Green pathways toward energy Storing

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Green pathways toward energy Storing

In our modern Society, the Energy demand is in constant evolution, as the growth rate of the population.
In 2015, the world population was estimated at 7.35 billion; it is forecast in a report from the United Nations to be close to 8.50 billion in 2030 [9], and is expected to approach 11 billion of persons in 2100, which raises questions on our energy supplies, as the fossil fuels are supposed to be fully consumed by 2230 [10] (from a report based on the fossil fuels consumption of year 2002, that may not be up-to-date).
In addition, a more concerning point is the (ever-growing) pollution generated by the use of those fossil fuels: in 2014, despite political/industrial efforts and awareness of society, CO2 emissions were superior to 30 000 Mt [11]. It must be noted, that the overall production mostly accounts from OECD countries (nearly 10 000 Mt per year), and was almost constant from 1971 to 2014, while China and others Asian country (except for those included in OECD) sharply raised their emissions.
Giving such observations, it is clear that, only green energies will fulfill the issues of both the pollution and the energy demand, but viable solutions must be proposed, in order to sustain the living trend of our society.

Actual Energy Sources

Use in percent

Energy is a central issue in our modern society. Concerns about energy supplies are growing, while in the meantime, fossil fuels reserves dry-up at an increasing rate. As depicted on Figure I.1, the global energy consumption has more than doubled, from 1971 to 2014. Nowadays, energy sources varies from fossil fuels (Oil, Coal and Natural gas, largest part), Biofuels and wastes (second largest part), Nuclear (third largest part), and greener energies such as hydraulic, solar, wind (which account in less than 4% worldwide [11]).
It can be clearly seen that Fossil fuels account for more than 80% of the overall energy production. As discussed above, in 2002, fossil fuels were supposed to be fully consumed by 2230 [10], but at this time, nearly 8,000 Mtoe of energy was produced from fossil fuels , when nearly 11,110 Mtoe was produced in 2014. Giving data extracted from [10] and [11], 1,596,000 Mtoe of fossil fuels were available on Earth, in 2002.
A simple calculation was made in order to better evaluate the date on which Earth is supposed to run out of fossil fuels ( fitting the Mtoe consumption of the past ten years, cumulated from 2002[11] give an equation, in which the available fossil fuels reserves, at the moment of 2002[10] can be introduced). Thanks to these calculations, the “extinction date” of the fossil fuels is supposed to be 2158 (72 years less than the prevision of [10]). If one needs to extend this period, two strategies exist:
– First, to lower the world’s energy consumption; this appears not easily feasible, because emerging countries such as China consume increasing levels of energy (as depicted on Figure 1) to expand and grow, being admitted that in addition, their population grows.
– Second, to rise the contribution of green energies (solar, wind, hydraulic, etc.), but also of nuclear energy (which is mandatory, because unlike most green energies, and like fossil fuels, nuclear power plants are not dependent on the environment (wind, sun), thus enabling to supply power peaks; hydraulic energies are only available for countries with seacoasts, or with large rivers, which narrow their use worldwide).

