Article pubs.acs.org/JPCC
Enhanced Electrocatalytic Performance of Pt3Pd1 Alloys Supported on CeO2/C for Methanol Oxidation and Oxygen Reduction Reactions Ammar Bin Yousaf,†,‡ M. Imran,† Nestor Uwitonze,† Akif Zeb,† Syed Javaid Zaidi,§ Tariq Mahmood Ansari,∥ Ghazala Yasmeen,*,∥ and Suryyia Manzoor∥ †
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China ‡ Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan § Center for Advanced Materials, Qatar University, Doha 2713, Qatar ∥ Institute of Chemical Sciences, Bahauddin Zakaria University, Multan 60800, Pakistan S Supporting Information *
ABSTRACT: Direct methanol fuel cell (DMFC) with noble metals based anode and cathode is a promising energy generator to portable power devices. However, the deterioration of catalyst performance suffered by CO poisoning, crossover of fuel from anode to cathode, and higher economical cost of such devices hinder their commercialization. Herein, all of the above issues have been neutralized and crossed the huge hump of faced challenges. Highly efficient, durable, and surfactant-free catalyst with ultralow Pt3Pd1 loadings supported on CeO2/C was synthesized. The ex-situ and in situ spectroelectrochemical techniques such as, CV, in situ FTIR, and online DEMS studies confirm the highly efficient activity of catalyst toward electro-oxidation of methanol. In addition, the critical and detailed analysis of RDE results prove the superiority of the present material for electro-reduction of oxygen along a cathode side. The as-synthesized catalyst has proven itself as a better substitute for commercial Pt/C catalyst, with enhanced and durable performance as anode and cathode material for DMFCs. The obtained remarkable performance of catalysts can be attributed to the accumulative effects of PtPd bimetallic NPs and the enhanced synergistic factors of CeO2 in a hybrid material.
1. INTRODUCTION
efforts have focused on the development of bimetallic structures in combination with Pt.7−12 Among various candidates, Pd has been identified as an auspicious choice to form bimetallic structures with Pt because they share the same crystal structure and almost identical lattice constant. These features are beneficial for developing bimetals with single crystallinity, which can exceedingly facilitate the improvement or collective effect of the catalytic performance due to a strong coupling among these two metals.13,14 Moreover, the strong metal and support interaction endows a number of characteristics that make them outstanding for catalysis due to the synergetic electronic effect.15,16 Among those, cerium oxide (CeO2) can highly enhance the performance of DMFCs mainly in acidic electrolytes by fast oxidation of organic molecules (i.e., methanol) at the anodic end, good oxygen storage capability, and oxygen transfer ability, which promote excellent cathodic efficiency and ease of poisoning by
Direct methanol fuel cells (DMFCs) have attracted attention as a source of clean, renewable, and effective energy devices in near and past scientific epoch. A great deal of work has been extensively carried out on DMFCs due to their exclusive characteristics such as their high specific energy, environmental safety, and portability.1,2 Accurate designing structure of unique nanoelectrocatalysts is one of the challenging tasks to overcome the slow kinetics of fuel oxidation at anode and the raised effect of depolarization during electro-reduction of oxygen at cathode. Platinum has been widely accepted as a model choice for meeting these demands due to its excellent property for adsorption and decomposition of methanol on the surface. However, the monometallic Pt catalyst may not satisfy the increasing demands of industrial applications at low costs due to its scarcity and high costs.3−6 Because only the exposed outmost few layers of Pt atoms are required for catalyzing the electrochemical reaction, it is also extremely suited to enhance the mass activity and reduce the vulnerability toward reaction poisoning e.g., CO poisoning for monometallic Pt catalyst. In order to overcome these limitations of DMFCs catalysts, great © XXXX American Chemical Society
Received: November 16, 2016 Revised: January 10, 2017
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DOI: 10.1021/acs.jpcc.6b11528 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
