rGO Electro-catalysts for Formic Acid Oxidation

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La2O3 Promoted Pd/rGO Electro-catalysts for Formic Acid Oxidation Hassan Ali,† Fehmida K. Kanodarwala,‡ Imran Majeed,† John Arron Stride,*,‡ and Muhammad Arif Nadeem*,† †

Catalysis and Nanomaterials Lab 27, Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan School of Chemistry, University of New South Wales, Sydney, New South Wales 2052, Australia

‡

S Supporting Information *

ABSTRACT: High activity, a low rate of CO poisoning, and long-term stability of Pd electro-catalysts are necessary for practical use as an anode material in direct formic acid fuel cells. Achieving a high degree of Pd nanoparticle dispersion on a carbon support, without agglomeration, while maintaining a facile electron transfer through the catalyst surface are two challenging tasks to be overcome in fulfilling this aim. Herein, we report the effect of addition of La/La-oxides on the efficiency of Pd nanoparticles supported on reduced graphene oxide (rGO) for formic acid electro-oxidation reaction. A series of electro-catalysts with different Pd−La molar ratios were successfully synthesized and characterized using a range of techniques including PXRD, XPS, TEM, FTIR, and Raman spectroscopy and then tested as anode materials for direct formic acid fuel cells. We explore that the lanthanum species (La/La-oxide) significantly promote the activity and stability of Pd catalyst toward electrocatalytic oxidation of formic acid. The metallic ratio is found to be critical, and the activity order of various catalysts is observed as follows; Pd30La70/rGO > Pd80La20/rGO > Pd70La30 rGO. The obtained mass specific activity for Pd30La70/rGO (986.42 A/g) is 2.18 times higher than that for Pd/rGO (451 A/g) and 16 times higher than that for Pd/C (61.5 A/g) at given onset peak potentials. The high activity and stability of the electrocatalysts are attributed to the uniform dispersion of Pd nanoparticles over the rGO support, as evidenced from TEM images. It is believed that the role of La species in promoting the catalyst activity is to disperse the catalyst particles during synthesis and to facilitate the electron transfer via providing a suitable pathway during electrochemical testing. KEYWORDS: palladium nanoparticles, fuel cell, graphene, lanthanum oxide, cyclic voltammetry (1.190 V). FA also has a lower crossover flux through Nafion, or the proton exchange membrane, than methanol and ethanol due to the repulsion offered by the membrane terminal groups, hence it facilitates proton transport within the anodic compartment of the fuel cell and high energy conversion.10 Although the energy density of FA is 2086 WhL−1 which is smaller than that of methanol (4690 WhL−1), it carries more energy per unit volume than methanol due to the fact that concentrated FA (20 M or 70 wt %) can be used as a fuel compared to a relatively low concentration of methanol (2 M).11 Another major advantage of FA use as a fuel is its production from environmental waste through biomass conversion processes.12 Various forms of carbon such as Vulcan XC-72 (carbon black), carbon nanotubes (CNTs), and porous carbon are being used as supports for Pd-based catalysts. These materials are useful to minimize the loading amount of the Pd metal without reducing its efficiency thus reducing the cost of catalyst for practical applications.13,14 Recently a new class of 2dimensional carbon nanostructures, namely graphene, have

1. INTRODUCTION Portable devices such as cellular phones, personal digital assistants, laptops, and so forth require power and energy; however, at the same time, the operational charging life of these power sources is not being increased in accordance with the consumer demand. The usage of liquid fuels in such devices has been an alternative and attractive field of research in the past decade.1,2 Initially, extensive work on direct methanol fuel cells (DMFCs) was carried out due to their high energy density, activity, efficient energy conversion, room temperature operation, and easy availability of fuel with negligible pollutant emissions.3,4 However, the commercial use of DMFCs is limited due to some critical problems such as (1) operation at limited concentration; (2) methanol crossover, which limits the use of high concentrations of methanol, generally less than 2 M;5 (3) the high cost of Pt (Pt is specific catalyst for DMFCs); (4) poor kinetics due to catalyst poisoning as a result of carbon intermediates produced during methanol oxidation, resulting in reduced fuel efficiency; and finally (5) low activity at room temperature.6−8 To overcome these issues, direct formic acid fuel cells (DFAFCs) have acquired attention in recent years. Formic acid (FA) is less toxic than other liquid fuels9 and has a high theoretical open circuit potential (1.450 V) as compared to proton exchange membrane fuel cells (1.229 V) and DMFCs © XXXX American Chemical Society

