Performance of titanium oxynitrides in the electrocatalytic oxygen evolution reaction

We report results of a detailed study on the activity and stability of titanium oxynitrides in the oxygen evolution reaction (OER) and their correlation with structure and (surface) composition of these materials, which had been reported as promising catalysts for this reaction and for the reverse oxygen reduction reaction (ORR). Combining electrochemical flow cell measurements with online mass spectrometry analysis (differential electrochemical mass spectrometry) allows us to separate catalytic O-2 evolution from other electrochemical reactions such as catalyst oxidation. Materials with different nitridation levels were fabricated via thermal treatment of titanium oxide in gaseous ammonia. The resulting materials, which cover a wide range of O:N ratios, were characterized by X-ray diffraction, elemental carbon-hydrogen-nitrogen analysis, and X-ray photoelectron spectroscopy, yielding information about bulk crystallinity, chemical composition, and surface species present, respectively. They show increasing oxidation current densities starting at about 1.2 V, with a peak maximum at 1.7 V. Online mass spectrometry analysis, however, reveals that this current results from oxidation of the electrode surface rather than from O-2 evolution, while O-2 evolution occurs only at potentials > 1.7 V. Increasing nitride contents were found to increase the electrochemical electrode oxidation reaction, while the onset of O-2 evolution is independent of the extent of nitridation. Consequences of these findings on the suitability of these materials as OER catalysts will be discussed. (C) 2016 Elsevier Ltd. All rights reserved.