Keywords: High energy ball milling; Spark plasma sintering; Densification mechanisms; Transmission electron microscopy.
Fossil fuels are the non-renewable and conventional source of energy formed over the millions of years ago from the buried, fossilized remains of plants and animals. They have high carbon content. Coal, oil and natural gas are all considered as the fossil fuels. Today more than 70 percent of the world’s energy is produced by fossil fuels. The process of producing energy from the fossil fuels involves the combustion of fuel which generates the heat energy which boils the water and produces stream.
These stream from water increases in pressure and spins the turbine. The turbine rotates the magnet enclosed in high speed generator and then electricity is produced. This process basically involves conversion of mechanical energy to thermal energy and the finally to electrical energy.
Though it produces major amount of energy, it has some disadvantages and have huge impact on environment. The fossil fuel has adverse effect on the climate change.
Combustion is a reaction with oxygen. During combustion of fossil fuels, the hydrocarbon molecules are converted into carbon di oxide and water and produces huge amount of heat. The reaction is
CxHy +O2 === CO2 +H2O
These production of large amount of carbon di oxide as byproduct traps the heat in atmosphere and it leads to climate change causing global warming. It also pollutes land, water and air due to the harmful pollutants emission from fossil fuel burning. These factors have led to alternative sources of energy which should possess higher efficiency than diesel or gas engines.
It should also eliminate pollution caused by fossil fuel burning and should be simple compared to conventional energy sources.
Fuel cells is one such device which possess major advantages compared to conventional energy conversion technologies such internal combustion engine or batteries. Compared to combustion engines, fuel cell efficiency is higher and can be used for decentralized power generation. It also promises low emission as fuel cell works mostly on hydrogen and oxygen. So, it also eliminates the Greenhouse gas effect. The fuel is extremely simple and operates without any moving parts which tend to exhibit longer life. Operating times are much longer than batteries which can be increased by increasing the amount of fuel. Various size and weights of fuel cell helps them in variety of applications compared to conventional energy conversion technologies.
A fuel cell is an electrochemical device which converts the chemical potential energy which is stored in molecular bonds of fuel into direct electrical energy. The main part of fuel cell is the proton conductive membrane. In Membrane electrode assembly, the membranes are placed between the electrodes. The electrodes must be porous because the gases from cathode and anode must reach the interface between electrode and membrane where the electrochemical reaction takes places at the catalyst surface. The membrane electrode assembly is placed in between the collector plate where they collect electrons and conduct electric current and separator plate where they separate gases from the adjacent cells. The bipolar plates are the plates which is used in multi cell configuration provides the pathway for gas flow where cathode of one cell is connected to anode of another cell.
The electrolyte membrane is the main component of fuel cell, where other components are designed according to the fuel cell. The stability, lifetime and efficiency of fuel cell depends on the type of electrolyte membrane. The main function of membrane is to act as electrolyte between the anode and cathode for conducting ions and it also separates reactant gases of fuel cell. The proton conducting membranes must ensure the properties like high conductivity, high water uptake capacity, low fuel crossover, good mechanical strength, chemical and thermal stability. The membrane should be cheaper and suitable for all fuels.
There are various types of fuel cell based on electrolyte and operating temperature. Table 1 explains the general classification of fuel cells. \nPolymer electrolyte membrane fuel cells (PEMFC) is a type of fuel cell in which protonic conductive polymer-based membrane is used as an electrolyte. It is known for its simplicity, fast start ups and operation at low temperature. During the operation of PEMFC, the hydrogen molecules which passed in anode region splits into hydrogen ions and electrons in the presence of catalyst (Pt). The hydrogen ions pass through the membrane and electrons moves through the external circuits and reaches the cathode. In cathode region, oxygen is passed and it reacts with hydrogen ions and electrons to form water.
Anode: H2 2H+ + 2e-
Cathode: ½ O2 + e- +2H+ H2O + Heat
Per fluorinated ionomers are the mostly used materials used for proton exchange membrane fuel cell. It possesses good proton conductivity, good mechanical properties and thermal stability. Nafion which is a perfluoro sulfonic acid (PFSA) membrane is one of the successful electrolytes used due to its high conductive to cations and chemical stability.
Nafion is a copolymer which consist of three regions. One of the regions is polytetrafluoroethylene (PTFE) which is backbone for nafion, second region is the side chains of -O-CF2-CF-O-CF2- which connects the first and third region, sulfonic acid ion clusters is the third region. Nafion shows phase separated structure consist of hydrophilic domains which is sulfonic group and hydrophobic domains which is poly tetrafluoroethylene. CITATION 1 \l 16393 (Mohammad Bagher Karimi, 2019)There are two main advantages for using nafion membrane in fuel cell. First nafion membrane is stable and possess good mechanical strength. Second due to the presence of high electronegativity atom fluorine which is attached to the same carbon where -SO3H group is attached, make the sulfonic acid group super acid. This acidic region enhances proton conductivity. Poly tetrafluoroethylene is chemically inert, stable, hydrophobic in nature and not electrically conductive.
The hydrophobic part of nafion is responsible for mechanical strength and hydrophilic part is responsible for proton conductivity. These hydrophilic sulfonic groups of nafion aggregated to form ionic clusters through which water and protons get transported. This region is responsible for ionic conductivity of nafion. In the presence of water, both hydrophobic and hydrophilic phases separate each other and hydrophilic phase forms amorphous shell around water clusters. This cluster formation depends on temperature and level of hydration and many other factors like size, shape of clusters and solvents. Generally, nafion is good proton conductivity at low temperature. At high temperature due to the water evaporation and restricted proton flow nafion shows poor conductivity. The nafion also exhibits good mechanical properties and thermal stability especially in high humid condition.
