(Invited) Fabrication and Operation Under the Same Conditions: Oxygen Reduction on Cathodically Deposited Manganese Oxide

Abstract

We present the data on the oxygen reduction (ORR) and oxygen evolution (OER) in an alkaline solution for manganese oxide electrocatalysts fabricated by means of cathodic electrocrystallization. The catalysts are deposited from a permanganate alkaline bath, and later operate in the same 1 M NaOH medium. The effect of the deposition potential is addressed. The goal of the study is to obtain carbon-free oxide electrocatalysts with satisfactory activitiy and stability. A qualitative difference of the electrochemical behavior is observed for oxides deposited at low (potentials above ~0.75 V RHE, region 1) and at higher overvoltage (potentials below ~0.75 V (RHE) region 2). Current efficiency is close to 40% in region 1, and increases up to 60% in region 2. All the samples consist of delta-MnO2 (birnessite), but their microstructure is essentially different for electrodeposition from regions 1 and 2. For the deposits fabricated in region 2 the birnessite structure is slightly distorted, while for the deposits prepared in region 1 a complete disordering along the layer stacking direction is observed. The voltammograms of oxides in supporting electrolyte solution undergo fast evolution under potential cycling, accompanied by a decrease of the total charge. Stabilized voltammograms for the samples fabricated in region 2 are featureless and demonstrate a pronounced Ohmic contribution, while for the samples deposited in region 1 a reversible redox behavior is observed, with broad current peaks and formal potential of ~ 0.85 V RHE. The total charge is proportional to the oxide loading in the interval under study (~ 10 – 90 mkg/cm2). Both types of samples do not exhibit reductive degradation at potentials above 0.7 V (RHE). ORR activities are estimated from the rotating disc electrode data in an O2-saturated solution. For oxides fabricated in region 1 the kinetic currents at 0.85 V RHE approach 40-50 A/g (for intermediate loadings), making them promising binder-free ORR catalysts. The ORR activity of oxides electrodeposited at higher overvoltages is an order of magnitude lower corroborating the influence of structure on the electrocatalytic activity. Even though the OER activities of electrodeposited oxides at 1.63 V RHE do not exceed 3 A/g, their remarkable stabilities both in the ORR and in the OER potential interval allows one to hope that these materials may be of interest as a component of hybrid bi-functional catalytic compositions together with other oxides more active in the OER. The authors are grateful to Yu.A. Velikodnyi and A.Ye. Baranchikov for their help with structural characterization of the samples. The work is supported by EraNetRus program, project (#270 NANO-Morf).