Trans. Nonferrous Met. Soc. China 25(2015) 2986-2993

Pd micro-nanoparticles electrodeposited on graphene/polyimide membrane for electrocatalytic oxidation of formic acid

Yan ZHANG, Qin WANG, Wei-chun YE, Jia-jia LI, Chun-ming WANG

College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China

Received 29 October 2014; accepted 28 May 2015

Abstract: A novel Pd electrocatalyst with flowerlike micro-nanostructures was synthesized by electrochemical deposition on a flexible graphene/polyimide (Gr/PI) composite membrane and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD). The Pd micro-nanoparticles were prepared on a COOH-CNTs/PI membrane as a comparative sample. The XRD and SEM investigations for Pd electrodeposition demonstrate that the particle size of Gr/PI composite membrane is smaller than that of COOH-CNTs/PI membrane, while the uniform and dense distribution of Pd micro-nanoparticles on the Gr/PI composite membrane is greater than that on the COOH-CNTs/PI membrane. The electrocatalytic properties of Pd/Gr/PI and Pd/COOH-CNTs/PI catalysts for the oxidation of formic acid were investigated by cyclic voltammetry (CV) and chronoamperometry (CA). It is found that the electrocatalytic activity and stability of Pd/Gr/PI are superior to those of Pd/COOH-CNTs/PI catalyst. This is because smaller metal particles and higher dense distribution desirably provide abundant catalytic sites and mean higher catalytic activity. Therefore, the Pd/Gr/PI catalyst has better catalytic performance for formic acid oxidation than the Pd/COOH-CNTs/PI catalyst.

Key words: Pd micro-nanoparticles; graphene/polyimide membrane; carboxyl carbon nanotubes/polyimide membrane; electro catalytic oxidation; formic acid; electrochemical deposition

1 Introduction

Recently, Pd-based nanomaterials are extensively used as anode electrocatalysts to catalyze formic acid oxidation in direct formic acid fuel cells (DFAFCs) [1-3]. However, the poor durability/stability of conventional Pd electrocatalysts has become a major obstacle for the commercialization of the DFAFC technology [4,5]. Although the reasons for the poor durability of Pd catalysts in DFAFCs are still not fully clear, some possibilities have been proposed: the aggregation of Pd micro-nanoparticles, the fast dissolution of Pd from the carbon support under the DFAFCs operating conditions, and CO accumulation at relatively high formic acid concentrations [4,5]. These difficulties warrant the development of highly durable and efficient Pd-based electrocatalysts for DFAFCs application.

The nanocomposite membrane based on carbon nanotubes (CNTs) and aromatic polyimides (PIs) is found to be an excellent support for loading electrocatalysts [6-8]. The membrane exhibits the excellent electrical property, chemical stability and high specific surface area of CNTs and the excellent thermal stability, dimensional stability, and mechanical properties of PIs [9-13]. However, in comparison with CNTs, grapheme (Gr), one-atom thick planar sheet of hexagonally arrayed sp² carbon atoms, has attracted tremendous scientific attention in recent years [14-19]. This two-dimensional (2D) material exhibits excellent physical and chemical properties, which makes it promising for potential applications in many technological fields, such as nanoelectronics, sensors, nanocomposites, batteries, supercapacitors and hydrogen storage [14]. Especially, graphene has potential application as a heterogeneous catalyst support in direct methanol fuel cells [20-23]. Additionally, the production cost of Gr in large quantities is much lower than that of CNTs [17,24]. So the Gr/PI material as substrate is believed to represent better corrosion resistance and chemical stability, higher specific surface area, enhanced mechanical performance and endurance of high potential (vs SCE) at low pH (pH<1) [13,25-28] than the CNTs/PI.

This work describes the preparation, characterization and electrocatalytic applications of Pd micro-nanoparticles electrodeposited on the Gr/PI nanocomposite membrane. The obtained film is slim, light and flexible, which makes it easily processed and recycled. The as-prepared Pd/Gr/PI catalyst shows higher electrocatalytic activity and stability than Pd/ COOH-CNTs/PI.

