Integrated platinum carbon nanotube based electrocatalyst for high efficiency proton exchange membrane fuel cells 1

1.3 Proton Exchange Membrane Fuel Cells PEMFCs Proton Exchange Membrane Fuel Cells are also known as polymer electrolyte membrane fuel cells or polymer electrolyte fuel cells PEFCs.. Un

Chapter 1 Introduction

1.1 Background

Fuel cells are deemed as one of the most important alternative power sources utilizing sustainable energy to replace conventional combustion generators A fuel cell is an energy conversion device that generates electricity directly from an electrochemical reaction, in which oxygen (or air) and fuels (e.g hydrogen) combine

to form water [1, 2] Fuel cells differ from ordinary batteries in that they are able to continuously produce electricity as long as fuels are supplied Moreover, fuel cells convert fuels directly into electricity via an electrochemical process that does not require fuel combustion Therefore fuel cells are intrinsically more efficient and environment friendly than combustion engines [1, 2]

1.2 Main Types of Fuel Cells

In the last two decades of the 20th Century, fuel cell technologies have achieved significant breakthrough in their applications when the world is facing the shortage of fossil fuels, coal and oil [2] Owing to the tremendous research effort on fuel cell technologies, several types of fuel cells have been developed for a variety of applications to meet future energy demand, including transportation, stationary, and portable electronic devices

In general, fuel cells can be categorized into five main types according to the specific electrolyte used The five main types of fuel cells are: proton exchange membrane fuel cells (PEMFC), alkaline fuel cells (AFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC) [1, 2]

The different characteristics of their respective electrolytes used in the cell configuration determine the particular materials and fuels required for them, as well

as their unique properties and applications Table 1.1 demonstrates the main types of fuel cells and their characteristics and applications [1] Among the five types of fuel cells, PEMFCs have the most promising potential for domestic uses and other applications in small-scale power generations due to their relatively low operating temperature and compact structure [1, 2]

Fuel Cell

Operating Temperatu

Vehicles and portable applications, lower power station

Alkaline

Used in military & space vehicles

~ 650 °C

H2, CO, CH4, other hydrocarb

on

>50%

Suitable for medium to large-scale power stations, up to

on

>50%

Suitable for all sizes of power stations, 2 kW

to multi-MW Table 1.1 Main types of fuel cells and their characteristics and applications

1.3 Proton Exchange Membrane Fuel Cells (PEMFCs)

Proton Exchange Membrane Fuel Cells are also known as polymer electrolyte membrane fuel cells or polymer electrolyte fuel cells (PEFCs) Unlike other types of fuel cells, a typical PEMFC uses a thin solid polymer membrane as its electrolyte This membrane is not only an electronic insulator but also an excellent conductor for hydrogen ions, i.e protons Owing to its thin and solid properties, the polymer membrane is able to greatly diminish the electrochemical corrosion of electrodes and improve the power densities compared to liquid electrolytes Furthermore, as it needs water for proton conduction, a PEMFC system usually operates at a relatively low temperature of about 80oC, which allows rapid start-ups of the system from ambient temperature These distinct advantages of PEMFCs make them particularly suitable for automotive and portable applications

the electrodes [1] Historically, the first major application of PEMFCs was to serve as

an auxiliary power source in NASA’s Gemini space flights in the 1960s [1] However, the early version of PEMFCs with a polystyrene sulfonate ion-exchange membrane as electrolyte was unable to give adequate power densities and required operational lifetime Thereafter the development of this technology remain stagnant for more than ten years until the polystyrene sulfonic acid membrane was replaced by Du Pont’s perflourosulfonic acid membrane (Nafion®) in the 1970s [1] The utilization of Nafion membrane in PEMFCs boosted the power densities by ten times and the lifetime from two thousand hours to one hundred thousand hours In the late 1980s and early 1990s, another technical breakthrough was achieved via the 10-fold reduction of platinum loading in PEMFC electrodes This achievement was first realized by using high surface area carbon particles as platinum support instead of using pure Pt black as electrocatalyst, and also by impregnating a small amount of Nafion ionomers into the catalyst layer of the porous gas diffusion electrode in the Gemini fuel cells [1]

1.3.2 Structure of PEMFCs

As shown in Fig 1.1 [3], a typical single PEMFC usually consists of four main components: tow current collectors, two gas diffusion layers, two catalyst layers and a solid polymer electrolyte membrane

Fig 1.1 Schematic diagrams of (a) a single PEMFC and (b) a 3-cell PEMFC stack [3]

Current Collectors

Current collectors, which are placed at two cell ends to collect current, are used to separate reactant gases, and provide mechanical support and gas flow channels (see Fig 1.1) [4] They are also called bipolar plates in multi-cell stacks when each plate is electrically connecting the anode of one cell to the cathode of the adjacent cell A desirable material for current collector must be electrically and thermally conductive,

as well as impermeable to gases It should also have high resistance to the reactant

  (a)

(b)

gases to avoid corrosion Presently graphite and stainless steel are the most commonly used materials for current collectors, which provide both high electronic conductivity and corrosion resistance Current research on current collectors has been focusing on developing novel materials with superior corrosion resistance and lower cost. 

