Electrocatalytic oxidation of ethylene glycol on palladium coated on 3D reduced graphene oxide aerogel paper in alkali media: Effects of carbon supports and hydrodynamic diffusion

Electrochimica Acta 212 (2016) 237–246

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Electrocatalytic oxidation of ethylene glycol on palladium coated on 3D reduced graphene oxide aerogel paper in alkali media: Effects of carbon supports and hydrodynamic diffusion Atiweena Krittayavathananon, Montree Sawangphruk* Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand

A R T I C L E I N F O

Article history: Received 13 March 2016 Received in revised form 22 June 2016 Accepted 29 June 2016 Available online 30 June 2016 Keywords: Pd electrocatalyst 3D graphene aerogel Electrocatalyst paper Enthylene glycol oxidation Direct ethylene glycol fuel cells

A B S T R A C T

To enhance Pd electro-catalysts in their electro-catalytic activity for C-C bond breaking and tolerance stability, Pd was electrodeposited on high-porosity 3D reduced graphene oxide (rGO) aerogel paper. For comparison, the Pd was also coated on other three different carbon material surfaces: bare carbon fiber paper (CFP), and CFP modified with either 2D graphene oxide (GO) and 2D rGO nanosheets. The Pd/3D rGO aerogel paper exhibits higher catalytic activity toward the electro-oxidation of ethylene glycol in alkaline media and higher excellent tolerance stability than other electrodes due to ultra-high porosity of the 3D rGO support leading to high active electrochemical surface area of the as-fabricated Pd catalyst electrode. In addition, the hydrodynamic diffusion of ethylene glycol to the cayalyst surface investigated in this work plays a major role to the catalytic activity of the as-prepared electrocatalyst. The Pd/3D rGO aerogel rotating disk electrode exhibits a high anodic current density of 267.8 mA cm
2 at 3000 rpm. This high-performance Pd/3D rGO aerogel may be practically used as the high-efficiency anode of direct ethylene glycol and other related alcohol fuel cells. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Direct ethylene glycol fuel cells (DEGFCs) are of interest and being studied as a candidate of direct alcohol fuel cells. This is because ethylene glycol (EG) has low toxicity, available in supply chain (not yet for methanol), inflammability, high boiling point of 198  C (64.7  C for methanol), and superior energy density (7.56 kWh dm
3) or high theoretical capacity of 4.8 Ah ml
1 (4.0 Ah ml
1 for methanol) [1–3]. DEGFCs are based on the oxidation of EG at the anode electrode for which EG is fed directly into the fuel cells. Whilst, oxygen gas is reduced to water at the cathode. In addition, the electro-oxidation of EG in alkaline solution is pretty fast providing higher anodic current densities than those of other related fuels e.g., methanol, glycerol, erythritol, and xylitol [3,4]. All these advantages make DEGFCs be a promising alternative device of alternative energy. An ideal complete electro-oxidation mechanism of EG on the metal electro-catalyst to CO2 is based on reaction (1) [3]. However,

* Corresponding author. E-mail addresses: [email protected], [email protected] (M. Sawangphruk). http://dx.doi.org/10.1016/j.electacta.2016.06.162 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

this reaction needs rather high activation energy for breaking C

C bond of EG. To the best of our knowledge, there is at present no electro-catalyst, which can entirely convert EG to CO2 and H2O leaving a great challenge in this research area. Alternatively, there are so many incomplete reactions taking places for the electrooxidation of EG. For example in reaction (2), one mole of EG is oxidized by 14 moles of OH
leading to 2 moles of CO32
, 10 moles of H2O, and 10 electrons. Other incomplete oxidation mechanisms of EG at the metal electro-catalysts in alkaline solution are based on reactions (3)–(5) below [5] having the intermediates, which can poison the catalyst surfaces; CH2OH-CH2OH + 10OH
! 2CO2 + 8H2O + 10e


(1)

CH2OH-CH2OH + 14OH
! 2CO32
+ 10H2O + 10e


(2)

CH2OH-CH2OH + 10OH
! COO
-COO
+ 8H2O + 8e


(3)

CH2OH-CH2OH + 8OH
! 2HCOO
+ 6H2O + 6e


(4)

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CH2OH-CH2OH + 5OH
! CH2OH-COO
+ 4H2O + 4e


