INTRODUCTION AND LITERATURE REVIEW
Fuel cell systems
1.1.1 Overview of fuel cell technologies
The origins of fuel cell technology can be traced back to the early nineteenth century, with significant contributions from Humphry Davy and Christian Friedrich Schürnbein In 1839, William Grove made a groundbreaking discovery by demonstrating the fundamental operating principle of fuel cells, which he referred to as a gas voltaic battery His experiments revealed that electricity could be generated from an electrochemical reaction between hydrogen and oxygen using a platinum catalyst, a principle that remains relevant to modern fuel cell systems.
Figure 1 1 A series of experiments of William Grove
(http://www.eniscuola.net/en/mediateca/william-grove1/)
A fuel cell is an electrochemical ―device‖ in which chemical energy is directly converted into electrical energy through electrochemical reactions of fuels (H 2 ,
CH 3 OH, CH 4 …) and oxidants (O2, air…) to form electricity and byproducts such as heat, water (a little amount of CO 2 in the case of direct methanol fuel cells) [7] Fuel cells do not contain energy inside, but can rather directly convert fuels into electricity, so as to produce electricity continuously as long as resources are supplied Unlike engines or conventional batteries, a fuel cell does not need recharging and produce only power and drinking water Thus, fuel cells have been regarded as clean and potential sources of electrical power for the future [8, 9]
Figure 1 2 The basic structure of a fuel cell system
(https://www.doitpoms.ac.uk/tlplib/fuel-cells/high_temp_sofc.php)
A fuel cell system comprises an electrolyte in contact with an anode (negative electrode) and a cathode (positive electrode) Various electrolytes, including acid, base, and molten salt, are utilized in fuel cells, with the Nafion membrane being a popular choice for allowing selective ion penetration while blocking electrons The catalyst layer may be positioned between the electrolyte and the electrodes or function as an electrode itself, often deposited on their surfaces Common catalysts include pure platinum or platinum alloys with metals such as nickel, ruthenium, and cobalt, as well as carbon-supported catalysts like Pt/C or Pt-M/C.
Fuel cell systems can generate a wide range of energy levels, from 1 Watt to 10 Megawatts, making them suitable for various applications throughout their lifespan At energy levels below 50 Watts, fuel cells are ideal for powering personal electronic devices like mobile phones and laptops In the 1 kW to 100 kW range, they can efficiently power both domestic and military vehicles, as well as public transportation and auxiliary power units (APUs) For larger applications, fuel cells in the 1 MW to 10 MW range are capable of providing grid-quality AC for distributed power use.
Figure 1 3 Applications of different fuel cells
Compare to other power devices, fuel cells possess several advantages (Figure 1 4):
High power conversion efficiency is crucial for transportation applications, especially as efficiency tends to improve at lower loads This characteristic is significant since internal combustion engines (ICE) often operate at reduced efficiency under low load conditions.
Fuel cell systems offer very low gas emissions, with pure hydrogen fuel achieving true zero-emission performance, as the only byproduct is water Even when using natural gas or petrol through a reforming process, CO2 emissions remain significantly lower than those from internal combustion engines (ICE) Additionally, fuel cells do not produce harmful nitrogen oxides (NOx) or sulfur oxides (SOx), making them a cleaner energy alternative.
Fuel cell systems operate with a low noise level due to their reliance on electrochemical reactions and the absence of moving parts While these technologies include essential components for cooling, power conversion, fueling, and air supply, the primary source of noise comes from the air compressor.
Fuel cell systems are designed as modular power generators, allowing for scalability and efficiency in power generation This flexibility enables the construction of systems that can deliver power ranging from just a few Watts to several Mega Watts, making them suitable for a wide range of applications.
Figure 1 4 Advantages of fuel cell systems compared to others generating power
(http://archive.siliconchip.com.au/cms/A_30527/article.html)
Fuel cell systems are categorized based on their electrolyte types, including Proton Exchange Membrane Fuel Cell (PEMFC), Direct Methanol Fuel Cell (DMFC), Solid Oxide Fuel Cell (SOFC), Alkaline Fuel Cell (AFC), and Phosphoric Acid Fuel Cell (PAFC) The most researched and widely used among these are PEMFC and DMFC, which offer advantages such as low operating temperatures around 80°C, high energy efficiency ranging from 50% to 80%, and rapid start-up times.
Table 1 1 Summary of main types of fuel cell systems [11]
PEMFC AFC PAFC MCFC ITSOFC TSOFC
Mobilized or Immobilized Potassium Hydroxide
Yes Yes Yes No No No
Components Carbon-based Carbon-based Carbon-based Stainless-based Ceramic Ceramic
Catalyst Platinum Platinum Platinum Nickel Perovskites Perovskites
Evaporative Evaporative Evaporative Gaseous Product Gaseous Product Gaseous Product
Process Gas + Independent Cooling Medium
Process Gas + Independent Cooling Medium
1.1.2 Proton Exchange Membrane Fuel Cell
Proton Exchange Membrane Fuel Cell (PEMFC) has been studied and employed in many fields like mobile phones, laptops, vehicles, transportation, military equipment
Major automotive manufacturers are utilizing Proton Exchange Membrane Fuel Cells (PEMFC) to produce clean and eco-friendly vehicles, which boast advanced features such as low operating temperatures ranging from 50 to 100°C, high energy conversion efficiencies of 50–60%, and nearly zero emissions compared to traditional cars.
Figure 1 5 Some commercialized cars using PEMFC
A proton-exchange membrane fuel cell system comprises an anode and a cathode, where hydrogen oxidation and oxygen reduction reactions occur, respectively These electrodes are separated by a gas-tight electrolyte membrane that permits only H+ ions to pass through The surface facing the electrolyte membrane is coated with a platinum nanocatalyst, facilitating the oxidation at the anode and reduction at the cathode Additionally, a conductor and a porous diffusion layer are positioned adjacent to the catalyst layer, enabling the flow of hydrogen and oxygen gases to the catalyst for reactions, while allowing water byproducts to be expelled outside.
