From electricity-generating fuel cells to chemical sensors, many devices rely on a flow of protons for operation. A range of materials have been developed to provide the necessary pathways for proton flow, but so far no single substance has ever combined all of the most desirable properties for a proton-conducting material. Kimoon Kim and colleagues from the Pohang University of Science and Technology in Korea have now developed a highly porous organic material that meets many of the important requirements for proton conduction.1

Fig. 1: Crystal structure of a cucurbit[6] uril array with a proton-conducting network of water and acid molecules (center)© 2011 Wiley-VCH

Kim and his team recently synthesized a porous organic material based on a ring-shaped macrocyclic molecule called cucurbit[6] uril. The molecule assembles into large arrays with a honeycomb-like structure pervaded by one-dimensional channels (see image). While studying the material’s crystal structure, the researchers noticed that these channels were filled with a hydrogen-bonded network of water and acid molecules, forming an excellent pathway for the flow of protons.

The cucurbit[6] uril-based material offers several potential advantages over alternative porous materials for proton-conduction applications, says Kim. “Most remarkably, our porous organic materials are very stable under highly humid conditions, even in water and acidic aqueous solutions, making it possible to increase or control proton conductivity,” he says. The level of proton conductivity can also be tuned by changing the acid inside the pores, the team found.

Perhaps the most promising property of the cucurbit[6] uril-based material is its strong directionality of proton flow — the difference in proton flow parallel and perpendicular to the pore direction is the largest obtained so far for any material. Such flow anisotropy could be particularly useful in fuel cells, where it would allow energy loss to be minimized while maximizing proton conduction efficiency.

The team is now looking to further improve the properties of their materials for real-world proton-conducting applications. “Currently, we are trying to develop materials that operate efficiently at temperatures above 100 °C by incorporating guest molecules that can conduct protons at such high temperatures into the channels of our materials,” says Kim. “We are also synthesizing new porous organic materials using organic molecules containing sulfonates and phosphonates in order to develop materials with even higher proton conductivity.”