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Cu catalysts for borohydride fuel cells
Accepted Manuscript Title: CoB/Cu and PtCoB/Cu catalysts for borohydride fuel cells Author: A. Balˇci¯ unait˙e Z. Sukackien˙e L. ˇ cien˙e A. Selskis E. Norkus ˇ Cinˇ Tamaˇsauskait˙e-Tamaˇsi¯ unait˙e Z. PII: DOI: Reference:
S0013-4686(16)32714-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.155 EA 28627
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-9-2016 23-12-2016 24-12-2016
Please cite this article as: A.Balˇci¯ unait˙e, Z.Sukackien˙e, L.Tamaˇsauskait˙e-Tamaˇsi¯ unait˙e, ˇ cien˙e, A.Selskis, E.Norkus, CoB/Cu and PtCoB/Cu catalysts for borohydride fuel ˇZ.Cinˇ cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.155 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
Decoration of CoB/Cu with Pt nanoparticles by galvanic displacement.
The electrocatalytic activity of CoB/Cu and PtCoB/Cu catalysts was investigated towards borohydride oxidation.
CoB/Cu and PtCoB/Cu catalysts are catalytically active towards borohydride oxidation and hydrolysis.
CoB/Cu and PtCoB/Cu catalysts for borohydride fuel cells
A. Balčiūnaitė, Z. Sukackienė, L. Tamašauskaitė-Tamašiūnaitė*, Ž. Činčienė, A. Selskis, E. Norkus
Department of Catalysis, Center for Physical Sciences and Technology, Saulėtekio av. 3, LT-10257, Vilnius, Lithuania
Abstract
CoB/Cu and PtCoB/Cu catalysts have been formed using the methods of electroless metal deposition and galvanic displacement. The activity of the PtCoB/Cu and CoB/Cu catalysts towards borohydride oxidation in alkaline solutions was studied by means of the cyclic voltammetry, chronoamperometry and chronopotentiometry. Optimal conditions of formation of catalysts have been determined and their electrocatalytic activity towards sodium borohydride oxidation has been evaluated. Pt nanoparticles sized 10-45 nm have been deposited on the CoB/Cu electrode. It has been determined that the PtCoB/Cu catalysts have enhanced electrocatalytic activity towards the oxidation of sodium borohydride as compared with that of Pt and CoB/Cu. Sodium borohydride oxidation current densities are ca. 10-12 and 4.1-4.5 times higher at the PtCoB/Cu catalysts with the Pt loadings in the range of 9.8 and 14.4 µgPt cm-2 than those at pure Pt and CoB/Cu catalysts, respectively. It has been found that CoB/Cu and PtCoB/Cu catalyze the hydrolysis of sodium borohydride. These catalysts are promising materials and can be used in alkaline sodium borohydride fuel cells as anodes.
Keywords: Platinum, cobalt, boron, copper, borohydride.
*
Corresponding author. Tel. +370 5 2661291, fax: +370 5 2649774. E-mail address: [email protected] (L. Tamašauskaitė-Tamašiūnaitė)
1. Introduction Direct borohydride fuel cells are of interest for development of fuel cell technologies. In these fuel cells the alkaline sodium borohydride solution is used as fuel [1-5]. It should be noted that sodium borohydride is stable in an alkaline medium, moreover, its anodic oxidation reaction products are not harmful to the environment and soluble in water. The working principle of this fuel cell is based on the reactions of borohydride oxidation on the anode and oxygen reduction on the cathode, which are described in Ref. [3]. The oxidation of sodium borohydride has been extensively studied on various catalytic substances, such as Au, Pt, Ni, Pd, Cu, etc., in order to create the most effective and suitable catalysts, which could be used in borohydride fuel cells as anode materials [6-22]. It is known that the platinum catalyst effectively catalyzes both the hydrolysis and oxidation reactions of sodium borohydride, however its application in fuel cells is restricted because of its expensiveness [6]. The mechanism of the sodium borohydride electro-oxidation reaction at Pt electrodes has been extensively investigated and reported [5,6,8,12,15-18]. Electrocatalysts, such as Pt, Pd, etc. possess good catalytic properties towards the oxidation and hydrolysis reactions of BH4ˉ ion and in some cases these electrode materials lead to high current densities and low or high Faradaic output when used in the DBFC anode [14]. Freitas et al. demonstrated that the latter strongly depends on the texture of the active layer [15], whereas the former also depends on the ratio between the fuel concentration and the catalyst loading [16,17]. Higher fuel cell efficiencies can be achieved on Pt and Pd electrodes, using low BH4ˉ concentrations and high anodic currents [19]. Celikkan et al. [13] investigated the catalytic activity of Pt, Ag, Pd and Ni metals towards the oxidation of BH4ˉ ions and determined that of all the studied metals Pt is distinguished for the highest activity towards BH4ˉ oxidation, while Ni is noted for the lowest one.
Although precious metals are distinguished for their high catalytic activity towards BH4ˉ ions oxidation, their commercial application is not viable due to their expensiveness. Therefore, investigations have been carried out to determine how to diminish the quantity of the precious metal in the catalyst simultaneously increasing its effectiveness. In recent years, alloys of precious metals (Pt, Au, Ag) with transition metals (Co, Cu, Fe) have been proposed and it has been shown that the bimetallic catalysts, such as Au-Ni [23-25], Au-Co [26-28], Au-Cu [29], Au-Zn [30], Pt-Ni [31], Pt-Co [31,32], Pt-Cu [33], Pt-Zn [34], Pd-Zn [35], AgCu [36], have higher electrocatalytic activity towards the oxidation of BH4- ions as compared to that of pure Au, Pt, Pd, Ag metals. The improved activity of Au-based or Pt-based catalysts has been attributed to AuM or PtM alloy formation and the change in Au or Pt electronic structure due to the presence of transition metals [37-46]. Herein we present a simple approach to prepare an efficient CoB/Cu and PtCoB/Cu catalysts with low Pt loadings for direct borohydride oxidation. The catalysts were prepared by means of the electroless deposition and galvanic displacement [23,24,27,28]. The electrocatalytic activity of the prepared CoB/Cu and PtCoB/Cu catalysts was investigated with respect to the electro-oxidation of sodium borohydride by means of the cyclic voltammetry and chrono-techniques. The surface morphology and composition of the samples were characterized using Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDX) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).
2. Experimental
2.1. Chemicals
H2PtCl6·6H2O (Alfa Aesar, 99.95 %), CoSO4·7H2O (Chempur, 99.5%), C4H8ONH∙BH3 (borane morpholine complex, Sigma-Aldrich, 97 %), NaBH4 (Sigma-Aldrich, 99.9 %, 96 wt.%) and NaOH (AnalaR NORMAPUR, 99 wt.%) were used as received and all solutions were prepared using deionized (Elix 3 Millipore) water. All chemicals were of analytical grade.
2.2. Preparation of catalysts
Electroless deposition of CoB alloy was performed on the copper foil (1x1cm) surface. The duration of electroless deposition of the CoB coatings on the copper surface was 100 min and the bath operated at pH 7 and a temperature of 30 oC [47]. The thickness of the CoB coatings obtained was ca. 1 μm. Pt nanoparticles were deposited on the prepared CoB/Cu electrodes by the galvanic displacement of platinum from 1 mM H2PtCl6 (denoted as a platinum(IV)-containing solution) for 10, 30 and 60 seconds. The prepared catalysts were used for sodium borohydride electro-oxidation measurements without any further treatment.