Received: August 2, 2016 Accepted: November 8, 2016

A

DOI: 10.1021/acsami.6b09645 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces actively engaged researcher’s interest for use in supercapacitors, nanocomposites, batteries, and hydrogen storage materials, largely due to their excellent electron mobility, thermal conductivity, mechanical strength, electrical conductivities (105−106 S/m), and most importantly huge surface area (∼2600 m2/g).15−17 One route to synthesis is from graphite oxide (GO) since it has plentiful functional groups,18 which enables excellent dispersion of nanoparticles on the graphene surface. This property of rGO reduces the loading amount of noble metal thus reducing the economy of the catalyst. Although many efforts have been devoted to improve the performance of the Pd-based catalysts used in fuel cells, it remains a challenge to develop reliable and versatile approaches to prepare Pd-based nanostructures with higher performance and better stability. Noble metals such as Pt are the most popular electrocatalysts for DFAFCs; Pt is also used as a primary catalyst to attain a high activity in the case of methanol and ethanol fuel cells. However, certain drawbacks of Pt limit its use in DFAFCs, such as CO poisoning by CO, produced an intermediate and reduces the electrocatalytic activity of Pt. Recent investigations suggest that Pd is a better catalyst for DFAFCs than Pt, due to its high efficiency and negligible CO poisoning.15,19 In a couple of reports, the activity of Pd has been found to be twice the magnitude as that of Pt black.9,11 Due to the higher oxidation potential of Pd, its oxides are stable.20 Addition of a second metal decreases the Pd-d band, thereby minimizing the chances of reaction with reaction intermediates.21 Various Pd based multicomponent catalysts such as Pd−TiO2,22 Pd−MoO3,19 Pd−WO323 and bimetallic catalysts Pd−Au,24 Pd−Pt,25 Pd− Ni,26 Pd−Ir,27 and Pd−Co20 have been studied, seeking to enhance the electrochemical activity and durability of the electro-catalysts. Herein, we aimed at obtaining a high dispersion of Pd nanoparticles, while controlling the particle size and shape for a highly active catalyst while maintaining its stability toward DFAFCs. For this purpose, we have explored the fact that the activity and stability of Pd/rGO catalysts can be amplified by the addition of La2O3. In this study, we report that La2O3 not only significantly enhances the activity of the Pd catalyst, but also helps to disperse nanoparticles over the rGO support. 1.1. Proposed Mechanism of Electro-oxidation of Formic Acid. The addition of lanthanum/lanthanum oxide in the catalyst increases its electrocatalytic activity by providing a suitable pathway and thus enhances the electron transfer during electrochemical testing. Recently, our group and another also revealed that the dispersion of metallic active sites on the support material can be increased by diluting it with alkali metal or lanthanide ions.28,29 The mechanism by which La enhances the electrocatalytic activity of Pd may follow two paths of electron transfer; either electrons are transformed from La to Pd through the graphene acting as a highly conductive pathway or directly from La to Pd;30 both possibilities are schematically shown in Figure 1.

