July 18, 2012
An electrochemical battery has its energy stored in the potential energy of the chemical reactions it contains. In a lead-acid battery, the net discharge reaction is
Pb + PbO2 + 2H2SO4 --> 2PbSO4 + 2H2O
The reaction has an enthalpy (ΔH°) of -549 kJ, per starting mole of lead (Pb); the negative sign indicating a reaction in which we get some energy. Lead and lead dioxide (PbO2) are somewhat heavy, so it takes quite a bit of mass to get this energy. There is, however, a lot of energy to be had. Since a watt is a joule per second, 549 kJ is about 150 watt-hour. In theory, a ten pound battery could run a hair dryer for about half an hour.
The volumetric energy density is constrained by the volume of the sulfuric acid (H2SO4), which is in a water solution. Right away, we take a volume penalty, since the 4.2 Molar sulfuric acid used is just a third H2SO4, and two-thirds water. Although lead-acid cells are rechargeable, once they're charged, that's the limit of your energy storage. Any extra energy you have can't be stored, so it's lost.
Instead of having huge electrochemical batteries, wouldn't it be nice if we could have a smaller device that converts the energy into a liquid without consuming the electrodes in the process? We could then pump, or pour, this energy into suitable storage vessels, and then put this liquid energy back into the cell when we need to extract some power. This is reminiscent of the energon cubes in the Transformers animated television series.
This science fiction is actually fact in a device called a flow battery; specifically, a redox flow battery. All the chemical reactions are contained in the solution, and the electrode materials are not consumed. The power you can get from such a flow battery depends in the electrode area, so our battery still needs some heft. However, the fluid volume depends only on the capacity of our storage tanks; and, the cost of a tank becomes less per unit volume as the tank volume increases.
Flow batteries are a possible solution to load balancing in renewable energy systems, such as wind and solar energy. Such energy sources function intermittently, at the whim of nature, so you need to store energy while the sun shines, so to speak, and use it at a later time. Flow batteries eliminate the need to have a multitude of cells, formed from expensive materials, in a parallel combination.
A team of scientists from Drexel University's A.J. Drexel Nanotechnology Institute has applied the flow battery concept to supercapacitors. In this case, the electrolyte is replaced by a slurry of carbon particles and electrolyte. Just as in standard supercapacitors, the energy is stored in an electric double layer at the charged carbon particles.[1-2]
This research project was led by Yury Gogotsi, director of the A.J. Drexel Nanotechnology Institute. I wrote about some of Gogotsi's previous work with supercapacitors in a previous article (Carbon Nano-Onion Supercaps, August 26, 2010). This flow capacitor research is published as an article in the July, 2012, issue of Advanced Energy Materials, a special issue devoted to battery materials.
As can be seen from the diagram, below, the device is simply made, with separate reservoirs for electron donor (+) and electron acceptor (-) materials. The flow materials are thick slurries of small carbon particles in the electrolyte liquids.[1-3] Uncharged, or partially-charged, slurry is pumped through the flow cell, which charges the carbon particles and deposits the charged slurry into reservoirs. When energy is needed, the process is reversed, and the charge flows through the external circuit attached to the flow capacitor.
|Structure of a conventional supercapacitor.|
There are separate ionic liquids that are negatively and positively charged. One disadvantage of current materials is that these ionic liquids decompose at voltages above a few volts.
(Via Wikimedia Commons, modified).
If conventional supercapacitors can be a guide, such a system would be robust and long-lived, since they do not have any performance degradation for a million charge-discharge cycles. There is still considerable work needed to make a battery system that's competitive with other energy storage options. Gogotsi's team is working on development of slurry compositions composed of a variety of carbon nanomaterials and electrolytes. Said Gogotsi,
|The Drexel University flow battery.|
(Diagram rendered by the author using Inkscape).
“We have observed very promising performance so far, being close to that of conventional packaged supercapacitor cells... However, we will need to increase the energy density per unit of slurry volume by an order of magnitude, and achieve it using very inexpensive carbon and salt solutions to make the technology practical."
- Volker Presser, Christopher R. Dennison, Jonathan Campos, Kevin W. Knehr, Emin C. Kumbur and Yury Gogotsi, "Electrochemical Flow Cells: The Electrochemical Flow Capacitor: A New Concept for Rapid Energy Storage and Recovery," Advanced Energy Materials, vol. 2, no. 7 (July, 2012), pp. 895-902.
