The preparation and performance of calcium carbide-derived carbon/polyaniline composite electrode material for supercapacitors

Calcium carbide (CaC₂)-derived carbon (CCDC)/polyaniline (PANI) composite materials are prepared by in situ chemical oxidation polymerization of an aniline solution containing well-dispersed CCDC. The structure and morphology of CCDC/PANI composite are characterized by Fourier infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscopy (TEM) and N₂ sorption isotherms. It has been found that PANI was uniformly deposited on the surface and the inner pores of CCDC. The supercapacitive behaviors of the CCDC/PANI composite materials are investigated with cyclic voltammetry (CV), galvanostatic charge/discharge and cycle life measurements. The results show that the CCDC/PANI composite electrodes have higher specific capacitances than the as grown CCDC electrodes and higher stability than the conducting polymers. The capacitance of CCDC/PANI composite electrode is as high as 713.4 F g⁻¹ measured by cyclic voltammetry at 1 mV s⁻¹. Besides, the capacitance retention of coin supercapacitor remained 80.1% after 1000 cycles.     

Chemical processing of double-walled carbon nanotubes for enhanced hydrogen storage

Double-walled carbon nanotubes (DWCNTs) were modified for enhanced hydrogen storage by employing a combination of two techniques: KOH activation for the formation of defects on DWCNT surfaces and loading of the DWCNTs with nanocrystalline Pd. The physical properties of the pristine DWCNTs and chemically modified DWCNTs were systematically characterised by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and Brunauer–Emmett–Teller (BET) surface area measurements. The amounts of hydrogen storage capacity were measured at ambient temperature and found to be 1.7, 2.0, 3.7, and 2.8 wt% for pristine DWCNTS, 2 wt% Pd DWCNTs, activated DWCNTs, and 2 wt% Pd activated DWCNTs, respectively. Hydrogen molecules could be adsorbed on defect sites created by chemical activation in DWCNTs through van der Waals forces. For Pd nanoparticle loaded DWCNTs, H₂ molecules could be dissociated into atomic hydrogen and adsorbed on defect sites. We found that the hydrogen storage capacity of DWCNTs can be significantly enhanced by chemical activation or loading with Pd nanoparticles.     

Hydrogen storage properties of Mg–50 vol.%V7.4Zr7.4Ti7.4Ni composite prepared by spark plasma sintering

Hydrogen storage properties of Mg–50 vol.%V7.4Zr7.4Ti7.4Ni composite prepared by spark plasma sintering were investigated based on the PCT measurements, kinetics and DSC estimations and microstructure observations. The results showed that the composite consisted of Mg phase and V-based solid solution, with a small amount of sintering phase at their interface, and could absorb and desorb hydrogen at 303 K and 573 K, with a maximum hydrogen storage capacity of 3.05 wt.% and 2.55 wt.%, respectively. At 573 K it was found that the Mg phase was the basis for the hydrogen absorption/desorption, but with the combination of the V-based solid solution its kinetics was greatly improved, and its hydrogen desorption temperature decreased by about 117 K, which made it possible for hydrogen desorption of Mg phase at 573 K. Meanwhile the sintering phase was considered to be a key factor in improving hydriding properties of the Mg phase, which might act as a catalyst and offer preferable paths for hydrogen diffusion from V-based solid solution to the Mg phase.     

Hydrogen storage in binary and ternary Mg-based alloys: A comprehensive experimental study

This study focused on hydrogen sorption properties of 1.5 μm thick Mg-based films with Al, Fe and Ti as alloying elements. The binary alloys are used to establish as baseline case for the ternary Mg–Al–Ti, Mg–Fe–Ti and Mg–Al–Fe compositions. We show that the ternary alloys in particular display remarkable sorption behavior: at 200 °C the films are capable of absorbing 4–6 wt% hydrogen in seconds, and desorbing in minutes. Furthermore, this sorption behavior is stable over cycling for the Mg–Al–Ti and Mg–Fe–Ti alloys. Even after 100 absorption/desorption cycles, no degradation in capacity or kinetics is observed. For Mg–Al–Fe, the properties are clearly worse compared to the other ternary combinations. These differences are explained by considering the properties of all the different phases present during cycling in terms of their hydrogen affinity and catalytic activity. Based on these considerations, some general design principles for Mg-based hydrogen storage alloys are suggested.     

