Polydopamine-functionalized multi-walled carbon nanotubes-supported palladium–lead bimetallic alloy nanoparticles as highly efficient and robust catalysts for ethanol oxidation

Abstract

High performance electrocatalysts of palladium–lead bimetallic alloy nanoparticles anchored onto polydopamine-functionalized multi-walled carbon nanotubes (PDA-MWCNTs) were fabricated by a facile one-step strategy. The PDA-MWCNTs, a superior electrocatalyst support, could be easily obtained through a mild and friendly method. Then, the palladium–lead alloy nanoparticles with different amounts of lead were uniformly anchored on the PDA-MWCNTs through a facile and surfactant-free one-step co-reduction approach. The morphologies, structures, compositional and electronic properties of the catalysts were analyzed by various techniques. As direct ethanol alkaline fuel cells (DEAFCs) catalysts, the new electrocatalysts exhibited improved electrocatalytic activities and enhanced electrochemical stabilities due to the synergistic effect of PDA and Pb as explained by the dispersive effect, bi-functional mechanism and d-band theory. In particular, the Pd₃Pb/PDA-MWCNTs, as the proposed catalyst, possesses a larger electrochemical active surface area, higher electrocatalytic activity, more negative onset oxidative potential and stability for ethanol electrooxidation compared to other as-prepared electrocatalysts. Therefore, the proposed electrocatalyst is a promising catalyst for DEAFCs and the PDA-MWCNTs is believed to have potential use in other fields as well.

---

1 Introduction

Direct liquid fuel cells, as a type of attractive energy conversion device for electronics vehicles, have attracted interest recently due to their high efficiency and low environmental pollution.^1,2 Among various types of liquid fuel, ethanol is a promising candidate because of its low toxicity, easy manipulation as well as high theoretical energy density. More importantly, ethanol can be easily produced in large quantities by the fermentation of biomass from agriculture and the organic fractioning of municipal solid wastes.^3,4 Besides, the kinetics of ethanol electrooxidation in alkaline medium is faster relative to in an acid medium.⁵ Therefore, direct ethanol alkaline fuel cells (DEAFCs) are more attractive and realistic with the rapid development of alkaline anion-exchange membranes.⁶

A highly active anode catalyst with high durability is extremely desired to fulfill the commercialization of DEAFCs. At present, Pd and Pd-based catalysts have emerged as attractive options for the application of DEAFCs owing the fact that Pd has a low price, a high abundance, high electrocatalytic activity and great resistance to intermediate products for ethanol oxidation in alkaline medium among various precious metals.^7–9 However, the activity of Pd for ethanol oxidation also needs to be enhanced before achieving practical application in DEAFCs. Alloying Pd with a second metal is an alternative effective approach to enhance electrocatalytic activity, which can reduce the loading of Pd and improve the electrocatalytic activity with a higher tolerance for the poisoning effect.¹⁰ In particular, among these metal elements, Pb has stronger promoting effects, such as the geometric effect, the electronic effect and the bifunctional mechanism.^10–13

Apart from the active metal regulation, a suitable support is also the key to enhance electrocatalytic activity. Multi-walled carbon nanotubes (MWCNTs) have been considered as a promising catalyst support system because of its high specific surface area, electrical conductivity and good chemical stability.¹⁴ As is well known, the performance of catalysts relies on the size, surface cleanness and dispersity.^15,16 Therefore, surface functionalization of MWCNTs is necessary to overcome the drawbacks of “pristine” MWCNTs used in such applications, such as insufficient binding sites for anchoring the nanoparticles (NPs) and poor dispersion. According to the literature, polydopamine (PDA) can interact with the surfaces of substrates by intensive covalent and noncovalent binding, which offers a new strategy for surfaces modification.^17,18 Consequently, PDA is a promising alternative to functionalize MWCNTs, combining advantages of both covalent and noncovalent functionalization.

Based on the above considerations, high performance electrocatalysts that combined the palladium–lead alloy NPs and the PDA functionalized MWCNTs (PDA-MWCNTs) were fabricated by a facile one-step strategy in the current work. The PDA-MWCNTs can be easily obtained through a mild and friendly method. Then, the palladium–lead alloy NPs, with different amounts of lead, were anchored onto the PDA-MWCNTs by a facile and surfactant-free one-step co-reduction approach. Systematic characterizations were carried out to obtain information on the morphologies, structures, composition and electronic properties for the as-prepared catalysts. More importantly, the new electrocatalysts were applied in DEAFCs and their electrocatalytic activity and durability was studied under half-cell conditions. Moreover, the synergistic effects of PDA and Pb toward ethanol electrooxidation were also explained by the dispersive effect, bi-functional mechanism and d-band theory according to the results of this work and corroborated by previous reports.

