Driving a liquid droplet with control of directional motion on a solid surface, by introducing a surface wettability gradient or external stimuli, has attracted considerable research attention. There still remain challenges, however, due to the slow response rate and slow speed of continuous liquid droplet motion on the structured surface. Here, an approach to continuously drive the underwater oil droplet with control of directional motion by the cooperative effects of an electric field and the gradient of a porous polystyrene microstructure is demonstrated. The gradient microstructure induces the liquid droplet to take on an asymmetrical shape, causing unbalanced pressure on both ends to orient the droplet for motion in a particular direction. Meanwhile, the electric field decreases the contact area and the corresponding viscous drag between the droplet and the gradient‐structured surface. Then, the unbalanced pressure pushes the underwater oil droplet to move directionally and continuously at a certain voltage. This work provides a new strategy to control underwater oil droplets and realize unidirectional motion. It is also promising for the design of new smart interface materials for applications such as electrofluidic displays, biological cell and particle manipulation, and other types of microfluidic devices.

Summary Converting CO 2 into value-added chemical fuels and functional materials by CO 2 reduction reaction (CO 2 RR) is conducive to achieving a carbon-neutral energy cycle. However, it is still challenging to efficiently navigate CO 2 RR toward desirable products. Herein, we report a facile strategy to extend product species in borate-containing molten electrolyte at a positively shifted cathodic potential with a high current density (e.g. 100 mA/cm 2 ), which can selectively electro-transform CO 2 into desired products (either CO or solid carbon nanofibers, respectively reaching a high selectivity of ∼90%). The borates can act as a controller of electrolyte alkalinity to buffer the concentration of sequentially generated O 2− during CO 2 RR, positively shifting the reduction potential of the captured CO 2 and concurrently extending the product species. The sustainable buffering effect is available under CO 2 atmosphere. Compared with borate-free electrolyte, the CO 2 conversion efficiency is over three times higher, while the electrolysis energy consumption is decreased by over 40%.

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