Highly active and durable flowerlike Pd/Ni(OH)₂ catalyst for the electrooxidation of ethanol in alkaline medium

Abstract

In this paper, a novel flowerlike Pd/Ni(OH)₂ catalyst is successfully synthesized, and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectrometry (EDX) elemental mapping. The results indicate that the flowerlike Pd nanoparticles possess a three-dimensional structure. These flowerlike Pd nanoparticles are uniformly dispersed on the surface of Ni(OH)₂. The incorporation of Ni(OH)₂ plays the key role in promoting the dissociative adsorption of water molecules and subsequent oxidative removal of poisonous intermediates on neighboring palladium sites. Because of the synergistic effects between the flowerlike Pd nanoparticles and Ni(OH)₂, the Pd/Ni(OH)₂ electrocatalyst exhibits high catalytic activity, excellent durability and superior CO poisoning tolerance for the electrooxidation of ethanol in comparison with commercial Pd/C.

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1. Introduction

In recent decades, direct ethanol fuel cells (DEFCs) have attracted growing attention as promising energy converters due to their high energy density, high efficiency and low, even zero, emissions. However, their barriers to commercial success are primarily three key elements. First, the catalytic activity of the catalyst has not yet reached the requirements of the practical application. Second, the stability of the electrocatalysts needs to be improved. Last, the high cost of electrocatalysts associated with precious metals should be reduced as much as possible. At present, Pt and Pt-based alloys are considered efficient electrocatalysts for the oxidation of ethanol.^1–8 But the high cost of Pt hinders large-scale manufacture and CO poisoning restricts the commercial development of the DEFCs.^9,10 Therefore, many researchers are committed to the development of non-platinum electrocatalysts for reactions of DEFCs.^11,12 Recently, Pd is considered as a promising alternative to Pt because of its relatively higher abundance in nature and lower cost than Pt, and its outstanding electrocatalytic activity for DEFCs. Nevertheless, there are still further efforts needing to develop highly active Pd catalyst for practical application in DEFCs. As we know that the catalytic activity of the catalysts could be influenced by many factors, such as the size (dimension) and shapes (morphology) of the nanoparticles.^13–16 Because the high electrochemical active surface areas of the catalysts are crucial for the development of high-performance electrocatalysts.

In addition, the durability is also another challenge for industrializing electrocatalysts for DEFCs.^17,18 In the anode, many novel metal electrocatalysts for ethanol oxidation reaction are very susceptible to poisoning by surface-adsorbed reaction intermediates such as CO_ads and CH₃CO_ads, thus lowering the catalytic activity and durability.^19,20 Currently, a conventional strategy to address this obstacle is via alloyed Pd with another metal. According to the relevant studies, during ethanol electrocatalytic oxidation reaction, oxophilic metals (such as Ru) assist in the dissociative adsorption of water molecules to form OH adspecies on their surface. These OH adspecies can promote the oxidation of poisoning intermediates on neighbor active sites and thereby facilitate their regeneration for further ethanol oxidation.^21–24 Although these binary alloys have exhibited improved electrocatalytic activity, they provide pretty limited gain in durability. Inspired by these, the Ni(OH)₂ is known to better facilitate water dissociation in alkaline electrolytes compared with oxophilic metals.^24–26 The synergistic effect between novel metal and Ni(OH)₂ can promote CO and CH₃CO_ads oxidation.²⁵ In light of these effects, we propose that the bifunctional interaction between metal Pd and Ni(OH)₂ may present a possible solution to the long-standing durability issue in ethanol electrooxidation reaction.

According to the above considerations, novel Pd/Ni(OH)₂ electrocatalyst was designed and fabricated for ethanol electrooxidation for DEFCs. Each component of the hybrids fulfills an important specific role, Pd serves as the active sites for ethanol oxidation, Ni(OH)₂ provides abundant OH_ads species to facilitate the oxidative remove the adsorbed poisoning species (such as CH₃CO_ads and CO) on the surface of the Pd layers. As a result of the special surface and synergistic effects, the Pd/Ni(OH)₂ exhibited significantly improved electrocatalytic activity for ethanol electrooxidation, intermediates poisoning tolerance and long-term cycling stability compared with the commercial Pd/C electrocatalyst.

