Membranes have an increasingly important role in alleviating water scarcity and the pollution of aquatic environments. Promising molecular-level design approaches are reviewed for membrane materials, focusing on how these materials address the urgent requirements of water treatment applications.
Chemistry at the nexus of water and energy
Our ability to harness reactions that absorb or release energy is often contingent on water as a mediator. We can appreciate this simply by considering the steam that drives our electricity-generating turbines, the rivers that flow through our hydroelectric plants, and the freshwater–saltwater interface from which we can harvest blue energy. Whether we split water (as plants do), make it (as a product of combustion) or just drink it, this compound is inexorably tied to energy. Chemistry is at the heart of these topics and this collection brings together content from across Nature Research that focuses on the chemistry of energy production and water treatment.
Graphene oxide membranes have previously been reported as potential ion-sieving devices which could prove useful for water purification among other applications. In this paper a simple alternative method is demonstrated to control the interlayer spacing by using the ions themselves. Analysis of the membranes when removed from aqueous solutions containing a specific cation (for example, Mg2+, Li+ or Na+) showed that the spacing between the graphene oxide sheets could be precisely controlled by changing the type of cation, with the smaller ions generating smaller interlayer spacing. These controlled interlayer spacings enabled ionic sieving, because the membranes with smaller interlayer spacing prevented larger cations from penetrating the membrane. Further experiments revealed that the cations that control the interlayer spacing are adsorbed at regions where oxide groups and aromatic rings coexist.
Few-layered, vertically aligned MoS2 films can efficiently harvest visible light for photocatalytic water disinfection, allowing >99.999% bacteria to be rapidly inactivated.
Ion permeation and selectivity of graphene oxide membranes with sub-nm channels dramatically alters with the change in interlayer distance due to dehydration effects whereas permeation of water molecules remains largely unaffected.
Scalable graphene-based membranes reject more than 80% NaCl under aggressive shear for more than 20 h and exhibit chlorine resistance.
Electro-oxidation of CNT Joule heaters can be eliminated through the application of sufficiently high a.c. frequencies, which enables their use as self-heating membranes in membrane distillation.
Solar energy can be used to evaporate water and generate steam, however this usually requires expensive optical concentrators. Ni et al. demonstrate a low-cost solar receiver based on thermal concentration that generates steam at 100 ∘C without the need for optical concentration.
Water treatment processes mostly rely on the use of membranes and filters, which have high pumping costs and require periodic replacement. Here, the authors describe an efficient membraneless method that induces directed motion of suspended colloidal particles by exposing the suspension to CO2.
Nanoparticles can act as absorbent materials for environmental clean-up due to their high surface-to-volume ratio, but subsequent removal can be difficult. Here, the authors report nanoparticles that aggregate upon UV radiation, allowing them to absorb pollutants from water and subsequently be removed in the aggregated state.
Carbon nanotubes have been proposed for many forms of water treatment, although ultrafiltration nanotube-based membranes with very high flow rates remain rare. Here, the authors fabricate a membrane delivering water permeability close to 30,000 litres per square meter per hour at 1 bar.
Spider-silk-mimicking microfibers often suffer from low efficiency and durability in water collection. Here, the authors fabricate robust microfibers with spindle cavity-knots and different topological fiber-networks with improved water-collecting performance
Zhong Lin Wang proposes a radically different way to harvest renewable energy from the ocean using nanogenerator networks.
Markku Kulmala calls for continuous, comprehensive monitoring of interactions between the planet’s surface and atmosphere.
The electrochemical oxidation and reduction of water and carbon dioxide are associated with the release or storage of energy. This Review reports the latest developments in the design and use of low-dimensional materials and their van der Waals heterostructures for electrocatalytic and photocatalytic hydrogen evolution and CO2 conversion.
Blue energy can be cleanly and renewably harvested from a salinity gradient. The large-scale viability of this non-intermittent source is restricted by certain challenges, including the inefficiency of present harvesting technologies. This Perspective describes how nanofluidics can afford membranes better able to convert chemical potentials to electrical potentials.
More than twenty 2D carbides, nitrides and carbonitrides of transition metals (MXenes) have been synthesized and studied, and dozens more predicted to exist. Highly electrically conductive MXenes show promise in electrical energy storage, electromagnetic interference shielding, electrocatalysis, plasmonics and other applications.
Overall water splitting using powdered photocatalysts is a promising approach to large-scale solar hydrogen production. This Review details recent developments in particulate photocatalysts for overall water splitting based on one- and two-step photoexcitation systems.
Theoretical limiting efficiencies play a critical role in determining technological viability and expectations for device prototypes. Here, the authors present a unified framework for photoelectrochemical device performance through which previous limiting efficiencies can be understood and contextualized.
Fuel cells running on hydrogen are attractive alternative power supplies. One approach to ensuring safe storage and transportation of the required hydrogen is its in situ release from a stable liquid such as inexpensive methanol. Lili Lin et al. report that platinum atomically dispersed on an α-molybdenum carbide substrate is a very effective catalyst for the production of hydrogen from methanol and water under mild conditions. Although long-term stability remains to be tested and optimized, the exceptional performance of this system—which is attributed to the active involvement of the substrate and the synergy between it and platinum—meets the requirements for state-of-the-art fuel-cell vehicle applications.
Karsten Meyer and colleagues report the first example of electrocatalytic reduction of H2O to H2 using a molecular uranium (III) coordination complex. The catalytic nature of this reaction is unusual, since the bonding strength of the uranyl (O=U(IV)=O) motif means that most uranium complexes react irreversibly with water to form uranyl-based species, rather than participating in reversible oxidation/reduction reactions inherent to the recovery of catalysts. The catalytic cycle is shown to involve a rare terminal U(IV)-OH complex. The possibility of developing uranium-based catalysts could be relevant to nuclear waste management strategies, as the mildly radioactive and depleted uranium produced as waste by the nuclear power industry could be a valuable resource.
