Boosting the capacitance of NiCo2O4 hierarchical structures on nickel foam in supercapacitors

A facile method of directly growing NiCo₂O₄ hybrid hierarchical nanostructures on nickel foam is developed by a hydrothermal and post heat-treatment method without using any surfactant, stabilizer or organic binder. Due to the rich porous nanostructures, relative large specific surface area (177.71 m² g^?1) of the NiCo₂O₄ hybrid structure and efficient electrical contact with the conductive nickel substrate, the NiCo₂O₄NF hybrid electrode shows significantly enhanced specific capacitance (3105.1 F g^?1 at 1 A g^?1), outstanding rate properties (1621.3 F g^?1 at 20 A g^?1 and 1191.5 F g^?1 at 50 A g^?1) and high energy density (95.26 Wh kg^?1). This facile and effective design method opens up new possibilities for producing binder-free electrodes in high-performance electrochemical supercapacitors and miniaturized devices.     

One step synthesis of rGO-Ni3S2 nano-cubes composite for high-performance supercapacitor electrodes

In the last decade, supercapacitors possessing high power density and cyclic stability have attracted great interests in various applications. Graphene-based composite electrodes are known as a promising candidate for supercapacitors due to synergistic effects. For the first time, in this work, we develop a simple one-step hydrothermal synthesis of graphene wrapped Ni₃S₂ nanocubes (rGO-Ni₃S₂) composite for high-performance and low-cost supercapacitor electrodes. The rGO-Ni₃S₂ electrode exhibits an ultrahigh specific capacity of 616 C g^?1 at the current density of 1 A g^?1 with excellent cycling durability of 92.7% after 5000 cycles, which is much better when compared with the counterpart without graphene (pure Ni₃S₂). We attribute the remarkable performance of the rGO-Ni₃S₂ electrode to the synergistic effects of the graphene as the conductive support and Ni₃S₂ cubics as the pseudocapacitive material. This work constitutes a step forward towards the development of low-cost and high-performance supercapacitors for the next generation of portable electronics.     

Reversible hydrogen sorption and kinetics of hydrogen storage tank based on MgH2 modified by TiF4 and activated carbon

By doping with 5 wt % TiF₄ and activated carbon (AC), onset and main dehydrogenation temperatures of MgH₂ significantly reduce (ΔT = 138 and 109 °C, respectively) with hydrogen capacity of 4.4 wt % H₂. Up-scaling to storage tank begins with packing volume and sample weight of 28.8 mL and ～14.5 g, respectively, and continues to 92.6 mL and ～60.5–67 g, respectively. Detailed hydrogen sorption mechanisms and kinetics of the tank tightly packed with four beds of MgH₂TiF₄-AC (～60.5 g) are investigated. De/rehydrogenation mechanisms are detected by three temperature sensors located at different positions along the tank radius, while hydrogen permeability is benefited by stainless steel mesh sheets and tube inserted in the hydride beds. Fast desorption kinetics of MgH₂TiF₄-AC tank at ～275–283 °C, approaching to onset dehydrogenation temperature of the powder sample (272 °C) suggests comparable performances of laboratory and tank scales. Hydrogen desorption (T = 300 °C and P(H₂) = 1 bar) and absorption (T = 250 °C and P(H₂) = 10–15 bar) of MgH₂TiF₄-AC tank provide gravimetric and volumetric capacities during the 1^st-2nd cycles of 4.46 wt % H₂ and 28 gH₂/L, respectively, while those during the 3^rd-15th cycles are up to 3.62 wt % H₂ and 23 gH₂/L, respectively. Due to homogeneous heat transfer along the tank radius, de/rehydrogenation kinetics superior at the tank center and degrading forward the tank wall can be due to poor hydrogen permeability. Particle sintering and/or agglomeration upon cycling yield deficient hydrogen content reproduced.     

