Owing to their high power density and superior cyclability relative to batteries, electrochemical double layer capacitors (EDLCs) have emerged as an important electrical energy storage technology that will play a critical role in the large-scale deployment of intermittent renewable energy sources, smart power grids, and electrical vehicles. Because the capacitance and charge-discharge rates of EDLCs scale with surface area and electrical conductivity, respectively, porous carbons such as activated carbon, carbon nanotubes and crosslinked or holey graphenes are used exclusively as the active electrode materials in EDLCs. One class of materials whose surface area far exceeds that of activated carbons, potentially allowing them to challenge the dominance of carbon electrodes in EDLCs, is metal-organic frameworks (MOFs). The high porosity of MOFs, however, is conventionally coupled to very poor electrical conductivity, which has thus far prevented the use of these materials as active electrodes in EDLCs. Here, we show that Ni(2,3,6,7,10,11-hexaiminotriphenylene) (Ni(HITP)), a MOF with high electrical conductivity, can serve as the sole electrode material in an EDLC. This is the first example of a supercapacitor made entirely from neat MOFs as active materials, without conductive additives or other binders. The MOF-based device shows an areal capacitance that exceeds those of most carbon-based materials and capacity retention greater than 90% over 10,000 cycles, in line with commercial devices. Given the established structural and compositional tunability of MOFs, these results herald the advent of a new generation of supercapacitors whose active electrode materials can be tuned rationally, at the molecular level.

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Mussels adhere to a variety of surfaces by depositing a highly specific ensemble of 3,4-dihydroxyphenyl-L-alanine (DOPA) containing proteins. The adhesive properties of Mytilus edulis foot proteins mfp-1 and mfp-3 were directly measured at the nano-scale by using a surface forces apparatus (SFA). An adhesion energy of order W Ϸ3 ؋ 10 ؊4 J/m 2 was achieved when separating two smooth and chemically inert surfaces of mica (a common alumino-silicate clay mineral) bridged or ''glued'' by mfp-3. This energy corresponds to an approximate force per plaque of Ϸ100 gm, more than enough to hold a mussel in place if no peeling occurs. In contrast, no adhesion was detected between mica surfaces bridged by mfp-1. AFM imaging and SFA experiments showed that mfp-1 can adhere well to one mica surface, but is unable to then link to another (unless sheared), even after prolonged contact time or increased load (pressure). Although mechanistic explanations for the different behaviors are not yet possible, the results are consistent with the apparent function of the proteins, i.e., mfp-1 is disposed as a ''protective'' coating, and mfp-3 as the adhesive or ''glue'' that binds mussels to surfaces. The results suggest that the adhesion on mica is due to weak physical interactions rather than chemical bonding, and that the strong adhesion forces of plaques arise as a consequence of their geometry (e.g., their inability to be peeled off) rather than a high intrinsic surface or adhesion energy, W. bioadhesion ͉ Mytilus edulis

The beak of the Humboldt squid Dosidicus gigas represents one of the hardest and stiffest wholly organic materials known. As it is deeply embedded within the soft buccal envelope, the manner in which impact forces are transmitted between beak and envelope is a matter of considerable scientific interest. Here, we show that the hydrated beak exhibits a large stiffness gradient, spanning two orders of magnitude from the tip to the base. This gradient is correlated with a chemical gradient involving mixtures of chitin, water, and His-rich proteins that contain 3,4-dihydroxyphenyl-L-alanine (dopa) and undergo extensive stabilization by histidyl-dopa cross-link formation. These findings may serve as a foundation for identifying design principles for attaching mechanically mismatched materials in engineering and biological applications.Living organisms are functional assemblages of different interconnected tissues. Not infrequently, tissues with highly disparate mechanical properties (e.g., bone and cartilage, shell and adductor muscle, nail and skin) are joined together (1). In practice, the joining of dissimilar materials can lead to high interfacial stresses and contact damage (2,3). In apparent contradiction to this, the contacts between mechanically mismatched biomolecular tissues are remarkably robust. Mechanical-property gradients are increasingly invoked as the principal reason for their mechanical performance. The dentino-enamel junction (4), arthropod exoskeleton (5), polychaete jaws, and mussel byssal threads (6) all exhibit such gradients. Optical properties in squid eyes have also been correlated to a protein-density gradient (7). Although much is known about the mechanical and biochemical properties of the separate tissues, surprisingly little has been done to explain how mixtures of macromolecules are adapted for incremental mechanical effects at interfaces.The beak of the Humboldt squid Dosidicus gigas is an example of a system with grossly mismatched tissues. It is composed of slightly offset apposing upper and lower parts that make no hard pivotal contact with one another and are set into a muscular buccal mass that controls