Enhanced electrocatalytic activity and stability of platinum, gold, and nickel oxide nanoparticles-based ternary catalyst for formic acid electro-oxidation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e8

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Enhanced electrocatalytic activity and stability of platinum, gold, and nickel oxide nanoparticlesbased ternary catalyst for formic acid electro-oxidation Gumaa A. El-Nagar a, Ahmad M. Mohammad a,b,* a b

Chemistry Department, Faculty of Science, Cairo University, Cairo 12613, Egypt Department of Chemical Engineering, Faculty of Engineering, The British University in Egypt, Cairo 11837, Egypt

article info

abstract

Article history:

The global interest to realize and commercialize the direct formic acid fuel cells has

Received 1 April 2014

motivated the development of efficient and stable anodes for the formic acid (FA) electro-

Received in revised form

oxidation (FAO). In this investigation, a ternary catalyst composed of Pt nanoparticles

22 May 2014

(PtNPs), Au nanoparticles (AuNPs) and nickel oxide nanoparticles (nano-NiOx), all were

Accepted 5 June 2014

sequentially electrodeposited onto the surface of a glassy carbon (GC) electrode, was rec-

Available online xxx

ommended for this reaction. The surface morphology investigation revealed the deposition of grain-shaped PtNPs (25 nm average particle size), and flower-shaped nanospheres (less

Keywords:

than 60 nm average particle size) of AuNPs and nano-NiOx. Interestingly, the ternary

Electrocatalysis

modified NiOx-Au-Pt/GC electrode has shown an outstanding electrocatalytic activity to-

Gold nanoparticles

wards the direct FAO, concurrently with a complete suppression for the indirect route. It

Platinum nanoparticles

further exhibited excellent stability that extended for 7 h of continuous electrolysis. While

Nickel oxide nanoparticles

PtNPs furnished a suitable base for FA adsorption, AuNPs played a significant role to

Direct formic acid fuel cells

interrupt the contiguity of the Pt surface sites, which is necessary for CO poisoning. On the

Third body

other hand, nano-NiOx acted as a catalytic mediator facilitating the charge transfer of FAO and the oxidative removal of CO at a lower potential. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have recently attracted a growing interest as viable candidates to replace batteries in portable power devices. As a consequence, the electrochemical oxidation of formic acid (FA) has been an

active topic for research because of prospective applications in PEMFCs. Indeed, FA has several advantages as a fuel for PEMFCs, which derived the significant motivation to commercialize the direct formic acid fuel cells (DFAFCs). These merits involve its eco-friendship and easiness of transportation and storage. In addition, FA has a much lower fuel crossover through the PEM than other fuels, and further

* Corresponding author. Chemistry Department, Faculty of Science, Cairo University, Cairo 12613, Egypt. E-mail addresses: [email protected], [email protected] (A.M. Mohammad). http://dx.doi.org/10.1016/j.ijhydene.2014.06.028 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: El-Nagar GA, Mohammad AM, Enhanced electrocatalytic activity and stability of platinum, gold, and nickel oxide nanoparticles-based ternary catalyst for formic acid electro-oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.06.028