Performance and durability of an anode-supported solid oxide fuel cell with a PdO/ZrO2 engineered (La0.8Sr0.2)0.95MnO3-δ-(Y2O3)0.08(ZrO2)0.92 composite cathode

PdO/ZrO₂ co-infiltrated (La_0.8Sr_0.2)_0.95MnO_3-δ-(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM-YSZ) composite cathode (PdO/ZrO₂+LSM-YSZ), which adsorbs more oxygen than equal amount of PdO/ZrO₂ and LSM-YSZ, is developed and used in Ni-YSZ anode-supported cells with YSZ electrolyte. The cells are investigated firstly at temperatures between 650 and 750 °C with H₂ as the fuel and air as the oxidant and then polarized at 750 °C under 400 mA cm^?2 for up to 235 h. The initial peak power density of the cell is in the range of 438–1207 mW cm^?2 at temperatures from 650 to 750 °C, corresponding to polarization resistance from 1.04 to 0.35 Ω cm². This result demonstrates a significant performance improvement over the cells with other kinds of LSM based cathode. The cell voltage at 750 °C under 400 mA cm^?2 decreases from initial 0.951 to 0.89 V after 170 h of current polarization and remains essentially stable to the end of current polarization. It is identified that the self-limited growth of PdO particles is responsible for the cell voltage decrease by reducing the length of triple phase boundary affecting the high frequency steps involved in oxygen reduction reaction in the cathode.     

Nanoporous (Pt1-xCox)3Al intermetallic compound as a high-performance catalyst for oxygen reduction reaction

Nanocatalysts that boost the sluggish kinetics of oxygen reduction reaction with a long-term durability are crucial for widespread use of low-temperature fuel cells. Here we report a nanoporous intermetallic compound typically composed of platinum–cobalt–aluminum intermetallic core with in-situ grown atomic-layer-thick Pt skin as a novel oxygen-reduction-reaction nanocatalyst with remarkably enhanced performance. Both Pt and Co atoms thermodynamically prefer to locate nearby Al element within face-centered cubic Pt₃Al matrix via the formation of strong PtAl and CoAl bonds, which not only enable synergistic ligand and compressive strain effects to moderately weaken the oxygen adsorption energy of Pt skin, but alleviate the evolution of surface Pt atoms to protect against the further dissolution of less-noble Co and Al. As a result, the nanoporous platinum–cobalt–aluminum nanocatalyst exhibits specific activity of 3.40 mA cm^?2_Pt and mass activity of 2.2 A mg^?1_Pt for the oxygen reduction reaction at 0.9 V versus reversible hydrogen electrode (～13- and ～20-fold enhancement relative to commercially available platinum nanoparticles supported carbon) with an exceptional durability, showing genuine potential as cathode catalyst in next-generation electrochemical energy conversion devices.     

Nitrogen doped mesoporous carbon supported Pt electrocatalyst for oxygen reduction reaction in proton exchange membrane fuel cells

Nitrogen doped mesoporous carbons are employed as supports for efficient electrocatalysts for oxygen reduction reaction. Heteroatom doped carbons favour the adsorption and reduction of molecular oxygen on Pt sites. In the present work, nitrogen doped mesoporous carbons (NMCs) with variable nitrogen content were synthesized via colloidal silica assisted sol-gel process with Ludox-AS40 (40 wt% SiO₂) as hard template using melamine and phenol as nitrogen and carbon precursors, respectively. The NMC were used as supports to prepare Pt/NMC electrocatalysts. The physicochemical properties of these materials were studied by SEM, TEM, XRD, BET, TGA, Raman, XPS and FTIR. The surface areas of 11 wt% (NMC-1) and 6 wt% (NMC-2) nitrogen doped mesoporous carbons are 609 m² g^?1 and 736 m² g^?1, respectively. The estimated electrochemical surface areas for Pt/NMC-1 and Pt/NMC-2 are 73 m² g^?1 and 59 m² g^?1, respectively. It is found that Pt/NMC-1 has higher ORR activity with higher limiting current and 44 mV positive onset potential shift compared to Pt/NMC-2. Further, the fuel cell assembled with Pt/NMC-1 as cathode catalyst delivered 1.8 times higher power density than Pt/NMC-2. It is proposed that higher nitrogen content and large pyridinic nitrogen sites present in NMC-1 support are responsible for higher ORR activity of Pt/NMC-1 and high power density of the fuel cell using Pt/NMC-1 cathode electrocatalyst. The carbon support material with high pyridinic content promotes the Pt dispersion with particle size less than 2 nm.     