2 Experiments

High-purity multi-walled carbon nanotubes (MWCNTs) were supplied by Chengdu Organic Chemicals Co. Ltd, Chinese Academy of Sciences (Chengdu, PR China). All analytically pure reagents were used as received without any further purification. Water used in the experiments was deionized and doubly distilled.

2.1 Synthetic and preparation procedures

The fabrication route of the new catalysts is summarized in Scheme 1. The specific steps of the preparation were shown as follows:

Scheme 1 Schematic of the synthesis process of the catalysts.

“Pristine” MWCNTs (50 mg) were ultrasonicated in Tris buffer (pH = 8.5, 100 mL) for 30 min in an ice–salt bath. Subsequently, dopamine hydrochloride solution (2 mg mL⁻¹, 10 mL) was added into the above dispersion and the reaction was vigorously stirred at 60 °C for 24 hours. In the end, the product was filtrated with a 400 nm nanofiltration membrane, washed by water and absolute ethanol several times and dried under reduced pressure at 60 °C overnight.

An appropriate amount of metal precursors (K₂PdCl₄ and Pb(NO₃)₂, appropriate ratios) were dissolved in water. The PDA-MWCNTs or MWCNTs (20 mg) were dispersed in H₂O (20 mL) with ultrasonic treatment to form a uniform suspension. Then, the appropriate metal precursor solution was added into the suspension under stirring and the mixture was stirred for 30 min at room temperature. Subsequently, excess amounts of 0.01 M NaBH₄ (freshly prepared) were added dropwise into the solution and the mixtures were stirred for 3 h at room temperature. After that, the black solid material was separated by filtration, washed repeatedly with water and absolute ethanol several times and finally dried in a vacuum at 60 °C for 12 h. The final catalysts were Pd/MWCNTs, Pd/PDA-MWCNTs, Pd₅Pb/PDA-MWCNTs, Pd₃Pb/PDA-MWCNTs and PdPb/PDA-MWCNTs. The weight percentage of Pd was approximately 15 wt% in all catalysts.

2.2 Sample characterization

Elemental analysis was conducted with a conventional combustion method (CHN, varioMLCRO) based on the burn-off mass of the sample and on the analysis of the evolved gases using a thermal conductivity detector. Inductively coupled plasma atomic emission spectrometer (ICP-AES) analysis was recorded on a Perkin Elmer instrument (Optima-4300DV). Raman spectra were performed using inVia Reinishaw confocal spectroscopy with 633 nm laser excitation. The X-ray powder diffraction (XRD) patterns were measured on a Rigaku D/max-2400 diffractometer using Cu-Kα radiation as the X-ray source. X-ray photoelectron spectroscopy (XPS) measurements were performed with a PHI-5702 X-ray photoelectron spectrometer. Transmission electron microscopy (TEM) images were obtained on a Tecnai G² F³⁰ electron microscope operating at 300 kV.

2.3 Electrochemical measurements

Electrochemical measurements were recorded using a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Factory, China) and carried out in a standard three-electrode cell at room temperature. A platinum wire was used as the counter electrode and an Hg/HgO electrode was used as the reference. The working electrode was a glassy carbon electrode (GCE) of 3 mm in diameter (surface area of 0.07065 cm²) that was carefully polished with Al₂O₃ powder (Aldrich, 0.05 mm) until a mirrored finish was obtained. In order to prepare the catalyst ink, 2 mg of the catalyst was dispersed into 2 mL solution of double distilled water, ethanol and 0.5 wt% Nafion solution (10 : 9 : 1) with ultrasonic treatment to produce a homogeneous suspension. Then, the well-dispersed suspension (5 μL) was drop-casted onto the GCE, which was then dried in air at room temperature.

