INTRODUCTION AND LITERATURE REVIEW
Fuel cell systems
1.1.1 Overview of fuel cell technologies
The early nineteenth century marked the inception of fuel cell concepts, first demonstrated by Humphry Davy This foundational work paved the way for Christian Friedrich Schürenbein's advancements in fuel cell systems In 1839, William Grove discovered the fundamental operating principle of fuel cells, which he referred to as a gas voltaic battery, through experiments that revealed electricity generation from an electrochemical reaction between hydrogen and oxygen using a platinum catalyst This enduring principle remains relevant in today's fuel cell technology.
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 positioned between an anode (negative electrode) and a cathode (positive electrode) Various electrolytes, including acid, base, and molten salt, are employed in fuel cells, with the Nafion membrane being particularly popular for its ability to allow ion passage while blocking electron flow Additionally, catalyst layers can be applied between the electrolyte and electrodes or used directly as electrodes, often in the form of pure platinum or platinum alloys with metals like nickel, ruthenium, and cobalt, as well as carbon-supported catalysts such as Pt/C or Pt-M/C.
Fuel cell systems can generate varying energy levels by connecting multiple fuel cells, ranging from 1 Watt to 10 Megawatts, making them suitable for diverse applications throughout their lifespan At energy levels below 50 Watts, fuel cells are ideal for powering personal electronic devices such as mobile phones and laptops In the 1 kW to 100 kW range, they can effectively 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 converting energy for distributed power uses, providing grid-quality AC electricity.
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, particularly because efficiency tends to improve at lower loads This characteristic is significant as internal combustion engines (ICE) operate less efficiently 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 alternative for energy generation.
Fuel cell systems operate quietly 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 used in the system.
Fuel cell systems are inherently modular power generators, allowing for scalable solutions that efficiently accommodate power levels ranging from several Watts to several Mega Watts.
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 electrolytes into several types, including Proton Exchange Membrane Fuel Cells (PEMFC), Direct Methanol Fuel Cells (DMFC), Solid Oxide Fuel Cells (SOFC), Alkaline Fuel Cells (AFC), and Phosphoric Acid Fuel Cells (PAFC) Among these, PEMFCs and DMFCs are the most researched and widely utilized due to their low operating temperature of approximately 80°C, high energy efficiency ranging from 50% to 80%, and rapid start-up capabilities.
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 car manufacturers are utilizing Proton Exchange Membrane Fuel Cells (PEMFC) to produce environmentally friendly vehicles that boast advanced features such as low operating temperatures of 50-100°C, high energy conversion efficiencies of 50-60%, and near-zero emissions compared to traditional cars.
Figure 1 5 Some commercialized cars using PEMFC
A proton-exchange membrane fuel cell system features an anode and cathode electrode, facilitating hydrogen oxidation and oxygen reduction reactions, respectively These electrodes are separated by a gas-tight electrolyte membrane that permits only H+ ions to pass through The surface of the electrolyte membrane is coated with a platinum nanocatalyst, which aids in the oxidation process at the anode and the reduction process at the cathode Additionally, a conductor and a porous diffusion layer are positioned adjacent to the catalyst layer, enabling the flow of oxygen and hydrogen gases to the catalyst for reaction, while allowing the resulting water product to be expelled outdoors.
The Proton Exchange Membrane Fuel Cell (PEMFC) stands out among fuel cells due to its high power density, generating more power per volume or weight, making it compact and lightweight with an operating temperature below 100 ºC for quick start-up These features, along with the ability to rapidly adjust power output, position the PEMFC as a leading choice for automotive applications Additionally, the solid electrolyte used in PEMFCs simplifies the sealing of anode and cathode gases, resulting in lower manufacturing costs It also offers enhanced 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 central membrane, measuring just a few hundred micrometers thick, serves as a crucial component This polymer solid electrolyte functions as a thin insulating layer and a barrier between the two electrodes, effectively facilitating proton transfer and enabling 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 towards the cathode, while the electrons travel through an external circuit to the cathode, generating electricity in the process.
