Synthesis and characterization of carbon supported pt and pt alloy nanoparticles as electrocatalysts material for proton exchange membrane fuel cell

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ----------------------------------- DO CHI LINH SYNTHESIS AND CHARACTERIZATION OF CARBON SUPPORTED PT AND PT ALLOY NANOPARTICLES AS ELECTROCATALYSTS MATERIAL FOR PROTON EXCHANGE MEMBRANE FUEL CELL Major: Metal Science Code: 62.44.01.29 SUMMARY OF MATERIALS SCIENCE DOCTORAL THESIS Hanoi – 2018 PREFACE Proton exchange membrane fuel cell (PEMFC), a potential renewable energy source in the near future, has been considerablely studied in the world. The advantages of the PEMFCs are low temperature operation, high conversion efficiency, fast startup, low temperature operation (<100°C), flexible power scalee and particularly friendly environment. The application of PEMFC focuses on three main areas: transportation, power supply for residential areas and power for portable electronic devices. Platinum is the ideal catalyst material for PEMFC due to its high catalytic activity for hydrogen oxidation (HOR) and oxygen reduction (ORR) reactions as well as high stability in low pH environments at cathode. However, Pt is precious and expensive, so the use of this material will greatly increase the cost of PEMFC. This is one of the major challenges that limit the commercialization of PEMFCs in the world. To reduce the Pt metallic catalyst content, development of nanotechnology has played an important role with research of dispersing Pt metallic particles on carbon supports. To disperse catalyst nano-particles means that the catalyst surface area is increased. In some reports, this area may reach to approcimately 120 m2/g. As a result, the activity of the catalyst material has been significantly improved and Pt metallic loading might dropped to 0.4 mg/cm2 while PEMFC properties change insignificantly. Using Pt alloy catalysts with cheaper metals as electrode material is another effective approche for reducing PEMFC costs. For anodic catalysts, many Pt alloys with alloying metals such as Ru, W, Sn, Pd have been studied. These studies have shown that using catalyst alloys may improve catalytic activity for hydrogen oxidation reaction (HOR) and CO poisoning in PEMFC. For cathode ctalysts, Pt-M catalyst alloys (with M transition metals such as Mn, Cr, Fe, Co and Ni) are the most widely studied due to activity for oxygen reduction reaction (ORR) higher than pure Pt. Alloying catalysts improve ORR activity towards reducing oxygen by direct 4-electron reaction withou H2O2 intermediate stage therefore catalytic activity of these alloys may be higher 3-5 times compared to pure Pt / C catalysts. In Vietnam, research on PEMFC fuel cells has not intensively been considered and there are few research groups being studying on direct methanol fuel cell. With desire to develop PEMFC area using direct hydrogen fuel, research on catalytic materials is essential. Therefore, the topic of the thesis was chosen as: “Synthesis and characterization of carbon supported Pt and Pt alloy nanoparticles as electrocatalysts material for proton exchange membrane fuel cell” Scope of thesis: - Research and development of Pt/C and Pt-M/C alloys high performence catalysts to apply in proton exchange membrane fuel cells using direct hydrogen fuel. - Research and development of single PEM fuel cell having high power density with active area of 5cm2. Main contents of thesis: - Introdution of fuel cell and studies on Pt catalysts and Pt alloy catalysts in PEMFC. - Research on synthesis of catalytic materials Pt/C 20% wt. by electroless deposition method and evaluating to influence of experimental parameters such as pH, temperature... in the synthesis process on the properties of catalyst Pt / C 