Incidents and political/ safety issues

As discussed before, usage of fossil fuels not only triggers pollution by their main combustion products (carbon dioxide, nitrous oxides, etc.), but also, major safety, pollution and health issues, because of accidents linked to their exploitation, transportation, storage and transformation. As presented in its corresponding safety data sheet [12], oil is carcinogen, toxic for reproduction, and heavily toxic for both aquatic and land life. Because of such harmful properties, oil spills always motivate great concerns, and local populations often are defiant to pipelines, constructed near their habitat and cultural places, as illustrated by the recent crisis at Cannonball, North Dakota, USA. In addition, the pollution generated by oil on aquatic life is not only due to spills but also to slow and chronical poisoning of the seas and ocean, from what is called “routine pollution”[13], when oil tankers proceeds to the cleaning of the oil tanks, but also from the rejection of oil from the ships engine rooms, and from offshore production sites.
In addition, the acute pollution, generated by large spills (such as the Exxon Valdez in 1989), is responsible for sudden death of the ecosystem in spilled area (300 000 birds died because of the spill). Another major catastrophe can be also cited, as it is the largest one in our decade: the explosion of the Deepwater Horizon Oil rig. Because of both conception and human errors, the oil rig exploded in April, 20th 2010, leading to a spill equivalent de 4.9 million of barrels (780 million of liters). Not only this explosion impacted the ecosystem (eight American national parks were threatened from spill), but also it severely impacted the fishing industry (with a net loss of 2.5 billions of dollars) and the tourism (loss of 3 billion).
But, safety issues not only concern fossil fuels; nuclear power plant are also responsible for catastrophic events, such as Chernobyl, where the official casualties number accounted for the event, was determined in a very broad range from ca. 4000 (as published by a report of the UN of 2005) to 100,000 – 400,000 (according to Greenpeace estimates). More recently, the Fukushima catastrophe also triggered safety and health issues, along with the emerging of new anti-nuclear policies around the world (as depicted in France, where the anti-nuclear movements gain importance and supporters). However, the nuclear toll is nowhere close to the fatalities caused by other energy production means. As published in [14], the deadliest energy source (including catastrophes around the world), was the hydroelectric, when in 1975, 30 dams in central china failed in short succession, killing around 230,000 peoples. In addition, as illustrated by Figure I.2, the second deadliest energy source is not nuclear power, but coal (fossil fuels), because of the death generated by pollution (in the US alone, 13,200 peoples die per year from fine particles emitted by coal power plants). This observation shows how the nuclear impact is over estimated by our society, and how the media truly relates the information concerning the real threat, from each energy sources (because for now, the safest energy power available is, indeed the nuclear, even though mankind still ignores how to handle nuclear wastes in the long-term).
Because of the resentment of the population toward nuclear electricity, government policies tend to phase out for nuclear power (as illustrated by Germany, with a zero nuclear objective for 2022), and replacements solutions are both needed for fossil fuels power plants and nuclear energy. This naturally triggers a huge research effort, all around the world.

Replacement solutions for the electrical power grid

The urge for green energies for the supply of our power grid is facing new challenges: (i) lowering the cost of these energies, to be as close as possible to the cost of fossil fuels power/nuclear power plants,
(ii) and finding ways to store this energy, in order to avoid any dependency of the power grid on the weather (for solar and wind energies for example). A screening of the available technologies is necessary to assess the feasibility and the usability of each available technology.

