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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. Platinum and Palladium are important catalysts for industrial applications. They exhibit similar catalytic properties and are widely used in fuel cells as catalysts for direct transformation of the chemical energy of a fuel into electricity by electrochemical reactions and are one of the vital enabling technologies for the change to a hydrogen based economy. Among several types of fuel cells that are available in the market, polymer electrolyte fuel cells or proton exchange membrane (PEFCs) have been considered as a capable future power source for zero emission vehicles. Platinum and palladium catalysts are been studied more frequently for hydrogen oxidation and hydrogen evolution reactions. As demand for fuel cell is increased day by day, research has been focused on improving the quality of catalyst used in fuel cell. Hydrogen oxidation and hydrogen evolution reactions in fuel cell are very important for electrochemical reaction from practical aspects and as well from point of view of hydrogen economy. As platinum electrocatalyst is expensive so there are efforts carried out to replace with less expensive catalyst such as palladium. This review investigates the mechanism of platinum catalyst for fuel cell and also their hydrogen reactions.
Currently the need for clean and viable energy sources have become a strong instigates force in continuing economic development. Proton exchange membrane (PEMs) fuel cells which act as clean energy converting devices have drawn plenty of attention in recent years due to its high efficiency, energy density and low emissions. Fuel cells are the energy converting devices with a high efï¬ciency and very low emission. Fuel cells have many vital applications in areas such as transportation, stationary power and micro power. The fuel cells are nothing but an electrochemical device that changes the chemical energy of a reaction into electrical energy. It employs various catalysts, however, platinum (Pt) based catalysts are the most used in fuel cells. As these catalysts are expensive in terms of cost, so a great deal of attempt has been implanted in the exploration of cost effective and active 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 layer(J Larminie and A Dicks, 2002). Thus the electrocatalysts and their corresponding catalyst layers play important roles in fuel cell performance. In current trend the most used catalyst is Platinum (Pt), as splitting of hydrogen molecule is easy in Proton Exchange Membrane fuel cell. However, splitting of oxygen molecule is relatively difficult which causes electricity losses. So, as no other alternative catalyst is developed Pt is the best available. The attainment of fuel cell technology depends largely on the type of the electrocatalysts and membrane used.
Noble metal catalystsÂ such as platinum, palladium, ruthenium and rhodium are widely used catalyst in fuel cell for different reactions as Pt is the most efficient catalyst and also they 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. Besides this nano sized particles have high surface area thus having more chances of 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(M.C. Daniel and D. Astruc, 2004, C. Cobley et al., 210, Sau and Rogach, 2010, X.M. Lu et al., 2009, Y. Xia et al., 2009, A. Tao et al., 2008a, R. Murray, 2008, Z. Peng and H. Yang, 2009, A. Tao et al., 2008b, J. Chen et al., 2009, S. Skrabalak et al., 2008, B. Wiley et al., 2007). Nanoparticle's unique property of larger surface to volume ratio makes them to aggregate with their bulk counterparts, as a reason they have gained high importance in catalytic applications(P. L. Freund and M. Spiro, 1985, L. N. Lewis, 1993, M. Moreno Manas and R. Pleixats, 2003, R. Narayanan and M. A. El Sayed, 2005). 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(M. Baldauf and D. M. Kolb, 1996, R. Narayanan and M. A. El Sayed, 2004, K.M. Bratlie et al., 2007). Both platinum and palladium are important catalysts for many industrial processes where they exhibit similar catalytic activities (C. R. Henry, 1998). 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 (M. Arenz et al., 2005, A. T. Bell, 2003, J. G. de Vries, 2006, D.Astruc, 2007). 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 (B. C. Gates et al., 2008).
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 has been research studies carried out on their properties in many reactions. Platinum (Pt) based electrocatalysts are usually applied in Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). 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 operations.
In reality the data regarding the fundamental factors such as state and property of carbon supported catalyst is unknown. The last decade showed an increasing interest in this particular subject. However, mainly the relationship was under study between characteristics of support (predominantly, the functional coverage of the surface) and dispersion state of resulting metal but not their catalytic properties. Besides, non-porous carbon blacks were mainly used in the scientiï¬c studies, whereas activated carbons are the typical supports in use. 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 makes traditional carbon supported catalysts a difï¬cult and unattractive topic for exploration.
