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Nanomaterials 2020, 10, 1407 13 of 26 electrolyzer. Water electrolyzers are expected to rank highly in the production of hydrogen in the coming years, for instance, in hydrogen-fueling stations. Water splitting can happen when applying electrical energy: H2O → H2 + 12O2 (4) In an electrolyzer, the above reaction is separated by an electrolyte (in liquid or solid form) into two half reactions. The HER occurs at the cathode: 2H+ +2e− →H2 (5) while the other half reaction, OER, occurs at the anode: 2H2O→O2 +4H+ +4e− (6) The hydrogen evolution reaction can happen in three elementary steps in acidic media. Reaction, referred to as the Volmer step, is the first step of the reaction, where the reduction of a proton on an active site of the catalyst surface takes place. Then, the evolution of molecular H2 follows, either through a second proton/electron transfer (Heyrovsky step) or through recombination of two hydrogen atoms adsorbed on the electrode surface (Tafel step). H3O+ + e− → Hads + H2O Hads + H3O+ + e− → H2 + H2O Hads + Hads → H2 (Volmer adsorption) (Heyrovsky desorption) (Tafel desorption) Tafel analysis is a very effective method to compare electrocatalytic activity and to elucidate the reaction mechanism of electrocatalysts. Analysis of the Tafel slope, i.e., the sensitivity of the electric current response to the applied potential can provide information associated with the rate determining steps of electrolysis. The experimentally observed Tafel slopes can be compared with the theoretically derived slopes assuming different rate-determining steps based on the microkinetic model. The kinetic parameters of the overall reaction, apparent Tafel slope, and reaction order, are determined by the rate-determining step and are subject to various criteria, such as the electrode material, surface crystal structure, electrolyte, overpotential, adsorbed species, and trace impurities [127]. Up to date, Pt and Pt-based materials are the most efficient electrocatalysts towards HER with excellent performance, since they exhibit almost zero onset potential and zero Gibbs free energy of hydrogen adsorption (∆GH) and Tafel slope around 30 mV/dec. In general, favorable electrocatalysts exhibit low overpotential, low Tafel slope values, and should be able to withstand high current density as well as be resistant to acidic media in which normally HER takes place. Different non-noble metal electrocatalysts have undergone intensive investigation over the past years, such as alloys [128,129], transition metal compounds MX (where M is Mo, W, Ni, Co, etc., and X is S, Se, P, C, N, etc.) [130–132], and carbonaceous nanomaterials [133]. Meanwhile, carbon nanomaterials feature tunable molecular structures, interesting composition chemistry, superior conductivity, and negligible environmental impact, and thus, they have been massively used as electrocatalysts toward HER. Even so, these catalysts still have not been able to minimize overpotential like their transition metal-based counterparts. This is why carbon nanomaterials have mostly been used as electrocatalytic supports. As a matter of fact, electrochemical activation and/or acidic treatment has proven to help effectively carbon nanomaterials such as CNTs [134,135] and fullerenes [136], to report outstanding catalytic performances for HER, comparable to that of Pt/C [135]. Proposed activation mechanisms for the enhanced catalytic activity are the increase in the number of oxygen functionalities, which was considered as the active sites for HER [134], or the “adjacent Tafel” mechanism [135], but still, further investigation is required. In order to reach a better understanding of this mechanism, the activation of CNHs was investigated by using different counter electrodes during CV measurements [137]. In a three-electrode cell, current passes between the working electrode and the counter electrode. In theory, the counter electrode can be ofPDF Image | Carbon Nanohorn-Based Electrocatalysts for Energy Conversion
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