Traditional design strategies for catalytic materials for HER rely on the volcano plot paradigm, where the metal-Had bond energy is used as a single activity-descriptor. However, the use of metal-Had energetics alone completely fails to predict the behavior of HER in alkaline electrolytes. We have persistently drawn attention to the importance of kinetic synergy (bifunctional nature) of the HER in alkaline electrolytes, where both the beneficial OHad–M and Had–M energetics are necessary for achieving a highly effective catalyst [1-4]. This is especially evident on metal surfaces, decorated with small clusters of Ni(OH)2, where an order of magnitude increase in catalytic activity for the HER can most often be achieved [5]. Furthermore, we have stressed the importance of spectator species in any satisfactory description of most common electrocatalytic reactions, with HER being no exception [6,7]. Nonetheless, both topics are still a subject of many academic discussions up to this day. In this presentation, we will focus on the electrochemical behavior of Ni in alkaline electrolyte. We argue that the experimentally obtained “intrinsic” HER activity values reported in the available literature are misleading possibly due to two main challenges: i) Ni has very complex surface chemistry (because of the metal-hydride formation during the HER and/or the partial coverage by various (hydr)oxide species, even in the HER potential region), and ii) analysis of the material intrinsic properties has most often not been conducted on well-defined systems. By overcoming these two challenges, we will show the role of the individual Ni surface species in the kinetics of the HER, identify the active sites involved in the reaction, and present strategies for controlling and manipulating the electrochemical interface to enhance the efficiency of Ni-based material for HER. Finally, we again highlight the importance of 2 major factors controlling the rate of HER in alkaline solutions on Ni-based catalysts: the availability of active sites on Ni electrode surface [the 1−Θad term] and the energetics of the activated water complex [the ΔG0# (H2O) term]. Through meticulous experimental design, we were able to isolate and examine these variables, revealing their distinct influence on reaction kinetics. We will discuss our findings in the broader context of the volcano plot for HER. References: [1] Stamenkovic, V.R., Strmcnik, D., Lopes, P.P. and Markovic, N.M., Nature Materials, 16 (2017) [2] Subbaraman, R., Tripkovic, D., Strmcnik, D., Chang, K.C., Uchimura, M., Paulikas, A.P., Stamenkovic, V.R., Markovic, N.M., Science, 334 (2011) [3] Subbaraman, R., Tripkovic, D., Chang, K.C., Strmcnik, D., Paulikas, A.P., Hirunsit, P., Chan, M., Greeley, J., Stamenkovic, V.R., Markovic, N.M., Nature Materials, 11 (2012) [4] Strmcnik, D., Lopes, P. P., Genorio, B., Stamenkovic, V. R. & Markovic, N. M., Nano Energy, 29 (2016) [5] Danilovic, N., Subbaraman, R., Strmcnik, D., Chang, K.C., Paulikas, A.P., Stamenkovic, V.R., Markovic, N.M., Angewandte Chemie International Edition, 124 (2012) [6] Stamenkovic, V.R., Fowler, B., Mun, B.S., Wang, G., Ross, P.N., Lucas, C.A., Markovic, N.M., Science, 315 (2007) [7] Strmcnik, D., Uchimura, M., Wang, C., Subbaraman, R., Danilovic, N., Van Der Vliet, D., Paulikas, A.P., Stamenkovic, V.R. and Markovic, N.M., 2013, Nature Chemistry, 5 (2013)

Electrochemical energy storage and conversion technologies, which include fuel cells, electrolyzers, batteries, photoelectrochemical devices are at the forefront of the transition to a sustainable future. Although they have all been in use for more than half a century, they are far from reaching their full potential as defined by the laws of thermodynamics. Their performance rests almost entirely on the electrochemical interface - the boundary between the electronic conductor (electrode) and the ionic conductor (electrolyte). The desire of both phases to reduce the surface energy as well as the appearance of electrochemical potential across the interface can manifest itself as the formation of unique (near)surface atom arrangements (e.g. surface relaxation or reconstruction), as significant differences in electrode composition close to the surface (e.g. segregation profile), via substrate-adsorbate covalent and non-covalent interactions, via formation of a passive film as well as ordering of solvent and/or electrolyte molecules several nm away from the surface. This extremely complex and sensitive "interfacial bridge", is a consequence of inherent incompatibility of two materials, brought into contact, and is very hard to control. However, to control it means to control the energy efficiency, power density, durability and safety – the most important metrics of any energy conversion and storage device. In this presentation we will discuss, how the chemical nature of non-covalently and covalently [1,2] adsorbed species as well as thicker passive films and their morphology at the electrochemical interface affect the individual terms of the common rate equation [1], including the free energy of adsorbed intermediates and adsorbed spectators, mass transport, availability of active sites and electronic and ionic resistivity for common electrocatalytic reactions in acid and alkaline aqueous as well as in non-aqueous media on a plethora of metal electrodes (Pt, Ir, Au, Ni, Cu) as well as carbon. We will draw parallels between HER, OER, HOR and ORR in electrolyzers [1], fuel cells [2] and Li-ion batteries [3,4]. The interphase properties will be discussed through the lens of deviations of modified electrode properties from its intrinsic properties. Examples of artificially modified interfaces [5,6] will be given to demonstrate our ability to tailor their activity, stability and selectivity to our liking. [1] Strmcnik, D. Lopes, P.P., Genorio, B., et al. Design principles for hydrogen evolution reaction catalyst materials, Nano Energy, 29, 29-36 (2016) [2] Strmcnik, D., Uchimura, M., Wang, C. et al. Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption. Nature Chem 5, 300–306 (2013) [3] Strmcnik, D., Castelli, I.E., Connell, J.G. et al. Electrocatalytic transformation of HF impurity to H2 and LiF in lithium-ion batteries. Nat Catal 1, 255–262 (2018) [4] Martins, M., Haering, D., Connell, J.G., et al. Role of Catalytic Conversions of Ethylene Carbonate, Water, and HF in Forming the Solid-Electrolyte Interphase of Li-Ion BatteriesACS Catalysis 13, 9289-9301 (2023) [5] Zorko, M. Martins, P.F.B.D., Connell, J.G. et al. Improved Rate for the Oxygen Reduction Reaction in a Sulfuric Acid Electrolyte using a Pt(111) Surface Modified with Melamine, ACS Applied Materials & Interfaces 13, 3369-3376 (2021) [6] Strmcnik, D., Escudero-Escribano, M., Kodama, K. et al. Enhanced electrocatalysis of the oxygen reduction reaction based on patterning of platinum surfaces with cyanide. Nature Chem 2, 880–885 (2010)

