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Use of PtAu/C electrocatalysts toward formate oxidation: electrochemical and fuel cell considerations

Use of PtAu/C electrocatalysts toward formate oxidation: electrochemical and fuel cell... Mater Renew Sustain Energy (2016) 5:15 DOI 10.1007/s40243-016-0079-8 OR IGINAL PAPER Use of PtAu/C electrocatalysts toward formate oxidation: electrochemical and fuel cell considerations 1 1 2 • • • Sirlane G. da Silva Ju´lio Ce´sar M. Silva Guilherme S. Buzzo 1 3 Almir O. Neto Moˆnica H. M. T. Assumpc¸a˜o Received: 24 June 2016 / Accepted: 18 August 2016 / Published online: 1 September 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract This study reports the use of PtAu/C electro- Introduction catalysts with different atomic ratios (90:10, 70:30 and 50:50) supported on Vulcan XC 72 carbon and prepared by The increasing in the atmospheric CO is attributed to the the sodium borohydride method toward formate electro- burning of fossil fuels for our energy needs. The present oxidation in alkaline media. The materials were charac- need for power for an average US lifestyle is *11 kW/ terized by X-ray diffraction, showing peaks characteristics person while a good guess appears to be a world average no of Pt and Au face-centered-cubic structures, and also by less than 4 kW/person [1–3]. The excess of CO in atmo- transmission electron micrographs that show the nanopar- sphere causes environmental pollution, green house effect ticles well dispersed on carbon and a mean particle size and it is found to be a major cause for the global warming between 4 and 5 nm for all electrocatalysts. Electrochem- process. In order to address these issues, different strategies ical experiments show PtAu/C as promising catalysts are being considered and most of them involve a deep toward formate oxidation, while single cell experiments change in the current energy supply. Thus, the develop- reveal PtAu/C 90:10 as the best material since it provides a ment of high-efficiency and renewable energy conversion power density higher than Pt/C. The incorporation of Au systems are the main issue [4–7]. could increase formate oxidation for more than one reason: Therefore, carbon capture such as storage technologies (i) a facilitated rupture of C–H bond; (ii) the Au/oxide and CO valorization are in the interest of many interface or (iii) by regenerating active sites. researchers and one promising way of CO valorization is the conversion of CO in valuable products (such as formic Keywords Direct formate fuel cell  PtAu/C acid and formate) and its use as fuels in direct liquid fuel electrocatalysts  Borohydride process, formate oxidation cells (DLFC) [8, 9]. DLFCs convert chemical energy into electricity and have been recognized as one of the most attractive alternative when compared to the H -proton exchange membrane fuel cell (PEMFC), since DLFC shows obvious advantages in terms of fuel storage, simple configuration, easy operation, safety, and transportation & Monica H. M. T. Assumpc ¸a ˜o [2, 3, 10, 11]. monicahelena@ufscar.br Actually, numerous organic, inorganic and bioorganic Instituto de Pesquisas Energeticas e Nucleares, fuels have been studied for using in DLFC such as IPEN/CNEN-SP, Av. Prof. Lineu Prestes, 2242 Cidade methanol, ethanol, glycerol, formic acid, formate, sodium ´ ˜ Universitaria, Sao Paulo, SP CEP 05508-900, Brazil borohydride, hydrazine, ascorbic acid and glucose. Nev- Faculdade de Filosofia e Ciencias Humanas de Goiatuba, ertheless, among these fuels, formic acid have being con- Campus Goiatuba, Rodovia GO-320, Jardim Santa Paula, sidered the best one considering its theoretical GO-320, s/n, Goiatuba, GO CEP 75600-000, Brazil electromotive force, less toxicity and low crossover Universidade Federal de Sao Carlos, UFSCar, Campus Lagoa [10, 12–14], Pt being the most active catalysts for its do Sino, Rodovia Lauri Simoes de Barros, Km 12, Buri, ˜ decomposition [15]. However, DLFC using formic acid has Sao Paulo, SP CEP 18290-000, Brazil 123 15 Page 2 of 8 Mater Renew Sustain Energy (2016) 5:15 several engineering challenges such as: (i) the oxidation alternative sources of energy which could contribute also to reactions are kinetically sluggish in acid media; (ii) the reduction of greenhouse emissions. Considering the exis- catalysts are susceptible to be poisoned and (iii) the envi- tent technologies, direct liquid fuel cell is a promising one ronment of the fuel cell is corrosive [12, 16]. and among the fuels, formate has been proved to be the In order to improve the use of formic acid, the use of best choice. formate in alkaline solutions has been intensely studied for According to the literature, formate oxidation catalysis fuel cell applications [3, 11, 17–20]. The change from the are limited and the main catalysts are still Pt and Pd-based acid media to the alkaline can dramatically improve ones [9, 25]. Thus, the catalytic properties of Pt-containing kinetics of the oxygen reduction reaction (ORR) and also nanostructures are strongly dependent on their morphology formate oxidation. Additionally, lower overpotential is and compositions and in accordance with Zhang et al [34] required to oxidize the fuel in alkaline environment and the combining Pt with other metal element(s) to form bi or anode catalyst does not poison during formate oxidation as multimetallic nanostructures could be efficient to tune the