Gas Diffusion Layers (GDLs)

Gas diffusion layers are a part of a PEMFC electrode and they are in contact with

current collectors They usually consist of a macroporous backing layer and a microporous gas diffusion layer [4] The macroporous backing layer is either carbon cloth or carbon paper, with thickness ranging from 100 to 300 μm And the microporous gas diffusion layer is usually composed of nanosized carbon blacks spread on the backing layer These high porosity media can provide mechanical support as well as pathways for electrons, gas and water in PEMFC electrodes When reactant gases flow out from the channels in current collectors and reach the gas diffusion layer, they are evenly distributed by the porous structure before reacting in the catalyst layer In addition, the carbon backing layer is the electronic connection between current collector and electrode Furthermore, GDLs also provide hydrophobicity for electrodes to facilitate liquid water removal by impregnating Polytetrafluoroethylene (PTFE) into them Hence the gas diffusion layer is a critical factor in electrode structure design to ensure efficient mass transport in PEMFC electrodes

Catalyst Layers (CLs)

Catalyst layers are attached to the microporous gas diffusion layers They are the most essential part of a PEMFC where the electrochemical reactions take place [4]

The electrochemical reactions in a PEMFC consist of two separate reactions: hydrogen oxidation reaction (HOR) occurring at anode and oxygen reduction reaction (ORR) at cathode At anode catalyst layer, gaseous hydrogen splits into two protons and the protons then pass through the electrolyte membrane to reach the cathode While at cathode catalyst layer, oxygen combines with these protons from anode and electrons from external circuit to form water and excess heat Typically, these two half-reactions would take place very slowly at PEMFC operating temperature Therefore electrocatalysts are necessary on both anode and cathode to increase the reaction rate of each half-reaction The best electrocatalyst for each reaction to date is noble metal platinum, a very expensive material In the 1970s and 1980s the catalyst layer of PEMFCs was made up of pure platinum black and PTFE suspension with a Pt loading up to 4 mg cm-2 [1] However, it was revealed in later research that it is the catalyst surface area that determines the reaction rate rather than catalyst weight Thereafter a significant Pt loading reduction to less than 0.4 mg cm-2 was achieved in the late 1990s by synthesizing carbon supported Pt catalyst for PEMFC applications [5] Porous carbon blacks are widely used as Pt support for their high surface ratio and excellent electrical conductivity In the most prevalent catalyst layer, composite catalysts consisting of Pt nanoparticles (4nm or smaller) supported on carbon black Vulcan XC72R (ca 40nm) are usually used with Nafion® ionomers impregnated The catalyst layers are usually very thin with a thickness of around 10−50 μm, containing electrochemically active regions where three phases − catalysts, ionomers, and reactant gases coexist In order to improve the utilization of Pt catalyst, optimum catalyst layer structure should be well-maintained to obtain maximum three-phase zones [4]

Polymer Electrolyte Membrane (PEM)

A polymer electrolyte membrane is a solid polymer film that separates the anode and cathode catalyst layers It is a pivotal component of a PEMFC as it not only permits protons to transport from anode to cathode but also insulates electrons to travel through that the free electrons can only reach cathode through external circuit, generating useful electricity It also separates fuel and oxidant gases from each other thus direct combustion of fuels can be avoided In addition, a good PEM material should remain chemical and mechanical stability in the hostile fuel cell environment

to ensure long-term operation durability The most commonly used PEM at present is Nafion® series invented by Dupont in 1960’s, due to its high proton conductivity and chemical inertness [2] The chemical structure of a typical Nafion PEM is composed

of a polytetrafluoroethylene (PTFE) chain and a side chain ending with sulphonic acid HSO3 A micro-view of the PEM structure is shown in Fig 1.2 The fluorocarbon chain usually has a repeating structural unit, i.e —[CF2–CF2]n—, where n is very large This chain can provide the PEM with good mechanical strength and chemical stability On the other hand, the sulphonic acid group HSO3 is highly hydrophilic and

is ionically bonded with a SO3- ion and a H+ ion This is why such structure is called ionomer When the PEM absorbs water and becomes hydrated, the ionically bonded