Though nafion is widely used membrane for fuel cell but it has some disadvantages like high cost, low conductivity at high temperature and low humid condition, high fuel crossover, water management operation at high temperature and environmental issues.
To overcome the disadvantages of nafion, several modification and strategy have been made by researchers. One of the effective ways is use of metal oxide which is hygroscopic in nature like silica, tungsten trioxide, zirconium oxide. These oxides are incorporated with polymers like polyaniline, sulfonated poly (ether ether ketone) (SPEEK) ,etcOne such studies involves incorporation of SiO2/SPEEK nanofiber impregnated with nafion. No dissolution and good adhesion between nanofiber and nafion matrix were confirmed in the final membrane. The presence of SiO2 which is hygroscopic in nature attracts more water compared to unmodified nafion. The proton conductivity of the membrane was also far better than the unmodified nafion due to the better water retention capacity and resistance to swelling. Due to these properties, the cell performance with power density of 170 mW/cm2 is also enhanced significantly compared to unmodified nafion at low temperature CITATION CLe13 \l 16393 (C. Lee, 2013) .Still there are problems in the modified membrane like low conductivity at high temperature, poor mechanical strength at long term.
Another approach is use of acid-based membrane which showed better performance by blocking fuel crossover. Basic polymers such as poly (ethylene oxide) (PEO), Poly vinyl alcohol (PVA) blends with strong acids such as sulfuric or phosphoric acids.
PVA is widely studied alternate membrane in the fuel cell due to its hydrophilicity, film forming nature,etc. Due to high affinity of water and excellent mechanical properties, PVA is the most attractive membrane used for Polymer exchange membrane fuel cell. The PVA act as a better fuel crossover barrier than nafion due to the hydrogen bonding which causes dense molecular packing structure.
Relative selectivity of membrane is reciprocal to the product of membrane area resistance and fuel permeability current density. The studies reported the fabrication of sulfonated layer of 1:1 wt% PVA/ Nafion membrane in between thin nafion achieved better selectivity and cell performance. Due to its thinner membrane and effective fuel barriers, it possesses greater advantages compared to uncast nafion. It is also reported that conductivity of membrane found in experiment seems to be apparent conductivity of membrane which is conductivity due to skin layer effect and intrinsic conductivity of bulk membrane. Functionalized PVA/nafion nanofiber shows higher ultimate tensile strength, yield strength and young modulus but less strains compared to nafion. This ensured better mechanical strength by nafion/PVA nanofiber matrix. This is also ensured ultrathin membrane as low-cost ion exchange membrane for fuel cell. It is observed that the PVA/membrane is good conductive under low humidity but at high humid condition the nafion is better conductive (PAPER 1-5)
PVA shows poor proton conductivity compared to nafion due to negatively charged ions absence and it also limits its mechanical strength. The chemical cross linking of PVA solves the absence of negatively charged ions. The most used crosslinking agent used is the presence of carboxylic and sulfonic acid group like sulfosuccinic acid (SSA),poly (acrylic acid -co-maleic acid )(PAA-PMA),etc. The water retention property, fuel permeability and protonic conductivity are based on the degree of cross linking of SSA with the PVA membrane. CITATION Yun12 \l 16393 (Yun-Sheng Ye, 2012) The studies show that up to the 17 wt% concentration of SSA in PVA/SSA, both proton conductivity and fuel permeability decreases but above it due to the high sulfonic acid group concentration (–SO3−H+). proton conductivity enhances and it also act as a fuel barrier. CITATION Dae05 \l 16393 (Dae Sik Kim, 14 January 2005)Various fillers like CNT, Graphene oxide have been used in ionic membranes to improve several properties like ionic conductivity, permeability, mechanical strength, etc. Graphene reinforced membrane has gained attention in various fuel cell applications due to its excellent mechanical, thermal properties, and physical properties. Graphene and CNT posseses similar conductivity but graphene has high surface area than CNT. This makes graphene a cheaper material for fuel cell. Graphene sheet are functionalized in which hydroxyl and epoxy group is attached to its basal plane and hydroxy and carbonyl group is attached to its edges. The oxygen attachment at the surface and edges makes graphene oxide a better dispersion in solvents and polymer which leads to long term stability. Graphene oxide is easy to hydrate due to the presence of functional groups like epoxy1,carboxyl and hydroxyl. CITATION TBa14 \l 16393 (T. Bayer, 2014) Another study reported that the conductivity of Graphene is increasing with humidity under all temperature condition. The conductivity increases upto 70C but above that temperature the conductivity reduces due to the partial reduction of proton conducting oxygen groups. It also reported that maximum power density of graphene oxide is 33.8 mW/cm2 at 30 °C is more than half the power density of nafion at the same condition. CITATION URF18 \l 16393 (U.R. Farooqui, 2018)The studies CITATION CWL13 \l 16393 (C.W. Lin, 2013)show that fuel crossover of Graphene oxide laminated nafion membrane is far less than the nafion. The minimum fuel crossover is due to the parallel orientation of graphene oxide. The graphene oxide on nafion surface with good adhesion is fabricated easily due to its highly ordered graphene oxide. The results shows GO laminated nafion has IEC value 0.99 meq/g , proton conductivity 2.35*10-2 Scm-1 which shows better fuel cell performance than nafion with higher power density. It possesses high protonic conductivity compared to nafion and its high conductive nature is attributed to the hydrogen bonds in Graphene Oxide. All these unique properties of graphene make it the most viable materials in energy applications.