Cobalt-Based Nanoscale Material: An Emerging Electrocatalyst for Hydrogen Production.

Modern civilization has been highly suffering from energy crisis and environmental pollutions. These two burning issues are directly and indirectly created from fossil fuel consumption and uncontrolled industrialization. The above critical issue can be solved through the proper utilization of green energy sources where no greenhouse gases will be generated upon burning of such materials. Hydrogen is the most eligible candidate for this purpose. Among various methods of hydrogen generation, electrocatalytic process is one of the most efficient methods because of easy handling and high efficiency. In these aspects Co-based nanomaterials are considered to be extremely significant as they can be utilized as efficient, recyclable and ideal catalytic system. In this article a series of Co-based nano-electrocatalysts has been discussed with proper structure-property relationship and their medium dependency. Therefore, such type of stimulating summary on recently reported electrocatalysts and their activity may be helpful for scientists of the corresponding field as well as for broader research communities. This can be inspiration for materials researchers to fabricate active catalysts for the production of hydrogen gas in room temperature.

Monodispersed Ni active sites anchored on N-doped porous carbon nanosheets as high-efficiency electrocatalyst for hydrogen peroxide sensing.

Metal active species combined with N-doped porous carbon nanosheets usually own excellent electrochemical activity and sensing performance owing to its unique microstructure and composition. In this work, monodispersed Ni active sites anchored on N-doped porous carbon nanosheets (Ni@N-PCN) were facilely prepared via rational metal-organic frameworks (MOFs) route. Firstly, zeolitic imidazolate frameworks-8 (ZIF-8) was in situ grown on physically-exfoliated graphene nanosheets (GN) with homogeneous sandwich-like structure (ZIF-8@GN). Secondly, nickel bonded ZIF-8@GN hybrids (Ni/ZIF-8@GN) were obtained by ionic exchange reaction, and then transformed into Ni@N-PCN by high-temperature pyrolysis. Benefiting from the monodispersed Ni active sites and highly reactive N-doped porous carbon nanosheets (N-PCN), the as-prepared Ni@N-PCN hybrids displayed superior catalytic performance toward hydrogen peroxide (H2O2) sensing. As a result, a highly sensitive electrochemical sensing platform for H2O2 was fabricated with low detection limit (0.032 µM), wide detection linearity (0.2-2332.8 µM), and high sensitivity (6085 µA cm-2 mM-1). Besides, the as-developed electrochemical sensing platform was successfully applied to detect H2O2 contents in biological medicine and food specimens with satisfied results. This study will provide effective guidance for the preparation of novel metal/N-doped carbon nanomaterials and establishment of high-performance electrochemical sensors.

Strongly coupled Mo2C and Ni nanoparticles with in-situ formed interfaces encapsulated by porous carbon nanofibers for efficient hydrogen evolution reaction under alkaline conditions.

Herein, strongly coupled Mo2C and Ni nanoparticles with in-situ formed interfaces encapsulated by porous carbon nanofibers (Ni-Mo2C-CNF) have been rationally fabricated via pyrolyzing electrospinning polyvinyl alcohol fibers containing hydrothermally obtained NiMoO4 under Ar atmosphere and applied as high-performance and stable electrocatalyst for HER in alkaline electrolytes. Powered by NiMoO4 as homologous bimetallic precursor, the Ni-Mo2C-CNF possesses numerous in-situ formed Ni-Mo2C interfaces, which facilitates the synergistic effect between Ni and Mo2C, improving the conductivity and thus boosting the electrocatalytic performance towards HER. In the meantime, the porous carbon nanofibers with well encapsulated Ni-Mo2C active components stacks, constituting conductive network, which promotes the mass transport, electron transfer, active sites exposure and electrocatalytic stability. As a result, the Ni-Mo2C-CNF features prominently in HER, as it demands a low overpotential of 196 mV but is able to stably yield the current density of 10 mA cm-2 with a small Tafel plot of 54.7 mV dec-1. The method demonstrated in our work to synthesize bimetallic heterostructured materials will offer valuable inspiration to construct promising non-precious electrocatalysts for diverse vital renewable energy applications.

