A general route via formamide condensation to prepare atomically dispersed metal-nitrogen-carbon electrocatalysts for energy technologies

Single-atom electrocatalysts (SAECs) have gained tremendous attention due to their unique active sites and strong metal–substrate interactions. However, the current synthesis of SAECs mostly relies on costly precursors and rigid synthetic conditions and often results in very low content of single-site metal atoms. Herein, we report an efficient synthesis method to prepare metal–nitrogen–carbon SAECs based on formamide condensation and carbonization, featuring a cost-effective general methodology for the mass production of SAECs with high loading of atomically dispersed metal sites. The products with metal inclusion were termed as formamide-converted metal–nitrogen–carbon (shortened as f-MNC) materials. Seven types of single-metallic f-MNC (Fe, Co, Ni, Mn, Zn, Mo and Ir), two bi-metallic (ZnFe and ZnCo) and one tri-metallic (ZnFeCo) SAECs were synthesized to demonstrate the generality of the methodology developed. Remarkably, these f-MNC SAECs can be coated onto various supports with an ultrathin layer as pyrolysis-free electrocatalysts, among which the carbon nanotube-supported f-FeNC and f-NiNC SAECs showed high performance for the O2 reduction reaction (ORR) and the CO2 reduction reaction (CO2RR), respectively. Furthermore, the pyrolysis products of supported f-MNC can still render isolated metallic sites with excellent activity, as exemplified by the bi-metallic f-FeCoNC SAEC, which exhibited outstanding ORR performance in both alkaline and acid electrolytes by delivering ∼70 and ∼20 mV higher half-wave potentials than that of commercial 20 wt% Pt/C, respectively. This work offers a feasible approach to design and manufacture SAECs with tuneable atomic metal components and high density of single-site metal loading, and thus may accelerate the deployment of SAECs for various energy technology applications

Universal Parameter Optimization of Density Gradient Ultracentrifugation Using CdSe Nanoparticles as Tracing Agents

Density gradient ultracentrifugation (DGUC) has recently emerged as an effective nanoseparation method to sort polydispersed colloidal NPs mainly according to their size differences to reach monodispersed fractions (NPs), but its separation modeling is still lack and the separation parameters’ optimization mainly based on experience of operators. In this paper, we gave mathematical descriptions on the DGUC separation, which suggested the best separation parameters for a given system. The separation parameters, including media density, centrifuge speed and time, which affected the separation efficiency, were discussed in details. Further mathematical optimization model was established to calculate and yield the “best” (optimized) linear gradient for a colloidal system with given size and density. The practical experiment results matched well with theoretical prediction, demonstrating the DGUC method, an efficient, practical, and predictable separation technique with universal utilization for colloid sorting

Understanding the “Tailoring Synthesis” of CdS Nanorods by O₂

Parameters such as solution concentrations and composition of the ambient atmosphere are known to be important in phase and morphology control in the solvothermal synthesis of CdS semiconductor nanorods (NRs), but a clear understanding of the underlying mechanisms involved is lacking. In this work, a series of experiments were performed to demonstrate that the key factor affecting the phase and morphology of CdS NRs is the amount of O₂ in the space above the reaction solution in the sealed vessel relative to the amount of precursors in solution: O₂-depleted conditions resulted in more cubic phase CdS and thick polycrystalline NRs with an aspect ratio usually less than 3, which have small blue shifts in band-edge emission and little surface trap emission, while O₂-rich conditions resulted in more hexagonal-phase CdS and slim single-crystal NRs, which have significantly blue shifted band-edge emission and relatively strong surface trap emission. Thus, increasing the amount of solution in the vessel, changing the ambient atmosphere from air to N₂, and increasing the reagent concentration all lower the molar ratio of O₂ to reagents and lead to more cubic phase and thicker NRs. The results indicate that the composition of the “empty” section of the reaction vessel plays as important a role as the composition of the liquid in determining the phase and morphology, something that has been overlooked in earlier work. A mechanism to explain the effect of oxygen on the nucleation and growth stages has been proposed on the basis of those results and further supported by shaking experiments and ZnS NR synthesis manipulation. The CdS NRs synthesized under different conditions showed obvious differences in photocatalytic activity, which indicated that controlling the synthetic process can lead to materials with tailored photocatalytic activity

Synthesis Mechanism Study of Layered Double Hydroxides Based on Nanoseparation

Colloidal layered double hydroxides (LDH) nanosheets were sorted by their lateral sizes using a density gradient ultracentrifuge separation technique. Composition investigations on these size-sorted nanosheets indicated that larger sheets had higher Mg:Al ratio than the smaller ones. Experiments using different Mg:Al feed ratios confirmed that high Mg:Al ratio induced fast sheet growth speed. Tracking the source of the Mg:Al spatial distribution difference in one batch of synthesis at the nucleation process revealed the coprecipitation-redissolution of Mg²⁺. Thus the discriminative separation of these nanosheets led to a new insight into the structure-composition relationship of LDH nanomaterials and more understanding on their formation mechanism