In order to reduce the widen gap between the existing and required knowledge on the preparation of carbon supported catalysts with platinum group metals, studies on Pt and Pd catalysts on different activated carbons are required. These studies were described by properties of catalysts on ï¬ve activated carbons especially selected to investigate a possible inï¬‚uence on porous structure of the support (L.B.Okhlopkova et al., 2000). The catalyst preparation was restricted to use chlorides of platinum and palladium as the starting metal complexes, their deposition on the supports by the adsorption method and reduction in hydrogen flow. Carbon supported Pt and Pd catalysts have been synthesized and used in PEM hydrogen fuel cell anodes (Y.Hu Cho et al., 2007). Electrocatalysts based on Pt and Pd deposited over charcoal has been prepared and tested by Escudero (M.J. Escudero et al., 2002) on the anodic side of PEM fuel cells. The assessment of the electrochemical activity of Pd catalysts on carbon in alkaline solutions for the oxidation of hydrogen and methanol was reported by Pattabiraman (R. Pattabiraman, 1997). Also the kinetics of hydrogen evolution reaction at palladium in alkaline solution has also been studied by Green and Britz. (T Green and D Britz, 1996). Palladium is used as a catalyst in the fuel cells to power vehicles such as cars and buses as it is one of the cheapest and readily available catalysts when compared with platinum. 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 use of platinum and palladium catalyst is immense in fuel cell research and Pt has been the most common choice of catalyst for both HOR and ORR reactions. The demand of these catalysts is largely defined by the unique nature of chemical processing operations. The production of a product in the pharmaceutical industry is usually counted 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.
Figure . Schematic representation of anode and cathode in a fuel cell (Anon)
Figure 1 shows the schematic figure of anode and cathode in a 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 in a PEM fuel cell. HOR and ORR reactions are important reactions particularly HOR is more as it is providing hydrogen as a source of energy fuel. But in this literature study, more focus is given on HOR reactions occurring in a PEM fuel cell. In the present technology, the mostly employed catalyst in PEM fuel cells are highly distributed Pt based nano particles. Also, Pt based catalysts are sensitive to various contaminants together with high cost. In need for new catalyst at low price and with properties similar as Pt based catalyst, research have taken various options such as using platinum group metal like Pd based catalyst. In spite of all these things, there has been no such alternate until date and research is still moving forward for new option, as the catalyst activities and stabilities of other catalyst to be used as a replacement are low to be practical. Another approach is to decrease Pt loading in a catalyst layer by using carbon supports and achieving elementary knowing of catalyst structures and correlating mechanisms of catalytic reactions. To design new catalysts there is a need of theoretical and practical approach that will explore the structure activity. 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) (A Zuttel et al., 2008). 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. In order 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 (N.M Markovic et al., 1997, N.M. Markovic and P.N. Ross, 2002, K.Kunimatsu et al., 2005, B.E.Conway and G.Jerkiewicz, 2000, J.Barber et al., 1998, M.C.Tavares et al., 2001).
Fuel cells offer efficient and virtually pollution free energy conversion and power generations. The reality that fossil fuels are getting extinct 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 (C.J.Zhong et al., 2008). 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 (approx. 30%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.
Electrocatalysis is an important aspect for fuel cells to generate hydrogen source before catalyst injection involve. Carbon monoxide derived 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 difficult to realise. Many types of electrocatalysts have been considered over years for their various applications to fuel cells. The nature of appropriate electrocatalysts is dependent on the nature of the fuel cell. The high temperature molten carbonate and solid oxide fuel cells (MCFC and SOFC) results in difficulties of thermal stability as well as compatibility with the electrolyte. Currently preferred electrocatalysts for the various cells are listed in Table 1.
Table 1. Electrocatalysts for the various cells (G.J.K.Acresa et al., 1997)
Pt/Au, Pt, Ag
Pt/Au, Pt, Ag
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. In general hydrogen fuel cell employs catalyst Pt which is rare and expensive. In addition to Pt catalyst there are few alternatives such as Pd and Rh. Most other catalyst cannot withstand fuel cells high acidic condition in the reaction for converting chemical energy into electrical power. There are only four elements which can resist to the corrosive environment. These elements are Platinum, palladium, gold and iridium. 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 with highly acidic solvents present in the chemical reaction within fuel cells. CarbonÂ supportedÂ platinumÂ is commonly used as anode and cathode electrocatalysts in low temperature fuel cell fuelled with hydrogen. The cost of Pt and the rarity of this catalyst have created a significant barrier for use of this type catalyst in fuel cell. Moreover,Â presence of catalyst platinumÂ in anode material is readily poisoned byÂ carbon monoxide (CO). However, Pt alone does not give sufficient activity for the ORR when used in cathode. So, to avoid this binary and ternaryÂ platinum basedÂ catalystsÂ and non-platinum based catalyst have been tested in low temperature fuel cells as an electrode material. The activity for the ORR of Palladium (Pd) is slightly lower than that of platinum (Pt) but by adding metals such asÂ Iron (Fe), the ORR activity of Pd can be achieved by using Pt. Contrary the activity for the HOR ofÂ PdÂ is considerably lower than that ofÂ Pt but by enumerating a small percentage of Platinum, the Hydrogen oxidation reaction activity of Pd attains same as that of Pt.
