Catalysts are materials that speed up chemical reactions and are widely used in the synthesis of chemicals. Noble metal catalysts are used for fuel cells as they are resistant to corrosion and oxidation in moist air. They also are employed in fuel cells to convert water into hydrogen Fuel cells, as devices for direct conversion of the chemical energy of a fuel into electricity by electrochemical reactions and are among the key enabling technologies for the transition to a hydrogen based economy. Of several different types of fuel cells under development today, polymer electrolyte fuel cells or Proton Exchange Membrane (PEFCs) have been recognized as a potential future power source for zero-emission vehicles. In recent decades, fuel cell technology has been undergoing revolutionary developments, with fundamental progress being the replacement of electrolyte solutions with polymer electrolytes, making the device more compact in size and higher in power density. Despite great success, fuel cells based on acidic polyelectrolyte still depend heavily on noble metal catalysts, predominantly platinum (Pt), thus increasing the cost and hampering the widespread application of fuel cells.
Metal noble catalyst
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Currently the demand for clean and sustainable energy sources have become a strong instigates force in continuing economic development. Proton exchange membrane (PEM) fuel cells which act as clean energy converting devices have drawn enough attention in recent years due to their high efficiency, energy density and low emissions. PEM fuel cells have several important application areas, including transportation, stationary power and micro-power. Fuel cells are nothing but an electrochemical device that converts the chemical energy of a reaction directly into electrical energy. Fuel cell catalysts, such as platinum (Pt) based catalysts and their associated catalyst are the most used catalysts in fuel cells. As these catalysts are costly so a great deal of effort has been put into the exploration of cost effective, active and stable fuel cell catalysts. In a PEM fuel cell, hydrogen oxidation reaction (HOR) takes place at the anode and the oxygen reduction reaction (ORR) take place at the cathode within the respective catalyst layers1. Thus the electrocatalysts and their corresponding catalyst layers play important roles in fuel cell performance. In current state of technology the most practical catalysts in PEM fuel cells are highly dispersed Pt based nanoparticles.
Noble metal catalystsÂ are resistant toÂ corrosion and oxidationÂ in moist air. Usually the noble metals are considered to be in order of increasing atomic number such as ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold. Nanomaterial based catalysts are commonly heterogeneous and can be broken up to increase catalytic process. Metal nanoparticles have very high surface area so there are always chances of increased catalytic reactions. Nobel metal nanomaterial's (NMNs) have been intensively pursued, not only for their fundamental scientiï¬c interest, but also for their many technological applications with size dependent chemical properties and optical as a type of trendy materials.2-13
The much larger surface-to-volume ratio of nanoparticles collate to their bulk counterparts has allure a great deal of attention for catalytic applications.14-17 This is because much smaller amount of materials can be used and higher catalytic activity can be achieved for catalysts of the same mass. Apart from the effects of size, the role of nanoparticle shape is also important in catalytic work which has been found to be highly dependent on the crystallographic planes that terminate the nanoparticle surface. 18-20
Both platinum and palladium are important catalysts for many industrial processes where they exhibit similar catalytic activities.21 Nanoparticles of platinum and palladium are heavily studied for various catalytic applications including hydrogenation reactions, carbon-carbon bond formation and oxidation and reduction reactions in fuel cells.22-25 As the demands for precious metals such as platinum and palladium are growing, there has been a major focus on the development of high performance catalysts. 26
Noble metal catalysts are well known for their high catalytic activities. Carbon supported platinum group metals have long been recognized as essential catalysts in organic synthesis and there is abundant literature on their properties in many reactions. Platinum (Pt) based electrocatalysts are usually employed in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). It is well known that the catalytic activity of the metal is mainly dependent on the particle size and shape distribution27 . Carbon supported platinum (Pt/C) is the best known electrocatalysts for both hydrogen oxidation and oxygen reduction in phosphoric acid fuel cells (PAFCs) and proton exchange membrane fuel cells (PEMFCs). The structure and proper dispersal of these platinum particles make low loading catalyst feasible for PAFCs and PEFCs operation.
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In view of the role played by carbon supported catalysts in the practice, fundamental studies on the factors inï¬‚uencing the state and properties of the active phase in these catalysts are still rather insufficient. The last decade showed an increasing interest in this subject. However, mainly the relationship was under study between the characteristics of the support (predominantly, the functional coverage of the surface) and the dispersion state of the resulting metal but not its catalytic properties. Besides, non-porous carbon blacks were mainly used in the scientiï¬c studies, whereas activated carbons are the typical supports in practice. Obviously, this may be explained by a too complex beginning of catalysts on the porous carbon supports. It is also true because of the scarcity of literature on this subject it makes traditional carbon supported catalysts a difï¬cult and unattractive topic for exploration.
So in order to reduce the widen gap between the existing and required knowledge on the preparation of carbon supported catalysts with platinum group metals, a collaborating study on Pt and Pd catalysts supported on different activated carbons is required. This was described by properties of catalysts on ï¬ve activated carbons specially selected to investigate a possible inï¬‚uence of the porous structure of the support28 . The catalyst preparation was restricted to use chlorides of platinum and palladium as the starting metal complexes, their deposition onto the supports by the adsorption method, and reduction in ï¬‚owing hydrogen. Carbon supported Pt and Pd catalysts have been synthesized and used in PEM hydrogen fuel cell anodes29 . Electrocatalysts based on Pt and Pd deposited onto charcoal have been prepared and tested by30 on the anodic side of PEM fuel cells 11. The assessment of the electrochemical activity of Pd catalysts on carbon in alkaline solutions for the oxidation of hydrogen and methanol was reported by31 . Also the kinetics of hydrogen evolution reaction at palladium in alkaline solution has also been studied in a paper32 . Palladium catalyst has a genuine use in 'green' energy, as a catalyst in hydrogen fuel cells. It is one of a number of metals starting to be used in the fuel cells to power a host of things in vehicles such as cars and buses. When a molecule of hydrogen first comes into contact with palladium, they are adsorbed on the surface but then they diffuse throughout the metal.