(XPS). TEM images and high-resolution transmission electron microscopic (HRTEM) images were carried out on a JEM2100F field emission electron microscope at an accelerating voltage of 200 kV. The XRD patterns of the samples were collected on a Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å), the operation voltage and current was maintained at 40 kV and 200 mA, respectively. XPS was performed at a PerkinElmer RBD upgraded PHI-5000C ESCA system. Synthesis of Pt3Pd1−CeO2/C Catalyst. The typical synthesis of Pt3Pd1−CeO2/C is a two-step process. In the first step of synthesis, CeO2/C was prepared as follows: 1 g of Vulcan carbon was ultrasonically well dispersed into water and then an appropriate amount of Ce(NO3)3. 6H2O was added as a cerium precursor into the prior carbon dispersion. All of the final solution was stirred for 30 min and then transferred into a 100 mL autoclave for hydrothermal treatment at 160 °C up to 8 h. After cooling for the desired reaction time, the solution was filtered and dried at 110 °C in oven, and CeO2/C was obtained. Afterward, in second step, Pt3Pd1 bimetallic alloy NPs were grafted onto the CeO2 surface. In order to obtain 2 wt % of both the metals PtPd, the composition of these two was varied by changing the concentration of their precursors H2PdCl4 and K2PtCl6 followed by being mixed with 0.12 g of CeO2/C product in 50 mL of H2O. The suspension was stirred overnight at room temperature to adsorb the PtPd ions over the base matrix. The suspension was heated at 60 °C to evaporate the water, and then the obtained material was treated for calcinations in a furnace for 3 h at 350 °C in an Ar/H2 environment to reduce the preadsorbed Pt and Pd ions at the surface with homogeneous bimetallic alloys. The final product obtained was designated as Pt3Pd1−CeO2/C and further used for characterizations and applications. Similarly, Pt1Pd1−CeO2/ C, Pt1Pd3−CeO2/C, Pt−CeO2/C, and Pd−CeO2/C catalysts were prepared by the same method. Electrochemical Measurements. Before each electrochemical experiment, a glassy carbon (GC) electrode (0.196 cm2 geometric surface are) was first polished with alumina slurries (Al2O3, 0.05 mm) on a polishing cloth to obtain a mirror finish, followed by sonication in 0.1 M HNO3, 0.1MH2SO4, and pure water for 10 min, successively. To prepare a catalyst-supported working electrode, 10 μL of 2 mg/ mL suspension in ethanol was drop-coated on the polished electrode surface by a microliter syringe, with a catalyst loading of 0.1 mg cm−2, followed by drying in vacuum at room temperature. Afterward, the catalyst was covered with a thin layer of Nafion (0.1 wt % in water, 5 mL) to ensure that the catalyst was tightly attached to the electrode surface during the electrochemical measurements. Voltammetry measurements were carried out with a CHI660D electrochemical workstation. The electrode prepared above was used as the working electrode. The Ag/AgCl (in 3 M KCl, aq.) combination, isolated in a double junction chamber, and a Pt coil were used as the reference and counter electrodes, respectively. All the measurements were performed in electrochemical experiments with respect to the standard values of reversible hydrogen electrode (RHE). Electrochemical experimental work was done by potential cycling and chronoamperometric methods for MOR studies in 0.1 M HClO4 + I M CH3OH solution with N2 purging at a scan rate of 50 mV s−1. Electrochemical in situ FTIR reflection spectroscopy measurements were carried out on a Nexus 870 spectrometer (Nicolet) equipped with a liquid nitrogen-cooled MCT-A
quick electro-oxidation of the poisoning species (i.e., CO) at lower potential values improve overall cell performances.17−20 The electrocatalytic activities of nanocatalysts highly depend on their size, composition, and exposed surface atoms. Asthe word electrocatalysis means the optimization of an electrode by means of careful design of the electrode material in the environments of the electrolyte,21 the ultrapure and clean surface of nanoparticles is critical for their application in electro catalysis.22 The activity of an electrocatalyst toward a specific reaction is marked by its capability to increase the faradic current of the reaction that ultimately accelerates the standard rate constant of the reaction. The lower electrochemical over potential and high voltage output are required for