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has a high theoretical open circuit potential of 1.45 V. Nevertheless, the development of anodic materials of high stability and electrocatalytic activity towards the FA electro-oxidation (FAO) remains challenging [1e12]. Conventionally, platinum (Pt) is assumed among the efficient electrocatalysts for FAO (the essential anodic reaction in DFAFCs). However, its catalytic activity deteriorates rapidly due to the accumulation of the poisoning CO intermediate produced as a result of the “non-faradaic” dissociation of FA [5,13e16]. This, unfortunately, reduces the active sites of Pt for FAO and delays the reaction kinetics. Hence, finding efficient and stable anodes for FAO was the incentive behind several investigations aiming to mitigate such poisoning effect [5,10,13,17e20]. Two different strategies can be thought to overcome the CO surface poisoning at Pt surfaces; either to impede the CO adsorption, and/or to facilitate its oxidative removal away from the Pt surface at low anodic potential. In this regard, the use of bimetallic catalyst could alter the electronic properties of Pt, in a way enhancing the oxidation of adsorbed CO at relatively lower potentials [5,17]. Alternatively, the deposition of metal oxides nanostructures on the Pt surfaces could provide the oxygen atmosphere necessary to facilitate the CO oxidation at low potential domain [5,13,17]. On the other hand, tailoring the Pt surface by metallic dopants such as gold nanoparticles (AuNPs), in a way interrupting the contiguity of the Pt surface active sites necessary for the CO adsorption, could overcome the CO poisoning [20]. Mechanistically, FAO proceeds effectively in a dualpathway procedure at Pt-based electrodes [5,13e17,21,22]. The first (direct) involves the dehydrogenation of FA to CO2, with formate anion as the reactive intermediate [5e8,13e24]. The other (indirect) path involves the oxidation of the poisonous CO, generated from the non-faradaic dehydration of FA, at higher anodic potentials [14,16,17,24]. This is CO, which degrades the catalytic activity of Pt with time. In the current study, a novel nanoparticles-based ternary catalyst composed of platinum nanoparticles (PtNPs), AuNPs and nickel oxide nanoparticles (nano-NiOx), all are sequentially deposited electrochemically onto a GC substrate, is suggested for efficient FAO. The role of AuNPs and nano-NiOx in the observed catalytic enhancement is discussed and the stability of the developed catalyst is as well investigated.

Experimental Electrode's pretreatment and modification Glassy carbon (d ¼ 3.0 mm) and polycrystalline Pt (d ¼ 1.6 mm) electrodes served as the working electrodes. An Ag/AgCl/KCl (sat) and a spiral Pt wire were used as reference and counter electrodes, respectively. Conventional procedure was applied to clean the Pt and GC electrodes as described elsewhere [5,13,17]. The electrodeposition of PtNPs was achieved in 0.2 M H2SO4 containing 1.0 mM H2PtCl6 solution using potential step electrolysis from 1 to 0.1 V vs. Ag/AgCl/KCl(sat) for 300 s according to the following electrochemical equation; Pt4þ þ 4e /Pt

(1)

The PtNPs loading was estimated from Faraday's law using the charge associated in the i-t curve obtained during deposition to be ca. 65 mg. However, the electrodeposition of AuNPs was carried out in 0.1 M H2SO4 containing 1.0 mM K[AuCl4] solution using potential step electrolysis from 1.1 to 0 V vs. Ag/AgCl/KCl(sat) for 350 s according to the following equation; Au3þ þ 3e /Au

(2)

The AuNPs loading was estimated similarly from the charge associated in the i-t curve obtained during deposition to be ca. 45 mg. On the other hand, the electrode's modification with nano-NiOx was achieved in two sequential steps. The first involved the electrodeposition of metallic nickel onto the working electrode from an aqueous solution of 0.1 M acetate buffer solution (ABS, pH ¼ 4.0) containing 1 mM Ni(NO3)2 by a constant potential electrolysis at 1 V vs. Ag/AgCl/KCl (sat.) for 60 s. 2þ

Ni

þ 2e /Ni

(3)

Next, the electrodeposited Ni was passivated (oxidized) in 0.1 M phosphate buffer solution (PBS, pH ¼ 7) by cycling the potential between 0.5 and 1 V vs. Ag/AgCl/KCl (sat) for 10 cycles at 200 mV s1 [5,17]. Ni þ xH2 O/NiOX þ 2xHþ þ 2xe

(4)

All of the chemicals used in this investigation were of analytical grade and used without further purification. The electrochemical behavior of a set of four electrodes will be compared in this investigation. These electrodes are basically the bare Pt bulk electrode, PtNPs-modified GC (Pt/GC), AuNPsmodified Pt/GC (AuePt/GC), and nano-NiOx modified AuePt/ GC (NiOx-Au-Pt/GC).