it does during oxidation of formic acid. Furthermore, for- structure and catalytic activity of Pt. Moreover, the creation mate salts are stable, have low toxicity, are easily handled of a hetero-(metal–metal) bond in well-defined Pt surfaces and relatively inexpensive and is a carbon-neutral fuel may induce significant changes in the electronic structure which can be produced from reduction of carbon dioxide and catalytic property of Pt. Thus, this paper describes the [18, 19, 21–23]. Moreover, Del Castillo et al. [4] affirmed use of PtAu/C electrocatalysts with different atomic ratios that electrochemical conversion of CO into formate is the (90:10, 70:30, and 50:50) aiming that Au could increase the most promising reaction to be used at commercial scale. formate oxidation since Au could favor the formate Following the literature, there has been only a handful of adsorption [35] and also because theoretically Au studies of HCOO oxidation on Pt. To the best of our nanoparticles would show enhanced electron density near knowledge, the earliest report of HCOO oxidation on Pt Fermi level, narrowed d-band and higher lying d-band was proposed in 1962 by Buck and Griffith [24]. More center relative to bulk Au [34]. recently, Jiang et al. [19] proposed a triple-path mechanism of formate oxidation on Pt electrode in alkaline media while John et al. [25] propose a dual mechanism which Experimental shows that mechanistic details of HCOO oxidation are still very much lacking in the literature. PtAu/C electrocatalysts with different atomic ratios (Pt:Au: Studies show that in alkaline media formate is relatively 90:10, 70:30, and 50:50) and with 20 wt% of metal loading stable on Pt and itself cannot be oxidized at mild conditions were prepared by the sodium borohydride reduction and this stability was attributed to the poor absorbability of method [36–39] using H PtCl 6H O (Aldrich) and 2 6 2 formate on Pt [19, 26, 27]. Nevertheless, Gu ¨ nther and HAuCl 3H O (Aldrich), as metallic precursors and Vulcan 4 2 Wetzel [28, 29] investigated HCOO electro-oxidation on XC72 (Cabot) as support. Firstly the support was dispersed Pt in pH range of 10–13.5 and low current densities were in an isopropyl alcohol/water solution (50/50, v/v) and put obtained for this process, but they observed a enhancement on stirring. After the metal sources were added and the of the electro-oxidation rate when the pH goes below 12.5 resultant solution was put on an ultrasonic bath for 10 min -1 [25]. and afterward, a solution of NaBH in 0.01 mol L NaOH In the past few years, formate oxidation has been taken was added (in one portion) and maintained on stirring for upon the metallic Pt or Pd moieties [23]. For example, 30 min. The final mixture was filtered, the solids washed Noborikawa et al. [22] used PdCu/C and Pd/C electrocat- with distillated water and then dried at 70 C for 2 h. alysts toward formate oxidation and found that the The X-ray diffraction (XRD) patterns were recorded in bimetallic material was better than just Pd/C. Moreover, the range of 2h = 20–90 with a step size of 0.05 and a Hsu et al. [30] used Au/Pd core–shell nanoparticles toward scan time of 2 s per step using a Rigaku diffractometer formate-based solutions, showing superior catalytic model Miniflex II with a Cu Ka radiation source activities. (0.15406 nm) while transmission electron microscopy Considering fuel cell experiments, within the past year (TEM) images were obtained using a JEOL transmission there have been at least two demonstrations of direct for- electron microscope model JEM-2100 operated at 200 kV. mate fuel cell, using dissolved potassium formate as fuel. The atomic ratios of Pt and Au were taken by energy Thus, formate has been proved to be readily oxidized in dispersive spectroscopy (EDS) using a JEOL—JSM6010 alkaline media as successfully demonstrated by Jiang and LA equipment. Wieckowski [31, 32] and Bartrom and Hann [33]. Electrochemical measurements were conducted at room As already mentioned above, the scientific-technological temperature using a potentiostat/galvanostat PGSTAT communities of modern society are looking intensively for 302N Autolab and a three-electrode electrochemical cell. A 123 Mater Renew Sustain Energy (2016) 5:15 Page 3 of 8 15 -1 platinum electrode and an Ag/AgCl (3.0 mol L KCl) [52] attributed the downshift of XRD patterns to the lattice electrode were used as the counter and reference elec- parameter expansion on the PtAu electrocatalysts. trodes, respectively. The work electrodes (geometric area Figure 2 shows TEM micrographs and histograms with of 0.5 cm with a depth of 0.3 mm) were prepared using mean diameter distribution for the different PtAu/C elec- the thin porous coating technique [40–42]. Characteriza- trocatalysts in study and also for Pt/C. The mean average tions were made using cyclic voltammetry conducted at a size was determined by counting 100 particles at different -1 -1 rate of 10 mV s in 2 mol L NaOH aqueous solution in regions of the different electrocatalysts [49, 53] and was -1 presence and absence of 1 mol L sodium formate and achieved in the range of about 4–5 nm for all PtAu/C and the amperometric i–t curves were recorded in the same Pt/C catalysts. In all images the nanoparticles are well electrolyte containing sodium formate at -0.55 V for dispersed on the substrate although some agglomerates can 1800 s. also be observed. The mean