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 from thermodynamic properties under specific conditions, such as pressure and temperature, when no net current is drawn Fuel cells consist of an anode compartment for fuel, typically gaseous hydrogen, and a cathode compartment for the oxidizing gas, usually oxygen from the air These compartments are separated by an electrolyte membrane that is electrically insulating and gas impermeable, allowing proton conduction Both sides of the membrane feature a catalyst layer made of platinum to facilitate the reactions.
Non-carbon support materials
To address the limitations of carbon supports, non-carbon materials such as Magnéli-Phase oxides (Ti n O 2n-1), titanium oxides, cerium oxide, niobium oxide, and tungsten oxides (WO x) have emerged as promising alternatives These materials offer superior properties, including high corrosion resistance, enhanced stability, and strong interactions with platinum nanocatalysts, making them suitable replacements for traditional carbon substrates.
Titanium-based oxides, particularly Titanium dioxide (TiO2), are gaining attention for their cost-effectiveness, high electrochemical stability, and compatibility with 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 maintains stable electrochemical active surface area (ECSA) and exhibits superior electrochemical activity for hydrogen oxidation and oxygen reduction reactions compared to traditional Pt/C catalysts at 80°C.
Son Truong Nguyen's research team 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 six hours The experimental findings demonstrated that Ti n O 2n-1 exhibited significantly better durability than carbon black in alkaline environments.
Ioroi et al developed a TiO x support and Pt/TiO x catalyst for proton exchange membrane fuel cells (PEMFC) focused on the oxygen reduction reaction (ORR) Their experiments demonstrated that the Pt/TiO x catalyst exhibited significantly higher stability compared to conventional catalysts at high potentials (greater than 1.0 V) Notably, the ORR activity of Pt/TiO x was found to be twice that of traditional Pt/XC-72 Additionally, after 10,000 cycles, the electrochemically active surface area (ECSA) of Pt/TiO x showed negligible change, while the Pt/XC-72 catalysts 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 support 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 commonly used as a conducting oxide, featuring a band gap of approximately 2.6 to 2.8 eV Research by Chhina et al demonstrated that Pt/WO3 catalysts exhibit superior stability for the oxygen reduction reaction (ORR) compared to Pt/XC-72 catalysts Additionally, B Rajesh et al confirmed that Pt/WO3 shows enhanced ORR activity relative to pure Pt; 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 Pt/WO3 particles.
Cui et al demonstrated that Pt/mesoporous WO3, with its high surface area, outperformed commercial 20 wt % Pt/C and 20 wt % PtRu/C catalysts for methanol oxidation reaction (MOR) at potentials between 0.5 – 0.7 V vs NHE The enhanced electrochemical activity of Pt/WO3 is attributed to the strong interaction between the Pt nanocatalyst and the WO3 support, leading to improved CO tolerance Additionally, Micoud et al found that the Pt/WO3 catalyst exhibited superior CO tolerance compared to Pt/C and PtRu/C electrocatalysts, as evidenced by the CO oxidation current at a low potential of 0.1 V vs RHE.
Optimizing the platinum (Pt) loading on catalyst supports is crucial for enhancing electrochemical activity in fuel cells Researchers aim to minimize Pt nanoparticle (NP) usage while maintaining electrocatalyst performance for commercialization Notably, the Pt/WO3 catalyst demonstrates that reducing Pt loading by over 50% can achieve electrochemical activity for methanol oxidation reaction (MOR) comparable to pure Pt, with spherical nanoparticles ranging from 50 nm to 150 nm in diameter Furthermore, the morphology and structure of the WO3 support significantly influence the electrochemical performance of Pt nanocatalysts in MOR applications.
A study demonstrated that micrometer-sized spheral WO3 support enhances the electrochemical activity and durability of Pt catalysts for the methanol oxidation reaction (MOR), outperforming both 20 wt % PtRu/Vulcan XC-72 and 20 wt % PtRu/C.
