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PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER
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PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER Demetrius W. Lima a, Fernanda Fiegenbaum a,*, Fernanda Trombetta b, Michele O. de Souza a, Emilse M.A. Martini a a
Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸alves, 9500, P.O. Box 15003, Porto Alegre 91501-970, Brazil b Universidade Federal do Rio Grande e FURG, Escola de Quı´mica e Alimentos, Campus Santo Ant^onio da Patrulha, ~o do Caı´, 125, Santo Ant^onio da Patrulha 95500-000, Brazil Rua Bara
article info
abstract
Article history:
Carbon-supported bimetallic nanoparticles, PtNi/C and PtMo/C, were synthesized using the
Received 16 September 2016
borohydride reduction method and were tested as cathode in the hydrogen evolution re-
Received in revised form
action (HER) employing acid aqueous solution of 3-triethylammonium-propanesulfonic
18 November 2016
acid tetrafluoroborate (TEA-PS.BF4) ionic liquid as the electrolyte. For comparison a com-
Accepted 25 November 2016
mercial carbon-supported Pt nanoparticles (Pt/C) has been tested. The morphology,
Available online xxx
composition and structure of the carbon-supported nanoparticles were characterized by EDX, XRD, and TEM. The results obtained by chronoamperometry, Tafel plots and elec-
Keywords:
trochemical impedance spectroscopy (EIS) exhibited an excellent catalytic effect and the
Hydrogen evolution reaction
same kinetics mechanism for all carbon-supported nanoparticles in HER studied. Results of
Bimetallic nanoparticles
Tafel analysis showed that the HER rate determining step is the VolmereHeyrovsky
TEA-PS.BF4 ionic liquid
mechanism. In the temperature range 25e80 C, the PtNi/C and PtMo/C cathodes presented
Water electrolysis in acid medium
lower activation energy and higher current density than system using the Pt/C cathode.
Kinetic mechanism
The PtMo/C cathode improves charge transfer kinetics and hydrogen adsorption, while the
Impedance spectroscopy
PtNi/C cathode facilitates the desorption step. These results demonstrate that the carbonsupported bimetallic nanoparticles PtNi/C and PtMo/C are responsible for increasing the catalytic performances of the HER when TEA-PS.BF4 aqueous solution is employed as electrolyte. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is one of the most preferable renewable energy carriers and ideal energy storage, since it has high energy density, and could be easily converted into various available energy forms. However, nowadays, 96% of hydrogen is
produced from hydrocarbon sources such as natural gas, oil and coal. Over time, this scenario must change, requiring the production of hydrogen from renewable sources, i.e., involving clean and simple processes in order to completely avoid carbon dioxide (CO2) production. Even if water electrolysis is a clean and renewable process which produces
* Corresponding author. E-mail address: [email protected] (F. Fiegenbaum). http://dx.doi.org/10.1016/j.ijhydene.2016.11.166 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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highly purified hydrogen it presents an important disadvantage related to the need of high energy consumption [1e4]. Decreasing the cathodic overpotential of hydrogen evolution is one of the goal to be attained to reduce the cost of electrolytic hydrogen production and then to turn realizable technological applications. The main factors that influence the efficiency of the water electrolysis are related to the electrolytes and electrodes properties. Recently, the 3-triethylammonium-propanesulfonic acid tetrafluoroborate (TEA-PS.BF4) ionic liquid, a € nsted and Lewis acidity, has been compound combining Bro shown to be a new efficient electrolyte having a specific conductivity around 0.25 mS cm1 for a 0.1 M aqueous solution at room temperature. Hydrogen evolution reaction (HER) conducted with TEA-PS.BF4 ionic liquid as electrolyte and using bulk electrodes of platinum, nickel, stainless steel 304 and glassy carbon as cathodes presented remarkable performance [5e7]. Recent research and development efforts have been focused on minimizing ohmic drop, lowering overpotential through the improvement of cell and electrode design and using new and cheaper cathode materials with higher electrocatalytic activity for HER [3,8e11]. Among the noble metals, the platinum (Pt) is regarded as an ideal catalyst for electrochemical hydrogen production, but its high cost and scarce resource definitely limit its extensive application in HER [12e14]. Some non-noble metals are used in the water electrolysis in acid medium, such as Ni, Mo, Cr, W, and Nb. For example, Nickel bulk, when combined with other metals, has been considered one of the most important electrocatalysts in the hydrogen production due to it is low cost, corrosion resistance and reduced overpotential [5,15e18]. However, the combining of BMI.BF4 ionic liquid with Mo electrodes in water electrolysis system showed higher current density and lower activation energy than the same system with Ni and Pt bulk electrodes [19]. An alternative to decrease costs of metal and to increase the electroactive area is to produce electrocatalysts containing metallic nanoparticles (NPs) on black carbon support. The most usual carbon support is the Vulcan XC-72R black carbon that presents high surface area, good electrical conductance, low impurity content and high chemical stability [20,21]. Nanoparticles are currently of great interests because of their noteworthy catalytic behaviours in comparison with that of the bulk material [22]. Analogously, bimetallic NPs have distinct properties than monometallic NPs [23,24]. In the case of water electrolysis, carbon-supported NPs increase the performance, providing active sites for the water dissociation, enhancing the electron transfer, assisting the Hþ adsorption and the HeH bond formation [8,25,26]. Herein, we report the hydrogen evolution reaction using carbonsupported bimetallic NPs (PtNi/C and PtMo/C) as cathode in presence of TEA-PS.BF4 acid aqueous solution. The behaviour of these materials was compared to carbon-supported Pt NPs (Pt/C) as cathode. The effectiveness of investigated cathode materials was evaluated through kinetics parameters obtained from chronoamperometry, Tafel analysis, and ac impedance spectroscopy. Based on the electrochemical
impedance spectroscopy (EIS) measurements, equivalent circuits are used to describe the processes in the cathode/ electrolyte interface.
