Bypassing Formation of Oxide Intermediate via Chemical Vapor Deposition for the Synthesis of an Mn-N-C Catalyst with Improved ORR Activity

A significant barrier to the commercialization of proton exchange membrane fuel cells (PEMFCs) is the high cost of the platinum-based oxygen reduction reaction (ORR) cathode electrocatalysts. One viable solution is to replace platinum with a platinum-group metal (PGM) free catalyst with comparable activity and durability. However, PGM-free catalyst development is burdened by a lack of understanding of the active site formation mechanism during the requisite high-temperature synthesis step, thus making rational catalyst design challenging. Herein we demonstrate in-temperature X-ray absorption spectroscopy (XAS) to unravel the mechanism of site evolution during pyrolysis for a manganese-based catalyst. We show the transformation from an initial state of manganese oxides (MnOx) at room temperature, to the emergence of manganese-nitrogen (MnN4) site beginning at 750 °C, with its continued evolution up to the maximum temperature of 1000 °C. The competition between the MnOx and MnN4 is identified as the primary factor governing the formation of MnN4 sites during pyrolysis. This knowledge led us to use a chemical vapor deposition (CVD) method to produce MnN4 sites to bypass the evolution route involving the MnOx intermediates. The Mn-N-C catalyst synthesized via CVD shows improved ORR activity over the Mn-N-C synthesized via traditional synthesis by the pyrolysis of a mixture of Mn, N, and C precursors.

A "Pre-Division Metal Clusters" Strategy to Mediate Efficient Dual-Active Sites ORR Catalyst for Ultralong Rechargeable Zn-Air Battery

To conquer the bottleneck of sluggish kinetics in cathodic oxygen reduction reaction (ORR) of metal-air batteries, catalysts with dual-active centers have stood out. Here, a "pre-division metal clusters" strategy is firstly conceived to fabricate a N,S-dual doped honeycomb-like carbon matrix inlaid with CoN₄ sites and wrapped Co₂ P nanoclusters as dual-active centers (Co₂ P/CoN₄ @NSC-500). A crystalline {Co^II ₂ } coordination cluster divided by periphery second organic layers is well-designed to realize delocalized dispersion before calcination. The optimal Co₂ P/CoN₄ @NSC-500 executes excellent 4e^- ORR activity surpassing the benchmark Pt/C. Theoretical calculation results reveal that the CoN₄ sites and Co₂ P nanoclusters can synergistically quicken the formation of *OOH on Co sites. The rechargeable Zn-air battery (ZAB) assembled by Co₂ P/CoN₄ @NSC-500 delivers ultralong cycling stability over 1742 hours (3484 cycles) under 5 mA cm^-2 and can light up a 2.4 V LED bulb for ≈264 hours, evidencing the promising practical application potentials in portable devices.

Achievements, Challenges, and Perspectives on Nitrogen Electrochemistry for Carbon-Neutral Energy Technologies

Unrestrained anthropogenic activities have severely disrupted the global natural nitrogen cycle, causing numerous energy and environmental issues. Electrocatalytic nitrogen transformation is a feasible and promising strategy for achieving a sustainable nitrogen economy. Synergistically combining multiple nitrogen reactions can realize efficient renewable energy storage and conversion, restore the global nitrogen balance, and remediate environmental crises. Here, we provide a unique aspect to discuss the intriguing nitrogen electrochemistry by linking three essential nitrogen-containing compounds (i.e., N₂ , NH₃ , and NO₃ ^- ) and integrating four essential electrochemical reactions, i.e., the nitrogen reduction reaction (N₂ RR), nitrogen oxidation reaction (N₂ OR), nitrate reduction reaction (NO₃ RR), and ammonia oxidation reaction (NH₃ OR). This minireview also summarizes the acquired knowledge of rational catalyst design and underlying reaction mechanisms for these interlinked nitrogen reactions. We further underscore the associated clean energy technologies and a sustainable nitrogen-based economy.

Atomically Dispersed Dual-Metal Site Catalysts for Enhanced CO2 Reduction: Mechanistic Insight into Active Site Structures

Carbon-supported nitrogen-coordinated single-metal site catalysts (i.e., M-N-C, M: Fe, Co, or Ni) are active for the electrochemical CO₂ reduction reaction (CO₂ RR) to CO. Further improving their intrinsic activity and selectivity by tuning their N-M bond structures and coordination is limited. Herein, we expand the coordination environments of M-N-C catalysts by designing dual-metal active sites. The Ni-Fe catalyst exhibited the most efficient CO2RR activity and promising stability compared to other combinations. Advanced structural characterization and theoretical prediction suggest that the most active N-coordinated dual-metal site configurations are 2N-bridged (Fe-Ni)N₆ , in which FeN₄ and NiN₄ moieties are shared with two N atoms. Two metals (i.e., Fe and Ni) in the dual-metal site likely generate a synergy to enable more optimal *COOH adsorption and *CO desorption than single-metal sites (FeN₄ or NiN₄ ) with improved intrinsic catalytic activity and selectivity.

Theory-Guided Regulation of FeN4 Spin State by Neighboring Cu Atoms for Enhanced Oxygen Reduction Electrocatalysis in Flexible Metal-Air Batteries

Iron, nitrogen-codoped carbon (Fe-N-C) nanocomposites have emerged as viable electrocatalysts for the oxygen reduction reaction (ORR) due to the formation of FeN_x C_y coordination moieties. In this study, results from first-principles calculations show a nearly linear correlation of the energy barriers of key reaction steps with the Fe magnetic moment. Experimentally, when single Cu sites are incorporated into Fe-N-C aerogels (denoted as NCAG/Fe-Cu), the Fe centers exhibit a reduced magnetic moment and markedly enhanced ORR activity within a wide pH range of 0-14. With the NCAG/Fe-Cu nanocomposites used as the cathode catalyst in a neutral/quasi-solid aluminum-air and alkaline/quasi-solid zinc-air battery, both achieve a remarkable performance with an ultrahigh open-circuit voltage of 2.00 and 1.51 V, large power density of 130 and 186 mW cm^-2 , and good mechanical flexibility, all markedly better than those with commercial Pt/C or Pt/C-RuO₂ catalysts at the cathode.