3 Results and discussion

The morphology and structure of the catalysts was examined using TEM (Fig. 1) one hundred nanoparticles based on the TEM images were randomly selected to determine the average particle sizes and the distribution histograms. It can be observed that the Pd NPs were non-uniformly attached on the surface of MWCNTs with a wide diameter (2–9 nm) and the mean sizes of NPs were estimated to be 5.4 nm. In addition, parts of small Pd NPs seriously aggregated to become large particles. In comparison, the Pd NPs and Pd–Pb NPs were equably anchored onto the surface of PDA-MWCNTs. The average diameters of the Pd NPs and Pd₃Pb NPs were calculated to be 4.2 nm and 3.5 nm, and they exhibited a narrow particle-size distribution (2–7 nm and 2–5.5 nm), respectively. In the high-magnification TEM (HRTEM) images of an individual nanoparticle, the fringe patterns with lattice spacing of Pd for Pd/MWCNTs and Pd/PDA-MWCNTs were clearly observed and about 0.226 nm correspond to the distance of (111) planes of Pd crystals with a face-center cubic (fcc) structure. Moreover, the spacing value of the (111) planes increased with the introduction of Pb and the Pd₃Pb shows a spacing value of 0.230 nm, confirming the formation of the Pd–Pd alloy NPs. The small mean particles size, narrow size distribution and good distribution of NPs for PDA-MWCNTs were attributed to the role of PDA, which can immobilize Pd precursors (PdCl₄²⁻) onto the PDA-MWCNTs surface through coordination and prevent NPs from growth and agglomeration during the reaction process.^19,20 The result of TEM indicated that PDA-MWCNTs, as a support, is better than MWCNTs for metal nanoparticle dispersion. Furthermore, the uniform dispersion of alloyed nanoparticles supported on PDA-MWCNTs provides the basis for the high performance of the catalyst.¹⁰ Table 1 shows the Pd and Pb contents by ICP-AES analysis and the Pd/Pb atomic ratios of the as-prepared catalysts.

Fig. 1 TEM images of (A) Pd/MWCNTs, (C) Pd/PDA-MWCNTs and (E) Pd₃Pb/PDA-MWCNTs (inset: HRTEM images of the Pd NPs crystal structure). Pd NPs size distribution histograms of (B) Pd/MWCNTs, (D) Pd/PDA-MWCNTs and (F) Pd₃Pb/PDA-MWCNTs.

Table 1 The Pd and Pb contents by ICP-AES analysis and the Pd/Pb atomic ratios of the as-prepared catalysts

Catalysts	Pd content (wt%)	Pb content (wt%)	Pd/Pb atomic ratios
Pd/MWCNTs	12.98	—	—
Pd/PDA-MWCNTs	14.41	—	—
Pd5Pb/PDA-MWCNTs	14.31	6.11	4.8
Pd3Pb/PDA-MWCNTs	14.29	9.78	2.8
PdPb/PDA-MWCNTs	14.14	24.77	1.1

---

Additional evidence of the MWCNTs functionalization was provided by Raman spectroscopy. Conventionally, as shown in Fig. 2A, MWCNTs and PDA-MWCNTs show three characteristic peaks at 1351 cm⁻¹, 1590 cm⁻¹ and 2706 cm⁻¹, assigned to D, G and G* bands, respectively. It is well-known that the D band is usually associated with defects and amorphous carbon impurities of carbon nanotubes, and the G band is attributed to the ordered sp² hybridized carbon network.^21,22 Therefore, the relative intensity ratio of the D band and G band (I_D/I_G) can be used to probe the effectiveness of functionalization. Interestingly, the analysis reveals that the functionalization process reduced the value of I_D/I_G from 0.73 for “pristine” MWCNTs to 0.58 for PDA-MWCNTs as shown in Fig. 2A. According to other functionalization processes, the results indicate that the diffuse defect sites of MWCNTs are covered by PDA or reacted with PDA.^8,19 Considering these conformational changes in the Raman spectra, the efficiency of MWCNTs functionalization achieved for PDA-MWCNTs is high. The N content, determined using the combustion CHN method, was 1.26 wt%. Fig. 2B displays the XRD patterns of Pd/PDA-MWCNTs, Pd₅Pb/PDA-MWCNTs, Pd₃Pb/PDA-MWCNTs and PdPb/PDA-MWCNTs, as well as Pd/MWCNTs for comparison. Simulated XRD patterns for the Pd phase using the Joint Committee Powder Diffraction Standard (JCPDS) of bulk Pd (No. 87-0645) is indicated by the solid bars in the bottom portion of the figure for comparison. All of the XRD profiles exhibited a broad peak around 26.3° that correspond to the (002) reflections of hexagonal graphite for MWCNTs.²¹ The other four diffraction peaks of the catalysts around 40.3°, 46.4°, 67.3° and 80.9° were attributed to the (111), (200), (220) and (311) crystalline planes of the Pd fcc lattices, respectively.²¹ Additionally, several small diffraction peaks were observed in the XRD pattern of the PdPb/PDA-MWCNTs, which is likely due to crystalline Pb or Pb oxides on the catalyst.¹⁰ However, no obvious diffraction peaks related to Pb or Pb oxides were observed in the Pd₃Pb/PDA-MWCNTs and Pd₅Pb/PDA-MWCNTs catalysts, which may be the Pb existing in amorphous phases.¹³ Moreover, the addition of Pb causes clear negative shifts of the Pd peaks for these catalysts, demonstrating that parts of the Pb atoms were alloyed with Pd.¹⁰

