Electroactive layers Modified electrodes

Bard, A.J. and Faulkner, L.R. (2001) Electrochemical Methods Electroactive Layers and Modified Electrodes, John Wiley and Sons, New York. 

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

Considerable potential exists to design surface modified electrodes which can mimic the behaviour of electronic components. For example, a rectifying interface can be produced by using two-layer polymer films on electrodes. The electroactive species in the layers have different redox potentials. Thus electron transfer between the electrode (e.g. platinum) and the outer electroactive layer is forced to occur catalytically by electron transfer mediation through the inner electroactive layer. 

The modified electrode showed a very good stability even at pH 9. Although at this pH a decrease in the redox peaks in CV is observed after 250 cycles, the PB layer is still highly electroactive and sufficient to catalyse the H2O2 reduction. In fact, stirred batch amperometric measurements of H202 (10 pm oil ) were carried out before and after the continuous cycling at pH 9, and the decrease of the signal was only 10% of the initial value. 

Mousty, C., Therias, S., Forano, C. and Besse, J. P. (1994). Anion-exchanging clay-modified electrodes - synthetic layered double hydroxides intercalated with electroactive organic-anions. J. Electro anal. Chem. 374, 63. 

We are investigating the effects of binding non-electroactive molecules to electrode surfaces. The attached layer will be sufficiently thin (ca. 1 monolayer) that electron transfer across the electrode/electrolyte interface will not be inhibited. However, other surface properties may be advantageously modified. For semiconductor electrodes, desirable changes include suppression of the photo-activated surface corrosion and shifts in the flatband potential. We are seeking to improve the performance of semiconductor liquid-junction solar cells by these means. 

The deliberate modification of electrode surfaces by coating with one or more layers of electroactive material has been used for a variety of purposes. Solar energy conversion, electrochromism, corrosion protection, and electrocatalysis are but a few of the applications which are currently of interest. The use of in situ Raman spectroscopic studies can help to determine the structural characteristics of electrode coatings at the molecular level and can provide information on the mechanisms of electrochemical reactions occurring at modified electrode surfaces. 

Chapter 14. Electroactive Layers and Modified Electrodes Initial conditions must also be supplied, for example,... 

---