Effect of metal content on the electrocatalytic activity of Au_xPd_y mixtures and their use in a glucose membraneless microfluidic fuel cell

Abstract

Au_xPd_y bimetallic mixtures with different elemental contents were synthesized on glassy carbon electrodes using electrochemical techniques, which are easy, quick, versatile and cheap. Pulse potential and staircase techniques such as cyclic voltammetry (Au₆₀Pd₄₀), square-wave voltammetry (Au₅₀Pd₅₀ and Au₃₅Pd₆₅) and second harmonic AC voltammetry (Au₁₅Pd₈₅) were used to easily change the metal proportion and reduce the Au content in the Au_xPd_y mixtures. Au₆₀Pd₄₀ exhibited the most negative potential (−0.4 V vs. NHE) towards the glucose electro-oxidation reaction. For this reason, it was used in the anode compartment of a microfluidic fuel cell and compared with single Au and Pd materials by cyclic voltammetry. Au₆₀Pd₄₀ showed a greater negative potential than that of the Au anode; meanwhile, Pd showed no electrocatalytic activity. The lattice parameters were calculated by X-ray diffraction patterns resulting in values of 3.83 and 4.03 Å for Au and Pd, respectively, and 3.94 Å for Au₆₀Pd₄₀, which provides evidence for the internal structural changes due to the incorporation of Pd to the Au matrix. The maximum power density obtained with a glucose membraneless microfluidic fuel cell (GMMFC) using 10 mM glucose and Au₆₀Pd₄₀ as the anode was 0.28 mW cm⁻².

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Introduction

Research in microfluidic fuel cells utilizing glucose as a fuel has shown growth in the last few years.^1–3 The main reason is related with its easy production and handling; besides, it has shown intuitive applications for implantable and portable fuel cell applications.⁴ In this context, microfluidic fuel cells are a class of fuel cells that operates without a membrane to separate the anode and cathode.⁵ A natural interface between the fuel and oxidant is formed due to the laminar flow regimen produced when they are introduced in a microchannel, allowing independent electrochemical reactions in each stream.⁶

Different catalytic materials have been evaluated in glucose microfluidic fuel cells, such as modified or immobilized enzymes,^7–10 noble metals like gold supported on vulcan carbon or multi-walled carbon nanotubes.^3,11,12 The AuPd mixture has been reported using different chemical synthesis methods. Some of the reported pathways are via chemical reduction methods,¹³ thermal chemical vapour deposition,¹⁴ reversible addition-fragmentation chain transfer (RAFT) polymerization,¹⁵ and electrodeposition using differential pulse amperometry.¹⁶ The AuPd mixture has been used for the electro-oxidation of formic acid,¹³ glycerol,¹⁷ and carbon monoxide;¹⁸ the hydrogen evolution reaction,¹⁹ and the electroreduction of hydrogen peroxide.¹⁶ The effect of the Au and Pd molar content in AuPd materials on the glucose electro-oxidation reaction has been recently investigated by Yan et al.²⁰ They found that introducing Au to the Pd lattice allowed an increasing tolerance to glucose by-products. Zhang et al.,¹³ as well as Park et al.,²¹ also observed that the electrocatalytic properties of AuPd are enhanced in terms of its stability (by decreasing CO poisoning) and activity for the electro-oxidation reaction of formic acid.

The miniaturization of membraneless fuel cells involves a decrease in the electrode size and hence, increases the difficulty for the incorporation of electrocatalysts in the electrode surface. Electrodeposition is an easy, cheap, quick and versatile synthesis method. The shape and the thickness of the film can be easily tuned and reproduced by controlling some experimental conditions, such as the deposition time, scan rate, the nature of the supporting electrolyte and the magnitude of the applied potential.²² We propose the use of staircase and pulse potential electrochemical techniques as tools for an easy and selective integration of the catalytic material into the microfluidic fuel cell. In this sense, Au_xPd_y mixtures with different mass ratios were first synthesized on glassy carbon electrodes in order to determine the effect of metal content on the glucose electro-oxidation reaction, and the subsequent synthesis of the best Au_xPd_y ratio on a metallized glass plate (Ti, Ni, Au, tri-layer) for its use as an anodic electrode array in a glucose membraneless microfluidic fuel cell (GMMFC).

