Scanning Electrochemical Microscopy operating modes

Uitto OD, White HS, Aoki K (2002) Diffusive-convective transport into a porous membrane. A comparison of theory and experiment using scanning electrochemical microscopy operated in reverse imaging mode. Anal Chem 74(17) 4577-4582. doi 10.1021/ac0256538... 

Most scanning electrochemical microscopy (SECM) experiments are conducted in the amperometric mode, yet microelectrodes have for many years been used as potentiometric devices. Not surprisingly, several SECM articles have described how the tip operated in the potentiometric mode. In this chapter we aim to present the background necessary to understand the differences between amperometric and potentiometric SECM applications. Since many aspects of SECM are covered elsewhere in this monograph, we have focused on the progress made in the held of potentiometric microelectrodes and presented it in the context of SECM experiments. Starting with an historical perspective, the key discoveries that facilitated the development and applications of micro potentiometric probes are highlighted. Fabrication techniques and recipes are reviewed. Basic theoretical principles are covered as well as properties and technical operational details. In the second half of the chapter, SECM potentiometric applications are discussed. There the differences between the conventional amperometric mode are developed and emphasized. 

Chemical sensitivity can be conferred to AFM by coating the tip with covalently linked monolayers which affect the tip-surface interaction the method is called chemical force microscopy [77]. Additional modulation of the piezo actuator operating in z-direction and evaluation of the force signal can be used to measure the adhesion force between a surface and a chemically modified AFM tip [78]. Metal coated AFM tips can be used in a scanning electrochemical microscopy (SECM, see p. 264) mode [79] in studies of crystal dissolution or growth where surface processes are associated with considerable fluxes of species. 

FIGURE 18.14 Schematic representation of different operation modes of SECM for screening electrocatalysts. Ej, tip potential ij, tip current E, substrate potential and i, substrate current. (Reprinted with permission from Amemiya, S., Bard, A.J., Fan, F.-R.F., Mirkin, M.V., and Unwin, P.R., Scanning electrochemical microscopy, An . Rev. Anal. Chem., 1, 95-131, 2008. Copyright 2008 Annual Reviews.)... 

FIGURE 8.16 Schematic representation of the SECM operating in the facilitated (A) and simple (B) IT feedback mode. (A) Potassium ions are transferred from the pipette into DCE by interfacial complexation with DB18C6 (Equation 8.26) and from DCE to the bottom aqueous layer by interfacial dissociation mechanism (Equation 8.27). (Reprinted from J. Electroanal. Chem., 439, Shao, Y. and Mirkin, M.V., Scanning electrochemical microscopy (SECM) of facilitated ion transfer at the liquid/liquid interface, 137-143, Copyright 1997, with permission from Elsevier Science S.A.) (B) Positive feedback is due to IT from the bottom (aqueous) layer into the organic phase. Electroneutrality in the bottom layer is maintained by reverse transfer of the common ion across the ITIES beyond the close proximity of the pipette where its concentration is depleted. 

Durability testing takes a long time in an operation environment, which is difficult as normally several thousand hours are necessary to obtain a meaningful conclusion. In the development of durability testing, some in situ and ex situ methods and techniques for material evaluation have been used, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), CV, EIS and so on. However, new electrochemical and/or physical techniques are desirable to gain a better understanding of durability failure modes and then improve fuel cell durability and reliability. 

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