Electrochemical profile of an asymmetric supercapacitor using carbon-coated LiTi2(PO4)3 and active carbon electrodes

In this work, we reported an asymmetric supercapacitor in which active carbon (AC) was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode in 1 M Li₂SO₄ aqueous electrolyte. The LiTi₂(PO₄)₃/AC hybrid supercapacitor showed a sloping voltage profile from 0.3 to 1.5 V, at an average voltage near 0.9 V, and delivered a capacity of 30 mAh g⁻¹ and an energy density of 27 Wh kg⁻¹ based on the total weight of the active electrode materials. It exhibited a desirable profile and maintained over 85% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibited an excellent rate capability, even at a power density of 1000 W kg⁻¹, it had a specific energy 15 Wh kg⁻¹ compared with 24 Wh kg⁻¹ at the power density about 200 W kg⁻¹.     

Characteristics of La0.8Sr0.2Ga0.8Mg0.2O3-δ-supported micro-tubular solid oxide fuel cells with LaCo0.4Ni0.6-xCuxO3-δ cathodes

In this study, micro-tubular solid oxide fuel cells (T-SOFCs) with extruded La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) electrolyte as the mechanical support and LaCo_0.4Ni_0.6O_3-δ (LCNO) or LaCo_0.4Ni_0.4Cu_0.2O_3-δ (LCNCO) as cathodes were prepared and characterized. Partial substitution of Cu for the Ni-ion positions in the LCNO lattices was found to significantly enhance the densification and accelerate the grain growth. The porosity-corrected electrical conductivity was significantly increased from 1275 S/cm for LCNO ceramic to 1537 S/cm for LCNCO ceramic, because the acceptor doping was compensated by the formation of hole carriers that produced additional polarons and significantly augmented the electrical conductivity. SOFCs with three configurations were built in this study, including Cell A that had a lanthanum-doped ceria (LDC) buffer layer inserted between the LSGM electrolyte and the LCNCO cathode, Cell B that used an LCNO-LSGM composite cathode, and Cell C that featured an LCNCO-LSGM composite cathode. Among the three cells, Cell C with 263 μm of LSGM electrolyte possessed the lowest Ohmic resistance of 0.89 Ω cm², a polarization resistance of 0.69 Ω cm², and the highest maximum power density of 178 mW cm^?2 at 750 °C.     

Bismuth doped TiO2 as an excellent photocathode catalyst to enhance the performance of microbial fuel cell

Bismuth impregnation on pure TiO₂ (BiTiO₂) was carried out and tested in microbial fuel cell (MFC) as photocathode catalyst. UV–Visible spectral observation confirmed higher catalytic activity of BiTiO₂ under visible light irradiation with reduced band gap of 2.80 eV as compared to pure TiO₂ (3.26 eV). Electrochemical impedance spectroscopy also showed two times higher exchange current density with lower charge transfer resistance for BiTiO₂ (1.90 Ω) than pure TiO₂ (3.95 Ω), thus confirming it as superior oxygen reduction reaction catalyst. MFC operated with BiTiO₂ could generate a maximum power density of 224 mW m^?2, which was higher than MFC with Pt as cathode catalyst (194 mW m^?2) and much higher than MFCs with TiO₂ catalyzed cathode (68 mW m^?2) and without any cathode catalyst (60 mW m^?2). The results thus promote Bi doped TiO₂ as a superior low-cost alternative to the costly Pt catalyst to take this MFC technology forward for field application.     

Synthesis of bimetallic iron ferrite Co0.5Zn0.5Fe2O4 as a superior catalyst for oxygen reduction reaction to replace noble metal catalysts in microbial fuel cell