Experimental Preparation of cathodes Three types of cathodes were studied: Pt/C (Pt EC-20-PTC, Carbon XC-72R), PtMo/C and PtNi/C with 20 wt.% metal loading on Vulcan XC-72R carbon support (Cabot, 240 m2 g1). The precursors were hexachloroplatinic acid (IV) hexahydrate (H2PtCl6$6H2O, Merck), nickel (II) chloride hexahydrate (NiCl2$6H2O, 98%, Synth) and ammonium molybdate tetrahydrate ((NH4)6Mo7O24$4H2O, 99.98%, Sigma-Aldrich). The Vulcan XC-72R carbon support was previously treated with HNO3 [21]. The syntheses were made using sodium borohydride (NaBH4, 95%, Vetec) similar to previously published [27]. The structure and morphology of the carbon-supported NPs were analyzed by transmission electron microscopy (TEM) JEOL JEM 1200ExII at 120 kV. The relative proportions of Pt:Ni and Pt:Mo were obtained by energy-dispersive X-ray spectroscopy (EDX) coupled with scanning electronic microscope (JEOL-JSM 5800) with a 20 kV incident electron beam. The X-ray diffractograms were achieved using a D500 Rigaku diffractometer (Siemens) operating at a scan rate of 0.05 s1 in the range of 2q ¼ 20e90 using CuKa radiation (1.54056 A). The accuracies of these measurements are 3e4%. The cathodes were made by brushing technique with 0.5 mgmetal cm2
Fig. 1 e X-ray diffraction of carbon-supported NPs: Pt/C, PtNi/C and PtMo/C.
Table 1 e The composition of the Pt/C, PtNi/C and PtMo/C determined by EDX. Carbon-supported NPs Pt/C PtMo/C PtNi/C
metal loading wt.%
Pt at.%
Mo at.%
Ni at.%
20 18 21
100 87 84
e 13 e
e e 16
Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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and 30 wt.% of Nafion® on diffusion layer - carbon cloth (30 wt.% treated Teflon, Fuel Cell Earth LLC e Woburn, Massachusetts, USA).
Electrochemical measurements For all experiments a 0.1 M TEA-PS.BF4 acid aqueous solution was used as electrolyte. The TEA-PS.BF4 ionic liquid was prepared according previously published reports [5]. Chronoamperometry, Tafel analysis and electrochemical impedance spectroscopy (EIS) were performed with an
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Autolab PGSTAT30 potentiostat, with GPES and FRA modules. These experiments were made in a cell with three electrodes, Pt net as counter electrode, Pt wire as the quasireference electrode (QRE-Pt) and carbon-supported NPs (Pt/C, PtNi/C and PtMo/C) as working electrode. Chronoamperometry was carried out at 2.0 V vs. QRE-Pt potential applied during 30 min and the temperature was controlled by a thermostatic bath in the range from 25 to 80 C, an overpotential of 2.0 V was applied due to the high current densities observed in previous studies [5]. The average current density was calculated by integration of the current density (j)
Fig. 2 e TEM micrographs and particle size distribution histograms (d ¼ average particle size) of the Pt/C (a), PtNi/C (b) and PtMo/C (c). Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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versus time (t), that represents the charge (Q). This value was divided by the total time (30 min). Tafel analysis were used to determinate the kinetic parameters that were obtained from Tafel plots, at a scan rate of 10 mV s1, according to (Eq. (1)): h ¼ a þ b log j
(1) 1
where b (V dec ) is the Tafel coefficient, which indicates the rate of change of the current density, j (mA cm2), with the applied overpotential, h (V). EIS measurements were carried out at 0.750 V (QRE-Pt), using a frequency range from 100 kHz to 0.1 Hz and an ac signal amplitude of 10 mV. The experimental data were fitted to the equivalent circuit by NOVA 1.11 software (MetrohmAutolab). It is possible to use the fitting and simulation tool to simplify a parallel combination of a resistor and a constant phase element (CPE) by converting the CPE to pseudocapacitance C, according to Eq. (2):
1
C¼
= Y0n $R
1
=n1
(2)
where C is the pseudocapacitance, Y0 is the admittance value of the constant phase element, R is the resistance value and n is the exponent of the constant phase element.
