Preparation of Carbon-Supported Ternary Nanocatalysts Palladium-Vanadium-Cobalt for Alcohol Electrooxidation

Carbon-supported nanocatalysts palladium-vanadium-cobalt (PdVCo) were synthesized via ethylene glycol (EG) reduction reaction and NaBH4-assisted reduction. e electrocatalytic performance for alcohol oxidation in alkaline solutions was investigated. e XRD and EDX results confirmed the incorporation of V and Co with Pd lattice to form the ternary nanocatalysts PdVCo in a single phase. e NaBH4-assisted EG reduction process exhibited highly dispersed nanoparticles with a uniform size, and the electrochemical surface area (ECSA) determined by cyclic voltammetry in 1M KOH was also superior. In electrocatalysis performance, the cyclic voltammetry (CV) and chronoamperometry (CA) results presented an excellent electrocatalytic activity and stability of the PdVCo-20EG-20NaBH4 sample in the alcohol electrooxidation as compared to other synthesized samples with the steady current of 52mA/cm and 21.9mA/cm in methanol and ethanol, respectively.


Introduction
e blooming interest in using Direct Alcohol Fuel Cells (DAFCs) as portable and mobile power sources is rooted in their desirable features of relatively small environmental footprint, compact system design, and higher volumetric energy densities compared with existing technologies. [1,2] DAFC relies on the oxidation of alcohol, such as methanol, ethanol, or glycerol, on a catalyst layer to form carbon dioxide (CO 2 ). Water (H 2 O) is consumed at the anode and produced at the cathode. Hydrogen ions formed during the oxidation of alcohol are transported across the proton exchange membrane to the cathode, where they react with oxygen (O 2 ) to produce water. Electrons are transported through an external circuit from the anode to the cathode, providing power to external devices.
Ethylene glycol reduction is common process for the preparation of metal nanoparticles, but this process needs to be assisted by high temperature or microwave irradiation to improve the reduced performance. [6,15,28] However, NaBH 4 is considered as a force reducing agent which can reduce metallic ions at room temperature, and the disadvantage of NaBH 4 process is the formation of irregular particle size. [29][30][31][32][33] As a consequence, the combination process of ethylene glycol (EG) and NaBH 4 can get better reduction to prepare the nanoparticles.
In this work, we prepared the carbon-supported ternary nanocatalysts palladium-vanadium-cobalt (PdVCo) by ethylene glycol reduction process and, the NaBH 4 -assisted reduction was also considered. e nanocatalysts PdVCo showed a comparable alcohol electrocatalytic oxidation activity and stability as compared to the original Pd catalyst.

