Balancing the Micro-Mesoporosity for Activity Maximization of N-Doped Carbonaceous Electrocatalysts for the Oxygen Reduction Reaction

Coexisting Fe single atoms and nanoparticles on hierarchically porous carbon for high-efficiency oxygen reduction reaction and Zn-air batteries

Fe single-atom catalysts still suffer from unsatisfactory intrinsic activity and durability for oxygen reduction reaction (ORR). Herein, the coexisting Fe single atoms and nanoparticles on hierarchically porous carbon (denoted as Fe-FeN-C) are prepared via a Zn5(OH)6(CO3)2-assisted pyrolysis strategy. Theoretical calculation reveals that the Fe nanoparticles can optimize the electronic structures and d-band center of Fe active center, hence reducing the reaction energy barrier for enhancing intrinsic activity. The Zn5(OH)6(CO3)2 self-sacrificial template not only can promote the formation of Fe single atoms, but also contributes to the construction of microporous/mesoporous/macroporous structures. Therefore, the obtained Fe-FeN-C exhibits impressive ORR activity with a half-wave potential of 0.921 V, which far exceeds Pt/C. With Fe-FeN-C as the cathode catalyst, the assembled Zn-air batteries delivered a maximum power density of 206 mW cm-2 and a long-cycle life over 400 h.

Highly Porous Polypyrrole (PPy) Hydrogel Support for the Design of a Co-N-C Electrocatalyst for Oxygen Reduction Reaction

Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts have emerged as one of the most promising platinum-group metal (PGM)-free cathode catalysts for oxygen reduction reaction (ORR). Among the various approaches to enhance the ORR performance of the catalysts, increasing the density of accessible active sites is of paramount importance. Thus, nitrogen-rich support with abundant porosity can be very propitious. Herein, we report a highly porous polypyrrole (PPy) hydrogel as a versatile support for the facile design of a Co-N-C electrocatalyst for ORR. The resulting Co-N-C catalyst with abundant micro- and mesoporous combinations demonstrates a half-wave potential (E1/2) of 0.825 V vs reversible hydrogen electrode (RHE) in O2-saturated 0.1M KOH with just 2.1 wt % Co content. The ORR performance reduces only 11 mV (E1/2) after 5000 cycles of accelerated durability test (ADT), portraying its excellent stability. The catalyst retains ≈83% of its original current during a short-term durability test at 0.8 V vs RHE for 25 h. Furthermore, the catalyst shows electron transfer approaching ≈4 with low H2O2 yield in the potential range 0.5-0.9 V vs RHE. This work provides a simple design strategy to synthesize M-N-C catalysts with increased accessible active site density and enhanced mass transport for ORR and other electrocatalytic applications.

Heterocyclic Modulated Electronic States of Alkynyl-Containing Conjugated Microporous Polymers for Efficient Oxygen Reduction

The oxygen reduction reaction (ORR) is a key process in green energy conversion technology. Heteroatom doping has been proven to be a prospective strategy to prepare metal-free carbon-based electrocatalysts, but such methods often suffer from uncontrollable catalyst frameworks and imprecise active sites. Herein, an organic heterocyclic strategy is adopted to modulate the charge redistribution of alkynyl-containing conjugated microporous polymers (CMPs) by introducing varied five-membered heterocyclic structures. Among these CMPs, the S, 2N-containing thiadiazole heterocyclic molecule (CMP-Tdz) with carbonized alginate materials (C_CA ) displays a remarkable quasi-four-electron-transfer ORR pathway, exhibiting an excellent half-wave potential (E_1/2 ) of 0.77 V, coupled with superior methanol tolerance and electrochemical stability, which are among the highest performance in the metal-free organic catalytic material systems. Density functional theory calculations prove that the high catalytic performance of these catalysts originates from the sp-hybridized C atom (site-2) which is activated by their adjacent heterocyclic structures. Importantly, the five-membered heterocyclic structures can also modulate the local charge distribution, and increase dipole moment, with significantly improved catalytic kinetics. This incorporation of chemically designed heterocyclic-containing alkynyl-CMPs provides a new approach to developing efficient metal-free carbon-based electrocatalysts for fuel cells.

Synthesis and Characterization of Dense Carbon Films as Model Surfaces to Estimate Electron Transfer Kinetics on Redox Flow Battery Electrodes

Redox flow batteries (RFBs) are a promising electrochemical technology for the efficient and reliable delivery of electricity, providing opportunities to integrate intermittent renewable resources and to support unreliable and/or aging grid infrastructure. Within the RFB, porous carbonaceous electrodes facilitate the electrochemical reactions, distribute the flowing electrolyte, and conduct electrons. Understanding electrode reaction kinetics is crucial for improving RFB performance and lowering costs. However, assessing reaction kinetics on porous electrodes is challenging as their complex structure frustrates canonical electroanalytical techniques used to quantify performance descriptors. Here, we outline a strategy to estimate electron transfer kinetics on planar electrode materials of similar surface chemistry to those used in RFBs. First, we describe a bottom-up synthetic process to produce flat, dense carbon films to enable the evaluation of electron transfer kinetics using traditional electrochemical approaches. Next, we characterize the physicochemical properties of the films using a suite of spectroscopic methods, confirming that their surface characteristics align with those of widely used porous electrodes. Last, we study the electrochemical performance of the films in a custom-designed cell architecture, extracting intrinsic heterogeneous kinetic rate constants for two iron-based redox couples in aqueous electrolytes using standard electrochemical methods (i.e., cyclic voltammetry, electrochemical impedance, and spectroscopy). We anticipate that the synthetic methods and experimental protocols described here are applicable to a range of electrocatalysts and redox couples.