The most vital application of platinum and palladium catalyst is in the pharmaceutical industry as these two noble metals are commonly the prime catalysts of choice. Their demand is largely defined by the unique nature of chemical processing operations. The production of a product in the pharmaceutical industry is usually expressed in pounds per year and in most other sectors of the chemical industry, production in terms of tons per year is the rule. Precious metals are used extensively as catalysts in a wide range of industrial chemical processes. They can be used in a homogeneous form, but more commonly they are heterogeneous. In many operations only a precious metal catalyst can provide the necessary speed or selectivity to the reaction, while in others these characteristics, together with a long catalyst life make the overall system the most cost effective choice.
In a PEM fuel cell, Hydrogen oxidation reaction (HOR) takes place at the anode and the Oxygen reduction reaction (ORR) take place at the cathode within the respective catalyst layers. But we will focus mainly on Hydrogen oxidation reactions. Thus electrocatalysts and their corresponding catalyst layers play critical roles in fuel cell performance. In our present state of technology, the most practical catalysts in PEM fuel cells are highly dispersed Pt based nanoparticles. However, Pt based catalysts have several drawbacks, such as high cost, sensitivity to contaminants and no tolerance for methanol oxidation (in a direct methanol fuel cell (DMFC) application), fewer completed four electron reduction reactions and Pt dissolution. In the search for alternative low cost non Pt catalysts, researchers have looked at several others, including supported platinum group metal (PGM) types such as Pd based catalysts. However, these approaches are as yet in the research stage, as the catalyst activities and stabilities are still low to be practical. Another approach is to reduce Pt loading in a catalyst or catalyst layer using alloying and carbon supports.
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Another significant challenge is gaining a fundamental understanding of fuel cell catalyst structures and their corresponding catalytic reaction mechanisms. Current approaches rely largely on trial and error. To design new, breakthrough catalysts, we need a well-defined theoretical approach. Theoretical studies will provide a platform for understanding catalyst performance and also exploring the structure activity relationship.
Usually any hydrogen based energy conversion plot depend on effective catalysts for oxidation and reduction of hydrogen which is nothing but Hydrogen oxidation reaction (HOR) and Hydrogen reduction reaction (HER)33 . Platinum based catalysts are stable and effective for both hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) under acidic conditions as it is found in a polymer electrolyte fuel cell. But as we know since Pt is rare and expensive there is a need for the development of electrodes made of cheaper materials to reduce the overall cost. To be able to design new electrodes for the hydrogen evolution or oxidation reactions, it may well prove essential to acquire insight into their mechanism at the atomic level 34-39 . Overall HOR/HER reaction H2 â†” 2(H+ + e-) taking place at an electrode with an electrolyte, involves three elementary reactions. In the ï¬rst step, H2 is dissociated and H adsorbed. This is achieved either by the Tafel reaction H2 â†’2H* (H* denotes hydrogen adsorbed on the surface) or by the Heyrovsky reaction H2 â†’H* + H+ + e- . The adsorbed H is then discharged, following the Volmer route H* â†’H+ + e- . Despite intensive research efforts it is still unclear which of the two pathways, Tafel-Volmer or Heyrovsky-Volmer dominates under different conditions even on the most studied electrode material, Pt.
Fuel cells offer efficient and virtually pollution free energy conversion and power generations. The reality that fossil fuels are finishing out and the certainty that pollution from using fossil fuels has become an issue of environmental concern to human health constitute two of the major driving forces for the increasing interest in the development of fuel cells 40 .
The auto industry is possibly the biggest market behind the massive investment in fuel cell development. This is clear as the price of oil is highly volatile and has been increasing in the past few years which are likely to continue. Additionally the harmful emissions of gases such as CO2, CO and other volatile organic compounds into the atmosphere cause serious environmental damage and produce 'greenhouse gases' that give rise to global warming. But in contrast fuel cell extract energy from fuel (40-70% efï¬ciency) more effectively than traditional internal combustion engines (âˆ¼30%efï¬ciency). Because of this condition along with the hydrogen's high efï¬ciency (from 40-70%) could eventually lead to the possibility of better utilization of both heat and electricity in fuel cells and thus make a signiï¬cant contribution to reducing atmospheric emissions.
Of all the different types of fuel cells available Proton exchange membrane fuels cells are being used most because of its advantages over conventional energy converting devices. A fuel cell is an electrochemical device that continuously and directly converts the chemical energy of externally supplied fuel and oxidant to electrical energy. Fuel cells are usually classified according to the type of electrolyte used. The five most common technologies are polymer electrolyte membrane fuel cells (PEM fuel cells or PEMFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). Unlike most other types of fuel cells, PEMFCs use a quasi-solid electrolyte, which is based on a polymer backbone with side chains possessing acid-based groups. The numerous advantages of this family of electrolytes make the PEM fuel cell particularly attractive for smaller-scale terrestrial applications such as transportation; home based distributed power, and portable power applications. The distinguishing features of PEMFCs include relatively low temperature (under 90Â°C) operation, high power density, a compact system, and ease in handling liquid fuel.
The nature of the electrolyte in any fuel cell type and the combine operating temperature are key features of consideration for effective catalysts. Also in addition to this the nature of the electrolyte also drives the distinctiveness of the dominant migrating ion, as illustrated in Table 1.
H2 + 2OH- â†’ 2H2O + 2e-
O2 + 2H2O + 4e- â†’ 4OH-
H2 â†’ 2H+ + 2e-
O2 + 4H+ + 4e- â†’ 2H2O
H2 â†’ 2H+ + 2e-
O2 + 4H+ + 4e- â†’ 2H2O
H2 + CO32- â†’ H2O + CO2 + 2e-
CO + CO32- â†’ 2CO2 + 2e-
O2 + 2CO2 + 4e- â†’ CO32-
Table 1. Fuel cell systems showing anodic and cathodic reactions and the dominant mode of ion transport in the electrolyte
Electrocatalysis is very important for the fuel cells. In order to generate hydrogen source before fuel cell is injected involves different catalysts. Carbon monoxide derive from reforming hydrocarbons acts as a poison for the anode electrocatalysts in the low temperature fuel cells and its removal from the fuel source is a demanding work for the fuel processing catalysts.Apart from illustrating electrocatalytic activity towards the electrode reactions (the fuel anode as well as the air or oxygen cathode), the electrocatalysts essentially be stable within the working cell. As far as for the alkaline fuel cell (AFC) this is comparably easy because many electrocatalytic materials are adequately stable in alkaline solutions. The certainty that the AFC is highly sensitive to the presence of CO2, either in the fuel stream or in the air stream, has reduced its application heavily to those situations where very pure oxygen and hydrogen can be supplied. For the fuel cells operating with acidic electrolytes, stability of the electrocatalysts is much more immense to realize. Many types of electrocatalysts have been considered over years for their various applications to fuel cells. The nature of appropriate electrocatalysts is precariously dependent on the nature of the fuel cell. The high temperature molten carbonate and solid oxide fuel cells (MCFC and SOFC) present difficulties of thermal stability as well as compatibility with the electrolyte. Currently preferred electrocatalysts for the various cells are listed in Table 2.