Figure 1. Schematic representation of proposed mechanism of electron transfer from support surface to metal. above solution. The resulting solution was stirred for 3 h at 35 °C, and the temperature again reduced to 10 °C. 100 mL of distilled water was added dropwise to this solution and heated for 1 h at a constant temperature of 90 °C. Another 80 mL of distilled water was added, followed by the addition of 20 mL of 35% H2O2 dropwise with effervescence. Finally, the mixed solution was cooled to room temperature, vacuum filtered, and washed with 5% HCl several times. A blackish brown colored graphene oxide (GO) product was obtained and dried at 70 °C for 8 h. Reduction of GO was carried out by adding 2.5 mL hydrazine monohydrate 65% to 1 mg/mL solution of GO in distilled water prepared by ultrasonication and kept at 110 °C for 2 h. The solution was allowed to cool to room temperature, vacuum filtered, washed with methanol and acetone, and air-dried to get rGO. Reduced GO was washed several times with deionized water and dried at 70 °C for further use. 2.2. Synthesis of Catalysts. A total 20 wt % metal loading was performed by sodium borohydride reduction method. Briefly, a homogeneous suspension of rGO in water was prepared by mixing 80 mg of rGO with 20 mL of distilled water and sonicated for 1 h. An appropriate amount of aqueous lanthanum nitrate solution was added dropwise with vigorous stirring. The mixture was then once again sonicated for 30 min. To this solution, the required amount of an aqueous solution of palladium acetate was added dropwise and stirred vigorously. The Pd ions were then reduced by the addition of freshly prepared sodium borohydride (metal/NaBH4 = 1/5) solution dropwise. The obtained slurry was subjected to vacuum filtration, washed, and dried at 60 °C. The Pd−La/rGO powder obtained was calcined at 300 °C for 3 h in a H2/Ar gas atmosphere before characterization and electrochemical applications. Various catalysts (PdxLay/rGO) were prepared by using this method, where x and y represents the nominal loading ratios of palladium and lanthanum, respectively. The same procedure was used to prepare 20 wt % Pd/ rGO and 20 wt % Pd/VC (Pd/C) as reference catalysts. 2.3. Physical Characterization. A X-ray diffractometer with Cu Kα radiation source PANalytical (model X’pert Pro origin Netherland) ((λ = 1.544206 Å) radiation generated at a speed of 0.015 s−1 at operating potential of 40 kV and 30 mA current radiation source was used for the X-ray diffraction pattern of rGO and catalysts. X-ray photoelectron spectroscopy (XPS) measurement was taken with the ESCALAB250Xi (Thermo Scientific, U.K.) with X-ray source Al Kα (energy 1486.68 eV) for elemental analysis and to know about the oxidation states of the loaded metals. The surface morphology, particle size, and distribution of metal particles on the surface of the support (rGO) were estimated through transmission electron microscopy (TEM). 2.4. Electrochemical Measurements. To perform the electrochemical experiments, the surface of the glassy carbon electrode (GCE) was cleaned by pasting alumina slurry (0.3 μm) onto the GCE surface. Briefly, 10 μL of alumina slurry was pasted on the electrode surface and allowed to stand overnight at room temperature, the electrode surface was then cleaned with ethanol and finally sonicated

2. EXPERIMENTAL SECTION 2.1. Synthesis of Reduced Graphene Oxide (rGO). Graphene oxide (GO) was synthesized by slight modification to the Hummers method.31,32 Briefly, a homogeneous solution was prepared by adding 25 mL concentrated sulfuric acid to a mixture containing 1.0 g of graphite powder and 0.5 g of sodium nitrate (NaNO3), while maintaining a temperature below 10 °C. A 10 mL of aqueous potassium permanganate (3.0 g) solution was added dropwise to the B


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ACS Applied Materials & Interfaces

Figure 2. PXRD pattern of Pd80La20/rGO (a), Pd70La30/rGO (b), Pd50La50/rGO (c), Pd30La70/rGO (d), Pd20La80/rGO (e), and Pd/rGO (f). in acetone for 15 min to obtain a mirror-clean GCE. In a typical catalyst loading procedure, 3 mg of the catalyst was suspended in 180 μL of 2-propanol and 20 μL Nafion 117 binder, and sonicated for 30 min to obtain a catalyst ink. Six μL of the resulting catalyst ink was spread on the surface of GCE and allowed to dry overnight at room temperature. Electrochemical experiments were performed on potentiostat/galvanostat (Biologic SP-300) at room temperature, with a three-electrode cell system; GCE (geometrical surface area 0.071 cm2) as the working electrode, platinum foil as the counter electrode and Ag/AgCl (saturated KCl) as the reference electrode. Electrochemical surface area (ECSA) measurements were carried out in aqueous H2SO4 (0.5 mol L−1) solution at a scan rate of 20 mVs−1. 0.25 mol L−1 H2SO4 was used as the supporting electrolyte for the electro-oxidation of 1.0 mol L−1 HCOOH (working solution). Cyclic voltametric experiments were performed for electrocatalytic activity measurements whereas for stability and durability measurements, chronoamperometric experiments were carried out in a mixture of 1.0 mol L−1 HCOOH and 0.25 mol L−1 H2SO4. Prior to the cyclic voltammetry run, the surface of working electrode loaded with Pd/rGO and PdxLay/rGO was cleaned by running the experiment between 0 to 1.0 V in 0.25 M H2SO4 at a scan rate of 50 mVs−1 for 10 cycles. CV measurements were carried in a mixed solution of 1.0 mol L−1 HCOOH and 0.25 mol L−1 H2SO4 in the working potential range of −0.3 to 1.0 V, whereas chronoamperometric experiments were carried out at a fixed potential of 0.3 V and 1.0 V for 3000 s in the same working solution. Prior to each measurement, high purity N2 gas was bubbled through the solution for 15 min in order to remove dissolved O2 in the electrolyte.

