Urea treatment of nitrogen-doped carbon leads to enhanced performance for the oxygen reduction reaction
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Kengo Shimanoe Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Fukuoka 8168580, Japan
Masayoshi Yuasa Department of Biological & Environmental Chemistry, Faculty of Humanity-Oriented Science and Engineering, Kinki University, Fukuoka 820-8555, Japan (Received 11 October 2017; accepted 9 April 2018)
Manufacturing of advanced functional materials should also rely on the green chemistry principles like utilization of natural renewable resources. Marine environment offers plenty of renewable raw materials like chitin and its derivative chitosan. The paper presents how urea treatment has influenced several textural, chemical, and electrocatalytic properties of N-doped activated carbons (N_ACs) obtained from chitosan and chitin. The materials were subjected to an activation procedure (with different activators) as well as nitrogenation by premixing the precursors with water solutions of urea. Raw and premixed precursors were carbonized in the temperature range of 700–800 °C. The urea treatment resulted in a spectacular increase in the nitrogen content by weight (up to 68%) and an improvement of the surface area (up to 42%) along with total/micro-/ mezo-pore volume (up to 49%). Some urea-modified N_ACs were capable of reducing oxygen in an alkaline solution as effectively as a Pt-loaded carbon material. The highest number of electrons transferred to O2 molecule was found to be equal to 3.76 for a chitosan derived sample. This ability of chitosan and chitin derived N-rich activated carbons was studied by means of the method named rotating ring disc electrode.
I. INTRODUCTION
Recently, porous carbon materials doped with heteroatoms, such as oxygen, nitrogen, and phosphorus, have attracted considerable attention to some electrochemical energy devices (supercapacitors, fuel cells, researchable batteries, and metal-air batteries) due to several unique properties of heteroatom-incorporating surface active sites.1,2 Nitrogen- and oxygen-containing surface groups are common functionalities of the carbon electrode materials, which can be generated through various activations.3–5 Apart from the porous structure, the activated carbons exhibit a complex surface chemistry that alters the electrochemical energy storage of the carbon.6,7 Besides oxygen, nitrogen is the most common heteroatom studied from the point of view of its effects on the catalytic8 and electrochemical performance9–11 of the activated carbons. When nitrogen-rich carbons are synthesized or modified at high temperatures, the majority of their nitrogen is present as pyrrolic (N-5), pyridinic a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.116
(N-6), quaternary (N-Q), or pyridinic-N-oxides (N-X).12 Besides negatively charged nitrogen in the pyridinic and pyrrolic arrangements, which are assumed to take part in the pseudocapacitive Faradaic reactions,13,14 the quaternary and pyridinic-N-oxides, when distributed in the pores accessible to ions, were found as important in e
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