Light-Augmented Multi-ion Interaction in MXene Membrane for Simultaneous Water Treatment and Osmotic Power Generation.

The mixing of wastewater and natural water releases abundant osmotic energy. Harvesting this energy could significantly reduce the energy and economic cost of water treatment, leading to sustainable wastewater treatment technology. Yet, such energy harvesting is highly challenging because it requires a material that is highly permeable to nontoxic ions while rejecting toxic ions in wastewater to reach high power density and prevent environmental pollution. In this work, we demonstrate that a light-augmented biomimetic multi-ion interaction in an MXene membrane can simultaneously realize high permeability of Na+ ions for enhanced osmotic power generation and high selectivity to heavy metal ions up to a ratio of 2050 for wastewater treatment. The Na+ permeability is enhanced by the photothermal effect of the MXene membrane. The transport of heavy metal ions, however, is suppressed because, under angstrom-confinement, heavy metal ions are strongly electrostatically repelled by the increased number of permeating Na+ ions. As a result, the membrane can stably generate osmotic power from simulated industrial wastewater, and the power density can be enhanced by 4 times under light illumination of approximate 1 sun intensity. This work highlights the importance of multi-ion interaction for the transport properties of ionic materials, which remains rarely investigated and poorly understood in previous studies.

Janus carbon nitride membrane for robust and enhanced nanofluidic power generation from wastewater.

Nanofluidic membranes have shown great promise in harvesting osmotic power. Yet, previous studies extensively focused on osmotic energy released by the mixing of seawater and river water, while there exist many other osmotic energy sources, such as the mixing of wastewater with other water. However, harvesting the osmotic power of wastewater is highly challenging because it requires the membranes to have environmental remediation capabilities to avoid pollution and biofouling, which has not been satisfied by previous nanofluidic materials. In this work, we demonstrate that a Janus carbon nitride membrane can be used for simultaneous power generation and water purification. The Janus structure of the membrane generates asymmetric band structure and therefore a built-in electric field, facilitating electron-hole separation. As a result, the membrane shows strong photocatalytic capability, which efficiently degrades organic pollutants and kills microorganisms. In particular, the built-in electric field also facilitates ionic transport, significantly promoting the osmotic power density up to 30 W/m2 under simulated sunlight illumination. The power generation performance can be robustly kept with or without the presence of pollutants. This study will shed light on the development of multi-functional power generation materials for the comprehensive utilization of industrial wastewater as well as domestic sewage.

Toward Scalable Nanofluidic Osmotic Power Generation from Hypersaline Water Sources with a Metal-Organic Framework Membrane.

Nanofluidic membranes have shown great promise in harvesting osmotic energy but its scalablity remains challenging since most studies only tested with a membrane area of ≈10-2  mm2 or smaller. We demonstrate that metal-organic-framework membranes with subnanometer pores can be used for scalable osmotic power generation from hypersaline water sources. Our membrane can be scaled up to a few mm2 , and the power density can be stabilized at 1.7 W m-2 . We reveal that the key is to improve the out-of-membrane conductance while keeping the membrane's charge selectivity, contradicting the previous conception that the ionic conductivity of the membrane plays the dominating role. We highlight that subnanometer pores are essential to ensure the charge selectivity in hypersaline water sources. Our results suggest the importance to engineer the interplay between the in-membrane and out-of-membrane ion transport properties for scalable osmotic power generation.

Bio-Inspired Salinity-Gradient Power Generation With UiO-66-NH2 Metal-Organic Framework Based Composite Membrane.

Salinity-gradient directed osmotic energy between seawater and river water has been widely considered as a promising clean and renewable energy source, as there are numerous river estuaries on our planet. In the past few decades, reverse electrodialysis (RED) technique based on cation-selective membranes has been used as the key strategy to convert osmotic energy into electricity. From this aspect, developing high-efficiency anion-selective membranes will also have great potential for capturing osmotic energy, however, remains systematically unexplored. In nature, electric eels can produce electricity from ionic gradients by using their "sub-nanoscale" protein ion channels to transport ions selectively. Inspired by this, here we developed a UiO-66-NH2 metal-organic framework (MOF) based anion-selective composite membrane with sub-nanochannels, and achieved high-performance salinity-gradient power generation by mixing artificial seawater (0.5 M NaCl) and river water (0.01 M NaCl). The UiO-66-NH2 metal-organic framework based composite membranes can be easily and economically fabricated with dense structure and long-term working stability in saline, and its performance of power generation can also be adjusted by pH to enhance the surface charge density of the MOF sub-nanochannels. This study will inspire the exploitation of MOFs for investigating the sub-nanochannel directed high-performance salinity-gradient energy harvesting systems based on anion-selective ion transport.

Spontaneous and Selective Potassium Transport through a Suspended Tailor-Cut Ti3C2Tx MXene Film.

Biological ion pumps selectively transport target ions against the concentration gradient, a process that is crucial to maintaining the out-of-equilibrium states of cells. Building an ion pump with ion selectivity has been challenging. Here we show that a Ti3C2Tx MXene film suspended in air with a trapezoidal shape spontaneously pumps K+ ions from the base end to the tip end and exhibits a K+/Na+ selectivity of 4. Such a phenomenon is attributed to a range of properties of MXene. Thanks to the high stability of MXene in water and the dynamic equilibrium between evaporation and swelling, the film keeps a narrow interlayer spacing of ∼0.3 nm when its two ends are connected to reservoirs. Because of the polar electrical structure and hydrophilicity of the MXene nanosheet, K+ ions experience a low energy barrier of ∼4.6 kBT when entering these narrow interlayer spacings. Through quantitative simulations and consistent experimental results, we further show that the narrow spacings exhibit a higher energy barrier to Na+, resulting in K+/Na+ selectivity. Finally, we show that the spontaneous ion transport is driven by the asymmetric evaporation of the interlayer water across the film, a mechanism that is similar to pressure driven streaming current. This work shows how ion transport properties can be facilely manipulated by tuning the macroscopic shape of nanofluidic materials, which may attract interest in the interface of kirigami technologies and nanofluidics and show potential in energy and separation applications.

Wetting ridge assisted programmed magnetic actuation of droplets on ferrofluid-infused surface.

Flexible actuation of droplets is crucial for biomedical and industrial applications. Hence, various approaches using optical, electrical, and magnetic forces have been exploited to actuate droplets. For broad applicability, an ideal approach should be programmable and be able to actuate droplets of arbitrary size and composition. Here we present an "additive-free" magnetic actuation method to programmably manipulate droplets of water, organic, and biological fluids of arbitrary composition, as well as solid samples, on a ferrofluid-infused porous surface. We specifically exploit the spontaneously formed ferrofluid wetting ridges to actuate droplets using spatially varying magnetic fields. We demonstrate programmed processing and analysis of biological samples in individual drops as well as the collective actuation of large ensembles of micrometer-sized droplets. Such model respiratory droplets can be accumulated for improved quantitative and sensitive bioanalysis - an otherwise prohibitively difficult task that may be useful in tracking coronavirus.