2 Experimental

2.1 Materials

4,4-oxydianiline (ODA) and 4,4-oxydiphthalic anhydride (ODPA) were purchased from Shanghai Chemical (Shanghai, China). N-methyl pyrrolidone (NMP) was redistilled. PdCl₂ was purchased from Sigma Aldrich Reagent Company and carboxyl carbon nanotubes (COOH-CNTs) were purchased from Chengdu Science and Technological Company. Graphene was synthesized by a modified Hummers and Offeman’s method [29].

2.2 Preparation of Gr/PI membrane

The preparation of Gr/PI membrane was as follows (Fig. 1). Firstly, a 1:1 molar ratio of ODA and ODPA were added into anhydrous NMP solvent and stirred for 24 h in the magnetic stirrer to obtain polyamic acid (PAA). Then, graphene was added into PAA solution and stirred fully. Thirdly, the solution containing graphene was paved onto a glass slide. Finally, the slide was evaporated at 60 °C for 3 h and sequentially heated up to 250 °C in 200 min. The Gr/PI membrane was obtained via separating from the glass slide. The COOH-CNTs/PI membrane was obtained under the same conditions.

2.3 Preparation of Pd/Gr/PI

Pd micro-nanoparticles were electrodeposited on the Gr/PI membrane in the aqueous solution of 5 mmol/L PdCl₂ + 0.1 mol/L NaCl. The deposition potential was 0.2 V and the deposition time was selected as 1000 s. The water was purified by Milli-Q pure water system (Millipore, electrical resistivity of 18.2 MΩ/cm). The Pd/COOH-CNTs/PI was prepared under the same process.

2.4 Characterization

The crystal structures of samples were identified by X-ray diffraction (XRD, RigakuD/Max-2400). The morphology and size of the material were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-944800, Japan).

Fig. 1 Schematic of fabrication and electrocatalytic application of Pd/Gr/PI film electrode

2.5 Electrochemical measurements

The electrochemical properties of the samples were measured by cyclic voltammetry in a standard three-electrode cell using A CHI 612E electrochemical workstation at room temperature. The as-prepared Gr/PI membrane was used as the working electrode. The preparation method of the working electrode has been detailed in the previous work [6,30]. A Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. To measure the formic acid electrooxidation reaction activity, cyclic voltammetry was performed between -0.2 and +0.8 V in a mixing solution containing 0.5 mol/L HCOOH and 0.5 mol/L H₂SO₄. The scan rate was set as 50 mV/s. And to evaluate the catalytic stability of catalysts, the chronoamperometry (CA) experiments were carried out in the solution of 0.5 mol/L H₂SO₄ + 0.5 mol/L HCOOH at a constant potential of 0.3 V.

3 Results and discussion

3.1 SEM and XRD analyses

Figures 2(a) and (c) show the surface morphology of COOH-CNTs/PI membrane and Gr/PI membrane, respectively. It can be observed that COOH-CNTs and Gr are dispersed uniformly in PI, what is more, the films’ surface are relatively flat. Controlling the size and dispersion of Pd micro-nanoparticles on the COOH- CNTs/PI and Gr/PI composite membrane is critical for their application in fuel cells [31]. The SEM analysis reveals that well defined flowerlike micro-nanostructure Pd particles, 0.9-1.2 μm in diameter, are uniformly dispersed on the COOH-CNTs/PI composite membrane surface (Fig. 2(b)). For Pd particles electrodeposited on the Gr/PI composite membrane surface, the particle size is from 0.5 to 0.85 μm (Fig. 2(d)). And the uniform and dense distribution of smaller Pd micro-nanoparticles on the Gr/PI composite membrane is greater than those on the COOH-CNTs/PI membrane. Therefore, the Gr/PI composite membrane is proved to be a better catalyst support than the COOH-CNTs/PI membrane.