Barczuk et al demonstrated that Pt/mesoporous WO3 catalysts, characterized by a high surface area and pore structure, showed superior electrochemical activity for methanol oxidation reactions (MOR) However, the solubility of tungsten oxide in acidic environments remains a significant challenge for its application in fuel cell technologies To enhance the durability of WO3, Raghuveer and Viswanathan developed Ti-doped WO3, which improved durability but resulted in decreased electronic conductivity.
Iridium dioxide (IrO2) is gaining attention as a non-carbon support in fuel cell technology due to its exceptional corrosion resistance, electrical conductivity, and stability in acidic and electrochemical environments The Pt/IrO2 catalyst has been investigated 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, attributed to the dissolution of Pt/IrO2 in electrochemical media, hinder its effectiveness Additionally, the agglomeration of IrO2 increases Ohmic resistance and impedes electron transport between Pt nanocatalysts To address these issues, a promising strategy involves incorporating Ir nanoparticles into IrO2 materials to enhance electrical conductivity.
Kong et al developed Pt/Ir x (IrO 2 ) 10-x electrocatalysts, with Pt/Ir 3 (IrO 2 ) 7 demonstrating the highest ECSA of 24.74 m²/g and exceptional ORR activity of approximately 21.71 mA/mg at 0.85 V This enhanced performance is attributed to the strong interaction between Ir nanoparticles and IrO 2, which improves the material's electronic conductivity Additionally, the Pt/Ir-IrO 2 catalyst outperforms the Pt/IrO 2 catalyst due to both enhanced electronic conductivity and the synergistic effects 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 widely used in various applications, including sensors, solar cells, and photocatalysis, due to its high electrochemical and chemical stability, controllable size and structure, affordability, and non-toxicity In fuel cell technology, TiO2 is valued for its durability in acidic and aqueous environments Additionally, TiO2 exists in three crystalline phases, with anatase being one of them.
Titanium dioxide (TiO2) exists in three phases: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic), with rutile being the most stable and commonly found in nature The metastable anatase and brookite phases can transform into rutile when heated, with the transition from anatase to rutile typically occurring around 600°C in ambient conditions Research shows that this conversion can happen at temperatures ranging from 400°C to 1200°C, influenced by synthesis methods and precursor materials Xu et al demonstrated the preparation of single-phase TiO2 with controlled structure and morphology through a hydrothermal process using nanotube H-titanate precursors During this process, the nanotubes were dispersed in acidic media, converting into ~3 nm anatase phase intermediates, which further transformed into rhombus-shaped anatase-TiO2 nanoparticles at pH values of 1 or higher, while nanorod rutile formed at pH values 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 material with a high surface area of 200 m²/g using a sol-gel method for proton exchange membrane fuel cells (PEMFC) Their experiments revealed the creation of anatase-TiO2 with small particle sizes of approximately 10 nm, which facilitated an effective distribution of platinum nanocatalysts on its surface Additionally, the TiO2 support demonstrated excellent durability in acidic environments.
W-doped TiO 2 material
Recent studies have highlighted the potential of W-doped TiO2 as an effective photocatalyst for decomposing toxic organic compounds For example, Sangkhun et al synthesized W-doped TiO2 using a solvothermal method with Na2WO4·2H2O and titanium tetrapropoxide (TTIP) as precursors Their findings revealed that the W-doped TiO2 exhibits both anatase and rutile structures with an approximate diameter of 10 nm, demonstrating high decomposition efficiency for BTEX compounds that rivals that of commercial TiO2 (P25).
Tian's group synthesized W-doped TiO2 as an effective photocatalyst for the decomposition of methyl orange The anatase form of W-doped TiO2 was prepared through a hydrothermal method using Ti(SO4)2 and Na2WO4·2H2O as precursors This material demonstrated a high surface area and excellent performance in degrading methyl orange.
In 2015, Oseghe et al developed a 0.1 wt % W-doped TiO2 material using a sol-gel method with Titanium (IV) isopropoxide and Sodium tungstate dehydrate as precursors The study found that increasing tungsten doping led to a reduction in particle size and an increase in 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 ammonium tungstate as precursors Their findings demonstrated that W-doped TiO2 significantly enhances the performance of solar cells, indicating its potential as an effective material for solar energy applications.