solution was initially investigated measuring the current density (j) vs. temperature (see Table 2). Results reported Table 2 show that for the evaluated temperature range 25e80 C, the PtNi/C and PtMo/C evidence a greater performance in the HER than the Pt/C, enhancing up to 22% the current density. Independently of the temperature, the PtNi/C cathode shows higher current densities than those registered for the other cathodes, revealing a good performance of this bimetallic NPs for the hydrogen evolution. Based on these preliminary results, which show remarkable behaviours of the carbon-supported bimetallic NPs, further studies were performed for a better understanding of the mechanism involved with theses cathodes in the HER in the presence of TEA-PS.BF4. Quasi-potentiostatic polarization measurements were carried out, providing Tafel plots (Fig. 3). The determination of Tafel parameters as Tafel coefficient (b), effective exchange current density (j0) and equilibrium potential (Eeq), listed in Table 3, enable to elucidate the kinetic mechanism for HER, to determine the reaction mechanism and the rate-determining step (rds). The Eeq values obtained for the cathodes PtNi/C, Pt/C and PtMo/C were 0.01 V, þ0.11 V and þ0.21 V, respectively. These results again suggest a better catalytic effect obtained with the PtNi/C cathode. For the HER conducted with an aqueous solution of 0.1 M TEA-PS.BF4 using carbon-supported NPs as cathode, the Tafel
Results and discussion Characterization of carbon-supported NPs Pt/C, PtMo/C and PtNi/C were analyzed by X-ray Diffraction. The corresponding diffractograms present the same peak characteristics of Pt, being a face-centred cubic structure, as seen in Fig. 1. The peaks at 2q ¼ 39.7 , 46.0 , 68.5 , 82.5 and 87.2 corresponds to the crystallographic planes (111), (200), (220), (311) and (220), respectively [27,28]. The peak of the carbon support corresponding to the plane (002) appears at 2q ¼ 25 [21,29]. No peaks of Ni or Mo metallic or oxides/hydroxides were observed, indicating the absence of segregated phases and suggesting the formation of metallic solid solutions [27,30]. The atomic composition determined by EDX measurements of the synthesized carbon-supported NPs ranged from 18 to 21 wt.% of total metal loading [27]. The bimetallic NPs compositions are 13 at.% Mo and 16 at.% Ni, in PtMo/C and PtNi/C, respectively, as reported in Table 1. TEM analysis (Fig. 2) revealed spherical metallic NPs (in black) with good dispersion on the carbon support (in grey). The average particle sizes (d) of NPs are 4 ± 1 nm (Pt/C), 7 ± 3 nm (PtNi/C) and 3.5 ± 0.4 nm (PtMo/C) confirming the synthesis of nanoparticles. Additional data around the characterization of carbon-supported NPs are presented in the Electronic Supplementary Material.
Table 2 e Effect of the temperature on current density (j) in HER for the cathodes Pt/C, PtNi/C and PtMo/C at 0.1 M TEA-PS.BF4 aqueous solution at ¡2.0 V vs. QRE-Pt. j (mA cm2)
T ( C)
25 30 40 50 60 70 80
Pt/C
PtMo/C
PtNi/C
76 83 99 110 118 123 134
89 103 116 112 125 135 139
97 106 120 125 137 148 149
Hydrogen evolution reaction The behaviour of the cathodes Pt/C, PtMo/C and PtNi/C in the hydrogen evolution reaction using 0.1 M TEA-PS.BF4 aqueous
Fig. 3 e Tafel plots for the HER on Pt/C, PtNi/C and PtMo/C. Operating conditions: 25 C, 0.1 M TEA-PS.BF4 aqueous solution, scan rate 10 mV s¡1.
Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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Table 3 e Kinetic parameters of the HER on cathodes Pt/C, PtNi/C and PtMo/C. Operating conditions: 25 C, 0.1 M TEA-PS.BF4 aqueous solution. Carbon-supported Ea NPs kJ mol1 Pt/C PtNi/C PtMo/C
8.7 6.9 6.4
Eeq V
b mV dec1
jo A cm2
0.54 þ0.09 þ0.22
39 47 43
1.4 105 6.4 106 2.7 105
coefficient values were calculated according to Equation (1). The b values, 39, 43 and 47 mV dec1 obtained for Pt/C, PtMo/C, and PtNi/C respectively, are very closed one to each other suggesting that all cathodes present the same kinetic mechanisms. These values, which are around 40 mV dec1 characterize a VolmereHeyrovsky mechanism illustrated by Equations (3) and (4) and presented in Fig. 4. Volmer : Hþ þ e þ NP/NP Hads
(3)
Heyrovsky : NP Hads þ Hþ þ e /NP þ H2
(4)
where NP represents the catalyst active free sites available for the HER, whereas NP-Hads represents adsorbed hydrogen atoms over the nanoparticles surface. According to the literature, through the usual VolmereHeyrovsky mechanism, the
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Volmer step is fast and is followed by the Heyrovsky step which is the rate-determining step [31e33]. Arrhenius-type dependence predicts an exponential increase of the current density with the increase of the temperature, which in a logarithmic form corresponds to Equation (5): ln j ¼ ln A Ea =RT
(5)
where j is the current density, A the pre-exponential factor, Ea the apparent activation energy, R the ideal gas constant and T the absolute temperature. The Ea values obtained in this study (see Table 3) were lower than those found in previously published reports for an equivalent system employing the same electrolyte (TEAPS.BF4 aqueous solutions) but using bulk metallic as cathodes [5,6]. The increasing order of Ea is PtMo/C < PtNi/C < Pt/C showing that, beside the NPs favourable effect on the Ea, their bimetallic composition increase this effect. The effective exchange current density (jo) for HER for Pt/C (1.4 105 A cm2) and for PtMo/C (2.7 105 A cm2) was higher than observed for PtNi/C (6.4 106 A cm2) confirming the correlation between the electrode composition and the influence on the efficiency in the HER. However, in comparison to published reference values, all studied cathodes presented higher exchange current density than systems with KOH aqueous electrolyte for HER [1,34,35].
Fig. 4 e Schematic representation of the Volmer-Heyrovsky mechanism for the HER.
Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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EIS measurements of HER EIS measurements were used to investigate the electrode/ electrolyte interface and the processes occurring on the cathodes surface immersed in the 0.1 M TEA-PS.BF4. Nyquist plots show three depressed semicircles for all cathodes, as shown in Fig. 5. Each depressed semicircle is related to one time constant, t (t ¼ R C, where R is the resistance and C is the pseudocapacitance) which is related to the relaxation rate of the HER when potential changes. The use of CPE is required due to the distribution of the relaxation times as a result of inhomogeneity of the cathode surface [25,36]. The impedance data were interpreted using two electric equivalent circuit (EEC) models, as presented in Fig. 6. The first EEC, named 1-EEC, is composed of by three parallel time
Fig. 6 e Equivalent circuits used for the HER: 1-EEC model represent fitting to Pt/C and PtNi/C and 2-EEC model represent fitting to PtMo/C.
constants, and the second EEC, named 2-EEC, is depicted by two parallel time constants and one serial time constant. The PtNi/C and Pt/C are better represented by the 1-EEC model while PtMo/C electrode is best described by 2-EEC model. The impedance parameters values are given in Table 4, namely, RS, R1, C1, t1, R2, C2, t2, R3, C3, t3. In both circuits, RS is the series resistance. This parameter is associated with the solution conductivity between the working and reference electrodes. For a better understanding, the time constant parameter (t) is used in the next discussion. At high frequencies, the process is related to hydrogen adsorption on the electrode surface followed by charge transfer kinetics and it is associated at the first time constant (t1) that is related with R1 and C1 [36]. R1 is the resistance related to the charge transfer of NP-Hads on the
Table 4 e The parameters obtained by fitting EIS experimental spectra recorded for different cathodes for the HER in TEA-PS.BF4 aqueous solution at 25 C, E ¼ ¡0.750 V vs. QRE-Pt. Circuit elements 2
Fig. 5 e Nyquist diagrams for the HER on cathodes Pt/C, PtNi/C, and PtMo/C. Operating conditions: 25 C, 0.1 M TEAPS.BF4, E ¼ ¡0.750 V vs. QRE-Pt.
RS/U cm R1/U cm2 C1a/mF cm2 n1 t1/103 s R2/U cm2 C2a/mF cm2 n2 t2/103 s R3/U cm2 C3a/mF cm2 n3 t3/s a
Carbon-supported NPs Pt/C
PtNi/C
PtMo/C
7.85 0.54 0.383 0.61 0.206 0.229 235.0 0.72 53.8 1.79 668.0 1.03 1.2
6.10 0.60 0.344 0.585 0.206 1.38 23.5 0.85 32.6 13.65 96.2 1.00 1.3
6.10 0.27 0.255 0.60 0.069 2.37 59.0 0.59 139.8 2.54 377.0 1.05 0.9
C1, C2 and C3 are pseudocapacitances.
Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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active sites of the cathode and C1 is the pseudocapacitance consequence of a charge arrangement at the electrode/electrolyte interface. These circuit elements correspond to impedance response at fast step in HER (Volmer step, Eq. (3)). It appears that the order of t1 is: PtMo/C < Pt/C ¼ PtNi/C. The PtMo/C cathode presents lower t1 due to higher j0 (2.7 105 A cm2) and lower Ea (6.4 kJ mol1) to the HER, showing a greater catalytic effect of this bimetallic NPs. This behaviour also can be observed by lower charge transfer resistance (R1). The higher value to t1 for Pt/C and PtNi/C can be attributed to the values of Ea and j0, which are 8.7 kJ mol1 and 6.4 106 A cm2, respectively. The second time constant (t2) shows the hydrogen desorption process which is related with R2 and C2. R2 is the second charge transfer resistance and C2 is the pseudocapacitance associated with modifications of the electrical double layer due to the reduction of Hþ and their desorption leading to H2 formation. These circuit elements correspond to impedance response at slow step in HER (Heyrovsky step, Eq. (4)). It appears that the following order of t2 is: PtNi/C < Pt/C < PtMo/C. PtNi/C cathode presents low t2 due to weak NP-Hads bond strength, evidencing the geometric effect caused by Ni in the PtNi alloy [15]. Ni-Hads bond strength is weaker than Pt-Hads and Mo-Hads bond strengths, respectively, as observed in the Volcano curve [35]. At low frequencies, the third time constant (t3) is in the same range for all studied cathodes and can be related to the electrode surface porosity response and it is related with R3 and C3 [34,36]. R3 is the resistance and C3 the pseudocapacitance associated to gas occlusion in the porous structure of the cathodes. All results of electrochemical measurements present in this paper (current density, apparent activation energy, Tafel parameters and impedance data) prove that the carbonsupported bimetallic NPs, PtNi/C and PtMo/C, are responsible for the catalytic effect observed when these materials are used as cathodes in the HER conducted with the in TEA-PS.BF4 aqueous solution.
Conclusion Carbon-supported bimetallic nanoparticles, PtNi/C and PtMo/ C, were synthesized for use as cathodes in the HER in the presence of TEA-PS.BF4 aqueous solution as electrolyte. Activation energy values of 8.7, 6.9, and 6.4 kJ mol1 were obtained for Pt/C, PtNi/C, and PtMo/C, respectively. These values are considered low compared to other electrolytes and/or to bulk electrodes. The HER conducted in all cathodes presented the same kinetic mechanism (VolmereHeyrovsky), showing that the cathode material determines the kinetic mechanism of HER responsible the decrease in activation energy. Nyquist plots revealed three depressed semicircles for all cathodes, indicating that three different processes occur. The Mo effect in the PtMo/C cathode improves charge transfer kinetics and hydrogen adsorption. The Ni facilitates the hydrogen desorption process in the PtNi/C cathode. Carbon-supported bimetallic NPs (PtNi/C and PtMo/C) in the TEA-PS.BF4 aqueous solution exhibited significantly improvement of the electrochemical performance when compared with Pt/C. All the results of this study demonstrate that the PtNi/C and PtMo/C are responsible for the catalytic effect observed.
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Acknowledgements The authors acknowledge the support of this work by the lise-CNPq, CNPq/FAPERGS/PRONEX. The authors INCT-Cata lise (CMM) of also thank the Centro de Microscopia e Microana the Universidade Federal do Rio Grande do Sul and Mr. Otelo J. Machado for XRD powder analysis.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.166.
