Summary of materials science doctoral thesis: Synthesis and characterization of carbon supported Pt and Pt alloy nanoparticles as electrocatalysts material for proton exchange membrane fuel

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.

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

- 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 Pt 3 M (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

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 80C

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 NaBH 4 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

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 Pt 3 M 1 /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

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 -

0 10 30 50 70 90 100 120 140 160 180 200 220 240

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

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 -

0 10 40 70 100 130 160 190 220 250 280 310 340