Tổng hợp xúc tác oxi hóa điện hóa trên cơ sở pt và chấm lượng tử graphen ứng dụng trong pin nhiên liệu sử dụng trực tiếp alcohol TT TIENG ANH

A-INTRODUCTION The necessity of the thesis Currently, the high cost of most catalysts in Direct Alcohol Fuel Cell (DAFC) is still a barrier that prevents this type of battery from being widely commercialized Catalytic studies in DAFC show that based-Pt catalysts have high activity in the electrochemical oxidation of alcohol and are being studied by scientists around the world Pt is currently being considered as a standard active phase for the development of new high-activity and stable catalysts Many works have announced that in order to improve the catalytic activity on the basis of Pt, Pt nanoparticles are often dispersed on carbon materials with high conductivity and surface area such as carbon fiber, carbon paper, graphene, graphene quantum dots (GQDs), etc The use of based- GQDs carrier offers the potential to enhance the catalyst performance for electrochemical reactions in DAFC since GQDs are dominant over traditional carrier forms such as carbon, graphene by the characteristics of electrical conductivity, non-toxicity, high surface area, adjustable surface functional groups On this basis, the current research direction is looking for new methods of synthesizing GQDs, carriers based on GQDs, catalysis based on Pt and GQDs, and dispersing nanoscale Pt on based-GQDs carrier , denaturing based-Pt and GQDs catalysts to improve properties, activity stability, energy conversion, environmental friendliness and reduce synthesis costs, leading to commercialization electrocatalysts for application in DAFC Research purpose and research task Within the framework of research directions of National Key Laboratory for Petrochemical and Refinery Technologies (PTNTĐ), the main objective of the thesis is: "Synthesis of electrochemical oxidation catalysts based on Pt and graphene quantum dots applied in Direct Alcohol Fuel Cell” The thesis is carried out by the scientific guidance of Prof Dr Vu Thi Thu Ha To achieve the goal, the thesis has implemented the following main contents: ✓ Research on synthesis of GQDs from carbon felt and the influence of some factors on the synthesis of GQDs; ✓ Research on synthesis of catalysts based on Pt and GQDs applied in ethanol oxidation reaction (EOR) and methanol oxidation reaction (MOR); Scientific and practical significance of the thesis In terms of science, the thesis has made certain contributions in researching the synthesis of GQDs from carbon felt, systematically surveying the affected factors the quality of the GQDs; dispersing Pt active phase on GQDs, and investigating the influence of Pt content on the catalytic activity At the same time, the thesis makes scientific contributions to the fabrication of a new and advanced carrier (GQDs-GO) that not only has a simple synthesis method but also has typical properties for GQDs and GO, researching dispersion Pt onto GQDs-GO, survey the amount of Pt content Notably, the catalytic modification of Pt on the GQDs-GO carrier has been synthesized, the properties and electrochemical activity are characterized The obtained results of the thesis have practical significance in making GQDs carrier from new materials and GQDs-GO by a simple method At the same time, increasing the efficiency of Pt-based electrochemical catalysts when dispersing on GQDs and GQDs-GO after investigating the Pt content and denaturation has practical significance as a new generation of catalysts with a high low amount of active phase, reduce the cost of catalyst synthesis, towards commercialization of these catalysts that apply in DAFC New contributions of the thesis ✓ The graphene quantum dot (GQDs) carrier has been successfully synthesized from an inexpensive, readily available carbon felt source, and at the same time, the affected factors are systematically investigated to the satisfactory synthesis of GQDs quality GQDs are 7-15 nm in size, consisting of 2-3 graphene layers, fluorescing yellow under UV irradiation at an excitation wavelength of 365 nm ✓ The influence of Pt content (theoretical calculation) is systematically investigated on GQDs and