(5)

The undesirable intermediates such as carbonate (reaction (2)), oxalate (reaction (3)), formate (reaction (4)), and glycolate (reaction (5)) ions typically occur in the electro-oxidation process of EG at the metal electrodes depending on the overpotential of each reaction. In order to achieve the complete reaction (1) yielding 10 electrons, the development of new highly active catalysts is needed. In the past, precious Pt electrodes were used as the electro-catalysts toward EG oxidation in acidic media at room temperature yielding only 5% [6,7]. On the other hand, this reaction on the carbon-supported Pd catalysts exhibits much higher yields [8]. Also, Pd is more abundant in nature, less expensive, and could more efficiently promote EG oxidation in alkaline media when compared with Pt electro-catalyst [5,9]. Therefore, Pd is considered to be a promising electro-catalyst for EG oxidation. There are a number of carbon materials used as the catalyst supports such as carbon plate, carbon aerogel, carbon nanotube, and graphene [10–14]. Graphene has attracted huge attention by researchers due to its outstanding properties, i.e. high surface area [15], fast electron transport, and excellent thermal and electrical

conductivities [16]. However, the restacking of graphene sheets thermodynamically stable can lead to the uncertainty of the 2D graphene performance as the catalyst support limiting the diffusion of the electrolytes [15]. To address the restacking problem of graphene sheets, in this work a three-dimensional (3D) reduced graphene oxide (rGO) [17] aerogel has been synthesized with diluted oxygen-containing groups to improve the ionic conductivity of graphene materials and assist the interaction with Pd catalysts. The 3D rGO aerogel, an interconnected porous structure of neighboring rGO sheets via diluted oxygen containing groups enhancing specific surface area and conductivity of the material [18], was coated on the carbon fiber paper (CFP) forming the 3D rGO aerogel paper. Note, the CFP is an electrically graphitic sheet of the randomly arranged short carbon fibers having high flexibility, conductivity, and electrochemical stability [19,20]. The as-fabricated 3D rGO paper plays an important role to the electro-catalytic activity of the metal catalysts coated on its surface. Here, the as-electrodeposited Pd on the 3D rGO aerogel paper exhibits high catalytic activity with rather high anodic current density and high poisoning tolerance toward EG oxidation in alkali media. To the best of our knowledge,

Fig. 1. SEM images of 3D rGO aerogel coated on CFP with different magnifications (a, c), and Pd electrodeposited on top layer of 3D rGO/CFP with different magnifications (b, d) as well as N2 adsorption/desorption isotherms of 3D rGO aerogel and 2D rGO sheets restacked (e), and the pore size distribution of such materials (f).

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the Pd/3D rGO paper anode, which has not yet been prepared and reported, provides the highest anodic current density and poisoning tolerance to carbonaceous intermediates. In addition, the effect of hydrodynamic diffusion of EG fuel to the as-synthesized metal electro-catalysts is for the first time investigated in this work using a rotating disk electrode (RDE). Surprisingly, the Pd/3D rGO RDE fabricated exhibits ultrahigh anodic current density of 267.8 mA cm
2 at 3000 rpm indicating that the hydrodynamic diffusion of alcohol fuels plays an important role to the performance of the anode of the direct alcohol fuel cells. 2. Experimental 2.1. Chemicals and Materials All chemicals used in this work were of analytical grade and used without further purification. The chemicals consisted of graphite powder (20–40 mm, Sigma–Aldrich), sulfuric acid (98.0%, QRec), hydrogen peroxide (30.0%, Merck), hydrazine hydrate (98%, Sigma–Aldrich) potassium permanganate (99.0%, Ajax Finechem), ethylene glycol (99.5%, QRec), palladium (II) nitrate dihydrate (40.0% Pd basis, Sigma–Aldrich) and acetone (99.5%, QRec). Graphitized carbon fiber paper (CFP) with the trade name of SIGRACET1 GDL 10 BA (thickness = 400 mm, electrical resistance <12 mV cm
2, and an areal weight of ca. 85 mg m
2) was purchased from SGL CARBON SE (Germany). For solution preparation, the ultrapure water used as the solvent was purified by using the Milli-Q system.