2.3. Characterization of catalysts
The morphology and composition of the fabricated catalysts were characterized using a SEM/FIB workstation Helios Nanolab 650 with an energy dispersive X-ray (EDX) spectrometer INCA Energy 350 XMax 20. The Pt metal loading in the PtCoB/Cu catalysts was estimated from ICP-OES measurements. The ICP optical emission spectra were recorded using an ICP optical emission spectrometer Optima 7000DV (Perkin Elmer).
2.4. Electrochemical measurements
All electrochemical measurements were performed with a Metrohm Autolab potentiostat (PGSTAT100) using the Electrochemical Software (Nova 1.6.013). Steady state linear sweep voltammograms (CVs) were recorded in a 1 M NaOH solution or that additionally containing 0.05 M NaBH4 at a temperature of 25 oC and a potential sweep rate of 10 mV s–1 from the stationary Es value in the anodic direction up to 1.623 V versus RHE. A three-electrode electrochemical cell was used for electrochemical measurements. Pure Pt foil, PtCoB/Cu and CoB/Cu catalysts with a geometric surface area of 2 cm2 were employed as working electrodes, an Ag/AgCl/KCl (3 M KCl) electrode was used as reference and a Pt sheet was used as a counter electrode. The electrode potentials are given with respect to the reversible hydrogen electrode (RHE). The presented current densities are normalized with respect to the geometric area of catalysts. The chronoamperometric measurements were carried out by, at first, holding the potential at open circuit for 10 s, and then stepping to 0.023 and 1.123 V versus RHE for 1800 s, respectively. Chronopotentiometric curves were recorded at 10 mA cm–2 for 180 s.
2.5. Kinetic studies of the catalytic hydrolysis of NaBH4
The amount of hydrogen generated was measured by using a MilliGascounter (Type MGC1 V3.2 PMMA, Ritter, Germany). In a typical measurement, the reaction solution containing NaBH4 and NaOH was thermostated in an airtight flask fitted with an outlet connected to the MilliGascounter for collection of the evolved H2 gas. Then, the PtCoB/Cu and CoB/Cu catalysts were immersed into the solution at a designated temperature to initiate hydrolysis. The rate of hydrogen generation was measured at different solution temperatures (30, 40, 50, 60 and 70 °C) in order to determine the activation energy.
3. Results and discussion
The CoB/Cu and PtCoB/Cu catalysts with low Pt loadings were prepared by a simple approach with the aim to use them as electrocatalysts for direct sodium borohydride oxidation. The catalysts were fabricated using electroless metal deposition and galvanic displacement. The cobalt-boron alloy layer, electrolessly deposited on the Cu surface, was used as a sublayer for deposition of Pt crystallites. As seen from the data in Fig. 1a, the electroless cobalt-boron alloy film, deposited onto the Cu surface, formed a layer of CoB alloy with the average size of crystallites ca. 1 µm. The Pt/CoB/Cu catalysts with Pt particles of a few nanometers in size were prepared onto the Cu surface by immersion of CoB/Cu electrode into a 1 mM H2PtCl6 solution for 10, 30 and 60 seconds, respectively, as testified in Fig. 1 (b-d). After immersion of the CoB/Cu electrode into a 1 mM H2PtCl6 solution at 25°C for 10 or 30 s, Pt nanoparticles sized 10 to 25 nm were deposited on the CoB/Cu surface. As seen, Pt nanoparticles were homogeneously dispersed on the surface of CoB/Cu (Fig. 1b,c). An increase in CoB/Cu electrode immersion time in a 1 mM H2PtCl6 solution up to 60 s results in deposition of larger Pt nanoparticles on the CoB/Cu surface. As depicted in Fig. 1d, the Pt nanoparticles were about 35 to 45 nm in size. The presence of Pt and Co was also confirmed by Energy Dispersive X-ray analysis. The data obtained are given in Table 1. Significant amounts of Co and much lower amounts of Pt deposited on the electrode surface were determined. According to the ICP-OES analysis of the composition of the prepared PtCoB/Cu catalysts, the Pt loadings were 9.8, 10.6 and 14.4 gPt cm-2 in the as-prepared catalysts after immersion of the CoB/Cu electrodes into the platinum(IV)-containing solution for 10, 30 and 60 s, respectively. The catalytic activities of the prepared CoB/Cu and different PtCoB/Cu catalysts were investigated with respect to the electro-oxidation of sodium borohydride. The cyclic voltammograms of the investigated catalysts were recorded in a deaerated 0.05 M NaBH4 + 1 M NaOH solution between -0.177 and 1.623 V at a sweep rate of 10 mV s–1. Figure 2 presents
the electrochemical behavior of the CoB/Cu catalyst in 0.05 M NaBH4 + 1 M NaOH (solid line) and 1 M NaOH (dashed line) at 25 °C. In the case of CoB/Cu in 1 M NaOH, during anodic scan only anodic peaks a1 and a2 are observed at lower potential values. These anodic peaks can be attributed to the formation of Co hydroxides/oxides in the alkaline medium at low potential values (Fig. 2, dashed line) [48-50]. The measured sodium borohydride oxidation current density values at CoB/Cu are significantly greater as compared to those, recorded in the background solution (1 M NaOH) at the latter catalyst. It is clearly seen that borohydride oxidation at the CoB/Cu electrode proceeds via different and potential-depending pathways in agreement with the data published in [49,50]. Two well-expressed anodic peaks A0 and A are observed in the CV recorded on the CoB/Cu electrode in a 0.05 M NaBH4 +1 M NaOH solution (Fig. 2, solid line). The measured oxidation current densities in the lowpotential region from -0.216 to 0.300 V (peak A0) recorded on CoB/Cu (Fig. 2, solid line) may be ascribed to the direct oxidation of borohydride anion as was confirmed by the authors in Ref. [51]. The authors show that for Co electrode at high borohydride concentrations ( 0.01 M), open circuit potentials (OCPs) could be due to the oxidation of borohydride on bare metal (-0.422 V vs. RHE). The measured OCP of CoB/Cu in a 0.05 M NaBH4 +1 M NaOH solution is around -0.027 V vs. RHE, which is lower than the thermodynamic one. Actually, on the ground of FTIR, EQCM and RDDE investigations of the overall borohydride oxidation process on various catalysts show that the mechanism of borohydride oxidation is rather complicated due to the formation of intermediates, adsorption phenomenon and influence of electrode potential, e.g., formation, adsorption and oxidation of BH3OH– [7,15-18,46,52-59]. It may be assumed that in the low-potential region up to < 0.3 V borohydride oxidation on the CoB/Cu surface proceeds via fully dissociative adsorption of BH4- [17]. It was shown that in the low-potential region direct BH4- electro-oxidation produces more electrons then the highpotential direct BH4- electro-oxidation [18].
