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Applied Surface Science 357 (2015) 994–999
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Enhanced formic acid electro-oxidation reaction on ternary Pd-Ir-Cu/C catalyst Jinwei Chen ∗ , Jie Zhang, Yiwu Jiang, Liu Yang, Jing Zhong, Gang Wang, Ruilin Wang ∗ College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 13 July 2015 Received in revised form 11 September 2015 Accepted 15 September 2015 Keywords: Formic acid Electro-oxidation Palladium Iridium Copper
a b s t r a c t Aim to further reduce the cost of Pd-Ir for formic acid electro-oxidation (FAEO), the Cu was used to construct a ternary metallic alloy catalyst. The prepared catalysts are characterized using XRD, TGA, EDX, TEM, XPS, CO-stripping, cyclic voltammetry and chronoamperometry. It is found that the Pd18 Ir1 Cu6 nanoparticles with a mean size of 3.3 nm are highly dispersed on carbon support. Componential distributions on catalyst are consistent with initial contents. Electrochemical measurements show that the PdIrCu/C catalyst exhibits the highest activity for FAEO. The mass activity of Pd in Pd18 Ir1 Cu6 /C at 0.16 V (vs. SCE) is about 1.47, 1.62 and 2.08 times as high as that of Pd18 Cu6 /C, Pd18 Ir1 /C and Pd/C, respectively. The activity enhancement of PdIrCu/C should be attributed to the weakened CO adsorption strength and the removal of adsorbed intermediates at lower potential with the addition of Cu and Ir. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Formic acid (HCOOH) has become a promising fuel [1–6] for direct liquid fuel cell due to its obvious advantages, such as less toxic, faster oxidation kinetics and lower crossover through Nafion membrane than methanol [7–9]. Pt and Pd are two primary catalytic metals for formic acid electro-oxidation (FAEO). But Pd catalysts have recently been found to possess superior performance for FAEO compared with Pt. It is generally accepted that the FAEO on Pd can mainly proceed through a direct dehydrogenation reaction mechanism to form CO2 [10] with less CO-like poisoning species generated, which is different from that of Pt with the main self-poisoning dehydration pathway [11,12]. However, the monometallic Pd may be rapidly deactivated owing to a spot of COads (the dehydration product) and the CO accumulation as a result of reduction of CO2 (the dehydrogenation product) at Hadsorbed/absorbed Pd surfaces [13]. Therefore, developing novel Pd-based catalysts with improved activity and durability is important for their practical applications. It has been demonstrated that the combination of Pd with a second metal M (M = Ni [14,15], Co [16], Fe [17], and Au [18,19]) greatly enhanced the catalytic performance of Pd. In our previous work [20], the PdIr catalyst showed lower onset and peak potentials and higher current density for FAEO than that of Pd. It was attributed
∗ Corresponding authors. Tel.: +86 28 85418018; fax: +86 28 85418018. E-mail addresses: [email protected] (J. Chen), [email protected] (R. Wang). http://dx.doi.org/10.1016/j.apsusc.2015.09.136 0169-4332/© 2015 Elsevier B.V. All rights reserved.
that the addition of Ir can produce more hydroxyl groups at lower potential to remove adsorbed CO intermediates [21]. However, the Ir element is precious metal and its usage should be reduced. To further upgrade the mass specific activity of noble metals, one promising strategy is to form a ternary catalyst with adding a third non-noble metal, by which the two addition elements may have synergistic effect to major catalyst [22]. Fortunately, we found Cu is a good candidate. Cu is a non-noble element and has been attracted great attention for Pd-based toward FAEO [23]. Therefore, in this work, we synthesized the ternary PdIrCu/C catalyst for the first time. Then the structural characterization and electrocatalytic activity of prepared catalysts were investigated. And the improvement mechanism of Ir and Cu to Pd for FAEO was discussed. 2. Experimental 2.1. Synthesis of catalysts Vulcan XC-72R carbon black was obtained from Cabot Company (Boston, USA). Certain amount of carbon black was pretreated in a concentrated nitric acid and sulfuric acid mixture at 40 ◦ C for 1 h under an ultrasonic condition to import oxygen-containing functional groups onto the surface of carbon. PdCl2 and H2 IrCl6 ·6H2 O were purchased from Shenyang Research Institute of Nonferrous Metals (Shenyang, China). All other reagents (AR, Chengdu Kelong Chemical, China) were used as received without further treatment. The functionalized carbon black was employed as catalyst support. Each mono-Pd/C, binary Pd18 Ir1 /C, Pd18 Cu6 /C and ternary
J. Chen et al. / Applied Surface Science 357 (2015) 994–999
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Pd18 Ir1 Cu6 /C were synthesized using PdCl2 , H2 IrCl6 ·6H2 O and Cu(NO3 )2 ·3H2 O as metal precursors. The metal loading of each catalyst is 20 wt%. The carbon supported Pd-based catalyst was prepared by an EG-assisted NaBH4 reduction method [24,25]. Briefly, 40 mg of pretreated carbon black and stoichiometric amount metal precursors were dispersed in 20 mL EG with sonication for 30 min, and then stirred for 2 h. A freshly prepared NaBH4 in EG solution was added dropwise into the above solution at 25 ◦ C and the resulting suspension was stirred for 3 h. The product was filtered, washed, and dried in a vacuum oven at 75 ◦ C for 12 h. For comparison, commercial Pt/C and Pd/C catalysts were purchased from Johnson Matthey (UK) and Sigma–Aldrich (USA), respectively. 