A low-cost electrochemically active oxygen reduction reaction (ORR) catalyst is obligatory for making microbial fuel cells (MFCs) sustainable and economically viable. In this endeavour, a highly active surface modified ferrite, with Co and Zn bimetal in the ratio of 1:1 (w/w), Co_0.5Zn_0.5Fe₂O₄ was synthesised using simple sol-gel auto combustion method. Physical characterisation methods revealed a successful synthesis of nano-scaled Co_0.5Zn_0.5Fe₂O₄. For determination of ORR kinetics of cathode, using Co_0.5Zn_0.5Fe₂O₄ catalyst, electrochemical studies viz. cyclic voltammetry and electrochemical impedance spectroscopy were conducted, which demonstrated excellent reduction current response with less charge transfer resistance. These electrochemical properties were observed to be comparable with the results obtained for cathode using 10% Pt/C as a catalyst on the cathode. The MFC using Co_0.5Zn_0.5Fe₂O₄ catalysed cathode could produce a maximum power density of 21.3 ± 0.5 W/m³ (176.9 ± 4.2 mW/m²) with a coulombic efficiency of 43.3%, which was found to be substantially higher than MFC using no catalyst on the cathode 1.8 ± 0.2 W/m³ (15.2 ± 1.3 mW/m²). Also, the specific power recovery per unit cost for MFC with Co_0.5Zn_0.5Fe₂O₄ catalysed cathode was found to be 4 times higher as compared to Pt/C based MFC. This exceptionally low-cost cathode catalyst has enough merit to replace costly cathode catalyst, like platinum, for scaling up of the MFCs.     

High energy density PbO2/activated carbon asymmetric electrochemical capacitor based on lead dioxide electrode with three-dimensional porous titanium substrate

A PbO₂/AC asymmetric electrochemical capacitor (AEC) with energy density as high as 49.4 Wh kg⁻¹, power density of 433.2 W kg⁻¹ and specific capacitance of 135.2 F g⁻¹ was fabricated with PbO₂ electrodeposited on three-dimensional porous titanium (3D-Ti/PbO₂) and activated carbon. The high electrochemical active surface of 3D-Ti/PbO₂ resulted in high specific capacity making it suitable for use as positive electrode in PbO₂/AC AEC. The fabricated AEC demonstrated good power performance with an energy density conservation of 30 Wh kg⁻¹ at power density of 2078 W kg⁻¹. The fabricated AEC also showed excellent cycling stability with capacitance retention of 99.2% after 1000 cycles.     

Porous Co3O4 decorated nitrogen-doped graphene electrocatalysts for efficient bioelectricity generation in MFCs

High-performance, low-cost, and robust oxygen reduction reaction (ORR) catalysts have played a very crucial role in the development of microbial fuel cells (MFCs). Herein, A novel in-situ Co₃O₄ nanoparticles (NPs) modified nitrogen-doped graphene with three-dimensional porous structure (3D GN-Co₃O₄) has been successfully synthesized and employed as an efficient ORR catalyst in MFCs. Benefiting from 3D porous architecture feature, highly intrinsic conductivity and synergistic effect between nitrogen-doped graphene and Co₃O₄ NPs, the 3D GN-Co₃O₄ as a cathode catalyst in alkaline condition realizes significantly enhanced electrochemical performance and outstanding cycling stability. Furthermore, the self-assembly of MFCs based on the 3D GN-Co₃O₄ cathode offers a high power density of 578 ± 10 mW m^?2, which is even comparable to the commercial Pt/C.     

Carbon supported nickel-phthalocyanine/MnOx as novel cathode catalyst for microbial fuel cell application

Performance of microbial fuel cells (MFCs) with carbon supported nickel phthalocyanine (NiPc)MnO_x composite (MFC-1) and nickel phthalocyanine (MFC-2) incorporated cathode was compared with a control MFC with non-catalysed carbon felt as cathode (MFC-3) and MFC-4 having Pt on cathode (as benchmark reference control). MFC-1 exhibited power density of 8.02 Wm^?3, which was four folds higher than control MFC-3 (2.08 Wm^?3) and 1.14 times higher than MFC-2 (6.97 Wm^?3). Coulombic efficiency of 30.3% obtained in MFC-1 was almost double of that obtained for control MFC-3 and it was 5.4% lesser as compared to MFC-4 (35.7%). Linear sweep voltammetry study of cathodes revealed that NiPc-MnO_x could enhance the electrocatalytic activity of oxygen reduction reaction (ORR) in comparison to control cathode. However, the power recovery from MFC-1 was noted little lower than what obtained from MFC-4 (10.58 Wm^?3), however the cost normalized power was two times higher than Pt catalyst on cathode. Thus, NiPc-MnO_x based catalyst developed in this study has potential to enhance ORR in cathodes of MFCs in order to harvest more power.     