- Britt Faulstick, "Making "Renewable" Viable: Drexel Engineers Develop New Technology for Grid-Level Electrical Energy Storage," Drexel University Press Release, July 10, 2012.
- Supplemental videos for ref. 1, Advanced Energy Materials, vol. 2, no. 7 (July, 2012).
Permanent Link to this article
Linked Keywords: Electrochemical battery; energy; potential energy; chemical reaction; lead-acid battery; discharge; enthalpy; joule; kJ; mole; lead (Pb); lead dioxide (PbO2); mass; watt; watt-hour; pound; hair dryer; volumetric energy density; volume; sulfuric acid (H2SO4); aqueous solution; water solution; Molar concentration; Molar; electrode; energon cube; Transformers; science fiction; flow battery; redox; electrical load balancing; renewable energy; wind power; solar energy; nature; Make hay while the sun shines; material; parallel circuit; scientist; Drexel University; A.J. Drexel Nanotechnology Institute; supercapacitor; electrolyte; slurry; carbon; electric double layer; Wikimedia Commons; Yury Gogotsi; Advanced Energy Materials; electron donor; electron acceptor; Inkscape; charge-discharge cycle; energy storage; nanomaterial.
Latest Books by Dev Gualtieri
Thanks to Cory Doctorow of BoingBoing for his favorable review of Secret Codes!
Blog Article Directory on a Single Page
- J. Robert Oppenheimer and Black Holes - April 24, 2017
- Modeling Leaf Mass - April 20, 2017
- Easter, Chicks and Eggs - April 13, 2017
- You, Robot - April 10, 2017
- Collisions - April 6, 2017
- Eugene Garfield (1925-2017) - April 3, 2017
- Old Fossils - March 30, 2017
- Levitation - March 27, 2017
- Soybean Graphene - March 23, 2017
- Income Inequality and Geometrical Frustration - March 20, 2017
- Wireless Power - March 16, 2017
- Trilobite Sex - March 13, 2017
- Freezing, Outside-In - March 9, 2017
- Ammonia Synthesis - March 6, 2017
- High Altitude Radiation - March 2, 2017
- C.N. Yang - February 27, 2017
- VOC Detection with Nanocrystals - February 23, 2017
- Molecular Fountains - February 20, 2017
- Jet Lag - February 16, 2017
- Highly Flexible Conductors - February 13, 2017
- Graphene Friction - February 9, 2017
- Dynamic Range - February 6, 2017
- Robert Boyle's To-Do List for Science - February 2, 2017
- Nanowire Ink - January 30, 2017
- Random Triangles - January 26, 2017
- Torricelli's law - January 23, 2017
- Magnetic Memory - January 19, 2017
- Graphene Putty - January 16, 2017
- Seahorse Genome - January 12, 2017
- Infinite c - January 9, 2017
- 150 Years of Transatlantic Telegraphy - January 5, 2017
- Cold Work on the Nanoscale - January 2, 2017
- Holidays 2016 - December 22, 2016
- Ballistics - December 19, 2016
- Salted Frogs - December 15, 2016
- Negative Thermal Expansion - December 12, 2016
- Verbal Cues and Stereotypes - December 8, 2016
- Capacitance Sensing - December 5, 2016
- Gallium Nitride Tribology - December 1, 2016
- Lunar Origin - November 27, 2016
- Pumpkin Propagation - November 24, 2016
- Math Anxiety - November 21, 2016
- Borophene - November 17, 2016
- Forced Innovation - November 14, 2016
- Combating Glare - November 10, 2016
- Solar Tilt and Planet Nine - November 7, 2016
- The Proton Size Problem - November 3, 2016
- Coffee Acoustics and Espresso Foam - October 31, 2016
- SnIP - An Inorganic Double Helix - October 27, 2016
- Seymour Papert (1928-2016) - October 24, 2016
- Mapping the Milky Way - October 20, 2016
- Electromagnetic Shielding - October 17, 2016
- The Lunacy of the Cows - October 13, 2016
- Random Coprimes and Pi - October 10, 2016
- James Cronin (1931-2016) - October 6, 2016
- The Ubiquitous Helix - October 3, 2016
- The Five-Second Rule - September 29, 2016
- Resistor Networks - September 26, 2016
- Brown Dwarfs - September 22, 2016
- Intrusion Rheology - September 19, 2016
- Falsifiability - September 15, 2016
- Fifth Force - September 12, 2016
- Renal Crystal Growth - September 8, 2016
- The Normality of Pi - September 5, 2016
- Metering Electrical Power - September 1, 2016
Deep Archive 2006-2008