Xianlang Chen

Integrating cobalt phosphide and cobalt nitride-embedded nitrogen-rich nanocarbons: high-performance bifunctional electrocatalysts for oxygen reduction and evolution

The demand for cost-effective bifunctional oxygen electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for application in rechargeable metal–air batteries and fuel cells operated in alkaline solutions has increased over the decades. We report for the first time an easy procedure for a unique nitrogen-rich sandwich-architectured catalyst (CoNP@NC/NG) as a highly efficient bifunctional electrocatalyst for ORR and OER. Physical characterizations confirmed the coexistence of Co2P and CoxN crystal phases in the nanostructure. The as-prepared CoNP@NC/NG exhibited potent bifunctional electrochemical performance with superior positive onset potential, large kinetic current density, and outstanding stability toward both ORR and OER, thereby showing excellent activities compared with Pt/C and state-of-the-art nonprecious catalysts. The excellent performance could have originated from the robust conjugation between the Co2P and CoxN crystal structures leading to a synergistic effect of the two interfaces, and the carbon shell also increased the number of nitrogen active sites. Moreover, the integrated structure of CoNP@NC/NG provided high electrical conductivity and facilitated electron transfer. Furthermore, the rechargeable zinc–air battery testing of CoNP@NC/NG-700 revealed good performance and long-term stability. The current work provided a new pathway to design bifunctional catalysts with multiple crystal phases for energy conversion and storage.

Tuning the confinement space of N-carbon shell-coated ruthenium nanoparticles: highly efficient electrocatalysts for hydrogen evolution reaction

Development of efficient and durable catalysts for the hydrogen evolution reaction (HER) in an alkaline system is vital for the transformation of renewable energy into hydrogen fuel. In this study, we report the difference in the activity of semi- and fully-encapsulated Ru catalysts based on the effect of confined space. The fully-encapsulated Ru catalyst with porous nitrogen-doped carbon (5.0% F-Ru@PNC-800) displayed outstanding HER performance, a low overpotential of only 28 mV at 10 mA cm−2, and excellent stability. The fully-encapsulated Ru catalyst performs better than the semi-encapsulated Ru catalyst (5.0% S-Ru@PNC-800). Density functional theory calculation revealed that the different space sizes of carbon layers affect the charge transfer of the Ru nanoparticles and the carbon surface, leading to different activities. This work demonstrates that the control of confined space is an important strategy for designing highly efficient catalysts for energy conversion.

Effect of graphene with nanopores on metal clusters

Porous graphene, which is a novel type of defective graphene, shows excellent potential as a support material for metal clusters. In this work, the stability and electronic structures of metal clusters (Pd, Ir, and Rh) supported on pristine graphene and graphene with different sizes of nanopores were investigated using first-principles density functional theory (DFT) calculations. Then, CO adsorption and oxidation on the Pd-graphene system were chosen to evaluate its catalytic performance. Graphene with nanopores can strongly stabilize the metal clusters and cause a substantial downshift of the d-band center of the metal clusters, thus decreasing CO adsorption. All binding energies, d-band centers, and adsorption energies show a linear change with the size of the nanopore: a bigger size of the nanopore corresponds to stronger bonding of metal clusters with graphene, lower downshift of the d-band center, and weaker CO adsorption. By using a suitable size nanopore, Pd clusters supported on graphene will have similar CO and O2 adsorption abilities, thus leading to superior CO tolerance. The DFT calculated reaction energy barriers show that graphene with nanopores is a superior catalyst for CO oxidation reaction. These properties can play an important role in instructing graphene-supported metal catalyst preparation to prevent the diffusion or agglomeration of metal clusters and enhance the catalytic performance.