Experimental

Electrochemical synthesis of Au_xPd_y mixtures on glassy carbon electrodes

Au_xPd_y mixtures with different metal ratios were synthesized using three different electrochemical techniques: cyclic voltammetry using a staircase technique, square-wave voltammetry and second harmonic AC voltammetry using pulse potential techniques. Briefly, an electrolytic solution was made using 2 mM HAuCl₄ (Sigma-Aldrich, 98%) and 3 mM Na₂PdCl₄ (Sigma-Aldrich, 99%) as the Au³⁺ and Pd²⁺ ion sources and 0.5 M H₂SO₄ (J. T. Baker, 99.7%) as the electrolyte. Glassy carbon plates (SPI Instruments®, 1.44 cm²) were used as working electrodes. Hg/Hg₂SO₄ sat. K₂SO₄ was used as the reference electrode and a Pt wire as the counter electrode. Experiments were performed using a standard three-electrode electrochemical cell through an AutoLab PGSTAT-30 Potentiostat/Galvanostatat at 25 °C. Au_xPd_y with the highest Au content (Au₆₀Pd₄₀) was electrochemically synthesized using cyclic voltammetry with a potential range from −0.68 to 1.3 V vs. NHE at 100 mV s⁻¹ for 20 cycles. Pulse potential techniques were used to increase the Pd concentration. Square-wave voltammetry was used at two potential ranges: the first was applied from 1 to −0.15 V (Au₅₀Pd₅₀) and the second potential range was from 0.78 to −0.15 V vs. NHE (Au₃₅Pd₆₅); the frequency (10 Hz), amplitude (0.05 V) and cycles (20) were kept as constant for both syntheses. Finally, a second harmonic AC voltammetry was used in order to increase the Pd content (Au₁₅Pd₈₅) due to the facility of this technique to produce dendritic Pd structures;²³ the initial potential and end potential were of 1 and −0.15 V vs. NHE, respectively. The frequency was maintained at 10 Hz. The amplitude was of 0.001 V rms, with a modulation and interval times of 2 and 4 s, respectively. This experiment was performed for one cycle only.

Electrochemical synthesis of Au, Pd and Au_xPd_y catalysts on the anode side of the GMMFC

After evaluating the electrocatalytic properties of the Au_xPd_y mixtures on glassy carbon electrodes to find the best metal ratio, we proceeded to synthesis this mixture as the anode side of the GMMFC. Likewise, bare gold and palladium were synthesised for comparison purposes. The anode electrode consisted of a glass slide plate covered with a Ti, Ni and Au, trilayer (1 cm²). The electrodes were also covered with a Vulcan carbon ink (Vulcan® XC72, Cabot) to avoid the gold effect (from the trilayer) in all experiments (see Scheme 1). The Vulcan ink was set to the trilayer by a spray technique using 3 mg of Vulcan® dispersed in isopropyl alcohol and Nafion® with a ratio of 1 mg Vulcan® : 75 μL alcohol : 15 μL Nafion®.

Scheme 1 (A) PDMS channel and electrode dimensions, (B) microfluidic cell design.

Au nanoparticles were obtained using a cyclic voltammetry technique for 20 cycles through an electrolytic solution of 2 mM HAuCl₄ (Sigma-Aldrich, 98%) and 0.5 M H₂SO₄ (99.7%, J. T. Baker). A potential between 0.283 to 1.383 V vs. NHE was applied with a scan rate of 100 mV s⁻¹. Pd nanoparticles were obtained using an electrolytic solution of 3 mM Na₂PdCl₄ (Sigma-Aldrich, 99%) in 0.5 M H₂SO₄. The potential range was between −0.067 to 1.383 V vs. NHE at 100 mV s⁻¹ for 20 cycles. All experiments were carried out in the presence of N₂ (99.999%, Infra) as an inert atmosphere at 25 °C.