239

2.2. Syntheses of 2D reduced graphene oxide (rGO) nanosheets and 3D rGO aerogel 2D rGO nanosheets were produced using a modified Hummers and Offeman method [21] with our modification previously reported ealswhere [19,22–25]. Briefly, 1 g of GO was dispersed in Mili-Q water (400 ml). Then, 0.5 M hydrazine hydrate was added into the suspension. The as-prepared suspension was heated at 90  C at atmospheric pressure for 2 days. The as-synthesized 2D rGO was washed with Mili-Q water for several times and collected by a vacuum filtration process. 3D rGO aerogel was synthesized by a hydrothermal reduction method [26,27]. Briefly, 1 g of GO was dispersed to Milli-Q water (400 ml) by ultrasonication for 30 min. Subsequently, 0.5 M hydrazine hydrate was added to the dispersed GO suspension and sealed in Teflon-lined autoclave at 180  C for 48 hr. The as-synthesized 3D rGO with was washed with Milli-Q water for several times in order to remove the residual agents and then filtrated by a vacuum filtration. The filtrated rGO powder was further dried by a freezing dry method giving a final 3D rGO aerogel product. 2.3. Fabrication of 3D rGO aerogel paper and electrodeposition of Pd electro-catalyst A 2.5-mg 3D rGO aerogel powder was dispersed and sonicated in 0.5 ml acetone. The 3D rGO dispersion was then sprayed on 2 cm2 CFP by our own established coating method [14,19,20,23,28] based on using an airbrush pen (Paasche Airbrush Company, USA)

Fig. 2. (a) Lower and (b) higher magnification TEM images of 3D rGO aerogel as well as (c) lower and (d) higher magnification HRTEM images of as-scrapped Pd electrodeposited on 3D rGO aerogel paper.

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with 0.3 mm brush nozzle. The pressure and temperature were 20 psi and 25  C, respectively. To electrodeposit Pd onto the asfabricated 3D rGO paper, the cyclic voltammetry (CV) technique was used by applying the cycling potential from
1 to 0 V vs. Hg/ Hg2SO4, K2SO4(sat'd) in 1 mM Pd(NO3)2 in 0.5 M H2SO4 at a scan rate of 0.1 V/s for 10 cycles. 2.4. Structural characterization and electrochemical evaluation The morphology of Pd/3D rGO aerogel paper was characterized using scanning electron microscopy (SEM; JSM35CF,) operated at 20 keV. Energy dispersive X-ray spectroscopy (EDX) was used to determine the elements on the as-prepared catalyst electrodes. The structure and oxidation state of Pd in the Pd/3D rGO paper were evaluated by X-ray absorption spectroscopy (XAS). Electrochemical activity of the as-prepared electro-catalysts was studied under ambient temperature (25  C) using a computercontrolled m-AUTOLAB II potentiostat (Eco-Chemie, Utrecht, The Netherlands) equipped with a FRA2 frequency response analyzer module running NOVA software under a three-electrode cell system including a working electrode, a Pt wire counter electrode, and a reference electrode. Three reference electrodes i.e. Hg/ Hg2SO4, (sat'd K2SO4), Ag/AgCl (3 M KCl), and Hg/HgO (1 M NaOH) were used in order to make sure that the effect of anions released from the reference electrodes do not play an important role to the catalytic activity of the as-prepared Pd electrodes. 3. Results and discussion 3.1. Physical Characterization A low magnification of SEM image in Fig. 1a shows 3D rGO aerogel coated on the CFP for which the fiber diameter of the CFP is about 10 mm. A higher magnification as shown in Fig. 1c shows the

