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Potassium ion batteries or capacitors are a promising technology for large-scale energy storage due to the abundant resource and low cost of potassium. However, the development of stable electrode materials with high capacity, capable rate ability, and excellent cycling stability remains a challenge. Herein, carbon dots modified reduced graphene oxides (LAP-rGO-CDs) are designed and synthesized via a rapid and green microwave-assisted method with l -Ascorbic acid 6-palmitate (LAP) as the reducing agent. LAP-rGO-CDs present enlarged interlayer spacing and faster ion transfer rate owing to the introducing of carbon dots. Serving as a potassium ion battery electrode, LAP-rGO-CDs showed a high specific capacity of 299 mAh g−1 at 1 A g−1 and excellent cycling stability. Moreover, a LAP-rGO-CDs//AC full carbon-based potassium ion capacitor is assembled and displays a maximum energy density of 119 Wh kg−1 and a power density of 5352 W kg−1. This work demonstrates the potential application of LAP-rGO-CDs for high-performance potassium ion storage.

Binary metal niobium oxides can offer a higher specific capacity compared to niobium pentoxide (Nb2O5) and thus are ideal anode candidates for lithium ion capacitors (LICs). However, their lower electronic conductivity limits their ability to achieve high energy and power densities. In this paper, one-dimensional (1D) copper niobate (CuNb2O6) nanowires are successfully prepared by electrospinning technology and then immobilized on two-dimensional (2D) reduced graphene oxide (rGO) nanosheets to form a unique 1D nanowire/2D nanosheet CuNb2O6/rGO structure. The 1D/2D CuNb2O6/rGO electrode exhibits a high specific capacity of 312.2 mAh g−1 at 100 mA g−1 as the anode of LICs. The proposed Li+ storage mechanism of the CuNb2O6 anode involves CuNb2O6 decomposition into lithium niobate (Li3NbO4) and copper (Cu) during the initial lithium insertion process. The intercalation-type Li3NbO4 will further serve as the host to Li+ and the inactive Cu phase will act as a conductive network for electron transportation. Furthermore, the energy density of the assembled CuNb2O6/rGO//activated carbon (CuNb2O6/rGO//AC) device could achieve a value as high as 92.1 Wh kg−1 and could thus be considered as a possible alternative electrode material for high energy and power LICs.

Electrocatalytic water splitting used for generating clean and sustainable hydrogen (H2) can be very promising to address current energy shortage and associated environmental issues. However, this methodology is severely impeded by the tardy oxygen evolution reaction (OER). Hence, designing a preferable kinetics and thermodynamics oxidation reaction that supersede OER is very significant for the energy-saving production of H2. Herein, hollow needle-like copper cobalt sulfide was constructed on carbon cloth (CuCo2S4/CC) as a bifunctional electrocatalyst to accelerate H2 generation and simultaneously convert ethanol into value-added acetic acid. Thanks to the synergistic effect and unique structure of Cu and Co, CuCo2S4/CC displays superior catalytic activity and durability in ethanol oxidation reaction (EOR) with a low potential of 1.38 V vs. RHE (@10 mA cm−2). Meanwhile, it exhibits excellent hydrogen evolution reaction (HER) performance. The homemade CuCo2S4/CC//CuCo2S4/CC ethanol–water electrolyser only demands a voltage of 1.59 V to deliver 10 mA cm−2, 150 mV less than that used for ordinary water splitting. This shows that the ethanol–water electrolyser elaborated here holds encouraging potential in the energy-saving production of H2 and oxidation of ethanol into value-added acetic acid. This present work may open the way for the rational design of other electrocatalysts for efficient biomass oxidation reaction and relevant H2 production applications.