planes of a single face centered cubic (fcc) structure of Pd metal. The interaction of Pd with La/La2O3 can be noticed by slight shifting of pure Pd peaks.34 The PXRD analysis revealed no obvious peak for La/La2O3, which is due to slight alloying of La with Pd and amorphous nature of La2O3. Functional groups present as active sites on the rGO sheets were analyzed by FTIR and Raman spectroscopies. The FTIR spectrum of rGO is shown in Figure S1 of Supporting Information. The spectrum of rGO is characterized by a strong and relatively broad carbonyl band at 1704 cm−1 accompanied by a band at 1618 cm−1, attributed to the aromatic CC bonds, which is rather stable in intensity and insensitive to the oxidation state of graphite. These values are concordant with literature values of CC bond and CO bonds.33,35 The broad absorption peak between 3000 to 3500 cm−1 is due to the acidic OH and moisture content. So graphene oxide formation is confirmed by its FTIR spectra, also functional groups present in it shows that it has sp2 as well as sp3 carbon atoms. The Raman spectrum of rGO is shown in Figure S2, which clearly shows the D and G bands. The D band is due to the distortion present in the graphitic plane, and it appears at 1337 cm−1 and G band refers to the presence of sp2 carbon atoms in rGO and it appears at 1571 cm−1. These D and G band values are in accordance with the literature values.36,37 Raman spectra of Pd30La70/rGO catalyst are shown in the Figure S3, which also shows a D band with a decrease in peak intensity as compared to pure rGO, due to the further reduction of rGO by the addition of NaBH4, thus reducing the distortion in the graphitic plane and increasing graphene structure. While the G band peak is shifted slightly due to the presence of metal oxides in the catalyst; metal oxide peaks can also be seen in between 400−600 cm−1 value. To investigate the surface morphology and nanoparticle distribution on the support surface, transmission electron

3. RESULTS AND DISCUSSIONS 3.1. Characterization. Figure 2 shows the XRD diffraction patterns of various PdxLay/rGO and Pd/rGO. The XRD pattern of rGO exhibits a typical broad peak at 2θ = 25° (hkl = 002), which is ascribed to disordered stacks of weakly crystalline rGO nanosheets.33 The XRD patterns of all PdxLay/rGO catalysts show the diffraction peaks at 2θ = 40° and 47° which correspond to the (111) and (200) crystalline C


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Figure 3. TEM micrographs (a−d) at different magnifications of the synthesized Pd30La70/rGO catalyst and corresponding particle size distribution (e−h).

carboxylic entities. The C 1s XPS spectrum of as-synthesized rGO is shown in Figure 4a, the assignment of various peaks depending upon the nature of the carbon are assigned as CC (284.78 eV), CC bonds (285.28 eV),38 CO (288.88 eV), CO bonds (286.88 eV) (including epoxide and hydroxyl bonds), COCOOH bonds (290.78 eV), and OCO (292.36 eV) (ester bonds). The CN peak (287.28 eV) in the samples is due to the nitrogen doping during hydrazine reduction of graphite oxide into graphene.39 XPS spectra of Pd 3d (Figure 4b) shows two well-resolved asymmetrical signals in the region of 335−342 eV corresponding to Pd (3d5/2) and Pd (3d3/2), with a spin−orbit splitting of about 5.5−6.0 eV. The Pd (3d5/2) signal gives three peaks at 335.47, 336.63, and 337.57 eV, which on the basis of their binding energies can be assigned to Pd0, (PdO, PdOads) and PdO2 species, respectively.40,41 The shifting of the binding energy for Pd (335 to 335.47 eV) is attributed to partial alloying of Pd with La. XPS spectra of La 3d Figure 4c shows well separated spin−orbit components, which are further split. The two multiple splittings (836.4 eV) in the La (3d) spectrum from the Pd30La70 sample are due to