2. Experimental section

Chemicals

Palladium chloride (PdCl₂, 99%), nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O), ammonium hydroxide, cetyltrimethylammonium bromide (CTAB, 99%), and ascorbic acid (AA, 99.7%) were used as received. C₂H₅OH, HCl and KOH were all of analytical grade and used without further purification. All the above chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd., China. Commercial Pd/C catalyst (JM 20% Pd) was purchased from Shanghai Hesen Electric Co., Ltd, China. Doubly distilled water was used throughout the work.

Apparatus

All electrochemical experiments were carried out with a CHI660 B electrochemical workstation (Shanghai Chenhua Instrument Plant, China) at room temperature. A glassy carbon electrode (GCE, 3 mm diameter) modified with as-prepared catalyst was used as working electrode. A saturated calomel electrode (SCE) and platinum wire were used as the reference electrode and the counter electrode, respectively. Transmission electron microscopy (TEM) characterization was performed on a TECNAI-G20 electron microscope with an accelerating voltage of 200 kV. Scanning electron microscope (SEM, S-4700) was used to determine the morphology of as prepared catalyst. X-ray diffraction (XRD) patterns of the catalyst was measured on a PANalytical X'Pert PRO MRD X-ray diffractometer using Cu Kα as the radiation source (λ = 1.54056 Å).

Synthesis of the as-prepared catalysts

First, a 22.56 mM H₂PdCl₄ solution was prepared by completely dissolving 0.2 g of PdCl₂ powder in 25 mL of 90.2 mM HCl solution and diluting the solution to a 50 mL of volumetric flask. The Pd/Ni(OH)₂ binary hybrids were prepared via a two-step solution method. In the first step, poured the 20 mL of doubly distilled water into the round-bottomed flask, the pH value of the solution was adjusted to 9 with NH₃·H₂O solution. Then, 100 mg of the CTAB and 20 mg of the Ni(NO₃)₂·6H₂O were added into the above mixture. The reaction mixture was heated at 85 °C for 3 h under vigorous stirring. At the end, the Ni(OH)₂ was evenly dispersed in the solution. Secondly, the solution of H₂PdCl₄ (1.0 mL, 22.56 mM) was quickly added into the above reaction mixture, followed by the addition of freshly prepared AA solution (3.0 mL, 0.01 M) under vigorous stirring, then the reaction continued for 20 min and cooled down to room temperature. After that, the catalyst was washed with doubly distilled water by repeating the centrifugation and re-dispersion procedure several times to remove impurities. Finally, the product was dispersed in 10 mL of doubly distilled water. The catalyst contains a total Pd loading 2.4 mg.

Electrochemical characterization

Electrochemical activities of the flowerlike Pd/Ni(OH)₂ were studied by making cyclic voltammogram (CV) measurements for electrooxidation of ethanol. First, 10 μL of as-prepared catalyst solution was added dropwise on the surface of the GCE and dried in an oven at 65 °C. To test the catalytic activity of the catalyst, the modified GCEs were tested in 1 M KOH + 1 M C₂H₅OH solution by CVs from −0.8 V to 0.3 V at a scan rate of 50 mV s⁻¹, chronoamperometric (CA) measurements were conducted at −0.25 V for 3600 s.

3. Results and discussion

The EDX spectra presented in Fig. 1 were performed to analyze the surface composition of the Pd/Ni(OH)₂ catalyst. It confirmed the presence of Pd and Ni in the Pd/Ni(OH)₂ catalyst. Moreover, the Pd/Ni mass ratios of spectrogram 8, 9 and 10 are 11.7, 11.8 and 11.1, respectively, as shown the Fig. 1(B–D). The Pd/Ni mass ratios of different parts are very close, implying that the Pd nanoparticles are evenly dispersed on Ni(OH)₂. The strong peak of the Si is attributed to the basal plane. To probe the spatial distribution of Ni and Pd species, elemental mapping by energy-dispersive spectroscopy (EDS) was performed under SEM (Fig. 2). The distribution of the Pd signal (Fig. 2(B)) corresponds well with the morphology of Pd nanocrystals (Fig. 2(A)). Ni and O species were also detected and mapped (Fig. 2C and D). In the Fig. 2, it can be seen that the Pd nanocrystal is evenly dispersed on the Ni(OH)₂.

Fig. 1 (A) SEM image of Pd/Ni(OH)₂ and (B, C, D) the corresponding EDX spectra of the spectrogram 8, 9, and 10.

