Understanding the Electrochemical Extraction of Lithium from Ultradilute Solutions.

The electrochemical extraction of lithium (Li) from aqueous sources using electrochemical means is a promising direct Li extraction technology. However, to this date, most electrochemical Li extraction studies are confined to Li-rich brine, neglecting the practical and existing Li-lean resources, with their overall extraction behaviors currently not fully understood. More still, the effect of elevated sodium (Na) concentrations typically found in most Li-lean water sources on Li extraction is unclear. Hence, in this work, we first understand the electrochemical Li extraction behaviors from ultradilute solutions using spinel lithium manganese oxide as the model electrode. We discovered that Li extraction depends highly on the Li concentration and cell operation current density. Then, we switched our focus on low Li to Na ratio solutions, revealing that Na can dominate the electrostatic screening layer, reducing Li ion concentration. Based on these understandings, we rationally employed pulsed electrochemical operation to restructure the electrode surface and distribute the surface-adsorbed species, which efficiently achieves a high Li selectivity even in extremely low initial Li/Na concentrations of up to 1:20,000.

Reversible pH-Gated MXene Membranes with Ultrahigh Mono-/Divalent-Ion Selectivity.

Artificial ion channel membranes hold high promise in water treatment, nanofluidics, and energy conversion, but it remains a great challenge to construct such smart membranes with both reversible ion-gating capability and desirable ion selectivity. Herein, we constructed a smart MXene-based membrane via p-phenylenediamine functionalization (MLM-PPD) with highly stable and aligned two-dimensional subnanochannels, which exhibits reversible ion-gating capability and ultrahigh metal ion selectivity similar to biological ion channels. The pH-sensitive groups within the MLM-PPD channel confers excellent reversible Mg2+-gating capability with a pH-switching ratio of up to 100. The mono/divalent metal-ion selectivity up to 1243.8 and 400.9 for K+/Mg2+ and Li+/Mg2+, respectively, outperforms other reported membranes. Theoretical calculations combined with experimental results reveal that the steric hindrance and stronger PPD-ion interactions substantially enhance the energy barrier for divalent metal ions passing through the MLM-PPD, and thus leading to ultrahigh mono/divalent metal-ion selectivity. This work provides a new strategy for developing artificial-ion channel membranes with both reversible ion-gating functionality and high-ion selectivity for various applications.

Oriented UiO-67 Metal-Organic Framework Membrane with Fast and Selective Lithium-Ion Transport.

Metal-organic frameworks (MOFs) membranes with high pore density and tunable pore size down to the subnanoscale exhibit great potential in ion separation when appropriately designed and prepared. By a washing-assisted secondary growing method, a well intergrown UiO-67 membrane with preferential growth along the [022] direction was synthesized on a polyvinylpyrrolidone (PVP)-modified AAO substrate. Because of the oriented growth of UiO-67 nanocrystals, highly interconnected ion-transporting channels are created throughout the UiO-67/AAO membrane capable of achieving an ultrahigh Li+ permeance of 27.01 mol m-2 h-1 as well as very decent Li+ /Mg2+ selectivity of up to 159.4. Molecular dynamics simulations reveal that the high selectivity is associated with the large disparity of the transport energy barrier between Li+ and Mg2+ , which is caused by different extents of ion dehydration in unique bimodal and oriented membrane channels.

Efficient metal ion sieving in rectifying subnanochannels enabled by metal-organic frameworks.

Biological ion channels have remarkable ion selectivity, permeability and rectification properties, but it is challenging to develop artificial analogues. Here, we report a metal-organic framework-based subnanochannel (MOFSNC) with heterogeneous structure and surface chemistry to achieve these properties. The asymmetrically structured MOFSNC can rapidly conduct K+, Na+ and Li+ in the subnanometre-to-nanometre channel direction, with conductivities up to three orders of magnitude higher than those of Ca2+ and Mg2+, equivalent to a mono/divalent ion selectivity of 103. Moreover, by varying the pH from 3 to 8 the ion selectivity can be tuned further by a factor of 102 to 104. Theoretical simulations indicate that ion-carboxyl interactions substantially reduce the energy barrier for monovalent cations to pass through the MOFSNC, and thus lead to ultrahigh ion selectivity. These findings suggest ways to develop ion selective devices for efficient ion separation, energy reservation and power generation.

Insight into the Reactivity of Carbon Structures for Nitrogen Reduction Reaction.

