Introduction

Direct alcohol fuel cells (DAFCs) have attracted considerable attention because of their interest as alternative power sources for vehicles and portable electronic devices (Badwal et al. 2015; Zhang et al. 2015, Hansen 2015). The use of low-molecular weight alcohols such as methanol or ethanol as energy sources has numerous advantages, since for example these compounds can be easily handled, stored, and transported. Nevertheless, there are limitations related to the application of DAFCs, including the high cost of the final devices, low levels of electrocatalytic activity, and reduced durability. Up to now, the most common electrocatalysts used in DAFCs contain platinum and palladium alloys because of the outstanding electrocatalytic activity of these materials. However, Pt cannot be used for large-scale applications owing to its high cost and toxicity by carbon monoxide (CO). Additionally, alkaline direct alcohol fuel cells present an important advantage having a basic electrolyte membrane which implies reaction kinetics of both the alcohol anodic oxidation and oxygen cathodic reduction faster than in acidic media, making it possible to use Pt-free metal catalysts (Xu et al. 2008; Jiang et al. 2010; Estudillo-Wong et al. 2013). Thus, alternative materials are needed (Shen and Xu 2006) and today researches are largely focused on Pd-based electrocatalysts for fuel cells. Moreover, it has been found that by allowing a second metal such as Fe, Co, Au, or Ni, the d-band of Pd can be modified (Su et al. 2013). For these reasons, bimetallic catalysts with alloy structures are an important research topic for application in low-molecular weight alcohols such as methanol or ethanol due to their enhanced activity and stability, different from those of the monometallic components (Su et al. 2013; Demirci 2007; Manzo-Robledo et al. 2003).

Metal nanoparticles display a considerable interest as catalysts in organic and inorganic reactions, as electrocatalysts in fuel cells or materials with novel electronic, optical, and magnetic properties. Consequently, many research works are dedicated to the development of synthetic methods for the size- and shape-selective preparation of noble metal nanoparticles (Roucoux et al. 2002; Durán Pachón and Rothenberg 2008; Tao and Habas 2008; Niu and Xu 2011; Chen and Liu 2011; Lu and Yin 2012; Sastry et al. 2005). Comparisons of the effect of synthesis method on the catalytic performance of metal nanoparticles when used as electrocatalysts for methanol or ethanol electro-oxidation in alkaline media have been reported (Zhao et al. 2014; Lin et al. 2009; Antolini 2007; Bandarenka and Koper 2013; Yu et al. 2010; Park and Sung 2006). Conventional chemical methods to obtain Pd or PdNi nanoparticles are based on the chemical reduction of metal transition salts such as PdCl2, Pd(NO3)2, NiCl2, NiSO4, and Ni(NO3)2 using NaBH4 as a reducing agent (Xu et al. 2007; Zhang et al. 2011; Amin et al. 2014a, b; Yang et al. 2014; Awasthi and Anindita 2010; Shen et al. 2010; Jin et al. 2012; Zhao et al. 2010). Alternative syntheses have been reported for the same precursors, like modified polyol reduction process in the presence of ethylene glycol or glycerol as reducing agents (Cai et al. 2013; Li et al. 2015). Other reducing agents such as hydrazine, formic acid, or dihydrogen have also been employed (Guo et al. 2012; Liu et al. 2009; Wei et al. 2011; Su et al. 2013). An interesting method that includes cathodic deposition using hydrogen dynamic bubble template allowed obtaining Pd and PdNi nanostructures (~10 nm) having three-dimensional hierarchical pores of interconnected dendrite walls (Li et al. 2013). An alternative method includes a conventional electrodeposition of Pd spherical nanostructures in the range of 140–350 nm and PdNi nanostructures of 10–26 nm in diameter (Yi et al. 2013). A similar method employing electrodeposition followed by galvanic replacement allowed obtaining spherical porous aggregates of 58 nm in size (Hosseini et al. 2013). A modified polyol process based on Pd(acac)2 as a precursor and 1,2-hexadecanediol as a reducing agent led to the formation of Pd nanoflowers (Yin et al. 2009). Pd(acac)2 precursor has been also used in a modified solution phase-based nanocapsule method in the presence of lithium triethylborohydride to generate Pd nanoparticles of 2–2.3 nm in size (Zhang et al. 2011). A non-conventional but still interesting method is the melt spinning technique and subsequent dealloying of Al75Pd10Ni15 precursor in 20 wt% NaOH to prepare Pd40Ni60 nanorods (Qi et al. 2011). Furthermore, the electron beam irradiation method using Pd(NO3)2·2H2O as a precursor and solvated electrons and H˙ radicals as reducing agents has been effectively used to prepare spherical Pd nanoparticles of 7.1–7.5 nm in size (Napoleão Geraldes et al. 2013).