Fig. 2 (A) SEM image of Pd/Ni(OH)₂ and its corresponding (B) Pd EDS mapping, (C) Ni EDS mapping and (D) O EDS mapping.

The SEM images of Pd/Ni(OH)₂ catalyst are shown in Fig. 3. As observed, well-dispersed Pd particles embed into the Ni(OH)₂ sheets. To further observe the morphology of Pd nanoparticles, the catalysts are characterized by TEM. As shown in Fig. 4(A and B), we can clear see that the Pd nanoparticles are very uniformly distributed on the Ni(OH)₂ sheets. As shown in Fig. 4(C and D), it is found that the size of the Pd nanoparticles is estimated to be 30 nm. Moreover, the Pd nanoparticles are not dense-structure and they have many interspace. The shape of the Pd nanoparticle is flowerlike, and exhibits a three-dimension structure. AA is one of the most important factors in controlling the morphology of the Pd nanoparticles. The specific and appropriate amount of AA is essential to grow the flower like shape of Pd nanoparticles, possibly due to the optimal kinetic growth controlled by the optimal AA concentrations. The three-dimensional interstitial structure as well as the interconnected framework may greatly increase the electrolyte-accessible surface area of flowerlike Pd nanoparticles, thereby enhancing its electrochemical activity and providing more active sites for ethanol electrooxidation. The shadows of the Fig. 4 are Ni(OH)₂ nanoparticles. The XRD, SEM and TEM images of pure Ni(OH)₂ nanoplates are displayed in ESI (Fig. S1–S4†).

Fig. 3 SEM images of Pd/Ni(OH)₂ with different magnification.

Fig. 4 TEM images of Pd/Ni(OH)₂ with different magnification.

Fig. 5 presents XRD patterns of the Pd/Ni(OH)₂ catalyst, as well as the commercial Pd/C catalyst for comparison. For the Pd/Ni(OH)₂ catalyst, the peak observed at 2θ = 19.2° is attributed to the (001) planes of the Ni(OH)₂. The peaks at around 39.7, 45.8, 67.7 and 81.3° are attributed to the (111), (200), (220) and (311) planes of Pd.^27–29 respectively, which is consistent with an fcc (face-centered cubic) crystalline structure of Pd. The diffraction peak of commercial Pd/C located at 23.3° is ascribed to the C (002) plane, and the diffraction peaks at 2θ = 40.1 and 46.3° correspond to the Pd(111) and Pd(200) lattice planes, respectively. Moreover, a broad peak at about 34.18° was observed for the commercial Pd/C sample, which was likely to related to PdO.³⁰

Fig. 5 XRD patterns of the commercial Pd/C and Pd/Ni(OH)₂ catalysts.

Fig. 6 shows CVs of the Pd/Ni(OH)₂ and commercial Pd/C catalysts in an alkaline medium of 1 M KOH solution from −0.8 to 0.3 V at a scan rate of 50 mV s⁻¹. For the Pd/Ni(OH)₂ and commercial Pd/C catalyst, a typical redox peak was observed at approximately −0.31 V vs. SCE in the cathodic scan, which was associated with palladium oxide. The electrochemical active surface area (ECSA) of the catalysts was determined from the coulombic charge for the reduction of palladium oxide, according to following eqn (1).³¹

m represents the mass of palladium loaded on the glassy carbon electrode surface. Q_H in mC represents reduction charge of the PdO that was measured by integration of the area under the CV reduction peak, an electrical charge of 0.43 mC cm⁻² was assumed for the reduction of the PdO monolayer. As shown in Fig. 6, the Pd/Ni(OH)₂ catalyst displays the higher reduction peak current density compared with the commercial Pd/C catalyst. The ECSA values of Pd/Ni(OH)₂ and commercial Pd/C catalysts are estimated to be 304.26 cm² mg⁻¹ and 64.92 cm² mg⁻¹, respectively, as shown Table 1.

Fig. 6 CVs of commercial Pd/C and Pd/Ni(OH)₂ catalysts in 1 M KOH solution at the scan rate of 50 mV s⁻¹.