diameter of the nanoparticles Direct formate fuel cell experiments, or in other words, is also shown in Table 1. The narrow size distribution experiments considering real conditions were taken using obtained could be attributed to the sodium borohydride asinglecellwith5 cm of area. The membrane electrode method of preparation [54]. assemblies (MEA) were prepared by hot pressing the Figure 3 shows the cyclic voltammetry of PtAu/C, Au/C -1 anode and the cathode to a pre-treated Nafion 117 and Pt/C electrocatalysts measured in 2.0 mol L NaOH membrane at 125 C for 3 min under a pressure of solution normalized by gram of metal. With reducing Pt -2 247 kgf cm . Prior to use, the membranes were exposed content there was also a reduction in the hydrogen -1 to 6 mol L KOH for 24 h as already proposed in our adsorption/desorption, characteristic region of Pt [55]. previous studies [42, 43]. The catalytic ink, used as anode and cathode was formulated in a way that Nafion com- prised about 35 % wt% of the total solid in the ink and 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 this was applied to a carbon cloth. All the cathodes and anodes were prepared using 2 mg of metal per cm .Fuel Pt/C cell performances were determined with polarization and power density curves using a potentiostat/galvanostat PGSTAT 302N Autolab. In all experiments the fuel cell was maintained at 60 C and the oxygen humidifier -1 maintained at 85 C with a flow of 150 mL min .The PtAu/C 90:10 -1 -1 fuel, 1.0 mol L formate and 2.0 mol L NaOH was -1 delivered at 1 mL min , based on the results of Bartrom et al. [33]. Moreover, a commercial Pt/C (BASF) was used as cathode in all tests. PtAu/C 70:30 Results and discussion Figure 1 shows the XRD patterns of the PtAu/C electro- catalysts. All XRD patterns showed a broad peak at 2h PtAu/C 50:50 about 25 assigned to the (022) reflection of the hexagonal structure of Vulcan XC 72 carbon [44, 45]. The face-cen- tered cubic systems of Pt were observed by five main peaks at 2h = 39,46,67,81 and 86 which, respectively, corresponds to (111), (200), (220), (311) and (222) planes, Au/C accordingly to JCPDF # 04 802 and JCPDF # 88 2334, respectively, and as already observed before [40, 41, 46]. Au/C electrocatalysts showed diffraction peaks at about 38,44,65,78 and 82, attributed to the (111), (200), (220). (311) and (222) planes, characteristics of the fcc 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 structure of Au [21, 43, 47, 48]. The PtAu/C electrocata- 2θ / Degree lysts showed diffractograms peaks shifted to lower 2h values when compared to Pt/C, suggesting the PtAu alloy Fig. 1 X-ray diffraction patterns for the Pt/C, PtAu/C and Au/C formation as already observed before [49–51]. Zhou et al. electrocatalysts Intensity / arbitrary units 15 Page 4 of 8 Mater Renew Sustain Energy (2016) 5:15 Fig. 2 TEM micrographs and PtAu/C 90:10 histograms of PtAu/C and Pt/C electrocatalysts 34 5678 9 10 Particle Diameter / nm PtAu/C 70:30 Particle Diameter / nm 20 PtAu/C 50:50 Particle Diameter / nm Pt/C 34 56 7 Particle Diameter / nm Frequency / % Frequency / % Frequency / % Frequency / % Mater Renew Sustain Energy (2016) 5:15 Page 5 of 8 15 5,5 Table 1 PtAu/C mean diameter sizes obtained by TEM images PtAu/C 90:10 5,0 PtAu/C 70:30 Pt:Au TEM d (nm) PtAu/C 50:50 4,5 Pt/C 100:0 4.2 4,0 Au/C 90:10 4.8 3,5 70:30 4.3 3,0 50:50 4.6 2,5 2,0 PtAu/C 90:10 1,5 PtAu/C 70:30 PtAu/C 50:50 1,0 Pt/C Au/C 0,5 0,0 0 5 10 15 20 25 30 time / min Fig. 5 Chronoamperometric measurements at -0.55 V vs Ag/AgCl -1 -1 for Pt/C and PtAu/C electrocatalysts in 2.0 mol L -1 NaOH ? 1.0 mol L sodium formate at room temperature -2 -3 indirect pathway of formate oxidation. Besides, during negative scan, the higher currents are due to the quicker -4 electro-oxidation of formate on the CO -free surface [18]. ads -0,8 -0,6 -0,4 -0,2 0,0 From Fig. 4, the oxidation current of formate starts at about E vs Ag/AgCl (V) -0.7 V for all electrocatalysts except Au/C. However, considering just this figure Pt/C seems to be the best Fig. 3 Cyclic voltammograms of Pt/C and PtAu/C electrocatalysts in -1 -1 material toward formate oxidation in alkaline solution 2.0 mol L NaOH at m = 10 mV s at room temperature because of the highest peak current at about -0.4 V. However, seeing chronoamperometric curves (Fig. 5), it is PtAu/C 90:10 possible to observe that PtAu/C 50:50 presents better sta- PtAu/C 70:30 bility than Pt/C and also a current density of about 50 % PtAu/C 50:50 Pt/C higher than Pt/C. Such as PtAu/C 50:50, PtAu/C 70:30 and Au/C PtAu/C 90:10 also show good stabilities and current den- sities higher than Pt/C. Thus, considering electrochemical experiments PtAu/C electrocatalysts seems to be promising 4 materials for direct formate fuel cells. In order to verify the results obtained in electrochemical experiments the real single cell measurements were done. Figure 6 shows the polarization and power density curves -1 -1 using 1.0 mol L sodium formate and 2 mol L NaOH -2 as fuel. For these measurements Pt/C, Au/C and PtAu/C in different atomic ratios were used as anodes and Pt/C Basf -0,8 -0,6 -0,4 -0,2 0,0 E vs Ag/AgCl (V) was used as cathode. The summarized results are shown in Table 2, showing the open circuit potential (OCV) and also Fig. 4 Cyclic voltammograms of Pt/C and PtAu/C electrocatalysts in the maximum power density (MPD). Among all the elec- -1 -1 2.0 mol L NaOH ? 