Square-Wave Pulse Deposition of Pt Nanoparticles for Ethanol Electrooxidation

Platinum (Pt) is often used as an electrocatalyst in the electrooxidation of ethanol. There have been many attempts to improve the catalytic properties of platinum-based catalysts to achieve satisfactory results. The solutions are manipulating the size and shape of Pt. In this study, Pt will be deposited using the square wave pulse deposition method at upper potential variations. The Pt samples were characterized by scanning electron microscopy (SEM), X-ray diffraction, energy dispersive X-ray, and electrochemical impedance spectroscopy. The results of SEM show that the morphology of Pt at potentials of 0.3 V, 0.5 V, and 1.0 V produces a dendritic nanothorn morphology, while 1.25 V and 1.50 V produce a nanoleaf morphology. The lowest charge transfer resistance value is at Pt 1.0V with the smallest size. The highest yield of ethanol electrooxidation was at Pt 1.0V reaching 8.82 mA/cm 2 .


Introduction
Air pollution and global warming due to fossil fuels are a concern for the whole world crisis environmental crisis.The solution is to utilize renewable resources whose gas emissions are not harmful to the environment.Direct ethanol fuel cell is one of the solutions offered to replace fossil fuels [1].The advantages offered are that the energy density produced is quite large, environmentally safer, easy to store, has low toxicity, and can be carried out at room temperature [2,3].
The problem in the process of obtaining electrical energy from ethanol electrooxidation lies in the catalyst.Catalyst development is needed for the process of breaking down the C-C bond in ethanol into CO2 gas [4].Platinum (Pt) is one of the catalysts of interest for the ethanol electrooxidation process.This is because Pt has high catalytic activity and can work at room temperature for the ethanol electrooxidation process [5].
Studies have found that Pt is effective as an electrocatalyst in ethanol electrooxidation [6][7][8][9][10].The effectiveness of the Pt-catalyst can be further increased by reducing the particle size to nanosize [11,12] and the morphology of the catalyst can be manipulated to increase the contact area of ethanol electrooxidation [13,14].Manipulating the size and shape of the Pt-catalyst seems promising to increase ethanol electrooxidation activity.One of the synthesis methods of nanoparticles that is easy to manipulate in terms of size and shape of the Pt-catalyst is square wave pulse deposition.
The Pt-based catalyst will be synthesized using the square wave pulse deposition method to obtain the expected size and morphology [6].Manipulation of size and morphology can be obtained using several factors, one of which is the applied potential in stimulator mode [6,15].To synthesize Pt-based catalysts with a variety of sizes and morphologies, the upper potential will be varied during the research process.

Materials and Method
The method used for the synthesis of this Pt-based catalyst is square wave pulse deposition at room temperature.The solution used is K2PtCl6 at 1 mM and dissolved in a 1.0 M H2SO4 electrolyte solution.Other chemicals used in the research are KCl, NaOH, and ethanol 96% with Merck analytical chemical specifications.Electrodeposition was carried out using a three-electrode Published by The Center for Science Innovation system (eDAQ ER 466 potentiostat).The working electrode used is fluorine-doped tin oxide (FTO) 0.5x0.3cm, the counter electrode is Pt wire, and the reference electrode is Ag/AgCl (KCl 3 M).The FTO was then washed with aquadest and ethanol (p.a) and then dried briefly.Electrodeposition will be carried out using a lower potential at -0.3 V and varying upper potentials at 0.3, 0.5, 1.0, 1.25, and 1.50 V for 10 minutes.The results of the Pt synthesis are then rinsed and dried for the characterization process using X-ray diffraction Cu Kα (XRD, PANalytical AERIS) to determine the Pt phase (with 2θ 20 o -70 o ), field emission scanning electron microscopy (FESEM, Thermo Scientific Quattro S) to observe the morphology formed, energy dispersive x-ray (EDX, EDAX Apollo X) to determine the formation of the Pt formed, electrochemical impedance spectroscopy (EIS) with electrochemical workstation (Corrtest CS310) at a frequency of 50 kHz to 0.1 kHz with a 0.5 M KCl solution.The ethanol electrooxidation test was then carried out using a solution of 1.0 M ethanol and 0.1 M NaOH.200), respectively, which were consistent with face-centered cubic [16][17][18].Other peaks that were originally recorded come from the FTO substrate [19] because of the thinness of the Pt layer.2010) states that the effective potential for Pt nucleation is -0.2 V; if more positive potential is applied, Pt will slowly nucleate [20,21], followed by Pt growth at nucleated Pt sites with a sharp tip.Pt particles will likely grow in the sharp tips of the morphologies because of the difference in concentration between different areas.Therefore, Pt growth is more directed towards the tip site at the beginning of Pt nucleation because more concentrated Pt at the tip site [22].The results of this growth form a dendritic nanothorn morphology, which can be observed in Fig. 2.