The Proton Exchange Membrane Fuel Cell (PEMFC) stands out among fuel cells due to its high power density, generating more power per volume and weight, making it compact and lightweight With an operating temperature below 100 ºC, PEMFCs enable rapid start-up and can quickly adjust power output, making them ideal for automotive applications Additionally, the solid electrolyte used in PEMFCs simplifies the sealing of anode and cathode gases, resulting in lower manufacturing costs This solid material also offers better resistance to orientation issues and corrosion, contributing to a longer lifespan for both the cell and stack.
Figure 1 6 Proton Exchange Membrane Fuel Cell (PEMFC)
(https://www.tech-etch.com/photoetch/fuelcell.html)
In a Proton Exchange Membrane Fuel Cell (PEMFC), the membrane, typically a few hundred micrometers thick, is centrally located within the fuel cell This polymer solid electrolyte serves as a thin insulating layer and a barrier between the two electrodes, enabling efficient proton transfer and achieving high current density.
The essence of the PEMFC is the energy transformation from the reaction between hydrogen and oxygen to generate electricity (1.1) and the byproduct is water
This above overall reaction happens in two spatially separated half reactions, occurring at the anode and the cathode electrode hand side and given by:
At the anode electrode, hydrogen molecules diffuse into the catalyst layer, where they split into hydrogen ions (protons) and electrons The hydrogen ions then permeate the electrolyte to reach the cathode, while the electrons travel through an external circuit to generate electricity at the cathode.
At the cathode electrode: Oxygen, usually in the form of air, is supplied and combines with the H + ions and the electrons to generate water (1.3)
The hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) generate a voltage difference in fuel cells, converting chemical energy into electrical energy This voltage can be determined under specific thermodynamic conditions without drawing net current A fuel cell consists of an anode compartment for fuel, typically gaseous hydrogen, and a cathode compartment filled with oxidizing gas, usually ambient oxygen These compartments are separated by a gas-impermeable, electrically insulating membrane known as the electrolyte, which conducts protons Both sides of the membrane feature a platinum catalyst layer to facilitate the reactions.
Non-carbon support materials
To address the challenges associated with carbon supports, the development of non-carbon materials such as Magnéli-Phase oxides (Ti n O 2n-1), titanium oxides, cerium oxide, niobium oxide, and tungsten oxides (WO x) has gained traction These materials are considered promising alternatives due to their superior properties, including high corrosion resistance, exceptional stability, and strong interaction with platinum nanocatalysts.
Titanium-based oxides, particularly Titanium dioxide (TiO2), are increasingly recognized for their cost-effectiveness, high electrochemical and chemical stability, and strong synergy with platinum (Pt) catalysts, making them ideal supports for long-term fuel cell operations For instance, research by Ioroi et al demonstrated that the Pt/Ti4O7 electrocatalyst exhibited minimal changes in electrochemical surface area (ECSA) and maintained high activity for both hydrogen oxidation and oxygen reduction reactions at 80°C, outperforming traditional Pt/C catalysts.
Son Truong Nguyen’s group synthesized Magnéli-Phase oxides (Ti n O 2n-1) as a support material for alkaline ethanol fuel cells through heat treatment of commercial anatase-TiO 2 in hydrogen at 1050°C for 6 hours Experimental results demonstrated that Ti n O 2n-1 exhibited superior durability compared to carbon black in alkaline environments.
Ioroi et al developed a TiO x support and a Pt/TiO x catalyst for use in PEMFCs targeting the oxygen reduction reaction (ORR) Their experimental findings revealed that the Pt/TiO x catalyst demonstrated significantly enhanced stability at high potentials (greater than 1.0 V) compared to conventional catalysts Additionally, the ORR activity of Pt/TiO x was found to be twice that of traditional Pt/XC-72 catalysts Notably, after 10,000 cycles, the electrochemically active surface area (ECSA) of Pt/TiO x showed minimal change, while Pt/XC-72 experienced an approximate 30% loss in ECSA.
50 % after 10.000 cycles Other studies of Krishnam’s group [36] and Geng’s group
Research has demonstrated that Ti n O 2n-1 materials exhibit superior durability compared to carbon supports in electrochemical environments However, the low surface area of Magnéli-Phase oxides (Ti n O 2n-1) limits their applicability in fuel cell technologies.
Tungsten trioxide (WO3) is a commercially available n-type semiconductor with a band gap of approximately 2.6 to 2.8 eV, often used as a conducting oxide Research by Chhina et al demonstrated that a Pt/WO3 catalyst exhibits superior stability for the oxygen reduction reaction (ORR) compared to Pt/XC-72 catalysts Additionally, B Rajesh et al confirmed that Pt/WO3 shows higher activity for ORR than pure platinum; however, its performance for the methanol oxidation reaction (MOR) remains inferior to that of the commercial Pt/C catalyst, primarily due to the agglomeration of the Pt/WO3 catalysts.
Cui et al demonstrated that Pt/mesoporous WO3, with its high surface area, exhibited superior activity for methanol oxidation reaction (MOR) compared to commercial 20 wt % Pt/C and 20 wt % PtRu/C catalysts at potentials of 0.5 – 0.7 V vs NHE The enhanced electrochemical activity of Pt/WO3 was attributed to the strong interaction between the Pt nanocatalyst and the WO3 support, which improved CO tolerance Additionally, Micoud et al found that the Pt/WO3 catalyst displayed significantly higher CO tolerance than Pt/C and PtRu/C electrocatalysts, as evidenced by the CO oxidation current observed at a low potential of 0.1 V vs RHE.
The amount of platinum (Pt) loading on catalyst supports significantly influences their electrochemical activity Researchers are focused on reducing Pt nanoparticle (NP) loading on substrates while maintaining high performance in electrocatalysts, which is essential for the commercialization of fuel cell technologies For instance, in the case of the Pt/WO3 catalyst, a reduction of over 50% in Pt loading can still achieve electrochemical activity for methanol oxidation reaction (MOR) comparable to that of pure Pt, especially when the Pt NPs have a spherical morphology ranging from 50 nm to 150 nm in diameter Additionally, the morphology and structure of the WO3 support directly affect the electrochemical performance of Pt nanocatalysts in MOR applications.