Physicochemical characterization of Au, Pd and Au_xPd_y electrocatalysts

The metal content of Au_xPd_y mixtures synthesized on glassy carbon was obtained using a Bruker S2Picofox X-ray fluorescence (XRF) spectrometer operated at 50 kV and 600 μA. The electrocatalysts synthesized on the anodic Ti, Ni, Au tri-layer were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer operated at 30 kV and 30 mA. A JEOL JSM-7401F field emission scanning electron microscope (FE-SEM) was used to investigate the particle size. The elemental analysis was investigated with a coupled OXFORD EDS analysis INCA-model.

Electrochemical characterization of Au, Pd and Au_xPd_y electrocatalysts

The presence of the electrocatalysts synthesized on glassy carbon and in the anode compartment of GMMFC was verified by electrochemical experiments (with a standard three-electrode electrochemical cell) using cyclic voltammetry in 0.5 M H₂SO₄ aqueous solution at a scan rate of 50 mV s⁻¹ for 10 cycles at 25 °C. The metals electrodeposited on the anode side and on the glassy carbon electrodes were used as working electrodes; Hg/Hg₂SO₄ saturated in K₂SO₄ was used as the reference electrode, and a Pt wire was used as the counter electrode. The electrocatalytic activity of Au_xPd_y mixtures were evaluated in terms of the 10 mM glucose electro-oxidation reaction (Reagent grade, Sigma-Aldrich) using 0.3 M KOH (88%, J. T. Baker) as the electrolyte and the saturated calomel electrode as reference for 10 cycles. The scan rate and temperature were 20 mV s⁻¹ and 25 °C, respectively.

Fabrication and testing setup of the microfluidic fuel cell

A detailed description on the fabrication can be found elsewhere.²⁴ Briefly, a glass slide (76 × 26 mm) was covered with Ti and Ni layers using metal sputtering deposition. Subsequently, photolithography was carried out on the metallic surface to define the conductive area for its consequent Au electrodeposition, showing a final electrode area of 1 cm² with an electrode separation of 500 μm (Scheme 1A and B). The fluids flow through a poly-dimethylsiloxane (PDMS, Sylgard 184, Dow Corning Inc.) channel, which was made by soft-lithography from a master (SU-8 photoresist pattern) to define the double “Y” shape of the microchannel resulting in dimensions of 2 mm wide, 75 μm high and 25 mm long (Scheme 1A).

Au (used as a target sample) and Au_xPd_y were used as the anode electrocatalysts and commercial Pt (20 wt%, E-TEK) was used as the cathode electrocatalyst (Scheme 1B). Commercial Pt was incorporated to the electrode substrate via ink painting employing the same procedure used for the Vulcan carbon. 10 mM D-(+)-glucose (Sigma-Aldrich) and oxygen (zero/UHP degree, Infra) were used as fuel and oxidant, respectively. Both glucose and oxygen were dissolved in separate reservoirs using 0.3 M KOH (78%, J. T. Baker) as the electrolyte. The fuel was deaerated using N₂ (99.999%, Infra) for 1 h. Likewise, the cathode solution was oxygenated for 1 h. The pressure-driven flow rate was 25 μL min⁻¹ and was regulated using a syringe pump (Cole Parmer, single-syringe infusion pump, 115 VAC). The voltage and current measurements were monitored using a BioLogic VSP Potentiostat/Galvanostat. The current and power densities reported in this study were calculated according to the geometric area of the electrodes in the microchannel (0.075 cm²).

Results and discussion

Electrochemical behaviour of Au_xPd_y with different metal ratio

The electrochemical responses of Au_xPd_y materials in an acidic medium (0.5 M H₂SO₄) are shown in Fig. 1. The cyclic voltammograms were labelled according to the Pd mass content obtained by XRF (Table 1). Four well-defined regions were found for the four compositions. The first region is attributed to the hydrogen adsorption/desorption region (0 to 0.3 V vs. NHE), the second region is related to the capacitive zone (0.3 to 0.6 V vs. NHE), the third region to the formation and reduction of Pd oxides (0.6 to 0.9 V vs. NHE), and the fourth region is attributed to the formation of Au oxides and their respective reduction (0.9 to 1.7 V vs. NHE). The electrochemical profiles also showed an increase in the charge attributed to the hydrogen adsorption/desorption zone, which is related to the higher content of Pd.