1.0 mol L sodium formate at -1 trocatalysts in study the PtAu/C 90:10 showed the best m = 10 mV s at room temperature -2 result, 8.3 mW cm while Pt/C showed a power density -2 of 7.8 mW cm . The Au/C electrocatalyst showed almost Figure 4 shows the cyclic voltammetry of PtAu/C, Au/C -1 no activity toward formate oxidation. Considering the and Pt/C electrocatalysts in presence of 1.0 mol L experiments using single cell, it is possible to affirm that sodium formate. During positive scan the curves were split increasing the Au content into the materials there is also a into two peaks while negative scan shows only one sharp decrease of power density as already observed in previous peak. It is considered that the peaks during positive scan studies using PdAu/C electrocatalysts [43]. However, in correspond to an oxidative pathway (which do not involve lower proportions, such as 90:10 Au proves to be a good CO ) and the oxidation of CO accompanied by an ads ads -1 -1 j (mA g metal) j (mA g metal) j (mA g-1 metal) 15 Page 6 of 8 Mater Renew Sustain Energy (2016) 5:15 1.0 platinum for the oxidation of formate. Additionally, Pt/C according to Hsu et al. [30], the impact of sodium formate Au/C PtAu/C 90:10 on Pd-based materials in formic acid would be different to 0.8 PtAu/C 70:30 what has been discovered on Pt/C based on the studies of PtAu/C 50:50 Guo et al. [57] and Gao et al. [10]. 0.6 Furthermore, Hsu et al. [58] working with Au/Pd nanoparticles toward formic acid oxidation observed that 0.4 the comparison between the Au/Pd NPs and Pd black suggests higher formate coverage on the Au/Pd nanopar- ticles, especially at higher potentials. Considering that Pt 0.2 and Pd show similar properties [26], the information revealed by Hsu shows that Au could contribute to the 0.0 absorbability of formate on PtAu/C electrocatalysts and 0 5 10 15 20 25 30 consequently increase its oxidation. In addition, Hsu et al. -2 Current Density (mA cm ) [30] affirm that the catalytic efficiency of formate oxidation depends on the rate of removing reaction intermediates Pt/C from the catalyst surface (regenerating more active sites) Au/C PtAu/C 90:10 and using simulation studies they suggested that Pd oxi- 7 PtAu/C 70:30 dation has been retarded and formatted consuming rate PtAu/C 50:50 6 could have been boosted on Au/Pd nanoparticles. Additionally, Gazsi et al. [35], working with the decomposition of formic acid and methyl formate on TiO doped with N and promoted with Au observed, using infrared spectroscopic measurements, that formate does 2 exist on Au particles. Thus, they affirm that Au nanopar- ticles are very active catalysts of the decomposition of formic acid at elevated temperature and this is attributed to the facilitation of the rupture of a C–H bond in the formate 0 5 10 15 20 25 30 species adsorbed on the Au or the Au/oxide interface. -2 Current Density (mA cm ) Moreover, the creation of a hetero-(metal–metal) bond in well-defined Pt surfaces could induce significant changes Fig. 6 Polarization and power density curves of a 5 cm direct -2 in the electronic structure and catalytic property of Pt and formate fuel cell with 2 mg metal cm in both anode and cathode at -1 -1 60 C and using 1.0 mol L sodium formate and 2.0 mol L NaOH Au nanoparticles could show enhanced electron density near Fermi level, narrowed d-band and higher lying d-band Table 2 Main results obtained using PtAu/C catalysts using a direct center [34]. formate fuel cell: open circuit potential (OCV) and maximum power As a result, considering the discussion above and also density (MPD) the results obtained, Au, in small quantities, contributes to -2 Catalyst OCV (V) MPD (mW cm ) the formate absorbability and consequently to its oxidation. Thus, the use of Au could increase formate oxidation by Au/C 0.41 1.3 more than one reason: (i) a facilitated rupture of C–H bond; Pt/C 0.88 7.8 (ii) the Au/oxide interface or (iii) by regenerating active PtAu/C 50:50 0.79 5.1 sites. PtAu/C 70:30 0.79 6.5 PtAu/C 90:10 0.92 8.3 Conclusions option as second metal in Pt and Pd binary compositions [21, 43]. The use of the sodium borohydride process for PtAu/C As already mentioned before, studies showed that for- electrocatalysts yields materials with mean particle size mate is relatively stable on Pt and itself cannot be oxidized between 4 and 5 nm. PtAu/C showed to be good alternative at mild conditions and this stability was attributed to the for Pt/C toward formate oxidation since they increase the poor absorbability of formate on Pt [19, 26, 27]. Never- formate absorbability and also its oxidation. Considering theless, Beden et al. 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Hsu, C., Huang, C., Hao, Y., Liu, F.: Impact of surface roughness 46. Cantane, D.A., Ambrosio, W.F., Chatenet, M., Lima, F.H.B.: of Au core in Au/Pd core–shell nanoparticles toward formic acid Electro-oxidation of ethanol on Pt/C, Rh/C, and Pt/Rh/C-based oxidation—experiment and simulation. J. Power Sources 243, electrocatalysts investigated by on-line DEMS. J. Electroanal. 343–349 (2013) Chem. 681, 56–65 (2012) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Use of PtAu/C electrocatalysts toward formate oxidation: electrochemical and fuel cell considerations

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Springer Journals
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Materials Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