Graphene-based structures have been widely reported as promising metal-free catalysts for nitrogen reduction reaction. To explain the reactivity origin, various structures have been proposed and debated, including defects, functional groups, and doped heteroatoms. This computational work demonstrates that these structures may evolve from one to another under electrochemical conditions, generating weakly coordinated carbons, which have been identified as the active sites for N2 adsorption and activation.

Self-Assembled Ruddlesden-Popper/Perovskite Hybrid with Lattice-Oxygen Activation as a Superior Oxygen Evolution Electrocatalyst.

The oxygen evolution reaction (OER) is pivotal in multiple gas-involved energy conversion technologies, such as water splitting, rechargeable metal-air batteries, and CO2 /N2 electrolysis. Emerging anion-redox chemistry provides exciting opportunities for boosting catalytic activity, and thus mastering lattice-oxygen activation of metal oxides and identifying the origins are crucial for the development of advanced catalysts. Here, a strategy to activate surface lattice-oxygen sites for OER catalysis via constructing a Ruddlesden-Popper/perovskite hybrid, which is prepared by a facile one-pot self-assembly method, is developed. As a proof-of-concept, the unique hybrid catalyst (RP/P-LSCF) consists of a dominated Ruddlesden-Popper phase LaSr3 Co1.5 Fe1.5 O10-δ (RP-LSCF) and second perovskite phase La0.25 Sr0.75 Co0.5 Fe0.5 O3-δ (P-LSCF), displaying exceptional OER activity. The RP/P-LSCF achieves 10 mA cm-2 at a low overpotential of only 324 mV in 0.1 m KOH, surpassing the benchmark RuO2 and various state-of-the-art metal oxides ever reported for OER, while showing significantly higher activity and stability than single RP-LSCF oxide. The high catalytic performance for RP/P-LSCF is attributed to the strong metal-oxygen covalency and high oxygen-ion diffusion rate resulting from the phase mixture, which likely triggers the surface lattice-oxygen activation to participate in OER. The success of Ruddlesden-Popper/perovskite hybrid construction creates a new direction to design advanced catalysts for various energy applications.

Production of Hydrogen Peroxide by Photocatalytic Processes.

Hydrogen peroxide (H2 O2 ) has received increasing attention because it is not only a mild and environmentally friendly oxidant for organic synthesis and environmental remediation but also a promising new liquid fuel. The production of H2 O2 by photocatalysis is a sustainable process, since it uses water and oxygen as the source materials and solar light as the energy. Encouraging processes have been developed in the last decade for the photocatalytic production of H2 O2 . In this Review we summarize research progress in the development of processes for the photocatalytic production of H2 O2 . After a brief introduction emphasizing the superiorities of the photocatalytic generation of H2 O2 , the basic principles of establishing an efficient photocatalytic system for generating H2 O2 are discussed, highlighting the advanced photocatalysts used. This Review is concluded by a brief summary and outlook for future advances in this emerging research field.

Super-Exchange Interaction Induced Overall Optimization in Ferromagnetic Perovskite Oxides Enables Ultrafast Water Oxidation.

Oxygen evolution reaction (OER) is crucial in many renewable electrochemical technologies including regenerative fuel cells, rechargeable metal-air batteries, and water splitting. It is found that abundant active sites with favorable electronic structure and high electrical conductivity play a dominant role in achieving high electrocatalytic efficiency of perovskites, thus efficient strategies need to be designed to generate multiple beneficial factors for OER. Here, highlighted is an unusual super-exchange effect in ferromagnetic perovskite oxide to optimize active sites and enhance electrical conductivity. A systematic exploration about the composition-dependent OER activity in SrCo1 x Rux O3- δ (denoted as SCRx) (x = 0.0-1.0) perovskite is displayed with special attention on the role of super-exchange interaction between high spin (HS) Co3+ and Ru5+ ions. Induced by the unique Co3+ -O-Ru5+ super-exchange interactions, the SCR0.1 is endowed with abundant OER active species including Co3+ /Co4+ , Ru5+ , and O2 2- /O- , high electrical conductivity, and metal-oxygen covalency. Benefiting from these advantageous factors for OER electrocatalysis, the optimized SCR0.1 catalyst exhibits a remarkable activity with a low overpotential of 360 mV at 10 mA cm-2 , which exceeds the benchmark RuO2 and most well-known perovskite oxides reported so far, while maintaining excellent durability. This work provides a new pathway in developing perovskite catalysts for efficient catalysis.