Boosting the energy storage performance of cobalt molybdate microspheres constructed from urotropin-induced ultrathin nanosheets

The CoMoO₄ microspheres constructed from ultrathin nanosheets were synthesized by means of a simple chemical precipitation using urotropin (C₆H₁₂N₄) as a soft template, which were used as the electrode materials for supercapacitors. It was found that the CoMoO₄ sample with 3 mmol of added urotropin (vs 1 mmol CoCl₂) exhibits the uppermost specific capacity value of 89 mAh/g at a current density of 0.25 A/g due to its unique mesoporous structure and large specific surface area. In addition, a hybrid supercapacitor was established by the CoMoO₄ microspheres (positive electrode) and activated carbon (AC, negative electrode). The obtained CoMoO₄//AC hybrid supercapacitor possesses a specific energy value of 34 Wh/kg at 375 W/kg. Most importantly, this device owns excellent cycle stability with no capacity decay observed after 10 000 cycles. These good electrochemical properties of CoMoO₄ microspheres are promising in the practical use fields.     

Single-step hydrothermal synthesis of WO3-MnO2 composite as an active material for all-solid-state flexible asymmetric supercapacitor

In present study, new strategy is employed to build composite nanostructure and asymmetric configuration to enhance the capacitive performance of supercapacitor device. The WO₃-MnO₂ composite with mesoporous structure is prepared by single-step hydrothermal method and used to gain superior electrochemical performance in asymmetric configuration. A binder-free and additive-less WO₃-MnO₂ composite electrode exhibits high specific capacitance of 609 F g^?1 at a scan rate of 5 mV s^?1. The flexible asymmetric supercapacitor device with WO₃-MnO₂ as a positive electrode and WO₃ as a negative electrode demonstrates stable operating potential window of 1.4 V with specific capacitance of 103 F g^?1 at a scan rate of 5 mV s^?1 and energy density of 24.13 Wh kg^?1 at power density of 915 W kg^?1. Furthermore, WO₃-MnO₂//WO₃ device exhibits good cycle life with capacity retention of 95% after 2500 cycles and excellent mechanical flexibility. These results reveal the potential of WO₃-MnO₂ composite electrode for fabrication of high-performance supercapacitors.     

Sulfur/alumina/polypyrrole ternary hybrid material as cathode for lithium-sulfur batteries

Recently, lithium-sulfur batteries (LSBs) have received extensive attention due to its high energy density of 2600 Wh kg^?1. At the same time, sulfur is earth-abundant, economical and non-poisonous. Nevertheless, the poor electrochemical performance restricts its commercial application, including the inferior cycling stability caused by the significant dissolution of lithium polysulfides and the low specific capacity because of the poor electrical conductivity of sulfur. In this work, we adopt a simple and amicable process to prepare sulfur/alumina/polypyrrole (S/Al₂O₃/PPy) ternary hybrid material to overcome these defects. In this strategy, each composition of the ternary hybrid material plays an essential role in cathode: alumina and PPy can provide strong adsorption for the dissolved intermediate polysulfides. Meanwhile, PPy also works as a conductive and flexible additive to expedite electron transport, and is coated on the surface of the as-prepared SAl₂O₃ composite by in situ chemical polymerization. The sulfur is encapsulated uniformly and perfectively by the two components, which is confirmed by field emission scanning electron microscope. The ternary hybrid material manifests good electrochemical performance as expected, and displays high initial discharge capacity of 1088 mA h g^?1 and a discharge capacity of 730 mA h g^?1 after 100 cycles at a current density of 200 mA g^?1. Besides, S/Al₂O₃/PPy also shows good rate capability. The synergy between alumina and PPy is the decisive factor, which gives rise to good electrochemical performance of cathode for high-performance LSBs.     