Layered double hydroxide-based electrocatalysts for the oxygen evolution reaction: identification and tailoring of active sites, and superaerophobic nanoarray electrode assembly

The electrocatalytic oxygen evolution reaction (OER) is a critical half-cell reaction for hydrogen production via water electrolysis. However, the practical OER suffers from sluggish kinetics and thus requires efficient electrocatalysts. Transition metal-based layered double hydroxides (LDHs) represent one of the most active classes of OER catalysts. An in-depth understanding of the activity of LDH based electrocatalysts can promote further rational design and active site regulation of high-performance electrocatalysts. In this review, the fundamental understanding of the structural characteristics of LDHs is demonstrated first, then comparisons and in-depth discussions of recent advances in LDHs as highly active OER catalysts in alkaline media are offered, which include both experimental and computational methods. On top of the active site identification and structural characterization of LDHs on an atomic scale, strategies to promote the OER activity are summarised, including doping, intercalation and defect-making. Furthermore, the concept of superaerophobicity, which has a profound impact on the performance of gas evolution electrodes, is explored to enhance LDHs and their derivatives for a large scale OER. In addition, certain operating standards for OER measurements are proposed to avoid inconsistency in evaluating the OER activity of LDHs. Finally, several key challenges in using LDHs as anode materials for large scale water splitting, such as the issue of stability and the adoption of membrane–electrode-assembly based electrolysers, are emphasized to shed light on future research directions.</p

Ultrathin Dendritic Pt₃Cu Triangular Pyramid Caps with Enhanced Electrocatalytic Activity

Here we report on the synthesis of novel dendritic Pt₃Cu triangular pyramid caps via a solvothermal coreduction method. These caps had three-dimensional caved structures with ultrathin branches, as evidenced by high-resolution transmission electron microscopy (HRTEM) and HAADF-STEM characterization. Tuning the reduction kinetics of two metal precursors by an iodide ion was believed to be the key for the formation of an alloyed nanostructure. Electro-oxidation of methanol and formic acid showed dramatically improved electrocatalytic activities and poison-tolerance for these nanoalloys as compared to commercial Pt/C catalysts, which was attributed to their unique open porous structure with interconnected network, ultrahigh surface areas, as well as synergetic effect of the two metallic components

Anti-buoyancy and unidirectional gas evolution by Janus electrodes with asymmetric wettability

The bubbles electrochemically generated by gas evolution reactions are commonly driven off the electrode by buoyancy, a weak force used to overcome bubble adhesion barriers, leading to low gas transporting efficiency. Herein, a Janus electrode with asymmetric wettability has been prepared by modifying two sides of a porous stainless-steel mesh electrode, with superhydrophobic polytetrafluoroethylene (PTFE) and Pt/C (or Ir/C) catalyst with well-balanced hydrophobicity, respectively; affording unidirectional transportation of as-formed gaseous hydrogen and oxygen from the catalyst side to the gas-collecting side during water splitting. “Bubble-free” electrolysis was realized when “floating” the Janus electrode on the electrolyte. Anti-buoyancy through-mesh bubble transportation was observed when immersing the electrode with PTFE side downward. The wettability gradient within the electrode endowed sticky states of bubbles on the catalyst side, resulting in efficient “bubble-free” gas transportation with 15 folds higher current density than submerged states

Synthesis and properties of stable sub-2-nm-thick aluminum nanosheets: Oxygen passivation and two-photon luminescence

The high reductivity of aluminum implies the utmost difficulty in achieving oxygen resistant ultrathin Al nanostructures. Herein, we demonstrate that sub-2 nm thick Al nanosheets with ambient stability can be synthesized through a facile wet chemical approach. Selective oxygen adsorption on the (111) facets of the face-centered cubic (fcc) Al has been revealed as the reason of controlling the morphology and stability of Al nanosheets, tailoring the thickness from 18 nm down to 1.5 nm. Within the (111) surface passivation, Al nanosheets have achieved satisfactory stability which ensures the possibility to study thickness-dependent localized surface plasmon resonance from visible to the Near-IR region, and significantly enhanced two-photon luminescence. This work demonstrates, for the first time, the feasibility in obtaining stable ultrathin nanostructures of Al metal, which paves the way toward optical application of Al as a sustainable plasmonic material; it also shows the great potential of the controllable synthesis for investigation of other active metal- based nanomaterials.</p