inter-linked 3D porous framework of 3D rGO sheets for which high porosity of the top layer is clearly observed. An inset photograph in Fig. 1c shows a hydrogel product put on the Al foil with a cylindrical morphology containing about 97.5% water, which is the product after the hydrothermal reduction process. Note, the water content (Ww) of the hydrogel was calculated by Ww = (Wh
Wdh)/Wh  100%, where Wh is the total weight of the hydrogel and Wdh is the weight of the hydrogel in the dry state[29]. Fig. 1b and d show lower and higher magnification FE-SEM images of Pd nanoparticles deposited on top layer of the 3D rGO paper with an average diameter of < 50nm: The Pd nanoparticles are agglomerated forming a cauliflower-liked shape. EDX spectrum, an inset image in Fig. 1b, shows two major peaks at 0.06 and 0.28 keV associated with Ka of carbon and oxygen atoms of rGO and CFP materials, respectively. A peak at 2.84 keV is from La of Pd atoms. Further quantitative investigation of the porosity and surface area of the 3D rGO material was also carried out by a N2 gas adsorption/desorption technique for which the 3D rGO shows the nitrogen adsorption-desorption type 4 isotherm with a hysteresis loop type 2 (IUPAC) (Fig. 1e). The interconnected networks of the as-prepared material exhibit an impressive structure due to its high specific BET surface area of 248.63 m2 g
1 when compared with 2D restacked rGO sheets having a slit-shaped mesopore with a hysteresis loop type 3 (IUPAC) and a ca. 10-fold lower specific surface area of 25.34 m2 g
1. Note, the pore size distribution of both 2D and 3D rGO materials are mesoporous with a pore diameter of ca. 5–8 nm (see Fig. 1f). TEM images of 3D-rGO aerogel with lower and higher magnifications are shown in Fig. 2a and b, respectively. It is clearly observed that 3D-rGO aerogel consists of many wrinkles, which are from its framework structure. Whilst, the TEM images of 2D-rGO nanosheet previously reported elsewhere by our group present almost smooth surface [25].

Fig. 3. Narrow XPS spectra of (a) C1 s and (b) O1 s of 3D rGO aerogel, (c) FTIR spectra of GO, 2D rGO, and 3D rGO, and (d) Pd LIII-edge adsorption spectra of Pd/3D rGO aerogel.

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TEM images of as-scrapped Pd, electrodeposited on the 3D rGO aerogel paper, are shown in Fig. 2c and d for which Fig. 2c shows Pd nanoparticles with the diameter of about 30 nm agglomerated. HRTEM image in Fig. 2d shows that the interplanar spacing of Pd nanocrystals is 2.22 Å, which is respect to the (111) plane of a typical face-centered cubic (fcc) lattice structure of Pd metal [13,14,24]. Fig. 3a and b show narrow scan C1 s XPS spectra of GO precursor and 3D rGO aerogel, respectively. The C1 s XPS spectra mainly display four different functional groups that correspond to carbon atom, i.e. the honey-comb lattice of C-C bond (284.9 eV) and sp2 atomic structure of C in C

N (285.5 eV), C¼O (287.1 eV), and O¼C¼O (288.4 eV). This result is well agreeable with previous report[30]. The C/O atomic ratio of the GO is approximately 1.37. After reduction process by the hydrothermal method with hydrazine hydrate, the intensity peaks of oxygen functional group are significantly reduced with much higher C/O of 10.48. Furthermore, the intensity peak of C

N of rGO aerogel noticeably increases, rising from 1.26 to 4.22 due to the hydrothermal reduction process with hydrazine solution. Fig. 3c shows FTIR spectra of GO, 2D rGO, and 3D rGO for which oxygen containing groups can clearly be seen in an FTIR spectrum of GO. Whilst, diluted oxygen containing groups are found on the 2D rGO, and 3D rGO spectra. Raman spectra of graphite, GO, 2D rGO and 3D rGO aerogel are also shown in Fig. S1 of the supporting information for which the results are in good agreement with other previous report [13,14]. To further understanding the catalytic activity of the aselectrodeposited Pd catalyst, the oxidation number of Pd was determined by XAS (Synchrotron technique). Pd LIII-edge adsorption spectra are shown in Fig. 3d. The edge energy of the Pd coated on top of 3D rGO paper is close to Pd metallic foil with zero

241

oxidation number (Pd0) [24,31], indicating that using the simple electrodeposition technique can provide pure Pd0 electro-catalyst without other Pd compounds. The intensity beyond edge of the sample peak is at 3173.6 eV caused by the electronic transition that rises from 2p to 4d band [32] and E0 is at 3173.4 eV calculated from Athena program [33]. 3.2. Electrochemical active surface area (ECSA) Fig. 4a shows CV analysis for the ECSA determination of the asprepared electrodes in 0.5 M H2SO4. Note, CFP itself shows no electrochemical response in sulfuric acid solution. All Pd-based electro-catalysts show an anodic peak between
0.4 and
0.5 V vs. Hg/Hg2SO4 associated with hydrogen desorption region [34] (see reaction (6)). Pd-Hads $ Pd + H+ + e


