Aqueous-phase electrochemical reduction of carbon dioxide requires an active, earth-abundant electrocatalyst, as well as highly efficient mass transport. Here we report the design of a porous hollow fibre copper electrode with a compact three-dimensional geometry, which provides a large area, three-phase boundary for gas–liquid reactions. The performance of the copper electrode is significantly enhanced; at overpotentials between 200 and 400 mV, faradaic efficiencies for carbon dioxide reduction up to 85% are obtained. Moreover, the carbon monoxide formation rate is at least one order of magnitude larger when compared with state-of-the-art nanocrystalline copper electrodes. Copper hollow fibre electrodes can be prepared via a facile method that is compatible with existing large-scale production processes. The results of this study may inspire the development of new types of microtubular electrodes for electrochemical processes in which at least one gas-phase reactant is involved, such as in fuel cell technology.

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a b s t r a c tSilicon carbide (SiC) membranes have shown large potential for applications in water treatment. Being able to make these membranes in a hollow fiber geometry allows for higher surface-to-volume ratios. In this study, we present a thermal treatment procedure that is tuned to produce porous silicon carbide hollow fiber membranes with sufficient mechanical strength. Thermal treatments up to 1500 1C in either nitrogen or argon resulted in relatively strong fibers, that were still contaminated with residual carbon from the polymer binder. After treatment at a higher temperature of 1790 1C, the mechanical strength had decreased as a result of carbon removal, but after treatments at even higher temperature of 2075 1C the SiC-particles sinter together, resulting in fibers with mechanical strengths of 30-40 MPa and exceptionally high water permeabilities of 50,000 L m À 2 h À 1 bar À 1 . Combined with the unique chemical and thermal resistance of silicon carbide, these properties make the fibers suitable microfiltration membranes or as a membrane support for application under demanding conditions.

Membrane separation under harsh conditions, such as high-p,T or in the presence of aggressive chemicals, requires a robust membrane support. In academia commonly ceramic disks are used for this purpose, but these disks posses a too low surface-area-to-volume ratio for practical applications. Ceramic hollow fibers potentially provide a much larger specific surface area, but applying a defect free thin selective layer on such structures is more intricate. Here we show the successful preparation of a thin polyamide layer on a thin porous hollow α-alumina fiber by interfacial polymerization of piperazine with trimesoyl chloride. Two aspects of the fabrication method are identified as particularly crucial for obtaining a high quality selective layer: i) the layer the ceramic surface should have a sufficient amount of hydroxyl groups for covalent attachment in order to avoid delamination, and ii) controlled drying steps are necessary to avoid local surplus or lack of liquid on the outer surface of the ceramic. To increase the hydroxyl group concentration, and to facilitate the presence of sufficient reactants in a large volume of small pores, the fibers have been coated with a layer of γ-alumina. Sufficiently long drying steps (20 mm) have been employed to avoid uneven drying over the length of the fiber. The obtained fibers show clean water fluxes in the range of 2-5 L m −2 h −1 bar −1 combined with a retention of Rose Bengal above 99%.