Electrochemically accessible surface area

A cobalt 5,10,15-tetrakis(l-methylpyridinio-4-yl)porphyrin/poly(sodium p-styrene sulfonate)/reduced graphene oxide composite (CoTPyP/PSS-RGO), was fabricated using an in situ solvothermal synthesis method [92]. It showed excellent catalytic activity for ORR, attributed by the authors to its large electrochemically accessible surface area, to the excellent electrical conductivity of PS S-RGO, and to the synergistic effect between CoTPyP and PSS-RGO. 

The catalyst layers (the cathode catalyst layer in particular) are the powerhouses of the cell. They are responsible for the electrocatalytic conversion of reactant fluxes into separate fluxes of electrons and protons (anode) and the recombination of these species with oxygen to form water (cathode). Catalyst layers include all species and all components that are relevant for fuel cell operation. They constitute the most competitive space in a PEFC. Fuel cell reactions are surface processes. A primary requirement is to provide a large, accessible surface area of the active catalyst, the so-called electrochemically active surface area (ECSA), with a minimal mass of the catalyst loaded into the structure. 

Thus the potential in the macropores is directly affected by transport and charging phenomena in the micropores accessible to electrolyte. These properties of active carbon render it a difficult material to use as an electrode. The large electrochemically active surface area leads to considerable double-layer charging currents, which tend to ob.scure faradic current features. The network of micropores in the electrode material might be expected to result in a significant ohmic effect, which would further impair the potential resolution (IR drop on electrode material) obtainable by PACE voltammetry. CV curves recorded with different masses (and sediment layer thicknesses) of powdered samples of selected carbons in various electrolyte solutions are presented in Fig. 8 as an example [194]. Where amounts of material were greater than 20 mg, the CVs recorded were of the same shape. 

The electric double-layer capacitance is almost linear to the accessible surface area of the electrolyte ions. Additionally, the chemically/electrochemically stability, electric conductivity, and adequate commercial price are necessary for the EDLC electrode, so the activated carbons are suitable as practical electrode materials. The EDLC has been commercialized as a memory back-up device since the 1970s because of its high cycle efficiency and its long cycle life. Recently, the EDLC is also being considered to be one of the promising systems for... 

In general, there are two methods to introduce the PA to the CL. The first one rehes on a highly PA-doped membrane. The PA is transferred between membrane and catalyst layer upon MEA preparation and/or cell assembly, respectively. The second method involves depositing (spraying, painting, etc.) PA directly onto the CL. The PA content in the membrane is one of the decisive factors for the selection of one of the two preparation methods. Overall, the CL needs to provide electron and proton transport pathways, gas accessibility, high catalytic activity towards HOR and ORR, a high electrochemically active surface area (ECSA) and has to withstand the harsh HT-PEMFC environment (acidity close to pH = 0, temperature up to 180 °C, and electrochemical potentials up to 1.5 V). 

The objective of catalyst layer design is twofold from a materials scientist s perspective, the objective is to maximize the electrochemically active surface area (ECSA) per unit volume of the catalytic medium Secsa, by (i) catalyst dispersion in nanoparticle form or as an atomistically thin film and (ii) optimization of access to the catalyst surface for electroactive species consumed in surface reactions. From a fuel cell developers point of view, the objective is to optimize pivotal performance metrics like voltage efficiency, energy density, and power density (or specific power) under given cost constraints and lifetime requirements. These performance objectives are achievable by integration of a highly active and sufficiently stable catalyst into a structurally well-designed layer. 

Layered materials are of special interest for bio-immobilization due to the accessibility of large internal and external surface areas, potential to confine biomolecules within regularly organized interlayer spaces, and processing of colloidal dispersions for the fabrication of protein-clay films for electrochemical catalysis [83-90], These studies indicate that layered materials can serve as efficient support matrices to maintain the native structure and function of the immobilized biomolecules. Current trends in the synthesis of functional biopolymer nano composites based on layered materials (specifically layered double hydroxides) have been discussed in excellent reviews by Ruiz-Hitzky [5] and Duan [6] herein we focus specifically on the fabrication of bio-inorganic lamellar nanocomposites based on the exfoliation and ordered restacking of aminopropyl-functionalized magnesium phyllosilicate (AMP) in the presence of various biomolecules [91]. 

In addition to the criticisms from Anderman, a further challenge to the application of SPEs comes from their interfacial contact with the electrode materials, which presents a far more severe problem to the ion transport than the bulk ion conduction does. In liquid electrolytes, the electrodes are well wetted and soaked, so that the electrode/electrolyte interface is well extended into the porosity structure of the electrode hence, the ion path is little affected by the tortuosity of the electrode materials. However, the solid nature of the polymer would make it impossible to fill these voids with SPEs that would have been accessible to the liquid electrolytes, even if the polymer film is cast on the electrode surface from a solution. Hence, the actual area of the interface could be close to the geometric area of the electrode, that is, only a fraction of the actual surface area. The high interfacial impedance frequently encountered in the electrochemical characterization of SPEs should originate at least partially from this reduced surface contact between electrode and electrolyte. Since the porous structure is present in both electrodes in a lithium ion cell, the effect of interfacial impedances associated with SPEs would become more pronounced as compared with the case of lithium cells in which only the cathode material is porous. 

---