Carbon-Supported Nanocatalyst Preparation. Carbon
Vulcan powder underwent two-step treatment to eliminate the metal traces. In the acid treatment, the carbon powder was soaked in 1 M HNO 3 for 10 hours, coupled with ultrasonic irradiation for 15 minutes per hour; the powder was washed several times with DI water and dried at 70°C overnight. Next, the carbon powder was soaked in 1 M NaOH for 1 hour, washed with DI water, and dried at 70°C overnight. After that, the powder was heated at 200°C for 2 hours to obtain the pretreated carbon powder. 0.8850 g PdCl 2 was dissolved in 100 mL 0.01 M HCl, 0.91000 g V 2 O 5 was dissolved in 100 mL 0.15 M NaOH, and 0.915 g Co(NO 3 ) 2 .6H 2 O was dissolved in 100 mL DI water to obtain precursor solution 0.05 M H 2 PdCl 4 , 0.05 M Na 3 VO 4 , and 0.05 M Co(NO 3 ) 2 , respectively. e reduction agents were EG and 0.01 M NaBH 4 . e carbon-supported nanocatalysts PdVCo at 20 wt% were prepared by the polyol reduction process. e precursor's solutions H 2 PdCl 4 , Na 2 VO 3, and Co(NO 3 ) 2 with the stoichiometry Pd : V : Co ratio of 1 : 1 : 1 were stirred in an ultrasonic condition; then, 100 mg pretreated Vulcan XC-72R carbon powder was added, and the mixture was ultrasonicated for 30 minutes. e reduction agents were dropped into the mixture, following four samples: (i) 15 mL EG, (ii) 20 mL EG, (iii) 15 mL EG + 20 mL NaBH 4 , and (iv) 20 mL EG + 20 mL NaBH 4 . e products were filtered, washed with DI water several times, and dried in an oven at 120°C for 2 hours. We then denoted the respective nanocatalyst samples: (i) PdVCo-15EG; (ii) PdVCo-20EG; (iii) PdVCo-15EG-20NaBH 4 ; and (iv) PdVCo-20EG-20NaBH 4 . e structure was characterized by X-ray diffraction by using a D8-Advance Diffractometer (Bruker) with Cu Kα radiation (λ �1.5406Å). e size and morphological features were analyzed by SEM with energy-dispersive X-ray (EDX) detector using a Hitachi S-4800 instrument, and transmission electron microscopy (TEM) was performed using a JEOL JEM 1400 microscope at 120 kV. Brunauer-Emmett-Teller specific surface area (S BET ) was determined by nitrogen adsorption measurement (Quantachrome Autosorb 1C) with an out-gas process at 200°C for 2 hours.

Electrochemical Measurements.
All the electrochemical measurements were carried out at room temperature under nitrogen atmosphere on potentiostat/galvanostat PGSTAT320N (Metrohm, AG) apparatus with a threeelectrode cell, including a glassy carbon electrode (GCE, 3 mm in diameter) as working electrode, a platinum wire as counter electrode, and a Ag/AgCl in 3.5 M KCl electrode as reference electrode. To prepare the working electrode, 2.5 mg nanocatalysts PdVCo were put into 1.0 mL ethanol and 25 μl PTFE solution; the mixture was then ultrasonicated for 1 hour to obtain a homogeneous suspension. 75 μl of suspension was dropped on GCE and dried at 40°C in 1 hour. e electrocatalytic behavior of synthesized nanocatalysts was studied by cyclic voltammetry (CV) and chronoamperometry (CA). In order to determine the electrochemical surface areas (ECSAs) of the nanocatalysts, the CV measurements were carried out in a solution of 1 M KOH at a scan rate of 50 mV/s from −1 V to 0.5 V. For the alcohol oxidation electroactivity, the CV measurements in a solution of 1 M CH 3 OH + 1 M KOH and 1 M C 2 H 5 OH + 1 M KOH were conducted from −0.8 V to 0.3 V with a scan rate of 50 mV/s. CA measurements were performed in the same alcohol-contained KOH solution at an applied voltage of −0.130 V or −0.150 V for an hour. Figure 1 shows the typically XRD pattern of carbon-supported nanocatalysts PdVCo with the metal loading of 20 wt% (sample PdVCo-20EG-NaBH 4 ). e characteristic peaks of face-center cubic crystalline Pd (JCPDS Card 00-005-0681) are observed with the planes (111), (200), and (220), suggesting the singlephase structure for all nanocatalysts. Compared to the original phase Pd (a � 3.8971Å), the characteristic peaks of PdVCo-20EG-NaBH 4 quietly shift toward higher 2θ, reflecting the incorporation of V and Co into the Pd lattice to form a ternary-alloy phase PdVCo. Moreover, the cubic lattice parameter of PdVCo-20EG-NaBH 4 is slightly decreased to a � 3.8618Å (detailed in Table S1) due to the smaller atom radius of Co and V. e broadening of the diffraction peaks is considered as the nanocrystalline characteristic of nanocatalysts PdVCo. e average crystallite size is calculated from the full width of the half maximum (FWHM) of highest intensity peak (111) through Debye-Scherrer equation (1) [13,20]:

