Article publié le 23/12/2015
Governments and industrials are currently focusing on reducing their CO2 emissions by using and producing cleaner energy. However, the spread of “Green energies” is facing one big challenge : energy storage. Indeed, energy storage seems to be a sine qua non for the development of a low carbon economy and electricity system.
If solar or wind power could be economically stored and released on demand, that could fundamentally change the world’s power dynamics. Energy storage can also supply more flexibility and balancing to the grid, providing a back-up plan to intermittent renewable energy, improving the management of distribution networks, reducing costs and improving efficiency. In these ways, it would ease the market introduction of renewable energies, accelerating the decarbonisation of the electricity grid, improvement of the security and efficiency of electricity transmission and distribution (reduce unplanned loop flows, grid congestion, voltage and frequency variations), stabilisation of market prices for electricity, while also ensuring a higher security of energy supply.
Energy storage is expected to grow from a one-third of a gigawatt in 2013 to 6 GW by 2017 and more than 40 GW by 2022.
The economic impact of energy storage is difficult to assess but seems to be ranging somewhere between US$90 billion and US$635 billion for 2025 alone. The trend will be greatly influenced by how fast electrical batteries will be applied to vehicles.
Even if this part is quite small compare to the total global energy market (US$6000 billion) — it will keep growing and getting more relevant in the next few years .
Currently, there is limited energy storage in the energy system, exclusively from pumped hydro-storage. Other forms of storage – batteries, flywheels, hydrogen or chemical storage - are hardly even used, or at a very early stage of development. After pumped hydro-storage, and hydrogen storage, the storage in batteries are the most advanced technology and not that well… Thanks to their promising performances, batteries are nowadays considered as the main topic of various researchers team around the world.
If batteries and energy storage systems are already part of our daily life as for instance in cell phone, laptops, and other portable devices all run on batteries, in a close future, some other application will soon required more efficient energy storage systems, like electric vehicles for example, than ever. But currently no technology exists yet for storing wind or photovoltaic energy with good performance. There is no technologycapable of meeting the demanding features requirements of the grid, either. Less than 1% of the grid storage is done by batteries over the world .
Thus, the main challenges for storage systems are technological : increasing capacity and improving efficiency of storage technologies.
Capacity and energy density are ones of the most important aspects of battery materials, as well as, the stability of the materials and the interaction between electrolyte and electrodes surface. Battery performance is affected by both the properties of the material and the way the electrodes are built.
History of electricity storage :
Alessandro Volta invented the first “battery” in early 1800. This first prototype was made of a superposition of copper and zinc coins, separated by a cardboard, the whole system soaked in salt water. This battery was not rechargeable. The first rechargeable lead–acid batteries, still widely used today, were invented in 1859 by the French physicist Gaston Planté and consisted of a spiral roll of two sheets of pure lead separated by a linen cloth, the whole machine immersed in a glass jar of sulfuric acid solution .
Since more than a century, scientists have focused their researches on battery to solve big energy challenges that are still pending today. The reason why battery technologies are still discussed today is that many interesting issues have not yet been solved, as for example the reduction of the costs and size, improvement of storage, lifetime… That’s why alternative technologies are under development - such as supercapacitors.
What is a supercapacitor :
Supercapacitors are an other kind of storage.
Supercapacitors - also called electrochemical capacitors - should be called electrostatic capacitors because electrostatics process occurs. They are composed of two electrodes separated by an ion-permeable membrane, an ionized electrolyte connecting both electrodes. When the electrodes are polarized by an applied voltage, ions in the electrolyte form electric double layers of opposite polarity to the electrode’s polarity.
Specifically, the energy storage in a battery or a supercapacitor is due to their ability to transfer and store called ions charged particles. Both devices have at their base an electrolyte : a mixture of positive and negative ions. In a battery, chemical reactions displace the ions from the electrolyte to the inside or outside of the atomic structure of the material composing the electrode, resulting in a change of oxidation state of the material, depending on whether the battery is charged or discharged. In contrast, in a supercapacitor, an electric field causes the ions to move to or from the electrode surface without redox reaction. Since the ions are only adsorbed or desorbed (attached or detached) electrodes with no chemical reaction, a supercapacitor can be charged and discharged rapidly and over again. But a battery stores charges by redox reaction in the volume of the materials, allowing it to store a large amount of energy, whereas it does not store the supercapacitor ions at the surface of the electrodes.[4bis]
This theoretical modeling of the electrochemical double layer phenomenon was initiated in the 1870s with the work of Helmholtz and completed by Gouy, Chapman and Stern’s work later.
