Current voltage responses

Abstract Described are the synthetic routes to precisely defined molecular wires which are of discrete length and constitution. They are fully conjugated systems and are expected to have nearly linear current-voltage response curves. Their ends are functionalized with molecular alligator clips, based on chalconides and isonitriles, for adhesion between proximal probes. Both solution and solid-phase approaches have been used to prepare these molecular wires that are based on oligofthiophene ethynylene)s and oligofphe-nylene ethynylene)s. Molecular device syntheses are also described that would be expected to have nonlinear current-voltage responses. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

The ultimate aim of the project was to see if rectification (asymmetric current-voltage responses) or gating could be achieved by use of the redox-active metallocene moieties. Not only did channel-like function prove difficult to establish but the potentials required to oxidize the Fe(II) center for example (ca. 720 mV), were beyond the limits of the experimental setup. Indeed, membrane stability at very high voltages (>200 mV) is questionable.41 With the small amounts of data obtained (Figure 14) it is difficult to substantiate whether any rectification is realized. 

Values in mV, Measurements made by analog differentiation of the current-voltage response... 

Figure 5. Photosensitization vs. photoelectrocatalytic reactions. At top, the illuminated current/voltage curve is placed negative of the current/voltage response on bare Pt in the dark. At bottom the current/voltage curve indicates an increase in reaction rate, but no change in the apparent emf of the reaction.

The current-voltage response of a 2 x 2 direct oxidation liquid-feed fuel cell operated with TMM, investigated over a range of concentrations, and with methanol under similar conditions is compared and illustrated in Fig 1.42. 

Fig. 10 Current voltage response of dye-sensitized cell measured at NREL [101]. This cell has a short-circuit current density of over 20 mA cm and an open circuit voltage of over 0.7 V. The relatively low slope of l-V near Voc indicates a significant series resistance, largely due to the sheet resistance of the conducting glass substrate.

Figure 4.4.6a shows a perturbing voltage sinusoid reflected about the dc current voltage response curve at Vfc and at some positive bias, fit the linear region, this reflection results in an undistorted current response, with V//being a constant (equal to the dc corrosion resistance). 

Precise measurements of the current responses at each of a number of harmonics can be used, in conjunction with Eq. (16), to evaluate the unknown terms in Eq. (11). A complete description of the faradic current-voltage response is sufficient to define the anodic partial current (the corrosion rate) at any potential the parameters needed are the forward and reverse Tafel coefficients, the free corrosion current, and the free corrosion potential. 

Methods to calculate time dependent current/voltage response for circuits whose parameters depend on voltage or current are well developed in theoretical electronics and free open source tools for numerical calculations are available, such as general electronic circuit simulator SPICE (Hageman [1993]). 

Figure 26.7 Current-voltage response of a single cell operating on CH4 and H2 [49].

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