SISTEMA ESTATAL ANTICORRUPCIÓN DE TAMAULIPAS

The goal of exploring electrode materials with better performance and higher efficiency can be achieved with knowledge of the basic science behind the electrocatalytic processes itself. Therefore, in order to study the structure and composition of solid electrode surfaces, the reduction/oxidation state of surface atoms, the adsorbate bonding capability, surface coverages and lifetimes on the surface of reaction intermediates become key questions. These can be addressed taking a macroscopic viewpoint of the experiment and varying parameters

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such as electrolyte composition, applied electrical potential, pressure, temperature, etc., under control.175 Combining these approaches with surface sensitive probes based on photon, electron, and ion spectroscopies aids enormously the exploration of electrochemical surface chemistry.176, 177

Usually, based on surface science and electrochemical kinetic measurements, electrocatalysts with high efficacy may be designed. A wide variety of materials, including single metals, alloys, carbons, oxides, amorphous materials, polymers, enzymes and so on, are used as materials in fuel cell applications. Those consisting of inorganic catalysts are relative simple to manufacture.1 Their anodes are usually made of single metals such as platinum or occasionally (to help with CO tolerance), Ru may be added supported on graphite178. The anode is supplied with the fuel (hydrogen or methanol for example) whilst the cathode (again usually Pt or a Pt alloy) is supplied with oxygen/air. Separating both of these electrodes will be a semi-permeable hydrated membrane usually consisting of the polyelectrolyte Nafion® which facilitates proton transport but prevents diffusion of fuel from anode to cathode.179 During the overall fuel cell reaction, energy is produced in the form of electric current as well as water and/or carbon dioxide. More complex fuel cells involving enzymes and living organisms are also becoming more popular in order to replace the expensive precious metals as electrocatalysts together with a drive to look for more environmentally friendly processes. Some typical electrocatalysts will be introduced according to their chemical nature as electrocatalytic systems.180

Firstly, dispersed and rough metal electrodes are the most widely utilised. From nanometer to micrometer, the size of crystalline metal clusters varies in the electrocatalyst dispersions.181 In heterogeneous reactions, the particle size and shape plays a significant role in the behaviour of the electrocatalyst.182, 183 The discovery of changes of electrocatalytic activity with time in aged fuel cell electrode electrocatalysts has been ascribed to both catalyst particle sintering and corrosion which decreases the overall surface area of the electrocatalyst.184 Corrosion of the graphite support has also been noted as an extensive problem in optimal fuel cell performance. Therefore, when new catalysts with improved performance (activity) and multi-functionality (selectivity) are designed, their size, shape, surface structure and environmental characteristics are taken into consideration. The preparation involves a wide range of processing, such as rough palladium nanoparticles and nanowires formed on highly ordered graphite (HOPG) by electrodeposition, fractal surface

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formation and highly rough palladium or platinum electrodes produced from electrochemical roughening by cyclic potential techniques.185-187

Secondly, dispersed metal-carbon electrodes are of special interest due to their unique electronic properties. These carbon materials exhibit electrical properties ranging from an essentially metallic material in disordered graphite or glassy carbon to a semi-metallic behaviour in high surface area carbons.188 Carbon surfaces are largely inert to electrochemical processes over a wide potential range although as mentioned earlier, prolonged exposure to electrooxidising conditions can result in slow degradation of the material. Hence, carbon electrodes are often used for manufacturing chemically modified electrodes. However, the surface chemistry of carbons is very complex since there are a range of surface defects present.189, 190 Also, some functional groups containing O- and OH- usually take part in the processes of physisorption and chemisorption.191 Due to their physicochemical properties for inhibiting the agglomeration of metal nanoparticles, carbon blacks are still used. Vulcan carbon is one of the well-known power source carbon supports, which contains 0.5 wt% of thiophene-like sulphur.192 The supported platinum electrocatalyst is not poisoned by the sulphur as the oxide layer on the surface of Pt NPs protects it by desulphurising the carbon whilst making membrane electrodes for fuel cells.193 Its large surface area (~250 m2·g-1) with micropores allows Pt NPs to readily distribute themselves equally through the material. Therefore, the requirement of maximising Pt surface accessibility to fuel is met. Since the contact between Pt and carbon is considered of great importance for tethering the particles, work has been undertaken trying to incorporate platinum nanoclusters during the synthesis of porous structures of conductive carbons.194 Furthermore, the surface chemistry and morphology of the carbon support must sustain its high dispersion over time.

Nowadays, a lot of researchers are focusing on electrocatalys is in low temperature proton exchange membrane fuel cells (PEMFC) .195 In order to achieve the desired reaction rate in fuel cells, the electrocatalyst usually needs to be a precious metal such as platinum although metal thrifting is more and more being used to reduce the total amount of Pt necessary to maintain a particular power output by combining a thin layer of platinum on top of a „cheaper‟ metal.196

Usually, platinum metal NPs are supported on electronically conductive carbons but recently, carbon nanotubes (CNTs) have become attractive alternatives to graphite197. The observation of the decomposition of hydrazine being more effective for

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iridium NPs supported on CNTs instead of graphite points to superior properties afforded by CNT supports.198 This is already impacting significantly in choosing the support material for hydrazine-fuelled space vehicles.199

Besides, metals such as Raney nickel for molecular hydrogen electrochemical oxidation200, metal alloys including platinum-tin for methanol electrooxidation201-203, and platinum- chromium for oxygen electrochemical reduction reaction (OERR)204, are typical electrocatalysts used in fuel cells. Finding the most active sites of the nanoscale materials and controlling their amorphousness and disorder and relating this to the catalytic activity is a key issue.205 It was discovered that the activity of Pt catalyst in the direct methanol fuel cell (DMFC) was improved by adding tin, in which methanol oxidation in sulphuric acid took place on carbon-supported Pt-Sn electrodes.49, 206 It has also been proved that platinum-based alloys, such as Ptα-Ruβ for methanol oxidation and Ptα-Crβ for the OER, are more efficient than pure platinum.207 Both of their interfacial domains are highly disordered. Since there are many choices of transition metals (and their oxides) which can be potential alloy materials, scientists are working extensively testing alternative alloy catalyst combinations with both lower costs and greater resilience.

Finally, there is another kind of chemically modified electrodes which can also meet certain requirements with specific electrocatalytic activities. By anchoring chemical functional groups, the catalytic properties of conducting substrates can be modified. The electrode surface formed would be more complex due to the interactions between the functional groups and the substrate. However, molecular complexes grafted onto the support can help electrochemically convert reactants to products by accelerating the electron transfer processes or immobilising oxidation-reduction couples.208, 209 They can be prepared by various conventional methods, such as adsorption (functionalised vinyl compounds on platinum)210, 211

, chemical reactions (metal porphyrin film covering an activated carbon electrode)212, polymerisation (pyrolytic graphite modified by polyvinylpyridine)213, and so on. Furthermore, new nanostructured materials with novel functions and special optical, magnetic, and catalytic properties have been designed and utilised for electrode assemblage and appear to hold out great potential.214-223

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