Highly efficient cobalt sulfide heterostructures fabricated on nickel foam electrodes for oxygen evolution reaction in alkaline water electrolysis cellsHighly efficient cobalt sulfide heterostructures fabricated on nickel foam electrodes for oxygen evolution reaction in alkaline water electrolysis cells Peer-Reviewed Journal Publication Δημοσίευση σε Περιοδικό με Κριτές 2025-06-232023enNon-noble metal electrocatalysts for the oxygen evolution reaction (OER) have recently gained particular attention. In the present work, a facile one-step electrodeposition method is applied in situ to synthesize cobalt sulfide nanostructures on nickel foam (NF) electrodes. For the first time, a systematic study is carried out on the impact of the Co/S molar ratio on the structural, morphological, and electrochemical characteristics of Ni-based OER electrodes by employing Co(NO3)2·6 H2O and CH4N2S as Co and S precursors, respectively. The optimum performance was obtained for an equimolar Co:S ratio (1:1), whereas sulfur-rich or Co-rich electrodes resulted in an inferior behavior. In particular, the CoxSy@NF electrode with Co/S (1:1) exhibited the lowest overpotential value at 10 mA cm−2 (0.28 V) and a Tafel slope of 95 mV dec−1, offering, in addition, a high double-layer capacitance (CDL) of 10.7 mF cm−2. Electrochemical impedance spectroscopy (EIS) measurements confirmed the crucial effect of the Co/S ratio on the charge-transfer reaction rate, which is maximized for a Co:S molar ratio of 1:1. Moreover, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) were conducted to gain insights into the impact of the Co/S ratio on the structural and morphological characteristics of the electrodes. Notably, the CoxSy@NF electrocatalyst with an equimolar Co:S ratio presented a 3D flower-like nanosheet morphology, offering an increased electrochemically active surface area (ESCA) and improved OER kinetics.This research has received funding from the European Union under grant agreement No 101099717–ECOLEFINS project. Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or European Innovation Council and SMEs Executive Agency (EISMEA) granting authority. Neither the European Union nor the granting authority can be held responsible for them. PL, NP acknowledge financial support from the European Union’s Horizon 2020 research and innovation program under grant agreement no 871124 Laserlab-Europe. The authors would like to acknowledge the HELLAS-CH national infrastructure (MIS 5002735) implemented under “Action for Strengthening Research and Innovation Infrastructures,” funded by the Operational Programme “Competitiveness, Entrepreneurship, and Innovation” (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).http://creativecommons.org/licenses/by/4.0/Surfaces64493–508Poimenidis_et_al_Surfaces_6(4)_2023.pdfChania [Greece]Library of TUC2025-06-23application/pdf3.1 MBfree Poimenidis Ioannis Ποιμενιδης Ιωαννης Papakosta Nikandra Loukakos Panagiotis A. Marnellos, Georges E Konsolakis Michail Κονσολακης Μιχαηλ MDPI Alkaline electrolysis Oxygen evolution reaction Electrodeposition on Ni foam Cobalt–sulfur molar ratio Thiourea