Fig. 2 (A) Raman spectra of (a) MWCNTs and (b) PDA-MWCNTs. (B) XRD patterns of (a) Pd/MWCNTs, (b) Pd/PDA-MWCNTs, (c) Pd₅Pb/PDA-MWCNTs, (d) Pd₃Pb/PDA-MWCNTs and (e) PdPb/PDA-MWCNTs.

The surface composition and chemical states of the as-prepared catalysts were analyzed by XPS and illustrated in Fig. 3. The N 1s peaks were obviously observed at 400.1 eV for Pd/PDA-MWCNTs and Pd₃Pb/PDA-MWCNTs while it does not appear for Pd/MWCNTs, further confirming that the MWCNTs were successfully functionalized through the PDA (Fig. 3A and B). In addition, the presence of metallic Pb was detected for Pd₃Pd/PDA-MWCNTs in the XPS survey spectra, and the main doublet of Pb 4f at 139.1 eV (Pb 4f_7/2) and 143.9 eV (Pb 4f_5/2) were characteristic of metallic Pb (Fig. 3A and C).^11,12 The stronger peaks, Pb(0), represent that more Pb was alloyed with Pd, which caused the slight change in lattice parameter of the catalyst (as exhibited in the XRD patterns).¹⁰ As presented in Fig. 3A and D, the Pd 3d signals were observed for all catalysts and divided into two pairs of asymmetric peaks (Pd 3d_5/2 and Pd 3d_3/2 peaks), indicating the metallic state of Pd was the main species in the as-prepared catalysts.⁸ It is worth noting that the peak of Pd 3d_3/2 on the Pd/PDA-MWCNTs negatively shifted about 0.4 eV compared to Pd/MWCNTs, which suggests a bigger electron-donating effect was rendered towards Pd NPs by the PDA-MWCNTs than that of MWCNTs.¹⁰ Thence, the electronic effect may promote the catalytic activity of the catalyst by altering its adsorptive property towards poisoning species.¹¹ Moreover, the peak of Pd 3d_3/2 on the Pd₃Pb/PDA-MWCNTs positively shifted 0.3 eV, in contrast to Pd/PDA-MWCNTs with the adding of Pb, which may be due to a synergetic action of Pb(0). The presence of Pb in Pd can increase the lattice distance and consequently lead to an increase in the d-orbital vacancy in the Pd₃Pb/PDA-MWCNTs catalyst. On account of the increase of d-orbital vacancy, the adsorption of both reactants and intermediates, in a modest manner, on the catalyst is weakened, which is beneficial for the electrooxidation of ethanol.¹² Therefore, the potential advantage of the Pd–Pb alloyed catalyst against poisoning from intermediates is improved.

Fig. 3 XPS of (A) survey spectra and high-resolution spectra of (B) N 1s, (C) Pb 4f, (D) Pd 3d for (a) Pd/MWCNTs, (b) Pd/PDA-MWCNTs and (c) Pd₃Pb/PDA-MWCNTs.

The electrochemical properties of the as-prepared catalysts were studied in KOH solution by cyclic voltammetry (CV). Fig. 4 shows the CV scans of the catalysts in a solution of 1 M KOH and the currents are normalized with respect to the electrode area. Pd/MWCNTs and Pd/PDA-MWCNTs manifest similar shapes of curves, whereas a different form emerges for the Pd–Pb alloy catalysts. In two typical processes of CV scans in KOH solution, the broad peaks appearing between −0.95 and −0.55 V can be ascribed to the hydrogen desorption/adsorption regions of the catalysts, which can be clearly observed for Pd/MWCNTs and Pd/PDA-MWCNTs.²³ With the addition of Pb atoms, the peaks of hydrogen desorption/adsorption are suppressed in diverse degrees and become less defined, indicating partial coating of Pd active sites by Pb atoms.¹⁰ Moreover, the anodic peaks in the potential region of −0.30 to −0.10 V originated from the redox of surface Pd oxide. Obviously, the surface redox peaks for all catalysts are distinctly shown and the intensity of the surface redox peaks also relies heavily on Pd/Pb atomic ratios, as displayed in Fig. 4. The geometrical property for the electrode material is used to illustrate by the electrochemically active surface area (ECSA). As described in previous research, the ECSA of the electrodes was measured by integrating the coulombic charge (Q_S) for the reduction of palladium oxide between −0.30 to −0.10 V. This is because a certain amount of hydrogen on the surface of NPs inevitably absorbs into the interior of the Pd lattice space; thus, it is impossible to quantitatively differentiate between the under-potential from deposited hydrogen and absorbed hydrogen.^24,25 The ECSA values can be estimated based on eqn (1).