Anisotropic Wetting of Droplets on Stripe-Patterned Chemically Heterogeneous Surfaces: Effect of Length Ratio and Deposition Position.

The equilibrium state of a droplet deposited on chemically heterogeneous surfaces is studied by using many-body dissipative particle dynamics. The length ratio covers 2 orders from 0.01 to 1 and allows a systematical inspection of the changes of the droplet shape, contact angle, and aspect ratio with this parameter. Moreover, a new parameter, global aspect ratio, is introduced to better characterize the distortion of the droplet. It is found that the droplet shape at the equilibrium stage strongly lies on the deposition position when the length ratio is beyond 0.1. Additionally, the lateral displacement is observed when depositing the droplet on the border of two stripes at large length ratios (over 0.1). On the other hand, the Cassie area fraction also has a significant effect on the wetting behaviors. When the droplet is driven by a body force with a 45° inclined angle to the stripes, the moving direction could be strictly in line with the force direction, deviating from the force direction, or totally in line with the stripes, depending on the length ratio.

Droplet Sliding: The Numerical Observation of Multiple Contact Angle Hysteresis.

Droplets sliding on surfaces always exhibit an advancing and a receding contact angle. When exerting different driving forces on the droplet to force it to slide at different velocities, the droplet would alter its shape to adapt to the new motion. Hence, different advancing/receding contact angles are likely to be observed, leading to the multiple contact angle hysteresis on a given surface. To verify this hypothesis, many-body dissipative particle dynamics is employed to perform the sliding simulation on both chemically homogeneous and heterogeneous surfaces. By ensuring the droplet sliding in uniform motions under different driving forces, the advancing/receding contact angles are recorded for analysis. Simulation results show that, for homogeneous surfaces, a larger driving force can result in both larger advancing contact angle and smaller receding contact angle, while for heterogeneous surfaces, increasing the driving force only results in smaller receding contact angles. For both cases, multiple contact angle hysteresis can be observed. These observations are contrary to the currently prevailing opinion, which believes that the contact angle hysteresis should be unique on given surfaces. Our findings would advance the understanding of wetting phenomena and possibly inspire new guidance for the design of functional interfaces.

Thermally Reduced Nanoporous Graphene Oxide Membrane for Desalination.

Graphene-based laminar membranes open new avenues for water treatment; in particular, reduced graphene oxide (rGO) membranes with high stability in aqueous solutions are gaining increased attention for desalination. However, the low water permeability of these membranes significantly limits their applications. In this study, the water permeability of thermally reduced GO membrane was increased by a factor of 26 times by creating in-plane nanopores with an average diameter of ∼3 nm and a high density of 2.89 × 1015 m-2 via H2O2 oxidation. These in-plane nanopores provide additional transport channels and shorten the transport distance for water molecules. Meanwhile, salt rejection of this membrane is dominated by both the Donnan effect and the size exclusion of the interspaces. Besides, the water permeability and salt rejection of the thermally reduced nanoporous GO membrane can also be simply tuned by adjusting the thermal treatment time and membrane thickness. Additionally, the fabricated membrane exhibited a relatively stable rejection of Na2SO4 during the long-term testing. This work demonstrates a novel and effective strategy for fabricating high-performance laminar rGO membranes for desalination applications.

Seawater-driven forward osmosis for pre-concentrating nutrients in digested sludge centrate.

Seawater-driven forward osmosis to enrich nutrients from sludge centrate and reduce membrane fouling is demonstrated. Due to enrichment and pH increase in the feed solution, without appropriate control measure, nutrient precipitation can occur directly on the membrane surface causing severe membrane fouling and reducing nutrient enrichment efficiency. Indeed without agitating the feed, there was less precipitation on the membrane surface, compared to with agitation. In addition, increase in the membrane area over permeate volume ratio significantly reduced the filtration time and nutrient precipitation. A novel technique to maintain the draw solution (DS) at an acidic condition was developed to improve nutrient enrichment and reduce membrane fouling. By using this technique and a high membrane surface to permeate volume ratio, nutrient enrichment similar to the theoretical efficiency was successfully demonstrated. Our technique reduced the filtration time to achieve 70% water recovery by over 90% (compared to unbuffered seawater as the DS, small membrane area, and feed agitation), as a result of significantly less membrane fouling. The amount of phosphorus precipitate on the membrane surface decreased by more than 10 times. The enrichment of ammonia and phosphorus as a function of water recovery was similar to the theoretical calculation, indicating negligible nutrient loss due to precipitation.

