Presently the demand for clean and sustainable energy beginnings have become a strong instigates force in go oning economic development. Proton exchange membrane ( PEM ) fuel cells which act as clean energy change overing devices have drawn adequate attending in recent old ages due to their high efficiency, energy denseness and low emanations. PEM fuel cells have several of import application countries, including transit, stationary power and micro-power. Fuel cells are nil but an electrochemical device that converts the chemical energy of a reaction straight into electrical energy. Fuel cell accelerators, such as Pt ( Pt ) based accelerators and their associated accelerator are the most used accelerators in fuel cells. As these accelerators are dearly-won so a great trade of attempt has been put into the geographic expedition of cost effectual, active and stable fuel cell accelerators. In a PEM fuel cell, H oxidization reaction ( HOR ) takes topographic point at the anode and the O decrease reaction ( ORR ) take topographic point at the cathode within the several accelerator layers1. Thus the electrocatalysts and their corresponding accelerator beds play of import functions in fuel cell public presentation. In current province of engineering the most practical accelerators in PEM fuel cells are extremely dispersed Pt based nanoparticles.
Baronial metal catalystsA are immune toA corrosion and oxidationA in damp air. Normally the baronial metals are considered to be in order of increasing atomic figure such as Ru, Rh, Pd, Ag, Os, Ir, Pt and gold. Nanomaterial based accelerators are normally heterogenous and can be broken up to increase catalytic procedure. Metallic element nanoparticles have really high surface country so there are ever opportunities of increased catalytic reactions. Nobel metal nanomaterial ‘s ( NMNs ) have been intensively pursued, non merely for their cardinal scientii¬?c involvement, but besides for their many technological applications with size dependent chemical belongingss and optical as a type of voguish materials.2-13
The much larger surface-to-volume ratio of nanoparticles collate to their majority opposite numbers has temptingness a great trade of attending for catalytic applications.14-17 This is because much smaller sum of stuffs can be used and higher catalytic activity can be achieved for accelerators of the same mass. Apart from the effects of size, the function of nanoparticle form is besides of import in catalytic work which has been found to be extremely dependent on the crystallographic planes that terminate the nanoparticle surface. 18-20
Both Pt and Pd are of import accelerators for many industrial procedures where they exhibit similar catalytic activities.21 Nanoparticles of Pt and Pd are to a great extent studied for assorted catalytic applications including hydrogenation reactions, carbon-carbon bond formation and oxidization and decrease reactions in fuel cells.22-25 As the demands for cherished metals such as Pt and Pd are turning, there has been a major focal point on the development of high public presentation accelerators. 26
Baronial metal accelerators are good known for their high catalytic activities. Carbon supported Pt group metals have long been recognized as indispensable accelerators in organic synthesis and there is abundant literature on their belongingss in many reactions. Platinum ( Pt ) based electrocatalysts are normally employed in proton exchange membrane fuel cells ( PEMFC ) and direct methyl alcohol fuel cells ( DMFC ) . It is good known that the catalytic activity of the metal is chiefly dependent on the atom size and form distribution27. Carbon supported Pt ( Pt/C ) is the best known electrocatalysts for both hydrogen oxidization and O decrease in phosphorous acid fuel cells ( PAFCs ) and proton exchange membrane fuel cells ( PEMFCs ) . The construction and proper dispersion of these Pt atoms make low lading accelerator executable for PAFCs and PEFCs operation.
In position of the function played by C supported accelerators in the pattern, cardinal surveies on the factors ini¬‚uencing the province and belongingss of the active stage in these accelerators are still instead deficient. The last decennary showed an increasing involvement in this topic. However, chiefly the relationship was under survey between the features of the support ( preponderantly, the functional coverage of the surface ) and the scattering province of the resulting metal but non its catalytic belongingss. Besides, non-porous C inkinesss were chiefly used in the scientii¬?c surveies, whereas activated Cs are the typical supports in pattern. Obviously, this may be explained by a excessively complex beginning of accelerators on the porous C supports. It is besides true because of the scarceness of literature on this topic it makes traditional C supported catalysts a difi¬?cult and unattractive subject for geographic expedition.
So in order to cut down the widen spread between the bing and required cognition on the readying of C supported accelerators with Pt group metals, a collaborating survey on Pt and Pd accelerators supported on different activated Cs is required. This was described by belongingss of accelerators on i¬?ve activated Cs specially selected to look into a possible ini¬‚uence of the porous construction of the support28. The accelerator readying was restricted to utilize chlorides of Pt and Pd as the get downing metal composites, their deposition onto the supports by the surface assimilation method, and decrease in i¬‚owing H. Carbon supported Pt and Pd accelerators have been synthesized and used in PEM H fuel cell anodes29. Electrocatalysts based on Pt and Pd deposited onto wood coal have been prepared and tested by30 on the anodal side of PEM fuel cells 11. The appraisal of the electrochemical activity of Pd accelerators on C in alkalic solutions for the oxidization of H and methyl alcohol was reported by31. Besides the dynamicss of H development reaction at Pd in alkalic solution has besides been studied in a paper32. Palladium accelerator has a echt usage in ‘green ‘ energy, as a accelerator in H fuel cells. It is one of a figure of metals get downing to be used in the fuel cells to power a host of things in vehicles such as autos and coachs. When a molecule of H foremost comes into contact with Pd, they are adsorbed on the surface but so they diffuse throughout the metal.
The most critical application of Pt and Pd accelerator is in the pharmaceutical industry as these two baronial metals are normally the premier accelerators of pick. Their demand is mostly defined by the alone nature of chemical processing operations. The production of a merchandise in the pharmaceutical industry is normally expressed in lbs per twelvemonth and in most other sectors of the chemical industry, production in footings of dozenss per twelvemonth is the regulation. Cherished metals are used extensively as accelerators in a broad scope of industrial chemical procedures. They can be used in a homogenous signifier, but more normally they are heterogenous. In many operations merely a cherished metal accelerator can supply the necessary velocity or selectivity to the reaction, while in others these features, together with a long accelerator life make the overall system the most cost effectual pick.
