Highly Active Ba0.5Sr0.5Co0.8Fe0.2O3-δ Single Material Electrode towards the Oxygen Evolution Reaction for Alkaline Water Splitting Applications

Perovskites have shown high electrocatalytic activity towards the oxygen evolution reaction (OER) in alkaline media.[1] Therefore they can represent potential new anode materials for alkaline electrolyzers.

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. Generally, the perovskite electronic properties are considered to be determined mostly by the B-site cations. Perovskites having the B sites occupied mostly by Co cations have shown outstanding performance towards the OER. Particularly, composite electrodes made of perovskites and carbon have shown the most promising activity for the OER. It is often reported that carbon is needed in the perovskite-based electrodes to improve the electrical connectivity between particles and agglomerates, even though several recent literature reports have shown that carbon might play a more complex role as just a simple conductive support.[2-5] Therefore, understanding why carbon can act as activity booster for the electrochemical activity of some perovskite oxides is crucial, both from a fundamental point of view and for the rational design of better perovskite-based electrocatalysts. Related to this, it has been also previously reported that carbon can act as activity booster for cobalt-based spinel catalysts.[6]

In this contribution we show that electronic effects play an important role in the OER catalytic activity of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d(BSCF) perovskite/carbon composite electrodes compared to the single material electrodes in alkaline environment. By using X-ray absorption near edge structure spectroscopy (XANES), changes in the local electronic structure of the perovskite have been observed when a composite perovskite/carbon electrode is developed. This finding provides a novel understanding of perovskite/carbon composite electrodes and thus opens new perspectives for the optimization of the composite electrodes or for tailoring new perovskite compositions.

 BSCF powders have been synthesized using a modified sol gel process.[7] To obtain single phase materials all the powder precursors have been treated in oxygen atmosphere at 1000 °C for 2h. Acetylene black carbon was treated in nitric acid at 80 ºC overnight in order to create surface oxygen-containing functional groups which were shown to improve and stabilize the AB catalytic activity in alkaline environment.[4] The electrodes were prepared by ultrasonicating BSCF or BSCF and acetylene black (AB; 5:1 weight ratio) in isopropanol and Na+-modified Nafion as binder; then optimized thin porous films were deposited on glassy carbon substrates which allows investigating electrochemical activity of the material by the thin film rotating disk electrode technique.

OER measurements have revealed a significant decrease of the oxygen evolution overpotential at a fixed steady-state current density for the composite BSCF/AB electrodes compared to the single BSCF electrode.  XANES measurements have shown that while identical spectra were recorded for the Sr, Fe and Ba absorption edges, a clear shift in the Co K-edge towards lower energies was observed for the BSCF/AB composite electrode compared to the BSCF single material electrode. Co K-edge spectra were also recorded for reference samples, i.e. Co₃O₄ and CoO. While for BSCF the Co K-edge corresponds to that of Co₃O₄, the composite electrode displays an adsorption edge much closer to that of CoO, indicating that the mean oxidation state of the Co cations is lower in BSCF/AB than in BSCF electrode.  Therefore, these results suggest that the change of the Co oxidation state in BSCF when the catalyst is coupled with carbon might account for the superior catalytic activity of the composite electrodes vs. the single material electrodes. The influence of the Co oxidation state in the BSCF structure on the OER catalytic activity will be discussed in this talk.

References

[1] E. Fabbri, A. Habereder, K. Waltar, R. Kotz, T. J. Schmidt, Catal. Sci. Technol. 4 (2014)  3800.

[2] T. Poux, F. S. Napolskiy, T. Dintzer, G. Keranguevena, S. Y. Istomin, G. A. Tsirlina, E. V. Antipov, E. R. Savinova, Catal. Today  189 (2012) 83.

[3] S. Malkhandi, P. Trinh, A. K. Manohar, K. C. Jayachandrababu, A. Kindler, G. K. Surya Prakash, S. R. Narayanan, J. Electrochem. Soc. 160 (2013) F943.

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

[5] R. Mohamed, E. Fabbri, P. Levecque, R. Kötz, T. J. Schmidt, O. Conrad,  J. Electrochem. Soc 162 (2015) F579.

[6] Y. Y. Liang, Y. G. Li, H. L. Wang, J. G. Zhou, J. Wang, T. Regier, H. J. Dai, Nat. Mater. 10 (2011) 780.

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