Both fuel-cell technology for power generation and metal-air batteries for energy storage require an efficient electrode for O₂ reduction. Such air electrodes are usually a Pt catalyst embedded in a porous carbon electrode. Despite having a high current density suitable for high-power applications such as vehicle drive systems, a number of issues with these electrodes may ultimately limit the use and lifetime of the fuel cell or the storage battery, despite recent improvements. For example, the cost of the Pt alone in a polymer membrane fuel cell for a small 100-kW passenger vehicle is substantially greater (at March 2008 prices) than the current cost of an entire 100-kW gasoline engine. Several technical issues also arise with the use of Pt catalysts. The Pt particles present in the composite electrode are not fixed in place, and a well-known drift phenomenon by which the particles diffuse and agglomerate over time ultimately diminishes the performance of the fuel cell. Further, Pt is very sensitive to deactivation in the presence of CO, either in the air supply or as a by-product from the use of methanol in the direct methanol fuel cell.

Other metallic electrode materials, such as cobalt and Ru/Pt alloys, have been explored to overcome some of these problems, but in all cases one or more of the issues remain. In particular, the sensitivity to CO is a particularly difficult problem to overcome. In this work, we have developed a Pt-free air electrode based on a nano-porous, intrinsically conductive polymer (ICP) multilayer structure that offers performance similar to that of Pt-catalyzed electrodes under parallel testing. Because the material is homogeneous, the drift issue is avoided and, being nonmetallic, the catalyst is not sensitive to CO poisoning.

—Winther-Jensen et. al (2008)

Prior efforts on using ICPs for catalytic electrodes were limited by low conductivity and efficiency, and the instability of the ICP in the environment required for the catalysis. To overcome those issues, researchers incorporated traditional catalytic centers—such as Pt—into ICPs, but found that the materials suffer from many of the same problems as the traditional Pt-C electrode.

The use of vapor phase polymerization (VPP) enabled the production of materials with high conductivity, improved stability and controllable porosity, improving the potential of such ICPs for electrocatalytic applications. This prompted the Monash team to re-examine PEDOT’s potential as an electrocatalytic material, despite the failure of earlier studies to demonstrate catalysis of oxygen reduction.

An air electrode must establish a high-surface area interface between the three active phases: air, the electrolyte, and the catalyst/conductor.

The designed structure allows access of the air stream from one side of the electrode to a high–surface area, electrochemically active layer of the PEDOT, which is simultaneously in contact with the electrolyte. The Goretex membrane provides a good, although not entirely optimized, starting point because it is highly porous at the micrometer level and, being hydrophobic, does not allow penetration of the aqueous electrolyte into the pores of the membrane. Because the electrical conductivity of PEDOT is still not high enough to provide a low-resistance path to the external circuit, a more electronically conductive under-layer was used. Coating one face of this membrane with a ~40-nm layer of gold provided the conductor layer without altering the structure of the membrane.

—Winther-Jensen et. al (2008)

The researchers tested Goretex/PEDOT electrodes with PEDOT thicknesses ranging from around 40 to 1,300nm; they found that the 400-nm coating is nearly optimal for the membrane pore size. The air electrodes were tested at various pH levels and potentials in a cell that allowed direct contact with air from one side and electrolyte from the other. Continuous operation in air was achieved at –0.3 V for more than 1,500 hours at pH = 1; 3 Ah/cm² of charge was passed during this test. Testing of the electrochemical characteristics of the electrode after 1500 hours showed no change as a result of this period of operation.

The team created a Pt-catalyzed version of its electrode by depositing a 45-nm Pt layer onto the gold layer. The magnitudes of the conversion currents delivered by the PEDOT electrode were comparable to those of Pt for the same geometrical (membrane) area. (See figure below.)

Steady-state measurements (each point after 1 hour of continuous operation) of the conversion current versus potential at different pH values (black line: 400-nm PEDOT/Goretex; gray line: 45-nm Pt/Goretex). (A) pH1, (B) pH 7, and (C) pH 13 for oxygen reduction from air. Click to enlarge. Credit: AAAS

The highest room-temperature current density the researchers achieved with membranes was ~6 mA/cm².

This would be sufficient for some metal/air batteries and a number of fuel-cell technologies including small direct methanol fuel cells, micro fuel cells, and the various biofuel cell concepts (9, 26). Higher–current density fuel-cell application of the PEDOT electrocatalyst concept would require extension of the three-phase interface into a thicker membrane structure.

...The electrode described here provides only a partial solution to some of the problems with the use of Pt discussed in the introduction, because Pt is also used in the anode (fuel) electrode in the fuel cell. However, the fundamental mode of catalysis at work in the present materials may be able to be extended to other reactions, such as the hydrogen oxidation reaction, by careful choice of the ICP. ICPs can be successfully used as a substitute for Pt in dye-sensitized solar cells for the I–/I –3 redox reaction. Thus, the development of the gas-ICP-electrolyte three-phase interface electrode reported here may provide a platform for a new generation of metal-free electrocatalysts.

—Winter-Jensen et. al (2008)

Resources

• Bjorn Winther-Jensen, Orawan Winther-Jensen, Maria Forsyth, Douglas R. MacFarlane (2008) High Rates of Oxygen Reduction over a Vapor Phase–Polymerized PEDOT Electrode. Science Vol. 321. no. 5889, pp. 671 - 674 doi: 10.1126/science.1159267