Physiochemical Characterization.
where d hkl is the average crystallite size; k is the constant depending on the crystallite shape (0.9); λ is the wavelength of copper Kα X-ray radiation (1.5406Å); β is the FWHM (in radian) of the most intense peak (in radian) and is determined by X'Pert HighScore Plus software; and θ is the diffraction angle. e average crystallite sizes of nanocatalysts PdVCo are gathered in Table S1. We observed that the crystallite sizes are around 5-6 nm. Although no Co and V peaks can be seen in XRD patterns, the EDX spectrum ( Figure 2) of nanocatalysts indicates clearly the presence of the elements V and Co in ternary alloy; the mass ratios of Pd : V:Co are shown in Table 1. e Co-content and especially V-content in two NaBH 4 -assisted reduction samples are higher than those in the other samples; the atomic ratios of Pd : V : Co in PdVCo-15EG-NaBH4 and PdVCo-20EG-NaBH4 are 1 : 0.35 : 0.59 and 1 : 0.55 : 0.86, respectively. e results revealed the beneficial support of NaBH 4 for trimetallic-alloy reduction. Figure 3 shows the TEM images and particle size distribution of the nanocatalysts. e synthesized nanoparticles PdVCo have a small and uniform size and are well dispersed on the carbon surface with a narrow particle size distribution of 3 nm to 9 nm. In case of EG reduction agent, the nanoparticles are found with an average particle diameter of 6.8 nm, and the particle size larger 6 nm occupied more than 50%. Rather, in case of NaBH 4 -assisted reduction agents, the nanoparticles look smaller; the major particle size lies in range 5 nm to 7 nm. e NaBH 4 assisted in the polyol reduction process that leads to little smaller and more uniformly sized nanoparticles. [29] Moreover, the incorporation of V and Co also supports a decrease of catalyst particle size and the modified-surface structure of Pd that leads to the surfaceadsorbed species, and the electronic effect contributes to the alcohol electrooxidation on the Pd nanoparticles. [31,34,35]. e BET surface area (S BET ) of nanocatalysts was evaluated by N 2 absorption and is detailed in Table 2. e increase of S BET , which was caused by the increase of EG, decreased the nanoparticle size. e smaller nanoparticle size can be favorable for the higher ECSA and catalytic activity for alcohol electrochemical reaction.