In a PEM fuel cell, Hydrogen oxidization reaction ( HOR ) takes topographic point at the anode and the Oxygen decrease reaction ( ORR ) take topographic point at the cathode within the several accelerator beds. But we will concentrate chiefly on Hydrogen oxidization reactions. Therefore electrocatalysts and their corresponding accelerator beds play critical functions in fuel cell public presentation. In our present province of engineering, the most practical accelerators in PEM fuel cells are extremely dispersed Pt based nanoparticles. However, Pt based accelerators have several drawbacks, such as high cost, sensitiveness to contaminations and no tolerance for methanol oxidization ( in a direct methyl alcohol fuel cell ( DMFC ) application ) , fewer completed four negatron decrease reactions and Pt disintegration. In the hunt for alternate low cost non Pt accelerators, research workers have looked at several others, including supported Pt group metal ( PGM ) types such as Pd based accelerators. However, these attacks are as yet in the research phase, as the accelerator activities and stablenesss are still low to be practical. Another attack is to cut down Pt lading in a accelerator or accelerator bed utilizing debasing and C supports.
Another important challenge is deriving a cardinal apprehension of fuel cell accelerator constructions and their corresponding catalytic reaction mechanisms. Current attacks rely mostly on test and mistake. To plan new, breakthrough accelerators, we need a chiseled theoretical attack. Theoretical surveies will supply a platform for understanding accelerator public presentation and besides researching the construction activity relationship.
Normally any H based energy transition secret plan depend on effectual accelerators for oxidization and decrease of H which is nil but Hydrogen oxidization reaction ( HOR ) and Hydrogen decrease reaction ( HER ) 33. Platinum based accelerators are stable and effectual for both hydrogen oxidization reaction ( HOR ) and hydrogen development 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 demand for the development of electrodes made of cheaper stuffs to cut down the overall cost. To be able to plan new electrodes for the H development or oxidization reactions, it may good turn out indispensable to get penetration into their mechanism at the atomic degree 34-39. Overall HOR/HER reaction H2 a†” 2 ( H+ + e- ) taking topographic point at an electrode with an electrolyte, involves three simple reactions. In the i¬?rst measure, H2 is dissociated and H adsorbed. This is achieved either by the Tafel reaction H2 a†’2H* ( H* denotes H adsorbed on the surface ) or by the Heyrovsky reaction H2 a†’H* + H+ + e- . The adsorbed H is so discharged, following the Volmer path H* a†’H+ + e- . Despite intensive research attempts it is still ill-defined which of the two tracts, Tafel-Volmer or Heyrovsky-Volmer dominates under different conditions even on the most studied electrode stuff, Pt.
Fuel cells offer efficient and virtually pollution free energy transition and power coevalss. The world that fossil fuels are completing out and the certainty that pollution from utilizing fossil fuels has become an issue of environmental concern to human wellness constitute two of the major drive forces for the increasing involvement in the development of fuel cells 40.
The car industry is perchance the biggest market behind the monolithic investing in fuel cell development. This is clear as the monetary value of oil is extremely volatile and has been increasing in the past few old ages which are likely to go on. Additionally the harmful emanations of gases such as CO2, CO and other volatile organic compounds into the ambiance cause serious environmental harm and green goods ‘greenhouse gases ‘ that give rise to planetary heating. But in contrast fuel cell pull out energy from fuel ( 40-70 % efi¬?ciency ) more efficaciously than traditional internal burning engines ( a??30 % efi¬?ciency ) . Because of this status along with the H ‘s high efi¬?ciency ( from 40-70 % ) could finally take to the possibility of better use of both heat and electricity in fuel cells and therefore do a signii¬?cant part to cut downing atmospheric emanations.
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 change overing devices. A fuel cell is an electrochemical device that continuously and straight converts the chemical energy of externally supplied fuel and oxidizer to electrical energy. Fuel cells are normally classified harmonizing to the type of electrolyte used. The five most common engineerings are polymer electrolyte membrane fuel cells ( PEM fuel cells or PEMFCs ) , alkalic fuel cells ( AFCs ) , phosphorous acid fuel cells ( PAFCs ) , liquefied 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 anchor with side ironss possessing acid-based groups. The legion advantages of this household of electrolytes make the PEM fuel cell peculiarly attractive for smaller-scale tellurian applications such as transit ; place based distributed power, and portable power applications. The separating characteristics of PEMFCs include comparatively low temperature ( under 90A°C ) operation, high power denseness, a compact system, and easiness in managing liquid fuel.
The nature of the electrolyte in any fuel cell type and the combine operating temperature are cardinal characteristics of consideration for effectual accelerators. Besides in add-on to this the nature of the electrolyte besides drives the peculiarity of the dominant migrating ion, as illustrated in Table 1.
H2 + 2OH- a†’ 2H2O + 2e-
O2 + 2H2O + 4e- a†’ 4OH-
H2 a†’ 2H+ + 2e-
O2 + 4H+ + 4e- a†’ 2H2O
H2 a†’ 2H+ + 2e-
O2 + 4H+ + 4e- a†’ 2H2O
H2 + CO32- a†’ H2O + CO2 + 2e-
CO + CO32- a†’ 2CO2 + 2e-
O2 + 2CO2 + 4e- a†’ CO32-
Table 1. Fuel cell systems demoing anodal and cathodic reactions and the dominant manner of ion conveyance in the electrolyte
Electrocatalysis is really of import for the fuel cells. In order to bring forth H beginning before fuel cell is injected involves different accelerators. Carbon monoxide derive from reforming hydrocarbons acts as a toxicant for the anode electrocatalysts in the low temperature fuel cells and its remotion from the fuel beginning is a demanding work for the fuel treating catalysts.Apart from exemplifying electrocatalytic activity towards the electrode reactions ( the fuel anode every bit good as the air or O cathode ) , the electrocatalysts basically be stable within the working cell. Equally far as for the alkaline fuel cell ( AFC ) this is comparably easy because many electrocatalytic stuffs are adequately stable in alkalic solutions. The certainty that the AFC is extremely sensitive to the presence of CO2, either in the fuel watercourse or in the air watercourse, has reduced its application to a great extent to those state of affairss where really pure O and H can be supplied. For the fuel cells runing with acidic electrolytes, stableness of the electrocatalysts is much more huge to recognize. Many types of electrocatalysts have been considered over old ages for their assorted 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 troubles of thermic stableness every bit good as compatibility with the electrolyte. Presently preferable electrocatalysts for the assorted cells are listed in Table 2.
