N‐, O‐, and S‐Tridoped Carbon‐Encapsulated Co9S8 Nanomaterials: Efficient Bifunctional Electrocatalysts for Overall Water Splitting

The development of highly active and stable earth‐abundant catalysts to reduce or eliminate the reliance on noble‐metal based ones in green and sustainable (electro)chemical processes is nowadays of great interest. Here, N‐, O‐, and S‐tridoped carbon‐encapsulated Co₉S₈ (Co₉S₈@NOSC) nanomaterials are synthesized via simple pyrolysis of S‐ and Co(II)‐containing polypyrrole solid precursors, and the materials are proven to serve as noble metal‐free bifunctional electrocatalysts for water splitting in alkaline medium. The nanomaterials exhibit remarkable catalytic performances for oxygen evolution reaction in basic electrolyte, with small overpotentials, high anodic current densities, low Tafel slopes as well as very high (nearly 100%) Faradic efficiencies. Moreover, the materials are found to efficiently electrocatalyze hydrogen evolution reaction in acidic as well as basic solutions, showing high activity in both cases and maintaining good stability in alkaline medium. A two‐electrode electrolyzer assembled using the material synthesized at 900 °C (Co₉S₈@NOSC‐900) as an electrocatalyst at both electrodes gives current densities of 10 and 20 mA cm^?2 at potentials of 1.60 and 1.74 V, respectively. The excellent electrocatalytic activity exhibited by the materials is proposed to be mainly due to the synergistic effects between the Co₉S₈ nanoparticles cores and the heteroatom‐doped carbon shells in the materials.     

Establishing a Theoretical Landscape for Identifying Basal Plane Active 2D Metal Borides (MBenes) toward Nitrogen Electroreduction

To achieve efficient ammonia synthesis via electrochemical nitrogen reduction reaction (NRR), a qualified catalyst should have both high specific activity and large active surface area. However, integrating these two merits into one single material remains a big challenge due to the difficulty in balancing multiple reaction intermediates. Here, it is demonstrated that the boron-analogues of MXenes, namely “MBenes”, could cope with the challenge and achieve the high activity and large reaction region simultaneously toward NRR. Using extensive density functional theory computations and taking 16 MBenes as representatives, it is identified that seven MBenes (CrB, MoB, WB, Mo₂B, V₃B₄, CrMnB₂, and CrFeB₂) not only have intrinsic basal plane activity for NRR with limiting potentials ranging from −0.22 to −0.82 V, but also possess superior capability of suppressing the competitive hydrogen evolution reaction. Particularly, different from the MXenes whose surface oxidation may block the active sites, once oxidized, these MBenes can catalyze NRR via the self-activating process, reducing O*/OH* into H₂O* under reaction conditions, and favoring the N₂ electroreduction. As a result, the exceptional activity and selectivity, high active area (≈10¹⁹ m⁻²), and antioxidation nature render these MBenes as pH-universal catalysts for NH₃ production without introducing any dopants and defects.     

Mesh‐on‐Mesh Graphitic‐C3N4@Graphene for Highly Efficient Hydrogen Evolution

Mesoporous materials have attracted considerable interest due to their huge surface areas and numerous active sites that can be effectively exploited in catalysis. Here, 2D mesoporous graphitic‐C₃N₄ nanolayers are rationally assembled on 2D mesoporous graphene sheets (g‐CN@G MMs) by in situ selective growth. Benefiting from an abundance of exposed edges and rich defects, fast electron transport, and a multipathway of charge and mass transport from a continuous interconnected mesh network, the mesh‐on‐mesh g‐CN@G MMs hybrid exhibits higher catalytic hydrogen evolution activity and stronger durability than most of the reported nonmetal catalysts and some metal‐based catalysts.     

Multifunctional Mo–N/C@MoS2 Electrocatalysts for HER,OER, ORR,and Zn–Air Batteries

Replacement of noble‐metal platinum catalysts with cheaper, operationally stable, and highly efficient electrocatalysts holds huge potential for large‐scale implementation of clean energy devices. Metal–organic frameworks (MOFs) and metal dichalcogenides (MDs) offer rich platforms for design of highly active electrocatalysts owing to their flexibility, ultrahigh surface area, hierarchical pore structures, and high catalytic activity. Herein, an advanced electrocatalyst based on a vertically aligned MoS₂ nanosheet encapsulated Mo–N/C framework with interfacial Mo–N coupling centers is reported. The hybrid structure exhibits robust multifunctional electrocatalytic activity and stability toward the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. Interestingly, it further displays high‐performance of Zn–air batteries as a cathode electrocatalyst with a high power density of ≈196.4 mW cm^?2 and a voltaic efficiency of ≈63 % at 5 mA cm^?2, as well as excellent cycling stability even after 48 h at 25 mA cm^?2. Such outstanding electrocatalytic properties stem from the synergistic effect of the distinct chemical composition, the unique three‐phase active sites, and the hierarchical pore framework for fast mass transport. This work is expected to inspire the design of advanced and performance‐oriented MOF/MD hybrid‐based electrocatalysts for wider application in electrochemical energy devices.     

