Pt nanoparticles supported on nitrogen-doped porous carbon as efficient oxygen reduction catalysts synthesized via a simple alcohol reduction method

Research Article Pt nanoparticles supported on nitrogen‑doped porous carbon as efficient oxygen reduction catalysts synthesized via a simple alcohol reduction method Naoki Tachibana1 · Yasuyuki Yukawa1 · Kazuo Morikawa1 · Masahiro Kawaguchi1 · Kengo Shimanoe2 Received: 3 December 2020 / Accepted: 4 February 2021 © The Author(s) 2021  OPEN Abstract Pt nanoparticles supported on nitrogen-doped porous carbon (NPC) were investigated as both a highly active catalyst for the oxygen reduction reaction (ORR) and a suitable porous support structure. Pt/NPC catalysts with loadings of 8.8–35.4 wt.% were prepared via a simple alcohol reduction method and exhibited homogeneously dispersed Pt nanoparticles with a small mean size ranging from 1.90 to 2.99 nm. X-ray photoelectron spectroscopy measurement suggested the presence of strong interactions between the Pt nanoparticles and NPC support. 27.4% Pt/NPC demonstrated high catalytic activity for the ORR in a rotating disk electrode system and was also effectively applied to a gas diffusion electrode (GDE). A GDE fabricated using the Pt/NPC with a fine pore network exhibited excellent performance, especially at high current densities. Specific activity of Pt/NPC and Pt/carbon black catalysts for the ORR correlated with the peak potential of adsorbed OH reduction on Pt, which was dependent on the particle size and support. Supplementary information The online version contains supplementary material available at https://doi.org/10.1007/s42452-021-04343 -8 * Naoki Tachibana, tachibana.naoki@iri‑tokyo.jp | 1Tokyo Metropolitan Industrial Technology Research Institute, 2‑4‑10, Aomi, Koto‑ku, Tokyo 135‑0064, Japan. 2Department of Advanced Materials Science and Engineering, Faculty of Engineering Sciences, Kyushu University, 6‑1, Kasugakoen, Kasuga‑shi, Fukuoka 816‑8580, Japan. SN Applied Sciences (2021) 3:338 | https://doi.org/10.1007/s42452-021-04343-8 Vol.:(0123456789) Research Article SN Applied Sciences (2021) 3:338 | https://doi.org/10.1007/s42452-021-04343-8 Graphic abstract Keywords Oxygen reduction · Pt nanoparticles · Nitrogen-doped carbon · Porous carbon 1 Introduction Electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media have attracted tremendous interest for their applications in alkaline fuel cells [1–4], metal–air batteries [5, 6], and chlor-alkali electrolysis with oxygen depolarized cathodes [7]. Recently, alkaline membrane fuel cells, which can be simplified and the C O2 poisoning problem for alkaline fuel cells is alleviated [8], have been reported to exhibit improved performance, which approaches that of proton exchange membrane fuel cells, due to advances in anion-exchange membranes [9]. At present, Pt and Ptalloy nanoparticles supported on carbon are universally available electrocatalysts for the ORR [10, 11]. Among carbon support materials, carbon blacks (CBs) are commonly used in gas diffusion electrodes (GDEs) for fuel cells and metal–air batteries because of their large surface area, good electric conductivity, and well-developed porous structure as well as low cost [12, 13]. Despite these advantages, the slow kinetics of CB-supported Pt nanoparticles for the ORR necessitates high loading levels of Pt, hindering the further deployment of the electrochemical energy conversion and storage devices. In order to improve the performance of Pt nanoparticle catalysts, decreasing the particle size and thereby increasing the surface area per unit mass is one of the reasonable strategies in heterogeneous catalyst fields [14]. However, many reports have demonstrated that the specific activity (current density per unit surface area) of Pt nanoparticles for the ORR decreases with decreasing particle size Vol:.(1234567890) between about 1 and 5 nm and mass activity (current density per unit mass) does not linearly increase in this size range [15–17]. The effects of small Pt size on the ORR activity may be predicted by considering the ratio of terrace, corner, and edge sites on the particles [18, 19]. Following the geometrical considerations, the distribution of the surface sites rapidly changes for the small nanoparticles, which may lead to the strong adsorption of oxygen species, such as OH and O, through an increased number of low-coordinated sites [20, 21]. The strongly adsorbed species on the surface inhibit the adsorption of molecular oxygen and intermediates for the ORR [22]. Thus, the state and coverage of the oxygen species is considered to be a key factor in determining the activity of Pt catalysts [13, 23, 24]. Besides, it is suggested that the ORR activity of Pt nanoparticle catalysts can be related to interparticle distances and mass transport of oxygen [25, 26]. Watanabe et al. have reported that, if the Pt interparticle distance is too close (< 20 nm), the diffusion field of the particles for O2 overlaps, resulting in decreased specific activity for the ORR [25]. It is widely recognized that support materials can significantly affect the catalytic activity of Pt nanoparticles [27–29]. Therefore, long-running efforts to develop various supports such as graphene [30, 31], carbon nanotubes [32, 33], CBs [34, 35], metal carbides [36, 37], and metal oxides [38] have been undertaken. Carbon supports with high graphitic nature have both excellent electrical conductivity and high mechanical strength, but the carbon lattice with strong sp2 chemical bonds does not readily SN Applied Sciences (2021) 3:338 | https://doi.org/10.1007/s42452-021-04343-8 permit the chemisorption of Pt on the basal planes [39]. Because nitrogen doping can introduce anchoring sites for Pt deposition, nitrogen-doped carbon has been used as a support for Pt catalysts over the past few decades [40–43]. Furthermore, it is suggested that surface nitrogen species such as pyridinic N in nitrogen-doped carbon increase the interactions between Pt catalysts and nitrogen-doped carbon supports, which may enhance the catalytic performance [42, 44, 45]. Nitrogen-doped porous carbon (NPC) supports with large surface area and high pore volume, which can act as a substrate for the effective dispersion of Pt nanoparticles and facilitate mass transport, would contribute to improving the ORR activity of Pt nanoparticles. In the present study, Pt nanoparticles supported on NPC were prepared via a surfactant-free alcohol reduction method and characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and N 2 gas adsorption. To evaluate the ORR activity, rotating disk electrode (RDE) measurements were conducted on the synthesized catalysts. Furthermore, a GDE was fabricated using Pt/NPC as an ORR catalyst and exhibited excellent performance, especially at high current densities. 2 Experimental section 2.1 Preparation of Pt nanoparticles supported on nitrogen‑doped porous carbon NPC was prepared via heat treatment using commercial (...truncated)