Impact of H-termination on the nitrogen reduction reaction of molybdenum carbide as an electrochemical catalyst.

Transition metal molybdenum (Mo) exhibits a strong capacity to adsorb nitrogen (N2), but the Mo-N2 interaction is too strong and thus it is difficult for ammonia (NH3) to be released from the catalyst surface. Bonding with nonmetals with strong electronegativity is helpful to weaken the Mo-N2 interaction, while the effect of hydrogen termination on catalyst surfaces needs to be evaluated given that the hydrogen evolution reaction (HER) is a key side reaction. This computational work aims to explore α-molybdenum carbide (Mo2C, orthorhombic phase) as an electrochemical catalyst for the full nitrogen reduction reaction (NRR). Our density functional theory (DFT) calculations focus on a (100) surface and demonstrate that (i) surface molybdenum and carbon can be terminated by hydrogen via the Volmer step and (ii) the NRR can occur on H-terminated Mo2C(100) with an energy requirement of 1.0-1.4 eV, depending on H-coverage. Although C-Mo bonding can remarkably reduce difficulty in NH3 release from a Mo-site, H-terminals result in performance deterioration. These results provide new insights into the development of NRR catalysts.

Domestic wastewater treatment by forward osmosis-membrane distillation (FO-MD) integrated system.

In this study, real domestic wastewater treatment by forward osmosis-membrane distillation (FO-MD) integrated system was investigated in laboratory scale. The integrated membrane system presented a good separation performance and the removal efficiency of most contaminants in the domestic wastewater was higher than 90%. High molecular weight contaminants were completely removed, while a few low molecular weight contaminants permeated through the membrane. The FO membrane fouling layer mainly consisted of organic substances like polysaccharides and proteins, and was very loose and could be effectively removed by rinsing the membrane surface with tap water. By comparison, the MD membrane fouling was mainly induced by inorganic salts and was not as severe as that of the FO membrane. During 120 h continuous operation, the FO-MD integrated system exhibited satisfying performance stability and maintained a high water yield and high product water quality. The results indicated the potential of the FO-MD integrated system for municipal wastewater treatment in coastal cities, water purification and desalination.

Incorporation of Homochirality into a Zeolitic Imidazolate Framework Membrane for Efficient Chiral Separation.

Homochiral metal-organic frameworks (MOFs) have gained much attention because of their chiral properties and disposition for chiral separation. However, the fabrication of high-quality homochiral MOF membranes remains challenging because of the difficulty in controlling growth of MOF membranes with chiral functionalities. A homochiral zeolitic imidazolate framework-8 (ZIF-8) membrane is reported for efficient chiral separation. The membrane is synthesized by incorporating a natural amino acid, l-histidine (l-His), into the framework of ZIF-8. The homochiral l-His-ZIF-8 membrane exhibits a good selectivity for the R-enantiomer of 1-phenylethanol over the S-enantiomer, showing a high enantiomeric excess value up to 76 %.

Zeolitic Imidazolate Framework/Graphene Oxide Hybrid Nanosheets as Seeds for the Growth of Ultrathin Molecular Sieving Membranes.

A defect-free zeolitic imidazolate framework-8 (ZIF-8)/graphene oxide (GO) membrane with a thickness of 100 nm was prepared using two-dimensional (2D) ZIF-8/GO hybrid nanosheets as seeds. Hybrid nanosheets with a suitable amount of ZIF-8 nanocrystals were essential for producing a uniform seeding layer that facilitates fast crystal intergrowth during membrane formation. Moreover, the seeding layer acts as a barrier between two different synthesis solutions, and self-limits crystal growth and effectively eliminates defects during the contra-diffusion process. The resulting ultrathin membranes show excellent molecular sieving gas separation properties, such as with a high CO2 /N2 selectivity of 7.0. This 2D nano-hybrid seeding strategy can be readily extended to the fabrication of other defect-free and ultrathin MOF or zeolite molecular sieving membranes for a wide range of separation applications.

"Periodic-table-style" paper device for monitoring heavy metals in water.

