Carbon supported nano Pt–Mo alloy catalysts for oxygen reduction in magnesium–air batteries

Abstract

In this work, carbon supported platinum–molybdenum alloy catalysts (Pt–Mo/C) with different Pt : Mo atomic ratios were synthesized via an impregnation reduction method at room temperature in aqueous solution. Their oxygen reduction reaction (ORR) performances were evaluated in magnesium–air batteries. Structural characterizations revealed that Pt–Mo alloy nanoparticles having an average size of 5 nm formed and distributed homogeneously on a carbon support. Electrochemical measurements showed that Pt–Mo/C catalysts exhibited better catalytic activity than Pt/C since a Pt-rich surface formed on the Pt–Mo alloy particles and Pt 5d-band vacancies increased due to the partial substitution of Mo for Pt. Among the Pt–Mo/C catalysts, Pt–Mo/C (3 : 1) showed the best performance with a maximum ORR current density of 21.14 mA cm⁻² in O₂-saturated NaCl solution and a specific discharge capacity of 1311 mA h g⁻¹ in Mg–air batteries, which are 164.9% and 11.8% higher than those of the Pt/C catalyst synthesized by using the same method (7.98 mA cm⁻² and 1156 mA h g⁻¹).

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Introduction

Nowadays, the excessive utilization of fossil fuels in industries and transportation fields as the major energy supply has caused severe global environmental and energy crises. Therefore, new energy sources with less CO₂ emission and pollution are greatly needed. As a series of such new energy sources, fuel cells have become promising candidates for solving the energy and environmental challenges in the 21st century. Metal–air batteries, a new branch of fuel cells, have attracted much attention due to their environmental friendliness and high efficiency.¹ However, their commercial application is still limited by several existing obstacles. One of the major problems for metal–air batteries is the sluggish kinetics and high over potential of the oxygen reduction reaction (ORR) taking place at the air cathode. Therefore, it is necessary to develop more efficient ORR catalysts.²

Platinum is the most active electrocatalyst for oxygen reduction reaction. However, due to the high cost and limited availability, the loading of Pt needs to be reduced for commercial applications. Alloying Pt with nonprecious transition metals is a valid approach, which not only reduces Pt usage but also enhances catalytic activity. Previous studies suggested that Pt–Co, Pt–Ni, Pt–Fe, Pt–Cu,³ Pt–Ru,⁴ Pt–Y,⁵ Pt–Zn,⁶ and Pt–Mn⁷ alloys all exhibit higher ORR activity than Pt alone. Furthermore, loading nanosized Pt particles on carbon materials with high specific surface area, such as carbon blacks, carbon nanotubes,⁸ carbon aerogels⁹ and graphene,¹⁰ also improves the catalytic activity of pure Pt.

It has been established in recent years that through alloying with Mo, both catalytic activity and stability of Pt for hydrogen oxidation reaction in traditional fuel cells can be enhanced.^11–19 However, the ORR catalytic activity of Pt–Mo alloy catalysts has not been well studied yet. Moreover, Pt–Mo alloys reported in previous studies were mostly prepared by arc melting of pure elements.^11–14 As to the preparation of carbon supported Pt-based catalysts, solution-based impregnation reduction method is widely used. However, it is particularly challenging to adopt this method to the synthesis of Pt–Mo/C alloy catalysts due to the large negative redox potential of the Moⁿ⁺/Mo⁰ couple and the low miscibility of Pt and Mo.¹⁵ In solution-based methods of the synthesis of Pt–Mo/C alloy catalysts, organic solvents have been mostly used, but they are not environmentally friendly.^13,16,17 Pt–Mo/C catalysts synthesized by impregnation reduction method also have Mo oxide species on the surface. To reduce the Mo oxide species and form a true alloy, heat treatment at high temperature have to be employed afterwards, which increases the cost.^18,19 Furthermore, in most reports, ORR activity tests on Pt-based catalysts were carried out in alkaline or acidic media. Few of them were performed in neutral solution such as sodium chloride solution, which is the most suitable electrolyte for magnesium–air batteries.²⁰

