BACKGROUND Synthetic membranes are used for desalination, dialysis, sterile filtration, food processing, dehydration of air and other industrial, medical, and environmental applications due to low energy requirements, compact design, and mechanical simplicity. New applications are emerging from the water-energy nexus, shale gas extraction, and environmental needs such as carbon capture. All membranes exhibit a trade-off between permeability—i.e., how fast molecules pass through a membrane material—and selectivity—i.e., to what extent the desired molecules are separated from the rest. However, biological membranes such as aquaporins and ion channels are both highly permeable and highly selective. Separation based on size difference is common, but there are other ways to either block one component or enhance transport of another through a membrane. Based on increasing molecular understanding of both biological and synthetic membranes, key design criteria for new membranes have emerged: (i) properly sized free-volume elements (or pores), (ii) narrow free-volume element (or pore size) distribution, (iii) a thin active layer, and (iv) highly tuned interactions between permeants of interest and the membrane. Here, we discuss the permeability/selectivity trade-off, highlight similarities and differences between synthetic and biological membranes, describe challenges for existing membranes, and identify fruitful areas of future research. ADVANCES Many organic, inorganic, and hybrid materials have emerged as potential membranes. In addition to polymers, used for most membranes today, materials such as carbon molecular sieves, ceramics, zeolites, various nanomaterials (e.g., graphene, graphene oxide, and metal organic frameworks), and their mixtures with polymers have been explored. Simultaneously, global challenges such as climate change and rapid population growth stimulate the search for efficient water purification and energy-generation technologies, many of which are membrane-based. Additional driving forces include wastewater reuse from shale gas extraction and improvement of chemical and petrochemical separation processes by increasing the use of light hydrocarbons for chemicals manufacturing. OUTLOOK Opportunities for advancing membranes include (i) more mechanically, chemically, and thermally robust materials; (ii) judiciously higher permeability and selectivity for applications where such improvements matter; and (iii) more emphasis on fundamental structure/property/processing relations. There is a pressing need for membranes with improved selectivity, rather than membranes with improved permeability, especially for water purification. Modeling at all length scales is needed to develop a coherent molecular understanding of membrane properties, provide insight for future materials design, and clarify the fundamental basis for trade-off behavior. Basic molecular-level understanding of thermodynamic and diffusion properties of water and ions in charged membranes for desalination and energy applications such as fuel cells is largely incomplete. Fundamental understanding of membrane structure optimization to control transport of minor species (e.g., trace-organic contaminants in desalination membranes, neutral compounds in charged membranes, and heavy hydrocarbons in membranes for natural gas separation) is needed. Laboratory evaluation of membranes is often conducted with highly idealized mixtures, so separation performance in real applications with complex mixtures is poorly understood. Lack of systematic understanding of methodologies to scale promising membranes from the few square centimeters needed for laboratory studies to the thousands of square meters needed for large applications stymies membrane deployment. Nevertheless, opportunities for membranes in both existing and emerging applications, together with an expanding set of membrane materials, hold great promise for membranes to effectively address separations needs.

http://science.sciencemag.org/content/sci/356/6343/eaab0530.full.pdf

Maximizing the right stuff: The trade-off between membrane permeability and selectivity

A new approach to flow battery design is demonstrated wherein diffusion-limited aggregation of nanoscale conductor particles at ∼1 vol % concentration is used to impart mixed electronic-ionic conductivity to redox solutions, forming flow electrodes with embedded current collector networks that self-heal after shear. Lithium polysulfide flow cathodes of this architecture exhibit electrochemical activity that is distributed throughout the volume of flow electrodes rather than being confined to surfaces of stationary current collectors. The nanoscale network architecture enables cycling of polysulfide solutions deep into precipitation regimes that historically have shown poor capacity utilization and reversibility and may thereby enable new flow battery designs of higher energy density and lower system cost. Lithium polysulfide half-flow cells operating in both continuous and intermittent flow mode are demonstrated for the first time.

Lithium Polysulfide Flow Batteries Enabled By Percolating Nanoscale Conductor Networks

New technologies are required to electrocatalytically convert carbon dioxide (CO2) into fuels and chemicals at near-ambient temperatures and pressures more effectively. One particular challenge is mediating the electrochemical CO2 reduction reaction (CO2RR) at low cell voltages while maintaining high conversion efficiencies. Anion exchange membranes (AEMs) in zero-gap reactors offer promise in this direction; however, there remain substantial obstacles to be overcome in tailoring the membranes and other cell components to the requirements of CO2RR systems. Here we review recent advances, and remaining challenges, in AEM materials and devices for CO2RR. We discuss the principles underpinning AEM operation and the properties desired for CO2RR, in addition to reviewing state-of-the-art AEMs in CO2 electrolysers. We close with future design strategies to minimize product crossover, improve mechanical and chemical stability, and overcome the energy losses associated with the use of AEMs for CO2RR systems. Carbon dioxide electroreduction is a promising approach to synthesize chemicals and fuels using renewable energy. This Review explores our understanding of anion exchange membranes — a key component of certain carbon dioxide electrolysers — and outlines approaches to design improved materials.

Designing anion exchange membranes for CO2 electrolysers

The CO2 reduction reaction (CO2RR) is a potential means of using renewable electricity to synthesize commodity chemicals and fuels. The CO2RR can be performed at industrially relevant product formation rates in an electrolyser, which must simultaneously manage the transport of electrons, water, CO2 and protons at a cathode. Gas diffusion electrodes (GDEs) and polymer electrolyte membranes are used to mediate these critical processes. Consequently, the design and development of GDEs and membranes tailored for the CO2RR is critical. In this Review, we discuss how the properties of GDEs and polymer electrolyte membranes affect CO2RR electrolysis. The CO2 reduction reaction can be used to produce carbon-neutral chemicals using renewable electricity. This Review presents material design strategies for controlling the transport of products and reactants in electrochemical reactors that convert CO2 emissions into chemicals and fuels.

Gas diffusion electrodes and membranes for CO2 reduction electrolysers

Solar hydrogen production from water is a sustainable alternative to traditional hydrogen production route using fossil fuels. However, there is still no existing large-scale solar hydrogen production system to compete with its counterpart. In this Review, recent developments of four potentially cost-effective pathways towards large-scale solar hydrogen production, viz. photocatalytic, photobiological, solar thermal and photoelectrochemical routes, are discussed, respectively. The limiting factors including efficiency, scalability and durability for scale-up are assessed along with the field performance of the selected systems. Some benchmark studies are highlighted, mostly addressing one or two of the limiting factors, as well as a few recent examples demonstrating upscaled solar hydrogen production systems and emerging trends towards large-scale hydrogen production. A techno-economic analysis provides a critical comparison of the levelized cost of hydrogen output via each of the four solar-to-hydrogen conversion pathways.

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Research advances towards large-scale solar hydrogen production from water

Optimization of membrane based nitrogen removal from natural gas

What are the microscopic events of colloidal membrane fouling

Efficient operation is crucial for the deployment of photoelectrochemical CO2 reduction devices for large-scale artificial photosynthesis. In these devices, undesired transport of CO2 reduction pro...

Johannes Lohaus

Papers

Optimization of membrane based nitrogen removal from natural gas

What are the microscopic events of colloidal membrane fouling

Rational Design of Ion Exchange Membrane Material Properties Limits the Crossover of CO2 Reduction Products in Artificial Photosynthesis Devices.

On charge percolation in slurry electrodes used in vanadium redox flow batteries

What are the microscopic events during membrane backwashing