microscopy (TEM) analysis was carried out. Figure 3 shows the TEM micrographs of as prepared catalyst Pd30-La70/rGO. The results clearly show the nanosheets of rGO, which are generally folded at the edges (Figure 3a). Figure 3(a−d) shows a uniform distribution of palladium and lanthanum/Lanthanum oxide nanoparticles on the support surface. There are two types of particles uniformly distributed over the rGO nanosheets. Larger nanoparticles that appear to be amorphous (their images have a blurred appearance) having an average diameter of 5−15 nm are of lanthanum oxide or the alloy of lanthanum and palladium, whereas the well dispersed smaller particles having an average diameter of approximately 3−7 nm are of palladium nanoparticles. These TEM micrographs also depict that a very small number of particles are agglomerated and as such, the TEM images are supportive to data obtained from PXRD. C 1s XPS is a useful tool to study the type of bonding in rGO. For rGO, peaks centered at 284−285 eV are mostly assigned to sp2 carbon species in aromatic rings, and peaks centered in the region of 286−289 eV are usually assigned to oxygenated organic species such as hydroxyl, epoxide, and D


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Figure 4. XPS spectra of C 1s (a), Pd 3d (b), La 3d (c), and O 1s (d).

only a single state of La3+ in La2O3. Figure 4d represents the XPS spectra of O 1s, which shows a singlet at 532.15 eV, which is a characteristic peak of O 1s. Energy dispersive X- ray (EDX) technique was used to further investigate the elemental composition of the Pd30La70/rGO catalyst. EDX mapping with inset showing atomic and weight % of various elements present in the Pd30La70/rGO is shown in Figure S4. 3.2. Electrochemical Studies. Generally, the oxidation of FA may follow a dual path mechanism,42,43 a direct path {(HCOOH → CO2 + 2H+ + 2e−) (dehydrogenation)} and an indirect path {(HCOOH → COads + H2O → CO2 + 2H+ + 2e−) (dehydration)}. However, the FAO over Pd based electrocatalysts follows the direct path dominantly.44 The dehydration path produces CO that decreases electrocatalytic performance of the catalyst by poisoning it at the surface (adsorption on

surface) and thus decreasing the number of active sites, thereby inhibiting further oxidation of FA. In this background, the efficiency/activity of the Pd based catalysts can be superior than Pt based catalysts due to a decrease in the dehydration reaction. The role of La/La2O3 is to enhance the electrocatalytic performance of Pd/rGO catalyst by increasing the electron density at Pd which reduces the catalytically inactive PdO layer and also by improving the electron transfer pathway. Electro-oxidation of FA over the Pd/C, Pd/rGO and PdxLay/ rGO catalysts surface was performed in an electrolyte medium of 1.0 mol L−1 HCOOH and 0.25 mol L−1 aqueous H2SO4, and the results are shown in Figure 5. CV curves of the synthesized catalysts show two peaks, one in the forward scan and the other in the reverse scan. This is in accordance with the reported patterns of electro-oxidation of FA.45−47 The current intensity E


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Figure 5. Cyclic voltammograms showing electro-oxidation of formic acid of different nominal concentration ratios of La−Pd/rGO catalysts. (a) Current density measurement for synthesized catalysts Pd30La70/rGO, Pd/rGO, and Pd/C; (b) mass specific activity curves for synthesized catalysts Pd30La70/rGO, Pd/rGO, and Pd/C; and (c) in 0.25 mol L−1 H2SO4/1.0 mol L−1 HCOOH solution at scan rate 50 mVs−1 and CV curves for ECSA determination in 0.5 mol L−1 H2SO4 at a scan rate 20 mVs−1 (d).

