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The development of earth-abundant-metal-based electrocatalysts with high efficiency and long-term stability for hydrogen evolution reaction (HER) is crucial for the clean and renewable energy application. Herein, we report a molten-salt method to synthesize Co-doped CaMn3O6 (CMO) nanowires (NWs) as effective electrocatalyst for HER. The as-obtained CaMn3−xCoxO6 (CMCO) exhibits a small onset overpotential of 70 mV, a required overpotential of 140 mV at a current density of 10 mA·cm−2, a Tafel slope of 39 mV·dec−1 in 0.1 M HClO4, and a satisfying long-term stability. Experimental characterizations combined with density functional theory (DFT) calculations demonstrate that the obtained HER performance can be attributed to the Co-doping which altered CMO’ s surface electronic structures and properties. Considering the simplicity of synthesis route and the abundance of the pertinent elements, the synthesized CMCO shows a promising prospect as a candidate for the development of earth-abundant, metal-based, and cost-effective electrocatalyst with superior HER activity. Our results also establish a strategy of rational design and construction of novel electrocatalyst toward HER by tailoring band structures of transition metal oxides (TMOs).

Two-dimensional (2D) layered MnO2 materials, composed of exotic electronic properties and accessible active sites with alkali metal ions, provide a comprehensive platform for developing catalysts w...

Transition metal carbides (TMCs) have attracted great interest due to their potential for application in energy conversion and electrocatalysis. However, their nanostructures are difficult to control, especially in the case of TMC nanodots since their growth process usually requires high carburization temperature which could cause easy agglomeration of small size nanoparticles. Here, we develop a microwave combustion method for rapid (∼2 min) synthesis of TMC nanodots with tunable size from ∼3 to ∼40 nm under ambient conditions. These TMC nanodots exhibit outstanding catalytic performance for the hydrogen evolution reaction (HER) with low onset overpotentials of 19 and 27 mV vs. RHE as well as small Tafel slopes of 39 and 45 mV dec−1 in H2SO4 and KOH, respectively. This work highlights a novel strategy for the design and synthesis of transition metal carbides.

Conducting polymer nanosheets are promising for many intriguing applications owing to their unique optical and electrical properties. However, it is still challenging to fabricate size- and thickness-tunable conducting polymer nanosheets with the feature of large scalability in a facile manner. Here, we developed a new approach for the massive synthesis of various conducting polymer nanosheets (polypyrrole (PPy), polyaniline (PANi), polyethylene dioxythiophene (PEDOT)) at a gas–solid interface between the monomeric gas and CuCl2·2H2O salt. The thickness of the as-obtained polymer nanosheets can be tuned from ∼2.1 to ∼8.8 nm. Furthermore, a highly conductive, flexible and large-area (∼60 cm2) PPy film with a transparency of ∼85% was prepared on the surface of the CuCl2·2H2O film. The PPy nanosheet-assembled free-standing electrode exhibited a high volumetric capacitance of ∼320 F cm−3 and great rate capability. This facile and cost-effective method could massively produce conducting polymer nanosheets, and is promising for potential practical applications.

Manganese oxides (δ-MnO2) with a layered structure are considered as potential electrode materials due to the presence of an extra ion transport channel between interlayers. However, their dissolution and structure collapse during the electrochemical charge and discharge process usually lead to rapid capacity fading. In this study, we designed a metal-ion (Ba2+, Sn2+, Co3+ and Ni2+)-doped two dimensional (2D) δ-MnO2 to improve the stability of the layered structure. Both the experimental and the theoretical calculations indicated that Co3+ and Ni2+-doped 2D δ-MnO2 (2D Co–KMnO and Ni–KMnO, respectively), obtained via a substitutional doping route (by replacing the Mn atom), could efficiently stabilize the layered structure. Moreover, the 2D Co–KMnO electrode material exhibited a high specific capacitance (342.7 F g−1) and excellent cycling performance, with 95.3% capacitance retention after 20 000 cycles at the current density of 5 A g−1. This study revealed the mechanism of the effect of the doping behavior of metal ions on the crystal stability of metal oxides, possibly paving a new path for the rational design of other robust functional nanomaterials.

Polypyrrole (PPy) is a highly conductive polymer and promising pseudocapacitive material for energy storage. However, the poor stability of PPy based supercapacitors (SCs) limits their applications. Here we demonstrate a facile dipping process to uniformly grow 4-butylbenzenesulfonate modified PPy film on a filter paper substrate. The flexible and free-standing PPy paper electrode demonstrated excellent cycling stability with capacitance retention of ~84.8% after 200,000 cycles. We believe that the 4-butylbenzenesulfonate modification could increase the order degree of PPy via layer-by-layer stacked α-α coupled chains, leading to a robust structure under the charging and discharging progress. This work pushes the cycling limitation of PPy based SCs and realizes their potentials for practical applications.

