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.     

Dimethoxymethane (DMM) electrooxidation on carbon-supported Pt-based nanosized catalysts for PEMFC

Previous studies on dimethoxymethane (DMM: CH₃OCH₂OCH₃) on platinum poly- and single crystals allowed us to propose a general mechanism of DMM electrooxidation. At the time, making electrodes for proton exchange membrane fuel cells (PEMFC) with nanoparticles (based on Pt) was encouraged. It is well known that the improvement of Pt activity for electrocatalysis is possible by modifying platinum with other metals able to increase the kinetics of specific steps of the reaction (activation of water for example). Nanosized PtM/C electrocatalysts have been synthesized by the Bönneman method and characterized for DMM electrooxidation. Voltammetry, in situ IRTF spectroscopy and fuel cell tests were carried out to better understand DMM oxidation reaction. Voltammetry and fuel cell tests showed that PtRuMo and PtRu are the most active catalysts at high potential, whereas PtSn and PtMo have a best activity at low potentials. In situ IR experiments allowed the observation of CO_ads and CO₂ bands.     

Electrochemical Phase Evolution of Metal‐Based Pre‐Catalysts for High‐Rate Polysulfide Conversion

In situ evolution of electrocatalysts is of paramount importance in defining catalytic reactions. Catalysts for aprotic electrochemistry such as lithium–sulfur (Li‐S) batteries are the cornerstone to enhance intrinsically sluggish reaction kinetics but the true active phases are often controversial. Herein, we reveal the electrochemical phase evolution of metal‐based pre‐catalysts (Co₄N) in working Li‐S batteries that renders highly active electrocatalysts (CoS_x). Electrochemical cycling induces the transformation from single‐crystalline Co₄N to polycrystalline CoS_x that are rich in active sites. This transformation propels all‐phase polysulfide‐involving reactions. Consequently, Co₄N enables stable operation of high‐rate (10 C, 16.7 mA cm^?2) and electrolyte‐starved (4.7 μL mg_S^?1) Li‐S batteries. The general concept of electrochemically induced sulfurization is verified by thermodynamic energetics for most of low‐valence metal compounds.     

Vacancy Engineering of Iron‐Doped W18O49 Nanoreactors for Low‐Barrier Electrochemical Nitrogen Reduction

The electrochemical nitrogen reduction reaction (NRR) is a promising energy‐efficient and low‐emission alternative to the traditional Haber–Bosch process. Usually, the competing hydrogen evolution reaction (HER) and the reaction barrier of ambient electrochemical NRR are significant challenges, making a simultaneous high NH₃ formation rate and high Faradic efficiency (FE) difficult. To give effective NRR electrocatalysis and suppressed HER, the surface atomic structure of W₁₈O₄₉, which has exposed active W sites and weak binding for H₂, is doped with Fe. A high NH₃ formation rate of 24.7 μg h^?1 mg_cat^?1 and a high FE of 20.0 % are achieved at an overpotential of only ?0.15 V versus the reversible hydrogen electrode. Ab initio calculations reveal an intercalation‐type doping of Fe atoms in the tunnels of the W₁₈O₄₉ crystal structure, which increases the oxygen vacancies and exposes more W active sites, optimizes the nitrogen adsorption energy, and facilitates the electrocatalytic NRR.     

Atypical Hybrid Metal–Organic Frameworks (MOFs): A Combinative Process for MOF‐on‐MOF Growth,Etching, and Structure Transformation

The structural, compositional, and morphological features of metal–organic frameworks (MOFs) govern their properties and applications. Construction of hybrid MOFs with complicated structures, components, or morphologies is significant for the development of well‐organized MOFs. An advanced route is reported for construction of atypical hybrid MOFs with unique morphologies and complicated components: 1) MOF‐on‐MOF growth of a 3D zeolitic imidazolate framework (ZIF) on a ZIF‐L template, 2) etching of a part of the 2D ZIF‐L template, and 3) structural transformation of 2D ZIF‐L into 3D ZIF. The formation of core–shell‐type MOF rings and plates is controlled by regulating the three processes. The formation route for the core–shell‐type MOF rings and plates was monitored by tracking changes in morphology, structure, and composition. Carbon materials prepared from the pyrolysis of the core–shell‐type hybrid MOFs displayed enhanced oxygen reduction reaction activities compared to their monomeric counterparts.     