effect of Pd for the oxidation of FA. As proposed in Figure 1, the electrons from the La/La2O3 can be directly transferred to palladium or through rGO and La species provide an electron transport pathway. Figure 5a, represents the cyclic voltammograms for different La−Pd/rGO catalysts at a scan rate of 50 mVs−1. The current activity order for various catalysts in this study is observed as Pd30La70/rGO > Pd80La20/rGO > Pd70La30/rGO > Pd50La50/rGO > Pd20La80/rGO. The maximum synergistic effect was observed in the Pd30La70/rGO catalyst, which shows the maximum current density compared to the other catalysts. Figure 5b shows a comparison of the electrocatalytic activity of Pd/C, Pd/rGO, and Pd30La70/rGO in terms of mass specific activity (current normalized to metal/Pd loading). The mass specific activity (MSA) for Pd30La70/rGO is 986.42 A/g, which is 2.18 times higher than Pd/rGO (451 A/g) and 16 times higher than Pd/C (61.5 A/g) at given onset peak potentials. Similarly, current density (current normalized to surface area of electrode) of Pd30La70/rGO is 694.56 A/m2, which is also 1.82 times larger than Pd/rGO (381 A/m2) and 4.46 times higher than Pd/C (155.8 A/m2), as shown in Figure 5c. Mass specific activity and current densities were recorded in the potential range of −0.3 to 1.0 V with a scan rate of 50 mVs−1. The forward peak current to backward peak current (If/ Ib) ratios are 1.11, 0.77, and 1.56 for Pd/rGO, Pd/C, and Pd30La70/rGO respectively. These (If/Ib) ratios indicate that in case of Pd30La70/rGO, the direct oxidation path of FA is

is higher in the forward scan than in reverse scan, which reflects higher activity and fast oxidation of adsorbed species on the catalyst surface. In the forward scan, the current density increases up to a maximum value (0.66 V, anodic peak, Ap) due to electron release during FA oxidation, then, decreases to a minimum with a small shoulder at 0.8 V which is attributed to the formation of oxide layer on Pd. In the reverse scan, the current density again increases due to simultaneous reduction of the oxidized Pd surface and oxidation of FA with a cathodic peak (Cp) at 0.56 V. This results in regeneration of the active sites of Pd, hence increase in activity of the catalyst to oxidize FA in both the forward and reverse scans.48 Same pattern of CV is followed by both the Pd/rGO and the Pd/C catalysts. A conspicuous difference among the activities of Pd30La70/rGO and of those of the reference Pd/C, Pd/rGO catalysts can be rationalized in Figure 5.49 The results depict that in the absence of La species, current activities of the reference Pd/C, Pd/rGO catalysts are much smaller as compared to Pd30La70/rGO. The higher current activity at Ap and Cp at relatively higher potentials indicates that the synthesized catalyst has a higher CO (if produced after longer periods of running the catalyst) tolerance. This is attributed to the strong synergistic electronic interactions between Pd, La, and reduced graphene oxide. In order to study the effect of La species, an electrode coated with La-oxide/rGO was used but no electro-oxidation of FA acid was observed which means that La species have no direct role in FA oxidation. In the present study La is clearly enhancing the F


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ACS Applied Materials & Interfaces Table 1. Comparison of Current Density and If/Ib of the Prepared Catalysts for Formic Acid Oxidation sample Pd/C Pd/rGO

Pd30La70/rGO

a

onset potential (V)

current density (A/m2)

offset potential (V)

current density at offset potential (A/m2)

If/Iba

0.1 0.2 (peak potential) 0.1 0.2 0.27 (peak potential) 0.3 0.5 0.66 (peak potential)

134 146 332 377 381 372.40 595.60 694.5

0.47

100.30

1.56

0.47

493

0.77

0.56

627

1.11

If forward peak current and Ib reverse peak current

Table 2. Comparison of ECSA and Mass Specific Activities of the Synthesized Catalysts for Formic Acid Oxidation with Metal Loading 1.26 mg cm−2 catalyst

potential limits E (V)

coulombic charge ( QH) (mC)

electrochemical active surface area (cm2)

specific electrochemical surface area ECSA (m2/g)

mass specific activity (A/g Pd)

Pd/C Pd/rGO Pd30La70/rGO

−0.164−0.215 −0.17−0.12 −0.18−0.07

3.50 7.12 10.9

16.42 33.87 51.97

18.25 37.60 57.80

61.50 451 986.42

(μC cm−2) = charge required to oxidize monolayer of hydrogen on the bright surface of noble metal (Pd, Pt).