Pt/Au, Pt, Ag
Pt/Au, Pt, Ag
Table 2. Electrocatalysts for the various cells
Platinum and platinum alloys are the most efficient catalysts for speeding up chemical reactions in hydrogen fuel cells. Platinum is the only metal that can withstand the acidic conditions inside such a cell but it is expensive and this has limited the broad, large scale applications of fuel cells. Furthermore, about 90 percent of the world's platinum supply comes from two countries South Africa and Russia.
The majority of hydrogen fuel cells use catalysts which are made of a rare and expensive metal Platinum. Of course there are few alternatives because most elements cannot encounter the fuel cell's highly acidic solvents existing in the reaction which converts hydrogen's chemical energy into electrical power. There are only four elements which can resist the corrosive process. These elements are Platinum, palladium, gold and iridium.
Usually hydrogen fuel cells reckons on the catalysts made of platinum, which is very expensive and also rare. Unfortunately there aren't many other options because most of the other elements cannot resist the corrosive process that converts hydrogen's chemical energy into electrical power. Platinum and iridium are up to the function but both as described earlier are rare and expensive. Although Palladium and gold are less expensive but these two elements are not able to cope the highly acidic solvents present in the chemical reaction within fuel cells.
CarbonÂ supportedÂ platinumÂ is commonly used as anode and cathode electrocatalysts in low temperatureÂ fuelÂ cellsÂ fuelled withÂ hydrogen. The cost ofÂ Platinum and the limited world supply is significant barriers to the widespread use of these types ofÂ fuel cells. Moreover,Â platinumÂ used as anode material is readily poisoned byÂ carbon monoxide, present in the reformate gas used as H2Â carrierÂ in the case ofÂ polymerÂ electrolyteÂ fuelÂ cells, and a byproduct ofÂ alcohol oxidationÂ in the case of direct alcoholÂ fuelÂ cells. In addition,Â PtÂ alone does not present satisfactory activity for theÂ oxygenÂ reductionÂ reaction when used as cathode material. For all these reasons, binary and ternaryÂ platinum-basedÂ catalystsÂ and non-platinum-basedÂ catalystsÂ have been tested asÂ electrodeÂ materials for low temperatureÂ fuelÂ cells.
Need of Pt and Pd catalyst
PalladiumÂ andÂ platinumÂ have very similar properties because they belong to the sameÂ groupÂ in the periodic table. The activity for theÂ oxygenÂ reductionÂ reaction (ORR) ofÂ PdÂ is only slightly lower than that ofÂ Pt and by addition of a suitable metal, such asÂ CoÂ orÂ Fe, the oxidation reduction reaction (ORR) activity ofÂ PdÂ can overcome that ofÂ Pt. Conversely, the activity for theÂ hydrogenÂ oxidationÂ reaction (HOR) ofÂ PdÂ is considerably lower than that ofÂ Pt, but by adding of a very small amount (5 at %) ofÂ Pt, the HOR activity ofÂ PdÂ attains that of pureÂ Pt.
A simplified illustration of the principle of fuel-cell operation is shown in Fig. 1. Polymer Electrolyte Fuel Cell (PEFCs) operates with a polymer electrolyte membrane that separates the fuel (hydrogen) from the oxidant (air or oxygen). Precious metal catalysts, essentially platinum (Pt) supported on carbon are used for both the oxidation of the fuel and reduction of the oxygen in a typical temperature range of 80-100Â°C.
Figure 1. Polymer Electrolyte Fuel Cell (PEFC)
Fuel cells are the energy converting devices with a high efï¬ciency and very low emission. For hydrogen gas fed fuel cells at their current technological stage, hydrogen production, storage and transportation are the major challenges in addition to cost, reliability and durability issues. Direct methanol fuel cells (DMFCs), using liquid and renewable methanol fuel, have been regard to be a favourable option in terms of fuel usage and feed strategies41 . When correlate to hydrogen fed fuel cells, DMFC uses a liquid methanol fuel, which is easily stored, transported and simpliï¬es the fuel cell system. The attainment of fuel cell technology depends largely on the type of the electrocatalysts and membrane used.
While fuel cell technology provides a covenant answer for the need to reduce pollution, the threat of oil depletion and the ambition of many countries to reduce foreign energy dependencies from developed countries still require some issues to be answered. Fuel cell manufacturing costs are still very high for extended consumer application. To add to this the boundless application of hydrogen fuel cells requires distributed generation and transport of hydrogen which is still very expensive. Unfortunately, hydrogen, being the lightest element, lacks the convenience of energy density, storage and widespread distribution of current fuels, i.e. gasoline, natural gas, etc. Presently, the portability of hydrogen for mobile applications does not present itself as a very feasible option due to its low density.
In the present review work, we study the catalysts used in fuel cells such as platinum and palladium catalysts, its working mechanisms of reactions for Pt and Pd catalysts towards the HER/HOR reactions.
The science of catalysis is encouraged by technology as it has been from the beginning. Some of the earliest known examples of controlled chemical transformations are catalytic 42 . For example, before the sixteenth century ether was made by distilling spirits in the presence of sulfuric acid. In 1746, nitric oxide was used as a catalyst in the lead chamber process for oxidation of sulfur dioxide to give sulfur trioxide in the manufacture of sulfuric acid. In 1781, acids were used to catalyze the conversion of starch into sugar. In 1817, H. Davy discovered that in the presence of platinum, mine gases were oxidized at low temperatures; he designed a safety lamp for miners in which the platinum glowed if the ï¬‚ame was extinguished 43 .