A study demonstrated that the prepared micrometer-sized spheral WO3 support significantly enhanced the electrochemical activity and durability of the Pt catalyst for the methanol oxidation reaction (MOR), outperforming both 20 wt % PtRu/Vulcan XC-72 and 20 wt % PtRu/C with similar micrometer size.
Barczuk et al demonstrated that catalysts with identical PtRu loading on various WO3 supports revealed that Pt/mesoporous WO3, characterized by its high surface area and pore structure, showed superior electrochemical activity for methanol oxidation reactions (MOR) However, the solubility of tungsten oxide in acidic media poses a significant challenge for its application in fuel cell technologies To enhance the durability of WO3 materials, Raghuveer and Viswanathan developed Ti-doped WO3, which improved durability but resulted in reduced electronic conductivity.
Iridium dioxide (IrO2) is gaining attention as a non-carbon support in fuel cell technology due to its exceptional corrosion resistance, high electrical conductivity, and stability in acidic and electrochemical environments The Pt/IrO2 catalyst has been explored for its potential as an electrocatalyst for the oxygen reduction reaction (ORR) However, challenges such as poor distribution of Pt nanocatalysts on the IrO2 surface and low electrocatalytic activity, primarily due to the dissolution of Pt/IrO2 in electrochemical media, hinder its effectiveness The low activity of Pt/IrO2 catalysts is also attributed to the agglomeration of IrO2, which increases Ohmic resistance and impedes electron transport between Pt nanocatalysts To address these issues, a promising strategy involves enhancing electrical conductivity by incorporating Ir nanoparticles into IrO2 materials.
Kong et al prepared Pt/Ir x (IrO 2 ) 10-x electrocatalysts, revealing that the Pt/Ir 3 (IrO 2 ) 7 variant achieved the highest electrochemical surface area (ECSA) of 24.74 m²/g and optimal oxygen reduction reaction (ORR) activity of approximately 21.71 mA/mg at 0.85 V The enhanced performance of this catalyst is attributed to the strong interaction between Ir nanoparticles and IrO 2, which improves its electronic conductivity Additionally, the Pt/Ir-IrO 2 catalyst outperforms the Pt/IrO 2 variant due to both enhanced electronic conductivity and the strong synergy between Pt and Ir nanoparticles However, the high cost and limited stability of iridium metal pose significant challenges for its use in fuel cell applications.
Titanium dioxide (TiO2) is extensively used in various applications, including sensors, solar cells, and photocatalysis, thanks to its high electrochemical and chemical stability, controllable size and structure, affordability, and non-toxicity In fuel cell technology, TiO2 is particularly valued for its durability in acidic and aqueous environments Additionally, TiO2 exists in three crystalline phases, with anatase being one of the most studied.
Titanium dioxide (TiO2) exists in three phases: rutile (tetragonal), brookite (orthorhombic), and anatase (tetragonal), with rutile being the most stable and commonly found in nature In contrast, anatase and brookite are metastable and can transform into rutile when heated The phase transition of TiO2 is influenced by synthesis conditions, particularly temperature and reaction time, with the anatase to rutile transition occurring around 600°C in ambient conditions Research indicates that this conversion can happen at temperatures ranging from 400 to 1200°C, depending on the synthesis route and precursors used Xu et al demonstrated the preparation of single-phase TiO2 with controlled structure and morphology through a hydrothermal method using nanotube H-titanate precursors at varying temperatures and pH levels In acidic media, the nanotube H-titanate transforms into ~3 nm anatase phase nanoparticles, which can further evolve into rhombus-shaped anatase-TiO2 at pH values of 1 or higher, while nanorod rutile forms at pH levels of 0.5 or lower.
Figure 1 12 Mechanism transition of nanotube H-titanate to single-phase TiO 2 - with the different morphology and structure [84]
Armstrong et al [85] developed TiO2 using a sol-gel method, achieving a high surface area of 200 m²/g, ideal for proton exchange membrane fuel cells (PEMFC) Their experiments revealed the formation of anatase-TiO2 with small particle sizes of approximately 10 nm, which facilitated an effective distribution of platinum nanocatalysts on the TiO2 surface Additionally, the TiO2 support demonstrated excellent durability in acidic environments.
W-doped TiO 2 material
Recent studies have focused on W-doped TiO2 for its effectiveness as a photocatalyst in decomposing toxic organic compounds For example, Sangkhun et al prepared W-doped TiO2 using Na2WO4·2H2O and titanium tetrapropoxide through a solvothermal method The findings revealed that the W-doped TiO2 features both anatase and rutile structures, with particle diameters around 10 nm Notably, the decomposition performance of W-doped TiO2 for BTEX compounds is comparable to that of commercial TiO2 (P25).
Tian’s group synthesized W-doped TiO2 as an effective photocatalyst for decomposing methyl orange The anatase form of W-doped TiO2 was prepared using a hydrothermal method with Ti(SO4)2 and Na2WO4·2H2O as precursors This material demonstrated a high surface area and excellent performance in the degradation of methyl orange.
In 2015, Oseghe et al developed a 0.1 wt % W-doped TiO2 using a sol-gel method with Titanium (IV) isopropoxide and Sodium tungstate dehydrate as precursors The study found that increasing tungsten content in the anatase W-doped TiO2 resulted in reduced particle size and an enhanced surface area ranging from 86.08 to 91.71 m²/g Additionally, the W-doped TiO2 demonstrated effective decomposition performance for 4-chloro-2-methylphenoxyacetic acid (MCPA).
In 2017, Y Xiao et al developed a 1000 ppm tungsten-doped titanium dioxide (W-doped TiO2) using a solvothermal method with titanium isopropoxide and (NH4)10W12O41·xH2O as precursors Their experimental findings demonstrated that W-doped TiO2 is a promising material for enhancing solar cell performance.
