Layered graphene oxide membranes (GOM) with densely packed sub-nanometer-wide lamellar channels show exceptional ionic and molecular transport properties. Mass and charge transport in existing materials follows their concentration gradient, whereas attaining anti-gradient transport, also called active transport, remains a great challenge. Here, we demonstrate a coupled photon-electron-ion transport phenomenon through the GOM. Upon asymmetric light illumination, cations are able to move thermodynamically uphill over a broad range of concentrations, at rates much faster than that via simple diffusion. We propose, as a plausible mechanism, that light irradiation reduces the local electric potential on the GOM following a carrier diffusion mechanism. When the illumination is applied to an off-center position, an electric potential difference is built that can drive the transport of ionic species. We further develop photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for active ion sieving and artificial photosynthesis on synthetic nanofluidic circuits.

Recent advances in materials science and nanotechnology have lead to considerable interest in constructing ion-channel-mimetic nanofluidic systems for energy conversion and storage. The conventional viewpoint suggests that to gain high electrical energy, the longitudinal dimension of the nanochannels (L) should be reduced so as to bring down the resistance for ion transport and provide high ionic flux. Here, counterintuitive channel-length dependence is described in nanofluidic osmotic power generation. For short nanochannels (with length L < 400 nm), the converted electric power persistently decreases with the decreasing channel length, showing an anomalous, non-Ohmic response. The combined thermodynamic analysis and numerical simulation prove that the excessively short channel length impairs the charge selectivity of the nanofluidic channels and induces strong ion concentration polarization. These two factors eventually undermine the osmotic power generation and its energy conversion efficiency. Therefore, the optimal channel length should be between 400 and 1000 nm in order to maximize the electric power, while balancing the efficiency. These findings reveal the importance of a long-overlooked element, the channel length, in nanofluidic energy conversion and provide guidance to the design of high-performance nanofluidic energy devices.

No abstract

Nanofluidic ion transport in nacre-like 2D layered materials attracts broad research interest due to subnanometer confined space and versatile surface chemistry for precisely ionic sieving and ultrafast water permeation. Currently, most of the 2D-material-based nanofluidic systems are homogeneous, and the investigations of proton conduction therein are restricted to symmetric transport behaviors. It remains a great challenge to endow the 2D nanofluidic systems with asymmetric proton transport characteristics and adaptive responsibilities. Herein, we report the asymmetric proton transport phenomena through a 2D nanofluidic heterojunction membrane under three different types of electrokinetic driving force, that is, the external electric field, the transmembrane concentration gradient, and the hydraulic pressure difference. The heterogeneous 2D nanofluidic membrane comprises of sequentially stacked negatively and positively charged graphene oxide (n-GO and p-GO) multilayers. We find that the preferential direction for proton transport is opposite under the three types of electrokinetic driving force. The preferential direction for electric-field-driven proton transport is from the n-GO multilayers to the p-GO multilayers, showing rectified behaviors. Intriguingly, when the transmembrane concentration difference and the hydraulic flow are used as the driving force, a preferred diffusive and streaming proton current is found in the reverse direction, from the p-GO to the n-GO multilayers. The asymmetric proton transport phenomena are explained in terms of asymmetric proton concentration polarization and difference in proton selectivity. The membrane-scale heterogeneous 2D nanofluidic devices with electrokinetically controlled asymmetric proton flow provide a facile and general strategy for potential applications in biomimetic energy conversion and chemical sensing.

Precise control of ion transport is a fundamental characteristic for the sustainability of life. It remains a great challenge to develop practical and high‐performance artificial ion‐transport system that can allow active transport of ions (protons) in an all solid‐state nanoporous material. Herein, we develop a Janus microporous membrane by combining reduced graphene oxide (rGO) and conjugated microporous polymer (CMP) for controllable photodriven ion transport. Upon light illumination, a net ionic current is generated from the CMP to the rGO side of the membrane, indicating that the rGO/CMP Janus membrane can realize photodriven directional and anti‐gradient ion transport. Analogously to the p‐n junction in photovoltaic devices, light is firstly converted into separated charges to trigger a transmembrane potential, which subsequently drives directional ion movement. For the first time, this method enables integration of a photovoltaic effect with an ionic field to drive active ion transport. With the advantages of scaled up production and easy fabrication, the concept of photovoltaic ion transport based on Janus microporous membrane may find wide application in energy storage and conversion, photodriven ion‐sieving, and water treatment.

No abstract

Low membrane conductivity originated from a high membrane thickness has long been the “Achilles heel” of the conventional polymeric membrane, greatly hampering the improvement of the output power density in osmotic power generation. Herein, we demonstrate a molecularly-thin two-dimensional (2D) covalent organic framework (COF) monolayer membrane, featured with ultimate thickness, high pore density, and tight pore size distribution, which performs as a highly efficient osmotic power generator. Despite the large pore size up to 3.8 nm and relatively low surface charge density of 2.2 mC m–2, the monolayer COF membrane exhibits a high osmotic current density of 16.7 kA m–2 and an output power density of 102 W m–2 under 50 times the NaCl salinity gradient (0.5 M/0.01 M). This superior power density could be further improved to 170 W m–2 in the real seawater/river water gradient system. When the large pore size and low surface charge density are considered, this superior performance is not expected. Computational studies further reveal that the ultimate membrane permeability originated from the high membrane porosity, rather than ion selectivity, plays a dominant role in the production of high current density, especially under high salinity. This work provides an alternative strategy to realize improved output power density in ultrapermeable membranes.

No abstract