Nanostructured metal-contained catalysts are one of the most widely used types of catalysts applied to facilitate some of sluggish electrochemical reactions. However, the high activity of these catalysts cannot be sustained over a variety of pH ranges. In an effort to develop highly active and stable metal-contained catalysts, various approaches have been pursued with an emphasis on metal particle size reduction and doping on carbon-based supports. These techniques enhances the metal-support interactions, originating from the chemical bonding effect between the metal dopants and carbon support and the associated interface, as well as the charge transfer between the atomic metal species and carbon framework. This provides an opportunity to tune the well-defined metal active centers and optimize their activity, selectivity and stability of this type of (electro)catalyst. Herein, recent advances in synthesis strategies, characterization and catalytic performance of single atom metal dopants on carbon-based nanomaterials are highlighted with attempts to understand the electronic structure and spatial arrangement of individual atoms as well as their interaction with the supports. Applications of these new materials in a wide range of potential electrocatalytic processes in renewable energy conversion systems are also discussed with emphasis on future directions in this active field of research.

Replacement of precious metal electrocatalysts with highly active and cost efficient alternatives for complete water splitting at low voltage has attracted a growing attention in recent years. Here, this study reports a carbon-based composite co-doped with nitrogen and trace amount of metallic cobalt (1 at%) as a bifunctional electrocatalyst for water splitting at low overpotential and high current density. An excellent electrochemical activity of the newly developed electrocatalyst originates from its graphitic nanostructure and highly active Co-Nx sites. In the case of carefully optimized sample of this electrocatalyst, 10 mA cm(-2) current density can be achieved for two half reactions in alkaline solutions-hydrogen evolution reaction and oxygen evolution reaction-at low overpotentials of 220 and 350 mV, respectively, which are smaller than those previously reported for nonprecious metal and metal-free counterparts. Based on the spectroscopic and electrochemical investigations, the newly identified Co-Nx sites in the carbon framework are responsible for high electrocatalytic activity of the Co,N-doped carbon. This study indicates that a trace level of the introduced Co into N-doped carbon can significantly enhance its electrocatalytic activity toward water splitting.

Zero-valent iron nanoparticles (nZVI) have been widely tested as they are showing significant promise for environmental remediation. However, many recent studies have demonstrated that their mobility and reactivity in subsurface environments are significantly affected by their tendency to aggregate. Both the mobility and reactivity of nZVI mainly depends on properties such as particle size, surface chemistry and bulk composition. In order to ensure efficient remediation, it is crucial to accurately assess and understand the implications of these properties before deploying these materials into contaminated environments. Many analytical techniques are now available to determine these parameters and this paper provides a critical review of their usefulness and limitations for nZVI characterisation. These analytical techniques include microscopy and light scattering techniques for the determination of particle size, size distribution and aggregation state, and X-ray techniques for the characterisation of surface chemistry and bulk composition. Example characterisation data derived from commercial nZVI materials is used to further illustrate method strengths and limitations. Finally, some important challenges with respect to the characterisation of nZVI in groundwater samples are discussed.

The development of ordered mesoporous carbon materials with controllable structures and improved physicochemical properties by doping heteroatoms such as nitrogen into the carbon framework has attracted a lot of attention, especially in relation to energy storage and conversion. Herein, a series of nitrogen-doped mesoporous carbon spheres (NMCs) was synthesized via a facile dual soft-templating procedure by tuning the nitrogen content and carbonization temperature. Various physical and (electro)chemical properties of the NMCs have been comprehensively investigated to pave the way for a feasible design of nitrogen-containing porous carbon materials. The optimized sample showed a favorable electrocatalytic activity as evidenced by a high kinetic current and positive onset potential for oxygen reduction reaction (ORR) due to its large surface area, high pore volume, good conductivity, and high nitrogen content, which make it a highly efficient ORR metal-free catalyst in alkaline solutions.

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Developing low cost, highly active and stable electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) using the same electrolyte has remained a major challenge. Herein, we report a novel and robust material comprised of nickel-cobalt nanoparticles coated on a porous nitrogen-doped carbon (NC) thin film synthesized via a two-step pulsed laser deposition technique. The optimized sample (NiCo/NC) achieved the lowest overpotentials of 176 mV and 300 mV at a current density of 10 mA cm for HER and OER, respectively. The optimized OER activity might be attributed to the available metal oxide nanoparticles with an effective electronic structure configuration and enhanced mass/charge transport capability. At the same time, the porous nitrogen doped carbon incorporated with cobalt and nickel species can serve as an excellent HER catalyst. As a result, the newly developed electrocatalysts manifest high current densities and strong electrochemical stability in overall water splitting, outperforming most of the previously reported non-precious metal-based catalysts.

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Identifying efficient non-precious metal catalysts for oxygen evolution reaction (OER) remains a great challenge. Here we report robust cobalt (oxide) nanoparticles deposited on porous nitrogen-doped carbon (N-carbon) film prepared by pulsed laser deposition under a reactive background gas which exhibit highly efficient OER performance with low overpotential and high stability.The global energy crisis has prompted intense research into the development of various types of sustainable energy conversion and storage systems.1 Splitting water is widely considered to be a critical step toward efficient renewable energy production, storage and usage. One of the major hurdles in making water electrolysis commercially more viable is the low efficiency of the anodic oxygen evolution reaction (OER) and the high cost of conventional OER catalysts such as IrO 2 and RuO 2 .2 Inexpensive and durable "noble metal-free" electrocatalysts such as non-precious metal and metal-free nanostructures, as well as their hybrids, have received much attention recently.3 Among different non-precious metals, cobalt-based materials are promising OER catalysts, however, the easy accumulation and low conductivity of pure cobalt oxides decreases the available active sites and limits charge transport during the oxidation process. 4 On the other hand, various carbon-based materials feature unique advantages due to their tunable molecular structures, abundance, and strong tolerance to acid/alkaline environments. The interplay between carbon and cobalt oxide nanoparticles can modify the overall physicochemical and electronic structures which make the resultant composites highly competitive to traditional cobalt-based electrocatalysts. 5Despite recent progress in developing non-precious metal (specifically cobalt) based hybrid materials, new OER electrocatalysts with low overpotentials and long-term stability are still needed. To overcome these challenges, synthesis techniques are required which ensure high control and tunability of morphology, structure and composition of multi-component materials. Physical vapour deposition (PVD) techniques allow high purity and control in the fabrication of coatings and thin films. In this context, pulsed laser deposition (PLD) is particularly versatile in the tuning of properties of deposited materials 6 which is based on ablating a target material by laser pulses to produce plasma of ejected species that can be deposited onto a substrate. The control of the ablation process permits the tuning of the growth mode and properties of the deposited films over a wide range.