Figure 3 shows the typical XRD patterns of Pd/COOH-CNTs/PI and Pd/Gr/PI catalysts, COOH- CNTs/PI and Gr/PI membranes. The diffraction peak observed at ~24.7° is attributed to the carbon materials. Furthermore, the diffraction peaks at 40°, 47°, 68°, 83° and 86° correspond to the Pd (111), (200), (220), (311) and (222) planes, respectively, which proves the typical character of a crystalline Pd face-centered cubic (FCC) pattern [32-37]. The XRD patterns indicate that Pd micro-nanoparticles electrodeposited on the two samples are both FCC crystals, while the peak intensity on the Gr/PI electrode is greater than that on the COOH-CNTs/PI electrode.

3.2 Electrochemical behaviors of Pd electrodeposition

The effect of the membrane on the Pd electrodeposition was investigated by cyclic voltammograms (CV) and chronoamperometic curve. Figure 4 shows the CV curves of the COOH-CNTs/PI and Gr/PI membranes electrodes in 0.1 mol/L NaCl and 5 mmol/L PdCl₂ solution at a scan rate of 10 mV/s. It can be seen that there are three coupled reduction-oxidation peaks on the COOH-CNTs/PI and Gr/PI membrane electrodes. A couple of reduction-oxidation peaks C1 and A1 belong to the reduction and oxidation of Pd, respectively. Also, the reduction of Pd starts from 0.45 V to peak C1 until the maximum peak current, and the cathode current achieves limiting diffusion current density and maintains stable until about 0.1 V. Another pair of slightly smaller peaks C2 and A2 should be attributed to the adsorption and desorption process of chloride ion. The most negative position of the pair of reduction-oxidation peaks C3 and A3 should be attributed to the adsorption and desorption process of H on Pd metal [38-40]. It is known that in the process of electrochemical deposition and dissolution of Pd on Pt and Au single crystal, PdCl₄^2- complex is very important [41,42], and the asymmetry of three coupled peaks is caused by the chloride ion and PdCl₄^2- complex adhering on Pd in the solution [43]. However, the relative negative shift of all reduction-oxidation peaks on Gr/PI electrode is caused by the differences of electrode surface. Besides, the current density on the Gr/PI electrode is greater than that on the COOH-CNTs/PI electrode due to the better conductivity of graphene.

Fig. 2 SEM images of blank COOH-CNTs/PI composition membrane (a), Pd catalysts electrodeposited on COOH-CNTs/PI membrane (b), blank Gr/PI composition membrane (c) and Pd catalysts electrodeposited on Gr/PI membrane (d)

Fig. 3 XRD patterns of Pd nanoflowers on COOH-CNTs/PI and Gr/PI membrane and their blank membranes

Fig. 4 CV curves measured for COOH-CNTs/PI and Gr/PI electrode in 0.1 mol/L NaCl +5 mmol/L PdCl₂ solution at scan rate of 10 mV/s

Various studies on the electrodeposited metals have shown that the shift of the growth process, from progressive to instantaneous, is brought about by a more negative potential for the electrolysis. According to the CV data of different electrodes shown in Fig. 4, the potential of 0.2 V (vs SCE) was chosen for the Pd electrochemical deposition. According to the model including diffusion-controlled growth of hemispherical particles proposed by SCHARIFKER and HILLS [44], the deposition transients for progressive and instantaneous nucleation can be described by the equations:

   (1)

  (2)

Figure 5 shows the dimensionless (J/J_max)² versus t/t_max plot and the theoretical curves of Pd electrodeposition in 0.1 mol/L NaCl + 5 mmol/L PdCl₂ solution on the Gr/PI electrode (J_max=0.135 mA, t_max=4.2 s) and COOH-CNTs/PI electrode (J_max=0.129 mA, t_max=4.8 s). It is clearly seen that the current transient on the Pd/COOH-CNTs/PI electrode and Pd/Gr/PI electrode correspond to the model involving instantaneous nucleation. Because the surface of COOH-CNTs has much attachment points which benefits for the nucleation of atoms and increases the nucleation rate at the initial stage. The current transient on Pd/Gr/PI fits relatively well with the theoretical curve for instantaneous nucleation.

