Introduction

Direct ethanol fuel cells (DEFCs) are an attractive option as power sources for mobile, stationary, and portable applications. Ethanol is produced in large quantities from biomass and is much less toxic than others combustibles. On the other hand, its complete oxidation to CO2 and water is not easy due to the difficulties in C–C bond breaking and the formation of intermediates that poison the platinum anode electrocatalysts (the term electrocatalyts as used herein means a catalyst for catalyzing oxidation and reduction electrode reactions) [1].

The global oxidation mechanism of ethanol in acid solution may be summarized in the following scheme of parallel reactions [2]:

$$ {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}}\, \to \,{\left[ {{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}}} \right]_{\text{ad}}}\, \to \,{{\text{C}}_{\text{1 ad}}},{{\text{C}}_{\text{2 ad}}}\, \to \,{\text{C}}{{\text{O}}_2}\,\left( {\text{total\ oxidation}} \right) $$
(1)
$$ {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}} \to {\left[ {{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}}} \right]_{\text{ad}}} \to {\text{C}}{{\text{H}}_3}{\text{CHO}} \to {\text{C}}{{\text{H}}_3}{\text{COOH }}\left( {\text{partial\ oxidation}} \right) $$
(2)

The formation of CO2 goes through two adsorbed intermediates C1 ad and C2 ad, which represent fragments with one and two carbon atoms, respectively. In direct ethanol fuel cell, the principal products formed are acetaldehyde and/or acetic acid [2].

The Pt/C electrocatalyst is commonly used as anode in low temperature fuel cells; however, platinum alone is not a good catalyst for the ethanol oxidation reaction (EOR), consequently efforts to mitigate the poisoning of Pt have been concentrated on the addition of cocatalysts to platinum [2].

In recent years, it is found that certain metal oxides, such as CeO2, RuO2, or SnO2, can enhance the catalytic activity for ethanol electro-oxidation through synergetic interaction with Pt in order to enhance the efficiency and reduce the catalyst price [3–5]. It was shown that an intermetallic PtSb had a significant activity for formic acid oxidation in relation to Pt/C [6]. Silva et al. [7] showed that the PtSn/C and PtSb/C supported intermetallics prepared by an alcohol reduction method were more active than the Pt/C electrocatalyst. Neto et al. [8] showed that Pt nanoparticles supported on C-Sb2O5.SnO2 (C-antimony tin oxide (ATO)) were more active for ethanol oxidation compared to Pt nanoparticles supported only on ATO-(Sb2O5.SnO2 ) or on carbon, while Lee et al. [9] showed that the activities of Pt/ATO for methanol and ethanol electro-oxidation were greater than those of Pt/C electrocatalyst. The enhancement of activity was attributed to better dispersion of Pt particles on the ATO support, as well as to the effects of SnO2 adjacent to Pt (bifunctional effect and/or the electronic effect).

Neto et al. [10] showed that PtSn/C is a good electrocatalysts for ethanol electro-oxidation reaction. Nevertheless, it is still necessary to improve further the performance with PtSn/C anodic catalysts because the principal products formed are acetaldehyde and/or acetic acid; consequently, the development of new electrocatalysts and new preparation methods are necessary. An alternative to the PtSn/C electrocatalysts for the ethanol oxidation reaction could be a ternary electrocatalyst. Lee et al. [11] showed that the incorporation of Mo to PtSn/C enhances further the catalytic activity for EOR. Ribeiro et al. [12] showed that PtRuNi/C prepared by an alcohol reduction process had superior performance for methanol oxidation at room temperature compared to commercial PtRu/C electrocatalyst. Spinacé et al. [13] showed that in a single cell at 100 °C, the PtSnRh/C electrocatalyst had superior performance compared to PtSn/C and PtRh/C electrocatalyst.

In this context, the aim of this work is to present a preliminary electrochemical study and direct ethanol fuel cell tests with PtSnSb/C prepared in a single step by an alcohol reduction process (coreduction of mixed ions) using ethylene glycol as solvent and reducing agent in the presence of carbon Vulcan XC72.