favored during both the scans. Enhanced mass specific activity and current density values of Pd30La70/rGO are due to evenly distributed smaller Pd nanoparticles (3−7 nm) presenting the enhanced ECSA and making intimate contact with the support for quick electron transfers. Electrochemical parameters as calculated from the cyclic voltametric studies of the Pd30La70/ rGO, Pd/rGO, and Pd/C are shown in Table 1. To evaluate the electrocatalytic performance of the catalyst, it is essential to take into consideration the active area of the electrode. The area of the electrode on which electron exchange takes place is known as active area, or electrochemical surface area (ECSA), and provides insight into the number of active sites.50 It is believed that catalysts having greater ECSA have enhanced electrocatalytic activity. Electrochemical surface area can be calculated either by CO striping51 or by hydrogen adsorption−desorption method.52 As the CO has little adsorption affinity on the Pd surface, in addition to special handling, the hydrogen adsorption−desorption charge was used to calculate the electrochemical surface areas of the synthesized catalysts.52,53 Electrochemical techniques such as cyclic voltammetry are generally used to measure the ECSA. The ECSA (m2/g) was measured by carrying out the CV experiments in 0.5 mol L−1 aqueous H2SO4 in a potential window of −0.3 to 1.0 V at a scan rate of 20 mVs−1, Figure 5d. Prior to running the cyclic voltammograms, the electrolyte solution was purged with N2 gas for 15 min. During the forward scan, the peak centered at −0.2 to 0.2 V is due to hydrogen desorption and the region above 0.5 V is due to the formation of Pd oxide layer, whereas, in the reverse scan, the reduction of the Pd-oxide layer occurred between 0.55 V and 0.45 V. This behavior of Pd/C, Pd/rGO, and Pd30La70/rGO catalysts in 0.5 mol L−1 aqueous H2SO4 is in accordance to literature.54,55 The ECSA was estimated by integrating the hydrogen desorption charge (QH) after subtracting electrode double layer charge (qH) from cyclic voltammetric experiment by the following scheme.54,56

ECSA(m 2/g) = Aec /Ag × W

where W = loading of metal (μg cm−2), Ag = geometric area of GCE. The obtained large ECSA for the catalyst is attributed to the highly dispersed Pd nanoparticles of smaller size on rGO. A larger ECSA infers that the catalyst has a large number of active sites. These results demonstrate that La−Pd nanoparticles supported on rGO are electrochemically more accessible, which is very important for electrocatalytic applications in fuel cells. The Pd30La70/rGO has an ECSA value of 57.8 m2 g−1 which is much higher than Pd/rGO (37.6 m2g−1) and Pd/C (18.25 m2g−1). High ECSAs also support the high electrocatalytic activity of the synthesized catalyst. Table 2 shows the ECSA and corresponding mass specific activity of the synthesized and reference catalysts. To test the stability of the catalyst against CO poisoning and that if any CO is produced during FA oxidation, CO striping experiment was carried out by bubbling CO for 15 min through the cell containing 0.5 mol L−1 H2SO4 at constant potential of −0.15 V. Prior to CV experiment, argon gas was bubbled through the electrochemical cell to remove any dissolved CO. Cyclic voltammogram of the synthesized catalyst (Pd30La70/ rGO) was recorded at a scan rate of 50 mVs−1 and potential range of −0.2 to 1.0 V. The observed peak in the forward scan centered at 0.71 V is due to the preadsorbed COads oxidation on the catalyst surface. This peak disappears in the second cycle which means the COads is oxidized almost completely in first cycle and synthesized catalyst has very little affinity with CO. It is inferred from CO stripping studies that during electrochemical oxidation of FA at Pd30La70/rGO, only dehydrogenation (direct) path is followed (Figure S5). Chronoamperometry studies were carried out to test the stability of the synthesized electro-catalysts. Chronoamperometric experiments were performed in an electrolyte mixture of 1 mol L−1 HCOOH and 0.25 mol L−1 aqueous H2SO4 at two different working potential values, i.e., 0.3 and 1.0 V (Figure 6a,b). These experiments were performed for up to 3000 s

Q H(mC) = Q ′H − qH /scan rate

where QH = Coulombic charge, Q′H = hydrogen desorption charge, qH = charge due to double layer of electrode, Aec (electrochemical surface area (cm2)) = QH/210 μC cm−2, 210 G


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Figure 6. Chronoamperometric curves of Pd30La70/rGO, Pd/rGO, and Pd/C at 0.3 V (vs Ag/AgCl) (a) at 1.0 V (vs Ag/AgCl) (b), in 0.25 mol L−1 H2SO4/1.0 mol L−1 HCOOH for 3000 s.