Polyoxometalate electrocatalysts based on earth-abundant metals for efficient water oxidation in acidic media
Electrochemical water oxidation in acidic media is a promising water-splitting technique, but typically requires noble metal catalysts. Now, two polyoxometalate salts based on earth-abundant metals have shown excellent catalytic performance for the oxygen evolution reaction. The barium salt of a cobalt-phosphotungstate polyanion outperformed the state-of-the-art IrO2 catalyst at pHs lower than 1.
Public trust demands greater openness from those whose research is used to set policy, argues Stefan Pfenninger.
Stop these promising photovoltaics from degrading, urge Yang Yang and Jingbi You.
Decades of experience must inform future initiatives, urge Gabriel Chan and colleagues.
Artificial intelligence can speed up research into new photovoltaic, battery and carbon-capture materials, argue Edward Sargent, Alán Aspuru-Guzikand colleagues.
The tunable bandgap of perovskites and their combination in multi-junction solar cells can afford highly efficient photovoltaic technologies. This Review reports the latest developments in tandem multi-junction perovskite solar cells and discusses prospects for this technology to achieve energy conversion efficiencies well beyond those attained by silicon-based cells.
The technological progress made since the industrial revolution has brought with it one of our greatest challenges: how to power our world while also minimizing environmental harm. This Perspective highlights the important role that quantum chemistry has in sustainable energy research.
The electric eel can generate electrical discharges of 100 watts to stun prey, but should you X-ray an eel, you wouldn't find a battery pack inside. Instead, thousands of cells called electrocytes are arranged along its body, each producing a small ion gradient and therefore a potential difference across them. Now, Michael Mayer and colleagues have developed a hydrogel-based system that mimics the electrocyte mechanism and could be used as a soft power source for robotics. They arrange sets of ion-selective hydrogels in series to generate ion gradients across a group of four hydrogel droplets. These droplets can either be arranged in series in a microfluidic set-up, or be stacked in parallel by folding up an array of hydrogels using origami principles. The net result is a power source that is able to generate voltages similar to those generated by the electric eel.
Converting oxygen-rich biomass into fuels requires the removal of oxygen groups through hydrodeoxygenation. MoS2 monolayer sheets decorated with isolated Co atoms bound to sulfur vacancies in the basal plane have now been synthesized that exhibit superior catalytic activity, selectivity and stability for the hydrodeoxygenation of 4-methylphenol to toluene when compared to conventionally prepared materials.
Vibronically coherent ultrafast triplet-pair formation and subsequent thermally activated dissociation control efficient endothermic singlet fission
Singlet fission — the conversion of one singlet exciton into two triplet excitons, could improve the efficiency of photovoltaic devices — but its mechanism is still to be fully understood. Now, in films of TIPS-tetracene, it has been shown that the formation of the triplet pair state, which has been proposed to mediate singlet fission, is ultrafast and vibronically coherent in this endothermic fission system.
It is still a great challenge to synthesize value-added products with two or more carbons directly from CO2. Now, a bifunctional catalyst composed of reducible metal oxides (In2O3) and zeolites (HZSM-5) is prepared and yields high selectivity to gasoline-range hydrocarbons (78.6%) with a high octane number directly from CO2 hydrogenation.
Direct hydrogenation of CO2 into liquid fuels can mitigate CO2 emissions and reduce the rapid depletion of fossil fuels. Here, the authors show an iron-based multifunctional catalyst that converts CO2to gasoline with high selectivity due to synergistic cooperation of multiple catalytic active sites.
Vertically co-oriented two dimensional metal-organic frameworks for packaging enhanced supercapacitive performance
Metal-organic frameworks are candidates for future energy storage materials, but are limited by poor conductivity and random crystal orientation on current collectors. Here, fabrication of electrodes containing uniformly oriented crystals supported by carbon nanowalls leads to improved electrochemical performance.
Graphene oxide membranes are promising materials for the separation of low molecular weight gases. Here, composite membranes comprising metal organic frameworks and graphene oxide show improved selectivity for the separation of hydrogen and carbon dioxide over graphene oxide alone.
Sodium-ion batteries are an appealing alternative to lithium-ion batteries because they use raw materials that are less expensive, more abundant and less toxic. The background leading to such promises is carefully assessed in terms of cell and battery production, as well as raw material supply risks, for sodium-ion and modern lithium-ion batteries.
Policy on the water-energy nexus
Hydropower plans in eastern and southern Africa increase risk of concurrent climate-related electricity supply disruption
Hydropower is critical to eastern and southern Africa but it is at risk from climate variability. Conway et al. examine river basins and rainfall variability to explore potential hydropower disruption for present and planned generation sites, highlighting the risks to supply and their spatial interlinkages.
Biofuels offer a sustainable alternative to fossil fuels but may need large land-use changes. This study combines ecosystem and economic models to explore land-use allocation and greenhouse gas emissions for a 32-billion-gallon Renewable Fuel Standard in the US.
Climate change and the vulnerability of electricity generation to water stress in the European Union
Climate change affects the availability of water for cooling thermoelectric power plants, causing curtailments in generation. This study models how future changes in water availability due to climate and water usage impacts power generation across the EU, and assesses different adaptation strategies.
A Low Carbon Fuel Standard seeks to regulate indirect land use change by including its related carbon emissions in the carbon intensity of biofuels. Khannaet al. show the economic cost of abatement achieved by including this factor is much larger than the social cost of carbon.