In this study, Pt–Mo binary alloys supported by carbon (Pt–Mo/C) as ORR catalyst were prepared via NaBH₄ impregnation reduction method in aqueous solution at room temperature. The Pt–Mo/C catalyst was characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) in O₂ saturated sodium chloride solution, and discharge curves of magnesium–air batteries. The test results showed that Pt–Mo/C alloy catalysts exhibited better catalytic properties than Pt/C while the catalytic activity dropped when Mo content in Pt–Mo exceeds certain threshold.

Experimental

Materials

H₂PtCl₆·6H₂O, Na₂MoO₄·2H₂O, NaBH₄, nitric acid, ethanol (99.5%) and NaCl were obtained from Sigma-Aldrich. 5% Nafion solution was purchased from Dupont and carbon sheets were bought from Shanghai Hesen Co. These chemicals were used as received. Carbon aerogels used as support for the catalysts were synthesized by adopting the method described in detail in the previous work.²¹ Mg sheets were supplied by the National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University.

Catalysts synthesis

Carbon supported Pt and Pt–Mo alloy catalysts (Pt/C, Pt–Mo/C) were synthesized by NaBH₄ impregnation reduction method. 100 mg carbon aerogels were added into a glass flask containing 60 mL distilled water and sonicated for 15 min to form uniform suspension. Subsequently, H₂PtCl₆ and Na₂MoO₄ water solutions were added to the suspension to give a loading of 20 wt% metal and Pt : Mo atomic ratios of 3 : 1, 2 : 1 or 1 : 1. Afterwards, the suspension was moved into an argon-filled glove box and 200 mg NaBH₄ dissolved in 50 mL water was added. After stirred for 4 h, the mixture was moved out of the glove box, centrifuged, washed with distilled water for several times, and dried at 40 °C for 4 h.

Characterization

Metallic phases were detected using X-ray diffraction on a D/max 2550VL/PCX diffractometer equipped with a Cu Kα radiation source at a scanning rate of 1° min⁻¹. Chemical composition of the catalysts was characterized by energy-dispersive spectroscopy (EDS) on a JSM 7600F scanning electron microscope (SEM) attached with an Oxford instruments EDS attachment. Particle size and structural features of the catalysts were observed using a JEM2100F transmission electron microscope operated at 200 kV. The TEM samples were dispersed in ethanol and sonicated for 20 min. Several drops of the supernatant liquid were deposited on 200 mesh copper grids covered with carbon. Surface chemical states of the elements were evidenced by X-ray photoelectron spectroscopy (XPS) using an AXIS ULTRA DLD photoelectron spectrometer, with a beam of monochromatic Al Kα as radiation source. Charging effects of the samples were corrected by referencing the binding energies to that of the C 1s peak at 284.6 eV.

Electrode preparation and electrochemical measurement

A glassy carbon electrode (GCE, I.D. 3 mm, O.D. 6 mm, CHI Co.) was polished with 1.0 and 0.3 μm Al₂O₃ to a mirror finish, then washed with dilute nitric acid, ethanol and distilled water before each experiment. Catalyst ink containing 5 mg catalysts, 1 mL ethanol, and 100 μL 5% Nafion solution was put in an ultrasound bath for 20 min. Afterwards, 10 μL of the ink was placed on tip of the GCE with a micropipette and evaporated at room temperature for 1 h to form a catalytic solid layer.

Cyclic voltammograms (CV) were measured on a CHI660C electrochemical workstation, in a typical 3-electrode system, where saturated calomel electrode (SCE, Hg/Hg₂Cl₂/sat. KCl) acted as reference electrode, a Pt wire was used as counter electrode and a 3 mm-diameter glassy carbon electrode covered with catalyst layer served as working electrode. The surface area of the working electrode was calculated to be 0.07 cm², which was used for calculating the current densities in CVs. O₂-Saturated 3.5 wt% NaCl solution (bubbled with oxygen for 30 min before testing), was employed as the electrolyte. Negative-going CV scans were conducted between −1.0 and +0.7 V vs. SCE at a scanning rate of 50 mV s⁻¹.