GQDs-GO carriers For the GQDs carrier, the catalyst containing 3%Pt (Pt-3(2.65)/GQDs) had the highest electrochemical activity in both acidic and alkaline medium; catalyst activity is 17.67 times higher (acid, MOR), 9.28 times higher (alkaline, MOR), 14.38 times higher (acid, EOR) and 7.14 times higher (alkaline, EOR) than Pt/rGO catalyst at the same conditions For the GQDs-GO carrier, the catalyst containing 9%Pt (Pt-9(6.63)/(GQDs-rGO)) is the best catalyst with high electrochemical activity and reactivity comparable to the Pt/GQDs in MOR and EOR Moreover, the catalysts also have high stability and reactivity in both EOR and MOR ✓ Successfully denatured Pt-9(6.63)/(GQDs-rGO) by Au Compared to the undenatured catalyst, the post-modified catalyst (Pt9(6.63)-Au/(GQDs-rGO)) has 2.5 times higher activity (MOR, acid); 1.95 times (MOR, alkaline); 1.2 times (EOR, acid); 3.1 times (EOR, alkaline) The catalytic activity also increased approximately 1.2 times in an acidic medium at EOR and MOR Compared to Pt11(9.81)/(GQDs-rGO), Pt-9(6.63)-Au/(GQDs-rGO) increased the activity by 3.6 times and 7.16 times in acidic medium; 4.13 times and 3.3 times in alkaline medium of EOR and MOR, respectively This highlights the role of synergy between Pt and Au as presented before The successful modification of the Pt/GQDs-GO catalyst by a small content of Au (2% by mass) contributed to enhancing the electrochemical catalytic efficiency, and at the same time, significantly reduced the number of precious metals used in the catalyst, leading to reduce the cost of catalytic synthesis for DAFC The role of Au is to reduce the adsorption of toxic intermediates or reaction products on the surface of the catalyst, acting synergistically with Pt to reduce the bond cleavage energy of adsorbed alcohol molecules on the catalyst sites In addition, Au can promote the conversion of CO to CO2 to enhance the tolerance of CO poisoning of the catalytic sites As a result, the Pt-Au-containing catalyst can remove the CO intermediates more easily The catalytic activity of Pt-9(6.63)-Au/(GQDs-rGO) was evaluated in the DAFC model as an anode catalyst The maximum power density of both DMFC and DEFC models when using AEM (135.39 and 41.69 mW cm-2, respectively), is about 10% higher than published works on AEMDMFC and AEM-DEFC models of Pt/C commercial catalysis at the same conditions Structure of the thesis The thesis consists of 167 pages, 22 tables, 62 figures, and graphs, are distributed into the following parts: Introduction (2 pages); Theoretical overview (43 pages); Experiments and research methods (15 pages); Results and discussion (89 pages); Conclusion (2 pages); New contributions of the thesis (2 pages); List of published scientific works (1 page); References (14 pages) includes 168 references *** B- KEY CONTENT OF THE THESIS CHAPTER OVERVIEW Graphene quantum dots (GQDs) are graphene disks with sizes in the 2-20 nm range GQDs not only exhibit physical and chemical properties similar to those of graphene, but also exhibit special physicochemical properties of quantum dots, including edge effects, nonzero bandgaps, and effects quantum confinement GQDs are capable of luminescence-based on the excitation wavelength Furthermore, compared with semiconductor quantum dots, GQDs show many advantages such as chemical inertness, biocompatibility, ease of fabrication, and low toxicity Currently, there are many methods to synthesize GQDs, including: top-down method, bottom-up method These two methods can be accomplished by physical, chemical, or chemical-physical processes In particular, the top-down method using chemical engineering is widely used due to its advantages of simplicity, efficiency and can be used in large-scale production Pt-based catalysts are widely used and popular in the application of DAFC anode catalyst materials because Pt is considered as a standard catalytic phase to develop a new catalyst with high and durable activity In particular, four types of catalysts are focused on research and application for fuel cells, including Pt catalysts dispersed on the carrier, alloy catalysts in the presence of Pt and nonPt, non-metallic catalysts, and shape and size-adjusted catalysts