Figure . Polymer Electrolyte Fuel Cell (PEFC) (R.Bashyam and P.Zelenay, 2006)
A simplified illustration of the Polymer exchange fuel cell operation is shown in Fig. 2. It operates with a polymer electrolyte membrane which divides the fuel hydrogen which is used from the oxygen. Novel metal catalysts, essentially platinum (Pt) supported on carbon are accepted for both the oxidation of hydrogen and reduction of the oxygen in a temperature between 80 to 100Â°C. 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) by using liquid and renewable methanol fuel have been regarded to be an obvious choice in terms of the fuel handling (S. Wasmus and A. Kuver, 1999). 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.
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. In addition, 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. At present, the portability of hydrogen for mobile applications does not represent itself as a very feasible option due to its low density. In the following literature 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.
Catalyst is a substance which accelerates the rate of access to counterbalance the chemical reaction without being appreciably consumed in it. A catalyst changes the rate but not the equilibrium of the reaction (J.H.Sinfelt, 1984). Catalysts may be in solids, liquids or even gases form. Most catalysts used in industrial technology are either solids or liquids form. Catalysis occurring in a gas or liquid phase is termed as homogeneous catalysis. This is termed as homogenous catalysis because of the closeness of the phase in which it occurs. Catalysis which occurs in a two phase mixture such as a gas and solid mixture is termed as heterogeneous catalysis; this phenomenon is also known as surface catalysis. The completion of a catalyst is analyzed by conditions of chemical kinetics, as a catalyst inï¬‚uences the rate of a chemical reaction and not the equilibrium state.
The science of catalysis is very vast and encouraged by revolution of technology. Examples of catalysis include different controlled chemical transformations which are catalytic in nature (A.Mittasch, 1939). From long ago, beginning of 16th century catalysis has been performed. In 1781, various acids were used to catalyze the change of starch into sugar. H. Davy in 1981 come across with theory that oxidization of mine gases at low temperature is possible in presence of Platinum catalyst (A.Mittasch and E. Thies, 1932).
The term catalysis was discovered by Berzelius in 1835. It is known that catalysts work, when they form chemical bonds with one or more reactants, thus opening up pathways to their conversion into products with regeneration of the catalyst. It can deduced that catalysis is nothing but cyclic reactants bond to give one form of the catalyst and then the products are decoupled from other form and the initial original form is regenerated. Without the presence of catalysts, various chemical reactions of importance would proceed so slowly that they could not even be detected although the reaction conditions (temperature and pressure) are thermodynamically favorable for the occurrence of the reactions. There are different types of catalysis as it can be either homogenous or heterogeneous catalysis. Some of the types of catalysis are mentioned below.
Homogeneous catalysis is usually linked with catalytic systems in which the substrates for a particular reaction and the components are coupled together in one phase. Usually they are brought closer in 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. In this type of catalysis the reactants are dispersed on the surface of catalyst and then subsequently adsorb through the formation of chemical bonds. As reaction if approaching, the product desorbs from the surface of catalyst and diffuses away. For solid heterogeneous catalysis, the surface area of the catalyst is demanding since it determines the availability of catalytic sites. Surface areas can be large or even less but more surface area is needed for good catalyst activity. The commonly approach to increase catalyst surface area is by the use of catalyst supports.
Electrocatalysis is very important from electrochemistry point of view, particularly in fuel cell engineering operations. Various metal containing catalysts are used to increase the rates of the half reactions that affect the fuel cell performance. One of the most common types of fuel cell electrocatalysts is based upon Pt nano sized particles. Nano sized Pt particles when comes contact with one of the electrodes in a fuel cell, it increases the rate of oxygen reduction to water.
