Mechanistic insights into the activation process in electrocatalytic ethanol oxidation by phosphomolybdic acid-stabilised palladium(0) nanoparticles (PdNPs@PMo₁₂)

Abstract

A Keggin type polyoxometalate (POM), phosphomolybdic acid (PMo₁₂), has been employed to encapsulate and stabilise pseudo-spherical Pd(0) nanoparticles (PdNPs). The resulting nanohybrid phosphomolybdic acid-stabilised PdNPs (PdNPs@PMo₁₂) were characterised by transmission electron microscopy (TEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and powder X-ray diffraction (XRD). The PdNPs@PMo₁₂ were used as an electrocatalyst for ethanol oxidation in alkaline media and the best electrocatalytic activity was assessed by applying an activation process. The electrochemical data (chronoamperometry, cyclic voltammetry and linear sweep voltammetry) allowed us to attribute the superior performance of the nanohybrid to the generation of Pd–OH_ads sites at the surface of the PdNPs@PMo₁₂, reinforced by the formation of a new molybdenum species on the surface of the electrocatalyst.

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Introduction

Today, continuing global demands for energy and limited traditional power sources make serious challenges for scientists to develop alternative clean energy technologies. In this regard, new kinds of technologies are becoming more appealing; they include fuel cells, high energy density batteries, supercapacitors, and so on.^1–4 Among them, fuel cells are devices that convert chemical energy of a fuel into electrical energy through a chemical reaction. They are designed as efficient sources of energy and environmentally friendly with little or no pollution. Among the various types of fuel cells, direct alcohol fuel cells (DAFCs) are fascinating due to their low operation temperature (<100 °C) and portable applications. However, the commercialisation of the fuel cell technology depends strongly on the design and development of new electrocatalyst materials that are capable of lowering their cost, while simultaneously enhancing their efficiency and increasing their durability.^5–10

Nanoscopic materials have found widespread attention because particles in the nanoscale range possess unique and applicable physical, optical, electronic and magnetic properties that often differ greatly from the corresponding bulk materials.^11,12 In this view, one of the wide applications of noble-metal nanoparticles (NPs), such as Pd and Pt, is in the field of electrocatalysis, concerning reactions that involve charge transfer at the interface between a solid catalyst and an electrolyte solution.^13–16

Among these noble-metal NPs, nanostructured Pd-based materials are the preferred candidate over Pt. Although the latter is undoubtedly the best electrocatalyst, Pd is a less expensive, more abundant, and more accessible element; and also has the potential to stimulate oxidation of low molecular weight alcohols, such as ethanol, especially in alkaline media.^17–21 However, rather than using the bulk metal, the use of PdNPs with high relative surface area can reduce the noble-metal loading of catalysts and further reduce the cost while at the same time increase the efficiency. Polyoxometalates (POMs), early row transition metal-oxygen anionic clusters, known to be effective reducing and stabilizing agents, have long been known to produce NPs, from Au, Ag, Pt and Pd to Fe₂O₃ etc. Recently, several green facile synthetic routes have been reported and summarised in the recent reviews.^12,22–26

Electrooxidation of alcohol by metal NPs@POM was surveyed by several researchers.^27–29 We recently reported PdNPs@PMo₁₂ nanohybrids as one of the most efficient alkaline media electrocatalysts for use in direct ethanol fuel cells (DEFCs).³⁰ In that study, we addressed a keynote, namely, the activation process that significantly increases the efficiency of electrocatalytic activity of PdNPs@PMo₁₂ for oxidation of ethanol. The activation process involved applying a constant potential step of −1.5 V for 300 s duration in phosphate buffer solution (pH 8.5). It was claimed that upon the activation process the following reaction (eqn (1)) would occur at the electrode–electrolyte interface that leads to the formation of Pd–OH_ads sites, which participate in the rate determining step of ethanol electrooxidation and thus significantly improve the efficiency of the reaction.

2H₂O + 2e⁻ → H₂ + 2OH⁻ (1)

In the research presented here within, we focus on the electrochemical evidence for the events that happen during the activation process. The ultimate goal is to understand the activation process at the molecular level in order to design electrocatalysts with the desired activity and selectivity for application in DAFCs.

