Intercalation makes the difference with TiS 2 : Boosting electrocatalytic water oxidation activity through Co intercalat
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Maryline Ralaiarisoa Humboldt Universitat zu Berlin, Institut für Physik & Integrative Research Institute for the Sciences Adlershof, Berlin 12489, Germany
Kyung Taek Cho Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne Valais, Sion 1951, Switzerland
Emad Oveisi Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland
Norbert Koch Humboldt Universitat zu Berlin, Institut für Physik & Integrative Research Institute for the Sciences Adlershof, Berlin 12489, Germany
Mohammad Khaja Nazeeruddin Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne Valais, Sion 1951, Switzerland (Received 10 August 2017; accepted 17 October 2017)
Intercalated and unmodified TiS2 nanomaterials were synthesized and characterized by UV-Visible-NIR spectroscopy, Powder X-Ray Diffraction, and X-Ray Photoelectron and Ultraviolet Photoelectron Spectroscopy. Photoelectron spectroscopy measurements indicated that CoS and Cu2S appeared to be intercalated between sheets of partially or fully oxidized TiS2, which could be solution processed on conductive oxide substrates. The materials were then applied toward water oxidation and evaluated by cyclic voltammetry, chronoamperometry, and impedance measurements. While unmodified TiS2 was not observed to perform well as an electrocatalyst with overpotentials .3 V in 1 M NaOH electrolyte, CoS intercalation was found to lower the overpotential by ;1.8–1.44 V at 10 mA/cm2. Conversely, Cu2S intercalation resulted in only a modest increase in performance (.2.3 V overpotential). Impedance measurements indicated that intercalation increased the series resistance in the as-prepared samples but decreased the series resistance in oxidized samples.
I. INTRODUCTION
Efficient conversion of water to hydrogen and oxygen with solar energy will become increasingly important in the future.1 The sun is the earth’s only energy input and it supplies all of humanity’s annual energy in a matter of hours. Therefore, whether the sunlight is directly utilized through photoelectrochemical methods or through solardriven electrochemical methods, identifying materials capable of efficiently catalyzing the oxygen evolution reaction (OER) is an important challenge for the development of solar fuel generators.2,3 This four-electron, four-proton reaction producing a diatomic gas is one of the most Contributing Editor: Artur Braun a) Address all correspondence to this author. e-mail: aron.huckaba@epfl.ch DOI: 10.1557/jmr.2017.431
important on earth. Indeed, the electrons used to make fuels of all sorts should likely originate from the OER.4,5 The thermodynamic potential of the four electron, four proton overall water splitting reaction, 2 H2O (l) ! 2H2 (g) 1 O2 (g), is E 0 5 1.23 V. At all pH values, larger overpotentials (potential above the thermodynamic potential required to overcome kin
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