Redox Potentials as Reactivity Descriptors in Electrochemistry

Abstract

A redox catalyst can be present in the solution phase or immobilized on the electrode surface. When the catalyst is present in the solution phase the process can proceed via inner-(with bond formation, chemical catalysis) or outer-sphere mechanisms (without bond formation, redox catalysis). For the latter, log k is linearly proportional to the redox potential of the catalysts, E°. In contrast, for inner-sphere catalyst, the values of k are much higher than those predicted by the redox potential of the catalyst. The behaviour of these catalysts when they are confined on the electrode surface is completely different. They all seem to work as inner-sphere catalysts where a crucial step is the formation of a bond between the active site and the target molecule. Plots of (log i) E versus E°give linear or volcano correlations. What is interesting in these volcano correlations is that the falling region corresponding to strong adsorption of intermediates to the active sites is not necessarily attributed to a gradual surface occupation of active sites by intermediates (Langmuir isotherm) but rather to a gradual decrease in the amount of M(II) active sites which are transformed into M(III)OH inactive sites due to the applied potential.

Enhancing the electrocatalytic activity of Fe phthalocyanines for the oxygen reduction reaction by the presence of axial ligands: Pyridine-functionalized single-walled carbon nanotubes

“…In all systems, the presence of two distinct redox processes are observed and attributed to the Fe(II)/(I) and Fe(III)/(II) reversible transitions. [24] , [86] Both Fe(II)/(I) and Fe(III)/(II) formal potentials of FeN4 are related to the electrocatalytic activity for several electrochemical reactions [23][24][25]28,34,[36][37][38][39] and it can be used as reactivity descriptors. In our case, the Fe(III)/(II) redox potential (highlighted in Figure 6B is the reactivity descriptor for ORR.…”

“…Therefore, they can act as mediators in electron transfer processes involving numerous reactants. [18][19][20][21][22][23][24][25] The FeN4 macrocycles are particularly interesting because of their structural similarity with the FeN4 active site in hemoproteins, comprising important biological catalysts such as cytochromes and peroxidases as well as the O2carrier hemoglobin. [26] Even though such unadulterated MN4 macrocycles lack long term stability in the corrosive environment of acidic fuel cells and even perhaps in AEM fuel cells, they are interesting as a structural model with clearly identified active centers.…”

Electrocatalytic Self-Assembled Nanoarchitectonics for Clean Energy Conversion Applications

“…The HER, HOR, OER, and ORR reactions occur through an internal sphere electron transfer process and involve electrocatalytic phenomena. Thus, their efficiency is directly related to the electrodic materials used as anode and cathode [3,9,10]. Therefore, it requires electrodes with specific activity for the reaction involved (Figure 1b-d).…”

“…Graphite electrodes modified by a simple process involving the physical adsorption process of intact MN4 complexes on their surface have made it possible to perform fundamental studies to establish reactivity descriptors for specific reactions such as ORR and, with this, advance in the search for the best catalyst for this reaction through a rational design [9,10] of the MN4 molecules. However, new experimental strategies and surface characterization techniques have allowed the development of more advanced electrode systems based on nanostructured systems of molecular catalysts as the main active component [6,[12][13][14].…”

Anthracene Substituted Co (II) and Cu (II) phthalocyanines; Preparations, Investigation of Catalytical and Electrochemical Behaviors

“…Phthalocyanines (Pcs) have had wide usage in different areas such as press inks, dyes, pigments and laser technology, beginning from their first synthesis early in the last century . Other areas of current interest include applications in liquid crystals, catalysis, dye‐sensitized solar cells, photodynamic therapy (PDT), biomedicine, electronic and optoelectronic devices, chemical sensors, nonlinear optical (NLO) materials …”