In the case when potential values are more positive than 1.423 V, anodic peak A1 is observed. This peak may be attributed to the formation of Co(III) surface compounds [48]. For comparison, electrooxidation of sodium borohydride was studied on a pure Pt electrode by recording a cyclic voltammogram in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC and sweeping the electrode potential at a rate of 10 mV s–1 from the stationary potential value Es in the anodic direction up to 1.623 V. The electrochemical behavior of pure Pt in an alkaline sodium borohydride solution is widely described in the literature [15-19]. The presented CV of pure Pt in Fig. 2 (the inset) demonstrates two anodic peaks: A0 in the lower potential region and A in the higher potential region. According to recent studies of the mechanism of borohydride oxidation on the Pt/C catalyst by means of differential electrochemical massspectrometry (DEMS), the borohydride oxidation on Pt/C occurs via different pathways in low and high potential regions [7]. It was determined that at low potentials (around 0 V vs. RHE), the gas generation was found to result from a mixed potential between the reactions of borohydride oxidation and hydrogen evolution. At higher potentials (above 0.6 V vs. RHE) the gas generation resulted almost only from the recombination of the deuterium atoms of NaBD4 [7]. Figure 3 presents the stabilized CVs (5th cycles) recorded on the PtCoB/Cu catalysts with the Pt loadings in the range from 9.8 to 11.4 µg cm-2 in the background solution of 1 M NaOH and that containing 0.05 M NaBH4 at 10 mV s-1. The PtCoB/Cu catalysts show an oxidation peaks a1 and a2 at lower potential values in a 1 M NaOH solution, which can be attributed to the formation of Co(OH)2 and CoO on the open sites of bare CoB surface (Fig. 3, dashed lines) [48-50]. Much higher anodic current densities were recorded at the PtCoB/Cu catalysts in the solution containing NaBH4 than those in the background solution (Fig. 3, solid lines). It is obvious that BOR on the PtCoB/Cu catalysts is also a potential-depended process. Fig. 3d depicts comparative anodic sweeping voltammograms (5th cycles) recorded on pure Pt, CoB/Cu and PtCoB/Cu catalysts. In all the voltammograms anodic peaks A0 and A are
observed. A comparison of sodium borohydride oxidation current densities at peak A, measured on Pt, CoB/Cu and PtCoB/Cu catalysts, suggests that sodium borohydride oxidation current densities are markedly higher on the PtCoB/Cu electrodes as compared to those on the CoB/Cu catalyst and pure Pt (Fig. 3d). Deposition of Pt crystallites on the CoB/Cu surface increases the activity of the CoB catalyst towards borohydride oxidation. Greater efficiency of PtCoB/Cu catalysts may be associated with the electrocatalytic activity of Pt nanoparticles deposited on the CoB/Cu surface, PtM alloy formation and changes in Pt electronic structure due to the presence of Co [36-44]. It has been determined that the borohydride oxidation current density values measured on the PtCoB/Cu catalysts with the deposited Pt loadings of 9.8 to 14.4 µgPt cm-2, are 10 to 12 times greater than those on pure Pt, and 4.1 to 4.5 times higher than those on the CoB/Cu electrode (Fig. 3d). The kinetics of the oxidation of borohydride on the CoB/Cu and PtCoB/Cu catalysts were investigated by cyclic voltammetry at various sweep rates. Figure 4 presents the anodic scans for CoB/Cu (a) and PtCoB/Cu with the Pt loading of 10.6 µgPt cm-2 (b) recorded in 0.05 M NaBH4 + 1 M NaOH at various potential sweep rates. The dependence of current densities recorded at the CoB/Cu and PtCoB/Cu under both borohydride oxidation peaks A0 and A on the sweep rate is given in Fig. 4c,d. As seen from the data in Fig. 4a and b, the borohydride oxidation peaks currents and peaks potentials increase with an increase in sweep rate. Upon increasing the sweep rate, the plots of oxidation peaks A0 and A current density versus sweep rate are linear with a correlation coeficient of 0.9787 and 0.9916, respectively, for CoB/Cu (Fig. 4c) and 0.9988 and 0.9827, respectively, for PtCoB/Cu (Fig. 4d). The data obtained confirmed that the kinetics of borohydride oxidation at potentials under the peaks A0 and A on the CoB/Cu and PtCoB/Cu catalysts are a diffusion-controlled process. In order to confirm the activity of catalysts towards hydrogen generation, the hydrolysis of NaBH4 was studied on the CoB/Cu and PtCoB/Cu catalysts. Fig. 5 depicts the dependence
of the volume of the evolved hydrogen on temperature obtained at the CoB/Cu (a) and PtCoB/Cu catalyst with the Pt loading of 14.4 µgPt cm-2 (b). As evident from the data obtained, the rate of catalytic hydrolysis of NaBH4 increases exponentially with an increase in temperature. Summarized NaBH4 catalytic hydrolysis data obtained on the CoB/Cu and PtCoB/Cu catalysts are presented in Table 2. The highest H2 evolution rates were obtained on the PtCoB/Cu and CoB/Cu catalysts at a temperature of 70 oC and are equal to 229.3 L min–1 gPt–1 and 3.0 L min-1gCo-1, respectively. In order to calculate the activation energy and Arrhenius constant, the Arrhenius dependence of lnk on 1/T was plotted using the data presented in Fig. 5 (a,b). The Arrhenius equation establishes the dependence of H2 evolution rate on temperature: k Ae Ea / RT ,
where k is the reaction rate constant, Ea is the activation energy (kJ mol-1), A is the Arrhenius constant, depending on the nature of reactants, T is the thermodynamic temperature, R is the mole gas constant (8.314 J mol-1 K-1). The activation energies of PtCoB/Cu and CoB/Cu catalysts calculated from the Arrhenius curves (Fig. 5c, d) are equal to ~ 20 and ~ 33 kJ mol–1, respectively (Table 2). The data obtained confirm that both the CoB/Cu and PtCoB/Cu catalysts effectively catalyze the catalytic hydrolysis from an alkaline sodium borohydride solution. Decoration of CoB/Cu with Pt nanoparticles results in a significantly greater hydrogen generation rate from an alkaline sodium borohydride solution as compared to that of pure CoB/Cu. In order to assess the catalytic characteristics and stability of CoB/Cu and PtCoB/Cu catalysts with different Pt loadings, chronoamperometric measurements were performed. The chronoamperometric curves were recorded on the CoB/Cu and PtCoB/Cu catalysts in a sodium borohydride solution at constant potential values of 0.023 and 1.123 V, respectively (Fig. 6). It is clear from the data in Fig. 6, that the current density values of investigated catalysts stabilize with time and remain constant. The measured BH4- ions oxidation current
density values on the PtCoB/Cu catalysts with different Pt loadings are higher than those on the pure CoB/Cu catalyst. The comparison of the PtCoB/Cu catalysts activities suggests that higher BH4- oxidation current density values were obtained at PtCoB/Cu with the Pt loading of 14.4 µg cm-2 (Fig. 6). Chronopotentiometric studies of these catalysts were also performed in a sodium borohydride solution at a constant current density of 10 mA cm-2 for 180 s. The open circuit potential values of catalysts are -0.169 to 0.205 V (Fig. 7). The difference in potential values between the potential measured under open circuit conditions and that measured at a constant current density, when t = 180 s, defines the activity and stability of the catalyst. A lower potential values difference defines a higher catalytic activity. It has been determined that the lowest difference in potential values between the potential under open circuit conditions and stationary potential value is in the case of the PtCoB/Cu (14.4 µgPt cm-2) catalyst and is equal to 0.0098 V, which suggests a higher activity of this catalyst. For the PtCoB/Cu (10.6 µgPt cm-2) catalyst this value is 0.0125 V, for CoB/Cu – 0.0131 V and PtCoB/Cu (9.8 µgPt cm2
) – 0.0177 V.
4. Conclusions
PtCoB/Cu catalysts have been formed using the electroless metal plating and galvanic displacement methods. Pt nanoparticles sized 10 to 45 nm have been deposited on the CoB/Cu electrode. It has been shown that the PtCoB/Cu catalysts have higher electrocatalytic activity towards the sodium borohydride oxidation reaction as compared to that of pure Pt or CoB/Cu catalysts. Sodium borohydride oxidation current density values measured on the PtCoB/Cu catalysts with Pt loadings in the range of 9.8 to 14.4 µgPt cm-2 are ca. 10 to 12 and 4.1 to 4.5 times higher than those on pure Pt and CoB/Cu electrodes, respectively.
