|CV of the Ppy(Lig) composite electrode. (A) Voltammograms recorded between 0.1 and 0.4 V. (B) Voltammograms recorded between 0.1 and 0.75 V versus Ag/AgCl, scan rates 5 to 25 mV s−1 (inner to outer). Milczarek and Inganäs. Click to enlarge.|
Grzegorz Milczarek from Poznan University of Technology (Poland), and Olle Inganäs from Linköping University (Sweden), have combined lignin derivatives, which are electronic insulators, with polypyrole, a conductive polymer, into an interpenetrating composite suitable for use as a battery cathode. Their research appears in the 23 March issue of the journal Science.
Milczarek and Inganäs describe how a class of organic compounds known as quinones allows the lignin derivatives to shed a proton and store this electric charge in its place. The polypyrrole is able to hold on to the loose proton until the charge is released and the proton returns to the quinone group of the lignin derivative.
Renewable energy systems based on intermittent sources require methods for power balancing over time, and thus some means of storage. Charge storage in organic polymers rarely gives energy and power densities, gravimetric or volumetric, that match the needs for secondary batteries and supercapacitors. This was one reason for the abandonment of efforts to make polymer batteries from conjugated polymers two decades ago, because inorganic insertion electrodes are superior. Widespread application of electrical power storage may require more abundant materials than those available in inorganics (which often require rare metals), and at a lower cost. Materials for charge storage are desired from easily accessible and renewable sources. Combining cellulose materials and conjugated polymers for charge storage has again attracted attention.
Biopolymers with redox functions are used in energy conversion processes in plants. The highly sophisticated structures designed to split water to oxygen and protons, in photosystem II in green plants, are made from protein structures combined with a manganese complex and use temporary proton storage on amino acid residues to accomplish this four-electron oxidation step. Electron and proton storage is found in the metabolism of plants and bacteria, where quinones are used as soluble electron/proton transport agents. With hydroquinone (Q/QH2), two electrons and protons are stored in a structure of 6 carbon and 2 oxygen atoms, an electronic charge density of 2 Faraday per 108 g, 1787 C/g, or 496 mAh/g. This is a favorable number compared with standard electrochemical systems; in lithiated carbon materials, a maximum doping level is 6 carbons per lithium, equivalent to 344 mAh/g and, in the olivine FePO4 system, 170 mAh/g. It is desirable to use the quinone redox function in electroactive materials to enhance charge-storage capacity.
...If lignin is incorporated into an electrode material with sufficient electronic and ionic conductivity to allow charge transport to and from the quinone site, it is possible to use this redox function for charge storage. We show that polypyrrole is suitable and that quinone electrochemistry and polypyrrole conductivity combine to create an electroactive conjugated polymer/biopolymer composite.—Milczarek and Inganäs
Upon investigating the electrochemical properties of the material, they determined that it had a volumetric charge density of 100 mAh/cm3.
The authors note that self discharge is a problem with these electrodes and will need further study. However, they added, there is room for optimization of the materials using different sources of processed lignins with varying loading, with varying charge densities, and with a possibility to improve upon present results.
We have demonstrated interpenetrating networks of lignosulfonate and polypyrrole that can be used for charge and energy storage. The use of the renewable biopolymer should lead to low-cost electrodes with improved safety and nontoxicity, operating in water. There is ample room for further developments to improve charge density and capacitance by searching through the universe of lignins.—Milczarek and Inganäs
Among biopolymers, they note, lignin is second only to cellulose in biosynthesis and makes up some 20 to 28% of wood.
Grzegorz Milczarek and Olle Inganäs (2012) Renewable Cathode Materials from Biopolymer/Conjugated Polymer Interpenetrating Networks, Science 335 (6075), 1468-1471. doi: 10.1126/science.1215159