The approach used in the present work involves investigating the corrosion protection properties of mixed inhibitors for copper and aluminium substrates in chloride-containing solutions, which serve as a benchmark for studies of the alloy AA2024, with Cu and Al being the main culprits for localized corrosion. A synergistic mixture of inhibitors could find potential applications in novel blending combinations, such as in cooling water as an inhibitor in closed systems or incorporated in various protective coatings as additives, nano-containers, etc. If possible, the protective inhibitor film should show irreversibility of inhibition which can be defined as the ability to, once formed, retains its protective properties when the concentration of the corrosion inhibitor decreases. This irreversibility of the protective properties is essential for long-term protection. Inhibitory action of organic molecules, 2-mercaptobenzimidazole (MBI) and octylphosphonic acid (OPA) and their binary combinations on aluminium, copper and aluminium alloy 2024-T3 was investigated in chloride environments by conventional electrochemical methods and surface-analytical techniques [1,2]. In addition, the influence of pre-treatment of the metal surface and the choice of solvent for liquid-phase deposition on adsorption of MBI and OPA was studied on individual metals, Al and Cu [3]. Although OPA is not an inhibitor for Cu, it can synergistically boost corrosion inhibition of copper when added to MBI. In contrast, a synergistic effect between MBI and OPA as corrosion inhibitors was not observed for AA2024-T3. The mechanism was proposed where the thickness and structure of the surface layer are dependent on the pH. For the sample exposed to MBI at pH 5.5, where the Cu2O is stable, a thin Cu(I)MBI film is formed. In contrast, when exposed to the mixture of MBI and OPA at a pH of 4, the amount of produced Cu+ ions is boosted, and a much thicker Cu(I)MBI film forms by dissolution-precipitation mechanism. This layer exhibits high inhibitory effectiveness on copper substrate. At even lower pH, the thick compact Cu(I)MBI film does not form due to intensive dissolution of the Cu2O underlayer, resulting in a voluminous product. The postulated mechanism is confirmed by electrochemical data and composition of the layer by X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy and focused-ion beam scanning electron microscopy with chemical analysis. Immersion of AA2024 in an OPA-containing solution caused significant localized corrosion, while no local electrochemical activity on AA2024 was detected in an MBI-containing solution, indicating that the MBI inhibitor was very effective against pitting corrosion. Figure shows FIB/SEM (cross-section) analysis of local corrosion induced by Al2CuMg phase after 24 h immersion of AA2024 in 3 wt.% NaCl containing 1 mM MBI. The chemical analysis employed at the cross-section (yellow rectangles) of the Al2CuMg revealed that the MBI layer reduces the dissolution rate of dealloying of this phase and the rate of oxygen reduction on the copper remnant sites. This study shows that the behaviour of each combination of inhibitor and metal substrate is unique and cannot be translated to the more complex system such as alloy. Therefore, a profound understanding of the inhibition mechanism of individual metals is a prerequisite for further investigation of the corrosion inhibition of aluminium alloys. Acknowledgements: The financial support of the project by the Slovenian Research Agency is acknowledged (grants No. P1-0134, P2-0393 and BI-US/22-24-140) is acknowledged. Barbara Kapun, BSc, is acknowledged for FIB-SEM-EDS analysis. References: [1] D.K. Kozlica, A. Kokalj, and I. Milošev, Corros. Sci., 182 (2021) 109082 [2] D.K. Kozlica, J. Ekar, J. Kovač, and I. Milošev, J. Electrochem. Soc., 168 (2021) 031504 [3] D.K. Kozlica, and I. Milošev, to be submitted. Figure 1

Terms such as “charge” and “oxidation state” appear frequently in the literature. The problem is that they are often viewed to be synonymous. However, they are fundamentally different concepts using distinct notations. The aim of the present discussion is to attract the attention of researchers from various fields of science in order to prevent further use of misleading interpretations.