Chemistry of Flow Batteries

Technology is progressing more and more with each passing day. In order to support these changes, advancements regarding power supply must also be made. At the moment, the trend is moving towards more efficient and sustainable sources of energy. The rechargeable, low-cost, and long-lasting flow battery seems to fit the bill perfectly. Of course, it is not as well known as the common lithium-ion battery because it is used on a larger scale rather than for small everyday items. It can be used to store excess electrical grid power which is then released during periods of high demand.

When you hear the word “battery”, the first thing that comes to mind is probably a package of AA or AAA batteries — you know, the ones with the adorable Energizer bunny on the front? Flow batteries are neither portable nor small. In fact, they consist of two liquid-filled tanks that are separated from the actual cell of the battery. These “tanks”, or electrolyte reservoirs, pump liquid electrolytes into two half cells separated by a membrane, as can be seen in the image below.

A reduction reaction takes place on one side and an oxidation reaction in the other, similar to the reactions within the fuel cells we mentioned in our last post. The membrane between the two half cells  keeps  the electrolytes separate, but is thin enough to allow certain ions to pass through in order to complete the redox reaction. Ions from both sides flow through the membrane and react with the electrodes on both sides of the cell, drawing energy from them. To store more power, the batteries can be stacked in a bipolar arrangement. At this point, they essentially provide unlimited electrical storage capacity. The only limit is the capacity of the electrolyte reservoirs.

Sounds pretty simple, right? Now for the interesting, and slightly more complex, part: how the actual energy storage takes place. The liquid electrolytes that flow through the cell are mixed with energy storing materials such as iron, vanadium, zinc, or bromine. Zinc-bromine flow batteries, for example, have a zinc anode in one half cell and a bromine cathode in the other. Aqueous zinc bromide is circulated through the two half cells. In discharge, (bottom image on right) a load is applied to the cell and the zinc metal on the anode is oxidized (Zn(s)↔ Zn2+(aq) + 2e) to form zinc ions and bromine is reduced to bromide ions at the cathode (Br2(aq)+ 2e ↔ 2Br(aq)). When the battery is completely discharged, the metal zinc on the anode dissolves in the electrolyte. It is stored there until the battery is recharged (top image on right), during which the reactions involving zinc and bromine are reversed. The zinc ions are reduced back to metal (Zn2+(aq) + 2e– ↔ Zn(s)), thus plating metallic zinc back onto the electrode. On the cathode side, bromide ions are oxidized into molecular bromine in the aqueous solution (2Br(aq) ↔ Br2(aq) + 2e) which combines with an oil to form a dense, oily liquid called a polybromide complex. As more polybromide complex is created and more zinc metal is plated onto the anode, the energy stored in the system increases. Because there are always fresh electrolytes in both half cells, the system is always ready to produce full power, even when the pumps are off. The electrodes in the zinc-bromine batteries don’t take part in thereactions but rather function as substrates, so repeated cycling won’t cause the electrode materials to deteriorate, as it would in most other rechargeable batteries.

So far, flow batteries seem like the best option available. The only problem is that they’re not quite available — not commercially, at least. Right now, the main issue is cost. The most commercially developed flow battery utilizes the rather expensive metal vanadium. For this reason, a team of Harvard researchers have recently developed a new type of metal-free flow battery that instead uses organic molecules called quinones, specially those found in rhubarb. According to Harvard author Roy Gordon, there is a limited number of metal ions that can be put into solution and used to store energy, none of which can store large amounts of renewable energy. For this reason, researchers have turned to organic molecules. The quinone used by the Harvard team is known as 9,10-anthraquinone-2,7-disulfonic acid (AQDS). What is unique about AQDS is its capacity for rapid reactions. It is able to undergo quick and reversible two-electron two-proton reduction on a glass-like carbon electrode in sulfuric acid. Although the small battery prototype has only run through about a hundred cycles, so far it has exhibited little to no losses. The model displays nearly the same performance as vanadium flow battery, but is much less expensive.

