Haoxiong Nan

Transition Metal Nitride Coated with Atomic Layers of Pt as a Low-Cost, Highly Stable Electrocatalyst for the Oxygen Reduction Reaction

The main challenges to the commercial viability of polymer electrolyte membrane fuel cells are (i) the high cost associated with using large amounts of Pt in fuel cell cathodes to compensate for the sluggish kinetics of the oxygen reduction reaction, (ii) catalyst degradation, and (iii) carbon-support corrosion. To address these obstacles, our group has focused on robust, carbon-free transition metal nitride materials with low Pt content that exhibit tunable physical and catalytic properties. Here, we report on the high performance of a novel catalyst with low Pt content, prepared by placing several layers of Pt atoms on nanoparticles of titanium nickel binary nitride. For the ORR, the catalyst exhibited a more than 400% and 200% increase in mass activity and specific activity, respectively, compared with the commercial Pt/C catalyst. It also showed excellent stability/durability, experiencing only a slight performance loss after 10,000 potential cycles, while TEM results showed its structure had remained intact. The catalysts outstanding performance may have resulted from the ultrahigh dispersion of Pt (several atomic layers coated on the nitride nanoparticles), and the excellent stability/durability may have been due to the good stability of nitride and synergetic effects between ultrathin Pt layer and the robust TiNiN support.

Binary transition metal nitrides with enhanced activity and durability for the oxygen reduction reaction

With a novel two-step approach, we prepared a low-cost, high-performance, binary transition metal nitride (BTMN) catalyst. An ammonia (NH3) complex of Ti and a transition metal was prepared in an organic solvent by the reaction of metal ions with ammonium; the complex then was dried in a vacuum oven, followed by nitridation in a tubular furnace under NH3 flow. The catalyst exhibited excellent activity towards the oxygen reduction reaction (ORR) in an alkaline medium and good ORR activity in an acidic medium. The effects of the doping elements (Fe, Co, and Ni), the doping concentration, and various nitriding temperatures on catalytic performance were intensively investigated. The onset potential of the Ti0.95Ni0.5N catalyst reached 0.83 V, with a limiting diffusion current density of 4 mA cm−2 (at a rotation speed of 1600 rpm) in 0.1 M HClO4 solution, which is the highest to date among the reported TiN-based electrocatalysts in an acidic medium. In 0.1 M KOH solution, the performance of this catalyst was almost comparable to that of commercial JM Pt/C; the diffusion current density reached 5.3 mA cm−2, and the halfway potential was only 71 mV inferior to that of commercial JM Pt/C. Furthermore, the catalyst showed high stability and only a slight drop in its current density after the durability test. All of these findings make our BTMN catalyst attractive for PEMFCs.

Three dimensional palladium nanoflowers with enhanced electrocatalytic activity towards the anodic oxidation of formic acid

Three-dimensional palladium nanoflowers (Pd-NF) composed of ultrathin Pd nanosheets were synthesized by a solvothermal approach. The Pd-NF catalyst shows 6.6- and 5.5-fold enhancements in mass activity and surface activity compared to normal palladium nanoparticles (Pd-NP) in the electro-oxidation of formic acid.

A core–shell Pd1Ru1Ni2@Pt/C catalyst with a ternary alloy core and Pt monolayer: enhanced activity and stability towards the oxygen reduction reaction by the addition of Ni

A core–shell structured catalyst, Pd1Ru1Ni2@Pt/C, with a ternary alloy as its core and a Pt monolayer shell was prepared using a two-stage strategy, in which Pd1Ru1Ni2 alloy nanoparticles were prepared by a chemical reduction method, and then the Pt monolayer shell was generated via an underpotential deposition method. It was found that the addition of Ni to the core played an important role in enhancing the catalysts oxygen reduction activity and stability. The optimal molar ratio of Pd : Ru : Ni was about 1 : 1 : 2; the catalyst with this optimal ratio had a half-wave potential approximately 65 mV higher than that of a PdRu@Pt/C catalyst, and its mass activity was up to 1.06 A mg−1 Pt, which was more than five times that of a commercial Pt/C catalyst. The catalysts structure and composition were characterized using X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The core–shell structure of the catalyst was demonstrated by the EDS mapping results and supported by the XPS results. We also performed a stability test that confirmed the catalysts superior stability in comparison to that of commercial JM Pt/C (20 wt% Pt).

A platinum monolayer core-shell catalyst with a ternary alloy nanoparticle core and enhanced stability for the oxygen reduction reaction

We synthesize a platinum monolayer core-shell catalyst with a ternary alloy nanoparticle core of Pd, Ir, and Ni. A Pt monolayer is deposited on carbon-supported PdIrNi nanoparticles using an underpotential deposition method, in which a copper monolayer is applied to the ternary nanoparticles; this is followed by the galvanic displacement of Cu with Pt to generate a Pt monolayer on the surface of the core. The core-shell Pd1Ir1Ni2@Pt/C catalyst exhibits excellent oxygen reduction reaction activity, yielding a mass activity significantly higher than that of Pt monolayer catalysts containing PdIr or PdNi nanoparticles as cores and four times higher than that of a commercial Pt/C electrocatalyst. In 0.1 M HClO4, the half-wave potential reaches 0.91 V, about 30 mV higher than that of Pt/C. We verify the structure and composition of the carbon-supported PdIrNi nanoparticles using X-ray powder diffraction, X-ray photoelectron spectroscopy, thermogravimetry, transmission electron microscopy, and energy dispersive X-ray spectrometry, and we perform a stability test that confirms the excellent stability of our core-shell catalyst. We suggest that the porous structure resulting from the dissolution of Ni in the alloy nanoparticles may be the main reason for the catalysts enhanced performance.