Experimental

Materials and methods

Palladium(II) chloride (PdCl₂), ethanol, sodium dihydrogen phosphate dihydrate (NaH₂PO₄·2H₂O), sodium hydroxide (NaOH), DMF (N,N-dimethylformamide), commercial Pd/C and sulphuric acid (H₂SO₄) were purchased from Merck (Darmstadt, Germany). Phosphomolybdic acid, H₃PMo₁₂O₄₀·xH₂O (PMo₁₂) and Nafion (5 wt% in lower aliphatic alcohols and water) were from Sigma-Aldrich. Deionised (DI) water was used throughout all the experiments. Phosphate buffer solution (PBS, 0.1 M) was made by dissolving NaH₂PO₄·2H₂O in DI water and adjusting the pH to 7 and 8.5 by 0.1 M NaOH (aq) solution.

High-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images were collected using a Tecnai S-Twin30, 300 keV, GIF-TRIDIEM. Energy-dispersive X-ray spectroscopy (EDS) data were obtained on the same instrument. X-Ray diffraction (XRD) pattern of the as-prepared catalyst was recorded on X-ray diffractometer (X'Pert Pro MPD) using Cu-Kα as the radiation source. Fourier transform infrared spectroscopy (FTIR) was performed on an AVATAR-370 spectrometer by pressed KBr pellets.

Electrochemical experiments were performed using a μ-Autolab type III electrochemical workstation. The experiments were carried out at room temperature (∼25 °C), using a conventional three-electrode set-up. Platinum wire (Azar Electrode Co.) served as the counter electrode and Ag/AgCl as the reference electrode (Azar Electrode Co.).

Synthesis of PdNPs@PMo₁₂

The catalyst, PdNPs@PMo₁₂ was prepared by the polyoxometalate assisted photochemical reduction method.²⁴ In a typical procedure, 9.6 mL of the PMo₁₂ (0.15 mM) and PdCl₂ (0.2 mM) mixed solution was placed into a cell and mixed with 0.4 mL isopropyl alcohol, then it was irradiated under a UV lamp (125 W high pressure mercury vapour lamp) for 2 h under continuous magnetic stirring. The reaction was done at a constant room temperature (∼25 °C) using a water circulating system. The colour of the solution changed from bright yellow (initial solution) to resulting greyish-brown colloidal solution, indicating the formation of PdNPs.^31,32 The resulting colloidal suspension of the PdNPs@PMo₁₂ was centrifuged at 14 000 rpm for 9 min. The PdNPs@PMo₁₂ precipitated at the bottom of the microtube and were dried at room temperature overnight.

Fabrication of working electrode

The working electrode was fabricated using the following procedure: catalyst ink (1 mg_Pd mL⁻¹) was prepared by dispersion of the as-prepared dried catalyst in 200 μL DI water. Then, 2 μL of the catalyst ink was loaded onto a clean pre-polished glassy carbon electrode (GCE, 0.03 cm² surface area, Azar Electrode Co.) and dried in air.

Electrochemical activation process

Chronoamperometry technique was employed for the electrochemical activation process. Activation of the PdNPs@PMo₁₂ catalyst was performed at the surface of the electrode by applying a constant potential step of −1.5 V for 300 s duration in nitrogen-bubbled PBS (pH 8.5) as the electrolyte. Ethanol electrooxidation was carried out just after the activation process.

Electrochemical measurements

Ethanol electrooxidation of the as-prepared catalyst was acquired by cyclic voltammetry at a scan rate of 50 mV s⁻¹ in a mixed solution of 1 M ethanol and 1 M NaOH (aq) as the electrolyte.

Linear sweep voltammetry (LSV) experiments were performed at a scan rate of 50 mV s⁻¹ in an electrolyte solution of 0.5 M H₂SO₄ (pH 0.3), 1.0 M NaOH (pH 14), 0.1 M PBS (pH 7) or 0.1 M PBS (pH 8.5).