20%wt. - To optimum process for synthesis of Pt/C highly active catalysts as electrode materials in PEMFC. - Research on synthesis of Pt-M/C alloy catalyst 20%wt. (M = Ni, Co and Fe) by electroless deposition method and characterization of alloy catalysts to select a suitable alloy catalyst for ORR at cathode in PEMFC. - Researching, designing, and fabricating components of a single PEM fuel cell with active area of 5cm2 and study on operating conditions for single fuel cell. CHAPTER 1. INTRODUCTION - Brief introduction on history, configuration, operation principle and application of PEMFC. - Describe the mechanisms and kinetics of hydrogen oxidation reaction and oxygen reduction reaction taking place on Pt / C catalysts in PEMFC - Introduction of history, research and development of catalytic materials serving as anode and cathode in PEMFC. CHAPTER 2. EXPERIMENTAL AND RESEARCH APPROCHES 2.1. Preparation of catalytic materials Pt and Pt3M (M = Ni, Co, Fe) on Vulcan XC-72 carbon supports. Pt/C catalyst with metallic content of 20%wt. is synthesized by electroless deposition using ethylene glycol and NaBH4 assisted ethylene glycol. In ethylene glycol preparation, Pt/C catalysts were synthesized at temperatures of 80°C and 140°C. In addition, the catalysts were synthesized in a mixture of ethylene glycol: water by ratios (EG: W) of 9:1, 7:1, 5:1, and 3:1 (in unit volume). In NaBH4 assisted ethylene glycol method, catalytic samples are synthesized in mixture solvents with varying pH values of 10; 7; 4 and 2 2.2. Preparation of catalyt ink In characterization of catalytic materials and MEA electrodes, catalytic particles were prepared into catalytic inks including Pt/C and Pt3M/C catalyst particles with metallic content of 20%wt into a mixture solvent. Catalyst composition includes metallic catalyst of 5mg, absolute ethanol of 4 ml, and Nafion solution 10% of 25 µl. 2.3. MEA preparation The MEA electrodes with a nafion membrane sandwiched between two symmetry diffusion layers coated with catalyst ink were prepared hot-pressing method. Catalysts were prepared by brushing catalyst ink onto a gas diffusion carbon paper with active area of 5 cm2. 2.4. Research approches 2.4.1. Physical methods Transmission Electron Microscopy TEM is used to evaluate the size and distribution of metallic catalyst particles while X-ray diffraction is used to evaluate the structure and alloying degree of Pt-M/C catalysts. Energy-Dispersive X-Ray Spectroscopy (EDX) were used to determine the purity of the synthesized Pt/C catalysts. 2.4.2. Electrochemical methods 2.4.2.1. Cyclic Voltammetry (CV) The electrochemical sample is holded into a Teflon mold with a working area of 1 cm2. The measurements were conducted in a three electrode cell with counter electrode as platinum and the reference electrode as saturated calomel. Electrochemical measurements were done in solution H2SO4 0.5 M and apparatus was PARSTAT2273 (EG & G -USA). To study catalytic activity, CV measurements were scanned in potential range of 0.0 – 1.3V (NHE) with scanning speed of 50 mV/s at room temperature. For durability test, samples were scanned in potential range of 0.5 - 1.2 V (NHE) with 1000 cycles and a scanning speed of 50 mV/s. After each 200 durability test cycles of test, the sample is measured by CV for repeating evaluation of catalytic activity. 