Optimizing the intermediates adsorbability and revealing the dynamic reconstruction of Co6Fe3S8 solid solution for bifunctional water splitting.

Co9S8 has been extensively studied as a promising catalyst for water electrolysis. Doping Co9S8 with Fe improves its oxygen evolution reaction (OER) performance by regulating the catalyst self-reconfigurability and enhancing the absorption capacity of OER intermediates. However, the poor alkaline hydrogen evolution reaction (HER) properties of Co9S8 limit its application in bifunctional water splitting. Herein, we combined Fe doping and sulfur vacancy engineering to synergistically enhance the bifunctional water-splitting performance of Co9S8. The as-synthesized Co6Fe3S8 catalyst exhibited excellent OER and HER characteristics with low overpotentials of 250 and 84 mV, respectively. It also resulted in the low Tafel slopes of 135 mV dec-1 for the OER and 114 mV dec-1 for the HER. A two-electrode electrolytic cell with Co6Fe3S8 used as both the cathode and anode produced a current density of 10 mA cm-2 at a low voltage of only 1.48 V, maintaining high stability for 100 h. The results of in/ex-situ experiments indicated that the OER process induced electrochemical reconfiguration, forming CoOOH/FeOOH active species on the catalyst surface to enhance its OER performance. Density functional theory (DFT) simulations revealed that Fe doping and the presence of unsaturated coordination metal sites in Co6Fe3S8 promoted H2O and H* adsorption for the HER. The findings of this study can help develop a strategy for designing highly efficient bifunctional water splitting electrocatalysts.

A Universal Approach for Sustainable Urea Synthesis via Intermediate Assembly at the Electrode/Electrolyte Interface.

Electrocatalytic C-N coupling process is indeed a sustainable alternative for direct urea synthesis and co-upgrading of carbon dioxide and nitrate wastes. However, the main challenge lies in the unactivated C-N coupling process. Here, we proposed a strategy of intermediate assembly with alkali metal cations to activate C-N coupling at the electrode/electrolyte interface. Urea synthesis activity follows the trend of Li+

Hydrogen-Rich Pyrolysis from Ni-Fe Heterometallic Schiff Base Centrosymmetric Cluster Facilitates NiFe Alloy for Efficient OER Electrocatalysts.

Binary metal nickel-iron alloys have been proven to have great potential in oxygen evolution reaction (OER) electrocatalysis, but there are still certain challenges in how to construct more efficient nickel-iron alloy electrocatalysts and maximize their own advantages. In this work, a heterometallic nickel-iron cluster (L = C64 H66 Fe4 N8 Ni2 O19 ) of Schiff base (LH3  = 2-amino-1,3-propanediol salicylaldehyde) is designed as a precursor to explore its behavior in the pyrolysis process under inert atmosphere. The combination of TG-MS, morphology, and X-ray characterization techniques shows that the Schiff base ligands in the heterometallic clusters produces a strong reductive atmosphere during pyrolysis, which enable the two 3d metals Ni and Fe to form NiFe alloys. Moreover, Fe2 O3 /Fe0.64 Ni0.36 @Cs carbon nanomaterials are formed, in which Fe2 O3 /Fe0.64 Ni0.36 is the potential active material for OER. It is also found that the centrosymmetric structure of the heterometallic Schiff base precursor is potentially related to the formation of the Fe2 O3 /Fe0.64 Ni0.36 alloy@carbon structures. The Fe2 O3 /Fe0.64 Ni0.36 @C-800 provides 274 mV overpotential in 1 m KOH solution at 10 mA cm-2 in OER. This work provides an effective basis for further research on Schiff base bimetallic doping-derived carbon nanomaterials as excellent OER electrocatalysts.

Innovative Electrochemical Strategies for Hydrogen Production: From Electricity Input to Electricity Output.

Renewable H2 production by water electrolysis has attracted much attention due to its numerous advantages. However, the energy consumption of conventional water electrolysis is high and mainly driven by the kinetically inert anodic oxygen evolution reaction. An alternative approach is the coupling of different half-cell reactions and the use of redox mediators. In this review, we, therefore, summarize the latest findings on innovative electrochemical strategies for H2 production. First, we address redox mediators utilized in water splitting, including soluble and insoluble species, and the corresponding cell concepts. Second, we discuss alternative anodic reactions involving organic and inorganic chemical transformations. Then, electrochemical H2 production at both the cathode and anode, or even H2 production together with electricity generation, is presented. Finally, the remaining challenges and prospects for the future development of this research field are highlighted.

A Substrate-Induced Fabrication of Active Free-Standing Nanocarbon Film as Air Cathode in Rechargeable Zinc-Air Batteries.