Results and discussion

Structural analysis

The crystallinity of the as-prepared PdNPs@PMo₁₂ was verified by powder X-ray diffraction (PXRD) analysis (see Fig. 1). There are five distinct reflections in the diffractogram at 40.02° (111), 46.49° (200), 68.05° (220), 81.74° (311) and 86.24° (222). These characteristic reflections can be indexed to the face centered cubic (fcc) structure of Pd (JCPDS: 87-0641).^31,33

Fig. 1 PXRD pattern of PdNPs@PMo₁₂.

FT-IR spectra of pure PMo₁₂ and PdNPs@PMo₁₂ are shown in Fig. 2. Several bands characteristic of the pure PMo₁₂ could be found at 1063, 961, 873 and 781 cm⁻¹ and that are attributed to the vibration (P–O_a), (Mo–O_d), (Mo–O_b–Mo) and (Mo–O_c–Mo) (Fig. 2a), respectively.^34–36

Fig. 2 FT-IR spectra of PMo₁₂ (solid line) and PdNPs@PMo₁₂ (dashed line).

The comparison of the spectra of PdNPs@PMo₁₂ with the pure PMo₁₂, indicate that except for some negligible differences due to Pd–PMo₁₂ surface interaction, both FT-IR spectra are the same, indicating the presence of the PMo₁₂ stabilising the PdNPs, presumably retaining its original molecular Keggin structure.

The TEM image of PdNPs@PMo₁₂ reveals that the particles possess an average diameter of ∼20 nm and are all surrounded by a deep (ca. 5–10 nm) layer (light contrast) (see Fig. 3a). This amorphous layer and the composition of the metallic particle at the atomic scale were studied further using HAADF-STEM and EDS. A cross-sectional EDS line scan profiles extracted from the HAADF-STEM image clearly confirms that the immediate area surrounding the PdNPs is covered by Mo elements, implying they are from PMo₁₂ structure (see Fig. 3b). It should be noted the EDS spectra was adjusted to determine only Pd and Mo elements.

Fig. 3 (a) TEM image of PdNPs@PMo₁₂. The inset in (a) corresponds to the particle size distribution histogram showing average diameter of 20 nm (from ∼125 NPs), (b) EDS line scan profile. The inset in (b) corresponds to the HAADF-STEM image of PdNPs@PMo₁₂.

Electrochemical investigations

Here, electrocatalytic ethanol oxidation is considered as a typical reaction. It is widely accepted that the cyclic voltammogram (CV) of this process is characterised by two anodic current peaks: one in the forward (i.e. anodic scan: I_f) and the other one in the backward sweeps (i.e. cathodic scan: I_b). The former is related to the oxidation of freshly chemisorbed ethanol and the latter, to the oxidation of residual carbonaceous species generated in the forward scan.^30,37 The criteria to evaluate the electrocatalyst performance are onset potential (E_op), mass current density peaks (J_p), and current peak potentials (E_p). It is desired to introduce such electrocatalyst with lower E_op value and higher I_f/I_b parameter.

Effect of activation process on the electrocatalytic performance of PdNPs@PMo₁₂. To examine the effect of activation process on the electrocatalytic performance of PdNPs@PMo₁₂, experiments were performed with and without activation process. Fig. 4 represents the CVs of the activated and non-activated PdNPs@PMo₁₂ electrodes in 1 M NaOH (aq) and 1 M ethanol solution. To interpret CVs in Fig. 4, the values of J_p, E_op and E_p are shown in Table 1. The forward peak mass current density (J_f) for the activated PdNPs@PMO₁₂ is 9.5 times larger than that of the non-activated PdNPs @PMo₁₂.

Fig. 4 Cyclic voltammograms (CVs) in 1.0 M NaOH and 1.0 M ethanol solution for non-activated PdNPs@PMo₁₂ (dashed line) and activated PdNPs@PMo₁₂ (solid line).

Table 1 Electrocatalytic ethanol oxidation properties of activated PdNPs@PMo₁₂ and non-activated PdNPs@PMo₁₂

	Eop^a (V)	Jf (mA mg^−1pd cm^−2)	Ef^b (V)	Jb (mA mg^−1Pd cm^−2)	Eb^c (V)	If/Ib
a Eop = onset potential.b Ef = forward current peak potential.c Eb = backward current peak potential.
Activated PdNPs@PMo12	−0.60	14 533	−0.26	11 678	−0.4	1.24
Non-activated PdNPs@PMo12	−0.45	1533	−0.12	3433	−0.4	0.45

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Moreover, I_f/I_b parameter, which is an important criterion to evaluate the performance of the catalysts, is ∼2.7 times higher for activated PdNPs@PMo₁₂ compared to the non-activated PdNPs@PMo₁₂.

