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Received Oct 18, 2017; Revised Mar 26, 2018; Accepted Apr 16, 2018
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1. Introduction
To date, Pt-based materials are preferably used as the electrocatalyst for fuel cell because of the low over potential and high current density [1–3], though Pt-based catalysts as precious metal are expensive and their kinetics of the oxygen reduction reaction (ORR) is sluggish.
To overcome the above problem, the nonmetal catalysts with similar activity of oxygen reduction reaction and superior long-time durability compared to Pt-C catalyst become the focus of proton exchange membrane fuel cell in recent years [4–6]. Among them, heteroatom-doped carbon has attracted intensive attention due to excellent catalytic performance, high stability, and low cost, such as nitrogen, sulfur, boron, phosphorus, fluorine, and so on [7–12]. These heteroatom-doped carbon materials emerged one after another for metal-free catalytic technology. Afterwards, dual or multiple doped carbon materials were studied for further enhancement in the catalytic activity due to the synergistic effect of different heteroatoms, such as N/S, N/B, N/P, N/I, N/B/P, and so on [13–17]. Meanwhile, researchers found that F-doped carbon materials showed the promising ORR performance because F atom affected greatly the electronic properties of carbon materials [9]. Few literatures about F-doped or codoped carbon materials were reported because of the difficult preparing process.
Polydopamine as a carbon resource to form carbon-coated materials has been investigated in the previous work because of its strong molecule designability and easy deposition on substrates [18] such as CNx electrocatalyst; N, S doped graphene; and so on [19, 20].
Based on these studies, we developed firstly a novel route to prepare F and other heteroatom codoped carbon-based catalyst and study the influence of the kind and the amount of heteroatom on the ORR activity.
2. Materials and Methods
First, carboxylic MWCNTs were dispersed uniformly in pH 8.5 Tris-HCl buffer solution through the ultrasonic method for 1 h. Then, the polydopamine was deposited on MWCNTs after the addition of 0.2 mg/mL of dopamine hydrochloride in the above solution under stirring continuously at room temperature for 24 h. Afterwards, CNTs-modified polydopmaine, named PDA-CNTs, were obtained by centrifugation, washing,...