Designing cost-effective and high-efficiency bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurred at air electrodes is vitally significant yet challenging for Zn-air batteries (ZABs). In this work, a zinc substrate induced fabrication is reported of free-standing nanocarbon hybrid film which shows good bifunctional activity and can be directly used as the air electrode in the rechargeable ZABs. The designed nanocarbon film in Zn-air battery provides a satisfactory power density of 185 mW cm-2 and cycling stability for 1200 h under the current density of 10 mA cm-2 . This hybrid film also gives a solid-state ZAB excellent flexibility with a power density of 160 mW cm-2 . The free-standing hybrid with abundant cobalt-nitrogen-carbon species coupled with porous architecture would be the original factor for its satisfactory performance of rechargeable ZABs. This work would pave an ideal way to design integrated electrode with high electrocatalytic performance towards electrochemical energy technologies.

Regulation of Morphology and Electronic Structure of NiSe2 by Fe for High Effective Oxygen Evolution Reaction.

With the development of hydrogen-energy economy, it is urgent for researchers to explore high effective non-noble metal electrocatalysts for oxygen evolution reaction (OER). Nickel-based selenides have good conductivity and easy to regulate, which make them to be a promising OER electrocatalysts. Hence, many researchers engineering the structure of Nickel-based selenides to further improve the OER performance. In this paper, NixFe1-x Se2 porous-nano-microspheres with different ratio were synthesized. Results confirm that Fe not only affects the number of active sites in NiSe2 , but also affects the intrinsic activity by forming lattice defects. Besides, introduction of Fe can change the redox ability of Ni cation and Se anion, thus, reducing the average valence state of Ni cation in NiOOH. As a result, the current density of OER is improved remarkably. When the current density reaches 10 mA cm-2 , the overpotential is only 285 mV.

Operando Identification of the Dynamic Behavior of Oxygen Vacancy-Rich Co3O4 for Oxygen Evolution Reaction.

The exact role of a defect structure on transition metal compounds for electrocatalytic oxygen evolution reaction (OER), which is a very dynamic process, remains unclear. Studying the structure-activity relationship of defective electrocatalysts under operando conditions is crucial for understanding their intrinsic reaction mechanism and dynamic behavior of defect sites. Co3O4 with rich oxygen vacancy (VO) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co3O4 and VO-rich Co3O4 as catalyst models to study the defect mechanism and investigate the dynamic behavior of defect sites during the electrocatalytic OER process by various operando characterizations. Operando electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) implied that the VO could facilitate the pre-oxidation of the low-valence Co (Co2+, part of which was induced by the VO to balance the charge) at a relatively lower applied potential. This observation confirmed that the VO could initialize the surface reconstruction of VO-Co3O4 prior to the occurrence of the OER process. The quasi-operando X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption fine structure (XAFS) results further demonstrated the oxygen vacancies were filled with OH• first for VO-Co3O4 and facilitated pre-oxidation of low-valence Co and promoted reconstruction/deprotonation of intermediate Co-OOH•. This work provides insight into the defect mechanism in Co3O4 for OER in a dynamic way by observing the surface dynamic evolution process of defective electrocatalysts and identifying the real active sites during the electrocatalysis process. The current finding would motivate the community to focus more on the dynamic behavior of defect electrocatalysts.

Coupling N2 and CO2 in H2O to synthesize urea under ambient conditions.

The use of nitrogen fertilizers has been estimated to have supported 27% of the world's population over the past century. Urea (CO(NH2)2) is conventionally synthesized through two consecutive industrial processes, N2 + H2 → NH3 followed by NH3 + CO2 → urea. Both reactions operate under harsh conditions and consume more than 2% of the world's energy. Urea synthesis consumes approximately 80% of the NH3 produced globally. Here we directly coupled N2 and CO2 in H2O to produce urea under ambient conditions. The process was carried out using an electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets. This coupling reaction occurs through the formation of C-N bonds via the thermodynamically spontaneous reaction between *N=N* and CO. Products were identified and quantified using isotope labelling and the mechanism investigated using isotope-labelled operando synchrotron-radiation Fourier transform infrared spectroscopy. A high rate of urea formation of 3.36 mmol g-1 h-1 and corresponding Faradic efficiency of 8.92% were measured at -0.4 V versus reversible hydrogen electrode.

BN Pairs Enriched Defective Carbon Nanosheets for Ammonia Synthesis with High Efficiency.

Electrochemical synthesis has garnered attention as a promising alternative to the traditional Haber-Bosch process to enable the generation of ammonia (NH3 ) under ambient conditions. Current electrocatalysts for the nitrogen reduction reaction (NRR) to produce NH3 are comprised of noble metals or transitional metals. Here, an efficient metal-free catalyst (BCN) is demonstrated without precious component and can be easily fabricated by pyrolysis of organic precursor. Both theoretical calculations and experiments confirm that the doped BN pairs are the active triggers and the edge carbon atoms near to BN pairs are the active sites toward the NRR. This doping strategy can provide sufficient active sites while retarding the competing hydrogen evolution reaction (HER) process; thus, NRR with high NH3 formation rate (7.75 µg h-1 mgcat. -1 ) and excellent Faradaic efficiency (13.79%) are achieved at -0.3 V versus reversible hydrogen electrode (RHE), exceeding the performance of most of the metallic catalysts.

Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting.

Layered double hydroxide (LDH)-based materials have attracted widespread attention in various applications due to their unique layered structure with high specific surface area and unique electron distribution, resulting in a good electrocatalytic performance. Moreover, the existence of multiple metal cations invests a flexible tunability in the host layers; the unique intercalation characteristics lead to flexible ion exchange and exfoliation. Thus, their electrocatalytic performance can be tuned by regulating the morphology, composition, intercalation ion, and exfoliation. However, the poor conductivity limits their electrocatalytic performance, which therefore has motivated researchers to combine them with conductive materials to improve their electrocatalytic performance. Another factor hampering their electrocatalytic activity is their large lateral size and the bulk thickness of LDHs. Introducing defects and tuning electronic structure in LDH-based materials are considered to be effective strategies to increase the number of active sites and enhance their intrinsic activity. Given the unique advantages of LDH-based materials, their derivatives have been also used as advanced electrocatalysts for water splitting. Here, recent progress on LDHs and their derivatives as advanced electrocatalysts for water splitting is summarized, current strategies for their designing are proposed, and significant challenges and perspectives of LDHs are discussed.

Recent Advances on Black Phosphorus for Energy Storage, Catalysis, and Sensor Applications.

As a new type of 2D semiconductor, black phosphorus (BP) possesses high charge-carrier mobility and theoretical capacity, thickness-dependent bandgap, and anisotropic structure, which has attracted tremendous attention since early 2014. To explore its full application in all aspects, studies based on BP nanostructures are swiftly expanding from the electronic field to energy storage and even biochemistry. The mechanism and application of BP in Li-/Na-ion battery anodes, oxygen evolution reaction/hydrogen evolution reaction catalysis, photocatalytic hydrogen production, and selective sensors are summarized. Based on the solid research on this topic, feasible improvements and constructive suggestions regarding these four fields are put forward.

In situ growth of cobalt@cobalt-borate core-shell nanosheets as highly-efficient electrocatalysts for oxygen evolution reaction in alkaline/neutral medium.

Development of efficient non-noble metal electrocatalysts for oxygen evolution reaction (OER) is still a crucial issue for renewable energy technologies. Herein, we report a core-shell-like catalyst of amorphous cobalt borate nanosheets grown on metallic cobalt deposited on Ti-mesh (Co@Co-Bi/Ti) by an in situ conversion strategy. Benefiting from the high activity, large surface area of Co-Bi nanosheets, outstanding electronic conductivity of metallic Co and the 3D structure inherited from Ti-mesh substrate, Co@Co-Bi/Ti shows high OER activity with a relatively small overpotential of 329 mV to obtain a current density of 10 mA cm-2, a low Tafel slope of 46 mV dec-1 and an accessible large current density of 500 mA cm-2 in alkaline solution. Besides, Co@Co-Bi/Ti exhibits good performance in a near-neutral medium. This study provides an effective pathway to improve the cobalt metal based materials.

Defect Chemistry of Nonprecious-Metal Electrocatalysts for Oxygen Reactions.

Oxygen electrocatalysis, including the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER), is a critical process for metal-air batteries. Therefore, the development of electrocatalysts for the OER and the ORR is of essential importance. Indeed, various advanced electrocatalysts have been designed for the ORR or the OER; however, the origin of the advanced activity of oxygen electrocatalysts is still somewhat controversial. The enhanced activity is usually attributed to the high surface areas, the unique facet structures, the enhanced conductivities, or even to unclear synergistic effects, but the importance of the defects, especially the intrinsic defects, is often neglected. More recently, the important role of defects in oxygen electrocatalysis has been demonstrated by several groups. To make the defect effect clearer, the recent development of this concept is reviewed here and a novel principle for the design of oxygen electrocatalysts is proposed. An overview of the defects in carbon-based, metal-free electrocatalysts for ORR and various defects in metal oxides/selenides for OER is also provided. The types of defects and controllable strategies to generate defects in electrocatalysts are presented, along with techniques to identify the defects. The defect-activity relationship is also explored by theoretical methods.

In Situ Exfoliated, Edge-Rich, Oxygen-Functionalized Graphene from Carbon Fibers for Oxygen Electrocatalysis.