Battery test

A 4 mm × 4 mm piece of carbon sheet coated with the as-prepared catalyst was used as the cathode of a Mg–air battery. Catalyst ink containing 300 μL ethanol, 400 μL distilled water, 100 μL Nafion solution and 7.5 mg as-prepared catalysts was added on the carbon sheet and dried at 60 °C for 4 h before battery test. A magnesium piece (20 × 20 × 2 mm) served as the anode and 3.5 wt% NaCl water solution acted as the electrolyte. Discharge curves of the Mg–air batteries were obtained on a LAND battery measurement system at a discharge rate of 5 mA cm⁻². The weights of anodes before and after discharge were measured to calculate the mass loss of the Mg anodes. After discharge, the Mg anodes were immersed into a 200 g L⁻¹ chromate acid solution with 10 g L⁻¹ silver nitrate at ambient temperature for 7 min to remove the discharge products before weighted.

Results

Physical characterization of Pt–Mo alloy electrocatalysts

XRD patterns of the Pt/C and Pt–Mo/C catalysts are shown in Fig. 1. All the XRD patterns only present diffraction peaks associated to the face-centered-cubic (fcc) Pt crystal structure. Diffraction peaks at around 2θ = 39.8, 46.2, 67.6 and 81.5 degrees correspond to Pt(111), Pt(200), Pt(220) and Pt(311) crystal planes, respectively, according to JCPDS card no. 70-2057. No peaks of carbon are observed in the XRD patterns since carbon aerogels are amorphous. Peaks of Pt in Pt–Mo/C catalysts shift slightly to higher angles compared to those of Pt in Pt/C, which is an evidence of lattice contraction caused by alloying with Mo. The extent of lattice contraction increases with increasing Mo content, as shown by the lattice constants listed in Table 1. According to previous research, the lattice contraction is an indication of partial substitution of Mo for Pt in Pt lattice.^22–24 Furthermore, broadening of Pt diffraction peaks in the XRD patterns is also observed, indicating that the grain sizes are in nanometer-scale range. Crystallite sizes of the catalysts were calculated from the broadening of (220) diffraction peak using the Scherrer equation (D = 0.89λ/B cos θ) since Pt(220) diffraction peaks are not affected by carbon support.^25,26 D is the crystallite size, λ the wavelength of radiation source (0.154056 nm), B the width at half height of the diffraction peak, and θ is the angle at which the peak reaches the maximum. The crystallite sizes of the catalysts are calculated to be 4–6 nm (Table 1), which increase slightly with increasing Mo content.

Fig. 1 XRD patterns of Pt/C and Pt–Mo/C catalysts.

Table 1 Data obtained from XRD and EDS analyses

Catalyst	Pt : Mo atomic ratio		Lattice parameter (Å)	Crystallite size by XRD (nm)
	Nominal	EDS
Pt/C	—	—	3.922	4.90
Pt–Mo/C (3 : 1)	75 : 25	77 : 23	3.910	5.31
Pt–Mo/C (2 : 1)	67 : 33	63 : 37	3.906	5.43
Pt–Mo/C (1 : 1)	50 : 50	56 : 44	3.902	5.74

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The elemental composition (Pt : Mo) in the Pt–Mo/C catalysts determined by EDS are also provided in Table 1. The atomic ratios of Pt and Mo are close to the nominal composition, suggesting effective reduction of Pt and Mo by the synthesis method used here.