Carbon nanotube/polyaniline composite as anode material for microbial fuel cells

A carbon nanotube (CNT)/polyaniline (PANI) composite is evaluated as an anode material for high-power microbial fuel cells (MFCs). Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) are employed to characterize the chemical composition and morphology of plain PANI and the CNT/PANI composite. The electrocatalytic behaviour of the composite anode is investigated by means of electrochemical impedance spectroscopy (EIS) and discharge experiments. The current generation profile and constant current discharge curves of anodes made from plain PANI, 1 wt.% and 20 wt.% CNT in CNT–PANI composites reveal that the performance of the composite anodes is superior. The 20 wt.% CNT composite anode has the highest electrochemical activity and its maximum power density is 42 mW m⁻² with Escherichia coli as the microbial catalyst. In comparison with the reported performance of different anodes used in E. coli-based MFCs, the CNT/PANI composite anode is excellent and is promising for MFC applications.     

Effect of carbon addition on hydrogen storage behaviors of Li–Mg–B–H system

Various carbon additives were mechanically milled with LiBH₄/MgH₂ composite and their hydrogen storage behaviors were investigated. It was found that most of the carbon additives exhibited prominent effect on the host material. Among the various carbon additives, purified single-walled carbon nanotubes (SWNTs) exhibited the most prominent effect on the kinetic improvement and cyclic stability of Li–Mg–B–H system. Results show that LiBH₄/MgH₂ composite milled with 10 wt.% purified SWNTs additive can release nearly 10 wt.% hydrogen within 20 min at 450 °C, which is about two times faster than that of the neat LiBH₄/MgH₂ sample. On the basis of hydrogen storage behavior and structure/phase investigations, the possible mechanism involved in the property improvement upon carbon additives was discussed.     

Influence of charging parameters on the effectiveness of electrochemical hydrogen storage in activated carbon

Carbonaceous material, if it is to compete with metallic hydride alloys as a hydrogen storage electrode in a reversible chemical power source, should demonstrate 2 key qualities. Firstly, it should exhibit a high hydrogen elecrosorption. Secondly, it should co-operate efficiently with the cathode under particular charging and discharge conditions. Based on this assumption, an investigation into the influence of charging conditions on storage efficiency of a lignin based active carbon electrode with high hydrogen storage capacity was undertaken. Current densities of up to 32 A/g and charging times ranging from 196 seconds to 48 h were used. The results show that it is possible to charge the electrode rapidly even for tens of seconds using adequately high current density. However, full exploitation of charge storage capability of the carbon material (585 mA h/g in the tested material [the equivalent of storing 2.17 wt% in gas hydrogen]), required significant overcharge and, therefore, was only possible at a very low coulombic efficiency – below 2%. The acceptable coulombic efficiency of the charge/discharging process – 60%, could only be reached provided that less than 50% of the maximum material sorption capacity was utilized.     

A study on the hydrogen storage capacity of Ni-plated porous carbon nanofibers

In this study, we prepared highly porous carbon-nanofiber-supported nickel nanoparticles as a promising material for hydrogen storage. The porous carbons were activated at 1050 °C, and the nickel nanoparticles were loaded by an electroless metal-plating method. The textural properties of the porous carbon nanofibers were analyzed using N₂/77 K adsorption isotherms. The hydrogen storage capacity of the carbons was evaluated at 298 K and 100 bar. It was found that the amount of hydrogen stored was enhanced by increasing nickel content, showing 2.2 wt.% in the PCNF-Ni-40 sample (5.1 wt.% and 6.4% of nickel content and dispersion rate, respectively) owing to the effects of the spill-over of hydrogen molecules onto the metal–carbon interfaces. This result clearly indicates that the presence of highly dispersed nickel particles can enhance high-capacity hydrogen storage.     