(6)

The oxide growth on the Pd electrode, which occurs at applied positive potentials from about 0 to 0.8 V vs. Hg/Hg2SO4, is as the following reactions (7)-(9): Pd + H2O ! Pd-(OHads) + H+ + e


(7)

Pd-(OHads) + H2O ! Pd-(OHads)2 + H+ + e


(8)

Pd-(OHads)2 ! PdOads + H2O

(9)

The reduction process can be observed in the cathodic peak at an onset potential of ca. 0.0 V vs. Hg/Hg2SO4 attributing to reduce the oxide formed on the catalyst surface as shown in reaction (10) [35]

Fig. 4. (a) CVs of Pd electro-catalysts electrodeposited on different carbon electrodes in 0.5 M H2SO4 at 0.05 V/s as well as (b) CVs, (c) chronoamperograms, and (d) Nyquist plots of the Pd-based catalyst electrodes in 1 M EG in 0.5 M KOH.

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PdOads + 2H+ +2e
! Pd + H2O

(10)

Thus the area of hydrogen desorbed region on the CV curves in Fig. 4a can be evaluated and converted to the number of Pd active sites relating to the charge passed for hydrogen atom-adsorbed monolayer [36], which associates to the electrochemical active surface area (ECSA). The ECSA was determined from Fig. 4a by a following equation, ECSA = (QH)/(210  m) where QH is the accumulated charge for the monolayer hydrogen adsorbed (mC), 210 is a constant defined as the required charge density for the molecules of hydrogen adsorbed on palladium with a homogeneous and single layer (mC cm
2) [37], and m represents the total amount of Pd loaded on the 3D rGO aerogel/CFP electrode (mg). As listed in Table 1, the ECSA of the Pd electro-catalyst is improved by the 3D rGO aerogel support. This is because the existence of a 3D network structure is beneficial for allowing the fast Pd2+ diffusion from the bulk electrolyte to the 3D rGO pores due to capillary force and successively getting reduced to be Pd metal [38]. The 3D rGO support is then an ideal place of active Pd catalysts (see the TGA result in Fig. S2 of the supporting information and Table 1). It was previously reported that the more active site of Pd electro-catalysts exhibited higher catalytic activity toward alcohol oxidation [13,14]. The charge density due to the monolayer hydrogen adsorbed at the Pd surface (mC cm
2) based on the geometrical surface area and the ECSA determined in this work are compared with other previous work based on Pd electro-catalysts (see Table 1). The ECSA of the Pd/3D rGO/CFP calculated is 96.26 m2 g
1, which is 1.05-, 4.02-, and 5.75-fold higher than those of Pd/2D rGO/CFP, Pd/GO/ CFP, and Pd/CFP, respectively. Also, this ECSA is rather high when compared with other previous report listed in Table 1. 3.3. Electro-catalytic performance of the as-electrodeposited Pd electro-catalysts toward ethylene glycol oxidation in alkali solution The catalytic activity of Pd electrodeposited on different supports (i.e. 3D rGO/CFP, 2D rGO/CFP, GO/CFP, and CFP) toward EG oxidation in alkaline solution was investigated by CV at 0.05 V/s. As shown in Fig. 4b, the electrochemical response presents an irreversible process appearing two main oxidation peaks in the forward and backward scans, respectively. In forward scan, the oxidation reaction presents approximately 0.20 V vs. Ag/AgCl for

the Pd/3D rGO/CFP, which is respect to an onset potential of EG oxidation at the Pd electrode surface [35]. Note, there is no significant difference in electrochemical results when using Ag/AgCl and Hg/HgO as the reference electrodes (see Fig. S3 of the supporting information). This means that Cl
from the reference electrode does not play an important role to the electro-catalytic activity of the Pd/3D rGO/CFP towards the ethylene glycol oxidation in alkali media. However, there is an unclear explanation on an oxidation peak of the backward scan at this stage. In the past, the backward oxidation peak was believed that it is due to the stripping oxidation of the remaining adsorbed carbonaceous species on the metallic catalyst surfaces [8,48–50]. Until recently, it was reported that the oxidation peak of methanol in the backward scan on the surfaces of Pt/C, unactivated PtRu/C, and activated PtRu/C is most possibly due to the oxidation of freshly chemisorbed alcohol fuel species, not the remaining adsorbed carbonaceous species [51]. In this work, Pd was also electrodeposited on fluorine-doped tin oxide (FTO) electrode without carbon element and tested toward the electrooxidation of 1 M EG in 0.5 M KOH. Ex-situ ATR FTIR measurements carried out shows the C