`Organometallic catalysts abide in central metal which is surrounded by organic and inorganic ligands. The metallic catalyst and the array of ligands determine the properties of the catalyst. The attainment of these metallic catalysts thrives upon the relative inactivity of catalyst modification by adjusting the ligand environment. The critical properties to be influenced here are the rate of reaction and the selectivity of the material. The following type of selectivity can be categorized to products as through chemoselectivity, regioselectivity, diastereoselectivity and enantioselectivity. The selectivity in catalysis is one of the most important factors to be governed carefully for good activity of catalyst. Selectivity can be controlled in several ways such as by structural, chemical, electronic, compositional, kinetic and energy considerations. Certain factors may be more important in homogeneous catalytic reactions rather than heterogeneous reactions and vice versa.
In chemoselectivity, when two dissimilar chemical functionalities are occurred like in example given below, an alkene and an aldehyde can be hydrogenated. Thus chemoselectivity draws the fact that whether the aldehyde or the alkene is being hydrogenated or when more than one reaction can take place for the same given substrate e.g. hydrogenation or hydroformylation.
Figure Selectivity of chemical conversions (Piet W.N.M. van Leeuwen, 2004)
In regioselectivity, as given in above reaction for the hydroformylation reaction, the formyl group can be linked to the primary carbon atom, secondary carbon atom and to the internal carbon atom which leads to the continuous and the branched product. In diastereoselectivity, the substrate contains a stereo genic center and this when combined with the catalyst can link the addition of dihydrogen as in the above example to give two diastereomers. The selectivity for either one is termed as diastereoselectivity. In enantioselectivity, the selectivity the substrate is in achiral form. Here the enantio pure or enantio enriched catalyst can rise to the formation of one specific product.
The catalytic activity is an effect of catalyst reactions which helps to measure how rapid a reaction can take place. These catalyst reactions are known as the rate of the catalytic reaction or a conversion under speciï¬ed circumstances. As described earlier, the selectivity is an amount of a catalyst property to link a reaction to defined products. Sometimes selectivity works as a product distribution. This is because catalysts normally lose selectivity during operation, thereby, catalyst is also determined in terms of longevity. The determination of a catalyst is the rate of loss of selectivity of a catalyst. In industrial applications, the stability is measured either as the rate of change of the required catalytic reaction or as the rate of change during which the temperature of the catalyst is increased to satisfy for the activity loss. Catalysts that lose its activity are often regenerated to bring back the original activity.
Promoters are substances responsible to increase a catalyst result during various chemical reactions to boost product rate. An inhibitor is a substance that retards a reaction as it acts as a blockage. Inhibitors occurring in a radical chain reaction can be in a radical scavenger that creates blockage for the chain reaction. In a catalyzed reaction an inhibitor could be a substance that adsorbs onto the metal surface making it less active site for substrate linked up.
Principle of catalysis
A catalyst reacts with either one or more reactants to form intermediary reactions that consequently results in the final reaction product. This process is nothing but regenerating the catalyst. The following is a common reaction scheme, where C exhibit the catalyst, A and B are reactants and Z is the product formed from the reaction of A and B:
A + C â†’ AC (1)
B + AC â†’ ABC (2)
ABC â†’ CZ (3)
CZ â†’ C + Z (4)
Although the catalyst is absorbed by reaction (1), it is simultaneously generated by reaction 4, so for the overall reaction:
A + B â†’ Z (5)
As a catalyst is regenerated again and again in a reaction, only some amounts of it are needed to increase the reaction rate.
Catalysts are more active when they are small sufficient to fall within the range of 1-100 nanometers (nm) in size. The active sites of a catalyst are more easily freely available in nano-scale which causes chemical reaction to work fast. All reactants combine to form a product I active site area of the catalyst, which is on the edge or surface of the catalyst.
Role of Surface Phenomena in catalysis
Surface science is always in relation with catalysis. The association is not with catalysis in general, but rather with the particular branch of the subject known as heterogeneous catalysis. The heterogeneous catalysis refers to the fact that the reactants are present in one phase and the catalyst in another, with the catalytic action occurring at the interface or surface between them.
Catalyst characterization techniques
Catalyst characterization is important in order to investigate the internal structure and properties of materials. Characterization can take various forms such as actual materials testing or analysis of a material. Different analysis techniques are used to visualize the internal structure and distribution of different elements associated within the material. Catalysts used in fuel cells are in the range of nano size range and are usually deposited on the high surface area. All these are done by magnification and internal visualization which is done with the help of microscope such as electron microscopy which includes Scanning Electron Microscope (SEM) and Transmission electron microscope (TEM).There are many different forms of microscopy, some of which will be discussed here. Microscopy is an excellent technique as it allows direct examination of the particles in question and shape and size information to be determined. However, there are some disadvantages such as the sample must usually be dried out. Also relatively few particles are examined and there is a real danger of unrepresentative sampling and microscopy cannot examine the size continuously so cannot be put in a production line.
