Membrane, electrochemically active

Guilminot E, Corcella A, Chatenet M, Maillard E, Chariot E, Berthome G, lojoiu C, Sanchez JY, Rossinot E, Claude E. 2007b. Membrane and active layer degradation upon PEMEC steady-state operation. J Electrochem Soc 154 B1106-B1114. 

The use of porous membranes as templates for electrode structures was pioneered by Martin and coworkers nearly 20 years ago, and this approach has since been extended to include numerous electrode compositions and geometries " and applications beyond energy storage, including sensing and separations. In this approach, chemical and electrochemical routes are used to fill in the cylindrical, uniform, unidirectional pores of a free-standing membrane with electrochemically active materials and... 

A possible reaction mechanism based on these results is shown in Scheme 6, where Pyc plays a dual catalytic role both in the ORR (dark reaction) and the SOR (light reaction). It is noteworthy that, since the Pyc is opaque, only [Ru(bpy)3] can be used to absorb the light in the membrane. The active Pyc site is reported to be an efficient catalyst for the ORR and hence, the purging O2 is essential for the formation of H2O2 during the reaction. The control experiment in pure H2O2 gave only -47% conversion with poor selectivity (Table 4). However, the assistance of Pyc and [Ru(bpy)3] in the SOR is supported by the indirect electrochemical studies. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

The anode layer of polymer electrolyte membrane fuel cells typically includes a catalyst and a binder, often a dispersion of poly(tetraflu-oroethylene) or other hydrophobic polymers, and may also include a filler, e.g., acetylene black carbon. Anode layers may also contain a mixture of a catalyst, ionomer and binder. The presence of a ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires a ionically conductive pathway to the cathode catalyst to generate electric current (16). 

The effect of pore diffusion is described through the term, 5, which represents a ratio of the kinetic rate to the diffusion rate in the electrode. In general increasing the value of, v, i.e., decreasing the diffusion rate relative to the kinetic rate, has the effect of causing a significant reduction in the local current densities and that more of the electrochemical activity of the electrode is focused closer to the membrane. This is a consequence of the reduced concentration of reactant away from the membrane due to, for example, a slower diffusion rate (lower diffusion coefficient). 

A porous anode and cathode are attached to each surface of the membrane, forming a membrane-electrode assembly, similar to that employed in SPE fuel cells. Electrochemical reactions (electron transfer-l-hydrogenation) occur at the interfaces between the ion exchange membrane and electrochemically active layers of electrodes. Electrochemical reductive HDH occurred at the interfaces between the ion exchange membrane and the cathode catalyst layer when an electrical current is applied between the electrodes ... 

Fig. 13.8 Change in the percentage of pentachlorophenol (PCP) removal for the electrochemical HDH of saturated aqueous solution using a Nation 117 membrane reactor. Active area 20 cm2. Cathode Pd or Pt/Ti mesh (2 mg Pd or Ptcm-2), Fe or Ni mesh. Anode Pt/Ti mesh (2mgPtcm-2). Anolyte Water. Flow rate 100ml min-1. Applied current density 10mAcm-2. Temperature 17 0.5°C...

So far, various studies focused on developing catalyst materials with improved ORR activity, but only few reported the stability and durability of ORR catalysts. The study of accelerated durability tests (ADT) in conjunction with electron microprobe analysis (BMPA), LEED, and XRD techniques on Pt-based al-loys ° observed hd metal dissolution, diffusion of 3bulk oxides on the surface, and migration and agglomeration of Pt. Yu et al. compared the durability and activity of PtCo/C with Pt/C catalysts. Throngh determination of the electrochemically active sniiace area, mass, and specific activities with respeet to the potential cycles, they found the overall cell performance of PtCo/C is higher than that of Pt/C. They also concluded that the observed dissolution of Co has no severe impact on the cell performanee or membrane conductance. Additionally, Popov et al studied the stabihty of Pt M/C for X = 1,3 and M = V, Fe, Ni, Co. ADT analyses revealed that Pt/C has the lowest activity when eompared to Pt-alloy catalysts, and that the metal dissolntion is lower for a Pt M ratio of 3 1 than compared to a 1 1 ratio. Also, Pt-Ni showed a lower dissolution rate than the other considered Pt-M alloys. 

The case study showed a successful method for manufacturing composite sorption materials for ion removal by electrosorption. The prepared electrosorption membranes that were electrochemically activated at 10 V and removed approximately 100% of Ni and >90% of Na" ", S04, and CP ions from a simulated nickel effluent solution. The energy consumption at... 

Relatively few applications can be found in this field that involve the coupling of microdialysis to biosensors, especially electrochemical ones. The major problem is always the need to exclude electrochemically active substances, which can be present in the external medimn, and, because of their low molecular weight, can easily cross the membrane of the microdialysis probe. Some attempts to overcome this problem have been reported by Csoregi [187], who coupled a microdialysis probe equipped with a polysulphone membrane with a 5000 Da cut-ofiF to a carbon paste electrode with wired GOD, thus excluding many interfering substances. Similarly, Rohm [188] has assembled a screen-printed biosensor for lactate based on an ultraviolet-polymerizable enzyme paste and then applied it in an FIA system for the on-hne monitoring of cell cultivation. 

Some direct indications of electrochemical activity on graphite were deduced from square wave voltammetry experiments, which were not possible when using cyclic voltammetry [225], However, on cystamine modified gold electrodes clear independent evidence of DET was shown [110,227]. To increase the local concentration of CDH the modified Au electrode was mounted with a dialysis membrane. 

The most widely employed types of biosensors are those that employ an oxidase to generate hydrogen peroxide. A classic example is the electrode containing an immobilized glucose oxidase that generates an amperometric signal related to the concentration of glucose present in the sample. However, biosensors of these types are often susceptible to interference from other electrochemically active solutes in the sample. A wide variety of techniques have been developed to circumvent or minimize this problem, for example, application of a semipermeable membrane above the... 

FIGURE 11-42 Lactose uptake in E. coli. (a) The primary transport of out of the cell, driven by the oxidation of a variety of fuels, establishes both a proton gradient and an electrical potential (inside negative) across the membrane. Secondary active transport of lactose into the cell involves symport of and lactose by the lactose transporter. The uptake of lactose against its concentration gradient is entirely dependent on this inflow of H", driven by the electrochemical gradient. 

The term transporter refers to a variety of membrane proteins with diverse functions and structures. Transporters can mediate either facilitated or active transport (Figure 9.1). Facilitated transport involves movement of drug across a membrane down an electrochemical gradient. Conversely, active transporters rely on energycoupling mechanisms (e.g., ATP hydrolysis) to move drug. These transporters can also create ion/solute gradients across membranes (secondary active transport), which in turn drive uphill membrane transport of drugs. 

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