How to use our Mobile Devices more Sustainably

Image Credit: Yutaka Tsutano, US. CC by 2.0.

To most people mobile devices like smartphones are so much more than simple everyday objects. In personal messages, music, notes and calendars they basically contain our entire lives.

Despite the deep relationship to our smartphone, many do not hesitate to replace it with the newer model as soon as they get the chance. A report from 2016 suggests that in the UK smartphones are replaced by a newer version every two years.

Many people are not ware what the costs of this tech consumerism are. Smartphones, laptops and other consumer electronics contain up to 92 different elements and the origin of some is very problematic.

One example is the metal cobalt which is used in the lithium-ion batteries powering our devices. Cobalt normally comes from mines in African countries like the Democratic Republic of  Congo (DRC). Cobalt miners in these countries have to endure harsh working conditions with little to no safety regulations to protect them. The cobalt mines in DRC have also been linked to the use of child labour.

Other metals in mobile devices that have the same issues are gold, tantalum and tin. Gold and tantalum are used in the electric circuits while tin is contained in touch screens.

These metals, cobalt, gold, tantalum and tin, are also called ”conflict minerals”. The reason is that their trade has been linked to funding killings and violence in the DRC and other places.

In addition, the large scale mining and extraction of the metals needed to make a mobile device causes pollution and devours large amounts of energy, which in turn fuels global warming. And it does not stop there. After extraction the metals are mostly transported to China where the devices are assembled often by workers with low pay and poor working conditions.

So, what can we do as consumers to make sure our beloved smartphones do not have a negative impact?

1) Don’t upgrade your device unless you really have to. Even though it is tempting to replace your device as soon as the new version is released, it is best to use your old device until it really stops working.

2) Try to repair your device when it is broken. The two things that are most likely to break in your smartphone are the battery and the screen. Both can be replaced. Even though many current phone models cannot be taken apart and repaired easily, you can still try to repair the device yourself using YouTube tutorials. You can also send it to a shop for repair.

3) When you do have to part with your device, donate it to charity or sell it for example to a refurbishing shop. A lot of environmental problems are caused by mobile device disposal. Recycling is difficult because electronci devices contain so many different elements. Often they are sent to countries like Ghana, Nigeria or Vietnam for recycling. The easiest way to extract the valuable metals like copper, gold and aluminium is to take the devices apart and burn them. This process releases toxic chemicals into the air and the soil.

4) Consider buying a refurbished phone. From refurbishing companies you can get sturdy and cheap smart phones. None of the problems discussed above regarding mining and disposal are associated with them.

5) When buying a new device think carefully about what you want. Greenpeace’s ”Guide to Greener Electronics” is very useful if you want to buy a phone that has been produced sustainably. The guide rated the 17 world leading mobile device producers in the categories, Energy, Resource Consumption and Chemicals. The most sustainable device in their rating was the Fairphone made by a Dutch company with a grade B. It was followed by Apple in place two with a grade B-. A relatively young company producing more sustainable mobile devices is Shiftphone. This German start-up focusses on repairability and extending the lifespan os consumer electronics.

What are Ionic Liquids and what are they good for?

Image: An ionic liquid tested for the application in lithium-ion batteries at Chalmers University of Technology Gothenburg and Uppsala University (Sweden).

Ionic liquids are basically salts with low melting points of under 100 degrees Celsius. Some might pause now because in school we are normally taught that salts have very high melting points. This is true for most – let us say – ”common” salts as for example table salt with the scientific name sodium chloride (NaCl). Sodium chloride has a melting point of 801 degrees Celsius which I guess can be considered a high melting point.

Salts consist of positive and negative ions that attract each other electrostatically which results in the formation of ionic bonds. This is how they form the compounds that we know as salts. Sodium chloride contains positive sodium ions and negative chloride ions. The strong attraction between the positive and negative charges causes the high melting points we observe in most salts.

Nevertheless, salts with low melting points and ionic liquids do exist. Why is that? The melting points of a salt are generally lower, the larger the ions are that it is made up from. For example, when changing the positive ion ( also called cation) in sodium chloride from sodium to the bigger rubidium ion, the salt rubidium chloride is obtained which has a melting point of 715 degrees Celsius. The melting point of rubidium chloride is 86 degrees Celsius lower than that of sodium chloride because the rubidium ion is bigger. When choosing even larger postive ions, for example the organic cation 1-ethyl-3-methyl-immidazolium, a salt called 1-ethyl-3-methyl-immidazolium chloride (a real tongue breaker) is obtained which has a melting point of only 89 degrees Celsius. This salt is an ionic liquid which melts under 100 degrees Celsius. We arrived at this ionic liquid simply by exchanging the positive ion (cation) with larger ones.