Results and Discussion
The electrodeposition process using different potentials produces different morphologies and sizes.In EU samples at 0.3 V and 0.5 V, the Pt particles experienced agglomeration and the morphology shown was nonuniform nanothorns.Agglomeration is caused by adhesion between particles, where the growth of particles on top of other particles (overlapping) occurs due to the force on a particle that can attract other particles [23].In addition, Pt synthesized with EU above 1.0 V can be oxidized to the oxide compound and form PtO2. The study states that Pt can be oxidized at a potential of 1.25 V and can form other larger compounds, like PtO2.nH2O.The more positive the potential applied, the larger the particle size will be formed [24].Therefore, the dendritic nano thorns on Pt with EU above 1.0 V form a nano leaf-like morphology.The study by Tian et al. (2006) states that lower potentials above -0.1 V will make nano thorns difficult to observe, while upper potentials exceeding 1.0 V will make the spines on nano thorns become blunt and form nanoleaf [25].The particle size of each EU variation Pt sample can be seen in Figure 2.
The EU variation Pt size is 63.0 nm for Pt0.3V, 68.0 nm for Pt0.5V, and 61.0 nm for P1.0V.On the other hand, particle sizes for Pt1.25V and Pt1.50V with nanoleaf morphology are 69.0 nm and 76.0 nm, respectively.Therefore, the best distribution and morphology are found in EU 1.0 V synthesis conditions.
Figure 3 shows the EDX spectrum results of the Pt catalyst synthesized on the FTO substrate.The peak signal that appears at 2.0 -2.5 keV is Pt.The study by Naderi et al. (2012) stated that Pt was detected at signals between 2.0 and 2.5 keV [26].Other elements such as Sn, Si, and F were detected in the FTO-glass substrate.
Figure 4 is a Nyquist plot of Pt with an upper potential of 0.3 V, 0.5V, 1.0V, 1.25V, and 1.50 V at a lower potential of -0.3 V. Based on these data, the smallest Rct is obtained at an upper potential of Pt1.0V.In the Nyquist plot, the Rs value, or solution resistance, in different samples has different values.This difference is due to the different morphology of Pt under EU variations, so the initial response to solution resistance will be different [27].The morphology of the sample in Figure 2 shows that Pt with an upper potential of 1.0 V has nano thorn dendritic morphology, but Pt synthesized above 1.0 V has nano leaf morphology that has a different surface area.The Nanoleaf morphologies in Figure 2 show a buildup of morphology that causes the surface area to not be as large as that of the dendritic nanothorn, so the initial response can affect the Rs value.Apart from that, the factor of small particle size and particle distribution influences the resistance of the samples [28,29].A smaller particle size has affected ionic movement in the test because the distance it travels will be smaller, which will increase the movement of the ions.The smaller the Rct value, the greater the electrical conductivity, which causes the electron transfer kinetics and catalytic process to happen faster [30].Upper potential variation consisting of Pt with EU 0.3 V, 0.5 V, 1.0 V, 1.25 V, and 1.50 V at EL -0.3 V produces an electric current of, respectively, 6.94 mA/cm 2 , 5.28 mA/ cm 2 , 8.82 mA/ cm 2 , 3.69 mA/ cm 2 , and 2.13 mA/ cm 2 .In Figure 5, the difference in upper potential gives variations in the yield of oxidized ethanol in the form of electric current density during the process of oxidizing ethanol to CO2.The higher the electric current value, the more ethanol is oxidized to CO2 because complete oxidation of ethanol will produce 12 e-.The mechanism that occurs during ethanol electrooxidation is as follows: [31] C1 pathway: C2 pathway: Two pathways of ethanol electrooxidation can occur during the ethanol electrooxidation process (the C1/C2 pathway).This second pathway has different results at the end of the process, where the C1 pathway will produce COads or CO2 and 12 e -, while the C2 pathway will produce acetic acid (CH3COOH) and 4 e - [31].Ethanol (C2H5OH), which is adsorbed on the electrode surface, can then be split or undergo C-C bond breaking to form an acetic acid phase, which requires low energy, or forms CO2, which requires large energy [32].A study from Ferre-Vilaplana states that the breaking of the C-C bond in ethanol begins to appear at a potential of 0.2 V and CO2 begins to form at a potential below 0.5 V [33].The difference in potential or the occurrence of a potential shift that is different from the reference indicates the presence of a different platinum lattice.It causes better dissociation to produce OHads [34].
The highest ethanol electrooxidation was Pt1.0V, with a value of 8.82 mA/cm 2 .This matches the best size and morphology of Pt1.0V compared to Pt with other upperpotential variations.This data is matched by the electrochemical surface area (EASA) value shown in Table 1.A large EASA value indicates that the active surface of the catalyst for the electrooxidation process will be higher.The formula of EASA used is: Which, QCO is the charge calculated during the EOR process, QPt is the charge released on the surface (210 µC/cm 2 ), and A is the surface area of the catalyst [35].
The stability of the electrocatalyst can be seen from the comparison value between If/Ib in Table 1.A large If/Ib value indicates that the stability of the electrocatalyst is higher than others [36].In Table 1, Pt1.0V has the largest If/Ib value, which indicates that Pt1.0V has the best poisoning tolerance.Therefore, Pt1.0V is Pt with optimum results with a more dispersed dendritic nanothorn morphology, a low Rct value, and the best ethanol electrooxidation activity.

