Conventional 3-Electrode Cells

If good adhesion is difficult to achieve, a cavity above the disk ean be made to hold the catalyst. It is preferred that the height of the cavity be adjustable so that variable catalyst layer thicknesses can be studied. 

A dilute aqueous acid solution (e.g., 0.1-1 M) is typically used as the electrolyte. The reason for using acid is that the fuel cell reactions involve protons (note this article is limited to reactions having protons as the ionic charge transport species). Sulfuric acid (H2SO4) has been used in many studies. However, sulfate anions (S04 ) can adsorb onto the surface of the Pt catalyst, which alters the reaction kinetics. Perchlorate anion (CIO4 ) from perchloric acid (HCIO4) does not adsorb onto the surface of Pt catalysts, and, therefore, could be a better choice. 

Since a perfluorosulfonic acid ionomer such as Nafion is normally used to make the catalyst layer, trifluoromethane sulfonic acid (CF3SO3H) is expected to behave more closely to the ionomer than the inorganic acids are. In addition, CF3SO3H and its anion (CF3SO3 ) do not adsorb onto the surface of the Pt catalyst and thus do not affect the reaction kinetics. 

At around 200 °C only a small percentage of H3PO4 dissociates into protons and H2P04 because of the lack of water. Meanwhile, some H3PO4 forms 

Another reason for the lower performance in H3PO4 is the low solubility and diffusivity of oxygen in it. They deerease with an inerease in the eoneentration of the H3PO4. In 85 wt% acid, the solubility and diffusion eoeffieient of oxygen are about an order of magnitude and fivefold lower, respeetively, than those in perfluorinated ionomers [42]. 

---

Conventional 3-electrode cells, commercial potentiostats with external feedback IR compensation, and procedures similar to those described previously were used for electrochemical experiments. Working electrodes were a PARC Model 9323 hanging-drop-mercury electrode (HDME, A = 0.019 cm ) or a highly polished glassy carbon disk electrode (GCE, A = 0.071 cm ). A Pt wire served as the counter electrode and the reference was a saturated calomel electrode (SCE). For the HDME, a fresh Hg drop was used for each experiment. A polishing method described previously was used for GCE " and was repeated prior to each voltammetric scan. Area of the GCE was estimated electrochemicaly by using the Randles-Sevcik equation and CV peak for the oxidation of ferrocene in acetonitrile (D = 2.4 X 10 cm s i). Experiments in all CTAB solutions were thermo-statted at 30.0 0.1 C for SDS the temperature was usually 25.0 0.1 C. 

Scale of Operation Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Most analyses are carried out in conventional electrochemical cells using macro samples however, microcells are available that require as little as 50 pL of sample. Microelectrodes, with diameters as small as 2 pm, allow voltammetric measurements to be made on even smaller samples. For example, the concentration of glucose in 200-pm pond snail neurons has been successfully monitored using a 2-pm amperometric glucose electrode. ... 

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

In the Antek Fluoride Analyzer, a pyrolysis furnace is combined with an ion-specific electrode cell (ISE). Table 8.9 compares this specific analyser to a conventional combustion bomb. 

The in situ Mossbauer experiments were conducted with 90% - Fe enriched 9 1 Ni/Fe oxyhydroxide films which were deposited in the fashion described above onto a gold on Melinex support(12) in a conventional electrochemical cell. Prior to their transfer into the in situ Mossbauer cell, the electrodes were cycled twice between 0 and 0.6 V vs. Hg/HgO,OH" in 1 M KOH. Two such films were used in the actual Mossbauer measurements in order to reduce the counting time. The in situ Mossbauer cell involved in these experiments was previously described. 

An electrode of surface area 100 pm or less is called a microelectrode and provides a means of decreasing the double-layer capacitance which can affect our coulometry experiments so badly. Microelectrodes are also useful when the cell considered is also tiny, as, for example, is the case when performing in vivo voltammetry (see next chapter) with biological samples. For example, a nerve ending is typically 10-100 pm in diameter, so electroanalytical experiments using a conventional electrode would be impossible. 