In electron microscopes, a beam of extremely energetic electrons is used to examine objects in a manner of fine scale. The advantage of using electron microscopy is that it provides information on composition, morphology and topography. An electron microscope uses electrons to illuminate a specimen and create an enlarged image. Electron microscopes have much greater resolving power than light microscopes. Some electron microscopes can magnify specimens up to 2 million times, while the best light microscopes are limited to magnifications of 2000 times. Both electron and light microscopes have resolution limitations, imposed by their wavelength. The electron microscope makes use of the wave nature of the electron and the fact that electric and magnetic fields of suitable geometry can act like lenses to refract, deflect and focus an electron beam. However, elaborate sample preparation is required and the method is slow. Electron microscopy is a high vacuum technique, though the development of environmental microscopes is reducing this limitation.
Scanning Electron microscopy (SEM)
This instrument is particularly used for the generation of topographical images and elemental information due to higher resolution. The scanning electron microscopy causes light with a wavelength of 0.12 Angstroms, resulting in resolution limit of 1 million diameters with exaggeration of 10x to 10,000x with nearly countless depth of field. In this method an electron beam is focused to 5-10 nm and is then deflected in a regular manner across the surface of the sample which is held at an angle to the beam. The low velocity secondary electrons that are emitted as a result are drawn toward a collector grid and fall onto a sensitive detector. An image is generated by scanning the beam across the surface. The eye interprets the image as truly three dimensional. The depth of the field is 300-500 times greater than for a light microscope.
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Figure . Scanning Electron microscopy (Anon).
Transmission electron microscopy
This technique is applied to thin samples or their replicas. The transmission electron microscopy (TEM) gives the advantage of increased magnification and resolution. In transmission electron microscopy an accelerated electron beam is passed through a thin sample 50 to 300 Angstrom. It is able to resolve single atoms in favourable circumstances. A two dimensional representation of the actual structure is obtained. Some techniques such as shadow casting and replication can support in several material investigations. Actually the purpose of the Transmission Electron Microscope (TEM) is to investigate the structure and composition of a sample in submicroscopic form.
Electrocatalyst characterization by using spectroscopy technique
X ray diffraction
This is the most widely used method to characterize different catalyst materials and it helps to reveal the various chemical composition and crystallographic structure of various catalyst materials. In this technique, normally X-rays interact with electrons in matter and are scattered in different directions by the electrons. In the above figure different X ray diffraction pattern is shown (a) For fresh catalysts (b) For Rh/Al2O3 (c) Pt/Al2O3 (d) Pd/Al2O3. In various different conditions, the alumina supported Rh, Pt and Pd catalysts reveals good activity in thermal reforming of methane such as (CH4/H2O/O2/Ar = 40/30/20/10) at temperature of 1123 K.
Figure 5. Various X-ray diffraction patterns (BT.Li et al., 2004)
In the Figure 5(a) the sharp peaks are selected to gamma alumina as no usual peak was observed due to the low metal loading of 0.3 by wt% in the catalysts (BT.Li et al., 2004). Likewise in the case of Rh/Al2O3, rhodium particles used were highly dispersed on the catalyst surface. It should be noted that catalytic activity is heavily relied on the size and distribution of the particles and also carbon support acts as supporter on the catalyst surface. With carbon acting as a supporter it provides high specific area required for high metal loading and it's very high conductivity helps in decreasing the resistance in electron transportation. Platinum based catalyst support has high intrinsic activity and good stability. So, for all these reasons only highly dispersed catalyst is widely used for fuel cell. But due to Pt high cost and availability different catalyst particles are used such as palladium.