Fig. 2 shows the TEM images of Pt/C, Pt–Mo/C (3 : 1), Pt–Mo/C (2 : 1) and Pt–Mo/C (1 : 1) catalysts. The nanoparticles are homogenously distributed on the carbon support with no evidence of agglomeration. After counting over 100 randomly chosen nanoparticles in TEM images, it is concluded that the average sizes for the nanoparticles in Pt/C, Pt–Mo/C (3 : 1), Pt–Mo/C (2 : 1) and Pt–Mo/C (1 : 1) are 4.70, 4.94, 4.98 and 5.34 nm, with 74.1%, 69.9%, 68.8% and 75.0% of the particles in the range of 4–6 nm, respectively. The particle sizes measured by TEM are in consistence with the average crystallite sizes determined from the corresponding XRD patterns, suggesting that most of the Pt or Pt–Mo particles are single crystals.¹⁸

Fig. 2 TEM images of Pt/C (a), Pt–Mo/C (3 : 1) (b), Pt–Mo/C (2 : 1) (c) and Pt–Mo/C (1 : 1) (d) catalysts.

The selected area electron diffraction (SAED) patterns of the four catalysts in Fig. 2 indicate that all the diffraction rings are consistent with fcc platinum phase. High resolution-transmission electron microscopy (HR-TEM) images of the catalysts revealed their crystalline nature. The spacing between adjacent lattice planes is around 0.23 nm, corresponding to (111) plane of fcc Pt.²⁷ No diffraction rings or d-spacing corresponding to metallic Mo, Mo oxide or Mo carbide structures were observed.

The surface chemical composition, chemical states of elements and intermetallic bonding information were analyzed by XPS. In Pt–Mo/C (3 : 1), Pt/Mo atomic ratio is 88 : 12, which is higher than that obtained from the EDS analysis (78 : 22), indicating the formation of a structure with Pt-rich and Mo-lean surface.

Narrow scan XPS spectrum of Pt 4f and Mo 3d orbitals are shown in Fig. 3. As to the Pt 4f spectrum, the 4f_7/2 peak (71.5 eV) and 4f_5/2 peak (74.9 eV) have a binding energy (BE) difference of 3.4 eV, indicating that Pt possesses a valence of zero according to the handbook of XPS.²⁸ The Mo 3d spectrum is relatively rough due to low content of Mo. However, two obvious peaks at 228.4 and 231.6 eV can be referred to 3d_5/2 and 3d_3/2, and the gap between the two peaks is 3.2 eV, which is a proof on the existence of zero-valence Mo according to the handbook of XPS. The absence of Mo oxide species may be attributed to the inert atmosphere in which the catalysts were synthesized. Furthermore, the peaks shift slightly compared to pure metal (+0.6 eV shift of Pt 4f peaks and +0.7 eV shift of Mo 3d peaks), which may be an evidence of the alloying effect, where d electron transfer from hyper-d-electronic Pt antibonding band to the deficient hypo-d-electronic Mo band.^29–31 Therefore, both contraction of lattice in XRD results and binding energy shifts in XPS spectrum indicate the formation of a Pt–Mo alloy.

Fig. 3 XPS spectra of Pt 4f (a) and Mo 3d (b) of the Pt–Mo/C catalyst.

Electrochemical characterization

Fig. 4 presents the cyclic voltammograms of Pt/C, Pt–Mo/C (3 : 1), Pt–Mo/C (2 : 1) and Pt–Mo/C (1 : 1) in O₂-saturated 3.5 wt% NaCl aqueous solution. The electrocatalytic oxygen reduction behavior of ORR catalysts is generally evaluated by the onset potentials where ORR reaction starts and the maximum current density at cathodic peak. The more positive the onset reduction potential is, the more easily ORR will occur and the larger the maximum current density is, the faster ORR will be.²¹ The onset potentials of the four catalysts are almost the same, at around −0.05 V. The maximum current density of Pt/C, Pt–Mo/C (3 : 1), Pt–Mo/C (2 : 1) and Pt–Mo/C (1 : 1) are 7.98, 21.14, 12.33 and 10.26 mA cm⁻², respectively. All the Pt–Mo/C catalysts display better ORR activity compared to Pt/C. Among the Pt–Mo/C catalysts, Pt–Mo/C (3 : 1) exhibits the best ORR activity, followed by Pt–Mo/C (2 : 1) and Pt–Mo/C (1 : 1), which means ORR activity decreases with increasing the content of Mo.