O (alcohol) stretching peak on the used and washed Pd/ITO after the backward scan. This result indicates that the backward scan cannot remove all of adsorbed C

O alcohol species on the surface of the Pd electro-catalyst (see Fig. S4 of the supporting information). Note that the oxidation peak in the backward scan in this work is pretty small when compared with all other previous work regarding to the electro-oxidation of EG listed in Table 2. It was reported that the onset potential and anodic current density of the alcohol oxidation in the forward scan are the gauges judging the catalytic activity of the electro-catalysts [40,51,52]. The onset potential provides a rough measure of the overpotential requirements of the EG oxidation reaction for which the Pd/3D rGO/CFP can oxidize the EG at lower potential when compared with other catalyst electrodes fabricated in this work and the related electrodes previously reported. This result clearly implies that the EG oxidation at the Pd/3D rGO/CFP has the lowest overpotential or activation energy leading to the highest catalytic activity among others. Further investigation at faster scan rates was also carried out. The Pd/3D rGO/CFP at each scan rate has higher forward peak

Table 1 Electrochemical active surface area (ECSA) of Pd electro-catalysts on different carbon supports. Catalyst electrodes

Pd/hollow carbon Pd nanowires Pd film Pd/CNT Pd/PPY–graphene Pd/modified carbon Pd-(NiFe)x-(NiFeO)y/C Pd–Fe2CoOx/C Pd/CNT Pd/rGO Pd black 10% Pd/C Pd nanodendrites/rGO Pd NPs (7.3 nm)/C Pd/DNA/Graphene Pd/rGO Pd/CNT Pd/3D rGO aerogel/CFP Pd/2D rGO/CFP Pd/GO/CFP Pd/CFP

Solutions

1 M KOH 1 M KOH 1 M KOH 0.5 M NaOH 0.5 M NaOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4

Weight (mg/cm2)

ECSA

Refs.

(mC/cm2)

(m2/g)

0.30 0.24 1.10 – 0.088 – – – – – 0.06 0.06 0.2

10.72 10.60 3.40 – – – – – – – – – –

8.80

0.17 – – 0.42 0.36 0.24 0.27

– – – 84.90 65.50 12.10 9.50

30.8 41.8 58.4 71.7 64.18 28.6 45.5 8.09 19.7 9.09 14.6 8.22 45.5 28.6 96.26 91.81 23.98 16.73

[39] [40] [40] [41] [42] [43] [44] [45] [41] [41] [8] [8] [8] [46] [47] [41] [41] This This This This

work work work work

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243

Table 2 Catalytic activities of Pd-based electro-catalysts toward EG oxidation reaction in alkaline media. Electro-catalysts

Fuels

Mass (mg)

If (mA/cm2)

Ref.

FeCo/Fe/Pd/C Pd/CeO2 Pd/NiO Pd/Co3O4 Pd/Mn3O4 P-MWCNT/Pd F-MWCNT/Pd S-MWCNT/Pd Pd/rGO nanodendrites Pd black SF/MWCNT/PdSn Pd/3D rGO RDE (3000 rpm)

0.5 M EG in 0.5 M KOH 1 M EG in 1 M KOH 1 M EG in 1 M KOH 1 M EG in 1 M KOH 1 M EG in 1 M KOH 0.5 M EG in 2 M KOH 0.5 M EG in 2 M KOH 0.5 M EG in 2 M KOH 0.5 M EG in 0.5 M KOH 0.5 M EG/0.5 M KOH 0.5 M EG in 0.5 M KOH 1 M EG in 2 M KOH

0.018 0.530 0.375 0.450 0.450 0.125 0.125 0.125 0.06 0.06 N/A 0.360

5.01 68.00 104.00 98.00 98.00 18.27 12.28 22.15 33.70 mA/mg 30.50 mA/mg 51.90  0.02 267.8