Pt/Au, Pt, Ag
Pt/Au, Pt, Ag
Table 2. Electrocatalysts for the assorted cells
Platinum and Pt metals are the most efficient accelerators for rushing up chemical reactions in H fuel cells. Platinum is the lone metal that can defy the acidic conditions inside such a cell but it is expensive and this has limited the wide, big graduated table applications of fuel cells. Furthermore, about 90 per centum of the universe ‘s Pt supply comes from two states South Africa and Russia.
The bulk of H fuel cells use accelerators which are made of a rare and expensive metal Platinum. Of class there are few options because most elements can non meet the fuel cell ‘s extremely acidic dissolvers bing in the reaction which converts H ‘s chemical energy into electrical power. There are merely four elements which can defy the caustic procedure. These elements are Platinum, Pd, gold and Ir.
Normally H fuel cells reckons on the accelerators made of Pt, which is really expensive and besides rare. Unfortunately there are n’t many other options because most of the other elements can non defy the caustic procedure that converts H ‘s chemical energy into electrical power. Platinum and Ir are up to the map but both as described earlier are rare and expensive. Although Palladium and gold are less expensive but these two elements are non able to get by the extremely acidic dissolvers present in the chemical reaction within fuel cells.
CarbonA supportedA platinumA is normally used as anode and cathode electrocatalysts in low temperatureA fuelA cellsA fuelled withA H. The cost ofA Platinum and the limited universe supply is important barriers to the widespread usage of these types ofA fuel cells. Furthermore, A platinumA used as anode stuff is readily poisoned byA C monoxide, nowadays in the reformate gas used as H2A carrierA in the instance ofA polymerA electrolyteA fuelA cells, and a by-product ofA intoxicant oxidationA in the instance of direct alcoholA fuelA cells. In add-on, A PtA entirely does non present satisfactory activity for theA oxygenA reductionA reaction when used as cathode stuff. For all these grounds, binary and ternaryA platinum-basedA catalystsA and non-platinum-basedA catalystsA have been tested asA electrodeA stuffs for low temperatureA fuelA cells.
Need of Pt and Pd accelerator
PalladiumA andA platinumA have really similar belongingss because they belong to the sameA groupA in the periodic tabular array. The activity for theA oxygenA reductionA reaction ( ORR ) ofA PdA is merely somewhat lower than that ofA Pt and by add-on of a suited metal, such asA CoA orA Fe, the oxidization decrease reaction ( ORR ) activity ofA PdA can get the better of that ofA Pt. Conversely, the activity for theA hydrogenA oxidationA reaction ( HOR ) ofA PdA is well lower than that ofA Pt, but by adding of a really little sum ( 5 at % ) ofA Pt, the HOR activity ofA PdA attains that of pureA Pt.
A simplified illustration of the rule of fuel-cell operation is shown in Fig. 1. Polymer Electrolyte Fuel Cell ( PEFCs ) operates with a polymer electrolyte membrane that separates the fuel ( H ) from the oxidizer ( air or O ) . Cherished metal accelerators, basically Pt ( Pt ) supported on C are used for both the oxidization of the fuel and decrease of the O in a typical temperature scope of 80-100A°C.
Figure 1. Polymer Electrolyte Fuel Cell ( PEFC )
Fuel cells are the energy change overing devices with a high efi¬?ciency and really low emanation. For H gas fed fuel cells at their current technological phase, H production, storage and transit are the major challenges in add-on to cost, dependability and lastingness issues. Direct methyl alcohol fuel cells ( DMFCs ) , utilizing liquid and renewable methyl alcohol fuel, have been regard to be a favorable option in footings of fuel use and provender strategies41. When correlative to hydrogen fed fuel cells, DMFC uses a liquid methyl alcohol fuel, which is easy stored, transported and simplii¬?es the fuel cell system. The attainment of fuel cell engineering depends mostly on the type of the electrocatalysts and membrane used.
While fuel cell engineering provides a compact reply for the demand to cut down pollution, the menace of oil depletion and the aspiration of many states to cut down foreign energy dependences from developed states still require some issues to be answered. Fuel cell fabrication costs are still really high for drawn-out consumer application. To add to this the unbounded application of H fuel cells requires distributed coevals and conveyance of H which is still really expensive. Unfortunately, H, being the lightest component, lacks the convenience of energy denseness, storage and widespread distribution of current fuels, i.e. gasolene, natural gas, etc. Soon, the portability of H for nomadic applications does non show itself as a really executable option due to its low denseness.
In the present reappraisal work, we study the accelerators used in fuel cells such as Pt and Pd accelerators, its working mechanisms of reactions for Pt and Pd accelerators towards the HER/HOR reactions.
The scientific discipline of contact action is encouraged by engineering as it has been from the beginning. Some of the earliest known illustrations of controlled chemical transmutations are catalytic 42. For illustration, before the 16th century quintessence was made by condensing liquors in the presence of sulphuric acid. In 1746, azotic oxide was used as a accelerator in the lead chamber procedure for oxidization of S dioxide to give sulfur trioxide in the industry of sulphuric acid. In 1781, acids were used to catalyse the transition of amylum into sugar. In 1817, H. Davy discovered that in the presence of Pt, mine gases were oxidized at low temperatures ; he designed a safety lamp for mineworkers in which the Pt glowed if the i¬‚ame was extinguished 43.
The term contact action was invented in 1835 by Berzelius. For illustration, agitations added in little sums were known to do possible the transition of works stuffs into intoxicant and there were legion illustrations of both decomposition and synthesis reactions that were seemingly caused by add-on of assorted liquids or by contact with assorted solids. Berzelius attributed catalytic action to ill dei¬?ned forces and the value of Ostwald ‘s more permanent dei¬?nition is that it identii¬?ed contact action as a phenomenon that was consistent with the emerging rules of physical chemical science. Now it is good recognized that accelerators function by organizing chemical bonds with one or more reactants, thereby opening up tracts to their transition into merchandises with regeneration of the accelerator. Catalysis is therefore cyclic reactants bond to one signifier of the accelerator, merchandises are decoupled from another signifier and the initial signifier is regenerated.