Effects of different palladium content loading on the hydrogen storage capacity of double-walled carbon nanotubes

The effects of different amounts palladium loading on the hydrogen sorption characteristics of double-walled carbon nanotubes (DWCNTs) have been investigated. The physical properties of the pristine DWCNTs and Pd/DWCNTs were systematically characterized by X-ray diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller surface area measurements. Pd nanoparticles were loaded on DWCNT surfaces for the dissociation of H₂ into atomic hydrogen, which spills over to the defect sites on the DWCNTs. When we use different Pd content, the particle size and dispersion will be different, which affects the hydrogen storage capacity of the DWCNTs. In this work, the hydrogen storage capacities were measured at ambient temperature and found to be 1.7, 1.85, 3.0, and 2.0 wt% for pristine DWCNTS, 1.0 wt%Pd/DWCNTs, 2.0 wt%Pd/DWCNTs, and 3.0 wt%Pd/DWCNTs, respectively. We found that the hydrogen storage capacity can be enhanced by loading with Pd nanoparticles and selecting a suitable content. Furthermore, the sorption can be attributed to the chemical reaction between the atomic hydrogen and the dangling bonds of the DWCNTs.     

Chemisorption,physisorption and hysteresis during hydrogen storage in carbon nanotubes

The question of chemisorption versus physisorption during hydrogen storage in carbon nanotubes (CNTs) is addressed experimentally. We utilize a powerful measurement technique based on a magnetic suspension balance coupled with a residual gas analyzer, and report new data for hydrogen sorption at pressures of up to 100 bar at 25 °C. The measured sorption capacity is less than 0.2 wt.%, and there is hysteresis in the sorption isotherms when multi-walled CNTs are exposed to hydrogen after pretreatment at elevated temperatures. The cause of the hysteresis is then studied, and is shown to be due to a combination of weak sorption – physisorption – and strong sorption – chemisorption – in the CNTs. Analysis of the experimental data enables us to calculate separately the individual hydrogen physisorption and chemisorption isotherms in CNTs that, to our knowledge, are reported for the first time here. The maximum measured hydrogen physisorption and chemisorption are 0.13 wt.% and 0.058 wt.%, respectively.     

Effect of C (graphite) doping on the H2 sorption performance of the Mg–Ni storage system

Binary Mg–Ni mixtures and ternary Mg–Ni–C (graphite) samples with fixed proportions of metals (Mg 85%–Ni 15% by weight) and amount of C increasing in increments of 5 wt % from 5 wt % to 15 wt % were prepared by high energy ball milling (BM) in Ar for t_BM = 2 h. The purpose of the study was to evaluate the effect of C addition on the reactivity, the sorption activation and the storage performance of the Mg–Ni system.     

Characterization of Mg–20 wt% Ni–Y hydrogen storage composite prepared by reactive mechanical alloying

Mg–20 wt% Ni–Y composite was successfully prepared by reactive mechanical alloying (RMA). X-ray diffraction (XRD) measurement showed that both MgH₂ and Mg₂NiH₄ co-exist in the milled composite. The composite exhibits excellent hydrogen sorption kinetics and does not need activation on the first hydrogen storage process. It can absorb 3.92 and 5.59 wt% hydrogen under 3.0 MPa hydrogen pressure at 293 and 473 K in 10 min, respectively, and desorb 4.67wt% hydrogen at 523 K in 30 min under 0.02 MPa hydrogen pressure. The equilibrium desorption pressure of the composite are 0.142, 0.051 and 0.025 MPa at 573, 543 and 523 K, respectively. The differential scanning calorimetry (DSC) measurement showed that dehydrogenation of Mg–20 wt% Ni–Y composite was depressed about 100 K comparing to that of milled pure MgH₂. It is deduced that both the catalysis effect of Mg₂Ni and YH₃ distributed in Mg substrate and the crystal defects formed by RMA are the main reason for improving hydrogen sorption kinetics of the Mg–20 wt% Ni–Y composite.     