Conclusion
Pt-based catalysts have been successfully synthesized using variations in voltage.Variations in the upper voltage result in morphological changes at an upper voltage above 1.0 V compared to a voltage of 1.0 V.This can occur because the Pt nanothorn growth process effectively occurs at an upper voltage of 1.0 V.The results of the EIS characterization also show that Pt1.0V produces the lowest Rct value due to its size, which produces smaller and more dispersed Pt particles.The best ethanol electrooxidation test was obtained on a Pt1.0V catalyst with a value of 8.82 mA/cm 2 .This result is supported by the best nanothorn size and morphology on the Pt1.0V catalyst and the lowest Rct value on the Pt1.0V catalyst.

Figure 1 .
Figure 1.XRD pattern of Pt The XRD pattern of platinum synthesized with upper potential is shown in Fig. 1.The XRD pattern recorded platinum peaks at 39.89 o and 46.25 o , corresponding to the planes (111) and (200), respectively, which were consistent with face-centered cubic[16][17][18].Other peaks that were originally recorded come from the FTO substrate[19] because of the thinness of the Pt layer.

Figure 2
Figure 2 shows the morphology of platinum synthesized at a lower potential (EL) of -0.3 V with variations in the upper potential (EU) of 0.3 V, 0.5 V, 1.0 V, 1.25 V, and 1.50 V.The applied potential will affect nucleation and growth.The initial nucleation of Pt starts with the reduction of Pt ions.The study by Malek et al. (2019) and Zhang et al. (2010) states that the effective

Figure 4 .
Figure 4. Nyquist plot of Pt electrocatalyst with various applied upper potential Figure 5 is a voltammogram of ethanol electrooxidation using a platinum-catalyst with variations in upper potential.Differences in upper potential produce different peaks of electric current.The results of ethanol electrooxidation are presented in Table1.

Figure 3 .
Figure 3. EDX spectrum of Pt nanocatalyst deposited in Eu 1.0 V and EL -0.3 V.

Table 1 .
1. Interpretation of ethanol electrooxidation in Pt with various applied upper potential