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10.1007/s40243-016-0079-8
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Mater Renew Sustain Energy (2016) 5:15 DOI 10.1007/s40243-016-0079-8 OR IGINAL PAPER Use of PtAu/C electrocatalysts toward formate oxidation: electrochemical and fuel cell considerations 1 1 2 • • • Sirlane G. da Silva Ju´lio Ce´sar M. Silva Guilherme S. Buzzo 1 3 Almir O. Neto Moˆnica H. M. T. Assumpc¸a˜o Received: 24 June 2016 / Accepted: 18 August 2016 / Published online: 1 September 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract This study reports the use of PtAu/C electro- Introduction catalysts with different atomic ratios (90:10, 70:30 and 50:50) supported on Vulcan XC 72 carbon and prepared by The increasing in the atmospheric CO is attributed to the the sodium borohydride method toward formate electro- burning of fossil fuels for our energy needs. The present oxidation in alkaline media. The materials were charac- need for power for an average US lifestyle is *11 kW/ terized by X-ray diffraction, showing peaks characteristics person while a good guess appears to be a world average no of Pt and Au face-centered-cubic structures, and also by less than 4 kW/person [1–3]. The excess of CO in atmo- transmission electron micrographs that show the nanopar- sphere causes environmental pollution, green house effect ticles well dispersed on carbon and a mean particle size and it is found to be a major cause for the global warming between 4 and 5 nm for all electrocatalysts. Electrochem- process. In order to address these issues, different strategies ical experiments show PtAu/C as promising catalysts are being considered and most of them involve a deep toward formate oxidation, while single cell experiments change in the current energy supply. Thus, the develop- reveal PtAu/C 90:10 as the best material since it provides a ment of high-efficiency and renewable energy conversion power density higher than Pt/C. The incorporation of Au systems are the main issue [4–7]. could increase formate oxidation for more than one reason: Therefore, carbon capture such as storage technologies (i) a facilitated rupture of C–H bond; (ii) the Au/oxide and CO valorization are in the interest of many interface or (iii) by regenerating active sites. researchers and one promising way of CO valorization is the conversion of CO in valuable products (such as formic Keywords Direct formate fuel cell  PtAu/C acid and formate) and its use as fuels in direct liquid fuel electrocatalysts  Borohydride process, formate oxidation cells (DLFC) [8, 9]. DLFCs convert chemical energy into electricity and have been recognized as one of the most attractive alternative when compared to the H -proton exchange membrane fuel cell (PEMFC), since DLFC shows obvious advantages in terms of fuel storage, simple configuration, easy operation, safety, and transportation & Monica H. M. T. Assumpc ¸a ˜o [2, 3, 10, 11]. monicahelena@ufscar.br Actually, numerous organic, inorganic and bioorganic Instituto de Pesquisas Energeticas e Nucleares, fuels have been studied for using in DLFC such as IPEN/CNEN-SP, Av. Prof. Lineu Prestes, 2242 Cidade methanol, ethanol, glycerol, formic acid, formate, sodium ´ ˜ Universitaria, Sao Paulo, SP CEP 05508-900, Brazil borohydride, hydrazine, ascorbic acid and glucose. Nev- Faculdade de Filosofia e Ciencias Humanas de Goiatuba, ertheless, among these fuels, formic acid have being con- Campus Goiatuba, Rodovia GO-320, Jardim Santa Paula, sidered the best one considering its theoretical GO-320, s/n, Goiatuba, GO CEP 75600-000, Brazil electromotive force, less toxicity and low crossover Universidade Federal de Sao Carlos, UFSCar, Campus Lagoa [10, 12–14], Pt being the most active catalysts for its do Sino, Rodovia Lauri Simoes de Barros, Km 12, Buri, ˜ decomposition [15]. However, DLFC using formic acid has Sao Paulo, SP CEP 18290-000, Brazil 123 15 Page 2 of 8 Mater Renew Sustain Energy (2016) 5:15 several engineering challenges such as: (i) the oxidation alternative sources of energy which could contribute also to reactions are kinetically sluggish in acid media; (ii) the reduction of greenhouse emissions. Considering the exis- catalysts are susceptible to be poisoned and (iii) the envi- tent technologies, direct liquid fuel cell is a promising one ronment of the fuel cell is corrosive [12, 16]. and among the fuels, formate has been proved to be the In order to improve the use of formic acid, the use of best choice. formate in alkaline solutions has been intensely studied for According to the literature, formate oxidation catalysis fuel cell applications [3, 11, 17–20]. The change from the are limited and the main catalysts are still Pt and Pd-based acid media to the alkaline can dramatically improve ones [9, 25]. Thus, the catalytic properties of Pt-containing kinetics of the oxygen reduction reaction (ORR) and also nanostructures are strongly dependent on their morphology formate oxidation. Additionally, lower overpotential is and compositions and in accordance with Zhang et al [34] required to oxidize the fuel in alkaline environment and the combining Pt with other metal element(s) to form bi or anode catalyst does not poison during formate oxidation as multimetallic nanostructures could be efficient to tune the it does during oxidation of formic acid. Furthermore, for- structure and catalytic activity of Pt. Moreover, the