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 support with a diameter of 50 nm using a multi sol-gel method followed by heat treatment Their findings demonstrated that the Pt/Ti0.7W0.3O2 catalyst exhibited superior durability and enhanced CO tolerance compared to commercial Pt/C catalysts.
Subban et al [102] developed Ti 0.7 W 0.3 O 2 as a support for Pt-based catalysts in Proton Exchange Membrane Fuel Cells (PEMFC), revealing that the Pt/Ti 0.7 W 0.3 O 2 catalyst demonstrated enhanced durability compared to the conventional Pt/C catalyst However, previous research involved complex synthesis methods and additional heat treatments with stabilizers and surfactants, leading to agglomeration of the Ti 0.7 W 0.3 O 2 nanoparticles, which poses a significant challenge for its application in fuel cell technology.
Ir-doped TiO 2 material
In 2014, Victor M Menensez et al developed an Ir-doped brookite TiO2 photocatalyst using a microwave-assisted hydrothermal method, varying the Iridium doping levels at 0.25, 0.5, 0.75, and 1.5 wt % The 0.5 wt % Ir-doped brookite TiO2 exhibited a rod-like morphology with an average diameter of 10 nm The findings demonstrated that this photocatalyst significantly outperformed commercial alternatives in decomposing acetaldehyde and toluene in gas flow However, there has been no research conducted on the preparation of Ir-doped TiO2 supports specifically for fuel cell technology.
Recent reports in Vietnam highlight advancements in fuel cell technology, particularly through the work of Dr Nguyen M Tuan at the Institutes of Physics, who focused on Direct Methanol Fuel Cells (DMFCs) by developing electrodes with carbon osmosis membranes for enhanced gas transport and conductivity Additionally, Nguyen Mau Cu and colleagues designed a Pt-M alloy (where M represents Co and Fe) for use in fuel cell systems However, research on M-doped TiO2 supports (with M being W and Ir) remains unexplored 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, and 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 Following this, a hydrolysis reaction occurs, 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 used to hydrolyze the initial precursors Subsequently, the mixture is concentrated to create a gel as the solvent is removed.
The products obtained from the sol-gel method typically undergo high-temperature treatment to decompose organic precursors, influencing the morphology and structure of the final oxides The size of the colloidal particles is affected by factors such as solution composition, pH, and temperature Additionally, variations in the active levels of metal alkoxides complicate the control of composition and homogeneity The necessity for high-temperature treatment presents a significant challenge in the sol-gel process.
The hydrothermal route is a widely used method for synthesizing TiO2 nanoparticles, employing water as a catalyst or reaction component at temperatures exceeding 100°C and under high pressure This technique is favored for its ability to produce well-distributed products and uniform nanoparticle sizes Typically conducted in a Teflon-lined autoclave at around 200°C and pressures below 100 bar, the process is influenced by factors such as temperature, time, pH, and precursor materials One of the significant advantages of the hydrothermal method is its capacity for tightly controlled diffusion within a closed system.
The hydrothermal route can be enhanced by combining it with other methods to improve reaction kinetics and develop new materials For example, Zhang et al [111] demonstrated that the microwave-assisted hydrothermal method could produce N-doped TiO2 in just 5 minutes, resulting in a large surface area and superior photocatalytic performance compared to traditional processes Overall, this technique is frequently employed to synthesize M-doped TiO2 with small particle sizes and high surface areas [88, 95, 98].
The solvothermal route is a synthesis method for chemical compounds that resembles the hydrothermal process, differing mainly in the use of organic solvents like methanol, ethanol, and 1,4-butanol under supercritical conditions By manipulating factors such as temperature, reaction time, pressure, solvent type, and starting precursors, the morphology and structure of TiO2 materials can be effectively controlled.