Thus, supercapacitors are more reactive than batteries because energy storage in supercapacitors is electrostatic which is completely different from the operation of a battery which is a chemical reaction, since there is no electrochemical reaction - which induces kinetics and distribution problems that slow down the process[4ter]. The supercapacitor can be recharged 10,000 times faster than traditional batteries and offer an extremely high power in a short period of time (Table 1).
It’s a high-capacity electrochemical capacitor with capacitance values in the range of around 100,000 µF up to 1.000 F with rated voltages in the range of 1.2 up to 3.8V .
They are able to be charged and to release their energy much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries, i.e. 1 000 000 cycles for usual supercapacitors instead of 400 cycles for the best batteries, which means a much more longer lifetime than battery’s one. But for a same given charge, there are around 10 times bigger than batteries .
If supercapacitors are not as known and used as batteries, it’s because they currently store less energy than a battery for the same size and the same weight (density and volume energy ) and they are more expensive by unit of energy stored. Nevertheless, thanks to their high charge capacity and long lifetime supercapacitors are a promising alternative to batteries.
The supercapacitor technology has been under development for about a decade but until now they have stored considerably less energy than batteries, limiting their application .
It is undoubtful that it is a promising technology with a lot of direct applications. Hong Kong institutions already understood this challenge few years ago and have been supporting a lot of research and educative projects relative to this field as the “HKTREE : Hong Kong Technology & Renewable Energy Events” which focused their main events on supercapacitors .
The Department of Applied Physics of The Hong Kong Polytechnic University (PolyU) has also improved by more than 30 times the capacity of supercapacitor compared to commercial capacitors of the same weight of active material (e.g. carbon powder). By using manganese dioxide (MnO2) , a kind of environmentally-friendly material in the supercapacitor, they significantly enhanced the performances of energy storage of capacitors, whereas its production cost decreased to HK$1 .
French innovation :
French researchers are among the most innovative in supercapacitor field .
As for example, Dr. Patrice Simon, Professor of Materials Science at the University Paul Sabatier in Toulouse, in the laboratory “Centre Interuniversitaire de Recherche et d’Ingénierie des Matériaux” CIRIMAT , worked on supercapacitors since 1996 and won in 2015, both the silver medal of the CNRS and the Rusnano prize with Yury Gogotsi for his work and discoveries in the field of supercapacitors .
Others french researchers, from the “Intégration de systèmes de gestion de l’énergie” team of the “Laboratoire d’analyse et d’architecture des systèmes” (LAAS-CNRS) in Toulouse working in collaboration with the “Institut National de Recherches Scientifiques” (INRS) in Quebec were recently recognized for their discovery on mini supercapacitors . Their work was published in Advanced Materials.
They managed to develop an electrode material whose energy density exceeds all the systems available. Including it in mini supercapacitor, it produces results similar to batteries, while retaining their particular advantages.
The new electrode is a 3D electrode made of gold, but its structure, which vastly increase the available surface area, is made as an extremely porous shape into which ruthenium oxide - which has excellent conductivity and rechargeability properties - has been inserted .
Because in the mini supercapacitors components are tiny (some millimeters) expensive materials -such as gold- can be used without increasing costs too much.
The energy density of this new micro-supercapacitor is 0.5 J/cm² - 1.000 times more than existing devices - and similar to current Li-ion micro-batteries density characteristics .
With their energy density, their long lifetime (almost infinite) , their high power and tolerance to temperature variations, these new micro-supercapacitors with macro storage energy could definitely rivaled with micro-batteries.
Such capacitors could be used for example in wearables, microcircuits and autonomous sensor networks. But before being included to instant-charging cell-phones and electric cars, it might take few more years.
If batteries and supercapacitors have a long history , human and financial resources committed to improve them have never been greater.
The increased funding will help advance batteries and supercapacitors but research is already accelerating because we apprehend in a more efficient way the molecular mechanisms that occur in batteries and supercapacitors.
Julie METTA, Chargée de mission scientifique - Hong Kong