creation mate salts are stable, have low toxicity, are easily handled of a hetero-(metal–metal) bond in well-defined Pt surfaces and relatively inexpensive and is a carbon-neutral fuel may induce significant changes in the electronic structure which can be produced from reduction of carbon dioxide and catalytic property of Pt. Thus, this paper describes the [18, 19, 21–23]. Moreover, Del Castillo et al. [4] affirmed use of PtAu/C electrocatalysts with different atomic ratios that electrochemical conversion of CO into formate is the (90:10, 70:30, and 50:50) aiming that Au could increase the most promising reaction to be used at commercial scale. formate oxidation since Au could favor the formate Following the literature, there has been only a handful of adsorption [35] and also because theoretically Au studies of HCOO oxidation on Pt. To the best of our nanoparticles would show enhanced electron density near knowledge, the earliest report of HCOO oxidation on Pt Fermi level, narrowed d-band and higher lying d-band was proposed in 1962 by Buck and Griffith [24]. More center relative to bulk Au [34]. recently, Jiang et al. [19] proposed a triple-path mechanism of formate oxidation on Pt electrode in alkaline media while John et al. [25] propose a dual mechanism which Experimental shows that mechanistic details of HCOO oxidation are still very much lacking in the literature. PtAu/C electrocatalysts with different atomic ratios (Pt:Au: Studies show that in alkaline media formate is relatively 90:10, 70:30, and 50:50) and with 20 wt% of metal loading stable on Pt and itself cannot be oxidized at mild conditions were prepared by the sodium borohydride reduction and this stability was attributed to the poor absorbability of method [36–39] using H PtCl 6H O (Aldrich) and 2 6 2 formate on Pt [19, 26, 27]. Nevertheless, Gu ¨ nther and HAuCl 3H O (Aldrich), as metallic precursors and Vulcan 4 2 Wetzel [28, 29] investigated HCOO electro-oxidation on XC72 (Cabot) as support. Firstly the support was dispersed Pt in pH range of 10–13.5 and low current densities were in an isopropyl alcohol/water solution (50/50, v/v) and put obtained for this process, but they observed a enhancement on stirring. After the metal sources were added and the of the electro-oxidation rate when the pH goes below 12.5 resultant solution was put on an ultrasonic bath for 10 min -1 [25]. and afterward, a solution of NaBH in 0.01 mol L NaOH In the past few years, formate oxidation has been taken was added (in one portion) and maintained on stirring for upon the metallic Pt or Pd moieties [23]. For example, 30 min. The final mixture was filtered, the solids washed Noborikawa et al. [22] used PdCu/C and Pd/C electrocat- with distillated water and then dried at 70 C for 2 h. alysts toward formate oxidation and found that the The X-ray diffraction (XRD) patterns were recorded in bimetallic material was better than just Pd/C. Moreover, the range of 2h = 20–90 with a step size of 0.05 and a Hsu et al. [30] used Au/Pd core–shell nanoparticles toward scan time of 2 s per step using a Rigaku diffractometer formate-based solutions, showing superior catalytic model Miniflex II with a Cu Ka radiation source activities. (0.15406 nm) while transmission electron microscopy Considering fuel cell experiments, within the past year (TEM) images were obtained using a JEOL transmission there have been at least two demonstrations of direct for- electron microscope model JEM-2100 operated at 200 kV. mate fuel cell, using dissolved potassium formate as fuel. The atomic ratios of Pt and Au were taken by energy Thus, formate has been proved to be readily oxidized in dispersive spectroscopy (EDS) using a JEOL—JSM6010 alkaline media as successfully demonstrated by Jiang and LA equipment. Wieckowski [31, 32] and Bartrom and Hann [33]. Electrochemical measurements were conducted at room As already mentioned above, the scientific-technological temperature using a potentiostat/galvanostat PGSTAT communities of modern society are looking intensively for 302N Autolab and a three-electrode electrochemical cell. A 123 Mater Renew Sustain Energy (2016) 5:15 Page 3 of 8 15 -1 platinum electrode and an Ag/AgCl (3.0 mol L KCl) [52] attributed the downshift of XRD patterns to the lattice electrode were used as the counter and reference elec- parameter expansion on the PtAu electrocatalysts. trodes, respectively. The work electrodes (geometric area Figure 2 shows TEM micrographs and histograms with of 0.5 cm with a depth of 0.3 mm) were prepared using mean diameter distribution for the different PtAu/C elec- the thin porous coating technique [40–42]. Characteriza- trocatalysts in study and also for Pt/C. The mean average tions were made using cyclic voltammetry conducted at a size was determined by counting 100 particles at different -1 -1 rate of 10 mV s in 2 mol L NaOH aqueous solution in regions of the different electrocatalysts [49, 53] and was -1 presence and absence of 1 mol L sodium formate and achieved in the range of about 4–5 nm for all PtAu/C and the amperometric i–t curves were recorded in the same Pt/C catalysts. In all images the nanoparticles are well electrolyte containing sodium formate at -0.55 V for dispersed on the substrate although some agglomerates can 1800 s. also be observed. The mean diameter of the nanoparticles Direct formate fuel cell experiments, or in other words, is also shown in Table 1. The