Heteroatom doping can effectively improve the electrochemical activity of carbon materials. In this paper, three kinds of fluorine-doped biomass carbon are synthesized using industrial hemp core as the precursor and Poly tetra fluoroethylene (PTFE), Diethylaminosulphur trifluoride (DAST) and Hydrofluoric acid (HF) as fluorinating agents, respectively. Among them, the PTFE-treated biomass carbon (PTFE-CHEMP) is wrapped by fluorine-containing nanotubes, which has the most defects, the most F doping amount and the optimum pore size, and is beneficial to the adsorption of K+. When used as the anode of potassium ion batteries (PIBs), the PTFE-CHEMP electrode can provide an average reversible capacity of 369.6 mAh g−1 (200 mA g−1) in 500 cycles. Even at a high current of 2000 mA g−1, it can still provide an excellent rate capability of 229.3 mAh g−1. Kinetic analysis verify that its excellent rate performance comes from the K+ storage mechanism dominated by surface-driven behavior. In addition, the constant current intermittent titration technique (GITT) shows that it still has the storage capacity for K+ at high potentials. This work not only provides a reference for subsequent research on fluorine-doped biomass carbon, but also provides a potential solution for the production of low-cost, high-capacity PIBs anodes.

Potassium ion hybrid capacitors have the advantages of high energy density, high power density, and low cost, and are expected to replace expensive lithium ion storage devices in large-scale applications. Here, an innovative nitrogen-rich biomass carbon (N-CHC) is prepared using urea as the etchant and hemp core as the raw material, which has a high specific surface area of 1185.3 m2 g−1 and nitrogen content of 8.56%. In addition, it shows a capacity of 442.4 mAh g−1 as a potassium ion half-cell anode at 30 mA g−1. Even under the high current test of 2000 mA g−1, N-CHC also shows excellent performance (175.0 mAh g−1). Further, the good rate performance is attributed to the large-scale capacitance control by analyzing reaction process kinetics. And the constant current intermittent titration technique (GITT) results show that N-CHC has a larger K+ diffusion coefficient than unetched biomass carbon. The maximum energy density of the full carbon-based hybrid capacitor composed of N-CHC anode and activated carbon cathode is 127.36 Wh kg−1, and the maximum power density is 2371 W kg−1.

A unique nanostructure electrode consisting of RuO 2 nanoparticles with ultra-fine diameter (1.9 nm) anchored on the surface of graphene nanosheets (GNS) and carbon nanotube (CNT) is prepared as a binder-free supercapacitor electrode through two-step electrochemical routes. At first, free-standing GNS and CNT (GC) are directly deposited on the surface of carbon fiber cloth (CFC) to form a cross-linked composite via a cathodic electrophoretic deposition method. Safranin, a kind of cationic organic dye, is introduced in this stage as a new-type dispersant to disperse GNS and CNT in water to form a suspension solution. After the following electro-deposition process, the as-prepared GC composite is uniform covered with RuO 2 nanoparticles. Benefiting from the combined advantages of GNS, CNT and ultra-fine RuO 2 nanoparticles in such a binder-free structure, the hybrid electrode exhibits a high specific capacitance up to 480.3 F g −1 (based on the total mass of GNS, CNT and RuO 2 ) and remarkable cycling stability (89.4% capacitance retention after 10000 cycles). Furthermore, the assembled symmetric supercapacitor exhibits a high energy density of 30.9 Wh kg −1 and power density of 14000 W kg −1 with excellent stability performance (92.7% capacitance retention after 10000 cycles). Thus, the remarkable performance of the resultant RuO 2 electrode has provided a rational design strategy for developing supercapacitors with high energy density.