Highly exposed surface pore-edge FeNx sites for enhanced oxygen reduction performance in Zn-air batteries

Atomically dispersed pore-edge FeNx sites anchored on porous carbon exhibit excellent activity and stability towards ORR. The assembled Zn-air battery presents a high peak power density (150 mW cm−2) and long-cycle stability (450 h).

Nitrogen-doped carbon derived from horse manure biomass as a catalyst for the oxygen reduction reaction

A massive amount of animal biomass is generated daily from livestock farms, agriculture, and food industries, causing environmental and ecological problems. The conversion of animal biomass into value-added products has recently gained considerable interest in materials science research. Herein, horse manure (HM) was utilized as a precursor for synthesizing nitrogen-doped carbons (NCs) via hydrothermal ammonia treatment and the post pyrolysis process. The ammonia concentration varied between 0.5, 1.0, and 1.5 M in the hydrothermal process. From the comprehensive characterization results, horse manure-derived nitrogen-doped carbons (HMNCs) exhibited an amorphous phase and a hierarchical nanoporous structure. The specific surface area decreased from 170.1 to 66.6 m2 g-1 as the ammonia concentration increased due to micropore deterioration. The nitrogen content was 0.90 atom% even with no ammonia treatment, indicating self-nitrogen doping. With hydrothermal ammonia treatment, the nitrogen content slightly enhanced up to 1.54 atom%. The electrocatalytic activity for the oxygen reduction reaction (ORR) of HMNCs in an alkaline solution was found to be related to nitrogen doping content and porous structure. The ORR activity of HMNCs mainly proceeded via a combination of two- and four-electron pathways. Although the ORR activity of HMNCs was still not satisfactory and comparable to that of a commercial Pt/carbon catalyst, it showed better long-term durability. The results obtained in this work provide the potential utilization of HM as a precursor for ORR catalysts and other related applications.

Enhanced Catalytic Performance of N-Doped Carbon Sphere-Supported Pd Nanoparticles by Secondary Nitrogen Source Regulation for Formic Acid Dehydrogenation

The development of catalysts with high selectivity, good catalytic activity, and excellent cycle performance is of significance for the application of formic acid (HCOOH, FA) as a hydrogen support. Herein, Pd is deposited on a series of N-doped carbons, which are prepared by cocarbonization of N-containing zeolite imidazole frameworks (ZIF-8) and other N/C sources (melamine, xylitol, urea, and glucose), for hydrogen generation from FA. The results demonstrate that the introduction of a secondary N/C source further affects the catalytic performance of Pd by adjusting the morphology, specific surface area, N content, and type of carbon. The effects of N atoms and the favorable reaction pathways of FA dehydrogenation were revealed by theoretical calculation. This work will improve the understanding of N doping on the decomposition mechanism of FA and provide a new approach for the rational design of metal-N-C materials.

Hollow and mesoporous lipstick-like nitrogen-doped carbon with incremented catalytic activity for oxygen reduction reaction

Hollow structure and pore size are considered to be crucial to the performance of nitrogen-doped carbon materials. In this paper, a lipstick-like hollow and mesoporous nitrogen-doped carbon (HNC-1000) material is prepared using a bottom-up template participation strategy. The images by scanning electron microscopy and transmission electron microscopy show that the precursor ZnO particles, the intermediate ZnO@ZIF-8 core-shell particles, and the target HNC-1000 particles all maintain a lipstick-like morphology, and HNC-1000 is a hollow nitrogen-doped carbon material. The specific surface area and pore size analyses show that the synthesized HNC-1000 has a very rich mesoporous structure with Vmeso+macro/Vtotal of 94.8% and mean mesopore size at 13.67 nm. X-ray photoelectron spectroscopy results show that the nitrogen in the catalyst HNC-1000 is mainly pyridine nitrogen and graphite nitrogen. The prepared HNC-1000 has excellent ORR catalytic activity with onset potential (0.98 V versus RHE), half-wave potential (0.85 V versus RHE), and limiting current density (5.51 mA cm^-2), which is comparable to that of commercial Pt/C (20 wt%) and superior to NC-1000 derived from pristine ZIF-8. HNC-1000 also has good stability and strong methanol tolerance, which is superior to commercial Pt/C catalyst. The improved performance of HNC-1000 is attributed to its hollow and mesoporous morphology. These findings demonstrate a stratage for the rational design and synthesis of practical electrocatalysts.