narrow size distribution experiments considering real conditions were taken using obtained could be attributed to the sodium borohydride asinglecellwith5 cm of area. The membrane electrode method of preparation [54]. assemblies (MEA) were prepared by hot pressing the Figure 3 shows the cyclic voltammetry of PtAu/C, Au/C -1 anode and the cathode to a pre-treated Nafion 117 and Pt/C electrocatalysts measured in 2.0 mol L NaOH membrane at 125 C for 3 min under a pressure of solution normalized by gram of metal. With reducing Pt -2 247 kgf cm . Prior to use, the membranes were exposed content there was also a reduction in the hydrogen -1 to 6 mol L KOH for 24 h as already proposed in our adsorption/desorption, characteristic region of Pt [55]. previous studies [42, 43]. The catalytic ink, used as anode and cathode was formulated in a way that Nafion com- prised about 35 % wt% of the total solid in the ink and 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 this was applied to a carbon cloth. All the cathodes and anodes were prepared using 2 mg of metal per cm .Fuel Pt/C cell performances were determined with polarization and power density curves using a potentiostat/galvanostat PGSTAT 302N Autolab. In all experiments the fuel cell was maintained at 60 C and the oxygen humidifier -1 maintained at 85 C with a flow of 150 mL min .The PtAu/C 90:10 -1 -1 fuel, 1.0 mol L formate and 2.0 mol L NaOH was -1 delivered at 1 mL min , based on the results of Bartrom et al. [33]. Moreover, a commercial Pt/C (BASF) was used as cathode in all tests. PtAu/C 70:30 Results and discussion Figure 1 shows the XRD patterns of the PtAu/C electro- catalysts. All XRD patterns showed a broad peak at 2h PtAu/C 50:50 about 25 assigned to the (022) reflection of the hexagonal structure of Vulcan XC 72 carbon [44, 45]. The face-cen- tered cubic systems of Pt were observed by five main peaks at 2h = 39,46,67,81 and 86 which, respectively, corresponds to (111), (200), (220), (311) and (222) planes, Au/C accordingly to JCPDF # 04 802 and JCPDF # 88 2334, respectively, and as already observed before [40, 41, 46]. Au/C electrocatalysts showed diffraction peaks at about 38,44,65,78 and 82, attributed to the (111), (200), (220). (311) and (222) planes, characteristics of the fcc 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 structure of Au [21, 43, 47, 48]. The PtAu/C electrocata- 2θ / Degree lysts showed diffractograms peaks shifted to lower 2h values when compared to Pt/C, suggesting the PtAu alloy Fig. 1 X-ray diffraction patterns for the Pt/C, PtAu/C and Au/C formation as already observed before [49–51]. Zhou et al. electrocatalysts Intensity / arbitrary units 15 Page 4 of 8 Mater Renew Sustain Energy (2016) 5:15 Fig. 2 TEM micrographs and PtAu/C 90:10 histograms of PtAu/C and Pt/C electrocatalysts 34 5678 9 10 Particle Diameter / nm PtAu/C 70:30 Particle Diameter / nm 20 PtAu/C 50:50 Particle Diameter / nm Pt/C 34 56 7 Particle Diameter / nm Frequency / % Frequency / % Frequency / % Frequency / % Mater Renew Sustain Energy (2016) 5:15 Page 5 of 8 15 5,5 Table 1 PtAu/C mean diameter sizes obtained by TEM images PtAu/C 90:10 5,0 PtAu/C 70:30 Pt:Au TEM d (nm) PtAu/C 50:50 4,5 Pt/C 100:0 4.2 4,0 Au/C 90:10 4.8 3,5 70:30 4.3 3,0 50:50 4.6 2,5 2,0 PtAu/C 90:10 1,5 PtAu/C 70:30 PtAu/C 50:50 1,0 Pt/C Au/C 0,5 0,0 0 5 10 15 20 25 30 time / min Fig. 5 Chronoamperometric measurements at -0.55 V vs Ag/AgCl -1 -1 for Pt/C and PtAu/C electrocatalysts in 2.0 mol L -1 NaOH ? 1.0 mol L sodium formate at room temperature -2 -3 indirect pathway of formate oxidation. Besides, during negative scan, the higher currents are due to the quicker -4 electro-oxidation of formate on the CO -free surface [18]. ads -0,8 -0,6 -0,4 -0,2 0,0 From Fig. 4, the oxidation current of formate starts at about E vs Ag/AgCl (V) -0.7 V for all electrocatalysts except Au/C. However, considering just this figure Pt/C seems to be the best Fig. 3 Cyclic voltammograms of Pt/C and PtAu/C electrocatalysts in -1 -1 material toward formate oxidation in alkaline solution 2.0 mol L NaOH at m = 10 mV s at room temperature because of the highest peak current at about -0.4 V. However, seeing chronoamperometric curves (Fig. 5), it is PtAu/C 90:10 possible to observe that PtAu/C 50:50 presents better sta- PtAu/C 70:30 bility than Pt/C and also a current density of about 50 % PtAu/C 50:50 Pt/C higher than Pt/C. Such as PtAu/C 50:50, PtAu/C 70:30 and Au/C PtAu/C 90:10 also show good stabilities and current den- sities higher than Pt/C. Thus, considering electrochemical experiments PtAu/C electrocatalysts seems to be promising 4 materials for direct formate fuel cells. In order to verify the results obtained in electrochemical experiments the real single cell measurements were done. Figure 6 shows the polarization and power density curves -1 -1 using 1.0 mol L sodium formate and 2 mol L NaOH -2 as fuel. For these measurements Pt/C, Au/C and PtAu/C in different atomic ratios were used as anodes and Pt/C Basf -0,8 -0,6 -0,4 -0,2 0,0 E vs Ag/AgCl (V) was used as cathode. The summarized results are shown in Table 2, showing the open circuit potential (OCV) and also Fig. 4 Cyclic voltammograms of Pt/C and PtAu/C electrocatalysts in the maximum power density (MPD). Among all the elec- -1 -1 2.0 mol L NaOH ? 