2.4.2.2. Linear scan voltammmetry (LSV) Activity improvements of PtM alloy catalysts for ORR were investigated by measuring LSV scanning. Samples were polarized from open circuit potential value to potential value of 0.7 V in 0.5 M H2SO4 solution with scanning speed of 1mV/s. 2.4.2.3. Evaluation of MEA properties by U-I polarization curve In the U-I polarization measurement, voltage of single cell was changed by using an external electric load. Current values were measured by ammeter and were recorded corresponding to each voltage value of single cell. The measurement was done from open circuit voltage to the voltage value of 0.4V. The recorded data of the single cell was used to plot a U-I graph by Excel software. CHAPTER 3. SYNTHESIS OF PT/C CATALYSTS BY ETHYLENE GLYCOL METHOD 3.1. Synthesis of Pt/C catalyst by electroless deposition using ethylene glycol In synthesis process of Pt/C catalysts with EG, temperature has a considerable influence on reduction of forming Pt particles on carbon supports. In this study, Pt/C catalyst was synthesized at 80oC. On TEM image, Pt particles on carbon support were not observed. In addiction, on the CV curve of this sample, there is not electrochemical peaks corresponding to reduction and oxidation of Pt metal. Therefore, at temperature of 80°C, formation of Pt metallic particles from precursor salt occurs slowly due to weak ethylene glycol reduction agent. Increase of temperature to 140oC, the reaction of forming Pt metallic particles becomes better. EDX analysis results confirmed the formation of Pt metal after reduction process. However, in this result presence of oxygen element is also observed. Thus, a small amount of PtO oxide was formed while forming of Pt/C catalysts. Figure 3.6. TEM picture and size distribution histogram of Pt/C catalyst particles synthesized at 1400C The formation of Pt catalyst particles on carbon support is also confirmed by TEM picture. For Pt/C catalysts, on TEM picture balack and small Pt particles appear uniformly on sphere carbon particles. A histogram of Pt particle size distribution measured 100 catalyst particles from TEM picture showed that at this synthesis condition, size of Pt metallic catalyst particles is mainly in the range of 3.0 - 4.5 nm and average particle size is about 3.8 nm (seeing on figure 3.6). Meanwhile, with the commercial catalyst sample, size of Pt catalyst particles is in range of 1.5-4.5nm with average particle size of 3.1nm. Figure 3.9. CV curves of Vulcan XC-72 carbon, commercial catalyst and synthesized catalyst Pt/C 20%wt. Figure 3.9 shows the CV curves of Vulcan XC-72 carbon commercial catalyst and synthesized catalyst Pt/C 20%wt. samples in 0.5M H2SO4 solution. By integrating, ESA values are calculated from H2 desorb/absorb peaks in the 0.0-0.4V potential range. ESA value of commercial and synthesized catalyst samples reach to 64.91 and 56.78 m2/ g, respectively. Figure 3.11 shows changes in ESA values of Pt/C commercial and synthesized catalyst samples after durability test. In general, after every 200 durability cycles, activity of catalyts decreases. After 1000 cycles, deacreses in ESA values of synthesized and commercial catalyst samples are 23.88 and 32.33%. Thus, compared with commercial sample, synthesized catalyst sample is more durable. Figure 3.11. Graph of changes in ESA values of commercial and synthesized catalyst samples after the durability test To reduce particle size and increase the surface area of Pt metallic catalyst particles, studies using solvent mixtures in synthesis of Pt/C catalyst were conducted. Pt/C catalysts were synthesized at 140° C with mixture solvents having different EG:W ratios of 9:1, 7:1, 5:1 and 3:1. The Pt/C catalysts synthesized with EG:W ratios of 9:1 and 7:1 have ESA values of about 72m2/g while the JM sample is about 64,91 m2/g. In addition, after 1000 cycles of durability test, these synthesized catalysts exhibite high durability. Therefore, Pt/C catalysts synthesized with EG:W ratios of 9:1 and 7:1 have high activity and durability. 