How could we store electricity efficiently, so that we could supply all the devices around us? By printing supercapacitators and batteries, says Valeria Nicolosi. The Dublin-based chemist is a pioneer of 2D nanomaterials ink to manufacture various energy storage device typologies.
Photo credit: Amber / Trinity College Dublin
Climate change and the decreasing availability of fossil fuels require society to move towards sustainable and renewable resources. As a result, we are observing an increase in renewable energy production from sun and wind, as well as the development of electric vehicles or hybrid electric vehicles. Because the sun does not shine during the night, wind does not blow on demand and we all expect to drive our car with at least a few hours of autonomy, energy storage systems are starting to play a larger part in our lives.
Batteries and supercapacitors are two very complementary types of energy storage devices. The first stores energy electrochemically, whilst the latter electrostatically. An electrochemical battery contains a positive electrode (connected to the battery's positive or + terminal), a negative electrode (connected to the negative or − terminal), and a chemical called an electrolyte in between them. During charging the positive active species is oxidised, while the negative is reduced. During this process the positive electrode gives up some of its ions (Lithium ions in Li-Ion Batteries) which move through the electrolyte to the negative electrode and remain there. The battery takes in and stores energy during this process. When the battery is discharging, the ions move back across the electrolyte to the positive electrode, producing the energy that powers the battery. In both cases, electrons flow in the opposite direction to the ions around the outer circuit. Electrons do not flow through the electrolyte: it's effectively an insulating barrier, so far as electrons are concerned.
Supercapacitors store charges physically (electrostatically) in electric double layers forming near the electrode/electrolyte interfaces. When the two electrodes of a supercapacitor are connected in an external current path, current flows until complete charge balance is achieved. The capacitor can then be returned to its charged state by applying voltage. Because the charge is stored physically, with no chemical or phase changes taking place, the process is fast and highly reversible, and the discharge-charge cycle can be repeated over and over again, virtually without limit.
Batteries so far have secured the biggest slice of the energy storage market, being able to deliver much higher energy densities. The battery energy storage system market is expected to reach 7.5 billion euro by 2022, growing at a CAGR of 37.0% between 2016 and 2022. Supercapacitors, on the other hand, hold one tenth of the electricity per unit of volume or weight as compared to batteries, but can achieve hundreds of times higher power densities, meaning that they can be charged and discharged extremely quickly. Compared to batteries, supercapacitors also have much longer lifetimes and cyclabilities (i.e. one cycle is equal to a full charge followed by a full discharge). Together these are the power technologies of choice for consumer electronics (portable devices, hand-held devices, etc.), traction (engine starting, security doors opening, etc.), automotive (electric cars, regenerative braking, active seat belt restraints, etc.) and industrial (uninterrupted power supply, security doors, forklifts, telecommunications, etc.).
To meet the ever-increasing requirements (higher energy density, higher power density, and longer lifetimes) for such a variety of very demanding applications, new materials and chemistries must be developed. In the highly-competitive Industry and Academic research arena, there are two paths to the technological advancement of these devices. Each path involves separate but critical figures-of-merit: (i) improve the power density of batteries and (ii), improve the energy density of supercapacitors.
Apart from the mere optimisation of the parameters involved in the operation of these devices, one of the most crucial challenges that scientists are facing is the poor mechanical and thermal stability of the materials used in the current technologies.
In fact, the accommodation of ions in the charging/discharging process - through a process called “ion intercalation chemistry” for batteries and through the formation of the electric double layer (EDL) in the porous electrodes for supercapacitors – is usually accompanied by enormous volume changes and structural failures in the host electrodes.
Moving from bulk materials to the nanoscale can significantly change electrode and electrolyte properties, and consequently their performance in devices for energy storage and conversion. Within this context, 2D graphene-like materials have attracted considerable interest in the last few years, offering considerable advantages towards more classical bulk materials. Our team at Trinity College Dublin is developing such kinds of new materials and chemistries. Especially, our research has been focusing on using a palette of 2D nanomaterials ink to manufacture various energy storage device typologies.
There are several potential advantages associated with the development of 2D nanomaterials-based batteries and supercapacitors. They include: better accommodation of strain of ion insertion/removal (nanomaterials have much stronger mechanical properties as compared to their bulk counterparts), improving cycle life, high surface area and high electrode/electrolyte contact area (leading to higher charge/discharge rates), short path lengths for electronic transport, permitting operation with low electronic conductivity or at higher power.
