Nanocomposite Hybrid Molecular Materials for Application in Solid-State Electrochemical Supercapacitors** By A. Karina Cuentas-Gallegos , Monica Lira-Cant, Nieves Casaæ-Pastor, and Pedro Gómez-Romero* 1. Introduction Energy-conversion devices such as fuel cells and renewable- energy devices, such as photovoltaic or dye-sensitized solar cells, will need the support of efficient energy-storage technologies. Rechargeable lithium batteries and electrochemical superca- pacitors are two of the most prominent alternatives in this respect. [1,2] Batteries are best suited to store relatively large amounts of charge (highenergy densities).However, the ion-dif- fusion processes are relatively slow, which is the reason why bat- teries score low in power-density tests, and why complementary devices related to conventional capacitors are being developed. Electrochemical supercapacitors fill in the gap between batter- ies and conventional capacitors, resulting in devices that provide higher power density than a battery and higher energy density than a conventional parallel-plate or double-layer capacitor. [3,4] Current research in electrochemical capacitors has been car- ried out with emphasis on the development of new electrode materials. In this line of work, we can find three kinds of elec- trode materials, namely, high-surface-area carbons, [5] metal oxides, [6±11] and conducting polymers. [12±15] But in addition to these conventional types, novel alternative materials such as hybrid organic±inorganic nanocomposites are being considered because of their potential for synergic behavior. In this respect we mention a recent report of supercapacitor electrodes based on conducting polymers and metal oxides, [16] and the recent communication by our group [17] of preliminary work on super- capacitors based on hybrid electrodes formed by conducting polymers and polyoxometalates (POMs), a work which is re- ported in detail in this article. POMs resemble clusters of metal oxides, from both structural and electronic points of view; they are formed by a small num- ber of metal centers (typically 6±18 tungsten or molybdenum moieties) coordinated by bridging oxygen atoms; they present well-known structures; [18] and they undergo reversible multi- electron reduction processes both electrochemically and pho- tochemically, similarly to quantum-sized oxide particles. [19,20] Nevertheless, their solubility derived from their molecular nature, has caused them to be ignored as active compounds for electrodes or for any kind of material where collective proper- ties were needed. POMs have been extensively studied from a chemical point of view and have been used in catalysis and photocatalysis, either as homogeneous catalyst or supported onto polymers. Some examples of POM-doped conducting or- ganic polymers (COPs) such as polyaniline (PAni) are known; these can be applied in catalysis and in energy storage. [21] The similarities between POMs and oxides are not limited to their composition and topology; their electrochemical and pho- tochemical behavior are also parallel. Thus, POMs can be elec- trochemically or photochemically reduced to form blue species. These reduced species are chemically and spectroscopically equivalent to tungsten or molybdenum bronzes in the form of colloidal semiconducting quantum dots, with the added advan- tage for POMs of a well-known structure that is stable in solu- tion. [20] One such structure is the Keggin structure, common to many heteropolyacids including all three studied here. In addi- tion to their reversible redox activity, these species present high proton conductivities in their solid (acidic) form. Furthermore, they represent the ultimate limit for the dispersion of oxide species, since all metal centers can be considered to be ªsur- faceº centers, in contact with an electrolyte. All of which makes them a priori good candidates for electrode materials for electrochemical supercapacitors. COPs, on the other hand, have been extensively studied as promising novel materials for use in rechargeable batteries [21±25] Adv. Funct. Mater. 2005, 15, 1125±1133 DOI: 10.1002/adfm.200400326  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1125 ± [*] Dr. P. Gómez-Romero,Dr. A. K. Cuentas-Gallegos, Dr. M. Lira-Cant,Dr. N. Casaæ-Pastor Institut de Ciencia de Materials de Barcelona (ICMAB) Campus UAB E-08193 Bellaterra (Spain) E-mail: pedro@icmab.es [**] This work was carried out within the framework of the ªXarxa Temàti- ca de Piles de Combustible de la Generalitat de Catalunyaº and the ªRed de Pilas de Combustible del CSICº and was partially funded by MCyT (Spain) (MAT2002-04529-C03). We also acknowledge a Ramon y Cajal contract for MLC and thank the ªConsejo Nacional de Ciencia y Tecnología de MØxico (CONACYT)º for a pre-doctoral fellowship to AKCG. Molecular hybrid materials formed from polyoxometalates dispersed in conducting polymers represent an innovative concept in energy storage. This work reports in detail the first practical realization of electrodes based on these materials for energy storage in electrochemical supercapacitors. The molecular hybrids PAni/H 4 SiW 12 O 40 , PAni/H 3 PW 12 O 40 , and PAni/H 3 PMo 12 O 40 (PAni: polyaniline) have been prepared electrochemically on platinum or carbon substrates, with PAni/H 3 PMo 12 O 40 being the proto- typical example presenting the best energy-storage performance in the series. This hybrid displays the combined activity of its organic and inorganic components to store and release charge in solid-state electrochemical capacitor cells, leading to a promis- ing value of 120 F g ±1 and good cyclability beyond 1000 cycles. FULL PAPER