2D metal-organic framework-based materials for electrocatalytic, photocatalytic and thermocatalytic applications

Ultrathin two-dimensional metal-organic frameworks (2D MOFs) have recently attracted extensive interest in various catalytic fields (e.g., electrocatalysis, photocatalysis, thermocatalysis) due to their ultrathin thickness, large surface area, abundant accessible unsaturated active sites and tunable surface properties. Besides tuning the intrinsic properties of pristine 2D MOFs by changing the metal nodes and organic ligands, one of the hot research trends is to develop 2D MOF hybrids and 2D MOF-derived materials with higher stability and conductivity in order to further increase their activity and durability. Here, the synthesis of 2D MOF nanosheets is briefly summarized and discussed. More attention is focused on summaries and discussions about the applications of these 2D MOFs, their hybrids and their derived materials as electrocatalysts, photocatalysts and thermocatalysts. The superior properties and catalytic performance of these 2D MOF-based catalysts compared to their 3D MOF counterparts in electrocatalysis, photocatalysis and thermocatalysis are highlighted. The enhanced activities of 2D MOFs, their hybrids and derivatives come from abundant accessible active sites, a high density of unsaturated metal nodes, ultrathin thickness, and tunable microenvironments around the MOFs. Views regarding current and future challenges in the field, and new advances in science and technology to meet these challenges, are also presented. Finally, conclusions and outlooks in this field are provided.

Incorporation of Pt-Cr nanoparticles into highly porous MOF-5 as efficient oxygen reduction electrocatalysts

Developing new materials that can enhance the efficiency of energy conversion and storage systems is critical to meeting the rising energy demand of low-carbon economies. Mesoporous materials have the advantages of large specific surface area and multiple channels, which can increase efficiency and flexibility in terms of energy and power density. An active catalyst for oxygen reduction reaction (ORR) based on Pt-Cr nanoparticles with ultralow Pt content (0.90 wt%) has been studied in this paper. In contrast, electrocatalyst Pt/Cr/NPC-900 exhibited an ORR activity with onset potential (E _o) of 1.01 V vs. RHE in an alkaline solution that was superior to commercial Pt/C (20 wt%) (0.96 V vs. RHE). The presence of metal oxides and optimal Pt content enhanced the ORR activity. Therefore, the synergistic effect of the high surface area increased charge transfer, and excellent structural stability can achieve significant ORR efficiency, which is conducive to excellent activity. These findings provide a new perspective for economical and practical ORR electrocatalysts to be designed and synthesized rationally.

Spinach-Derived Porous Carbon Nanosheets as High-Performance Catalysts for Oxygen Reduction Reaction

Biomass-derived porous carbon materials are effective electrocatalysts for oxygen reduction reaction (ORR), with promising applications in low-temperature fuel cells and metal-air batteries. Herein, we developed a synthesis procedure that used spinach as a source of carbon, iron, and nitrogen for preparing porous carbon nanosheets and studied their ORR catalytic performance. These carbon sheets showed a very high ORR activity with a half-wave potential of +0.88 V in 0.1 M KOH, which is 20 mV more positive than that of commercial Pt/C catalysts. In addition, they showed a much better long-term stability than Pt/C and were insensitive to methanol. The remarkable ORR performance was attributed to the accessible high-density active sites that are primarily from Fe-N _x moieties. This work paves the way toward the use of metal-enriching plants as a source for preparing porous carbon materials for electrochemical energy conversion and storage applications.

3D honeycombed cobalt, nitrogen co-doped carbon nanosheets via hypersaline-protected pyrolysis towards efficient oxygen reduction

The broad application of metal-air batteries and fuel cells have been greatly limited due to their slow kinetics of oxygen electrodes involving the oxygen reduction reaction (ORR), and therefore the development of high-efficient, low-cost and high-reserve ORR electrocatalysts is of great significance. Herein, a hypersaline-protected pyrolysis strategy is presented for preparing 3D honeycombed cobalt, nitrogen co-doped carbon nanosheets (Co/N-CNS) by using eco-friendly biomass as a carbon and nitrogen source. During the hypersaline-protected pyrolysis, the pyridinic nitrogen-rich biomass facilitates the formation of highly active Co/N active sites among the resultant Co/N-CNS, while the templating-washing-drying cyclic utilization of salts creates honeycombed pore structures among the Co/N-CNS. Due to the structural features of honeycombed pores and uniform distributed active sites, the Co/N-CNS catalyst offers excellent ORR activity, high durability and methanol-tolerant performance in an alkaline electrolyte. As a demonstration, a primary Zn-air battery using the Co/N-CNS cathode delivers a high power density and excellent operating stability beyond that of commercial Pt/C cathode.

Understanding the roles of amorphous domains and oxygen-containing groups of nitrogen-doped carbon in oxygen reduction catalysis: toward superior activity

The secret to high ORR activity lies in tuning the oxygen functionalities and the amount of graphitic vs. amorphous domains in nitrogen-doped carbons.