Carbon fiber electrodes cylinder

Cylindrical diffusion is observed, for example, during diffusion to a thin metallic wire or a carbon fiber electrode. Impedance of cylindrical electrodes was studied by Fleischmann et al. [165,166] and Jacobsen and West [167]. The partial differential equation describing diffusion to a cylinder, written for the oscillation concentration of an Ox form, is [144]... 

Carbon fibers have not been studied as extensively as GC or graphite, and in most cases the fiber is pretreated. Thus most of the electrochemical properties of fibers are discussed in the next section, Preparation. A few general points are useful here, dealing with size and resistance. Since the majority of carbon fibers are 5-15 pm in diameter, they will exhibit nonplanar diffusion under most conditions, whether they are used as disks or as cylinders. For example, VDt for a typical analyte (D = 5 x 10 6 cm2/s) equals 2.2 pm at 10 ms. This is a significant fraction of a typical fiber diameter, so diffusion will become nonplanar even at short times. Thus any experiment lasting more than a few milliseconds will deviate from a response predicted for planar diffusion. Note that the deviation depends on whether the fiber end is used as a disk electrode or an exposed fiber is used as a cylinder, but quantitative theories have been presented for both cases [48]. 

Both disk-shaped and cylinder electrodes can be fashioned with metal microwires and carbon fibers. Two practical considerations that must be kept in mind when working with small wires and fibers are their resistivity and their delicacy. The former consideration demands that only short sections of the microwire or fiber be used for electrode construction, while the latter mainly represents a significant source of frustration for the novice electrode maker. A 5-/ m-radius platinum wire at times seems to break just because you ve looked at it With a little practice, however, handling these small materials becomes straightforward. 

Ultramicroelectrodes can also greatly benefit modem microseparation techniques such as open-tube liquid chromatography or capillary-zone electrophoresis (CZE) (73). For example, cylinder-shaped carbon or copper fibers can be inserted into the end of the CE separation capillary (e.g., see Fig. 3.26). Such alignment of the working electrode with the end of the capillary represents a challenge in combining electrochemistry with CZE. 

---