Fig. 5 Dimensionless current transients for chronoamperometic deposition of Pd micro-nanoparticles on Gr/PI electrode and COOH-CNTs/PI electrode in 0.1 mol/L NaCl +5 mmol/L PdCl₂ solution at 0.2 V (t=1000 s)

Fig. 6 Cyclic voltammograms of Pd/COOH-CNTs/PI (a) and Pd/Gr/PI (c) electrodes in 0.5 mol/L H₂SO₄ solution at different scan rates, dependency of anodic and cathodic peak currents to square root of scan rates of Pd/COOH-CNTs/PI (b) and Pb/Cr/PI (d) electrodes

3.3 Electro-oxidation of formic acid

Figures 6(a) and (c) show the CVs of Pd catalyst on the COOH-CNTs/PI and Gr/PI electrodes, respectively, in 0.5 mol/L H₂SO₄ solution at a scan rate range of 10-90 mV/s. It is evident that the Pd/COOH-CNTs/PI catalyst shows the oxidation peak A1 and the reduction peak C1 of the Pd. The Pd/Gr/PI catalyst shows the oxidation peak a1 and the reduction peak c1 of the Pd and the hydrogen adsorption (H_ad) peak a2. According to the calculation, the obtained results are shown in Figs. 6(b) and (d). The peak current increases linearly with the square root of the scan rates. This indicates that the electrocatalytic oxidation of formic acid on the Pd/COOH-CNTs/PI and Pd/Gr/PI electrodes is a diffusion-controlled process in 0.5 mol/L H₂SO₄ [45]. The catalytic area of Pd catalyst electrodeposited on the electrode is theoretically calculated by the following equation [46]:

                                (3)

where R is the gas constant (8.314 J/(K·mol)), T is the experimental temperature, I_p refers to the peak current, v is the scan speed, A refers to the catalytic electrode area (here for 9 mm²), n refers to the electron transfer number in the process of reduction, and F is the Faraday constant. The τ^* values of Pd/COOH-CNTs/PI and Pd/Gr/PI are calculated to be 1.24×10^-9 and 1.24×10^-8 mol/cm², respectively. This indicates that the Pd/Gr/PI has higher electrode catalytic activity than the Pd/COOH-CNTs/PI. It is accessible that Pd micro-nanoparticles on the Gr/PI are more efficient catalysts in comparison with that on the COOH-CNTs/PI used as the catalyst support.

Figure 7 shows the cyclic voltammograms of Pd/Gr/PI and Pd/COOH-CNTs/PI catalysts in a solution of 0.5 mol/L HCOOH + 0.5 mol/L H₂SO₄ at room temperature. The forward scans of the CVs for the formic acid oxidation are characterized by a strong current peak A at 0.3 V and a shoulder peak B at 0.6 V. According to the previous Refs. [1,32,33,47] about Pd-based electrocatalysts for formic acid, peak A can be attributed to the direct oxidation of formic acid to form CO₂ (dehydrogenation path), while the peak B is related to the oxidation of the formic acid with the formation of intermediate CO generated from the dissociative adsorption step (dehydration path).

Fig. 7 Cyclic voltammograms for formic acid oxidation on Pd/COOH-CNTs/PI and Pd/Gr/PI catalysts recorded at 50 mV/s in 0.5 mol/L HCOOH + 0.5 mol/L H₂SO₄ solution

The oxidation process of formic acid can be expressed as two ways [48,49]: the direct oxidation way (Pathway I) and indirect oxidation way (Pathway II). Pathway I:

HCOOH→HCOO_ad+H⁺+e→CO₂+2H⁺+2e

Pathway II:

HCOOH→CO_ad+H₂O→CO₂+2H⁺+2e

As can be seen, the peak potentials of the formic acid oxidation at the two catalysts are similar, but the current density of Pd/Gr/PI electrode is higher than that of Pd/COOH-CNTs/PI electrode. The oxidation peak current density for the Pd/Gr/PI catalyst is 11.3 mA/cm², which is higher than 10.6 mA/cm² for the Pd/COOH- CNTs/PI. This result indicates that the electrocatalytic activity of Pd/Gr/PI is obviously higher than that of Pd/ COOH-CNTs/PI. This is because smaller metal particles and higher dense distribution desirably provide abundant catalytic sites and mean higher catalytic activity [31]. As a result, the Pd/Gr/PI catalysts have better catalytic performance for the oxidation of formic acid.