Graphene is the best-known 2D system. But hundreds of other inorganic layered materials exist, all displaying a wide range of properties. These materials can commonly be exfoliated directly in liquids producing semiconducting, metallic and insulating inks. This exfoliation strategy consists in mixing the bulk, layered crystal (like graphite and its analogues) with a suitable organic solvent, and exposing it to ultrasonic waves (using a simple ultrasonic bath like the one used to clean jewels). Such ultrasonic waves generate cavitation bubbles that collapse into high-energy jets, breaking up the layered crystallites and producing exfoliated nanosheets (1). Suitable solvents are those whose surface energy is similar to that of the layered material.
Liquid exfoliation brings considerable advantages: it represents a cheap, easy, versatile, scalable, and sustainable route for production of 2D nanosheets. In addition, access to suspensions of nanosheets permits processing in ways that would otherwise be difficult or impossible. For example, processing from liquids allows the deposition of individual nanosheets on surfaces and the formation of thin or free-standing films; it facilitates mixing with other nanomaterials to form composites with polymers (i.e. plastics), facilitating processing by using standard technologies. This is of crucial importance, considering that most of the object and devices we use in our normal file are made of plastics. The incorporation of 2D nanomaterials in such polymers makes them mechanically more robust and gives us the possibility to add to the plastic the type of functionalities that we want, turning them into semiconducting or metallic plastics (conventional plastics are insulating).
ROLLING HIS SMARTPHONE
Dispersions of semiconducting, metallic and insulating 2D materials can in turn be used as the building blocks for manufacturing a wide range of electronic devices. Our group manufactured various energy storage device typologies : for instance, classical coin-cell batteries (2) and ultra-thin, transparent, flexible supercapacitors produced by conventional roll-to-roll technologies for the portable electronics industry. Remarkably, the later device is ultra-thin, transparent and highly flexible (all extremely important properties for the smart, hand-held technology of the future. - imagine a battery of this type embedded into a smart phone that can be rolled up and stored in your pocket). Also, these devices are importantly all-solid-state and do not contain any trace of classical flammable electrolytes. This is particularly relevant in sight of the safety concern we currently have in relation to the flammable and toxic electrolytes used in commercial batteries.
We have all heard of the hundreds of smartphones suddenly being effecting by battery swelling and exploding; an All Nippon Airways Boeing 787 flight made an emergency landing at Logan International Airport in the USA after a battery fault caused fire; the Airbus A380 had similar electrical problems caught at an early stage and a UPS plane crash landed with its cargo of lithium-ion batteries on fire causing 2 deaths from asphyxiation. All these accidents were caused by the high corrosion, toxicity and flammability of the electrolytes used, coupled with the high instability of lithium under normal conditions.
Our research sought to replace classical dangerous and poorly temperature-resistant liquid electrolytes with ultra-thin layers of solid/gel electrolytes. Indeed, these can be easily handled without spillage of hazardous liquid, they cause low internal corrosion, they have flexibility in packaging and withstand much wider temperature ranges. This is particularly relevant from the industrial point of view, especially in relation to the temperature at which these devices are usually exposed to (-60ºC-+120°C), and given the flammability issues of the electrolytes used for commercial energy storage devices at present.
In addition to this novel work, we have pioneered custom formulation of 2D nanomaterials inks used to manufacture fully operational ink-jet printed transparent solid-state supercapacitors. This in-house developed technology becomes particularly relevant in the context of the emergent multibillion-dollar industry of micro-electro-mechanical systems (MEMS) and constantly downsized complementary metal oxide semiconductor (CMOS) electronics – both bring crucial for the smart, portable electronics industry.
To operate independently, such devices require an on-board power source. While batteries and supercapacitors seem ideal choices for power delivery in these devices, today common researchable batteries and supercapacitors - typically using liquid electrolytes - are not applicable for these electronic devices due to the restrictions on their size and on-chip design - and the inherent risk of liquid electrolyte leakage. The technology developed by our group thus represents an ideal solution for facile miniaturization and good flexibility for the design of efficient standalone micro-/nano-electronic devices.
Under our ERC Consolidator Grant 3D2DPrint the possibility of producing 3D printed geometrically complex all-solid-state micro-energy storage devices directly embedded into fully operational fully 3D printed products is being sought with the main advantage of offering considerable advantages in terms of performance, variety of form factors, lower manufacturability costs and wider range of personal, mobile and bespoke applications that have the potential of revolutionise our lives in a very unique and powerful way.
(1) A. Pokle et al., Nano Energy, 30, 18, 2016.
(2) C. Zhang et al., Nano Energy, 39, 151, 2017.
Solar or wind-powered energy is not always available. This creates the need for devices capable of storing this kind of electricity and then supply it on demand. What about batteries and supercapacitators? Research on this type of systems are in full swing.
In 2012, Valeria Nicolosi became an ERC Research Professor at the School of Chemistry and at the School of Physics, Trinity College Dublin. She is a leading international expert in the creation of adaptive nanostructures and nanodevices.