Fuel cells and metal-air batteries have attracted the attention of many researchers because of their superior power storage capacity. Pt and Pt-based alloys supported on carbon black (CB) are widely used on air electrodes in the batteries. However, the electrocatalytic activity for oxygen reduction reaction (ORR) is still insufficient, which is a barrier to the further commercial deployment of the batteries. Pt supported on nitrogen-doped graphene has attracted attention because the N functionalities modified nucleation and growth kinetics during catalyst nanoparticle deposition, which results in smaller catalyst particle size and increased catalyst particle dispersion, and increased support/catalyst chemical binding, which results in enhanced durability.¹ However, the stacking of graphene inflicted by van der Waals forces would significantly reduce the number of active sites. Furthermore, for optimal utilization of the available reaction sites, large pore volumes in catalyst supports play an essential role in gas mass-transport by acting as gas-supplying channels in the catalyst layer. Our group recently reported that highly porous nitrogen-doped carbon nanoparticles (NC) based on CB and cyanamide have a fine pore network that provides efficient access for mass transport.²In this study, for improving the oxygen reduction activity, we used NC as a Pt catalyst support and measured the electrocatalytic activity of synthesized Pt/NC for ORR.

 NC was synthesized by catalyst-free thermal treatments using CB and cyanamide, and have large pore volume (2.23 cm³g⁻¹) and high surface area (1141 m²g⁻¹).² Pt nanoparticles supported on NC were prepared by a simple alcohol-reduction method using methanol and dinitro-diamine platinum (II) solution as a reducing agent and platinum source. The Pt content in the catalysts was fixed at 30 wt%. Compared to Pt/CB (mean diameter = 3.2 nm), the smaller Pt nanoparticle size (mean diameter = 2.2 nm) and improved nanoparticle dispersion on NC were observed in TEM images, because the incorporated nitrogen sites on the NC would provide a strong metal–support interaction to anchor efficiently Pt precursor on NC and to stabilize the Pt nanoclusters. Electrochemical measurements were executed using two types of electrodes, rotating disk electrode (RDE) and gas diffusion electrode (GDE). Linear sweep voltammograms at a scan rate of 20 mV s^—1 for Pt/NC and Pt/CB are shown in Fig. 1. The kinetic current density value of 6.13 mA cm^—2 at 0.80 V vs. RHE for Pt/NC calculated from a mass-transport correction of RDE is about twice as high as that for Pt/CB (3.33 mA cm^—2). The steady-state current density—voltage curves of the three-electrode cells installed with the fabricated GDEs were measured in 8.0 M KOH under an O₂ flow (100 mL min^—1). The GDE performance for ORR of Pt/NC exhibited higher than that of Pt supported on non-doped CB at the measured current density (<300 mA cm^—2). The current density value of 52 mA cm^—2 at 50 mV vs. Hg/HgO (8.0 M KOH) for Pt/NC obtained from the iR-corrected polarization curve is approximately two times higher than that for Pt/CB (25 mA cm^—2). No obvious potential drop at the measured current density for Pt/NC was observed, suggesting the use of NC as a catalyst support was effective in the GDE system because the hierarchical pore structure of the NC would provide both excellent electron transfer and reactant transport rate towards ORR.

Figure caption

Fig. 1. Linear sweep voltammograms of Pt/NC and Pt/CB at a rotation rate of 1600 rpm measured by RDE in O₂-saturated 0.1 M KOH.

Reference

1. 1. Y. Zhou et al., Energy Environ. Sci., 3, 1437 (2010).

2. N. Tachibana, et al., Carbon, 115, 515 (2017).