2.2. Characterization of catalysts The X-ray diffraction (XRD) analyses were carried out on a DX-2000 X-ray diffractometer (Dandong Ltd., China). The thermogravimetric analysis (TGA) was carried out in a Q50 Instruments (TA, USA) in the presence of air following the variation of the percentage weight loss in the range between room temperature and 1073 K at 10 K min−1 of heating rate. The composition of the PdIrCu nanoparticles (NPs) was studied by JSM-7500F SEM (JEOL, Japan) with energy-dispersive X-rays (EDX) analysis. TEM images were obtained using a FEI, TECNAI G2 microscope (USA). X-ray photoelectron spectroscopy (XPS) also was used to analyze the as-attained samples. All electrochemical measurements were carried out in a conventional three-electrode electrochemical cell using cyclic voltammetry (CV) and chronoamperometry (CA) techniques on a CHI 760B (Shanghai Chenhua Instruments Ltd., China). The counter electrode was a graphite electrode and the reference electrode was a saturated calomel electrode (SCE). All potentials reported in this paper were referred to the SCE. A glassy carbon electrode (GCE, 3, surface area 7.0 mm2 ) deposited with a thin film of the prepared catalysts served as the working electrode. A 5.0 mg sample of the prepared catalyst was dispersed in a solution containing 1.0 mL of deionized water, 1.0 mL of isopropanol, and 50 L of a 5 wt% Nafion solution using 30 min of ultrasonification to form a uniform suspension. A 5 L sample of the dispersed catalyst suspension was pipetted onto the glassy-carbon substrate. The calculated loading of metal was 35 g cm−2 . The CVs and CAs were carried out in 0.5 M HCOOH + 0.5 M H2 SO4 solution deaerated by ultra-pure argon for 20 min before measurements. For CO-stripping voltammetry, a working electrode began with bubbling CO (>99.9% purity) in solution for 20 min at 0.05 V. Then, the dissolved CO was removed from the electrolyte by bubbling Ar for 30 min while maintaining the potential at 0.05 V. Finally, the CO-stripping voltammograms were recorded by scanning the potential from −0.2 to 0.8 V at 50 mV s−1 . 3. Results and discussion 3.1. Physical characterization of prepared catalysts Fig. 1 shows the XRD patterns of the prepared catalysts. The peak assigned to crystalline Ir or Cu is not identified. The first diffraction peak located at about 24.8◦ in all XRD patterns is associated to the (0 0 2) of carbon support. The other four peaks of homemade Pd/C catalyst (curve a) at 2 values of 39.4◦ , 45.8◦ , 66.8◦ and 80.4◦ are characteristic of face-centered cubic (fcc) crystalline Pd, corresponding of the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively. From the curve (b) to (d), it can be observed that the PdIr/C, PdCu/C and PdIrCu/C catalysts also show the Pd fcc lattice structure, but the four specific diffraction peaks of Pd shift to the higher 2 angles. These results indicate that Ir or Cu enter into the Pd crystal lattice. The PdCu and PdIrCu have obvious shift reflects more
Fig. 1. XRD patterns of the (a) Pd/C, (b) PdIr/C, (c) PdCu/C and (d) PdIrCu/C catalysts.
replacement of Pd atoms with smaller sized Cu atoms and higher PdCu alloy degree. TGA measurement was employed to specify the carbon contents. TGA curve of PdIrCu/C catalyst (Fig. 2A) reveals three different regions. The first region is the slight decrease at temperature below 400 K attributed to the evaporation of absorbed water [26]. The second region at 400–750 K is mainly associated to the reaction between C and O2 to form CO2 . The last region, at temperature beyond 750 K showed a stable zone associated with the real metal loading on carbon resulting in 19.5%. It can be estimated that the weight percent of carbon is around 80% which is consistent with the initial carbon contents. The components of PdIrCu NPs are investigated by EDX (Fig. 2B). The atomic ratio (AR) of Pd:Ir:Cu of the PdIrCu NPs is 18.2:1:5.1 which is close to the initial ratio. TEM characterization is more intuitive and reliable for the investigation of metal NPs. Fig. 3 shows TEM images of the prepared catalysts and their corresponding particle size distribution histograms. The ∼30 nm spherical particles are carbon blacks and the dark dots are metal NPs supported on the carbon black. The average particle sizes of the Pd/C, PdIr/C, PdCu/C and PdIrCu/C catalysts are 3.8, 3.7, 4.1 and 3.3 nm, respectively. As we know, charge transfer between the two metals can alter the electronic structure of alloy, resulting in a shift of d-band center. Ex situ XPS analysis was carried out to evaluate both core level binding energy and the near-surface composition on Pd-based catalysts. Fig. 4 shows the XPS spectra (A) and Pd core-level spectra (B) of the prepared catalysts. It can be observed that the Pd3d3/2 core-level binding energy (B.E.) of PdIr/C, PdCu/C and PdIrCu/C are 340.80, 340.68 and 340.69 eV, a negative shift of 0.17, 0.29 and 0.28 eV with respect to that on Pd/C (340.97 eV), respectively. This corelevel B.E. shift could be related to partial electron transfer from Cu to Pd. The charge transfer contributes to a lowered d-band center, resulting in weakened adsorption strength of reactive and intermediate species. And the addition of Cu has notable electron transfer. Furthermore, the XPS data can also be used to analyze approximately the compositions of surface layers. The AR of Pd:M for PdIr, PdCu and PdIrCu are estimated to be 17.8:1, 18.6:6, and 17.9:1:6.1, respectively. 3.2. Electrocatalytic activities for formic acid oxidation The CVs in Fig. 5(A) were obtained in 0.5 M H2 SO4 solution. Each the 5th cycle of four catalysts in sulfuric acid was chose for comparison. At the potential region below 0 V, the CVs are featured with adsorption and desorption of Had on Pd. The first anodic H desorption peak shifts negatively in Pd-based binary and ternary
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Fig. 2. TGA curve (A) and SEM-EDX (B) of PdIrCu/C catalyst.