Table 3. Comparison of Reported Electrochemical Activities of Pd-Based Catalysts for FA Oxidation with Current Study catalyst

surface area (ECSA) m2/g

mass activity A/g metal

reference

Pt/Ni(OH)2−NiOOH/Pd Pd−Sn−INNs Pd−Mo2C/CNTs Pd/CMG Pd1Ni1−NNs/RGO Pd/C−C Pd/C-k7 Pd/FCNT Pd/GN1 Pd−Au (3:1)/PDDA-xGNP Pd/CMK-8-I cubic chain-like Pd nanostructures Pd30La70/rGO

94.84 18.6 72.4 155.6 98.2 34.6 26.12 55.3 162.56 58 28.7

510 553.37 845 112.36 604.3 841 363.2 746.1 300 580 486.4 540 986.42

201554 201559 201319 201160 201552 201561 201462 201363 201164 201465 201249 201366 present work

57.8

response which showed a smaller current density compared to 1.0 V because complete electro-oxidation of FA does not occur at this potential. Stable current vs time curves were obtained at 0.3 V for all three catalysts as negligible amounts of poisoning species were produced at this potential value. A literature comparison of electrochemical activities of some previously reported Pd-based catalysts for FA oxidation is shown in Table 3, and it is clear that the catalyst activity values reported in this manuscript surpass most of the previously reported catalysts.

which aimed to test the stability and tolerance of catalysts against the intermediate poisoning species.53,54 The current vs time curves recorded for Pd/C, Pd/rGO, and Pd30La70/rGO are shown in Figure 6a,b. The current density decreases sharply at the start of both experiments carried out at 0.3 and 1.0 V, followed by a stable current at the polarization point. The current decay for the FA oxidation indicates slow deactivation over time which indicates a little adsorption of carbonaceous intermediates over Pd/C, Pd/rGO, and Pd 30La 70 /rGO catalysts.57,58 Chronoamperometric experiments at 1.0 V show much higher tolerance of Pd30La70/rGO against poisonous intermediates compared to Pd/rGO and Pd/C. The current density of Pd30La70/rGO catalyst was reduced to 189.6 A/m2 (20.6% of the initial value of 919 A/m2) and becomes stabilized after 3000 s. In case of the Pd/rGO catalyst, the current density decreased to 84.1 A/m2 (15.6% of the initial value) after 3000 s and for Pd/C catalyst, it was decreased to 3.36 A/m2 (2.9% of the initial value) after 3000 s. The initial values of the current density were taken at 10 s to avoid hydrogen desorption current. These results indicate a very high activity and stability for the Pd30La70/rGO catalyst. This high stability of Pd30La70/rGO compared to Pd/rGO and Pd/C is due to the additional role of La species which facilitates and enhances the electron transfer during electrochemical testing. A similar behavior was also observed at 0.3 V current−time

4. CONCLUSIONS In this study, PdxLay in various metallic ratios electro-catalysts supported on rGO sheets were fabricated by conventional chemical reduction method. The synthesized catalysts were structurally and morphologically characterized via a range of techniques and tested as electro-catalysts for formic acid oxidation. During electrochemical testing, it was revealed that the metallic ratio of Pd and La in the catalyst is important and optimized to achieve the best catalytic activity. The obtained activity order for various catalysts is as follows; Pd30La70/rGO > Pd80La20/rGO > Pd70La30 rGO. The obtained mass specific activity for Pd30La70/rGO (986.42 A/g) is 2.18 times higher than for Pd/rGO (451 A/g) and 16 times higher than Pd/C (61.5 A/g) at given onset peak potentials. The current study also divulged that the presence of lanthanum/lanthanum oxide H


Research Article


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in the catalyst was found to enhance the catalytic activity which is attributed to its role in fine dispersion of the Pd particles on the support and to promoting the electron transfer during electrochemical reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09645. FTIR and Raman spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.A.S.). *Phone: +92-51-9064-2062. E-mail: [email protected] (M.A.N.). ORCID

Muhammad Arif Nadeem: 0000-0003-0738-9349 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Higher Education Commission (HEC) of Pakistan (No. 20-2704/NRPU/R&D/HEC/12). REFERENCES

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rGO Electro-catalysts for Formic Acid Oxidation

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