The recent prediction and subsequent creation of electron vortex beams in a number of laboratories occurred after almost 20 years had elapsed since the recognition of the physical significance and potential for applications of the orbital angular momentum carried by optical vortex beams. A rapid growth in interest in electron vortex beams followed, with swift theoretical and experimental developments. Much of the rapid progress can be attributed in part to the clear similarities between electron optics and photonics arising from the functional equivalence between the Helmholtz equations governing the free space propagation of optical beams and the time-independent Schrödinger equation governing freely propagating electron vortex beams. There are, however, key di↵erences in the properties of the two kinds of vortex beams. This review is concerned primarily with the electron type, with specific emphasis on the distinguishing vortex features: notably the spin, electric charge, current and magnetic moment, the spatial distribution as well as the associated electric and magnetic fields. The physical consequences and potential applications of such properties are pointed out and analysed, including nanoparticle manipulation and the mechanisms of orbital angular momentum transfer in the electron vortex interaction with matter.

Non-platinum group metal (non-PGM) oxygen reduction reaction (ORR) catalysts have been widely reported, but their application in proton exchange membrane fuel cells (PEMFCs) is challenging due to their poor performance in acidic environments. Here [BMIM][NTf2] ionic liquid (IL) modification of microporous ZnCoNC catalysts (derived from ZIF-ZnCo) is investigated to study their behavior in PEMFCs and to elucidate the catalytic mechanisms in practical operation. The high O2 solubility of ILs enhance the

One-dimensional (1D) single crystal AgPt alloy nanostructures show excellent catalytic activities and reasonable stability toward the oxygen reduction reaction (ORR) in fuel cell operation.

A clear understanding of catalytic activity enhancement mechanisms in fuel cell operation is necessary for a full degree translation of the latest generation of non-Pt/C fuel cell electrocatalysts into high performance electrodes in proton exchange membrane fuel cells (PEMFCs). In this work, PtNi nanowire (NW) array gas diffusion electrodes (GDEs) are fabricated from Pt nanowire arrays with Ni impregnation. A 2.84-fold improvement in the ORR catalytic activity is observed for the PtNi NW array GDE (cf. the Pt NW array GDE) using half-cell GDE measurement in 0.1 M HClO4 aqueous electrolyte at 25°C, in comparison to only 1.07-fold power density recorded in PEMFC single cell test. Ionomer is shown to significantly increase electrochemically active surface area of the GDEs, but the PtNi NW array GDE suffers from Ni ion contamination at a high temperature, contributing to decreased catalytic activities and limited improvement in operating PEMFCs. ASSOCIATED CONTENT Supporting Information. Contains supporting SEM, SEM-EDX, XRD, XPS, ex-situ GDE and single cell testing data (1 file .docx) AUTHOR INFORMATION

A general orbital angular momentum (OAM) mode selection principle is put forward involving the rotationally symmetric superposition of chiral states. This principle is not only capable of explaining the operation of spiral zone plate holograms and suggesting that naturally occurring rotationally symmetric patterns could be inadvertent sources of vortex beams, but more importantly, it enables the systematic and flexible generation of structured OAM waves in general. This is demonstrated both experimentally and theoretically in the context of electron vortex beams using rotationally symmetric binary amplitude chiral sieve masks. 42.50.Tx Vortices are common to all wave phenomena [1], including tornadoes at the large scale and superfluid helium at the small scale. The prototypical example of these is the optical vortex beam [2], followed recently by the electron vortex beam [3][4][5] and there are early studies of neutron [6] and atom vortex beams [7]. Vortex beams are characterized by a phase singularity described by an exp ℓ azimuthal phase factor, where θ is the azimuthal angle and ℓ stands for the winding number and is also called the topological charge. Vortex beams are of special interest because of their quantized OAM of ℓħ per particle [1] and have led to various applications in the contexts of super-resolution microscopy [8], nano-manipulation [9], astronomy [10] and crystallography [11]. In contrast to the intrinsic (spin) angular momentum, OAM can be very large and, as such, vortex beams can lead to new physics [12,13] and have potential in multiplex free-space communication [14] and quantum information [15]. PACS:The establishment of a toolbox for flexible vortex beam generation is the key to the development of science and technology involving vortex beams. If the vortex beam wavefunctions are known exactly, they can be generated either by direct phase manipulation (or wavefront shaping) [16] using, for example, spiral phase plates [3,6,17], spinto-orbital angular momentum convertors [18], and by phase encoding techniques through diffraction involving computer-generated holograms (CGHs) [19], including fork grating [4,5] and spiral zone plates [20][21][22]. Other techniques require case by case analysis to identify the nature of the vortex beams produced, as in the case involving the Aharonov-Bohm effect experienced by a charged particle in a suitable magnetic field [23,24]. More recent methods make use of photon sieves [25][26][27] and Vogel spiral arrays [28,29] as diffractive elements for vortex beam generation.Here we put forward a general principle for vortex beam generation in rotationally symmetric systems. We examine the role of rotational symmetry, not only for the purpose of generating a specific vortex wavefunction, but also for the essential symmetry elements that must be possessed by any vortex-related state by virtue of its characteristic azimuthal phase factor exp ℓ . The principle would enable us to understand, in a novel deconstructive manner, the generation of individual pure vorte...