The term catalysis was invented in 1835 by Berzelius. For example, ferments added in small amounts were known to make possible the conversion of plant materials into alcohol and there were numerous examples of both decomposition and synthesis reactions that were apparently caused by addition of various liquids or by contact with various solids. Berzelius attributed catalytic action to ill deï¬ned forces and the value of Ostwald's more lasting deï¬nition is that it identiï¬ed catalysis as a phenomenon that was consistent with the emerging principles of physical chemistry. Now it is well recognized that catalysts function by forming chemical bonds with one or more reactants, thereby opening up pathways to their conversion into products with regeneration of the catalyst. Catalysis is thus cyclic reactants bond to one form of the catalyst, products are decoupled from another form and the initial form is regenerated.
Catalyst is a substance which increases the rate of access to equilibrium of a chemical reaction without being considerably consumed itself. A catalyst changes the rate but not the equilibrium of the reaction.
Catalysts may be solids, liquids or even gases. Most catalysts used in industrial technology are either solids or liquids. Catalysis occurring in a single gas or liquid phase is referred to as homogeneous catalysis because of the uniformity of the phase in which it occurs. Catalysis occurring in a multiphase mixture such as a gas-solid mixture is referred to as heterogeneous catalysis, commonly this is surface catalysis. The performance of a catalyst is measured largely by criteria of chemical kinetics, as a catalyst inï¬‚uences the rate and not the equilibrium of a reaction. There are two types of catalyst namely homogenous and heterogeneous catalysis.
Homogeneous catalysis associate to a catalytic system in which the substrates for a reaction and the catalyst components are brought together in one phase, most often the liquid phase. In other words, in this the catalyst is in the same phase as the reactants. Typically everything will be present as a gas or contained in a single liquid phase. In this low temperature is required and separation are tricky. Some advantages of homogenous catalysis on an industrial scale are -
Ease of heat dissipation from exothermic reactions
in lab scale it is easier to study the mechanism of reaction
But there are also some disadvantages of homogeneous catalysis on an industrial scale-
Scale-up can be costly, difficult, and dangerous
Separation is required
Heterogeneous catalysis involves the use of a catalyst in a different phase from the reactants. Typical examples involve aÂ solidÂ catalyst with the reactants as eitherÂ liquids or gases. In this high temperature is required and design and optimization is tricky.
A "catalyst" can be added to the reactants in a various form, the catalyst precursor, which has to be brought into an active form ("activated"). During the catalytic cycle the catalyst might be present in many intermediate forms when we look more closely at the molecular level. An active catalyst will pass a number of times through this cycle of states; in this sense the catalyst remains unaltered. The number of times that a catalyst goes through this cycle is the turnover number. The turnover number (TON) is the total number of substrate molecules that a catalyst converts into product molecules. The turnover frequency (TOF) is the turnover number in a certain period of time. Substrates are present in larger amounts than the catalyst because when we report on catalytic reactions the ratio of substrate to catalyst is an important figure.
An inhibitor is a substance that retards a reaction. In a radical chain reaction an inhibitor may be a radical scavenger that interrupts the chain. In a metal catalyzed reaction an inhibitor could be a substance that adsorbs onto the metal making it less active or blocking the site for substrate co-ordination. We also talk about a poison, a substance that stops the catalytic reaction. A poison may kill the catalyst. The catalyst dies, we say, after which it has to be regenerated wherever possible.
`Organometallic catalysts consist of a central metal surrounded by organic (and inorganic) ligands. Both the metal and the large variety of ligands determine the properties of the catalyst. The success of organometallic catalysts lies in the relative ease of catalyst modification by changing the ligand environment. Crucial properties to be influenced are the rate of the reaction and the selectivity to certain products.
The following types of selectivity can be distinguished in a chemical reaction:
When two chemically different functionalities are present such as an alkene and an aldehyde in the example in Figure 2 which both can be hydrogenated, the chemoselectivity tells us whether the aldehyde or the alkene is being hydrogenated; or when more than one reaction can take place for the same substrate e.g. hydrogenation or hydroformylation.
Figure 2 Selectivity of chemical conversions.
As in the example shown for the hydroformylation reaction, the formyl group can be attached to the primary, secondary or the terminal carbon atom, internal carbon atom, which leads respectively to the linear and the branched product.
In this the substrate contains a stereo genic center and this together with the catalyst can direct the addition of dihydrogen in the example to give two diastereomers, the selectivity for either one is called the diastereoselectivity
In this the substrate is achiral in this instance but the enantio-pure or enantio-enriched catalyst may give rise to the formation of one specific product enantiomer.
The catalytic activity is a property of a catalyst that measures how fast a catalytic reaction takes place and may be deï¬ned as the rate of the catalytic reaction, a rate constant, or a conversion (or temperature required for a particular conversion) under speciï¬ed conditions. The selectivity is a measure of the property of a catalyst to direct a reaction to particular products. There is no single deï¬nition of selectivity but it is sometimes deï¬ned as a ratio of activities such as the ratio of the rate of a desired reaction to the sum of the rates of all the reactions that deplete the reactants. Selectivity may also be represented simply as a product distribution. Because catalysts typically lose activity and/or selectivity during operation, they are also assessed in terms of stability and lifetime. The stability of a catalyst is a measure of the rate of loss of activity or selectivity. In practical terms the stability might be measured as a rate of deactivation, such as the rate of change of the rate of the desired catalytic reaction or as the rate at which the temperature of the catalyst would have to be raised to compensate for the activity loss. Catalysts that have lost activity are often treated to bring back the activity.
Stability of Electrocatalysts
Platinum is widely used as a catalyst for chemical reactions. The most important use of platinum is in vehicles, as a catalytic converter, facilitating the complete combustion of unburned hydrocarbon passing through the exhaust 20. Platinum monolayer electrocatalysts offer a seriously reduced platinum content while affording considerable possibilities for enhancing their catalytic activity and stability. These electrocatalysts comprise a monolayer of platinum on carbon supported metal or metal alloy nanoparticles. The platinum monolayer approach has several unique features, such as high platinum utilization and enhanced activity, making it very attractive for practical applications with their potential for resolving the problems of high platinum content and low efficiency apparent in conventional electrocatalysts 44 .