Another advantageous chemical preparation method is based on the use of organometallic precursors. This method was developed by Chaudret for the synthesis of transition metal nanoparticles (Amiens et al. 2013). The main advantage of using such metal precursors is their facile decomposition that can be achieved in organic solution, in the presence of different stabilizing agents and under mild conditions. In this way, the control of the particles’ size, shape, and surface state can be achieved which facilitates the generation of monodisperse systems. Bimetallic nanoparticles with variable metal ratios can also be prepared by this organometallic approach (Costa et al. 2014). For instance, Ni, Pt, and Ni x Pt (x = 1, 2, 3) nanoparticles stabilized by hexadecylamine (HDA) were prepared and evaluated as electrocatalysts for the hydrogen evolution reaction (HER). The presence of donor ligands such as amines did not block electron transfer for the HER reaction, and excellent electrocatalytic activity on HER was obtained (Domínguez-Crespo et al. 2009, 2012). Moreover, the mean diameter of the Ni x Pt nanostructures in the range of 2–5 nm appeared to correlate with the platinum amount and the interaction between the metallic nanoparticles and stabilizer HDA (Domínguez-Crespo et al. 2012). The composition of NiPt catalysts influenced strongly the electrocatalytic activity for HER.

In continuation of our prior studies on the electrochemical properties of bimetallic nanoparticles, here we further explore the interest of organometallic Pd, Ni, and PdNi nanoparticles containing different Pd:Ni molar ratios with the aim to determine how this metal composition can affect the catalytic performance of such nanomaterials in methanol oxidation reaction under alkaline conditions.

Experimental section

Synthesis of metal nanoparticles

The bimetallic NiPd nanoparticles with different Ni:Pd molar ratios (50:50, 90:10, and 10:90) in the presence of an excess of hexadecylamine (HDA) were prepared under inert atmosphere using vacuum line techniques by co-decomposition of tris(dibenzylideneacetone)dipalladium(0), Pd2(dba)3, and bis(1,5-cyclooctadiene)-nickel(0), Ni(cod)2, as previously reported (Costa et al. 2014). The as-prepared catalyst powders were mixed with carbon Vulcan in order to obtain supported active material (10 % wt. Pd x Ni y /C).

TEM analysis

The size and morphology of the obtained nanostructures were determined using a JEOL JEM 2010 transmission electron microscope. TEM samples were prepared by dropping each crude colloidal solution onto a carbon-covered copper grid.

Electrode preparation and electrochemical experiments

The electrochemical measurements were performed potentiostatically (Versastat 3) in a three-electrode standard electrochemical cell. A carbon rod and saturated Calomel electrode (SCE) were used as counter (CE) and reference electrodes, respectively. A glassy carbon (GC) disk electrode with 4 mm in diameter, polished to a mirror-like finish with a 0.05 µm alumina suspension before each experiment, was used as the substrate for the catalyst ink. The ink was prepared by dispersing 4.0 mg of electrocatalyst in 0.1 % vol. solution (5 wt % Nafion) in ultrasound bath for 30 min. The same procedure was carried out for the preparation of the carbon Vulcan ink free of catalyst. An aliquot of 6.3 μL of this ink was deposited onto the GC surface and dried under argon atmosphere. The methanol concentration was xM MetOH + 0.5 M NaOH (x = 0.01 M, 0.1 M, 0.5 M and 1 M). The temperature varied from 20 to 60 °C, using a solution of 0.5 M methanol and 0.5 M NaOH. For all experiments, gas argon was used to eliminate oxygen from electrolyte. All reagents were used without further purification.