However, the published catalysts in general still use large amounts of Pt, Pt-based, and GQDs catalysts, in particular, have not been focused on developing in this potential application Most of the catalysts using GQDs at present just stop at applications in hydrogen fuel cells, DMFCs, or are selected as catalytic carrier applied in cathode electrodes of DAFC Almost no works have published research results on GQDs-based carrier-based Pt catalysts at low Pt content, applied as anode electrodes in DAFC In Vietnam, there are a lot of research groups that have experimented with manufacturing GQDs, but the synthesis process is complicated, with high technical requirements, and mainly focuses on application directions in optical processing, manufacturing photovoltaic cells, semiconductor materials without any publication related to catalytic synthesis for DAFC based on GQDs Therefore, the thesis aims to research the fabrication of GQDs, Pt-based catalysis, and GQDs carrier, application in EOR and MOR, towards catalysis for the anode electrode of the DAFC model, opening the potential developmental direction that has high scientific and practical value This direction is completely consistent with the development trend of new materials technology and new energy in the world, as well as the development orientation of PTNTĐ in recent years CHAPTER EXPERIENCE 2.1 Instruments, chemicals and equipment Chemicals and supplies are sourced from Sigma Aldrich, Merk, FuelcellStore (USA), China, and Vietnam The thesis uses specialized equipment such as a transducer ultrasonic vibration device, electrochemical analysis equipment system, etc 2.2 Synthetic method of carriers and catalysts 2.2.1 Synthesis of GQDs carrier The method of synthesizing GQDs was carried out by oxidation reaction cutting carbon felt powder, using a mixture of concentrated HNO3 and concentrated H2SO4 at 120ºC, 12 hours After purification by membrane dialysis The GQDs were lyophilized and redispersed in DI water at a concentration of mg/mL and stored in a sample cabinet avoiding light 2.2.2 Synthesis of Pt/GQDs catalysts The synthesis of Pt/GQDs was carried out by chemical method using a reducing agent, NaBH4 at 55oC, for hours The catalysts were dispersed in deionized water with a concentration of 0.5 mg.mL-1 The catalysts are denoted as Pt-x(y)/GQDs (x is the theoretically calculated Pt content, y is the Pt content determined by the ICP-MS analytical method) 2.2.3 Synthesis of GQDs-GO carrier The reaction mixture includes carbon felt powder, HNO3 and H2SO4 After reacting at 120ºC for 12 hours, the product is neutralized to remove excess acid, crystallize the salt obtained GQDs-GO The GQDs-GO solution was lyophilized and redispersed in DI water at a concentration of mg/mL 2.2.4 Synthesis of Pt/GQDs-rGO catalysts The catalyst was dispersed in deionized water with a concentration of 0.5 mg.mL-1 The catalyst is denoted as Ptx(y)/(GQDs-rGO) (x is the theoretical Pt content, y is the Pt content determined by the ICP-MS method) 2.2.5 Method of catalytic modification on the basis of Pt/(GQDsrGO) catalyst Au was selected for the modification of Pt/(GQDs-rGO) catalyst The theoretical Au content was 2% compared with the GQDs-GO carrier, the reducing agent is EG The concentration of final solid product was 0.5 mg.mL-1 The post-modification catalysts were denoted as Pt-x(y)-Au/(GQDs-rGO) 2.3 Methods to evaluate physicochemical characterization of catalysts The physicochemical properties of the catalysts were determined by methods such as UV-Vis, PL, FT-IR, TEM, HAADF-STEM, XPS, ICP-MS, EDX and Raman 2.4 Methods to evaluate the electrochemical characterization A PGS- ioc- HH12Potentiostat/Galvanostat having a typical three-electrode cell is chosen to conduct electrochemical experiments at ambient temperature The electrochemical active surface area (ECSA) of the catalysts were determined based on cyclic potential current (CV) measurements at room temperature in the two medium H2SO4 0.5 M and NaOH 0.5 M, with a scan rate of 50 mV.s-1 Electrochemical characterization in EOR and MOR was evaluated by cyclic potential current (CV) scanning with a potential sweep rate of 50 mV.s-1: in acidic medium (C2H5OH