Fuel cells are the shortest route to convert the chemical energy stored in molecular hydrogen to electrical energy. Since a fuel cell is an electrochemical device, therefore, electrochemical methods are account to play important roles in characterizing the fuel cell and its various components such as the catalyst, membrane and electrode. The electrochemical characterization methods include potential step, potential sweep, potential cycling and rotating disk electrode. Some techniques acquired from these methods are also used for fuel cell characterization. An electrochemical reaction basically involves the steps such as transport of the reactants to the surface of the electrode, adsorption of the reactants onto the surface of the electrode, charge transfer through either oxidation or reduction on the surface of the electrode and transport of the products from the electrode surface. The desire of the electrochemical characterizations is to determine the details of the various steps and likewise the characterization are carried out.
As this electrochemical conversion does not depend on the heat of combustion, some fuel cells may have a high limiting efficiency than the Carnot cycle which operates conventional "heat engine" power plants. Molecular hydrogen is regarded to be the most promising of chemical fuels in terms of reducing our dependence on fossil fuels, but the development of cheap, efficient, fuel-cell systems has not yet been realized on commercial basis to a large extend. Currently, fuel cells are based on the heterogeneous, breakdown splitting of hydrogen on a platinum surface but these fuel cells have the understandable problem that Pt is scarce and expensive 45-46 . Till now few improvements in efficiency have been gained in over the years, so a new model for fuel-cell catalysis is required to generate a fuel cell based economy. Fuel cell development might be seized in an entirely new direction by the introduction of molecular catalysts capable of working in homogeneous solutions. As molecular catalysts have the authority of being highly variable in terms of design and solution phase catalysis is important because it enables us to directly observe the details of the mechanism in the below (Figure 3).
Figure 3. Direct observation of catalysis in fuel cell47 .
A fuel cell is a device that converts the chemical energy in a fuel to electrical energy through electrochemical reactions. All the electrochemical reactions are the oxidation of the hydrogen and reduction of the oxygen. The electrochemical reactions, reduction and oxidation, are catalyzed by the cathode and the anode. However, catalysis in fuel cell systems is not bind only to these electrochemical reactions. In real systems there are different chemical reactions and associated catalysis associating to processing the fuel to a form suitable for the fuel cells removal of contaminants which damage the electrocatalytic activity of the electrode which convert the fuel hydrogen.
Types of Fuel Cell
Currently there are six fuel cells which are in research stages. All this cell systems are given below in Table 3. For the fuel cell types characterization and nomenclature of these is by the electrolyte and the correlate operating temperature. All these features manage the necessity of the electrocatalysts which control the reactions. The Direct methanol fuel cell stands alone in involving a carbonaceous fuel (methanol) which is fed directly to the anode, whereas all others use hydrogen as the anode fuel, either as a hydrogen rich gas mixture or as a pure gas.
Fuel Cell Type
Operating Temperature in Â°C
Proton exchange membrane
Solid proton conducting polymer
Lithium carbonate mixture
Table 3. Different Fuel cells.
Application of fuel cell
Fuel cell has wide application because of its rich properties as clean source of energy. It is widely used in Automobiles. In fact, nowadays almost every car manufacturer has developed at least one prototype vehicle and many have already gone through several generations of fuel cell vehicles. It is also used in automotive vehicles like Scooters and bicycles. Fuel cell powered scooters and bicycles using either hydrogen stored in metal hydrides or methanol in direct methanol fuel cells.
Proton Exchange Membrane Fuel cell
Figure 3. Diagram of Proton Exchange Membrane Fuel Cell (PEMFC)
The above Figure illustrates diagrammatic representation of Proton Exchange Membrane Fuel cell. The conversion of chemical energy to electrical energy in a PEM fuel cell occurs through a direct electrochemical reaction and it takes place without combustion. The important part of a PEM fuel cell is membrane electrode assembly (MEA) which consists of a polymer electrolyte in contact with an anode and a cathode on either side. In order to precede the mechanism in PEMFC, the membrane present must conduct hydrogen ions (protons) and separate either gas to pass to the other side of the cell. In the above Figure, it can be seen that hydrogen is delivered through the flow field channel of the anode plate to the anode in one the side of the cell. On the other side of the cell, oxygen from the air is delivered through the channeled plate to the cathode. H2 is decomposed into positively charged protons and negatively charged electrons at the anode whereas the electrons. Positively charged protons pass through the polymer electrolyte membrane (PEM) to the cathode, whereas the negatively charged electrons travel along an external circuit to the cathode, creating an electrical current.
In PEM fuel cell, a polymer membrane is present. It is sealed to gases but it conducts protons so it is known as proton exchange membrane fuel cell. The membrane that acts as the electrolyte is hold tight between the two porous, electrically conductive electrodes which are made of carbon cloth or carbon ï¬ber paper. At the interface between the porous electrode and the polymer membrane there is a layer with catalyst particles, typically platinum supported on carbon. Electrochemical reactions happen at the surface of the catalyst at the interface between the electrolyte and the membrane. Hydrogen, which is fed on one side of the membrane, splits into its primary constituent's protons and electrons. In this splitting of Hydrogen molecule is quite easy using a platinum catalyst. Each hydrogen atom consists of one electron and one proton. Protons travel through the membrane, whereas the electrons travel through electrically conductive electrodes, through current collectors, and through the outside circuit where they perform useful work and come back to the other side of the membrane. At the catalyst sites between the membrane and the other electrode they meet with the protons that went through the membrane and oxygen that is fed on that side of the membrane. Water is created in the electrochemical reaction and then pushed out of the cell with excess ï¬‚ow of oxygen. The result of these simultaneous reactions is current of electrons through an external circuit direct electrical current.