While W-doped TiO2 has been extensively studied for photocatalytic applications, research on its preparation for electrochemical applications, particularly in fuel cell technology, remains limited Deli Wang and colleagues synthesized Ti0.7W0.3O2 with a diameter of 50 nm using a multi sol-gel method followed by heat treatment Their findings revealed that the Pt/Ti0.7W0.3O2 catalyst demonstrated superior durability and CO tolerance compared to conventional Pt/C catalysts.
Subban et al [102] developed Ti 0.7 W 0.3 O 2 as a support for Pt-based catalysts in PEMFC applications, demonstrating that the Pt/Ti 0.7 W 0.3 O 2 catalyst offers enhanced durability compared to the conventional Pt/C catalyst However, previous studies faced challenges due to complex synthesis methods and heat treatments involving stabilizers and surfactants, which led to the agglomeration of Ti 0.7 W 0.3 O 2 nanoparticles, hindering its potential in fuel cell technology.
Ir-doped TiO 2 material
In 2014, Victor M Menensez et al developed an Ir-doped brookite TiO2 photocatalyst for the degradation of acetaldehyde and toluene using a microwave-assisted hydrothermal method, varying the Iridium doping from 0.25 to 1.5 wt.% The optimal 0.5 wt.% Ir-doped brookite TiO2 exhibited a rod-like structure with an average diameter of 10 nm and demonstrated superior photocatalytic performance compared to commercial alternatives However, there is currently no research on the application of Ir-doped TiO2 as a support material in fuel cell technology.
Recent reports in Vietnam highlight advancements in fuel cell technology, particularly the work of Dr Nguyen M Tuan from the Institutes of Physics, who is focusing on direct methanol fuel cells (DMFCs) by developing electrodes with carbon osmosis membranes that enhance gas transport and conductivity Additionally, Nguyen Mau Cu and colleagues have designed a platinum-manganese alloy for use in fuel cell systems However, the exploration of M-doped titanium dioxide supports, specifically those incorporating tungsten and iridium, remains an area yet to be investigated in Vietnam.
Methods for synthesizing M-doped TiO 2 materials
The sol-gel process involves creating an oxide network through polycondensation reactions of molecular precursors in a liquid medium These precursors can be inorganic salts, such as chlorides, nitrates, or sulfates, or organic derivatives like metal alkoxides Initially, the metal alkoxide is transformed into a sol, which consists of dispersed colloidal particles in a liquid Subsequently, hydrolysis reactions occur, leading to the formation of internal linkages and a pore network within the liquid phase.
The hydrolysis reaction of alkoxide salts happens in water or alcohol media
Ti(OR) 4 + 4H 2 O Ti(OH) 4 + 4ROH (hydrolysis) (R is alkyl)
In addition to water and alcohol, both acidic and alkaline environments can be employed to hydrolyze the initial precursors Subsequently, the suspension is concentrated to create a gel as the solvent is removed.
The products obtained through the sol-gel method require high-temperature treatment to decompose organic precursors, which significantly influences the morphology and structure of the resulting materials The size of colloidal particles is affected by factors such as solution composition, pH, and temperature Additionally, variations in the activity levels of metal alkoxides complicate the control over the composition and homogeneity of the oxides produced This necessity for high-temperature processing presents a significant challenge in the sol-gel synthesis process.
The hydrothermal method employs water as a catalyst or reaction component at temperatures exceeding 100 °C and under high pressure, typically several bars This technique is favored for its ability to produce well-distributed products and uniformly sized nanoparticles Titanium dioxide (TiO2) materials are often synthesized in a Teflon-lined autoclave, operating at approximately 200 °C and pressures below 100 bar Key factors influencing the hydrothermal process include temperature, time, pH, and the choice of starting precursors A significant advantage of this method is the precise control over diffusion within a closed system.
The hydrothermal route, when combined with other techniques, can enhance reaction kinetics and facilitate the development of new materials For example, Zhang et al demonstrated the effectiveness of the microwave-assisted hydrothermal method in creating N-doped TiO2 with a rapid preparation time of just 5 minutes, resulting in a material with a large surface area and superior photocatalytic performance compared to traditional methods This process has proven to be effective for producing M-doped TiO2 with small particle sizes and high surface areas.
The solvothermal route is a chemical synthesis method akin to the hydrothermal process, utilizing an organic solvent like methanol, ethanol, or 1,4-butanol under supercritical conditions in a stainless steel autoclave By manipulating factors such as temperature, reaction time, pressure, solvent choice, and precursor materials, the morphology and structure of TiO2 can be precisely controlled.
The solvothermal method effectively produces homogeneous particles and metastable materials at low temperatures For example, Yin et al successfully synthesized N-doped TiO2 (TiO2-xNy) using this technique, allowing for precise control over the structure and morphology of the materials, which displayed three distinct structures: anatase, rutile, and brookite.
Liu et al [116] synthesized N-doped TiO2 in various structures, including anatase, rutile, and mixed phases, using a solvothermal method with TiCl3-hexamethylene tetramine as precursors and alcohol as the solvent Their findings revealed that the anatase structure transformed into rutile when different solvents like methanol, ethanol, 1-propanol, and 1-butanol were used A pure anatase phase was formed at pH levels between 1 and 2, while mixtures of anatase with rutile or brookite emerged at pH levels from 7 to 10 in methanol The solvothermal approach produced purified products due to the low dielectric constant of the organic solvent, which minimized anion generation [117] Additionally, Byranvand et al [118] employed the solvothermal method to manipulate the morphology, structure, and particle size of TiO2 materials.
The precipitation method is a technique for producing oxides by creating a homogeneous liquid phase through the careful adjustment of temperature, pH, and surfactant concentrations in either acidic or alkaline environments This process typically involves two key stages: nucleation, where stable, small particles are formed, and subsequent particle development or agglomeration By effectively managing the reaction kinetics, well-distributed particles can be achieved, enhancing the overall quality of the final product.