[58] [59] [59] [59] [59] [60] [60] [60] [8] [8] [55] This work

current density than other electrodes (see an inset image in Fig. 4b). For example at 0.05 V/s, the Pd/3D rGO/CFP has 60.70 mA cm
2, which is 1.26-, 1.80-, and 2.64-fold higher than those of Pd/2D rGO/CFP, Pd/GO/CFP, and Pd/CFP electrodes, respectively. In addition, the oxidation peak current density in the forward scan of all electrodes has a linear relationship with the square root of the scan rate. It indicates that, apart from the fast reaction kinetics of the Pd/3D rGO/CFP, the diffusion of EG to the electrode may also play an important role [53]. The tolerance stability of the as-prepared catalyst electrodes was also investigated using chronoamperometry. By applying 0.25 V vs. Ag/AgCl through the catalyst electrodes in 1 M EG in 0.5 M KOH, the Pd/3D rGO/CFP has higher anodic current density than that of Pd coated on other supports over 2500 s and after that reaching a pseudo-steady state [54] (see Fig. 4c). The catalyst stability was also studied by the CV over 100 cycles (see Fig. S5) for

which the Pd/3D rGO/CFP Exhibits 54 mA cm
2 after 100 cycles, which is 1.38-, 1.86-, and 2.34-fold higher than those of the Pd/2D rGO/CFP, Pd/GO/CFP, and Pd/CFP electrodes, respectively. As a result, it can be concluded here that the Pd/3D rGO/CFP has higher poisoning tolerance than others. It is also necessary to note here that overall the Pd electro-catalysts coated on GO and rGO materials have higher anodic current density than the Pd/CFP. This implies that the carbonaceous intermediate species on the catalyst surfaces basically inhibiting the catalytic activity and stability of the Pd catalysts can react with the hydroxyl groups of GO and rGO and spill over to produce CO2 releasing the ECSA of the Pd. Therefore, the improved stability of the Pd-based electro-catalysts can be ascribed to the facilitated hydroxyl groups of GO and rGO. To further investigate the reaction kinetics of the EG oxidation at the Pd-based electrodes, the electrochemical impedance spectroscopy (EIS) was carried out in 1 M EG in 0.5 M KOH at a

Fig. 5. (a) CVs and (b) Nyquist plots of Pd/3D rGO aerogel/CFP in 1 M EG in different concentrations of KOH (i.e. 0.5, 1, 2, 3 and 4 M) at 0.05 V/s as well as (c) CVs and (d) Nyquist plots of Pd/3D rGO/CFP in different concentrations of EG (i.e., 0.2, 0.5, 1, 2 and 3 M) in 2 M KOH.

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potential of 0.25 V vs. Ag/AgCl from 10 mHz to 1 MHz. Fig. 4d shows Nyquist plots of all as-prepared electrodes (high frequency on the left bottom of the plots) for which the Pd/3D rGO/CFP has smaller hemisphere than Pd/2D rGO/CFP, Pd/GO/CFP, and Pd/CFP, respectively indicating that it has lower charge transfer resistance or in another word faster electron transfer. The Nyquist spectra in Fig. 4d are well fitted with an equivalent RS(RCTCdl) circuit[55], where Rs is the electrolyte resistance, RCT is the charge transfer resistance, and Cdl is the double layer capacitance. Rs observed is about 8 V for all electrodes. RCT is typically relied on the arc diameter for which the fitted RCT values are 47.6, 47.8, 59.1, and 60.1 V for Pd/3D rGO/CFP, Pd/2D rGO/CFP, Pd/3D rGO/CFP, Pd/GO/ CFP, and Pd/CFP, respectively. 3.4. Effects of EG and KOH concentrations

Fig. 6. (a) CVs of Pd/3D rGO aerogel/CFP in 1 M EG in 2 M KOH at 0.05 V/s at different rotation speeds from 0 to 4000 rpm and (b) chronoamperograms at 0 and 3000 rpm over 15000 s.

