(Invited) Unraveling the Oxygen Reduction Reaction Mechanism and Activity of d-Band Perovskite Electrocatalysts for Low Temperature Alkaline Fuel Cells

Perovskites have recently shown the potentials of relatively high electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline media.[1] Therefore they can represent potential low cost cathode materials for low temperature alkaline fuel cell applications.

The basic perovskite oxide structure can be represented as ABO₃, where A is the larger cation, such as a rare earth or an alkaline earth element, and B is the smaller cation, generally a transition metal. The ABO₃ structure can accommodate cation substitution in a wide range by partial substitution of either the A and the B-site cation with another element giving (A_xA`<sub>1-x</sub>)(B<sub>y</sub>B`_1-y)O₃ compositions. Such substitution leads to modification of the perovskite band structure which in turn modifies the electrical, optical and magnetic properties of the oxides, and, thus, may also have a significant effect on their electrocatalytic activity.

Generally, the perovskite electronic properties are considered to be determined mostly by the B-site cation. When the B-site cation is a transition metal, the major contribution to the material physical properties derives from the B-site cation d-band electrons; for this reason perovskites having B-site transition metals are generally regarded as d-band perovskites.[2] However, the A-site cation can also play an important role in the physicochemical properties of d-band perovskites. Its size determines whatever the crystal structure is deviated from the ideal cubic form and doping the A-site with an aliovalent element can lead to the formation of oxygen vacancies or electron holes.

It has recently been shown that the electronic structure of perovskites (total number of occupied states) can be correlated to the oxygen adsorption energy on their surface.[3] The surface binding energy of oxygenated species is generally considered as one of a most crucial parameters for the catalytic activity toward the ORR.[4] Following the Sabatier principle,[5] the best perovskite catalyst would present an electronic (surface) structure which leads to an optimum surface-oxygen interaction energy able to maximize the overall ORR rate.

In the present contribution, different perovskite series have been taken into account systematically varying either the B-site or the A-site cation. This results in a tailored electronic (surface) structure, and thus in different ORR activities which have been investigated by rotating disk electrode (RDE) technique.

Perovskite powders have been synthesized using a modified sol gel process.[6] To obtain single phase materials all the powder precursors have been treated in oxygen atmosphere at high temperature (between 700-1000 °C), and the surface area has been subsequently measured by Brunauer-Emmett-Teller (BET) analysis. As generally reported in the literature, most of the prepared perovskites show low surface area (around 10 m²g^-1).[7] Furthermore, some of them present a relatively low conductivity. To overcome these issues, several studies on perovskite oxide catalysts use different carbons as additives. However, carbon is known to show a not negligible activity towards ORR in alkaline media reducing O₂ to hydroperoxide via a 2-electron process. Therefore, the addition of carbon should be systematically studied in order to verify whether the carbon plays only a simple role as conductive enhancer in the catalytic layer or if it is actually involved in the ORR kinetics.

In the present work we have investigated the ORR mechanism and activity of composite electrodes made of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) perovskite and acetylene black carbon by thin film rotating ring disk electrode (RRDE) technique. The RRDE technique allows a fundamental understanding of the ORR mechanism discriminating between a four or a two-electron process. The ratio of BSCF and acetylene black has been systematically varied and the onset potential for the ORR and the hydroperoxide formation has been determined for each composition. The RRDE measurements showed that composite electrodes possess a more positive onset potential for ORR and a lower hydroperoxide production in the whole tested potential range compared to pure BSCF and acetylene black, clearly pointing towards a synergistic effect between BSCF and acetylene black.

References

[1] R. F. Savinell, Nat. Chem. 3 (2011) 501.

[2] T. Wolfram and S. Ellialtioglu in “Electronic and Optical Properties of d-Band Perovskites”, Cambridge University Press, 2006.

[3] A. Vojvodic and J. K. Nørskov, Science 334 (2011) 1355-1356.

[4] J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard, H. Jónsson, J. Phys. Chem. B 108 (2004) 17886-17892.

[5] P. Sabatier, Berichte der deutschen chemischen Gesellschaft, 1911, 44, 1984-2001.

[6] E. Fabbri, R. Mohamed, P. Levecque, O. Conrad, R. Kötz, T. J. Schmidt, ChemElectroChem 1 (2014) 338–342.

[7] E. Fabbri, R. Mohamed, P. Levecque, O. Conrad, R. Kötz, T. J. Schmidt, ACS Catalysis 4 (2014) 1061−1070