Metal (Ni,Co)‐Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) along with hydrogen evolution reaction (HER) have been considered critical processes for electrochemical energy conversion and storage through metal‐air battery, fuel cell, and water electrolyzer technologies. Here, a new class of multifunctional electrocatalysts consisting of dominant metallic Ni or Co with small fraction of their oxides anchored onto nitrogen‐doped reduced graphene oxide (rGO) including Co‐CoO/N‐rGO and Ni‐NiO/N‐rGO are prepared via a pyrolysis of graphene oxide and cobalt or nickel salts. Ni‐NiO/N‐rGO shows the higher electrocatalytic activity for the OER in 0.1 m KOH with a low overpotential of 0.24 V at a current density of 10 mA cm^?2, which is superior to that of the commercial IrO₂. In addition, it exhibits remarkable activity for the HER, demonstrating a low overpotential of 0.16 V at a current density of 20 mA cm^?2 in 1.0 m KOH. Apart from similar HER activity to the Ni‐based catalyst, Co‐CoO/N‐rGO displays the higher activity for the ORR, comparable to Pt/C in zinc‐air batteries. This work provides a new avenue for the development of multifunctional electrocatalysts with optimal catalytic activity by varying transition metals (Ni or Co) for these highly demanded electrochemical energy technologies.     

Laser Assisted Solution Synthesis of High Performance Graphene Supported Electrocatalysts

Simple, yet versatile, methods to functionalize graphene flakes with metal (oxide) nanoparticles are in demand, particularly for the development of advanced catalysts. Herein, based on light‐induced electrochemistry, a laser‐assisted, continuous, solution route for the simultaneous reduction and modification of graphene oxide with catalytic nanoparticles is reported. Electrochemical graphene oxide (EGO) is used as starting material and electron–hole pair source due to its low degree of oxidation, which imparts structural integrity and an ability to withstand photodegradation. Simply illuminating a solution stream containing EGO and metal salt (e.g., H₂PtCl₆ or RuCl₃) with a 248 nm wavelength laser produces reduced EGO (rEGO, oxygen content 4.0 at%) flakes, decorated with Pt (≈2.0 nm) or RuO₂ (≈2.8 nm) nanoparticles. The RuO₂–rEGO flakes exhibit superior catalytic activity for the oxygen evolution reaction, requiring a small overpotential of 225 mV to reach a current density of 10 mA cm^?2. The Pt–rEGO flakes (10.2 wt% of Pt) show enhanced mass activity for the hydrogen evolution reaction, and similar performance for oxygen reduction reaction compared to a commercial 20 wt% Pt/C catalyst. This simple production method is also used to deposit PtPd alloy and MnO_x nanoparticles on rEGO, demonstrating its versatility in synthesizing functional nanoparticle‐modified graphene materials.     

Flexible Solid-State Direct Ethanol Fuel Cell Catalyzed by Nanoporous High-Entropy Al-Pd-Ni-Cu-Mo Anode and Spinel (AlMnCo)3O4 Cathode

As in many other electrochemical energy-converting systems, the flexible direct ethanol fuel cells rely heavily on high-performance catalysts with low noble metal contents and high tolerance to poisoning. In this work, a generic dealloying procedure to synthesize nanoporous multicomponent anodic and cathodic catalysts for the high-performance ethanol fuel cells is reported. On the anode side, the nanoporous AlPdNiCuMo high-entropy alloy exhibits an electrochemically active surface area of 88.53 m² g⁻¹_Pd and a mass activity of 2.67 A mg⁻¹_Pd for the ethanol oxidation reaction. On the cathode side, the dealloyed spinel (AlMnCo)₃O₄ nanosheets with no noble metals demonstrate a comparable catalytic performance as the standard Pt/C for the oxygen reduction reaction, and tolerance to high concentrations of ethanol. Equipped with such anodic and cathodic catalysts, the flexible solid-state ethanol fuel cell is able to deliver an ultra-high energy density of 13.63 mWh cm⁻² with only 3 mL ethanol, which is outstanding compared with other similar solid-state energy devices. Moreover, the solid-state ethanol fuel cell is highly flexible, durable and exhibits an inject-and-run function.     