Main components and materials
The main purpose of the membrane in PEM fuel cells is to transport protons from the anode to the cathode. The membrane polymers present have sulfonic groups which facilitate the transport of protons. The other activity includes holding the fuel and oxidant separated that prevents mixing of the two gases and withstanding harsh conditions, including active catalysts, high temperatures fluctuations and reactive radicals. Thus, the ideal polymer must have excellent proton conductivity, chemical and thermal stability, strength, flexibility, low gas permeability, low cost, and good availability. Many different membranes have been tested for commercial use in PEM fuel cells. The membranes are commonly polymers modified to include ions, especially sulfonic groups. These hydrophilic ionic components are the key for allowing proton transport across the membrane 48 .
For hydrogen oxidation reaction (HOR) and oxidation reduction reaction (ORR) Platinum has been considered as best catalyst. Though there might be great difference between the HOR and ORR reactions when using the same catalyst. Still great efforts are taken in research towards developing different catalyst material but still platinum is the best available. In many PEMFCs, the anode and the cathodes use same platinum catalyst. Commonly, the platinum catalyst is formed into small particles on a surface of considerably larger particles that act as a supporter. This is nothing but carbon powder and mostly used carbon based powder is Vulcan XC72Â® (by Cobalt). This way the platinum is highly divided and spread out, so that a very high proportion of the surface area will be in contact with the reactant, resulting in a great reduction of the catalyst loading with an increase in power 49 .
A PEM fuel cell consists of an electrolyte compressed between two electrodes. At the surfaces of the two electrodes, two electrochemical reactions take place. At the anode, hydrogen oxidation reaction occurs over which hydrogen gas passes, whereas oxidation reduction reaction occurs at the anode over which the oxygen passes. The electrode reaction occurring are as given below,
H2 â†’ 2H+ + 2e-
Corresponding to an anode potential Ea0 = 0 V (under standard conditions) versus SHE.
1/2O2 + 2H+ + 2e- â†’ H2O
Corresponding to a cathode potential Ec0 = 1.229 V (under standard conditions) versus SHE. Therefore, the overall reaction of the fuel cell is
H2 + 1/2O2 â†’H2O
With the equilibrium standard electromotive force calculated to be 1.229 V.
Hydrogen Oxidation Reaction
At the anode, hydrogen is stripped of its electrons and becomes protons and electrons. For electrochemical reactions, even if a simple one electron reaction is not that simple and is always with a reaction mechanism involving several steps.
The overall reaction rate depends on the slowest elementary reaction, which is called the rate determining step. The steps of H2 oxidation on Pt electrode include the following:
H2 + Pt â†’ Pt-H2
Pt-H2 â†’ Pt-Hads
Pt-Hads â†’ Pt + H+ + e-
Platinum based catalysts are widely used as the anodic electrode material for hydrogen oxidation. The HOR on Pt catalysts has lower oxidation over potential and a higher kinetic rate. The apparent exchange current density of the HOR has been calculated to be i0anode = 0.1 Acm-2 which is high when compared with ORR i0cathode = 6 ÂµAcm-2 which is obtained from the (EIS) electrochemical impedance spectroscopy measurements done by Wagner50 . This proves the extreme fast reaction kinetics of HOR. The table shows the exchange current densities of the hydrogen evolution reaction at different electrode materials in aqueous 1 M H2SO4 solution at ambient temperature.
Exchange current density
1.0 Ã- 10-3
8.0 Ã- 10-4
2.5 Ã- 10-4
2.0 Ã- 10-4
7.0 Ã- 10-6
Table 4. Exchange current densities of the HOR reactions at different electrode materials in aqueous 1M H2SO4 solution at ambient temperature51 .
But for many practical applications, the presence carbon monoxide (CO) traces in the hydrogen gas mixture produced by the reforming of other fuels is must. Carbon monoxide can strongly adsorb on the Pt catalyst in the anode. The adsorbed CO even mere traces (10 ppm) blocks the catalytically active area, thereby relatively decreasing its reactivity and causing "CO poisoning". Due to this, anode catalyst in PEM fuel cells has to show not only high catalytic activity toward hydrogen oxidation but also enhanced activity in the presence of CO. The alternative option for CO tolerant catalysts has been a demanding task in the successful development of more efficient PEMFC systems.
Electrocatalysis of Hydrogen Oxidation Reaction
The electrocatalysis of the HOR is one of the important areas in fuel cell applications. In general, electrocatalysis can be considered a specific type of heterogeneous catalysis whereby reactants and products adsorb onto the catalyst surface during the reaction process. Hydrogen is an important material and product in chemical industries and has been investigated as a new clean energy source for many decades 52-54 . With the development of proton exchange membrane (PEM) fuel cell technology, in which hydrogen is used as a fuel, the chemical energy stored in this hydrogen can be electrochemically converted to electric energy with zero emissions and high efficiency. From early 1990s, the advantages of PEM fuel cells, including low emissions, high energy efficiency and high power density have attracted world-wide research and development in many important application areas, including automotive engines, stationary power generation stations, and portable power devices55 . The major cost of a PEM fuel cell is the platinum (Pt) based catalysts. Based upon the current technological stage, these Pt-based catalysts for both the cathodic oxygen reduction reaction (ORR) and the anodic hydrogen oxidation reaction (HOR) are the most practical catalysts in terms of catalytic activity and lifetime stability.
With respect to fuel cell catalysis, most research has been focused on cathode ORR catalysts development as the ORR kinetics are much slower than the anodic HOR kinetics, in other words, the fuel cell voltage drop polarized by load is due mainly to the cathode ORR over potential56 . However, in some cases the over-potential of the anodic HOR can also contribute a non-negligible portion of the overall fuel cell voltage drop57 . Therefore, the catalytic HOR on the fuel cell anode catalyst is also very important of potential use of hydrogen as a future fuel. Apart from its importance in fuel cell applications, hydrogen electrooxidation catalysis is also a model system for the fundamental understanding of electrochemical kinetics and electrochemical surface science58-59 .
Undoubtedly the hydrogen evolution/oxidation reaction (HER/HOR) is the simplest and most widely studied electrochemical process. Almost all the basic laws of electrode kinetics and the concepts of electrocatalysis were developed and verified by below two reactions.
The following describes the kinetics and mechanisms of the electro-catalyzed HOR on different electrode materials, including platinum group metals, carbides, and transition metals. Despite its wide range of topics, the main purpose of this chapter is to provide a fundamental understanding of the electrocatalysis of the HOR, the most important reaction other than the ORR in the PEM hydrogen fuel cell.