M + H2SO4 0.5 M) potential range from to V In alkaline medium (C2H5OH M +NaOH 0.5 M) potential range from -0.8 to 0.5 V Catalytic stability was evaluated by CA measurement at unchanged potential The stability of the catalytic activity was evaluated by the decrease in current density according to the number of CV cycles in the corresponding electrochemical solution The measurement is repeated 1200 cycles in acidic medium and 400 cycles in alkaline medium The activity of the catalyst was selected from among the investigated catalysts, and the activity was evaluated through the power density value of the DAFC model with the electrode area of 10 cm2 (3.3 × 3.3 cm), by polarimetric measurement at 50oC, using two types of ion exchange membranes including: proton exchange membrane (PEM) and anion exchange membrane (AEM) In which, the cathode used is a commercial cathode with the composition of Pt/Carbon black coated on carbon cloth, density of mgPt.cm-2, charged with O2 gas at a pressure of bar The catalyst coated anode electrode was selected with a catalytic density of 1.0 mgPt.cm-2 CHAPTER RESULTS AND DISCUSSION 3.1 Research on synthesis of graphene quantum dots from carbon felt The SEM results of the carbon felt (Figure 3.1) show that the carbon felt has a long, thin, densely interwoven, fibrous structure with a diameter of ⸟10-20 µm Figure 3.1 SEM images (a,b) of carbon felt Figure 3.2 Raman spectra of carbon felt Raman spectra results of carbon felt (Figure 3.2) show that there are D bands and G bands at positions ≈ 1300 cm-1 and 1600 cm-1 Ratio ID/IG > characterizes the typical disordered structure of graphite materials 3.1.1 Investigate some affected factors to the synthesis of GQDs 3.1.1.1 Effect of reaction time The TEM images (Figure 3.3) show that the hours reaction time is not enough to oxidize the carbon felt to inform GQDs While 12 hours and 24 hours reaction time, GQDs crystal grains with sizes in the range of 7-15 nm are formed hours 12 hours 24 hours Figure 3.3 TEM images of GQDs at different times Figure 3.4 Raman, IR, PL spectra of GQDs at different reaction times (reaction temperature 120ᵒC) On Raman spectra, the synthesized sample at hours has a rather low characteristic peak represented for graphene, while the intensity of these peaks for the remaining samples is much higher Similarly, the IR spectra of this sample does not observe the vibrations characteristic for the typical functional groups of GQDs, the oxidation reaction has not yet taken place strongly enough to form graphene materials The IR results of the 12 hours sample and the 24 hours sample were not significantly different These results is similar when comparing the fluorescence results of the samples at h, 12 h and 24 h Thus, the reaction time of 12 hours is suitable for the oxidation process to cut the carbon felt to form GQDs and save the cost of material synthesis 3.1.2 Effect of reaction temperature Figure 3.5 Raman, IR, PL spectra of GQDs at different reaction temperatures (reaction time 12 hours) Raman spectra of the sample prepared at a reaction temperature of 120ᵒC showed the appearance of rather high intensity D and G bands At this temperature, the obtained sample contains characteristic oscillations (IR spectra), with high intensity and clarity of functional groups characteristic of GQDs structure The fluorescence density of the synthesized sample at 120ᵒC reached the highest value (Figure 3.5) Thus, the temperature for the synthesis of GQDs is 120ᵒC 3.1.3 Research on refining process Figure 3.6 TEM images of the pre-refined product (SP1) Observing the TEM images of SP1 appeared spherical crystals of rather small size, below 20 nm, thin sheet materials, few overlapping layers, needle-shaped crystalline materials with size quite small size, a few nm Figure 3.7 UV-Vis, IR spectra of SP1 SP1 is a mixture of many different types of graphene products, which may include: graphene oxide, GQDs, unreacted carbon buffer, salt generated from neutralization reaction of residual HNO3 and H2SO4, etc This mixture is required to refine GQDs Table 3.1 Effect of purification conditions on the luminescence of GQDs Time (h) Luminescence in static Luminescence in