Green energies

Hydropower: This Energy is clean and renewable, and works on a very simple basis, from converting mechanical energy of the flowing water to electricity, by placing turbines on the water path [15]. It is also reversible, (when considering hydroelectricity from dams) thanks to the possibility of pumping back the water into the dam, when the electricity production is above the consumption, thus allowing the water to be passed again through the turbines when the energy demand on the grid rises. Another kind of hydroelectricity production that is not very common, is the wave power plant (as the one situated on the estuary of the Rance, in France, which can produce up to 240 MW of electricity, which is equivalent to the electric consumption of Rennes, France). However, this kind of hydraulic power plants are still in their infancy, because the waves movements is not yet fully understood, thus this technology is not optimized.
The main disadvantages of such power plants are: the initial cost of the facility (much higher for coal power plants, for the same power yield), dependence on precipitations, changes in stream regimens (huge impact on wildlife), flooding, etc..
Still these technologies are among the cleanest ways, with the weakest impact on the environment, but suffer from a huge drawback: these power plants can only be installed on coasts (for wave power grids), or in mountainous areas, with rivers (for conventional dam power hydraulic power plants), which restrain their establishment in most countries, where coasts, rivers and mountains are rare (or nonexistent).
Solar power: Harvesting the power of the sun has become one of the greatest objective of the century. However, before the recent advances, with the finding of Organic Photovoltaic (OPV), this way of producing energy was not interesting, because, the production of solar panels was far more expensive than burning fossil fuels [15]. Also, the cost of OPV power plants is supposed to be matching those of conventional energy power plants by 2020, thanks to the research effort by the scientific community (the cost of thermal solar power plants is also supposed to match this trend [16]). As for now, two main problems remain for photovoltaic power plants: first their initial cost, which triggers a huge cost for its energy production: 0.2 $/KWh in 2007 (When fossil fuels power plants cost around 0.7 $/KWh) [16], second : the dependability of the electricity production on the weather (clouds or dust stop most of the suns radiations), but also on the time (no sun radiation during night time).
Wind Power: the generation of Electricity from the power of wind is pretty simple and is similar to the principle of hydraulic electricity production: the wind flow is converted into mechanical energy through a turbine, which produces electricity. However, unlike hydraulic systems and like the solar power, the electricity produced by wind power plants is weather-dependent (on whether there is wind or not), but also raises a lot problems, concerning the visual pollution triggered from such installations, their location (near forests, protected areas), and interference with nature (especially with birds; they interfere with their migration), and with signals such as radio and TV.
Geothermal Power: The principle of geothermal energy is based on the temperature gradient induced from the surface to a point under the surface [15] (usually 2000 meters underground), where the temperature is greater than at the surface. This gradient can be used directly (household heating), or in association with turbines: cold water is pumped under the surface, gets warmer (eventually evaporate), and returns at the surface with more energy, which can be converted, thanks to turbines, into electricity. For example, in Chevilly-Larue (France), a geothermal plant allows the heating and the supply of hot water of 21,000 households. The drawbacks of this way of producing energy is linked with the rarity of the available sites (geothermal energy can be found along plate boundaries), but also to issues concerning corrosion of the tubes deep under the surface, and can sometimes pull hazardous compounds trapped deep under the surface (hydrogen sulfide, mercury, ammonia, arsenic).
Biomass:
Biomass energy refers to the use of natural sources such as wood, food crops, but also residues from other industries such as agriculture and forestry residues, oil rich algae, municipal and industrial wastes.
According to Javid Mohtasham [15], biomass feed for power generation are paper mill residues, lumber mill scrap and municipal waste. But the biomass can also generate fuels, as corn grain (for ethanol production), and soybeans (for biodiesel). However, like the fossil fuels, most biomass generates carbon dioxide when consumed, and some also generate others greenhouse gases. Also, a major drawback that have risen lately is the competition between food crops and energy crops, which triggered a loss in food production. In conclusion, it is preferable to use biomass for both power and biofuel production, instead of fossil fuels, but the biomass cannot be used as the unique green energy sources, as it will eventually lead to major issues (such as population starving); it must therefore be considered in association with the others green energies sources, and used wisely, in order to complete a sustainable transition from fossil fuels.

Solution for large scale energy storing

As discussed above, the versatility of the electricity production via green energies (PV, wind, etc.) suffers a huge drawback: storage systems must be used in order to store electricity. When the weather allows it (strong winds for wind energy, high sunshine for PV), these technologies must be used for production and store the produced energy and during peak consumption (or when the conditions for electricity productions are not fulfilled: nighttime for PV, lack of wind for solar). In this perspective, some solutions are already available in order to chemically store electricity and some examples will be presented below.
– Low Temperature electrolysis: As presented by Badwal et al. [17], water bounds can be split and both hydrogen and oxygen can be produced at temperatures below 100°C. The oxygen production is not relevant as oxygen is present in wide proportion in ambient atmosphere, but the advantage of producing hydrogen from water is the formation of ultra-pure hydrogen (a purity that is often required for current fuel cell systems) at the end of the electrolysis. Others advantages can also be cited: on site production, and low operating temperature (even at room temperatures). Strong drawbacks exist however, such as poor efficiency (50-55%, [17]: 4.26 Wh of energy are required to produce 2.94 Wh of hydrogen), and high costs of the electrolyzer (as for now, their catalysts comprise expensive platinum group metal (PGM) materials.
– High Temperature electrolysis: this time, the operating temperature ranges from 700°C to 800°C. the advantages of such technique is to gain in yield compared to low-temperature water electrolysis (4.03 Wh of energy are required to produce 2.94 Wh of hydrogen). Even though this gain can appear small, on a large scale, this could be the discriminating factor between LT and HT electrolysis. However, as pointed out by Badwal et al, this technology suffers from strong drawbacks: high temperature management, high investment and operating costs, poor lifetime and durability issues. It must be noted that for both LT and HT techniques, a huge issue is yet to be solved: the finding of a proper hydrogen storage technology. Hydrogen in both its gas and liquid form have a very low viscosity and an extremely high diffusivity [18], rendering leakages very hard to prevent, even on “leak-tight” systems, tested on nitrogen, and on leak-free systems (as hydrogen diffuses faster than any gases through materials); this increases both its transportation and storage costs, but also raises safety issues when a leak is present (extremely dangerous, as hydrogen is explosive).
– Redox flow batteries: as for fuel cells and electrolyzers, such systems are of great interest, thanks to a virtual unlimited capacity (the capacity is only limited by the tank volume) [19]. RFB systems are reversible: when too much electricity is present on the electrical grid, the battery can be charged; when peaks of consumption occur, the battery can be discharged. Their advantages are numerous: no self-discharge (the two electrolytes are stored in different tanks, and when electricity production is needed, pumps are switched on, both on the anodic and cathodic sides, which allows the discharge of the system), an easy monitoring of the State of Charge (SoC) of the Battery, via the cell potential, deep discharged does not affect the cell morphology nor its performances, and RFB are capable of rapid response, which allows them to follow the peak energy demand. Some drawbacks however exist: low power and energy density (not suitable for mobile applications), and precipitations problems that occur below 15°C and above 35°C, which triggers a strict monitoring of the temperature. Various technologies exist such as Fe-Cr, VRB system (Vanadium/Vanadium battery: V(V)/V(VI) reaction at the positive; V(III)/V(II) at the negative). This technologies is currently used in various countries and a system of 5 MW (36 GJ) was installed in China, connected to the Woniushi wind power farm on May 23rd, 2013).
– Hybrid Flow Battery: These systems have very similar characteristics to the RFB devices. The only difference is that on one side (usually the negative side), the soluble materials are replaced by a solid, electrochemically active, electrode. Thus, the overall capacity of the battery only relies on the size of the installation. HFB systems have some advantages compared to RFB systems, such as a decreased size for the negative compartment (solid matter allow greater energy density by volume than saturated aqueous electrolytes), but suffers on the other hand of parasitic reactions (commons HFB systems usually comprises Zn at the negative side [17], and it is commonly known that Zn, in aqueous solutions, is not stable and slowly reacts with water, and oxygen traces).
– Li-Sulfur: As presented by Badwal et al. [17], Li-S systems have come of great interest, thanks to
their high theoretical energy density: 1672 mAh g-1. In a fully-packaged prototype, Li-S system can operate up to 700 Wh kg-1 which is of high interest concerning energy storage. However, some issues remain such as solubility problems for the end discharge product (Li2S), and redox shuttle of
polysulfides from the positive to the negative side which triggers capacity losses (polysulfide: intermediate discharge products). However, those issues can (in principle) be solved thanks to additives, which put Li-S batteries as the next evolution for lithium-based batteries, with higher specific energy than Li-ion. However, due to the relatively restrained lithium resources worldwide, this technology is not suitable for large scale energy storing, but rather for smaller applications which require high energy densities.
– Metal-air systems provide the highest theoretical energy density (for non-aqueous Li-air : 5200 Wh
kg-1 [8], for non-aqueous Na air : 1105 Wh kg-1 [17]); they however face strong challenges and are still in their infancy. For both technologies, the discharge product is an insulant (hard to re-oxidize) and a strong oxidant, which reacts readily with the positive electrode components [17] (carbon support, binder, solvent, salt, etc.). Also, the solvents that are commonly used in non-aqueous Li batteries are highly flammable, which raises a lot of safety issues, and the cyclability of the negative electrode is uncertain (metallic lithium forms dendrites during recharge, which can lead to cell failures by shortcuts). It must be noted that despite the large gap between the specific energy of Li-air and Na-air systems, Na-air are more attractive because sodium can be found in large amount anywhere around the globe, while lithium resources are smaller and located in specific locations (58% of the world resources are in Bolivia, and 27% in China), which might trigger geopolitical issues. Thus, an extensive amount of research is necessary in order to solve these problems, in order to build a practical system.

Toward high energy systems for gasoline replacement in Mobile application

Fuel cells

Among Fuel cells, the most interesting technology for electric vehicle are Proton Exchange Fuel Cells. Such systems work with hydrogen at the negative electrode (oxidation of hydrogen) and oxygen at the positive electrode (reduction of oxygen harvested in air). The electrolyte of those systems are polymer resins (usually Nafion® or perfluorinated membranes), and their characteristics are very attractive [20] (low temperature operation, high current densities, tolerant to shocks and vibrations, no emission of NOx or CO, and only bi-product: water). However, as for now, this technology suffers from a major drawback: a practical systems uses platinum-based catalysts on both the negative and positive electrodes and the MEA (Membrane-Electrode Assembly) represents on itself 80% of the cost of the stack. Moreno et al. [20], stated that, in 2013, the US department of Energy estimated the cost of PEMFC at $55/KWh, and that in order to be competitive, the end price of PEMFC has to go below $30/KWh. In order to do so, both the cost of the electrodes, the GDL (Gas Diffusion Layers) and the membrane have to be reduced. On the one hand, in the opinion of the authors, the determinant factor for the GDL and membrane costs is the volume, thus by enlarging their production, costs will be greatly reduced. On the other hand, the cost of the electrode is purely dependent on the cost of the catalyst (platinum). Several approaches are already investigated (non-PGM catalysts, core-shell platinum catalysts, alloying platinum with cheaper metals (palladium, Cobalt, etc.), but this not an easy task. Also, those systems operate on very pure hydrogen (hydrogen from petroleum reformates contains CO, which is one of the strongest poison for the negative electrodes), which also puts a hold on the use of such technologies. Hydrogen storage is also particularly demanding for transportation: one solution is the use of hydrides tanks [21], and especially magnesium hydrides tanks [21, 22]. Yet, this way of storing hydrogen suffers of the reversibility of the system (it is easier to adsorb hydrogen (exothermal) than to desorb it (endothermal), but a tank prototype has been designed lately which has proven a very good efficiency (around 90% [23]) which, makes magnesium hydrides tanks a relevant technology for PEMFC-powered vehicles, even though their mass has to be reduced (their mass-percentage of H2 stored increased) to meet the needs. High-pressure H2 storage (700 bar) in composite tanks is another option, but its usage imply severe safety measures in a practical vehicle, as hydrogen is highly explosive.

Table of contents :

Chapter I:Non aqueous Li-O2 batteries
I. Green pathways toward energy Storing
a. Actual Energy Sources
b. Replacement solutions for the electrical power grid
c. Toward high energy systems for gasoline replacement in Mobile application
II. The Li-O2 systems
a. Discovery of Li-oxygen batteries
b. ORR and OER mechanisms in aprotic medium
c. Effect of water
d. Issues of Li-O2 cathodes and improvement paths
Conclusion
References
Chapter II :Experimental section
I. Electrochemistry
a. Washing of the glassware
b. Chemicals
c. Three-electrodes setup
d. Differential Electrochemical Mass Spectrometry
e. Full cell experiment
f. Electrochemical testing
II. Characterization techniques
a. Differential Electrochemical Mass Spectrometry
b. Raman Spectroscopy
c. Field-Emission Gun-Scanning Electron Microscopy (FEG-SEM)
d. X-Ray Energy Dispersive Spectrometer (X-EDS)
e. UV-Visible Spectrophotometry
f. X-ray Photoelectron Spectroscopy (XPS)
g. Ellipsometry
h. X-ray Diffraction
References
Chapter III: High Surface Area Carbon-based Materials for High Energy Li-O2 cathodes: Advantages
and Drawbacks
I. Materials
a. Morphologies
b. Surface areas
c. Raman Spectroscopy
II. Electrochemical properties
a. Half Cells
b. Full cells
c. DEMS
d. Comparative Raman
Conclusion
References
Chapter IV: Redox Shuttles: necessary additives for OER enhancement in Li-O2 batteries.
I. Redox shuttle screening for Li-O2 cathodes
a. Redox Shuttles: the ideal behavior
b. Unfruitful selection of potential redox mediators
c. Useful compounds to be used as redox shuttles in Li-O2 batteries
II. OER enhancement mechanisms of Co(II)-Po and Co-salen – How the Co salen is beneficial for
both the OER and ORR
a. Enhancement pathway of the OER for Co(II)-Po and Co-Salen
b. ORR Homogeneous catalysis of Co-Salen
Conclusions
References

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