Also, the negative ions (also called anions) – the chloride ion in our case – can be exchanged with larger ones. When switching from the chloride ion to the organic anion bis(trifluoro methane sulfonyl) imide (another tongue breaker), the melting point can be lowered even more. When putting together the anion bis(trifluoro methane sulfonyl) imide with the cation 1-ethyl-3-methyl-immidazolium a salt with a melting point of -15 degrees Celsius is obtained. This salt (I will spare you the name) is a room temperature ionic liquid (RTIL). This is indeed a salt which is liquid at room temperature.

So, why do chemists make liquid salts? I admit a salt that melts at a lower temperature than water is pretty cool, but are they good for anything? To answer this question, we have to first look at the properties. The most important properties of ionic liquids are that they can conduct electricity due to their mobile ions and that they are stable even at high temperature which also means that they are non-flammable and cannot easily catch fire. Especially, their good high-temperature staibility makes ionic liquids interesting for many applications, for example as high-temperature lubricants in machines or as solvents for chemical reactions in the industry. Due to the conduction of electricity, ionic liquids are also tested in solar cells and batteries.

I have talked to Manfred Kerner, a PhD student in the Applied Physics Group at Chalmers University of Technology in Gothenburg (Sweden) who is working on the application of ionic liquids in batteries. Kerner is trying to use ionic liquids as the electrolyte (= the liquid placed between two battery electrodes) in lithium-ion batteries. One big problem lithium-ion batteries face is the high flammability of commonly used electrolytes which is a part of the reason for many youtoube videos showing laptops on fire. Ionic liquids can be a solution to this problem because they are stable at high temperatures and do not catch fire, while they are able to conduct electricity. These properties are especially useful for high-temperature lithium-ion batteries which run at temperatures higher than room temperature and are popular in hybrid electric vehicles.

Nevertheless, Kerner admits that there are still some hurdles to overcome until ionic liquids can be broadly applied in batteries. One example is their high cost. Another is that ionic liquids strongly attract water from air which damages the electrode materials of lithium-ion batteries. For this reason the use of controlled water-free environments such as gloveboxes is necessary. In addition, ionic liquids are not stable in contact with all the possible battery electrode materials which limits their use.

Despite these drawbacks some ionic liquids are already on the market as electrolytes for batteries according to Kerner. However, they are so far mainly limited to research applications. Nevertheless, some day in the future we might find ionic liquids in our own laptop or car batteries.

New magnets for wind power plants

Image credit: Johan Cedervall, 2017. CC BY-SA IGO 3.0. This material is a strong magnet investigated for the use in wind power plants and other applications.

Almost everyone has seen magnets in action when sticking a birthday card or the drawing of a child to a refrigerator. Some might have even used one when taking a compass on a hike. There is one more important application for magnets that most of us have seen: wind power plants. The generators inside wind power plants use very strong magnets to create electricity. These magnets are normally made from the elements neodymium, iron and boron.

While iron and boron are are relatively easy to extract and separate from their minerals (or stones), neodymium is a rare-earth element. There are in total 17 rare-earth elements and despite their name, they are actually quite plentiful in the Earth`s crust. The problem is that these elements tend to occur together in the same minerals which makes their extraction and separation extremely difficult and expensive. In addition, China stands for 85 % of all rare-earth element production which provides a monopoly position enabling it to dictate prices and make other countries dependent. Besides, there are strong environmental as well as health and safety concerns about the mining procedures used in China.

For these reasons research is carried out to find new neodymium-free or rare-earth-free, strong magnets. I have spoken to Johan Cedervall, a PhD student in the Inorganic Chemistry group at Uppsala University, who is trying to make new rare-earth-free magnets for wind power plants and other applications. Cedervall is working on magnetic materials composed of iron, boron, phosphorus and silicon, all abundant materials which are relatively easy to extract from their minerals.

To produce his magnets, Cedervall melts the elements together in an electric arc (basically a permanent, artificial lightning) at 1500 to 2000 °C (2732 to 3632 °F) under argon atmosphere. This procedure results in a highly magnetic, grey compound as seen in the image above. According to Cedervall, his materials are generally a bit less magnetic and easier to demagnetize than conventional neodymium-based magnets, which is a disadvantage. But they are also much cheaper thanks to the absence of neodymium.

Nevertheless, Cedervall says, that the goal is to find even stronger rare-earth-free magnets than his for the use in wind power plants. The problem is that weaker magntic materials have to be used in larger amounts to reach the effect of a neodymium-based compound. For this reason, the search for strong, rare-earth-free magnets is ongoing.

Cedervall believes, that we will see a shift to rare-earth-free or rare-earth-lean (containing smaller amounts of neodymium) magnets in the near future. Wind power plants are being built at a rapid pace all over the world at the moment. To maintain this development, industry will sooner or later be forced to turn to alternative magnets. The German company Enercon has, in fact, already implemented neodymium-free technology into their wind turbines. In addition, magnetic materials called ferrites, for example strontium iron oxide, have already started to replace neodymium in other applications.

When asked which results of his research he is most proud off, Cedervall answers, that these are actually not related to wind power plants. All magnets lose their magnetic properties at a certain temperature when they are heated. This point is called Curie temperature which is between 500 and 600 °C (932 and 1112 °F) for Cedervall`s materials. He has found that this point can be tuned by the gradual substitution of iron with cobalt. A higher cobalt content decreases the Curie temperature, while a lower one increases it. This result could be interesting for building magnetic refrigerators that are safer and more environmentally friendly. The details of this application are a story for another time.

Making batteries with sulfur

Image credit: Ben Mills.


The yellow powder in the image above is the element sulfur, which I am sure many people have seen before in school science lessons. Normally in chemistry classes, we are mainly taught about how bad the compounds made from this element smell. Examples are hydrogen sulfide with the distinct smell of rotten eggs and sulfur oxides. But could there be more to this powder? To find out, I have talked to Dr Matthew Lacey at Ångström Advanced Battery Center who is trying to build rechargeable batteries from sulfur.

Lacey is working on lithium-sulfur batteries (Li-S batteries) which consist of sulfur as the positive electrode (cathode) and lithium metal as the negative electrode (anode). When discharging (or using) these batteries, lithium and sulfur react to form the compound lithium sulfide and the process is reversed during charge. The advantage is the light weight of both sulfur and lithium, which leads to a very high energy density by weight. This gravimetric energy density of the batteries themselves could be up to two or three times higher than for conventional lithium-ion batteries according to Lacey. In addition, sulfur is abundant as well as a biproduct of the oil industry and, for these reasons, very cheap.

Nevertheless, Lacey admits that Li-S batteries also still have some drawbacks compared to conventional lithium-ion batteries. During discharge, sulfur dissolves in the electrolyte (the liquid placed between positive and negative electrode), where it can exist as a large number of different compounds called ”polysulfides”. These polysulfides can react with each other or be transported to the lithium foil (negative electrode) and react there. Especially, the reaction between polysulfides lithium results in very short lifespans for Li-S batteries. Strangely, the dissolution of sulfur in the electrolyte is an important part of how Li-S batteries work, but at the same time it is the source of severe problems. Another disadvantage is that, although the energy density per weight is high, the energy density per volume of Li-S batteries is rather low since lithium and sulfur are light, but have low densities.

Lacey is mainly working on a part of the battery called the ”binder”. In order to build a Li-S battery, sulfur and conductive carbon powder have to be attached to a conductive substrate, the current collector. Binders are the glue that stick sulfur and carbon powder to the current collector surface. Together these components form the positive electrode. Lacey is trying to find new binders that can do more than just be the glue of the electrode. The battery scientist wants his binders to interact with the polysulfides in the electrolyte (described above) to extend the lifetime of Li-S batteries. Lacey and his co-workers have been quite successful in this quest during the past years and more about it can be read in this recently published article.

The battery scientist is also proud about a new method called ”intermittent current interruption” that he developed during the past years. This technique makes it possible to track the resistance in batteries over long periods of time and can be used for all kinds of batteries, not only Li-S batteries.

Despite the challenges, Airbus Defence and Space are developing Li-S batteries for the Zephyr “High-Altitude Pseudo Satellite”. Li-S batteries are the only rechargeable batteries with a high enough energy density per weight to run this ”pseudo satellite” when sunlight is not available in the shadow of Earth. In addition, the UK based company OXIS Energy is working towards the commercialization of Li-S batteries.

Nevertheless, Lacey believes that due the low cost of lithium-ion batteries, they will keep dominating most markets such as electric cars and portable electronics in the foreseeable future. But, he also says, that Li-S batteries are still interesting for applications where battery volume and cost are less important. Examples are defence or space where Li-S batteries could power soldier equipment or satellites. Hybrid electric trucks could be another future market for Li-S batteries with battery volume being less of a problem for trucks than for smaller cars.




What is artificial photosynthesis?

Plants are truly amazing. To produce energy they basically ingest sunlight, water and carbon dioxide. As a result energy is chemically stored in sugars like glucose. (Scientists call them carbohydrates.) We all have heard about this process in school, photosynthesis. The carbohydrates can be further modified by the plants into fats or proteins. Animals like humans rely on all three as food, but also on the oxygen produced as a byproduct. In fact, there was no oxygen in the Earth atmosphere before the first cyanobacteria invented photosynthesis. These cyanobacteria later evolved into the chloroplasts inside plant cells where photosynthesis takes place.

Photosynthesis is very effective in transforming the energy of sunlight into chemical energy in sugars without creating any toxic or polluting waste. For this reason scientists today are trying to artificially create photosynthesis. The goal of these systems is to produce hydrogen or other fuels for engines and electricity. Another advantage would be that carbon dioxide released by the use of fossil fuels could be ”mopped up” from the atmosphere by artificial photosynthesis.

The main difference between artificial and natural photosynthesis is that plants produce carbohydrates, fats and proteins while humans are looking for suitable fuels that can power airplanes or cars. These fuels should ideally resemble fossil fuels and thus enable the use of already existing combustion motors. For this reason, chemists are trying to create different end-products than plants while using the same energy source (sun) and building blocks (carbon dioxide and water).

Plants use their chlorophyll to capture the sunlight while a collection of enzymes and proteins uses this energy to split water molecules into hydrogen, electrons and oxygen. Hydrogen and electrons then form carbohydrates (sugars) with the carbon dioxide and oxygen is released.

For artificial photosynthesis, scientists are mainly interested in the first two steps above. Capturing sunlight is the easy part, as there are plenty of solar-power systems available. Splitting water, however, is trickier and the main challenge. Water is a very stable compound and catalysts are required to initiate the splitting reaction. Catalysts are materials that can accelerate chemical reactions, without being depleted in the process. A great amount of research is being carried out  in order to find suitable catalysts for artificial photosynthesis. Among recently published very succesful catalysts were cobalt-based materials. Nevertheless, these systems still require more work and research in order to be optimized for commercialization.

Theoretically, the produced hydrogen could be directly used as a fuel. However, right now it is still more practical to transform hydrogen and carbon dioxide to fuels that closely resemble fossil fuels. This last step can be carried out either with the help of bacteria or other inorganic catalysts like copper. The conversion makes it possible to use the products of artificial photosynthesis in already existing car and airplane engines.

Maybe it will not take too long before we are able to drive cars with fuels directly produced with sunlight and carbon dioxide.

Why look at tin oxide in lithium-ion batteries?

tin oxide in LIBs new new

With my PhD thesis just published and my defense taking place in only a few weeks, I decided to dedicate a blog post to it. So, for those of you who wonder what I have been doing the past four and half years, pay attention, here is the answer.

The price of renewable energy is falling rapidly making it cheaper than fossil fuel based options in many parts of the world. The storage of the produced electricity in batteries has become one bottle neck for the use of renewable energy technology. It is not only large scale grid storage, but also energy storage for electric vehicles that poses a challenge. One problem is that today`s batteries cannot store enough energy for these applications which leads for example to a low range for electric vehicle batteries.

The batteries that can store most energy per weight and volume are lithium-ion batteries thanks to the low atomic weight of lithium. In such a battery lithium ions travel through an electrolyte between two electrodes which are called the postive electrode (cathode) and the negative electrode (anode). This principle is called a ”rocking chair battery” and it can be seen in the upper left image of the infographic.

When the battery is in use (or discharged) lithium is turned into lithium ions at the negative electrode (anode). The ions are released into the electrolyte and travel to the postive electrode (cathode) where they are incorporated. At the same time electrons are released from the the anode via an external circuit where they can power an electric device. This process is reversed during charge when an external electrical power is applied that forces electrons and ions to migrate into the reversed direction.

I have studied the negative electrode or anode of lithium-ion batteries. In portable electronics like mobile phones and laptops graphite represents the most common anode material. However, it can only incorporate a limited amount of lithium which results in a relatively low energy density (also called capacity). Graphite can store 372 mAh/g of energy when incorporating lithium which is not enough for an efficient use in electric vehicles. Therefore anode materials with higher energy densities are desirable to achieve longer battery ranges.

One possible alternative is tin which has a capacity of 991 mAh/g. However, when using tin oxides even higher energy densities can be obtained, with 1271 mAh/g for tin(II) oxide and 1491 mAh/g for tin(IV) oxide. The theoretical capacities can be seen in the upper right picture in the infographic. The most famous issue associated with tin based materials is the rapid loss of energy density during battery operation which is caused by volume changes the material undergoes during charge and discharge. However, in my work I have  paid special attention to some other challenges when using tin(IV) oxide as a negative electrode material in lithium-ion batteries.

Despite the high theoretical energy density (1491 mAh/g) described above, tin(IV) oxide usually shows practical energy densities below 1000 mAh/g when used in lithium-ion batteries. I have studied this phenomenon and found that extremely small tin(IV) oxide particles (< 10 nm = one ten thousandth mm) have to be employed to obtain the full energy density. The reason for this is that lithium moves very slowly through tin(IV) oxide. In smaller particles the distances that lithium needs to travel are much shorter which means that more lithium and with that energy can be stored or extracted in short timespans.

Another strategy explored to improve the transport of lithium in tin(IV) oxide was battery operation at high temperatures. The reason for this was that heat accelerates the movement of ions, atoms and molecules. In other words, it increases the speed of the lithium traveling through tin(IV) oxide. The lower image of the infographic shows that the energy density of tin(IV) oxide was larger when cycling at 60 degrees Celsius (140 degrees Fahrenheit) instead of room temperature, increasing with about 30 % over 60 charge-discharge-cycles. Therefore, using tin(IV) oxide at high temperatures in lithium-ion batteries could be a good strategy to improve its performance. This finding is especially valuable for the possible use in batteries for hybrid electric vehicles where manufacturers are striving to reduce the number of cooling systems. A battery that could operate at 60 degrees Celsius would be a huge advantage here. Nevertheless, many more tests need to be conducted before such an application.

Other battery materials like the electrolyte (between the two electrodes) can, however, not withstand operating temperatures above 60 degrees Celsius. Thus, we tried to use an alternative, more heat resistant (thermally stable) electrolyte to use tin(IV) oxide at a higher temperature, i.e. 80 degrees Celsius (176 degrees Fahrenheit). This electrolyte was a so-called ionic liquid, which is defined as a salt that is liquid at room temperature. Unfortunately, the ionic liuid electrolyte proved to be unstable towards the reactions happening during battery operation despite its generally good thermal stability. As a consequence, the search for suitable high temperature electrolytes will have to continue in the future.

In summary, it seems that the battery performance of tin(IV) oxide as a negative electrode material in lithium-ion batteries can be significantly improved at high temperatures. Nevertheless, more suitable electrolytes will have to be found and more tests need to be conducted in order to make this strategy practically feasible. Another approach to obtain higher energy denisties is the use of extremely small tin(IV) oxide particles below 10 nm.

What will come after lithium-ion batteries?

Image credit: Ronnie Mogensen, 2017. The materials ”Prussian white” (left) and ”Prussian blue” (right) are studied for the use in new battery technologies. CC BY-SA IGO 3.0.

With the dawn of electric vehicles and plummeting costs for renewable energies, battery technology is playing an ever more important role in our lives. Today lithium-ion batteries (LIBs) are being used in electric vehicles and for stationary power storage of renewable energy, for example in the Tesla powerwall. They can also be found in portable electronics such as laptops and mobile phones.

The problem is that lithium is a rather rare element with an abundance of only 0.0017 % in the Earth crust, most of which is found in South America. This inherent shortage combined with the high demand have lead to an increase of the lithium price by 14 % in 2016 with the price surges most likely continuing in the future. It is even doubtful if Earth has enough lithium resources to support the transformation from the fossil fuel-based society we are today to one depending mainly on renewable energy.

I have talked to two scientists who are trying to tackle this problem, Assistant Professor Dr. Reza Younesi and PhD student Ronnie Mogensen from the Ångström Advanced Battery Center (Uppsala University, Sweden). They are working on new kinds of batteries called sodium-ion batteries (SIBs) and magnesium-ion batteries (MIBs). These batteries rely on the use of sodium or magnesium instead of lithium, but work based on the same ”rocking-chair-principle”. In these batteries, metal-ions travel between two different electrodes during charge and discharge. The main difference is the nature of the travelling metal-ions, i.e. sodium-ions are used in SIBs, magnesium-ions in MIBs and lithium-ions in LIBs.

The huge advantage of these new batteries is, according to Younesi, the high abundance of sodium (2.3 % in Earth crust, atomic number 11) and magnesium (2.9 % in Earth crust, atomic number 12) as compared to lithium (0.0017 % in Earth crust, atomic number 3). In addition, large amounts of sodium can be found dissolved in sea water. Their good abundance enables the production of cheaper batteries from sodium and magnesium.

Nevertheless, Younesi admits that these battery technologies also have some drawbacks. Both sodium and magnesium are for example heavier than lithium, which is indicated by their higher atomic numbers. This leads to an overall lower energy density of these materials in batteries. This means that sodium-ion and magnesium-ion batteries need to be much heavier than lithium-ion batteries in order to store the same amount of energy.

Another challenge is the search for new electrode materials as not all state-of-the-art-materials used in lithium-ion batteries work for sodium- or magnesium-based systems. In addition, electrodes in lithium-ion batteries usually contain other scarce, toxic and expensive elements like cobalt, nickel or manganese. But, Younesi says, in order to truly produce cheap batteries, even the electrodess have to be made from abundant and inexpensive raw materials. For this reason one focus of Younesis and Mogensens research are iron- and carbon-based electrodes for sodium-ion batteries.

The most promising material they are studying at the moment together with researcher Dr. William Brant is called ”Prussian white”. It is derived from ”Prussian blue”, a compound that you might remember from high school chemistry lessons for its bright blue colour. It can be seen to the right in the image above. Prussian blue is a salt containing potassium, iron, carbon and nitrogen. In Prussian white the potassium has been replaced with sodium making it suitable for the use in sodium-ion batteries. It also gives the material a more ”whitish” colour as seen to the left in the photograph above. According to Mogensen, the beauty of this compound is that it only consists of very abundant elements (i.e. sodium, iron, carbon and nitrogen) and that it is  non-toxic.

In the future, Younesi sees great potential for sodium- and magnesium-ion batteries to replace lithium-ion batteries in stationary applications such as the energy storage in solar and wind power plants or ”home wall” batteries. The reason for this is that the higher weight of sodium and magnesium is less important in these applications than in portable electronics or cars.

In fact, sodium-ion batteries have already been commercialized in other fields. The battery company Faradion based in Sheffield (UK) produces sodium-ion batteries, e.g. for electric bicycles. Other steps have been taken in France by the Centre Nationale de la Recherche Scientifique (CNRS) where a sodium-ion battery prototype has been built that can fit and run in a conventional laptop. Also, Younesi and Mogensen in Sweden are not missing out on this rapid development. They are working towards the commercialization of their Prussian white material in sodium-ion batteries. With such a high innovation speed, it seems only a matter of time until we will see a wide-spread utilization of these new battery technologies.

A short history of solar cells

Image Credit: U.S. Department of Agriculture, 2011, CC by 2.0.

The photocoltaic effect

In 1839 the photovoltaic effect was first observed by the French physicist Alexandre Edmond Becquerel (the father of Henri Becquerel – yes, they are related).  This phenomenon occurs when two different materials are in close contact with each other. When light hits one of the materials, energy is consumed and electrons are lifted to an excited state where they have a higher energy than in the ground state. As a result an electric field is formed along the contact to the second material. This field applies a force on the excited electrons and can force them into an external electrical load where their energy can be used to power an electronic device.

The photoconductivity of selenium

The English engineer Willoughby Smith experimented on selenium and found in 1873 that the normally insulating material becomes electronically conductive when exposed to light. This phenomenon is called photoconductivity. William Grylls Adams and Richard Evans Day discovered three years later that selenium can also produce electricity when light is shone on it. The first solar cell was finally created in 1883 by the American engineer Charles Fritts when he coated selenium with a thin layer of gold. His results were reproduced and confirmed later by the German engineer Werner von Siemens. Nevertheless, these prototype cells could only convert about 1 % of sunlight into electricity (in other words they had an efficiency of 1 %) and the phenomenon could not be well understood at the time. For these reasons solar cells were not developed further back then.

The photoelectric effect

The photoelectric effect was first observed by the German physicist Heinrich Hertz. This effect occurs when solid materials emit free electrons under exposure to light (or other electromagnetic radiation like X-rays). Modern silicon solar cells rely on this phenomenon to create electricity from sunlight. Albert Einstein later received the Nobel Prize in Physics for explaining the photoelectric effect in detail. (No, he did not get it for the relativity theory.)

The silicon solar cell

Daryl Chapin, Calvin Fuller and Gerald Pearson from the Bell Laboratories (New Jersey, US) discovered in the early 1950s that silicon is much better at converting sunlight into power than selenium. This lead to the first practical silicon solar cell being demonstrated in 1954 which showed an efficiency of 6 %. The first commercial silicon solar cells entered the market 1956, but they were still very expensive and not very succesful at first. The situation changed with the dawn of spaceflight where solar cells were used by NASA to power satellites like Vangguard 1 from 1958 onwards. This application enabled further research which resulted in lower prices for solar power. Nevertheless, it was not before 1982 that the first solar park was installed in California (US). Today silicon solar cells reach an efficiency of 15 – 20 %. Due to climate change, smog and pollution, solar cell technology has received a lot of interest during the past years as a clean alternative to burning of fossil fuels like coal. Now we are at a point where the cost for silicon solar cells if falling rapidly which could make them even more popular in the future.

The future of solar cell technology

Despite the historically low prices of silicon solar cells, they might soon encounter some serious competition as new materials and concepts are being studied. One contender could be perovskite solar cells (discovered 2009 in Japan) which promise a cheaper and simpler manufacturing process than silicon solar cells. In addition, dye-sensitized solar cells, which were discovered already in the 1960s and rely on the photovoltaic effect, could eventually prove another low-cost competitor. Up to now these new technologies are less efficient than silicon solar cells, but especially perovskite solar cells are on a good way to reach an efficiency of 15 – 20 % within just a few years. Another strategy could be the combination of perovskite solar cells and silicon solar cells in hybrid modules. It remains exciting to see which road solar cell technology will take next.

A new, old material could transform solar cell technology

Image: Kári Sveinbjörnsson, 2017. Two perovskite solar cells from his research compared in size to a coin of one Swedish krona. CC BY-SA IGO 3.0.

In 2009 a Japanese researcher found out that a material called perovskite could convert sun light into electricity similar to silicon solar cells. Interestingly, this kind of material had already been known for quite some time. Perosvkite is the name of not only one particular material, but of different compositions sharing the same crystal structure, the perovskite structure. The first perovskites were discovered as early as 1839 in the Ural Mountains and were named after the Russian mineral expert Lev Perovski who first studied them. The new perovskite that has received a lot of attention for its possible use in solar cells is called methylammonium lead iodide, quite a tongue twister, which is why I will refer to it as perovskite.

I have talked to Kári Sveinbjörnsson, a PhD student in the Physical Chemistry Group at Uppsala University (Sweden), who is working on perovskite solar cells. He says that a great advantage of perovskite solar cells is the cheaper manufacturing processes. Producing silicon from sand (which consists of silicon dioxide) consumes large amounts of energy as temperatures over 2000 degrees Celsius are required. In addition, silicon in solar cells needs to be extremely pure which leads to a total of 14 manufacturing steps. Producing perovskite solar cells, on the other hand, requires lower number of  steps and much less energy. A solution of the perovskite is dropped on an electronically conductive glas substrate. It is then put into a rotating machine, called a spin coater, to evenly distribute the solution. This is carried out at 100 degrees Celsius to evaporate the liquid solvent.

One big drawback of perosvikite solar cells is their poor stability according to Sveinbjörnsson. Perovskite is especially sensitive to moisture in air. Therefore, it has a much shorter lifespan than silicon solar cells and can not yet compete with their lifetime of 20 to 30 years. However, a lot of research is being carried out to change that. Another disadvantage is the presence of the toxic metal lead in the structure (indicated by the full name methylammonium lead iodide). For this reason scientists are trying to substitute lead with tin or bismuth, two less harmful metals. However, this exchange comes at a cost as the substitution of lead decreases the efficiency of the solar cells.

The extremely fast development of their efficiency has resulted in great interest for perovskite solar cells. While only a few percent of sunlight energy were converted into electricity with the first perovskite cells in 2009, we are today at an efficiency of 22 %, not far from the 25 % of silicon cells. This is an incredibly rapid increase within only eight years. At the moment a lot of research is focussed on the upscaling of the production process. It can be seen in Sveinbjörnsons image above that perovskite cells for research are not much bigger than one Swedish krona coins. Therefore, upscaling is important to make perovskite cells large enough to cover big roof parts.

Perovskite solar cells need one transparent electric contact on the top, facing the sun, and one electric contact at the bottom (which does not have to be transparent) in order to create electricity. Normally, thin layers of gold or silver are used for this task, but they can easily disintegrate and start to move through the entire solar cell which lowers its efficiency and stability. The most important part of Sveinbjörnssons research has been carried out on this issue in a collaboration with Aalto University (Finland) and École Polytechnique Fédérale de Lausanne (EPFL, Switzerland). In this work so-called carbon nanotubes, in other words extremely thin threads made of carbon, were used as the bottom electronic contact instead of silver or gold which resulted in more stable cells. This approach could eventually lead to cheaper perovskite cells with longer lifespans.

To the question when we will be able to buy commercialized perovskite solar cells, Sveinbjörnsson answers that the falling prices of silicon solar cells make predictions difficult. But he believes that we could see the first large-scale perovskite modules with 15 to 20 % effeciency within the next five years. Some more research revealed that there are already a few companies working towards the commercialization of perovskite solar cells. One approach is for example to combine perovskite with silicon by putting a thin perovskite cell on top of a silicon cell. This way the total efficiency can be increased as silicon absorbs the red part of the sunlight and perovskite the blue-green part. It remains interesting to see when and in which form perovskite will be commercialized in solar cells in the next years.

Is the future of the lithium-ion battery green?

Image credit: Stéven Renault, 2014, CC BY-SA IGO 3.0.

Lithium-ion batteries (LIBs) are normally considered an environmentally friendly technology, for example in electric vehicles where they are replacing climate gas spewing combustion engines. A widely unknown fact is that the production of these batteries causes a lot of pollution and climate gas emission too. I have talked to Dr. Stéven Renault, a researcher at Ångström Advanced Battery Center, Uppsala University (Sweden), who is trying to tackle this problem regarding the manufacturing process of LIBs.

Materials currently used in commercial LIBs are graphite and metal oxides such as cobalt oxide and manganese oxide. They are obtained via mining and subsequent chemical extraction. These processes create a lot of climate gas emission, pollution and waste. Metal oxides like cobalt oxide are also mainly mined in central Africa where unethical working standards and child labour are involved. In addition, it is very difficult to recycle lithium from these batteries when they reach the end of their life time.

The goal of Renaults research is to develop batteries from organic materials with a completely sustainable life cycle, in other words a ”green” battery. This means that the battery materials are produced from renewable sources like plants using environmentally friendly extraction methods. The contained lithium can also be easily recycled at the end of the batterys life span. This approach also focusses on using non-edible plants, for example trees, or parts of plants like the stems of crops to avoid conflicts with food production.

Nevertheless, Renault admits that green batteries still have some drawbacks, for example low energy densities. For this reason, they cannot yet compete with state-of-the-art materials (like graphite, cobalt oxide and manganese oxide) in mobile phones, laptops and electric vehicles. But the situation might change in the future if the lithium price keeps rising and lithium recycling becomes more attractive. In 2016 alone the price of lithium has increased by 14 %. Another issue is the lack of studies regarding battery safety. (We all have seen the images of burning mobile phones and laptops.)

To the question, if it is possible to buy these green batteries yet, Renault says that a Japanese company had unsuccesfully tried to commercialize them. But the competition of the standard batteries was too strong. Now a German company is working on green batteries for niche-markets where energy density matters less than in portable electronics and cars. This is where we will probably see the commercialization within the next two years.

Renaults own greatest accomplishment is the development of dilithium benzene diacrylate, a compound for use in LIBs that can be extracted from pine resins or alfalfa. When the end of the battery life cycle is reached lithium carbonate can be retrieved via ”thermal desctruction”, in other words burning, of the material. The lithium carbonate can then be used together with a precursor (benzene diacrylic acid) extracted from pine resins or alfalfa to create new dilithium benzene diacrylate to be used in another battery. This recycling process is shown in detail by the image above.

It will be very interesting to see when and in which form green batteries will be commercialized. Maybe we will be able to buy them for certain applications not too long from now.