh at al [20], Shan al al [21], Kuila at al [22], Antolini at al [23]. On the other hand, the partial or total substitution of platinum with palladium seems a promising way to reduce Pt content as Pd have very similar properties. Altolini et al [24]. Moreover, the cost of palladium (Pd) is about three times lower than that of platinum and it is also fifty times abundant on the earth than Pt. Moreover, Pd shows similar electro-catalytic activity for both the hydrogen oxidation and the oxygen reduction reaction (ORR). Maximov et al [25] and Rao at el. [26]. Recently, the supported Pt and/or Pd alloy electrocatalysts were found to be more electrochemically active due to their enhanced catalytic properties relative to individual carbon supported Pt or Pd electrocalatyst. Yucong et al [27], Adnan et al [28], Tan et al [29], Unni et al [30], Zhang et al [31], Rao et al [32], Lobato et al [33], Dutta et al [34], Zhang et al [35]. Generally metal nanoparticle gets agglomerated due to their surface energy. Xie et al [36] and Guilminot et al [37]. As a result the electrochemical durability (hydrogen oxidation and ORR) decreases of anode and cathode catalyst over time. This can be improved by alloying of Pt or Pd with other transition metals. Wang et al [38]. In addition, the supported (on carbon black or FGP) bimetallic electrocatalyst enhance the CO tolerance as compared to commercial Pt/C electrocatalyst, which is one of the most important desired properties of PEMFC. Gold (Au) nanomaterial is a special metal in view of its inertness nature and its high catalytic activity for oxygen reduction reaction. Carbon supported Au-based nanostructure materials have been extensively evaluated for the electrocatalyst oxidation and reduction. Park et al [39]. The enhanced ORR activity was observed for the carbon supported bimetallic (PtAu) electrocatalyst. This can be attributed to the modification of Pt nanoparticles by Au cluster, which lead to an increase in Pt oxidation potential and thus to significant enhancement in catalytic durability for ORR. Zhang et al [40]. Lee at al. [41] reported that the functionalized graphene supported Pt and/or Pt-alloy electrocatalyst show superior electro-catalytic activities as compared to Pt/C and Pt/MWCNT for PEMFC applications. However, from the practical point of view, the fuel cell test is the ultimate evaluation criterion for the supported bimetallic electrocatalyst. For example, Seger and Kamat reported a single-cell PEMFC performance using Pt/GP or Pt electrocatalyst at cathode whereas at anode commercial Pt/C was used. They have reported that the Pt/GP as a cathode electrocatalyst showed a maximum power density of 0.161 W·cm-2 as compared to 0.091 W·cm-2 for an unsupported Pt electrocatalyst. Lee et al [42]. The similar kind of support effect was reported, where functionalize graphene supported electrocatalyst was evaluated only as the cathode electrocatalyst for PEMFC. Jafri et al [43] and Ha et al [44]. It is found that the research work is being conducted on functionalized graphene but the work on FGP supported bimetallic electrocatalyst for fuel cell is feeble. Moreover, the potential of the FGP in
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PEMFC is still under exploited. There is hardly any report with FGP supported PdAu bimetallic electrocalatyst replacing the both anode and cathode catalysts of PEMFC. This paper reports the synthesis of Pd/FGP, Au/FGP and bimetallic PdAu/FGP electrocatalysts by NaBH4 reduction techniques. The synthesis and characterization of FGP is reported elsewhere. Ghosh et al [20]. The synthesized electrocatalysts were characterized thoroughly and evaluated in single cell PEMFC using both anode and cathode catalyst. Moreover, the performances of the as synthesized electrocatalyst were compared with the commercial Pt/C. The electrochemical stability test was performed for ORR using PdAu/FGP and compared with commercial Pt/C. 2. Experimental 2.1. Material Industrial grade natural graphite (NG) was procuredlocally. NaOH, NaBH4, dimethylformamide (DMF), and citricacid (CA) were procured from Merck, India. PdCl2(38-40wt.%) and aqueous solution of HAuCl4 (1 mM) were purchased from Spectrochem and Merck, India, respectively. NaOH and NaBH4, were procured from Merck, India. Nafion solution 5wt.% and Nafion-117 membrane was procured from DuPont, USA. Toray carbon paper (TGP-H120) and 20 wt.% Pt/C (Vulcan XC-72) were purchased from ElectroChem, USA. The purity of gases usedwas 99.99% and Millipore water was used whereverrequired. The in-house synthesized FGP is used as an electrocatalyst support. The details synthesis and characterization of FGP is described elsewhere. Ghosh et al [20]. 2.2. Methods 2.2.1