Electrooxidation of Hydrogen
Mechanism of the Hydrogen Oxidation Reaction
The overall reactions of anodic hydrogen oxidation in acidic and alkaline mediums may be expressed by following two equations;
Â½ H2 â†’ H+ + e- (1)
Â½ H2 + OH- â†’ H2O + e- (2)
The hydrogen oxidation reaction may occur by the following three sequential steps:
1) Adsorption step:
In this the hydrogen molecule diffuses from the electrolyte to the electrode, then adsorbs on the electrode surface to form surface species (H2, ad):
H2 â†’ H2, sol â†’ H2, adsorp (3)
2) Hydration/ionization step:
In this the adsorbed hydrogen forms adsorbed H atoms (Hadsorp) through process (a) or (b):
(a) Tafel-Volmer route
H2, adsorption â†’2Hadsorption (Tafel reaction) (4)
Hadsorp â†’ H+ + e- (5)
Hadsorp + OH- â†’ H2O + e- (6)
This is Volmer reaction in alkaline medium
(b) Heyrovsky-Volmer route:
(b) Heyrovsky-Volmer route:
H2, adsorp â†’ Hadsorp.H+ + e- â†’ Hadsorp + H+ + e- (7)
(This is Heyrovsky reaction in acidic medium)
H2, adsorp + OH- â†’ Hadsorp.H2O + e- â†’ Hadsorp + H2O + e- (8)
(The above equation is Heyrovsky reaction in alkaline medium)
Hadsorp â†’ H+ + e- (9)
(Volmer reaction in acidic medium)
Hadsorp + OH- â†’ H2O + e- (10)
(Volmer reaction in alkaline medium)
3) Desorption step:
In this the products, such as H+ and H2O are desorbed and then transported into the electrolyte. In each step of the above routes, the overall reaction rate can be controlled by a step which is sufficiently slow compared with the others. This is the rate determining step (rds). The rate determining steps have been identified for several mechanisms:
(a-1) The slow Volmer-rapid Tafel mechanism (the slow-discharge mechanism):
H2 â†’ 2Hadsorp, Hadsorp â†’ H+ + e- (rds) (11)
(a-2) The rapid Volmer-slow Tafel mechanism (the slow combination or the catalytic mechanism):
H2 â†’ 2Hadsorp (rds), Hadsorp â†’ H++ e- (12)
Had + OH- â†’ H2O + e- (13)
(b-1) The slow Volmer-rapid Heyrovsky mechanism:
H2, adsorp â†’ Hadsorp.H+ + e- â†’ Hadsorp + H+ + e- (rds) (14)
H2, adsorp + OH- â†’ Hadsorp.H2O + e- â†’ Hadsorp + H2O + e-(rds) (15)
(b-2) The rapid Volmer-slow Heyrovsky mechanism:
H2, adsorp (rds) â†’ Hadsorp.H+ + e- â†’ Hadsorp + H+ + e- (16)
H2, adsorp + OH- (rds) â†’ Hadsorp.H2O + e- â†’ Hadsorp + H2O + e- (17)
In the past, the majority of the basic laws and concepts in electrode kinetics were developed and verified by Tafel60 , Volmer61 and Frumkin62 using the hydrogen electrode. Two important reaction mechanisms are well recognized and experimentally validated. The first is the Volmer-Tafel mechanism, shown in Equations (11-13). The other, which is more important for the hydrogen electrode, is the Heyrovsky-Volmer mechanism, expressed in Equations (14-17).
Thermodynamic Considerations for the Hydrogen Electrode Reaction
The conventional thermodynamic relations for the hydrogen electrode reaction are as follows;
EH = -
âˆ†G = Â½ ÂµH2 - ÂµH+ = (Â½ ÂµÂ° H2 - ÂµÂ°H+) + RT ln PH2Â½ / aH+
EH = EÂ°H - - ln (PH2Â½ / aH+)
Where âˆ†G is the Gibbs free energy, ÂµH2 and ÂµH+ are the chemical potentials for H2 and H+ , aH+ is the activity of the proton, F is the Faraday constant and aH+ has a practical significance through the conventional definition of pH. EÂ°H is defined as the standard hydrogen electrode potential with EÂ°H = 0 V at standard conditions63 .
Electrocatalysis of Hydrogen Oxidation
The electrocatalysis of the HOR is one of the important areas in fuel cell applications. Most importantly electrocatalysis may be contemplating a specific type of heterogeneous catalysis by which reactants and products adsorb onto the catalyst surface during the reaction process. The reactants get activated by interaction with the catalyst surface and are rapidly selectively converted to adsorbed products.
Since the catalyzed electrochemical reaction occurs at the catalyzed electrode interface, the intrinsic kinetic rate of an electrochemical reaction (measured by the exchange current density) strongly depends on the potential difference between the catalyst surface and the electrolyte and as well as on the kind of catalyst and its surface morphology. For electrode reactions, the exchange current density can vary from about 10-3 A.cm-2 at a platinum electrode to 10-12 A.cm-2 at a mercury electrode for the anode reaction (HOR)64 . Under normal conditions, the HOR on Pt is approximately 5 to 7 magnitudes more rapid than the ORR and has one of the fastest known specific rate constants in aqueous solutions65 . Since the pioneering work of the 1960s, when the dependence of hydrogen adsorption upon the crystallographic orientation of the platinum surface was discovered, the study of the relationship between electrochemical activity and surface structure has been the main theme of electrochemical research66 .
Platinum and Platinum Group Metals (Pt, Pd, Ru, Rh, Ir and Os)
Adsorption and desorption of reactants on a catalyst surface can directly affect its catalytic ability so it is usually expressed as a "Volcano curve" plotted to correlate the exchange current density of the HOR with the enthalpy of hydrogen adsorption. If the enthalpy is too small, a slow adsorption kinetic will result and limit the rate of the overall reaction, on the other hand, if the enthalpy is too high then desorption of hydrogen becomes difficult.