Pollution treatment, a problem our world is being deeply puzzled by, has required large amounts of energy during enrichment and degradation. However, some pollutants, for instance organics in wastewater, could offer an energy supply but instead are wasted. Here we report an energy harvesting galvanic cell, built by using a Pt foil as an anode and 2D K+-intercalated MnO2 as a cathode, which combines both dye degradation in wastewater and energy harvesting during the degradation process. Owing to the galvanic effect, this cell could accelerate the degradation rate and indicate the progress of degradation. Different kinds of organics could be degraded and produce energy in this cell with a stable open-circuit voltage (0.45 V). Magnification and imitation of this strategy offer a new chance to harvest waste energy in other exothermic reactions.

Two-dimensional (2D) transition-metal nitrides just recently entered the research arena, but already offer a potential for high-rate energy storage, which is needed for portable/wearable electronics and many other applications. However, a lack of efficient and high-yield synthesis methods for 2D metal nitrides has been a major bottleneck for the manufacturing of those potentially very important materials, and only MoN, Ti4N3, and GaN have been reported so far. Here we report a scalable method that uses reduction of 2D hexagonal oxides in ammonia to produce 2D nitrides, such as MoN. MoN nanosheets with subnanometer thickness have been studied in depth. Both theoretical calculation and experiments demonstrate the metallic nature of 2D MoN. The hydrophilic restacked 2D MoN film exhibits a very high volumetric capacitance of 928 F cm–3 in sulfuric acid electrolyte with an excellent rate performance. We expect that the synthesis of metallic 2D MoN and two other nitrides (W2N and V2N) demonstrated here will pro...

Paper-like electrodes with high conductivity and flexibility hold great potential for assembling high-performance flexible electronic devices. In this study, a flexible conductive film was fabricated via vacuum filtration using highly conductive MoO3−x ultralong nanobelts. This film has low sheet resistance of 5.1 Ω sq−1 and exhibits a stable three-dimensional structure even under 1000 times of bending test. Significantly, this free-standing film has a high volumetric capacitance of 652 F cm−3. Moreover, a symmetric device based on this electrode demonstrates good cycling stability with a capacitance retention of 85.7% after 25000 cycles. We anticipate that this strategy can also be applied to other three-dimensional flexible porous films based on one-dimensional conductive nanostructure, which could open up new opportunities for energy storage and conversion.

Developing eco-friendly and low-cost electronics is an effective strategy to address the electronic waste issue. In this study, transparent cellulose nanopaper (T-paper) and polylactic acid (PLA) electret were used to construct a biodegradable and transparent paper-based electret nanogenerator. The nanogenerator could be assembled with paper products to form a self-powered smart packaging system without impairing the appearance, due to the high transparency and desirable output performance. Furthermore, the self-degradation property in the natural soil of the nanogenerator is demonstrated, indicating that the nanogenerator is recycled and will not pollute the environment. We anticipate that this study will provide new insights to develop eco-friendly power source and paper-based electronics.

The introduction of oxygen vacancy for transition metal oxide is an effective method to tune their conductivity. Here, microwave combustion method is used to quickly synthesize reduced metal oxides and graphene hybrid composite in several minutes. Among these as-synthesized composites, the reduced orthorhombic Nb2O5 and graphene oxide (rNb2O5/rGO) composite demonstrates great electrochemical performance with a reversible specific capacity of 726.2 C g−1 at 2 mV s−1 in organic electrolyte as well as a good capacity retention of 87% after 3000 cycles at 10 A g−1. It is believed that this strategy can be a common route to quickly produce oxygen vacancy in oxides and satisfy much more application requirements even for the possibility of industrialization.

The development of portable electronics strongly requires flexible, lightweight, and inexpensive energy-storage devices with high power density, long cycling stability, and high reliability. In this work, we prepare a flexible solid-state electrochemical capacitor using cross-linked hierarchical porous carbon network as electrode material via electrospinning and carbonization process. This device can reversibly deliver a maximum energy density of 10.18 W h/kg with excellent cycling stability which achieves 95% capacitance retention after 20000 charge/discharge cycles. Moreover, it also demonstrates outstanding mechanical flexibility and excellent capacitance retention even when the device is repeatedly bended 10000 cycles under 90°. All of these results suggest its promising perspective in flexible energy storage device.