H+ ions are relatively weakly attracted to the SO3- group As a result, the H+ ions are able to move through concentration gradient within the well-hydrated regions of PEM, making this material a very good proton conductor The proton conductivity of this PEM is strongly correlated with its water content, thus making humidification of the fuel gases a requirement during cell operation Another requirement of using this PEM is that the operating temperature is limited to the boiling temperature of water to maintain its liquid water content However, this requirement can lead to a severe mass

transport limitation when excess water accumulates in electrode and blocks gas diffusion pores Therefore water management is a very important topic in current PEMFC research, especially in developing high-temperature PEM materials as well as optimizing electrode structure [2]

Fig 1.2 Chemical structure of Nafion membrane [2]

1.3.3 Basic Thermodynamics and Electrochemistry of PEMFCs

As shown previously, a typical PEMFC consists of three core parts: two electrodes (anode and cathode) sandwiched with a solid polymer electrolyte membrane between them The combination of anode, cathode and membrane corresponds to the heart of a PEMFC, known as membrane electrode assembly (MEA) In a working PEMFC, hydrogen fuel flows into the anode catalyst layer through gas diffusion layer, and it is then split by platinum catalysts into two electrons and two protons (hydrogen ions) The protons can pass through the polymer electrolyte membrane to cathode catalyst layer, whereas the electrons have to travel from the anode to cathode through an external circuit consuming the power generated

by the cell Again with the help of platinum catalysts, the protons and electrons combine with oxygen within the cathode catalyst layer, producing pure water The

overall electrochemical process of a PEMFC is called "reverse hydrolysis", corresponding to the opposite reaction of hydrolyzing water to form hydrogen and oxygen The illustration of this process is shown below in Fig 1.3

Fig 1.3 Illustration of electrochemical processes in PEMFCs [2]

As shown in Fig 1.3, the electrochemical reactions occurring at anode and

cathode in a PEMFC can be illustrated as Eq 1-1 to 1-3 [2]:

Anode: H2 2H+ + 2e- (1-1) Cathode: 2H+ + 2e- + 1/2O2 H2O (1-2) Overall: H2 + 1/2O2 H2O + electricity + heat (1-3) According to Eq 1-3, the electrochemical energy in hydrogen fuel is directly converted into electricity through the PEMFC reaction, producing pure liquid water and heat as the only by-products As an electrochemical energy converter, a PEMFC must obey the laws and principles of thermodynamics

Open Circuit Voltage

In the H2-O2 reaction of a PEMFC (Eq 1-3), only the free energy part (△G) of

the total enthalpy (△H) can be converted into electricity due to irreversible entropy

E 0 = E θ – RT In[p H (p O ) 1/2 ]/zF (1-8)

In practice, the open circuit potential is usually about 0.2 V lower than the

theoretical value One main reason causing this deviation is the formation of hydrogen peroxide as an intermediate of the oxygen reduction reaction Another factor that attributes to it lies in the diffusion of hydrogen from anode to cathode through polymer electrolyte membrane As a result, the open circuit potential is difficult to determine purely by estimation from the theoretical equations shown above [4]

Theoretical Efficiency

The efficiency of any energy conversion device is defined as the ratio between useful energy output and energy input [6] A simplified way to compare the efficiencies of energy conversion devices is to examine their maximum theoretical efficiencies For the H2-O2 reaction in PEMFCs, it is the changes in Gibbs free energy

of water formation that are converted into electrical energy [4] The maximum efficiency for a PEMFC can be calculated based on the changes in Gibbs free energy

△G and the changes in enthalpy △H of the reaction:

η max = △G/ △H (1-9)

The total electrochemical energy can be obtained from any electrochemical reaction

is determined by the total Gibbs free energy change △G, which is approximately -237

kJ mol-1 for the H2-O2 reaction at the standard-state condition If the process is reversible, all the Gibbs free energy will be converted into electrical energy In practice, however, the process is usually not reversible and some Gibbs free energy will be released as heat The enthalpy change △H represents the total thermal energy

available from the reaction, and its value is varied depending on whether the product water is in vapor or in liquid phase If the produced water is in liquid phase, then △H

is higher due to the release of heat from water condensation The higher △H value is

called higher heating value (HHV) with the amount of -286 kJ mol-1, and the lower

△H value is about -241 kJ mol-1, denoted as lower heating value (LHV) Assuming that all of the Gibbs free energy can be converted into electrical energy, the maximum theoretical efficiency of a PEMFC is given by:

η max = △G/ △H = -237 / -286 = 83% (1-10)