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PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER Demetrius W. Lima a, Fernanda Fiegenbaum a,*, Fernanda Trombetta b, Michele O. de Souza a, Emilse M.A. Martini a a
Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸alves, 9500, P.O. Box 15003, Porto Alegre 91501-970, Brazil b Universidade Federal do Rio Grande e FURG, Escola de Quı´mica e Alimentos, Campus Santo Ant^onio da Patrulha, ~o do Caı´, 125, Santo Ant^onio da Patrulha 95500-000, Brazil Rua Bara
article info
abstract
Article history:
Carbon-supported bimetallic nanoparticles, PtNi/C and PtMo/C, were synthesized using the
Received 16 September 2016
borohydride reduction method and were tested as cathode in the hydrogen evolution re-
Received in revised form
action (HER) employing acid aqueous solution of 3-triethylammonium-propanesulfonic
18 November 2016
acid tetrafluoroborate (TEA-PS.BF4) ionic liquid as the electrolyte. For comparison a com-
Accepted 25 November 2016
mercial carbon-supported Pt nanoparticles (Pt/C) has been tested. The morphology,
Available online xxx
composition and structure of the carbon-supported nanoparticles were characterized by EDX, XRD, and TEM. The results obtained by chronoamperometry, Tafel plots and elec-
Keywords:
trochemical impedance spectroscopy (EIS) exhibited an excellent catalytic effect and the
Hydrogen evolution reaction
same kinetics mechanism for all carbon-supported nanoparticles in HER studied. Results of
Bimetallic nanoparticles
Tafel analysis showed that the HER rate determining step is the VolmereHeyrovsky
TEA-PS.BF4 ionic liquid
mechanism. In the temperature range 25e80 C, the PtNi/C and PtMo/C cathodes presented
Water electrolysis in acid medium
lower activation energy and higher current density than system using the Pt/C cathode.
Kinetic mechanism
The PtMo/C cathode improves charge transfer kinetics and hydrogen adsorption, while the
Impedance spectroscopy
PtNi/C cathode facilitates the desorption step. These results demonstrate that the carbonsupported bimetallic nanoparticles PtNi/C and PtMo/C are responsible for increasing the catalytic performances of the HER when TEA-PS.BF4 aqueous solution is employed as electrolyte. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is one of the most preferable renewable energy carriers and ideal energy storage, since it has high energy density, and could be easily converted into various available energy forms. However, nowadays, 96% of hydrogen is
produced from hydrocarbon sources such as natural gas, oil and coal. Over time, this scenario must change, requiring the production of hydrogen from renewable sources, i.e., involving clean and simple processes in order to completely avoid carbon dioxide (CO2) production. Even if water electrolysis is a clean and renewable process which produces
* Corresponding author. E-mail address: [email protected] (F. Fiegenbaum). http://dx.doi.org/10.1016/j.ijhydene.2016.11.166 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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highly purified hydrogen it presents an important disadvantage related to the need of high energy consumption [1e4]. Decreasing the cathodic overpotential of hydrogen evolution is one of the goal to be attained to reduce the cost of electrolytic hydrogen production and then to turn realizable technological applications. The main factors that influence the efficiency of the water electrolysis are related to the electrolytes and electrodes properties. Recently, the 3-triethylammonium-propanesulfonic acid tetrafluoroborate (TEA-PS.BF4) ionic liquid, a € nsted and Lewis acidity, has been compound combining Bro shown to be a new efficient electrolyte having a specific conductivity around 0.25 mS cm1 for a 0.1 M aqueous solution at room temperature. Hydrogen evolution reaction (HER) conducted with TEA-PS.BF4 ionic liquid as electrolyte and using bulk electrodes of platinum, nickel, stainless steel 304 and glassy carbon as cathodes presented remarkable performance [5e7]. Recent research and development efforts have been focused on minimizing ohmic drop, lowering overpotential through the improvement of cell and electrode design and using new and cheaper cathode materials with higher electrocatalytic activity for HER [3,8e11]. Among the noble metals, the platinum (Pt) is regarded as an ideal catalyst for electrochemical hydrogen production, but its high cost and scarce resource definitely limit its extensive application in HER [12e14]. Some non-noble metals are used in the water electrolysis in acid medium, such as Ni, Mo, Cr, W, and Nb. For example, Nickel bulk, when combined with other metals, has been considered one of the most important electrocatalysts in the hydrogen production due to it is low cost, corrosion resistance and reduced overpotential [5,15e18]. However, the combining of BMI.BF4 ionic liquid with Mo electrodes in water electrolysis system showed higher current density and lower activation energy than the same system with Ni and Pt bulk electrodes [19]. An alternative to decrease costs of metal and to increase the electroactive area is to produce electrocatalysts containing metallic nanoparticles (NPs) on black carbon support. The most usual carbon support is the Vulcan XC-72R black carbon that presents high surface area, good electrical conductance, low impurity content and high chemical stability [20,21]. Nanoparticles are currently of great interests because of their noteworthy catalytic behaviours in comparison with that of the bulk material [22]. Analogously, bimetallic NPs have distinct properties than monometallic NPs [23,24]. In the case of water electrolysis, carbon-supported NPs increase the performance, providing active sites for the water dissociation, enhancing the electron transfer, assisting the Hþ adsorption and the HeH bond formation [8,25,26]. Herein, we report the hydrogen evolution reaction using carbonsupported bimetallic NPs (PtNi/C and PtMo/C) as cathode in presence of TEA-PS.BF4 acid aqueous solution. The behaviour of these materials was compared to carbon-supported Pt NPs (Pt/C) as cathode. The effectiveness of investigated cathode materials was evaluated through kinetics parameters obtained from chronoamperometry, Tafel analysis, and ac impedance spectroscopy. Based on the electrochemical
impedance spectroscopy (EIS) measurements, equivalent circuits are used to describe the processes in the cathode/ electrolyte interface.