Electrochemical measurements The electrochemical characterization methods are very powerful and sensitive to traces from the active ingredients, and can distinguish firmly between the different types of surface atoms. The electrocatalytic activity of the modified electrodes toward FAO was examined in an aqueous solution of 0.3 M FA solution (pH ¼ 3.5). The pH was adjusted by adding a proper amount of NaOH. As the use of highly acidic solutions would diminish the stability of nickel oxide (albeit at slow kinetics), the current study was conducted at slightly acidic pH, which lied within the stability domain of nickel oxide [5,17,25]. Moreover, at this pH an appreciable amount of FA was ionized to formate anion (about one third). This would enhance the ionic conductivity in the solution, thus reduce the resistance polarization, in addition to compressing the thickness of the diffusion layer [5,17]. Cyclic voltammetry (CV) was performed in a conventional two-compartment threeelectrode glass cell. Current densities were calculated on the basis of the real surface area of the working electrodes. All measurements were performed at room temperature (z25  C) using an EG&G potentiostat (model 273A) operated with Echem 270 software.

Please cite this article in press as: El-Nagar GA, Mohammad AM, Enhanced electrocatalytic activity and stability of platinum, gold, and nickel oxide nanoparticles-based ternary catalyst for formic acid electro-oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.06.028

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Materials characterization A field emission scanning electron microscope, FE-SEM, (QUANTA FEG 250) coupled with an energy dispersive X-ray spectrometer (EDS) unit was employed to evaluate the electrode's morphology and bulk composition. X-ray diffraction, XRD, (PANalytical, X’Pert PRO) operated with Cu target (l ¼ 1.54Å) was used to identify the crystallographic structure of nano-NiOx.

Results and discussions Electrochemical and materials characterization Fig. 1a depicts clearly a typical characteristic CV of a clean polycrystalline Pt electrode in alkaline conditions at a scan rate of 100 mV s1; the Pt oxidation, extending over a wide range of potential, is coupled with the (Pt/PtO) reduction peak at ca. 0.27 V. In addition, the hydrogen adsorption/desorption peaks are shown in the potential range from 0.6 to 0.8 V. Similar features have been appeared on Pt/GC electrode, albeit the small negative shift in the potential of PtO/Pt peak (see Fig. 1b). With further modification with AuNPs, the characteristic peaks of Au appeared at ca. 0.2 V (oxide formation) and 0.07 V (oxide reduction), in addition to those of Pt (Fig. 1c). This indicates the exposure of both metals (Pt and Au) to the surface and that the deposition of AuNPs did not entirely cover the Pt surface atoms. The decrease in the Pt surface area (revealed from the decrease in the intensities of the Pt oxide reduction peak at ca. 0.34 V and the hydrogen adsorption/desorption peaks in the potential range from 0.6 to 0.8 V) supports this further. Interestingly, upon modifying the AuePt/GC electrode with nano-NiOx (Fig. 1d), a further decrease in the intensities of the reduction peak of the Pt oxide and the hydrogen adsorption/desorption peaks was observed. In parallel, a decrease in the surface area of AuNPs (compare the peak intensities of Au oxide reduction at ca. 0.07 V) was noticed, which suggests the electrodeposition of nano-NiOx on both of PtNPs and AuNPs. The surface