Synthesis of Pd/FGP, Au/FGP, and PdAu/FGPelectrocatalysts
The Pd/FGP (20wt.%) is prepared using precipitation method via NaBH4 reduction method using FGP, 20wt.% Palladium chloride (II) (PdCl2), and H2O. Initially, as-synthesized FGP was dispersed in ultra-pure millipore water by ultasonication for 1 h to make FGP slurry. Then, the Pd metal precursor was separately dissolved in water using ultrasonication. The metal precursor was added drop-wise to the FGP slurry under continuous sonication. The pH of the mixture was adjusted at around 12 using 1 M NaOH solution. The 1 M NaBH4 was added into the above mixture as a reducing agent under continuous stirring for 5 h and slow heating upto 90°C. The resulting mixture was treated with N2 for 30 min. Then the treated solution was filtered, washed copiously with water followed by ethanol, and dried in vacuum oven at 80°C for overnight to get Pd/FGP (20wt.%) electrocatalyst. The similar process was followed to synthesize Au/FGP and PdAu/FGP using aqueous HAuCl4 as metal precursor by similar NaBH4 reduction techniques as discussed earlier. The 20wt.% metal (Pd and/or Au) loading was kept for all the electrocatalysts. In case of PdAu electrocatalyst, the metal loading was calculated 1:1 (Pt:Au) ratios. 2.3. Characterization 2.3.1
Characterization of Pd/FGP, Au/FGP, and PdAu/FGP electrocatalyst
Various physico-chemical characterization techniques were used to characterize the developed FGP supported metal (Pd and/or Au) electrocatalyst. The surface morphologies were studied using TEM (Transmission electron microscopy, JEOL, JEM 2100, Japan). The XRD (X-ray diffractometer, Bruker, USA) measurements were performed using CuKa source at a scan rate of 0.05°s-1. Ex-situ electrochemical performance of the developed electrocatalysts were evaluated using rotating disc electrode (RDE) for oxygen reduction reaction (ORR) and cyclic voltammetry (CV) for hydrogen adsorption/desorption (HAD) using three electrode assembly (CHI 600E, USA). All the potentials were referenced to the standard hydrogen electrode (SHE).
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2.3.1.1 The oxygen reduction reaction analyses using linear sweep voltammetry The oxygen reduction reaction (ORR) was carried out using rotating disc electrode (RDE) by linear sweep voltammetry (LSV). The LSV was conducted using three electrodes RDE cell assembly connected to a potentiostat/galvanostat. The Ag/AgCl and Pt were used as reference and counter electrodes, respectively. The glassy carbon disc electrode (3 mm diameter) was used as working electrode. The pretreatment of working electrode was done by polishing the electrode surface using 0.05 µm alumina paste and then washing with ethanol and ultrapure water under sonication. The electrocatalyst slurry was prepared by mixing around 5 mg of Pt/C (commercial) or Pd/FGP or Au/FGP or PtAu/FGP powder with 5wt.% nafion-isopropyl alcohol (1:4) solution using sonication for 30 min. The electrocatalyst slurry was then drop cast on the surface of the pretreated glassy carbon electrode and dried to obtain the working electrode. An aqueous solution of 0.5M H2SO4, saturated with O2, was used as electrolyte. The LSV analysis was conducted using RDE at a scan rate of 5 mV·s-1 at a constant rotation speed of 1600 rpm. 2.3.1.2 Hydrogen adsorption/desorption analyses using cyclic voltammetry The electrochemical hydrogen adsorption/desorption (HAD) activity of the developed electrocatalysts were conducted by cyclic voltammetry (CV) technique at room temperature. The conventional three electrode cell assembly was used for the HAD study. The Ag/AgCl and Pt electrodes were used as reference and counter electrodes, respectively. The working electrode was prepared by the developed electrocatalyst with metal (Pd and/or Au) loading of 0.4 mg·cm-2. The working electrode was prepared using toray carbon paper (TGP-H-120) and catalyst ink. The catalyst ink was prepared by mixing of required amount of catalyst, 5wt.