Enhanced hydrogen adsorption in ordered mesoporous carbon through clathrate formation

Ordered mesoporous carbons were synthesized with a soft-template approach and modified with a water and tetrahydrofuran mixture having a H₂O/THF molar ratio of 17:1 as potential adsorbent media for hydrogen storage. Hydrogen adsorption equilibrium on the carbon adsorbents was measured gravimetrically at 270 K and hydrogen pressures up to 163 bar. Enhanced hydrogen adsorption was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF due to the combined effects of hydrogen adsorption on the carbon surface and formation of a binary H₂–H₂O–THF clathrate. Hydrogen adsorption capacities on the carbon adsorbents doped with 0.5 wt.%, 0.75 wt.% of H₂O/THF, and the pure carbon at 270 K and 163 bar are 0.747 wt.%, 0.646 wt.% and 0.585 wt.%, respectively. The hydrogen adsorption isotherms on all the doped carbon adsorbents are of typical Type III and can be well correlated by the Freundlich equation. A desorption hysteresis loop was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF, which was probably caused by the pore size difference during the adsorption and desorption steps.     

Processing analysis of the ternary LiNH2–MgH2–LiBH4 system for hydrogen storage

In this article, we investigate the ternary LiNH₂–MgH₂–LiBH₄ hydrogen storage system by adopting various processing reaction pathways. The stoichiometric ratio of LiNH₂:MgH₂:LiBH₄ is kept constant with a 2:1:1 molar ratio. All samples are prepared using solid-state mechano-chemical synthesis with a constant rotational speed, but with varying milling duration. Furthermore, the order of addition of parent compounds as well as the crystallite size of MgH₂ are varied before milling. All samples are intimate mixtures of Li–B–N–H quaternary hydride phase with MgH₂, as evidenced by XRD and FTIR measurements. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150 °C. Furthermore, it is observed that the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm. The as-synthesized hydrides exhibit two main hydrogen release temperatures, one around 160 °C and the other around 300 °C. The main hydrogen release temperature is reduced from 310 °C to 270 °C, while hydrogen is first reversibly released at temperatures as low as 150 °C with a total hydrogen capacity of ∼6 wt.%. Detailed thermal, capacity, structural and microstructural properties are discussed and correlated with the activation energies of these materials.     

Hydrogen storage of the Mg–C composites

The effect of different kinds of carbon on the hydrogen sorption kinetics by magnesium–carbon composites was analyzed. To prepare magnesium-based composites by ball milling, graphite and carbon nanomaterials (hereinafter CNM) obtained by the electroexplosion technique were used. Phase composition and structure state of the as-milled and hydrogenated magnesium–carbon and magnesium–nickel–carbon composites have been investigated. It was found the crystallite size in the Mg–CNM composite is smaller in comparison with the magnesium–graphite and magnesium–graphite–nickel mixtures. The CNM additives to magnesium essentially improve the hydrogen sorption kinetics. It results in a reduction of hydrogen sorption temperature. The noticeable hydrogen absorption took place already at a temperature of 363 K. The hydrogen capacity was about 5 wt% for magnesium ball milled with CNM additives.     

Equilibrium,kinetics and enthalpy of hydrogen adsorption in MOF-177

Metal–organic framework (MOF-177) was synthesized, characterized and evaluated for hydrogen adsorption as a potential adsorbent for hydrogen storage. The hydrogen adsorption equilibrium and kinetic data were measured in a volumetric unit at low pressure and in a magnetic suspension balance at hydrogen pressure up to 100 bar. The MOF-177 adsorbent was characterized with nitrogen adsorption for pore textural properties, scanning electron microscopy for morphology and particle size, and X-ray powder diffraction for phase structure. The MOF-177 synthesized in this work was found to have a uniform pore size distribution with median pore size of 12.7 Å, a higher specific surface area (Langmuir: 5994 m²/g; BET: 3275 m²/g), and a higher hydrogen adsorption capacity (11.0 wt.% excess adsorption, 19.67 wt.% absolute adsorption) than previously reported values on MOF-177. Freundlich equation fits well the hydrogen adsorption isotherms at low and high pressures. Diffusivity and isosteric heat of hydrogen adsorption were estimated from the hydrogen adsorption kinetics and equilibrium data measured in this work.     