Electrochemical Behaviors.
In order to estimate the electrochemical surface area (ECSA), the CV measurement was performed in 1 M KOH at a scan rate of 50 mV/s ( Figure 4). Commonly, the CV curve of Pd-based nanocatalysts in KOH media presents three regions: (i) a potential region lower than −0.7 V assigned to the oxidation of the absorbed and the absorption of hydrogen from the catalyst surface; (ii) a potential region from −0.7 to −0.2 V corresponding to the formation of palladium hydroxides; and (iii) a potential region from −0.2 to 0.4 V relating to the palladium oxidation. For the reverse scan, the reduction of palladium oxide (Pd-O layers) to Pd 0 appeared around −0.35 V. [10,27,33] e ECSA can be estimated by PdO reduction peak in CVs according to the following equation [27]: where Q s is the total charge (mC) determined by integrating current peak of palladium oxide reduction, Q C is the charge needed for the reduction of PdO monolayer, and m is the Pd loading in mg. e ECSA was also determined by the Coulombic charge corresponding to the oxide reduction peak at a negative potential of −0.36 V (vs. Ag/AgCl) in Figure 4. e calculated ECSAs are detailed in Table 3, e PdVCo-15EG-20NaBH 4 (282 cm 2 /mg with Q C � 0.581 mC/ cm 2 ) and PdVCo-20EG-20NaBH 4 (375 cm 2 /mg with Q C � 0.741 mC/cm 2 ) catalysts exhibit higher ECSAs than the EG-reduced catalyst. e alcohol electrooxidation was performed by cyclic voltammetry on the nanocatalysts PdVCo in alkaline medium. e cyclic voltammograms are shown in Figures 5 and  6. Although mechanisms of electrooxidation of methanol and ethanol are different, their cyclic voltammograms exhibit the same motif with two well-defined peaks in each curve. [2] In the forward scan, the oxidation peak can be assigned to the oxidation of freshly chemisorbed species coming from methanol or ethanol adsorption. e oxidation peak in the reverse scan associated with the removal of Journal of Chemistry carbonaceous species (mostly in the form of linearly bonded Pd�C�O) is not completely oxidized in the forward scan compared to the oxidation of freshly chemisorbed species. [36][37][38][39][40] e magnitude of the peak current in the forward scan indicates the electrocatalytic activity of the electrocatalysts for alcohol oxidation. Furthermore, the ratio of the forward anodic peak current density (I f ) to the reverse anodic peak current density (I b ), I f /I b , is used to determine the catalyst's tolerance to the intermediate carbonaceous species accumulated on the electrode surface. [41,42] A higher ratio of I f /I b promotes a better electrocatalytic activity and resistance to CO poisoning on the electrode surface. e potential and current density of the alcohol oxidation peak in the forward scan and I f /I b ratio are detailed in Table 3. Figure 5 displays the cyclic voltammetry on nanocatalysts PdVCo at a scan rate of 50 mV/s in the solution of 1 M CH 3 OH + 1 M KOH. We observed that the current density became higher with the increase of reduction agent. Particularly, the highest current density (86.7 mA/cm 2 ) can be found in the mixture of reduction agent (sample PdVCo-20EG-20NaBH 4 ), which can be attributed to the fact that the NaBH 4 assisted in the EG reduction process with highly dispersed nanoparticles with a uniform size (4-5 nm) and the highest electrochemical surface area. [29] Moreover, the I f /I b ratio varies among the PdVCo nanocatalysts. For EG reduction agent, the ratios are almost 2.5; for NaBH 4assisted reduction, they increased significantly. e ratios for PdVCo-15EG-20NaBH 4 and PdVCo-20EG-20NaBH 4 are 2.82 and 2.98, respectively. On the other hand, the onset potential of PdVCo nanocatalysts looked more negative than those of sample Pd-20EG-20NaBH 4 ( Figure S3 and Table S2). e more negative onset potential can suggest the enhancement of Co tolerance and increase of electrocatalytic performance as well as stability. [31,33,43] In sum, the data from CV measurements indicated that the nanocatalyst PdVCo-20EG-NaBH 4 exhibited superior catalytic activity for methanol oxidation.
e CVs of the nanocatalysts in a solution of 1 M KOH + 1 M C 2 H 5 OH are shown in Figure 6. Two well-defined oxidation peaks can be observed in the five CV curves.
e PdVCo-20EG-20NaBH 4 catalyst shows the best catalytic activity and poisoning tolerance among the five catalysts, as evidenced by its largest peak current and i f /i b ratio. It is apparent that the size effect promotes the higher catalytic activity of nanocatalysts PdVCo for the alcohol electrooxidation.
In order to evaluate the long-term activity of the alcohol oxidation reaction on nanocatalysts PdVCo, we performed the CA measurements for 3600 s at −0.130 V (vs. Ag/AgCl) in solution of 1 M KOH + 1 M CH 3 OH (shown in Figure 7)

Conclusion
e ternary-alloy nanocatalysts PdVCo were successfully prepared by EG reduction, and the atoms V and Co were incorporated into Pd lattice to form a single phase. Moreover, the NaBH 4 assisted not only in enhancing V-content and Co-content but also in slightly decreasing nanoparticle size, which led to a better electrocatalysis performance for alcohol oxidation.
e PdVCo-20EG-20NaBH 4 exhibited superior catalytic activity and stability as compared to the other synthesized samples with the steady current of 52 mA/ cm 2 in methanol and 21.9 mA/cm 2 in ethanol.

Data Availability
e data used to support the findings of this study are included within the supplementary information files.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.