The electrochemical method is a versatile technique for producing thin film and porous materials at low temperatures By manipulating factors such as potential, current density, temperature, and pH, it is possible to create materials with desired properties.
Methods for preparing Pt-based catalyst
The polyol route has been widely utilized to design metal nanocatalysts such as Pt,
The use of multifunctional alcohols like ethylene glycol or glycerol, which have high boiling points, enables the reduction of metal ions (Au, Ag, and Pd) to metal nanoparticles This method allows for precise control over the particle size, structure, and morphology of the resulting nanoparticles.
The chemical reduction route is an effective method for producing metals from their salts in liquid media, utilizing reducing agents such as borohydride, formaldehyde, or formic acid Sodium borohydride (NaBH4) is preferred due to its strong reducing capabilities, with a high reduction potential of 1.24 V in alkaline conditions, compared to the reduction potentials of metals, which range from -0.5 V to 1.0 V This method allows for the generation of platinum nanoparticles with diameters of approximately 2-3 nm, while weaker reducing agents like hydrazine can produce larger particles around 40 nm Its simplicity and flexibility make the chemical reduction route widely adopted in metal nanoparticle synthesis.
Objectives of thesis research
Graphitic carbon, the standard catalyst support in fuel cells, faces degradation in harsh operating environments, leading to catalyst agglomeration and reduced electrochemical surface area, which diminishes performance and lifespan To address these challenges, recent research has focused on corrosion-resistant materials such as nanofibers, carbon nanotubes, and graphene, although they still struggle to prevent carbon oxidation Consequently, there is growing interest in non-carbon support materials, including nitrides, carbides, mesoporous silicas, electronically conducting polymers, Magnéli-phase oxides, titanium oxides, cerium oxide, niobium oxide, and tungsten oxides, which offer superior corrosion resistance and enhanced stability with platinum nanocatalysts However, previous studies highlight challenges such as complex synthesis and the need for heat treatment and stabilizers, which can lead to agglomeration and reduced surface area or conductivity, hindering the integration of these materials into fuel cell technology.
In light of the current challenges facing the commercialization of Proton Exchange Membrane Fuel Cells (PEMFC) and Direct Methanol Fuel Cells (DMFC), our study aims to address the critical issue of catalysts We seek to enhance the efficiency and durability of low-temperature fuel cells by developing more active and stable electrocatalytic materials for both cathode and anode electrodes Our innovative approach utilizes a novel nanostructured Ti 0.7 M 0.3 O 2 support that leverages an electronic transfer mechanism to improve performance.
Ti 0.7 M 0.3 O 2 to Pt that can modify the surface electronic structure of Pt, owing to a shift in the d-band center of the surface Pt atoms Furthermore, another benefit of
Ti 0.7 M 0.3 O 2 is the high stability of Pt/Ti 0.7 M 0.3 O 2 , which is attributable to the strong metal support interaction (SMSI) between Pt and Ti 0.7 M 0.3 O 2 , as well as the enhanced the inherent structural, chemical stability and the corrosion resistance of the TiO 2 - based in acidic and oxidative environments We expect that this new approach opens a reliable path to the discovery of advanced concept in designing a new catalyst that can replace the traditional catalytic structure and motivate further researches in the field
The primary goal of this research is to identify and develop novel nanostructured Ti0.7M0.3O2 supports, where M represents W or Ir, to serve as high-performance catalysts for Proton Exchange Membrane Fuel Cells (PEMFC) and Direct Methanol Fuel Cells (DMFC) This dissertation addresses various aspects related to the synthesis and optimization of these advanced materials.
1.7.1 High conductivity and surface area of Ti 0.7 W 0.3 O 2 mesoporous nanostructure support for Pt toward enhanced methanol oxidation in DMFC
- Synthesis of Ti 0.7 W 0.3 O 2 nanoparticles via a one-pot solvothermal process
- Characterization of the novel structure Ti 0.7 W 0.3 O 2
-Deposition of Platinum nano-forms on as-synthesized Ti 0.7 W 0.3 O 2
- Electrochemical properties of the 20 wt % Pt/Ti 0.7 W 0.3 O 2 catalyst
1.7.2 New Ir-doped TiO 2 nanostructure supports for Platinum: enhance catalytic activity and catalytic durability for fuel cells
- Synthesis of the new nanostructure Ti 0.7 Ir 0.3 O 2 via hydrothermal process
- Characterization of the new Ti 0.7 Ir 0.3 O 2 nanostructure support
-Deposition of Pt nano-forms on the new Ti 0.7 Ir 0.3 O 2 nanostructure support
- Novel Ti 0.7 Ir 0.3 O 2 nanorod prepared by facile hydrothermal process: A promising non-carbon support for Pt in PEMFC
- Advanced nanoelectrocatalyst of Pt nanoparticles supported on robust
Ti 0.7 Ir 0.3 O 2 NPs as a promising catalyst for fuel cells
- High conductivity of novel Ti 0.9 Ir 0.1 O 2 support for Pt as a promising catalyst for low-temperature fuel cell applications
Scheme 1 1 Traditional approach to improve the performance of catalyst at low- temperature fuel cells
Scheme 1 2 New approaches to improve the performance of catalyst for low- temperature fuel cells
MATERIALS AND EXPERIMENT
Materials
Table 2 1 Materials for this research
Tungsten (VI) chloride (WCl6) ≥ 99.9% Sigma – Aldrich, USA Iridium (III) chloride trihydrat (IrCl 3 3H 2 O) ≥ 99.9% Sigma – Aldrich, USA Chloroplatinic acid hydrate (H 2 PtCl 6 xH 2 O) ≥ 99.9% Sigma – Aldrich, USA
Sodium borohydride (NaBH 4 ) ≥ 98.0% Merck, Belgium
Sodium hydroxide (NaOH) 1N Merck, Belgium
Hydrochloric acid (HCl) 37.0% Merck, Belgium
Sulfuric acid (H 2 SO 4 ) 98.0% Merck, Belgium
Titanium tetrachloride (TiCl 4 ) 99.0% Aladdin, China
Nafion 117 solution ~5% Sigma – Aldrich, USA
Experimental procedure
Titanium dioxide (TiO2) is widely favored in various applications due to its non-toxicity, affordability, chemical and redox stability, and exceptional corrosion resistance These properties make it an ideal choice for use in photocatalysts, solar cells, and photocatalytic water splitting technologies.
