Graphite Nanoarrays-Confined Fe and Co Single-Atoms within Graphene Sponges as Bifunctional Oxygen Electrocatalyst for Ultralong Lasting Zinc-Air Battery

Trifunctional electrocatalysts based on a bimetallic nanoalloy and nitrogen-doped carbon with brush-like heterostructure

Trifunctional ORR/OER/HER catalysts are emerging for various sustainable energy storage and conversion technologies. For this function, employing materials with 1D structures leads to catalysts having limited surface area and structural robustness. Instead of 1D catalysts, heterostructured catalysts (i.e., catalysts consisting of interfaces created by combining diverse structural components) have attracted much attention due to their high efficiency. We have fabricated a directly grown 1D-1D heterostructured bimetallic N-doped carbon trifunctional catalyst based on Fe/Co bimetallic-organic frameworks, forming nanobrushes (FeCoNC-NB) with improved resistance to collapsing and substantial numbers of exposed active sites. The secondary 1D structure of this design contributes to creating interparticle conductive networks. By combining the brush-like heterostructure, FeCo alloy active sites, and N-doped carbon as support and for encapsulation of the metal, the catalyst features a high ORR Eonset value (1.046 V), low OER overpotential (363 mV), and comparable HER overpotential (254 mV) in alkaline electrolyte. Zn-air batteries with FeCoNC-NB demonstrate a power density of 195 mW cm-2 and a superior battery life of up to 350 h. Self-powered FeCoNC-NB-based water electrolyzers as energy conversion devices are also demonstrated. This work drives the progress of trifunctional catalysts based on heterostructured nonprecious metal N-doped carbon for energy storage and conversion developments.

Review of Carbon Support Coordination Environments for Single Metal Atom Electrocatalysts (SACS)

This topical review focuses on the distinct role of carbon support coordination environment of single-atom catalysts (SACs) for electrocatalysis. The article begins with an overview of atomic coordination configurations in SACs, including a discussion of the advanced characterization techniques and simulation used for understanding the active sites. A summary of key electrocatalysis applications is then provided. These processes are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR). The review then shifts to modulation of the metal atom-carbon coordination environments, focusing on nitrogen and other non-metal coordination through modulation at the first coordination shell and modulation in the second and higher coordination shells. Representative case studies are provided, starting with the classic four-nitrogen-coordinated single metal atom (MN4 ) based SACs. Bimetallic coordination models including homo-paired and hetero-paired active sites are also discussed, being categorized as emerging approaches. The theme of the discussions is the correlation between synthesis methods for selective doping, the carbon structure-electron configuration changes associated with the doping, the analytical techniques used to ascertain these changes, and the resultant electrocatalysis performance. Critical unanswered questions as well as promising underexplored research directions are identified.

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

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.

Graphene-Supported Atomically Dispersed Metals as Bifunctional Catalysts for Next-Generation Batteries Based on Conversion Reactions

Next-generation batteries based on conversion reactions, including aqueous metal-air batteries, nonaqueous alkali metal-O₂ and -CO₂ batteries, alkali metal-chalcogen batteries, and alkali metal-ion batteries have attracted great interest. However, their use is restricted by inefficient reversible conversion of active agents. Developing bifunctional catalysts to accelerate the conversion reaction kinetics in both discharge and charge processes is urgently needed. Graphene-, or graphene-like carbon-supported atomically dispersed metal catalysts (G-ADMCs) have been demonstrated to show excellent activity in various electrocatalytic reactions, making them promising candidates. Different from G-ADMCs for catalysis, which only require high activity in one direction, G-ADMCs for rechargeable batteries should provide high activity in both discharging and charging. This review provides guidance for the design and fabrication of bifunctional G-ADMCs for next-generation rechargeable batteries based on conversion reactions. The key challenges that prevent their reversible conversion, the origin of the activity of bifunctional G-ADMCs, and the current design principles of bifunctional G-ADMCs for highly reversible conversion, have been analyzed and highlighted for each conversion-type battery. Finally, a summary and outlook on the development of bifunctional G-ADMC materials for next-generation batteries with a high energy density and excellent energy efficiency are given.

