Composite Electrode Materials

As previously discussed, the poor cyclability of conductive polymer materials may hinder their application as stand-alone electrode materials in ESs. Compositing conductive polymers with carbon-based active materials may overcome these challenges while also increasing the electronic conductivity of the active layer. For example, combining psuedocapacitive polymer materials with CNTs [41-43] and graphene [44-46] are currently the most extensively investigated composite formulations for ES applications. 

While CNTs possess limited surface areas compared to other microporous carbon blacks, they maintain rigid mechanically robust structures that can result in favorable three-dimensional electrode architectural configurations. The network arrangement can provide a porous structure that can readily facilitate the transport of electrolyte species and provide highly electronically conductive pathways to the redox centers of the active materials. The structure, porosity, and pore size distribution of this scaffold-like architecture can also be tailored by using CNTs with varying diameters, surface properties, and wall thicknesses or by modifying preparation techniques. 

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Explain clearly how SECM images the microdistribution of the electrochemical activity of composite electrode materials. 

The values of specific capacitance and cell resistance obtained with symmetric two-electrode capacitors built with a-Mn02/CNTs composite electrodes are presented in Table 8.3. The addition of nanotubes to a-Mn02 causes a drastic decrease of cell resistance and an increase of specific capacitance referred to the mass of a-Mn02 H20. However, when the specific capacitance is referred to the total mass of the composite electrode material, to be realistic, a CNTs loading higher than 10-15 wt% does not improve the electrodes performance. Therefore, 10-15 wt% of CNTs conducting additive seems to be an optimal amount both on the point of view of electrodes capacitance... 

This is an area of much practical importance for research and development in electrolyzer technology more work is currently required for elucidation of the behavior of high-area porous and composite electrode materials with regard especially to the values of Tafel slope and conditions under which low b values can be achieved for H2, O2, and CI2 evolution reactions, thus minimizing activation overpotential energy losses in high current-density operations. 

Equation 1 is a simplification because the double-layer capacitance has been shown to be a function of potential, solution composition, electrode material, electrode pretreatment, concentration of adsorbed species, and duration of contact between solution and electrode (S, 2) ... 

C. Zanardi, F. Terzi, L. Pigani, A. Heras, A. CoUina, J. Lopez-PaUacios, and R. Seeber, Development and characterization of a novel composite electrode material consisting of poly (3,4-ethylenediox-ythiophene) including Au uanoparticles, Electrochim. Acta, 53, 3916-3923 (2008). 

For graphene/CP composite films, the goal of combining the materials has been both to obtain a mechanically more robust material and to combine the attractive properties of the individual components to obtain a superior material. As discussed above, graphene/CP composite materials can be S3mthesized by a range of different methods. In this section, electropolymerization of graphene/CP composite electrode materials and the direct use of such electrodes in the field of supercapacitors and electrochromic devices will be briefly summarized. 

Wu Z-S, Zhou G, Yin L-C, Ren W, Li F, Cheng H-C (2012) Graphene/metal oxide composite electrode materials for energy storage. NanoEnerg 1 107-131... 

At the same time, a fundamental understanding of supercapacitor design, operation, performance, and component optimization led to improvements of supercapacitor performance, particularly increasing their energy density. To further increase energy density, more advanced supercapacitors called pseudocapacitors, in which the electroactive materials are composited with carbon particles to form composite electrode materials, were developed. The electrochemical reaction of the electroactive material in a pseudocapacitor takes place at the interface between the electrode and electrolyte via adsorption, intercalation, or reduction-oxidation (redox) mechanisms. In this way, the capacitance of the electrode and the energy density can be increased significantly. 

Composite electrode material of Mn and Fe metal oxides with ratio ranging from 3.5 1 to 4.5 1 for use in ESs. Metal oxides react to form gel and are subsequently dried in supercritical CO2 to form powder. An electrode with a weight composition of 15 to 60% carbon and 40 to 80% metal oxide nanoparticles achieved nunimum specific capacitance of 500 F/g at 1 mV/s in IM KOH electrolyte. 

Development and investigation of composite electrode materials, integrating their combined benefits and overcoming associated challenges... 

Zheng, L.P. et al. The preparation and performance of calcium carbide-deiived carbon/polyaniline composite electrode material for supercapacitors. Journal of Power Sources 195,1747-1752, 2010. 

Parametric studies are possible by systematically changing, for example, temperature, pH, electrolyte composition, electrode materials, separators, etc. 

Hierarchically porous monolithic LiFeP04/carbon composite electrode materials for high power lithium ion batteries. Chem. Mater., 21 (21), 5300-5306. 

Electrode modification of these bulk materials is the major topic to be treated in the next sections. We will see that the range of modification that has been considered is quite large, with the deposition of metal film electrodes, the electrode texturation (for increasing the electrode surface area with respect to the geometric one), the functionalization with organic monolayers or polymeric multilayers, or the elaboration of composite electrode materials for the development of sensors for possible application in the field of environmental analysis. 

Zanardi C, Terzi E, Pigani L, Heras A, Colina A, Lopez-Palacios J, Seeber R (2008) Development and characterisation of a novel composite electrode material consisting of poly (3,4-ethylenedioxythiophene) including Au nanoparticles. Electrochim Acta 53 3916-3923... 

In addition to the film resistance around the particles of active material, there may be a contact resistance due to imperfect contact between the electrode and the current collector [70]. This contact resistance is separate from interparticle contact resistance in the bulk of the electrode, which is included in the electrical conductivity a of the composite electrode material. Contact resistances at the current collector would have a different effect on the current distribution than a film resistance, because this contact resistance would occur only at the boundary of the electrode rather than being distributed throughout the entire surface area of active material like the film resistance. The ohmic potential drop from the contact resistance is treated by simply subtracting IRconiaa from the ceU potential. 

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