purification dynamic refining 12 No luminescence No luminescence 24 No luminescence Luminescent (GQDs-1) 48 72 96 120 No luminescence No luminescence Luminescent Luminescent (GQDs-5) 10 Luminescent Luminescent Luminescent Luminescent Figure 3.22 HAADF-STEM of Pt-9(6.63)/(GQDs-rGO): Pt (b), C (c), O (d) IR spectra allows to partially determine the functional groups present in Pt/(GQDs-rGO) and carrier structure at different Pt content From the results of IR spectra, the absorption bands of GQDs-GO and Pt/(GQDs-rGO) at 3300-3500 cm-1 represent the stretching vibrations of the O-H bond Figure 3.23 IR spectra of the catalysts On the IR spectra of the catalysts, there are not O-H (at 1350 cm1 ) and C-O-C (at 1135 cm-1) vibration compared with GQDs-GO It proves that the Pt reduction process has partially removed these functional groups on the material surface after the catalytic synthesis To better understand the state of existence of Pt in various catalysts, four representative catalysts are analyzed by XPS (Figure 3.24) XPS spectra of Pt-9(6.63)/(GQDs-rGO) almost only show the appearance of oxidation number Pt0 (accounting for 77.73%) of Pt in this catalyst (Pt4+ accounts for only 22.27%), 55.46% higher than the Pt0 content in Pt-11(9.81)/(GQDs-rGO) This result predicts the high electrochemical activity of the catalyst in the electrochemical oxidation reaction for Pt-9(6.63)/(GQDs-rGO) 19 Pt-3(2.79)/(GQDs-rGO) Pt-7(5.80)/(GQDs-rGO) Pt-9(6.63)/(GQDs-rGO) Pt-11(9.81)/(GQDs-rGO) Figure 3.24 XPS Pt spectra of the catalysts Figure 3.25 present in more detail the states of C, O, and Pt that exist in Pt-9(6.63)/(GQDs-rGO) Figure 3.25 XPS survey (a), C 1s (b), O 1s (c), Pt 4f (d) spectra of Pt-9(6.63)/(GQDs-rGO) As shown in Figure 3.25(b), C1s is divided into three peaks corresponding to C= C (284.75 eV), C-O-C (285.34 eV) and O-C= O (288.55 eV) Figure 3.25(c) O1s is decomposed into two separate 20 peaks at 531eV (C=O) and 533eV (C-O/C-O-C respectively These results suit with the results obtained previously from the IR spectra (Figure 3.23) 3.3.3 Evaluation of the electrochemical activity of the catalysts For the electrochemical oxidation of methanol and ethanol, Pt9(6.63)/(GQDs-rGO) also showed activity, per unit mass of the active phase Pt, much higher than that of the remaining catalysts in both medium H2SO4 0.5 M + MeOH M NaOH 0.5 M + MeOH M Figure 3.26 CV curves of catalysts Specifically, for MOR, in alkaline medium, the activities of the catalysts are arranged in increasing order as follows: Pt1(0.98)/(GQDs-rGO) (7964) < Pt-3(2.79)/(GQDs-rGO) (15701) < Pt-5(4.44)/(GQDs-rGO) (16108) < Pt-7(5.80)/(GQDs-rGO) (20264) < Pt-11(9.81)/(GQDs-rGO) (21337) < Pt-9(6.63)/(GQDs-rGO) (36041 mA.mgPt-1) In acidic medium, Pt-1(0.98)/(GQDs-rGO) (2260) < Pt-3(2.79)/(GQDs-rGO) (6360) < Pt-5(4.44)/(GQDs-rGO) (6590) < Pt-11(9.81)/(GQDs-rGO) (6968) < Pt-7(5.80)/(GQDs-rGO) (6596) < Pt-9(6.63)/(GQDs-rGO) (18920) Similar to that in MOR, in EOR, Pt-9(6.63)/(GQDs-rGO) exhibits outstanding electrochemical activity when the IF value reaches 22046 mA.mgPt-1, almost times higher than the activity properties of Pt1(0.98)/(GQDs-rGO) (3836 mA.mgPt-1), 1.8 times Pt5(4.44)/(GQDs-rGO), 1.3 times Pt-7(5.80)/(GQDs-rGO) 1.3 times Pt- 7(5.80)/(GQDs-rGO) and nearly 1.4 times higher than that of Pt11(9.81)/(GQDs-rGO) Pt-11(9.81)/(GQDs-rGO) (16537 mA.mgPt-1) This result is completely similar to TEM results presented previously (Figure 3.21) 21 NaOH 0.5 M + EtOH M H2SO4 0.5 M + EtOH M Figure 3.27 CV curves of catalysts 3.3.4 Evaluation of the activity stability of the catalysts It was found that, in all mediums, the electrochemical activity of Pt-9(6.63)/(GQDs-rGO) was always the highest among the catalysts, in addition, the activity stability was also the best The results of the evaluation of the activity stability are shown from Figure 3.28 to Figure 3.31 a In H2SO4 0.5 M + EtOH M Pt-3(2.79)/(GQDs-rGO) Pt-7(5.80)/(GQDs-rGO) Pt-11(9.81)/(GQDs-rGO) Pt-9(6.63)/(GQDs-rGO) Figure 3.28 The activity stability of the catalysts The activity of Pt-9(6.63)/(GQDs-rGO) is almost unchanged after many reaction cycles 22 b In NaOH 0.5 M + EtOH M It was found that, after 400 s, the residual current density