The efficient catalysis of the four-electron oxidation of water to molecular oxygen (oxygen evolution reaction, OER) is a central challenge for the development of devices for the conversion of electrical energy, ideally from renewable sources, into storable chemical energy (e.g. by electrolysis or “artificial leaves”). Up to now, a plethora of earth-abundant, non-toxic catalysts (e.g. Ni/Fe, Co, Mn oxides)¹ for OER is known. Most of these possess high activities and stabilities under strongly alkaline pH conditions. In contrast, the only convincing materials for OER in acidic media are based on the scarce elements Ru and Ir, while especially Ni and Co based electrodes suffer from corrosion and/or show little activity.²

Inspired by the oxygen evolving complex (OEC) of Photosystem II, the biological catalyst for this reaction, several manganese oxide (MnO_x) polymorphs have been tested as heterogeneous water oxidation (electro-)catalysts. Here we found that amorphous layered or tunnelled manganese oxides show good activities and stabilities also for strong acidic reaction media.³ Recently, we developed a route to directly prepare coatings of amorphous MnO_x on free-standing carbon based electrode supports (e.g. carbon-fiber-paper, CFP) by a simple, scalable redox deposition approach.⁴

This presentation will deal with a study where the electrocatalytic performance of such MnO_x/CFP-anodes was tested under different electrolyte conditions. A special focus was laid on the influence of the pH on activity and stability. Additionally, the MnO_x/CFP-electrodes were characterized before and after electrolysis by means of XRD, SEM/EDX, vibrational- and X-ray absorption spectroscopy. Details of the preparation, characterization, electrocatalytic performance and corrosion stability will be discussed together with possible reasons for the different behavior of the electrocatalyst at different pHs. We will report on an efficient catalyst for OER at acidic and near-neutral solution, a pH range highly desirable for (photo-)electrochemical devices, such as polymer electrolyte membrane (PEM) electrolysers or artificial leaves.

References

¹ N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337–365.

² L. C. Seitz, C. F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H. Y. Hwang, J. K. Nørskov and T. F. Jaramillo, Science, 2016, 353, 1011–1014.

³ C. E. Frey and P. Kurz, Chem. A Eur. J., 2015, 21, 14958–14968.

⁴ J. Melder, W. L. Kwong, D. Shevela, J. Messinger and P. Kurz, ChemSusChem, 2017, 4491–4502.