Fig. 1 Cyclic voltammograms in a saturated-N₂ acidic medium of Au_xPd_y with different mass content: (A) 60 : 40, (B) 50 : 50, (C) 35 : 65 and (D) 15 : 85 on glassy carbon. Scan rate: 50 mV s⁻¹.

Table 1 XRF analysis for the determination of mass content of AuPd mixtures

System	Au content/mass%	Pd content/mass%
AuPd A	59	41
AuPd B	48	52
AuPd C	36	64
AuPd D	15	85

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Electrocatalytic activity of Au_xPd_y mixtures toward glucose oxidation

The electrocatalytic activity of the different Au_xPd_y mixtures was tested in the glucose electro-oxidation reaction using 10 mM D-(+)-glucose (Sigma-Aldrich) and the results are shown in Fig. 2.

Fig. 2 Cyclic voltammograms in basic medium of Au_xPd_y with different mass content: (A) 60 : 40, (B) 50 : 50, (C) 35 : 65 and (D) 15 : 85 for the glucose electrooxidation reaction using glassy carbon as substrate. Scan rate: 20 mV s⁻¹.

As it is well known, Au is a good electrocatalyst for the glucose oxidation reaction.¹² In general, potential shifts towards more anodic values as function of palladium content were observed. The potentials for the first oxidation reaction were of −0.41, −0.35, −0.35 and 0.30 V vs. NHE for Au₆₀Pd₄₀, Au₅₀Pd₅₀, Au₃₅Pd₆₅ and Au₁₅Pd₈₅, respectively. This peak on the Au surfaces is related to the first oxidation of glucose to gluconolactone.²⁵ In contrast, the onset potential for glucose oxidation reaction is associated with the potential value at which the OH species are adsorbed onto the metallic surface;²⁶ apparently the presence of Pd in the catalyst contributes to the formation of a higher quantity of M − OH sites.²⁷ This leads to remarkably high current densities for the Au₆₀Pd₄₀ and Au₅₀Pd₅₀ materials at lower potentials during the glucose oxidation process. In order to determine the best anode electrode for the microfluidic fuel cell, we consider the glucose oxidation potential value, the current density related with the first oxidation process and the simplicity of the technique. In this sense, we decided to use the most common electrochemical technique: cyclic voltammetry, which exhibited more negative potential than the other techniques.

Electrochemical synthesis of Au, Pd and Au_xPd_y on the anodic compartment of the GMMFC

Au₆₀Pd₄₀, Au and Pd were synthesised on the Ti–Ni–Au/Vulcan anodes. The electrochemical profiles of these materials are shown in Fig. 3 (the values for 1, 5, 10, 15, and 20 cycles are presented). In all three cases, an increase in the current was observed due to a change in the conductivity of the electrode surface attributed to the formation of metallic Au, Pd and Au₆₀Pd₄₀ mixture or an increase in the surface area due to the formation of nanoparticles. Peak IV related to the Au zero valent formation in Fig. 3-Au suffers a positive shift in the potential between the first and 20^th cycle. This shift is attributed to the energy required for nucleation and crystal growth, which is higher than for successive cycles. In the first cycle, energy is required for the formation of nuclei and crystal growth at the same time, which is higher than the succeeding cycles. As the number of cycles increase, energy lowers because the consumed energy is mostly employed for the crystal growth. The same behaviour was observed for Pd (peak IV) and for Au₆₀Pd₄₀ (peak VIII for Pd and VII for Au). Returning to the case of Au, peaks branded as I and II can be related to the formation of Au oxides, and peak III to the reduction of the Au oxides.²⁸ In the case of Pd (Fig. 3-Pd), peaks labelled as I, II, V, and VI are related to the hydrogen adsorption/desorption region. Meanwhile, peak III is related to the formation of PdH_x,²⁹ and peak IV to the reduction of Pd ions to metallic Pd.³⁰ As a consequence, for Au₆₀Pd₄₀ mixture (Fig. 3-AuPd), peaks I, II, IX and X are associated to the Pd hydrogen adsorption/desorption region. Peaks labelled as III–V correspond to the formation of Pd oxides (peak III) and Au oxides (IV and V). Peak labelled as VI corresponds to the reduction of the Au oxides, peak VII to the formation of metallic Au and peak VIII to the formation of metallic Pd.