catalysts, suggesting the weakened adsorption energy due to a downshift of Pd d-band center. Since bulk Pd can absorb hydrogen, the real electrochemical active surface area (EASA) must be estimated by CO-stripping measurements [27]. CVs with and without COads on prepared electrodes are shown in Fig. 5(B). The EASA of Pd/C, PdIr/C, PdCu/C and PdIrCu/C electrodes are estimated using the stripping peak area to be 1.31, 1.46, 1.32 and 1.40 cm2 , respectively. Even if the NPs size of PdIrCu is smaller than that of Pd, the PdIrCu electrode does not show significant large EASA. This is attributed to the decrease adsorption strength and quantity of CO on Pd-based catalysts. It will be further proved by the CO oxidation potential. The onset potential of CO oxidation on PdIr/C
and PdCu/C shifts negatively by 20 mV compared to that on Pd/C, and the PdIrCu/C has a 120 mV negative shift. The peak potentials of CO oxidation on Pd-based catalysts also have negative shift. Fig. 6 shows the activity of different catalysts for FAEO. Fig. 6(A) shows CVs (only present forward scan curves) of four prepared catalysts for FAEO. It can be observed that the Pd/C and PdCu/C have the similar onset and peak potential of FAEO, at around −0.1 V and 0.22 V, respectively. But the current density (current normalized to geometrical area of GCE) of PdCu/C is higher than that of Pd/C (58.6 vs. 49.3 mA cm−2 ). The addition of Ir will further promote the electrocatalytic activity for FAEO. It can be observed that the onset
Fig. 3. TEM images of (a) Pd/C, (b) PdIr/C, (c) PdCu/C and (d) PdIrCu/C. Insets show size distributions of carbon supported nanoparticles.
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Fig. 4. (A) XPS spectra and (B) Pd core-level spectra for tested Pd-based catalysts.
Fig. 5. (A) CVs and (B) CO-stripping voltammograms of prepared catalysts in 0.5 M H2 SO4 solution at a scan rate of 50 mV s−1 .
Fig. 6. (A) CVs of formic acid oxidation on the catalysts in 0.5 M H2 SO4 solution containing 0.5 M HCOOH with a scan rate of 50 mV s−1 ; (B–C) the current normalized to the EASA and the Pd loading, respectively; (D) CAs in 0.5 M H2 SO4 solution containing 0.5 M HCOOH at 0.15 V up to 1000 s.
potential of FAEO on PdIr and PdIrCu shift negatively by 50 mV compared to that on Pd and PdCu, which locate around −0.15 V. The peak current density of PdIr and PdIrCu/C catalysts are 53.8 and 76.6 mA cm−2 , respectively.
For avoiding the particle size and area effect, specific activity of prepared catalysts were estimated by normalizing current to EASA. The results are presented in Fig. 6(B). It can be observed that the Pd-based catalysts still have higher specific activity at the
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Table 1 Comparison of catalytic activity of the catalysts. Sample
EASA (cm2 )
Onset potential (V vs. SCE)
Current density (mA cm−2 )
Specific activity (mA cm−2 )
Mass activity (mA mg−1 ) (@0.2 V) Pd
C-Pt/C C-Pd/C H-Pd/C PdIr/C PdCu/C PdIrCu/C
– – 1.31 1.46 1.32 1.4
0.05 −0.1 −0.1 −0.15 −0.1 −0.15
– – 49.3 53.8 58.6 76.6
– – 2.6 2.58 3.1 3.9
470 (@0.66 V) 1240 1430 1720 2050 2900
potential region between −0.1 V and 0.16 V. The peak specific activity of PdIrCu/C catalyst is 3.9 mA cm−2 , which is much higher than that of Pd/C (2.6 mA cm−2 ). It indicates that the activity enhancement of PdIrCu/C should not be attributed to the particle size or area effect. As to the utilization of precious metal in Pd-based catalysts, the mass activity of Pd (current normalized to Pd loading) is shown in Fig. 6(C). The mass activity of Pd in PdIrCu/C cata, which is about 1.47, 1.62 and 2.08 times lyst is 2900 mA mg−1 Pd higher than that of PdCu/C, PdIr/C and Pd/C, respectively. Moreover, as to the mass activity of PdIr in PdIrCu/C vs. PdIr/C is 2620 vs. 1550 mA mg−1 . To further evaluate the activity of Pd-based PdIr catalysts, commercial Pt/C and Pd/C were employed as reference catalyst. The inset of Fig. 6(C) presents a typical CV curve of FAEO on C-Pt/C catalyst [28,29], in which the reaction proceeds slowly to reach a plateau at 0.3 V. This corresponds to formic acid oxidation through the dehydrogenation path, but the coverage by COads simultaneously continues to grow and then causes only relatively small currents. At potentials more positive than 0.55 V, the reaction is significantly accelerated because of the oxidation of COads . An anodic peak emerges at 0.66 V with current density of 470 mA mg−1 . The C-Pd/C catalyst exhibits higher activity than Pt C-Pt/C. It shows an anodic peak at 0.2 V with current density of 1240 mA mg−1 . The homemade Pd/C presents slight higher activity Pd than C-Pd/C. Moreover, it can be observed that the IrCu/C has no activity for FAEO. It can be concluded that the Pd-based catalysts present higher activity than C-Pt/C and C-Pd/C. All the comparative electrochemical data are