- Email: [email protected]
Novel carbon nanotube supported [email protected]@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method
Journal Pre-proof Novel carbon nanotube supported Co@Ag@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method Omer Faruk Er, Aykut Caglar, Berdan Ulas, Hilal Kivrak, Arif Kivrak PII:
S0254-0584(19)31237-4
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122422
Reference:
MAC 122422
To appear in:
Materials Chemistry and Physics
Received Date: 20 August 2019 Revised Date:
31 October 2019
Accepted Date: 7 November 2019
Please cite this article as: O.F. Er, A. Caglar, B. Ulas, H. Kivrak, A. Kivrak, Novel carbon nanotube supported Co@Ag@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122422. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Novel carbon nanotube supported Co@Ag@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method Omer Faruk Er1, Aykut Caglar1, Berdan Ulas1, Hilal Kivrak1*, Arif Kivrak2 1
Van Yuzuncu Yil University, Faculty of Engineering, Department of Chemical Engineering, Van 65000, Turkey 2
Van Yuzuncu Yıl University, Department of Chemistry, Faculty of Sciences, 65080 Van, Turkey
*E-mail: [email protected],
Acknowledgements Hilal Kivrak would like to thank for the financial support for The Scientific and Technological Research Council of Turkey TUBITAK projects (project no: 114M879 and 114M156).
ABSTRACT At present, monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts are prepared by NaBH4 co-reduction method. Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method. These catalysts are characterized via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The characterization results show that all catalysts are successfully synthesized at desired molar composition. Furthermore, Ag and Co addition change electronic state of catalyst. Formic acid electrooxidation activities of these catalysts are measured by cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) to investigate the effect of Ag and Co addition through the formic acid electrooxidation activity. Considering the bimetallic catalysts, Ag20@Pd80/CNT exhibits better catalytic activity than the one of other bimetallic catalysts. It is clear that the addition of Ag to Pd improves electrocatalytic activity due to synergetic effects. Furthermore, [email protected]@Pd79.98/CNT has the best activity, lowest charge transfer resistance (Rct), and a long term stability. EIS and CA results are in a good agreement with CV results. [email protected]@Pd79.98/CNT is a promising catalyst for Direct formic acid fuel cells.
Keywords: Ag; Pd; Co; formic acid; sequential reduction
1. INTRODUCTION As the world population grows rapidly, demand for energy is increasing. According to the United States Energy Information Administration (EIA), the demand for energy will increase about 48% up to 2040. Increased energy demand could be provided by fuel cells that have a significant place among renewable energy sources. Fuel cells are classified according to the electrolyte used, namely phosphoric acid fuel cell (PAFC), solid oxide fuel cells (SOFC), molten carbonate fuel cells (MCFC), alkaline fuel cells (AFC), and proton exchange membrane fuel cells (PEMFC) [1]. Among these fuel cells, PEMFC is advantageous due to easy installation, clean, high efficiency, zero emissions, better power density, and low cellular temperature. PAFC, SOFC, MCFC, and AFC have disadvantages such as low efficiency and power density, high operating temperature in fuel cells and high generation cost. PEMFC could be classified according to fuel as H2-PEM, methanol-PEM, ethanol-PEM, and formic acid-PEM [2]. PEMFC employ water as a by-product (temperature below > 100 °C) to obtain electricity from oxygen and hydrogen [3]. Recently, direct liquid fuel cells fed by H2, methanol, ethanol, and formic acid (FA) with proton exchange membrane were investigated by researchers due to their unique properties [4]. Among these fuel cells, the direct formic acid fuel cells (DFAFCs) are distinguished by their superior advantages due to (i) low toxicity, (ii) rapid oxidation kinetics, (iii) renewability, and (iv) high energy density at low temperatures. In addition, FA is harmless and nonflammable. Recently, research efforts have increased in the field of DFAFC. DFAFCs were studied extensively with Pt and its alloys. However, Pt has some disadvantages such as poisoning by CO. CO reduces the activity of the catalyst by binding with strong bonds to surface sites [5, 6]. Latest developments pointed out Pd have higher activity than Pt. During the electrooxidation of FA, CO2 is formed by dehydrogenation (FA
H2 + CO2) and CO is formed by
dehydration (FA
CO + H2O). CO quickly inhibits the Pt surface and hinders
dehydrogenation, whereas direct dehydrogenation acts on the Pd surface during FAEO. In a recent study, DFAFC was intensely studied with Pd and its alloys with the aim to increase the activity of the catalyst and to increase CO resistance of catalyst. For example, Tengyue et al. [7] reported that dendritic Pd is much more stable and has higher activity than commercial black Pd. In another study, the effect of Co addition to Pd through the FA oxidation was investigated. Considering the Pd-based bimetallic catalysts, second metal addition such as Cu [8], Ni [9], Pt [10], Au [11, 12], Ru [13], Bi [14], Mn [15], and Fe [16] was also investigated for FA oxidation to increase the activity and stability. Table 1 shows the maximum peak values of Pd and its alloys used in DFAFC compiled from the literature.
Table 1: Maximum peak values for FAEO compiled from literature Catalyst
Preparation
PdBi Nanodots Pd/MWCNT
Reproducible wet-chemical Method Ultrasonic to Form Uniform Catalyst ink Method Polyol Method In-situ Method Typical wet-chemical Method One-step Salt Reduction Method One-step Solvothermal Route Method One-pot Reaction Method Simple Hydrogen Co-reduction Process Method Two-step Chemical Reduction Process Method NaBH4 Reduction Method NaBH4 Reduction Method NaBH4 Reduction Method
Pd-Co/OMC Pd/C-S Pd1Bi1/NG PdNiCu/C Pd4Sn NCN PdBi0.05/rGO PdPtNi/MWCNT Ru@Pd/MWCNT PdAuCo/MWCN PdAg/C Pd/CNT
Maximum Peak mA*mg-1
Referenc e
1628.5 491.0
[14] [17]
185 1622 2025.5 792 850.47 1340 2230.5 1757 1630 467.4 974.80
[18] [19] [20] [21] [22] [23] [24] [25] [26] [1] [27]
Herein, monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts are prepared by NaBH4 co-reduction method. Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method. These all catalysts are characterized via XRD, XPS, and TEM. To investigate the effect of Ag and Co addition, FA oxidation activities of these catalysts are measured by CV, CA, and EIS.
2. EXPERIMENTAL 2.1. Materials and Equipments Potassium tetrachloropalladate (II) (K2PdCI4, 99.99%), silver nitrate (AgNO3, ≥99.0%), cobalt (II) nitrate hexahydrate ( Co(NO3)2 6H2O, 98%), formic acid (HCCOH, ≥95% ) sodium borohydride (NaBH4, >99%), carbon nanotube multi-walled (MWCNT, 98%) were purchased from Sigma-Aldrich and used as received. H2SO4 was supplied from Sigma-Aldrich. Nafion 117 solution (5%) was obtained from Aldrich. Potentiostat, Ag/AgCl reference electrode, and Pt wire electrodes were purchased from CH Instruments. Deionized water was distilled by water purification system (Milli-Q Water Purification System). All glassware were washed with acetone and copiously rinsed with distilled water.
2.2. Preparation of catalysts and working electrodes 2.2.1. Preparation of monometallic catalysts Pd/CNT catalyst was synthesized with NaBH4 reduction method. The metal loading on the carbon support was 10 wt%. Pd metal precursor (K2PdCI4) was dissolved in pure water and then CNT was added. These mixtures were stirred for two hours. NaBH4 was used for metal
reduction. After adding NaBH4, stirring was continued for half an hour and filtered. Finally, these catalysts were washed completely and dried at 85 oC for 12 hours.
2.2.2. Preparation of bimetallic catalysts by co-reduction method The bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts were prepared by NaBH4 co-reduction method. The Pd loading on the carbon support was 5 wt%. The calculated Pd, Co, and Ag precursors for desired Pd:Ag, Pd:Co, and Ag:Pd ratios were added to pure water and CNT was added. This mixture was stirred for 2 hours and NaBH4 was added for the reduction of metal precursors. After adding NaBH4, this mixture was stirred for a further half hour and filtered. After complete washing, this mixture was dried at 85 oC for 12 hours.
2.2.3. Preparation of bimetallic and trimetallic catalysts by sequential reduction method The bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag: Pd ratios were prepared by NaBH4 sequential-reduction method. To prepare Ag@Pd/CNT catalysts, Pd/CNT catalyst was prepared. Then, the calculated amount of Ag precursors for desired Ag:Pd ratios were added to pure water and Pd/CNT catalyst was added. Following this, mixture was stirred for 2 hours and NaBH4 was added for the reduction of metal precursors. After adding NaBH4, this mixture was stirred for a further half hour and filtered. After complete washing, this mixture was dried at 85 o
C for 12 hours.
