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Tatyana Alexandrovna Kryukova (Figure 1), a Russian scientist and electrochemist, made important contributions to electroanalytical chemistry (Figure 2), particularly working in close collaboration with Professor Aleksandr Naumovich Frumkin, who was the greatest Russian scientist in the area of electrochemistry. Kryukova is particularly remembered for developing the theory of polarographic maxima, which were observed as a sharp increase in the current produced upon polarographic measurements under some conditions (Figure 3). These current peaks originated from tangential movements (rotation) of a mercury droplet electrode, then stimulating diffusion in the depletion layer and current increase. Kryukova experimentally observed and theoretically explained the formation and then inhibition of these peaks upon adsorption of organic substances (mostly surfactants) on a mercury droplet electrode. It should be noted that for the first time, the effect of surfactants on polarographic measurements was reported in the 1920s in the laboratory of Professor Jaroslav Heyrovský (polarography inventor and Nobel Prize laureate in 1959), and the study of this effect was published in 1931. However, the study of the surfactant effect performed by Heyrovský was only fragmental. Then, the credit for a detailed explanation of the reasons for the polarographic maxima origin and a systematic study of this effect belongs to Kryukova.

In 1949, Kryukova discovered another very unusual phenomenon, later named as “Kryukova effect” (Figure 4). This effect was observed as a sudden decrease in the current at very negative potentials upon polarographic reduction of anionic species, for example, persulfate or dichromate anions, particularly when a very diluted supporting electrolyte was present in the analyte solution. This current minimum disappeared when the electrolyte concentration was increased. Later, in 1952, Frumkin and G. M. Florianovich (a graduate student at that time) theoretically explained the effect observed by Kryukova as the repulsion of redox anions from the negatively charged electrode surface, as predicted by the Frumkin theory of 1933. This is exactly why the effect was only observed for anionic redox species particularly with very negative potentials, providing a negative charge at the working electrode. As expected, the high concentration of the supporting electrolyte was screening the electrostatic interaction between the negative Hg droplet electrode and the negative redox-anions, then eliminating the current decrease.

It should be noted that the electrochemical study of persulfate ions when the “Kryukova effect” was observed, had not only gained theoretical interest demonstrating a fundamental electrostatic effect at polarized electrodes, but it was also practically important as a part of the Russian uranium project because they were used as a reagent in the separation of uranium isotopes.

Kryukova published many important research pa

The general concept of fuel cells starts from the experiments of British physicist William Grove who published the first results on fuel cells in 1839. He used hydrogen and oxygen as a fuel and oxidizer, respectively, reacting on platinum catalytic electrodes and generating electric power. However, his research was considered only as scientific proof of the process reversed to the water electrolysis with no practical importance. Indeed, the cell invented by Grove produced a very small current and voltage over a short time. Obviously, after the concept demonstration, some engineering had to be done for improving the cell efficiency to make it feasible for practical use.

During the late 1880s, two British chemists, Ludwig Mond and his assistant Carl Langer (Figure 1), developed a fuel cell with a longer service life with improved geometry of the catalytic electrodes and flow channels (Figure 2). They used the known scientific concept from Grove's cell, but with the improved engineering. Their fuel cell generated 6 amps per square foot current density and 730 mV voltage. The cell operated with coal-derived gas as a fuel and air (actually oxygen in the air) as an oxidizer. The cell was filled with diluted sulfuric acid and included thin perforated platinum electrodes separated with a porous nonconducting membrane. The first engineered fuel cell was demonstrated and patented in 1889. Note that Ludwig Mond and Carl Langer were the first to introduce the term “fuel cell” which is commonly used now.

The author declares that he has no conflict of interest.

John Alfred Valentine Butler was the first to connect the kinetic electrochemistry built up in the second half of the twentieth century with the thermodynamic electrochemistry that dominated the first half. John Alfred Valentine Butler had, to his credit, not only the first exponential relation between current and potential (1924) but also (along with R.W. Gurney) the introduction of energy-level thinking into electrochemistry (1951).

However, Butler was not alone in this study and therefore it is necessary to give credit also to Max Volmer, a great German surface chemist, and his student (at that time) Erdey-Gruz. Butler's very early contribution in 1924 and the Erdey-Gruz and Volmer contribution in 1930 form the basis of phenomenological kinetic electrochemistry. The resulting famous Butler-Volmer equation is very important in electrochemistry.

The author declares no conflict of interest.

Hydrogen evolution and oxidation reactions (HER/HOR) are the most fundamental reactions in electrocatalysis. Despite the practical significance, the mechanisms of HER/HOR in aqueous solutions are still elusive. Various theories have been proposed to rationalize the pH effect, cation effect, and structure effect of HER/HOR but none of them can explain all observations. In this review, we discuss four schools of thought for the HER/HOR, focusing on the strengths and shortcomings of each hypothesis and highlighting the magnitude of electrochemical interface structure in hydrogen electrocatalysis.

Passive films are essential for the longevity of metals and alloys in corrosive environments. A great deal of research has been devoted to understanding and characterizing passive films, including their chemical composition, uniformity, thickness, porosity, and conductivity. Many characterization techniques are conducted under vacuum, which do not portray the true in-service environments passive films will endure. Scanning electrochemical probe microscopy (SEPM) techniques have emerged as necessary tools to complement research on characterizing passive films to enable the in situ extraction of passive film parameters and monitoring of local breakdown events of compromised films. Herein, we review the current research efforts using scanning electrochemical microscopy, scanning electrochemical cell microscopy (or droplet cell measurements), and local electrochemical impedance spectroscopy techniques to advance the knowledge of local properties of passivated metals. The future use of SEPM for quantitative extraction of local film characteristics within in-service environments (i.e., with varying pH, solution composition, and applied potential) is promising, which can be correlated to nanostructural and microstructural features of the passive film and underlying metal using complementary microscopy and spectroscopy methods. The outlook on this topic is highlighted, including exciting avenues and challenges of these methods in characterizing advanced alloy systems and protective surface films.