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coverage (q) of Au and NiOx in the NiOx-Au-Pt/GC modified electrode was calculated to be z 30 and 35%, respectively [5]. On the other hand, a redox couple appeared (in Fig. 1d) at ca. 0.4e0.5 V corresponding to the Ni(OH)2/NiOOH reversible transformation [5,10,26]. The appearance of this redox peak couple along with the Au oxide and Pt oxide reduction peaks at the NiOx-Au-Pt/GC modified electrode suggests the simultaneous exposure of nano-NiOx, AuNPs, PtNPs at the surface. The effort was committed next to evaluate the morphology, composition and crystal structure of the different electrodes involved in this investigation. Fig. 2 depicts the FE-SEM images for (a) bare Pt, (b) Pt/GC, (c) AuePt/GC, and (d) NiOx-Au-Pt/GC electrodes. The bare Pt electrode (Fig. 2a) has appeared featureless immediately right after mechanical polishing and electrochemical treatment. However, grained shaped PtNPs (25 nm average particle size) have appeared at the Pt/GC electrode (Fig. 2b). Interestingly, the deposition of AuNPs on top of PtNPs resulted in spherical flower-decorated nano-agglomerates that homogeneously covered a significant portion of the GC surface (Fig. 2c). The image infers about the intensive deposition of AuNPs on PtNPs, as the average particle size increased. However, the deposition of AuNPs did not cover entirely the PtNPs, as revealed from Fig. 1c, where both the Au and Pt surfaces were exposed to the solution. Interestingly, the deposition of nanoNiOx on the AuePt/GC modified electrode resulted in a further increase in the average particle size of (ca. 60 nm, Fig. 2d), which may infer about the deposition of nano-NiOx on PtNPs and/or AuNPs. The XRD investigation confirmed the deposition of Pt in the Pt/GC (Fig. 3a), Au/Pt/GC (Fig. 3b) and NiOx-AuPt/GC (Fig. 3c) electrodes in a face-centered cubic (fcc) structure, where the all the typical characteristic peaks of Pt (1 1 0), (2 0 0), (2 2 0), and (3 1 1) appeared [5]. No obvious change was noticed upon the deposition of AuNPs onto the Pt/GC electrode. We could only observe a small shift in the Pt characteristic peaks, which might result with PteAu alloying. However, interestingly, Fig. 3c depicts two new diffraction peaks at 2q equal to 38 and 64.5 , corresponding to nickel oxide (presumably b-NiOOH phase) [5,17]. The major broad peak appeared at 2q ¼ 25 is associated to the carbon support (0 0 2). On the other hand, the EDS analysis of the different electrodes confirmed the deposition of the different ingredients in the catalyst and assisted in the calculations of their relative ratios (see Tables 13 and insets in Fig. 3).

Electrocatalytic activity towards FAO

Fig. 1 e The CVs in 0.5 M KOH for (a) bare Pt, (b) Pt/GC, (c) AuePt/GC and (d) NiOx-Au-Pt/GC modified electrode at 100 mV s¡1.

Fig. 4 shows the CVs of the FAO at the bare Pt (a), Pt/GC (b), AuePt/GC (c), and NiOx-Au-Pt/GC (d) electrodes in an aqueous solution of 0.3 M FA (pH ¼ 3.5). For the bare Pt (Fig. 4a) and Pt/ GC (Fig. 4b) electrodes, two oxidation peaks are observed in the forward scan, at ca. 0.30 V (assigned to the direct oxidation of FA to CO2, Idp) and at ca. 0.65 V (assigned to the oxidation of the poisonous COads species to CO2, Iind p ) [5,13,15,17,20]. The COads refers to the adsorbed CO resulted from the “non-faradaic” dissociation of FA. At low potential (ca. 0.30 V), the measured current corresponds to the FAO at Pt sites via the dehydrogenation pathway. At high potential (ca. 0.65 V), the COads is oxidized by PteOH (formed at ca. 0.50 V), which

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Fig. 2 e FE-SEM images of (a) bare Pt, (b) Pt/GC, (c) AuePt/GC and (d) NiOx-Au-Pt/GC modified electrode.