Experimental

The atomic composition of the electrocatalysts prepared by an alcohol reduction process was chosen to be close to PtSn/C (50:50), the most active composition for the ethanol electro-oxidation [10], so PtSn/C electrocatalysts (Pt/Sn atomic ratio 50:50) and PtSnSb/C electrocatalysts (Pt:Sn:Sb atomic ratios of 50:45:05, 50:40:10 and 50:10:40) were prepared with metal loading of 20 wt.% using H2PtCl6·H2O (chloroplatinic acid, Aldrich), Sb(OOCCH3) (antomony triacetate, Aldrich), and SnCl2·2H2O (stannous chloride, Aldrich) as metal sources, ethylene glycol (Merck) as the solvent and reducing agent and carbon Vulcan XC72 as the carbon support. In the alcohol reduction process procedure, the metal sources were dissolved in ethylene glycol/water (75/25, v/v) and carbon Vulcan XC72 support was added. The resulting mixtures were sonicated and were refluxed for 3 h under open atmosphere. The mixtures were filtered and the solids washed with water and dried at 70 °C for 2 h.

The atomic ratios were obtained by energy-dispersive X-ray analysis using a scanning electron microscope Phillips XL30 working at 20 kV and equipped with EDAX DX4 microanalyzer.

The synthesized materials were characterized by X-ray diffraction (XRD), with a Rigaku diffractometer model Miniflex II using Cu Kα radiation source (l = 0.15406 nm). The diffractograms were recorded in the range of 2θ = 20–90° with a step size of 0.05° and a scan time of 2 s per step.

The morphology, distribution, and size of the nanoparticles were observed with a JEOL electron microscope model JEM-2100 operated at 200 kV.

The electrochemical activity of the electrocatalysts for ethanol oxidation were tested by cyclic voltammetry and chronoamperometry experiments (performed at 25 °C with 1.0 mol L−1 of ethanol in 0.5 mol L−1 H2SO4 solutions saturated with N2) using a Microquimica (model MQPG01, Brazil) potenciostat/galvanostat and the thin porous coating technique [14]. The reversible hydrogen electrode was employed as reference system and the counter electrode was a Pt plate.

The DEFC performances were determined in a single cell with an area of 5 cm2. The temperature was set to 100 °C for the fuel cell and 80 °C for the oxygen humidifier. The fuel was 2 mol L−1 ethanol solution feed at approximately 2 mL min−1, the oxygen flow was regulated at 500 mL min−1 and pressure of 2 bar. Polarization curves were obtained using a TDI RBL 488 electronic load. The membrane electrode assemblies were prepared by hot pressing of a pretreated Nafion® 117 membrane placed between either PtSn/C, PtSb/C, or PtSnSb/C electrocatalysts prepared in this work as anode (1 mgPt cm−2 catalyst loading) and a 20 wt.% Pt/C E-TEK as cathodic catalyst (1 mgPt cm−2 catalyst loading) at 125 °C for 2 min under pressure of 225 Kgf cm−2.

Results and discussion

PtSn/C, PtSb/C, and PtSnSb/C were prepared in a single step by an alcohol reduction process (coreduction of mixed ions) using ethylene glycol as solvent and reducing agent in the presence of carbon Vulcan XC72. The Pt/Sn, Pt/Sb, and Pt/Sn/Sb atomic ratios of the obtained electrocatalysts were similar to the nominal atomic ratios and the mean crystallite sizes determined using Scherrer equation [10] were in the range of 2–4 nm (Table 1).

Table 1 Pt/Sn, Pt/Sb, and PtSn/Sb atomic ratios and mean particle size of the prepared electrocatalysts
Full size table

The X-ray diffractograms with all synthesized electrocatalysts are shown in Fig. 1. In all difractograms, a broad peak at about 25° can be clearly seen associated with the Vulcan XC72 support and four peaks at approximately 2θ = 40°, 47°, 67°, and 82°, which are associated with the (111), (200), (220), and (311) planes, respectively, of the face-centered cubic (fcc) structure characteristic of platinum and platinum alloys [10]. In the diffractograms of PtSn/C, PtSb/C, and PtSnSb/C, electrocatalysts two very broad peaks were also observed at approximately 2θ = 34° and 52° that could be identified as a SnO2 (cassiterite) [10] and/or Sb2O5.SnO2 phases. For all PtSnSb/C electrocatalysts, the diffraction peaks of Pt (fcc) phase are shifted to small angles compared to Pt/C electrocatalyst, which indicates some lattice expansion and that part of Sb and/or Sn atoms could be incorporated into the fcc lattice.