the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR) in DMFCs.23,24 Recently, much effort has been devoted to develop active catalyst comprising Pt based materials for DMFCs. Shen et al. studied the common anodic behavior of CeO2-supported Pt catalysts for electro-oxidation of alcohols,25 Gutiérrez et al. tested the previous material with modification of intermixed PdPt bimetallic catalysts for common electrochemical analysis of DMFCs in alkaline medium, Bera et al.26 and Scibioh et al.27 tried to evaluate the mutual chemistry of highly loaded Pt based catalysts in combination with CeO2 for heterogeneous catalysis, and Qu et al.28 provided the anode behavior of different geometries of CeO2-supported Pt catalyst proposing the effect of the CeO2 matrix on tolerating the CO poisoning issues of DMFCs. So far, the development of smart material exhibiting remarkable performance as both the anode and cathode material for DMFCs in acidic medium is pending. Moreover, the complete analysis of methanol oxidation from surface interfacial chemistry to the molecular and atomic level for asdeveloped materials has not been seen until now. In addition, the excellent retarding behavior of methanol crossover from anode to cathode by methanol tolerance has not been shown by any of the documented electrocatalysts. To this end, to meet all the above challenges, we have developed a series of nanoelectrocatalysts by grafting PdPt bimetallic alloy nanoparticles (NPs) at carbon-supported CeO2 nanospheres via facile surfactant-free calcined route. The overall composite material comprises PdPt/CeO2/C heteronanostructure. The asdeveloped nanocomposite material has been tested as anode and cathode material for DMFCs. The anodic activity for the oxidation of methanol has been investigated by detailed and modern electrochemical techniques such as in situ FTIR spectroscopy and online differential electrochemical mass spectrometry (DEMS) along with CV and CA measurements in order to investigate the complete oxidation of methanol to CO2. Additionally, the cathode performance of composite material is measured in the presence or absence of methanol in acidic electrolyte medium to evaluate the ORR and methanoltolerant ORR activities.
2. EXPERIMENTAL SECTION Reagents. Ce(NO3)3·6H2O, H2PdCl4 (99.9% metal basis, from Alfa Aesar, K2PtCl6 (40%) was purchased from Fluka and CH3OH (99.9%) was purchased from J.T. Baker. Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were of analytical-reagent grade. Characterizations. The crystallographic structure, morphology, and element compositions of the synthesized catalyst were studied by transmission electron microscopy (TEM), Xray diffraction (XRD), and X-ray photoelectron spectroscopy B
DOI: 10.1021/acs.jpcc.6b11528 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. (A) Schematic representation of Pt3Pd1−CeO2/C catalysts synthesis, in first-step simultaneous adsorption of Pt and Pd ions on the surface of CeO2 substrate, while leading to the coreduction of PtPd homogeneous bimetallic alloys via annealing in a second step. Red/white balls are CeO2 structure, magenta balls are Pt atoms, and gray balls represent Pd atom. (B) XRD patterns of as prepared Pt3Pd1−CeO2/C and all PtPd-CeO2/C catalysts series along with Pt-CeO2/C and Pd-CeO2/C catalysts. (C) (111) diffraction peak positions against mole fraction of Pd, to evaluate the degree of alloying among PtPd bimetallic systems (data extracted from XRD patterns).
is transferred electron number, F is the Faraday constant (96485 C mol−1), C0 is the bulk concentration of O2 ((1.1 × 10−6 mol cm−3), D0 is the diffusion coefficient of O2 in 0.1 M HClO4, and ν is the kinematic viscosity of the electrolyte.47 Tefal plots and slope were obtained from the following masstransport correction formula:48
detector. A CaF2 prism was used as the IR window. An IR cell with a thin layer configuration between the electrode and this IR window was approached by pushing the electrode against the window before FTIR measurement. The in situ FTIR spectra were collected using multistepped FTIR spectroscopy (MSFTIR) procedures. The resulting spectra were reported as the relative change in reflectivity and calculated as follows: R(ES) − R(E R ) ΔR = R R (E R )
Jk = (1)
(3)
where i is the obtained current, and id is the diffusion limited current, afterward by plotting E (V/vs RHE) vs Jk.