Co@Ag@Pd/CNT catalysts were prepared by NaBH4 sequential-reduction method. Firstly, Pd/CNT catalyst was prepared and then Ag20@Pd80/CNT catalyst was prepared by sequential reduction method. Finally, necessary amount of Co precursor was dissolved in water and mixed
with Ag20@Pd80/CNT catalyst. Obtained mixture was reduced with NaBH4, filtered, washed and dried. 2.2.4 Preparation of working electrodes Glassy carbon electrode was polished by alumina before electrode preparation. 5 mg of catalyst was dispersed in 1 mL of Aldrich 5% Nafion solution. As a result, a catalyst ink was obtained. Following this, 5 µL of the catalyst ink was dropped on glassy carbon electrode. Finally, the electrode was dried at room temperature to remove the solvent.
2.3. Materials Characterization Techniques Pd/CNT, Ag20@Pd80/CNT, and [email protected]@Pd79.98/CNT catalysts were characterized by ICP-MS, XRD, XPS, TEM. The atomic molar ratios of the synthesized catalysts were checked by using ICP‐MS (Agilent 7800). XRD measurements of these catalysts were analyzed via using a PANalytical Empyrean device-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.54056 Å). Transmission electron microscopy (TEM) was run on a Hitachi HighTech HT7700 transmission electron microscope operating at 120 kV to obtain particle size and surface metal distribution. The distribution and particle size of these catalysts were defined using TEM. XPS analysis was performed with a Specs-Flex instrument to determine the oxidation state of these catalysts. 2.4. Electrochemical measurements CV, CA, and EIS measurements were performed in 0.5 M H2SO4 + 1 M FA solution to investigate the FA oxidation activities of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction, bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction,
Co@Ag@Pd/CNT catalysts prepared at varying Co:Ag:Pd atomic ratios. These measurements were performed by using CHI 660E electrochemical potentiostat in a three-electrode system. Monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts are prepared by NaBH4 co-reduction method. Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method. These all catalysts are characterized via XRD, XPS, and TEM. To investigate the effect of Ag and Co addition, FA oxidation activities of these catalysts are measured by CV, CA, and EIS. The working electrode was a glassy carbon electrode (diameter=3 mm) modified with catalyst sample. Pt wire and Ag/AgCl (3 M KCl) electrode were employed as counter electrode and reference electrode, respectively. FA oxidation activities of these catalysts were analyzed in CV at -0.23 V to 1 V potential range at 50 mV s-1 scan rate. Stability of these catalysts was investigated at 1000 s and -0.1 V via CA. The electrochemical resistance of these catalysts was obtained with EIS at about 316 kHz and 0.046 Hz to 5 mV amplitude at -0.1 V. 3. RESULTS AND DISCUSSION 3.1 Characterization Pd/CNT, Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT catalysts were characterized by ICP‐MS.
XRD,
XPS,
and
TEM.
Elemental
compositions
of
Pd/CNT
and
[email protected]@Pd79.98/CNT catalysts were determined by ICP-MS. The Ag:Pd and Co:Ag:Pd ratios of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT were found as 18.9:81.1 and 0.1: 22.3: 77.6. As can be seen from the ICP‐MS results, the metal ratios of these catalysts are very close
to
the
targeted
ratios.
XRD
patterns
of
Pd/CNT,
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT are given in Fig. 1. The diffraction patterns of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT resemble that of Pd/CNT. It can be reasonably explained by the greater Pd ratio compared to other used metals. The peak at 25.9° was clearly assigned to reflection
of
the
C
(0
0
2)
plane.
For
the
Pd/CNT,
Ag20@Pd80/CNT,
and
[email protected]@Pd79.98/CNT, the diffraction peaks at ca. 39o, 45o, 66o, and 79o were due to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of face-center cubic (fcc) Pd, respectively (JCPDS no. 46-1043). The peaks at ca. 43o for Pd/CNT, Ag20@Pd80/CNT, and [email protected]@Pd79.98/CNT could be attributed the PdO (1 1 0) facets [28]. Besides, a slight shift (0.1o) was observed for Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT due to electronic state change [26].
C (0 0 2) Pd (1 1 1)
PdO (1 1 0)
Pd (2 2 0) Pd (3 1 1)
Pd (2 0 0)
Intensity (a.u.)
Pd/CNT Ag20@Pd80/CNT [email protected]/CNT
0
20
40
60
80
100
2 Theta (deg.)
Figure 1. XRD patterns of Pd/CNT, Ag20@Pd80/CNT, and [email protected]@Pd79.98/CNT catalysts.
TEM images of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT catalysts are given in Figure 2. The morphology and particle size of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT were confirmed by using TEM. One could note that the majority of nanoparticles scatter uniformly on the
CNT
support
without
agglomeration.
Moreover,
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT catalysts were dispersed in a narrow scale in terms of their size. The average particle size of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT were found as 16 and 25 nm, respectively. TEM images of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT indicate that nanoparticles were well fastened to the exterior walls of CNT. XPS
was
used
to
determine
the
oxidation
state
of
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT catalysts. Binding energies of all peaks were determined by taking C1 s binding energy (284.6 eV) as reference. The core level spectra were displayed in Fig. 3. The deconvolution Co 2p survey of [email protected]@Pd79.98/CNT was not given due to excessive noise observed. It could be seen from the general survey of [email protected]@Pd79.98/CNT that C 1s, O 1s, Pd 3d, and Ag 3d spectra were detected. Pd 3d spectra of Ag20Pd80/CNT and [email protected]@Pd79.98/CNT showed that the main component of these catalysts was Pdo. The Pd 3d3/2 peaks of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT appeared at 341.0 and 340.7 eV, whereas Pd 3d5/2 peaks were at 335.5 and 335.4 eV. As can be seen from Table 2, Ag 3d3/2 and Ag 3d5/2 peak were revealed at the 374.1 and 368.0 eV for Ag20@Pd80/CNT. For [email protected]@Pd79.98/CNT, these were observed at 373.8 and 368.0 eV. The observed shift in Ag 3d3/2 with the addition of Co indicate that structures have a significant effect on the electronic state
of
the
catalysts.
The
main
components
for
both
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT were Ago and Pdo, respectively. However, it is thought that AgO, AgNO3,
PdO,
and
PdCl2
are
also
present
in
both
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT. This can be attributed to the inadequate reduction of Pd and Ag salts with the preparation method.
(b)
(a)
60
% Frequency
50 40 30 20 10 0 5
10
15
20
25
30
35
Particle size (nm)
(c )
(d)
25
% Frequency
20
15
10
5
0 10
20
30
40
50
Particle size (nm)
Figure 2. (a, c)TEM images and particle size distribution histograms of (b, d) Ag20 @Pd80/CNT and [email protected]@Pd79.98/CNT, respectively .
40
400
a)
600
Pd 3d 5/2
Ag 3d5/2
b)
300
Pd 3d 3/2
Intensity (a.u.)
Intensity (a.u.)
500
350
400 300 200
250
Ag 3d3/2
200 150 100
100
50
0
0
-100 348 346 344 342 340 338 336 334 332 330 328
-50 376
374
372
Binding Energy (eV) 100000
c) 80000
40000 Pd 3d Ag 3d
20000
O 1s
368
366
364
Pd 3d5/2 Pd 3d3/2
800
Intensity (a.u.)
Intensity ( a.u.)
d)
1000
C 1s
60000
370
Binding Energy (eV)
Co 2p
600 400 200 0
0 -200
0
200
400
600
800
1000 1200 1400
Binding Energy (eV)
700
e)
-200 346
344
342
340
338
336
334
332
330
328
Binding Energy (eV)
Ag 3d5/2
600
Intensity (a.u.)
500 400
Ag 3d3/2
300 200 100 0 -100 380 378 376 374 372 370 368 366 364 362 360
Binding Energy (eV)
Figure 3. XPS spectra of a) Pd 3d, and b) Ag 3d for Ag20 @Pd80/CNT and XPS spectra of c) general survey, d) Pd 3d, and e) Ag 3d of [email protected]@Pd79.98/CNT catalysts.
Table 2. Pd 3d and Ag 3d binding energy of Ag20 @Pd80/CNT and [email protected]@Pd79.98/CNT catalysts Catalysts
Species
Pd 3d Ag20 @Pd80 Ag 3d
Pd 3d [email protected] @Pd79.98 Ag 3d
Binding Energy (eV) 335.5 337.6 341.0 343.7 366.8 369.6 368.0 373.1 374.1 335.4 336.9 340.7 342.1 368.0 369.7 373.8
Possible Chemical State Pd PdCl2 Pd PdO AgO AgNO3 Ag AgO2 Ag Pd PdO Pd PdO Ag AgNO3 Ag
Relative Intensity (%) 41.6 13.7 33.6 11.1 13 4.5 49.1 8.9 24.5 37.5 15.8 33.8 12.9 53.6 13.0 33.4
References
[29] [30] [31] [32] [33] [34] [35] [36] [37] [29] [31] [31] [31] [38] [34] [39]
.