releases most Pt sites for FA oxidation. The relative peak current intensities indicate the level of Pt surface poisoning by COads. In the backward cathodic-going scan, most of the poisonous intermediates have been released to expose a clean Pt surface for FAO through the dehydrogenation pathway. Therefore, the current intensity in the backward scan increases largely (peak Ib in Fig. 4a). The electrocatalytic preference of the Pt/GC (compared to bulk Pt) towards FAO is inferred from the ratio of the current intensities of the direct (Idp) to the indirect (Iind p ) peaks of FAO. This ratio (Idp/Iind p ) increased from 0.19 at the bare Pt to 2.6 at the Pt/GC electrode (i.e, about 14 times). Interestingly, the AuePt/GC electrode (Fig. 4c) shows a sharp increase in Idp concurrently with a corresponding d ind decrease in Iind p , and the (Ip/Ip ) ratio increased to 3.6 (i.e, about 19 times that at the bare Pt). Actually, the Pt surface poisoning with CO requires the existence of three adjacent Pt surface atoms with certain spacing. Interruption of this contiguity or even the atomic spacing would impede the CO adsorption and surface poisoning. That is actually what AuNPs did where they tailored the Pt surface against the CO adsorption, a phenomenon known as the third-body effect [10,20]. Therefore, the enhancement with AuNPs is principally originated geometrically [20]. Nevertheless, the AuePt/GC electrode suffered from instability concerns, where the electrocatalytic efficiency deteriorated with time (will be next discussed). Therefore, a further treatment is required to improve the catalytic deterioration. In this regard, nano-NiOx was promising in improving

the stability of several electrodes in multiple electrochemical reactions [5,17,19,26]. It worth mentioning that neither Au nor NiOx alone are catalytically active for FAO [17]. Surprisingly, with further modification of the AuePt/GC electrode with nano-NiOx (NiOx-Au-Pt/GC electrode, Fig. 4d), the degree of enhancement towards FAO was outstanding, where a significant increase in Idp was observed with a complete absence of the indirect oxidation peak. Furthermore, the Idp exceeded the backward cathodic peak current (Ib) and the ratio of Idp/Ib increased from ca. 0.09 (at bare Pt) to ca. 1.1 (at NiOx-Au-Pt/GC electrode), in a fashion similar to that reported at Pd substrates, for which FAO proceeds with no CO formad ind d tion. Table 4 lists comparisons for Idp, Iind p , Ip/Ip , Ip/Ib and the real surface areas of the investigated electrodes. The role nano-NiOx played in this enhanced catalytic activity of FAO is questioned. Did it interrupt further the contiguity of the Pt surface atoms and consequently the enhancement was geometrically-based? Did it modify the electronic structure of the Pt surface atoms in a way facilitating the oxidative removal of CO at low potential domain? To elaborate that, the oxidation of CO adlayer (chemisorbed from FA at open circuit potential) is investigated at the four investigated electrodes and the data are presented in Fig. 5. As clearly seen in Fig. 5a, the CO oxidation occurred at 0.78 V at the Pt/GC electrode, which is slightly shifted to 0.80 V at the AuePt/GC electrode with a slight decrease in the peak current intensity (Fig. 5b). The decrease in the peak current density outlines the resistance increase of the AuePt/GC electrode to

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Fig. 3 e XRD pattern for (a) Pt/GC, (b) AuePt/GC and (c) NiOx-Au-Pt/GC electrodes. Insets depict the corresponding EDS for the same electrodes.

adsorb CO. This supports the assumption that the deposition of AuNPs at the Pt/GC electrode assisted in retardation of CO adsorption rather than to facilitate its oxidative removal. On the other hand, the deposition of nano-NiOx on AuePt/GC (Fig. 5c) electrode invoked a significant negative shift in the onset of CO oxidation (~0.2 V in comparison to that of the AuePt/GC electrode), and the peak current intensity was further decreased. This recommends a major change in the

Table 1 e Composition of the Pt/GC electrode revealed from EDS measurement. Element CK OK Pt L Total

Atomic content, At.%

Weight content, Wt.%

Measurements error, %

17.75 3.65 21.15 100.00

17.75 1.15 81.10 100.00

±0.0445 ±0.0025 ±0.7149 e

Table 2 e Composition of the AuePt/GC electrode revealed from EDS measurement. Element CK OK Pt L Au L Total