3.2. Synthesis of Pt/C catalysts by NaBH4 assisted ethylene glycol In order to reduce temperature of synthesis process as well as to improve properties of Pt / C catalyst material, study on synthesis of Pt/C catalysts by NaBH4 assisted ethylene glycol method was conducted. Catalyst samples were synthesized with pH values of 2, 4, 7, 10 and 12. Table 3.5 summarizes average size values of the synthesized Pt catalyst particles at these pH values. From the obtained results, it is clear that when pH of mixture solvent decreases, size of the synthesized catalyst particles also decreases. With pH value of 4, the synthesized catalyst sample have the highest ESA value of 78.88 m2/g. The Pt/C synthesized catalyst at pH of 4 has also moderate durability with a decrease in ESA value after 1000 cycles of test only of 15.58%. Table 3.5. Average particle size of Pt/C catalyst samples synthesized at different pH values pH 2 4 7 10 12 Average size (nm) 2.6 2.5 3.0 3.2 5.0 3.3. Synthesis procedure of Pt/C catalyst Figure 3.30. Synthesis procedure of catalyst Pt/C 20 %wt CHAPTER 4 – SYNTHESIS AND CHARACTERIZATION OF PT-M/C ALLOY CATALYSTS (M = NI, CO, FE) Alloy catalysts in research have a molar ratio Pt: M of 3: 1 (with M as Ni, Co and Fe transition metals). Precursors of M metals include NiCl2, CoCl2 and FeCl2 corresponding to the Pt3Ni1/C, Pt3Co1/C and Pt3Fe1/C catalysts. The reduction process for deposition of metals to form alloys is done with NaBH4 reduction agent. 4.1. Characterization of Pt3M1/C synthesized alloy catalysts Figure 4.2 expresses X-ray diffraction patterns of Pt/C, Pt3Ni1/C, Pt3Co1/C and Pt3Fe1/C catalyst samples. On diffraction pattern of Pt/C sample, there are reflection peaks at different angle values (Fig. 4.2a). For Pt metal, from standard atlas, X-ray result confirms that Pt structure is face centred cubic. The diffraction peaks of the Pt/C catalyst are wide and low intensity. This proves that Pt synthesized catalyst particles are very small size and therefore can have a very large active surface area. With presence of M metals, diffraction peaks of Pt3M1 alloy particles change considerably. On the diffraction patterns of Pt3Ni1, Pt3Co1 and Pt3Fe1 alloys, structure of Pt-M alloys are still face centred cubic (Fig. 4.2b, Fig. 4.2c and Fig. 4.2d). The reflection peaks of (111) and (200) faces still appear with large peak root, but angle value 2θ is slightly shifted in compared to peaks of Pt/C catalyst. Specific peaks of Ni, Co and Fe metals and their oxides are not found. Therefore, after deposition, alloying of Pt with M metals happends to form a solid solution. Atoms of M metals have entered Pt crysstall and randomly replaced to some positions of Pt atom and caused Pt lattice deformation. This deformation may shorten Pt-Pt bond in the lattice. As a result, reflection peaks on X-ray diffraction patterns are slightly shifted. Different types of M metals lead to different shifts of angle 2θ. The shift is more clear for lattice of the Pt-Ni alloy catalyst. Thus, Pt3Ni1/C alloy catalyst may have the highest alloying degree of three synthesized alloy catalysts. a. Pt/C b. Pt3Ni1/C c. Pt3Co1/C d. Pt3Fe1/C Figure 4.2. X – ray diffraction pattern of catalyst samples (a) Pt/C, (b) Pt3Ni1/C, (c) Pt3Co1/C and (d) Pt3Fe1/C Figure 4.2 is LSV graph of Pt3Ni1/C, Pt3Co1/C and Pt3Fe1/C alloy catalyst samples in H2SO4 0.5M. The LSV curves of the alloy catalyst samples are shifted to the left of the LSV curve of the Pt/C sample meaning increase of current density. This may be due to the higher ORR reaction rates taking place on the alloy catalyst samples Faculty of Chemistry, HUS, VNU, D8 ADVANCE-Bruker - Sample M38 00-001-1194 (D) - Platinum - Pt - Y: 77.32 % - d x by: 1. - WL: 1.5406 - Cubic - a 3.91610 - b 3.91610 - c 3.91610 - alpha 90.000 - beta 90.000 - gamma 90.000 - Face-centered - Fm-3m (225) - 4 - 60.0567 - File: Thai VKHVL mau M38.raw - Type: 2Th/Th locked - Start: 20.000 ° - End: 70.010 ° - Step: 0.030 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 17 s - 2-Theta: 20.000 ° - Theta: 10.000 ° - Chi: 0 L in ( C p s) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 2-Theta - Scale 20 30 40 50 60 70 d = 1. 