3.2. Electrochemical Evaluation CV, CA, and EIS measurements were performed in 0.5 M H2SO4 + 1 M FA solution to investigate the FA oxidation activities of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction, bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction, Co@Ag@Pd/CNT catalysts prepared at varying Co:Ag:Pd atomic ratios. Firstly, FA oxidation were taken by CV in 0.5 M H2SO4 + 1 M FA solution at -0.23 V to 1.0 V potential and a scan rate of 50 mV s-1. Fig. 4 shows electro-oxidation measurements of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction,
bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction. Table 3 shows forward, reverse, and maximum current densities of the peaks obtained from the CV. Monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction, bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction were tested in 0.5 M H2SO4 and 0.5 M H2SO4 + 1 M FA solution, respectively (Fig. 4). When using a second metal together with Pd, the current density is clearly observed to increase. The catalyst was observed to have an increase in the current density as the Ag ratio decreased (Fig.4 and Table 3). As the atomic ratio of Ag increases, it leads to agglomeration of Ag atoms on the Pd-active site on the catalyst surface, thus reducing the synergistic effect between Pd and Ag [1]. The highest performance among the bimetallic was for Ag20 @Pd80/CNT catalysts (892.81 mA/mg Pd respectively) (Table 3). Due to the fact that the current intensities are close to each other, the If/Ir ratios of these catalysts were examined. In terms of current densities, forward current (If) and reverse current (Ir), the highest performance among the bimetallics was obtained from Ag20 @Pd80/CNT.
Pd Pd50Ag50
60
Pd50Co50
40
a
120
b
80
Pd50@Ag50
Current ( mA/mg Pd )
Current ( mA/mg Pd )
80
Ag50@Pd50
20 0 -20
40 0 Ag90@Pd10 Ag80@Pd20
-40
Ag70@Pd30 Ag60@Pd40 Ag50@Pd50
-80
Ag10@Pd90 Ag20@Pd80
-40
-120
-60
-160
Ag30@Pd70 Ag40@Pd60 Pd50@Ag50 Pd
-0,2
-80 -0,4
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Potential ( V vs Ag/AgCI )
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
Potential ( Vvs Ag/AgCI )
a
700
Pd50Co50
500
Pd50@Ag50 400
Ag50@Pd50
300 200
Ag80@Pd20
800
Current ( mA/mg Pd )
Current ( mA/mg Pd )
Ag90@Pd10
Pd Pd50Ag50
600
d
1000
Ag70@Pd30 Ag60@Pd40 Ag50@Pd50
600
Ag10@Pd90 Ag20@Pd80 Ag30@Pd70 Ag40@Pd60
400
Pd50@Ag50 Pd
200
100
0 0
-200 -0,4
-100 -0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
Potential ( V vs Ag/AgCI )
Potential ( V vs Ag/AgCI )
Figure 4. Cyclic voltammograms of a) Pd/CNT, Pd50Ag50/CNT, Pd50Co50/CNT, Pd50@Ag50/CNT, and Ag50@Pd50/CNT catalysts and b) Ag@Pd/CNT catalysts at varying Ag:Pd atomic ratios in 1 M H2SO4 solution at 50 mV s-1 scan rate; Cyclic voltammograms of c) Pd/CNT, Pd50Ag50/CNT, Pd50Co50/CNT, Pd50@Ag50/CNT, and Ag50@Pd50/CNT catalysts and b) Ag@Pd/CNT catalysts at varying Ag:Pd atomic ratios in 1 M H2SO4 + 0.5 M FA solution at 50 mV s-1 scan rate
Table 3. Measured peaks for CV results of Pd/CNT, Pd50Ag50/CNT, Pd50Co50/CNT, Pd50@Ag50/CNT, and Ag@Pd/CNT catalysts at varying Ag:Pd atomic ratios CNT supported Catalysts Pd
Ef (V)
0.19
Forward Peak If (mA/mg Pd) 426.23
Er (V)
0.16
Reverse Peak Ir (mA/mg Pd)
If/Ir
Peak Current Density (mA/mg Pd)
710.89
0.60
404.66
Pd50Ag50
0.14
619.81
0.19
554.31
1.12
599.02
Pd50Co50
0.10
221.12
0.14
226.68
0.98
194.49
Pd50@Ag50
0.16
682.60
0.21
607.24
1.12
649.90
Ag50@Pd50
0.19
656.17
0.23
502.47
1.31
630.78
Ag90@Pd10
0.16
116.54
0.20
103.14
1.13
81.46
Ag80@Pd20
0.16
203.08
0.23
282.09
0.72
172.33
Ag70@Pd30
0.15
58.20
0.30
45.49
1.28
25.96
Ag60@Pd40
0.17
241.37
0.25
189.16
1.28
210.74
Ag40@Pd60
0.20
472.89
0.26
509.04
0.93
449.58
Ag30@Pd70
0.20
837.97
0.23
844.77
0.99
794.94
Ag20@Pd80
0.22
921.57
0.25
943.81
0.98
892.81
Ag10@Pd90
0.20
933.33
0.21
1038.54
0.90
910.84
Additionally, trimetallic Co@Ag@Pd/CNT catalysts were synthesized on Ag20@Pd80/CNT catalyst which exhibited the highest performance among the bimetallics for FA oxidation. Co@Ag@Pd/CNT catalysts were synthesized by sequential reduction using the NaBH4 reduction method. Firstly, 0.5 M H2SO4 measurements, then FA oxidation measurements were taken by CV in 0.5 M H2SO4 + 1 M FA solutions between -0.23 V-1 V potential at a scan rate of 50 mVs-1. Table 4, Fig. 5, and Fig. 5 show the current density and values obtained from the CV. Fig. 5 shows the Co addition reduced the difference between adsorption/desorption in FA oxidation with Pd. It is evident that the Ag20@Pd80/CNT catalyst causes aggregation around the
active sites as the amount of Co increases, leading to a decrease in the current density values (Table 4). As the amount of Co increases, the transfer of electrons (synergistic effect) [1] between Pd/Ag is inhibited and this causes the reduction peak to be higher than the oxidation peak in FA oxidation (Fig. 8a). The highest current density was obtained for [email protected] @Pd79.98/CNT catalyst and was measured as 931.29 mA/mg Pd. This value indicates that a low amount of Co is more effective for FA oxidation.
60
80
a
60
20 0 [email protected]@Pd79.99 [email protected]@Pd79.98
-20
[email protected]@Pd79.85 [email protected]@Pd79.49
-40
[email protected]@Pd69.9 [email protected]@Pd46.50 [email protected]@Pd32.72
-60
[email protected]@Pd20.56
Current ( mA/mg Pd )
Current ( mA/mg Pd )
40
Pd Ag20@Pd80
b
[email protected]@Pd79.98
40 20 0 -20 -40
[email protected]@Pd11.81 [email protected]@Pd9.73
-80
[email protected]@Pd7.20
-0,4
-0,2
0,0
0,2
0,4
0,6
Potential ( V vs Ag/AgCI )
0,8
1,0
-60 -0,4
-0,2
0,0
0,2
0,4
0,6
Potential ( V vs Ag/AgCI )
0,8
1,0
1000
c
1000
[email protected]@Pd79.99
Pd Ag20@Pd80
d
[email protected]@Pd79.98
[email protected]@Pd79.85
800
800
[email protected]@Pd79.49 [email protected]@Pd69.9 [email protected]@Pd46.50 [email protected]@Pd32.72
600
[email protected]@Pd20.56 [email protected]@Pd11.81 [email protected]@Pd9.73
400
[email protected]@Pd7.20
200
Current ( mA/mg Pd )
Current ( mA/mg Pd )
[email protected]@Pd79.98
600
400
200
0
0 -0,4
-0,2
0,0
0,2
0,4
0,6
0,8
-0,2
1,0
0,0
0,2
0,4
0,6
0,8
Potential ( V vs Ag/AgCI )
Potential ( Vvs Ag/AgCI )
Figure 5. Cyclic voltammograms of a) Co@Ag@Pd/CNT and b) Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in 1 M H2SO4 solution at 50 mV s-1 scan rate, c) Cyclic voltammograms of Co@Ag@Pd/CNT and b) Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in 1 M H2SO4 + 0.5 M FA at 50 mV s-1 scan rate
Table 4. Measured peak values in CV results on 10% Ag@[email protected]/CNT catalysts Forward Peak Ef (V) Reverse Peak Er (V) Peak Current Density Catalysts If/Ir If (mA/mg Ir (mA/mg Pd) (mA/mg Pd) Pd) 0.22 0.24 [email protected] @Pd79.99 800.90 759.49 1.05 781.35 [email protected] @Pd79.98 [email protected] @Pd79.85 [email protected] @Pd79.49 [email protected] @Pd69.90 [email protected] @Pd46.5 [email protected] @Pd32.72 [email protected] @Pd20.56
961.85 756.26 753.05 715.95 373.60 394.89 655.65
0.20 0.20 0.24 0.23 0.20 0.17 0.20
869.83 750.39 749.85 651.74 363.46 320.11 560.61
0.22 0.23 0.26 0.25 0.23 0.22 0.25
1.11
931.29
1.01
732.75
1.004
738.45
1.10
694.32
1.03
355.97
1.23
369.97
1.17
629.33
1,0
0.14
99.65
[email protected] @Pd11.81 [email protected] @Pd9.73
0.12
50.52
0.32
60.64
0.31
27.89
1.64
70.48
1.81
29.24
Stability of Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts was measured by CA at 0.1 V. Fig. 6a shows the CA results for Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT catalysts measured in 0.5 M H2SO4 + 1 M FA . Current values were normalized by Pd amount and given Fig. 6a. Furthermore, for the stability evaluation, the retention of current value of 1000 s to the initial value was used by normalizing chronomaperommogram by dividing current values to initial current values and given in Figure 6b. Figure 6 a and Figure 6b clearly shows that Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT have higher stability than Pd/CNT catalyst.