Measurement of physical surface area of catalyst
BET method is a very effective method to measure the catalyst surface area. As surface area of solid catalyst surface is a "key" point, so it is very important to determine and control the catalyst surface area. Besides this the porosity and surface area are also very vital to know the catalyst particle structure formation and applications of all the materials. The most widely used for surface area determination is the BET method, which is known as Brunauer, Emmett and Teller in 1938 (S.Brunauer et al., 1938). BET equations helps to study adsorption of gases by a solid material and helps to understand the correlation between the surface area and adsorbed gas molecules. The BET method is continuation of Langmuir adsorption theory for monolayer molecule adsorption to multilayer adsorption and it uses the hypothesis that no more than one atom can occupy one site and the atoms do not interact between adsorption layers. The resulted adsorption and desorption isotherms is used to define the amount of gas molecules adsorbed to a surface, according to the BET equation (S.Brunauer et al., 1938),
In the above equation the symbols P stands for the equilibrium pressure, P0 is the saturated vapor pressure, V is the total volume of molecules added and Vm stands for the number of molecules in the monolayer. From the experiment, a series of Pressure and Volume can be achieved. Plotting ( v/s) results a straight line. The slope and intercept of the line are obtained by using and respectively, from which Vm can be obtained. This BET method is mostly used in surface catalysis science for the determination of surface areas of solids molecules by physical adsorption of gaseous molecules.
Bi metallic catalysts
Bi metallic catalysts are extremely important for active fuel cell operations. Apart from the case of PEMFC anodes which operate on hydrogen, is mono-metallic low loading Pt used. In the case of hydrogen, the reaction is very rapid. However, in the case of fuel cell which run on methanol or reformate gas (H2 and CO), bi-metallic catalysts need to be used. One of the most common choices of bi-metallic catalysts is Pt/Ru. In the case of the cathode, where O2 is reduced (ORR), Pt is the most commonly used catalyst. Catalyst stability is of greater relevance for the cathode than the anode, as in a fuel cell cathode operates at very positive potentials, thus increasing the chance of corrosion of the catalyst and support. In the case of the core-shell catalyst, the core exists of one metal component that is surrounded by a layer of the active catalyst. This layer of catalyst typically starts with an alloy from the surface of which the unstable component is leached.
Catalysis working and reaction energetics
Catalysts work by providing a different mechanism that involves different type of transition state theory and lower activation of energy. Thereupon, more molecular collisions have the energy required to attain the transition state. Catalysts help in increasing the rate of reactions that would sometimes be slowed or reduced by a kinetic barrier. The catalyst may also sometimes increase reaction rate or selectivity at lower temperatures as shown in figure 6 and can be seen with a Boltzmann energy outline illustration.
Figure 6. Prolife potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction (Anon).
In the catalyzed elementary reaction, catalysts never change the extent of a reaction and they too have no aftermath on the chemical equilibrium of a reaction. This is due to the second law of thermodynamics as the catalyst introduced to the chemical reaction consequence in reaction to change to the different equilibrium which gives energy. Energy formation is a result of spontaneous reactions occurring that depends upon Gibbs free energy. This energy is produced only if there is no energy obstruction. Eliminating the catalyst would result in reaction by generating energy. Hence, a catalyst that may change the equilibrium would be a constant motion like machine, a contradiction to the laws of thermodynamics (A.Robertson, 1970). If a catalyst is not taking part in any equilibrium changed, then it must be absorb as the reaction is moving forward and thus acting as a reactant. The SI formulate unit for determining the catalytic activity of a catalyst is the katal, measured in a mole per second. The productivity of a catalyst is characterized by the turn over number (TON) and the catalytic activity by the turn over frequency (TOF) which is the TON per time unit. The catalyst fastens the transition state much more than stabilizing the original material objective. It lessens the kinetic blockade by reducing the change in energy between initial material and transition state.
Typical catalytic materials
The chemical characteristic of catalysts is as varying as catalysis itself, although some observations can be made. Proton acids are the most extensively used catalysts for various applications, especially for the reactions which involves water, including hydrolysis and its reverse. Multi-functional solids are usually very catalytically active such as graphitic carbon and nanoparticles. Transition metals are mostly used to catalyze redox reactions. For organic synthesis, late transition metals are used such as platinum, palladium, ruthenium, rhodium, gold and iridium.
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 (K.M. Bratlie et al., 2007). 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 (R.R.Adzic and J. Zhang, 2007).
Fuel cells are the electrochemical devices that convert the chemical energy stored in molecular hydrogen to electrical energy. As described earlier it is a chemical device thus 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 method accommodates several steps such as the potential step, potential sweep, potential cycling and rotating disk electrode. Fuel cell characterization also depends on some of these mentioned steps. An electrochemical reaction basically involves the different steps such as transfer of the reactants to the electrode surface, adsorption of the reactants on the surface of the electrode, charge distribution through oxidation or reduction on electrode surface and transport of the products from an electrode surface. The necessity of an electrochemical characterisation is to determine the details of different step 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 (B. C. H. Steele and A. Heinzel, 2001, M. L. Perry and T. F. Fuller, 2002). 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 6).