Metal-free electrocatalysts have been extensively developed to replace noble metal Pt and RuO2 catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in fuel cells or metal-air batteries. These electrocatalysts are usually deposited on a 3D conductive support (e.g., carbon paper or carbon cloth (CC)) to facilitate mass and electron transport. For practical applications, it is desirable to create in situ catalysts on the carbon fiber support to simplify the fabrication process for catalytic electrodes. In this study, the first example of in situ exfoliated, edge-rich, oxygen-functionalized graphene on the surface of carbon fibers using Ar plasma treatment is successfully prepared. Compared to pristine CC, the plasma-etched carbon cloth (P-CC) has a higher specific surface area and an increased number of active sites for OER and ORR. P-CC also displays good intrinsic electron conductivity and excellent mass transport. Theoretical studies show that P-CC has a low overpotential that is comparable to Pt-based catalysts, as a result of both defects and oxygen doping. This study provides a simple and effective approach for producing highly active in situ catalysts on a carbon support for OER and ORR.

Edge-selectively phosphorus-doped few-layer graphene as an efficient metal-free electrocatalyst for the oxygen evolution reaction.

The development of efficient, stable and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is the key for water splitting. Carbon-based nanomaterials have found significant applications as metal-free OER electrocatalysts. In this study, for the first time, edge-selectively phosphorus-doped graphene (G-P) was synthesised for OER electrocatalysts. The G-P catalyst reached a current density of 10 mA cm-2 at a small overpotential of 330 mV for the OER with a Tafel slope as low as 62 mV dec-1, which is superior to most of the carbon-based electrocatalysts reported to date.

Polyaniline-Reduced Graphene Oxide Hybrid Nanosheets with Nearly Vertical Orientation Anchoring Palladium Nanoparticles for Highly Active and Stable Electrocatalysis.

We report a nearly vertical reduced graphene oxide (VrGO) nanosheet coupled with polyaniline (PANI) for supporting palladium (Pd) nanoparticles. The PANI-coupled VrGO (PANI@VrGO) nanosheet is prepared by a simple one-step electrodeposition technique ,and Pd nanoparticles are anchored on the support of PANI@VrGO through the spontaneous redox reaction of PANI with a palladium salt. The designed PANI@VrGO nanosheet efficiently exposes the surface of rGO sheets and stabilizes metal nanoparticles. Consequently, the Pd/PANI@VrGO electrocatalyst exhibits high catalytic activity and excellent durability for alcohol oxidation reaction. The proposed nanoarchitecture offers a new pathway to greatly promote the performances of rGO in various applications; moreover, this work provides a powerful and universal synthetic strategy for such an architecture.

Three-Dimensional Nitrogen-Doped Reduced Graphene Oxide-Carbon Nanotubes Architecture Supporting Ultrafine Palladium Nanoparticles for Highly Efficient Methanol Electrooxidation.

A three-dimensional (3D) nitrogen-doped reduced graphene oxide (rGO)-carbon nanotubes (CNTs) architecture supporting ultrafine Pd nanoparticles is prepared and used as a highly efficient electrocatalyst. Graphene oxide (GO) is first used as a surfactant to disperse pristine CNTs for electrochemical preparation of 3D rGO@CNTs, and subsequently one-step electrodeposition of the stable colloidal GO-CNTs solution containing Na2 PdCl4 affords rGO@CNTs-supported Pd nanoparticles. Further thermal treatment of the Pd/rGO@CNTs hybrid with ammonia achieves not only in situ nitrogen-doping of the rGO@CNTs support but also extraordinary size decrease of the Pd nanoparticles to below 2.0 nm. The resulting catalyst is characterized by scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Catalyst performance for the methanol oxidation reaction is tested through cyclic voltammetry and chronoamperometry techniques, which shows exceedingly high mass activity and superior durability.

Palladium nanoparticles supported on vertically oriented reduced graphene oxide for methanol electro-oxidation.

Reduced graphene oxide (rGO) is a promising support material for nanosized electrocatalysts. However, the conventional stacking arrangement of rGO sheets confines the electrocatalysts between rGO layers, which decreases the number of catalytic sites substantially. We report here a facile synthesis of vertically oriented reduced graphene oxide (VrGO) through cyclic voltammetric electrolysis of graphene oxide (GO) in the presence of Na2 PdCl4 . Experiments without Pd nanoparticles or with a low loading amount of Pd nanoparticles results in the deposition of rGO parallel to the electrodes. The vertical orientation of Pd/rGO nanoflakes causes a remarkable enhancement of the catalytic activity toward methanol electro-oxidation. The mass activity (620.1 A gPd (-1) ) of Pd/VrGO is 1.9 and 6.2 times that of Pd/flat-lying rGO (331.8 A gPd (-1) ) and commercial Pd/C (100.5 A gPd (-1) ), respectively. Furthermore, the Pd/VrGO catalyst shows excellent resistance to CO poisoning. This work provides a simple wet-chemical method for VrGO preparation.