% nafion solution, and isopropyl alcohol under continuous sonication for 15 min. Then the dispersed catalyst ink was sprayed on the Toray carbon paper. Subsequently, the catalyst coated carbon paper was dried in hot air oven for overnight. An aqueous solution of 0.5 M H2SO4, saturated with N2, was used as an electrolyte for CV analysis at a scan rate of 50 mV·s-1. Electrochemical surface area (ESA), one of the most important parameters for evaluating the activity of electrocatalysts, was measured from the CV plots. The active ESA is determined as per the procedure reported elsewhere Ghosh et al [20]. The charge associated with the hydrogen adsorption on Pt, which can be found out by integrating the hydrogen adsorption peak in the potential range of 0.04-0.4 V (SHE) for evaluating the electrochemical surface area. Chaparro et al [46]. 2.3.2
PEMFC performance evaluation
A PEMFC experimental setup was developed in the laboratory to study the performance of developed electrocatalyst in the real environment of the fuel cell. The membrane electrode assembly (MEA) was developed using Toray carbon paper (TGP-H-120) coated with catalyst layer and nafion 115 membrane using hot press technique at 110 oC under the pressure of 45 kg.cm-2 for 3 min. The prepared MEA was installed inside the in-house fabricated single cell PEMFC set-up, having active area of around 6.25 cm2. The catalyst loading on anode and cathode were kept as 0.2 mg·cm-2 and 0.4 mg·cm-2, respectively. The hydrogen and oxygen flow rates were maintained at 0.5 and 1.0 lpm, respectively. The output current and voltage were measure with the help of two digital multimeters. A rheostat was used as a variable load. I-V performance of the PEMFC was recorded at around 70 oC. All the parameters were kept same for analyzing different developed electrocatalysts.
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3. Result and discussions 3.1. XRD analyses of Pd/FGP, Au/FGP, and PdAu/FGP The XRD diffraction patterns of synthesized Pd/FGP, Au/FGP, and PdAu/FGP are shown in the fig. 1, and compared with commercial Pt/C diffraction pattern. The broad diffraction peak at 2θ value of around 26.5o is associated with the graphitized carbon C(002) for all the catalysts. The synthesized Pd/FGP electrocatalyst shows face center cubic (fcc) crystalline Pd diffraction of (111), (200), and (220) at 2θ of 39.95, 46.05, and 68.01, respectively (JCPDS 65-2867) [3]. The peak at 2θ of 38.11, 44.51, and 66.31 corresponds to Au (111), (200), and (220) crystal planes, respectively (JCPDS 65-2870). The diffraction peaks of PdAu/graphene were located between the expected positions of pure Pd (no. 65-2867) and Au (no. 65-2870). This XRD phenomenon has also been observed in other bimetallic alloy nanostructures. Huang et al [47] and Lu et al [48]. The shift of each peak of PdAu/FGP electrocatalyst to lower 2θ value compared to Pd/FGP and to higher 2θ value compared to Au/FGP. It indicates that a single phase bimetallic electrocatalyst is formed on the FGP support due to the incorporation of Au atoms into the Pd lattice, which leads to the formation of PdAu alloy. Wang et al [3]. The separate diffraction peaks of Pd and Au were not observed in the XRD diffraction of PdAu/FGP due to the similar crystalline structure of Pd and Au. The Pd, Au, and PdAu average crystallite sizes were calculated using broad diffraction peak of (111) by Scherrer’s equation. Ghosh et al [20]. The average crystallite sizes of Pd and Au, and PdAu particle for Pd/FGP, Au/FGP, and PdAu/FGP were about 4.1, 5.4 and 4.4 nm, respectively, which can be further supported by TEM analyses. Moreover, the XRD crystallite size is a mass average, which is inherently larger than the TEM average counted by number.