This method enables the production of homogeneous particles and metastable materials at low temperatures For example, Yin et al successfully synthesized N-doped TiO2 (TiO2-xNy) using a solvothermal approach, which allows for precise control over the structure and morphology of the N-doped TiO2 materials, resulting in the formation of anatase, rutile, and brookite structures.
Liu et al [116] developed N-doped TiO2 with various structures, including anatase, rutile, and mixed phases, using a solvothermal method with TiCl3-hexamethylene tetramine as precursors and alcohol as a solvent Their findings revealed that the anatase structure transformed into rutile when different solvents, such as methanol, ethanol, 1-propanol, and 1-butanol, were used Single-phase anatase was formed at pH values 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 use of organic solvents with low dielectric constants, 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 involves producing oxides through the formation of a homogeneous liquid phase, which is influenced by temperature, pH, and surfactant concentration in either acidic or alkaline conditions This process typically includes two key stages: nucleation, where stable and small particles are generated, and the subsequent development or agglomeration of these particles By effectively managing the reaction kinetics, well-distributed particles can be achieved.
The electrochemical method offers a versatile approach for preparing thin film and porous materials at low temperatures By manipulating factors such as potential, current density, temperature, and pH, it is possible to produce tailored materials effectively.
Methods for preparing Pt-based catalyst
The polyol route has been widely utilized to design metal nanocatalysts such as Pt,
The method utilizes multifunctional alcohols with high boiling temperatures, such as ethylene glycol or glycerol, to reduce metal ions like Au, Ag, and Pd to their metallic forms This approach enables 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 the preferred choice due to its strong reducing capability, with a high reduction potential of 1.24 V in alkaline conditions, making it more effective than weaker agents like hydrazine This method enables the synthesis of platinum nanoparticles with diameters of approximately 2-3 nm, in contrast to the larger 40 nm particles produced by hydrazine Its simplicity and flexibility contribute to the widespread use of this technique in metal production.
Objectives of thesis research
Graphitic carbon, the standard catalyst support material, is electrically conductive but degrades over time in fuel cells, leading to catalyst agglomeration and reduced electrochemical surface area, which diminishes cell performance and lifespan Recent research has focused on developing corrosion-resistant materials, including nanofibers, carbon nanotubes, and graphene, which exhibit lower corrosion rates but still fail to prevent carbon oxidation effectively Consequently, non-carbon support materials such as nitrides, carbides, mesoporous silicas, electronically conducting polymers, Magnéli-Phase oxides, titanium oxides, cerium oxide, niobium oxide, tungsten oxides, and metal-doped TiO2 are gaining attention for their ability to prevent carbon corrosion These materials show promise due to their superior corrosion resistance and strong interactions with Pt nanocatalysts, enhancing stability However, challenges such as complex synthesis processes and the need for heat treatment and stabilizers have led to agglomeration, resulting in low surface area and electrical conductivity, hindering their application in fuel cell technology.
This study aims to address the significant challenges in the commercialization of Proton Exchange Membrane Fuel Cells (PEMFC) and Direct Methanol Fuel Cells (DMFC), particularly focusing on catalyst issues Our objective is to enhance the efficiency and durability of low-temperature fuel cells by identifying more active and stable electrocatalytic materials for both cathode and anode electrodes We propose a novel approach utilizing a Ti 0.7 M 0.3 O 2 nanostructure 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 discover and enhance new nanostructured Ti0.7M0.3O2 (where M = W, Ir) supports to serve as high-performance catalysts for Proton Exchange Membrane Fuel Cells (PEMFC) and Direct Methanol Fuel Cells (DMFC) This dissertation emphasizes several key aspects of this development.
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 for its non-toxic nature, affordability, chemical and redox stability, and exceptional corrosion resistance These properties make it a versatile material for numerous applications, including photocatalysts, solar cells, and photocatalytic water splitting.
The low electrical conductivity of TiO2 significantly limits its application in fuel cell technology To address this issue, doping TiO2 with transition metals has emerged as an effective strategy to enhance its electronic conductivity, as well as to improve 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 exhibit co-catalyst activity, further contributing to the performance of fuel cells.