Catalyst is a substance which increases the rate of entree to equilibrium of a chemical reaction without being well consumed itself. A accelerator changes the rate but non the equilibrium of the reaction.
Catalysts may be solids, liquids or even gases. Most accelerators used in industrial engineering are either solids or liquids. Catalysis happening in a individual gas or liquid stage is referred to as homogenous contact action because of the uniformity of the stage in which it occurs. Catalysis happening in a multiphase mixture such as a gas-solid mixture is referred to as heterogenous contact action, normally this is surface contact action. The public presentation of a accelerator is measured mostly by standards of chemical dynamicss, as a accelerator ini¬‚uences the rate and non the equilibrium of a reaction. There are two types of accelerator viz. homogeneous and heterogenous contact action.
Homogeneous contact action
Homogeneous contact action associate to a catalytic system in which the substrates for a reaction and the accelerator constituents are brought together in one stage, most frequently the liquid stage. In other words, in this the accelerator is in the same stage as the reactants. Typically everything will be present as a gas or contained in a individual liquid stage. In this low temperature is required and separation are slippery. Some advantages of homogeneous contact action on an industrial graduated table are –
Ease of heat dissipation from exothermal reactions
in lab graduated table it is easier to analyze the mechanism of reaction
But there are besides some disadvantages of homogenous contact action on an industrial scale-
Scale-up can be dearly-won, hard, and unsafe
Separation is required
Heterogeneous contact action
Heterogeneous contact action involves the usage of a accelerator in a different stage from the reactants. Typical illustrations involve aA solidA accelerator with the reactants as eitherA liquids or gases. In this high temperature is required and design and optimisation is slippery.
A “ accelerator ” can be added to the reactants in a assorted signifier, the accelerator precursor, which has to be brought into an active signifier ( “ activated ” ) . During the catalytic rhythm the accelerator might be present in many intermediate signifiers when we look more closely at the molecular degree. An active accelerator will go through a figure of times through this rhythm of provinces ; in this sense the accelerator remains unchanged. The figure of times that a accelerator goes through this rhythm is the turnover figure. The turnover figure ( TON ) is the entire figure of substrate molecules that a accelerator converts into merchandise molecules. The turnover frequence ( TOF ) is the turnover figure in a certain period of clip. Substrates are present in larger sums than the accelerator because when we report on catalytic reactions the ratio of substrate to accelerator is an of import figure.
An inhibitor is a substance that retards a reaction. In a extremist concatenation reaction an inhibitor may be a extremist scavenger that interrupts the concatenation. In a metal catalyzed reaction an inhibitor could be a substance that adsorbs onto the metal doing it less active or barricading the site for substrate co-ordination. We besides talk about a toxicant, a substance that stops the catalytic reaction. A toxicant may kill the accelerator. The accelerator dies, we say, after which it has to be regenerated wherever possible.
`Organometallic accelerators consist of a cardinal metal surrounded by organic ( and inorganic ) ligands. Both the metal and the big assortment of ligands determine the belongingss of the accelerator. The success of organometallic accelerators lies in the comparative easiness of accelerator alteration by altering the ligand environment. Crucial belongingss to be influenced are the rate of the reaction and the selectivity to certain merchandises.
The undermentioned types of selectivity can be distinguished in a chemical reaction:
When two chemically different functionalities are present such as an olefine and an aldehyde in the illustration in Figure 2 which both can be hydrogenated, the chemoselectivity tells us whether the aldehyde or the olefine is being hydrogenated ; or when more than one reaction can take topographic point for the same substrate e.g. hydrogenation or hydroformylation.
Figure 2 Selectivity of chemical transitions.
As in the illustration shown for the hydroformylation reaction, the formyl group can be attached to the primary, secondary or the terminal C atom, internal C atom, which leads severally to the additive and the bifurcate merchandise.
In this the substrate contains a stereo genic centre and this together with the accelerator can direct the add-on of dihydrogen in the illustration to give two diastereomers, the selectivity for either one is called the diastereoselectivity
In this the substrate is achiral in this case but the enantio-pure or enantio-enriched accelerator may give rise to the formation of one specific merchandise enantiomorph.
The catalytic activity is a belongings of a accelerator that measures how fast a catalytic reaction takes topographic point and may be dei¬?ned as the rate of the catalytic reaction, a rate invariable, or a transition ( or temperature required for a peculiar transition ) under specii¬?ed conditions. The selectivity is a step of the belongings of a accelerator to direct a reaction to peculiar merchandises. There is no individual dei¬?nition of selectivity but it is sometimes dei¬?ned as a ratio of activities such as the ratio of the rate of a coveted reaction to the amount of the rates of all the reactions that deplete the reactants. Selectivity may besides be represented merely as a merchandise distribution. Because accelerators typically lose activity and/or selectivity during operation, they are besides assessed in footings of stableness and life-time. The stableness of a accelerator is a step of the rate of loss of activity or selectivity. In practical footings the stableness might be measured as a rate of inactivation, such as the rate of alteration of the rate of the desired catalytic reaction or as the rate at which the temperature of the accelerator would hold to be raised to counterbalance for the activity loss. Catalysts that have lost activity are frequently treated to convey back the activity.
Stability of Electrocatalysts
Platinum is widely used as a accelerator for chemical reactions. The most of import usage of Pt is in vehicles, as a catalytic convertor, easing the complete burning of unburned hydrocarbon go throughing through the exhaust 20. Platinum monolayer electrocatalysts offer a earnestly reduced Pt content while affording considerable possibilities for heightening their catalytic activity and stableness. These electrocatalysts comprise a monolayer of Pt on C supported metal or metal metal nanoparticles. The Pt monolayer attack has several alone characteristics, such as high Pt use and enhanced activity, doing it really attractive for practical applications with their possible for deciding the jobs of high Pt content and low efficiency apparent in conventional electrocatalysts 44.
Fuel cells are the shortest path to change over the chemical energy stored in molecular H to electrical energy. Since a fuel cell is an electrochemical device, hence, electrochemical methods are history to play of import functions in qualifying the fuel cell and its assorted constituents such as the accelerator, membrane and electrode. The electrochemical word picture methods include possible measure, possible expanse, possible cycling and revolving disc electrode. Some techniques acquired from these methods are besides used for fuel cell word picture. An electrochemical reaction fundamentally involves the stairss such as conveyance of the reactants to the surface of the electrode, surface assimilation of the reactants onto the surface of the electrode, charge transportation through either oxidization or decrease on the surface of the electrode and conveyance of the merchandises from the electrode surface. The desire of the electrochemical word pictures is to find the inside informations of the assorted stairss and likewise the word picture are carried out.
As this electrochemical transition does non depend on the heat of burning, some fuel cells may hold a high modification efficiency than the Carnot rhythm which operates conventional “ heat engine ” power workss. Molecular H is regarded to be the most promising of chemical fuels in footings of cut downing our dependance on fossil fuels, but the development of cheap, efficient, fuel-cell systems has non yet been realized on commercial footing to a big extend. Presently, fuel cells are based on the heterogenous, breakdown splitting of H on a Pt surface but these fuel cells have the apprehensible job that Pt is scarce and expensive 45-46. Till now few betterments in efficiency have been gained in over the old ages, so a new theoretical account for fuel-cell contact action is required to bring forth a fuel cell based economic system. Fuel cell development might be seized in an wholly new way by the debut of molecular accelerators capable of working in homogenous solutions. As molecular accelerators have the authorization of being extremely variable in footings of design and solution stage contact action is of import because it enables us to straight detect the inside informations of the mechanism in the below ( Figure 3 ) .
Figure 3. Direct observation of contact action 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 oxidization of the H and decrease of the O. The electrochemical reactions, decrease and oxidization, are catalyzed by the cathode and the anode. However, contact action in fuel cell systems is non bind merely to these electrochemical reactions. In existent systems there are different chemical reactions and associated contact action tie ining to treating the fuel to a signifier suitable for the fuel cells remotion of contaminations which damage the electrocatalytic activity of the electrode which convert the fuel H.
Types of Fuel Cell
Presently there are six fuel cells which are in research phases. All this cell systems are given below in Table 3. For the fuel cell types word picture and terminology of these is by the electrolyte and the correlative operating temperature. All these characteristics manage the necessity of the electrocatalysts which control the reactions. The Direct methyl alcohol fuel cell stands entirely in affecting a carbonous fuel ( methyl alcohol ) which is fed straight to the anode, whereas all others use H as the anode fuel, either as a H rich gas mixture or as a pure gas.
Fuel Cell Type
Operating Temperature in A°C
Potassium hydrated oxide
Proton exchange membrane
Solid proton carry oning polymer
Lithium carbonate mixture
Stabalised zirconium oxide
Table 3. Different Fuel cells.
Application of fuel cell
Fuel cell has broad application because of its rich belongingss as clean beginning of energy. It is widely used in Automobiles. In fact, nowadays about every auto maker has developed at least one paradigm vehicle and many have already gone through several coevalss of fuel cell vehicles. It is besides used in automotive vehicles like Water scooters and bikes. Fuel cell powered scooters and bikes utilizing either H stored in metal hydrides or methyl alcohol in direct methyl alcohol 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 transition of chemical energy to electrical energy in a PEM fuel cell occurs through a direct electrochemical reaction and it takes topographic point without burning. The of import portion 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 predate the mechanism in PEMFC, the membrane present must carry on H ions ( protons ) and separate either gas to go through to the other side of the cell. In the above Figure, it can be seen that H is delivered through the flow field channel of the anode home base to the anode in one the side of the cell. On the other side of the cell, O from the air is delivered through the channeled home base to the cathode. H2 is decomposed into positively charged protons and negatively charged negatrons at the anode whereas the negatrons. Positively charged protons pass through the polymer electrolyte membrane ( PEM ) to the cathode, whereas the negatively charged negatrons travel along an external circuit to the cathode, making 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 C fabric or C i¬?ber paper. At the interface between the porous electrode and the polymer membrane there is a bed with accelerator atoms, typically platinum supported on C. Electrochemical reactions happen at the surface of the accelerator at the interface between the electrolyte and the membrane. Hydrogen, which is fed on one side of the membrane, splits into its primary component ‘s protons and negatrons. In this splitting of Hydrogen molecule is rather easy utilizing a Pt accelerator. Each H atom consists of one negatron and one proton. Protons travel through the membrane, whereas the negatrons travel through electrically conductive electrodes, through current aggregators, and through the outside circuit where they perform utile work and come back to the other side of the membrane. At the accelerator sites between the membrane and the other electrode they meet with the protons that went through the membrane and O that is fed on that side of the membrane. Water is created in the electrochemical reaction and so pushed out of the cell with extra i¬‚ow of O. The consequence of these coincident reactions is current of negatrons through an external circuit direct electrical current.
Main constituents and stuffs
The chief intent 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 conveyance of protons. The other activity includes keeping the fuel and oxidizer separated that prevents blending of the two gases and defying rough conditions, including active accelerators, high temperatures fluctuations and reactive groups. Therefore, the ideal polymer must hold first-class proton conduction, chemical and thermic stableness, strength, flexibleness, low gas permeableness, low cost, and good handiness. Many different membranes have been tested for commercial usage in PEM fuel cells. The membranes are commonly polymers modified to include ions, particularly sulfonic groups. These hydrophilic Attic constituents are the key for leting proton conveyance across the membrane 48.
For H oxidization reaction ( HOR ) and oxidization decrease reaction ( ORR ) Platinum has been considered as best accelerator. Though there might be great difference between the HOR and ORR reactions when utilizing the same accelerator. Still great attempts are taken in research towards developing different accelerator stuff but still Pt is the best available. In many PEMFCs, the anode and the cathodes use same Pt accelerator. Normally, the Pt accelerator is formed into little atoms on a surface of well larger atoms that act as a protagonist. This is nil but C pulverization and largely used C based pulverization is Vulcan XC72A® ( by Cobalt ) . This manner the Pt is extremely divided and spread out, so that a really high proportion of the surface country will be in contact with the reactant, ensuing in a great decrease of the accelerator lading with an addition 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 topographic point. At the anode, H oxidization reaction occurs over which H gas base on ballss, whereas oxidization decrease reaction occurs at the anode over which the O base on ballss. The electrode reaction happening are as given below,
Anode Chemical reaction:
H2 a†’ 2H+ + 2e-
Matching to an anode possible Ea0 = 0 V ( under standard conditions ) versus SHE.
Cathode Chemical reaction:
1/2O2 + 2H+ + 2e- a†’ H2O
Matching to a cathode possible Ec0 = 1.229 V ( under standard conditions ) versus SHE. Therefore, the overall reaction of the fuel cell is
H2 + 1/2O2 a†’H2O
With the equilibrium criterion electromotive force calculated to be 1.229 V.
Hydrogen Oxidation Reaction
At the anode, H is stripped of its negatrons and becomes protons and negatrons. For electrochemical reactions, even if a simple one negatron reaction is non that simple and is ever with a reaction mechanism affecting several stairss.
The overall reaction rate depends on the slowest simple reaction, which is called the rate finding measure. The stairss of H2 oxidization on Pt electrode include the followers:
H2 + Pt a†’ Pt-H2
Pt-H2 a†’ Pt-Hads
Pt-Hads a†’ Pt + H+ + e-
Platinum based accelerators are widely used as the anodal electrode stuff for H oxidization. The HOR on Pt accelerators has lower oxidization over possible and a higher kinetic rate. The evident exchange current denseness of the HOR has been calculated to be i0anode = 0.1 Acm-2 which is high when compared with ORR i0cathode = 6 AµAcm-2 which is obtained from the ( EIS ) electrochemical electric resistance spectrometry measurings done by Wagner50. This proves the utmost fast reaction dynamicss of HOR. The tabular array shows the exchange current densenesss of the H development reaction at different electrode stuffs in aqueous 1 M H2SO4 solution at ambient temperature.
Exchange current denseness
1.0 A- 10-3
8.0 A- 10-4
2.5 A- 10-4
2.0 A- 10-4
7.0 A- 10-6
Table 4. Exchange current densenesss of the HOR reactions at different electrode stuffs in aqueous 1M H2SO4 solution at ambient temperature51.
But for many practical applications, the presence C monoxide ( CO ) hints in the H gas mixture produced by the reforming of other fuels is must. Carbon monoxide can strongly adsorb on the Pt accelerator in the anode. The adsorbed CO even mere hints ( 10 ppm ) blocks the catalytically active country, thereby comparatively diminishing its responsiveness and doing “ CO toxic condition ” . Due to this, anode accelerator in PEM fuel cells has to demo non merely high catalytic activity toward H oxidization but besides enhanced activity in the presence of CO. The alternate option for CO tolerant accelerators has been a demanding undertaking in the successful development of more efficient PEMFC systems.
Electrocatalysis of Hydrogen Oxidation Reaction
The electrocatalysis of the HOR is one of the of import countries in fuel cell applications. In general, electrocatalysis can be considered a specific type of heterogenous contact action whereby reactants and merchandises adsorb onto the accelerator surface during the reaction procedure. Hydrogen is an of import stuff and merchandise in chemical industries and has been investigated as a new clean energy beginning for many decennaries 52-54. With the development of proton exchange membrane ( PEM ) fuel cell engineering, in which H is used as a fuel, the chemical energy stored in this H can be electrochemically converted to electric energy with zero emanations and high efficiency. From early 1990s, the advantages of PEM fuel cells, including low emanations, high energy efficiency and high power denseness have attracted global research and development in many of import application countries, including automotive engines, stationary power coevals Stationss, and portable power devices55. The major cost of a PEM fuel cell is the Pt ( Pt ) based accelerators. Based upon the current technological phase, these Pt-based accelerators for both the cathodic O decrease reaction ( ORR ) and the anodal H oxidization reaction ( HOR ) are the most practical accelerators in footings of catalytic activity and life-time stableness.
With regard to fuel cell contact action, most research has been focused on cathode ORR catalysts development as the ORR dynamicss are much slower than the anodal HOR dynamicss, in other words, the fuel cell electromotive force bead polarized by burden is due chiefly to the cathode ORR over potential56. However, in some instances the over-potential of the anodal HOR can besides lend a non-negligible part of the overall fuel cell electromotive force drop57. Therefore, the catalytic HOR on the fuel cell anode accelerator is besides really of import of possible usage of H as a hereafter fuel. Apart from its importance in fuel cell applications, H electrooxidation contact action is besides a theoretical account system for the cardinal apprehension of electrochemical dynamicss and electrochemical surface science58-59.
Undoubtedly the H evolution/oxidation reaction ( HER/HOR ) is the simplest and most widely studied electrochemical procedure. Almost all the basic Torahs of electrode dynamicss and the constructs of electrocatalysis were developed and verified by below two reactions.
The undermentioned describes the dynamicss and mechanisms of the electro-catalyzed HOR on different electrode stuffs, including Pt group metals, carbides, and passage metals. Despite its broad scope of subjects, the chief intent of this chapter is to supply a cardinal apprehension of the electrocatalysis of the HOR, the most of import reaction other than the ORR in the PEM H fuel cell.
Electrooxidation of Hydrogen
Mechanism of the Hydrogen Oxidation Reaction
The overall reactions of anodal H oxidization in acidic and alkalic mediums may be expressed by following two equations ;
A? H2 a†’ H+ + e- ( 1 )
A? H2 + OH- a†’ H2O + e- ( 2 )
The H oxidization reaction may happen by the undermentioned three consecutive stairss:
1 ) Adsorption measure:
In this the H molecule diffuses from the electrolyte to the electrode, so adsorbs on the electrode surface to organize surface species ( H2, ad ) :
H2 a†’ H2, colloidal suspension a†’ H2, adsorp ( 3 )
2 ) Hydration/ionization measure:
In this the adsorbed H signifiers adsorbed H atoms ( Hadsorp ) through procedure ( a ) or ( B ) :
( a ) Tafel-Volmer path
H2, surface assimilation a†’2Hadsorption ( Tafel reaction ) ( 4 )
And ( B )
Hadsorp a†’ H+ + e- ( 5 )
Hadsorp + OH- a†’ H2O + e- ( 6 )
This is Volmer reaction in alkalic medium
( B ) Heyrovsky-Volmer path:
( B ) Heyrovsky-Volmer path:
H2, adsorp a†’ Hadsorp.H+ + e- a†’ Hadsorp + H+ + e- ( 7 )
( This is Heyrovsky reaction in acidic medium )
H2, adsorp + OH- a†’ Hadsorp.H2O + e- a†’ Hadsorp + H2O + e- ( 8 )
( The above equation is Heyrovsky reaction in alkalic medium )
Hadsorp a†’ H+ + e- ( 9 )
( Volmer reaction in acidic medium )
Hadsorp + OH- a†’ H2O + e- ( 10 )
( Volmer reaction in alkalic medium )
3 ) Desorption measure:
In this the merchandises, such as H+ and H2O are desorbed and so transported into the electrolyte. In each measure of the above paths, the overall reaction rate can be controlled by a measure which is sufficiently slow compared with the others. This is the rate finding measure ( rds ) . The rate finding stairss have been identified for several mechanisms:
( a-1 ) The slow Volmer-rapid Tafel mechanism ( the slow-discharge mechanism ) :
H2 a†’ 2Hadsorp, Hadsorp a†’ H+ + e- ( rds ) ( 11 )
( a-2 ) The rapid Volmer-slow Tafel mechanism ( the slow combination or the catalytic mechanism ) :
H2 a†’ 2Hadsorp ( rds ) , Hadsorp a†’ H++ e- ( 12 )
Had + OH- a†’ H2O + e- ( 13 )
( b-1 ) The slow Volmer-rapid Heyrovsky mechanism:
H2, adsorp a†’ Hadsorp.H+ + e- a†’ Hadsorp + H+ + e- ( rds ) ( 14 )
H2, adsorp + OH- a†’ Hadsorp.H2O + e- a†’ Hadsorp + H2O + e- ( rds ) ( 15 )
( b-2 ) The rapid Volmer-slow Heyrovsky mechanism:
H2, adsorp ( rds ) a†’ Hadsorp.H+ + e- a†’ Hadsorp + H+ + e- ( 16 )
H2, adsorp + OH- ( rds ) a†’ Hadsorp.H2O + e- a†’ Hadsorp + H2O + e- ( 17 )
In the yesteryear, the bulk of the basic Torahs and constructs in electrode dynamicss were developed and verified by Tafel60, Volmer61 and Frumkin62 utilizing the H electrode. Two of import reaction mechanisms are good recognized and by experimentation validated. The first is the Volmer-Tafel mechanism, shown in Equations ( 11-13 ) . The other, which is more of import for the H electrode, is the Heyrovsky-Volmer mechanism, expressed in Equations ( 14-17 ) .
Thermodynamic Considerations for the Hydrogen Electrode Reaction
The conventional thermodynamic dealingss for the H electrode reaction are as follows ;
EH = –
a?†G = A? AµH2 – AµH+ = ( A? AµA° H2 – AµA°H+ ) + RT ln PH2A? / aH+
EH = EA°H – – ln ( PH2A? / aH+ )
Where a?†G is the Gibbs free energy, AµH2 and AµH+ are the chemical potencies for H2 and H+ , aH+ is the activity of the proton, F is the Faraday changeless and aH+ has a practical significance through the conventional definition of pH. EA°H is defined as the criterion H electrode possible with EA°H = 0 V at standard conditions63.
Electrocatalysis of Hydrogen Oxidation
The electrocatalysis of the HOR is one of the of import countries in fuel cell applications. Most significantly electrocatalysis may be contemplating a specific type of heterogenous contact action by which reactants and merchandises adsorb onto the accelerator surface during the reaction procedure. The reactants get activated by interaction with the accelerator surface and are quickly selectively converted to adsorbed merchandises.
Since the catalyzed electrochemical reaction occurs at the catalyzed electrode interface, the intrinsic kinetic rate of an electrochemical reaction ( measured by the exchange current denseness ) strongly depends on the possible difference between the accelerator surface and the electrolyte and every bit good as on the sort of accelerator and its surface morphology. For electrode reactions, the exchange current denseness can change from about 10-3 A.cm-2 at a Pt electrode to 10-12 A.cm-2 at a quicksilver electrode for the anode reaction ( HOR ) 64. Under normal conditions, the HOR on Pt is about 5 to 7 magnitudes more rapid than the ORR and has one of the fastest known specific rate invariables in aqueous solutions65. Since the pioneering work of the sixtiess, when the dependance of H surface assimilation upon the crystallographic orientation of the Pt surface was discovered, the survey of the relationship between electrochemical activity and surface construction has been the chief subject of electrochemical research66.
Platinum and Platinum Group Metals ( Pt, Pd, Ru, Rh, Ir and Os )
Adsorption and desorption of reactants on a accelerator surface can straight impact its catalytic ability so it is normally expressed as a “ Volcano curve ” plotted to correlate the exchange current denseness of the HOR with the heat content of H surface assimilation. If the heat content is excessively little, a slow surface assimilation kinetic will ensue and restrict the rate of the overall reaction, on the other manus, if the heat content is excessively high so desorption of H becomes hard.
Consequently, this H desorption measure will go a rate finding measure within the overall reaction. As a consequence an intermediate value in the heat content of Hydrogen surface assimilation on a accelerator is required in order for it to be an active accelerator. As shown in below Figure 5 Pt and Pt-group metals all have intermediate values of H surface assimilation and show high catalytic activities67. Platinum group metals including Pt, Pd, Ru, Rh, Ir and Os have long been known as accelerators for both the ORR and the HOR68. On Pt group metal surfaces the chemosorption of H can easy take the adsorbed O with the formation of H2O at room temperature which does non normally occur on other passage metals as a consequence they bind oxygen excessively strongly69. Another feature of Pt group metals is their ability to disassociate H2 in the presence H2O.