Fig. 4 CV curves of electrodes with different catalysts in O₂-saturated 3.5 wt% NaCl aqueous solution.

Battery test

Fig. 5 presents the discharge curves of magnesium–air batteries using Pt/C and Pt–Mo/C catalysts at a discharge rate of 5 mA cm⁻². The operating voltages for magnesium–air batteries using Pt/C, Pt–Mo/C (3 : 1), Pt–Mo/C (2 : 1) and Pt–Mo/C (1 : 1) are 1.25 V, 1.34 V, 1.32 V and 1.32 V. The specific discharge capacities of magnesium–air batteries using those catalysts are 1156 mA h g⁻¹ for Pt/C, 1311 mA h g⁻¹ for Pt–Mo/C (3 : 1), 1244 mA h g⁻¹ for Pt–Mo/C (2 : 1) and 1210 mA h g⁻¹ for Pt–Mo/C (1 : 1), respectively. Thus, all Pt–Mo/C catalysts perform better ORR properties than Pt/C with higher voltage and larger discharge capacity. Among the Pt–Mo/C catalysts, Pt–Mo/C (3 : 1) shows the best performance, followed by Pt–Mo/C (2 : 1) and Pt–Mo/C (1 : 1), which is in consistent with the CV results.

Fig. 5 Discharge curves of magnesium–air batteries using different Pt-based catalysts.

Discussion

The relatively better performance of Pt–Mo/C catalysts over Pt/C can be attributed to the following two reasons: one is the Pt segregation to the surface of Pt–Mo alloy; the other is the change of the electronic nature of Pt by alloying with Mo. Pt atoms tend to segregate to the surface of Pt–Mo alloy, which has been proved by density functional theory (DFT) studies^32,33 and test results in previous research.¹¹ Such Pt-rich surface is advantageous for improving the catalytic performance per unit mass since Mo is less active in ORR than Pt. On the other hand, metallic bond between Pt and Mo is formed in the Pt–Mo alloy. The electronic structures of Pt and Mo atoms are 5d⁹6s¹ and 4d⁵5s¹. Pt owns more d electrons than Mo, hence some d electrons of Pt move into d electronic-orbital of Mo, resulting in the increase of Pt 5d-band vacancies. The Pt 5d-band vacancies can lead to improvement in ORR kinetics since the vacancies enhance the chemical adsorption of O₂ on Pt surface as well as weaken the O–O bond in O₂, making O₂ easier to be reduced to OH⁻.^33,34 However, since Mo is less active in ORR than Pt, increasing Mo content may lead to the decrease of the percentage of high-activity O₂ adsorption center on surface of the catalyst, which is believed to be the main reason accounting for the dropping of catalytic activity with further increasing the Mo content.

Conclusions

In summary, we synthesized carbon supported Pt–Mo alloy catalysts via an environmentally-friendly and low-cost approach for the oxygen reduction reaction (ORR) in Mg–air batteries. The results showed that Pt and Mo were effectively reduced and nano sized Pt–Mo alloy particles were formed with a Pt-rich surface and a Mo-rich core. The Pt–Mo alloy particles distributed homogeneously on the carbon support and their average size increased slightly with the increasing Mo content. The Pt–Mo/C catalysts all performed higher ORR activity than that of Pt/C, while the catalytic activity dropped with increasing the Mo content. The enhancement of catalytic activity can be attributed to both Pt segregation toward the particle surface and the increase of the density of Pt 5d-band vacancies caused by the Mo addition. The decrease of catalytic performance with increasing Mo content is due to the decrease of Pt at the particle surface, which acts as the high-activity O₂ adsorption center on the surface.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 51274140) and Research Fund for the Doctoral Program of Higher Education of China (No. 20110073130001).

Notes and references

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