To evaluate the catalytic stability of catalysts, the chronoamperometry (CA) experiments were carried out in the solution of 0.5 mol/L H₂SO₄ + 0.5 mol/L HCOOH at a constant potential of 0.3 V, as shown in Fig. 8. The current densities for the formic acid oxidation reaction on the Pd/COOH-CNTs/PI and Pd/Gr/PI catalysts show the decay initially and reach a stable value after being polarized at 0.3 V for 600 s. The current decay for the HCOOH oxidation reaction indicates the slow deactivation of Pd-based electrocatalysts by the adsorption of CO or CO-like intermediates [4,5]. Nonetheless, the stable current density for the formic acid oxidation reaction on the Pd/Gr/PI is about 0.27 mA/cm², significantly higher than that on the Pd/COOH-CNTs/PI of 0.14 mA/cm². It indicates that the Pd/Gr/PI catalyst has a much better stability toward the process of formic acid catalytic oxidation. But compared with Refs. [50-52], the catalytic stability of Pd/C is greater than that of Pd/Gr/PI catalyst.

Fig. 8 Current density time plots for formic acid oxidation on Pd/COOH-CNTs/PI and Pd/Gr/PI catalysts measured at 0.3 V

4 Conclusions

1) A novel Pd nano-electrocatalyst for DFAFCs was fabricated by electrodeposition on the surface of Gr/PI film.

2) The XRD and SEM analyses demonstrate that the particle size of Gr/PI composite membrane is smaller than that of COOH-CNTs/PI membrane, while the uniform and dense distribution of Pd micro-nanoparticles on the Gr/PI composite membrane is greater than that on the COOH-CNTs/PI membrane.

3) The electrochemical experiments demonstrate that the electrocatalytic activity and stability of Pd/Gr/PI are superior to those of Pd/COOH-CNTs/PI catalyst. This is because smaller metal particles and higher dense distribution desirably provide abundant catalytic sites and mean higher catalytic activity.

4) The flexible Gr/PI composite membrane is proved to be a better catalyst support than the COOH-CNTs/PI membrane. It is expected that further applications of the proposed electrodes will be explored in future.

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Pd微纳米粒子在石墨烯/聚酰亚胺薄膜上的电沉积及其电催化氧化甲酸

张 焱，王 琴，叶为春，李佳佳，王春明

兰州大学 化学化工学院，兰州 730000

摘  要：利用电化学沉积法在柔韧的石墨烯/聚酰亚胺(Gr/PI)复合膜上制备一种新的花状、微纳结构的Pd电催化剂。作为对比研究，同时在羧基化碳纳米管/聚酰亚胺(COOH-CNTs/PI)上制备Pd微纳米粒子催化剂。对合成的Pd/Gr/PI和Pd/COOH-CNTs/PI进行X射线衍射(XRD)和扫描电子显微镜(SEM)分析。XRD和SEM分析结果表明：当沉积条件相同时，相对于在COOH-CNTs/PI膜上制备的Pd微纳米粒子，在Gr/PI复合膜上电沉积得到的Pd微纳米粒子尺寸更小，分布密度更大，且分布更均匀。通过循环伏安法(CV)和计时电流法(CA)研究这两种催化剂对甲酸的电催化氧化性能。结果表明：在Gr/PI电极上沉积得到的Pd的催化效率更高、稳定性更好。这是由于在Gr/PI电极上沉积得到的Pd颗粒尺寸更小，分布密度更大，催化活性点位更多，因而对甲酸的催化氧化效果更好。

关键词：Pd 微纳米粒子；Gr /PI薄膜；COOH-CNTs/PI薄膜；电催化氧化；甲酸；电化学沉积

(Edited by Mu-lan QIN)

Foundation item: Project (51372106) supported by the National Natural Science Foundation of China

Corresponding author: Chun-ming WANG; Tel: +86-931-8912596; Fax: +86-931-8912582; E-mail: wangcm@lzu.edu.cn

DOI: 10.1016/S1003-6326(15)63925-5