Experimental Preparation of cathodes Three types of cathodes were studied: Pt/C (Pt EC-20-PTC, Carbon XC-72R), PtMo/C and PtNi/C with 20 wt.% metal loading on Vulcan XC-72R carbon support (Cabot, 240 m2 g1). The precursors were hexachloroplatinic acid (IV) hexahydrate (H2PtCl6$6H2O, Merck), nickel (II) chloride hexahydrate (NiCl2$6H2O, 98%, Synth) and ammonium molybdate tetrahydrate ((NH4)6Mo7O24$4H2O, 99.98%, Sigma-Aldrich). The Vulcan XC-72R carbon support was previously treated with HNO3 [21]. The syntheses were made using sodium borohydride (NaBH4, 95%, Vetec) similar to previously published [27]. The structure and morphology of the carbon-supported NPs were analyzed by transmission electron microscopy (TEM) JEOL JEM 1200ExII at 120 kV. The relative proportions of Pt:Ni and Pt:Mo were obtained by energy-dispersive X-ray spectroscopy (EDX) coupled with scanning electronic microscope (JEOL-JSM 5800) with a 20 kV incident electron beam. The X-ray diffractograms were achieved using a D500 Rigaku diffractometer (Siemens) operating at a scan rate of 0.05 s1 in the range of 2q ¼ 20e90 using CuKa radiation (1.54056 A). The accuracies of these measurements are 3e4%. The cathodes were made by brushing technique with 0.5 mgmetal cm2
Fig. 1 e X-ray diffraction of carbon-supported NPs: Pt/C, PtNi/C and PtMo/C.
Table 1 e The composition of the Pt/C, PtNi/C and PtMo/C determined by EDX. Carbon-supported NPs Pt/C PtMo/C PtNi/C
metal loading wt.%
Pt at.%
Mo at.%
Ni at.%
20 18 21
100 87 84
e 13 e
e e 16
Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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and 30 wt.% of Nafion® on diffusion layer - carbon cloth (30 wt.% treated Teflon, Fuel Cell Earth LLC e Woburn, Massachusetts, USA).
Electrochemical measurements For all experiments a 0.1 M TEA-PS.BF4 acid aqueous solution was used as electrolyte. The TEA-PS.BF4 ionic liquid was prepared according previously published reports [5]. Chronoamperometry, Tafel analysis and electrochemical impedance spectroscopy (EIS) were performed with an
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Autolab PGSTAT30 potentiostat, with GPES and FRA modules. These experiments were made in a cell with three electrodes, Pt net as counter electrode, Pt wire as the quasireference electrode (QRE-Pt) and carbon-supported NPs (Pt/C, PtNi/C and PtMo/C) as working electrode. Chronoamperometry was carried out at 2.0 V vs. QRE-Pt potential applied during 30 min and the temperature was controlled by a thermostatic bath in the range from 25 to 80 C, an overpotential of 2.0 V was applied due to the high current densities observed in previous studies [5]. The average current density was calculated by integration of the current density (j)
Fig. 2 e TEM micrographs and particle size distribution histograms (d ¼ average particle size) of the Pt/C (a), PtNi/C (b) and PtMo/C (c). Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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versus time (t), that represents the charge (Q). This value was divided by the total time (30 min). Tafel analysis were used to determinate the kinetic parameters that were obtained from Tafel plots, at a scan rate of 10 mV s1, according to (Eq. (1)): h ¼ a þ b log j
(1) 1
where b (V dec ) is the Tafel coefficient, which indicates the rate of change of the current density, j (mA cm2), with the applied overpotential, h (V). EIS measurements were carried out at 0.750 V (QRE-Pt), using a frequency range from 100 kHz to 0.1 Hz and an ac signal amplitude of 10 mV. The experimental data were fitted to the equivalent circuit by NOVA 1.11 software (MetrohmAutolab). It is possible to use the fitting and simulation tool to simplify a parallel combination of a resistor and a constant phase element (CPE) by converting the CPE to pseudocapacitance C, according to Eq. (2):
1
C¼
= Y0n $R
1
=n1
(2)
where C is the pseudocapacitance, Y0 is the admittance value of the constant phase element, R is the resistance value and n is the exponent of the constant phase element.
