Sulfur-bridge ligands altering the microenvironment of single-atom CoN3S sites to boost the oxygen reduction reaction.

We report here an asymmetric N,S-coordinated cobalt-based single-atom catalyst with sulfur (S)-bridge ligands (Co-N/S-C) for the oxygen reduction reaction (ORR). The Co-N/S-C exhibits a half-wave potential (E1/2) of 0.908 V versus RHE, outperforming most state-of-the-art ORR catalysts. Theoretical calculations indicate that the CoN3SC10-S moiety facilitates the ORR kinetics by optimizing the adsorption of intermediates. This work provides new insights into the design of single-atom catalysts for electrocatalysis through heteroatom-bridge ligand engineering.

Stable and High-performance Flow H2-O2 Fuel Cells with Coupled Acidic Oxygen Reduction and Alkaline Hydrogen Oxidation Reactions.

Conventional H2-O2 fuel cells suffer from the low output voltage, insufficient durability, and high-cost catalysts (e.g., noble metals). Herein, this work reports a conceptually new coupled flow fuel cell (CF-FC) by coupling asymmetric electrolytes for acidic oxygen reduction reaction and alkaline hydrogen oxidation reaction. By introducing an electrochemical neutralization energy, the newly-developed CF-FCs possess a significantly increased theoretical open-circuit voltage. Specifically, a CF-FC based on a typical transition metal single-atom Fe-N-C cathode catalyst demonstrates a high electricity output up to 1.81 V and durability with an ultrahigh retention of 91% over 110 h, far superior to the conventional fuel cells (usually, < 1.0 V, < 50% retention over 20 h). The output performance can even be significantly enhanced easily by connecting multiple CF-FCs into the parallel, series, or combined parallel-series connections at a fractional cost of that for the conventional H2-O2 fuel cells, showing great potential for large-scale practical applications. Thus, this study provides a platform to transform conventional fuel cell technology through the rational design and development of advanced energy conversion and storage devices by coupling different electrocatalytic reactions.

In Situ Electrochemical Restructuring B-Doped Metal-Organic Frameworks as Efficient OER Electrocatalysts for Stable Anion Exchange Membrane Water Electrolysis.

Metal organic frameworks (MOFs) are promising as effective electrocatalysts toward oxygen evolution reaction (OER). However, the origin of OER activity for MOF-based electrocatalysts is still unclear because of their structure reconstruction during electrocatalysis process. Here, a novel MOF (B-MOF-Zn-Co) with spherical superstructure is developed by hydrothermal treatment of zeolitic imidazolate framework-Zn, Co (ZIF-Zn-Co) using boric acid. The resultant B-MOF-Zn-Co shows high OER activity with a low overpotential of 362 mV at 100 mA cm-2. Remarkably, B-MOF-Zn-Co displays excellent stability with only 3.6% voltage delay over 300 h at 100 mA cm-2 in alkaline electrolyte. Surprisingly, B-MOF-Zn-Co thoroughly transforms into B-doped CoOOH (B-CoOOH) during electrolysis process, which is served as actual active material for high OER electrocatalytic performance. The newly-formed B-CoOOH possesses lower energy barrier of potential-determining step (PDS) for OOH* formation compared with CoOOH, benefiting for high OER activity. More importantly, B-MOF-Zn-Co based anion exchange membrane water electrolytic cell (AEMWE) demonstrates continuously durable operation with stable current density of 200 mA cm-2 over 300 h, illustrating its potential application in practice water electrolysis. This work offers an in situ electrochemical reconstruction strategy for the development of stable and effective OER electrocatalysts toward practice AEMWE.

Fe/Co dual metal catalysts modulated by S-ligands for efficient acidic oxygen reduction in PEMFC.

Here, we report a conceptual strategy for introducing spatial sulfur (S)-bridge ligands to regulate the coordination environment of Fe-Co-N dual-metal centers (Spa-S-Fe,Co/NC). Benefiting from the electronic modulation, Spa-S-Fe,Co/NC catalyst showed remarkably enhanced oxygen reduction reaction (ORR) performance with a half-wave potential (E1/2) of 0.846 V and satisfactory long-term durability in acidic electrolyte. Combined experimental and theoretical studies revealed that the excellent acidic ORR activity with a remarkable stability observed for Spa-S-Fe,Co/NC is attributable to the optimal adsorption-desorption of ORR oxygenated intermediates achieved through charge-modulation of Fe-Co-N bimetallic centers by the spatial S-bridge ligands. These findings provide a unique perspective to regulate the local coordination environment of catalysts with dual-metal-centers to optimize their electrocatalytic performance.

Facile Synthesis of FeS@C Particles Toward High-Performance Anodes for Lithium-Ion Batteries.

High energy density batteries with high performance are significantly important for intelligent electrical vehicular systems. Iron sulfurs are recognized as one of the most promising anodes for high energy density lithium-ion batteries because of their high theoretical specific capacity and relatively stable electrochemical performance. However, their large-scale commercialized application for lithium-ion batteries are plagued by high-cost and complicated preparation methods. Here, we report a simple and cost-effective method for the scalable synthesis of nanoconfined FeS in porous carbon (defined as FeS@C) as anodes by direct pyrolysis of an iron(III) p-toluenesulfonate precursor. The carbon architecture embedded with FeS nanoparticles provides a rapid electron transport property, and its hierarchical porous structure effectively enhances the ion transport rate, thereby leading to a good electrochemical performance. The resultant FeS@C anodes exhibit high reversible capacity and long cycle life up to 500 cycles at high current density. This work provides a simple strategy for the mass production of FeS@C particles, which represents a critical step forward toward practical applications of iron sulfurs anodes.

Chemical Approaches to Carbon-Based Metal-Free Catalysts.

Highly active and durable catalysts play a key role in clean energy technologies. However, the high cost, low reserves, and poor stability of noble-metal-based catalysts have hindered the large-scale development of renewable energy. Owing to their low cost, earth abundance, high activity, and excellent stability, carbon-based metal-free catalysts (CMFCs) are promising alternatives to precious-metal-based catalysts. Although many synthetic methods based on solution, surface/interface, solid state, and noncovalent chemistries have been developed for producing numerous CMFCs with diverse structures and functionalities, there is still a lack of effective approaches to precisely control the structures of active sites. Therefore, novel chemical approaches are needed for the development of highly active and durable CMFCs that are capable of replacing precious-metal catalysts for large-scale applications. Herein, a comprehensive and critical review on chemical approaches to CMFCs is given by summarizing important advancements, current challenges, and future perspectives in this emerging field. Through such a critical review, our understanding of CMFCs and the associated synthetic processes will be significantly increased.