presented in Table 1. To further investigate the activity and short term stability of the catalysts, CA curves of the four samples toward FAEO were conducted at 0.15 V for a period of time. As shown in Fig. 6(D), at the beginning the potentiostatic currents decreased rapidly for all catalysts due to the formation of double layer capacitance. The following current decrease should originate from the loss of surface active sites caused by the adsorption of intermediate species on the catalyst surface [30]. In the region of activation polarization, the PdIrCu/C exhibits the highest current density, and its final mass activity which is 3.5 times as high as that for Pd/C. So far, it is demonstrated that PdIrCu/C exhibits best catalytic activity for FAEO. As to the enhancement mechanism of Ir and Cu to Pd for FAEO, it might be explained by the weakened CO adsorption strength and the removal of adsorbed intermediates at lower potential. The weakened CO adsorption strength should be mainly caused by the addition of Cu. The XPS analysis shows that the Pd3d3/2 core-level binding energy of PdCu-based catalyst has about 0.3 eV negative shifts, resulting in a lower d-band center and weakens adsorption strength of intermediate species. The CO-stripping analysis further confirmed the decrease adsorption quantity of CO on PdCu-based catalysts. Furthermore, the adsorbed COads still need be oxidized to CO2 (Eq. (2)). H2 Oads -(Ir, CuorPd) → OHads -(Ir, CuorPd) + H+ + e−
(1)
COads -Pd + OHads -(Ir, CuorPd) → CO2 + H+ + e−
(2)
The oxidation of COads needs the oxygen-containing species like OHads which can be generated by the activation of H2 O (Eq. (1)). The activation of H2 O on the Pd surface is difficult, and relatively high positive potential are needed. However, the addition of Ir and Cu will create more oxygen-containing species at lower potential [31]. It also can be confirmed by CO-stripping analysis that the peak potential of CO oxidation on PdIr and PdCu catalysts is more negative than that of Pd. Moreover, it can be observed that the Ir has a significant effect on the onset and peak potential of CO oxidation. Therefore, we believe that the Ir has main effect on the removal of adsorbed intermediates at lower potential. In summary, the high electrocatalytic activity of PdIrCu/C should be attributed to the electronic effect and bi-functional mechanism. 4. Conclusions In this work, mono-Pd/C, binary PdIr/C, PdCu/C and ternary PdIrCu/C catalysts were synthesized. It is found that the PdIrCu NPs have a mean size of 3.3 nm with a narrow size distribution and a high dispersion on carbon support. The addition of Ir and Cu to Pd brings the best catalytic activity of PdIrCu/C catalyst for FAEO. The mass activity of Pd in PdIrCu/C at 0.16 V is about 1.47, 1.62 and 2.08 times higher than that of PdCu/C, PdIr/C and Pd/C, respectively. The activity enhancement of PdIrCu/C should be attributed to the weakened CO adsorption strength and the removal of adsorbed intermediates at lower potential. The lower cost and higher performance of PdIrCu/C catalyst make it a good choice for further direct formic acid fuel cell. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC, 21306119), the Provincial Natural Science Foundation of Sichuan (2013FZ0034, 2013JY0150), the Outstanding Young Scientist Foundation of Sichuan University (2013SCU04A23). References [1] J. Chang, L. Feng, C. Liu, W. Xing, X. Hu, An effective Pd-Ni2P/C anode catalyst for direct formic acid fuel cells, Angew. Chem. Int. Ed. 53 (2014) 122–126. [2] L. Zhang, S.-I. Choi, J. Tao, H.-C. Peng, S. Xie, Y. Zhu, Z. Xie, Y. Xia, Pd-Cu bimetallic tripods: a mechanistic understanding of the synthesis and their enhanced electrocatalytic activity for formic acid oxidation, Adv. Funct. Mater. 24 (2014) 7520–7529. [3] Q. Huayu, H. Huajie, W. Xin, Design and synthesis of palladium/graphitic carbon nitride/carbon black hybrids as high-performance catalysts for formic acid and methanol electrooxidation, J. Power Sources 275 (2015) 734–741. [4] J. Chen, G. Wang, X. Wang, C. Jiang, S. Zhu, R. Wang, Synthesis of highly dispersed Pd nanoparticles with high activity for formic acid electro-oxidation, J. Mater. Res. 28 (2013) 1553–1558. [5] J. Chen, G. Wang, X. Wang, X. Yang, S. Zhu, R. Wang, Effect of NaBH4 concentration and synthesis temperature on the performance of Pd/C catalyst for formic acid electro-oxidation, Mater. Express 3 (2013) 176–180. [6] S.R. Hosseini, R. Hosseinzadeh, S. Ghasemi, N. Farzaneh, Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic Pt-Cu nanoparticles and its application for formic acid oxidation, Inter. J. Hydrogen Energy 40 (2015) 2182–2192. [7] A. Nouralishahi, A.M. Rashidi, Y. Mortazavi, A.A. Khodadadi, M. Choolaei, Enhanced methanol electro-oxidation reaction on Pt-CoOx/MWCNTs hybrid electro-catalyst, Appl. Surf. Sci. 335 (2015) 55–64.
