Scientific Method / Science & Exploration

Organic hydrogel outperforms typical carbon supercapacitors

Water makes it cheaper and means devices can be made with an inkjet printer.

Hydrogels come in a variety of forms, based on the materials that are mixed with water.

Supercapacitors complement batteries in energy storage and delivery schemes both large and small, as they can provide quick bursts of power. They already help Honda’s fuel cell vehicle FCX accelerate. But supercapacitors hold less energy per volume than a typical battery, so they have limited storage capacity.

Changing the electrode material can boost the capacitance, thus improving the energy density. Yi Cui and Zhenan Bao of Stanford University have made a hydrogel (water-based gel) using a conducting polymer. When used as electrodes in a supercapacitor, the new material has a capacitance about three times greater than a typical carbon supercapacitor. It’s also cheap to build and operate.

Typical supercapacitors are made from two closely spaced, porous carbon electrodes that charge and discharge quickly. Negative ions from the electrolyte collect inside the pores in the positive electrode, while positive ions gather in the negative electrode. That ion separation stores energy as a potential difference between the two electrodes.

Besides storing ions, supercapacitors made from conducting polymers have an additional form of capacitance called pseudocapacitance. Electrons transferred to and from the polymer strands build extra charge along the chains.

One common conducting polymer is a string of many molecules of aniline, a ringed structure with a nitrogen hanging off. The researchers connected strands of this polymer with a phosphate-covered sugar derived from plants to form the new hydrogel material. The hydrogel looks like foam under a scanning electron microscope. Its 3D branches create a porous structure that provides the material with more surface area to hold ions and charges.

To make the hydrogel supercapacitor, the researchers coated two carbon sheets with the new polymer gel and soaked the electrodes in a dilute solution of sulfuric acid. Then they connected each electrode in a circuit and measured how much charge the supercapacitor could hold.

Aniline worked very well in hydrogel form. The device retained 93 percent of its capacitance when charged quickly with ten times the standard current density. Typical electrodes made from polymerized aniline lose 25-40 percent of their capacitance when charged at higher power. The new material also retained 83 percent of its capacitance when charged at high power for 10,000 cycles—ten times more charging cycles than other polyaniline electrodes.

The researchers say this new material is easier to synthesize than other polyaniline-based hydrogels. Previous conducting polymer hydrogels were limited because they started with a block of non-conducting polystyrene soaked in aniline. A special molecule triggers the chain-forming reaction, or polymerization, of aniline. That creates threads of conducting polyaniline winding through the non-conducting hydrogel.

Because this new material is made mostly from polyaniline, the entire structure can conduct electricity. And its synthesis only requires mixing two solutions: one containing the aniline and the sugar linker, the other containing the triggering molecule. Once mixed, the hydrogel sets within three minutes.

Things are much simpler physically, too. The polymer can be synthesized using inkjet printing or spray coating each solution on a surface. That means it’s easy to make it on a large scale for possible energy storage applications—or even on a tiny scale for microelectronics, the researchers say.

The electrolyte is another factor that makes this conducting polymer supercapacitor attractive for low cost, yet high performance, energy devices. This hydrogel uses a cheaper water-based electrolyte compared to the organic ionic liquids used in carbon supercapacitors.

Proc. Natl. Acad. Sci., 2012. DOI: 10.1073/pnas.1202636109 (About DOIs).

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