Orthorhombic molybdenum trioxide (α-MoO3) is a layered oxide with promising performance as electrode material for Li-ion capacitors. In this study, we show that expansion of the interlayer spacing (by ~0.32 A) of the structure along the b-axis, introduced by partial reduction of α-MoO3 and formation of MoO3−x (x=0.06–0.43), results in enhanced diffusion of Li ions. Binder-free hybrid electrodes made of MoO3−x nanobelts and carbon nanotubes show excellent electrical conductivity. The combination of increased interlayer spacing and enhanced electron transport leads to high gravimetric and volumetric capacitances of about 420 F/g or F/cm3 and excellent cycle life of binder-free MoO3−x electrodes.

Two-dimensional atomic crystals, such as two-dimensional oxides, have attracted much attention in energy storage because nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. However, current strategies are largely limited to intrinsically layered compounds. Here we report a general strategy that uses the surfaces of water-soluble salt crystals as growth templates and is applicable to not only layered compounds but also various transition metal oxides, such as hexagonal-MoO3, MoO2, MnO and hexagonal-WO3. The planar growth is hypothesized to occur via a match between the crystal lattices of the salt and the growing oxide. Restacked two-dimensional hexagonal-MoO3 exhibits high pseudocapacitive performances (for example, 300 F cm−3 in an Al2(SO4)3 electrolyte). The synthesis of various two-dimensional transition metal oxides and the demonstration of high capacitance are expected to enable fundamental studies of dimensionality effects on their properties and facilitate their use in energy storage and other applications., Two-dimensional atomic crystals are known to be effective electrode materials for energy storage applications. Here, the authors report the preparation of various two-dimensional metal oxides, including those which do not have a layered parent structure, via a salt templating strategy.

Band gap engineering was achieved by in situ doping method for high electrical conductivity and chemical activity of MnO2. By in situ releasing and adsorption during electrodeposition, Al3+ with close ion radius to Mn4+ could replace the position of Mn4+ in MnO2. The in situ doping process brings impurity level in MnO2 and changes the energy band structure. The narrower band gap of MnO2 after Al doping with higher electron concentration on conduction band could improve the conductivity of MnO2. The specific capacitance of Al-doped MnO2 achieves 430.6 F g−1 which is almost 2.5 times of the capacitance of initial MnO2 (177 F g−1), shedding light on its practical applications.

Transition-metal phosphides (TMPs) are considered as promising non-noble electrochemical catalysts for hydrogen evolution reaction (HER). Their highly active sites are located on certain facets, and single crystalline two-dimensional (2D) structures enable them to expose the most active facets for HER. However, the synthesis of single crystalline 2D TMPs is still a challenge owing to their intrinsically non-layered structures. Herein, we demonstrate the synthesis of various single crystalline 2D TMPs (Co2P, MoP2, Ni12P5 and WP2) by a salt-templating method. The as-synthesized 2D Co2P exhibited efficient electrocatalytic ability for HER with an overpotential of 41 mV at 10 mA cm−2 and a Tafel slope of 35 mV dec−1 in 0.5 M H2SO4 solution. We expect that the synthesis of 2D TMPs reported here will open the way to expand the family of 2D materials for electrocatalysis and beyond.

Intercalation capacitance is emerging as a promising approach to increasing energy density of electrochemical capacitors. However, the number of materials capable of intercalating ions other than small H+ and Li+ is limited. Here we report a K+ preintercalated hydrogenated MoO3 (KyMoO3−x) with enhanced conductivity and widened interlayer gaps. Different cations (Mg2+, Na+, K+ and Li+) were able to intercalate along the (010) facet of KyMoO3−x with high intercalation capacitance (374 F/cm3 at 0.5 A/g in 5 M LiCl). The cation intercalation behavior and the origin of the high capacitance have been systematically studied. Using artificial seawater as electrolyte, high volumetric capacitance (188 F/cm3 at 0.5 A/g) and good rate handling were also achieved. The KyMoO3−x electrode also showed a similar capacitance (196 F/cm3 at 0.5 A/g) and cycling performance (89% of initial capacitance maintained after 10,000 cycles) in natural seawater. When KyMoO3−x was fabricated into a symmetric pseudocapacitor with natural seawater as the electrolyte, the energy density and power density of the electrode reached 0.91 mWh/cm3 and 6.5 W/cm3, respectively. The impressive results indicate KyMoO3−x is a promising high-power handling pseudocapacitor electrode material.

Flexible, cross-linked N-doped carbon nanofiber network (CLCF) is successfully prepared via electrospinning and carbonization process. CLCF shows a specific capacitance of 175 F/g at the current density of 50 A/g with an excellent rate capability of 78.5% of its initial capacitance from 0.5 A/g to 50 A/g under the mass loading of 1.2 mg/cm2. Supercapacitor device based on CLCF exhibits high energy density of 5.9 W h/kg with a power density of 1200 W/kg, as well as excellent cycling stability which achieves 106% capacitance retention after 20,000 charge–discharge cycles. Solid-state supercapacitor device displays outstanding mechanical flexibility and no obvious capacitance loss can be observed even when the device was bended to 180°. Moreover, as scaffold for supporting PANi, capacitance retention of CLCF/PANi can reach 72% with the current density range from 0.5 to 50 A/g. All the results suggest that CLCF has great potential for application not only as supercapacitor electrode but also as scaffold for supporting capacitive materials.