Mo-/Co-N-C Hybrid Nanosheets Oriented on Hierarchical Nanoporous Cu as Versatile Electrocatalysts for Efficient Water Splitting

Designing robust and cost-effective electrocatalysts based on Earth-abundant elements is crucial for large-scale hydrogen production through electrochemical water splitting. Here, nitrogen-doped carbon engrafted Mo₂N/CoN hybrid nanosheets that are seamlessly oriented on hierarchical nanoporous Cu scaffold (Mo-/Co-N-C/Cu), as highly efficient electrocatalysts for alkaline hydrogen evolution reaction are reported. The constituent heterostructured Mo₂N/CoN nanosheets work as bifunctional electroactive sites for both water dissociation and adsorption/desorption of hydrogen intermediates while the nitrogen-doped carbon bridges electron transfers between electroactive sites and interconnective Cu current collectors by making use of Mo-/Co-N-C bonds and intimate C/Cu contacts at interfaces. As a consequence of unique architecture having electroactive sites to be sufficiently accessible, self-supported nanoporous Mo-/Co-N-C/Cu hybrid electrodes exhibit outstanding electrocatalysis in 1 m KOH, with a negligible onset overpotential and a low Tafel slope of 47 mV dec⁻¹. They only take overpotential of as low as 230 mV to reach current density of 1000 mA cm⁻². When coupled with their electro-oxidized derivatives that mediate efficiently the oxygen evolution reaction, the alkaline water electrolyzer can achieve ≈100 mA cm⁻² at 1.622 V in 1 m KOH electrolyte, ≈0.343 V lower than the device constructed with commercially available Pt/C and Ir/C nanocatalysts immobilized on nanoporous Cu electrodes.     

PdAg Nanorings Supported on Graphene Nanosheets: Highly Methanol‐Tolerant Cathode Electrocatalyst for Alkaline Fuel Cells

Due to the high costs, slow reaction kinetics, and methanol poisoning of platinum‐based cathode catalysts, designing and exploring non‐Pt or low‐Pt cathode electrocatalysts with a low cost, high catalytic performance, and high methanol‐tolerance are crucial for the commercialization of fuel cells. Here, a facile method to fabricate a system of PdAg nanorings supported by graphene nanosheets is demonstrated; the fabrication is based on the galvanic displacement reaction between pre‐synthesized Ag nanoparticles and palladium ions. X‐ray diffraction and high‐resolution transmission electron microscopy show that the synthesized PdAg nanocrystals exhibit a ring‐shaped hollow structure with an average size of 27.49 nm and a wall thickness of 5.5 nm. Compared to the commercial Pd–C catalyst, the PdAg nanorings exhibit superior properties as a cathode electrocatalyst for oxygen reduction. Based on structural and electrochemical studies, these advantageous properties include efficient usage of noble metals and a high surface area because of the effective utilization of both the exterior and interior surfaces, high electrocatalytic performance for oxygen reduction from the synergistic effect of the alloyed PdAg crystalline phase, and most importantly, excellent tolerance of methanol crossover at high concentrations. It is anticipated that this synthesis of graphene‐based PdAg nanorings will open up a new avenue for designing advanced electrocatalysts that are low in cost and that exhibit high catalytic performance for alkaline fuel cells.     

2D,Metal-Free Electrocatalysts for the Nitrogen Reduction Reaction

Environmentally friendly ammonia production is important for addressing the carbon emissions and substantial energy consumption that are currently associated with the chemical industry. In recent decades, many achievements are made in this area; however, low production yield, poor selectivity, and unsatisfactory Faradaic efficiency hinder large-scale applications. 2D, metal-free electrocatalysts stand out from other candidates because of their physical, electronic, and chemical properties. In this study, recent developments of 2D-based electrochemical materials for converting dinitrogen into ammonia in ambient conditions are systematically reviewed. First, recent unique progress and challenges on novel 2D electrocatalysts for the nitrogen reduction reaction are summarized. Then, various synthetic strategies for electrochemical materials and the influence of these methods have on the intrinsic material performance are highlighted. Last, by comparing current engineering strategies, electrochemical tests, and computational calculations, the opportunities, critical issues, and scientific challenges for 2D nanomaterials as stable, efficient catalysts, are analyzed. On the basis of this comparison, technology solutions are provided and rational principles for future studies are proposed.