Table 1 Catalytic performance of the catalysts for ethanol oxidation in alkaline solution

Samples	ECSA (cm^2 mg^−1)	If (mA mg^−1)	If(after 500 cycles) (mA mg^−1)
Pd/Ni(OH)2	304.26	2532.72	1224.06 (48.33%)
Pd/C	64.92	572.31	55.06 (9.62%)

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The electrochemical catalytic activities of the catalysts were measured by CV in an alkaline solution containing 1 M KOH + 1 M C₂H₅OH at a scan rate of 50 mV s⁻¹, and the CV results are shown in Fig. 7. In the forward scan, the oxidation peak corresponds to the oxidation of the freshly chemisorbed species coming from ethanol adsorption. The onset potential of the Pd/Ni(OH)₂ catalyst is about −0.62 V, 100 mV more negative than that of the commercial Pd/C catalyst, indicating that the ethanol is more easily to be oxidized at a lower potential on the Pd/Ni(OH)₂ catalyst. The peak in the backward sweep at about −0.38 V is related to the removal of carbonaceous species not completely oxidized in the forward scan. The incompletely oxidized carbonaceous species, such as CO and CH₃CO_ads, could accumulate on the electrode and poison the electrode.^32–37 It can be observed that the forward peak current densities are 2532.72 mA mg⁻¹ and 572.31 mA mg⁻¹ for Pd/Ni(OH)₂ and commercial Pd/C, respectively, as shown Table 1. To compare their intrinsic catalytic activity towards the ethanol oxidation, the current densities are normalized to the mass of Pd. The results show that Pd/Ni(OH)₂ has the higher catalytic activity toward ethanol oxidation. The peak current density on Pd/Ni(OH)₂ is approximately 4.4 times higher than that on commercial Pd/C catalysts. The improvement in ethanol oxidation is mainly ascribed to the factor that the hydroxide to facilitate the oxidative removal of carbonaceous poisons on adjacent metal sites and help to enhance the ethanol oxidation process.^24,38 Additionally, this interstitial flowerlike structure may lead to a higher ECSA value that contribute to the presence of more active sites of Pd/Ni(OH)₂ catalyst, which significantly enhanced its electrocatalytic activity for ethanol oxidation. The results indicate that the Pd/Ni(OH)₂ catalyst shows the superior activity as compared to commercial Pd/C catalyst. Moreover, in order to investigate the effect of the content of Pd in the catalysts on the catalytic performance for ethanol electro-oxidation, the different catalysts have been synthesized. As demonstrated later, the best electrocatalyst for ethanol oxidation reaction is Pd/Ni(OH)₂, as shown Fig. S5 (ESI†). The pure Ni(OH)₂ has no catalytic activity (Fig. S6, ESI†).

Fig. 7 CV curves of commercial Pd/C and Pd/Ni(OH)₂ catalysts in solution containing 1 M KOH + 1 M C₂H₅OH at the scan rate of 50 mV s⁻¹.

The long-term stability and durability of the Pd/Ni(OH)₂ and commercial Pd/C catalysts were further investigated by chronoamperometric (CA) curves measurements in a 1 M C₂H₅OH + 1 M KOH solution at −0.25 V (vs. SCE) for 3600 s, as shown in Fig. 8(A). For the two catalysts, the current density decreased rapidly in the first 400 s, which is ascribed to the poisoning of the catalysts by the accumulation of poisonous intermediates species such as CO and CH₃CO_ads formed during the ethanol oxidation reaction, and then the current decay gradually slowed down at longer times. Moreover, it was noticeable that the current density of the Pd/Ni(OH)₂ catalyst maintained a relative higher value than commercial Pd/C catalyst during the whole test time, which demonstrated that the Pd/Ni(OH)₂ catalyst displayed a better stability towards the ethanol electrooxidation reaction compared with the commercial Pd/C catalyst.

Fig. 8 (A) CA curves of ethanol electrooxidation on commercial Pd/C, and Pd/Ni(OH)₂ catalysts at −0.25 V in 1 M KOH + 1 M C₂H₅OH. (B) The change of the maximum specific peak current density with increasing cycle number for the commercial Pd/C and Pd/Ni(OH)₂ (500 cycles).

