Researcher: Şimşek, Kaan
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Şimşek, Kaan
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Publication Metadata only Pyridinic nitrogen induced compressed bilayer graphene for oxygen reduction reaction(Elsevier Sci Ltd, 2023) Cankaya, Mehmet; Titus, Charles James; Lee, Sang Jun; Nordlund, Dennis; Ogasawara, Hirohito; Tekin, Adem; Department of Computer Engineering;Department of Chemistry; Solati, Navid; Kahraman, Abdullah; Şimşek, Kaan; Kaya, Sarp; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Engineering; College of SciencesDespite the emergence of nitrogen-doped graphene as a noble-metal free electrocatalyst for oxygen reduction reaction, its participation in the electrochemical conversion mechanism is not well -established. In the present study, functionalities of the nitrogen species on the oxygen reduction activ-ity of bilayer graphene were investigated by combining atom-specific X-ray spectroscopy, Raman spec-troscopy, and density functional theory calculations with electrochemical activity tests in alkaline media. Among various nitrogen species, pyridinic nitrogen as the dominant species improved the electro-chemical activity of bilayer graphene, which was followed by graphene bilayers doped with graphitic nitrogen in majority. Polarization curves revealed a significantly high electrocatalytic oxygen reduction activity of the nitrogen-doped bilayer graphene where the pyridinic nitrogen was the major dopant. This improved activity was confirmed by the lowest overpotential and Tafel slope (78.9 mV/dec). The enhanced interaction of graphene bilayers doped with pyridinic nitrogen is shown to be the main reason for this improvement.(c) 2023 Elsevier Ltd. All rights reserved.Publication Metadata only Compositional alteration of low dimensional nickel sulfides under hydrogen evolution reaction conditions in alkaline media(Elsevier Ltd, 2024) Department of Computer Engineering;Department of Chemistry; Solati, Navid; Karakaya, Cüneyt; Şimşek, Kaan; Kaya, Sarp; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Engineering; College of SciencesDeveloping cost-effective electrodes for electrochemical water splitting with minimal kinetic losses remains a primary challenge in water electrolysis. Nickel-based dichalcogenides, particularly sulfides, have emerged as promising candidates for the hydrogen evolution reaction (HER). However, the stability of various nickel sulfide (NixSy) phases under cathodic potentials in alkaline electrolytes is subject to debate. In this study, we investigate the HER activity and durability of both 2-dimensional (2D) and 3-dimensional (3D) NixSy nanostructures with diverse chemical compositions. Through cross-correlation analysis of spectroscopic and electrochemical characterization data, we elucidate the transformation behavior of these materials under operating conditions. Our findings reveal that NiS2 tends to undergo conversion to nickel hydroxide (Ni(OH)2), while Ni-rich phases like Ni3S4 and NiS exhibit greater stability. Furthermore, we demonstrate that in 3D nanostructures, the initial formation of a protective hydroxide layer on the surface inhibits further sulfide transformation. This nickel sulfide/hydroxide composite shows an improved HER activity with lower onset potential (−0.25 V), Tafel slope (105 mV s−1), and charge transfer resistance (17.3 Ω).