Carbon materials, especially graphene nanosheets (GNS) and/or multi-walled carbon nanotube (MWNT), have been widely used as electrode materials for supercapacitor due to their advantages of higher specific surface area and electronic conductivity, but the relatively low specific capacitance thus results in low energy density hindering their large applications. On the contrary, MnO2 exhibits higher energy density but poor electrical conductivity. In order to obtain high performance supercapacitor electrode, here, combining the advantages of these materials, we have designed a facile two-step strategy to prepare 3D MnO2-GNS-MWNT-Ni foam (MnO2-GM-Ni) electrode. First, GNS and MWNT is wrapped on the surface of Ni foam (GM-Ni) via a “dip & dry” method by using an organic dye as a co-dispersant. Then, by using this 3D GM-Ni as substrate, MnO2 nanoflakes are in-situ supporting on the surface of GNS and MWNT through a hydrothermal reaction. The specific capacitances of MnO2-GM-Ni electrode reach as high as 470.5 F g−1 at 1 A g−1. Furthermore, we have successfully fabricated an asymmetric supercapacitor with MnO2-GM-Ni and GM-Ni as the positive and negative electrodes, respectively. The MnO2-GM-Ni//GM-Ni asymmetric supercapacitor exhibits a maximum energy density of 35.3 Wh kg−1 at a power density of 426 W kg−1 and also a favorable cycling performance that 83.8% capacitance retention after 5000 cycles. These results show manageable and high-performance which offer promising future for practical applications.

Recently, direct borohydride fuel cell (DBFC) has been considering as a promising energy conversion devices. During the development of DBFC, reducing the use of noble metals and increasing the anode performance are the hot topic in recent researches. In this article, reduced graphene oxide nanosheets deposit on C@TiC coaxial nanowire array (rGO-C@TiC) by means of a combine method of chemical vapor deposition and electrodeposition is chosen as 3D current collector for Pd nanoparticles deposition. The morphology and crystal structure of the as-obtained 3D electrode is checked with FESEM, TEM, EDS, and XRD. Results claim that the as-prepared 3D electrode exhibits a mushroom-like structure with the mean diameter size of Pd is 5.32 nm. Their catalytic ability for NaBH4 electro-oxidation is evaluated in a three electrode system by using the method of cycle voltammetry and chronoamperometry, proving that the 3D Pd-rGO-C@TiC electrode has a higher catalytic performance. The oxidation current density of 1.35 A cm−2 mg−1Pd is achieved at −0.6 V. Furthermore, a direct borohydride-hydrogen peroxide fuel cell (DBHPFC) is assembled by using the as-prepared Pd-rGO-C@TiC electrode and a Pd/CFC electrode as anode and cathode catalyst, respectively, and a maximum power density of 68.5 mW cm−2 is obtained. In addition, the assembled DBHPFC shows excellent higher performance based on the mass activity basis (1427.1 W g−1) among those reported literatures, indicating that our Pd-rGO-C@TiC could be acted as a promising cost-effective and ponderable alternative catalyst for NaBH4 electrooxidation.

Potassium-ion batteries (PIBs) have huge advantages in terms of price and resource abundance, and are ideal substitutes for lithium-ion batteries (LIBs), but there is currently little research on their high-temperature fields. In this work, the sulfur-doped biomass carbon (CHP/S) is synthesized from biomass hemp stalks as the carbon source and sulfur powder as the sulfur source, with a high sulfur content of 18.6%. Sulfur mainly exist in the form of bonded sulfur (S-C, S-O) and have quite favorable stability. As a PIBs anode, it exhibits a specific capacity of 589 mAh g−1 (30 mA g−1) at 60 °C, which is 1.5 times the capacity at 25 °C. Electrochemical analysis and kinetic analysis indicate that the increased capacity mainly comes from the “arousal effect” of sulfur under high temperature conditions, and an increased temperature has a positive effect on the absorption and desorption performance of the material. In addition, the full cell assembled with Perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) cathode shows greater capacity (~80 mAh g−1) at 60 °C than at 25 °C. Unfortunately, the electrolyte decomposition problem caused by high temperature is easily dangerous. The result is of great significance to the research of high-temperature PIBs.