Fig. 1. XRD patterns of Pd/FGP, Au/FGP, and PdAu/FGP
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3.2. TEM analyses of Pd/FGP, Au/FGP, and PdAu/FGP The TEM images of the synthesized electrocatalysts are shown in the fig. 2 and compared with micrograph of commercial Pt/C electrocatalyst. The fig. 2 (a, b. and c) shows that the Pd and/or Au nanoparticles are dispersed on the funtionalized graphene (FGP) support material. It may be noted that the carbon and FGP support were having same loading (20 wt.%) of metal electrocatalyst (Pt or Pd and/or Au). The hogh surface area support material is favourable for better dispersion of metal nanoparticles.The TEM images of Pd/FGP, Au/FGP, and PdAu/FGP shows that the Pd and/or PdAu nanoparticles are well distributed as compared to Pt/C. It may be due to the very less surface area of carbon (249.46 m2.g-1) as compared to FGP (910.32 m2.g-1). Ghosh et al [20]. The distribution of Pt on the carbon black shows large agglomeration of Pt particles at many places compared to Pd and/or Au particles on functionalized graphene support. Moreover, it can be seen that the Pd and PdAu alloy nanoparticles are comparatively well dispersed on FGP as compared to Au nanoparticles.
Fig 2. TEM surface morphology of (a) Pd/FGP; (b) Au/FGP; (c) PdAu/FGP; and (d) Pt/C.
3.3. Oxygen reduction reaction (ORR) analyses of Pd/FGP, and (b) PdAu/FGP Figure 3 shows the oxygen reduction reaction (ORR) analyses of the synthesized electrocatalysts (Pd/FGP, Au/FGP, and PdAu/FGP) and results were compared with commercial Pt/C electrocatalyst. The LSV was conducted in the potential range 1.1 V to 0.1 V at a scan rate 5 mV.s-1 and at 1600 r.p.m. The initial region is the kinetic control region and the oxygen reduction current density is not affected by the rate of mass transfer. The mixed control region can be seen in the plots (0.746-0.897 V for Pt/C; 0.754-0.933 V for Pd/FGP; 0.633-0.866 V for Au/FGP; and 0.751-0.921 V for PdAu/FGP), where the reduction current depends upon the rare of mass transport and kinetic of electron transport. The diffusion control region showing defined current plateau towards lower potential from beyond 0.746 V for Pt/C, 0.754 V for Pd/FGP, 0.633 V for Au/FGP, and 0.751 V for PdAu/FGP. Rao et al [26] and Trongchuankil et al [49]. The ORR of PdAu/FGP was started gradually at a potential around 1.1 V and reached a limiting plateau at a potential around 0.75 V. The mixed control region can be seen in the voltammograms between 0.75 and 0.92, the diffusion control is started beyond 0.75 V towards lower potential. The ORR diffusion current density at around 0.5 V for PtAu/FGP electrocatalyst is observed around 5.8 mA.cm-2, which is marginally higher
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that Pd/FGP current density (5.5 mA.cm-2) and significantly higher that Au/FGP (4.6 mA.cm-2) and Pt/C ( 5.3 mA.cm-2) electrocatalysts (fig.3). It may be noted that the diffusion controlled region is slightly deviated from the standard trend due to solution impurity and noise in the instrument. Leon et al [50]. It can be seen that though the 50% Pd was replaced by Au but the ORR performance of the PdAu/FGP is almost maintained. The similar electrocatalytic activity of the Pd and/or PdAu can also be explained based on the synergetic effect of the Pd and Au in ORR. It can be seen that the Au/FGP electrocatalyst is less active for ORR as compared to Pd/FGP, PdAu/FGP, and Pt/C electrocatalyst, because Au is not capable of providing adsorptyion sites for the formation of Au-OH. Teliska et al [51]. Moreover, the Au in PtAu electrocatalyst can facilitate the breaking of Pd-OH thus allowing Pd to act efficiently in the ORR electrocatalyst activity. Therefore, in comparision with Pd/FGP, Au/FGP, and Pt/C, the higher ORR activity was observed for the PdAu/FGP electrocatalyst (fig.3). Ma et al [52].