Results and discussion

Characterization of NiPd nanoparticles

Pd, Pd90Ni10, Pd50Ni50, Pd10Ni90, and Ni nanoparticles were obtained from the decomposition of Pd2(dba)3 and Ni(cod)2 precursors under dihydrogen atmosphere in the presence of 10 molar equivalents of HDA in toluene. Well-dispersed, semi-spherical, and regular size metal nanoparticles with average sizes of Pd (5.4 nm), Pd90Ni10 (6.1 nm), Pd50Ni50 (4.2 nm), Pd10Ni90 (5.1 nm), and Ni (6.3 nm) were previously characterized by TEM (Costa et al. 2014). Additional TEM characterization is shown in Fig. 1, and the average particle size is in the order Ni > Pd10Ni90 > Pd > Pd90Ni10 > Pd50Ni50. As previously discussed regarding Pd, Pt, and Ni nanoparticles (Ramírez et al. 2004; Costa et al. 2014; Ramírez et al. 2007; Cordente et al. 2001), the large amount of HDA allows good stabilization of the nanoparticles. The purification of the nanoparticles was performed by repeated cycles of precipitation with cold pentane in an ice bath; however, the excess of HDA was not completely removed due to the simultaneous precipitation of HDA. When the purification was performed at room temperature, a more efficient removal of HDA was achieved, but it also decreases the amount of nanoparticles recovered. The Pd:Ni molar ratio was determined by ICP OES and it is very close to the theoretical ratio for all the bimetallic nanoparticles (0.9:9.1; 4.9:5.1, and 8.9:1.1 for Pd10Ni90, Pd50Ni50, and Pd90Ni10, respectively) (Costa et al. 2014). The excess of HDA did not allow us to obtain a description of the Pd and Ni metal domain distribution in the isolated nanoparticles by chemical mapping using the STEM-XEDS technique. The bimetallic nanostructure of the PdNi nanoparticles prepared by the co-decomposition of the organometallic precursors could be assessed after their immobilization on silica as presented in (Costa et al. 2014). Analysis of many compositional maps revealed segregation of nickel at the nanoparticle surface, which was also in agreement with the XPS results, showing the presence of nickel oxide on the nanoparticles’ surface (Costa et al. 2014). The segregation of Pd and Ni domains is expected in nickel−palladium disordered alloys because of the mismatch of the atomic size of Ni and Pd.

Fig. 1
figure 1

Size and morphology of the as-prepared nanoparticles stabilized by HDA: Ni, Ni90Pd10, Ni50Pd50, Ni10Pd90, and Pd

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Electrochemical analysis

Figure 2 reports the current versus potential profiles obtained by cyclic voltammetry at a scan rate of 50 mV s−1 in alkaline conditions for the samples in this study. It is important to note that the shapes of these curves are very different for monometallic palladium (Fig. 2a) and nickel (Fig. 2c) nanoparticle systems, representing the redox process corresponding only to Pd or Ni, as expected. During surface polarization at the intermediate bimetallic nanoparticle materials, named Pd x –Ni y /C (Fig. 2b), some differences can be observed: (i) the intensity of the typical peak of Pd in alkaline conditions decreases as a consequence of the presence of nickel at 10 and 50 %; (ii) at 90 % of nickel, such a peak exhibits an onset to more negative potentials; and (iii) typical peaks of nickel are only visible at Pd10Ni90/C.

Fig. 2
figure 2

i–E characteristics obtained during cyclic voltammetry at 50 mV s−1 in 0.5 M NaOH solution. Starting potential at open circuit toward positive direction. a Pd/C, b Pdx–Niy/C, and c Ni/C