The CoB/Cu and PtCoB/Cu catalysts are also catalytically active towards sodium borohydride hydrolysis. The highest H2 evolution rates were obtained on the PtCoB/Cu catalyst with the Pt loading of 14.4 µgPt cm-2 and the CoB/Cu catalyst at a temperature of 70 o
C and are equal to 229.3 L min-1gPt-1 and 3.0 L min-1gCo-1, respectively. The PtCoB/Cu
catalysts formed are promising and catalytically active materials, which can be used as anodes in direct borohydride fuel cells.
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Figure captions
Fig. 1. FESEM images of the as-prepared CoB/Cu (a) and PtCoB/Cu catalysts (b-d). The catalysts were prepared by immersion of Cu surface in the electroless cobalt plating solution at 30 oC for 100 min, followed by its immersion in 1 mM H2PtCl6 at room temperature for 10 (b), 30 (c) and 60 (d) s. Fig. 2. Stabilized CVs (5th cycles) of the CoB/Cu catalyst recorded in 1 M NaOH (dashed line) and in 0.05 M NaBH4 + 1 M NaOH (solid line) at 25 oC at 10 mV s-1. CoB/Cu catalysts were obtained by immersion of Cu into an electroless cobalt plating solution at 30 oC for 100 min. The inset demonstrates a CV recorded on a pure Pt electrode in a 0.05 M NaBH4 + 1 M NaOH solution. Fig. 3. Stabilized CVs (5th cycles) of the PtCoB/Cu catalysts with the Pt loading of 9.8 (a), 10.6 (b) and 14.4 (c) µg cm-2 recorded in 1 M NaOH (dashed line) and in 0.05 M NaBH4 + 1 M NaOH (solid line) at 25 oC at 10 mV s-1. (d) Positive-potential going scans (5th cycles) recorded on CoB/Cu (solid line), Pt (pink line) and PtCoB/Cu catalysts in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC at 10 mV s-1. Fig. 4. Anodic scans of the CoB/Cu (a) and PtCoB/Cu with the Pt loading of 10.6 µgPt cm-2 (b) catalysts at various sweep scans in 0.05 M NaBH4 + 1 M NaOH. c, d – plots of peaks A0 and A current density versus sweep rate on the same catalysts. Fig. 5. H2 generation from 15 ml 0.05 M NaBH4 + 1 M NaOH catalyzed by the CoB/Cu (a) and PtCoB/Cu with the Pt loading of 14.4 µgPt cm-2 (b) catalysts at different solution temperatures. The Arrhenius plots calculated from the rates of NaBH4 hydrolysis in the same solution for CoB/Cu (c) and PtCoB/Cu (d). Fig. 6. Chronoamperometric curves of CoB/Cu and PtCoB/Cu catalysts with the Pt loadings of 9.8, 10.6 and 14.4 µg cm-2 recorded in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC, when E = 0.023 (a) and 1.123 (b) V, t = 1800 s.
Fig. 7. Chronopotentiometric curves of CoB/Cu and PtCoB/Cu catalysts with the Pt loadings of 9.8, 10.6 and 14.4 µg cm-2, recorded in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC, when j = 10 mA cm-2, t = 180 s.
Table 1. Surface atomic composition of the CoB/Cu and PtCoB/Cu catalysts by EDX analysis. The Pt loadings in the catalysts were determined by ICP-OES. The catalysts are the same as in Fig. 1. Catalysts
Elements. at.%
Pt loading, g cm-2
Pt
Co
Cu
a
–
96.75
3.25
-
b
0.53
95.79
3.69
9.8
c
0.61
95.46
3.93
10.6
d
0.98
95.28
3.74
14.4
Table 2. Data of hydrogen generation from catalytic hydrolysis of NaBH4 using the CoB/Cu and PtCoB/Cu catalysts in a 0.05 M NaBH4 + 1 M NaOH solution (v = 15 ml), at different temperatures.
Catalyst (metal loading)
CoB/Cu (934.1 µgCo cm-2)
PtCoB/Cu (14.4 µgPt cm-2)
Ea, kJ mol
32.92
20.10
-1
o
H2 generation rate, L
T, C min-1 gmetal-1 30
0.629
40
1.014
50
1.227
60
1.785
70
3.023
30
90.532
40
110.258
50
142.812
60
193.712
70
229.279
(a)
(b)
(c)
(d)
Fig. 1.
15
CoB/Cu
D
Pt
80 10
j / mA cm-2
60
A0
A0
B
5
40
A C
0 0.0 0.4 0.8 1.2 1.6 A1 A
a2
20
a1
0
1 M NaOH 1 M NaOH + 0.05 M NaBH4
-20
0.0
0.4
0.8
E vs. RHE / V
Fig. 2.
1.2
1.6
80
80
(a)
A
A0
A0 40
j / mA cm-2
40
j / mA cm-2
A
(b)
a2 a1 0
a2 a1 0
1 M NaOH + 0.05 M NaBH4 -40
1 M NaOH + 0.05 M NaBH4 -40
1 M NaOH
0.0
0.4
0.8
1.2
1 M NaOH
1.6
0.0
0.4
(c)
A
A0
1.2
1.6
E vs. RHE / V
E vs. RHE / V 80
0.8
80
(d)
A A0
PtCoB/Cu
60
j / mA cm-2
j / mA cm-2
40 a2 a1 0
1 M NaOH + 0.05 M NaBH4 -40
1 M NaOH
40
20
CoB/Cu Pt 9.8 gPt cm-2
0
10.6 gPt cm-2 14.4 gPt cm-2
-20 0.0
0.4
0.8
1.2
1.6
E vs. RHE / V
0.0
0.4
0.8
E vs. RHE / V
Fig. 3.
1.2
1.6
80
(a) CoB/Cu A0
(c)
-1
80
20 mVs 30 mVs-1 40 mVs-1 -1 50 mVs 60 mVs-1
60 40 20
for peak A
y=0.1546x+4.999 R2=0.9916
40
A
jp / mA cm-2
j / mA cm-2
y=0.1057x+61.871 2 R =0.9787
60
20 0 -2
100
for peak A0
(b) PtCoB/Cu (10.6 gPt cm ) A0
0 100
(d)
for peak A0 y=0.6918x+60.211 2 R =0.9988
90
80
A
60
80
40
70
20
for peak A y=0.4597x+44.275 R2=0.9827
60
0 0.0
0.4
0.8
1.2
1.6
10
20
30
40
v / mV s-1
E vs. RHE / V
Fig. 4
50
60
70
8.4 100
8.0 80
60
40 30 °C 40 °C 50 °C 60 °C 70 °C
20
ln k / ml min -1 g -1
H 2 generated volume / ml
(c)
(a) CoB/Cu
7.6 7.2 6.8 6.4
0
6.0 0
20
40
60
80
0.0029
0.0030
0.0031
0.0032
0.0033
0.0032
0.0033
T -1/ K-1
t / min 12.6 (b) Pt/CoB/Cu (14.4 gPt cm-2)
100
(d)
80
60
40
30 °C 40 °C 50 °C 60 °C 70 °C
20
0
ln k / ml min -1 g -1
H 2 generated volume / ml
12.4 12.2 12.0 11.8 11.6 11.4 11.2 0
20
40
60
80
0.0029
0.0030
0.0031
T -1/ K-1
t / min
Fig. 5
(a) 0.023 V
80
j / mA cm-2
70 60 50 40 30
-2
Pt/CoB/Cu (gPtcm ) 20
-2
Pt/CoB/Cu (gPtcm ) -2
10
Pt/CoB/Cu (gPtcm ) CoB/Cu
0 0
300
600
900
1200
1500
1200
1500
t/s (b) 1.123 V
40
j / mA cm-2
30
20
10
0 0
300
600
900
t/s
Fig. 6
E vs. RHE / V
-0.15 -0.16
CoB/Cu PtCoB/Cu (gPtcm-2)
-0.17
PtCoB/Cu (gPtcm )
-2 -2
PtCoB/Cu (gPtcm ) -0.18 -0.19 -0.20
0
30
60
90
t/s
Fig. 7
120
150
180
S0013-4686(16)32714-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.155 EA 28627
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-9-2016 23-12-2016 24-12-2016
Please cite this article as: A.Balˇci¯ unait˙e, Z.Sukackien˙e, L.Tamaˇsauskait˙e-Tamaˇsi¯ unait˙e, ˇ cien˙e, A.Selskis, E.Norkus, CoB/Cu and PtCoB/Cu catalysts for borohydride fuel ˇZ.Cinˇ cells, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.155 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
Decoration of CoB/Cu with Pt nanoparticles by galvanic displacement.