Fig.3. LSV for ORR of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP at a scan rate of 5 mV.s-1 using RDE at 1600 rpm (O2 saturated 0.5 M H2SO4).
3.4. Hydrogen adsorption/desorption (HAD) analyses of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP
Fig.4. Cyclic voltammetry for HAD of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP at a scan rate of 50 mV.s-1 (H2 saturated 0.5 M H2SO4).
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The cyclic voltammetry analyses of the synthesized electrocatalysts (Pd/FGP, Au/FGP, and PdAu/FGP) are shown in fig.4 and compared with the commercial Pt/C electrocatalyst. Hydrogen adsorption/desorption can clearly be seen in the fig.4 for Pt/C, Pd/FGP, and PdAu/FGP in the potential range of 0.04 V to 0.4 V. Chaparro et al [46]. The high electrochemical surface area is favourable for the PEMFC hydrogen oxidation and oxygen reduction reaction. Therefore, the charge associated with the peak of hydrogen adsorption on Pt or Pd and/or Au was found out by integrating the hydrogen adsorption peak of cyclic voltamograms for the determining the electrochemical surface area (ESA) as per the procedure explained in the section 2.3.1.2. The ESA for hydrogen adsorption of Pt/C and Pd/FGP were determined as 60.32 m2.g-1pt and 43.4 m2.g-1pt, respectively. The ESA of Pd/FGP was found to be higher than 45.8 % as compared to commercial Pt/C. This can be attributed to the high surface area and very good electrical conductivity of the functionalized graphene (FGP) support as compared to carbon black support. The CV plots display characteristics of a polycrystalline Au electrode, showing the formation of Au surface oxide in the anodic region and a cathodic peak around 1.1 V corresponding to the subsequent reduction of Au oxides. However, no significant hydrogen adsorption/desorption peaks were observed for the Au/FGP electrocatalyst. Selthil et al [53] and Khosravi et al [54]. However, for PdAu/FGP electrocatalyst, the characteristic peak of Au is significantly reduced and the characteristic peaks of Pd are clearly observed (fig.4). Therefore, it can be suggested that the surface of the bimetallic nanoparticles has a Pd-rich composition and Au is preferentially distributed. The ESA of PdAu/FGP electrocatalyst was calculated (51.27 m2.g-1pt) for hydrogen adsorption on Pd polycrystalline, assuming that hydrogen is adsorbed on the Pd surface not on Au. The ESA of the synthesized electrocatalyst was in increasing order of Pd/FGP (60.32 m2.g-1pt) > PdAu/FGP (51.27 m2.g-1pt) > commercial Pt/C (43.4 m2.g-1pt). It can be seen that though 50% of the Pd loading was replaced by Au in PdAu/FGP electrocatalyst, the ESA was found to be higher than Pt/C electrocatalyst. Therefore, it can be used for the evaluation in real PEMFC environment for the better understanding of the electrocatalyst activity, which is discussed in the subsequent section. 3.5. Fuel Cell Performance evaluation using synthesized electrocatalysts
Fig.5. PEMFC performance of Pt/C, Pd/FGP, Au/FGP, and PdAu/FGP (filled symbol for current density and cell voltage; hollow symbol for current density and power density).