where Q_S is the coulombic charge (μC) determined by integrating the current peak of PdO reduction, Q_C is the proportionality constant used to relate charge with area and m is the catalyst loading (g). A charge value of 4.05 C m⁻² was assumed for the reduction of the palladium oxide monolayer. The ECSA values were calculated to be 11.8, 13.7, 25.3, 49.8 and 38.2 m² g⁻¹ for Pd/MWCNTs, Pd/PDA-MWCNTs, Pd₅Pb/PDA-MWCNTs, Pd₃Pb/PDA-MWCNTs and PdPb/PDA-MWCNTs, respectively. The ECSA of Pd/PDA-MWCNTs was larger than that of Pd/MWCNTs, owing to the role of PDA-MWCNTs. With incorporation of Pb into the catalysts, the Pd–Pb NPs become smaller and more Pd atoms were expose on the NPs surface. However, the excess of lead atoms will occupy the sites of the palladium atoms. Therefore, the Pd₃Pb/PDA-MWCNTs had the largest ECSA among all the as-prepared catalysts. According to above results, the PDA-MWCNTs was confirmed as a better electrochemical catalyst support than MWCNTs and the appropriate Pd/Pb atomic ratio was 3 : 1.

Fig. 4 CVs of (a) Pd/MWCNTs, (b) Pd/PDA-MWCNTs, (c) Pd₅Pb/PDA-MWCNTs, (d) Pd₃Pb/PDA-MWCNTs and (e) PdPb/PDA-MWCNTs in 1 M KOH solution at a scan rate of 50 mV s⁻¹.

The electrooxidation of ethanol by the as-prepared electrocatalysts was evaluated by CV under half-cell conditions. Fig. 5 presents the CVs of ethanol oxidation and the magnified curves between −0.7 and −0.3 V on the catalysts in nitrogen-saturated KOH (1 M) solution containing ethanol (1 M) at a scan rate of 50 mV s⁻¹. As clearly observable, two well-defined peaks were present. The peaks in the forward scans corresponded to ethanol oxidation, whereas the peaks in the backward scans were attributed to the further electrooxidation of incompletely oxidized carbonaceous species.²⁶ Generally, the magnitude of the anodic peak current (I_f) and the onset potentials (E_s) in the forward scan was used to directly evaluate the catalytic activity of the catalyst. In order to facilitate the comparison of the electrocatalytic activities of the catalysts, the I_f (normalized with respect to the electrode area) and E_s in the forward scan are depicted along with various catalysts, as listed in Table 2 and shown in Fig. 6. As seen, the I_f of Pd/PDA-MWCNTs was 37.8 mA cm⁻², which was higher than that of Pd/MWCNTs (16.4 mA cm⁻²). In particular, with the increment of Pb content in the catalysts, the value of I_f firstly increases from 73.1 mA cm⁻² (Pd₅Pb/PDA-MWCNTs) to 135.1 mA cm⁻² (Pd₃Pb/PDA-MWCNTs) and then declines to 110.9 mA cm⁻² (PdPb/PDA-MWCNTs), demonstrating a gradually improved electrocatalytic activity. Similarly, the E_s for the as-prepared electrocatalysts presented the same trend as I_f. The E_s for ethanol electrooxidation on the Pd/PDA-MWCNTs (−0.52 V) was 80 mV more negative than that of Pd/MWCNTs (−0.44 V). The E_s first negatively shifted from −0.58 V (Pd₅Pb/PDA-MWCNTs) to −0.62 V (Pd₃Pb/PDA-MWCNTs), then positively shifted to −0.59 V (PdPb/PDA-MWCNTs). As is well known, the negative shift of E_s indicates the significant enhancement in the kinetics of the ethanol electrooxidation reaction.⁴ In the light of the results of I_f and E_s, PDA-MWCNTs was obviously a better support compared to MWCNTs with the same metal component in the catalyst. Moreover, the Pd catalytic activity in ethanol electrooxidation could be promoted by adding Pb, while excessive Pb probably leads to over-coverage of Pd active sites.^10,27 Furthermore, the Pd₃Pb/PDA-MWCNTs, which possessed the highest I_f and most negative E_s among all the as-prepared electrocatalysts, exhibited the best catalytic activity for ethanol electrooxidation. Therefore, the catalyst activity for ethanol electrooxidation achieved the best results when the Pd/Pb atomic ratio in this catalytic system is 3 : 1.