Accordingly, this hydrogen desorption step will become a rate determining step within the overall reaction. As a result an intermediate value in the enthalpy of Hydrogen adsorption on a catalyst is required in order for it to be an active catalyst. As shown in below Figure 5 Pt and Pt-group metals all have intermediate values of hydrogen adsorption and display high catalytic activities67 . Platinum group metals including Pt, Pd, Ru, Rh, Ir and Os have long been known as catalysts for both the ORR and the HOR68 . On platinum group metal surfaces the chemisorption of hydrogen can easily remove the adsorbed oxygen with the formation of water at room temperature which does not commonly occur on other transition metals as a result they bind oxygen too strongly69 . Another characteristic of platinum group metals is their ability to dissociate H2 in the presence H2O.
Figure 5. Volcano plot for electrocatalysis of the hydrogen reaction, in terms of (logi0) as a function of the enthalpy of hydrogen adsorption on various catalysts
Platinum shows the eminent exchange current density in all the platinum group metals that are the most electro catalytically active electrode materials for the hydrogen oxidation reaction70 . The extensive effort in HOR electrocatalysis has been attracted on understanding rate dependency on the atomic scale morphology of a platinum single crystal surface. Virtually all the early kinetic studies of the HER/HOR were carried out on polycrystalline platinum electrodes71 . And to some extend kinetics studies were also carried on platinum single crystals that had poorly defined surface structures72 . In the early studies, the kinetics of the HER/HOR on Pt (hkl) was reported to be insensitive to the surface crystallography. Now only the catalysis studies on well-defined Pt single-crystal electrodes clearly demonstrated that the HER/HOR kinetics on Pt (hkl) vary with a crystal face that has "structure sensitivity"73 . This sensitivity is mainly caused by the structure-sensitive adsorption of hydrogen and electrolyte anions.
Supported Pt Catalysts
Since catalyst performance for the HOR is strongly dependent on the total active surface area, supported catalysts have been developed to maximize the catalyst surface area. When drawn comparison to bulk Pt catalysts, supported catalysts show higher activity and stability due to fine dispersion, high utilization, and stable nanoscale metallic particles.
Till now, carbon black materials are the most commonly used carbon supports for PEM fuel cell catalysts. Supported catalysts have several advantages over unsupported catalysts such as they have relatively higher stability than unsupported catalysts in terms of agglomeration under fuel cell operating conditions. The good electric conductivity of the carbon support allows electron transfer from catalytic sites to the conductive carbon electrodes and then to the external circuit and the small dimensions of catalyst particles (nanoscale) dispersed on a carbon support can maximize the contact area between catalyst and reagents74 .
Various carbon blacks show different physical and chemical properties and they are usually manufactured by pyrolyzing hydrocarbons. Both these properties have strong effects on the properties of the supported metal catalysts such as morphology, metal particle size, stability, size distribution and dispersion. And so in order to optimize the performance of the catalyst, a suitable carbon black support has to be down selected taking into consideration properties such as specific area, porosity, morphology, corrosion resistance, etc. One of the type of carbon support used is Nanostructured carbon such as carbon nanotubes (CNTs)75 . Carbon nanotubes are a compact kind of material for catalyst support in fuel cell catalysis applications because of their unique electrical and structural properties. Various studies have shown that Carbon nanotubes are superior to carbon blacks as catalyst supports for PEM fuel cells76 . Another example of novel catalyst support is the ultra-thin nanostructured (NS) film system. In this catalyst support (NS) film system consists of a uniquely structured thin film composed of highly oriented, densely packed crystalline organic whiskers in which the catalyst particles are deposited by vacuum coating methods (Figure 6).
Figure 6. SEM pictures of Pt coated nanostructured whiskers. To the Left: the plane view at 10,000 magnification; To the right: 45Â° angle view at 150,000 magnification77 .
Due to the definite and highly controllable nature of the oriented thin film the process for making a catalyst supported on this thin film inherently results in extremely high degrees of catalyst uniformity and though this uniformity may not be achievable for the heterogeneous surface of a conventional carbon black support78 .
Controllable synthesis of noble metal
Case of Palladium
The palladium hydrogen (Pd-H) system has engaged a lot of activities as it is one of the first of all metal hydrogen systems and has been investigated extensively . It has been found that Pd shows definite catalyst activity for hydrogen generation so that the adsorption of hydrogen on different Pd metal surface has been the subject of both experimental and theoretical investigation [2, 3]. Palladium nanomaterial is well-known for its remarkable capacity in hydrogen absorption so they are mostly used in automobile pollutants, water treatments, and hydrogenation reactions and in different organic reactions. (Rwf) Palladium nanomaterial also plays a vital role in fuel cell technology. Recent study further concedes that Pd Nano-materials exhibit good surface-enhanced Raman scattering (SERS) and sensing activity .
In all of these applications, the size and shape of Pd nanomaterials are still critical parameters in order to maximize their performance. Therefore, controllable synthesis of Pd nanomaterials is highly desired for tailoring their catalytic properties and also a prerequisite for achieving their high performance in various catalytic applications. At present, high-quality Pd nanomaterials with different shapes have been facilely obtained through a kinetics-controlled or thermally controlled process.
Case of Platinum nanomaterials
Because of their high catalytic activity, Pt nanomaterials have been widely applied in many ï¬elds including fuel cells, sensors, and the petroleum and automotive catalysis .Given that Pt is a precious and rare metal; most of the recent efforts have emphasized the reduction of platinum utilization through increasing the catalytic efï¬ciency of Pt catalysts . A number of studies reveal that the catalytic reactivity of platinum nanomaterials depends highly on their morphology, and therefore the design of novel platinum nanomaterials with unique morphologies has been greatly intensiï¬ed due to their potential for enhanced and new properties and applications in the last decades .
Recent signiï¬cant development in nanomaterials synthesis has led to the formation of various kinds of nanomaterials with controlled size, shape, composition, inter-particle interaction and hybrid. These functional nanomaterials provide good opportunity for developing highly active catalysts for fuel cell reactions. Generally speaking, among them, Pt and Pt-based nanomaterials are still the most effective electrocatalysts for fuel cell applications, which catalyze hydrogen or small molecule oxidation at anode and ORR at cathode. Herein, from the perspective of enhancing the electro activity of catalysts, some recent push in developing high-efï¬ciency Pt-based nano electrocatalysts for fuel cell applications are highlighted.