Figure 5. Volcano secret plan for electrocatalysis of the H reaction, in footings of ( logi0 ) as a map of the heat content of H surface assimilation on assorted accelerators
Pt ( hkl )
Platinum shows the high exchange current denseness in all the Pt group metals that are the most electro catalytically active electrode stuffs for the H oxidization reaction70. The extended attempt in HOR electrocatalysis has been attracted on understanding rate dependence on the atomic graduated table morphology of a Pt individual crystal surface. Virtually all the early kinetic surveies of the HER/HOR were carried out on polycrystalline Pt electrodes71. And to some extend dynamicss surveies were besides carried on Pt individual crystals that had ill defined surface structures72. In the early surveies, the dynamicss of the HER/HOR on Pt ( hkl ) was reported to be insensitive to the surface crystallography. Now merely the contact action surveies on chiseled Pt single-crystal electrodes clearly demonstrated that the HER/HOR dynamicss on Pt ( hkl ) vary with a crystal face that has “ construction sensitiveness ” 73. This sensitiveness is chiefly caused by the structure-sensitive surface assimilation of H and electrolyte anions.
Supported Pt Catalysts
Since accelerator public presentation for the HOR is strongly dependent on the entire active surface country, supported accelerators have been developed to maximise the accelerator surface country. When drawn comparing to bulk Pt accelerators, supported accelerators show higher activity and stableness due to all right scattering, high use, and stable nanoscale metallic atoms.
Till now, C black stuffs are the most normally used C supports for PEM fuel cell accelerators. Supported accelerators have several advantages over unsupported accelerators such as they have comparatively higher stableness than unsupported accelerators in footings of agglomeration under fuel cell runing conditions. The good electric conduction of the C support allows electron transportation from catalytic sites to the conductive C electrodes and so to the external circuit and the little dimensions of accelerator atoms ( nanoscale ) dispersed on a C support can maximise the contact country between accelerator and reagents74.
Assorted C inkinesss show different physical and chemical belongingss and they are normally manufactured by pyrolyzing hydrocarbons. Both these belongingss have strong effects on the belongingss of the supported metal accelerators such as morphology, metal atom size, stableness, size distribution and scattering. And so in order to optimise the public presentation of the accelerator, a suited C black support has to be down selected taking into consideration belongingss such as specific country, porousness, morphology, corrosion opposition, etc. One of the type of C support used is Nanostructured C such as C nanotubes ( CNTs ) 75. Carbon nanotubes are a compact sort of stuff for accelerator support in fuel cell contact action applications because of their alone electrical and structural belongingss. Assorted surveies have shown that Carbon nanotubes are superior to carbon inkinesss as accelerator supports for PEM fuel cells76. Another illustration of fresh accelerator support is the ultra-thin nanostructured ( NS ) movie system. In this accelerator support ( NS ) movie system consists of a unambiguously structured thin movie composed of extremely oriented, dumbly jammed crystalline organic beards in which the accelerator atoms are deposited by vacuity surfacing methods ( Figure 6 ) .
Figure 6. SEM images of Pt coated nanostructured beards. To the Left: the plane position at 10,000 magnification ; To the right: 45A° angle position at 150,000 magnification77.
Due to the definite and extremely governable nature of the oriented thin movie the procedure for doing a accelerator supported on this thin movie inherently consequences in highly high grades of accelerator uniformity and though this uniformity may non be accomplishable for the heterogenous surface of a conventional C black support78.
Controllable synthesis of baronial metal
Case of Palladium
The Pd H ( Pd-H ) system has engaged a batch of activities as it is one of the first of all metal H systems and has been investigated extensively [ 1 ] . It has been found that Pd shows definite accelerator activity for H coevals so that the surface assimilation of H on different Pd metal surface has been the topic of both experimental and theoretical probe [ 2, 3 ] . Palladium nanomaterial is well-known for its singular capacity in H soaking up so they are largely used in car pollutants, H2O interventions, and hydrogenation reactions and in different organic reactions. ( Rwf ) Palladium nanomaterial besides plays a critical function in fuel cell engineering. Recent survey farther concedes that Pd Nano-materials exhibit good surface-enhanced Raman sprinkling ( SERS ) and feeling activity.
In all of these applications, the size and form of Pd nanomaterials are still critical parametric quantities in order to maximise their public presentation. Therefore, governable synthesis of Pd nanomaterials is extremely desired for orienting their catalytic belongingss and besides a requirement for accomplishing their high public presentation in assorted catalytic applications. At present, high-quality Pd nanomaterials with different forms have been facilely obtained through a kinetics-controlled or thermally controlled procedure.
Case of Platinum nanomaterials
Because of their high catalytic activity, Pt nanomaterials have been widely applied in many i¬?elds including fuel cells, detectors, and the crude oil and automotive contact action.Given that Pt is a cherished and rare metal ; most of the recent attempts have emphasized the decrease of Pt use through increasing the catalytic efi¬?ciency of Pt accelerators. A figure of surveies reveal that the catalytic responsiveness of Pt nanomaterials depends extremely on their morphology, and hence the design of fresh Pt nanomaterials with alone morphologies has been greatly intensii¬?ed due to their possible for enhanced and new belongingss and applications in the last decennaries.
Recent signii¬?cant development in nanomaterials synthesis has led to the formation of assorted sorts of nanomaterials with controlled size, form, composing, inter-particle interaction and loanblend. These functional nanomaterials provide good chance for developing extremely active accelerators for fuel cell reactions. By and large talking, among them, Pt and Pt-based nanomaterials are still the most effectual electrocatalysts for fuel cell applications, which catalyze H or little molecule oxidization at anode and ORR at cathode. Herein, from the position of heightening the electro activity of accelerators, some recent push in developing high-efi¬?ciency Pt-based nano electrocatalysts for fuel cell applications are highlighted.