However, the actual efficiency of a real fuel cell is lower than this theoretical value shown above due to various irreversible losses under certain operating conditions It is also proportional to the cell potential that corresponds to the cell output power [4]

Denote E HHV as the potential corresponding to hydrogen’s higher heating value, or the thermoneutral potential:

E HHV = -△H / zF =1.48 V (1-11)

The actual efficiency of a working PEMFC can be estimated as the ratio of the cell

potential E and the potential corresponding to hydrogen’s higher heating value E HHV:

by the cell [6] The deviation from the equilibrium value is called overpotential or

polarization, denoted as V There are three major overpotentials causing different

irreversible voltage losses during cell operation: activation overpotential, ohmic overpotential and mass transport overpotential Therefore, the output voltage of a single fuel cell can be determined by the sum of the open circuit potential and the three voltage losses:

E = E 0 + V act + V ohm + V mt (1-13)

Where V act is the activation polarization, V ohm is the ohmic polarization and V mt is the mass transport polarization

Activation overpotential is caused by the slow charge transfer rate at the interface between electrocatalyst and electrolyte This voltage loss is utilized to activate the electron transfer in the reaction Activation overpotential dominates particularly at low current density region where the ohmic and mass transport losses are negligibly small It can be mostly attributed to the sluggish cathode reaction whereas the contribution of anode activation polarization is negligible The main factors that affect activation overpotential are the electrochemical kinetics of cathode reaction, including reaction temperature, catalyst surface area, reaction Tafel slope, exchange current density and so forth [4]

As current density increases, ohmic overpotential becomes predominant for the voltage losses occurring in the PEMFC It can be ascribed to the overall ohmic resistance of the PEMFC, including the electrical resistance of electrodes and other cell components as well as the ionic resistance of PEM Usually the PEMFC components are made up of materials with very high electrical conductivity so that the ohmic resistance of a PEMFC is mainly derived from the relatively much slower ionic conductivity of the electrolyte Ohmic resistance can be reduced by decreasing membrane thickness or increasing ionic conductivity of electrolyte However, thinning membrane thickness is limited by losing the mechanical strength required for the PEM To alleviate the ohmic loss from ionic resistance of PEM, fuel gases are usually humidified to sustain high water content in the electrolyte [4]

Mass transport overpotential arises from mass transport limitations of reactant gases in PEMFC electrodes, mainly occurring at large current densities When the supply of reactant gases is not rapid enough to meet the fast reaction rate at large current densities, a part of the generated energy is lost to drive mass transport process, yielding a corresponding loss in output voltage This loss is mostly determined by the structure characteristics of GDLs and CLs, as well as the concentration and pressure

of reactant gases In addition, mass transport overpotential can also occur when water accumulates in the electrode pores and obstructs the diffusion paths or dilutes the reactants, thus exacerbating the voltage loss [4]

Polarization curve is a characteristic diagnostic technique to evaluate fuel cell performance, illustrating a plot of cell potential vs current density Polarization curve

is usually obtained by increasing cell current starting from open circuit potential and recording I-V measurements at prescribed potential or current intervals A typical polarization curve of a PEMFC is shown in Fig 1.4 As can be seen in Fig 1.4, the cell potential decreases with increasing current densities and three distinct regions of a PEMFC polarization curve are noticeable according to the characteristics of voltage losses At low current densities, a dramatic voltage drop is drawn from open circuit potential due to the activation overpotential As current density increases, the polarization curve shows a nearly linear region at intermediate current densities, where voltage loss is dominated by ohmic overpotential At large current densities, the decrease of cell potential deviates from linear relationship with current and aggravates dramatically till the maximum current limit is reached This additional sharp voltage drop stems from the mass transport overpotential, as described in previous sections In general, polarization curve measurement is commonly

Fig 1.4 A typical polarization curve of PEM fuel cell

performed as the first characterization step to evaluate fuel cell performance for its convenience and ease of data acquisition and interpretation [1]

1.3.4 Applications of PEMFCs

PEMFCs are particularly attractive for applications ranging from low (less than 1 kW) to intermediate (up to 50 kW) power levels, due to their inherent advantages such as low emissions, high efficiency, low operation temperature and rapid start-up

as described in the above introduction The main driving force for the commercialization of PEMFCs is from the automotive industry Presently the world’s major automobile manufacturers, such as General Motors, DaimlerChrysler, Toyota Motor Corporation, Ford and etc., have been developing PEMFC powered vehicles for their commercial viability to replace combustion engines, since the Canadian fuel cell company Ballard invented the first automobile powered by a PEMFC in 1993