To further investigate the long-term stability of the as-prepared catalyst, the CVs scans of 500 cycles were performed on the catalysts in an alkaline medium containing 1 M KOH + 1 M C₂H₅OH at a scan rate of 50 mV s⁻¹, and the corresponding change of forward peak current density is shown in Fig. 8(B). The catalytic activity of the Pd/Ni(OH)₂ catalyst has a drastic increase in the initial cycles and the maximum peak current density appears at about the 20^th cycle. After 160 cycles, the peak current density almost remains stable with increasing cycle number, indicating long-term cycle stability for ethanol electrooxidation. In addition, the Pd/Ni(OH)₂ catalyst exhibits greatly enhanced catalytic activity and durability compared with commercial Pd/C catalyst. After 500 cycles, the Pd/Ni(OH)₂ still has a higher peak current density. The conservation rate of the specific peak current density of the Pd/Ni(OH)₂ is 48.33% of the maximum value, which is much higher than that of the commercial Pd/C (9.62%) catalyst, as shown Table 1. These results indicate that the Pd/Ni(OH)₂ catalyst has excellent cycling stability for ethanol oxidation. This suggests that the presence of Ni(OH)₂ is contributed to enhancing the stability and poisoning tolerance of Pd.

Here, high ability of CO anti-poisoning of Pd/Ni(OH)₂ and commercial Pd/C catalysts were also investigated, and the results shown in Fig. 9. The catalytic activity of Pd/Ni(OH)₂ toward CO oxidation was contrasted with commercial Pd/C catalyst in solution of 1 M KOH at room temperature. Firstly, in order to achieve the maximum coverage of CO at the Pd centers, high purity CO was bubbled into the electrolyte solution for 10 min while keeping the electrode potential at 0.1 V. Fig. 9 shows two consecutive CVs of Pd/Ni(OH)₂ and commercial Pd/C catalysts recorded from −0.8 to 0.3 V at 50 mV s⁻¹ in CO saturated solution of 1 M KOH. As shown in Fig. 9(A), a remarkably larger CO oxidation peak can be clearly seen in the initial forward scan in the 1^st cycle, indicating that the Pd/Ni(OH)₂ have a large ECSA and good oxidation activity for CO. During the 2^nd cycle, the CV does not show any peak for CO oxidation, indicating that the CO was oxidized completely during the 1^st scan. However, for the commercial Pd/C catalyst, we can observe a small peak for CO oxidation during the 2^nd cycle, as shown in Fig. 9(B). In addition, in the initial forward scan of CO stripping, the onset potential (−0.43 V) of CO oxidation on the Pd/Ni(OH)₂ is obviously more negative than that on the commercial Pd/C (−0.31 V) catalyst, and we can conclude that the Pd/Ni(OH)₂ has a much better CO oxidation ability than the commercial Pd/C catalyst, as shown in Fig. 9(C). The above results show that the introduction of Ni(OH)₂ can facilitate the removal of CO from the catalyst surface as the Ni(OH)₂ which can provide enough OH_ads species to oxidize CO. Therefore, the Pd/Ni(OH)₂ catalyst has better catalytic activity and durability for ethanol electrooxidation reaction than commercial Pd/C catalyst.

Fig. 9 CO stripping curves on (A) Pd/Ni(OH)₂ and (B) commercial Pd/C catalysts recorded in 1 M KOH solution at the scan rate of 50 mV s⁻¹, (C) the comparison of the CO stripping voltammograms of the Pd/Ni(OH)₂ and commercial Pd/C catalysts.

4. Conclusions

In summary, we have designed and fabricated the flowerlike Pd/Ni(OH)₂ catalyst for ethanol electrooxidation. The flowerlike Pd nanoparticles exhibited the three-dimensional interstitial structure, which may greatly increase the electrolyte-accessible surface area of flowerlike Pd nanoparticles. And that every flowerlike Pd nanoparticles are evenly dispersed on the surface of the Ni(OH)₂. The introduction of Ni(OH)₂ as a co-catalyst can greatly facilitate the dissociative adsorption of water molecules and subsequently assists in the oxidative removal of carbonaceous poison. Through collective synergy, the two functional components of the hybrid materials together achieve excellent ethanol oxidation reaction activity. Compared with the commercial Pd/C catalyst, the Pd/Ni(OH)₂ exhibits a significantly enhanced electrocatalytic activity, cycling stability and CO poisoning tolerance for ethanol electrooxidation. The results suggest that Pd/Ni(OH)₂ should be considered a good electro-catalyst material for alkaline direct ethanol fuel cells.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51373111), State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14321k

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