In addition, the E_f for the activated catalyst is clearly more negative than that observed for the non-activated one; while, the E_b values are comparable in both cases. Also, the E_op for the activated PdNPs@PMo₁₂ is more negative than for the non-activated PdNPs@PMo₁₂, indicating that the reaction is facilitated on the activated catalyst.

Also, the stability of the activated and non-activated PdNPs@PMo₁₂ are examined by chronoamperometry at a potential of −0.2 V in 1 M NaOH + 1 M ethanol and the results are shown in Fig. 5. It is obvious that a gradual decrease with time occurs in the oxidation current. In comparison, the current for non-activated catalyst decay to approximately zero value, however, in the case of activated catalyst it is seen that the current reaches a constant value of 40 μA.

Fig. 5 Chronoamperograms of activated and non-activated PdNPs@PMo₁₂ at −0.2 V in 1.0 M NaOH and 1.0 M ethanol solution.

All of these lines of evidence confirm that the activation process has a considerable effect on electrocatalytic activity of the catalyst. Now, a critical question arises: what is the mechanism involved in the activation process?

From this point on our research focussed on using electrochemical techniques to gain further insight into the processes occurring during the activation step.

Chronoamperometry experiments. In order to elucidate the mechanism of the activation process, several chronoamperometry experiments (similar to activation process) have been performed using GCE and PdNPs@PMo₁₂ modified GCE in PBS (pH 8.5) at the applied potential of −1.5 V. A quick glance at Fig. 6a reveals much higher currents (∼100 times) for PdNPs@PMo₁₂ modified GCE which indicates the key role of the PdNPs@PMo₁₂ in the activation process.

Fig. 6 (a) and (b) Chronoamperograms of different modified electrodes at −1.5 V in PBS (pH 8.5).

To distinguish between the role of Pd and PMo₁₂, it was necessary to perform further experiments. In this regard, chronoamperometry experiments have been performed (at the same conditions as before) using PMo₁₂ and PdNPs@PMo₁₂ modified GCEs. It should be noted that because the PMo₁₂ is highly soluble in aqueous solutions; a Nafion coating has been applied to stabilise the PMo₁₂ at the surface of GCE. To account for the probable effects of Nafion coating on the obtained currents, PdNPs@PMo₁₂ was also covered with Nafion. Fig. 6b shows the chronoamperograms of Nafion-coated PMo₁₂ and PdNPs@PMo₁₂-modified GCEs. Here, the obtained current with PdNPs@PMo₁₂ is only about 2.5 times higher than that of PMo₁₂; thus, it can be inferred that both the Pd and PMo₁₂ play key role in the activation process.

Cyclic voltammetry experiments. Based on the results of chronoamperometry experiments, further studies were accomplished using cyclic voltammetry to clarify the possible events that may occur for Pd and/or PMo₁₂ during the activation process. To this end, two hypotheses can be considered: first, it is possible that OH⁻ species that are created at the catalyst surface during the activation process facilitate the electrocatalytic ethanol oxidation by a mechanism that is discussed later in Section A; second, it is possible that the structure of PMo₁₂ changes during this process and some new Mo-based compounds are produced; for which the details are explained in Section B.
A: Hypothesis I. Fig. 7 shows the CV of the PdNPs@PMo₁₂-modified GCE in the activation solution (PBS, 0.1 M, pH 8.5) in the potential range between 0.6 and −1.6 V. It is seen that by sweeping the potential toward negative values, an abrupt rise in the current is observed at around ∼−1.12 V which corresponds to hydrogen evolution reaction (HER).^38,39 It is necessary to mention that during HER in neutral and alkaline media, OH⁻ ions are created at the electrode surface (eqn (1)). These OH⁻ species are captured within the nanoscopic cavities of the catalyst and entrapped in such a way that they cannot be removed even by washing.

