In the quest for sustainable energy and advanced materials, scientists are turning to a class of substances that might seem mundane at first glance but hold almost magical properties: liquid salts.
Imagine a substance that can store the sun's heat to power cities through the night, enable the production of cleaner metals, or form the heart of next-generation batteries. This isn't science fiction—it's the exciting reality of liquid salts research being explored by scientists worldwide. At the forefront of these discoveries, over 100 researchers from across the globe gathered in Ningbo, China, for a special Faraday Discussion titled "Liquid Salts for Energy and Materials" in May 2016. This conference shed light on how these remarkable substances are shaping the future of energy and technology 6 .
When we hear the word "salt," we typically think of the white crystals we sprinkle on food. But in scientific terms, salts are a broad class of compounds formed when positively charged ions (cations) bond with negatively charged ions (anions). The liquid salts that researchers are so excited about come in several varieties, each with unique properties and applications.
What unites these diverse materials is their ionic nature—they consist of positively and negatively charged ions that form what scientists call a "coulombic continuum," a sea of charged particles that can efficiently conduct electricity and heat 6 . This fundamental property makes them invaluable for a wide range of applications, from energy storage to materials production.
| Type | Composition | Temperature Range | Key Features | Primary Applications |
|---|---|---|---|---|
| Traditional Molten Salts | Simple inorganic salts (e.g., sodium chloride, lithium chloride) | High temperatures (often several hundred °C) | Excellent thermal stability, high ionic conductivity | Metal production, nuclear reactors, thermal energy storage |
| Ionic Liquids | Organic ions with delocalized charges | Room temperature | Extremely low vapor pressure, tunable properties | Green solvents, catalysts, electrochemical devices |
| Deep Eutectic Solvents | Mixtures forming lower-melting-point compounds | Wide range, including room temperature | Biodegradable components, low cost | Biomass processing, electrochemistry, material synthesis |
Operate at high temperatures with excellent thermal stability for industrial applications.
Liquid at room temperature with tunable properties for specialized applications.
Environmentally friendly solvents with biodegradable components.
One of the most remarkable features of liquid salts is their incredible range of working temperatures. As conference chair Professor G. Z. Chen noted, these materials can operate "from glowing red to cryogenic conditions" 6 . This broad temperature adaptability means different liquid salts can be selected for specific applications:
High-temperature molten salts glow red-hot and can handle the extreme conditions needed for industrial metal processing or concentrated solar power.
Room-temperature ionic liquids remain liquid without special heating or cooling systems, making them practical for everyday applications.
Cryogenic liquid salts operate at extremely low temperatures, opening possibilities for specialized chemical processes and advanced materials.
One of the most promising applications of liquid salts lies in solving our energy challenges. As we transition away from fossil fuels, we need better ways to store and convert renewable energy.
Concentrated solar power plants use mirrors to focus sunlight, generating intense heat that can be stored for use when the sun isn't shining. Molten salts are the perfect medium for this job, as they can retain thermal energy efficiently over long periods. According to research presented at the conference, the thermal conductivity of these salts is crucial—it determines how much heat can be stored and how quickly it can be extracted to do useful work like generating electricity 4 .
The Ningbo conference featured exciting developments in electrochemical energy storage. Professor Chen's group presented work on high-energy supercapatteries—hybrid devices that combine the best features of batteries and supercapacitors—using ionic liquids containing LiClO₄ as electrolytes 5 . These advanced energy storage devices could lead to:
| Fluid Type | Example Materials | Thermal Conductivity Range | Temperature Application | Uncertainty in Data |
|---|---|---|---|---|
| Molten Salts | Sodium chloride, nitrate mixtures | Moderate (varies significantly) | 100-500°C and above | Up to 275% spread for some salts |
| Molten Metals | Iron, copper, gallium | High (10-100 W m⁻¹ K⁻¹) | High temperature applications | Up to 70% spread for molten iron |
| Conventional Fluids | Water, ethylene glycol | Low to moderate | Ambient to moderate temperatures | Well-characterized |
Beyond energy applications, liquid salts are revolutionizing how we produce and process materials, often with significant environmental benefits.
Traditional metal production methods can be energy-intensive and polluting. Liquid salts offer cleaner alternatives. At the conference, researchers presented the FFC Cambridge process—a method for directly electrochemically reducing metal oxides to pure metals in molten salts 6 . This approach could make titanium production (traditionally a dirty process) much cleaner and more efficient.
via the Hall-Héroult process
through electrolysis of molten LiCl
using the FFC Cambridge and Kroll processes
One of the most captivating presentations came from Dr. A. R. Kamali, who explained a novel method for producing graphene in molten LiCl. When water vapor is introduced into the system, it leads to H⁺ ions that reduce on the cathode, forming hydrogen atoms that intercalate into graphite. These atoms then combine to form H₂ molecules with enough kinetic energy to exfoliate graphite into graphene 6 . This process could make high-quality graphene production more scalable and affordable, with applications in everything from electronics to composite materials.
| Material Category | Specific Examples | Function in Research |
|---|---|---|
| Primary Salts | Lithium chloride (LiCl), sodium nitrate (NaNO₃), potassium nitrate (KNO₃) | Form the base medium for processes; chosen for melting point, conductivity, and chemical stability |
| Additives | Magnesium chloride (MgCl₂), nickel/nickel oxide | Modify properties; enable specific reactions like oxygen removal in metal recycling |
| Electrode Materials | Graphite, various metals | Serve as anodes and cathodes in electrochemical processes |
| Specialty Ionic Liquids | Custom-synthesized organic salts | Provide tailored environments for specific chemical reactions |
| Biomass Feedstocks | Lignin, other plant materials | Raw materials for conversion into valuable chemicals in ionic liquid solvents |
Despite their enormous potential, liquid salts research faces several challenges that scientists are working to overcome:
Accurately determining properties like thermal conductivity at high temperatures is notoriously difficult due to errors from convection, radiation, and corrosion 4 .
Unlike gases and solids, we lack well-validated general models for how heat conducts through liquids 4 .
The Faraday Discussion in Ningbo highlighted several promising research directions, including developing nanofluids to enhance heat capacity and thermal conductivity, improving molecular dynamics simulations, and creating better measurement techniques 4 .
The research presented at the Liquid Salts for Energy and Materials Faraday Discussion reveals a compelling picture: these remarkable substances offer powerful solutions to some of our most pressing energy and environmental challenges. From storing solar heat to enabling greener metal production and powering advanced batteries, liquid salts are proving to be versatile tools in building a more sustainable future.
As Professor John T. S. Irvine noted in his perspective article for the conference, liquid salts have "high potential rewards" for addressing both energy and materials challenges 3 . The "magic" of these materials lies not in mystery, but in their fundamental ionic nature—the same coulombic continuum that Michael Faraday himself identified nearly two centuries ago 6 . As research continues, we're finding ever more innovative ways to harness this property for the benefit of society, truly powering our future with molten magic.