Hydrogen storage in rapidly solidified and crystallized Mg–Ni-(Y,La)–Pd alloys

Amorphous-crystalline composite ribbons of quaternary Mg–Ni–(Y,La)–Pd alloys are produced via rapidly solidification and used as precursors for creating nanocrystalline hydrogen storage materials. The resulting materials demonstrate relatively high hydrogen capacity of around 4.5 mass% H and excellent absorption/desorption kinetics at 573 K. Additionally, the alloys demonstrate reversible hydrogen storage at 473 K. A composition of Mg₈₅Ni₁₀Y_2.5Pd_2.5 fully absorbs and desorbs 4.6 mass% H in 90 min. The cyclability of the quaternary alloys demonstrates good stability, with little loss in maximum capacity through 8–10 cycles. This has been attributed to the improved stability of the nanocrystalline structure attained via the Y and La additions. Thermodynamically, the enthalpy of the hydrogen absorption reaction is reduced by 5 kJ/mol in the quaternary alloys, compared to Mg-MgH₂; while the entropy of reaction is also reduced.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Hydrogenation and degradation of Mg–10 wt% Ni alloy after cyclic hydriding–dehydriding

Hydrogenation and degradation properties of Mg–10 wt% Ni hydrogen storage alloys were investigated by cyclic hydriding–dehydriding tests. Mg–10 wt% Ni alloy was synthesized by rotation-cylinder method (RCM) under 0.3% HFC-134a/air atmosphere and their hydrogenation and degradation properties were evaluated by pressure-composition-isotherm (PCI) measurement. Hydrogen storage capacities gradually increased following 160 hydriding–dehydriding cycles and thereafter started to decrease. Measured maximum hydrogen capacity of Mg–10 wt% Ni alloy is 6.97 wt% at 623 K. Hydriding and dehydriding plateau pressure were kept constant for whole cycles. Reversible hydrogen capacity started to descend after 280 hydriding–dehydriding cycles. The lamellar eutectic structure of Mg–Ni alloy consists of Mg-rich α$\alpha$-phase and β$\beta$-_Mg2Ni${\mathrm{Mg}}_2\mathrm{Ni}$. It is assumed that the lamellar eutectic structure enhances hydrogenation properties.     

Effect of polyaniline on hydrogen absorption–desorption properties and discharge capacity of AB3 alloy

A series of AB₃/PANI composites were prepared by adding polyaniline (PANI) into La_0.7Mg_0.25Ti_0.05Ni_2.975Co_0.525 (AB₃) hydrogen storage alloy, which was prepared by magnetic levitation melting, and the composites were investigated by XRD and SEM. The effects of PANI concentration on the hydrogen absorption–desorption properties and discharge capacities of AB₃ alloy were systematically studied by P–C–T isotherms and LAND battery test system, respectively. The results indicated that the addition of PANI did not change the hydrogen absorption capacity of AB₃ alloy distinctly, while the hydrogen desorption plateau pressure of AB₃ alloy decreased firstly, and then increased with the increase in the PANI concentration, the minimum plateau pressures of 0.022, 0.1, 0.321 and 0.55 MPa were obtained with PANI concentration of 2 wt% at 25, 50, 80 and 100 °C, respectively. It was theoretically verified by the changes in enthalpy and entropy of AB₃/PANI hydrides dehydrogenation which were calculated by Van’t Hoff equation. In the present paper, the phenomenon that PANI improved the hydrogen absorption kinetics of AB₃ alloy was found; the influence of PANI concentration on hydrogen absorption kinetics of AB₃ alloy was more apparent at higher temperature. The activation energies of dissolved hydrogen in AB₃/PANI hydrides were calculated from hydrogen absorption kinetics and the Arrhenius equation. LAND experiments demonstrated that, the AB₃/PANI electrodes composites possessed higher cycling discharge capacity retention rates than AB₃ electrode alloy.