Molecular Engineering toward High-Crystallinity Yet High-Surface-Area Porous Carbon Nanosheets for Enhanced Electrocatalytic Oxygen Reduction

Carbon-based nanomaterials have been regarded as promising non-noble metal catalysts for renewable energy conversion system (e.g., fuel cells and metal-air batteries). In general, graphitic skeleton and porous structure are both critical for the performances of carbon-based catalysts. However, the pursuit of high surface area while maintaining high graphitization degree remains an arduous challenge because of the trade-off relationship between these two key characteristics. Herein, a simple yet efficient approach is demonstrated to fabricate a class of 2D N-doped graphitized porous carbon nanosheets (GPCNSs) featuring both high crystallinity and high specific surface area by utilizing amine aromatic organoalkoxysilane as an all-in-one precursor and FeCl3 ·6H2 O as an active salt template. The highly porous structure of the as-obtained GPCNSs is mainly attributed to the alkoxysilane-derived SiOx nanodomains that function as micro/mesopore templates; meanwhile, the highly crystalline graphitic skeleton is synergistically contributed by the aromatic nucleus of the precursor and FeCl3 ·6H2 O. The unusual integration of graphitic skeleton with porous structure endows GPCNSs with superior catalytic activity and long-term stability when used as electrocatalysts for oxygen reduction reaction and Zn-air batteries. These findings will shed new light on the facile fabrication of highly porous carbon materials with desired graphitic structure for numerous applications.

In Situ Construction of FeCo Alloy Nanoparticles Embedded in Nitrogen‑Doped Bamboo-like Carbon Nanotubes as a Bifunctional Electrocatalyst for Zn-Air Battery

The rational design and exploration of low-cost, highly efficient, and robust bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts are essential for the application of zinc-air batteries....

Accelerating Triple Transport in Zinc-Air Batteries and Water Electrolysis by Spatially Confining Co Nanoparticles in Breathable Honeycomb-Like Macroporous N-Doped Carbon

Rational engineering electrode structure to achieve an efficient triple-phase contact line is vital for applications such as in zinc-air batteries and water electrolysis. Herein, a facile "MOF-in situ-leaching and confined-growth-MOF" strategy is developed to construct a breathable trifunctional electrocatalyst based on N-doped graphitic carbon with Co nanoparticles spatially confined in an inherited honeycomb-like macroporous structure (denoted as Co@HMNC). The unique orderly arranged macroporous channels and the "ships in a bottle" confinement effect jointly expedite the triple transport, endowing the catalysts with fast reaction kinetics. As a result, the obtained Co@HMNC catalyst presents superb trifunctional performance with a positive half-wave potential (E_1/2 ) of 0.90 V for oxygen reduction reaction (ORR), and low overpotentials of 318 and 51 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) at 10 mA cm^-2 , respectively. The Co@HMNC-based liquid Zn-air battery reaches a large specific capacity of 859 mA h g_Zn ^-1 , a high-power density of 198 mW cm^-2 , and long-term stability for 375 h, suggesting its promise for actual applications.

Cooperativity in supported metal single atom catalysis

The discussion concerning cooperativity in supported single-atom (SA) catalysis is often limited to the metal-support interaction, which is certainly important, but which is not the only lever for modifying the catalytic performance. Indeed, if the interaction between the SA and the support, which can be seen as a solid ligand presenting its own specificities that fix the first coordination sphere of the metal, plays a central role as in homogeneous catalysis, other factors can strongly contribute to modification of the activity, selectivity and stability of SAs. Therefore, in this mini-review, we briefly summarize the importance of the support (oxide, carbon or a second metal) in SA photo- electro- and thermal-catalysis (support-assisted operation), and concentrate on other types of cooperativities that in some cases enable previously impossible reaction pathways on supported metal SAs. This includes topics that are not specific to SA catalysis, such as metal-ligand or heterobimetallic cooperativity, and cooperativity which is SA-specific such as nanoparticle-SA or mixed-valence SA cooperativity.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.