General Design of Aligned-Channel Porous Carbon Electrodes for Efficient High-Current-Density Gas-Evolving Electrocatalysis.

Gas-evolving reactions (GERs) are important in many electrochemical energy conversion technologies and chemical industries. The operation of GERs at high current densities is critical for their industrial implementation but remains challenging as it poses stringent requirements on the electrodes in terms of reaction kinetics, mass transfer, and electron transport. Here the general and rational design of self-standing carbon electrodes with vertically aligned porous channels, appropriate pore size distribution, and high surface area as supports for loading a variety of catalytic species by facile electrodeposition are reported. These electrodes simultaneously possess high intrinsic activity, large numbers of active sites, and efficient transport highways for ions, gases, and electrons, resulting in significant performance improvements at high current densities in diverse GERs such as urea oxidation, hydrogen evolution, and oxygen evolution reactions, as well as overall urea/water electrolyzers. As an example, the carbon electrode decorated with Ni(OH)2 demonstrates a record-high current density of 1000 mA cm-2 at 1.360 V versus the reversible hydrogen electrode, largely outperforming the conventional nickel foam-based counterpart and the state-of-the-art electrodes.

Sulfur-Tuned Main-Group Sb-N-C Catalysts for Selective 2-Electron and 4-Electron Oxygen Reduction.

The selective oxygen reduction reaction (ORR) is important for various energy conversion processes such as the fuel cells and metal-air batteries for the 4e- pathway and hydrogen peroxide (H2O2) electrosynthesis for the 2e- pathway. However, it remains a challenge to tune the ORR selectivity of a catalyst in a controllable manner. Herein, an efficient strategy for introducing sulfur dopants to regulate the ORR selectivity of main-group Sb-N-C single-atom catalysts  is reported. Significantly, Sb-N-C with the highest sulfur content follows a 2e- pathway with high H2O2 selectivity (96.8%) and remarkable mass activity (96.1 A g-1 at 0.65 V), while the sister catalyst with the lowest sulfur content directs a 4e- pathway with a half-wave potential (E1/2 = 0.89 V) that is more positive than commercial Pt/C. In addition, practical applications for these two 2e-/4e- ORR catalysts are demonstrated by bulk H2O2 electrosynthesis for the degradation of organic pollutants and a high-power zinc-air battery, respectively. Combined experimental and theoretical studies reveal that the excellent selectivity for the sulfurized Sb-N-Cs is attributed to the optimal adsorption-desorption of the ORR intermediates realized through the electronic structure modulation by the sulfur dopants.

Highly Durable and Efficient Seawater Electrolysis Enabled by Defective Graphene-Confined Nanoreactor.

Direct seawater electrolysis is a promising technology for massive green hydrogen production but is limited by the lack of durable and efficient electrocatalysts toward the oxygen evolution reaction (OER). Herein, we develop a core-shell nanoreactor as a high-performance OER catalyst consisting of NiFe alloys encapsulated within defective graphene layers (NiFe@DG) by a facile microwave shocking strategy. This catalyst needs overpotentials of merely 218 and 276 mV in alkalized seawater to deliver current densities of 10 and 100 mA cm-2, respectively, and operates continuously for 2000 h with negligible activity decay (1.0%), making it one of the best OER catalysts reported to date. Detailed experimental and theoretical analyses reveal that the excellent durability of NiFe@DG originates from the formation of the built-in electric field triggered by the defective graphene coating against chloride ions at the electrode/electrolyte interface, thus protecting the active NiFe alloys at the core from dissolution and aggregation under harsh operation conditions. Further, a highly stable and efficient seawater electrolyzer is assembled with the NiFe@DG anode and the Pt/C cathode to demonstrate the practicability of the catalysts.

High-Pressure Synthesis of Single-Crystalline SnS Nanoribbons.

