25 ORIGINAL RESEARCH ARTICLE A study of electrocatalytic ethanol oxidation of nanoporous PtSi alloy Nali Lu1,3, Yao Li2,3, Lei Zhang3,4, Yong Fang3,4, Bin Qian3,4, Zhida Han3,4*, Xuefan Jiang3,4 1 School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, China 2 College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China 3 School of Physics and Electronic Engineering, Changshu Institute of Technology, Changshu 215500, China. E-mail: han@cslg.edu.cn 4 Jiangsu Key Laboratory of Advanced Functional Materials, Changshu 215500, China ABSTRACT In recent years, nanoporous alloys have presented the advantages of a large specific surface area, low density, and simple operation, and they have been widely used in the fields of catalysis, magnetism, and medicine. Nanoporous Pt-Si alloy was prepared by melt-spun and chemical dealloying, and was characterized by X-ray diffraction, X-ray photoelec- tron spectroscopy, scanning electron microscope, and transmission electron microscopy. Pt-Si alloys possess a three-di- mensional bicontinuous structure and an average size of 5 nanometers. Compared with commercial Pt/C catalysts, nanoporous Pt-Si alloys exhibit excellent electrocatalytic activity and stability in ethanol-catalyzed oxidation reactions. It is taken into consideration to be a promising catalyst in direct ethanol fuel cells. Keywords: Dealloying; Nanoporous; Electrocatalysis; Ethanol Oxidation Reaction Characterization and Application of Nanomaterials (2021) Volume 4 Issue 1 doi:10.24294/can.v4i1.1325 ARTICLE INFO Received: 14 December 2020 Accepted: 3 February 2021 Available online: 10 February 2021 COPYRIGHT Copyright © 2021 Nali Lu, et al. EnPress Publisher LLC. This work is licensed under the Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by- nc/4.0/ 1. Introduction The fuel cell is an energy conversion device, which can directly convert the energy generated by fuel through chemical reactions into electric energy, and only water is generated in the entire process[1]. A fuel cell combines the best characteristics of an internal combustion engine and battery, like an internal combustion engine that can run only with fuel without any mechanical conversion process. Fuel cells are similar to those under load conditions[2]. Direct ethanol fuel cell (DEFC) uses ethanol as fuel, which has attracted more and more attention. Eth- anol, as a small alcohol organic molecule, converts the chemical energy into electrical energy, has higher capacity density, low toxicity, and is convenient for storage and transportation, as well as the advantages of large-scale preparation of biomass products, it is widely used in mobile devices such as automobiles, mobile phones and computers[3–5]. The re- search of Wang et al.[6] shows that the permeability of ethanol through the electrolytic membrane is lower than that of methanol. Under acidic conditions, the reaction process of ethanol is as follows: Total reaction: C2H5OH+3O2 → 2CO2+3H2O Anode process: C2H5OOH+3H2O → 2CO2+12H++12e– Cathode process: 3O2+12H++12e– → 6H2O The reaction mechanism of ethanol is very complex, involving 12 e transfers, accompanied by the fracture of the C–C bond, the interme- 26 diate products dominated by COads are generated, which gather on the surface of the catalyst and pro- duce poisoning phenomenon that reduces the activity of the catalyst. In practical application, the catalytic oxidation of ethanol requires a high overpotential. Therefore, developing new anode catalysts with high activity and high stability has become a research hotspot. The existing research shows that the im- provement of the performance of Pt-based catalysts is due to the shortening of Pt metal bonds, the en- hancement of bond energy, and the easier adsorption of oxygen after alloying; the microstructure between materials is changed, the alloying effect of different elements, the specific surface area increases, and the active sites increase. To mix Pt with other non-noble metals[7–9], and the microstructure of Pt is optimized to prepare nano Pt catalyst with controllable mor- phology[10], which can prevent the aggregation of Pt particles due to the synergistic effect between bime- tallic metals and is helpful to improve the stability of the catalyst[11,12]. Based on the different properties of different elements, the dealloying method uses chemical or electrochemical methods[13] to remove the more ac- tive components in the alloy. Then the remaining components finally obtain more stable elements through diffusion and aggregation to form the frame- work of nanometal materials[13–15]. In recent years, Chen et al.