Figure 5 shows the single cell PMFC performance evaluation of the synthesized Pd/FGP, Au/FGP, and PdAu/FGP electrocatalyst at both anode and cathode electrodes and compared with commercial Pt/C. It can be seen that the PEMFC performance of Pd/FGP and PdAu/FGP is better than the Pt/C and Au/FGP. Au/FGP is not suitable for the hydrogen adsorption on Au polycrystalline surface as discussed in the section 3.4. Therefore, Au/FGP is giving very
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less power output as compared to Pt/C electrocatalyst. However, the maximum power densities of the Pt/C, Pd/FGP, and PdAu/FGP were obtained as 373, 439, and 414 mW.cm-2, respectively. Furthermore, the probable reason for the better performance of Pd/FGP and PdAu/FGP as compared to Pt/C can be understand by the results of ORR and HAD analysis discussed earlier. The Pt mass specific activity was determined as 0.62, 0.73, and 1.38 W.mg-1 for Pt/C, Pd/FGP, and PdAu/FGP, respectively. It can be seen that the Pt mass specific activity of PdAu/FGP electrocatalyst at the maximum power density is around 1.89 times higher than Pd/FGP and around 2.2 times higher than commercial Pt/C. This may be attributed to the improvement of the utilizations of Pd-Au metal nanostructure. The interaction between the Pt and Au may also contribute to its high performance. Moreover, the electrochemical stability of PdAu/FGP was conducted using ORR and results compared with commercial Pt/C, which are discussed in the subsequent section. 3.6. Electrochemical stability analyses of different electrocatalyst An electrochemical stability test of the bimetallic PdAu/FGP and Pt/C electrocalatyst towards ORR was conducted by continuously potential cycling between 1 V to 0.1 V at a scan rate of 5 mV.s-1 and in an O2 saturated 0.5 M H2SO4 at 1600 rpm. The Linear sweep voltammetry of PdAu/FGP and Pt/C was conducted upto 5000 cycles and the results are reported for 1st, 2500th, and 5000th cycles (fig.6). Table 1 shows that the comparison of decreasing of the mass specific activity at 0.6 V with respect to 1st scan. At a given potential of 0.6 V, the ORR mass activity on PdAu/FGP decreases 0.93% and 5.86% for 2500th scan and 5000th scan, respectively. Whereas, for the Pt/C the mass specific activity is decreases 2.79% and 13.66% after 2500th and 5000th cycles, respectively. It can be seen (table 1) that the electrocatalyst performances of the PdAu/FGP and Pt/C are more or less similar for 2500th scan cycle. However, for a long run (after 5000th cycle) the PdAu/FGP outperformed as compared to Pt/C. It is evident that the interaction between metal (Pd and/or Au) and support (FGP) was improved due to the presence of functional groups on FGP. Moreover, metal nanoparticles inherently show a strong tendency to agglomerate due to their high specific surface energy. The bimetallic electrocatalysts (PdAu/FGP) prevents metal aggregation due to their lower specific surface energy. In turn, the lower specific surface energy of bimetallic electrocatalysts also prohibits the catalysts from aggregation to form larger nanoparticles during the electrochemical measurements, thus leading to an enhanced durability. Zhang et al [35].
Fig.6. LSVs of (a) bimetallic PtAu/FGP catalysts; and (b) Pt/C in 0.5 M H2SO4 saturated with O2 using RDE (5 mV. s-1 and 1600 rpm).
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Table 1. Comparison in Mass Activity of the PdAu/FGP and Pt/C during the stability test. 1st scan Mass activity at 0.6 V
2500th Scan
5000th scan
PdAu/FGP
Pt/C
PdAu/FGP
Pt/C
PdAu/FGP
113.12
107.5
112.08
104.58
106.86
94.58
Pt/C
0.93%
2.79%
5.86%
13.66%
(mA. mg-1 of metal) % decrease in mass activity with respect to 1st scan