a
1,1
(b)
Pd Ag20@Pd80
280
[email protected]@Pd79.98
240
Relative Current
Current (mA/mg Pd)
320
200 160 120 80
1,0
Pd Ag20@Pd80
0,9
[email protected]@Pd79.98
0,8 0,7 0,6 0,5 0,4
40
0,3
0
200
400
600
Time (seconds)
800
1000
0,2 0
200
400
600
800
1000
Time (s)
Figure 6. (a) Chronoamperomograms of ) Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in solution of 0.5 M H2SO4 + 1 M FA at -0.1 V and (b) Chronoamperomograms at normalized currents of Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in solution of 0.5 M H2SO4 + 1 M FA at -0.1 V
EIS results were measured on Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in 0.5 M H2SO4 + 1 M FA. Figure 7 shows the Nyquist Plot data plotted according to the data from the EIS results. Nyquist Plot is known as semicircles and the diameter of these semicircles gives information about the electrocatalytic activity of the catalyst employed and its transfer resistance [7]. Accordingly, when the semicircle diameter diminishes, one could observe that electron transfer resistance decrease and the electro-catalytic activity of the catalyst increases. Figure 7 shows that the transfer resistance load is in the order Pd/CNT > Ag20@Pd80/CNT > [email protected]@Pd79.98/CNT. As a result, [email protected]@Pd79.98/CNT catalyst was found to have the highest electro-catalytic activity, which is agreement with CV and CA results.
Pd Ag20@Pd80
10000
[email protected]@Pd79.98
-Z'' / ohm
8000
6000
4000
2000
0 0
2000
4000
6000
8000
10000
12000
Z' / ohm
Figure 7. Nyquist graphs of Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts at -0.1 V in 0.5 M H2SO4 + 1 M FA.
4. CONCLUSION In conclusion, the study of synthesis of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT via NaBH4 co-reduction method and Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method, their characterization and eployment in FA oxidation led to the following conclusions and insights: •
Monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT, and Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios could be easily prepared from the co-reduction and sequential reduction of corresponding palladium, silver and cobalt precursors by NaBH4 reduction method.
•
Characterization results revealed that Co and Ag addition changes the surface electronic state.
•
FA oxidation measurements revealed that Co and Ag addition improves the electrochemical activity. This phenomenon could be attributed to the electronic state change of catalysts by the addition of second or third metal.
•
The activity of Ag50Pd50/CNT catalyst prepared via NaBH4 sequential -reduction method is gretare than the activity of Ag50Pd50/CNT via NaBH4 co-reduction method. One could note preparation order improves electrochemical activity.
•
The addition of Co improves the electrochemical activity of Ag19.98Pd79.98Co0.072/CNT catalyst. Further increase in Co loading leads to decrease in the electrocatalytic activity due to aggregation and blocking of the surface active sites. Ag19.98Pd79.98Co0.072/CNT catalyst has a great potential as an alternative fuel cell anode catalyst.
Acknowledgments The CHI 660E potentiostat employed in electrochemical measurements was purchased from the Scientific and Technological Research Council of Turkey (TUBITAK) project (project no: TUBITAK 113Z249). The chemicals were purchased from Van Yuzuncu Yıl BAP project (project no: FBA-2018-7152). Characterization measurements were also purchased from Van Yuzuncu Yıl BAP project (project no: FBA-2018-7152).These characterization measurements as ICP-MS, XRD, and TEM were performed at DAYTAM.
REFERENCES [1] B. Ulas, A. Caglar, O. Sahin, H. Kivrak, Composition dependent activity of PdAgNi alloy catalysts for formic acid electrooxidation, Journal of colloid and interface science, 532 (2018) 47-57. [2] S.J. Peighambardoust, S. Rowshanzamir, M. Amjadi, Review of the proton exchange membranes for fuel cell applications, International journal of hydrogen energy, 35 (2010) 9349-9384. [3] A. Hermann, T. Chaudhuri, P. Spagnol, Bipolar plates for PEM fuel cells: A review, International journal of hydrogen Energy, 30 (2005) 1297-1302. [4] W. Qian, D.P. Wilkinson, J. Shen, H. Wang, J. Zhang, Architecture for portable direct liquid fuel cells, Journal of power sources, 154 (2006) 202-213. [5] Z. Liu, L. Hong, M.P. Tham, T.H. Lim, H. Jiang, Nanostructured Pt/C and Pd/C catalysts for direct formic acid fuel cells, Journal of Power Sources, 161 (2006) 831-835. [6] P. Hong, F. Luo, S. Liao, J. Zeng, Effects of Pt/C, Pd/C and PdPt/C anode catalysts on the performance and stability of air breathing direct formic acid fuel cells, International journal of hydrogen energy, 36 (2011) 8518-8524. [7] T. Ma, C. Li, T. Liu, Q. Yuan, Size-controllable synthesis of dendritic Pd nanocrystals as improved electrocatalysts for formic acid fuel cells’ application, Journal of Saudi Chemical Society, (2018). [8] V. Mazumder, M. Chi, M.N. Mankin, Y. Liu, O.n. Metin, D. Sun, K.L. More, S. Sun, A facile synthesis of MPd (M= Co, Cu) nanoparticles and their catalysis for formic acid oxidation, Nano letters, 12 (2012) 1102-1106. [9] Y. Jin, J. Zhao, F. Li, W. Jia, D. Liang, H. Chen, R. Li, J. Hu, J. Ni, T. Wu, Nitrogen-doped graphene supported palladium-nickel nanoparticles with enhanced catalytic performance for formic acid oxidation, Electrochimica Acta, 220 (2016) 83-90. [10] B. Gralec, A. Lewera, Catalytic activity of unsupported Pd-Pt nanoalloys with low Pt content towards formic acid oxidation, Applied Catalysis B: Environmental, 192 (2016) 304-310. [11] Y. Suo, I.-M. Hsing, Synthesis of bimetallic PdAu nanoparticles for formic acid oxidation, Electrochimica Acta, 56 (2011) 2174-2183. [12] G. Zhang, Y. Wang, X. Wang, Y. Chen, Y. Zhou, Y. Tang, L. Lu, J. Bao, T. Lu, Preparation of Pd–Au/C catalysts with different alloying degree and their electrocatalytic performance for formic acid oxidation, Applied Catalysis B: Environmental, 102 (2011) 614-619.
[13] E. Hasheminejad, R. Ojani, J.B. Raoof, A rapid synthesis of high surface area PdRu nanosponges: Composition-dependent electrocatalytic activity for formic acid oxidation, Journal of energy chemistry, 26 (2017) 703-711. [14] H. Xu, K. Zhang, B. Yan, J. Wang, C. Wang, S. Li, Z. Gu, Y. Du, P. Yang, Ultra-uniform PdBi nanodots with high activity towards formic acid oxidation, Journal of Power Sources, 356 (2017) 27-35. [15] M.A. Matin, J.-H. Jang, Y.-U. Kwon, PdM nanoparticles (M= Ni, Co, Fe, Mn) with high activity and stability in formic acid oxidation synthesized by sonochemical reactions, Journal of Power Sources, 262 (2014) 356-363. [16] Y. Zhou, C. Du, G. Han, Y. Gao, G. Yin, Ultra-low Pt decorated PdFe alloy nanoparticles for formic acid electro-oxidation, Electrochimica Acta, 217 (2016) 203-209. [17] J. Chang, S. Li, L. Feng, X. Qin, G. Shao, Effect of carbon material on Pd catalyst for formic acid electrooxidation reaction, Journal of Power Sources, 266 (2014) 481-487. [18] E. Cazares-Avila, E. Ruiz-Ruiz, A. Hernandez-Ramirez, F. Rodriguez-Varela, M. Morales-Acosta, D. Morales-Acosta, Effect of OMC and MWNTC support on mass activity of PdCo catalyst for formic acid electro-oxidation, International Journal of Hydrogen Energy, 42 (2017) 30349-30358. [19] S. Yao, G. Li, C. Liu, W. Xing, Enhanced catalytic performance of carbon supported palladium nanoparticles by in-situ synthesis for formic acid electrooxidation, Journal of Power Sources, 284 (2015) 355-360. [20] H. Xu, B. Yan, K. Zhang, J. Wang, S. Li, C. Wang, Y. Du, P. Yang, S. Jiang, S. Song, N-doped graphenesupported binary PdBi networks for formic acid oxidation, Applied Surface Science, 416 (2017) 191-199. [21] S. Hu, F. Munoz, J. Noborikawa, J. Haan, L. Scudiero, S. Ha, Carbon supported Pd-based bimetallic and trimetallic catalyst for formic acid electrochemical oxidation, Applied Catalysis B: Environmental, 180 (2016) 758-765. [22] Y. Gong, X. Liu, Y. Gong, D. Wu, B. Xu, L. Bi, L.Y. Zhang, X. Zhao, Synthesis of defect-rich palladiumtin alloy nanochain networks for formic acid oxidation, Journal of colloid and interface science, 530 (2018) 189-195. [23] R. Liu, F. Liu, D. Fu, Y. Bai, G. Han, Y. Tian, M. Li, Y. Xiao, Y. Li, One-pot synthesis of PdBi/reduced graphene oxide catalyst under microwave irradiation used for formic acid electrooxidation, Catalysis Communications, 46 (2014) 146-149. [24] J.-M. Zhang, R.-X. Wang, R.-J. Nong, Y. Li, X.-J. Zhang, P.-Y. Zhang, Y.-J. Fan, Hydrogen co-reduction synthesis of PdPtNi alloy nanoparticles on carbon nanotubes as enhanced catalyst for formic acid electrooxidation, International Journal of Hydrogen Energy, 42 (2017) 7226-7234. [25] X.-J. Zhang, J.-M. Zhang, P.-Y. Zhang, Y. Li, S. Xiang, H.-G. Tang, Y.-J. Fan, Highly active carbon nanotube-supported Ru@ Pd core-shell nanostructure as an efficient electrocatalyst toward ethanol and formic acid oxidation, Molecular Catalysis, 436 (2017) 138-144. [26] H. Kivrak, D. Atbas, O. Alal, M.S. Çögenli, A. Bayrakceken, S.O. Mert, O. Sahin, A complementary study on novel PdAuCo catalysts: Synthesis, characterization, direct formic acid fuel cell application, and exergy analysis, International Journal of Hydrogen Energy, (2018). [27] A. Caglar, T. Sahan, M.S. Cogenli, A.B. Yurtcan, N. Aktas, H. Kivrak, A novel Central Composite Design based response surface methodology optimization study for the synthesis of Pd/CNT direct formic acid fuel cell anode catalyst, International Journal of Hydrogen Energy, (2018). [28] J. Lin, T. Mei, M. Lv, C.a. Zhang, Z. Zhao, X. Wang, Size-controlled PdO/graphene oxides and their reduction products with high catalytic activity, RSC Advances, 4 (2014) 29563-29570.