[16] proposed a reverse dealloying method to prepare nanosilver. To selective remove the inert component Au in Au-Ag alloy. Meanwhile, thiourea plays a role in promoting gold dissolution and pas- sivating silver in the process of forming the porous silver framework[16]. According to Liu et al.[17], by changing the time sequence of dealloying, a series of nanoporous metal compound products with bimodal porous metals and single peak pores can be synthe- sized[17]. At present, Pt and 3d transition metals M (such as Ti, Cr, V, Mn, Fe, Co, Ni, Cu[18–20]) form al- loys and are loaded on carbon carrier, which can ef- fectively improve the stability of catalysts. Relevant literature reports Pt-Sn[21], Pt-W[22], and other alloy catalysts also show CO tolerance and are excellent catalysts[21,22]. Considering the small radius of the Si atom, the introduction of the Si atom reduces the distance between Pt-Pt and enhances the catalytic ac- tivity. In addition, Si is relatively stable under acidic conditions and can obtain better stability. Therefore, that alloying Pt and Si expects to improve the elec- trocatalytic performance of the material. Although studies have shown that bulk Pt-Si alloy[23] has great electrocatalytic oxidation activity for methanol and carbon monoxide, nano-scale Pt-Si alloy and its cat- alytic performance have not been reported[23]. In this study, we successfully prepared a 3D framework nanoporous PtSi (NP PtSi) binary alloy through the combination of melt rapid quenching technology and dealloying. Meanwhile, it also stud- ied its electrocatalytic oxidation activity and stability for ethanol. 2. Experiment 2.1 Sample preparation The purity of metal materials Pt, Si and Al is more than 99.99%, and the corresponding stoichio- metric ratio is 9:3:88. Then calculate their respective elemental mass. Put the prepared materials into a wa- ter-cooled copper crucible electric arc furnace, and inject Ar gas to melt repeatedly 3–4 times to obtain Pt9Si3Al88 alloy ingot. Cut the melted Pt9Si3Al88 alloy ingot into small pieces, put it into the fired quartz glass tube and melt the material slowly to obtain the alloy strip with basically the same thickness. Weigh about 50 mg of the strip and put it in 5 wt% HCl solution to produce lots of bubbles at the beginning. When the reaction is slow, put it into a constant tem- perature water bath at 50 ℃ Celsius for 48 h, take out the sample, centrifuge and wash it repeatedly 5–6 times. Finally, it obtains the NP-PtSi by drying in a vacuum drying oven for 24 h. 2.2 Characterization of materials We used the SA-HF3 X-ray powder diffractom- eter (XRD) of Rigaku Company from Japan to ana- lyze the crystal structure of the materials. Detecting the chemical composition and surface atomic state of the material by the X-ray photoelectron spectros- copy (XPS) of Axis Ultra, which is a wholly-owned subsidiary of Shimadzu group in Japan. The Sigma 27 scanning electron microscope (SEM) of ZEISS Com- pany in Germany was used to observe the micromor- phology of the material, the accelerating voltage is 20 kV. TecnaiG220S-TWIN transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) of American FEI Company are used, which not only could detect the microstructure of materials but also obtain the crystal plane spacing of atoms. The electrochemical proper- ties of materials are measured by the electrochemical workstation of model 760E of Shanghai Chenhua Instrument Co., Ltd. 2.3 Electrode preparation and electrochemi- cal test Weigh 1 mg of NP-PtSi and 1.5 mg of carbon black with an electronic balance into a 2 ml centri- fuge tube, and put 980 μL absolute