The latest PEMFC vehicle developed by DaimlerChrysler and Ballard has grown to the six generation It has the capability of traveling 150 km with one fill of fuel and its highest speed can reach up to 140 km/h [1] However, barriers to the commercialization of PEMFCs to date remain formidable due to their exiguous fuel sources and high economic costs One main problem lies in that expensive platinum must be used as catalyst for hydrogen oxidation reaction at low temperature Moreover, the platinum catalyst is extremely sensitive to CO contained in the hydrogen fuel Therefore intensive research on advanced PEMFC electrocatalysts with higher electrocatalytic performance and lower cost are pressingly needed to provide competitive price and reliable performance for PEMFCs [4]

1.4 Electrocatalytic Material Studies of PEMFCs

The electrochemical reactions that take place in PEMFCs are the hydrogen oxidation reaction (HOR) at anode and the oxygen reduction reaction (ORR) at cathode, respectively Although these reactions are thermodynamically favorable, they cannot occur spontaneously at normal conditions due to their sluggish kinetics, especially for the ORR A suitable electrocatalyst is thus necessary to speed up the reaction kinetics to meet the requirement for practical uses of PEMFCs In the state-of-the-art PEMFCs, platinum (Pt)-based catalysts are the most effective electrocatalysts for both HOR and ORR, owing to the high electrochemical activity and stability of platinum under PEMFC operating conditions However, these Pt-based catalysts consisting of noble metals are too expensive to make commercially viable PEMFCs; hence extensive research over the past several decades has been focusing on reducing Pt loading and improving Pt utilization in PEMFC electrodes

To achieve these aims, novel electrocatalytic materials are developed mainly through

two strategies: Pt-based alloy catalysts and Pt catalysts on carbon support In the following sections, an overview of supported Pt catalysts will be given including Pt catalysts supported on carbon black (CB) and carbon nanotube (CNT) as well as their diverse synthesis methods

1.4.1 Carbon Supported Pt Electrocatalysts for ORR

Platinum has been used as the electrocatalyst for PEMFC reactions since the invention of the first prototype of PEMFC It is believed that Pt has the best electrochemical activity on the oxygen reduction reaction (ORR) over any other pure metal In earlier PEMFC models, pure Pt black was used as the electrocatalyst in PEMFC electrodes with a high Pt loading of 4 mg cm-2 [1] It is only till late 1990’s that a significant Pt loading reduction to less than 0.4 mg cm-2 was achieved by synthesizing supported Pt catalysts as PEMFC electrocatalyst [5] Practical electrocatalysts for current PEMFCs are typically nanosized Pt particles dispersed on high-surface-area carbon supports The most commonly used support material is carbon black Vulcan XC-72R (VXC72R), which has an average particle size of 30

nm and a high surface area of 250 m2 g-1 [4] Despite of the significant Pt loading reduction by utilizing such composite catalysts, the costly amount of Pt catalyst in state-of-the-art PEMFCs remains one of the major barriers for PEMFC commercialization Thus further reducing Pt loading has been a main driving force for PEMFC research in recent years There are a variety of synthesis methods for Pt/C composite catalysts, including wet-chemical processes and physical vapor deposition (PVD) techniques Each method has its particular advantages over the others and also disadvantages that limit its ubiquitous application A general overview of the most prevailing synthesis methods for carbon supported Pt catalysts is presented below

Particular emphasis will be given to the sputter-deposition technique that was adopted

in this study

Chemical Precipitation

Chemical precipitation method for supported Pt catalyst synthesis has been used for several decades [7] The main advantage of this method is that it can prepare nanosized Pt catalysts at low temperatures In principle, the preparation process is basically reducing a Pt salt solution by adding a reducing agent into it Typically, the precursor Pt salt is firstly reduced to Pt metallic state The reduced Pt particles precipitate out of solution and deposit onto support particles when carbon supports are mixed in the solution Aqueous Pt salt solution such as H2PtCl6 is commonly used as precursor solution and sodium borohydride and hydrazine hydrate are popular reducing agents Continuous stirring must be conducted during Pt reduction to ensure sufficient mixing of the precursor solution and the reducing agent After reduction process, the supported Pt catalysts will undergo a series of filtration, washing and stoving processes In general, the average Pt particle sizes synthesized through chemical precipitation method are 2−5 nm [8, 9] The main disadvantages of this method are its time-consuming preparation process and incomplete Pt reduction that additional reduction process under H2 may be required for the as-synthesized Pt/C catalysts

Colloidal

The colloidal method is a common synthesis method for present Pt/C catalysts in PEMFCs [10] It is similar to the chemical precipitation method whereas it can provide size control of the catalyst particles by adding a capping agent to prevent their