Figure 7. Direct observation of catalysis in fuel cell (S. Ogo, 2010).
A fuel cell is a device that transforms the chemical energy in a fuel to electrical energy through electrochemical way. 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.
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. 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 2.
Table 2. Fuel cell systems showing anodic and cathodic reactions and the dominant mode of ion transport in the electrolyte (G.J.K.Acresa et al., 1997)
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-
Types of Fuel Cell
Of all the different types of fuel cells available Proton exchange membrane fuels cells are being used most due of its advantages over conventional energy converting devices. A fuel cell is a device which converts the chemical energy of intake fuel supply and oxygen to electricity. Fuel cells are distinguished depending on the type of electrolyte used. Different types of fuel cell are available in market such as Polymer electrolyte membrane fuel cell, direct formic acid fuel cell, alkaline fuel cell and direct methanol fuel cell. In comparison with other types of fuel cells, Polymer electrolyte membrane fuel cells use a solid electrolyte which depends on a polymer with side chains maintaining acid based groups. The copious advantages this electrolyte makes the PEM fuel cell attractive for smaller scale earthy applications such as for transportation purposes in vehicles and acting as a source of power for various portable powers. The characterizing components of PEM fuel cell include relatively low temperature (under 90Â°C) operation, high power density and easiness in handling the fuel.
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.
Table 3. Different Fuel cells (G.J.K.Acresa et al., 1997)
Fuel Cell Type
Operating Temperature in Â°C
Proton exchange membrane
Solid proton conducting polymer
Lithium carbonate mixture
Proton Exchange Membrane Fuel cell
The Figure 8 illustrates Proton Exchange Membrane fuel cell (PEMFC). The change from chemical to an electrical energy happens through a direct electrochemical reaction in a PEM fuel cell and it takes place without combustion. The important part of a PEM fuel cell is membrane electrode assembly (MEA) which exists of a polymer electrolyte that is in connection with an anode and a cathode on both sides of the fuel cell. In order to precede the mechanism in PEMFC, the membrane present carry hydrogen ions (protons) and split the gas to penetrate to the other side of the cell. In the above Figure, it can be seen that hydrogen is transferred through the flow network of the anode plate to the anode in one the side of the cell. From other side in fuel cell, oxygen is supplied through the controlled plate to the cathode. H2 is dissolved at the anode into positively charged protons and negatively charged electrons. Positively charged protons moves towards the cathode, while the negatively charged electrons travel along an external circuit to the cathode, thus producing an electrical current in Proton exchange membrane fuel cell.
Figure 8. Diagram of Proton Exchange Membrane Fuel Cell (Anon)
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 present in PEM fuel cells is to carry protons from the anode to the cathode. The membrane polymers present have sulfonic groups which eases the transport of protons. Another activity includes holding the fuel and oxidant separated that prevents mixing of the two gases and withstanding harsh conditions, including active catalysts, reactive radicals and high temperatures fluctuations. The best polymer should have good conductivity for protons, low permeability for gases, good thermal stability and low cost. Many different membranes have been tested for commercial use in this type of fuel cell. 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 (MK.Kadirov et al., 2005).
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 almost all the Proton exchange membrane fuel cell, the anode and the cathodes use same platinum catalyst. Commonly, the platinum catalyst used is in nano sized 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). Like this only platinum is highly divided in nano sized particles and spread out, so that a maximum available surface area will interact with the reactant resulted in a degradation in catalyst load (J.Larminie and A.Dicks).
Membrane electrode Assemblies (MEA)
A membrane electrode assembly (MEA) is a bunch of stack of proton exchange membranes (PEMs) catalyst and flat plate electrode used in a fuel cell. A typical PEM fuel cell diagram is shown below showing the MEA. Two methods are used to form the MEA of a Proton Exchange membrane fuel cell. One method is by using applicable approaches to enhance the carbon supported catalyst to a permeable and conductive material, such as carbon paper or cloth called a gas diffusion layer (GDL) while another method is to form the electrode directly onto the membrane. Electrocatalysis in fuel cells needs the efficient interconnection of the cluster of catalyst particles with transport pathways for protons, electrons and gases. These inter penetrating nanoscale percolation networks must be optimized to produce acceptable performance of the electrode.
Figure 9. MEA in PEM fuel cell.