solution was initially investigated measuring the current density (j) vs. temperature (see Table 2). Results reported Table 2 show that for the evaluated temperature range 25e80 C, the PtNi/C and PtMo/C evidence a greater performance in the HER than the Pt/C, enhancing up to 22% the current density. Independently of the temperature, the PtNi/C cathode shows higher current densities than those registered for the other cathodes, revealing a good performance of this bimetallic NPs for the hydrogen evolution. Based on these preliminary results, which show remarkable behaviours of the carbon-supported bimetallic NPs, further studies were performed for a better understanding of the mechanism involved with theses cathodes in the HER in the presence of TEA-PS.BF4. Quasi-potentiostatic polarization measurements were carried out, providing Tafel plots (Fig. 3). The determination of Tafel parameters as Tafel coefficient (b), effective exchange current density (j0) and equilibrium potential (Eeq), listed in Table 3, enable to elucidate the kinetic mechanism for HER, to determine the reaction mechanism and the rate-determining step (rds). The Eeq values obtained for the cathodes PtNi/C, Pt/C and PtMo/C were 0.01 V, þ0.11 V and þ0.21 V, respectively. These results again suggest a better catalytic effect obtained with the PtNi/C cathode. For the HER conducted with an aqueous solution of 0.1 M TEA-PS.BF4 using carbon-supported NPs as cathode, the Tafel
Results and discussion Characterization of carbon-supported NPs Pt/C, PtMo/C and PtNi/C were analyzed by X-ray Diffraction. The corresponding diffractograms present the same peak characteristics of Pt, being a face-centred cubic structure, as seen in Fig. 1. The peaks at 2q ¼ 39.7 , 46.0 , 68.5 , 82.5 and 87.2 corresponds to the crystallographic planes (111), (200), (220), (311) and (220), respectively [27,28]. The peak of the carbon support corresponding to the plane (002) appears at 2q ¼ 25 [21,29]. No peaks of Ni or Mo metallic or oxides/hydroxides were observed, indicating the absence of segregated phases and suggesting the formation of metallic solid solutions [27,30]. The atomic composition determined by EDX measurements of the synthesized carbon-supported NPs ranged from 18 to 21 wt.% of total metal loading [27]. The bimetallic NPs compositions are 13 at.% Mo and 16 at.% Ni, in PtMo/C and PtNi/C, respectively, as reported in Table 1. TEM analysis (Fig. 2) revealed spherical metallic NPs (in black) with good dispersion on the carbon support (in grey). The average particle sizes (d) of NPs are 4 ± 1 nm (Pt/C), 7 ± 3 nm (PtNi/C) and 3.5 ± 0.4 nm (PtMo/C) confirming the synthesis of nanoparticles. Additional data around the characterization of carbon-supported NPs are presented in the Electronic Supplementary Material.
Table 2 e Effect of the temperature on current density (j) in HER for the cathodes Pt/C, PtNi/C and PtMo/C at 0.1 M TEA-PS.BF4 aqueous solution at ¡2.0 V vs. QRE-Pt. j (mA cm2)
T ( C)
25 30 40 50 60 70 80
Pt/C
PtMo/C
PtNi/C
76 83 99 110 118 123 134
89 103 116 112 125 135 139
97 106 120 125 137 148 149
Hydrogen evolution reaction The behaviour of the cathodes Pt/C, PtMo/C and PtNi/C in the hydrogen evolution reaction using 0.1 M TEA-PS.BF4 aqueous
Fig. 3 e Tafel plots for the HER on Pt/C, PtNi/C and PtMo/C. Operating conditions: 25 C, 0.1 M TEA-PS.BF4 aqueous solution, scan rate 10 mV s¡1.
Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166
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Table 3 e Kinetic parameters of the HER on cathodes Pt/C, PtNi/C and PtMo/C. Operating conditions: 25 C, 0.1 M TEA-PS.BF4 aqueous solution. Carbon-supported Ea NPs kJ mol1 Pt/C PtNi/C PtMo/C
8.7 6.9 6.4
Eeq V
b mV dec1
jo A cm2
0.54 þ0.09 þ0.22
39 47 43
1.4 105 6.4 106 2.7 105
coefficient values were calculated according to Equation (1). The b values, 39, 43 and 47 mV dec1 obtained for Pt/C, PtMo/C, and PtNi/C respectively, are very closed one to each other suggesting that all cathodes present the same kinetic mechanisms. These values, which are around 40 mV dec1 characterize a VolmereHeyrovsky mechanism illustrated by Equations (3) and (4) and presented in Fig. 4. Volmer : Hþ þ e þ NP/NP Hads
(3)
Heyrovsky : NP Hads þ Hþ þ e /NP þ H2
(4)
where NP represents the catalyst active free sites available for the HER, whereas NP-Hads represents adsorbed hydrogen atoms over the nanoparticles surface. According to the literature, through the usual VolmereHeyrovsky mechanism, the
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Volmer step is fast and is followed by the Heyrovsky step which is the rate-determining step [31e33]. Arrhenius-type dependence predicts an exponential increase of the current density with the increase of the temperature, which in a logarithmic form corresponds to Equation (5): ln j ¼ ln A Ea =RT
(5)
where j is the current density, A the pre-exponential factor, Ea the apparent activation energy, R the ideal gas constant and T the absolute temperature. The Ea values obtained in this study (see Table 3) were lower than those found in previously published reports for an equivalent system employing the same electrolyte (TEAPS.BF4 aqueous solutions) but using bulk metallic as cathodes [5,6]. The increasing order of Ea is PtMo/C < PtNi/C < Pt/C showing that, beside the NPs favourable effect on the Ea, their bimetallic composition increase this effect. The effective exchange current density (jo) for HER for Pt/C (1.4 105 A cm2) and for PtMo/C (2.7 105 A cm2) was higher than observed for PtNi/C (6.4 106 A cm2) confirming the correlation between the electrode composition and the influence on the efficiency in the HER. However, in comparison to published reference values, all studied cathodes presented higher exchange current density than systems with KOH aqueous electrolyte for HER [1,34,35].
Fig. 4 e Schematic representation of the Volmer-Heyrovsky mechanism for the HER.
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EIS measurements of HER EIS measurements were used to investigate the electrode/ electrolyte interface and the processes occurring on the cathodes surface immersed in the 0.1 M TEA-PS.BF4. Nyquist plots show three depressed semicircles for all cathodes, as shown in Fig. 5. Each depressed semicircle is related to one time constant, t (t ¼ R C, where R is the resistance and C is the pseudocapacitance) which is related to the relaxation rate of the HER when potential changes. The use of CPE is required due to the distribution of the relaxation times as a result of inhomogeneity of the cathode surface [25,36]. The impedance data were interpreted using two electric equivalent circuit (EEC) models, as presented in Fig. 6. The first EEC, named 1-EEC, is composed of by three parallel time
Fig. 6 e Equivalent circuits used for the HER: 1-EEC model represent fitting to Pt/C and PtNi/C and 2-EEC model represent fitting to PtMo/C.