The low electrical conductivity of TiO2 poses a significant challenge for its application in fuel cell technology To address this issue, doping titania with transition metals has proven to be an effective strategy, enhancing both its electronic conductivity and the electrochemical activity and durability of platinum-based catalysts Additionally, various support materials, such as Ti0.7Mo0.3O2, Ti0.7Ru0.3O2, and Ti0.7W0.3O2, also demonstrate co-catalyst activity, further improving performance in fuel cells.
When selecting dopant metals for TiO2, it is crucial to choose those with ion radii similar to Ti4+ (0.605 Å), such as W6+ (0.60 Å), to minimize lattice distortion Tungsten doping can enhance the electrical conductivity of TiO2 by releasing up to two electrons per dopant atom Studies have shown that Ti0.7W0.3O2 exhibits high durability and superior CO tolerance compared to carbon-supported materials However, the agglomeration of Ti0.7W0.3O2 nanoparticles poses a challenge, as it significantly reduces the electrochemical surface area (ECSA) by hindering the distribution of platinum nanoparticles on its surface Furthermore, the complex synthesis process of Ti0.7W0.3O2 is also regarded as a significant barrier to its production.
In this dissertation, W-doped TiO 2 was synthesized using a single-step solvothermal process, at low temperature as a low energy consuming fabrication technique that did not employ a surfactant or stabilizer (Figure 2 1)
Figure 2 1 Process for preparing W-doped TiO 2
To synthesize W-doped TiO2, 0.238 g of WCl6 was dissolved in 50 mL of absolute ethanol to create a uniform solution, followed by the addition of 0.155 mL of TiCl4, which was stirred for 15 minutes This solution, containing 12 mM WCl6 and 28 mM TiCl4 with a W:Ti molar ratio of 3:7, was then placed in a Teflon-lined autoclave to investigate the effects of reaction temperature and time on the synthesis process The resulting suspension was washed with water and centrifuged multiple times until a neutral pH of 7 was achieved Finally, the precipitates were dried overnight at 80°C for subsequent electrochemical and textural analyses.
Table 2 2 The effect of reaction temperature on the synthesis of W-doped TiO 2
Numerical order TiCl 4 (ml) WCl 6 (g) Reaction times
Table 2 3 The effect of reaction time on synthesis of W-doped TiO 2
Numerical order TiCl 4 (ml) WCl 6
2.2.2 Synthesis of 20 wt % Pt/Ti 0.7 W 0.3 O 2 catalyst
Finding an effective method to deposit small-diameter metal catalysts with optimal distribution on supports for improved electrocatalytic activity remains a significant challenge It is crucial to develop a fast and efficient approach for enhancing electrocatalysts in fuel cell applications Over the last two decades, microwave chemistry has evolved from a laboratory curiosity to a widely adopted synthesis technique in academic and industrial settings globally This technology facilitates new synthesis pathways, promotes the use of environmentally friendly solvents, and produces cleaner products that require less purification Key benefits of microwave heating include reduced reaction times, lower energy consumption, and increased product yields Additionally, microwave-assisted methods can be integrated with other green chemistry strategies to enhance their appeal Previous studies indicate that the microwave-assisted polyol method using ethylene glycol, which features rapid and uniform heating alongside moderate reduction, is an effective approach for depositing platinum nanoparticles on supports, thereby improving the electrocatalytic activity of Pt-based catalysts for fuel cells.
Figure 2 2 Schematic drawing for synthesis Pt/Ti 0.7 W 0.3 O 2 catalyst via microwave-assisted polyol route
In this study, Platinum nanoparticles (NPs) were anchored onto a Ti0.7W0.3O2 support using microwave-assisted polyol synthesis The process began with the addition of 110 mg of Ti0.7W0.3O2 into 25 mL of ethylene glycol, which was magnetically stirred until fully dispersed The suspension was then ultrasonicated for 30 minutes and cooled to 5 °C Following this, 2.818 mL of 0.05 M H2PtCl6 was added and stirred for 20 minutes, with the pH adjusted to 11 using NaOH The mixture was transferred to a microwave oven for a reduction reaction at 160 °C for 2 minutes at 240 W Finally, the product was rinsed with acetone and distilled water, then dried at 80 °C overnight for further analysis.
2.2.3 Synthesis of Ir-doped TiO 2
Research on synthesizing iridium (Ir) doped titanium dioxide (TiO2) is limited, primarily focusing on low iridium loading for photocatalytic applications However, the potential of Ir-doped TiO2 as a support material in electrocatalysts remains unexplored and warrants further investigation In this study, Ir-doped TiO2 nanostructures were synthesized using a one-step hydrothermal method with TiCl4 and IrCl3 precursors This energy-efficient process does not require surfactants or stabilizers, resulting in nanomaterials with a large surface area, making it a viable green fabrication technique.
In the synthesis process, 0.2117 g of IrCl3·3H2O was dissolved in 50 mL of purified water, and the pH was adjusted using 37% hydrochloric acid Next, 0.155 mL of TiCl4 was added to the solution, maintaining a Ti:Ir molar ratio of 7:3, and the mixture was stirred for 5 minutes The resulting solution was then transferred into a Teflon-lined autoclave with a stainless steel shell, where reactions were conducted under varying times, temperatures, and pH levels to identify optimal synthesis conditions After the reaction, the suspension was allowed to cool to room temperature, rinsed thoroughly with purified water, and the precipitates were dried overnight for subsequent analysis.