of Pt9(6.63)/(GQDs-rGO) reached 18564 mA.mgPt-1; 2.16 times higher than Pt-11(9.81)/(GQDs-rGO) (8582 mA.mgPt-1); 2.65 times Pt7(5.80)/(GQDs-rGO) (8582 mA.mgPt-1); 2.2 times Pt-3(2.79)/(GQDsrGO) (8268 mA.mgPt-1) Pt-7(5.80)/(GQDs-rGO) Pt-3(2.79)/(GQDs-rGO) Pt-9(6.63)/(GQDs-rGO) Pt-11(9.81)/(GQDs-rGO) Figure 3.29 The activity stability of the catalysts c In H2SO4 0.5 M + MeOH M Similar to EOR, under the condition of continuous scanning of 1200 cycles at acidic medium, the current densities of the remaining catalysts are arranged in the same increasing order: Pt3(2.79)/(GQDs-rGO) (24.26%) < Pt-7(5.80)/(GQDs-rGO) (28.55%) < Pt-11(9.81)/(GQDs-rGO) (30.23%) < Pt-9(6.63)/(GQDs-rGO) (60.92%) The results of durability analysis after 1200 cycles are shown in Figure 3.30 23 Pt-3(2.79)/(GQDs-rGO) Pt-7(5.80)/(GQDs-rGO) Pt-9(6.63)/(GQDs-rGO) Pt-11(9.81)/(GQDs-rGO) Figure 3.30 The activity stability of the catalysts d In NaOH 0.5 M + MeOH M Pt-3(2.79)/(GQDs-rGO) Pt-7(5.80)/(GQDs-rGO) Pt-9(6.63)/(GQDs-rGO) Pt-11(9.81)/(GQDs-rGO) Figure 3.31 The activity stability of the catalysts 24 The decrease in current density after 400 continuous scanning cycles of the catalysts (Figure 3.31) is arranged in ascending order as follows: Pt-9(6.63)/(GQDs-rGO) (42.01%) < Pt-11(9.81)/(GQDsrGO) (42.57%) < Pt-7(5.80)/(GQDs-rGO) (44.45%) < Pt3(2.79)/(GQDs-rGO) (55.88%) Therefore, to explain for the stability results obtained above, the four catalysts were subjected to TEM images (Figure 3.32) As for Pt-9(6.63)/(GQDs-rGO), there is no significant change in the microstructure morphology and size (like the other catalysts), but only the precipitation phenomenon catalyst capacitor Therefore, Pt9(6.63)/(GQDs-rGO) has the highest activity and stability of the two medium in both EOR and MOR Pt-3(2.79)/(GQDs-rGO) After 1200 cycles After 400 cycles Pt-7(5.80)/(GQDs-rGO) After 1200 cycles After 400 cycles Pt-9(6.63)/(GQDs-rGO) After 1200 cycles After 400 cycles Pt-11(9.81)/(GQDs-rGO) After 1200 cycles After 400 cycles Figure 3.32 TEM images of the catalysts after many cycles 25 3.3.5 Research on Pt-9(6.63)/(GQDs-rGO) catalytic modification 3.3.5.1 Characteristic results of catalysts after modification Pt-9(6.63)/(GQDs-rGO) Pt-9(6.63)-Au/(GQDs-rGO) Figure 3.33 TEM images of catalysts before (a) and after modification (b); HAADF-STEM elemental mapping images of Pt-9(6.63)-Au/(GQDs-rGQ) (c,d,e,f,g) The dispersion of Au, Pt particles on the carrier of Pt-9(6.63)Au/(GQDs-rGQ) is clarified in Figure 3.33 c,d,e,f,g that show the Au and Pt particles are dispersed quite uniformly on the surface of the GQDs-GO carrier Figure 3.34 IR spectra of the catalysts IR spectra show that the denaturation process has kept the basic structure of the carrier when it is observed that the number of waves appearing on Pt-9(6.63)-Au/(GQDs-rGQ) almost coincides with the wavenumber that appeared on the GQDs-GO carrier and Pt9(6.63/(GQDs-rGQ) 26 The results on the state of existence of the elements present in the post-denaturation catalyst are clarified through the results of XPS analysis The survey spectra (Figure 3.35a) did not show the presence of foreign elements in the catalyst's composition Pt exists in two oxidation state Pt(0) (Figure 3.35d) The Pt-9(6.63)Au/(GQDs-rGQ) did not significantly change the state of existence of C and O in the catalyst structure Specifically, Figure 3.35b and Figure 3.35d show that C1s is divided into three functional groups, assigned to C = C (284 eV), C – O (286 eV) and O – C = O (289 eV) O1s is divided into two different peaks, corresponding to C = O (532,3 eV) and C – O/C – O – C (533 eV) similar to the state that exists in Pt-9(6.63)/(GQDs-rGO) before denaturation Figure 3.35 Results of XPS survey (a), XPS C 1s (b) and O1s (c), Pt 4f (d), Au 4f (e) of Pt-9(6.63)-Au/(GQDs-rGQ) 27 3.3.6 Evaluation results of electrochemical activity of the catalyst after modification In EOR, Pt-9(6.63)-Au/(GQDs-rGQ) with total active phase content of Pt and Au (according to ICP analysis results is 8.2%) exhibited higher activity than 3.6 times in acidic medium and 10 times in alkaline medium when compared with Pt-11(9.81)/(GQDsrGO) containing 9.81%Pt Moreover, with only 2% Au (by mass) modified, the ethanol electrochemical catalytic activity increased by 1.2 times with the unmodified catalyst in the acidic medium and 3.1 times in the alkaline medium H2SO4 0.5 M + EtOH M NaOH 0.5 M + EtOH M Figure 3.36 CV curves of catalysts H2SO4 0.5 M + MeOH M NaOH 0.5 M + MeOH M Figure 3.37 CV curves of catalysts Similar to the case of ethanol (Figure 3.36), Pt-9(6.63)Au/(GQDs-rGQ) was 2.6 times more active than Pt-9(6.63)/(GQDsrGQ) in acidic medium and 1.9 times in alkaline medium (for MOR) 28 3.3.6.1 Evaluation of the stable activity of Pt-9(6.63)-Au/(GQDsrGQ) after modification In acidic medium, after 1200 cycles, these quantities decrease to17245 mA.mgPt-Au-1 and 35166 mA.mgPt-Au-1, respectively, which is about 29% reduction (Table 3.2) Table 3.2 Electrochemical results of Pt-9(6.63)-Au/(GQDs-rGQ) after 1200 cycles in acidic medium % IFx compared to IFmax EtOH MeOH EtOH MeOH EtOH MeOH 10 8937 41264 0.99 8.73 89.35 82.69 50 24159 49902 0.89 0.80 100.00 100.00 100 21654 47996 0.94 0.80 89.63 96.18 200 21171 47806 0.10 0.83 87.63 95.80 400 21134 46259 1.02 0.81 87.48 92.70 600 20448 45561 1.00 0.84 84.64 91.30 800 19006 45221 0.98 0.84 78.67 90.62 1000 18192 42961 0.94 0.81 75.30 86.09 1200 17245 35166 0.89 0.80 71.38 70.47 Table 3.3 Electrochemical results of Pt-9(6.63)-Au/(GQDs-rGQ) after 400 cycles in alkaline medium Cycles Chu kỳ IF (mA.mgPt-Au-1) IF (mA.mgPt-Au-1) IF/IB IF/IB % IFx compared to IFmax EtOH MeOH EtOH MeOH EtOH MeOH 10 60064 61819 5.49 21.15 96.98 87.85 50 68371 70369 4.82 11.96 100.00 100.00 100 52947 66407 4.24 13.87 77.44 94.37 150 50260 56760 4.57 11.87 73.51 80.66 200 46349 52369 4.61 12.02 67.79 74.42 250 43340 51292 4.49 13.06 63.39 72.89 300 40900 44705 4.49 12.87 59.82 63.53 350 38076 42292 4.77 12.98 55.69 60.10 400 34507 35487 0.48 13.58 50.47 50.43 In alkaline medium (Table 3.3), after 400 scan cycles, the peak current density of the scanning direction decreased to 34507 29 mA.mgPt-Au-1 and 35487 mA.mgPt-Au -1 respectively, equivalent to a decrease of about 49,53 and 49,57% In addition, Pt-9(6.63)-Au/(GQDs-rGQ) was also evaluated and its stability explained through the TEM characteristic results after many continuous reaction cycles (Figure 3.38) Before scanning After 1200 cycles After 400 cycles Figure 3.38 TEM images of the catalyst after many cycles After 400 scanning cycles in alkaline medium or after 1200 scanning cycles in acidic medium, the active phase particles are not changed to the structural state, but only agglomeration occurs 3.3.6.2 Application of Pt-9(6.63)-Au/(GQDs-rGQ) as anode electrode in DAFC model Table 3.4 Comparison of maximum power density of two fuel cell models using Pt-9(6.63)-Au/(GQDs-rGQ) as the anode catalyst Anode Characteristics Fuel cell models DMFC DEFC PEM AEM PEM AEM Maximum power 102.55 135.39 30.56 41.69 -2 density, mW.cm Open circuit potential, V 0.76 0.8 0.57 0.74 The maximum power density value is about 10% higher than the published results on the same AEM-DMFC on Pt/C catalysis *** 30 CONCLUSION ✓ The affected factors in the synthesis method of GQDs have been systematically studied, including temperature, reaction time, and purification conditions The synthesized GQDs products are mainly in the range of 7-15 nm, consisting of 2-3 layers of graphene, fluorescing yellow under UV irradiation at 365 nm excitation wavelength Conditions for making GQDs from carbon felt include 120oC reaction temperature, 12 hours reaction time, 24 hours purification under dynamic conditions ✓ Successfully synthesized catalytic materials based on Pt dispersed on GQDs with 1%, 3%, 8%, and 20%Pt theoretical content At the same time, the influence of Pt content on the activity of catalysts was investigated in MOR and EOR Accordingly, Pt3(2.65)/GQDs (3%Pt content) has the highest activity in both acidic and alkaline mediums In acidic medium, the corresponding current density (IF) values in the MOR and EOR of the catalyst reached 13512 mA.mgPt-1 and 4717 mA.mgPt-1, respectively, lower in alkaline medium, reaching 49670 mA.mgPt-1 and 16363 mA.mgPt-1, respectively Not only the activity is 14.38 times higher (in acidic medium) and 7.14 times higher (in alkaline medium) than the Pt/rGO catalyst, Pt-3(2.65)/GQDs