where R(ES) and R(ER) are the single-beam spectra collected at sample potential ES and reference potential ER. Whereas the DEMS setup used in this study is a HidenHPR40 DSA Bench top-membrane inlet gas analysis system, mass signals are collected 20 points/s. The mass signal for CO2 produced has been calibrated by oxidative stripping of a saturated CO adlayer preadsorbed at 0.06 V, mass calibration constant k = QF/Q mass is found to be 3.65 × 106 mA/Torr. The same above glassy carbon (prior used for MOR) was used as the working electrode fixed on a rotating apparatus for ORR measurements. The ORR electrochemical measurements were performed in 0.1 M HClO4 as the electrolyte. O2 was bubbled to electrolyte for 10 min prior to each experiment and a flow of O2 was maintained over the electrolyte during the measurements in order to ensure O2 saturation. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were collected at a scan rate of 10 mV s−1. Electron transfer number (n) of ORR was calculated by the following equation:
3. RESULTS AND DISCUSSION We have developed a capping agent free, clean Pt3Pd1−CeO2/ C catalyst by a hydrothermal treatment and reduced method, the flow diagram of fabricating Pt3Pd1−CeO2/C catalyst is shown in Figure 1A. In this approach, Pt3Pd1 ions were first adsorbed on the surface of prior synthesized CeO2/C by direct contact of their precursors. Afterward, in a second step the adsorbed Pt3Pd1 alloy NPs were reduced by pyrolysis in a H2/ Ar environment. The atomic composition among PtPd bimetallic system was tuned within restricting their accumulative presence of 2 wt % in all the synthesized catalysts. Three different catalysts with atomic ratios of, Pt1.5%:Pd0.5%, Pt1%:Pd1% and Pt0.5%:Pd1.5% designated as Pt3Pd1/CeO2/C, Pt1Pd1/CeO2/ C, and Pt1Pd3/CeO2/C, respectively, were synthesized in this study. The real Pt and Pd loadings in all the catalysts were also measured by ICP-MS analysis, the detailed Pt and Pd molar and weight percentage (wt %) detail for all catalysts is provided in Table S1. Moreover, for the critical comparison, Pt−CeO2/C and Pd− CeO2/C catalysts were also synthesized with 2 wt % Pt and 2 wt % Pd weight percentage. All the catalysts were employed for complete studies of DMFCs including anode as well as cathode material tests. As the whole series of synthesized catalysts were economically favorable in light of this advantage, the catalysts
1 1 1 1 1 = + = + 1/2 J JL JK JK Bω B = 0.62nFC0D0 2/3v−1/6
(i × id) id − i
(2)
J is the measured current density, JK and JL are the kinetic and diffusion-limiting current densities, ω is the angular velocity, n C
DOI: 10.1021/acs.jpcc.6b11528 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. TEM and HRTEM images of Pt3Pd1-CeO2/C catalyst. (A,B) Low magnification TEM images of Pt3Pd1−CeO2/C catalyst and (C) HRTEM image of Pt3Pd1−CeO2/C catalyst; inset: FFT patterns of CeO2 and Pt3Pd1 bimetallic alloys extracted from HRTEM micrograph.