Fig. 7 Cyclic voltammogram (CV) of non-activated PdNPs@PMo₁₂ in 0.1 M PBS (pH 8.5).

Now, it is useful to point to the electrocatalytic ethanol oxidation mechanism on Pd in alkaline media, for which the successive reactions can be expressed as follows:

Pd + CH₃CH₂OH ↔ Pd–(CH₃CH₂OH)_ads (2)

Pd–(CH₃CH₂OH)_ads + 3OH⁻ → Pd–(CH₃CO)_ads + 3H₂O + 3e⁻ (3)

Pd–(CH₃CO)_ads + Pd–OH_ads → Pd–CH₃COOH + Pd (4)

Pd–CH₃COOH + OH⁻ → Pd + CH₃COO⁻ + H₂O (5)

It is generally accepted that the eqn (4) is the rate determining step, so that, it is important for Pd–OH_ads species to be available during the ethanol oxidation. These species are obtained by chemisorption of OH⁻ ions at the surface of Pd.^37,40,41 The activation process plays a key role on this stage as it can produce huge amounts of OH⁻ ions which are readily adsorbed at the surface of neighbouring PdNPs to form pre-adsorbed Pd–OH_ads.^42,43

In order to eliminate the effect of PMo₁₂ layer on catalytic activity of the PdNPs@PMo₁₂ after activation treatment in this hypothesis, the behaviour of a commercial Pd/C catalyst was also investigated. Fig. 8 compares the catalytic activity of activated and non-activated Pd/C catalyst toward ethanol electrooxidation and confirms the positive effect of the activation process on the Pd/C catalyst.

Fig. 8 CVs in 1.0 M NaOH and 1.0 M ethanol solution for non-activated Pd/C (dashed line) and activated Pd/C (solid line).

To obtain further information on the role of HER in the activation process, the behaviour of PdNPs@PMo₁₂-modified GCE towards this reaction was investigated over several pH values using linear sweep voltammetry (LSV) and the results are shown in Fig. 9. In acidic media (0.5 M H₂SO₄) the HER involves the consumption of H⁺ and production of gaseous H₂ which do not contribute in the activation process; on the other hand, based on eqn (1), in neutral and basic media the overall reaction promotes the mass transfer of OH⁻ to the surface of PdNPs. Fig. 9 compares the behaviour of the catalyst at pH 7, 8.5 and 14, respectively. It is obvious that the rate of HER in potentials lower than ∼−1.5 V (which is the applied potential in the activation step) is highest at pH 8.5 which means that the hydroxide ions are generated more efficiently in this condition. Also, more basic solutions are not proposed for activation process as they can prompt a chemical reaction for PMo₁₂ and do not show any significant difference on HER efficiency at the applied potential. These results are in complete accordance with our experimental evidences for the optimised operating potential and pH for activation step (not shown).

Fig. 9 Linear sweep voltammetry (LSV) for non-activated PdNPs@PMo₁₂ in 0.5 M H₂SO₄ (pH 0.3), 1.0 M NaOH (pH 14), 0.1 M PBS (pH 7), 0.1 M PBS (pH 8.5).

B: Hypothesis II. To investigate the second claim, whether or not the PMo₁₂ maintain its original molecular structure during the activation process, the CVs of Nafion coated PMo₁₂ modified GCE was obtained before and after applying the activation process (Fig. 10). For PMo₁₂, it is difficult to obtain well-defined redox peaks in pure aqueous solvent because of hydrolysis of PMo₁₂. However, PMo₁₂ is stabilised by addition of equal amounts of some organic solvents. Thus, cyclic voltammetry is generally performed in mixed organic-aqueous solvents containing acid (50% v/v water : DMF solution containing 0.5 M H₂SO₄).^44,45 Also, it is important to note that because the PMo₁₂ is highly soluble in water, a Nafion coating has been applied to stabilise the PMo₁₂ at the surface of GCE. Fig. 10a shows the CVs of PMo₁₂-modified GCE, three pair of redox peaks (1, 2 and 3) at E_1/2 of about 0.31, 0.19 and −0.06 V corresponded to a two-electron redox reversible reaction for each step, which can be explained by the following equations:

