Oxygen Reduction Reaction Activity in Non-Precious Single-Atom (M-N/C) Catalysts-Contribution of Metal and Carbon/Nitrogen Framework-Based Sites

Why Does the Performance of Nitrogen-Doped Carbon Electrocatalysts Decrease in Acidic Conditions?

Nitrogen-doped carbon has emerged as a promising low-cost and durable alternative to platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells. However, its catalytic activity decreases significantly in acidic electrolytes, limiting the practical applications. Here, we report the degradation mechanisms of nitrogen-doped carbon catalysts, focusing on the acid-base equilibrium of pyridinic nitrogen (pyri-N), which serves the primary active site. We found that the electrochemical hydrogenation of pyri-N to pyri-NH, coupled with oxygen adsorption, is a critical process. Although this reaction occurs at higher potentials in basic electrolytes, it shifts to lower potentials in acidic environments due to the protonation and stabilization of pyri-N. These results demonstrate that the decrease of the catalytic activity in acidic electrolytes is tied to the basicity of pyri-N. By controlling the basicity of pyri-N, specifically its pKa, a guideline for enhancing the ORR and other electrode reactions has been established.

Why Do Weak-Binding M-N-C Single-Atom Catalysts Possess Anomalously High Oxygen Reduction Activity?

Single-atom catalysts (SACs) with metal-nitrogen-carbon (M-N-C) structures are widely recognized as promising candidates in oxygen reduction reactions (ORR). According to the classical Sabatier principle, optimal 3d metal catalysts, such as Fe/Co-N-C, achieve superior catalytic performance due to the moderate binding strength. However, the substantial ORR activity demonstrated by weakly binding M-N-C catalysts such as Ni/Cu-N-C challenges current understandings, emphasizing the need to explore new underlying mechanisms. In this work, we integrated a pH-field coupled microkinetic model with detailed experimental electron state analyses to verify a novel key step in the ORR reaction pathway of weak-binding SACs─the oxygen adsorption at the metal-nitrogen bridge site. This step significantly altered the adsorption scaling relations, electric field responses, and solvation effects, further impacting the key kinetic reaction barrier from HOO* to O* and pH-dependent performance. Synchrotron spectra analysis further provides evidence for the new weak-binding M-N-C model, showing an increase in electron density on the antibonding π orbitals of N atoms in weak-binding M-N-C catalysts and confirming the presence of N-O bonds. These findings redefine the understanding of weak-binding M-N-C catalyst behavior, opening up new perspectives for their application in clean energy.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Operando Fe dissolution in Fe-N-C electrocatalysts during acidic oxygen reduction: impact of local pH change

Atomic Fe in N-doped C (Fe-N-C) catalysts provide the most promising non-precious metal O2 reduction activity at the cathodes of proton exchange membrane fuel cells. However, one of the biggest remaining challenges to address towards their implementation in fuel cells is their limited durability. Fe demetallation has been suggested as the primary initial degradation mechanism. However, the fate of Fe under different operating conditions varies. Here, we monitor operando Fe dissolution of a highly porous and >50% FeN x electrochemical utilization Fe-N-C catalyst in 0.1 M HClO4, under O2 and Ar at different temperatures, in both flow cell and gas diffusion electrode (GDE) half-cell coupled to inductively coupled plasma mass spectrometry (ICP-MS). By combining these results with pre- and post-mortem analyses, we demonstrate that in the absence of oxygen, Fe cations diffuse away within the liquid phase. Conversely, at -15 mA cm-2 geo and more negative O2 reduction currents, the Fe cations reprecipitate as Fe-oxides. We support our conclusions with a microkinetic model, revealing that the local pH in the catalyst layer predominantly accounts for the observed trend. Even at a moderate O2 reduction current density of -15 mA cm-2 geo at 25 °C, a significant H+ consumption and therefore pH increase (pH = 8-9) within the bulk Fe-N-C layer facilitate precipitation of Fe cations. This work provides a unified view on the Fe dissolution degradation mechanism for a model Fe-N-C in both high-throughput flow cell and practical operating GDE conditions, underscoring the crucial role of local pH in regulating the stability of the active sites.