•
Co@Ag@Pd/CNT catalysts were successfully synthesized via NaBH4 sequential reduction method
•
Co@Ag@Pd/CNT catalysts has high formic acid electrooxidation activity
•
Co and Ag promotion to Pd enhances the formic acid electrooxidation activity.
Cover Letter
April 2019 Dear editor Enclosed please find the research article manuscript entitled Novel carbon nanotube supported Co@Ag@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method co-authored by Omer Faruk Er, Aykut Caglar, Berdan Ulas, Hilal
Kivrak, Arif Kivrak submitted for publication in International Journal of environmental science and technology. I, Hilal Kivrak, will be handling the correspondence about the article. The authors confirm that this submitted manuscript has not been published previously that it is not under consideration for publication elsewhere. This work described has not been published previously (except in the form of an abstract or as part of a published lecture or academic thesis), that it is not under consideration for publication elsewhere, that its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out. Submission also implies that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, without the written consent of the Publisher. Thanking in advance, for your and the reviewers efforts for the evaluation of the manuscript. Sincerely yours; Assoc. Prof. Dr. Hilal DEMİR KIVRAK Chemical Engineering Department Yuzuncu Yıl University Van, Turkey : e-mail: [email protected]
S0254-0584(19)31237-4
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122422
Reference:
MAC 122422
To appear in:
Materials Chemistry and Physics
Received Date: 20 August 2019 Revised Date:
31 October 2019
Accepted Date: 7 November 2019
Please cite this article as: O.F. Er, A. Caglar, B. Ulas, H. Kivrak, A. Kivrak, Novel carbon nanotube supported Co@Ag@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/ j.matchemphys.2019.122422. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
Novel carbon nanotube supported Co@Ag@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method Omer Faruk Er1, Aykut Caglar1, Berdan Ulas1, Hilal Kivrak1*, Arif Kivrak2 1
Van Yuzuncu Yil University, Faculty of Engineering, Department of Chemical Engineering, Van 65000, Turkey 2
Van Yuzuncu Yıl University, Department of Chemistry, Faculty of Sciences, 65080 Van, Turkey
*E-mail: [email protected],
Acknowledgements Hilal Kivrak would like to thank for the financial support for The Scientific and Technological Research Council of Turkey TUBITAK projects (project no: 114M879 and 114M156).
ABSTRACT At present, monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts are prepared by NaBH4 co-reduction method. Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method. These catalysts are characterized via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The characterization results show that all catalysts are successfully synthesized at desired molar composition. Furthermore, Ag and Co addition change electronic state of catalyst. Formic acid electrooxidation activities of these catalysts are measured by cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) to investigate the effect of Ag and Co addition through the formic acid electrooxidation activity. Considering the bimetallic catalysts, Ag20@Pd80/CNT exhibits better catalytic activity than the one of other bimetallic catalysts. It is clear that the addition of Ag to Pd improves electrocatalytic activity due to synergetic effects. Furthermore, [email protected]@Pd79.98/CNT has the best activity, lowest charge transfer resistance (Rct), and a long term stability. EIS and CA results are in a good agreement with CV results. [email protected]@Pd79.98/CNT is a promising catalyst for Direct formic acid fuel cells.
Keywords: Ag; Pd; Co; formic acid; sequential reduction
1. INTRODUCTION As the world population grows rapidly, demand for energy is increasing. According to the United States Energy Information Administration (EIA), the demand for energy will increase about 48% up to 2040. Increased energy demand could be provided by fuel cells that have a significant place among renewable energy sources. Fuel cells are classified according to the electrolyte used, namely phosphoric acid fuel cell (PAFC), solid oxide fuel cells (SOFC), molten carbonate fuel cells (MCFC), alkaline fuel cells (AFC), and proton exchange membrane fuel cells (PEMFC) [1]. Among these fuel cells, PEMFC is advantageous due to easy installation, clean, high efficiency, zero emissions, better power density, and low cellular temperature. PAFC, SOFC, MCFC, and AFC have disadvantages such as low efficiency and power density, high operating temperature in fuel cells and high generation cost. PEMFC could be classified according to fuel as H2-PEM, methanol-PEM, ethanol-PEM, and formic acid-PEM [2]. PEMFC employ water as a by-product (temperature below > 100 °C) to obtain electricity from oxygen and hydrogen [3]. Recently, direct liquid fuel cells fed by H2, methanol, ethanol, and formic acid (FA) with proton exchange membrane were investigated by researchers due to their unique properties [4]. Among these fuel cells, the direct formic acid fuel cells (DFAFCs) are distinguished by their superior advantages due to (i) low toxicity, (ii) rapid oxidation kinetics, (iii) renewability, and (iv) high energy density at low temperatures. In addition, FA is harmless and nonflammable. Recently, research efforts have increased in the field of DFAFC. DFAFCs were studied extensively with Pt and its alloys. However, Pt has some disadvantages such as poisoning by CO. CO reduces the activity of the catalyst by binding with strong bonds to surface sites [5, 6]. Latest developments pointed out Pd have higher activity than Pt. During the electrooxidation of FA, CO2 is formed by dehydrogenation (FA
H2 + CO2) and CO is formed by
dehydration (FA
CO + H2O). CO quickly inhibits the Pt surface and hinders
dehydrogenation, whereas direct dehydrogenation acts on the Pd surface during FAEO. In a recent study, DFAFC was intensely studied with Pd and its alloys with the aim to increase the activity of the catalyst and to increase CO resistance of catalyst. For example, Tengyue et al. [7] reported that dendritic Pd is much more stable and has higher activity than commercial black Pd. In another study, the effect of Co addition to Pd through the FA oxidation was investigated. Considering the Pd-based bimetallic catalysts, second metal addition such as Cu [8], Ni [9], Pt [10], Au [11, 12], Ru [13], Bi [14], Mn [15], and Fe [16] was also investigated for FA oxidation to increase the activity and stability. Table 1 shows the maximum peak values of Pd and its alloys used in DFAFC compiled from the literature.
Table 1: Maximum peak values for FAEO compiled from literature Catalyst
Preparation
PdBi Nanodots Pd/MWCNT
Reproducible wet-chemical Method Ultrasonic to Form Uniform Catalyst ink Method Polyol Method In-situ Method Typical wet-chemical Method One-step Salt Reduction Method One-step Solvothermal Route Method One-pot Reaction Method Simple Hydrogen Co-reduction Process Method Two-step Chemical Reduction Process Method NaBH4 Reduction Method NaBH4 Reduction Method NaBH4 Reduction Method
Pd-Co/OMC Pd/C-S Pd1Bi1/NG PdNiCu/C Pd4Sn NCN PdBi0.05/rGO PdPtNi/MWCNT Ru@Pd/MWCNT PdAuCo/MWCN PdAg/C Pd/CNT
Maximum Peak mA*mg-1
Referenc e
1628.5 491.0
[14] [17]
185 1622 2025.5 792 850.47 1340 2230.5 1757 1630 467.4 974.80
[18] [19] [20] [21] [22] [23] [24] [25] [26] [1] [27]
Herein, monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts are prepared by NaBH4 co-reduction method. Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method. These all catalysts are characterized via XRD, XPS, and TEM. To investigate the effect of Ag and Co addition, FA oxidation activities of these catalysts are measured by CV, CA, and EIS.