PMo₁₂O₄₀³⁻ + 2e⁻ + 2H⁺ ↔ H₂PMo₁₂O₄₀³⁻ (6)

H₂PMo₁₂O₄₀³⁻ + 2e⁻ + 2H⁺ ↔ H₄PMo₁₂O₄₀³⁻ (7)

H₄PMo₁₂O₄₀³⁻ + 2e⁻ + 2H⁺ ↔ H₆PMo₁₂O₄₀³⁻ (8)

Fig. 10 Cyclic voltammograms (CVs) of Nafion coated PMo₁₂ in 50% (v/v) water–DMF solution containing 0.5 M H₂SO₄: (a) before activation step (dashed line), (b) after activation step (solid line).

Further reversible four-electron process can be assigned to peak 4. Also, peak 5 is related to an irreversible process.^46–48 The absences of all of characteristic waves after the activation process in Fig. 10b highly reinforce our second hypothesis. The same behaviour was also observed when PMo₁₂ is in the nanohybrid structure (Fig. 11a). It is noteworthy that the redox peaks (4 and 5) of PMo₁₂ overlap with hydrogen evolution area and are difficult to distinguished; therefore, the three first peaks are considered. A comparison of Fig. 11a and b indicate the complete disappearing of all three characteristic peaks and again verifies the second postulation.

Fig. 11 Cyclic voltammograms (CVs) of PdNPs@PMo₁₂ in 50% (v/v) water–DMF solution containing 0.5 M H₂SO₄: (a) before activation step (dashed line), (b) after activation step (solid line), inset: magnified of the CVs.

Another issue which strongly reinforce this hypothesis is that in alkaline media, PMo₁₂ is not stable; thus it may decompose to some new Mo-based compounds and change the structure of the nanohybrid.^47,49,50

Herein, we think that the possible reason for the superior catalytic activity of the nanocatalyst after applying the activation process can be attributed to the fragments issued from the decomposition of PMo₁₂ units.

Now, a fundamental question arises: which one of the hypotheses is really responsible for the unique performance of the activation process? Indeed, the increase in the nanocatalyst activity is related to a bifunctional mechanism. Also, a final point that must not be ignored is that the Mo-based materials are active hydrogen evolution reaction catalysts.^51,52 Therefore, upon production of these compounds during the activation process, the HER may also being fortified simultaneously, and the pre-adsorbed Pd–OH_ads could be formed more efficiently according to hypothesis I.

Conclusion

Nanohybrid phosphomolybdic acid-stabilised palladium nanoparticles (PdNPs@PMo₁₂) were synthesised using a previously reported experimental procedure.³⁰ The as-prepared nanohybrid was employed as an electrocatalyst for the ethanol electrooxidation reaction. We found that the electrocatalytic activity of the as-prepared nanocatalyst was significantly increased by applying an activation step. The focus of our current research has been to uncover what happens during this activation process. To conclude, electrochemical experiments – involving chronoamperometry, cyclic voltammetry and linear sweep voltammetry – were employed to trace this process and to illuminate the possible events that may occur during the activation process. Based on our research, two possible mechanisms could occur. On the one hand, it is possible that HER occurs at the surface of the catalyst and facilitates the formation of Pd–OH_ads sites. These Pd–OH_ads species are major participants in rate determining step of ethanol electrooxidation mechanism; so that, their presence significantly facilitates the oxidation reaction (hypothesis I). On the other hand, it is possible that some new Mo-based compounds with enhanced electrocatalytic activity are generated (hypothesis II). Our results suggest that the real function of the activation process is based on a synergistic effect between these two hypotheses. To shed more light on this issue we are currently investigating the use of hybrid nanoparticles for the HER in more detail.

Acknowledgements

This work was supported by a Grant-in-Aid for Renewable Energy Organization of Iran. This work has also been funded in part by the Marie Curie Intra-European Fellowship 328985-COCOPOPS and the ERC-Starting Grant 239931-NANOPUZZLE. The authors would like to acknowledge the use of The Advanced Microscopy Laboratory (LMA-INA).

Notes and references

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