Figure 2 3 Procedure for preparing Ir-doped TiO 2 Table 2 4 The effect of reaction time on the synthesis of Ir-doped TiO 2
Table 2 5 The effect of reaction temperature on synthesis of Ir-doped TiO 2
Table 2 6 The effect of pH value on synthesis of Ir-doped TiO 2
2.2.4 Synthesis of Pt/Ti 0.7 Ir 0.3 O 2 catalyst
Enhancing catalytic activity can be achieved by minimizing particle size, which boosts surface area and decreases loading requirements Typically, platinum catalysts are synthesized using reducing agents like NaBH4 and ethylene glycol (EG).
[152, 153] NaBH 4 has a strong reduction ability in the liquid phase at low temperature
While NaBH4 is effective, its primary limitation lies in the challenge of controlling particle size and dispersion In contrast, using ethylene glycol (EG) as a reducing agent produces platinum (Pt) catalysts with smaller particle sizes and improved dispersion, as EG also acts as a stabilizer for nanoparticles However, its weaker reduction capability necessitates higher reaction temperatures and specific pH levels in the solution.
Figure 2 4 Schematic drawing for synthesizing Pt/Ti 0.7 Ir 0.3 O 2 catalyst
In this experiment, we utilized a modified chemical reduction method with sodium borohydride (NaBH4) as the reducing agent to synthesize Pt/Ti0.7Ir0.3O2 catalysts Previous studies indicate that this approach, which combines NaBH4's strong reducing properties with the excellent dispersion capabilities of ethylene glycol, is effective for catalyst preparation.
Pt nanocatalysts were synthesized with a small size and uniform distribution on a support A solution of 3.0 mL of 0.05 M aqueous H2PtCl6 was prepared by mixing it with 25 mL of purified water and 0.5 mL of ethylene glycol, followed by pH adjustment to 11 using NaOH Subsequently, 110 mg of Ti0.7Ir0.3O2 powders were ultrasonicated in this solution for 5 minutes to create a uniform suspension Next, 3 mL of 0.05 mM aqueous NaBH4 was added to the suspension and stirred for 2 hours at room temperature After the reaction, the mixture was centrifuged and washed multiple times with purified water, and the resulting precipitates were dried at 80°C overnight for further analysis.
Characterization techniques
X-ray diffraction (XRD) is a technique used to provide a structural characterization of a sample material utilizing X‐ray beams XRD is used to identify crystalline materials with known diffraction patterns or to determine the structure of newly developed materials XRD is based on the interaction of a monochromatic X-ray beam with the crystal lattice Diffraction occurs when irradiation by electromagnetic waves interact with a regular array of scattering centers that have a spacing similar in size to the wavelength of the radiation [15]
X-ray diffraction (XRD) measurements were performed on D2 PHASER (Brucker – US) using Cu K radiation with the 2range from 20 o – 80 o at a scan rate of 2 o /min to confirm the structure of catalyst support and nanocatalyst in this work
X-ray photoelectron spectroscopy (XPS) is a technique to analyze the chemical state and elemental composition of material primarily at the top surface region between 0.5 to 5 nm [40] of a sample, with a possibility for higher probing depths In XPS, an X- ray beam irradiates the material and emits the core-level electrons The binding energy of the electrons can be determined by detecting the kinetic energy of the emitted electrons from the material Each electron of an element has its own set of electron binding energies The measured kinetic energies, therefore, provides elemental identification XPS sampling is commonly conducted under ultra-high vacuum conditions involving a source of radiation and an electron energy analyzer The electron energy analyzer determines the kinetic energy of the emitted electrons from the specimen For XPS, the radiation source is from Al Kα or Mg Kα X-rays
Figure 2 5 The basic principle of XPS
X-ray photoelectron spectroscopy (XPS) operates on the principle that ejected electrons create characteristic peaks in a spectrum based on their intensity Electrons that escape the surface without energy loss provide distinct peaks, while those that undergo inelastic scattering contribute to the low kinetic energy background By utilizing standard binding energy values, researchers can identify unknown elements and their oxidation states on a surface Changes in valence charge density or oxidation lead to increased binding energy, offering insights into the chemical environment and valence states of atoms Quantitative analysis typically involves a curve-fitting process to ascertain atomic percentages.
2.3.3 Scanning electron microscopy with energy dispersive X-ray spectroscopy
To analyze the average elemental composition of catalyst support and the loading of platinum nanoparticles (Pt NPs) on the support, Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX) was conducted using a JEOL EDX-JSM 6500 F at an accelerating voltage of 15 kV The elemental maps obtained from EDX provided insights into the distribution of platinum on the support material.
2.3.4 Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HR-TEM)
The particle sizes of the support materials and nanocatalysts were analyzed using transmission electron microscopy (TEM) with an FEI-TEM-2000 microscope at an accelerating voltage of 3800 kV Specimens were prepared by ultrasonically suspending them in ethanol, followed by transfer to a copper grid and drying in an oven.
High-resolution transmission electron microscopy (HR-TEM) was utilized to analyze the morphology and size of the catalyst support and platinum (Pt) nanoparticles The measurements were conducted using a JEOL-JEM 1400 microscope, operating at an accelerating voltage of 3800 V.
2.3.5 Brunauer Emmett Teller (BET) surface area analysis
The BET technique is widely used to measure the specific surface area of materials by adsorbing gas molecules onto solid surfaces This method builds upon the Langmuir adsorption theory, extending it from monolayer to multilayer adsorption Adsorption occurs when gas molecules adhere to a solid surface, and variations in gas concentration on the solid can help determine the surface area of materials For assessing the surface area and pore size of catalyst supports, nitrogen adsorption/desorption isotherms are conducted at 77K using the NOVA 1000e Before BET measurements, specimens are degassed and dried at 250°C for three hours to remove any water molecules trapped in the meso/micropores of the catalyst support.
