Investigation of Conducting Polymers As Binding Agents for Oer/ORR Electrode Characterization

Investigation of the electrochemical performance of catalytic materials for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) has been the topic of intense research due to the different outcomes depending on the preparation of the electrode and the lack of standard characterization techniques to ease the evaluation of diverse materials.

Most of the materials studied include perovskites, pyrochlores, spinels, among other structures. Nafion® and Polyvinylidene fluoride (PVDF) binding agents suspended in aqueous solutions and organic solvents, respectively, are widely employed for the characterization of catalyst powder samples. Nafion® is a copolymer with ionic properties due to ion hoping between adjacent sulfonic groups. Typical catalyst supports using Nafion® involve small amounts of Nafion 5%wt diluted in water and alcohols to ensure a homogeneous dispersion [1]. In the case of PVDF, which is a non-reactive thermoplastic polymer, the catalyst powder is mixed with 10 to 20% PVDF, active carbon (acetylene black, AB) and dispersed in n-methyl pyrolidone (NMP). In the latter, AB provides with enough electronic conductivity needed for the electrochemical characterization. Using carbon as a conductive backbone is a very common practice; however several disadvantages arise from its utilization, such as: 1) lack of a reliable method for the determination of real electrochemical surface area, 2) participation of carbon during ORR might overestimate the real performance of the catalyst, and 3) carbonaceous materials tend to oxidize during the oxygen evolution reaction. Therefore, the determination of a binding agent that is capable of providing sufficient electronic conductivity and high chemical and mechanical stability during both OER and ORR can serve as advancement for the investigation of electrochemical properties for the catalytic powder oxides.

In the present work, a set of conducting polymers are used as binding agents and compared with Nafion-based supports. The conducting polymers include Polyaniline and poly(9,9-dioctylfluorene-co-fluorenone (PFM) conducting polymers were evaluated as binding agents in alkaline media for OER. Both of these polymers have been evaluated as negative electrode binders for Li-ion batteries, and have shown improved cycleability and high energy density [2]. The catalytic material used for the evaluation was lead ruthenium pyrochlore, Pb₂Ru₂O_7-δ. Preliminary results, (Fig. 1) show it is possible to have an enhanced performance of the pyrochlore by the use of a conducting polymer binder during anodic reactions. The performance for ORR was not greatly influenced by the presence of a conducting polymer binder. A well-defined redox peak ascribed to Ru^4+/5+ with high reversibility was observed for PFM support. This peak is also present in the case of Nafion®, but it is only noticeable during the first cycle, and diminishes with further cycling.

A systematic study of the influence of the conductive polymer binders on the OER performance in alkaline media will be presented.

Figure 1. Oxygen evolution on lead ruthenium pyrochlore supported in PFM (blue) and Nafion (pink). For comparison, PFM with no catalyst (red) and bare glassy carbon electrode (black) are also presented.

Acknowledgements

This work is supported by the IGERT-NSF fellowship and partially funded by the US Department of Energy/NETL, Oak Ridge Institute for Science and Education (ORISE) fellowship.

References

[1] S. Malkhandi, B. Yang, A. Manohar, A. Manivannan, S. Prakash, S.R. Narayan, Electrocatalytic Properties of nanocrystalline calcium-doped lanthanum cobalt oxide for bifunctional oxygen electrodes, J. Phys. Chem. Lett., 2012, 3, 967-972.  

[2] G. Liu, et al. Polymers with Tailored Electronic Structure for High Capacity Lithium Battery Electrodes, Adv. Mater. 2011, 23, 4679–4683.