Electrochemical contributions: Julius Tafel (1862–1918)

Julius Tafel (Figure 1) was a Swiss chemist and electrochemist. Tafel started his scientific career working on the field of organic chemistry with Hermann Emil Fischer, but soon changed his interests to electrochemistry after his work with Wilhelm Ostwald. Then, Tafel's work was concentrated on the electrochemistry of organic compounds and relation between rates of electrochemical reactions and applied overpotentials. Tafel's name is presently associated with many electrochemical terms: Tafel equation, Tafel slope, Tafel rearrangement, and Tafel mechanism of hydrogen evolution. The Tafel equation and the corresponding Tafel plot (Figure 2) in electrochemical kinetics are relating the rate of an electrochemical reaction (in terms of the current density [i] to the overpotential [η] applied). The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification. Indeed, it represents a simplified version of the theoretically derived Butler–Volmer equation (Figure 2) when the overpotentials are rather high (|η| > 0.1 V; Tafel region). For a large overpotential (anodic or cathodic), one part of the Butler–Volmer equation becomes negligible while the second part can be transformed to the Tafel equation. The Tafel slope (A) shows how much the overpotential needs to be increased to increase the reaction rate (which is current in electrochemistry) by 10-fold. In a simple case of a one-electron transfer electrochemical reaction, the Tafel slope is determined by the symmetry factors (αa and αc), which are usually ca. 0.5, translating to a Tafel slope (A) of 120 mV. The Tafel equation, empirically derived from his experiments with electrochemical evolution of H2, laid the background for a new scientific area of electrochemical kinetics. Tafel is also credited for the discovery of the catalytic mechanism of hydrogen evolution (the Tafel mechanism), construction of a new kind of hydrogen coulometer used in his study of H2 evolution. Also, he demonstrated that hydrocarbons with isomerized structures can be generated upon electrochemical reduction of the respective acetoacetic esters (named Tafel rearrangement) (Figure 3). This was an important method for the synthesis of certain hydrocarbons from alkylated ethyl acetoacetate, a reaction accompanied by the rearrangement reaction of the alkyl group. The author declares no conflict of interest.