PNAS - Hierarchical nanostructured conducting polymer hydrogel with high electrochemical activity
Conducting polymer hydrogels represent a unique class of materials that synergizes the advantageous features of hydrogels and organic conductors and have been used in many applications such as bioelectronics and energy storage devices. They are often synthesized by polymerizing conductive polymer monomer within a nonconducting hydrogel matrix, resulting in deterioration of their electrical properties. Here, we report a scalable and versatile synthesis of multifunctional polyaniline (PAni) hydrogel with excellent electronic conductivity and electrochemical properties. With high surface area and three-dimensional porous nanostructures, the PAni hydrogels demonstrated potential as high-performance supercapacitor electrodes with high specific capacitance (about 480 F·g-1), unprecedented rate capability, and cycling stability (about 83% capacitance retention after 10,000 cycles). The PAni hydrogels can also function as the active component of glucose oxidase sensors with fast response time (about 0.3 seconds) and superior sensitivity (∼16.7 μA·mM-1). The scalable synthesis and excellent electrode performance of the PAni hydrogel make it an attractive candidate for bioelectronics and future-generation energy storage electrodes.
Bao and Cui made the gel by binding long chains of the organic compound aniline together with phytic acid, found naturally in plant tissues. The acid is able to grab up to six polymer chains at once, making for an extensively cross-linked network.
"There are already commercially available conducting polymers," said Bao, "but they all form a uniform film without any nanostructures."
In contrast, the new gel's cross-linking makes for a complex, sponge-like structure. The hydrogel is marked with innumerable tiny pores that expand the gel's surface area, increasing the amount of charge it can hold, its ability to sense chemicals, and the rapidity of its electrical response.
Still, the gel can be easily manipulated. Because the material doesn't solidify until the last step of its synthesis, it can be printed or sprayed as a liquid and turned into a gel after it's already in place – meaning that manufacturers should be able to construct intricately patterned electrodes at low cost.
"You can't print Jell-O," said Cui. "But with this technique, we can print it and make it Jell-O later."
The material's unusual structure also gives the gel what Cui referred to as "remarkable electronic properties."
Most hydrogels are tied together by a large number of insulating molecules, reducing the material's overall ability to pass electrical current. But phytic acid is a "small-molecule dopant" – meaning that when it links polymer chains, it also lends them charge. This effect makes the hydrogel highly conductive.
The gel's conductance is "among the best you can get through this kind of process," said Cui. Its capacity to hold charge is very high, and its response to applied charge is unusually fast.
The substance's similarity to biological tissues, its large surface area and its electrical capabilities make it well suited for allowing biological systems to communicate with technological hardware.
The researchers envision it being used in everything from medical probes and laboratory biological sensors to biofuel cells and high-energy density capacitors.
"And all it's made of are commercially available ingredients thrown into a water solution," said Bao.
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