In current scenario, in PEMFCs, the necessary functionality is attained by the formation of 'thin-film composite' electrodes with two or more networks of Pt layered carbon and pores. However, path to the Pt is not ideal and mass transport linked voltage losses occur at high current densities. Membrane electrode assembly (MEA) in proton exchange membrane fuel cell determines the flow of gases and water between the cell and the catalyst. The materials for this type of application are centered on carbon cloth usually wet proofed with polytetrafluoroethylene or Teflonâ„¢ and sometimes filled with carbon blacks or graphite particles(M.F.Mathias et al., 2003).
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 = 0 V (under standard conditions) versus SHE.
O2 + 2H+ + 2e- â†’ H2O
Corresponding to a cathode potential = 1.229 V (under standard conditions) versus SHE. Therefore, the overall reaction of the fuel cell is
H2 + O2 â†’H2O
With the equilibrium standard electromotive force calculated to be 1.229 V.
Overall HOR/HER reaction taking place at anode at is H2 â†” 2(H+ + e-) taking place at an electrode with an electrolyte. It involves three different elementary reactions. In the ï¬rst step, H2 is dissociated and then is 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.
Increasing catalytic activity
Platinum is most efficient component used in PEM fuel cell as a catalyst and almost all existing PEM fuel cells use Pt particles on permeable carbon supports to drive both hydrogen oxidation and oxygen reduction reactions. Since cost of Pt is very expensive so use of Pt/C catalysts are not practical. The U.S. Department of Energy predicts that Pt based catalysts should use nearly four times less platinum than is used in existing PEM fuel cell, in order to represent a reasonable alternative to internal combustion engines (Anon, 2007). Subsequently by doing this the catalytic activity of platinum is increased by a factor of four that would be sufficient to achieve similar performance. By optimizing the shape and extend of the platinum particles the performance of platinum catalysts can be significantly increased. The surface area of the catalyst available is increased by decreasing the particles size but recent studies have showed the some more approach to make long advancement to the performance of catalyst. It has been figured that high key faces of Pt nanoparticles (Miller keys with large integers, as Pt (730)) gives a high density of reactive sites for oxygen reduction in comparison to Pt based nanoparticles (N.Tian et al., 2007). Another way of boosting the activity of Pt based catalyst is to mix it with different metals such as nickel and palladium. Stamenkovic reported that the surface of Pt3Ni (111) has a higher oxygen reduction activity in comparison to pure Pt (111) by a factor of 10 (V.R.Stamenkovic et al., 2007). By reducing the sensitivity of catalyst to many impurities such as carbon mono-oxide (CO) present in fuel source the performance of catalyst can be increased. Certain ppm of carbon monoxide can poison Pt catalyst, thereby, decreasing its activity. Currently, H2 gas is expensive to mass produce by either electrolysis or by any alternative process. Wang revealed in a study that a cube shaped Pt nano sized particles with (100) faces effect a fourfold increase in oxygen reduction activity when compared to randomly faceted platinum nanoparticles of similar size (C.Wang et al.). In addition, researchers have been investigating methods of diminishing the CO content of hydrogen to avoiding poisoning of the catalysts. Also it has been reported that RuPt core shell nano sized particles are significantly effective at oxidizing CO to form CO2 (S.Alayoglu et al.).
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 = 0.1 Acm-2 which is high when compared with ORR = 6 ÂµAcm-2 which is obtained from the (EIS) electrochemical impedance spectroscopy measurements done by Wagner (N.Wagner et al., 1998). 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.
Table 4.Exchange current densities of the HOR reactions at different electrode materials in aqueous 1M H2SO4 solution at ambient temperature (H.Wendt et al., 2005)
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
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 (CO) 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.
Electrocatalytic 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 (BE. Conway and JOM. Bockris, 1957, NM.Markovic, 2003, H.Wendt, 1990). 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 1990's, 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 devices (H.Li et al., 2008). 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 reverence to fuel cell catalysis, most of the research has been focused on cathode ORR catalysts development, as the ORR kinetics are slower than the anodic HOR kinetics (E.Antolini, 2003). However, in some cases the over-potential of the anodic HOR can also contribute a non-negligible portion of the overall fuel cell voltage drop (C.Song et al., 2007). 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 science (H Wendt, 1990, BE.Conway, 1999). Undoubtedly the hydrogen evolution/oxidation reaction (HER/HOR) is the simplest and most widely studied electrochemical process. 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.
Electro-oxidation of Hydrogen
The overall reactions of anodic hydrogen oxidation in acidic and alkaline mediums may be expressed by following two equations;
H2 â†’ H+ + e- (1)
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 (