ethanol and 20 μL 5 wt% Nafion with a pipette gun respectively into a centrifuge tube. Then put into an ultrasonic cleaner for about 1h to obtain the catalyst suspension. Com- mercial Pt/C catalyst with the same concentration was prepared by the same method. Firstly, the glassy carbon electrode (GCE, diam- eter 3 mm) was applied on the foot skin with 0.05 μm polishing powder shall be polished and cleaned with ultrapure water, and then placed in the mixed solution of 0.1 mol/L KCl + 1 mol/L potassium ferricyanide to measure the redox potential differ- ence. When the value is less than 70 mV, it shall be washed with ultrapure water and dry naturally for standby. All electrochemical tests were carried out in a three-electrode system. Before the reaction, intro- ducing saturated N2, removing O2 from the solution, and the reaction temperature was 25 degrees Celsius. 3. Results and discussion 3.1 Characterization of NP-PtSi 3.1.1 XRD characterization of NP-PtSi Figure 1 is the XRD diagram of Pt9Si3Al88 al- loy before and after dealloying. It shows from the diagram that the diffraction peak of PtSiAl alloy is very complex, and it is difficult to distinguish the al- loy phase of PtSiAl, and there is no diffraction peak corresponding to pure Pt, Al, and Si, indicating the formation of Pt-Si alloy. NP-PtSi sample has three diffraction peaks, i.e. 2θ = 40.140°, 46.599°, and 68.317°, corresponding to (111), (200), (220) crystal planes of face-centered cubic PtSi. Compared with the standard pure Pt card (JCPDS 04-0802), it finds that the diffraction peak of each crystal plane devi- ates slightly to a high angle, which may be because Si atoms with small atomic radius are added into the alloy to replace some Pt atoms, which reduces the distance between Pt-Pt and finally forms Pt-Si alloy. Figure 1. XRD spectra of Pt9Si3Al88 alloy before and af- ter dealloying. Standard spectra: Pt (JCPDS04-0802), Si (JCPDS35-1158), Al (JCPDS 04-0787). 3.1.2 XPS characterization of NP-PtSi Figure 2(a) is the full spectrum of NP-PtSi. By that, we can see the peaks of Pt 4f, Si 2f, O 1s, C 1s, and other elements, in which O 1s and C 1s belong to water or carbon dioxide physically or chemically adsorbed on the material surface, which fits with XPS Peak Fit[13]. Figure 2(b) is the peak diagram of Pt 4f. There are two peaks at 71.2 eV and 74.6 eV, corresponding to Pt and Pt2+ in the metal state. The results show that some Pt is oxidized to Pt2+, but it mainly exists in the form of Pt in a metal state, which is consistent with the internal XRD test results of the material. Figure 2(c) is the sub-peak diagram of Si 2p. Both Si0 and Si4+ can be detected, and the binding energy belongs to Si0 at 99.5 eV; the binding energy belongs to Si4+ at 103.2 eV. On the surface, Si mainly exists in the form of Si4+. It also can observe some metallic Si. 28 Figure 2. (a) XPS full spectrum of NP-PtSi; (b) Pt 4f peak dia- gram; (c) Si 2p peak diagram. 3.1.3 SEM, TEM and HRTEM characteriza- tion of NP PtSi Figure 3(a) is the SEM diagram of Pt9Si3Al88 strip after dealloying, which shows that it forms a uniform nanoporous structure composed of nanopores and ligaments. The pore size is 5–20 nm, the specific surface area increases, and the active sites of an electrochemical reaction in the material increase. Figure 3(b) is the cross-sectional view of NP-PtSi, from which the uniform pores can be seen. This confirms that the fine nanoporous structure is obtained in the whole sample. The TEM diagram can observe the microstructure of the sample from the nano size, as shown in Figure 3(c). The uniformly distributed bright white areas can be seen more clear- ly from the TEM diagram. Compared with the SEM diagram, the structure with pores in the multi-level pores is more significant. It is conducive to improv- ing the kinetics of oxygen diffusion. Figure 3(d) is the HRTEM diagram of NP-PtSi, and it is clear that the continuous lattice diffraction fringes. Figure 3. Characterization diagram of NP-PtSi alloy. (a) (b) SEM diagram; (c) TEM diagram; (d) HRTEM diagram. 