Supercapacitive properties of activated carbon electrode using ammonium based proton conducting electrolytes

In this study, we demonstrated the usefulness of proton conducting electrolytes (such as ammonium thiocyanate (NH₄SCN) and ammonium nitrate (NH₄NO₃)) for electrochemical energy storage devices using activated carbon (AC) as the electrode material. The cyclic voltammetry analysis revealed the presence of rectangular shaped cyclic voltammograms indicating the presence of electrical double layer capacitance in AC electrode using NH₄SCN and NH₄NO₃ electrolytes. The mechanism of charge-storage in AC electrode using the proton conducting electrolytes has been studied in detail using electrochemical impedance spectroscopy (Nyquist and Bode plots). The galvanostatic charge-discharge analysis revealed that a maximum specific capacitance of AC electrode using NH₄SCN and NH₄NO₃ electrolytes was found to be 136.75 mF cm^?2 and 113.38 mF cm^?2 at a current density of 0.5 mA cm^?2. This study would open a new avenue for the use of ammonium based proton conducting electrolytes for supercapacitor applications.     

Energy and exergy analyses of hybrid photocatalytic hydrogen production reactor for CuCl cycle

The present paper concerns electrochemical, energy, exergy and exergoeconomic analyses of a hybrid photocatalytic-based hydrogen production reactor which is capable of replacing the electrolysis sub-system of the CuCl thermochemical cycle. Several operating parameters, such as current density, reactor temperature, ambient temperature and electrode distance, are varied to study their effects on the hydrogen production rate, the cost of hydrogen production and energy and exergy efficiencies. The present results show that the voltage drops across the anolyte solution (sol 1), catholyte solution (sol 2), an anode, cathode, and cation exchange membrane vary from 0.005 to 0.016 V, 0.004–0.013 V, 1.67–2.168 V, 0.18–0.22 V and 0.06–0.19 V, respectively with an increase in current density from 0.5 to 1.5 A/cm². The energy and exergy efficiencies of the hybrid photocatalytic hydrogen production reactor decrease from 5.74 to 4.54% and 5.11 to 4.04%, respectively with an increase in current density.     

The synthesis of ZnO/SrTiO3 composite for high-efficiency photocatalytic hydrogen and electricity conversion

Heterogeneous ZnO–SrTiO₃ nanocomposites were synthesized via a facile hydrothermal method. The highest H₂ production rate, 1317.44 μmol g^?1 within 5 h under solar-light irradiation was achieved for the Zn/Sr ratio of 9:1 of the ZnO–SrTiO₃. More interestingly though the ZnO–SrTiO₃ is a semiconducting system, it could be used as an electrolyte in the low temperature solid oxide fuel cell without electronic conducting short circuiting problem. This device displayed an open circuit voltage of 1.14 V and reached the maximum power density of 564 mW cm^?2 at 550 °C. These results are attributed to the enhanced separation of the charge-hole pairs by the heterogeneous structure of ZnO–SrTiO₃ nanocomposites, which possess obvious both heterogeneously ionic and semiconduction. This new discovery indicates a good promising candidate for both solar and hydrogen energy conversions.     

Dual functions of activated carbon in a positive electrode for MnO2-based hybrid supercapacitor

Utilizing the dual functions of activated carbon (AC) both as a conductive agent and an active substance of a positive electrode, a hybrid supercapacitor (AC-MnO₂&AC) with a composite of manganese dioxide (MnO₂) and activated carbon as the positive electrode (MnO₂&AC) and AC as the negative electrode is fabricated, which integrates approximate symmetric and asymmetric behaviors in the distinct parts of 2 V operating windows. MnO₂ in the positive electrode and AC in the negative electrode together form a pure asymmetric structure, which extends the operating voltage to 2 V due to the compensatory effect of opposite over-potentials. In the range of 0-1.1 V, both AC in the positive and negative electrode assemble as a symmetric structure via a parallel connection which offers more capacitance and less internal resistance. The optimal mass proportions of electrodes are calculated though a mathematical process. In a stable operating window of 2 V, the capacitance of AC-MnO₂&AC can reach 33.2 F g⁻¹. After 2500 cycles, maximum energy density is 18.2 Wh kg⁻¹ with a 4% loss compared to the initial cycle. The power density is 10.1 kW kg⁻¹ with an 8% loss.