Fig. 1
figure 1

X-ray diffractograms of the Pt/C, PtSn/C (50:50), PtSb/C (50:50)/C, PtSnSb/C (50:45:05), PtSnSb/C (50:40:10), PtSnSb/C (50:10:40), and Sb2O5.SnO2

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Figure 2 shows transmission electron microscopy (TEM) images with all synthesized electrocatalysts. All electrocatalysts showed a good distribution of the nanoparticles on the carbon support. The TEM image of PtSnSb/C (50:45:05) electrocatalyst (Fig. 2c) showed nanoparticles on the carbon support with particle sizes of 3.5 ± 2.0 nm, while that PtSnSb/C (50:40:10), PtSnSb/C (50:10:40), PSn/C (50:50), and PtSb/C (50:50) showed particles sizes of 2.8 ± 2.0, 3.0 ± 2.0, 2.0 ± 1.0, and 2.5 ± 2.0 nm, respectively. The particles sizes of the PtSn/C were smaller than those of PtSnSb/C and PtSb/C; however, the morphologies were not significantly changed. TEM micrographs of PtSn/C, PtSnSb/C, and PtSb/C are in agreement with XRD result.

Fig. 2
figure 2

TEM images of the electrocatalysts (a) PtSn/C (50:50), (b) PtSb/C (50:50), (c) PtSnSb/C (50:45:05), (d) PtSnSb/C (50:40:10), and (e) PtSnSb/C(50:10:40)

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Figure 3a shows the cyclic voltammetry with all synthesized electrocatalysts obtained in a half cell at a scan rate of 10 mVs−1 in a potential range of 0.05–0.8 V in absence of ethanol, while that the Fig. 3b shows the cyclic voltammetry results in the presence of 1.0 mol L−1 of ethanol and 0.5 mol L−1 of H2SO4. The current values for cyclic voltammetry and chronoamperometry studies were normalized per gram of platinum considering that ethanol adsorption and dehydrogenation occur only on platinum sites at room temperature [10].

Fig. 3
figure 3

(a) Cyclic voltammetry of PtSn/C (50:50), PtSb/C (50:50) and PtSnSb/C (50:45:05), PtSnSb/C (50:40:10), and PtSnSb/C (50:10:40) electrocatalysts obtained in absence of ethanol. (b) Cyclic voltammetry results in presence of 1.0 mol L−1 of ethanol and 0.5 mol L−1 of H2SO4

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PtSb/C and all PtSnSb/C electrocatalysts showed that the hydrogen adsorption region is greatly reduced by the presence of Sb in comparison with PtSn/C (Fig. 3a). PtSb/C (50:50) and PtSnSb/C (50:10:40) electrocatalysts also showed an increase in the current values in the double layer (0.4–0.8 V) in relation to PtSn/C (50:50), which may be attributed to the formation of antimony or tin oxide species. The ethanol electro-oxidation started at approximately 0.25 V for PtSnSb/C (50:45:05) and PtSn/C (50:50) electrocatalysts (Fig. 3b), while that for PtSnSb/C (50:40:10), PtSnSb/C (50:10:40), and PtSb/C (50:50) the ethanol electro-oxidation started at approximately 0.40 V. In the potential range of interest for a direct ethanol fuel cell (0.2–0.5 V), the current values of the PtSnSb/C (50:45:05) and PtSn/C (50:50) electrocatalysts were higher than of the others eletrocatalysts prepared; however, PtSnSb/C (50:45:05) electrocatalyst showing the best performance for ethanol oxidation. The better performance of PtSnSb/C (50:45:05) for ethanol electro-oxidation is due to the bifunctional effects. The presence of the tin oxides and antimony oxides in the electrocatalysts promotes the formation of oxygenated species in a potential region less positive than the pure platinum and these species play a crucial role in the oxidation of adsorbed intermediates releasing the actives sites of Pt [8].