The electrocatalytic activity of CoB/Cu and PtCoB/Cu catalysts was investigated towards borohydride oxidation.
CoB/Cu and PtCoB/Cu catalysts are catalytically active towards borohydride oxidation and hydrolysis.
CoB/Cu and PtCoB/Cu catalysts for borohydride fuel cells
A. Balčiūnaitė, Z. Sukackienė, L. Tamašauskaitė-Tamašiūnaitė*, Ž. Činčienė, A. Selskis, E. Norkus
Department of Catalysis, Center for Physical Sciences and Technology, Saulėtekio av. 3, LT-10257, Vilnius, Lithuania
Abstract
CoB/Cu and PtCoB/Cu catalysts have been formed using the methods of electroless metal deposition and galvanic displacement. The activity of the PtCoB/Cu and CoB/Cu catalysts towards borohydride oxidation in alkaline solutions was studied by means of the cyclic voltammetry, chronoamperometry and chronopotentiometry. Optimal conditions of formation of catalysts have been determined and their electrocatalytic activity towards sodium borohydride oxidation has been evaluated. Pt nanoparticles sized 10-45 nm have been deposited on the CoB/Cu electrode. It has been determined that the PtCoB/Cu catalysts have enhanced electrocatalytic activity towards the oxidation of sodium borohydride as compared with that of Pt and CoB/Cu. Sodium borohydride oxidation current densities are ca. 10-12 and 4.1-4.5 times higher at the PtCoB/Cu catalysts with the Pt loadings in the range of 9.8 and 14.4 µgPt cm-2 than those at pure Pt and CoB/Cu catalysts, respectively. It has been found that CoB/Cu and PtCoB/Cu catalyze the hydrolysis of sodium borohydride. These catalysts are promising materials and can be used in alkaline sodium borohydride fuel cells as anodes.
Keywords: Platinum, cobalt, boron, copper, borohydride.
*
Corresponding author. Tel. +370 5 2661291, fax: +370 5 2649774. E-mail address: [email protected] (L. Tamašauskaitė-Tamašiūnaitė)
1. Introduction Direct borohydride fuel cells are of interest for development of fuel cell technologies. In these fuel cells the alkaline sodium borohydride solution is used as fuel [1-5]. It should be noted that sodium borohydride is stable in an alkaline medium, moreover, its anodic oxidation reaction products are not harmful to the environment and soluble in water. The working principle of this fuel cell is based on the reactions of borohydride oxidation on the anode and oxygen reduction on the cathode, which are described in Ref. [3]. The oxidation of sodium borohydride has been extensively studied on various catalytic substances, such as Au, Pt, Ni, Pd, Cu, etc., in order to create the most effective and suitable catalysts, which could be used in borohydride fuel cells as anode materials [6-22]. It is known that the platinum catalyst effectively catalyzes both the hydrolysis and oxidation reactions of sodium borohydride, however its application in fuel cells is restricted because of its expensiveness [6]. The mechanism of the sodium borohydride electro-oxidation reaction at Pt electrodes has been extensively investigated and reported [5,6,8,12,15-18]. Electrocatalysts, such as Pt, Pd, etc. possess good catalytic properties towards the oxidation and hydrolysis reactions of BH4ˉ ion and in some cases these electrode materials lead to high current densities and low or high Faradaic output when used in the DBFC anode [14]. Freitas et al. demonstrated that the latter strongly depends on the texture of the active layer [15], whereas the former also depends on the ratio between the fuel concentration and the catalyst loading [16,17]. Higher fuel cell efficiencies can be achieved on Pt and Pd electrodes, using low BH4ˉ concentrations and high anodic currents [19]. Celikkan et al. [13] investigated the catalytic activity of Pt, Ag, Pd and Ni metals towards the oxidation of BH4ˉ ions and determined that of all the studied metals Pt is distinguished for the highest activity towards BH4ˉ oxidation, while Ni is noted for the lowest one.
Although precious metals are distinguished for their high catalytic activity towards BH4ˉ ions oxidation, their commercial application is not viable due to their expensiveness. Therefore, investigations have been carried out to determine how to diminish the quantity of the precious metal in the catalyst simultaneously increasing its effectiveness. In recent years, alloys of precious metals (Pt, Au, Ag) with transition metals (Co, Cu, Fe) have been proposed and it has been shown that the bimetallic catalysts, such as Au-Ni [23-25], Au-Co [26-28], Au-Cu [29], Au-Zn [30], Pt-Ni [31], Pt-Co [31,32], Pt-Cu [33], Pt-Zn [34], Pd-Zn [35], AgCu [36], have higher electrocatalytic activity towards the oxidation of BH4- ions as compared to that of pure Au, Pt, Pd, Ag metals. The improved activity of Au-based or Pt-based catalysts has been attributed to AuM or PtM alloy formation and the change in Au or Pt electronic structure due to the presence of transition metals [37-46]. Herein we present a simple approach to prepare an efficient CoB/Cu and PtCoB/Cu catalysts with low Pt loadings for direct borohydride oxidation. The catalysts were prepared by means of the electroless deposition and galvanic displacement [23,24,27,28]. The electrocatalytic activity of the prepared CoB/Cu and PtCoB/Cu catalysts was investigated with respect to the electro-oxidation of sodium borohydride by means of the cyclic voltammetry and chrono-techniques. The surface morphology and composition of the samples were characterized using Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDX) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).
2. Experimental
2.1. Chemicals
H2PtCl6·6H2O (Alfa Aesar, 99.95 %), CoSO4·7H2O (Chempur, 99.5%), C4H8ONH∙BH3 (borane morpholine complex, Sigma-Aldrich, 97 %), NaBH4 (Sigma-Aldrich, 99.9 %, 96 wt.%) and NaOH (AnalaR NORMAPUR, 99 wt.%) were used as received and all solutions were prepared using deionized (Elix 3 Millipore) water. All chemicals were of analytical grade.
2.2. Preparation of catalysts
Electroless deposition of CoB alloy was performed on the copper foil (1x1cm) surface. The duration of electroless deposition of the CoB coatings on the copper surface was 100 min and the bath operated at pH 7 and a temperature of 30 oC [47]. The thickness of the CoB coatings obtained was ca. 1 μm. Pt nanoparticles were deposited on the prepared CoB/Cu electrodes by the galvanic displacement of platinum from 1 mM H2PtCl6 (denoted as a platinum(IV)-containing solution) for 10, 30 and 60 seconds. The prepared catalysts were used for sodium borohydride electro-oxidation measurements without any further treatment.
2.3. Characterization of catalysts
The morphology and composition of the fabricated catalysts were characterized using a SEM/FIB workstation Helios Nanolab 650 with an energy dispersive X-ray (EDX) spectrometer INCA Energy 350 XMax 20. The Pt metal loading in the PtCoB/Cu catalysts was estimated from ICP-OES measurements. The ICP optical emission spectra were recorded using an ICP optical emission spectrometer Optima 7000DV (Perkin Elmer).
