Atomically dispersed transition metal active sites have emerged as one of the most important fields of study because they display promising performance in catalysis and have the potential to serve as ideal models for fundamental understanding. However, both the preparation and determination of such active sites remain a challenge. The structural engineering of carbon- and nitrogen-coordinated metal sites (M-N-C, M = Fe, Co, Ni, Mn, Cu, etc.) via employing new heteroatoms, e.g., P and S, remains challenging. In this study, carbon nanosheets embedded with nitrogen and phosphorus dual-coordinated iron active sites (denoted as Fe-N/P-C) were developed and determined using cutting edge techniques. Both experimental and theoretical results suggested that the N and P dual-coordinated iron sites were favorable for oxygen intermediate adsorption/desorption, resulting in accelerated reaction kinetics and promising catalytic oxygen reduction activity. This work not only provides efficient way to prepare well-defined single-atom active sites to boost catalytic performance but also paves the way to identify the dual-coordinated single metal atom sites.

Boosting Oxygen Reduction of Single Iron Active Sites via Geometric and Electronic Engineering: Nitrogen and Phosphorus Dual Coordination

Multifunctional Transition Metal-Based Phosphides in Energy-Related Electrocatalysis

Rechargeable Zn-air batteries are under intensive studies because of their high-energy density, low cost, and safety. However, their wide application is prevented by several remaining technical issues, one of which is the lack of suitable bifunctional cathodic catalysts for oxygen reduction reaction (ORR) during discharging and oxygen evolution reaction (OER) during charging. Due to low material cost and wide distribution, carbon-based materials may serve as promising electrocatalysts, while doping heteroatoms such as nitrogen or boron can effectively enhance their catalytic activity. Herein, we pyrolyze a metal-organic framework containing Zn, N, and B as the precursor to synthesize dual-doped and metal-free porous carbon materials as efficient ORR/OER bifunctional electrocatalysts. The surface area of obtained carbon materials can be greatly enhanced by pyrolysis under H 2 -containing atmosphere. In addition, N and B are evenly distributed within the carbon materials due to the crystalline MOF precursor. The resultant carbon materials exhibit high ORR and OER catalytic activities in both half-cell and single-cell battery measurements. Our study has demonstrated for the first time that MOFs can be used as precursors to synthesize metal-free ORR/OER bifunctional cathodic electrocatalysts with great potential in rechargeable Zn-air batteries.

A metal-free ORR/OER bifunctional electrocatalyst derived from metal-organic frameworks for rechargeable Zn-Air batteries

Developing a scalable approach to construct efficient and multifunctional electrodes for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) is an urgent need for overall water splitting and zinc-air batteries. In this work, a freestanding 3D heterostructure film is synthesized from a Ni-centered metal-organic framework (MOF)/graphene oxide. During the pyrolysis process, 1D carbon nanotubes formed from the MOF link with the 2D reduced graphene oxide sheets to stitch the 3D freestanding film. The results of the experiments and theoretical calculations show that the synergistic effect of the N-doped carbon shell and Ni nanoparticles leads to an optimized film with excellent electrocatalytic activity. Low overpotentials of 95 and 260 mV are merely needed for HER and OER, respectively, to reach a current density of 10 mA cm-2 . In addition, a high half-wave potential of 0.875 V is obtained for the ORR, which is comparable to that of Pt/RuO2 and ranks among the top of non-noble-metal catalysts. The use of an "all-in-one" film as the electrode leads to excellent performance of the homemade water electrolyzer and zinc-air battery, indicating the potential of the film for practical applications.

A Freestanding 3D Heterostructure Film Stitched by MOF-Derived Carbon Nanotube Microsphere Superstructure and Reduced Graphene Oxide Sheets: A Superior Multifunctional Electrode for Overall Water Splitting and Zn-Air Batteries.

The resplendent prospect of water splitting hydrogen production technology makes the development of efficient and stable hydrogen/oxygen evolution reactions (HER/OER) bifunctional catalysts become urgent. Herein, inspired by the density function theory (DFT) calculation result that Ru-dopants have a climbing effect on both OER and HER processes, we construct a Ru doped three-dimensional flower-like bimetallic phosphide on nickel foam (Ru-NiCoP/NF) derived from Co leaf-like zeolitic imidazolate framework (Co ZIF-L), effectively driving both OER (216mV@20 mA cm−2) and HER (44mV@10 mA cm−2) in 1 M KOH solutions. The overall water splitting device assembled by using Ru-NiCoP/NF as both anode and cathode shows an ultralow cell voltage of 1.515 V to obtain 10 mA cm−2. Interestingly, almost 100 % Faradic yield is achieved for the overall water splitting. This work represents a significant addition to exploring a new class of transition metal phosphides with outstanding performance in producing hydrogen via electrochemical water electrolysis.

Ru-doped 3D flower-like bimetallic phosphide with a climbing effect on overall water splitting

A Tannic Acid–Derived N-, P-Codoped Carbon-Supported Iron-Based Nanocomposite as an Advanced Trifunctional Electrocatalyst for the Overall Water Splitting Cells and Zinc–Air Batteries

Highly efficient and stable bifunctional electrocatalysts for ORR and OER are essential for fabricating rechargeable zinc–air batteries. Herein, we prepared metal-free N,P-codoped hydrophilic carbon nanosheets (NPC-“Li”) as effective bifunctional electrocatalysts using LiCl as the medium, in which phytic acid was used as the source of phosphorous and carbon, and ammonia atmosphere as the source of nitrogen. Furthermore, micrometer-sized agglomerations of carbon materials were significantly restrained using LiCl. NPC-“Li” exhibits superior bifunctional catalytic activity for both ORR and OER. In addition, the NPC-“Li”-based primary zinc–air battery shows a high energy density of 908 Wh kgZn−1, and the rechargeable battery shows excellent charge–discharge cycle efficiency.

Bing Yuan

Papers

A Tannic Acid–Derived N-, P-Codoped Carbon-Supported Iron-Based Nanocomposite as an Advanced Trifunctional Electrocatalyst for the Overall Water Splitting Cells and Zinc–Air Batteries

Using lithium chloride as a medium to prepare N,P-codoped carbon nanosheets for oxygen reduction and evolution reactions