Fig. 5 CVs of (A) (a) Pd/MWCNTs, (b) Pd/PDA-MWCNTs, (c) Pd₅Pb/PDA-MWCNTs, (d) Pd₃Pb/PDA-MWCNTs and (e) PdPb/PDA-MWCNTs. (B) Magnified curves between −0.7 and −0.3 V in nitrogen-saturated 1 M KOH + 1 M C₂H₅OH solution at a scan rate of 50 mV s⁻¹.

Table 2 Comparison of electrocatalytic activity with different catalysts

Catalysts	Es (V)	If (mA m^−2)	Is (mA m^−2)
Pd/MWCNTs	−0.44	16.4	2.8
Pd/PDA-MWCNTs	−0.52	37.8	3.7
Pd5Pb/PDA-MWCNTs	−0.58	73.1	5.8
Pd3Pb/PDA-MWCNTs	−0.62	135.1	9.2
PdPb/PDA-MWCNTs	−0.59	110.9	6.9

---

Fig. 6 Peak current density (I_f) and the onset potentials (E_s) in the forward scan of (a) Pd/MWCNTs, (b) Pd/PDA-MWCNTs, (c) Pd₅Pb/PDA-MWCNTs, (d) Pd₃Pb/PDA-MWCNTs and (e) PdPb/PDA-MWCNTs in nitrogen-saturated 1 M KOH + 1 M C₂H₅OH solution.

The anti-poisoning abilities and long-term activities of the electrocatalysts were further explored by chronoamperometry (CA) and the CA curves were carried out in a nitrogen-saturated KOH (1 M) and ethanol (1 M) solution under a constant potential of −0.20 V. As shown in Fig. 7, all CA curves present a rapidly decaying current density at first, which may be caused by the formation of intermediates and poisoning species during the ethanol oxidation, and then becomes relatively stable.²⁸ However, the current density decay of Pd/PDA-MWCNTs was much slower than that of Pd/MWCNTs during the whole test. In addition, the decay rate was lower with Pb in the catalysts. The decrease in current density for Pd₃Pb/PDA-MWCNTs was the slowest during the entire test. Moreover, it was clear that the quasi-steady current value (I_s, summarized in Table 2) of Pd₃Pb/PDA-MWCNTs (9.2 mA cm⁻²) was also higher than that of Pd/MWCNTs (2.8 mA cm⁻²), Pd/PDA-MWCNTs (3.7 mA cm⁻²), Pd₅Pb/PDA-MWCNTs (5.8 mA cm⁻²) and PdPb/PDA-MWCNTs (6.9 mA cm⁻²). The results of CA tests proved that the Pd₃Pb/PDA-MWCNTs had a higher catalytic activity and better stability for ethanol oxidation than the other as-prepared catalysts, which was in agreement with the results of CV. In order to further study the role of the PDA, the electrochemical and electrocatalytic activity of the PDA-free catalytic system and Pd₃Pb-MWCNTs were investigated for comparison (Fig. S1, Table S1†). Apparently, the PDA-MWCNTs-based catalysts had high catalytic activity and good stability.

Fig. 7 Chronoamperometric curves of (a) Pd/MWCNTs, (b) Pd/PDA-MWCNTs, (c) Pd₅Pb/PDA-MWCNTs, (d) Pd₃Pb/PDA-MWCNTs and (e) PdPb/PDA-MWCNTs for ethanol electrooxidation at −0.20 V in nitrogen-saturated 1 M KOH + 1 M C₂H₅OH solution.

