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We consider compositional and structural factors which can affect the activity of bixbyite alpha-Mn2O3 towards the oxygen reduction reaction (ORR) and the stability of this oxide in alkaline solution. We compare electrochemistry of undoped, Fe and Al-doped alpha-Mn2O3 with bixbyite structure and braunite Mn7SiO12 having bixbyite-related crystal structure, using the rotating disk electrode (RDE), the rotating ring-disk electrode (RRDE), and cyclic voltammetry (CV) techniques. All manganese oxides under study are stable in the potential range between the ORR onset and ca. 0.7 V vs. Reversible Hydrogen Electrode (RHE). It is found that any changes introduced in the bixbyite structure and/or composition of alpha-Mn2O3 lead to an activity drop in both the oxygen reduction and hydrogen peroxide reactions in this potential interval. For the hydrogen peroxide reduction reaction these modifications also result in a change in the nature of the rate-determining step. The obtained results confirm that due to its unique crystalline structure undoped alpha-Mn2O3 is the most ORR active (among currently available) Mn oxide catalyst and favor the assumption of the key role of the (111) surface of alpha-Mn2O3 in the very high activity of this material towards the ORR. (C) 2020 Elsevier Ltd. All rights reserved.

Coulomb blockade is a fundamental phenomenon in physics enabling transfer of individual electrons one by one into electrically isolated nanostructures such as nanowires or quantum dots and thereby creation of sources of single electrons. Nowadays, solid-state single-electron sources are key elements of the emerging new technologies of quantum information processing and single-electron electronics. Moreover, advanced research in free-electron quantum optics and developments in electron microscopy require the point sources of free electrons in vacuum, which can be controlled on a single electron level. However, up to now, single-electron vacuum guns were not realized in practice. The problems to be solved include the formation of stable tip-shaped heterostructured emitters and control of liberated electrons in time and energy domain. Here we overcome these challenges by creating a field emission (FE) electron source based on a carbon nanowire coupled to an ultra-sharp diamond tip by a tunnel junction. Using energy spectroscopy, we directly observe Coulomb oscillations of the electron Fermi level in the nanowire at room temperature and FE currents up to 1 $\mu$A. We reveal that the oscillations are suppressed at high FE current either by Joule heating or due to the high tunneling resistance, depending on the nanowire charging time, ranging from about 25 fs to 1 ps. We anticipate that the combination of introduced carbon single-electron sources with laser-induced gating is highly promising for the creation of coherent ultrashort free-electron bunches of interest for low-energy electron holography and ultrafast electron or X-ray imaging and spectroscopy., Comment: 25 pages, 10 figures