Two-dimensional tin monosulfide (SnS) is attractive for the development of electronic and optoelectronic devices with anisotropic characteristics. However, its shape-controlled synthesis with an atomic thickness and high quality remains challenging. Here, we show that highly crystalline SnS nanoribbons can be produced via high-pressure (0.5 GPa) and thermal treatment (400 °C). These SnS nanoribbons have a length of several tens of micrometers and a thickness down to 5.8 nm, giving an average aspect ratio of ∼30.6. The crystal orientation along the zigzag direction and the in-plane structural anisotropy of the SnS nanoribbons are identified by transmission electron microscopy and polarized Raman spectroscopy, respectively. An ionic liquid-gated field-effect transistor fabricated using the SnS nanoribbon exhibits an on/off current ratio of >103 and a field-effect mobility of ∼0.7 cm2 V-1 s-1. This work provides a unique way to achieve one-dimensional growth of SnS.

Amorphous/crystalline heterophase electrocatalysts: synthesis, applications and perspectives.

Phase engineering is arising as an effective strategy to regulate the properties, functions and applications of nanomaterials. In particular, amorphous/crystalline (a/c) heterophase nanostructures with enriched active sites, unsaturated coordination structures and abundant phase boundaries have exhibited some intriguing properties in various catalytic applications. This review briefly summarizes the recent advances on the synthetic strategies (e.g., wet-chemical synthesis, thermal annealing, electrochemical strategy, ultrafast heating method and other strategies) and electrocatalytic applications (e.g., water splitting, oxygen reduction reaction, carbon dioxide reduction reaction, nitrogen reduction reaction and organic electrooxidation reaction) of a/c heterophase catalysts. Finally, some challenges and personal perspectives for the a/c heterophase electrocatalysts are provided.

Sb2S3-templated synthesis of sulfur-doped Sb-N-C with hierarchical architecture and high metal loading for H2O2 electrosynthesis.

Selective two-electron (2e-) oxygen reduction reaction (ORR) offers great opportunities for hydrogen peroxide (H2O2) electrosynthesis and its widespread employment depends on identifying cost-effective catalysts with high activity and selectivity. Main-group metal and nitrogen coordinated carbons (M-N-Cs) are promising but remain largely underexplored due to the low metal-atom density and the lack of understanding in the structure-property correlation. Here, we report using a nanoarchitectured Sb2S3 template to synthesize high-density (10.32 wt%) antimony (Sb) single atoms on nitrogen- and sulfur-codoped carbon nanofibers (Sb-NSCF), which exhibits both high selectivity (97.2%) and mass activity (114.9 A g-1 at 0.65 V) toward the 2e- ORR in alkaline electrolyte. Further, when evaluated with a practical flow cell, Sb-NSCF shows a high production rate of 7.46 mol gcatalyst-1 h-1 with negligible loss in activity and selectivity in a 75-h continuous electrolysis. Density functional theory calculations demonstrate that the coordination configuration and the S dopants synergistically contribute to the enhanced 2e- ORR activity and selectivity of the Sb-N4 moieties.

A General Strategy to Remove Metal Aggregates toward Metal-Nitrogen-Carbon Catalysts with Exclusive Atomic Dispersion.

Metal- and nitrogen-doped nanocarbons (M-N-Cs) are promising alternatives to precious metals for catalyzing electrochemical energy conversion processes. However, M-N-Cs synthesized by high-temperature pyrolysis frequently suffer from compositional heterogeneity with the simultaneous presence of atomically dispersed M-Nx sites and crystalline metal nanoparticles (NPs), which hinders the identification of active sites and rational optimization in performance. Herein, a universal and efficient strategy is reported to obtain both precious- and nonprecious-metal-based M-N-Cs (M = Pt, Fe, Co, Ni, Mn, Cu, Zn) with exclusive atomic dispersion by making use of ammonium iodide as the etchant to remove excessive metal aggregates at high temperature. Taking Pt-N-C as a proof-of-concept demonstration, the complete removal of Pt NPs in Pt-N-C enables clarification on the contributions of the atomic Pt-Nx moieties and Pt NPs to the catalytic activity toward the hydrogen evolution reaction. Combined electrochemical measurements and theoretical calculations identify that the atomic Pt-Nx moieties by themselves possess negligible activity, but they can significantly boost the activity of the Pt NPs via the synergistic effect.