If a paper-based analytical device (µ-PAD) could be made by printing indicators for detection of heavy metals in chemical symbols of the metals in a style of the periodic table of elements, it could be possible for such µ-PAD to report the presence and the safety level of heavy metal ions in water simultaneously and by text message. This device would be able to provide easy solutions to field-based monitoring of heavy metals in industrial wastewater discharges and in irrigating and drinking water. Text-reporting could promptly inform even nonprofessional users of the water quality. This work presents a proof of concept study of this idea. Cu(II), Ni(II), and Cr(VI) were chosen to demonstrate the feasibility, specificity, and reliability of paper-based text-reporting devices for monitoring heavy metals in water.

Mechanism of enhanced removal of quinonic intermediates during electrochemical oxidation of Orange II under ultraviolet irradiation.

The effect of ultraviolet irradiation on generation of radicals and formation of intermediates was investigated in electrochemical oxidation of the azo-dye Orange II using a TiO2-modified ß-PbO2 electrode. It was found that a characteristic absorbance of quinonic compounds at 255 nm, which is responsible for the rate-determining step during aromatics degradation, was formed only in electrocatalytic oxidation. The dye can be oxidized by either HO radicals or direct electron transfer. Quinonic compounds were produced concurrently. The removal of TOC by photo-assisted electrocatalytic oxidation was 1.56 times that of the sum of the other two processes, indicating a significant synergetic effect. In addition, once the ultraviolet irradiation was introduced into the process of electrocatalytic oxidation, the degradation rate of quinonic compounds was enhanced by as much as a factor of two. The more efficient generation of HO radicals resulted from the introduction of ultraviolet irradiation in electrocatalytic oxidation led to the significant synergetic effect as well as the inhibiting effect on the accumulation of quinonic compounds.

Novel D-π-A hemicyanine dye as photoinitiators for in situ hydrogel formation and DLP printing.

The field of biofabrication imposes stringent requirements on the polymerization activity and biosafety of photopolymeric hydrogel systems. In this investigation, we designed and synthesized four hemicyanine dyes with a D-π-A structure specifically tailored for biofabrication purposes. These novel dyes, incorporating carbazole (CZ), triphenylamine (TPA), anthracene (AN), and benzodithiophene (BDT) as electron donors, along with heterocyclic salt (IN) as electron acceptors, were prepared using a straightforward synthesis method. The absorption maxima of ANIN, CZIN, and TPAIN exceeded 500 nm, rendering them suitable co-initiators for the free radical photopolymerization of acrylates under green-red light exposure facilitated by light-emitting diodes (LEDs) and the co-initiator iodonium salt (ION). Notably, CZIN and TPAIN, due to their robust dye absorption and efficient electron transfer to ION, functioned as high-performance photosensitizers. Meanwhile, BDTIN, with its strong and broad absorption range (400-600 nm), enhanced the accuracy of visible light photopolymerization. These dyes exhibit characteristics such as facile synthesis, heightened photo stability, and non-toxicity and also demonstrate the ability to discern the alkalinity of a solution to some extent. Furthermore, we explored the application of these hemicyanine dyes in 3D printing, showing potential to enhance printing resolution in DLP 3D printing (digital light process 3D printing).

Electricity generation from carbon dioxide adsorption by spatially nanoconfined ion separation.

Selective ion transport underpins fundamental biological processes for efficient energy conversion and signal propagation. Mimicking these 'ionics' in synthetic nanofluidic channels has been increasingly promising for realizing self-sustained systems by harvesting clean energy from diverse environments, such as light, moisture, salinity gradient, etc. Here, we report a spatially nanoconfined ion separation strategy that enables harvesting electricity from CO2 adsorption. This breakthrough relies on the development of Nanosheet-Agarose Hydrogel (NAH) composite-based generators, wherein the oppositely charged ions are released in water-filled hydrogel channels upon adsorbing CO2. By tuning the ion size and ion-channel interactions, the released cations at the hundred-nanometer scale are spatially confined within the hydrogel network, while ångström-scale anions pass through unhindered. This leads to near-perfect anion/cation separation across the generator with a selectivity (D-/D+) of up to 1.8 × 106, allowing conversion into external electricity. With amplification by connecting multiple as-designed generators, the ion separation-induced electricity reaching 5 V is used to power electronic devices. This study introduces an effective spatial nanoconfinement strategy for widely demanded high-precision ion separation, encouraging a carbon-negative technique with simultaneous CO2 adsorption and energy generation.