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Enhanced formic acid electro-oxidation reaction on ternary Pd-Ir-Cu/C catalyst Jinwei Chen ∗ , Jie Zhang, Yiwu Jiang, Liu Yang, Jing Zhong, Gang Wang, Ruilin Wang ∗ College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 13 July 2015 Received in revised form 11 September 2015 Accepted 15 September 2015 Keywords: Formic acid Electro-oxidation Palladium Iridium Copper
a b s t r a c t Aim to further reduce the cost of Pd-Ir for formic acid electro-oxidation (FAEO), the Cu was used to construct a ternary metallic alloy catalyst. The prepared catalysts are characterized using XRD, TGA, EDX, TEM, XPS, CO-stripping, cyclic voltammetry and chronoamperometry. It is found that the Pd18 Ir1 Cu6 nanoparticles with a mean size of 3.3 nm are highly dispersed on carbon support. Componential distributions on catalyst are consistent with initial contents. Electrochemical measurements show that the PdIrCu/C catalyst exhibits the highest activity for FAEO. The mass activity of Pd in Pd18 Ir1 Cu6 /C at 0.16 V (vs. SCE) is about 1.47, 1.62 and 2.08 times as high as that of Pd18 Cu6 /C, Pd18 Ir1 /C and Pd/C, respectively. The activity enhancement of PdIrCu/C should be attributed to the weakened CO adsorption strength and the removal of adsorbed intermediates at lower potential with the addition of Cu and Ir. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Formic acid (HCOOH) has become a promising fuel [1–6] for direct liquid fuel cell due to its obvious advantages, such as less toxic, faster oxidation kinetics and lower crossover through Nafion membrane than methanol [7–9]. Pt and Pd are two primary catalytic metals for formic acid electro-oxidation (FAEO). But Pd catalysts have recently been found to possess superior performance for FAEO compared with Pt. It is generally accepted that the FAEO on Pd can mainly proceed through a direct dehydrogenation reaction mechanism to form CO2 [10] with less CO-like poisoning species generated, which is different from that of Pt with the main self-poisoning dehydration pathway [11,12]. However, the monometallic Pd may be rapidly deactivated owing to a spot of COads (the dehydration product) and the CO accumulation as a result of reduction of CO2 (the dehydrogenation product) at Hadsorbed/absorbed Pd surfaces [13]. Therefore, developing novel Pd-based catalysts with improved activity and durability is important for their practical applications. It has been demonstrated that the combination of Pd with a second metal M (M = Ni [14,15], Co [16], Fe [17], and Au [18,19]) greatly enhanced the catalytic performance of Pd. In our previous work [20], the PdIr catalyst showed lower onset and peak potentials and higher current density for FAEO than that of Pd. It was attributed
∗ Corresponding authors. Tel.: +86 28 85418018; fax: +86 28 85418018. E-mail addresses: [email protected] (J. Chen), [email protected] (R. Wang). http://dx.doi.org/10.1016/j.apsusc.2015.09.136 0169-4332/© 2015 Elsevier B.V. All rights reserved.
that the addition of Ir can produce more hydroxyl groups at lower potential to remove adsorbed CO intermediates [21]. However, the Ir element is precious metal and its usage should be reduced. To further upgrade the mass specific activity of noble metals, one promising strategy is to form a ternary catalyst with adding a third non-noble metal, by which the two addition elements may have synergistic effect to major catalyst [22]. Fortunately, we found Cu is a good candidate. Cu is a non-noble element and has been attracted great attention for Pd-based toward FAEO [23]. Therefore, in this work, we synthesized the ternary PdIrCu/C catalyst for the first time. Then the structural characterization and electrocatalytic activity of prepared catalysts were investigated. And the improvement mechanism of Ir and Cu to Pd for FAEO was discussed. 2. Experimental 2.1. Synthesis of catalysts Vulcan XC-72R carbon black was obtained from Cabot Company (Boston, USA). Certain amount of carbon black was pretreated in a concentrated nitric acid and sulfuric acid mixture at 40 ◦ C for 1 h under an ultrasonic condition to import oxygen-containing functional groups onto the surface of carbon. PdCl2 and H2 IrCl6 ·6H2 O were purchased from Shenyang Research Institute of Nonferrous Metals (Shenyang, China). All other reagents (AR, Chengdu Kelong Chemical, China) were used as received without further treatment. The functionalized carbon black was employed as catalyst support. Each mono-Pd/C, binary Pd18 Ir1 /C, Pd18 Cu6 /C and ternary
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Pd18 Ir1 Cu6 /C were synthesized using PdCl2 , H2 IrCl6 ·6H2 O and Cu(NO3 )2 ·3H2 O as metal precursors. The metal loading of each catalyst is 20 wt%. The carbon supported Pd-based catalyst was prepared by an EG-assisted NaBH4 reduction method [24,25]. Briefly, 40 mg of pretreated carbon black and stoichiometric amount metal precursors were dispersed in 20 mL EG with sonication for 30 min, and then stirred for 2 h. A freshly prepared NaBH4 in EG solution was added dropwise into the above solution at 25 ◦ C and the resulting suspension was stirred for 3 h. The product was filtered, washed, and dried in a vacuum oven at 75 ◦ C for 12 h. For comparison, commercial Pt/C and Pd/C catalysts were purchased from Johnson Matthey (UK) and Sigma–Aldrich (USA), respectively. 