Vacancy-Enhanced Sb-N4 Sites for the Oxygen Reduction Reaction and Zn-Air Battery

With the advantages of a Fenton-inactive characteristic and unique p electrons that can hybridize with O2 molecules, p-block metal-based single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) have tremendous potential. Nevertheless, their undesirable intrinsic activity caused by the closed d10 electronic configuration remains a major challenge. Herein, an Sb-based SAC featuring carbon vacancy-enhanced Sb-N4 active centers, corroborated by the results of high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure, has been developed for an incredibly effective ORR. The obtained SbSA-N-C demonstrates a positive half-wave potential of 0.905 V and excellent structural stability in alkaline environments. Density functional theory calculations reveal that the carbon vacancies weaken the adsorption between Sb atoms and the OH* intermediate, thus promoting the ORR performance. Practically, the SbSA-N-C-based Zn-air batteries achieve impressive outcomes, such as a high power density of 181 mW cm-2, showing great potential in real-world applications.

Atomically Dispersed Dual-Metal ORR Catalyst with Hierarchical Porous Structure for Zn-Air Batteries

The metal-nitrogen-carbon (M-N-C)-based catalysts are promising to replace PGM (platinum group metal) to accelerate oxygen reduction reaction due to their excellent electrocatalytic performance. However, the inferior intrinsic activity and poor active site density confining further improvement in their performance. Modulating the electronic structure and reasonably designing the pore structure are widely acknowledged effective strategies to boost the activity of the M-N-C catalysts. However, it is a great challenge to form abundant pores to regulate the electronic structure via the facile method. Herein, a hierarchical, porous dual-atom catalyst FeNi-NPC-1000 has been architectured by the Na2CO3 template method and bimetallic doping modification strategy. Benefitting from the optimized pore and electronic structure, the as-prepared FeNi-NPC-1000 possesses a high specific surface area (1412.8 m2 g-1) and improved ORR activity (E1/2 = 0.877 V vs RHE), which is superior to that of Pt/C (E1/2 = 0.867 V vs RHE). With the evidence of AC-STEM, XAS, and DFT, the FeNi-N8-C moiety is proven to be the key active site to realize high-efficiency ORR catalysis. When assembled it as an air cathode of ZABs, FeNi-NPC-1000 displays superior discharge performance (Pmax = 367.1 mW cm-2) and a stable battery long-life. This article will provide a new strategy for designing dual-metal atomic catalysts applied in metal-air batteries.

Chemical Kinetic Method for Active-Site Quantification in Fe-N-C Catalysts and Correlation with Molecular Probe and Spectroscopic Site-Counting Methods

Mononuclear Fe ions ligated by nitrogen (FeNx) dispersed on nitrogen-doped carbon (Fe-N-C) serve as active centers for electrocatalytic O2 reduction and thermocatalytic aerobic oxidations. Despite their promise as replacements for precious metals in a variety of practical applications, such as fuel cells, the discovery of new Fe-N-C catalysts has relied primarily on empirical approaches. In this context, the development of quantitative structure-reactivity relationships and benchmarking of catalysts prepared by different synthetic routes and by different laboratories would be facilitated by the broader adoption of methods to quantify atomically dispersed FeNx active centers. In this study, we develop a kinetic probe reaction method that uses the aerobic oxidation of a model hydroquinone substrate to quantify the density of FeNx centers in Fe-N-C catalysts. The kinetic method is compared with low-temperature Mössbauer spectroscopy, CO pulse chemisorption, and electrochemical reductive stripping of NO derived from NO2- on a suite of Fe-N-C catalysts prepared by diverse routes and featuring either the exclusive presence of Fe as FeNx sites or the coexistence of aggregated Fe species in addition to FeNx. The FeNx site densities derived from the kinetic method correlate well with those obtained from CO pulse chemisorption and Mössbauer spectroscopy. The broad survey of Fe-N-C materials also reveals the presence of outliers and challenges associated with each site quantification approach. The kinetic method developed here does not require pretreatments that may alter active-site distributions or specialized equipment beyond reaction vessels and standard analytical instrumentation.

Atomic metal coordinated to nitrogen-doped carbon electrocatalysts for proton exchange membrane fuel cells: a perspective on progress, pitfalls and prospectives

Proton exchange membrane fuel cells require reduced construction costs to improve commercial viability, which can be fueled by elimination of platinum as the O2 reduction electrocatalyst. The past 10 years has seen significant developments in synthesis, characterisation, and electrocatalytic performance of the most promising alternative electrocatalyst; single metal atoms coordinated to nitrogen-doped carbon (M-N-C). In this Perspective we recap some of the important achievements of M-N-Cs in the last decade, as well as discussing current knowledge gaps and future research directions for the community. We provide a new outlook on M-N-C stability and atomistic understanding with a set of original density functional theory simulations.