Role of N in Transition-Metal-Nitrides for Anchoring Platinum-Group Metal Atoms toward Single-Atom Catalysis.

Single-atom catalysts (SACs) with a maximum atom utilization efficiency have received growing attention in heterogeneous catalysis. The supporting substrate that provides atomic-dispersed anchoring sites and the local electronic environment in these catalysts is crucial to their activity and stability. Here, inspired by N-doped graphene substrate, the role of N is explored in transition metal nitrides for anchoring single metal atoms toward single-atom catalysis. A pore-rich metallic vanadium nitride (VN) nanosheet is fabricated as one supporting-substrate example, whose surface features abundant unsaturated N sites with lower binding energy than that of widely used N-doped graphene. Impressively, it is found that this support can anchor nearly all platinum-group single atoms (e.g., platinum, palladium, iridium, and ruthenium), and even be extendable to multiple SACs, i.e., binary (Pt/Pd) and ternary (Pt/Pd/Ir). As a proof-of-concept application for hydrogen production, Pt-based SAC (Pt1 -VN) performs excellently, exhibiting a mass activity up to 22.55 A mg-1 Pt at 0.05 V and a high turnover frequency value close to 0.350 H2 s-1 , superior to commercial platinum/carbon catalyst. The catalyst's durability can be further improved by using binary (Pt1 Pd1 -VN) SAC. This work provides inexpensive and durable nitride-based support, giving a possible pathway for universally constructing platinum-group SACs.

The promises, challenges and pathways to room-temperature sodium-sulfur batteries.

Room-temperature sodium-sulfur batteries (RT-Na-S batteries) are attractive for large-scale energy storage applications owing to their high storage capacity as well as the rich abundance and low cost of the materials. Unfortunately, their practical application is hampered by severe challenges, such as low conductivity of sulfur and its reduced products, volume expansion, polysulfide shuttling effect and Na dendrite formation, which can lead to rapid capacity fading. The review discusses the Na-S-energy-storage chemistry, highlighting its promise, key challenges and potential strategies for large-scale energy storage systems. Specifically, we review the electrochemical principles and the current technical challenges of RT-Na-S batteries, and discuss the strategies to address these obstacles. In particular, we give a comprehensive review of recent progresses in cathodes, anodes, electrolytes, separators and cell configurations, and provide a forward-looking perspective on strategies toward robust high-energy-density RT-Na-S batteries.

Engineering the Morphology and Microenvironment of a Graphene-Supported Co-N-C Single-Atom Electrocatalyst for Enhanced Hydrogen Evolution.

Graphene-supported single-atom catalysts (SACs) are promising alternatives to precious metals for catalyzing the technologically important hydrogen evolution reaction (HER), but their performances are limited by the low intrinsic activity and insufficient mass transport. Herein, a highly HER-active graphene-supported Co-N-C SAC is reported with unique design features in the morphology of the substrate and the microenvironment of the single metal sites: i) the crumpled and scrolled morphology of the graphene substrate circumvents the issues encountered by stacked nanoplatelets, resulting in improved exposure of the electrode/electrolyte interfaces (≈10 times enhancement); ii) the in-plane holes in graphene preferentially orientate the Co atoms at the edge sites with low-coordinated Co-N3 configuration that exhibits enhanced intrinsic activity (≈2.6 times enhancement compared to the conventional Co-N4 moiety), as evidenced by detailed experiments and density functional theory calculations. As a result, this catalyst exhibits significantly improved HER activity with an overpotential (η) of merely 82 mV at 10 mA cm-2 , a small Tafel slope of 59.0 mV dec-1 and a turnover frequency of 0.81 s-1 at η = 100 mV, ranking it among the best Co-N-C SACs.

A Silicon Monoxide Lithium-Ion Battery Anode with Ultrahigh Areal Capacity.