Flow batteries have a promising future, though they are not widely available at the moment and there are still unresolved issues regarding cost and production. Eventually, large quantities of energy will need to be stored and conserved in order to sustain our population and the great demand for power. Flow batteries will undoubtedly provide the best and most efficient solution.

The Chemistry of Hydrogen Fuel Cells

In cities like Beijing, citizens have to live every day with a suffocating layer of smog in the atmosphere. Fortunately for most Americans, this is not a very prominent issue. However, with the immense amount of pollution produced from the industries that continuously provide for the needs and luxuries of Americans, it could be.

Pollution has been a problem ever since global warming was coined as a term. There are so many ways to contribute to this problem that is growing at such a fast rate that there seems to be no significant way to stop it.

One small step towards solving this problem is the implementation of fuel cells in vehicles. Fuel cells, specifically hydrogen fuel cells, are one of the most environmentally friendly power sources available. The byproducts of their electricity-making process are solely water and heat! Although frequently compared to the battery, technically speaking, the fuel cell is an electrochemical energy conversion device. It converts hydrogen and oxygen into water, producing energy in the form of electricity in the process. Now, how does energy enter into the equation?

This energy actually comes from the reverse of a relatively familiar process – electrolysis. Electrolysis uses electricity to separate a molecule into its original components. By sending an electric current into a solution through an electrolyte, which ionizes when dissolved in a solvent, the flow of ions is stimulated and allows for the non-spontaneous reaction (the break-up of the molecule) to occur. In 1839, a Welsh scientist named Sir William Robert Grove reversed this process and generated electricity and water from hydrogen. He called his creation a gas voltaic battery, now known today as a hydrogen fuel cell.

There are many types of fuel cells that serve different purposes, but they all have the same general setup. In a hydrogen fuel cell hydrogen atoms enter at the anode, where their electrons are stripped by an oxidation reaction: 2H2 –> 4H++ 4e-. As a result, the hydrogen atoms are ionized and carry a positive charge. The electrons then travel through a wire where they produce a direct electric current (DC) output. In some fuel cells, the positively charged hydrogen ions move through the electrolyte (represented by the proton exchange membrane pictured below) and join with oxygen molecules that enter from the cathode and the electrons returning from the wire. Other fuel cells have the oxygen molecules pick up the electrons first before moving through the electrolyte to the cathode, where the electrons combine with the hydrogen ions to form water, as can be seen in the reduction reaction O2 + 4H+ + 4e- –> 2H2O. In this process, water and heat are formed as a result, meaning that the reaction is exothermic. All these steps can be summarized in the net, or “redox” reaction: 2H2 + O2 => 2H2O + energy. Both products are released from the exhaust and are harmless in terms of pollution.

Graphic credit to Marc Marshall, Schatz Energy Research Center/

Not only are fuel cells eco-friendly, but, in comparison with batteries, they have the ability to last much longer. Unlike batteries, fuel cells can be used as long as there is access to hydrogen and oxygen. Batteries, on the other hand, can’t be refueled. The one exception is rechargeable batteries, but even these will eventually die. Additionally, since fuel cells create electricity chemically, they are not subject to the thermodynamic laws that greatly restrict efficiency. Fuel cells are, thus, more efficient than batteries.

There are six main types of fuel cells: polymer exchange membrane (PEMFC), solid oxide (SOFC), alkaline (AFC), molten-carbonate (MCFC), phosphoric acid (PAFC), and direct methanol (DMFC). They each have advantages and disadvantages and hold different futures in the ever-changing world of technology. For more specific information on each type, check out this link.

Let’s explore one of them. Utilizing the simplest reactions, PEMFC is one of the more promising of the fuel cell family and will most likely be used in homes and transportation. It consists of an anode, a cathode, an electrolyte, and a catalyst, and follows the aforementioned description of how a fuel cell works. Hydrogen gas enters the fuel cell on the anode side and is split into H+ ions and electrons once it comes into contact with the catalyst. The electrons move through the anode to the external electric circuit where they produce an electric current, then return to the cell on the cathode side, where oxygen gas is pumped through. Because of oxygen’s high electronegativity, it attracts and pulls the H+ ions through the exchange membrane and forms, along with the electrons that return to the cell, water molecules. This reaction will produce about 0.7 volts. To increase this voltage to a more useful level, several fuel cells are layered on top of each other and connected by bipolar plates, forming a fuel-cell stack. Try out the simulation here!

Now for how they work in cars! Fuel cell vehicles consist of five distinct components: the fuel cell stack, electric motor, high-output battery, hydrogen storage tank, and power control unit. The fuel cell stack converts the highly pressurized hydrogen gas (to increase driving range) stored in the hydrogen storage tank with the oxygen from the air into electricity, which powers the electric motor. Compared to a conventional internal combustion engine, the electric motor is much quieter, more efficient, and more smooth. The high-output battery stores energy that is generated from regenerative braking and provides supplemental power to the electric motor. Lastly, the power control unit controls and oversees the flow of electricity.

It is clear that hydrogen fuel cells have a bright future ahead, Of course, there are disadvantages to every new breakthrough in science, but with time, those are ensured to be addressed. Even now, these issues are being acknowledged and improved. Recently, an article from USA Today detailed Toyota’s advancement in fuel cell technology with an increase in range and shortening of the time it takes to refuel. With improvements already being seen, this technology is sure to make its way into the transportation and home infrastructure in no time. So keep your eyes peeled for any mention of fuel cells!

The Chemistry of Fusion Bombs

In the previous article, we learned about how nuclear fusion occurs naturally, specifically in stars throughout the universe. Now that we understand the very extreme set of conditions that must be present for fusion to occur, we can shift to the oppositeend of the spectrum: man-made fusion. As we have already learned, the fusion reaction gives off a great amount of energy which can be used for many practical purposes that would benefit our daily lives. Throughout the ages, it seems that it has been humanity’s goal to harness ever increasing power for its own benefit which could also destroy humanity altogether. Whether it is a bomb, or an alternative energy source in the form of a nuclear fusion reactor, those inventors who are able to achieve both are definitely making a great contribution to all mankind as long as humans are seeking peaceful cohabitation at all times.

Let’s start off with the largest nuclear fusion bomb ever detonated. The device officially designated RDS-220, known to its designers as Big Ivan, and nicknamed in the west Tsar Bomba was the largest nuclear weapon ever constructed or detonated. The nickname Tsar Bomba is areference to a famous Russian tradition for making gigantic artifacts for show. The bomb was detonatedin 1961 on Novaya Zemlya islands in Arctic Russia. This three-stage weapon was actually a 100 megaton bomb design, but the uranium fusion stage tamper of the tertiary (and possibly the secondary) stage(s) was replaced by one(s) made of lead. This reduced the yield by 50% by eliminating the fast fissioning of the uranium tamper by the fusion neutrons, and eliminated 97% of the fallout (1.5 megatons of fission, instead of about 51.5 Mt), yet still proved the full yield design. (http://www.tsarbomba.org/)

The result was the “cleanest” weapon ever tested with 97% of the energy coming from fusion reactions. The effect of this bomb at full yield on global fallout would have been tremendous. It would have increased the world’s total fission fallout since the invention of the atomic bomb by 25%. Despite the very substantial burst height of 4,000 m (13,000 ft) the vast fireball reached down to the Earth, and swelled upward to nearly the height of the release plane. The blast pressure below the burst point was 300 PSI, six times the peak pressure experienced at Hiroshima in 1945. The flash of light was so bright that it was visible at a distance of 1,000 kilometers, despite cloudy skies.

As we discussed in the first blog post, there are two types of nuclear fusion reactors that have been developed. They include the magnetic confinement reactor and the inertial confinement reactor. In the late 1940’s it seemed an irresolvable problem to scientists as how to enclose the plasma, since any contact to the reaction container wall would let the surface layer of the wall evaporate and cool the plasma rapidly, causing the fusion cease. In 1951, Lyman Spitzer had the idea to enclose the plasma in a magnetic cage. As the plasma is ionized, it consists of charged particles (positive ions and electrons) that can be influenced by a magnetic field. Their trajectory has two components: a circular motion at right angles to the magnetic field and a linear motion across the magnetic field. The Tokamak was invented by Soviet physicist Igor Tamm and Andrei Sakharov in 1952. Toroidal magnetic fields were used to avoid the particles that escaped at the poles of the magnetic field. However, a toroidal magnetic field was not able to hold the plasma in an equilibrium force balance because the field strength decreased from the inside to the outside of the toroidal field with the effect that the particles drift towards the wall. Therefore, the field lines may not take a circular course about the axis of the torus, but need to be helically looped. The scientists were enthusiastic and predicted in 1955 that in 20 years time, nuclear fusion would provide us with limitless energy. However, the magnetic confinement turned out to be much trickier than assumed. To this day, experiments and tests are being run in nuclear fusion facilities in France to prevent particles from leaving the magnetic field. (http://www.world-nuclear.org/info/Current-and-Future-Generation/Nuclear-Fusion-Power/

Now that we know that fusion is the future of clean and virtually unlimited power for all of our electrical needs, we can move forward into our next topic: Fusion in the Future, and specifically, cold fusion. Get excited to really focus on the chemistry concepts involved with this unique type of energy creation. Until then, keep reading to find out!

Pons, Fleishmann, and their Cold Fusion Success

After the completion of their cold fusion success, Pons and Fleishmann went to the United States department of Energy to request more funding for further research. However, they were denied because others who had attempted to replicate their results had not been successful. Cold fusion, they concluded, was impossible and the experiment carried out by the two electrochemists was flawed. In the last blog post we discussed the details of the experiment, and today we will explain some theories as to why these false results were obtained.

A research team at the California Institute of Technology extensively attempted to reproduce the results of Pons and Fleishmanns experiment to no avail. They then attempted to implement different experimental errors to see if they could reproduce the same results. When the Caltech team did not put a stirrer in the tester cell, temperature differences within the cell led to false temperature recordings, which led to data that was similar to that of Pons and Fleishmanns data. These researchers at Caltech also suggested that the helium neutrons which suggested to Pons and Fleishmann that there was nuclear fusion transpiring could have simply been atmospheric traces of helium which are natural.

Some researchers also believe that Dr. Pons’ son, who helped by watching over the lab some days, might have turned off the electrical current briefly. This, Dr. Lewis of Caltech says, will cause the chemical reaction on the surface of the palladium to cease, and the excess deuterium to “bubble out” causing a fire hazard and thus, exhibited a pseudo-raise in the temperature of the cell.

Many even suggested that Pons and Fleishmann outright lied about their data. There was a great deal of mystery surrounding the experimental set-up, and the two electrochemists purposely shielded their experiment from testing and criticism by the scientific community. If you want to see for yourself how suspicious (or innocent) Dr. Pons and Dr. Fleishmann were, you can watch this video of the electrochemists at a press conference, addressing their critics. Notice how well they avoid the questions — it’s more like politics than science.

Chemistry of Cold Fusion

In our group’s previous post, we talked about nuclear fusion in the past but now its back to the future, with cold fusion, (and specifically the Pons-Fleishmann experiement)!

Cold fusion was a concept thought up by Stanley Pons and Martin Fleishmann in 1989. These electrochemists claimed that they had been able to achieve nuclear fusion at room temperature, and dubbed this miracle cold fusion. Had these claims been true, nuclear fusion would have been significantly more feasible for more widespread use. However, when other scientists attempted to replicate this experiment, they were not able to obtain the same of level success, and cold fusion came to be regarded as a myth. Why were other scientists not able to get the same result, you ask? Well lets look at the experiment in detail.

Stanley Pons (University of Utah) and Martin Fleishmann (University of Southampton) hypothesized that nuclear fusion may occur if electrolysis was carried out with deuterium in palladium metal. Electrolysis is simply the process by which a chemical reaction is instigated using an electrical current. Pons and Fleishmann believed that electrolyzed deuterium would have a very high compression ratio and level of mobility, which would allow it to undergo nuclear fusion, and thus produce exorbitant amounts of energy. Palladium naturally experiences a chemical reaction on its surface which causes it to absorb large amounts of hydrogen into its metal. Fleishmann and Pons believed that if enough deuterium (an isotope of hydrogen) atoms were absorbed at once, they may undergo nuclear fusion. To test this theory, they put a palladium cathode in a calorimeter with heavy water, or deuterium oxide (contains more of the deuterium isotope than regular water). Through electrolysis, the deuterium oxide broke down into its elemental components, and the deuterium was absorbed into the palladium. This electrical current was applied over several weeks, and the heat change was measured. For most of the experiment the temperature remained stable at around 30 degrees celsius, but then the temperature would suddenly rise to 50 degrees celsius for two or more days at a time, although the input energy would not change. Thus, during these phases the calculated output energy was significantly higher than the input energy

It is widely believed that the Pons-Fleishmann experiment was flawed, especially in its sources of experimental error. Based on how the experiment was just outlined, what do you think are the sources of experimental error? Check to see if you guessed correctly when we reveal the answer in our group’s next post!

What Makes a Rocket Soar?

Since the launch of Sputnik 1 by the Soviet Union on 4 October 1957, the world has been entranced by the idea of traveling in deep space. For the past six decades, humanity has watched all manner of rocket and shuttle be launched into space for missions in low earth orbit, to the very edge of our solar system. However, despite the grand ambitions of generations past and present, wide scale space travel and exploration is still not commercially viable, instead relying primarily on government funded space agencies, like the US’s own NASA. Part of the problem is the complicated, not to mention expensive, process of creating and launching a rocket into outer space. This blog post will cover the chemical basis of that exact process, describing in detail the various fuels that make a rocket zoom.

Rocket technology does not consist of one piece, and neither does a rocket’s propulsion system. To get a rocket into orbit, or out of Earth’s influence altogether, requires a combination of different fuels and propellants to best suit the situation. In the following section, I will cover some of the most common fuel sources, as well as some interesting experimental ones.

Liquid-Based

Liquid Oxygen and Liquid Hydrogen

One of the most popular, and simple, of the conventional fuels is a combination of liquid hydrogen and liquid oxygen, which are kept compressed and cooled in storage buildings on-site until needed at the time of launch, as seen in the white dome in the leftmost edge of the image on this page, where NASA describes in detail some of its transfer and storage methods for this cryogenic fuel. An interesting point to note is that, at least in the case of the liquid hydrogen, no active pump in necessary to transport the fuel. A small amount is simply allowed to vaporize, creating a pressure differential that pushed the liquid fuel out of the tank, through the piping, and into the waiting rocket.

The liquid oxygen and hydrogen are then combined in the rocket. From there, it is a simple matter of igniting the two, setting off a reduction-oxidation (redox) reaction. As might be expected, the oxygen is the oxidizer, and the hydrogen is the reducing agent. Written in expanded form, the reaction proceeds as such.

Hydrogen is oxidized (gives up electrons)

 H2 → 2H+ + 2e

Oxygen is reduced (gains electrons)

O2 + 4e → 2O2-

Combined, they yield a net reaction of:

2H2 + O2 → 2H2O

This reaction, however, is very spontaneous under elevated temperatures. As anyone who has ever ignited hydrogen will tell you, given the right conditions, it more explodes (or pops) than burns. It is this controlled explosion that powers the first state of the rocket’s flight. From then on, however, different means of propulsion come into play, one of the most prominent of which is solid fuel rocket boosters.

Other forms of liquid propellant include using kerosene instead of hydrogen and using a combination of nitrogen tetroxide and hydrazine as the fuel, but they all rely on the same fundamental process of a redox reaction.

Solid-Based

Solid-fuel based rockets come in a wide variety of configurations, depending on the scale, cost, safety, and performance needed for a given operation. They are useful in providing boosts to the main liquid-based propulsion systems of some rockets (i.e. the Space Shuttle), hence the name of solid rocket boosters. While the exact fuel varies, one of the most common general fuel types for large scale rockets (the kind needed to launch spacecraft) is a powdered metal, usually magnesium or aluminum, that is oxidized by a strong oxidizing agent, such as ammonium nitrate or ammonium perchlorate. These are called composite propellants. An example reaction goes as follows:

10Al + 6NH4ClO4 → 4Al2O3 + 2AlCl3 +                                                 12H2O + 3N2

The aluminum is oxidized, yielding the high energy output of the reaction, and the ammonium chlorate acts as the oxidizer, easily accepting the aluminum’s electrons.

Other forms of solid rocket propellants include conventional black powder, adding high explosives to composite propellants, a powdered zinc-sulfur mixture, and using sugar as the oxidized material, amongst others.

 Other Forms of Propulsion/The Future:

One promising form of rocket propulsion lies in ion-based systems, pursued by space agencies up to and including NASA. In an ion-based system, the careful position of electrodes is used to accelerate charged particles (the ions) to speeds that can reach a significant fraction of the speed of light. Theoretically, given enough time, the rocket and/or vehicle can accelerate with an upper bound of the speed of its ion exhaust, but other factors (i.e. time, particles in space) make the top functional speed of the rocket significantly slower, but still faster, depending on scale, than the current batch of propellant technologies. Already, ion propulsion is used to make minute adjustments in a rocket’s (or other space vehicle’s) trajectory and/or orientation, but the time of ion propulsion as the primary source of acceleration is still a ways away.

This system relies on a constant source of electrical power, usually solar or nuclear, given the constraints of a system with limited resources in outer space. It also requires a computer and a source of particles to ionize to operate, conditions unique to this system of propulsion.

The future is promising for ion propulsion technology, given the high theoretical velocity vehicles using it can reach and its relatively fuel-cheap design. If may well be that ion propulsion systems come to dominate the industry for space vehicle propulsion, but for now, we just have to rely on conventional fuels, be they cryogenic liquids or metallic powders.

Looking to the Future

From the earliest, most primitive form of “solar panels,” plants, to the contemporary quantum dot solar cells, the harvesting of energy provided by our greatest resource, the sun, has evolved tremendously. Throughout this blog, we have covered quite a bit. We talked about the photovoltaic effect, the premise for the chemistry behind solar panels, semiconductors, especially silicon the most common material for solar panels, solar cell efficiency (link 3rd post), and even some of the cutting edge solar cell research happening right now including perovskites (link 4th post)and quantum dot solar cells (link 5th post).

 Due to the nature of solar technology, the role of chemists is irreplaceable. Chemists must design new dyes, develop materials for better electron transport, and they must also characterise each new material to ensure that the energy of the electron ‘fits in’ with the other materials – the rest of the solar cell. Just like a car engine, if one part doesn’t fit, the whole thing doesn’t work. In this way, chemists, along with physicists and engineers have to work together to find the best materials which fit together to ensure that the solar cell is working as best as possible.

Can solar power change the world? For some communities in developing countries, it already has. Scientists must continue to work with businesses, economists, architects, designers and a whole host of other professions to make sure that solar cells are practical, cost effective and appealing. Solar cells will continue to make a large contribution to reducing the world’s dependency on fossil fuels, closing the poverty gap and changing the world. In one hour, more sunlight falls on the earth then is used by the entire population of our planet in a year. It is the most readily available renewable source of energy we have access to, and it is up to us to utilize it the best way possible.

 So we leave you with a deeper understanding of the mechanisms of a photovoltaic cell, the variety of solar panels, and this awesome (at least we think it is) TED Talk by Joe Jordan, a NASA Researcher about “The Solar Window of Opportunity.”