Figure 3. XPS spectra of Pt3Pd1-CeO2/C catalyst. Survey scans of inset: pi chart of weight percentage of each component (A), the Pt 4f spectrum (B), the Pd 3d spectrum (C), the Ce 3d spectrum (D), and the O 1s spectrum (E) of Pt3Pd1−CeO2/C catalyst.
the difference in atomic size (Pt > Pd) and also an indication that both the NPs are well-mixed homogeneously and result in the formation of alloy NPs. The morphology and structure of the obtained nanocomposite was analyzed by TEM and HRTEM images. Lowmagnification TEM micrographs have shown that the assynthesized nanocomposite was successfully prepared with precise nanometric average size of 8−10 nm (Figure 2A), the dark colored tiny spheres represent the PtPd bimetallic nanoparticles (NPs) while the comparatively larger spheres show the CeO2 (Figure 2B). The intimate contact of the nanocomposite with carbon support matrix and successive grafting of the PtPd bimetallic alloy NPs onto CeO2 surfaces with their crystal lattice confirmation were measured by HRTEM analysis. The HRTEM results indicate the homogeneous formation of nanosized CeO2 and PtPd over the CeO2 surface comprising a nanocomposite material that is in contact with the carbon support. In the nanocomposite, CeO2 is present with the lattice spacing values of 0.27 nm corresponding to the (100) and (111) planes of the CeO2, whereas the bimetallic PtPd NPs exhibit the d-spacing values of
have higher and highest activities compared to the well-known 20 wt % Pt/C. The structure and crystalline phase of obtained samples were measured by X-ray diffraction (XRD) patterns. Figure 1B shows the characteristic reflections of CeO2 (JCPDS no. 34-0394) with diffraction peaks at 28.54° and 33.02° correspond to the (111) and (100) planes. PtPd-CeO2/C shows Pt peaks of a polycrystalline fcc structure indexed to Pt (JCPDS, card no 04−0802). The patterns corresponding to the bimetallic Pt3Pd1 sample show only the features of Pt, though the diffraction peaks of the bimetallic material are shifted to higher angles compared with that of their monometallic existence, indicating the formation of an alloyed structure. The diffraction peak values for (111) plane vary roughly linearly as a function of apparent composition of Pt−Pd alloys NPs. This trend clearly suggests the formation of alloys of Pt and Pd, resulting in an increase of the unit cell lattice parameter through an increase in the mole fraction of Pd (Figure 1C). Moreover, the wide diffraction peaks of Pt−Pd corresponding to peaks in PtPd-CeO2/C samples indicate that the size of bimetallic alloys NPs grafted onto CeO2/C was quite small. These results could be attributed to the addition of Pd to Pt for D
DOI: 10.1021/acs.jpcc.6b11528 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. CO oxidation and MOR performance. (A) Electrochemical CO oxidation of Pt3Pd1−CeO2/C, Pt−CeO2/C catalysts compared with commercial 20% Pt/C catalyst, in 0.1 M HClO4; the scan rate is 50 mV s−1. (B) MOR performance of Pt3Pd1−CeO2/C, Pt−CeO2/C, and Pt/C catalysts, in 0.1 M HClO4 + 1 M CH3OH at a scan rate of 50 mV s−1. (C) Mechanism of MOR on the surface of Pt3Pd1−CeO2/C catalysts. (D) Comparison of measured specific and mass activities at positive peak (If max) potential for all catalysts compared with 20% Pt/C catalyst.
electrons come from CeO2 support, which is clear evidence of strong interaction between bimetallic PtPd and CeO2 support. The bonding of cerium with oxygen species in cerium oxide can be examined by deconvolution of the O 1s spectrum of a highresolution XPS scan of oxygen in the catalyst. The O 1s spectrum exhibits three dominant featured peaks at 530.1, 531.5, and 532.8 eV corresponding to lattice (Olatt), adsorbed (Oads), and surface (Osurf) oxygen species, respectively (Figure 3E). Among these peaks, Olatt is apparently dominant over the others, providing the useful information on oxygen bonding with metallic cations of the oxides such as Celatt species within the CeO2 structure. Oxygen peaks of Oads and Osurf with comparatively lower intensities suggest the presence of adsorbed oxygen of O2/OH− and surface C−O/H2O on the surface of carbon. The weight percentage of each component is 1.5 wt % Pt, 0.5 wt % Pd, 3 wt % CeO2, and 95 wt % C in the Pt3Pd1−CeO2/C sample (Figure 3A, inset), respectively. Electrochemical Measurements, MOR and ORR Activities. The as-developed composite materials were tested for complete studies of DMFCs as, anode and cathode catalyst by methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). In addition, hybrid nanocatalysts were also investigated in terms of methanol tolerant ORR performance. The methanol tolerant ORR activity has shown that the material is highly sensitive to its surrounding environment. In the nitrogen environment, it behaves as anode material, while during oxygen flows it behaves as cathode catalyst in the same medium. The basic electrochemical behavior of composites was measured in acidic electrolyte for the evaluation of electrochemical active surface areas (ECSA).