As is known, the adsorption of the intermediary CO species is an important reason for the poisoning of Pd-based catalysts. Therefore, the anti-poisoning effect of CO on the as-prepared catalysts was investigated (Fig. S2, Table S2†). Obviously, the onset potential of CO oxidation (E^co_onset) and the oxidation peak for CO (E^co_f) on Pd₃Pb/PDA-MWCNTs were more negative than that of the other as-prepared catalysts. It exhibited a significant increase in the anti-poisoning ability owing to the introduction of PDA and Pb.²⁹ Moreover, the forward scan I_f for Pd₃Pb/PDA-MWCNTs was 128.1 mA cm⁻² after 500 cycles and showed only a slight loss of current density compared with the initial cycle. The Pb content for Pd₃Pb/PDA-MWCNTs was 9.59 wt%, suggesting this was enough for a strong interaction between alloy NPs and PDA.

According to the results of this work and previous literature, the improved catalytic activity and good stability of the proposed catalyst (Pd₃Pb/PDA-MWCNTs) may be mainly attributed to the following reasons: (1) the role of PDA, the small sizes and good dispersion of NPs that are anchored on the PDA-MWCNTs (verified by TEM), which makes the large ECSA. In addition, the PDA-MWCNTs-based catalysts were well dispersed and can be uniformly drop-casted onto the working electrode surface because of the good hydrophilicity of PDA, which was conducive to ethanol oxidation. Thence, the small size and high dispersion of the NPs supported on PDA-MWCNTs, large ECSA and good dispersion determined the electrocatalytic activities and stabilities of the fuel cells. (2) The incorporation of Pb into the catalysts can act as a key factor for improving the catalytic activity and enhancing long-term durability towards ethanol electrooxidation. Because Pb, as second metal, can activate water at lower potentials than Pd, the activated water can oxidize the adsorbed intermediate, which releases Pd active sites, resulting in improving catalytic activity.¹³ And the introduction of Pb can induce abundant oxygenated species and promote oxidative removal of CO_ad through a bifunctional mechanism.¹⁰ Especially, the OH_ads species, as the key species catalyzing the ethanol electrooxidation reaction, can be generated on Pb atoms by the discharge of OH⁻ in alkaline solution, which contributes to the increase in the oxidation currents of ethanol at lower electrode potentials.³⁰ Then, the OH_ads species on Pb atoms can rapidly react with the intermediate species adsorbed on the Pd sites, which is the rate-determining step of the ethanol electrooxidation reaction process on the surface of the Pd electrode in alkaline media.³¹ Therefore, the Pd active sites can be liberated for further adsorption of the reactive species. Moreover, the d-band center of Pd may be shifted up after Pb is combined with Pd, which may promote the good catalytic performance of the catalyst.¹³ However, the excess of Pb atoms would not alloy with Pd and occupy the Pd sites, which is not conducive to ethanol electrooxidation. Therefore, Pd₃Pb/PDA-MWCNTs is the proposed catalyst based on the synergistic effects of PDA and Pb.

4 Conclusions

In conclusion, the high performance electrocatalysts of palladium–lead alloy NPs anchored on PDA-MWCNTs were fabricated by a facile one-step strategy. The PDA-MWCNTs, a superior electrocatalyst support, can be easily obtained through a mild and friendly method. Then, palladium–lead alloy NPs with different amounts of lead were uniformly immobilized onto the PDA-MWCNTs through a facile and surfactant-free one-step co-reduction approach. To obtain the information of the morphologies, structures, compositional and electronic properties for the as-prepared catalysts, systematic characterizations were carried out. Moreover, the new electrocatalysts were used as DEAFCs catalyst and exhibited improved electrocatalytic activities and enhanced electrochemical stabilities. In particular, the Pd₃Pb/PDA-MWCNTs, as the proposed catalyst, possessed a larger ECSA, higher electrocatalytic activity, more negative onset oxidative potential and higher stability for ethanol electrooxidation compared to the other as-prepared electrocatalysts. Based on the previous reports and the results of this work, the superior performance of the proposed catalyst toward ethanol electrooxidation may be ascribed to the synergistic effect of PDA and Pb, as explained by the dispersive effect, bi-functional mechanism and d-band theory. Therefore, the proposed electrocatalysts are promising catalysts for direct alcohol fuel cells and PDA-MWCNTs is believed to have potential use in other fields as well.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (lzujbky-2015-16 and lzujbky-2015-17).