To further investigate the effects of EG and KOH concentrations on the catalytic activity of the as-electrodeposited Pd electrocatalysts, the CVs of the Pd/3D rGO/CFP in 1 M EG in different KOH concentrations (0.5-4.0 M) at a scan rate of 0.05 V/s are shown in Fig. 5a. The CVs clearly show that increasing KOH concentrations from 0.5 to 2 M (at pH between 13.67 and 14.60) can significantly improve the electro-oxidation of EG by reducing the onset forward oxidation potential or in another word reducing the overpotential of the EG oxidation. Note, there is no significant improvement for higher concentrations (3 and 4 M). In addition to CV, EIS result carried out and shown in Fig. 5b do support well the CV result. 2 M was therefore considered as the optimum concentration hereafter. Subsequently, EG concentrations were varied from 0.2-3.0 M. Fig. 5c shows that reducing EG concentrations from 3 to 0.2 M, the onset oxidation potential of the forward scan is shifted to more negative potential,
0.07,
0.09,
0.12,
0.16 and
0.20 V vs. Ag/ AgCl for 3, 2, 1, 0.5, and 0.2 M, respectively. This is because the decreasing iR drop due to higher conductivity of the solutions when reducing concentrations of insulating EG chemical in KOH electrolyte [56]. The anodic peak current density rises sharply when the EG concentration increases from 0.2 to 1 M and rather stable at 1–3 M. Nyquist plots well support the CV results since RCT values at high concentrations of EG (1–3 M) are much lower than those at 0.2 and 0.5 M (see Fig. 5d) [57]. As the results, 1 M EG in 2 M KOH was considered to be the optimum concentration providing the highest catalytic activity of the Pd/3D rGO aerogel paper with an anodic peak current density of 125.92 mA cm
2.

GO was produced by a modified Hummers method and then used as a precursor for producing 3D rGO aerogel using a hydrothermal reduction with hydrazine hydrate and subsequent freezing dry method. The 3D rGO aerogel was spray-coated on the CFP and used as the catalyst support. A simple electrodeposition method was then used to fabricate the Pd/3D rGO aerogel paper and employed as the electro-catalyst toward EG oxidation in alkali media. Effects of the material supports including 3D rGO/CFP, 2D rGO/CFP, GO/CFP, CFP and electrolyte concentrations as well as hydrodynamic diffusion were systematically investigated. The ECSA of the Pd/3D rGO/CFP calculated is 96.26 m2 g
1, which is 1.05-, 4.02-, and 5.75-fold higher than those of Pd/2D rGO/CFP, Pd/ GO/CFP, and Pd/CFP, respectively. The Pd/3D rGO/CFP also provides higher anodic current density, lower overpotential, and higher poisoning tolerance when compared with other three electrode catalysts. 1 M EG in 2 M KOH finely tuned is an optimum concentration providing the highest catalytic activity of the Pd/3D rGO aerogel/CFP with a maximum anodic peak current density of 125.92 mA cm
2. More interestingly, the Pd/3D rGO RDE fabricated exhibits a much higher anodic current density of 267.8 mA cm
2 at 3000 rpm indicating that the hydrodynamic diffusion of EG plays an important role to the performance of the electro-catalyst electrode.

3.5. Effect of hydrodynamic diffusion

Acknowledgement

In order to study the hydrodynamic effect, the Pd was electrodeposited on 3D rGO aerogel rotating disk electrode (RDE) toward EG oxidation at different rotating speeds of 0– 4000 rpm. Here, 5 ml of 3D rGO aerogel (10 mg) in acetone (0.5 ml) was dropped on the 3 mm GC RDE electrode before electrodeposition of Pd using the cyclic voltammetry method at a potential range from
1 to 0 V vs. Ag/AgCl in 1 mM palladium (II) nitrate in 0.5 M H2SO4 at a scan rate of 0.1 V/s for 10 cycles. In Fig. 6a, by increasing the rotation speed of the catalyst electrode from 0 to 3000 rpm, the as-fabricated Pd/3D rGO aerogel RDE provides the anodic current densities at the onset potential of 163.2, 208.6, 222.3, 242.0, 267.8 mA cm
2 for 0, 500, 1000, 2000, and 3000 rpm, respectively. This indicates that the hydrodynamic diffusion introduced by the RDE can overcome the diffusion limit and significantly increase the catalytic performance of the as-fabricated Pd electro-catalyst. Note, the same experiment carried out at 4000 rpm presents the same peak current density as that at 3000 rpm indicating that 3000 rpm is the optimum rotation speed.

This work was financially supported by the Thailand Research Fund and Vidyasirimedhi Institute of Science and Technology (RSA5880043).

4. Conclusions

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