Challenges of the transition towards renewable energies
The de-fossilisation of the energy sector is a goal that has been agreed by 195 countries as per the Paris agreements. To ensure this, fossil sources used to produce energy must be left behind and renewable alternatives must become the main energy source. To achieve this, an uninterrupted power supply according to consumer demand must be ensured.
Ideally, this must be a power mix with a low CO2 footprint provided by a well-established, stable and reliable electrical grid. Another cornerstone to reducing fossil sources dependence is the electrification of the transport sector. Additionally, autonomous households that can rely on solar energy all year round are key for the de-fossilisation of developing regions.
Yet, the intermittent nature of renewable sources (e.g. sunlight is limited in the winter months and the amount of energy from the wind depends on its variable speed) imposes some challenges that must be addressed. To overcome this hurdle, alternatives for energy storage must be found, which itself is not an easy task.
The most common way to store energy is by taking advantage of gravity. This is the case of hydropower, which harnesses the energy from falling water using turbines, mills, hammers or other mechanical devices. Modern hydroelectric plants are extremely efficient and they can convert up to 90% of the mechanical energy into electricity.
However, the amount of energy provided by gravity is not very high compared to, for example, that found in chemical bonds. For this reason, electrochemical energy storage systems are taking off rapidly.
In fact, it is no secret that the market potential is growing and, according to one report by Prescient & Strategic Intelligence Private Limited published on Research and Markets, it has been projected that electrochemical energy storage will reach 51.4 GW of installed capacity by 2023, most of which is driven by the transportation sector.
What are electrochemical energy storage systems?
Batteries and electrical double-layer capacitors (EDLC), also known as super- or ultracapacitors, have been gaining increasing attention in the past decade since they are the logical replacement for internal combustion engines in automobiles. However, they can also be used as support for the power grid or in combination with solar home systems. But what is a supercapacitor and how does it differ from a battery?
EDLCs store energy electrostatically by adsorption of positively or negatively charged particles at the interface between an electrolyte (an organic or aqueous solution that provides ionic conductivity) and the electrode material. Between the electrodes, there is a separator, which is a physical barrier that permits the ion movement and prevents electrical shorting.
When a supercapacitor is charged, the positively charged ions move towards the negative electrode and the negatively charged ions move towards the positive electrode (figure 1). Since the power and energy density of an EDLC essentially depends on the surface area of the material, it is common to use activated carbon for both the positive and negative electrodes. This material has surface areas greater than 1500 m2/g and a large proportion of fine micropores (we are talking in the range of a couple of nanometres).
Activated carbon is an attractive material in this application not only because of its large surface areas but also because it can be chemically modified with functional groups containing heteroatoms (e.g. carboxylic or amino groups). These groups interact with the charged particles, increasing the storage capacity by means of reversible reduction-oxidation reactions.
Due to the electrostatic character of the energy storage mechanism, EDLCs can release the stored energy in a very short period of time. Thus, they have a very high specific power (power = energy/time) and low specific energy (figure 3). EDLCs are usually found in applications where a large amount of power is needed (e.g. wind turbines to adjust the blade position or in hybrid, electric and fuel cell vehicles, incl. automobiles, buses, and trains).
Conversely to EDLCs, batteries store energy in chemical bonds. In other words, electrical energy is converted into chemical energy. Lithium-ion batteries are currently the ‘go-to’ technology for several applications due to their high energy density (high energy content stored in a small volume), long calendar and cycle life as well as a relatively broad operating temperature range.
Lithium-ion batteries consist of several cells stacked in modules and connected in series to increase current, or in parallel to increase voltage.
Every single cell is composed of:
- a positive and a negative electrode
- the electrolyte (a mixture of lithium salts dissolved in an organic solvent)
- a separator, permeable for Li-ions.
Contrary to EDLCs, the active materials of the positive and negative electrodes in Li-ion cells are different. In the negative electrode, graphite is the most common material. Lithium titanate is an alternative to graphite but it is not as widespread.