2.4. Electrochemical measurements
All electrochemical measurements were performed with a Metrohm Autolab potentiostat (PGSTAT100) using the Electrochemical Software (Nova 1.6.013). Steady state linear sweep voltammograms (CVs) were recorded in a 1 M NaOH solution or that additionally containing 0.05 M NaBH4 at a temperature of 25 oC and a potential sweep rate of 10 mV s–1 from the stationary Es value in the anodic direction up to 1.623 V versus RHE. A three-electrode electrochemical cell was used for electrochemical measurements. Pure Pt foil, PtCoB/Cu and CoB/Cu catalysts with a geometric surface area of 2 cm2 were employed as working electrodes, an Ag/AgCl/KCl (3 M KCl) electrode was used as reference and a Pt sheet was used as a counter electrode. The electrode potentials are given with respect to the reversible hydrogen electrode (RHE). The presented current densities are normalized with respect to the geometric area of catalysts. The chronoamperometric measurements were carried out by, at first, holding the potential at open circuit for 10 s, and then stepping to 0.023 and 1.123 V versus RHE for 1800 s, respectively. Chronopotentiometric curves were recorded at 10 mA cm–2 for 180 s.
2.5. Kinetic studies of the catalytic hydrolysis of NaBH4
The amount of hydrogen generated was measured by using a MilliGascounter (Type MGC1 V3.2 PMMA, Ritter, Germany). In a typical measurement, the reaction solution containing NaBH4 and NaOH was thermostated in an airtight flask fitted with an outlet connected to the MilliGascounter for collection of the evolved H2 gas. Then, the PtCoB/Cu and CoB/Cu catalysts were immersed into the solution at a designated temperature to initiate hydrolysis. The rate of hydrogen generation was measured at different solution temperatures (30, 40, 50, 60 and 70 °C) in order to determine the activation energy.
3. Results and discussion
The CoB/Cu and PtCoB/Cu catalysts with low Pt loadings were prepared by a simple approach with the aim to use them as electrocatalysts for direct sodium borohydride oxidation. The catalysts were fabricated using electroless metal deposition and galvanic displacement. The cobalt-boron alloy layer, electrolessly deposited on the Cu surface, was used as a sublayer for deposition of Pt crystallites. As seen from the data in Fig. 1a, the electroless cobalt-boron alloy film, deposited onto the Cu surface, formed a layer of CoB alloy with the average size of crystallites ca. 1 µm. The Pt/CoB/Cu catalysts with Pt particles of a few nanometers in size were prepared onto the Cu surface by immersion of CoB/Cu electrode into a 1 mM H2PtCl6 solution for 10, 30 and 60 seconds, respectively, as testified in Fig. 1 (b-d). After immersion of the CoB/Cu electrode into a 1 mM H2PtCl6 solution at 25°C for 10 or 30 s, Pt nanoparticles sized 10 to 25 nm were deposited on the CoB/Cu surface. As seen, Pt nanoparticles were homogeneously dispersed on the surface of CoB/Cu (Fig. 1b,c). An increase in CoB/Cu electrode immersion time in a 1 mM H2PtCl6 solution up to 60 s results in deposition of larger Pt nanoparticles on the CoB/Cu surface. As depicted in Fig. 1d, the Pt nanoparticles were about 35 to 45 nm in size. The presence of Pt and Co was also confirmed by Energy Dispersive X-ray analysis. The data obtained are given in Table 1. Significant amounts of Co and much lower amounts of Pt deposited on the electrode surface were determined. According to the ICP-OES analysis of the composition of the prepared PtCoB/Cu catalysts, the Pt loadings were 9.8, 10.6 and 14.4 gPt cm-2 in the as-prepared catalysts after immersion of the CoB/Cu electrodes into the platinum(IV)-containing solution for 10, 30 and 60 s, respectively. The catalytic activities of the prepared CoB/Cu and different PtCoB/Cu catalysts were investigated with respect to the electro-oxidation of sodium borohydride. The cyclic voltammograms of the investigated catalysts were recorded in a deaerated 0.05 M NaBH4 + 1 M NaOH solution between -0.177 and 1.623 V at a sweep rate of 10 mV s–1. Figure 2 presents
the electrochemical behavior of the CoB/Cu catalyst in 0.05 M NaBH4 + 1 M NaOH (solid line) and 1 M NaOH (dashed line) at 25 °C. In the case of CoB/Cu in 1 M NaOH, during anodic scan only anodic peaks a1 and a2 are observed at lower potential values. These anodic peaks can be attributed to the formation of Co hydroxides/oxides in the alkaline medium at low potential values (Fig. 2, dashed line) [48-50]. The measured sodium borohydride oxidation current density values at CoB/Cu are significantly greater as compared to those, recorded in the background solution (1 M NaOH) at the latter catalyst. It is clearly seen that borohydride oxidation at the CoB/Cu electrode proceeds via different and potential-depending pathways in agreement with the data published in [49,50]. Two well-expressed anodic peaks A0 and A are observed in the CV recorded on the CoB/Cu electrode in a 0.05 M NaBH4 +1 M NaOH solution (Fig. 2, solid line). The measured oxidation current densities in the lowpotential region from -0.216 to 0.300 V (peak A0) recorded on CoB/Cu (Fig. 2, solid line) may be ascribed to the direct oxidation of borohydride anion as was confirmed by the authors in Ref. [51]. The authors show that for Co electrode at high borohydride concentrations ( 0.01 M), open circuit potentials (OCPs) could be due to the oxidation of borohydride on bare metal (-0.422 V vs. RHE). The measured OCP of CoB/Cu in a 0.05 M NaBH4 +1 M NaOH solution is around -0.027 V vs. RHE, which is lower than the thermodynamic one. Actually, on the ground of FTIR, EQCM and RDDE investigations of the overall borohydride oxidation process on various catalysts show that the mechanism of borohydride oxidation is rather complicated due to the formation of intermediates, adsorption phenomenon and influence of electrode potential, e.g., formation, adsorption and oxidation of BH3OH– [7,15-18,46,52-59]. It may be assumed that in the low-potential region up to < 0.3 V borohydride oxidation on the CoB/Cu surface proceeds via fully dissociative adsorption of BH4- [17]. It was shown that in the low-potential region direct BH4- electro-oxidation produces more electrons then the highpotential direct BH4- electro-oxidation [18].