Atomic content, At.%

Weight content, Wt.%

Measurements error, %

89.53 3.24 3.17 4.06 100.00

42.23 2.04 24.30 31.43 100.00

±0.1218 ±0.003 ±0.1966 ±0.2542 e

electronic structure of the Pt surface atoms, which weakens the PteCO bonding and facilitates its oxidative removal at lower potential [5,17]. Alternatively, the belief that nano-NiOx may further facilitate the charge transfer during the CO oxidation via a catalytic mediation with the Ni(OH)2/Ni(OOH) reversible transformation cannot be excluded [5,17]. We can, therefore, assume that the catalytic enhancement appeared at the NiOx-Au-Pt/GC electrode was bi-functional, where both the geometrical effect (third body) and the electronic influence participated.

Stability concerns Actually, the electrode's stability concerns any fuel cell manufacturer exactly the same as electrode's catalytic efficiency. This is basically an issue of saving money, where a stable electrode with a moderate efficiency will stay longer

Table 3 e Composition of the NiOx-Au-Pt/GC electrode revealed from EDS measurement. Element CK OK Pt L Au L Ni K Total

Atomic content At.%

Weight content, Wt.%

Measurements error, %

80.51 5.42 5.25 4.47 4.35 100.00

29.27 2.70 39.09 21.30 7.64 100.00

±0.0811 ±0.0044 ±0.3823 ±0.1850 ±0.0069 e

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Ib

a

25

Ib

b

14 12

Ipind

15

10

Ipind

I pd

10

I / m A c m -2

I / m A cm

20

8 6 4 2

5

Ipd

0

0

-2 -200

0

200

400

600

-200

0

E / mV vs. Ag/AgCl/KCl (sat.)

Ib

c

600

Ib

30

Ipind I / m A c m -2

15

I / m A c m -2

400

Ipd

d

Ipd

20

200

E / mV vs. Ag/AgCl/KCl (sat.)

10

5

20

10

0

0 -200

0

200

400

600

-200

0

E / mV vs. Ag/AgCl/KCl (sat.)

200

400

600

E / mV vs. Ag/AgCl/KCl (sat.)

Fig. 4 e The CVs in 0.3 M HCOOH (pH ¼ 3.5) for (a) bare Pt, (b) Pt/GC, (c) AuePt/GC and (d) NiOx-Au-Pt/GC modified electrode at 100 mV s¡1.

1.5

-2

1.0

I / m A cm

than unstable electrode of excellent efficiency. In this investigation, the modification of the AuePt/GC electrode with nano-NiOx is intended to improve the catalytic stability of the AuePt/GC electrode, which actually has an excellent catalytic activity towards FAO but this activity does not last for long time of continuous electrolysis. The stability of the four investigated electrodes was evaluated by chronoamperometry measurements (see Fig. 6) at a potential of 0.3 V for prolonged electrolysis time (up to about 7 h).

b c 0.5

a

0.0

Table 4 e Summary of electrochemical measurements. Electrodes Pt bare Pt/GC AuePt/GC NiOx-Au-Pt/GC

Iind Idp/Iind Idp/Ib Real surface Idp p p area/cm2 mA cm2 mA cm2 2.4 9.7 19.4 33.6

12.2 3.7 5.3 e

0.19 2.6 3.7 ∞

0.1 0.7 0.9 1.1

0.09 0.224 0.162 0.117

As estimated from the charge consumed during hydrogen desorption peaks in Fig. 1 using a reported value of 210 mC cm2 [22].

-200

0

200

400

600

800

1000

1200

E / mV vs. Ag/AgCl/KCl (sat.)

Fig. 5 e The CO stripping in 0.5 M H2SO4 at (a) Pt/GC, (b) AuePt/GC and (c) NiOx-Au-Pt/GC at 100 mV s¡1. Before measurements, CO was adsorbed from 0.5 M FA at the open circuit potential. Current densities were calculated using the real surface area.