Porous NiCo2S4 Nanoneedle Arrays with Highly Efficient Electrocatalysis Anchored on Carbon Cloths as Self-Supported Hosts for High-Loading Li-S Batteries.

Lithium-sulfur (Li-S) batteries have attracted all-time attention because of their supernormal high energy density and low cost, whereas they are still plagued by the severe polysulfide shuttling and sluggish sulfur redox reaction kinetics. Moreover, poor sulfur electrochemical utilization and rapid capacity degradation are top concerns in the high-loading Li-S batteries, which severely hinder their practical applications. Herein, a completely novel porous nanoneedle array NiCo2S4 electrocatalyst grown on a nitrogen-sulfur-doped carbon cloth (NSCC) (NiCo2S4@NSCC) is constructed as a 3D self-supported sulfur host for high-loading Li-S batteries, in which the highest sulfur loading reaches 4.9 mg cm-2. The as-prepared NiCo2S4@NSCC with a typical sulfur loading of around 2.0 mg cm-2 provides a high discharge capacity of 1223 mA h g-1 at 0.2 C and long-term cycle stability with a low capacity decay of 0.046% per cycle over 500 cycles at 1 C. Additionally, NiCo2S4@NSCC/S with a high sulfur loading of 4.9 mg cm-2 delivers an excellent reversible areal capacity of 4.4 mA h cm-2 g over 50 cycles. Noting that such superior electrochemical performance of NiCo2S4@NSCC/S with high-loading sulfur is mainly attributed to high electronic conductivity and the abundant porous structure of NSCC to transport electrons and ions fastly and accommodate sulfur as well as robust absorbability and the outstanding catalytic effect of NiCo2S4 to accelerate the capture and conversion of the polysulfide intermediate. Predictably, this work can provide a guideline to efficiently and rationally design the structure of metal-based compounds with catalytic functions for various applications.

A novel cobalt and nitrogen co-doped mesoporous hollow carbon hemisphere as high-efficient electrocatalysts for oxygen reduction reaction.

Exploring a cheap catalyst with effective activity for oxygen reduction reaction (ORR) to replace precious metal electrocatalysts has gained tremendous attention for several decades. In this study, we designed and synthesized cobalt and nitrogen supported on mesoporous hollow carbon hemisphere (Co/N/HCHs) nanocomposites by a facile and economical approach. Semisphere-shaped mesoporous hollow carbon is self-generated using silica particles as template, followed by a pyrolysis-etching process; and exhibits high electrical conductivity and high specific surface. The unique porous structure of carbon provides significant number of the abundant defective sites and shortens the mass transfer pathway, leading to a greatly enhanced electrocatalytic activity with mainly 4e- reduction. Moreover, the synergistic effects of large electrochemically active areas and good electrical conductivity, resulting from the introduction of Co and N heteroatom, are the main reason for displaying outstanding ORR activity with a high half-wave potential of 0.8 V and the electron transfer numbers of 3.89. Furthermore, an excellent long-term stability (the current density retention of 87.0%) and superb methanol tolerance in alkaline medium are achieved. Undoubtedly, this demonstrates a potential way to strategically design the non-precious metal doped carbon catalysts for wider practical applications.

Mo2C-Ni modified carbon microfibers as an effective electrocatalyst for hydrogen evolution reaction in acidic solution.

In this work, carbon microfibers modified with Mo2C and metallic Ni (Mo2C-Ni-CMF0.2) was successfully synthesized by one-step strategy and demonstrated that it is efficient and stable low-cost electrocatalyst for hydrogen evolution reaction (HER) in acidic conditions. The as-obtained Mo2C-Ni-CMF0.2 shows excellent HER activity with a low overpotential (131 mV) to reach current density of 10 mA cm-2, a small Tafel plot (34.1 mV dec-1) and remarkable stability. The carbonized cotton fibers has a pore structure and a large specific surface area, which is beneficial for improving electrocatalytic activity. Moreover, cotton fibers also provide a carbon source for the formation of Mo2C during carbonization. The catalyst owes better activity to the electrion transmission facilitated by the synergy between Mo2C and Ni, and the adsorption of H+ based on pore structure.