2. EXPERIMENTAL 2.1. Materials and Equipments Potassium tetrachloropalladate (II) (K2PdCI4, 99.99%), silver nitrate (AgNO3, ≥99.0%), cobalt (II) nitrate hexahydrate ( Co(NO3)2 6H2O, 98%), formic acid (HCCOH, ≥95% ) sodium borohydride (NaBH4, >99%), carbon nanotube multi-walled (MWCNT, 98%) were purchased from Sigma-Aldrich and used as received. H2SO4 was supplied from Sigma-Aldrich. Nafion 117 solution (5%) was obtained from Aldrich. Potentiostat, Ag/AgCl reference electrode, and Pt wire electrodes were purchased from CH Instruments. Deionized water was distilled by water purification system (Milli-Q Water Purification System). All glassware were washed with acetone and copiously rinsed with distilled water.
2.2. Preparation of catalysts and working electrodes 2.2.1. Preparation of monometallic catalysts Pd/CNT catalyst was synthesized with NaBH4 reduction method. The metal loading on the carbon support was 10 wt%. Pd metal precursor (K2PdCI4) was dissolved in pure water and then CNT was added. These mixtures were stirred for two hours. NaBH4 was used for metal
reduction. After adding NaBH4, stirring was continued for half an hour and filtered. Finally, these catalysts were washed completely and dried at 85 oC for 12 hours.
2.2.2. Preparation of bimetallic catalysts by co-reduction method The bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts were prepared by NaBH4 co-reduction method. The Pd loading on the carbon support was 5 wt%. The calculated Pd, Co, and Ag precursors for desired Pd:Ag, Pd:Co, and Ag:Pd ratios were added to pure water and CNT was added. This mixture was stirred for 2 hours and NaBH4 was added for the reduction of metal precursors. After adding NaBH4, this mixture was stirred for a further half hour and filtered. After complete washing, this mixture was dried at 85 oC for 12 hours.
2.2.3. Preparation of bimetallic and trimetallic catalysts by sequential reduction method The bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag: Pd ratios were prepared by NaBH4 sequential-reduction method. To prepare Ag@Pd/CNT catalysts, Pd/CNT catalyst was prepared. Then, the calculated amount of Ag precursors for desired Ag:Pd ratios were added to pure water and Pd/CNT catalyst was added. Following this, mixture was stirred for 2 hours and NaBH4 was added for the reduction of metal precursors. After adding NaBH4, this mixture was stirred for a further half hour and filtered. After complete washing, this mixture was dried at 85 o
C for 12 hours.
Co@Ag@Pd/CNT catalysts were prepared by NaBH4 sequential-reduction method. Firstly, Pd/CNT catalyst was prepared and then Ag20@Pd80/CNT catalyst was prepared by sequential reduction method. Finally, necessary amount of Co precursor was dissolved in water and mixed
with Ag20@Pd80/CNT catalyst. Obtained mixture was reduced with NaBH4, filtered, washed and dried. 2.2.4 Preparation of working electrodes Glassy carbon electrode was polished by alumina before electrode preparation. 5 mg of catalyst was dispersed in 1 mL of Aldrich 5% Nafion solution. As a result, a catalyst ink was obtained. Following this, 5 µL of the catalyst ink was dropped on glassy carbon electrode. Finally, the electrode was dried at room temperature to remove the solvent.
2.3. Materials Characterization Techniques Pd/CNT, Ag20@Pd80/CNT, and [email protected]@Pd79.98/CNT catalysts were characterized by ICP-MS, XRD, XPS, TEM. The atomic molar ratios of the synthesized catalysts were checked by using ICP‐MS (Agilent 7800). XRD measurements of these catalysts were analyzed via using a PANalytical Empyrean device-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.54056 Å). Transmission electron microscopy (TEM) was run on a Hitachi HighTech HT7700 transmission electron microscope operating at 120 kV to obtain particle size and surface metal distribution. The distribution and particle size of these catalysts were defined using TEM. XPS analysis was performed with a Specs-Flex instrument to determine the oxidation state of these catalysts. 2.4. Electrochemical measurements CV, CA, and EIS measurements were performed in 0.5 M H2SO4 + 1 M FA solution to investigate the FA oxidation activities of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction, bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction,
Co@Ag@Pd/CNT catalysts prepared at varying Co:Ag:Pd atomic ratios. These measurements were performed by using CHI 660E electrochemical potentiostat in a three-electrode system. Monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts are prepared by NaBH4 co-reduction method. Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method. These all catalysts are characterized via XRD, XPS, and TEM. To investigate the effect of Ag and Co addition, FA oxidation activities of these catalysts are measured by CV, CA, and EIS. The working electrode was a glassy carbon electrode (diameter=3 mm) modified with catalyst sample. Pt wire and Ag/AgCl (3 M KCl) electrode were employed as counter electrode and reference electrode, respectively. FA oxidation activities of these catalysts were analyzed in CV at -0.23 V to 1 V potential range at 50 mV s-1 scan rate. Stability of these catalysts was investigated at 1000 s and -0.1 V via CA. The electrochemical resistance of these catalysts was obtained with EIS at about 316 kHz and 0.046 Hz to 5 mV amplitude at -0.1 V. 3. RESULTS AND DISCUSSION 3.1 Characterization Pd/CNT, Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT catalysts were characterized by ICP‐MS.
XRD,
XPS,
and
TEM.
Elemental
compositions
of
Pd/CNT
and
[email protected]@Pd79.98/CNT catalysts were determined by ICP-MS. The Ag:Pd and Co:Ag:Pd ratios of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT were found as 18.9:81.1 and 0.1: 22.3: 77.6. As can be seen from the ICP‐MS results, the metal ratios of these catalysts are very close
to
the
targeted
ratios.
XRD
patterns
of
Pd/CNT,
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT are given in Fig. 1. The diffraction patterns of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT resemble that of Pd/CNT. It can be reasonably explained by the greater Pd ratio compared to other used metals. The peak at 25.9° was clearly assigned to reflection
of
the
C
(0
0
2)
plane.
For
the
Pd/CNT,
Ag20@Pd80/CNT,
and
[email protected]@Pd79.98/CNT, the diffraction peaks at ca. 39o, 45o, 66o, and 79o were due to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of face-center cubic (fcc) Pd, respectively (JCPDS no. 46-1043). The peaks at ca. 43o for Pd/CNT, Ag20@Pd80/CNT, and [email protected]@Pd79.98/CNT could be attributed the PdO (1 1 0) facets [28]. Besides, a slight shift (0.1o) was observed for Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT due to electronic state change [26].
C (0 0 2) Pd (1 1 1)
PdO (1 1 0)
Pd (2 2 0) Pd (3 1 1)
Pd (2 0 0)
Intensity (a.u.)
Pd/CNT Ag20@Pd80/CNT [email protected]/CNT
0
20
40
60
80
100
2 Theta (deg.)
Figure 1. XRD patterns of Pd/CNT, Ag20@Pd80/CNT, and [email protected]@Pd79.98/CNT catalysts.
TEM images of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT catalysts are given in Figure 2. The morphology and particle size of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT were confirmed by using TEM. One could note that the majority of nanoparticles scatter uniformly on the
CNT
support
without
agglomeration.
Moreover,
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT catalysts were dispersed in a narrow scale in terms of their size. The average particle size of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT were found as 16 and 25 nm, respectively. TEM images of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT indicate that nanoparticles were well fastened to the exterior walls of CNT. XPS
was
used
to
determine
the
oxidation
state
of
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT catalysts. Binding energies of all peaks were determined by taking C1 s binding energy (284.6 eV) as reference. The core level spectra were displayed in Fig. 3. The deconvolution Co 2p survey of [email protected]@Pd79.98/CNT was not given due to excessive noise observed. It could be seen from the general survey of [email protected]@Pd79.98/CNT that C 1s, O 1s, Pd 3d, and Ag 3d spectra were detected. Pd 3d spectra of Ag20Pd80/CNT and [email protected]@Pd79.98/CNT showed that the main component of these catalysts was Pdo. The Pd 3d3/2 peaks of Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT appeared at 341.0 and 340.7 eV, whereas Pd 3d5/2 peaks were at 335.5 and 335.4 eV. As can be seen from Table 2, Ag 3d3/2 and Ag 3d5/2 peak were revealed at the 374.1 and 368.0 eV for Ag20@Pd80/CNT. For [email protected]@Pd79.98/CNT, these were observed at 373.8 and 368.0 eV. The observed shift in Ag 3d3/2 with the addition of Co indicate that structures have a significant effect on the electronic state
of
the
catalysts.