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Once again, the initial current densities in the data of Fig. 6 arrange the four investigated electrodes in the expected activity sequence, where the bare Pt electrode (Fig. 6a) represents the lowest catalytic efficiency while the ternary modified electrode (NiOx-Au-Pt/GC) exhibits the highest catalytic efficiency. Moreover, the catalytic stability of the bare Pt (Fig. 6a), Pt/GC (Fig. 6b), and AuePt/GC (Fig. 6c) electrodes deteriorates significantly with time (note the sharp decays in current density with time). Fortunately, this decay in current and deterioration in the catalytic stability disappeared effectively at the ternary modified NiOx-Au-Pt/GC electrode (Fig. 6d). The deterioration of the bare Pt, Pt/GC, and AuePt/GC electrodes may originate from the accumulation of COads on the electrodes surface, which may induce a surface reconstruction of the Au and/or Pt atoms. This reconstruction may restore the contiguity of the Pt sites required for the CO poisoning. On the other hand, the surface detachment of PtNPs and AuNPs from the electrode may explain the observed behavior. However, for the NiOx-Au-Pt/GC electrode, the existence of nano-NiOx ensures a mediated FAO and/or oxidative removal of COads (if any), therefore, the possibility of surface reconstruction decreases. It also makes sense that the deposition of nano-NiOx worked against the detachment of PtNPs and AuNPs. This was verified experimentally by measuring the CV in 0.5 M KOH at the NiOx-Au-Pt/GC electrode after the prolonged (7 h) electrolysis and comparing it with that of Fig. 1d. Interestingly, there was actually no significant change in the Pt and Au characteristic peaks, indicating no detachment of Pt and Au atoms.

Conclusion A ternary catalyst composed of PtNPs, AuNPs, and nano-NiOx (all were electrochemically deposited on the surface of a GC substrate) was developed for the FAO. The electrocatalytic activity of four different electrodes (bare Pt, Pt/GC, AuePt/GC, and NiOx-Au-Pt-GC) was compared and analyzed. Morphologically, PtNPs (grained shaped e 25 nm average particle size) were deposited on the GC substrate and AuNPs then nanoNiOx were deposited next in a flower sphere agglomerates. 16 d

14

I / mA cm-2

12 c

10 8 b

6 4 a

2 0

0

2

4

6

t/h

Fig. 6 e Chronoamperometric measurements for (a) bare Pt, (b) Pt/GC, (c) AuePt/GC and (d) NiOx-Au-Pt/GC electrodes at 0.3 V in 0.3 M FA (pH ¼ 3.5).

7

The Pt/GC electrode exhibited a higher electrocatalytic activity towards FAO than the bare Pt electrodes, which was revealed from the increase of the Idp/Iind p ratio from 0.19 at the bare Pt to 2.6 at the Pt/GC electrode. The electrocatalytic enhancement was improved after the deposition of AuNPs, and the Idp/Iind p ratio increased to 3.6. The deposition of nano-NiOx on top of the AuePt/GC electrodes induced an outstanding catalytic enhancement with a complete disappearance of the indirect oxidation route of FAO. The existence of Pt surface atoms was proved essential for the FAO, while AuNPs assisted in resolving the CO surface poisoning. On the other hand, nanoNiOx is believed to act as a catalytic mediator, which facilitates the charge transfer during the direct FAO thus, prevents the deterioration of the catalytic activity of the anode. Interestingly, the ternary modified NiOx-Au-Pt-GC electrode exhibited a unique stability for a prolonged time of continuous electrolysis.

Appendix A. Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.ijhydene.2014.06.028.

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Please cite this article in press as: El-Nagar GA, Mohammad AM, Enhanced electrocatalytic activity and stability of platinum, gold, and nickel oxide nanoparticles-based ternary catalyst for formic acid electro-oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.06.028

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Please cite this article in press as: El-Nagar GA, Mohammad AM, Enhanced electrocatalytic activity and stability of platinum, gold, and nickel oxide nanoparticles-based ternary catalyst for formic acid electro-oxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.06.028