Fig. 3 Cyclic voltammograms for the electrochemical synthesis of Au, Pd and the AuPd mixture on the anode compartment using a Ti, Ni, Au tri-layer as substrate. Scan rate: 100 mV s⁻¹.

Physicochemical characterization of Au, Pd and Au₆₀Pd₄₀ in the anodic compartment of GMMFC

The XRD patterns for Au, Pd and Au₆₀Pd₄₀ using graphite as the substrate are shown in Fig. 4. Only some of the characteristic crystallographic planes of the different materials were identified due to the poor crystallinity that they exhibited. In the case of Pd, the peaks related to (111) and (311) were located at 40.04 and 81.7 Å, respectively. Au exhibited (111) and (220) planes, found at 38.24 and 64.72 Å, respectively. The Au₆₀Pd₄₀ mixture exhibited the (111) and (220) crystallographic planes, which are characteristic of Au and Pd in their zero-valent form and were found at 39.15 and 66.3°, respectively. The graphite peaks were labelled in grey colour and correspond to the following peaks: (100), (101), (102), (004), (103), (110) and (112).

Fig. 4 X-ray diffraction patterns for Pd, Au and Au₆₀Pd₄₀ synthesized by cyclic voltammetry. The (111) is marked with a (*). The graphite peaks are marked in grey colour.

The (111) plane of the Au₆₀Pd₄₀ mixture was located between the characteristic degrees of the Au and Pd, suggesting changes in the internal structure of the mixture. In this sense, the lattice parameters were calculated from the (111) plane resulting in 3.83, 4.03 and 3.94 Å, for the Pd, Au and Au₆₀Pd₄₀ respectively, concluding that the internal distance between the crystallographic network for the Au₆₀Pd₄₀ is different than the Au and Pd. This behaviour is explained by Toda et al.³¹ in terms of alloy formation. Additional experiments are needed to confirm the nature of the mixture in terms of the interactions between both metals.

An approximation of the crystallite sizes was conducted applying Scherrer's equation to the (111) plane resulting in sizes of 44, 16 and 33 nm for the Au, Pd and Au₆₀Pd₄₀ electrocatalysts, respectively. SEM images are illustrated in Fig. 5 at two magnifications, 10k and 50k×. All three cases showed uniform films, as can be observed at 10k×. The Au film exhibited the existence of semi-spherical Au particles with a low presence of aggregates (Fig. 5 at 50k×). The Pd films showed the existence of semi-spherical Pd particles with sharp-pointed growths. Finally, the Au₆₀Pd₄₀ mixture showed the presence of AuPd nanoflowers and the existence of semi-spherical particles. The particle shapes for the three materials can be related to the applied potential range.

Fig. 5 FE-SEM micrographs for Au, Pd and Au₆₀Pd₄₀ synthesized by cyclic voltammetry (left, lower magnifications and right, higher magnification).

Electrochemical characterization of Au, Pd and Au₆₀Pd₄₀ electrocatalysts in the anodic compartment of GMMFC

The electrochemical responses of Au, Pd and Au₆₀Pd₄₀ in an acidic medium (0.5 M H₂SO₄) are shown in Fig. 6. In a potential range from 1.3 to 1.7 V vs. NHE (Fig. 6-Au), the formation of Au oxides occurs, and at 1.16 V, their respective reduction. For the case of Pd (Fig. 6-Pd), its three typical regions were observed, i.e. the hydrogen adsorption/desorption region in an potential interval from 0 to 0.3 V vs. NHE, the double layer region from 0.3 to 0.6 V, and finally, the formation of Pd oxides and their respective reduction from 0.6 to 1.5 V vs. NHE. For the Au₆₀Pd₄₀ electrocatalyst, a combination of the signals from the last two cases was observed. From 0 to 0.3 V the typical hydrogen adsorption/desorption region of Pd was observed. From 0.3 to 0.6 V, the double layer region was located. From 0.6 to 1 V, the formation of Pd oxides occurs and from 1.3 to 1.5 V, the formation of Au oxides occurred. The reduction of Pd oxides was located at 0.6 V and the reduction of Au oxides at 1.1 V.