Selecting dopant metals for TiO2 typically hinges on their ion radii similarity to Ti4+ (r ion (Ti4+) = 0.605 Å, r ion (W6+) = 0.60 Å) to minimize lattice distortion Tungsten doping in TiO2 lattices can yield up to two electrons per dopant atom, thereby enhancing the electrical conductivity of TiO2 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 by hindering the distribution of Pt nanoparticles on its surface, significantly reducing the electrochemical surface area (ECSA) Furthermore, the complex synthesis process of Ti0.7W0.3O2 is regarded as a 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, creating a uniform solution Following this, 0.155 mL of TiCl4 was added and stirred for 15 minutes, resulting in a precursor solution with concentrations of 12 mM WCl6 and 28 mM TiCl4, maintaining a W:Ti molar ratio of 3:7 This solution was then placed in a Teflon-lined autoclave to investigate the effects of varying reaction temperatures and times on the synthesis process The resulting suspension underwent multiple washes with water and was collected via centrifugation 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 good distribution on supports for improved electrocatalytic activity remains a significant challenge in fuel cell applications The development of a fast and efficient synthesis technique is crucial Over the past two decades, microwave chemistry has evolved from a laboratory novelty to a widely adopted synthesis method in academic and industrial settings This technology facilitates new synthesis pathways, utilizes environmentally friendly solvents, and produces cleaner products that require less purification Microwave heating offers several advantages for greener syntheses, including reduced reaction times, lower energy consumption, and higher product yields Combining microwave-assisted technology with other green chemistry strategies enhances its appeal for sustainable synthesis Previous studies indicate that using microwave-assisted polyol with ethylene glycol effectively deposits Pt nanoparticles on supports, significantly 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 research, Platinum nanoparticles (NPs) were anchored on Ti 0.7 W 0.3 O 2 support through a microwave-assisted polyol synthesis method Initially, 110 mg of Ti 0.7 W 0.3 O 2 was mixed with 25 mL of ethylene glycol and magnetically stirred until fully dispersed The suspension underwent ultrasonication for 30 minutes and was then cooled to 5 °C Subsequently, 2.818 mL of 0.05 M H2PtCl 6 was added to the suspension, which was stirred for 20 minutes before adjusting the pH to 11 using NaOH The mixture was then placed in a microwave oven, where the reduction reaction occurred at 160 °C for 2 minutes with a power setting of 240 W Finally, the product was thoroughly rinsed with acetone and distilled water, followed by drying at 80 °C overnight for further analysis.
2.2.3 Synthesis of Ir-doped TiO 2
Research on synthesizing iridium-doped titanium dioxide (Ir-doped TiO2) is limited, with most studies 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 This study presents the synthesis of Ir-doped TiO2 nanostructures through a one-step hydrothermal method using TiCl4 and IrCl3 precursors This energy-efficient process avoids surfactants and stabilizers, resulting in nanomaterials with a large surface area, aligning with green fabrication techniques.
To synthesize the desired compound, 0.2117 g of IrCl3·3H2O was dissolved in 50 mL of purified water, and the pH was adjusted using 37% hydrochloric acid Subsequently, 0.155 mL of TiCl4 was added to achieve a Ti:Ir molar ratio of 7:3, followed by stirring for 5 minutes The resulting solution was transferred to a Teflon-lined autoclave with a stainless steel shell, where the reaction was conducted under varying times, temperatures, and pH levels to determine optimal synthesis conditions After the reaction, the suspension was allowed to cool to room temperature and was thoroughly rinsed with purified water The precipitates were then dried overnight for further 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 increases 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
The use of NaBH4 presents challenges in controlling particle size and dispersion In contrast, employing ethylene glycol (EG) as a reduction agent produces platinum (Pt) catalysts with smaller particle sizes and enhanced dispersion, as EG effectively stabilizes nanoparticles However, its limited 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 study, we utilized a modified chemical reduction method employing sodium borohydride (NaBH4) as a reducing agent to synthesize Pt/Ti0.7Ir0.3O2 catalysts Previous research has demonstrated 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 optimal size and distribution on a support by dissolving 3.0 mL of a 0.05 M aqueous H2PtCl6 solution in a mixture of 25 mL purified water and 0.5 mL ethylene glycol, with the pH adjusted 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 Afterward, 3 mL of 0.05 mM aqueous NaBH4 was added to the suspension, which was stirred for 2 hours at room temperature The resulting mixture was then centrifuged and washed multiple times with purified water, and the 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 escape the surface and produce characteristic peaks in the electron intensity spectrum Inelastic scattering leads to energy loss for some electrons, contributing to the low kinetic energy background Standard binding energy values for elements allow for the identification of unknown surface elements and their oxidation states An increase in binding energy, resulting from the loss of valence charge density or oxidation, provides insights into the chemical environment and valence state of atoms For quantitative analysis, a curve-fitting process is typically employed to ascertain atomic percentages.
