The electro-catalysts for oxygen reduction reaction (ORR) on air-cathode have a decisive impact on metal-air batteries and fuel cells. Much effort has been invested in finding a cost effective electro-catalyst, enabling a decrease in the ORR over-potential and enhancing the power source device discharge performance.^[1-2]

Manganese-oxides (MnO_x) are particularly interesting as non-precious ORR electro-catalysts candidates due to their rich oxidation states, chemical compositions and their variety of crystal structures^[1-3]. In order to broaden our understanding on the catalytic mechanism of Manganese-Oxide and its surface chemistry during ORR, we performed unique in-situ electrochemical surface stress (ESS) measurements^[4].

The in-situ ESS response was measured on an Au/MnO_x electrode in both Ar and O₂ saturated environments. A complex stress response was measured during the electrochemical scan caused by various crystal structure transitions^[5]. The presence of oxygen resulted in a less compressive stress response during the ORR; suggesting that part of the activated sites on the surface are reduced and re-oxidized (Mn⁴⁺↔Mn³⁺) during the ORR. These observations evidently support the proposed ORR catalytic mechanism^[3,5].

In addition, an overall tensile trend was recorded during multiple cycling, pouring some light on the poor mechanical stability of the MnO_x film. This insight has major implications on the applicability of MnO_x as a catalyst on practical carbon based electrodes.

References:

1. R. Mainar, L.C. Colmenares , O. Leonet , F. Alcaide , J.J. Iruin , S. Weinberger , V. Hacker , E. Iruin , I. Urdanpilleta , J.A. Blazquez., Manganese oxide catalysts for secondary zinc air batteries: from electrocatalytic activity to bifunctional air electrode performance, Electrochimica Acta 217 (2016) 80–91.
2. Li, M. Gong, Y. Liang, J. Feng, J.-E. Kim, H. Wang, G. Hong, B. Zhang, and H. Dai., Advanced zinc-air batteries based on high-performance hybrid electrocatalysts, Nature Commun., 4 , (2013), 1805-1811.
3. A. Stoerzinger, M. Risch, B. Han and Y. Shao-Horn, Recent insights into manganese oxides in catalyzing oxygen reduction kinetics, ACS Catal., 5, (2015), 6021-6031.
4. Ha, J.L. Oberst, Z. Zeng, T. T.H. Hoang, Y. Cohen, D.J. Wetzel, R.G. Nuzzo, J. Greeley and A.A. Gewirth, In Situ Surface Stress Measurement and Computational Analysis Examining the Oxygen Reduction Reaction on Pt and Pd., Electrochimica Acta, 260, (2018), 400-406.
5. Gorlin, B. Lassalle-Kaiser, J. D. Benck, S. Gul, S. M. Webb, V. K. Yachandra, J. Yano and T. F. Jaramillo, In situ X-ray absorption spectroscopy investigation of a bifunctional manganese oxide catalyst with high activity for electrochemical water oxidation and oxygen reduction, J. Am. Chem. Soc., 135, (2013), 8525-8534.