The cathode materials are usually cobalt, manganese, or nickel‐based layered and spinel materials. During charging, lithium ions travel from the positive through the electrolyte to the negative electrode and the opposite occurs during discharge. When the ions reach the negative electrode, they are inserted between the graphene layers of graphite, building intercalation compounds of the form of LiC6, resulting in a (theoretical) charge capacity of 372 Ah/kg (in practice, the maximum capacity measured has been 335 Ah/kg corresponding to a stoichiometry of Li0.9C6).
The energy and power densities are two of the most important criteria when it comes to selecting the type of energy storage system. In the case of lithium-ion batteries, this is determined by the amount of lithium available, which in turn depends on the material used in the positive electrode.
NMC and NCA (lithium nickel cobalt aluminium oxide) batteries are the leading technologies in electromobility since they provide the highest energy densities of all available chemistries.
NMC batteries last many charge-discharge cycles, thus they are preferred in vehicles that have been designed in such a way that the battery needs to be charged once daily (e.g. hybrid and plug-in hybrid electric cars). This ensures also a long calendar life for the battery.
NCA batteries are lighter than NMC batteries and have a higher energy density but their cycle life is shorter. Tesla has taken advantage of this technology and has provided a vehicle that can run for more than 600 km with one battery charge. This means that, in the case of a traditional driver that uses their vehicle to drive short distances in a city, the battery must be recharged approximately once a week.
Is the future of e-mobility green?
One milestone that must be considered in the development of energy-dense electrochemical energy storage systems with a long cycling life is the development of efficient materials with high capacities and charge-discharge rates. Furthermore, these materials must be sustainable and environmentally friendly.
In the case of the EDLCs, the activated carbon used in the electrodes can be produced from renewable resources. Several successful trials have been conducted with sugars, cotton, or honey. However, using these kinds of raw materials leads to the ‘food vs. fuel’ dilemma. To overcome this, agricultural residues such as fruit stones, nutshells or even corn cobs can be used to develop activated carbon with properties that are comparable or superior to those of fossil sources.
With respect to lithium-ion batteries, one of their downsides is the use of lithium. Lithium has the advantage that it has a very low reduction potential, or a high tendency to donate electrons, compared to other metals and it is the lightest metal that exists. Yet, it is a relatively rare material. In fact, the largest reserves are in the Americas (Bolivia, Chile, Argentina and the United States), China and Australia. On the other hand, most of the processing takes place in Asia.
Consequently, the production costs are remarkably high and the CO2 footprint for the manufacture of a 1 kWh Li-ion battery can be compared to that of the combustion of 35 litres of gasoline . Additionally, other metals used in the electrodes, such as cobalt, are also in limited supply. Thus, a more sustainable and environmentally friendly alternative is needed. Sodium, being also an alkali metal, has similar properties to lithium and it is one of the most abundant elements (just consider the saltwater of the oceans).
However, sodium has a much larger ionic size than lithium, which makes it harder for the sodium ion to be inserted into the graphite structure. Amorphous carbon materials obtained from biomass have a less organised structure than graphite, which provides more active sites for the insertion of the sodium ions. For this reason, bio-based carbon materials can play a determining role in the development of sodium-ion batteries.
Carbon materials are a fundamental piece in the development of electrochemical energy storage systems. Yet, these carbon materials need not be fossil-based. By carbonising agricultural residues and any other type of sustainable biomass, bio-based materials are obtained that have the potential to make the future of electro-mobility green.
 Budde-Meiwes, H.; Drillkens, J.; Lunz, B.; Muennix, J.; Rothgang, S.; Kowal, J.; Sauer, D. U. A review of current automotive battery technology and future prospects. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 2013, 227, 761–776, doi:10.1177/0954407013485567.
 Larcher, D.; Tarascon, J.-M. Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 2015, 7, 19–29, doi:10.1038/nchem.2085.
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