1.0 mol L sodium formate at -1 trocatalysts in study the PtAu/C 90:10 showed the best m = 10 mV s at room temperature -2 result, 8.3 mW cm while Pt/C showed a power density -2 of 7.8 mW cm . The Au/C electrocatalyst showed almost Figure 4 shows the cyclic voltammetry of PtAu/C, Au/C -1 no activity toward formate oxidation. Considering the and Pt/C electrocatalysts in presence of 1.0 mol L experiments using single cell, it is possible to affirm that sodium formate. During positive scan the curves were split increasing the Au content into the materials there is also a into two peaks while negative scan shows only one sharp decrease of power density as already observed in previous peak. It is considered that the peaks during positive scan studies using PdAu/C electrocatalysts [43]. However, in correspond to an oxidative pathway (which do not involve lower proportions, such as 90:10 Au proves to be a good CO ) and the oxidation of CO accompanied by an ads ads -1 -1 j (mA g metal) j (mA g metal) j (mA g-1 metal) 15 Page 6 of 8 Mater Renew Sustain Energy (2016) 5:15 1.0 platinum for the oxidation of formate. Additionally, Pt/C according to Hsu et al. [30], the impact of sodium formate Au/C PtAu/C 90:10 on Pd-based materials in formic acid would be different to 0.8 PtAu/C 70:30 what has been discovered on Pt/C based on the studies of PtAu/C 50:50 Guo et al. [57] and Gao et al. [10]. 0.6 Furthermore, Hsu et al. [58] working with Au/Pd nanoparticles toward formic acid oxidation observed that 0.4 the comparison between the Au/Pd NPs and Pd black suggests higher formate coverage on the Au/Pd nanopar- ticles, especially at higher potentials. Considering that Pt 0.2 and Pd show similar properties [26], the information revealed by Hsu shows that Au could contribute to the 0.0 absorbability of formate on PtAu/C electrocatalysts and 0 5 10 15 20 25 30 consequently increase its oxidation. In addition, Hsu et al. -2 Current Density (mA cm ) [30] affirm that the catalytic efficiency of formate oxidation depends on the rate of removing reaction intermediates Pt/C from the catalyst surface (regenerating more active sites) Au/C PtAu/C 90:10 and using simulation studies they suggested that Pd oxi- 7 PtAu/C 70:30 dation has been retarded and formatted consuming rate PtAu/C 50:50 6 could have been boosted on Au/Pd nanoparticles. Additionally, Gazsi et al. [35], working with the decomposition of formic acid and methyl formate on TiO doped with N and promoted with Au observed, using infrared spectroscopic measurements, that formate does 2 exist on Au particles. Thus, they affirm that Au nanopar- ticles are very active catalysts of the decomposition of formic acid at elevated temperature and this is attributed to the facilitation of the rupture of a C–H bond in the formate 0 5 10 15 20 25 30 species adsorbed on the Au or the Au/oxide interface. -2 Current Density (mA cm ) Moreover, the creation of a hetero-(metal–metal) bond in well-defined Pt surfaces could induce significant changes Fig. 6 Polarization and power density curves of a 5 cm direct -2 in the electronic structure and catalytic property of Pt and formate fuel cell with 2 mg metal cm in both anode and cathode at -1 -1 60 C and using 1.0 mol L sodium formate and 2.0 mol L NaOH Au nanoparticles could show enhanced electron density near Fermi level, narrowed d-band and higher lying d-band Table 2 Main results obtained using PtAu/C catalysts using a direct center [34]. formate fuel cell: open circuit potential (OCV) and maximum power As a result, considering the discussion above and also density (MPD) the results obtained, Au, in small quantities, contributes to -2 Catalyst OCV (V) MPD (mW cm ) the formate absorbability and consequently to its oxidation. Thus, the use of Au could increase formate oxidation by Au/C 0.41 1.3 more than one reason: (i) a facilitated rupture of C–H bond; Pt/C 0.88 7.8 (ii) the Au/oxide interface or (iii) by regenerating active PtAu/C 50:50 0.79 5.1 sites. PtAu/C 70:30 0.79 6.5 PtAu/C 90:10 0.92 8.3 Conclusions option as second metal in Pt and Pd binary compositions [21, 43]. The use of the sodium borohydride process for PtAu/C As already mentioned before, studies showed that for- electrocatalysts yields materials with mean particle size mate is relatively stable on Pt and itself cannot be oxidized between 4 and 5 nm. PtAu/C showed to be good alternative at mild conditions and this stability was attributed to the for Pt/C toward formate oxidation since they increase the poor absorbability of formate on Pt [19, 26, 27]. Never- formate absorbability and also its oxidation. Considering theless, Beden et al. [56] showed the activity of gold and fuel cell experiments, PtAu/C 90:10 showed the highest Cell Potential (V) -2 Power Density (mW cm ) Mater Renew Sustain Energy (2016) 5:15 Page 7 of 8 15 14. Li, Y., Wu, H., He, Y., Liu, Y., Jin, L.: Performance of direct power density. By the results, the use of Au could increase formate-peroxide fuel cells. J. Power Sources 287, 75–80 (2015) formate oxidation by facilitating the rupture of C–H bond, 15. Ojeda, M., Iglesia, E.: Formic acid dehydrogenation on Au-based the Au/oxide interface or by regenerating active sites. catalysts at near-ambient temperatures. Angew. Chem. Int. Ed. 48, 4800–4803 (2009) Acknowledgments The authors wish to thank Laborato ´ rio de 16. Bartrom, A.M., Haan, J.L.: The direct formate fuel cell with an Microscopia do Centro de Cie ˆncias e Tecnologia de Materiais alkaline anion exchange membrane. J. 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