In the case when potential values are more positive than 1.423 V, anodic peak A1 is observed. This peak may be attributed to the formation of Co(III) surface compounds [48]. For comparison, electrooxidation of sodium borohydride was studied on a pure Pt electrode by recording a cyclic voltammogram in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC and sweeping the electrode potential at a rate of 10 mV s–1 from the stationary potential value Es in the anodic direction up to 1.623 V. The electrochemical behavior of pure Pt in an alkaline sodium borohydride solution is widely described in the literature [15-19]. The presented CV of pure Pt in Fig. 2 (the inset) demonstrates two anodic peaks: A0 in the lower potential region and A in the higher potential region. According to recent studies of the mechanism of borohydride oxidation on the Pt/C catalyst by means of differential electrochemical massspectrometry (DEMS), the borohydride oxidation on Pt/C occurs via different pathways in low and high potential regions [7]. It was determined that at low potentials (around 0 V vs. RHE), the gas generation was found to result from a mixed potential between the reactions of borohydride oxidation and hydrogen evolution. At higher potentials (above 0.6 V vs. RHE) the gas generation resulted almost only from the recombination of the deuterium atoms of NaBD4 [7]. Figure 3 presents the stabilized CVs (5th cycles) recorded on the PtCoB/Cu catalysts with the Pt loadings in the range from 9.8 to 11.4 µg cm-2 in the background solution of 1 M NaOH and that containing 0.05 M NaBH4 at 10 mV s-1. The PtCoB/Cu catalysts show an oxidation peaks a1 and a2 at lower potential values in a 1 M NaOH solution, which can be attributed to the formation of Co(OH)2 and CoO on the open sites of bare CoB surface (Fig. 3, dashed lines) [48-50]. Much higher anodic current densities were recorded at the PtCoB/Cu catalysts in the solution containing NaBH4 than those in the background solution (Fig. 3, solid lines). It is obvious that BOR on the PtCoB/Cu catalysts is also a potential-depended process. Fig. 3d depicts comparative anodic sweeping voltammograms (5th cycles) recorded on pure Pt, CoB/Cu and PtCoB/Cu catalysts. In all the voltammograms anodic peaks A0 and A are
observed. A comparison of sodium borohydride oxidation current densities at peak A, measured on Pt, CoB/Cu and PtCoB/Cu catalysts, suggests that sodium borohydride oxidation current densities are markedly higher on the PtCoB/Cu electrodes as compared to those on the CoB/Cu catalyst and pure Pt (Fig. 3d). Deposition of Pt crystallites on the CoB/Cu surface increases the activity of the CoB catalyst towards borohydride oxidation. Greater efficiency of PtCoB/Cu catalysts may be associated with the electrocatalytic activity of Pt nanoparticles deposited on the CoB/Cu surface, PtM alloy formation and changes in Pt electronic structure due to the presence of Co [36-44]. It has been determined that the borohydride oxidation current density values measured on the PtCoB/Cu catalysts with the deposited Pt loadings of 9.8 to 14.4 µgPt cm-2, are 10 to 12 times greater than those on pure Pt, and 4.1 to 4.5 times higher than those on the CoB/Cu electrode (Fig. 3d). The kinetics of the oxidation of borohydride on the CoB/Cu and PtCoB/Cu catalysts were investigated by cyclic voltammetry at various sweep rates. Figure 4 presents the anodic scans for CoB/Cu (a) and PtCoB/Cu with the Pt loading of 10.6 µgPt cm-2 (b) recorded in 0.05 M NaBH4 + 1 M NaOH at various potential sweep rates. The dependence of current densities recorded at the CoB/Cu and PtCoB/Cu under both borohydride oxidation peaks A0 and A on the sweep rate is given in Fig. 4c,d. As seen from the data in Fig. 4a and b, the borohydride oxidation peaks currents and peaks potentials increase with an increase in sweep rate. Upon increasing the sweep rate, the plots of oxidation peaks A0 and A current density versus sweep rate are linear with a correlation coeficient of 0.9787 and 0.9916, respectively, for CoB/Cu (Fig. 4c) and 0.9988 and 0.9827, respectively, for PtCoB/Cu (Fig. 4d). The data obtained confirmed that the kinetics of borohydride oxidation at potentials under the peaks A0 and A on the CoB/Cu and PtCoB/Cu catalysts are a diffusion-controlled process. In order to confirm the activity of catalysts towards hydrogen generation, the hydrolysis of NaBH4 was studied on the CoB/Cu and PtCoB/Cu catalysts. Fig. 5 depicts the dependence
of the volume of the evolved hydrogen on temperature obtained at the CoB/Cu (a) and PtCoB/Cu catalyst with the Pt loading of 14.4 µgPt cm-2 (b). As evident from the data obtained, the rate of catalytic hydrolysis of NaBH4 increases exponentially with an increase in temperature. Summarized NaBH4 catalytic hydrolysis data obtained on the CoB/Cu and PtCoB/Cu catalysts are presented in Table 2. The highest H2 evolution rates were obtained on the PtCoB/Cu and CoB/Cu catalysts at a temperature of 70 oC and are equal to 229.3 L min–1 gPt–1 and 3.0 L min-1gCo-1, respectively. In order to calculate the activation energy and Arrhenius constant, the Arrhenius dependence of lnk on 1/T was plotted using the data presented in Fig. 5 (a,b). The Arrhenius equation establishes the dependence of H2 evolution rate on temperature: k Ae Ea / RT ,
where k is the reaction rate constant, Ea is the activation energy (kJ mol-1), A is the Arrhenius constant, depending on the nature of reactants, T is the thermodynamic temperature, R is the mole gas constant (8.314 J mol-1 K-1). The activation energies of PtCoB/Cu and CoB/Cu catalysts calculated from the Arrhenius curves (Fig. 5c, d) are equal to ~ 20 and ~ 33 kJ mol–1, respectively (Table 2). The data obtained confirm that both the CoB/Cu and PtCoB/Cu catalysts effectively catalyze the catalytic hydrolysis from an alkaline sodium borohydride solution. Decoration of CoB/Cu with Pt nanoparticles results in a significantly greater hydrogen generation rate from an alkaline sodium borohydride solution as compared to that of pure CoB/Cu. In order to assess the catalytic characteristics and stability of CoB/Cu and PtCoB/Cu catalysts with different Pt loadings, chronoamperometric measurements were performed. The chronoamperometric curves were recorded on the CoB/Cu and PtCoB/Cu catalysts in a sodium borohydride solution at constant potential values of 0.023 and 1.123 V, respectively (Fig. 6). It is clear from the data in Fig. 6, that the current density values of investigated catalysts stabilize with time and remain constant. The measured BH4- ions oxidation current
density values on the PtCoB/Cu catalysts with different Pt loadings are higher than those on the pure CoB/Cu catalyst. The comparison of the PtCoB/Cu catalysts activities suggests that higher BH4- oxidation current density values were obtained at PtCoB/Cu with the Pt loading of 14.4 µg cm-2 (Fig. 6). Chronopotentiometric studies of these catalysts were also performed in a sodium borohydride solution at a constant current density of 10 mA cm-2 for 180 s. The open circuit potential values of catalysts are -0.169 to 0.205 V (Fig. 7). The difference in potential values between the potential measured under open circuit conditions and that measured at a constant current density, when t = 180 s, defines the activity and stability of the catalyst. A lower potential values difference defines a higher catalytic activity. It has been determined that the lowest difference in potential values between the potential under open circuit conditions and stationary potential value is in the case of the PtCoB/Cu (14.4 µgPt cm-2) catalyst and is equal to 0.0098 V, which suggests a higher activity of this catalyst. For the PtCoB/Cu (10.6 µgPt cm-2) catalyst this value is 0.0125 V, for CoB/Cu – 0.0131 V and PtCoB/Cu (9.8 µgPt cm2
) – 0.0177 V.
4. Conclusions
PtCoB/Cu catalysts have been formed using the electroless metal plating and galvanic displacement methods. Pt nanoparticles sized 10 to 45 nm have been deposited on the CoB/Cu electrode. It has been shown that the PtCoB/Cu catalysts have higher electrocatalytic activity towards the sodium borohydride oxidation reaction as compared to that of pure Pt or CoB/Cu catalysts. Sodium borohydride oxidation current density values measured on the PtCoB/Cu catalysts with Pt loadings in the range of 9.8 to 14.4 µgPt cm-2 are ca. 10 to 12 and 4.1 to 4.5 times higher than those on pure Pt and CoB/Cu electrodes, respectively.
The CoB/Cu and PtCoB/Cu catalysts are also catalytically active towards sodium borohydride hydrolysis. The highest H2 evolution rates were obtained on the PtCoB/Cu catalyst with the Pt loading of 14.4 µgPt cm-2 and the CoB/Cu catalyst at a temperature of 70 o
C and are equal to 229.3 L min-1gPt-1 and 3.0 L min-1gCo-1, respectively. The PtCoB/Cu
catalysts formed are promising and catalytically active materials, which can be used as anodes in direct borohydride fuel cells.