2.3.3 Scanning electron microscopy with energy dispersive X-ray spectroscopy
The average elemental composition of catalyst support and the loading of platinum nanoparticles (Pt NPs) on the support were analyzed using Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX) on a JEOL EDX-JSM 6500 F system at an accelerating voltage of 15 KV Elemental mapping obtained from EDX provided insights into the dispersion 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 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 ultrasonic suspension 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 This analysis was 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 It expands upon the Langmuir adsorption theory, addressing multilayer adsorption Adsorption involves gas molecules adhering to a solid's surface, with varying concentrations of the adsorbate on the adsorbent under different conditions, which helps determine the solid's surface area For analyzing the surface area and pore size of catalyst supports, nitrogen adsorption/desorption isotherms are conducted at 77K using a NOVA 1000e Before BET measurements, samples are degassed and dried at 250°C for three hours to remove water molecules from the meso/micropores of the catalyst support.
The electrical conductivity of catalyst support materials was evaluated using a standard four-point probe technique with a Jandel MWP-6 instrument Prior to testing, Ti 0.7 M 0.3 O 2 (M=W, Ir) powder was compressed into pellets measuring approximately 10 mm in diameter and 1 mm in thickness, utilizing steel dies in a hydraulic press at a pressure of around 300 MPa To ensure accurate electrical conductivity measurements, the four-point probe system was meticulously positioned on the pellet at three distinct locations for each sample to obtain average data.
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 reported on 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 consistent electrode value of 0.22 mg-Pt/cm², which allowed us to perform a backward calculation to determine the weight of samples combined 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 )
To ensure optimal contact between the rotating disk electrode (RDE) surface and the acid solution, a cylinder glass equipped with a Styrofoam sparger was utilized, effectively preventing bubble formation on the RDE surface during the bubbling gas process.
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 utilized a scan rate of 50 mV/s within a potential window of 0 to 1.10 V to assess the electrochemical surface area (ECSA) of catalysts in a nitrogen-purged 0.5 M H2SO4 electrolyte solution Additionally, methanol electro-oxidation experiments were conducted using a nitrogen-purged solution of 10 v/v % CH3OH/0.5 M H2SO4 at the same scan rate To evaluate the durability of the 20 wt % Pt/Ti0.7W0.3O2 catalyst, chronoamperometry measurements were performed at a potential of 0.7 V under nitrogen purging conditions.
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 specified points while recording the current in the cycling region The resulting current reflects the extent of cathodic or anodic reactions occurring Initially, the voltage is increased from low to high and then returned to the starting point, with the transition point where the voltage ceases to rise and begins to fall referred to as the "switching" potential.
The CV procedures are highly dependent on the specific application, as illustrated by a typical cyclic voltammogram (CV) for platinum supported on carbon black in an 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 platinum is oxidized to PtOH and subsequently to PtOx, and (v) the reduction of PtO back to platinum.
In sulfuric acid solution, H + is reversibly adsorbed to form a monolayer at potentials between 0.05 and 0.4 V via the reaction: