Science Buddies: "Ask an Expert"

I have a question about a project that I am working on at my high school and in my off time. The premise is this: To use an endothermic reaction to store energy, and then release that energy in a electrochemical reaction (can't remember what it's called) like what goes on inside a battery. Essentially making a rechargeable battery that absorbs heat to recharge itself? In theory this is possible as you are just converting energy from heat to chemical to electrical. The chemical reactions would all be ionic bonds, the endothermic with a high activation energy and a low output energy, storing the heat energy. With the "battery" part of the reaction needing a low activation energy and a high output energy, undoing the work of the endothermic reaction and releasing that energy in the form of electrical current Although I am learning chemistry rather quickly through sites like Kahn academy, Crash course, & Google, as well as through several teachers at affiliate high-schools, What I need is chemistry help as my main knowledge lies in the electrical fields. Any help, confirmation of ideas, or willingness to work as a team or become a professional contact for me would be greatly appreciated.

Hello highschoolengineer,

The Ask An Expert people aren't organized to make commitments to team work or any long-term arrangement. We will be happy to answer specific questions as you pose them.

The idea that you describe sounds very interesting as you state it, but I'm no chemistry expert, so I can't think of an example of the battery cycle you're describing. Can you give an example of some reaction that would enable the cycle? The endothermic reactions I am familiar with are such as dissolving sugar in water. The end result is a state with higher entropy than the starting state. How would the other half of the cycle drive the state back to low entropy while extracting energy?

Good luck, WW

Wendellwiggins, Thank you for your answer. These are exactly the type of problems I am having. I do know that there are examples of cycling reactions, where the product of the reaction forms the reactants of a second chemical reaction, which products in turn are the reactants for the original reaction. The thing that seems to trigger each of the halves of the cycle reaction are the temperatures as they reach the activation energy of that half. Also I don't think dissolving sugar in water is an endothermic reaction as you are just dissolving something and as far as I know there is no reaction happening, but correct me if I'm wrong. It may absorb heat but it is not an actual chemical reaction. A good example of one would be what goes on inside an instant ice pack. When you "snap" the ice pack you mix the ammonium chloride and water inside - Causing the following chemical reaction, which absorbs heat to make the pack cold in an endothermic reaction.
NH4Cl(s) + H2O(l) ----> NH4+(aq) + Cl-(aq) + H2O+(l) .
In my idea of the battery the products of an endothermic reaction would either be, or be converted into, the reactants for the electrochemical current producing reaction. The products of this reaction would then be used ad the reactants of the endothermic reaction. Forming a "cycling chemical reaction" that converted heat energy into electrical energy. The exact opposite of an electrical resistor.
Also I'm not sure how exactly entropy factors into this, but my understanding of entropy is limited at best. Could you please explain?

highschoolengineer,

It seems clear that you've thought about your problem a lot, but, as you say, much remains to be settled.

You are right that dissolving sugar isn't strictly a chemical reaction in the sense that new molecular species are formed. However, the sugar molecules become surrounded by water molecules that are very weakly bound to them by the charge attraction between the OH groups of the sugar molecules and the water molecules. Some more info. on this reaction is at http://www.weironline.net/chemcentral/book/chap15.pdf.

The cooling occurs because heat has to be absorbed to move a sugar molecule from being attached to other sugars in the crystal. This happens anyway because there are just so many more ways that a sugar molecule can be in solution than the number of ways it can be attached to the sugar crystal. This movement toward the most abundant molecular arrangements is a primary aspect of what's called entropy. That explanation probably doesn't make much sense all by itself. There are many books and web sites that discuss entropy, so I'll leave it at that.

A simple type of battery that has some features of what you describe can be made as follows. Get a membrane that is porous to chlorine ions but not sodium. Place a NaCl solution on each side of the membrane. If the solutions on each side are the same concentration, no net flow of chloride ions goes across the membrane. If you connect an electrical supply to make one side of the membrane positive and the other negative, the chloride ions will flow from the positive side to the negative side because of their negative electrical charge (the charging reaction). Then after the concentration of chloride ions builds up on one side, turn off the supply. Now more chloride ions will flow back to the low concentration side than the reverse direction and provide a current for an electrical circuit (the discharging reaction). Several web pages discuss this idea, for example http://en.wikipedia.org/wiki/Concentration_cell. Most directly to your point, if the solution on one side of the membrane were heated, the increased energy of the hot chloride ions could drive them across the membrane, but this effect would be small compared to the electrically driven migration.

One more idea to think about. In a conventional chemical-reaction battery, the process is directed by providing for a single reaction pair that absorbs an electron on one side and releases it on the other half of the pair. If you build a system in which you just move heat around such as in a steam engine, the efficiency with which you can extract work is limited by the laws of thermodynamics. The upper limit of efficiency may be much lower than achievable in an A -> B chemical reaction. See http://en.wikipedia.org/wiki/Heat_engine.

Well, I hope this gives you some food for thought. The scope of all the issues you have to consider is so large that I'll leave it to you to track down some of the learning materials already out there. We can't tackle any of them in detail here. Please don't be discouraged just because you have much to learn. You seem to have a pretty good grasp of several ideas already. So far as any deadlines you have, the trick will be to narrow your attention down to something you can do in the time given.

Good luck, WW

highschoolengineer,

I think I finally get your idea. I don't have the background to know the range of different chemical reactions that nature provides, so I can't make a list of reactions that might be good candidates for a thermally driven battery. I did remember, however, one line of work that's very close to your idea.

Please look at http://sadoway.mit.edu/wordpress/wp-con ... me/145.pdf. In Section 2 of that paper is a discussion of "the thermally regenerative battery, known as the bimetallic cell." As in your idea, one half of the reaction cycle is driven thermally, and the other half is driven by electric current. The reaction, like dissolving sugar, is not strictly a chemical reaction, but otherwise I think it's what you had in mind.

The senior author of that paper is a Professor at MIT and founder of a company trying to commercialize liquid metal batteries. See http://www.ambri.com.

I think the entire paper will provide you a lot of ideas to chew on. I hope you have some fun with it. WW

Thank you for your reply. The paper had some great ideas and taught me some things about battery testing, electrodes and electrolites. It also gave me a bit of Déjà vu as I had just watched the ted talk that the mit professor that wrote the peper did the day before. I will continue on my project, searching out solutions and posting "smaller" questions to this forum as they arise. Thank you.