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On the other hand, typical current–potential characteristics showing methanol oxidation reaction (MOR) into carbon dioxide were observed during polarization at 5 mV s−1 using cyclic voltammetry in 0.5 M NaOH and different methanol concentrations for all catalysts; for simplicity, Fig. 3 shows the profiles obtained at Pd/C, whereas the inset of this figure shows the current–potential characteristic of a palladium electrode in a solution free of methanol (curve (a)) and carbon Vulcan deposited on the surface of glassy carbon electrode (curve (b)). As can be observed, the carbon Vulcan ink (prepared as mentioned in the experimental section) deposited onto glassy carbon did not affect the corresponding methanol oxidation in the potential-window interval (ca. −0.6 to 0.0 V/SCE). On the other hand, it is well known that Ni possesses inferior electrochemical activity than Pd; however, it is also expected that nanostructured bimetallic alloys might combine the properties of the individual constituents due to some kind of composition and reorganization at nanometric scale. Then for the Pd-Ni material, the accumulation of nickel species on the surface is possible (Costa et al. 2014; Tong et al. 2012; Wu et al. 2011); as a consequence, the electro-induced interfacial reaction might be altered (vide infra). Thus, although it is possible to assume that methanol oxidation is carried out, the magnitude of the current increases as a function of palladium content, as can be discerned from Fig. 4. In this figure, the faradic current variation as a function of methanol concentration is depicted and is in the order Pd > Pd90Ni10 > Pd50Ni50 > Pd10Ni90 > Ni, within experimental error. However, with a low methanol concentration, this behavior is altered as nickel sites could interact in a different way when methanol is better distributed (dilute) in the vicinity of electrode–electrolyte interface in order to interact with the OH ions. This behavior was also verified with respect to temperature at constant concentration of methanol as shown in Fig. 5; then, the activation energy for the MOR is altered in temperature by the material in turn increasing the interaction in the catalyst active sites, and decreasing as a function of palladium loading (Estudillo-Wong et al. 2013). From these results, we can assume that nickel added to palladium in the nanoparticles influences the OH–methanol surface interaction and decreases the activity for alcohol oxidation by modifying the reaction selectivity toward, probably, the oxygen evolution reaction (OER) (Estudillo-Wong et al. 2013). From this assumption, the OER anodic current increases as nickel content increases for the bimetallic Pd90Ni10 and Pd50Ni50 (see curves (a) and (b) in the inset of Fig. 5) until reaching the oxidation of nickel at Pd10Ni90 and then inhibiting the evolution of oxygen as a secondary reaction (see curve (c) in the inset of Fig. 5). Therefore, during polarization in presence of alcohol, OH ions are selectively adsorbed on nickel sites, promoting a major conversion toward the OER (at more anodic potentials) instead of methanol oxidation at high and moderate Ni concentration and in the catalyst free of palladium. This last assumption might be linked with the mechanism of formation of the bimetallic nanoparticles from Ni(cod)2 and Pd2(dba)3 since under experimental conditions (with dihydrogen) Pd2(dba)3 decomposes to generate Pd(0) nanoparticles which may then catalyze the decomposition of Ni(cod)2 to produce bimetallic nanoparticles containing palladium at the core and an excess of nickel atoms on the surface (Costa et al. 2014; Cordente et al. 2001; Ramírez et al. 2004).

Fig. 3
figure 3

Typical profiles corresponding to methanol oxidation in alkaline conditions, presented here for Pd/C. The observed peaks are assigned to the CO2 formation at the electrode interface. The inset shows a the current–potential (i–E) profile for Pd in a solution free of methanol and b the profile (i–E) corresponding to carbon Vulcan (free of catalyst) deposited onto glassy carbon. Supporting electrolyte is 0.5 M NaOH; starting potential was fixed at −1.1 V in the positive direction; scan rate is 5 mV s−1

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Fig. 4
figure 4

Current versus concentration variation profiles with respect to Ni loading during methanol oxidation reaction. Data were obtained at maximum current during the positive-going scan. The scan rate was fixed at 5 mV s−1

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Fig. 5
figure 5

Current versus temperature variation profiles with respect to Ni loading during the methanol oxidation reaction. Data were obtained at maximum current during the positive-going scan. The scan rate was fixed at 5 mV s−1. Inset current–potential profiles in the OER zone

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Conclusions

The methanol oxidation reaction was investigated using Pd–Ni-based nanoparticles ranging from 4.2 to 6.3 nm in size prepared from organometallic precursors. Redox characteristics including Pd to Ni were observed during cyclic voltammetry experiments in the alkaline environment. It was found that methanol oxidation is possible in all set of catalysts evaluated. However, for this case of study, the presence of nickel-(oxide) species and the possible formation of a core–shell system associated with palladium induce electronic and structural modification during the transition from nickel to palladium that modifies the OH ions’ surface adsorption interaction, reaction kinetic performance, and the selectivity conversion toward the interfacial coupled OER–MOR in the alkaline environment.