Microbial biofuel cells were the earliest biofuel cell technology to be developed, as an alternative to conventional fuel cell technology. The concept and performance of several microbial biofuel cells have been summarized in recent review chapters." The most fuel-efficient way of utilizing complex fuels, such as carbohydrates, is by using microbial biofuel cells where the oxidation process involves a cascade of enzyme-catalyzed reactions. The two classical methods of operating the microbial fuel cells are (1) utilization of the electroactive metabolite produced by the fermentation of the fuel substrate " and (2) use of redox mediators to shuttle electrons from the metabolic pathway of the microorganism to the electrodes. ... 

The conventional dye cell uses I /I2 as the redox couple, and no other known redox couple works nearly as well [49]. The use of most other redox couples, and most substrates besides Sn02, is expected to accelerate the recombination reactions relative to the conventional dye cell and thus diminish Voc, independent of the mechanistic model (see Section VI). Because this would obscure what we were looking for, the relationship between and Voc, we used only the reduced half of the redox couple in order to minimize recombination rates and maximize Voc. Versus vacuum, the work functions of the clean electrodes are = 4.3 eV [88], 8 02 = 4.8 eV [88], = 5.1 eV [89], and 1 =... 

Electrodes or arrays of electrodes can be produced by these techniques with complex or fine patterns that would be very difficult to fabricate with conventional bulk electrode materials. Films can commonly be reproducibly formed in quite pure chemical form (with a caveat based upon interfering chemical effects that may arise from diffusion of adhesion-promoting intermediate layers through the thin film to the exposed surface). Film electrodes lend themselves to inexpensive fabrication of disposable electrodes, cells, or eventually complete integrated measurement devices in large quantities that would be economically infeasible with conventional materials, due to either the complexities of fabrication or the cost of materials. 

A few special technical problems arise when attempting to mount film electrodes into conventional electrochemical cells due to difficulties in defining the electrode area and in making electrical connection to the active surface. These constraints usually result in cells that have been somewhat modified, although the same principles that were discussed in Chapter 9 still apply. In this final section we address some of these practical difficulties. 

Because of the difficulties described earlier, electroanalytical studies are usually performed separately from radical generation studies. But a flat cell has been designed [26] (Fig. 29.19) to permit simultaneous monitoring of the electrochemical and EPR response of a free-radical system (SEEPR). The auxiliary electrode extends along the edges of the working electrode, which diminishes the problems of iR drops and provides better uniformity of current density than is possible with conventional electrode placement. This cell is used primarily for short-term (on the order of seconds) electrochemical experiments, such as... 

At low light flux, the semiconductor sensitization is constrained to one electron routes, since the valence band hole is annihilated by a single electron transfer. Presumably after decarboxylation the resulting alkyl radical can be reduced to the observed monodecarboxylate more rapidly than it can transfer a second electron to form the alkene. In a conventional electrochemical cell, in contrast, the initially formed radical is held at an electrode poised at the potential of the first oxidation so that two-electron products cannot be avoided and alkene is isolated in fair chemical yield. Other contrasting reactivity can be expected for systems in which the usual electrochemistry follows multiple electron paths. 

Electropolymerization and cyclic voltammetric experiments are performed with an EG G PARC, Model 173 potentiostat equipped with a Model 175 universal programmer and a Model 179 digital coulometer in conjunction with a Kipp and Zonen BD 91 XY/t recorder. All experiments are carried out using a conventional three-electrode cell. Instrumental setup for amperometric measurements ... 

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its "hole companion" as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

Wetproofed porous electrodes, applied previously in fuel cells, have also been tested as cathodes for electrosynthesis from gaseous and liquid starting materials with limited solubility in water. The reagent is supplied through the hydrophobic electrode, which is in contact with an aqueous electrolyte. They present some attractive advantages over conventional electrodes because they ... 

A CH Instruments 800B electrochemical analyzer (Austin, TX) was used for all electrochemical measurements and electrode preparation. Glassy carbon (GC) electrodes (3 mm diameter, geometrical area 0.07 cm2) were purchased from CH Instruments (Austin, TX) and polished with polishing alumina solution (BAS, West Lafayette, IN). All electrochemical measurements were conducted in a conventional three-electrode cell using Ag/AgCl (3M KC1) (CH Instruments,... 

The EQCM comprises a quartz crystal oscillator, in which one of the Au exciting electrodes is also exposed to the solution and acts as the working electrode in a conventional (here, three electrode) cell. Provided any surface film is rigidly coupled to the underlying electrode changes in inertial mass (Am) of the electrode result in crystal resonant frequency changes (A/) that are described by the Sauerbrey equation [11] ... 

---