Silicon monoxide (SiO) is an attractive anode material for next-generation lithium-ion batteries for its ultra-high theoretical capacity of 2680 mAh g-1. The studies to date have been limited to electrodes with a relatively low mass loading (< 3.5 mg cm-2), which has seriously restricted the areal capacity and its potential in practical devices. Maximizing areal capacity with such high-capacity materials is critical for capitalizing their potential in practical technologies. Herein, we report a monolithic three-dimensional (3D) large-sheet holey graphene framework/SiO (LHGF/SiO) composite for high-mass-loading electrode. By specifically using large-sheet holey graphene building blocks, we construct LHGF with super-elasticity and exceptional mechanical robustness, which is essential for accommodating the large volume change of SiO and ensuring the structure integrity even at ultrahigh mass loading. Additionally, the 3D porous graphene network structure in LHGF ensures excellent electron and ion transport. By systematically tailoring microstructure design, we show the LHGF/SiO anode with a mass loading of 44 mg cm-2 delivers a high areal capacity of 35.4 mAh cm-2 at a current of 8.8 mA cm-2 and retains a capacity of 10.6 mAh cm-2 at 17.6 mA cm-2, greatly exceeding those of the state-of-the-art commercial or research devices. Furthermore, we show an LHGF/SiO anode with an ultra-high mass loading of 94 mg cm-2 delivers an unprecedented areal capacity up to 140.8 mAh cm-2. The achievement of such high areal capacities marks a critical step toward realizing the full potential of high-capacity alloy-type electrode materials in practical lithium-ion batteries.

Electronic Structure Regulation of Single-Atom Catalysts for Electrochemical Oxygen Reduction to H2 O2.

Electrochemical synthesis of hydrogen peroxide (H2 O2 ) via the 2-electron oxygen reduction reaction (ORR) has emerged as a promising alternative to the energy-intensive anthraquinone process and catalysts combining high selectivity with superior activity are crucial for enhancing the efficiency of H2 O2 electrosynthesis. In recent years, single-atom catalysts (SACs) with the merits of maximum atom utilization efficiency, tunable electronic structure, and high mass activity have attracted extensive attention for the selective reduction of O2 to H2 O2 . Although considerable improvements are made in the performance of SACs toward the 2-electron ORR process, the principles for modulating the catalytic properties of SACs by adjusting the electronic structure remain elusive. In this review, the regulation strategies for optimizing the 2-electron ORR activity and selectivity of SACs by different methods of electronic structure tuning, including the altering of the central metal atoms, the modulation of the coordinated atoms, the substrate effect, and alloy engineering are summarized. Finally, the challenges and future prospects of advanced SACs for H2 O2 electrosynthesis via the 2-electron ORR process are proposed.

Iodine-Doping-Induced Electronic Structure Tuning of Atomic Cobalt for Enhanced Hydrogen Evolution Electrocatalysis.

The development of strategies for tuning the electronic structure of the metal sites in single-atom catalysts (SACs) is the key to optimizing their activity. Herein, we report that iodine doping within the carbon matrix of a cobalt-nitrogen-carbon (Co-N-C) catalyst can effectively modulate its electronic structure and catalytic activity toward the hydrogen evolution reaction (HER). The iodine-doped Co-N-C catalyst shows exceptional HER activity in acid with an overpotential of merely 52 mV at 10 mA cm-2, a small Tafel slope of 56.1 mV dec-1, making it among the best SACs based on both precious and nonprecious metals. Moreover, this catalyst possesses a high turnover frequency (TOF) value of 1.88 s-1 (η = 100 mV), which is about 1 order of magnitude larger than that (0.2 s-1) of the iodine-free counterpart. Experimental and theoretical studies demonstrate that the introduction of iodine dopants lowers the chemical oxidation state of the Co sites, resulting in the optimized hydrogen adsorption and facilitated HER kinetics. This work provides an alternative strategy to regulate the electronic structure of SACs for improved performance.

Design of Aligned Porous Carbon Films with Single-Atom Co-N-C Sites for High-Current-Density Hydrogen Generation.

Metal- and nitrogen-doped carbon (M-N-C) materials as a unique class of single-atom catalysts (SACs) have increasingly attracted attention as the replacement of platinum for the hydrogen evolution reaction (HER); however, their employment as HER electrodes at high current densities of industrial level remains a grand challenge. Herein, an aligned porous carbon film embedded with single-atom Co-N-C sites of exceptional activity and stability at high current densities is designed. Within the film, the atomic CoNx moieties exhibit high intrinsic activity, while the multiscale porosity of the carbon frameworks with vertically aligned microchannels afford facilitated mass transfer under the conditions of high production rate and ultrathick electrodes. Moreover, the superwetting properties of the film promote electrolyte wetting and ensure the timely removal of the evolving H2 gas bubbles. The as-designed film can work as an efficient HER electrode to deliver 500 and 1000 mA cm-2 in acid at overpotentials of 272 and 343 mV, respectively, and can operate uninterruptedly and stably at 1000 mA cm-2 for at least 32 h under static conditions. These findings pave the road toward the rational design of SACs with improved activity and stability at high current densities in gas-evolving electrocatalytic processes.

Single atom electrocatalysts supported on graphene or graphene-like carbons.

Electrocatalysis plays an essential role in diverse electrochemical energy conversion processes that are vital for improving energy utilization efficiency and mitigating the aggravating global warming challenge. The noble metals such as platinum are generally the most frequently used electrocatalysts to drive these reactions and facilitate the relevant energy conversion processes. The high cost and scarcity of these materials pose a serious challenge for the wide-spread adoption and the sustainability of these technologies in the long run, which have motivated considerable efforts in searching for alternative electrocatalysts with reduced loading of precious metals or based entirely on earth-abundant metals. Of particular interest are graphene-supported single atom catalysts (G-SACs) that integrate the merits of heterogeneous catalysts and homogeneous catalysts, such as high activity, selectivity, stability, maximized atom utilization efficiency and easy separation from reactants/products. The graphene support features a large surface area, high conductivity and excellent (electro)-chemical stability, making it a highly attractive substrate for supporting single atom electrocatalysts for various electrochemical energy conversion processes. In this review, we highlight the recent advancements in G-SACs for electrochemical energy conversion, from the synthetic strategies and identification of the atomistic structure to electrocatalytic applications in a variety of reactions, and finally conclude with a brief prospect on future challenges and opportunities.

Bacteria-Derived Biological Carbon Building Robust Li-S Batteries.

Lithium sulfur (Li-S) batteries are attracting increasing interest for high-density energy storage. However, the practical application is limited by the rapid capacity fading over repeated charge/discharge cycles which is largely attributed to the formation and shuttling of soluble polysulfide species. To address these issues, we develop a hierarchical structure composite with triple protection strategy via graphene, organic conductor PEDOT, and nitrogen and phosphorus codoped biological carbon to encapsulate sulfur species (GOC@NPBCS). This unique hierarchical structure can effectively immobilize the sulfur species while at the same time improve the electrical conductivity and ensure efficient lithium ion transport to enable excellent Li-S battery performance. In particular, the biological carbon derived from natural bacteria features inherent nitrogen and phosphorus codoping with a strong absorption to lithium polysulfides, which can greatly suppress the dissolution and shuttling of polysulfides that are responsible for rapid capacity fading. With these synergistic effects, the GOC@NPBCS cathode exhibits exceptionally stable cycling stability (an ultralow capacity fading rate of 0.045% per cycle during 1000 cycles at the current rate of 5 C), high specific capacity (1193.8 mAh g-1 at 0.5 C based on sulfur weight), and excellent rate capability.

Double-negative-index ceramic aerogels for thermal superinsulation.

Ceramic aerogels are attractive for thermal insulation but plagued by poor mechanical stability and degradation under thermal shock. In this study, we designed and synthesized hyperbolic architectured ceramic aerogels with nanolayered double-pane walls with a negative Poisson's ratio (-0.25) and a negative linear thermal expansion coefficient (-1.8 × 10-6 per °C). Our aerogels display robust mechanical and thermal stability and feature ultralow densities down to ~0.1 milligram per cubic centimeter, superelasticity up to 95%, and near-zero strength loss after sharp thermal shocks (275°C per second) or intense thermal stress at 1400°C, as well as ultralow thermal conductivity in vacuum [~2.4 milliwatts per meter-kelvin (mW/m·K)] and in air (~20 mW/m·K). This robust material system is ideal for thermal superinsulation under extreme conditions, such as those encountered by spacecraft.

A Highly Active Star Decahedron Cu Nanocatalyst for Hydrocarbon Production at Low Overpotentials.

The electrochemical carbon dioxide reduction reaction (CO2 RR) presents a viable approach to recycle CO2 gas into low carbon fuels. Thus, the development of highly active catalysts at low overpotential is desired for this reaction. Herein, a high-yield synthesis of unique star decahedron Cu nanoparticles (SD-Cu NPs) electrocatalysts, displaying twin boundaries (TBs) and multiple stacking faults, which lead to low overpotentials for methane (CH4 ) and high efficiency for ethylene (C2 H4 ) production, is reported. Particularly, SD-Cu NPs show an onset potential for CH4 production lower by 0.149 V than commercial Cu NPs. More impressively, SD-Cu NPs demonstrate a faradaic efficiency of 52.43% ± 2.72% for C2 H4 production at -0.993 ± 0.0129 V. The results demonstrate that the surface stacking faults and twin defects increase CO binding energy, leading to the enhanced CO2 RR performance on SD-Cu NPs.

Microwave-Assisted Rapid Synthesis of Graphene-Supported Single Atomic Metals.

Graphene-supported single atomic metals (G-SAMs) have recently attracted considerable research interest for their intriguing catalytic, electronic, and magnetic properties. The development of effective synthetic methodologies toward G-SAMs with monodispersed metal atoms is vital for exploring their fundamental properties and potential applications. A convenient, rapid, and general strategy to synthesize a series of monodispersed atomic transition metals (for example, Co, Ni, Cu) embedded in nitrogen-doped graphene by two-second microwave (MW) heating the mixture of amine-functionalized graphene oxide and metal salts is reported here. The MW heating is able to simultaneously induce the reduction of graphene oxide, the doping of nitrogen, and the incorporation of metal atoms into the graphene lattices in one simple step. The rapid MW process minimizes metal diffusion and aggregation to ensure exclusive single metal atom dispersion in graphene lattices. Electrochemical studies demonstrate that graphene-supported Co atoms can function as highly active electrocatalysts toward the hydrogen evolution reaction. This MW-assisted method provides a rapid and efficient avenue to supported metal atoms for wide ranges of applications.

Efficient Water-Splitting Electrodes Based on Laser-Induced Graphene.

Electrically splitting water to H2 and O2 is a preferred method for energy storage as long as no CO2 is emitted during the supplied electrical input. Here we report a laser-induced graphene (LIG) process to fabricate efficient catalytic electrodes on opposing faces of a plastic sheet, for the generation of both H2 and O2. The high porosity and electrical conductivity of LIG facilitates the efficient contact and charge transfer with the requisite electrolyte. The LIG-based electrodes exhibit high performance for hydrogen evolution reaction and oxygen evolution reaction with excellent long-term stability. The overpotential reaches 100 mA/cm2 for HER, and OER is as low as 214 and 380 mV with relatively low Tafel slopes of 54 and 49 mV/dec, respectively. By serial connecting of the electrodes with a power source in an O-ring setup, H2 and O2 are simultaneously generated on either side of the plastic sheet at a current density of 10 mA/cm2 at 1.66 V and can thereby be selectively captured. The demonstration provides a promising route to simple, efficient, and complete water splitting.