4. Conclusions Functionalized graphene (FGP) supported bimetallic (PdAu) electrocatalyst was developed using NaBH4 reduction techniques successfully. The average crystallite sizes of Pd and/or Au particle for Pd/FGP, Au/FGP, and PdAu/FGP electrocatalysts were found to be 4.1, 5.4 and 4.4 nm, respectively. The TEM morphology analyses confirmed the better dispersion of metal nanoparticles (Pd and/or Au) over FGP support as compared Pt nanoparticles on carbon black support (fig.2). The better electrochemical activity was observed for the developed Pd/FGP and PdAu/FGP electriocatalysts as compared to commercial Pt/C electrocatalyst. The ORR diffusion current density (at around 0.5 V) of PtAu/FGP electrocatalyst was found to be around 10% higher than to that of commercial Pt/C. The cyclic voltammetry ESA of the synthesized electrocatalyst was in increasing order of Pd/FGP (60.32 m2.g-1pt) > PdAu/FGP (51.27 m2.g-1pt) > commercial Pt/C (43.4 m2.g-1pt). It can be seen that though 50% of the Pd loading was replaced by Au in PdAu/FGP electrocatalyst, the ESA was found to be higher than Pt/C electrocatalyst. The single cell PEMFC maximum power density of the Pt/C, Pd/FGP, and PdAu/FGP electrocatalysts were obtained as 373, 439, and 414 mW.cm-2, respectively. It can be seen that the PEMFC maximum power density increased around 11% and 6 % for PdAu/FGP and Pd/FGP, respectively as compared to commercial Pt/C. The Pt mass specific activity was determined as 0.62, 0.73, and 1.38 W.mg-1 for Pt/C, Pd/FGP, and PdAu/FGP, respectively. It can be seen that the Pt mass specific activity of PdAu/FGP electrocatalyst at the maximum power density is around 1.89 times higher than Pd/FGP and around 2.2 times higher than commercial Pt/C. The developed bimetallic electrocatalyst (PdAu/FGP) shows higher stability under stimulated electrochemical environment (upto 5000th cycle in ORR fig.6.). The percentage degradation of mass specific activity for PdAu/FGP was more than two times lower than commercial Pt/C for ORR (Table1). It can be attributed that PdAu/FGP can be an alternative low cost electrocatalyst as compared to the commercial Pt/C electrocatalys. References [1] M. Kani et al. J. Chen, R. Wang, J. Energy. Chem. 27(4) (2018) 1124-1139. [2] C.-W. Liu, H.-S. Chen, C.-M. Lai, J.-N. Lin, L.-D. Tsai, K.-W. Wang, ACS Appl. Mater. Interfaces 6 (2014) 1589−1594. [3] M. Wanga, X. Qin, K. Jiang, Y. Dong, M. Shao, W.-B. Cai, J. Phys. Chem. C 121 (2017) 3416-3423. [4] X. Li, L. An, X. Chen, N. Zhang, D. Xia, W. Huang,W. Chu, Z. Wu, Sci. Rep. 3 (2013) 1-5. [5] H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Appl. Catal., B 56 (2005) 9-35. [6] B. Lim, M. J. Jiang, P. H. C. Camargo, E. C. Cho, J. Tao, X. M. Lu,Y. M. Zhu and Y. N. Xia, Science 324 (2009) 1302-1305. [7] J. K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard and H. Jonsson, J. Phys. Chem. B 108 (2004) 17886-17892. [8] S. Zhang, Y. Shao, G. Yin, Y. Lin, J. Mater. Chem. A. 1 (2013) 4631-4641. [9] Y.-H. Cho, B. Choi, Y.-H. Cho, H.-S. Park, Y.-E. Sung, Electrochem. Commun. 9 (2007) 378–381 [10] E. Antolini, S. C. Zignani, S. F. Santos, E. R. Gonzalez, Electrochim. Acta 56 (2011) 2299–2305 [11] E. Antolini, Energy Environ. Sci. 2 (2009) 915-931. [12] J.-H.Wee, K.-Y. Lee, S.H. Kim, J. Power Sources 165 (2007) 667-677. [13] K. H. Kangasniemi, D. A. Condt, T. D. Jarvi, J. Electrochem. Soc. 151 (2004) E125-132. [14] M. X. Wang, F. Xu, H. F. Sun, Q. Liu, K. Artyushkova, E. A. Stach, J. Xie, Electrochim. Acta 56 (2011) 2566-1573. [15] A. A. Kulikovsky, J. Electrochem. Soc. 158 (2011) B957-B962. [16] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 306 (2004) 666-669. [17] A. K. Geim, Science 324 (2009) 1530-1534. [18] J. Liu, Y. Xue, M. Zhang, L. Dai, MRS Bulletin. 37 (2012) 1265-1272. [19] M. Zhang, Z. Yan, Q. Sun, J. Xie, J. Jing, New J. Chem. 36 (2012) 2533–2540. [20] A. Ghosh, S. Basu, A.Verma, Fuel Cells 13 ( 2013) 355-363. [21] C. Shan, H. Yang, D. Han, Q. Zhang, A. Ivaska, L. Niu, Langmuir 25 (2009 12030-12033. [22] T. Kuila, S. Bose, A. K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Prog. Mater. Sci. 57 (2012) 1061-1105.
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