0.22 nm corresponding to the (111) planes of the PtPd single phase (Figure 2C). Along with the calculation of lattice spacing values, the fast Fourier transform (FFT) patterns (inset in Figure 2C) are also taken to strengthen the claim of crystallite formation of as-synthesized nanocomposite material. The XPS studies were employed to investigate the surface oxidation states of metallic species such as Pt, Pd, and Ce in the composite. The full range survey spectrum contains all expected metals at their corresponding specific binding energy positions (Figure 3A). The Pt 4f deconvoluted spectrum of 4f5/2 and 4f7/2 with corresponding binding energies positions appeared at 74.6 and 71.8 eV, respectively, with an asymmetric nature, exhibiting the metallic Pt(0) state (Figure 3B).27 Similarly, the Pd 3d deconvoluted scan containing two spin−orbit doublets of 3d5/2 and 3d3/2 electrons for Pd appeared at 335.1 and 340.3 eV, respectively, corresponding to the metallic Pd(0) state (Figure 3C).29 The electron transfer between Pt and Pd in bimetallic Pt3Pd1 NPs can be examined by comparing positions of their corresponding metallic peaks in XPS spectra with standard monometallic Pt(0) and Pd(0) states. The binding energy values for Pt shifted toward lower values, while for those Pd shifted to higher values as compared to the standard values, suggesting that electron transfer occurred from Pd to Pt due to the higher electronegativity of Pt (2.28) compared to Pd (2.20).29 The deconvolution of Ce 3d is resolved into Ce3+ and Ce4+ valent states with different compartments such as at 906.76, 900.72, 885.58, and 880.64 eV for Ce3+ and 916.66, 910.62, 903.99, 989.60, 888.53, and 883.40 eV for Ce4+ state (Figure 3D).16 The molar ratio of Ce3+/Ce4+ was estimated to be ca. 0.94 in CeO2. The above electron transference authentications can also be rationally presumed that these E
DOI: 10.1021/acs.jpcc.6b11528 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Electrochemical in situ FTIR and online DEMS studies for MOR. (A,B) In situ FTIR response during MOR over Pt3Pd1−CeO2/C and Pt−CeO2/C catalysts at different potentials from 0.05 to 0.80 V, in 0.1 M HClO4 + 1 M CH3OH. (C−E) Online DEMS response of Pt3Pd1−CeO2/ C and Pt−CeO2/C catalysts at different potentials of 0.40 V, 0.50 and 0.60 V, time course of 600 s with reaction current and mass signal m/z = 44 from CO2.
differential electrochemical mass spectrometry (DEMS) for in situ molecular level studies of MOR. The CV measurements were carried out between 0.05 to 1.2 V vs RHE with a scan rate of 50 mV/s. As shown in (Figures 4B and S3), each of these CVs includes a methanol oxidation peak during the forward scan at around 0.7 to 1.1 V and another anodic peak during the reverse scan attributed to the removal of incompletely oxidized carbonaceous species such as CO formed in the forward scan. The onset potential of methanol oxidation observed with Pt3Pd1−CeO2 appeared in the range of 0.40−0.50 V vs RHE (∼0.40 or