2.2. Characterization of catalysts The X-ray diffraction (XRD) analyses were carried out on a DX-2000 X-ray diffractometer (Dandong Ltd., China). The thermogravimetric analysis (TGA) was carried out in a Q50 Instruments (TA, USA) in the presence of air following the variation of the percentage weight loss in the range between room temperature and 1073 K at 10 K min−1 of heating rate. The composition of the PdIrCu nanoparticles (NPs) was studied by JSM-7500F SEM (JEOL, Japan) with energy-dispersive X-rays (EDX) analysis. TEM images were obtained using a FEI, TECNAI G2 microscope (USA). X-ray photoelectron spectroscopy (XPS) also was used to analyze the as-attained samples. All electrochemical measurements were carried out in a conventional three-electrode electrochemical cell using cyclic voltammetry (CV) and chronoamperometry (CA) techniques on a CHI 760B (Shanghai Chenhua Instruments Ltd., China). The counter electrode was a graphite electrode and the reference electrode was a saturated calomel electrode (SCE). All potentials reported in this paper were referred to the SCE. A glassy carbon electrode (GCE, 3, surface area 7.0 mm2 ) deposited with a thin film of the prepared catalysts served as the working electrode. A 5.0 mg sample of the prepared catalyst was dispersed in a solution containing 1.0 mL of deionized water, 1.0 mL of isopropanol, and 50 L of a 5 wt% Nafion solution using 30 min of ultrasonification to form a uniform suspension. A 5 L sample of the dispersed catalyst suspension was pipetted onto the glassy-carbon substrate. The calculated loading of metal was 35 g cm−2 . The CVs and CAs were carried out in 0.5 M HCOOH + 0.5 M H2 SO4 solution deaerated by ultra-pure argon for 20 min before measurements. For CO-stripping voltammetry, a working electrode began with bubbling CO (>99.9% purity) in solution for 20 min at 0.05 V. Then, the dissolved CO was removed from the electrolyte by bubbling Ar for 30 min while maintaining the potential at 0.05 V. Finally, the CO-stripping voltammograms were recorded by scanning the potential from −0.2 to 0.8 V at 50 mV s−1 . 3. Results and discussion 3.1. Physical characterization of prepared catalysts Fig. 1 shows the XRD patterns of the prepared catalysts. The peak assigned to crystalline Ir or Cu is not identified. The first diffraction peak located at about 24.8◦ in all XRD patterns is associated to the (0 0 2) of carbon support. The other four peaks of homemade Pd/C catalyst (curve a) at 2 values of 39.4◦ , 45.8◦ , 66.8◦ and 80.4◦ are characteristic of face-centered cubic (fcc) crystalline Pd, corresponding of the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively. From the curve (b) to (d), it can be observed that the PdIr/C, PdCu/C and PdIrCu/C catalysts also show the Pd fcc lattice structure, but the four specific diffraction peaks of Pd shift to the higher 2 angles. These results indicate that Ir or Cu enter into the Pd crystal lattice. The PdCu and PdIrCu have obvious shift reflects more
Fig. 1. XRD patterns of the (a) Pd/C, (b) PdIr/C, (c) PdCu/C and (d) PdIrCu/C catalysts.
replacement of Pd atoms with smaller sized Cu atoms and higher PdCu alloy degree. TGA measurement was employed to specify the carbon contents. TGA curve of PdIrCu/C catalyst (Fig. 2A) reveals three different regions. The first region is the slight decrease at temperature below 400 K attributed to the evaporation of absorbed water [26]. The second region at 400–750 K is mainly associated to the reaction between C and O2 to form CO2 . The last region, at temperature beyond 750 K showed a stable zone associated with the real metal loading on carbon resulting in 19.5%. It can be estimated that the weight percent of carbon is around 80% which is consistent with the initial carbon contents. The components of PdIrCu NPs are investigated by EDX (Fig. 2B). The atomic ratio (AR) of Pd:Ir:Cu of the PdIrCu NPs is 18.2:1:5.1 which is close to the initial ratio. TEM characterization is more intuitive and reliable for the investigation of metal NPs. Fig. 3 shows TEM images of the prepared catalysts and their corresponding particle size distribution histograms. The ∼30 nm spherical particles are carbon blacks and the dark dots are metal NPs supported on the carbon black. The average particle sizes of the Pd/C, PdIr/C, PdCu/C and PdIrCu/C catalysts are 3.8, 3.7, 4.1 and 3.3 nm, respectively. As we know, charge transfer between the two metals can alter the electronic structure of alloy, resulting in a shift of d-band center. Ex situ XPS analysis was carried out to evaluate both core level binding energy and the near-surface composition on Pd-based catalysts. Fig. 4 shows the XPS spectra (A) and Pd core-level spectra (B) of the prepared catalysts. It can be observed that the Pd3d3/2 core-level binding energy (B.E.) of PdIr/C, PdCu/C and PdIrCu/C are 340.80, 340.68 and 340.69 eV, a negative shift of 0.17, 0.29 and 0.28 eV with respect to that on Pd/C (340.97 eV), respectively. This corelevel B.E. shift could be related to partial electron transfer from Cu to Pd. The charge transfer contributes to a lowered d-band center, resulting in weakened adsorption strength of reactive and intermediate species. And the addition of Cu has notable electron transfer. Furthermore, the XPS data can also be used to analyze approximately the compositions of surface layers. The AR of Pd:M for PdIr, PdCu and PdIrCu are estimated to be 17.8:1, 18.6:6, and 17.9:1:6.1, respectively. 3.2. Electrocatalytic activities for formic acid oxidation The CVs in Fig. 5(A) were obtained in 0.5 M H2 SO4 solution. Each the 5th cycle of four catalysts in sulfuric acid was chose for comparison. At the potential region below 0 V, the CVs are featured with adsorption and desorption of Had on Pd. The first anodic H desorption peak shifts negatively in Pd-based binary and ternary
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Fig. 2. TGA curve (A) and SEM-EDX (B) of PdIrCu/C catalyst.