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Figure captions
Fig. 1. FESEM images of the as-prepared CoB/Cu (a) and PtCoB/Cu catalysts (b-d). The catalysts were prepared by immersion of Cu surface in the electroless cobalt plating solution at 30 oC for 100 min, followed by its immersion in 1 mM H2PtCl6 at room temperature for 10 (b), 30 (c) and 60 (d) s. Fig. 2. Stabilized CVs (5th cycles) of the CoB/Cu catalyst recorded in 1 M NaOH (dashed line) and in 0.05 M NaBH4 + 1 M NaOH (solid line) at 25 oC at 10 mV s-1. CoB/Cu catalysts were obtained by immersion of Cu into an electroless cobalt plating solution at 30 oC for 100 min. The inset demonstrates a CV recorded on a pure Pt electrode in a 0.05 M NaBH4 + 1 M NaOH solution. Fig. 3. Stabilized CVs (5th cycles) of the PtCoB/Cu catalysts with the Pt loading of 9.8 (a), 10.6 (b) and 14.4 (c) µg cm-2 recorded in 1 M NaOH (dashed line) and in 0.05 M NaBH4 + 1 M NaOH (solid line) at 25 oC at 10 mV s-1. (d) Positive-potential going scans (5th cycles) recorded on CoB/Cu (solid line), Pt (pink line) and PtCoB/Cu catalysts in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC at 10 mV s-1. Fig. 4. Anodic scans of the CoB/Cu (a) and PtCoB/Cu with the Pt loading of 10.6 µgPt cm-2 (b) catalysts at various sweep scans in 0.05 M NaBH4 + 1 M NaOH. c, d – plots of peaks A0 and A current density versus sweep rate on the same catalysts. Fig. 5. H2 generation from 15 ml 0.05 M NaBH4 + 1 M NaOH catalyzed by the CoB/Cu (a) and PtCoB/Cu with the Pt loading of 14.4 µgPt cm-2 (b) catalysts at different solution temperatures. The Arrhenius plots calculated from the rates of NaBH4 hydrolysis in the same solution for CoB/Cu (c) and PtCoB/Cu (d). Fig. 6. Chronoamperometric curves of CoB/Cu and PtCoB/Cu catalysts with the Pt loadings of 9.8, 10.6 and 14.4 µg cm-2 recorded in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC, when E = 0.023 (a) and 1.123 (b) V, t = 1800 s.
Fig. 7. Chronopotentiometric curves of CoB/Cu and PtCoB/Cu catalysts with the Pt loadings of 9.8, 10.6 and 14.4 µg cm-2, recorded in a 0.05 M NaBH4 + 1 M NaOH solution at 25 oC, when j = 10 mA cm-2, t = 180 s.
Table 1. Surface atomic composition of the CoB/Cu and PtCoB/Cu catalysts by EDX analysis. The Pt loadings in the catalysts were determined by ICP-OES. The catalysts are the same as in Fig. 1. Catalysts
Elements. at.%
Pt loading, g cm-2
Pt
Co
Cu
a
–
96.75
3.25
-
b
0.53
95.79
3.69
9.8
c
0.61
95.46
3.93
10.6
d
0.98
95.28
3.74
14.4
Table 2. Data of hydrogen generation from catalytic hydrolysis of NaBH4 using the CoB/Cu and PtCoB/Cu catalysts in a 0.05 M NaBH4 + 1 M NaOH solution (v = 15 ml), at different temperatures.
Catalyst (metal loading)
CoB/Cu (934.1 µgCo cm-2)
PtCoB/Cu (14.4 µgPt cm-2)
Ea, kJ mol
32.92
20.10
-1
o
H2 generation rate, L
T, C min-1 gmetal-1 30
0.629
40
1.014
50
1.227
60
1.785
70
3.023
30
90.532
40
110.258
50
142.812
60
193.712
70
229.279
(a)
(b)
(c)
(d)
Fig. 1.
15
CoB/Cu
D
Pt
80 10
j / mA cm-2
60
A0
A0
B
5
40
A C
0 0.0 0.4 0.8 1.2 1.6 A1 A
a2
20
a1
0
1 M NaOH 1 M NaOH + 0.05 M NaBH4
-20
0.0
0.4
0.8
E vs. RHE / V
Fig. 2.
1.2
1.6
80
80
(a)
A
A0
A0 40
j / mA cm-2
40
j / mA cm-2
A
(b)
a2 a1 0
a2 a1 0
1 M NaOH + 0.05 M NaBH4 -40
1 M NaOH + 0.05 M NaBH4 -40
1 M NaOH
0.0
0.4
0.8
1.2
1 M NaOH
1.6
0.0
0.4
(c)
A
A0
1.2
1.6
E vs. RHE / V
E vs. RHE / V 80
0.8
80
(d)
A A0
PtCoB/Cu
60
j / mA cm-2
j / mA cm-2
40 a2 a1 0
1 M NaOH + 0.05 M NaBH4 -40
1 M NaOH
40
20
CoB/Cu Pt 9.8 gPt cm-2
0
10.6 gPt cm-2 14.4 gPt cm-2
-20 0.0
0.4
0.8
1.2
1.6
E vs. RHE / V
0.0
0.4
0.8
E vs. RHE / V
Fig. 3.
1.2
1.6
80
(a) CoB/Cu A0
(c)
-1
80
20 mVs 30 mVs-1 40 mVs-1 -1 50 mVs 60 mVs-1
60 40 20
for peak A
y=0.1546x+4.999 R2=0.9916
40
A
jp / mA cm-2
j / mA cm-2
y=0.1057x+61.871 2 R =0.9787
60
20 0 -2
100
for peak A0
(b) PtCoB/Cu (10.6 gPt cm ) A0
0 100
(d)
for peak A0 y=0.6918x+60.211 2 R =0.9988
90
80
A
60
80
40
70
20
for peak A y=0.4597x+44.275 R2=0.9827
60
0 0.0
0.4
0.8
1.2
1.6
10
20
30
40
v / mV s-1
E vs. RHE / V
Fig. 4
50
60
70
8.4 100
8.0 80
60
40 30 °C 40 °C 50 °C 60 °C 70 °C
20
ln k / ml min -1 g -1
H 2 generated volume / ml
(c)
(a) CoB/Cu
7.6 7.2 6.8 6.4
0
6.0 0
20
40
60
80
0.0029
0.0030
0.0031
0.0032
0.0033
0.0032
0.0033
T -1/ K-1
t / min 12.6 (b) Pt/CoB/Cu (14.4 gPt cm-2)
100
(d)
80
60
40
30 °C 40 °C 50 °C 60 °C 70 °C
20
0
ln k / ml min -1 g -1
H 2 generated volume / ml
12.4 12.2 12.0 11.8 11.6 11.4 11.2 0
20
40
60
80
0.0029
0.0030
0.0031
T -1/ K-1
t / min
Fig. 5
(a) 0.023 V
80
j / mA cm-2
70 60 50 40 30
-2
Pt/CoB/Cu (gPtcm ) 20
-2
Pt/CoB/Cu (gPtcm ) -2
10
Pt/CoB/Cu (gPtcm ) CoB/Cu
0 0
300
600
900
1200
1500
1200
1500
t/s (b) 1.123 V
40
j / mA cm-2
30
20
10
0 0
300
600
900
t/s
Fig. 6
E vs. RHE / V
-0.15 -0.16
CoB/Cu PtCoB/Cu (gPtcm-2)
-0.17
PtCoB/Cu (gPtcm )
-2 -2
PtCoB/Cu (gPtcm ) -0.18 -0.19 -0.20
0
30
60
90
t/s
Fig. 7
120
150
180