The Effect of Deposition Potential on the Electrodeposition of Platinum Nanoparticles for Ethanol Electrooxidation

Platinum (Pt) nanoparticles were successfully prepared using square wave pulse deposition technique by varying the upper potential. The X-ray energy dispersive spectrum pattern confirmed the formation of Pt nanoparticles on the fluorine-doped tin oxide coated glass substrate. The results of scanning electron microscopy showed that the potential of 0.60 V was able to produced a large number of Pt particles with a unique morphology. The ethanol electrooxidation test conducted using cyclic voltammetry showed that Pt 0.60v has the lowest charge transfer resistance value showing high catalytic activity which could be associated to the increase of particle number and its active sites that activated the redox reactions in the system.


Introduction
In recent years, direct ethanol fuel cells (DEFCs) and direct methanol fuel cells (DMFCs) have high possibility to replace conventional fossil fuels and reduce energy crisis and green house gas emissions. Compared to DMFCs, DEFCs demonstrates remarkable advantages, such as safer and easier to store and transport as well as lower costs for producing and handling ethanol fuel [1,2]. Moreover, ethanol significantly has higher energy density than methanol (6.28 kWh/L for ethanol vs 4.82 kWh/L for methanol) [3]. Demirci and M. Li et al., also reported that ethanol has a high specific energy which is 8.01 kWh·kg -1 comparable to gasoline [4,5].
In DEFCs, an active electrocatalyst with high activity was required to break the C-C bond in ethanol [6] so the complete ethanol oxidation occured at lower overpotential. These reactions aimed to improve cell performace, ethanol conversion rate, and fuel efficiency [7]. Platinum (Pt), has been reported, as an electrocatalyst that provides a large number of active coordination sites and relatively high selectivity to promoted inter-carbon alcohol bonds cleavage [8]. Pt catalyst activity can also be increased by modifying the structure during synthesis [9]. One of the method that can be used to modify the structure of Pt is electrodeposition. This method commonly used due to its simple process, low cost, and the ability to regulate the growth of the material to be synthesized [10,11]. The electrodeposition technique has several controllable parameters including the concentration of the precursor, the supporting electrolyte, the time, and the deposition potential. According to the previous studies, the potential used in electrodeposition process can affect the morphology and electrochemical properties [12]. In this study, Pt nanoparticles was synthesized through electrodeposition by varying the potential, thus it was expected the morphology, impedance, and catalytic performance of ethanol electrodeposition can be increased.

Materials and Method
The method used to synthesize Pt nanoparticles is a square wave pulse deposition carried out at room temperature in a salt solution of K2PtCl6 1 mM and a Published by The Center for Science Innovation supporting electrolyte of H2SO4 1 M. The materials used for this study consisted of K2PtCl6, H2SO4, KCl, C2H5OH 96%, and NaOH. All the materials were purchased from PT Merck Indonesia. While for the substrate used fluorine-doped tin oxide (FTO) coated glass 10 ohm/sq purchased from NSG Pilkington. The electrodeposition process is controlled by eDAQ ER466 which is connected to three electrode systems: Pt wire as a counter electrode, Ag/AgCl (3 M) as a reference electrode, and FTO as a working electrode. The electrodeposition of Pt nanoparticles occured at a lower potential -0.5 V and different upper potentials of 0.30, 0.60, and 1.00 V for 10 minutes deposition time. After the electrodeposition process is completed, the sample is rinsed with aquadest and dried. The samples were labelled as Pt0.30V, Pt0.60V, and Pt1.00V, respectively. The synthesized samples were subsequently characterized by X-ray Diffraction (XRD) (SmartLab Rigaku Co. Ltd., Japan) using the Cu-Kα radiation source to identify the presence of a Pt nanoparticle phase in the sample, scanning electron microscopy was connected with energy dispersive X-ray (SEM-EDX) (FEI Inspect F50) to observe the morphology and elemental composition of the sample, electrochemical impedance spectroscopy (EIS) (Corrtest CS310) at a frequency of 50 kHz to 0.1 Hz in a 0.5 M KCl solution to determine the electrochemical properties of the sample. Ethanol electrooxidation activity in sample was elucidated by cyclic voltammetry (CV) in 0.1 M NaOH and 1 M ethanol with potential range appproximately around -0.75 to 0.75 V and scan rate 25 mV/s. XRD results of Pt samples are shown in Fig. 1. All diffractograms confirmed the formation of Pt nanoparticles on the FTO surface. According to the observed data, the peak that apperead at 2Ɵ = 39.92 ̊ came from Pt metal [13][14][15]. that Pt0.30V is shaped like parallel leaves, short and small and overlapped each other. The morphological development at Pt0.60V was gradually formed and becoming more visible with the increasing protrude of pointed branches as well as the number of particles. While at Pt1.00V the shaped of the parallel leaves was clearly visible, elongated and there were several branches. These figure demonstrated the different upper potential did affect the particle growth. However, based on previous experiment, nucleation occured in potential range -0.40 V to -2.00 V, while 0.30 V to 0.70 V contributed to the particle growth process [16].

Results and Discussion
SEM characterization showed the best morphological result is result is Pt0.60V because of the smoother shape and tapered branches resulting in many sites that can increase conductivity [17]. The EDX spectrum shown in Fig. 3 confirmed the platinum nanoparticles formation on the FTO surface. These data indicated the sample has Pt elemental composition which is characterized by the apperance of a signal at 2 to 2.5 keV. In electrochemical processes, electrocatalytic properties are influenced by the electrode. EIS was performed to characterize the properties of synthesized Pt electrocatalyst interface with different upper potentials. Semicircle-shaped of Nyquist plot represented electron transfer resistance and linear shapes represented surface diffusion processes [18]. The smaller Rct (semicircle-shaped) indicated faster electron transfer kinetics. In general, the higher potential during synthesis, the greater Rct [19]. Fig. 4 show Nyquist plot of the synthesized Pt nanoparticles at different upper potentials. It is known that the smallest Rct was produced by Pt0.60V, followed by Pt0.30V and Pt1.00V. This indicated the highest electron transfer kinetics came from Pt0.60V, with longer particle shape such as parallel leaves. These results showed the different upper potentials during the sample synthesis process affect the current density. In the forward scan, the ethanol oxidation peak of the three Pt electrocatalyst appeared in the range of -0.60 to -0.30 V. While in the backward scan, three peaks appeared at -0.28 V which was associated with the oxidation of the adsorbed intermediate species. Futhermore, all Pt electocatalysts have reduction peak in the backward scan around -0.10 V associated with Pt oxide reductions. In general, all Pt electrocatalysts showed an oxidation initiation potential at -0.57 V. In short, Pt0.60V has significant oxidation current, indicating the highest catalytic activity when compared to other electrocatalysts.   The value of current density ratio between forward and backward oxidation peak can determined the tolerance of Pt electrocatalyst to carbonaceous intermediates that was produced during ethanol electrooxidation reactions. The smaller ratio between the backward oxidation peak and forward scan exhibited stronger poisoning-tolerance for the ethanol electrooxidation [20]. Based on data shown in Table  1, Pt0.60V has the lowest jb/jf value, followed by Pt0.30V and Pt1.00V. Thus, Pt0.60V has the best poisoning-tolerance compared to the other two samples.

Conclusion
Pt nanoparticles with parallel leaves and tapered branches shaped was successfully synthesized through square wave pulse deposition by varying the upper potential. Pt0.60V was discovered to have the fastest electron transfer kinetics, higher ethanol electrooxidation current density, low jb/jf ratio value that produced the highest poisoning-tolerance and ethanol oxidation activity of the electrocatalysts.