A three-dimensional porous copper film is directly deposited on Cu foam by an electrodeposition method using hydrogen bubbles as dynamic template (denoted as Cu/Cu foam). Its electrocatalytic activity toward hydrazine electrooxidation is tested by linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. Compared with Cu foam, hydrazine electrooxidation on the Cu/Cu foam electrode shows that the onset oxidation potential displays a ~100 mV negative shift, the current density at −0.6 V raises about 14 times, the apparent activation energy and the charge transfer resistance reduce significantly. The increasing electrocatalytic performance for hydrazine electrooxidation is mainly caused by the highly porous structure of the Cu/Cu foam electrode which can provide a large surface area and make electrolyte access the electrocatalyst surfaces more easily. Hydrazine electrooxidation on the Cu/Cu foam electrode proceeds through a near 4-electron process.

A novel three-dimensional (3D) porous nickel-cobalt (Ni-Co) film on nickel foam is successfully prepared and further used as an efficient anode for direct urea-hydrogen peroxide fuel cell (DUHPFC). By varying the cobalt/nickel mole ratios into 0%, 20%, 50%, 80% and 100%, the optimized Ni-Co/Ni foam anode with a ratio of 80% is obtained in terms of the best cell performance among five anodes. Effects of the KOH and urea concentrations, the flow rate and operation temperature on the fuel cell performance are investigated. Results show DUHPFC with the 3D Ni-Co/Ni foam anode exhibits a higher performance than those reported direct urea fuel cells. The cell gives an open circuit voltage of 0.83 V and a peak power density as high as 17.4 and 31.5 mW cm−2 at 20 °C and 70 °C, respectively, when operating on 7.0 mol L−1 KOH and 0.5 mol L−1 urea as the fuel at a flow rate of 15 mL min−1. Besides, when the human urine is directly fed as the fuel, direct urine-hydrogen peroxide fuel cell reaches a maximum power density of 7.5 mW cm−2 with an open circuit voltage of 0.80 V at 20 °C, showing a good application prospect in wastewater treatment.

Nanoporous palladium supported on the carbon coated titanium carbide (C@TiC) nanowire arrays (Pd NP/C@TiC) are successfully prepared by a facile chemical vapor deposition of three-dimensional (3D) C@TiC substrate, followed by electrochemical codeposition of Pd–Ni and removal of Ni via dealloying. The structure and morphology of the obtained Pd NP/C@TiC electrodes are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) are used to examine the catalytic performances of the electrodes for H2O2 electroreduction in H2SO4 solution. The Pd NP/C@TiC electrode exhibits a largely effective specific surface area owing to its open nanoporous structure allowing the full utilization of Pd surface active sites. At the potential of 0.2 V in 2.0 mol L−1 H2O2 and 2 mol L−1 H2SO4 solutions, the reduction current density reaches 3.47 A mg−1, which is significantly higher than the catalytic activity of H2O2 electroreduction achieved previously with precious metals as catalysts.

To meet the ever-increasing need for high-efficiency energy storage in modern society, porous carbon materials with large surface areas are typically employed for electrical double-layer capacitors to achieve high gravimetric performances. However, their poor volumetric performances come from low packing density and/or high pore volume resulting in poor volumetric capacitance, which would limit their further applications. Here, a novel and one-step molten salt synthesis of a three-dimensional, densely nitrogen-doped porous carbon (NPC) material by using low-cost and eco-friendly tofu as the nitrogen-containing carbon source is proposed. Hierarchically porous carbon with a specific surface area of 1202 m2 g−1 and a high nitrogen content of 4.72 wt% and a bulk density of ∼0.84 g cm−3 is obtained at a carbonation temperature of 750 °C. As the electrode material for a supercapacitor, the NPC electrode shows both ultra-high specific volumetric and gravimetric capacitances of 360 F cm−3 and 418 F g−1 at 1 A g−1 (based on a three-electrode system), respectively, and excellent cycling stability without capacitance loss after 10 000 cycles at a high charge current of 10 A g−1 in KOH electrolyte. Moreover, the as-assembled symmetric supercapacitor exhibits not only an excellent cycling stability with 97% capacitance retention after 10 000 cycles, but also a high volumetric energy density up to 27.68 W h L−1 at a current density of 0.2 A g−1, making this new method highly promising for compact energy storage devices with simultaneous high volumetric/gravimetric energy and power densities.