constants, and the second EEC, named 2-EEC, is depicted by two parallel time constants and one serial time constant. The PtNi/C and Pt/C are better represented by the 1-EEC model while PtMo/C electrode is best described by 2-EEC model. The impedance parameters values are given in Table 4, namely, RS, R1, C1, t1, R2, C2, t2, R3, C3, t3. In both circuits, RS is the series resistance. This parameter is associated with the solution conductivity between the working and reference electrodes. For a better understanding, the time constant parameter (t) is used in the next discussion. At high frequencies, the process is related to hydrogen adsorption on the electrode surface followed by charge transfer kinetics and it is associated at the first time constant (t1) that is related with R1 and C1 [36]. R1 is the resistance related to the charge transfer of NP-Hads on the
Table 4 e The parameters obtained by fitting EIS experimental spectra recorded for different cathodes for the HER in TEA-PS.BF4 aqueous solution at 25 C, E ¼ ¡0.750 V vs. QRE-Pt. Circuit elements 2
Fig. 5 e Nyquist diagrams for the HER on cathodes Pt/C, PtNi/C, and PtMo/C. Operating conditions: 25 C, 0.1 M TEAPS.BF4, E ¼ ¡0.750 V vs. QRE-Pt.
RS/U cm R1/U cm2 C1a/mF cm2 n1 t1/103 s R2/U cm2 C2a/mF cm2 n2 t2/103 s R3/U cm2 C3a/mF cm2 n3 t3/s a
Carbon-supported NPs Pt/C
PtNi/C
PtMo/C
7.85 0.54 0.383 0.61 0.206 0.229 235.0 0.72 53.8 1.79 668.0 1.03 1.2
6.10 0.60 0.344 0.585 0.206 1.38 23.5 0.85 32.6 13.65 96.2 1.00 1.3
6.10 0.27 0.255 0.60 0.069 2.37 59.0 0.59 139.8 2.54 377.0 1.05 0.9
C1, C2 and C3 are pseudocapacitances.
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active sites of the cathode and C1 is the pseudocapacitance consequence of a charge arrangement at the electrode/electrolyte interface. These circuit elements correspond to impedance response at fast step in HER (Volmer step, Eq. (3)). It appears that the order of t1 is: PtMo/C < Pt/C ¼ PtNi/C. The PtMo/C cathode presents lower t1 due to higher j0 (2.7 105 A cm2) and lower Ea (6.4 kJ mol1) to the HER, showing a greater catalytic effect of this bimetallic NPs. This behaviour also can be observed by lower charge transfer resistance (R1). The higher value to t1 for Pt/C and PtNi/C can be attributed to the values of Ea and j0, which are 8.7 kJ mol1 and 6.4 106 A cm2, respectively. The second time constant (t2) shows the hydrogen desorption process which is related with R2 and C2. R2 is the second charge transfer resistance and C2 is the pseudocapacitance associated with modifications of the electrical double layer due to the reduction of Hþ and their desorption leading to H2 formation. These circuit elements correspond to impedance response at slow step in HER (Heyrovsky step, Eq. (4)). It appears that the following order of t2 is: PtNi/C < Pt/C < PtMo/C. PtNi/C cathode presents low t2 due to weak NP-Hads bond strength, evidencing the geometric effect caused by Ni in the PtNi alloy [15]. Ni-Hads bond strength is weaker than Pt-Hads and Mo-Hads bond strengths, respectively, as observed in the Volcano curve [35]. At low frequencies, the third time constant (t3) is in the same range for all studied cathodes and can be related to the electrode surface porosity response and it is related with R3 and C3 [34,36]. R3 is the resistance and C3 the pseudocapacitance associated to gas occlusion in the porous structure of the cathodes. All results of electrochemical measurements present in this paper (current density, apparent activation energy, Tafel parameters and impedance data) prove that the carbonsupported bimetallic NPs, PtNi/C and PtMo/C, are responsible for the catalytic effect observed when these materials are used as cathodes in the HER conducted with the in TEA-PS.BF4 aqueous solution.
Conclusion Carbon-supported bimetallic nanoparticles, PtNi/C and PtMo/ C, were synthesized for use as cathodes in the HER in the presence of TEA-PS.BF4 aqueous solution as electrolyte. Activation energy values of 8.7, 6.9, and 6.4 kJ mol1 were obtained for Pt/C, PtNi/C, and PtMo/C, respectively. These values are considered low compared to other electrolytes and/or to bulk electrodes. The HER conducted in all cathodes presented the same kinetic mechanism (VolmereHeyrovsky), showing that the cathode material determines the kinetic mechanism of HER responsible the decrease in activation energy. Nyquist plots revealed three depressed semicircles for all cathodes, indicating that three different processes occur. The Mo effect in the PtMo/C cathode improves charge transfer kinetics and hydrogen adsorption. The Ni facilitates the hydrogen desorption process in the PtNi/C cathode. Carbon-supported bimetallic NPs (PtNi/C and PtMo/C) in the TEA-PS.BF4 aqueous solution exhibited significantly improvement of the electrochemical performance when compared with Pt/C. All the results of this study demonstrate that the PtNi/C and PtMo/C are responsible for the catalytic effect observed.
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Acknowledgements The authors acknowledge the support of this work by the lise-CNPq, CNPq/FAPERGS/PRONEX. The authors INCT-Cata lise (CMM) of also thank the Centro de Microscopia e Microana the Universidade Federal do Rio Grande do Sul and Mr. Otelo J. Machado for XRD powder analysis.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.166.
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Please cite this article in press as: Lima DW, et al., PtNi and PtMo nanoparticles as efficient catalysts using TEA-PS.BF4 ionic liquid as electrolyte towards HER, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.166