The chronoamperometry curves of all synthesized electrocatalysts (at 0.5 V) are shown in Fig. 4. PtSnSb/C (50:10:40) and PtSb/C (50:50) electrocatalysts showed a great initial current drop and a very low performance for ethanol oxidation at room temperature in relation to the others electrocatalysts. The current values obtained for PtSnSb/C (50:45:05) electrocatalysts were higher than those obtained for PtSn/C (50:50) and PtSnSb/C (50:40:10) in agreement with cyclic voltammetry experiments.

Fig. 4
figure 4

Current–time curves at 0.5 V in 1 mol L−1 ethanol solution in 0.5 mol L−1 H2SO4 for PtSn/C (50:50), PtSb/C (50:50), PtSnSb/C (50:45:05), PtSnSb/C (50:40:10), and PtSnSb/C (50:10:40)

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Figure 5 shows the performances of single fuel cell with all synthesized electrocatalysts. The open circuit voltage of the fuel cell containing PtSnSb/C (50:45:05) and PtSn/C (50:50) electrocatalyst were 0.81 and 0.76 V, respectively, while the corresponding value for PtSb/C (50:50) was approximately 0.50 V. PtSnSb/C (50:45:05) electrocatalysts showed higher the open circuit voltage with relation to others electrocatalysts prepared indicating an increased cell performance.

Fig. 5
figure 5

I–V curves and the power density at 100 °C of a 5 cm2 DEFC using PtSb/C (50:50), PtSn/C (50:50), and PtSnSb/C (50:45:05) electrocatalysts anodes (1 mgPt cm−2 catalyst loading) and Pt/C E-TEK electrocatalyst cathode (1 mgPt cm−2 catalyst loading, 20 wt.% Pt loading on carbon), Nafion® 117 membrane, ethanol (2.0 mol L−1), and oxygen pressure (2 bars)

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The maximum power density of PtSnSb/C electrocatalyst (40 mW cm−2) is slightly higher than that with PtSn/C electrocatalyst (37 mW cm−2). The maximum power density of PtSnSb/C and PtSn/C electrocatalyst also were greater than that of PtSb/C electrocatalyst (12 mW cm−2). Curiously, similar results were observed for PtSnSb/C (50:45:05) prepared in this work by an alcohol reduction process using Pt, Sn, and Sb metals salts as precursors (40 mW cm−2) and for Pt/C-Sb2O5.SnO2/(38 mW cm−2) [8] where Pt nanoparticles were deposited by the same methodology on a physical mixture of carbon and Sb2O5.SnO2 (∼10% Sb2O5 and ∼90% SnO2). Thus, these catalysts have similar compositions and probably structures (Pt fcc phase and Sn and Sb oxides).

The single-cell tests demonstrate that the addition of Sb into PtSn catalyst can promote its activity for EOR in agreement with cyclic voltammetry and chronoamperommetry experiments. The enhanced performance of PtSnSb/C (50:45:05) catalyst could be associated to the presence of Sn and Sb oxides that favors the oxidation of adsorbed intermediates by the bifunctional mechanism. Also, the incorporation of Sb into the SnO2 lattice (Sb2O5.SnO2) could increase of its electrical conductivity [8, 9].

Conclusions

The alcohol reduction process showed to be an effective method for producing in a single step of PtSnSb/C electrocatalyst for ethanol oxidation. The X-ray diffractograms of PtSn/C and PtSnSb/C electrocatalysts showed the presence of Pt (fcc) and SnO2 or Sb2O5.SnO2 phases. The results of TEM for all electrocatalysts showed a good distribution of the nanoparticles on the carbon support. The electrochemical experiments showed that PtSnSb/C(50:45:050) electrocatalyst was more active than PtSn/C (50:50) and PtSb/C (50:50) electrocatalysts for ethanol oxidation at room temperature and the tests in a single DEFC at 100 °C also confirm these results. The increase of performance of PtSnSb/C electrocatalyst could be attributed to the effect of tin and antimony oxides adjacent to Pt sites, which favors the oxidation of adsorbed intermediates by the bifunctional mechanism. The presence of Sb2O5 could also increase of electrical conductivity of to SnO2 phase.