Searching for anode materials with fast Li+ intercalation kinetics to mitigate the dynamic imbalance with the rapid capacitive cathode is the fundamental strategy to achieve high energy-power density of Li-ion capacitors (LICs). Hence, NiNb2O6 hollow nanoparticles are first synthesized by hydrothermal method and then sealed with a protective N-doped carbon shell to achieve the target NiNb2O6@NC composite. As the anode of LICs, the NiNb2O6@NC sample exhibits a high capacity of 435.8 mAh g−1 at 50 mA g−1 and excellent rate performance of 47.8% capacity retention with the current density increased to 2 A g−1. The lithium storage mechanism of NiNb2O6 is proposed as a combined conversion reaction and an intercalation reaction, which the NiNb2O6 decomposes into Ni metal and LixNb2O5 during the 1st discharge/charge process. The irreversible Ni would not contribute to the reversible capacity and will improve the electric conductivity for fast transport of electrons and the LixNb2O5 will act as the host for lithium ions by means of the intercalation mechanism. Furthermore, the assembled NiNb2O6@NC//activated carbon LICs exhibit a maximum energy density of 123.9 Wh kg−1 and the capacity retention reaches 86.1% after 5000 cycles. The superior electrochemical performances indicate that the rationally-designed NiNb2O6@NC is expected to be a potential anode material for LICs.

As a mainstream energy storage device, lithium-ion batteries have always been the focus of scientific researchers. As the most promising anode material, lithium metal has the advantages of high capacity and low reduction potential. Unfortunately, lithium dendrites can cause serious safety hazards during cycling. In this study, biomass carbon with a large number of straight holes was embedded with nickel particles as the anode framework. Nickel induces lithium to complete nucleation and growth in the pores and inhibits dendrite growth. Solid electrolyte interface (SEI) film growth, dead lithium, and electrode breakage all occur within the holes to a limited extent. Benefited from this internal reaction, the electrode obtains better electrochemical and cycling performances. The nickel-modified electrode was stably cycled for 1370 h at a current density of 5 mA/cm2 with a capacity of 2 mAh/cm2. Moreover, more massive capacity lithium storage (5 mAh/cm2) was tried, and the cycle was stable for 630 h. Using the same biomass carbon matrix, the pores were filled with sulfur powder as the positive electrode. After 100 periods of the full cell operation, the capacity retention rate is 85%.

The electrochemical reduction of dysprosium(III) was studied on W and Cu electrodes in eutectic LiCl–KCl by transient electrochemical methods. Cyclic voltammogram and current reversal chronopotentiogram results demonstrated that dysprosium(III) was directly reduced to dysprosium (0) on the W electrode through a single-step process with the transfer of three electrons. Electrochemical measurements on the Cu electrode showed that different Cu–Dy intermetallics are formed. Moreover, the thermodynamic properties of Cu–Dy intermetallic compounds were estimated by open circuit chronopotentiometry in a temperature range of 773–863 K. Using the linear polarization method, the exchange current density (j0) of dysprosium in eutectic LiCl–KCl on the Cu electrode was estimated, and the temperature dependence of j0 was studied to estimate the activation energies associated with Dy(III)/Cu5Dy and Dy(III)/Cu9/2Dy couples. In addition, potentiostatic electrolysis was conducted to extract dysprosium on the Cu electrode, and five Cu–Dy intermetallic compounds, CuDy, Cu2Dy, Cu9/2Dy, Cu5Dy and Cu0.99Dy0.01 were identified by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Meanwhile, the change of dysprosium(III) concentration was monitored using inductively coupled plasma-atomic emission spectrometry, and the maximum extraction efficiency of dysprosium was found to reach 99.2%.