MnO2-Based Bifunctional Oxygen Catalyst for Rechargeable Metal/Air Batteries: The Effect of K­­+ Intercalation

Extensive research has been undertaken to make intermittent clean and renewable energy sources more reliable. Bifunctional oxygen cathodes which can catalyze both ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) are the backbone of rechargeable metal-air batteries as well as regenerative fuel cells [1-3]. Although noble metal family elements and alloys such as Pt , Pt-Au  and Pt-Co  are known as the best catalysts for ORR in alkaline media, their poor electrocatalytic activity toward OER  as well as high price, comparing to non-PGM (non-precious group metal) compounds, limit their usage as a cost effective and active catalyst in bifunctional oxygen cathodes [2, 4]. Manganese oxides have been vastly employed as a robust cost-effective multifunctional and environmental friendly electrode material in battery industry, from primary to rechargeable metal-air batteries, as well as alkaline fuel cells and capacitors [3]. The electrolytic manganese dioxide (γ-MnO₂) is known as the most electrochemically active crystallographic form of MnO₂ for ORR in alkaline media with tafel slope of 40 mV dec^-1 and a low overpotential of -375 mV [1, 5]. However, poor OER electrocatalytic activity of MnO_x-based oxides in alkaline media diminish the hope of finding an exclusive bi-functional catalyst for both ORR and OER [1].

Our research is aimed at the development of highly active MnO₂-based catalysts for both ORR and OER with long cycle life by adding non-precious metal compounds as co-catalysts and intercalating potassium ions into the catalyst layer. The mechanisms for OER and ORR of the mixed catalysts and the role of K⁺are investigated by a combination of surface characterization methods and electrochemical techniques.

In the present work, LaCoO₃ were synthesized via co-precipitation methods. MnO₂-based Gas diffusion electrodes (GDE) were then prepared using the method described elsewhere [1]. In order to intercalate the potassium ions into the catalyst layer, a simple yet effective method has been used. The GDEs were kept in in 6M KOH solution at open cicuit potential while rotating the sample at 400 RPM for 6 days at 313 K.

Cyclic voltammetry tests were performed in O₂ saturated 6 M KOH at 293 K to investigate the electrocatalytic activity of the MnO₂-LaCoO₃ catalyst for both OER and ORR. The longer-term durability of the electrodes was also investigated by performing 100 repeated OER-ORR voltammetric cycles. Electron energy loss spectroscopy (EELS) has been also used to further analyze the catalyst layer and study the Mn valance changes during both OER and ORR. X-ray photoelectron spectroscopy (XPS) was also used to confirm the existence of intercalated K⁺ions in the catalyst layer.

Interestingly, huge enhancement is observed in the OER/ORR performance of the activated catalyst comparing to the fresh electrodes without any K⁺ activation. Fig 1-a and 1-b show the OER and ORR voltammograms, respectively, of the fresh and activated MnO₂-LaCoO₃ GDEs as well as fresh MnO₂ electrodes.  The OER overpotential for MnO₂-LaCoO₃ decreases to 193 from 367 mV after intercalating potassium ions in the catalyst layer (Fig. 1-a). In the ORR region, activated MnO₂-LaCoO₃ electrodes still provides the lowest ORR overpotential of -321 mV (Fig. 1-b), which is far better than the ORR behavior reported in the literature for Core-Corena Bifunctional Catalyst (CCBC), needle-like MnO_x thin film,  CoMn₂O₄ and even nanostructured Mn oxide thin film [1].

Moreover, the activated catalyst performs almost without any degradation in OER region while suffering up to 61% decrease in the ORR current density at -250 mV (vs MOE) after 100 cycles of severe durability tests. Our results indicate that even 12 hrs of activation in 6 M KOH could enhance the electrocatalytic activity of the degraded catalyst after severe cycling to some point, mentioned before as “healing effect” [1].  The EELS analysis reveals that the Mn valance decreases during severe cycling from about 4 to 2.7 for the cycle No. 1 and 100, respectively. This confirms that the loss in the ORR electrocatalytic activity of the activated sample after 100 cycles is associated with the transformation of MnO₂ to Mn₃O₄during severe cycling.

References:

[1] P.H. Benhangi, A. Alfantazi, E. Gyenge, Electrochimica Acta, 123 (2014) 42-50.

[2] J. Ludwig, J. Power Sources, 155 (2006) 23-32.

[3] A. Serov, A. Aziznia, P.H. Benhangi, K. Artyushkova, P. Atanassov, E. Gyenge, Journal of Materials Chemistry A, (2013).

[4] Y. Chabre, J. Pannetier, Prog. Solid State Ch., 23 (1995) 1-130.

[5] E.L. Gyenge, J.-F. Drillet, J. Electrochem. Soc., 159 (2012) F23-F34.