Fig. 6 Cyclic voltammograms in an acidic medium showing the electrical response of Au, Pd and Au₆₀Pd₄₀ synthesized by cyclic voltammetry using the tri-layer anode as substrate.

The electrochemically active surface area (ECSA) was calculated through the peaks related to the reduction of the Au oxides (for Au and Au₆₀Pd₄₀ cases) and the hydrogen desorption region for the case of Pd using the theoretical values of 559 and 424 μC cm⁻², respectively.^32,33 The calculated values were of 6.98 (Au), 2.69 (Au in Au₆₀Pd₄₀ mixture) and 3.94 cm² (Pd).

The electrocatalytic activity during the 10 mM glucose electro-oxidation reaction in 0.3 M KOH is shown in Fig. 7A. The Pd incorporation in the Au lattice modified the electrochemical behaviour for glucose electro-oxidation (black line) in which the successive glucose oxidations were shifted between Au and Au₆₀Pd₄₀ can be observed in Fig. 7A. In the case of Au, the reaction of interest took place at −0.31 V vs. NHE. Meanwhile, for the Au₆₀Pd₄₀ electrocatalyst, the reaction was carried out at −0.41 V vs. NHE. The current density for Au₆₀Pd₄₀ was 0.3 mA cm⁻².

Fig. 7 (A) Cyclic voltammograms of Au, Pd and Au₆₀Pd₄₀ during the 10 mM glucose electro-oxidation reaction using 0.3 M KOH as electrolyte; scan rate: 20 mV s⁻¹. (B) Polarization and power density curves in 10 mM glucose for the membraneless microfluidic fuel cells equipped with Au and Au₆₀Pd₄₀ anodes at 25 °C.

Performance of the electrocatalysts in a glucose microfluidic fuel cell

Fig. 7B shows the polarization and power density curves for the microfluidic fuel cell operated with Au and Au₆₀Pd₄₀ as anode electrocatalysts in the presence of 10 mM glucose and Pt/V XC72 as the cathode with O₂-saturated 0.3 M KOH. It is interesting to note that the open circuit potential in both cases show similar values, around 525 mV. The current density in the microfluidic fuel cell operated with Au₆₀Pd₄₀ as the anode increased from 1.5 to 2.0 mA cm⁻². The power density (normalized by the geometrical area) of Au₆₀Pd₄₀ is slightly larger than that obtained with Au/C as anode. These results show the enhancement of electrocatalytic activity for the Au₆₀Pd₄₀ material.

Conclusions

Pd content is critical in the electrocatalytic activity for the glucose electro-oxidation reaction. In this work, we explored its use as the bimetallic component in Au electrocatalysts. The glucose mechanism was affected by the bimetallic composition. By increasing the Pd content (Au₃₅Pd₆₅), the glucose mechanism started to exhibit similar current density for glucose and gluconolactone as the principal by-product, which means that both oxidations were carried out in an almost 1 : 1 ratio. When the Pd metal concentration was much higher than that of Au (Au₁₅Pd₈₅), the peak related to the glucose and gluconolactone oxidation overlapped. At intermediate concentrations (Au₅₀Pd₅₀), Au showed a higher effect than Pd, which was observed as an increase in the current density toward gluconolactone oxidation due to the high content of (111) defects. The highest Au concentration was Au₆₀Pd₄₀ electrocatalyst, which showed well-defined peaks related to both glucose and gluconolactone oxidations. This material also showed the most negative potential toward glucose oxidation. Therefore, Au₆₀Pd₄₀ was electrodeposited onto a glucose membraneless microfluidic fuel cell, showing better performance than that obtained using a typical Au electrocatalyst as anode. In summary, AuPd materials showed high activity toward the glucose electro-oxidation reaction and can be easily electrochemically synthesized in the anodic compartment of membraneless microfluidic fuel cells avoiding the use of Nafion® as a binder and hence decreasing the cell resistance.

Acknowledgements

The authors gratefully acknowledge the financial support of ANR-CONACYT 2011 (Grant 163114).

Notes and references

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