Highly Porous Nitrogen and Phosphorus Dual-Doped Carbon with Increased Phosphorus Content As an Efficient Oxygen Reduction Electrocatalyst

Abstract

Fuel cells and metal-air batteries have attracted the attention of many researchers because of their high power density and large energy density. Carbon materials are typically used as catalyst supports and conductive agents at the cathodes in fuel cells and metal-air batteries. However, pure carbon exhibits low electrocatalytic activity for the oxygen reduction reaction (ORR) at the cathodes. Single doping of heteroatoms, such as nitrogen and phosphorus, has been investigated as a simple method to improve the ORR activity of carbon materials. Moreover, compared with single doping, dual doping of nitrogen and phosphorus can further enhance the ORR performance due to the synergetic effect of the dopants [1]. Acetylene blacks (ABs) are readily available and widely used in industry for pigments, reinforcing rubber, and battery electrodes. However, the content of heteroatoms doped into AB by conventional heat treatment is low [2]. Recently, mechanochemical treatments have received increasing attention because they are more robust and simpler than conventional methods. Some studies have been conducted on solid-gas mechanochemical treatment under an air atmosphere as a nitrogen doping method for graphite and graphite-based materials [3]. In this study, nitrogen and phosphorus dual-doped carbon (NPC) was synthesized by a two-step method: solid-gas mechanochemical treatment using a planetary ball mill at a rotation speed of 600 rpm for 1 h to prepare nitrogen-doped carbon (NC), followed by heat treatment with ammonium dihydrogen phosphate (ADP) at 800 °C for 12 h to dope the NC with phosphorus. For comparison, phosphorus-doped carbon (PC) was synthesized by heat treatment of AB with ADP at 800 °C for 12 h. X-ray photoelectron spectroscopy was used to identify the doping states and concentrations of the samples. NPC has high concentrations of nitrogen (1.60 at%) and phosphorus (1.91 at%). In contrast, PC has a low phosphorus concentration (0.14 at%). Scanning electron microscopy revealed that mechanochemical treatment using a ball mill broke the particles and reduced their size from 35 nm in AB to approximately 10 nm in NC and NPC. The surface areas of NC and NPC significantly increased to 424 m 2 g −1 and 540 m 2 g −1 , respectively, compared with that of AB (64.7 m 2 g −1 ), mainly due to particle size reduction. Furthermore, NC and NPC exhibit high porosity with a maximum at 3.6 nm (Fig. 1a). The primary pores of NC and NPC constitute spaces between the particles. Electrochemical measurements were performed using a rotating disk electrode (RDE) in O 2 -saturated 1.0 M KOH. Linear sweep voltammetry curves of samples at an RDE rotation speed of 1600 rpm are shown in Fig. 1b. NC exhibits a much higher kinetic current density of 1.71 mA cm −2 at −0.15 V vs. Hg/HgO calculated from a mass transport correction of RDE than AB (0.04 mA cm −2 ), suggesting that the doped nitrogen and high porosity in NC significantly promote the ORR process. Furthermore, NPC shows 2.4 times higher kinetic current density of 4.14 mA cm −2 than NC, indicating that additional phosphorus doping into NC can enhance the ORR activity. Owing to the highly porous structure in NC, phosphorus atoms can be effectively doped into the carbon matrix during heat treatment, which may lead to the generation of abundant active sites for the ORR in NPC. References [1] D. Liu et al., Adv. Mater. , 31 , 1 (2019). [2] J. Chang et al., J. Power Sources , 239 , 94 (2013). [3] Y. Kado et al., J. Electrochem. Soc. , 166 , A2471(2019). Acknowledgments This work was partially supported by Grant-in-Aid for Scientific Research (KAKENHI), Grant Number JP 23K13700 from the Japan Society for the Promotion of Science (JSPS). Figure 1