The electrical conductivity of catalyst support materials was measured using a Jandel MWP-6 four-point probe technique Prior to testing, Ti 0.7 M 0.3 O 2 (with M being W or Ir) powder was formed into pellets approximately 10 mm in diameter and 1 mm thick under 300 MPa pressure using steel dies To ensure accurate conductivity readings, the probe system was meticulously positioned on the pellet at three distinct locations for each sample to obtain average values.
2.3.7 Electrode preparation and electrochemical measurements
An EC-Lab Electrochemistry instrument from Bio-Logic SAS was used for electrochemical analysis, employing a saturated calomel electrode (SCE) as the reference electrode, a platinum piece as the counter electrode, and a 5-mm-diameter glassy carbon disk as the working electrode All potential measurements were referenced to the normal hydrogen electrode (NHE) scale.
The procedure for catalyst ink/slurry preparations involved:
Preparing the 0.5 % Nafion by diluting 5 % commercial Nafion with ethanol absolute 99.9 %
In our experiment, we maintained a standard electrode value of 0.22 mg-Pt/cm², which allowed us to perform a backward calculation to determine the weight of samples mixed with a Nafion solution.
0.22 mgPt/cm 2 * 0.1964 cm 2 (RDE geometric area) = 0.043208 mg-Pt
500 μL/7μL * 0.043208 mg-Pt = 3.086286 mgPt (because we used 7 μL to drop in RDE surface and prepare the catalyst ink in 500 μL solutions)
Because the samples were not pure Pt (except for Pt Black), the number of samples were 3.086286 mg/x (x is the Pt percentage of samples)
So 3.086286/x mg samples are mixed with 500 μL 0.5 % Nafion solutions
Ultrasonicating the catalyst ink to create a homogeneous suspension for about 3-4 hours and keeping the temperature below 30 °C to avoid evaporation of samples
The procedure for H 2 SO 4 preparation involved:
Taking a clean volumetric 2000 mL flask, and fill it half with ultra-pure water
Measuring an exact 54.35 mL H 2 SO 4 p.a and pouring it slowly to the volumetric flask
Adding ultra-pure water until the volume was exactly 2000 mL
Putting in magnetic stirrer to stir it for 2-3 hours
2.3.7.3 Cleaning the equipment (EC cell, RE, CE)
The procedure for cleaning the equipment involved:
For electrochemical cell, just washing normally like other glass equipment and at the end rinsing with ultra-pure water several times, after that drying in the oven
For RE, saturated calomel electrode, just rinsing several times with ultra- pure water before and after each experiment SCE was stored in a saturated
For CE, ultrasonicating in low concentration of HNO 3 solution, and then repeating the sonication using ultra-pure water several times to remove HNO 3 from CE surface
2.3.7.4 Dropping the catalyst ink on the RDE surface
The procedure for dropping the catalyst ink on RDE surface involved:
Cleaning the RDE surface using a wetted tissue (wetting agent methanol high purity 99.9%) to make sure previous catalyst was no longer there
Mixing Al 2 O 3 polishing powder (0.05 μm) with ultra-pure water on a special cloth and then polishing the RDE surface by that cloth (which already have
Al 2 O 3 slurry) with movement changed from clockwise to counterclockwise Doing it for around 2 minutes
Ultrasonicating the polished RDE in ultra-pure water for 5 minutes
Taking out the RDE, cleaning the surface using the tip of a tissue (to avoid pressure on a clean-polished surface)
Dropping 7 μL of catalyst ink (which was considered homogeneous by prior sonication)
Drying in RT for 5 minutes and putting in the oven 80 °C for 5 minutes
2.3.7.5 Preparation of electrochemical test compartment
The procedure for electrochemical test compartment preparation involved:
Putting 200 mL H 2 SO 4 0.5 M inside the four-hole cell
One hole for RDE, another one for SCE, another one for Pt-plate (1x1 cm 2 )
The experiment utilized a cylinder glass with a Styrofoam sparger at the bottom to effectively facilitate the bubbling gas, ensuring optimal contact between the RDE surface and the acid solution while preventing bubble formation on the surface.
The compartment and all the set-up of electrochemical test experiments were well described in Figure 2.6:
Figure 2 6 Three-electrode electrochemical cell for measuring polarization curve
To start with, the catalyst electrode was activated about 50 cycles at a scan rate of
The study investigated the electrochemical surface area (ECSA) of catalysts using a scan rate of 50 mV/s in a N2-purged 0.5 M H2SO4 electrolyte solution, within a potential window of 0 to 1.10 V Additionally, methanol electro-oxidation was examined in a N2-purged 10 v/v % CH3OH/0.5 M H2SO4 solution at the same scan rate To assess the durability of the 20 wt % Pt/Ti0.7W0.3O2 catalyst, chronoamperometry measurements were conducted at a potential of 0.7 V in a nitrogen-purged environment.
10 v/v % CH3OH/0.5 M H2SO4 solution for 60 min
2.3.8.1 Cyclic voltammetry (CV) and electrochemical surface area (ECSA) determination
This technique enables the acquisition of both qualitative and quantitative data regarding catalysts and their electrochemical reactions It involves cycling the potential between two points while recording the current within the cycling region The current observed reflects the degree of cathodic or anodic reactions occurring Initially, the voltage is increased from low to high and then decreased back to the starting point, with the transition point where the voltage begins to drop referred to as the "switching" potential The selected potential range is crucial for accurate analysis.
The CV procedures are highly dependent on the specific application, as illustrated by a typical cyclic voltammogram (CV) for platinum (Pt) supported on carbon black in a sulfuric acid (H2SO4) medium This CV reveals five distinct potential regions: (i) the hydrogen adsorption region, (ii) the hydrogen desorption region, (iii) the double-layer region, (iv) the Pt oxidation region, where the Pt surface undergoes oxidation to form PtOH and subsequently PtOx, and (v) the reduction of PtO back to metallic Pt.
In sulfuric acid solution, H + is reversibly adsorbed to form a monolayer at potentials between 0.05 and 0.4 V via the reaction: