Iron and nitrogen codoped carbons (Fe–N–C) have attracted increasingly greater attention as electrocatalysts for oxygen reduction reaction (ORR). Although challenging, the synthesis of Fe–N–C catalysts with highly dispersed and fully exposed active sites is of critical importance for improving the ORR activity. Here, we report a new type of graphitic Fe–N–C catalysts featuring numerous Fe single atoms anchored on a three-dimensional simple-cubic carbon framework. The Fe–N–C catalyst, derived from self-assembled Fe3O4 nanocube superlattices, was prepared by in situ ligand carbonization followed by acid etching and ammonia activation. Benefiting from its homogeneously dispersed and fully accessible active sites, highly graphitic nature, and enhanced mass transport, our Fe–N–C catalyst outperformed Pt/C and many previously reported Fe–N–C catalysts for ORR. Furthermore, when used for constructing the cathode for zinc–air batteries, our Fe–N–C catalyst exhibited current and power densities comparable to those of the state-of-the-art Pt/C catalyst.Keywords: Fe−N−C catalyst; oxygen reduction reaction; Self-assembly; single atom; zinc−air battery;

Metal–air batteries have a theoretical energy density that is much higher than that of lithium-ion batteries and are frequently advocated as a solution toward next-generation electrochemical energy storage for applications including electric vehicles or grid energy storage. However, they have not fulfilled their full potential because of challenges associated with the metal anode, air cathode, and electrolyte. These challenges will have to be properly resolved before metal–air batteries can become a practical reality and be deployed on a large scale. Here we survey the current status and latest advances in metal–air battery research for both aqueous (e.g., Zn–air) and nonaqueous (e.g., Li–air) systems. An overview of the general technical issues confronting their development is presented, and our perspective on possible solutions is offered.

Sodium-ion batteries are potential low-cost alternatives to current lithium-ion technology, yet their performances still fall short of expectation due to the lack of suitable electrode materials with large capacity, long-term cycling stability, and high-rate performance. In this work, we demonstrated that ultrasmall (∼5 nm) iron selenide (FeSe2) nanoparticles exhibited a remarkable activity for sodium-ion storage. They were prepared from a high-temperature solution method with a narrow size distribution and high yield and could be readily redispersed in nonpolar organic solvents. In ether-based electrolyte, FeSe2 nanoparticles exhibited a large specific capacity of ∼500 mAh/g (close to the theoretical limit), high rate capability with ∼250 mAh/g retained at 10 A/g, and excellent cycling stability at both low and high current rates by virtue of their advantageous nanosizing effect. Full sodium-ion batteries were also constructed from coupling FeSe2 with NASICON-type Na3V2(PO4)3 cathode and demonstrated impressive capacity and cycle ability.Keywords: full battery; Iron selenide; nanosizing effect; sodium-ion battery; ultrasmall nanoparticles;

•Core-sheath organic-inorganic hybrid was investigated for CO2RR for the first time•Excellent activity, selectivity, and stability were reported for CO2 reduction to CO•Experimental results were rationalized by DFT calculationsGrowing worldwide consumption of fossil fuels has resulted in an increasing concentration of atmospheric CO2 and profound climate changes. Electrocatalytic CO2 reduction to liquid or gaseous fuels is ideal for high-density renewable energy storage and could represent an attractive prospect for CO2 capture. However, it still remains a grand challenge to develop efficient and stable electrocatalysts for CO2 reduction. Most current research attention has focused on inorganic candidates such as Au, Ag, and Cu, but the potential of organic phthalocyanines or porphyrins has not been adequately explored. These molecular catalysts generally suffer from large overpotentials, insufficient faradic efficiency, and poor durability. In this work, we report a hybrid material consisting of a carbon nanotube core and cobalt polyphthalocyanine sheath as a highly efficient and stable catalyst for the selective conversion of CO2 to CO with a performance unmatched by those of other organic CO2 reduction electrocatalysts.Electrochemical reduction of CO2 represents a possible solution for transforming atmospheric CO2 to value-added chemicals such as CO or hydrocarbons, but so far it has been hampered by the lack of suitable electrocatalysts. In this work, we design a type of organic-inorganic hybrid material by template-directed polymerization of cobalt phthalocyanine on carbon nanotubes for a high-performance CO2 reduction reaction. Compared with molecular phthalocyanines, the polymeric form of phthalocyanines supported on the conductive scaffold exhibits an enlarged electrochemically active surface area and improved physical and chemical robustness. Experimental results show that our hybrid electrocatalyst can selectively reduce CO2 to CO with a large faradic efficiency (∼90%), exceptional turnover frequency (4,900 hr−1 at η = 0.5 V), and excellent long-term durability. These metrics are superior to those of most of its organic or inorganic competitors. Its high electrocatalytic activity is also supported by density functional theory calculations.Download high-res image (220KB)Download full-size image

•A N,B-codoped carbon nanocage is reported for high efficiency multifunctional electrocatalyst.•DFT calculations reveal the catalytic compatibility of the N,B-codoping for ORR/OER/HER.•A primary zinc-air battery is assembled presenting a maximum power density of 320 mW cm−2.Nanocarbon materials recognized as effective and inexpensive catalysts for independent electrochemical reactions, are anticipated to possess a broader spectrum of multifunctionality toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). A rational design of trifunctional nanocarbon catalyst requires balancing the heteroatoms-doping and defect-engineering to afford desired active centers and satisfied electric conductivity, which however is conceptually challenging while desires in-depth research both experimentally and theoretically. This work reports a N,B-codoped graphitic carbon nanocage (NB-CN) with graphitic yet defect-rich characteristic as a promising trifunctional electrocatalyst through a facile thermal pyrolysis assisted in-situ catalytic graphitization (TPCG) process. Density functional theory (DFT) calculations are conducted, for the first time, to demonstrate that the best performance for ORR/OER and HER can be originated from the configuration with B meta to a pyridinic-N, which presents a minimum theoretical overpotential of 0.34 V for ORR, 0.39 V for OER, and a lowest Gibbs free-energy (ΔGads) of 0.013 eV for HER. A primary zinc-air battery is assembled presenting a maximum power density of 320 mW cm−2 along with excellent operation durability, evidencing great potential in practical applications.The N,B-codoped defect-rich graphitic carbon nanocages (NB-CN) are prepared through a technique of in situ vapor deposition. The NB-CN exhibits efficient multifunctional catalytic activity for ORR, OER and HER, shows potential practical performances in the fields of metal-air battery, fuel cell and water splitting.Download high-res image (200KB)Download full-size image

Transition metal carbides are promising electrode materials for electrochemical energy storage, yet to unveil their full potential requires judicious structural engineering at the nanoscale. In this study, we report a chrysanthemum-inspired nanoscale design to prepare a three-dimensional hierarchical molybdenum carbide hybrid. It consisted of an ensemble of numerous nanoflakes protruding out from the center, each formed by ultra-small (∼2 nm) α-MoC1−x nanoparticles uniformly supported on a N-doped carbonaceous support. Such a hybrid material has enlarged surface areas, shortened ionic diffusion length, great mechanical robustness, and buffer room for electrode volume change. Owing to the three-dimensional hierarchical arrangement, this hybrid material exhibits impressive performance toward active lithium-ion storage. It delivers a large reversible capacity of >1000 mA h g−1, great rate capacity with significant capacity at 10 A g−1, and excellent cycling stability with >95% capacity retention after 100 cycles at 500 mA g−1. Most impressively, we demonstrate that the structural integrity of the hybrid microflower is largely preserved even after prolonged cycling.

The reversible electrochemical storage of Li+ and Na+ ions is the operating basis of secondary lithium-ion and sodium-ion batteries. In recent years, there has been rapid growth in the search for appropriate electrode materials. Nevertheless, the development of host materials for active and durable electrochemical storage of both Li+ and Na+ ions remain challenging. In this study, we report a facile solvothermal method to prepare hierarchical assemblies of thin SnS2 nanosheets in N-methyl-2-pyrrolidone. The as-prepared product has an expanded layered structure due to the presence of organic intercalates. Mild annealing restores the normal 2H-SnS2 phase with the hierarchical architecture preserved. When annealed SnS2 was evaluated as the anode material of lithium-ion batteries, it exhibited large capacity in excess of 1200 mA h g−1 and decent short-term cycling stability. It was further coated with a thin carbon layer as the physical and electrical reinforcement, which led to a much improved cycle life at both low and high current rates. Moreover, carbon coated SnS2 also demonstrated a large capacity (∼600 mA h g−1) and decent cycling stability as the anode material of sodium-ion batteries.

Reversible electrochemical storage of alkali metal ions is the basis of many secondary batteries. Over years, various electrode materials are developed and optimized for a specific type of alkali metal ions (Li+, Na+, or K+), yet there are very few (if not none) candidates that can serve as a universal host material for all of them. Herein, a facile solvothermal method is developed to prepare VS2 nanosheet assemblies. Individual nanosheets are featured with a few atomic layer thickness, and they are hierarchically arranged with minimized stacking. Electrochemical measurements show that VS2 nanosheet assemblies enable the rapid and durable storage of Li+, Na+, or K+ ions. Most remarkably, the large reversible specific capacity and great cycling stability observed for both Na+ and K+ are extraordinary and superior to most existing electrode materials. The experimental results of this study are further supported by density functional theory calculations showing that the layered structure of VS2 has large adsorption energy and low diffusion barriers for the intercalation of alkali metal ions.

AbstractIncreasing CO2 concentration in the atmosphere is believed to have a profound impact on the global climate. To reverse the impact would necessitate not only curbing the reliance on fossil fuels but also developing effective strategies capture and utilize CO2 from the atmosphere. Among several available strategies, CO2 reduction via the electrochemical or photochemical approach is particularly attractive since the required energy input can be potentially supplied from renewable sources such as solar energy. In this Review, an overview on these two different but inherently connected approaches is provided and recent progress on the development, engineering, and understanding of CO2 reduction electrocatalysts and photocatalysts is summarized. First, the basic principles that govern electrocatalytic or photocatalytic CO2 reduction and their important performance metrics are discussed. Then, a detailed discussion on different CO2 reduction electrocatalysts and photocatalysts as well as their generally designing strategies is provided. At the end of this Review, perspectives on the opportunities and possible directions for future development of this field are presented.

AbstractConventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO2 into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.

AbstractConventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO2 into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.

Down to the wire: A silicon nanowire array photocathode mediates photoelectroreductive transformation of maleic and oxalic acids into building-block chemicals. In their Communication on page 7181 ff., Y. G. Li et al. report a large photovoltage generated from the semiconductor/electrolyte junction of the photocathode, which is used to offset the external bias required by electroreduction. The photoelectrosynthetic approach is selective for succinic and glyoxylic acids.

Auf Draht:Eine Siliciumnanodraht-Photokathode vermittelt die photoelektroreduktive Überführung von Malein- und Oxalsäuren in als Bausteine geeignete Chemikalien. In der Zuschrift auf S. 7287 ff. beschreiben Y. G. Li et al. eine vom Halbleiter-Elektrolyt-Kontakt der Photokathode erzeugte große Photospannung, mit der die externe Vorspannung, die für die Elektroreduktion benötigt wird, verschoben wird. Diese Photoelektrosynthese liefert selektiv Bernstein- und Glyoxalsäure.

Most electrocatalysts for the ethanol oxidation reaction suffer from extremely limited operational durability and poor selectivity toward the CC bond cleavage. In spite of tremendous efforts over the past several decades, little progress has been made in this regard. This study reports the remarkable promoting effect of Ni(OH)2 on Pd nanocrystals for electrocatalytic ethanol oxidation reaction in alkaline solution. A hybrid electrocatalyst consisting of intimately mixed nanosized Pd particles, defective Ni(OH)2 nanoflakes, and a graphene support is prepared via a two-step solution method. The optimal product exhibits a high mass-specific peak current of >1500 mA mg−1Pd, and excellent operational durability forms both cycling and chronoamperometric measurements in alkaline solution. Most impressively, this hybrid catalyst retains a mass-specific current of 440 mA mg−1 even after 20 000 s of chronoamperometric testing, and its original activity can be regenerated via simple cyclic voltammetry cycles in clean KOH. This great catalyst durability is understood based on both CO stripping and in situ attenuated total reflection infrared experiments suggesting that the presence of Ni(OH)2 alleviates the poisoning of Pd nanocrystals by carbonaceous intermediates. The incorporation of Ni(OH)2 also markedly shifts the reaction selectivity from the originally predominant C2 pathway toward the more desirable C1 pathway, even at room temperature.

The layered molybdenum disulfide (MoS2) nanostructured materials are of great interest for electrochemical energy storage and conversion and electrocatalytic water splitting. However, they still exhibit very limited performance because of their limited active sites. To create more efficient MoS2 materials, herein, we develop a simple yet efficient approach to a unique column-like MoS2 superstructure composed of edge-terminated MoS2 nanosheets (CLET MoS2). These MoS2 nanosheets as building blocks with fully exposed active edges are oriented in a preferred manner, rendering CLET MoS2 that exhibits excellent electrochemical performance in both lithium ion storage and hydrogen evolution reaction (HER). Compared with that of commercial MoS2, these hierarchical MoS2 superstructures possess much higher specific capacity and superior cycling performance for lithium ion storage and excellent electrocatalytic activity and stability for HER with a very low Tafel slope of 39 mV decade–1, showing their great potential applications in lithium ion batteries and water splitting.

Rechargeable sodium-ion batteries have been an active area of research over the past several years. While a great deal of attention is now focused on the development and evaluation of single electrode materials, much less is paid to their combined performance in full batteries. Most full batteries currently available suffer from rapid capacity fading under extended cycling. In this study, we prepare ultra-small, poorly crystalline FeOx nanoparticles supported on carbon nanotubes as the anode material for sodium-ion batteries. It exhibits excellent half-cell performances; and, when combined with a Prussian blue cathode material, it leads to iron-based full batteries. Our prototypes have a working voltage of ∼2 V, specific energy density of ∼136 W h kg−1 and, most impressively, outstanding cycling stability at both low and high current rates with negligible capacity loss. Owing to their low material and fabrication cost, long cycle life and high efficiency, we believe that these iron-based sodium-ion batteries would be highly appealing toward the stationary energy storage.

Over recent years, tremendous efforts have been invested in the search and development of active and durable semiconductor materials for photoelectrochemical (PEC) water splitting, particularly for photoanodes operating under a highly oxidizing environment. CuWO4 is an emerging candidate with suitable band gap and high chemical stability. Nevertheless, its overall solar-to-electricity remains low because of the inefficient charge separation process. In this work, we demonstrate that this problem can be partly alleviated through designing three-dimensional hierarchical nanostructures. CuWO4 nanoflake arrays on conducting glass are prepared from the chemical conversion of WO3 templates. Resulting electrode materials possess large surface areas, abundant porosity and small thickness. Under illumination, our CuWO4 nanoflake array photoanodes exhibit an anodic current density of ∼0.4 mA/cm2 at the thermodynamic potential of water splitting in pH 9.5 potassium borate buffer — the largest value among all available CuWO4-based photoanodes. In addition, we demonstrate that their performance can be further boosted to >2 mA/cm2 by coupling with a solution-cast BiVO4 film in a heterojunction configuration. Our study unveils the great potential of nanostructured CuWO4 as the photoanode material for PEC water oxidation.Keywords: charge separation; CuWO4; heterojunction; nanoflakes; photoelectrochemical water oxidation

Increasing attention has now been focused on the photoelectrochemical (PEC) hydrogen evolution as a promising route to transforming solar energy into chemical fuels. Silicon is one of the most studied PEC electrode materials, but its performance is still limited by its inherent PEC instability and electrochemical inertness toward water splitting. To achieve significant PEC activities, silicon-based photoelectrodes usually have to be coupled with proper cocatalysts, and thus, the formed semiconductor–cocatalyst interface presents a critical structural parameter in the rational design of efficient PEC devices. In this study, we directly grow nanostructured pyrite-phase nickel phosphide (NiP2) cocatalyst films on textured pn+-Si photocathodes via on-surface reaction at high temperatures. The areal loading of the cocatalyst film can be tailored to achieve an optimal balance between its optical transparency and electrocatalytic activity. As a result, our pn+-Si/Ti/NiP2 photocathodes demonstrate a great PEC onset potential of 0.41 V versus reversible hydrogen electrode (RHE), a decent photocurrent density of ∼12 mA/cm2 at the thermodynamic potential of hydrogen evolution, and an impressive operation durability for at least 6 h in 0.5 M H2SO4. Comparable PEC performance is also observed in 1 M potassium borate buffer (pH = 9.5) using this device.Keywords: acidic and neutral electrolytes; NiP2; on-surface synthesis; photoelectrochemical hydrogen evolution; silicon photocathode

Advances in lithium ion batteries would facilitate technological developments in areas from electrical vehicles to mobile communications. While two-dimensional systems like MoS2 are promising electrode materials due to their potentially high capacity, their poor rate capability and low cycle stability are severe handicaps. Here, we study the electrical, mechanical, and lithium storage properties of solution-processed MoS2/carbon nanotube anodes. Nanotube addition gives up to 1010-fold and 40-fold increases in electrical conductivity and mechanical toughness, respectively. The increased conductivity results in up to a 100× capacity enhancement to ∼1200 mAh/g (∼3000 mAh/cm3) at 0.1 A/g, while the improved toughness significantly boosts cycle stability. Composites with 20 wt % nanotubes combine high reversible capacity with excellent cycling stability (e.g., ∼950 mAh/g after 500 cycles at 2 A/g) and high rate capability (∼600 mAh/g at 20 A/g). The conductivity, toughness, and capacity scale with nanotube content according to percolation theory, while the stability increases sharply at the mechanical percolation threshold. We believe that the improvements in conductivity and toughness obtained after addition of nanotubes can be transferred to other electrode materials, such as silicon nanoparticles.Keywords: anode; mechanical; network; percolating

The development of nonprecious metal based electrocatalysts for hydrogen evolution reaction (HER) has received increasing attention over recent years. Previous studies have established Mo2C as a promising candidate. Nevertheless, its preparation requires high reaction temperature, which more than often causes particle sintering and results in low surface areas. In this study, we show supporting Mo2C nanoparticles on the three-dimensional scaffold as a possible solution to this challenge and develop a facile two-step preparation method for ∼3 nm Mo2C nanoparticles uniformly dispersed on carbon microflowers (Mo2C/NCF) via the self-polymerization of dopamine. The resulting hybrid material possesses large surface areas and a fully open and accessible structure with hierarchical order at different levels. MoO42– was found to play an important role in inducing the formation of this morphology presumably via its strong chelating interaction with the catechol groups of dopamine. Our electrochemical evaluation demonstrates that Mo2C/NCF exhibits excellent HER electrocatalytic performance with low onset overpotentials, small Tafel slopes, and excellent cycling stability in both acidic and alkaline solutions.Keywords: hybrid electrocatalyst; hydrogen evolution reaction; molybdenum carbide; polydopamine; three-dimensional hierarchical structure;

The search for high-capacity, low-cost electrode materials for sodium-ion batteries is a significant challenge in energy research. Among the many potential candidates, layered compounds such as MoS2 have attracted increasing attention. However, such materials have not yet fulfilled their true potential. Here, we show that networks of liquid phase exfoliated MoS2 nanosheets, reinforced with 20 wt % single-wall carbon nanotubes (SWNTs), can be formed into sodium-ion battery electrodes with large gravimetric, volumetric, and areal capacity. The MoS2/SWNT composite films are highly porous, electrically conductive, and mechanically robust due to its percolating carbon nanotube network. When directly employed as the working electrode, they exhibit a specific capacity of >400 mAh/g and volumetric capacity of ∼650 mAh/cm3. Their mechanical stability allows them to be processed into free-standing films with tunable thickness up to ∼100 μm, corresponding to an areal loading of 15 mg/cm2. Their high electrical conductivity allows the high volumetric capacity to be retained, even at high thickness, resulting in state-of-the-art areal capacities of >4.0 mAh/cm2. Such values are competitive with their lithium-ion counterparts.Keywords: carbon nantube percolation; free-standing membranes; liquid phase exfoliation; MoS2 nanosheets; sodium-ion batteries; volumetric and areal capacities

The development of non precious metal based electrocatalysts for the hydrogen evolution reaction (HER) holds a decisive key to a spectrum of energy conversion technologies. Previous studies have established layered molybdenum chalcogenides as promising candidates. In this work, we prepared ultrathin MoS2(1–x)Se2x alloy nanoflakes with monolayer or few-layer thickness and fully tunable chemical composition for maximum HER activity. Spectroscopic characterizations corroborate the progressive evolution of their structures and properties as x increases from 0 to 1 without any noticeable phase separation. In particular, it is evidenced that the introduction of selenium continuously modulates the d band electronic structure of molybdenum, probably leading to tuned hydrogen adsorption free energy and consequently electrocatalytic activity. Electrochemical measurements show that all MoS2(1–x)Se2x nanoflakes are highly active and durable for HER with small overpotentials in the range of 80–100 mV and negligible activity loss up to 10000 cycles. Most importantly, alloyed nanoflakes, especially with the chemical composition of MoSSe, exhibit improved performance in comparison to either MoS2 or MoSe2. Given their overall similar nanoflake morphologies, we believe such improvements reflect the higher intrinsic activity of alloyed catalysts with the hydrogen adsorption free energy closer to thermoneutral.Keywords: alloys; electrocatalysis; hydrogen evolution reaction; molybdenium chalcogenides; ultrathin nanoflakes

Research on sodium ion batteries has recently been revived. Attention is now placed on the development of high-capacity and stable electrode materials at low costs. Among them, compounds operating on the conversion mechanism represent a promising class of anode materials. Unfortunately, they are generally plagued by poor electrical conductivity and large volume changes during repeated cycling. In this study, we exploit a new type of composite material made of copper phosphide and Super P carbon black (CuP2/C) as a potential anode candidate. The final products consisted of crystalline CuP2 cores coated with carbon black nanoparticles on the surface. Electrochemical measurements and multiple ex situ studies demonstrate that CuP2/C composites are capable of fast and reversible sodiation and desodiation based on the conversion mechanism. They deliver a large capacity in excess of 500 mA h g−1, high rate capability and decent short-term cycling stability. Our study suggests that these transition metal phosphides with a suitable carbon coating may hold great opportunities as anode materials for sodium ion batteries for effective and economical energy storage.

There have been growing efforts to search for active, robust and cost-effective electrocatalysts for the oxygen evolution reaction (OER). Among the different candidates, Ni–Fe layered double hydroxides (LDHs) hold great promise due to their high activity closely approaching or even outperforming that of the precious metal benchmark in alkaline media. Here, we show that their activity can be further promoted when forming ultrathin LDH nanosheets intercalated with molybdate ions via an exfoliation-free hydrothermal method. In 1 M KOH, these nanosheets exhibit about 3-fold higher OER current density than regular NiFe LDH nanosheets, which was believed to be mostly contributed by their higher available density of electrochemically active sites associated with the ultrathin thickness. The great activity is also accompanied by remarkable durability at different current density levels. Finally, we demonstrate that these ultrathin nanosheets can also be directly grown on Ni foam for achieving significant current densities.

The hydrogen evolution reaction plays a decisive role in a range of electrochemical and photoelectrochemical devices. It requires efficient and robust electrocatalysts to lower the reaction overpotential and minimize energy consumption. Over the last decade, we have witnessed a rapid rise in new electrocatalysts, particularly those based on non-precious metals. Some of them approach the activity of precious metal benchmarks. Here, we present a comprehensive overview of the recent developments of heterogeneous electrocatalysts for the hydrogen evolution reaction. Detailed discussion is organized from precious metals to non-precious metal compounds including alloys, chalcogenides, carbides, nitrides, borides and phosphides, and finally to metal-free materials. Emphasis is placed on the challenges facing these electrocatalysts and solutions for further improving their performance. We conclude with a perspective on the development of future HER electrocatalysts.

Silicon (Si) is an important material in photoelectrochemical (PEC) water splitting because of its good light-harvesting capability as well as excellent charge-transport properties. However, the shallow valence band edge of Si hinders its PEC performance for water oxidation. Generally, thanks to their deep valence band edge, metal oxides are incorporated with Si to improve the performance, but they also decrease the transportation of carriers in the electrode. Here, we integrated a ferroelectric poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] layer with Si to increase the photovoltage as well as the saturated current density. Because of the prominent ferroelectric property from P(VDF–TrFE), the Schottky barrier between Si and the electrolyte can be facially tuned by manipulating the poling direction of the ferroelectric domains. The photovoltage is improved from 460 to 540 mV with a forward-poled P(VDF–TrFE) layer, while the current density increased from 5.8 to 12.4 mA/cm2 at 1.23 V bias versus reversible hydrogen electrode.Keywords: dipole; ferroelectric; photoelectrochemical; silicon; surface polarization; water oxidation

•One of few reports about polymer materials for aprotic supercapacitors.•Hybridizing with Ketjenblack NPs improves the cycling stability of polymer.•Composite materials have large energy and power density.•Organic electrode materials are potentially renewable and low cost.Currently very few non-aqueous pseudocapacitor electrode materials are available. Organic matter based electrodes hold a great promise but remain largely unexplored. In this work, Ketjenblack carbon supported polyanthraquinone with different carbon to polymer ratios was prepared and evaluated as the pseudocapacitor electrode material in acetronitrile. These composite electrodes exhibit large specific capacitance up to 650 F/g and specific capacity up to 270 mAh/g when normalized to the polymer mass, which is much greater than other non-aqueous supercapacitor materials. They also have improved cycling stability with more than 85% capacitance retention after 1000 cycles as a result of the diminished quinone solubility in the electrolyte. When paired with a graphene-based positive electrode in asymmetric supercapacitors, composite electrodes demonstrate significant energy density up to 45.5 Wh/kg and power density up to 21.4 kW/kg. Their great supercapacitive performance, together with their low costs as well as structural and property diversities, makes them highly appealing for future energy storage applications.

Zinc–air is a century-old battery technology but has attracted revived interest recently. With larger storage capacity at a fraction of the cost compared to lithium-ion, zinc–air batteries clearly represent one of the most viable future options to powering electric vehicles. However, some technical problems associated with them have yet to be resolved. In this review, we present the fundamentals, challenges and latest exciting advances related to zinc–air research. Detailed discussion will be organized around the individual components of the system – from zinc electrodes, electrolytes, and separators to air electrodes and oxygen electrocatalysts in sequential order for both primary and electrically/mechanically rechargeable types. The detrimental effect of CO2 on battery performance is also emphasized, and possible solutions summarized. Finally, other metal–air batteries are briefly overviewed and compared in favor of zinc–air.

Although great recent efforts have been invested to improve the performance of supercapacitors, these energy storage devices still fall short of meeting our expectations because of their limited working voltage, insufficient cycle life, and high manufacturing cost. Here, we report the facile preparation of cobalt hexacyanoferrate (CoHCFe) nanoparticles, which have an analogous structure to Prussian blue but with many vacant ferricyanide sites. In 0.5 M Na2SO4, CoHCFe exhibits specific capacitance of >250 F/g, excellent rate capability, and ultrahigh cycling stability with capacitance retention of 93.5% after 5000 cycles. Furthermore, CoHCFe was paired up with a carbon black modified graphene (mRGO) negative electrode to form asymmetric supercapacitors. They deliver a wide working voltage of ∼2.4 V in Na2SO4, large energy density and power density. Given its high electrochemical performance, chemical robustness, environmental benignity, ease of preparation and low cost, CoHCFe as well as other Prussian blue analogues clearly deserve more attention for future energy storage applications.Keywords: asymmetric supercapacitors; cobalt hexacyanoferrate; energy storage; Prussian blue

Transition metal carbides are promising electrode materials for electrochemical energy storage, yet to unveil their full potential requires judicious structural engineering at the nanoscale. In this study, we report a chrysanthemum-inspired nanoscale design to prepare a three-dimensional hierarchical molybdenum carbide hybrid. It consisted of an ensemble of numerous nanoflakes protruding out from the center, each formed by ultra-small (∼2 nm) α-MoC1−x nanoparticles uniformly supported on a N-doped carbonaceous support. Such a hybrid material has enlarged surface areas, shortened ionic diffusion length, great mechanical robustness, and buffer room for electrode volume change. Owing to the three-dimensional hierarchical arrangement, this hybrid material exhibits impressive performance toward active lithium-ion storage. It delivers a large reversible capacity of >1000 mA h g−1, great rate capacity with significant capacity at 10 A g−1, and excellent cycling stability with >95% capacity retention after 100 cycles at 500 mA g−1. Most impressively, we demonstrate that the structural integrity of the hybrid microflower is largely preserved even after prolonged cycling.

Zinc–air is a century-old battery technology but has attracted revived interest recently. With larger storage capacity at a fraction of the cost compared to lithium-ion, zinc–air batteries clearly represent one of the most viable future options to powering electric vehicles. However, some technical problems associated with them have yet to be resolved. In this review, we present the fundamentals, challenges and latest exciting advances related to zinc–air research. Detailed discussion will be organized around the individual components of the system – from zinc electrodes, electrolytes, and separators to air electrodes and oxygen electrocatalysts in sequential order for both primary and electrically/mechanically rechargeable types. The detrimental effect of CO2 on battery performance is also emphasized, and possible solutions summarized. Finally, other metal–air batteries are briefly overviewed and compared in favor of zinc–air.

Rechargeable sodium-ion batteries have been an active area of research over the past several years. While a great deal of attention is now focused on the development and evaluation of single electrode materials, much less is paid to their combined performance in full batteries. Most full batteries currently available suffer from rapid capacity fading under extended cycling. In this study, we prepare ultra-small, poorly crystalline FeOx nanoparticles supported on carbon nanotubes as the anode material for sodium-ion batteries. It exhibits excellent half-cell performances; and, when combined with a Prussian blue cathode material, it leads to iron-based full batteries. Our prototypes have a working voltage of ∼2 V, specific energy density of ∼136 W h kg−1 and, most impressively, outstanding cycling stability at both low and high current rates with negligible capacity loss. Owing to their low material and fabrication cost, long cycle life and high efficiency, we believe that these iron-based sodium-ion batteries would be highly appealing toward the stationary energy storage.

Research on sodium ion batteries has recently been revived. Attention is now placed on the development of high-capacity and stable electrode materials at low costs. Among them, compounds operating on the conversion mechanism represent a promising class of anode materials. Unfortunately, they are generally plagued by poor electrical conductivity and large volume changes during repeated cycling. In this study, we exploit a new type of composite material made of copper phosphide and Super P carbon black (CuP2/C) as a potential anode candidate. The final products consisted of crystalline CuP2 cores coated with carbon black nanoparticles on the surface. Electrochemical measurements and multiple ex situ studies demonstrate that CuP2/C composites are capable of fast and reversible sodiation and desodiation based on the conversion mechanism. They deliver a large capacity in excess of 500 mA h g−1, high rate capability and decent short-term cycling stability. Our study suggests that these transition metal phosphides with a suitable carbon coating may hold great opportunities as anode materials for sodium ion batteries for effective and economical energy storage.

There have been growing efforts to search for active, robust and cost-effective electrocatalysts for the oxygen evolution reaction (OER). Among the different candidates, Ni–Fe layered double hydroxides (LDHs) hold great promise due to their high activity closely approaching or even outperforming that of the precious metal benchmark in alkaline media. Here, we show that their activity can be further promoted when forming ultrathin LDH nanosheets intercalated with molybdate ions via an exfoliation-free hydrothermal method. In 1 M KOH, these nanosheets exhibit about 3-fold higher OER current density than regular NiFe LDH nanosheets, which was believed to be mostly contributed by their higher available density of electrochemically active sites associated with the ultrathin thickness. The great activity is also accompanied by remarkable durability at different current density levels. Finally, we demonstrate that these ultrathin nanosheets can also be directly grown on Ni foam for achieving significant current densities.

The hydrogen evolution reaction plays a decisive role in a range of electrochemical and photoelectrochemical devices. It requires efficient and robust electrocatalysts to lower the reaction overpotential and minimize energy consumption. Over the last decade, we have witnessed a rapid rise in new electrocatalysts, particularly those based on non-precious metals. Some of them approach the activity of precious metal benchmarks. Here, we present a comprehensive overview of the recent developments of heterogeneous electrocatalysts for the hydrogen evolution reaction. Detailed discussion is organized from precious metals to non-precious metal compounds including alloys, chalcogenides, carbides, nitrides, borides and phosphides, and finally to metal-free materials. Emphasis is placed on the challenges facing these electrocatalysts and solutions for further improving their performance. We conclude with a perspective on the development of future HER electrocatalysts.