The
main
components
for
both
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT were Ago and Pdo, respectively. However, it is thought that AgO, AgNO3,
PdO,
and
PdCl2
are
also
present
in
both
Ag20@Pd80/CNT
and
[email protected]@Pd79.98/CNT. This can be attributed to the inadequate reduction of Pd and Ag salts with the preparation method.
(b)
(a)
60
% Frequency
50 40 30 20 10 0 5
10
15
20
25
30
35
Particle size (nm)
(c )
(d)
25
% Frequency
20
15
10
5
0 10
20
30
40
50
Particle size (nm)
Figure 2. (a, c)TEM images and particle size distribution histograms of (b, d) Ag20 @Pd80/CNT and [email protected]@Pd79.98/CNT, respectively .
40
400
a)
600
Pd 3d 5/2
Ag 3d5/2
b)
300
Pd 3d 3/2
Intensity (a.u.)
Intensity (a.u.)
500
350
400 300 200
250
Ag 3d3/2
200 150 100
100
50
0
0
-100 348 346 344 342 340 338 336 334 332 330 328
-50 376
374
372
Binding Energy (eV) 100000
c) 80000
40000 Pd 3d Ag 3d
20000
O 1s
368
366
364
Pd 3d5/2 Pd 3d3/2
800
Intensity (a.u.)
Intensity ( a.u.)
d)
1000
C 1s
60000
370
Binding Energy (eV)
Co 2p
600 400 200 0
0 -200
0
200
400
600
800
1000 1200 1400
Binding Energy (eV)
700
e)
-200 346
344
342
340
338
336
334
332
330
328
Binding Energy (eV)
Ag 3d5/2
600
Intensity (a.u.)
500 400
Ag 3d3/2
300 200 100 0 -100 380 378 376 374 372 370 368 366 364 362 360
Binding Energy (eV)
Figure 3. XPS spectra of a) Pd 3d, and b) Ag 3d for Ag20 @Pd80/CNT and XPS spectra of c) general survey, d) Pd 3d, and e) Ag 3d of [email protected]@Pd79.98/CNT catalysts.
Table 2. Pd 3d and Ag 3d binding energy of Ag20 @Pd80/CNT and [email protected]@Pd79.98/CNT catalysts Catalysts
Species
Pd 3d Ag20 @Pd80 Ag 3d
Pd 3d [email protected] @Pd79.98 Ag 3d
Binding Energy (eV) 335.5 337.6 341.0 343.7 366.8 369.6 368.0 373.1 374.1 335.4 336.9 340.7 342.1 368.0 369.7 373.8
Possible Chemical State Pd PdCl2 Pd PdO AgO AgNO3 Ag AgO2 Ag Pd PdO Pd PdO Ag AgNO3 Ag
Relative Intensity (%) 41.6 13.7 33.6 11.1 13 4.5 49.1 8.9 24.5 37.5 15.8 33.8 12.9 53.6 13.0 33.4
References
[29] [30] [31] [32] [33] [34] [35] [36] [37] [29] [31] [31] [31] [38] [34] [39]
.
3.2. Electrochemical Evaluation CV, CA, and EIS measurements were performed in 0.5 M H2SO4 + 1 M FA solution to investigate the FA oxidation activities of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction, bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction, Co@Ag@Pd/CNT catalysts prepared at varying Co:Ag:Pd atomic ratios. Firstly, FA oxidation were taken by CV in 0.5 M H2SO4 + 1 M FA solution at -0.23 V to 1.0 V potential and a scan rate of 50 mV s-1. Fig. 4 shows electro-oxidation measurements of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction,
bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction. Table 3 shows forward, reverse, and maximum current densities of the peaks obtained from the CV. Monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT catalysts prepared via co-reduction, bimetallic catalysts Ag@Pd/CNT catalysts at varying Ag:Pd ratios synthesized by sequential reduction were tested in 0.5 M H2SO4 and 0.5 M H2SO4 + 1 M FA solution, respectively (Fig. 4). When using a second metal together with Pd, the current density is clearly observed to increase. The catalyst was observed to have an increase in the current density as the Ag ratio decreased (Fig.4 and Table 3). As the atomic ratio of Ag increases, it leads to agglomeration of Ag atoms on the Pd-active site on the catalyst surface, thus reducing the synergistic effect between Pd and Ag [1]. The highest performance among the bimetallic was for Ag20 @Pd80/CNT catalysts (892.81 mA/mg Pd respectively) (Table 3). Due to the fact that the current intensities are close to each other, the If/Ir ratios of these catalysts were examined. In terms of current densities, forward current (If) and reverse current (Ir), the highest performance among the bimetallics was obtained from Ag20 @Pd80/CNT.
Pd Pd50Ag50
60
Pd50Co50
40
a
120
b
80
Pd50@Ag50
Current ( mA/mg Pd )
Current ( mA/mg Pd )
80
Ag50@Pd50
20 0 -20
40 0 Ag90@Pd10 Ag80@Pd20
-40
Ag70@Pd30 Ag60@Pd40 Ag50@Pd50
-80
Ag10@Pd90 Ag20@Pd80
-40
-120
-60
-160
Ag30@Pd70 Ag40@Pd60 Pd50@Ag50 Pd
-0,2
-80 -0,4
0,0
0,2
0,4
0,6
0,8
1,0
1,2
Potential ( V vs Ag/AgCI )
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
Potential ( Vvs Ag/AgCI )
a
700
Pd50Co50
500
Pd50@Ag50 400
Ag50@Pd50
300 200
Ag80@Pd20
800
Current ( mA/mg Pd )
Current ( mA/mg Pd )
Ag90@Pd10
Pd Pd50Ag50
600
d
1000
Ag70@Pd30 Ag60@Pd40 Ag50@Pd50
600
Ag10@Pd90 Ag20@Pd80 Ag30@Pd70 Ag40@Pd60
400
Pd50@Ag50 Pd
200
100
0 0
-200 -0,4
-100 -0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
Potential ( V vs Ag/AgCI )
Potential ( V vs Ag/AgCI )
Figure 4. Cyclic voltammograms of a) Pd/CNT, Pd50Ag50/CNT, Pd50Co50/CNT, Pd50@Ag50/CNT, and Ag50@Pd50/CNT catalysts and b) Ag@Pd/CNT catalysts at varying Ag:Pd atomic ratios in 1 M H2SO4 solution at 50 mV s-1 scan rate; Cyclic voltammograms of c) Pd/CNT, Pd50Ag50/CNT, Pd50Co50/CNT, Pd50@Ag50/CNT, and Ag50@Pd50/CNT catalysts and b) Ag@Pd/CNT catalysts at varying Ag:Pd atomic ratios in 1 M H2SO4 + 0.5 M FA solution at 50 mV s-1 scan rate
Table 3. Measured peaks for CV results of Pd/CNT, Pd50Ag50/CNT, Pd50Co50/CNT, Pd50@Ag50/CNT, and Ag@Pd/CNT catalysts at varying Ag:Pd atomic ratios CNT supported Catalysts Pd
Ef (V)
0.19
Forward Peak If (mA/mg Pd) 426.23
Er (V)
0.16
Reverse Peak Ir (mA/mg Pd)
If/Ir
Peak Current Density (mA/mg Pd)
710.89
0.60
404.66
Pd50Ag50
0.14
619.81
0.19
554.31
1.12
599.02
Pd50Co50
0.10
221.12
0.14
226.68
0.98
194.49
Pd50@Ag50
0.16
682.60
0.21
607.24
1.12
649.90
Ag50@Pd50
0.19
656.17
0.23
502.47
1.31
630.78
Ag90@Pd10
0.16
116.54
0.20
103.14
1.13
81.46
Ag80@Pd20
0.16
203.08
0.23
282.09
0.72
172.33
Ag70@Pd30
0.15
58.20
0.30
45.49
1.28
25.96
Ag60@Pd40
0.17
241.37
0.25
189.16
1.28
210.74
Ag40@Pd60
0.20
472.89
0.26
509.04
0.93
449.58
Ag30@Pd70
0.20
837.97
0.23
844.77
0.99
794.94
Ag20@Pd80
0.22
921.57
0.25
943.81
0.98
892.81
Ag10@Pd90
0.20
933.33
0.21
1038.54
0.90
910.84
Additionally, trimetallic Co@Ag@Pd/CNT catalysts were synthesized on Ag20@Pd80/CNT catalyst which exhibited the highest performance among the bimetallics for FA oxidation. Co@Ag@Pd/CNT catalysts were synthesized by sequential reduction using the NaBH4 reduction method. Firstly, 0.5 M H2SO4 measurements, then FA oxidation measurements were taken by CV in 0.5 M H2SO4 + 1 M FA solutions between -0.23 V-1 V potential at a scan rate of 50 mVs-1. Table 4, Fig. 5, and Fig. 5 show the current density and values obtained from the CV. Fig. 5 shows the Co addition reduced the difference between adsorption/desorption in FA oxidation with Pd. It is evident that the Ag20@Pd80/CNT catalyst causes aggregation around the
active sites as the amount of Co increases, leading to a decrease in the current density values (Table 4). As the amount of Co increases, the transfer of electrons (synergistic effect) [1] between Pd/Ag is inhibited and this causes the reduction peak to be higher than the oxidation peak in FA oxidation (Fig. 8a). The highest current density was obtained for [email protected] @Pd79.98/CNT catalyst and was measured as 931.29 mA/mg Pd. This value indicates that a low amount of Co is more effective for FA oxidation.