catalysts, suggesting the weakened adsorption energy due to a downshift of Pd d-band center. Since bulk Pd can absorb hydrogen, the real electrochemical active surface area (EASA) must be estimated by CO-stripping measurements [27]. CVs with and without COads on prepared electrodes are shown in Fig. 5(B). The EASA of Pd/C, PdIr/C, PdCu/C and PdIrCu/C electrodes are estimated using the stripping peak area to be 1.31, 1.46, 1.32 and 1.40 cm2 , respectively. Even if the NPs size of PdIrCu is smaller than that of Pd, the PdIrCu electrode does not show significant large EASA. This is attributed to the decrease adsorption strength and quantity of CO on Pd-based catalysts. It will be further proved by the CO oxidation potential. The onset potential of CO oxidation on PdIr/C
and PdCu/C shifts negatively by 20 mV compared to that on Pd/C, and the PdIrCu/C has a 120 mV negative shift. The peak potentials of CO oxidation on Pd-based catalysts also have negative shift. Fig. 6 shows the activity of different catalysts for FAEO. Fig. 6(A) shows CVs (only present forward scan curves) of four prepared catalysts for FAEO. It can be observed that the Pd/C and PdCu/C have the similar onset and peak potential of FAEO, at around −0.1 V and 0.22 V, respectively. But the current density (current normalized to geometrical area of GCE) of PdCu/C is higher than that of Pd/C (58.6 vs. 49.3 mA cm−2 ). The addition of Ir will further promote the electrocatalytic activity for FAEO. It can be observed that the onset
Fig. 3. TEM images of (a) Pd/C, (b) PdIr/C, (c) PdCu/C and (d) PdIrCu/C. Insets show size distributions of carbon supported nanoparticles.
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Fig. 4. (A) XPS spectra and (B) Pd core-level spectra for tested Pd-based catalysts.
Fig. 5. (A) CVs and (B) CO-stripping voltammograms of prepared catalysts in 0.5 M H2 SO4 solution at a scan rate of 50 mV s−1 .
Fig. 6. (A) CVs of formic acid oxidation on the catalysts in 0.5 M H2 SO4 solution containing 0.5 M HCOOH with a scan rate of 50 mV s−1 ; (B–C) the current normalized to the EASA and the Pd loading, respectively; (D) CAs in 0.5 M H2 SO4 solution containing 0.5 M HCOOH at 0.15 V up to 1000 s.
potential of FAEO on PdIr and PdIrCu shift negatively by 50 mV compared to that on Pd and PdCu, which locate around −0.15 V. The peak current density of PdIr and PdIrCu/C catalysts are 53.8 and 76.6 mA cm−2 , respectively.
For avoiding the particle size and area effect, specific activity of prepared catalysts were estimated by normalizing current to EASA. The results are presented in Fig. 6(B). It can be observed that the Pd-based catalysts still have higher specific activity at the
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Table 1 Comparison of catalytic activity of the catalysts. Sample
EASA (cm2 )
Onset potential (V vs. SCE)
Current density (mA cm−2 )
Specific activity (mA cm−2 )
Mass activity (mA mg−1 ) (@0.2 V) Pd
C-Pt/C C-Pd/C H-Pd/C PdIr/C PdCu/C PdIrCu/C
– – 1.31 1.46 1.32 1.4
0.05 −0.1 −0.1 −0.15 −0.1 −0.15
– – 49.3 53.8 58.6 76.6
– – 2.6 2.58 3.1 3.9
470 (@0.66 V) 1240 1430 1720 2050 2900
potential region between −0.1 V and 0.16 V. The peak specific activity of PdIrCu/C catalyst is 3.9 mA cm−2 , which is much higher than that of Pd/C (2.6 mA cm−2 ). It indicates that the activity enhancement of PdIrCu/C should not be attributed to the particle size or area effect. As to the utilization of precious metal in Pd-based catalysts, the mass activity of Pd (current normalized to Pd loading) is shown in Fig. 6(C). The mass activity of Pd in PdIrCu/C cata, which is about 1.47, 1.62 and 2.08 times lyst is 2900 mA mg−1 Pd higher than that of PdCu/C, PdIr/C and Pd/C, respectively. Moreover, as to the mass activity of PdIr in PdIrCu/C vs. PdIr/C is 2620 vs. 1550 mA mg−1 . To further evaluate the activity of Pd-based PdIr catalysts, commercial Pt/C and Pd/C were employed as reference catalyst. The inset of Fig. 6(C) presents a typical CV curve of FAEO on C-Pt/C catalyst [28,29], in which the reaction proceeds slowly to reach a plateau at 0.3 V. This corresponds to formic acid oxidation through the dehydrogenation path, but the coverage by COads simultaneously continues to grow and then causes only relatively small currents. At potentials more positive than 0.55 V, the reaction is significantly accelerated because of the oxidation of COads . An anodic peak emerges at 0.66 V with current density of 470 mA mg−1 . The C-Pd/C catalyst exhibits higher activity than Pt C-Pt/C. It shows an anodic peak at 0.2 V with current density of 1240 mA mg−1 . The homemade Pd/C presents slight higher activity Pd than C-Pd/C. Moreover, it can be observed that the IrCu/C has no activity for FAEO. It can be concluded that the Pd-based catalysts present higher activity than C-Pt/C and C-Pd/C. All the comparative electrochemical data are presented in Table 1. To further investigate