also exhibits high stability and durability activity in MOR and EOR ✓ In particular, the advanced carrier line (GQDs-GO), containing both GQDs and GO is synthesized by a simple method, the precursors are cheap, readily available, and capable of increasing the total scale easily On this type of carrier, the effect of Pt content was studied systematically The ideal Pt content on GQDs-GO is 9% (theoretical calculation, Pt-9(6.63)/(GQDs-rGO)), achieve high efficiency in EOR and MOR in the two reaction medium In EOR, the IF catalyst reached 19822 mA.mgPt-1 (acidic medium), 22046 mA.mgPt-1 (alkaline medium) In the MOR, the catalyst achieve IF of 19920 mA.mgPt-1 (acidic medium), 36041 mA.mgPt-1 (alkaline medium) Moreover, the catalysts kept their activity over 50% after 1200 cycles in acidic medium and 400 cycles in alkaline medium; equivalent to Pt/GQDs catalyst in terms of both durability, stability and activity 31 • ✓ Successfully modified Pt-9(6.63)/(GQDs-rGO) by Au (Pt9(6.63)-Au/(GQDs-rGO)) Compared with the unmodified catalyst, the post-modified catalyst has 2.5 times higher activity (MOR, acid); 1.95 times (MOR, alkaline); 1.2 times (EOR, acid); 3.1 times (EOR, alkaline) In terms of the stability of electrochemical activity, Pt9(6.63)-Au/(GQDs-rGO) maintained the IF current density reaching 71.38 % (EOR) and 70.47% (MOR) compared to that of Pt-9(6.63)Au/(GQDs-rGO) after 1200 CV cycles scanning in acidic medium After 400 CV cycles of scanning in an alkaline medium, this catalyst still maintains the current density value of 50.47 % (EOR), 50.43 % (MOR), compared to the maximum value The Pt-9(6.63)Au/(GQDs-rGO) was evaluated for its activity in DAFC models as an anode catalyst When using cation exchange membrane (PEM), the maximum power densities of DMFC and DEFC were 102.55 mW.cm-2 and 30.56 mW.cm-2, respectively When using anion exchange membrane (AEM), the DMFC model has a maximum power density of 135.39 mW.cm-2, the DEFC model reaches 41.69 mW.cm-2, about 10% higher than the AEM-DMFC and AEM-DEFC published models, using Pt/C commercial catalysis *** C- LIST OF PUBLISHED WORKS OF THE AUTHOR RELATING TO THE THESIS Published works in the country Lam Thi Tho, Nguyen Quang Minh, Vu Thi Thu Ha, Synthesis of graphene quantum dots from carbon felt applies as catalysts for ethanol oxidation reaction, Journal of Chemistry, 57(2e1,2), 31-35 Vu Thi Thu Ha, Lam Thi Tho, Nguyen Bich Ngoc, Study on impacts of some factors to synthesis and purification of graphene quantum dots from carbon felt, Vietnam Journal of Catalysis and Adsorption, 8(4), 2019, 95-100 Lam Thi Tho, Nguyen Minh Dang, Vu Thi Thu Ha, Aplication of the modified Pt-based on Graphene Quantum Dots in DMFC and DEFC, Journal of Chemistry and Application, 4(59), 2021 32 • Scientific patents Vu Thi Thu Ha, Nguyen Quang Minh, Lam Thi Tho, Nguyen Thi Thao, Nguyen Bich Ngoc, Patent: “Method for preparation of a platinum-dispersed catalyst on a graphene quantum dots carrier for direct alcohol fuel cell and the catalyst obtained by this method”, Application number 1-2019-01994 dated 22/04/2019 Decision on acceptance of valid application No 40467/QD-SHTT dated 22/05/2019 Thu Ha Thi Vu, Quang Minh Nguyen, Tho Thi Lam, Thao Thi Nguyen, Patent: ”Method of preparing catalyst based on Platium dispersed on carrier containing mixture of reduced graphene oxide and graphene quantum dot for direct alcohol fuel cell and catalyst obtained by this method”, Patent application at the United States Patent and Trademark Office, application number 16856022, dated 23/04/2019 33 ... Figure 3.13 shows the XPS spectra of Pt- 3(2.65)/GQDs and Pt8 (7.01)/GQDs Pt- 8(7.01)/GQDs: survey Pt- 3(2.65)/GQDs: survey Pt- 8(7.01)/GQDs: Pt 4f Pt- 3(2.65)/GQDs: Pt 4f Figure 3.13 XPS spectra of catalysts... mA.mgPt-1) In acidic medium, Pt- 1(0.98)/(GQDs-rGO) (2260) < Pt- 3(2.79)/(GQDs-rGO) (6360) < Pt- 5(4.44)/(GQDs-rGO) (6590) < Pt- 11(9.81)/(GQDs-rGO) (6968) < Pt- 7(5.80)/(GQDs-rGO) (6596) < Pt- 9(6.63)/(GQDs-rGO)... density of Pt9 (6.63)/(GQDs-rGO) reached 18564 mA.mgPt-1; 2.16 times higher than Pt- 11(9.81)/(GQDs-rGO) (8582 mA.mgPt-1); 2.65 times Pt7 (5.80)/(GQDs-rGO) (8582 mA.mgPt-1); 2.2 times Pt- 3(2.79)/(GQDsrGO)