References

1. C. Laberty-Robert, K. Valle, F. Pereira and C. Sanchez, Chem. Soc. Rev., 2011, 40, 961–1005.
2. E. Reddington, A. Sapienza, B. Gurau, R. Viswanathan, S. Sarangapani, E. S. Smotkin and T. E. Mallouk, Science, 1998, 280, 1735–1737.
3. S. Song and P. Tsiakaras, Appl. Catal., B, 2006, 63, 187–193.
4. J. Ma, J. Wang, G. Zhang, X. Fan, G. Zhang, F. Zhang and Y. Li, J. Power Sources, 2015, 278, 43–49.
5. L. Jiang, A. Hsu, D. Chu and R. Chen, Int. J. Hydrogen Energy, 2010, 35, 365–372.
6. J. R. Varcoe, R. C. T. Slade, E. Lam How Yee, S. D. Poynton, D. J. Driscoll and D. C. Apperley, Chem. Mater., 2007, 19, 2686–2693.
7. S. Li, H. Yang, R. Ren, J. Ma, J. Jin and J. Ma, J. Power Sources, 2015, 294, 360–368.
8. S. Li, H. Yang, Z. Dong, S. Guo, J. Zhao, G. Gou, R. Ren, J. Huang, J. Jin and J. Ma, Catal. Sci. Technol., 2013, 3, 2303–2310.
9. Y. Wang, S. Ma, Y. Su and X. Han, Chem.–Eur. J., 2015, 21, 6084–6089.
10. P. Wu, Y. Huang, L. Zhou, Y. Wang, Y. Bu and J. Yao, Electrochim. Acta, 2015, 152, 68–74.
11. Y. Huang, Y. Guo, Y. Wang and J. Yao, Int. J. Hydrogen Energy, 2014, 39, 4274–4281.
12. D. Chen, Y. Zhao, Y. Fan, X. Peng, X. Wang and J. Tian, J. Mater. Chem. A, 2013, 1, 13227–13232.
13. Y. Wang, T. S. Nguyen, X. Liu and X. Wang, J. Power Sources, 2010, 195, 2619–2622.
14. J. M. Schnorr and T. M. Swager, Chem. Mater., 2011, 23, 646–657.
15. W. Li, X. Zhao and A. Manthiram, J. Mater. Chem. A, 2014, 2, 3468–3476.
16. Q. Wang, Y. Liao, H. Zhang, J. Li, W. Zhao and S. Chen, J. Power Sources, 2015, 292, 72–77.
17. H. Lee, N. F. Scherer and P. B. Messersmith, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 12999–13003.
18. W. Zong, Y. Hu, Y. Su, N. Luo, X. Zhang, Q. Li and X. Han, J. Microencapsulation, 2016, 33, 257–262.
19. X. Gu, W. Qi, X. Xu, Z. Sun, L. Zhang, W. Liu, X. Pan and D. Su, Nanoscale, 2014, 6, 6609–6616.
20. W. Ye, J. Yu, Y. Zhou, D. Gao, D. Wang, C. Wang and D. Xue, Appl. Catal., B, 2016, 181, 371–378.
21. S. Li, Z. Dong, H. Yang, S. Guo, G. Gou, R. Ren, Z. Zhu, J. Jin and J. Ma, Chem.–Eur. J., 2013, 19, 2384–2391.
22. Y. Zhang, H. He, C. Gao and J. Wu, Langmuir, 2009, 25, 5814–5824.
23. S. Guo, S. Dong and E. Wang, Energy Environ. Sci., 2010, 3, 1307–1310.
24. S.-S. Li, A.-J. Wang, Y.-Y. Hu, K.-M. Fang, J.-R. Chen and J.-J. Feng, J. Mater. Chem. A, 2014, 2, 18177–18183.
25. K. Kakaei and M. Dorraji, Electrochim. Acta, 2014, 143, 207–215.
26. H. Na, L. Zhang, H. Qiu, T. Wu, M. Chen, N. Yang, L. Li, F. Xing and J. Gao, J. Power Sources, 2015, 288, 160–167.
27. Y. Huang, J. Cai, S. Zheng and Y. Guo, J. Power Sources, 2012, 210, 81–85.
28. F. A. Ksar, L. Ramos, B. Keita, L. Nadjo, P. Beaunier and H. Remita, Chem. Mater., 2009, 21, 3677–3683.
29. Q. Liu, J. Fan, Y. Min, T. Wu, Y. Lin and Q. Xu, J. Mater. Chem. A, 2016, 4, 4929–4933.
30. T. Gunji, T. Tanabe, A. J. Jeevagan, S. Usui, T. Tsuda, S. Kaneko, G. Saravanan, H. Abe and F. Matsumoto, J. Power Sources, 2015, 273, 990–998.
31. Z. X. Liang, T. S. Zhao, J. B. Xu and L. D. Zhu, Electrochim. Acta, 2009, 54, 2203–2208.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17089g

---