the activity and short term stability of the catalysts, CA curves of the four samples toward FAEO were conducted at 0.15 V for a period of time. As shown in Fig. 6(D), at the beginning the potentiostatic currents decreased rapidly for all catalysts due to the formation of double layer capacitance. The following current decrease should originate from the loss of surface active sites caused by the adsorption of intermediate species on the catalyst surface [30]. In the region of activation polarization, the PdIrCu/C exhibits the highest current density, and its final mass activity which is 3.5 times as high as that for Pd/C. So far, it is demonstrated that PdIrCu/C exhibits best catalytic activity for FAEO. As to the enhancement mechanism of Ir and Cu to Pd for FAEO, it might be explained by the weakened CO adsorption strength and the removal of adsorbed intermediates at lower potential. The weakened CO adsorption strength should be mainly caused by the addition of Cu. The XPS analysis shows that the Pd3d3/2 core-level binding energy of PdCu-based catalyst has about 0.3 eV negative shifts, resulting in a lower d-band center and weakens adsorption strength of intermediate species. The CO-stripping analysis further confirmed the decrease adsorption quantity of CO on PdCu-based catalysts. Furthermore, the adsorbed COads still need be oxidized to CO2 (Eq. (2)). H2 Oads -(Ir, CuorPd) → OHads -(Ir, CuorPd) + H+ + e−
(1)
COads -Pd + OHads -(Ir, CuorPd) → CO2 + H+ + e−
(2)
The oxidation of COads needs the oxygen-containing species like OHads which can be generated by the activation of H2 O (Eq. (1)). The activation of H2 O on the Pd surface is difficult, and relatively high positive potential are needed. However, the addition of Ir and Cu will create more oxygen-containing species at lower potential [31]. It also can be confirmed by CO-stripping analysis that the peak potential of CO oxidation on PdIr and PdCu catalysts is more negative than that of Pd. Moreover, it can be observed that the Ir has a significant effect on the onset and peak potential of CO oxidation. Therefore, we believe that the Ir has main effect on the removal of adsorbed intermediates at lower potential. In summary, the high electrocatalytic activity of PdIrCu/C should be attributed to the electronic effect and bi-functional mechanism. 4. Conclusions In this work, mono-Pd/C, binary PdIr/C, PdCu/C and ternary PdIrCu/C catalysts were synthesized. It is found that the PdIrCu NPs have a mean size of 3.3 nm with a narrow size distribution and a high dispersion on carbon support. The addition of Ir and Cu to Pd brings the best catalytic activity of PdIrCu/C catalyst for FAEO. The mass activity of Pd in PdIrCu/C at 0.16 V is about 1.47, 1.62 and 2.08 times higher than that of PdCu/C, PdIr/C and Pd/C, respectively. The activity enhancement of PdIrCu/C should be attributed to the weakened CO adsorption strength and the removal of adsorbed intermediates at lower potential. The lower cost and higher performance of PdIrCu/C catalyst make it a good choice for further direct formic acid fuel cell. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC, 21306119), the Provincial Natural Science Foundation of Sichuan (2013FZ0034, 2013JY0150), the Outstanding Young Scientist Foundation of Sichuan University (2013SCU04A23). References [1] J. Chang, L. Feng, C. Liu, W. Xing, X. Hu, An effective Pd-Ni2P/C anode catalyst for direct formic acid fuel cells, Angew. Chem. Int. Ed. 53 (2014) 122–126. [2] L. Zhang, S.-I. Choi, J. Tao, H.-C. Peng, S. Xie, Y. Zhu, Z. Xie, Y. Xia, Pd-Cu bimetallic tripods: a mechanistic understanding of the synthesis and their enhanced electrocatalytic activity for formic acid oxidation, Adv. Funct. Mater. 24 (2014) 7520–7529. [3] Q. Huayu, H. Huajie, W. Xin, Design and synthesis of palladium/graphitic carbon nitride/carbon black hybrids as high-performance catalysts for formic acid and methanol electrooxidation, J. Power Sources 275 (2015) 734–741. [4] J. Chen, G. Wang, X. Wang, C. Jiang, S. Zhu, R. Wang, Synthesis of highly dispersed Pd nanoparticles with high activity for formic acid electro-oxidation, J. Mater. Res. 28 (2013) 1553–1558. [5] J. Chen, G. Wang, X. Wang, X. Yang, S. Zhu, R. Wang, Effect of NaBH4 concentration and synthesis temperature on the performance of Pd/C catalyst for formic acid electro-oxidation, Mater. Express 3 (2013) 176–180. [6] S.R. Hosseini, R. Hosseinzadeh, S. Ghasemi, N. Farzaneh, Synthesis of poly (2-Methoxyaniline)/sodium dodecyl sulfate film including bimetallic Pt-Cu nanoparticles and its application for formic acid oxidation, Inter. J. Hydrogen Energy 40 (2015) 2182–2192. [7] A. Nouralishahi, A.M. Rashidi, Y. Mortazavi, A.A. Khodadadi, M. Choolaei, Enhanced methanol electro-oxidation reaction on Pt-CoOx/MWCNTs hybrid electro-catalyst, Appl. Surf. Sci. 335 (2015) 55–64.
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