Metal–Organic Framework‐Derived Carbon Nanorods Encapsulating Bismuth Oxides for Rapid and Selective CO2 Electroreduction to Formate

Carbon dioxide (CO₂) conversion is promising in alleviating the excessive CO₂ level and simultaneously producing valuables. This work reports the preparation of carbon nanorods encapsulated bismuth oxides for the efficient CO₂ electroconversion toward formate production. This resultant catalyst exhibits a small onset potential of ?0.28 V vs. RHE and partial current density of over 200 mA cm^?2 with a stable and high Faradaic efficiency of 93 % for formate generation in a flow cell configuration. Electrochemical results demonstrate the synergistic effect in the Bi₂O₃@C promotes the rapid and selective CO₂ reduction in which the Bi₂O₃ is beneficial for improving the reaction kinetics and formate selectivity, while the carbon matrix would be helpful for enhancing the activity and current density of formate production. This work provides effective bismuth‐based MOF derivatives for efficient formate production and offers insights in promoting practical CO₂ conversion technology.     

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.     

Fabrication of a Hierarchical Ni(OH)2@Ni3S2/Ni Foam Electrode from a Prussian Blue Analogue-Based Composite with Enhanced Electrochemical Capacitive and Electrocatalytic Properties

Developing high-efficiency, cost-effective, and durable electrodes is significant for electrochemical capacitors and electrocatalysis. Herein, a 3D bifunctional electrode consisting of nickel hydroxide nanosheets@nickel sulfide nanocubes arrays on Ni foam (Ni(OH)₂@Ni₃S₂/NF) obtained from a Prussian blue analogue-based precursor is reported. The 3D higher-order porous structure and synergistic effect of different compositions endow the electrode with large specific surface area, facile ion/electron transport path, and improved conductivity. As a result, the Ni(OH)₂@Ni₃S₂/NF electrode exhibits a high specific capacity of 211 mA h g⁻¹ at a current density of 1 A g⁻¹ and 73 % capacity retention after 5000 cycles at 5 A g⁻¹. Moreover, the Ni(OH)₂@Ni₃S₂/NF electrode has superior electrocatalytic activity for the hydrogen evolution reaction with low overpotentials of 140 and 210 mV at current densities of 10 and 100 mA cm⁻², respectively. The synthetic strategy for the unique higher-order porous structure can be extended to fabricate other composite materials for energy storage and conversion.     

Hatted 1T/2H-Phase MoS2 on Ni3S2 Nanorods for Efficient Overall Water Splitting in Alkaline Media

A new hatted 1T/2H-phase MoS₂ on Ni₃S₂ nanorods, as a bifunctional electrocatalyst for overall water splitting in alkaline media, is prepared through a simple one-pot hydrothermal synthesis. The hat-rod structure is composed mainly of Ni₃S₂, with 1T/2H-MoS₂ adhered to the top of the growth. Aqueous ammonia plays an important role in forming the 1T-phase MoS₂ by twisting the 2H-phase transition and expanding the interlayer spacing through the intercalation of NH₃/NH₄⁺. Owing to the special “hat-like” structure, the electrons conduct easily from Ni foam along Ni₃S₂ to MoS₂, and the catalyst particles maintain sufficient contact with the electrolyte, with gaseous molecules produced by water splitting easily removed from the surface of the catalyst. Thus, the electrocatalytic performance is enhanced, with an overpotential of 73 mV, a Tafel slope of 79 mV dec⁻¹, and excellent stability, and the OER demonstrates an overpotential of 190 mV and Tafel slope of 166 mV dec⁻¹.