3.2 Electrochemical performance test of NP-PtSi In 0.5 mol/L H2SO4 deoxidized solution, the sta- ble CV curve of NP-PtSi alloy is shown in Figure 4 (a), and commercial Pt/C catalysts can be compared. In the whole potential scanning range, NP-PtSi and commercial Pt/C catalysts show the same CV curve characteristics, including hydrogen adsorption and desorption zone: –0.25–0.1 V vs. SCE; electric double layer area: 0.1–0.35 V vs. SCE; high potential region: 0.35–1.2 V vs. SCE. In the hydrogen adsorption and desorption region, the peak of NP-PtSi alloy becomes wider and shifts to high potential compared with com- mercial Pt/C, which is due to the increase of hydrogen adsorption strength on Pt due to the interaction between Pt and Si. Compared with commercial Pt/C catalyst, NP-PtSi shows a significant current peak at about 0.45 V vs. SCE, which shows that lots of OH species adsorb on the surface of the catalyst. Figure 4. Cyclic voltammetric curves of Pt/C and NP PtSi catalysts in deaeration solution. (a) 0.5 mol/L H2SO4 solution; (b) 0.5 mol/ L H2SO4 + 1 mol/L CH3CH2OH mixed solution. 29 It believes that the activity of the catalyst is re- lated to the electrochemical activity area (ECSA). We use Origin software to calculate the desorption peak area of H, and the formula is ECSA = QH/(m × c). In the formula, Q is the Coulomb charge mC ∙cm–2 in the H region, m is the Pt load, mg∙cm–2, and c is the charge density adsorbed by the monolayer of H of Pt, which is 0.210 mC∙cm–2. It shows in Fig- ure 4(a) that the electrochemically active area of NP-PtSi is much larger than that of commercial Pt/C, indicating that the number of active sites of NP-PtSi catalyst after dealloying is increased, which is con- ducive to the catalytic oxidation of ethanol. Figure 4(b) shows the CV curve of NP-PtSi and commercial Pt/C catalyst in the mixed solution of 0.5 mol/L H2SO4 + 1 mol/L CH3CH2OH. During the potential positive scanning, there are two oxida- tion peaks. During the negative scanning, there is a reduction peak. The Pt-based catalyst surface will adsorb -OH and be oxidized to PtO. During the neg- ative scanning, PtO is restored. The oxidation peak (I) is about 0.7 V, which is usually taken into con- sideration as the characteristic peak of ethanol oxi- dation to CO2. It is one of the basis for judging the performance of the catalyst. The oxidation peak (Ⅱ) is about 1.1 V, which is related to the secondary ox- idation of intermediate products. The peak onset po- tential is also a key index for evaluating the activity of the catalyst. The peak onset potential of NP-PtSi alloy is negative compared with that of commercial Pt/C catalyst, which shows that the catalyst can im- prove the catalytic oxidation kinetics of ethanol and has good application value. We further explored the stability of NP-PtSi al- loy and commercial Pt/C catalyst by the potentiostat method. Figure 5 is the I-t curve with a constant potential of 0.7 V. It shows from the figure that at the beginning, the current density of NP-PtSi and Pt/ C catalyst decreased sharply due to the formation of electric double-layer capacitance. The subsequent decrease in current is due to the loss of surface ac- tivity sites caused by the adsorption of oxygen-con- taining intermediate species on the catalyst surface. After 500 s, the current density tends to be stable, which attributes to the continuous reduction of cat- alyst concentration during the reaction. Although the current density of both decreased to varying de- grees within the entire 3,000 s, the current density of NP-PtSi is always higher than that of commercial Pt/ C catalyst, indicating that the addition of Si makes NP-PtSi alloy show better stability than commercial Pt/C catalyst. Figure 5. Potentiostatic I-t curve of Pt/C and NP-PtSi catalysts in mixed solution of 0.5 mol/L H2SO4 + 1 mol/L CH3CH2OH (constant potential is 0.7 V). 4. Conclusion Nanoporous Pt-Si alloy was prepared by the combination of melt rapid quenching technology and dealloying. NP-PtSi alloy has a three-dimensional bi-continuous structure, with an average pore size of 5 nm and a large specific surface area. 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