60
80
a
60
20 0 [email protected]@Pd79.99 [email protected]@Pd79.98
-20
[email protected]@Pd79.85 [email protected]@Pd79.49
-40
[email protected]@Pd69.9 [email protected]@Pd46.50 [email protected]@Pd32.72
-60
[email protected]@Pd20.56
Current ( mA/mg Pd )
Current ( mA/mg Pd )
40
Pd Ag20@Pd80
b
[email protected]@Pd79.98
40 20 0 -20 -40
[email protected]@Pd11.81 [email protected]@Pd9.73
-80
[email protected]@Pd7.20
-0,4
-0,2
0,0
0,2
0,4
0,6
Potential ( V vs Ag/AgCI )
0,8
1,0
-60 -0,4
-0,2
0,0
0,2
0,4
0,6
Potential ( V vs Ag/AgCI )
0,8
1,0
1000
c
1000
[email protected]@Pd79.99
Pd Ag20@Pd80
d
[email protected]@Pd79.98
[email protected]@Pd79.85
800
800
[email protected]@Pd79.49 [email protected]@Pd69.9 [email protected]@Pd46.50 [email protected]@Pd32.72
600
[email protected]@Pd20.56 [email protected]@Pd11.81 [email protected]@Pd9.73
400
[email protected]@Pd7.20
200
Current ( mA/mg Pd )
Current ( mA/mg Pd )
[email protected]@Pd79.98
600
400
200
0
0 -0,4
-0,2
0,0
0,2
0,4
0,6
0,8
-0,2
1,0
0,0
0,2
0,4
0,6
0,8
Potential ( V vs Ag/AgCI )
Potential ( Vvs Ag/AgCI )
Figure 5. Cyclic voltammograms of a) Co@Ag@Pd/CNT and b) Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in 1 M H2SO4 solution at 50 mV s-1 scan rate, c) Cyclic voltammograms of Co@Ag@Pd/CNT and b) Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in 1 M H2SO4 + 0.5 M FA at 50 mV s-1 scan rate
Table 4. Measured peak values in CV results on 10% Ag@[email protected]/CNT catalysts Forward Peak Ef (V) Reverse Peak Er (V) Peak Current Density Catalysts If/Ir If (mA/mg Ir (mA/mg Pd) (mA/mg Pd) Pd) 0.22 0.24 [email protected] @Pd79.99 800.90 759.49 1.05 781.35 [email protected] @Pd79.98 [email protected] @Pd79.85 [email protected] @Pd79.49 [email protected] @Pd69.90 [email protected] @Pd46.5 [email protected] @Pd32.72 [email protected] @Pd20.56
961.85 756.26 753.05 715.95 373.60 394.89 655.65
0.20 0.20 0.24 0.23 0.20 0.17 0.20
869.83 750.39 749.85 651.74 363.46 320.11 560.61
0.22 0.23 0.26 0.25 0.23 0.22 0.25
1.11
931.29
1.01
732.75
1.004
738.45
1.10
694.32
1.03
355.97
1.23
369.97
1.17
629.33
1,0
0.14
99.65
[email protected] @Pd11.81 [email protected] @Pd9.73
0.12
50.52
0.32
60.64
0.31
27.89
1.64
70.48
1.81
29.24
Stability of Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts was measured by CA at 0.1 V. Fig. 6a shows the CA results for Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT catalysts measured in 0.5 M H2SO4 + 1 M FA . Current values were normalized by Pd amount and given Fig. 6a. Furthermore, for the stability evaluation, the retention of current value of 1000 s to the initial value was used by normalizing chronomaperommogram by dividing current values to initial current values and given in Figure 6b. Figure 6 a and Figure 6b clearly shows that Ag20@Pd80/CNT and [email protected]@Pd79.98/CNT have higher stability than Pd/CNT catalyst.
a
1,1
(b)
Pd Ag20@Pd80
280
[email protected]@Pd79.98
240
Relative Current
Current (mA/mg Pd)
320
200 160 120 80
1,0
Pd Ag20@Pd80
0,9
[email protected]@Pd79.98
0,8 0,7 0,6 0,5 0,4
40
0,3
0
200
400
600
Time (seconds)
800
1000
0,2 0
200
400
600
800
1000
Time (s)
Figure 6. (a) Chronoamperomograms of ) Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in solution of 0.5 M H2SO4 + 1 M FA at -0.1 V and (b) Chronoamperomograms at normalized currents of Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in solution of 0.5 M H2SO4 + 1 M FA at -0.1 V
EIS results were measured on Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts in 0.5 M H2SO4 + 1 M FA. Figure 7 shows the Nyquist Plot data plotted according to the data from the EIS results. Nyquist Plot is known as semicircles and the diameter of these semicircles gives information about the electrocatalytic activity of the catalyst employed and its transfer resistance [7]. Accordingly, when the semicircle diameter diminishes, one could observe that electron transfer resistance decrease and the electro-catalytic activity of the catalyst increases. Figure 7 shows that the transfer resistance load is in the order Pd/CNT > Ag20@Pd80/CNT > [email protected]@Pd79.98/CNT. As a result, [email protected]@Pd79.98/CNT catalyst was found to have the highest electro-catalytic activity, which is agreement with CV and CA results.
Pd Ag20@Pd80
10000
[email protected]@Pd79.98
-Z'' / ohm
8000
6000
4000
2000
0 0
2000
4000
6000
8000
10000
12000
Z' / ohm
Figure 7. Nyquist graphs of Pd/CNT, Ag@Pd/CNT, and Co@Ag@Pd/CNT catalysts at -0.1 V in 0.5 M H2SO4 + 1 M FA.
4. CONCLUSION In conclusion, the study of synthesis of monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT via NaBH4 co-reduction method and Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios are prepared via NaBH4 sequential reduction method, their characterization and eployment in FA oxidation led to the following conclusions and insights: •
Monometallic Pd/CNT, bimetallic Pd50Ag50/CNT, Pd50Co50/CNT, Ag50Pd50/CNT, and Ag@Pd/CNT catalysts at varying Ag:Pd ratios and Co@Ag@Pd/CNT catalysts at varying Co:Ag:Pd atomic ratios could be easily prepared from the co-reduction and sequential reduction of corresponding palladium, silver and cobalt precursors by NaBH4 reduction method.
•
Characterization results revealed that Co and Ag addition changes the surface electronic state.
•
FA oxidation measurements revealed that Co and Ag addition improves the electrochemical activity. This phenomenon could be attributed to the electronic state change of catalysts by the addition of second or third metal.
•
The activity of Ag50Pd50/CNT catalyst prepared via NaBH4 sequential -reduction method is gretare than the activity of Ag50Pd50/CNT via NaBH4 co-reduction method. One could note preparation order improves electrochemical activity.
•
The addition of Co improves the electrochemical activity of Ag19.98Pd79.98Co0.072/CNT catalyst. Further increase in Co loading leads to decrease in the electrocatalytic activity due to aggregation and blocking of the surface active sites. Ag19.98Pd79.98Co0.072/CNT catalyst has a great potential as an alternative fuel cell anode catalyst.
Acknowledgments The CHI 660E potentiostat employed in electrochemical measurements was purchased from the Scientific and Technological Research Council of Turkey (TUBITAK) project (project no: TUBITAK 113Z249). The chemicals were purchased from Van Yuzuncu Yıl BAP project (project no: FBA-2018-7152). Characterization measurements were also purchased from Van Yuzuncu Yıl BAP project (project no: FBA-2018-7152).These characterization measurements as ICP-MS, XRD, and TEM were performed at DAYTAM.
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Co@Ag@Pd/CNT catalysts were successfully synthesized via NaBH4 sequential reduction method
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Co@Ag@Pd/CNT catalysts has high formic acid electrooxidation activity
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Co and Ag promotion to Pd enhances the formic acid electrooxidation activity.
Cover Letter
April 2019 Dear editor Enclosed please find the research article manuscript entitled Novel carbon nanotube supported Co@Ag@Pd formic acid electrooxidation catalysts prepared via sodium borohydride sequential reduction method co-authored by Omer Faruk Er, Aykut Caglar, Berdan Ulas, Hilal
Kivrak, Arif Kivrak submitted for publication in International Journal of environmental science and technology. I, Hilal Kivrak, will be handling the correspondence about the article. The authors confirm that this submitted manuscript has not been published previously that it is not under consideration for publication elsewhere. This work described has not been published previously (except in the form of an abstract or as part of a published lecture or academic thesis), that it is not under consideration for publication elsewhere, that its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out. Submission also implies that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, without the written consent of the Publisher. Thanking in advance, for your and the reviewers efforts for the evaluation of the manuscript. Sincerely yours; Assoc. Prof. Dr. Hilal DEMİR KIVRAK Chemical Engineering Department Yuzuncu Yıl University Van, Turkey : e-mail: [email protected]