Ultrathin two-dimensional (2D) crystals have been predicted to have high electrochemical activity because nearly all active atoms are exposed to the electrolytes, which offers great potential for energy storage. However, to construct layered structure metal oxides, simplifying the synthetic methods and improving the electronic conductivity remain a challenge. Herein, we synthesized 2D vanadium doped manganese oxides through a facile hydrothermal method. Vanadium dopant is also used as a template agent for the formation of nanosheet-shaped MnO2, further leading to high specific surface area as well as significant enhancement of the electronic conductivity, as confirmed by the first-principle calculations and four-point probe method. For the sake of a shortened ion transport distance and enhanced electronic conductivity, V-doped MnO2 nanosheets display an excellent electrochemical performance as a supercapacitor electrode.

The Al-doped α-MnO2 was prepared by hydrothermal method could promote the conductivity and improve the capacitance, which demonstrated by theoretical calculation and experiments. The Al doped α-MnO2 electrode with a specific capacitance of about 213 F g−1 and 146 F cm−3 under the high mass loading of ~4 mg cm−2 had excellent cycling performance that of ~91% maintained after 15,000 cycles. In addition, different doping amount of Al as contrast were discussed, indicating that through modification and optimization of the chemical composition of the α-MnO2 based electrode material by Al doping could greatly improve the electrochemical behavior and to achieve higher specific capacitance. Also, we believed that the notable cycling life and high mass loading of Al doped α-MnO2 electrode material was beneficial to the application of supercapacitors. As an energy storage device, a high energy density about 2.42 mW h cm−3 with power density of 5.72 W cm−3 could be achieved.

Pseudocapacitors, which store charge through fast reversible redox reactions occuring at the surface of the electrode, could offer higher specific capacitance than electrochemical double-layer capacitors (EDLCs). However, the prime pseudocapacitive phase, transition metal oxides usually have poor electronic conductivity, limiting their practical application. Here, we fabricated highly conductive HxMoO3−y nanobelts as electrode materials for pseudocapacitors. This electrode has demonstrated great electrochemical activity in various aqueous cation solutions with the highest volumetric capacitance of over 350 F cm−3. Significantly, this electrode has high specific capacitance as well as excellent cycling stability in sea water. Moreover, this hybrid composite also exhibits remarkable ion electrosorptive capacity (4 mg g−1) as a desalination electrode.

Activated carbon (AC) was prepared via carbonizing melaleuca bark in an argon atmosphere at 600 °C followed with KOH activation for high-rate supercapacitors. This AC electrode has a high capacitance of 233 F g(-1) at a scan rate of 2 mV s(-1) and an excellent rate capability of ∼80% when increasing the sweep rate from 2 to 500 mV s(-1). The symmetric supercapacitor assembled by the above electrode can deliver a high energy density of 4.2 Wh kg(-1) with a power density of 1500 W kg(-1) when operated in the voltage range of 0-1 V in 1 M H2SO4 aqueous electrolyte while maintaining great cycling stability (less than 5% capacitance loss after 10 000 cycles at sweep rate of 100 mV s(-1)). All the outstanding electrochemical performances make this AC electrode a promising candidate for potential energy storage application.

Molybdenum carbide (Mo2C) has been widely applied in energy conversion, electrocatalysis, and other electronic devices, but its nanostructure with certain morphology and porous structure is tough to control. In this work, 1D or 2D porous Mo2C nanostructures can be synthesized by carburizing cobalt-based zeolitic imidazolate framework (ZIF-67) cladding MoO3 nanowires or nanosheets hybrid structure under high temperature. The obtained 2D porous Mo2C shows a low onset overpotential (η = 25 and 36 mV) and a small Tafel slope (40 and 47 mV dec−1) in 0.1 m HClO4 and 0.1 m KOH as well as great stability. This work highlights a new strategy for the design and synthesis of porous nanostructure Mo2C electrocatalysts.

A visible-light driven photocatalytic fuel cell (PFC) system comprised of Ag/TiO2-NTs photoanode and Cu2O/TiO2/Ti photocathode was established for providing a self-sustained and energy-saving way for simultaneous wastewater treatment and energy recovery

A ternary Ag/SiO2/TiO2 nanotube array film was synthesized by a two-step approach. The dramatically enhanced photocatalytic activity of the film was evaluated via the inactivation of pathogenic bacteria under visible light irradiation.