3 8 2 d = 1 .9 5 2 d = 2 .2 3 1 Faculty of Chemistry, HUS, VNU, D8 ADVANCE-Bruker - Sample 33 00-001-1194 (D) - Platinum - Pt - Y: 83.27 % - d x by: 1. - WL: 1.5406 - Cubic - a 3.91610 - b 3.91610 - c 3.91610 - alpha 90.000 - beta 90.000 - gamma 90.000 - Face-centered - Fm-3m (225) - 4 - 60.0567 - File: Thai VKHVL mau 33.raw - Type: 2Th/Th locked - Start: 20.000 ° - End: 70.010 ° - Step: 0.030 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 12 s - 2-Theta: 20.000 ° - Theta: 10.000 ° - Chi: 0.0 L in ( C p s ) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 2-Theta - Scale 20 30 40 50 60 70 d = 1 .4 0 2 d = 1 .9 3 2 d = 2 .2 5 7 Faculty of Chemistry, HUS, VNU, D8 ADVANCE-Bruker - Sample 41 00-001-1194 (D) - Platinum - Pt - Y: 93.80 % - d x by: 1. - WL: 1.5406 - Cubic - a 3.91610 - b 3.91610 - c 3.91610 - alpha 90.000 - beta 90.000 - gamma 90.000 - Face-centered - Fm-3m (225) - 4 - 60.0567 - File: Thai VKHVL mau 41.raw - Type: 2Th/Th locked - Start: 20.000 ° - End: 70.010 ° - Step: 0.030 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 12 s - 2-Theta: 20.000 ° - Theta: 10.000 ° - Chi: 0.0 L in ( C p s) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 2-Theta - Scale 20 30 40 50 60 70 d = 1 .3 8 4 d = 1 .9 7 2 d = 2 .2 6 1 than the Pt pure metal. Thus, activity for the ORR of Pt3M1/C alloy samples is much higher than that of Pt/C sample. From LSV measurements, the current density values at the 0.9V (i@0.9V) of the catalyst samples are summarized in Table 4.5 which show that activity of Pt3Ni1/C catalyst is the highest. Figure 4.8. LSV curves of alloying catalysts: Pt3Ni1/C, Pt3Co1/C and Pt3Fe1/C Table 4.5. Current density at potential of 0.9V (NHE) of Pt-M/C alloying catalyst samples Samples Pt/C Pt3Ni1/C Pt3Co1/C Pt3Fe1/C Current density i@0.9V (mA/cm 2 ) - 35.1 - 349.3 - 264.1 - 83.2 4.2. Influence of Ni content on performance of PtNi/C alloy catalysts In order to find optimal composition of PtNi/C alloy catalyst materials, research on effect of Ni content in alloy catalyst materials were conducted. PtNi/C alloy catalysts with different atomic ratios of Pt:Ni of 3:1; 2:1; 1:1; 1:2 and 1:3 was synthesized by electroless deposition. The results from CV curves show that activity of alloy catalyst samples decreases when Ni metallic content in sample increases. This decrease may be explained by comparison of activity for ORR of various metals. As increasing Ni content, amount of Ni metal on catalyst surface increases meaning that amount of Pt metallic catalysts decreases. Because activity of Pt metal for ORR is much higher than that of Ni metal, on whole surface of catalytic particle the rate for ORR may be decreased. In addition, at equilibrium potential, a strong bond of Ni metal with O and the group containing OH adsorbed on the surface slows down the rate of proton exchange steps in mechanism of ORR. Therefore, the rate for ORR on Ni surface would be decreased meaning that rate for ORR on PtNi/C is also decreased. When increasing Ni content, durability of PtNi/C alloy catalysts is considerably influenced. To be optimum between activity and economy effectiveness, Pt1Ni1/C particles may be potential catalysts served as cathode electrode in PEMFC. Table 4.6. ESA values of PtNi/C catalyst samples with different composition Sample Pt:Ni 3:1 2:1 1:1 1:2 1:3 ESA (m 2 /g) 76.14 66.16 52.41 42.87 42.52 4.3. Influence of heat treatment on performance of PtNi/C alloy catalysts. In synthesis of alloy catalysts for ORR, heat treatment is an important stage needed to be investigated for improving properties of catalytic material. To investigate the effect of heat treatment on